Charge-transfer phase transition and ferromagnetism of iron mixed-valence complexes (n-C_nH_2n+1)₄N[Fe^IIFe ^III(dto)₃] (n = 3-6; dto = C₂O ₂S₂)

The iron mixed-valence complex, (n-C₃H₇) ₄N[Fe^IIFe^III(dto)₃] (dto = dithiooxalato) shows a charge-transfer (CT) phase transition at T_CT = 122.4 K. In the vicinity of T_CT, the spin state changes from Fe ^II (S = 2) - Fe^III (S = 1/2) (high-temperature phase: HTP) to Fe^II (S = 0) - Fe^III (S = 5/2) (low-temperature phase: LTP) accompanied by a charge transfer between Fe^II and Fe ^III. This complex also undergoes a ferromagnetic transition at 7 K in the LTP. In order to investigate the mechanism of the CT phase transition and the ferromagnetism, we have systematically synthesized (n-C_nH _2n+1)₄N[Fe^IIFe^III-(dto) ₃] (n = 3-6), and have investigated their physical properties by magnetic susceptibility, powder X-ray diffraction measurements, and ESR spectroscopy. The compounds (n-C_nH_2n+1) ₄N[Fe^IIFe^III(dto)₃] (n = 3-6) display ferromagnetic phase transitions at 7 K, 7 K (& 13 K), 19.5 K, and 22 K, respectively. For n = 3 and 4, the CT phase transitions take place at T _CT ≈ 120 K and T_CT ≈ 140 K, respectively. For n = 5 and 6, on the other hand, the CT phase transition does not occur, and the spin configuration of Fe^II (S = 2) and FeIII (S = 1/2) corresponding to the HTP for n = 3 and 4 is stable between 2 K and 300 K. The cation size of (n-C_nH_2n+1)₄N⁺ (n = 3-6) acts as an effective internal pressure which induces the CT phase transition and the ferromagnetic ordering in the [Fe^IIFe^III(dto) ₃]^-_∞ layer. We also discuss the mechanism of the CT phase transition and the ferromagnetism induced by the charge-transfer interaction between Fe^II and Fe^III.