Three‐dimensional hydrogen‐bonded structures in the guanidinium salts of the monocyclic dicarboxylic acids rac‐trans‐cyclohexane‐1,2‐dicarboxylic acid (2:1, anhydrous), isophthalic acid (1:1, monohydrate) and terephthalic acid (2:1, trihydrate)

The structures of bis(guanidinium) rac‐trans‐cyclohexane‐1,2‐dicarboxylate, 2CH₆N₃⁺·C₈H₁₀O₄²⁻, (I), guanidinium 3‐carboxybenzoate monohydrate, CH₆N₃⁺·C₈H₅O₄⁻·H₂O, (II), and bis(guanidinium) benzene‐1,4‐dicarboxylate trihydrate, 2CH₆N₃⁺·C₈H₄O₄²⁻·3H₂O, (III), all reveal three‐dimensional hydrogen‐bonded framework structures. In anhydrous (I), both guanidinium cations form classic cyclic R₂²(8) N—H...O,O′_carboxylate and asymmetric cyclic R₂¹(6) hydrogen‐bonding interactions, while one cation forms an unusual enlarged cyclic interaction with O‐atom acceptors of separate ortho‐related carboxylate groups graph set R₂²(11)]. Cations and anions also associate across inversion centres, giving cyclic R₄²(8) motifs. In the 1:1 guanidinium salt, (II), the cation forms two separate cyclic R₂¹(6) interactions, one with a carboxyl O‐atom acceptor and the other with the solvent water molecule. The structure is unusual in that both carboxyl groups form short interanion O...H...O contacts, one across a crystallographic inversion centre O...O = 2.483 (2) Å] and the other about a twofold axis of rotation O...O = 2.462 (2) Å], representing shared sites on these elements for the single acid H atom. The water molecule links the cation–anion ribbon structures into a three‐dimensional framework. In (III), the repeating molecular unit comprises a benzene‐1,4‐dicarboxylate dianion which lies across a crystallographic inversion centre, two guanidinium cations and two solvent water molecules (each set related by twofold rotational symmetry), and a single water molecule which lies on a twofold axis. Each guanidinium cation forms three types of cyclic interaction with the dianions: one R₂¹(6), the others R₃²(8) and R₃³(10) (both of these involving the water molecules), giving a three‐dimensional structure through bridges down the b‐cell direction. The water molecule at the general site also forms an unusual cyclic R₂²(4) homodimeric association across an inversion centre O...O = 2.875 (2) Å]. The work described here provides further examples of the common cyclic guanidinium–carboxylate hydrogen‐bonding associations, as well as featuring other less common cyclic motifs.