The 2D hybrid inorganic–organic solids herein under study were prepared by intercalation of organic molecules between neighboring layers of ferrous nitroprusside. In the formed solid, the organic molecules (4-methylpyridine, 4-acetylpyridine, 4-pyridinecarboxyaldehyde, and 4-vinylpyridine) are found occupying the axial coordination sites of the iron atom linked at the equatorial N ends of the CN ligands in the pseudo-octahedral [Fe(CN)₅NO] block. The resulting 3D framework of these hybrid solids is supported by intermolecular dispersive, C–H⋯π and dipole–dipole interactions in the interlayer region, with a contribution from dipolar coupling between unlinked NO and CN ligands from neighboring layers. Such intermolecular interactions modulate the coordination geometry of the iron atom. The local crystal field sensed by the latter is such that on material cooling and then on heating, a spin crossover transition with pronounced thermal hysteresis is observed. Both the transition temperatures and hysteresis are determined by the nature of the intercalated molecule. The mentioned structural features and the thermally induced spin transition were studied using the refined crystal structures, magnetic measurements, Mössbauer, Raman, and IR spectra, and DSC data. These experimental studies were complemented by computational calculations of the iron atom coordination environment. To the best of our knowledge, this is the first study on the preparation and characterization of this series of hybrid 2D solids, and of their thermally induced spin crossover effect.

This contribution reports the thermally induced spin crossover (SCO) in the titled series of hybrid solids from the magnetic (SQUID), DSC, IR, Raman, and Mössbauer data. The onset temperature for the high-to-low spin transition follows the order Cl (217 K) > Br (210 K) > I (155 K). This order is inverse to the one known for the halogen-bond propensity, Cl < Br < I. Their crystal structure for the high-spin (HS) phase was solved and refined from the powder XRD data, complemented with the IR spectra and TG curves. For the low-spin (LS) phase, the crystal structure was obtained by periodic DFT calculations, relaxing the room temperature (HS) structure. At room temperature, the CN–ON distance for these dangling ligands in the interlayer region follows the order Cl (5.03 Å) < Br (5.08 Å) < I (5.70 Å). The repulsive electrostatic interaction between these charge centers is responsible for a certain kinetic effect observed in the recorded magnetic data. This interaction was also detected in the recorded Mössbauer spectra. For the 3X-pyridine analogs, the thermally induced spin-crossover was observed only for X = F. This study discusses the nature of that difference in the behavior between the two subseries from three probes for the stabilization energy (SE) of the LS phase. Notably, (1) the effect of the substituent position on the deformation of the iron atom coordination environment; (2) the Mössbauer quadrupole splitting (Δ_QS) value; and (3) the onset of the a_1g → e_g(e_g) band absorption in the UV-vis-NIR spectrum. For a non-distorted octahedron, the value of SE coincides with 10Dq (the energy splitting between e_g and t_2g orbitals for the iron atom). This study is part of an effort to rationalize those structural features affecting the SCO in 2D ferrous nitroprussides.

Thermally induced spin crossover in the Fe(3-hydroxypyridine) ₂ [M(CN) ₄ ] series with M = Ni, Pd, and Pt is reported. The role of the distortion of the iron atom coordination environment in the possibility of observing the spin crossover is explored.

The charge density accumulated at the CN 5σ orbital, via π-back bonding interaction, determines the crystal field splitting for the Fe(ii) atom and the temperature where the spin transition is observed.