Developing preorganized iron catalyst for the selective coupling reaction of carbon dioxide with epoxide

Abstract

Carbon dioxide has received much attention as an abundant, nontoxic and renewable C1 source. One of the promising areas is a coupling reaction of $CO_2$ and epoxides to produce industrially important commodity chemicals, polycarbonates and cyclic carbonates. Cyclic carbonates have potential applications in various indus-tries as the intermediate of organic synthesis, solvents of electrolyte and precursors for polycarbonates. In order to work with the effective and selective conversion of $CO_2$ to cyclic carbonate, we have designed a new type of a ligand scaffold that can accommodate a preorganized iron center. Specifically, we are developing a new catalyst based on understanding the relationship between the geometry of a metal center and the catalytic activity. We have synthesized a series of iron and aluminum complexes supported by tri-anionic tetradentate $C_s$ symmetric $NO_3^{3-}$ ligand (1a, $NO_3H_3$ = (E)-2,2'-(((2-hydroxybenzylidene)amino)methylene)diphenol) and its analogue sub-stituted with a nitro group $(5-NO_2)NO_3^{3-}$ (1b, $(5-NO_2)NO_3H_3$= (E)-2,2'-(((2-hydroxy-5-nitrobenzylidene)amino)methylene)diphenol) in order to accommodate pre-organized rigid structure for 6-coordinate species. A series of iron and aluminum complexes, ${(NO_3)Fe(THF)}_2 (Fe-1a), {(5-NO_2)NO_3}Fe}_2 (Fe-1b), {(NO_3)Al}_2 (Al-1a)$ and ${(5-NO_2)NO_3}Al (Al-1b)$ were fully characterized including X-ray crystallog-raphy. Solid state structures revealed the dimeric nature of Fe-1a, Fe-1b and Al-1a where the available binding sites for each metal center are occupied by bridging phenolate oxygen atoms, while Al-1b exhibits mononuclear 5-coordinate environment. The coupling reaction of $CO_2$ with propylene oxide (PO) was investigated using Fe-1a, Fe-1b, Al-1a and Al-1b as a catalyst. Among these complexes, Fe-1a exhibited highest catalytic activity for the selective synthesis of propylene carbonate. In order to achieve the high catalysis with Fe-1a, several varia-bles including pressure, temperature and co-catalyst/catalyst ratio were adjusted. Under the optimized reaction conditions, Fe-1a showed unusual $4,400 (h^{-1})$ TOF values toward propylene oxide.