The water–boryl radical as a proton-coupled electron transfer reagent for carbon dioxide, formic acid, and formaldehyde — Theoretical approach

Abstract

The thermochemistry of the hydrogen atom transfer reactions from the H 2 O-BX 2 radical system (X = H, CH 3 , NH 2 , OH, F) to carbon dioxide, formic acid, and (or) formaldehyde, which produce hydroxyformyl, dihydroxymethyl, and hydroxymethyl radicals, respectively, were investigated theoretically at ROMP2/6-311+G(3DF,2P)//UB3LYP/6-31G(D) and UG3(MP2)-RAD levels of theory. Surprisingly, in the cases of a strong Lewis acid (X = H, CH 3 , F), the spin transfer process from the water-boryl radical to the carbonyl compounds was barrier-free and associated with a dramatic reduction in the B-H bond dissociation energy (BDE) relative to that of isolated water-borane complexes. Examining the coordinates of these reactions revealed that the entire hydrogen atom transfer process is governed by the proton-coupled electron transfer (PCET) mechanism. Hence, the elucidated mechanism has been applied in the cases of weak Lewis acids (X = NH 2 , OH), and the variation in the accompanied activation energy was attributed to the stereoelectronic effect interplaying in CO 2 and HCOOH compared with HCHO. We ascribed the overall mechanism as a SA-induced five-center cyclic PCET, in which the proton transfers across the so-called complexationinduced hydrogen bond (CIHB) channel, while the SOMO B -LUMO C=O= interaction is responsible for the electron migration process. Owing to previous reports that interrelate the hydrogen-bonding and the rate of proton-coupled electron-transfer reactions, we postulated that "the rate of the PCET reaction is expected to be promoted by the covalency of the hydrogen bond, and any factor that enhances this covalency could be considered an activator of the PCET process." This postulate could be considered a good rationale for the lack of a barrier associated with the hydrogen atom transfer from the water-boryl radical system to the carbonyl compounds. Light has been shed on the water-boryl radical reagent from the thermodynamic perspective.Key words: -boryl radical, complexation-induced hydrogen bond (CIHB), proton-coupled electron transfer (PCET), B-H bond dissociation energies (BDEs).Résumé : Faisant appel à des calculs théoriques effectués aux niveaux ROMP2/6-311+G(3DF,2P)//UB3LYP/6-31G(D) et UG3(MP2)-RAD de la théorie, on a effectué une étude de la thermochimie des réactions de transfert d'atomes d'hydrogène de systèmes radicalaires H 2 O-BX 2 vers le dioxyde de carbone, l'acide formique et (ou) le formaldéhyde qui conduisent respectivement à la formation de radicaux hydroxyformyle, dihydroxyméthyle et hydroxyméthyle. D'une façon surprenant, dans les cas d'acides de Lewis forts (X = H, CH 3 , F) les processus de transferts de spin des radicaux eau-boryle vers les composés carbonylés ne comportent aucune barrière et ils sont associés à une réduction dramatique de l'énergie de la dissociation de la liaison (EDL) B-H relative à celle des complexes eau-boranes isolés. L'examen des coordonnées de la réaction de ces réactions révèle que, le processus de transfert de l'atome d'hydrogène est gouverné par le mé...