Published on Web 10/05/2002
Activation of Dimethyl Zirconocene by Methylaluminoxane (MAO)sSize Estimate for Me-MAO- Anions by Pulsed Field-Gradient NMR Dmitrii E. Babushkin*,† and Hans-Herbert Brintzinger*,‡ Contribution from the BoreskoV Institute of Catalysis, Pr. Ak.LaVrent’eVa 5, 630090 NoVosibirsk, Russia, and UniVersita¨ t Konstanz, D-78457 Konstanz, Germany Received May 6, 2002
Abstract: In a study of the reaction system MAO/(C5H5)2ZrMe2, the size of the ion pair [(C5H5)2Zr(µMe)2AlMe2]+ [Me-MAO]- was determined by pulsed field-gradient NMR of its cationic moiety. A mean effective hydrodynamic radius of 12.2-12.5 Å, determined from diffusion rates in benzene solution at different zirconocene and MAO concentrations, indicates that the ion pair remains associated even at the lowest concentrations studied. At elevated concentrations, aggregation to ion quadruples or higher aggregates is indicated by an apparent size increase and by shifts of the C5H5 and Me 1H NMR signals. The equilibrium constant for the reaction [(C5H5)2ZrMe+‚‚‚Me-MAO-] + 1/2Al2Me6 h [(C5H5)2Zr(µ-Me)2AlMe2]+ [Me-MAO]changes at different Al/Zr ratios; this indicates that MAO contains various species that produce Me-MAOanions with different Lewis basicities. The volume of the Me-MAO- anion suggests that it contains 150200 Al atoms.
Introduction
Even though polymerization of alkenes with methylaluminoxane-activated metallocene catalysts has been the object of substantial interest for more than two decades, the structure of methylaluminoxane (MAO) and the mechanisms of its cocatalytic action are still unclear.1 It is generally assumed that some particularly Lewis acidic sites of the MAO cluster are capable of abstracting a Cl- or Me- unit from a zirconocene chloride or methyl precursor to generate an ion pair of general type (C5H5)2ZrMe+ Me-MAO-.1d Recent studies have provided some details with regard to the structure and the nature of the zirconocene species that appear in the system MAO/(C5H5)2ZrMe2. Two of them, a weak Lewis acid adduct of the type (C5H5)2Zr(Me)Me f Al(MAO) (type I) and an ion pair containing the dinuclear cation [{(C5H5)2ZrMe}2(µ-Me)]+ (type II), predominate at relatively low [Al]/[Zr] ratios but disappear when the excess of MAO is increased.2 At ratios of Al/Zr > 200 (as are typical for active catalyst systems) only two other species are observed, an ion pair of the cation [(C5H5)2Zr(µ-Me)2AlMe2]+ with some MeMAO- counteranions (type III)2,3 and a species containing a * Corresponding authors. E-mail: (D.E.B.)
[email protected]; (H.-H.B.)
[email protected]. † Boreskov Institute of Catalysis. ‡ Universita ¨ t Konstanz. (1) (a) Sinn, H. Macromol. Symp. 1995, 97, 27. (b) Sinn, H., Kaminsky, W., Hoker, H., Eds. Alumoxanes; Macromolecular Symposia 97; Huthig & Wepf: Heidelberg, Germany, 1997. (c) Sinn, H.; Schimmel, I.; Ott, M.; von Thienen, N.; Harder, A.; Hagendorf, W.; Heitmann, B.; Haupt, E. Metalorganic Catalysts for Synthesis and Polymerization; Kaminsky, W., Ed.; Springer-Verlag: Berlin, 1999; p 105. (d) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (2) Babushkin, D. E.; Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P. Macromol. Chem. Phys. 2000, 201, 558. 10.1021/ja020646m CCC: $22.00 © 2002 American Chemical Society
Scheme 1
(C5H5)2ZrMe+ cation in contact with a CH3 group of a stable Me-MAO- anion (type IV),2 from which III arises in an equilibrium reaction with Al2Me6 (Scheme 1). For an understanding of these catalyst systems, it would be highly desirable to elucidate the nature both of the MAO4 and of the Me-MAO- species present in these equilibria. Pulsed field-gradient (PFG) NMR methods, which allow the determination of translational diffusion coefficients of a molecule or ion, would in principle be ideal to estimate the molecular dimensions of the Me-MAO- anions, which are necessarily produced when dimethyl zirconocene complexes are transformed to cationic species. Since signals of the Me-MAO- moieties cannot be identified, however, in the 1H NMR spectra of these reaction systems (because of their overlap with signals of neutral MAO), we have measured the rate of diffusion of the cationic (3) (a) Tritto, I.; Donetti, R.; Sacchi, M. C.; Locatelli, P.; Zannoni, G. Macromolecules 1997, 30, 1247. (b) Pedeutour, J. N.; Cramail, H.; Deffieux, A. J. Mol. Catal. 2001, 176, 87. (c) Wieser, U.; Brintzinger, H.-H. Organometallic Catalysts and Olefin Polymerization; Blom, E., Follestad, A., Rytter, E., Tilset, M., Ystenes, M., Eds.; Springer-Verlag: Berlin, 2001; p 3. (4) For the molecular size of MAO, an average diameter of 19.4 ( 0.4 Å has recently been estimated from 1H spin-lattice relaxation times: Hansen, E. W.; Blom, R.; Kvernberg, P. O. Macromol. Chem. Phys. 2001, 202, 2880. J. AM. CHEM. SOC. 2002, 124, 12869-12873
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Babushkin and Brintzinger
Table 1. Relative Reciprocal Diffusion Coefficients and Effective Hydrodynamic Radii of Species III, Chemical Shifts and Line Widths of the 1H NMR Signals of Its (C H ) Zr(µ-Me) AlMe + Moiety, and Concentration Ratios of Species III and IV at Different Zirconocene and MAO 5 5 2 2 2 Concentrationsa
sample
[Zr]tot, mmol/L
[Al]tot, mol/L
1 2 3 4 5 6 7 8
12.0 2.6 1.36 0.048 7.17 3.5 1.54 0.81
0.72 0.72 0.72 0.72 0.43 0.21 0.43 0.43
a
Al/Zr
Dref/DIII, ±0.03
RIII, ±0.2
δ(III-C5H5), ppm ± 0.002
δ(III-Me), ppm ± 0.002
∆ω (III-C5H5), Hz ± 0.1
∆ω (III-Me), Hz ± 0.1
60 280 530 15 000 60 60 280 530
1.88 1.86 1.85 1.59 1.74 1.65 1.67 1.63
14.4 14.3 14.2 12.2 13.3 12.7 12.8 12.5
5.466 5.418 5.414 5.418 5.442 5.423 5.405 5.403
-0.611 -0.634 -0.637 -0.640 -0.632 -0.647 -0.650 -0.649
1.03 0.46 0.42 0.64 0.84 0.65 0.45 0.43
1.18 0.90 0.89 1.18 0.94 0.89 0.88
[IV]/[III]
5.37 3.23 2.28
