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

Song, Fuquan, Lancaster, Simon J., Cannon, Roderick D., Schormann, Mark, Humphrey, Simon M., Zuccaccia, Cristiano, Macchioni, Alceo and Bochmann, Manfred ORCID: https://orcid.org/0000-0001-7736-5428 (2005) 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. Organometallics, 24 (6). pp. 1315-1328. ISSN 0276-7333

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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)M delta+(CH2SiMe3)(mu-Me)B-delta-(C6F5)(3) and the ion pairs [(SBI)MCH2SiMe3+center dot center dot center dot 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 gamma-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 degrees C (at 300 MHz), while in the zirconium analogue cooling to below -60 degrees C is required. It was shown that chain swinging involves a 180 degrees 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)(mu-Me)B(C6F5)(3) exists in solution as mononuclear zwitterions as expected, [(SBI)ZrCH2SiMe3+center dot center dot center dot B(C6F5)(4)(-)] forms ion quadruples ([Zr] approximate to 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" k(ex)), as determined by variable-temperature NMR spectroscopy, decreases with decreasing metallocene concentration. For [(SBI)ZrCH2SiMe3+center dot center dot center dot B(C6F5)(4)(-)] at 25 degrees C and [Zr] = 2 mM, k(ex) = 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 (k(p)[M] approximate to 10(4) 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 k(ex) ("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 caalyst resting states in preference to anion coordination.

Item Type:	Article
Faculty \ School:	Faculty of Science > School of Chemistry
UEA Research Groups:	Faculty of Science > Research Groups > Synthetic Chemistry (former - to 2017) Faculty of Science > Research Groups > Chemistry of Light and Energy Faculty of Science > Research Groups > Chemistry of Materials and Catalysis
Depositing User:	Rachel Smith
Date Deposited:	16 Jun 2011 13:53
Last Modified:	24 Oct 2022 02:27
URI:	https://ueaeprints.uea.ac.uk/id/eprint/32757
DOI:	10.1021/om049248d

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