Fe[S₂CNC₄H₈]₂I: A quasi‐two‐dimensional XY antiferromagnet Available to Purchase

The magnetic properties of Fe[S₂CNC₄H₈]₂I have been studied, the first pentacoordinate Fe(III) system of this type with a cyclic terminal group, rather than an alkyl group, to be examined in detail. The susceptibility of a polycrystalline sample is of Curie–Weiss form above 30 K, with ḡ=2.16 ±0.02 and S=3/2 and with θ=−0.75±1.0 K. Thus the unusual intermediate spin state occurs here as in earlier examples. The susceptibility can be fit quite well down to about 6 K assuming that axial and rhombic crystal field distortions occur, represented by D[Ŝ²_z − S(S + 1)/3] and E[Ŝ²_x − Ŝ²_y] terms in the spin Hamiltonian, and incorporating exchange interactions in a mean‐field approximation. Fitted parameters are D/k=9.3± 0.5 K, E/k=−0.6±0.2 K, g_x=2.23± 0.05, g_y= 2.19 ± 0.05, g_z=2.04 ± 0.05, and zJ/k=−0.23₅ ±0.02 K. At low temperatures the susceptibility exhibits a broad maximum with T(χ_max)=3.23± 0.04 K and χ_max =0.376±0.004 emu/mol. An antiferromagnetic transition at T_c=2.21±0.01 K is inferred from the position of a maximum in (∂χ/∂T), an ordering temperature in reasonable agreement with earlier Mössbauer results. The ratio T_c/T(χ_max) =0.68±0.01 is suggestive of quasi‐two‐dimensionality. In the region of the maximum an acceptable fit according to a 2D‐ XY model with plausible parameter values can be achieved, but only with a ferromagnetic interlayer exchange interaction about 28% as strong as the leading antiferromagnetic intralayer interaction. Isotherms of M vs H show somewhat unusual curvature, the explanation of which is unclear.