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The structures of two ammonium salts of 3-carb­oxy-4-hy­droxy­benzene­sulfonic acid (5-sulfosalicylic acid, 5-SSA) have been determined at 200 K. In the 1:1 hydrated salt, ammonium 3-carb­oxy-4-hy­droxy­benzene­sulfonate mono­hydrate, NH4+·C7H5O6S·H2O, (I), the 5-SSA monoanions give two types of head-to-tail laterally linked cyclic hydrogen-bonding associations, both with graph-set R44(20). The first involves both carb­oxy­lic acid O—H...Owater and water O—H...Osulfonate hydrogen bonds at one end, and ammonium N—H...Osulfonate and N—H...Ocarb­oxy hydrogen bonds at the other. The second association is centrosymmetric, with end linkages through water O—H...Osulfonate hydrogen bonds. These conjoined units form stacks down c and are extended into a three-dimensional framework structure through N—H...O and water O—H...O hydrogen bonds to sulfonate O-atom acceptors. Anhydrous tri­ammonium 3-carb­oxy-4-hy­droxy­benzene­sulfonate 3-carboxyl­ato-4-hy­droxy­benzene­sulfonate, 3NH4+·C7H4O6S2−·C7H5O6S, (II), is unusual, having both dianionic 5-SSA2− and monoanionic 5-SSA species. These are linked by a carb­oxy­lic acid O—H...O hydrogen bond and, together with the three ammonium cations (two on general sites and the third comprising two independent half-cations lying on crystallographic twofold rotation axes), give a pseudo-centrosymmetric asymmetric unit. Cation–anion hydrogen bonding within this layered unit involves a cyclic R33(8) association which, together with extensive peripheral N—H...O hydrogen bonding involving both sulfonate and carboxy/carboxylate acceptors, gives a three-dimensional framework structure. This work further demonstrates the utility of the 5-SSA monoanion for the generation of stable hydrogen-bonded crystalline materials, and provides the structure of a dianionic 5-SSA2− species of which there are only a few examples in the crystallographic literature.

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The structures of the anhydrous proton-transfer compounds of the sulfa drug sulfamethazine with 5-nitro­salicylic acid and picric acid, namely 2-(4-amino­benzene­sulfonamido)-4,6-di­methyl­pyrimidinium 2-hy­droxy-5-nitro­benzoate, C12H15N4O2S+·C7H4NO4, (I), and 2-(4-amino­benzene­sulfonamido)-4,6-di­methyl­pyrimidinium 2,4,6-tri­nitro­phenolate, C12H15N4O2S+·C6H2N3O7, (II), respectively, have been determined. In the asymmetric unit of (I), there are two independent but conformationally similar cation–anion heterodimer pairs which are formed through duplex inter­molecular N+—H...Ocarboxylate and N—H...Ocarboxylate hydrogen-bond pairs, giving a cyclic motif [graph set R22(8)]. These heterodimers form separate and different non-associated substructures through aniline N—H...O hydrogen bonds, one one-dimensional, involving carboxyl­ate O-atom acceptors, the other two-dimensional, involving both carboxyl­ate and hydroxy O-atom acceptors. The overall two-dimensional structure is stabilized by π–π inter­actions between the pyrimidinium ring and the 5-nitro­salicylate ring in both heterodimers [minimum ring-centroid separation = 3.4580 (8) Å]. For picrate (II), the cation–anion inter­action involves a slightly asymmetric chelating N—H...O R21(6) hydrogen-bonding association with the phenolate O atom, together with peripheral conjoint R12(6) inter­actions between the same N—H groups and O atoms of the ortho-related nitro groups. An inter-unit amine N—H...Osulfone hydrogen bond gives one-dimensional chains which extend along a and inter-associate through π–π inter­actions between the pyrimidinium rings [centroid–centroid separation = 3.4752 (9) Å]. The two structures reported here now bring to a total of four the crystallographically characterized examples of proton-transfer salts of sulfa­meth­azine with strong organic acids.

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The structures of the 1:1 hydrated proton-transfer compounds of isonipecotamide (piperidine-4-carboxamide) with oxalic acid and adipic acid are three-dimensional hydrogen-bonded constructs involving several different types of enlarged water-bridged cyclic associations.
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