The connection between hydride affinity and local electrophilicity: a key factor in designing frustrated Lewis pairs for reversible H2 activation

Abstract

We present a clear relationship between hydride affinity and the condensed-to-boron electrophilicity index for 27 intramolecular aminoborane frustrated Lewis pairs (FLPs), employing Density Functional Theory calculations. First, the Gibbs free energy change's dependence of H₂ splitting on the hydride affinity of the FLPs is explored, identifying strong linear correlations for aromatic and aliphatic nitrogen substituents. Additionally, we found that the condensed-to-boron electrophilicity index's inverse and logarithm show strong linear relationships with hydride affinity across all FLP systems, generating a direct connection between a purely theoretical reactivity index with an experimental property like the hydride affinity. Using these models, we applied our findings to an experimentally tested FLP system, predicting hydride affinity values that yield ΔG (ΔH) for H₂ splitting of −0.2 (−8.2) and 0.9 (−9.4) kcal mol⁻¹ with the inverse and logarithmic relationships, respectively, which are close to the DFT calculated values: 1.3 (−6.0) kcal mol⁻¹ and closely matching the experimental value of −1.7 ± 4.0 (−8 ± 1) kcal mol⁻¹. These results confirm the predictive power of our models, which can be used to guide the rational design of new aminoborane FLPs to be employed in reversible H₂ activation and metal-free hydrogenation of unsaturated compounds.