Proton-transfer compounds of isonipecotamide with the aromatic dicarboxylic acids 4-nitrophthalic, 4,5-dichlorophthalic, 5-nitroisophthalic and terephthalic acid.

The structures of the 1:1 proton-transfer compounds of isonipecotamide (piperidine-4-carboxamide) with 4-nitrophthalic acid [4-carbamoylpiperidinium 2-carboxy-4-nitrobenzoate, C(6)H(13)N(2)O(8)(+)·C(8)H(4)O(6)(-), (I)], 4,5-dichlorophthalic acid [4-carbamoylpiperidinium 2-carboxy-4,5-dichlorobenzoate, C(6)H(13)N(2)O(8)(+)·C(8)H(3)Cl(2)O(4)(-), (II)] and 5-nitroisophthalic acid [4-carbamoylpiperidinium 3-carboxy-5-nitrobenzoate, C(6)H(13)N(2)O(8)(+)·C(8)H(4)O(6)(-), (III)], as well as the 2:1 compound with terephthalic acid [bis(4-carbamoylpiperidinium) benzene-1,2-dicarboxylate dihydrate, 2C(6)H(13)N(2)O(8)(+)·C(8)H(4)O(4)(2-)·2H(2)O, (IV)], have been determined at 200 K. All salts form hydrogen-bonded structures, viz. one-dimensional in (II) and three-dimensional in (I), (III) and (IV). In (I) and (III), the centrosymmetric R(2)(2)(8) cyclic amide-amide association is found, while in (IV) several different types of water-bridged cyclic associations are present [graph sets R(4)(2)(8), R(4)(3)(10), R(4)(4)(12), R(3)(3)(18) and R(6)(4)(22)]. The one-dimensional structure of (I) features the common `planar' hydrogen 4,5-dichlorophthalate anion, together with enlarged cyclic R(3)(3)(13) and R(4)(3)(17) associations. In the structures of (I) and (III), the presence of head-to-tail hydrogen phthalate chain substructures is found. In (IV), head-to-tail primary cation-anion associations are extended longitudinally into chains through the water-bridged cation associations, and laterally by piperidinium-carboxylate N-H···O and water-carboxylate O-H···O hydrogen bonds. The structures reported here further demonstrate the utility of the isonipecotamide cation as a synthon for the generation of stable hydrogen-bonded structures. An additional example of cation-anion association with this cation is also shown in the asymmetric three-centre piperidinium-carboxylate N-H···O,O' interaction in the first-reported structure of a 2:1 isonipecotamide-carboxylate salt.