Spin crossover in Hofmann-like coordination polymers. Effect of the axial ligand substituent and its position

This contribution reports the preparation and thermally induced spin crossover (SCO) in the Hofmann-like Fe(3-hydroxypyridine)₂[M(CN)₄] series, where M = Ni, Pd, and Pt. These three 2D coordination polymers crystallize with a monoclinic unit cell, in the C2/m space group (No. 12). Their crystal structures were solved and refined from powder XRD patterns. The thermally induced SCO in that series was studied from magnetic measurements (SQUID), DSC curves, and Mössbauer, IR, and Raman spectra. In Hofmann-like analog solids, pyridine and its analogs are the ligands of choice to observe SCO, particularly for 3-substituted pyridines. For 4-substituted derivatives, SCO in tetracyanide-based coordination polymers is rare or not observed. The nature of that regularity is discussed in this contribution, from NBO calculations, XPS, IR, Raman, UV-vis-NIR, and Mössbauer spectra. The mentioned regularity is related to the distortion of the iron atom coordination polyhedron. A significant distortion is unfavorable for the formation of a strong coordination bond of the axial organic ligand to the iron atom, and at the same time, it reduces the available energy gap to stabilize the low-spin electronic configuration for the iron atom, making the SCO not possible. XPS, IR, Raman, UV-vis, and Mössbauer spectra provided conclusive clues in that sense.