4,4′-(1,4-phenylene)dipyridinium sulfonic acid triflimidate: As an efficient ionic liquid catalyst for the facile synthesis of pyrido-dipyrimidine derivatives

In the present study, a novel Brønsted-acidic ionic liquid, namely 4,4′-(1,4-phenylene)dipyridinium sulfonic acid triflimidate ([HO₃S-Py-Ph-Py-SO₃H][(CF₃SO₃)₂N]₂), was synthesized and characterized as a bi-functional catalyst. The characterization was conducted using various analytical techniques, including Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), carbon-13 nuclear magnetic resonance (¹³C NMR), proton nuclear magnetic resonance (¹H NMR), Fluorine-19 nuclear magnetic resonance (¹⁹F NMR), mass spectrometry and Carbon, Hydrogen and Nitrogen (CHN) elemental analysis. Subsequently, the catalytic performance of [HO₃S-Py-Ph-Py-SO₃H][(CF₃SO₃)₂N]₂ in the synthesis of pyrido[2,3-d:6,5-d']dipyrimidine derivatives was investigated under optimized conditions (8 mol% of catalyst, H₂O, 25 °C). The results indicated its effectiveness and broad performance in this reaction (1a-16a, 1–5 min, 88–98 %). This high performance was owing to the role of two pairs of acidic and basic sites (i.e., SO₃H and (CF₃SO₃)₂Nˉ) of the catalyst in promoting the synthetic pathway according to the proposed mechanism. Additional significant aspects of this process encompass the use of inexpensive and readily available primary reactants for catalyst production, the potential for catalyst recovery and reuse for up to five cycles without any significant changes to its catalytic properties, reduced reaction times, high product yields, notable values of turnover number (TON: 11.00 to 12.25) and turnover frequency (TOF: 2.20 to 12.25 min⁻¹), moderate reaction conditions, easy separation, and the resolution of prior challenges encountered in the synthesis of these compounds. Meanwhile, three new derivatives were synthesized and characterized (14a, 15a and 16a).