The Role of the Redox Non-Innocent Hydroxyl Ligand in the Activation of O₂ Performed by [Ni(H)(OH)]⁺

Abstract

The cationic complex [Ni(H)(OH)](+) was previously found to activate dioxygen and methane in gas phase under single collision conditions. These remarkable reactivities were thought to originate from a non-classical electronic structure, where the Ni-center adopts a Ni(II), instead of the classically expected Ni(III) oxidation state by formally accepting an electron from the hydroxo ligand, which formally becomes a hydroxyl radical in the process. Such radicaloid oxygen moieties are envisioned to easily react with otherwise inert substrates, mimicking familiar reactivities of free radicals. In this study, the reductive activation of dioxygen by [Ni(H)(OH)](+) to afford the hydroperoxo species was investigated using coupled cluster, multireference ab initio and density functional theory calculations. Orbital and wave function analyses indicate that O-2 binding tranforms the aforementioned non-classical electronic structure to a classical Ni(III)-hydroxyl system, before O-2 reduction takes place. Remarkably, we found no evidence for a direct involvement of the radicaloid hydroxyl in the reaction with O-2, as is often assumed. The function of the redox non-innocent character of the activator complex is to protect the reactive electronic structure until the complex engages O-2, upon which a dramatic electronic reorganization releases internal energy and drives the chemical reaction to completion.