π-d interaction-based molecular magnets: Role of sulfur-to-selenium substitution

The crystal structure and physical properties of the three conducting molecular magnets are discussed. (DMET)₂FeBr₄ is composed of alternating stacks of quasi-one-dimensional donor sheets and square lattice magnetic anion sheets. This salt undergoes an spin density wave (SDW) transition of the donor layer at 40 K and an antiferromagnetic transition of Fe ³⁺ spins on the anion layer at 3.7 K. The one-to-one correspondence of the anomalies appearing on the magnetization curves with those on the magnetoresistance supports the presence of the π-d interaction. The all-sulfur analog (EDTDM)₂FeBr₄ shows, besides similar behaviors as the DMET salt, insulator-to-metal transition of the ground state by applying the pressure, accompanied with a large negative magnetoresistance. (EDS-TTF)₂FeBr₄ shows little π-d interaction despite the presence of close Se-Br contacts, showing the importance of the intermolecular orbital overlap between the π- and d-components.