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(CF)] ( ) reacts with main group hydrides H-BPin and H-SiEt in dichloromethane solution at -70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N-CH = 2-(CHBu )-6-(CHBu )pyridine anion; Pin = OCMeCMeO). 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)AuCF}(μ-H)] ( ) and (C^N-CH)AuH(CF) (), together with spectroscopically detected [EtSi-OEt]. The activation of dihydrogen also involves heterolytic H-H bond cleavage but requires a higher temperature (-20 °C). H activation proceeds in two mechanistically distinct steps: the first leading to plus [H(OEt)], the second to protonation of one of the C^N pyridine ligands and reductive elimination of CFH. By comparison, formation of gold hydrides by cleavage of suitably activated C-H bonds is very much more facile; the reaction of 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.