Theoretical Study of POCOP-Pincer Iridium(III)/Iron(II) Hydride Catalyzed Hydrosilylation of Carbonyl Compounds: Hydride Not Involved in the Iridium(III) System but Involved in the Iron(II) System

The catalytic hydrosilylation of carbonyl compounds by two POCOP-pincer transition-metal hydrides, (POCOP)1r(H)(acetone)(+) (1A-acetone) and (POCOP)Fe(H)(PMe3)(2) (1B) (POCOP = 2,6-bis(dibutyl-/ diisopropylphosphinito)phenyl), was theoretically investigated to determine the underlying reaction mechanism. Several plausible mechanisms were analyzed using density functional theory calculations. The 1A-acetone-catalyzed hydrosilylation of carbonyl compounds proceeds via the ionic hydrosilylation pathway, which is initiated by the nucleophilic attack of the eta(1)-silane metal adduct by carbonyl substrate. This attack results in the heterolytic cleavage of the Si-H bond and the generation of a siloxy carbenium ion paired with a neutral iridium dihydride, [(POCOP)Ir(H)(2)][R3SiOCHR'](+) followed by transfer of hydride from the metal center to the siloxy carbenium ion to yield the silyl ether product. The activation energy of the turnover-limiting step was calculated as similar to 15.2 kcal/mol. This value is energetically more favorable than those of other pathways by as much as 22.6 kcal/mol. The most energetically favorable process for the hydrosilylation of carbonyl compound catalyzed by POCOP-pincer iron hydride 1B was determined as the carbonyl precoordination pathway, which involves the initial coordination of the carbonyl substrate to the metal center and subsequent migratory insertion into the M-H bond to give the alkoxide intermediate. This intermediate then undergoes M-O/Si-H sigma-bond metathesis to yield the silyl ether product. The ionic hydrosilylation pathway requires an activation energy that is similar to 30.0 kcal/mol higher than that of the carbonyl precoordination pathway. Our calculation results indicate that the hydride moiety is not involved in the POCOP-pincer iridium(III) hydride 1A-acetone-catalyzed hydrosilylation of carbonyl compounds but is involved in the POCOP-pincer iron(II) hydride 1B-catalyzed process.