pH-dependent assembly of metal–organic hybrid compounds based on octamolybdates and a new flexible multidentate ligand

Abstract

Six novel molybdenum hybrid materials based on a flexible multidentate ligand, namely (H₃tib)₄(β-Mo₈O₂₆)₃·5H₂O (1), (H₃tib)₂[Cu(H₂O)₂(γ-Mo₈O₂₆)₂]·2H₂O (2), [Cu(Htib)₂(δ-Mo₈O₂₆)]·4H₂O (3), [Cu₂(tib)₂(H₂O)₂(γ-Mo₈O₂₆)]·H₂O (4), (H₃tib)₂[Co(H₂O)₂(γ-Mo₈O₂₆)₂]·2H₂O (5), and Zn(tib)(MoO₄)·3H₂O (6) (where tib = 1,3,5-tris(imidazol-1-ylmethyl)benzene), have been synthesized at different pH values under hydrothermal conditions. Their structures have been determined by single-crystal X-ray diffraction analysis and further characterized by PXRD, TGA, IR spectra, and electrochemistry. Compound 1 is a supramolecule with classical β-Mo₈O₂₆ cages. Compounds 2 and 5 exhibit a rare one-dimensional (1D) molybdenum oxide duplex chain. We have not yet found any other examples of the two adjacent 1D molybdenum oxide chains linked together through the centers of the Cu/Co cations. In 3, the Cu^II cations are bridged by Htib⁺ ligands and (δ-Mo₈O₂₆)⁴⁻ anions to form an interesting two-dimensional (2D) layer. In 4, the Cu^IIcations, tib ligands and (γ-Mo₈O₂₆)⁴⁻ anions are connected to form a 2D layer, which is further reinforced by the tib ligands through an Mo–N bond. Compound 6 is a 2D coordination polymer possessing a high thermal stability. The structural diversities of 2–4 reveal that the pH value of the reaction system, the metal coordination preference and the ligand configuration play a crucial role in the assembly of POM-based complexes.