The stepwise construction of polyoxovanadate-based Zn-MOF nanoparticles with Lewis acid sites for the one-pot synthesis of N-heterocycles in air

Abstract

The condensation of aldehydes with o-phenylenediamine or 2-aminobenzamide followed by oxidation is an efficient method for synthesizing benzimidazoles and quinazolinones. From a green chemistry perspective, developing a catalyst capable of preparing these heterocyclic compounds via one-pot aerobic oxidation is highly desirable. The catalysts involved in this process must possess both acid catalytic and oxidation catalytic sites. Based this, we employed polyoxovanadates (POVs) as building blocks owing to their ability to activate oxygen, and obtained the POV-based MOF Zn₄(azpy)₈(V₂O₆)₄ (1) (azpy: 4,4′-azopyridine), in which Zn²⁺ exhibits a six-coordinate configuration, via hydrothermal synthesis. By adjusting the reaction temperature, the configuration of Zn²⁺ was converted to four-coordinated, and Zn₈(azpy)₈(V₂O₇)₄·H₂O (2) was obtained, successfully constructing Lewis acid sites. In addition, as increasing the specific surface area of the catalyst will improve its catalytic efficiency, we incorporated the less-polar solvent acetonitrile to modulate the particle size of 2 during the synthesis, and ultimately obtained 2-NP nanoparticles with a diameter of approximately 50 nm. In the catalytic synthesis of benzimidazoles and quinazolinones, 2-NP demonstrated outstanding performance, effectively activating oxygen from air to produce singlet oxygen at room temperature for the swift formation of benzimidazoles. Additionally, at 90 °C, the singlet oxygen can be further oxidized to generate superoxide radicals, facilitating the formation of quinazolinones. The above strategy provides a new guiding principle for the selective synthesis of MOF catalysts.