Elsevier

Journal of Organometallic Chemistry

Volume 795, 15 October 2015, Pages 40-44
Journal of Organometallic Chemistry

Synthesis, structure and bonding of a digold complex with bridging triphenylstannyl ligands

Dedicated to Professor Hubert Schmidbaur on the occasion of his 80th birthday.
https://doi.org/10.1016/j.jorganchem.2015.01.021Get rights and content

Highlights

  • The new digold–ditin complex [Au(PPh3) (μ–SnPh3)]2 contains two bridging SnPh3 groups.

  • DFT analysis shows the Au–Sn bonding involves delocalized four center: Two electron bond.

  • The two Au(PPh3) groups are joined by a strong Au–Au bond.

Abstract

The reaction of Au(PPh3)Ph with HSnPh3 yielded the new digold–ditin complex, [Au(PPh3)(μ–SnPh3)]2, 6 in 52% yield. Benzene was also formed. Sn2Ph6 is a major coproduct that was formed by the degradation of 6. Compound 6 was characterized structurally by a single-crystal X-ray diffraction analysis. The molecule contains two Au(PPh3) groups that are joined by a strong Au–Au bond (2.5590(5) Å in length) that is bridged by two SnPh3 groups. The metal–metal bonding was analyzed by DFT Mo calculations. The Au–Sn bonding is represented by the HOMO which is a four center: two electron bond.

Graphical abstract

The new digold–ditin complex, [Au(PPh3) (μ–SnPh3)]2, 6 was synthesized and characterized by a single-crystal X-ray diffraction analysis. The molecule contains a strong Au–Au bond (2.5590(5) Å in length) that is bridged by two SnPh3 groups.

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Introduction

Interest in the organometallic chemistry of gold has grown rapidly in recent years following the discoveries that gold nanoparticles supported on metal oxides exhibit surprisingly high activity for the catalytic oxidation of CO and selected hydrocarbons [1]. Studies have also shown bimetallic catalysts containing gold exhibit even better activity for oxidation catalysis [2]. Tin is well known for its ability to serve as a modifier of heterogeneous metal catalysts [3]. A recent study has shown that Au/SnO2 catalyst exhibits oxidation activity comparable to Au/TiO2, one of the most active catalytic gold oxidation systems [4]. Gold complexes can also perform novel forms of dual catalysis homogeneously when combined with other metals, most notably with palladium [5].

There are very few examples of organometallic gold–tin complexes in the literature. Examples with terminally-coordinated tin ligands include Au(PMe2Ph)2(SnCl3), 1, Au–Sn = 2.881(1) Å [6] and Au(PPh3)[Sn{N(p-tol)SiMe2}3SiMe], 2 [7], see Scheme 1.

Compounds [Au4(PPh3)4(μ-SnCl3)2], 3[8] and Au4(PPh3)4[μ-SnCB10H11]2, 4[9] and the dianion compound [Bu3NH]2[(PPh3)Au(μ-SnB11H11)]2, 5[10] have bridging Sn ligands, Scheme 2.

We have recently found that aryl containing gold phosphine complexes, Au(PPh3)Ar, Ar = Ph, Np, Py, react with certain dirhenium [11] and triosmium [12] carbonyl complexes to yield electronically unsaturated complexes containing bridging aryl ligands. In the present work, we have investigated the reaction of Au(PPh3)Ph with HSnPh3. The principal product is a new digold–ditin complex [Au(PPh3) (μ-SnPh3)]2, 6 that contains two bridging SnPh3 ligands. The results of our studies of the synthesis, structural characterization and bonding in this compound are described in this report.

Section snippets

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. 1H NMR and 31P{1H} NMR were recorded on a Varian Mercury 400 spectrometer operating at 400.1 and 161.9 MHz respectively. 31P{1H} NMR spectra were referenced externally by using 85% ortho-H3PO4. Solid state 119Sn cross-polarization magic angle spinning (CP-MAS) spectra were collected on a Bruker Avance III-HD

Reaction of HSnPh3 with Au(PPh3)Ph

30.0 mg (0.026 mmol) of Au(PPh3)Ph was added to 18.6 mg (0.052 mmol) of HSnPh3 dissolved in 10 mL of benzene. The solution was allowed to stir for 7 h at room temperature. A red-orange solution formed and the solvent was then removed in vacuo. The residue was extracted in methylene chloride and separated by TLC by using hexane solvent to elute a yellow band of [Au(PPh3)(μ-SnPh3)]2, 6 18.8 mg (52%) and Sn2Ph6, 8.6 mg (46%). The orange crystals of pure 6 can be physically separated from the

Crystallographic analysis

Orange single crystals of 6 crystallize together with colorless crystals of Sn2Ph6 (a decomposition product) upon slow evaporation of solvent from a solution in methylene chloride at 15 °C. An orange crystal of 6 was glued onto the end of a thin glass fiber. X-ray intensity data were measured by using a Bruker SMART APEX CCD-based 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

Computational details

Density functional theory (DFT) calculations were performed with the Amsterdam Density Functional (ADF) suite of programs [16] by using the PBEsol functional [17] with valence quadruple-ζ + 4 polarization function, relativistically-optimized (QZ4P) basis sets for the gold, tin, phosphorus, carbon and hydrogen atoms with frozen cores. The molecular orbitals for compound 6 and their energies were determined by geometry-optimized calculations with scalar relativistic corrections that were

Results and discussion

The reaction of Au(PPh3)Ph with HSnPh3 yielded the new digold–ditin complex, [Au(PPh3)(μ-SnPh3)]2, 6 in 52% yield. Sn2Ph6 was a major coproduct that was obtained in 46% yield. Sn2Ph6 was subsequently found to be formed by the degradation of 6. The formation of benzene was observed when the reaction was performed in an NMR tube in CD2Cl2 solvent. Compound 6 was characterized structurally by a single-crystal X-ray diffraction analysis. An ORTEP diagram of its molecular structure is shown in Fig. 1

Conclusions

Compound 6 is a dimer of the formula unit “Au(PPh3)(SnPh3)”. The molecule is held together by a strong direct Au–Au σ-bonding interaction and two bridging SnPh3 ligands. The Au–Sn bonding is best described by a four center – two electron bond having a node along the Au–Au vector.

Acknowledgments

This research was supported by the following grants from the National Science Foundation: CHE-1111496 and CHE-1048629. Many thanks to Professor John Fackler for helpful discussions.

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