Exchange coupling in a bis(heterodinuclear) [CuIINiII]2 and a linear heterotrinuclear complex CoIIICuIINiII. Synthesis, structures and properties

Abstract

Two heterometallic complexes [{Cu^II(HLOX)Ni^II(N₃)}₂] and [(tmtacn)Co^III(μ-OH)Cu^II(LOX)Ni^II(OH₂)₂][ClO₄]₂ (tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) containing the same [Cu^IINi^II] core embedded in an unsymmetrical dicompartmental imine-oxime ligand H₄LOX have been synthesized and characterized. Their crystal structures show that the Cu^II resides at the N(oxime)₂O(phenolate)₂ site and assumes a planar geometry. The Ni^II is six-co-ordinated and bound to an N₂O₄ donor array comprising two iminonitrogens, two phenolate oxygens and two axially co-ordinated H₂O molecules. The cobalt(III) is low spin and six-co-ordinated. In DMF solution at 10 K the EPR spectra of the complexes exhibit a spin-doublet ground state with “inverted” g values which demonstrate the delocalization of the unpaired electron over the CuNi core. Magnetic susceptibility measurements over the range 2–290 K confirm that the paramagnetic nickel(II) and copper(II) centres are antiferromagnetically coupled, with values for the exchange coupling constant J through the phenolate oxygens of −115 cm⁻¹ and −130 cm⁻¹ respectively. Considering the dimer as a single tetranuclear unit, the coupling constant J′ through the path Cu–N–O–Ni is very small (≈1 cm⁻¹) but positive (ferromagnetic coupling).