Exploring the metal-free catalytic reduction of CO2 to methanol with saturated adamantane scaffolds of phosphine-borane frustrated Lewis pair: A DFT study

Abstract

Frustrated Lewis pair (FLP) chemistry is an alternative strategy in the area of metal-free organic catalysis. This study reports adamantane as FLP molecular scaffold used to design new FLP (ambiphilic molecule) for the metal-free catalytic reduction of CO2 to methoxy borane (CH3O[B]) via hydroboration process. The DFT [B97D/6-31G(d,p), (SMD,benzene)//B3PW91/6-31G(d,p),SDD] calculations have been employed to examine the reaction. The saturated hydrocarbon showed a marked improvement in the energetics of the reduction process compared to the unsaturated phenylene FLPs employed for such reactions. The saturated scaffold improves the Lewis acid and base characters in adamantyl-derived FLPs. The gauche arrangement of Lewis pairs in the adamantyl FLP system is critical to lower the energy barriers in the potential energy surfaces compared to the unsaturated phenylene FLPs. The adamantyl scaffold FLP lowered the activation barriers by ∼6.0 kcal/mol in the key steps of the reduction of CO2 with the reducing agent compared to the phenylene FLPs. These calculated results corroborate the activation of the reducing agent and carbon dioxide to function as efficient catalysis in all steps. The enhancement in the Lewis characters of FLPs with adamantane scaffold was elucidated with conceptual density functional theory (CDFT) calculations.