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(C(6)F(5))](+) (1(+)) reacts with main group hydrides H–BPin and H–SiEt(3) in dichloromethane solution at –70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N–CH = 2-(C(6)H(3)Bu(t))-6-(C(6)H(4)Bu(t))pyridine anion; Pin = OCMe(2)CMe(2)O). 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)AuC(6)F(5)}(2)(μ-H)](+) (2(+)) and (C^N–CH)AuH(C(6)F(5)) (5), together with spectroscopically detected [Et(3)Si–OEt(2)](+). The activation of dihydrogen also involves heterolytic H–H bond cleavage but requires a higher temperature (–20 °C). H(2) activation proceeds in two mechanistically distinct steps: the first leading to 2 plus [H(OEt(2))(2)](+), the second to protonation of one of the C^N pyridine ligands and reductive elimination of C(6)F(5)H. 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(2) 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.