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)

Abstract

The structures of bis(guanidinium) rac-trans-cyclohexane-1,2-dicarboxylate, 2CH6N3+烸H10O42-, (I), guanidinium 3-carboxybenzoate monohydrate, CH6N3+烸H5O4-爲O, (II), and bis(guanidinium) benzene-1,4-dicarboxylate trihydrate, 2CH6N3+烸H4O42-糈2O, (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) ŝ and the other about a twofold axis of rotation [O...O = 2.462 (2) ŝ, 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) ŝ. 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.