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 (Ka), 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.