Supramolecular architectures of metal-oxalato complexes containing purine nucleobases

The reaction of purine nucleobases (adenine, 3-methyladenine and 9-methylguanine) with a metallic salt in the presence of potassium oxalate yields three compounds with formulae {Cd(μ-ox)(H₂O)(Hade)]·H₂O}_n (1), {Cu(μ-ox)(H₂O)(3Meade)]·H₂O}_n (2) and Cu(ox)(H₂O)₂(9Megua)]·2.5H₂O (3). Crystal structures of compounds 1-2 consist of 1D zig-zag chains in which cis-M(H₂O)(nucleobase)]²⁺ fragments are linked by bis-bidentate oxalato ligands. In compound 1, the nucleobase is coordinated through the minor groove N3 atom, and the resulting non-canonical 7H-adenine tautomer is stabilized by non-covalent interactions involving more basic N9 and N7 sites. In compound 2, the mutagenic 3-methyladenine is attached to the metal atoms by means of the imidazole N7 atom. The dissimilar binding pattern of the nucleobases produces significant differences in the supramolecular architectures of compounds 1 and 2 which are essentially governed by an extensive network of non-covalent interactions such as hydrogen bonded adenine-adenine base pairs, hydration of the nucleobases, carboxylato-nucleobase associations, and face-to-face π-π stacking. The model 9-methylguanine nucleobase of compound 3 exhibits its usual coordination mode through the major groove N7 atom to form two monomeric (Cu(ox)(H₂O)₂(9Megua)] units which are held together by means of Watson-Crick like hydrogen bonds between the guanine moieties and the inorganic frameworks generating almost planar tetrameric metal-organic aggregates. The 3D packing of the complex entities affords an open structure containing voids which are filled by decameric (H₂O)₁₀ clusters. Variable-temperature magnetic susceptibility measurements of compound 2 show the occurrence of antiferromagnetic intrachain interactions in good agreement with the structural features of its 1D metal-oxalato framework.