Base-free phosphine-sulfonate nickel benzyl complexes

Article abstract of DOI:10.1021/om800741w

Base-free [2-PR₂-4-Me-benzenesulfonate)]Ni(η³- benzyl) complexes (R = 2-OMe-Ph (4a), Cy (4b)) were prepared and tested for ethylene polymerization. 4a produces low-molecular-weight polyethylene (M _n = 1300) that contains 10 Me branches/10³ C and terminal and internal olefin units. The polymer yield and structure are unaffected by the ethylene pressure (60-300 psi). 4b is less active than 4a and produces a polymer with higher M_n, fewer branches, and a higher fraction of terminal olefin units.

Full text of DOI:10.1021/om800741w

Copyright 2008
Volume 27, Number 19, October 13, 2008
American Chemical Society
Communications
Base-Free Phosphine-Sulfonate Nickel Benzyl Complexes
Xiaoyuan Zhou, Se´bastien Bontemps, and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
ReceiVed July 31, 2008
norborenes,6 N-vinylpyrrolidinone and N-isopropylacrylamide,7
and copolymerize vinyl acetate with carbon monoxide.8 Analo-
gous [PO]NiR species are of interest because nickel catalysts
are often more reactive and are less expensive than palladium
catalysts and thus are more attractive for practical applications.9,10
In-situ-generated [PO]NiR species were reported to oligo-
merize ethylene to C4-C20 R-olefins11 and to polymerize
ethylene.2a Rieger showed that discrete (2-PAr2-benzene-
sulfonate)Ni(Ph)(PPh3) complexes (Ar ) Ph, 2-Me-Ph, 2-OMe-
Summary: Base-free [2-PR2-4-Me-benzenesulfonate)]Ni(η3-ben-
zyl) complexes (R ) 2-OMe-Ph (4a), Cy (4b)) were prepared
and tested for ethylene polymerization. 4a produces low-
molecular-weight polyethylene (Mn ) 1300) that contains 10
Me branches/103 C and terminal and internal olefin units. The
polymer yield and structure are unaffected by the ethylene
pressure (60-300 psi). 4b is less actiVe than 4a and produces
a polymer with higher Mn, fewer branches, and a higher fraction
of terminal olefin units.
Ph) polymerize ethylene to low-molecular-weight (Mw
)
1000-4000), moderately branched (15-25 branches/103 C)
polymers in the presence of phosphine scavengers (B(C6F5)3
or Ni(COD)2, where COD ) 1,5-cyclooctadiene).12 Low activi-
ties were observed in the absence of a scavenger. [PO]NiR
catalysts are inhibited by methyl methacrylate and poisoned by
Palladium alkyl complexes that contain o-phosphinoarene-
sulfonate ligands ([PO]-) catalyze the copolymerization of
ethylene and polar monomers to linear functionalized polyeth-
ylenes. In-situ-generated and discrete [PO]PdR species copo-
lymerize ethylene with carbon monoxide,1 alkyl acrylates,2
acrylonitrile,3 vinyl ethers,4 vinyl fluoride,5 functionalized
(6) (a) Liu, S.; Borkar, S.; Newsham, D.; Yennawar, H.; Sen, A.
Organometallics 2007, 26, 210. (b) Skupov, K. M.; Marella, P. R.; Hobbs,
J. L.; McIntosh, L. H.; Goodall, B. L.; Claverie, J. P. Macromolecules 2006,
39, 4279.
(7) Skupov, K. M.; Piche, L.; Claverie, J. P. Macromolecules 2008, 41,
2309.
(8) Kochi, T.; Nakamura, A.; Ida, H.; Nozaki, K. J. Am. Chem. Soc.
2007, 129, 7770.
(9) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000,
100, 1169. (b) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 429. (c) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV.
2003, 103, 283.
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Soc. 1995, 117, 6414. (b) Liu, W.; Malinoski, J. M.; Brookhart, M.
Organometallics 2002, 21, 2836. (c) Malinoski, J. M.; Brookhart, M.
Organometallics 2003, 22, 5324. (d) Daugulis, O.; Brookhart, M. Orga-
nometallics 2002, 21, 5926. (e) Kim, J. S.; Pawlow, J. H.; Wojcinski, L. M.,
II; Murtuza, S.; Kacker, S.; Sen, A. J. Am. Chem. Soc. 1998, 120, 1932.
(11) Murray, R. E. U.S. Patent 4,689,437, 1987.
* E-mail: rfjordan@uchicago.edu.
(1) (a) Drent, E.; Dijk, R.; Ginkel, R.; Oort, B.; Pugh, R. I. Chem.
Commun. 2002, 964. (b) Hearley, A. K.; Nowack, R. J.; Rieger, B.
Organometallics 2005, 24, 2755. (c) Newsham, D. K.; Borkar, S.; Sen, A.;
Conner, D. M.; Goodall, B. L. Organometallics 2007, 26, 3636. (d) Bettucci,
L.; Bianchini, C.; Claver, C.; Suarez, E. J. G.; Ruiz, A.; Meli, A.;
Oberhauser, W. Dalton Trans. 2007, 5590.
(2) (a) Drent, E.; Dijk, R.; Ginkel, R.; Oort, B.; Pugh, R. I. Chem.
Commun. 2002, 744. (b) Drent, E.; Pello, D. H. L. European Patent 0632084,
1995. (c) Kochi, T.; Yoshimura, K.; Nozaki, K. Dalton Trans. 2006, 25.
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10.1021/om800741w CCC: $40.75
2008 American Chemical Society
Publication on Web 09/11/2008

4822 Organometallics, Vol. 27, No. 19, 2008
Communications
Scheme 1
Scheme 2
methyl acrylate.2a,12 Similar SHOP-type nickel catalysts that
contain phosphine-enolate, phosphine-phenolate, or related
ligands have been studied extensively.13,14
We are interested in base-free [PO]NiR complexes to simplify
mechanistic studies and avoid the use of scavengers, which, in
addition to sequestering L ligands of [PO]M(R)(L) species,
might also react with the [PO]- ligand.15 Here we report the
syntheses of base-free [PO]Ni(η3-CH2Ph) complexes and their
behavior in ethylene polymerization.
We first prepared [PO-OMe]Ni(Ph)(PPh3) (3; [PO-OMe]- )
2-P(2-OMe-Ph)2-4-Me-benzenesulfonate) for use as a bench-
mark PPh3-stabilized catalyst. Following Rieger’s synthesis of
the parent benzenesulfonate analogue,12 the zwitterion [PO-
OMe]H (1)16 was deprotonated with NaH to afford Na[PO-
OMe] (2), which was reacted with trans-Ni(PPh3)2(Ph)Cl17 to
afford 3 in 87% yield (Scheme 1). The 31P{1H} NMR spectrum
of 3 contains doublets for the PPh3 (δ 17.1) ligand and the P(2-
Figure 1. Molecular structure of 4a. Hydrogen atoms are omitted.
Selected bond distances (Å) and angles (deg): Ni(1)-P(1) 2.163(1),
Ni(1)-O(1) 1.938(3), Ni(1)-C(22) 1.949(4), Ni(1)-C(23) 2.066(4),
Ni(1)-C(24) 2.191(4), S(1)-O(1) 1.499(3), S(1)-O(2) 1.443(3),
S(1)-O(3) 1.446(3); O(1)-Ni(1)-P(1) 99.34(8), P(1)-Ni(1)-C(22)
96.8(1), O(1)-Ni(1)-C(24) 91.7(1), C(22)-Ni(1)-C(24) 71.8(2).
2
OMe-Ph)2 unit (δ -4.1) with a large Jpp value (283 Hz)
characteristic of a trans arrangement of the phosphines.
Bazan showed that the reaction of Na[2-PPh2-benzoate],
benzyl chloride, and Ni(COD)2 produces [κ2-P,O-2-PPh2-
C6H4CO2]Ni(CH2Ph) in high yield.18 We used this method to
(13) Review: Kuhn, P.; Semeril, D.; Matt, D.; Chetcuti, M. J.; Lutz, P.
Dalton Trans. 2007, 515.
synthesize [PO]Ni(η3-CH2Ph) complexes (Scheme 2). The
reaction of 2 with benzyl chloride and Ni(COD)2 gives [PO-
OMe]Ni(η3-CH2Ph) (4a) in 29% yield as a deep-red powder.
The analogous reaction of in-situ-generated [PO-Cy]Li ([PO-
Cy]- ) 2-PCy2-4-Me-benzenesulfonate) gives the deep-red
product [PO-Cy]Ni(η3-CH2Ph) (4b) in 11% yield after multiple
recrystallizations.
The molecular structures of 4a and 4b were determined by
X-ray diffraction (Figures 1 and 2).19 Complex 4a features an
approximately square-planar Ni center. The [PO]Ni chelate ring
adopts a boat conformation, with an angle of 129.7° between
the S(1)-O(1)-Ni(1)-P(1) and S(1)-C(1)-C(7)-P(1) planes.
(14) Leading references: (a) Peuckert, M.; Keim, W. Organometallics
1983, 2, 594. (b) Starzewski, K. A. O.; Witte, J. Angew. Chem., Int. Ed.
1987, 26, 63. (c) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123.
(d) Kuhn, P.; Semeril, D.; Jeunesse, C.; Matt, D.; Neuburger, M.; Mota, A.
Chem.sEur. J. 2006, 12, 5210. (e) Soula, R.; Broyer, J. P.; Llauro, M. F.;
Tomov, A.; Spitz, R.; Claverie, J.; Drujon, X.; Malinge, J.; Saudemont, T.
Macromolecules 2001, 34, 2438. (f) Soula, R.; Saillard, B.; Spitz, R.;
Claverie, J.; Llaurro, M. F.; Monnet, C. Macromolecules 2002, 35, 1513.
(g) Gibson, V. C.; Tomov, A.; White, A. J. P.; Williams, D. J. Chem.
Commun. 2001, 719. (h) Heinicke, J.; Koehler, M.; Peulecke, N.; He, M.;
Kindermann, M. K.; Keim, W.; Fink, G. Chem.sEur. J. 2003, 9, 6093. (i)
Monteiro, A. L.; de Souza, M. O.; de Souza, R. F. Polym. Bull. 1996, 36,
331. (j) Pietsch, J.; Braunstein, P.; Chauvin, Y. New J. Chem. 1998, 22,
467. (k) Heinicke, J.; He, M.; Dal, A.; Klein, H. F.; Hetche, O.; Keim, W.;
Flo¨rke, U.; Haupt, H. Eur. J. Inorg. Chem. 2000, 431. (l) Heinicke, J.;
Peulecke, N.; Kindermann, M. K.; Jones, P. G. Z. Anorg. Allg. Chem. 2005,
631, 67.
(15) (a) Chen, Y.; Boardman, B. M.; Wu, G.; Bazan, G. C. J. Organomet.
Chem. 2007, 692, 4745. (b) Chen, Y.; Wu, G.; Bazan, G. C. Angew. Chem.,
Int. Ed. 2005, 44, 1108. (c) Kim, Y. H.; Kim, T. H.; Lee, B. Y.;
Woodmansee, D.; Bu, X.; Bazan, G. C. Organometallics 2002, 21, 3082.
(d) Lee, B. Y.; Bu, X.; Bazan, G. C. Organometallics 2001, 20, 5425.
(16) (a) McIntosh, L. H., III; Allen, N. T.; Kirk, T. C.; Goodall, B. L.
Can. Pat. Appl. 2556356, 2007. (b) Vela, J.; Lief, G. R.; Shen, Z.; Jordan,
R. F. Organometallics 2007, 26, 6624.
j
(19) (a) Crystal data for 4a: C28H27NiO5PS, M ) 565.24, triclinic, P1,
a ) 9.566(2) Å, b ) 9.877(2) Å, c ) 15.129(4) Å, R ) 104.407(4)°, ꢀ )
95.245(4)°, γ ) 112.580(4)°, V ) 1250.2(5) Å3, Z ) 2, T ) 100 K, Mo
KR radiation (0.710 73 Å), absorption coefficient 0.962 mm-1, 12 010
reflections collected, 4416 independent reflections, Rint ) 0.0313; R indices
[I > 2σ(I)] R1 ) 0.0488, wR2 ) 0.1136; R indices (all data) R1 ) 0.0687,
wR2 ) 0.1204. (b) 4b crystallizes with two independent molecules in the
unit cell, which have similar structures; one molecule is shown in Figure
2. Crystal data for 4b: 2C26H35NiO3PS + C7H8, M ) 1126.69, triclinic,
j
P1, a ) 10.524(3) Å, b ) 14.941(4) Å, c ) 17.791(5) Å, R ) 87.493(5)°,
(17) Zeller, A.; Herdtweck, E.; Strassner, T. Eur. J. Inorg. Chem. 2003,
1802.
(18) (a) Komon, Z. J. A.; Bu, X.; Bazan, G. C. J. Am. Chem. Soc. 2000,
122, 12379. (b) Rojas, R. S.; Wasilke, J.; Wu, G.; Ziller, J. W.; Bazan,
G. C. Organometallics 2005, 24, 5644.
ꢀ ) 85.024(5)°, γ ) 81.311(5)°, V ) 2753(1) Å3, Z ) 2, T ) 100 K, Mo
KR radiation (0.710 73 Å), absorption coefficient 0.868 mm-1, 26 770
reflections collected, 9780 independent reflections, Rint ) 0.0370; R indices
[I > 2σ(I)] R1 ) 0.0514, wR2 ) 0.0973; R indices (all data) R1 ) 0.0877,
wR2 ) 0.1075.

Communications
Organometallics, Vol. 27, No. 19, 2008 4823
Ethylene polymerization results for 3, 4a, and 4b at 25 °C
and 60-300 psi of ethylene pressure are summarized in Table
1. Comparison of entry 1 versus 2 and entry 4 versus 5 shows
that 4a is ca. 4 times more active and produces polymers with
slightly higher Mn compared to 3. Both 3 and 4a produce
polyethylenes with ca. 10 methyl branches per 1000 carbon
atoms, which corresponds to 0.5-1 Me branch/chain. Longer
branches were not observed by 13C NMR.25 The polymers
produced by 3 and 4a contain terminal and internal olefin units,
1
with the terminal/internal ratio being higher for 3 than 4a. H
and 13C NMR analysis of polymers produced by 3 shows that
the internal olefins are mainly 2-olefins (85%), with small
amounts of 2+-olefins (15%), and that the E/Z ratio is ca. 1/1.16b
For the polymers produced by 4a, 67% of the internal olefins
are 2-olefins (E/Z ) 1/1). Increasing the ethylene pressure from
60 to 300 psi results in a higher yield and Mn for 3 but has
little influence on the performance of 4a (entries 7 and 8). The
polymer microstructures are not influenced by the pressure in
either case. The ethylene polymerization behavior of 4a is very
similar to that of (2-PAr2-benzenesulfonate)Ni(Ph)(PPh3) com-
plexes in the presence of phosphine scavengers.12
Comparison of entry 2 versus 3 and entry 5 versus 6 shows
that 4b is ca. 10 times less active but produces polymer with
higher Mn compared to 4a. The polymer produced by 4b is
highly linear (1 Me branch/1000 C) with a high level (91%) of
terminal unsaturation. The small fraction of internal olefins
contains only 2-olefins.
NMR monitoring of the reaction of 4a with ethylene (30
equiv) at 25 °C shows that ethylene is immediately and rapidly
polymerized but that a minimal amount of 4a is consumed,
showing that the initial insertion is slower than subsequent
insertions. For all three catalysts, polymer yields increase by
only a factor of 2-3 when the polymerization time is extended
from 2 to 18 h (entries 1-3 vs 4-6), indicating that significant
catalyst deactivation occurs under these conditions.
These results are consistent with the mechanism in Scheme
3, which is analogous to that established for related (phosphi-
nosulfonamide)nickel catalysts.26 The lack of a pressure de-
pendence of the polymer yield, Mn, and microstructure for 4a
suggests that the catalyst resting state is the alkyl olefin adduct
II and that chain transfer occurs by ꢀ-H transfer to metal
followed by associative olefin exchange of III or ꢀ-H transfer
to monomer and olefin exchange of IV. 2,1-Insertion of III
would generate secondary alkyl species V, which can insert
ethylene to form Me branches, undergo chain transfer to form
internal olefins, or undergo further chain walking, leading to
2+-olefins.
Figure 2. Molecular structure of 4b. Hydrogen atoms are omitted.
Selected bond distances (Å) and angles (deg): Ni(1)-P(1) 2.149(1),
Ni(1)-O(1) 1.922(3), Ni(1)-C(1) 1.919(4), Ni(1)-C(2) 2.023(4),
Ni(1)-C(3) 2.260(4), S(1)-O(1) 1.477(3), S(1)-O(2) 1.442(3),
S(1)-O(3) 1.442(3); O(1)-Ni(1)-P(1) 98.10 (8), P(1)-Ni(1)-C(1)
98.9 (1), O(1)-Ni(1)-C(3) 96.1(1), C(1)-Ni(1)-C(3) 69.8(2).
The methoxy group of the axial 2-OMe-Ph ring [O(5)] sits above
an axial coordination site, but the NisO distance (Ni(1)-O(5),
3.25 Å) is too long for a significant Ni-O interaction.20 The
benzyl group is coordinated in a η3 fashion with the methylene
group cis to the phosphine. The Ni-Cmethylene distance is ca.
0.24 Å shorter than the Ni-Cortho distance, as expected based
on the greater trans influence of the phosphine compared to the
sulfonate ligand and the greater negative charge on the meth-
ylene carbon compared to the o-carbon of the benzyl anion.21
Similar features were observed for other η3-benzyl nickel
complexes.18a,22 The structure of 4b is similar to that of 4a
except that the [PO]Ni chelate ring in 4b adopts an envelope
conformation, with O(1) lying 0.81 Å out of the Ni(1)-P(1)-
C(13)-C(8)-S(1) plane.
The NMR spectra of 4a and 4b show that the η3-benzyl
structures are retained in solution. The 13C NMR -CH2Ph
resonances (4a, δ 25.8, JCP ) 8 Hz; 4b, δ 18.1, JCP ) 9 Hz)
are typical for η3-benzyl nickel complexes (δ 15-35) and are
downfield from the position expected for η1-benzyl nickel
1
complexes (ca. δ 10).18,22,23 Additionally, the JCH value (157
Hz) for the -CH2Ph groups of 4a and 4b is indicative of η3
coordination. The ambient temperature 13C NMR spectra contain
one set of 2-OMe-Ph resonances for 4a and one set of Cy
resonances for 4b, indicative of fast inversion of the [PO]Ni
1
rings. Also, the ambient temperature H NMR spectra of both
complexes contain one doublet for the -CH2Ph hydrogens (4a,
JHP ) 5.0 Hz; 4b, JHP ) 3.5 Hz) and one set of -CH2Ph
resonances, which shows that in both cases the edges of the
benzyl ligand are equivalent on the NMR time scale. These
observations can be accounted for by fast η3/η1-benzyl isomer-
ization or fast suprafacial shifting of the [PO]Ni unit across the
η3-benzyl ligand.22c,23,24
(25) Galland, G. B.; de Souza, R. F.; Mauler, R. S.; Nunes, F. F.
Macromolecules 1999, 32, 1620.
(26) Rachita, M. J.; Huff, R. L.; Bennett, J. L.; Brookhart, M. J. Polym.
Sci., Part A 2000, 38, 4627.
(27) PPh3 might also induce chain transfer by the formation of
[PO]Ni(R)(ethylene)(PPh3) species as suggested in refs 14d and 14i.
(28) Two isomers and two modes of insertion are possible for [PO]Ni(R)-
(ethylene) species II in Scheme 3. As a result of trans-influence effects,
the cis-P,R-II isomer shown in Scheme 3 is probably more stable than trans-
P,R-II, in which the alkyl is trans to the PR2 unit, but the ethylene insertion
barrier is probably higher for cis-P,R-II than for trans-P,R-II. A similar
situation arises in other unsymmetrical catalysts. It has been proposed that
in such systems chain growth proceeds by isomerization of the more stable
(LL′)M(R)(olefin) isomer to the less stable isomer, followed by migratory
insertion. Thus, chain growth in Scheme 3 may occur by isomerization of
cis-P,R-II to trans-P,R-II followed by insertion to form I; however, further
studies are required to fully understand the chain growth mechanism in
this system. For discussions of this issue, see refs 13 and 14d and (a) Jenkins,
J. C.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 5827. (b) Haras, A.;
Anderson, G. D. W.; Michalak, A.; Rieger, B.; Ziegler, T. Organometallics
2006, 25, 4491. (c) Michalak, A.; Ziegler, T. Organometallics 2003, 22,
2069. (d) Chan, M. S. W.; Deng, L.; Ziegler, T. Organometallics 2000, 19,
2741.
(20) Σ(Ni and O covalent radii) ) 1.94 Å; Σ(Ni and O van der Waals
radii) ) 3.15 Å. Radii are taken from Webelements. See: http://www.we-
belements.com/.
(21) Bushby, R. J.; Tytko, M. P. J. Organomet. Chem. 1984, 270, 265.
(22) (a) Lee, B. Y.; Bazan, G. C.; Vela, J.; Komon, Z. J. A.; Bu, X.
´
J. Am. Chem. Soc. 2001, 123, 5352. (b) Albers, I.; Alvarez, E.; Ca´mpora,
J.; Maya, C. M.; Palma, P.; Sa´nchez, L. J.; Passaglia, E. J. Organomet.
Chem. 2004, 689, 833. (c) Ascenso, J. R.; Carrondo, M. A.; Dias, A. R.;
Gomes, P. T.; Piedade, M. F. M.; Roma˜o, C. C. Polyhedron 1989, 8, 2449.
(23) (a) Shim, C. B.; Kim, Y. H.; Lee, B. Y.; Dong, Y.; Yun, H.
Organometallics 2003, 22, 4272. (b) S, S.; Joe, D. J.; Na, S. J.; Park, Y.;
Choi, C. H.; Lee, B. Y. Macromolecules 2005, 38, 10027.
(24) (a) Carmona, E.; Paneque, M.; Poveda, M. L. Polyhedron 1989, 8,
285. (b) Becker, Y.; Stille, J. K. J. Am. Chem. Soc. 1978, 100, 845. (c)
Cotton, F. A.; Marks, T. J. J. Am. Chem. Soc. 1969, 91, 1339.

4824 Organometallics, Vol. 27, No. 19, 2008
Communications
Table 1. Ethylene Polymerization Results
c
entry
catalyst
P(C2H4) (psi)
time (h)
yield (g)
activityb
Mn
Me branchesd
Ter./Int.e
1a
2a
3a
4a
5a
6a
7f
3
60
60
60
60
60
60
300
300
2
2
2
18
18
18
2
1.2
5.6
0.4
2.4
9.5
1.3
2.2
6.5
30
140
11
6.5
26.3
3.6
55
760
1300
2100
920
1200
2100
1300
1500
9
11
1
7
10
1
68/32
35/65
91/9
78/22
32/68
91/9
4a
4b
3
4a
4b
3
12
11
69/31
36/64
8f
4a
2
163
a Polymerization conditions for entries 1-6: Fisher-Porter glass pressure bottle, 20 µmol catalyst, 60 psi of ethylene pressure, T ) 25 °C, solvent )
25 mL of toluene and 5 mL of CH2Cl2 (added to dissolve compound 4a). b (g of PE)(mmol of Ni)-1 h-1 c Determined by 1H NMR assuming that each
.
chain contains one CdC unit. d Number of Me branches per 103 C, determined by 13C NMR. e Ratio of terminal olefins to internal olefins.
f Polymerization conditions for entries 7 and 8: 300 mL glass-lined stainless steel Parr autoclave equipped with a water cooling loop, a thermocouple,
and a magnetically coupled stirrer and controlled by a Parr 4842 controller. Other conditions were the same as entries 1-6.
Scheme 3a
to the active nickel species.12-14 PPh3 may also displace olefin
from III, resulting in the observed lower Mn and higher terminal/
internal olefin ratio for 3 versus 4a.27 The lower activity and
higher Mn observed for 4b compared to 4a probably reflect the
difference in the trans influence of the diaryl- and dialkylphos-
phino units in these catalysts. The strong trans influence of the
PCy2 unit of 4b should disfavor structures in which an alkyl or
hydride ligand is trans to the PCy2 unit, raising the barriers for
insertion and chain transfer.14h,28 The differences in the overall
electron-donating ability and steric bulk of -PCy2 and -PAr2
may also be important.
This work shows that base-free [PO]Ni(η3-benzyl) complexes
are readily accessible and function as single-component ethylene
polymerization catalysts. Studies of the reactions of these and
related [PO]Ni(η3-benzyl) species with polar monomers are in
progress.
Acknowledgment. This work was supported by the U.S.
Department of Energy (Grant DE-FG-02-00ER15036). We
thank Jacqueline DeFoe for assistance with the synthesis of
1 and Ian Steele for assistance with X-ray crystallography.
Supporting Information Available: Experimental procedures,
characterization data for new compounds, and crystallographic data
for 4a and 4b including CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
a
R ) polymer chain.
The lower polymer yield and Mn observed for PPh3 complex
3 compared to base-free 4a reflect competitive binding of PPh3
OM800741W