Mössbauer and magnetic properties of mononuclear, homo- and hetero-dinuclear complexes

Abstract

A series of mononuclear, homo- and hetero-dinuclear transition metal complexes with compartmental, potentially hexadentate, Schiff bases has been synthesized and characterized by IR, Mössbauer and magnetic measurements. The mononuclear and homo-dinuclear complexes have been obtained by reaction of the preformed ligands, derived by the condensation of 2,3-dihydroxy-benzaldehyde and, respectively, 1,2-diaminoethane (H 4L A), 1,2- diamino-2-methylpropane (H 4L B), 1,3-diamino-2,2-dimethylpropane (H 4L C) or 1,2-diaminobenzene (H 4L D) with the appropriate metal salts, or by template condensation of the formyl and the appropriate amine precursor. The hetero-dinuclear complexes have been obtained by reaction of the mononuclear complexes with the desired metal salt, in the presence of base. By using iron(II) acetate, mononuclear and homo-dinuclear iron(III) complexes have also been prepared. Mononuclear, homo- and hetero-dinuclear complexes have also been prepared for the ligands obtained by reaction of ethylenediamine with 3-methoxy (H 2L E) or 3 ethoxy-salicylaldehyde (H 2L F), respectively. For the iron-containing compounds, the site occupancy, the oxidation state of the metal ion and the coordination geometry were verified by Mössbauer spectroscopy. The Mössbauer effect parameters of the mononuclear complexes are typical for octahedral high-spin iron(III) derivatives containing a single iron site. The presence of substituents on the ligand does not affect the isomer shift values, while it has a marked influence on the quadrupole splitting. Its value increases from 0.77 mm/s for the complex with H 4L B to 0.98 mm/s for the derivative with H 4L D. In the same way for the hetero-dinuclear compounds containing copper(II)-iron(II1) and nickel(II)-iron(III) ions, the δ values do not change while large differences are evident in the quadrupole splitting. The Mössbauer spectra of the homo-dinuclear complexes exhibit a doublet with a large linewidth that can only be fit as a superimposition of two different doublets, one inside the other. The parameters obtained for the inner and the outer doublet are very similar to those of the mononuclear and of the hetero-dinuclear derivatives, respectively. In the di-iron derivative with the ligand H 4L D, the outer iron is only partially oxidized, and variable amounts of a distorted octahedral high-spin iron(II) species can always be detected in the Mössbauer spectra. The magnetic properties of this class of complexes have been studied. In particular the magnetic susceptibility of the mononuclear Fe, homodinuclear Fe 2, heterodinuclear CuFe and CuMn derivatives have been measured down to 2.5 K and analyzed by means of appropriate models. The coupling for the dinuclear compounds was found to be antiferromagnetic in nature.