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.