Basicity-Controlled C-H Bond Activation by a Structurally Characterized Ni(III)-Hydroxo Complex.

Abstract

The selective oxidation of strong C-H bonds remains a central challenge in synthetic chemistry, in part due to the elusive nature of active oxidants and their underlying mechanisms. Herein, we report the isolation and complete characterization of a room-temperature-stable mononuclear Ni(III)-hydroxo complex, [Na(15c5)][Ni(PS3″)(OH)] ([Na(15c5)][2]), supported by a tris(benzenethiolato)phosphine ligand derivative. The X-ray crystallographic structure of 2 reveals a trigonal bipyramidal Ni(III) center, in which the coordinated hydroxo ligand is stabilized by secondary coordination sphere interactions. Complex 2 displays hydrogen atom transfer (HAT) reactivity toward strong C-H bonds, including that in cyclohexane (BDE = 99.5 kcal mol-1). Kinetic studies with various C-H substrates reveal a strong linear correlation between log(k2) and substrate pKa, but a poor correlation with C-H bond dissociation energies, indicating an asynchronous PCET pathway with a transition state predominantly governed by proton transfer (PT). The O-H bond dissociation free energy of a resulting Ni(II)-aqua species was estimated to be 96.6-100.3 kcal mol-1 based on thermodynamic data. A semiempirical free energy analysis following the approach of Barman et al. (Proc. Natl. Acad. Sci. U.S.A. 2021, 118, e2108648118) gives a best-fit x value of 0.18 (R2 = 0.99), where x = 1 indicates synchronous PCET and lower values reflect greater PT character in the transition state. These findings underscore the critical role of basicity in modulating PCET reactivity and establish complex 2 as a rare, well-defined Ni(III)-OH oxidant capable of strong C-H bond activation at low redox potential.