Study on the valence fluctuation and the magnetism of an iron mixed-valence complex, (n-C4H9)4N[FeIIFeIII(mto)3](mto = C2O3S)

In the case of iron mixed-valence complexes whose spin states are situated in the spin-crossover region, conjugated phenomena coupled with spin and charge are expected. In general, the Fe site coordinated by six S atoms is in the low-spin state, while the Fe site coordinated by six O atoms is in the high-spin state. From this viewpoint, we have synthesized and investigated physical properties for an monothiooxalato-bridged (mto = C2O3S) iron mixed-valence complex, (n-C4H9)4N[FeIIFeIII(mto)3], consisting of FeIIIO3S3 and FeIIO6 octahedra, which behaves as a ferrimagnet with its magnetic transition temperature of TN = 38 K and Weiss temperature of θ = −93 K. From the analysis of 57Fe Mössbauer spectra of 57Fe enriched complexes, (n-C4H9)4N[57FeIIFeIII(mto)3] and (n-C4H9)4N[FeII57FeIII(mto)3], the charge transfer between FeII and FeIII exists in the paramagnetic phase. Considering the time window of 57Fe Mössbauer spectroscopy, the time scale of the valence fluctuation is at least slower than 10−7 s. In order to confirm the valence fluctuation between FeII and FeIII, we investigated the dielectric constant and found an anomalous enhancement attributed to the Fe valence fluctuation between 170 and 250 K.

Graphical abstractIron mixed-valence complex, (n-C4H9)4 N[FeIIFeIII(mto)3] (mto = monothiooxalato), behaves as a ferrimagnet with its magnetic transition temperature of TN = 38 K and Weiss temperature of θ = −93 K. From the analysis of 57Fe Mössbauer spectra of (n-C4H9)4 N[FeIIFeIII(mto)3] in which FeII or FeIII was substituted with 57Fe and the dielectric constant, the valence fluctuation between FeII and FeIII exists in the paramagnetic phase.Figure optionsDownload full-size imageDownload as PowerPoint slide