The Catalytic Role of N-Heterocyclic Carbene in a Metal-Free Conversion of Carbon Dioxide into Methanol: A Computational Mechanism Study

Abstract

A density functional theory study at the M05-2X(IEFPCM, THF)/6-311+G**//M05-2X/6-31G* level has been conducted to gain insight into the catalytic mechanism of the first metal-free N-heterocyclic carbene (NHC)-catalyzed conversion of carbon dioxide into methanol. Among the various examined reaction pathways, we found that the most favorable leads to the experimentally detected intermediates, including formoxysilane (FOS), bis(silyl)acetal (BSA), silylmethoxide (SMO), and disiloxane (DSO). However, our study also revealed that formaldehyde (CH₂O), generated from the dissociation of BSA into DSO and CH₂O via a mechanism somewhat similar to the Brook rearrangement, should be an inevitable intermediate, although it was not reported by the experimentalists. When NHC catalyzes the reactions of CO₂/FOS/CH₂O with silane, there are two activation modes. It was found that NHC prefers to activate Si−H bonds of silane and push electron density to the H atoms of the Si−H bonds in favor of transferring a hydridic atom of silane to the electrophilic C center of CO₂/FOS/CH₂O. This holds true in particular for the NHC-catalyzed reactions of silane with FOS/CH₂O to produce BSA/SMO. The preferred activation mode can operate by first passing an energetically unfavorable NHC−silane local minimum via π−π interactions or by directly crossing a transition state involving three components simultaneously. The activation mode involving initial coordination of NHC with the electrophilic C atom of CO₂/FOS/CH₂O is less favorable or inoperable. The predicted catalytic mechanism provides a successful interpretation of the experimental observation that phenylsilane is more efficient than diphenylsilane in performing the conversion.