Redox-Active Ligands Permit Multielectron O₂ Homolysis and O-Atom Transfer at Exceptionally High-Valent Vanadyl Complexes

A five-coordinate chlorovanadium species supported by two redox-active N-phenyl aminophenol ligands was prepared. Experimental and computational data support formulation of this complex as [(Phap)(Phisq)VIVCl], containing one dianionic [Phap]2- amidophenolate and one monoanionic [Phisq]•- iminosemiquinonate radical. Exposure of [(Phap)(Phisq)VIVCl] to O2 readily cleaves the O═O bond to generate [(Phisq)(Phibq)VIV(O)Cl], containing an [Phibq] iminobenzoquinone, so the 2e- oxidation is entirely ligand centered. [(Phisq)(Phibq)VIV(O)Cl] is reduced by net H2 abstraction from 9,10-dihydroanthracene, or in reactions with main-group nucleophiles, such as PPh3 and Me2S, which form a new bond to oxygen and regenerate [(Phap)(Phisq)VIVCl]. Accordingly, the dioxygenase-type O2 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-1 in [(Phisq)(Phibq)VIV(O)Cl]. Reported V≡O BDEs in redox-innocent vanadyl complexes typically fall in the range of 120-170 kcal mol-1. 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 [(Phisq)(Phibq)VIV-(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.