Sulfonyl-bridged (copper-immobilized nickel ferrite) with activated montmorillonite, [(NiFe2O4@Cu)SO2(MMT)]: a new class of magnetically separable clay nanocomposite systems towards Hantzsch synthesis of coumarin-based 1,4-dihydropyridines†
In this study, the synthesis of a new class of magnetic clay-based nanocomposites by bridging of sulfonyl groups between copper-immobilized nickel ferrite (NiFe2O4@Cu) and activated montmorillonite is described. Synthesis of the clay nanocatalyst was carried out via the activation of montmorillonite by ClSO3H to afford sulfonated montmorillonite. The modified montmorillonite was then reacted with copper-layered nickel ferrite to afford the magnetic clay nanocomposite [(NiFe2O4@Cu)SO2(MMT)]. Next, the characterization of porous materials was carried out using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller and vibrating sample magnetometer analyses. The obtained results showed that the clay nanocomposite containing a sulfonyl-bridge has a large surface area and magnetic properties versus the prepared one without the sulfonyl groups as (NiFe2O4@Cu)(MMT). The nanostructured clay had excellent catalytic activity towards the Hantzsch synthesis of coumarin-based 1,4-dihydropyridines via one-pot and three-component condensation of 4-hydroxycoumarin, aromatic aldehydes and ammonia. All reactions were carried out in water as a green and economic green solvent within 10–45 min to affords 1,4-DHPs in high to excellent yields. Reusability of the clay nanocomposite was investigated for six consecutive cycles without significant loss of catalytic activity. Based on this study, therefore, sulfonated montmorillonite could be considered an excellent support for the immobilization of magnetic materials.