Synthesis, Ion Aggregation, Alkyl Bonding Modes, and Dynamics of 14-Electron Metallocenium Ion Pairs [(SBI)MCH₂SiMe₃+···X-] (M = Zr, Hf): Inner-Sphere (X = MeB(C₆F₅)₃) versus Outer-Sphere (X = B(C₆F₅)₄) Structures and the Implications for “Continuous” or “Intermittent” Alkene Polymerization Mechanisms

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Abstract

The new mixed-alkyl metallocene complexes (SBI)M(Me)CH2SiMe3 (M = Zr, Hf) are accessible by the successive treatment of (SBI)MCl2 with Me3SiCH2MgCl and MeMgCl in toluene (SBI = rac-Me2Si(1-Ind)2). Reaction with B(C6F5)3 or CPh3+[B(C6F5)4]- in toluene or toluene/difluorobenzene affords (SBI)Mδ+(CH2SiMe3)(μ-Me)Bδ-(C6F5)3 and the ion pairs [(SBI)MCH2SiMe3+···B(C6F5)4-], respectively. Both types of compounds are thermally stable in aromatic solvents at ambient temperature. Whereas in the MeB(C6F5)3- complexes the alkyl ligand points away from the metal and tight anion coordination forms the familiar inner-sphere ion pair, in the B(C6F5)4- salts the alkyl ligand adopts a conformation that enables agostic bonding to a γ-CH3 group. Here, and by implication in M-polymeryl species of similar steric requirements, agostic interactions are preferred over anion coordination, leading to an outer-sphere ion pair structure. This alkyl bonding mode retards the −SiMe3 rotation, which for M = Hf is slow on the NMR time scale at −20 °C (at 300 MHz), while in the zirconium analogue cooling to below −60 °C is required. It was shown that chain swinging involves a 180° rotation of the alkyl ligand about the Zr−C bond. Measurements of diffusion coefficients by pulsed field gradient spin−echo (PGSE) techniques suggest that while (SBI)Zr(CH2SiMe3)(μ-Me)B(C6F5)3 exists in solution as mononuclear zwitterions as expected, [(SBI)ZrCH2SiMe3+···B(C6F5)4-] forms ion quadruples ([Zr] ≈ 2 mM), rising to hextuples at higher concentration. The relative positions of cations and anions depend on the ion pair concentration; higher aggregates make it difficult to assign specific anion positions. The rate of ion pair symmetrization (“anion exchange” kex), as determined by variable-temperature NMR spectroscopy, decreases with decreasing metallocene concentration. For [(SBI)ZrCH2SiMe3+···B(C6F5)4-] at 25 °C and [Zr] = 2 mM, kex ≈ 500 ± 170 s-1; this value represents the upper limit of anion mobility expected under catalytic conditions where concentrations are typically 100 times lower. Ion pair symmetrization rates are therefore at least 1 order of magnitude slower than the growth of the number-average molecular weight of polypropene chains (kp[M] ≈ 104 s-1 at [M] = 0.59 mol L-1) generated with tetraarylborate-based (SBI)Zr and other high-activity catalysts at identical temperatures. It is suggested that while for slower, inner-sphere ion pair catalysts the rate of 1-alkene consumption is commensurate with kex (“continuous” chain propagation mechanism), high-activity catalysts may operate by a mechanism where the anion does not bind to the metal center and so does not limit the rate of monomer enchainment. In such a situation, agostic metal−alkyl interactions form the catalyst resting states in preference to anion coordination.

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