Antiferromagnetic ordering of the FeBr4− d-spins via strong π–d interaction in a semiconducting EDT-CP-TTFVS·FeBr4 salt

Abstract

The 1 : 1 cation-radical salts of the bent donor molecules ethylenedioxy(-ethylenedithio-)trimethylene-tetrathiafulvalenothioquinone-1,3-dithiolemethides [EDO-CP-TTFVS (1) and EDT-CP-TTFVS (2)] with an FeX₄⁻ (X = Cl, Br) ion, 1·FeX₄ and 2·FeX₄, were obtained as black, plate-like single crystals. Three of them, 1·FeCl₄, 2·FeCl₄ and 2·FeBr₄, were prepared by electrochemical oxidation of 1 and 2 in the presence of Bu₄N·FeX₄ (X = Cl, Br), while 1·FeBr₄ was prepared by chemical oxidation of 1 with FeBr₃. All of these salts show semiconducting behaviour. The 2·FeX₄ salts have relatively good electrical conductivities of about 0.1 S cm⁻¹ at room temperature, which is around 100 times higher than those of 1·FeX₄. The crystal structures of these four salts are very similar, in that the donor molecules are strongly dimerized and the dimers construct one-dimensional columns along the a-axis. The main difference in the donor layers between 1·FeX₄ and 2·FeX₄ is their intradimer overlap modes, which are the “ring-over-bond” and “ring-over-atom” types, respectively. The counter-anions FeX₄⁻ are located close to the central part of the bent donor molecules as expected, and form the one-dimensional, uniform chain along the a-axis. There are close contacts between the donor molecules and the FeX₄⁻ ions in both 1·FeX₄ and 2·FeX₄ although with significant differences in both contact number and contact distance. The Fe(III) d-spins of the FeX₄⁻ ions are subject to antiferromagnetic interactions, and antiferromagnetic ordering is found at around 8 K and 5 K for 1·FeBr₄ and 2·FeBr₄, respectively. The magnetic-exchange interactions between π-electrons of the donor and the d-spins of the FeX₄⁻ ions are calculated by an extended Hückel method, which demonstrates that the indirect π–d interactions play an important role in the antiferromagnetic ordering in 1·FeBr₄ and 2·FeBr₄.