Enhanced Proton-Coupled Electron-Transfer Reactivity by a Mononuclear Nickel(II) Hydroxide Radical Complex

The synthesis, characterization, and reactivity of a NiOH core bearing a tridentate redox-active ligand capable of reaching three molecular oxidation states is presented in this paper. The reduced complex [LNiOH]^2- was characterized by single-crystal X-ray diffraction analysis, depicting a square-planar NiOH core stabilized by intramolecular H-bonding interactions. Cyclic voltammetry measurements indicated that [LNiOH]^2- can be reversibly oxidized to [LNiOH]^- and [LNiOH] at very negative reduction potentials (-1.13 and -0.39 V vs ferrocene, respectively). The oxidation of [LNiOH]^2- to [LNiOH]^- and [LNiOH] was accomplished using 1 and 2 equiv of ferrocenium, respectively. Spectroscopic and computational characterization suggest that [LNiOH]^2-, [LNiOH]^-, and [LNiOH] are all Ni^II species in which the redox-active ligand adopts different oxidation states (catecholate-like, semiquinone-like, and quinone-like, respectively). The NiOH species were found to promote H-atom abstraction from organic substrates, with [LNiOH]^- acting as a 1H⁺/1e^- oxidant and [LNiOH] as a 2H⁺/2e^- oxidant. Thermochemical analysis indicated that [LNiOH] was capable of abstracting H atoms from stronger O-H bonds than [LNiOH]^-. However, the greater thermochemical tendency of [LNiOH] reactivity toward H atoms did not align with the kinetics of the PCET reaction, where [LNiOH]^- reacted with H-atom donors much faster than [LNiOH]. The unique stereoelectronic structure of [LNiOH]^- (radical character combined with a basic NiOH core) might account for its enhanced PCET reactivity.