N-H∙∙∙O, O-H∙∙∙O hydrogen-bonded supramolecular sheet formation in the bis(2-aminoanilinium) fumarate, 3-methylanilinium hydrogen fumarate and 4-chloroanilinium hydrogen fumarate salts.

In bis(2-aminoanilinum) fumarate, 2C₆H₉N₂⁺·C₄H₂O₄²⁻, (I), the asymmetric unit consists of two aminoanilinium cations and one fumarate dianion, whereas in 3-methylanilinium hydrogen fumarate, C₇H₁₀N⁺·C₄H3O₄⁻, (II), and 4-chloroanilinium hydrogen fumarate, C₆H₇ClN⁺·C₄H3O₄⁻, (III), the asymmetric unit contains two symmetry-independent hydrogen fumate anions and anilinium cations with a slight difference in their geometric parameters; the two salts are isostructural. In (II) and (III), the carboxylic acid H atoms of the anions are disordered across both ends of the anion, with equal site occupancies of 0.50. Both the 4-chloroanilinium cations of (III) are disordered over two orientations with major occupancies fixed at 0.60 in each case. The hydrogen fumarate anions of (II) and (III) form one-dimensional anionic chains linked through O-H∙∙∙O hydrogen bonds. Salts (II) and (III) form two-dimensional supramolecular sheets built from R₄⁴16), R₄⁴(18), R₅⁵(25) and C₂²(14) motifs extending parallel to the (010) plane, whereas in (I), an (010) sheet is formed built from two R₄³(13) motifs, two R₂²(9) motifs and an R₄⁴(18) motif.