Rational design of FLP catalysts for reversible H2 activation: A DFT study of the geometric and electronic effects

Frustrated Lewis pairs (FLPs) emerge as a new type of bifunctional metal-free catalysts for reversible H₂ activation, which is important for the storage and liberation of H₂ or further controllable utilizing chemical fuels via hydrogenation/dehydrogenation. Herein, a DFT study was conducted to understand the geometric factors and electronic effects of FLPs on reversible H₂ activation. The Lewis base group mainly contributes to the proton attachment, and influences the kinetics of the H₂ activation. The Lewis acid group mainly relates to the hydride attachment, and affects more significantly on the thermodynamics of H₂ activation. The dimer and quenched structure of FLPs also have a degree of influence on the performance of catalyzed H₂ activation. A series of FLPs with para-substituted phenyl derivatives as LA groups were designed and evaluated. The results indicate that the variation of LA groups has significant impact on thermodynamic energy of dihydrogen adducts but insignificant effect on kinetics. Moreover, we found the thermodynamic energy of products has a good linear relationship with Hammett substituent constants. The solvent effect on H₂ activation was also studied, and polar solvent is beneficial for zwitterionic products. These results should provide deeper insight to understand the relation between FLPs structure and reactivity, which is critical for rational design of more efficient FLPs catalysts for reversible H₂ activation.