Fang Shaoqin, Chen Hongcai, Wei Haiyan
Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Jiangsu Provincial Key Laboratory for NSLSCS, Nanjing Normal University Nanjing 210097 China
RSC Adv. 2018 Mar 2;8(17):9232-9242. doi: 10.1039/c7ra13486j. eCollection 2018 Feb 28.
Using density functional theory computations, we investigated in detail the underlying reaction mechanism and crucial intermediates present during the reduction of carbon dioxide to methane with silanes, catalyzed by the cationic Ir-pincer complex ((POCOP)Ir(H)(acetone), POCOP = 2,6-bis(dibutylphosphinito)phenyl). Our study postulates a plausible catalytic cycle, which involves four stages, by sequentially transferring silane hydrogen to the CO molecule to give silylformate, bis(silyl)acetal, methoxysilane and the final product, methane. The first stage of reducing carbon dioxide to silylformate is the rate-determining step in the overall conversion, which occurs the direct dissociation of the silane Si-H bond to the C[double bond, length as m-dash]O bond of a weakly coordinated Ir-CO moiety, with a free energy barrier of 29.5 kcal mol. The ionic S2 outer-sphere pathway in which the CO molecule nucleophilically attacks at the η-silane iridium complex to cleave the η-Si-H bond, followed by the hydride transferring from iridium dihydride [(POCOP)IrH] to the cation [O[double bond, length as m-dash]C-OSiMe], is a slightly less favorable pathway, with a free energy barrier of 33.0 kcal mol in solvent. The subsequent three reducing steps follow similar pathways: the ionic S2 outer-sphere process with silylformate, bis(silyl)acetal and methoxysilane substrates nucleophilically attacking the η-silane iridium complex to give the ion pairs [(POCOP)IrH] [HC(OSiMe)], [(POCOP)IrH] [CH(OSiMe)(SiMe)], and [(POCOP)IrH] [CHO(SiMe)], respectively, followed by the hydride transfer process. The rate-limiting steps of the three reducing stages are calculated to possess free energy barriers of 12.2, 16.4 and 22.9 kcal mol, respectively. Furthermore, our study indicates that the natural iridium dihydride [(POCOP)IrH] generated along the ionic S2 outer-sphere pathway could greatly facilitate the silylation of CO, with a potential energy barrier calculated at a low value of 16.7 kcal mol.
我们使用密度泛函理论计算方法，详细研究了阳离子铱钳形配合物（(POCOP)Ir(H)(丙酮)，POCOP = 2,6 - 双（二丁基次膦酰基）苯基）催化硅烷将二氧化碳还原为甲烷过程中的潜在反应机理和关键中间体。我们的研究提出了一个合理的催化循环，该循环包括四个阶段，通过依次将硅烷氢转移到CO分子上，生成甲硅烷基甲酸酯、双（甲硅烷基）缩醛、甲氧基硅烷和最终产物甲烷。将二氧化碳还原为甲硅烷基甲酸酯的第一阶段是整个转化过程中的速率决定步骤，它通过硅烷Si - H键直接解离到弱配位的Ir - CO部分的C = O键上发生，自由能垒为29.5 kcal/mol。离子型S2外层球途径中，CO分子亲核进攻η - 硅烷铱配合物以裂解η - Si - H键，随后氢化物从二氢铱[(POCOP)IrH]转移到阳离子[O = C - OSiMe]上，这是一条稍不利的途径，在溶剂中的自由能垒为33.0 kcal/mol。随后的三个还原步骤遵循类似的途径：离子型S2外层球过程，甲硅烷基甲酸酯、双（甲硅烷基）缩醛和甲氧基硅烷底物分别亲核进攻η - 硅烷铱配合物，生成离子对[(POCOP)IrH][HC(OSiMe)]、[(POCOP)IrH][CH(OSiMe)(SiMe)]和[(POCOP)IrH][CHO(SiMe)]，随后是氢化物转移过程。计算得出这三个还原阶段的速率限制步骤分别具有12.2、16.4和22.9 kcal/mol的自由能垒。此外，我们的研究表明，沿着离子型S2外层球途径生成的天然二氢铱[(POCOP)IrH]可以极大地促进CO的甲硅烷基化，计算得出的势能垒低至16.7 kcal/mol。
