Heterolytic bond activation at gold: evidence for gold(iii) H-B, H-Si complexes, H-H and H-C cleavage

The coordinatively unsaturated gold(iii) chelate complex [(C^N-CH)Au(C₆F₅)]⁺ (1 ⁺) reacts with main group hydrides H-BPin and H-SiEt₃ in dichloromethane solution at -70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N-CH = 2-(C₆H₃Bu )-6-(C₆H₄Bu )pyridine anion; Pin = OCMe₂CMe₂O). In the presence of an external base such as diethyl ether, heterolytic cleavage of the silane H-Si bond leads to the gold hydrides [{(C^N-CH)AuC₆F₅}₂(μ-H)]⁺ (2 ⁺) and (C^N-CH)AuH(C₆F₅) (5), together with spectroscopically detected [Et₃Si-OEt₂]⁺. The activation of dihydrogen also involves heterolytic H-H bond cleavage but requires a higher temperature (-20 °C). H₂ activation proceeds in two mechanistically distinct steps: the first leading to 2 plus [H(OEt₂)₂]⁺, the second to protonation of one of the C^N pyridine ligands and reductive elimination of C₆F₅H. By comparison, formation of gold hydrides by cleavage of suitably activated C-H bonds is very much more facile; e.g. the reaction of 1·OEt with Hantzsch ester is essentially instantaneous and quantitative at -30 °C. This is the first experimental observation of species involved in the initial steps of gold catalyzed hydroboration, hydrosilylation and hydrogenation and the first demonstration of the ability of organic C-H bonds to act as hydride donors towards gold.