Hydrogen-bonding in two pyridinium salts of [Mo$_2$O$_4$Cl$_4$(μ$_2$-dmsH)]$^{3−}$ complex (dmsH$^−$ = a half-neutralized form of 2,2-dimethylsuccinic acid)

Reactions of a mononuclear molybdenum(V) starting material, (PyH)$_5$[MoOCl$_4$(H$_2$O)]$_3$Cl$_2$, with 2,2-dimethylsuccinic acid in the presence of base afforded two products, (PyH)$_3$[Mo$_2$O$_4$Cl$_4$(μ$_2$-dmsH)]·1/2CH$_3$CN (1) and (PyH)$_4$[Mo$_2$O$_4$Cl$_4$(μ$_2$-dmsH)]Cl (2). As revealed by the X-ray structure analysis, the half-neutralized form of the dicarboxylic acid, the dmsH$^−$ ion, coordinated to the well-known {Mo$_2$O$_4$}$^{2+}$ core in the syn-syn bidentate bridging manner. In both compounds, the non-ionized terminus of the ligand, the COOH function, participated in hydrogen-bonding interactions. The incorporation of the chloride counteranion in 2, prevented the formation of the common “carboxylic acid dimer” which was observed for 1. Instead, a hydrogen-bonded linkage of the COOH function with the chloride occurred