Theoretical and experimental investigation of novel iron(II)-based spin crossover compounds

Two novel spin crossover (SCO) compounds, namely [Fe(INMe)(pyN4)]Br2, and [Fe(IMMe)(pyN4)](OTf)2, where pyN4 = 2,6-Bis(1′,3′-diamino-2′methyl-prop-2′yl)pyridine, INMe = isonicotinic acid methyl ester, IMMe = N-methyl-imidazole, and OTf = triflate, are characterized here both from experimental and theoretical viewpoints. In particular, we apply various density functionals and basis sets to obtain optimized geometries for low- (LS) and high-spin (HS) states, vibrational spectra, LS-HS splittings, and temperature-dependent UV/vis spectra. While geometries and spectra are in good agreement with experimental data, the well-known spin pairing problem makes it difficult to compute accurate LS-HS splitting energies and enthalpies. Based on TD-DFT calculations, the capacity of the compounds for use as reversibly photo-switchable molecules is discussed.