Synthesis and Structural Investigation of Stable Zirconacyclopentanes Which Bear Additional Functional Groups

The reactions of zirconocene-alkyne complexes Cp‘₂Zr(L)(η²-Me₃SiC₂SiMe₃) {Cp‘₂ = (η⁵-C₅H₅)₂:  L = THF, 1; L = pyridine, 2; Cp‘₂ = rac-ebthi, 3, [ebthi = 1,2-ethylene-1,1‘-bis(η⁵-tetrahydroindenyl)]} with 2 equiv (or excess) of bicyclo[2.2.1]hepta-2,5-diene (NBD) in THF at 50 °C result in five-membered zirconacyclopentane complexes Cp₂Zr(C₁₄H₁₆) (4) and rac-(ebthi)Zr(C₁₄H₁₆) (5) (C₁₄H₁₆ means two coupled NBDs) in good yields. NMR spectra of complexes 4 and 5 show the C₂ symmetry of the molecules, indicating that the two NBD components in the complexes are in exo−trans−exo conformation. An X-ray crystallographic structure determination of 5 confirms that the two NBD fragments are exo−trans−exo linked to form a racemate. Both optical isomers are present in a 1:1 ratio in the crystals. Reactions of 1 and 2 with 1 equiv of NBD have also been investigated, and only zirconacyclopentane complex 4 was isolated from the reactions. The analogous zirconacyclopentane complexes Cp₂Zr(C₂₂H₂₀) (6) and rac-(ebthi)Zr(C₂₂H₂₀) (7) (C₂₂H₂₀ means two coupled BNBDs) are obtained from the reactions of 1 (or 2) and 3 with an excess of 1,4-dihydro-1,4-methanonaphthalene (BNBD), respectively. The X-ray structure of the orange complex 7 shows that the geometry of the Zr coordination sphere can be described as a distorted tetrahedron and similar to that of 5. The zirconacyclopentane complex rac-(ebthi)Zr(C₁₄H₁₅N) (9), bearing a free N coordinating site and a double bond, is formed from the coupling reaction of rac-(ebthi)Zr(C₇H₇N) (8) with NBD. All these reactions are stereospecific, and all new complexes are characterized by NMR, MS, and elemental analysis.