Department of Chemistry, University of California, Berkeley, California 94720, United States.
Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California, Berkeley, California 94720, United States.
J Am Chem Soc. 2021 May 12;143(18):6990-7001. doi: 10.1021/jacs.1c01487. Epub 2021 Apr 29.
Electrocatalysis enables the construction of C-C bonds under mild conditions via controlled formation of carbon-centered radicals. For sequences initiated by alkyl halide reduction, coordinatively unsaturated Ni complexes commonly serve as single-electron transfer agents, giving rise to the foundational question of whether outer- or inner-sphere electron transfer oxidative addition prevails in redox mediation. Indeed, rational design of electrochemical processes requires the discrimination of these two electron transfer pathways, as they can have outsized effects on the rate of substrate bond activation and thus impact radical generation rates and downstream product selectivities. We present results from combined synthetic, electroanalytical, and computational studies that examine the mechanistic differences of single electron transfer to alkyl halides imparted by Ni metal-ligand cooperativity. Electrogenerated reduced Ni species, stabilized by delocalized spin density onto a redox-active tpyPY2Me polypyridyl ligand, activates alkyl iodides via outer-sphere electron transfer, allowing for the selective activation of alkyl iodide substrates over halogen atom donors and the controlled generation and sequestration of electrogenerated radicals. In contrast, the Ni complex possessing a redox-innocent pentapyridine congener activates the substrates in an inner-sphere fashion owning to a purely metal-localized spin, thereby activating both substrates and halogen atom donors in an indiscriminate fashion, generating a high concentration of radicals and leading to unproductive dimerization. Our data establish that controlled electron transfer via Ni-ligand cooperativity can be used to limit undesired radical recombination products and promote selective radical processes in electrochemical environments, providing a generalizable framework for designing redox mediators with distinct rate and potential requirements.
电催化能够在温和条件下通过控制碳中心自由基的形成来构建 C-C 键。对于由烷基卤化物还原引发的序列，配位不饱和的 Ni 配合物通常作为单电子转移试剂，这就提出了一个基本问题，即在氧化加成中是优先发生外层或内层电子转移。实际上，电化学过程的合理设计需要区分这两种电子转移途径，因为它们会对底物键活化的速率产生巨大影响，从而影响自由基生成速率和下游产物选择性。我们提出了综合合成、电分析和计算研究的结果，这些研究考察了 Ni 金属-配体协同作用对烷基卤化物单电子转移的机理差异。通过将离域自旋密度稳定到氧化还原活性的 tpyPY2Me 多吡啶配体上，电生成的还原 Ni 物种激活烷基碘化物通过外层电子转移，允许选择性地激活烷基碘化物底物而不是卤原子供体，并控制电生成自由基的生成和隔离。相比之下，具有氧化还原惰性的五吡啶同系物的 Ni 配合物以内球方式激活底物，因为它具有纯金属局域化的自旋，从而以不可区分的方式激活底物和卤原子供体，生成高浓度的自由基并导致非生产性的二聚化。我们的数据表明，通过 Ni-配体协同作用控制电子转移可以用来限制不需要的自由基重组产物，并在电化学环境中促进选择性自由基过程，为设计具有不同速率和电位要求的氧化还原介质提供了一个通用框架。
