Metal ion interactions with nucleic acid bases in the tripodal tris(2-aminoethyl)amine (tren) ligand-system: Crystal structures of [Ni(tren)(adeninato)(ClO 4)] · adenine, [Cu(tren)(9-ethylguanine)] · (ClO 4) 2, [{Ni(tren)(H 2O)} 2(hypoxanthinato)] · (ClO 4) 3 · 1.5H 2O, [Ni(tren)(hypoxanthinato)(H 2O)] · ClO 4 · 2H 2O, and [{Ni(tren)(H 2O)} 2(hypoxanthinato)] 2 · (NO 3) 6 · 4.5H 2O

Abstract

Reaction of tris(2-aminoethyl)amine (tren) and Cu(ClO 4) 2, Ni(ClO 4) 2, or Ni(NO 3) 2 with purine nucleobases (adenine, 9-ethylguanine, and hypoxanthine) gave five ternary tren–metal ion–nucleobase complexes, [Ni(tren)(adeninato)(ClO 4)] · adenine ( 1), [Cu(tren)(9-ethylguanine)] · (ClO 4) 2 ( 2), [{Ni(tren)(H 2O)} 2(hypoxanthinato)] · (ClO 4) 3 · 1.5H 2O ( 3), [Ni(tren)(hypoxanthinato)(H 2O)] · ClO 4 · 2H 2O ( 4), and [{Ni(tren)(H 2O)} 2(hypoxanthinato)] 2 · (NO 3) 6 · 4.5H 2O ( 5), where 1 and 4 were prepared under pH 8–9 while 2, 3, and 5 were under pH 6–7, and their crystal structures were determined by X-ray diffraction. In the adenine complex 1, the tren-capped metal ion binds to the base through N(9) with the formation of an intramolecular interligand hydrogen bond between the amino group of tren and N(3) of the base, and the 9-ethylguanine complex 2 involves the metal bonding to N(7) of the base with the formation of an intramolecular hydrogen bond between the amino group of tren and O(6) of the base. In the hypoxanthine complexes 3 and 5, the metal ions bind to N(7) in addition to N(9), forming three intramolecular hydrogen bonds, two involving O(6) with the amino nitrogen of tren and the aqua ligand and one between the aqua ligand and N(3) of the base, whereas the hypoxanthine complex 4 involves the metal bonding solely to N(9) with the formation of an intramolecular hydrogen bond between the aqua ligand and N(3) of the base, where N(7) as well as N(1) are hemi-protonated. The absence of the metal–N(7) bonding for adenine in 1 while the metal bonding to N(7) specific for guanine derivatives in 2, 3 and 5 are rationalized in terms of interligand interactions.