X-ray diffraction study of the crystal structures of pyrazine-2,5-dicarboxylic acid dihydrate and its magnesium complex

Pyrazine-2,5-dicarboxylic acid dihydrate, C4H2N2(COOH)(2). 2H(2)O, is triclinic, P1, a = 5.255(1) Angstrom, b = 6.907(1) Angstrom, c = 6.951(1) Angstrom, alpha = 119.(3)degrees, beta = 100.81(3)degrees, gamma = 99.06(3)degrees, Z = 1. The structure is composed of molecular layers consisting of 2,5-pzdc and water molecules held by a system of hydrogen bonds. The interlayer spacing of 3.2 Angstrom indicates interactions of the van der Waals type. Magnesium pyrazine-2,5-dicarboxylate hexahydrate, [Mg(H2O)(6)][C4H2N2(COO)(2)], is monoclinic, P2(1)/c, a = 8.016(1) Angstrom, b = 6.858(1) Angstrom, c = 11.278(1) Angstrom, beta = 99.52(3)degrees, Z = 2. Each magnesium atom is surrounded by six water molecules located at the apices of an almost regular octahedron with mean d(Mg-O) = 2.060(1) Angstrom. The 2,5-pzdc anions are planar with interatomic distances and angles characteristic for the pyrazine ring. A number of hydrogen bonds link the water molecules coordinated to Mg with carboxylic oxygens of the acid molecules. Their lengths range from 2.51 Angstrom to 3.07 Angstrom.