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(H2O)6X2 · 2ade] (with X = Cl− (1), Br− (2), ClO4- (3)), [Mn(H2O)4 · (ade)2 · (ClO4)2] (4), and [Mn2(H2O)6 · (ade)2 · (NO3)4 · 2H2O] (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(H2O)4 · (ade)2]2+ units, with the ade molecules directly coordinated to the metal centres through the N(3) atoms, while compound 5 consists of [Mn2(H2O)6 · (ade)2]4+ 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.

Graphical abstractThe synthesis and crystal structure of the compounds [Mg(H2O)6X2 · 2ade] (with X = Cl− (1), Br− (2), ClO4- (3)), [Mn(H2O)4 · (ade)2 · (ClO4)2] (4), and [Mn2(H2O)6 · (ade)2 · (NO3)4 · 2H2O] (5) (where ade = adenine) are reported. Compounds 1–3 consist of self-assembled hexaaquo Mg(II) ions, ade molecules and anions. Compounds 4 and 5 represent a rare example of direct Mn–N binding involving isolated nucleobases and the first one containing ade as ligand. The detailed analysis of non-covalent interactions in all the compounds prove that the anions play a crucial role in determining the non-covalent stabilization of the minor 7H-adenine tautomer.Figure optionsDownload full-size imageDownload as PowerPoint slide