Bulky alkyls and a hydridoalkyl of zirconium(IV): influence of steric constraints upon (i) conformation and the Zr–C rotational barrier and (ii) the Zr–C bond length. X-Ray crystal and molecular structure of [zr(η-C5H5)2{CH(SiMe3)2}Ph]

Abstract

Variable temperature ¹H n.m.r. spectra of [Zr(η-C₅H₅)₂{CH(SiMe₃)₂]}R](R = Me, Prⁿ, CH₂SiMe₃, Ph, Cl, or H) in [²H₈]toluene show that the activation free energy for rotation about the Zr–CH(SiMe₃)₂ bond is exceptionally high [11·6–15·3 kcal mol^–1: R = H (a dimer in vapour or in C₆H₆) > Cl > alkyl > Ph] with a preference to a low temperature conformation in which the two sets of SiMe₃ and η-C₅H₅ groups are diastereotopic; this is established for the crystalline state (X-ray) in the case of R = Ph [Zr–Ph, 2·324(7)Å], in which the Zr–C(sp³) bond is very long [2·329(6)Å], cf. [Zr(η-C₅H₅)₂(CH₂MMe₃)₂][M = C, 2·29(1); or Si; 2·279(4)Å, respectively].