NNC-Scorpionate Zirconium-Based Bicomponent Systems for the Efficient CO₂ Fixation into a Variety of Cyclic Carbonates

Two new derivatives of the bis­(3,5-dimethyl­pyrazol-1-yl)­methane modified by introduction of organosilyl groups on the central carbon atom, one of which bearing a chiral fragment, have been easily prepared. We verified the potential utility of these compounds through the reaction with [Zr­(NMe₂)₄] for the preparation of novel zirconium complexes in which an ancillary bis­(pyrazol-1-yl)­methanide acts as a robust monoanionic tridentate scorpionate in a κ³-NNC chelating mode, forming strained four-membered heterometallacycles. These κ³-NNC-scorpionate zirconium amides were investigated as catalysts in combination with tetra-n-butylammonium bromide as cocatalyst for CO₂ fixation into five-membered cyclic carbonate products. The study has led to the development of an efficient zirconium-based bicomponent system for the selective cycloaddition reaction of CO₂ with epoxides. Kinetics investigations confirmed apparent first-order dependence on the catalyst and cocatalyst concentrations. In addition, this system displays very broad substrate scope, including mono- and disubstituted substrates, as well as the challenging biorenewable terpene derived limonene oxide, under mild and solvent-free conditions.