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-2(mu-CH3)(PMe2Ar')(2)][NTf2], [Au-2(mu-CH3)(XPhos)(2)] [NTf2], and [Au-2(mu-CH3)((t)BuXPhos)(2)][NTf2] {Ar' = C6H3-2,6-(C6H3-2,6-Me)(2), C6H3-2,6-(C6H2-2,4,6-Me)(2), C6H3-2,6-(C6H3-2,6-Pr-i)(2), or C6H3-2,6-(C6H2-2,4,6-Pr-i)(2); XPhos = 2-dicyclohexylphosphino-2',4',6'-triisopro-pylbiphenyl; (t)BuXPhos = 2-di-tert-butylphosphino-2',4',6'-triisopro-pylbiphenyl; NTf2 = bis(trifluoromethyl sulfonylimide)}. The gem-digold methyl complexes are synthesized through reaction between Au(CH3)L and Au(L)(NTf2) {L = phosphines listed above}. For [Au-2(mu-CH3)(XPhos)(2)] [NTf2] and [Au-2(mu-CH3)((t)BuXPhos)(2)] [NTf2], solid-state X-ray structures have been elucidated. The rate of ethane formation from [Au-2(mu-CH3)(PMe2Ar')(2)][NTf2] 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(mu-CH3)(PMe2Ar')(2) ][NTf2] to form Au(PR2Ar')(NTf2) and Au(CH3)(PMe2Ar'), (2) phosphine migration from a second equivalent of [Au-2(mu-CH3)(PMe2Ar')(2)][NTf2] aided by binding of the Lewis acidic [Au(PMe2Ar')](+), formed in step 1, to produce [Au-2(CH3)(PMe2Ar')][NTf2] and [Au-2(PMe2Ar')](+), and (3) recombination of [Au-2(CH3)(PMe2Ar')][NTf2] and Au(CH3)(PMe2Ar') to eliminate ethane.

Automated summary

Organometallic gold complexes are used as catalyst precursors in C-C bond formation reactions. This study investigates C-C coupling to form ethane from various phosphine-ligated gem-digold(I) methyl complexes, showing that the rate of ethane formation increases with decreasing steric bulk of the phosphine substituent.