α-iminocarboxamidato-nickel(II) ethylene polymerization catalysts [3]

Full text of DOI:10.1021/ja004191h

5352
J. Am. Chem. Soc. 2001, 123, 5352-5353
Scheme 1a
r-Iminocarboxamidato-Nickel(II) Ethylene
Polymerization Catalysts
Bun Yeoul Lee, Guillermo C. Bazan,* Javier Vela,
Zachary J. A. Komon, and Xianhui Bu
Departments of Chemistry and Materials
UniVersity of California, Santa Barbara, California 93106
ReceiVed December 7, 2000
Nickel-based olefin polymerization and oligomerization cata-
lysts have attracted considerable recent attention.1,2 Depending
on the ancillary ligand framework, these catalysts participate in
chain-walking reactions,3,4 tolerate polar functionalities on the
monomer,5 and may even be used in water.6 These properties
allow for the synthesis of materials with unique topologies7 and
could enable new industrial processes.2
a i) KH; ii) Ni(η3-CH2C6H5)Cl(PMe3); iii) 2 B(C6F5)3.
During our efforts at developing tandem catalytic processes,8
we discovered that the reactivity of SHOP-type catalysts such as
[(C6H5)2PC6H4C(O)O-κ2P,O]Ni(η3-CH2CMeCH2)9 increases con-
siderably upon addition of B(C6F5)3. Carbonyl coordination to
the borane gives [(C6H5)2PC6H4C(O-B(C6F5)3)O-κ2P,O]Ni(η3-
CH2CMeCH2) and removes electron density from nickel. This
“activation” by action of a Lewis acid on a site removed from
the monomer insertion trajectory prompted our attention. The
more common situation reduces to methyl abstraction and coor-
dination of the resulting borate anion.10 Borate dissociation from
the metal is a generally accepted requirement for olefin insertion.11
It occurred to us that metal activation by formation of carbonyl
adducts could form the basis of a new strategy for designing novel
nickel olefin polymerization catalysts such as 1-3. Resonance
structures I and II illustrate the loss of electron density at nickel.
Figure 1. ORTEP drawing of 4 drawn at 30% probability. Hydrogen
atoms not shown for clarity.
transfer to the monomer.1,12 The η3-benzyl fragment was selected,
instead of the more frequently used methallyl, because it displays
faster rates of initiation.13
Typical Schiff-base condensation14 of primary arylamines with
N-aryl pyruvamides15 yields R-iminocarboxamides. As shown in
Scheme 1, carboxamide deprotonation with 1.0 equiv KH,
followed by reaction with Ni(η3-CH2C6H5)Cl(PMe3),16 results in
the clean formation of the R-iminocarboxamide complexes 4 (R1
) R2 ) H), 5 (R1 ) CHMe2, R2 ) H), and 6 (R1 ) R2 ) CHMe2).
Recrystallization from benzene by slow diffusion of pentane vapor
at room temperature (4 and 5), or pentane at -30 °C (6), affords
analytically pure 4-6 in 70-80% yields.
Structural characterization of 4 (Figure 1) reveals a distorted
square-planar geometry (interplane angle of NNiN and CNiP
planes: 42.2°) with a trans relationship between PMe3 and the
imine nitrogen. The distance between Ni and the carboxamide-N
is shorter (1.936(2) Å) than that between Ni and the imine-N
(2.001(3) Å); the C-O distance (1.243 (4) Å) is consistent with
a double bond between these two atoms. In 6, it is the
carboxamide oxygen that coordinates to nickel (Figure 2), and
the ligand environment is strictly square-planar (interplane angle
of NNiO and CNiP planes: 4.3°). The C-O distance in 6 is longer
(1.302(3) Å) than that of 4, consistent with a reduced π interaction.
Single crystals of 5 suitable for X-ray diffraction studies are
not available at this stage. However, the 31P NMR chemical shifts
in C6D6 are sensitive to the carboxamide binding mode. In 4
(N-bound), one observes a signal at -24.6 ppm, while for 6
(O-bound) the signal appears at -10.0 ppm. For 5, the PMe3
R-Iminocarboxamide ligands were chosen because they can be
readily prepared and because the size of the substituents on nitro-
gen can be varied to modulate steric effects. Of interest to us
was to control the size of the substituents on the pseudoaxial sites,
since blocking these sites in other nickel catalysts reduces chain
(1) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995,
117, 6414.
(2) Reviews: (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV.
2000, 100, 1169. (b) Mecking, S. Coord. Chem. ReV. 2000, 203, 325.
(3) Mo¨hring, V. M.; Fink, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 1001.
(4) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999,
283, 2059.
(5) (a) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.;
Grubbs, R. H.; Banselben, D. A. Science 2000, 287, 460.
(6) (a) Held, A.; Bauers, F. M.; Mecking, S. Chem. Commun. 2000, 301.
(b) Bauers, F. M.; Stefan Mecking, S. Macromolecules 2001, 34, 1165.
(7) Cotts, P. M.; Guan, Z.; McCord, E.; McLain, S. Macromolecules 2000,
33, 6945.
(8) (a) Komon, Z. J. A.; Bu, X.; Bazan, G. C. J. Am. Chem. Soc. 2000,
122, 1830. (b) Barnhart, R. W.; Bazan, G. C.; Mourney, T. J. Am. Chem.
Soc. 1998, 120, 1082.
(9) Bonnet, M. C.; Dahan, F.; Ecke, A.; Keim, W.; Schultz, R. P.;
Tkatchenko, I. Chem. Commun. 1994, 615.
(10) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391.
(11) Substantial debate continues on this mechanistic point. For relevant
discussion, see: Beck, S.; Lieber, S.; Schaper, F.; Geyer, A.; Brintzinger,
H.-H. J. Am. Chem. Soc. 2001, 123, 1483.
(12) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc.
1996, 118, 11664. (b) Deng, L.; Woo, T. K.; Cavallo, L.; Margl, P. M.; Ziegler,
J. Am. Chem. Soc. 1997, 119, 6177. (c) Svejda, S. A.; Johnson, L. K.;
Brookhart, J. Am. Chem. Soc. 1999, 121, 10634.
(13) Komon, Z. J. A.; Bu, X.; Bazan, G. C. J. Am. Chem. Soc. 2000, 122,
12379.
(14) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Mad-
dox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.; Stro¨m-
berg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 8728.
(15) Wang, C.-Z.; Mallat, T.; Baiker, A. Tetrahedron: Asymmetry 1997,
8, 2133.
(16) Carmona, E.; Paneque, M.; Poveda, M. L. Polyhedron 1989, 8, 285.
10.1021/ja004191h CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/11/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 22, 2001 5353
Table 1. Reactivity toward Ethylenea
entry
compound
[Ni]b activityc
0.33
0.33 1500 oligomers
Mw
Mw/Mn branchesd Tm
1
2
3
4
5
6
7
4
1
2
3
0
0.33
0.33
550 119000
350 508000
510 533000
850 349000
430e 299000
25
9
33
71
125
124
123
122
122
3 + 2.5 B(C6F5)3 0.33
3 + 2.5 B(C6F5)3 0.10
3 + 2.5 B(C6F5)3 0.10
6
71
2.3
2.8
104
106
a Polymerization conditions: 30 mL of toluene, 100 psig ethylene,
10 min. b Concentration in mM. c kg product/(mol Ni‚h). d Determined
by 1H NMR, and corresponds to the number of branches/1000 carbons.19
e Reaction carried out with 50 psig ethylene.
Figure 2. ORTEP drawing of 6 drawn at 30% probability. Hydrogen
atoms not shown for clarity.
resonance structure II is therefore required to account for these
structural trends.
A series of ethylene polymerization studies is summarized in
Table 1. No reaction is observed by using 4 (entry 1). Similar
results are obtained with 5 and 6. In the case of 1, ethylene is
consumed, quickly giving oligomers, as determined by GC/MS
(entry 2). The product is a mixture of 1-alkenes, internal olefins,
and 1-alkene dimers. The fraction corresponding to 1-alkenes is
described by a Schultz-Flory distribution.17 Use of 2 and 3 leads
to polymer formation (entries 3 and 4). The polymer obtained
with 3 is of higher molecular weight and is described by a
narrower molecular-weight distribution (PDI). Comparison of
entries 4 and 5 shows that adding 2.5 equiv of B(C6F5)3 increases
the activity and decreases the PDI. Lower nickel concentration
gives rise to a further PDI narrowing (entries 5 and 6).18 Reducing
the pressure (entries 6 and 7) results in a drop in activity. Highest
activities are obtained for 1, probably because there is no
entrapment of the catalyst within the polymer precipitate.
Analysis by 1H NMR spectroscopy19 shows a branched polymer
structure.1 Branching is encouraged by the bulkier ligand environ-
ment of 3, relative to 2 (Table 1, entries 3 vs 4). Examination by
13C NMR spectroscopy using established pulse sequences20 shows
a distribution of branching sizes similar to other nickel catalysts.4
For the polymer in entry 5 one observes a majority of methyl
branches (∼60%), a minor component of ethyl branches (∼10%)
and the remainder approximately equal quantities of propyl, butyl,
amyl, and long-chain branches. Lower melting points are observed
with increased branching.
In summary, the synthesis and characterization of compounds
1-3 show that borane attachment to a site removed from the
ethylene insertion trajectory can be used to activate nickel catalysts
for olefin polymerization. It is interesting to note the similarities
in structure/reactivity relationships to Brookhart’s diimine cata-
lysts.1 In particular, hindering axial sites leads to substantially
higher molecular weight product and the production of a branched
polymer structure. The extent to which mechanistic similarities
exist between compounds 2 and 3 and other nickel catalysts and
how a more open reaction site leads to advantages/disadvantages
for polymer synthesis are the subjects of ongoing studies.
Figure 3. ORTEP drawing of 3 drawn at 30% probability. Hydrogen
atoms not shown for clarity.
resonance occurs at -22.2 ppm, which strongly suggests an
N-bound carboxamide, as shown in Scheme 1.
Addition of 2 equiv of B(C6F5)3 to solutions of 4-6 in benzene
results in the immediate precipitation of Me3P-B(C6F5)3 and the
1
formation of 1-3. H NMR spectra of the products show the
formation of two isomers, the ratio of which depends on the ligand
environment (1:1, 3:1, 2:1 for 1, 2, and 3, respectively). The
upfield shift of the aromatic protons on the benzyl ligand from 7
to 8 to 5.6 to 7 ppm indicates η3-coordination.16 In addition, the
11B NMR signal from B(C6F5)3 changes from 59 to 0.7 ppm,
consistent with the formation of a B-O adduct. Our current
thinking is that the sets of isomers arise from pseudorotamers of
the benzyl ligand, as shown by A and B below.
Acknowledgment. We are grateful to the Department of Energy for
financial assistance. B.Y.L. is thankful to Korea Science and Engineering
Foundation for partial financial support.
Examination of 6 raises the possibility of borane coordination
to the carboxamide nitrogen, rather than oxygen. To resolve this
uncertainty, single crystals of one isomer of 3 suitable for X-ray
diffraction studies were obtained by slowly allowing layered
hexane solutions of 6 and B(C6F5)3 to diffuse at room temperature.
As shown in Figure 3, compound 3 is the oxygen adduct. The
two aryl rings are perpendicular to the nickel square plane and
are “pushed” slightly toward nickel by the bulky fluorinated rings.
Relative to the N-bound carboxamide in 4, there is an elongation
of the C-O distance (1.243(4) in 4 versus 1.288(6) Å in 3) and
a contraction of the C-N distance (1.399(4) in 4 versus 1.288(6)
Å in 3). There are nearly identical bond distances between the
Ni and the two N atoms (d(Ni-N(carboxamide)) ) 1.909(4) Å,
d(Ni-N(imine)) ) 1.942(4) Å). A substantial contribution from
Supporting Information Available: Complete details for the syn-
thesis of all compounds, polymerization procedures, and the crystal-
lographic studies of 3, 4, and 6 (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA004191H
(17) (a) Flory, P. J. J. Am. Chem. Soc. 1940, 62, 1561. (b) Schulz, G. V.
Z. Phys. Chem., Abt. B 1935, 30, 379. (c) Schulz, G. V. Z. Phys. Chem., Abt.
B 1939, 43, 25.
(18) Narrower PDI values are the result of better temperature control under
more dilute conditions.
(19) Gates, D. P.; Svejda, S. A.; Onate, E.; Killian, C. M.; Johnson, L. K.;
White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320.
(20) Galland, G. B.; de Sousa, R. F.; Mauler, R. S.; Nunes, F. F.
Macromolecules 1999, 32, 1620.