Metallocene derivatives of early transition elements. Part 3. Synthesis, characterisation, conformation, and rotational barriers [for the Zr–C(sp3) bond] of the zirconium(IV) complexes [Zr(η-C5H4R)2{CH-(SiMe3)2}Cl] and the crystal and molecular structures of the t-butyl and trimethylsilyl complexes (R = CMe3 or SiMe3)

Abstract

The complexes [Zr(η-C₅H₄R)₂{CH(SiMe₃)₂}Cl] have been prepared from the appropriate metallocene dichloride (R = Me, Et, Prⁱ, Bu^t, or SiMe₃) and an equimolar portion of Li[CH(SiMe₃)₂] in diethyl ether. [The same method failed to yield the hafnium analogues (for R = H or SiMe₃).] Except for R = Et, these are white or pale yellow sharp-melting crystalline solids which have been characterised by elemental analysis and i.r., ¹H, and ¹³C n.m.r. spectra. Variable-temperature ¹H n.m.r. spectra show that (i) at low temperature (coalescence temperature, T_c, –4 to 18 °C) the preferred conformation has diastereotopic pairs of SiMe₃[of CH(SiMe₃)₂] and η-C₅H₄R groups, and (ii)ΔG^‡ for rotation about the Zr–C(sp³) bond is in the range 59.8 to 65.6 kJ mol^–1(R = SiMe₃ > Et > Bu^t > Prⁱ≈ Me > H). The ring cyclopentadienyl ¹³C n.m.r. signals are each split into a doublet at –30 °C but are observed as three distinct sharp singlets at 60 °C. The compounds [Zr(η-C₅H₄R)₂{CH(SiMe₃)₂}Cl](R = Bu^t or SiMe₃) are isostructural, crystallising in the space group P2₁/n, with cell constants, for Z= 4, being a= 10.496(6), b= 15.250(8), c= 18.272(9)Å, β= 100.48(5)° for R = Bu^t, and a= 10.525(5), b= 15.320(7), c= 19.064(8)Å, β= 98.73(4)° for R = SiMe₃. There is considerable strain within the substitued cyclopentadienyl ligands, as exemplified by the distinct spread in the range of Zr–C(π) distances and the significant deviation from the cyclopentadienyl plane of the –XMe₃ moiety. The remainder of the molecule, however, does not seem to be influenced by any steric constraints with the Zr–Cl bonds, of 2.452(2) and 2.447(1)Å(for X = C and Si respectively), and the Zr–C(σ) bonds, of 2.324(8) and 2.327(3)Å, being within previously observed limits. The conformation in the crystal corresponds to that found by low-temperature n.m.r. spectroscopy.