Neutral bimetallic nickel(II) phenoxyiminato catalysts for highly branched polyethylenes and ethylene-norbornene copolymerizations

Article abstract of DOI:10.1021/om800208f

The synthesis and characterization of novel bimetallic, neutrally charged dinickel 2,7-diimino-1,8-dioxynaphthalene polymerization catalysts is reported. Ethylene polymerizations as well as ethylene-co-norbornene copolymerizations display increased cataly

Full text of DOI:10.1021/om800208f

2166
Organometallics 2008, 27, 2166–2168
Neutral Bimetallic Nickel(II) Phenoxyiminato Catalysts for Highly
Branched Polyethylenes and Ethylene-Norbornene
Copolymerizations
Brandon A. Rodriguez, Massimiliano Delferro, and Tobin J. Marks*
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed March 6, 2008
as ∼3.1 Å,5 and initial observations on ethylene polymerization
Summary: The synthesis and characterization of noVel bimetal-
lic, neutrally charged dinickel 2,7-diimino-1,8-dioxynaphthalene
polymerization catalysts is reported. Ethylene polymerizations
as well as ethylene-co-norbornene copolymerizations display
increased catalytic actiVity, methyl branch formation, and
comonomer enchainment selectiVity Versus the monometallic
analogues. Furthermore, these systems turn oVer in the absence
of cocatalyst under mild conditions.
and copolymerization characteristics. It will be seen that these
catalysts exhibit non-negligible cooperativity effectssthe first
reported for a group 10 metalsmanifested in enhanced poly-
merization activity, enhanced methyl chain branching, and
enhanced comonomer incorporation under mild reaction condi-
tions and not requiring a cocatalyst.6
The remarkable enchainment cooperativity effects displayed
by single-site group 4 bimetallic olefin polymerization catalysts
include significantly enhanced activity, chain branching, and
comonomer enchainment selectivity.1 Moreover, these effects
roughly scale inversely with the intermetallic distance and are
evident in both constrained geometry1 and aryloxyiminato2
group 4 catalysts (e.g., Ti2, FI2-Zr2, respectively). Since studies
to date have focused exclusively on group 4 metals, the question
arises as to whether such cooperativity effects are limited to
early transition metals or might be more pervasive. To explore
this issue, we focused on Ni(II) complexes, which are active
olefin polymerization catalysts,3 as exemplified by the Ni
phenoxyiminates of Grubbs, which afford LDPEs having
moderate molecular weights and 10-55 branches/1000 C
atoms.4 This general ligand architecture confers distinctive
electronic, steric, and catalytic characteristics on the metal
center, and we report here the synthesis of binuclear 2,7-diimino-
1,8-dioxynaphthalene Ni(II) catalysts FI2-Ni2-A and FI2-Ni2-
B, in which rigid ligation enforces Ni · · · Ni distances as small
* Corresponding author. E-mail: t-marks@nortwestern.edu.
(1) (a) Review: Li, H.; Marks, T. J. Proc. Nat. Acad. Sci. 2006, 103,
15295–15302, and references therein. (b) Li, H.; Stern, C. L.; Marks, T. J.
Macromolecules 2005, 38, 9015–9027. (c) Li, H.; Li, L.; Schwartz, D. J.;
Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 14756–14768. (d)
Li, H.; Li, L.; Marks, T. J. Angew. Chem., Int. Ed. 2004, 43, 4937–4940.
(e) Guo, N.; Li, L.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 6542–6543.
(2) Salata, M. R.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 12–13.
(3) (a) Domski, G. J.; Rose, J. M.; Coates, G. W.; Bolig, A. D.;
Brookhart, M. Prog. Polym. Sci. 2007, 32, 30–92. (b) McCord, E. F.;
McLain, S. J.; Nelson, L. T. J.; Ittel, S. D.; Tempel, D.; Killian, C. M.;
Johnson, L. K.; Brookhart, M. Macromolecules 2007, 40 (3), 410420. (c)
Zhang, L.; Brookhart, M.; White, P. S. Organometallics 2006, 25 (8), 1868–
1874. (d) Jenkins, J. C.; Brookhart, M. J. J. Am. Chem. Soc. 2004, 126,
5827–5842. (e) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103,
283–316. (f) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000,
100, 1169–1203. (g) Desjardins, S. Y.; Cavell, K. J.; Hoare, J. L.; Skelton,
B. W.; Sobolev, A. N.; White, A. W.; Keim, W. J. Organomet. Chem.
1997, 544, 163–174. (h) Huang, Y.; Tang, G.; Jin, G.; Jin, G. Organome-
tallics 2008, 27 (2), 259–269.
2
The sodium salt of ligand FI2-H2 was obtained by treating
2,7-di(2,6-diisopropylphenyl)imino-1,8-dihydroxynaphtha-
lene2 with NaH in THF. The bimetallic catalysts FI2-Ni2-A and
FI2-Ni2-B were prepared as shown in Scheme 1 (for details,
see Supporting Information). The imine protons in the Ni2FI2-A
1H NMR spectrum exhibit a characteristic JPH ≈ 9 Hz,
4
corresponding to PMe3 coordination trans to the ketimine
(5) Ni-Ni ) 3.09 Å in the crystal structure of a Ni2L2(OPMe3)2
thermolysis product (Rodriguez, B. A.; Delferro, M.; Marks, T. J.,
unpublished results).
(6) For binuclear Ni(II) catalysts having less rigid ligation, longer
intermetallic distances, and minimal cooperative polymerization effects, see:
(a) Chen, Q.; Yu, J.; Huang, J. Organometallics 2007, 26, 617–625. (b)
Hu, T.; Tang, L.; Li, X.; Li, Y.; Hu, N. Organometallics 2005, 24, 2628–
2632. (c) Zhang, D.; Jin, G. Organometallics 2003, 22, 2851–2854. (d)
U.S. Patent 0270811, 2006.
(4) (a) Waltman, A.; Younkin, T.; Grubbs, R. H. Organometallics 2004,
23, 5121–5123. (b) Younkin, T. R.; Connor, E. F.; Henderson, J. I.;
Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460–
462. (d) Wang, C.; Friedrich, S. K.; Younkin, T. R.; Li, R. T.; Grubbs,
R. H.; Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149–
3151.
10.1021/om800208f CCC: $40.75
2008 American Chemical Society
Publication on Web 04/24/2008

Communications
Organometallics, Vol. 27, No. 10, 2008 2167
Scheme 1. Synthesis of Binuclear Catalysts FI2-Ni2-A and FI2-Ni2-B and Mononuclear Catalyst FI2TMS-Ni
Table 1. Ethylene and Ethylene-co-Norbornene Polymerization Data for Nickel FI2-Ni2, FI2-Ni, and FI Catalysts
polymer
yield (g)
d
entry
catalyst
cocatalyst
comonomer
Mw
Mw/Mn
total Me/1000 Ce
mp, °Cf
activg
comon incorph
1
2
3
4
5
6
7
8
FI2-Ni2-Aa
FI2-Ni2-Ba
FI2-Ni2Bj
FI2-Ni2-Bk
FI-Ni-Aa
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
0.663
0.684
0.631
0.566
0.167
0.175
0.141
0.103
10 300
10 100
10 700
10 900
11 700
10 500
11 200
6000
2.6
2.6
2.6
2.6
2.5
2.5
2.6
2.7
2.7
80
93
92
86
52
54
40
102
105
68
66
68
68
93
97
98
60
61
7.1
7.4
6.8
6.2
3.6
3.7
3.3
0.2
0.4
FI-Ni-Ba
FI2TMS-Nia
FI2-Ni2-Ab
FI2-Ni2-Bb
FI-Ni-Ab
9
0.196
7000
i
10
11
12
13
14
15
FI-Ni-Bb
i
FI2-Ni2-Ac
FI2-Ni2-Bc
FI-Ni-Ac
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
norbornene
norbornene
norbornene
norbornene
0.558
0.504
0.072
0.066
66 400
65 800
63 200
64 000
5.2
4.5
2.3
2.1
34
38
9
107
106
124
124
1.3
1.2
0.3
0.3
9
11
3
FI-Ni-Bc
11
3
a Polymerizations carried out with 10 µmol of catalyst and 2 equiv of cocatalyst/Ni at 25 °C for 40 min in 25 mL of toluene at 7.0 atm of ethylene.
b Polymerizations carried out with 20 µmol of catalyst at 25 °C for 2 h in 25 mL of toluene at 7.0 atm of ethylene. c Polymerizations carried out with
20 µmol of catalyst and 2 equiv of cocatalyst/Ni at 25 °C for 90 min in 25 mL of toluene and 225 equiv of norbornene at 7.0 atm of ethylene. d GPC vs
polyethylene standard; uncorrected. e By 1H NMR. f Determined by DSC. g kg polyethylene/mol of Ni · h atm. h Molar percentage by 13C NMR. i No
polymer obtained. j Polymerizations carried out with 10 µmol of catalyst and 2 equiv of cocatalyst/Ni at 25 °C for 60 min in 25 mL of toluene at 7.0
atm of ethylene. k Polymerizations carried out with 10 µmol of catalyst and 2 equiv of cocatalyst/Ni at 25 °C for 90 min in 25 mL of toluene at 7.0 atm
ethylene.
1
1
(confirmed by 2D H – H NOESY).7 Close proximity of the
(Scheme 1). Stepwise Ni incorporation can be monitored by
integration of the now inequivalent isopropyl and imine H
Ni-CH3 group and the methyls of one iPr group is also detected.
1
4
1
In contrast, JPH ≈ 6 Hz and the 1D H NOESY indicate cis
PPh3 binding in FI2-Ni2-B.7 The 31P singlets in both complexes
are consistent with the proposed FI2-Ni2-A and FI2-Ni2-B
symmetries. For control experiments, mononuclear FI-Ni-A and
FI-Ni-B were synthesized by reaction of the corresponding
monosalicylaldiminate sodium salt4 with the aforementioned
Ni(II) precursors. In both monometallic complexes, the PR3 (R
NMR resonances. These monometallic complexes are designed
to probe the nature and extent of Ni-Ni cooperativity effects
on polymerization.
Room-temperature ethylene homopolymerizations using the
present catalysts were carried out in the presence of the
phosphine scavenger/cocatalyst Ni(cod)2 under conditions mini-
mizing mass transport and exotherm effects (see Supporting
Information for details).1,2 Bimetallic FI2-Ni2-A and FI2-Ni2-B
afford polyethylenes with molecular weights comparable to those
produced by the mononuclear analogues and with polydisper-
sities consistent with single-site processes (Table 1). However,
the bimetallic catalysts exhibit a 2-fold greater polymerization
activity along with increased methyl (and only methyl; see
) Me, Ph) ligand is bound trans to the ketimine group (4JPH
≈
9 Hz; see Supporting Information for data). A second mono-
metallic control complex was prepared by reaction of 1.0 equiv
of the Ni(II) precursor with the disodium salt of FI2-H2,
followed by addition of TMS-Cl in situ to yield FI2(TMS)-Ni
(7) Zhang, L.; Brookhart, M.; White, P. S. Organometallics 2006, 25
(8), 1868–1874.

2168 Organometallics, Vol. 27, No. 10, 2008
Scheme 2. Branch Formation Pathways in FI2-Ni2 Mediated Ethylene Homopolymerization
Communications
1
below) branching. The branch density by H NMR8 is ∼2×
carbons R, ꢀ, and γ to the branches at δ 21.0, 35.0, 39.1, 28.2,
and 31.5 ppm, respectively. In contrast, note that the polyeth-
ylenes derived from the bimetallic catalysts contain almost
exclusively methyl branches. Thus, the 13C NMR spectra of
the FI2-Ni2-A/FI2-Ni2-B-derived products exhibit, in addition
to backbone resonances, only four prominent signals, assign-
able12 to methyl branches and carbons R, ꢀ, and γ to the branch
at δ 21.0, 39.1, 28.2, and 31.5 ppm, respectively. The extent of
ethyl branching amounts to e1% of the total branching (see
Supporting Information). While further mechanistic experiments
will be required to define additional aspects of the branch-
forming pathways, at this stage it appears likely that the presence
of the second Ni center suppresses insertion pathway 2 versus
pathway 1 (Scheme 2) via an interplay of Ni2-associated steric
and electronic/coordination factors.
that achieved by the mononuclear catalysts under identical
reaction conditions and is confirmed by depressed DSC-
determined melting points (Table 1). In the absence of a
cocatalyst, the mononuclear systems do not produce polyeth-
ylene.10 In contrast, the present bimetallic catalysts produce
polyethylenes with increased branching densities and concur-
rently depressed melting points, albeit at somewhat reduced
polymerization rates versus the cocatalyzed polymerizations
(Table 1).11 This particular productivity difference between FI2-
Ni2-A, FI2-Ni2-B, and the mononuclear analogues may reflect
phosphine dissociation-related steric and electronic factors.
Typically, equilibria between such phosphine-coordinated and
uncoordinated species heavily favor the former;3d however the
proximate bulky phosphine ligands in FI2-Ni2-A and FI2-Ni2-B
may favor phosphine dissociation.
Ethylene + norbornene copolymerizations were also inves-
tigated, and modest comonomer enchainment levels are achieved
with the present mononuclear catalysts, in accord with results
for comparable mononuclear systems11 (Table 1). In marked
contrast, ethylene + norbornene copolymerizations mediated
by binuclear FI2-Ni2-A and FI2-Ni2-B proceed with 3-4 times
greater activity and achieve 3-4 times greater selectivity for
comonomer enchainment, while product molecular weights are
comparable (as in the homopolymerization cases).
These results show that, for single-site d8 Ni(II) aryloxyimi-
nato ethylene polymerization catalysts, a proximate catalytically
active Ni site substantially increases activity, degree of and
selectivity for methyl group branching, and comonomer incor-
poration selectivity versus the mononuclear analogues. Further-
more, these binuclear catalysts produce highly branched poly-
ethylenes in the absence of a cocatalyst. Further studies are
underway to better define the scope and mechanisms of these
and related processes, including low-temperature NMR to
identify any intermetallic agostic effects.
Control homopolymerization experiments were also carried
out with catalyst FI2TMS-Ni, having a single Ni center bound
to the FI2 ligand. The results (Table 1) indicate comparable
polymerization activities and branch densities to that of the
mononuclear catalysts FI-Ni-A and FI-Ni-B under identical
conditions and argue that neither the FI2 ligand nor steric bulk
alone ensures enhanced homopolymerization activity or branch-
ing. While most data in Table 1 are the result of 40 min
polymerization trials, ethylene polymerizations were also carried
out for 60 and 90 min, with four of the catalysts, and the results
verify continuing polymerization activity beyond 40 min.
Homopolymerizations at higher temperatures (g40 °C), with
or without cocatalyst, yield minimal polymer. Rather, bis-
chelating Ni2(FI2)2L2 complexes are identified, reminiscent of
the analogous mononuclear phenoxyiminato catalysts.4
In regard to homopolymer microstructure, the 13C NMR
spectra of the polyethylenes produced by all of the monometallic
catalysts exhibit five prominent non-polyethylene backbone
resonances assignable12 to methyl branches, ethyl branches, and
Acknowledgment. Financial support by DOE (86ER13511)
is acknowledged. M.D. thanks U. of Parma (Italy) for a study
leave. We thank Mr. M. Salata for helpful discussions and
Dr. E. Szuromi and Prof. R. F. Jordan (U. of Chicago) for
hospitality in GPC measurements.
(8) (a) Polyethylene branching densities were quantified by 1H NMR
spectroscopy using the methyl group:overall carbon (methyl + methylene
+ methine) ratios8b and are reported as branches per 1000 carbons. (b)
Gates, D. P.; Svejda, S. A.; Onate, E.; Killian, C. M.; Johnson, L. K.; White,
P. S.; Brookhart, M. Macromolecules 2000, 33, 2320–2334.
(9) Van Soolingen, J.; Verkruijsse, H. D.; Keegstra, M. A.; Brandsma,
L. Synth. Commun. 1990, 20, 3153.
(10) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Waltman, A. W.;
Grubbs, R. H. Chem. Commun. 2003, 18, 2272–2273.
Supporting Information Available: Details of catalyst synthe-
sis, polymerization experiments, and polymer characterization;
NMR spectra of representative polymer samples. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Hwang, S.; Grubbs,
R. H.; Roberts, W. P.; Litzau, J. J. J. Polym. Sci. Part A: Polym. Chem.
2002, 40, 2842–2854.
(12) Jurkiewicz, A.; Eilberts, N. W.; Hsieh, E. T. Macromolecules 1999,
32 (17), 5471–5476.
OM800208F