STRUCTURE AND DYNAMICS OF ZIRCONOCENE COMPLEXES IN THE SOLID-STATE STUDIED BY C-13 CP MAS NMR-SPECTROSCOPY

High-resolution one- and two-dimensional solid-state CP /MAS spectra of microcrystalline bent metallocene complexes of the type (η5-RCp)2ZrL2 have been obtained at various temperatures (L = Cl, R = H (1), Me (2), Et (3),tBu (4); L2 = s-cis-η4-C4H6, R-H (5); L2 = s-cis- η4-C4H4Me2, R-H (6); L2-s-cis- η4-C4H5Me, R = H (7); L2 = s-cis-η4-C4H6, R =tBu (8); L2 = s-cis-η4-C4H6, R = 1,1-dimethylpentyl (9); L2 = s-cis-η4-C4H5Me, R =tBu (10)). Complexes 1-4 possess C2 molecular symmetry, while the overall symmetry of complexes 5 and 6 is close to Cs. In 7-9 there is no element of symmetry, and 10 yields diastereoisomers (95:5). Variable-temperature MAS spectra give evidence forRCp rotation and diene topomerization as fluxional processes occurring in the solid state. Due to steric interaction with the methyl protons in 7 the rotation of the upper η5-bonded cyclopentadienyl ring was frozen out at T= 110 K (ΔG* ≈ 5.5 kcal/mol) and the five13C chemical shifts of this species cover a range of more than 15 ppm. Due to less steric interaction the activation barrier for rotation of the lower Cp ring is smaller (ΔG* ≈ 4 kcal/mol). Upon introduction of alkyl substituents at the η5-bonded rings the rotation barrier is significantly increased: in 2 it is around 17 kcal/mol. In contrast, movement of the η4-coordinated diene is largely independent of alkyl substituents and in 5-10 the barrier for this process is around 14 kcal/mol. © 1990, American Chemical Society. All rights reserved.