Redox-Active Ligand-Mediated Oxidative Addition and Reductive Elimination at Square Planar Cobalt(III): Multielectron Reactions for Cross-Coupling

Square planar cobalt(III) complexes with redox-active amidophenolate ligands are strong nucleophiles that react with alkyl halides, including CH2Cl2, under gentle conditions to generate stable square pyramidal alkylcobalt(III) complexes. The net electrophilic addition reactions formally require 2e− oxidation of the metal fragment, but there is no change in metal oxidation state because the reaction proceeds with 1e− oxidation of each amidophenolate ligand. Although the four-coordinate complexes are very strong nucleophiles, they are mild outer-sphere reductants. Accordingly, addition of alkyl- or phenylzinc halides to the five-coordinate organometallic complexes regenerates the square planar starting materials and extrudes C−C coupling products. The net 2e− reductive elimination reaction also occurs without a oxidation state change at the cobalt(III) center. Together these reactions comprise a complete, well-defined cycle for cobalt Negishi-like cross-coupling of alkyl halides with organozinc reagents.

带有氧化还原活性酰胺酚配体（amidophenolate ligands）的方形平面型钴(III)配合物（square planar cobalt(III) complexes）为强亲核试剂，可在温和条件下与包括二氯甲烷（CH₂Cl₂）在内的卤代烷烃反应，生成稳定的四方锥型烷基钴(III)配合物。该净亲电加成反应形式上要求金属片段经历2e⁻氧化，但金属中心的氧化态并未发生改变，因为反应通过每个酰胺酚配体发生1e⁻氧化得以进行。尽管这类四配位配合物是极强的亲核试剂，却同时属于温和的外层球还原剂（outer-sphere reductants）。据此，将烷基或芳基卤化锌加至该五配位有机金属配合物中，可再生得到方形平面型起始原料，并释放出C-C偶联产物。该净2e⁻还原消除反应同样在钴(III)中心未发生氧化态变化的条件下进行。上述一系列反应共同构成了一套完整且明确的循环，用于实现卤代烷烃与有机锌试剂的钴基类根岸（Negishi）交叉偶联。