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Bridged zirconocenes bearing seven-membered rings dichlorodimethylsilylenebis(2-methyl-4-phenyl-4-H-azulenyl)zirconium (3a) and dichlorodimethylsilylenebis(2-methyl-4-phenyl-4-H-5,6,7,8-tetrahydroazulenyl)zirconium (4) were synthesized. A silylene-bridged ligand was obtained by the reaction of 2-methylazulene, phenyllithium, and dichlorodimethylsilane. After lithiation of the ligand by n-butyllithium, reaction with zirconium tetrachloride gave a rac and meso mixture of 3. The structure was confirmed by X-ray crystallographic analysis. It was found that both of the phenyl groups at the 4-position are oriented outside toward the metal center and the seven-membered ring is not planar. Furthermore, reaction with hydrogen in the presence of platinum oxide gave 4. When activated with methylaluminoxane, these complexes showed high catalytic activity for polymerization of propene to give isotactic polypropylene.

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Photochemical reaction of a rac/meso mixture of bridged bis-azulenyl zirconocenes was investigated. Irradiation of the rac/meso mixture of dichlorodimethylsilylenebis(2-methyl-4-phenyl-4H-azulenyl)zirconium resulted in a rapid conversion of only the meso isomer to give a novel bridged metallocene with a bis-azulenyl ligand cross-linked by a cyclobutylene ring, which was produced by intramolecular [2+2] cycloaddition, and the rac isomer was stable toward photoirradiation. The structure of the cyclobutylene-bridged metallocene derived from meso-dichlorodimethylsilylenebis(2-methyl-4-(2-fluoro-4-biphenylyl)-4H-azulenyl)zirconium was determined by X-ray crystallographic analysis. The structure shows a bond formation between two azulenyl rings at the 7- and 8-position, respectively, to form a four-membered ring. The solubility of cyclobutylene-bridged metallocenes in solvent was found to be much higher than those of rac/meso isomers. After photoirradiation of a rac/meso mixture, rac isomers were easily isolated.

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