Isopolymolybdate-induced organic–inorganic hybrid assemblies with copper ions and bichelate-bridging ligands

Abstract

The heptamolybdate [NH₄]₆[Mo₇O₂₄], as a common polyoxometalate (POM) precursor with dynamic configuration transformation in aqueous solution by changing the pH, has been introduced into the hydrothermal reaction system containing copper ions and bichelate-bridging ligands, leading to the isolation of seven new organic–inorganic hybrid compounds [Cu^I₂Lo₂][Mo₆O₁₉] (1), [Cu^II₂Lo₂][β-Mo₈O₂₆]·2H₂O (2), [Cu^II₄Lo₂(pypz)₄][α-Mo₈O₂₆]·3H₂O (3), [Cu^II₄Lp₄(H₂O)₄][α-Mo₈O₂₆][β-Mo₈O₂₆]·4H₂O (4), [Cu^I₄Lp₃][β-Mo₈O₂₆]·H₂O (5), [Cu^I₄Lp₄][α-Mo₈O₂₆] (6), and [Cu^I₂Lp₂(Emim)₂][β-Mo₈O₂₆] (7) (bis(3-(2-pyridyl)pyrazole-1-ylmethyl)benzene = bppmb, Lo = 1,2-bppmb, Lp = 1,4-bppmb, pypz = 3-(2-pyridyl)pyrazole, Emim = 1-ethyl-3-methylimidazolium). All compounds were characterized by elemental analysis, IR spectroscopy, TG analysis, powder X-ray diffraction and single-crystal X-ray diffraction analysis. The structural analyses show that the isopolymolybdate clusters act as a type of “molecular adaptor”, which modulate the coordination mode of metal–organic subunits, leading to a series of new metal–organic secondary building units, such as the “Möbius-Ring”-type binuclear unit, “S”-type binuclear unit, butterfly-type unit, helical chain and mesomeric helical chain. As a result, the POM anions and the metal–organic subunits have suitable matching modes to form new hybrid self-assemblies. The photocatalytic properties of compounds 1–7 for the degradation of Rhodamine-B (RhB) under UV irradiation were investigated. All compounds showed good catalytic activity. Furthermore, the luminescent properties of compounds 1–7 have also been explored, indicating an organic ligand-based emission mode for all compounds.