Preparation and Characterization of a Formally Ni<sup>IV</sup>–Oxo Complex with a Triplet Ground State and Application in Oxidation Reactions
Deepika G. Karmalkar, Virginia A. Larson, Deesha D. Malik, Yong‐Min Lee, Mi Sook Seo, Jin Kim, Dovydas Vasiliauskas, Jason Shearer, Nicolai Lehnert, Wonwoo Nam
Abstract
High-valent first-row transition-metal–oxo complexes are important intermediates in biologically and chemically relevant oxidative transformations of organic molecules and in the water splitting reaction in (artificial) photosynthesis. While high-valent Fe– and Mn–oxo complexes have been characterized in detail, much less is known about their analogues with late transition metals. In this study, we present the synthesis and detailed characterization of a unique mononuclear terminal Ni–O complex. This compound, [Ni(TAML)(O)(OH)]3–, is characterized by an intense charge-transfer (CT) band around 730 nm and has an St = 1 ground state, as determined by magnetic circular dichroism spectroscopy. From extended X-ray absorption fine structure (EXAFS), the Ni–O bond distance is 1.84 Å. Ni K edge XAS data indicate that the complex contains a Ni(III) center, which results from an unusually large degree of Ni–O π-bond inversion, with one hole located on the oxo ligand. The complex is therefore best described as a low-spin Ni(III) complex (S = 1/2) with a bound oxyl (O•–) ligand (S = 1/2), where the spins of Ni and oxyl are ferromagnetically coupled, giving rise to the observed St = 1 ground state. This bonding description is roughly equivalent to the presence of a Ni–O single (σ) bond. Reactivity studies show that [Ni(TAML)(O)(OH)]3– is a strong oxidant capable of oxidizing thioanisole and styrene derivatives with large negative ρ values in the Hammett plot, indicating its electrophilic nature. The intermediate also shows high reactivity in C–H bond activation of hydrocarbons with a kinetic isotope effect of 7.0(3) in xanthene oxidation.