pH-Dependent assembly of three novel inorganic–organic hybrids based on different isopolymolybdates and CuI/II(bbx)n units

Abstract

Three novel inorganic–organic hybrid compounds based on different types of isopolymolybdates and Cu^II(bbx) complexes, namely, [Cu₂^II(bbx)₂(β-Mo₈O₂₆)]·2H₂O (1), [Cu₂^II(bbx)₄(γ-Mo₈O₂₆)] (2) and H₂[Cu₂^I(bbx)₂(Mo₂O₈)] (3) (bbx = 1,3-bis(1-imidazolyl)benzene), have been successfully synthesized by adjusting the pH value under hydrothermal conditions. Compound 1, which was hydrothermally prepared at pH = 3.5, exhibits a unique 3D (2,5,6)-connected net with a (4³·6⁴·8²·10)₂(4⁴·6⁸·8³)(4)₂ topology, constructed by sexadentate Mo₈O₂₆)⁴⁻ anions coordinated to Cu^II ions of one-dimensional [Cu^II(bbx)]_n chains. When the pH value was adjusted to 4.5, a structurally different complex 2 was obtained. Compound 2 shows a 3D (2,5)-connected framework with an (8⁶·12⁴)₂(8)₅ topology, constructed by bidentate γ-(Mo₈O₂₆)⁴⁻ polyanions linking 2D layers composed of [Cu^II(bbx)₂]_n. Moreover, when the pH value was adjusted to 7.5, an interesting complex 3 was obtained. Compound 3 shows a 2D (4)-connected framework with a (4⁴·6²) topology, constructed by bidentate (Mo₂O₈)⁴⁻ polyanions linking [Cu₂^I(bbx)₂]_n ladder-like double-chains. The (Mo₂O₈)⁴⁻ unit has never been observed before, which is composed of two rare distorted {MoO₅} tetragonal pyramids connected via edge-sharing. All compounds were characterized by elemental analysis, TG analysis, powder X-ray diffraction, single-crystal X-ray diffraction analysis and solid diffuse reflectance spectroscopy. In addition, the electrochemical properties of compounds 1–3 have been investigated in 1 M H₂SO₄ aqueous solution. Moreover, compounds 1 and 3 have definite catalytic activities to activate peroxymonosulfate (PMS) in the reduction of methylene blue (MB).