Structural and dynamic features of bis[2-(2-furyl)indenyl]zirconium derivatives

Article abstract of DOI:10.1021/om0104518

Treatment of 2-indanone with 2-lithio-5-methylfuran, followed by hydrolysis, dehydration, deprotonation with n-butyllithium, and transmetalation, gave bis[2-(5-methyl-2-furyl)indenyl]-zirconium dichloride (6a). The dynamic 600 MHz ¹H NMR spectroscopic analysis in the temperature range between 253 and 128 K revealed the presence of two metallocene conformational isomers in a 60:40 ratio. It is likely that this observed feature just originates from freezing out the indenyl-furyl rotation. The corresponding activation barrier of 6a was determined at ΔG?_rot = 7.0 ± 0.4 kcal mol^-1 by a line shape analysis. A series of analogous 5-methyl-2-furyl- and 5-methyl-2-thienyl-substituted bis(indenyl)zirconium dibromides and dichlorides show analogous conformational behavior and almost identical rotational activation energies. In the bis[2-(5-methyl-2-furyl)indenyl]ZrR₂ series the metallocene rotational barrier can apparently be frozen on the 600 MHz ¹H NMR time scale for R = CH₂CMe₃ and R = CH₂Ph. The di(neopentyl)Zr complex 7b shows a metallocene rotational barrier of ΔG?_rot(Cp)(213 K) = 9.0 ± 0.3 kcal mol^-1, and for the di(benzyl)Zr complex 7c a ΔG?_rot(Cp) value of 11.4 ± 0.3 kcal mol^-1 was obtained at 243 K. In the cases of 7b and 7c only chiral rac-type metallocene conformers were found in solution within the limits of the accuracy of the ¹H NMR method. The complexes bis[2-(5-ethyl-2-furyl)indenyl]ZrCl₂ (8) and bis[2-(5-methyl-2-furyl)indenyl]Zr(neopentyl)₂ (7b) were characterized by X-ray crystal structure analyses. In the solid state both complexes exhibit chiral rac-like metallocene frameworks.