Very large counteranion modulation of cationic metallocene polymerization activity and stereoregulation by a sterically congested (perfluoroaryl)fluoroaluminate

Full text of DOI:10.1021/ja963834j

2582
J. Am. Chem. Soc. 1997, 119, 2582-2583
Scheme 1
Very Large Counteranion Modulation of Cationic
Metallocene Polymerization Activity and
Stereoregulation by a Sterically Congested
(Perfluoroaryl)fluoroaluminate
You-Xian Chen, Charlotte L. Stern, and Tobin J. Marks*
Department of Chemistry, Northwestern UniVersity
EVanston, Illinois 60208-3113
ReceiVed NoVember 5, 1996
Perfluoroaryl boranes1 and borates2 are currently of great
scientific and technological interest as efficient abstractors/
cocatalysts for generating highly active, cationic early transition
metal olefin polymerization catalysts.3 Recent studies suggest
that many of the properties of such catalysts are intimately
connected with the nature of the relatively strong cation-anion
ion pairing4 and that it would be of great interest to investigate
the properties of other main group fluoroarylmetals differing
in size, shape, and latent ligational characteristics. We com-
municate here the unusual structural and cocatalytic features
of a new, stable (perfluoroaryl)aluminate anion, tris(2,2′,2′′-
nonafluorobiphenyl)fluoroaluminate (PBA-).5 Noteworthy fea-
tures include very large ion pairing/metallocenium ancillary
ligand structural effects on olefin polymerization activity and
stereoselectivity.
Under a variety of conditions, reaction of (2-nonafluorobi-
phenyl)lithium6 with AlCl3 leads to a compound having the
composition Li+(C12F9)3AlF-, which presumably results from
aryl fluoride activation by strongly Lewis acidic, transient “tris-
(perfluorobiphenyl)aluminum” (Scheme 1). Ion exchange me-
tathesis with Ph3CCl yields the corresponding trityl (perfluo-
robiphenyl)fluoroaluminate, Ph3C+PBA-, which was characterized
by standard spectroscopic and analytical techniques,7 as well
as by single-crystal X-ray diffraction,8 revealing unassociated
trityl cations and sterically congested chiral (C3-symmetric)
(fluoroaryl)aluminate anions (Scheme 1).
Reaction of Ph3C+PBA- with metallocene dialkyls in toluene
cleanly generates the corresponding cationic complexes (eq 1),
(1) (a) Wu, Z.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1995,
117, 5867-5868. (b) Pellecchia, C.; Pappalardo, D.; Oliva, L.; Zambelli,
A. J. Am. Chem. Soc. 1995, 117, 6593-6594. (c) Temme, B.; Erker, G.;
Karl, J.; Luftmann, H.; Fro¨hlich, R.; Kotila, S. Angew. Chem., Int. Ed. Engl.
1995, 34, 1755-1757. (d) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem.
Soc. 1994, 116, 10015-10031. (e) Bochmann, M.; Lancaster, S. J.;
Hursthous, M. B.; Malik, K. M. A. Organometallics 1994, 13, 2235-2243.
(f) Gillis, D. J.; Tudoret, M.-J., Baird, M. C. J. Am. Chem. Soc. 1993, 115,
2543-2545. (g) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc.
1991, 113, 3623-3625.
(2) (a) Chien, J. C. W.; Tsai, W.-M.; Rausch, M. D. J. Am. Chem. Soc.
1991, 113, 8570-8571. (b) Yang, X.; Stern, C. L.; Marks, T. J. Organo-
metallics 1991, 10, 840-842. (c) Ewen, J. A.; Elder, M. J. Eur. Pat. Appl.
426637, 1991; Chem. Abstr. 1991, 115, 136987c, 136988d. (d) Hlatky, G.
G.; Upton, D. J.; Turner, H. W. U.S. Pat. Appl. 459921, 1990; Chem. Abstr.
1991, 115, 256897v.
(3) For recent reviews, see: (a) Bochmann, M. J. Chem. Soc., Dalton
Trans. 1996, 255-270. (b) Brintzinger, H.-H.; Fischer, D.; Mu¨lhaupt, R.;
Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-
1170. (c) Soga, K,, Terano, M., Eds. Catalyst Design for Tailor-Made
Polyolefins; Elsevier: Tokyo, 1994. (d) Mo¨hring, P. C.; Coville, N. J. J.
Organomet. Chem. 1994, 479, 1-29. (e) Marks, T. J. Acc. Chem. Res. 1992,
25, 57-65.
1
which were characterized by standard H/13C/19F NMR and
analytical techniques.7,9 Interestingly, the 19F-Al NMR data7
suggest some degree of M+- - -F-Al- interaction, which is
confirmed by the molecular structure of 5 (see below) and which
qualitatively appears to diminish with increasing ancillary ligand
steric bulk.10 The crystal structure of complex 5 (Scheme 2)11
reveals cation-anion pairing via a nearly linear Zr- - -F-Al
bridge ( Zr-F-Al ) 175.4(4)°) with Zr-F and Al-F dis-
tances of 2.123(6) and 1.780(6) Å, respectively. These can be
compared to terminal and bridging Zr-F distances of
1.93(1) and 2.11(1) Å, respectively, in [Cp′′2ZrF]2(µ-F)+B(C6F4-
TBS)4- 12 and Al-F ) 1.682(5) Å in PBA-. The CGCZrCH3
+
(4) (a) Chen, Y.-X.; Stern, C. L.; Yang, S.; Marks, T. J. J. Am. Chem.
Soc. 1996, 118, 12451-12452. (b) Deck, P. A.; Marks, T. J. J. Am. Chem.
Soc. 1995, 117, 6128-6129. (c) Giardello, M. A.; Eisen, M. S.; Stern, C.
L.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 12114-12129. (d) Jia, L.;
Yang, X.; Ishihara, A.; Marks, T. J. Organometallics 1995, 14, 3135-
3137. (e) Chien, J. C. W.; Song, W.; Rausch, M. D. J. Polym. Sci., Part A:
Polym. Chem. 1994, 32, 2387-2393. (f) Eisch, J.; Pombrik, S. I.; Zheng,
G.-X. Organometallics 1993, 12, 3856-3863. (g) Siedle, A. R.; Lamanna,
W. M.; Newmark, R. A.; Stevens, J.; Richardson, D. E.; Ryan, M.
Makromol. Chem., Macromol. Symp. 1993, 66, 215-224. (h) Herfert, N.;
Fink, G. Makromol. Chem. 1992, 193, 773.
(5) (a) Communicated in part: Chen, Y.-X.; Marks, T. J. Abstracts of
Papers; 212th National Meeting of the American Society, Orlando, FL,
August 25-29, 1996; INOR 379. (b) One patent briefly claims
Ph3C+Al(C6F5)4- as a cocatalyst for syndiospecific polymerization: Elder,
M. J.; Ewen, J. A. Eur. Pat. Appl. EP 573,403, 1993; Chem. Abstr. 1994,
121, 0207d.
(7) Experimental and characterization details are given in the Supporting
Information.
(8) Space group P1h; a ) 12.179(5) Å, b ) 12.473(5) Å, c ) 18.334(5)
Å, R ) 99.21(3)°, â ) 94.88(3)°, γ ) 108.82° at -120 °C; V ) 2574(1)
Å3; Z ) 2; R(F), Rw(F) ) 0.072, 0.053. Important bond distances (Å):
Al-F ) 1.682(5), Al-C(1) ) 2.027(9), Al-C(13) ) 2.019(10), Al-C(25)
) 2.009(9).
(9) While too thermally unstable to isolate, complex 4 can be generated
in situ and characterized spectroscopically.7
(10) δ 19F-Al: -138.11 (Cp2ZrCH3+PBA-), -138.69 (CGCZrMe+-
PBA-), -143.38 (Cp′′2ZrCH3+PBA-), -144.23 ((CpTMS )2ZrCH3+PBA-),
2
-155.78 (Cp′2ZrCH3+PBA-), -176.81 (Ph3C+PBA-).
(11) Space group P21/c; a ) 18.461(9) Å, b ) 13.934(6) Å, c ) 23.85-
(1) Å, â ) 108.34(4)° at -120 °C; V ) 5822(4) Å3; Z ) 4; R(F), Rw(F) )
0.071, 0.056. Important bond distances (Å) and angles (deg): Al-F )
1.780(6), Zr-F ) 2.123(6), Zr-C(16) ) 2.21(1), Zr-F-Al ) 175.4(4).
(12) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J. Organometallics, in
press.
(6) Penton, D. E.; Park, A. J.; Shaw, D.; Massey, A. G. J. Organomet.
Chem. 1964, 2, 437-446.
S0002-7863(96)03834-6 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 10, 1997 2583
Table 1. Ethylene Polymerization Activities with Metallocene/Ph3C+PBA- Catalysts and Polymer Propertiesa
d
Tp
(°C)
µmol of
cat.
reaction
time (min) yield (g)
polymer
activityb (g of polymer/
Tm
∆Hu
(cal/g)
c
entry
catalyst
(mol cat‚atm‚h))
Mw
Mw/Mn
(°C)
1
2
3
4
5
6
7
Cp2ZrMe2 (1b)
Cp′′2ZrMe2 (2)
25
25
25
25
25
60
110
20
20
15
15
15
30
30
20
30
2.0
0.67
10
30
5.0
0
0
0.18
0.54
1.15
0
0.20
0.20
1.80 × 104
1.08 × 106
6.90 × 106
0
5.46 × 105
1.26 × 106
8.97 × 104
6.0
5.6
4.6
139.4
142.3
138.0
40.5
29.5
53.9
(CpTMS )2ZrMe2 (3)
2
Cp′2ZrMe2 (4)
CGCMMe2 (5, 6)
CGCTiMe2 (6)
CGCTiMe2 (6)
1.33 × 104
2.05 × 106
3.9
3.1
139.2
142.5
19.5
24.4
8.00 × 104
2.05 × 106
a Carried out at 1.0 atm of ethylene in 50 mL of toluene on a high-vacuum line. See Supporting Information for polymerization procedures.
b Reproducibility between runs ) 10-15%. c GPC relative to polystyrene standards. DSC from the second scan.
d
Scheme 2
(2) kcal/mol for 5) without permutation of diastereotopic Cp′′
Me groups in 5 and 6 (∆Gq > 20.5(4) kcal/mol for 5) or indenyl
fragments in 7 (∆Gq > 20.8(4) kcal/mol).
Perhaps the most interesting aspect of 1-7 chemistry is the
remarkable sensitivity of olefin polymerization characteristics
(Table 1) to ion pairing as inferred from ancillary ligand bulk,
structural data, and δ19F-Al.10 Thus, while Cp2ZrCH3+PBA-
and CGCZrCH3+PBA- exhibit negligible ethylene polymeri-
zation activity at 25 °C and 1.0 atm, increasing ancillary ligand
bulk effects dramatic, and to our knowledge unprecedented,1-4
increases in polymerization activity which roughly parallel
trends in δ 19F-Al. Furthermore, CGCMCH3+ polymerization
characteristics are markedly temperature-dependent, with 6-me-
diated polymerization at 60 and 110 °C affording ultrahigh
molecular weight polyethylene. In regard to anion effects on
metrical parameters are similar to those in CGCZr-
chiral cation stereoregulation, propylene polymerization (1.0 atm
of C3H6, 15 µmol of catalyst, 50 mL of toluene, 60 °C) mediated
by rac-Me2Si(Ind)2ZrMe+B(C6F5)4- (B(C6F5)4- is known to be
weakly coordinating2b,12) yields products of low isotacticity
([mm] ) 47%, [mr] ) 33%, and [rr] ) 20%),14a while under
the same conditions, the strongly ion-paired PBA- analogue
produces highly isotactic polypropylene ([mmmm] ) 98%).14b
The present findings complement and support recent observa-
tions of Eisch and co-workers15 that weakening of ion pairing
enhances propylene polymerization activity at the expense of
syndioselectivity and of Herfert and Fink4h that syndiospecificity
is sensitive to solvent polarity.
- 13
CH3+CH3B(C6F5)3
.
The ion pairing interplay of PBA- chirality and cation
stereochemistry is evident in the NMR spectra as is the strength
of the Zr- - -F-Al interaction. In contrast to the seven 19F
signals observed in Ph3C+PBA-, all of the cationic complexes
exhibit nine, indicative of restricted internal C6F5 rotation but
free anion rotation (at 25 °C) about the M- - -F-Al axes. In
the C2V-symmetric metallocenium cations of 1 and 2 (enanti-
omers A), anion dissymmetry renders the Cp ligands diaste-
These results considerably expand what is known about the
consequences of strong cation-anion interactions for group
4-mediated olefin polymerization. For anions that coordina-
tively “intrude” into the cation coordination sphere, the effects
can be dramatic.
reotopic. Broadening and coalescence of the signals at higher
temperatures (∆Gq ) 16.4(2) kcal/mol in 1b) can be associated
with anion stereomutation. With diastereotopic 1,2-Me2 Cp
substitution, 2 exhibits four Cp Me signals at 25 °C, indicating
dissymmetry with respect to the Cp centroid-Zr-Cp centroid
plane and that perpendicular (cf. A). On raising the temperature,
broadening and collapse of this pattern to two Me signals is
observed, with ∆Gq ) 16.9(2) kcal/mol. A barrier comparable
to that in 1b (suggesting anion racemization) and the lack of
significant additional symmetrization (∆Gq > 19.9(3) kcal/mol
at 87 °C) argues that anion dissociation (eq 2) has a higher
barrier in these systems than in analogous metallocenium
(fluoroaryl)borates.1d,4a,c,12 Indeed, 5-7 each exist as pairs of
unequally populated diastereomers (B and C; enantiomers not
shown) which undergo spectroscopic exchange (∆Gq ) 15.8-
Acknowledgment. This research was supported by the U.S.
Department of Energy (DE-FG 02-86 ER 13511). Y.X.C. thanks Akzo-
Nobel Chemicals for a postdoctoral fellowship.
Supporting Information Available: Details of Ph3C+PBA- and
catalyst synthesis, characterization and X-ray experimental details, and
tables of positional and thermal parameters, bond lengths and angles,
and labeled drawings (64 pages). See any current masthead page for
ordering and Internet access instructions.
JA963834J
(14) (a) 2.97 × 106 g of polymer/(mol‚cat‚atm‚h), Mw ) 2.34 × 104,
Mw/Mn ) 3.59, amorphous. (b) 1.63 × 104 g of polymer/(mol‚cat‚atm‚h),
Mw ) 6.97 × 104, Mw/Mn ) 2.40, Tm ) 145 °C.
(13) Fu, P. -F.; Wilson, D. J.; Rudolph, P. R.; Stern, C. L.; Marks, T. J.,
unpublished results.
(15) Eisch, J. J.; Pombrik, S. I.; Gu¨rtzgen, S.; Rieger, R.; Vzick, W. In
ref 3c, pp 221-235.