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 [Cu 3 (H 2 O) 3 (α‐AsW 9 O 33 ) 2 ] 12− ( 1 ), [Cu 3 (H 2 O) 3 (α‐SbW 9 O 33 ) 2 ] 12− ( 2 ), [Cu 3 (H 2 O) 3 (α‐SeW 9 O 33 ) 2 ] 10− ( 3 ), and [Cu 3 (H 2 O) 3 (α‐TeW 9 O 33 ) 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 Cu 2+ 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 S T = 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 Cu 2+ site symmetry, the unpaired electrons are shown to occupy the 3d x 2 − y 2 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 . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)