Facile synthesis of a dimeric titanium(IV) complex with terminal TiO moieties and its application as a catalyst for the cycloaddition reaction of CO₂ to epoxides

In this report, facile and exclusive synthesis of dimeric titanium(IV) complex with a terminal TiO moiety from the reaction between novel pyridine-based tridentate ligand (LH₂) and Ti(O-i-Pr)₄ under the bubbling of wet air is presented. Alternatively, the same dimeric Ti complex was obtained via wet air bubbling of monomeric LTi(O-i-Pr)₂ or addition of the same equiv. of H₂O into LTi(O-i-Pr)₂. All compounds including LH₂ and two titanium complexes were characterized by single crystal X-ray analyses. Newly synthesized terminal oxo-titanium compound is the first example of structurally characterized dimeric terminal oxo-titanium compound having no TiO→Ti bonds. Two titanium complexes were used as effective catalysts for the cycloaddition of CO₂ to propylene oxide in the presence of various kinds of cocatalysts such as n-Bu₄PBr, n-Bu₄NI, n-Bu₄NBr, n-Bu₄NCl, PPNCl, and DMAP.