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- A nd 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.
Bibliographical noteFunding Information:
This work was supported by a grant from the National Science Foundation (CHE-1900380 to N.L. and J.S.). This work was also supported by the NRF of Korea through CRI (NRF-2021R1A3B1076539 to W.N.) and the Basic Science Research Program (NRF-2020R1I1A1A01074630 to Y.M.L. and NRF-2019R1I1A1A01055822 to M.S.S.). V.A.L. acknowledges support from a University of Michigan Rackham Predoctoral Fellowship.
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