Solvent-mediated pseudo-quadruple hydrogen-bond motifs in three lamotrigine–carboxylic acid complexes

A resonance-assisted intra-molecular hydrogen bond in compounds containing 2-hy-droxy-3,5-di-nitro-benzoic acid and its various deprotonated forms: redetermination of several related structures

A large number of structural determinations of compounds containing 2-hy-droxy-3,5-di-nitro-benzoic acid (I) and its various deprotonated forms, 2-hy-droxy-3,5-di-nitro-benzoate (II) or 2-carb-oxy-4,6-di-nitro-phenolate (III), are biased. The reason for the bias follows from incorrectly applied constraints or restraints on the bridging hydrogen, which is involved in the intra-molecular hydrogen bond between the neighbouring carb-oxy-lic/carboxyl-ate and oxo/hy-droxy groups. This hydrogen bond belongs to the category of resonance-assisted hydrogen bonds. The present article suggests corrections for the following structure determinations that have been published in Acta Crystallographica: DUJZAK, JEVNAA, LUDFUL, NUQVEB, QIQJAD, SAFGUD, SEDKET, TIYZIM, TUJPEV, VABZIJ, WADXOR, YAXPOE [refcodes are taken from the Cambridge Structural Database [CSD; Groom et al. (2016 ▸). Acta Cryst. B72, 171-179]. The structural features of the title mol-ecules in all the retrieved structures, together with structures that contain 3,5-di-nitro-2-oxidobenzoate (IV), are discussed. Attention is paid to the localization of the above-mentioned bridging hydrogen, which can be situated closer to the O atom of the carboxyl-ate/carb-oxy-lic group or that of the hy-droxy/oxo group. In some cases, it is disordered between the two O atoms. The position of the bridging hydrogen seems to be dependent on the pKa (base) although with exceptions. A stronger basicity enhances the probability of the presence of a phenolate (III). The present article examines the problem of the refinement of such a bridging hydrogen as well as that of the hydrogen atoms involved in the hy-droxy and primary and secondary amine groups. It appears that the best model, in many cases, is obtained by fixing the hydrogen-atom position found in the difference electron-density map while refining its isotropic displacement parameter.

Lamotriginium crotonate and lamotriginium salicylate ethanol monosolvate: the role of solvent molecules in the packing organization

Two lamotriginium salts, namely lamotriginium crotonate [systematic name: 3,5-diamino-6-(2,3-dichlorophenyl)-1,2,4-triazin-2-ium but-2-enoate, C9H8Cl2N5 +·C4H5O2 −, (III)] and lamotriginium salicylate [systematic name: 3,5-diamino-6-(2,3-dichlorophenyl)-1,2,4-triazin-2-ium 2-hydroxybenzoate ethanol monosolvate, C9H8Cl2N5 +·C7H5O3 −·C2H5OH, (IV)] present extremely similar centrosymmetric hydrogen-bonded A...L...L...A packing building blocks (L is lamotriginium and A is the anion). The fact that salicylate salt (IV) is (ethanol) solvated, while crotonate salt (III) is not, has a profound effect on the way these elemental units aggregate to generate the final crystal structure. Possible reasons for this behaviour are analyzed and the hypothesis raised checked against similar structures in the literature.