Ligand Mobility and Solution Structures of the Metallocenium Ion Pairs [Me2C(Cp)(fluorenyl)MCH2SiMe3+···X−] (M = Zr, Hf; X =

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• 作者：Carlos Alonso-Moreno ; Simon J. Lancaster ; Joseph A. Wright ; David L. Hughes ; Cristiano Zuccaccia ; Andrea Correa ; Alceo Macchioni ; Luigi Cavallo ; Manfred Bochmann
• 刊名：Organometallics
• 出版年：2008
• 出版时间：November 10, 2008
• 年：2008
• 卷：27
• 期：21
• 页码：5474-5487
• 全文大小：507K
• 年卷期：v.27,no.21(November 10, 2008)
• ISSN：1520-6041

文摘

The mixed-alkyl metallocene complexes (IPCF)M(Me)(CH₂SiMe₃) (M = Zr, Hf; IPCF = Me₂C(C₅H₄)(C₁₃H₈)) were synthesized by the reaction of (IPCF)M(Me)Cl (M = Zr, Hf) with Me₃SiCH₂MgCl. The crystal structures of (IPCF)Zr(CH₂SiMe₃)₂, (IPCF)HfMe₂, and (IPCF)Zr(Me)Cl were determined by X-ray diffraction. The kinetics of site epimerization of the ion pairs (IPCF)M(CH₂SiMe₃)(μ-Me)B(C₆F₅)₃ and [(IPCF)MCH₂SiMe₃⁺···B(C₆F₅)₄⁻] (M = Zr, Hf) were studied by variable-temperature NMR spectroscopy, while the solution ground-state structures of the ion pairs [LZrCH₂SiMe₃⁺···B(C₆F₅)₄⁻] (L = SBI, IPCF; SBI = rac-Me₂Si(Ind)₂) were probed experimentally by ¹⁹F,¹H HOESY NMR spectroscopy and theoretically by DFT and molecular dynamics calculations. They reveal differences in the strength of anion interactions between the SBI and IPCF systems which may be significant for their catalytic activity. The tetraarylborate salts are stabilized by agostic interactions to ligand Si−Me moieties, with Hf > Zr. The exchange rates of both the MeB(C₆F₅)₃⁻ and the B(C₆F₅)₄⁻ compounds increase with increasing ion pair concentration. This acceleration is also seen on addition of excess [Ph₃C][B(C₆F₅)₄]. Pulsed-field gradient spin−echo (PGSE) NMR measurements indicated that both [(IPCF)ZrCH₂SiMe₃⁺···B(C₆F₅)₄⁻] and [(SBI)ZrCH₂SiMe₃⁺···B(C₆F₅)₄⁻] were present mainly as ion quadruples in toluene-d₈/1,2-F₂C₆H₄ (8/2 in volume) at millimolar concentrations and, notably, their aggregation increased to a similar extent with the addition of an excess of [Ph₃C][B(C₆F₅)₄]. The results demonstrate the formation of mixed-ion aggregates of the type {[(L)MR⁺···X⁻][CPh₃⁺···X⁻]_n}. However, whereas the site epimerization rates k_ex of the system (SBI)ZrMe(CH₂SiMe₃)/[Ph₃C][B(C₆F₅)₄] continue to increase linearly with the total ion concentration, for (IPCF)ZrMe(CH₂SiMe₃)/[Ph₃C][B(C₆F₅)₄] mixtures k_ex reaches a plateau at ca. 400 s⁻¹ (at 20 °C). Measurement of site epimerization rates as a function of ion pair concentration [(A⁺)_x(B⁺)_1−xX⁻] therefore provides evidence for the existence of a rate-limiting barrier in the IPCF system, while it is absent in others.