Metal ion interactions with nucleobases in the tripodal tris(2-aminoethyl)amine (tren) ligand-system. Crystal structures of [Zn(tren)(adeninato)] ClO4, [Cd(tren)(adeninato)] ClO4, [Ni(tren)(adeninato)(ClO4)], [Zn(tren)(hypoxanthinato)] ClO4 · H2O, [{Cd(tren)}2(hypoxanthinato)]·(ClO4)3·0.5H2O, [Ni(tr