Mixed-ligand complexes of Zn(II), Cd(II) and Cu(II) with triethanolamine and p-nitrobenzoic acid: Syntheses and crystal structures

Abstract

Three new mixed-ligand complexes of Zn, Cd and Cu with triethanolamine (TEA) and p-nitrobenzoic acid (NBA) have been synthesized and structurally characterized by single-crystal X-ray diffraction as compounds with the compositions [Cu2(NBA)2TEA](NBA)(5H2O), [Zn(NBA)2TEA] and [Cd(NBA)2TEA], respectively.The Zn complex crystallized in the monoclinic space group Cc. The coordination polyhedron around the zinc atom corresponds to a highly distorted octahedron of the ZnNO5 type. The TEA ligand is tetradentately coordinated to the cation, thus forming three five-membered chelate rings. In addition, two NBA anions are bonded to the Zn2+-cation in the monodentate mode.The Cd complex crystallized in the orthorhombic space group Pbca. The cadmium atom is seven-coordinated with one monodentate NBA and one bidentate NBA anion, and a TEA ligand in the tetradentate mode.The formation of a dinuclear Cu complex, crystallizing in triclinic space group, is unexpected. The copper(II) ions are five-coordinated, forming distorted tetragonal pyramids, and are O,O′ bridged by the carboxyl group of an NBA ligand and an alkoxide group of the TEA ligand. In addition, the deprotonated TEA ligand coordinates one Cu center in a tetradentate fashion; the coordination sphere of the other Cu cation is completed by a monodentate NBA and aqua ligands.Thus, the 3 divalent metal cations form complexes of very different structures and compositions with the same NBA and TEA ligands because size criteria and crystal field effects play a role.