Reactive Intermediates of the Catalytic Carbomagnesation Reaction:  Isolation and Structures of [Cp₂ZrEt]₂(μ-ethene), [Cp₂Zr(ethene)(L)] (L = THF, Pyridine), and [(indenyl)₂Zr(ethene)(THF)] and of Metallacycles with Norbornene^†

The reaction between Cp₂ZrCl₂ and 2 equiv of EtMgCl in THF at 0 °C results in the formation of the binuclear complex [Cp₂Zr(Et)]₂(μ-ethene) (1) in good yield. An X-ray crystallographic structure determination of 1 shows that the bridging ethene can be considered as the dianion C₂H₄^2- which is asymmetrically coordinated to the two neighboring Cp₂Zr(Et) centers. The analogous complex [(Me₃Si-Cp)₂Zr(Et)]₂(μ-ethene) (2) was isolated from the reaction of (Me₃Si-Cp)₂ZrCl₂ and 2 equiv of EtMgCl. The mononuclear orange compound [Cp₂Zr(ethene)(THF)] (3) was obtained by reaction of Cp₂ZrCl₂ and Et₂Mg(dioxane) in THF below 0 °C. In the ¹H NMR spectrum of 3 in THF-d₈ at 0 °C only two singlets were observed (5.46 and 0.51 ppm, respectively), corresponding to the Cp and the ethene protons. Furthermore, its structural motif in the solid state was determined by X-ray crystallography at −90 °C. The closely related complexes [(indenyl)₂Zr(ethene)(THF)] (4) and [Cp₂Zr(ethene)(pyridine)] (5) were analogously prepared in good yield. The X-ray structure of the dark red complex 4 shows that the zirconium atom, the two carbons of ethene, and the oxygen donor atom of THF lie in a single plane. The C−C bond distance of the coordinated ethene is elongated to 1.451(5) Å. Complex 3 undergoes a fast reaction with norbornene to form the metallacycle 6, which was characterized by NMR spectroscopy in solution and by X-ray diffraction analysis in the solid state as well. The complexes 1, 3, and 6 are isolated compounds which are involved in the Cp₂ZrX₂-catalyzed carbomagnesation reaction of norbornene with EtMgX.