Immobilized Cu(II) and Co(II) Schiff base complexes on the surface of functionalized magnetized multiwalled carbon nanotubes: Novel catalysts for oxidation and solvent-free pseudo six-component condensation reaction

Abstract

In this study, copper(II) and cobalt(II) Schiff-base complexes have been covalently anchored on magnetized multiwalled carbon nanotubes separately (Cu(II)/Schiff base@MWCNT-Fe3O4/SiO2 & Co(II)/Schiff base@MWCNT-Fe3O4/SiO2). The new synthesized nano materials have been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FT-IR), energy-dispersive X-ray spectroscopy (EDX), and also elemental analysis techniques. The two obtained nanostructures applied as catalysts in oxidation reactions. The investigations revealed that Cu(II)/Schiff base@MWCNT-Fe3O4/SiO2 (catalyst A) catalyzed oxidation of styrene in the presence of tert-butyl hydro peroxide (TBHP), as oxidant, yielded styrene oxide (73%) as the main product with 94% conversion and Co(II)/Schiff base@MWCNT-Fe3O4/SiO2 (catalyst B) in the presence of H2O2 and imidazole, got mostly benzaldehyde (67%) with 100% conversion. In the next step the wide-efficiency of the novel Cu(II)/Schiff base@MWCNT-Fe3O4/SiO2 examined to prepare 4-(hetro)aryl-3,5-di(alkyl/aryl)-1,4,7,8-tetrahydropyrazolo[3,4-b:4′,3′-e]pyridines via pseudo six-component condensation-cyclization process of β-ketoesters, aldehydes, hydrazine hydrate, and ammonium acetate in completely green and economic conditions in the absence of solvent at room temperature. Among various advantages of the reported procedures, the main feature is simple recoverability utilizing an internal magnet and reusability in several cycles without activity loss of the nanocatalysts.