Electronic Structural Analysis of Copper(II)–TEMPO/ABNO Complexes Provides Evidence for Copper(I)–Oxoammonium Character

Copper/aminoxyl species are proposed as key intermediates in aerobic alcohol oxidation. Several possible electronic structural descriptions of these species are possible, and the present study probes this issue by examining four crystallographically characterized Cu/aminoxyl halide complexes by Cu K-edge, Cu L2,3-edge, and Cl K-edge X-ray absorption spectroscopy. The mixing coefficients between Cu, aminoxyl, and halide orbitals are determined via these techniques with support from density functional theory. The emergent electronic structure picture reveals that Cu coordination confers appreciable oxoammonium character to the aminoxyl ligand. The computational methodology is extended to one of the putative intermediates invoked in catalytic Cu/aminoxyl-driven alcohol oxidation reactions, with similar findings. Collectively, the results have important implications for the mechanism of alcohol oxidation and the underlying basis for cooperativity in this co-catalyst system.

铜/硝酰（aminoxyl）物种被提出为好氧醇氧化反应中的关键中间体。针对这类物种，目前已存在多种可行的电子结构描述方案，本研究通过对四种经晶体学表征的铜/硝酰卤化物配合物开展铜K边、铜L2,3边以及氯K边X射线吸收光谱（X-ray absorption spectroscopy）测试，对该问题展开探究。借助上述光谱技术并结合密度泛函理论（density functional theory）的计算支持，本研究确定了铜、硝酰配体与卤离子轨道间的轨道混合系数。所揭示的电子结构图景表明，铜配位作用可赋予硝酰配体显著的氧铵（oxoammonium）特征。本研究进一步将该计算方法拓展至被援引作为铜/硝酰介导的醇氧化催化反应中一类假定中间体的分析，得到了一致的结论。综上，本研究结果对于阐明醇氧化反应的反应机理，以及该共催化体系中协同效应的内在本质均具有重要的学术意义。