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, [

Reaction between nucleobases (adenine, hypoxanthine, cytosine, and uracil), tris(2-aminoethyl)amine (tren) and M(ClO4)2 (M2+ = Zn2+, Cd2+, Ni2+ or Cu2+) under pH 8–9 gave eight ternary tren-metal ion-nucleobase complexes, among which seven crystal structures, [Zn(tren)(adeninato)] · ClO4 (1), [Cd(tren)(adeninato)] · ClO4 (2), [Ni(tren)(adeninato)(ClO4)] (3), [Zn(tren)(hypoxanthinato)] · ClO4 · H2O (4), [{Cd(tren)}2(hypoxanthinato)] · (ClO4)3 · 0.5 H2O (5), [Ni(tren)(uracilato)(H2O)] · ClO4 · (0.5H2O)2 (7), and [Cu(tren)(uracilato)] · ClO4 · 0.5H2O (8) were determined by X-ray diffraction. In each of the adenine complexes with Zn2+ (1), Cd2+ (2), and Ni2+ (3), among which 1 and 2 are isostructural to each other, a tren-capped metal ion binds to an adeninato ligand through N(9) with the formation of an intramolecular interligand hydrogen bond between the amino nitrogen of tren and N(3) of the base. In the hypoxanthine complexes 4 and 5, a tren-capped Zn2+ ion in 4 binds to a hypoxanthinato ligand through N(9) with the formation of an intramolecular N(tren)–H⋯N(3) hydrogen bond, while in 5 two tren-capped Cd2+ ions bind to a hypoxanthinato ligand, one through N(7) with the formation of an intramolecular N(tren)–H⋯O(6) hydrogen bond and the other through N(9) to form an intramolecular N(tren)–H⋯N(3) hydrogen bond. In each of the uracil complexes with Ni2+ (7) and Cu2+ (8), a tren-capped metal ion binds to an uracilato ligand through N(1) with the formation of an intracomplex O(aqua)–H⋯O(2) hydrogen bond in 7 or an intramolecular N(tren)–H⋯O(2) hydrogen bond in 8. Tren-capped metal ion-bonded nucleobases act as building blocks for the assembly of supramolecular structures, by versatile hydrogen bonding and/or stacking abilities of nucleobases. The significance of intramolecular interligand interaction as a factor that affects metal-binding site(s) on nucleobases is emphasized.

Graphical abstractTreatment of nucleobases (adenine, hypoxanthine, cytosine, and uracil), tris(2-aminoethyl)amine (tren), and M(ClO4)2 (M = Zn2+, Cd2+, Ni2+, or Cu2+) under alkaline conditions gave seven ternary tren-metal ion-nucleobase complexes, whose crystal structures were determined by X-ray diffraction. In each complex, a tren-capped metal ion binds (or tren-capped metal ions bind) to N(9) of an adeninato ligand, N(9) or N(7) + N(9) of a hypoxanthinato ligand, and N(1) of an uracilato ligand. In all of these complexes, an intramolecular interligand hydrogen bond is (or hydrogen bonds are) commonly formed.Figure optionsDownload full-size imageDownload as PowerPoint slide