Catalytic copolymerization of CO and ethylene with a charge neutral palladium(II) zwitterion

Article abstract of DOI:10.1021/ja017011s

The synthesis of a zwitterionic Pd(II) complex supported by an anionic bis(phosphino)borate ligand, Ph₂B(CH₂PPh₂)₂ (abbreviated as [Ph₂BP₂]), is reported. The new complex, [Ph₂BP₂]PdMe(THF), is active for CO and ethylene copolymerization. The copolymerization activity and polyketone molecular weight for the neutral, zwitterionic system are compared with those for the cationic systems [R₂E(CH₂PPh₂)₂PdMe(THF)][B(C₆F₅)₄] where ER₂ = SiPh₂ and CH₂. Surprisingly, the more electron rich zwitterionic system is a catalyst of activity comparable to that of the more conventional cationic systems. Copyright

Full text of DOI:10.1021/ja017011s

Published on Web 04/23/2002
Catalytic Copolymerization of CO and Ethylene with a Charge Neutral
Palladium(II) Zwitterion
Connie C. Lu and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received September 6, 2001
Late transition-metal polymerization catalysts currently enjoy
widespread scrutiny in academia and industry,1 partly because their
low oxophilicity relative to early metal systems is desirable for
polar comonomer incorporation.2 The cationic polymerization
systems are frequently generated by methide or halide abstraction
with an activator such as a Lewis acidic borane.3 We are pursuing
an alternate approach whereby charge-neutral zwitterions incorpo-
rating a borate counteranion might themselves serve as polymer-
ization catalysts.4,5 Aside from eliminating the need for a cocatalyst,
Figure 1. Comparing a neutral palladium zwitterion with its prototypical
cationic relatives.
one might anticipate significant differences between zwitterionic
Scheme 1
and traditionally cationic systems due to (i) differences in their
relative electrophilicities, (ii) differences in donor ligand lability,
and (iii) reduced or completely eliminated ion-pairing effects in
the zwitterionic systems. To begin to understand the utility of
zwitterionic systems in polymerization catalysis, well-defined
comparative model studies are required.6
In this communication, we describe a zwitterionic palladium(II)
alkyl complex supported by the anionic ligand [Ph2B(CH2PPh2)2]-
(abbreviated as [Ph2BP2]).7 This system serves as a charge neutral
relative to the highly active class of cationic, phosphine-supported
Table 1. Copolymerization Results for Catalysts 2, 7, and 8a
palladium CO/ethylene copolymerization catalysts such as [(dppp)-
Pd(Me)(solv)]+ (dppp ) bis-diphenyl(phosphino)propane)).8,9 We
specifically address whether a related zwitterionic palladium system
is active for CO and ethylene copolymerization by comparing its
reactivity to that of cationic systems supported by structurally
similar bidentate phosphines (Figure 1). Notably, neutral group 10
systems supported by anionic LX-type ligands are generally
regarded as poor catalysts for CO/ethylene copolymerization.8b,10
Preparation of the target palladium alkyl complex proceeds first
by reaction of an acetonitrile solution of the bis(phosphino)borate
ligand, [ASN][Ph2BP2] (ASN ) 5-azonia-spiro[4.4]nonane),7 with
a benzene solution of (tmeda)PdMe2 (tmeda ) N,N,N′,N′-tetra-
methylethylenediamine). The resulting product, [ASN][(Ph2BP2)-
PdMe2] (1), is then protonated by the ammonium salt [HNiPr2Et]-
[BPh4] to cleanly generate the target zwitterion, [Ph2BP2]PdMe-
(THF) (2), as a light-peach solid (Scheme 1).
With the solvento adduct 2 in hand, we examined the scope of
its reactivity under CO and ethylene gas. A light yellow solution
results when a dilute THF solution (ca. 10 µM) of 2 is exposed to
a CO atmosphere. The unstable product, [Ph2BP2]Pd(C(O)Me)(CO)
(3), is related to a cationic derivative, [(dppp)Pd(C(O)Me)(CO)]-
[B(3,5-(CF3)2-C6H3)4], recently reported by Brookhart and co-
workers.11 On the basis of their relative carbonyl stretching
frequencies, it is clear that the palladium center of the zwitterion
is more electron-rich than in the cationic derivative.12 At 30 psi of
CO and 30 psi of ethylene, strictly alternating polyketone material
rapidly precipitates from solution (established by 13C NMR and
MALDI-TOF). The activity of catalyst 2 was examined at higher
cat.
activity (TON)b
yield (g polymer)c
M (103)d
M (103)d
M /M
w
n
w
n
2
7
8
39 ( 1
35 ( 2
45 ( 1
0.36 ( 0.01
0.32 ( 0.02
0.42 ( 0.01
138 ( 1
130 ( 1
190 ( 1
112 ( 3
99 ( 2
1.3
1.3
1.5
130 ( 5
a Conditions: 9.3 × 10-6 mol Pd catalyst in 10 mL THF; 100 psi CO;
100 psi ethylene; 23 °C; 1 h. b TON values are expressed as kg of polymer
per mol catalyst per h. c Average mass of polymer obtained from eight
independent runs. d Determined by GPC using polystyrene standards for
calibration (1,3-cresol; 1 mL/min; 120 °C; duplicate runs).
pressures (200 psi, vide infra), and calculated TON values (kg
polymer mol-1 catalyst h-1) establish that it is a very active catalyst
for polyketone production at room temperature (Table 1).
Interestingly, the reactivity of catalyst 2 with CO gas, in the
absence of ethylene, is concentration-dependent. At high concentra-
tions of 2 (ca. 20 mM), exposure to an atmosphere of CO produces
an orange solution (31P NMR, singlet at 22 ppm),13 which turns
red (two doublets at 11 and 30 ppm) upon replacing the CO
atmosphere with dinitrogen. The final red product is accessible by
other routes (vide infra), and its crystallographic characterization
establishes it to be the dimeric Pd(I) complex, {[Ph2BP2]Pd}2
(4),14,15 shown in Scheme 2. An alternative method for cleanly
generating dimeric 4 results from exposure of 2 to ethylene gas in
the absence of CO. In this case, the color change to red is rapid.
Monitoring this transformation under ethylene at low temperature
(from -78 to -10 °C) establishes one observable intermediate,
assigned as an insertion product, [Ph2BP2]Pd(C2H5)(H2CdCH2) (5).
A related cationic species, [(dppp)Pd(C2H5)(H2CdCH2)][B(3,5-
(CF3)2-C6H3)4], has also been reported by Brookhart and co-
workers.11 Storage of intermediate 5 at -10 °C under excess
* To whom correspondence should be addressed. E-mail: jpeters@caltech.edu.
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J. AM. CHEM. SOC. 2002, 124, 5272-5273
10.1021/ja017011s CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
ionic [Ph2BP2] system is essentially as active as the conventional
dppp and (Ph2SiP2) systems that are based upon cationic palladium.
Ongoing work includes comparative kinetic studies of the elemen-
tary insertion processes relevant to polymerization for both cationic
and zwitterionic palladium systems.19
Acknowledgment. We thank the NSF (CHE-0132216) and the
Dreyfus Foundation for funding. Dr. Michael Day and Lawrence
Henling are acknowledged for crystallographic assistance, Mona
Shahgholi for MALDI-TOF, and Rapra Technology Limited for
GPC analyses of polyketone.
Supporting Information Available: Experimental protocols; crys-
tallographic data for complex 4 (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) For two recent reviews see: (a) Ittel, S. D.; Johnson, L. K.; Brookhart,
M. Chem ReV. 2000, 100, 1169-1203. (b) Mecking, S. Coord Chem.
ReV. 2000, 203, 325-351.
(2) Younkin, T. R.; Conner, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs,
R. H.; Bansleben, D. A. Science 2000, 287, 460-462.
(3) (a) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391-1434. (b)
Lee, B. Y.; Bazan, G. C.; Vela, J.; Komon, Z. J. A.; Bu, X. H. J. Am.
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ethylene effects the catalytic production of butenes. On warming,
the only new species detectable by 31P NMR is the red dimer 4.
The expected product of â-hydride elimination, [Ph2BP2]Pd(H)(L)
(6) (L ) THF or ethylene), was not observed. Furthermore, an
independent attempt to generate the hydride 6 by addition of H2 to
2, while generating dimeric 4 quantitatively, offered no evidence
for a detectable hydride intermediate. Considered collectively, the
data suggests that the conversion of 2 to 4 under ethylene may
occur as follows (Scheme 2): initial ethylene insertion and rapid
â-hydride elimination, followed by a second ethylene insertion to
give the observable intermediate 5. Intermediate 5 undergoes further
chemistry with ethylene to generate butenes catalytically. At higher
temperatures, a bimolecular path competes in which the unobserv-
able hydride 6, generated by â-hydride elimination, reacts with a
palladium alkyl, such as 5, to produce alkane and the dimeric
palladium(I) species 4. Notably, gas analysis of a reaction mixture
from the conversion of 2 to 4 under ethylene showed no evidence
for hydrogen production. Bimolecular loss of H2 from a hydride
intermediate such as 6 does not occur.
Having established some of the comparative reaction chemistry
between charge neutral 2 and its cationic dppp counterpart, [(dppp)-
Pd(Me)(solv)][B(C6F5)4], we sought to compare their activities for
CO/ethylene copolymerization. The THF adduct complex, [(dppp)-
Pd(Me)(THF)][B(C6F5)4] (7), was thus prepared, and its copolym-
erization activity was measured (Table 1). To our surprise, under
analogous conditions, zwitterionic 2 proved to be a slightly better
copolymerization catalyst (Table 1). To examine whether the
slightly increased activity and higher-weight polymers of the
[Ph2BP2] system were perhaps due to the difference in relative
charge between the palladium centers in 2 and 7, we sought a second
comparison. A cationic complex that is structurally similar to neutral
2, [(Ph2SiP2)Pd(Me)(THF)][B(C6F5)4] (8), was prepared using the
neutralphosphinechelatePh2Si(CH2PPh2)2(abbreviatedas(Ph2SiP2)).16
Under analogous conditions, cationic 8 proved to be a slightly better
catalyst than zwitterionic 2 (Table 1), indicating that the phenyl
substituents incorporated within the ligand backbones of 2 and 8
may also contribute to slight differences in reactivity by comparison
to the dppp system 7. Likewise, others have observed significant
increases in activity for CO copolymerization with ethylene and
propene by incorporating methyl groups onto the central carbon of
bis(phosphino)propane ligands.17,18
(4) For a review on zwitterions in organometallic chemistry, see: Chauvin,
R. Eur. J. Inorg. Chem. 2000, 577-591.
(5) For leading papers dealing with the reactivity of inorganic zwitterions,
see: (a) Amer, I.; Alper, H. J. Am. Chem. Soc. 1990, 112, 3674-3676.
(b) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T. J. Am. Chem.
Soc. 1992, 114, 8863-8869. (c) Dai, C.; Marder, T. B.; Robins, E. G.;
Yufit, D. S.; Howard, J. A. K.; Scott, A. J.; Clegg, W. Chem. Commun.
1998, 1983-1984. (d) Winter, R. F.; Hornung, F. M. Inorg. Chem. 1997,
36, 6197-6204.
(6) (a) Piers, W. E. Chem. Eur. J. 1998, 4, 13-18. (b) Bochmann, M. Top.
Catal. 1999, 7, 9-22. (c) Piers, W. E.; Sun, Y.; Lee, L. W. M. Top. Catal.
1999, 7, 133-143.
(7) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100-5101.
(8) (a) Sen, A. Acc. Chem. Res. 1993, 26, 303-310. (b) Drent, E.; Budzelaar,
P. H. M. Chem. ReV. 1996, 96, 663-681.
(9) For leading references on this topic, see: (a) Mul, W. P.; Drent, E.; Jansens,
P. J.; Kramer, A. H.; Sonnemans, M. H. W. J. Am. Chem. Soc. 2001,
123, 5350-5351. (b) Shultz, C. S.; DeSimone, J. M.; Brookhart, M.
Organometallics 2001, 20, 16-18. (c) Murtuza, S.; Harkins, S. B.; Sen,
A. Macromolecules 1999, 32, 8697-8702. (d) Margl, P.; Ziegler, T. J.
Am. Chem. Soc. 1996, 118, 7337-7344. (e) Svensson, M.; Matsubara,
T.; Morokuma, K. Organometallics 1996, 15, 5568-5576.
(10) Unlike classical LX-type ligands that are regarded as three-electron donors,
the [Ph2BP2] ligand is regarded as a four-electron donor ligand in the
zwitterionic depiction.
(11) (a) Shultz, C. S.; Ledford, J.; DeSimone, J. M.; Brookhart, M. J. Am.
Chem. Soc. 2000, 122, 6351-6356. (b) Tempel, D. J.; Johnson, L. K.;
Huff, L. M.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2000, 122,
6686-6700.
(12) Complex 3 shows two very intense bands at 1694 cm-1 {ν(C(O)Me}
and 2108 cm-1 {ν(CO)} in CH2Cl2. The stretches reported for [(dppp)-
Pd(C(O)Me)(CO)][B(3,5-(CF3)2-C6H3)4] in CH2Cl2 are 1715 cm-1
{ν(C(O)Me} and 2130 cm-1 {ν(CO)} (ref 11a).
(13) We suspect the orange species initially produced to be the dimeric
palladium(I) species {[Ph2BP2]Pd(µ-CO)}2.
(14) X-ray data for (4‚3CH2Cl2), MW ) 1594.38, red plate, collection
temp ) 98 K, monoclinic, space group ) P21/c, a ) 22.389(3) Å, b )
20.690(2) Å, c ) 16.3816(18) Å, â ) 104.361(5)°, V ) 7265.2(14) Å3,
Z ) 4, R1 ) 0.0606 [I > 2σ(I)], GOF ) 2.522 (see Supporting Information).
(15) A related, dicationic species, [{(dppp)Pd}2][OTf]2, has been previously
obtained by hydrogenation of (dppp)Pd(OTf)2. See Budzelaar, P. H. M.;
van Leeuwen, W. N. M.; Roobeek, C. F.; Orpen, G. A. Organometallics
1992, 11, 23-25.
(16) Ph2Si(CH2PPh2)2 is a neutral ligand designed as a close structural model
for the anionic borato ligand [Ph2BP2] on square planar complexes;
Thomas, J. C.; Peters, J. C. 2001. Manuscript in preparation.
(17) (a) Keijsper, J. J.; van der Made, A. W. Eur. Pat. Appl. 0454270, 1991.
(b) van Doorn, J. A.; Meijboom, N.; Snel, J. J. M.; Wife, R. L. Eur. Pat.
Appl. 0300583, 1988. (c) Baardman, F.; Blecker, E. P. P.; van Broekhoven,
M. B. H.; Crijnen-van Beers, M. B.; Drent, E.; Dulles, E. H. F.; Jager,
W. W.; Jubb, J.; Van der Made, A. W.; Scheerman, P.; de With, J.
Eur. Pat. Appl. 0743336, 1996.
(18) According to ref 17c, exchanging the central CMe2 unit for SiMe2 in the
bis(phosphino)propane ligand backbone does not significantly alter the
copolymerization rate or the polymer molecular weight.
(19) For example, low-temperature measurement of the ethylene insertion rate
for neutral [Ph2BP2]Pd(CH3)(CH2dCH2) (kobs ≈ 1.2 × 10-4; -45.8 °C
in CH2Cl2) is surprisingly similar to that reported for [(dppp)Pd(C(O)Me)-
(CO)][B(3,5-(CF3)2-C6H3)4] (ref 11a) under analogous conditions
(kobs ≈ 4.9 × 10-4; -45.6 °C).
In summary, the zwitterionic Pd(II) complex 2 is a very active
catalyst for the copolymerization of CO and ethylene at ambient
temperature. Most significant is that a more electron-rich, zwitter-
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