Metal-Nucleobase Interactions in Magnesium(II) and Manganese(II) Complexes with Adenine: Influence of the Anion on the Non-Covalent Stabilization of 7H-Adenine Tautomer

The synthesis and crystal structure of the compounds [Mg(H(2)O)(6)X(2) center dot 2ade] (with X = Cl(-) (1), Br(-) (2), ClO(4)(-) (3)), [Mn(H(2)O)(4) center dot (ade)(2) center dot (ClO(4))(2)] (4), and [Mn(2)(H(2)O)(6) center dot (ade)(2) center dot (NO(3))(4) center dot 2H(2)O] (5) (where ade = adenine) are reported. Compound 1, 2 and 3 consist of hexaaquo ions, ade molecules and chloride (1), bromide (2) or perchlorate (3) anions self-assembled through an extended network of hydrogen bonding interactions. Compound 4 is made up of [Mn(H(2)O)(4) center dot (ade)(2)](2+) units, with the ade molecules directly coordinated to the metal centres through the N(3) atoms, while compound 5 consists of [Mn(2)(H(2)O)(6) center dot (ade)(2)](2+) dimers, with the ade molecules directly coordinated through the N(3) and N(9) atoms. They both represent a rare example of direct Mn-N binding, characterized at solid-state, involving isolated nucleobases and the first one containing ade as ligand. In compounds 1, 2 and 4 the most basic imidazole N9 atom is not protonated and not involved in metal coordination, being engaged in strong hydrogen-bond interactions with coordinated water molecules, that determine the non-covalent stabilization of the minor 7H-adenine tautomer.