CO and ethylene migratory insertion reactions and copolymerization involving palladium complexes of a NiN2S2 metallodithiolate ligand

Article abstract of DOI:10.1021/om0605783

Neutral ((Ni-1)Pd(CH₃)(Cl)) and cationic ([(Ni-1)Pd(CH ₃)(OEt₂)][BAr′₄]) complexes containing the strongly electron-donating metallodithiolate ligand Ni-1 ([(N,N′-bis(2-mercaptoethyl)-N,N′-diazacyclooctane]nicke

Full text of DOI:10.1021/om0605783

Organometallics 2007, 26, 783-792
783
Articles
CO and Ethylene Migratory Insertion Reactions and
Copolymerization Involving Palladium Complexes of a NiN2S2
Metallodithiolate Ligand
Marilyn V. Rampersad, Erik Zuidema, Jan Meine Ernsting,
Piet W. N. M. van Leeuwen,* and Marcetta Y. Darensbourg*,†
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843, and
Anorganisch Chemisch Laboratorium, J. H. Van’t Hoff Research Institute,
UniVersiteit Van Amsterdam, Van’t Hoff Institute for Molecular Sciences,
Nieuwe Achtergracht 166, NL 1018 WV Amsterdam, The Netherlands
ReceiVed June 29, 2006
Neutral ((Ni-1)Pd(CH3)(Cl)) and cationic ([(Ni-1)Pd(CH3)(OEt2)][BAr′4]) complexes containing the
strongly electron-donating metallodithiolate ligand Ni-1 ([(N,N′-bis(2-mercaptoethyl)-N,N′-diazacyclo-
octane]nickel(II), (S(CH2)2N((CH2)3)2N(CH2)2S)Ni) have been prepared. Carbonylation reactions with
these Pd-alkyl complexes were investigated by IR and high-pressure NMR spectroscopy. Carbon-13
NMR spectroscopic studies showed that the rates of CO insertion to form the derivatives (Ni-1)Pd(C(O)-
CH3)(Cl) and [(Ni-1)Pd(C(O)CH3)(CO)]+ were immeasurably fast at -80 °C in CD2Cl2, in contrast to
slower rates for Pd metal based diphosphine and diimine analogues. It was further shown that displacement
of the terminally bound CO by ethylene in the acetylated derivative [(Ni-1)Pd(C(O)CH3)(CO)]+ was
slow, attributable to the high binding affinity of CO toward Pd2+ in the presence of the electron-rich
nickel dithiolate ligand. Bulk copolymerization studies of CO/ethylene in the presence of the cationic
catalyst precursor [(Ni-1)Pd(CH3)(OEt2)][BAr′4] find alternating polyketones. The catalytic efficiency is
solvent-dependent: CH2Cl2 > CH3CN > THF. From low-pressure and bulk copolymerization studies
the resting state of the catalyst was determined to be the open-chain intermediate [(Ni-1)Pd(C(O)R)-
(CO)]+ rather than the â- and γ-keto chelate complexes [(LkL)Pd(CH2CH2(O)R3)]+ and [(LkL)-
Pd(C(O)CH2CH2(O)R3)]+ that were found for diimines and diphosphines.
contains a Cys-Gly-Cys motif which binds nickel(II) in an N2S2
diamido, dithiolato square plane. The NiN2S2 complex is
Introduction
2-
The efficacy of a catalyst depends on the metal as well as
the surrounding ligands, which provide the steric and electronic
environment necessary to influence catalyst performance in
terms of rate and selectivity.1-2 Therefore, important techno-
logical advances in biomimetic or industrial catalyst develop-
ment and modification rely on a fundamental understanding of
the ligand properties and their effects on the metal center during
catalysis.1-4
Inspired by a heterobimetallic biological catalyst, acetyl CoA
synthase (ACS), we have endeavored to develop cis-dithiolato
NiN2S2 metal complexes as a new class of ligands for organo-
metallic chemistry.5-8 The active site of the ACS enzyme
coordinated through the sulfurs to a second nickel which is
catalytically active in sequential C-C and C-S coupling
reactions.8-9 This natural NiN2S2 ligand joins a collection of
synthetic analogues which have distinctive steric, electronic, and
structural properties, as established through derivatives of
tungsten tetracarbonyl.10 These ligands may be neutral NiN2S2
or dianionic NiN2S22- donors, both of which were found to be
better electron-donating ligands than classical diphosphines,
diimines, and dithioether ligands. They have a unique stereo-
chemical orientation with respect to the coordinated metal, as
shown in Figure 1 for the heterobimetallic complex (Ni-1)Pd-
(6) Darnault, C.; Volbeda, A.; Kim, E. J.; Legrand, P.; Verne`de, X.;
Lindahl, P. A.; Fontecilla-Camps, J. C. Nat. Struct. Biol. 2003, 10, 271-
279.
† Fax: (979) 845-0158. E-mail: marcetta@mail.chem.tamu.edu.
(1) (a) van Leeuwen, P. W. N. M. Homogeneous Catalysis, Understand-
ing the Art; Kluwer Academic: Dordrecht, The Netherlands, 2004. (b) van
Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes. P. Chem.
ReV. 2000, 100, 2741-2769.
(2) Sen, A. Catalytic Synthesis of Alkene-Carbon Monoxide Copolymers
and Cooligomers; Kluwer Academic: Dordrecht, The Netherlands, 2003.
(3) Ledford, J.; Shultz, S. C.; Gates, D. P.; White, P. S.; DeSimone, J.;
Brookhart, M. Organometallics 2001, 20, 5266-5276.
(4) Tolman, C. A. Chem. ReV. 1977, 77, 313-348.
(5) Dokov, T. I; Iverson, T. M; Seravalli, J.; Ragsdale, S. W.; Drennan,
C. L. Science 2002, 298, 567-572.
(7) Svetlitchnyi, V.; Dobbek, H.; Meyer-Klaucke, W.; Meins, T.; Thiele,
B.; Romer, P.; Huber, R.; Meyer, O. Proc. Natl. Acad. Sci. U.S.A. 2004,
101, 446-451.
(8) Webster, C. E.; Darensbourg, M. Y.; Lindahl, P. A.; Hall, M. B. J.
Am. Chem. Soc. 2004, 126, 3410-33411.
(9) Amara, P.; Volbeda, A.; Fontecilla-Camps, J. C.; Field, M. J. J. Am.
Chem. Soc. 2005, 127, 2776-2785.
(10) Rampersad, M. V.; Jeffery, S. P.; Golden, M. L.; Lee, J.; Reiben-
spies, J. H.; Darensbourg, D. J.; Darensbourg, M. Y. J. Am. Chem. Soc.
2005, 127, 17323-17334.
10.1021/om0605783 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 01/10/2007

784 Organometallics, Vol. 26, No. 4, 2007
Rampersad et al.
Figure 1. Butterfly core defined by the (µ-SR)2 bridge or hinge
in (Ni-1)Pd(CH3)(Cl).
(CH3)(Cl).10,11 This unique “hinge-angle” or “butterfly” feature
might allow for stereo- and regioselective substrate addition.
The µ-SR bridge feature formed between the NiN2S2 chelate
and a Pd2+ metal center, displayed in the two views in Figure
1, is a direct analogue of the dinickel active site in ACS.5-7 An
added functionality of metallodithiolate ligands is their ability
to serve as hemilabile ligands.12 Bond dissociation between
sulfur and the catalytic metal provides an open site on an
otherwise coordinatively saturated metal center. Hemilability
may also account for stabilization of the catalytically active
metal during oxidation state changes which require different
coordination numbers.
The heterobimetallic complex (Ni-1)Pd(CH3)(Cl) (Figure 1)
was designed to mimic structural and functional features of the
ACS enzyme active site as well as to serve as a catalyst or
catalyst precursor in a probe reaction in C-C coupling reactions,
specifically CO/olefin copolymerization.11 Elegant mechanistic
studies by Sen,13-15 Brookhart,16 Drent,17 and van Leeuwen18
over the past decade have provided substantial mechanistic
information that highlighted the key steps of CO/ethylene
chemistry and delineated the formation of perfectly alternating
polyketone (Figure 2), as catalyzed by amine and phosphine
derivatives of PdII. By analogy we expect that the NiN2S2 ligand
might take the place of a diphosphine or diimine ligand. In
preliminary studies this analogy was demonstrated for the methyl
migration/CO insertion reaction given in eq 1.11
Figure 2. Mechanism proposed by Brookhart et al. for the copoly-
merization of CO/C2H4 employing palladium diimine catalysts.16
et al. have shown that double CO insertion is thermodynamically
unfavorable and that double insertion of ethylene is retarded
by the high affinity of PdII metal centers for CO.13-15
Ligand effects on this reaction have been explored with a
variety of diphosphines and diimines. With the diphosphine
complex [(dppp)Pd(CH3)][OTf]- (dppp ) bis(diphenylphos-
phino)propane), Drent and co-workers found evidence that the
resting states of the catalysts under mild conditions are the R,â-
keto chelate complexes.17 They suggested that CO replacement
of the O donor in the γ-keto chelate reduces steric crowding,
facilitates the side-on approach of ethylene, and results in more
facile CO/C2H4 exchange and C2H4 insertion.17 In contrast,
Brookhart and co-workers posited that the relatively flat
phenanthroline ligand provided less steric crowding and facili-
tated the direct ketone displacement by ethylene.16 Thus, in this
case the resting state of the catalyst is the open-chain acetyl
carbonyl [(o-phen)Pd(C(O)R)(CO)]+, attributable to the much
larger binding affinity of CO over ethylene for the metal center
(Figure 2).
In addition to the ligand dependence of resting states of the
catalyst during CO/C2H4 copolymer formation, there exist
differences between the mechanistic route to CO addition in
neutral (L2)Pd(CH3)(X) and cationic [(L2)Pd(CH3)]+[X]- metal
complexes.17,18 These differences may derive from the ligand’s
steric and electronic components as well as the charge and
coordination geometry of the overall complex.18 In order to place
the NiN2S2 ligands more clearly in the scheme of such a
catalytically significant reaction, high-pressure 13C NMR studies
were conducted in order to monitor the carbonylation reaction
of the neutral (Ni-1)Pd(CH3)(X) and [(Ni-1)Pd(CH3)]+[X]-
metal complexes. As it relates to CO/olefin copolymerization,
the reactivity of the acetylated derivative [(Ni-1)Pd(C(O)CH3)-
(CO)]+ with ethylene is also described along with pre-
liminary results regarding the catalytic activity of [(Ni-1)Pd-
(CH3)(OEt2]+ in the CO/ethylene copolymerization reaction with
varying ethylene concentrations and solvents. With these results,
the nickel dithiolate ligand is compared to the classical L2
ligands.
In CO/olefin copolymer catalysis the migratory insertion of
CO into a Pd-alkyl bond (eq 2) is followed by ethylene binding
and migratory insertion into a Pd-acetyl bond (eq 3).2,15,19 Sen
Experimental Section
(11) Rampersad, M. V.; Jeffery, S. P.; Reibenspies, J. H.; Ortiz, C. G.;
Darensbourg, D. J.; Darensbourg, M. Y. Angew. Chem., Int. Ed. 2005, 44,
1217-1220.
(12) Phelps, A. L.; Rampersad, M. V.; Fitch, S. B.; Darensbourg, D. J.;
Darensbourg, M. Y. Inorg. Chem. 2006, 45, 119-126.
(13) Lai, T. W.; Sen, A. Organometallics 1984, 3, 866-870.
(14) Chen, J. T.; Sen, A. J. Am. Chem. Soc. 1985, 106, 1506.
(15) Sen, A.; Chen, J. T.; Vetter, W. M.; Whittle, R. R. J. J. Am. Chem.
Soc. 1987, 109, 148.
Definitions: cod ) cyclooctadiene, HBAr′4 ) [H(OEt2)2][(3,5-
(CF3)2C6H3)4B], TMEDA ) tetramethylethylenediamine, Ni-1 )
(N,N′-bis(2-mercaptoethyl)-N,N′-diazacyclooctane)nickel(II).
General Methods and Materials. All manipulations were
performed on a double-manifold Schlenk vacuum line under an
(16) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996,
118, 4746-4764.
(17) Mul, W. P.; Oosterbeek, H.; Beitel, G. A.; Kramer, G.; Drent. E.
Angew. Chem., Int. Ed. 2000, 39, 1858-1851.
(18) Ru¨lke, R. E.; Delis, J. G. P.; Groot, A. M.; Elsevier, C. J.; van
Leeuwen, P. W. N. M.; Vrieze, K.; Goubitz, K.; Schenk, H. J. Organomet.
Chem. 1996, 508, 109-120.
(19) Drent. E.; Budzelaar, P. H. M. Chem. ReV. 1996, 663-681.

Pd Complexes of a Metallodithiolate Ligand
Organometallics, Vol. 26, No. 4, 2007 785
atmosphere of nitrogen or in an argon-filled glovebox. Solvents
were reagent grade and were predried and deoxygenated according
to published procedures under a N2 atmosphere.20 The complexes
(TMEDA)Pd(CH3)2,21 H(OEt2)2BAr′4,22 (cod)Pd(CH3)(Cl),23 Ni-1,24
room temperature before the solution was concentrated to about 5
mL, resulting in a red precipitate. The slurry was washed with
pentane (2 × 20 mL) and then dried under vacuum to yield a red-
pink solid of 0.32 g (80%) of (Ni-1)Pd(CH3)(Cl)‚CH2Cl2. (The
CH2Cl2 solvent molecule was observed in the X-ray diffraction
study.) Anal. Calcd (found) for C12H25N2S2NiPdCl3: C, 26.2 (27.0);
H, 4.94 (4.73); N, 5.86 (5.26). Included in the calculated elemental
analysis is a CH2Cl2 solvent molecule that was observed in the
X-ray diffraction study of the crystals. UV-vis (in CH2Cl2 solution;
11
and (Ni-1)Pd(CH3)2 were synthesized according to published
procedures. All other chemicals were purchased from Aldrich
Chemical Co. and used as received. All compounds were stored
under an argon atmosphere at -45 °C. The CO gas used was filtered
through a Drierite column before use.
Physical Measurements. Canadian Microanalytical Services,
Ltd., Delta, British Columbia, Canada, performed elemental
analyses. Mass spectral analyses were done at the Laboratory for
Biological Mass Spectroscopy at Texas A&M University. Electro-
spray ionization mass spectra were recorded using an MDS-Series
QStar Pulsar instrument with a spray voltage of 5 eV. MALDI-
TOF measurements were performed on an ABI Voyager-DE STR
spectrometer, operating in positive reflection mode. Dithranol (1,8,9-
trihydroxyanthracene) was used as the matrix, and the polymer
was dissolved in hexafluoro-2-propanol ((CF3)2CHOH. Note that
(CF3)2CHOH is a highly toxic and corrosive solvent that is
destructive to tissues, membranes, eyes, skin, and respiratory tract
and should be used with extreme care). Infrared spectra were
recorded on a Bruker Optics 6021 FTIR spectrometer with a DTGS
detector. 13C NMR studies were carried out on an INOVA 500
MHz Varian spectrometer with a 1 cm high-pressure sapphire NMR
tube. N.B.: care must be taken to ensure there are no scratches or
microfractures on the tube, which may provide a nucleation point
for explosion during pressurization. The tube was cleaned by
consecutive rinsings with CH2Cl2 and by a gentle scrub using a
modified pipe cleaner. Visible/UV spectra were recorded in CH2-
λ
max, nm (ꢀ, M-1 cm-1)): 232 (18 010), 262 (13 810), 302 (8560),
526 (460). Mass spectrum (CH3CN solution; m/z (% abundance
relative to base peak at 100%)): 413 [(Ni-1)Pd(CH3)]+ (100%),
454 [(Ni-1)Pd(CH3)(CH3CN)]+ (22%).
Preparation of [(N,N′-Bis(2-mercaptoethyl)-N,N′-diazacyclo-
octane)nickel(II)]palladium(Acetyl)(Chloride), (Ni-1)Pd(C(O)-
CH3)(Cl). Under argon, a sample of solid (Ni-1)Pd(CH3)(Cl) (0.030
g, 0.066 mmol) was placed in a degassed and flame-dried Schlenk
tube and cooled to -78 °C in a dry ice/acetone bath. Approximately
5 mL of CH2Cl2 was added to the flask, followed by vigorous
sparging of CO gas for 5 min. The solution was stirred for an
additional 30 min at -78 °C. IR (CH2Cl2; ν(CdO), cm-1): 1692
(s). 13C NMR (CD2Cl2; δ, ppm): 225 (s, C(O)).
Copolymerization of CO and Ethylene. Samples of (Ni-1)Pd-
(CH3)2 and HBAr′4 were added to a flame-dried degassed vial, in
which 15 mL of CH2Cl2 was added. The solution was cooled to
-78 °C and stirred for 10 min before being transferred via an
injection port to a stainless steel 300 mL Parr autoclave at room
temperature. The autoclave was predried at 80 °C under vacuum
for 8 h and cooled to room temperature prior to use. The vial was
washed with an additional 10 mL of CH2Cl2, and the washings
were added to the autoclave. The autoclave was charged with CO
gas followed by ethylene; the contents were heated at the desired
temperature for the designated time. Copolymerization was initiated
at the time of monomer addition and was stopped by removing the
autoclave from its heating mantle, thereby cooling to room
temperature. The remaining gases were vented in the fume hood.
The polymer was extracted from the catalyst by precipitation with
an acidic methanol solution (95% CH3OH and 5% concentrated
HCl). The grayish to white polymer was collected via filtration,
dried, and weighed. IR ((CF3)2CHOH); ν(CdO), cm-1): 1704 (s).
13C NMR (400 MHz, (CF3)2CHOH/CDCl3, 25 °C; δ, ppm): 220.0
(s, C(O)), 43 (s, R-CH2).
13C NMR Study of CO Addition to [(Ni-1)Pd(CH3)(OEt2)]-
[BAr′4]. To a degassed flame-dried Schlenk tube was added
[(Ni-1)Pd(CH3)(OEt2)][BAr′4] (0.053 g, 0.039 mmol), to which 2
mL of anhydrous CD2Cl2 was added under Ar. The red solution
was transferred via a plastic cannula into a high-pressure sapphire
NMR tube that had been flushed with Ar for 15 min. With an Ar
purge, the stainless steel cap was placed on the top of the tube
simultaneously as the plastic cannula was removed. The sample
was cooled to -80 °C in a dry ice/acetone bath and pressurized to
a total of 8 bar with isotopically labeled 13CO gas a few minutes
prior to transporting the sample to the NMR spectrometer (probe
precooled to -80 °C). Approximately 5 min after CO addition,
the NMR tube was wiped dry on the outside, shaken once, and
inserted into the precooled NMR instrument. Consecutive 13C NMR
spectra were recorded over the range -80 to +60 °C.
Cl2 on a Hewlett-Packard HP8552A diode array spectrometer. 13
C
NMR analysis of the polyketone was carried out with
an INOVA 400 MHz Varian spectrometer in a 1:1 mixture of
(CF3)2CHOH and CDCl3.
Preparation of (NiN2S2)PdR2 Complexes. (a) [(N,N′-Bis(2-
mercaptoethyl)-N,N′-diazacyclooctane)nickel(II)]palladium-
(Methyl)(Diethyl Ether), [(Ni-1)Pd(CH3)(OEt2)][BAr′4]. Under
an argon atmosphere solids of the red (Ni-1)Pd(CH3)2 (0.030 g,
0.070 mmol) and white HBAr′4 (0.071 g, 0.070 mmol) were added
to a flame-dried and degassed Schlenk tube, which was immediately
placed in a -30 °C dry ice/CH3CN ice bath. A 3:1 v/v mixture of
CH2Cl2 and diethyl ether was added to the solids, taking care to
dissolve all of the reactants (CO exposure to unreacted (Ni-1)Pd-
(CH3)2 results in decomposition to Pd black). The orange-red
solution was stirred for 2 h at -30 °C, followed by the evaporation
of the solvent under vacuum to yield an orange-red solid, yield
0.068 g (72%). UV-vis (in CH2Cl2 solution; λmax, nm (ꢀ, M-1
cm-1)): 236 (24 710), 272 (15 770), 280 (15 435), 302 (9730), 330
(5830), 396 (1440), 518 (564). Mass spectrum (CH3CN solution;
m/z (% abundance relative to base peak at 100%)): 413 [(Ni-1)-
Pd(CH3)]+ (97%), 454 [(Ni-1)Pd(CH3)(CH3CN)]+ (100%).
(b) [(N,N′-Bis(2-mercaptoethyl)-N,N′-diazacyclooctane)nickel-
(II)]palladium(Methyl)(Chloride), (Ni-1)Pd(CH3)(Cl). Under N2,
20 mL of CH2Cl2 was added to a degassed sample of (cod)Pd-
(CH3)(Cl) (0.20 g, 0.75 mmol). To this colorless solution was added
a purple solution of Ni-1 (0.22 g, 0.75 mmol dissolved in 30 mL
of CH2Cl2). The resulting red-pink solution was stirred for 5 h at
13C NMR Study of CO Addition to (Ni-1)Pd(CH3)(Cl). By a
procedure almost identical with that for the preparation of
[(Ni-1)Pd(CH3)(OEt2)][BAr′4] for CO addition, the red-pink solid
(Ni-1)Pd(CH3)(Cl) (0.018 g, 0.040 mmol) was dissolved in 2 mL
of anhydrous CD2Cl2 in a flame-dried degassed flask under an argon
atmosphere. The 20 mM solution was transferred to the sapphire
NMR tube (degassed thoroughly in a large Schlenk tube via vacuum
and refilled with Ar at 1 bar), which was cooled to -80 °C and
charged to 9 bar with isotopically labeled 13CO gas a few minutes
prior to transporting the sample to the NMR instrument (probe
(20) Gordon, A. J; Ford, R. A. The Chemist’s Companion; Wiley: New
York, 1972; pp 429-436.
(21) De Graaf, W.; Boersma, J.; Smeets, J. J.; Spek, A. L.; van Koten,
G. Organometallics 1989, 8, 2907-2917.
(22) Brookhart, M.; Grant, M.; Volpe, A. F. Organometallics 1992, 11,
3920-3922.
(23) Ru¨lke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769-5778.
(24) Mills, D. K.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem.
1990, 29, 4364-4366.
(25) Musie, G.; Farmer, P. J.; Tuntulani, T.; Reibenspies, J. H.;
Darensbourg, M. Y. Inorg. Chem. 1996, 35, 2176-2183.

786 Organometallics, Vol. 26, No. 4, 2007
Rampersad et al.
Figure 3. 13C NMR study of CO uptake by the [(Ni-1)Pd(CH3)(OEt2)]+ complex: (a) spectrum at -80 °C; (b) monitoring of the effect
of raising the temperature from -70 to 60 °C in a high-pressure sapphire NMR tube at 7 bar of 13CO. Asterisks indicate minor impurities.
Pd0 metal are products of a well-known thermal decomposition
reaction of L2Pd(CH3)2 complexes (L2 ) diimine) by release
of ethane.21 As a solid, the (Ni-1)Pd(CH3)2 complex is stable
at low temperature.
precooled to -80 °C) (N.B.: the total pressure in the tube is 9 bar
(8 bar of CO + 1 bar of Ar)). Approximately 5 min after CO
addition, the 13C NMR spectra were recorded over the range -80
to +30 °C.
13C NMR Study of CO and Ethylene Uptake by [(Ni-1)Pd-
(CH3)(OEt2)][BAr′4]. A sample of [(Ni-1)Pd(CH3)(OEt2)][BAr′4]
(0.057 g, 0.042 mmol) dissolved in 2 mL of CD2Cl2 was prepared
in a manner identical with that above. The 21 mM sample was
pressurized with 7 bar of 13CO gas approximately 5 min prior to
recording the 13C NMR spectrum at -80 °C (NMR instrument
precooled to -80 °C). For the addition of ethylene, the NMR tube
was depressurized to 1 bar in a fume hood and charged with 4 bar
of ethylene at -80 °C in a dry ice/acetone bath (total pressure 5
bar). Consecutive 13C NMR spectra were recorded over the range
-80 to +20 °C.
Our original report of the (Ni-1)Pd(CH3)Cl complex derived
from the unplanned reactivity of (Ni-1)Pd(CH3)2 with CH2Cl2
solvent.11 A direct and facile preparation of (Ni-1)Pd(CH3)(Cl)
in 95% yield was adopted from the method of van Leeuwen
and co-workers by metallodithiolate ligand substitution of the
cyclooctadiene in (cod)Pd(CH3)(Cl) (eq 4).23 Mass spectral
Results and Discussion
analysis of the product in acetonitrile found that the base peak
was [(Ni-1)Pd(CH3)]+ with a mass to charge ratio of 413, while
at 22% the acetonitrile adduct [(Ni-1)Pd(CH3)(CH3CN)]+ was
seen.
According to the experimental protocol laid out by Brookhart
et al.,16 the monomethyl derivative [(Ni-1)Pd(CH3)(OEt2)]-
[BAr′4] was prepared from (Ni-1)Pd(CH3)2 by the acid release
of CH4. This complex is an extremely useful precursor that is
stable as a red solid at low temperature. Mass spectral analysis
of the product dissolved in acetonitrile found that the base peak
(100%) is the acetonitrile adduct [(Ni-1)Pd(CH3)(CH3CN)]+,
with a mass to charge ratio of 454. The stability of [(Ni-1)Pd-
(CH3)(OEt2)][BAr′4] in CH2Cl2 solution was monitored by
UV-vis spectroscopy at room temperature. Spectral changes
of the d-d transitions were observed after 2 h, with more promi-
nent changes occurring overnight, showing a dramatic increase
in absorbance with a concurrent shift in the absorptions of the
starting complex at 396 and 518 nm (orange-red) to 402 and
522 nm (opaque dark purple) (see the Supporting Information).
These spectroscopic features are consistent with the formation
13C NMR Study of CO Addition to [(Ni-1)Pd(CH3)-
(OEt2)][BAr′4]. In order to study the migratory insertion
possibilities of [(Ni-1)Pd(CH3)(OEt2)]+, a 20 mM sample of
the complex as its [BAr′4]- salt was prepared at room temper-
ature, placed in a high-pressure sapphire NMR tube, and cooled
to -80 °C prior to the addition of 13CO gas to 7 bar (eq 5).
of trimetallic complexes such as the dark purple [(Ni-1)2Pd]2+
-
(Cl)2 (λmax at 408 (5245), 522 nm (3730 M-1 cm-1) in
CH3OH) that has been spectroscopically and structurally
characterized.11 The trimetallic complex [Ni-1]2Pd[BAr′4]2 and
The 13C NMR spectrum was recorded within 5 min of CO

Pd Complexes of a Metallodithiolate Ligand
Organometallics, Vol. 26, No. 4, 2007 787
Figure 4. Possible reaction pathways for CO exchange in [(Ni-1)Pd(C(O)CH3)(CO)]+.
addition and transport of the sample to the NMR instrument
that was precooled to -80 °C.
pathway is expected to be the favored route to ligand exchange
(path A), as it involves a five-coordinate, square-pyramidal, 18-
electron intermediate species (Figure 4). This is similar to a
ligand exchange process confirmed to be associative by
Brookhart and co-workers, in which the alkyl carbonyl com-
plex (o-phen)Pd(CH3)(12CO) underwent rapid exchange with
13CO.16
13C NMR Study of CO Addition to (Ni-1)Pd(CH3)(Cl). The
same procedure as described above was applied to CO addition
to the neutral complex (Ni-1)Pd(CH3)(Cl) (eq 6). A 20 mM
Carbon-13 NMR spectra were recorded at -80 °C and at 10
°C increments as the temperature was increased to 60 °C (Figure
3). At -80 °C the 13C NMR spectrum displayed three sharp
signals in the CO region (Figure 3a). The large resonance at
185 ppm is assigned to free 13CO dissolved in CD2Cl2. The
signal at 225 ppm is assigned to the carbonyl carbon of the
acetyl group (COa), coordinated to the palladium metal center,
while the resonance at 176 ppm, upfield from free 13CO, is
assigned to carbon of a terminally bound CO ligand (COt).
Resonances with asterisks represent minor impurities. The
assignments are in accord with that of the complex [(o-phen)-
Pd(C(O)CH3)(CO)]+, reported by Brookhart and co-workers.16
When the sample was warmed, the resonance of the acetylated
carbonyl derivative observed at -80 °C did not grow in
intensity. This suggested that CO/CH3+ migratory insertion was
complete within 5 min. The fast rate of CO reaction and
insertion with [(Ni-1)Pd(CH3)(OEt2)]+ at -80 °C thus prevented
observation of the presumed intermediate, [(L2)Pd(CH3)(CO)]+,
which was observable with the diphosphine and diimine ligands.
Quantitative studies with such ligands have determined rate
constants for migratory insertion in a diphosphine derivative,
[(dppp)Pd(CH3)(CO)]+ (kobs ) 4.5 × 10-5 s-1 (∆Gq ) 61.7(1)
kJ/mol, -81.7 °C)), and in a diimine derivative, [(o-phen)Pd-
(CH3)(CO)]+ (kobs ) 2.5 × 10-4 s-1 (∆Gq ) 64.5 kJ/mol, -66
°C)).16,20 Thus, it is evident that the rate of methyl migration
on the palladium metal center is much more rapid in the presence
of the metallodithiolate ligand as compared to the rate for the
classical ligands in similar cationic complexes.
sample was pressurized with 8 bar of isotopically labeled 13CO
gas at -80 °C, and the 13C NMR spectrum was recorded in the
precooled NMR spectrometer probe within 5 min after CO
addition.
As shown in Figure 5a, the resonance for free 13CO dissolved
in solution is observed at 185 ppm and a signal at 226 ppm is
assigned to the acetyl, COa. Notably, CO addition to similar
neutral complexes of the type L2Pd(CH3)(Cl) (L ) diimine)
also forms acetylated derivatives, L2Pd(C(O)CH3)(Cl).26 Con-
sistently, the infrared spectrum of the acetylated (Ni-1)Pd(C(O)-
CH3)(Cl) shows a ν(CdO) stretch at 1692 cm-1, in agreement
with the ν(CdO) stretching frequency (1690 cm-1) of the
(bipy)Pd(C(O)CH3)(Cl) complex.18
When the solution was warmed from -70 to 20 °C, the C-13
resonance assigned to the carbon of the acetyl ligand did not
intensify (Figure 5b). This signifies that CO addition to the
neutral complex (Ni-1)Pd(CH3)(Cl) was complete at -80 °C
within the same time frame as that of the cationic deriva-
tive [(Ni-1)Pd(CH3)(OEt2)]+; i.e., we are unable to measure
events that happen during the first 5 min from the time of
mixing of CO and the NiPd derivatives. Interestingly, the rate
13CO Exchange in [(Ni-1)Pd(C(O)CH3)(COt)]+. As shown
in Figure 3b, as the sample of [(Ni-1)Pd(C(O)CH3)(COt)]+ in
CD2Cl2 under 13CO is warmed, the terminally bound COt
resonance at 176 ppm disappears with concomitant broadening
of the free 13CO signal at 185 ppm. This observation is
interpreted as ligand exchange between the terminally bound
COt and free 13CO dissolved in solution. Over the same
temperature range the acetyl signal remains relatively sharp and
only shows a slight upfield shift to 223 ppm.
Terminal COt exchange may occur via dissociative or
associative pathways. Ligand dissociation via Ni-S or
Pd-CO bond breaking (path B or C in Figure 4) would result
in the formation of a 14-electron, open-site intermediate.
Because of greater activation barriers to bond breaking, however,
such processes are unlikely at low temperature. An associative
(26) van Asselt, R.; Gielens, E. C. G.; Ru¨lke, R. E.; Elsevier, C. J. J.
Chem. Soc., Chem. Commun. 1993, 1203-1205.

788 Organometallics, Vol. 26, No. 4, 2007
Rampersad et al.
Figure 5. 13C NMR spectra of CO uptake by (Ni-1)Pd(CH3)(Cl): (a) spectrum at -80 °C; (b) monitoring of the effect of raising the
temperature from -70 to 30 °C in a high-pressure sapphire NMR tube at 8 bar of 13CO. Asterisks indicate Ni(CO)4 impurity (see text).
of CO/CH3 migratory insertion with the (bipy)Pd(CH3)(Cl)
complex is reported to be much slower: kr ) 4.05 × 10-4 s-1
at 20 °C.18
We have assigned the large resonance observed at 192 ppm
to the carbonyl ligands of Ni(CO)4,27 an unfortunate impurity
from a reaction between 13CO gas trapped in a gas regulator
composed of a nickel alloy. That the Ni(CO)4 formation was
not the result of a decomposition reaction formed by oxidation
of the thiolate donors to form disulfide, reducing Ni2+ to Ni0 in
the presence of CO, was explored by the following experiment.
Addition of 13CO to the free metallodithiolate ligand over an
extended period did not result in Ni(CO)4 formation. The 13C
NMR spectrum of the sample after being depressurized of CO
followed by a slight argon purge at 20 °C showed a significant
decrease in the resonances for free 13CO and Ni(CO)4 dissolved
in solution. The carbonyl acetyl signal, however, remained sharp
Figure 6. Possible reaction pathways for CO addition/insertion in
and unchanged, further affirming that the (Ni-1)Pd(C(O)CH3)-
(Ni-1)Pd(CH3)(Cl).
(Cl) species does not undergo deinsertion and CO exchange.
Other interesting features observed in the 13C NMR spectra are
a small resonance at 227 ppm and a slight line broadening of
the acetyl signal that occurs as the temperature is raised. This
will be addressed later.
co-workers,18 with precedence in work by Natile and co-
workers.28 In addition, five-coordinate intermediates through
associated routes have been observed by Brookhart and co-
workers when excess CO is added to [(o-MeO-dppe)NiMe-
(OEt2)]+ to form a methyl dicarbonyl complex, [(o-MeO-dppe)-
NiMe(CO)2]+, which undergoes insertion to form [(o-MeO-
dppe)Ni(C(O)Me)(CO)2]+.29
Possible Mechanisms of CO Addition/Migratory Insertion
to (Ni-1)Pd(CH3)(Cl). The observation of an acetyl group
within the neutral complex (Ni-1)Pd(C(O)CH3)(Cl) raised
questions regarding the CO addition reaction and the enhanced
rate of migratory insertion as compared to the rate for (L2)Pd-
(CH3)(Cl) (L2 ) diimines). Dissociative routes involving the
Cl- ion as proposed for (L2)Pd(CH3)(Cl) complexes are unlikely,
since Cl- dissociation at such low temperatures (-80 °C) is
not expected to be rapid. An additional dissociative route could
involve the hemilabile property of the NiN2S2 ligand, as has
been established for NiN2S2W(CO)4 complexes.12 These mech-
anisms are expressed in Figure 6, as dissociative paths A and
B. A more attractive alternative is the associative path shown
in Figure 6, path C, as proposed by van Leeuwen and
In our studies, some support for the pentacoordinate species
comes from infrared studies of CO addition to the (Ni-1)Pd-
(CH3)(Cl) complex in CH2Cl2 held at -80 °C (see the
Supporting Information). An infrared spectrum following 5 min
of CO purge displayed ν(CdO) bands at 1692 and 1663 cm-1
.
When the temperature was raised to 22 °C, a terminal ν(CtO)
stretch at 2041 cm-1 was observed. This band showed a
predictable shift when the solution was purged with 13CO and
(28) (a) Fanizzi, F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.;
Tiripicchio, A.; Pacchioni, G. J. Chem. Soc., Chem. Commun. 1992, 333-
335. (b) Fanizzi, F. P.; Lanfranchi, M.; Natile, G.; Tiripicchio, A. Inorg.
Chem. 1994, 33, 3331-3339.
(29) Shultz, S. C.; DeSimone, J. M.; Brookhart, M. J. Am. Chem. Soc.
2001, 123, 9172-9173.
(27) Hill, A. M.; Levason, W.; Webster, M.; Albers, I. Organometallics
1997, 16, 5641-5647.

Pd Complexes of a Metallodithiolate Ligand
Organometallics, Vol. 26, No. 4, 2007 789
Figure 7. Low-field 13C NMR spectra between -70 and 20 °C of the CO acetyl resonance of (Ni-1)Pd(C(O)CH3)(Cl): (a) normal resolution;
(b) resolution enhancement by a sine bell function (sb ) 0.130 s).
was also shown to disappear when CO gas was removed.
Whether the terminal CO band was due to a pentacoordinate
palladium species, i.e., (Ni-1)Pd(C(O)CH3)(CO)(Cl), or some
other carbonyl species, such as the four-coordinate (η1-NiN2S2-
Pd(C(O)CH3)(CO)(Cl)) cannot be determined. It should be noted
that the 13C NMR spectrum of the isotopically labeled product
did not show resonances in the 190 ppm region that would
suggest Pd0 or Ni0 as tetracarbonyls. Hence, at this point no
conclusion about the identity of the product with the ν(CtO)
stretch at 2041 cm-1 can be made.
Temperature-Dependent 13C NMR Spectra of (Ni-1)Pd-
(C(O)CH3)(Cl). The 13C NMR spectrum at -80 °C of the
(Ni-1)Pd(C(O)CH3)(Cl) complex shows two resonances in the
acetyl region, a major one at 226 ppm and a smaller one at 227
Figure 8. Ball-and-stick representation of an overlay of the
ppm. As seen in Figure 7a, warming the sample in 10 °C
molecular structures of (Ni-1)Pd(CH3)(Cl) and [(o-phen)Pd(C(O)-
increments led to both broadening and a shift of the signal,
ultimately yielding one signal at 225 ppm. Figure 7b also
CH3)(CO)]+, illustrating the orientation of the acetyl (CdO) group
perpendicular to PdS2CCl and parallel to the Ni-1 ligand. Hydrogen
atoms were added on the Ni-1 ligand and the CH3 group of the
acetyl to illustrate the steric influence of the Ni-1 ligand.
displays a resolution enhancement of the signals (produced from
a sine bell function),30 which shows that the distinctiveness of
the two resonances disappeared at ca. 0 °C. It should be noted
that when the sample is cooled back to -30 °C, both carbonyl
signals reappear, indicating reversibility.
Assuming that the temperature dependence of the acetyl
resonance reflects signals from a single molecule and intramo-
lecular rearrangements, its interpretation may benefit from
Figure 9. Stick drawings illustrating isomerization involving either
(a) buckling of the NiN2S2 ligand and Pd(C(O)CH3(Cl) unit at the
sulfur hinges or (b) acetyl carbonyl bond rotation.
examination of the solid-state structure of (Ni-1)Pd(CH3)(Cl)
and the known molecular structures of several acetylpalladium
complexes.16,18 The latter show that the acetyl (CdO) ligand is
oriented perpendicular to the PdL2C2 plane square plane, as
shown in the structure of the [(o-phen)Pd(C(O)CH3)(CO)]+
complex (Figure 8). Superimposed on this structure is that of
(Ni-1)Pd(CH3)(Cl). If one assumes that the orientation of the
acetyl (CdO) group is similar to that of the (o-phen)Pd
derivative, two isomeric forms of the (Ni-1)Pd(C(O)CH)3(Cl)
are possible. The “up” positions the CdO group in close contact
with the NiN2S2 ligand. Alternatively, the CdO group may be
oriented downward from the NiN2S2 ligand (Figure 9). Assum-
ing these structural forms have a significant lifetime in solu-
tion, then the VT NMR results could reflect the intercon-
version between isomers, represented by the stick drawings in
Figure 9.
While the mechanism of this site exchange is unknown,
equilibration could occur via a buckling of the NiN2S2 ligand
with respect to the palladium square plane (Figure 9), a process
which has precedence in a VT NMR study of the [Ni-1*]-
W(CO)4 complex.10 Alternatively, a rotation around the
Pd-(C(O)Me) bond could also result in the equilibration
(Figure 9).
13C NMR Study of CO and Ethylene Uptake by [(Ni-1)-
Pd(CH3)(OEt2)][BAr′4]. As background for the CO/olefin
(30) Mann, B. E.; Akitt, J. W. In NMR and Chemistry, 4th ed.; Thornes,
N., Ed. CRC Press: Boca Raton, FL, 2000; pp 161-162.

790 Organometallics, Vol. 26, No. 4, 2007
Rampersad et al.
Figure 10. 13C NMR spectral monitoring of the reaction of ethylene and CO with [(Ni-1)Pd(C(O)CH3)CO)]+ in CD2Cl2: (a) -20 °C; (b)
0 °C; (c) 20 °C.
copolymerization studies, the reaction progress of 13CO and
ethylene gas uptake by [(Ni-1)Pd(CH3)(OEt2)]+ was monitored
by 13C NMR spectroscopy. A 21 mM sample of [(Ni-1)Pd(CH3)-
(OEt2)]+ in CD2Cl2 was pressurized with 8 bar of 13C-labeled
CO gas at -80 °C, and the 13C NMR spectrum was re-
corded within a 5 min period, with results as described in eq 5.
Two resonances characteristic of the acetyl carbonyl com-
plex [(Ni-1)Pd(C(O)CH3)(CO)]+ appeared at 225 ppm (the
acetyl CdO carbon) and 176 ppm (the CO). In addition,
resonances for the free 13CO and for the Ni(CO)4 impurity were
evident.
(Cc(O)CH3)(η2-CH2dCH2)]+ intermediate and are more likely
due to differences in insertion products, [(Ni-1)Pd(Cc(O)Rpoly)-
(CO)]+.
At 20 °C resonances are observed at 208, 207, and 206 ppm
and are assigned to the carbon signals from ketone-like end
groups after three consecutive CO/ethylene insertions in
[(Ni-1)Pd(C(O)CH3)CO)]+, forming a growing polymer chain
(Figure 10c). This is consistent with a decrease in the concentra-
tion of free 13CO in solution: i.e., the resonance at 184 ppm.
No change was observed in the intensity of the resonance at
192 ppm assigned to Ni(CO)4.
While it was maintained at -80 °C, the sample was
depressurized to 1 bar followed by addition of ethylene to a
total pressure of 5 bar (eq 7). The 13C NMR spectrum was
Since no spectral changes were observed at temperatures
lower than -20 °C, it can be concluded that ethylene does not
easily replace CO. The appearance of new signals at T > -20
°C suggests that a CO/CH2dCH2 exchange has occurred, along
with some ethylene insertion. As may be implied from Figure
2, ethylene displacement of CO in the resting state of the catalyst
[(o-phen)Pd(C(O)CH3)(CO)]+, to form the ethylene acetyl
complex [(o-phen)Pd(C(O)Me)(η2-CH2CH2)]+, is thermody-
namically uphill.16 Therefore, it is reasonable that a metal center
with a very electron rich ligand will prefer to have a good
π-acceptor ligand such as CO rather than ethylene, as evidenced
by the presence of a Pd-bound terminal CO-trapped species
during the reaction. Hence, the resting state of the catalyst with
a metallodithiolate ligand is the species [(Ni-1)Pd(C(O)CH3)-
(CO)]+ (Figure 11a) and, due to the high binding affinity of
CO for the Pd2+ metal center, ethylene insertion is slow. The
absence of resonances in the 240 ppm region (Figure 10)
indicate the five-membered â-chelate (Figure 11c) and six-
membered γ-chelate intermediates (Figure 11d) are not prevalent
in the CO/C2H4 reaction. This result is in contrast to what was
noted under mild reaction conditions with the diphosphine Pd
metal based systems.17
recorded at -80 °C and at temperature increases of 10 °C
increments. When the sample was warmed, minor spectral
changes were observed. Two resonances are seen in the acetyl
CdO carbon region, at 224 and 225 ppm. Brookhart and co-
workers have shown that the acetyl CdO carbon resonance of
[(o-phen)Pd(C(O)CH3)(η2-CH2dCH2)]+, 217 ppm, is positioned
ca. 6 ppm upfield relative to the carbon acetyl resonance of
[(o-phen)Pd(C(O)CH3)(CO)]+ (223 ppm).16 The changes ob-
served in our study are too minor to argue for a [(Ni-1)Pd-

Pd Complexes of a Metallodithiolate Ligand
Organometallics, Vol. 26, No. 4, 2007 791
Figure 11. Proposed mechanistic cycle for the CO/C2H4 coupling reaction.
Table 1. Effect of [CO] and [Ethylene] on Productivitya
CO/Ethylene Copolymerization with [(Ni-1)Pd(CH3)-
(OEt2)][BAr′4]. Having shown that CO and ethylene insertion
can occur with [(Ni-1)Pd(CH3)(OEt2)]+, bulk copolymerization
CO
pressure
(psi)
CH2dCH2
pressure
(psi)
g of PK/
g of Pdb
run
yield (g)
productivityc
studies were conducted. The catalytic species was formed in a
CH2Cl2 solution by reacting the (Ni-1)Pd(CH3)2 and HBAr′4
precursors in predried glassware prior to transfer into the
stainless steel high-pressure Parr autoclave. Pressurization with
CO was followed by ethylene. Over the course of 24 h at room
temperature, a light gray to white product, characteristic of
polyketone, precipitated from the reaction mixture. Polyketones
are known to be soluble in cresol and (the highly toxic)
hexafluoro-2-propanol, permitting analysis by NMR, IR, and
mass spectroscopy. The solution infrared spectrum of the
polymer in (CF3)2CHOH has a distinctive ν(CdO) stretch at
1704 cm-1 in the CdO region (the KBr pellet has ν(CdO) 1692
cm-1). The room-temperature 13C NMR spectrum of the
copolymer displays two resonances at 220 and 43 ppm,
corresponding to the CdO moiety and CH2 groups from
ethylene insertion, respectively (see the Supporting Information).
These assignments correspond to those reported in the litera-
ture.13,31
1
2
3
100
100
100
100
200
300
0.1
0.3
0.4
9.4
45.8
60.0
0.4
1.9
2.5
a PK ) polyketone. Conditions: volume of solvent, 25 mL of CH2Cl2;
[catalyst] ) 0.056 mmol; temperature, 30 °C; run time, 24 h. b g of
polyketone/g of Pd. c Productivity ) g of polyketone/((g of Pd) h).
ketonic end group at 2.17 ppm, slightly upfield from the
resonance of the CH2 groups at 2.73 ppm, consistent with
literature reports.32 The MALDI-TOF mass spectral analysis of
the copolymer also confirmed that one of the end groups is a
ketone (see the Supporting Information). The polymer was
characterized by the presence of the cationic (adducts of
potassium ion) oligomers, which differ from each other by the
mass of one repeat unit (m/z 56). On the basis of the integration
of the ketonic CH3 group relative to the -CH2CH2 repeat unit,
the average molecular mass of the polymer was computed to
be approximately 3700 g/mol, corresponding to ca. 65 repeat
units.
Effect of Monomer Concentration and Solvent on Poly-
ketone Productivity with [(Ni-1)Pd(CH3)(OEt2)][BAr′4]. To
examine the effect of monomer concentration or solvent type
on polyketone formation, the following experiments were car-
ried out. Approximately 50-75 mg (0.037-0.056 mmol) of
[(Ni-1)Pd(CH3)(OEt2)]+ catalyst in 25 mL of solvent was added
to the stainless steel high pressure autoclave held at 30 °C with
varying ethylene monomer concentration while the CO pressure
was kept constant over a 24 h period. Solvent dependence was
also explored.
A red residue found in the opened autoclave was collected
after several polymer runs. Mass spectral analysis indicated it
to be the red trimetallic species, [(Ni-1)2Pd]2+. As shown in eq
8, its presence derives from chain termination with acid release
The data in Table 1 illustrate that an increase in ethylene
monomer concentration increases the polymer yield. These
results are consistent with those of Brookhart and co-workers,
in that a greater concentration of ethylene is required to shift
the reaction equilibrium toward the ethylene-bound side.22
Copolymerization reactions conducted in more donating solvents
such as CH3CN and THF showed lower production than in
CH2Cl2 (Table 2), suggesting that better coordinating solvents
of the polymer, which also produces [(Ni-1)Pd2+], presumably
as a solvate. In such a case redistribution to form the stable
trimetallic species [(Ni-1)2Pd]2+ is reasonable.
End group analysis of the polymer according to the 1H NMR
spectrum shows a small resonance of the CH3 group from the
(32) Wu, T. K.; Ovenall, D. W.; Hoen, H. H. In Applications of Polymer
Spectroscopy, 1st ed.; Brame, E. G., Ed.; Academic Press: New York, 1978;
p 19.
(31) Zhao, A. X.; Chien, J. C. W. J. Polym. Sci., Polym. Chem. 1992,
30, 2735.

792 Organometallics, Vol. 26, No. 4, 2007
Rampersad et al.
Table 2. Effect of Solvent on Productivitya
solvent time (h) yield of PK (g) g of PK/g of Pdb productivityc
complexes [(L2)Pd(CH3)]+(X)- the CO uptake and insertion
reaction is faster in the (Ni-1)PdII metal complexes.
(3) Low-pressure NMR studies of CO/ethylene (ca. 5 bar)
addition to [(Ni-1)Pd(CH3)(OEt2)]+ demonstrate fundamental
steps in the catalysis of alternating polyketone.
CH2Cl2
CH3CN
THF
24
24
24
0.056
0.035
0.031
14.1
8.9
7.8
0.6
0.4
0.3
(4) The results of in situ NMR spectral studies as well as
bulk copolymerization studies can be interpreted in terms of
the now classic mechanism proposed for the [(L2)Pd(CH3)-
(OEt2)]+ (L ) diimine) catalysts.16
a PK ) polyketone. Conditions: volume of solvent, 25 mL; [catalyst]
) 0.037 mmol; temperature, 30 °C; run time, 24 h; CO/CH2dCH2 (100
psi/200 psi). b g of polyketone/g of Pd. c Productivity ) g of polyketone/
((g of Pd) h).
The palladium-based reaction chemistry observed in this study
is readily ascribed to the presence of an L2 ligand with better
electron donating ability than the traditional diphosphine and
diimine ligands. This conclusion is consistent with the previously
established relative donor ability of NiN2S2, diimines, and
diphosphines toward W(CO)4. This conclusion is also reason-
able, as CO will have a greater binding affinity for palladium
in the presence of better electron donating ligands. Ancillary
to this is the lack of keto chelate structures in the (NiN2S2)Pd/
CO/olefin products. The low productivities in CO/olefin copo-
lymerization observed in the (NiN2S2)Pd system suggest that,
as supporting ligands in catalysis, nickel dithiolates would be
better suited to reaction chemistry involving oxidation state
increases at the attached metal. We also note that reactions
involving sulfur-bridged bimetallics may serve as homogeneous
analogues to sulfided surface catalysts.
compete with ethylene and CO for the open site, again consistent
with the studies of Brookhart and co-workers on the (diimine)-
Pd2+ system.16 Other non-coordinating solvents, while ideal for
the above reasons, are not practical for copolymerization with
our approach, because of low catalyst solubility. Interestingly,
regardless of the solvent the 13C NMR analysis showed that
the copolymer remains alternating.
In comparison to a current diphosphine-based industrial
catalyst, (diMe-Si-AXPHS)Pd2+ (diMe-Si-AXPHS ) di-o-
anisylphosphinodimethylsilane), which produces a TOF of
40 000 g of polyketone with 50 bar of CO/ethylene at 90 °C in
methanol,33 the [(Ni-1)Pd(CH3)(OEt2)]+ catalyst was much less
efficient, producing a TOF of 2.5 g at 30 °C under 7 bar of
CO/ethylene. The low productivity is likely due to the very
stable resting state complex [(Ni-1)Pd(C(O)CH3)(CO)][BAr′4].
Summary
Acknowledgment. We acknowledge support from the
National Science Foundation (Grant No. CHE 01-11629 and
CHE 06-16695 for this work and Grant No. CHE 98-07975 for
the X-ray diffractometer and crystallographic computing sys-
tem), the Robert A. Welch Foundation, and the National
Institutes of Health for a Ruth L. Kirschstein-NRSA fellowship
(to M.V.R., F31 GM073350-01).
The salient features of the nickel dithiolate ligand palladium
derivatives as CO/olefin copolymerization catalysts are as
follows.
(1) [(Ni-1)Pd(CH3)(OEt2)]+ is readily generated from
(Ni-1)Pd(CH3)2 by addition of HBAr′4, demonstrating the
reactivity of the Pd(CH3)2 precursor and the stability of the Ni-1
ligand-palladium interaction in the presence of acid.
(2) CO addition to the [(Ni-1)Pd(CH3)(OEt2)]+ cation results
in CO uptake, CO/CH3 migratory insertion, and the production
of [(Ni-1)Pd(C(O)CH3)(CO)]+ at the time of mixing at -80
°C. A similar rate of CO uptake is observed for (Ni-1)Pd(CH3)-
(Cl), with the product being (Ni-1)Pd(C(O)CH3)(Cl). In com-
parison to traditional neutral (L2)Pd(CH3)(X) and cationic metal
Supporting Information Available: Additional UV-vis, in-
frared, MALDI-TOF, and 13C NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM0605783
(33) van Doorn, J. A.; Meijboom, N.; Snel, J. J M.; Wife, R. L. Eur.
Pat. Appl. 1989, EP 300, 583, 1989.