Two types of novel tetra-iron substituted sandwich-type arsenotungastates with supporting lanthanide pendants

Abstract

Two classes of novel tetra-iron substituted sandwich-type arsenotungastates (ATs) with supporting lanthanide (Ln) pendants KNa₂ [Ln(H₂O)₇][Fe₄(H₂O)₁₀(B-β-AsW₉O₃₃)₂]·21H₂O [Ln = La^III (1), Pr^III (2), Nd^III (3), Sm^III (4)] and [Ln(H₂O)₈]₂[Fe₄(H₂O)₈(L-thr)₂(B-β-AsW₉O₃₃)₂]·20H₂O [Ln = La^III (5), Pr^III (6), Nd^III (7), Sm^III (8), Eu^III (9), Gd^III (10), Tb^III (11), Dy^III (12), Er^III (13)] (L-thr = L-threonine) have been synthesized by the hydrothermal reaction of the [As₂W₁₉O₆₇(H₂O)]¹⁴⁻ precursor with Fe³⁺ cations and Ln³⁺ cations in the presence of L-thr or L-leucine and L-alanine, and further characterized by elemental analyses, IR spectra and single-crystal X-ray diffraction. Structural analyses indicate that 1–4 display the inorganic 2-D sheet architecture constructed from tetra-iron sandwiched AT [Fe₄(H₂O)₁₀(B-β-AsW₉O₃₃)₂]⁶⁻ fragments by bridging [Ln(H₂O)₇]³⁺ cations whereas the molecular structures of the isostructural 5–13 consist of an organic–inorganic hybrid tetra-iron substituted sandwich-type AT [Fe₄(H₂O)₈(L-thr)₂(B-β-AsW₉O₃₃)₂]⁶⁻ fragment and two pendant [Ln(H₂O)₈]³⁺ cations. As far as we know, 1–4 represent the rare inorganic 2-D extended ATs based on transition-metal substituted sandwich-type polyoxometalate units and Ln linkers and 5–13 are the first Fe–Ln heterometallic ATs with amino acid ligands. The solid state photoluminescence (PL) measurements of 9 and 11 have been performed at room temperature. The PL emission of 9 is mainly derived from the characteristic ⁵D₀ → ⁷F₂ (J = 4–0) transitions of the Eu^III cations whereas the PL behavior of 11 stems from the common contribution of the ⁵D₄ → ⁷F_J (J = 5–3) transitions of the Tb^III ions and oxygen-to-metal (O → W) charge-transfer transitions of AT segments. The thermogravimetric (TG) analyses of 1–4 and 6–12 have been investigated.