Cobaltocenium [1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazolidyl: Synthesis, Structure, and Magnetic Properties

Cobaltocene, [CoII(η5-C5H5)2], readily reduced [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (2) to the [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazolidyl radical anion (1) which was isolated in the form of the thermally stable salt [CoIII(η5-C5H5)2][1] (5) in 93 % yield. The XRD structure of 5 displays layers of radical anions 1, with S···N contacts of 358.2 and 366.0 pm between neighboring species. The salt was further characterized by ESR spectroscopy in the solid state as well as in solution. The temperature dependence of its magnetic susceptibility (χ) in the range of 2–300 K with a maximum at (9.7 ± 0.5) K indicates antiferromagnetic ordering of the spin system. The magnetic structure of 5 was analyzed in terms of dimeric exchange integrals calculated by a spin-unrestricted broken-symmetry approach at the DFT/UB3LYP/6-31+G(d) level of theory and was found to be 2D. Further analysis of the χ(T) dependence was performed by using the low- and high-temperature series expansion methods for the S = 1/2 square lattice antiferromagnetic Heisenberg model. The exchange interactions between radical anions 1 were estimated to be J = (–4.2 ± 0.7) cm–1. With the general form of the Van Vleck equation for a [3 × 4] grid of spins 1/2, the best fit of the experimental χ(T) was achieved with J1 = –5.0 cm–1 and J2 = –0.23 cm–1. Thus, the exchange interactions in 5 are stronger than those observed for previously studied salts of 1. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)