Antiferromagnetic tetranuclear manganese complex: Wheel or dicubane?

Abstract

[Mn 4(anca) 4(Htea) 2(dbm) 2] · 2.5Et 2O ( 1) (dbm − is the mono-anion of dibenzoylmethane, anca − is the anion of 9-anthracenecarboxylic acid and Htea is the dianion of triethanolamine) has been characterized through X-ray crystallographic and magnetic susceptibility techniques. Complex 1 is comprised of an alkoxy-bridged [ Mn 2 III Mn 2 II O 4 ] wheel-shaped core with an across-wheel Mn II⋯O distance of ∼2.7 Å. Variable-temperature dc magnetic susceptibility data were collected between 300 K and 1.8 K with an applied dc field of 0.1 T. Complex 1 exhibits a gradual decrease in magnetic susceptibility with decreasing temperature which is indicative of antiferromagnetic exchange interactions. Least squares fitting employing the Kambe approach with one J-value yielded an S = 1 ground state and fitting parameters of g = 1.8 and J = −2.1 cm −1. A fit with the Kambe approach assuming two different J-values, where one represents an across-wheel Mn II⋯Mn II interaction, did not improve on the fit. Analysis was also carried out on the variable temperature dc susceptibility and magnetization versus field data (5, 4, 3, 2 and 1 T in the 1.8–4.0 K range) using a full matrix diagonalization approach Preliminary calculations have yielded fitting parameters of g = 2.2, J = −2.8 cm −1 and D = −9.2 cm −1. In this case the 900 × 900 matrix included terms for the single-ion zero-field interactions, HDVV and antisymmetric exchange interactions as well as Zeeman interactions for the [ Mn 2 III Mn 2 II ] complex.