Effect of Heteroatoms on Field-Induced Slow Magnetic Relaxation of Mononuclear Fe^III ( S = 5/2) Ions within Polyoxometalates

In this paper, the synthesis and magnetic properties of mononuclear Fe^III-containing polyoxometalates (POMs) with different types of heteroatoms, TBA₇H₁₀[(A-α-XW₉O₃₄)₂Fe] (II^X, X = Ge, Si; TBA = tetra- n-butylammonium), are reported. In these POMs, mononuclear highly distorted six-coordinate octahedral [FeO₆]^9- units are sandwiched by two trivacant lacunary units [A-α-XW₉O₃₄]^10- (X = Ge, Si). These POMs exhibit field-induced slow magnetic relaxation based on the single high-spin Fe^III magnetic center ( S = 5/2). Combining experiment and ab initio calculations, we investigated the effect of heteroatoms of the lacunary units on the field-induced slow magnetic relaxation of these POMs. By changing the heteroatoms from Si (II^Si) to Ge (II^Ge), the coordination geometry around the Fe^III ion is mildly changed. Concretely, the axial Fe-O bond length in II^Ge is shortened compared with that in II^Si, and consequently the distortion of the [FeO₆]^9- unit in II^Ge from the ideal octahedral coordination geometry becomes larger than that in II^Si. The effective demagnetization barrier of II^Ge (11.4 K) is slightly larger than that of II^Si (9.2 K). Multireference ab initio calculations predict zero-field splitting parameters in good agreement with experiment. Although the differences in the coordination geometries and magnetic properties of II^Ge and II^Si are quite small, ab initio calculations indicate subtle changes in the magnetic anisotropy which are in line with the observed magnetic relaxation properties.