Iron(III) spin-crossover compounds [Fe(pap)(2)]ClO(4) (1), [Fe(pap)(2)]BF(4) (2), [Fe(pap)(2)]PF(6) (3), [Fe(qsal)(2)]NCS (4), and [Fe(qsal)(2)]NCSe (5) (Hpap=2-(2-pyridylmethyleneamino)phenol and Hqsal=2-[(8-quinolinylimino)methyl]phenol) were prepared and their spin-transition properties investigated by magnetic susceptibility and Mössbauer spectroscopy measurements. The iron(III) compounds exhibited spin transition with thermal hysteresis. Single crystals of the iron(III) compounds were obtained as suitable solvent adducts for X-ray analysis, and structures in high-spin (HS) and low-spin (LS) states were revealed. Light-induced excited-spin-state trapping (LIESST) effects of the iron(III) compounds were induced by light irradiation at 532 nm for 1-3 and at 800 nm for 4 and 5. The activation energy E(a) and the low-temperature tunneling rate k(HL)(T-->0) of iron(III) LIESST compound 1 were estimated to be 1079 cm(-1) and 2.4x10(-8) s(-1), respectively, by HS-->LS relaxation experiments. The Huang-Rhys factor S of 1 was also estimated to be 50, which was similar to that expected for iron(II) complexes. It is thought that the slow relaxation in iron(III) systems is achieved by the large structural distortion between HS and LS states. Introduction of strong intermolecular interactions, such as pi-pi stacking, can also play an important role in the relaxation behavior, because it can enhance the structural distortion of the LIESST complex.

Iron(III) spin-crossover compounds with H3-OMe-salRen ligands [Fe(3-OMe-salRen)2]ClO4 (1–5) were prepared and characterized by single-crystal X-ray diffraction, Mössbauer spectra, magnetic susceptibilities, and electronic spectra, where 3-OMe-salRen is a tridentate ligand derived from 3-methoxysalicylaldehyde and N-R-ethylenediamine (R = H, Me, Et, Pr, and Bu for 1, 2, 3, 4, and 5). The structures of compounds 1, 2, 3, and 5 at 90 and 298 K, and that of compound 4 at 298 K were determined. Compounds 1, 2, 3, and 5 exhibited a spin transition depending on temperature; the transition temperatures for 1, 2, 3, and 5 were >400, 360, 196, and 223 K, respectively. Compound 4 was in the high-spin state in the temperature range of 5 to 400 K. σ–π or π–π interactions exist in compounds 1–5, and the structures of the compounds were made clear in both the high-spin and low-spin states. Compound 3 exhibited a spin transition with thermal hysteresis.

www.chemeurj.org C E U J E D 1 5 ( 1 4 ) 3 3 0 5 -3 6 2 0 ( 2 0 0 9 ) · I S S N 0 9 4 7 -6 5 3 9 · V o l . 1 5 · N o . 1 4 · 2 0 0 9 Review Structure Formation Principles and Reactivityof Organolithium Compounds C. Strohmann et al.