Synthesis and Reactivity of Thermally Stable Zirconocene (II) Species Precursors

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Reactions of tris (indenyl) zirconium hydride with a variety of alkynes were investigated and evaluated the reactivity of the zirconium complex as zirconocen (II) equivalent. The reaction of tris (indenyl) zirconium hydride with internal alkynes afforded the corresponding zirconacyclopentadienes in high yields. Interestingly, the reaction with terminal arylalkynes produced the2,5-diarylzirconacycles with high selectivity.A reaction of thus obtained bis (indenyl) zirconacyclopentadienes with titanium tetrachloride afforded the corresponding fluorene derivatives via coupling of the diene moiety with the indenyl ligand. This reaction was applicable for alkyl-or aryl-substituted, and bicyclic zirconacyclopentadienes and furnished the substituted fluorenes. Instead of the indenyl ligand, zirconacyclopentadienes having substituted cyclopentadienyl ligands were also applied for the reaction and afforded the corresponding indene derivatives. A comparison of the structural parameters of the zirconacyclopentadienes with the related ones suggested that the coupling reactivity was strongly affected by the distance between the indenyl or cyclopentadienyl ligands and the diene moiety of the zirconacyclopentadienes. The most reactive titanacyclopentadienes have the shortest distance. In addition, the reactivity was increased with increasing the steric bulkiness of the ligand. Titanium tetrachloride played an important role as an oxidant in the reaction.