Interactions of aromatic carboxylic acids with quinolin-8-ol (oxine): synthesis and the crystal structures of the proton-transfer compounds with the nitro-substituted benzoic acids

Proton-transfer compounds of quinolin-8-ol (oxine) with the nitro-substituted aromatic carboxylic acids 2-nitrobenzoic acid, [(C9H8NO+)(C7H4NO4-).H2O] (1), 3-nitrobenzoic acid, [(C9H8NO+)(C7H4NO4)(-)] (2), 4-nitrobenzoic acid, [(C9H8NO+)(C7H4NO4-)(C7H5NO4)] (3), 3,5-dinitrobenzoic acid, [(C9H8NO+)(2)(C7H3N2O6-)(2). 3H(2)O] (4), 5-nitrosalicylic acid, [(C9H8NO+) (C7H4NO5-)] (5) and 3,5-dinitrosalicylic acid, [(C9H8NO+)(C7H3N2O7-)] (6) have been prepared and characterized by using both infrared spectroscopy and single-crystal X-ray diffraction methods [( 3), (4) and (6)]. In all compounds, protonation of the quinoline nitrogen occurs together with primary hydrogen-bonding interactions involving this group and the carboxylate group of the acid, while further peripheral associations, in the case of the trihydrate (4), also involving the water molecules, result predominantly in simple chain polymeric structures.