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

The 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 ...).