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

Abstract

The catalytic hydrosilylation of carbonyl compounds by two POCOP-pincer transition-metal hydrides, (POCOP)­Ir­(H)­(acetone)⁺ (<b>1A-acetone</b>) and (POCOP)­Fe­(H)­(PMe₃)₂ (<b>1B</b>) (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 <b>1A-acetone</b>-catalyzed hydrosilylation of carbonyl compounds proceeds via the ionic hydrosilylation pathway, which is initiated by the nucleophilic attack of the η¹-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 ∼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 <b>1B</b> 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 σ-bond metathesis to yield the silyl ether product. The ionic hydrosilylation pathway requires an activation energy that is ∼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 <b>1A-acetone</b>-catalyzed hydrosilylation of carbonyl compounds but is involved in the POCOP-pincer iron­(II) hydride <b>1B-</b>catalyzed process