Organometallics 1996, 14, 5446-5449
5446
exo,exo-Bis(isodicyclopentadienyl)titanium and -zirconium Dichlorides: Conformational Characterization of the Homogeneous Ziegler Catalyst Precursors in Solution Cornelia Fritze, Markus Knickmeier, and Gerhard Erker* Organisch-Chemisches Institut der Universitat Miinster, Corrensstrasse 40, 0-48149 Miinster, Germany
Florence Zaegel, Bernard Gautheron, and Philippe Meunier Universiti de Bourgogne, Laboratoire de Synthke et 8Electrosynthkse Organomitalliques, Faculti des Sciences “Gabriel”, 6, Boulevard Gabriel, 21000-D~on, France
Leo A. Paquette Evans Chemical Laboratories, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210-1173 Received February 8, 1995@ Summary: exo,exo-Bis(isodicyclopentadieny1)titanium dichloride (1)exhibits a chiral C2-symmetric structure in solution that is characterized by a bis-1ateral:anti orientation of the annulated bicyclo[2.2.llheptene moieties at the bent-metallocene wedge. This was shown by dynamic NMR spectroscopy of complex 1 in [Dddichloromethane solution. The activation energy for the conformational inversion of complex 1 is AG*m,t(198K) = 9.8 f 0.4 kcal mol-’. The bent-metallocene rotational barrier of the analogous zirconium complex (2) is much lower (-6 kcal mol-’). Complexes 1 and 2 were both used to generate active homogeneous Ziegler catalyst systems for a-olefin polymerization.
Introduction The group 4 metals can very selectively be coordinated t o the exo or endo faces of the isodicyclopentadienyl ligand system, respectively, by carefully selecting the reaction c0nditions.l Several reprdsentative examples of such group 4 bent-metallocene complexes were characterized by X-ray diffkaction.132 For the (isodicyclopentadieny1)MCpXz complexes two rotamers were observed, one where the annulated bicyclo[2.2.llheptene structural unit is oriented toward the open front part of the bent-metallocene wedge (A) and another where the annulated norbornene moiety occupies a lateral sector (B).Consequently, three conformations of the corresponding (isodicyclopentadieny1)zMClz complexes can be envisaged, having the annulated bicyclic ring systems arranged in a Cz-symmetricbis-lateralanti (C), a Czv-symmetric bis-central:syn, or a lateraycentral (symmetry group Cd arrangement. @Abstractpublished in Advance ACS Abstracts, October 1, 1995. (1)Paquette, L.A.; Moriarty, K. J.; Meunier, P.; Gautheron, B.; Crocq, V. Organometallics 1988,7, 1873. Paquette, L.A.; Moriarty, K. J.; Meunier, P.; Gautheron, B.; Sornay, C.; Rogers, R. D.; Rheingold, A. L. Organometallics 1989, 8, 2159. Sornay, C.; Meunier, P.; Gautheron, B.; O’Doherty, G. A.; Paquette, L. A. Organometallics 1991, 10,2082.Zaegel, F.;Gallucci, J. C.; Meunier, P.; Gautheron, B.; Sivik, M. R.; Paquette, L. A. J . Am. Chem. Soc. 1994,116,6466. (2)Gallucci, J . C.; Gautheron, B.; Gugelchuk, M.; Meunier, P.; Paquette, L. A. Organometallics 1987,6,15. Zaegel, F.;Meunier, P.; Gautheron, B.; Gallucci, J. C.; Paquette, L. A. Organometallics, submitted for publication.
0276-7333l95l2314-5446~09.QQlQ
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A
B
0 C
Of the ex0 ,em-(isodicyclopentadieny1)zMClz complexes 1 (M = Ti) and 2 (M = Zr) in the crystal state, the
structures of the bis-1ateral:anti bent-metallocene conformers were determined by X-ray diffraction.2 For remotely related group 4 bent-metallocene complexes, examples of all three conformations have been found in ~ ~ also been shown the solid state and in s o l ~ t i o n .It~has that X-ray crystal structure analyses can only serve to identify possible structural types.5 Which of these is present in solution (i.e. in the absence of crystal-packing forces) and might determine the chemistry of a particu(3) Kriiger, C.; Nolte, M.; Erker, G.; Thiele, S.2.Nabrforsch. 1992, 47B, 995. See also: Benn, R.; Grondey, H.; Nolte, R.; Erker, G. Organometallics 1988,7, 777. Erker, G.; Nolte, R.; Tainturier, G.; Rheingold, A. Organometallics 1989,8,454. Erker, G.; Nolte, R.; Kriiger, C.; Schlund, R.; Benn, R.; Grondey, H.; Mynott, R. J . Organomet. Chem. 1989,364,119.Benn, R.; Grondey, H.; Erker, G.; Aul, R.; N o h , R. Organometallics 1990,9,2493. (4)For selected examples see: Petersen, J. L.; Dahl, L. F. J . Am. Chem. Soc. 1976, 97, 6422. Dusausoy, Y.;Protas, J.; Renaut, P.; Gautheron, B.; Tainturier, G. J. Organomet. Chem. 1978,157, 167. Luke, W.D.; Streitwieeer, A. J . Am. Chem. SOC.1981,103,3241 and references cited therein. Howie, R. A,; McQuillan, G. P.; Thompson, D. W. J. Organomet. Chem. 1984,268,149.Howie, R. A.; McQuillan, G. P.; Thompson, D. W.; Lock, G. A. J. Organomet. Chem. 1986,303, 213. Antinolo, A.; Lappert, M. F.; Singh, A.; Winterborn, D. J . W.; Engelhardt, L. M.; Raston, C.; White, A. H.; Carty, A. J.; Taylor, N. J. J . Chem. Soc., Dalton Trans. 1987,1463. Okuda, J . J . Organomet. Chem. 1988,356, C43. Broussier, R.; Da Rold, A.; Gautheron, B.; Dromzee, Y.; Jeannin, Y.Inorg. Chem. 1990,29,1817. Mallin, D. T.; Rausch, M. D.; Mintz, E. A.; Rheingold, A. L. J . Organomet. Chem. 1990,381,35.Winter, C.H.; Dobbs, D. A.; Zhou, X.-X. J . Organomet. Chem. 1991, 403, 145. See also: Janiak, C.; Schumann, H. Adu. Organomet. Chem. 1991,33,291. Okuda, J . Top. Curr. Chem. 1991, 160,97 and references cited therein. (5)Erker, G.; Aulbach, M.; Knickmeier, M.; Wingbermiihle, D.; Krtiger, C.; Nolte, M.; Werner, S.J . Am. Chem. SOC.1993,115,4590. Krtiger, C.;Lutz, F.; Nolte, M.; Erker, G.; Aulbach, M. J . Organomet. Chem. 1993,452,79.
1995 American Chemical Society
Notes
Organometallics, Vol. 14, No. 11, 1995 5447
1 " " / " " 1 " " 1 " " l " " J " " I " " l ~ " ' ~ " " I " " ~ " " ~
6 0
5 5
5 0
4 5
4 0
3 5
3 0
2 5
2 0
1 5
1 0
ppm
Figure 1. Dynamic lH NMR spectra of 1 in [Dzldichloromethane(600MHz) at 300 K (top), 213 K (center), and 173 K (bottom) (the asterisk denotes residual solvent CHDClZ). lar system cannot, however, be deduced from the solidstate structural analysis, since the crystallization process serves as a kinetic exit for the rapidly equilibrating metallocene conformers. In such an open system any information about the equilibrium situation of the bentmetallocene conformations is lost. We have recently shown that in such cases the conformational equilibration can be sometimes studied by dynamic NMR spectroscopy and that in selected cases it is even possible t o obtain information about the properties of bentmetallocene catalysts by such ~ t u d i e s .Therefore, ~ we have tried to learn about the solution structures and behavior of complexes 1 and 2 by using this method.
Results and Discussion Complexes 1 and 2 were prepared according to a literature procedure2 by treating 2 molar equiv of (isodicyclopentadieny1)lithiumwith titanium trichloride (followed by oxidation) and zirconium tetrachloride, respectively, in tetrahydrofuran at high temperature. Under the conditions of thermodynamic control, only the exo,exo-(isodicyclopentadieny1)zMClzproducts (M = Ti, Zr) are obtained.
M=Ti M=Zr
1
ent-1
2
ent-2
K in [Daldichloromethane) in a 1:2:2:2:1:1:2 ratio, and six 13C signals (150.8MHz, 300 K,observed at 6 145.8, 129.1,107.4,48.0,41.9,and 28.3 ppm). However, this spectral appearance does not reflect the actual molecular symmetry of 1in solution, as it is in fact the result of an equilibration process of metallocene conformations which is fast on the NMR time scale. Complex 1 exhibits dynamic NMR spectra upon lowering the temperature. Below coalescence (T,= 198 K) 11 'H NMR resonances of equal intensity are observed (see Figure 1). This indicates that the molecular symmetry of 1 in solution is CZ.Complex 1thus exhibits two symmetryequivalent isodicyclopentadienyl ligands in an orientation that results in a chemical differentiation of the hydrogen atoms of the H-exo, H-endo, bridgehead-H, and Cp-H pairs, as is typically found in the bis-lateral: anti conformation (Ctype). As expected, the analogous splitting of the respective 13C NMR resonance (with the exception of the isochronous bridgehead carbon atom signals) is also observed at sufficiently low temperatures. At 200 K one observes 13C NMR resonances of the Cz-symmetric complex 1 a t 6 152.6,134.7,129.1, 109.9,104.6,46.6,40.7(double intensity), 27.7,and 26.4 ppm. We thus conclude that, within the accuracy of the NMR measurement, complex 1 exhibits a single CZsymmetric bent-metallocene conformation in solution that is detected when the rapid 1=+ ent-1 equilibration becomes frozen on the NMR time scale. From the coalescence of the pair of bridgehead hydrogens (173K, 6 3.32,3.12ppm; T,= 198 K) and independently of the pair of hydrogen signals at 6 1.04h.25ppm (173K) an activation barrier of AG*,,t(198 K) = 9.8f 0.4kcal mol-l ~
The titanium complex 1 exhibits lH and 13C NMR a t ambient temperature that would be compatible with an overall C2,-sy"etric structure. The alleged presence of the two symmetry planes would result in a total of seven lH NMFt resonances, which are observed at Q 6.26, 6.05,3.31,1.87, 1.37,1.23,and 1.09ppm (600MHz, 300
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(6) Binsch, G. In Dynamic Nuclear Magnetic Resonance Spectroscopy; Jackman, L. M., Cotton, F. A., Eds.; Academic Press: New York, 1975; p 45-81. Binsch, G.; Kessler, H. Angew. Chem. 1980,92,445; Angew. Chem., Int. E d . Engl. 1980, 19,411. Green, M. L. H.; Wong, L. L.; Sella, A. Organometallics 1992,11, 2660 and references cited therein. Gutowsky, H. S. In Dynamic Nuclear Magnetic Resonance Spectroscopy; Jackman, L. M., Cotton, F. A,, Eds.; Academic Press: New York, 1975; pp 1-21. See also: Gunther, H. NMR-Spectroscopie; Thieme: Stuttgart, Germany, 1992; pp 303-350.
5448 Organometallics, Vol. 14, No. 11, 1995 was estimated6 for the equilibration (1--L ent-1) of the enantiomeric conformations of the exo,exo-(isodicyclopentadieny1)zTiClz complex 1in solution at the coalescence temperature. The analogous rotational barrier of the zirconium complex 2 is much lower. The rapid equilibration of conformations could not be frozen out on the 'H NMR time scale. At all temperatures seven lH NMR resonances were observed (6 6.28,5.89,3.30,1.83,1.45,1.38, and 1.01 ppm at 253 Kin [Dzldichloromethane solution); only at 173 K (600 MHz) was some line broadening monitored that was not due t o viscosity effects. Even the chemical shift difference in the 13C NMR spectrum at 14 T (150.8 MHz) was not sufficient to allow observation of the low-temperature limiting spectrum below the coalescence (six "high temperature" 13C NMR signals of 2 are observed at 253 K d 142.3, 122.3, 103.3,48.9, 40.9, and 28.2 ppm). Only the quarternary Cp resonance signal at d 142.3 becomes broad below 213 K. If one assumes a separation of the resulting pair of the corresponding 13C NMR signals in the static lowtemperature spectrum for 2 similar to that observed for 1(Ad = 20 ppm, i.e. Av RZ 3000 Hz), this would lead to an estimate of the activation barrier of the 2 * ent-2 rearrangement of < 7 kcal mol-l (in [D2ldichloromethane solution). Complexes 1 and 2 were activated with methylalumoxane7 to give highly reactive homogeneous Zieglertype propene polymerization catalysts. The 2/MAO catalyst (Zr:Al 1400) produces atactic polypropylene in toluene solution (2 bar propene pressure, 0 "C, 4.65 kg of polymer (mmol of Zr1-l h-l bar-' obtained; -40 "C, 220 g of polymer (mmol of Zr1-l h-l bard1; stereochemical analysis by 13CNMR methyl pentad analysis;8 M,, = 13 000 (0 "C) and 150 000 (-40 "C), respectively). The corresponding titanium Ziegler catalyst 1/MAO is much less active. In the temperature range between -50 and +50 "C it exhibits a broad activity maximum at ca. -10 "C (1:MAO ratio of the individual experiments between 900 and 1200; activity (g of polymer (mmol of Ti)-l h-l bar-l): 0.7 (-50 "C), 7.6 (-29 "C), 21.8 (-11 "C), 10.9 (+12 "C), 3.2 (+30 "C), 2.3 (+50 "C)). M,, decreased from -120 000 t o -20 000 within this temperature range. Over the whole temperature range the stereochemistry is exclusively determined by chain end contr01,~similar to the behavior observed previously by Ewen using simple Cp2Ti-derived catalyst systems.8b At low temperature this stereocontrol is rather efficient; the isotactic polymer obtained at the 1 M O catalyst at -50 "C in toluene exhibits only three 13C NMR methyl signals at d 21.9, 21.6, and 20.8 ppm in a 82: 11:7 intensity ratio, representing the mmmm, mmmr, and mmrm methyl pentad resonances, respectively. Statistical analysis of this spectrum leads to a prob(7) Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99 and references cited therein. (8) (a) Bovey, F. A.; Tiers, G. V. D. J . Polym. Sci. 1960, 44, 173. Sheldon, R. A,;Fueno, T.; Tsuntsuga, R.; Kurukawa, J. J . Polym. Sci., Part B; Polym. Lett. 1965, 3, 23. Zambelli, A.; Locatelli, P.; Bajo, G.; Bovey, F. A. Macromolecules 1975, 8, 1565. Farina, M. Top. Stereochem. 1987,17,1. (b)Ewen, J. A. J.Am. Chem. SOC. 1984,106,6355. Ewen, J. A. Stud. Surf. Sci. Catal. 1986,25, 271. Ewen, J. A,; Elder, M. J.; Jones, R. L.; Curtis, S.; Cheng, - H. N. Stud. Surf. Sci. Catal. 1990, 56, 439. (9) (a) Inoue, J.; Itabashi, Y.; Chujo, R.; Doi, Y. Polymer 1984, 25, 1640. (b) Erker. G.: Nolte. R.: Tsav. Y.-H.: Kriieer. C. Aneew. Chem. 1989,101,642;Angew.Chem.;Int.Ed. Engl. l969,28,628T (c) Erker, G.; Nolte, R.; Aul, R.; Wilker, S.; Kriiger, C.; Noe, R. J.Am. Chem. SOC.1991, 113, 7594.
Notes ability of finding a polypropylene m pentad a t the 1 M O catalyst under these conditions of u = 0.95. Increasing the polymerization temperature results in a rapid decrease of the selectivity, although also at higher temperatures the stereochemistry exclusively is chain end controlled. At -29 "C the mmmm methyl pentad signal has dropped t o 70% intensity (a = 0.91) (-11 "C, 32%mmmm, u = 0.75) and the atactic situation is reached at +12 "C (5% mmmm, CJ = 0.48). The remarkable feature of this series of polymerization reactions at the 1MAO catalyst is that the maximum nonselectivity does not represent the limiting hightemperature situation. Increasing the polymerization temperature leads to a further reduction of the mmmm 13C NMR methyl resonance intensity (+30 "C, -1% mmmm, o = 0.40; +50 "C, no detectable mmmm intensity, u = 0.35). This means that the system has crossed the atactic borderline (a = 0.5) and entered into the syndiotactic polypropylene regime just from an increase in the polymerization temperature.1° The overall stereocontrolhas remained completely chain end controlled, even at +50 "C. This study has shown that exo,exo-bis(isodicyc1opentadienylltitanium dichloride exhibits a single conformation in solution that is best described as of the bis1ateral:anti metallocene conformational type (C;see above). This chiral C2-symmetric structure in solution is probably very similar to the molecular geometry found of 1 in the solid state by X-ray crystallography.2 The enantiomerization barrier of 1 is rather high at AG*,,t = 10 kcal mol-'. The exo,exo-bis(isodicyclopentadieny1)zirconium dichloride homolog (2) is probably also C2symmetric, but it exhibits a much lower enantiomerization barrier. If the metallocene conformation in the active catalyst systems derived from 1and 2 is also CZ, this chirality information is not transferred onto the growing polypropylene chain, either from the titanium complex 1or the zirconium system 2. This may be due to an extreme lateral arrangement of the bulky annulated ring systems (similar to that found for l and 2 in the solid state2). We find it remarkable how different the stereochemical behaviors of the 1MAO and 2 M A O catalyst systems are during propene polymerization. Of course, this conformational study does not give any direct evidence or explanation as to why the zirconium catalyst ( W O ) provides no stereocontrol at all whereas the titanium system (1MAO) is able to exert a pronounced chain end stereocontrol in the investigated temperature interval, ranging from rather highly isotactic polypropylene block polyme$ to slightly syndiotactic polypropylene formation. Chain end stereocontrolprobably requires that the re- vs. si-face coordination of the incoming a-olefin monomer is determined by the precise conformational arrangement of the adjacent chiral M-CH2CH(CHdR (R = polymer chain) a-ligand system. It may be that the rotational barrier of the M-CH2 linkage in these catalyst systems also exhibits a pronounced dependence on the size of the central metal atom, similar to what is observed for the rotation around the M-Cp vector in 1 and 2. Then it would be expected that the chain end chirality information associated with the zirconium (10)Erker, G.; Fritze, C. Angew. Chem. 1992, 104, 204; Angew. Chem., Int. Ed. Engl. 1992, 31, 199. See also: Resconi, L.; Abis, L.; Franciscono, G. Macromolecules 1992,25, 6814.
Notes
Organometallics, Vol. 14, No. 11, 1995 5449
catalyst system is much less persistent than that of the titanium catalyst, and consequently the zirconiumcontaining homogeneous Ziegler catalyst (here !Z/MAO) is expected to be much less stereoselective than the corresponding titanium system (l/MAO) even outside the enantiomorphic site control regime. A detailed investigation about the remarkable temperature dependence of the isotactic vs syndiotactic chain end controlled polypropylene formation a t this and other titanocendmethylalumoxane catalyst systems is currently being carried out in our laboratory. The results of that study will be described in a forthcoming publication.
Experimental Section Complexes 1 and 2 were prepared as described in the literature.2 The dynamic NMR spectra of complexes 1 and 2 were measured on a Bruker AC 200 P and a Varian Unity Plus 600 NMR spectrometer. The activation barriers were determined at the coalescence temperature of the respective pair of signals using the Gutowsky-Holm approximation.6
Propene polymerization reactions were carried out in a Buchi glass autoclave at 2 bar of propene pressure in toluene solvent, analogous to the procedure previously described in detai1.5p9J0 The molecular weights of the polymers were determined with an Ubbelohde viscosimeter (Schott AVS 440) in decalin solution at 135 "C. The 13C NMR spectra of the polymers were recorded in 1,2,4-trichlorobenzene/[Dslbenzene (4:l) solution at 350 K. The methyl pentad signals were integrated by a curve-fitting procedure as previously described and then subjected to a statistical treatment to determine the partition between enantiomorphic site control (0)and chain end stereocontrol (1 - o)as well as the isotacticity (a)and, respectively, syndiotacticity (1- a) of the obtained polymer sample under chain end control. A detailed description of this procedure can be found in the l i t e r a t ~ r e . ~ , ~ '
Acknowledgment. This work was supported by the Fonds der Chemischen Industrie, the Alfried Krupp von Bohlen und Halbach-Stiftung, and Hoechst AG. OM950102B
