Bis(cyclooctatetraenyl)zirconium: X-ray Crystal Structure and Solution and Solid-State NMR Spectra

The structure of bis(cyclooctatetraenyl)zirconium, Zr(C₈H₈)₂, has been investigated by variable-temperature solution and solid-state NMR spectroscopy and by X-ray crystallography. The solution ¹H and ¹³C NMR data show that all 16 protons and all 16 carbon atoms are equivalent on the NMR time scale even at −100 °C. No line broadening due to decoalescence of a fluxional process was noted, and the spectra are thus consistent with the original proposal that Zr(C₈H₈)₂ adopts a symmetric sandwich structure. However, the CPMAS ¹³C NMR spectrum contains two resonances and the X-ray crystallographic results unambiguously reveal a structure with one η⁸− and one η⁴−C₈H₈ ring. The exchange barrier for the fluxional process that makes the two C₈H₈ rings equivalent has been estimated to be <7.5 kcal mol⁻¹ in solution and >13.5 kcal mol⁻¹ in the solid state. A second fluxional process which involves 1,2-shifts of the η⁴−C₈H₈ ring has a much lower barrier that has been estimated to be <5.5 kcal mol⁻¹ in the solid state. Neither electronic nor steric factors clearly favor the asymmetric structure over the symmetric sandwich alternative. Similar conclusions are drawn for the structure of the hafnium analogue Hf(C₈H₈)₂. Crystal data for Zr(C₈H₈)₂ at −75 °C: monoclinic; space group Pn; a = 7.049 (5), b = 7.695 (4), c = 11.217 (10)Å; β = 95.08 (7)°; V = 606 (1)ų; Z = 2; R_F = 0.054; R_wF = 0.057 for 153 variables and 1023 independent reflections with I > 2.58σ(I).