A 3D supramolecular framework with electron-deficient-carboxylate-functionalized [γ-Mo8O26]-subunits for catalytic epoxidation of alkenes and electrochemical detection of sunset yellow

Abstract

Using a one-pot assembly method, a 4,4-connected polyoxometalate (POM)-based metal-organic framework (POMOF) H₂[Ni(pbba)(Mo₈O₂₆)(H₂O)₂]·6H₂O (1, H₂pbbaBr₂ = 1,1′- (1,4-phenylene-bis(methylene))-bis(pyridine-3-carboxylicacid)dibromo), was successfully synthesized and characterized by PXRD, IR spectroscopy and single crystal X-ray diffraction. Structural analyses showed that the carboxylic groups of the electron-deficient pbba ligand coordinated with the Mo⁶⁺ centers of the {γ-Mo₈O₂₆} cluster to facilitate the transformation of its configuration from {γ-Mo₈O₂₆} to {β-Mo₈O₂₈}. The {β-Mo₈O₂₈} anions and [Ni(H₂O)₂]²⁺ cations are interconnected alternately to form a one-dimensional inorganic chain [{β-Mo₈O₂₈}-Ni]_n. The carboxylic groups of the pbba ligand then bridge the Ni²⁺ cations and the {β-Mo₈O₂₈} anions, ultimately extending the 1D chains into 4,4-conected 2D layer. Due to the positive charge at the N sites of the pbba ligand, the 2D networks further assemble into a “split-level” 3D supramolecular framework, adhering to the charge matching principle and stabilized by the hydrogen bond interactions. Complex 1 serves as an effective heterogeneous catalyst for the selective epoxidation of olefins. It has demonstrated remarkable efficiency in catalyzing the epoxidation of cis-cyclooctene, achieving a high conversion rate of 97 % with >99 % selectivity. Complex 1 displays good recoverability and high stability, enabling it to reused up to five times without a significant decline in activity. The mechanism of the catalytic reaction was also explored. In addition, 1-body modified carbon paste electrode (1-CPE) was used for the detection of sunset yellow with good anti-interference and low detection limits.