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 polyoxotungstates [Cu3(H2O)3(α‐AsW9O33)2]12− (1), [Cu3(H2O)3(α‐SbW9O33)2]12− (2), [Cu3(H2O)3(α‐SeW9O33)2]10− (3), and [Cu3(H2O)3(α‐TeW9O33)2]10− (4) with the intention of examining the possible influence of the diamagnetic heteroatoms X (X = AsIII, SbIII, SeIV, TeIV) 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 Cu3 belts in 1−4. Variable‐temperature, W‐band (ca. 94 GHz) EPR spectra reveal that the Cu3 units have ST = 1/2 as the spin ground state with the excited state ST = 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 ST = 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 3dx2−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 Cu3 belt. Our results suggest that the heteroatoms, containing lone pairs, allow the fine tuning of the magnetic parameters of 1−4. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)