Anhydrous 1:1 proton‐transfer compounds of isonipecotamide with picric acid and 3,5‐dinitrosalicylic acid: 4‐carbamoylpiperidinium 2,4,6‐trinitrophenolate and two polymorphs of 4‐carbamoylpiperidinium 2‐carboxy‐4,6‐dinitrophenolate

The structures of the anhydrous 1:1 proton‐transfer compounds of isonipecotamide (piperidine‐4‐carboxamide) with picric acid and 3,5‐dinitrosalicylic acid, namely 4‐carbamoylpiperidinium 2,4,6‐trinitrophenolate, C 6 H 13 N 2 O + ·C 6 H 2 N 3 O 7 − , (I), and 4‐carbamoylpiperidinium 2‐carboxy‐4,6‐dinitrophenolate [two forms of which were found, the monoclinic α‐polymorph, (II), and the triclinic β‐polymorph, (III)], C 6 H 13 N 2 O + ·C 7 H 3 N 2 O 7 − , have been determined at 200 K. All three compounds form hydrogen‐bonded structures, viz. one‐dimensional in (II), two‐dimensional in (I) and three‐dimensional in (III). In (I), the cations form centrosymmetric cyclic head‐to‐tail hydrogen‐bonded homodimers [graph set R 2 2 (14)] through lateral duplex piperidinium–amide N—H...O interactions. These dimers are extended into a two‐dimensional network structure through further interactions with phenolate and nitro O‐atom acceptors, including a direct symmetric piperidinium–phenol/nitro N—H...O,O cation–anion association [graph set R 1 2 (6)]. The monoclinic polymorph, (II), has a similar R 1 2 (6) cation–anion hydrogen‐bonding interaction to (I) but with an additional conjoint symmetrical R 1 2 (4) interaction as well as head‐to‐tail piperidinium–amide N—H...O,O hydrogen bonds and amide–carboxyl N—H...O hydrogen bonds, giving a network structure which includes large R 4 3 (20) rings. The hydrogen bonding in the triclinic polymorph, (III), is markedly different from that of monoclinic (II). The asymmetric unit contains two independent cation–anion pairs which associate through cyclic piperidinium–carboxyl N—H...O,O′ interactions [graph set R 1 2 (4)]. The cations also show the zigzag head‐to‐tail piperidinium–amide N—H...O hydrogen‐bonded chain substructures found in (II), but in addition feature amide–nitro and amide–phenolate N—H...O associations. As well, there is a centrosymmetric double‐amide N—H...O carboxyl bridged bis(cation–anion) ring system [graph set R 4 2 (8)] in the three‐dimensional framework. The structures reported here demonstrate the utility of the isonipecotamide cation as a synthon with previously unrecognized potential for structure assembly applications. Furthermore, the structures of the two polymorphic 3,5‐dinitrosalicylic acid salts show an unusual dissimilarity in hydrogen‐bonding characteristics, considering that both were obtained from identical solvent systems.