Molecule-based Magnets: Ferro- and Antiferromagnetic Interactions in Copper(II)-polyorganosiloxanolate Clusters

The magnetic behavior of the clusters [(PhSiO2)6Cu6(O2SiPh)6]·6EtOH (1), Na4[(PhSiO2)12Cu4]·8BuOH (2), and K4[(C2H2SiO2)12Cu4]·6BuOH (3) was studied by combined magnetic susceptibility measurements and variable-temp. EPR techniques (9.25 and 245 GHz). The six Cu(II) ions in the core of 1, which approaches 6/mmm symmetry, are ferromagnetically coupled as a result of the geometry at the bridging siloxanolate O atoms (Cu-O-Cu = 91.5-94.6°; J = -42 cm-1). The ground S = 3 spin state is split in zero field mainly due to anisotropic exchange contributions (D = 0.30 cm-1). Notably, both the magnitude and the sign of the zero-field splitting parameter were detd. from HF-EPR spectra. Large antiferromagnetic Cu-Cu interactions (J ∼ 200 cm-1) and an S = 0 ground state were detected in the tetranuclear clusters 2 and 3 as a consequence of the larger Cu-O-Cu angles. The results are relevant to the search for new mol.-based magnetic materials.