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Article

Half-Lantern Pt(II) and Pt(III) Complexes. New Cyclometalated Platinum Derivatives

by
Violeta Sicilia
1,*,
Pilar Borja
2 and
Antonio Martín
2
1
Departamento de Química Inorgánica, Escuela de Ingeniería y Arquitectura de Zaragoza, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC—Universidad de Zaragoza, Edificio Torres Quevedo, Campus Río Ebro, Zaragoza 50018, Spain
2
Departamento de Química Inorgánica, Facultad de Ciencias, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC—Universidad de Zaragoza, Pedro Cerbuna 12, Zaragoza 50009, Spain
*
Author to whom correspondence should be addressed.
Inorganics 2014, 2(3), 508-523; https://doi.org/10.3390/inorganics2030508
Submission received: 7 July 2014 / Revised: 4 August 2014 / Accepted: 5 August 2014 / Published: 26 August 2014
(This article belongs to the Special Issue Organoplatinum Complexes)
Browse Figures
Versions Notes

Abstract

:
The divalent complex [{Pt(bzq)(μ-L)}2] (1) [Hbzq = benzo[h]quinolone, HL = CF3C4H2N2SH: 4-(trifluoromethyl)pyrimidine-2-thiol] was obtained from equimolar amounts of [Pt(bzq)(NCMe)2]ClO4 and 4-(trifluoromethyl)pyrimidine-2-thiol with an excess of NEt3. The presence of a low intensity absorption band at 486 nm (CH2Cl2), assignable to a metal-metal-to-ligand charge transfer transition (1MMLCT) [dσ*(Pt)2→π*(bzq)], is indicative of the existence of two platinum centers located in close proximity because the rigidity of the half-lantern structure allows the preservation of these interactions in solution. Compound 1 undergoes two-electron oxidation upon treatment with halogens X2 (X2: Cl2, Br2 or I2) to give the corresponding dihalodiplatinum (III) complexes [{Pt(bzq)(μ-L)X}2] (L = CF3C4H2N2S-κN,S; X: Cl 2, Br 3, I 4). Complexes 2–4 were also obtained by reaction of 1 with HX (molar ratio 1:2, 10% excess of HX) in THF with yields of about 80% and compound 2 was also obtained by reaction of [{Pt(bzq)(μ-Cl)}2] with HL (4-(trifluoromethyl)pyrimidine-2-thiol) in molar ratio 1:2 in THF, although in small yield. The X-ray structures of 2 and 3 confirmed the half-lantern structure and the anti configuration of the molecules. Both of them show Pt–Pt distances (2.61188(15) Å 2, 2.61767(16) Å 3) in the low range of those observed in Pt2(III,III)X2 half-lantern complexes.

1. Introduction

Half-lantern compounds of platinum with two four-bond bridging groups have been known for a long time (Scheme 1). Cationic complexes derived from Cisplatin and analogues relevant to antitumor activity have been widely investigated over the last 40 years [1]. Bridging ligands for dinuclear cis-diaminoplatinum complexes include mainly carboxylate, α-pyridonate, amidates, and anionic nucleobases [1,2]. Some examples of divalent Pt2(II,II) complexes are cis-[Pt2(μ-OAc)2(NH3)4]2+ [3,4], ht-[Pt2(μ-C5H4NO)2(NH3)4]2+ (C5H4NO = α-pyridonate) [5] or ht-cis-[(NH3)2Pt(1-MeC−-N3,N4)2Pt(NH3)2](NO3)2 (1-MeC: 1-Methylcytosine) [6], which exhibit Pt–Pt distances in the range 2.9–3.0 Å, indicating the absence of a formal metal-metal bond between the two d8 metal ions [7]. The dimers usually stack in the crystal lattice to form infinite chains [1,3,4], with Pt–Pt separations of ~3.15 Å and hydrogen bonding between the oxygen atoms of the bridging ligands and the protons of the amine ligands, stabilizing the intermolecular interactions.
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Scheme 1. Schematic structures and four-bond bridging groups.
Scheme 1. Schematic structures and four-bond bridging groups.
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Neutral Pt2 (II,II) complexes with bulkier monodentate ancillary ligands [2] and carboxylate-bridging groups, such as [Pt2(µ-OAc)2(η1-OAc)2(PPh3)2] [8], show a increase of the metal-metal distance of up to 3.10 Å. In spite of this, in [Pt2Cl2(µ-O2CMe)2(PMe2Ph)2] the 31P and 195Pt NMR data [J(Pt–P') ≈ 30 Hz; J(Pt–Pt') ≈ 1000 Hz)] strongly support M···M interactions [9]. In complexes with pyridine-2-chalcogenolates (2-PyE−, E = O, S, Se) [2], when E = O, binuclear half-lantern complexes are formed [Pt2Cl2(µ-PyE)2(PR3)2] but no M···M interactions are detected from the NMR spectra. As the chalcogen size increases, monomeric complexes are predominantly formed. Four-bond ligands with a larger bite size, such as xanthates, dithiocarbamates, or dithiophosphates, render exclusively mononuclear complexes [2]. Therefore, the metal-metal separation is significantly affected by the bite size and orientation of the bridging ligands, as well as by the bulkiness of the ancillary ones. Planar di- or tert-dentated polyimines and cyclometalated ligands do not hinder the face-to-face orientation of the metal planes, allowing metal-metal interactions to exist (Scheme 2a,b). Many of these half-lantern compounds exhibit luminescence from triplet metal-metal-to-ligand charge transfer (3MMLCT) excited states. The MMLCT transition involves charge transfer between a filled Pt–Pt antibonding (dσ*) orbital and an empty ligand-based π* orbital [dσ*(Pt)2→π*(L)] of the polyimine or the cyclometalated ligand.
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Scheme 2. Schematic Pt2(II,II) and Pt2(III,III) structures with diimines and cyclometalated ligands.
Scheme 2. Schematic Pt2(II,II) and Pt2(III,III) structures with diimines and cyclometalated ligands.
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Considering half-lantern complexes containing diimines, Kato and co-workers were able to isolate and characterize the syn and anti isomers of [Pt2(pyt)2(bipy)2][PF6]2 from the mixture of both, obtained from [PtCl2(bipy)], pyridine-2-thiol and NH4PF6 [10]. The syn isomer has a head-to-headconfiguration of two bridging pyt and Pt···Pt separation of 2.923(1) Å. The anti isomer shows a head-to-tail arrangement of them and inter-metallic distance of 2.997(1) Å.
Tzeng and co-workers prepared the dinuclear diimine complexes syn- or anti-[Pt2(μ-NS)2(dtbpy)2]2+ dtbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, NHS = pyridine-2-thiol (Hpyt), 2-mercaptobenzothiazole (HNS2), 2-mercaptobenzimidazole (HN2S), 2-mercaptobenzoxazole (HNOS)) by reaction of the mononuclear derivative [Pt(dtbpy)Cl2] with the corresponding HNS in the presence of NaOMe [11,12]. The anti-[Pt2(pyt)2(dtbpy)2]2+ isomer shows a Pt···Pt distance of 2.917(2) Å and also shows photoluminescence at room temperature in the solid state (λmax = 606 nm) [12]. The syn- or anti-[Pt2(μ-NS)2(dtbpy)2]2+ (NS = NS2, N2S, NOS) exhibit intramolecular Pt···Pt distances of 2.9727–3.0079 Å, which lead to Pt···Pt and π-π interactions in the solid state and, additionally, intermolecular Pt···Pt and π-π contacts in the syn isomers.
Changing diimines by C, N-cyclometalated fragments allowed neutral half-lantern Pt(II) complexes, such as complexes [Pt2(C^N)2(pyt)2] (Hpyt = pyridine-2-thiol, HC^N = 2-phenylpyridine (Hppy), 2-(p-tolyl)pyridine (Hptpy), 2-(2-thienyl)pyridine (Hthpy), benzo[h]quinolone (Hbzq)) to be prepared from (NBu4)[Pt(C^N)Cl2] and Hpyt with an excess of tributylamine (NBu3) [13,14]. These dinuclear complexes have a rigid dinuclear framework and only exhibit head-to-tail conformation due to the different trans influence of C and N of the C^N ligand, whereas the pyt group prefers the N-coordination at the trans position to C (Scheme 2b). Complexes [Pt2(C^N)2(pyt)2] (C^N = ppy, ptpy, thpy, bzq) display short Pt···Pt distance (2.82–2.88 Å), suggesting the existence of strong Pt···Pt interactions, and exhibit red luminescence at room temperature both in solid state (λmax = 633–711 nm) and in solution (λmax = 650–715 nm) derived from a 3MMLCT origin.
The half-lantern Pt(II) complexes [{Pt(bzq)(μ-C7H4NS2-κN,S)}2]·Me2CO and [{Pt(bzq)(μ-C7H4NOS-κN,S)}2] (Hbzq: benzo[h]quinolone; C7H4NS2: 2-mercaptobenzothiazolate, C7H4NOS: 2-mercaptobenzoxazolate) were prepared selectively (yield ca. 80%) by reaction of [Pt(bzq)(NCMe)2]ClO4 and KC7H4NYS (Y = S, O) in 1:1 molar ratio in acetone/methanol (2:1) at room temperature [15,16]. Complex [{Pt(bzq)(μ-C7H4NOS-κN,S)}2] and the analogous [{Pt(ppy)(μ-C7H4NOS-κN,S)}2] were also obtained by one-pot reaction between [{Pt(C^N)(µ-Cl)}2] (C^N, bzq, ppy) and NOSH in a ratio of 1.0:4.2 in THF at r.t. in the presence of NaOAc in yields of 32%–35% [17]. All three complexes show an anti configuration, with the N of the bridging groups coordinated at the trans position to C, and intermetallic distances of 2.91 Å, 2.97 Å (bzq) and 3.02 Å (ppy). The bzq complexes show significant π-π interactions that are absent in the ppy one. These compounds were isolated as orangish-red solids, which exhibit a weak absorption at ~500 nm and intense orangish-red photoluminescence at room temperature both in solid state (λmax = 665–691 nm) and in toluene solution (λmax = 660–677 nm) [15,16]. TD-DFT studies on complexes [{Pt(bzq)(μ-C7H4NYS-κN,S)}2] (Y = S, O) [15,16] proved the 1,3MMLCT character of their lower energy absorption and emission, which is affected by the strength of the π···π interactions. Complexes [{Pt(pbt)(μ-C7H4NYS-κN,S)}2] (pbt: 2-phenylbenzothiazole; Y = S: 2-mercaptobenzothiazolate, Y = O: 2-mercaptobenzoxazolate) were prepared later by a similar procedure with yields of 60%–70% [18].
The half-lantern Pt2(II,II) complexes show complex and interesting redox chemistry [19]. Oxidation of cis-diaminoplatinum Pt2 (II,II) leads to oligomeric mixed-valence “platinum blues” with oxidation states varying +2.25, +2.5 and 3. Apart from these, discrete cis-diaminoplatinum (III) dimers, such as, ht-[Pt2(μ-C5H4NO)2(NH3)4X2]2+ (C5H4NO = α-pyridonate, X = NO3, NO2, Cl, Br) [20,21], ht-cis-[(OH2)(NH3)2Pt(1-MeC−-N3,N4)2-Pt(NH3)2(OH2)] (ClO4)4·H2O [22], or cis-[Pt2(μ-OAc)2X2(NH3)4]2+ (X = Cl, Br) [4] are known, with bond distances of ~2.6 Å. The diamagnetic nature of these compounds and the shortening of the Pt−Pt distance with respect to the Pt2(II,II) are consequences of the formation of a metal-metal σ bond.
Oxidation of C, N-cyclometalated Pt2(II,II) compounds do not lead to mixed-valence oligomeric species but rather to discrete Pt2(III,III) dimers [13] (Scheme 2c). By way of example, complexes [{Pt(bzq)(μ-C7H4NYS-κN,S)}2] (Y = S, O) undergo two-center, two-electron oxidation in their reactions with halogens (X2 = Cl, Br, I) [15,16] to give [{Pt(bzq)(μ-C7H4NYS-κN,S)X}2] (Y = S, O; X = Cl, Br, I) which show Pt−Pt distances (2.63 to 2.68 Å), about 10% shorter than their starting complexes (2.91, 2.97 Å). The half-lantern Pt2(III,III)X2 compounds (X = Cl, Br, I) seem to be quite stable, being the final products of diverse kinds of reactions. The dichloro compounds [Pt2(C^N)2(µ-pyt)2Cl2] (Hpyt = pyridine-2-thiol, HC^N = 2-phenylpyridine (Hppy), 2-(p-tolyl)pyridine (Hptpy), 2-(2-thienyl)pyridine (Hthpy), benzo[h]quinolone (Hbzq)) were the final products of the reactions between (NBu4)[Pt(C^N)Cl2] and Hpyt in the absence of a base [13]; otherwise, in the presence of NBu3, the divalent compounds [Pt2(C^N)2(µ-pyt)2] were formed. Compound [Pt2(ppy)2(µ-pyt)2Cl2], which shows a Pt-Pt distance of 2.6150(8) Å, was also obtained by reaction of the corresponding Pt2(II,II) derivative with HCl or by recrystallization of Pt2(II,II) from chloroform [14].
Pt2(III,III)Cl2 derivatives can also be obtained by using the dinuclear complexes [{Pt(C^N)(µ-Cl)}2] as starting materials. Examples of this synthetic route are the synthesis of [{Pt(ppy)(µ-N^S)Cl}2] (HN^S = 2-mercaptobenzimidazole (mbzimH), 5-nitro-2-mercaptobenzimidazole (NO2-mbzimH), 2-mercaptobenzothiazole (mbztzH), 2-mercaptobenzoxazole (mbzoxzH), 2-mercaptoimidazoline (mimH)) by reaction of [{Pt(C^N)(µ-Cl)}2] and HN^S in CHCl3 at room temperature [23]. The Pt−Pt distances in these trivalent dinuclear complexes range from 2.6185(14) Å to 2.6445(3) Å.
Carboxylate-bridged Pt2(III,III)Cl2 derivatives such as [{Pt(C^N)(µ-O2CR)Cl}2] (C^N = 2,5-bis(4-ethoxyphenyl)pyridine-H, R = Me [24], 1-methoxy-4-(2-pyridyl)benzo[g] phenantrene-H, R = Ph [25,26] could be obtained by reaction of the mononuclear [Pt(C^N)Cl(DMSO)] with acetic acid [24] or by reaction of the dinuclear [{Pt(C^N)(µ-Cl)}2] with AgO2CPh [25,26]. The metal-metal distances in these di-μ-carboxylate complexes are 2.5730(3) Å [24] and 2.5952(6) Å [26], both of which are quite shorter than those observed in the above described complexes containing two N^S bridging groups.
None of the described half-lantern Pt2(III,III) complexes are luminescent in the visible region. In general, the d7−d7 complexes are no emitter, both in solution and in solid state, there being just a few exceptions, for example, [Pt2(µ-pop)4X2]4− (pop = P,P-pyrophosphite,P2O5H22−, X = Cl, Br, SCN, or py) [27,28], which exhibit red luminescence in an alcohol glass or in solid state at low temperature and [{Pt(κ2-As,C-C6H3-5-CHMe2-2-AsPh2)2X}2] (X = Cl, Br, I, CN) [29] that emit in the visible to NIR region even at room temperature.
In the course of our research on half-lantern Pt(II) complexes, we have prepared a new Pt2(II,II) derivative, [{Pt(bzq)(μ-L)}2] [Hbzq = benzo[h]quinolone, HL = CF3C4H2N2SH: 4-(trifluoromethyl)pyrimidine-2-thiol] and the two-electron-oxidized dihalodiplatinum (III) complexes [{Pt(bzq)(μ-L)X}2] (L = CF3C4H2N2S-κN,S; X: Cl, Br, I). In spite of the similarities of [{Pt(bzq)(μ-CF3C4H2N2S-κN,S)}2] with compounds [{Pt(bzq)(μ-C7H4NYS-κN,S)}2] (Y = S, O) [15,16], it showed not luminescence in the visible region and, as expected, the Pt2(III,III)X2 neither did.

2. Results and Discussion

The divalent complex [{Pt(bzq)(μ-L)}2] (1) [Hbzq = benzo[h]quinolone, HL = CF3C4H2N2SH: 4-(trifluoromethyl)pyrimidine-2-thiol] was obtained by refluxing equimolar amounts of [Pt(bzq)(NCMe)2]ClO4 and 4-(trifluoromethyl)pyrimidine-2-thiol with an excess of NEt3 in acetone-methanol. Compound 1 precipitated in the reaction mixture from which it was separated and obtained as a pinkish-red, air-stable solid in a good yield (92%) (see Scheme 3, path i, and Experimental section). No single crystals of 1 could be grown for X-ray purposes, but evidences from other techniques lead us to propose the structure represented in Scheme 3. The dinuclear nature of 1 is revealed from its mass spectrum that shows two important peaks at m/z 1105.1 (97%) and 925.2 (100%) corresponding to [M]+ and ([M-CF3-C4H2N2S]+ respectively. The electronic absorption spectrum of 1 recorded in CH2Cl2 (Figure 1) also provided some structural information. It shows a low intensity band centered at 486 nm (CH2Cl2), which can be tentatively assigned to a metal-metal-to-ligand charge transfer transition, (1MMLCT) [dσ*(Pt)2→π*(bzq)] as in the analogous half-lantern compounds [{Pt(bzq)(μ-C7H4NYS-κN,S)}2] (Y = O, 480 nm CH2Cl2; Y = S, 487 nm CH2Cl2) [15,16]. The presence of this band due to Pt···Pt interactions is indicative of the existence of two platinum centers located in close proximity. The rigidity of the half-lantern structure would allow the preservation of these interactions in solution. The presence of only one set of signals corresponding to the bzq and the bridging CF3C4H2N2S-κN,S ligands, indicates that compound 1 exists as one single, symmetric isomer, most probably the anti one, as observed in the previously reported analogous compounds [{Pt(bzq)(μ-C7H4NYS-κN,S)}2] (Y = O, S). The scarce solubility of 1 in most common solvents precludes obtaining more information about its structure in solution by other means, such as a NOE spectrum.
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Scheme 3. Reaction scheme and numbering for NMR purposes.
Scheme 3. Reaction scheme and numbering for NMR purposes.
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As expected from the presumed short Pt−Pt distance, compound 1 undergoes two-electron oxidation upon treatment with halogens X2 (X2 = Cl2, Br2 or I2) to give the corresponding dihalodiplatinum (III) complexes [{Pt(bzq)(μ-L)X}2] (L = CF3C4H2N2S-κN,S; X: Cl 2, Br 3, I 4) as yellowish-orange, orange and pinkish-red solids, respectively, in very high yield (Scheme 3, path ii and Experimental section). The IR and 1H-NMR spectra of 2–4 are very similar and show small changes with respect to the starting complex 1 ones; the different colors of these complexes are revealed in their UV-vis spectra (Figure 1). The X-ray studies confirmed that these Pt(III) complexes are isostructural (see below). Complexes 2–4 were also obtained by reaction of 1 with HX (molar ratio 1:2, 10% excess of HX) in THF with yields of about 80% (Scheme 3 path iii and Experimental section).
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Figure 1. Normalized absorption spectra in CH2Cl2 solution (10−4 M) of 1–4 at room temperature.
Figure 1. Normalized absorption spectra in CH2Cl2 solution (10−4 M) of 1–4 at room temperature.
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Compound 2 was also obtained by reaction of [{Pt(bzq)(μ-Cl)}2] with HL (4-(trifluoromethyl)pyrimidine-2-thiol) in molar ratio 1:2 in THF (Scheme 3 path iv and Experimental section). This pathway rendered compound 2 with a moderate yield, as previously observed in the synthesis of the analogous compounds [{Pt(2-PhPy)(μ-L)Cl}2] (HL = 2-mercaptobenzimidazole, 5-nitro-2-mercaptobenzimidazole, 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptoimidazoline) [23]. The use of [{Pt(bzq)(μ-Cl)}2] as starting material, has important limitations: (a) it exclusively renders the dichloro compound, preventing comparison of dichloro with dibromo and diiodo derivatives and (b) it proceeds with low yield, probably because side reactions occurred simultaneously.
The X-ray structures of 2 and 3 are shown in Figure 2 and Figure 3 respectively and a selection of bond distances and angles is listed in Table 1. They confirmed the expected half-lantern structure and the anti configuration of the molecule. Each Pt (III) center has a distorted octahedral environment with the axial positions occupied by an halogen atom (Cl 2, Br 3) and the other Pt(III) center, and with the X−Pt−Pt angles being close to 174°. Compounds 2 and 3 show Pt-Pt distances (2.61188(15) Å 2, 2.61767(16) Å 3), similar to that of [Pt2(ppy)2(µ-pyt)2Cl2] (2.6150(8) Å) [14], all three being in the low range of those observed in Pt2(III,III)X2 half-lantern complexes [15,16]. The shortening of the Pt-Pt distance in 2 and 3 with respect to those of the analogous compounds (X = Cl: 2.6420(3) Å B1, 2.6383(3) Å B2; Br: 2.6435(4) Å C1, 2.6671(9) Å C2) [15,16] seems to be related to the NCS angle values of the 4-membered bridges. These angles are close to 120° in 2 and 3 (120.47(22)° 2, 121.23(24)° 3) and clearly smaller than those in B1(128.4(4)°), C1(129.03(52)°), B2 (128.2(5)°) and C2 (131.56(00)°). In addition, the Pt-Pt distances in 2 and 3 were shorter in the dichloro than in the dibromo derivative, in accordance with the larger trans-influence of Br with respect to Cl [30], as was previously observed in [Pt2(P2O5H2)4X2]4− (X = Cl, Br, I) [31], [Pt2(µ-κAs,κC-C6H3-5-CHMe2-2-AsPh2)4X2] (X = Cl, Br, I) [29], [Pt2(µ-κAs,κC-C6H3-5-Me-2-AsPh2)4X2] (X = Cl, Br, I) [32] or [{Pt(bzq)(μ-C7H4NYS-κN,S)X}2] (Y = O, S; X = Cl, Br, I) [15,16]. The Pt(III)−X distances are in the range of those found in other complexes with these kinds of ligands. In these dinuclear Pt2(III,III)X2 complexes, the two platinum coordination planes are almost parallel, with a small interplanar angle [9.79(5)° 2, 9.76(6)° 3] and the Pt-Pt line is almost perpendicular to the two Pt square coordination planes, the biggest angle being 5.44(4)° (Pt1−Pt2 line and Pt1 plane for compound 2). The crystal packing of these two complexes does not show intermolecular interactions among neighbor molecules.
Table
Table 1. Selected Bond Distances (Å) and angles (deg) of 2 and 3.
Table 1. Selected Bond Distances (Å) and angles (deg) of 2 and 3.
Bonds and angles2 CH2Cl23·CH2Cl2
Pt(1)−C(1)2.015(3)2.013(3)
Pt(1)−N(1)2.090(2)2.092(2)
Pt(1)−Nμ-N^S2.151(2)2.165(3)
Pt(1)−Sμ-N^S2.3016(7)2.2995(8)
Pt(1)− X(1)2.4305(7)2.5593(3)
Pt(1)−Pt(2)2.61188(15)2.61767(16)
Pt(2)−C(14)2.015(3)2.015(3)
Pt(2)−N(2)2.078(2)2.080(3)
Pt(2)−Nμ-N^S2.151(2)2.156(3)
Pt(2)−Sμ-N^S2.3036(7)2.3047(8)
Pt(2)− X(2)2.4381(7)2.5692(3)
C(1)−Pt(1)−N(1)81.44(11)81.55(11)
N(1)−Pt(1)−Nμ-N^S93.43(9)93.37(10)
C(1)−Pt(1)−Sμ-N^S96.52(8)96.25(9)
Nμ-N^S−Pt(1)−Sμ-N^S88.46(6)88.70(7)
C(14)−Pt(2)−N(2)81.68(11)81.56(12)
N(2)−Pt(2)−Nμ-N^S93.87(9)93.69(10)
C(14)−Pt(2)−Sμ-N^S96.35(9)96.31(9)
Nμ-N^S−Pt(2)−Sμ-N^S88.13(7)88.48(7)
C(1)−Pt(1)− X(1)86.94(8)86.29(9)
N(1)−Pt(1)− X(1)88.94(6)88.73(7)
Nμ-N^S−Pt(1)− X(1)91.44(6)92.14(7)
Sμ-N^S−Pt(1)− X(1)86.47(2)86.71(2)
Pt(2)−Pt(1)− X(1)173.389(18)173.901()
C(14)−Pt(2)− X(2)89.04(8)88.22(9)
N(2)−Pt(2)− X(2)88.51(6)88.71(7)
Nμ-N^S−Pt(2)− X(2)91.36(6)92.34(7)
Sμ-N^S−Pt(2)− X(2)88.25(2)88.09(2)
Pt(1)−Pt(2)− X(2)175.352(1)175.567(9)
Differently to the analogous compounds [{Pt(bzq)(μ-C7H4NOS-κN,S)}2] (A1) and [{Pt(bzq)(μ-C7H4NS2-κN,S)}2] (A2), compound 1 showed no luminescence either in solid or in solution and, as expected, neither did the d7–d7 compounds 2–4.
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Figure 2. Molecular Structure of compound 2. Ellipsoids are drawn at their 50% probability level; solvent molecules and hydrogen atoms were omitted for clarity.
Figure 2. Molecular Structure of compound 2. Ellipsoids are drawn at their 50% probability level; solvent molecules and hydrogen atoms were omitted for clarity.
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Inorganics 02 00508 g003 1024
Figure 3. Molecular Structure of compound 3. Ellipsoids are drawn at their 50% probability level; solvent molecules and hydrogen atoms were omitted for clarity.
Figure 3. Molecular Structure of compound 3. Ellipsoids are drawn at their 50% probability level; solvent molecules and hydrogen atoms were omitted for clarity.
Inorganics 02 00508 g003

3. Experimental section

General procedures and materials. Elemental analyses were carried out with a Perkin Elmer (Waltham, MA, USA) 2400 CHNS analyzer. IR spectra were recorded on a Perkin-Elmer (Waltham, MA, USA) Spectrum 100 FT-IR Spectrometer (ATR in the range 250–4000 cm−1). Mass spectral analyses were performed with a Microflex MALDI-TOF Bruker or an Autoflex III MALDI-TOF Bruker (Madison, WI, USA), instruments. NMR spectra were recorded on a Bruker (Madison, WI, USA) AV-400 spectrometer using the standard references: SiMe4; J is given in Hz and assignments are based on 1H-1H-COSY experiments. Absorption spectra were recorded on a Thermo Electron Corporation (Waltham, MA, USA) evolution 600 spectrophotometer and diffuse reflectance UV-vis (DRUV) spectra were recorded on a Thermo Electron Corporation (Waltham, MA, USA) Evolution 600 spectrophotometer equipped with a Praying Mantis integrating sphere. The solid samples were homogeneously diluted with silica. The mixtures were placed in a homemade cell equipped with quartz window.
The starting material [Pt(bzq)(NCMe)2]ClO4 was prepared, as described elsewhere [33]. (CF3)C4H2N2SH was used as purchased from Aldrich (St. Louis, MO, USA).
[{Pt(bzq)(µ-(CF3)C4H2N2S-κN,S)}2] (1). A yellowish-orange suspension of [Pt(bzq)(NCMe)2]ClO4 (0.251 g, 0.453 mmol) in acetone (20 mL) was treated with a solution of (CF3)C4H2N2SH (0.082 g, 0.453 mmol) in methanol (10 mL) and NEt3 (0.5 mL). The mixture was stirred and refluxed for 2 h and then was concentrated to about 15 mL. The resulting pinkish-red solid was filtered-off, washed with MeOH (2 × 3 mL) and Et2O (2 × 5 mL) and dried, 1. Yield: 0.231 g, 92%. Anal. Calcd for C36H20F6N6Pt2S2: C, 39.13; H, 1.82; N, 7.61; Found: C, 39.10; H, 1.99; N, 7.55; 1H-NMR (CD2Cl2, 400.16 MHz, 298K): δ 9.10 (2H, d, 3J(H6',H5') = 5.7; H6’), 7.97 (2H, dd, 3J(H4,H3) = 7.9, 4J(H4,H2) = 1.0, H4), 7.93 (2H, dd, 3J(H2,H3) = 5.2; H2), 7.32 (2H, dd, H3), 7.20 (4H, m, H9, H5'), 7.16 (4H, AB, 3J(H5,H6) = 8.6, H5, H6), 6.86 (2H, d, 3J(H7,H8) = 7.5; H7), 6.64 (2H, dd, 3J(H8,H9) = 7.5, H8) ppm; 19F-NMR (CD2Cl2, 376 MHz, 298K): δ −70.9 (s, CF3) ppm; MS (MALDI+): m/z 925.2 ([M − CF3-C4H2N2S]+, 100%), 1105.1 (M+, 97%); IR (cm−1): 1620 d, 1575 m, 1545 m, 1450 d, 1426 m, 1405 m, 1328 f, 1314 mf, 1205 f, 1178 m, 1144 f, 1113 mf, 1087 f, 1053 m, 998 m, 928 m, 841 f, 823 f, 762 m, 751 m, 730 f, 710 f, 671 f, 654 m, 526 m, 469 m, 288 m, 281 m, 264 m.
[{Pt(bzq)(µ-(CF3)C4H2N2S-κN,S)Cl}2] (2). (Method A) A solution of Cl2 in CCl4 0.25M (0.602 mL, 0.151 mmol) was added to a pinkish-red suspension of 1 (0.1513 g, 0.137 mmol) in THF (4 mL). The resulting orange mixture was stirred for 4 h and concentrated to 2 mL. Addition of Et2O (5 mL) to the residue rendered a yellowish-orange solid that was filtered-off and washed with Et2O (2 × 4 mL), 2. Yield: 0.134 g, 83%. Anal. Calcd for C36H20Cl2F6N6Pt2S2: C, 36.77; H, 1.71; N, 7.15; Found: C, 36.65; H, 2.06; N, 7.08; 1H-NMR (CD2Cl2, 400.16 MHz, 298K): δ 10.00 (2H, d, 3J(H6',H5') = 6.1; H6'), 8.13 (2H, dd, 3J(H2,H3) = 5.6, 4J(H2,H4) = 1.0, 3J(Pt,H2) = 22.5, H2), 8.06 (2H, dd, 3J(H4,H3) = 8.1, 4J(H4,H2) = 1.0, H4), 7.55 (2H, dd, H3), 7.40 (2H, d, H5’), 7.29 (4H, AB, 3J(H5,H6) = 8.8, H5, H6), 7.04 (2H, dd, 3J(H9,H8) = 7.5; 4J(H9,H7) = 0.8 Hz, 3J(Pt,H9) = 31.6, H9), 6.93 (2H, d, 3J(H7,H8) = 8.0; H7), 6.75 (2H, dd, H8) ppm; 19F-NMR (CD2Cl2, 376 MHz, 298K):δ −70.6 (s, CF3) ppm. IR (cm−1): 1621 d, 1586 m, 1575 m, 1549 m, 1454 d, 1436 m, 1407 m, 1343 f, 1332 f, 1322 mf, 1205 f, 1185 m, 1142 f, 1116 mf, 928 m, 844 m, 830 mf, 819 m, 764 m, 731 f, 711 f, 674 f, 654 m, 514 m, 467 m, 438 m. (Method B) A solution of HCl in H2O 1M (299 µL, 0.299 mmol) was added to a pinkish-red suspension of 1 (0.150 g, 0.136 mmol) in THF (10 mL) and the mixture was stirred for 51 h in the darkness. During this time, a yellowish-orange solid was precipitated. The solvent was evaporated to ca. 2 mL and Et2O (10 mL) was added to the reaction mixture. The yellowish-orange solid 2·THF was filtered-off, washed with Et2O (2 × 5 mL) and dried to the air. Yield: 0.1368 g, 81%. (Method C) A yellow solution of C5H3N2F3S (0.062 g, 0.348 mmol) in THF (20 mL) was added drop by drop to a yellow suspension of [{Pt(bzq)(µ-Cl)}2] (0.142 g, 0.174 mmol) in THF (10 mL). Then, the mixture was stirred for 70 h to give a yellow solution that was evaporated to dryness. Addition of acetone (15 mL) to the residue rendered a yellowish- orange solid, 2, that was filtered-off, washed with acetone (2 × 5 mL) and Et2O (2 × 5 mL) and dried to the air. Yield: 0.0341 g, 24%.
[{Pt(bzq)(µ-(CF3)C4H2N2S-κN,S)Br}2] (3). (Method A) was prepared in the same way as 2, but using Br2 (0.149 mmol 7.66 µL, d = 3.119 g/mL), 1 (0.150 g, 0.136 mmol). Orange solid 3, Yield: 0.154 g, 90%. Anal. Calcd for Br2C36H20F6N6Pt2S2: C, 34.19; H, 1.59; N, 6.65; Found: C, 33.80; H, 1.61; N, 6.71. 1H-NMR (CD2Cl2, 400.16 MHz, 298K): δ 10.14 (2H, d, 3J(H6',H5') = 6.0; H6'), 8.19 (2H, dd, 3J(H2,H3) = 5.5, 4J(H2,H4) = 1.3, 3J(Pt,H2) = 22.2, H2), 8.05 (2H, dd, 3J(H4,H3) = 7.8,4J(H4,H2) = 1.3, H4), 7.56 (2H, dd, H3), 7.38 (2H, d, H5'), 7.29 (4H, AB, 3J(H5,H6) = 8.8 Hz, H5, H6), 7.00 (2H, dd, 3J(H9,H8) = 7.6; 4J(H9,H7) = 0.7, 3J(Pt,H9) = 31.7, H9), 6.90 (2H, d, 3J(H7,H8) = 8.0; H7), 6.74 ppm (2H, dd, H8); 19F-NMR (CD2Cl2, 376 MHz, 298K):δ −70.6 (s, CF3) ppm. IR (cm−1): 1621 d, 1586 m, 1575 m, 1549 m, 1454 m, 1436 m, 1407 m, 1343 f, 1332 f, 1322 mf, 1205 f, 1185 f, 1142 f, 1116 mf, 1053 m, 928 m, 844 f, 830 mf, 819 f, 764 f, 731 f, 711 f, 674 f, 654 m, 514 m, 467 m, 438 m, 271 m. (Method B) A solution of HBr in H2O 1M (296 µL, 0.296 mmol) was added to a pinkish-red suspension of 1 (0.149 g, 0.135 mmol) in THF (20 mL) and the mixture was stirred for 72 h in the darkness, while an orange solid was precipitated. The solvent was evaporated to ca. 2 mL and Et2O (10 mL) was added to the reaction mixture. The orange solid 3·THF was filtered-off, washed with Et2O (2 × 5 mL) and dried to the air. Yield: 0.127 g, 70%.
[{Pt(bzq)(µ-(CF3)C4H2N2S-κN,S)I}2] (4). (Method A) was prepared in the same way as 2, but using a solution of I2 (0.038 g, 0.151 mmol) in THF (4 mL), a suspension of 1 (0.152 g, 0.137 mmol) in THF (4 mL). Pinkish-red solid 4, Yield: 0.162 g, 87%. Anal. Calcd for C36H20I2F6N6Pt2S2: C, 31.82; H, 1.48; N, 6.19; Found: C, 31.58; H, 1.32; N, 6.28. 1H-NMR (CD2Cl2, 400.16 MHz, 298K): δ 10.34 (2H, d, 3J(H6',H5') = 6.0; H6'), 8.28 (2H, dd, 3J(H2,H3) = 5.4, 4J(H2,H4) = 1.1, H2), 8.05 (2H, dd, 3J(H4,H3) = 8.0, 4J(H4,H2) = 1.1, H4), 7.57 (2H, dd, H3), 7.35 (2H, d, H5'), 7.30 (4H, AB, 3J(H5,H6) = 8.7 Hz, H5, H6), 6.93 (2H, dd, 3J(H9,H8) = 7.5; 4J(H9,H7) = 0.8, 3J(Pt,H9) = 31.4, H9), 6.86 (2H, d, 3J(H7,H8) = 7.8; H7), 6.71 (2H, dd, H8) ppm; 19F-NMR (CD2Cl2, 376 MHz, 298K): δ-70.6 (s, CF3) ppm; IR (cm−1): 1620 d, 1585 m, 1574 m, 1546 m, 1453 m, 1436 m, 1407 m, 1332 mf, 1322 mf, 1205 f, 1187 m, 1158 m, 1138 f, 1116 mf, 928 m, 845 m, 829 mf, 819 m, 763 f, 731 f, 709 f, 673 f, 653 m, 512 m, 482m, 467 m, 438 m. (Method B) A solution of HI in H2O 1M (292 µL, 0.292 mmol) was added to a pinkish-red suspension of 1 (0.147 g, 0.133 mmol) in THF (10 mL) and the mixture was stirred for 26 h in absence of light. During this time, a reddish-garnet solid was precipitated. The solvent was evaporated to ~2 mL and 10 mL of Et2O was added. The reddish-garnet solid 4·THF was filtered-off, washed with Et2O (2 × 5 mL) and dried. Yield: 0.156 g, 82%.
X-ray Structure Determinations. Crystal data and other details of the structure analyses are presented in Table 2. Suitable crystals for X-ray diffraction studies were obtained by slow diffusion of n-hexane into concentrated solutions of the complexes in 3 mL of CH2Cl2. Crystals were mounted at the end of quartz fibres. The radiation used in both cases was graphite monochromated MoKα (λ = 0.71073 Å). X-ray intensity data were collected on an Oxford Diffraction Xcalibur diffractometer. The diffraction frames were integrated and corrected from absorption using the CrysAlis RED program [34]. The structures were solved by Patterson and Fourier methods and refined by full-matrix least squares on F2 with SHELXL-97 [35]. All non-hydrogen atoms were assigned anisotropic displacement parameters and refined without positional constraints. All hydrogen atoms were constrained to idealized geometries and assigned isotropic displacement parameters equal to 1.2 times the Uiso values of their attached parent atoms. Full-matrix least-squares refinement of these models against F2 converged to final residual indices given in Table 2. CCDC 1011979–1011980 contain the supplementary crystallographic data for compounds 2·CH2Cl2 and 3·CH2Cl2[36].
Table
Table 2. Crystal data and structure refinement for complexes [{Pt(bzq)(µ-(CF3)C4H2N2S-κN,S)Cl}2]·CH2Cl2 (2·CH2Cl2), [{Pt(bzq)(µ-(CF3)C4H2N2S-κN,S)Br}2]·CH2Cl2 (3·CH2Cl2).
Table 2. Crystal data and structure refinement for complexes [{Pt(bzq)(µ-(CF3)C4H2N2S-κN,S)Cl}2]·CH2Cl2 (2·CH2Cl2), [{Pt(bzq)(µ-(CF3)C4H2N2S-κN,S)Br}2]·CH2Cl2 (3·CH2Cl2).
Parameters2·CH2Cl23·CH2Cl2
C37H22Cl4F6N6Pt2S2C37H22Br2Cl2F6N6Pt2S2
Mt [g mol−1]1260.711349.63
T [K]100(2)100(2)
λ [Å]0.710730.71073
crystal systemtriclinictriclinic
space groupP-1P-1
a [Å]11.3613(3)11.4798(2)
b [Å]11.8938(3)11.8668(2)
c [Å]14.2579(3)14.4711(3)
α [°]90.003(2)88.942(2)
β [°]98.289(2)81.111(2)
γ [°]101.170(2)78.410(2)
V [Å3]1869.64(8)1907.83(6)
Z22
ρ [g cm−3]2.2392.349
μ [mm−1]7.9429.737
F(000)11921264
2 θ range [°]4.21–28.884.19–28.88
no. of reflns collected4104041763
no. of unique reflns89619166
R(int)0.03020.0318
final R indices [I > 2θ(I)] a
R10.02070.0232
wR20.04460.0516
R indices (all data)
R10.02500.0277
wR20.04650.0534
Goodness-of-fit on F2 b1.0241.058
a R1 = ∑(|Fo| − |Fc|)/∑|Fo|, wR2 = [∑w(Fo2− Fc2)2/∑w(Fo2)2]1/2. b Goodness-of-fit = [∑w(Fo2− Fc2)2/(nobs− nparam)]1/2.

4. Conclusions

The divalent complex [{Pt(bzq)(μ-L)}2] (1) [Hbzq = benzo[h]quinolone, HL = CF3C4H2N2SH: 4-(trifluoromethyl)pyrimidine-2-thiol] was obtained from equimolar amounts of [Pt(bzq)(NCMe)2]ClO4 and 4-(trifluoromethyl)pyrimidine-2-thiol with an excess of NEt3. The electronic absorption spectra of 1 shows a low intensity band at 486 nm (CH2Cl2) assignable to a metal-metal-to-ligand charge transfer transition, (1MMLCT) [dσ*(Pt)2→π*(bzq)], due to the presence of two platinum centers located in close proximity.
As expected, compound 1 undergoes two-center two-electron oxidation upon treatment with halogens X2 (X2: Cl2, Br2 or I2) to give the corresponding dihalodiplatinum (III) complexes [{Pt(bzq)(μ-L)X}2] (L = CF3C4H2N2S-κN,S; X: Cl 2, Br 3, I 4) in a very high yield. Complexes 2–4 were also obtained by reaction of 1 with HX (molar ratio 1:2, 10% excess of HX) and compound 2 even by reaction of [{Pt(bzq)(μ-Cl)}2] and HL (4-(trifluoromethyl)pyrimidine-2-thiol) in 1:2 molar ratio, albeit in moderate-low yield. The use of [{Pt(bzq)(μ-Cl)}2] as the starting material has significant limitations: it exclusively renders the dichloro compound preventing the comparison of dichloro with dibromo and diyodo derivatives, and it proceeds with a low yield, probably because of side reactions occurring simultaneously.
The X-ray structures of 2 and 3 confirmed the half-lantern structure and the anti configuration of the molecules. Both of them show Pt−Pt distances (~2.61 Å) in the low range of those observed in Pt2(III,III)X2 half-lantern complexes, and their values are in accordance with the larger trans-influence of Br with respect to Cl. The Pt-Pt distance in Pt2(III,III)X2 compounds also seems to be related to the NCS angle values of the 4-member bridges, these being close to 120° in 2 and 3, which is clearly less than that observed in other dichloro- and dibromo-compounds compounds (~129°) exhibiting longer intermetallic separations (~2.64 Å).
Compounds 1–4 showed not luminescence in the visible region.

Acknowledgments

This work was supported by the Spanish MICINN/FEDER (Project CTQ2008-06669-C02), the Spanish Ministerio de Economía y Competitividad, MEC (Project CTQ2012-35251) and the Gobierno de Aragón (Grupo Consolidado: Química Inorgánica y de los Compuestos Organometálicos). Pilar Borja acknowledges the support of a FPI grant from the Spanish government.

Author Contributions

Violeta Sicilia: Responsible for research publication. Pilar Borja: Research performer. phD student. Antonio Martín: Crystallographer.

Conflicts of Interest

The authors declare no conflict of interest.

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MDPI and ACS Style

Sicilia, V.; Borja, P.; Martín, A. Half-Lantern Pt(II) and Pt(III) Complexes. New Cyclometalated Platinum Derivatives. Inorganics 2014, 2, 508-523. https://doi.org/10.3390/inorganics2030508

AMA Style

Sicilia V, Borja P, Martín A. Half-Lantern Pt(II) and Pt(III) Complexes. New Cyclometalated Platinum Derivatives. Inorganics. 2014; 2(3):508-523. https://doi.org/10.3390/inorganics2030508

Chicago/Turabian Style

Sicilia, Violeta, Pilar Borja, and Antonio Martín. 2014. "Half-Lantern Pt(II) and Pt(III) Complexes. New Cyclometalated Platinum Derivatives" Inorganics 2, no. 3: 508-523. https://doi.org/10.3390/inorganics2030508

APA Style

Sicilia, V., Borja, P., & Martín, A. (2014). Half-Lantern Pt(II) and Pt(III) Complexes. New Cyclometalated Platinum Derivatives. Inorganics, 2(3), 508-523. https://doi.org/10.3390/inorganics2030508

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