New Coordination Polymers of Zinc(II), Copper(II) and Cadmium(II) with 1,3-Bis(1,2,4-triazol-4-yl)adamantane

Abstract

The new coordination polymers (CPs) [Zn(tr₂ad)Cl₂]_n, {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n, [Cd₂(tr₂ad)Cl₄]_n, {[Cu(tr₂ad)(NO₃)](NO₃)}_n and {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n were obtained in the form of air- and moisture-stable microcrystalline powders by the solvothermal reactions of zinc(II), copper(II) and cadmium(II) chlorides or nitrates with the ligand 1,3-bis(1,2,4-triazol-4-yl)adamantane (tr₂ad). Investigation of the thermal behaviour assessed the thermal stability of these CPs, with [Cd₂(tr₂ad)Cl₄]_n starting to decompose only around 365 °C. As retrieved by powder X-ray diffraction, while [Zn(tr₂ad)Cl₂]_n features 1-D chains along which the metal centre shows a tetrahedral geometry and the spacer is exo-bidentate, the other CPs contain 2-D double-layers in which the metal ions possess an octahedral stereochemistry and the linker is exo-tetradentate. A comparative structural analysis involving known coordination compounds containing the tr₂ad ligand enabled us to disclose (i) the versatility of the ligand, as far as the coordination modes are concerned; (ii) the variability in crystal structure dimensionality, ranging from 1-D to 3-D; (iii) the fact that, to the best of our knowledge, [Zn(tr₂ad)Cl₂]_n is the first Zn^II-based CP containing the tr₂ad spacer.

1. Introduction

Since the discovery that metal ions and organic ligands can act as connectors and spacers, respectively, to generate infinite frameworks [1], the chemistry of coordination polymers (CPs) [2,3,4,5], including the subclass of metal–organic frameworks (MOFs) [6,7,8,9,10], has recorded a rapid growth, due to the plethora of functional properties they were found to possess. One of the main advantages of CPs and MOFs is the possibility to modulate their chemical composition, crystal structure and functional properties through a modification of the metal ion and/or the organic spacer. In view of their potential applications, CPs and MOFs appear as interesting platforms which may offer sustainable solutions in fields of major economical, technological and environmental importance, e.g., gas storage and separation [11], catalysis [12], luminescence [13,14], conductivity [15], magnetism [16], sensing [17,18,19] and biomedicine [20]. The successful preparation of CPs has generally relied on organic ligands from the class of poly(carboxylic) acids [21,22,23], pyrazines and bipyridines [21,22,23,24], phosphonic acids [25] and poly(azoles) [26,27,28].

Among the nitrogen-donor ligands from the class of poly(azoles), attention has been paid also to 1,2,4-triazolyl derivatives, due to their electron-donating ability and rich coordination chemistry. As a representative example, they can provide the N¹,N²-bridging between two adjacent metal ions [29] in the same manner as pyrazolates do [30]. Based on the coordination modes they can adopt, 1,2,4-triazolyl ligands have been exploited in building up polynuclear and polymeric coordination compounds [31,32,33,34,35]. This is also the case of the ditopic ligand 1,3-bis(1,2,4-triazol-4-yl)adamantane (tr₂ad, Scheme 1) which, although at present less explored, provides an attractive platform for crystal engineering.

Aiming at enlarging and diversifying the library of tr₂ad-based coordination frameworks, we report hereafter on the synthesis, thermal behavior and structural characterization of the five new compounds [Zn(tr₂ad)Cl₂]_n, {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n, [Cd₂(tr₂ad)Cl₄]_n, {[Cu(tr₂ad)(NO₃)](NO₃)}_n and {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n. The crystal and molecular structures of the anhydrous and trihydrate tr₂ad ligand are also described.

2. Results and Discussion

2.1. Synthesis and Preliminary Characterization

A detailed description of the synthesis of the tr₂ad ligand, including analytical details on the intermediates never reported before, is provided in the Supporting Information.

Several screening reactions, involving the adoption of synthetic conditions differing in solvent, metal-to-ligand ratio, temperature, and/or time, were carried out in order to successfully obtain microcrystalline batches of the tr₂ad-based CPs [Zn(tr₂ad)Cl₂]_n, {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n, [Cd₂(tr₂ad)Cl₄]_n, {[Cu(tr₂ad)(NO₃)](NO₃)}_n and {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n. Scheme 2 shows the synthetic conditions fruitfully used for their isolation.

Compounds [Zn(tr₂ad)Cl₂]_n and {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n were isolated by carrying out a solvothermal reaction among zinc(II) chloride dihydrate and anhydrous copper(II) chloride, respectively, and tr₂ad in the 2:1 molar ratio (DMF, 150 °C, 24 h). Also, the formation of compounds [Cd₂(tr₂ad)Cl₄]_n, {[Cu(tr₂ad)(NO₃)](NO₃)}_n and {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n required the application of solvothermal conditions, reacting anhydrous cadmium(II) chloride, copper(II) nitrate hemipentahydrate and cadmium(II) nitrate tetrahydrate, respectively, with tr₂ad in the 1:1 molar ratio (methanol, 100 °C, 24 h). All the compounds were isolated, in reasonable yields (55–70%), in the form of air- and moisture-stable microcrystalline powders, insoluble in water and in most common organic solvents (see Section 3.2).

The IR spectrum of the tr₂ad ligand (Figure S1, Supplementary Materials) shows a strong absorption band at 1517 cm⁻¹, which is assigned to the stretching vibration of the triazolyl ring [36]. In the case of the title CPs, this absorption is shifted towards higher wavenumbers (1551–1539 cm⁻¹), as a consequence of the ligand coordination to the metal ions (Figure 1). The medium-intensity broad bands centered around 3400 cm⁻¹ in the IR spectra of compounds {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n and {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n (Figure 1), characteristic of the O–H stretching vibration, witness the presence of water molecules.

A deeper analysis of the IR spectra of {[Cu(tr₂ad)(NO₃)](NO₃)}_n and {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n (Figure 1) allows to differentiate among the uncoordinated and coordinated forms of the nitrate anion. Indeed, the strong bands located at 1439 and 1282 cm⁻¹ for {[Cu(tr₂ad)(NO₃)](NO₃)}_n, and at 1478 and 1275 cm⁻¹ for {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n, assigned to the asymmetric and symmetric stretching vibrations of the nitrate group, together with the presence of two very weak bands, at 1755 and 1733 cm⁻¹ for {[Cu(tr₂ad)(NO₃)](NO₃)}_n and at 1748 and 1717 cm⁻¹ for {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n, suggest the presence of μ₂:η² nitrate anions [37,38]. At variance, the bands observed at 1394 and 1346 cm⁻¹ for {[Cu(tr₂ad)(NO₃)](NO₃)}_n and at 1374 and 1339 cm⁻¹ for {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n, together with the band centered at 1073 cm⁻¹ in {[Cu(tr₂ad)(NO₃)](NO₃)}_n and at 1076 cm⁻¹ in {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n, can be ascribed to the asymmetric and symmetric stretching modes of uncoordinated nitrate anions [39].

2.2. Thermal Behaviour

Thermogravimetric analyses (TGAs) were performed on the five compounds from 30 °C to 700 °C under a flow of nitrogen. The resulting TGA curves are gathered in Figure 2. Compound [Zn(tr₂ad)Cl₂]_n is stable up to 350 °C, temperature at which a slow decomposition begins. In the temperature range 30–120 °C, {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n undergoes a weight loss of ca. 15%, which reasonably corresponds to the evolution of four water molecules per formula unit (calculated weight loss 15.1%).

After solvent loss, no further weight loss is observed up to the decomposition onset at 325 °C. Upon heating, compound {[Cu(tr₂ad)(NO₃)](NO₃)}_n does not undergo any weight loss up to 250 °C, the temperature at which decomposition starts. To the best of our knowledge, the only known 2-D coordination polymers containing the tr₂ad ligand of which the thermal behavior have been studied are [Cu^II₂(tr₂ad)₄](Mo₈O₂₆), [Cu^II₄(μ₄-O)(tr₂ad)₂(MoO₄)₃]·7.5H₂O [40] and [Cu₃(tr₂ad)₄(H₂O)₄](SiF₆)₃·16H₂O [41], which decompose at 310 °C, 240 °C and 190 °C, respectively. Compound [Cd₂(tr₂ad)Cl₄]_n displays the highest thermal robustness, peaking up to 365 °C. Until this temperature, no weight loss is observed. Finally, compound {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n undergoes a weight loss of ca. 3.5% in the range 30–150 °C, which reasonably corresponds to the release of one water molecule per formula unit (calculated weight loss 3.4%). After this event, no further weight losses are observed up to the decomposition onset at 330 °C. To the best of our knowledge, in no case the thermal behavior of the known Cd^II 2-D coordination polymers containing the tr₂ad ligand has been investigated, so that a comparison cannot be carried out. For the title compounds, at the end of the heating process, black residues, possibly containing carbonaceous species, have been recovered.

2.3. Crystal and Molecular Structures

Tr₂ad crystallizes in the monoclinic space group P2₁/n. The asymmetric unit contains one tr₂ad molecule in general position. Figure S2a shows the Ortep drawing at 40% probability level. Due to the lack of conventional hydrogen-bond donors, the crystal structure of tr₂ad only features a network of weak CH···N interactions, with shortest C···N distances of 3.350(2) Å. Both triazole and adamantane CH groups act as unconventional hydrogen bond donors, and most of these non-bonding interactions are directional. Two pairs of such CH···N interactions, together with a slipped π/π interaction among adjacent triazole rings (centroid-centroid distance 3.82 Å, slippage angle 6.6°), concur to the formation of tr₂ad centrosymmetric dimers (Figure 3a). Such self-association is reminiscent of the pairing of 1,3,5-triphenyladamantane molecules prompted by weak CH···π interactions [42].

The hydrate ligand tr₂ad·3H₂O crystallizes in the orthorhombic space group Pnma. The asymmetric unit contains half of a tr₂ad molecule and half of a H₂O molecule, both situated across a mirror plane (Wyckoff letter c), and one water molecule in general position. Figure S2b shows the Ortep drawing at 30% probability level. The primary intermolecular interactions in the crystal structure are conventional OH···N hydrogen bonds (O···N = 2.896(3), 2.932(3) Å) involving all the triazole nitrogen atoms as acceptors. These interactions assemble the tr₂ad and water molecules (in a 1:2 ratio) into 1-D strips along the crystallographic a-axis (Figure 3b). Additional water molecules establish bridges between the strips through pairs of symmetry-equivalent OH···O bonds (O···O = 2.764(2) Å) (Figure 3b). The 2-D hydrogen-bond connectivity comprises water trimers H₂O···H–O–H···OH₂ linked to four triazole-N sites. Overall, the crystal structures of tr₂ad and tr₂ad·3H₂O reveal the potentiality of triazole-N¹,N² atoms as efficient hydrogen-bond acceptors.

Compound [Zn(tr₂ad)Cl₂]_n crystallizes in the orthorhombic space group P2₁2₁2₁. The asymmetric unit contains one Zn^II ion, one tr₂ad ligand and two chloride anions, all in general positions. The metal centre shows a ZnCl₂N₂ tetrahedral stereochemistry (Figure 4a; the Figure caption collects the values of the bond distances and angles at the metal ion), defined by two chloride anions and the nitrogen atoms of the triazole rings of two tr₂ad ligands. The ligands are exo-bidentate (μ₂-κN¹:κN^1′) and bridge neighbouring Zn^II ions along 1-D polymeric chains (Figure 4b) of pitch 11.120(4) Å parallel to the [001] crystallographic direction (this occurrence rationalizing the preferred orientation pole; see Section 3.3). The chains pack in the ab plane defining a rectangular motif (Figure 4c). Non-bonding interactions of the kind C–H···N (C···N 3.2 Å) and C–H···Cl involving both chloride anions (C···Cl 3.5–3.7 Å) are at work within the chains and between nearby chains, respectively. No empty volume is present [43].

Compound {[Cu(tr₂ad)Cl]Cl·4H₂O}_n crystallizes in the monoclinic space group P2₁/m. The asymmetric unit is composed by half of a metal centre (on an inversion centre, Wyckoff letter b), half of a tr₂ad ligand, two halves of a chloride anion and two halves of a water molecule (all on mirror planes, Wyckoff letter e), as well as one water molecule (in general position). The Cu^II ions are hexa-coordinated in trans-CuCl₂N₄ octahedral geometry defined by the nitrogen atoms of four tr₂ad ligands and one of the two independent chloride anions (Figure 5a; the main bond distances and angles at the metal ions are reported in the Figure caption). µ-coordination by triazole rings and coordinated chloride anions brings about the formation of 1-D helices of metal ions (Figure 5b) with pitch 3.5863(2) Å (half of the b-axis) running along the crystallographic direction [010]. The tr₂ad ligands, which are overall exo-tetradentate (μ₄-κN¹:κN²:κN^1′:κN^2′), connect the helices along the crystallographic direction [001], bringing about the formation of 2-D double-layers parallel to the bc crystallographic plane (Figure 5c). The layers pack, staggered, along the a-axis. The second independent chloride anion and one of the four independent water molecules occupy the rhombic cavities formed by the ligands within the double-layers (Figure 5c) and are involved in a HO–H···Cl non-bonding interaction (O···Cl 2.58(3) Å; Figure S7a). The other three water molecules are located in the inter-layer space and, by means of hydrogen bonds (O···O 2.44(2), 2.91(2), 3.07(2) Å; Figure S3a), define a 1-D supramolecular chain running parallel to the b-axis. Finally, the double layers are reinforced by C–H···Cl interactions (C···Cl 3.3–3.4 Å; Figure S3b). No empty volume is envisaged [43].

Compound [Cd₂(tr₂ad)Cl₄]_n crystallizes in the triclinic space group P-1. The asymmetric unit equals the formula unit, i.e., it contains two cadmium(II) ions, four chloride anions and one tr₂ad ligand, all in general positions. Both independent metal centres are hexa-coordinated and show an octahedral stereochemistry, though of different kind, namely: cis-CdN₂Cl₄ and CdNCl₅ (Figure 6a; the main bond distances and angles at the metal ions are reported in the Figure caption). Three of the four chloride anions bridge adjacent metal centres, while the fourth one behaves as a terminal ligand. The tr₂ad spacer is exo-tridentate (μ₃-κN¹:κN²:κN^1′). The reciprocal disposition of cations and anions brings about the formation of 1-D polymeric strands (Figure 6b) running along the [100] crystallographic direction. The tr₂ad linkers bridge nearby strands leading to the formation of 2-D double-layers parallel to the (01-1) plane and packing, staggered, along the [011] direction (Figure 6c). The reciprocal disposition of the spacers within a layer brings about the formation of intra-layer rhombic cavities, in which the terminal chloride anions are directed (Figure 6c). This structural motif is analogous to that found in {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n (see above). Weak intra- and inter-layer C–H···Cl interactions (C···Cl 3.3–3.7 Å) are present. No empty volume is observed [43].

Compound {[Cu(tr₂ad)(NO₃)](NO₃)}_n crystallizes in the orthorhombic space group Pnma. The asymmetric unit is composed by one Cu^II ion, two halves of nitrate anions and half of a tr₂ad spacer, all lying on mirror planes (Wyckoff letter h). The metal centre is hexa-coordinated in trans-CuN₄O₂ stereochemistry (Figure 7a; the main bond distances and angles at the metal ions are reported in the Figure caption), defined by four tr₂ad linkers and one of the two independent nitrate anions. The latter bridges (μ₂-κO¹:κO²) nearby metal centres 3.54(2) Å apart, while the other nitrate anion is not coordinated. The tr₂ad ligand is exo-tetradentate (μ₄-κN¹:κN²:κN^1′:κN^2′). µ₂-bridging of the nitrate anions and triazole rings brings about the formation of 1-D chains running along the [100] direction (Figure 7b).

Nearby chains are connected along [010] by the tr₂ad spacers within 2-D polymeric double-layers parallel to the (001) plane and packing, staggered, along the [001] direction (Figure 7c; this occurrence explains the preferred orientation pole—see Section 3.3). The reciprocal disposition of the tr₂ad linkers within a layer brings about the formation of intra-layer rhombic cavities, in which the not coordinated nitrate anions are located (Figure 7c) and involved in C–H···O non-bonding interactions (C···O 2.6–3.2 Å). The structural motif is analogous to that found in the Cu^II and Cd^II compounds described above. No empty volume is observed [43].

Compound {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n crystallizes in the monoclinic space group C2/c. The asymmetric unit contains two halves of Cd^II ions (one on an inversion centre, Wyckoff position b, the other one on a two-fold axis, Wyckoff position e), two nitrate anions, one tr₂ad ligand and one water molecule—all in general positions. The metal centres are hexa-coordinated in trans-CdN₄O₂ octahedral stereochemistry defined by four ligands and two nitrate anions (Figure 8a; the main bond distances and angles at the metal ions are reported in the Figure caption). One of the two independent nitrate anions bridge (μ₂-κO¹:κO²) neighbouring metal centres, while the other one is not coordinated. The tr₂ad spacer is exo-tetradentate (μ₄-κN¹:κN²:κN^1′:κN^2′). µ₂-Coordination of the nitrate anions and triazole rings is responsible for the formation of 1-D chains parallel to the [001] crystallographic direction (Figure 8b). Adjacent chains are bridged along the [010] direction to yield 2-D double-layers parallel to the bc plane and packing, staggered, along the a-axis (Figure 8c; this occurrence explains the preferred orientation pole—see Section 3.3). The reciprocal disposition of the tr₂ad linkers within a layer brings about the formation of intra-layer rhombic cavities, in which the not coordinated nitrate anions are located (Figure 8c). The structural motif is analogous to that found in the Cu^II and Cd^II compounds described above. The water molecules are located in the inter-layer space (Figure 8c) and are involved in C–H···O non-bonding interactions (C···O 2.9–3.2 Å) with adjacent tr₂ad ligands. No empty volume is observed [43].

2.4. Comparative Structure Analysis

A search in the Cambridge Structural Database (v 2020.1) for coordination compounds containing the tr₂ad ligand has revealed the existence of 22 coordination polymers.

Table 1. Main structural properties of the known coordination polymers containing the tr₂ad ligand. Abbreviations: M = metal ion; OC = octahedral; SP = square planar; SQP = square pyramidal; TB = trigonal bipyramidal; TD = tetrahedral; TP = trigonal planar; H₃btc = 1,3,5-benzenetricarboxylic acid; H₄adtc = 1,3,5,7-adamantane-tetracarboxylic acid; ^a M–F.

CSD Code	Molecular Formula	M Stereochemistry	M Geometry	M–N (Å)	M–X (Å)	Tr2ad Apticity	Dim.	Ref.
COVFIE	[Cu2(tr2ad)4](Mo8O26)	CuN5	SP	1.968–2.247	-	Exo-bidentate Exo-tridentate	2-D	[40]
COVFOK	[Cu4(tr2ad)2(µ4-O)(MoO4)3]·7.5H2O	CuN2O3	TB	2.078, 2.093	1.923–2.028	Exo-tetradentate (2×)	2-D	[40]
		CuN2O3	TB	2.005	1.937–2.090
		CuN2O3	TB	2.009	1.937–2.067
COVFUQ	[Cu2(tr2ad)2](Mo2O7)∙H2O	CuN3	TP	1.928–2.002	-	Exo-tridentate (2×)	1-D	[40]
		CuN3	TP	1.934–2.000	-
KEMLEV	[Cu(tr2ad)(SO4)]∙3H2O	trans-CuN4O2	OC	2.009–2.012	2.406	Exo-tetradentate	2-D	[41]
KEMLIZ	[Cu3(tr2ad)4(H2O)2(SO4)2](SO4)·28H2O	CuN6	OC	2.024–2.365	-	Exo-tetradentate	2-D	[41]
		CuN4O	SQP	1.995–2.223	1.958	Exo-tridentate
KEMLOF	[Cu3(tr2ad)4(H2O)2](SiF6)3·16H2O	CuN6	OC	2.001-2.413	-	Exo-tetradentate	2-D	[41]
		cis-CuN4O2	OC	1.988–2.269	1.987, 2.554	Exo-tridentate
PODMAX	[Cu4(tr2ad)2(btc)2(µ3-OH)2]	CuNO4	SQP	2.411	1.915–1.999	Exo-tridentate	3-D	[44]
		CuN2O3	TB	1.989–2.010	1.953–2.165
SERCUP	[Cu4(tr2ad)2(H-adtc)2(OH)2(H2O)2]·4H2O	CuNO5	OC	1.981	1.905–2.685	Exo-tridentate	2-D	[45]
		cis-CuN2O4	OC	1.989–2.334	1.957–2.691
		CuN2O3	SQP	2.005–2.271	1.950–1.996
		CuNO4	SQP	1.987	1.923–2.279
TUGSUK	[Cu3(tr2ad)4(SO4)(H2O)3](SO4)2·34H2O	CuN6	OC	1.989–2.295	-	Exo-tetradentate (2×) Exo-tridentate (2×)	3-D	[46]
		cis-CuN4O2	OC	1.990–2.250	1.989, 2.408
		CuN4O	SQP	1.957–2.291	1.989
TUGTAR	[Cu2(µ-OH)(tr2ad)2](NO3)3·4H2O	CuN6	OC	2.026–2.527	-	Exo-tetradentate (2×)	3-D	[46]
		cis-CuN4O2	OC	1.982–2.212	1.921–2.788
		trans-CuN2O4	OC	2.000	1.909–2.686
ILUFEB	[Cd3(tr2ad)3](SeCN)6	CdN6	OC	2.289–2.370	-	Exo-tridentate (4×)	3-D	[47]
		CdN6	OC	2.332–2.353	-
KEHDEI	[Cd2(tr2ad)2(H2O)4](CdBr4)2	cis-CdN4O2	OC	2.266–2.343	2.377–2.382	Exo-bidentate (4×)	3-D	[48]
		cis-CdN4O2	OC	2.273–2.323	2.370–2.406
		CdBr4	TD	-	2.567–2.629
KEHDIM	[Cd3(tr2ad)2I6]	CdN6	OC	2.305–2.403	-	Exo-bidentate (2×)	2-D	[48]
		CdNI3	TD	2.283	2.744–2.756
TUGSIY	[Cd3(tr2ad)3(µ-NCS)3](NCS)3	CdN1−xSx (0 < x < 1)	OC	2.296–2.473	2.662	Exo-tetradentate (3×)	2-D	[46]
		CdN6	OC	2.312–2.405
TUGSOE	[Cd3(tr2ad)6](NO3)6·22H2O	CdN6	OC	2.324–2.356	-	Exo-tridentate (3×)	3-D	[46]
		CdN6	OC	2.297–2.381
HUSQES	[Mo2(tr2ad)O6)]·6H2O	cis-MoN2O4	OC	2.366, 2.375	1.702–1.932	Exo-tetradentate	3-D	[49]
LUYRII	[Mo2(tr2ad)O6)]·H2O	cis-MoN2O4	OC	2.405	1.714–1.910	Exo-tetradentate	2-D	[50]
UZAKIQ	[Mo2(tr2ad)F2O5]	MoN2O3F	OC	2.399	1.710–1.918	Exo-tetradentate	1-D/2-D	[51]
					1.917 a
		MoN2O3F	OC	2.380	1.706–1.914
					1.934 a
VEPDEB	{Fe3(tr2ad)4[Au(CN)2]2}[Au(CN)2]4·8H2O	FeN6	OC	1.910–1.970	-	Exo-tetradentate (2×) Exo-tridentate	3-D	[52]
		FeN6	OC	1.975–1.987	-
KEHDOS	[Ag(tr2ad)](NO3)·H2O	AgN4	TD	2.185–2.502	-	Exo-tetradentate (2×)	3-D	[48]
		AgN4	TD	2.185–2.528	-
WEJWAL	[Ag(tr2ad)](ClO4)	AgN3	TP	2.191–2.347	-	Exo-tridentate	2-D	[53]
WEJWEP	[Ag2(tr2ad)2(VO2F2)2]·H2O	AgN3O	TD	2.209–2.464	2.558	Exo-tetradentate	1-D	[53]

collects key structural aspects (coordination sphere and geometry at the metal ion, tr₂ad ligand hapticity, polymer dimensionality) of these compounds. The following observations can be carried out:

• Among the CPs retrieved in the literature (Table 1), 14 contain Cu^II or Cd^II, while the others feature Cu^I, Ag^I, Mo^II or Fe^II. Hence, to the best of our knowledge, [Zn(tr₂ad)Cl₂]_n is the first example of Zn^II-based coordination compound containing the tr₂ad ligand.
• As regards the stereochemistry at the metal ion, apart from [Cd₃(tr₂ad)₂I₆], in which one of the two independent Cd^II ions shows a tetrahedral geometry, in all the known Cd^II CPs the metal centre adopts an octahedral geometry, as in [Cd(tr₂ad)Cl₄]_n and {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n. On the other hand, while in {[Cu(tr₂ad)Cl]Cl·4H₂O}_n and {[Cu(tr₂ad)(NO₃)](NO₃)}_n the metal ion is in octahedral stereochemistry, in the known CPs the Cu^II coordination number varies in the range 4–6, with different coordination geometries associated (Table 1).
• Upon comparing the values of the M–N distances (M = Cu^II or Cd^II, N = tr₂ad nitrogen atom), it appears (Figure S4) that the novel materials share distances comparable with those of the literature CPs.
• Apart from [Zn(tr₂ad)Cl₂]_n, all the other compounds studied in this work are 2-D coordination polymers characterized by the same structural motif (see Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8). At variance, Table 1 shows that in the known compounds the dimensionality ranges from 1-D to 3-D. Interestingly, the structural motif observed in the title Cu^II and Cd^II derivatives, with rhombic cavities within 2-D strands, is shown also by COVFIE, COVFOK, KEHDEI, KEMLEV and TUGSIY, containing Cu^II or Cd^II ions, and UZAKIQ, containing the Mo^II ion.

3. Materials and Methods

3.1. General

All reagents and solvents were purchased from Sigma-Aldrich (Darmstadt, Germany) and used as received, without further purification. The ligand 1,3-bis(1,2,4-triazol-4-yl)adamantane (tr₂ad) was synthesized by the acid-catalyzed condensation reaction of 1,3-diaminoadamantane and N,N-dimethylformamide azine, according to an already reported method [46]. A detailed description regarding the preparation of the intermediates (Scheme S1), on which no details have ever been reported before, is provided in the Supplementary Materials. NMR spectra (DMSO-d₆, δ, ppm) were recorded on a Bruker 400 MHz spectrometer. The IR spectra were recorded from 4000 to 650 cm⁻¹ with a PerkinElmer Spectrum 100 instrument (Perkin-Elmer, Shelton, CT, USA) by attenuated total reflectance on a CdSe crystal. Elemental analyses (carbon, hydrogen, and nitrogen %) were performed with a Fisons Instruments 1108 CHNS-O elemental analyzer (Thermo Scientific, Waltham, MA, USA). Before the analytical characterization was carried out, all the samples were dried under vacuum (50 °C, ~0.1 Torr) until a constant weight was reached. Thermogravimetric analyses (TGAs) were carried out under a N₂ flow (25 mL/min), in the temperature range 30–700 °C and with a heating rate of 5 °C/min, using a PerkinElmer STA 6000 simultaneous thermal analyzer (Perkin-Elmer, Shelton, CT, USA).

3.2. Synthesis of the Tr₂ad-Based CPs

3.2.1. Synthesis of [Zn(tr₂ad)Cl₂]_n

Tr₂ad (0.054 g, 0.2 mmol) was dissolved in N,N-dimethylformamide (DMF) (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, ZnCl₂∙2H₂O (0.017 g, 0.1 mmol) was added, and the resulting solution was introduced into a high-pressure glass tube and heated at 150 °C for 24 h. Slow cooling of the solution to room temperature, followed by partial slow evaporation of the solvent, afforded a white solid which was filtered off, washed twice with DMF, dried under vacuum and identified as [Zn(tr₂ad)Cl₂]. Yield: 55%. [Zn(tr₂ad)Cl₂] is insoluble in alcohols, acetone, acetonitrile, chlorinated solvents, DMF, dimethylsulfoxide (DMSO) and water. Elem. anal. calc. for C₁₄H₁₈Cl₂N₆Zn (FW = 406.65 g/mol): C, 41.35; H, 4.46; N, 20.67%. Found: C, 40.95; H, 4.23; N, 20.35%. IR (cm⁻¹): 3160(w), 3110(m) ν(C–H_aromatic), 2930(m), 2870(m) ν(C–H_aliphatic), 1539(s) ν(C=N), 1387(m), 1329(m), 1192(vs), 1110(m), 1029(vs), 973(w), 883(m), 934(m), 786(w), 728(m), 680(m), 656(vs).

3.2.2. Synthesis of {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n

Tr₂ad (0.054 g, 0.2 mmol) was dissolved in DMF (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, CuCl₂ (0.013 g, 0.1 mmol) was added, and the resulting solution was introduced into a high-pressure glass tube and heated at 150 °C for 24 h. Slow cooling of the solution to room temperature, followed by slow partial evaporation of the solvent, afforded a light blue solid which was filtered off, washed twice with DMF, dried under vacuum and identified as [Cu(tr₂ad)Cl]Cl∙4H₂O. Yield: 65%. [Cu(tr₂ad)Cl]Cl∙4H₂O is insoluble in alcohols, acetone, acetonitrile, chlorinated solvents, DMF, DMSO and water. Elem. anal. calc. for C₁₄H₂₆Cl₂CuN₆O₄ (FW = 476.91 g/mol): C, 35.26; H, 5.49; N, 17.62%. Found: C, 34.85; H, 5.33; N, 17.25%. IR (cm⁻¹): 3500–3200(br) ν(H–O), 3200–3000(w) ν(C–H_aromatic), 2921(m), 2864(w) ν(C–H_aliphatic), 1543(s) ν(C=N), 1346(s), 1207(vs), 1086(vs), 1045(w), 1021(w), 883(m), 842(w), 786(w), 728(m), 680(m).

3.2.3. Synthesis of [Cd₂(tr₂ad)Cl₄]_n

Tr₂ad (0.054 g, 0.2 mmol) was dissolved in methanol (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, CdCl₂ (0.036 g, 0.2 mmol) was added, and the resulting solution was introduced into a high-pressure glass tube and heated at 100 °C for 24 h. The white precipitate which was formed was filtered off, washed three times with hot methanol, dried under vacuum and identified as [Cd₂(tr₂ad)Cl₄]. Yield: 65%. [Cd₂(tr₂ad)Cl₄] is insoluble in alcohols, acetone, acetonitrile, chlorinated solvents, DMF, DMSO and water. Elem. anal. calc. for C₁₄H₁₈Cd₂Cl₄N₆ (FW = 637.00 g/mol): C, 26.40; H, 2.85; N, 13.19%. Found: C, 26.33; H, 2.92; N, 12.85%. IR (cm⁻¹): 3143(w), 3121(m) ν(C–H_aromatic), 2915(m), 2858(w) ν(C–H_aliphatic), 1737(m), 1534(m) ν(C=N), 1371(m), 1253(m), 1191(vs), 1105(m), 1069(s), 1046(vs), 990(m), 883(m), 866(m), 850(m), 791(w), 730(m), 681(m), 658(m).

3.2.4. Synthesis of {[Cu(tr₂ad)(NO₃)](NO₃)}_n

Tr₂ad (0.054 g, 0.2 mmol) was dissolved in methanol (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, Cu(NO₃)₂·2.5H₂O (0.037 g, 0.2 mmol) was added, and the resulting solution was introduced into a high-pressure glass tube and heated at 100 °C for 24 h. A green precipitate was formed, which was filtered off, washed three times with hot methanol, dried under vacuum and identified as [Cu(tr₂ad)(NO₃)](NO₃). Yield: 70%. [Cu(tr₂ad)(NO₃)](NO₃) is insoluble in alcohols, acetone, acetonitrile, chlorinated solvents, DMF, DMSO and water. Elem. Anal. calc. for C₁₄H₁₈CuN₈O₆ (FW = 457.95 g/mol): C, 36.72; H, 3.96; N, 24.47%. Found: C, 36.35; H, 3.73; N, 24.19%. IR (cm⁻¹): 3136(m), 3016(vw) ν(C–H_aromatic), 2916(m), 2856(w) ν(C–H_aliphatic), 1755(vw), 1733(vw), 1551(s) ν(C=N), 1439(s) ν_asym(coordinated NO₃), 1394(vs) ν_asym(uncoordinated NO₃), 1346(vs) ν_sym(uncoordinated NO₃), 1329(vs), 1282(vs) ν_sym(coordinated NO₃), 1210(s), 1180(m), 1119(w), 1089(s), 1073(m), 1054(m), 1019(s), 867(vs), 826(m), 787(w), 738(m), 716(w), 681(m).

3.2.5. Synthesis of {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n

Tr₂ad (0.054 g, 0.2 mmol) was dissolved in methanol (5 mL) and the obtained solution was left under stirring at room temperature for 5 min. Then, Cd(NO₃)₂·4H₂O (0.047 g, 0.2 mmol) was added, and the resulting solution was introduced into a high pressure glass tube and heated at 100 °C for 24 h. The white precipitate which was formed was filtered off, washed three times with hot methanol, dried under vacuum and identified as [Cd(tr₂ad)(NO₃)](NO₃)∙H₂O. Yield: 60%. [Cd(tr₂ad)(NO₃)](NO₃)∙H₂O is insoluble in alcohols, acetone, acetonitrile, chlorinated solvents, DMF, DMSO and water. Elem. Anal. calc. for C₁₄H₂₀CdN₈O₇ (FW = 524.83 g/mol): C, 32.04; H, 3.84; N, 21.35%. Found: C, 31.87; H, 3.55; N, 20.98%. IR (cm⁻¹): 3418(br) ν(H–O), 3098(w), 3043(w) ν(C–H_aromatic), 2916(m), 2869(w), 2853(w) ν(C–H_aliphatic), 1748(vw), 1717(vw), 1633(w), 1543(m) ν(C=N), 1478(s) ν_asym(coordinated NO₃), 1374(vs) ν_asym(uncoordinated NO₃), 1339(vs) ν_sym(uncoordinated NO₃), 1301(vs), 1275(vs) ν_sym(coordinated NO₃), 1208(vs), 1174(m), 1110(w), 1076(m), 1037(vs), 998(vs), 851(m), 828(m), 789(w), 731(s), 683(s).

3.3. X-ray Diffraction Structural Analysis

3.3.1. Structural Analysis of tr₂ad and tr₂ad·3H₂O

The X-ray diffraction data of tr₂ad (colorless prism with dimensions of 0.27 × 0.22 × 0.20 mm) and tr₂ad·3H₂O (colorless prism with dimensions of 0.33 × 0.16 × 0.13 mm) were collected at 173 K on a Bruker APEXII area-detector diffractometer (Bruker, Billerica, MA, USA) equipped with a sealed X-ray tube (Mo-Kα radiation, λ = 0.71073 Å). The data were corrected for Lorentz-polarization effects and for the effects of absorption (multi-scans method). The crystal structures were solved by direct methods and refined against F² using the programs SHELXS-97 or SHELXL-2018/1 [54,55]. The non-hydrogen atoms were assigned anisotropic thermal displacement parameters. All the hydrogen atoms were located in difference Fourier maps and then refined freely with isotropic thermal displacement parameters and with soft similarity restraints applied to O–H bond lengths in the structure of tr₂ad·3H₂O.

Crystal data for tr₂ad, FW = 270.34 g mol⁻¹: monoclinic, P2₁/n, a = 8.9372(4) Å, b = 8.7877(5) Å, c = 16.8366(8) Å, β = 97.888(2)°, V = 1309.79(11) ų, Z = 4, ρ = 1.371 g cm⁻³, F(000) = 576, R1 = 0.039, wR2 = 0.095 [I > 2σ(I)] and R1 = 0.053, wR2 = 0.1032 (all data) for 2656 data and 253 parameters in the 4.9–52.8° 2θ range. CCDC No. 2034960.

Crystal data for tr₂ad·3H₂O, FW = 324.39 g mol⁻¹: orthorhombic, Pnma, a = 9.6759(10) Å, b = 16.3052(8) Å, c = 10.0229(11) Å, V = 1581.3(3) ų, Z = 4, ρ = 1.363 g cm⁻³, F(000) = 696, R1 = 0.049, wR2 = 0.087 [I > 2σ(I)] and R1 = 0.110, wR2 = 0.104 (all data) for 1656 data and 164 parameters in the 4.8–52.8° 2θ range. CCDC No. 2034961.

3.3.2. Structural Analysis of the Coordination Polymers

Powdered samples (~50 mg) of the five CPs were deposited in the cavity of a silicon free-background sample-holder 0.2 mm deep (Assing S.r.l., Monterotondo, Italy). Powder X-ray diffraction (PXRD) data acquisitions were carried out with a Bruker AXS D8 Advance vertical-scan θ:θ diffractometer (Bruker, Billerica, MA, USA), equipped with a sealed X-ray tube (Cu-Kα, λ = 1.5418 Å), a Bruker Lynxeye linear position-sensitive detector, a filter of nickel in the diffracted beam and the following optical components: primary beam Soller slits (aperture 2.5°), fixed divergence slit (aperture 0.5°), anti-scatter slit (aperture 8 mm). The generator was set at 40 kV and 40 mA. Preliminary PXRD analyses to unveil the purity and crystallinity of the samples were performed in the 2θ range 3.0–35.0°, with steps of 0.02° and time per step of 1 s. PXRD acquisitions for the assessment of the crystal structure were performed in the 2θ range 5.0–105.0°, with steps of 0.02° and time per step of 10 s. After a standard peak search, enabling us to assess the maximum position of the 20–25 lower-angle peaks, indexing was performed applying the Singular Value Decomposition approach [56] implemented in TOPAS-R V3 [57]. The space groups were assigned based on the systematic absences. The crystallographically independent portion of the tr₂ad ligand and nitrate anion were described using rigid bodies built up through the z-matrix formalism, assigning average values to the bond distances and angles (For tr₂ad: C_ad/_tz–C_ad = 1.55 Å, C_tz–N_tz, N_tz–N_tz = 1.36 Å, C–H = 0.95 Å; Triazole Internal and External Bond Angles = 108 and 126°; Angles at the C_ad Atoms = 109.5°. For the Nitrate Anion: N–O = 1.30 Å; O–N–O = 120°.). The structures were solved working in the real space with the Simulated Annealing approach [58], as implemented in TOPAS-R V3. Structures refinement was carried out with the Rietveld method [59], as implemented in TOPAS-R V3. The background was modelled through a polynomial function of the Chebyshev type. An isotropic thermal factor [B_iso(M)] was refined for the metal centres; the isotropic thermal factor of lighter atoms was calculated as B_iso(L) = B_iso(M) + 2.0 (Ų). The peak profile was modelled trough the Fundamental Parameters Approach [60]. The anisotropic shape of the peaks was modelled with the aid of spherical harmonics in all the cases. A correction was applied for preferred orientation adopting the March-Dollase model [61] in the case of [Zn(tr₂ad)Cl₂]_n and {[Cu(tr₂ad)(NO₃)](NO₃)}_n (along the [001] pole), as well as of {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n (along the [100] pole). The final Rietveld refinement plots are shown in Figures S5–S9.

Crystal data for [Zn(tr₂ad)Cl₂]_n, FW = 406.65 g mol⁻¹: orthorhombic, P2₁2₁2₁, a = 14.6240(3) Å, b = 10.1054(2) Å, c = 11.1204(1) Å, V = 1643.40(5) ų, Z = Z’ = 4, ρ = 1.64 g cm⁻³, F(000) = 832.0, R_Bragg = 0.051, R_p = 0.057 and R_wp = 0.078, for 4801 data and 46 parameters in the 9.0–105.0° 2θ range. CCDC No. 2038425.

Crystal data for {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n, FW = 476.91 g mol⁻¹: monoclinic, P2₁/m, a = 14.5644(9) Å, b = 7.1726(4) Å, c = 10.9798(6) Å, β = 122.820(3)°, V = 963.9(1) ų, Z = 4, Z’ = 2, ρ = 1.64 g cm⁻³, F(000) = 494.0, R_Bragg = 0.019, R_p = 0.028 and R_wp = 0.040, for 4951 data and 71 parameters in the 6.0–105.0° 2θ range. CCDC No. 2038423.

Crystal data for [Cd₂(tr₂ad)Cl₄]_n, FW = 637.00 g mol⁻¹: triclinic, P-1, a = 6.9425(2) Å, b = 12.2352(3) Å, c = 12.6513(2) Å, α = 115.621(1)°, β = 90.837(2)°, γ = 101.165(2)°, V = 944.74(4) ų, Z = Z’ = 2, ρ = 2.24 g cm⁻³, F(000) = 616.0, R_Bragg = 0.050, R_p = 0.049 and R_wp = 0.068, for 4901 data and 68 parameters in the 7.0–105.0° 2θ range. CCDC No. 2038421.

Crystal data for {[Cu(tr₂ad)(NO₃)](NO₃)}_n, FW = 457.95 g mol⁻¹: orthorhombic, Pnma, a = 7.0648(4) Å, b = 10.7226(5) Å, c = 22.495(1) Å, V = 1704.0(2) ų, Z = 8, Z’ = 4, ρ = 1.79 g cm⁻³, F(000) = 940.0, R_Bragg = 0.025, R_p = 0.037 and R_wp = 0.048, for 4951 data and 73 parameters in the 6.0–105.0° 2θ range. CCDC No. 2038424.

Crystal data for {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n, FW = 524.83 g mol⁻¹: monoclinic, C2/c, a = 23.181(1) Å, b = 11. 3867(2) Å, c = 15.486(1) Å, β = 108.956(5)°, V = 3866.1(3) ų, Z = Z’ = 8, ρ = 1.80 g cm⁻³, F(000) = 2112.0, R_Bragg = 0.047, R_p = 0.079 and R_wp = 0.110, for 4901 data and 53 parameters in the 7.0–105.0° 2θ range. CCDC No. 2038422.

4. Conclusions

In this work, we have described the synthesis and solid-state characterization of the novel coordination polymers (CPs) [Zn(tr₂ad)Cl₂]_n, {[Cu(tr₂ad)Cl]Cl∙4H₂O}_n, [Cd₂(tr₂ad)Cl₄]_n, {[Cu(tr₂ad)(NO₃)](NO₃)}_n and {[Cd(tr₂ad)(NO₃)](NO₃)∙H₂O}_n [tr₂ad = 1,3-bis(1,2,4-triazol-4-yl)adamantane], isolated as air- and moisture-stable microcrystalline powders by means of solvothermal reactions. As assessed by thermogravimetric analysis, the five CPs show an appreciable thermal stability. As retrieved by powder X-ray diffraction, while [Zn(tr₂ad)Cl₂]_n features 1-D chains, the other compounds contain 2-D double-layers. A comparative structural analysis involving known CPs built up with the tr₂ad ligand unveiled the coordination modes versatility of the ligand and the crystal structure dimensionality variability. Work can be anticipated in the functional characterization of these CPs as heterogeneous catalysts for cutting-edge organic reactions.