Synthesis, crystal structure and magnetic properties of the spin crossover system [Fe(pq)3]2+

Three new compounds formulated (ClO4)2[Fe(pq)3] (1), (BF4)2[Fe(pq)3] · EtOH (2) and {(ClO4)[MnCr(C2O4)3][Fe(pq)2(H2O)2]} (3), where pq is 2,2′-pyridylquinoline, have been synthesised and characterised. Despite the different crystal packing exhibited by 1 and 2, the cationic species [Fe(pq)3]2+ are structurally quite similar. At 293 K, the Fe–N bond lengths are characteristic of the iron(II) in the high-spin state. In contrast to 1, 2 undergoes a continuous spin transition. Indeed, at 95 K its structure experiences a noticeable change in the Fe–N bonds and angles, i.e. the Fe–N bonds shorten by 0.194 Å on the average. The magnetic behaviour confirms that 1 is fully high-spin in the 4–300 K temperature range while 2 shows a spin transition centred at T1/2 = 150 K. The corresponding enthalpy, entropy and interaction parameter are ΔH = 7.49 kJ mol−1, ΔS = 50 J K−1 mol−1and Γ = 1.35 kJ mol−1. Compound 3 has been obtained as a microcrystalline powder. The magnetic properties of 3 point at the occurrence of ferromagnetic coupling below 100 K and the onset of a ferromagnetic ordering below 10 K (Weiss constant equal to 6.8 K). The Mössbauer spectra of 3 show the occurrence of a magnetic order at T ⩽ 4.2 K.

The crystal structure of the [Fe(pq)3]2+ complex cation has been studied for the ClO4- and BF4- salts. The former is high-spin in the whole range of temperature while the latter undergoes a continuous spin transition. Although, their molecular structures are similar the corresponding crystal packing strongly differs as the BF4- salt contains an additional EtOH. The use of the non-innocent extended [MnCr(C2O4)3]− affords the compound[Fe(pq)2(H2O)2][ClO4][MnCr(ox)3] · 4H2O whose structure, magnetic properties and Mössbauer spectra have been investigated.Figure optionsDownload as PowerPoint slide