Phase Competition and Weak Hydrogen Bonding in the Giant Hysteresis of an S = 1/2 Nickel Dithiolene Complex: Combined Structural and Magnetic Studies

Abstract

The room-temperature (RT) crystal structure of the colbaltocinium salt of the S = 1/2 nickel dithiolene complex [Ni(tfadt) 2 ] –• (tfadt, 2-trifluoromethylacrylonitrile-1,2-dithiolate) is characterized by uniform one-dimensional spin chains of radical anions, separated from each other by the [Cp 2 Co] + cations. Both entities exhibit disorder affecting one CF 3 and one Cp ring. Upon being cooled from RT (phase A), two successive structural phase transitions lead to tetramerized spins chains with a low-temperature singlet ground state (phase C) with an associated ordering of the CF 3 and Cp moieties. An intermediate tetramerized C′ phase is observed during the cooling sweep, in competition with a dimerized chain structure (phase B) upon warming, affording an apparent giant thermal hysteresis loop of 50 K. The phase succession in temperature at atmospheric pressure is established on the basis of the correlation between the temperature dependence of the magnetic susceptibility and crystal structure data collections and resolutions at seven different temperatures. The behavior of this cobaltocinium salt contrasts strongly with the ferricinium analog, which exhibits successive A ↔ B ↔ C transitions upon being cooled from the regular A phase. A model based on the temperature evolution of the Gibbs energy for the two compounds is proposed to account for those differences. Comparison of the structural data between the salts in the B and C′ phases demonstrates the important role of weak C–H···F,N hydrogen-bond interactions in the stabilization of the different phases depending on the nature of the metallocenium cation.