Thermally-induced spin-crossover (SCO) in Fe(4-ethynylpyrdine)2[Fe(CN)5NO]. Why is the SCO observed in 2D ferrous nitroprussides despite the NO–NC repulsive interaction in the interlayer region?

Pillared ferrous nitroprussides form an interesting series of coordination polymers where the spin crossover (SCO) behavior has been observed for several pillar organic molecules. This contribution reports such a reversible spin transition for the titled composition. The high-to-low spin transition (HS → LS) involves an electron density redistribution in the iron atom and its coordination environment, which is probed by magnetic (SQUID) measurements, IR, Raman, Mössbauer spectra, and DSC curves. The Mössbauer spectra recorded at 300 and 5 K disclose why in pillared ferrous nitroprussides, the SCO is possible despite the repulsive NO–NC electrostatic interaction in the interlayer region and the low electron density found at the CN5σ orbital of the equatorial ligands. On the sample cooling and related unit cell contraction, the NO–NC repulsive interaction enhances, and electron density is returned from the NO group and axial CN to the iron atom in the nitroprusside ion. This effect is particularly reinforced during the HS → LS transition. The increase of the electron density at the iron atom results in a stronger π-back donation interaction with the equatorial CNs to increase the charge accumulated at their CN5σ orbitals. This makes possible the formation of a stronger Fe–NC coordination bond, increasing the 10Dq value, and contributing to stabilizing the LS state of the iron atom. Such concerted electron density redistribution mechanisms, supported by IR and Mössbauer spectra, make possible the SCO in pillared ferrous nitroprussides. No previous study has been reported in that sense.