Redox-Active Ligands Permit Multielectron O Homolysis and O-Atom Transfer at Exceptionally High-Valent Vanadyl Complexes.

A five-coordinate chlorovanadium species supported by two redox-active -phenyl aminophenol ligands was prepared. Experimental and computational data support formulation of this complex as [(ap)(isq)VCl], containing one dianionic [ap] amidophenolate and one monoanionic [isq] iminosemiquinonate radical. Exposure of [(ap)(isq)VCl] to O readily cleaves the O═O bond to generate [(isq)(ibq)V(O)Cl], containing an [ibq] iminobenzoquinone, so the 2e oxidation is entirely ligand centered. [(isq)(ibq)V(O)Cl] is reduced by net H abstraction from 9,10-dihydroanthracene, or in reactions with main-group nucleophiles, such as PPh and MeS, which form a new bond to oxygen and regenerate [(ap)(isq)VCl]. Accordingly, the dioxygenase-type O activation and O-atom transfer cycling are a direct consequence of ligand redox noninnocence and covalency in the vanadium─aminophenol bonding. The reactions with O-atom donor and acceptor substrates establish a V≡O BDE of 73 ± 14 kcal mol in [(isq)(ibq)V(O)Cl]. Reported V≡O BDEs in redox-innocent vanadyl complexes typically fall in the range of 120-170 kcal mol. Unlike later 3d metals, where M═O species are typically high energy and activated by, for instance, occupancy of M-O π* antibonding MOs, the exceptionally weak V≡O bond in [(isq)(ibq)V-(O)Cl] reflects stabilization of the reduced product. Thus, this research highlights an alternative pathway to generating strong oxidants that are not strong outer-sphere electron acceptors, with implications for the design of early metal catalysts for aerobic oxidations of weak O-atom acceptors or strong X-H bonds.