Sterically hindered phosphine and phosphonium-based activators and additives for olefin polymerization

Article abstract of DOI:10.1039/b911489k

The ability of phosphonium borates of the form [R₃PH][B(C ₆F₅)₄], R₂PHC₆F ₄BF(C₆F₅)₂ and R₂PHC ₄H₈OB(C₆

Full text of DOI:10.1039/b911489k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Sterically hindered phosphine and phosphonium-based activators and additives
for olefin polymerization
Jenny S. J. McCahill, Gregory C. Welch and Douglas W. Stephan*†
Received 12th June 2009, Accepted 30th July 2009
First published as an Advance Article on the web 25th August 2009
DOI: 10.1039/b911489k
The ability of phosphonium borates of the form [R3PH][B(C6F5)4], R2PHC6F4BF(C6F5)2 and
R2PHC4H8OB(C6F5)3 as well as the phosphine-boranes R2PC6F4B(C6F5)2 to activate
CpTiMe2(NPtBu3) for olefin polymerization was examined via both stoichiometric reactions and
catalytic performance. In general these activators resulted in highly active ethylene polymerization
catalysts, despite the generation of liberated phosphine donors. Independent experiments in which
phosphines were added to the catalyst systems revealed the expected decrease in activity for small
phosphines. However in the case of sterically encumbered phosphines, a marked increase in activity was
observed. The cause of this increase is considered in the context of the concept of “frustrated Lewis
pairs”.
one of the C6F5 rings affording zwitterionic species of the form
Introduction
R2PHC6F4BF(C6F5)2 (R2 = Cy2 4, Mes2 5, tBu(Mes) 6, tBu2
7, Fig. 1(a)). Similarly, the species Mes2PHC6F4BCl(C6F5)2 8,45
Over the last 25 years a great deal of research has focused on the de-
velopment of homogeneous catalysts for olefin polymerization.1-6
In these efforts, the primary focus has been on the nature of the
metal complex. Thus ligand design has played a principal role in
many investigations.1-6 Indeed, over the last 10 years, we employed
Cy3PC6F4BF(C6F5)2 9, subsequent treatment of which with a
Grignard reagent affords the corresponding phosphine-boranes,
R2PC6F4B(C6F5)2 (R2 = Mes2 10, tBu(Mes) 11, tBu2 12). Al-
ternatively, an alternative avenue to phosphonium borates, in-
this approach to develop active olefin polymerization catalysts
volves the reactions of sterically encumbered phosphines with
based on Ti-phosphinimide complexes.7-10 A primary example of
(THF)B(C6F5)3 giving the zwitterions R2PHC4H8OB(C6F5)3(R =
these systems is the species (tBu3PN)2TiMe2 which upon activation
Mes 13, tBu 14). (Fig. 1(b)). These phosphonium borates are of
is remarkably active under commercially relevant conditions.9
potential interest as activators as these species are expected to ac-
Typically the activation strategy for olefin polymerization catalysts
tivate dialkyl-titanium catalysts via protonation of an alkyl group,
has been the use of large excesses of methylaluminoxane (MAO).
affording the titanium cation and generating free phosphine. This
behavior closely parallels that of ammonium cations [R3NH]+.46
However in some cases liberated amine from ammonium-based
In combination with a metal dihalide species, this reagent acts
as both an alkylating and alkyl abstraction reagent to generate
cationic metal alkyl species which are catalytically active. Alter-
natively, B(C6F5)3, [Ph3C][B(C6F5)4] or [HNRR¢2][B(C6F5)4] have
been commonly employed to react with metal-dialkyl derivatives
to effect alkyl abstraction. While in general the nature of the
cocatalyst has drawn lesser attention than the metal-precursors,
the research groups of Marks,11-19 Piers20-29 and others30-34 have
developed a variety of Al and B-based cocatalysts and probed the
impact of size and charge distribution of the anion on catalyst
activity.
In what is seemingly unrelated chemistry, we have recently re-
viewed a series of reactions of bulky phosphines with B(C6F5)3.35-39
Such “frustrated Lewis pairs” (FLPs) offer unique synthetic
routes to a number of phosphonium borates40-42 and phosphine-
boranes41 that could serve as activators for olefin polymerization
catalysts. For example, phosphonium borate salts of the form
[R3PH][B(C6F5)4] R = Cy 1, Mes43 2, tBu44 3) are readily prepared.
Alternatively, heating bulky secondary phosphines with B(C6F5)3
results in the substitution of P for F at the para-position of
Department of Chemistry & Biochemistry, University of Windsor, Windsor,
Ontario, Canada N9B 3P4
† Current address: Department of Chemistry, University of Toronto,
80 St. George St. Toronto, Ontario, Canada M5S 3H6, email:
dstephan@chem.utoronto.ca
Fig. 1 Preparative routes to phosphonium-borates and phosphine-
boranes.
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activators has been shown to coordinate to the metal cation
supressing activity to some extent.46 While small phosphines have
been shown to sequester the Ti cation as [CpTiMe(NPtBu3)(PR3)]+
(R = Me, Bu, Ph, C6H4Me) (Fig. 2),47 the present phosphonium-
borates will liberate sterically encumbered phosphines. Such
donors have been shown not to coordinate to main group Lewis
acids, affording uniquely reactive FLPs. In this manuscript we
explore the viability of these phosphonium borate and phosphine-
borane species for use as activators. The impact on the activity of
the resulting catalysts is examined.
PhCMe), 6.09 (s, 5H, Cp), 2.07 (s, 3H, PhCMe), 1.18 (d of d, 3JHP
= 14 Hz, 30 Hz, tBu, TiMe). 11B NMR (C6D5Br): -16.7 (s). 13C
1
NMR (C6D5Br): 149.07 (s, quat, Ph), 148.66 (d, JCF = 236 Hz,
1
1
CF), 138.44 (d, JCF = 245 Hz, CF), 136.56 (d, JCF = 242 Hz,
CF), 128.81 (s, CH, Ph), 127.96 (s, CH, Ph), 136.03 (s, CH, Ph),
116.10 (s, Cp), 61.29 (s, TiMe), 52.53 (s, quat, Ph3CMe), 41.19 (d,
1JCP = 41 Hz, quat, tBu3), 30.51 (s, Ph3CMe), 28.93 (s, tBu) 19F
3
NMR (C6D5Br): -132.29 (s, 8F, o-C6F5), -162.67 (t, 4F, JFF
=
20 Hz, p-C6F5), -166.47 (t, 8F, JFF = 17 Hz, m-C6F5). 31P{ H}
3
1
NMR (C6D5Br): 55.9 (s).
Generation of [CpTiMe(NPtBu3)][R2P(C6F4)BMe(C6F5)2]
(R = Mes 16, tBu 17)
These compounds were prepared in a fashion similar to that
described for 15 employing the appropriate activator. 16: Yield
115 mg (78%). 1H NMR (C6D5Br): 6.71 (s, 4H, C6H2), 6.12 (s, 5H,
Cp), 2.32 (s, 12H, C6H2Me-2,6), 2.16 (s, 6H, C6H2Me-4), 1.18 (s,
3H, BMe), 1.14 (br s, 27H, tBu), 0.85 (s, 3H, TiMe). 11B NMR
Fig. 2 Complexation of [CpTiMe(NPtBu3)]+ by small phosphines.
1
(C6D5Br): -14.6 (br s). 13C{ H} NMR (C6D5Br) partial: 148.54
Experimental
(dm, 1JCF = 250 Hz, CF), 147.15 (dm, 1JCF = 250 Hz, CF), 142.61
2
(d, JCP = 12 Hz, quat, Mes), 138.02 (s, quat, Mes), 137.76 (dm,
General considerations
1
1JCF = 245 Hz, CF), 136.67 (dm, JCF = 240 Hz, CF), 130.13
All preparations were performed under an atmosphere of dry
O2–free N2 employing either Schlenk-line techniques or a Vacuum
Atmospheres inert atmosphere glove box. Solvents were purified
employing Grubbs-type column systems manufactured by Inno-
vative Technologies or were distilled from the appropriate drying
(s, CH, Mes), 114.13 (s, Cp), 52.80 (br s, TiMe), 41.24 (br, tBu),
3
28.71 (br, tBu), 22.64 (d, JCP = 18 Hz, C6H2Me-2,6), 20.92 (s,
C6H2Me-4), 10.50 (br s, BMe). 19F NMR (C6D5Br): -132.24 (br,
6F, o-C6F5, C6F4), -135.37 (br, 2F, C6F4), -164.08 (br, 2F, p-C6F5),
-166.47 (br, 4F, m-C6F5). 19F NMR (C6D5Br, 243K): -132.57 (m,
5F, o-C6F5, C6F4), -133.52 (s, 1F, C6F4), -133.73 (s, 1F, C6F4),
-136.10 (s, 1F, C6F4), -164.06 (m, 2F, p-C6F5), -166.66 (m, 4F,
1
agents under N2. 1H and 13C{ H} NMR spectra were recorded on
Bruker Avance 300 and 500 spectrometers. Deuterated benzene,
toluene and methylene-chloride were purchased from Cambridge
Isotopes Laboratories, vacuum distilled from the appropriate
drying agents and freeze–pump–thaw degassed (3 times). Trace
amounts of protonated solvents were used as references, and
1
m-C6F5). 31P{ H} NMR (C6D5Br): 50.6 (br, PtBu3), -50.3 (br,
1
PMes2). 31P{ H} NMR (C6D5Br, 243K): 49.0 (br, PtBu3), -52.0 (t,
3JPF = 37 Hz, PMes2).
1
1H and 13C{ H} NMR chemical shifts are reported relative to
17
1
1
SiMe4. 31P{ H}, 11B{ H}, and 19F NMR spectra were referenced
to external 85% H3PO4, BF3·Et2O, and CFCl3, respectively. NMR
data were acquired at 300 K unless otherwise noted. Ethylene
was purchased from BOC gases and was degassed and dried
Yield 115 mg (92%). 1H NMR (C6D5Br): 6.12 (s, 5H, Cp), 1.23 (d,
18H, 3JHP = 13 Hz, PtBu2), 1.12 (s, 3H, BMe), 1.12 (d, 27H, 3JHP
= 14 Hz, PtBu3), 0.85 (s, 3H, TiMe). 11B NMR (C6D5Br): -14.5
1
(br s). 13C{ H} NMR (C6D5Br): 148.78 (dm, 1JCF = 250 Hz, CF),
˚
over Q5 copper deoxygenation material and 3 A molecular
138.46 (dm, 1JCF = 250 Hz, CF), 137.61 (dm, 1JC-F = 245 Hz, CF),
sieves. MeOH was purchased from Aldrich Chemical Co. HCl
was purchased from EM Science; all were used as received.
B(C6F5)3, [Ph3C][B(C6F5)4] and AliBu3 (TiBAl) were generously
donated by Nova Chemicals Corp. and were used without further
purification. CpTiMe2(NPtBu3),7 [R3PH][B(C6F5)4] (R = Cy 1,
Mes43 2, tBu44 3), R2PHC6F4BF(C6F5)2 (R = Cy 4, Mes 5, tBu 7; R2
= tBuMes 6), Mes2PHC6F4BCl(C6F5)2 8,45 Cy3PC6F4BF(C6F5)2
9, R2PC6F4B(C6F5)2 (R = Cy 10, tBu 12; R2 = tBuMes 11) and
R2PHC4H8OB(C6F5)3 (R = Mes 13, tBu 14) were prepared as
previously reported.35,36,40-42
1
114.19 (s, Cp), 53.04 (s, TiMe), 41.14 (d, JCP = 42 Hz, PtBu3),
1
2
32.61 (d, JCP = 27 Hz, PtBu2), 30.46 (d, JCP = 14 Hz, PtBu2),
29.19 (s, PtBu3), 11.20 (s, BMe). 19F NMR (C6D5Br): -124.58 (br,
1F, C6F4), -131.09 (br, 1F, C6F4), -132.38 (br, 4F, o-C6F5), -132.76
(br, 2F, C6F4), -160.99 (br, 2F, p-C6F5), -166.06 (br, 4F, m-C6F5).
19F NMR (C6D5Br, 243K): -123.66 (s, 1F, C6F4), -132.20 (m, 4F,
o-C6F5), -132.60 (m, 1F, C6F4), -133.13 (m, 1F, C6F4), -133.56
(m, 1F, C6F4), -160.76 (br, 2F, p-C6F5), -164.26 (m, 4F, m-C6F5).
1
3
31P{ H} NMR (C6D5Br): 50.8 (PtBu3), 21.2 (br d, JPF = 90 Hz,
PtBu2). 31P{ H} NMR (C6D5Br, 243K): 50.1 (PtBu3), 17.6 (d,
1
3JPF = 95 Hz, PtBu2).
Generation of [CpTiMe(NPtBu3)][B(C6F5)4] 15
This species was generated using varying activators and thus
only one preparation is detailed. To an orange solution of
[Cy3PH][B(C6F5)4] (0.057 g, 0.059 mmol) in C6D5Br (0.4 mL)
was added dropwise a solution of CpTiMe2(NPtBu3) (0.021 g,
0.058 mmol) in C6D5Br (0.3 mL). The solution was stirred
for 5 min. Quantitative product formation was observed by
NMR spectroscopy. 15: 1H NMR (C6D5Br): 7.18–7.09 (m, 15 H,
Generation of [CpTiMe(THF)(NPtBu3)] [R2P(C6F4)BMe(C6F5)2]
(R = Mes 18, tBu 19)
The species 16 or 17 were dissolved in THF (5 mL) at 25 ◦C,
filtered and characterized by NMR spectroscopy. The solution was
stirred for 5 min, the volatiles removed in vacuo, and the residue
1
redissolved for NMR characterization. 18: H NMR (C6D5Br):
8556 | Dalton Trans., 2009, 8555–8561
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6.69 (s, 4H, C6H2), 6.09 (s, 5H, Cp), 3.61 (br s, 4H, THF), 2.25 (s,
12H, C6H2Me-2,6), 2.16 (s, 6H, C6H2Me-4), 1.59 (br s, 4H, THF),
1.22 (s, BMe), 1.11 (d, 3JHP = 14 Hz, tBu), 0.88 (s, TiMe). 11B NMR
CpTiMe2(NPtBu3) solution (1.0 mL) was injected followed im-
mediately with the injection of the B(C6F5)3 solution (1.5 mL) into
the reactor vessel. The mixture was stirred at 1500 5 rpm at 30 ◦C
under 2 atm of dynamic ethylene flow for 10 min. The temperature
and ethylene flow rate were recorded manually at regular intervals.
After 10 min, the polymerization was stopped by closing the
ethylene inlet valve and venting the reactor, the stirring was
stopped, and the reactor was disassembled. The contents of the
reactor were emptied into a 4 L beaker containing approximately
100 mL of 10% HCl (v/v) in MeOH. The precipitated polymer
was collected by filtration, washed with toluene and acetone, and
dried under vacuum overnight at 25 ◦C. The resulting polymer
was weighed and polymerization activity calculated.
1
(C6D5Br): -14.9 (br s). 13C{ H} NMR (C6D5Br): 149.28 (dm,
1
1JCF = 240 Hz, CF), 148.80 (dm, JCF = 240 Hz, CF), 147.22
1
2
(dm, JCF = 245 Hz, CF), 142.71 (d, JCP = 16 Hz, quat, Mes),
137.90 (s, quat, Mes), 137.32 (dm, 1JCF = 245 Hz, CF), 136.68 (dm,
1JCF = 250 Hz, CF), 130.21 (s, CH Mes), 114.17 (s, Cp), 112.86
(d, 1JCP = 70 Hz, quat, Mes), 76.11 (br s, THF), 52.19 (s, TiMe),
40.96 (d, 1JCP = 41 Hz, tBu), 28.97 (s, tBu), 22.88 (s, THF), 22.78
(d, 3JCP = 16 Hz, C6H2Me-2,6), 21.08 (s, C6H2Me-4), 10.86 (br s,
BMe). 19F NMR (C6D5Br): -131.31 (d, 3JFF = 24 Hz, 4F, o-C6F5),
-131.60 (m, 2F, C6F4), -134.86 (m, 2F, C6F4), -163.62 (m, 2F,
1
p-C6F5), -166.15 (m, 4F, m-C6F5). 31P{ H} NMR (C6D5Br): 51.1
(PtBu3), -50.0 (t, 3JPF = 37 Hz, PMes2).
Molecular modeling calculations49
Energy minimization calculations were performed by employing
the MMX and MM2 options of the Cache Software system. Initial
coordinates and geometric parameters were taken from X-ray
data.
19
1H NMR (C6D5Br): 6.09 (s, 5H, Cp), 3.64 (br s, 4H, THF), 1.63
(br s, 4H, THF), 1.23 (d, 18H, 3JHP = 12 Hz, PtBu2), 1.21 (s, 3H,
3
BMe), 1.15 (d, 27H, JHP = 14 Hz, PtBu3), 0.93 (s, 3H, TiMe).
11B NMR (C6D5Br): -15.0 (br s). 13C{ H} NMR (C6D5Br): 149.13
1
Results and discussion
(dm, 1JCF = 250 Hz, CF), 137.57 (dm, 1JCF = 245 Hz, CF), 137.61
1
(dm, JCF = 250 Hz, CF), 113.28 (s, Cp), 68.40 (s, THF), 52.25
It is well established that early metal alkyl cations which act as
polymerization catalysts can be generated by protonolysis of a
methyl group from a dialkyl-catalyst precursor.1-6,19 In probing the
viability of the phosphonium-borates [R3PH][B(C6F5)4] (R = Cy
1, Mes 2, tBu 3) and R2PHC6F4BF(C6F5)2 (R = Cy 4, Mes 5, tBu 7;
R2 = tBuMes 6) to effect protonolysis to generate such a polymer-
ization catalyst, 1:1 stoichiometric combinations of these species
and CpTiMe2(NPtBu3) were monitored by NMR spectroscopy.
In the case of 1/CpTiMe2(NPtBu3) or 2/CpTiMe2(NPtBu3), gas
evolution was apparent upon mixing, while 31P NMR data revealed
complete generation of the free phosphine as evidenced by the
signals at 11.1 and 35.5 ppm for Cy3P and Mes3P, respectively.31P
(s, TiMe), 41.19 (d, 1JCP = 44 Hz, PtBu3), 32.53 (d, 1JCP = 28 Hz,
PtBu2), 29.37 (d, 2JCP = 16 Hz, PtBu2), 29.01 (s, PtBu3), 10.83 (s,
BMe). 19F NMR (C6D5Br): -124.97 (m, 1F, C6F4), -131.89 (m, 1F,
C6F4), -132.03 (d, 4F, 3JFF = 22 Hz, o-C6F5), -132.22 (s, 1F, C6F4),
-132.43 (dd, 1F, 3JFP = 113 Hz, 3JFF = 23 Hz, C6F4), -160.99 (t,
2F, 3JFF = 23 Hz, p-C6F5), -166.06 (t, 4F, 3JFF = 24 Hz, m-C6F5).
31P{ H} NMR (C6D5Br): 50.9 (PtBu3), 19.6 (dd, JPF = 120 Hz,
1
3
3JPF = 20 Hz PtBu2).
Polymerization protocol
For comparable results, routine standards were run regularly to
ensure reproducibility. The polymerizations were performed in a
1 L Buchi reactor system. All polymerizations were performed
in duplicate to ensure reproducibility. The average activities of at
least duplicate runs are reported. Following assembly, the reactor
vessel and solvent storage unit were refilled with nitrogen via
4 refill/evacuation cycles over at least 90 min. Approximately
600 mL of toluene was transferred to the solvent storage container
from the purification column. The solvent was purged with dry
nitrogen for 20 min and then transferred to the reactor vessel
by differential pressure. The solvent was stirred at 1500 5 rpm
and the temperature was kept constant at 30 2 ◦C. The system
was then exposed to ethylene via five vent/refill cycles. The pre-
catalyst,48 cocatalysts40-42 and scrubber stock solutions were freshly
prepared as previously reported and loaded into syringes in a glove
box, then transferred to the reactor immediately before injection
to limit the possibility of catalyst decomposition.
1
and H NMR spectroscopy were consistent with the generation
of the species, [CpTiMe(NPtBu3)][B(C6F5)4] 15.7 The inability of
the liberated sterically bulky phosphines to coordinate to the Ti-
cations is noteworthy and is considered (vide infra). Although
the species 15 was previously reported to be unstable,7 it proved
to be stable in bromobenzene, permitting spectroscopic charac-
terization. Not surprisingly, attempts to isolate this species were
unsuccessful as a result of the extreme air and moisture sensitivity
of this coordinatively unsaturated cation. Similar sensitivities of
related cations have been previously noted.7,9,10,47
The analogous experiment employing 3/CpTiMe2(NPtBu3)
showed 31P NMR signals attributable to 1:1 ratio of tBu3P and
[tBu3PH]+, as well as resonance attributable to the phosphin-
imide ligand in the previously reported methyl-bridged dimer,
[{Cp(NPtBu3)TiMe}2(m-Me)][B(C6F5)4] (Fig. 3).47 These data sug-
gest that 3 is less efficient at activation, allowing CpTiMe2(NPtBu3)
to intercept the generated cation. This is attributed to the greater
basicity of the phosphine.
Sample injection sequence procedure
In
a
similar fashion, the stoichiometric reaction of
The sequence was the same for all polymerizations with the
appropriate substitution of the activator; thus only one example
is detailed. A prepared solution of triisobutylaluminium (TiBAl)
(3.0 mL) was injected into the reaction vessel through the catalyst
injection inlet and allowed to stir for 5 min. The prepared
CpTiMe2(NPtBu3) with the phosphonium-fluoroborate 5 was
monitored by NMR spectroscopy. This reaction gave rise to a
complex mixture of products. However, NMR data suggested
incomplete deprotonation of the PH moiety, as well as some
B–F for B–Me exchange. In related experiments, combination
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Table 1 Ethylene polymerization activity with varying activatorsa
Activator
Activity
Activator
Activity
B(C6F5)3
[Ph3C][B(C6F5)4]
[Me2PhNH][B(C6F5)4]
1
2
3
4
5
8800
7500
3000
6600
6000
330
6
5000
2900
600
4800
9200
17900
14500
100
7
8
10
11
12
13
14
4500
11600
Fig. 3 Activation of CpTiMe2(NPtBu3) by phosphonium borates 1–3.
a Polymerizations were performed using the catalyst/cocatalyst combina-
tion CpTiMe2(NPtBu3)/B(C6F5)3 in 600 mL toluene (5 mmol/L), 30 ◦C,
20 equiv iBu3Al, 1 equiv B(C6F5)3, 2 atm C2H4, 10 min. Activity given in g
PE/mmol/h/atm.
of the Cy3PC6F4BF(C6F5)2 9 and CpTiMe2(NPtBu3) showed
no reaction whereas addition of 9 to independently gener-
ated [CpTiMe(NPtBu3)][B(C6F5)4] showed evidence of immediate
B–F/Me exchange. This confirms that such exchange requires the
presence of the highly Lewis acidic Ti-cation. It is noteworthy that
this reactivity does not preclude the use of these phosphonium-
fluoroborate as activators. Marks and coworkers50-51 have recently
reported the use of [Ph3C][FM(C6F5)3], where M = B and Al, as
co-catalysts for olefin polymerization, despite the formation of
species containing bridging Zr–F–Zr fragments in stoichiometric
reactions of these activators with Me2C(Cp)(Flu)ZrMe2 in the
absence of olefin.
An alternative strategy to activate dialkyl-catalyst precursors
is methyl abstraction. Thus the stoichiometric reactivity of
the phosphine-boranes of the form R2PC6F4B(C6F5)2 were also
probed. In the case of the reactions of 10 (R = Mes) and 12
(R = tBu) with CpTiMe2(NPtBu3), clean formation of the salts
[CpTiMe(NPtBu3)][R2P(C6F4)BMe(C6F5)2] (R = Mes 16, tBu 17)
was observed. For compound 17, the 1H NMR showed signals at
1.22 and 0.88 ppm, corresponding to the B–Me and Ti–Me groups,
respectively. The assignment of these resonances was confirmed
by 1H–13C HSQC experiments which correlated the 1H NMR
signals of the B–Me and Ti–Me groups to 13C NMR resonances
at 10.7 and 50.4, respectively. A 1H–1H EXSY experiment showed
these methyl groups do not exchange. The 31P NMR spectrum
of 17 showed a slight broadening of the doublet at 21.2 ppm
(P–F coupling) attributed to the P of the phosphine-borate. Upon
addition of excess THF to the above reaction mixtures the species
[CpTiMe(THF)(NPtBu3)] [R2P(C6F4)BMe(C6F5)2] (R = Mes 18,
tBu 19) are immediately formed (Fig. 4). The 31P NMR peaks
sharpen and are better resolved. For example, the 31P resonance for
the anion of 19 was resolved to a doublet of doublets, characteristic
of the independently generated anionic phosphino-borate where
coupling to the o-fluorines of the C6F4 bridge is observed. These
results suggest that no significant interaction between the Ti center
in 17 and the P on the borate anion.
CpMe2Ti(NPtBu3) to effect ethylene polymerization. These poly-
◦
merizations were performed at 30 C for 10 min using a catalyst
concentration of 5 mmol/L in approximately 600 mL of toluene
and an ethylene pressure of 2 atm. TiBAL (20 equiv) was
used as a scavenger. The ratio of Ti:activator used was 1:1
unless otherwise noted. For a given activator the polymerization
were performed in at least duplicate to ensure reproducibility
within 10% of the reported activity. Standard runs employing
CpTiMe2(NPtBu3)/B(C6F5)3 were performed regularly to ensure
systematic and procedural reproducibility. It has been previously
documented that activation of this catalyst as described herein,
results in a single-site polymerization producing linear high
molecular weight polyethylene with PDIs ranging from 1.2–
2.6.7-10 To ensure valid and reproducible activities, polymerizations
were performed for 10 min. These conditions are known to
result in high molecular weight PE (> 1,300,000 g/mol).7-10
Indeed, attempts to obtain GPC data for the polymers from the
present polymerizations were unsuccessful due to solubility and
instrumental limitations. Nonetheless, the activity data provide a
basis for comparison of the cocatalyst efficacy.
Initially, the phosphonium borates 1–3 were employed as
activators in reaction with CpTiMe2(NPtBu3) (Table 1). In the case
of 1 and 2, the activities observed were slightly less than that seen
when B(C6F5)3 or [Ph3C][B(C6F5)4] were used. Nonetheless, 1 and
2 gave rise to significantly higher activity than that derived from
[Me2PhNH][B(C6F5)4], inferring that the bulky phosphonium salts
were protic enough to effect Ti–C bond cleavage and that the
liberated phosphines were bulky enough to deter coordination
to Ti.
In contrast, the catalyst generated using 3 as the co-catalyst
gave rise to much lower activity. Considering that the liberated
phosphine, tBu3P, has a cone angle of 182◦, which is between those
of Cy3P (170◦) and Mes3P (212◦),52 it seems unlikely that tBu3P
coordinates to the Ti cation. Instead, a far more likely explanation
is that 3 is less effective at protonation of a Ti–C bond as a result of
the significantly lower acidity of the PH moiety.53 This observation
is consistent with the stoichiometric reactivity described above.
Similar trends were observed employing phosphonium-
borates that incorporate a C6F4 linker between B and P;
R2PHC6F4BX(C6F5)2 (X = F; R = Cy 4, Mes 5, tBuMes 6, tBu 7,
X = Cl, R = Mes 8). The activator 5 gave rise to the highest activity
while 7 like 3, resulted in a much less active catalyst. Comparing
2 and 5, the greater activity resulting from 5 can be ascribed
to the increase in PH acidity as a result of the presence of the
Fig. 4 Activation of CpTiMe2(NPtBu3) by phosphine-boranes 10, 12.
The above series of phosphonium borates and phosphine-
boranes were also employed as coacatalysts to activate
8558 | Dalton Trans., 2009, 8555–8561
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Table 2 Ethylene polymerization activity with added phosphinea
electron-withdrawing C6F4 fragment. Indeed generally, it appears
that the activity increases with the acidity of the PH moiety,
consistent with the importance of efficient cation generation
in the determination of activity. It is also noteworthy that the
inclusion of B–F bonds in 4–7 does not have a deleterious effect
on the catalyst activity, despite the complex reactivity observed
in stoichiometric reactions. In these cases, clearly the Ti cation
reacts much faster with ethylene than B–F affording an active
polymerization catalyst. In contrast, incorporation of B–Cl in 9
results in a significant decrease in catalyst turnover. Presumably
in the latter case the Ti cation can be deactivated by capture of
chloride. This view is also consistent with the greater bond strength
of B–F versus B–Cl.
Equiv.
PR3
PEt3
PCy3
PMes3
PtBu3
0
2
10
20
50
14000
9700
7400
4900
0
14000
11000
22000
24000
7300
14000
16900
19200
17200
13400
14000
26200
48400
31700
27800
a Polymerizations were performed using the catalyst/cocatalyst combina-
tion CpTiMe2(NPtBu3)/B(C6F5)3 in 600 mL toluene (3 mmol/L), 30 ◦C,
20 equiv. iBu3Al, 1 equiv. B(C6F5)3, 2 atm C2H4, 10 min. Activity given
in g PE/mmol/h/atm.
The phosphonium alkoxy-borates Mes2PHC4H8OB(C6F5)3 13
tBu2PHC4H8OB(C6F5)3 14 were also evaluated as activators. The
species 13 was highly effective while the latter species proved to
be a very poor activator. Although the reduced acidity of the PH
moiety in 14 may also account for ineffective activation, the high
activity derived from use of 13 reflects the improved solubility of
the resulting anion in hydrocarbon solvent. Moreover, the activity
derived from 13 supports the view that the B–O bond are resistant
to abstraction by the Ti cation, at least under the conditions
employed herein. In related systems, alcohol adducts of B(C6F5)3
have been employed as Brønsted acids and shown to be capable
of cleaving metal–alkyl bonds of Cp2ZrMe2 to generate the active
olefin polymerization catalyst, [Cp2ZrMe][ROB(C6F5)].54-55
In contrast to the salts above, the phosphine-borane acti-
vators, Mes2PC6F4B(C6F5)2 10, tBuMesPC6F4B(C6F5)2 11 and
tBu2PC6F4B(C6F5)2 12 act via methyl abstraction rather than
protonation. The activities resulting from these activators correlate
with increasing basicity of the P centers, thus following the
order 10 < 11 < 12. Although seemingly counter-intuitive,
it appears that an electron rich P center in the methylborate
anions [R2PC6F4B(Me)(C6F5)2], facilitates polymerization of the
Ti cation. This suggests that the presence of electrostatic approach
of the sterically encumbered phosphine to the Ti cation crowds,
but fails to bind to the Ti center. This crowding weakens the
Ti–methyl–borate interaction, effecting an increase in the average
cation–anion separation, facilitating polymerization activity.
complexes have been previously prepared, isolated and fully
characterized.47
The analogous experiments employing 2, 10, 20 and 50
equivalents of the much more sterically demanding phosphines
PCy3, PMes3 and PtBu3 resulted surprisingly in increased activity.
Addition of 20 equivalents of PCy3 or 10 equivalents of PtBu3
resulted in significant increases in the activity to over 24.0 ¥
103 and 48.0 ¥ 103 g PE/mmol of catalyst/atm/h, respectively.
In subsequent polymerizations using increased equivalents of
phosphine, the activity began to decline.
The above observations are initially surprising and appear
counter-intuitive as the addition of a donor is expected to
sequester Ti cations and preclude polymerization. However, it
is noteworthy that previous efforts to isolate complexes of the
form [CpTi(NPtBu3)Me(PR3)][MeB(C6F5)3] with sterically bulky
phosphines (R = Mes, tBu) were unsuccessful and indeed no
evidence of Ti–P binding was observed.47 Thus, it appears that
despite the absence of a direct bonding interaction, the presence
of phosphine alters the environment of the active site.
It is reasonable to propose that an equilibrium involving
the electrostatic attraction of the Ti-cation and the sterically
demanding Lewis base results in some degree of association
in solution. While the nature of the molecular orbitals of the
cation is well understood,48 we have probed the steric interactions
of a sterically demanding phosphine approaching the cation
employing molecular mechanics calculations.49 Models based on
crystallographic data for the two fragments [CpTi(NPtBu3)Me]+
and PtBu3 were employed to calculate the total energy as a
function of approach of PtBu3 on a vector towards the vacant
coordination site on Ti. (Fig. 5). These computations reveal that
Additives
It is demonstrated above that phosphonium-borate and
phosphine-borane activators are effective despite the libera-
tion of phosphine donors. This was ascribed to the steric
bulk of these systems. This prompted the examination of im-
pact of added phosphine on catalytic activity. Polymerizations
were performed employing the precatalyst–activator combination
CpTiMe2(NPtBu3)/B(C6F5)3. Immediately prior to the addition
of cocatalyst 2, 10, 20 and 50 equivalents of PEt3 were added to
reaction mixtures. While the polymerization activity diminished
with increasing concentration of phosphine, some activity did per-
sist in the presence of up to 20 equivalents of phosphine (Table 2).
Addition of 50 equivalents completely quenched the catalyst activ-
ity, and no polymer was isolable. These observations suggest that
although ethylene appears to compete to some degree with phos-
phine donors, ultimately with increased phosphine concentration
the catalytically active species is sequestered as the base-stabilized
Ti cationic complex [CpTi(NPtBu3)Me(PR3)][MeB(C6F5)3]. Such
˚
the minimum energy corresponds to a Ti/P separation of 4.2 A
and support the view that steric demands preclude Ti–P bonding
for sterically encumbered PtBu3. It is noteworthy that previous
computational studies have shown the most significant energy
barrier to insertion of ethylene into the growing polymer chain is
cation–anion separation.48 Thus it is proposed that the affiliation
of the sterically demanding phosphine and the Ti-cation may
crowd the anion, resulting in greater anion–cation separation, and
thus increased activity. Although PMes3 is even more sterically
shielded, it is much less basic, and this appears to diminish the
positive impact on activity.
Interestingly and in marked contrast, the corresponding poly-
merization employing [Ph3C][B(C6F5)4] as the activator and
20 equivalents of tBu3P resulted in a slight decrease in activity
to 8700 g PE/mmol of catalyst/atm/h compared to the case in
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Fig. 5 Plot of total energy vs Ti ◊ ◊ ◊ P distance and space-filling diagram
of the minimum energy conformation of [CpTiMe(NPtBu3)]+ and PtBu3
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˚
which no phosphine was added. It must be noted that trityl cation
has been shown to react with PtBu3 to give the product of para-
attack [tBu3P(C6H4)CHPh2][B(C6F5)4]56 whereas PtBu3 does not
react with B(C6F5)3. The reduced activity with larger excesses of
phosphine, presumably reflects the competitive formation of this
product.
It is also instructive to note the analogy between the current
polymerization experiments and main group FLPs, where the
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Conclusions
Herein it has been demonstrated that phosphonium-borates and
phosphine-boranes that incorporate sterically encumbered P cen-
ters can be utilized to effect the activation of an early metal ethylene
polymerization pre-catalyst. The former activators operated via
protonation of a metal–alkyl bond while the latter abstract methyl
to form a methyl-borate anion. In general these activators give
highly active catalysts despite the liberation or initial provision of
a free phosphine. Furthermore, rather than having a detrimental
effect on activity, excess equivalents of sterically encumbered
phosphines enhance polymerization activity, although this effect
is activator dependent. This observation points to the impact
of steric congestion on electrostatic donor–acceptor attraction.
These latter observations suggest that metal-based FLPs are
uniquely reactive. The reactivity of such combinations continues
to be the subject of study in our laboratories and will be reported
in due course.
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37 J. S. J. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem., Int.
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