The preparation of bis(pentamethylcyclopentadienyl)zinc and bis(trimethylsilylcyclopentadienyl)zinc, and their molecular structures determined by gas electron diffraction

Abstract

The syntheses of two derivatives of dicyclopentadienylzinc, (C5Me5)2Zn(I) and (C5H4SiMe3)2Zn(II), Me = CH3, are described. The gas electron diffraction patterns were recorded with nozzle temperatures of about 130 °C (I) and 100 °C (II). Molecular models with two η5-bonded or two η3-bonded rings are not compatible with the GED data. The molecular structure of I is (η5-C5Me5)Zn(η1-C5Me5) with Zn-C(η5) = 228(2) pm and Zn-C(η1) = 204(6) pm. The former is similar to the Zn-C(η5) bond distance in (η5-C5H5)ZnCH3. The latter appears to be longer than the Zn-C(σ) bond distances in (C5H5)ZnCH3and Zn(CH3)2. The angle between the Zn-C(η1) bond and the C5ring plane is 84(4)°. The best fit between calculated and experimental intensities is obtained with CC bond distances in the η1-bonded ring corresponding to localized single and double bonds. The electron diffraction data of (II) are consistent with a similar model with the (η5-C5H4SiMe3)Zn and Me3Si groups attached to the same carbon atom, C(5), of the η1-bonded ring. The presence of other isomers can, however, not be ruled out. NMR spectra of both compounds show that exchange of η5- and η1-bonded rings is rapid on the NMR time-scale at room temperature, but 13C spectra of II show that this exchange is frozen out at - 37 °C.