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-tBuC(6)H(3))(2)pyridine 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, 1.15x10(-3)Lmol(-1)s(-1) at 221K, 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(sp(2))-C(sp(2)) elimination from three-coordinate ions [(Ar-1)(Ar-2)AuL](+) is almost barrier-free, particularly if L=phosphine.