Magnetic properties of lone-pair-containing, sandwich-type polyoxoanions: A detailed study of the heteroatomic effect

Magnetic susceptibility and high-frequency EPR spectroscopy investigations were carried out on the isostructural family of tricopper(II)-substituted, sandwich-type polyoxo-tungstates [Cu-3(H2O)(3)(alpha-AsW9O33)(2)](12-) (1), [Cu-3(H2O)(3)(alpha-SbW9O33)(2)](12-) (2), [Cu-3(H2O)(3)(alpha-SeW9O33)(2)](10-) (3), and [Cu-3(H2O)(3)(alpha-TeW9O33)(2)](10-) (4) with the intention of examining the possible influence of the diamagnetic heteroatoms X (X = As-III, Sb-III, Se-IV, Te-IV) on the magnetic exchange properties of the three Cu2+ ions in the central belt. Magnetic susceptibility data indicate a single antiferromagnetic spin exchange constant J for the triangular Cu-3 belts in 1-4. Variable-temperature, W-band (ca. 94 GHz) EPR spectra reveal that the Cu-3 units have S-T = 1/2 as the spin ground state with the excited state S-T = 3/2 located within a few cm(-1). We also determined the g matrix components for both the ST = 1/2 and 3/2 states. The zero-field parameter D has been measured for the S-T = 3/2 state, and its sign is shown to be positive for 1-4. From the analysis of the g matrix components for the molecular and the local Cu2+ site symmetry, the unpaired electrons are shown to occupy the 3d(x2-y2) orbitals. The J values change from -1.36 to -1.04 cm(-1) for 1 and 2, and from -1.52 to -1.48 cm(-1) for 3 and 4, respectively. The g values follow a similar trend, but the D value for 4 implies a role of through-space as well as through-bond sources for the magnetic interactions in the Cu-3 belt. Our results suggest that the heteroatoms, containing lone pairs, allow the fine tuning of the magnetic parameters of 1-4. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.