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 (<BO>6a</BO>). 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 <BO>6a</BO> was determined at ΔG^&thermod;<INF>rot</INF> = 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]ZrR<INF>2</INF> series the metallocene rotational barrier can apparently be frozen on the 600 MHz ¹H NMR time scale for R = CH<INF>2</INF>CMe<INF>3</INF> and R = CH<INF>2</INF>Ph. The di(neopentyl)Zr complex <BO>7b</BO> shows a metallocene rotational barrier of ΔG^&thermod;<INF>rot(Cp)</INF>(213 K) = 9.0 ± 0.3 kcal mol^-1, and for the di(benzyl)Zr complex <BO>7c</BO> a ΔG^&thermod;<INF>rot(Cp)</INF> value of 11.4 ± 0.3 kcal mol^-1 was obtained at 243 K. In the cases of <BO>7b</BO> and <BO>7c</BO> 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]ZrCl<INF>2</INF> (<BO>8</BO>) and bis[2-(5-methyl-2-furyl)indenyl]Zr(neopentyl)<INF>2</INF> (<BO>7b</BO>) were characterized by X-ray crystal structure analyses. In the solid state both complexes exhibit chiral rac-like metallocene frameworks.