Structural studies in metal–purpurate complexes. Part 1. Crystal structures of potassium purpurate trihydrate and ammonium purpurate monohydrate (murexide)

Abstract

The crystal structures of the title compounds, [K][L]·3H₂O(1) and [NH₄][L]·H₂O(2)(L = purpurate, [C₈H₄N₅O₆]^–) have been determined at 295 K by X-ray diffraction and refined by full-matrix least squares to R 0.059 (517 ‘observed’ reflections) and 0.080 (874 ‘observed’ reflections) respectively. Crystals of both are orthorhombic. For (1)(space group Ibca), a= 21.75(1), b= 16.512(3), c= 7.4304(7)Å. Z= 8; for (2)(space group Pbnn), a= 4.5688(8), b= 15.309(5), c= 17.011 (4)Å, Z= 4. In both structures, the anion has crystallographic symmetry 2, the angles at the central nitrogen being 126.3(7)(1) and 125.8(4)°(2). The two approximately planar barbiturate rings of each anion are not coplanar with each other, the interaction between neighbouring carbonyl groups causing torsion about the central nitrogen–carbon bonds. In both structures anion–cation interaction and hydrogen bonding is extensive.