NEW METAL-BINDING MODES FOR 5-AMINOOROTIC ACID - PREPARATION, CHARACTERIZATION AND CRYSTAL-STRUCTURES OF ZINC(II) COMPLEXES

Treatment of ZnCl2 with 2 equivalents of 5-aminoorotic acid ,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, respect ively. A good yield (ca. 70%) of pure 2 can be obtained by the reactio n of Zn(O2CMe)(2) . H2O and 2 equivalents of H4L in refluxing water. T he crystal structure of 1 consists of neutral octahedral [Zn(H2L)(H2O) (2)] units which form polymer chains along the b axis; H2L2- behaves a s 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 a nd forming two five-membered chelate rings. The H-1 NMR spectra of 1 i n (CD3)(2)SO at 290 and 310 K suggest that its solid-state structure i s not retained in solution. Slow crystallization of 1 or 2 from dmso s olutions yielded crystals of the monomeric octahedral complex [Zn(H3L) (2)(dmso)(2)(H2O)(2)] 3 the structure of which was solved by single-cr ystal X-ray crystallography. The monoanion H3L- utilizes only one carb oxylate oxygen for metal binding in the centrosymmetric complex 3. The difference in anionic charge and co-ordination mode between H2L2- and H3L- leads to different hydrogen-bonded supramolecular structures for 1 and 3. The IR and H-1 NMR spectra of the prepared complexes are dis cussed.