Wingtip-Dictated Cyclometalation of N‑Heterocyclic Carbene Ligand Framework and Its Implication toward Tunable Catalytic Activity

Cyclometalative C–H activation within an N-heterocyclic carbene (NHC) ligand framework has been found to be wingtip-dictated under a competitive environment with two different N-substituents, namely, N-phenyl and N-pyridyl. It shows preferential C–H bond activation at one of the wingtip groups guided by the electronic nature of the C–H bond involved and the ring size of the metallacycle formed. Electrochemical analyses suggested that the resulting cyclometalated iridium­(III) and ruthenium­(II) complexes exhibit disparate electronic properties exerted by the different cyclometalating group (phenyl vs pyridyl). An implication of this electronic modulation in tunable catalytic activity was demonstrated in a model transfer hydrogenation reaction.