Two-Electron Redox Tuning of Cyclopentadienyl Cobalt Complexes Enabled by the Phenylenediamide Ligand

Achieving multielectron activity at first-row transition-metal complexes has important implications for homogeneous catalysis using earth-abundant metals. Here, we report a family of cobalt–phenylenediamide complexes that undergo reversible 2e– oxidation regardless of the ligand substituents, enabling unprecedented multielectron redox tuning over 0.5 V and, in each case, affording the dicationic Co(III)-benzoquinonediimine species. The neutral complexes are best described as delocalized systems with π-bonding in the metallocycle, consistent with a closed-shell singlet ground state predicted by density functional theory (DFT) calculations. Our DFT results also predict an ECE pathway for 2e– oxidation (ECE = electrochemical step, chemical step, electrochemical step), where the first 1e– step involves redox-induced electron transfer to yield a Co(II) intermediate. Disruption of the metallocycle bonding in this state enables a change in the coordination geometry through association of an addition ligand, which is critical for accessing the potential inversion. The electronic properties of the phenylenediamide ligand govern whether the second electron is lost from the ligand or metal, providing a remarkable example of tunable 2e– behavior at first-row systems.

在第一过渡系金属配合物中实现多电子反应，对于采用丰产金属的均相催化研究具有重要意义。本文报道了一类钴-苯二胺基配合物，该类配合物不受配体取代基影响，均可发生可逆两电子氧化反应，可在0.5 V的电位区间内实现前所未有的多电子氧化还原调控，且在每种反应条件下均能得到双阳离子型Co(III)-苯醌二亚胺物种。中性配合物最优可被描述为金属环内存在π成键的离域体系，这与密度泛函理论（DFT）计算预测的闭壳层单重态基态结果相一致。我们的DFT计算结果还预测了两电子氧化的ECE路径（ECE即电化学步骤-化学步骤-电化学步骤），其中第一步单电子氧化过程涉及氧化还原诱导的电子转移，生成Co(II)中间体。该状态下金属环的成键作用被破坏，使得额外配体缔合后配位几何构型发生改变，这是实现电势反转的关键因素。苯二胺基配体的电子性质决定了第二个电子是从配体还是金属中心失去，这为第一过渡系金属体系中可调谐的两电子反应行为提供了一个极具代表性的研究实例。