4,4′-trimethylenebi(sulfo)pyridinium tetrachloroaluminate hybrid salt as a useful acidic-basic catalyst for the synthesis of phenazine derivatives

Here, we present the production method of 4,4′-trimethylenebi(sulfo)pyridinium tetrachloroaluminate (TMBSPTCA) organic-inorganic hybrid salt as an eco-friendly, acidic-basic heterogeneous catalyst for the synthesis of benzo[a]pyrano[2,3-c]phenazine via one-pot solvent-free multicomponent domino reaction of 2-hydroxynaphthalene-1,4‑dione (1 mmol), benzene-1,2-diamine (1 mmol), malononitrile (1 mmol), and aromatic aldehydes (1 mmol) by generating C[sbnd]C, C[dbnd]C, C[sbnd]O, C[sbnd]N and C[dbnd]N bonds. The XRD (X-ray diffraction), FT-IR (Fourier-transform infrared spectroscopy), ¹³C NMR, ¹H NMR, Mass, TGA (thermal gravimetric analysis), FE-SEM (field emission scanning electron microscopy), and EDX (energy dispersive spectrometer), Ultraviolet–visible (UV–Vis) spectroscopy approved the framework of this hybrid salt. The structural results revealed that the catalyst is in the form of interconnected quasi-spherical clusters with a size of 50–150 nm with a dominant crystalline nature. Good thermal stability up to 600 °C, high purity, and its remarkable acidic property with a Hammett acidity function (H₀) of 1.42 are other features of the synthesized hydride salt. Eye-catching optimal conditions (60 °C, solvent-free, and 8 mol% TMBSPTCA) compared to previous protocols, rapid production of derivatives 1a-13a in a short time with excellent yields (7 to14 min, 92 to 98 %), reasonable values ​​of turnover number (TON: 11.50 to 12.25) and turnover frequency (TOF: 0.82 to 1.75 min⁻¹), the performance of the catalyst in the role of an acid-base duel with sulfonic acid and tetrachloroaluminate active centers, and the ability to easily recycle the catalyst (5 times) are the prominent advantages of this work.