Molecular Recognition of Adeninium Cations on Anionic Metal−Oxalato\nFrameworks:  An Experimental and Theoretical Analysis

Abstract

Reactions of adenine with water-soluble oxalato complexes at acidic pH give the compounds (1<i>H</i>,9<i>H</i>-ade)₂[Cu(ox)₂(H₂O)] (<b>1</b>) [H₂ade = adeninium cation (1+), ox = oxalato ligand (2−)] and (3<i>H</i>,7<i>H</i>-ade)₂[M(ox)₂(H₂O)₂]·2H₂O\n[M(II) = Co (<b>2</b>), Zn (<b>3</b>)]. The X-ray single crystal analyses show that the supramolecular architecture of all compounds\nis built up of anionic sheets of metal−oxalato−water complexes and ribbons of cationic nucleobases among them\nto afford lamellar inorganic−organic hybrid materials. The molecular recognition process between the organic and\nthe inorganic frameworks determines the isolated tautomeric form of the adeninium cation found in the crystal\nstructures: the canonical 1<i>H</i>,9<i>H</i> for compound <b>1</b>, and the first solid-state characterized 3<i>H</i>,7<i>H</i>-adeninium tautomer\nfor compounds <b>2</b> and <b>3</b>. Density functional theory calculations have been performed to study the stability of the\nprotonated nucleobase forms and their hydrogen-bonded associations by comparing experimental and theoretical\nresults.