Novel trityl activators with new weakly coordinating anions derived from C6F41,2-[B(C6F5)2] 2: Synthesis, strussctures, and olefin polymerization behavior

Article abstract of DOI:10.1021/om0001974

The weakly coordinating onions {C₆F₄-1,2-[B(C₆F₅)₂] ₂(μ-OR)}^- (R = CH₃, C₆F₅) were prepared as their trityl salts (2-OR) and fully characterized.

Full text of DOI:10.1021/om0001974

Organometallics 2000, 19, 1619-1621
1619
Novel Tr ityl Activa tor s w ith New Wea k ly Coor d in a tin g
An ion s Der ived fr om C6F 4-1,2-[B(C6F 5)2]2: Syn th esis,
Str u ctu r es, a n d Olefin P olym er iza tion Beh a vior
V. Clifford Williams,† Geoffry J . Irvine,† Warren E. Piers,*,†,‡ Zengmin Li,§
Scott Collins,*,§ William Clegg,| Mark R. J . Elsegood,| and Todd B. Marder
Departments of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, Alberta, Canada T2N 1N4, University of Waterloo, Waterloo,
Ontario, Canada N2L 3G1, University of Newcastle, Newcastle upon Tyne NE1 7RU, U.K.,
and University of Durham, South Road, Durham DH1 3LE, U.K.
Received March 1, 2000
Summary: The weakly coordinating anions {C6F4-1,2-
[B(C6F5)2]2(µ-OR)}- (R ) CH3, C6F5) were prepared as
their trityl salts (2-OR) and fully characterized. Sto-
ichiometric reactions with Cp2ZrMe2 yield dimeric (3-
OR) or monomeric (4-OR) metallocenium ions; the latter
are highly effective cocatalysts for ethylene polymeriza-
tion.
methide anion is not chelated by the two borane centers
and the resultant anion is comprised of a equilibrium
mixture of two exchanging species. One of these contains
a neutral B-CH3 moiety which is capable of complexing
to cationic zirconocenes in the same way “Al(CH3)3”
binds these species.7 Accordingly, we sought to incor-
porate anions which can be bound by the diborane in a
chelating fashion, leading to more delocalization of the
negative charge8 and weaker ion pairing.
Trityl salts of weakly coordinating borate1 and alu-
minate2 counteranions are highly effective stoichiomet-
ric activators for olefin polymerization reactions. Unlike
the isoelectronic, but neutral, borane family of initia-
tors,2,3 alkide abstraction using trityl-based activators
is irreverisible, and ion-ion interactions are generally
weaker for metallocenium complexes with [B(ArF)4]-
type counteranions as opposed to [RB(C6F5)3]-.4 To the
extent that ion-ion interactions affect the catalyst’s
activity, stability, and, to a lesser extent, ability to
stereoregulate,2 anion engineering is an important
endeavor for catalyst improvements.
Trityl salts of F- and OR- (R ) CH3, C6F5) were
prepared straightforwardly via the reactions shown in
eqs 1 and 2. Diborane 1 abstracts F- from [Ph3C]+[BF4]-,
Recently we reported the synthesis and characteriza-
tion of the perfluorinated o-phenylene diborane C6F4-
1,2-[B(C6F5)2]2 (1).5 Although this compound is an
effective activator without modification,6 the character
of the methide ion produced leads to cation-anion
interactions which attenuate catalyst activity in com-
parison to B(C6F5)3. Essentially, this arises because the
to give 2-F as an orange solid in 95% yield with BF3 as
the only byproduct. In an even more atom-economical
reaction,9 1 reacts directly with trityl ethers in CH2Cl2
to afford 2-OCH3 and 2-OC6F 5 in >90% yield.
Compounds 2 were characterized by NMR spectros-
copy and elemental analysis; in addition, the solid-state
structures of 2-OCH 3 and 2-OC6F 5 were determined
crystallographically. Full details can be found in the
Supporting Information. The primary tool for determin-
ing solution structure for these compounds is 19F NMR
spectroscopy. For 2-F and 2-OCH3, the spectra are
simple and consistent with C2v symmetry for the anion,
suggesting 1 is indeed chelating the anion. The 11B NMR
data corroborates this assignment, since only one reso-
† University of Calgary.
‡ Phone: 403-220-5746. FAX: 403-289-9488. email: wpiers@ucal-
gary.ca.
§ University of Waterloo.
| University of Newcastle.
University of Durham.
(1) (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. (c) J ia, L.; Yang, X.; Stern, C. L.; Marks, T. J .
Organometallics 1997, 16, 842.
(2) Chen, Y.-X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J . J .
Am. Chem. Soc. 1998, 120, 6287.
(3) (a) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1991,
113, 3623. (b) Ewen, J . A.; Edler, M. J . Chem. Abstr. 1991, 115,
136998g, 256895t. (c) Spence, R. E. v. H.; Piers, W. E.; Sun, Y.; Parvez,
M.; MacGillivray, L. R.; Zaworotko, M. J . Organometallics 1998, 17,
2459. (d) Luo, L.; Marks, T. J . Top. Catal. 1999, 7, 97 and references
therein. (e) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 345.
(4) (a) Deck, P. A.; Beswick, C. L.; Marks, T. J . J . Am. Chem. Soc.
1998, 120, 1772. (b) Sun, Y.; Spence, R. E. v H.; Piers, W. E.; Parvez,
M.; Yap, G. P. A. J . Am. Chem. Soc. 1997, 119, 5132.
(5) Williams, V. C.; Piers, W. E.; Clegg, W.; Collins, S.; Marder, T.
B. J . Am. Chem. Soc. 1999, 121, 3244.
(7) Bochmann, M.; Lancaster, S. L. Angew. Chem., Int. Ed. Engl.
1994, 33, 1634.
(8) (a) J ia, L.; Yang, X.; Stern, C.; Marks, T. J . Organometallics 1994,
13, 3755. (b) Marks, T. J .; J ia, L.; Yang, X. U.S. Patent 5,447,895, 1995
(Northwestern University). (c) Lancaster, S. J .; Walker, D. A.; Thorn-
ton-Pett, M.; Bochmann, M. Chem. Commun. 1999, 1533.
(9) Shriver, D. F.; Biallas, M. J . J . Am. Chem. Soc. 1967, 89, 1078.
(6) Williams, V. C.; Dai, C.; Li, Z.; Collins, S.; Piers, W. E.; Clegg,
W. C.; Elsegood, M. R. J .; Marder, T. B. Angew. Chem., Int. Ed. Engl.
1999, 38, 3695.
10.1021/om0001974 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 03/25/2000

1620 Organometallics, Vol. 19, No. 9, 2000
Communications
dihedral angle is 19.9(2)°,5 these atoms are essentially
coplanar (along with O) in anions 2. Notably, the
geometry about oxygen is trigonal planar (the sums of
the angles are 360.0 and 359.9° for 2-OC6F 5 and
2-OCH 3, respectively) in contrast to the trigonal-
pyramidal ground-state structures found for the related
alkyloxonium salts [R3O]+[A]-. The energy surface
connecting these two geometries is soft;11 presumably,
bulky groups about O will favor planar geometries.12
Alternatively, the bite angle of 1 may require that O
assumes an sp2 hybridization in order to form strong σ
bonds to the two boron atoms.
The stoichiometric reactivity of compounds 2 with 1
or 2 equiv of Cp2ZrMe2 was investigated to assay the
stability of the resulting ion pairs. Clean reactions were
observed in each case, although in the case of 2-F , it is
clear that F- transfer to zirconium is facile (eq 3). This
F igu r e 1. ORTEP diagram for the anionic portion of
2-OC6F 5. The F atoms on the B-C6F5 rings and the OC6F5
ring have been removed for clarity. Selected bond distances
(in Å; values in brackets are the analogous values for
2-OCH3): B(1)-C(1), 1.600(10) [1.614(2)]; B(2)-C(6), 1.572-
(10) [1.622(2)]; B(1)-O(1), 1.693(8) [1.568(2)]; B(2)-O(1),
1.644(9) [1.562(2)]; O(1)-C(7), 1.386(8) [1.4485(19)]. Se-
lected bond angles (in deg): B(1)-O(1)-B(2), 115.4(5)
[117.72(12)]; B(1)-O(1)-C(7), 120.8(5) [121.27(13)]; B(2)-
O(1)-C(7), 123.8(5) [120.93(13)]; B(1)-C(1)-C(6), 115.4-
(6) [113.27(14)]; B(2)-C(6)-C(1), 116.8(6) [112.77(14)].
is particularly indicated by the highly characteristic 19
F
NMR spectral fingerprint for the [1‚Me]- anion,6 which
forms when free diborane 1 is liberated after the F-
transfer event. The presence of the dimeric µ-F cation2,13
is evidenced by a signal in the 19F NMR spectrum at
-90.4 ppm.
nance is observed between 0 and 9 ppm, the region
expected for four-coordinate boron centers with a formal
-0.5 charge. For 2-OC6F 5, evidence for fluxionality is
apparent in the room-temperature 19F spectrum. Vari-
able-temperature experiments show that this process
is associated with restricted rotation of the boron C6F5
rings, since the patterns present for the backbone
fluorines and those associated with the OC6F5 group
indicate that the C2v symmetry of the anion is main-
tained at all temperatures. Presumably, the hindered
rotation of the -C6F5 substituents on boron is due to
In contrast, the µ-OR anions are stable in the presence
of both [(Cp2ZrMe)2(µ-Me)]+ 14 (eq 4a) and [Cp2Zr(S)Me]+
(eq 4b) in C6D5Br, with no OR- transfer to Zr15 observ-
able over time periods on the order of days in each case.
the bulkier character of the pentafluorophenoxide group
-
in comparison to F- or OCH3
.
This stability appears to be thermodynamic in nature.
Gentle heating (60 °C) of solutions of the ion pair
3-OCH3 resulted in decomposition of the dimeric cation
through loss of methane,16 but 19F NMR spectroscopy
showed that the anion remained intact throughout.
Similarly, heating solutions of the monomeric ion pair
4-OCH3 gives no evidence for decomposition at 40 °C.
The molecular structure of the anion in 2-OC6F 5 is
shown in Figure 1 along with selected metrical param-
eters; in brackets, the analogous data for 2-OCH3 are
given for comparison.10 Confirming the solution struc-
tural assignments, in both of these compounds, OR is
bound to both borane centers in a symmetrical fashion.
Unlike the free diborane, in which the B-C-C-B
(11) (a) Lambert, J . B. Top. Stereochem. 1971, 6, 19. (b) Rauk, A.;
Andose, J . D.; Frick, W. G.; Tang, R.; Mislow, K. J . Am. Chem. Soc.
1971, 93, 6507.
(12) A related compound with less bulky groups at boron, [(µ-CH3-
CO2)(BF2)2(µ-OCH3)], crystallizes with two molecules in the unit cell,
one of which is planar at the µ-OCH3 oxygen and the other pyrami-
dal: Binder, H.; Matheis, W.; Deiseroth, H.-J .; Fu-Son, H. Z. Natur-
forsch., B 1983, 38, 554.
(10) Crystal data for 2-OC6F 5: C57H19B2Cl3F29O, 0.20 × 0.19 × 0.06
mm, triclinic, P1h, a ) 12.2241(15) Å, b ) 12.8975(16) Å, c ) 18.801(2)
Å, R ) 80.166(2)°, â ) 72.091(2)°, γ ) 70.812(2)°, V ) 2655.9(6) Å3, Z
) 2, FW ) 1398.69, Dcalcd ) 1.749 g cm-3, θ range for data collection
1.68-25.00°, Mo KR radiation, λ ) 0.710 73 Å, T ) 160(2) K, 15 208
measured reflections, 8865 unique, 3565 reflections with Inet > 2.0σ-
(Inet), µ ) 0.319 mm-1, min/max transmission 0.939 and 0.981, final R
indices R1 ) 0.0812 and wR2 ) 0.2135, GOF ) 0.969, 36 restraints,
848 parameters. Crystal data for 2-OCH3: C51H20B2Cl2F24O, 0.60 ×
0.35 × 0.35 mm, monoclinic, P21/n, a ) 13.5900(2) Å, b ) 19.3540(2)
Å, c ) 18.5439(2) Å, â ) 101.517(2)°, V ) 4779.23(10) Å3, Z ) 4, FW
) 1197.19, Dcalcd ) 1.664 g cm-3, θ range for data collection 2.00-
29.11°, Mo KR radiation, λ ) 0.710 73 Å, T ) 160(2) K, 39 886
measured reflections, 11 847 unique, 8912 reflections with Inet > 2.0σ-
(Inet), µ ) 0.271 mm-1, final R indices R1 ) 0.0446 and wR2 ) 0.1158,
GOF ) 1.061, 15 restraints, 733 parameters. The restraints were
applied to geometry and anisotropic displacement parameters of atoms
in disordered solvent molecules, to aid their refinement. There were
no restraints on the cations and anions.
(13) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10015.
(14) (a) Bochmann, M.; Lancaster, S. L. Angew. Chem., Int. Ed. Engl.
1994, 33, 1634. (b) Yang, X.; Stern, C. L.; Marks, T. J . Organometallics
1991, 10, 840. (c) Beck, S.; Prosenc, M.-H.; Brintzinger, H. H.; Goretzki,
R.; Herfert, N.; Fink, G. J . Mol. Catal. 1996, 111, 67. (d) Ko¨hler, K.;
Piers, W. E.; Xin, S.; Feng, Y.; Bravakis, A. M.; J arvis, A. P.; Collins,
S.; Clegg, W.; Yap, G. P. A.; Marder, T. B. Organometallics 1998, 17,
3557.
(15) Siedle, A. R.; Newmark, R. A.; Lamanna, W. M.; Schroepfer, J .
N. Polyhedron 1990, 9, 301.
(16) Bochmann, M.; Cuenca, T.; Hardey, D. T. J . Organomet. Chem.
1994, 484, C10.

Communications
Organometallics, Vol. 19, No. 9, 2000 1621
Ta ble 1. P olym er iza tion of Eth ylen e w ith Cp 2Zr Me2 a n d Va r iou s Coca ta lystsa
entry
cocatalyst
scrubbing agentb
t (min)
tield (g)
activityc
Mn
Mw/ Mn
1
2
B(C6F5)3
MAD
MAD
MAD
MAD
MAD
MAD
MAD
MAD
MAD
PMAO
20
20
30
30
10
30
30
30
30
30
3.39
6.49
24.42
24.51
8.85
2.02
0.00
3.07
20.83
4.20
10.6
20.2
24.4
24.5
26.6
4.18
nil
6.36
43.15
8.40
272
344
244
2.18
2.02
2.06
[Ph3C][B(C6F5)4]
[Ph3C][B(C6F5)4]
[Ph3C][B(C6F5)4]
[Ph3C][B(C6F5)4]
1
2-F
2-OMe
2-OC6F 5
PMAO
3d
4d
5d
6
249
250
1.92
2.04
7
8
200
240
1.92
1.72
9
10e
a
For a typical procedure consult ref 21. Conditions: toluene 500 mL, 1000 rpm, 30 °C, 14 psi of C2H4 with [Cp2ZrMe2] ) [cocatalyst]
) 2 µM unless otherwise noted. MAD ) MeAl(BHT)2 with [MAD] ) 300 µM. c Activity in 106 g of PE (mol of Zr)-1 h-1 atm-1
.
b
Polymerizations conducted at 29 psi of C2H4. e Polymerization conducted at 25 °C and 14 psi of C2H4 and with [PMAO] ) 2.0 mM (Al:Zr
d
) 1000:1).
At 60 °C, decomposition begins, but the primary path
involves transfer of a -C6F5 ring to zirconium,17 rather
than OCH3-. This contrasts with the observation of a
highly complex product mixture formed upon reaction
lysts in a preliminary fashion. We are currently at-
tempting to more accurately define the nature of the
ion-ion interactions in these ion pairs, preparing next-
generation derivatives of 1, and carrying out more
advanced olefin polymerization screening experiments.
18
of (C6F5)2BOCH3 with Cp2ZrMe2, generated from
initial products arising from rapid CH3/OCH3 ex-
change between zirconium and the monomeric borinic
Ack n ow led gm en t. Financial support for this work
was provided by the Natural Sciences and Engineering
Council of Canada’s Strategic Projects program (STR-
192869) and the EPSRC (UK). Technical and in-kind
support from Nova Chemicals of Calgary, Alberta,
Canada, is also acknowledged. W.E.P. thanks the Alfred
P. Sloan Foundation for a Research Fellowship (1996-
2000).
ester.15 Finally, diborane 1 rapidly abstracts OCH3
-
from Cp2Zr(OCH3)2;19 although the cation in the result-
ing species is not very stable,20 the 19F and 11B NMR
spectra show clean formation of the µ-OCH3 anion.
The efficacy of these trityl salts as ethylene polym-
erization initiators using Cp2ZrMe2 was examined with
a view to comparison with standard borane- and borate-
based activators. The results of these experiments,
performed using a scrubbing agent which does not
interact chemically with the initiators,6 are given in
Table 1.21 As predicted on the basis of the activation
chemistry in the absence of monomer, 2-F is ineffectual
as an activator. Alkoxide derivatives 2-OCH3 and
2-OC6F 5, however, are active; the latter compound gives
activities which are more than double those found for
[Ph3C]+[B(C6F5)4]-, generally the most effective stoi-
chiometric activator found to date. The combination of
steric protection of the alkoxide oxygen lone pair and
extensive delocalization of the anion’s negative charge
accounts for its superior performance as a weakly
coordinating anion.
Su p p or tin g In for m a tion Ava ila ble: Experimental and
spectroscopic details for 2-F , 2-OCH3, 2-OC6F 5, 3-OR, and
4-OR, as well as tables of crystal data, atomic coordinates,
bond lengths and angles, anisotropic displacement parameters,
and hydrogen atom coordinates for 2-OCH3 and 2-OC6F 5. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM0001974
(21) A typical experimental procedure is as follows. A 1 L autoclave
was dried under vacuum (10-2 mmHg) at 100 °C for several hours and
refilled with dry N2 after cooling to 30 °C. The vessel was charged with
450 mL of dry and deoxygenated toluene under positive pressure of
N2 and then saturated with ethylene monomer at the required
pressure. A solution of MAD in toluene (10 mL) was introduced using
a small sampling vessel, overpressurized with N2, to give a final
concentration of 300 µM. After the mixture was stirred for 1 h, solutions
of the cocatalyst and Cp2ZrMe2 in toluene (20 mL each) were
sequentially introduced in the same manner so as to give a final
concentration of 2 µM for each. Ethylene uptake was monitored using
a calibrated mass flow meter; constant monomer flow was observed
typically within 5 min after catalyst introduction. The temperature
was controlled (to ca. ( 2 °C) using an external cooling jacket connected
to a recirculating heating/cooling bath and was monitored by an RTD
sensor placed in a thermocouple well in contact with the reactor
contents. Polymerizations were conducted for a time period corre-
sponding to 20-30 min, following attainment of steady-state conditions
(i.e., constant T and ethylene flow) in the reactor, and polymerizations
were quenched by the addition of MeOH through an overpressurized
sample vessel. Polymer samples were isolated by filtration, washed
with MeOH, and dried in vacuo at 80 °C for 24 h prior to weighing.
In summary, we have prepared a series of trityl salts
containing new weakly coordinating anions and exam-
ined their behavior as ethylene polymerization cocata-
(17) As evidenced by the production of Cp2Zr(CH3)C6F5.14d
(18) Generated from the borane HB(C6F5)2 and CH3OH: Parks, D.
J .; Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17, 5492.
(19) Gray, D. R.; Brubaker, C. H., J r. Inorg. Chem. 1971, 10, 2143.
(20) (a) Cationic alkoxide zirconocenes with bulky alkyl20b,c or
donor20d substituents are known, but those with less bulky groups tend
to form dimers and undergo other side reactions. (b) Collins, S.; Koene,
B. E.; Ramachandran, R.; Taylor, N. J . Organometallics 1991, 10, 2092.
(c) Hong, Y.; Kuntz, B. A.; Collins, S. Organometallics 1993, 12, 964.
(d) Wu, Z.; J ordan, R. F.; Petersen, J . L. J . Am. Chem. Soc. 1995, 117,
5867.