Intramolecular ion-ion interactions in Zwitterionic metallocene olefin polymerization catalysts derived from 'tucked-in' catalyst precursors and the highly electrophilic boranes XB(C6F5)2 (X = H5 C6F5)

Article abstract of DOI:10.1021/ja970140h

The reactions of so called 'tuck-in' permethyl zirconocene compounds Cp*(η⁵-η¹-C₅Me₄CH₂)ZrX (X = Cl(1a), C₆H₅ (1b), CH₃ (1c)) with the highly electrophilic boranes HB(C₆F₅)₂ and B(C₆F₅)₃ are described. The products are zwitterionic olefin polymerization catalysts. Reactions with 1a and 1b yielded single products cleanly, but reactions with tuck-in methyl starting material 1c gave mixtures. Spectroscopic and structural studies showed that the electrophilic zirconium center in the product zwitterions was stabilized by a variety of mechanisms. In the products of reaction between 1a and 1b with HB(C₆F₅)₂, Cp*[η⁵,η¹-C₅Me₄CH₂B(C₆F₅)₂(μ-H)]ZrX (X = Cl (2a), 74%), C⁶H⁵ (2b, 62%)), the metal is chelated by a pendant hydridoborate moiety. Chloride product 2a was characterized crystallographically. In the reaction of B(C⁶F⁵)³ with 1a, the fluxional zwitterionic product Cp*[η⁵-C⁵Me⁴CH₂B(C₆F₅)₃]ZrCl (3a, 84%) is stabilized by a weak donor interaction between one of the ortho-fluorine atoms of the -CH₂B^- (C₆F₅)₃ counteranion and the zirconium center (Zr-F = 2.267(5) A?). In the product of the reaction between lb and B(C₆F₅)₃, Cp*[η⁵-C₅Me₄CH₂B(C₆F₅)₃]ZrC₆H₅ (3b, 82%), a similar ortho- fluorine interaction was found in a yellow kinetic product (y-3b), which converted upon heating gently to a thermodynamic orange polymorph (o-3b) in which the zirconium center is compensated via an agostic interaction from an ortho C-H bond of the phenyl group and an interaction between the methylene group of the -CH₂B (C₆F₅)₃ counteranion. These compounds were both characterized by X-ray crystallography. Zwitterion o-3b reacts with H₂ to form the zwitterionic hydride Cp*[η⁵-C₅Me₄CH₂B(C₆F₅)₃]ZrH (4, 77%), characterized by NMR spectroscopy and X-ray crystallography to reveal a return to the ortho-fluorine mode of stabilization. Compounds 2a, 3a, o-3b, and 4 were all found to be active ethlyene polymerization catalysts; the chloride derivatives required minimal amounts of methylaluminoxane (MAO) to alkylate the zirconium center. Polymerization data are discussed in light of the structural findings for the catalysts employed.

Full text of DOI:10.1021/ja970140h

5132
J. Am. Chem. Soc. 1997, 119, 5132-5143
Intramolecular Ion-Ion Interactions in Zwitterionic Metallocene
Olefin Polymerization Catalysts Derived from “Tucked-In”
Catalyst Precursors and the Highly Electrophilic Boranes
XB(C6F5)2 (X ) H, C6F5)
Yimin Sun,† Rupert E. v. H. Spence,‡ Warren E. Piers,*,†,1 Masood Parvez,† and
Glenn P. A. Yap§
Contribution from the Department of Chemistry, The UniVersity of Calgary,
2500 UniVersity DriVe N.W., Calgary, Alberta T2N 1N4, Canada, NOVA Research &
Technology Corp., 2928 16th Street N.E., Calgary, Alberta T2E 7K7, Canada, and Windsor
Molecular Structure Center, Department of Chemistry and Biochemistry, UniVersity of Windsor,
Windsor, Ontario N9B 3P4, Canada
ReceiVed January 16, 1997X
Abstract: The reactions of so called “tuck-in” permethyl zirconocene compounds Cp*(η5-η1-C5Me4CH2)ZrX (X )
Cl (1a), C6H5 (1b), CH3 (1c)) with the highly electrophilic boranes HB(C6F5)2 and B(C6F5)3 are described. The
products are zwitterionic olefin polymerization catalysts. Reactions with 1a and 1b yielded single products cleanly,
but reactions with tuck-in methyl starting material 1c gave mixtures. Spectroscopic and structural studies showed
that the electrophilic zirconium center in the product zwitterions was stabilized by a variety of mechanisms. In the
products of reaction between 1a and 1b with HB(C6F5)2, Cp*[η5,η1-C5Me4CH2B(C6F5)2(µ-H)]ZrX (X ) Cl (2a),
74%), C6H5 (2b, 62%)), the metal is chelated by a pendant hydridoborate moiety. Chloride product 2a was
characterized crystallographically. In the reaction of B(C6F5)3 with 1a, the fluxional zwitterionic product Cp*[η5-
C5Me4CH2B(C6F5)3]ZrCl (3a, 84%) is stabilized by a weak donor interaction between one of the ortho fluorine
atoms of the -CH2B-(C6F5)3 counterion and the zirconium center (Zr-F ) 2.267(5) Å). In the product of the
reaction between 1b and B(C6F5)3, Cp*[η5-C5Me4CH2B(C6F5)3]ZrC6H5 (3b, 82%), a similar ortho-fluorine interaction
was found in a yellow kinetic product (y-3b), which converted upon heating gently to a thermodynamic orange
polymorph (o-3b) in which the zirconium center is compensated via an agostic interaction from an ortho C-H bond
of the phenyl group and an interaction between the methylene group of the -CH2B-(C6F5)3 counteranion. These
compounds were both characterized by X-ray crystallography. Zwitterion o-3b reacts with H2 to form the zwitterionic
hydride Cp*[η5-C5Me4CH2B(C6F5)3]ZrH (4, 77%), characterized by NMR spectroscopy and X-ray crystallography
to reveal a return to the ortho-fluorine mode of stabilization. Compounds 2a, 3a, o-3b, and 4 were all found to be
active ethlyene polymerization catalysts; the chloride derivatives required minimal amounts of methylaluminoxane
(MAO) to alkylate the zirconium center. Polymerization data are discussed in light of the structural findings for the
catalysts employed.
Introduction
ligand set and the ion-ion interactions present between this
highly electrophilic cation and its counteranion.7 Although the
affect of ion pairing on activity is not intimately understood,
generally strong ion-ion interactions lead to lower polymeri-
zation activity as the counterion blocks the olefin monomer from
the active site of the catalyst. Consequently, the nature of the
cocatalyst or activator employed to generate the active cations
responsible for catalysis often has a significant effect on the
behavior of the resultant catalyst system.8 The two most
important classes of catalyst activators are methylaluminoxane
Polyolefins produced by group 4 metal metallocene based
technologies are gaining an increasing share of the worldwide
plastics market.2 After a decade of intense research activity in
both industrial and academic laboratories, the details of the
means by which these catalysts mediate the polymerization
reaction are now relatively well understood.3 Fourteen-electron
cationic species4 are commonly presumed to be responsible for
olefin enchainment while activity5 and stereoregularity6,3b are
modulated by the steric/electronic properties of the ancillary
(5) (a) Piccolrovazzi, N.; Pino, P.; Consiglio, G.; Sirone, A.; Moert, M.
Organometallics 1990, 9, 3098. (b) Lee, I. K.; Gauthier, W. J.; Ball, J. M.;
Iyengary, B.; Collins, S. Organometallics 1992, 11, 2115. (c) Collins, S.;
Gauthier, W. J.; Holden, D. A.; Kuntz, B. A.; Taylor, N. J.; Ward, D. G.
Organometallics 1991, 10, 2061.
(6) (a) Herzog, T. A.; Zubris, D. L.; Bercaw, J. E. J. Am. Chem. Soc.
1996, 118, 11988. (b) Gilchrist, J. H.; Bercaw, J. E. J. Am. Chem. Soc.
1996, 118, 12021.
(7) (a) Richardson, D. E.; Alameddin, N. G.; Ryan, M. F.; Hayes, T.;
Eyler, J. R.; Siedle, A. R. J. Am. Chem. Soc. 1996, 118, 11244. b) Deck,
P. A.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 6128. (c) Siedle, A. R.;
Newmark, R. A. J. Organomet. Chem. 1995, 497, 119. (d) Vizzini, J. C.;
Chien, J. C. W.; Gaddam, N. B.; Newmark, R. A. J. Polym. Sci. A, Polym.
Chem. 1994, 32, 2049. (e) Giardello, M. A.; Eisen, M. S.; Stern, C. L.;
Marks, T. J. J. Am. Chem. Soc. 1993, 115, 3326. (f) Jia, L.; Yang, X.;
Ishihara, A.; Marks, T. J. Organometallics 1995, 14, 3135.
† The University of Calgary.
‡ NOVA Research & Technology Corp.
§ University of Windsor.
X Abstract published in AdVance ACS Abstracts, May 1, 1997.
(1) Author to whom correspondence may be addressed. FAX: 403-289-
9488. E-mail: wpiers@chem.ucalgary.ca.
(2) Thayer, A. M. Chem. Eng. News 1995, 73 (37), 15.
(3) (a) Mohring, P. C.; Coville, N. J. J. Organomet. Chem. 1994, 479,
1. (b) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (c) Bochmann, M. J.
Chem. Soc., Dalton Trans. 1996, 255. d) Marks, T. J. Acc. Chem. Res.
1992, 25, 57.
(4) (a) Jordan, R. F. AdV. Organomet. Chem. 1991, 32, 325. (b) Jordan,
R. F.; Bradley, P. K.; LaPointe, R. E.; Taylor, D. F. New J. Chem. 1990,
14, 505.
S0002-7863(97)00140-6 CCC: $14.00 © 1997 American Chemical Society

Intramolecular Ion-Ion Interactions in Zwitterionic Catalysts
J. Am. Chem. Soc., Vol. 119, No. 22, 1997 5133
Scheme 1. Reactions of Cp2Zr(CH3)2 with HB(C6F5)2
(MAO)9 based and those yielding polyfluorinated borate coun-
terions (e.g., B(C6F5)3,10 [HNR3]+[B(C6F5)4]-,11 or [Ph3C]+[B-
(C6F5)4]- 12). Given the impact ion-ion interactions can have
on polymerization activity, a detailed understanding of these
contacts is necessary for the rational design of new catalysts
and/or new activators.13
While little is known regarding [Cp2MR]+[MAO]- interac-
tions,14 several classes of [Cp2MR]+[borate]- contacts have been
structurally and/or spectroscopically established. For example,
when cationic metallocenes with borate counterions of the
general formula [XB(C6F5)3]- (X ) C6F5, H, or an abstracted
alkyl group) are generated in the absence of Lewis base donors,
the borate counterion may stabilize the highly Lewis acidic metal
center via a fluorine atom lone pair,15 a three-center two-electron
µ-hydride16 or methyl bridge,17 or the arene π-system of an
abstracted benzyl group.18
One strategy for controlling ion-ion interactions is to prepare
zwitterionic catalysts wherein the charges are physically sepa-
rated by covalent attachment of a borate counterion within the
ancillary structure of the supporting ligand of the catalyst.
Hlatky and Turner19 had early success using this tactic,
producing highly active zwitterionic catalysts incorporating
borate- and carborane-type counterions. More recently the
groups of Bochmann20 and Erker21 have taken diverse synthetic
approaches toward zwitterionic catalysts. Although these
compounds exhibit ethylene polymerization activities akin to
non-zwitterionic analogs, quantitative comparison studies to
show that higher activities can be achieved by enforced charge
separation are yet to be done.
two competing pathways by which HB(C6F5)2 reacts with simple
zirconocene bis-alkyls (Scheme 1).16a One involved loss of
alkane to produce borane-stabilized alkylidene derivatives of
zirconocene, while the other was an alkyl/hydride exchange
reaction24 followed by complexation of HB(C6F5)2 to the newly
formed Zr-H moiety, giving borate derivatives. While interest-
ing, this chemistry ran counter to the goal of forming zwitte-
rionic compounds capable of polymerizing olefins in the absence
of further cocatalysts.
We decided to utilize the alkyl/hydride exchange reaction to
our advantage and treat group 4 metallocene compounds with
tethered alkyl ligands with HB(C6F5)2 to prepare zwitterionic
compounds. One family of compounds with the requisite
tethered alkyl functionality are the so-called “tucked-in” per-
methyl metallocenes in which one of the C5Me5 (Cp*) methyl
groups has undergone metallation and is σ bound to the metal
center. Several such compounds are known, including the tuck-
in chloro, phenyl, and methyl derivatives Cp*(C5Me4CH2)ZrX
(X ) Cl (1a),25 C6H5 (1b),26 CH3 (1c)). Herein we describe
the reactions of these tuck-in precursors with both HB(C6F5)2
and the more common activator B(C6F5)3. The zwitterionic
products provide structural insights into cation-anion interac-
tions and are highly active olefin polymerization catalysts which
do not require other cocatalysts.
Our initial attempts to prepare zwitterionic catalysts for olefin
polymerization involved hydroboration of olefinic groups at-
tached to the cyclopentadienyl ligand framework of conventional
metallocene catalysts22 with use of the highly electrophilic
borane reagent [HB(C6F5)2]2.23 During the course of these
studies, we found that HB(C6F5)2, in addition to hydroborating
the pendant olefin, reacted with the Zr-C bonds of alkyl
compounds containing these ligands. Separate studies revealed
(8) For example, the contrasting results found in the following references
may be attributed to counterion effects. a) Scollard, J. D.; McConville, D.
H. J. Am. Chem. Soc. 1996, 118, 10008. b) Scollard, J. D.; McConville, D.
H.; Payne, N. C.; Vittal, J. J. Macromolecules 1996, 29, 5241.
(9) Sinn, H.; Kaminsky, W. AdV. Organomet. Chem. 1980, 18, 99.
(10) (a) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.
(b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623.
(11) Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840.
(12) (a) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1992, 434,
C1. (b) Chien, J. C. W.; Tsai, W. M.; Rausch, M. D. J. Am. Chem. Soc.
1991, 113, 8570.
(13) Chen, Y.-X.; Stern, C. L.; Yang, S.; Marks, T. J. J. Am. Chem.
Soc. 1996, 118, 12451.
(14) Harlan, C. J.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1995,
117, 6465.
(15) Horton, A. D.; Orpen, A. G. Organometallics 1991, 10, 3910.
(16) (a) Spence, R. E. v. H.; Parks, D. J.; Piers, W. E.; MacDonald, M.;
Zaworotko, M. J.; Rettig, S. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 1230.
(b) Sun, Y.; Piers, W. E.; Rettig, S. J. Organometallics 1996, 15, 4110.
(17) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
10015.
Results and Discussion
Reactions of Tucked-In Zirconocenes with HB(C6F5)2.
Tuck-in precursors 1a and 1b were prepared according to
literature procedures25,26 by thermal elimination of RH from
Cp*2Zr(Cl)CH2CMe3 and Cp*2Zr(C6H5)2, respectively. The
tuck-in methyl compound 1c was prepared straightforwardly
from chloro complex 1a and MeLi and isolated as a red powder
in 83% yield.
The chloro and phenyl tuck-in compounds reacted rapidly
and cleanly with 1 equiv of HB(C6F5)2 and 100% regioselec-
tivity for the metallated Zr-C bond (eq 1), yielding zwitterionic
(18) (a) Bochmann, M.; Lancaster, S. J.; Hursthouse, M. B.; Abdul Malik,
K. M. Organometallics 1994, 13, 2235. (b) Bochmann, M.; Lancaster, S.
J. Makromol. Chem. Rapid Commun. 1993, 14, 807. (c) Bochmann, M.;
Lancaster, S. J. Organometallics 1993, 12, 633. (d) Pellecchia, C.; Immirzi,
A.; Grassi, A.; Zambelli, A. Organometallics 1994, 13, 4473.
(19) Hlatky, G. G.; Turner, H. W.; Eckman, R. R. J. Am. Chem. Soc.
1989, 111, 2728.
(20) Bochmann, M.; Lancaster, S. J.; Robinson, O. B. J. Chem. Soc.,
Chem. Commun. 1995, 2081.
(21) (a) Ruwwe, J.; Erker, G.; Fro¨hlich, R. Angew. Chem., Int. Ed. Engl.
1996, 35, 80. (b) Temme, B.; Karl, J.; Erker, G. Chem. Eur. J. 1996, 2,
919. (c) Temme, B.; Erker, G.; Karl, J.; Luftmann, H.; Fro¨hlich, R.; Kotila,
S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1755.
complexes 2a and 2b in excellent yield. (All new compounds
were characterized by 1H, 13C, 19F, and 11B NMR spectroscopy;
these data are collected in Tables 1 and 2.) For compounds 2,
(22) Spence, R. E. v. H.; Piers, W. E. Organometallics 1995, 14, 4617.
(23) (a) Parks, D. J.; Spence, R. E. v. H.; Piers, W. E. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 809. (b) Piers, W. E.; Spence, R. E. v. H. U.S.
Patent 5,496,960, March 5, 1996 (University of Guelph).
(24) Marsella, J. A.; Caulton, K. G. J. Am. Chem. Soc. 1982, 104, 2361.
(25) Tjaden, E. B.; Stryker, J. M. J. Am. Chem. Soc. 1993, 115, 2083.
(26) Schock, L. E.; Brock, C. P.; Marks, T. J. Organometallics 1987, 6,
232.

5134 J. Am. Chem. Soc., Vol. 119, No. 22, 1997
Sun et al.
Table 1. 1H and 13C{1H} NMR Data for Isolated New Compoundsa
a All spectra were recorded in C6D6 at room temperature unless otherwise indicated. b Unless indicated, carbons without protons attached to them
were generally not observed. c Low-temperature spectra recorded in C7D8. d Sample not stable in solution long enough to accumulate a quality 13
NMR spectrum.
C
the diastereotopic relationship between protons and carbons with
the same connectivity of the metallated Cp* ring is retained,
indicating the presence of a strong interaction between the
hydridoborate counterion and the zirconium center as depicted.
The 19F NMR spectra for both compounds 2 show that the C6F5
groups are diastereotopic as well. The chemical shifts for the
11B nuclei of -25.4 and -24.9 ppm are diagnostic for the
tetracoordinate, anionic environment of borates.27 Broad reso-
nances at about 0.4 ppm for the µ-H protons are at a comparable
chemical shift to that observed for those of the dihydridoborate
complex Cp2Zr[(µ-H)2B(C6F5)2]2 (0.38 ppm).16a The resonances
due to the Cp* ligand for 2a and 2b are shifted upfield by about
0.2-0.4 ppm relative to tuck-in compounds 1; this shift is
common to all of the new compounds described herein and
signals that boration of the metallated methylene has occurred.
In phenyl complex 2b, resonances for the protons and carbons
of the phenyl group are chemically inequivalent, suggesting
restricted rotation about the Zr-Cipso bond, a notion supported
by the broadening and coalescence of related signals in the 1H
spectrum at elevated temperatures. One of the ortho CH groups
in the phenyl substituent resonates at higher field than normal
(C-H, 6.04 ppm; C-H, 104.2 ppm). A NOESY experiment
1
allowed for precise assignment of all the resonances in the H
NMR spectrum (Table 1). Based on a crosspeak between the
ortho proton at 6.04 ppm and the broad peak at 0.4 ppm for the
µ-H of the hydridoborate moiety, the upfield ortho CH group
must occupy the endo position of the metallocene wedge. The
high-field chemical shift does not appear to be due to an agostic
interaction between this ortho CH and zirconium as no
supporting evidence for such an interaction was found in the
IR or the NMR spectra of 2b. A similar upfield shift in the
tuck-in phenyl derivative 1b was rationalized on the basis of
hindered rotation of the ligand fixing one of the ortho C-H
groups in an unusual magnetic environment.26
A strong chelating borate interaction between -B-H- and
the zirconium center was confirmed in an X-ray structural
analysis of 2a. An ORTEP diagram of the molecule is shown
in Figure 1. Selected bond lengths for crystallographically
characterized molecules in this paper are given in Table 3;
pertinent angles are found in Table 4. For 2a, the hydrogen
atom bridging boron and zirconium was located on the differ-
ence Fourier map and fixed at this position. The Zr-H distance
(27) Kidd, R. G. In NMR of Newly Accessible Nuclei, Volume 2; Laszlo,
P., Ed.; Academic Press: New York, 1983; Vol. 2.

Intramolecular Ion-Ion Interactions in Zwitterionic Catalysts
J. Am. Chem. Soc., Vol. 119, No. 22, 1997 5135
Table 2. 19F and 11B NMR Data for New Compoundsa
the HB(C6F5)2 reagent is able to add to the Zr-C bonds in 1a-b
to form borate-stabilized zwitterions relatively free of internal
strain.
19F NMRb
o-F
m-F
p-F
11B NMR
Stabilization of electrophilic metal centers by B-H bonds
has precedent in related compounds from our lab,16 in cationic
zirconocenes containing carborane counterions,19,29 and in
neutral scandium derivatives incorporating dicarbollide ancillary
ligands.30 In some instances, this interaction is weak enough
to engender high lability of the counterion and high reactivity
associated with the electrophilic center. The chelating nature
of the borate interaction in complexes 2 appears to render this
B-H interaction relatively non-labile as both 2a and 2b serve
as poor ethylene polymerization catalysts (Vide infra).
Reaction of the tuck-in methyl zirconocene 1c with 1 equiv
of HB(C6F5)2 was not as clean as the above described chemistry.
This reaction appears to be complicated by a lack of regiose-
lectivity the borane reagent exhibits when presented with the
two Zr-C bonds in 1c. The minor product (≈40%) is
analogous to 2a and 2b, based on a characteristic pattern for
2a
2b
-130.5
-131.1
-129.1
-130.4
-140.9
-128.6
-128.7(2)
-130.6
-134.1
-167.7
-163.7
-157.6
-158.4
-158.5
-159.2
-159.1
-155.9(2)
-156.4
-25.4
-164.1
-24.9
-13.8
3a
-162.0
-159.1
-159.5
-163.2
-163.5
-163.8
-164.4
-165.7
-160.7
-163.7
-164.4
-162.6
-155.7
-156.5
-163.0
-163.5
-164.5
3ac
o-3b
-134.1
-127.1
-131.8
-148.0(br)
-133.1
-127.2
-129.3
-130.1
-134.1
-143.3(br)
-159.4
-157.1(2)
-159.7
-13.3
-13.4
o-3bd
4
-159.2
-158.9
-159.5
-161.1
4e
2
the methylene protons (δ 3.33, d, JHH ) 14.1 Hz; δ 2.36, dd,
3JHH ) 6.5 Hz). The major product (eq 2) likely arises from
a All values are chemical shifts in C6D6. 19F spectra were referenced
to C6F6 at -163.0 ppm; 11B spectra were referenced to BF3‚Et2O at
0.0 ppm. b Many of the fluorine signals were broadened at room
temperature. c -61 °C. d -72 °C. e -85 °C.
initial reaction with the terminal Zr-CH3 group rather than the
17
tucked-in methylene group since CH3B(C6F5)2 was also
observed in the medium at short reaction times; it subsequently
was used up in an unknown reaction that formed the major
product in the final mixture. We do not know the structure of
the major compound, but it appears to contain a Zr(CH3)C6F5
structural unit31 based on a characteristic triplet for Zr-CH3 at
0.22 ppm (JH-F ) 8.5 Hz), which we have observed in other
complexes in which C6F5 transfer to zirconium has occurred.
A similar lack of regioselectivity, with its corresponding
complexity, was found in the reactions of tuck-in methyl
complex 1c with B(C6F5)3. This chemistry was therefore not
pursued further.
Reaction of Cp*(η5,η1-C5Me4CH2)ZrCl with B(C6F5)3.
Given the propensity of the HB(C6F5)2 reagent to yield
compounds with strong, potentially deleterious, hydridoborate
metal interactions in the zwitterionic products, we performed
10a
analogous chemistry using B(C6F5)3 as an alkyl abstracting
Figure 1. ORTEP drawing of 2a.
agent instead. As in the reactions with HB(C6F5)2, the tuck-in
chloride and phenyl starting materials gave clean products, with
selective reaction at the metallated methylene group. Thus,
treatment of 1a with 1 equiv of B(C6F5)3 in hexanes gave a
yellow precipitate whose microanalysis suggested a formula for
the expected zwitterion, 3a. As a solid, the compound is quite
stable and may be prepared conveniently in multigram quantities
by this method. In solution, however, its behavior is complex
is typical for such bonds as compared with related hydridoborate
compounds,28 but the B-H distance of 1.53 Å is somewhat
elongated due to the constraints of the chelate structure. The
chelating borate linkage is positioned such that the bridging
hydride occupies one of the coordination sites available in the
plane bisecting the Cp donors; the Cl-Zr-H angle is 99.6°.
Metrical parameters for the Cp* ring to which the borate
counterion is attached are for the most part undistorted compared
with the unsubstituted Cp* ring in the molecule. Worthy of
particular note, however, is the fact that C(18) is tilted away
from zirconium out of the plane defined by the five cyclopen-
tadienyl carbons by only 2°, compared to tilt angles of ≈4-6°
for the rest of the ring methyl groups in the molecule. Thus,
(29) Crowther, D. J.; Borkowsky, S. L.; Swenson, D.; Meyer, T. Y.;
Jordan, R. F. Organometallics 1993, 12, 2897.
(30) Bazan, G. C.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1993,
12, 2126.
(31) (a) For example, reaction of Cp2Zr(CH3)2 with CH3B(C6F5)2 initially
gives a cationic species, [Cp2ZrCH3]+[(CH3)2B(C6F5)2]-; C6F5 transfer to
zirconium occurs over the course of about an hour in solution, giving Cp2-
Zr(CH3)C6F5 and (CH3)2BC6F5 as the products (ref 31b). Transfer of C6F5
groups thus appears to be facile from R2B(C6F5)2 type counterions, an
example of which could arise from initial H/CH3 exchange in the reaction
of 1c with HB(C6F5)2. (b) Piers, W. E. Unpublished results.
(28) For example, Zr-Have and B-Have in the compound Cp2Zr[(µ-H)2B-
(C6F5)2]2 are 2.07 and 1.24 Å, respectively. Sun, Y.; Piers, W. E.; Parvez,
M. Unpublished results.

5136 J. Am. Chem. Soc., Vol. 119, No. 22, 1997
Sun et al.
Table 3. Selected Bond Lengths (Å) for 2a, 3a, y-3b, o-3b, and 4
2a
3a
y-3b
o-3b
4
Zr(1)-Cl(1)
2.413(3)
2.517(9)
2.507(10)
2.477(8)
2.510(8)
2.488(9)
1.66(1)
1.42(1)
1.42(1)
1.41(1)
1.42(1)
1.43(1)
2.01
Zr(1)-Cl(1)
2.404(3)
2.589(8)
2.542(8)
2.494(8)
2.505(9)
2.488(8)
1.67(1)
1.42(1)
1.42(1)
1.42(1)
1.46(1)
1.41(1)
2.267(5)
1.414(9)
1.317(9)
3.821
Zr-C(76)
Zr-C(5)
Zr-C(1)
Zr-C(4)
Zr-C(2)
Zr-C(3)
C(6)-B(1)
C(1)-C(5)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(1)-C(2)
Zr-F(65)
2.227(6)
2.461(5)
2.511(5)
2.476(5)
2.546(5)
2.560(5)
1.654(8)
1.433(8)
1.415(8)
1.428(8)
1.445(7)
1.441(8)
2.414(3)
1.415(7)
1.344(7)
3.330
Zr-C(76)
2.195(8)
2.341(6)
2.444(6)
2.487(6)
2.566(6)
2.585(6)
1.705(9)
1.418(9)
1.431(9)
1.407(9)
1.419(9)
1.428(8)
Zr(1)-H(1)
1.860
Zr(1)-C(14)
Zr(1)-C(16)
Zr(1)-C(13)
Zr(1)-C(15)
Zr(1)-C(17)
C(18)-B(1)
C(13)-C(14)
C(13)-C(17)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
Zr(1)-H(1)
B(1)-H(1)
Zr(1)-C(1)
Zr(1)-C(2)
Zr(1)-C(5)
Zr(1)-C(3)
Zr(1)-C(4)
C(6)-B(1)
C(1)-C(5)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(1)-C(2)
Zr(1)-F(1)
F(1)-C(22)
F(2)-C(23)
Zr(1)-C(6)
Zr-C(5)
Zr(1)-C(1)
Zr(1)-C(5)
Zr(1)-C(4)
Zr(1)-C(2)
Zr(1)-C(3)
C(6)-B(1)
C(3)-C(4)
C(2)-C(3)
C(1)-C(5)
C(1)-C(2)
C(4)-C(5)
Zr(1)-F(1)
F(1)-C(22)
F(2)-C(23)
Zr(1)-C(6)
2.546(7)
2.501(7)
2.475(7)
2.463(7)
2.463(7)
1.66(1)
1.395(9)
1.409(9)
1.418(9)
1.438(9)
1.415(9)
2.237(4)
1.414(7)
1.320(8)
3.726
Zr-C(1)
Zr-C(4)
Zr-C(2)
Zr-C(3)
C(6)-B(1)
C(1)-C(2)
C(1)-C(5)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
1.53
C(65)-F(65)
C(64)-F(64)
Zr-C(6)
Zr-C(6)
Zr-C(71)
2.878
2.683
Zr-C(71)
2.917
and spectroscopic evidence suggested the reaction is to some
degree reversible.
Solid-state structural analysis of 3a indeed revealed a
stabilizing dative interaction between an ortho fluorine group
and the zirconium center (Figure 3). Fluorine(1) occupies a
coordination site in the metallocene wedge (Cl(1)-Zr(1)-F(1)
) 95.0(1)°), with a Zr(1)-F(1) distance of 2.267(5) Å. This
value is shorter than the Zr-F separations of between 2.322(2)
and 2.534(3) Å in other zirconocene cations stabilized via lone
pair donation from a fluorine17,21 but still substantially longer
than the 1.98(1) Å distance found in Cp2ZrF2.33 In addition,
the F(1)-C(22) distance of 1.414(9) Å is significantly elongated
in comparison to the other C-F bonds in the molecule; the
F(2)-C(23) length of 1.317(9) Å included in Table 3 is
representative of these bonds.
There are no chelating ion-ion interactions present in 3a that
are strong enough to impose dissymmetric environments on the
groups with the same connectivity at room temperature in
solution. For example, at room temperature, the proton NMR
spectrum of freshly prepared, analytically pure crystals of 3a
exhibited broad featureless peaks, including a peak at 1.74 ppm
more typical of a Cp* ligand in an unreacted tuck-in complex.
As the solution samples were cooled, all the peaks broadened
until a spectrum compatible with a structure featuring some type
of zirconium-borate interaction was observed at 233 K. At
Alkyl or aryl fluoride coordination to metals outside of the
context of group 4 metallocene cations is quite rare,34 and
generally occurs in entropically favored chelating situations such
as that observed here. Even chelating FfM donor bonds are
intrinsically weak, and as alluded to above, the Zr-F contact
observed in the solid state structure of 3a is labile in solution.
The lability of the aryl fluoride ligand in 3b can possibly be
attributed to the strain associated with the chelating arrangement
in this complex. In order for fluorine coordination to zirconium
to occur in 3a, the carbon bonded to boron (labeled C(6) in the
ORTEP) must tilt out of the plane defined by the Cp carbons
away from the zirconium center by 21°. This contrasts with
the negligible tilt angle found in 2a, which did not have to
deform in order for the hydridoborate to stabilize the zirconium
center via chelation. In comparison, ortho-fluorine coordination
in a zwitterionic complex reported recently by Erker et al.21a
featuring a -B(C6F5)3 group bonded directly to a Cp carbon
atom (rather than with an intervening CH2 group as in 3a)
appears qualitatively to be less prone to dissociation than the
present example. In Erker’s compound, fluorine coordination
is maintained in solution even at room temperature, possibly
because the size of the chelate is more suited to the alignment
of the C-F dipole moment toward the electropositive zirconium
center (C-F-Zr ) 150.6(2)°) than in the chelate in 3a, which
is larger by one carbon (C(22)-F(1)-Zr(1) ) 124.6(5)°). It
therefore appears that in the class of zwitterionic catalysts with
borate counterions affixed to a Cp donor, the chelate ring size
can have dramatic affects on the strength of the Zr-F interaction
and thus the extent to which this particular mode of cation
stabilization occurs.
1
this temperature, one major species was present. Its H NMR
spectrum consisted of a singlet for the Cp* ligand (1.34 ppm)
and four singlets at 1.28, 1.51, 1.65, and 1.90 ppm for the
chemically inequivalent methyl groups of the metallated Cp
donor; broad resonances at 3.43 and 2.77 ppm which emerged
upon further cooling were attributed to the diastereotopic CH2B
protons. Other minor compounds were also present, and further
spectral changes which we do not fully understand occurred as
the sample was cooled to 193 K.
Variable-temperature 19F NMR spectra were also complicated.
At room temperature, sharp signals for a small amount (≈5%)
32
of free B(C6F5)3 punctuated three very broad peaks in the
ortho, meta, and para regions of the spectrum. Lowering the
temperatures resulted in the transformation of the broad signals
into a complex pattern of multiplets; the spectrum obtained at
-61 °C is shown in Figure 2. Six separate resonances for the
meta fluorines were observed along with three for the para
fluorine atoms in the region of the spectrum where these groups
are normally found. One of the resonances due to the ortho
fluorines appears far upfield from the others at -167.7 ppm
and is consistent with the presence of a species in which one
fluorine comes into close contact with the zirconium center.17,21
Reaction of Cp*(η5,η1-C5Me4CH2)ZrC6H5 with B(C6F5)3.
The outcome of this reaction was found to be chemically more
complex than that discussed above, but the results are illustrative
(32) 19F resonances for Very carefully dried B(C6F5)3 appear at -128.78
ppm (ortho), -141.86 ppm (para), and -160.14 ppm (meta). Minute
amounts of water and/or donor solvents such as THF tend to broaden and
shift these resonances significantly, particularly that of the para fluorines.
(33) Bush, M. A.; Sim, G. A. J. Chem. Soc. A 1971, 2225.
(34) Kulawiec, R. J.; Crabtree, R. H. Coord. Chem. ReV. 1990, 99, 89.

Intramolecular Ion-Ion Interactions in Zwitterionic Catalysts
J. Am. Chem. Soc., Vol. 119, No. 22, 1997 5137

5138 J. Am. Chem. Soc., Vol. 119, No. 22, 1997
Sun et al.
Figure 4. ORTEP drawing of y-3b.
Figure 2. 19F NMR spectrum of zwitterionic choride 3a at -61 °C.
Figure 5. ORTEP drawing of o-3b.
Scheme 2. Reaction of Tuck-In Phenyl 1b with B(C6F5)3
Figure 3. ORTEP drawing of 3a.
of the subtleties of intramolecular ion-ion interactions in these
zwitterionic systems. As in the reaction of tuck-in chloride with
B(C6F5)3, admixing 1b and the borane in hexanes led to a yellow
precipitate that was virtually insoluble in this solvent. If this
powder was allowed to remain in the mother liquor and heated
gently, large, dark-orange crystals began to form polka dots in
the mass of yellow precipitate on the bottom of the flask.
Eventually, most of the yellow solid converted to the orange
product, o-3b, a visually stunning transformation. Alternatively,
the yellow product (y-3b) could be isolated immediately.
Dissolution of this material in C6D6 resulted in an immediate
color change to that characteristic of the orange product. The
1H NMR spectra of these particular samples revealed the
presence of free hexane in addition to the signals for the orange
zwitterion product. Thus, it was not possible to assess the
structural differences, if any, between the yellow and orange
forms of 3b by NMR spectroscopy.
crystals of o-3b were cultivated by more conventional recrys-
tallization techniques. The molecular structures of y-3b and
o-3b are shown in Figures 4 and 5, respectively, and comparable
metrical data are given in Tables 3 and 4. Scheme 2 summarizes
the chemistry involved based on these structure determinations.
As the NMR experiments suggested, the structure of y-3b
contains a molecule of hexane in the lattice, but in many ways
is similar to that of 3a in that the zirconium center is again
stabilized by dative coordination from an ortho-fluorine atom.
The Zr-F(65) of 2.414(3) Å distance in y-3b is ≈0.15 Å longer
than that in 3a, allowing the dipole of the C-F bond to align
itself toward the positive charge on zirconium more effectively
(Zr-F(65)-C(65) ) 165.2(3)°). The Zr-F attraction is at-
tenuated in y-3b because the metal is also receiving electron
Fortunately, X-ray quality crystals of both the kinetic y-3b
and the thermodynamic product o-3b were obtainable. Those
for the kinetic product were grown by slow diffusion of a layered
hexane solution of tuck-in phenyl complex 1b into a settled
hexane suspension of B(C6F5)3 at room temperature while

Intramolecular Ion-Ion Interactions in Zwitterionic Catalysts
J. Am. Chem. Soc., Vol. 119, No. 22, 1997 5139
density from an agostic C-H interaction as suggested by the
distortions evident in the phenyl group; the steric bulk of the
phenyl group relative to a chloride ligand is also a factor. The
phenyl group lies in the plane bisecting the Cp donors and the
ipso carbon is σ bound in one of the exo positions of the
metallocene girdle. The agostic phenyl interaction is implicated
by the much smaller Zr-C(76)-C(71) angle of 106.0(5)° in
comparison to the Zr-C(76)-C(75) angle of 138.0(6)°, bringing
C(71)-H into much closer proximity to the zirconium center
(Zr-C(71) ) 2.92 Å) than C(75). A final feature of this
structure worth noting is the limited extent to which C(6) is
perturbed from the plane of the five Cp carbons. Unlike 3a,
(Vide supra) the metallated carbon in y-3b is tilted slightly
toward the metal by about 6°.
The structure of o-3b is strikingly different from that of the
kinetic product discussed above. As in y-3b, the zirconium
center is satiated by an ortho C-H agostic interaction, but as
manifested by the structural parameters, the interaction is
stronger in o-3b. The phenyl group is canted more severely
toward the endo coordination site (Zr-C(76)-C(71) ) 94.5-
(5)°; Zr-C(76)-C(75) ) 149.3(6)°) than in y-3b, and the
contact distance of 2.683 Å between Zr and C(71) is signifi-
cantly shorter and more comparable to similar data observed in
other â-agostic zirconocene complexes.19,35 Evidence that the
agostic interaction is maintained in solution is sketchy. The
agostic structure could not be frozen out at low temperature;
signals for the ortho and meta protons of the phenyl group
remained averaged at all temperatures examined. Further, the
averaged C-Hortho coupling constant of 155 ( 1 Hz, while lower
than the C-Hmeta (165 ( 1 Hz) and the C-Hpara (171 ( 1 Hz)
J values, is not diagnostic.36
The biggest difference between 3a or y-3b and the orange
polymorph of 3b is the absence of a close Zr-F contact in the
latter compound. Rather, a second stabilizing feature present
in o-3b involves donation from C(6) to the electron-deficient
zirconium center. This interaction is related to the bridging
methyl interactions observed in cation-like metallocenes isolated
upon treatment of Cp2Zr(CH3)2 complexes with B(C6F5)3.
Borate ions of the form RCH2B-(C6F5)3, because of the
differences in electronegativity between B and C, exhibit some
charge localization on the carbon bonded to boron. The carbon
atom thus is capable of donating electron density to vacant
orbitals on zirconium when counterions of this type are
present.16,37
Several lines of structural evidence indicate that the abstracted
metallated carbon is engaged in a stabilizing interaction with
the metal center in o-3b. Unlike the compounds discussed
above, the metallated carbon in 3b is tilted severely out of the
plane defined by the Cp carbons toward the zirconium atom
by 24°, bringing C(6) to within 2.878 Å of zirconium (compared
to a distance of 3.33 Å for the same parameter in y-3b). Indeed,
bonding of the entire borate-substituted Cp ring is severely
distorted from normal, symmetric η5 bonding. The Zr-C(5)
distance is only 2.341(6) Å, compared with an average Zr-C
separation of 2.519 Å in the unsubstituted Cp* ligand. The
Zr-C(1) and Zr-(C5) bonds are slightly longer at 2.444(6) and
2.487(6) Å, respectively, while bonds to C(2) and C(3) are
longer still (Table 3). The tilted bonding mode of this Cp ligand
is a consequence of the attraction between the zirconium center
and C(6). This interaction is also supported by the alignment
of the C(6)-B vector with zirconium (Zr-C(6)-B ) 170.7°),
Scheme 3. Fluxional Process Exchanging Diastereotopic
Groups in o-3b
pointing directly at the remaining exo coordination site on the
zirconium (compared to an angle of 35.8° for the same parameter
in y-3b). A further consequence of this interaction is the
elongation of the B-C(6) bond (to 1.707(9) Å) in comparison
with the same bond in compounds 2a, 3a, and y-3b. The
alignment Zr-C(6)-B also argues against significant agostic
C-H character to this interaction which, if present, should give
lower values for this angle. The close to normal CH coupling
constant of 121 ( 1 Hz (averaged) observed for the methylene
CH bonds in o-3b supports minimal direct C-H bond involve-
ment in the Zr-C(6) contact.
In comparison to the non-chelating Zr-Cbridging separations
of 2.549-2.640 Å observed for compounds of general formula
Cp′2Zr+Me--MeB-(C6F5)3,17,18a the Zr-C(6) distance is quite
long, suggesting that constraining the counterion through
covalent attachment to the Cp* ring attenuates this type of ion-
ion interaction. Indeed, in benzene or toluene solution, 1H NMR
spectra of o-3b are consistent with an averaged structure at all
temperatures examined, i.e., all Cp and phenyl protons with the
same connectivity were exchanging on the NMR time scale even
at -80 °C. At this temperature, the signals were severely
broadened but had not undergone complete coalescence. The
proposed process by which these groups are exchanged is
depicted in Scheme 3. Dissociation of the Zr---CH2B-(C6F5)3
interaction leads to a symmetric structure, averaging the
environments of related protons. The variable-temperature 19
F
NMR spectra are entirely consistent with this hypothesis. At
room temperature, the three C6F5 groups are equivalent; at -80
°C the spectrum shows signals for three separate pentafluo-
rophenyl groups (Table 2).
The process in Scheme 3 is essentially analogous to the ion-
pair dissociation mechanism put forward by Marks et al. to
1
partially account for exchange phenomena observed in the H
NMR spectra of non-zwitterionic complex Cp′2Zr+Me--MeB--
(C6F5)3 (Cp′ ) 1,2-Me2C5H3).7b,17 The barrier for ion-pair
dissociation at 80 °C in this non-zwitterionic compound is on
the order of 18 kcal mol-1; since ∆Sq for this process is positive,
this barrier rises as temperature decreases. While we were
unable to calculate a barrier to the analogous process in the
zwitterionic complex o-3b with the available data, clearly it is
substantially lower than that found for ion-pair dissociation in
non-zwitterionic compounds, showing that the strategy of
sequestering the counteranion through the design of zwitterionic
catalysts can lead to weaker ion-ion interactions.
Note that the other process by which methyl groups were
exchanged in the non-zwitterionic systems, namely B-CH3/
Zr-CH3 exchange based on B-CH3 bond cleavage, cannot
exchange the methylene protons in the zwitterionic compounds
discussed here. Nonetheless this process also appears to occur
in zwitterions 3 as evidenced by the regeneration of the tuck-in
starting materials 1, with concomitant formation of the adduct
Me3P‚B(C6F5)3,38 upon treatment of compounds 3a and o-3b
with PMe3 (eq 4). This result is consistent with the observations
concerning the solution behavior of 3a (Vide supra), which likely
proceeds via loss of B(C6F5)3 from a structural variant of 3a
(35) Jordan, R. F.; Bradley, P. K.; Baenziger, N. C.; LaPointe, R. E. J.
Am. Chem. Soc. 1990, 112, 1289.
(36) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem.
1988, 36, 1.
(37) Radius, U.; Silverio, S. J.; Hoffmann, R.; Gleiter, R. Organometallics
1996, 15, 3737.
32
akin to that observed in o-3b. No evidence for free B(C6F5)3

5140 J. Am. Chem. Soc., Vol. 119, No. 22, 1997
Sun et al.
was found for solutions of analytically pure o-3b by 19F
spectroscopy, indicating that for this compound the equilibrium
is shifted far toward the zwitterion.
The structural attributes of the kinetic y-3b and thermody-
namic o-3b products suggest that the B(C6F5)3 reagent attacks
the Zr-Ctuck-in bond from the underside of the metallated carbon
(path a, Scheme 4) rather than the top (path b). Although attack
from the top would presumably lead directly to the thermody-
namically favored polymorph o-3b, the borane kinetically
prefers the electron density associated with the Zr-Ctuck-in bond,
giving y-3b. These observations may have some bearing on
the mode of abstraction of methyl groups from zirconium by
B(C6F5)3 in non-zwitterionic catalysts. Paths a and b in Scheme
4 correspond to two extremes originally proposed by Marks et
al.17 for attack of B(C6F5)3 on Cp2Zr(CH3)2, namely, attack of
a Zr-C σ bond (I) or the sterically less hindered direct attack
on the methyl carbon (II), respectively.
Figure 6. ORTEP drawing of 4.
Scheme 4. Trajectories of Borane Attack on Tuck-In Phenyl
1b
Phenyl zwitterion o-3b is stable for several weeks in the solid
state (-35 °C, dark), but is generally a highly reactive species.
Reactivity patterns are similar to those observed in other
organometallic early metal metallocenes, both neutral and
cationic. Like many formally 14 electron metallocenes, σ-bond
metathesis reactions with aromatic C-H bonds and H-H are
facile.39 In toluene, the material was converted quite rapidly
(t1/2 for loss of o-3b was about 20 min) to a mixture of tolyl
isomers with elimination of benzene. Similarly, reaction with
dihydrogen rapidly leads to expulsion of benzene and formation
of a hydride derivative, 4 (eq 5). This highly reactive compound
remains a zwitterion as evidenced by the 11B chemical shift of
-13.4 ppm. At room temperature, 1H NMR spectra are
broadened by an exchange process and are near the coalescence
point. Lowering the temperature resulted in the emergence of
familiar patterns for a structure with arrested rotation of the
metallated ring. From the coalescence behavior of the meth-
ylene protons, a rate constant of 418(2) s-1 (288 K) was
calculated for the process that exchanges these groups, corre-
sponding to a barrier of 13.4 ( 3 kcal mol-1. Although the 1H
NMR spectra are consistent with a process similar to that shown
in Scheme 3 above, the 19F NMR spectra at various temperatures
make it clear that hydride 4 is stabilized by a dative FfZr
interaction as in 3a and y-3b rather than the B-CH2---Zr
Coulombic attraction found in o-3b. At -55 °C, six separate
resonances for the ortho fluorines were observed in a similar
pattern to that described above for the low-temperature 19F
spectra for 3a, including an upfield shifted signal at -147.1
ppm.
Yellow crystals of hydride 4 were obtainable via slow reaction
of a settled suspension of o-3b with H2 in hexanes; the molecular
structure of this compound is shown in Figure 6. As can be
seen, the structure is similar to that of 3a, with Zr(1)-F(1) a
relatively short 2.237(4) Å and Zr(1)-F(1)-C(22) equal to
129.3(4)°. The C(6) tilt angle is again away from the zirconium
center to the tune of +14°. The hydrogen attached to zirconium
was found on the difference Fourier map and fixed in its
decomposed in benzene solution in about an hour even in the
presence of an atmosphere of dihydrogen, but as a solid, it may
be stored at low temperature for several days. The compound
(38) This compound is almost totally insoluble in aromatic solvents and
was identified by separate synthesis and comparison of the 1H NMR spectra
(δ ≈ 0.43 ppm in C6D6). Other phosphine adducts of B(C6F5)3 have been
studied: Bradley, D. C.; Harding, I. S.; Keefe, A. D.; Motevalli, M.; Zheng,
D. H. J. Chem. Soc., Dalton Trans. 1996, 3931.
(39) (a) Jordan, R. F.; Guram, A. S. Organometallics 1990, 9, 2116. (b)
Jordan, R. F.; Bajgur, C. S.; Dasher, W. E.; Rheingold, A. L. Organome-
tallics 1987, 6, 1041. (c) Jordan, R. F.; LaPointe, R. E.; Bradley, P. K.;
Baenziger, N. Organometallics 1989, 8, 2892. (d) Guo, Z. Y.; Bradley, P.
K.; Jordan, R. F. Organometallics 1992, 11, 2690. (e) Thompson. M. E.;
Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.;
Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203. f) Watson,
P. L. J. Am. Chem. Soc. 1983, 105, 6491.

Intramolecular Ion-Ion Interactions in Zwitterionic Catalysts
J. Am. Chem. Soc., Vol. 119, No. 22, 1997 5141
Table 5. Ethylene Polymerization with Zwitterionic Catalystsa
transport limited44 under the conditions we employed. Unfor-
tunately, the data surrendered by these polymerization experi-
ments do not allow for an assessment of the relative activities
of zwitterionic versus non-zwitterionic catalysts, which appear
to be more or less equal performers under these conditions. More
sophisticated experiments are necessary to address this question.
Polymerizations with fluorine-stabilized 3a/MAO or 4 un-
derwent rapid color changes upon exposure to ethylene from
yellow to the darker orange color associated with the CH2B-
(C6F5)3-stabilized complex o-3b. This observation suggests that
during the polymerization, when the alkyl group on zirconium
is the bulky polymer chain, this latter mode of compensation
for the zirconium center is preferred over fluorine donation and
is consistent with the observations made concerning how
structures change with different wedge ligands.
The polymer samples produced in these experiments had
relatively low molecular weights in comparison to those reported
for the cation-like systems,17 likely a function of the different
standards employed in the GPC analyses of the polymers
(polystyrene17 vs polyethylene). Polydispersities found for the
polyethylenes are closer to reported values and show that the
Mw/Mn for the zwitterions are between 1.5 and 2.0. The higher
value of 4.1 for the o-3b/MAO initiated polymerization is
probably a reflection of incomplete alkylation of the phenyl
complex and the presence of two different active catalysts. In
support of this notion, a comparison of the Mn and Mw values
for the 3a/MAO and the o-3b/none entries suggests that the
polymer sample for the o-3b/MAO run is essentially a composite
of polymers from the former two catalysts.
cat co-cat. PE
catalyst
2a
3a
o-3b
o-3b
(mg) (5 eq) (mg) 106 Ab 10-3Mn 10-3Mw Mw/Mn
15 MAO
22 0.0022 50
180
142
118
36
3.6
1.5
4.1
1.8
3.0
2.4
9.4
15 MAO 138 1.0
20 MAO 117 0.67
95
29
20
7.4
16
20 none
19 none
15 none
217 1.3
223 1.3
206 1.5
4
23
non-Zwitc
Cp*2ZrCl2
40
136
9
MAO
10
.0049 14
a Conditions: 25 °C, 15 psi ethylene, toluene solution. b Activity
units: (g pol/MolZr‚atm‚h). c [Cp*2ZrMe][MeB(C6F5)3].
position. The Zr-H distance of 1.86 Å compares to that of
2.00(5) Å found in the only other “base-free” cationic zirconium
hydride crystallographically characterized.17,40 Stable cationic
zirconium hydrides are still rare; the reactivity of this compound
is under active study.
Clearly, the factors which determine how the electrophilic
zirconium centers are compensated electronically in these
systems are subtle. Probably the most important determinant
is the steric bulk of the substituent on zirconium. When this
ligand is smaller, such as a chloride or hydride, an ortho fluorine
is able to coordinate to the zirconium without undue steric
interactions between this ligand and the pentafluorophenyl
groups of the counterions. The phenyl ligand, however, is of
sufficiently large size to disrupt this mode of bonding, and
although accessible, the ortho-fluorine structure is unstable with
respect to the other stabilizing motif. Isomerization to allow
donation of electron density from the δ-CH2B(C6F5)3 to
zirconium swings the bulky B(C6F5)3 group away from the
molecular core, lessening steric interactions. The fact that the
phenyl group is capable of partially replenishing the electron
density lost through disruption of the fluorine donation is also
a factor that tips the thermodynamic balance in favor of o-3b
in this particular system. Upon treatment with hydrogen, the
yellow color characteristic of the ortho-fluorine-stabilized
structure reappears, and this is again the thermodynamically
preferred state of the compound.
Conclusions
We have prepared several zwitterionic olefin polymerization
catalysts from the electrophilic boranes XB(C6F5)2 (X ) H and
C6F5) and tuck-in complexes of zirconium. The “base-free”
nature of these zwitterions renders them highly active toward
polymerization of ethylene, with comparable activities to
analogous non-zwitterionic catalysts. To the extent that cationic
zirconocenes are never completely base-free in condensed
media, the zirconium centers in these compounds are sustained
electronically via a variety of intramolecular interactions. The
strongest (and most damaging from the point of view of
polymerization activity) is the chelating borate-type linkage
observed in products 2 derived from HB(C6F5)2 and the tuck-
in precursors. Three separate types of intramolecular stabilizing
interactions were observed in zwitterionic complexes obtained
from B(C6F5)3: â-agostic interactions for the phenyl compounds,
chelating dative fluorine to zirconium bonds from an ortho-
fluorine atom of the pendant borate moiety, and donation from
the carbon attached to boron in the counterion. These interac-
tions differ in importance depending on the steric properties of
the reactive ligand in the metallocene wedge.
Ethylene Polymerization with Zwitterionic Catalysts.
Each of the new zwitterionic compounds described above was
tested for suitability as an ethylene polymerization catalyst. All
polymerizations were performed at room temperature under 15
psi of ethylene in toluene following procedures modelled after
those described by Marks et al.41 Polymerization data re
collected in Table 5. Hydridoborate chelated complexes 2a and
2b42 were poor catalysts, not surprising given the rather strong
ion-ion interaction in these compounds. Activities for the other
zwitterionic catalysts, alkylated with small amounts of MAO,
rivalled those found for non-zwitterionic Marks-type complexes.
For comparison, we carried out some experiments using the
[Cp*2ZrMe]+[MeB(C6F5)3]- system under our conditions. In
another control experiment, the lack of significant activity
observed for Cp*2ZrCl2 activated similarly with 5 equiv of
MAO43 illustrates the self-activating, single-component nature
of these catalysts. Indeed, o-3b appears to perform much better
in the absence of MAO. The activities reported in Table 5
represent a lower limit since the reactions were undoubtly mass
Experimental Section
General Procedures. All operations were performed under a
purified argon atmosphere in an Inert Atmosphere glovebox or on high-
vacuum lines with standard techniques.45 Solvents were purified as
follows: toluene was distilled from sodium benzophenone ketyl and
stored over “titanocene”;46 tetrahydrofuran (THF) was predried with
activated (10-4 Torr, 200 °C, 3 h) 3 Å molecular sieves, distilled from
and stored over sodium benzophenone ketyl; hexanes were distilled
from lithium aluminum hydride (Aldrich) and stored over “titanocene”;
(40) Yang, X.; Stern, C. L.; Marks, T. J. Angew. Chem., Int. Ed. Engl.
1992, 31, 1375.
(41) Giardello, M. A.; Eisen, M. S.; Stern, C. L.; Marks, T. J. J. Am.
Chem. Soc. 1995, 117, 12114.
(42) Data for 2a/MAO only are given in Table 5; the 2b/MAO system
had poorer activity and the polymer sample obtained therefrom was not
analyzed.
(43) Of course, much higher Al/Zr ratios are required to obtain the true
activity data for the Cp*2ZrCl2 catalyst precursor.
(44) Janiak, C.; Versteeg, U.; Lange, K. C. H.; Weimann, R.; Hahn, E.
J. Organomet. Chem. 1995, 501, 219.
(45) Burger, B. J.; Bercaw, J. E. Experimental Organometallic Chemistry;
Wayda, A. L.; Darensbourg, M. Y., Eds.; ACS Symposium Series 357;
American Chemical Society: Washington, DC, 1987.
(46) Marvich, R. H.; Brintzinger, H. H. J. Am. Chem. Soc. 1971, 93,
2046.

5142 J. Am. Chem. Soc., Vol. 119, No. 22, 1997
Sun et al.
Table 6. Summary of Data Collection and Structure Refinement Details for 2a, 3a, y-3b, o-3b, and 4
2a
3a
y-3b
o-3b
4
formula
fw
cryst syst
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
space group
Z
C32H30BF10ClZr
742.06
monoclinic
16.680(4)
10.091(2)
19.388(3)
C38H29BF15ClZr
908.11
monoclinic
11.977(7)
18.835(5)
16.390(7)
C47H41BF15Zr
992.83
C44H34BF15Zr
949.74
monoclinic
10.074(1)
19.313(3)
21.108(3)
C38H30BF15Zr
873.66
monoclinic
11.802(3)
18.438(2)
16.524(3)
triclinic
11.3399(2)
13.7300(2)
16.6497(2)
95.604(1)
102.559(1)
101.829(1)
2448.95(6)
P1h
108.32(2)
103.45(4)
94.758(2)
97.98(2)
3097(1)
P21/n
4
3595(2)
P21/c
4
4092.6(9)
P21/c
4
3560(1)
P21/c
4
2
F(000)
1496
1.591
0.523
0.046
0.042
1816
1.677
0.487
0.060
0.065
1006
1.346
0.312
1912
1.541
0.369
1752
1.630
0.416
0.044
0.033
d
calc, mg m-3
µ, mm-1
R
Rw
R1
wR2
gof
0.0577
0.1642
1.080
0.0646
0.1429
1.193
1.72
3.69
1.89
dichloromethane was distilled from CaH2; benzene-d6 was dried
sequentially over activated 3 Å sieves and “titanocene” and stored in
the glovebox; and other NMR solvents were dried analogously to the
perprotio solvents.
Compounds HB(C6F5)2,23 Cp*(η5,η1-C5Me4CH2)ZrCl, and Cp*(η5,η1-
C5Me4CH2)ZrPh were prepared according to published procedures.25,26
B(C6F5)3 was purchased from Boulder Scientific Co., and methylalu-
minoxane (MAO) was obtained from Akzo Nobel as a 7.1% (wt Al)
toluene solution. Ethylene was obtained from Matheson and purified
by passage through a Matheson Oxiclear disposable purifier (Model
No. DGP-250-R1). Other materials were puchased from Aldrich and
used as received.
2993 (sh), 2950 (sh), 2916 (m), 1748 (br,m), 1643 (m), 1514 (s),
1468 (s), 1382 (m), 1277 (m), 1089 (s), 971 (s).
Synthesis of Cp*[η5-C5Me4CH2B(C6F5)2(µ-H)]ZrPh (2b). Pentane
(15 mL) was condensed into an evacuated flask containing tuck-in
phenyl 1b (118 mg, 0.270 mmol) and HB(C6F5)2 (93 mg, 0.269 mmol)
with liquid nitrogen. The reaction mixture was warmed to room
temperature and stirred for 1 h. A voluminous yellow precipitate was
formed during this period, isolated by filtration, and washed by back
distillation of pentane. Yield of 2b: 132 mg, 0.168 mmol, 62%. Anal.
Calcd for C38H35F10BZr: C, 58.24; H, 4.50. Found: C, 58.35; H, 3.96.
IR (KBr, cm-1): 2988 (sh), 2955 (sh), 2910 (m), 1767 (sh), 1742 (m),
1641 (m), 1513 (s), 1464 (s), 1380 (m), 1279 (m), 1113 (sh), 1088 (s),
971 (s).
Reaction of Cp*(η5,η1-C5Me4CH2)ZrMe (1c) with HB(C6F5)2.
Pentane (10 mL) was condensed into an evacuated flask containing
tuck-in methyl complex 1c (20 mg, 0.053 mmol) and HB(C6F5)2 (18
mg, 0.052 mmol) with liquid nitrogen. The reaction mixture was stirred
for 1 h at -78 °C, warmed to room temperature, and stirred for another
hour. The pentane was removed under reduced pressure, leaving an
orange powder that was examined by 1H NMR spectroscopy. 1H NMR,
2c (C6D6): 3.33 (d, 1H, 2JH-H ) 14.1 Hz, CHHB); 2.36 (dd, 1H, 3JH-H
) 6.5 Hz, CHHB); 1.56 (s, 15H, C5(CH3)5); 1.93, 1.92, 1.31, 1.30 (s,
12H, C5(CH3)4); Zr-CH3 and Zr-H-B resonances not found. Major
product: 1.85, 1.64, 1.49, 0.22 (t, JH-F ) 8.5 Hz).
Synthesis of Cp*[η5-C5Me4CH2B(C6F5)3]ZrCl (3a). Hexanes (8
mL) were condensed into an evacuated flask containing 1a (103 mg,
0.260 mmol) and B(C6F5)3 (133 mg, 0.260 mmol) at -78 °C. The
reaction mixture was stirred for 20 min at -78 °C, warmed to room
temperature, and stirred for another hour. The voluminous yellow
precipitate formed was isolated by filtration and washed with hexanes.
Yield of compound 3a: 190 mg, 0.217 mmol, 84% yield. Anal. Calcd
for C38H29F15ClBZr: C, 50.26; H, 3.22. Found: C, 50.30; H, 3.11. IR
(KBr, cm-1): 2981 (sh), 2961 (sh), 2918 (m), 1642 (m), 1515 (s), 1455
(s), 1385 (m), 1272 (m), 1084 (s), 1063 (sh), 982 (s), 942 (m), 930
(m).
Synthesis of Cp*[η5-C5Me4CH2B(C6F5)3]ZrPh (o-3b). Hexanes
(15 mL) were condensed into an evacuated flask containing 1b (299
mg, 0.680 mmol) and B(C6F5)3 (350 mg, 0.684 mmol) at -78 °C. The
reaction mixture was warmed to room temperature and stirred for 1 h,
resulting in a yellow precipitate. The yellow slurry was heated at 60
to 55 °C for hours without stirring during which time the yellow
precipitate converted to orange crystals. These crystals were isolated
by filtration and washed with cold hexanes. Yield of o-3b: 530 mg,
0.558 mmol, 82%. Anal. Calcd for C44H34F15BZr: C, 55.64; H, 3.55.
Found: C, 54.53; H, 3.29. IR (KBr, cm-1): 2954 (sh), 2906 (w), 2867
(sh), 1642 (w), 1514 (s), 1458 (s), 1272 (w), 1083 (m), 977 (m).
NMR spectra were recorded on Bruker AM200, 300, or WH400
MHz spectrometers. 13C NMR assignments were made by using the
HMQC pulse sequence,47 which we have found to be useful for
detecting resonances for carbons broadened by quadrupolar nuclei and
for determining JCH values. 19F NMR spectra were referenced externally
to C6F6 at -163.0 ppm relative to CFCl3 at 0.0 ppm.48 11B NMR spectra
were referenced externally to BF3‚Et2O at 0.0 ppm. Elemental analyses
were performed in house on a Control Equipment Corporation Model
440 Elemental Analyzer. GPC analyses of polyethylene samples were
carried out at the Nova Chemicals Technology Center, Calgary, Alberta
on a Waters 150C GPC instrument under the following conditions:
Columns, Shodex AT 103,4,5,6 Å; mobile phase, 1,2,4-trichlorobenzene;
temperature, 140 °C; flow rate, 1.0 mL/min; concentration, 0.1% (w/
v). Molecular weights were calibrated against the standard PE105K
obtained from American Polymer Standards Corporation.
Synthesis of Cp*(η5,η1-C5Me4CH2)ZrMe (1c). Benzene (10 mL)
was condensed into an evacuated flask containing Cp*(η5,η1-C5Me4-
CH2)ZrCl (1a, 572 mg, 1.44 mmol) and solid MeLi (42 mg, 1.44 mmol)
at -78 °C. The reaction mixture was warmed to room temperature
and stirred for 2 h. The benzene was removed in Vacuo, and the residue
was extracted with hexane (15 mL). The solvent was pumped away,
and 1c was isolated as a red powder by filtration. Yield: 450 mg,
83%. 1H NMR (C6D6): δ 1.83 (s, 15H, C5(CH3)5), 1.86, 1.70, 1.58,
1.42 (s, 12H, C5(CH3)4CH2), 2.11, 1.63 (d, J ) 6.6 Hz, 2H, C5-
(CH3)4CH2), -0.94 (s, 3H, CH3).
Synthesis of Cp*[η5-C5Me4CH2B(C6F5)2(µ-H)]ZrCl (2a). Benzene
(15 mL) was condensed into an evacuated flask containing tuck-in
chloride 1a (200 mg, 0.505 mmol) and HB(C6F5)2 (167 mg, 0.483
mmol) at -78 °C. The reaction was warmed to room temperature and
stirred for 40 min. The benzene was removed under vacuum and the
vessel charged with hexanes (25 mL). The suspension was cooled to
-78 °C and filtered cold to afford yellow crystals of compound 2a
(277 mg, 0.373 mmol) in 74% yield. Anal. Calcd for C32H30F10-
ClBZr: C, 51.80; H, 4.08. Found: C, 51.55; H, 3.26. IR (KBr, cm-1):
Reaction of Cp*(η5,η1-C5Me4CH2)ZrMe (1c) with B(C6F5)3.
Pentane (10 mL) was condensed into an evacuated flask containing 1c
(20 mg, 0.053 mmol) and B(C6F5)3 (18 mg, 0.052 mmol) with liquid
(47) Bax, A.; Subramanian, S. J. Magn. Reson. 1986, 67, 565.
(48) NMR and the Periodic Table; Harris, R. K., Mann, B., Eds.;
Academic Press: New York, 1978; p 98.

Intramolecular Ion-Ion Interactions in Zwitterionic Catalysts
J. Am. Chem. Soc., Vol. 119, No. 22, 1997 5143
nitrogen. The reaction mixture was stirred for 1 h at -78 °C and
warmed to room temperature with stirring for 1 h, resulting in a light
orange clear solution. Pentane was removed in Vacuo, and powder
was obtained. The powder was dissolved in C6D6 to give a red solution
6. Crystals of 2a were grown from hexane solution at -78 °C. Crystals
of 3a were obtained by slow diffusion of hexane into a concentrated
toluene solution of zwitterion at room temperature. Slow reaction of
o-3b with H2 in hexanes yielded yellow nuggets of hydride 4. Suitable
crystals were placed in glass capillaries, sealed, and mounted onto a
Rigaku AFC6S diffractometer. Measurements were made with use of
graphite monochromated Mo KR radiation at -103, -73, and -103
°C for 2a, 3a, and 4, respectively. The structures were solved by direct
methods and refined by full-matrix least-squares calculations. The non-
hydrogen atoms were refined anisotropically; hydrogen atoms were
included at geometrically idealized positions with C-H ) 0.95 Å and
were not refined. Exceptions were the H-atom bridging Zr and B in
2a and the hydride ligand in 4, which were located from a difference
map and included at that position. At convergence, R and wR were
0.046 and 0.042 for 2a, 0.060 and 0.065 for 3a, and 0.044 and 0.033
for 4. The final difference maps were essentially featureless. All
calculations were performed with use of the TEXAN49 crystallographic
software package of Molecular Structure Corporation.
1
and the H NMR spectrum was recorded.
Reaction of Cp*[η5-C5Me4CH2B(C6F5)3]ZrPh (o-3b) with PMe3.
Zwitterion o-3b (20 mg, 0.053 mmol) was loaded into a sealable 5-mm
NMR tube and suspended in C6D6 (≈0.7 mL). The sample was
degassed by 2 freeze-pump-thaw routines and PMe3 (1 equiv) was
condensed into the sample at -78 °C. The tube was flame sealed and
upon warming to room temperature a white precipitate was formed;
1
this material was centrifuged to the end of the tube. The H NMR
spectrum was recorded and showed quantitative formation of tuck-in
phenyl complex 1b. The white precipitate, highly insoluble in C6D6,
was shown to be the adduct Me3P‚B(C6F5)3 by preparation of an
authentic sample as follows. Hexanes (10 mL) and 1 equiv of PMe3
were condensed into an evacuated flask containing B(C6F5)3 (354 mg,
0.692 mmol) at -78 °C. The reaction was warmed to room temperature
and sonicated for 0.5 h, resulting in a white precipitate that was isolated
by filtration.
X-ray Crystallography for y-3b and o-3b. General: These
structures were solved from data collected on the Siemens SMART
system on crystals loaded into sealed glass capillaries. In each case,
unit-cell parameters were calculated from reflections obtained from 60
data frames collected at different sections of the Ewald sphere. The
structures were solved by direct methods, completed by subsequent
Fourier syntheses, and refined with full-matrix least-squares methods.
All scattering factors and anomalous dispersion coefficients are
contained in the SHELXTL 5.03 program library. y-3b: Crystals of
the kinetically formed yellow polymorph y-3b were obtained by
layering a solution of tuck-in phenyl complex 1b in hexanes on top of
a settled hexanes suspension of B(C6F5)3. Slow diffusion at room
temperature resulted in large yellow crystals at the interface. No
symmetry higher than triclinic was evident from the diffraction data
and unit-cell parameters. The E-statistics strongly suggested the centric
option and solution in P1h yielded chemically reasonable and compu-
tationally stable results. A trial application of semiempirical absorption
correction based on redundant data at varying effecitve azimuthal angles
yielded Tmax/Tmin at unity and was ignored. A molecule of cocrystallized
n-hexane solvent, disordered along the molecular polar axis, was located
at the inversion center and refined isotropically. All non-hydrogen
atoms were refined with anisotropic displacement coefficients, while
all hydrogen atoms were treated as idealized contributions. o-3b:
Crystals of the orange thermodynamic polymorph of 3b were grown
from hexanes by slow cooling of a solution heated to 50 °C for 30
min. The systematic absences in the diffraction data and the determined
unit-cell parameters were uniquely consistent for the space group P1h/n.
Semiempirical absorption corrections were applied based on redundant
data at varying effective azimuthal angles. Non-hydrogen and hydrogen
atoms were treated as above.
Synthesis of Cp*[η5-C5Me4CH2B(C6F5)3]ZrH (4). Hexanes (10
mL) were condensed into an evacuated flask containing Cp*[η5-C5-
Me4CH2B(C6F5)3]ZrPh (113 mg, 0.119 mmol) at -78 °C. While being
warmed to room temperature, the reaction mixture was stirred under 1
ato pressure of H2 for 1 h. The resulting yellow precipitate was isolated
by filtration and washed once with a portion of cold hexanes. Yield
of 4: 80 mg, 0.092 mmol, 77%. Anal. Calcd for C38H30F15BZr: C,
52.24; H, 3.46. Found: C, 51.59; H, 1.91. IR (KBr, cm-1): 2959
(sh), 2917 (w), 2849 (sh), 1644 (m), 1515 (s), 1459 (s), 1383 (w), 1275
(w), 1085 (s), 977 (s).
Reaction of Cp*[η5-C5Me4CH2B(C6F5)3]ZrPh with Toluene-d8.
Cp*[η5-C5Me4CH2B(C6F5)3]ZrPh was dissolved in toluene-d8 in a flame-
sealed NMR tube. Overnight the color of the solution changed from
orange to red. The 1H NMR spetrum clearly showed the formation of
C6H5D. The Cp resonances of the spectrum could not be interpreted.
General Procedures for Ethylene Polymerization. Data collected
in Table 5 for ethylene polymerizations are the average of at least two
runs. Polymerizations were carried out with use of the following
procedures, based on procedures reported by Marks et al.42 Method
A: In a drybox, a 100 mL reaction flask equipped with a large magnetic
stirring bar and a septum-covered stopcock sidearm was charged with
metallocene catalyst (10-30 mg) and toluene (100 mL). If required,
MAO was injected dropwise Via a syringe at this point into the toluene
solution with stirring. An aliquot of this solution (50 mL) was
transfered to another 100 mL reaction flask. The two flasks were
attached to a vacuum line, and the solutions were degassed by 2 freeze-
pump-thaw routines and equilibrated at the desired temperature under
vacuum with use of an external constant temperature bath. Gaseous
ethylene was next introduced with rapid stirring, and the pressure was
maintained at 15 psi. After a measured time interval, the polymerization
was quenched by the addition of acidified methanol (3 mL). Methanol
(100 mL) was added and the polymer was collected by filtration, washed
with methanol (10 mL), and dried in Vacuo overnight. Method B:
For polymerizations with catalysts derived from tuck-in methyl
derivative 1c, the procedures are the same as above except that the
solutions of 1c were saturated with 15 psi of ethylene after the solution
was degassed and equilibrated at the desired temperature. Polymeriza-
tions were initiated by injecting 1 equiv of B(C6F5)3 into the reaction
flask through the septum via syringe. Quenching and workup were
carried out as described above. Method C: For polymerizations with
zwitterion 4 as the catalyst, the procedures were as above, except that
4 was generated in situ by treating a solution of 3b with dry H2 for 5
min. The hydrogen was removed by exposing the solution to vacuum
for a few seconds three times in a row. Polymerization was initiated
by admitting 15 psi of ethylene into the vessel; quenching and workup
were carried out as described above.
Acknowledgment. This work was generously supported by
the NOVA Research & Technology Corporation of Calgary,
Alberta, NSERC of Canada’s Research Partnerships office (CRD
program), and the University of Calgary. W.E.P. thanks the
Alfred P. Sloan Foundation for a Research Fellowship (1996-
98) and Mr. Leon Neumann and Mr. Sunil Chaudry of the Nova
Technical Center for GPC measurements on the polyethylene
samples.
Supporting Information Available: Full listings of crystal-
lographic data for compounds 2a, 3a, y-3b, o-3b, and 4 (72
pages). See any current masthead page for ordering and Internet
access instructions.
JA970140H
X-ray Crystallographic Analysis of 2a, 3a, and 4. A summary
of crystal data and refinement details for all structures is given in Table
(49) Crystal Structure Analysis Package, Molecular Structure Corporation,
1985 and 1992.