Zirconocene complexes as catalysts for the cycloaddition of CO₂ to propylene oxide

Six zirconocene derivatives were systematically designed; one of the cyclopentadienyl (Cp) ligands of zirconocene dichloride [Cp₂ZrCl₂] was silylated and this group was then incrementally increased in size; furthermore, one derivative with a 4,4-disilylbiphenyl bridge between the two zirconocene fragments was prepared. One zirconium complex was characterized by single-crystal X-ray analysis. All zirconium complexes were effective catalysts for the cycloaddition of CO₂ to propylene oxide. Two complexes containing a biphenyl group attached to a SiMe₂ group showed higher activity than the other complexes. In addition, the dinuclear complex containing the biphenylenebis(dimethylsilyl) bridge showed the highest activity among all the zirconium complexes reported. Furthermore, these complexes are the first examples of zirconocene catalytic systems for the cycloaddition reaction of CO₂ to propylene oxide.