Frustrated Lewis pairs: encounter complexes in an ionic liquid

Frustrated Lewis Pairs (FLPs) are combinations of bulky Lewis acids and bases, which cannot form acid-base complexes due to steric hindrance. They are capable of metal-free H2 activation, by H2 interacting with an "encounter complex" - a transient association of a Lewis acid and base. In benzene, encounter complexes are impossible to detect by simple NMR studies, but were detected by neutron scattering on SANDALS, combined with modelling and structure refinement with EPSR, as reported by our group (L. C. Brown, et al., Chem Commun, 2018, 54, 8689). We found that ionic liquids may stabilise encounter complexes, increasing their lifetime and concentration compared to benzene. Here, we study a phosphine/borane FLP in an ionic liquid, aiming to probe the encounter complex structure, compare it to that in benzene, and use this knowledge to optimise a system for metal-free hydrogenations.