Orotate Complexes, II¹⁾ Preparation and Crystal Structures of Calcium and Zinc Orotate(2‐) Hydrates

Abstract

From aqueous solutions containing orotate (dianion of 1 ) and calcium or zinc ions in the molar ratio of 2:1, metal orotate (2‐) hydrates are readily obtained in crystalline form at pH 9.0 and 5.4, respectively. According to an X‐ray crystal structure analysis, calcium orotate tetrahydrate forms a one‐dimensional coordination polymer with each orotate ligand coordinated to no less than three neighboring calcium atoms. In this cation triple, the orotate(2‐) anion acts as a carboxylate‐O/amide‐N‐chelating group for the central calcium ion, but also as a carboxylate‐O/O‐chelating group for the first neighboring calcium ion, and as a terminal amide‐O donor for the second. Each calcium ion is thus octacoordinated and linked to three orotate(2‐) anions and three water molecules. The fourth water molecule is only engaged in hydrogen bonding. The new zinc orotate(2‐) phase features Zn(Or)(H 2 O) 4 units. For the hexacoordinate zinc atom bearing four water molecules the orotate(2‐) ligand also acts as an O/N‐chelating group. Unlike a discrete Zn(Or)(H 2 O) 4 H 2 O phase reported earlier, the lattice of the new phase contains further disordered water of crystallization, with the overall composition approaching Zn(Or) 5.5 H 2 O. The apparent stability of the M(Or) units probably reflects the well‐documented carrier function of the orotate ligand for certain divalent metal ions in biological systems.