CO2-to-Cyclic Carbonate Transformation at Ambient Pressure Using a Mechanochemically Prepared Ti(III) Catalyst

A readily accessible Ti(III) complex, [Li(thf)₄][Ti(^MesPDA)₂] (4) (^MesPDA = N, N′-bis(2,4,6-trimethylphenyl)-o-phenylenediamide), has been synthesized mechanochemically, offering a sustainable route to Ti(III) species. Complex 4, combined with [NBu₄I] (TBAI), efficiently catalyzes the cycloaddition of CO₂ (1 bar) with terminal and internal epoxides, bio-resourced diepoxides, and the tri-substituted limonene oxide. This work represents the first example of a Ti(III)-based catalyst capable of efficiently synthesizing cyclic carbonates from CO₂ and epoxides under atmospheric pressure, and demonstrates the superior catalytic activity of Ti(III) compared to its Ti(IV) analog [Ti(^MesPDA)₂] (3). A detailed mechanistic study is conducted through density functional theory analysis to explore the catalytic pathways mediated by Ti(III) and Ti(IV). Key steps include epoxide coordination to Ti, iodide-mediated ring-opening to form a Ti-alkoxide species, and CO₂ insertion into the Ti–O bond. The superior performance of Ti(III) arises from its lower oxophilicity, which weakens Ti–O interactions, enhancing CO₂ insertion. Electron Paramagnetic Resonance (EPR) spectroscopy confirms epoxide coordination and supports the formation of a Ti(III)-alkoxide intermediate. Further evidence for the latter species comes from X-ray crystallography of the Ti(IV)-alkoxide analog [Li(4-crown-12)₂][Ti(^MesPDA)₂(OⁱPr)] (17), providing indirect structural evidence. These findings advance the development of Ti(III) systems in CO₂ valorization and highlight the key role of oxidation state in catalytic efficiency.