Thermochemistry and Tunneling Enhance the PCET Reactivity of Ni and Cu Complexes With Redox-Active Ligands

Abstract

Herein, we report the synthesis, characterization, and reactivity of Cu-acetate and Ni-acetate complexes bound by tridentate redox-active ligands capable of reaching three molecular oxidation states. The reduced complexes ([XMOAc]2–) were characterized by single-crystal X-ray diffraction analysis, depicting metal-acetate cores stabilized by intramolecular H-bonding interactions. Cyclic voltammetry measurements indicate that [XMOAc]2– can be reversibly oxidized to [XMOAc]– and [XMOAc], and that the reduction potentials varied depending on the ligand substitution. The oxidation of [XMOAc]2– to [XMOAc]– and [XMOAc] was accomplished using 1 and 2 equiv of ferrocenium, respectively. The proton-coupled electron transfer reactivity of the oxidized complexes was analyzed in detail. Thermochemical analysis determined that the [XMOAc] species had higher affinities toward H atom equivalents than [XMOAc]–, and that the systems with electron-withdrawing groups were more prone to undergo reductive protonation. The kinetics for the reactions of [XMOAc]– and [XMOAc] with TEMPOH (1-hydroxy-2,2,6,6-tetramethylpiperidine) showed that the [XNiOAc]– complexes depict enhanced reaction rates. These findings contradict our thermochemical analysis (i.e., complexes with higher thermodynamic tendency to accept H atoms usually react faster) and are explained by tunneling effects. Further analysis of the PCET reactions provided further evidence that nonthermodynamic factors govern the PCET reactivity of the Ni-semiquinone oxidants.