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"Double Activation" of Constrained Geometry and ansa-Metallocene Group 4 Metal Dialkyls: Synthesis, Structure, and Olefin Polymerization Study of Mono- and Dicationic Aluminate Complexes Strong organo-Lewis acids such as B(C6F5)3 and a number of its derivatives1 play vital roles in generating highly active, single-site homogeneous olefin polymerization catalysts.2 In sharp contrast, the aluminum analogue, Al(C6F5)3,3 has attracted much less attention, despite its higher alkide affinity.4-6 Bochmann et al.5 have disclosed that, unlike relatively stable Cp22ZrMe+MeB(C6F5)3- complexes7 derived from methide abstraction from the zirconocene dimethyl by B(C6F5)3,8 the aluminum analogue undergoes very facile C6F5-transfer to Zr above 0 C to form Cp2ZrMe(C6F5), resulting in diminished polymerization activity. Cocatalytic systems derived from Al(C6F5)3 in combination with a second component for high-temperature, homogeneous solution olefin polymerization processes have been previously described.9 We communicate here the very unusual cocatalytic features of Al(C6F5)3.10 These attributes include the unprecedented "double activating" ability of Al(C6F5)3 for the formation of dicationic group 4 constrained geometry11 and ansa-metallocene12 bis-aluminate complexes. In contrast to the B(C6F5)3 activation, use of multiple equivalents of Al(C6F5)3 substantially enhances exothermicities and efficiencies of olefin polymerization catalyzed by the constrained geometry and ansa-metallocene catalysts. While donor-stabilized dicationic group 4 metal (M) complexes with a general formula of [Cp2MD2]2+X-2 (D = neutral donor ligand; X = anion) are known,13 isolation and characterization of dicationic structures absent of donor ligands are challenging and of great interest. Green et al.14 recently reported NMR spectroscopic evidence for the formation of a dication-like zirconocene stabilized by metal-arene interactions by treating (p-MeC6H4CMe2Cp)2ZrMe2 with 2 equiv of B(C6F5)3 at -60 C in CD2Cl2 which reverts to the monocationic species and neutral B(C6F5)3 above -40 C in solution. Most recently, Stephan et al.15 reported a crystallographically characterized non-Cp bis-borate-zwitterionic complex [(tBu3P=N)2Ti{-MeB(C6F5)3}2] generated from the reaction of [(tBu3P=N)2TiMe2 with an excess of B(C6F5)3 in CH2Cl2. The formation of such a species in the presence of excess B(C6F5)3 was considered to be a catalyst deactivation pathway, as the bis-borate adduct exhibits negligible polymerization activity while the corresponding mono-borate adduct is a very active catalyst. Although the reaction of B(C6F5)3 with either the constrained geometry titanium dimethyl Me2Si(5-Me4C5)(t-BuN)TiMe2 (CGC-TiMe2) or the ansa-metallocene dimethyl rac-Me2Si(5-Ind)2ZrMe2 (SBI-ZrMe2) proceeds rapidly and quantitatively in hydrocarbon solvents to produce the corresponding monocationic complexes, reaction with an excess of B(C6F5)3 does not affect the abstraction of the second CH3- group.16 This behavior is likewise observed for bis-Cp-type dimethyl zirconocenes7 and Cp-based titanocenes.15 However, unlike the reaction of Cp2ZrMe2 with Al(C6F5)3, reaction of CGC-TiMe2 and SBI-ZrMe2 with 1 equiv of Al(C6F5)3 proceeds cleanly in hydrocarbon solvents to produce the corresponding stable and isolable cationic complexes CGC-TiMe(-Me)Al(C6F5)3 (1) and SBI-ZrMe(-Me)Al(C6F5)3 (2), respectively.17 The substantially enhanced solution stability of these complexes (t1/2 = 5 and 16 days for 1 and 2, respectively, at room temperature) versus the bis-Cp analogue is attributable to stronger anion coordination to these sterically more open and coordinatively more unsaturated metal centers having ansa-bridged ligation. Singly activated species 1 and 2 exhibit lower olefin polymerization efficiencies than the borane analogues. The crystal structure of complex 118 reveals the pseudo-tetrahedral coordination sphere about Ti. The Ti-CH3 (bridging) distance is 2.332(3) Å which is longer than the Ti-CH3 (terminal) distance by 0.235 Å. The Ti-H3C-Al vector is nearly linear with an angle of 169.0(2). Another noteworthy feature of complex 1 is that two of the bridging methyl hydrogens exhibit relatively close contacts to Ti, with Ti-H distances of 2.21(3) and 2.24(3) Å and acute Ti-C(bridging)-H angles of 71(2) and 73(2), indicative of -agostic interactions, compared to a nonbonding distance of 2.36 (3) Å for the third methyl hydrogen atom. The most striking feature of the abstractive chemistry of Al(C6F5)3 is its ability to effect the removal of the second metal-methyl groups to form the corresponding dicationic bis-aluminate complexes CGC-Ti[(-Me)Al(C6F5)3]2 (3) and SBI-Zr[(-Me)Al(C6F5)3]2 (4).17 Thus, addition of a second equivalent of Al(C6F5)3 to a toluene solution of 1 or 2 causes an immediate color change from yellow green 1 to orange 3 or from yellow 2 to deep red 4. NMR spectroscopic data of 3 and 4 are consistent with symmetry changes of the complexes from previously C1-symmetric 1 and 2 to Cs-symmetric 3 and to C2-symmetric 4, as a result of bis-aluminate adduct formation.17 -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- A crystallographic study confirms the structure of the doubly activated dicationic bis-aluminate complex 4 (Figure 1)19 in which the two Al(C6F5)3 groups are almost symmetrically bound to the two bridging methyl groups. The two Zr-H3C-Al vectors are close to linearity with angles of 163.3(2) and 169.7(1). The di-ionic character of 4 is unambiguously established by the Zr-CH3 distances (2.431(2) Å and 2.454(2) Å) which are both substantially longer than the Zr-CH3 (terminal) distances of 2.24(2) Å in an F-bridged aluminate complex SBI-ZrMe+(PBA)- 20 and of 2.223(6) Å in (Me5C5)2ZrCH3+CH3B(C6F5)3-,7 and by the relatively "normal" Al-CH3 distances (2.084(2) Å and 2.059(2) Å), compared to the average Al-C (aryl) distances (2.001(2) Å and 2.012(2) Å) in 4 and the Al-CH3 distance (2.033(3) Å) in 1. The positions of the hydrogen atoms of the -methyl groups in 4 were located and refined, and two of the three bridging methyl hydrogens of each -methyl group were slightly closer to the Zr center than the third by 0.17 Å, indicative of weak Zr-methyl -agostic interactions. To investigate the influence of the catalyst double activation on polymerization characteristics, ethylene and 1-octene were copolymerized at 140 C using CGC-TiMe2 and SBI-ZrMe2 activated with one or multiple equivalents of B(C6F5)3 and Al(C6F5)3, respectively.16 In varying the B(C6F5)3:pre-catalyst ratio from 1 to 4, polymerization characteristics are not noticeably affected, nor are the polymer properties (exotherm: 0.2-2.1 C; efficiency: 1.22-1.43 g polymer/g Ti; MW: 76.5-66.1 K; density: 0.900-0.897). In contrast, variation of the Al(C6F5)3:pre-catalyst ratio from 1 to 4 causes substantially increased both initial polymerization exothermicity (from 0.3 to 30.6 C with the same amount of pre-catalyst or less) and overall polymerization efficiency (from 0.32 to 2.40 g polymer/g Ti).21 Similar low-density elastomers were produced in all cases but with noticeably higher molecular weights (by ~50% with narrow PDI of 1.96-2.06), compared with polymers produced using B(C6F5)3 activation. Likewise, similar polymerization behavior is observed with catalyst SBI-ZrMe2, which, however, produced relatively high-density polymers (d = 0.926). Multiple equivalents of Al(C6F5)3 also very effectively activate rac-dimethylsilane-bis(2-methyl-4-phenylindenyl)zirconium(II)-1,4-diphenyl-1,3-butadiene22 in a 0.25/0.125 mol activator/pre-catalyst ratio to produce isotactic polypropylene of Tm = 157.8 C with 4.46 × 106 g polymer/g Zr efficiency at a 70 C polymerization temperature, compared to 0.14 × 106 g polymer/g Zr polymerization efficiency when activated with B(C6F5)3 under similar conditions. -------------------------------------------------------------------------------- Figure 1 Molecular structure of 4. The F atoms of the anion portion are not labeled for clarity. -------------------------------------------------------------------------------- It is tempting to suggest that double activation lowers the initiation/propagation barriers via a pathway in which olefin inserts into either lengthened Ti-Me bond, followed by migration of the Al moiety back to the Lewis basic -carbon (A). Alternatively, one can also speculate on the equilibrium formation of an ion pair (B) having an -Me bridged dinuclear anion [(C6F5)3Al-CH3-Al(C6F5)3]- under high-temperature polymerization conditions. Although such species have not been detected in the activation chemistry of group 4 complexes, such anions paired with tantalocene cations have been observed and characterized by X-ray diffraction analysis in the activation of group 5 metallocenes.23 -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- Unlike B(C6F5)3, Al(C6F5)3 is capable of producing the dicationic constrained geometry and ansa-metallocene group 4 complexes which are far more efficient olefin polymerization catalysts than the corresponding mono-cationic catalysts * In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed. Current address: Department of Chemistry, Colorado State University, Fort Collins, CO 80523. E-mail: eychen@lamar.colostate.edu. 1. For recent reviews, see: (a) Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391-1434.[Full text - ACS] (b) Lupinetti, A. J.; Strauss, S. H. Chemtracts: Inorg. Chem. 1998, 26, 565-595. (c) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345-354. 2. For a special review issue, see: Gladysz, J. A., Ed. Chem. Rev. 2000, 100, 1167-1682.[Full text - ACS] 3. Biagini, P.; Lugli, G.; Abis, L.; Andreussi, P. U.S. Pat. 5,602269, 1997. 4. Computing the methide affinity of B(C6F5)3 and Al(C6F5)3 using the B3LYP/6-31G(d) level for energies and HF/3-21G for geometries gives affinities of -135.6 and -146.3 kcal/mol, respectively. Storer, J. W. (Dow Chemical). Personal communication. 5. For an -Me complex derived from Cp2ZrMe2, see: Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908-5912.[Full text - ACS] 6. For a zwitterionic complex derived from a diene complex, see: Cowley, A. H.; Hair, G. S.; McBurnett, B. G.; Jones, R. A. Chem. Commun. 1999, 437-438. 7. Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015-10031. 8. (a) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245-250. (b) Massey, A. G.; Park, A. J.; Stone, F. G. A. Proc. Chem. Soc. 1963, 212-212. 9. (a) Chen, E. Y.-X.; Kruper, W. J.; Roof, G. PCT Int. Appl. WO 00/09515, 2000. (b) Chen, E. Y.-X.; Kruper, W. J.; Roof, G.; Schwartz, D. J.; Storer, J. W. PCT Int. Appl. WO 00/09514, 2000. 10. For patent applications, see: (a) Chen, E. Y.-X. PCT Int. Appl. WO 00/09524, 2000. (b) Chen, E. Y.-X.; Kruper, W. J.; Nickias, P. N.; Wilson, D. R. PCT Int. Appl. WO 00/09523, 2000. International filing date: June 11, 1999. 11. (a) Shapiro, P. J.; Cotter, W. D.; Schaefer, W. P.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1994, 116, 4623-4640. (b) Okuda, J. Comments Inorg. Chem. 1994, 16, 185-205. (c) Canich, J. M.; Hlatky, G. G.; Turner, H. W. PCT Appl. WO 9200333, 1992. (d) Stevens, J. C.; Timmers, F. J.; Wilson, D. R.; Schmidt, G. F.; Nickias, P. N.; Rosen, R. K.; Knight, G. W.; Lai, S. Eur. Pat. Appl. EP 416 815-A2, 1991. (e) Piers, W. E.; Shapiro, P. J.; Bunnel, E. E.; Bercaw, J. E. Synlett. 1990, 2, 74-84. 12. (a) Herrmann, W. A.; Rohrmann, J.; Herdtweck, E.; Spaleck, W.; Winter, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 1511-1512. (b) Wild, F. R. W. P.; Wasiucionek, M.; Huttner, G.; Brintzinger, H.-H. J. Organomet. Chem. 1985, 288, 63-67. 13. (a) Bondar, G. V.; Aldea, R.; Levy, C. J.; Jaquith, J. B.; Collins, S. Organometallics 2000, 19, 947-949.[Full text - ACS] (b) Bosch, B. E.; Erker, G.; Fröhlich, R.; Meyer, O. Organometallics 1997, 16, 5449-5456.[Full text - ACS] (c) Hollis, T. K.; Robinson, N. P.; Bosnich, B. J. Am. Chem. Soc. 1992, 114, 5464-5466. (d) Jordan, R. F.; Echols, S. F. Inorg. Chem. 1987, 26, 383-386. (e) Lasser, W.; Thewalt, U. J. Organomet. Chem. 1986, 301, 69-77. 14. Green, M. L. H.; Sassmannshausen, J. Chem. Commun. 1999, 115-116. 15. Guérin, F.; Stephan, D. W. Angew. Chem., Int. Ed. 2000, 39, 1298-1300. 16. See Supporting Information for experimental and characterization details. 17. Extra caution should be exercised when handling Al(C6F5)3 material due to its thermal and shock sensitivity. Complete synthetic procedures and characterization details are given in the Supporting Information. 18. See Supporting Information for crystallographic data for 1. 19. See Supporting Information for crystallographic data for 4. 20. Chen, E. Y.-X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 6287-6305.[Full text - ACS] 21. No scavengers were used for current studies, and presumably some Al(C6F5)3 was consumed as scavenger. In a related study (see ref 9), when a scavenger was used, the efficiency of the polymerization with multiple equivalents of Al(C6F5)3 can be greater than 4.0 × 106 g polymer/g Ti. 22. Chen, E. Y.-X.; Campbell, R. E.; Devore, D. D.; Green, D. P.; Patton, J. T.; Soto, J.; Wilson, D. R. PCT Int. Appl. WO 99/46270. 23. Chen, E. Y.-X.; Abboud, K. A. Organometallics 2000, 19, 5541-5543.[Full text - ACS] |
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