These findings reveal how the populations can be radically enhanced using ionic liquid solvents.
Frustrated Lewis Pairs (FLPs) are combinations of bulky Lewis acids and bases, which cannot form acid-base complexes due to steric hindrance. They have attracted immense interest in the last decade, because they have the capability of metal-free hydrogen activation. “Metal-free hydrogen activation is of great interest to the chemistry community. Traditional hydrogenation catalysts, based on metals such as platinum or palladium, are expensive and pose threat of heavy metal pollution. FLP systems, designed around abundant main group elements, eradicate both issues" explains Dr Swadzba-Kwasny.
Since this first report in 2006, there has been an explosion of interest in these systems for hydrogenation reactions and activation of other small molecules, described comprehensively here by Doug Stephan, the pioneer of FLP chemistry. In addition to hydrogenation and small molecule activation FLP chemistry also spans organic chemistry, transition metal chemistry, enzyme models, polymers and materials, and surface chemistry.
The mechanism of FLP-catalysed hydrogen activation is particularly interesting. Mechanistic studies based on kinetics of hydrogenation, supported by computational investigation, suggest that FLP activation of hydrogen requires that the Lewis acid and base components cooperatively associate in solution, in order to pre-form the catalytically active site before their interaction with H2 molecule. However, these so-called 'encounter complexes' were found to be elusive associations by weak intermolecular forces, with hardly any direct spectroscopic evidence for their existence - both due to their low concentrations and transient nature.
This in itself has caused some 'frustration' to experimental scientists, left reliant on indirect evidence.
In work published in Chemical Communications, chemists at the QUILL Research Centre in the School of Chemistry and Chemical Engineering at Queen's University Belfast, teamed up with members of the ISIS Disordered Materials Group, to directly study the liquid structure of a Frustrated Lewis Pair solutions using the instrument SANDALS. Their findings provided the first direct observational evidence for formation and population of associated 'encounter complexes' in solution.
Solution structure using SANDALS
First, the team used the unique capabilities offered at ISIS Neutron and Muon Source to study liquid structure, combining experimental neutron scattering data collected on SANDALS with modelling and structure refinement with EPSR. These have been applied to a model FLP solution in benzene. From the EPSR model, fitted to experimental data, the relative distributions of Lewis acid and base sites in solution were tracked, identifying that a greater than average proportion of acids and bases were involved in pair correlations at distances around 8 Å - correlating with solvent-separated Lewis acid/base pair, proposed by an earlier DFT study. Furthermore, the proportion of Lewis acid and base compounds contributing to associated FLPs in solution, compared to entire Lewis acids and bases population, was counted, giving a direct measure of the number of active catalytic sites present.
This provides the first direct experimental demonstration of FLP formation in solution, corresponds to predictions from earlier DFT structural modelling and - crucially - enables populations of catalytically active sites to be determined.
Promoting encounter complex formation in solution
Having demonstrated that the FLP 'encounter complexes' can be characterised in benzene using neutron scattering, the researchers then demonstrated how the population of 'encounter complex' can be enhanced using ionic liquids. Using the ionic liquid 1‑decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, commonly abbreviated as [C10mim][NTf2], as a solvent, the team reported the first observation of an FLP encounter complex by simple NMR spectroscopic experiment. Signature new signals demonstrated the presence of long-lived FLP associated pairs that are not observed in conventional solvents, such as benzene. “These results demonstrate that ionic liquids can stabilise FLP 'encounter complex' formation, generating significantly higher concentrations of long-lived associated pairs in solutions, in perspective leading to enhanced catalytic activity compared to conventional solvents" summarises Dr Holbrey.
The challenge of understanding these more complex FLP in ionic liquid solutions is one that is actively underway in collaboration with ISIS Neutron and Muon Source staff in the Disordered Materials group and the ISIS Deuteration Facility. “We are also screening FLP solutions in ionic liquids for simple hydrogenation experiments, and in parallel we work towards developing FLPs that are themselves ionic liquids, rather than merely solutes in ionic liquid solvents" adds Dr Swadzba-Kwasny.
Read the full research publication here: L. C. Brown, J. M. Hogg, M. Gilmore, L. Moura, S. Imberti, S. Gärtner, H. Q. N. Gunaratne, R. J. O'Donnell, N. Artioli, J. D. Holbrey and M. Swadźba-Kwaśny, “Frustrated Lewis pairs in ionic liquids and molecular solvents – a neutron scattering and NMR study of encounter complexe"Chem. Commun., 2018, Advance Article, DOI: 10.1039/C8CC03794A
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