Group 4 octahedral benzamidinate complexes: Syntheses, structures, and catalytic activities in the polymerization of propylene modulated by pressure

Article abstract of DOI:10.1021/ja020575r

The synthesis and structural X-ray diffraction studies for some benzamidinate ligations and several group 4 benzamidinate complexes are presented. The use of the cis-octahedral C₂-symmetry compounds was studied to shed light on the conceptual applicability of these complexes as potential catalysts for the stereoregular polymerization of propylene. We demonstrate that the stereoregular polymerization of propylene catalyzed by early-transition metal octahedral benzamidinate complexes, activated with either MAO or B(C₆F₅)3 as cocatalysts, can be modulated by pressure (from atactic to isotactic through elastomers). The different effects in the polymerization process such as the nature of solvent or cocatalyst, temperature, pressure, molar ratio catalyst: cocatalyst, and the relationship between the symmetry of the complex and the polymer microstructure have been investigated. When the complex [4-CH₃-C₆H₄C (NTMS)₂]₂ZrMe₂ (9) was activated with MAO, it was found to be a good catalyst for the polymerization of propylene, at atmospheric pressure, producing an oily polymer resembling an atactic polypropylene. Being activated with B(C₆F₅)3, complex 9 produces a highly isotactic (mmmm = 98%) product. Likewise, when the polymerization of propylene was performed with complex 9 and MAO at high pressure (liquid propylene), a highly stereoregular polymer was also obtained. Larger activities and stereoregularities were achieved by performing the reaction in CH₂Cl₂ as compared to toluene. Contrary to complex 9, at atmospheric pressure the complex [4-CH₃-C₆H₄C (NTMS)₂]₂TiMe₂ (10) is not active either in CH₂Cl₂ or in toluene. At high pressure, complex 10 produces elastomeric polypropylene. Activities of the isolobal complexes [C₆H₄C(NTMS)₂]₂- ZrMe₂ (11) and [C₆H₄C(NTMS)₂]₂TiMe₂ (12) were found to be larger than those of complexes 9 and 10, respectively. Contrary to the structures of the elastomeric polypropylenes described in the literature, the obtained elastomers are characterized by frequent alternation of the isotactic domains with stereodefects. The stereoregular errors are formed by the intramolecular epimerization of the growing chain at the last inserted unit. The epimerization reaction was corroborated through the isomerization of alkenes.

Full text of DOI:10.1021/ja020575r

Published on Web 01/31/2003
Group 4 Octahedral Benzamidinate Complexes: Syntheses,
Structures, and Catalytic Activities in the Polymerization of
Propylene Modulated by Pressure
Victoria Volkis,† Elza Nelkenbaum,† Anatoli Lisovskii,† Gil Hasson,† Rafi Semiat,‡
Moshe Kapon,† Mark Botoshansky,† Yoav Eishen,† and Moris S. Eisen*,†
Contribution from the Department of Chemistry and Institute of Catalysis Science and
Technology, Technion, Israel Institute of Technology, Haifa 32000, Israel, and Department of
Chemical Engineering, Technion, Israel Institute of Technology, Haifa 32000, Israel
Received April 22, 2002 ; E-mail: chmoris@tx.technion.ac.il
Abstract: The synthesis and structural X-ray diffraction studies for some benzamidinate ligations and several
group 4 benzamidinate complexes are presented. The use of the cis-octahedral C2-symmetry compounds
was studied to shed light on the conceptual applicability of these complexes as potential catalysts for the
stereoregular polymerization of propylene. We demonstrate that the stereoregular polymerization of
propylene catalyzed by early-transition metal octahedral benzamidinate complexes, activated with either
MAO or B(C6F5)3 as cocatalysts, can be modulated by pressure (from atactic to isotactic through elastomers).
The different effects in the polymerization process such as the nature of solvent or cocatalyst, temperature,
pressure, molar ratio catalyst:cocatalyst, and the relationship between the symmetry of the complex and
the polymer microstructure have been investigated. When the complex [4-CH3-C6H4C(NTMS)2]2ZrMe2 (9)
was activated with MAO, it was found to be a good catalyst for the polymerization of propylene, at
atmospheric pressure, producing an oily polymer resembling an atactic polypropylene. Being activated
with B(C6F5)3, complex 9 produces a highly isotactic (mmmm ) 98%) product. Likewise, when the
polymerization of propylene was performed with complex 9 and MAO at high pressure (liquid propylene),
a highly stereoregular polymer was also obtained. Larger activities and stereoregularities were achieved
by performing the reaction in CH2Cl2 as compared to toluene. Contrary to complex 9, at atmospheric pressure
the complex [4-CH3-C6H4C(NTMS)2]2TiMe2 (10) is not active either in CH2Cl2 or in toluene. At high pressure,
complex 10 produces elastomeric polypropylene. Activities of the isolobal complexes [C6H4C(NTMS)2]2-
ZrMe2 (11) and [C6H4C(NTMS)2]2TiMe2 (12) were found to be larger than those of complexes 9 and 10,
respectively. Contrary to the structures of the elastomeric polypropylenes described in the literature, the
obtained elastomers are characterized by frequent alternation of the isotactic domains with stereodefects.
The stereoregular errors are formed by the intramolecular epimerization of the growing chain at the last
inserted unit. The epimerization reaction was corroborated through the isomerization of alkenes.
Introduction
These studies resulted in the discovery of metallocenes as a
new Ziegler-Natta type of homogeneous “single-site” catalysts
Metal-mediated olefin polymerization catalysts have experi-
enced a vast growth since the pioneering work of Ziegler and
Natta, which showed that systems such as TiCl4/AlClEt2 catalyze
the polymerization of ethylene to high-density polyethylene and
propylene to stereoregular polypropylene.1 Successful applica-
tion of the Ziegler-Natta catalysts has stimulated intense
academic and industrial research activity focused on understand-
ing the relationships between the structure, activity, selectivity
of the catalysts, and the properties of the obtained polymers.
for the polymerization of R-olefins.2,3 These metallocenes being
activated with either methylalumoxane (MAO) or perfluorinated
boron cocatalysts provide high activities, high stereoregularities,
narrow polydispersities of the polymers, and control over the
(2) (a) Kaminsky, W. Olefin Polymerization Catalyzed by Metallocenes. In
AdVances in Catalysis; Gates, B. C., Kno¨zinger, H., Eds.; Academic
Press: San Diego, 2002; Vol. 46, p 89. (b) Marks, T. J., Stevens, J. C.,
Eds. Topics in Catalysis; Baltzer: Basel, Switzerland, 1999; Vol. 15 and
references therein. (c) Mu¨lhaupt, R. In Polypropylene: An A-Z Reference;
Karger-Kocsis, J., Ed.; Kluwer Academic Publisher: Dordrecht, 1999; p
454. (d) Kaminsky, W.; Arndt, M. AdV. Polym. Sci. 1997, 127, 143 and
references therein. (e) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996,
255 and references therein. (f) Brintzinger, H. H.; Fisher, D.; Mu¨lhaupt,
R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34,
1143 and references therein.
(3) (a) Alt, H. G.; Ko¨pll, A. Chem. ReV. 2000, 100, 1205 and references therein.
(b) Coates, G. W. Chem. ReV. 2000, 100, 1223 and references therein.
(c) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000,
100, 1253. (d) Alt, H. G. J. Chem. Soc., Dalton Trans. 1999, 1703 and
references therein. (e) Kaminsky, W. J. Chem. Soc., Dalton Trans. 1998,
1413.
† Department of Chemistry and Institute of Catalysis Science and
Technology, Technion.
‡ Department of Chemical Engineering, Technion.
(1) (a) Ziegler, K.; Holzkamp, H.; Breil, H.; Martin, H. Angew. Chem., Int
Ed. Engl. 1955, 67, 54. (b) Natta, G. Angew. Chem., Int. Ed. Engl. 1956,
68, 393. (c) Catalytic Olefin Polymerization; Keii, T., Soga, K., Eds.;
Elsevier: Amsterdam, 1990. (d) Fink, G.; Mu¨lhaupt, R.; Brintzinger, H.
H. Ziegler Catalysis; Springer-Verlag: Heidelberg, 1994. (e) Transition
Metals and Organometallics as Catalysts for Olefin Polymerization;
Kaminsky, W., Sinn, H., Eds.; Springer-Verlag: Berlin, 1987 and references
therein.
9
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J. AM. CHEM. SOC. 2003, 125, 2179-2194
2179

A R T I C L E S
Volkis et al.
polymeric architecture.2-4 In addition, comonomer incorporation
and long chain branching can be modulated to a significant
extent by suitable adjustment of both catalyst and cocatalyst
structures’ design.5
In general, these catalytic systems, as represented by A,
contain many required motifs for an active polymerization
catalyst, including a suitable spectator ancillary ligation (Ln),
an electron-deficient and coordinatively unsaturated metal center
(M), an effective weekly coordinative counteranion/cocatalyst
(X-), and the appropriate temperature, solvent, and pressure
conditions.
A tremendous effort has been made in designing new
complexes with different ancillary ligations of the metals of
groups 3-13 as suitable precursors for the polymerization of
R-olefins.8 The wide variety of carbon-,9 oxygen-,10 and
nitrogen-based ligands11,12 has been described. Most attention
has been directed to chelating di(amido) ligands,13 some of
whose group 4 transition metal complexes induce the living
polymerization of R-olefins.14 Recently, chelating alkoxo,15
boratabenzene,16 chelating salicilaldiminato,15,17 acetylaceto-
nate,18 phosphinoamines,19 and chelating amidinate20 compounds
have received attention as ancillary ligands in the synthesis of
potential Ziegler-Natta catalysts.
Within the amidinate-based ligands, a variety of benzamidi-
nate group 4-containing complexes have been described. These
(8) (a) Ittel, S. D.; Johnson, L. K.; Brookhardt, M. Chem. ReV. 2000, 100,
1169. (b) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428 and references therein.
(9) (a) Bochmann, M.; Lancaster, S. J. Organometallics 1993, 12, 663. (b)
Pellecchia, C.; Grassi, A.; Immirzi, A. J. Am. Chem. Soc. 1993, 115, 1160.
(c) Ewart, S. W.; Sarsfield, M. J.; Jeremic, D.; Tremblay, T. L.; Williams,
E. F.; Baird, M. C. Organometallics 1998, 17, 1502. (d) Pellecchia, C.;
Immirzi, A.; Grassi, A.; Zambelli, A. Organometallics 1993, 12, 4473.
(10) (a) Shrock, R. R.; Baumann, R.; Reid, S. M.; Goodman, J. T.; Stumpf, R.;
Davis, W. M. Organometallics 1999, 18, 3649. (b) Mack, H.; Eisen, M. S.
J. Chem. Soc., Dalton Trans. 1998, 917. (c) Gielens, E. E. C. G.; Tiesnitsch,
J. Y.; Hessen, B.; Teuben, J. H. Organometallics 1998, 17, 1652. (d) Chen,
Y.-X.; Fu, P.-F.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16, 5958.
(e) Sernetz, F. G.; Mu¨lhaupt, R.; Fokken, S.; Okuda, J. Macromolecules
1997, 30, 1562. (f) Bei, X.; Swenson, D. C.; Jordan, R. F. Organometallics
1997, 16, 3282.
The majority of the catalysts belong to groups 3 and 4
metallocene derivatives, although some are know to contain one
cyclopentadienyl ring and one pendant ligand.6 The tacticity of
the polymers varies predictably with the structure of the
metallocenes. Hence, C2- or C1-symmetry complexes are
expected to produce isotactic poly R-olefins, whereas C2V
induces the formation of atactic polymers.7 Because in the
polymerization of R-olefins the ligation around the metal center
is crucial in determining the activity and stereospecificity of
the catalysts, affecting both electronic and steric properties, a
growing interest in the development of noncyclopentadienyl
ligations has been stimulated.8
(11) (a) Kempe, R. Angew. Chem., Int. Ed. 2000, 39, 468 and references therein.
(b) Gade, L. H. Chem. Commun. 2000, 173.
(12) (a) Goodman, J. T.; Schrock, R. R. Organometallics 2001, 20, 5205. (b)
Schrock, R. R.; Bonitatebus, P. J., Jr.; Schrodi, Y. Organometallics 2001,
20, 1056. (c) O’Connor, P. E.; Morrison, D. J.; Steeves, S.; Burrage, K.;
Berg, D. J. Organometallics 2001, 20, 1153. (d) Yoshida, Y.; Matsui, S.;
Takagi, Y.; Mitani, M.; Nakano, T.; Tanaka, H.; Kashiwa, N.; Fujita, T.
Organometallics 2001, 20, 4793. (e) Danie`le, S.; Hitchcock, P. B.; Lappert,
M. F.; Merle, P. G. J. Chem. Soc., Dalton Trans. 2001, 13. (f) Mehrkho-
davandi, P.; Bonitatebus, P. J., Jr.; Schrock, R. R. J. Am. Chem. Soc. 2000,
122, 7841. (g) Skinner, M. E. G.; Cowhig, D. A.; Mountford, P. Chem.
Commun. 2000, 1167. (h) Nomura, K.; Oya, K.; Imanishi, Y. Polymer 2000,
41, 2755. (i) Lee, C. H.; La, Y.-H.; Park, J. W. Organometallics 2000, 19,
344 and references therein.
(13) (a) Ziniuk, Z.; Goldberg, I.; Kol, M. Inorg. Chem. Commun. 1999, 2, 549.
(b) Liang, L.-C.; Schrock, R. R.; Davis, W. M.; McConville, D. H. J. Am.
Chem. Soc. 1999, 121, 5797. (c) Graf, D. D.; Schrock, R. R.; Davis, W.
M.; Stumpf, R. Organometallics 1999, 18, 843. (d) Schrock, R. R.;
Baumann, R.; Reid, S. M.; Goodman, J. T.; Stumpf, R.; Davis, W. M.
Organometallics 1999, 18, 3649. (e) Gue´rin, F.; McConville, D. H.; Vittal,
J. J.; Yap, G. A. P. Organometallics 1998, 17, 5172. (f) Gibson, V. C.;
Kimberley, V. S.; White, A. J. P.; Williams, D. J.; Howard, P. Chem.
Commun. 1998, 313. (g) Scollard, J. D.; McConville, D. H.; Vittal, J. J.
Organometallics 1997, 16, 4415. (h) Mack, H.; Eisen, M. S. J. Organomet.
Chem. 1996, 525, 81.
(4) (a) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (b) Metz,
M. V.; Schwartz, D. J.; Stem, C. L.; Nickias, P. N.; Marks, T. J. Angew.
Chem., Int. Ed. 2000, 39, 1312. (c) Beswick, C. L.; Marks, T. J. J. Am.
Chem. Soc. 2000, 122, 10358 and references therein. (d) Lanza, G.; Fragala`,
I. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 12764 and references
therein. (e) Beswick, C. L.; Marks, T. J. Organometallics 1999, 18, 10358,
2410. (f) Williams, V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J.;
Collins, S.; Marder, T. B. J. Am. Chem. Soc. 1999, 121, 3244. (g) Deck,
P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 1772. (h)
Chen, Y.-X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J. J. Am. Chem.
Soc. 1998, 120, 6287. (i) Li, L.; Marks, T. J. Organometallics 1998, 17,
3996. (j) Piers, W. E.; Chivers, T. Chem. Soc. ReV. 1997, 26, 345. (k)
Temme, B.; Erker, G.; Karl, J.; Luftmann, H.; Frohlich, R.; Kotila, S.
Angew. Chem., Int. Ed. Engl. 1995, 34, 1755.
(5) (a) Fan, W.; Leclerc, M. K.; Waymouth, R. M. J. Am. Chem. Soc. 2001,
123, 9555. (b) Kim, J. S.; Wojcinski, L. M., II; Liu, S.; Sworen, J. C.; Sen,
A. J. Am. Chem. Soc. 2000, 122, 5668. (c) Leclerc, M. K.; Waymouth, R.
M. Angew. Chem., Int. Ed. 1998, 37, 922. (d) Jin, J. H.; Uozumi, T.; Sano,
T.; Teranishi, T.; Soga, K.; Shiono, T. Macromol. Rapid Commun. 1998,
19, 337. (e) Arndt, N.; Kaminsky, W.; Schauwienold, A. M.; Weingarten,
U. Macromol. Chem. Phys. 1998, 199, 1135. (f) Uozumi, T.; Myyazawa,
K.; Sano, T.; Soga, K. Macromol. Rapid Commun. 1997, 18, 883.
(6) (a) Brown, S. J.; Gao, X.; Harrison, D. G.; Koch, L.; Spence, R. E. H.;
Yap, G. P. A. Organometallics 1998, 17, 5445. (b) McKnight, A. L.;
Massod, M. A.; Waymouth, R. M.; Straus, D. A. Organometallics 1997,
16, 2879. (c) Chen, Y.-X.; Marks, T. J. Organometallics 1997, 16, 3649.
(d) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16,
842. (e) Chen, Y. X.; Fu, P. F.; Stern, C. L.; Marks, T. J. Organometallics
1997, 16, 5958. (f) Chen, Y.-X.; Stern, C. L.; Yang, S.; Marks, T. J. J.
Am. Chem. Soc. 1996, 118, 12451. (g) Hightower, J. W., Delglass, W. N.,
Iglesia, E., Bell, A. T., Eds.; Studies in Surface Science and Catalysis;
Elsevier: Amsterdam, 1996; Vol. 101, pp 1-729. (h) Tetsunosuke, S.;
Tadashi, A.; Norihide, I. Makromol. Rapid Commun. 1996, 17, 9. (i)
Ribeiro, M. R.; Deffleux, A.; Portella, M. F. Ind. Eng. Chem. Res. 1997,
36, 1224. (j) Galan-Fereres, M.; Koch, T.; Hey-Hawkins, E.; Eisen, M. S.
J. Organomet. Chem. 1999, 580, 145. (k) Shapiro, P. J.; Brunel, E.;
Schaefer, W. P.; Bercaw, J. E. Organometallics 1990, 9, 867. (l) Shapiro,
P. J.; Cotter, W. D.; Schaefer, W. P.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 1994, 16, 4623. (m) Carpenetti, D. W.; Kloppenburg, L.; Kupec,
J. T.; Petersen, J. L. Organometallics 1996, 15, 255.
(14) (a) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics
2001, 20, 3017. (b) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem.
Soc. 2000, 122, 10706. (c) Baumann, R.; Davis, W. M.; Schrock, R. R. J.
Am. Chem. Soc. 1997, 119, 3830. (d) Scollard, J. D.; McConville, D. H. J.
Am. Chem. Soc. 1996, 118, 10008 and references therein.
(15) (a) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Matsukawa, N.;
Takagi, Y.; Tsuru, K.; Nitabaru, M.; Nakano, T.; Tanaka, H.; Kashiwa,
N.; Fujita, T. J. Am. Chem. Soc. 2001, 123, 6847. (b) Matsukawa, N.;
Matsui, S.; Mitani, M.; Saito, J.; Tsuru, K.; Fujita, T. J. Mol. Catal. A:
Chem. 2001, 169, 99. (c) Saito, J.; Mitani, M.; Mohri, J.; Yoshida, Y.;
Matsui, S.; Ishii, S.; Kojoh, S.; Kashiwa, N.; Fujita, T. Angew. Chem., Int.
Ed. 2001, 40, 2918. (d) Sobota, P.; Przybylak, K.; Urko, J.; Jerzykiewicz,
L. B.; Pombiero, A. J. L.; da Silva, M. F. C. G.; Szczegot, K. Chem.-Eur.
J. 2001, 7, 951. (e) Nakayama, Y.; Watanabe, K.; Ueyama, N.; Nakamura,
A.; Harada, A.; Okuda, J. Organometallics 2000, 19, 2498. (f) Okuda, J.;
Fokken, S.; Kleinhenn, T.; Spaniol, T. P. Eur. J. Inorg. Chem. 2000, 1321.
(16) (a) Pellecchia, C.; Proto, A.; Longo, P.; Zambelli, A. Macromol. Chem.
Rapid Commun. 1991, 12, 663. (b) Pellecchia, C.; Proto, A.; Longo, P.;
Zambelli, A. Macromol. Chem. Rapid Commun. 1992, 13, 277.
(17) (a) Tian, J.; Hustad, P. D.; Coates, G. W. J. Am. Chem. Soc. 2001, 123,
5134. (b) Tian, J.; Coates, G. W. Angew. Chem., Int. Ed. 2000, 39, 3626.
(c) Mitani, M.; Furuyama, R.; Mohri, J.; Saito, J.; Ishii, S.; Terao, H.;
Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2002, 124, 7888. (d) Saito, J.;
Mitani, M.; Mohri, J..; Ishii, S.; Yoshida, Y.; Matsugi, T.; Kojoh, S.;
Kashiwa, N.; Fujita, T. Chem. Lett. 2001, 567.
(7) (a) Averbuj, C.; Tish, E.; Eisen, M. S. J. Am. Chem. Soc. 1998, 120, 8640.
(b) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem. Soc.
1988, 110, 6255. (c) Catalyst Design for Tailor-Made Polyolefins; Soga,
K., Terrano, M., Eds.; Elsevier: Tokyo, 1994. (d) Mo¨hring, P. C.; Coville,
N. J. J. Organomet. Chem. 1994, 479, 1. (e) Ewen, J. A. J. Am. Chem.
Soc. 1984, 106, 6355.
(18) Shmulinson, M.; Galan-Fereres, M.; Lisovskii, A.; Nelkenbaum, E.; Semiat,
R.; Eisen, M. S. Organometallics 2000, 19, 1208.
(19) Koch, T.; Blaurock, S.; Hey-Hawkins, E.; Galan-Fereres, M.; Plat, D.; Eisen,
M. S. J. Organomet. Chem. 2000, 595, 126.
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2180 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003

Group 4 Octahedral Benzamidinate Complexes
A R T I C L E S
systems are in principle devoted mainly to the use of N,N′-bis-
(trimethylsilyl)benzamidinate ligation. The benzamidinate ligands
can be considered as a steric equivalent of Cp or Cp* (Cp )
C5H5; Cp* ) C5Me5), although they are unique in their
electronic properties.20,21 The anionic moiety [R-C(NSiMe3)2]-
is a four-electron donor, promoting a higher electrophilicity at
the metal center, as compared to the six electrons of the
cyclopentadienyl ligands. The possibility to simply modify both
steric bulk and electronic properties of the benzamidinate-based
ligands, through changes in either the organic substituents at
the nitrogen atom and/or different functional groups at the
aromatic ring, makes these ligands very attractive for synthesis
of various organometallic complexes and their corresponding
utilization as catalytic precursors in the polymerization of
R-olefins.
Previously, others and we have investigated the polymeriza-
tion of R-olefins catalyzed by several group 4 chelating
benzamidinate complexes.22 The bis(benzamidinate) dichloride/
dialkyl group 4 complexes are normally obtained as a mixture
of racemic C2-symmetry cis-octahedral structures, and, when
activated with methylalumoxane (MAO), these complexes were
found to be active catalytic precursors for the polymerization
of ethylene, propylene, the oligomerization of 1,5-hexadiene,
and the isomerization of internal and terminal olefins.7a,22 For
propylene, performing the polymerization at atmospheric pres-
sure produced polypropylene as an oil, despite the expected
isotactic polymer.23 These results raise conceptual questions
regarding the applicability of cis-octahedral C2-symmetry
complexes to the stereospecific polymerization of R-olefins.
Here, we present the synthesis and structural X-ray diffraction
studies on several group 4 benzamidinate complexes. We
introduce the application of these complexes as new catalytic
precursors for the highly stereoregular polymerization of pro-
pylene, which can be modulated by pressure. To our knowledge,
this is the first example of stereoregular polymerization of
R-olefins catalyzed by early-transition metal octahedral com-
plexes, modulated by pressure (from atactic to isotactic through
elastomers). We present a thorough study disclosing the different
effects, such as pressure, temperature, and nature of solvents in
the polymerization process and in the properties of the polymers
obtained. Moreover, we introduce a new type of elastomeric
polypropylene that is facile to design and to tailor. The
mechanism for the formation of the various types of polypro-
pylene (atactic, isotactic, and elastomeric) at the different
conditions is discussed.
Results and Discussion
The goal of this investigation was to examine the scope,
relationship between symmetry of the complex and polymer
microstructure, stereoregularity, metal sensitivity, monomer
concentration, substituent effect at the ancillary ligation, tem-
perature, solvent, cocatalyst, polymerization time, and to propose
a plausible mechanism for the polymerization of propylene
promoted by racemic mixtures of cis-octahedral complexes. This
study represents an extension of the unique reactivities of this
type of complex in the polymerization of propylene, when
activated by either MAO or perfluoroaryl borane cocatalysts.
In the following presentation of the results, we focus on the
synthesis of the ligands and group 4 metal complexes, and
on the different parameters affecting the polymerization of
propylene toward the formation of either atactic, isotactic, or
elastomeric polymers. We report the X-ray single-crystal
diffraction studies for the benzamidinate ligands and the
corresponding group 4 complexes. In addition, we present
evidence, through the isomerization of alkenes, for the epimer-
ization of the last inserted monomer as the major mechanistic
pathway, responsible for the formation of polypropylene with
elastomeric properties. We start the presentation of the results
with various synthetic pathways of different ancillary benza-
midinate ligations followed by the synthesis and characterization
of the corresponding group 4 complexes.
Synthesis of Benzamidinate Ligands. The preparation of
the lithium benzamidinate was carried out through the reaction
between benzonitrile (or p-tolunitrile) and the lithium salt LiN-
(SiMe3)2.24
(20) (a) Jayaratne, K. C.; Sita, L. R. J. Am. Chem. Soc. 2001, 123, 10754. (b)
Keaton, R. J.; Jayaratne, K. C.; Henningsen, D. A.; Koterwas, L. A.; Sita,
L. R. J. Am. Chem. Soc. 2001, 123, 6197. (c) Jayaratne, K. C.; Sita, L. R.
J. Am. Chem. Soc. 2000, 122, 958. (d) Littke, A.; Sleiman, N.; Bensimon,
C.; Richeson, D. S.; Yap, G. P. A.; Brown, S. J. Organometallics 1998,
17, 446. (e) Gomez, R.; Duchateau, R.; Chernega, A. N.; Teuben, J. N.;
Edelmann, F. T.; Green, M. L. H. J. Organomet. Chem. 1995, 491, 153.
(21) (a) Hagadorn, J. R.; Arnold, J. Organometallics 1998, 17, 1355. (b)
Hagadorn, J. R.; Arnold, J. J. Chem. Soc., Dalton Trans. 1997, 3087. (c)
Hagadorn, J. R.; Arnold, J. J. Am. Chem. Soc. 1996, 118, 893. (d) Edelmann,
F. T. Coord. Chem. ReV. 1994, 137, 403 and references therein. (e)
Edelmann, F. T. Top. Curr. Chem. 1996, 179, 113 and references therein.
(f) Hagadorn, J. R.; Arnold, J. Organometallics 1994, 13, 4670.
(22) (a) Richter, J.; Edelmann, F. T.; Noltemeyer, M.; Schmidt, H.-G.;
Shmulinson, M.; Eisen, M. S. J. Mol. Catal. 1998, 130, 149. (b) Walter,
D.; Fischer, R.; Friedrich, F.; Gebhardt, P.; Go¨rls, H. Chem. Ber. 1996,
129, 1389. (c) Walter, D.; Fischer, R.; Go¨rls, H.; Koch, J.; Scheweder, B.
J. J. Organomet. Chem. 1996, 508, 13. (d) Duchateau, R.; van Wee, C. T.;
Meetsma, A.; van Duijnen, P. T.; Teuben, J. H. Organometallics 1996,
15, 2279. (e) Gomez, R.; Green, M. L. H.; Haggitt, J. L. J. Chem. Soc.,
Dalton Trans. 1996, 939. (f) Flores, J. C.; Chien, J. C. W.; Rausch, M. D.
Organometallics 1995, 14, 1827. (g) Flores, J. C.; Chien, J. C. W.; Rausch,
M. D. Organometallics 1995, 14, 2106. (h) Herscovics-Korine, D.; Eisen,
M. S. J. Organomet. Chem. 1995, 503, 307. (i) Gomez, R.; Duchateau, R.;
Chernega, A. N.; Meetsma, A.; Edelmann, F. T.; Teuben, J. H.; Green, M.
L. H. J. Chem. Soc., Dalton Trans. 1995, 217. (j) Gomez, R.; Green, M.
L. H.; Haggitt, J. L. J. Chem. Soc., Chem. Commun. 1994, 2607.
(23) For a preliminary result, see: Volkis, V.; Shmulinson, M.; Averbuj, C.;
Lisovskii, A.; Edelmann, F. T.; Eisen, M. S. Organometallics 1998, 17,
3155.
In the absence of a strong coordinating polar solvent, the
dimeric structures 1 or 2 are the major products obtained (eq
1), in addition to some trimeric species, although when the
reaction is carried out in the presence of TMEDA (tetrameth-
ylethylenediamine), as the coordinated solvent, the monomeric
(24) (a) Wedler, M.; Kno¨sel, F.; Noltemeyer, M., Edelmann, F. T. J. Organomet.
Chem. 1990, 388, 21. (b) Stalke, D.; Wedler, M.; Edelmann, F. T. J.
Organomet. Chem. 1992, 431, C1. (c) Edelmann, F. T. J. Organomet. Chem.
1992, 426, 295. (d) Dick, D. G.; Duchateau, R.; Edema, J. H.; Gambarotta,
S. Inorg. Chem. 1993, 32, 1959. (e) Barker, J.; Barr, D.; Barnett, N. D. R.;
Clegg, W.; Cragg-Hine, I.; Davidson, M. G.; Davies, R. P.; Hodson, S.
M.; Howard, J. A. K.; Kilner, M.; Lehmann, C. W.; Lopez-Solera, I.;
Mulwey, R. E.; Raithby, P. R.; Snaith, R. J. Chem. Soc., Dalton Trans.
1997, 95.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2181

A R T I C L E S
Volkis et al.
complexes [R-PhC(NSiMe3)2Li(TMEDA)] (R ) CH3 (3); H (4))
are obtained as the sole products (eq 2).
The low-temperature X-ray analysis of 2 (Figure 1, Tables 1
and 10) shows a dimeric benzamidinate structure with two
distorted-tetrahedral lithium cations. Each lithium atom is
π-bonded by one bidentate benzamidinate moiety, σ-bonded to
the second benzamidinate nitrogen atom, and σ-bonded to a
nitrogen of a benzonitrile molecule. The core units are comprised
by a four-membered ring Li-N-C-N and a six-membered ring
Li-N-C-N-Li-N. The planes formed by the two η3-
diazaallyl motifs make a dihedral angle of almost 90°, forming
a distorted C2V-symmetry complex. The four C-N bond lengths
in both amidinates are close to the average of the CdN [1.30
Å] and C-N [1.36 Å] bond lengths, as compared to similar
protonated N-substituted phenyl ligands.25 This result suggests
that the anion in complex 2 is best described as a system
containing two diazaallyl moieties, one of which acts as a double
bridge of two lithium atoms.
The molecular structure of complex 4 (Figure 2, Tables 2,
10) shows a slightly distorted tetrahedral monomeric lithium
atom (angle between the two planes containing the metal )
93.3 (4)°) attached in an almost symmetrical mode to both
amidine and TMEDA moieties [Li-N(5) ) 2.033 (6), Li-N(13)
) 2.019 (6), Li-N(18) ) 2.120 (6), Li-N(23) ) 2.116 (7) Å].
The similar electronic delocalization throughout the amidine
moiety is observed by the similar C-N distances [N(5)-C(6)
) 1.331 (4), C(6)-N(13) ) 1.325 (4) Å]. Interestingly, there
is almost no electronic interaction between the aromatic phenyl
ring and the diazaalyl section as exhibited by the torsional angle
(94.2°).
Figure 1. ORTEP diagram (50% probability ellipsoid) of complex 2.
Table 1. Selected Bond Lengths [Å] and Angles [deg] for
Complex 2
Li(1)-N(13)
Li(1)-N(35)
Li(1)-N(22)
Li(1)-N(30)
Li(1)-C(23)
Li(1)-Li(2)
Li(2)-N(5)
Li(2)-N(30)
Li(2)-N(43)
Li(2)-N(22)
Li(2)-C(23)
2.055(4)
2.086(4)
2.104(4)
2.126(4)
2.392(4)
2.465(5)
1.995(4)
2.069(4)
2.088(4)
2.364(4)
2.408(4)
N(13)-Li(1)-N(35)
N(13)-Li(1)-N(22)
N(35)-Li(1)-N(22)
N(13)-Li(1)-N(30)
N(35)-Li(1)-N(30)
N(22)-Li(1)-N(30)
N(5)-Li(2)-N(30)
N(5)-Li(2)-N(43)
N(30)-Li(2)-N(43)
N(5)-Li(2)-N(22)
N(30)-Li(2)-N(22)
N(43)-Li(2)-N(22)
118.53(18)
122.39(19)
111.28(18)
121.56(18)
106.25(17)
64.88(11)
116.67(18)
120.7(2)
110.84(17)
140.8(2)
61.16(11)
92.61(15)
Synthesis of Zr and Ti Bis(benzamidinate) Complexes. The
reaction of MCl4 (M ) Zr, Ti) with 2 equiv of the appropriate
ligands 3 and 4 takes place at room temperature in toluene to
give pale yellow (Zr) and brown-red (Ti) solutions, from which
analytically pure compounds 5-8 were isolated. For the
zirconium complexes, high yields (88-89%) are achieved,
whereas for the corresponding titanium complexes, lower yields
are obtained (60-70%). The solid X-ray characterization of
complexes 721b,22d,i and 826 has been already reported. It is
noteworthy that complexes 7 and 8 can be also attained by the
reaction of the MCl4 (M ) Zr, Ti) and 1 equiv of the
corresponding benzamidinate lithium dimer 2, whereas com-
plexes 5 and 6 can be prepared in lower yields, when they are
formed using the corresponding dimer 1. Alkylation of com-
plexes 5-8 with 2 equiv of MeLi‚LiBr in ether yields yellow
dimethyl complexes of zirconium (9 and 11) or wine-red
dimethyl complexes of titanium (10 and 12) in quantitative
yields (98-99%) (eq 3).
The crystalline structures of complexes 1121b,22d and 1227 have
been reported in the literature. The crystallographic data for
complexes 9 and 10 are given in Table 10. Selected bond lengths
and angles are listed in Tables 3 and 4, and their molecular
structures are shown in Figures 3 and 4, respectively. The low-
temperature X-ray analysis of complex 9 shows that the central
Zr atom is octahedral-bonded to two benzamidinate ligands and
two terminal methyl groups. The octahedral complex is formed
(25) For a comparison to a similar isostructural complex and to various lithium
benzamidinate complexes, see: (a) Eisen, M. S.; Kapon, M. J. Chem. Soc.,
Dalton Trans. 1994, 3507. (b) Lisovskii, A.; Botoshansky, M.; Eisen, M.
S. J. Chem. Soc., Dalton Trans. 2001, 1692 and references therein, and ref
24.
(26) Roesky, H. W.; Meller, B.; Noltemeyer, M.; Schmidt, H.-G.; Scholz, U.;
Sheldrick, G. M. Chem. Ber. 1988, 121, 1403.
(27) Thiele, K.-H.; Windisch, H.; Windisch, H.; Edelmann, F. T.; Kilimann,
U.; Noltemeyer, M. Z. Anorg. Allg. Chem. 1996, 622, 713.
9
2182 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003

Group 4 Octahedral Benzamidinate Complexes
A R T I C L E S
Table 3. Selected Bond Lengths [Å] and Angles [deg] for
Complex 9
Zr(1)-N(1)
Zr(1)-C(1)
Zr(1)-N(4)
Zr(1)-C(11)
Zr(1)-N(3)
Zr(1)-C(2)
Zr(1)-N(2)
Zr(1)-C(3)
2.227(4)
2.246(5)
2.295(4)
2.672(5)
2.234(4)
2.248(6)
2.320(4)
2.578(5)
N(1)-Zr(1)-N(3)
N(1)-Zr(1)-N(4)
N(1)-Zr(1)-N(2)
N(4)-Zr(1)-N(2)
N(3)-Zr(1)-C(1)
N(1)-Zr(1)-C(1)
N(1)-Zr(1)-C(2)
N(3)-Zr(1)-C(2)
C(1)-Zr(1)-N(4)
C(1)-Zr(1)-C(2)
N(3)-Zr(1)-N(4)
C(2)-Zr(1)-N(4)
N(3)-Zr(1)-N(2)
C(2)-Zr(1)-N(2)
91.1(2)
122.3(2)
59.34(14)
172.23(14)
97.4(3)
144.5(2)
91.3(2)
143.2(2)
143.2(2)
101.8(3)
59.3(2)
Figure 2. ORTEP diagram (50% probability ellipsoid) of complex 4.
89.0(2)
113.93(14)
98.7(2)
Table 2. Selected Bond Lengths [Å] and Angles [deg] for
Complex 4
Li-N(13)
Li-N(5)
Li-N(23)
Li-N(18)
N(5)-C(6)
C(6)-N(13)
2.019(6)
2.033(6)
2.116(7)
2.120(6)
1.331(5)
1.325(4)
Table 4. Selected Bond Lengths [Å] and Angles [deg] for
Complex 10
Ti-N(14)
Ti-C(19)
Ti-N(5)
2.124(2)
2.120(4)
2.137(2)
2.515(3)
1.321(3)
1.321(3)
N(13)-Li-N(5)
N(13)-Li-N(23)
N(5)-Li-N(23)
N(13)-Li-N(18)
N(5)-Li-N(18)
N(23)-Li-N(18)
69.14(19)
119.6(3)
122.5(3)
136.1(3)
128.6(3)
86.4(2)
Ti-C(6)
N(5)-C(6)
C(6)-N(14)
N(14)-Ti-N(14)#1
N(14)-Ti-C(19)#1
N(14)-Ti-C(19)
C(19)#1-Ti-C(19)
N(14)-Ti-N(5)#1
N(14)#1-Ti-N(5)#1
C(19)#1-Ti-N(5)#1
C(19)-Ti-N(5)#1
N(14)-Ti-N(5)
94.88(12)
91.75(17)
153.16(13)
94.0(3)
108.32(8)
63.17(8)
90.06(12)
97.89(13)
63.17(8)
in such a way that one carbon atom from the methyl groups
(C(1)) and one nitrogen atom from the chelating benzamidinate
N(5)#1-Ti-N(5)
168.37(12)
rings ZrCN2 are almost symmetric: the C-N and Zr-N bond
lengths are respectively equal (N(2)-C(11) ) 1.345 (6), N(1)-
C(11) ) 1.325 (6), N(3)-C(3) ) 1.336 (6), N(4)-C(3) ) 1.322
(6), Zr-N(1) ) 2.227 (4), Zr-N(2) ) 2.320 (4), Zr-N(3) )
2.234 (4), Zr-N(4) ) 2.295 (4) Å). By comparison to other
benzamidinate complexes, the Zr-N(1) to Zr-N(4) distances
are somewhat longer than those in the dimeric compound
[C6H5C(NSiMe3)2ZrCl3]2 (2.14 and 2.19 Å),28 while they are
similar to those in the monomeric complexes [C6H5C(NSiMe3)2-
ZrCl2] (2.20 and 2.24 Å)21b,22d,i and [C6H5C(NSiMe3)2ZrMe2]
(2.24 and 2.31 Å).21b,22d The benzamidinate moieties are located
almost in one plane with the ZrCN2 units, with the corresponding
angles Zr-C(3)-C(4) and Zr-C(11)-C(12) of 168.7 (3) and
170.6 (4)°, respectively.
The X-ray study of the dimethyl titanium bis(benzamidinate)
complex (10) shows that the Ti atom is octahedrally distorted
surrounded by two chelating benzamidinate ligands and two
terminal methyl groups. The methyl carbon atom (C(19)) and
the nitrogen atom (N(14)) are in the axial positions [C(19)-
Ti-N(14) ) 153.16 (13)°], while the second methyl group and
the remaining nitrogen atoms occupy the equatorial position
[C(19)-Ti-N(14)# ) 91.75 (17), C(19)-Ti-N(5) ) 90.06
unit (N(4)) are in the axial positions, similarly as in complex
10, producing a C2-symmetric complex. The two four-membered
(28) Fenske, D.; Hartmann, E.; Dehnicke, K. Z. Naturforsch. 1988, 43b,1611.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2183

A R T I C L E S
Volkis et al.
Figure 3. ORTEP diagram (50% probability ellipsoid) of complex 9.
Figure 5. ORTEP diagram (50% probability ellipsoid) of complexes 10*
(a) and 13 (b).
Table 5. Selected Bond Lengths [Å] and Angles [deg] for
Complexes 10*/13
Ti-N(1)
2.106(4)
2.117(5)
2.139(11)
2.150(4)
Ti-N(2)
Ti-C(15)
Ti-C(15A)
N(1)#1-Ti-N(1)
N(1)#1-Ti-N(2)
N(1)-Ti-N(2)
167.3(2)
107.35(17)
63.50(16)
96.2(2)
98.5(3)
88.9(6)
N(2)-Ti-N(2)#1
N(1)-Ti-C(15)#1
N(2)-Ti-C(15)#1
N(1)-Ti-C(15)
89.8(3)
N(2)-Ti-C(15)
153.2(3)
98.3(12)
120.3(10)
70.8(10)
131.1(10)
113.5(10)
75(2)
C(15)#1-Ti-C(15)
N(1)-Ti-C(15A)#1
N(1)-Ti-C(15A)
N(2)-Ti-C(15A)
N(2)#1-Ti-C(15A)
C(15A)#1-Ti-C(15A)
reddish color were observed. The X-ray analysis of this latter
complex “10*” (Figure 5a) gave almost the same crystal data
(unit cell parameters, Table 10) as compared to those of 10,
but with different bond lengths and angles, due to the partial
(22%) cocrystallization of the additional complex 13 (Table 5).
Complex 13 is formed by the elimination of two methane
molecules, which are produced from the symmetric C-H
activation of the methyl group in the TMS moieties with the
metal-methyl bonds (eq 4) (Figure 5b). The C-H activation
of the TMS groups in complex 13 induces the elongation of
the Ti-C bond length [Ti-C(15A) ) 2.15 (4) Å] and the
shortening of the Si-N bond length [Si(1A)-N(1) ) 1.562 (10)
Å], as compared to the corresponding bond distances in complex
10 [Ti-C(19) ) 2.120 (4), Si(1)-N(5) ) 1.750 (2) Å]. The
Figure 4. ORTEP diagram (50% probability ellipsoid) of complex 10.
(12), N(5)-Ti-N(14) ) 63.17 (8)°]. As a result, the C-N bond
lengths of the two chelate units are equidistant (C-N ) 1.321
(3) Å), similar to those in the corresponding unsubstituted
benzamidinate complex.21b
During the selection of the best crystal of complex 10 for
the X-ray analysis, solitary crystals with a more pronounced
9
2184 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003

Group 4 Octahedral Benzamidinate Complexes
A R T I C L E S
activation of C-H moieties in titanium alkyl complexes
containing TMS-amido groups has been reported in the litera-
ture.29
Figure 6. ORTEP diagram (50% probability ellipsoid) of triazine 15.
Table 6. Selected Bond Lengths [Å] and Angles [deg] for
Complex 15
N(1)-C(1)
N(2)-C(1)
N(2)-C(2)
C(1)-C(3)
C(2)-C(9)
1.336(5)
1.333(5)
1.344(4)
1.469(6)
1.479(8)
C(1)-N(1)-C(1)#1
C(1)-N(2)-C(2)
N(2)-C(1)-N(1)
N(2)-C(1)-C(3)
N(1)-C(1)-C(3)
N(2)-C(2)-N(2)#1
116.6(6)
115.4(5)
123.9(5)
117.7(4)
118.4(4)
124.7(6)
1
material that was found to be silent in H NMR. This powder
is paramagnetic, exhibiting, at room temperature, an ESR signal
(X-band) centered at 3360 G and ∆H ) 12 G, with a g value
of 2.0017 characteristic for a Hf(III) complex.30 One of the most
intriguing questions in the formation of the Hf(III) complex is
the elucidation of the reducing agent entity. From the diethyl
ether washing solutions of the powder material, we were able
to crystallize the organic compound tris-p-tolyl-1,3,5-triazine
(15), the product of the trimerization of p-tolunitrile. The
formation of this triazine indicates that Hf(III) induces the
reverse reaction as indicated in eq 2, yielding p-tolunitrile and
the lithium salt LiN(TMS)2. The former compound trimerizes
to tris-p-tolyl-1,3,5-triazine. The Hf(III) presumably is obtained
by the reduction of HfCl4‚2THF with [4-CH3-PhC(NSiMe3)2-
Li(TMEDA)], as observed for similar Ti and Zr complexes.31
Interestingly, from the low-temperature X-ray analysis (Figure
6, Table 6), the triazine was found to be a polymorphic
compound of the recent structure published in the literature.32
The difference between the two crystalline materials resides in
the packing disposition of the molecules between the main plain
of the triazine ring with respect to the a0b plane. X-ray cell
parameters and refinement data are presented in Table 10.
Synthesis of Hf Bis(benzamidinate) Complex. An attempt
to achieve the corresponding hafnium bis(benzamidinate)
dichloride complex was intended following a similar protocol,
as described for the other lighter isolobal group 4 elements (eq
3), between the lithium salt of the ligand and HfCl4‚2THF,
resulting in an oily yellow mixture of products containing
TMEDA. Dissolving the oily compound in toluene and remov-
ing under vacuum all of the liquids afford the production of a
glassy yellow solid. The 1H and 13C NMR of this solid indicate
the formation of the expected complex [(4-CH3C6H4C(NSiMe3)2)2-
HfCl2] (14). Elemental analysis shows that complex 14 contains
additional products. Dissolving the mixture in a minimum
amount of toluene followed by the vacuum transfer of large
amounts of hexane affords the precipitation of a yellow powder
(30) Fakhr, A.; Mugnier, Y.; Gautheron, B.; Laviron, E. J. Organomet. Chem.
1986, 302, C7 and references therein.
(31) Ray, B.; Gueta-Neyroud, T.; Kapon, M.; Eichen, Y.; Eisen, M. S.
Organometallics 2001, 20, 3044.
(29) (a) Putzer, M. A.; Neumu¨ller, B.; Dehnicke, K. Z. Anorg. Allg. Chem. 1998,
624, 1087. (b) Putzer, M. A.; Magull, J.; Goesmann, H.; Neumu¨ller, B.;
Dehnicke, K. Chem. Ber. 1996, 129, 1401.
(32) Thalladi, V. R.; Muthuraman, M.; Nagia, A.; Desiraju, G. R. Acta
Crystallogr. 1999, C55, 698.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2185

A R T I C L E S
Volkis et al.
Figure 7. 13C NMR of the polypropylene obtained catalyzed by complex 9 at room temperature.
Catalytic Polymerization Studies. Polymerization reactions
were performed with the dimethyl complexes 9-12 activated
by MAO or B(C6F5)3. Because of the large differences in
polymer properties obtained with these complexes at the various
reaction conditions, the polymerization results are presented
separately for complexes 9-12.
octahedral geometry, suggesting that, when activated by MAO,
theoretically an isotactic polymer, as obtained with B(C6F5)3,
should also be expected.35
Polymerization of Propylene at Atmospheric Pressure
Catalyzed with Complex 9. This complex was found to be a
good precursor for the polymerization of propylene at atmo-
spheric pressure at 25 °C. When activated with MAO (Al:Zr
molar ratio 250:1) in toluene, complex 9 produces large amounts
(8.1 × 105 gPP/mol Zr‚h) of an oily polymer with medium
molecular weight Mw ) 38 000, resembling an atactic polypro-
pylene. 13C NMR tryad and pentad analysis of this polymer
shows that, contrary to the 10 expected signals for the methyl
groups of an atactic polypropylene, only two major signals are
observed in the region between 19.5 and 21.9 ppm (Figure 7).33
A very similar spectrum has been recently observed in the
characterization of polypropylene polymerized by R-diimine
nickel complexes.34 The two major signals belong to the
corresponding mr and rr tryads. A very elegant analysis of
similar type of polymers has been recently published.34 The
small signals in the region of 15 to 19 ppm and the signals
between 29 and 41 ppm corroborate the presence of 2,1 and
1,3 misinsertions. Moreover, it is possible to conclude that â-Me
elimination is the major termination mechanism, as represented
by the signals observed at 146 and 113 ppm, the signals between
22 and 26 ppm for the vinylic CH (C1), CH2 (C2), and the
terminal carbons of the isobutylene unit (C3, C4, and C5) in
Figure 7, respectively.33
The “atactic polymer”, obtained with MAO, can be rational-
ized by two plausible mechanisms besides the 2,1 insertions:
(1) formation of a “cationic” five-coordinated trigonal bypyra-
mid complex (B) or a square planar pyramid intermediate (C),
as a new active complex, with almost no stereodifferentiation
at the propylene insertion, and (2) an intramolecular epimer-
ization reaction of the growing polypropylene chain at the last-
inserted monomeric unit (Scheme 1).36,37
When precursor 9 was activated with B(C6F5)3 (molar ratio
B:Zr ) 1), under the same reaction conditions as with MAO, a
highly isotactic polypropylene was obtained (mmmm ) 98%),
contrary to the results obtained with MAO (eq 5). The activity
of this catalytic system (1.2 × 105 gPP/mol Zr‚h) is somewhat
lower than the activity of complex 9 activated with MAO. It is
important to point out that complex 9 has a C2-symmetry
(33) Resconi, L.; Piemontesi, F.; Franciscono, G.; Abis, L.; Fiorani, T. J. Am.
Chem. Soc. 1992, 114, 1025.
(34) McCord, E. F.; McLain, S. J.; Nelson, L. T. J.; Arthur, S. D.; Coughlin, E.
B.; Ittel, S. D.; Johnson, I. K.; Tempel, D.; Killian, C. M.; Brookhart, M.
Macromolecules 2001, 34, 362.
9
2186 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003

Group 4 Octahedral Benzamidinate Complexes
A R T I C L E S
Scheme 1. Proposed Mechanism for the Intramolecular Epimerization of the Growing Polypropylene Chain at the Last-Inserted Monomeric
Unit
Table 7. Data for the Polymerization of Propylene Promoted by Complex 9 Using Different Solvents, Temperature, and MAO Amounts
c
T
°C
P
atm
activitya
A × 10-5
mmmm
%
Mp
°C
entry
solvent
Al:Zr
250
Mn
Mw
MWD
1
toluene
25
9.2
1.1b
261 000
36 000
10 750
23 500
58 100
10 200
10 700
149 700
26 000
3000
440 000
86 000
26 750
42 470
82 600
18 900
26 700
271 000
51 300
9300
1.69
2.35
2.49
1.81
1.42
1.85
2.49
1.81
1.96
3.10
86d
11
90
96
98
86
90
96
98
7
142
oil
2
3
4
5
6
7
8
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
250
400
1000
250
250
25
25
25
0
25
50
25
9.2
9.2
9.2
5.1
9.2
2.2
7.5
7.9
0.5
2.2
146
147
149
138
146
152
154
oil
250
B(C6F5)3
17.0
9.2
26.6
2.8e
a gr PP/mol Zr‚h. b Mixture of isotactic (50%) and atactic (50%) fractions of polypropylene. c Measured by 13C NMR. d Isotacticity of the isotactic
fraction. e 70% atactic and 30% isotactic.
The elucidation of which of the two plausible mechanisms
is responsible for the production of an “atactic polypropylene”
was achieved by performing the reactions under pressure (liquid
propylene). For the first mechanism, none or small changes in
the stereoregularity of the polymer will be predicted, whereas
for the second mechanism, a highly isotactic polymer will be
expected.
of the monomer and the termination of the polymer chain,
allowing larger molecular weights and narrower molecular
weight distributions, as compared to the polymers obtained in
CH2Cl2.
High-Pressure Polymerization of Propylene Catalyzed by
Complex 9. When the reaction catalyzed by 9 activated with
MAO is carried out at high pressure, a highly stereoregular
polymer is formed. A summary of the polymerization results is
presented in Table 7. To acquire the best experimental condi-
tions, we have started the polymerizations by studying the effect
of the solvent. By performing the polymerization in dichlo-
romethane, larger activities, stereoregularities, and melting points
of the polymers were observed, as compared to the results
exhibited in toluene (entries 1 and 2). This increase in activity
is plausible as a consequence of the polarity of the CH2Cl2,
causing a greater charge separation between the cationic
benzamidinate alkyl complex and the MAO anion, encouraging
the insertion of the monomer, as compared to toluene.
For toluene, a putative π-bond of the ring to the cationic
center of complex (E) possibly takes place forming a cationic
η6-toluene compound.38 This intermediate inhibits the insertion
The polymerization of propylene under higher pressure in
CH2Cl2 yields an isotactic polypropylene with small amounts
of stereodefects, while in toluene a mixture of isotactic and
atactic products was obtained. The formation of two different
polypropylene fractions in toluene can testify about the presence
of two different active catalytic species. The nature of the
intermediate responsible for the formation of the atactic fraction
has not been fully elucidated yet. On the basis of NMR
experiments, it seems that, in toluene, one of the benzamidinate
ligations opens to a η1 coordination, and the solvent occupies
the vacant site. Interestingly, similar heteroallylic ligations
(alkoxysilyl-imido) have exhibited dynamic behaviors.39
Up to a molar ratio of Al:Zr ) 400, the activity of the catalytic
complex enhances with an increase in the MAO amount (entries
(35) Recently a C2-symmetry titanium complex has been shown to be active
for the polymerization of propylene forming highly syndiotactic polymer
that is formed through the chain-end mechanism. See ref 17a,b.
(36) (a) Busico, V.; Cipullo, R.; Caporaso, P.; Angeloni, G.; Segre, A. L. J.
Mol. Catal. A: Chem. 1998, 128, 53. (b) Busico, V.; Cipullo, R.; Monaco,
G.; Vacatello, M. Macromolecules 1997, 30, 6251. (c) Busico, V.; Brita,
D.; Caporaso, L.; Cipullo, R.; Vacatello, M. Macromolecules 1997, 30,
3971. (d) Busico, V.; Caporaso, L.; Cipullo, R.; Landriani, L.; Angelini,
G.; Margonelli, A.; Segre, A. L. J. Am. Chem. Soc. 1996, 118, 2105. (e)
Leclerc, M.; Brinzinger, H. H. J. Am. Chem. Soc. 1996, 118, 9024 and
references therein. (f) Busico, V.; Cipullo, R.; Corradini, P.; Landriani, L.;
Vacatello, M.; Segre, A. L. Macromolecules 1995, 28, 1887. (g) Busico,
V.; Cipullo, R. J. Am. Chem. Soc. 1994, 116, 9329.
(38) (a) Lancaster, S. J.; Robinson, O. B.; Bochmann, M.; Coles, S. J.;
Hursthouse, M. B. Organometallics 1995, 14, 2456. (b) Gillis, D. J.;
Tudoret, M. J.; Baird, M. C. J. Am. Chem. Soc. 1993, 115, 2543. (c) Eisen,
M. S.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 10358. (d) Eisen, M. S.;
Marks, T. J. Organometallics 1992, 11, 3939. (e) Solari, E.; Floriani, C.;
Chiesi-Villa, A.; Guastini, C. J. Chem. Soc., Chem. Commun. 1989, 1747.
(39) (a) Duchateau, R.; Brussee, E. A. C.; Meetsma, A.; Teuben, J. H.
Organometallics 1997, 16, 5506. (b) Duchateau, R.; Tuinstra, T.; Brussee,
E. A. C.; Meetsma, A.; van Duijnen, P. T.; Teuben, J. H. Organometallics
1997, 16, 3511.
(37) The additional “allylic mechanism” for the epimerization has been proposed,
see: Resconi, L. J. Mol. Catal. 1999, 146, 167. This mechanism was found
to be not a major pathway due to the lack of internal vinylidenic and
isobutenyl groups.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2187

A R T I C L E S
Volkis et al.
Table 8. Data for the Polymerization of Propylene Promoted by Complex 10 with MAO
T
°C
P
atm
activityb
A × 10-5
mmmm
%
type of
polymer
entry
solvent
Al:Tia
Mn
Mw
MWD
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
CH2Cl2
CH2Cl2
toluene
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
1000
1000
1000
1000
1000
1000
1000
1000
1000
300
25
25
25
1.0
9.2
1.0
9.2
1.5
3.0
4.5
7.0
17.0
9.2
9.2
9.2
9.2
9.2
9.2
9.2
0.0
1.9
0.0
1.9
0.6
0.3
0.6
1.0
1.7
0.4
0.4
0.4
0.2
0.3
2.6
3.9
55 400
51 300
113 800
93 700
1.51
1.83
18
elastomer
25
21
26
23
22
22
18
20
19
20
22
21
13
14
elastomer
solidc
elastomer
elastomer
elastomer
oil
elastomer
elastomer
elastomer
elastomer
elastomer
elastomer
elastomer
-25
-10
0
10
50
25
25
25
25
102 700
79 700
68 900
46 700
57 400
48 400
55 400
47 700
43 300
59 800
67 900
163 000
138 100
117 600
82 300
89 700
74 300
86 500
81 300
114 300
116 700
115 500
1.57
1.73
1.71
1.76
1.56
1.53
1.56
1.71
2.64
1.95
1.70
300d
300e
300f
500
25
25
25
2000
3000
a Moles of catalyst ) 7.8 × 10-6
.
b gr PP/mol Ti‚h. c Solid with no elastomeric properties and low solubility in hot trichlorobenzene although enough
to measure mmmm content. d 15.6 × 10-6 mol of catalyst. e 31.2 × 10-6 mol of catalyst. f 46.8 × 10-6 mol of catalyst.
2-4). Further augmentation of MAO concentration does not
influence the polymerization rate, but rather the properties of
the polypropylene. Likewise, by raising the Al:Zr ratio, the
melting points, molecular weights, and isotacticities of the
polymers increase with a concomitant reduction in the molecular
weight distribution (MWD). By performing the reaction at
elevated temperatures, higher activities, stereoregularities, and
molecular weights of polypropylene are achieved (entries 5, 6,
and 7). These results suggest that an increase of the temperature
induces faster insertion rates as compared to the rate of the
epimerization, resulting in higher isotacticities.
Worthy of note is that in the polymerization of propylene,
performed by complex 9 activated with B(C6F5)3, in toluene
(entry 8), two polymeric fractions (70% atactic and 30% highly
isotactic) are attained as is observed with the MAO. In CH2-
Cl2, no polymerization is observed.
Comparing the stereoregularities of the polymers obtained
at atmospheric or at high pressure, when complex 9 was
activated by MAO or B(C6F5)3, it is remarkably the substantial
effect of the counterion. When B(C6F5)3 is used at atmospheric
1-octene.23,40 Because the polymerization of 1-octene is ex-
pressure in toluene, a highly isotactic polymer is obtained, while
two polymer fractions are accountable at higher monomer
concentrations. The opposite effect is observed for MAO
allowing large isotacticities at high pressures, but only in
dichloromethane. Therefore, the stereoregularity of polypropy-
lene promoted by the system “9 + MAO” can be tailored as
presented in eq 6.
It is important to point out that 13C NMR analysis of the chain
ends of the polymers, obtained at high monomer concentration,
also shows that the â-methyl elimination is the exclusive
termination chain mechanism for these catalytic systems.
Statistically, we can deduce that at atmospheric pressure, the
epimerization is observed every small number of insertions,
forming a -mmrrmmrrmmrrmmrr- microstructure responsible
for the two major signals with similar integration, and the almost
absent signal for the mmmr pentad, as presented in Figure 7.
The lack of 2,1 and 1,3 misinsertion signals in the 13C NMR
for the polymers obtained at high pressures indicates the rapidity
of the 1,2 insertion, as compared to the misinsertions.
A further endorsement that the mechanism presented in
Scheme 1 is responsible for the epimerization of the last inserted
unit was obtained by reacting complex 9 with MAO and
tremely slow, it will be expected that if the epimerization is an
operative pathway, a â-hydrogen elimination can take place from
either the two methyl groups or the methylene group attached
to the polymer chain (D in Scheme 1). In the first route, no
changes in the alkene are expected, but in the second pathway,
an isomerization of the double bond will be observed. The fact
that 1-octene was isomerized to a mixture of octenes displays
strong evidence for the formation of the intermediate complex
D in Scheme 1.
Polymerization of Propylene Promoted by the Titanium
Benzamidinate Complex 10. When the polymerization of
propylene is performed using the titanium complex 10, in most
cases, an elastomeric polypropylene is formed with properties
depending on the polymerization conditions. Results for the
polymerization of propylene catalyzed by complex 10 activated
with MAO are presented in Table 8.
Table 8 shows that, contrary to complex 9, complex 10 is
not active in the polymerization of propylene at atmospheric
pressure either in CH2Cl2 or in toluene (entries 1 and 3). The
solvent-metal interaction seems to be much stronger in titanium
(40) Averbuj, C.; Eisen, M. S. J. Am. Chem. Soc. 1999, 121, 8755.
9
2188 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003

Group 4 Octahedral Benzamidinate Complexes
A R T I C L E S
The 13C NMR spectrum (Figure 8) of the polymers shows
that at 50 °C, large signals are present in the region between
32 and 45 ppm. These signals belong to the 2,1 and 1,3
misinsertions of propylene forming ethyl and butyl domains at
the polymer chain.2f,34,42a The increase of misinsertions observed
with the elevation of the temperature accounts for the formation
of the various forms of polypropylene (isotactic, elastomeric,
and oily).
It is important to point out the effect of the amount of MAO.
The higher the MAO amount is, the better the polymerization
activity is, although beyond the Al:Ti ratio of 300 a plateau is
reached, regarding the molecular weight of the polymers (entries
10, 14-16). The effect of the amount of the catalyst was
measured by keeping the Al:Ti ratio constant, but increasing
the amount of the catalyst (entries 10-13). Hence, similar
activities are always observed regardless of the higher concen-
trations of the catalyst, presumably due to the formation of
inactive dimeric species.
The different types of polymers that are achieved under
pressure at room temperature by either complexes 9 (highly
isotactic) or 10 (highly epimerized and a large number of 2,1
and 1,3 insertions) can be rationalized due to the R-agostic
interactions for both complexes. The strong interactions in
complex 9 inhibit the â-hydrogen elimination (less epimeriza-
tions), whereas for the titanium complex 10, this interaction
has been shown to be much weaker.41b This effect also accounts
for the absence of 2,1 misinsertions promoted with complex 9.
The strong interaction seems to be responsible for the occupation
of the active coordination site by the hydrogen preventing the
2,1 misinsertion.
Influence of Substituents at the Benzamidinate Ligands
on the Catalytic Activity of Complexes 11 and 12. A
conceptual question regarding the catalytic activity of the
benzamidinate complexes involves the effect of different groups
at the aromatic ring. To shed some light, we have prepared and
studied complexes 11 and 12, which are isolobal to complexes
9 and 10, respectively. In the former complexes, a hydrogen
atom is at the para-position of the aromatic rings. The
polymerization results obtained for the unsubstituted benza-
midinate complexes (11, 12) under similar reaction conditions
are presented in Table 9.
Comparison of the activities for the zirconium complexes 9
and 11 (Tables 7 and 9) shows that for the unsubstituted complex
11, the activity in toluene is larger than that of complex 9 in
either toluene or CH2Cl2. Larger activity is also observed for
the unsubstituted titanium complex 12 as compared to complex
10 (Tables 8 and 9).
The polypropylene obtained with complex 11 is characterized
by a high molecular weight (Mw of ∼1.9 × 106). The molecular
weights of the polymers obtained with complex 12 in CH2Cl2
are much lower than those obtained by complex 10, whereas in
toluene the contrary is observed (compare entries 2, 4, and 14
in Table 8 with entries 8, 5, and 7 of Table 9, respectively). A
plausible elucidation for the increase in activity of the unsub-
stituted complexes (11 and 12) in the polymerization of
propylene, as compared to the complexes with the substituted
benzamidinate ligands (9 and 10), relates to the difference in
the electronic effects of both ligations on the cationic metal
Figure 8. 13C NMR spectra of polypropylene obtained at different
temperatures by complex 10 activated with MAO. The temperature of the
polymerization is indicated at each spectrum.
than in zirconium because of the effective charge at the metal
center, indicating that at low pressure for 10 both solvents inhibit
almost completely the monomer insertion. At high monomer
concentration, 10 activated by MAO is an active precursor in
both solvents (entries 2 and 4). At -25 °C, a solid polypropylene
is formed. This polymer contains almost no 2,1 or 1,3 misin-
sertions (Figure 8a) and is almost immiscible in hot trichlo-
robenzene. The steady augmentation of the temperature up to
25 °C leads to a continuous increase of the polymerization rate,
an increase in the number of the 2,1 and 1,3 misinsertions, and
a decrease in the polymer molecular weight (entries 5-8 and
Figure 8b-e), with no major change in the mmmm content. In
this range of temperatures (-10-25 °C), elastomeric polymers
are formed.41 At 50 °C, a small change in the activity is
observed, while the molecular weight of the polymer continues
to decrease producing a viscous oil (entry 9) (Figure 8f).
(41) For elastomeric polymerization using metallocenes, see: (a) Tagge, C. D.;
Kravchenko, R. L.; Lal, T. K.; Waymouth, R. M. Organometallics 1999,
18, 380 and references therein. (b) Lin, S.; Hauptman, E.; Lal, T. K.;
Waymouth, R. M.; Quan, R. W.; Ernst, A. B. J. Mol. Catal. A: Chem.
1998, 136, 23 and references therein. (c) Maciejewski Petoff, J. L.; Agoston,
T.; Lal, T. K.; Waymouth, R. M. J. Am. Chem. Soc. 1998, 120, 11316 and
references therein. (d) Kravchenko, R.; Masood, A.; Waymouth, R. M.;
Myers, C. I. J. Am. Chem. Soc. 1998, 120, 2039. (e) Hu, Y.; Krejchi, M.
T.; Shah, C. D.; Myers, C. L.; Waymouth, R. M. Macromolecules 1998,
31, 6908. (f) Carlson, E. D.; Krejchi, M. T.; Shah, C. D.; Terakawa, T.;
Waymouth, R. M.; Fuller, G. G. Macromolecules 1998, 31, 5343. (g) Bruce,
M. D.; Waymouth, R. M. Macromolecules 1998, 31, 2707. (h) Maciejewski
Petoff, J. L.; Bruce, M. D.; Waymouth, R. M.; Masood, A.; Lal, T. K.;
Quan, R. W.; Behrend, S. J. Organometallics 1997, 16, 5909. (i) Bruce,
M. D.; Coates, G. W.; Hauptman, E.; Waymouth, R. M.; Ziller, J. W. J.
Am. Chem. Soc. 1997, 119, 11174. (j) Coates, G. W.; Waymouth, R. M.
Science 1995, 267, 217. (k) Llinas, G. H.; Dong, S. H.; Mallin, D. T.;
Rausch, M. D.; Lin, Y. G.; Winter, H. H.; Chien, J. C. W. Macromolecules
1992, 25, 1242. (l) For recent formation of elastic polypropylene with
metallocenes due to a large number of stereoregular insertions “mistakes”,
see: Dietrich, U.; Hackmann, M.; Rieger, B.; Klinga, M.; Laskela¨, M. J.
Am. Chem. Soc. 1999, 121, 4348. (m) For a recent publication regarding
η3 f η1 dynamic coordination and the effect in the polymerization of
propylene, see: ref 19, Plat, D. The ReactiVity of Group IV Aminophosphine
Complexes; Ph.D. Thesis; Technion, Israel, 2000 and Shaviv, E.; Sc, M.
Heteroallylic Early Transition Metal Complexes; Thesis; Technion, Israel,
2000. (n) For elastomeric polypropylene obtained by other group 4
complexes, see refs 18, 19, and 31.
(42) (a) NMR of Polymers; Bovey, F. A., Miray, P. A., Eds.; Academic Press:
San Diego, CA, 1996. (b) Margl, P.; Deng, L.; Ziegler, T. Organometallics
1998, 17, 933.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2189

A R T I C L E S
Volkis et al.
Table 9. Data for the Polymerization of Propylene Promoted by Benzamidinate Complexes 11 and 12a
c
activityb
mmmm
%
type of
polymer
entry
cat.
solvent
Al:M
Ax10-5
Mn
Mw
3100
611 600
1 982 000
169 900
310 100
211 700
35 100
MWD
1
2
3
4
5
6
7
8
11
11
11
12
12
12
12
12
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
CH2Cl2
500
1000
2000
500
1000
2000
500
2.2
12.1
9.3
1600
380 200
866 100
92 400
125 000
137 000
12 000
1.96
1.61
1.2
1.84
2.48
1.54
2.91
2.70
15
20
19
12
17
27
9
elastomer
elastomer
solid
elastomer
elastomer
elastomer
elastomer
elastomer
2.2
28.5
14.0
1.8
1000
9.9
10 200
27 500
11
a 25 °C, 9.2 atm. b gr PP/mol M‚h. c Measured by 13C NMR.
Table 10. Crystal Data and Refinement Details for Complexes 2, 4, 9, 10, 10*/13 and 15
complex
2
4
9
10
10*/13
15
empirical formula
formula weight
temperature
wavelength
crystal system
C40H56Li2N6Si4
747.15
230(2) K
0.7107 Å
monoclinic
P21/n
C19H39LiN4Si2
386.66
230(2) K
0.71070 Å
monoclinic
P21/c
C30H56N4Si4Zr
676.37
210(2) K
0.71073 Å
triclinic
C15H28N2Si2Ti0.50 C14.78H24.97N2Si2Ti0.50 C12H10.50N1.50
316.52
310.85
175.72
230.0(1) K
0.71069 Å
monoclinic
I2/a
200.0(1) K
0.71069 Å
monoclinic
I2/a
220.0(1) K
0.71070 Å
orthorhombic
Pmn21
space group
unit cell dimensions
P1h
a ) 11.4006(2) Å a ) 10.6540(4) Å a ) 11.1578(8) Å
a ) 20.1172(5) Å a ) 20.039(10) Å
a ) 21.630(4) Å
R ) 90°
b ) 24.2914(4) Å b ) 13.4360(6) Å b ) 11.8659(9) Å
â ) 98.75 (6)°
c ) 16.7460(3) Å c ) 18.1020(8) Å c ) 15.8740(11) Å c ) 21.9639(5) Å c ) 21.829(10) Å
R ) 90°
R ) 100.1940(10)° R ) 90°
R ) 90°
R ) 90°
b ) 8.7559(2) Å
b ) 8.678(4) Å
b ) 9.458(2) Å
â ) 90°
â ) 91.6800(18)° â ) 106.3370(10)° â ) 95.2520(10)° â ) 94.72(5)°
c ) 4.5734(9) Å
γ ) 90°
γ ) 90°
γ ) 90°
γ ) 98.7370(10)°
1939.2(2) Å3
2, 1.158 Mg/m3
0.430 mm-1
720
γ ) 90°
γ ) 90°
volume
4583.60(14) Å3
4, 1.083 Mg/m3
0.162 mm-1
1600
2590.14(19) Å3
4, 0.992 Mg/m3
0.146 mm-1
848
3852.57(16) Å3
8, 1.091 Mg/m3
0.369 mm-1
1368
3783(3) Å3
8, 1.092 Mg/m3
0.375 mm-1
1333
935.5(7) Å3
4, 1.248 Mg/m3
0.074 mm-1
372
Z, calculated density
absorption coefficient
F(000)
crystal size
0.15 × 0.3 ×
0.15 × 0.20 ×
0.50 × 0.40 ×
0.04 × 0.18 ×
0.15 × 0.23 ×
0.18 × 0.04 ×
0.5 mm
0.30 mm
0.40 mm
0.37 mm
0.31 mm
0.01 mm
θ range for data
collection
1.49-25.50°
1.89-25.03°
1.37-26.40°
2.51-25.05°
2.04-25.04°
2.15-30.50°
limiting indices
0 g h g 13,
0 g k g 28,
-19 g l g 19
8089/8089
0 g h g 12,
0 g k g 15,
-21 g l g 21
4558/4558
-13 g h g 13,
-10 g k g 14,
-19 g l g 19
10 962/7543
-23 g h g 18,
0 g k g 10,
-26 g l g 17
3406/3406
[R(int) ) 0.0000] [R(int) ) 0.0428]
99.7%
-23 g h g 18,
0 g k g 10,
0 g l g 26
0 g h g 29,
0 g k g 13,
-52 g l g 6
10 623/2438
[R(int) ) 0.0760]
91.5%
reflections
3432/3344
collected/unique
completeness to
highest θ
[R(int) ) 0.0000] [R(int) ) 0.0000] [R(int) ) 0.0604]
99.7%
99.6%
99.6%
100.0%
refinement method
full-matrix least-
squares on F 2
full-matrix least-
squares on F 2
4558/0/308
full-matrix least-
squares on F 2
7539/0/426
full-matrix least-
squares on F 2
3406/0/199
full-matrix least-
squares on F 2
3344/0/222
full-matrix least-
squares on F 2
2438/0/155
data/restraints/parameters 8089/0/693
goodness-of-fit on F 2
1.034
0.989
1.028
1.099
1.044
1.096
final R indices [I > 2σ(I)] R1 ) 0.0444,
wR2 ) 0.1023
R1 ) 0.0648,
wR2 ) 0.1475
R1 ) 0.1485,
wR2 ) 0.1715
0.276 and
R1 ) 0.0608,
wR2 ) 0.1413
R1 ) 0.1038,
wR2 ) 0.1892
0.458 and
R1 ) 0.494,
wR2 ) 0.1361
R1 ) 0.0711,
wR2 ) 0.1450
0.352 and
R1 ) 0.0927,
wR2 ) 0.1697
R1 ) 0.1758,
wR2 ) 0.1919
0.455 and
R1 ) 0.0657,
wR2 ) 0.1270
R1 ) 0.2407,
wR2 ) 0.1980
0.195 and
R indices (all data)
R1 ) 0.0757,
wR2 ) 0.1115
largest diff. peak and hole 0.175 and
-0.234 e Å-3
-0.222 e Å-3
-0.547 e Å-3
-0.224 e Å-3
-0.230 e Å-3
-0.174 e Å-3
center. The induction Taft constants (σ*), which quantitatively
characterize the induction effect of an aliphatic substituent in
an aromatic hydrocarbons, are +0.49 for the H atom and 0.00
for the CH3 group.43 Hence, the methyl group has a more
negative inductive effect, that is, greater electron-donor proper-
ties, than the hydrogen atom, increasing the electron density at
the phenyl ring and decreasing the positive charge at the metal
center. The reduction of the cationic character at the metal center
induces a weaker bond between an incoming molecule of
propylene, reducing the polymerization activity of the complex.
The augmentation of the electron density at the metal center
in complex 9 inhibits not only the polymerization rate, but also
the rate of the epimerization reaction (slowing down the
â-hydrogen elimination), resulting in larger domains of the
isotactic pentads, and therefore highly isotactic polymers. For
the titanium complex 10, besides the inhibition rate, no restrain
takes place for the epimerization reaction, allowing the produc-
tion of elastomeric polymers.
Structure of the Elastomeric Polypropylenes. As already
mentioned above, in the polymerization of propylene at higher
monomer concentrations using the complexes 10, 11, and 12,
an elastomeric polypropylene was formed. It is essential and
fundamental to compare the chemical structures of the obtained
elastomers with the structure of the elastomeric polypropylenes,
described in the literature.
(43) (a) Taft, R. W. J. Phys. Chem. 1960, 64, 1805. (b) March, J. AdVanced
Three types of elastomeric polypropylene are known: (i) a
large molecular weight atactic polymer,44 (ii) polymers with
Organic Chemistry, 3rd ed.; Wiley-Interscience: New York, 1985; Chapter
9.
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Group 4 Octahedral Benzamidinate Complexes
A R T I C L E S
alternating isotactic and atactic blocks,40a-k and (iii) polymers
obtained by the “dual-side” mechanism.40l To elucidate the
structure of the obtained elastomeric polypropylene, its 13C
NMR spectra was compared with the spectrum of the “atactic
oil” and the isotactic polypropylenes, prepared by the zirconium
complex 9. On the basis of the NMR data, for all of the samples,
the statistical lengths of the isotactic blocks between two
neighboring epimerization stereodefects were calculated. Ac-
cording to these calculations, the length of an isotactic block
before a stereodefect was found to be between 35 and 45 CH3
groups for an isotactic polymer (mmmm ≈ 90%) and between
7 and 8 CH3 groups for the atactic sample (mmmm ≈ 7%). For
the elastomeric polymers obtained with complex 10, the length
of the isotactic block was calculated to be between 11 and 27
CH3 groups. Thus, for the elastomeric polypropylene, the length
of the statistical isotactic fragment between two stereodefects
was found to be shorter than that obtained for an isotactic
polymer and longer than that observed for an atactic sample.
Therefore, it is plausible to assume that the elastomeric
polypropylene prepared with these complexes differs, in prin-
ciple, from the other three above-mentioned types of elastomers.
Thus, contrary to the high molecular weight atactic elastomer,43
our samples have an isotactic structure, in which longer isotactic
domains are disposed between stereodefects. Simultaneously,
different from the alternating isotactic-atactic block elas-
tomers,40 where the alternating isotactic and atactic domains
have commensurate lengths, our products are characterized by
frequent alternation of the isotactic domains with many stereo-
defects. As compared to the polymers obtained through the
“dual-side” mechanism, which exhibit large mmmm contents
(up to 72%),40l our polymers reside with lower stereoregularities.
In the “dual-site” mechanism, each site has a different symmetry,
and, by the back-skipping of the polymeric chain, two stere-
ochemistries for the insertions are possible. In our complexes,
a back-skipping of the polymer chain will induce the same
symmetry, thus inducing the same type of stereoregular inser-
tion. The stereoregular errors are formed because of two
competing reactions, the insertion of propylene and the intramo-
lecular epimerization of the growing chain at the last inserted
unit. The rate correlation of these two reactions influences the
amount and length of the isotactic domains between the
stereoerrors.
We have utilized atomic force microscopy (AFM) to dis-
criminate between the crystalline and the amorphous areas in
the homogeneous polymer. The use of AFM to characterize the
morphology polymer samples on the nanometer scale is well
documented in the literature.45 Topography, phase imaging, and
lateral force measurements are efficient tools to examine
polymer structure and properties on the nanometer scale. Here,
topographic and phase imaging techniques were used to
Figure 9. AFM pictures of different polypropylenes. (a) Isotactic polymer
from Table 7, entry 4. (b) Elastomeric polymer from Table 8, entry 2. (c)
Atactic polymer fraction from Table 7, entry 8.
characterize the morphology of the different types of polypro-
pylene samples.
(44) Resconi, L.; Jones, R. L.; Rheingold, A. L.; Yap, G. P. A. Organometallics
1996, 15, 998.
Samples of the different polymers were spin-coated on glass,
and then left to dry at ambient temperature and atmosphere.
Figure 9a-c depicts typical phase images of isotactic, elasto-
meric, and atactic polypropylene samples, respectively. Topo-
graphic images taken simultaneously fail to resolve any
significant detail within the entire scan range, an indication that
the differences in phase shift originate from differences in tip-
surface interactions and not from topographical differences. The
sample of the isotactic polymer consists of a mosaic of
crystalline islands (brighter spots in the image) embedded in
(45) (a) Haeringen, D. T. V.; Varga, J.; Ehrenstein, G. W.; Vancso, G. J. J.
Polym. Sci., Part B: Polym. Phys. 2000, 38, 672. (b) Ivanov, D. A.; Nysten,
B.; Jonas, A. M. Polymer 1999, 40, 5899. (c) Xu, K.; Gusev, A. I.; Hercules,
D. M. Surf. Interface Anal. 1999, 27, 659. (d) Orefice, R. L.; Brennan, A.
Mater. Res. 1998, 1, 19. (e) Pfau, A.; Janke, A.; Heckmann, W. Surf.
Interface Anal. 1999, 27, 410. (f) Tomasetti, E.; Legras, R.; Nysten, B.
Nanotechnology 1998, 9, 305. (g) Albrecht, T. R.; Dovek, M. M.; Lang,
C. A.; Gru¨tter, P.; Quate, C. F.; Kuan, S. N. J.; Frank, C. W.; Pease, R. F.
W. J. Appl. Phys. 1988, 64, 1178. (h) Landman, U.; Luedke, W. D.; Nitzan,
A. Surf. Sci. 1989, 10, L177. (i) Wawkuschewski, A.; Cantow, H.-J.;
Magonov, S. N.; Sheiko, S.; Mo¨ller, M. Polym. Bull. 1993, 31, 693. (j)
Wawkuschewski, A.; Cantow, H.-J.; Magonov, S. N. AdV. Mater. 1994, 6,
476.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2191

A R T I C L E S
Volkis et al.
Figure 10. 13C NMR spectrum of complex 9 (top) and the corresponding
cationic complex obtained by the stoichiometry reaction with the cocatalyst
B(C6F5)3 (bottom).
an amorphous phase (darker areas) of the same material. The
elastomeric polymer is composed of long interpenetrated and
branched fibrils of crystalline character that interconnect
amorphous regions. This phase image provides further support
to the concept of microheterogeneous crystallinity of the
elastomeric phase, caused by different crystalline domains that
are characterized by a slightly different melting point. This set
of heterogeneous crystalline phases is claimed to yield a broad
and undefined melting zone. The atactic polymer is predomi-
nantly amorphous with a low density of crystalline domains.
Activation of the Benzamidinate Complexes by the Dif-
ferent Cocatalysts. The polymerization of propylene promoted
by the benzamidinate complexes was carried out using either
MAO or B(C6F5)3 as cocatalysts. To shed some light on the
structure of the active cationic complexes, we have followed
the changes of the benzamidinate complexes when reacted with
Figure 11. 19F NMR spectra of complex 9 with B(C6F5)3 at room
temperature (a) and after heating (b).
11a), the spectra show two sets of signals. The signals marked
1, 2, and 3 belong to the formed cationic complex, whereas the
other three signals belong to an intermediate unsolved structure.
By heating and cooling the mixture, a reaction is observed
achieving a quantitative conversion toward the cationic complex
(Figure 11b). The chemical shift difference between the m- and
p-19F of the anion CH3B(C6F5)3 can also be indicative of the
anion coordination.47
Hence, values of ∆δ (m,p-F) between 3 and 6 ppm indicate
coordination, whereas values below 3 ppm indicate noncoor-
dination. For the Zr complex, a difference of ∆δ (m,p-F) )
2.28 ppm is observed, indicative for a noncoordinating ion pair,
corroborating the results obtained in the polymerization of
propylene by complex 9 in CH2Cl2. The 19F NMR spectra of
complex 10 with an equimolar amount of the cocatalyst B(C6F5)3
show the immediate formation of at least two inactive com-
pounds (six new signals besides the starting cocatalyst) with
no resemblance in the chemical shifts to other active or
nonactive B(C6F5)3-containing metallocene complexes.46
the B(C6F5)3 cocatalyst under olefin starving conditions, by 13
and 19F NMR spectroscopy.
C
The 13C NMR spectra of complex 9 and its mixture with an
equivalent amount of B(C6F5)3 in BrC6D5 are shown in Figure
10. The signal of the Zr-CH3 (43 ppm) moves downfield (62.3
ppm) upon the reaction with the cocatalyst as expected for the
formation of a cationic metal-alkyl group.4,46 A larger shift was
observed for the corresponding titanium complex 10, showing
a displacement from 70 to 110 ppm in its cationic form.
It is important to point out that the spectrum presented in
Figure 10b for the cationic complex was obtained after heating
the reaction mixture to 90 °C for 2 h to achieve a complete
conversion. For complex 10, the shift of the methyl group is
obtained regardless of any heating. Despite the similar character
of the active species formed under activation of the complexes
with B(C6F5)3, they behave totally different in the polymerization
of propylene. Thus, the activity of the cationic complex 9 at
high pressure is rather small at room temperature, but it increases
if it undergoes a preliminary heating stage and recooling to the
reaction temperature. On the contrary, the precursor 10 activated
with B(C6F5)3 is inactive in the polymerization, regardless of
any heating.
Conclusions
Several early-transition octahedral metal complexes contain-
ing the chelating benzamidinate ligation were prepared. These
complexes exhibit a C2-symmetry as displayed in their X-ray
diffraction studies. The reaction of the compounds with a
cocatalyst produces the corresponding cationic complexes that
are active catalysts for the polymerization of propylene.
Depending on the nature of the complexes, either highly
stereoregular or elastomeric polymers can be obtained. More-
over, the type of polymer can be modulated by the pressure of
the monomer. The elastomeric polymers are characterized by
an alternation of short isotactic domains with stereodefects that
are encountered by the epimerization of the growing polymeric
chain at the last monomeric insertion. The use of changing
The 19F NMR spectra of complex 9 with an equimolar amount
of the cocatalyst, B(C6F5)3, before and after heating, are shown
in Figure 11a and b, respectively. At room temperature (Figure
(46) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
10015. (b) Deck, P. A.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 6128.
(c) Wu, Z.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1995, 117,
5867. (d) Erker, G.; Ahlers, W.; Frohlich, R. J. Am. Chem. Soc. 1995,
117, 5853. (e) Bochmann, M.; Lancaster, S. J.; Robinson, B. O. J. Chem.
Soc., Chem. Commun. 1995, 2081.
(47) (a) Bochmann, M.; Green, M. L. H.; Powell, A. K.; Saâmannshausen, J.;
Triller, M. U.; Wocadlo, S. J. Chem. Soc., Dalton Trans. 1999, 43. (b)
Horton, A. D.; de With, J.; Linden, J. v. d.; Weg, H. v. d. Organometallics
1996, 15, 2672.
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Group 4 Octahedral Benzamidinate Complexes
A R T I C L E S
monochromatized Mo KR radiation. The crystals were placed into a
Parathon-N oil in a glovebox. Single crystals were mounted on the
diffractometer under a stream of cold N2 at 220 K. Data collection and
reduction and cell refinements were carried out with the Nonius software
package.52 The structure solution was carried out by the SHELXS 9753
and SHELXSL 9754 software packages, respectively. The ORTEP
program incorporated in the TEXRAY structure analysis package was
used for molecular graphics.55 Relevant crystallographic information
is summarized in Table 10.
pressure to obtain a different block homo-copolymer is under
investigation and will be reported shortly.
Experimental Section
General Procedure. All manipulations were carried out with the
exclusion of oxygen and moisture using flamed Schlenk-type glassware
and high vacuum (10-5 Torr) techniques. For storage of air-sensitive
materials, a nitrogen-filled “Vacuum Atmospheres” glovebox with a
medium capacity recirculator (1-2 ppm O2) was used. The gases (argon,
nitrogen, propylene) were purified by passage through a MnO oxygen
removal column and a Davison 4 Å molecular sieves column.
Analytically pure solvents were distilled under argon from potassium
benzophenone ketyl (tetrahydrofuran), Na/K alloy (diethyl ether,
hexane), Na (toluene), or P2O5 (dichloromethane). All solvents for
vacuum line manipulations were stored in a vacuum over Na/K alloy.
Nitrile compounds (Aldrich) and TMEDA were degassed and freshly
distilled under argon. Methylalumoxane cocatalyst (Witco) was prepared
from a 30% suspension in toluene by vacuum evaporation of the solvent
at 25 °C/10-5 Torr. Tris(pentafluorophenyl)boron cocatalyst was
prepared according to the literature.48 HfCl4‚2THF, ZrCl4, and TiCl4
were purchased and used as received (Aldrich). NMR spectra were
recorded on Bruker AM-200 and AM-400 spectrometers. Chemical
Synthesis of p-CH3C6H4C(NSiMe3)2Li‚TMEDA (3). To a well
stirred solution of 5.42 g (32.5 mmol) of LiN(TMS)2 in 50 mL of
hexane at 0 °C was added 3.93 g (33.6 mol) of 4-methylbenzonitrile
by a syringe in an argon flow. The mixture was stirred at room
temperature for 3 h, and then 5.0 g (42.0 mmol) of TMEDA was added.
The red-brown solution formed was stirred for an additional 3 h. After
the reaction mixture was cooled overnight at -50 °C, a crystalline
material precipitated that was cold filtered and dried under high vacuum
to obtained 11.6 g (86%) of red-brown crystals of 3: mp 112 °C (dec).
1H NMR (C6D6, 200.13 MHz): δ ) 7.22 (d, 3J ) 8 Hz, 2H, Ph), 6.92
3
(d, J ) 8 Hz, 2H, Ph), 2.1 (s, 3H, CH3-Ph), 1.98 (s, 12H, CH3-N),
1.74 (s, 4H, CH2-N), 0.08 (s, 18H, TMS). 13C NMR (C6D6, 50 MHz):
δ ) 182.0 (N-C-N), 138.9, 137.2, 128.9, 128.1, 126.8 (Ph), 56.5
(CH2-N), 45.6 (CH3-N), 21.4 (CH3-Ph), 3.0 (TMS).
1
shifts for H NMR and 13C NMR were referenced to internal solvent
Synthesis of Ligand C6H5C(NSiMe3)2Li‚TMEDA (4). Complex 4
resonances and reported relative to SiMe4. The polypropylene NMR
experiments were conducted in 1,2,4-trichlorobenzene at 130 °C with
20% DMSO-d6. The NMR experiments of the benzamidinate complexes
were conducted on Teflon valve-sealed tubes after vacuum transfer of
the solvent in a high vacuum line. 19F NMR experiments were
performed in toluene, and the chemical shifts were reported relative to
CCl3F. Complexes 1,25 2,25 5,23 6,23 7,21b,22d,i 8,21b,22d,i 11,21b,22d and
1221b,22d were prepared as described in the literature.
Melting points of isotactic polypropylene were determined on a
differential scanning calorimeter (DSC). Three runs (first heating,
cooling, second heating) were performed at a heating rate of 10 °C/
min in the temperature range 30-190 °C. The second-heating melting
peak temperature was taken as a melting point.
Molecular weights of polymers were determined either by viscosi-
metric measurements49 or by the GPC method. The latter was performed
on a Waters-Alliance 2000 instrument using 1,2,4-trichlorobenzene
(HPLC) as the mobile phase at 150 °C.
AFM measurements were performed on a DI3100 AFM machine
(Veeco) equipped with an acoustic shield and vibration isolation table
(TMC) using Si tips.
For complexes 9 and 13, X-ray diffraction experiments were carried
out with a Philips PW1100/20 four-circle diffractometer. Because of
the extreme sensitivity of all synthesized complexes to oxygen and
moisture, a suitable crystal for X-ray analysis was placed inside a
glovebox in dry and degassed Parathon oil (Du-Pont). It was then
mounted on a diffractometer, where it met a cold stream of nitrogen at
200 K. Reflections were collected, and intensities were corrected in
the usual way except for absorption. The structure was solved by direct
methods,50 and refinement, based on F, was made by block-diagonal
least-squares.51 All non-hydrogen atoms were refined anisotropically
in two blocks. Hydrogen atoms were included at calculated positions
and refined isotropically in one block. The d(C-H) used was 1.08 Å.
Measurement conditions and structure refinement are summarized in
Table 10.
was prepared according to a modified literature procedure.24
A
suspension solution of 6.5 g (38.9 mmol) of LiN(TMS)2 in 100 mL of
diethyl ether was reacted for 1 h with 4.62 g (39.8 mmol) of TMEDA.
The reaction mixture was cooled to -10 °C, and 4.01 g (38.9 mol) of
benzonitrile was added via a syringe under an argon flow. The reaction
was stirred for 10 h at room temperature, and a crystalline product
began to precipitate. Slow cooling of the reaction mixture to -40 °C
overnight induced the precipitation of light yellow crystals, which were
cold filtered and vacuum-dried to obtain 13.2 g (88%) of ligand 4. For
spectroscopic characterization, see ref 25.
Synthesis of [4-CH3-C6H4C(NTMS)2]2ZrMe2 (9). 1.03 g (1.4
mmol) of complex 5 was dissolved in 50 mL of diethyl ether, and
2.16 mL of a 1.3 M solution of methyllithium in hexane (2.8 mmol)
was added dropwise at -78 °C. The solvents were completely removed
in a vacuum, the residue was redissolved in hexane (30 mL), and the
resulting solution was filtered through a thin layer of dry Celite filter-
aid to remove insoluble particles. After the solution was cooled
overnight at -35 °C, pale yellow crystals were cold filtered and
vacuum-dried to obtain 0.95 g (98%) of complex 9. Anal. Calcd for
C30H56N4Si4Zr (676.36): C, 53.3; H, 8.4; N, 8.3. Found: C, 52.8; H,
1
3
8.2; N, 7.6. mp 135 °C (dec). H NMR (C6D6): δ ) 7.16 (d, J ) 8
Hz, 2H, Ph), 7.12 (d, 3J ) 8 Hz, 2H, Ph), 1.99 (s, 3H, CH3-Ph), 1.04
(s, 3H, CH3-Zr), 0.17 (s, 18H, TMS). 13C NMR (C6D6): δ ) 184
(N-C-N), 138.8 (ipso carbon), 137.3, 130.0, 126.1 (Ph), 44.7 (CH3-
Zr), 21.2 (CH3-Ph), 2.3 (TMS).
Synthesis of [4-CH3-C6H4C(NTMS)2]2TiMe2 (10). In a manner
similar to that described for the preparation of complex 9, 1.68 g (2.5
mmol) of complex 6 was dissolved in 50 mL of diethyl ether and was
reacted with 3.85 mL of 1.3 M (5.0 mmol) methyllithium solution in
hexane. The reaction mixture was stirred for 8 h, and the solvents were
vacuum evaporated. Next 30 mL of hexane was vacuum transferred to
the residue, and LiCl was removed by filtration. Cooling the solution
overnight at -40 °C resulted in the formation of brown crystals, which
were cold filtered and vacuum-dried, producing 1.45 g (92%) of
For compounds, 2, 4, 10, and 15, X-ray crystallographic experiments
were carried out on a Nonius-Kappa CCD diffractometer with graphite-
(52) Nonius 1997, Kappa CCD Collect Program for data collection and HKL,
Schalepack and Denzo (Otwinowski & Minor, 1997) software package for
data reduction and cell refinement.
(53) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(54) Sheldrick, G. M. SHELXL97, Program for the refinement of crystal
structures; University of Go¨ttingen, Germany, 1997.
(55) Molecular Structure Corporation 1999, ORTEP, TEXRAY Structure
Analysis Package, MSC, 3200, Research Forest Drive, The Woodlands,
TX 77381.
(48) Massey, A. G.; Park, A. J.; Stone, F. G. A. J. Organomet. Chem. 1964, 2,
245.
(49) Allcock, H. R.; Lampe, F. W. Contemporary Polymer Chemistry, 2nd ed.;
Prentice Hall: NJ, 1990; Chapter 15.
(50) Sheldrick, G. M. SHELXS 86, Program for the solution of crystal
structures; University of Go¨ttingen, 1986.
(51) Sheldrick, G. M. SHELX 76, Program for crystal structure refinement;
Chemical Laboratory, Cambridge, 1976
9
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A R T I C L E S
Volkis et al.
complex 10. Anal. Calcd for C30H56N4Si4Ti (632): C, 56.96; H, 8.86;
N, 8.86. Found: C, 55.23; H, 8.08; N, 9.43. mp 98 °C (dec). 1H NMR
(C6D6, 200.13 MHz): δ ) 7.25 (d, J ) 7 Hz, 2H, Ph), 6.85 (d, J ) 7
Hz, 2H, Ph), 2.0 (s, 6H, CH3-Ph), 1.90 (s, 6H, CH3-Ti), 0.15 (s, 18H,
TMS). 13C NMR (C6D6, 50 MHz): δ ) 189 (N-C-N), 139.1, 136.8,
128.6, 126.6 (Ph), 70.2 (CH3-Ti), 21.6 (CH3-Ph), 1.9 (TMS).
Synthesis of [4-CH3-C6H4C(NTMS)2]2HfCl2 (14). In a glovebox
was charged a swivel frit with 0.573 g (1.6 mmol) of the dimeric ligand
1 prepared in diethyl ether25 without TMEDA, and 1.49 g (3.2 mmol)
of HfCl4‚2THF. Next 50 mL of toluene was vacuum transferred, and
the reaction mixture was stirred for 16 h at room temperature. All
volatiles were vacuum evaporated, and 40 mL of toluene was vacuum
transferred to allow the separation of LiCl. The inorganic salt was
filtered, and toluene was vacuum evaporated to obtain a yellow oily
material. The oil was washed several times with diethyl ether, and
evaporation of the solvent allowed the formation of 0.22 g (34%) of a
yellow solid. 1H and 13C NMR analysis of the solid indicate the
formation of complex 14, although elemental analysis shows that
additional materials are present, which are silent in NMR; see text
bath, and the stirring began. The pressure in the reactor was measured
by means of a manometer. After being stirred for a certain period, the
mixture was quenched by exhausting the unreacted propylene, followed
by the fast introduction of 40 mL of CH3OH/HCl (1:1). Polypropylene
was filtered, washed with water until neutral solution was obtained,
washed with acetone, and dried under vacuum. The organic fraction
from the filtrate solution was separated, dried, and the solvent was
evaporated. In most cases, this solution shows no atactic fraction but
when appreciated the amount of it was inserted in the corresponding
table.
Purification of Polypropylene from Aluminum Salts. Quenching
the polymerization reactions, in which MAO was used as a cocatalyst,
by MeOH:HCl produces aluminum salts that are absorbed into the
polymer. These inorganic salts are difficult to remove by just washing
the polymer. We have been able to clean most of the salts by dissolving
the polymer in a hot solution of either 1,2,4-trichlorobenzene, Decaline,
or p-xylene (120 °C). The hot solution is filtered, and the filtrate is
poured into cold acetone to reprecipitate the polypropylene. The latter
was then washed with acetone in the Soxhlet apparatus to remove any
traces of high boiling point solvents, followed by drying under vacuum.
1
above. H NMR (CDCl3, 200.13 MHz): δ ) 7.2-7.05 (m, 4H, Ph),
2.3 (s, 3H, CH3-Ph), 0.08 (s, 18H, TMS). 13C NMR (CDCl3, 50
MHz): δ ) 187.1 (N-C-N), 137.6, 136.4, 128.9, 126.8 (Ph), 2.1
(TMS).
Acknowledgment. This research was supported by the
Ministry of Science, Culture and Sport, by the Fund for the
Promotion of Research at the Technion, and the Entrepreneurial
Mitchel Fund for Research at the Technion. M.S.E. thanks the
Humboldt Foundation for support during the preparation of the
manuscript. A.L. thanks the Ministry of Immigration for the
KAMEA Fellowship. We thank Dr. Alexander Shames from
the Ben-Gurion University at the Neguev, Beer Sheva, Israel
for the ESR experiments. We thank Prof. Arnon Siegman from
the Department of Material Engineering, Technion, Israel.
Propylene Polymerization Experiments. The catalytic polymeri-
zation of propylene was studied using complexes 9-12. Cationic
complexes were prepared by activation of these precatalysts by either
MAO or B(C6F5)3. Polymerization at atmospheric pressure was carried
out in a 100 mL flamed round-bottom flask and in NMR J-Young tubes.
Experiments at higher pressures were performed either in a 100 mL
heavy-wall glass or in stainless steal reactors. The reactor was charged
inside a glovebox with a certain amount of the catalytic precursor,
cocatalyst, and a magnetic stirrer. The reactor was connected to a high
vacuum line, and, after introducing the solvent (7-10 mL) via a syringe
under an argon flow, it was frozen at liquid nitrogen temperature and
pumped-down. For the polymerization at atmospheric pressure, the
reactor was warmed to the reaction temperature, and propylene was
used to back fill the vessel, and it was all maintained at 1.0 atm with
a mercury manometer. When polymerization was performed at higher
pressures, to the frozen reactor was vacuum transferred a certain amount
of liquid propylene. The temperature was then raised in a thermostated
Supporting Information Available: Additional crystallo-
graphic details, tables of all bond distances, angles, and
anisotropic thermal parameters (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA020575R
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2194 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003