Novel composite of nano-silica, l-tartaric acid, and magnesium bismuthate (SiO2/L-Tar/MgBi2O4): A versatile recoverable promoter for solvent-free synthesis of 5-(indole)-chromeno[2,3-b]pyridines

In the present work, commercial amorphous nano-silica has been covered by l-tartaric acid and MgBi₂O₄ to obtain the nanocomposite (SiO₂/L-Tar/MgBi₂O₄) which has been characterized by Fourier transform infrared (FT-IR), Field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDAX), and X-ray diffraction (XRD) techniques. In continue, it has been utilized as an efficient, heterogeneous, and recoverable catalyst for the green and pseudo four-component synthesis of diverse 5-(indole)-chromeno[2,3-b]pyridines from the reaction of various salicylaldehydes, indoles, and malononitrile, in 1:1: 2 molar ratio, under solvent-free conditions at 50 °C. The recovery of the nanocatalyst was also examined within two runs successfully. The recovered inorganic-organic nanohybrid has been identified through FESEM and EDAX techniques. The structural parameters and thermodynamic stability of SiO₂/L-Tar complexes have been investigated by density functional theory (DFT) calculations. Additionally, the nature of intermolecular hydrogen bonds has been studied by natural bond orbital (NBO) analysis. Short reaction times, simple and facile work-up procedures, and good yields are the advantages of this producer.