Abstract
Two new mononuclear iron(II) compounds (1) and (2) of the general formula [Fe(L)2](BF4)2·nCH3CN (L = 4-(2-bromoethyn-1-yl)-2,6-bis(pyrazol-1-yl)pyridine, n = 1 for (1) and n = 2 for compound (2)), were synthesized. The room temperature crystallization afforded concomitant formation of two different solvent analogues: compound (1) exhibiting triclinic P-1 and compound (2) monoclinic C2/c symmetry. Single-crystal X-ray studies confirmed the presence of the LS (low-spin) state for both compounds at 180 K and of the HS (high-spin) state for compound (2) at 293 K, in full agreement with the magnetic investigations for both solvent polymorphs. Compound (1) exhibits spin transition above 293 K followed by subsequent solvent liberation, while the spin transition of (2) takes already place at 237 K. After complete solvent removal from the crystal lattice, compound (1d) (the desolvated polymorph derived from (1)) exhibits spin transition centered at 342 K accompanied by a thermal hysteresis loop, while the analogous compound (2d) (the desolvated derivate of compound (2)) remains blocked in the HS state over all the investigated temperature range.
Highlights
Spin crossover (SCO) can be considered as a unimolecular transformation upon which switching entities are reversibly transformed between the low-spin (LS) and high-spin (HS) state
3d4 –3d7 electronic configuration, whereby the spherical symmetry of d-orbitals was broken into the eg and t2g orbital subsets by the tetragonal bipyramidal environment of the N6 coordination environment
From the thermodynamic point of view, the SCO can be described as a case of electronic bistability, which is associated with the existence of two minima of the Gibbs energy [8]
Summary
Spin crossover (SCO) can be considered as a unimolecular transformation upon which switching entities (e.g., molecular complexes or coordination polymers) are reversibly transformed between the low-spin (LS) and high-spin (HS) state. The transition of spin states is an entropy-driven process, which can be triggered by external parameters such as temperature [1], pressure [2], magnetic [3] or electric field [4,5], and by irradiation with the visible/near—infrared [6] or even X-ray [7] range. 3d4 –3d7 electronic configuration, whereby the spherical symmetry of d-orbitals was broken into the eg and t2g orbital subsets by the tetragonal bipyramidal environment of the N6 coordination environment. From the thermodynamic point of view, the SCO can be described as a case of electronic bistability, which is associated with the existence of two minima of the Gibbs energy [8]. When the Materials 2016, 9, 585; doi:10.3390/ma9070585 www.mdpi.com/journal/materials
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