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

[Image: see text] 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(2)(μ-CH(3))(PMe(2)Ar′)(2)][NTf(2)], [Au(2)(μ-CH(3))(XPhos)(2)][NTf(2)], and [Au(2)(μ-CH(3))((t)BuXPhos)(2)][NTf(2)] {Ar′ = C(6)H(3)-2,6-(C(6)H(3)-2,6-Me)(2), C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Me)(2), C(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr)(2), or C(6)H(3)-2,6-(C(6)H(2)-2,4,6-(i)Pr)(2); XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl; (t)BuXPhos = 2-di-tert-butylphosphino-2′,4′,6′-triisopropylbiphenyl; NTf(2) = bis(trifluoromethyl sulfonylimide)}. The gem-digold methyl complexes are synthesized through reaction between Au(CH(3))L and Au(L)(NTf(2)) {L = phosphines listed above}. For [Au(2)(μ-CH(3))(XPhos)(2)][NTf(2)] and [Au(2)(μ-CH(3))((t)BuXPhos)(2)][NTf(2)], solid-state X-ray structures have been elucidated. The rate of ethane formation from [Au(2)(μ-CH(3))(PMe(2)Ar′)(2)][NTf(2)] 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(2)(μ-CH(3))(PMe(2)Ar′)(2)][NTf(2)] to form Au(PR(2)Ar′)(NTf(2)) and Au(CH(3))(PMe(2)Ar′), (2) phosphine migration from a second equivalent of [Au(2)(μ-CH(3))(PMe(2)Ar′)(2)][NTf(2)] aided by binding of the Lewis acidic [Au(PMe(2)Ar′)](+), formed in step 1, to produce [Au(2)(CH(3))(PMe(2)Ar′)][NTf(2)] and [Au(2)(PMe(2)Ar′)](+), and (3) recombination of [Au(2)(CH(3))(PMe(2)Ar′)][NTf(2)] and Au(CH(3))(PMe(2)Ar′) to eliminate ethane.