Thermally induced spin transition in Fe(pyrazine)[Fe(CN)₅NO]

Transition metal nitroprussides form a family of coordination polymers with interesting physical properties. From their three-dimensional (3D) phases, by inducing rupture of the axial T–NC bond through organic molecules (L) with a high ability to form a T–L coordination bond, pillared transition metal nitroprussides can be obtained. For monodentate molecules, for example, pyridine and its derivatives, the pillars are formed in the interlayer region by pairs of molecules coupled through their dipole moments, and the resulting solid has a two-dimensional (2D) structure with T(L)₂[Fe(CN)₅NO] as the formula unit. In this contribution, we report the use of a bidentate molecule, pyrazine, to obtain pillared ferrous nitroprusside. According to the refined crystal structure, this solid has a 3D framework, which results from the pyrazine molecule (L) coordinating to two iron atoms to afford Fe(pyrazine)[Fe(CN)₅NO]. This material shows spin-crossover (SCO) behavior with thermal hysteresis of about 40 K, which is associated with relatively large structural changes, for instance, a reversible variation of 12% in the unit cell volume. The thermally induced spin transition is accompanied by a notable color change of the material. The main features of this thermally activated spin transition in the material under study are discussed herein on the basis of its structural characterization at 100 and 300 K, magnetic and differential scanning calorimetry (DSC) measurements, and Raman and Mössbauer spectra for both the low- and high-spin phases. The reversibility of this thermally activated spin transition has been verified by magnetic, Mössbauer, X-ray powder diffraction (XRD), and infrared (IR) data. To the best of our knowledge, this is the first report on the preparation and characterization of Fe(pyrazine)[Fe(CN)₅NO] and its thermally induced SCO behavior.