Synthesis, characterization, and ethylene oligomerization action of [(C6H5)2 PC6H4C(O-B(C6F5)3) O-Κ2P,O] Ni(η3-CH2C6H5)

Full text of DOI:10.1021/ja002042t

J. Am. Chem. Soc. 2000, 122, 12379-12380
12379
and the distribution of alkene products shifts to higher molecular
weights with increasing reaction temperature. In view of the rising
importance of late metal catalysts in olefin polymerization9 and
oligomerization10,11 reactions, we also examine this molecular
weight dependence.
Synthesis, Characterization, and Ethylene
Oligomerization Action of
[(C6H5)2PC6H4C(O-B(C6F5)3)O-K2P,O]Ni(η3-CH2C6H5)
Zachary J. A. Komon, Xianhui Bu, and Guillermo C. Bazan*
Addition of bis(cyclooctadiene)nickel to a mixture containing
benzyl chloride and sodium 2-(diphenylphosphino)benzoate in
THF gives {[(C6H5)2PC6H4(µ-CO2)-κ3P,O,O′]Ni(η1-CH2C6H5)}2
(32 in eq 1) in 89% yield. The dimeric nature of 32 and coordina-
tion mode of the carboxylate functionalities were confirmed by
a single-crystal X-ray diffraction study (Supporting Information).
Departments of Chemistry and Materials
UniVersity of California, Santa Barbara, California 93106
ReceiVed June 8, 2000
We recently reported that [(C6H5)2PC6H4C(O-B(C6F5)3)O-κ2P,O]-
Ni(η3-CH2CMeCH2) (1) and {[(η5-C5Me4)SiMe2(η1-NCMe3)]TiMe}-
{MeB(C6F5)3}1 (2) form a well-matched pair of homogeneous
initiators for the preparation of branched polyethylene using only
ethylene.2 This tandem process can be tuned such that the 1-butene
produced by the nickel species is inserted into the growing poly-
ethylene chain at the titanium site. It is possible to obtain a linear
correlation between the Ni/Ti ratio and the branching content of the
polymer chain. Compound 1 is obtained by addition of B(C6F5)3
to [(C6H5)2PC6H4C(O)O-κ2P,O]Ni(η3-CH2CMeCH2), which forms
part of the family of Ni compounds used in the SHOP process3,4
and which was originally reported by Keim.5 Carbonyl coordina-
tion to the borane6 is important to raise the affinity of Ni for
ethylene so that it matches the consumption rate at Ti.
1
Treatment of /232 with B(C6F5)3 gives [(C6H5)2PC6H4C(O-
B(C6F5)3)O-κ2P,O]Ni(η3-CH2C6H5) (4 in eq 2). The nuclearity
of 4 and the hapticity of the benzyl ligand were confirmed by
structural characterization (Figure 1).
One drawback of 1 is that the methallyl functionality is less
reactive than the propagating species, resulting in only a small
fraction of the Ni centers participating in the polymerization
process.7 It seemed logical that substitution of methallyl for the
isoelectronic η3-benzyl fragment8 would result in a complex that
would initiate more efficiently and would therefore offer more
control in tandem polymerizations.
We report here on the synthesis and characterization of the
benzyl analogue of 1. We show that a faster initiation is indeed
observed; however, the oligomerization sequence shows unex-
pected features. Specifically, the K factor, defined by
More ethylene is consumed in the first hour when starting with
4, relative to the reaction with 1 (Table 1, entries 3 vs 4 and 7 vs
8). The ratio of 1-butene to 1-hexene is not affected, consistent
with 1 and 4 giving rise to the same catalytic species. Therefore,
initiation is faster with the benzyl ligand. The larger quantity of
1-alkene produced by 4/C2H4 per unit time increases the prob-
ability of secondary reactions, resulting in a higher fraction of
2-alkyl-1-alkenes and internal olefins (entries 3 and 4). Selectivity
for R-olefins improves under more dilute reaction conditions
(entries 4 and 5). Tandem polymerization of ethylene (P(C2H4)
) 3 atm, T ) 20 °C, [Ni]:[Ti] ) 1:100) carried out with 2 and
4 produces a polymer with 9.2% branching. No branching could
be detected by 13C NMR spectroscopy12 under the same conditions
by using 1/2/C2H4.
K ) (moles Cn+2 olefin)/(moles Cn olefin) ) RP/(RP + RCT
)
where RP and RCT are the rates of propagation and chain
termination, respectively, is independent of monomer pressure
(1) (a) Lai, S.-Y.; Wilson, S. R.; Knight, G. W.; Stevens, J. C.; Chun, P.-
W. S. U.S. Patent 5,272,236, 1993. (b) McKnight, A. L.; Waymouth, R. M.
Chem. ReV. 1998, 98, 2587.
(9) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169. (b) Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc.
1999, 121, 10634. (c) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am.
Chem. Soc. 1998, 120, 4049. (d) Britovsek, G. J. P.; Bruce, M.; Gibson, V.
C.; Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.;
Redshaw, C.; Solan, G. A.; Stro¨mberg, S.; White, A. J. P.; Williams, D. J. J.
Am. Chem. Soc. 1999, 121, 8728.
(2) (a) Komon, Z. J. A.; Bu, X.; Bazan, G. C. J. Am. Chem. Soc. 2000,
122, 1830. (b) Barnhart, R. W.; Bazan, G. C.; Mourey, T. J. Am. Chem. Soc.
1998, 120, 1082.
(3) Freitas, E. R.; Gum, C. R. Chem. Eng. Prog. 1979, 75, 73.
(4) Keim, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 235.
(5) Bonnet, M. C.; Dahan, F.; Ecke, A.; Keim, W.; Schultz, R. P.;
Tkatchenko, I. J. Chem. Soc., Chem. Commun. 1994, 615.
(6) Parks, D. J.; Piers, W. E.; Parvez, M.; Atencio, R.; Zaworotko, M. J.
Organometallics 1998, 17, 1369.
(10) (a) Lappin, G. R.; Nemec, L. H.; Sauer, J. D.; Wagner, J. D. In Kirk-
Othmer Encyclopedia of Chemical Technology; Kroschwitz, J. I., Howe-Grant,
M., Eds.; John Wiley & Sons: New York, 1996; Vol. 17, p 839. (b) Parshall,
G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; John Wiley and Sons:
New York, 1992. (c) Ittel, S. D. In Metalorganic Catalysts for Synthesis and
Polymerization; Kaminsky, W., Ed.; Springer-Verlag: Heidelberg, 1999; p 616.
(11) (a) Tsuji, S.; Swenson, D. C.; Jordan, R. F. Organometallics 1999,
18, 4758. (b) Small, B. L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 7143.
(c) Bansleben, D. A.; Friedrich, S. K.; Younkin, T. R.; Grubbs, R. H.; Wang,
C.; Li, R. T. W.O. Patent 9842665, 1998.
(7) Slow initiation has been reported previously for other organonickel
complexes. See: (a) Wang, W.; Friedrich, S.; Younkin, T. R.; Li, R. T.;
Grubbs, R. H.; Bansleben, D. A.; Day, M. W. Organometallics 1998, 17,
3149. (b) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123. (c) Klabunde,
U.; Mulhaupt, R.; Herskovitz, T.; Janowicz, A. H.; Calabrese, J.; Ittel, S. D.
J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 1989.
(12) Galland, G. B.; de Souza, R. F.; Mauler, R. S.; Nunes, F. F.
Macromolecules 1999, 32, 1620.
(8) Carmona, E.; Paneque, M.; Poveda, M. Polyhedron 1989, 8, 285.
10.1021/ja002042t CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/23/2000

12380 J. Am. Chem. Soc., Vol. 122, No. 49, 2000
Table 1. Oligomerization of C2H4 by 1 and 4a
Communications to the Editor
R-olefin distributionb
% 1-butene % 1-hexene
92
olefin distributionb
entry
pre-cat.
P (atm)
[Ni] (µM)
activityc
% R-olefin
% 2-alkyl-1-alkene
% internal
1
2
3
4
5
6
7
8d
1
4
1
4
4
1
1
4
1
1
3
3
3
3
7
7
125
56
1070
430
2110
2540
630
8
97
95
79
31
89
97
97
97
3
5
12.5
125
92
90
90
91
92
90
91
8
10
10
9
17
55
9
4
14
2
125
12.5
12.5
12.5
12.5
8
3
1840
14500
10
9
3
3
a Reaction conditions: toluene, 1 h, 20 °C. b Mole percent determined by 1H NMR spectroscopy. c Kilograms of ethylene consumed per mole of
precatalyst per hour. d Reaction time was limited to 10 min to control temperature changes from reaction exothermicity.
Figure 2. Graph of ln(kCT/kP) vs T-1 for 4/C2H4.
From the Eyring equation one obtains:
kCT
[C4H8]
∆HqP -∆HqCT
∆SqCT -∆SqP
1
ln
)ln
)
+
( k )
(T)(
) (
)
R
R
P
[C2nH4n]
∑
(
)
ng3
Figure 1. ORTEP view of 4, showing atom-numbering scheme. Thermal
ellipsoids at 30% probability level. Hydrogen and fluorine atoms omitted.
A plot of ln[ratio of 1-butene to all higher R-olefins] against 1/T
should therefore yield the difference in activation enthalpy for
propagation and chain transfer (∆HqP - ∆HqCT) and the difference
Table 2. Oligomerization of C2H4 by 4a
in activation entropies (∆SqCT-∆Sq ).15 Indeed, as shown in Figure
temp (°C) % 1-buteneb % higher R-olefinsb kCT/kP K-factor
P
2, a linear correlation is observed over a 69 °C range from which
we obtain ∆Hq - ∆Hq ) 4.8(2) kcal/mol and ∆Sq - ∆Sq
1
21
47
60
70
97
93
89
87
80
3
7
11
13
20
32
13
0.030
0.071
0.11
P
CT
CT
P
) -11(1) eu.16 The enthalpy factor thus favors chain transfer;
8.1
however, since ∆Sq > ∆SqCT, the rate of propagation increases
6.7
4.0
0.13
P
0.20
more quickly with increasing temperature.
In summary, the initiation rate for nickel complexes such as 1
can be increased by substitution of the η3-methallyl fragment for
the isoelectronic η3-benzyl ligand. Tandem catalysis by 4/2/C2H4
produces polymers with significantly more branching than the
1/2/C2H4 combination. Interestingly, the oligomerization reaction
using 4 exhibits a K factor that is pressure independent and
increases with increasing temperature. This trend is due to a larger
(less negative) entropy of activation for propagation. In other
words, while propagation and chain transfer are second-order
processes, the transition state for chain transfer is more organized.
a Solvent is toluene. Reaction time is 10 min. P(C2H4) ) 5 atm.
1
[Ni] ) 25.0 µM. b Determined by H NMR spectroscopy, average of
multiple runs.
Entries 2, 5, and 8 in Table 1 show that increasing the ethylene
pressure at a constant temperature increases the rate of monomer
consumption but does not alter the 1-butene to 1-hexene ratio.
Higher concentrations do not affect the chain length distribution
(entries 4 and 5). Thus, the propagation and chain transfer steps
are first order in monomer and catalyst. A similar order is
observed with the iron catalyst derived from {[(2-ArNdC(Me))2-
C5H3N]FeCl2} (Ar ) 2-C6H4Et) and methylaluminoxane.11b
Table 2 shows that, at a constant pressure, the ratio of 1-butene
to higher 1-alkenes decreases with increasing temperature.13 The
K factor, determined by integrating the 1H NMR signals of
1-butene, 1-hexene, and 1-octene, and by GC for 1-decene,
1-dodecene, and 1-tetradecene, is constant as a function of chain
length. These observations, together with the pressure indepen-
dence of K, indicate that the ratio of 1-butene to the sum of all
the higher 1-alkenes corresponds to the ratio of the rate constant
for chain transfer (kCT) to the rate constant for propagation (kP)
for the nickel-butyl species. Since a Shulz-Flory distribution14
is observed, this kCT/kP ratio is maintained for longer chain species.
Acknowledgment. The authors are grateful to the Department of
Energy, the ACS PRF, and Equistar Chemicals LP for financial assistance.
Supporting Information Available: Complete details for experi-
mental procedures and the crystallographic studies of 3 and 4 (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org.
JA002042T
(14) (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.
(15) Similar analyses have been used to determine ∆∆Hq and ∆∆Sq for
enantioselective reactions. (a) Heller, D.; Buschmann, H. Top. Catal. 1998,
5, 159. (b) Kagan, H. B. Recl. TraV. Chim. Pays-Bas 1995, 114, 203. (c)
Buschmann, H.; Scharf, H.-D.; Hoffmann, N.; Esser, P. Angew. Chem., Int.
Ed. Engl. 1991, 30, 477.
(16) Errors were calculated from modified equations derived in: Morse,
P. M.; Spencer, M. D.; Wilson, S. R.; Girolami, G. S. Organometallics 1994,
13, 1646.
(13) Data in Table 2 obtained under short reaction times. Under these condi-
tions there is negligible formation of 1-alkene dimers and 2-alkenes (<5%).