Alkali-regulated Fe6 and Fe18 molecular clusters and their structural transformation

Abstract

As a kind of metastable aggregation between iron ions and iron hydroxide precipitates, Fe clusters can only be isolated under narrow synthesis conditions. Here, two Fe clusters with different structural models, [Fe₆(H₂thmmg)₆] (Fe₆) and [Fe₁₈O₈(OH)₈(H₂thmmg)₁₀] (Fe₁₈), are constructed based on a rare N-tris(hydroxymethyl)methylglycine (H₅thmmg) ligand. The cyclic Fe₆ can be visualized as a “six-vane windmill” comprised of 6 Fe³⁺ and 6 H₂thmmg³⁻ ligands. Unlike the cyclic configuration of Fe₆, the Fe₁₈ cluster features a bricky structure like a “Chinese knot” coordinated by three species, H₂thmmg³⁻, OH⁻ and O²⁻. It was obtained by adding more triethylamine to the reactive solution of Fe₆ which yielded more coordinated species OH⁻ and O²⁻. Moreover, the conversion from Fe₆ to Fe₁₈ was successfully achieved by adding more triethylamine, realizing nuclearity enlargement and structural regulation of Fe clusters. Furthermore, the magnetic properties of Fe₆ and Fe₁₈ exhibit antiferromagnetism. This work reports two novel alkali-regulated Fe clusters and further realizes the transformation of nuclearity and structure, which provides some reference for the structural predesign and adjustment of metal nanoclusters.