Metal-free catalytic conversion of CO2 into methanol: local electrophilicity as a tunable property in the design and performance of aniline-derived aminoborane-based FLPs

Abstract

A deeper computational mechanistic study of an environmentally friendly metal-free CO₂ reduction process towards obtaining methanol is presented, employing a previously tested kind of intramolecular frustrated Lewis pair (2-[bis(R)boryl]-N,N-dimethylaniline) as the catalyst and H₂ as the reducing agent. The Lewis acid strength of the electrophilic boron atom was adjusted to facilitate hydride release by changing the R group, using electron-donating groups (EDGs) based on methylated aryls (Mes and Mes′) and electron-withdrawing groups (EWGs) based on fluorinated alkyls (CF₃ and PFtB) and aryls (FMes and C₆F₅), to analyse its effect on both the H₂ splitting and CO₂ hydrogenation processes. The acidity of boron was measured from the local electrophilicity index obtained using conceptual density functional theory, where an excellent correlation with the Gibbs free energy of the H₂ splitting process was found (R² = 0.95), indicating that the higher the acid power of boron is, the more exergonic the H₂ activation process is. The reversibility of H₂ activation directly impacts the CO₂ and formic acid hydrogenations, where the less exergonic the H₂ splitting process is, the lower the activation energies for these hydrogenation processes are. To obtain methanol at the end, methanediol dehydration forming formaldehyde is crucial, because methanediol has a high energetic barrier, hindering the catalytic cycle from being more efficient. Conversely, formaldehyde can be easily hydrogenated to methanol in the same way as CO₂ and formic acid. Finally, the catalytic activity in each case was analysed in terms of the energetic span model, where the local electrophilicity index condensed on boron shows a good linear correlation with the logarithm of the relative turnover frequency (R² = 0.91), indicating that this reactivity index can be employed to guide the design of optimal catalytic systems to increase its catalytic activity, opening new routes directing future experiments in the field.