Bimetallic compounds of trans-cyclohexane-1,2-diamine-NNN′N′-tetra-acetate(cdta): structural and magnetic characterization of [(H2O)4Cu(cdta)Ni]·3H2O and [(H2O)5Ni(cdta)Cu]·H2O

Abstract

The crystal structure of the complex [(H₂O)₄Cu(cdta)Ni]·3H₂O (1)(cdta =trans-cyclohexane-1,2-diamine-NNN′N′-tetra-acetate) has been determined by X-ray methods. It crystallizes in the triclinic space group P with Z= 2 in a cell of dimensions a= 8.685(4), b= 11.415(6), c= 12.841(4)Åα= 96.43(4), β= 96.60(3), γ= 102.23(6)°. Least-squares refinement of 2 368 reflections [I > 2.5σ(I)] and 316 parameters gave a final R= 0.036. The molecular structure consists of heterobimetallic units in which the Cu^II ion occupies a ‘hydrated’ octahedral site and the Ni^II ion a ‘chelated’ one. The two metal ions are linked through a bridging oxygen belonging to a carboxylate group. The molecular structure of [(H₂O)₅Ni(cdta)Cu]·H₂O (2) is described, based on that determined previously for the isostructural zinc–copper compound. Magnetic properties of these compounds are analyzed in terms of isolated (Cu^II, Ni^II) ion pairs. Whereas there is no exchange coupling detected down to 4 K for the nickel–copper compound (2), an antiferromagnetic coupling (J=–30.6 cm^–1) is found for the copper–nickel one, (1). This difference is discussed based on comparison of the structural features of the bridging network and co-ordination sites.