Spin crossover phenomenon of μ-bpt dinuclear one- dimensional complexes with different bridge ligands

Department of Chemistry, National Taiwan University
○Szu-Miao Chen Yu Wang Sheu Chou Fu Chen Ko Wa Lee Gene Hsiang

[{FeII(μ-bpt)(NCS)}2(bipy) CH3OH] (bpt = 3,5-bis(pyridin-2-yl)-1,2,4-triazole) synthesized successfully by one-step solvo-thermal syntheses represents the first example of an one dimensional dinuclear iron(II) spin-crossover compound which exhibits an abrupt two-step spin transition at 130 K. The zigzag chain structure containing a dinuclear Fe complexes bridged by 4,4'-bipy ligand. It has only one unique Fe site in the asymmetric unit. However, when it undergoes a spin transition, the spin transition corresponds to only half of the total amount of Fe content. The unit cell doubled at and below 130 K, thus two Fe centers can be different, namely a [HS -LS] pair of Fe(II) dinuclear complexes in the asymmetric unit. The LIESST phenomenon was observed with proper light irradiation (532 nm, power 10 mW cm-2) at 25 K. It is characterized with XRD, XANES, IR spectra and SQUID. The typical change of ~0.2 Å in Fe-N bond lengths is also observed during the LIESST process.
The other similar one-dimensional zigzag dinuclear Fe(II) complex bridged by a shorter ligand, pyrazine, [{FeII(μ-bpt)(NCS)}2(pyrazine)] is also synthesized and characterized. The complex shows quite different magnetic property, in which the spin transition takes place gradually at Tc ~120 K. The structures of this complex both at HS and at LS state will be presented. The comparison between these two dinuclear complexes as well as other similar structures will be discussed.

Keywords: LIESST, spin crossover, thermal relaxation