Aloisia KING1, John HOLBREY1, Ma?gorzata SWAD?BA-KWA?NY1
1Queens University Belfast, Belfast, United Kingdom
Frustrated Lewis acid/base pairs (FLPs) have been extensively investigated in the literature because of interest in their potential to replace platinum group metal catalysts for small molecule activation. However, limited solubility in typical reaction solvents results in low concentrations of FLP catalytic sites. We reasoned that the concentration of active FLP sites could be increased by designing suitable ionic liquid FLP systems which do not require a solvent. Moreover because irreversible splitting and binding of hydrogen by the Lewis acid and base components of an FLP results in formation of a hydrido-anion and a protonated cation, it should be possible to reverse-engineer ionic liquid FLP systems by selection of appropriate combinations of protic cations and hydrido-anions.
Inspired by the previously reported organic salt FLP system formed from N?methylacridinium salts as Lewis acids with 2,6-lutidine as the Lewis base that was restricted in activity due to limited solubility of the acridinium salts in dichloromethane,1 we set about examining whether suitable anion selection could be used to develop analogues of the reported N?methylacridinium salts/2,6-lutidine pairs that are ionic liquids, and to investigate whether these would function as neat liquids for hydrogen activation.
We report here the first study on N-alkylacridinium bis(trifluoromethanesulfonyl)imide salt/2,6-lutidine mixtures, demonstrating that the acridinium salt/lutidine acid-base pair forms an ionic liquid FLP, that the ionic liquid splits hydrogen and that the ionic liquid characteristics are retained in the corresponding protonated H-lutidinium salt/dihydroacridine mixture which offers the potential to tune and optimise hydrogen activation by shifting the equilibrium shown in Scheme 1.
1. E. R. Clark and M. J. Ingleson, Angew. Chem., Int. Ed., 2014, 53, 11306–11309.