Copper(II) and nickel(II) chelates with dihydrogen Trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetate(2−) ion (H 2CDTA 2−). Synthesis, XRD structure and properties of [Cu(H 2CDTA)]·H 2O and [Ni(H 2CDTA)(H 2O)·4H 2O

Abstract

Stoichiometric reactions of metal hydroxycarbonates with the acid trans-1,2-cyclohexanediaminotetraacetic acid (H 4CDTA) in water under reduced pressure yielded [Cu(H 2CDTA)]·H 2O ( I) and [Ni(H 2CDTA) (H 2O)]·4H 2O ( II). Both compounds were characterized by TG-DTA analysis, spectral properties (IR, reflectance and RSE) and X-ray diffraction. In I the copper(II) atom exhibits a distorted square-base coordination (type 4+1) by chelation of one H 2CDTA 2− ligand through two N and two O (carboxylate) at the square base and one O (carboxylic) at the apex of the coordination polyhedron; a second carboxymethyl group of H 2CDTA 2− remains free. In II the H 2CDTA 2− chelating ligand also plays a quinquedentate role, but one water molecule achieves the slightly distorted octahedral coordination of the nickel(II) atom. Appropriate comparisons with the structure of [M(H 2EDTA)(H 2O)] (M = Ni, Cu complexes suggest that the steric constraints in the H 2CDTA ligand promotes the distorted five-coordination of the Cu II chelate in I as well as the hydration of the nearly octahedral Ni II derivative ( II). The double protonation of the ligand H 2CDTA 2− is carried out over different kinds of chelate rings, G and R, for Cu II and Ni II M(CDTA) derivatives, respectively (where G and R, indicate metal-glycinate rings nearly coplanar or perpendicular to the plane MNN, respectively).