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 [Au2(μ-CH3)­(PMe2Ar′)2]­[NTf2], [Au2(μ-CH3)­(XPhos)2]­[NTf2], and [Au2(μ-CH3)(tBuXPhos)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-iPr)2, or
C6H3-2,6-(C6H2-2,4,6-iPr)2; XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl; tBuXPhos = 2-di-tert-butylphosphino-2′,4′,6′-triisopropylbiphenyl;
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 [Au2(μ-CH3)­(XPhos)2]­[NTf2] and [Au2(μ-CH3)­(tBuXPhos)2]­[NTf2], solid-state
X-ray structures have been elucidated. The rate of ethane formation
from [Au2(μ-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 [Au2(μ-CH3)­(PMe2Ar′)2]­[NTf2] to form Au­(PR2Ar′)­(NTf2) and
Au­(CH3)­(PMe2Ar′), (2) phosphine migration
from a second equivalent of [Au2(μ-CH3)­(PMe2Ar′)2]­[NTf2] aided
by binding of the Lewis acidic [Au­(PMe2Ar′)]+, formed in step 1, to produce [Au2(CH3)­(PMe2Ar′)]­[NTf2] and [Au2(PMe2Ar′)]+, and (3) recombination of
[Au2(CH3)­(PMe2Ar′)]­[NTf2] and Au­(CH3)­(PMe2Ar′) to eliminate
ethane.