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

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• 作者：Pablo Ríos ; Amor Rodríguez ; Joaquín López-Serrano
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：September 2, 2016
• 年：2016
• 卷：6
• 期：9
• 页码：5715-5723
• 全文大小：640K
• 年卷期：0
• ISSN：2155-5435

文摘

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.