Pd(II), Pd(III) and Pd(IV) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 44 Years (1980–2023) of NMR and Single Crystal X-ray Studies

Pazderski, Leszek; Abramov, Pavel A.

doi:10.3390/cryst13101482

Open AccessReview

Pd(II), Pd(III) and Pd(IV) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 44 Years (1980–2023) of NMR and Single Crystal X-ray Studies

by

Leszek Pazderski

^1,*

and

Pavel A. Abramov

²

¹

Faculty of Chemistry, Nicolaus Copernicus University, 7 Gagarina, 87100 Toruń, Poland

²

Nikolaev Institute of Inorganic Chemistry, Russian Academy of Sciences, 3 Acad. Lavrentiev Ave., Novosibirsk 630090, Russia

^*

Author to whom correspondence should be addressed.

Crystals 2023, 13(10), 1482; https://doi.org/10.3390/cryst13101482

Submission received: 1 August 2023 / Revised: 22 September 2023 / Accepted: 27 September 2023 / Published: 11 October 2023

(This article belongs to the Special Issue Cyclometallation of Platinum Group Elements: Between Complexes and Organometallics)

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Abstract

:

In this paper, a review on Pd(II), Pd(III), and Pd(IV) cyclometallated compounds with 2-arylpyridines (2-phenylpyridine, 2-benzylpyridine, 2-benzoylpyridine, 2-phenoxypyridine, 2-phenylsulfanylpyridine, 2-anilinopyridine, 2-(naphth-1-yl)pyridine, 2-(naphth-2-yl)pyridine, and their derivatives) and their analogues (2-phenylquinoline and 7,8-benzoquinoline) with 174 references is presented. A total of 672 species, containing κ²-N(1),C(6′)*-palladium (Pd(II), Pd(III), Pd(IV)) or analogous moiety (i.e., chelated by nitrogen of the pyridine-like ring and the deprotonated ortho-carbon of the phenyl-like ring) and thus possessing a character intermediate between metal complexes and organometallics, studied in the years 1980–2023 by NMR spectroscopy and/or single crystal X-ray diffraction (202 X-ray structures, for 186 species), are described. The biological or catalytic activity and luminescence properties of these species, as well as their possible applications as advanced materials were studied and are also quoted.

Keywords:

palladium compounds; 2-phenylpyridine; cyclometallation; ¹⁵N NMR; X-ray

1. Introduction

The aza aromatic compound 2-phenylpyridine (2ppy) is known to coordinate transition metal ions in two ways: as a monodentate N(1)-donor or as a bidentate N(1),C(6′)*-chelating agent 2ppy* (2ppy* = monoanionic form of 2ppy, deprotonated in the phenyl side group at the ortho-carbon C(6′)*). In the latter case, it forms Pd(II)-2ppy* cyclopalladated compounds, which can be regarded as either complexes or organometallics (due to the presence of both palladium-nitrogen and palladium-carbon bonds), usually upon the presence of some other, auxiliary ligands (inorganic and/or organic) which complete the square-planar (d⁸) coordination sphere. The same κ²-N(1),C(6′) coordination mode is observed also for 2ppy* derivatives containing substituents in the pyridine and/or in the phenyl ring (denoted generally as 2PPY*) and results in a variety of Pd(II)-2PPY* compounds (Scheme 1).

The same κ²-N(1),C(6′) coordination of Pd(II) is observed for such analogues of 2ppy* as 2-benzylpyridine*, 2-benzoylpyridine*, 2-phenoxypyridine*, 2-phenylsulfanylpyridine*, 2-anilinopyridine*, etc. (type A; Scheme 2), while those of κ²-N(1),C(2′) or κ²-N(1),C(3′) are noted for 2-(naphth-1-yl)pyridine* and 2-(naphth-2-yl)pyridine*, respectively (type B; Scheme 3), including their variously substituted derivatives. These 2PPY* analogues (denoted generally as 2ArPY*) yield some Pd(II)-2ArPY* species.

Similar Pd(II) chelation has also been described for analogues of 2ArPY* containing a pyridine-like ring (PY^#), especially quinoline, and an aryl ring (Ar), linked by a single bond (e.g., in 2-phenylquinoline*) or fused (e.g., in 7,8-benzoquinoline*) together with their variously substituted derivatives (denoted generally as ArPY^#*). In both cases, the coordination mode is nearly the same, but the numbering of metal-bonded atoms is different (N(1) and C(6′) in 2-phenylquinoline*; N(1) and C(10) in 7,8-benzoquinoline*), resulting in many Pd(II)-ArPY^#* species (Scheme 4).

All of these organic ligands, i.e., 2ppy*, 2PPY*, 2ArPY*, and ArPY^#*, are further generally named as “2-arylpyridines*”.

The majority of Pd(II) compounds with 2-arylpyridines* are mononuclear (monomeric), having the general formulae [Pd(2-arylpyridine*)L¹L²] (in case of L¹ = L², i.e., identical L ligands: [Pd(2-arylpyridine*)L₂]) or [Pd(2-arylpyridine*)(LL)]. However, many di-, oligo- and polynuclear forms are known as well—the most abundant are dimeric species in which two Pd(II) ions are bridged by two bidentate X ligands (e.g., halides, hydroxides, acetates, etc.), with the general formula [Pd(2-arylpyridine*)(µ-X)]₂; slight variations in composition (e.g., [Pd(2-arylpyridine*)(µ-XX)_0.5]₂ when the bridge is one tetradentate XX ligand, and [Pd(2-arylpyridine*)Y(µ-X)_0.5]₂ when the bridge is one bidentate X ligand, while Y is any monodentate ligand) are also possible.

The most popular examples of such dimeric molecules are [Pd(2ppy*)(µ-Cl)]₂ and [Pd(2ppy*)(µ-CH₃COO)]₂, shown below (Scheme 5); the other dimers have more or less similar structures (with possible cisoid or transoid orientation of the respective nitrogen atoms), although they can significantly differ in their three-dimensional distribution of all of their ligands in space. Depending on the type of bridge, both 2-arylpyridine* ligands are most often in the same plane (e.g., for X = Cl, OH), which results in nearly flat dimeric structures or in two distinct, but nearly parallel, planes (e.g., for X = CH₃COO).

Besides the above Pd(II) compounds, some Pd(III) and Pd(IV) species with 2-arylpyridines* are also known. Their formulae and detailed structures will be described separately below. Generally, all Pd(III)-(2-arylpyridine*) species are dinuclear or polynuclear, whereas the Pd(IV)-(2-arylpyridine*) ones are mononuclear.

It must be underlined here that the bidentate coordination mode of 2-arylpyridine* that we are concerned with is also widely observed for many other transition metal ions; however, this review is focused at the palladium compounds only.

The main aim of this review is the summary of NMR and X-ray structural data. However, many of the species noted herein exhibit biological (anti-tumour and/or anti-microbial) and catalytic activity, as well as luminescence (both fluorescence and phosphorescence) or interesting physicochemical properties, which allow for some practical applications; these features are also described in this review, at least in the most important cases.

The increasing interest in palladium compounds with 2-arylpyridine* can be illustrated by the numbers of compounds for which single crystal X-ray structures were published (in total, 202) every year, as presented in Figure 1.

The year-to-year fluctuations can be eliminated by calculating the numbers of compounds for which X-ray structures were published for each five-year period, being as follows: 1991–1995: 3, 1996–2000: 4, 2001–2005: 12, 2006–2010: 59, 2011–2015: 61, 2016–2020: 48, and from 2021: already 15 (in 2.5 years). This tendency clearly exhibits the growth of research intensity on the concerned compounds.

2. Reviewed Data

2.1. Pd(II) Compounds

The total number of Pd(II) species reviewed here is 557, including 172 X-ray structures (for 161 species, either neutral or ionic; various solvates/hydrates are treated here as distinct chemicals).

2.1.1. Pd(II)-2ppy* Compounds

A total of 277 Pd(II)-2ppy* compounds were reviewed, including 100 X-ray structures (for 91 species).

Mononuclear Pd(II)-2ppy* Compounds

A total of 240 Pd(II)-2ppy* mononuclear (monomeric) compounds with various auxiliary ligands, having the general formula [Pd(2ppy*)L¹L²] (in case of L¹ = L², i.e., identical L ligands: [Pd(2ppy*)L₂]) or [Pd(2ppy*)(LL)], as shown in Scheme 1 (left or right, respectively; R¹ = R² = H), are reported; 237 of them were characterised by NMR spectroscopy and/or by single crystal X-ray diffraction (exceptionally also by X-ray powder diffraction with Rietveld refinement, resulting in the full X-ray structure) [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94]. They are listed in Table 1, together with the solvents used upon the NMR studies and the CCDC reference codes for the respective 68 X-ray structures (and 68 species).

In cases when the sum of the electric charges at an auxiliary ligand(s) is different from −1 (usually 0), the concerned Pd(II)-2ppy* compound is cationic (usually +1 charge) and the relevant anion is presented in a separate column, Counterion; otherwise (the sum of electric charges at an auxiliary ligand(s) being −1) the Pd(II)-2ppy* molecule is electrically neutral. Some anionic Pd(II)-2ppy* compounds (with -1 charge, due to the sum of ligands’ charges being −2) are also known, being, however, rare. Nevertheless, in the general formulae using the symbols L, LL, etc., the electric charges were neglected for clarity.

Moreover, the compounds’ biological (BIO) and catalytic (CAT) activity, as well as luminescence properties (LUM1, LUM2, LUMx; for detailed explanation see Section 4.3 Luminescence) or possible applications as advanced materials (MAT) are indicated. In a few cases, they were studied for some compounds despite the lack of the respective X-ray or NMR data; such species have been also included in Table 1.

The same notations are used in analogous Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9 and Table 10.

Table 1. NMR and/or X-ray studied monomeric [Pd(2ppy*)L¹L²] (in particular, [Pd(2ppy*)L₂]) and [Pd(2ppy*)(LL)] compounds (L¹, L², L—monodentate ligands; LL—bidentate ligands).

L¹	L²	LL	Counterion	NMR Solvent	X-ray (CCDC)	Geometry	Activity
Cl ⁽¹⁾ Cl	Cl ⁽¹⁾ Cl		[Pd(2-phenylpyridine)(dimethyl sulfoxide)₂]^{+ (1)} [Te^IV(2-phenylpyridine)₂Cl]⁺	DMF-d₇+DMSO-d₆ [1] DMSO-d₆ [2]	PUBXAO ^(a) [2]
CO	Cl			CD₂Cl₂ [3]
NH₃	Cl			CDCl₃, DMSO-d₆ [4,5,6]			BIO [5]
CN	CN		K⁺	CD₃COCD₃ [7]
CN	CN		(CH₃CH₂CH₂CH₂)₄N⁺	CD₃OD, CD₃COCD₃ [8,9]			LUM1 [8,9]
¹³CN	¹³CN		(CH₃CH₂CH₂CH₂)₄N⁺	CD₂Cl₂ [9]
CN ⁽²⁾	CN ⁽²⁾		Tl^{+ (2)}	CD₃OD [9]	REJHOF [9]		LUM1 [9]
		SNSS		unknown [10]
		SNSNH		unknown [10]
		n-pentane-2,4-dionate ⁽³⁾		CDCl₃ [11,12,13]			LUM2 [12]
		1-phenyl-n-butane-1,3-dionate-κ²-O,O ⁽⁴⁾		CDCl₃ [13]	POKSAK [13]	trans(O^CH3O,N)
		1,1,1,5,5,5-hexafluoro-n-pentane-2,4-dionate ⁽³⁾		CDCl₃ [14]
dimethyl sulfoxide ⁽¹⁾	dimethyl sulfoxide ⁽¹⁾		[Pd(2-phenylpyridine*)Cl₂]^{− (1)}	DMF-d₇+DMSO-d₆ [1]
dimethyl sulfoxide	Cl			DMSO-d₆ [15]			CAT [15]
		1,4,7-trithiacyclononane-κ³-S,S,S	PF₆⁻	CD₃NO₂ [16]	VIYXOR [16] ^(5A)
		1-oxa-4,7-dithiacyclononane-κ²-S,S	PF₆⁻	CD₃NO₂ [17]	WOMQOH [17]
methylamine	Cl			CDCl₃ [5]			BIO [5]
isopropylamine	Cl			CDCl₃ [4]
tert-butylamine	Cl			CDCl₃ [4,5]			BIO [5]
		ethane-1,2-diamine-κ²-N,N	Cl⁻	DMSO-d₆ [18]	MAHPIW [19]		BIO [18] LUM1 [19]
		ethane-1,2-diamine	ClO₄⁻	CD₃CN [20,21]
		ethane-1,2-diamine	CF₃SO₃⁻	DMSO-d₆ [22]
		N,N,N’,N’-tetramethylethane-1,2-diamine	CF₃SO₃⁻	CDCl₃ [22]
		propane-1,3-diamine	Cl⁻	DMSO-d₆ [18]			BIO [18]
		cis-cyclohexane-1,2-diamine	Cl⁻	DMSO-d₆ [18]			BIO [18]
		trans-cyclohexane-1,2-diamine	Cl⁻	DMSO-d₆ [18]			BIO [18]
acetonitrile	acetonitrile		ClO₄⁻	CD₃CN [23]			LUM2 [23]
acetonitrile-N	trifluoroacetate-O			CD₃CN [24]	AFABOA [24]	trans(N,N)
		diethylaminocarbodithioate		CDCl₃ [13]
		pyrrolidine-1-carbodithioate-S,S		CDCl₃ [25]	XUYPOZ [25]
		piperidine-1-carbodithioate-S,S		CD₂Cl₂ [26]	JUPNIT [26]
		1,2-dicyanoethane-1,2-dithiolate	(CH₃CH₂CH₂CH₂)₄N⁺	CD₃CN [27]
		O,O’-diethyldithiophosphate		CDCl₃ [13,28]
		O,O’-di-n-propyldithiophosphate		CDCl₃ [28]
		O,O’-di-n-butyldithiophosphate		CDCl₃ [28]
		O,O’-di-sec-butyldithiophosphate		CDCl₃ [28]
		glycinate		CD₃OD [29]
		L-alaninate		CD₃OD [29]
		D-valinate		CD₃OD [29]
		D-leucinate		CD₃OD [29]
		L-prolinate		CD₃OD [29]
		2-formylphenolate-κ²-O,O		CDCl₃ [13]	POKRUD [13]	trans(O^phenolate,N)
		2-(phenyliminomethyl)phenolate-κ²-N,O		CDCl₃ [13]	YABMEV [30]	trans(N,N)	LUM2 [30]
		2-(4-chlorophenyliminomethyl)phenolate-κ²-N,O		CDCl₃ [13]	ASEGUB [30,31]	trans(N,N)	LUM2 [30]
		2-(naphth-1-yliminomethyl)phenolate		CDCl₃ [30]			LUM2 [30]
		2-((4-diphenylaminophenyl)iminomethyl)-4-methoxyphenolate-κ²-N^imine,O^phenolate		CDCl₃ [32]	NUPROH [32]	trans(N,N)	MAT [32]
		2-((4-(4-diphenylaminophenyl)phenyl)iminomethyl)-4-methoxyphenolate-κ²-N^imine,O^phenolate		CDCl₃ [32]	NUPRUN [32]	trans(N,N)	MAT [32]
		2-((4-(2-(4-diphenylaminophenyl)vinyl)phenyl)iminomethyl)-4-methoxyphenolate-κ²-N^imine,O^phenolate		CDCl₃ [32]	NUPDOT [32]	trans(N,N)	MAT [32]
		2-aminobenzenethiolate		CDCl₃ [31]			LUM2 [31]
		N-(4-n-hexoxybenzylidene-2-olate)-4-n-hexylaniline-κ²-N,O^phenolate		CDCl₃ [33]	UXILET [34]	trans(N,N)
		1,5-diphenylbiguanide	CH₃COO⁻	DMSO-d₆ [35]
		1,5-bis(4-tert-butylphenyl)diphenylbiguanide-κ²-N²,N⁴	CH₃COO⁻	DMSO-d₆ [35]	KEMFEQ [35]
		1,5-bis(3,5-difluorophenyl)biguanide	CH₃COO⁻	DMSO-d₆ [35]
		1,5-bis(3,5-dichlorophenyl)biguanide	CH₃COO⁻	DMSO-d₆ [35]
		1,5-bis(4-bromophenyl)diphenylbiguanide	CH₃COO⁻	DMSO-d₆ [35]
		1,5-bis(3,5-dimethoxyphenyl)biguanide	CH₃COO⁻	DMSO-d₆ [35]
		1,5-diphenylbiguanidate		DMSO-d₆ [35]
		1,5-bis(4-tert-butylphenyl)diphenylbiguanidate-κ²-N²,N⁴		DMSO-d₆ [35]	KEMDOY ^(b) [35]
		1,5-bis(3,5-difluorophenyl)biguanidate-κ²-N²,N⁴		DMSO-d₆ [35]	KEMDEO ^(b) [35]
		1,5-bis(3,5-dichlorophenyl)biguanidate-κ²-N²,N⁴		DMSO-d₆ [35]	KEMFAM ^(b) [35]
		1,5-bis(4-bromophenyl)diphenylbiguanidate		DMSO-d₆ [35]
		1,5-bis(3,5-dimethoxyphenyl)biguanidate		DMSO-d₆ [35]
		2-formylpyrrolate-κ²-N,O		CDCl₃ [13]	POKSEO [13]	trans(N,N)	LUM2 [31]
pyridine	pyridine		ClO₄⁻	DMSO-d₆ [36]
pyridine pyridine-d₅	Cl Cl			CDCl₃ [4,6]			BIO [5]
pyridine-d₅	Br						BIO [5]
pyridine-d₅	I						BIO [5]
pyridine-d₅	n-decanoate			CDCl₃+C₅D₅N [37]
pyridine-d₅	benzoate			CDCl₃ [37]
pyridine-d₅	4-methylbenzoate			CDCl₃ [37]
pyridine-d₅	4-fluorobenzoate			CDCl₃ [37]
pyridine-d₅	4-trifluoromethylbenzoate			CDCl₃ [37]
pyridine-d₅	4-chlorobenzoate			CDCl₃ [37]
pyridine-d₅	4-bromobenzoate			CDCl₃ [37]
pyridine-d₅	4-methoxybenzoate			CDCl₃ [37]
pyridine-d₅	4-phenoxybenzoate			CDCl₃ [37]
pyridine-d₅	4-acetylbenzoate			CDCl₃ [37]
pyridine-d₅	4-trifluoroacetylbenzoate			CDCl₃+C₅D₅N [38]
pyridine-d₅	4-cyanobenzoate			CDCl₃ [37]
pyridine-d₅	4-nitrobenzoate			CDCl₃ [37]
pyridine-N	saccharinate-N			CDCl₃ [39]	DAQNAM [39]	trans(N^py,N)	LUM2 [39]
2-methylpyridine	Cl			CDCl₃ [4,6]
3-methylpyridine	Cl			CDCl₃ [6]
4-methylpyridine	Cl			CDCl₃ [4,6]
2,3-dimethylpyridine	Cl			CDCl₃ [6]
2,4-dimethylpyridine	Cl			CDCl₃ [6]
2,6-dimethylpyridine-N	F			CDCl₃ [40]	FULWUF [40]	trans(N,N)
2,6-dimethylpyridine	Cl			CDCl₃ [6]			BIO [5]
2,6-dimethylpyridine	I			CDCl₃ [40]
3,5-dimethylpyridine	Cl			CDCl₃ [5,6]			BIO [5]
2,4,6-trimethylpyridine-N	Cl			CDCl₃ [6]	DUBLAP [6]	trans(N,N)
2-ethylpyridine	Cl			DMSO-d₆ [41]
4-tert-butylpyridine	F			CDCl₃, CD₂Cl₂ [40]
4-tert-butylpyridine	I			CDCl₃ [40]
2-phenylpyridine-N	trifluoroacetate-O			CDCl₃, CD₃COCD₃, CD₃CN [24]	AFABIU [24]	trans(N,N)
2-phenylpyridine-N	tridecafluoro-n-heptanoate-O			CDCl₃, CD₃COCD₃, CD₃CN [24]	AFABEQ [24]	trans(N,N)
		2-phenylpyridine* ⁽⁶⁾		CDCl₃ [42,43,44,45,46,47]	ZUNYIS ^(c) [48]	trans(C,N)
2-aminopyridine-N^py	Cl			DMSO-d₆ [41]	AMUNED [41]	trans(N,N)
2-amino-3-methylpyridine-N^py	Cl			DMSO-d₆ [41]	AMUNAZ [41]	trans(N,N)
2,6-diaminopyridine	Cl			DMSO-d₆ [41]
2-acetamido-3-methylpyridine	Cl			DMSO-d₆ [41]
		pyridine-2-carboxylate-κ²-N,O			WOVHEX [49]	trans(N,N)
		2-(thiophen-2-yl)pyridine* ⁽⁷⁾		CDCl₃ [42]
		bis(pyrid-2-yl)methanediol	CH₃COO⁻	D₂O, CD₃OD [50]
		bis(pyrid-2-yl) ketone	CH₃COO⁻	D₂O [50]
		2,2′-bipyridine	CF₃SO₃⁻	DMSO-d₆ [22,51]			BIO [51] LUM1 [51]
		4,4′-dimethyl-2,2′-bipyridine	CF₃SO₃⁻	DMSO-d₆ [22]
		4,4′-di-n-nonyl-2,2′-bipyridine	CF₃SO₃⁻	CDCl₃ [22]
		1,10-phenanthroline	CF₃SO₃⁻	DMSO-d₆ [22,51]			BIO [51] LUM2 [51]
		1,10-phenanthroline	NO₃⁻	CD₃OD [52]			LUM1 [52]
		1,10-phenanthroline	PF₆⁻	CD₃COCD₃ [53]
Cl		2,9-dimethyl-1,10-phenanthroline-κ²-N,N’		CD₃COCD₃ [53]	EKEDEG [53] ^(5B)
H₂O		2,9-dimethyl-1,10-phenanthroline-κ²-N,N’	PF₆⁻	CD₃COCD₃ [53]	EKEDOQ ^(d)[53] ^(5C)
		4,7-dimethyl-1,10-phenanthroline	CF₃SO₃⁻	DMSO-d₆ [22]
		4,7-diphenyl-1,10-phenanthroline	PF₆⁻	CD₃COCD₃ [53]
Cl		2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline		CD₃COCD₃ [53]	^(5B)
		dipyrido[f,h]quinoxaline	NO₃⁻	DMSO-d₆ [52]			LUMx [52]
		6,7-dicyanodipyrido[f,h]quinoxaline	NO₃⁻	DMSO-d₆ [52]			LUMx [52]
		dipyrido[a,c]phenazine	NO₃⁻	DMSO-d₆ [52]			LUMx [52]
quinoline	saccharinate			CDCl₃ [39]			LUM1 [39]
		quinolin-8-olate-κ²-N,O		CDCl₃ [13,54]	PERWIU [55]	trans(N,N)	LUMx [54]
		5-formyl-quinolin-8-olate					LUMx [54]
		5-(n-dodecylaminomethyl)quinolin-8-olate		CDCl₃ [54]			LUMx [54]
		5-(n-dodecyliminomethyl)quinolin-8-olate		CDCl₃ [54]			LUMx [54]
		5-(n-dodecanoylamide)quinolin-8-olate		CDCl₃ [54]			LUMx [54]
acridine	saccharinate			CDCl₃ [39]			LUM1 [39]
		7,8-benzoquinoline* ⁽⁸⁾		CDCl₃ [42]
		2,3-bis(pyrid-2-yl)pyrazine	ClO₄⁻	CD₃CN, DMSO-d₆ [56,57]			LUM1 [57]
2-aminopyrimidine-N^pm	Cl			DMSO-d₆ [41]	AMUNON [41]	trans(N,N)
2-acetamidopyrimidine	Cl			DMSO-d₆ [41]
1,8-naphthyridine-N	Cl			CD₂Cl₂ [58]	XAKKOM ^(e) [58]	trans(N,N)
7-azaindole	Cl			CD₂Cl₂ [58]
		6,7-dimethyl-2,3-bis(pyrid-2-yl)quinoxaline	ClO₄⁻	DMSO-d₆ [57]			LUM1 [57]
		dipyrido[f,h]quinoxaline	NO₃⁻	DMSO-d₆ [52]
		6,7-dicyanodipyrido[f,h]quinoxaline	NO₃⁻	DMSO-d₆ [52]
		dipyrido[a,c]phenazine	NO₃⁻	DMSO-d₆ [52]
		5-(2,4,6-trimethylphenyl)dipyrrinate		CD₂Cl₂, C₆D₆ [59]			LUM2 [59]
		5-(4-cyanophenyl)dipyrrinate-κ²-N,N′		CD₂Cl₂ [59]	OJATEZ [59]		LUM2 [59]
2-aminobenzothiazole-N^bzth	Cl			DMSO-d₆ [41]	AMUNIH [41]	trans(N,N)
2-acetamidobenzothiazole	Cl			DMSO-d₆ [41]
		1-(pyrrolidin-1-yl)-1,1-bis(pyrid-2-yl)ethane-κ²-N^py,N^py^′	B(C₆F₅)₄⁻	CD₂Cl₂ [60]	BIFDIG ^(f) [60]
		1-(pyrrolidin-1-ium-1-yl)-1,1-bis(pyrid-2-yl)ethane	2B(C₆F₅)₄⁻	CD₂Cl₂ [60]
		1-(pyrrolidin-1-ium-1-yl)-1,1-bis(pyrid-2-yl)ethane-κ²-N^py,N^py’	2SbF₆⁻	CD₂Cl₂ [60]	BIFDOM ^(c) [60]
N-(4-dimethylaminobenzyl)-2-(N-formyl-(4-dimethylaminobenzyl)amino)ethylamine-N^ethylamine	Cl			CDCl₃ [61]	IGERES [61]	trans(N,N)
		1,3-dimethyl-2,4-dione-5-(2,5-dimethylphenyliminomethyl)-1,2,3,4-tetrahydropyrimidin-6-olate		CDCl₃ [62]			CAT [62]
		1,3-dimethyl-2,4-dione-5-(2,6-diisopropylphenyliminomethyl)-1,2,3,4-tetrahydropyrimidin-6-olate-κ²-N^imine,O^olate		CDCl₃ [62]	XOFKEL [62]	trans(N,N)	CAT [62]
		1,3-dimethyl-2,4-dione-5-(2-methylsulfanylphenyliminomethyl)-1,2,3,4-tetrahydropyrimidin-6-olate-κ²-N^imine,O^olate		CDCl₃ [62]	XOFKIP [62]	trans(N,N)	CAT [62]
		(pyrazolate-1-yl)₂BH₂		CDCl₃ [63]
		(pyrazolate-1-yl)₂BH(pyrazolate-1-yl)-κ²-N²,N^2′		CDCl₃ [63]	HINBAH ^(d) [63]
trimethylphosphine	Cl			CDCl₃ [64]
trimethylphosphine-P	N₃			CDCl₃ [64]	GAJMUA [64]	trans(P,N)
trimethylphosphite	Cl			CDCl₃ [5]			BIO [5]
triethylphosphine	Cl			CDCl₃ [65]
tri-n-propylphosphine	Cl			CDCl₃ [64]
tri-n-propylphosphine	N₃			CDCl₃ [64]
dimethylphenylphosphine	succinimidate			CD₃COCD₃ [66]
methyldiphenylphosphine	succinimidate			CD₃COCD₃ [66]
triphenylphosphine-P	Cl			CDCl₃ [5,15]	PORNUG [67]	trans(P,N)	BIO [5], CAT [15,67]
triphenylphosphine-P	O,O’-dimethylthiophosphate-S			CDCl₃, THF-d₈ [68]	SEMDEU [68]	trans(P,N)
triphenylphosphine-P	4-methylphenylsulfonate-O			CDCl₃ [67]	PORNEQ [67]	trans(P,N)	CAT [67]
triphenylphosphine	succinimidate			CD₃COCD₃ [66]			CAT [69]
triphenylphosphine	maleimidate			CDCl₃ [69]			CAT [69]
triphenylphosphine	phthalimidate			CDCl₃ [69]			CAT [69]
triphenylphosphine-P	4,5-dichlorophtalimidate-N			CDCl₃ [70]	UMAPAB [70]	trans(P,N)
tris(4-fluorophenyl)phosphine	succinimidate			CD₃COCD₃ [66]			CAT [69]
tris(4-fluorophenyl)phosphine	maleimidate			CDCl₃ [69]			CAT [69]
tris(4-fluorophenyl)phosphine	phthalimidate			CDCl₃ [69]			CAT [69]
tris(4-methoxyphenyl)phosphine	succinimidate			CD₃COCD₃ [66]			CAT [69]
tris(4-methoxyphenyl)phosphine	maleimidate			CDCl₃ [69]			CAT [69]
tris(4-methoxyphenyl)phosphine	phthalimidate			CDCl₃ [69]			CAT [69]
(2-formylphenyl)diphenylphosphine	Cl			CDCl₃ [71]
		(2-formylphenyl)diphenylphosphine	CF₃SO₃⁻	CDCl₃ [71]
((oxydiphenylphosphino)methyl)diphenylphosphine	Cl			DMSO-d₆ [41]
(2-(oxydiphenylphosphino)ethyl)diphenylphosphine	Cl			DMSO-d₆ [41]
		(diphenylphosphinomethyl)diphenylphosphinolate	NO₃⁻	DMSO-d₆ [41]
		(2-diphenylphosphinoethyl)diphenylphosphinolate	NO₃⁻	DMSO-d₆ [41]
		(2-(methyliminomethyl)phenyl)diphenylphosphine-κ²-P,N	PF₆⁻	CDCl₃ [72]	CUBVEA [72]	trans(P,N)
		(2-(ethyliminomethyl)phenyl)diphenylphosphine	PF₆⁻	CDCl₃ [72]
		(2-(n-propyliminomethyl)phenyl)diphenylphosphine	PF₆⁻	CDCl₃ [72]
		(2-(iso-propyliminomethyl)phenyl)diphenylphosphine	PF₆⁻	CDCl₃ [72]
		(2-(tert-butyliminomethyl)phenyl)diphenylphosphine	PF₆⁻	CDCl₃ [72]
		(2-(phenyliminomethyl)phenyl)diphenylphosphine	PF₆⁻	CDCl₃ [72]
		(2-(methylaminoiminomethyl)phenyl)diphenylphosphine	PF₆⁻	CDCl₃ [72]
		2-diphenylphosphinobenzoate-κ²-P,O		CDCl₃ [73,74]	EQIRIF ^(g) [73]	trans(P,N)	CAT [74]
		N-isopropyl-2-diphenylphosphinobenzamide	PF₆⁻	CDCl₃ [73]
		N-phenyl-2-diphenylphosphinobenzamide	PF₆⁻	CDCl₃ [73]
		N-isopropyl-2-diphenylphosphinobenzamidate-κ²-P,N		CDCl₃ [73]	EQISEC [73]	trans(P,N)
		N-phenyl-2-diphenylphosphinobenzamidate		CDCl₃ [73]
(2-(isopropylaminocarbonyl)phenyl)diphenylphosphine	Cl			CDCl₃ [73]
(2-(phenylaminocarbonyl)phenyl)diphenylphosphine	Cl			CDCl₃ [73]
(2-(isopropylaminocarbonyl)phenyl)diphenylphosphine	acetate			CDCl₃ [73]
(2-(phenylaminocarbonyl)phenyl)diphenylphosphine	acetate			CDCl₃ [73]
2-(diphenylphosphinoxymethyl)pyridine			PF₆⁻	CD₃COCD₃ [75]
		2-(diphenylphosphinoamino)pyridine-κ²-N^py,P	PF₆⁻	CD₃COCD₃ [75]	JAJBOM [75]	trans(P,N)
		2-(diphenylphosphinoazanido)pyridine		CDCl₃ [75]
		CH₃CH₂OCOCHCOCH₂P(C₆H₅)₃ ^(9,10)	ClO₄⁻	CD₂Cl₂ [76]
		CH₃OCOC(NC₆H₅)C(P(C₆H₅)₃)COOCH₃ ^(11A)	ClO₄⁻	CDCl₃, CD₂Cl₂ [77]
		C₆H₅NHCSC(P(C₆H₅)₃)COOCH₃ ^(11B)	ClO₄⁻	CD₂Cl₂ [78]
C₆H₅NHCSC(P(C₆H₅)₃)COOCH₃ ^(11B)	triphenylphosphine		ClO₄⁻	CD₂Cl₂ [78]
		NCC(P(C₆H₅)₃)C(COOCH₃)CHCOOCH₃ ^(11C)	ClO₄⁻	CDCl₃ [78]
C(COOCH₃)C(COOCH₃)P(C₆H₅)₃ ^(11D)	triphenylphosphine		ClO₄⁻	CD₂Cl₂ [79]
		2-(N,P,P-triphenylphosphorimidoyl)phenyl-κ²-C¹,N		CDCl₃ [80]	LOQTIW [80]	trans(C,N)
1,3,5-triaza-7-phosphadamantane-P	phthalimidate-N			CDCl₃ [81]	IJEYAZ [81]	trans(P,N)	CAT [81]
1,3,5-triaza-7-phosphadamantane	saccharinate			CDCl₃ [81]			CAT [81]
1,3-bis(2,4,6-trimethylphenyl)imidazolidin-2-ylidene	Cl			CDCl₃ [61]
1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene-C²	Cl			CDCl₃ [61]	IGERIW [61]	trans(C,N)
1,3-bis(4-dimethylaminobenzyl)imidazolidin-2-ylidene	Cl			CDCl₃ [61]
1-methyl-3-n-butylimidazol-2-ylidene	Cl			CDCl₃ [82]			CAT [82]
1-methyl-3-n-butylimidazol-2-ylidene-C²	Br			CDCl₃ [82]	MUBQUX [82]	trans(C,N)	CAT [82]
1-methyl-3-n-butylimidazol-2-ylidene	I			CDCl₃ [82]			CAT [82]
1-methyl-3-n-butylimidazol-2-ylidene-C²	phthalimidate-N			CDCl₃ [82]	MUBYAL [82]	trans(C,N)
1-methyl-3-n-butylimidazol-2-ylidene-C²	saccharinate-N			CDCl₃ [82]	MUBRAE [82]	trans(C,N)	CAT [82]
1-methyl-3-(2-carboxyethyl)imidazol-2-ylidene	Cl			CDCl₃ [83]			CAT [83]
1-methyl-3-(2-methoxycarbonylethyl)imidazol-2-ylidene-C²	I			CDCl₃ [83]	MIXMIS [83]	trans(C,N)	CAT [83]
1-methyl-3-(2-ethoxycarbonylethyl)imidazol-2-ylidene-C²	Br			CDCl₃ [83]	MIXMOY [83]	trans(C,N)	CAT [83]
1-methyl-3-(2-(3-trimethoxysilyl-n-propyl)oxycarbonylethyl)imidazol-2-ylidene	Cl			CDCl₃ [83]
1-methyl-3-(2-benzoxycarbonylethyl)imidazol-2-ylidene	Cl			CDCl₃ [83]			CAT [83]
1-methyl-3-(2,4,6-trimethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [84]			CAT [84]
1-methyl-3-(2,3,5,6-tetramethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [84]			CAT [84]
1-methyl-3-(2,3,4,5,6-pentamethylbenzyl)imidazol-2-ylidene-C²	Br			CDCl₃ [84]	IGUVOW [84]	trans(C,N)	CAT [84]
1-n-butyl-3-phenylimidazol-2-ylidene	I			CDCl₃ [83]
1-n-butyl-3-(2,4,6-trimethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [85]			CAT [85]
1-n-butyl-3-(2,3,5,6-tetramethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [85]			CAT [85]
1-n-butyl-3-(2,3,4,5,6-pentamethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [85]			CAT [85]
1-allyl-3-(2,4,6-trimethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [86]			CAT [86]
1-allyl-3-(2,3,5,6-tetramethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [86]			CAT [86]
1-allyl-3-(2,3,4,5,6-pentamethylbenzyl)imidazol-2-ylidene-C²	Br			CDCl₃ [86]	POHHOL [86]	trans(C,N)	CAT [86]
1-phenyl-3-(2-carboxyethyl)imidazol-2-ylidene-C²	Cl			CDCl₃, DMSO-d₆ [83]	MIXNAL ^(c) [83]	trans(C,N)	CAT [83]
1-phenyl-3-(2-methoxycarbonylethyl)imidazol-2-ylidene	I			CDCl₃ [83]			CAT [83]
1-phenyl-3-(2-ethoxycarbonylethyl)imidazol-2-ylidene	Br			CDCl₃ [83]			CAT [83]
1-phenyl-3-(2-benzoxycarbonylethyl)imidazol-2-ylidene-C²	Cl			CDCl₃ [83]	MIXMUE [83]	trans(C,N)	CAT [83]
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene-C²	Cl			CDCl₃ [87]	KACMUZ [87]	trans(C,N)	CAT [87]
1,3-bis(2,4,6-trimethylphenyl)-4,5-dicyanoimidazol-2-ylidene-C²	Cl			CDCl₃ [87]	KACNAG ^(h) [87]	trans(C,N)	CAT [87]
1-(2,4,6-trimethylphenyl)-3-(2,4,6-trimethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [88]			CAT [88]
1-(2,4,6-trimethylphenyl)-3-(2,3,5,6-tetramethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [88]			CAT [88]
1-(2,4,6-trimethylphenyl)-3-(2,3,4,5,6-pentamethylbenzyl)imidazol-2-ylidene	Br			CDCl₃ [88]			CAT [88]
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene-C²	Cl			CD₂Cl₂ [89]	GADLEE ^(c) [89]	trans(C,N)	CAT [89]
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene	4-methylbenzenethiolate			C₆D₆ [90,91]
1,3-diisopropylbenzimidazol-2-ylidene-C²	Br			CDCl₃ [92]	QEZFEJ [92]	trans(C,N)
1-(2-methoxyethyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)benzimidazol-2-ylidene	Cl			CDCl₃ [93]			CAT [93]
1-(2,4,6-trimethylbenzyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)benzimidazol-2-ylidene	Cl			CDCl₃ [93]			CAT [93]
1-(2,3,5,6-tetramethylbenzyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)benzimidazol-2-ylidene	Cl			CDCl₃ [93]			CAT [93]
1-(2,3,4,5,6-pentamethylbenzyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)benzimidazol-2-ylidene	Cl			CDCl₃ [93]			CAT [93]
1-(2-methoxyethyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)-5,6-dimethylbenzimidazol-2-ylidene	Cl			CDCl₃ [93]			CAT [93]
1-(2,4,6-trimethylbenzyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)-5,6-dimethylbenzimidazol-2-ylidene	Cl			CDCl₃ [93]			CAT [93]
1-(2,3,5,6-tetramethylbenzyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)-5,6-dimethylbenzimidazol-2-ylidene	Cl			CDCl₃ [93]			CAT [93]
1-(2,3,4,5,6-pentamethylbenzyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)-5,6-dimethylbenzimidazol-2-ylidene	Cl			CDCl₃ [93]			CAT [93]
		bis(1-n-hexyl-1H-imidazol-2-ylidene-3-yl)methane	CF₃SO₃⁻	CDCl₃ [94]			BIO [94]

⁽¹⁾ This compound is an ionic pair [Pd(2ppy*)Cl₂]⁻ [Pd(2ppy*)(dimethyl sulfoxide)₂]⁺, thus it appears in two distinct rows of Table 1. ⁽²⁾ This ionic compound ([Pd(2ppy*)(CN)₂]⁻ Tl⁺) can also be regarded as pentacoordinated [PdTl(2ppy*)(CN)₂] neutral molecule. ⁽³⁾ n-pentane-2,4-dionate anionic ligand (−1 charge) is also called acetylacetonate. ⁽⁴⁾ 1-phenyl-n-butane-1,3-dione anionic ligand (−1 charge) is also called benzoylacetonate. ^(5A–5C) Pentacoordinated complexes (with chromophores: ^5A CNS₃, ^5B CN₃Cl, ^5C CN₃O). ⁽⁶⁾ This is [Pd(2ppy*)₂] compound. ⁽⁷⁾ 2-(thiophen-2-yl)pyridine* is N(1),C(3′)*-chelating, monoanionic form of 2-(thiophen-2-yl)pyridine, deprotonated in the thiophen side group at the carbon C(3′). ⁽⁸⁾ This [Pd(2ppy*)(7,8-benzoquinoline*)] compound is present also in Table 7. ⁽⁹⁾ CH₃CH₂OCOCHCOCH₂P(C₆H₅)₃ is a neutral ylide ligand having (−1) charge at CH– and (+1) charge at P. ⁽¹⁰⁾ Mixture of two isomers differing in the position of –COOCH₂CH₃ or –COCH₂P(C₆H₅)₃ moieties in respect to pyridine and phenyl rings of 2-phenylpyridine*. ^(11A–11D) CH₃OCOC(NC₆H₅)C(P(C₆H₅)₃)COOCH₃ ^(11A), C₆H₅NHCSC(P(C₆H₅)₃)COOCH₃ ^(11B), NCC(P(C₆H₅)₃)C(COOCH₃)CHCOOCH₃ ^(11C), and C(COOCH₃)C(COOCH₃)P(C₆H₅)₃ ^(11D) are neutral ylide ligands having (−1) charge at –C– (P-bonded) and (+1) charge at P. ^(a) Benzene solvate. ^(b) Dimethylformamide solvate. ^(c) Dichloromethane solvate. ^(d) Diethyl ether solvate. ^(e) Hemihydrate. ^(f) Chloroform solvate. ^(g) Monohydrate. ^(h) Acetonitrile solvate.

Dinuclear and Oligonuclear Pd(II)-2ppy* Compounds

A total of 37 Pd(II)-2ppy* di- and oligonuclear compounds were NMR- and/or X-ray-studied (32 X-ray structures for 23 species) [1,13,19,24,31,33,36,39,44,45,47,53,57,58,66,67,68,70,78,83,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119].

A total of 32 of these molecules are dinuclear [1,13,19,24,31,33,39,44,45,47,53,66,67,68,70,78,83,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118]. 26 of them, analogous to those shown at Scheme 5, correspond to the dimeric [Pd(2ppy*)(µ-X)]₂ general formula [1,13,19,24,31,33,36,39,44,45,47,53,57,58,66,67,68,70,78,83,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118], while 2 correspond to the [Pd(2ppy*)(µ-XX)_0.5]₂ [57,58] one and 2 to that of [Pd(2ppy*)Y(µ-X)_0.5]₂ [36,118]. Additionally, 2 compounds exhibiting the coordination number 5 have the untypical formula [Pd(2ppy*)(LL)(µ-Cl)_0.5]₂ (LL = 2,9-dimethyl- or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) [53]. All of these dimers are listed in Table 2, including 27 X-ray structures (for 18 species).

Table 2. NMR- and/or X-ray-studied dimeric [Pd(2ppy*)(µ-X)]₂, [Pd(2ppy*)(µ-XX)_0.5]₂, or [Pd(2ppy*)Y(µ-X)_0.5]₂ compounds (X—bridging bidentate ligand, XX—bridging tetradentate ligand, Y—monodentate ligand).

μ−X or μ−XX	Y	Counterion	NMR Solvent	X-ray (CCDC)	Geometry	Activity
Cl			DMF-d₇ [1]	SOHDUO [95] SOHDUO 01 [96] SOHDUO 02 [19] SOHDUO 03 [97] ^{all (1A)}		LUM1 [19]
OH				KEXXIV [98] KEXXIV 01 [99] ^{both (1B)}
acetate-O,O			CDCl₃ [33,44,45,47,95,100,101,102,103,104,105,106]	XEMQIQ [107] XEMQIQ 01 [108] XEMQIQ 02 [109] XEMQIQ 03 [67] XEMQIQ 04 [19] XEMQIQ 05 [110] ^{all (2)} COJBEJ ^{(2) (a)} [109]		BIO [110] LUM1 [19]
½ 3,3-dimethylglutarate			CD₂Cl₂ [58]
trifluoroacetate-O,O			CDCl₃, CD₂Cl₂ [19,24]	MAHNUG ⁽²⁾ [19]		LUM1 [19]
benzoate			CDCl₃, CD₂Cl₂ [103]
4-nitrobenzoate			CDCl₃, CD₂Cl₂ [103]
succinimidate-N,O			CDCl₃, CD₃COCD₃ [45,66]	XOTVIL ⁽²⁾ [66]	trans(N,N)	CAT [45]
2,2,3,3-tetramethylsuccinimidate			CDCl₃ [45]
maleimidate			CD₃COCD₃ [66]
phtalimidate			CD₃COCD₃ [66]
4,5-dichlorophtalimidate			CDCl₃ [70]
saccharinate-N,O^CO			CDCl₃ [39]	DAQMUF ^{(2) (b)} [39]	trans(N,N)	CAT [39] LUM1 [39]
½ 4,4′-bipyridine	NO₃		DMSO-d₆ [36]
½ Cl	2,9-dimethyl-1,10-phenanthroline-κ²-N,N	PF₆⁻	CD₃COCD₃ [53]	EKEDUW ⁽³⁾ [53]
½ Cl	2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline-κ²-N,N	PF₆⁻	CD₃COCD₃ [53]	⁽³⁾
N,N’-bis(pyrid-4-ylmethyl)oxalamide-N^py1,N^py2		2ClO₄⁻	CD₃COCD₃ [111]	HEYDOF ^{(4) (c)} [111]
pyrazolate-N¹,N²				OGONOO ⁽²⁾ OGONUU ^{(2) (b)} OGOPAC ^{(2) (d)} [112]
½ 2,2′,3,3′-tetra(pyrid-2-yl)-6,6′-biquinoxaline		2ClO₄⁻	DMSO-d₆ [57]			LUM1 [57]
pyridine-2-thiolate			CDCl₃ [13]			LUM2 [31]
pyrimidine-2-thiolate			CDCl₃ [31]			LUM2 [31]
1-methylimidazol-2-thiolate			CDCl₃ [31]			LUM2 [31]
benzimidazol-2-thiolate			CDCl₃ [31]			LUM2 [31]
5,5-diethyl-1,2,5,6-tetrahydropyrimidin-4-olate-N³,O⁴			DMSO-d₆ [113]	JUCFEU ^{(2) (e)} [113]	trans(N,N)	BIO [113]
O,O’-dimethylthiophosphate-S			CDCl₃, DMSO-d₆, DMSO-d₆+D₂O solid [68]	SEMDIY ^(1B) [68]		CAT [68]
hydrogenphosphonate			DMF-d₇ [114]
diethylphosphonate-P,O			CDCl₃, CD₃CN [114,115,116]	BUFYOS ^(1B) [116]	trans(P,N)
di(n-butyl)phosphonate-P,O			CDCl₃ [117]	SIDBUE ^(1B) [117]	trans(P,N)
½ 1,2-bis(diethylphosphino)ethane-P,P	N₃		CDCl₃ [118]	IGOWIL ^{(2) (b)} [118]	trans(N,N)
SC(NHC₆H₅)C(P(C₆H₅)₃)CN ⁽⁵⁾		2ClO₄⁻	DMSO-d₆ [78]
1-methyl-3-(2-carboxylatoethyl)imidazol-2-ylidene-C²,O			CD₃OD [83]	MIXMEO ^{(4) (f)} [83]	trans(C,N)	CAT [83]
1-phenyl-3-(2-carboxylatoethyl)imidazol-2-ylidene			CD₃OD [83]			CAT [83]

^(1A,1B) Both 2ppy* ligands are nearly in the same plane, exhibiting cisoid ^(1A) or transoid ^(1B) geometry (concerns position of their nitrogen atoms). ⁽²⁾ Both 2ppy* ligands are in different but nearly parallel planes (lying on the same side of the Pd-Pd axis), exhibiting transoid geometry (in respect to the position of their nitrogen atoms). ⁽³⁾ Pentacoordinated complex (CN₃Cl chromophore). ⁽⁴⁾ Both 2ppy* ligands are in highly non-parallel planes. ⁽⁵⁾ SC(NHC₆H₅)C(P(C₆H₅)₃)CN is a zwitterionic ligand having (−1) charge at S and (+1) charge at P (also some mesomeric ylide forms are possible). ^(a) Hg(C₆F₅)₂ solvate. ^(b) Dichloromethane solvate. ^(c) Diethyl ether dimethylformamide solvate. ^(d) Chloroform solvate. ^(e) Dimethyl sulfoxide solvate. ^(f) Toluene solvate.

Additionally, 5 Pd(II)-2ppy* compounds (including 5 X-ray structures) contain more than two palladium atoms, their molecular formulae being as follows: [Pd(2ppy*)(µ-4,4′-bipyridine)]₄(NO₃)₄ (NMR in DMSO-d₆) [36]; [Pd(2ppy*)_0.5(2ppy-N)_0.5(diethyl phosphonate-P)_0.5(µ-hydrogen phosphite-O,O)_0.5]₄ (NMR in CD₃CN; X-ray: ZUMSUY and ZUMSOS for 2ppy solvate) [114]; [Pd(2ppy*)(µ-1,3,5-triazine-2,4,6-trithiolate-N¹,N³,N⁵,S²,S⁴,S⁶)_0.33]₉ (NMR in CD₂Cl₂; X-ray: MIRGUR for benzene dichloromethane methanol solvate); [Pd(2ppy*)(µ-1,3,5-triazine-2,4,6-triolate-N¹,N³,N⁵,O²,O⁴,O⁶)_0.2(µ-1,3,5-triazine-2-ol-4,6-diolate-N³,N⁵,O⁴,O⁶)_0.2]₁₀ (X-ray: MIRGOL for benzene chloroform dichloromethane methanol solvate); and [Pd(2ppy*)(µ-1,3,5-triazine-2,4,6-triolate-N¹,N³,N⁵,S²,S⁴,S⁶)_0.33]₁₂ (NMR in CD₂Cl₂; X-ray: MIRGEB for acetone dichloromethane solvate) [119]. The details of Pd(II) linking in these oligonuclear (tetra-, nona-, deca-, and dodecanuclear) species can be found in the original papers.

2.1.2. Pd(II)-2PPY* Compounds

A total of 53 Pd(II)-2PPY* compounds were reviewed, including 15 X-ray structures (for 15 species).

Mononuclear Pd(II)-2PPY* Compounds

Besides Pd(II)-2ppy* compounds, 31 Pd(II)-2PPY* mononuclear (monomeric) species with various auxiliary ligands, having the general formula [Pd(2PPY*)L¹L²] (in case of L¹ = L², i.e., identical L ligands: [Pd(2PPY*)L₂]) or [Pd(2PPY*)(LL)], as shown at Scheme 1 (left or right, respectively; R¹, R² ≠ H), were reported and characterised by NMR spectroscopy and/or by single crystal X-ray diffraction [17,21,37,38,50,64,92,120,121,122,123,124,125]. They are listed in Table 3, including 10 X-ray structures (for 10 species).

3 of them contain 2ppy* derivatives that are substituted only in the pyridine ring (R¹ = 6-phenyl-) [21,120,121], while 28—only in the phenyl ring (R² = 3- and 4-methyl-; 3- and 4-fluoro-; 3-chloro-; 4-bromo-; 4-methoxy-; 4-formyl-) [17,37,38,50,64,92,122,123,124,125]. In contrast, there are no compounds which have 2ppy* derivatives with substituents in both the pyridine and the phenyl ring.

Table 3. NMR- and/or X-ray-studied monomeric [Pd(2PPY*)L¹L²] (in particular, [Pd(2PPY*)L₂]) and [Pd(2PPY*)(LL)] compounds (2PPY* = a-R¹-2-(b-R²-phenyl)pyridine*, other than 2ppy*, where R¹ and R² are substituents in the pyridine ring and the phenyl ring, respectively; a = 3–6, b = 2–5; L¹, L², L—monodentate ligands; LL—bidentate ligands).

R¹	R²	L¹	L²	LL	Counterion	NMR Solvent	X-ray (CCDC)	Geometry	Activity
6-phenyl	H			ethane-1,2-diamine	ClO₄⁻	CD₃CN [21]
6-phenyl	H			6-phenyl-2-phenylpyridine* ^(1,2)		CDCl₃ [120]
6-phenyl	H	triphenylphosphine-P	Cl			CD₂Cl₂ [121]	MAWSOW ^(a) [121]	trans(P,N)
H	3-methyl			2-(3-methylphenyl)pyridine* ⁽³⁾		CDCl₃ [37]
H	3-methyl	pyridine-d₅	benzoate			CDCl₃ [37]
H	3-methyl	pyridine-d₅	4-chlorobenzoate			CDCl₃ [37]
H	3-methyl	pyridine-d₅	4-bromobenzoate			CDCl₃ [37]
H	4-methyl	2,4,6-tris(trifluoromethyl)phenyl	H₂O			CDCl₃ [122]
H	4-methyl			bis(pyrid-2-yl)methanediol	CH₃COO⁻	CD₃OD [123]
H	4-methyl			bis(pyrid-2-yl) ketone	CH₃COO⁻	CD₃OD [123]
H	4-methyl	trimethylphosphine	Cl			CDCl₃ [64]
H	4-methyl	trimethylphosphine-P	N₃			CDCl₃ [64]	GAJNAH [64]	trans(P,N)
H	4-methyl	triethylphosphine	Cl			CDCl₃ [64]
H	4-methyl	triethylphosphine	N₃			CDCl₃ [64]
H	4-methyl	dimethylphenylphosphine	Cl			CDCl₃ [64]
H	4-methyl	dimethylphenylphosphine	N₃			CDCl₃ [64]
H	4-methyl	triphenylphosphine	2,4,6-tris(trifluoromethyl)phenyl			CDCl₃ [122]
H	4-methyl	1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene-C²	N₃			CDCl₃ [124]	KENTEE ^(b) [124]	trans(N,N)	CAT [124]
H	4-methyl	1,3-diisopropylbenzimidazol-2-ylidene-C²	Br			CDCl₃ [92]	QEZFIN [92]	trans(C,N)
H	3-fluoro			2-(3-fluorophenyl)pyridine* ⁽⁴⁾		CDCl₃ [38]
H	4-fluoro			bis(pyrid-2-yl)methanediol	CH₃COO⁻	D₂O, CD₃OD [50]
H	4-fluoro			bis(pyrid-2-yl) ketone	CH₃COO⁻	D₂O [50]
H	3-chloro			2-(3-chlorophenyl)pyridine* ⁽⁵⁾		CDCl₃ [38]
H	4-bromo			n-pentane-2,4-dionate-κ²-O,O ⁽⁶⁾		CDCl₃ [125]	SOFCAT [125]
H	4-bromo	1,3-bis(4-methylphenyl)imidazol-2-ylidene-C²	Cl			CDCl₃ [125]	SOFBEW [125]	trans(C,N)	CAT [125] LUMx [125]
H	4-bromo	1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene-C²	Cl			CDCl₃ [125]	SOFBOG [125]	trans(C,N)	CAT [125] LUMx [125]
H	4-bromo	1,3-bis(4-methoxyphenyl)imidazol-2-ylidene-C²	Cl			CDCl₃ [125]	SOFBIA ^(a) [125]	trans(C,N)	CAT [125] LUMx [125]
H	4-methoxy			bis(pyrid-2-yl)methanediol	CH₃COO⁻	D₂O, CD₃OD [50]
H	4-methoxy			bis(pyrid-2-yl) ketone	CH₃COO⁻	D₂O [50]
H	4-formyl			1,4,7-trithiacyclononane-κ³-S,S,S	PF₆⁻	CD₃NO₂ [17]	WOMREY [17]	⁽⁷⁾
H	4-formyl			1-oxa-4,7-dithiacyclononane-κ²-S,S	PF₆⁻	CD₃NO₂ [17]	WOMRAU [17]

⁽¹⁾ 6-phenyl-2-phenylpyridine* is N(1),C(3′)*-chelating, monoanionic form of 2,6-diphenylpyridine, deprotonated in one phenyl side group at the carbon C(3′) (while C(3″) in the second phenyl side group remains protonated and does not coordinate metal atom). ⁽²⁾ This is [Pd(6-phenyl-2-phenylpyridine*)₂] compound. ⁽³⁾ This is [Pd(2-(3-methylphenyl)pyridine*)₂] compound. ⁽⁴⁾ This is [Pd(2-(3-fluorophenyl)pyridine*)₂] compound. ⁽⁵⁾ This is [Pd(2-(3-chlorophenyl)pyridine*)₂] compound. ⁽⁶⁾ n-pentane-2,4-dionate anionic ligand (-1 charge) is also called acetylacetonate. ⁽⁷⁾ Pentacoordinated complex (CNS₃ chromophore). ^(a) Dichloromethane solvate. ^(b) Tetrahydrofuran solvate.

Dinuclear Pd(II)-2PPY* Compounds

A total of 22 Pd(II)-2PPY* dinuclear compounds, all corresponding to the dimeric [Pd(2PPY*)(µ-X)]₂ general formula, were NMR- and/or X-ray-studied [17,19,44,50,64,100,123,126,127,128,129].

1 of them contains 2ppy* derivatives substituted only in the pyridine ring (R¹ = 3-methyl-) [44,126], 20—only in the phenyl ring (R² = 2-, 3- and 4-methyl-; 4-fluoro-; 4-trifluoromethyl-; 2- and 4-chloro-; 4-bromo-; 4-methoxy-; 4-formyl-) [17,19,50,64,100,123,127,128], and 1—in both rings [129].

All these dimers are listed in Table 4, including 5 X-ray structures (for 5 species).

Table 4. NMR- and/or X-ray-studied dimeric [Pd(2PPY*)(µ-X)]₂ compounds (2PPY* = a-R¹-2-(b-R²-phenyl)pyridine*, other than 2ppy*, where R¹ and R² are substituents in the pyridine ring and the phenyl ring, respectively; a = 3–6, b = 2–5; X—bidentate bridging ligand).

R¹	R²	μ−X	NMR Solvent	X-ray (CCDC)	Geometry	Activity
3-methyl	H	acetate	CDCl₃+pyridine-d₅, CD₃COCD₃ [44,126]
H	2-methyl	acetate	CDCl₃ [100,127]
H	3-methyl	acetate	CDCl₃ [100]
H	4-methyl	acetate	CDCl₃ [100,123]	MAHPAO ⁽¹⁾ [19]		LUM1 [19]
H	4-methyl	trifluoroacetate	CD₂Cl₂ [19]	MAHPOC ⁽¹⁾ [19]		LUM1 [19]
H	4-methyl	1-ethyl-1,2,3,4-tetrazole-5-thiolate	CDCl₃ [64]
H	4-methyl	1-isopropyl-1,2,3,4-tetrazole-5-thiolate-N⁴,S	CDCl₃ [64]	GAJNOV ^{(1) (a)} [64]	trans(S,N)
H	4-methyl	1-n-butyl-1,2,3,4-tetrazole-5-thiolate	CDCl₃ [64]
H	4-methyl	1-allyl-1,2,3,4-tetrazole-5-thiolate	CDCl₃ [64]
H	4-methyl	pyridine-2-thiolate	CDCl₃ [128]			LUM1 [128]
H	4-fluoro	acetate	CDCl₃ [50]
H	4-trifluoromethyl	acetate	CD₂Cl₂ [17]	WOMSAV ⁽¹⁾ [17]
H	2-chloro	acetate	CDCl₃ [100]
H	4-chloro	acetate	CDCl₃ [100]
H	4-bromo	acetate	CDCl₃ [100]
H	2-methoxy	acetate	CDCl₃ [100]
H	3-methoxy	acetate	CDCl₃ [100]
H	4-methoxy	acetate	CDCl₃ [50,100]
H	4-formyl	acetate	CD₂Cl₂ [17]	WOMQUN ⁽¹⁾ [17]
H	2-nitro	acetate	CDCl₃ [100]
H	3-nitro	acetate	CDCl₃ [100]
6-(2,3-didodecyloxyphenyl) ⁽²⁾	2,3-didodecyloxy ⁽²⁾	Cl	CDCl₃ [129]

⁽¹⁾ Both 2ppy* ligands are nearly in the same plane, exhibiting transoid geometry (in respect to the position of their nitrogen atoms). ⁽²⁾ 6-(2,3-didodecyloxyphenyl)-2-(2,3-didodecyloxyphenyl)pyridine* is N(1),C(6′)*-chelating, monoanionic form of 2,6-bis(2,3-didodecyloxyphenyl)pyridine, deprotonated in one phenyl side group at the carbon C(6′) (while C(6″) in the second phenyl side group remains protonated and does not coordinate metal atom). ^(a) Dichloromethane solvate.

2.1.3. Pd(II)-2ArPY* Compounds

A total of 65 Pd(II)-2ArPY* compounds were reviewed, including 12 X-ray structures (for 11 species).

Mononuclear Pd(II)-2ArPY* Compounds

Besides Pd(II)-2ppy* and Pd(II)-2PPY* compounds, 52 Pd(II)-2ArPY* mononuclear (monomeric) species with various auxiliary ligands, having the general formula [Pd(2ArPY*)L¹L²] (in case of L¹ = L², i.e., identical L ligands: [Pd(2ArPY*)L₂]) or [Pd(2ArPY*)(LL)], as shown at Scheme 2 and Scheme 3 (left or right, respectively), were reported and characterised by NMR spectroscopy and/or by single crystal X-ray diffraction [10,11,12,29,33,50,70,74,81,82,92,122,123,130,131,132,133,134,135,136,137,138,139,140,141]. They are listed in Table 5, including 7 X-ray structures (for 7 species).

50 of them contain 2ArPY* type A ligands (2-benzylpyridine* [10,29,74,92,122,130,131,132,133], 2-(1-methylbenzyl)pyridine* [134], 2-(1-phenylbenzyl)pyridine* [135,136], 2-benzoylpyridine* and 2-(3-methylbenzoyl)pyridine* [11,12,33,50,70,81,82,123,137], 2-phenoxypyridine* [138], 2-phenylsulfanylpyridine* [138], and 2-anilinopyridine* [139]), while 2 of them contain 2ArPY* type B ligands (2-(naphth-1-yl)pyridine* [140] and 2-(naphth-2-yl)pyridine* [141]).

Table 5. NMR- and/or X-ray-studied monomeric [Pd(2ArPY*)L¹L²] (in particular, [Pd(2ArPY*)L₂]) and [Pd(2ArPY*)(LL)] compounds (2ArPY* ≠ 2PPY*; L¹, L², L—monodentate ligands; LL—bidentate ligands); R¹ and R² are substituents in the pyridine ring and the aryl ring, respectively.

Parent 2ArPY* Ring System	R¹	R²	L¹	L²	LL	Counterion	NMR Solvent	X-ray (CCDC)	Geometry	Activity
2-benzylpyridine*	H	H			SNSNH		unknown [10]
	H	H			thiosulfate	P(C₆H₅)₃(CH₂C₆H₅)⁺	CDCl₃ [130]
	H	H			n-pentane-2,4-dionate ⁽¹⁾		CDCl₃ [131]
	H	H			glycinate		CD₃OD+DMSO-d₆ [29]
	H	H			L-alaninate		CD₃OD [29]
	H	H			D-valinate		CD₃OD [29]
	H	H			D-leucinate		CD₃OD, DMSO-d₆ [29]
	H	H			L-prolinate-κ²-N,O		CD₃OD [29]	TUMLAO [29]	trans(N,N)
	H	H	2,4,6-tris(trifluoromethyl)phenyl	H₂O			CDCl₃ [122]
	H	H	pyridine	Cl			CDCl₃ [132]
	H	H	4-methylpyridine	Cl			CDCl₃ [132]
	H	H	3,4-dimethylpyridine	Cl			CDCl₃ [132]
	H	H	3,5-dimethylpyridine	Cl			CDCl₃ [131,132]
	H	H	2,4,6-trimethylpyridine	Cl			CDCl₃ [132]
	H	H	4-methoxycarbonylpyridine	Cl			CDCl₃ [132]
	H	H	4-dimethylaminopyridine	Cl			CDCl₃ [132]
	H	H			2,2′-bipyridine	NO₃⁻	CD₃CN [133]			LUM1 [133]
	H	H			1,10-phenanthroline	NO₃⁻	CD₃CN [133]			LUM1 [133]
	H	H	methyldiphenylphosphine	Cl			CDCl₃ [132]
	H	H	triphenylphosphine	Cl			CDCl₃ [132]
	H	H			2-diphenylphosphinobenzoate-κ²-P,O		CDCl₃ [74]	FESCEO ^(a) [74]	trans(P,N)	CAT [74]
	H	H	1,3-diisopropylbenzimidazol-2-ylidene	Br			CDCl3 [92]
2-(1-methylbenzyl)pyridine*	H	H			n-pentane-2,4-dionate ⁽¹⁾		CDCl₃ [134]
	H	H	pyridine	Cl			CDCl₃ [134]
	H	H			2,2′-bipyridine	BF₄⁻	CDCl₃ [134]
	H	H	triphenylphosphine	Cl			CDCl₃ [134]
	H	H			bis(diphenylphosphino)methane	BF₄⁻	CDCl₃ [134]
	H	H			n-pentane-2,4-dionate ⁽¹⁾		CDCl₃ [135]
	H	H			N-2,2,2-trichloro-1,1-dimethylethoxycarbonyl-L-leucine		DMSO-d₆ [136]		(R)/(S)
2-benzoylpyridine*	H	H			n-pentane-2,4-dionate ⁽¹⁾		CDCl₃ [11,12,137]
	H	H			N-(4-n-hexoxybenzylidene-2-olate)-4-n-hexylaniline-κ²-N,O^phenolate		CDCl₃ [33]	WOSYEI [33]	trans(N,N)
	H	H	3,5-dimethylpyridine	Cl			CDCl₃ [137]
	H	H			pyridine-2-carboxylate		CDCl₃ [123]
	H	H			bis(pyrid-2-yl)methanediol	CH₃COO⁻	D₂O [50,123]
	H	H			bis(pyrid-2-yl) ketone	CH₃COO⁻	D₂O [50,123]
	H	H	triphenylphosphine	phtalimidate			CDCl₃ [70]
	H	H	triphenylphosphine	4,5-dichlorophtalimidate			CDCl₃ [70]
	H	H	triphenylphosphine	saccharinate			CDCl₃ [70]
	H	H	1,3,5-triaza-7-phosphaadamantane-P	phthalimidate-N			CDCl₃ [81]	IJEYED [81]	trans(P,N)	CAT [81]
	H	H	1,3,5-triaza-7-phosphaadamantane	saccharinate			CDCl₃ [81]			CAT [81]
	H	H	1-methyl-3-n-butylimidazol-2-ylidene	Cl			CDCl₃ [82]			CAT [82]
	H	H	1-methyl-3-n-butylimidazol-2-ylidene	Br			CDCl₃ [82]			CAT [82]
	H	H	1-methyl-3-n-butylimidazol-2-ylidene	I			CDCl₃ [82]
	H	H	1-methyl-3-n-butylimidazol-2-ylidene-C²	saccharinate-N			CDCl₃ [82]	MUBQOR [82]	trans(C,N)
	H	3-methyl			bis(pyrid-2-yl)methanediol	CH₃COO⁻	D₂O [50,123]
	H	3-methyl			bis(pyrid-2-yl) ketone	CH₃COO⁻	D₂O [50,123]
	H	3-methyl			hydroperoxy(bis(pyrid-2-yl))methanol	CH₃COO⁻	D₂O [50]
2-phenoxypyridine*	H	H			n-pentane-2,4-dionate ⁽¹⁾		CDCl₃ [138]
2-phenylsulfanylpyridine*	H	H			n-pentane-2,4-dionate ⁽¹⁾		CDCl₃ [138]
2-anilinopyridine*	H	H	acetonitrile	acetonitrile		BF₄⁻	CDCl₃+CD₃CN [139]
(2-(naphth-1-yl)pyridine*	H	H	pyridine-N	Cl			CD₂Cl₂ [140]	KOXWAX [140]	trans(N,N)
(2-(naphth-2-yl)pyridine*			pyridine-N	Cl			CDCl₃ [141]	ZABFEN [141]	trans(N,N)

⁽¹⁾ n-pentane-2,4-dionate anionic ligand (−1 charge) is also called acetylacetonate. ^(a) Diethyl ether solvate.

Dinuclear Pd(II)-2ArPY* Compounds

A total of 13 Pd(II)-2ArPY* dinuclear compounds, all corresponding to the dimeric [Pd(2ArPY*)(µ-X)]₂ general formula, were NMR- and/or X-ray-studied [33,70,82,123,131,134,136,137,138,141,142,143,144,145].

Among them, 12 molecules contain 2ArPY* type A ligands (2-benzylpyridine* [131], 2-(1-methylbenzyl)pyridine* [134], 2-(1-phenylbenzyl)pyridine* [136,142], 2-benzoylpyridine* and 2-(3-methylbenzoyl)pyridine* [33,70,82,123,137], 2-phenoxypyridine* [138,143,144], and 2-phenylsulfoxylpyridine* [145]), while 1 contains 2ArPY* type B ligand (2-(naphth-2-yl)pyridine* [141]). All of these dimers are listed in Table 6, including 5 X-ray structures (for 4 species).

Table 6. NMR- and/or X-ray-studied dimeric [Pd(2ArPY*)(µ-X)]₂ compounds (2ArPY* ≠ 2PPY*; X—bidentate bridging ligand); R¹ and R² are substituents in the pyridine ring and the aryl ring, respectively.

Parent 2ArPY* Ring System	R¹	R²	μ−X	NMR Solvent	X-ray (CCDC)	Geometry
2-benzylpyridine*	H	H	acetate	CD₂Cl₂ [131]
2-(1-methylbenzyl)pyridine*	H	H	Cl	CDCl₃ [134]
2-(1-phenylbenzyl)pyridine*	H	H	Cl	DMSO-d₆ [136]	YAPXEU ⁽¹⁾ [136]
2-(1-phenylbenzyl)pyridine*	H	H	acetate-O,O	DMSO-d₆ [136,142]	NOGKUQ ^{(2) (a)} [136,142]
2-benzoylpyridine*	H	H	OH	CDCl₃ [82]
	H	H	acetate	CDCl₃ [33,123,137]
	H	H	phtalimidate-N,O	CDCl₃ [70]	UMAPEF ^{(2) (a)} [70]	trans(N,N)
	H	H	4,5-dichlorophtalimidate	CDCl₃ [70]
	H	H	saccharinate	CDCl₃ [70]
	H	3-methyl	acetate	CDCl₃ [123]
2-phenoxypyridine*	H	H	acetate-O,O	CDCl₃ [138,143,144]	ASOGAR [143] ASOGAR 01 [144] ^{both (2)}
2-phenylsulfoxylpyridine*	H	H	acetate	CDCl₃ [145]
2-(naphth-2-yl)pyridine*	H	H	Cl	DMSO-d₆ [141]

⁽¹⁾ Both 2ArPY* ligands are nearly in the same plane, exhibiting transoid geometry (in respect to the position of their nitrogen atoms). ⁽²⁾ Both 2ArPY* ligands are in different but nearly parallel planes (lying on the same side of the Pd-Pd axis), exhibiting transoid geometry (in respect to the position of their nitrogen atoms). ^(a) Dichloromethane solvate.

2.1.4. Pd(II)-ArPY^#* Compounds

A total of 162 Pd(II)-ArPY^#* species were reviewed, including 45 X-ray structures (for 44 species).

Mononuclear Pd(II)-ArPY^#* Compounds

Besides Pd(II)-2ppy*, Pd(II)-2PPY*, and Pd(II)-2ArPY* compounds, 128 mononuclear (monomeric) Pd(II)-ArPY^#* species with various auxiliary ligands, having the general formula [Pd(ArPY^#*)L¹L²] (in case of L¹ = L², i.e., identical L ligands: [Pd(ArPY^#*)L₂]) or [Pd(ArPY^#*)(LL)], as shown in Scheme 4 (left or right, respectively), were reported and characterised by NMR spectroscopy and/or by single crystal X-ray diffraction [7,9,10,11,12,13,14,16,17,18,20,21,22,23,30,31,33,38,39,40,42,45,46,51,67,69,71,73,74,75,76,78,79,92,103,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166]. All are listed in Table 7, including 29 X-ray structures (for 28 compounds).

Among them, 5 compounds contain 2-phenylquinoline* [20,51,67,146], 112—unsubstituted 7,8-benzoquinoline* [7,9,10,11,12,13,14,16,17,18,21,22,23,30,31,33,38,39,40,42,45,46,51,69,71,73,74,75,76,78,79,92,103,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166], and 11—variously substituted derivatives of 7,8-benzoquinoline* (7-methyl-, 7-chloro-, 7-methoxy-, 7-isopropoxy-, 7-formyl-, 7-cyano-, 7-nitro) [154,156].

Table 7. NMR- and/or X-ray-studied monomeric [Pd(ArPY^#*)L¹L²] (in particular, [Pd(ArPY^#*)L₂]) and [Pd(ArPY^#*)(LL)] compounds (ArPY^#* ≠ 2PPY*, 2ArPY*; L¹, L², L—monodentate ligands; LL—bidentate ligands); R¹ and R² are substituents in the pyridine-like ring and the aryl ring, respectively.

Parent ArPY^#* Ring System	R¹	R²	L¹	L²	LL	Counterion	NMR Solvent	X-ray (CCDC)	Geometry	Activity
2-phenylquinoline*	H	H			n-pentane-2,4-dionate ⁽¹⁾		CDCl₃ [146]
	H	H			ethane-1,2-diamine	ClO₄⁻	CD₃CN [20]
	H	H			2,2′-bipyridine	CF₃SO₃⁻	DMSO-d₆ [51]			BIO [51] LUM2 [51]
	H	H			1,10-phenanthroline	CF₃SO₃⁻	DMSO-d₆ [51]			BIO [51] LUM2 [51]
	H	H	triphenylphosphine-P	4-methylphenylsulfonate-O			CDCl₃ [67]	PORNIU ^(a) [67]	trans(P,N)	CAT [67]
7,8-benzoquinoline*	H	H	CN	CN		K⁺	CD₃COCD₃ [7]			LUM1 [9]
	H	H	CN	CN		(CH₃CH₂CH₂CH₂)₄N⁺	CD₃COCD₃ [9]
	H	H	¹³CN	¹³CN		(CH₃CH₂CH₂CH₂)₄N⁺	CD₂Cl₂ [9]
	H	H	CN	CN		Tl⁺	CD₃OD [9]	REJHIZ [9] ⁽²⁾		LUM1 [9]
	H	H			SNSNH		unknown [10]
	H	H			n-pentane-2,4-dionate ⁽¹⁾		CDCl₃ [11,12,13]
	H	H			1-phenyl-n-butane-1,3-dionate ⁽³⁾		CDCl₃ [13]
	H	H			1,1,1,5,5,5-hexafluoro-n-pentane-2,4-dionate ⁽¹⁾		CDCl₃ [14]
	H	H			1,4,7-trithiacyclononane-κ³-S,S,S	PF₆⁻	CD₃NO₂ [16]	VIYXUX [16] ⁽⁴⁾
	H	H			1-oxa-4,7-dithiacyclononane	PF₆⁻	CD₃NO₂ [17]
	H	H			ethane-1,2-diamine	Cl⁻	DMSO-d₆ [18]
	H	H			ethane-1,2-diamine	ClO₄⁻	CD₃CN [21]
	H	H			ethane-1,2-diamine	ClO₄⁻	DMF-d₇ [147]
	H	H			ethane-1,2-diamine	CF₃SO₃⁻	DMSO-d₆ [22]
	H	H			N,N,N’,N’-tetramethylethane-1,2-diamine	CF₃SO₃⁻	CDCl₃ [22]
	H	H	acetate	acetate		(CH₃CH₂CH₂CH₂)₄N⁺	CD₂Cl₂ [148]
	H	H	acetonitrile	acetonitrile		ClO₄⁻	CD₃CN [23]			LUM2 [23]
	H	H	acetonitrile	trifluoroacetate			CD₃CN [149]
	H	H			diethylaminocarbodithioate		CDCl₃ [13,150]
	H	H			O,O’-diethyldithiophosphate		CDCl₃ [13]
	H	H			2-formylphenolate		CDCl₃ [13]
	H	H			2-(phenyliminomethyl)phenolate-κ²-N,O		CDCl₃ [13]	POKSIS [13] POKSIS 01 [30]	trans(N,N)	LUM2 [30]
	H	H			2-(4-chlorophenyliminomethyl)phenolate-κ²-N,O)		CDCl₃ [13]	POKSOY [13]	trans(N,N)	LUM2 [30]
	H	H			2-(naphth-1-yliminomethyl)phenolate		CDCl₃ [30]			LUM2 [30]
	H	H			2-aminobenzenethiolate		CDCl₃ [31]			LUM2 [31]
	H	H			N-(4-n-hexoxybenzylidene-2-olate)-4-n-hexylaniline		CDCl₃ [33]
	H	H			1-methoxynaphth-8-yl		CDCl₃ [151]
	H	H			2-formylpyrrolate-κ²-N,O		CDCl₃ [13]	ASEHEM [31]	trans(N,N)	LUM2 [31]
	H	H	pyridine	Cl			CDCl₃ [103]
	H	H	pyridine	acetate			CDCl₃ [103]
	H	H	pyridine-N	succinimidate-N			CDCl₃ [45]	ANOJOD [45]	trans(N^succ,N)
	H	H	pyridine-N	saccharinate-N			CDCl₃ [39]	DAQNEQ ^(a) [39]	trans(N^py,N)	LUM2 [39]
	H	H	pyridine-d₅	4-trifluoroacetylbenzoate			CDCl₃+pyridine-d₅ [38]
	H	H	2-methylpyridine	Cl			DMSO-d₆ [152]			BIO [152]
	H	H	3-methylpyridine-N	Cl			DMSO-d₆ [152]	FIBRIV [152]	trans(N,N)	BIO [152]
	H	H	2,6-dimethylpyridine-N	F			CDCl₃ [40]	FULXAM ^(a) [40]	trans(N,N)
	H	H	2,6-dimethylpyridine	I			CDCl₃ [40]
	H	H	2-aminopyridine	Cl			DMSO-d₆ [152]			BIO [152]
	H	H			2-aminomethylpyridine	Cl⁻	DMSO-d₆ [152]			BIO [152]
	H	H	2-amino-3-methylpyridine	Cl			DMSO-d₆ [152]			BIO [152]
	H	H	2-amino-5-chloropyridine-N	Cl			DMSO-d₆ [152]	NIJXIR [152]	trans(N,N)	BIO [152]
	H	H			2-phenylpyridine* ⁽⁵⁾		CDCl₃ [42]
	H	H			2-(thiophen-2-yl)pyridine* ⁽⁶⁾		CDCl₃ [42]
	H	H			2,2′-bipyridine	CF₃SO₃⁻	DMSO-d₆ [22,51]			BIO [51] LUM1 [51]
	H	H			4,4′-dimethyl-2,2′-bipyridine	CF₃SO₃⁻	DMSO-d₆ [22]
	H	H			4,4′-di-n-nonyl-2,2′-bipyridine	CF₃SO₃⁻	CDCl₃ [22]
	H	H			1,10-phenanthroline	CF₃SO₃⁻	DMSO-d₆ [22,51]			BIO [51] LUM2 [51]
	H	H	quinoline	saccharinate			CDCl₃ [39]			LUM1 [39]
	H	H			quinolin-8-olate		CDCl₃ [13]
	H	H	acridine	saccharinate			CDCl₃ [39]			LUM1 [39]
	H	H			7,8-benzoquinoline* ⁽⁷⁾		CDCl₃ [42,46]	TAXHAE [153]	trans(C,N)
	H	H	1H-imidazole	Cl			DMSO-d₆ [152]			BIO [152]
	H	H	2-aminothiazole	Cl			DMSO-d₆ [152]			BIO [152]
	H	H	2-aminobenzimidazole	Cl			DMSO-d₆ [152]			BIO [152]
	H	H	2-aminobenzothiazole-N	Cl			DMSO-d₆ [152]	NIJXEN [152]	trans(N,N)	BIO [152]
	H	H			N-(2-(pyrid-2-yl)phenyl)benzenesulfonamidate		CDCl₃ [154]
	H	H			N-(2-(pyrid-2-yl)phenyl)-4-methylbenzenesulfonamidate		CDCl₃ [154]
	H	H			N-(2-(pyrid-2-yl)phenyl)-4-tert-butyl-benzenesulfonamidate		CDCl₃ [154]
	H	H			N-(2-(pyrid-2-yl)phenyl)-4-phenylbenzenesulfonamidate		CDCl₃ [154]
	H	H			N-(2-(pyrid-2-yl)phenyl)-4-fluorobenzenesulfonamidate		CDCl₃ [154]
	H	H			N-(2-(pyrid-2-yl)phenyl)-4-chlorobenzenesulfonamidate		CDCl₃ [154]
	H	H			N-(2-(pyrid-2-yl)phenyl)-4-cyanobenzenesulfonamidate		CDCl₃ [154]
	H	H			N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154,155]
	H	H			N-(2-(pyrid-2-yl)phenyl)-4-nitrobenzenesulfonamidate		CDCl₃ [154]
H	H			N-(2-(4-methylpyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
H	H			N-(2-(4-trifluoromethylpyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
H	H			N-(2-(4-bromopyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
H	H			N-(2-(4-methoxypyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
H	H			N-(2-(4-methoxycarbonylpyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
H	H			(pyrazolate-1-yl)₂B(pyrazolate-1-yl)₂		CDCl₃ [156,157]
H	H	tricyclohexylphosphine-P	4-methoxyphenylethynyl-C			CD₂Cl₂ [158]	VUYKAE [158]	trans(C,N)
H	H	dimethylphenylphosphine	1-methoxynaphth-8-yl			CDCl₃ [151]
H	H	diphenylphosphine	Cl			CDCl₃ [159]
H	H	(phenylethynyl)diphenylphosphine	Cl			CDCl₃ [159]
H	H	triphenylphosphine-P	F			CDCl₃ [40]	FULXEQ ^(a) [40]	trans(P,N)
H	H	triphenylphosphine	I			CDCl₃ [40]
H	H	triphenylphosphine	succinimidate			CDCl₃ [69]			CAT [69]
H	H	triphenylphosphine	maleimidate			CDCl₃ [69]			CAT [69]
H	H	triphenylphosphine-P	phthalimidate-N			CDCl₃ [69]	FELQET [69]	trans(P,N)	CAT [69]
H	H	tris(4-fluorophenyl)phosphine	succinimidate			CDCl₃ [69]			CAT [69]
H	H	tris(4-fluorophenyl)phosphine	maleimidate			CDCl₃ [69]			CAT [69]
H	H	tris(4-fluorophenyl)phosphine	phthalimidate			CDCl₃ [69]			CAT [69]
H	H	tris(4-methoxyphenyl)phosphine	succinimidate			CDCl₃ [69]			CAT [69]
H	H	tris(4-methoxyphenyl)phosphine	maleimidate			CDCl₃ [69]			CAT [69]
H	H	tris(4-methoxyphenyl)phosphine	phthalimidate			CDCl₃ [69]			CAT [69]
H	H	(2-formylphenyl)diphenylphosphine	Cl			CDCl₃ [71]
H	H		T	(2-formylphenyl)diphenylphosphine	CF₃SO₃⁻	CDCl₃ [71]
H	H			2-diphenylphosphinobenzoate-κ²-P,O		CDCl₃ [73,74]	EQIROL ^(b) [73]	trans(P,N)	CAT [74]
H	H			N-isopropyl-2-diphenylphosphinobenzamide-κ²-P,O	PF₆⁻	CDCl₃ [73]	EQIRUR ^(a) [73]	trans(P,N)
H	H			N-phenyl-2-diphenylphosphinobenzamide	PF₆⁻	CDCl₃ [73]
H	H			N-isopropyl-2-diphenylphosphinobenzamidate		CDCl₃ [73]
H	H			N-phenyl-2-diphenylphosphinobenzamidate		CDCl₃ [73]
H	H	(2-(isopropylaminocarbonyl)phenyl)diphenylphosphine-P	Cl			CDCl₃ [73]	EQIREB ^(c) [73]	trans(P,N)
H	H	(2-(phenylaminocarbonyl)phenyl)diphenylphosphine	Cl			CDCl₃ [73]
H	H	(2-(isopropylaminocarbonyl)phenyl)diphenylphosphine	acetate			CDCl₃ [73]
H	H	(2-(phenylaminocarbonyl)phenyl)diphenylphosphine	acetate			CDCl₃ [73]
H	H			2-(diphenylphosphinoxymethyl)pyridine	PF₆⁻	CD₃COCD₃ [75]
H	H			2-(diphenylphosphinoamino)pyridine	PF₆⁻	CD₃COCD₃ [75]
H	H			2-(diphenylphosphinoazanido)pyridine		CDCl₃ [75]
H	H			CH₃CH₂OCOCHCOCH₂P(C₆H₅)₃ ^(8,9)	ClO₄⁻	CD₂Cl₂ [76]
H	H			C₆H₅NHCSC(P(C₆H₅)₃)COOCH₃ ^(10A)	ClO₄⁻	CD₂Cl₂ [78]
H	H			NCC(P(C₆H₅)₃)C(COOCH₃)CHCOOCH₃ ^(10B)	ClO₄⁻	CDCl₃ [78]
H	H	C(COOCH₃)C(COOCH₃)P(C₆H₅)₃ ^(10C)	triphenylphosphine		ClO₄⁻	CD₂Cl₂ [79]
H	H			1-((diphenylphosphinomethyl)diphenylphosphino)-1-(4-chlorobenzoyl)methyl	ClO₄⁻	DMSO-d₆ [160]			CAT [159]
H	H			1-((diphenylphosphinomethyl)diphenylphosphino)-1-(4-bromobenzoyl)methyl	ClO₄⁻	DMSO-d₆ [160]			CAT [159]
H	H			1-((diphenylphosphinomethyl)diphenylphosphino)-1-(4-methoxybenzoyl)methyl	ClO₄⁻	DMSO-d₆ [160]			CAT [159]
H	H			1-((diphenylphosphinomethyl)diphenylphosphino)-1-(4-nitrobenzoyl)methyl	ClO₄⁻	DMSO-d₆ [160]			CAT [159]
H	H	(diphenylphosphorylamino)diphenylphosphine	Cl			CDCl₃ [161]
H	H			(diphenylphosphorylazanido)diphenylphosphine		CDCl₃ [161]
H	H			1-((diphenylphosphorylmethyl)diphenylphosphino)-1-(4-nitrobenzoyl)methylidene-κ²-C^methyl,O^CO		CDCl₃ [162]	CALWIZ ^(d) [162]	trans(C,N)
H	H			2-(diisopropylphosphinomethyl)pyridine	CH₃COO⁻	CD₂Cl₂ [163]
H	H			7-(diisopropylphosphinomethyl)-1,8-naphthyridine-2-one-κ²-N⁸,P	CH₃COO⁻	CD₂Cl₂ [163]	VAHTAD ^(a) [163]	trans(P,N)
H	H			7-(diisopropylphosphinomethyl)-1,8-naphthyridine-2-one-κ²-N⁸,P	B(3,5-bis(trifluoromethyl)phenyl)₄⁻	CD₂Cl₂ [163]	VAHSUW ^(a) [163]	trans(P,N)
H	H	1-methyl-3-(dibenzofuran-4-yl)imidazol-2-ylidene-C²	I			CDCl₃ [164]	HEXTIR [164]	trans(C,N)
H	H	1-isopropyl-3-(2-hydroxy-2-methylpropyl)imidazol-2-ylidene-C²	Cl			CD₂Cl₂ [165]	LUXYUZ [165]	trans(C,N)
H	H			1-isopropyl-3-(2-oxy-2-methylpropyl)imidazol-2-ylidene		CD₃CN [165]			CAT [165]
H	H	1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene	Cl			CDCl₃ [166]			CAT [166]
H	H	1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene-C²	Cl			CDCl₃ [166]	ZIGPEN [166]	trans(C,N)	CAT [166]
H	H	1,3-diisopropylbenzimidazol-2-ylidene-C²	Br			CDCl₃ [92]	QEZFOT [92]	trans(C,N)
H	7-methyl			N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
H	7-chloro			N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
H	7-methoxy			N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
H	7-isopropoxy			N-(7,8-benzoquinolin-10-yl)-4-methoxybenzenesulfonamidate-κ²-N^bqn’,N^amidate		CDCl₃ [156]	CAMSAM [156]	trans(N^amidate,N^bqn)
H	7-isopropoxy			N-(7,8-benzoquinolin-10-yl)-4-methoxybenzenesulfonamidate-κ²-N^bqn’,N^amidate		CDCl₃ [156]	CAMTAN ^(a) [156]	trans(N^bqn’,N^bqn)
	H	7-formyl			N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
	H	7-cyano			N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
	H	7-nitro			N-(2-(pyrid-2-yl)phenyl)benzenesulfonamidate		CDCl₃ [154]
	H	7-nitro			N-(2-(pyrid-2-yl)phenyl)-4-tert-butylbenzenesulfonamidate		CDCl₃ [154]
	H	7-nitro			N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]
	H	7-nitro			N-(2-(4-methylpyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		CDCl₃ [154]

⁽¹⁾ n-pentane-2,4-dionate anionic ligand (−1 charge) is also called acetylacetonate. ⁽²⁾ This ionic compound ([Pd(7,8-benzoquinoline*)(CN)₂]⁻ Tl⁺) can also be regarded as pentacoordinated [PdTl(7,8-benzoquinoline*)(CN)₂] neutral molecule. ⁽³⁾ 1-phenyl-n-butane-1,3-dione anionic ligand (−1 charge) is also called benzoylacetonate. ⁽⁴⁾ Pentacoordinated complex (CNS₃ chromophore). ⁽⁵⁾ This [Pd(7,8-benzoquinoline*)(2ppy*)] compound is present also in Table 1. ⁽⁶⁾ 2-(thiophen-2-yl)pyridine* is N(1),C(3′)*-chelating, monoanionic form of 2-(thiophen-2-yl)pyridine, deprotonated in the thiophen side group at the carbon C(3′). ⁽⁷⁾ This is [Pd(7,8-benzoquinoline*)₂] compound. ⁽⁸⁾ CH₃CH₂OCOCHCOCH₂P(C₆H₅)₃ is a neutral ylide ligand having (−1) charge at –CH– and (+1) charge at P. ⁽⁹⁾ Mixture of two isomers differing in the position of –COOCH₂CH₃ or -COCH₂P(C₆H₅)₃ moieties in respect to pyridine and phenyl rings of 7,8-benzoquinoline*. ^(10A–10C) C₆H₅NHCSC(P(C₆H₅)₃)COOCH₃ ^(10A), NCC(P(C₆H₅)₃)C(COOCH₃)CHCOOCH₃ ^(10B), and C(COOCH₃)C(COOCH₃)P(C₆H₅)₃ ^(10C) are neutral ylide ligands having (−1) charge at –C– (P-bonded) and (+1) charge at P. ^(a) Dichloromethane solvate. ^(b) Chloroform solvate. ^(c) Methanol solvate. ^(d) Acetone solvate.

Dinuclear and Oligonuclear Pd(II)-ArPY^#* Compounds

A total of 34 Pd(II)-ArPY* di- and oligonuclear compounds were reported; 32 of them were NMR- and/or X-ray-studied [13,31,33,39,40,44,45,47,78,99,102,103,128,148,149,150,154,155,156,157,158,163,167,168,169].

A total of 30 Pd(II)-ArPY* molecules are dinuclear [13,31,33,39,40,44,45,47,78,99,102,103,128,148,149,150,154,155,156,157,158,167,168,169]. 29 of them correspond to the dimeric [Pd(ArPY*)(µ-X)]₂ general formula [13,31,33,39,40,44,45,47,78,99,102,103,128,148,149,150,154,155,156,157,158,167,168,169], while 1 corresponds to the [Pd(ArPY*)(µ-XX)_0.5]₂ one [102,103]. On the other hand, all of them contain a 7,8-benzoquinoline* ring system, i.e., unsubstituted 7,8-benzoquinoline* (25 species) [13,31,33,39,40,44,45,47,78,99,102,103,128,148,149,150,154,155,156,157,158,167,168,169] or its variously substituted derivatives (5 species: 7-methyl-, 7-chloro-, 7-iodo-, 7-isopropoxy-, 7-formyl-) [102,103,156]. All of these dimers are listed in Table 8, including 12 X-ray structures (for 12 species).

Table 8. NMR- and/or X-ray-studied dimeric [Pd(ArPY^#*)(µ-X)]₂ or [Pd(ArPY^#*)(µ-XX)_0.5]₂ compounds (ArPY^#* ≠ 2PPY*, 2ArPY*; X—bidentate bridging ligand, Y—monodentate ligand); R¹ and R² are substituents in the pyridine-like ring and the aryl ring, respectively.

Parent ArPY^#* Ring System	R¹	R²	μ−X or µ−XX	Counterion	NMR Solvent	X-ray (CCDC)	Geometry	Activity
7,8-benzoquinoline	H	H	Cl		DMSO-d₆ [103,156,157,167]			BIO [152]
	H	H	I		CDCl₃ + pyridine-d₅ [40]
	H	H	OH			COHNET ⁽¹⁾ [99]
	H	H	acetate-O,O		CDCl₃, CD₂Cl₂ [33,44,45,47,102,103,148,150,154,155,156,157,167,168,169]	YULCUE ⁽²⁾ [45,102,103,148,169] WEFFIY ^{(2) (a)} [148,169] PEMVEK ^{(2) (b)} [148]		CAT [102]
	H	H	acetate-d₃		CDCl₃ [102,103]
	H	H	propionate		CD₂Cl₂ [103]
	H	H	n-hexanoate		CDCl₃ [167]
	H	H	benzoate		CDCl₃ [103]
	H	H	½ benzene-1,3-bis(2,2-dimethylpropionate)		CDCl₃ [102,103]			CAT [102]
	H	H	trifluoroacetate-O,O		CD₂Cl₂ [149]	EBOGUA ⁽²⁾ [149]
	H	H	4-fluorobenzoate		CDCl₃, CD₂Cl₂ [103]
	H	H	4-bromobenzoate		CDCl₃ [103]
	H	H	4-acetylbenzoate		CD₂Cl₂ [103]
	H	H	4-nitrobenzoate		CD₂Cl₂ [103]
	H	H	succinimidate-N,O		CD₂Cl₂, CD₂Cl₂+C₆D₆ [45]	ANOJIX ^{(2) (c)} [45]	trans(N,N)	CAT [45,69]
	H	H	maleimidate					CAT [69]
	H	H	phthalimidate					CAT [69]
	H	H	saccharinate		CDCl₃ [39]			CAT [39] LUM1 [39]
	H	H	pyridine-2-thiolate		CDCl₃ [13,128]			LUM1 [128] LUM2 [31]
	H	H	pyrimidine-2-thiolate		CDCl₃ [31]			LUM2 [31]
	H	H	1-methylimidazol-2-thiolate-S,N³		CDCl₃ [31]	ASEHAI ^{(2) (d)} [31]		LUM2 [31]
	H	H	benzimidazol-2-thiolate		CDCl₃ [31]			LUM2 [31]
	H	H	2H-2-oxo-7-((pyrid-2-yl)methyl)(methyl)aminomethyl)-1,8-naphthyridin-1-ide-N¹,N⁸		CDCl₃ [158]	VUYKEI ⁽²⁾ VUYHAB ^{(2) (a)} VUYGUU ^{(2) (e)} [158]
	H	H	µ-2H-2-oxo-7-(bis((pyrid-2-yl)methyl)aminomethyl)-1,8-naphthyridin-1-ide-N¹,N⁸		CDCl₃ [158]	VUYGOO ^{(2) (a)} [158]
	H	H	SC(NHC₆H₅)C(P(C₆H₅)₃)CN ⁽³⁾	2ClO₄⁻	DMSO-d₆ [78]
	H	H	diethylphosphonate-P,O			EBOHEL ⁽¹⁾ [149]	trans(P,N)
	H	7-methyl	acetate		CD₂Cl₂ [103]
	H	7-chloro	acetate		CDCl₃, CD₂Cl₂ [103]
	H	7-iodo	acetate		CD₂Cl₂ [103]
	H	7-isopropoxy	acetate		CDCl₃ [156]
	H	7-formyl	acetate		CD₂Cl₂ [103]

⁽¹⁾ Both ArPY^#* ligands are nearly in the same plane, exhibiting transoid geometry (in respect to the position of their nitrogen atoms). ⁽²⁾ Both ArPY^#* ligands are in different but nearly parallel planes (lying on the same side of the Pd-Pd axis), exhibiting transoid geometry (in respect to the position of their nitrogen atoms). ⁽³⁾ SC(NHC₆H₅)C(P(C₆H₅)₃)CN is a zwitterionic ligand having (−1) charge at S and (+1) charge at P (also some mesomeric ylide forms are possible). ^(a) Dichloromethane solvate. ^(b) 1,1,2,2-tetracyanoethene dichloromethane solvate. ^(c) Dichloromethane diethyl ether solvate. ^(d) Chloroform solvate. ^(e) Dichloromethane tetrahydrofuran solvate.

Additionally, 4 Pd(II)-ArPY^#* compounds (and 4 X-ray structures) contain more than two palladium atoms, their molecular formulae being as follows: [Pd(7,8-benzoquinoline*)(µ-acetate)₂Pd(µ-acetate)(µ-Cl)Pd(7,8-benzoquinoline*)] (X-ray structure YULDEP) [102,103]; [(Pd(7,8-benzoquinoline*)(acetate))₂(Pd(7,8-benzoquinoline*))₂(µ³-7-diisopropylphosphinomethyl-1,8-naphthyridine-2-one-P,N¹,O))₂] (NMR in CD₂Cl₂; X-ray: VAHTUX for cyclohexane toluene solvate); [(Pd(7,8-benzoquinoline*))₄(µ⁴-(7-diisopropylphosphinomethyl-1,8-naphthyridine-2-one-P,N¹,N⁸,O))₂](B(C₆H₅)₄)₂ (NMR in CD₂Cl₂; X-ray: VAHVAF for cyclohexane tetrahydrofuran solvate); and [(Pd(7,8-benzoquinoline*))₄(µ⁴-(7-diisopropylphosphinomethylidene-1,8-naphthyridine-2-one-8-ide-P,N¹,N⁸,O))₂] (X-ray: VAHVEJ for diethyl ether solvate) [163]. The details of Pd(II) linking in these oligonuclear (tri- and tetranuclear) species are in the original papers.

2.2. Pd(III) Compounds

The Pd(III) compounds with 2-arylpyridines* are much more rare than the Pd(II) ones, nevertheless, 27 species were reviewed; all were NMR- and/or X-ray-studied (14 X-ray structures for 9 species) [44,45,102,103,148,167,168].

5 of them are with 2ppy* or its derivatives [44,45,103], while 22 are with 7,8-benzoquinoline* or its derivatives [44,45,102,103,148,167,168]; in the case of the other 2-arylpyridines*, no examples have been found.

None of these molecules are mononuclear, whereas 23 are dinuclear [44,45,102,103,148,167,168] (22 exhibit the dimeric general formula [Pd(2-arylpyridine*)Y(µ-X)]₂ (in one case with different Y¹, Y² ligands in both halves of the dimer: [Pd(7,8-benzoquinoline*)Cl(µ-succinimidate)₂Pd(7,8-benzoquinoline*)(acetate)] [45]), while 1 exhibits the [Pd(2-arylpyridine*)Y(µ-XX)_0.5]₂ one), and 4 are polynuclear [167,168].

All 23 Pd(III)-(2-arylpyridine*) dimers are electrically neutral molecules. They are listed in Table 9, including 5 X-ray structures.

Table 9. NMR- and/or X-ray-studied dimeric Pd(III) compounds with 2-arylpyridines*, of the general formula [Pd(2-arylpyridine*)Y(µ-X)]₂ and [Pd(2-arylpyridine*)Y(µ-XX)_0.5]₂ (X—bridging bidentate ligand, XX—bridging tetradentate ligand, Y—monodentate ligand); R¹ and R² are substituents in the pyridine or pyridine-like ring and the aryl ring, respectively.

Parent 2-arylpyridine* Ring System	R¹	R²	Y	µ−X or µ−XX	NMR Solvent	X-ray (CCDC)	Activity
2-phenylpyridine*	H	H	acetate-O	acetate-O,O	CD₂Cl₂ [44,45]	CUHQAY ^{(1) (a)} [44,45]	CAT [44,45]
	H	H	4-nitrobenzoate	Benzoate	CD₂Cl₂ [103]
	H	H	Cl	2,2,3,3-tetramethylsuccinimidate	CD₂Cl₂ [45]
	H	H	acetate	2,2,3,3-tetramethylsuccinimidate	CD₂Cl₂ [45]
	3-methyl	H	acetate	acetate	CD₂Cl₂ [44]
7,8-benzoquinoline*	H	H	F	acetate-O,O		PAHREX ^{(1) (a)} [167]
	H	H	Cl	acetate-O,O	CD₂Cl₂ [102,103,148,167]	YULDAL ^{(1) (b)} [102,103]	CAT [102]
	H	H	Cl	n-hexanoate	CD₂Cl₂ [167]
	H	H	Br	acetate-O,O	CDCl₃ [102]
	H	H	trifluoromethyl	acetate	CD₂Cl₂ [148]
	H	H	acetate-O	acetate-O,O	CD₂Cl₂ [44,102,103,148]	YULDIT ^{(1) (a)} [102,103]
	H	H	acetate	acetate-d₃	CD₂Cl₂ [102,103]
	H	H	propionate	propionate	CD₂Cl₂ [103]
	H	H	Cl	½ benzene-1,3-bis(2,2-dimethylpropionate)	CDCl₃ [102,103]
	H	H	Cl	benzoate	CD₂Cl₂ [103]
	H	H	Cl	4-fluorobenzoate	CD₂Cl₂ [103]
	H	H	Cl	4-bromobenzoate	CD₂Cl₂ [103]
	H	H	Cl	4-acetylbenzoate	CD₂Cl₂ [103]
	H	H	Cl	4-nitrobenzoate	CD₂Cl₂ [103]
	H	H	½ Cl,½ acetate	succinimidate	CD₂Cl₂ [45]
	H	7-methyl	Cl	acetate	CD₂Cl₂ [103]
	H	7-chloro	F	acetate-O,O	CD₂Cl₂ [168]	PIVGEI ^{(1) (a)} [168]
	H	7-chloro	Cl	acetate	CD₂Cl₂ [103]
	H	7-iodo	Cl	acetate	CD₂Cl₂ [103]

⁽¹⁾ Both 2-arylpyridine* ligands are in different but nearly parallel planes (lying on the same side of the Pd-Pd axis), exhibiting transoid geometry (in respect to the position of their nitrogen atoms). ^(a) Dichloromethane solvate. ^(b) Dichloromethane iodobenzene solvate.

All 4 Pd(III) polynuclear species are ionic, containing [Pd(7,8-benzoquinoline*)(µ-acetate-O,O)]_n^a+ [167,168], [Pd(7,8-benzoquinoline*)(µ-n-hexanoate)]_n^a+ [167], or [Pd(7-chloro-7,8-benzoquinoline*)(µ-acetate)]_n^a+ [168] (a = n or n/2) cations and F⁻, PF₆⁻, or BF₄⁻ anions in various amounts [167,168].

Their exact molecular formulae are as follows: [Pd(7,8-benzoquinoline*)(µ-acetate-O,O)]_nF_n (NMR in CD₂Cl₂ [167,168]; X-ray: PAHRIB for dichloromethane solvate [167]) or [Pd(7,8-benzoquinoline*)(µ-acetate-O,O)]_nF_2n/3(PF₆)_n/3 (X-ray: PIVFUX for dichloromethane solvate [168]), and [Pd(7,8-benzoquinoline*)(µ-n-hexanoate)]_n(BF₄)_n (NMR in CD₂Cl₂ [167]), as well as [Pd(7,8-benzoquinoline*)(µ-acetate-O,O)]_nF_n/2 (NMR in CD₂Cl₂ [167,168]; X-ray: PAHSOI, PAHSOI 01, PAHSOI 02, PAHSOI 03, PAHSOI 04, PAHSOI 05 for dichloromethane solvate [167]) and [Pd(7-chloro-7,8-benzoquinoline*)(µ-acetate)]_nF_n/2 (NMR in CD₂Cl₂ [168]; X-ray: PIVGAE for dichloromethane solvate [168]). The two latter compounds contain [Pd(7,8-benzoquinoline*)(µ-acetate-O,O)]_n^(n/2)+ and [Pd(7-chloro-7,8-benzoquinoline*)(µ-acetate)]_n^(n/2)+ cations, in which the formal palladium oxidation state is +2.5, suggesting the simultaneous presence of Pd(II) and Pd(III) atoms.

2.3. Pd(IV) Compounds

The Pd(IV) compounds with 2-arylpyridines* are also more rare than the Pd(II) ones, but more abundant than those of Pd(III). Thus, 88 compounds were reviewed; all were NMR- and/or X-ray-studied (16 X-ray structures for 16 species) [37,38,44,45,46,47,48,50,80,123,148,154,155,156,157,165,170,171].

A total of 26 of them are with 2ppy* (23 of [Pd(2ppy*)₂L¹L²] type and 3 of [Pd(2ppy*)ABCD] type; for explanation see below) [37,38,44,45,46,47,48,50,80,165,170], while 12 with 2PPY* ≠ 2ppy* (R¹ = H, R² = 3-methyl-, 4-methyl-, 3-fluoro-, 4-fluoro-, 3-chloro-, 4-methoxy-) [37,38,50,123] and 4 with 2ArPY* ≠ 2PPY^* (2-benzylpyridine* derivatives: 2-(1-hydroxybenzyl-1-olate)pyridine* and 2-(1-hydroxy-3-methylbenzyl-1-olate)pyridine*; in these ligands both methylene bridge hydrogens are substituted by two OH groups (one deprotonated), so they are dianionic and tridentate, chelating Pd(IV) by N(1), C(6′) and O) [50,123]. Then, a total of 46 of them are with ArPY^#* (30 with 7,8-benzoquinoline and 16 with its substituted derivatives: 7-methyl-, 7-chloro-, 7-methoxy-, 7-isopropoxy-, 7-formyl-, 7-cyano-, and 7-nitro) [37,38,46,148,154,155,156,157,165,171].

All of Pd(IV)-(2-arylpyridine*) compounds, listed in Table 10 (including 16 X-ray structures for 16 species) are mononuclear (monomeric).

A total of 35 of them have the [Pd(2-arylpyridine*)₂L¹L²] (in particular, [Pd(2-arylpyridine*)₂L₂]) general formula (Table 10 top) [37,38,44,45,46,47,48,170], whereas none exhibits the [Pd(2-arylpyridine*)₂(LL)] one (thus, Pd(IV) compounds containing two 2-arylpyridine* ligands and one chelating LL ligand remain unknown). Then, in all cases both 2-arylpyridine* ligands are, for a given Pd(IV) compound, identical (no [Pd(2-arylpyridine*)¹(2-arylpyridine*)²L¹L²] species were reported). Moreover, these [Pd(2-arylpyridine*)₂L¹L²] molecules are always electrically neutral.

In contrast, 53 compounds correspond to many types of the [Pd(2-arylpyridine*)ABCD] general formula, where A, B, C, and D denote variable numbers of mono-, bi- or tridentate ligands—in various combinations resulting in the coordination number 6 and in the electric charges ranging from 0 to +2 (Table 10 bottom) [50,80,123,148,154,155,156,157,165,171].

Table 10. NMR- and/or X-ray-studied monomeric Pd(IV) compounds with 2-arylpyridines*, of the general formulae [Pd(2-arylpyridine*)₂L¹L²] (in particular, [Pd(2-arylpyridine*)₂L₂]) (top; L¹, L², L denote monodentate ligands) or [Pd(2-arylpyridine*)ABCD] (bottom; A, B, C, D denote mono-, bi- or tridentate ligands in various numbers and combinations). R¹ and R² are substituents in the pyridine or pyridine-like ring and the aryl ring, respectively.

[Pd(2-arylpyridine*)₂L¹L²]
Parent 2-arylpyridine* Ring System	R¹	R²	L¹		L²		NMR Solvent		X-ray (CCDC)	Geometry	Activity
2-phenylpyridine*	H	H	Cl		Cl		CDCl₃ [47,170]
	H	H	acetate-O		acetate-O		CDCl₃, CD₃COCD₃, CD₃COOD [38,44,45]		LUFQIN ^(a) [38] ZUNYOY ^(b) [48]	trans(O,N^2ppyA), trans(C^2ppyA, N^2ppyB)	CAT [44,45]
	H	H	acetate ⁽¹⁾		acetate-d₃ ⁽¹⁾		CD₃COCD₃ [38]
	H	H	acetate-d₃		acetate-d₃		CDCl₃ [38]
	H	H	acetate		Cl		DMSO-d₆ [38]
	H	H	acetate-d₃		Cl		DMSO-d₆ [38]
	H	H	n-decanoate		n-decanoate		CDCl₃ [37]
	H	H	n-decanoate-O		acetate-O		CDCl₃ [38]		LUFQOT [38]	trans(O^acetate, N^2ppyA), trans(C^2ppyA, N^2ppyB)
	H	H	benzoate		benzoate		CDCl₃ [37]
	H	H	4-methylbenzoate		4-methylbenzoate		CDCl₃ [37]
	H	H	4-fluorobenzoate		4-fluorobenzoate		CDCl₃ [37]
	H	H	4-trifluoromethylbenzoate		4-trifluoromethylbenzoate		CDCl₃ [37]
	H	H	4-chlorobenzoate		4-chlorobenzoate		CDCl₃ [37]
	H	H	4-bromobenzoate		4-bromobenzoate		CDCl₃ [37]
	H	H	4-methoxybenzoate		4-methoxybenzoate		CDCl₃ [37]
	H	H	4-phenoxybenzoate		4-phenoxybenzoate		CDCl₃ [37]
	H	H	4-acetylbenzoate		4-acetylbenzoate		CDCl₃ [37]
	H	H	4-trifluoroacetylbenzoate		4-trifluoroacetylbenzoate		CD₃COCD₃ [38]
	H	H	4-cyanobenzoate		4-cyanobenzoate		CDCl₃ [37]
	H	H	4-nitrobenzoate-O		4-nitrobenzoate-O		CDCl₃ [37]		QAVCAS ^(c) [37]	trans(O,N^2ppyA), trans(C^2ppyA,N^2ppyB)
	H	H	trifluoromethylsulfonyl		Cl		DMSO-d₆ [46]
	H	H	4-methylphenylsulfonyl		Cl		CD₂Cl₂ [46]
	H	H	benzylsulfonyl		Cl		CD₂Cl₂ [46]
	H	H	4-fluorophenylsulfonyl		Cl		CD₂Cl₂ [46]
	H	H	4-methoxyphenylsulfonyl		Cl		CD₂Cl₂ [46]
	H	H	succinimidate-N		Cl		CDCl₃,DMSO-d₆ [170]		PITZOI ^(b) [170]	trans(Cl,N^2ppyA), trans(C^2ppyA,N^2ppyB)
	H	3-methyl	benzoate		benzoate		CDCl₃ [37]
	H	3-methyl	4-chlorobenzoate		4-chlorobenzoate		CDCl₃ [37]
	H	3-methyl	4-bromobenzoate		4-bromobenzoate		CDCl₃ [37]
	H	3-methyl	4-acetylbenzoate		4-acetylbenzoate		CDCl₃ [38]
	H	3-fluoro	4-acetylbenzoate		4-acetylbenzoate		CDCl₃ [38]
	H	3-chloro	4-acetylbenzoate		4-acetylbenzoate		CDCl₃ [38]
7,8-benzoquinoline*	H	H	acetate		acetate		CD₃COCD₃ [38]
	H	H	acetate		Cl		DMSO-d₆ [38]
	H	H	n-decanoate		n-decanoate		CDCl₃ [37]
	H	H	nonadecafluoro-n-decanoate		nonadecafluoro-n-decanoate		CD₃COCD₃ [38]
	H	H	4-trifluoroacetylbenzoate		4-trifluoroacetylbenzoate		CD₃COCD₃ [38]
	H	H	4-methylphenylsulfonyl		Cl		CD₂Cl₂ [46]
	*[Pd(2-arylpyridine)ABCD]**
Parent2-arylpyridine*ring system	R¹	R²	A	B	C	D	Counterion	NMR Solvent	X-ray (CCDC)	Geometry	Activity
2-phenylpyridine*	H	H	hydroxybis(pyrid-2-yl)methanolate			OH	CH₃COO⁻	D₂O [50]
	H	H	2-(N,P,P-triphenylphosphorimido-P-yl)phenyl		acetate	acetate		CD₂Cl₂ [80]
	H	H	1-isopropyl-3-(2-oxy-2-methylpropyl)imidazol-2-ylidene		succinimidate	Br		CD₃CN [165]
	H	4-methyl	hydroxy(bis(pyrid-2-yl))methanolate			Cl	Cl⁻	D₂O [123]
	H	4-methyl	hydroxy(bis(pyrid-2-yl))methanolate			Br	Br⁻	D₂O [123]
	H	4-methyl	hydroxy(bis(pyrid-2-yl))methanolate ⁽²⁾			OH ⁽²⁾	CH₃COO⁻	D₂O [50,123]
	H	4-methyl	hydroxy(bis(pyrid-2-yl))methanolate			H₂O	2Cl⁻	D₂O [123]
	H	4-methyl	hydroxy(bis(pyrid-2-yl))methanolate			H₂O	2Br⁻	D₂O [123]
	H	4-methyl	hydroxy(bis(pyrid-2-yl))methanolate-κ³-N,N,O			H₂O	2CF₃COO⁻	D₂O [123]	APADAX ^(d) [123]	trans(N,N^bqn), trans(N,C^bqn)
	H	4-fluoro	hydroxy(bis(pyrid-2-yl))methanolate ⁽²⁾			OH ⁽²⁾	CH₃COO⁻	D₂O [50]
	H	4-methoxy	hydroxy(bis(pyrid-2-yl))methanolate ⁽²⁾			OH ⁽²⁾	CH₃COO⁻	D₂O [50]
2-(1-hydroxybenzyl-1-olate)pyridine*-κ³-N,C,O	H	H	hydroxy(bis(pyrid-2-yl))methanolate-κ³-N,N,O			none	CH₃COO⁻	D₂O, CD₃COOD [123]	APACIE ^(e) [123]	trans(N,N^bqn), trans(N,C^bqn)
	H	H	hydroxy(bis(pyrid-2-yl))methanolate			none	CF₃COO⁻	DMSO-d₆ [123]
	H	H	hydroxy(bis(pyrid-2-yl))methanolate			none	BF₄⁻	DMSO-d₆ [123]
	H	H	oxy(bis(pyrid-2-yl))methanolate-κ³-N,N,O			none		CD₃OD, CF₃CH₂OD [123]	APACUQ ^(f) [123]	trans(N,N^bqn), trans(N,C^bqn)
	H	3-methyl	hydroxy(bis(pyrid-2-yl))methanolate			none	CH₃COO⁻	D₂O [50,123]
	H	3-methyl	hydroxy(bis(pyrid-2-yl))methanolate			none	CF₃COO⁻	DMSO-d₆ [123]
	H	3-methyl	hydroxy(bis(pyrid-2-yl))methanolate			none	BF₄⁻	DMSO-d₆ [123]
	H	3-methyl	oxy(bis(pyrid-2-yl)methanolate-κ³-N,N,O			none		CD₃OD [123]	APADEB ^(g) [123]	trans(N,N^bqn), trans(N,C^bqn)
7,8-benzoquinoline *	H	H	acetate-O	acetate-O	trifluoromethyl-C	H₂O		CDCl₃, CD₂Cl₂ [148,171]	OPOQAM [171] WEFFEU ^(c) [148]	trans(O^acetate,N^bqn)(O^H2O,C^bqn)
	H	H	N-(2-(pyrid-2-yl)phenyl)benzenesulfonamidate			F	BF₄⁻	CD₃CN [154]
	H	H	N-(2-(pyrid-2-yl)phenyl)-4-methylbenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H	N-(2-(pyrid-2-yl)phenyl)-4-tert-butylbenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H		N-(2-(pyrid-2-yl)phenyl)-4-phenylbenzenesulfonamidate		F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H		N-(2-(pyrid-2-yl)phenyl)-4-fluorobenzenesulfonamidate		F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H		N-(2-(pyrid-2-yl)phenyl)-4-chlorobenzenesulfonamidate		F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H	N-(2-(pyrid-2-yl)phenyl)-4-cyanobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CD₃CN [154]
	H	H	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		acetonitrile	F	BF₄⁻	CD₃CN [155]
	H	H	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		pyridine	F	BF₄⁻	CD₃CN [154]
	H	H	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate-κ²-N^py,N^amidate		F	F		DMSO-d₆ [154,155]	JOHGOD ^(h) [154,155]	trans(N^py,N^bqn), trans(F,C^bqn)
	H	H	N-(2-(pyrid-2-yl)phenyl)-4-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H	N-(2-(4-methylpyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H	N-(2-(4-trifluoromethylpyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H	N-(2-(4-bromopyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H	N-(2-(4-methoxypyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H	N-(2-(4-methoxycarbonylpyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	H	(pyrazolate-1-yl)₃B(pyrazolate-1-yl)-κ³-N^pz1,N^pz2,N^pz3			F	CF₃SO₃⁻	CD₃CN, DMSO-d₆ [156,157]	CAMSIU [156]	trans(N^pz1,N^bqn), trans(N^pz2,C^bqn)	MAT [156]
	H	H	(pyrazolate-1-yl)₃B(pyrazolate-1-yl)-κ³-N^pz1,N^pz2,N^pz3			pyridine-N	2CF₃SO₃⁻	CDCl₃, CD₃CN [157]	OPETOU ⁽ⁱ⁾ [157]	trans(N^pz1,N^bqn), trans(N^pz2,C^bqn)
	H	H	(pyrazolate-1-yl)₃B(pyrazolate-1-yl)-κ³-N^pz1,N^pz2,N^pz3			4-methylpyridine-N	2CF₃SO₃⁻	CD₃CN [156,157]	CAMSEQ ⁽ⁱ⁾ [156]	trans(N^pz1,N^bqn), trans(N^pz2,C^bqn)
	H	H	(pyrazolate-1-yl)₃B(pyrazolate-1-yl)			4-cyanopyridine	2CF₃SO₃⁻	CD₃CN [156,157]
	H	H	1-isopropyl-3-(2-oxy-2-methylpropyl)imidazol-2-ylidene-κ²-C²,O		Cl	Cl		CD₃CN [165]	LUXZAG ^(j) [165]	trans(C,N), trans(Cl,C)
	H	H	1-isopropyl-3-(2-oxy-2-methylpropyl)imidazol-2-ylidene		succinimidate	Br		CD₃CN [165]
	H	7-methyl	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-methyl	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		pyridine	F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-chloro	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-chloro	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		pyridine	F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-methoxy	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-methoxy	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		pyridine	F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-isopropoxy	N-(7,8-benzoquinolin-10-yl)-4-methoxybenzenesulfonamidate			F	BF₄⁻	CDCl₃ [156]
	H	7-formyl	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-formyl	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		pyridine	F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-cyano	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-cyano	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		pyridine	F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-nitro	N-(2-(pyrid-2-yl)phenyl)benzenesulfonamidate			F	BF₄⁻	CD₃CN [154]
	H	7-nitro	N-(2-(pyrid-2-yl)phenyl)-4-tert-butylbenzenesulfonamidate			F	BF₄⁻	CD₃CN [154]
	H	7-nitro	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-nitro	N-(2-(pyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate		pyridine	F	BF₄⁻	CDCl₃, CD₃CN [154]
	H	7-nitro	N-(2-(4-methylpyrid-2-yl)phenyl)-2-nitrobenzenesulfonamidate			F	BF₄⁻	CD₃CN [154]

⁽¹⁾ Two isomers, differing in the positions of CH₃COO and CD₃COO ligands. ⁽²⁾ Two isomers, differing probably in geometry (trans(O,O) versus cis(O,O)). ^(a) Benzene solvate. ^(b) Chloroform solvate. ^(c) Dichloromethane solvate. ^(d) Monohydrate. ^(e) Methanol solvate. ^(f) Heptahydrate. ^(g) Decahydrate. ^(h) Acetonitrile solvate. ⁽ⁱ⁾ Diethyl ether acetonitrile solvate. ^(j) Acetone solvate.

3. Discussion

3.1. Discussion of Single Crystal X-ray Structures

3.1.1. Pd(II) Compounds

Mononuclear Pd(II) Compounds

Nearly all mononuclear (monomeric) Pd(II) compounds with 2-arylpyridines* have the coordination number 4 and square-planar geometry. The only exclusions are: [Pd(2ppy*)(1,4,7-trithiacyclononane-κ³-S,S,S)]⁺, [Pd(2-(4-formylphenyl)pyridine*)(1,4,7-trithiacyclononane-κ³-S,S,S)]⁺, and [Pd(7,8-benzoquinoline*)(1,4,7-trithiacyclononane-κ³-S,S,S)]⁺ cations in their hexafluorophosphate salts (VIYXOR, WOMREY, and VIYXUX) [16,17], as well as [Pd(2ppy*)(2,9-dimethyl-1,10-phenanthroline-κ²-N,N’)(H₂O)]⁺ cation in its hexafluorophosphate salt (EKEDOQ), and the neutral molecules of [Pd(2ppy*)(2,9-dimethyl-1,10-phenanthroline-κ²-N,N’)Cl] (EKEDEG) and [Pd(2ppy*)(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)Cl] (no X-ray structure) [53]; all of them exhibit the coordination number 5. Thus, in cases of L¹ ≠ L² or unsymmetrical LL ligands, two geometric isomers are possible which differ in the position of both their donor atoms versus the nitrogen of the pyridine (or pyridine-like) ring and the metallated carbon of the phenyl (or, more generally, aryl) ring (trans/cis isomerism).

In Table 1, Table 3, Table 5, and Table 7, it is indicated, for monomeric Pd(II)-(2-arylpyridine*) compounds with known X-ray structures, which donor atom (or the whole donor moiety) of the auxiliary ligand(s) is trans to the nitrogen of the pyridine (or pyridine-like) ring (column Geometry); this notation unambiguously determines the type of a geometric isomer for square-planar complexes/organometallics.

The comparison of these X-ray structures exhibits that the studied Pd(II)-(2-arylpyridine*) compounds which possess various elements as donor atoms of the auxiliary ligand(s) usually adopt the following geometries:

trans(N,N)—instead of trans(F,N): FULWUF and FULXAM [40]; instead of trans(O,N): POKSEO, POKSIS, POKSIS 01 and POKSOY [13,30], AFABOA, AFABIU and AFABEQ [24], TUMLAO [29], YABMEV, ASEGUB and ASEHEM [30,31], NUPROH, NUPRUN and NUPDOT [32], WOSYEI [33], UXILET [34], WOVHEX [49], PERWIU [55], and XOFKEL and XOFKIP [62]; instead of trans(Cl,N): DUBLAP [6], AMUNED, AMUNAZ, AMUNON and AMUNIH [41], XAKKOM [58], IGERES [61], KOXWAX [140], ZABFEN [141], and FIBRIV, NIJXIR and NIJXEN [152].

trans(C,N)—instead of trans(O,N): CALWIZ [162]; instead of trans(Cl,N): IGERIW [61], MIXNAL and MIXMUE [83], KACMUZ and KACNAG [87], GADLEE [89], SOFBEW, SOFBOG and SOFBIA [125], LUXYUZ [165], and ZIGPEN [166]; instead of trans(N,N): ZUNYIS [48], LOQTIW [80], MUBYAL, MUBRAE and MUBQOR [82], and TAXHAE [153]; instead of trans(Br,N): MUBQUX [82], MIXMOY [83], IGUVOW [84], POHHOL [86], and QEZFEJ, QEZFIN and QEZFOT [92]; instead of trans(I,N): MIXMIS [83] and HEXTIR [164]; instead of trans(P,N): VUYKAE [158].

trans(P,N)—instead of trans(F,N): FULXEQ [40]; instead of trans(O,N): PORNEQ and PORNIU [67], EQIRIF, EQIROL and EQIRUR [73], and FESCEO [74]; instead of trans(Cl,N): PORNUG [67], EQIREB [73], and MAWSOW [121]; instead of trans(N,N): GAJMUA and GAJNAH [64], FELQET [69], UMAPAB [70], CUBVEA [72], EQISEC [73], JAJBOM [75], IJEYAZ and IJEYED [81], and VAHTAD and VAHSUW [163]; instead of trans(S,N): SEMDEU [68].

Hence, generally, the less electronegative (less electron-acceptor) element is preferred to be positioned trans to the pyridine (or pyridine-like) nitrogen. The exceptions are the X-ray structures VUYKAE, i.e., trans(C,N)-[Pd(7,8-benzoquinoline*)(tricyclohexylphosphine-P)(4-methoxyphenylethynyl-C)] [158] (and not the trans(P,N) isomer—although carbon is more electronegative than phosphorus) and KENTEE, i.e., trans(N,N)-[Pd(2-(4-methylphenyl)pyridine*)(1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene-C²)(N₃)] [122] (and not the trans(C,N) isomer—although nitrogen is more electronegative than carbon).

The most important structural parameters for the monomeric Pd(II)-(2-arylpyridine*) compounds are the Pd-N and Pd-C bond lengths, as well as the N-Pd-C bond angles. They were usually given in original papers but can also be deduced from the respective CIF files, which have been used as a primary source for this review. All of these values, associated to the CCDC reference codes given in Table 1, Table 3, Table 5, and Table 7, together with indication which elements (donor atoms) of the auxiliary ligands are in trans position in respect to the metal-bonded N and C atoms, are listed in Table 11.

In the majority of monomeric Pd(II)-(2-arylpyridine*) compounds, the Pd-N bonds are longer than those of Pd-C, which is well-reflected by comparison of their mean bond lengths, averaged for the above 114 X-ray structures (corresponding to 113 compounds), after preliminary averaging of these parameters for a given Pd(II) species (when two or more slightly differing, crystallographically inequivalent molecules are present in the crystal lattice): 2.060 Å versus 1.997 Å. Similarly, the range of Pd-N bond lengths (1.999–2.146 Å) also corresponds to higher values than that for Pd-C (1.951–2.067 Å), despite their partial overlapping.

The N-Pd-C bond angles vary within the 79.7–90.1° range, with a mean value of 81.8°.

Dinuclear Pd(II) Compounds

Independently on the accurate molecular formulae of the dinuclear Pd(II)-(2-arylpyridine*) compounds ([Pd(2-arylpyridine*)(µ-X)]₂, [Pd(2-arylpyridine*)(µ-XX)_0.5]₂ or [Pd(2-arylpyridine*)Y(µ-X)_0.5]₂ dimers), they may have various geometries, depending mainly on the type of bridge between both of the palladium atoms.

In some Pd(II) dimers, both 2-arylpyridine* ligands are nearly in the same plane, the well-known examples being the chloride- and hydroxide-bridged species [Pd(2ppy*)(μ-Cl)]₂ (X-ray structures SOHDUO, SOHDUO 01, SOHDUO 02, SOHDUO 03 [19,95,96,97]) and [Pd(2ppy*)(μ-OH)]₂ (X-ray structures KEXXIV, KEXXIV 01 [98,99]). In many others, both 2-arylpyridine* ligands lie in two distinct planes which are, however, nearly parallel, as exemplified by the acetate-bridged species [Pd(2ppy*)(µ-acetate-O,O)]₂ (X-ray structures XEMQIQ, XEMQIQ 01, XEMQIQ 02, XEMQIQ 03, XEMQIQ 04, XEMQIQ 305 [19,67,107,108,109,110]) and [Pd(7,8-benzoquinoline*)(µ-acetate-O,O)]₂ (X-ray structure YULCUE [45,102,103,148,169]). In the latter case, both bridging X ligands are cis-orientated to each other, so the two 2-arylpyridine* ligands lie on the same side of the Pd-Pd axis.

Examples of both types of Pd(II) dimers are shown in Figure 2 and Figure 3.

Nevertheless, within each above geometric type, the Pd(II)-(2-arylpyridine*) dimers may have a cisoid (like [Pd(2ppy*)(μ-Cl)]₂) or a transoid (like [Pd(2ppy*)(µ-OH)]₂, [Pd(2ppy*)(µ-acetate-O,O)]₂ and [Pd(7,8-benzoquinoline*)(µ-acetate-O,O)]₂) orientation of both nitrogen atoms (see Scheme 5). Generally, the transoid orientation seems to be more abundant than the cisoid one (in fact, the latter was found only for one X-ray-studied compound, i.e., [Pd(2ppy*)(μ-Cl)]₂).

The attribution of the known X-ray structures for each geometric type and the cisoid or transoid orientation are indicated by the relevant footnotes in Table 2, Table 4, Table 6, and Table 8.

The other Pd(II)-(2-arylpyridine*) dimeric compounds usually belong to one of the two above geometric types, although a few molecules cannot be attributed to any of them due to the oblique orientation of their 2-arylpyridine* ring systems.

Besides this, in nearly all dimers, the Pd(II) atoms have the coordination number 4 and square-planar geometry; the only two exceptions are [Pd(2ppy*)(2,9-dimethyl-1,10-phenanthroline-κ²-N,N)(µ-Cl)_0.5]₂⁺ and [Pd(2ppy*)(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)(µ-Cl)_0.5]₂⁺ cations in their hexafluorophosphate salts (the former: EKEDUW, the latter: no X-ray structure), revealing the coordination number 5 [53]. Similarly to the monomeric Pd(II) compounds, there is a clear preference for trans(N,N) configuration (DAQMUF [39], ANOJIX [45], XOTVIL [66], UMAPEF [70], and JUCFEU [113]), trans(P,N) (BUFYOS [116], SIDBUE [117], and EBOHEL [149]), or trans(C,N) (MIXMEO [83]) instead of trans(O,N), as well as for the trans(S,N) one (GAJNOV [64]) instead of trans(N,N); thus, again, the less electronegative (less electron-acceptor) element is preferred to be positioned trans to the pyridine (or pyridine-like) nitrogen. The only exception is the X-ray structure IGOWIL, i.e., trans(N,N)-[Pd(2ppy*)(N₃)(µ-1,2-bis(diethylphosphino)ethane-P,P)_0.5]₂ [118], which is not the trans(P,N) isomer (although nitrogen is more electronegative than phosphorus).

The most important structural parameters for the dimeric Pd(II)-(2-arylpyridine*) molecules are, again, the Pd-N and Pd-C bond lengths, as well as the N-Pd-C bond angles, which are listed in Table 12.

In the majority of dinuclear Pd(II)-(2-arylpyridine*) molecules, the Pd-N bonds are, again, longer than those of Pd-C, as exhibited by the comparison of their mean bond lengths, averaged for the above 39 dimers: 2.033 Å versus 1.990 Å. In this case, the preliminary averaging of bond lengths (and also angles) was done in two stages: first, for both 2-arylpyridine* ligands inside the dimeric moiety (and, optionally, for slightly differing, crystallographically inequivalent complex molecules—if present) of a given X-ray structure, then for various X-ray structures corresponding to the same compound (e.g., SOHDUO, SOHDUO 01, SOHDUO 02, SOHDUO 03 for [Pd(2ppy*)(μ-Cl)]₂, etc.). On the other hand, however, the range of Pd-N bond lengths (1.900–2.129 Å) is similar to that for Pd-C (1.892–2.140 Å).

The N-Pd-C bond angles vary within the 77.0–91.0° range, with a mean value of 82.4°.

Oligonuclear Pd(II) Compounds

For oligonuclear Pd(II)-(2-arylpyridine*) compounds, there are several X-ray structures (ZUMSUY and ZUMSOS [114], MIRGUR, MIRGOL and MIRGEB [119], YULDEP [102,103], and VAHTUX, VAHVAF and VAHVEJ [163]). As they concern species with various numbers of palladium atoms (3, 4, 9, 10, or 12) and highly different auxiliary ligands, it is difficult to describe them as one group of species.

3.1.2. Pd(III) Compounds

In the dinuclear Pd(III)-(2-arylpyridine*) compounds (mainly [Pd(2-arylpyridine*)Y(µ-X)]₂ dimers), the X bridges are most often carboxylates, while Y monodentate ligands are various monoanions (usually halides or carboxylates); thus, they are neutral molecules. Their X-ray structures, however, are known only for acetate-bridged species (CUHQAY [44,45], YULDAL and YULDIT [102,103], PAHREX [167], and PIVGEI [168]), as already shown in Table 9.

In all of these X-ray-studied dimers, the Pd(III) coordination number is 6, with direct Pd-Pd linking and octahedral geometry around each palladium atom. Both Pd(III) ions and both auxiliary ligands Y (fluorides, chlorides, or acetates) are approximately co-linear, forming the axial axis of the whole dimeric molecule; from the viewpoint of each palladium atom, the equatorial positions are occupied by the N- and C-donor atoms of the respective 2-arylpyridine* ring system and the oxygen atoms of the bridging acetates, resulting in the untypical (owing to the participation of another palladium atom) chromophores PdCNO₂F, PdCNO₂Cl, or PdCNO₃ (for Y = F, Cl or O, respectively).

In consequence, both 2-arylpyridine* ligands lie in two distinct but nearly parallel planes. Due to the fact that the bridging acetates are cis-orientated to each other, the two 2-arylpyridine* ligands lie on the same side of the Pd-Pd axis. However, in respect to the position of nitrogen atoms, they always exhibit transoid orientation [44,45,102,103,167,168].

An example of such a Pd(III) dimer is shown in Figure 4.

The most important structural parameters for the dimeric Pd(III)-(2-arylpyridine*) molecules are, again, the Pd-N and Pd-C bond lengths, as well as the N-Pd-C bond angles, which are listed in Table 13.

Again, in these Pd(III)-(2-arylpyridine*) dimers, the Pd-N bonds are longer than those of Pd-C, as proven by comparison of their mean values (averaged for all five compounds in a way analogous to the Pd(II) dimers): 2.013 Å versus 1.992 Å, and their respective ranges: 2.002–2.027 Å versus 1.978–2.006 Å.

The N-Pd-C bond angles vary within the 81.6–82.9° range, with a mean value of 82.5°.

For polynuclear Pd(III)- and Pd(2.5)-(2-arylpyridine*) compounds, there are several X-ray structures (PAHRIB, PIVFUX, PAHSOI, PAHSOI 01, PAHSOI 02, PAHSOI 03, PAHSOI 04, PAHSOI 05, and PIVGAE) [167,168]. Generally, these are [Pd(2-arylpyridine*)(µ-acetate-O,O)]_n chains (with +n or +n/2 electric charge), in which the Pd(III) or Pd(+2.5) atoms form an infinite (-Pd-Pd-)_n wire.

3.1.3. Pd(IV) Compounds

All mononuclear (monomeric) Pd(IV) compounds with 2-arylpyridines* have the coordination number 6 and octahedral geometry, which allows for many possible geometric isomers.

In the case of the [Pd(2-arylpyridine*)₂L¹L²] type, the X-ray structures are known only for compounds containing 2ppy* (QAVCAS [37], LUFQIN and LUFQOT [38], ZUNYOY [48], and PITZOI [170]), as already shown in Table 10 (top). They exhibit a preference to form trans(X,N^2ppyA),trans(C^2ppyA,N^2ppyB) isomers, where X = O, Cl is the donor atom in one of the L¹, L² auxiliary ligands. Such an arrangement of donor atoms corresponds to the cis orientation of L¹ and L²; no trans(L¹,L²) isomers were described. On the other hand, as already mentioned, no X-ray structures (and, generally, compounds) of the [Pd(2-arylpyridine*)₂(LL)] type were reported.

In the case of the [Pd(2-arylpyridine*)ABCD] type, the X-ray structures are known for compounds containing various 2-arylpyridines*, although surprisingly not for 2ppy* (APACIE, APACUQ, APADAX and APADEB [123], WEFFEU [148], JOHGOD [154,155], CAMSEQ and CAMSIU [156], OPETOU [157], LUXZAG [165], and OPOQAM [171]). Due to the large variety of auxiliary ligands, there are many isomeric possibilities; nevertheless, when donor atoms at the A, B, C, and D sites are different and the steric reasons allow for the formation of distinct stereomers, the trans(N,N) geometry is preferred instead of trans(F,N) (JOHGOD [154,155]) and the trans(C,N) geometry is preferred instead of trans(Cl,N) or trans(O,N) (LUXZAG [165])—following an analogous tendency for Pd(II) compounds.

The most important structural parameters for the monomeric Pd(IV)-(2-arylpyridine*) compounds are, again, the Pd-N and Pd-C bond lengths, as well as the N-Pd-C bond angles, which are listed in Table 14.

Similarly to the Pd(II) monomers, in both types of the Pd(IV)-(2-arylpyridine*) species (treated together), the Pd-N bonds are, again, longer than those of Pd-C, as proven by the comparison of their mean values, averaged for the above 16 X-ray structures (corresponding to 16 compounds): 2.050 Å versus 2.022 Å. In this case, the averaging of the structural parameters took into account not only the possible presence of two slightly differing, crystallographically inequivalent molecules in the crystal lattice but also the fact that, in [Pd(2-arylpyridine*)₂L¹L²] molecules, there are two (and not one, like there usually is in the Pd(II) compounds) 2-arylpyridine* ligands. Furthermore, the range of Pd-N bond lengths (2.012–2.154 Å) also corresponds to higher values than that for Pd-C (1.985–2.068 Å), despite their partial overlapping.

The N-Pd-C bond angles vary within the 80.5–87.4° range, with a mean value of 83.0°.

3.2. Discussion of ¹⁵N NMR Spectra

Besides routine ¹H and/or ¹³C (and, optionally, ¹⁹F or ³¹P) NMR spectra, some Pd(II)-(2-arylpyridine*) compounds (in fact, only the Pd(II)-2ppy* ones) were studied by ¹⁵N NMR [1,6]. Their ¹⁵N chemical shifts (in respect to neat nitrometane), together with the ¹⁵N coordination shifts (i.e., differences from the ¹⁵N chemical shift for free 2ppy measured in the same or at least in a similar solvent) are listed in Table 15.

The ¹⁵N NMR chemical shifts for the above compounds are within a relatively narrow range, from ca. −147 to ca. −141 ppm. In all cases, the Pd(II) coordination of 2ppy* leads to a large decrease in this parameter (comparing to 2ppy), reflecting a strong ¹⁵N shielding phenomenon and resulting in a significant low-frequency (i.e., upfield) shift of the ¹⁵N signal (thus, the Δ^15N_coord values are negative). The absolute magnitude of this effect is ca. 68–74 ppm [1,6].

In two reviews by Pazderski [173,174], covering Pd(II) complexes or organometallics with various aza aromatic ligands (like azines, e.g., pyridine derivatives, etc.), this phenomenon was identified as typical and dependent on the type of donor atom in the trans position in respect to the Pd(II)-bonded nitrogen. However, for the presently reviewed Pd-2ppy* compounds, the absolute magnitudes of the ¹⁵N NMR coordination shifts are relatively similar, although the geometries are variable: trans(Cl,N) in [Pd(2ppy*)Cl₂]⁻ and [Pd(2ppy*)(µ-Cl)]₂ [1]; trans(S,N) in [Pd(2ppy*)(dimethyl sulfoxide)₂]⁺ [1]; trans(N,N) in [Pd(2ppy*)LCl], where L = NH₃, pyridine, 2-, 3-, and 4-methylpyridine, 2,3-, 2,4-, 2,6-, and 3,5-dimethylpyridine, and 2,4,6-trimethylpyridine (the latter proven by the X-ray structure DUBLAP) [6].

4. Applications

4.1. Biological Activity

A total of 34 of the reviewed compounds (Pd(II) only) were studied in respect to their biological activity [5,18,51,94,110,113,152]. Generally, all these reports concerned anti-tumour properties, but one also described anti-bacterial action [152]. The respective data are summarized in Table 16.

4.2. Catalytic Activity

A total of 94 of the reviewed compounds (Pd(II)—91, Pd(III)—2, Pd(IV)—1) were exhibited to have catalytic activity [15,39,44,45,62,67,68,69,74,81,82,83,84,85,86,87,88,89,93,102,124,125,160,165,166]. Their application as catalysts is summarized in Table 17.

4.3. Luminescence

A total of 63 of the reviewed compounds (Pd(II) only) were studied in detail in respect to their luminescence [8,9,12,19,23,30,31,39,51,52,54,57,59,125,128,133]. An important parameter for luminescence differentiation is the lifetime, i.e., the average time that a molecule remains in an excited state prior to returning to the ground state by emitting a photon. In this review, we have arbitrarily chosen the lifetime of 10 µs as a border between long- and short-living excited forms; the former correspond to phosphorescence, while the latter—to either phosphorescence or fluorescence (it is often difficult to distinguish both phenomena: although they have different mechanisms, their identification may be ambiguous, even taking into account the fact that phosphorescence lifetimes are principally longer than the fluorescence ones).

A total of 25 of the reviewed compounds exhibited luminescence with lifetimes above 10 µs (LUM1) [8,9,19,39,51,52,57,128,133], while 29 had lifetimes below 10 µs (LUM2) [12,23,30,31,39,51,59]; unfortunately, for 11 species, this principal parameter was not determined (LUMx) [52,54,125]. The detailed data describing their chemical character, together with the maximal quantum yields (for a given group of compounds, rounded to 1%), are summarized, in the order of their references, in Table 18.

4.4. Advanced Materials

The Pd(III) and Pd(2.5) polynuclear species of the general formula [Pd(ArPY^#*)(µ-X)]_nY_a (ArPY^# = 7,8-benzoquinoline, 7-chloro-7,8-benzoquinoline; X = acetate, n-hexanoate; Y = F, PF₆, BF₄; a = n, n/2), forming infinite one-dimensional chains, exhibit optical and electronic anisotropy, as well as semi-conductivity (Pd(III) species) or metallic conductivity (Pd(2.5)). This is why they can be used in photovoltaic cells and molecular sensors [167,168]. The monomeric compounds and their electropolymerized thin films [Pd^II(2ppy*)(LL)] (LL = 2-((4-diphenylaminophenyl)iminomethyl)-, 2-((4-(4-diphenylaminophenyl)phenyl)iminomethyl)-, and 2-((4-(2-(4-diphenylaminophenyl)vinyl)phenyl)iminomethyl)-4-methoxyphenolate) are also highly photoconductive [32].

Another application was described for [Pd(7,8-benzoquinoline*)((pyrazolate-1-yl)₃B(pyrazolate-1-yl))F]CF₃SO₃; it is relatively easy to synthesize the corresponding [Pd(7,8-benzoquinoline*)((pyrazolate-1-yl)₃B(pyrazolate-1-yl))¹⁸F]⁺ cation, it being a strong electrophilic fluorinating agent (while conventional ¹⁸F-fluoride chemistry is dominated by nucleophilic fluorination reactions), which allows for the introduction of ¹⁸F isotope into many biologically active organic molecules (e.g., estrone, converted to 3-deoxy-3-fluoroestrone, etc.) [156]. Such ¹⁸F-labeled aromatic compounds, obtained at the late stage of the synthetic route, can be applied as positron emission tomography (PET) tracers, being useful from the viewpoint of clinical PET imaging.

5. Conclusions

Large numbers of reports (>170), described compounds (>670), and single crystal X-ray structures (>200) indicate that the Pd(II), Pd(III), and Pd(IV) compounds with 2-arylpyridines* are interesting from the chemical, spectroscopic, and structural viewpoint. These species exhibit a specific (N,C) coordination mode that is alternative to classical, monodentate complexation by nitrogen; due to the fact that all contain both palladium-nitrogen and palladium-carbon bonds, they can be regarded as either complexes or organometallics.

The majority of Pd(II)-(2-arylpyridine*) compounds are mononuclear (monomeric) or dinuclear (mainly dimeric), exhibiting the coordination number 4 and square-planar geometry around the palladium atom(s). In the case of monomeric compounds, having the general formula [Pd(2-arylpyridine*)L¹L²] or [Pd(2-arylpyridine*)(LL)], upon the presence of different monodentate (L¹ ≠ L²) or unsymmetrical bidentate (LL) auxiliary ligands, the appearance of geometric isomers differing in the position of various donor atoms versus the nitrogen of the pyridine (or pyridine-like) ring or the metallated carbon of the phenyl (or aryl) ring, is possible. Analogous stereomers can appear for the [Pd(2-arylpyridine*)(µ-X)]₂ or [Pd(2-arylpyridine*)(µ-XX)_0.5]₂ dimers (if the respective X or XX bridges are unsymmetric), as well as for those of [Pd(2-arylpyridine*)Y(µ-X)_0.5]₂; this isomerism is enhanced when the donor atoms of the bridging X or XX ligands are different. In all cases, however, there is an evident preference (with a few exceptions) to form such stereomers, in which the auxiliary ligands that have less electron-acceptor properties (thus, usually containing less electronegative elements as donor atoms) are trans-positioned to the pyridine (or pyridine-like) nitrogen. Besides this, in all dimeric molecules, the transoid or cisoid orientation of both 2-arylpyridine* ring systems may occur, with the predominance of the former one.

The Pd(III)-(2-arylpyridine*) compounds are mainly dimeric, having the general formula ([Pd(2-arylpyridine*)Y(µ-X)]₂; the coordination number of both palladium atoms is 6 (including the presence of Pd-Pd direct bonding), while their geometry is octahedral. There are also some Pd(III) and Pd(2.5) polymeric species, and these form infinite one-dimensional wires. The Pd(IV)-(2-arylpyridine*) species are only monomeric, exhibiting the coordination number 6 and octahedral geometry.

A total of 13 Pd(II)-2ppy* compounds were studied by ¹⁵N NMR. Comparing them to free 2ppy*, the formation of palladium-nitrogen bonding results in a ca. 68–74 ppm decrease in the chemical shift of the pyridine nitrogen. This phenomenon reflects a strong ¹⁵N shielding effect at the Pd(II)-bonded N atom.

About 200 Pd-(2-arylpyridine*) compounds (predominantly the Pd(II) ones) exhibit some specific activities, especially biological and/or catalytic activities, and luminescence properties. These open the way for their application as anti-tumour or anti-bacterial drugs and catalysts in organic syntheses, and as materials for the production of organic light-emitting diodes. A few others can be applied as advanced materials in the production of photovoltaic cells and molecular sensors, and as electrophilic fluorinated agents that are suitable for preparation of the ¹⁸F-labeled compounds, used in positron emission tomography. The aim of this review was not only the collection in one place of spectroscopic and diffractometric data of the discussed palladium compounds, but also the presentation of their increasing applications in various branches of medicine and industry.

Author Contributions

Conceptualization, L.P.; methodology, L.P.; software, L.P.; validation, L.P. and P.A.A.; formal analysis, L.P. and P.A.A.; investigation, L.P. and P.A.A.; resources, L.P. and P.A.A.; data curation, L.P. and P.A.A.; writing—original draft preparation, L.P.; writing—review and editing, L.P. and P.A.A.; visualization, L.P.; supervision, L.P.; project administration, L.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data are available in the quoted papers.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Scheme 1. Pd(II)-2PPY* compounds (R¹, R²—any substituents) with L¹, L² monodentate ligands or LL bidentate ligand.

Scheme 2. Pd(II)-2ArPY* compounds with 2ArPY* of the type A (Z = CH₂ in 2-benzylpyridine*, CO in 2-benzoylpyridine*, O in 2-phenoxypyridine*, S in 2-phenylsulfanylpyridine*, NH in 2-anilinopyridine*; R¹, R²—any substituents) with L¹, L² monodentate ligands or LL bidentate ligand.

Scheme 3. Pd(II)-2ArPY* compounds with 2ArPY* of the type B (2-(naphth-1-yl)pyridine* (top), 2-(naphth-2-yl)pyridine* (bottom)) with L¹, L² monodentate ligands or LL bidentate ligand.

Scheme 4. Pd(II)-ArPY^#* compounds (ArPY^#* = 2-phenylquinoline* (top) and 7,8-benzoquinoline* (bottom)) with L¹, L² monodentate ligands or LL bidentate ligand.

Scheme 5. Examples of dimeric [Pd(2ppy*)(µ-LL)]₂ molecules: [Pd(2ppy*)(µ-Cl)]₂ (cisoid orientation of nitrogen atoms) and [Pd(2ppy*)(µ-CH₃COO)]₂ (transoid orientation of nitrogen atoms); analogous structures appear for various 2PPY*, 2ArPY*, and ArPY^#* ligands.

Figure 1. Annual numbers of Pd(II), Pd(III), and Pd(IV) compounds with 2-arylpyridines* for which single crystal X-ray structures were published in the years 1991–2023.

Figure 2. The X-ray structure of [Pd(2ppy*)(μ-Cl)]₂ (SOHDUO [95]), an example of Pd(II)-(2-arylpyridine*) dimer with both 2-arylpyridine* ligands positioned nearly in the same plane.

Figure 3. The X-ray structure of [Pd(2ppy*)(μ-CH₃COO)]₂ (XEMQIQ [107]), an example of Pd(II)-(2-arylpyridine*) dimer with both 2-arylpyridine* ligands positioned in distinct but nearly parallel planes.

Figure 4. The X-ray structure of [Pd(2ppy*)(CH₃COO)(μ-CH₃COO)]₂ (CUHQAY [44,45]), an example of Pd(III)-(2-arylpyridine*) dimer; both 2-arylpyridine* ligands are positioned in distinct but nearly parallel planes.

Table 11. Pd-N and Pd-C bond lengths [Å], as well as N-Pd-C bond angles [°] in monomeric Pd(II)-(2-arylpyridine*) compounds.

X-ray (CCDC)	Pd-N	Pd-C	N-Pd-C
[Pd(2ppy)L¹L²] and [Pd(2ppy)(LL)]
PUBXAO [2]	2.03(1)	2.02(1)	81.5(4)
REJHOF [9]	2.040(9)	2.067(8)	81.1(3)
POKSAK [13]	2.009(1)	1.964(2)	81.78(7)
VIYXOR [16]	2.061(2)	2.029(2)	81.14(7)
WOMQOH [17]	2.054(2)	2.009(2)	81.7(1)
MAHPIW [19]	2.0196(9)	1.985(1)	81.63(4)
AFABOA [24]	2.019(9)	1.97(1)	82.0(4)
XUYPOZ [25]	2.040(6)	2.018(6)	81.4(2)
JUPNIT [26]	2.066(4) 2.045(3) 2.050(4)	2.010(4) 2.011(4) 2.016(5)	81.1(2) 81.2(1) 81.6(2)
POKRUD [13]	2.012(2) 2.007(3)	1.965(4) 1.958(4)	81.5(1) 81.3(1)
YABMEV [30]	2.026(3)	2.001(3)	81.0(1)
ASEGUB [30,31]	2.026(1)	2.004(2)	80.84(7)
NUPROH [32]	2.037(3)	2.011(3)	81.0(1)
NUPRUN [32]	2.017(4) 2.026(4)	1.992(4) 1.991(4)	81.1(2) 80.7(2)
NUPDOT [32]	2.081(9)	2.037(9)	81.2(4)
UXILET [34]	2.025(2)	1.995(3)	81.0(1)
KEMFEQ [35]	2.039(2)	2.001(2)	81.16(9)
KEMDOY [35]	2.030(7)	2.029(8)	80.7(3)
KEMDEO [35]	2.047(2)	2.003(3)	81.3(1)
KEMFAM [35]	2.044(3)	2.010(3)	81.5(1)
POKSEO [13]	2.013(4) 2.007(3)	1.977(4) 1.980(5)	81.1(2) 81.8(2)
DAQNAM [39]	2.039(5)	1.984(6)	81.0(2)
FULWUF [40]	2.017(3)	1.960(3)	82.1(1)
DUBLAP [6]	2.036(2)	1.980(2)	81.49(8)
AFABIU [24]	2.012(2) 2.013(2)	1.969(3) 1.970(3)	81.2(1) 81.2(1)
AFABEQ [24]	2.017(3)	1.973(3)	81.7(1)
ZUNYIS [48]	2.121(3) 2.136(3)	1.992(4) 1.993(4)	80.7(1) 80.8(2)
AMUNED [41]	2.014(2)	1.954(3)	81.36(9)
AMUNAZ [41]	1.999(3)	1.951(3)	81.9(1)
WOVHEX [49]	2.008(3)	2.003(3)	81.0(1)
EKEDEG [53]	2.033(4)	1.982(4)	81.1(2)
EKEDOQ [53]	2.030(3) 2.025(3)	1.965(3) 1.973(3)	81.2(1) 81.5(1)
PERWIU [55]	2.010(2)	2.001(2)	80.90(9)
AMUNON [41]	2.031(2)	1.987(2)	81.34(7)
XAKKOM [58]	2.030(3) 2.032(3)	1.983(3) 1.981(3)	81.4(1) 81.5(1)
OJATEZ [59]	2.050(2)	1.983(2)	80.8(1)
AMUNIH [41]	2.035(2) 2.034(2) 2.033(2)	1.979(3) 1.983(3) 1.974(3)	81.6(1) 81.4(1) 81.5(1)
BIFDIG [60]	2.020(1)	2.002(2)	81.08(6)
BIFDOM [60]	2.018(3)	2.000(3)	80.8(1)
IGERES [61]	2.032(4)	1.983(5)	80.9(2)
XOFKEL [62]	2.028(3)	2.012(4)	80.8(1)
XOFKIP [62]	2.033(4)	1.993(5)	80.6(2)
HINBAH [63]	2.04(1)	1.995(9)	79.7(4)
GAJMUA [64]	2.086(2)	2.019(3)	81.1(1)
PORNUG [67]	2.095(2)	2.020(3)	80.89(8)
SEMDEU [68]	2.089(4) 2.080(3)	2.023(5) 2.025(5)	80.6(2) 81.1(2)
PORNEQ [67]	2.085(2)	1.985(3)	81.1(2)
UMAPAB [70]	2.091(1)	2.008(1)	81.25(5)
CUBVEA [72]	2.087(3)	1.993(5)	80.8(2)
EQIRIF [73]	2.081(2)	1.996(3)	81.22(9)
EQISEC [73]	2.112(2)	2.007(2)	80.73(8)
JAJBOM [75]	2.093(2) 2.094(2)	2.005(2) 2.000(2)	81.1(1) 81.3(1)
LOQTIW [80]	2.095(2)	1.992(2)	81.27(7)
IJEYAZ [81]	2.092(1)	2.006(1)	81.36(5)
IGERIW [61]	2.081(2)	1.990(3)	81.13(9)
MUBQUX [82]	2.085(6) 2.079(6) 2.095(6) 2.098(6)	2.009(9) 2.01(1) 2.002(9) 2.006(9)	81.7(3) 82.0(3) 81.8(3) 81.8(3)
MUBYAL [82]	2.070(1)	1.994(2)	81.44(6)
MUBRAE [82]	2.091(1)	2.002(2)	80.54(6)
MIXMIS [83]	2.096(6) 2.089(6)	2.013(8) 1.988(7)	81.2(3) 81.0(3)
MIXMOY [83]	2.087(4)	1.996(5)	81.4(2)
IGUVOW [84]	2.084(2)	1.980(3)	81.3(1)
POHHOL [86]	2.089(3) 2.097(3)	1.998(4) 1.993(3)	81.3(1) 81.5(1)
MIXNAL [83]	2.070(3)	1.998(4)	82.0(1)
MIXMUE [83]	2.075(2)	1.993(3)	81.61(8)
KACMUZ [87]	2.095(5)	1.970(4)	80.8(2)
KACNAG [87]	2.088(3)	1.994(4)	81.3(1)
GADLEE [89]	2.084(3) 2.068(3)	1.985(4) 1.980(4)	80.8(1) 81.6(1)
QEZFEJ [92]	2.087(3)	1.969(3)	81.4(1)
[Pd(2PPY)L¹L²] and [Pd(2PPY)(LL)] (2PPY* = a-R¹-2-(b-R²-phenyl)pyridine* ≠ 2ppy*, a = 3–6, b = 2–5)
MAWSOW [121]	2.103(2)	2.001(2)	80.99(8)
GAJNAH [64]	2.089(2)	2.013(3)	80.9(1)
KENTEE [124]	2.047(2)	2.013(3)	80.82(9)
QEZFIN [92]	2.082(3) 2.095(3)	2.013(4) 2.002(4)	81.8(1) 81.3(1)
SOFCAT [125]	2.006(7) 2.031(5)	1.959(7) 1.96(1)	81.5(3) 82.5(3)
SOFBEW [125]	2.071(2)	1.990(2)	81.28(8)
SOFBOG [125]	2.083(3)	1.985(3)	80.9(1)
SOFBIA [125]	2.071(2)	1.993(3)	81.8(1)
WOMREY [17]	2.066(2)	2.009(2)	81.26(9)
WOMRAU [17]	2.063(4)	2.001(4)	81.3(2)
[Pd(2ArPY)L¹L²] and [Pd(2ArPY)(LL)] (2ArPY* ≠ 2PPY, 2ppy)
TUMLAO [29]	2.025(4)	1.983(6)	88.8(2)
FESCEO [74]	2.106(2)	2.003(3)	89.43(8)
WOSYEI [33]	2.029(5)	1.987(7)	87.5(2)
IJEYED [81]	2.088(2)	1.996(3)	88.8(1)
MUBQOR [82]	2.077(1)	1.985(1)	90.05(6)
KOXWAX [140]	2.017(2)	1.992(3)	81.0(1)
ZABFEN [141]	2.041(4)	1.982(6)	81.7(2)
[Pd(ArPY^#)L¹L²] and [Pd(ArPY^#)(LL)] (ArPY^#* ≠ 2ArPY, 2PPY, 2ppy*)
PORNIU [67]	2.143(3)	1.987(3)	81.0(1)
REJHIZ [9]	2.076(3)	2.025(4)	82.0(1)
VIYXUX [16]	2.079(3)	2.023(3)	81.9(1)
POKSIS [13]	2.037(2)	2.026(4)	82.0(1)
POKSIS 01 [30]	2.039(3)	2.026(5)	82.2(2)
POKSOY [13]	2.037(1)	2.016(1)	82.06(5)
ASEHEM [31]	2.021(3)	1.993(3)	82.6(1)
ANOJOD [45]	2.049(2)	1.990(1)	82.50(7)
DAQNEQ [39]	2.042(5)	1.995(6)	82.5(2)
FIBRIV [152]	2.040(2)	1.991(2)	82.65(7)
FULXAM [40]	2.032(2)	1.977(2)	82.89(7)
NIJXIR [152]	2.044(2)	1.986(2)	82.34(7)
TAXHAE [153]	2.136(4) 2.133(3) 2.141(3) 2.146(4)	2.007(4) 2.007(4) 1.995(4) 2.001(4)	81.6(1) 81.6(1) 81.4(1) 81.3(1)
NIJXEN [152]	2.040(3)	1.985(3)	82.0(1)
VUYKAE [158]	2.137(1)	2.043(1)	80.86(5)
FULXEQ [40]	2.070(2)	2.013(3)	82.57(9)
FELQET [69]	2.098(3)	2.022(4)	82.1(1)
EQIROL [73]	2.081(4)	2.009(5)	82.1(2)
EQIRUR [73]	2.096(2)	2.018(3)	82.0(1)
EQIREB [73]	2.081(4)	2.002(4)	82.1(2)
CALWIZ [162]	2.063(3)	1.983(5)	82.6(2)
VAHTAD [163]	2.127(5)	2.003(6)	81.3(2)
VAHSUW [163]	2.122(2)	2.002(2)	82.1(1)
HEXTIR [164]	2.097(1)	2.006(1)	82.37(5)
LUXYUZ [165]	2.085(3)	2.000(3)	82.3(1)
ZIGPEN [166]	2.085(4)	1.988(5)	82.1(2)
QEZFOT [92]	2.090(4)	1.976(4)	83.1(2)
CAMSAM [156]	2.071(2)	1.975(2)	81.61(9)
CAMTAN [156]	2.026(3)	2.010(4)	82.0(1)

Table 12. Pd-N and Pd-C bond lengths [Å], as well as N-Pd-C bond angles [°] in dimeric Pd(II)-(2-arylpyridine*) compounds.

X-ray (CCDC)	Pd-N	Pd-C	N-Pd-C
[Pd(2ppy)(µ-X)]₂, [Pd(2ppy)(µ-XX)_0.5]₂ and [Pd(2ppy*)Y(µ-X)_0.5]₂
SOHDUO [95]	2.006(4) 2.013(5)	1.973(5) 1.977(4)	81.2(2) 80.9(2)
SOHDUO 01 [96]	2.011(5) 2.006(6)	1.974(6) 1.972(5)	80.9(2) 81.3(2)
SOHDUO 02 [19]	2.0134(9) 2.015(1)	1.987(1) 1.987(1)	81.22(4) 81.25(4)
SOHDUO 03 [97]	2.005(2)	2.005(3)	81.2(1)
KEXXIV [98]	2.09(1) 2.09(1) 2.07(2)	2.08(1) 2.07(1) 2.08(1)	77.0(5) 77.2(6) 77.4(6)
KEXXIV 01 [99]	2.08(1) 2.04(1) 2.08(1)	2.02(1) 1.99(1) 2.05(1)	78.6(5) 80.2(5) 77.7(5)
XEMQIQ [107]	2.002(3) 2.011(3)	1.969(3) 1.966(3)	81.4(1) 81.6(1)
XEMQIQ 01 [108]	2.006(2) 1.961(2)	1.960(3) 2.002(3)	81.7(1) 81.3(1)
XEMQIQ 02 [109]	2.009(2) 2.005(2)	1.969(2) 1.966(2)	81.69(8) 81.54(8)
XEMQIQ 03 [67]	1.962(4) 1.965(4)	1.997(4) 1.998(4)	81.3(1) 81.1(2)
XEMQIQ 04 [19]	2.0052(6) 2.0116(7)	1.9616(8) 1.9566(7)	81.68(3) 81.84(3)
XEMQIQ 05 [110]	2.002(3) 2.011(2)	1.961(3) 1.968(3)	81.7(1) 81.5(1)
COJBEJ [109]	1.990(7) 1.983(8)	1.978(8) 1.989(6)	80.6(3) 81.0(3)
MAHNUG [19]	2.009(3) 1.993(3) 1.998(4) 2.005(2) 2.001(4) 2.008(3)	1.958(3) 1.963(3) 1.976(4) 1.960(4) 1.960(3) 1.968(4)	81.0(1) 81.3(1) 81.6(1) 81.2(2) 81.5(1) 81.3(1)
XOTVIL [66]	2.023(3) 2.028(3)	1.964(5) 1.970(3)	81.6(2) 81.5(1)
DAQMUF [39]	2.018(3) 2.013(3)	1.963(4) 1.963(4)	82.0(1) 82.0(1)
EKEDUW [53]	2.056(7) 2.036(5)	1.972(8) 1.976(5)	81.9(3) 81.2(2)
HEYDOF [111]	2.008(4) 1.998(6)	2.004(5) 2.002(8)	81.0(2) 81.1(2)
OGONOO [112]	2.03(1) 2.027(7) 2.03(1) 2.03(1)	1.97(1) 1.97(1) 1.973(8) 1.970(7)	81.5(4) 81.5(4) 81.4(3) 81.5(4)
OGONUU [112]	2.027(4) 2.007(4) 2.026(4) 2.025(4)	1.998(5) 2.004(4) 1.985(5) 1.990(4)	81.6(2) 81.2(2) 81.9(2) 81.8(2)
OGOPAC [112]	2.025(3) 2.026(3) 2.026(3) 2.007(4)	1.985(4) 1.990(4) 1.997(4) 2.005(3)	81.9(1) 81.8(1) 81.5(1) 81.3(1)
JUCFEU [113]	2.021(5) 2.024(6)	1.957(7) 1.962(9)	81.2(3) 81.3(3)
SEMDIY [68]	2.06(1)	2.05(1)	80.9(4)
BUFYOS [116]	2.11(1) 2.109(9)	1.985(9) 2.001(8)	81.1(4) 81.7(4)
SIDBUE [117]	2.102(2) 2.103(2)	1.998(2) 1.996(2)	81.54(8) 81.45(7)
IGOWIL [118]	1.999(3)	2.091(4)	80.7(1)
MIXMEO [83]	2.074(5) 2.058(5)	1.987(6) 1.993(6)	82.2(2) 81.8(2)
[Pd(2PPY)(µ-X)]₂, [Pd(2PPY)(µ-XX)_0.5]₂ and [Pd(2PPY)Y(µ-X)_0.5]₂ (2PPY = a-R¹-2-(b-R²-phenyl)pyridine* ≠ 2ppy*, a = 3–6, b = 2–5)
MAHPAO [19]	2.016(2)	1.956(2)	81.84(8)
MAHPOC [19]	1.974(8) 1.999(6) 2.028(6) 1.982(7)	1.924(7) 1.924(7) 1.892(8) 1.946(5)	79.6(3) 81.1(3) 80.3(3) 81.0(3)
GAJNOV [64]	2.062(4) 2.056(5)	1.991(6) 1.990(4)	81.5(2) 81.5(2)
WOMSAV [17]	2.009(4) 2.017(4)	1.948(5) 1.959(7)	81.6(2) 81.8(2)
WOMQUN [17]	2.015(3) 2.014(3)	1.955(4) 1.970(4)	81.7(1) 81.9(1)
[Pd(2ArPY)(µ-X)]₂, [Pd(2ArPY)(µ-XX)_0.5]₂ and [Pd(2ArPY)Y(µ-X)_0.5]₂ (2ArPY ≠ 2PPY, 2ppy)
YAPXEU [136]	2.031(3)	1.983(3)	88.2(1)
NOGKUQ [136,142]	1.966(7) 1.991(6) 1.963(6) 1.983(6)	2.028(6) 1.993(6) 2.025(6) 1.980(4)	88.7(3) 88.1(2) 89.4(2) 87.3(2)
UMAPEF [70]	2.039(2) 2.021(3)	1.972(3) 1.971(3)	91.0(1) 89.6(1)
ASOGAR [143]	1.91(2) 1.90(2)	2.14(2) 2.08(3)	87.6(7) 86.3(8)
ASOGAR 01 [144]	1.988(4) 2.000(4)	1.992(4) 1.981(5)	88.5(2) 88.8(2)
[Pd(ArPY^#)(µ-X)]₂, [Pd(ArPY^#)(µ-XX)_0.5]₂ and [Pd(ArPY^#)Y(µ-X)_0.5]₂ (ArPY^# ≠ 2ArPY, 2PPY, 2ppy*)
COHNET [99]	2.066(4)	2.058(6)	81.4(2)
YULCUE [45,102,103,148,169]	1.999 2.001	1.999 2.001	82.5 83.1
WEFFIY [148,169]	2.021(4) 2.028(4) 2.033(3) 2.030(4) 2.021(4) 2.031(4) 2.027(3) 2.023(4) 2.024(4) 2.022(4) 2.032(3) 2.034(4)	1.972(4) 1.948(4) 1.977(4) 1.971(4) 1.989(4) 1.967(4) 1.965(4) 1.970(4) 1.972(4) 1.973(4) 1.967(4) 1.975(4)	82.6(2) 82.7(2) 82.7(2) 82.6(2) 82.7(2) 82.9(2) 82.5(2) 82.8(2) 82.5(2) 82.8(2) 82.5(2) 82.8(2)
PEMVEK [148]	1.999(4) 2.007(3)	1.999(3) 1.987(4)	82.2(1) 82.3(1)
EBOGUA [149]	2.024(2) 2.022(2) 2.027(2) 2.021(2)	1.968(2) 1.967(2) 1.964(2) 1.969(2)	82.70(9) 82.86(8) 82.62(8) 82.54(9)
ANOJIX [45]	2.029(2) 2.033(2)	1.967(3) 1.973(2)	82.68(9) 82.82(9)
ASEHAI [31]	2.073(4) 2.066(3)	1.997(4) 2.006(4)	82.6(1) 82.1(1)
VUYKEI [158]	2.054(1) 2.046(1)	1.988(1) 1.991(1)	82.04(6) 82.32(6)
VUYHAB [158]	2.044(4) 2.055(4)	1.989(6) 1.995(4)	82.3(2) 82.5(2)
VUYGUU [158]	2.065(5) 2.058(4)	1.992(4) 1.993(6)	81.9(2) 82.7(2)
VUYGOO [158]	2.039(5) 2.063(7)	1.999(8) 1.985(6)	82.1(3) 82.2(3)
EBOHEL [159]	2.129(4) 2.122(4)	2.004(6) 2.004(6)	82.7(2) 82.6(2)

Table 13. Pd-N and Pd-C bond lengths [Å], as well as N-Pd-C bond angles [°] in dimeric Pd(III)-(2-arylpyridine*) compounds.

X-ray (CCDC)	Pd-N	Pd-C	N-Pd-C
[Pd(2-arylpyridine)Y(µ-X)]₂ and [Pd(2-arylpyridine)Y(µ-XX)_0.5]₂
CUHQAY [44,45]	2.002(3) 2.004(4) 2.003(4) 2.010(3)	1.989(4) 1.988(4) 1.978(4) 1.992(4)	81.8(1) 81.6(2) 81.9(2) 81.7(1)
PAHREX [167]	2.004(4) 2.016(2)	1.990(2) 1.997(4)	82.8(1) 82.7(1)
YULDAL [102,103]	2.017(3)	1.999(3)	82.9(1)
YULDIT [102,103]	2.002(4) 2.009(5)	2.006(5) 1.988(5)	82.8(2) 82.2(2)
PIVGEI [168]	2.027(5) 2.025(4)	1.981(5) 1.983(6)	82.8(2) 82.6(2)

Table 14. Pd-N and Pd-C bond lengths [Å], as well as N-Pd-C bond angles [°] in monomeric Pd(IV)-(2-arylpyridine*) compounds.

X-ray (CCDC)	Pd-N	Pd-C	N-Pd-C
[Pd(2-arylpyridine*)₂L¹L²]
LUFQIN [38]	2.017	2.068	81.05
ZUNYOY [48]	2.150(1) 2.025(2)	2.002(2) 2.000(2)	80.54(7) 81.64(7)
LUFQOT [38]	2.154(3) 2.033(3)	1.985(4) 2.004(2)	80.9(1) 80.8(1)
QAVCAS [37]	2.137(3) 2.018(2)	2.013(2) 2.019(3)	80.92(9) 81.6(1)
PITZOI [170]	2.145(4) 2.038(3)	2.021(4) 2.028(4)	81.0(1) 81.4(1)
[Pd(2-arylpyridine*)ABCD]
APADAX [123]	2.034(3)	2.018(3)	82.3(1)
APACIE [123]	2.099(6)	2.064(7)	87.3(3)
APACUQ [123]	2.028(2)	2.038(4)	87.4(1)
APADEB [123]	2.013(3)	2.004(3)	86.3(1)
OPOQAM [171]	2.022(3) 2.042(3)	1.997(4) 2.005(4)	83.7(1) 83.3(2)
WEFFEU [148]	2.025(4) 2.029(5)	2.000(5) 1.998(6)	83.0(2) 82.9(2)
JOHGOD [154,155]	2.012(4)	2.008(5)	82.8(2)
CAMSIU [156]	2.031(3)	2.021(3)	82.8(1)
OPETOU [157]	2.028(4)	2.025(5)	82.6(2)
CAMSEQ [156]	2.030(3)	2.038(3)	82.7(1)
LUXZAG [165]	2.105(2)	2.025(2)	81.97(7)

Table 15. ¹⁵N chemical shifts (in respect to CH₃NO₂, in ppm—δ^15N) and ¹⁵N coordination shifts (Δ^15N_coord) for [Pd(2ppy*)L¹L²] compounds (L¹, L²—monodentate ligands).

L¹	L²	Counterion	NMR Solvent	¹⁵N Chemical Shift	¹⁵N Coordination Shift
Cl ⁽¹⁾	Cl ⁽¹⁾	[Pd(2-phenylpyridine*) (dimethyl sulfoxide)₂]^{+ (1)}	DMF-d₇+DMSO-d₆ (3:1, 243 K)	−145 [1]	−73 ^(a)
dimethyl sulfoxide ⁽¹⁾	dimethyl sulfoxide ⁽¹⁾	[Pd(2-phenylpyridine*) Cl₂]^{− (1)}	DMF-d₇+DMSO-d₆ (3:1, 243 K)	−141 [1]	−69 ^(a)
µ-Cl ⁽²⁾	µ-Cl ⁽²⁾		DMF-d₇ solid	−146 [1] −147 [1]	−74 ^(a) none
NH₃	Cl		DMSO-d₆	−142.0 [6]	−70.1 ^(b)
pyridine	Cl		CDCl₃	−145.1 [6]	−70.2 ^(c)
2-methylpyridine	Cl		CDCl₃	−145.0 [6]	−70.1 ^(c)
3-methylpyridine	Cl		CDCl₃	−144.7 [6]	−69.8 ^(c)
4-methylpyridine	Cl		CDCl₃	−144.7 [6]	−69.8 ^(c)
2,3-dimethylpyridine	Cl		CDCl₃	−144.2 [6]	−69.3 ^(c)
2,4-dimethylpyridine	Cl		CDCl₃	−144.3 [6]	−69.4 ^(c)
2,6-dimethylpyridine	Cl		CDCl₃	−143.6 [6]	−68.7 ^(c)
3,5-dimethylpyridine	Cl		CDCl₃	−143.8 [6]	−68.9 ^(c)
2,4,6-trimethylpyridine	Cl		CDCl₃	−143.5 [6]	−68.6 ^(c)

⁽¹⁾ This compound is an ionic pair [Pd(2ppy*)Cl₂]⁻ [Pd(2ppy*)(dimethyl sulfoxide)₂]⁺, thus it appears in two distinct rows of Table 15. ⁽²⁾ This is [Pd(2ppy*)(µ-Cl)]₂ dimer. ^(a) Vs. 2-phenylpyridine in DMF-d₇: −72 ppm [1]. ^(b) Vs. 2-phenylpyridine in DMSO-d₆: −71.9 ppm [6,172]. ^(c) Vs. 2-phenylpyridine in CDCl₃: −74.9 ppm [6,172].

Table 16. Summary of biological activity studies for the reviewed Pd(II) compounds.

Reference	General Formula	Auxiliary Ligand(s) (If Other Than Halides)	Cancer Cells	Non-Cancerous Cells
[5]	[Pd^II(2-phenylpyridine)L^ACl] [Pd^II(2-phenylpyridine)L^BX] (X = Cl, Br, I)	L^A = NH₃, methylamine, tert-butylamine, pyridine-d₅, 2,6- and 3,5-dimethylpyridine, trimethylphosphite, triphenylphosphine L^B = pyridine-d₅	murine leukemiaL1210, P388
[18]	[Pd^II(2-phenylpyridine*)(LL)]⁺	LL = ethane-1,2- and propane-1,3-diamine, cis- and trans-cyclohexane-1,2-diamine	murine leukemia L1210
[51]	[Pd^II(ArPY^#*)(LL)]⁺ ArPY^# = 2-phenylpyridine, 2-phenylquinoline, 7,8-benzoquinoline	LL = 2,2′-bipyridine, 1,10-phenanthroline	cervix HeLa
[94]	[Pd^II(2-phenylpyridine*)(LL)]⁺	LL = bis(1-n-hexyl-1H-imidazol-2-ylidene-3-yl)methane	murine colorectum CT26
[110]	[Pd^II(2-phenylpyridine*)(µ-X)]₂	X = acetate	bladder T-24; cervix HeLa; lung NCI-H460; ovary SK-OV-3	hepatocytes HL-7702
[113]	[Pd^II(2-phenylpyridine*)(LL)]	LL = 5,5-diethyl-1,2,5,6-tetrahydropyrimidin-4-olate	breast MCF-7; colon HT-29; prostate DU-145, PNT-1A
[152]	[Pd^II(7,8-benzoquinoline)LCl] ⁽¹⁾ [Pd^II(7,8-benzoquinoline)(µ-Cl)]₂	L = 2-aminopyridine, 2-amino-3-methylpyridine, 1H-imidazole, 2-aminobenzothiazole ⁽¹⁾	colon HCT116; ovary A2780

⁽¹⁾ Besides compounds tested in respect to their anti-tumour activity, anti-bacterial properties (against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus) were also studied for nine [Pd^II(7,8-benzoquinoline*)LCl] neutral molecules (L = 2-methylpyridine, 3-methylpyridine, 2-aminopyridine, 2-amino-3-methylpyridine, 2-amino-5-chloropyridine, 1H-imidazole, 2-aminothiazole, 2-aminobenzimidazole, 2-aminobenzothiazole) and one [Pd^II(7,8-benzoquinoline*)(LL)]⁺ cation (LL = 2-aminomethylpyridine) [152].

Table 17. Summary of catalytic activity studies for the reviewed Pd(II), Pd(III), and Pd(IV) compounds.

Reference	General Formula	Auxiliary Ligand(s) (If Other Than Halides)	Reactants	Main Product(s)
[15]	[Pd^II(2-phenylpyridine*)LCl]	L = dimethyl sulfoxide, triphenylphosphine	(hetero)aryl chlorides/bromides/iodides, phenylboronic acid	phenyl(hetero)arenes ^(a)
[39]	Pd^II(ArPY^#*)(µ-X)]₂ ArPY^# = 2-phenylpyridine, 7,8-benzoquinoline	X = saccharinate	4-(4-methylphenylsulfonyl)coumarine, ⁽¹⁾ arylboronic acid	4-arylcoumarine ^{(1) (a)}
[44]	[Pd^III(2-phenylpyridine)X(µ-X)]₂ [Pd^IV(2-phenylpyridine)₂L ₂]	X, L = acetate	2-phenylpyridine, (diacetoxyiodo)benzene	2-(2-acetoxyphenyl)pyridine and/or 2,6-bis(2-acetoxyphenyl)pyridine
[45]	1) [Pd^III(2-phenylpyridine)X(µ-X)]₂ [Pd^IV(2-phenylpyridine)₂L ₂] 2) [Pd^II(ArPY^#*)(µ-X)]₂ ArPY^# = 2-phenylpyridine, 7,8-benzoquinoline	1) X, L = acetate 2) X = succinimidate	1) 2-phenylpyridine, (diacetoxyiodo)benzene 2) 2-phenylpyridine or 7,8-benzoquinoline, N-chlorosuccinimide	1) 2-(2-acetoxyphenyl)pyridine and/or 2,6-bis(2-acetoxyphenyl)pyridine 2) 2-(2-chlorophenyl)pyridine or 10-chloro-7,8-benzoquinoline
[62]	[Pd^II(2-phenylpyridine*)(LL)]	LL = 1,3-dimethyl-2,4-dione-5-(2,5-dimethylphenyliminomethyl)-, 1,3-dimethyl-2,4-dione-5-(2,6-diisopropylphenyliminomethyl)- and 1,3-dimethyl-2,4-dione-5-(2-methylsulfanylphenyliminomethyl)-1,2,3,4-tetrahydropyrimidin-6-olate	aryl bromides, phenylboronic acid	phenylarenes ^(a)
[67]	[Pd^II(2-phenylpyridine)L^ACl] [Pd^II(ArPY^#)L^AL^B] ArPY^# = 2-phenylpyridine, 2-phenylquinoline	L^A = triphenylphosphine; L^B = 4-methylphenylsulfonate	2-bromo-6-methoxynaphthalene or 4-bromoisobutylbenzene or 3-bromobenzophenone, ethylene	2-vinyl-6-methoxynaphthalene or 4-vinylisobutylbenzene or 3-vinylbenzophenone^(b)
[68]	[Pd^II(2-phenylpyridine*)(µ-X)]₂	X = O,O’-dimethylthiophosphate	methylparathion,⁽²⁾ H₂O	4-nitrophenol
[69]	[Pd^II(ArPY^#)L¹L²] ArPY^# = 2-phenylpyridine, 7,8-benzoquinoline [Pd^II(7,8-benzoquinoline)(µ-X)]₂	L¹ = triphenylphosphine, tris(4-fluorophenyl)phosphine, tris(4-methoxyphenyl)phosphine; L², X = succinimidate, maleimidate, phthalimidate	1) aryl bromides, phenylboronic acid 2) 4-acetylphenyl bromide, phenylacetylene	1) phenylarenes ^(a) 2) 4-acetylphenylethynylbenzene ^(c)
[74]	[Pd^II(ArPY^#*)(LL)] ArPY^# = 2-phenylpyridine, 2-benzylpyridine, 7,8-benzoquinoline	LL = 2-diphenylphosphinobenzoate	(hetero)aryl chlorides/bromides, phenylboronic acid or 4-methylphenylboronic acid	phenyl(hetero)arenes of 4-methylphenyl(hetero)arenes ^(a)
[81]	[Pd^II(2ArPY*)L¹L²] 2ArPY = 2-phenylpyridine, 2-benzoylpyridine	L¹ = 1,3,5-triaza-7-phosphadamantane; L² = phthalimidate, saccharinate	5-iodo-2′-deoxyuridine, benzofuran-2-boronic acid	5-(benzofuran-2-yl)--2′-deoxyuridine ^(a)
[82]	[Pd^II(2ArPY*)L¹L²] 2ArPY = 2-phenylpyridine, 2-benzoylpyridine	L¹ = 1-methyl-3-n-butylimidazol-2-ylidene; L² = Cl, Br, I, saccharinate	9-bromophenanthrene, (hetero)arylboronic acids	9-(hetero)arylphenanthrenes
[83]	[Pd^II(2-phenylpyridine)L^ACl] [Pd^II(2-phenylpyridine)L^BBr] [Pd^II(2-phenylpyridine)L^CI] [Pd^II(2-phenylpyridine)(µ-X)]₂	L^A = 1-methyl- and 1-phenyl-3-(2-carboxyethyl)imidazol-2-ylidene as well as 1-methyl and 1-phenyl-3-(2-benzoxycarbonylethyl)imidazol-2-ylidene L^B = 1-methyl- and 1-phenyl-3-(2-ethoxycarbonylethyl)imidazol-2-ylidene; L^C = 1-methyl- and 1-phenyl-3-(2-methoxycarbonylethyl)imidazol-2-ylidene; X = 1-methyl- and 1-phenyl-3-(2-carboxylatoethyl)imidazol-2-ylidene	mesitylene and its derivatives, ethyl propiolate or phenylacetylene	various alkenyl derivatives of mesitylene and its derivatives
[84]	[Pd^II(2-phenylpyridine*)LBr]	L = 1-methyl-3-(2,4,6-trimethylbenzyl)-, 1-methyl-3-(2,3,5,6-tetramethylbenzyl)- and 1-methyl-3-(2,3,4,5,6-pentamethylbenzyl)imidazol-2-ylidene	aryl bromides, phenylboronic acid	phenylarenes ^(a)
[85]	[Pd^II(2-phenylpyridine*)LBr]	L = 1-n-butyl-3-(2,4,6-trimethylbenzyl)-, 1-n-butyl-3-(2,3,5,6-tetramethylbenzyl)- and 1-n-butyl-3-(2,3,4,5,6-pentamethylbenzyl)imidazol-2-ylidene	aryl bromides, phenylboronic acid	phenylarenes ^(a)
[86]	[Pd^II(2-phenylpyridine*)LBr]	L = 1-allyl-3-(2,4,6-trimethylbenzyl)-, 1-allyl-3-(2,3,5,6-tetramethylbenzyl)- and 1-allyl-3-(2,3,4,5,6-pentamethylbenzyl)imidazol-2-ylidene	aryl bromides, phenylboronic acid	phenylarenes ^(a)
[87]	[Pd^II(2-phenylpyridine*)LCl]	L = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 1,3-bis(2,4,6-trimethylphenyl)-4,5-dicyanoimidazol-2-ylidene	aryl chlorides/bromides, n-butyl acrylate	n-butyl arylacrylates ^(d)
[88]	[Pd^II(2-phenylpyridine*)LBr]	L = 1-(2,4,6-trimethylphenyl)-3-(2,4,6-trimethylbenzyl)-, 1-(2,4,6-trimethylphenyl)-3-(2,3,5,6-tetramethylbenzyl)- and 1-(2,4,6-trimethylphenyl)-3-(2,3,4,5,6-pentamethylbenzyl)imidazol-2-ylidene	aryl chlorides/bromides, phenylboronic acid	phenylarenes ^(a)
[89]	[Pd^II(2-phenylpyridine*)LCl]	L = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene	1) N-(4-chlorophenyl)pyrrole, 3-methoxyphenylboronic acid 2) 4-methylphenylmagnesium bromide, 4-methoxychlorobenzene 3) thiophen-2-ylmagnesium bromide, 2,6-dimethylchlorobenzene 4) 4-methylphenylzinc chloride/bromide, 4-methoxybromobenzene 5) n-butylzinc bromide, (3-bromo-n-propyl)benzene	1) N-(4-(3-methoxyphenyl)phenyl)pyrrole ^(a) 2) 4-methyl-4′-methoxy-1,1′-biphenyl ^(e) 3) 2-(2,6-dimethylphenyl)thiophene ^(e) 4) 4-methyl-4′-methoxy-1,1′-biphenyl ^(f) 5) various alkylbenzene derivatives ^(f)
[93]	[Pd^II(2-phenylpyridine*)LCl]	L = 1-(2-methoxyethyl)-, 1-(2,4,6-trimethylbenzyl)-, 1-(2,3,5,6-tetramethylbenzyl)- and 1-(2,3,4,5,6-pentamethylbenzyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)benzimidazol-2-ylidene as well as 1-(2-methoxyethyl)-, 1-(2,4,6-trimethylbenzyl)-, 1-(2,3,5,6-tetramethylbenzyl)- and 1-(2,3,4,5,6-pentamethylbenzyl)-3-(((+)-2-methyl-5-isopropylcyclohexyl)oxymethyl)-5,6-dimethylbenzimidazol-2-ylidene	1) 4-methylphenyl or 4-methylcarbonylphenyl chloride, phenylboronic acid2) phenyl or 4-methylcarbonylphenyl bromide, styrene	1) 4-methyl- or 4-methylcarbonyl-1,1′-biphenyl ^(a) 2) 1,2-diphenylethene or 1-phenyl-2-(4- methylcarbonylphenyl)ethene ^(b)
[102]	[Pd^II(7,8-benzoquinoline)(µ-X)]₂ [Pd^II(7,8-benzoquinoline)(µ-XX)_0.5]₂ [Pd^III(7,8-benzoquinoline*)Cl(µ-X)]₂	X = acetate; XX = benzene-1,3-bis(2,2-dimethylpropionate)	7,8-benzoquinoline, (dichloroiodo)benzene or N-chlorosuccinimide	10-chloro-7,8-benzoquinoline
[124]	[Pd^II(2-(4-methylphenyl)pyridine*)L¹L² ]	L¹ = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; L² = N₃	4-acetylphenyl chloride/bromide, phenylboronic acid	4-acetyl-1,1′-biphenyl ^(a)
[125]	[Pd^II(2-(4-bromophenyl)pyridine*)LCl]	L = 1,3-bis(4-methylphenyl)- 1,3-bis(2,4,6-trimethylphenyl)- and 1,3-bis(4-methoxyphenyl)imidazol-2-ylidene	4-hydroxymethylphenyl chloride/bromide, phenylboronic acid	4-hydroxymethyl-1,1′-biphenyl and/or 4-formyl-1,1′-biphenyl ^(a)
[160]	[Pd^II(7,8-benzoquinoline*)(LL)]⁺	LL = 1-((diphenylphosphinomethyl)diphenylphosphino)-1-(4-chlorobenzoyl)-, 1-((diphenylphosphinomethyl)diphenylphosphino)-1-(4-bromobenzoyl)-, 1-((diphenylphosphinomethyl)diphenylphosphino)-1-(4-methoxybenzoyl)- and 1-((diphenylphosphinomethyl)diphenylphosphino)-1-(4-nitrobenzoyl)methyl	(hetero)aryl chlorides/bromides/iodides, phenylboronic acid	phenyl(hetero)arenes ^(a)
[165]	[Pd^II(7,8-benzoquinoline*)(LL)]	LL = 1-isopropyl-3-(2-oxy-2-methylpropyl)imidazol-2-ylidene	2-phenylpyridine, 3-methyl-2-phenylpyridine or 7,8-benzoquinoline, N-bromosuccinimide	2-(2-bromophenyl)pyridine, 3-methyl-2-(2-bromophenyl)pyridine or 10-bromo-7,8-benzoquinoline
[166]	[Pd^II(7,8-benzoquinoline*)LCl]	L = 1,3-bis(2,4,6-trimethylphenyl)- and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene	1) aryl chlorides, primary/secondary amines 2) (hetero)aryl chlorides, aryl ethyl ketones	1) secondary/tertiary arylamines 2) aryl 2-(1-((hetero)aryl)ethylketones

⁽¹⁾ Coumarin is the common name for 2H-1-benzopyran-2-one. ⁽²⁾ Methylparathion is the common name for dimethoxy-(4-nitrophenoxy)-sulfanylidene-λ⁵-phosphane. ^(a) So-called Suzuki–Miyaura cross-coupling. ^(b) So-called Heck cross-coupling. ^(c) So-called Sonogashira cross-coupling. ^(d) So-called Mizoroki-Heck cross-coupling. ^(e) So-called Kumada–Tamao–Corriu cross-coupling. ^(f) So-called Negishi cross-coupling.

Table 18. Summary of luminescence studies for the reviewed Pd(II) compounds.

Reference	General Formula	Auxiliary Ligand(s) (If Other Than Halides)	Lifetime	Maximal Quantum Yield
[8]	[Pd^II(2-phenylpyridine*)L₂]⁻	L = CN	>10 µs	40%
[9]	[Pd^II(ArPY^#*)L₂]⁻ ArPY^# = 2-phenylpyridine, 7,8-benzoquinoline	L = CN	>10 µs	not given
[12]	[Pd^II(7,8-benzoquinoline*)(LL)]	LL = n-pentane-2,4-dionate	<10 µs	<40%
[19]	1) [Pd^II(2-phenylpyridine)(LL)]⁺ 2) [Pd^II(2-phenylpyridine)(µ-Cl)]₂ 3) [Pd^II(2PPY*)(µ-X)]₂ 2PPY = 2-phenylpyridine, 2-(4-methylphenyl)pyridine	1) LL = ethane-1,2-diamine 2) none 3) X = acetate, trifluoroacetate	1) >10 µs 2) >10 µs 3) >10 µs	1) not given 2) not given 3) 80%
[23]	[Pd^II(ArPY^#*)L₂]⁺ ArPY^# = 2-phenylpyridine, 7,8-benzoquinoline	L = acetonitrile	<10 µs	not given
[30]	[Pd^II(ArPY^#*)(LL)] ArPY^# = 2-phenylpyridine, 7,8-benzoquinoline	LL = 2-(phenyliminomethyl)-, 2-(4-chlorophenyliminomethyl)- and 2-(naphth-1-yliminomethyl)phenolate	<10 µs	1%
[31]	1) [Pd^II(ArPY^#)(LL)] 2) [Pd^II(ArPY^#)(µ-X)]₂ ArPY^# = 2-phenylpyridine, 7,8-benzoquinoline	1) LL = 2-aminobenzenethiolate, 2-formylpyrrolate 2) X = pyridin-2-thiolate, pyrimidine-2-thiolate, 1-methylimidazol-2-thiolate, benzimidazol-2-thiolate	1) <10 µs 2) <10 µs	1) 1% 2) 3%
[39]	1) [Pd^II(ArPY^#)L¹L²] 2) [Pd^II(ArPY^#)(µ-X)]₂ ArPY^# = 2-phenylpyridine, 7,8-benzoquinoline	1) L¹ = pyridine, quinoline, acridine; L² = saccharinate 2) X = saccharinate	1) <10 µs or >10 µs 2) >10 µs	1% 1%
[51]	[Pd^II(ArPY^#*)(LL)⁺ ArPY^# = 2-phenylpyridine, 2-phenylquinoline, 7,8-benzoquinoline	LL = 2,2′-bipyridine, 1,10-phenanthroline	<10 µs or >10 µs	not given
[52]	[Pd^II(2-phenylpyridine*)(LL)]⁺	L = 1,10-phenanthroline, dipyrido- and 6,7-dicyanodipyrido[f,h]quinoxaline, dipyrido[a,c]phenazine	>10 µs or not given	not given
[54]	[Pd^II(2-phenylpyridine*)(LL)]	LL = quinolin-8-olate; 5-formyl-, 5-(n-dodecylaminomethyl)-, 5-(n-dodecyliminomethyl)- and 5-(n-dodecanoylamide)quinolin-8-olate	not given	1%
[57]	1) [Pd^II(2-phenylpyridine)(LL)]⁺ 2) [Pd^II(2-phenylpyridine)(µ-XX)_0.5]₂	1) LL = 2,3-bis(pyrid-2-yl)pyrazine, 6,7-dimethyl-2,3-bis(pyrid-2-yl)quinoxaline2) XX = 2,2′,3,3′-tetra(pyrid-2-yl)-6,6′-biquinoxaline	1) >10 µs 2) >10 µs	not given not given
[59]	[Pd^II(2-phenylpyridine*)(LL)]	LL = 5-(2,4,6-trimethylphenyl)- and 5-(4-cyanophenyl)dipyrrinate	<10 µs	not given
[125]	[Pd^II(2-(4-bromophenyl)pyridine*)LCl]	L = 1,3-bis(4-methylphenyl)-, 1,3-bis(2,4,6-trimethylphenyl)- and 1,3-bis(4-methoxyphenyl)imidazol-2-ylidene	not given	4%
[128]	[Pd^II(ArPY^#*)(µ-X)]₂ ArPY^# = 2-(4-methylphenyl)pyridine, 7,8-benzoquinoline	X = pyridine-2-thiolate	>10 µs	not given
[133]	[Pd^II(2-benzylpyridine*)(LL)]⁺	LL = 2,2′-bipyridine, 1,10-phenanthroline	>10 µs	not given

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Pazderski, L.; Abramov, P.A. Pd(II), Pd(III) and Pd(IV) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 44 Years (1980–2023) of NMR and Single Crystal X-ray Studies. Crystals 2023, 13, 1482. https://doi.org/10.3390/cryst13101482

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Pazderski L, Abramov PA. Pd(II), Pd(III) and Pd(IV) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 44 Years (1980–2023) of NMR and Single Crystal X-ray Studies. Crystals. 2023; 13(10):1482. https://doi.org/10.3390/cryst13101482

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Pazderski, Leszek, and Pavel A. Abramov. 2023. "Pd(II), Pd(III) and Pd(IV) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 44 Years (1980–2023) of NMR and Single Crystal X-ray Studies" Crystals 13, no. 10: 1482. https://doi.org/10.3390/cryst13101482

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Pazderski, L., & Abramov, P. A. (2023). Pd(II), Pd(III) and Pd(IV) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 44 Years (1980–2023) of NMR and Single Crystal X-ray Studies. Crystals, 13(10), 1482. https://doi.org/10.3390/cryst13101482

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Pd(II), Pd(III) and Pd(IV) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 44 Years (1980–2023) of NMR and Single Crystal X-ray Studies

Abstract

1. Introduction

2. Reviewed Data

2.1. Pd(II) Compounds

2.1.1. Pd(II)-2ppy* Compounds

Mononuclear Pd(II)-2ppy* Compounds

Dinuclear and Oligonuclear Pd(II)-2ppy* Compounds

2.1.2. Pd(II)-2PPY* Compounds

Mononuclear Pd(II)-2PPY* Compounds

Dinuclear Pd(II)-2PPY* Compounds

2.1.3. Pd(II)-2ArPY* Compounds

Mononuclear Pd(II)-2ArPY* Compounds

Dinuclear Pd(II)-2ArPY* Compounds

2.1.4. Pd(II)-ArPY#* Compounds

Mononuclear Pd(II)-ArPY#* Compounds

Dinuclear and Oligonuclear Pd(II)-ArPY#* Compounds

2.2. Pd(III) Compounds

2.3. Pd(IV) Compounds

3. Discussion

3.1. Discussion of Single Crystal X-ray Structures

3.1.1. Pd(II) Compounds

Mononuclear Pd(II) Compounds

Dinuclear Pd(II) Compounds

Oligonuclear Pd(II) Compounds

3.1.2. Pd(III) Compounds

3.1.3. Pd(IV) Compounds

3.2. Discussion of 15N NMR Spectra

4. Applications

4.1. Biological Activity

4.2. Catalytic Activity

4.3. Luminescence

4.4. Advanced Materials

5. Conclusions

Author Contributions

Funding

Data Availability Statement

Conflicts of Interest

References

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2.1.4. Pd(II)-ArPY^#* Compounds

Mononuclear Pd(II)-ArPY^#* Compounds

Dinuclear and Oligonuclear Pd(II)-ArPY^#* Compounds

3.2. Discussion of ¹⁵N NMR Spectra