Spin crossover in Fe(4X-isonicotinate)2[Fe(CN)5NO] with X = methyl and ethyl: the effect of the 4X substituent size on the spin transition

This contribution reports the spin-crossover behavior of two 2D ferrous nitroprussides formed by incorporating 4-methyl isonicotinate and 4-ethyl isonicotinate as organic pillars (L), labeled MIso and EIso, respectively. The pillars between adjacent layers are formed by pairs of molecules (L) linked to axial coordination sites of the iron atom: Fe(L)₂[Fe(CN)₅NO]. These two Hofmann-like frameworks show thermally induced spin-crossover (SCO) in the 80–150 K temperature range. The corresponding crystal structures for their high-spin (HS) and low-spin (LS) phases were solved and refined from powder XRD patterns. The solids formed with these two pillar molecules crystallize in a monoclinic unit cell in the P2₁ space group. For MIso, where SCO involves pronounced magnetic hysteresis, the HS → LS transition is accompanied by a unit cell volume contraction of approximately 4.94%. When L = EIso, having a narrow magnetic hysteresis loop, there is a resulting unit cell volume contraction of 1.45%. The magnetic and structural study of SCO in these two solids was complemented by DSC curves and IR, Raman, and Mössbauer spectra. The observed thermally induced spin transition shows a marked dependence on the size of the substituent present in the pillar molecule. The nature of this dependence is discussed in this contribution.