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Article

Trinuclear and Cyclometallated Organometallic Dinuclear Pt-Pyrazolato Complexes: A Combined Experimental and Theoretical Study

by
Zhichun Shi
1,
Fengyu Li
2,
Hong Zhao
2,
Indranil Chakraborty
1,
Zhongfang Chen
2,* and
Raphael G. Raptis
1,*
1
Department of Chemistry, Biochemistry and Biomolecular Sciences Institute, Florida International University, Miami, FL 33199, USA
2
Department of Chemistry, University of Puerto Rico, San Juan, PR 00931, USA
*
Authors to whom correspondence should be addressed.
Chemistry 2023, 5(1), 187-200; https://doi.org/10.3390/chemistry5010016
Submission received: 19 December 2022 / Revised: 23 January 2023 / Accepted: 24 January 2023 / Published: 29 January 2023
(This article belongs to the Special Issue Coordination Chemistry: Current Developments and Future Perspectives — A Themed Issue in Honor of Professor Spyros P. Perlepes on the Occasion of His 70th Birthday)
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Versions Notes

Abstract

:
Two differently substituted pyrazole ligands have been investigated with regard to the topology of their Pt complexes: upon deprotonation, two mononuclear 1:2 PtII-pyrazole complexes—one of the sterically unhindered 4-Me-pzH and one of the bulky 3,5-tBu-pzH (pzH = pyrazole)—yield the corresponding 1:2 PtII-pyrazolato species; the former a triangular, trinuclear metallacycle (1), and the latter a dinuclear, half-lantern species (2) formed via the unprecedented cyclometallation of a butyl group. Stoichiometric oxidation of the colorless PtII2 complex produces the deep-blue, metal–metal bonded PtIII2 analog (3) with a rarely encountered unsymmetrical coordination across the Pt-Pt bond. All three complexes have been characterized by single crystal X-ray structure determination, 1H-NMR, IR, and UV-vis-NIR spectroscopic methods. The XPS spectra of the PtII2 and PtIII2 species are also reported. Density functional theory calculations were carried out to investigate the electronic structure, spectroscopic properties, and chemical bonding of the new complexes. The calculated natural population analysis charges and Wiberg bonding indices indicate a weak σ-interaction in the case of 2 and a formal Pt-Pt single bond in 3.

Graphical Abstract

1. Introduction

Metal–metal interactions in multinuclear systems determine the course of their chemical reactions, with implications in industrial and biological catalysis, the construction of functional materials, as well as in the understanding of fundamental chemical principles (e.g., metal–metal bonding). In addition to metal–metal distance and orientation, the role of peripheral ligands is often critical to the reaction outcome, as the steric bulk of groups proximal to the metal allows or prevents the formation of certain products, and the energy of the metal-based frontier orbitals is tuned by the resonance and inductive effects exerted by these groups. Two-electron oxidative addition/reductive elimination reactions across dinuclear or trinuclear late transition metal centers continue to attract considerable interest [1,2,3]; they can lead to either homovalent or mixed-valent products, depending on the distance between and relative orientation of the redox centers. For example, symmetric two-electron-two-center (2e-2c) oxidative addition across “face-to-face” AuI2, PtII2, or AuIPtIIAuI, has led to homovalent products, AuII2, PtIII2, and AuIIPtIIAuII, respectively, containing metal–metal bonds (Scheme 1A–C) [4,5,6,7]. On the other hand, when the bridging ligands tilt the coordination planes of the metal centers relative to each other, stepwise 2e-1c reduction of a triangular AuIII3Cl6 complex led to unsymmetrical, mixed-valent AuIAuIII2Cl4 and AuI2AuIIICl2 products (Scheme 1D). At the same time, a similar photochemical process has been reported for an AuIII2Cl4 generating AuIAuIIICl2 and AuI2 [8,9,10,11,12]. Mixed-valent 2e-1c oxidative addition products also result in the “face-to-face” AuI2 systems in which the Au-centers are widely separated [13]. In the case of a PtII2 complex with tilted coordination planes, both two- and four-electron oxidation products, Scheme 1E,F [14], have been obtained, whereas either symmetrical metal–metal bonded, or unsymmetrical two-electron oxidation products have been characterized for the analogous systems, E with M = Ru [15], and F with Rh and Ir [16,17].
In 2006, Umakoshi et al. [18] prepared a delocalized mixed-valent PtIII2PtII complex, namely, pyrazolato-bridged platinum cyclic trimer [Pt3(μ-pz)6Br2], by the two-electron oxidation of its yet not structurally characterized homovalent PtII3 precursor. This finding prompted us to reinvestigate dinuclear and trinuclear PtII-complexes with tilted coordination planes, maintained by bridging pyrazolates [19]. We have employed two pyrazole ligands containing alkyl substituents: one presenting no steric hindrance to the donor N atoms, and one containing bulky tert-butyl groups, the latter forcing a close contact between methyl groups and the N-coordinated metal (Scheme 2). Here, we present the synthesis, structural and spectroscopic characterization, and theoretical studies of three new complexes involving two pyrazole ligands: 4-Me-pzH and 3,5-tBu2-pzH (pzH = pyrazole), the latter capable of cyclometallating via its tBu group. Specifically, the triangular complex [PtII(μ-4-Me-pzH)2]3, 1, the dinuclear orthometallated complex [PtII(μ-3,5-tBu2-pz)(κ2-N,C-1-H-5-tBu-3-CMe2CH2-pzH)]2, 2, and its two-electron oxidation product [PtIII2(μ-3,5-tBu2-pz)2(κ2-N,C-1-H-3-CH2Me2CH2-5-tBu-pzH)(κ2-N,C-3-CCH2Me2-5-tBu-pz)Cl], 3, are discussed.

2. Results and Discussion

2.1. Synthesis and Characterization

Complex 1 was prepared by the same method used for the synthesis of the analogous Pd-complex, [PdII(μ-3-Ph-pz)2]3 [20]. Deprotonation of the pyrazole ligands of trans-[PtCl2(4-Me-pzH)2] initiated the cyclization of the homoleptic trimer (Scheme 3A). However, the analogous reaction involving 3,5-tBu2-pzH resulted in the cyclometallated dimer 2 (Scheme 3B). Complex 3 was prepared by the oxidation of 2 by one equivalent of the oxidizing agent (Scheme 3C). The orientation of pyrazolido anion electron donor orbitals favors the formation of triangular species, as long as bulky 3,5-pyrazole substituents do not sterically hinder these species; these tendencies are well documented in the literature [21,22,23]. In that light, the formation of 1 upon pyrazole deprotonation by a base is unexceptional. In contrast, whereas non-triangular products were expected upon deprotonation of trans-[PtCl2(3,5-tBu-pzH)2], the cyclometallation of a butyl group is noteworthy: cyclometallation reactions involving activated C-H bonds (typically of aromatic rings or heterocycles) have been reported for 4d and 5d transition metals [24,25,26], including platinum [27,28,29]. However, to the best of our knowledge, cyclometallation of a saturated aliphatic group has not been hitherto reported, even though the activation of C-H bonds by platinum is well established in the literature [30,31]. Oxidative addition to a diplatinum(II) complex containing bridging pyrazolates and chelating/orthometallated ligands, similar to the oxidation of 2 to 3 here, has recently been reported also by others [14]. Complexes 1–3 were structurally characterized by single-crystal X-ray crystallography. Selected distances and angles pertaining to 1–3 are listed in Table 1.
Complex 1 crystallized in the triclinic space group P-1 with two molecules of 1 and one-half interstitial acetone solvent molecule per asymmetric unit. The two crystallographically independent molecules of 1 do not differ statistically from each other; both show minor deviation from ideal D3h symmetry (Figure 1). The existence of a single set of resonances for the six pyrazolido ligands in its 1H-NMR spectrum shows that the trimeric structure persists in solution (Figure S1). The Pt centers are in an approximate square planar N4 environment with the Pt atoms deviating by 0.14–0.15 Å from the best-fit planes of the four N atoms in the direction away from the center of the metallacyclic ring. The intramolecular Pt...Pt distances average 3.0511(6) Å, being statistically indistinguishable from the 3.048(1) Å distance of the corresponding unsubstituted pyrazole complex, [Pt(μ-pz)2]3 [32], and quite similar to the average Pd...Pd distances of 3.054(1) Å in [Pd(μ-3-Ph-pz)2]3, 3.0471(3) Å in [Pd(μ-pz)2]3, and 3.0458(4) Å in [Pd(μ-4-Me-pz)2]3 (the ionic radii of PtII and PdII differ by 0.06 Å) [20,33,34].
Complex 2 crystallized in the chiral orthorhombic space group P212121 with a whole molecule of C2 symmetry per asymmetric unit. The structure consists of two Pt atoms, two bridging 3,5-tBu2-pyrazolato groups, and two chelating cyclometallated 1-H-3-CMe2CH2-5-tBu-pzH ligands, the latter forming five-membered chelates with each Pt center (Figure 2). The Pt atoms are approximately square-planar with an N3C-coordination environment. The μ-3,5-tBu2-pz ligands bridge the metals unsymmetrically, one pyrazole leaning towards one Pt atom (Pt-N = 2.026(7), 2.157(7) Å) and the other leaning the opposite way (Pt-N = 2.151(7), 2.034(7) Å). The solution 1H-NMR spectrum of 2 (Figure 3) is consistent with its solid-state structure. There are four resonances for the cyclometallated butyl-groups: two singlets for the diastereotopic Me groups (1.26 and 0.84 ppm) and two doublets for the diastereotopic, geminal H atoms of CH2 groups (2.28 and 1.65 ppm); the 195Pt satellites are not observed in this ambient temperature spectrum due to the broadening attributed to the coordination of three quadrupolar N atoms. The Pt...Pt distance of 2.9290(5) Å in 2 is significantly longer than the one determined in a related Pt-(μ-3,5-tBu2-pz)2-Pt complex, 2.8343(6) Å, containing also 2-(2,4-difluorophenyl)pyridyl chelating ligands [35]. Inspection of a molecular model of 2 shows that the platinum coordination planes are slightly bent to bring the chelating ligands closer to each other than they might have been in an ideal square planar arrangement. In contrast, a much shorter approach between the chelating ligands is found in complex 3 (vide infra). Both observations point to Coulombic repulsion between the Pt centers as the more likely explanation for this distortion, rather than the steric repulsion between the chelating cyclometallated ligands.
Complex 3 crystallized in the triclinic space group P-1 with two molecules per asymmetric unit, accompanied by four interstitial H2O molecules at chemically insignificant sites. The structure of 3 retains the basic features of 2, but with a Pt-Pt separation of 2.584(3) Å and 2.586(2) Å, corresponding to a formal single metal–metal bond and one chloride coordinated trans to it (Figure 4). The C1 molecular symmetry of 3 is reflected in its 1H-NMR spectrum (Figure 5) showing a doubling of the number of resonances recorded for 2, in addition to a downfield shift of all resonances, consistent with the increase in its oxidation state. Electroneutrality requires the presence of a crystallographically invisible proton on one of the two non-coordinated N atoms of 3; this proton is evident in the 1H-NMR by a broad resonance at 8.63 ppm whose integrated area corresponds to one H atom per molecule of 3. The absence of paramagnetically shifted resonances in 3 is consistent with its PtIII2 assignment and the presence of a Pt-Pt bond of 2.585 Å. The latter bond length is shorter than the corresponding unsupported bonds of 2.694(1) Å, 2.6964(5) Å, and 2.726 Å [36,37,38] reported earlier, but within the range of several ligand-bridged diplatinum(III) species [39,40]. The PtIII2 oxidation state assignment is further supported by a comparison of the 4f electron binding energies of 2 and 3 determined by X-ray photoelectron spectroscopy (Figure S2) and a comparison with the corresponding binding energies of PtIV species reported in the literature (Table 2). The experimental XPS peaks of 3 are deconvoluted into two equal components, attributed to its two distinct Pt sites, both with higher binding energies than those of 2 and lower than the literature values for PtIV compounds [41,42].
A structural comparison of 2 and 3 shows that the shortening of the separation between the two metal centers, brought about by the formation of a Pt-Pt bond, is accompanied by a decrease in the dihedral angles formed between the μ-3,5-tBu2-pz ligands from 102.5° in 2 to 92.1° and 95.9° in 3. The pyrazole–pyrazole dihedral angles of both 2 and 3 are more acute than the 110.6° (average) angle of the less sterically hindered 1. This agrees with the earlier observation that the Pt-Pt separation in a series of Pt-(μ-3,5-R2-pz)2-Pt complexes increases as the steric bulk of the bridging ligands decreases [35]. The electronic spectra of 1, 2, and 3 each contain an intense UV band with λmax at 225–230 nm, attributed to π-π* transitions. However, compound 3 shows five additional bands spanning the UV to NIR range—361 nm, 531 nm, 587 nm, 761 nm, and 818 nm—attributed to states arising from the Pt-Pt bonding manifold (Figure S4). To further probe the bonding in the complexes, density functional theory (DFT) calculations were carried out.

2.2. Computational Studies

The optimized geometries agree with the corresponding experimental X-ray structural values of 1–3 (Table 2). We ascertained that all optimized geometries exhibited no imaginary frequency. The computed bond length is slightly larger (0.02–0.10 Å) than the experimental data, since the molecules were optimized in the gas phase, and there is no interaction with other complex units as in the crystalline phase.
The DFT-simulated infrared spectra (IR) of the three complexes are shown in Figure S3. The infrared (IR) spectrum of complex 1 is clearly distinguished from those of 2 and 3 due to its different structure. Complexes 2 and 3 have approximately the same IR distribution, as they share similar structural features, except for the additional Cl atom in 3. A calculated mode of 286.6 cm−1, assigned to Pt-Cl stretching in complex 3, falls outside the experimentally accessible spectral window. No Pt-Pt interaction mode is identified in complex 2.
The HOMO-LUMO gaps of the Pt complexes computed by BP86 functional are 0.26, 2.74, and 0.94 eV for 1, 2, and 3, respectively. The frontier molecular orbitals of Pt complex 1, 2, and 3 are presented in Figure S5. The dz2 orbitals of Pt atoms dominate the HOMO of complexes 1 and 2, and Pt d-orbitals also contribute to their HOMO-1, HOMO-2, LUMO, LUMO+1, and LUMO+2. However, for complex 3, the dz2 orbitals of Pt and the p orbital of Cl contribute to the LUMO, the π orbital of pyrazole, d orbital of Pt, and p orbital of Cl contribute to the HOMO, and the π orbital of pyrazole mainly contribute to the HOMO-1, HOMO-1, LUMO+1, and LUMO+2. The LUMO of 2 consists of dxy and dxz of Pt atoms, and the overlap of the two orbitals exhibit a σ-bonding character, resulting in a shorter Pt-Pt distance than 1.
To help understand the bonding characteristics of these complexes, we calculated the natural population analysis (NPA) charges and the Wiberg bond index (WBI) based on natural bonding orbital (NBO) computations at the BP86/6–31G*~dz level of theory. The Pt atoms are positively charged (ca. 0.43, 0.44, and 0.47 |e| in 1; 0.35 |e| in 2; and 0.46, 0.56 |e| in 3), and the N/C/Cl atoms are negatively charged (for details see Table S4). Correspondingly, the natural electron configurations for Pt, N, Cl, and C (bonded to Pt) atoms are listed in Table S4. Pt atoms transfer considerable charge (ca. ~0.5 e) to the N/C/Cl atoms of all three complexes. The computed WBIs (Table S5) for Pt-Pt in 3 (0.28) are strikingly larger than that in 1 (0.08~0.09) and 2 (0.06), contributing to the shorter Pt-Pt distance in 3; the WBIs for Pt-N are comparable in the three complexes. These indicate that partial bonds form between Pt atoms and their surrounding Pt/N/C/Cl atoms, along with a weak σ-interaction in 2 and a formal Pt-Pt single bond in 3.
QTAIM topological analysis of the electronic density [43,44,45] gave further details of the bonding in the three Pt complexes. For simplicity, we substituted the methyl groups by H, and the BP86/6–31G*~dz optimized results provide almost the same structural parameters as the initial configurations. Figure 6 depicts the simplified complexes’ molecular graphs (at BP86/6–31G*~dz) representing Pt-Pt/Pt-N/Pt-C interactions. The bond critical points (BCPs) between Pt atoms for all three complexes lead to 3, 0, and 1 bond paths for the 1, 2, and 3, respectively. The larger 0.05 au ρbcp electron densities at the Pt-Pt bond critical points (BCPs) for complex 3 compared to the value of 0.02 au for complex 1 suggest stronger bonds, consistent with the shorter Pt-Pt distance in complex 3. Note that there is no BCP between Pt and Pt in complex 2, leading to the longer Pt-Pt distance compared to complex 3. In addition, the low electron densities ρbcp (0.11~0.13 au, 0.09~0.14 au, and 0.08~0.14 au in 1, 2, and 3, respectively), as well as negative Laplacian ∇2ρbcp (−0.11~−0.14 au, −0.05~−0.15 au, and −0.04~−0.13 au for 1, 2 and 3, respectively), located at the Pt-N/Pt-C/Pt-Cl BCPs, indicate ionic interactions and limited contributions to the total stability.

3. Materials, Methods, and Computational Details

Commercial reagents—K2PtCl4, 4-Me-pyrazole, pivaloylmethane, and hydrazine—were used as received. 3,5-Di-tert-butyl-pyrazole (3,5-tBu2-pzH) was prepared by refluxing equivalent amounts of dipivaloylmethane and hydrazine in 95% EtOH. Trans-[PtCl2(4-Me-pzH)2] and trans-[PtCl2(3,5-tBu2-pzH)2] were prepared quantitatively by stoichiometric addition of two equivalents of 4-Me-pzH, or 3,5-tBu2-pzH, to K2PtCl4 in MeOH/H2O and characterized by X-ray structure determination (Tables S1–S3). Solid p-Cl-C6H4-ICl2 was prepared by bubbling gaseous Cl2 through a solution of p-Cl-C6H4-I in toluene and collecting the product by filtration after washing with toluene and diethylether (Caution! The reaction should be carried out under a fume hood with a solution of a base trapping excess Cl2). Solvents were purified by standard methods [46]. 1H-NMR spectra were recorded with a Bruker Avance DPX-400 spectrometer. 13C-NMR resonances could not be safely distinguished from baseline noise due to solubility limitations. The electronic spectra of the complexes in solution were recorded on a Varian CARY 500 spectrophotometer in the 40,000–4000 cm−1 (250–2500 nm) range. Elemental analyses were performed by Galbraith Laboratories, Inc., Knoxville, TN.
[PtII(μ-4-Me-pz)2]3, 1: To a CH3CN solution (50 mL) of trans-[PtCl2(4-Me-pzH)2] (230 mg, 0.53 mmol) was added Et3N (162 mg, 1.60 mmol), and the solution was refluxed for 8 h. Yellow solid precipitated and was removed by filtration while the solution was still hot. The colorless filtrate was concentrated under air, yielding a white microcrystalline solid, which was collected and air-dried; Yield, 55 mg (29%). Colorless crystals of 1, suitable for X-ray analysis, were grown from MeOH/CH3COCH3. Anal. Calcd. for C24N12H30Pt3: C, 26.89; H, 2.82; N, 15.68%. Found: C, 27.14; H, 2.79; N, 15.93%. 1H-NMR (CD3OD, δ, ppm, 400 MHz, 293 K): 7.88 (12H, s, pz-H3,5), 2.06 (18H, s, CH3). UV-vis-NIR (CH2Cl2, λmax[nm]/ε[M−1cm−1]): 229/14200.
[PtII(μ-3,5-tBu2-pz)(κ2-N,C-1-H-5-tBu-3-CMe2CH2-pzH)]2, 2: To a CHCl3 solution (50 mL) of trans-[PtCl2(3,5-tBu2-pzH)2] (313 mg, 0.5 mmol) was added Et3N (302 mg, 3.0 mmol) and the solution was refluxed for 10 h. After filtration, the filtrate was concentrated under the air. Colorless crystals of 2, suitable for X-ray analysis, were collected and washed with MeOH; Yield, 127 mg (46%). Anal. Calcd. for C44N8H76Pt2: C, 47.73; H, 6.93; N, 10.12%. Found: C, 47.85; H, 6.55; N, 10.21%. 1H-NMR (CD2Cl2, δ, ppm, 400 MHz, 293 K): 8.20 (2H, s, N-H), 6.01(2H, s, μ-pz-H4), 5.63 (2H, κ2-pz-H4), 2.28 (2H, d, 2JH-H = 8.8 Hz, Pt-CH2), 1.65 (2H, d, 2JH-H = 20.0 Hz, C(CH2)), 1.58 (18H, s, C(CH3)3), 1.44 (18H, tBuμ-pz), 1.26 (6H, C(CH2)2), 1.11 (18H, tBuκ-pzH), 0.84 (6H, C(CH2)2). UV-vis-NIR (CH2Cl2, λmax[nm]/ε[M−1cm−1]): 225/16,500.
[PtIII2(μ-3,5-tBu2-pz)2(κ2-N,C-1-H-3-CMe2CH2-5-tBu-pzH)(κ2-N,C-3-CMe2CH2-5-tBu-pz)Cl], 3: To a colorless CH2Cl2 solution of 2 (110.7 mg, 0.1 mmol) was added p-Cl-C6H4-ICl2 (236.9 mg, 0.2 mmol). The solution turned black immediately, and the reaction was allowed for 0.5 h, followed by filtration. Single crystals of 3 were grown by slow diethyl ether vapor diffusion into the filtrate. Yield, 100.35 mg (90%). 1H-NMR (CD2Cl2, δ, ppm): 8.63 (1H, s, N-H, w1/2 = 4.32 Hz), 6.14 (1H, s, μ-pz-H4), 5.92 (1H, s, μ-pz-H4), 5.85 (1H, d, pz-H4pzH, 4JH-H(N) = 2.2 Hz), 5.55 (1H, s, pz-H4pz), 1.62 (3H, s, Pt-CCH3), 1.47 (9H, s, tBu), 1.43 (9H, s, tBu), 1.40 (3H, s, Pt-CCH3), 1.33 (9H, s, C(CH3)3), 1.31 (9H, s, C(CH3)3), 1.27 (3H, s, Pt-CCH3), 1.22 (9H, s, C(CH3)3), 1.17 (9H, s, C(CH3)3), 0.98 (2H, s, Pt-CH2), 0.48 (2H, s, Pt-CH2). UV-vis-NIR (CH2Cl2, λmax[nm]/ε[M−1cm−1]): 229/32,700, 361/6300, 531/600, 587/1450, 761/1440, 818/450.
X-ray diffraction data were collected with a Bruker AXS SMART 1K CCD diffractometer [47], using graphite-monochromated Mo-Kα radiation at ambient temperature from single crystals mounted atop glass fibers at random orientation. Data were corrected for Lorentz and polarization effects [48]. The structures were solved employing the SHELXTL-direct methods program and refined by full-matrix least-squares on F2 [49]. Crystallographic details for 1, 2, and 3 are summarized in Table 3.
The Gaussian 09 software package was employed throughout our density functional theory (DFT) computations [50]. Full geometry optimizations for the three complexes were carried out using the BP86 functional [51,52]. The 6–31G* basis set for C, N, H, and Cl atoms and a double-ζ basis set (LanL2DZ) with the effective core potential (ECP) for Pt (denoted here by 6–31G*∼dz) were used. All the optimized geometries were characterized as true local minima by harmonic vibrational frequency analysis at the same theoretical level. Atomic charges were based on the Natural Population Analysis (NPA) of Weinhold et al. [53]. To gain more insights into the chemical bonding, we performed a quantum theory of atoms in molecules (QTAIM) [43,44,45] study, using the all-electron basis set (6–31G* for C, N, Cl, and H; double zeta plus polarization function basis set, Douglas–Kroll–Hess for Pt) [54] by AIM2000 software [55]. Natural bond orbital NBO population analysis was used to describe the details of chemical bonding in the systems studied.

4. Conclusions

Differences in the steric bulk of peripheral substituents between the two pyrazoles employed here determine the topology of the resulting pyrazolato products, yielding upon deprotonation the new trinuclear homoleptic complex 1, or the dinuclear 2. Platinum(II) complexes involving less sterically crowded, 3,5-Me2-pzH and 3-tBu-pzH ligands have been employed in the stepwise construction of multinuclear heterometallic complexes via straightforward coordination chemistry [56,57]. In contrast, the bulky 3,5-tBu2 groups employed here apparently prevent the formation of a trimeric ring, leaving the Pt center coordinatively unsaturated. The latter satisfies the four-coordination requirement of PtII via the unprecedented cyclometallation of a tert-butyl group (C-H BDE of ~100 kcal/mol), suggesting possible further applications of 3,5-tBu2-pzH in C-H activation chemistry and catalysis. Dinuclear half-lantern PtII complexes, with even longer Pt…Pt separation than 2, have been studied in detail with regard to their tunable visible luminescence [35,58,59,60,61,62,63]. In contrast, compound 2 does not luminesce; this is tentatively attributed to the proximity to Pt atoms of the (cyclometallated) C-H group, whose vibrational modes can quench the excited state. The facile oxidative addition of complex 2 to a PtIII2 product was expected. However, the unsymmetrical addition of chloride across the Pt-Pt single bond of 3, while not unprecedented [14], is a rare example of this type of reactivity. The chemistry of five-coordinate (i.e., coordinatively unsaturated) PtIII centers, such as one of the two Pt centers of 3, remains unexplored, to date.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemistry5010016/s1, Figure S1: 1H NMR of 1; Figure S2: XPS spectra of 2 and 3; Figure S3: Calculated and observed FT-IR spectra of 1, 2 and 3; Figure S4: UV-vis-NIR spectra of 1, 2 and 3; Figure S5: Important molecular orbitals of 1, 2 and 3; Figure S6: ORTEP diagram of 1; Figure S7: ORTEP diagram of 2; Figure S8: ORTEP diagram of 3; Figure S9: ORTEP diagram of trans-[PtCl2(3,5-tBu2-pzH)2]; Figure S10: ORTEP diagram of trans-[PtCl2(4-Me-pzH)2]; Table S1: Crystallographic data for trans-[PtCl2(3,5-tBu2-pzH)2] and trans-[PtCl2(4-Me-pzH)2]; Table S2: Selected bond lengths and angles for [PtCl2(3,5-tBu2-pzH)2]; Table S3: Selected bond lengths(Å) and angles(º) for [PtCl2(4-Me-pzH)2]; Table S4: Computed NBA charges and natural electron configurations of the selected atoms 1, 2 and 3; Table S5: Computed WBI Pt-Pt/Pt-N/Pt-C/Pt-Cl bonds in the three simplified Pt complexes; Table S6: Selected bond lengths and bond angles for 1; Table S7: Selected bond lengths and bond angles for 2; Table S8: Selected bond lengths and bond angles for 3; Crystallographic data are available free of charge via the Internet at CCDC numbers: 2221215 (1) 2221216 (2), 2221217 (3), 2221218 ([PtCl2(3,5-tBu2-pzH)2]), and 2221219 ([PtCl2(4-Me-pzH)2]).

Author Contributions

Conceptualization, R.G.R.; methodology, H.Z. and F.L.; formal analysis, Z.S., H.Z. and I.C.; investigation, Z.S., F.L. and H.Z.; resources, R.G.R. and Z.C.; writing—original draft preparation, Z.S., F.L. and R.G.R.; writing—review and editing, R.G.R. and Z.C.; visualization, Z.S., F.L. and I.C.; supervision, R.G.R. and Z.C.; project administration, R.G.R.; funding acquisition, R.G.R. and Z.C. All authors have read and agreed to the published version of the manuscript.

Funding

This material is based upon work supported by the National Science Foundation under Grant No. NSF-DMR-2122078. Work in UPR has been supported by the National Science Foundation under Grant No. NSF-EPS-1010094.

Data Availability Statement

Not applicable.

Acknowledgments

R.G.R. is grateful to Johnson Matthey Co. for a generous gift of potassium tetrachloroplatinate.

Conflicts of Interest

The authors declare no conflict of interest.

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Chemistry 05 00016 sch001 550
Scheme 1. Oxidative addition/reductive elimination across dinuclear and trinuclear complexes.
Scheme 1. Oxidative addition/reductive elimination across dinuclear and trinuclear complexes.
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Chemistry 05 00016 sch002 550
Scheme 2. Two pyrazole ligands employed here and cyclometallation mode.
Scheme 2. Two pyrazole ligands employed here and cyclometallation mode.
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Chemistry 05 00016 sch003 550
Scheme 3. Synthesis of complexes 1 (A), 2 (B) and 3 (C).
Scheme 3. Synthesis of complexes 1 (A), 2 (B) and 3 (C).
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Chemistry 05 00016 g001 550
Figure 1. Two ball-and-stick views of the structure of 1. Color code: C, black; N, blue; Pt, grey. H atoms are not shown for clarity.
Figure 1. Two ball-and-stick views of the structure of 1. Color code: C, black; N, blue; Pt, grey. H atoms are not shown for clarity.
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Chemistry 05 00016 g002 550
Figure 2. Two ball-and-stick views of the structure of 2. Color code: C, black; H, pink; N, blue; Pt, grey. H atoms, except for two N-H, are not shown for clarity.
Figure 2. Two ball-and-stick views of the structure of 2. Color code: C, black; H, pink; N, blue; Pt, grey. H atoms, except for two N-H, are not shown for clarity.
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Chemistry 05 00016 g003 550
Figure 3. 1H NMR of 2 in CD2Cl2 at 293 K. Solvent peaks are identified by solid dot (CHDCl2) or star (H2O). Peak integrations in green. Protons, except N-H, are omitted for clarity. Inset: Ball-and-stick diagram of 2 with proton labels.
Figure 3. 1H NMR of 2 in CD2Cl2 at 293 K. Solvent peaks are identified by solid dot (CHDCl2) or star (H2O). Peak integrations in green. Protons, except N-H, are omitted for clarity. Inset: Ball-and-stick diagram of 2 with proton labels.
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Chemistry 05 00016 g004 550
Figure 4. Two ball-and-stick views of the structure of 3. Color code: C, black; H, pink; N, blue; Pt, grey; Cl, green. H atoms, except for N-H, are not shown for clarity.
Figure 4. Two ball-and-stick views of the structure of 3. Color code: C, black; H, pink; N, blue; Pt, grey; Cl, green. H atoms, except for N-H, are not shown for clarity.
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Figure 5. 1H NMR of 3 in CD2Cl2 at 293 K. Solvent peaks are identified by solid dot (CHDCl2) or star (H2O). Peak integrations in green. Protons, except N-H, are omitted for clarity. Inset: Ball-and-stick diagram of 3 with proton labels.
Figure 5. 1H NMR of 3 in CD2Cl2 at 293 K. Solvent peaks are identified by solid dot (CHDCl2) or star (H2O). Peak integrations in green. Protons, except N-H, are omitted for clarity. Inset: Ball-and-stick diagram of 3 with proton labels.
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Figure 6. Molecular graphs of the simplified (a) 1 (b) 2, and (c) 3 configurations. Ring and cage BCPs are omitted for clarity. The red spheres at the center of the interatomic bond lines denote the bond critical points (BCP).
Figure 6. Molecular graphs of the simplified (a) 1 (b) 2, and (c) 3 configurations. Ring and cage BCPs are omitted for clarity. The red spheres at the center of the interatomic bond lines denote the bond critical points (BCP).
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Table
Table 1. Selected experimental and calculated distances (Å) and angles (°) for 1, 2, and 3, calculated data in italics.
Table 1. Selected experimental and calculated distances (Å) and angles (°) for 1, 2, and 3, calculated data in italics.
123
Pt-Pt3.0355(5)–3.0758(6)
3.110, 3.114, 3.117		2.9290(5)
3.031		2.584(3), 2.586(2)
2.640
Pt-N(μ-pz)2.006(5)–2.023(5)
2.044–2.047		2.026(6)–2.157(6)
2.060–2.235		2.026(7)–2.222(7)
2.068–2.312
Pt-N(κ2-pzH)-1.961(8)
1.995, 1.996		1.973(7)–2.015(7)
2.000, 2.017
Pt-C-2.048(10), 2.034(10)
2.074, 2.077		2.060(8)–2.078(9)
2.089, 2.100
Pt-Cl--2.352(3), 2.346(3)
2.434
Pt-N-N113.4(4)–116.3(4)
113.3–115.7		108.9(5)–130.1(6)
110.9–112.6		101.7(5)–133.7(6)
101.8–111.6
N-Pt-N (cis-pz)86.6(2)–93.0(2)
81.8–89.2		89.2(3)–89.3(3)
88.2		87.6(3)–89.4(3)
88.1–91.1
N-Pt-N (trans)170.3(2)–172.8(2)
168.2–175.7		172.2(3)–172.3(3)
175.3, 175.7		168.1(3)–174.5(3)
170.2, 175.0
N-Pt-C-79.3(4), 80.1(4)
79.2, 79.3		80.0(3)–81.2(3)
80.5, 80.6
Pt-Pt-Cl--174.72(8), 174.91(7)
173.556
pz-Pt-Pt-pz96.9–121.7 (average 110.6)
94.2, 98.1, 103.4		102.6
105.3		92.1, 95.9
92.4
Table
Table 2. Electron binding energies for 2, 3, and two PtIV compounds.
Table 2. Electron binding energies for 2, 3, and two PtIV compounds.
Compound4f7/2 (eV)4f5/2 (eV)
272.675.4
3 (PtN3CCl)73.176.4
3 (PtN3C)74.477.7
H2PtCl675.2 a78.6 a
[Pt(oxa)(OH)2(dachex)]75.6 a79.0 a
a Data from references [41,42].
Table
Table 3. Crystallographic data for 1, 2, and 3.
Table 3. Crystallographic data for 1, 2, and 3.
1·0.5CH3COCH323·2H2O
FormulaC25.5H33N12O0.5Pt3C44H76N8Pt2C44H41ClN8O2Pt2
Crystal size, mm30.08 × 0.06 × 0.050.40 × 0.30 × 0.200.22 × 0.18 × 0.10
fw1100.911107.301173.75
Space groupP-1 (No. 2)P212121 (No. 19)P-1 (No. 2)
a, Å13.734(3)12.373(1)11.643(9)
b, Å13.837(2)18.696(2)21.040(2)
c, Å17.734(3)21.343(2)23.064(13)
α, °74.55(1)9064.77(7)
β, °81.87(1)9083.98(6)
γ, °74.81(1)9082.48(8)
V, Å33125.5(9)4937.2(8)5059(7)
Z244
T, K298(2)298(2)298(2)
ρcalcd, g cm−32.341.491.54
reflctns collected/2θmax11,453/51.0029,004/52.0028,262/50.00
Unique reflctns/I > 2σ(I)12,088/10,0309589/874317,509/13,447
No. of params/restraints753/0513/461071/0
μ(Mo Kα), mm−113.4335.6965.617
F(000)203222082336
R1 a/All data0.0285/0.03890.0311/0.04020.0471/0.0773
wR2 b (I > 2σ(I))0.06630.06980.1245
Goodness of fit c1.0571.1841.039
a I > 2σ(I). R1 = Σ||Fo| − |Fc||/Σ|Fo|. b wR2 = [Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]]1/2, where w = 1/σ2(Fo2) + (aP)2 + bP, P = (Fo2 + 2Fc2)/3. c GoF = [Σ[w(Fo2 − Fc2)2]/(n − p)]1/2.
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Shi, Z.; Li, F.; Zhao, H.; Chakraborty, I.; Chen, Z.; Raptis, R.G. Trinuclear and Cyclometallated Organometallic Dinuclear Pt-Pyrazolato Complexes: A Combined Experimental and Theoretical Study. Chemistry 2023, 5, 187-200. https://doi.org/10.3390/chemistry5010016

AMA Style

Shi Z, Li F, Zhao H, Chakraborty I, Chen Z, Raptis RG. Trinuclear and Cyclometallated Organometallic Dinuclear Pt-Pyrazolato Complexes: A Combined Experimental and Theoretical Study. Chemistry. 2023; 5(1):187-200. https://doi.org/10.3390/chemistry5010016

Chicago/Turabian Style

Shi, Zhichun, Fengyu Li, Hong Zhao, Indranil Chakraborty, Zhongfang Chen, and Raphael G. Raptis. 2023. "Trinuclear and Cyclometallated Organometallic Dinuclear Pt-Pyrazolato Complexes: A Combined Experimental and Theoretical Study" Chemistry 5, no. 1: 187-200. https://doi.org/10.3390/chemistry5010016

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