Site-specific metal bonding to xanthine in the tripodal tris(2-aminoethyl)amine (tren) ligand-system: Crystal structures of [Zn(tren)(H 2xan)] · ClO 4 · H 2O, [Cd(tren)(H 2xan)] · ClO 4 · H 2O, [Ni(tren)(H 2xan)(H 2O)] · ClO 4 · H 2O, and [Cu 2{(μ-Hxan)Cu(tren)} 2(μ-Hxan) 2(H 2O) 2] · 10H 2O (H 3xan = xanthine)

Abstract

Reaction between xanthine, tris(2-aminoethyl)amine (tren) and M(ClO 4) 2 (M = Zn 2+, Cd 2+, Ni 2+ or Cu 2+) under alkaline conditions gave five ternary tren–metal ion–xanthine complexes, among which crystal structures of [Zn(tren)(H 2xan)] · ClO 4 · H 2O ( 1), [Cd(tren)(H 2xan)] · ClO 4 · H 2O ( 2), [Ni(tren)(H 2xan)(H 2O)] · ClO 4 · H 2O ( 3), and [Cu 2{(μ-Hxan)Cu(tren)} 2(μ-Hxan) 2(H 2O) 2] · 10H 2O ( 4) (H 3xan = neutral xanthine) were determined by X-ray diffraction. In the Zn 2+ ( 1) and Cd 2+ ( 2) complexes, which are isostructural to each other, the tren-capped metal ion binds to xanthine through N(7) with the formation of an intramolecular interligand hydrogen bond between the amino nitrogen of tren and O(6) of the base. The Ni 2+ complex ( 3) involves the metal bonding to N(7) of the base with the formation of two intramolecular hydrogen bonds between O(6) of the base and the amino group of tren and the aqua ligand. The tetranuclear Cu 2+ complex ( 4) forms a paddle-wheel type of centrosymmetrical dicopper tetraxanthinate structure, where the two copper ions are bridged by four xanthine ligands through N(3)/N(9) chelation and, in addition, two among the four xanthine ligands are metalated each by the tren-capped copper ion through N(7) with the formation of an intramolecular hydrogen bond between the amino group of tren and O(6) of the base. The significance of intramolecular interligand interaction as a factor that affects site-specific metal bonding is emphasized. A review of metal bonding properties of so far reported xanthine–metal complexes is given.