The first purely inorganic polyoxotungstates constructed from dimeric tungstoantimonate-based iron–rare-earth heterometallic fragments

Abstract

The rational self-assembly of a trilacunary Keggin precursor with Fe³⁺ and RE³⁺ (rare-earth) ions via a conventional aqueous solution method has led to a series of novel tungstoantimonate-based Fe–RE heterometallic derivatives [Pr(H₂O)₈][Pr(H₂O)₆][Fe₄(H₂O)₁₀(B-β-SbW₉O₃₃)₂]·16H₂O (1) and [RE(H₂O)₇]₂[Fe₄(H₂O)₁₀(B-β-SbW₉O₃₃)₂]·22H₂O [RE = Tb^III (2), Dy^III (3), Lu^III (4), Y^III (5)], which have been further structurally characterized by elemental analysis, IR spectroscopy, powder X-ray diffraction (PXRD), single-crystal X-ray diffraction and thermogravimetric (TG) analysis. To the best of our knowledge, 1–5 represent the first purely inorganic Fe–RE heterometallic tungstoantimonates based on the trilacunary Keggin [B-β-SbW₉O₃₃]⁹⁻ unit. In 1, the tetra-Fe^III substituted sandwich-type unit [Fe₄(H₂O)₁₀(B-β-SbW₉O₃₃)₂]⁶⁻ is built up of two trilacunary Keggin subunits [B-β-SbW₉O₃₃]⁹⁻ sandwiching a rhomb-like {Fe^III₄} cluster, and then the disordered Pr^III cations work as bridges that link adjacent sandwich-type units [Fe₄(H₂O)₁₀(B-β-SbW₉O₃₃)₂]⁶⁻, giving rise to the first 2-D extended sheet based on tungstoantimonate fragments and Fe–RE heterometallic cations. 2–5 are isomorphic and exhibit a discrete structure composed of a tetra-Fe^III substituted sandwich-type unit [Fe₄(H₂O)₁₀(B-β-SbW₉O₃₃)₂]⁶⁻ and two supporting [RE(H₂O)₈]³⁺ cations. The electrochemical and electrocatalytic properties of 1 and 2 have been investigated in detail in a 0.5 mol L⁻¹ Na₂SO₄ + H₂SO₄ supporting electrolyte, and both 1 and 2 illustrate obvious electrocatalytic activity toward the reduction of NO₂⁻ and BrO₃⁻.