The Active Encounter Complex in Frustrated Lewis Pair Chemistry

30 August 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


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, Ka, 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.


Frustrated Lewis Pairs
Small-molecule activation
Main-group chemistry

Supplementary materials

Supporting Information for The Active Encounter Complex in Frustrated Lewis Pair Chemistry
This file contains the methods, data and spectra for the manuscript The Active Encounter Complex in Frustrated Lewis Pair Chemistry.


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