Unravelling the versatile metal binding modes of adenine: Looking at the recognition roles of deaza- and aza-adenines in mixed-ligand metal complexes

To better understand the extreme versatility of adenine as a ligand, the metal binding patterns of some closely related N-ligands have been carefully analysed. All the selected ligands comply with the requirement of having at least one N-heterocyclic atom in each cycle of the purine skeleton. The N-ligands are: (a) deaza-adenines [7-azaindole (H7azain), 4-azabenzimidazole (H4abim), 5-azabenzimidazole (H5abim), 7-deaza-adenine (H7deaA), purine (Hpur)] (b) adenine isomers [2-aminopurine (H2AP), 4-aminopyrazolo[3,4-d]pyrimidine (H4app), 7-amino[1,2,4]triazolo[1,5-a]pyrimidine (7atp)] and (c) aza-adenines [2,6-diaminopurine (Hdap), 8-aza-adenine (H8aA)]. The different molecular recognition patterns are reviewed on the basis of the available structural information in the Cambridge Structural Database, version 5.33 (updated Nov. 2012). No restraints have been placed concerning the neutral or charged species of these N-ligands or the metal ions involved in the coordination. Attention is paid to the proton tautomerism and its metal binding patterns, highlighting many examples where the molecular recognition pattern is carried out by the cooperation of a metal-N bond and an intra-molecular inter-ligand H-bonding interaction.