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Synthesis of [Pt(SnBut3)(IBut)(μ-H)]2, a Coordinatively Unsaturated Dinuclear Compound which Fragments upon Addition of Small Molecules to Form Mononuclear Pt−Sn Complexes Anjaneyulu Koppaka,† Veeranna Yempally,† Lei Zhu,† George C. Fortman,† Manuel Temprado,*,‡ Carl D. Hoff,*,† and Burjor Captain*,† †

Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering, Universidad de Alcalá, Madrid 28801, Spain

‡

S Supporting Information *

ABSTRACT: The reaction of Pt(COD)2 with one equivalent of tri-tert-butylstannane, But3SnH, at room temperature yields Pt(SnBut3)(COD)(H)(3) in quantitative yield. In the presence of excess But3SnH, the reaction goes further, yielding the dinuclear bridging stannylene complex [Pt(SnBut3)(μSnBut2)(H)2]2 (4). The dinuclear complex 4 reacts rapidly and reversibly with CO to furnish [Pt(SnBut3)(μ-SnBut2)(CO)(H)2]2 (5). Complex 3 reacts with N,N′-di-tert-butylimidazol2-ylidene, IBut, at room temperature to give the dinuclear bridging hydride complex [Pt(SnBut3)(IBut)(μ-H)]2 (6). Complex 6 reacts with CO, C2H4, and H2 to give the corresponding mononuclear Pt complexes Pt(SnBut3)(IBut)(CO)(H)(7), Pt(SnBut3)(IBut)(C2H4)(H)(8), and Pt(SnBut3)(IBut)(H)3 (9), respectively. The reaction of IBut with the complex Pt(SnBut3)2(CO)2 (10) yielded an abnormal Pt-carbene complex Pt(SnBut3)2(aIBut)(CO) (11). DFT computational studies of the dimeric complexes [Pt(SnR3)(NHC)(μ-H)]2, the potentially more reactive monomeric complexes Pt(SnR3)(NHC)(H) and the trihydride species Pt(SnBut3)(IBut)(H)3 have been performed, for NHC = IMe and R = Me and for NHC = IBut and R = But. The structures of complexes 3−8 and 11 have been determined by X-ray crystallography and are reported.

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“saw horse” geometry, the core structure of which showed little change when it was converted to the distorted octahedral dihydride complex 2. Related investigations of the coordination chemistry of But3SnH with transition metals such as cobalt,24 nickel,24 and iron25,26 have been reported earlier by our group. The current work began as part of an investigation of possible reaction mechanisms for the formation of complex 2, one of which is shown in Scheme 1 below. As described in the Results section, it proved possible to isolate and structurally characterize complex 3 by controlled addition of only one equivalent of But3SnH to Pt(COD)2. Further work to try to isolate and structurally characterize complex A has been unsuccessful to date. A possible reason for that may be that, in the absence of trapping agents, complex A may dimerize or form cluster complexes as suggested by the current work. Successful isolation of the first reaction product Pt(SnBut3)(COD)(H), 3, however, suggested that since the COD ligand is known to be weakly held, it might provide a route to explore new complexes of the “Pt(SnBut3)(H)” fragment allowing comparison to our earlier work based on the “Pt(SnBut3)2” moiety. Comparison of the reaction of a distannyl metal complex to a related stannyl

INTRODUCTION The chemistry of inorganic and organometallic complexes containing transition metal−main group metal bonds has experienced a resurgence of interest in recent years. In particular, due to potential applications as catalysts or materials, complexes containing bonds between transition metals and organotin compounds which display novel architectures played a prominent role in that renaissance.1−14 The use of simple inorganic tin compounds as modifiers and activators of heterogeneous catalysts has a long history.15−17 More recently, this has been extended to homogeneous systems.13,14,18−21 The ability of pendant groups on tin to play a prominent role in exerting steric pressure was recently demonstrated by us in the reaction chemistry of a series of mononuclear Pt−Sn compounds Pt(SnR3)2(CNBut)2 (where R = But, Mes, Pri, Ph). Oxidative addition of H2 and H2-D2 scrambling reactions were shown to depend critically on the nature of R, and this was attributed primarily to structural changes in the parent complexes enforced by steric pressure exerted by the stannyl ligands.22,23 For example, only for the highly sterically crowded complex Pt(SnBut3)2(CNBut)2, 1 was H2 gas activation at room temperature to yield the dihydride complex Pt(SnBut3)2(CNBut)2(H)2, 2 observed. This occurred rapidly and reversibly both in solution and in the solid state since starting complex 1 was distorted away from an ideal square planar geometry to a © 2015 American Chemical Society

Received: October 21, 2015 Published: December 16, 2015 307

DOI: 10.1021/acs.inorgchem.5b02441 Inorg. Chem. 2016, 55, 307−321

Article

Inorganic Chemistry

Scheme 1. Possible Mechanism for Formation of Complexes 1 and 2 via Oxidative Addition of But3SnH to Pt(COD)2 to Produce Complex 3

1 mL of freshly distilled toluene was added dropwise to 103 mg of Pt(COD)2 (0.250 mmol) over a period of 20 min at room temperature with continuous stirring. The solvent was allowed to evaporate overnight. The black oily residue was washed with dichloromethane solvent several times until the solution became colorless, yielding 34 mg of 4 (19% yield). Spectral data for 4: 1H NMR (toluene-d8, rt, in ppm): δ 1.58 (s, 3JSn−H = 64 Hz, 36 H, SnBut2), 1.50 (s, 3JSn−H = 60 Hz, 54 H, SnBu3t), −3.71(s, 1JPt−H = 905 Hz, 2JSn−H = 33 Hz, 4 H, hydride). Mass Spec. EI/MS m/z: 1383 (M+ − But), 1326 (M+ − 2But). The isotope pattern is consistent with the presence of two platinum and four tin atoms. Synthesis of [Pt(SnBut3)(μ-SnBut2)(D)2]2, 4-d4, from [Pt(SnBut3)(μ-SnBut2)(H)2]2, 4. A 7.0 mg (0.005 mmol) amount of crystalline 4 was charged into an NMR tube containing 0.6 mL of toluene-d8 solvent. The NMR tube was sealed under argon and heated in a water bath for 20 min to dissolve 4. D2 gas was purged through solution for 5 min, and 1H NMR indicated complete consumption of starting 4 in quantitative yield to form 4-d4. Evaporation of solvent in a glovebox overnight obtained 6.9 mg of crystalline 4-d4 (98%) as determined by mass spectrometry which showed an increase of +4 in value. Synthesis of [Pt(SnBut3)(μ-SnBut2)(CO)(H)2]2, 5. In a 10 mL Schlenk tube, under an atmosphere of argon, 25 mg (0.017 mmol) of 4 dissolved in 3 mL of hexane/dichloromethane (1:1 mixture) was transferred. The reaction mixture was colorless and turbid. The solution was purged with CO gas (1 atm) at room temperature for 10 min. During that time, the solution turned dark yellow. The reaction mixture was filtered and placed in an ice bath under a stream of CO gas to obtain yellow crystals of 5 which were washed with small portions of hexane to give 20 mg (79% yield). 1H NMR (toluene-d8, rt, in ppm): δ 1.69 (s, 36 H, SnBut2), 1.45 (s, 3JSn−H = 61 Hz, 54 H, SnBut3), −12.77 (s, 1JPt−H = 702 Hz, 2JSn−H = 160 Hz, 4H, hydride). IR νCO (cm−1 in hexane): 2027(vs), 2008 (m). Elemental anal. calcd: C, 33.72; H, 6.33%. Found: C, 34.18; H, 5.92%. Conversion of [Pt(SnBut3)(μ-SnBut2)(CO)(H)2]2, 5, to [Pt(SnBut3)(μ-SnBut2)(H)2]2, 4, in solution. In a 10 mL Schlenk tube, 10.0 mg of 5 (0.0067 mmol) was dissolved in 2 mL of hexane. During this time, the color of the solution changed from yellow to colorless, and after slow evaporation of solvent in a glovebox, 9.0 mg (93% yield) of crystalline 4 was obtained. Synthesis of [Pt(SnBut3)(IBut)(μ-H)]2, 6. In a glovebox, under an atmosphere of argon, 30.0 mg of But3SnH (0.103 mmol) dissolved in 3 mL of freshly distilled hexane was added to 40.0 mg of Pt(COD)2 (0.097 mmol). The reaction mixture immediately turned dark brown. The reaction mixture was stirred at room temperature for an additional 10 min and 17.0 mg of IBut (0.094 mmol) dissolved in 0.5 mL of toluene

hydride complex is of fundamental interest but also presents the possibility of developing new reactivity patterns due to incorporation of the reactive hydride ligand. This paper describes new oxidative addition or ligand addition reactions based primarily on study of the reactivity of complex 3 and allows us to further probe the steric and electronic consequences on stannane coordination to transition metals and activation of small molecules.

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EXPERIMENTAL SECTION

General Data. Unless otherwise indicated, all reactions were performed under an atmosphere of argon. Reagent grade solvents were dried by standard procedures and were freshly distilled prior to use. Infrared spectra were recorded on a Nicolet 380 FT-IR spectrophotometer. 1H NMR were recorded on Bruker 300 and 400 spectrometer operating at 300.13 and 399.99 MHz, respectively. Elemental analyses were performed by Columbia Analytical Services (Tucson, AZ). Mass spectrometric measurements performed by a direct-exposure probe using electron impact ionization (EI) were made on a VG 70S instrument at the University of South Carolina. IBut (N,N′-di-tertbutylimidazol-2-ylidene) was purchased from Strem chemicals and used without further purification. Ethylene, CO, and H2 gases of 99.9% purity were purchased from Airgas South and used without further purification. Bis(1,5-cyclooctadiene)platinum, Pt(COD) 2 , 2 7 trans-Pt(SnBut3)2(CO)2,23 and tri-tert-butylstannane, But3SnH,25,26 were prepared according to the published procedures. Synthesis of Pt(SnBut3)(COD)(H), 3. In a glovebox, under an atmosphere of argon, 25 mg of But3SnH (0.086 mmol) dissolved in 3 mL of freshly distilled hexane was added to 30.0 mg of Pt(COD)2 (0.073 mmol). The reaction mixture immediately turned dark brown. The reaction mixture was stirred at room temperature for an additional 10 min and then filtered and the resulting solution evaporated under argon gas flow. The yellowish-brown solid residue was redissolved in diethyl ether and placed in −20 °C freezer overnight, which gave crystalline Pt(SnBut3)(COD)(H), 3, covered with black oily residue. After washing the crystalline product with isopropyl alcohol (3 × 0.3 mL), 13.2 mg (30% yield) of 3 was obtained. Spectral data for 3: 1H NMR (C6D6, rt, in ppm): δ 5.80 (m, 2JPt−H = 51 Hz, 3JSn−H = 5.0 Hz, 2 H, CH-cod), 5.45 (m, 2JPt−H = 45 Hz, 3JSn−H = 7 Hz, 2 H, CH-cod), 1.82−1.51 (broad m, 8 H, CH2-cod), 1.53 (s, 3JSn−H = 52 Hz, 27 H, SnBut3), −3.74 (s, 1JPt−H = 1222 Hz, 2JSn−H = 48 Hz, 1 H, hydride). The compound is very sensitive, and reliable mass and elemental data could not be obtained. Synthesis of [Pt(SnBut3)(μ-SnBut2)(H)2]2, 4. In a glovebox, under an atmosphere of argon, 583 mg of But3SnH (2.000 mmol) dissolved in 308


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Inorganic Chemistry solution was added to the dark brown reaction mixture. The reaction mixture immediately turned red. The reaction was stirred for 30 min, after which time the solvent was reduced to ca. 1 mL. A solid precipitated from solution and was filtered and washed with 3 × 3.0 mL of pentane to yield 48 mg (yield 74%) of a cherry red powder. Spectral data for 6: 1H NMR (toluene-d8, rt, in ppm): δ 6.60 (s, 4H, imid), 1.93 (s, 36H, C(CH3)3), 1.34 (s, 3JSn−H = 50 Hz, 54H, C(CH3)3), −5.37 (s, 1JPt−H = 652 Hz, 2JSn−H = 199 Hz, 2JSn−H = 56 Hz, 2H, hydride). Elemental anal. calcd: C, 41.45; H, 7.26; N, 4.20%. Found: C, 41.47; H, 7.41; N, 4.09%. Synthesis of Pt(SnBut3)(IBut)(CO)(H), 7. A 25 mL Schlenk tube was charged with 30 mg of 6 (0.023 mmol) and dissolved in 4.0 mL of toluene. The red solution was placed under an atmosphere of ca. 10 psi CO and shaken vigorously. The resulting solution turned light orange immediately. The solvent was removed under reduced pressure, yielding a precipitate which was filtered and then washed with 3 × 0.25 mL heptane yielding a tan powder, 28.6 mg (90% yield). Spectral data for 7: 1 H NMR (C6D6, rt, in ppm): δ 6.60 (s, 2H, imid), 1.67 (s, 3JSn−H = 52 Hz, 27 H, SnBut3), 1.37 (s, 18 H, C(CH3)3), −2.48 (s, 1JPt−H = 892 Hz, 2 JSn−H = 18 Hz, 1H, hydride). IR νCO (cm−1 in hexane): 1979(vs). IR νPt−H (cm−1 in hexane): 2104(m). Mass Spec. EI/MS m/z: 637 (M+ − But). Synthesis of Pt(SnBut3)(IBut)(C2H4)(H), 8. A 25 mL Schlenk tube was charged with 30.0 mg (0.023 mmol) of 6, which was subsequently dissolved in 4.0 mL of toluene. The red solution was exposed to ethylene (1 atm) and shaken vigorously. The resulting solution turned light yellow immediately. The solvent was removed under a slow flow of ethylene gas to give a light yellow powder of 8, 26.6 mg (85% yield). Spectral data for 8: 1H NMR (toluene-d8, rt, in ppm): δ 6.69 (s, 2H, imid), 2.56 (s, 2JPt−H = 43 Hz, 4H, ethylene), 1.56 (s, 3JSn−H = 50 Hz, 27 H, SnBut3), 1.31 (s, 18 H, C(CH3)3), −3.89 (s, 1JPt−H = 705 Hz, 2JSn−H = 47 Hz, 1H, hydride). NMR and IR Studies of Reaction of [Pt(SnBut3)(IBut)(H)]2, 6, with H2 and D2 Gas. In a typical procedure, a solution of 137 mg of 6 was dissolved in 3 mL of toluene-d8 and filtered into a small Schlenk tube. A sample of 0.5 mL of this solution was loaded into a screw cap NMR tube and closed under an argon atmosphere. Both the NMR and Schlenk tubes were taken from the glovebox. The Schlenk tube containing the remaining 2.5 mL of solution was taken to a manifold and evacuated and filled with H2 gas in four purge cycles. In a separate Schlenk tube fitted with an adapter to hold the screw cap of the NMR tube above the open NMR tube, the Schlenk tube containing the NMR tube was evacuated and filled with H2 for four purge cycles. A sample of 0.7 mL of the stock solution of 6 was transferred by syringe to the NMR tube in the second Schlenk tube. The screw cap fitted with a Teflon coated silicone septa was closed on the NMR tube at approximately ambient atmospheric pressure. Once the NMR tube was closed the atmosphere of the Schlenk tube holding was switched to argon, the NMR tube was removed, inserted into a Dewar flask of liquid N2, and quickly sealed off with a torch. While sealing the tube, both the Dewar and the NMR tube itself were kept behind a safety shield. Caution! Care must be taken in sealing of f NMR tubes under H2 atmosphere. The solution was warmed and all material redissolved giving a clear orange solution. Analogous procedures were used to seal off glass tubes under vacuum containing 1 atm D2 or utilizing NMR tubes equipped with a pressure valve to study the reaction at 40 psi pressure, 2.7 atm above atmospheric pressure of 3.7 atm absolute pressure H2 or D2. In addition to NMR experiments, attempts to determine νPt−H were made using the ∼1 mL of solution that remained of the original stock solutions. Comparison of FTIR data for both H and D did not lead to clear assignments of the Pt− H stretch due to the weak and possibly broad nature of these absorptions. The NMR spectrum shows a new hydride peak at −4.44 ppm (1JPt−H = 725 Hz, 2JSn−H= 128 Hz) which has approximate integration for 3H when compared to the broad olefinic region of the IBut group. Synthesis of Pt(SnBut3)2(aIBut)(CO), 11. In a glovebox, under an atmosphere of argon, 30.0 mg of Pt(SnBut3)2(CO)2, 10, (0.036 mmol) dissolved in 3 mL of freshly distilled hexane was added to a 20 mL vial. To the above green-colored solution, 14.0 mg (0.078) of IBut was added at room temperature and stirred vigorously for 1 h. The reaction mixture turned orange, and the solvent was evaporated in a glovebox at room

temperature overnight to obtain orange crystals of compound 11. The crystals were washed with 3 × 1 mL portions of hexane to yield 30.0 mg of pure 11 (85% yield). Spectral data for 11: 1H NMR (C6D6, rt, in ppm): δ 7.45 (d, 4JPt−H = 12 Hz, 1H, imid), 6.31 (d, 3JPt−H = 27 Hz, 1H, imid), 1.52 (s, 3JSn−H = 46.7 Hz, 54 H, SnBut3), 1.47 (s, 9 H, C(CH3)3), 0.82 (s, 9 H, C(CH3)3). IR νCO (cm−1 in hexane): 2011(vs). Elemental anal. calcd: C, 43.96; H, 7.58; N, 2.85%. Found: C, 44.20; H, 7.46; N, 2.84%. Crystallographic Analyses. Colorless crystals of Pt(SnBut3)(COD)(H), 3, suitable for diffraction analysis were grown by slow evaporation of a solution of 3 in hexane solvent under a slow stream of argon gas at 0 °C (ice−water bath). Colorless crystals of [Pt(SnBut3)(μSnBut2)(H)2]2, 4, were grown by slow evaporation of a solution of 4 in hexane solvent under a slow stream of argon gas at 0 °C (ice-water bath). Light yellow crystals of [Pt(SnBut3)(μ-SnBut2)(CO)(H)2]2, 5, were grown by slow evaporation of a solution of 5 in a CH2Cl2/hexane solvent mixture under a slow stream of CO gas at room temperature. Redcolored crystals of [Pt(SnBut3)(IBut)(μ-H)]2, 6, were grown by slow evaporation of a toluene solution at room temperature in a glovebox. Yellow crystals of Pt(SnBut3)(IBut)(CO)(H), 7, were grown by evaporation of hexane solution of 6 at 0 °C under a slow stream of the CO gas. Light yellow crystals of Pt(SnBut3)(IBut)(C2H4)(H), 8, were grown by slow evaporation of hexane solution under a stream of ethylene gas in an ice bath (0 °C). Orange-colored crystals of Pt(SnBut3)2(aIBut)(CO), 11, were grown by evaporation of hexane solution in a glovebox at room temperature. The data crystals for 3, 5, 6, and 8 were mounted onto the end of a thin glass fiber using Paratone-N for data collection at 100 K under N2. The data crystals for 4, 7, and 11 were glued onto the end of a thin glass fiber. X-ray intensity data were measured by using a Bruker SMART APEX2 CCD-based diffractometer using Mo Kα radiation (λ = 0.71073 Å).28 The raw data frames were integrated with the SAINT+ program by using a narrow-frame integration algorithm.28 Corrections for Lorentz and polarization effects were also applied with SAINT+. An empirical absorption correction based on the multiple measurement of equivalent reflections was applied using the program SADABS. All structures were solved by a combination of direct methods and difference Fourier syntheses and refined by full-matrix least-squares on F2, by using the SHELXTL software package.29,30 Crystal data, data collection parameters, and results of the analyses are listed in Tables S3 and S4. Compounds 3 and 4 crystallized in the monoclinic crystal system. The systematic absences in the intensity data identified the unique space group P21/n. The hydride ligand in 3 was located from the difference map but refined on its positional parameters with a fixed isotropic thermal parameter. For compound 4 with Z = 2, there is half a formula equivalent of the molecule present in the asymmetric unit that has crystallographic center of inversion symmetry. Compound 5 crystallized in the triclinic crystal system. The space group P1̅ was assumed and confirmed by the successful refinement and solution of the structure. The CO−Pt−Pt−CO core in the molecule is disordered over two orientations which were refined with fixed site-occupancy factors in the ratio 50:50. The two orientations are mirror images of each other, and the CO−Pt−Pt−CO core as seen in Figure 1 is offset by an angle of 12.07°. Atoms Pt1 and Pt1* lie above and below the Sn1−Sn2−Sn1*− Sn2* plane at a distance of 0.4411(4) Å, while atoms Pt2 and Pt2* lie above and below the Sn1−Sn2−Sn1*−Sn2* plane at a distance of 1.0020(4) Å. The carbon atoms of the two But groups attached to atom Sn2 are also disordered. The disorder components in the But groups were located from the difference map and refined with fixed siteoccupancy factors in the ratio 50:50. The hydride ligand was not located and included in the refinement. There is still minor disorder present in the structure which was not accounted for due to satisfactory low R values (R1 = 3.91%) during the final stages of the refinement cycles. Compound 6 crystallized in the triclinic system. The space group P1̅ was assumed and confirmed by the successful refinement and solution of the structure. The hydride ligand was located from the difference map but refined on its positional parameters with a fixed isotropic thermal parameter. Compound 7 crystallized in the monoclinic system. The space group P21/c was confirmed on the basis of the systematic absences in the data. With Z = 8, there were two molecules in the structure, and 309


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Inorganic Chemistry

Figure 1. An ORTEP31 of the disordered CO−Pt−Pt−CO core in 5.

Figure 2. An ORTEP31 of the molecular structure of Pt(SnBut3)(COD)(H), 3, showing 50% probability thermal ellipsoids. Selected interatomic distances (Å) and angles (deg) are as follows: Pt(1)−Sn(1) = 2.5984(2); Pt(1)−H(11) = 1.45(3); Pt(1)−C(1) = 2.289(2); Pt(1)− C(2) = 2.324(2); Pt(1)−C(5) = 2.245(2); Pt(1)−C(6) = 2.266(2); Sn(1)−H(11) = 2.611; Sn(1)−Pt(1)−H(11) = 74(1); Sn(1)−Pt(1)− C(1) = 160.11(6); Sn(1)−Pt(1)−C(5) = 101.70(5).

the hydride ligand was located from the difference map but refined on its positional parameters with a fixed isotropic thermal parameter. Compound 8 crystallized in the triclinic system. The space group P1̅ was confirmed on the basis of the systematic absences in the data. The hydride ligand was located from the difference map but refined on its positional parameters with a fixed isotropic thermal parameter. Compound 11 crystallized in the orthorhombic crystal system. The space group P212121 was confirmed on the basis of the systematic absences in the data. Computational Details. Electronic structure calculations were carried out using the PBE0 density functional32 with the D3(BJ) empirical dispersion correction33 as implemented in the Gaussian 09 suite of programs.34 All calculations were performed with the def2TZVP basis set35,36 and the MWB60,37 and MWB4638 pseudopotentials for Pt and Sn and the triple-ζ quality basis 6-311G(d,p) for other elements. Geometry optimizations were performed without any symmetry restrictions, and all stationary points were optimized by computing analytical energy gradients. The obtained minima were characterized by performing energy second derivatives, confirming them as minima by the absence of negative eigenvalues of the Hessian matrix of the energy. Transition states were characterized by single imaginary frequency, whose normal mode corresponded to the expected motion. Computed electronic energies were corrected for zero-point energy, thermal energy, and entropic effects to determine ΔH°(298 K) and ΔG°(298 K) values.

Reaction of Pt(COD)2 with Excess But3SnH: Synthesis and Structure of [Pt(SnBut3)(μ-SnBut2)(H)2]2, 4. The reaction of Pt(COD)2 and an excess of But3SnH was investigated at room temperature in the absence of other added ligands in an attempt to isolate the proposed complex A {Pt(SnBut3)2(H)2} as shown in Scheme 1. However, the complex isolated proved to be the unexpected dinuclear product [Pt(SnBut3)(μ-SnBut2)(H)2]2, 4. Complex 4 is produced in low but reproducible yields (19%), and its structure in the solid state is shown in Figure 3. Compound 4 is a dinuclear complex, with a Pt−Pt vector that is bridged by two SnBut2 groups. The core structure of the diplatinum complex contains two terminal SnBut3 ligands, two bridging μ-SnBut2, and four hydride ligands. The Pt···Pt distance

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RESULTS Reaction of Pt(COD)2 with One Equivalent of But3SnH: Synthesis and Structure of Pt(SnBut3)(COD)(H), 3. The reaction of Pt(COD)2 and one equivalent of But3SnH at room temperature and −78 °C in the absence of additional ligands results in displacement of one COD ligand from Pt and oxidative addition of the stannane to form the Pt(II) complex Pt(SnBut3)(COD)(H), 3. Complex 3 is sensitive to air, moisture, and temperature and as described below readily reacts by either ligand displacement or oxidative addition accompanied by a loss of the weakly bound COD ligand. An ORTEP drawing of 3 is shown in Figure 2. Structurally, complex 3 resembles Pt(COD)(Me)(GePh3)39 obtained by methane elimination in the reaction of Ph3GeH and Pt(COD)Me2. The structure of complex 3 can be described as being roughly square planar with the H and SnBut3 ligands being cis. The presence of a hydride ligand in 3 was confirmed by 1H NMR, which showed appropriate one-bond coupling to platinum (1JPt−H = 1222 Hz) and two-bond coupling to tin (2JSn−H = 48 Hz).

Figure 3. An ORTEP31 of the molecular structure of [Pt(SnBut3)(μSnBut2)(H)2]2, 4, showing 50% probability thermal ellipsoids. Selected interatomic distances (Å) and angles (deg) are as follows: Pt(1)−Sn(1) = 2.6426(3); Pt(1)−Sn(2) = 2.6257(2); Pt(1)−Sn(2*) = 2.6297(3); Pt(1)···Pt(1)* = 3.0678(3); Sn(1)−Pt(1)−Sn(2) = 126.32(1); Sn(1)− Pt(1)−Sn(2)* = 125.05(1); Pt(1)−Sn(2)−Pt(1)* = 71.43(1). Hydride ligands are not shown. 310


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Inorganic Chemistry

The CO ligands are trans to each other and positioned perpendicular to the Pt−Sn−Pt−Sn plane minimizing steric interactions with the bulky SnBut3 ligands. The Pt···Pt distance in 4, 3.0678(3) Å, lengthens to 3.1894(6) Å by the addition of two CO’s to generate 5. Complex 5 is related to the complex Pt2(CO)2(GePh3)2(μ-GePh2),39 which is a 30 electron Pt(II) complex with a Pt−Pt bond bridged by two GePh2 moieties but does not contain the four hydride ligands present in 5. The Pt−Pt bond distance reported for Pt2(CO)2(GePh3)2(μ-GePh2) is 2.8394(3) Å, which is significantly shorter than the Pt···Pt distance of 3.1894(6) Å in 5 and also that of 3.0678(3) Å in 4. The 1H NMR resonances of the hydride ligands for compound 5 appear upfield at −12.77 ppm (1JPt−H = 702 Hz, 2JSn−H = 160 Hz). Appropriately, the infrared spectrum shows two peaks at 2027(vs) and 2008(m) cm−1. Reaction of 3 with One Equivalent of IBut. Synthesis and Structure of [Pt(SnBut3)(IBut)(μ-H)]2, 6. The reaction of 3 with the bulky NHC carbene ligand, N,N′-di-tert-butylimidazol-2-ylidene (IBut), produced the red dimeric complex [Pt(SnBut3)(IBut)(μ-H)]2, 6. Complex 6 is stable for short periods of time in air in the solid state but decomposes slowly in solution at room temperature upon exposure to air. Complex 6 was characterized by a combination of 1H NMR, elemental, and single crystal X-ray diffraction analyses. An ORTEP showing the molecular structure of 6 is shown in Figure 5.

in 4 of 3.0678(3) Å is in the range where some Pt−Pt interaction is indicated, but assignment of a bond order in such systems is difficult, as discussed in more detail in a later section. The position of the hydrides in 4 cannot be assigned crystallographically, and the presence in this complex of some weak Sn··· H interaction as that predicted for complex 9 in a later section cannot be excluded. Platinum dinuclear complexes similar to 4 have been reported in the literature,40−42 and there is precedent for conversion of stannane to stannylene ligands by cleavage of a Sn-alkyl bond.43,44 The tetrahydride nature of compound 4 was verified by making the deuteride complex 4-d4 and comparing the mass spectrum of 4 and 4-d4, which showed a difference of 4 mass units. A plausible scenario for the formation of 4 would involve initial formation of 3 followed by its conversion to proposed complex A {Pt(SnBut3)2(H)2} as shown in Scheme 1. Conversion of A to 4 involves cleavage of one of the SnBut bonds in addition to the uptake of additional H atoms from an unknown source. Since the yield of 4 is low, no effort was made to further elucidate the mechanism or to unequivocally identify the source of the additional H atoms. The detailed mechanism of the reaction producing 4 is not established, and its isolation was as a result of attempts to isolate {Pt(SnBut3)2(H)2} (complex A in Scheme 1) which have so far been unsuccessful. Reversible Addition of CO to 4 Producing [Pt(SnBut3)(μ-SnBut2)(CO)(H)2]2, 5. At room temperature in toluene solution, complex 4 readily adds two equivalents of CO in a rapid and reversible reaction to yield [Pt(SnBut3)(μ-SnBut2)(CO)(H)2]2, 5, as shown in eq 1. [Pt(SnBu t 3)(μ‐SnBu t 2)(H)2 ]2 + 2CO ⇌ [Pt(SnBu t 3)(μ‐SnBu t 2)(CO)(H)2 ]2

(1)

The addition of CO did not result in displacement of the hydride ligands in converting 4 to 5. Exposure of solutions of 5 to vacuum results in CO loss regenerating 4 with no evidence of loss of dihydrogen. The structure of complex 5 is shown in Figure 4.

Figure 5. An ORTEP31 of the molecular structure of [Pt(SnBut3)(IBut)(μ-H)]2, 6, showing 50% probability thermal ellipsoids. Selected interatomic distances (Å) and angles (deg) are as follows: Pt(1)··· Pt(1)* = 2.6715(1); Pt(1)−H(1) = 1.76(3); Pt(1)−H(1)* = 1.85(3); Pt(1)−Sn(1) = 2.5934(1); Pt(1)−C(1) = 2.022(2); C(1)−Pt(1)− Sn(1) = 97.62(5); C(1)−Pt(1)−H(1)* = 96.3(8); C(1)−Pt(1)−H(1) = 175.7(9); C(1)−Pt(1)−Pt(1)* = 137.00(5); Sn(1)−Pt(1)−H(1) = 81.9(9); Sn(1)−Pt(1)−H(1)* = 165.5(8); Sn(1)−Pt(1)−Pt(1)* = 125.38(1); Pt(1)−H(1)−Pt(1)*= 95(1); H(1)−Pt(1)−H(1)* = 85(1).

Figure 4. An ORTEP31 of the molecular structure of [Pt(SnBut3)(μSnBut2)(CO)(H)2]2, 5, showing 50% probability thermal ellipsoids. Selected interatomic distances (Å) and angles (deg) are as follows: Pt(1)−Sn(1) = 2.6300(4); Pt(1)−Sn(2) = 2.6361(4); Pt(1)−Sn(2*) = 2.8226(3); Pt(1)···Pt(1)* = 3.1894(6); Sn(1)−Pt(1)−Sn(2) = 113.22(1); Sn(1)−Pt(1)−Sn(2)* = 133.54(1); Pt(1)−Sn(2)−Pt(1)* = 71.41(1). Hydride ligands are not shown.

The Pt···Pt distance of 2.6715(1) Å in 6 is similar to that reported by Stone et al. for the related compound [Pt(SiR3)PR3(μ-H)]2,45 with a Pt···Pt distance of 2.692 Å, but is markedly shorter than in complexes 4 and 5. The question of whether or not there is a Pt−Pt bond in complex 6 is discussed in the Computational Studies section. 311


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The 1H NMR spectrum of 6 at room temperature showed a resonance at −5.37 ppm assigned to the hydride, which showed appropriate one-bond coupling to Pt (1JPt−H = 652 Hz) and two sets of two-bond couplings to Sn (2JSn−H = 199 and 56 Hz). These are in keeping with each hydride ligand being both trans and cis to the SnBut3 groups, respectively. The nature that the hydride ligand is a symmetric bridge is confirmed by the observance of a weak 2 × 1JPt−H = 1304 Hz (1JPt−H = 652 Hz) coupling value indicating equal coupling to two active 195Pt nuclei. As shown in Supporting Information Figure S1, the 1H NMR spectrum of dimer 6 shows little change as temperature is lowered to −60 °C. The central peaks remain relatively sharp as the temperature is lowered; however, the coupling to Pt becomes broadened. A proposed mechanism for the formation of 6 is shown in Scheme 2. The first step involves replacement of the COD ligand in compound 3 by IBut to yield presumably the proposed intermediate monomeric 14 electron Pt(II) complex Pt(SnBut3)(IBut)(H), labeled M in Scheme 2. Product 6 is the result of dimerization of the proposed monomeric intermediate M to yield the bridging hydrido dimer. Computational studies of the structure of “M” as well as the thermodynamics of dimer formation are reported in a later section. Efforts to isolate and characterize compound M have so far proven unsuccessful. Attempts to isolate analogs of M with other NHC ligands are in progress. Reaction of 6 with CO to Form Pt(SnBut3)(IBut)(CO)(H), 7. The reactivity of 6 with small molecules was first investigated with CO gas. A red solution of 6 in toluene immediately turned light yellow upon exposure to 1 atm of CO gas at room temperature. The bridging dihydride was cleaved to produce two equivalents of the monomeric carbon monoxide adduct Pt(SnBut3)(IBut)2(CO)(H), 7, in near quantitative yield (90%). The addition of CO to compound 6 is irreversible as evacuation of a solution of 7 does not regenerate 6. Compound 7 is stable at room temperature in the solid state and can be handled for brief periods of time in air. The structure of complex

7 is shown in Figure 6. Compound 7 has a distorted square planar geometry with a C(12)−Pt(1)−Sn(1) bond angle of 95.8(2)°

Figure 6. An ORTEP31 of the molecular structure of Pt(SnBut3)(IBut2)(CO)(H), 7, showing 30% probability thermal ellipsoids. Selected interatomic distances (Å) and angles (deg) are as follows: Sn(1)−Pt(1) = 2.6101(3); Pt(1)−C(1) = 2.090(4); Pt(1)−H(1) = 1.59(2); Pt(1)−C(12) = 1.864(5); C(12)−O(1) = 1.140(7); Sn(1)− H(1) = 2.618; Sn(1)−Pt(1)−C(1) = 159.0(1); Sn(1)−Pt(1)−H(1) = 73(1); Sn(1)−Pt(1)−C(12) = 95.8(2); C(1)−Pt(1)−H(1) = 86(1); C(1)−Pt(1)−C(12) = 105.2(2); C(12)−Pt(1)−H(1) = 168(1).

and a C(1)−Pt(1)−C(12) bond angle of 105.2(2)°. As would be expected based on steric factors, the IBut and SnBut3 groups are trans to each other. The 1H NMR spectrum shows a single hydride resonance peak at −2.48 ppm with appropriate couplings to platinum (1JPt−H = 892 Hz) and tin (2JSn−H = 18 Hz). The IR spectrum of 7 in hexane solution shows a strong peak for the Pt-bound carbonyl group at 1979 cm−1 and a medium intensity peak for ν(Pt−H) stretching at a frequency of 2104 cm−1, which is within the reported range for Pt complexes with terminal hydride ligands of 2050−2120 cm−1.46,47 Reaction of 6 with Ethylene Gas to Form Pt(SnBut3)(IBut)(η2-C2H4)(H), 8. The addition of ethylene gas to a toluene solution of 6 results in a rapid color change from red to yellow to furnish a mononuclear Pt complex Pt(SnBut3)(IBut)(η2-C2H4)312


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NMR studies as a function of H 2 pressure, complex concentration, and temperature. Even NMR tubes sealed under a H2 atmosphere showed signs of slow decomposition over time with small peaks due to unidentified products. However, the major results of the NMR studies as well as computational studies described later are consistent with the dominant reaction channel of a rapidly established equilibrium shown in Scheme 3. Analysis of the reaction mixture by 1H NMR at room temperature indicated the disappearance of the hydride resonance peaks corresponding to complex 6, and the appearance of a new hydride resonance that correspond to a single species with all the hydride ligands equivalent on the NMR time scale, see Figure 8.

(H), 8. In contrast to the addition of CO, the addition of ethylene to 6 is reversible in both solution and the solid state. In the presence of a vacuum or by purging argon gas, the dimeric complex 6 is regenerated from 8 as determined by NMR spectroscopy. Complex 8 was characterized by a combination of 1 H NMR and single crystal X-ray diffraction. Crystals suitable for X-ray diffraction analysis were grown under an ethylene atmosphere and the crystal structure of the 16 electron complex 8 is shown in Figure 7. The ethylene group is bound to the Pt-

Figure 7. An ORTEP31of the molecular structure of Pt(SnBut3)(IBut)(η2-C2H4)(H), 8, showing 50% probability thermal ellipsoids. Selected interatomic distances (Å) and angles (deg) are as follows: Pt(1)−Sn(1) = 2.6292(4); Pt(1)−H(1) = 1.59(5); Pt(1)−C(1) = 2.077(4); Pt(1)− C(12) = 2.161(6); Pt(1)−C(13) = 2.162(5); C(13)−C(12) = 1.411(8); Sn(1)−H(1) = 2.470; Sn(1)−Pt(1)−C(1) = 146.6(1); C(1)−Pt(1)−H(1) = 81(2); Sn(1)−Pt(1)−H(1) = 66(2). Figure 8. 1H NMR spectrum showing the hydride region in toluene-d8 for (a) compound 6 and (b) a solution of 6 under a 3.7 atm H2 atmosphere. Only a trace of starting material 6 is present, and this spectrum corresponds to the limiting high temperature spectrum of 9.

metal center in a classic η2 π-bond. The CC bond distance (C12−C13) of η2-bound ethylene in compound 8 is 1.411(8) Å, which is close to the literature reported CC bond length of η2bound ethylene in mononuclear Pt complexes.48−50 As was the case for 7, the IBut and SnBut3 groups are trans to each other. The proton resonance of the hydride ligand appears at −3.89 ppm with appropriate one-bond coupling to Pt (1JPt−H = 705 Hz) and two-bond coupling to Sn (2JSn−H = 47 Hz). Reaction of 6 with H2 to Form Pt(SnBut3)(IBut)(H)3, 9. Exposing a toluene solution of compound 6 to H2 gas results in a change in color of the solution from red to red-orange. This change is readily reversible upon evacuation. Attempts to isolate and crystallize the product have so far been unsuccessful. Characterization of the product of hydrogenation is based on

On the basis of the structures of the products of CO and C2H4 addition to complex 6 (Figures 6 and 7), the expected product of H2 addition to 6 would be a related mononuclear complex containing a molecular hydrogen and a hydride ligand in a trans arrangement. However, the failure to detect HD coupling in the 1 H NMR spectrum of the reaction mixture of complex 6 with D2 (see below) appears to rule out such a stable static structure containing a molecular hydrogen complex. It is conceivable that this complex once formed may easily oxidatively add the H2 ligand to the metallic center yielding a trihydride species. In fact,

Scheme 3. Proposed Reaction Pathway for H2 Addition to 6

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computational and additional experimental studies on these and related systems are planned. NMR Studies of Reaction of 6 with D2. The equivalent nature of the three hydride ligands in 9 in the NMR spectroscopic studies at room temperature does not rule out the possibility that product 9 could conceivably be a complex containing one hydrido ligand in rapid exchange with a coordinated molecular dihydrogen ligand. In that regard, very recently Conejero and co-workers51 have proposed the formation of [Pt(NHC)2(H2)H]+ complexes (NHC = N,N′dimesityl-4,5-dimethylimidazol-2-ylidene (IMes*), N,N′-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr)) that readily dissociate H2 in equilibrium to form the corresponding monohydrides [Pt(NHC)2H]+. Such a complex Pt(SnBut3)(IBut)(H)(H2) might react with D2 to form several isotopomers including those containing coordinated HD, for example Pt(SnBut3)(IBut)(D)(HD). Such a limiting form would be expected to display a relatively large H−D coupling constant in the 1H NMR spectrum. No evidence indicative of formation of a molecular HD complex showing large HD coupling was observed at room temperature or at −40 °C. Furthermore, no HD coupling was detected, indicating that the H and D ligands are far apart. This spectroscopic result provides evidence against the room temperature static structure of 9 being Pt(SnBut3)(IBut)(H2)H. However, the possible role of this tautomer in the exchange process is discussed later in the Computational Studies section. Reaction of IBut with trans-Pt(SnBut3)2(CO)2, 10. Formation of trans-Pt(SnBut3)2(aIBut)(CO), 11 (aIBut = “Abnormal” Carbene Ligand). In an attempt to prepare a complex analogous to complex 7 but with the hydrido ligand replaced by a second SnBut3 group, IBut and trans-Pt(SnBut3)2(CO)2, 10,23 were reacted anticipating displacement of a CO ligand by IBut as shown in eq 2.52

DFT calculations described in a later section predict that this transformation occurs with a very low activation barrier. The most stable computed structure for this complex, 9, is formally a “trihydride” but in addition contains a three center Pt−H−Sn interaction as depicted in Scheme 3. The 1H NMR spectrum in the hydride region at room temperature for the starting dimer 6 is shown in Figure 8a. As stated previously, the bridging hydrides of this dimer show up at −5.37 ppm with a set of one-bond coupled 195Pt satellites (1JPt−H = 652 Hz) and two sets of two-bond coupled 119Sn/117Sn satellites (2JSn−H = 199 Hz and 2JSn−H = 56 Hz). The humps at −3.8 and −7.0 ppm show 2J values of 1304 Hz [2 × 1JPt−H (652 Hz) = 1304 Hz], indicating that the hydride is coupled to two equivalent 195Pt NMR active nuclei. The 1H NMR spectrum in the hydride region at room temperature for the product formed after H2 addition (3.7 atm) to complex 6 is shown in Figure 8b. Product 9 shows a new signal at −4.44 ppm which displays only a set of one-bond coupled 195Pt satellites (1JPt−H = 725 Hz) and one set of two-bond coupled 119 Sn/117Sn satellites (2JSn−H= 128 Hz). Here, the 2J coupling to 195 Pt and a set of 2JSn−H, as in the spectrum of 6, are now absent, indicating the structure to be a mononuclear Pt complex. This resonance integrates for approximately 3H when compared to the broad olefinic region of the IBut group, indicating it to be a trihydride species. As depicted in Scheme 3, an equilibrium between complex 6 and a mononuclear product 9 is proposed, and accordingly, only when the NMR tube was pressurized with 3.7 atm of H2, is nearly complete conversion of 6 to 9 seen. At lower pressures of H2, this conversion is not complete, and the 1 H NMR resonances for both complex 6 and 9 are observed. The observance of both signals in the 1H NMR spectrum at intermediate pressures of H2 clearly indicates that H2 addition to complex 6 at room temperature is slower than the NMR time scale. As expected for an equilibrium where a dinuclear complex is cleaved to yield two equivalents of a mononuclear product (Scheme 3), a decrease in absolute concentration also leads to a greater product conversion of dimer 6 to 9. H2 addition to complex 6 is also favored at higher temperatures, indicating a favorable entropic term for this transformation. As the temperature is lowered, the resonance signal at −4.44 ppm corresponding to complex 9 broadens and additional hydride peaks start to appear in the 1H NMR spectra particularly in concentrated solution, which seem to be due to formation of dinuclear complexes of an as yet unknown structure. Detailed analysis of this more complex behavior observed at low temperatures would be speculative at this point and is beyond the scope of this paper. However, there is evidence supporting formulation of product 9 as a mononuclear trihydride which on the NMR time scale has three equivalent hydride ligands. In spite of the apparent simplicity of the room temperature 1H NMR spectrum of product 9 as well as its reversible formation upon the addition or evacuation of dihydrogen to 6, at this time the authors are not able to assign the static structure based on experimental data alone. In computational studies described in a later section, no single structure with three symmetrically equivalent hydride ligands was found. Alternatively, DFT calculations appear to support a highly fluxional system with several local minima and relatively low barriers for their interconversion. The structure proposed for product 9 shown in Scheme 3 is the lowest of all the minima obtained computationally and is in keeping with the structure determined by X-ray crystallography for complexes 7 and 8. More detailed

trans‐Pt(SnBu t 3)2 (CO)2 + IBu t → CO + trans‐Pt(SnBu t 3)2 (IBu t)(CO)

(2)

The addition of IBut to a toluene solution of 10 resulted in a color change from green to orange at room temperature. Evaporation of solvent yielded an orange-colored, air stable complex corresponding to the formula Pt(SnBut3)2(IBut)(CO). However, the crystal structure shown in Figure 9 indicated a complex trans-Pt(SnBut3)2(aIBut)(CO), 11, where the NHC ligand presents an “abnormal” carbene coordination. Carbene coordination results in IBut being mutually cis to the two bulky SnBut3 groups, and the “abnormal” coordination clearly reduces steric strain in the system. Complex 7 contains only two bulky groups (IBut and one SnBut3), and these are trans to each other with the hydride being trans to CO. The relationship between 7 and 11 is that the sterically undemanding H ligand in 7 has been replaced by a second bulky SnBut3 group which forces “abnormal” coordination of the NHC. This result is in keeping with a number of reports on “abnormal” NHC coordination in the literature.53−55 As previously proposed,53 the formation of complex 11 can be envisioned to proceed via intermolecular activation of the alkenyl C−H bond of the imidazole moiety of the NHC to yield a transient hydride Pt(IV) complex and subsequent hydrogen migration from the Pt to the carbene carbon of the coordinated NHC. Computational Studies. DFT calculations were performed for complex 6 and the proposed intermediate M in Scheme 2 in an effort to gather information about the structure of the 314


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which has been assigned in the literature to contain an η2coordination of Ph3SnH to the Mn center in a metal, hydrogen, tin three-center bond.57 As a reference, a typical Sn−H bond distance would be on the order of 1.71 Å.58 Steric interaction in this case is computed to lead to a situation intermediate between a formal Pt(0) η2-stannane complex and a Pt(II) stannyl hydride complex for intermediate M. The optimized structures of both MMe and M are clearly different than that previously determined by X-ray crystallography for the related cationic mononuclear 14electron Pt(II) hydride complex [Pt(IPr)2H]+ presenting the two bulkiest IPr ligands in a trans arrangement.51 The computed structures of the two dimeric complexes 6 and 6Me and a summary with selected metric parameters are shown in Figure 11. The calculated geometry for complex 6 is in good agreement with the experimental structure obtained by X-ray crystallography shown in Figure 5, with differences between the calculated and experimental bond lengths and angles generally lower than 0.03 Å and 2°, respectively. This gives some measure of confidence to the computed structure of other complexes for which there are no experimental data. The main differences between the computed structures of 6 and 6Me are a widening of the C−Pt−Sn angle (96.0 in 6 vs 84.2 o in 6Me) as the SnBut3 group is forced away from IBut due to steric repulsion. There is consequently also a shortening of the Sn···H distance from 3.085 Å for 6Me compared to 2.859 Å for 6. Qualitative examination of the two structures in Figure 11 appears to show the onset of steric distortion in the dimeric complex upon replacement of the Me groups with But. Thermochemical data for the dimerization reaction (eq 3) were computed for both M and MMe as shown in Table 1.

Figure 9. An ORTEP31 of the molecular structure of transPt(SnBut3)2(aIBut)(CO), 11, showing 30% probability thermal ellipsoids. Selected interatomic distances (Å) and angles (deg) are as follows: Pt(1)−Sn(1) = 2.7261(3); Pt(1)−Sn(2) = 2.7248(3); Pt(1)− C(12) = 1.855(5); Pt(1)−C(1) = 2.053(4); C(12)−O(1) = 1.139(6); Sn(1)−Pt(1)−Sn(2) = 161.96(1); C(1)−Pt(1)−Sn(1) = 93.7(1); C(1)−Pt(1)−C(12) = 173.1(2); Sn(1)−Pt(1)−C(12) = 87.6(1).

monomeric tricoordinate compound M and to determine the thermodynamic parameters for the dimerization process where species 6 is presumably formed. Moreover, the model complexes [Pt(SnMe3)(IMe)(μ-H)]2, 6Me, and Pt(SnMe3)(IMe)(H), MMe, in which all the But substituents on both the NHC and stannyl ligands in 6 and intermediate M were replaced by methyl groups were also computed to check the influence of the steric hindrance imposed by the bulky But groups in the dimerization reaction. The computed minimum energy structures of monomer M and the corresponding model complex MMe were initiated with the range of geometries shown in Scheme 4 as starting points. These included the ligands in various possible configurations of T-shaped as well as Y-shaped structures.

2Pt(SnR3)(NHC)(H) → [Pt(SnR3)(NHC)(μ‐H)]2

(3)

The energetics of formation of the less crowded system 6Me are more favorable than for 6 on the order of ∼15 kcal·mol−1 with respect to both ΔH° and ΔG° since the entropies of dimerization are computed to be similar and highly unfavorable. The computed similar entropic changes are at first sight surprising, but it must be kept in mind that there is more compression of the C−Pt−Sn angle for 6Me than for 6. This is what allows the more favorable enthalpy of formation of 6Me in the first place; the ligands do not begin to intermesh with subsequent reduced rotational freedom in 6Me, allowing it to adopt an enthalpically favored nearly square planar geometry. For the more sterically encumbered 6, the favorable enthalpic geometry for the dimer cannot be attained, and so the entropic factors are similar for the two complexes. The computed Gibbs energy of dimerization for 6 of −13 kcal·mol−1 at 298 K yields a Keq value of ∼2.5 × 1010. This value is not sufficiently high to rule out participation of the monomeric species M as a reactive intermediate in diluted conditions. Ligand addition reactions to dimeric 6 producing mononuclear products such as 7−9 could conceivably occur by either dissociation to mononuclear intermediate M or by direct addition to the intact dimer 6. This prompted investigation of the nature of bonding in the dimer 6 to see if orbitals were available for direct reaction with incoming ligands, in keeping with the unsaturation nature of the molecule. As shown in Figure 12, the LUMO and LUMO+1 are antibonding and bonding orbitals respectively between the two platinum centers of the dimer and present vacant frontier orbitals for ligand addition in the same plane (LUMO) or above and below the plane defined by the Pt−H−Pt−H core (LUMO+1).

Scheme 4. Starting Geometries for the Calculations of the Pt(SnR3)(NHC)(H) Species (MMe: NHC = IMe, R = Me; M: NHC = IBut, R = But)

Independent of starting geometry, all of them converged to the structures shown schematically in Figure 10. Complex MMe has a distorted T-shaped structure and somewhat surprisingly, the two bulkiest groups (SnMe3 and IMe) are mutually cis with a coordination vacancy in the metal in trans position to the stannyl ligand probably due to the large trans influence of this ligand.56 Complex M has a distorted Y-shaped structure as the C−Pt−Sn angle widens in going from MMe (107.4°) to M (133.6°) due to an increase in the steric pressure. Also as a consequence, the Sn−Pt−H angle narrows from 69.0° for MMe to 57.7° for M. There is a significant decrease in the computed Sn···H distance which shortens from 2.473 to 2.204 Å in going from MMe to M. The value of 2.204 Å is close to the value of 2.16 Å reported for the MeCp(CO)2Mn(H)SnPh3 complex 315


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Figure 10. (a) Optimized structures of monomeric intermediate M (left) and the model compound MMe (right). (b) Schematic summary of optimized structures of monomeric intermediate M (in red) and the model compound MMe (in blue). Bond lengths in Å and angles in degrees.

Figure 11. Optimized structures of complexes 6 (top) and 6Me (bottom). Selected distances (Å) and angles (deg) are tabulated for 6 and (6Me): Pt(1)···Pt(1*) = 2.687 (2.650); Pt(1)−H(1) = 1.699 (1.724); Pt(1)−H(1*) = 1.894 (1.861); Pt−Sn = 2.624 (2.597); Pt−C = 2.016 (1.984); Sn−H = 2.859 (3.085); C−Pt−Sn = 96.0 (84.2); C(1)−Pt(1)−H(1*) = 101.4 (102.3); C(1)−Pt(1)−H(1) = 175.3 (173.0); C(1)−Pt(1)···Pt(1*) = 140.0 (142.7); Sn(1)−Pt(1)−H(1) = 79.7 (88.7); Sn(1)−Pt(1)−H(1*) = 162.0 (173.5); Sn(1)−Pt(1)··· Pt(1*) = 124.0 (133.1); Pt(1)−H(1)−Pt(1*) = 96.7 (95.2); H(1)− Pt(1)−H(1*) = 83.2 (84.8). The hydrogen atoms (other than the bridging H’s) are omitted for clarity. The optimized structure of 6 is not symmetrical, and the values shown are the average between the two different possibilities. For additional data comparing the computed and experimental structures, see Table S1.

Figure 12. Selected molecular orbital diagrams of the LUMO and LUMO+1 computed for 6 (isovalue = 0.03) showing the presence of vacant frontier orbitals which depict coordinative unsaturation in the dimeric complex.

An additional area of interest in complex 6 was the nature of bonding in the dimeric structure which includes bridging hydride ligands as well as the possibility of the presence of a direct Pt−Pt bond between the two metals as indicated by the short Pt···Pt distance of 2.6715(1) Å. Several computational studies on related systems have led to conclusions that there is no bond, a single bond, or a double bond present between the two metallic centers.39,45 Delineation of the bonding in such systems can be problematic, and resolving this question is beyond the scope of this paper. Nevertheless several key orbitals are shown in Figure 13 relevant to this question. Orbitals HOMO−20 and HOMO−17 both contain significant electron density associated with the 3-centered two-electron bridging hydride ligands. HOMO−17 also contains limited overlap indicative of direct Pt−Pt interaction. HOMO−13 presents what could be interpreted as a Pt−Pt π-bond involving

Table 1. Computed Thermochemical Data for the Dimerization Reaction (eq 3) of M and MMe at 298.15 K R

NHC

ΔH° (kcal·mol−1)

ΔS° (cal·mol−1·K−1)

ΔG° (kcal·mol−1)

Me But

IMe IBut

−45.8 −31.1

−59.1 −60.9

−28.2 −13.0

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Figure 14. Optimized structures of two isomers of complex 9: Pt(SnBut3)(IBut)(H)3, 9a, and Pt(SnBut3)(IBut)(H2)(H), 9b. The hydrogen atoms (other than the hydrides and bound molecular H2) are omitted for clarity.

planar pyramid geometry with one hydride in axial position, 9e, all of them containing the two bulkiest ligands, IBut and SnBut3, in a cis arrangement were found computationally, but they were calculated to be 10.1, 12.1, and 19.4 kcal·mol−1 enthalpically less stable than isomer 9a respectively. The optimized structures of these three other isomers are shown in Figure S2 in the Supporting Information. No minima corresponding to a trigonal bipyramid or additional square planar pyramid structures were found. The optimized geometry of complex 9a shows that oxidative addition of H2 is complete and presents a short Sn···H distance of 2.161 Å ,which seems to suggest the presence of residual Sn···H bonding interaction indicating that some degree of reductive elimination to form a Sn−H bond has been produced in this species. This behavior has been previously observed in related complexes.57,59 Moreover, this fact is also supported by the 1H NMR spectrum of 9 (see above, Figure 8) that shows large twobond coupling to tin (2JSn−H = 128 Hz). Thus, according to all of these observations, complex 9a can be formally described as containing an η2-coordination of But3SnH to a Pt(II) center in a three-center two-electron bond. Dissociation of But3SnH from 9a was computed to proceed with a ΔG°(298 K) of 25.9 and 45.7 kcal·mol−1 to form the cis and trans isomer of the tricoordinate species Pt(IBut)(H)2, respectively. In addition to species 9a, another minimum corresponding to a molecular H2 complex, 9b, was found, and its structure is also shown in Figure 14. The geometry of isomer 9b is in keeping with the structure determined by X-ray crystallography for compounds 7 and 8. However, 9b was computed to be 4.8 kcal· mol−1 less stable in terms of Gibbs energy at 298.15 K than isomer 9a. The transition state connecting both 9a and 9b complexes, 9aTS9b, was located, and its computed structure and a summary of selected metric parameters are shown in Figure S2 and Table S2 in the Supporting Information. The Gibbs energy of activation at 298.15 K for the conversion of 9a to 9b was computed to be only 5.1 kcal·mol−1. Although this transformation occurs readily, the computed ΔG° value of 4.8 kcal· mol−1 for the isomerization reaction converting 9a to 9b is sufficiently high so that detectable amount of complex 9b at room temperature do not accumulate in keeping with the failure to detect it in the NMR studies of the reaction of 6 with H2 (vide supra). However, once 9b is formed, it can easily lose the weakly bound molecular H2 ligand regenerating monomeric species M as it can be evidenced by the thermochemical values computed for this dissociation reaction [ΔH°(298 K) = 5.9 kcal·mol−1;

Figure 13. Selected molecular orbital diagrams computed for 6 (isovalue = 0.03) showing interaction between the two metallic fragments.

primarily the dxy orbitals of the Pt centers. However, HOMO−8 which is also an occupied orbital appears to correspond essentially to the antibonding combination of HOMO−13. This will serve to negate the bonding interaction between the two Pt atoms. In balance, the view presented by these calculations appears to be that the major interaction between both metallic moieties for complex 6 reside in the bridging hydride ligands. Computational studies were also performed to assist in interpretation of the NMR results for the reaction of 6 with H2 described earlier where NMR studies suggested cleavage of 6 by 2 mol of H2 to form 2 mol of a mononuclear product, 9. The lowest two minima found for the proposed complex 9 are shown in Figure 14. A summary with selected bond lengths and angles for both species is collected in Table S2 in the Supporting Information. Furthermore, two other dihydrogen hydride complexes, 9c and 9d, and a trihydride presenting a square 317


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Inorganic Chemistry Scheme 5. Computed Mechanism for H Atom Exchange in Complex 9a

a

ΔG°(298 K) values in kcal·mol−1 are shown.

Scheme 6

ΔG°(298 K) = −1.3 kcal·mol−1]. Then, intermediate M in principle can react with an additional H2 molecule to revert to complex 9b or dimerize to form complex 6. In the optimized structure of complex 9a, the three H atoms are not equivalent, but a single resonance was observed for all the hydride ligands in the 1H NMR spectrum of 9 acquired at room temperature and below. A low energy pathway was computed which would allow rapid scrambling and thus average out the H atoms on the NMR time scale at room temperature as shown in Scheme 5. The equivalency of the three hydrides on the NMR time scale is proposed to occur through different indistinguishable structures of isomers 9a or 9b. To achieve the

interconversions 9a → 9a or 9b → 9b, the transition states 9aTS9a or 9bTS9b, respectively, have to be overcome. These two stationary points present a η2-stannane ligand coordinated in trans position to the IBut ligand (9aTS9a) or a η2-H2 ligand in the cis position to the IBut ligand (9bTS9b) both perpendicular to the coordination plane of the platinum center. Likewise, as previously stated, the reversible transformation between 9a and 9b proceeds readily through transition state 9aTS9b. The structures of the transition states 9aTS9a, 9aTS9b, and 9bTS9b are shown in Figure S2 and Table S2 in the Supporting Information. 318


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Inorganic Chemistry Recent reports60 on the interchange of D atoms in transPt(PBut3)2(D)2 have interpreted the equivalency of the D atoms in the NMR spectrum as being due to concerted motion of a D− Pt−D rotor perpendicular to the P−Pt−P axis. The highest computed barrier for this process was 4.9 kcal·mol−1. However, in the case of complex 9, the barrier calculated for this rotation, ΔG‡(298 K) = 21.1 kcal·mol−1, is much higher due to the big steric hindrance imposed by the bulky IBut ligand. The structure of the transition state found for this process is shown in Figure S2 in the Supporting Information, as 9aTSrot9a. The computed thermochemistry in the gas phase for the H2 addition reactions to 6, is in qualitative agreement with the experimental findings. While quantitative value of the equilibrium constant was not determined, it was observed that H2 addition to 6 to form 9 was favored at high temperature. The computed thermochemical data for H2 addition to 6 forming 2 mol of 9, is computed to be endothermic, but to have a slightly favorable entropy of reaction (ΔH°(298 K) = 7.7 kcal·mol−1 and ΔS°(298 K) = 6.8 cal·mol−1·K−1). The entropic driving force of H2 gas addition is not surprising considering the favorable entropy associated with cleaving the sterically congested dimer 6.

monomer M in this system have been so far unsuccessful due to its ready dimerization, which has a favorable free energy of formation and presumably low barrier for its formation. In spite of having a dimeric structure, complex 6 provides a ready source of the putative reactive fragment “Pt(SnBut3)(IBut)(H),” and its reactions with CO and C2H4 are rapid to form mononuclear complexes 7 and 8. The reactivity of 6 with small molecules extends to dihydrogen, where complex 9 is formed in a rapid and reversible addition of H2. Due to the ready reductive elimination of H2 from 9, it could not be isolated and crystallographically characterized, but NMR studies characterize product 9 as a mononuclear trihydride having three H atoms equivalent on the NMR time scale. The structure predicted for this novel complex based on DFT calculations presents three inequivalent hydrides, and one of them exhibits a short Sn···H distance of 2.161 Å and can be therefore formulated formally as a Pt(II) η2-stannane dihydride complex. Furthermore, computational calculations support that this isomer is in dynamic equilibrium with a monohydride−dihydrogen Pt(II) complex with relatively low barriers for their interconversion, which seems to explain the apparent equivalency of the three H atoms on the NMR time scale at room temperature. To our knowledge, Pt−carbene complexes with three terminal hydrides are unknown; no precedent examples exist for comparison. However, there is one literature reported example of trihydrido platinum(IV) complex mer-Pt(PEt3)2(C6H5)(H)3, which was observed as a byproduct from the reduction reaction of trans-[Pt(C6H5)(MeOH)(PEt3)2]+.69 Transition metal− NHC complexes with dihydrides are known.70−77 Recently, Nolan et al. prepared a dihydride complex Pd(IPr)(H)2(PCy3) with the IPr NHC ligand from the 14 electron Pd(0) complex.73 Crabtree et al. also reported a similar dihydride Ir-NHC carbene cationic complex Ir[(2-py)(CH2)n(C3H2N2)R]L2H2.77 The reversible nature of dihydrogen activation in this system is reminiscent of our recent report of reversible oxidative addition of dihydrogen at Pt(SnBu t 3 ) 2 (CNBu t ) 2 , forming Pt(SnBut3)2(CNBut)2(H)223 which compares to Pt(SnBut3)(IBut)(H)3. Both complexes upon addition of H2 are formally Pt(IV). The steric pressure in Pt(SnBut3)2(CNBut)2(H)2 is reduced in Pt(SnBut3)(IBut)(H)3 since one of the SnBut3 ligands has been replaced in the trihydride system. This is partially compensated for by the increased steric pressure of the IBut ligand. Additional work is in progress to further study the role of steric pressure in controlling reactivity in Pt−Sn and other transition metal complexes in which bulky tin ligands alter their complex geometry.

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DISCUSSION The main focus of this work was exploration of synthetic chemistry centering on reactions of Pt(SnBut3)(COD)(H), 3, as shown in Scheme 6. Due to lability of the COD ligand and its ready displacement by ligand substitution and oxidative addition reactions, a number of new mononuclear and dinuclear complexes containing the platinum bulky tin moiety Pt− SnBut3 have been prepared and crystallographically characterized. Starting from complex 3, the addition of excess But3SnH results in oxidative addition converting the Pt(II) monomer to the Pt(IV) bridging stannylene tetrahydride complex 4. This complex reversibly adds CO yielding 5 without apparent loss of the hydride ligands as dihydrogen. The addition of only one equivalent of But3SnH to 3 followed by the addition of two equivalents of CNBut provides the pathway to formation of Pt(SnBut3)2(CNBut)2(H)2 as reported previously.23 The addition of CO rather than CNBut as a trapping agent for the Pt(SnBut3)2(H)2 proposed intermediate (complex A in Scheme 1) results in elimination of dihydrogen and formation of Pt(SnBut3)2(CO)2 complex 10.23 Treatment of 10 with IBut results in formation of Pt(SnBut3)2(aIBut)(CO), 11, in which IBut displays an “abnormal” carbene coordination due to high steric hindrance. The most exciting results from this work evolved from the reaction of 3 with IBut. The strength of the Pt−NHC bond is sufficient to displace the remaining COD in 3 to generate the proposed transient intermediate “Pt(SnBut3)(IBut)(H),” M, which dimerizes to form [Pt(SnBut3)(IBut)(μ-H)]2, 6. The proposed tricoordinate intermediate most likely has a “T” shape as has been observed or postulated in previous reports.61−68 Computational studies indicate that the approximately T-shaped sterically truncated model complex “Pt(SnMe3)(IMe)(H),” MMe, would have the relatively bulky stannane and NHC ligands mutually cis and the H ligand trans to IMe and cis to SnMe3 as shown in Figure 10. This geometrical arrangement would require little reorganization to dimerize since the vacant position in the metal and the hydride ligand are mutally cis. The computed structure of the tricoordinated species M retains that arrangement, but the angle between the stannane and the NHC is necessarily widened, making the Sn···H smaller. Efforts to detect

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02441. NMR spectra and computational details (PDF) Crystallographic information (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: c.hoff@miami.edu. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 319


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Inorganic Chemistry

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ACKNOWLEDGMENTS The National Science Foundation (CHE-1300206) and the Spanish Ministry of Economy and Competitiveness (CTQ201236966) are gratefully acknowledged for support of this work.

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