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

NiIII (1-ox) and CuIII (2-ox) species, supported
by a bis-amidate-dioxime ligand scaffold,
were synthesized via one-electron oxidation of NiII (1) and CuII (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 NiII and CuII complexes adopt a similar geometry around the
metal center, while the CuIII complex exhibited significantly
shorter metal–ligand bond distances in the solid state relative
to CuII. 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 NiIII/NiII redox couple is anodically shifted by ca.
350 mV compared to the CuIII/CuII 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
NiIII/CuIII 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.

以双酰胺-二肟配体骨架负载的三价镍（NiIII，1-ox）与三价铜（CuIII，2-ox）物种，通过在-40℃甲醇体系中以硝酸铈铵对二价镍（NiII，1）和二价铜（CuII，2）进行单电子氧化而合成得到。上述物种通过包括X射线吸收光谱在内的多种光谱技术进行了全面表征。X射线结构分析显示，二价镍与二价铜配合物在金属中心周围具有相似的几何构型，而固态下的三价铜配合物的金属-配体键长相较于二价铜显著更短。X射线吸收近边结构（X-ray absorption near-edge structure, XANES）研究表明，在归一化吸光度为0.5时，样品1（8343.42 eV）与1-ox（8344.07 eV）之间存在0.65 eV的能量偏移；而将样品2（8979.40 eV）氧化为2-ox（8981.09 eV）则产生了1.65 eV的偏移，证实了其氧化态发生了单单位变化。电化学分析结果显示，相较于三价铜/二价铜的氧化还原电位，三价镍/二价镍的氧化还原对发生了约350 mV的阳极偏移。进一步研究了1-ox与2-ox与烟酰胺腺嘌呤二核苷酸磷酸（NADPH）类似物BNAH的反应活性，动力学分析证实其反应遵循氢转移（HT）路径。研究发现1-ox的反应速率约为2-ox的11倍。两个反应均表现出较高的一级动力学同位素效应（1-ox：7.3；2-ox：11.2）。此外，还考察了1-ox与2-ox与TEMPOH的反应动力学，结果表明其遵循协同质子-电子转移（CPET）机制，且1-ox的反应速率显著高于2-ox。相较于2-ox，1-ox所增强的HT/CPET反应活性可归因于其更高的氧化还原驱动力。本研究揭示了一种涉及三价镍/三价铜物种的独特氢转移机制，这与众多金属-氧体系中所观察到的传统范式截然不同——后者的限速步骤为氢原子转移，随后伴随快速电子转移。