Rotation-Triggered Transmetalation on a Heterobimetallic Cu/Al N-Phosphine-Oxide-Substituted Imidazolylidene Complex

A novel strategy for the preparation of heterobimetallic N-heterocyclic carbene (NHC) complexes is demonstrated using N-phosphine-oxide-substituted imidazolylidenes (PoxIms). In these heterobimetallic Cu/Al complexes, the Cu and Al centers can be either completely separated or brought near each other via the rotation of the N-phosphinoyl group in the PoxIm ligands. Triggered by this rotation, transmetalation to exchange the Cu-O^tBu and Al-C₆F₅ bonds occurs on in situ-generated Cu/Al PoxIm complexes, and the Cu-C₆F₅ and Al-O^tBu bonds are formed in excellent yield. On the basis of the results of mechanistic studies, including the isolation/in situ observation of key complexes and theoretical calculations, a plausible reaction mechanism for an intramolecular transmetalation is proposed to proceed via an activation complex that includes the simultaneous coordination of the phosphinoyl oxygen atom to the Cu as well as the Al centers. Furthermore, the formation of carbon-carbon bonds between Al(C₆F₅)₃ and allyl bromide mediated/catalyzed by Cu/Al PoxIm complexes is demonstrated. Upon the consecutive transfer of three C₆F₅ groups from a single molecule of Al(C₆F₅)₃, allyl pentafluorobenzene derivatives were obtained. The present results demonstrate the role of phosphine oxide in the activation of organoaluminum reagents for the transmetalation between Cu(I) complexes bearing NHCs as well as the benefit of constructing an intramolecular system based on a heterobimetallic complex to achieve efficient transmetalation by programming the encounter of two organometallic fragments.