Structurally crosslinked HEC/ECH–MnO₂ nanocomposite as a sustainable green catalyst for the synthesis of symmetric bis-thiazoles featuring diphenoxyalkane linkers

Crosslinked hydroxyethyl cellulose/epichlorohydrin MnO₂ (HEC/ECH–MnO₂) nanocomposite films were prepared by in situ solution-casting of MnO₂ nanoparticles into a crosslinked (HEC) with (ECH) at five loadings (5–25 wt.%). MnO₂ nanoparticles were chosen for their availability, low cost, and redox-active Mn ² ⁺/Mn ³ ⁺/Mn⁴⁺ surface; embedding them in a crosslinked biopolymer matrix suppressed aggregation and made recovery simple. Fourier transform infrared spectra (FTIR) absorptions at 738 and 518 cm⁻ ¹ confirmed Mn–O bond formation; X-Ray diffraction (XRD) showed the MnO₂ phase remained intact with a mean grain size of 28 nm (Debye–Scherrer); field emission scanning electron microscope (FESEM) indicated a uniform particle distribution throughout the film. The films were applied as base catalysts in the cyclo-condensation of bis(hydrazinecarbothioamides) with hydrazonoyl chlorides, yielding bis-hydrazonothiazoles with diphenoxyalkane linkers in 90–96% isolated yield under optimized conditions (20 mol% catalyst, 1,4-dioxane, 120°C, 5 h), compared with 71–80% using triethylamine. The catalyst was recovered by filtration and reused four times with under 3% yield loss. The efficient utility of a crosslinked HEC/ECH–MnO₂ nanocomposite as a heterogeneous base catalyst for bis-thiazole synthesis, and the results highlight its potential as a recyclable, biodegradable option for sustainable organic synthesis.