Crystal structure, thermal analysis and IR spectroscopic investigation of (C6H9N2)H2XO4 (X = As, P)

Crystals of 2-amino-4-methylpyridinium dihydrogenmonoarsenate (C6H9N2)H2AsO4 and 2-amino-4-methylpyridinium dihydrogenmonophosphate (C6H9N2)H2PO4 have been prepared and grown at room temperature. These materials are isotypic with the following unit cell dimensions (C6H9N2)H2AsO4: a = 12.4415(5) Å, b = 6.8224(3) Å, c = 11.3524(5) Å, Z = 4, V = 963.60(6) Å3; (C6H9N2)H2PO4: a = 12.4410(9) Å, b = 6.7165(3) Å, c = 11.3417(5) Å, Z = 4, V = 925.09(10) Å3. The common space group is Pnma. The structure of these compounds has been determined by X-ray data collection on single crystals of (C6H9N2)H2AsO4 and (C6H9N2)H2PO4. Due to the strong hydrogen-bond network connecting the H2XO4 groups, the anionic arrangement must be described as a linear organization. The chains composed by the (H2XO4)nn− macroanion spread along the b-direction, approximately centered by x = 0 and 1/2. All atoms of the structure, except one oxygen atom, are located in the mirror planes situated at y = 1/4 and 3/4, imparting an internal mirror symmetry to the anionic and the cationic entities. The linear macroanions are crossed by organic cations lying in mirrors perpendicular to the b-direction; this atomic arrangement is then described by a three-dimensional network of hydrogen bonds, built up by two types, O–H⋯O bonds inside the chains and N–H⋯O bonds linking adjacent chains. The thermal properties of both compounds are investigated as well as the IR properties supported by group theoretical analyses.