Can a decrease in anti-aromaticity increase the dihydrogen activation ability of a frustrated phosphorous/borane Lewis pair?: a DFT study

Abstract

The mechanism of dihydrogen activation has been theoretically investigated by means of DFT calculation. An experimentally synthesized bridged P/B frustrated Lewis pair (FLP) and two designed FLPs are used for this purpose. The model FLPs 2 and 3 are more efficient than FLP 1 for H2 activation as revealed by the thermochemical and kinetic data. A significant amount of electron density is transferred from H2 molecule to the FLPs at the transition states (TSs) during the process of H2 activation, and this is greater at the corresponding TSs of FLPs 2 and 3 than that of FLP 1. The NICS(0) and NICS(1zz) of the boron heterocycle at the FLPs 2 and 3, and at the corresponding TSs and the product geometries of H2 activation demonstrate that the anti-aromatic character of the rings in the FLPs is remarkably reduced at the TSs and finally at the products and that is most likely responsible for enhanced activity of FLPs 2 and 3 by decreasing the activation barrier.