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 Huckel method, which demonstrates that the indirect π–d interactions play an important role in the antiferromagnetic ordering in 1·FeBr4 and 2·FeBr4.