Solid and solution study of tetranuclear zinc citrates with N-donor chelates

Abstract

Tetranuclear zinc(II) citrates with N-heterocycle ligand, [Zn4(Hcit)2(phen)4(H2O)4]·2NO3·10H2O (1) and [Zn4(Hcit)2(bpy)4(H2O)6]·2NO3·12H2O (2) (H4cit = citric acid, phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine), were synthesized from aqueous solution and characterized by Infrared, Powder X-ray diffraction, fluorescence spectra, and X-ray structural analyses. Complexes 1 and 2 are centrosymmetric tetranuclear species with two crystallographically independent Zn(II) ions, which display different coordination geometries, while the citrate ligand shows similar coordination modes and bridges three different Zn ions. In 1, the Zn1 is five-coordinate by three oxygens from two citrates and two nitrogens from phen. There exist weak interactions between Zn1 and O4a atom of β-carboxy group of citrate (Zn1⋯O4a = 2.895 Å). Zn2 is six-coordinate by two oxygens from one citrate, two nitrogens from phen, and two water molecules. Zn1 and Zn2 are bridged by one citrate to form a dinuclear unit, which is further extended to a tetranuclear unit. In 2, two citrates bridge two pairs of symmetry-related Zn(II) ions into a tetranuclear unit. The structural analyses of 1 and 2 indicate that different tetranuclear species are caused by arrangement of citrates with different steric hindrance of N-chelates. 1H and 13C NMR spectra indicate that 2 remains coordinated in D2O. Moreover, both 1 and 2 show strong fluorescent emission in dilute solution and solid state.