Synthesis and characterisation of chelating polycarboxylate ligands capable of forming intermolecular, complementary triple hydrogen bonds

The reaction between the dianhydride of ethylenedinitrilotetraacetic acid (edta) 1 and aminouracil derivatives was utilised to synthesize bifunctional, chelating ligands capable of co-ordinating to a metal centre via the edta backbone, while simultaneously being able to form complementary intermolecular hydrogen bonds via the uracil moieties. 5-Aminouracil 2, 5-aminoorotic acid 3, 5,6-diaminouracil 4 and 5,6-diamino-2-thiouracil 5 were treated with 1 in dry dmf or dmso to give functionalised dicarboxamide derivatives H₂L¹-H₂L⁴ in 15-90% yield. The 5,6-diaminouracils 4 and 5 reacted with the dianhydride exclusively via the 5-amino position. The reaction of H₂¹-H₂L⁴ with basic metal salts [e.g. KVO₃ and Zn(O₂CMe)₂] in aqueous solutions led to the formation of metal complexes of the anionic ligands L¹-L⁴: K[VO₂L¹]·5H₂O, [Zn(OH₂)L¹]·4H₂O and [Zn(OH₂)L³]·5H₂O were characterized by single-crystal X-ray crystallography. The solid state structures of these complexes show that the uracil moieties are situated on pendant side-arms. The high degree of rotational freedom of these hydrogen-bonding groups makes this class of metal complex promising in terms of specific binding to water-soluble biomolecules having complementary hydrogen-bonding sites.