Comparing the Electronic Structure and Hydride Atom Transfer Reactivities of Nickel(III) vs Cu(III) Complexes.

Ni () and Cu () species, supported by a bis-amidate-dioxime ligand scaffold, were synthesized via one-electron oxidation of Ni () and Cu () using ceric ammonium nitrate in methanol at -40 °C. These species were extensively characterized by various spectroscopic tools, including X-ray absorption spectroscopy. X-ray structural analysis revealed that Ni and Cu complexes adopt a similar geometry around the metal center, while the Cu complex exhibited significantly shorter metal-ligand bond distances in the solid state relative to Cu. X-ray absorption near-edge structure (XANES) studies showed an energy shift of 0.65 eV at normalized 0.5 absorption between (8343.42 eV) and (8344.07 eV), whereas oxidation of (8979.40 eV) to (8981.09 eV) resulted in a shift of 1.65 eV, confirming a one-unit oxidation state change. The electrochemical analysis demonstrated that the Ni/Ni redox couple is anodically shifted by ca. 350 mV compared to the Cu/Cu potential. The reactivity of and with BNAH, an NADPH analog, were further analyzed, and kinetic analysis confirmed a hydride transfer (HT) pathway. The reaction of was found ca. 11 times faster than that of . Both reactions exhibited a high primary kinetic isotope effect (: 7.3; : 11.2). Additionally, the kinetics of and were examined with TEMPOH, indicating a concerted proton-electron transfer (CPET) mechanism. The reaction rate of was significantly higher than that of . The enhanced HT/CPET reactivity of relative to is attributed to its greater redox driving force. This work highlights a distinct HT mechanism involving Ni/Cu species, diverging from the conventional paradigm observed in many metal-oxo systems, where a rate-limiting hydrogen atom transfer is followed by a rapid electron transfer.