Opposite effects of added AsPh3 reveal a drastic mechanistic switch in RhI/AuI transmetalations via Rh–Au bonded intermediates

Abstract

In contrast with the previously reported decelerating effect of added L (L = AsPh3) on the Rf/Pf exchange reaction between [Au(Pf)L] and trans-[Rh(Rf)(CO)L2] (Pf = C6F5; Rf = C6F3Cl2-3,5), the Rf/Cl exchange between [AuClL] and trans-[Rh(Rf)(CO)L2] 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 AsPh3. 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.