Mechanistic Studies on the Selective Reduction of CO₂ to the Aldehyde Level by a Bis(phosphino)boryl (PBP)-Supported Nickel Complex

This work describes a thorough investigation of the mechanism of a highly selective hydrosilylation of CO2 to the formaldehyde level catalyzed by a bis­(phosphino)­boryl (PBP)­Ni­(II) complex in the presence of B­(C6F5)3. CO2 activation by insertion into the Ni–H bond of the catalyst precursor 2 is shown to occur very easily, because of the trans influence exerted by the boryl ligand. During catalysis, the limiting step is B­(C6F5)3 dissociation from the active species (PBP)­Ni–OCHO·B­(C5F6)3 (4), which controls the amount of free borane that can lead to over-reduction to methane. Free borane activates the silane by formation of [R3Si–H···B­(C6F5)3], which can then transfer the silylium (R3Si+) fragment to the oxygen atoms of the Ni formate and Ni acetal intermediates. The ion pair [(PBP)­Ni]­[HB­(C6F5)3] (5) is the key species that activates CO2 in the catalytic cycle (and silylformate in a second step) with [HB­(C6F5)3]− as the source of hydride. Hydride transfer to [(PBP)­Ni–OCO]+ is virtually barrierless, whereas hydride transfer to [(PBP)­Ni–OCHOSiR3]+ has the second-highest energy barrier of the process (25.2 kcal mol–1). Therefore, the (PBP)Ni framework is instrumental in both reduction steps of the catalysis and controls the selectivity of the reaction by sequestering B­(C6F5)3.本工作系统研究了双(膦基)硼基(PBP)-镍(II)配合物（bis(phosphino)boryl (PBP)-Ni(II) complex）在三(五氟苯基)硼[B(C₆F₅)₃]存在下，催化二氧化碳(CO₂)高选择性硅氢化至甲醛水平的反应机理。研究表明，得益于硼基配体的反式效应，催化剂前驱体2的Ni-H键发生二氧化碳插入活化的过程极易进行。催化过程中的决速步为活性物种(PBP)-Ni–OCHO·B(C₅F₆)₃ (4) 中B(C₆F₅)₃的解离，该步骤决定了游离硼烷的含量，而游离硼烷可引发产物过度还原为甲烷。游离硼烷通过形成[R₃Si–H···B(C₆F₅)₃]加合物活化硅烷，随后可将硅正离子(R₃Si⁺)片段转移至甲酸镍与缩醛镍中间体的氧原子上。离子对[(PBP)Ni][HB(C₆F₅)₃] (5) 是催化循环中活化二氧化碳（第二步活化甲硅烷基甲酸酯）的关键物种，其中[HB(C₆F₅)₃]⁻为氢负离子源。氢负离子向[(PBP)-Ni–OCO]⁺的转移过程几乎无能垒，而氢负离子向[(PBP)-Ni–OCHOSiR₃]⁺的转移则是该反应的第二高能垒步骤（25.2 kcal·mol⁻¹）。因此，(PBP)Ni骨架在催化的两步还原过程中均发挥关键作用，并通过捕获B(C₆F₅)₃调控反应的选择性。