Bis(NHC)-Pd(II) and Pd(0) complexes supported on magnetic SBA-15 for the selective aerobic oxidation of benzyl alcohols to benzaldehydes and reduction of nitroarenes

Abstract

The present study describes the synthesis, characterization, and use of two new supported magnetic Pd-NHC catalysts based on Pd(II) and Pd(0). A composite of magnetite-silica core shells and SBA-15 was successively treated with (3-aminopropyl) triethoxysilane (APTES), cyanuric chloride (CC), imidazole, 2-bromopyridine, and trans-[Pd(Cl)2(SMe2)2] complex. The obtained supported bis(NHC)-Pd(II) complex, Fe3O4@SiO2@SBA-AP-CC-bis(NHC)-Pd(II), was characterized by Fourier transform infrared spectroscopy (FT-IR), X‐ray diffraction spectroscopy (XRD), thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), energy‐dispersive X‐ray analysis (EDX), and Brunauer-Emmett-Teller surface area measurement (BET). The catalytic potential of the supported bis(NHC)-Pd(II) complex was explored for the selective preparation of benzaldehydes from the corresponding benzyl alcohols. The Pd(0) catalyst, Fe3O4@ SiO2@SBA-AP-CC-bis(NHC)-Pd(0), was obtained via reduction of the Pd(II)-catalyst with NaBH4. The obtained Pd(0)-catalyst efficiently reduced nitroarenes to anilines in the presence of hydrazine hydrate. The Pd contents of the Pd(II)-catalyst (2.83%wt.; 0.266 mmol/g) and Pd(0)-catalyst (2.26%wt., 0.212 mmol/g) were measured by atomic absorption spectroscopy. Both reactions are conducted under optimal reaction conditions of the solvent, the base, and the catalyst dosage. The catalysts enjoy easy magnetic separation, recyclability over eight runs without significant loss activity, and noticeable stability. They showed negligible palladium leaching (0.83% in the oxidation of benzyl alcohol and 0.1% in the reduction of nitroarenes).