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

A family of five new cyanido-bridged binuclear Fe1II-NC-Fe2II complexes, (PY5OMe)FeIINCFeII(dppe)CpMen(OTf)2•CH3CN (CpMen = alkyl cyclopentadienyl, n = 0 (1), 1 (2), 3 (3), 4 (4), 5 (5); dppe = 1, 2-bis(diphenylphosphino)ethane; PY5OMe = 2,6-bis((2-pyridyl)methoxymethane)pyridine; and OTf = CF3SO3–) have been synthesized and well characterized. Among them, Fe1II located at the N-terminal of the cyano group is the spin transition site, and the C-terminal Fe2II-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 Fe1II. The spin transition temperature (T1/2) 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.