New metal-binding modes for 5-aminoorotic acid: preparation, characterization and crystal structures of zinc(II) complexes

Abstract

Treatment of ZnCl₂ with 2 equivalents of 5-aminoorotic acid (5-amino-2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid, H₄L) and 2 equivalents of NaOH in water–methanol yielded a mixture of crystals and powder of complexes [{Zn(H₂L)(H₂O)₂}_n] 1 and [Zn(H₃L)₂(H₂O)₄] 2, respectively. A good yield (ca. 70%) of pure 2 can be obtained by the reaction of Zn(O₂CMe)₂·2H₂O and 2 equivalents of H₄L in refluxing water. The crystal structure of 1 consists of neutral octahedral [Zn(H₂L)(H₂O)₂] units which form polymer chains along the b axis; H₂L^2– behaves as a bis(bidentate) bridging ligand co-ordinating to two zinc atoms via the amino nitrogen, the oxygen of the neutral carboxamide group, the deprotonated carboxamide nitrogen and one of the carboxylate oxygens and forming two five-membered chelate rings. The ¹H NMR spectra of 1 in (CD₃)₂SO at 290 and 310 K suggest that its solid-state structure is not retained in solution. Slow crystallization of 1 or 2 from dmso solutions yielded crystals of the monomeric octahedral complex [Zn(H₃L)₂(dmso)₂(H₂O)₂] 3 the structure of which was solved by single-crystal X-ray crystallography. The monoanion H₃L^– utilizes only one carboxylate oxygen for metal binding in the centrosymmetric complex 3. The difference in anionic charge and co-ordination mode between H₂L^2– and H₃L^– leads to different hydrogen-bonded supramolecular structures for 1 and 3. The IR and ¹H NMR spectra of the prepared complexes are discussed.