Highly selective solvent free catalysis of CO2 and CS2 fixation under mild conditions using electronically varied zinc complexes

Abstract

A set of new generation N/O donors, maleonitrile tethered, tetradentate heteroscorpionate modified Schiff base type ligands, 2-((E)-2-hydroxybenzylideneamino)-3-(pyridin-2-ylmethylamino)maleonitrile (H₂L^H), 2-((E)-2-hydroxy-5-nitrobenzylideneamino)-3-(pyridin-2-ylmethylamino)maleonitrile (H₂L^NO2), and 2-((E)-2-hydroxy-3-methoxybenzylideneamino)-3-(pyridin-2-ylmethylamino)maleonitrile (H₂L^OMe) were synthesized. These ligands were reacted with a stoichiometric amount of zinc acetate to afford corresponding mononuclear Zn complexes [Zn(L^H)] (1^H), [Zn(L^NO2)] (1^NO2), and [Zn(L^OMe)] (1^OMe) with excellent yields. All metal complexes and ligands were fully characterized using all possible spectroscopic tools, including single-crystal X-ray structure analysis in some cases. The ¹H NMR spectral feature was well explained in terms of the electronic effect of the ligand's substitution at the phenyl ring. Novel monomeric Zn complexes supported by heteroscorpionate ligands were explored as effective catalysts to convert CO₂ and CS₂ to cyclic carbonates and thiocarbonates, respectively, via epoxide ring opening under mild and solvent-free conditions where tetrabutylammonium bromide (TBAB) was used as co-catalyst. The synthesized catalyst activates CO₂ and CS₂ at ambient conditions, whereas most of the reported catalysts require harsh conditions. Electronically, the Zn center in 1^H was found to have the highest Lewis acidity, enabling better catalytic activity with respect to other Zn complexes. The probable mechanisms of catalytic activity for CO₂ and CS₂ activation were also explained. Mechanistically, the {Zn–Br} unit serves as a Lewis acidic platform that stabilizes the activated epoxide intermediate after ring opening by bromide ion and provides suitable nucleophiles for a further attack at the CO₂/CS₂ center. Exclusive cyclic carbonate/thiocarbonate formation was explained as the cyclization of activated CO₂/CS₂ product occurring at the Zn center before it is released as an acyclic intermediate.