Synthesis and X-ray structural characterization of novel binuclear complexes of copper(II), nickel(II), and cobalt(II) with an octadentate ligand, N,N′,N″,N‴tetrakis(2-aminoethyl)-1,4,8,11-tetra-azacyclotetradecane

Abstract

The ligand N,N′,N″,N‴-tetrakis(2-aminoethyl)-1,4,8,11-tetra -azacyclotetradecane(taec) and its binuclear metal complexes, M₂(taec)X_nY_4–n′ were synthesized and characterized for M = Cu, Ni, or Co and where X and Y are anions (Cl^– Br^–, ClO₄^–, or OH^–). The crystal structures of Cu₂(taec)(ClO₄)₄(1), Cu₂(taec)Br(ClO₄)₃(2), and Ni₂(taec)(ClO₄)₄(3) were determined by single-crystal X-ray analysis. In all the complexes each taec molecule incorporates two metal ions, each of which is coordinated by two ring- and two pendant-nitrogen atoms. Complexes (1) and (3) adopt similar structures, different to (2). In (1) each copper is weakly co-ordinated by perchlorate oxygen forming an elongated square pyramid. In (2), the bromide ion is bound by both copper atoms. This structure accords with the proposed structure deduced from the unusually high stability of the Cu–Br linkage against hydrolysis in water. In (3) the co-ordination geometry is essentially square planar, and compatible with the observed diamagnetism and visible spectrum. From the magnetic data it was concluded that Ni₂(taec)Cl₄·H₂O consists of a diamagnetic nickel and a paramagnetic five- or six-co-ordinate nickel. All the cobalt(II) complexes are in the high-spin state. They are inert towards oxidation to cobalt(III), probably because of the difficulty of achieving six-co-ordination. In Co₂(taec)(OH)(PF₆)₃ and Co₂(taec)(OH)(ClO₄)₃ the hydroxide seems to be linked to both cobalt atoms, judging by the magnetic data.