Hydrogen bonding in the anhydrous 1:1 proton-transfer compounds of isonipecotamide with nitro-substituted benzoic acids: the salts of the three isomeric mononitrobenzoic acids and of 3,5-dinitrobenzoic acid.

The structures of the anhydrous 1:1 proton-transfer compounds of isonipecotamide (piperidine-4-carboxamide) with the three isomeric mononitro-substituted benzoic acids and with 3,5-dinitrobenzoic acid, namely 4-carbamoylpiperidinium 2-nitrobenzoate, (I), 4-carbamoylpiperidinium 3-nitrobenzoate, (II), and 4-carbamoylpiperidinium 4-nitrobenzoate, (III), all C(6)H(13)N(2)O(+)·C(7)H(4)NO(4)(-), and 4-carbamoylpiperidinium 3,5-dinitrobenzoate, C(6)H(13)N(2)O(8)(+)·C(7)H(3)N(2)O(6)(-), (IV), respectively, have been determined at 200 K. All the salts form hydrogen-bonded structures, viz. three-dimensional in (I), two-dimensional in (II) and (III), and one-dimensional in (IV). Featured in the hydrogen bonding of three of these [(I), (II) and (IV)] is the cyclic head-to-head amide-amide homodimer motif [graph set R(2)(2)(8)] through a duplex N-H...O association, the dimer then giving structure extension via either piperidinium or amide H-atom donors and carboxylate O-atom and, in some examples [(II) and (IV)], nitro O-atom acceptors. In (I), the centrosymmetric amide-amide homodimers are expanded laterally through N-H...O hydrogen bonds via cyclic R(4)(2)(8) interactions, forming ribbons which extend along the c cell direction. These ribbons incorporate the 2-nitrobenzoate cations through centrosymmetric cyclic piperidine-carboxylate N-H...O associations [graph set R(4)(4)(12)], giving interconnected sheets in the three-dimensional structure. In (II), in which no amide-amide homodimer is present, duplex piperidinium-amide N-H...O homomolecular hydrogen-bonding associations [graph set R(2)(2)(14)] give centrosymmetric head-to-tail dimers. Structure extension occurs through hydrogen-bonding associations between both the amide H-atom donors and carboxylate and nitro O-atom acceptors, as well as a three-centre piperidinium-carboxylate N-H...O,O' cyclic R(1)(2)(4) association, giving the two-dimensional network structure. In (III), the centrosymmetric amide-amide dimers are linked through the two carboxylate O-atom acceptors of the anions via bridging piperidinium N-H...O,O'...H-N(amide) hydrogen bonds, giving the two-dimensional sheet structure which features centrosymmetric cyclic R(4)(4)(12) associations. In (IV), the amide-amide dimer is also centrosymmetric, with the dimers linked to the anions through amide-nitro N-H...O interactions. The piperidinium groups extend the structure into one-dimensional ribbons via N-H...O(carboxylate) hydrogen bonds. The structures reported here further demonstrate the utility of the isonipecotamide cation in molecular assembly. They also highlight the efficacy of the cyclic R(2)(2)(8) amide-amide hydrogen-bonding homodimer motif in this process and provide an additional homodimer motif type in the head-to-tail R(2)(2)(14) association.