Study of Intermolecular Interaction for the Spin-Crossover Iron(II) Compounds

The thermal spin transitions of iron(II) spin-crossover compounds [Fe(PM-iPA)2(NCS)2] (1) and [Fe(PM-iPA)2(NCSe)2] (2) (PM-iPA = N-(2ʹ-pyridylmethyl)isopropylamine) have occurred at T1/2 = 267 K and 376 K without thermal hysteresis. No light-induced excited spin state trapping (LIESST) effect was observed for compounds 1 and 2 even at 5 K. The iron(II) spin-crossover compounds [Fe(PM-L)2(NCX)2] (PM-A = N-(2ʹ-pyridylmethyl)-aniline, PM-BiA = N-(2ʹpyridylmethyl)-4-aminobiphenyl, PM-TeA = N-(2ʹ-pyridylmethyl)-4-aminoterphenyl, PM-PEA = N-(2ʹ-pyridylmethyl)-4-(phenylethynyl)aniline, and PM-AzA = N(2ʹ-pyridylmethyl)-4-(phenylazo)aniline; X = S and Se) with ligands having (pi)-systems have exhibited the LIESST effect, and the critical LIESST temperature, Tc (LIESST), has been observed. The compounds 1 and 2 are crystallized at Pnna and C2/c at 298 K, respectively, and the space groups of the compounds remained unchanged until 100 K, although the compounds show the spin transition. The molecular packing structure and thermodynamic parameters of the spin transitions calculated from the magnetic susceptibility curves suggest that compounds 1 and 2 have no strong intermolecular interactions between the complexes, while the compounds with (pi)-system ligands form (pi)–(pi) intra- and intermolecular interactions between the ligands. Our conclusion is that the intermolecular interactions play an important role in trapping a light-induced metastatble state.