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

Two new mononuclear Fe(II) polymorphs, [(C2H5)4N]2[Fe(py3C-OEt)(NCS)3]2 (1) and [(C2H5)4N][Fe(py3C-OEt)(NCS)3] (2) (py3C-OEt = tris(pyridin-2-yl)ethoxymethane), have been synthesized characterized by single-crystal X-ray diffraction, magnetic photomagnetic measurements, detailed variable-temperature infrared spectroscopy. The molecular structure, in both complexes, is composed of the same anionic [Fe(py3C-OEt)(NCS)3]? complex (two units for 1 one unit 2) generated coordination to metal center tridentate py3C-OEt tripodal ligand three terminal ?N-SCN coligands. Magnetic studies revealed that polymorph 2 displays a high-spin (HS) state over entire studied temperature range (300–10 K), while exhibits an abrupt complete spin crossover (SCO) transition at ca. 132.3 K, structural characterizations which, performed 295 100 show strong modification, resulting from thermal evolutions Fe–N bond lengths distortion parameters (? ?) FeN6 sphere, agreement with presence HS low-spin (LS) states respectively. This has also confirmed evolution maximum absorbance ?(NCS) vibrational bands recorded 200–10 K. In signature metastable photoinduced observed using photoinfrared spectroscopy, leading similar T(LIESST) relaxation (LIESST light-induced excited spin-state trapping) 70