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 ...).