Studies of the Structures and Bonding of Gold-Bridged Dirhenium Carbonyl Cluster Complexes

The compounds Re2(CO)8(μ-AuPPh3)2, 1, a dimer of Re(CO)4(μ-AuPPh3), and ax,ax-Re2(CO)8(PPh3)2 were obtained from UV−vis radiationinduced decarbonylation of the compound Re(CO)5[Au(PPh3)]. Compound 1 contains two rhenium atoms bridged by two AuPPh3 groups. The complex has 32 valence electrons and is formally unsaturated by the amount of two electrons. The Re−Re bond distance in 1 is unusually short (Re−Re = 2.9070(3) Å), as found by a single-crystal structural analysis. The nature of the metal−metal bonding in 1 was investigated by DFT computational analyses, which have provided evidence not only for σ-bonding but also significant complementary π-bonding directly between the two rhenium atoms. The electronic structure of Re2(CO)8(μ-H)2, 2, was similarly analyzed and is compared with that of 1. Compound 1 is intensely colored due to low-energy, metal-based electronic transitions between the HOMO and HOMO-2 and the LUMO. Compound 1 reacts with I2 to yield Re2(CO)8(μ-AuPPh3)(μ-I), 3, and the known compound Re2(CO)8(μ-I)2, 4, by substitution of the bridging AuPPh3 groups with bridging iodide ligands. Compound 3 is electronically saturated, 34 valence electrons, and contains a formal Re−Re single bond: Re−Re = 3.2067(5) Å. Compound 3 was also in a high yield (83%) from the reaction of Re2(CO)8(μ-H)(μ-CHCHC4H9) with Au(PPh3)I. The Re− Re bonding in compounds 3, 4, and Re2(CO)10 was also analyzed computationally, and this bonding was compared with their bonding in 1 and 2. ■ INTRODUCTION The similarities between the hydrogen atom and the Au(PPh3) group are well-known. The two species are effectively isolobal and both contain only one valence electron. H and the Au(PPh3) group are well-known to bridge metal−metal bonds effectively in polynuclear metal complexes. There are a number of hydride-bridged metal carbonyl cluster complexes, such as Re2(CO)8(μ-H)2, A, 4 Os3(CO)10(μ-H)2, 5 B, Re4(CO)12(μ-H)4, 6 C, and higher nuclearity cluster complexes, such as Pt2Re3(CO)9(P-t-Bu3)3(μ-H)6, 7 and [Ru3(CO)8(μ3-CMe)(μ-H)2(μ3-H)]2, 8 that have attracted interest because they are formally electronically unsaturated. Unsaturated metal cluster complexes are of interest because they exhibit higher reactivity than their electronically saturated counterparts. A few unsaturated triosmium carbonyl cluster complexes containing the bridging Au(PR3) group(s) have also been prepared (e.g., Os3(CO)10(μ-AuPEt3)2, 11 Os3(CO)10(μ-AuPPh3)(μ-H), 12 and Os3(CO)10(μ-AuPPh3)(μ-Ph) ) that are related to Os3(CO)10(μ-H)2. We have now prepared and characterized the new dirhenium complex Re2(CO)8(μ-AuPPh3)2, 1, that contains two bridging Au(PPh3) groups. The two rhenium atoms contain a total of 32 valence electrons, leaving the metal atoms formally unsaturated by the amount of two electrons. In accord with this, the Re−Re distance in 1 is unusually short. The metal−metal bonding in 1 was investigated by DFT computational analyses, which have provided evidence for strong bonding directly between the two rhenium atoms. For comparisons, the electronic structure of Re2(CO)8(μ-H)2, 2, was also investigated. Reactions of 1 with I2 were investigated and were found to provide the new electronically saturated compound Re2(CO)8(μ-AuPPh3)(μ-I), 3, and the previously reported complex Re2(CO)8(μ-I)2, 4. 14 Compound 3 was prepared independently in a high yield from the reaction of Re2(CO)8(μ-H)(μ-CHCHC4H9) with Au(PPh3)I. The Re−Re bonding in 3 and 4 was also investigated by computational methods. Details of these studies are provided in this report. ■ EXPERIMENTAL SECTION General Data. Reagent grade solvents were dried by the standard procedures and were freshly distilled prior to use. Infrared spectra were recorded on a Thermo Nicolet Avatar 360 FT-IR spectrophotometer. Room-temperature H NMR and P{H} NMR were recorded on a Bruker Avance/DRX 400 NMR spectrometer operating at 400.3 and 162.0 MHz, respectively. Positive/negative ion mass spectra were recorded on a Micromass Q-TOF instrument by using electrospray (ES) ionization or electron impact (EI) ionization. UV−vis spectra Received: October 15, 2013 Published: November 21, 2013 Article pubs.acs.org/Organometallics © 2013 American Chemical Society 7540 dx.doi.org/10.1021/om4010127 | Organometallics 2013, 32, 7540−7546 D ow nl oa de d by T E X A S A & M U N IV C O L G S T A T IO N o n A ug us t 3 0, 2 01 5 | h ttp :// pu bs .a cs .o rg P ub lic at io n D at e (W eb ): N ov em be r 21 , 2 01 3 | d oi : 1 0. 10 21 /o m 40 10 12 7 were recorded on a (Agilent/Varian Cary 50) UV−vis spectrometer in methylene chloride solvent at a concentration of 0.46 × 10−4 M. Re(CO)5(AuPPh3), 15 Re2(CO)8(μ-H)(μ-CHCHC4H9), and Au(PPh3)I 17 were prepared by the previously reported procedures. Product separations were performed by TLC in air on Analtech 0.25 mm silica gel 60 Å F254 glass plates. Synthesis of Re2(CO)8(μ-AuPPh3)2, 1. A 45.0 mg (0.0572 mmol) portion of Re(CO)5Au(PPh3) was dissolved in 25 mL of benzene and irradiated for 30 min, and the solution turned to brown. The solvent was removed in vacuo. The residue was separated by TLC by using a 4:1 hexane/methylene chloride (v/v) solvent mixture to yield in order of elution: (1) a colorless band of Re2(CO)10, 0.7 mg (4% yield), (2) a colorless band identified as Re2(CO)8(PPh3)2, 16 6.7 mg (21% yield), and (3) a red band of Re2(CO)8(μ-AuPPh3)2, 1, 7.6 mg (18% yield). Spectral data for 1: IR νCO (cm −1 in methylene chloride): 2065(w), 2028(s), 1976(vs), 1942(m), 1917(s). H NMR (CD2Cl2, in ppm): δ = 7.20−7.36 (m, 30H, Ph). P NMR (CD2Cl2, in ppm): δ = 76.17 (s). Mass Spec. EI/MS m/z: 1514, M, 1486, M − CO. The isotope distribution pattern is consistent with the presence of two gold atoms and two rhenium atoms. The UV−vis absorption spectrum in CH2Cl2 solvent: λ max = 327 nm, ε = 6692 cm −1 M−1, λ max = 420 nm, ε = 2820 cm−1 M−1, and λ max = 493 nm, ε = 5836 cm −1 M−1. Synthesis of Re2(CO)8(μ-AuPPh3)(μ-I), 3. A 3.2 mg (0.0126 mmol) portion of I2 was added to 19.0 mg (0.0125 mmol) of 1 in 10 mL of benzene. The solution was stirred at room temperature for 15 min. During this time, the solution turned from orange to yellow. The solvent was removed in vacuo, and the products were separated by TLC by using a 4:1 (v/v) hexane/methylene chloride solvent mixture to yield in order of elution: (1) a yellow band of 1.9 mg of Re2(CO)8(μ-AuPPh3)(μ-I), 3, (13% yield), and (2) a colorless band of Au(PPh3)I, 4.9 mg (33% yield). Spectral data for 3: IR νCO (cm −1 in methylene chloride): 2094(w), 2064(m), 2005(s), 1972(m), 1935(m). P NMR (CD2Cl2, in ppm): δ = 76.79 (s). Mass Spec. EI/MS m/z: 1182, M, 1154, M − CO. The isotope distribution pattern is consistent with the presence of one gold atom and two rhenium atoms. Alternative Synthesis of 3. A 45.0 mg (0.0661 mmol) portion of Re2(CO)8(μ-H)(μ-CHCHC4H9) was added to 38.8 mg (0.0661 mmol) of Au(PPh3)I in 20 mL of hexane. The solution was refluxed for 30 min. After cooling, the solvent was removed in vacuo and the product was isolated by TLC by using hexane solvent to give a yellow band of 3, 65.0 mg (83% yield). Synthesis of Re2(CO)8(μ-I)2, 4, from the reaction of 1 with I2. A 6.6 mg (0.026 mmol) portion of I2 was added to a solution of 20.0 mg (0.0125 mmol) of 1 in 10 mL of benzene. The solution was stirred at room temperature for 15 min. During this time, the solution turned from orange to colorless. The solvent was removed in vacuo, and the residual was separated by TLC by using a 4:1 (v/v) hexane/ methylene chloride solvent mixture to give two products in order of elution: (1) a colorless band containing 3.6 mg of Re2(CO)8(μ-I)2, 14 4 (32% yield), and (2) a colorless band of Au(PPh3)I, 4.0 mg (26% yield). Spectral data for 4: IR νCO (cm −1 in methylene chloride): 2107(m), 2030(s), 1997(m), 1960(m). Mass Spec. EI/MS m/z: 850, M. Reaction of 3 with I2. A 3.2 mg (0.0126 mmol) portion of I2 was added to 30.0 mg (0.0125 mmol) of 3 that was dissolved in 10 mL of benzene. The solution was then stirred for 15 min at room temperature. During this time, the color of the solution turned from yellow to colorless. The solvent was removed in vacuo. The products were isolated by TLC by using a 4:1 hexane/methylene chloride (v/v) solvent mixture to give two products in order of elution: (1) a colorless band that contained 5.0 mg of 4 (23% yield) and (2) a colorless band of Au(PPh3)I, 9.8 mg (66% yield). Crystallographic Analyses. Red crystals of 1 suitable for X-ray diffraction analyses were obtained from a methylene chloride/hexane solution by slow evaporation of a solvent at 25 °C. Yellow crystals of 3 suitable for X-ray diffraction analyses were obtained from a benzene/ octane solution by slow evaporation of a solvent at 15 °C. X-ray intensity data were measured by using a Bruker SMART APEX CCDbased diffractometer by using Mo Kα radiation (λ = 0.71073 Å). The raw data frames were integrated with the SAINT+ program by using a narrow-frame integration algorithm. Corrections for Lorentz and polarization effects were also applied using 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 were refined by full-matrix least-squares on F by using the SHELXTL software package. Crystal data, data collection parameters, and results of the analyses are available in the Supporting Information. Computational Details. Density functional theory (DFT) calculations were performed with the Amsterdam Density Functional (ADF) suite of programs by using the PBEsol functional with the valence quadruple-ζ + 4 polarization function, relativistically optimized (QZ4P) basis sets for rhenium and gold, the valence triple-ζ + 2 polarization function (TZ2P) basis set for iodide, and double-ζ function (DZ) basis sets for the phosphorus, carbon, oxygen, and hydrogen atoms with nonfrozen cores. The molecular orbitals for 1−4 and Re2(CO)10 and their energies were determined by geometry-optimized calculations, with scalar relativistic corrections, that were initiated by using the atom positional parameters as determined from the crystal structure analyses. Electron densities at the bond critical points and Mayer bond orders were calculated by using the Bader quantum theory of atoms in molecules (QTAIM) model. Natural bond orbital (NBO) analyses were performed using the GENNBO 6.0 package embedded in ADF 2013. Time-dependent DFT (TDDFT) calculations were performed for models in the gas phase by using the PBEsol functional with the same basis sets. Figure 1. ORTEP diagram of the molecular structure of Re2(CO)8(μAuPPh3)2, 1, showing 40% thermal ellipsoid probability. The hydrogen atoms are omitted for clarity. Selected interatomic bond distances (Å) and angles (deg) are as follows: Re1−Au1 = 2.7914(2), Re1−Au1* = 2.7977(2), Re1−Re1* = 2.9070(3), Au1−P1 = 2.3308(11); Au1− Re1−Au1* = 117.320(6), Re1−Au1−Re1* = 62.681(6), P1−Au1− Re1 = 145.57(3), P1−Au1−Re1 = 147.68(3). Organometallics Article dx.doi.org/10.1021/om4010127 | Organometallics 2013, 32, 7540−7546 7541 D ow nl oa de d by T E X A S A & M U N IV C O L G S T A T IO N o n A ug us t 3 0, 2 01 5 | h ttp :// pu bs .a cs .o rg P ub lic at io n D at e (W eb ): N ov em be r 21 , 2 01 3 | d oi : 1 0. 10 21 /o m 40 10 12 7