Fe(L)2[M(CN)4] with L = 4-Ethyl Isonicotinate and M = Ni, Pd, Pt.─A Series of Hofmann-Type Frameworks with Spin Crossover: The Dominant Role of the Fe–NC Coordination Bond

This contribution reports the incorporation of an organic pillar (L), 4-ethyl isonicotinate, between adjacent layers of the series of 2D coordination polymers, Fe[M(CN)4], with M = Ni, Pd, and Pt. The resulting metal–organic frameworks (MOFs), Fe(L)2[M(CN)4], show thermally induced spin crossover (SCO) in the 150–250 K temperature range. In these frameworks, the pillars are formed by a pair of molecules (L) linked to the iron atoms in axial coordination sites. This series of hybrid inorganic–organic solids series preserves the 2D character of the inorganic building blocks because the interactions between the pairs of organic pillars are of a physical nature. These types of solids are also known as Hofmann-like frameworks. Their crystal structure was solved and refined from powder XRD patterns recorded at 100 and 300 K, corresponding to the low-spin (LS) and high-spin (HS) phases, respectively. The HS and LS phases crystallize with a triclinic unit cell in the p-1 (#2) space group. The HS→ LS transition involves a unit cell volume contraction of about 6%. The spin transition on the sample cooling and warming was studied from XRD data; magnetic (SQUID) measurements; DSC curves; and IR, Raman, and Mössbauer spectra. No similar study for this MOF series has been reported.