Interconversion of (η 3 ‐Allyl)(η 4 ‐conjugated‐diene)(η 5 ‐cyclopentadienyl)zirconium and ‐hafnium Stereoisomers. Evidence for the simultaneous rearrangement of diene and allyl ligands

Reaction of (η 3 ‐allyl)(cyclopentadienyl)zirconium(4+) chloride with (2,3‐dimethylbutadiene)magnesium or (2‐phenylbutadiene)magnesium gave the (η 3 ‐allyl)(η 4 ‐conjugated diene)(cyclopcntadienyl)zirconium complexes 3c and 3d , respectively. The analogous reaction between (η/ 3 ‐allyl)(cyclopentadienyl)hafnium(4+) chloride with (buta‐diene)magnesium afforded (η 3 ‐allyl)(η 4 ‐butadiene)(η 5 ‐cyclopentadienyl)hafnium 3e . Photolysis of the complexes 3 produced their stereoisomers 4 . The open‐chain π ligands in 3 open themselves towards the apical Cp ligand, in 4 they both are turned around by 180°. The Gibbs activation energy of the 4 → 3 rearrangement of the Hf complex does not deviate significantly from those of the Zr‐containing systems ( 4c → 3e : Δ G ≠ rearr. (−10°) = 81.3 ± 1.3 kJ/mol; 4d → 3d : Δ G ≠ (‐10°) = 85.7 ± 1.3 kJ/mol; 4e → 3e : Δ G ≠ (‐5°) = 84.4 ± 1.3 kJ/mol). A modified ex‐tended‐ Hückel theory (MEHT) was used in order to simulate the thermally induced rearrangement of the diene ligand in 3 / 4 . The calculations indicate that there should be a combinational interconversion of both the diene and allyl ligand. A diene inversion becomes energetically favoured, if a simultanous allyl rotation occurs. The transition state 9 lies ca. 90 kJ/mol above the optimized geometry of 4a and 115kJ/mol above 3a . Orbital considerations show that a spatially different ‘valence’ orbital of the [Zr(allyl)Cp] fragment 6 in comparison with [ZrCp 2 ] 7 causes the conformational preference of the diene ligand in 3a with a higher rearrangement barrier.