A unique heteropentanuclear CuII2CoIICoIII2 complex, synthesised from metallic Cu and Co acetate in the presence of triethanolamine. Magnetic properties and a strong H-bond stabilised lattice

Abstract

Reaction of zerovalent copper with cobalt(II) acetate and triethanolamine (H₃Tea) in DMF solution in air yields the unique pentanuclear mixed-valence complex [Cu^II₂Co^IICO^III₂(O₂CMe)₄(H₂Tea)₂(Tea)₂]·2(HO₂CMe), 1, which has been characterized by a range of spectroscopic methods and structurally by X-ray crystallography. In the centrosymmetric unit of 1 five metal ions are linked together by eight oxygen atoms of the four triethanolamine ligands and by two acetate anions in the sequence Cu(II)–Co(III)–Co(II)–Co(III)–Cu(II), with the metal–metal separations being 2.823(2) and 2.964(1) Å for Cu(II)···Co(III) and Co(III)···Co(II), respectively. Discrimination between Co(II) and Co(III) atoms is based on bond length considerations; the Co(III) is low spin. The lattice hydrogen-bonded network involving the non-coordinating acetic acid molecules links the pentanuclear units together and results in the formation of an extended one-dimensional structure. Magnetic measurements for 1 showed a marked decrease of the χT value at low temperature, indicative of antiferromagnetic interactions between magnetic centres. A fit was attempted assuming an effective S  = 1/2 ground state for Co(II) and an Ising-type anisotropy, considering only intramolecular interactions. The obtained value for J_z of 36 cm⁻¹ must originate from the efficient role of the diamagnetic Co(III) in transmitting the interaction between Cu(II) and Co(II) below 40 K.