Reductive Elimination Leading to C−C Bond Formation in Gold(III) Complexes: A Mechanistic and Computational Study

The factors affecting the rates of reductive C−C cross‐coupling reactions in gold(III) aryls were studied by using complexes that allow easy access to a series of electronically modified aryl ligands, as well as to gold methyl and vinyl complexes, by using the pincer compounds [(C^N^C)AuR] (R=C 6 F 5 , CH=CMe 2 , Me and p ‐C 6 H 4 X, where X=OMe, F, H, t Bu, Cl, CF 3 , or NO 2 ) as starting materials (C^N^C=2,6‐(4′‐ t BuC 6 H 3 ) 2 pyridine dianion). Protodeauration followed by addition of one equivalent SMe 2 leads to the quantitative generation of the thioether complexes [(C^N‐CH)AuR(SMe 2 )] + . Upon addition of a second SMe 2 pyridine is displaced, which triggers the reductive aryl−R elimination. The rates for these cross‐couplings increase in the sequence k (vinyl)> k (aryl)≫ k (C 6 F 5 )> k (Me). Vinyl−aryl coupling is particularly fast, 1.15×10 −3  L mol −1  s −1 at 221 K, whereas both C 6 F 5 and Me couplings encountered higher barriers for the C−C bond forming step. The use of P( p ‐tol) 3 in place of SMe 2 greatly accelerates the C−C couplings. Computational modelling shows that in the C^N‐bonded compounds displacement of N by a donor L is required before the aryl ligands can adopt a conformation suitable for C−C bond formation, so that elimination takes place from a four‐coordinate intermediate. The C−C bond formation is the rate‐limiting step. In the non‐chelating case, reductive C(sp 2 )−C(sp 2 ) elimination from three‐coordinate ions [(Ar 1 )(Ar 2 )AuL] + is almost barrier‐free, particularly if L=phosphine.