Affixing Luminescent Hexabromidomanganate(II) with a Poly(melamine-formaldehyde) Network: A Bifunctional Catalyst for CO ₂ -Epoxide Cycloaddition Reactions

The hydrobromide adduct of poly(melamine-formaldehyde) resin PMF·HBr is prepared for converting bromide (Br–) to bromidomanganate(II) counterion by reacting with MnBr2 salt. This results in the affixation of the hexabromidomanganate(II) anion (MnBr64–) with the PMF resin, forming (H4PMF)[MnBr6]. The transformation of counterion Br– of PMF·HBr to MnBr64– is evident from the observance of reddish luminescence (λem = 623 nm) and other analytical techniques such as UV–vis, HRMS, PXRD, and XPS. The bromidomanganate(II) anionic complex serves as an excellent catalyst for the CO2 cycloaddition reactions to epoxides, resulting in cyclic carbonate products. The reaction shows first-order kinetics with rate constants k = 4.3 × 10–5 s–1 at 40 °C, 6.5 × 10–5 s–1 at 60 °C, and 8.2 × 10–5 s–1 at 80 °C. (H4PMF)[MnBr6] acts as an efficient heterogeneous catalyst that can be used repeatedly without much loss of activity [first cycle (91% conversion), second cycle (86% conversion), third cycle (85% conversion), and fourth cycle (83% conversion)]. A further study of the catalytic activity of the bromidomanganate(II)-based homogeneous catalyst (Bu4N)2[MnBr4] revealed that 1 mol % of the catalyst could show 99% conversion of epichlorohydrin to its corresponding cyclic carbonate in a very mild condition, viz., room temperature and 1 bar CO2 pressure.