Spin Crossover and High-Spin State in Fe(II) Anionic Polymorphs Based on Tripodal Ligands

Two new mononuclear Fe(II) polymorphs, [(C2H5)(4)N](2)[Fe(py(3)C-OEt)(NCS)(3)](2) (1) and [(C2H5)(4)N][Fe(py(3)C-OEt)(NCS)(3)] (2) (py(3)C-OEt = tris(pyridin-2-yl)ethoxymethane), have been synthesized and characterized by single-crystal X-ray diffraction, by magnetic and photomagnetic measurements, and by detailed variable-temperature infrared spectroscopy. The molecular structure, in both complexes, is composed of the same anionic [Fe(py(3)C-OEt)(NCS)(3)](-) complex (two units for 1 and one unit for 2) generated by coordination to the Fe(II) metal center of one tridentate py(3)C-OEt tripodal ligand and three terminal kappa N-SCN coligands. Magnetic studies revealed that polymorph 2 displays a high-spin (HS) state over the entire studied temperature range (300-10 K), while complex 1 exhibits an abrupt and complete spin crossover (SCO) transition at ca. 132.3 K, the structural characterizations of which, performed at 295 and 100 K, show a strong modification, resulting from the thermal evolutions of the Fe-N bond lengths and of the distortion parameters (Sigma and Theta) of the FeN6 coordination sphere, in agreement with the presence of HS and low-spin (LS) states at 295 and 100 K, respectively. This thermal transition has been also confirmed by the thermal evolution of the maximum absorbance for nu(NCS) vibrational bands recorded in the temperature range 200-10 K. In 1 the signature of a metastable photoinduced HS state has been observed using photomagnetic and photoinfrared spectroscopy, leading to a similar T(LIESST) relaxation temperature (LIESST = light-induced excited spin-state trapping) of 70 K.

Automated summary

Two new mononuclear Fe(II) polymorphs with the same anionic complex have been synthesized and characterized. While polymorph 2 exhibits a high-spin state throughout the temperature range, complex 1 undergoes a complete spin crossover transition at around 132.3 K.