Coordination copolymerization of severely encumbered isoalkenes with ethylene: Enhanced enchainment mediated by binuclear catalysts and cocatalysts

Article abstract of DOI:10.1021/ja052995x

This contribution describes the implementation of the binuclear organotitanium constrained geometry catalysts (CGCs), (μ-CH ₂CH₂-3,3′){(η⁵-indenyl)[1-Me ₂Si(^tBuN)](TiMe₂)}₂[EBICGC(TiMe ₂)₂; Ti₂] and (μ-CH₂-3,3′) {(η⁵-indenyl)[1-Me₂Si(^tBuN)](TiMe ₂)}₂[MBICGC(TiMe₂)₂; C1-Ti ₂], in combination with the bifunctional bisborane activator 1,4-(C₆F₅)₂BC₆F₄B(C ₆F₅)₂ (BN₂) in ethylene + olefin copolymerization processes. Specifically examined are the classically poorly responsive 1,1-disubstituted comonomers, methylenecyclopentane (C), methylenecyclohexane (D), 1,1,2-trisubstituted 2-methyl-2-butene (E), and isobutene (F). For the first three comonomers, this represents the first report of their incorporation into a polyethylene backbone via a coordination polymerization process. C and D are incorporated via a ring-unopened pathway, and E is incorporated via a novel pathway involving 2-methyl-1 -butene enchainment in the copolymer backbone. In ethylene copolymerization, Ti ₂ + BN₂ enchains ～2.5 times more C, ～2.5 times more D, and ～2.3 times more E than the mononuclear catalyst analogue [1-Me₂Si(3-ethylindenyl)-(^tBuN)]TiMe₂ (Ti ₁) + B(C₆F₅)₃ (BN) under identical polymerization conditions. Polar solvents are found to weaken the catalyst-cocatalyst ion pairing, thus influencing the comonomer enchainment selectivity.

Full text of DOI:10.1021/ja052995x

Published on Web 09/29/2005
Coordination Copolymerization of Severely Encumbered
Isoalkenes with Ethylene: Enhanced Enchainment Mediated
by Binuclear Catalysts and Cocatalysts
Hongbo Li,† Liting Li,† David J. Schwartz,† Matthew V. Metz,† Tobin J. Marks,*,†
Louise Liable-Sands,‡ and Arnold L. Rheingold‡
Contribution from the Department of Chemistry, Northwestern UniVersity, EVanston,
Illinois 60208-3113, and Department of Chemistry and Biochemistry, UniVersity of California,
San Diego, La Jolla, California 92093-0332
Received May 7, 2005; E-mail: t-marks@northwestern.edu
Abstract: This contribution describes the implementation of the binuclear organotitanium “constrained
geometry catalysts” (CGCs), (µ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2[EBICGC(TiMe2)2; Ti2]
and (µ-CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2[MBICGC(TiMe2)2; C1-Ti2], in combination with the
bifunctional bisborane activator 1,4-(C6F5)2BC6F4B(C6F5)2 (BN2) in ethylene + olefin copolymerization
processes. Specifically examined are the classically poorly responsive 1,1-disubstituted comonomers,
methylenecyclopentane (C), methylenecyclohexane (D), 1,1,2-trisubstituted 2-methyl-2-butene (E), and
isobutene (F). For the first three comonomers, this represents the first report of their incorporation into a
polyethylene backbone via a coordination polymerization process. C and D are incorporated via a ring-
unopened pathway, and E is incorporated via a novel pathway involving 2-methyl-1-butene enchainment
in the copolymer backbone. In ethylene copolymerization, Ti2 + BN2 enchains ∼2.5 times more C, ∼2.5
times more D, and ∼2.3 times more E than the mononuclear catalyst analogue [1-Me2Si(3-ethylindenyl)-
(tBuN)]TiMe2 (Ti1) + B(C6F5)3 (BN) under identical polymerization conditions. Polar solvents are found to
weaken the catalyst-cocatalyst ion pairing, thus influencing the comonomer enchainment selectivity.
reported4 that the -CH2CH2- bridged bimetallic catalysts, Ti2
and Zr2, as well as binuclear cocatalysts, B2 and BN2,4,6 exhibit
significant nuclearity effects in terms of chain branch formation
and comonomer enchainment selectivity versus their mono-
nuclear counterparts (Chart 1). Generally, CGCZr catalysts
Introduction
Enzymes achieve superior reactivity and selectivity, in part
due to their efficacy in creating high local reagent concentrations
and special, conformationally advantageous active site-substrate
proximities/interactions.1 In this regard, the possibility of unique
and more efficient catalytic transformations based on cooperative
effects between adjacent active centers in multinuclear transition
metal complexes is currently under intense investigation.2 For
single-site olefin polymerization catalysts,3-5 we recently
(3) For recent reviews of single-site olefin polymerization, see: (a) Kaminsky,
W. J. Polym. Sci., Part B: Polym. Chem. 2004, 42, 3911-3921. (b) Gibson,
V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283-316. (c) Pedeutour,
J.-N.; Radhakrishnan, K.; Cramail, H.; Deffieux, A. Macromol. Rapid
Commun. 2001, 22, 1095-1123. (d) Gladysz, J. A. Chem. ReV. 2000, 100
(special issue on Frontiers in Metal-Catalyzed Polymerization). (e) Marks,
T. J.; Stevens, J. C. Top. Catal. 1999, 15, and references therein. (f)
Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed.
1999, 38, 428-447. (g) Kaminsky, W.; Arndt, M. AdV. Polym. Sci. 1997,
127, 144-187. (h) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255-
270. (i) Brintzinger, H.-H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170. (j)
Catalyst Design for Tailor-Made Polyolefins; Soga, K., Terano, M., Eds.;
Elsevier: Tokyo, 1994. (k) Marks, T. J. Acc. Chem. Res. 1992, 25, 57-
65.
(4) For studies of binuclear metallocenes, see: (a) Li, H.; Li, L.; Marks, T. J.
Angew. Chem., Int. Ed. 2004, 37, 4937-4940. (b) Wang, J.; Li, H.; Guo,
N.; Li, L.; Stern, C. L.; Marks, T. J. Organometallics 2004, 23, 5112-
5114. (c) Guo, N.; Li, L.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 6542-
43. (d) Li, H.; Li, L.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J.
Am. Chem. Soc. 2003, 125, 10788-10789. (e) Noh, S. K.; Lee, J.; Lee, D.
J. Organomet. Chem. 2003, 667, 53-60. (f) Li, L.; Metz, M. V.; Li, H.;
Chen, M.-C.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am. Chem.
Soc. 2002, 124, 12725-12741. (g) Abramo, G. P.; Li, L.; Marks, T. J. J.
Am. Chem. Soc. 2002, 124, 13966-13967. (h) Green, M. L. H.; Popham,
N. H. J. Chem. Soc., Dalton Trans. 1999, 1049-1059 and references
therein. (i) Spaleck, W.; Kuber, F.; Bachmann, B.; Fritze, C.; Winter, A.
J. Mol. Catal. A: Chem. 1998, 128, 279-287. (j) Yan, X.; Chernega, A.;
Green, M. L. H.; Sanders, J.; Souter, J.; Ushioda, T. J. Mol. Catal. A: Chem.
1998, 128, 119-141. (k) Soga, K.; Ban, H. T.; Uozumi, T. J. Mol. Catal.
A: Chem. 1998, 128, 273-278. (l) Bochmann, M.; Cuenca, T.; Hardy, D.
T. J. Organomet. Chem. 1994, 484, C10-C12.
† Northwestern University.
‡ University of California, San Diego.
(1) (a) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Chem. ReV. 2004,
104, 561-588. (b) Krishnan, R.; Voo, J. K.; Riordan, C. G.; Zahkarov, L.;
Rheingold, A. L. J. Am. Chem. Soc. 2003, 125, 4422-4423. (c) Bruice, T.
C. Acc. Chem. Res. 2002, 35, 139-148. (d) Bruice, T. C.; Benkovic, S. J.
Biochemistry 2000, 39, 6267-6274 and references therein. (e) O’Brien,
D. P.; Entress, R. M. N.; Matthew, A. C.; O’Brien, S. W.; Hopkinson, A.;
Williams, D. H. J. Am. Chem. Soc. 1999, 121, 5259-5265. (f) Carazo-
Salas, R. E.; Guarguaglini, G.; Gruss, O. J.; Segref, A.; Karsenti, E.; Mattaj,
L. W. Nature 1999, 400, 178-181. (g) Menger, F. M. Acc. Chem. Res.
1993, 26, 206-212 and references therein. (h) Page, M. I. In The Chemistry
of Enzyme Action; Page, M. I., Ed.; Elsevier: New York, 1984; pp 1-54.
(2) (a) Sammis, G. M.; Danjo, H.; Jacobsen, E. N. J. Am. Chem. Soc. 2004,
126, 9928-9929. (b) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates,
G. W. J. Am. Chem. Soc. 2003, 125, 11911-11924. (c) Trost, B. M.; Mino,
T. J. Am. Chem. Soc. 2003, 125, 2410-2411. (d) Jacobsen, E. N. Acc.
Chem. Res. 2000, 33, 421-431. (e) Molenveld, P.; Engbersen, J. F. J.;
Reinhoudt, D. N. Chem. Soc. ReV. 2000, 29, 75-86. (f) Konsler, R. G.;
Karl, J.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 10780-10781. (g)
Molenveld, P.; Kapsabelis, S.; Engbersen, J. F. J.; Reinhoudt, D. N. J.
Am. Chem. Soc. 1997, 119, 2948-2949. (h) Mathews, R. C.; Howell, D.
H.; Peng, W.-J.; Train, S. G.; Treleaven, W. D.; Stanley, G. G. Angew.
Chem., Int. Ed. Engl. 1996, 35, 2253-2256. (i) Sawamura, M.; Sudoh,
M.; Ito, Y. J. Am. Chem. Soc. 1996, 118, 3309-3310.
9
14756
J. AM. CHEM. SOC. 2005, 127, 14756-14768
10.1021/ja052995x CCC: $30.25 © 2005 American Chemical Society

Catalyst/Cocatalyst Enhanced Enchainment
A R T I C L E S
Chart 1
Chart 2
produce low-Mw polyolefins with low activity and with low
R-olefin coenchainment efficiency,5 while CGCTi catalysts
produce high-Mw polyolefins with high activity and high
R-olefin coenchainment efficiency. In a previous communication,4d
we reported that the binuclear organotitanium catalyst Ti2 +
binuclear activators significantly enhances incorporation of
sterically encumbered isobutene (F) in ethylene copolymeriza-
tions. This observation raises the intriguing question of just how
general this unusual selectivity pattern is and whether it can be
extended to even more severely encumbered monomers. Herein
we report a full discussion of the implementation of binuclear
catalysts and cocatalysts in such copolymerization processes,
including results with a new -CH2- bridged binuclear catalyst
(C1-Ti2).
We previously reported an unusual example of a d0/fn
metallocene-mediated â-alkyl elimination process: ring-opening
Ziegler polymerization (ROZP; eq 1 in Chart 2)7 of the strained
methylenecycloalkanes, methylenecyclopropane (A), and meth-
ylenecyclobutane (B) in which sequential double-bond insertions
and â-alkyl shift ring openings afford polyolefins with exo-
methylene functionalities along the backbone (G and H). For
those monomers having less or no ring strain in cases such as
methylenecyclopentane (C) and methylenecyclohexane (D),
isomerization to the thermodynamically more stable internal
cycloolefins (I and J, respectively) is known to occur under
reaction conditions mediated by a variety of mononuclear d0/fn
metallocene catalysts.
(5) (a) Chum, P. S.; Kruper, W. J.; Guest, M. J. AdV. Mater. 2000, 12, 1759-
1767. (b) McKnight, A. L.; Waymouth, R. M. Chem. ReV. 1998, 98, 2587-
2598. (c) Harrison, D.; Coulter, I. M.; Wang, S. T.; Nistala, S.; Kuntz, B.
A.; Pigeon, M.; Tian, J.; Collins, S. J. Mol. Catal. A: Chem. 1998, 128,
65-77. (d) Soga, K.; Uozumi, T.; Nakamura, S.; Toneri, T.; Teranishi, T.;
Sano, T.; Arai, T.; Shiono, T. Macromol. Chem. Phys. 1996, 197, 4237-
4251. (e) Devore, D. D.; Timmers, F. J.; Hasha, D. L.; Rosen, R. K.; Marks,
T. J.; Deck, P. A.; Stern, C. L. Organometallics 1995, 14, 3132-3134. (f)
Lai, S. Y.; Wilson, J. R.; Knight, G. W.; Stevens, J. C. WO-93/08221,
1993. (g) Canich, J. M.; Hlatky, G. G.; Turner, H. W. PCT Appl. WO-92/
00333, 1992; Canich, J. M. Eur. Patent Appl. EP 420 436-A1, 1991 (Exxon
Chemical Co.). (h) Devore, D. D. European Patent Application EP-514-
828-A1, November 25, 1992.
(6) Similar bifunctional cocatalysts: (a) Lewis, S. P.; Henderson, L. D.;
Chandler, B. D.; Parvez, M.; Piers, W. E.; Collins, S. J. Am. Chem. Soc.
2005, 127, 46-47. (b) Lewis, S. P.; Taylor, N. J.; Piers, W. E.; Collins, S.
J. Am. Chem. Soc. 2003, 125, 14686-14687. (c) Metz, M. V.; Schwartz,
D. J.; Stern, C. L.; Marks, T. J.; Nickias, P. N. Organometallics 2002, 21,
4159-4168. (d) Lancaster, S. J.; Rodriguez, A.; Lara-Sanchez, A.; Hannant,
M. D.; Walker, D. A.; Hughes, D. H.; Bochmann, M. Organometallics
2002, 21, 451-453. (e) McAdon, M. H.; Nickias, P. N.; Marks, T. J.;
Schwartz, D. J. WO9906413A1, February 11, 1999. (f) Metz, M. V.;
Schwartz, D. J.; Stern, C. L.; Nickias, P. N.; Marks, T. J. Angew. Chem.,
Int. Ed. 2000, 39, 1312-1316. (g) Williams, V. C.; Piers, W. E.; Clegg,
W.; Elsegood, M. R. J.; Collins, S.; Marder, T. B. J. Am. Chem. Soc. 1999,
121, 3244-3245. (h) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J.
Organometallics 1997, 16, 842-857.
(7) (a) Jensen, T. R.; O’Donnell, J. J., III; Marks, T. J. Organometallics 2004,
23, 740-754. (b) Jia, L.; Yang, X. M.; Seyam, A. M.; Albert, I. D. L.; Fu,
P. F.; Yang, S. T.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 7900-7913.
(c) Jia, L.; Yang, X. M.; Yang, S. T.; Marks, T. J. J. Am. Chem. Soc.
1996, 118, 1547-1548. (d) Yang, X. M.; Seyam, A. M.; Fu, P. F.; Marks,
T. J. Macromolecules 1994, 27, 4625-4626. (e) Yang, X. M.; Jia, L.;
Marks, T. J. J. Am. Chem. Soc. 1993, 115, 3392-3393.
9
J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14757

A R T I C L E S
Li et al.
Several methylenecycloalkane derivatives have been reported
to undergo polymerization/copolymerization via ring-unopened
pathways, yielding polymers with saturated hydrocarbon rings
appended to the polyolefin backbone.7a,8 Such an architecture
of saturated hydrocarbon rings arrayed along a polymer chain
is expected to afford significantly altered physical properties
because the bulky cycloalkane rings should frustrate the
tendency to coil tightly and should, thus, increase the average
chain length between entanglements.9-11 Intensive research
efforts have focused on producing this type of polymeric
product,10,11 and the saturated hydrocarbon rings are typically
created in the polymer backbone via heterogeneous hydrogena-
tion of aromatic-functionalized macromolecules, such as
polystyrene11b-d and polyindene.11a Compared with their un-
saturated precursors, saturated ring-functionalized polymers
should have lower dielectric constants, lower refractive indices,
lower water absorption, and greater optical transparency.10b
However, such inefficient two-step processes require relatively
harsh hydrogenation conditions and frequently suffer from
incomplete hydrogenation and chain scission. For these reasons,
a single-step homogeneous catalytic polymerization process
represents an attractive approach to accessing this challenging
macromolecular structure class.3 Nevertheless, in the methyl-
enecycloalkane family, only methylenecyclopropane derivatives
have so far been reported to be effective comonomers for this
type of polymerization, reflecting the highly constricted geom-
etries of three-membered rings, which presumably reduce steric
hindrance to CdC enchainment. Since the substantial strain of
the three-membered ring can potentially compromise polymer
stability, monomers such as C or D with minimal ring strain
(6.5 and 0 kcal/mol for C and D, respectively), when incorpo-
rated into a polyethylene chain in a ring-unopened geometry,
are expected to enhance the polymer thermal and chemical
stability and promote the aforementioned entanglement proper-
ties.7a,10
bimetallic “constrained geometry complexes” (CGC) (µ-CH2-
CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 (Ti2) and (µ-
CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 (C1-Ti2), the
monometallic analogue [1-Me2Si(3-ethylindenyl)(tBuN)] TiMe2
(Ti1) for comparative purposes, and the new binuclear bisborane
cocatalyst 1,4-(C6F5)2BC6F4B(C6F5)2 (BN2). The ethylene + C,
ethylene + D, and ethylene + E copolymerization characteristics
with various catalysts/cocatalyst combinations are then examined
in detail. It will be seen that this new family of CGCTi catalysts
can efficiently incorporate these sterically encumbered comono-
mers into polyethylene backbones, and that the consequence of
increasing catalyst and cocatalyst nuclearity is to dramatically
enhance selectivity for comonomer enchainment in these
copolymerizations.
Results
The goal of this study was to investigate nuclearity effects
for severely encumbered isoalkene enchainment in ethylene
copolymerizations using the coordinatively “open” and highly
reactive CGCTi (constrained geometry catalyst) core structures
and to explore selectivity effects for isoalkene incorporation in
polyethylene backbones arising from cooperative effects be-
tween proximate single-site catalytic centers. Thus, the new
bimetallic constrained geometry catalysts (CGC), Ti2, C1-Ti2,
the monometallic CGC complex, Ti1, and the binuclear bis-
borane cocatalyst, 1,4-(C6F5)2BC6F4B(C6F5)2 (BN2), were syn-
thesized for this purpose. It will be seen that the effect of
increasing catalyst and cocatalyst nuclearity is to significantly
enhance sterically encumbered comonomer incorporation in the
copolymerizations, producing new and unusual polyolefin
copolymers.
I. Synthesis and Characterization of Bimetallic Metal-
locene Complexes, EBICGC(TiMe2)2 (Ti2) and MBICGC-
(TiMe2)2 (C1-Ti2).12 The ligand (µ-CH2CH2-3,3′)[1-(Me2-
SiNHtBu)indenyl]2 (EBICGCH2), synthesized according to the
literature procedure,4f consists of two diastereomers (RR,SS) and
Here, we report that with coordinatively more open CGCTi+
catalysts, sterically hindered monomers C and D, can now be
incorporated into the polyethylene backbone in a ring-unopened
fashion to afford macromolecules K and L, respectively, rather
than simply undergoing double-bond migration. More interest-
ingly, the even more sterically encumbered 1,1,2-trisubstituted
monomer 2-methyl-2-butene (E) can also be enchained to form
copolymer M. We report here the synthesis, characterization,
and ethylene/isoalkene copolymerization characteristics of the
catalysts which efficiently effect these transformationss
1
(RS, SR) in an approximate 1:1 ratio as indicated by H and
13C NMR spectroscopy. Similar to the synthesis of the Zr
analogue, bimetallic metallocene complex EBICGC(TiMe2)2
(Ti2) was synthesized via the methodology outlined in Scheme
1. The first step is the synthesis of the bimetallic amido complex
EBICGC[Ti(NMe2)2]2 (1) via the protodeaminative reaction of
the free (3,3′-CH2CH2)[1-Me2SiNHtBu)ind]2 ligand (EBICGCH2)
with Ti(NMe2)4 in refluxing toluene, with constant removal of
the evolved HNMe2 byproduct. The bimetallic product consists
of two diastereomers (RS, SR) and (SS, RR) (1:1.3 or 1.3:1 ratio)
as indicated by 1H NMR spectroscopy. Both diastereomers have
relatively low solubility in toluene and benzene and are almost
completely insoluble in pentane. Attempts to isolate significant
quantities of the pure diastereomers by fractional crystallization
were unsuccessful. Bimetallic amido complex 1 was character-
ized by standard spectroscopic and analytical techniques, and
one diastereomer (RS, SR) by X-ray diffraction (vide infra).
Reaction of 1 with excess AlMe3 at room temperature cleanly
forms the bimetallic metallocene tetramethyl complex Ti2
(Scheme 1), which can be purified by repeated washing with
pentane, and was characterized by conventional spectroscopic
and analytical techniques. Both diastereomers (RS, SR) and (SS,
(8) (a) Takeuchi, D.; Anada, K.; Osakada, K. Macromolecules 2002, 35, 9628-
9633. (b) Takeuchi, D.; Osakada, K. Chem. Commun. 2002, 646-467. (c)
Takeuchi, D.; Kim, S.; Osakada, K. Angew. Chem., Int. Ed. 2001, 14, 2685-
2688.
(9) (a) Kulshrestha, A. K.; Talapatra, S. In Handbook of Polyolefins; Vasile,
C., Ed; Marcel Dekker: New York, 2000. (b) McKnight, A. L.; Waymouth,
R. M. Macromolecules 1999, 32, 2816-2825. (c) Natori, I.; Imaizumi,
K.; Yamagishi, H.; Kazunori, M. J. Polym. Sci., Part B: Polym. Phys.
1998, 36, 1657-1668. (d) Natori, I. Macromolecules 1997, 30, 3696-
3697. (e) Cherdron, H.; Brekner, M.-J.; Osan, F. Angew. Makromol. Chem.
1994, 223, 121-133. (f) James, D. E. In Encyclopedia of Polymer Science
and Engineering; Marks, H. F., Bikales, N. M., Overberger, C. G., Menges,
G., Eds.; Wiley-Interscience: New York, 1985; Vol. 6.
(10) (a) Zhao, J.; Hahn, S. F.; Hucul, D. A.; Meunier, D. M. Macromolecules
2001, 34, 1737-1741. (b) Hucul, D. A.; Hahn, S. F. AdV. Mater. 2000,
12, 1855-1858.
(11) (a) Hahn, S. F.; Hillmyer, M. A. Macromolecules 2003, 36, 71-76. (b)
Ness, J. S.; Brodil, J. C.; Bates, F. S.; Hahn, S. F.; Hucul, D. A.; Hillmyer,
M. A. Macromolecules 2002, 35, 602-609. (c) Gehlsen, M. D.; Weimann,
P. A.; Bates, F. S.; Mays, J. J. Polym. Sci., Part B: Polym. Phys. 1995,
33, 1527-1536. (d) Gehlsen, M.; Bates, F. S. Macromolecules 1993, 26,
4122-4127.
(12) Detailed synthetic and characterization data can be found in the Supporting
Information.
9
14758 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005

Catalyst/Cocatalyst Enhanced Enchainment
A R T I C L E S
Scheme 1. Synthetic Routes to Complexes Ti2 and C1-Ti2
Scheme 2. Synthetic Route to Complex Ti1
Scheme 3. Synthetic Route to Complex BN2
Scheme 4. Preparation of [(C5H5)2ZrMe+]2{Me2-1,4-C6F4[B(C6F5)2]2}2-
RR) (1:1.3 or 1.3:1 ratio) are present in the product. The
solubility of either diastereomer in toluene, benzene, and pentane
is rather low, even at elevated temperatures. Furthermore, the
complexes begin to decompose above 80 °C in solution, which
also thwarts recrystallization.
ethylindenyl)(tBuN)TiCl2, and subsequent reaction with MeLi
affords metallocene dimethyl complex 1-Me2Si(3-ethylindenyl)-
(tBuN)TiMe2 (Ti1). Complex Ti1 was characterized by standard
spectroscopic and analytical techniques and by X-ray diffraction
(vide infra).
Methylene-bridged bimetallic complex MBICGC(TiMe2)2
(C1-Ti2) was similarly synthesized from the ligand (µ-CH2-
3,3′)[1-(Me2SiNHtBu)indenyl]2 (MBICGCH2; Scheme 2).4a The
synthetic conditions are similar to those outlined for the Ti2
synthesis above, except that longer reaction times are required
in the metalation step, presumably due to the steric encumbrance.
II. Synthesis of [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe2
(Ti1). The monometallic metallocene complex [1-Me2Si(3-
ethylindenyl)(tBuN)]TiMe2 (Ti1) was synthesized as a mono-
nuclear control for studies of binuclear cooperativity in ethylene
polymerizations and copolymerizations. The ligand (1-Me2-
SiNHtBu)(3-ethyl)indene was synthesized according to the
literature procedure.4f The monometallic CGC complex [1-Me2-
Si(3-ethylindenyl)(tBuN)]TiMe2 (Ti1) was synthesized via pro-
todeaminative methodology similar to that for the monometallic
zirconium complex [1-Me2Si(3-ethylindenyl)(tBuN)]ZrMe2 (Zr1;
Scheme 2). Thus, the monometallic Ti amido complex [1-Me2-
Si(3-ethylindenyl)(tBuN)]Ti(NMe2)2 (2) was synthesized via
reaction of the free (1-Me2SiNHtBu)(3-ethyl)indene ligand with
Ti(NMe2)4 in refluxing toluene under constant removal of
HNMe2. The reaction of 2 with excess Me3SiCl at room
temperature then cleanly affords dichloro complex 1-Me2Si(3-
III. Synthesis of Binuclear Bisborane Cocatalyst 1,4-
(C6F5)2BC6F4B(C6F5)2 (BN2). The synthesis of binuclear bis-
borane 1,4-(C6F5)2BC6F4B(C6F5)2 (BN2) was accomplished by
heating 1,4-C6F4(SnMe3)2 with 6.0 equiv of (C6F5)2BCl in
toluene for 72 h (Scheme 3). Alternatively (Method II), the neat
reagents without solvent can be used to shorten the reaction
1
time to 24 h.12 As judged by H and 19F NMR spectroscopy,
alkyl/chloride exchange occurs initially to afford 2.0 equiv of
(C6F5)2BMe and 1,4-C6F4(SnMe2Cl)2; the latter then reacts with
the additional (C6F5)2BCl to yield 1,4-(C6F5)2BC6F4B(C6F5)2
(BN2) and 2.0 equiv of Me2SnCl2 (Scheme 3). Purification of
the crude reaction mixture is relatively straightforward; 1,4-
(C6F5)2BC6F4B(C6F5)2 (BN2) is insoluble in pentane, while
(C6F5)2BMe, Me2SnCl2, and the excess (C6F5)2BCl are pentane-
soluble and can be readily washed away.
IV. Synthesis of [(C5H5)2ZrMe+]2{Me2-1,4-C6F4[B-
(C6F5)2]2}2-. Reaction of BN2 with 2.0 equiv of (C5H5)2ZrMe2
results in the clean, instantaneous formation of the bimetallic
ion pair [(C5H5)2ZrMe+]2{Me2-1,4-C6F4[B(C6F5)2]2}2- (Scheme
4). This indicates that any intermediate abstraction product
resulting from reaction of bisborane with 1.0 equiv of (C5H5)2-
ZrMe2 is sufficiently Lewis acidic to abstract a methide anion
9
J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14759

A R T I C L E S
Li et al.
Table 1. Summary of the Crystal Structure Data for Complexes 1
and Ti1
Table 2. Selected Bond Distances (Å) and Angles (deg) for
EBICGC[Ti(NM2)2]2 (1)
complex
formula
formula weight
crystal dimensions
crystal system
a, Å
1
Ti1
Bond Distances
C40H68N6Si2Ti2
784.98
C19H31NSiTi
349.44
Ti(1)-N(1)
Ti(1)-N(3)
Ti(1)-C(2)
Ti(1)-C(8)
N(1)-C(12)
Si(1)-C(2)
1.994(4) Ti(1)-N(2)
1.914(4) Ti(1)-C(1)
2.353(5) Ti(1)-C(3)
2.599(5) Ti(1)-C(9)
1.497(6) N(2)-C(16)
1.878(5) Si(1)-N(1)
1.940(4)
2.395(4)
2.516(5)
2.512(5)
1.465(7)
1.732(4)
0.50 × 0.45 × 0.35
0.11 × 0.16 × 0.21
monoclinic
22.9273(14)
12.1450(7)
14.1744(8)
90
100.7690(10)
90
3877.4(4)
C2/c
triclinic
8.3171(2)
9.8177(2)
13.8667(4)
83.5443(10)
82.2403(11)
83.5394(8)
1111.62(8)
P1h
b, Å
c, Å
Angles
R, deg
N(3)-Ti(1)-N(2)
N(2)-Ti(1)-N(1)
N(1)-Si(1)-C(11)
N(1)-Si(1)-C(10)
C(12)-N(1)-Ti(1)
102.0(2)
N(3)-Ti(1)-N(1)
N(1)-Si(1)-C(2)
C(2)-Si(1)-C(11)
C(12)-N(1)-Si(1)
Si(1)-N(1)-Ti(1)
C(16)-N(2)-Ti(1)
104.7(2)
â, deg
105.9(2)
117.1(14)
115.5(3)
128.8(3)
94.6(2)
108.4(2)
126.5(3)
103.5(2)
124.8(4)
γ, deg
V, Å3
space group
Z value
2
8
C(16)-N(2)-C(17) 109.1(5)
D
calc, mg/m3
1.173
198(2)
4.46
Mo KR
1.49 to 28.05
241
4676, 0.0593
0.1142
0.2631
1.197
153(2)
5.01
Mo KR
1.81 to 28.28
323
4652, 0.0282
0.0534
0.1099
C(17)-N(2)-Ti(1)
C(18)-N(3)-Ti(1)
C(1)-C(2)-C(3)
C(3)-C(2)-Si(1)
C(1)-C(9)-C(20)
125.8(4)
119.5(4)
105.4(2)
126.5(3)
127.7(4)
C(18)-N(3)-C(19) 110.9(5)
temp, K
C(19)-N(3)-Ti(1)
C(1)-C(2)-Si(1)
C(1)-C(9)-C(8)
C(8)-C(9)-C(20)
127.6(4)
119.6(3)
106.5(4)
125.1(4)
µ, cm-1
radiation
2θ range, deg
No. of parameters
intensities (unique, Ri)
R
wR2
from a second neutral metallocene dialkyl. The displacements
in the 19F chemical shifts of BN2 upon formation of [(C5H5)2-
ZrMe+]2{Me2-1,4-C6F4[B(C6F5)2]2}2- (i.e., upon bisanion for-
mation) are similar to those observed for B(C6F5)3,13e with the
exception of the 19F resonance arising from the central C6F4
ring of BN2. This resonance is displaced 15 ppm upfield upon
ion pair formation, relative to the more usual upfield perturba-
tions of ca. 5 ppm observed for the B(C6F5)2 for ortho-fluorine
resonances,13 qualitatively indicating that a relatively large
amount of electron density is transferred to the central C6F4
ring in the (C5H5)2ZrMe+]2{Me2-1,4-C6F4[B(C6F5)2]2}2- prod-
uct.
Figure 1. The molecular structure and atom numbering scheme for the
binuclear complex EBICGC[TiNMe2)2]2 (1). Thermal ellipsoids are drawn
at the 50% probability level. A single enantiomer is shown.
tially longer than the Ti-NMe2 bonds (Ti(1)-N(2) ) 1.940(4)
Å, Ti(1)-N(3) ) 1.914(4) Å), likely owing to steric constraints
in addition to the decreased basicity15 of the (Me2Si)tBuN
moiety. The sum of bond angles around ring carbon atom C(2)
is 351.5°, indicating that the C(2)-Si(1) bond vector is displaced
appreciably from the ring plane because of the constrained
geometry. As expected, the Me2Si< bridge forces the ind plane
to tilt, increasing the ring centroid-Ti-N angle and making
the structure more opensthe N(1)-Ti(1)-centroid(1) angle is
106.58(6)°. The carbon atoms of the Cp ring do not exhibit
equal bonding distances to the Ti center. The average bond
lengths of Ti-C1/Ti-C2 and Ti-C3/Ti-C8/Ti-C9 are 2.374
and 2.542 Å, respectively. The difference ∆ (Ti-C3/Ti-C8/
Ti-C9) - (Ti-C1/Ti-C2) is 0.168 Å, 0.037 Å greater than
that value found for the more symmetrical Cp ligand in [(η5-
C5Me4)SiMe2(N-t-Bu)]TiCl2, which is 2.436 - 2.305 ) 0.131Å,14f
indicating a substantially more “slipped” coordination of the
Cp ligand in 1.
Molecular Structures of the Complexes EBICGC[Ti-
(NMe2)2]2 (Ti1), [1-Me2Si(3-Ethylindenyl)(tBuN)]TiMe+MeB-
(C6F5)3-, and [(C5H5)2ZrMe+]2{Me2-1,4-C6F4[B(C6F5)2]2}2-
:
A. Bimetallic Complex EBICGC[Ti(NMe2)2]2 (1). A summary
of crystal structure data for complex 1 is presented in Table 1,
and selected bond distances and angles for 1 are summarized
in Table 2. The crystal structure of complex 1 (Figure 1) reveals
an inversion center with a CGCTi unit located on either side of
the ethylenebis(indenyl) fragment and with the two π-coordi-
nated indenyl rings in a diastereomeric relationship. As can be
seen from Figure 1, the crystal consists of a single diastereomer
(SR, RS). The sum of the bond angles around nitrogen atom
N(1) is 358.8°, indicating that atoms Si(1), N(1), C(12), and
Ti(1) are essentially coplanar, which is also true for the atoms
surrounding dimethylamido atoms N(2) and N(3). Such coplanar
structures suggest non-negligible π-bonding between the Ti and
N atoms involving the N atom lone pair electrons.14 Neverthe-
t
less, the BuN-Ti bond (Ti(1)-N(1)) is 1.994(4) Å, substan-
(14) (a) Christopher, J. N.; Jordan, R. F.; Petersen, J. L.; Young, V. G., Jr.
Organometallics 1997, 16, 3044-3050. (b) Diamond, G. M.; Jordan, R.
F.; Petersen, J. L. Organometallics 1996, 15, 4030-4037. (c) Christopher,
J. N.; Diamond, G. M.; Jordan, R. F.; Petersen, J. L. Organometallics 1996,
15, 4038-4044. (d) Diamond, G. M.; Jordan, R. F.; Petersen, J. L.
Organometallics 1996, 15, 4045-4053. (e) Diamond, G. M.; Jordan, R.
F.; Petersen, J. L. J. Am. Chem. Soc. 1996, 118, 8024-8033. (f) Carpenetti,
D. W.; Kloppenburg, L.; Kupec, J. T.; Petersen, J. L. Organometallics 1996,
15, 1572-1581. (g) Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava,
R. C. Metal and Metalloid Amides; Ellis Horwood: Chichester, West
Sussex, U.K., 1980; pp 500-502. (h) Bradley, D. C.; Chisholm, M. H.
Acc. Chem. Res. 1976, 9, 273-280.
(13) (a) Song, F.; Lancaster, S. J.; Cannon, R. D.; Schormann, M.; Humphrey,
S. M.; Zuccaccia, C.; Macchioni, A.; Bochmann, M. Organometallics 2005,
24, 1315-1328. (b) Hannig, F.; Frohlich, R.; Bergander, K.; Erker, G.;
Petersen, J. L. Organometallics 2004, 23, 4495-4502. (c) Beck, S.; Lieber,
S.; Schaper, F.; Geyer, A.; Brintzinger, H. H. J. J. Am. Chem. Soc. 2001,
123, 1483-1489. (d) Chase, P. A.; Piers, W. E.; Patrick, B. O. J. Am.
Chem. Soc. 2000, 122, 12911-12912. (e) Yang, X.; Stern, C. L.; Marks,
T. J. J. Am. Chem. Soc. 1994, 116, 10015-10031. (f) Yang, X.; Stern, C.
L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623-2625. (g) Bochmann,
M.; Lancaster, S. J.; Hursthouse, M. B.; Abdul Malik, K. M. Organome-
tallics 1994, 13, 2235-2243. (h) Stahl, N. G.; Salata, M. R.; Marks, T. J.
J. Am. Chem. Soc. 2005, 127, 10898-10909.
(15) Barlos, K.; Huebler, G.; Noth, H.; Wanninger, P.; Wiberg, N.; Wrackmeyer,
B. J. Magn. Reson. 1978, 31, 363-376.
9
14760 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005

Catalyst/Cocatalyst Enhanced Enchainment
A R T I C L E S
Table 3. Selected Bond Distances (Å) and Angles (deg) for Ti1
Table 4. Summary of the Crystal Structure Data for Complexes 2
and 3
Bond Distances
complex
formula
formula weight
crystal dimensions
crystal system
a, Å
2
3
Ti(1)-N(1)
Ti(1)-C(18)
Ti(1)-C(5)
Ti(1)-C(4)
Si(1)-N(1)
Si(1)-C(12)
N(1)-C(14)
1.9300(15) Ti(1)-C(19)
2.110(2)
C44H39BF15NSiTi
953.56
C76H42B2F24Zr2
1615.19
0.28 × 0.26 x 0.010
triclinic
11.40(1)
11.52(1)
15.24(1)
67.97(4)
88.4(1)
66.31(4)
1682(2)
P1h
2.102(2) Ti(1)-C(2)
2.3430(17)
2.2869(16)
2.4940(17)
1.859(2)
2.4120(17) Ti(1)-C(1)
2.5428(17) Ti(1)-C(3)
1.7503(15) Si(1)-C(13)
1.8640(17) Si(1)-C(2)
1.493(2)
0.12 × 0.17 × 0 0.22
triclinic
11.0774(8)
12.8788(9)
16.3857(12)
90.2610(10)
97.9910(10)
110.6800(10)
2162.2(3)
P1h
1.869(2)
b, Å
c, Å
Angles
R, deg
N(1)-Ti(1)-C(19) 108.85(8)
C(19)-Ti(1)-C(18) 101.58(9)
N(1)-Si(1)-C(12) 115.72(9)
N(1)-Si(1)-C(1)
C(14)-N(1)-Ti(1) 130.84(12)
C(2)-C(1)-C(5)
C(5)-C(1)-Si(1)
C(2)-C(3)-C(10)
N(1)-Ti(1)-C(18) 109.15(9)
â, deg
N(1)-Si(1)-C(13) 115.69(9)
C(13)-Si(1)-C(12) 108.27(11)
C(14)-N(1)-Si(1) 126.64(12)
γ, deg
V, Å3
93.64(7)
space group
Z value
Si(1)-N(1)-Ti(1)
C(2)-C(1)-Si(1)
C(2)-C(3)-C(4)
C(4)-C(3)-C(10)
102.51(7)
120.21(13)
107.00(15)
125.56(16)
2
1
104.65(14)
125.65(12)
126.97(16)
D
calc, mg/m3
1.465
153(2)
3.24
Mo KR
1.26 to 28.30
243
10009, 0.0379
0.0794
0.1556
1.595
153(2)
4.21
Mo KR
1.46 to 22.97
279
2922, 0.0539
0.0842
temp, K
µ, cm-1
radiation
2θ range, deg
No. of parameters
intensities (unique, Ri)
R
wR2
Table 5. Selected Bond Distances (Å) and Angles (deg) for 2
Bond Distances
Ti(1)-N(1)
Ti(1)-C(18)
Ti(1)-C(5)
Ti(1)-C(4)
Si(1)-N(1)
Si(1)-C(12)
N(1)-C(14)
B(1)-C(20)
B(1)-C(32)
1.898(2) Ti(1)-C(19)
2.090(3) Ti(1)-C(1)
2.333(2) Ti(1)-C(2)
2.535(2) Ti(1)-C(3)
1.762(2) Si(1)-C(13)
1.855(3) Si(1)-C(1)
1.503(3) C(19)-B(1)
1.650(4) B(1)-C(26)
1.656(4) C(32)-C(37)
2.320(2)
2.246(2)
2.320(2)
2.484(2)
1.855(3)
1.861(2)
1.675(3)
1.639(4)
1.387(3)
Figure 2. The molecular structure and atom numbering scheme for the
mononuclear complex (3-CH3CH2-indenyl)[1-SiMe2(tBuN)]TiMe2 (Ti1).
Thermal ellipsoids are drawn at the 50% probability level. A single
enantiomer is shown.
Angles
N(1)-Ti(1)-C(18)
C(18)-Ti(1)-C(1)
C(18)-Ti(1)-C(19) 100.50(10) N(1)-Si(1)-C(13)
104.47(11) N(1)-Ti(1)-C(1)
125.73(10) N(1)-Ti(1)-C(19)
77.79(8)
109.58(9)
114.40(13)
B. Monometallic Complex Ti1. A summary of crystal
structure data for dimethyl complex Ti1 is compiled in Table
1, and selected bond distances and angles for Ti1 are summarized
in Table 3. The solid-state structure of monometallic Ti1 is
illustrated in Figure 2, and it can be seen that the three methyl
groups on the tert-butyl amido group are disordered. As
expected, the metrical parameters in Table 4 suggest that the
Me2Si< bridge again forces the ind plane to tilt, rendering the
structure more open; the N(1)-Ti(1)-centroid(1) is 109.55-
(6)°, comparable to the N(1)-Ti(1)-centroid(1) value in similar
CGCTi- structures: 105.5° in [(η5-C5H4)SiMe2(tBuN)]Ti-
(NMe2)2;14f 108.3° in [(η5-C9H5-2-NMe2)SiMe2(tBuN)TiCl2;16a
103.85° in [(η5-C5Me4)SiMe2(η1-NNMe2)]Ti(NMe2)2;16b 107.3°
in [(η5:η1-C5H4CMe2)SiMe2(tBuN)]TiCl2;16c 106.3° in (+)-(R)-
[η:5η1-(Ind)SiMe2-(S)-NCHMePh]TiCl2.16d Similar to bimetallic
complex 1, the sum of the bond angles around bridge-connected
nitrogen atom N(1) in Ti1 is close to 360°, indicating the atoms
around N(1) are essentially coplanar, again suggesting strong
Ti-N bonding,14 presumably involving π-donation.14g,h Com-
pared to bimetallic dimethylamido complex 1, both the Ti-
N(1) and Ti-C(ring) bond lengths in dimethyl Ti1 are signifi-
N(1)-Si(1)-C(12)
N(1)-Si(1)-C(1)
C(12)-Si(1)-C(1)
C(14)-N(1)-Ti(1)
B(1)-C(19)-Ti(1)
C(26)-B(1)-C(32)
C(26)-B(1)-C(19)
C(32)-B(1)-C(19)
114.09(13) C(13)-Si(1)-C(12) 111.49(15)
92.40(10) C(13)-Si(1)-C(1)
112.65(13) C(14)-N(1)-Si(1)
132.56(16) Si(1)-N(1)-Ti(1)
169.04(18) C(26)-B(1)-C(20)
110.46(12)
125.08(16)
102.22(10)
113.8(2)
113.8(2)
103.5(2)
104.8(2)
112.5(2)
108.5(2)
C(20)-B(1)-C(32)
C(20)-B(1)-C(19)
C(37)-C(32)-B(1)
127.2(2)
cantly shorter. Thus, Ti(1)-N(1) is 1.994(4) Å in 1 and
1.9300(15) Å in Ti1; Ti(1)-C(9) is 2.512(4) Å in 1, and the
corresponding Ti(1)-C(3) contact in Ti1 is 2.4940(17) Å. The
expanded bond distances in 1 are no doubt because the Ti center
is more electron-rich, presumably due to Ti-N bonding
involving the dimethylamido nitrogen lone pairs.
C. Activated Ti Complex [1-Me2Si(3-Ethylindenyl)(tBuN)]-
TiMe+MeB(C6F5)3 (2). A summary of crystal structure data
-
for complex 2 is given in Table 4; selected bond distances and
angles for 2 are summarized in Table 5, and the molecular
structure is shown in Figure 3. The Ti-Me(terminal) distance
(2.090(3) Å) is 0.01-0.02 Å shorter than the Ti-Me distances
in neutral Ti1 (2.102(2) and 2.1100(2) Å) due to the increased
electrophilic character, while it is 0.230 Å shorter than the Ti-
Me(bridging) separation (2.320(2) Å), reflecting the largely ionic
character of the ion pair interaction.13,17,18 The metrical param-
eters in 2 are similar to those in (η5-Me4C5)Me2Si(tBuN)TiMe+-
MeB(C6F5)3-, except the Ti- Me(bridging) distance:19 it is
2.320(2) Å in 2, while 2.364(2) Å in Me2Si(η5-Me4C5)(tBuN)-
(16) (a) Klosin, J.; Kruper, W. J., Jr.; Nickias, P. N.; Roof, G. R.; De Waele,
P.; Abboud, K. A. Organometallics 2001, 20, 2663-2665. (b) Park, J. T.;
Yoon, S. C.; Bae, B.-J.; Seo, W. S.; Suh, I.-H.; Han, T. K.; Park, J. R.
Organometallics 2000, 19, 1269-1276. (c) Feng, S.; Klosin, J.; Kruper,
W. J., Jr.; McAdon, M. H.; Neithamer, D. R.; Nickias, P. N.; Patton, J. T.;
Wilson, D. R.; Abboud, K. A.; Stern, C. L. Organometallics 1999, 18,
1159-1167. (d) McKnight, A. L.; Masood, M. A.; Waymouth, R. M.;
Straus, D. A. Organometallics 1997, 16, 2879-2885.
9
J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14761

A R T I C L E S
Li et al.
Table 6. Selected Bond Distances (Å) and Angles (deg) for 3
Bond Distances
Zr(1)-C(1)
Zr(1)-C(11)
F(1)-C(14)
F(11)-C(26)
C(1)-C(2)
C(13)-C(14)
C(25)-C(26)
C(13)-B(1)
C(25)-B(1)
2.51(1)
2.26(1)
1.36(1)
1.37(1)
1.41(2)
1.35(2)
1.39(2)
1.70(2)
1.65(2)
Zr(1)-C(6)
2.54(1)
2.52(1)
1.37(2)
1.41(2)
1.40(2)
1.36(2)
1.64(2)
1.67(2)
Zr(1)-C(12)
F(6)-C(20)
C(1)-C(2)
C(9)-C(10)
C(19)-C(20)
C(12)-B(1)
C(19)-B(1)
Angles
C(1)-Zr(1)-C(8)
C(11)-Zr(1)-C(12)
F(1)-C(14)-C(13)
C(3)-C(4)-C(5)
C(12)-B(1)-C(13)
C(12)-B(1)-C(25)
C(13)-B(1)-C(25)
98.8(5)
92.6(5)
123(1)
108(1)
113(1)
104(1)
112(1)
C(1)-Zr(1)-C(12)
Zr(1)-C(12)-B(1)
F(1)-C(14)-C(15)
C(13)-C(14)-C(15)
C(12)-B(1)-C(19)
C(13)-B(1)-C(19)
C(19)-B(1)-C(25)
88.3(5)
161.3(9)
112(1)
125(1)
114(1)
102(1)
112(1)
Figure 3. The molecular structure and atom numbering scheme for the
-
mononuclear ion pair [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe+MeB(C6F5)3
(2). Thermal ellipsoids are drawn at the 50% probability level.
TiMe+MeB(C6F5)3-. The difference in the Ti-Me(bridging)
separation probably reflects differences in steric interactions
between the MeB(C6F5)3- anion and the various substituted Cp
ligands. Additionally, the Ti-N and Ti-C(Cp ring) bond
distances in 2 are substantially shorter than those in neutral
Ti1: Ti(1)-N(1) ) 1.898(2) Å in 2 versus 1.930(2) Å in Ti1;
Ti(1)-C(1) is 2.246(2) Å in 2 versus 2.287(2) Å in Ti1, which
is doubtless due to the increased electrophilic character of
cationic center. The B(1)-C(19)-Ti(1) bond angle ) 169.04-
(18)° indicates that the cation-anion bridge is essentially linear.
The B-Me distance (1.675 (3) Å) in 2 is typical of a B-Me-
(bridging) bond lengths for B(C6F5)3-derived ion pairs.13
D. Activated Bimetallic Complex [(C5H5)2ZrMe+]2{Me2-
1,4-C6F4[B(C6F5)2]2}2- (3). A summary of crystal structure data
for complex 3 is given in Table 4, and selected bond distances
and angles for 3 are collected in Table 6. The crystal structure
is shown in Figure 4. The two zirconocenium cations are
centrosymmetrically disposed on opposite sides of the arene
plane, presumably reflecting repulsive steric and electrostatic
interactions. The structural features about the zirconocenium
fragments are similar to those in mononuclear analogues, such
-
as Cp2ZrMe+MeB(C6F5)3 (I)20 and (C5R5)2ZrMe+MeB(C6
Figure 4. The molecular structure and atom numbering scheme for
[(C5H5)2ZrMe+]2{Me2-1,4-C6F4[B(C6F5)2]2}-2 (3). Thermal ellipsoids are
drawn at the 50% probability level.
F5)3 complexes13e [C5R5 ) η5-1,2-Me2C5H3 (II), η5-1,3-
-
(SiMe3)2C5H3 (III), and η5-C5Me5 (IV)]. The Zr-Me(terminal)
distance (2.26(1)Å) is 0.26 Å shorter than the Zr-Me(bridging)
separation (2.52(1) Å). The Zr-C-B angle (161.3°) is some-
what smaller than that in I (169.1°). The increased bending of
the Zr-C-B angle in complex 3 is likely due to the greater
steric bulk of the Me2-1,4-C6F4[B(C6F5)2]22- dianion versus the
(2.52(1) Å) is similar to those in I (2.556(2) Å) and in II (2.549-
(3) Å), but shorter than those in III and IV (2.625(5) and 2.640-
(7) Å, respectively). Similarly, the Zr-C(Cp) distances in 3 are
similar to those in I, but slightly shorter than those in II-IV.
The shorter Zr-Me and Zr-C(Cp) distances in 3 likely reflect
the reduced steric requirements of C5H5- versus substituted Cp-
ligands. The influence of the charge on the structure of the
metallocene unit of 3 can be assessed by comparison of the
structures of 3 and the neutral precursor Cp2ZrMe2 (V). Due to
the increased electrophilic character of 3, the Zr-Me(terminal)
contact in 3 (2.26(1) Å) is presumably shorter than the
corresponding distance in V (2.277(5) Å). The metrical param-
-
MeB(C6F5)3 monoanion. The Zr-Me(bridging) distance in 3
(17) (a) Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M.-C.; Roberts, J.
A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448-1464. (c) Beswick,
C. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 10358-10370. (d) Deck,
P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 1772-
1784.
(18) (a) Lanza, G.; Fragala, I. L.; Marks, T. J. Organometallics 2002, 21, 5594-
5612. (b) Lanza, G.; Fragala, I. L.; Marks, T. J. Organometallics 2001,
20, 4006-4017. (c) Lanza, G.; Fragala, I. L.; Marks, T. J. J. Am. Chem.
Soc. 2000, 122, 12764-12777.
(19) Fu, P.; Marks, T. J. Unpublished results.
2-
eters for the Me2-1,4-C6F4[B(C6F5)2]2 dianion are similar to
(20) Guzei, I. A.; Stockland, R. A.; Jordan, R. F. Acta Crystallogr. 2000, C56,
635-636.
those found for the MeB(C6F5)3 anions.13
-
9
14762 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005

Catalyst/Cocatalyst Enhanced Enchainment
A R T I C L E S
Table 7. Ethylene + Isoalkene Copolymerization Results for Catalysts Ti2 and Ti1 with Cocatalysts BN2 and BNa
a Polymerizations carried out on high vacuum line at 24 °C under 1.0 atm ethylene pressure. b Gram polymer/[(mol cationic metallocene)‚atm‚h]. c From
GPC versus polystyrene standards. d Mole percentage, calculated from 13C NMR spectra.22
Methylenecycloalkane Homopolymerization Studies. Be-
fore attempting copolymerizations, the homopolymerizations of
methylenecycloalkanes C and D were surveyed with catalysts
Ti1 + BN1 and Ti2 + BN2. In both cases, negligible yields of
homopolymers are obtained on quenching the reaction mixture
with MeOH. Only isomerization products I and J are detected.
The sterical hindrance of the comonomer doubtless renders
multiple enchainment sluggish.
Ethylene Copolymerization Studies. With the coordinatively
open and reactive CGCTi catalysts, sterically hindered C and
D are successfully incorporated into polyethylene backbones
in a ring-unopened regiochemistry as will be discussed below.
More interestingly, the severely sterically hindered monomer,
1,1,2-trisubstituted monomer E, also undergoes copolymeriza-
tion with ethylene, albeit with formation of an unusual alterna-
tive microstructure (see below). For all of these encumbered
comonomers, it will be seen that the binuclear catalysts/
cocatalysts significantly enhance the comonomer enchainment
selectivity versus the mononuclear analogues under identical
reaction conditions. Similarly, as in the isobutene case reported
previously,4d CGCZr catalysts (Zr1 or Zr2) in combination with
any of the aforementioned cocatalysts introduce negligible
quantities of methylenecycloalkane comonomers into the poly-
ethylene products.
ane copolymers via coordination polymerization. Figure 5A
shows the 13C NMR spectrum of the ethylene + C copolymer
(Table 7, entry 3). Assignments have been made by comparison
to the reported spectrum of an ethylene + F copolymer,4d model
compound 1,1-dimethylcyclopentane,22a and from DEPT results
(Figure 5B). The disappearance of peaks with chemical shifts
in the δ ∼45-46 ppm region in the 45° DEPT spectrum (Figure
5B) indicates that they are quaternary carbons, with all others
carbons being secondary. Thus resonances a, a′, and b can be
assigned as quaternary carbons joining cyclopentane ring and
polymer backbone (depending on the number of ethylene units
enchained between the incorporated cyclopentanes, the chemical
shifts of the different quaternary carbons vary slightly: peak b,
having a single ethylene unit between two rings, a′, having two
ethylene units between the adjacent two rings, a, having three
or more ethylene units between the two rings22c). Carbons along
the polyethylene chain adjacent to the quaternary carbons (peaks
e, f, and g are assigned to the microstructures of enchained
cyclopentanes separated by two or more ethylene units; c and
d are assigned to the microstructures of enchained cyclopentanes
separated by a single ethylene unit) have chemical shifts similar
to those in the corresponding ethylene + isobutene copolymer
chains4d and can be assigned accordingly. Carbon resonances
within the cyclopentane ring are assigned based on the model
compound 1,1-dimethylcyclopentane;22a peak j has an almost
identical chemical shift to the carbon at the same position in
the model compound, while the chemical shifts of resonances
h and i are shifted ∼3 ppm to high field, presumably reflecting
the different influence of methyl versus polyethylene substitu-
ents. The 13C NMR data reveal that most of the enchained
methylenecyclopentane moieties are separated either by a single
When olefins C and D are used as ethylene comonomers,
the CGCTi catalysts cleanly incorporate the hindered comono-
mers in a ring-unopened regiochemistry to yield copolymers K
and L, respectively (see Table 7 for data summary).21 To our
knowledge, this is the first report of the formation of ethylene
+ methylenecyclopentane and ethylene + methylenecyclohex-
(21) (a) B(C6F5)3 does not initiate cationic methylenecycloalkane polymerization
in toluene,21c and the present copolymerizations with ethylene are incon-
sistent with a cationic pathway.21b,c (b) Baird, M. C. Chem. ReV. 2000,
100, 1471-1478 and references therein. (c) Barsan, F.; Karam, A. R.;
Parent, M. A.; Baird, M. C. Macromolecules 1998, 31, 8439-8447.
(22) (a) Breitmaier, E.; Voelter, W. Carbon-13 NMR Spectroscopy; VCH
Publishers: Weinheim, Germany, 1987. (b) Spectral Database for Organic
Compounds, SDBS. (c) The assignment is based on relative intensity
changes of these peaks at different incorporation levels.
9
J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14763

A R T I C L E S
Li et al.
ethylene unit or by two or more ethylene units, and there are
no detectable microstructures with adjacent comonomer units,
even under neat comonomer polymerization conditions (Table
8, entries 5 and 6). Note that in the spectrum of Figure 5C there
are no features in the olefinic region, confirming the predomi-
nance of the ring-unopened enchainment pathway (ring-
unopened structure G and H). Figure 5D shows the 13C spectrum
of the copolymer derived from copolymerization of ethylene
+ neat C (without toluene as the polymerization solvent; Table
8, entry 6), in which the comonomer incorporation level is so
great that the resonances of the -CH2CH2- segments are
greatly diminished. Note that the percentage of the microstruc-
ture consisting of enchained methylenecyclopentane units
separated by a single ethylene unit in the copolymer increases
correspondingly. Figure 6 shows the 13C spectrum of the
ethylene + D copolymer (Table 7, entry 7). Combined with
the spectral information from the DEPT spectrum (Figure 6B),
the ethylene + E copolymer,4d and the model compound 1,1-
dimethylcyclohexane,22a all the peaks can be assigned analo-
gously to the ethylene + C case.
To further increase the comonomer steric hindrance, 1,1,2-
trisubstituted olefin E was also investigated in ethylene co-
polymerization experiments. The 13C spectrum of the copolymer
reveals a unique microstructure assigned to M (Table 7, entry
12), as shown in Figure 7. From the microstructural information
provided by the DEPT spectrum (Figure 7B), the ethylene +
isobutene copolymer,4d and the model compound 3-ethyl-3-
methylheptane,22b all resonances can be assigned using the same
arguments as for the other copolymers above.
Table 7 summarizes data for ethylene copolymerizations with
isoalkenes C-F mediated by the various catalyst/cocatalyst
combinations. The copolymer product molecular weight and
polymerization activities decrease moderately with increasing
catalyst/cocatalyst nuclearity or increasing comonomer concen-
tration. The polymer polydispersities are around 2.0, which is
typical for single-site polymerizations. The comonomer con-
sumed during the copolymerization was typically ∼5% of the
total comonomer employed, thus maintaining the comonomer
concentration essentially constant during the course of the
polymerization. It can be seen from the incorporation levels
compiled in Table 7 that the comonomer reactivity ordering
for constant catalyst is C > D ∼ F . E. This ordering largely
parallels increasing monomer steric hindrance. While C, D, and
E are in the same general range of reactivity, E is by far the
least reactive among all comonomers investigated, presumably
reflecting the pronounced steric encumbrance engendered by
trisubstitution of the double bond and resulting in a different
copolymerization pathway than traversed by the other three
comonomers (see Discussion below).
Regarding nuclearity effects, it is found that under identical
reaction conditions the Ti2 + BN2 catalyst incorporates ∼2.5
times more C, ∼2.5 times more D, ∼2.3 times more E, and
∼5 times more F than the mononuclear Ti1 + BN analogue.
Table 8 summarizes the ethylene + C copolymerization results
achieved by different catalysts under a variety of polymerization
conditions. It can be seen that at higher comonomer incorpora-
tion levels, the binuclear enhancement effects on incorporation
are diminished. Thus, Table 8 entry 4 versus 1 indicates that
the binuclear catalyst/cocatalyst achieves ∼2.5 times greater
comonomer incorporation selectivity than the mononuclear
Figure 5. 13C NMR spectra (100 MHz, C2D2Cl4, 120 °C) of ethylene +
methylenecyclopentane copolymers showing spectral assignments: (A)
sample of Table 7, entry 3; (B) sample of Table 8, entry 6, DEPT spectrum;
(C) Table 7, sample of entry 3, full spectrum; (D) sample of Table 8, entry
6.
9
14764 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005

Catalyst/Cocatalyst Enhanced Enchainment
A R T I C L E S
Table 8. Ethylene + Methylenecyclopentane Copolymerization Results for Catalysts Ti2, C1-Ti2, and Ti1 with Cocatalysts BN2 and BNa
3
comonomer
µ
mol of
reaction
polymer
yield (g)
activity
(105)b
10-
comonomer
incorporation (%)d
Tg
C)
c
c
entry
catalyst
cocatalyst
concentrated (M)
catalyst
time (min)
Mw
Mw/Mn
(
°
1
2
Ti1
BN
1.6
1.6
10
5
10
5
10
5
10
5
5
5
5
5
8
8
5
5
5
5
0.67
0.61
0.55
0.49
0.57
0.45
0.51
0.44
0.82
0.61
8.0
7.3
6.6
5.9
4.3
3.4
6.1
5.3
9.8
7.3
503
342
285
186
203
158
447
325
738
245
2.4
2.1
1.9
2.3
2.0
1.8
2.5
2.2
2.5
2.3
8.3
13.3
14.8
20.4
23.2
33.8
19.5
27.4
15.9
19.6
-33.4
-27.8
-27.0
-25.3
-21.7
-17.5
-24.0
-22.6
-26.0
-25.2
Ti2
BN
3
Ti1
BN2
BN2
BN
BN2
BN
BN2
BN
BN2
1.6
4
Ti2
1.6
5
Ti1
neat
neat
1.6
6
Ti2
7
C1-Ti2
C1-Ti2
Ti1
8
1.6
9e
10e
1.6
10
5
Ti2
1.6
a Polymerizations carried out on high vacuum line at 24 °C under 1.0 atm ethylene pressure. b Gram polymer/[(mol cationic metallocene)‚atm‚h]. c From
GPC versus polystyrene standards. d Calculated from 13C NMR spectra.22 e Polymerizations carried out in chlorobenzene.
Figure 6. 13C NMR spectrum (100 MHz, C2D2Cl4, 120 °C) of an ethylene
+ methylenecyclohexane copolymer showing spectral assignments: (A)
sample of Table 7, entry 7; (B) sample of Table 7, entry 7, DEPT spectrum.
Figure 7. 13C NMR spectrum (100 MHz, C2D2Cl4, 120 °C) of an ethylene
+ 2-methyl-2-butene copolymer showing spectral assignments: (A) sample
of Table 7, entry 12; (B) Table 7, entry 11, DEPT spectrum.
analog, while entry 6 versus 5 indicates that, when the
incorporation level reaches ∼30%, the binuclear catalyst/
cocatalyst achieves only ∼1.4 times greater incorporation
selectivity than the mononuclear analogue. This saturation effect
doubtlessly reflects the high percentage of microstructure at this
point having comonomers separated by single ethylene units
and the severe steric and kinetic constraints on further isoalkene
enrichment. The observed absence of microstructure with
adjacent incorporated comonomer units reflects the steric
impediments of the bulky comonomer, and thus, in principle,
the maximum readily achievable comonomer incorporation level
is ∼50% (the absence of C-C diads precludes accurate
estimation of monomer reactivity ratios based on NMR data).
To further explore the correlation between catalyst structure
and polymerization behavior, methylene-bridged C1-Ti2 was
also synthesized and employed in catalytic studies. It can be
seen from entry 4 versus 8 in Table 8 that C1-Ti2 + BN2
incorporates ∼1.4 times more methylenecyclopentane than does
Ti2 + BN2, presumably because the diminished achievable Ti-
Ti distance enhances cooperative effects and increases the
comonomer enchainment selectivity (see more below).23
To investigate the role of ion pairing on the observed
nuclearity effects, the more polar solvent, chlorobenzene24 (ꢀ
) 5.68), was used as the copolymerization medium (Table 8,
entries 9 and 10). It can be seen that, in C6H5Cl, the binuclear
9
J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14765

A R T I C L E S
Li et al.
ably due to steric constraints, the next monomer (either ethylene
or C) insertion is unfavorable, and chain termination becomes
a competitive pathway, thus plausibly affording the observed
trisubstituted double bond endgroup. The aforementioned
observation of the absence of adjacent enchained comonomer
units (no enchained regioerrors) also supports the above
mechanism. Landis reported a similar observation in terms of
the relationship between insertion regioerrors and chain termina-
tion in 1-hexene polymerization; essentially every insertion
regioerror is committed to chain termination.26
The ethylene + C copolymers are amorphous and exhibit a
single glass transition temperature (Tg), as measured by DSC
(Table 8). Tg values are observed between -17 and -33 °C.
As a reference, Tg is -73 °C for polyisobutene and -93 °C
for amorphous polyethylene,9a,27 while the Tg values of the
present copolymers are higher than those of ethylene-isobutene
copolymers at comparable molecular weights and comonomer
incorporation levels,28 reflecting the influence of the bulky
enchained rings.10 It is found that Tg increases as the comonomer
incorporation level increases, reasonably because there is a
greater proportion of more ordered alternating microstructures
at higher comonomer incorporation levels, thus leading to higher
Tg values,29 in agreement with previous observations on ethylene
+ isobutene copolymers.28
Figure 8. 1H NMR spectrum (400 MHz, C2D2Cl4, 120 °C) of an ethylene
+ methylenecyclopentane copolymer (sample of Table 7, entry 3) in the
olefinic endgroup region, showing spectral assignments.
Scheme 5. Ethylene + Methylenecyclopentane (C)
Copolymerization Chain Termination Pathway
Ti2 + BN2 catalyst incorporates only ∼1.2 times more C than
mononuclear Ti1 + BN, compared with ∼2.5 times enhance-
ment observed in entry 1 versus 4 in Table 8. Thus, the binuclear
enhancement of comonomer enchainment is significantly di-
minished in the more polar, ion pairing weakening solvent. The
copolymerization activities in C6H5Cl are increased moderately
versus those in toluene, in sharp contrast to the significant
activity enhancement in C6H5Cl reported before for CGCZr
catalysts.4a
In regard to chain transfer pathways, the 1H spectrum of the
ethylene + methylenecyclopentane copolymer (Figure 8) indi-
cates that the there are two types of detectable endgroups. One
set is a vinyl group (peaks A and C; because of the high Mw,
the peaks are somewhat broad and peak splitting is not
resolved),9a,25 likely derived from â-H transfer when the last
inserted monomer is ethylene. The other endgroup is a trisub-
stituted double bond (peak B), assignable to the product of
methylenecyclopentane 2,1-insertion, followed by â-H elimina-
tion, as shown in Scheme 5 (assignment based on the model
compound 1-ethyl-1-cyclopentene22b). When methylenecyclo-
pentane undergoes 1,2-insertion, such chain termination pro-
cesses cannot occur due to the lack of a â-H and a thermody-
namically favorable â-C transfer ring-opening pathway. Thus,
methylenecyclopentane 1,2-insertion can be followed by chain
propagation. However, when 2,1-insertion errors occur, presum-
Discussion
I. Sterically Encumbered Methylenecycloalkane Copo-
lymerization. Compared with our previously reported methyl-
enecyclopropane (A) and methylenecyclobutane (B) ring-
opening polymerization processes,7b the methylenecycloalkanes
investigated here, methylenecyclopentane (C) and methylenecy-
clohexane (D), both follow ring-unopened insertion pathways.
The different polymerization pathways can be most readily
ascribed to the differences in ring strain energies of the different
monomers. Thus, A and B have sizable ring string energies,
which would be released in chain propagation via a ring-opening
process. For those monomers with less or no strain energy, such
as C and D, the ring-opening pathway is a thermodynamically
unfavorable process (eq 2).
(23) The corresponding Zr analogue C1-Zr2 exhibits greater 1-hexene incor-
poration selectivity than does C2-Zr2, and the crystal structures of the
different bridged precatalysts show the minimum possible metal-metal
distance in the methylene-bridged complex is ∼1.3 Å shorter than in the
-CH2CH2- bridged analogue, presumably facilitating the cooperative
enchainment effects. Li, H.; Stern, C. T.; Marks, T. J. Macromolecules, in
press.
(24) Light alkenes have similar solubilities in toluene and chlorobenzene. For
example, the solubility of ethylene is reported to be 0.117 mol/L in
toluene24g and 0.118 mol/L in chlorobenzene24h under the present polym-
erization conditions (25 °C, 1 atm). (a) Yang, S. H.; Huh, J. H.; Jo, W. H.
Macromolecules 2005, 38, 1402-1409. (b) Chen, M.-C.; Roberts, J. A.
S.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 4605-4625. (c) Forlini, F.;
Tritto, I.; Locatelli, P.; Sacchi, M. C.; Piemontesi, F. Makromol. Chem.
Phys. 2000, 201, 401-408. (d) Forlini, F.; Tritto, I.; Locatelli, P.; Sacchi,
M. C.; Piemontesi, F. Makromol. Chem. Phys. 2000, 201, 401-408. (e)
Kleinschmidt, R.; Griebenow, Y.; Fink, G. J. Mol. Catal. A-Chem. 2000,
157, 83-90. (f) Coevoet, D.; Cramail, H.; Deffieux, A. Makromol. Chem.
Phys. 1999, 200, 1208-1214. (g) Atiqullah, M.; Hammawa, H.; Hamid,
H. Eur Polym. J. 1998, 34, 1511-1520. (h) Sahgal, A.; La, H. M.; Hayduk,
W. Can. J. Chem. Eng. 1978, 56, 354-357.
These reactions can be analyzed in terms of a normal â-alkyl
elimination reaction, a reverse of CdC bond insertion, which
is therefore estimated to be ∼13 kcal/mol endothermic,7c,30
(26) Liu, Z.; Somsook, E.; White, C. B.; Rosaaen, K. A.; Landis, C. R. J. Am.
Chem. Soc. 2001, 123, 11193-11207.
(27) (a) Brandrup, J.; Immergut, E. H. Eds. Polymer Handbook, 2nd ed.;
Wiley: New York, 1975; Chapter 111, pp 143-144, and also, for PE,
Chapter V, p 16. (b) Ferry, J. D. Viscoelastic Properties of Polymers, 2nd
ed.; Wiley: New York, 1970; Tables 12-111.
(28) Shaffer, T. D.; Canich, J. M.; Squire, K. R. Macromolecules 1998, 31,
5145-5147. Here, ethylene + isobutene copolymerizations mediated by a
N-modified mononuclear CGCTi catalyst result in significant isobutene
enchainment in cases where the feed is very isobutene-rich (isobutene:
ethylene up to 150:1).
(25) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification
of Organic Compounds, 5th ed.; Wiley: New York, 1991; pp 215, 237-
238.
9
14766 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005

Catalyst/Cocatalyst Enhanced Enchainment
A R T I C L E S
Scheme 6. Ethylene + 2-Methyl-2-butene (E) Copolymerization
Insertion Pathway
coupled with release of the cycloalkane ring strain (27, 26, 6.5,
and 0 kcal/mol for cyclopropane, cyclobutane, cyclopentane,
and cyclohexane, respectively).31 Hence, the net result is ∆H
∼ 13 + (-27) ∼ -14; 13 + (-26) ∼ -13; 13 + (-6.5) ∼
+6.5; and +13 kcal/mol in these cases for n ) 0-3, respectively
(eq 2).
For monomers C and D, different catalysts lead to different
catalytic products. It has been reported that (C5H5)2ZrMe+MeB-
(C6F5)3- initially undergoes reaction with monomer C to form
an η3-allyl complexes, as in eq 3,32 without insertion. We
-
previously reported that, with (C5H5)2ZrMe+MeB(C6F5)3 as
III. Binuclear Catalyst/Cocatalyst Enhancement of Co-
monomer Enchainment. Among the three comonomers inves-
tigated, under identical reaction conditions, binuclear catalysts
Ti2 + BN2 and C1-Ti2 + BN2 incorporate significantly greater
levels of comonomer than does the mononuclear Ti1 + BN
analogue, and a shorter bridge connecting the two Ti centers
enhances the binuclear effects. It is plausible that the coordina-
tion of comonomer to a cationic metal center is stabilized by
secondary, possibly agostic interactions,35 with the proximate
cationic metal center, which may facilitate/stabilize comonomer
capture/binding and enhance the subsequent enchainment prob-
ability. It is likely that olefin insertion proceeds via reversible
alkene association at such electrophilic catalytic centers, fol-
lowed by irreversible migratory insertion, and in such a two-
step process, the alkene association equilibrium constant strongly
depends on cocatalyst and alkene structure as well as solvent.36-38
We suggest that the adjacent cationic metal center in the
binuclear catalysts displaces the bulky comonomer coordination/
dissociation equilibrium to the right, while the rate constant for
coordinated comonomer migratory insertion may be comparable
for mononuclear and binuclear catalysts (e.g., Scheme 7).36 It
has also been reported that polar solvents can compete for/
coordinate to electrophilic metal centers and weaken/replace
agostic interactions,39 which are proposed here to be central to
the observed binuclear effects. The present copolymerization
results in a more polar, ion pair weakening medium24 suggest
the proposed agostic interactions can also be weakened by polar
C6H5Cl (eq 4).
the catalyst, C and D are eventually converted to the thermo-
dynamically more stable internal olefins I and J.7b When
CGCZr+ catalysts are employed, the same isomerization reac-
tions occur to again yield internal olefins I and J as products.
When C or D and ethylene are both present, neither (C5H5)2-
-
ZrMe+MeB(C6F5)3 nor CGCZr+ catalysts are capable of
incorporating the comonomer into the polymer backbone, while
when CGCTi+ catalysts are used, ring-unopened copolymers
K and L are produced. This difference in the catalytic
comonomer incorporation selectivity was previously observed
in our work on isobutene copolymerization: CGCTi+ catalysts
incorporate significant quantities of isobutene into polyethylene
backbones, while CGCZr+ catalysts do not. One plausible
argument is that tighter ion pairing in CGCZr versus CGCTi
structures33 leads to lower reactivity in terms of bulky comono-
mer enchainment.
II. Sterically Encumbered 1,1,2-Trisubstituted Isoalkene
Copolymerization. In the copolymerization of ethylene and
2-methyl-2-butene, only excess comonomer, 2-methyl-2-butene
(E), is detected in the reaction mixture mother liquor after
removal of the polymeric product by filtration, with no 2-methyl-
1-butene (N) isomerization product detected. From a thermo-
dynamic point of view, E is more stable than N by ∼1.8 kcal/
mol,34 and the E h N equilibrium therefore lies to the left, as
in Scheme 6. Nevertheless, if there is rapid E h N equilibration
and N is much more reactive with respect to enchainment than
E, it will be captured and inserted selectively, with all of the E
draining away through N. Thus, if E were activated at the
catalyst center and enchained under the normal reaction condi-
tions, the observed N-derived microstructure would be formed.
To further confirm the above suggested pathway, N was
examined as comonomer in ethylene copolymerization. The 13
C
NMR spectrum of the copolymer produced from ethylene + N
is indistinguishably derived from ethylene + E, further sup-
porting the proposed mechanism (Scheme 6).
Summary
We have synthesized and characterized the binuclear orga-
notitanium “constrained geometry catalysts” (CGCs) (µ-CH2-
(29) Johnston, N. W. J. Macromol. Sci., ReV. Macromol. Chem. 1976, C14,
215-250.
(35) (a) Scherer, W.; McGrady, G. S. Angew. Chem., Int. Ed. 2004, 43, 1782-
1806. (b) Prosenc, M. H.; Brintzinger, H. H. Organometallics 1997, 16,
3889-3894. (c) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29,
85-93. (d) Prosenc, M. H.; Janiak, C.; Brintzinger, H. H. Organometallics
1992, 11, 4036-4041. (e) Cotter, W. D.; Bercaw, J. E. J. Organomet. Chem.
1991, 417, C1-C6. (f) Krauledat, H.; Brintzinger, H. H. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 1412-1413. (g) Piers, W. E.; Bercaw, J. E. J. Am.
Chem. Soc. 1990, 112, 9406-9407. (h) Brookhart, M.; Green, M. L. H.;
Wong, L. L. Prog. Inorg. Chem. 1988, 36, 1-124. (i) Clawson, L.; Soto,
J.; Buchwald, S. L.; Steigerwald, M. L.; Grubbs, R. H. J. Am. Chem. Soc.
1985, 107, 3377-3378.
(30) (a) King, W. A.; Marks, T. J. Inorg. Chim. Acta 1995, 229, 343-354. (b)
Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701-7715.
(31) (a) Isaacs, N. S. Physical Organic Chemistry; Wiley: New York, 1987;
pp 282-291. (b) McMillan, D. F.; Golden, D. M. Annu. ReV. Phys. Chem.
1982, 33, 493-532. (c) Benson, S. W. Thermochemical Kinetics, 2nd ed.;
Wiley: New York, 1976; appendix.
(32) Horton, A. D. Organometallics 1996, 15, 2675-2677.
(33) Luo, L.; Marks, T. J. in ref 3e, pp 97-106.
(34) Lide, D. R.; Kehiaian, H. V. CRC Handbook of Thermophysical and
Thermochemical Data; CRC Press: Boca Raton, FL.
9
J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14767

A R T I C L E S
Li et al.
Scheme 7. Binuclear Catalysts Facilitate Bulky Comonomer Enchainment
CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 [EBICGC-
(TiMe2)2;Ti2] and (µ-CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)]-
(TiMe2)}2 [EBICGC(TiMe2)2; C1-Ti2] together with the
bifunctional bisborane activator 1,4-(C6F5)2BC6F4B(C6F5)2 (BN2)
for ethylene + hindered isoalkene copolymerization processes.
Specifically examined are the poorly responsive comonomers
1,1-disubstituted isobutene, methylenecyclopentane, methyl-
enecyclohexane, and 1,1,2-trisubstituted 2-methyl-2-butene. For
the latter three comonomers, we report for the first time that
these can be incorporated in large percentages into polyethylene
backbones via coordination polymerization processes. Large
increases in comonomer enchainment efficiency into the poly-
ethylene microstructure are observed versus the corresponding
mononuclear catalyst [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe2
(Ti1) + B(C6F5)3 (BN) under identical polymerization condi-
tions. Solvent polarity also plays significant role in binuclear
ion pairing and in comonomer enchainment selectivity.
(36) (a) Casey, C. P.; Tunge, J. A.; Lee, T.-Y.; Fagan, M. A. J. Am. Chem. Soc.
2003, 125, 2641-2651. (b) Landis, C. R.; Rosaaen, K. A.; Uddin, J. J.
Am. Chem. Soc. 2002, 124, 12062-12063. (c) Casey, C. P.; Lee, T.-Y.;
Tunge, J. A.; Carpenetti, D. W., II. J. Am. Chem. Soc. 2001, 123, 10762-
10763. (d) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 85-
93.
(37) (a) Stoebenau, E. J., III; Jordan, R. F. J. Am. Chem. Soc. 2004, 126, 11170-
11171. (b) Stoebenau, E. J., III; Jordan, R. F. J. Am. Chem. Soc. 2003,
125, 3222-3223. (c) Carpentier, J.-F.; Maryin, V. P.; Luci, J.; Jordan, R.
F. J. Am. Chem. Soc. 2001, 123, 898-909.
(38) Brandow, C. G.; Mendiratta, A.; Bercaw, J. E. Organometallics 2001, 20,
4253-4261.
(39) (a) Bouwkamp, M. W.; de Wolf, J.; del Hierro Morales, I.; Gercama, J.;
Meetsma, A.; Troyanov, S. I.; Hessen, B.; Teuben, J. H. J. Am. Chem.
Soc. 2002, 124, 12956-12957. (b) Kawabe, M.; Murata, M. Macromol.
Chem. Phys. 2001, 202, 2440-2446. (c) Rybtchinski, B.; Konstantinovsky,
L.; Shimon, L. J. W.; Vigalok, A.; Milstein, D. Chem.sEur. J. 2000, 6,
3287-3292. (d) Carr, N.; Mole, L.; Orpena, A. G.; Spencer, G. L. J. Chem.
Soc., Dalton Trans. 1992, 18, 2653-2662. (e) Peng, T. S.; Gladysz, J. A.
J. Am. Chem. Soc. 1992, 114, 4174-4181. (f) Agbossou, S. K.; Bodner,
G. S.; Patton, A. T.; Gladysz, J. A. Organometallics 1990, 9, 1184-1191.
(g) Schmidt, G. F.; Brookhart, M. J. Am. Chem. Soc. 1985, 107, 1443-
1444.
Acknowledgment. This research was supported by grants
from NSF (CHE-0415407) and DOE (86ER13511). L.L. thanks
Dow Chemical Company for a postdoctoral fellowship. We
thank Dr. J. Wang for helpful discussions.
Supporting Information Available: Details of catalyst, co-
catalyst syntheses, polymerization experiments, and crystal
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA052995X
9
14768 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005