Al-, Nb-, and Ta-based perfluoroaryloxide anions as cocatalysts for metallocene-mediated Ziegler-Natta olefin polymerization

Article abstract of DOI:10.1021/om990946l

Effective cocatalysts need not contain metalloid/metal-carbon bonds and the properties of the resulting metallocenium cation-anion pairs are sensitive to both the counteranion core structure and the metallocene ancillary litigation. The perfluoroaryloxide

Full text of DOI:10.1021/om990946l

Organometallics 2000, 19, 1625-1627
1625
Al-, Nb-, a n d Ta -Ba sed P er flu or oa r yloxid e An ion s a s
Coca ta lysts for Meta llocen e-Med ia ted Ziegler -Na tta
Olefin P olym er iza tion
Yimin Sun, Matthew V. Metz, Charlotte L. Stern, and Tobin J . Marks*
Department of Chemistry, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208-3113
Received December 1, 1999
Summary: Perfluoroaryloxide salts of the formula Ph3C+-
ber of available cocatalysts is, however, limited and has
so far impeded a broad, systematic study of anion
effects. We communicate here the synthesis, character-
ization, and cocatalytic features of a new series of
sterically encumbered metalloid and transition metal
cocatalysts/counteranions based on the pentafluorophe-
noxide group, C6F5O-. We report that effective cocata-
lysts need not contain metalloid/metal-carbon bonds
and that the properties of the resulting metallocenium
cation-anion pairs are sensitive to both the counter-
anion core structure and the metallocene ancillary
ligation.7
-
-
Al(OC6F5)4 and Ph3C+M(OC6F5)6 (M ) Nb, Ta) have
been synthesized and characterized. In combination with
sterically encumbered zirconocene dimethyl complexes,
they yield active ethylene polymerization catalysts.
Current generation abstractors/cocatalysts for homo-
geneous single-site olefin polymerization1 are typically
group 13-based and contain metal/metalloid-carbon
3
bonds, e.g., methylalumoxane (MAO),2 MArxFAr3-x
,
M(C6F4-2-C6F5)3,4 MAr4 (M ) B, Al).5 Due to their
structurally well-defined chemical nature and high
cocatalytic activity, the perfluoroarylboranes, -borates,
and -aluminates are of considerable current scientific
and technological interest. Results to date reveal that
many of the properties of such catalyst systems are
intimately connected with the nature of the cation-
anion interaction in ways that are not well-understood.
The size, shape, electronic structure, and ligational
characteristics of the anion can have a profound effect
on catalyst activity, thermal stability, chain transfer
characteristics, stereoselection, etc.2a,3a-c,4,5a,6 The num-
F-
Reaction of LiAlH4 with HOC6F5 affords Li+Al(OC6-
F5)4-,8 and subsequent metathesis with Ph3CCl yields
the corresponding trityl tetrakis(pentafluorophenoxo)-
aluminate, Ph3C+Al(OC6F5)4 (1) (eq 1).9 All new com-
-
LiAlH4 + 4HOC6F5 toluene, 85 °C8
-H2
LiAl(OC6F5)4 Ph CCl8
(1)
3
Ph3C+Al(OC6F5)4
-
1
(1) (a) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428-447. (b) Marks, T. J ., Stevens, J . C., Eds. Topics
Catal. 1999, 15 (special issue). (c) J ordan, R. F., Ed. J . Mol. Catal.
1998, 128 (special issue). (d) Kaminsky, W.; Arndt, M. Adv. Polym.
Sci. 1997, 127, 144-187. (e) Bochmann, M. J . Chem. Soc., Dalton
Trans. 1996, 255-270. (f) Brintzinger, H.-H.; Fischer, D.; Mu¨lhaupt,
R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995,
34, 1143-1170. (g) Soga, K.; Terano, M. In Catalyst Design for Tailor-
Made Polyolefins; Soga, K., Terano, M., Eds.; Elsevier: Tokyo, 1994.
(h) Mo¨hring, P. C.; Coville, N. J . J . Organomet. Chem. 1994, 479, 1-29.
(2) (a) Coevoet, D.; Cramail, H.; Deffieux, A. Macromol. Chem. Phys.
1998, 199, 1459-1464, and references therein. (b) Reddy, S. S.;
Sivaram, S. Prog. Polym. Sci. 1995, 20, 309-367. (c) Harlan, C. J .;
Bott, S. G.; Barron, A. R. J . Am. Chem. Soc. 1995, 117, 6465-6474.
(d) Sishta, C.; Hathorn, R.; Marks, T. J . J . Am. Chem. Soc. 1992, 114,
1112-4. (e) Pasynkiewicz, S. Polyhedron 1990, 9, 429-453.
(3) (a) Luo, L.; Marks, T. J . In ref 1b, pp 97-106. (b) Li, L.; Marks,
T. J . Organometallics 1998, 17, 3996-4003. (c) Deck, P. A.; Beswick,
C. L.; Marks, T. J . J . Am. Chem. Soc. 1998, 120, 1772-1784. (d) Piers,
W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345-354. (e) Pellecchia,
C.; Pappalardo, D.; Oliva, L.; Zambelli, A. J . Am. Chem. Soc. 1995,
117, 6593-6594. (f) Temme, B.; Erker, G.; Karl, J .; Luftmann, H.;
Fro¨hlich, R.; Kotila, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1755-
1757. (g) Yang, X. M.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc.
1994, 116, 10015-10031. (h) Bochmann, M.; Lancaster, S. J .; Hurst-
hous, M. B.; Malik, K. M. A. Organometallics 1994, 13, 2235-2243.
(4) (a) Chen, Y. X.; Metz, M. V.; Li, L. T.; Stern, C. L.; Marks, T. J .
J . Am. Chem. Soc. 1998, 120, 6287-6305. (b) Chen, Y. X.; Stern, C.
L.; Yang, S. T.; Marks, T. J . J . Am. Chem. Soc. 1996, 118, 12451-
12452.
pounds were characterized by conventional spectro-
scopic and analytical methodology.9 Compound 1 is
thermally stable at 25 °C in CD2Cl2 solution for days
without noticeable decomposition. The pentachlorides
MCl5 (M ) Nb, Ta) readily undergo reaction with
LiOC6F5 in Et2O at 25 °C to afford crystalline [Li-
(OEt2)n]+{[M(OC6F5)4(µ2-OC6F5)2]2Li}‚C7H8 (Scheme 1;
M ) Nb, n ) 4 (2a ); M ) Ta, n ) 3 (2b)) salts.9,10
Diffraction results for 2a reveal the quasi-octahedral
(6) (a) Lanza, G.; Fragala`, I. L.; Marks, T. J . J . Am. Chem. Soc. 1998,
120, 8257-8258. (b) Rhodes, B.; Chien, J . C. W.; Rausch, M. D.
Organometallics 1998, 17, 1931-1933. (c) Shiomura, T.; Asanuma, T.;
Inoue, N. Macromol. Rapid Commun. 1996, 17, 9-14. (d) Eisch, J . J .;
Pombrik, S. I.; Gurtzgen, S.; Rieger, R.; Vzick, W. In ref 1g, pp 221-
235. (e) Giardello, M. A.; Eisen, M. S.; Stern, C. L.; Marks, T. J . J .
Am. Chem. Soc. 1995, 117, 12114-12129.
(7) Communicated in part: Sun, Y.; Stern, C. L.; Marks, T. J .
Abstracts of Papers; 217th National Meeting of the American Chemical
Society, Anaheim, CA, April, 1999; American Chemical Society:
Washington, DC, 1999; INOR 14.
(8) The synthesis of related fluoralkoxide complex Li+[Al[OC(Ph)-
(CF3)2]4]- has been reported: Barbarich, T. J .; Handy, S. T.; Miller, S.
M.; Anderson, O. P.; Grieco, P. A.; Strauss, S. H. Inorg. Chim. Acta.
1996, 3776.
(9) See Supporting Information for full synthetic and characteriza-
tion details.
(5) (a) J ia, L.; Yang, X.; Stern, C. L.; Marks, T. J . Organometallics
1997, 16, 842-857. (b) Chien, J . C. W.; Tsai, W.-M.; Rausch, M. D. J .
Am. Chem. Soc. 1991, 113, 8570-8571. (c) Yang, X. M.; Stern, C. L.;
Marks, T. J . Organometallics 1991, 10, 840-842. (d) Ewen, J . A.; Elder,
M. J .; Ewen, J . A.; Elder, M. J . European Patent Appl. p 426637, 1991.
(e) Hlatky, G. G.; Upton, D. J .; Turner, H. W. U.S. Patent Appl. p
459921, 1990.
(10) The synthesis of related fluoroalkoxide complexes Ph3C+[Nb-
(OCH(CF3)2)6]- and Li+[Nb(OCH(CF3)2)6]- has been reported: Rock-
well, J . J .; Kloster, G. M.; DuBay, W. J .; Grieco, P. A.; Shriver, D. F.;
Strauss, S. H. Inorg. Chim. Acta 1997, 195-200.
(11) Amor, J . I.; Burton, N. C.; Cuenca, T.; Go´mez-Sal, P.; Royo, P.
J . Organomet. Chem. 1995, 485, 153-160.
10.1021/om990946l CCC: $19.00 © 2000 American Chemical Society
Publication on Web 04/04/2000

1626 Organometallics, Vol. 19, No. 9, 2000
Sch em e 1. P r ep a r a tion of Com p lexes 2 a n d 3
Communications
nature of the group 5 centers and square-planar Li+
coordination (Scheme 1). Compounds 2 are cleanly
converted with Ph3CCl to the corresponding mono-
metallocenes, e.g., (C5Me4H)2ZrMe2 and (C5Me5)2ZrMe2,
is significantly different. Thus, reaction of 3a with 1.0
equiv of each of these metallocenes in toluene-d8 initially
results in red oily products, typical of more ionic (more
loosely ion-paired) species,5a as well as (C5Me4H)2ZrMe-
(OC6F5) or (C5Me5)2ZrMe(OC6F5), respectively, with the
latter assignments confirmed by NMR spectral com-
parison with authentic samples generated by C6F5OH
addition to the neutral dimethyl complexes.9 On stand-
ing at -20 °C, the oily material slowly converts to the
nuclear, thermally stable Ph3C+M(OC6F5)6 (M ) Nb,
-
(3a ); M ) Ta (3b)) salts9,10 in high yield. Diffraction
analysis reveals six-coordinate metal centers with pro-
nounced nonlinearlity of the M-O-C6F5 linkages and
dispersion in Nb-O distances. For example, the Nb-
O-C(aryl) angle ranges from 135.3(2)° to 170.6(2)° and
the Nb-O distance from 1.913(2) to 2.009(2) Å.
The reactivity and cocatalytic characteristics of 3 were
investigated with respect to a series of metallocene
dimethyls. At room temperature, facile C6F5O- transfer
from Nb/Ta to Zr/Ti is observed for coordinatively more
open complexes such as (C5H5)2ZrMe2, which reacts
readily to form, as assessed by 1H/19F NMR spectral
comparison with authentic samples,11 (C5H5)2Zr(OC6F5)2,
Ph3CCH3, and presumably MeNb(OC6F5)4, as well as
unidentified minor byproducts.11 Reaction of 3a with 1.0
equiv of rac-Me2Si(Ind)2ZrMe2 proceeds in a similar
fashion with formation of rac-Me2Si(Ind)2ZrMe(OC6F5).9
In contrast, the behavior toward sterically encumbered
metallocenium ion pair [(C5Me5)2ZrOC6F5]+Nb(OC6F5)6
-
(4; Scheme 2), characterized by diffraction and featuring
both unexceptional (C5Me5)2ZrR+ metrical parameters3f
and a relatively short Zr+‚‚‚F-C(aryloxide) contact
(2.342(2) Å) and a correspondingly elongated F-C bond
distance (1.383(4) vs 1.345(4) Å (av)).2e,f,5c,12 This result
argues for the intermediacy of [(C5Me5)2ZrMe]+Nb-
(OC6F5)6-, as does observation of [(C5Me5)2ZrMe(THF-
d8)]+Nb(OC6F5)6- (5)13 in reaction of 3a with (C5Me5)2-
ZrMe2 in THF-d8 and the olefin polymerization activity
(vide infra). The instability of [(C5Me5)2ZrMe]+Nb-
(OC6F5)6- may reflect the aforementioned deformability
of the Nb-O-C6F5 linkages, which would facilitate Nb-
O-C6F5 interaction with, and subsequent C6F5O- trans-
fer to, the electrophilic Zr center.
(12) For some related M+‚‚‚F-C examples, see: (a) Sun, Y.; Spence,
R. E.v. H.; Piers, W. E.; Parvez, M.; Yap, G. P. A. J . Am. Chem. Soc.
1997, 119, 5132-5143. (b) Ruwwe, J .; Erker, G.; Fro¨hlich, R. Angew.
Chem., Int. Ed. Engl. 1996, 35, 80-83. (c) Horton, A. D.; Orpen, A. G.
Organometallics 1991, 10, 3910-3918.
The above results imply that phenoxide transfer from
3 is facile for sterically open metallocenium systems,
explaining why such preactivated catalysts are inactive
toward olefin polymerization. However, spectroscopic
(13) 1H and 19F NMR for 5 (THF-d8, 23 °C): δ 2.01 (s, 30 H, C5Me5),
0.40 (s, 3 H, Zr-Me). 19F NMR (THF-d8, 23 °C): δ -157.26 (d, 2 F, o-F,
J F-F ) 18.1 Hz), -165.38 (tr, 2 F, p-F, J F-F ) 20.6 Hz), -169.14 (tr, 1
F, m-F, J F-F ) 22.9 Hz).

Communications
Organometallics, Vol. 19, No. 9, 2000 1627
Sch em e 2. P r op osed Rea ctivity P a tter n of (C5Me5)2Zr Me2 w ith Coca ta lyst 3a
Ta ble 1. Eth ylen e P olym er iza tion Activities a n d P olym er P r op er itiesa
µmol
of cat.
polymer
yield (g)
activity
g PE/atm‚mol‚h
b
entry
precursor
cocatalyst
rxn, time (s)
Mn
Mw/Mn
Tmc (°C)
1
2
3
4
5
6
7
8
(C5Me5)2ZrMe2
(C5Me5)2ZrMe2
(C5Me5)2ZrMe2
(C5Me5)2ZrMe2
(C5Me4H)2ZrMe2
(C5Me4H)2ZrMe2
(C5Me4H)2ZrMe2
(C5Me4H)2ZrMe2
Ph3C+Al(OC6F5)4- (1)
Ph3C+Nb(OC6F5)6- (3a )
Ph3C+Ta(OC6F5)6- (3b)
13.5
1.27
1.32
1.76
1.81
1.73
1.75
1.73
30
30
30
30
30
30
30
30
0.441
0.498
0.561
0.636
0.471
0.549
0.739
0.729
3.9 × 106
6.0 × 106
6.5 × 106
4.3 × 106
3.1 × 106
4.1 × 106
5.0 × 106
5.1 × 106
62 200
45 800
79 800
88 200
127 000
106 000
62 500
33 200
2.77
3.32
2.48
2.15
1.91
3.30
4.03
7.60
139.8
138.7
136.9
137.9
137.2
136.8
136.5
135.9
Ph3C+B(C6F5)4
-
Ph3C+Al(OC6F5)4- (1)
Ph3C+Nb(OC6F5)6- (3a )
Ph3C+Ta(OC6F5)6- (3b)
Ph3C+B(C6F5)4
-
a
Carried out at 1.0 atm of ethylene pressure in 100 mL of toluene on a high-vacuum line at 25 °C. See Supporting Information and ref
4a for polymerization procedures. GPC relative to polystyrene standards. c DSC trace from the second scan.
b
observation of ion pair 5 prompted studies of in situ (C5-
Me5)2ZrMe2 and (C5Me4H)2ZrMe2 activation in the pres-
ence of olefin (as is commonly performed for unstable
B(C6F5)4--based catalysts).5 Under such conditions,
reaction of 1 and 3 with (C5Me5)2ZrMe2 and (C5Me4H)2-
ZrMe2 at 25 °C affords efficient ethylene polymerization
catalysts (Table 1; procedure of refs 3c, 4a), with
activities and product polymer properties comparable
to those of Ph3C+B(C6F5)4--activated systems under
identical polymerization conditions.5 One striking and
unprecedented feature of the present catalysts is that
high ethylene polymerization activity is not paralleled
by propylene polymerization activity. To our knowledge,
this is the first report of such an anion-modulated
metallocene reactivity pattern.1,6 However, propylene
incorporation in ethylene + propylene copolymerizations
is observed. The ability of the present catalyst systemsto
differentiate CH2dCH2 and CH2dCHCH3 at the activa-
tion step may be related to secondary interactions of
the anion phenoxide ligands with the cationic metal-
locene centers. These results demonstrate that neither
group 13 elements nor metal/metalloid-C bonds are
necessary for effective metallocene olefin polymerization
cocatalysts. Additional steric and electronic modifica-
tions are the subject of continuing research.
Ack n ow led gm en t. This research was supported by
the U.S. Department of Energy (DE-FG 02-86 ER13511).
Y.S. thanks Dow Chemical Co. for a postdoctoral fel-
lowship. We thank Dr. P. N. Nickias of Dow for gpc
analyses.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and X-ray structural data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM990946L