Order-Disorder Transition Coupled with Magnetic Bistability in the Ferricinium Salt of a Radical Nickel Dithiolene Complex

International audienceThe ferricinium salt of the anionic, S = 1/2, dithiolene complex [Ni(tfadt)2]- (where tfadt is the asymmetrically substituted 2-(trifluoromethyl)acrylonitrile-1,2-dithiolate) crystallizes at room temperature (rt) into uniform chains of dithiolene complexes, separated by ferricinium cations. Both ionic entities are characterized by disorder affecting one CF3 and one Cp moiety. Above 250 K, this compound displays a Curie-type behavior. At lower temperatures, two first-order transitions around 249 and 137 K are revealed by susceptibility measurements and the observation of hysteresis effects. Crystal structure determinations performed at 230 and 120 K show that the high-temperature transition is associated with an ordering of the CF3 and Cp groups together with a dimerization of the anionic stacks that thus induces a drop of the susceptibility. The second, low-temperature transition leads to a tetramerization of these nickel dithiolene stacks now in a complete diamagnetic state while the remaining susceptibility originates from the sole ferricinium contribution. The first order character of the 249 K transition with its associated bistable behavior is likely correlated with the structural order−disorder transition, an original behavior in this class of materials where bistability is most often associated with a strengthening of interstack intermolecular interactions (hydrogen bonding, π−π interactions ...)