Diverse Ligand-Functionalized Mixed-Valent Hexamanganese Sandwiched Silicotungstates with Single-Molecule Magnet Behavior

Under hydrothermal conditions, replacement of the water molecules in the [Mn^III₄Mn^II₂O₄(H₂O)₄]⁸⁺cluster of mixed-valent Mn₆sandwiched silicotungstate [(B-α-SiW₉O₃₄)₂Mn^III₄Mn^II₂O₄(H₂O)₄]¹²⁻(1 a) with organic N ligands led to the isolation of five organic–inorganic hybrid, Mn₆-substituted polyoxometalates (POMs) 2–6. They were all structurally characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis, diffuse-reflectance spectroscopy, and powder and single-crystal X-ray diffraction. Compounds 2–6 represent the first series of mixed-valent {Mn^III₄Mn^II₂O₄(H₂O)_4−n(L)_n} sandwiched POMs covalently functionalized by organic ligands. The preparation of 1–6 not only indicates that the double-cubane {Mn^III₄Mn^II₂O₄(H₂O)_4−n(L)_n} clusters are very stable fragments in both conventional aqueous solution and hydrothermal systems and that organic functionalization of the [Mn^III₄Mn^II₂O₄(H₂O)₄]⁸⁺cluster by substitution reactions is feasible, but also demonstrates that hydrothermal environments can promote and facilitate the occurrence of this substitution reaction. This work confirms that hydrothermal synthesis is effective for making novel mixed-valent POMs substituted with transition-metal (TM) clusters by combining lacunary Keggin precursors with TM cations and tunable organic ligands. Furthermore, magnetic measurements reveal that 3 and 6 exhibit single-molecule magnet behavior.