Molecular recognition of adeninium cations on anionic metal-oxalato frameworks:: An experimental and theoretical analysis

Reactions of adenine with water-soluble oxalato complexes at acidic pH give the compounds (1H,9H-ade)(2)[Cu(ox)(2)(H2O)] (1) [H(2)ade = adeninium cation (1+), ox = oxalato ligand (2-)] and (3H,7H-ade)(2)[M(ox)(2)(H2O)(2)]center dot 2H(2)O [M(II) = Co (2), Zn (3)]. The X-ray single crystal analyses show that the supramolecular architecture of all compounds is built up of anionic sheets of metal-oxalato-water complexes and ribbons of cationic nucleobases among them to afford lamellar inorganic-organic hybrid materials. The molecular recognition process between the organic and the inorganic frameworks determines the isolated tautomeric form of the adeninium cation found in the crystal structures: the canonical 1H,9H for compound 1, and the first solid-state characterized 3H,7H-adeninium tautomer for compounds 2 and 3. Density functional theory calculations have been performed to study the stability of the protonated nucleobase forms and their hydrogen-bonded associations by comparing experimental and theoretical results.