Solvent-free CO2 cycloaddition by one- and two-dimensional sodium coordination polymers based on hydrazonic ligands

We successfully synthesized and characterized four new coordination polymers of sodium: sodium 4-hydroxy-3-{[2-(pyridine-4-carbonyl)hydrazinylidene]methyl}benzene-1-sulfonate (NaL^4-Py), sodium 3-[(2-benzoylhydrazinylidene)methyl]-4-hydroxybenzene-1-sulfonate (NaL^Ph), sodium 3-[(2-acetylhydrazinylidene)methyl]-4-hydroxybenzene-1-sulfonate (NaL^Me), and sodium 3-[(2-carbamoylhydrazinylidene)methyl]-4-hydroxybenzene-1-sulfonate (NaL^NH2). These polymers were synthesized through a 1:1 condensation reaction of hydrazide derivatives with sodium salicylaldehyde-5-sulfonate monohydrate (NaSalSO₃) and were thoroughly characterized using various analytical techniques. The compounds NaL^Me and NaL^Ph, crystallized in the P $\bar{1}$ and P2₁/n space groups, respectively, forming one-dimensional polymers. On the other hand, NaL^NH2 and NaL^4-Py crystallized in the Cmca and P $\bar{1}$ space groups, respectively, forming two-dimensional coordination polymers. The coordination environment of the sodium cation differed among the compounds, with the imine nitrogen atom interfering in NaL^4-Py and NaL^Ph. The sulfonate group exhibited various coordination modes, resulting in the different dimensionalities of the polymers. Compounds NaL^4-Py and NaL^Ph acted as heterogeneous Lewis acid catalysts for the solvent-free cycloaddition reaction of epichlorohydrine and CO₂ into valued cyclic carbonate. High selective conversion (68 %) of epichlorohydrin to cyclic carbonate was obtained at mild conditions, 60 °C and 1 bar CO₂, after 24 h of reaction.