Temperature- and Pressure-Induced Switching of the Molecular Spin State of an Orthorhombic Iron(III) Spin-Crossover Salt

The [(TPA)Fe-III(TCC)](+) complex [TPA = tris(2-pyridylmethyl)amine, TCC2- = 3,4,5,6-tetrachlorocatecholate dianion] forms a molecular solid with the SbF6- anion, which exhibits an incomplete S = 1/2 <-> S = 5/2 thermal spin-crossover process centered at 250 K. Single-crystal X-ray diffraction, Raman spectroscopy, and magnetic measurements of this orthorhombic polymorph were performed to delineate the spin-state thermo- and piezoswitching in relation to the structural and elastic properties. Indeed, a pressure-induced spin cross-over (centered at P-1/2 approximate to 1.65 kbar, T = 294 K) concomitant with unit-cell contraction was observed by applying a hydrostatic pressure to the single crystal. Under ambient conditions, the piezo-induced transition, like that induced thermally, is relatively gradual, which probably reflects a similar mechanism in response to both stimuli. Except for a 47 K shift, the spin-crossover curves of the isostructural SbF6- and PF6- salts compare well. They are discussed with respect to structural anisotropy and pressure effects.