The Active Encounter Complex in Frustrated Lewis Pair Chemistry

Sustainable catalysts based on main-group elements have emerged as alternatives to expensive and environmentally unfriendly precious metal systems. Frustrated Lewis pairs (FLPs) are precluded from forming a classical Lewis adduct, and have displayed remarkably versatile reactivity in the fields of small-molecule activation and catalysis. The initial reaction of the acid, base and small molecule (e.g. H₂) is formally termolecular, but the viability of this reaction is rationalised by the pre-association of the acid and base in an encounter complex. However, there is no experimental methodology to study the active encounter complex, i.e. the pre-associated complex that is in the correct orientation for small-molecule activation. Here we show that the charge-transfer band between PMes₃ and B(C₆F₅)₃ can be analysed by supramolecular techniques to provide the key thermodynamic parameter, K_a, for the active encounter complex. We also demonstrate that a higher concentration of active encounter complex in solution leads to a faster activation of hydrogen. This method enables researchers to directly probe the complex that underpins FLP small-molecule activation and subsequent catalysis, and will aid the design of more active sustainable catalysts.