Ligand mobility and solution structures of the metallocenium ion pairs [Me2C(Cp)(fluorenyl)MCH2SiMe3+···X−] (M = Zr, Hf; X = MeB(C6F5)3, B(C6F5)4)

Carlos Alonso-Moreno, Simon J. Lancaster, Joseph A. Wright, David L. Hughes, Cristiano Zuccaccia, Adriana Correa, Alceo Macchioni, Luigi Cavallo, Manfred Bochmann

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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 kex 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 kex 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.
Original languageEnglish
Pages (from-to)5474-5487
Number of pages14
Issue number21
Publication statusPublished - 30 Sep 2008

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