Zerovalent Palladium and Nickel Complexes of Heterocyclic Carbenes: Oxidative Addition of Organic Halides, Carbon-Carbon Coupling Processes, and the Heck Reaction

Article abstract of DOI:10.1021/om9809771

Zerovalent carbene complexes of Pd containing the 1,3,4,5-tetramethylimidazol-2-ylidene ligand (tmiy) have been synthesized. Pd(COD)(alkene) (COD = cyclooctadiene) reacts with the nucleophilic carbene tmiy to produce the complexes Pd(tmiy)₂(alkene) (alkene = maleic anhydride (MAH) (2), tetracyanoethylene (TCNE) (3)). Spectroscopic studies on the complexes provide strong evidence of the almost purely donor nature of the carbene ligand. Oxidative addition of hydrocarbyl halide and dihalide substrates to 2 and 3 yield the Pd^II derivatives Pd(tmiy)₂(Ph)I (4), Pd(tmiy)₂I₂ (5), Pd(tmiy)₂(4-nitrophenyl)I (6), and Pd(tmiy)₂Br₂ (7). The zerovalent Ni complex Ni(tmiy)₂ was produced in situ from the reaction of Ni(COD)₂ with tmiy, and the oxidative addition of organic halides yields Ni(tmiy)₂(o-tolyl)Br (8), Ni(tmiy)₂-(Me)I (9), and Ni(tmiy)₂I₂ (10). X-ray crystal structures of complexes 8 and 10 are reported which reveal square-planar coordination with the carbene ligands inclined at significant angles to the coordination planes of the complexes. Metal to ligand bond distances are all indicative of the electron density induced on the metal center by the donor carbene ligands. Halide abstraction from 6 in the presence of n-butyl acrylate leads to migratory insertion of the olefin and elimination of the Heck coupling product. Under stoichiometric reaction conditions hydrocarbyl-imidazolium ions are also produced as byproducts. Selected complexes were tested as catalysts for the Heck reaction (Pd) and Suzuki coupling (Pd, Ni) and were found to be highly active. The observed reaction behavior is interpreted, and a mechanism for the Heck coupling is provided.

Full text of DOI:10.1021/om9809771

1596
Organometallics 1999, 18, 1596-1605
Zer ova len t P a lla d iu m a n d Nick el Com p lexes of
Heter ocyclic Ca r ben es: Oxid a tive Ad d ition of Or ga n ic
Ha lid es, Ca r bon -Ca r bon Cou p lin g P r ocesses, a n d th e
Heck Rea ction
David S. McGuinness and Kingsley J . Cavell*
School of Chemistry, University of Tasmania, GPO Box 252-75,
Hobart, Tasmania 7001, Australia
Brian W. Skelton and Allan H. White
Department of Chemistry, University of Western Australia,
Nedlands, Western Australia 6907, Australia
Received December 3, 1998
Zerovalent carbene complexes of Pd containing the 1,3,4,5-tetramethylimidazol-2-ylidene
ligand (tmiy) have been synthesized. Pd(COD)(alkene) (COD ) cyclooctadiene) reacts with
the nucleophilic carbene tmiy to produce the complexes Pd(tmiy)2(alkene) (alkene ) maleic
anhydride (MAH) (2), tetracyanoethylene (TCNE) (3)). Spectroscopic studies on the complexes
provide strong evidence of the almost purely donor nature of the carbene ligand. Oxidative
addition of hydrocarbyl halide and dihalide substrates to 2 and 3 yield the PdII derivatives
Pd(tmiy)2(Ph)I (4), Pd(tmiy)2I2 (5), Pd(tmiy)2(4-nitrophenyl)I (6), and Pd(tmiy)2Br2 (7). The
zerovalent Ni complex Ni(tmiy)2 was produced in situ from the reaction of Ni(COD)2 with
tmiy, and the oxidative addition of organic halides yields Ni(tmiy)2(o-tolyl)Br (8), Ni(tmiy)2-
(Me)I (9), and Ni(tmiy)2I2 (10). X-ray crystal structures of complexes 8 and 10 are reported
which reveal square-planar coordination with the carbene ligands inclined at significant
angles to the coordination planes of the complexes. Metal to ligand bond distances are all
indicative of the electron density induced on the metal center by the donor carbene ligands.
Halide abstraction from 6 in the presence of n-butyl acrylate leads to migratory insertion of
the olefin and elimination of the Heck coupling product. Under stoichiometric reaction
conditions hydrocarbyl-imidazolium ions are also produced as byproducts. Selected
complexes were tested as catalysts for the Heck reaction (Pd) and Suzuki coupling (Pd, Ni)
and were found to be highly active. The observed reaction behavior is interpreted, and a
mechanism for the Heck coupling is provided.
In tr od u ction
complexes have been shown to have considerable po-
tential as efficient catalysts for Heck (Pd) and Suzuki
coupling (Pd, Ni)3-6 and hydrosilylation (Rh),7 appar-
ently owing to the stability imparted on the active
species by the strong donor heterocyclic carbene ligands.
The carbene ligand appears to play a role similar to that
for tertiary phosphine ligands, although it probably does
not undergo dissociation equilibria typical of the phos-
phines. It is generally accepted that reactions such as
Heck and Suzuki coupling proceed via a Pd0 f PdII cycle
when they are catalyzed by simple bis(phosphine)
complexes, the active species being Pd(phosphine)2,
although there has been some speculation on the
involvement of PdIV intermediates when ligands such
Many transition-metal-catalyzed processes have been
shown to proceed via an initial oxidative addition to the
metal, often to a d10 (Ni0, Pd0) center stabilized by
ancillary ligands. Examples of this include the Heck
reaction, hydrocyanation, telomerization, Suzuki cou-
pling, and hydrosilylation.1,2 Two major roles of the
ancillary ligand are (i) to stabilize reactive intermediates
or transition states and (ii) to influence the activity and
selectivity through electronic and steric effects. Thus,
tertiary phosphine ligands have found widespread use
in homogeneous catalysis due to their ability to stabilize
different oxidation states and to effect the electronic and
steric properties of the metal. Heterocyclic carbene
(3) Herrmann, W. A.; Elison, M.; Fischer, J .; Ko¨cher, C.; Artus, G.
R. J . Angew. Chem. 1995, 107, 2602.
* To whom correspondence should be addressed. E-mail:
Kingsley.Cavell@utas.edu.au.
(1) Herrmann, W. A.; Cornils, B. E. Applied Homogeneous Catalysis
with Organometallic Compounds; VCH: Weinheim, Germany, 1996;
Vol. 1.
(2) Herrmann, W. A.; Cornils, B. E. Applied Homogeneous Catalysis
with Organometallic Compounds; VCH: Weinheim, Germany, 1996;
Vol. 2.
(4) Herrmann, W. A.; Reisinger, C.-P.; Spiegler, M. J . Organomet.
Chem. 1998, 557, 93.
(5) Enders, D.; Gielen, H.; Raabe, G.; Runsink, J .; Teles, J . H. Chem.
Ber. 1996, 129, 1483.
(6) Herrmann, W. A.; Gerstberger, G.; Spiegler, M. Organometallics
1997, 16, 2209.
(7) Herrmann, W. A.; Goosen, L. J .; Ko¨cher, C.; Artus, G. R. J .
Angew. Chem. 1996, 108, 2980.
10.1021/om9809771 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 04/03/1999

Zerovalent Pd and Ni Complexes of Carbenes
Organometallics, Vol. 18, No. 9, 1999 1597
Sch em e 1. Syn th esis of P d 0 Com p lexesa
as phosphapalladacycles are employed.8-11 Similar in-
volvement of a Pd0 active species in the Heck reaction
catalyzed by Pd carbene complexes was indicated by the
results of Herrmann.12 Herrmann found noticeable
induction periods for PdII complexes. The complexes,
however, were rapidly activated on addition of a reduc-
ing agent, and when Pd0 complexes were used as
starting materials, no induction period was observed at
all. Our recent mechanistic studies suggest that the
active species in the Heck reaction, catalyzed by carbene
complexes, is a 14-electron Pd0(carbene)2 complex such
as 1, which contains the tetramethylimidazol-2-ylidene
a
Legend: (i) tmiy, THF, -75 to +20 °C, alkene ) MAH;
(ii) tmiy, THF, -70 to +20 °C, alkene ) TCNE.
subsequent reactions with a number of substrates to
afford divalent complexes through oxidative addition.
Furthermore, we present the synthesis of NiII carbene
complexes via oxidative addition to Ni(tmiy)2. Crystal
structures of two Ni carbene complexes are reported.
The variable-temperature reaction behavior of an aryl-
PdII complex in the stoichiometric Heck coupling is
reported, and the mechanism of the Heck reaction,
catalyzed by Pd carbene complexes, is proposed. Ad-
ditionally, the new complexes are evaluated as catalysts
for Heck and Suzuki coupling reactions.
ligand (tmiy).13 The catalytic cycle is initiated by oxida-
tive addition of an organic halide to 1 to yield a
(hydrocarbyl)PdII intermediate. A theoretical study on
the Heck reaction with a Pd carbene catalyst also shows
oxidative addition to Pd0(carbene)2 to be a favorable
reaction.14
Although oxidative addition of halogens to Ni0 carbene
complexes has previously been carried out,15 the addi-
tion of organic halides (RX) has not been reported, and
Ni diaminocarbene complexes containing a simple aryl
group are not known. Likewise, oxidative addition of RX
to a Pd0 carbene complex is limited to our initial report
on the formation of [Pd(tmiy)2(4-acetophenonyl)Br] from
Pd(tmiy)2 (produced in situ) and 4-bromoacetophe-
none.13 It was thus of interest to us to prepare zero-
valent carbene complexes of palladium and nickel and
to investigate the addition of organic halides to such
species. We have previously reported new routes to
methylpalladium complexes through ligand displace-
ment reactions.13,16 Oxidative addition was expected to
provide yet another route to hydrocarbyl carbene com-
plexes applicable to a broader range of hydrocarbyl
groups and metals. Furthermore, we have shown that
introduction of a methyl group into PdII carbene com-
plexes has a remarkable activating affect in the Heck
reaction, presumably by providing a facile route to the
Pd0 active species.13,17 Introduction of hydrocarbyl groups
into Ni carbene complexes was expected to have a
similar effect.
Resu lts a n d Discu ssion
Zer ova len t P a lla d iu m Com p lexes. Initial attempts
to prepare bis(carbene) Pd0 alkene complexes focused
on displacement of the dba ligand (dba ) dibenzylidene-
acetone) from Pd(dba)2 by addition of free carbene
(tmiy), followed by addition of an electron-deficient
alkene such as tetracyanoethylene (TCNE), maleic
anhydride (MAH), or diethyl fumarate (DEF). The
expected complexes Pd(tmiy)2(alkene) formed, but the
synthesis was complicated by a side reaction between
the free carbene and displaced dba.17 The addition of
carbenes across activated double bonds such as occur
in dba is a facile reaction18,19 and appears to take place
before all the carbene can coordinate to the metal center.
To overcome the problem with dba, 2 equiv of tmiy
was added to the Pd(0) complexes Pd(COD)(MAH) and
Pd(COD)(TCNE)20 in THF. The reaction proceeded
smoothly to afford the bis(carbene) complexes Pd(tmiy)2-
(MAH) (2) and Pd(tmiy)2(TCNE) (3) through displace-
ment of the COD ligand (Scheme 1). Complex 2 slowly
decomposes in solution and after several days in the
solid state when exposed to air. In contrast, 3 resists
prolonged refluxing in DCM solution and the solid
appears indefinitely stable in air. Complex 3 is ex-
tremely insoluble in all common solvents, and as a result
only the 1H NMR spectrum, which shows resonances
at 2.11 and 3.72 ppm due to the tmiy ligand, could be
obtained in solution. The 13C CP/MAS spectrum shows
peaks at 8.6, 35.2, and 126.3 ppm due to the carbene
C-CH3, N-CH3 and CdC carbons, respectively. The
We report here the synthesis and characterization of
novel zerovalent carbene complexes of Pd and their
(8) Herrmann, W. A.; Brossmer, C.; O¨ fele, K.; Reisinger, C.-P.;
Priermeier, T.; Beller, M.; Fischer, H. Angew. Chem., Int. Ed. Engl.
1995, 34, 1844.
(9) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J . Am.
Chem. Soc. 1997, 119, 11687.
(10) Dyker, G.; Kellner, A. J . Organomet. Chem. 1998, 555, 141.
(11) Shaw, B. L. New J . Chem. 1998, 77.
(12) Herrmann, W. A.; Elison, M.; Fischer, J .; Ko¨cher, C.; Artus, G.
R. J . Angew. Chem., Int. Ed. Engl. 1995, 34, 2371.
(13) McGuinness, D. S.; Green, M. J .; Cavell, K. J .; Skelton, B. W.;
White, A. H. J . Organomet. Chem. 1998, 565, 165.
(14) Albert, K.; Gisdakis, P.; Ro¨sch, N. Organometallics 1998, 17,
1608.
(15) Lappert, M. F.; Pye, P. L. J . Chem. Soc., Dalton Trans. 1977,
2172.
(16) Green, M. J .; Cavell, K. J .; Skelton, B. W.; White, A. H. J .
Organomet. Chem. 1998, 554, 175.
(17) McGuinness, D. S. Synthesis and Reaction Chemistry of Hy-
drocarbylpalladium Carbene Complexes; Honours Thesis, University
of Tasmania, Hobart, Tasmania, Australia, 1997.
(18) Enders, D.; Breuer, K.; Runsink, J .; Teles, J . H. Liebigs Ann.
Chem. 1996, 2019.
(19) Enders, D.; Breuer, K.; Raabe, G.; Runsink, J .; Teles, J . H.;
Melder, J .-P.; Ebel, K.; Brode, S. Angew. Chem., Int. Ed. Engl. 1995,
34, 1021.
(20) Itoh, K.; Ueda, F.; Hirai, K.; Ishii, Y. Chem. Lett. 1977, 877.

1598 Organometallics, Vol. 18, No. 9, 1999
McGuinness et al.
Ta ble 1. Sp ectr oscop ic Da ta for Selected
(d on or )2P d (a lk en e) Com p lexes
possibility exists that 3 is dimeric or oligomeric and is
more complex than the simple structure shown in
Scheme 1. There is evidence for this in the LSI mass
spectrum, which shows a cluster at m/z 965 correspond-
ing to the [2M - H]+ ion along with the [M - H]+ cluster
at m/z 481. As well as η2-olefin coordination, the TCNE
ligand could also coordinate through the nitrile groups.
An oligomeric structure has also been suggested for the
precursor complex Pd(COD)(TCNE).20
Bon d in g in Com p lexes 2 a n d 3. An important
aspect of these imidazole-carbene complexes is the
nature of the carbene to metal bonding. It has been
stated that the carbenes are strong donor ligands with
little back-bonding from metal to carbene occurring.6,12,21
Direct comparisons have been made between these
carbenes and phosphine ligands.22,23 To provide insight
into the carbene-metal bonding in the (carbene)2Pd0-
(alkene) complexes we have monitored the effect of
coordination on the alkene. In general, the bonding of
alkenes to transition metals can be represented by two
extremes, described as pure donor bonding (I) and pure
acceptor bonding (II). The true nature of the bonding
1H NMR 13C NMR
Pd complex
IR (cm-1
)
(ppm)a
(ppm)b
(tmiy)2Pd(TCNE)
(tmiy)2Pd(MAH)
(PPh3)2Pd(TCNE)c
(PPh3)2Pd(MAH)c
2208, 2122 (ν(CtN))
1749, 1706 (ν(CdO))
2230 (ν(CtN))
3.34
4.00
4.07
4.40f
38.2
56.3
1794, 1730 (ν(CdO))
(tBuNC)2Pd(TCNE)d 2226 (ν(CtN))
(tBuNC)2Pd(MAH)d 1805, 1732
(COD)Pd(TCNE)e
(COD)Pd(MAH)e
2265, 2225 (ν(CtN))
1802, 1731 (ν(CdO))
a
b
Olefinic protons (in CDCl3). Olefinic carbons (in CDCl3).
c Reference 26. d Reference 27. e Reference 20. f NMR run in acetone-
d6.
ligand, are shifted upfield to 38.2 ppm in 2, and the
CdO groups of the coordinated MAH resonate at 175.5
ppm in 2 compared to 164.3 ppm in free MAH. These
changes are consistent with a significant decrease in
double-bond character of the olefin. The effect of back-
bonding is also visible in the IR spectrum of 2, which
shows ν(CO) at 1749 and 1706 cm-1, compared to 1850
and 1770 cm-1 for the free ligand. Significant back-
bonding from the Pd center to TCNE is also evident and
is manifest in the IR spectrum of 3, which shows ν(CtN)
peaks at 2208 and 2122 cm-1 compared to 2261 and
2227 cm-1 in the free ligand.
A comparison of spectroscopic data for 2 and 3 with
those for a selection of (donor)2Pd(MAH) and (donor)2Pd-
(TCNE) complexes (Table 1)20,26,27 provides further
evidence of the Pd to olefin back-bonding and hence
strong donor characteristics of these carbenes. Thus 2
and particularly 3 could be envisaged as approaching
PdII derivatives, although both complexes undergo reac-
tions typical of Pd0 (vide infra).
for most complexes lies somewhere between I and II,
and the relative importance of each structure is influ-
enced by the olefin, the metal, and the other ligands
associated with the metal. It has been shown that “soft”,
electron-rich metal centers in combination with strongly
electron withdrawing substituents on the olefin greatly
increase the importance of II.24 In view of the electron
density induced on the metal center by heterocyclic
carbene ligands,6,21,25 it is expected that strong Pd to
alkene back-bonding would be present in 2 and 3, and
consequently structure II would play an important role.
In particular, this will be the case if little or no back-
bonding occurs from the metal to the carbene ligands
themselves. Therefore, very large differences in the IR
and NMR spectra of the free and coordinated olefin
ligands in 2 and 3 would be indicative of marked donor
properties of heterocyclic carbene ligands and demon-
strate an absence of Pd to carbene back-bonding.
Oxid a tive Ad d ition Rea ction s of 2 a n d 3. Oxida-
tive addition to 2 was initially attempted using phenyl
iodide as the substrate. At ambient temperature no
reaction is observed and 2 is recovered unchanged. Upon
heating to ca. 60 °C oxidative addition does occur, but
the product undergoes further reaction at this temper-
ature and a complex mixture of products is obtained.
The mixture was characterized by NMR and mass
spectroscopy, which show the presence of the expected
complex Pd(tmiy)2(Ph)I (4) plus the diiodide Pd(tmiy)2I2
(5) and the 2-phenyl-1,3,4,5-tetramethylimidazolium
iodide salt. Additionally, analysis of the crude reaction
mixture by GC-MS reveals the formation of biphenyl.
To explain these products, the reaction sequence
shown in Scheme 2 is proposed. Oxidative addition of
phenyl iodide would afford complex 4, which could then
decompose via elimination of the phenylimidazolium
ion, yielding Pd0 and other unidentified product(s) from
the second carbene moiety. We have previously shown
that this is a facile mode of decomposition for cationic
methylpalladium carbene complexes,13 and we have
shown here that it may also occur from neutral species.
It is possible that the diiodide complex 5 and biphenyl
may be formed via a PdIV intermediate, but it is most
likely that they result from an intermolecular exchange
reaction of 4 in which phenyl/halide exchange occurs,
resulting in complex 5 and a bis(phenyl) complex, from
1
The H NMR spectrum of 2 consists of resonances at
2.03 and 3.60 ppm for the C-CH3 and N-CH3 protons
of the tmiy ligand, respectively, along with a peak at
3.34 ppm that represents the MAH protons. Comparison
with the proton positions in free MAH (7.05 ppm)
indicates a large upfield shift upon coordination to Pd.
This may be attributed to the strong shielding afforded
by π-back-bonding from the Pd center which somewhat
lessens the double-bond character in MAH. This effect
is just as pronounced in the 13C NMR spectrum, where
the CdC resonances of MAH, 136.6 ppm in the free
(21) Fro¨hlich, N.; Pidun, U.; Stahl, M.; Frenking, G. Organometallics
1997, 16, 442.
(22) O¨ fele, K.; Herrmann, W. A.; Mihalios, D.; Elison, M.; Herdtweck,
E.; Scherer, W.; Mink, J . J . Organomet. Chem. 1993, 459, 177.
(23) Lappert, M. F. J . Organomet. Chem. 1975, 100, 139.
(24) Malatesta, L.; Cenini, S. Zerovalent Compounds of Metals;
Academic Press: London, 1974.
(26) Fitton, P.; McKeon, J . E. Chem. Commun. 1968, 4.
(27) Otsuka, S.; Yoshida, T.; Tatsuno, Y. J . Am. Chem. Soc. 1971,
93, 6462.
(25) O¨ fele, K.; Herrmann, W. A.; Mihalios, D.; Elison, M.; Herdtweck,
E.; Scherer, W.; Mink, J . J . Organomet. Chem. 1993, 459, 177.

Zerovalent Pd and Ni Complexes of Carbenes
Organometallics, Vol. 18, No. 9, 1999 1599
Sch em e 2
Sch em e 3. P ostu la ted Mech a n ism for Ar yl
Exch a n ge Lea d in g to Bia r yl F or m a tion for th e
System P d (P R3)2(Ar )2/Ar ′-I28,29
particularly sterically demanding and a cis arrangement
of the carbene ligands is expected, as previously found
for the 1,3-dimethylimadazol-2-ylidene (dmiy) ana-
logue.3 Consistent with our previous suggestion that the
iodide ligand in these complexes exhibits lability, in
CDCl3 solution 5 undergoes halide exchange, yielding
Pd(tmiy)2ICl and Pd(tmiy)2Cl2. As a result, three closely
spaced peaks are observed for the N(CH3) and C(CH3)
methyl groups in the 1H NMR spectrumscorresponding
to 5 and the two chloro complexes.
The TCNE complex 3 is less reactive than the MAH
analogue 2, as might be expected from the greater Pd
to olefin back-bonding and consequent stronger binding
of the olefin ligand. With hydrocarbyl halides no reac-
tion was observed under conditions in which the product
would be stable. However, 3 reacted with bromine at
ambient temperature to yield Pd(tmiy)2Br2 (7) in ca.
50% yield. The 1H NMR spectrum shows only two peaks
for the methyl groups (2.09 and 3.85 ppm), indicating
only one isomer is present, probably the cis isomer. The
13C NMR spectrum likewise shows only one resonance
for each chemically different carbon of tmiy. The carbene
carbon appears at 157.8 ppm, which is similar to that
in 5 (158.9 ppm). This is somewhat upfield from those
in the aryl complexes 4 (167.1 ppm) and 6 (175.6 ppm)
and probably reflects the greater electron donation from
the carbene to the metal in 5 and 7.
which biphenyl could reductively eliminate. A spectro-
scopic investigation of the reaction mixture failed to
detect any evidence of PdIV species. Furthermore, it has
been shown for related phosphine systems that forma-
tion of biaryls results not from a PdIV intermediate but
via an intermolecular exchange reaction similar to that
postulated above.28,29 It was shown that the intermo-
lecular exchange reaction requires dissociation of a
ligand to give the three-coordinate complex III, which
then forms the aryl-bridged dimer IV, from which ligand
exchange can occur (Scheme 3). It seems unlikely that
dissociation of a carbene ligand would occur in the
present system. It is possible, however, that the iodide
ion could reversibly dissociate to give a cationic three-
coordinate species, which could then react in a manner
similar to III.
To limit side reactions, an activated aryl halide was
used, allowing milder conditions to be employed. Thus,
the oxidative addition of 4-iodonitrobenzene to 2 was
carried out at room temperature to produce Pd(tmiy)2-
(4-nitrophenyl)I 6 in 51% yield (reaction 1). The ex-
Oxid a tive Ad d ition to Ni(tm iy)2. Displacement of
the COD ligand from Ni(COD)2 and Pt(COD)2 has been
reported by Arduengo and co-workers as a route to 14-
electron bis(carbene) M0 complexes.30 This method was
regarded as a suitable starting point to study oxidative
addition reactions of Ni0. Thus, Ni(COD)2 was treated
with 2 equiv of tmiy in THF to afford Ni(tmiy)2, which
was not isolated but rather was reacted in situ.
When the THF solution of Ni(tmiy)2 was treated with
o-tolyl bromide, the complex Ni(tmiy)2(o-tolyl)Br (8) was
isolated as an air-stable orange powder in 71% yield
(reaction 2). Complex 8 represents one of only two
examples of a hydrocarbyl Ni complex containing a
diaminocarbene ligand such as tmiy.31 Importantly, this
result shows that Ni0 carbene complexes readily un-
dergo oxidative addition to give a stable hydrocarbyl NiII
fragment, the first step in many established catalytic
reactions. A trans arrangement of the ligands is sug-
pected trans coordination of the carbene ligands is
suggested by the H NMR spectrum, which shows only
1
two sharp singlets for the tmiy methyl groups (2.04 and
3.87 ppm), indicating that both carbenes are in equiva-
lent environments. Likewise, only one peak for each
chemically different carbon of tmiy is observed in the
13C NMR spectrum. Complex 6 slowly decomposes in
CD2Cl2 solution (t1/2 ≈ 3 days) to yield the 2-(4-
nitrophenyl)-1,3,4,5-tetramethylimidazolium ion as the
major product along with metallic Pd.
Oxidative addition of dihalides to the Pd0 complexes
is a facile reaction. Thus, treatment of 2 with iodine
yields Pd(tmiy)2I2 (5) in 96% yield. Only one isomer is
present according to NMR data. The tendency of the
plane of the carbene ligand to sit at an angle to the Pd
coordination plane means that the tmiy ligand is not
(28) Ozawa, F.; Fujimori, M.; Yamamoto, T.; Yamamoto, A. Organo-
metallics 1986, 5, 2144.
(29) Ozawa, F.; Hidaka, T.; Yamamoto, T.; Yamamoto, A. J . Organo-
met. Chem. 1987, 330, 253.
(30) Arduengo, A. J .; Gamper, S. F.; Calabrese, J . C.; Davidson, F.
J . Am. Chem. Soc. 1994, 116, 4391.
(31) Sellmann, D.; Allmann, C.; Heinmann, F.; Knoch, F.; Sutter,
J . J . Organomet. Chem. 1997, 541, 291.

1600 Organometallics, Vol. 18, No. 9, 1999
McGuinness et al.
gested for 8 by the NMR spectra. The tmiy methyl
groups give rise to only two peaks in the 1H NMR
spectrum at 2.00 and 4.05 ppm, and only single peaks
are observed for the tmiy carbons in the 13C NMR
spectrum.
A crystal structure determination was undertaken for
complex 8, which exhibits a trans arrangement of the
carbene ligands and a distorted-square-planar core
geometry (Figure 1, Table 2). The Ni-Ctmiy distance in
8 is longer than in other NiII diaminocarbene complex-
es,6,32-34 probably reflecting the high electron density
induced on the Ni by two tmiy ligands and the tolyl
group. The insignificance of π-back-bonding in the
Ni-Ctmiy bond is confirmed by the Ni-Ctmiy distance,
which is within the range for normal Ni-C(alkyl/aryl)
bonds. The Ni-Ctolyl distance (1.947(3) Å) is typical of
those in the more electron rich NiL2RAr type complexes
(1.933(16)-1.950(12) Å).35-41 Considering the relatively
low trans influence of the bromide ligand, this feature
is probably also related to the high electron density
induced on the Ni center by the carbene ligands,
reducing σ-donation from the tolyl group. We have
previously observed a rather long M-C bond in the
methylpalladium complex Pd(dmiy)2(Me)Cl for the same
reason.13,17 Steric congestion around the Ni center is
relieved by rotation of the carbene ligand planes from
the plane of coordination. Dihedral angles of the carbene
ligands relative to the coordination plane are 70.71(9)
and 74.87(9)°, respectively. Here, this correlates with
relief of intermethyl substituent clashes, although such
twists are a characteristic feature of heterocyclic car-
bene complexes.5,6,12,42,43 The planar tolyl group is
likewise rotated by 63.7(1)° from the plane of coordina-
tion.
F igu r e 1. Projection of 8, normal to the coordination
plane. Thermal ellipsoids with 20% probability are shown
for the non-hydrogen atoms, hydrogen atoms having arbi-
trary radii of 0.1 Å.
Ta ble 2. Selected Bon d Len gth s (Å), Bon d An gles
(d eg), a n d In ter p la n a r An gles (d eg) in
Ni(tm iy)2(o-tolyl)Br a
Bond Lengths
Ni-C(1)
Ni-C(11)
N-C(21)
1.947(3)
1.919(3)
1.911(3)
Ni-Br
C(21)-N(22)
C(21)-N(25)
2.4317(6)
1.346(4)
1.356(4)
Bond Angles
Br-Ni-C(1)
Br-Ni-C(11)
Br-Ni-C(21)
175.20(9)
88.56(8)
89.72(9)
C(1)-Ni-C(11)
C(1)-Ni-C(21)
C(11)-Ni-C(21)
92.4(1)
89.3(1)
178.2(1)
Planes
I: C(n1),N(n2),C(n3),C(n4),N(n5)
II: C(1),C(11),Br,C(21)
III: C(1),C(2),C(3),C(4),C(5),C(6)
ø2 ) 4, 18
ø2 ) 596
ø2 ) 13
Interplanar Angles
I/II
70.71(9)
74.87(9)
III/II
63.7(1)
a
Esd’s are given in parentheses; see Figure 1 for atomic
numbering.
salt was the major product; thus, the oxidative addition
product was not obtained pure. However, formation of
the NiII complex 9 was verified by a peak at -0.77 ppm
in the 1H NMR spectrum, corresponding to the Ni-
coordinated methyl group. The high-field position of this
methyl group reflects the high electron density on Ni
coordinated to two tmiy ligands. Similar Ni-Me chemi-
cal shifts were observed in electron-rich complexes
containing the strongly basic PEt3 ligand.44,45 The LSI
mass spectrum of the product mixture is dominated by
the imidazolium ion at m/z 139, but a small cluster at
m/z 321 corresponding to [M - I]+ appears along with
Oxidative addition of methyl iodide to Ni(tmiy)2 was
attempted at a variety of temperatures in THF solution.
Although a NiII complex formed, it rapidly decomposed
via elimination of the 1,2,3,4,5-pentamethylimidazolium
ion (reaction 3). Even when the reaction and workup
(38) Gutie´rrez, E.; Hudson, S. A.; Monge, A.; Nicasio, M. C.;
Paneque, M.; Carmona, E. J . Chem. Soc., Dalton Trans. 1992, 2651.
(39) Desjardins, S. Y.; Cavell, K. J .; J in, H.; Skelton, B. W.; White,
A. H. 1996, 515, 233.
(40) Miki, K.; Tanaka, M.; Kasai, N.; Wada, M. J . Organomet. Chem.
1988, 352, 385.
were conducted at less than -50 °C, the imidazolium
(32) Sellmann, D.; Prechtel, W.; Knoch, F.; Moll, M. Organometallics
1992, 11, 2346.
(33) Sellmann, D.; Prechtel, W.; Knoch, F.; Moll, M. Inorg. Chem.
1993, 32, 538.
(34) Hoberg, H.; Burkhart, G.; Kru¨ger, C.; Tsay, Y.-H. J . Organomet.
Chem. 1981, 222, 343.
(35) Chen, J .; Kai, Y.; Kasai, N.; Wada, M.; Kumazoe, M. Bull. Chem.
Soc. J pn. 1991, 64, 2802.
(41) Coronas, J . M.; Muller, G.; Rocamora, M.; Miravitlles, C.;
Solans, X. J . Chem. Soc., Dalton Trans. 1985, 2333.
(42) Herrmann, W. A.; Elison, M.; Fischer, J .; Ko¨cher, C.; Artus, G.
R. J . Chem. Eur. J . 1996, 2, 772-780.
(43) Fehlhammer, W. P.; Bliss, T.; Kernbach, U.; Bru¨dgam, I. J .
Organomet. Chem. 1995, 490, 149.
(36) Qichen, H.; Minzhi, X.; Yanlong, Q.; Weihua, X.; Meicheng, S.;
Youqi, T. J . Organomet. Chem. 1985, 287, 419.
(37) Klabunde, U.; Tulip, T. H.; Roe, D. C.; Ittel, S. D. J . Organomet.
Chem. 1987, 334, 141.
(44) Yamamoto, T.; Kohara, T.; Yamamoto, A. Bull. Chem. Soc. J pn.
1981, 54, 1720.
(45) Yamamoto, T.; Kohara, T.; Yamamoto, A. Bull. Chem. Soc. J pn.
1981, 54, 2010.

Zerovalent Pd and Ni Complexes of Carbenes
Organometallics, Vol. 18, No. 9, 1999 1601
be regarded as the first step in the Heck reaction if a
Pd0 f PdII cycle is active. Following this first step,
coordination and migratory insertion of an olefin into
the Pd-aryl bond is required before â-hydride elimina-
tion can release the product. Recent theoretical results
of Albert et al. with the system Pd(carbene)2(Ph)Br
suggest insertion occurs via halide abstraction and
subsequent olefin coordination.14 In an attempt to
identify intermediates in such an insertion mechanism,
Pd(tmiy)2(4-nitrophenyl)I 6 was treated with AgBF4 and
1 equiv of n-butyl acrylate in CD2Cl2 at -50 °C.
Abstraction of iodide is verified by a downfield shift in
the NCH3 resonance of tmiy and an upfield shift in the
aromatic protons, although coordination of the n-butyl
acrylate was not observed. Even when the halide
abstraction is carried out at -80 °C, using Ag[(3,5-
(CF3)2C6H3)4B] to increase the solubility of the cationic
complex, only signals due to free n-butyl acrylate are
observed. When the solution is warmed to -30 °C,
signals due to the 2-(4-nitrophenyl)-1,3,4,5-tetrameth-
ylimidazolium ion V begin to appear with minor amounts
of the ion VII. At -20 °C signals due to n-butyl (E)-4-
nitrocinnamate (VIII) emerge together with those from
the 1,3,4,5-tetramethylimidazolium ion X. It is apparent
that under the conditions of this stoichiometric experi-
ment the migratory insertion and elimination steps are
fast and that the concentration of the intermediate with
coordinated alkene very low. We propose the reactions
shown in Scheme 4 to explain the formation of these
products.
F igu r e 2. Projection of 10 (centrosymmetric), normal to
the coordination plane.
Ta ble 3. Selected Bon d Len gth s (Å), Bon d An gles
a
(d eg), a n d In ter p la n a r An gles (d eg) in Ni(tm iy)2I2
Bond Lengths
Ni-C(1)
Ni-I
1.897(3)
2.5180(3)
C(1)-N(2)
C(1)-N(5)
1.349(4)
1.347(3)
Bond Angles
I-Ni-C(1)
Ni-C(1)-N(2)
90.79(8)
127.7(2)
Ni-C(1)-N(5)
N(2)-C(1)-N(5)
127.3(2)
104.9(2)
Planes
I: C(1),N(2),C(3),C(4),N(5)
II: C(1)2I2
ø2 ) 2.0
Interplanar Angle
I/II
86.97(8)
a
Esd’s are given in parentheses; see Figure 2 for atomic
numbering.
others at m/z 306 ([M - MeI]+) and m/z 182 ([Ni-
(tmiy)]+). No molecular ion could be observed for 9.
The mode of decomposition of 9 is a notable result,
as it shows that hydrocarbyl NiII complexes are also
susceptible to the carbene elimination reaction we have
observed for hydrocarbyl Pd complexes.13,17 Nickel is a
commonly used metal in catalysis, and hydrocarbyl-
metal complexes are considered to be intermediates in
a number of catalytic cycles, so that this observation
has important implications with respect to possible
catalyst deactivation.
Initial halide abstraction from 6 would yield the
cationic complex 11. This complex then partially de-
composes via the arylimidazolium elimination reaction
to give 2-(4-nitrophenyl)-1,3,4,5-tetramethylimidazolium
(V) and Pd0. We have noted previously that the hydro-
carbyl-carbene elimination reaction is most facile for
cationic complexes.13 This is consistent with the much
lower temperature at which it occurs for the cationic
complex 11 compared to its neutral parent 6. Formation
of compound VII is explained by coordination and rapid
migratory insertion of the olefin to afford the alkyl
intermediate VI, from which product elimination would
give VII. At -20 °C it seems â-hydride elimination
becomes competitive, resulting in formation of n-butyl
(E)-4-nitrocinnamate (VIII) and the unstable intermedi-
ate hydride complex IX. Decomposition of IX would then
produce 1,3,4,5-tetramethylimidazolium (X) and Pd0.
The species X can be considered as a proton trap, and
The zerovalent Ni complex reacts with halogens in a
manner similar to that for the Pd0 complexes. Thus,
when Ni(tmiy)2 is treated with iodine in THF, the NiII
complex Ni(tmiy)2I2 (10) is obtained as violet crystals
1
its appearance, clearly identified by H NMR, provides
after recrystallization from DCM/ether. Only one isomer
is indicated by NMR spectroscopy. A trans arrangement
is the most likely structure.6
evidence for the intermediate IX. The intermediates VI
and IX are not observed. However, each product in
1
Scheme 4 has been identified by H NMR and MS of
The results of a room-temperature, single-crystal
X-ray structure determination of 10 show the solid-state
Ni environment to be a centrosymmetric square-planar
trans array, with half of the molecule comprising the
asymmetric unit of the structure (Figure 2, Table 3).
The tmiy ligand plane lies quasi-normal (interplanar
dihedral angle 86.97(8)o) to the (Ni)C2I2 coordination
plane. The Ni-Ctmiy distance is rather shorter (ca. 0.02
Å) than in 8 but is still within the range for normal Ni-
C(alkyl/aryl) bonds, confirming that there is no signifi-
cant Ni-C π-bonding in 10.
the NMR solution (full spectroscopic data are provided
in the Experimental Section).
The relative proportions of each product depend on
the reaction conditions. When the experiment is con-
ducted at room temperature, the main products are the
migratory insertion/â-hydride elimination Heck product
VIII along with the 1,3,4,5-tetramethylimidazolium ion.
Only a small amount of 2-(4-nitrophenyl)-1,3,4,5-tetra-
methylimidazolium product is formed, and signals due
to VII are not observed. It appears that â-hydride
elimination is greatly accelerated at higher tempera-
tures and can compete with the carbene elimination
Mech a n istic Asp ects of th e Heck Rea ction . Oxi-
dative addition of 4-iodonitrobenzene to Pd0(tmiy)2 may

1602 Organometallics, Vol. 18, No. 9, 1999
McGuinness et al.
Sch em e 4a
Sch em e 5
although insertion via a five-coordinate species cannot
be ruled out. On the basis of this new experimental
information we propose that the mechanism for the
Heck reaction catalyzed by Pd-carbene complexes is
that shown in Scheme 5 and is essentially identical with
that predicted by the theoretical work of Albert et al.14
The likely active species is the 14-electron Pd0 complex
Pd(tmiy)2, to which oxidative addition of an aryl halide
occurs to yield the PdII aryl complex. Dissociation of the
halide ligand is accompanied by olefin coordination,
insertion, and â-hydride elimination to release the
product and give (after recoordination of the halide)
Pd(tmiy)2HX. Reductive elimination of HX in the pres-
ence of base then regenerates the active Pd0 species.
aR ) COOBu. The counterion BF4 is excluded from the
-
scheme. Formation of V, VII, and X is accompanied by the
formation of other unidentified product(s) from the second
carbene moiety.
Su zu k i Cou p lin g. The Suzuki coupling of 4-bromo-
acetophenone with phenylboronic acid was investigated
with selected examples of the new complexes. The
application of Pd complexes with chelating heterocyclic
dicarbene ligands in Suzuki coupling has been reported
by Herrmann et al,6 but 1 mol % catalyst was used such
that turnover numbers of only ca. 100 were obtained.
In contrast, the (aryl)PdII complex 6 and the Pd0
complex 3 catalyze the coupling with high turnover
numbers. For example, using 2 × 10-3 mol % of 6 a
turnover number of 28 000 is obtained after 24 h. We
believe the high activity of 6 is a further example of the
activating influence that the hydrocarbyl group has in
Pd carbene complexes of this type, by providing a facile
route to the Pd0 active species.13,17 Whereas in the Heck
reaction the Pd0 species is generated by olefin insertion
into the Pd-C bond followed by â-hydride elimination
and reductive elimination of HX, in Suzuki coupling the
arylboronic acid could react with the (aryl)Pd complex,
giving a bis(aryl) complex from which the biaryl coupling
product and a Pd0 active species are formed. In contrast,
dihalide complexes are not so readily reduced, requiring
a significant induction period before they become active
and leading to a loss of catalyst activity. The Pd0
complex 3 is even more active, giving a turnover number
of 55 700 with 1 × 10-3 mol % catalyst. In the case of 3
it is likely that the high activity results from its ability
to undergo oxidative addition (under the forcing condi-
tions of the Heck reaction, 120 °C) and hence undergo
the first and possibly rate-determining step in the
catalytic cycle.
reactions, suggesting that under catalytic Heck condi-
tions (120 °C, appropriate base, excess substrate) 6
should give rise to an effective Heck catalyst. Impor-
tantly, this was found to be the case. Under Heck
conditions complex 6 forms an efficient catalyst for the
coupling of 4-iodonitrobenzene with butyl acrylate to
yield n-butyl (E)-4-nitrocinnamate, the same product
that was obtained from the stoichiometric modeling
experiments. The E geometry of VII is verified by the
1
16 Hz coupling of the olefinic protons in the H NMR
spectrum. Turnover numbers were high, and there was
little evidence of decomposition to Pd0. Complex 6
(amount of catalyst 1.7 × 10-3 mol %) and the Pd0
complex 3 (2.1 × 10-3 mol %) also catalyzed the coupling
of n-butyl acrylate with 4-bromoacetophenone to pro-
duce n-butyl (E)-4-formylcinnamate with TONs of ca.
50 000.
These results provide experimental evidence that the
mechanism of olefin insertion into the Pd-aryl bond
during the Heck reaction is one in which halide abstrac-
tion from the PdII complex, formed from the oxidative
addition of an aryl halide to a Pd0 complex, allows
coordination and insertion of the olefin. In the absence
of silver salts no insertion is observed at lower temper-
atures. The elevated temperatures (120 °C)13 required
for the insertion step in the absence of a halide
scavenger may be necessary for bromide dissociation
before alkene coordination and insertion can proceed,

Zerovalent Pd and Ni Complexes of Carbenes
Organometallics, Vol. 18, No. 9, 1999 1603
The Suzuki coupling reaction is also catalyzed by Ni-
carbene complexes, although not as efficiently as ob-
served with Pd. The use of Ni(tmiy)2I2 (3 × 10-2 mol %)
as a catalyst precursor resulted in a 19% conversion of
aryl halide after 24 h, corresponding to a turnover
number of 630. When Ni(tmiy)2(o-tolyl)Br is used under
the same conditions, a 58% conversion is obtained (TON
) 1930). Thus, introduction of an aryl group into the
catalyst precursor leads to a large increase in activity,
although not as pronounced as in the Pd case. The lower
activity of the Ni complex relative to its Pd analogue
does not appear to result from decomposition of the
catalyst, as there was no observation of Ni0 deposits
even after reaction times of 24 h.
attempt to elucidate the detailed mechanism of this
decomposition reaction.
Exp er im en ta l Section
Gen er a l Com m en ts. Unless otherwise stated, all manipu-
lations were carried out using standard Schlenk techniques
or in a nitrogen glovebox. All solvents for use in an inert
atmosphere were purified by standard procedures and distilled
under nitrogen immediately prior to use.47 Nuclear magnetic
resonance (NMR) spectra were recorded at ambient temper-
ature unless otherwise stated, and peaks are labeled as singlet
(s), doublet (d), triplet (t), multiplet (m), and broad (br).
Elemental analysis, MS, and GC-MS were carried out by the
Central Science Laboratory, University of Tasmania. Pd(COD)-
(MAH), Pd(COD)(TCNE),20 Ni(COD)2 and tmiy49 were pre-
48
pared as described in the literature.
Su m m a r y
Str u ctu r e Deter m in a tion s. Room-temperature spheres of
data were measured using a Bruker AXS CCD instrument,
with proprietary SMART, SAINT, and SADADS data process-
ing (2θmax ) 58°, ω-scans; monochromatic Mo KR radiation, λ
) 0.710 73 Å, T ca. 298 K). Nt total reflections were reduced
to Nr unique reflections (Rint cited) and No reflections with |Fo|
> 4σ|Fo| being considered “observed” and used in the full-
matrix least-squares refinement. An “empirical” absorption
correction was applied. Anisotropic thermal parameters were
Zerovalent bis(carbene)Pd(olefin) complexes have been
prepared and fully characterized. Although these com-
plexes may be considered as having PdII character, they
undergo oxidative addition reactions typical of Pd0. In
this regard there are similarities to related phosphine
complexes such as Pd(PR3)2 and Pd(PR3)(olefin) as
previously highlighted.16,23,25 This similarity is in part
supported by the present work, which shows carbenes
may play a role similar to that of phosphine ligands in
stabilizing and activating the metal toward oxidative
addition. The oxidative addition of organic halides was
carried out to yield PdII carbene complexes, providing
a new route to Pd hydrocarbyl carbene complexes.
Likewise, a stable NiII hydrocarbyl diaminocarbene
complex was prepared via oxidative addition to a Ni0
center stabilized by the tmiy ligand. These results give
valuable mechanistic insights into reactions catalyzed
by Pd/Ni carbene complexes. The subsequent steps in
the Heck reactionsinsertion of an olefin into the Pd-
aryl bond and â-hydride elimination of the products
have been demonstrated using complex 6. Other cata-
lytic reactions, for instance, Suzuki coupling and
telomerization,17,46 may likewise be formulated as pro-
ceeding through a M0 f MII cycle when catalyzed by
carbene-stabilized Pd/Ni complexes.
refined for the non-hydrogen atoms, (x,y,z,Uiso
) being refined
H
for 10 and constrained at estimated values for 8. Conventional
residuals R and Rw (statistical weights) at convergence are
cited. Neutral atom complex scattering factors were employed;
computation used the XTAL 3.4 program system.50
Cr ysta l/Refin em en t Da ta . 8: C21H31BrN4Ni, Mr ) 478.1,
orthorhombic, space group Pbca (D12h5, No. 61), a ) 15.553(3)
Å, b ) 16.941(3) Å, c ) 16.941(3) Å, V ) 4464(2) Å3. Dc(Z ) 8)
) 1.423 g cm-3, F(000) ) 1984, µMo ) 26.7 cm-1, specimen 0.23
× 0.20 × 0.12 mm, Tmin,max ) 0.59, 0.86, Nt ) 28 825, Nr
)
)
5659 (Rint ) 0.062), No ) 3794, R ) 0.035, Rw ) 0.043, nυ
244; |∆Fmax| ) 0.6 e Å-3
.
10: C14H24I2N4Ni, Mr ) 280.4, monoclinic, space group P21/c
(C52h, No. 14), a ) 10.026(2) Å, b ) 10.204(2) Å, c ) 8.208(1) Å,
â ) 102.902(2)°, V ) 979.0 Å3, Dc(Z ) 2) ) 1.902 g cm-3, F(000)
) 540, µMo ) 41 cm-1, specimen 0.39 × 0.28 × 0.26 mm, Tmin,max
) 0.48, 0.80, Nt ) 10 969, Nr ) 2463 (Rint ) 0.023), No ) 2128,
R ) 0.027, Rw ) 0.039, nυ ) 98, |∆Fmax| ) 0.7 e Å-3
.
P r ep a r a tion of Com p ou n d s. P d (tm iy)2(MAH) (2). To a
stirred (-75 °C) suspension of Pd(COD)(MAH) (200 mg, 0.64
mmol) in 20 mL of THF was added dropwise over 10 min a
solution of tmiy (160 mg, 1.29 mmol) in 20 mL of THF. The
resulting deep red solution was stirred for 30 min at ca. -50
°C before being warmed to room temperature. The solution
was filtered through Celite, the THF removed in vacuo, and
the solid washed with THF/ether (3 mL/10 mL) and twice with
ether (10 mL) to afford a red-pink powder. Yield: 250 mg
(86%). Anal. Calcd for C18H26N4O3Pd‚0.25THF (indicated by
NMR): C, 48.47; H, 5.99; N, 11.90. Found: C, 48.17; H, 6.37;
N, 11.66. MS (LSIMS; m/z): 354, [M - (MAH)]+ (62%); 125,
Although comparisons have been made between phos-
phine and carbene ligands, there are, however, clear
differences between the ligands. One is a distinction in
degreescarbenes appear to be stronger donors than
even the most basic phosphines, thus greatly reducing
the possibility of ligand (carbene) dissociation, a com-
mon process for many phosphine complexes. The second
major distinction is the reactivity of the coordinated
carbene, possibly caused by the lack of back-donation
from the metal to the carbene. We have demonstrated
that hydrocarbyl (and hydride) complexes decompose via
attack of the hydrocarbyl group on the carbene with
elimination of imidazolium salts and subsequent reduc-
tion of the metal. This appears to be a common mode of
carbene loss for PdII and NiII hydrocarbyl complexes and
indicates a significant pathway for catalyst deactivation,
as hydrocarbyl species are expected to be intermediates
in many catalytic cycles. We are currently conducting
combined theoretical and kinetic investigations in an
1
[tmiyH]+ (100%). H NMR (200 MHz, CDCl3): δ 3.60 (s, 12H,
NCH3), 3.34 (s, 2H, HCdCH), 2.03 (s, 12H, CCH3). 13C NMR
(50 MHz, CDCl3): δ 185.5 (NCN), 175.5 (CdO), 125.3 (Cd
C
tmiy), 38.3 (CdCMAH), 35.7 (NCH3), 9.7 (CCH3). IR (KBr): 1749,
1706 cm-1 (ν(CO)).
(47) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, U.K., 1996.
(48) Herrmann, W. A.; Salzer, A. Synthetic Methods of Organo-
metallic and Inorganic Chemistry; Georg Thieme Verlag: New York,
1996; Vol. 1.
(46) McGuinness, D. S.; Cavell, K. J ., Unpublished results. Methyl-
palladium carbene complexes have been found to be effective catalysts
for the telomerization of butadiene with methanol to selectively yield
1-methoxy-2,7-octadiene.
(49) Kuhn, N.; Kratz, T. Synthesis 1993, 561.
(50) Hall, S. R., King, G. S. D., Stewart, J . M., Eds. The Xtal 3.4
User’s Manual; University of Western Australia: Lamb, Perth, Aus-
tralia, 1995.

1604 Organometallics, Vol. 18, No. 9, 1999
McGuinness et al.
P d (tm iy)2(TCNE) (3). This compound was prepared in the
same manner as for 2 (from 228 mg, 0.665 mmol of Pd(COD)-
(TCNE) and 166 mg, 1.34 mmol of tmiy) except for a modified
workup: approximately half of the THF was removed in vacuo
and 10 mL of ether added to complete precipitation. The solid
was collected by filtration, washed with ether (3 × 10 mL),
and dried in vacuo to yield an orange powder. Yield: 273 mg
(85%). Anal. Calcd for C20H24N8Pd: C, 49.75; H, 5.01; N, 23.20.
Found: C, 49.53; H, 4.81; N, 23.08. MS (LSIMS; m/z): 965,
[2M - H]+ (3%); 605, [M - H + (tmiy)]+ (8%); 481, [M - H]+
(40%); 354, [M - (TCNE)]+ (100%). 1H NMR (400 MHz,
CD2Cl2): δ 3.72 (s, 12H, NCH3), 2.11 (s, 12H, CCH3). 13C CP/
MAS NMR: δ 126.3 (CdCtmiy), 35.2 (NCH3), 8.6 (CCH3). IR
(KBr): 2208, 2122 cm-1 (ν(CN)).
complete precipitation. The solvent was decanted off and the
powder washed twice with acetone/ether (1 mL/6 mL) and
again with 15 mL of ether. Drying in vacuo gave an off-white
powder. Yield: 46 mg (51%). Anal. Calcd for C20H28N5O2IPd‚
0.1(acetone) (indicated by NMR): C, 40.00; H, 4.73; N, 11.49.
Found: C, 40.01; H, 4.48; N, 11.05. MS (LSIMS; m/z): 604,
mixed cluster [MH]+ and [M - I + tmiy]+ (14%); 480, mixed
cluster [M - I]+ and [Pd(tmiy)3]+ (6%); 354, [Pd(tmiy)2]+ (11%);
246, [2-(4-nitrophenyl)-1,3,4,5-tetramethylimidazolium]+ (100%).
1H NMR (200 MHz, CD2Cl2): δ 7.62 (d, J ) 9 Hz, 2H, Ph H),
7.42 (d, J ) 9 Hz, 2H, Ph H), 3.87 (s, 12H, NCH3), 2.04 (s,
12H, CCH3). 13C NMR (100 MHz, CD2Cl2): δ 175.6 (NCN),
163.9 (CNO2), 136.6 (PhC), 132.5 (PhC), 124.4 (CdCtmiy), 119.4
(PhC), 34.6 (NCH3), 8.4 (CCH3). IR (KBr): 1652, 1335 cm-1
(ν(NO2)). In DCM solution 6 was allowed to fully decompose
and found to consist of the 2-(4-nitrophenyl)-1,3,4,5-tetra-
methylimidazolium ion and other unidentified product(s) from
the second carbene moiety. MS (LSIMS; m/z): 246, [M]+
Rea ction of 2 w ith P h en yl Iod id e. In a typical experi-
ment Pd(tmiy)2(MAH) (91 mg, 0.20 mmol) was dissolved in
15 mL of THF and phenyl iodide (23 µL, 0.21 mmol) syringed
in. The solution was heated to near reflux for 30 min, during
which time Pd0 deposits form; these were cooled and filtered
through Celite to yield a homogeneous yellow solution. Re-
moval of the solvent in vacuo led to a mixture of yellow and
white solids. The product was washed with 3 mL of THF to
leave a white solid that was composed mainly of Pd(tmiy)2-
(Ph)I (4) with some 2-phenyl-1,3,4,5-tetramethylimidazolium
iodide. MS (LSIMS; m/z): 605, [M - Ph + tmiy]+ (5%); 555,
[M - I + tmiy]+ (2.6%); 481, [M - Ph]+ (2.2%); 431, [M - I]+
(1.8%); 354, [M - PhI]+ (4%); 201, [2-phenyl-1,3,4,5-tetra-
methylimidazolium]+ (100%); 125, [tmiyH]+ (68%). 1H NMR
of 4 (200 MHz, DMSO-d6): δ 7.21-6.60 (m, br, 5H, Ph H), 3.86
(s, br, 12H, NCH3), 2.07 (s, br, 12H, CCH3). 13C NMR of 4 (100
MHz, DMSO-d6): δ 167.1 (NCN), 136.2, 134.8, 130.6, 129.5
(PhC), 125.8 (CdC), 34.6 (NCH3), 8.5 (CCH3). The THF
washings were found to contain a mixture of the starting
complex, Pd(tmiy)2I2 (5), the imidazolium salt, and some
oxidative addition product 4. The ratio of products obtained
depended on the conditions employed. Thus, when the system
was heated for a prolonged time (ca. 2 h), the major product
was 5 along with the imidazolium salt. Complex 5 was
1
(100%); 125, [tmiyH]+ (41%). H NMR (200 MHz, CD2Cl2): δ
8.47 (d, J ) 9 Hz, 2H, Ph H), 8.14 (d, J ) 9 Hz, 2H, Ph H),
3.62 (s, 6H, NCH3), 2.36 (s, 6H, CCH3).
P d (t m iy)2Br 2 (7). To a DCM (20 mL) suspension of Pd-
(tmiy)2(TCNE) (108 mg, 0.22 mmol) was added bromine (12
µL, 0.23 mmol) dropwise and the solution stirred for 40 min.
The solution was filtered through Celite, the DCM removed
in vacuo, and the dark residue extracted with THF (4 × 5 mL).
The combined extracts were put under vacuum until ca. 5 mL
remained, and the product was precipitated by the addition
of 10 mL of ether. The solvent was decanted off and the solid
washed twice with 10 mL of ether. The crude product was then
taken up in 3 mL of DCM and precipitated by vapor diffusion
of ether to yield deep red analytically pure crystals. Yield
(crude): 61 mg (53%). Anal. Calcd for C14H24N4Br2Pd: C, 32.68;
H, 4.70; N, 10.89. Found: C, 32.60; H, 4.48; N, 10.70. MS
(LSIMS; m/z): 513, [MH]+ (1.4%); 433, [M - Br]+ (17%); 354,
1
[Pd(tmiy)2]+ (4%); 125, [tmiyH]+ (100%). H NMR (400 MHz,
CD2Cl2): δ 3.85 (s, 12H, NCH3), 2.09 (s, 12H, CCH3). 13C NMR
(100 MHz, CD2Cl2): δ 157.4 (NCN), 125.7 (CdC), 35.4 (NCH3),
8.6 (CCH3).
1
identified by comparison to the spectra (MS, H/13C NMR) of
an authentic sample prepared by oxidative addition of I2 to 2
Ni(tm iy)2(o-tolyl)Br (8). To a THF (20 mL, -70 °C)
solution of Ni(COD)2 (342 mg, 1.24 mmol) was added a solution
of tmiy (315 mg, 2.54 mmol) in 20 mL of THF. After the
mixture was warmed to ambient temperature, o-tolyl bromide
(0.15 mL, 1.25 mmol) was syringed in and the solution stirred
for 1 h. The solution was filtered through Celite to yield a
homogeneous orange solution, from which the THF was
removed in vacuo until ca. 10 mL remained. The product was
precipitated by the addition of 15 mL of hexane, the solvent
decanted off, and the solid washed with hexane (25 mL) to
yield an orange powder after drying in vacuo. Crystals suitable
for X-ray analysis were grown by vapor diffusion of hexane
into a THF solution of the complex. Yield: 420 mg (71%). Anal.
Calcd for C21H31N4BrNi: C, 52.76; H, 6.54; N, 11.72. Found:
C, 52.53; H, 6.54; N, 11.65. MS (LSIMS; m/z): 397, [M - Br]+
(86%); 306, [Ni(tmiy)2]+ (93%); 215, [tmiy + tolyl]+ (100%); 182,
[Ni(tmiy)]+ (45%); 125, [tmiyH]+ (97%). 1H NMR (400 MHz,
DMSO-d6, 60 °C): δ 7.50 (s, br, 1H, Ph H), 6.49 (d, J ) 7 Hz,
2H, Ph H), 6.43 (d, J ) 7 Hz, 1H, Ph H), 4.05 (s, 12H, NCH3),
2.61 (s, 3H, PhCH3), 2.00 (s, 12H, CCH3). 13C NMR (50 MHz,
CDCl3): δ 180.2 (NCN), 156.0, 143.3, 138.4, 126.8, 122.3, 121.0
(PhC), 124.6 (CdCtmiy), 35.8 (NCH3), 26.2 (PhCH3), 9.4 (CCH3).
Rea ction of Ni(tm iy)2 a n d Meth yl Iod id e. In a typical
experiment tmiy (168 mg, 1.35 mmol) in 20 mL of THF was
added to Ni(COD)2 (182 mg, 0.662 mmol) dissolved in 20 mL
of THF (-60 °C) and the solution warmed to ambient tem-
perature. The solution was then cooled again (-60 °C) and
methyl iodide (43 µL, 0.69 mmol) syringed in. After it was
stirred for 20 min at -40 °C, the solution was filtered through
Celite and the solvent removed in vacuo until ca. 10 mL
remained. Hexane (20 mL) was added to precipitate the
product, and the solvent was decanted off. The solid was
(vide infra). MS (LSIMS of imidazolium salt; m/z): 201 [M]+
1
(100%). H NMR (400 MHz, DMSO-d6): δ 7.67 (t, J ) 7 Hz,
2H, Ph H), 7.47 (t, J ) 7 Hz, 2H, Ph H), 7.03 (d, J ) 7 Hz, 1H,
Ph H), 3.68 (s, 6H, NCH3), 2.04 (s, 6H, CCH3). GC-MS of the
reaction solution showed peaks due to biphenyl (m/z 154) and
2-phenyl-1,4,5-trimethylimidazole (m/z 185) resulting from
pyrolysis of the imidazolium salt on the heated injector/column.
P d (tm iy)2I2 (5). To a solution (0 °C) of Pd(tmiy)2(MAH) (54
mg, 0.12 mmol) in 5 mL of DCM was added dropwise over 1 h
a DCM solution (10 mL) of iodine (35 mg, 0.14 mmol). The
orange solution was filtered through Celite and washed
through with 5 mL of DCM. The solvent was removed in vacuo
and the residue washed with DCM/ether (2 mL/6 mL) and
further with ether (2 × 10 mL) before drying in vacuo to afford
an orange powder. Yield: 70 mg (96%). Anal. Calcd for
C14H24N4I2Pd‚0.5DCM (indicated by NMR): C, 26.75; H, 3.87;
N, 8.61. Found: C, 26.88; H, 3.58; N, 8.14. MS (LSIMS; m/z):
607, mixed cluster [M]+ and [M - I + tmiy]+ (17%); 481, [M -
1
I]+ (100%); 354, [M - 2I]+ (50%). H NMR (200 MHz, DMSO-
d6): δ 3.75 (s, 12H, NCH3), 2.07 (s, 12H, CCH3). 13C NMR (50
MHz, DMSO-d6): δ 158.9 (NCN), 126.1 (CdC), 35.9 (NCH3),
8.8 (CCH3).
P d (tm iy)2(4-n itr op h en yl)I (6). A DCM solution (10 mL)
of Pd(tmiy)2(MAH) (68 mg, 0.15 mmol) and 4-iodonitrobenzene
(43 mg, 0.17 mmol) was stirred at ambient temperature for 3
h, during which time the solution changes from dark red to a
lighter orange-red. The solution was filtered through Celite
and washed through with 5 mL of DCM and the solvent
removed in vacuo until ca. 2 mL remained. Acetone (5 mL)
was added and the solvent removed in vacuo until a precipitate
began to form, at which point 5 mL of ether was added to

Zerovalent Pd and Ni Complexes of Carbenes
Organometallics, Vol. 18, No. 9, 1999 1605
washed with 10 mL of hexane and dried in vacuo to yield a
light orange solid. The 1H NMR and MS data indicated that
the crude product consisted of Ni(tmiy)2(Me)I (9) and the
1,2,3,4,5-pentamethylimidazolium ion as the major products
(ca. 1:2) along with other unidentified decomposition products
(the imidazolium assignments were made by comparison to
an authentic sample). All attempts at purification resulted in
further decomposition. MS (LSIMS; m/z): 321, [M - I]+ (8%);
306, [Ni(tmiy)2]+ (34%); 182, [Ni(tmiy)]+ (16%); 139, [1,2,3,4,5-
pentamethylimidazolium]+ (100%). 1H NMR (200 MHz,
CD2Cl2): δ 4.13 (s, 12H, NCH3 (9)), 3.71 (s, 6H, NCH3 (imid)),
2.76 (s, 3H, C2CH3 (imid)), 2.25 (s, 6H, CCH3 (imid)), 2.06 (s,
12H, CCH3 (9)), -0.77 (s, 3H, NiCH3).
Ni(tm iy)2I2 (10). Ni(tmiy)2 was produced in situ as for 8
(227 mg, 0.825 mmol of Ni(COD)2 and 209 mg, 1.68 mmol of
tmiy), and an iodine (210 mg, 0.827 mmol) solution in 20 mL
of THF was slowly syringed in at 0 °C. The solution was stirred
at ambient temperature for 30 min and filtered through Celite
and the solvent removed in vacuo until ca. 5 mL remained.
Ether (10 mL) was added and the precipitate collected by
filtration and washed with DCM/ether (2 mL/10 mL) followed
by ether (2 × 10 mL) and dried in vacuo to yield an orange-
brown powder. The product was purified by vapor diffusion of
ether into a DCM (2 mL) solution to afford analytically pure
violet crystals suitable for X-ray analysis. Yield (crude): 324
mg (70%). Anal. Calcd for C14H24N4I2Ni: C, 29.98; H, 4.31; N,
9.99. Found: C, 30.04; H, 4.37; N, 10.01. MS (LSIMS; m/z):
557, mixed cluster [M]+ and [M - I + (tmiy)]+ (38%); 433, [M
- I]+ (13%); 306, [Ni(tmiy)2]+ (34%); 125, [tmiyH]+ (100%). 1H
NMR (200 MHz, CD2Cl2): δ 4.02 (s, 12H, NCH3), 1.94 (s, 12H,
CCH3). 13C NMR (50 MHz, CD2Cl2): δ 169.4 (NCN), 125.5
(CdC), 34.2 (NCH3), 8.1 (CCH3).
VIII and butyl acrylate which are considerably more abun-
dant.) At -20 °C the formation of n-butyl (E)-4-nitrocinnamate
(VIII) is observed along with the 1,3,4,5-tetramethylimid-
azolium ion X, which were both indentified by comparison the
spectral data of authentic samples. The NMR solution was
analyzed by MS (ESI; m/z): 125 [tmiyH]+ (9%); 246 [2-(4-
nitrophenyl)-1,3,4,5-tetramethylimidazolium]+ (15%); 251
[C13H17O4N]+ (VII-imidazolium ring) (100%).
Heck Cou p lin g. In a typical run, a 100 mL two-necked
flask fitted with an air-cooled condenser and a septum was
charged with 4-bromoacetophenone (4.98 g, 25 mmol) and
anhydrous sodium acetate base (2.29 g, 28 mmol) and degassed
by successive vacuum-nitrogen cycles. N,N-Dimethylacet-
amide (25 mL), n-butyl acrylate (5 mL, 27 mmol) and bis-
(ethylene glycol) butyl ether (1000 µL, internal standard) were
then injected, and the mixture was heated to 120 °C by means
of an oil bath. A solution of the catalyst dissolved in DCM was
then injected. After the desired time, a sample of the solution
(500 µL) was taken, washed with 5% HCl (5.00 mL). and
extracted with 2.50 mL of DCM. The DCM extracts were
analyzed by gas chromatography.
Su zu k i Cou p lin g. A two-necked 100 mL flask equipped
with a reflux condenser and septum was charged with 4-bromo-
acetophenone (1.99 g, 10 mmol), phenylboronic acid (1.34 g,
11 mmol), and potassium carbonate (2.76 g, 20 mmol) before
being put under an atmosphere of N2. Toluene (20 mL) was
injected and the solution brought to reflux in an oil bath with
vigorous stirring. A solution of the catalyst (DCM or THF, 0.2
mL) was then injected and the solution left for 24 h, after
which time bis(ethylene glycol) butyl ether was added (200 µL,
internal standard) and the solution analyzed by GC-FID.
Olefin In ser tion Exp er im en ts w ith P d (tm iy)2(4-n itr o-
p h en yl)I. In a typical experiment, an NMR sample of 6 in
CD2Cl2 was treated with 1 equiv of n-butyl acrylate and a
slight excess of AgBF4 at -50 °C. A 1H NMR spectrum of the
product at -50 °C showed resonaces due to complex 11 (400
MHz, CD2Cl2, -50 °C): δ 7.68 (d, J ) 8 Hz, 2H, Ph H), 7.54
(d, J ) 8 Hz, 2H, Ph H), 3.91 (s, 12H, NCH3), 2.06 (s, 12H,
CCH3). At -30 °C peaks due to 2-(4-nitrophenyl)-1,3,4,5-
tetramethylimidazolium (V) begin to appear, which were
identified by comparison to those resulting from decomposition
of 6 (vide supra), along with peaks due to VII (400 MHz,
CD2Cl2, -30 °C): δ 8.35 (d, J ) 9 Hz, 2H, Ph H), 7.83 (d, J )
9 Hz, 2H, Ph H), 3.86 (s, 6H, NCH3), 2.74 (t, J ) 7 Hz, 1H,
C2CH). (The 7 Hz doublet expected for the benzylic protons of
VII was not observed and is probably obscured by one of the
several other peaks in the expected region. Likewise, in each
of species VII and VIII the methylene signals of the butyl
group overlap with those of remaining n-butyl acrylate, and
hence a complex pattern is observed in which assignment of
signals cannot be made. The signals due to the CH3 protons
of the butyl group in VII are obscured by the strong signals of
Ack n ow led gm en t. We are indebted to the Austra-
lian Research Council for financial support and for
providing an Australian Postgraduate Award for D.S.M.
We also acknowledge the generosity of J ohnson-Matthey
for providing a loan of palladium chloride. The staff of
the Central Science Laboratory (University of Tasma-
nia) are gratefully acknowledged for their assistance in
a number of the instrumental techniques, particularly
Dr. Evan Peacock for conducting solid-state NMR
experiments.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, atomic coordinates with isotropic displacement
parameters, bond lengths, bond angles, anistropic displace-
ment parameters, and H-atom coordinates with isotropic
displacement parameters for compounds 8 and 10. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM9809771