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) [H2ade = adeninium cation (1+), ox = oxalato ligand (2−)] and (3H,7H-ade)2[M(ox)2(H2O)2]·2H2O
[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.