Unusual solidification and phosphate binding to benzimidazolecations in the presence of water

Abstract

Reaction of bis(benzimidazolonium)alkane cations with phosphate ions in aqueous medium leads to the formation of solid masses with a water content as high as 99%. Optical microscopy shows this to arise from formation of a network of fibrous crystals. A crystal structure determination suggests this to arise from the coincidence of the directions of hydrogen bonding interactions between phosphate anions and stacking interactions between benzimidazole cations. Potentiometric titrations show that phosphate binds to benzimidazole cations in 50% aqueous alcohol solution at pH < 7. Similar studies on diammonium cations show binding only at pH > 7. It is argued that this is a reflection of the importance of matching the pK_a values of hydrogen bond donors and acceptors. In the case of favourable matching, hydrogen bond donors can successfully compete for anions with water.