A New Class of Remote N-Heterocyclic Carbenes with Exceptionally Strong sigma-Donor Properties: Introducing Benzo[c]quinolin-6-ylidene

We make the case for benzo[c]quinolin-6-ylidene (1) as a strongly electron-donating carbene ligand. The facile synthesis of 6-trifluoromethanesulfonylbenzo[c]quinolizinium trifluoromethanesulfonate (2) gives straightforward access to a useful precursor for oxidative addition to low-valent metals, to yield the desired carbene complexes. This concept has been achieved in the case of [Mn(benzo[c]quinolin-6-ylidene)(CO)(5)](+) (15) and [Pd(benzo[c]quinolin-6-ylidene)(PPh3)(2)(L)](2+) L=THF (21), OTf (22) or pyridine (23). Attempts to coordinate to nickel result in coupling products from two carbene precursor fragments. The CO IR-stretching-frequency data for the manganese compound suggests benzo[c]quinolin-6-ylidene is at least as strong a donor as any heteroatom-stabilised carbene ligand reported.