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

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)V^IVCl], containing one dianionic [^Phap]^2- amidophenolate and one monoanionic [^Phisq]^•- iminosemiquinonate radical. Exposure of [(^Phap)(^Phisq)V^IVCl] to O₂ readily cleaves the O═O bond to generate [(^Phisq)(^Phibq)V^IV(O)Cl], containing an [^Phibq] iminobenzoquinone, so the 2e^- oxidation is entirely ligand centered. [(^Phisq)(^Phibq)V^IV(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)V^IVCl]. 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)V^IV(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)V^IV-(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.