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=C6F5, CH=CMe2, Me and p-C6H4X, where X=OMe, F, H, tBu, Cl, CF3, or NO2) as starting materials (C^N^C=2,6-(4′-tBuC6H3)2pyridine dianion). Protodeauration followed by addition of one equivalent SMe2 leads to the quantitative generation of the thioether complexes [(C^N-CH)AuR(SMe2)]+. Upon addition of a second SMe2 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(C6F5)>k(Me). Vinyl−aryl coupling is particularly fast, 1.15×10−3 L mol−1 s−1 at 221 K, whereas both C6F5 and Me couplings encountered higher barriers for the C−C bond forming step. The use of P(p-tol)3 in place of SMe2 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(sp2)−C(sp2) elimination from three-coordinate ions [(Ar1)(Ar2)AuL]+ is almost barrier-free, particularly if L=phosphine.