Metal ion interactions with nucleobases in the tripodal tris(2-aminoethyl)amine (tren) ligand-system. Crystal structures of [Cu(tren)(adeninato)]·ClO4, [Ni(tren)(9-ethylguanine–0.5H)(H2O)]2·(ClO4)2.5·(ClO3)0.5 and [{Cu(tren)}2(hypoxanthinato)]·(ClO4)3