Lewis Acid Promoted Hydrogenation of CO2 and HCOO- by Amine Boranes: Mechanistic Insight from a Computational Approach.

Abstract

We employ quantum chemical calculations to study the hydrogenation of carbon dioxide by amine boranes, NMe3BH3 (Me3AB) and NH3BH3 (AB) weakly bonded to a bulkier Lewis acid, Al(C6F5)3 (LA). Additionally, computations have also been conducted to elucidate the mechanism of hydrogenation of carbon dioxide by Me3AB while captured between one Lewis base (P(o-tol3), LB) and two Lewis acids, Al(C6F5)3. In agreement with the experiments, our computational study predicts that hydride transfer to conjugated HCO2-, generated in the reaction of Me3AB-LA with CO2, is not feasible. This is in contrast to the potential hydrogenation of bound HCO2H, developed in the reduction of CO2 with AB-LA, to further reduced species like H2C(OH)2. However, the FLP-trapped CO2 effortlessly undergoes three hydride (H-) transfers from Me3AB to produce a CH3O- derivative. DFT calculations reveal that the preference for a H- abstraction by an intrinsically anionic formate moiety is specifically dependent on the electrophilicity of the 2 e- reduced carbon center, which in particular is controlled by the electron-withdrawing capability of the associated substituents on the oxygen. These theoretical predictions are justified by frontier molecular orbitals and molecular electrostatic potential plots. The global electrophicility index, which is a balance of electron affinity and hardness, reveals that the electrophilicity of the formate species undergoing hydrogenation is twice the electrophilicity of the ones where hydrogenation is not feasible. The computed activation energies at M06-2X/6-31++G(d,p) closely predict the observed reactivity. In addition, the possibility of a dissociative channel of the frustrated Lewis pair trapped CO2 system has been ruled out on the basis of predominantly high endergonicity. Knowledge of the underlying principle of these reactions would be helpful in recruiting appropriate Lewis acids/amine boranes for effective reduction of CO2 and its hydrogenated forms in a catalytic fashion.