Molecule-based magnets: Ferro- and antiferromagnetic interactions in copper(II)-polyorganosiloxanolate clusters

The magnetic behavior of the clusters [(PhSiO(2))(6)Cu-6(O(2)SiPh)(6)]. 6EtOH (1), Na-4[(PhSiO(2))(12)Cu-4]. 8(n)BuOH (2), and K-4[(C2H3SiO2)(12)Cu-4]. 6(n)BuOH (3) has been investigated by combined magnetic susceptibility measurements and variable-temperature EPR techniques (9.25 and 245 GHz). The six copper(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 oxygen atoms (Cu-O-Cu = 91.5-94.6 degrees; J = -42 cm(-1) with H = J Sigma(i=1)(6)Si . S-i+1, S-7 = S-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 have been determined from HF-EPR spectra. Large antiferromagnetic Cu-Cu interactions (J similar to 200 cm(-1)) and an S = 0 ground state have been detected in the tetranuclear clusters 2 and 3 as a consequence of the larger Cu-O-Cu angles. The results presented in the paper are relevant to the search for new molecule-based magnetic materials.