Opposite Effects of Added AsPh Reveal a Drastic Mechanistic Switch in Rh/Au Transmetalations via Rh-Au Bonded Intermediates.

In contrast with the previously reported decelerating effect of added L (L = AsPh) on the Rf/Pf exchange reaction between [Au(Pf)L] and -[Rh(Rf)(CO)L] (Pf = CF; Rf = CFCl-3,5), the Rf/Cl exchange between [AuClL] and -[Rh(Rf)(CO)L] is accelerated by addition of an excess of L. By combining experimental data and microkinetic modeling, with DFT calculations, the unexpected existence of two cooperative Rf/Cl exchange mechanisms is demonstrated. The opposite kinetic effects of L addition, from negative in the Rf/Pf exchange process, opposing L dissociation in an octahedral rhodium intermediate, to positive in the Rf/Cl exchange, opening an L-catalyzed alternative pathway via tricoordinate gold intermediates, explain the Janus effect of AsPh. The three transmetalation pathways involve a metal redox-insertion step with accessible activation barrier, producing intermediates with Rh-Au bonds. Whereas our previously reported Rf/Pf exchange implied Rh(I) oxidation by Au(I), the Rf/Cl exchange mechanism involves Au(I) oxidation by Rh(I). Further support is provided by NBO studies, which reveal remarkable electronic donations from the oxidized metal in each case forging the M-M' covalent interaction in the intermediates yielded by the redox-insertion step.