Supramolecular nanotubes constructed from 3d-4f heterometallic sandwiched polyoxotungstate dimers

By the hydrothermal technique, a series of organic-inorganic hybrid polyoxometalates (POMs) H_x(H₂en)_y [Fe₂Ln(β-PW₁₀O₃₇)₂(Tart)]·nH₂O (Ln = La³⁺, (x, y, n) = (5, 3, 9) for 1; Ln = Ce³⁺, (x, y, n) = (5, 3, 7) for 2; Ln = Sm³⁺, (x, y, n) = (3, 4, 25) for 3; Ln = Tb³⁺, (x, y, n) = (3, 4, 21) for 4; H₂Tart = tartaric acid) have been obtained and structurally characterized by elemental analyses, IR spectroscopy, thermogravimetric analyses and single-crystal X-ray diffraction. The common structural characteristics of 1-4 consist of a [Fe₂Ln(β-PW₁₀O₃₇)₂(Tart)]^9- unit, in which transition metal and lanthanide ions co-exist in the vacant site of divacant [β-PW₁₀O₃₇]^9- with the assistance of Tart ligands. It should be pointed out that 1-4 represent the first 3d-4f heterometallic cluster sandwiched phosphotungstate dimers, [Fe₂Ln(β-PW₁₀O₃₇)₂(Tart)]^9-, linked by Fe-O-Ln-O-Fe bonds. Furthermore, 3 and 4 exist as rare supramolecular nanotubes built from the organic-inorganic hybrid 3d-4f POM units. Also, the photochromism and optical properties of 1-4 have been studied.