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

Ni^III (1-ox) and Cu^III (2-ox) species, supported by a bis-amidate-dioxime ligand scaffold, were synthesized via one-electron oxidation of Ni^II (1) and Cu^II (2) 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^II and Cu^II complexes adopt a similar geometry around the metal center, while the Cu^III complex exhibited significantly shorter metal-ligand bond distances in the solid state relative to Cu^II. X-ray absorption near-edge structure (XANES) studies showed an energy shift of 0.65 eV at normalized 0.5 absorption between 1 (8343.42 eV) and 1-ox (8344.07 eV), whereas oxidation of 2 (8979.40 eV) to 2-ox (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^III/Ni^II redox couple is anodically shifted by ca. 350 mV compared to the Cu^III/Cu^II potential. The reactivity of 1-ox and 2-ox with BNAH, an NADPH analog, were further analyzed, and kinetic analysis confirmed a hydride transfer (HT) pathway. The reaction of 1-ox was found ca. 11 times faster than that of 2-ox. Both reactions exhibited a high primary kinetic isotope effect (1-ox: 7.3; 2-ox: 11.2). Additionally, the kinetics of 1-ox and 2-ox were examined with TEMPOH, indicating a concerted proton-electron transfer (CPET) mechanism. The reaction rate of 1-ox was significantly higher than that of 2-ox. The enhanced HT/CPET reactivity of 1-ox relative to 2-ox is attributed to its greater redox driving force. This work highlights a distinct HT mechanism involving Ni^III/Cu^III 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.