Synthesis and Crystal Structures of Copper(II) Complexes with Schiff Base Ligands

Abstract

Schiff bases containing N, O/S donor atoms and their metal complexes have received considerable attention owing to their intriguing structural motifs and industrial, antifungal, and biological applications. Recently, dinuclear copper(II) complexes, [Cu2(L)2(CH3COO)](ClO4) (L = 2benzoylpyridine S-methyldithiocarbazate) and [Cu2L2(SO4)] (L = di-2-pyridyl ketone N(4),N(4)-(butane-1,4-diyl)thiosemicarbazone) in which sulfur atom from Schiff bases ligand together with acetate or sulfate oxygen atoms bridges the two copper(II) ions, respectively, were reported. Although many metal(II)-Schiff base complexes have been reported, dinuclear Cu(II) complexes consist of Schiff base and other the second ligand, especially, dinuclear complexes linked through the other second ligand except the solvent molecule or the counter anion of metal salt as starting material have been little published. In this context, we reported dinuclear Cu(II) complexes of an acetyl pyridine based dithiocarbamate or 4-phenyl-3-thiosemicarbazide with benzilic acid (H2BA) as second ligand. For metal ions, benzilic acid can provide a variety of chelating and/or bridging coordination modes displayed by the carboxylic or hydroxy groups. Many frameworks constructed by BA or HBA with transition metal ions or rare earth ions have been reported, mainly using hydrothermal synthetic method. In this study, we extended these systems to a dinuclear compound of copper(II)-benzilate with 2-acetylpyridine/2-benzoylpyridine based benzhydrazide (Scheme 1) in order to study the crystal structure as part of our longstanding interest in synthesizing and extending the dimensionality of coordination compounds with mixed N,O/S coordination spheres. The molecular structures of 1 and 2 were determined using single crystal X-ray diffraction techniques. The crystallographic data and refinement parameters are listed in Table 1. Selected bond parameters are listed in Table 2. The molecular structure of complex 1 contains dinuclear [Cu2 (apb)2(HBA)(ClO4)] in which two enolate oxygen atoms and bidentate bridging HBA anion bridge the two copper(II) centers and lattice water molecule (Fig. 1a). Each of the two copper atoms in [Cu2(apb)2(HBA)(ClO4)] has different coordination environments. Cu1 adopts a five-coordinate square-pyramidal (τ = 0.27, the geometric parameter τ = |β − α|/60, where β and α are the two largest angles around the central atom; τ = 0 and 1 for the perfect square pyramidal and trigonal bipyramidal geometries, respectively.) with a N2O3 donor. The pyridine nitrogen (N1), the azomethine nitrogen atom (N2) and the enolate oxygen atom (O1) together with the carboxyl oxygen atom (O(3)) from HBA ligand comprise the basal plane of the square-pyramid whereas the enolate oxygen atom (O2) of another ligand occupies the apical position. The maximum displacement of them from the coordination plane is 0.005 (4) A (N2). Cu1 atom displaces 0.022(1) A out of the plane. The behavior of apb results in the formation of two five-membered rings around Cu1 atom. Two planes [Cu1−N1−C5−C6−N2 and Cu1−N2−N3−C8−O1] are nearly planar with mean deviation of 0.048(5) and 0.018(4) A, respectively, the dihedral angle between them being 2.3(1)°. The environment around Cu2 atom can be best described as a distorted octahedral geometry in a N2O4 manner. One