Reductive C-C Coupling from Molecular Au(I) Hydrocarbyl Complexes: A Mechanistic Study.

Organometallic gold complexes are used in a range of catalytic reactions, and they often serve as catalyst precursors that mediate C-C bond formation. In this study, we investigate C-C coupling to form ethane from various phosphine-ligated gem-digold(I) methyl complexes including [Au(μ-CH)(PMeAr')][NTf], [Au(μ-CH)(XPhos)][NTf], and [Au(μ-CH)(BuXPhos)][NTf] {Ar' = CH-2,6-(CH-2,6-Me), CH-2,6-(CH-2,4,6-Me), CH-2,6-(CH-2,6-Pr), or CH-2,6-(CH-2,4,6-Pr); XPhos = 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl; BuXPhos = 2-di--butylphosphino-2',4',6'-triisopropylbiphenyl; NTf = bis(trifluoromethyl sulfonylimide)}. The gem-digold methyl complexes are synthesized through reaction between Au(CH)L and Au(L)(NTf) {L = phosphines listed above}. For [Au(μ-CH)(XPhos)][NTf] and [Au(μ-CH)(BuXPhos)][NTf], solid-state X-ray structures have been elucidated. The rate of ethane formation from [Au(μ-CH)(PMeAr')][NTf] increases as the steric bulk of the phosphine substituent Ar' decreases. Monitoring the rate of ethane elimination reactions by multinuclear NMR spectroscopy provides evidence for a second-order dependence on the gem-digold methyl complexes. Using experimental and computational evidence, it is proposed that the mechanism of C-C coupling likely involves (1) cleavage of [Au(μ-CH)(PMeAr')][NTf] to form Au(PRAr')(NTf) and Au(CH)(PMeAr'), (2) phosphine migration from a second equivalent of [Au(μ-CH)(PMeAr')][NTf] aided by binding of the Lewis acidic [Au(PMeAr')], formed in step 1, to produce [Au(CH)(PMeAr')][NTf] and [Au(PMeAr')], and (3) recombination of [Au(CH)(PMeAr')][NTf] and Au(CH)(PMeAr') to eliminate ethane.