Spin-crossover in the Fe(4X-pyridine)₂[Fe(CN)₅NO] series with X = Cl, Br, and I. Role of the distortion for the iron atom coordination environment

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.