Ligand electronic effects on late transition metal polymerization catalysts

Article abstract of DOI:10.1021/om048988j

The synthesis and polymerization properties of substituted palladium (Pd)(II)-α-diimine complexes were investigated. The ligand electronic structure effects on the molecular weight, polymer branching topology, tolerance to polar monomers, and polar monomer incorporation of late transition metal α-diimine olefin polymerization catalysts were also investigated. The ligand electronic structure of the catalysts had a significant effect on the topology of the polyethylene formed. NMR chemical shift analysis of the complexes and IR analysis of the CO stretching frequencies of the carbonyl complexes showed that substitution did not effect the electrophilicity of the metal center.

Full text of DOI:10.1021/om048988j

Organometallics 2005, 24, 1145-1155
1145
Ligand Electronic Effects on Late Transition Metal
Polymerization Catalysts
Christopher Popeney and Zhibin Guan*
Department of Chemistry, University of California, 516 Rowland Hall,
Irvine, California 92697-2025
Received December 21, 2004
A series of bis-(aryl)-R-diimine ligands were synthesized bearing a range of electron-
donating and -withdrawing substituents to systematically investigate the ligand electronic
effects on late transition metal olefin polymerization catalysts. Their palladium(II) complexes
were prepared and characterized. Electronic perturbations were verified by analysis of the
1H and 13C NMR chemical shifts of the corresponding methyl chloride complexes and the
CO stretching frequencies of the corresponding cationic carbonyl complexes, which were
found to correlate strongly with the Hammet substituent constant (σp) of the substituent on
the ligand. The palladium(II) complexes of the functionalized R-diimine ligands were
employed in the polymerization of ethylene and the copolymerization of ethylene with methyl
acrylate. For ethylene homopolymerization, higher molecular weight was obtained with
catalysts bearing more strongly electron-donating ligands. It was observed for the first time
that the ligand electronic structure of the catalysts had a significant effect on topology of
the polyethylene formed. More dendritic polyethylene was obtained with catalysts bearing
more strongly electron-withdrawing ligands. This provides a fundamentally different
approach (catalyst approach) to control polyolefin branching topology, which complements
our previous strategy of controlling polymer topology by polymerization conditions. For the
copolymerization of ethylene with methyl acrylate, strong correlations were observed in the
incorporation ratio of methyl acrylate; catalysts bearing strongly electron-donating ligands
afforded copolymers with higher incorporation of the polar comonomer. These catalysts also
exhibited far greater activity in the presence of polar monomers than catalysts bearing more
weakly donating ligands possessing withdrawing substituents, which were deactivated
completely when methyl acrylate concentrations were sufficiently high.
Introduction
insertion polymerization mechanism. One attractive
feature of this system is its ability to prepare hyper-
branched to dendritic polyethylene under conditions of
low ethylene pressure.6 The reduced oxophillicity of
palladium endows the catalysts with the ability to
produce copolymers with a variety of polar-functional-
ized olefins, allowing the preparation of complex func-
tional polymer architectures in simple, one-pot synthe-
ses.10
Recent interest in the development of late transition
metal catalysts for the polymerization of olefins has
been spurred by their high functional group tolerance
and their ability to control polymer topology.1-9 The Pd-
(II) aryl-substituted R-diimine systems have received
much attention because they allow precise control of
polymer topology through simple variation of olefin feed
concentration.6-9 Branched polymer structures obtained
by these catalysts are attributed to an isomerization
mechanism, or “chain walking” of the catalyst along the
polymer chain, operating in addition to the migratory
Since Brookhart’s seminal discovery,3,4 tremendous
efforts have been spent in both industrial and academic
laboratories on improving late transition metal catalyst
systems. Many structural modifications, including steric
tuning, changing the ligand backbone structure, and
changing the chelating heteroatoms, have been pursued
on the archetype R-diimine ligands.1,2 However, an
important parameter in catalyst design, the ligand
electronic structure, has not been systematically inves-
tigated. Ligand electronic effects have been observed in
many catalytic systems. For example, in hydrocarbon
oxidation using iron porphyrin catalysts, a good Ham-
mett correlation was observed for the oxidation product
yields and the Hammett constants for the substituents
on the porphyrin ligands.11 In asymmetric catalysis,
* To whom correspondence should be addressed. E-mail:
zguan@uci.edu.
(1) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100,
1169-1203, and references therein.
(2) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283-
315, and references therein.
(3) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414-6415.
(4) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc.
1996, 118, 267-268.
(5) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am.
Chem. Soc. 1998, 120, 888-899.
(6) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999,
283, 2059-2062.
(7) Cotts, P. M.; Guan, Z.; McCord, E.; McLain, S. Macromolecules
2000, 33, 6945-6952.
(8) Guan, Z. Chem. Eur. J. 2002, 8, 3086-3092.
(9) Guan, Z. J. Polym. Sci. A: Polym. Chem. 2003, 41, 3680-3692.
(10) Chen, G.; Ma, X. S.; Guan, Z. J. Am. Chem. Soc. 2003, 125,
6697-6704.
10.1021/om048988j CCC: $30.25 © 2005 American Chemical Society
Publication on Web 02/12/2005

1146 Organometallics, Vol. 24, No. 6, 2005
Popeney and Guan
Chart 1
Chart 2
enantioselectivity was influenced by the ligand elec-
tronic structure of the catalysts.12,13 A remarkable
demonstration of ligand electronic tuning for improving
catalytic activity is the development of the Grubbs
ruthenium carbene metathesis catalysts.14,15 For po-
lymerization catalysis, significant ligand electronic ef-
fects were observed in a number of systems.16-21 For
example, Waymouth and co-workers discovered a pro-
found ligand electronic effect on stereoselectivity in
propylene polymerization using their “oscillating” zir-
conocene catalysts.18 Coates and co-workers reported an
order of magnitude increase of polymerization rate by
placing a cyano group on the â-diiminate ligand for CO2/
epoxide copolymerization using â-diiminate zinc alkox-
ide catalysts.21
Scheme 1
Scheme 2
For R-diimine-based late transition metal complexes,
ligand electronic effects manifested in a few reactions.
For example, in C-H activation chemistry on arenes
using R-bis(aryl)diimine complexes of platinum, electron-
donating substituents on the ligand were found to
increase the rate of activation.22 Others have shown that
incorporation of electron-donating or -withdrawing sub-
stituents onto R-diimine ligands affects their σ-donating
ability to Pd(II) as well as the Lewis acidity of the
metal.23,24 For olefin polymerizations, Brookhart and co-
workers observed an increase in polar olefin incorpora-
tion for increasingly electron-donating ligands.5 Despite
these studies, a thorough and systematic account of
electronic effects on late transition metal R-diimine
olefin polymerization catalyst systems has yet to be
reported.
In this study, the preparation and polymerization
properties of six new substituted Pd(II)-R-diimine com-
plexes, 1-3 and 5-7 (Chart 1), are reported. In par-
ticular, the resulting effects upon polymer molecular
weight, polymer branching topology, tolerance to polar
monomers, and polar monomer incorporation are de-
scribed. A specific interest we have in mind is to
investigate the ligand electronic effects on the topology
for polymers formed with the catalysts. Polymer branch-
ing topology is an important molecular parameter for
controlling polymer physical properties. Our group has
been developing efficient strategies for controlling poly-
mer topology by using transition metal catalyzed po-
lymerization.8,9 Our previous strategy for controlling the
topology of ethylene homopolymers and copolymers
using the Brookhart Pd(II) catalyst system is based on
the monomer concentration effect on polymerization
kinetics.6,10 In this study, we would like to examine the
ligand electronic effects on polymerization kinetics and
the resulting polymer topology. Our hypothesis is that
the perturbation of electronic structure of the catalyst
system should change the relative rate constants for
insertion and chain walking (kins/kwalking), which will
manifest in the resulting polymer topology.
Results and Discussion
A. Synthesis and Spectroscopic Studies of Sub-
stituted Pd(II) Complexes. Synthesis of Substi-
tuted r-Diimine Ligands. Substituents were placed
in the para-aryl position on the R-diimine ligands, where
the substituent could influence the donating ability of
the ligand while not changing sterics at the metal
center. The presence of ortho-isopropyl groups on the
aryl rings of the ligands was necessary to block associa-
tive chain transfer during polymerization in order to
produce high molecular weight polymer.3 In the first
step, corresponding substituted anilines possessing
ortho-isopropyl groups were prepared for the R-diimine
ligand synthesis.
Nitro- and dimethylamino-substituted anilines, 8 and
9 (Chart 2), were prepared using literature methods.25
The p-chloride 11 was prepared from 10 by an oxidative
chlorination route (Scheme 1).26
The remaining anilines were synthesized by Pd-
catalyzed isopropylation of the 2,6-dibrominated para-
substituted anilines. Palladium-catalyzed Kumada cou-
(11) Nappa, M. J.; Tolman, C. A. Inorg. Chem. 1985, 24, 4711-4719.
(12) RajanBabu, T. V.; Casalnuovo, A. L. Pure Appl. Chem. 1994,
66, 1535-1542.
(13) RajanBabu, T. V.; Casalnuovo, A. L.; Ayers, T. A.; Nomura, N.;
Jin, J.; Park, H.; Nandi, M. Curr. Org. Chem. 2003, 7, 301-316.
(14) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18-29.
(15) Love, J. A.; Sanford, M. S.; Day, M. W.; Grubbs, R. H. J. Am.
Chem. Soc. 2003, 125, 10103-10109.
(16) Lanza, G.; Fragala, I. L.; Marks, T. J. J. Am. Chem. Soc. 2000,
122, 12764-12777.
(17) Deming, T. J. J. Am. Chem. Soc. 1997, 119, 2759-2760.
(18) Lin, S.; Hauptman, E.; Lal, T. K.; Waymouth, R. M.; Quan, R.
W.; Ernst, A. B. J. Mol. Catal. A: Chem. 1998, 136, 23-33.
(19) Kamigaito, M.; Lal, T. K.; Waymouth, R. M. J. Polym. Sci. A:
Polym. Chem. 2000, 38, 4649-4660.
(20) Alcazar-Roman, L. M.; O’Keefe, B. J.; Hillmyer, M. A.; Tolman,
W. B. Dalton Trans. 2003, 3082-3087.
(21) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew.
Chem., Int. Ed. 2002, 41, 2599-2602.
(22) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2002, 124, 1378-1399.
(25) Carver, F. J.; Hunter, C. A.; Livingstone, D. J.; McCabe, J. F.;
Seward, E. M. Chem. Eur. J. 2002, 8, 2848-2859.
(26) Vyas, P. V.; Bhatt, A. K.; Ramachandraiah, G.; Bedekar, A. V.
Tetrahedron Lett. 2003, 44, 4085-4088.
(23) Gasperini, M.; Ragaini, F.; Cenini, S. Organometallics 2002,
21, 2950-2957.
(24) Gasperini, M.; Ragaini, F. Organometallics 2004, 23, 995-1001.

Ligand Electronic Effects on Palladium-R-Diimine
Organometallics, Vol. 24, No. 6, 2005 1147
Scheme 3
Scheme 4
Scheme 5
pling of Me- and OMe-substituted dibromoanilines 12
and 13 with isopropenylmagnesium bromide (Scheme
2) was effective in constructing the diisopropenyl ad-
ducts 14 and 15.27 Synthesis of 13 was achieved by the
reaction of para-anisidine with an ammonium tribro-
mide salt.28
Because the CF3 group is sensitive to the Grignard
reagent used in Kumada coupling, Suzuki coupling with
isopropenylboronic acid 18 (Scheme 3) was utilized to
give the CF3-substituted diisopropenyl adduct 20.29
Hydrogenation of the crude diisopropenylated anilines
14, 15, and 20 was sluggish, presumably due to the
poisoning of the Pd/C catalyst by the formed anilines,
but the reaction was driven by periodically replenishing
the catalyst. Hydrogenated anilines 16, 17, and 21 were
used directly for the preparation of the corresponding
R-diimines.
Electron rich R-diimine ligands 22-24 were prepared
using mild condensation conditions: methanol at reflux
with catalytic formic acid (Scheme 4).30 Preparation of
R-diimines 25-27 from electron-deficient anilines re-
quired azeotropic water removal from benzene using 5
Å molecular sieves for dehydration and 10-camphorsul-
fonic acid (CSA) as catalyst. Recrystallization of the
ligands in methanol or ethanol afforded high-purity
ligand in all cases except for diimine 23.
gishly so the corresponding Pd(II) complexes 1 and 2
were prepared by an in situ transmetalation-complex-
ation protocol from the more reactive precursor Pd-
(PhCN)2Cl2.32 The two methods are illustrated in Scheme
5.
1H and 13C NMR analysis of the Pd-bound methyl
groups (Pd-Me) in 1-7 indicated clear electronic effects.
Figure 1 illustrates chemical shift trends versus the
Hammett substituent constants (σp) of the ligand sub-
stituents.33 Resonances were overall shifted downfield
for complexes bearing withdrawing ligands and upfield
for donating ligands. It is interesting to note an inver-
sion point in the dependence of 1H chemical shift upon
the Hammet constant of the substituents. The most
electron-rich complexes 6 and 7 exhibited resonances
in 1H NMR spectra that appeared more downfield than
expected. We propose this results from two competing
effects on the proton chemical shift: one is electro-
negativity effect and the second is magnetic anisotropy
effect. As shown in X-ray crystal structures by Brookhart
for a complex similar to 4,34 the methyl group is in the
magnetic deshielding region of the two phenyl rings that
are roughly perpendicular to the coordination plane. For
the complexes containing strong electron-donating groups
Synthesis and Spectroscopic Studies of Pd(II)-
(r-diimine)MeCl Complexes 1-7 and [Pd(II)(r-
diimine)Me(CO)]+ Salts. Complexation of electron-
rich and neutral R-diimines to Pd(II) proceeded by the
displacement of 1,5-cyclooctadiene (COD) from Pd-
(COD)MeCl to afford complexes 3 and 5-7.31 The
electron-poor ligands 25 and 26 displaced COD slug-
(27) Bumagin, N. A.; Luzikova, E. V. J. Organomet. Chem. 1997,
532, 271-273.
(28) Kajigaeshi, S.; Kakinami, T.; Inoue, K.; Kondo, M.; Nakamura,
(31) Salo, E. V.; Guan, Z. Organometallics 2003, 22, 5033-5046.
(32) Vanasselt, R.; Gielens, E. E. C. G.; Rulke, R. E.; Vrieze, K.;
Elsevier: C. J. J. Am. Chem. Soc. 1994, 116, 977-985.
(33) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165-
195.
(34) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.;
Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686-6700.
H.; Fujikawa, M.; Okamoto, T. Bull. Chem. Soc. Jpn. 1988, 61, 597-
599.
(29) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11,
513-519.
(30) Tom Dieck, H.; Svoboda, M.; Greiser, T. Naturforschung 1981,
36B, 823-832.

1148 Organometallics, Vol. 24, No. 6, 2005
Popeney and Guan
1
Figure 1. Chemical shifts for Pd-Me resonances of complexes 1-7 in H NMR (left) and 13C NMR (right).
Scheme 6
(6 and 7), the electron-rich aromatic rings lead to larger
deshielding magnetic anisotropy effects from their ring
current, which counterbalances the electronegativity
effect and makes the 1H chemical shift more downfield
than expected. In support of this proposal, resonances
of the methine protons (CHMe2) of the isopropyl groups
of complexes 6 and 7, clearly within the aromatic
deshielding region, were also unexpectedly downfield.
used to quantitatively represent electronic effects in this
system of Pd(II) complexes.
B. Ethylene Polymerization Studies Using the
Pd(II)-r-Diimine Catalysts. Polyethylene Molecu-
lar Weight (MW). In this study a direct in situ
activation of the methyl chlorides 1-7 was used.35 To
initiate polymerization, complexes 1-7 were activated
in the presence of ethylene by the addition of excess
NaBAF. The molecular weight of polyethylene obtained
by catalysts 1-7 under 1 atm ethylene pressure and
room temperature was measured by multiangle light
scattering (MALS) analysis (Table 1). Catalyst 6 is
omitted since it gave unusually low activities and low
molecular weight (MW) polymer, which we attribute to
the previously mentioned inability to completely purify
the ligand. The polymerizations were run at relatively
long time to ensure the polymers achieved the maxi-
mum molecular weights. A significant dependence on
substituent was seen in the data, with an overall trend
of higher MW polymer produced from catalysts bearing
more electronic-donating ligands. The dependence of
polymerization rate (turnover frequency, TOF) on sub-
stituent follows a similar trend. It should be noted that
these observed trends might be partially complicated
by the catalyst stability. It was generally observed that
the more electron-deficient catalysts are less thermally
stable. Conversely, the more electron-rich catalysts are
more thermally stable and tend to afford higher polym-
erization productivities. The polymer MW is the result
of complex interplay of monomer insertion, chain trans-
fer, and potential catalyst decomposition. For a stable
catalyst, the polymer MW is primarily controlled by the
relative rates of monomer insertion and chain transfer
(kins/kCT). The similar trend in the dependence of MW
and polymerization rate on ligand substituent suggests
that the influence of ligand electronics on MW is
primarily through the effect on insertion rate, kins. A
more electron-donating ligand may better stabilize the
transition state for monomer insertion than for chain
transfer, resulting in a more productive catalyst that
produces higher MW.
The IR spectrometric measurement of CO stretching
frequencies of metal carbonyl compounds is a well-
known method of establishing the relative electrophi-
licity of a metal center. This technique was recently
employed to measure electronic effects in the study of
a Pt(II) system for C-H activation,22 which screened
several N-aryl R-diimine ligands spanning a similar σ
range as in our study. Reaction of carbon monoxide to
solutions of 1-7 and sodium tetrakis(3,5-bis(trifluoro-
methyl)phenyl)borate (NaBAF) afforded the desired [Pd-
(R-diimine)carbonylmethyl]+BAF- salts 28-34 (Scheme
6). NMR and IR characterizations confirmed the struc-
ture of the carbonyl complexes by the existence of the
Pd-Me group shifted slightly downfield in 1H NMR
compared to complexes 1-7, the presence of BAF-
1
signals in the H and 13C NMR, and the clearly visible
CO IR stretching band from 2129 to 2140 cm-1. The
possibility of methyl migration to form an acyl complex
was ruled out by the absence of an acyl carbonyl
resonance in 13C NMR, the lack of an acyl methyl signal
in the 1H NMR, and the presence of a CO ligand signal
in the 13C NMR.
The electron-donating ligands led to red-shifted CO
stretching frequencies, in accordance with a more
electron-rich metal capable of stronger M f CO π
donation. The range in νCO for complexes 28-34, about
10 cm-1, is similar to the range reported for the Pt(II)-
R-diimine system.22 When the stretching frequencies
were plotted versus the Hammet constant (σ), as shown
in Figure 2, a linear correlation was obtained. Despite
the large dihedral angle between the aryl rings and the
R-diimine backbone,3 the substituents are still able to
significantly influence the electronic properties of the
ligands and complexes. The quality of the correlation
between the CO stretching frequency and substituent
constant confirms that the Hammet constants can be
Polyethylene Branching Topology. The branching
density (B) is rather constant for the polyethylenes
made with different catalysts (Table 1). Our previous

Ligand Electronic Effects on Palladium-R-Diimine
Organometallics, Vol. 24, No. 6, 2005 1149
as expressed by smaller Rg values.6,7,10 A clear depen-
dence of polymer branching topology on substituent can
be seen from the data in Figure 3. At constant MW, the
most electron-deficient catalyst 1 afforded polymer with
appreciably smaller value of Rg than polymer made by
the most electron-rich catalyst 7. Given the relatively
constant branching density, the change in molecular
size at constant MW can only be accounted for by the
change in polymer branching topology. Therefore, at
exactly the same polymerization conditions, the poly-
mers made by the more electron-deficient catalysts are
more dendritic than those made by the electron-rich
catalysts.
We propose that the change in polymer topology is
due to the influence of catalyst electronic structure on
the polymerization kinetics. As discussed previously for
ethylene polymerization using the prototype Pd(II)-R-
diimine catalyst 4,6-10 the polymer branching topology
is controlled by the competition between ethylene inser-
tion rate and chain walking rate. Because ethylene
insertion and chain walking have zero- and inverse first-
order dependence on ethylene concentration (or pres-
sure), respectively,34 changing ethylene pressure will
alter the relative rates for the two events, which
subsequently results in a change in polyethylene branch-
ing topology. At low ethylene pressure, the chain walk-
ing process becomes faster than the insertion process.
After each ethylene insertion, the Pd(II) catalyst can
walk randomly on the polymer chain and the next
insertion can occur at a random site on the polymer.
Fine mechanistic studies by Brookhart and co-workers
have also shown that the Pd(II) catalyst can easily walk
through a tertiary carbon center.34 The random walk
of catalyst followed by monomer insertion in a statistical
manner leads to the formation of dendritic polymer
Figure 2. Plot of carbonyl ligand stretching frequencies
versus Hammett substituent constant for complexes 28-
34.
Table 1. Ethylene Polymerization Dataa
c
d
entry catalyst
σ
Bb
Mn
Mw/Mn
TONe
TOFf
1
2
3
4
5
6
1
2
3
4
5
7
0.78 100 133
1.81
1.44
1.41
1.42
1.40
1.36
280
8800
16
420
540
0.54
0.23
0
95 286
98 307
95 358
94 271
95 462
11 000
30 000 1700
-0.17
-0.83
12 000
580
23 000 1300
a Polymerization conditions: 1.0 atm ethylene, 25 °C, 20 h, 10.0
µmol catalyst, 2 equiv NaBAF, 40 mL CH2Cl2. b B ) branches per
1000 carbon. c Mn ) number-averaged MW in kg/mol, Mw
)
weight-averaged MW in kg/mol. d Polydispersity index. e TON )
turnover number (moles of ethylene polymerized per mole of
catalyst). f TOF ) turnover frequency (TON per hour).
6-10
topology at low monomer concentrations.
In our current investigation with fixed polymerization
conditions, the ligand electronic effects on the polym-
erization kinetics (rate constant, k) must account for the
change of polymer topology. Our data suggest that the
more electron-rich catalyst (7) leads to a higher kins
/
kwalking ratio and, therefore, affords a more linear
polymer. Conversely, the more electron-deficient cata-
lyst (1) should have a lower kins/kwalking ratio and affords
a more dendritic polymer. Our proposal offered in the
last section to explain the trend of MW could be used
to rationalize these effects on polymer topology as well.
A more electron-donating ligand may better stabilize the
transition state for monomer insertion than for chain
walking, which should result in a higher kins/kwalking ratio
and afford a more linear polymer. To the best of our
knowledge, this is the first report of ligand electronic
effects on polymer topology for olefin polymerization
catalysts. This provides a fundamentally different ap-
proach (catalyst approach) to control polyolefin branch-
ing topology, which complements our previous strategy
of controlling polymer topology by polymerization condi-
tions.6,10
Figure 3. Logarithmetic plot of Rg versus Mn for polyeth-
ylenes made by catalysts 1, 3, and 7, at 25 °C under general
conditions and 20 h reaction times.
studies have shown that while the overall branching
density B is relatively independent of reaction condi-
tions,4-6 the topology varies significantly with olefin
concentration (or pressure), ranging from relatively
linear with short branches to hyperbranched and den-
dritic. To determine the polymer branching topology,
multiangle light scattering coupled to size exclusion
chromatography (SEC) was employed to analyze the
polyethylenes.6,7,10 A logarithmic plot of polymer radius
of gyration (Rg) as a function of MW, or Mw specifically,
is shown in Figure 3. Compared at constant MW, more
dendritic polymers should have smaller molecular size
C. Copolymerization Studies with Methyl Acry-
late Using the Pd(II)-r-Diimine Catalysts. Incor-
poration Ratio of Polar Monomer. Copolymeriza-
tions of ethylene with methyl acrylate (MA) were carried
out by adding a known volume of MA to the polymeri-
zation mixture prior to catalyst activation. The reactions

1150 Organometallics, Vol. 24, No. 6, 2005
Popeney and Guan
Scheme 7
Table 2. Copolymerization Activities (TOF),
Incorporation Ratios (Ir), and Mn for Copolymers
at Varying Initial Concentrations of MA (in mol/L)
under Copolymerization Conditionsa
as a stable resting state, which is illustrated in Scheme
7.5 Opening of the chelate by ethylene to give intermedi-
ate 41 is necessary for polymerization to continue, an
unfavorable process, with measured Keq values far less
than unity.5 The strength of chelation, and the resulting
ease of chelate opening, should depend on the electron-
donating ability of the ligand. An electrophilic Pd(II)
should increase the strength of the Pd-O bond, mani-
fested as increased metal oxophilicity.36 The chelate
intermediate is stabilized in complexes bearing electron-
deficient ligands, leading to a decrease in Keq. The
greatly reduced activity of catalysts such as 1 occurred
in conjunction with low molecular weights, likely the
consequence of a low polymerization rate. Electron-rich
metals of catalysts such as 7 have a decreased tendency
to bind to electronegative groups, such as ester func-
tionalities, making chelate opening more favorable and
copolymerization faster overall.
TOFb
Ir (%)
Mn (kg/mol)
catalyst 0c 0.12 0.58 1.67 0.12 0.58 1.67 0.12 0.58 1.67
d
1
2
3
4
5
6
7
220 10
0
0
0
0.1
0.5
-
-
-
-
6.4
17
33
-
-
2000 40 <5
1.2
2.4
5.7 -
1800 130 30 <5 0.7
2600 130 10 <5 0.5
2.0 -
8.6 5.4
2.2 6.2 56
1600 160 50
20 0.7
920 170 30 < 5 0.6
2400 180 60 20 0.9
2.6 4.9 62 15
2.9 6.1 41 12
3.6 8.4 66 10
0.56
4.7
2.4
a Copolymerization conditions: general conditions followed for
6.5 h reaction times with an added 0.5, 2.5, or 8.0 mL of MA. b TOF
values of “<5” means a trace of recovered polymer (0.01 g or less),
while “0” indicates negligible polymer. c Ethylene homopolymer-
izations run for the same duration (6.5 h) shown for comparison.
d Data not available.
were run at room temperature and 1 atm ethylene
pressure. Data for copolymerizations are shown in Table
2. The incorporation ratio (Ir) was calculated from H
Conclusions
1
A series of bis-(aryl)-R-diimine ligands having a range
of electron-donating and -withdrawing substituents
were successfully synthesized with the purpose to probe
the ligand electronic effects on late transition metal
olefin polymerization catalysts. The corresponding Pd-
(II)(R-diimine)MeCl complexes 1-7 and [Pd(II)(R-di-
imine)Me(CO)]+BAF- complexes 28-34 were then pre-
pared and characterized. NMR chemical shift analysis
of complexes 1-7 and IR analysis of the CO stretching
frequencies of the carbonyl complexes 28-34 confirmed
that substitution does affect the electrophilicity of the
metal center in a systematic manner. Complexes pos-
sessing ligands with donating substituents exhibited CO
stretching bands that were shifted toward lower fre-
quencies. The trend correlated well to the Hammet
constant (σ) of the substituent.
Ethylene homopolymerizations and MA copolymer-
izations were carried out at room temperature and 1
atm of ethylene pressure using the series of complexes
1-7. Electronic perturbations due to substitution on the
R-diimine ligands were found to affect polymer proper-
ties such as molecular weight and branching topology.
Overall, catalysts with electron-donating ligands were
found to afford polymer with higher MW. It was
observed for the first time that the ligand electronic
structure of catalysts had a significant effect on topology
of the polyethylene formed. More dendritic polyethylene
was obtained with catalysts bearing more strongly
electron-withdrawing ligands. More dramatic electronic
effects were observed for ethylene copolymerization with
MA using these catalysts. Electron-donating ligands led
NMR analysis by the known method.4 A strong depen-
dence of Ir on substituent was observed for the catalysts.
The catalysts bearing more electron-donating ligands
exhibited significantly higher incorporation ratios than
less electron-rich analogues. For example, for copolym-
erization at initial MA concentration of 0.12 M, the Ir
increased from 0.1 to 0.9 mol % as the ligand substituent
was changed from -NO2 (1) to -NMe2 (7). The trend
was general over the three concentrations of MA exam-
ined, and deactivation of the catalysts was observed in
the order 1, 2, 3, then 4 as MA concentration increased.
This trend has also been observed previously in two
related R-diimine catalysts.5 The olefin group of MA is
a poorer σ-donor compared to ethylene. Presumably,
decreasing the electrophilicity of the metal center
increases the binding of the electron-deficient olefin
(MA) relative to ethylene, which leads to an increase in
the incorporation of MA.
Catalyst Tolerance to Polar Comonomer. We
have observed significant differences in catalytic activity
and polymer molecular weight obtained in copolymer-
izations using catalysts 1-7, and the results are shown
in Table 2. Both TON and TOF define the total
monomer turnover and were calculated using an aver-
aged monomer molecular weight based on the value of
Ir. Electron-deficient catalysts were apparently “poi-
soned” at sufficient MA concentrations, affording a
negligible amount of polymer. It has been shown that
all Pd(II)-R-diimine catalysts exhibit a considerable
decrease in activity in the presence of polar comonomers
such as MA; however those bearing strongly electron-
donating ligands were affected to a lesser degree. The
existence of an ester chelate complex, such as 40,
following polymer insertion into MA has been proposed
(35) Schmid, M.; Eberhardt, R.; Klinga, M.; Leskela, M.; Rieger, B.
Organometallics 2001, 20, 2321-2330.
(36) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; 3rd ed.; John Wiley and Sons: New York, 2001.

Ligand Electronic Effects on Palladium-R-Diimine
Organometallics, Vol. 24, No. 6, 2005 1151
not only to higher incorporation ratios of MA in copoly-
mers but also to a decreased degree of deactivation in
the presence of MA. Higher catalytic activity toward
copolymerization was observed in these more electron-
rich catalysts, along with the production of higher MW
copolymer.
2,6-Dibromo-4-methoxyaniline (13).28 To a solution of
p-anisidine (2.00 g, 16.2 mmol) in 150 mL of CH2Cl2 and 50
mL of methanol were added 6.05 g of CaCO3 (60.4 mmol) and
12.98 g of benzyltrimethylammonium tribromide (33.29 mmol).
The resulting deep purple suspension was stirred for 2 h and
then filtered. The filtrate was concentrated in vacuo and
extracted into CH2Cl2 with dilute HCl added to break up
emulsions. The organic phases were combined and washed
with brine. The product was purified by flash chromatography
through silica gel and with 1:1 hexanes/CH2Cl2 to afford 13
as a yellow-orange solid (3.10 g, 70%): 1H NMR (500 MHz,
CDCl3) δ 3.72 (s, 3H), 4.19 (br s, 2H), 7.01 (s, 2H); 13C NMR
(125 MHz, CDCl3) δ 56.6, 109.6, 118.5, 136.6, 152.5.
Experimental Section
General Considerations. The catalyst handling was car-
ried out in a Vacuum Atmospheres glovebox filled with
nitrogen. All other moisture- and air-sensitive reactions were
carried out in flame-dried glassware using magnetic stirring
under a positive pressure of argon or nitrogen. Removal of
organic solvents was accomplished by rotary evaporation and
is referred to as concentrated in vacuo. Flash column chro-
matography was performed using forced flow on EM Science
230-400 mesh silica gel. NMR spectra were recorded on
Bruker DRX400 and DRX500 FT-NMR instruments. Proton
and carbon NMR spectra were recorded in ppm and were
referenced to indicated solvents at indicated temperature, if
different from ambient. Data were reported as follows: chemi-
cal shift, multiplicity (s ) singlet, d ) doublet, t ) triplet, q )
quartet), integration, and coupling constant(s) in hertz (Hz).
Multiplets (m) were reported over the range (ppm) at which
they appear at the indicated field strength. IR spectra were
recorded on a Midac Prospect PRS-102 spectrometer. Elemen-
tal analysis (for new compounds) was preformed by Atlantic
Microlab, Norcross, GA.
Materials. Toluene, tetrahydrofuran (THF), diethyl ether,
and dichloromethane were purified by passing through solvent
purification columns following the method introduced by
Grubbs and are referred to as dry.37 Unless otherwise stated,
all other solvents and reagents were purchased from com-
mercial suppliers and used as received. Anilines 8 and 9,25
unsubstituted ligand, and Pd complex 1,3,4 isopropenylboronic
acid (18),38 Pd(COD)MeCl,39 and sodium tetrakis(3,5-bis-
(trifluoromethyl)phenyl)borate40 (NaBAF) were prepared by
known literature procedures. MA was uninhibited by passing
through oven-dried basic alumina prior to use in polymeriza-
tions.
ArNdC(Me)-C(Me)dNAr, Ar
≡
2,6-iPr2-4-MeC6H2-
(
MeN∧N, 22).27,30 A round-bottom flask was charged with 3.00
g (11.3 mmol) of 2,6-dibromo-4-methylaniline (12) and 150 mg
of PdCl2(dppf) and placed under high vacuum for 20 min. The
flask was then filled with nitrogen and 40 mL of dry THF and
then stirred at rt until all solid had dissolved. The flask was
then cooled to -78 °C, and to this solution was transferred
90.6 mL (45.3 mmol) of a solution of isopropenylmagnesium
bromide (0.5 M) in THF with stirring. Upon completion of
addition, the mixture was allowed to warm to rt and then
heated to reflux for 20 h. The mixture was cooled and carefully
quenched with 50 mL of 2 M HCl. Aqueous NaOH was added
until basic, and the mixture was extracted into diethyl ether.
The organic layer was then washed with brine before being
dried over Na2SO4 and concentrated. The crude oil (14) was
dissolved in 30 mL of methanol and 250 mg of 10% by wt Pd/C
was added. The mixture was stirred under hydrogen for 24 h.
The catalyst was replaced by filtering through Celite, adding
a new 250 mg quantity of Pd/C, and placed under a hydrogen
atmosphere for another 24 h. The replacement procedure was
repeated one last time before filtering after 3 days total
reaction time. The mixture was concentrated in vacuo, ex-
tracted in diethyl ether, dried over Na2SO4, and concentrated
again to give crude 16. The concentrated oil was then dissolved
in 30 mL of methanol, and to this solution were added two
drops of formic acid and 295 µL of 2,3-butanedione (290 mg,
3.36 mmol). The solution was then heated to reflux and stirred
for 3 days. The mixture was concentrated in vacuo, and the
residue was recrystallized from methanol to give 22 as pale
yellow-green crystals (260 mg). Flash chromatography of the
concentrated mother liquor afforded an additional 142 mg of
22 (16%) and 324 mg of monocondensed byproduct: 1H NMR
(400 MHz, CDCl3) δ 1.15 (d, 12H, J ) 6.9 Hz), 1.17 (d, 12H, J
) 7.0 Hz), 2.05 (s, 6H), 2.35 (s, 6H), 2.68 (septet, 4H, J ) 6.9
Hz), 6.96 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 16.5, 21.36,
22.7, 23.0, 28.4, 123.7, 132.7, 134.9, 143.7, 168.5. Anal. Calcd
for C30H44N2: C, 83.28; H, 10.25; N, 6.47. Found: C, 83.04; H,
10.30; N, 6.45.
4-Chloro-2,6-diisopropylaniline (11).26 To a solution of
10.0 mL (9.40 g, 53.0 mmol) of 2,6-diisopropylaniline (10) and
13.3 mL of 12.0 N HCl (159 mmol) in isobutanol (40 mL) was
added 6.31 mL (55.7 mmol) of 30% aqueous H2O2. The mixture
was stirred and heated at reflux for 20 h. The solvent was
removed in vacuo, ethyl acetate was added (100 mL), and the
solution was washed in water. After extracting twice more with
ethyl acetate (50 mL) the organic fractions were combined and
washed once with brine. The mixture was concentrated and
separated by flash chromatography on silica with 6:1 hexanes/
CH2Cl2 to give 10 as a reddish oil (7.46 g, 33%). NMR spectra
were found to match a literature report.41
(37) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.
(38) Braun, J.; Normant, H. Bull. Soc. Chim. Fr. 1966, 2557.
(39) Rulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.;
Vanleeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769-
5778.
(40) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992,
11, 3920-3922.
(41) Dombroski, M. A.; Eggler, J. F. (Pfizer Inc., USA) PCT Int. Appl.
Wo, 1998; p 81.

1152 Organometallics, Vol. 24, No. 6, 2005
Popeney and Guan
ArNdC(Me)-C(Me)dNAr, Ar ≡ 2,6-iPr2-4-OMeC6H2
dried 5 Å molecular sieves. The mixture was heated to reflux
and stirred for 2 days. The mixture was concentrated in vacuo,
and diethyl ether (20 mL) was added. The resultant slurry was
filtered and washed again with diethyl ether and dried to
afford ligand 25 as a yellow-green solid (4.516 g, 49%): 1H
NMR (500 MHz, CD2Cl2) δ 1.20 (d, 12H, J ) 6.8 Hz), 1.27 (d,
12H, J ) 6.9 Hz), 2.09 (s, 6H), 2.73 (septet, 4H, J ) 6.9 Hz),
8.07 (s, 4H); 13C NMR (125 MHz, CDCl3) δ 17.5, 22.7, 23.1,
119.8, 136.9, 145.3, 152.1, 168.3. Anal. Calcd for C28H38N4O4:
C, 67.99; H, 7.74; N, 11.33. Found: C, 67.52; H, 7.79; N, 10.81.
(
OMeN∧N, 23).27,30 A round-bottom flask was charged with 3.51
g (12.5 mmol) of 13 and 150 mg of PdCl2(dppf) and placed
under high vacuum for 20 min. The flask was then filled with
nitrogen and 40 mL of dry THF and then stirred at rt until
all solid had dissolved. The flask was then cooled to -78 °C,
and to this solution was transferred 100.0 mL (50.0 mmol) of
a solution of isopropenylmagnesium bromide (0.5 M) in THF
with stirring. Upon completion of addition, the mixture was
allowed to warm to rt and then heated to reflux for 20 h. The
mixture was cooled and carefully quenched with 50 mL of 2
M HCl. Aqueous NaOH was added until basic, and the mixture
was extracted into diethyl ether. The organic layer was then
washed with brine before being dried over Na2SO4 and
concentrated. The crude oil (15) was dissolved in 30 mL of
methanol, and 250 mg of 10% by wt Pd/C was added. The
mixture was stirred under hydrogen for 24 h. The catalyst was
replaced by filtering through Celite, adding a new 250 mg
quantity of Pd/C, and placed under a hydrogen atmosphere
for 24 h. The replacement procedure was repeated one last
time before filtering after 3 days total reaction time. The
mixture was concentrated in vacuo, extracted in diethyl ether,
dried over Na2SO4, and concentrated again to give crude 17.
The concentrated oil was then dissolved in 20 mL of methanol,
and to this solution were added two drops of formic acid and
172 µL of 2,3-butanedione (1.96 mmol), based on an estimated
aniline purity of 80%. The solution was then heated to reflux
and stirred for 3 days. The mixture was concentrated in vacuo,
and the residue was recrystallized from methanol to give 23
as pale yellow-green crystals. NMR analysis indicated the
existence of an impurity that was not removed by additional
recrystallization attempts (192 mg, 19%): 1H NMR (500 MHz,
CDCl3) δ 1.16 (d, 12H, J ) 6.9 Hz), 1.17 (d, 12H, J ) 7.0 Hz),
2.05 (s, 6H), 2.70 (septet, 4H, J ) 6.9 Hz), 3.83 (s, 6H), 6.96
(s, 2H); 13C NMR (125 MHz, CDCl3) δ 16.6, 22.67, 22.95, 28.7,
55.3, 108.6, 136.6, 139.8, 156.3, 169.3. Anal. Calcd for
C30H44N2O2: C, 77.54; H, 9.54; N, 6.03. Found: C, 76.82; H,
9.48; N, 6.04.
ArNdC(Me)-C(Me)dNAr, Ar ≡ 2,6-iPr2-4-CF3C6H2
(
CF3N∧N, 26).29 A round-bottom flask was charged with 2.60
g (8.15 mmol) of 2,6-dibromo-4-(trifluoromethyl)aniline (19)
and 500 mg of Pd(PPh3)4 (5.3 mol %) and placed under high
vacuum for 20 min. The flask was filled with argon and 100
mL of 1,4-dioxane. To this solution was transferred a solution
of 2.10 g of 18 (24.5 mmol) in 30 mL of ethanol and 25 mL of
2 M aqueous K2CO3. The mixture was heated to reflux with
stirring for 24 h, filtered through a Celite and silica plug, and
extracted into diethyl ether. The organic layer was dried over
Na2SO4 and concentrated to give a pale greenish oil. The crude
oil (20) was dissolved in 30 mL of methanol, and 250 mg of
10% by wt Pd/C was added. The mixture was stirred under
hydrogen for 24 h. The catalyst was replaced by filtering
through Celite, adding a new 250 mg quantity of Pd/C, and
placed under a hydrogen atmosphere for 24 h. The replacement
procedure was repeated one last time before filtering after 3
days total reaction time. The mixture was concentrated in
vacuo, extracted in diethyl ether, dried over Na2SO4, and
concentrated again to give crude 21. The concentrated mixture
yielded 1.72 g of brownish oil that was dissolved in 120 mL of
benzene. To this solution were added 50 mg of 10-CSA and
232 µL of 2,3-butanedione (227 mg, 2.64 mmol), based on an
estimated aniline purity of 80%. The flask was then fitted with
a medium Soxhlet extractor which was filled halfway with
oven-dried 5 Å molecular sieves. The mixture was heated to
reflux and stirred for 3 days. The mixture was concentrated
in vacuo, and the residue was recrystallized from methanol
to give 26 as pale yellow crystals (425 mg). Further condensa-
tion of the recovered mother liquor by adding an additional
161 µL of 2,3-butanedione afforded 381 mg of 26 after 3 days
to give a total yield of 26%: 1H NMR (500 MHz, CDCl3) δ 1.18
(d, 12H, J ) 6.9 Hz), 1.23 (d, 12H, J ) 6.9 Hz), 2.07 (s, 6H),
2.71 (septet, 4H, J ) 6.9 Hz), 7.40 (s, 2H); 13C NMR (125 MHz,
CDCl3) δ 16.7, 22.36, 22.73, 28.7, 120.3, 125.9, 126.2, 135.7,
148.9, 168.2. Anal. Calcd for C30H38N2F6: C, 66.65; H, 7.08;
N, 5.18. Found: C, 66.53; H, 7.11; N, 5.10.
ArNdC(Me)-C(Me)dNAr, Ar ≡ 2,6-iPr2-4-NMe2C6H2
(
NMe2N∧N, 24).30 To a solution of 9 (6.69 g, 30.4 mmol) in
methanol (20 mL) was added 1.07 mL (1.05 g, 12.1 mmol) of
2,3-butanedione and five drops of formic acid. The mixture was
heated to reflux for 3.5 h with stirring. The solution was
concentrated in vacuo, and the residue was washed with
methanol (2 × 20 mL) and filtered. The resulting solid was
recrystallized in methanol to afford 24 as a bright orange solid
(3.142 g, 21%): 1H NMR (500 MHz, CDCl3) δ 1.17 (d, 12H, J
) 6.7 Hz), 1.18 (d, 12H, J ) 6.8 Hz), 2.06 (s, 6H), 2.72 (septet,
4H, J ) 6.9 Hz), 2.95 (s, 12H), 6.62 (s, 4H); 13C NMR (125
MHz, CDCl3) δ 17.1, 23.2, 23.5, 41.9, 109.0, 136.4, 138.3, 148.1,
169.7. Anal. Calcd for C32H50N4: C, 78.31; H, 10.27; N, 11.42.
Found: C, 78.55; H; 10.36; N, 11.34.
ArNdC(Me)-C(Me)dNAr, Ar ≡ 2,6-iPr2-4-ClC6H2 (ClN∧N,
27). To a solution of 11 (1.72 g, 8.12 mmol) in 120 mL of
benzene was added 321 µL of 2,3-butanedione (315 mg, 3.65
mmol) followed by 150 mg of 10-camphorsulfonic acid. The
flask was then fitted with a medium Soxhlet extractor which
was filled halfway with oven-dried 5 Å molecular sieves. The
mixture was heated to reflux and stirred for 2 days. The
mixture was concentrated in vacuo, and the residue was
recrystallized from ethanol to give 27 as pale yellow crystals
(0.981 g, 51%): 1H NMR (400 MHz, CDCl3) δ 1.15 (d, 12H, J
) 6.9), 1.17 (d, 12H, J ) 6.9 Hz), 2.04 (s, 6H), 2.65 (septet,
ArNdC(Me)-C(Me)dNAr, Ar ≡ 2,6-iPr2-4-NO2C6H2
(
NO2N∧N, 25). To a solution of 8 (9.187 g, 41.33 mmol) in 120
mL of benzene was added 1.63 mL of 2,3-butanedione (1.61 g,
18.7 mmol) followed by 200 mg of 10-camphorsulfonic acid.
The flask was then fitted with a medium Soxhlet extractor
with condenser, and the extractor was filled halfway with oven-

Ligand Electronic Effects on Palladium-R-Diimine
Organometallics, Vol. 24, No. 6, 2005 1153
4H, J ) 6.9 Hz), 7.12 (s, 4H); 13C NMR (100 MHz, CDCl3) δ
16.5, 22.35, 22.68, 28.6, 123.2, 129.1, 137.0, 144.4, 168.7. Anal.
Calcd for C28H38N2Cl2: C, 71.02; H, 8.09; N, 5.92. Found: C,
71.08; H, 8.11; N, 5.86.
(ClN∧N)PdMeCl (3).31 To a solution of ligand 27 (400 mg,
0.845 mmol) in dry CH2Cl2 (20 mL) was added 213 mg (0.807
mmol) of Pd(COD)MeCl. After stirring the mixture for 2 days
at room temperature, it was concentrated in vacuo to give a
red-orange residue that was washed with hexanes (2 × 5 mL).
The remaining solid was then dried in vacuo for 2 days to yield
the complex 3 as an orange solid (476 mg, 89%): 1H NMR (500
MHz, CDCl3) δ 0.53 (s, 3H), 1.16 (t, 12H, J ) 6.9 Hz), 1.34 (d,
6H, J ) 6.8 Hz), 1.41 (d, 6H, J ) 6.8 Hz), 2.04 (s, 3H), 2.06 (s,
3H), 3.00 (septet, 2H, J ) 6.9 Hz), 3.02 (septet, 2H, J ) 6.9
Hz), 7.19 (s, 2H), 7.24 (s, 2H); 13C NMR (125 MHz, CDCl3) δ
3.4, 19.8, 21.2, 22.96, 23.22, 23.54, 23.59, 28.8, 29.2, 123.9,
124.5, 132.7, 133.5, 139.93, 140.01, 140.04, 140.60, 169.8,
174.4. Combustion (C, H, N): Anal. Calcd for C29H41PdN2Cl3:
C, 55.25; H, 6.56; N, 4.44. Found: C, 54.66; H, 6.48; N, 4.08.
LRMS (ES/MS, m/z in MeCN): Anal. Calcd for C29H41PdN2-
Cl3: 634 (M - Cl- + MeCN). Found: 634.
(
NO2N∧N)PdMeCl (1).32 To a -35 °C solution of (PhCN)2-
PdCl2 (513 mg, 1.34 mmol) in 5.0 mL of dry CH2Cl2 was added
454 µL (598 mg, 3.34 mmol) of tetramethyltin under argon.
This mixture was stirred at -35 °C for 2.5 h, after which a
solution of ligand 25 (551 mg, 1.114 mmol) in 3.0 mL of dry
CH2Cl2 was added. The reaction mixture was kept at low
temperature for 15 min then warmed to rt. The mixture was
stirred overnight and then placed directly on a silica gel
column for purification by flash chromatography with 5:4:1
hexanes/CH2Cl2/EtOAc as the mobile phase. The orange frac-
tion was concentrated and dried in vacuo overnight to give 1
as an orange solid (533 mg, 74%): 1H NMR (500 MHz, CDCl3)
δ 0.55 (s, 3H), 1.23 (d, 6H, J ) 7.8 Hz), 1.25 (d, 6H, J ) 7.3
Hz), 1.42 (d, 6H, J ) 6.8 Hz), 1.48 (d, 6H, J ) 6.7 Hz), 2.09 (s,
3H), 2.13 (s, 3H), 3.05 (septet, 2H, J ) 6.9 Hz), 3.09 (sep, 2H,
J ) 6.9 Hz), 8.14 (s, 2H), 8.19 (s, 2H); 13C NMR (125 MHz,
CDCl3) δ 4.5, 20.6, 21.9, 23.3, 23.7, 23.9, 24.0, 29.6, 30.0, 119.9,
120.5, 140.5, 141.3, 146.8, 147.0, 147.5, 147.9, 169.9, 174.7.
Combustion (C, H, N): Anal. Calcd for C29H41PdN4O4Cl: C,
53.46; H, 6.34; N, 8.60. Found: C, 53.45; H, 6.49; N, 8.28.
LRMS (ES/MS, m/z in MeCN): Anal. Calcd for C29H41PdN4O4-
Cl: 656 (M - Cl- + MeCN). Found: 656.
(
MeN∧N)PdMeCl (5).31 To a solution of ligand 22 (245 mg,
0.566 mmol) in dry CH2Cl2 (20 mL) was added 140 mg (0.528
mmol) of Pd(COD)MeCl. After stirring the mixture for 18 h at
room temperature, it was concentrated in vacuo to give a dark
brown residue, which was washed with hexanes (2 × 5 mL).
The remaining solid was then dried in vacuo for 2 days to yield
the complex 5 as an orange solid (284 mg, 85%): 1H NMR (500
MHz, CDCl3) δ 0.52 (s, 3H), 1.15 (dd, 12H, J ) 6.9 Hz, 1.9
Hz), 1.33 (d, 6H, J ) 6.9 Hz), 1.41 (d, 6H, J ) 6.9 Hz), 2.02 (s,
3H), 2.04 (s, 3H), 2.34 (s, 3H), 2.37 (s, 3H), 3.02 (septet, 4H, J
) 6.7 Hz), 7.01 (s, 2H), 7.05 (s, 2H); 13C NMR (125 MHz,
CDCl3) δ 3.4, 20.0, 21.48, 21.89, 22.04, 23.61, 23.87, 24.2, 28.8,
29.3, 124.5, 125.0, 136.6, 137.5, 138.0, 138.7, 169.7, 174.3.
Combustion (C, H, N): Anal. Calcd for C31H47PdN2Cl: C, 63.15;
H, 8.03; N, 4.75. Found: C, 63.03; H, 7.93; N, 4.74. LRMS (ES/
MS, m/z in MeCN): Anal. Calcd for C31H47PdN2Cl: 594 (M -
Cl- + MeCN). Found: 594.
(
CF3N∧N)PdMeCl (2).32 To a -35 °C solution of (PhCN)2-
PdCl2 (255 mg, 0.666 mmol) in 5.0 mL of dry CH2Cl2 was added
241 µL (318 mg, 3.20 mmol) of tetramethyltin under argon.
After stirring at -35 °C for 2.5 h a solution of ligand 26 (300
mg, 0.549 mmol) in 2.0 mL of dry CH2Cl2 was added. The
reaction mixture was warmed to rt and stirred overnight, then
placed directly on a silica gel column for purification by flash
chromatography and eluted with 5:4:1 hexanes/CH2Cl2/EtOAc.
The orange fraction was concentrated and dried in vacuo
overnight to give 2 as an orange solid (312 mg, 81%): 1H NMR
(500 MHz, CDCl3) δ 0.50 (s, 3H), 1.19 (t, 12H, J ) 6.9 Hz),
1.37 (d, 6H, J ) 6.8 Hz), 1.43 (d, 6H, J ) 6.8 Hz), 2.05 (s, 3H),
2.07 (s, 3H), 3.05 (septet, 2H, J ) 6.9 Hz), 3.08 (septet, 2H, J
) 6.9 Hz), 7.46 (s, 2H), 7.53 (s, 2H); 13C NMR (125 MHz,
CDCl3) δ 3.5, 20.2, 21.6, 23.12, 23.41, 23.71, 23.78, 28.9, 29.4,
120.8, 121.5, 123.04, 123.37, 125.20, 125.53, 129.18, 129.43,
130.13, 130.39, 139.19, 139.94, 144.40, 144.51, 170.0, 174.6.
Combustion (C, H, N): Anal. Calcd for C31H41PdN2ClF6: C,
53.38; H, 5.92; N, 4.02. Found: C, 53.50; H, 6.10; N, 3.92.
LRMS (ES/MS, m/z in MeCN): Anal. Calcd for C31H41PdN2-
ClF6: 702 (M - Cl- + MeCN). Found: 702.
(
OMeN∧N)PdMeCl (6).31 To a solution of ligand 23 (150 mg,
0.323 mmol) in dry CH2Cl2 (20 mL) was added 81 mg (0.31
mmol) of Pd(COD)MeCl. After stirring the mixture for 26 h at
room temperature, it was concentrated in vacuo to give a dark
red-orange residue, which was washed with hexanes (2 × 5
mL). The remaining oil was then dried in vacuo overnight. The
residue was dissolved in dry CH2Cl2 (1 mL) and precipitated
from hexanes (5 mL) and dried once again to yield the complex
6 as a red-orange solid (284 mg, 85%, 90% estimated purity).
An attempt at purification by flash chromatography through
silica gel as for 1 and 2 was not successful: 1H NMR (400 MHz,
CDCl3) δ 0.53 (s, 3H), 1.12 (dd, 12H, J ) 6.9 Hz, 1.9 Hz), 1.28
(d, 6H, J ) 6.9 Hz), 1.42 (d, 6H, J ) 6.9 Hz), 2.02 (s, 3H), 2.04
(s, 3H), 3.04 (septet, 2H, J ) 6.7 Hz), 3.06 (septet, 2H, J ) 6.7
Hz), 3.83 (s, 3H), 3.85 (s, 3H), 6.76 (s, 2H), 6.78 (s, 2H); 13C
NMR (100 MHz, CDCl3) δ 2.8, 19.5, 21.0, 23.00, 23.19, 23.42,
23.58, 28.6, 29.1, 54.9, 55.2, 108.6, 109.1, 135.16, 135.20,
139.30, 139.97, 158.17, 158.65, 169.8, 174.5. LRMS (ES/MS,
m/z in MeCN): Anal. Calcd for C31H47PdN2O2Cl: 626 (M -
Cl- + MeCN). Found: 626.

1154 Organometallics, Vol. 24, No. 6, 2005
Popeney and Guan
6.8 Hz), 1.31 (d, 6H, J ) 6.8 Hz), 1.38 (d, 6H, J ) 6.8 Hz),
2.28 (s, 3H), 2.39 (s, 3H), 2.67 (septet, 2H, J ) 6.9 Hz), 2.83
(septet, 2H, J ) 6.8 Hz), 7.38-7.49 (m, 6H); 13C NMR (125
MHz, CDCl3) δ 9.9 (Pd-Me), 20.0, 22.2, 23.30, 23.55, 23.80,
23.87, 29.83, 29.90, 125.5, 130.10, 130.54, 136.8, 138.02,
138.48, 142.6, 173.8 (CO), 175.8, 183.3; IR (CH2Cl2) 2135.4
cm-1 [ν(CO)].
1
[(MeN∧N)PdMe(CO)]+, 32: H NMR (500 MHz, CD2Cl2) δ
(
NMe2N∧N)PdMeCl (7).31 To a solution of ligand 24 (805.6
0.84 (s, 3H, Pd-Me), 1.19 (d, 6H, J ) 6.9 Hz), 1.24 (d, 6H, J )
6.9 Hz), 1.29 (d, 6H, J ) 6.8 Hz), 1.36 (d, 6H, J ) 6.8 Hz),
2.25 (s, 3H), 2.36 (s, 3H), 2.40 (s, 6H), 2.62 (septet, 2H, J )
6.8 Hz), 2.80 (septet, 2H, J ) 6.9 Hz), 7.17 (s, 2H), 7.18 (s,
2H); 13C NMR (125 MHz, CD2Cl2) δ 9.7 (Pd-Me), 20.2, 21.43,
21.47, 22.4, 23.18, 23.52, 23.72, 23.77, 29.66, 29.75, 125.95,
126.06, 136.7, 139.8, 140.28, 140.47, 174.1 (CO), 176.2, 183.7;
IR (CH2Cl2) 2133.7 cm-1 [ν(CO)].
mg, 1.64 mmol) in dry CH2Cl2 (20 mL) was added 414 mg (1.56
mmol) of Pd(COD)MeCl. After stirring the mixture overnight
at room temperature, it was concentrated in vacuo to give a
dark brown residue, which was washed with hexanes (3 × 10
mL). The remaining solid was then dried in vacuo for 2 days
to yield the complex 7 as a dark brown solid (885 mg, 84%):
1H NMR (400 MHz, CDCl3) δ 0.53 (s, 3H), 1.15 (d, 6H, J ) 6.9
Hz), 1.16 (d, 6H, J ) 6.9 Hz), 1.34 (d, 6H, J ) 6.8 Hz), 1.42 (d,
6H, J ) 6.8 Hz), 2.01 (s, 3H), 2.03 (s, 3H), 2.98 (s, 6H), 2.99 (s,
6H), 3.05-3.09 (m, 2H), 6.57 (s, 2H), 6.58 (s, 2H); 13C NMR
(100 MHz, CDCl3) δ 2.8, 19.5, 20.9, 23.1, 23.3, 23.7, 28.5, 29.1,
40.6, 40.7, 107.3, 107.5, 132.6, 138.7, 139.2, 149.5, 169.7, 174.4.
Combustion (C, H, N): Anal. Calcd for C33H53PdN4Cl: C, 61.20;
H, 8.25; N, 8.65. Found: C, 60.13; H, 8.11; N, 8.65. LRMS (ES/
MS, m/z in MeCN): Anal. Calcd for C33H53PdN4Cl: 652 (M -
Cl- + MeCN). Found: 652.
1
[(OMeN∧N)PdMe(CO)]+, 33: H NMR (500 MHz, CD2Cl2)
δ 0.85 (s, 3H, Pd-Me), 1.20 (d, 6H, J ) 6.8 Hz), 1.25 (d, 6H, J
) 6.8 Hz), 1.29 (d, 6H, J ) 6.8 Hz), 1.37 (d, 6H, J ) 6.8 Hz),
2.26 (s, 3H), 2.37 (s, 3H), 2.64 (septet, 2H, J ) 6.8 Hz), 2.82
(septet, 2H, J ) 6.9 Hz), 3.85 (s, 3H), 3.86 (s, 3H), 6.87 (s,
2H), 6.88 (s, 2H); 13C NMR (125 MHz, CD2Cl2) δ 9.9 (Pd-Me),
20.3, 22.4, 23.15, 23.46, 23.66, 23.69, 29.99, 30.08, 55.8, 110.6,
125.5, 131.5, 136.3, 138.8, 140.4, 160.59, 160.82, 174.0 (CO),
176.8, 184.3; IR (CH2Cl2) 2133.4 cm-1 [ν(CO)].
1
[(NMe2N∧N)PdMe(CO)]+, 34: H NMR (500 MHz, CD2Cl2)
δ 0.84 (s, 3H, Pd-Me), 1.19 (d, 6H, J ) 6.9 Hz), 1.24 (d, 6H, J
) 6.9 Hz), 1.29 (d, 6H, J ) 6.8 Hz), 1.36 (d, 6H, J ) 6.8 Hz),
2.25 (s, 3H), 2.35 (s, 3H), 2.66 (septet, 2H, J ) 6.9 Hz), 2.85
(septet, 2H, J ) 6.8 Hz), 3.02 (s, 12H), 6.60 (s, 2H), 6.61 (s,
2H); 13C NMR (125 MHz, CD2Cl2) δ 9.6 (Pd-Me), 20.1, 22.2,
23.25, 23.55, 23.69, 23.76, 29.96, 30.12, 40.52, 40.55, 107.96,
108.07, 128.5, 133.6, 138.1, 139.6, 151.25, 151.39, 174.3 (CO),
176.3, 184.1; IR (CH2Cl2) 2129.8 cm-1 [ν(CO)].
General Procedure for [Pd(diimine)Me(CO)]BAF For-
mation. To an oven-dried vial charged with 35.0 µmol of one
of complexes 1-7 and NaBAF (1.0 equiv) was added 5.0 mL
of dry CH2Cl2 under an atmosphere of CO. The solution was
stirred for 10 min under constant addition of CO, leading to a
darkening and an increase in turbidity of the solution. The
mixture was filtered through Celite and concentrated in vacuo
to afford the complexes 28-34, respectively, as yellow to deep
blue solids. Spectral data for the BAF counterion can be found
elsewhere and will not be repeated in the spectroscopic data
listed below for the cationic carbonyl complexes of 28-34.5
[(NO2N∧N)PdMe(CO)]+, 28: 1H NMR (500 MHz, CD2Cl2) δ
0.94 (s, 3H, Pd-Me), 1.29 (d, 6H, J ) 6.8 Hz), 1.34 (d, 6H, J )
6.8 Hz), 1.38 (d, 6H, J ) 6.8 Hz), 1.45 (d, 6H, J ) 6.8 Hz),
2.34 (s, 3H), 2.45 (s, 3H), 2.72 (septet, 2H, J ) 6.9 Hz), 2.87
(septet, 2H, J ) 6.8 Hz), 8,29 (s, 4H); 13C NMR (125 MHz,
CD2Cl2) δ 11.3 (Pd-Me), 21.0, 22.96, 23.02, 23.26, 23.57, 30.36,
119.7, 121.31, 121.36, 139.6, 141.3, 142.5, 146.6, 148.97,
149.28, 173.1 (CO), 177.0, 184.2; IR (CH2Cl2) 2139.8 cm-1
[ν(CO)].
General in-Situ-Activated Polymerization Procedure.
An evacuated oven-dried 100 mL two- or three-neck flask fitted
with a septum and ethylene feed was charged with 40 mL of
dry toluene. Temperature was controlled by placement in an
external bath as needed. Ethylene was added to fill the vacuum
and slowly ran through the flask and out the bubbler to
maintain ambient pressure. After 20 min with rapid stirring,
2.0 mL of a 0.0050 M solution (10.0 µmol) of complexes 1-7
in dry CH2Cl2 and 3.0 mL of NaBAF slurry (2 equiv) in dry
CH2Cl2 were then added to the mixture. The mixture was
allowed to stir at a given temperature for an allotted time
period, depending on experiment. Upon quenching with tri-
ethylsilane (0.5 mL),42 the solution was filtered through a
Celite and silica gel plug and concentrated in vacuo. Cold
methanol (40 mL) was then added, and the mixture was chilled
for 30 min at -35 °C. The suspensions were then decanted
and placed under soft vacuum overnight with heating (70 °C)
to dry the polymer.
[(CF3N∧N)PdMe(CO)]+, 29: 1H NMR (500 MHz, CD2Cl2) δ
0.91 (s, 3H, Pd-Me), 1.25 (d, 6H, J ) 6.8 Hz), 1.30 (d, 6H, J )
6.9 Hz), 1.34 (d, 6H, J ) 6.9 Hz), 1.42 (d, 6H, J ) 6.8 Hz),
2.31 (s, 3H), 2.42 (s, 3H), 2.71 (septet, 2H, J ) 6.9 Hz), 2.86
(septet, 2H, J ) 6.8 Hz), 7.68 (s, 2H), 7.69 (s, 2H); 13C NMR
(125 MHz, CD2Cl2) δ 10.8 (Pd-Me), 20.7, 22.82, 22.98, 23.29,
23.58, 30.08, 30.11, 122.90, 122.93, 138.5, 139.3, 140.15,
140.75, 173.4 (CO), 176.8, 184.1; IR (CH2Cl2) 2139.4 cm-1
[ν(CO)].
Molecular Weight and Topology Experiments. The
above procedure for in-situ-activated polymerizations was
followed using an external bath set at 25 °C. Reactions were
run for 20 h and worked up in the manner described in the
general procedure. Polymer samples were collected and char-
acterized by SEC-MALS as described below.
Copolymerizations with Methyl Acrylate. The above
procedure for in-situ-activated polymerizations was followed
using an external bath set at 25 °C, except a known volume
(either 0.5, 2.5, or 8.0 mL) of MA was added prior to catalyst
activation with NaBAF. Reactions were run for 6.5 h and then
quenched and worked up as in the general procedure. The MA
[(ClN∧N)PdMe(CO)]+, 30: H NMR (500 MHz, CD2Cl2) δ
1
0.90 (s, 3H, Pd-Me), 1.21 (d, 6H, J ) 6.8 Hz), 1.25 (d, 6H, J )
6.9 Hz), 1.30 (d, 6H, J ) 6.8 Hz), 1.37 (d, 6H, J ) 6.9 Hz),
2.29 (s, 3H), 2.40 (s, 3H), 2.63 (septet, 2H, J ) 6.8 Hz), 2.78
(septet, 2H, J ) 6.9 Hz), 7.38 (s, 2H), 7.39 (s, 2H); 13C NMR
(125 MHz, CD2Cl2) δ 10.5 (Pd-Me), 20.5, 22.7, 23.01, 23.31,
23.58, 29.99, 30.05, 125.5, 126.00, 126.06, 135.9, 136.34,
136.53, 139.2, 140.8, 173.6 (CO), 176.9, 184.3; IR (CH2Cl2)
2137.1 cm-1 [ν(CO)].
1
incorporation ratio was calculated from H NMR analysis as
done before in previous studies of MA-copolymers.4 Samples
were further characterized by SEC-MALS as described below.
The catalyst TON was calculated using the average monomer
[(HN∧N)PdMe(CO)]+, 31: 1H NMR (500 MHz, CD2Cl2) δ
0.85 (s, 3H, Pd-Me), 1.22 (d, 6H, J ) 6.8 Hz), 1.27 (d, 6H, J )
(42) Gottfried, A. C.; Brookhart, M. Macromolecules 2003, 36, 3085-
3100.

Ligand Electronic Effects on Palladium-R-Diimine
Organometallics, Vol. 24, No. 6, 2005 1155
molecular weight, based on the MA incorporation ratio using
the formula
the polymer eluted from the SEC column. A 30 cm column was
used (Polymer Laboratories PLgel Mixed C, 5 µm particle size)
to separate polymer samples. The mobile phase was THF, and
the flow rate was 0.5 mL/min. Both the column and the
differential refractometer were held at 35 °C. A 60 µL sample
of a 2 mg/mL solution was injected into the column. ASTRA
4.7 from Wyatt Technology was used to acquire data from the
18 scattering angles (detectors) and the differential refracto-
meter. The Mn and Mw data were obtained by following
classical light scattering treatments.
m
TON )
nc(M) (1 - Ir) + MMAIr)
where m is the yield of polymer, nc is the catalyst load, Ir is
the incorporation ratio, and M) and MMA are the molecular
weights of ethylene and MA, respectively.
SEC-MALS Characterization of Polymers.6,7 All the
polymers were characterized by size-exclusion chromatography
(SEC) coupled to a multiangle light scattering detector (MALS)
for obtaining both polymer MW (Mn and Mw) and Rg. Measure-
ments were made on highly dilute fractions eluting from a SEC
consisting of a HP Aglient 1100 solvent delivery system/auto
injector with an online solvent degasser, temperature-con-
trolled column compartment, and an Aglient 1100 differential
refractometer. A Dawn DSP 18-angle light scattering detector
(Wyatt Technology, Santa Barbara, CA) was coupled to the
SEC to measure both the MW and sizes for each fraction of
Acknowledgment. We thank the National Science
Foundation (DMR-0135233) and the University of Cali-
fornia at Irvine for partial financial support. We thank
the laboratories of Professors Richard A. Chamberlin,
Larry E. Overman, Scott D. Rychnovsky, Kenneth J.
Shea, and Keith A. Woerpel for use of lab instrumenta-
tion and chemicals.
OM048988J