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.