Spin-crossover in [Fe(Quinazoline)2][Fe(CN)5NO]. Evidence of its framework flexibility

The titled material shows a thermally induced spin-crossover (SCO) in the 75-150 K temperature region, with a narrow hysteresis of about 3 K. The recorded DSC data show a similar small hysteresis. Such behavior suggests that its framework has high flexibility, making possible the reversible spin transition on the sample cooling and warming, with slight kinetic effects. The crystal structure of this material was solved and refined from powder XRD patterns, complemented with information from IR, Raman, TG, and Mossbauer data. It crystallizes with a monoclinic unit cell in the P-21 space group. Its framework is formed by stacked [Fe(Quinazoline)(2)][Fe(CN)(5)NO] layers. The organic molecule occupies the axial coordination sites for the iron atom linked to the N ends of equatorial CN ligands. In the interlayer region, organic ligands from adjacent layers remain coupled through weak dipolar and dispersive forces and a pi center dot center dot center dot pi interaction. Such relatively weak forces between adjacent layers explain the high framework flexibility inferred from the magnetic and DSC data. The crystal structure for the low temperature (low spin) phase was obtained by periodic DFT calculations using the VASP package. The results herein discussed contribute to understanding the role of the solid framework flexibility on the spin-crossover behavior in Hofmann-like 2D coordination polymers.