Insight into catalytic reduction of CO to methane with silanes using Brookhart's cationic Ir(iii) pincer complex.

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.