Metal ion binding modes of hypoxanthine and xanthine versus the versatile behaviour of adenine

The metal coordination patterns of hypoxanthine, xanthine and related oxy-purines have been reviewed on the basis of the structural information available in the Cambridge Structural Database (CSD), including also the most recent reports founded in SciFinder. Attention is paid to the metal ion binding modes and interligand interactions in mixed-ligand metal complexes, as well as the possibilities of metal binding of the exocyclic-O atoms. The information in CSD is also reviewed for the complexes of adenine in cationic, neutral and anionic forms with every metal ion. In contrast to the scarce structural information about hypoxanthine and related complexes, large structural information is available for adenine complexes with a variety of metals that reveals some correlations between the crystal-chemical properties of metal ions. Three aspects are studied in deep: the coordination patterns, the interligand interactions influencing the molecular recognition in mixed-ligand metal complexes and the connectivity between metals for different adenine species, thus supporting its unique versatility as ligand. When possible, the overall behaviour showed by adenine metal complexes is discussed according to the HSAB Pearson criteria and the tautomeric behaviour observed for each protonated species of adenine. The differences between the roles of adenine and the referred oxypurines ligands are underlined.