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

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 60 0 MHz H-1 NMR spectroscopic analysis in the temperature range between 253 a nd 128 K revealed the presence of two metallocene conformational isomers in a 60:40 ratio. It is likely that this observed feature just originates fro m freezing out the indenyl-furyl rotation. The corresponding activation bar rier of 6a was determined at AG(rot)(double dagger) = 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 dichlor ides show analogous conformational behavior and almost identical rotational activation energies. In the bis[2-(5-methyl-2-furyl)indenyl]ZrR2 series th e metallocene rotational barrier can apparently be frozen on the 600 MHz H- 1 NMR time scale for R = CH2CMe3 and R = CH2Ph. The di(neopentyl)Zr complex 7b shows a metallocene rotational barrier of DeltaG(rot(cp))(double dagger )(213 K) = 9.0 +/- 0.3 kcal mol(-1), and for the di(benzyl)Zr complex 7C a DeltaG(rot(cp))(double dagger) value of 11.4 +/- 0.3 kcal mol(-1) was obtai ned at 243 K. In the cases of 7b and 7c only chiral rac-type metallocene co nformers were found in solution within the limits of the accuracy of the H- 1 NMR method. The complexes bis[2-(5-ethyl-2-furyl)indenyl]ZrCl2 (8) and bi s[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.