Quantifying interactions in the active encounter complex of frustrated Lewis pairs

Sustainable catalysts based on main-group elements, such as frustrated Lewis pairs (FLPs), have emerged as alternatives to precious metal systems. The initial reaction of the Lewis acid, Lewis base and small molecule (e.g. H₂) is formally termolecular, but the reaction is rationalised by the pre-association of the acid and base in an encounter complex. Here we show that the charge-transfer band between P(mes)₃ and B(C₆F₅)₃ can be analysed by supramolecular UV-vis spectroscopic techniques to provide the key thermodynamic parameter, the association constant (K_a), for the active encounter complex, i.e. the pre-associated complex that is specifically in the correct orientation for small-molecule activation. 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.