Sequential Synthesis of 3d–3d′–4f Heterometallic Heptanuclear Clusters in between Lacunary Polyoxometalates

In this work, we have successfully created several unprecedented discrete 3d–3d′–4f heterotrimetallic clusters in between lacunary polyoxometalates (POMs). By the three-step sequential introduction of metal cations into a trivacant lacunary POM TBA₄H₆[A-α-SiW₉O₃₄] (TBA = tetra-<i>n</i>-butylammonium) in organic media, five kinds of sandwich-type POMs with double-diamond-shaped 3d–3d′–4f heptanuclear clusters (<b>III</b>_{<b>FeM4Ln2</b>}, TBA_<i>n</i>H_<i>m</i>­[FeM₄­{Ln­(L)₂}₂­O₂­(A-α-SiW₉O₃₄)₂], M = Mn³⁺, Cu²⁺; Ln = Gd³⁺, Dy³⁺, Lu³⁺; L = acac (acetylacetonate), hfac (hexafluoroacetylacetonate)) were successfully synthesized for the first time. By introduction of two [Ln­(L)₂]⁺ units on the ends of pentanuclear clusters [FeMn₄O₁₈(OH)₂]^23– and [FeCu₄O₁₈(OH)₂]^27–, the magnetic interactions between Mn³⁺–Mn³⁺ and Cu²⁺–Cu²⁺ could be modulated. Among a series of the heterometallic heptanuclear compounds, <b>III</b>_{<b>FeMn4Lu2</b>} exhibited the slow magnetic relaxation characteristic for a single-molecule magnet under the zero applied magnetic fields