A New Class of Remote N-Heterocyclic Carbenes with Exceptionally Strong σ-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. Classy ligand: Benzo[c]quinolin‐6‐ylidene is introduced as a new, strongly electron‐donating, carbene ligand. Synthetic routes to metal complexes of this ligand and experimental evidence for its strong donor properties are outlined (see figure).