Dialkyl and Methyl-Alkyl Zirconocenes: Synthesis and Characterization of Zirconocene-Alkyls That Model the Polymeryl Chain in Alkene Polymerizations

Zirconocene precatalysts with sterically bulky alkyl groups were designed as model systems for the
propagating species in zirconocene-catalyzed alkene polymerization. Specialty alkyllithium reagents Li(CH2CEt3) and Li(CH2CMe2CH2Ph) were prepared and utilized in the synthesis of dialkyl and methyl-alkyl zirconocenes of the form CpCp*ZrR2, Cp2Zr(CH3)(R), and CpCp*Zr(CH3)(R) (Cp = (η5-C5H5);
Cp* = (η5-C5Me5); R = CH2CMe3, CH2SiMe3, CH2CEt3, CH2CMe2CH2Ph). These new zirconocene
alkyls were isolated and fully characterized by NMR spectroscopy and in some cases by X-ray diffraction.
The molecular structures determined display the bent-sandwich coordination mode common for
zirconocenes. The steric influence of the alkyl group on the observed structural parameters is reflected
in slightly expanded C−Zr−C or C−Zr−Cl angles in the equatorial plane and long zirconium−alkyl
bond distances.