Regulable Spin-Crossover Behaviors of a Family of Cyanido-Bridged Binuclear Iron(II) Compounds with a Thermally Induced Ultrafast High–Low Spin State Interconversion

Abstract

A family of five new cyanido-bridged binuclear Fe1 II -NC-Fe2 II complexes, [(PY 5 OMe)Fe II NCFe II (dppe)CpMe n ]-(OTf) 2 •CH 3 CN (CpMe n = alkyl cyclopentadienyl, n = 0 (1), 1 (2), 3 (3), 4 (4), 5 (5); dppe = 1, 2-bis(diphenylphosphino)ethane; PY 5 OMe = 2,6-bis((2-pyridyl)methoxymethane)pyridine; and OTf = CF 3 SO 3 −) have been synthesized and well characterized. Among them, Fe1 II located at the N-terminal of the cyano group is the spin transition site, and the C-terminal Fe2 II -fragment serves as an electron donor. Magnetic studies demonstrate that the electron-donating ability of the C-terminal substituents has a significant influence on the spin-state transition behavior of Fe1 II . The spin transition temperature (T 1/2 ) of the complexes can be regulated by the methyl number in the Cp ligand at the C-terminal. With the increase of the methyl substituents of the cyclopentadiene ligand, the spin transition area shifts toward a lower temperature. And this conclusion is also applicable to desolvated 1−5. Furthermore, a thermally induced ultrafast high−low spin interconversion (∼10 −12 s) around room temperature has been observed in complex 3, confirmed by IR and UV−vis−NIR spectra, the time scales of which being 10 −12 and 10 −14 s, respectively. This discovery provides more possibilities for the design of molecular devices with higher performance.