1,3‐Bis(2′‐hydroxyethyl)imidazolium ionic liquids : correlating structure and properties with anion hydrogen bonding ability

A series of 1,3‐bis(2′‐hydroxyethyl)imidazolium ionic liquids is reported where 1 H NMR chemical shift values and thermal stabilities ( T d ), as determined by thermogravimetric analysis, are correlated with the hydrogen bonding capability of various anions ([Cl − ], [Br − ], [CF 3 CO 2 − ], [NO 2 − ], [MsO − ], [NO 3 − ], [TfO − ], [BF 4 − ], [NTf 2 − ], and [PF 6 − ]). Use of anions with the strongest hydrogen bonding capability, such as chloride [Cl − ], bromide [Br − ], and trifluoroacetate [CF 3 CO 2 − ], led to the furthest observed downfield chemical shift values in DMSO‐d 6 and the poorest thermal stabilities ([CF 3 CO 2 − ] < 200 °C). Thermal stabilities in excess of 350 °C and upfield chemical shift values were observed for ionic liquids , which employed the weakly coordinating triflate [OTf − ], tetrafluoroborate [BF 4 − ], or bis(trifluoromethylsulfonyl)imide [NTf 2 − ] anion. Optimized structures of selected ionic liquids , as determined by density functional theory calculations at the B3LYP/6‐31G + (d,p) level, indicated that the anion preferred to be located above the imidazolium ring and in close proximity to the hydroxyl groups. Calculated dissociation energies ( Δ E) and a comparison of key bonding distances (C2―H, (C2)H···X, O―H, and (O)H···X) also confirmed this structural preference. Copyright © 2013 John Wiley & Sons, Ltd.