Study of the catalytic activity of methylene-bridged dicationic –SO3H-functionalized imidazolium phosphomolybdate hybrids for the one-pot sequential synthesis of 3-substituted indoles

Abstract

Herein, we aimed to synthesize three organic–inorganic hybrids (DILPOM-1/2/3), developed from the assembly of –SO₃H-functionalized dicationic imidazolium ionic liquids having variable methylene connecting spacers with inorganic 12-molybdophosphoric acid. The hybrids DILPOM-1/2/3 were 2-methyl-1-(3-(2-methyl-3-sulfo-1H-imidazol-3-ium-1-yl)propyl)-3-sulfo-1H-imidazol-3-ium phosphomolybdate, 2-methyl-1-(4-(2-methyl-3-sulfo-1H-imidazol-3-ium-1-yl)butyl)-3-sulfo-1H-imidazol-3-ium phosphomolybdate, and 2-methyl-1-(12-(2-methyl-3-sulfo-1H-imidazol-3-ium-1-yl)dodecyl)-3-sulfo-1H-imidazol-3-ium phosphomolybdate. The incorporation of various lengths of connecting spacers made their thermal stability window wider, as verified by thermogravimetric analysis, and thus, they could be used as efficient catalysts in high-temperature reactions. The extent of hydrogen bond interactions within the –SO₃H group and the PMo₁₂O₄₀³⁻ anion strategically controlled the acidic sites and together with the methylene linkages provide a heterogeneous nature to the hybrids, making them easily recyclable and thermally stable acid catalysts. The hybrid with the longest methylene connecting spacer (n = 12) was found to be more efficient as an acid catalyst than the shorter ones in the solvent-free Michael-like addition of indole with chalcones obtained via Claisen–Schmidt condensation.