π-d Interaction-based molecular magnets

The crystal structures and physical properties of molecular magnets developed in our group are reviewed. (1) (DMET)₂FeBr₄ and its analogues are composed of alternating stacks of quasi-one-dimensional donor sheets and square lattice magnetic anion sheets. These salts undergo an SDW transition of the donor layer and an antiferromagnetic transition of Fe³⁺ spins on the anion layer. The one-to-one correspondence of the anomalies appearing on the magnetization curves and the magnetoresistance supports the presence of the π-d interaction. On applying pressure, a large negative magnetoresistance is observed for the all-sulfur compound (EDTDM)₂FeBr₄ in the marginal region of the SDW and metallic ground states. (2) (BDH-TTP)[M(isoq)₂(NCS)₄] (M = Cr, Fe) show bulk weak ferromagnetism at 7.6 K. The donor cation radicals (S = 1/2) and anions (S = 3/2 (Cr), 5/2 (Fe)) form ferrimagnetic chains with close intermolecular S⋯S contacts, which are then antiferromagnetically coupled through the π-π overlap of the ligands and inter-chain S⋯S contacts of the donors. The non-collinear alignment of the molecular axes of adjacent anions is responsible for the canted spin structure.