New bidentate trans-chelating N-heterocyclic carbene ligands for palladium

Article abstract of DOI:10.1021/om100890q

A family of imidazolium salts of the type [BnN(CH₂CH ₂CH₂^RIm)₂]A·2[Cl] (Bn = CH₂Ph; ^RIm = 1-methylimidazole (1a), 1-tert-butylimidazole (1b), 1-benzylimidazole (1c), 1-methylbenzimidazole (1d)), which contain a tertiary amine linking two imidazolium groups, has been synthesized. These imidazolium salts can be deprotonated with Ag₂O to generate the Ag carbene complexes [{BnN(CH₂CH₂CH₂ ^RIm)₂}Ag].[AgCl₂] (^RIm = 1-methylimidazole (2a), 1-tert-butylimidazole (2b), 1-benzylimidazole (2c), 1-methylbenzimidazole (2d)). In the solid state 2d exists as an unusual tetramer, which consists of an [Ag₂Cl₄]^2- core bridging two Ag(NHC) cations. Subsequent reaction of the Ag complexes with PdCl₂(MeCN)₂ generates Pd species of the type {BnN(CH ₂CH₂CH₂^RIm)₂}PdCl ₂ (^RIm = 1-methylimidazole (3a), 1-tert-butylimidazole (3b), 1-benzylimidazole (3c), 1-methylbenzimidazole (3d)), which is a rare example of a family of Pd complexes that contain a bidentate trans-chelating N-heterocyclic carbene ligand. Compounds 3a and 3c were crystallographically characterized by X-ray crystallography and contain unusual 12-membered metallacycles. DFT calculations suggest that the preference for trans binding of the ligand is related to conformational effects of the linker. Compound 3b reacts with excess MeI to form {BnN(CH₂CH₂CH ₂^tBuIm)₂}PdI₂ (5b), a reaction in which we believe a Pd(IV) intermediate is generated. Compound 5b was crystallographically characterized. Compounds 3a-d are all active catalysts for the Heck reaction, and 3a can also catalyze the Suzuki reaction.

Full text of DOI:10.1021/om100890q

ARTICLE
pubs.acs.org/Organometallics
New Bidentate Trans-Chelating N-Heterocyclic Carbene Ligands
for Palladium
†
†
†
†
,†
James D. Blakemore, Matthew J. Chalkley, Joy H. Farnaby, Louise M. Guard, Nilay Hazari,*
†
†,‡
†
Christopher D. Incarvito, Eddie D. Luzik, Jr., and Hee Won Suh
†
The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
‡
The Department of Chemistry and Chemical Engineering, University of New Haven, 300 Boston Post Road, West Haven,
Connecticut 06516, United States
S Supporting Information
b
ABSTRACT:
R
R
A family of imidazolium salts of the type [BnN(CH CH CH Im) ] 2[Cl] (Bn = CH Ph; Im = 1-methylimidazole (1a), 1-tert-
2
2
2
2
3
2
butylimidazole (1b), 1-benzylimidazole (1c), 1-methylbenzimidazole (1d)), which contain a tertiary amine linking two imidazolium
groups, has been synthesized. These imidazolium salts can be deprotonated with Ag O to generate the Ag carbene complexes
2
R
R
[
1
{BnN(CH CH CH Im) }Ag] [AgCl ] ( Im = 1-methylimidazole (2a), 1-tert-butylimidazole (2b), 1-benzylimidazole (2c),
-methylbenzimidazole (2d)). In the solid state 2d exists as an unusual tetramer, which consists of an [Ag Cl ] core bridging two
2
2
2
2
3
2
2
ꢀ
2 4
Ag(NHC) cations. Subsequent reaction of the Ag complexes with PdCl (MeCN) generates Pd species of the type
2
2
R
2 2 2 2 2
R
{
BnN(CH CH CH Im) }PdCl ( Im = 1-methylimidazole (3a), 1-tert-butylimidazole (3b), 1-benzylimidazole (3c), 1-methyl-
benzimidazole (3d)), which is a rare example of a family of Pd complexes that contain a bidentate trans-chelating N-heterocyclic
carbene ligand. Compounds 3a and 3c were crystallographically characterized by X-ray crystallography and contain unusual
1
2-membered metallacycles. DFT calculations suggest that the preference for trans binding of the ligand is related to conformational
tBu
effects of the linker. Compound 3b reacts with excess MeI to form {BnN(CH CH CH Im) }PdI (5b), a reaction in which we
2
2
2
2
2
believe a Pd(IV) intermediate is generated. Compound 5b was crystallographically characterized. Compounds 3aꢀd are all active
catalysts for the Heck reaction, and 3a can also catalyze the Suzuki reaction.
’
INTRODUCTION
by their o-xylyl-lined carbene (ii) on the basis of solubility, in
most cases separation of cis and trans isomers has not been
The isolation and crystallographic characterization of ther-
19
achieved. Another problem in the preparation of trans-span-
ning bidentate NHC complexes was first encountered by Dano-
poulos and co-workers, who found that their ligand (iii) formed a
mixture of inseparable monomeric, dimeric, and oligomeric Pd
mally stable N-heterocyclic carbenes (NHCs) by Arduengo
et al. initiated a considerable amount of research into the
organometallic chemistry of these versatile ligands.
discovery, a wide range of different NHC ligands have been
prepared, including systems which incorporate NHCs in chelate
and pincer scaffolds.
coordinate to most transition metals and can be utilized in a
1
2
ꢀ4
Since this
11
species upon metalation. Recently, Burgess et al. successfully
5
ꢀ7
It has been shown that these ligands
prepared a family of trans-spanning bidentate NHC ligands (iv),
9
which feature a diamide linker. This was one of the first families
number of important catalytic reactions, particularly using
of trans-spanning bidentate NHC ligands, and Pd complexes
supported by these ligands were able to catalyze the Heck
reaction, albeit with low activity. In general, it appears that
3ꢀ5,7
Pd.
Surprisingly, given the plethora of research into biden-
7
tate chelating NHC ligands for Pd, only a handful are known to
coordinate in a trans fashion (NHCꢀMꢀNHC close to
although there are many examples of tridentate pincer ligands
8
ꢀ18
20ꢀ29
1
80°).
Selected examples are shown in Figure 1. The first
coordinated to Pd, which feature trans carbenes,
it is
example (i) was described in 2000 by RajanBabu et al. and
contains two dinapthyl groups. This ligand readily coordinated
difficult to synthesize trans-chelating bidentate NHC ligands.
Furthermore, the electronic or steric properties of the ligand
cannot be easily varied.
8
to Pd but gave a mixture containing the carbene coordinated in
both a cis and trans fashion; a problem which has been observed
1
3,19
in other systems.
Although Baker and co-workers were able
Received: September 15, 2010
Published: March 18, 2011
to separate the cis and trans isomers of a Pd complex supported
r 2011 American Chemical Society
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|Organometallics 2011, 30, 1818–1829

Organometallics
ARTICLE
Figure 1. Previously synthesized bidentate trans-spanning NHC ligands.
Given that bidentate trans-chelating phosphines have been
known since the 1970s and have found applications in many
30
catalytic reactions of Pd, our aim was to synthesize a series of
easily accessible bidentate NHC ligands which coordinate to Pd
in a trans fashion. Here, we build on pioneering work by
Douthwaite and co-workers exploring flexible bis-NHC-amido
1
4,31
ligands
to synthesize a family of new bidentate trans-chelat-
ing NHC ligands which contain a flexible linker incorporating a
tertiary amine. The ligands readily bind to Pd(II) to form
complexes which feature an unusual 12-membered palladacycle
and are active catalysts for the Heck and Suzuki reactions.
Figure 2. Douthwaite’s bis-NHC-amine (v) and bis-NHC-amido (vi)
and our target trans-chelating NHC ligands (vii).
intermolecular exchange between the NHCꢀAg groups. In all
cases the benzylic protons on the central amine appear as a singlet.
It has previously been observed that silver carbenes can adopt a
’
RESULTS AND DISCUSSION
1
7,32
variety of different binding modes in the solid state.
Single
Ligand Synthesis and Metalation. Recently Douthwaite and
co-workers synthesized a family of bis-NHC-amine (v) and bis
crystals of 2d were grown from a saturated solution of dichlor-
omethane at room temperature, and the structure is shown in
Figure 3. In the solid state 2d exists as an unusual tetramer, which
NHC-amido ligands (vi) and showed that they coordinate to Pd(II)
14,31
2ꢀ
(
Figure 2).
In almost all cases the amine or amido ligand was
consists of an [Ag Cl ] core bridging between two Ag(NHC)
2
4
bound to Pd to form a tridentate pincer type ligand, except for one
crystallographically confirmed example with a sterically bulky amine
substituent and one example with a bulky substituent on the
imidazole, which was not crystallographically characterized. We
postulated that by increasing the chain length of the linker from two
carbons to three carbons and by utilizing a sterically bulky sub-
stituent on the amine, it would be possible to shut down coordina-
tion of the amine and generate a rare family of trans-chelating NHC
ligands (vii) with easily varied properties (Figure 2).
Our synthesis of the new family of ligands is straightforward
and high-yielding (Scheme 1). The synthesis allows for easy
modification of the substituent on the imidazole ring, which we
have demonstrated, and in principle it should also be possible to
change the substituent on the central amine. Starting from
commercially available benzylamine and ethyl acrylate, we per-
formed a Michael addition to give the known ester (a). Reduc-
cations. There is a short AgꢀAg distance (2.9968(11) Å)
between the silver atoms of the cation and anion, which is
consistent with a weak interaction. In contrast, the AgꢀAg
distance in the anionic core is 3.5706(8) Å, which is longer than
14
3
3
the sum of the van der Waals radii of two silver atoms (3.44 Å).
Although this tetrameric Ag moiety has been observed in several
1
7,32,34ꢀ39
Ag(NHC) complexes with Br or I as the halide,
best of our knowledge it is only the second example with Cl as the
to the
34
halide and the first containing a bidentate ligand. The carbene
ligand binds to the Ag with a trans geometry(C(1)ꢀAg(1)ꢀC(29)
= 162.52(15)°), and there is no binding from the central amine unit.
The AgꢀC distances of 2.055(3) and 2.057(4) Å are consistent
with those observed in other silver carbenes.
Reaction between 2aꢀd and [PdCl (MeCN) ] resulted in
2
2
clean transmetalation of the carbene and generated the novel Pd
complexes 3aꢀd, with a trans-chelating NHC ligand. The
relatively high yields observed for carbene transmetalation
suggest that, in contrast to other systems, oligomerization is
tion of the ester with LiAlH generated the bis(alcohol) (b),
4
which was converted into the bis(chloride) (c) using SOCl . The
2
11,14
key step in the synthesis was an S 2 reaction between the
not a significant problem.
This may be related to the
N
bis(chloride) (c) and a substituted imidazole to generate the
imidazolium ligand precursors 1aꢀd. Interestingly, in our system
this reaction was significantly slower than in Douthwaite’s system
with a two-carbon linker, and it was critical to perform the
metalations being performed at low concentration. Furthermore,
only one isomer is formed, whereas other systems give a mixture
1
1,13
of cis and trans species or mono- and binuclear complexes.
1
The H NMR spectra of 3aꢀd suggest that the complexes
14
reaction at high concentration. The imidazolium salts 1aꢀd
possess C symmetry in solution or are undergoing exchange
s
were deprotonated using Ag O to generate the Ag carbenes
processes similar to those for the Ag species 2aꢀd, as even at low
temperature only three resonances were observed for the twelve
NCH CH CH N protons.
2
1
13
2
aꢀd. These species were characterized by H and C NMR
spectroscopy, high-resolution mass spectrometry (HRMS), and
2
2
2
1
elemental analysis. The H NMR spectrum is consistent with Cs
The X-ray structures of 3a and 3c were determined (Figures 4
and 5, respectively) and unequivocally confirm trans chelation of
the bis-NHC ligand with no coordination from the central amine
(PdꢀN distances are approximately 4.35 Å in 3a and 3.90 Å in
3c). The C(2)ꢀPd(1)ꢀC(18) bond angle in 3a is 177.01(13)°,
while the corresponding angles in 3c are 174.9(2) and 177.1(2)°
symmetry or an exchange process which equilibrates the methy-
lene protons in the two NCH CH CH N groups so that only
2
2
2
three resonances are observed. The exchange process could
involve inversion of the amine nitrogen and interconversion of
conformations by twisting about CꢀC and CꢀN bonds, or even
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Organometallics
ARTICLE
a
Scheme 1
a
Legend: (i) MeOH, room temperature, 5 days, 99%; (ii) LiAlH
4
, THF, 0 °C, 24 h, 85%; (iii) SOCl
2
, CH
2
Cl
2
, room temperature, 1 h, 92%; (iv)
imidazole, dioxane, 65 °C, 3ꢀ24 h, 82ꢀ94%; (v) Ag
2
O, CH
2
Cl
2
, room temperature, 24 h, 70ꢀ98%; (vi) PdCl
2 2 2 2
(MeCN) , CH Cl , room temperature, 3
h, 82ꢀ96%.
(
two independent molecules are present in the unit cell). These
PdꢀNHC coordination plane, and as a result all three benzyl
groups are pointing in the same direction.
are some of the largest bite angles reported for Pd complexes with
30
trans-spanning ligands. The complexes feature an unprece-
dented 12-membered palladacycle and the expected square-
planar geometry around Pd. The bond lengths and distances
are unexceptional, and the carbene ligands are oriented approxi-
mately perpendicular to the coordination plane, which is con-
sistent with the structure of other trans-spanning bis-NHC
In order to further understand the factors that cause the trans
isomer to be preferred, we performed density functional theory
(DFT) calculations. Initially we looked at the model systems cis-
and trans-(NHC) PdCl (viiiꢀxi), which contain two carbenes
2
2
which are not linked (Figure 6). In all cases the trans isomer is
lower in energy than the cis isomer, but the difference in energy is
surprisingly small and does not even rise significantly as the steric
bulk on the ligand is increased (Table 1). However, as the steric
bulk of the NHC ligand increases, the five-membered imidazole
ring tilts out of the coordination plane of Pd. This is shown for
the trans isomers of viiiꢀxi in Figure 7 (and in the Supporting
Information for the corresponding cis isomers of viiiꢀxi). In the
case of both the trans and cis isomers the tilt increases as the steric
bulk on the NHC increases. This suggests that electronically it is
preferable to have the imidazole ligand coplanar with the
coordination plane around Pd, but sterically it becomes prefer-
able to have the imidazole ring almost normal to the
coordination plane.
1
0,12,14
ligands.
Both structures possess C symmetry because
1
the benzyl group is canted toward one of the propyl arms, but
a fluxional process similar to those mentioned above presumably
gives rise to the C -symmetric NMR spectra. Interestingly, in the
s
structure of 3c two different orientations of the amine nitrogen
benzyl group are observed (as drawn in parts a and b of Figure 5,
this corresponds to the amine nitrogen benzyl group pointing
into or out of the page), whereas in the structure of 3a only one
orientation is present. This suggests that the flexible linker is able
to support several different conformations, while the carbene
ligands remain coordinated. In the structure of 3c the central
amine is below the PdꢀNHC coordination plane and the benzyl
group on the amine is directed above the plane. The benzyl
substituents on the imidazole rings are also directed above the
In contrast to the results from the unlinked case, calculations
on the linked systems trans- and cis-xii and trans- and cis-3a
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Organometallics
revealed that the cis isomer is significantly higher in energy. The
optimized structure of trans-3a was a staggering 42.7 kJ mol
lower in energy than the optimized structure of cis-3a (Figure 8).
ARTICLE
40
Oxidation and Catalysis. In recent years there has been
significant interest in the synthesis of Pd(III) and Pd(IV)
complexes, and they have been proposed as intermediates in
ꢀ
1
42ꢀ44
Similarly, the calculations on trans- and cis-xii revealed that the
several catalytic cycles.
In general NHC ligands are con-
ꢀ
1
trans isomer was 42.0 kJ mol lower in energy. This indicates
that the large difference in energy between the two structures is
not related to the orientation of the benzyl group on the central
amine or the methyl groups on the imidazole. We conclude that
conformational effects relating to the orientation of the linker are
responsible for the preference of 3aꢀd to adopt a trans geometry.
Interestingly, it has already been demonstrated that shorter
sidered to be excellent donors, and we were interested in
oxidizing our Pd(II) complexes 3aꢀd. Electrochemical studies
show that compounds 3aꢀd all display irreversible oxidations
40
between 1.4 and 1.7 V versus NHE (Figure 9). In contrast, no
oxidation of our starting material PdCl (MeCN) is seen at
2
2
potentials up to 2 V versus NHE, suggesting that our ligands
make the oxidation of Pd(II) more facile in comparison with a
commonly occurring Pd precatalyst. We believe that the oxida-
tion is metal-centered, because the Ag complexes 2aꢀd show no
sign of significant oxidation, suggesting that the ligand is not
41
linkers favor cis geometries, and on the basis of these results
and our current work we suggest that a linker of at least six atoms
is required to favor trans coordination.
40
readily oxidized. At this stage there is no obvious explanation
for the changes in oxidation potentials on the basis of the
electronic effects of the substituents on the ligand. Surprisingly,
compounds 3aꢀd are also remarkably resistant to reduction, as
no reduction was seen electrochemically. In contrast, PdCl -
2
(
MeCN) is easily reduced to Pd black.
Further support for our hypothesis that the new ligands make
2
oxidation of Pd(II) easier was obtained from the reaction of 3b
with excess MeI. At 50 °C the diiodide species 5b and MeCl are
generated. The reaction presumably proceeds through oxidative
addition of MeI to Pd(II) to generate an unstable Pd(IV) species,
which then reductively eliminates MeCl (Scheme 2). NMR-scale
experiments indicated the formation of an unsymmetrical inter-
mediate, which we postulate as the mixed Pd(II) halide species
(
3
4b). Reaction of 4b with MeI was clearly faster than reaction of
b, as significant concentrations of 4b did not build up in the
reaction mixture. Reductive elimination from the Pd(IV) is
presumably facile, as no evidence for the proposed Pd(IV)
intermediate was observed spectroscopically. Reaction between
3
b and EtI was significantly slower but also resulted in the
Figure 3. Molecular structure of 2d. Hydrogen atoms and solvent
molecules have been removed for clarity. Selected bond lengths (Å) and
formation of 5b and EtCl. This is consistent with our proposal of
oxidative addition. The X-ray structure of 5b was determined
angles (deg): Ag(1)ꢀAg(2)
.5706(8), Ag(1)ꢀC(1) = 2.055(3), Ag(1)ꢀC(29) = 2.057(4), Ag-
2)ꢀCl(1) = 2.3150(15), Ag(2)ꢀCl(2) = 2.3546(12), Ag(2)ꢀCl(2A)
=
2.9968(11), Ag(2)ꢀAg(2A)
=
(
Figure 10) and again shows trans chelation of the bis-NHC
3
(
ligand, with no coordination from the central amine (the PdꢀN
distance is approximately 4.69 Å). As expected, the PdꢀI bonds
in 5b (Pd(1)ꢀI(1) = 2.6164(7), Pd(1)ꢀI(2) = 2.6288(8) Å) are
significantly longer than the PdꢀCl bonds in either 3a
(Pd(1)ꢀCl(1) = 2.3217(7), Pd(1)ꢀCl(2) = 2.3230(7) Å) or
=
2.9551(14); C(1)ꢀAg(1)ꢀC(29) = 162.52(15), Ag(2)ꢀAg(1)ꢀC-
(
1) = 102.52(11), Ag(2)ꢀAg(1)ꢀC(29) = 94.92(11), Ag(1)ꢀAg-
2)ꢀAg(2A) = 130.832(14), Ag(1)ꢀAg(2)ꢀCl(1) = 80.09(4),
(
Ag(1)ꢀAg(2)ꢀCl(2) = 83.69(3), Ag(1)ꢀAg(2)ꢀCl(2A) = 151.35(3).
Figure 4. Molecular structure of 3a: (a) view down the Cl(1)ꢀPdꢀCl(2) axis; (b) view down the C(2)ꢀPdꢀC(18) axis. Hydrogen atoms and solvent
molecules have been removed for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)ꢀCl(1) = 2.3217(7), Pd(1)ꢀCl(2) = 2.3230(7),
Pd(1)ꢀC(18) = 2.018(4), Pd(1)ꢀC(2) = 2.021(4); Cl(1)ꢀPd(1)ꢀCl(2) = 177.73(3), C(2)ꢀPd(1)ꢀC(18) = 177.01(13), C(2)ꢀPd(1)ꢀCl(1) =
91.63(8), C(2)ꢀPd(1)ꢀCl(2) = 89.20(8), C(18)ꢀPd(1)ꢀCl(1) = 90.37(8), C(18)ꢀPd(1)ꢀCl(2) = 88.88(8).
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Organometallics
ARTICLE
Figure 5. Molecular structure of 3c: (a) first independent molecule in the unit cell; (b) second independent molecule in the unit cell. Hydrogen atoms
have been removed for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)ꢀCl(1) = 2.2969(14), Pd(1)ꢀCl(2) = 2.2988(16), Pd(1)ꢀC(8) =
2
.033(5), Pd(1)ꢀC(24) = 2.034(5); Cl(1)ꢀPd(1)ꢀCl(2) = 178.41(6), C(8)ꢀPd(1)ꢀC(24) = 174.9(2), C(8)ꢀPd(1)ꢀCl(1) = 92.50(16),
C(8)ꢀPd(1)ꢀCl(2) = 87.12(16), C(24)ꢀPd(1)ꢀCl(1) = 91.03(16), C(24)ꢀPd(1)ꢀCl(2) = 89.45(16); Pd(2)ꢀCl(3) = 2.2981(16), Pd(2)ꢀCl(4)
2.2993(18), Pd(2)ꢀC(41) = 2.016(7), Pd(2)ꢀC(57) = 2.016(6); Cl(3)ꢀPd(2)ꢀCl(4) = 178.49(6), C(41)ꢀPd(2)ꢀC(57) = 177.1(2),
C(41)ꢀPd(2)ꢀCl(3) = 89.92(19), C(41)ꢀPd(2)ꢀCl(4) = 89.76(19), C(57)ꢀPd(2)ꢀCl(3) = 90.49(17), C(57)ꢀPd(2)ꢀCl(4) = 89.90(17).
=
Figure 6. Compounds whose structures were optimized using DFT.
Table 1. Relative Energies of the Optimized Structures of
n-butyl acrylate using the conditions outlined by Douthwaite and
14
viiiꢀxi
compd
co-workers (Table 2). The maximum yield obtained was 80%,
and no further conversion was observed at longer reaction times.
Surprisingly, the substituent on the imidazole only made a minor
difference in catalytic activity. However, the possibility that the
ligand is decoordinating from the metal and that the reaction is
catalyzed by colloidal Pd seems unlikely, as there was no increase
in activity at lower catalyst loadings, which is a common feature of
-1 a
b
rel energy (kJ mol )
tilt of NHC (deg)
trans-viii
cis-viii
trans-ix
cis-ix
0
0, 0
2.51
0
23.2, 23.1
67.1, 67.1
68.8, 72.0
68.3, 68.3
73.2, 75.2
90.0, 90.0
69.7, 69.8
1.21
0
0
14
reactions that are catalyzed by Pd .
trans-x
cis-x
The results described in Table 2 used previous literature
1.16
0
14
conditions for the Heck reaction, and we were interested in
trans-xi
cis-viii
optimizing the conditions for our catalyst. A number of different
parameters were varied using 3a as the catalyst, including
temperature, the absence or presence of the tetra-n-butylammo-
nium bromide additive, the nature of the base, and the number of
equivalents of n-butyl acrylate (Table 3). Our results (entries 1
and 2) indicated that, although the reaction could be run at lower
temperatures, there was a significant decrease in rate as the
temperature was decreased, suggesting that in a fashion analo-
gous to that for other systems higher temperatures are required
to achieve reasonable reaction rates. Surprisingly, our highest
yield with 1.4 equiv of n-butyl acrylate was only 80% and we
noticed that, even though it was present in excess, all of the
n-butyl acrylate had been consumed at the end of the reaction.
Increasing the amount of acrylate to 2 equiv resulted in an
increased yield (entry 4), and changing the base from sodium
acetate to sodium carbonate resulted in a yield of 100%. It has
previously been reported that the addition of tetraalkylammonium
bromide salts results in increased catalytic activity in the Heck
2.67
a
ꢀ1 b
Lowest energy isomer set to 0 kJ mol . Dihedral angle between Pd
coordination plane and the plane of the imidazole ring.
3
c (Pd(1)ꢀCl(1) = 2.2969(14), Pd(1)ꢀCl(2) = 2.2988(16),
Pd(2)ꢀCl(3) = 2.2981(16), Pd(2)ꢀCl(4) = 2.2993(18) Å).
In the structure of 5b the plane formed by the five-membered
NHC ring is exactly perpendicular to the Pd coordination
plane, whereas there is some deviation from 90° in the structures
of 3a,c. The exact reason for this difference is unclear, but
it adds further support to our proposal that the ligand is highly
flexible.
Previously, it has been shown that Pd species with both
bidentate trans-spanning phosphine and NHC ligands are com-
8,14,17,18,45,46
petent catalysts for the Heck reaction.
We were
interested in testing our complexes as catalysts. Initially it was
demonstrated that 3aꢀd all couple 4-bromoacetophenone with
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ARTICLE
Figure 7. DFT optimized structures of trans-viiiꢀxi. H atoms are omitted for clarity.
Figure 8. DFT optimized structures of trans-3a and cis-3a. H atoms are omitted for clarity.
aryl iodides and aryl bromides but is unable to couple aryl
chlorides. The coupling of aryl iodides is more efficient than
that of aryl bromides. Electron-rich substituents which are less
likely to undergo the Heck reaction are tolerated for aryl iodides
but do not undergo efficient coupling with aryl bromides. Overall
the performance of our catalyst for the Heck reaction is better
than that of the Pd bidentate trans-spanning NHC system
8
described by RajanBabu and co-workers and is comparable with
14
that described by Douthwaite et al. All systems are vastly
4
8
inferior to state of the art catalysts for the Heck reaction, which
presumably reflects the difficulty in performing oxidative addi-
tion to a square-planar Pd complex with a trans-spanning
bidentate ligand.
It has also been demonstrated that Pd complexes with
bidentate trans-spanning NHC ligands can be used as catalysts
1
6,17
for the Suzuki reaction.
Using the optimized conditions
17
described by Smith and co-workers, we were able to demon-
strate that 3a is an active catalyst for the Suzuki reaction
(Table 5). The substrate scope is again limited to aryl bromides
and iodides, with no reaction observed with aryl chlorides. Our
system is less active than Smith’s, which is also capable of
Figure 9. Cyclic voltammograms for 3a (red, dashed/dotted line,
bottom), 3b (black, solid line, top), 3c (blue, dashed line, second from
top), and 3d (pink, dotted line, second from bottom) in MeCN.
4
7
17
reaction, and in our case removal of the additive also results in
lower activity (entries 4 and 6).
The generality of our catalyst for the Heck reaction was
investigated by exploring the substrate scope (Table 4). Com-
pound 3a can efficiently couple n-butyl acrylate with a variety of
coupling aryl chlorides. At this stage the reasons for the
differences between systems are unclear, as there is almost no
mechanistic information about how complexes with bidentate
trans-spanning NHC ligands undergo the fundamental steps
associated with the Suzuki reaction.
1
823
dx.doi.org/10.1021/om100890q |Organometallics 2011, 30, 1818–1829

Organometallics
ARTICLE
Scheme 2
Table 3. Results for the Optimization of the Conditions for
Heck Reaction between n-Butyl Acrylate and 4-Bromoaceto-
phenone Using 3a as the Catalyst
temp
amt of acrylate
(equiv) time (h) yield (%)
a
entry (°C) additive
base
1
2
3
4
5
6
120
140
140
140
140
140
yes
yes
yes
yes
yes
no
NaOAc
NaOAc
NaOAc
1.4
1.4
2
64
4
80
67
4
93
NEt
Na CO
NaOAc
3
2
4
22
2
3
2
4
100
39
2
1
4
a
The average of three runs determined by H NMR using an internal
standard; only the trans isomer was observed. No increase in yield was
observed if the reaction was left for longer times. Conditions: all
reactions used 0.1 mol % 3a, base (0.46 mmol), 4-bromoacetophenone
Figure 10. Molecular structure of 5b. Hydrogen atoms and solvent
molecules have been removed for clarity. Selected bond lengths (Å) and
angles (deg): Pd(1)ꢀI(1) = 2.6164(7), Pd(1)ꢀI(2) = 2.6288(8),
Pd(1)ꢀC(5) = 2.051(4), Pd(1)ꢀC(21) = 2.059(5); I(1)ꢀPd(1)ꢀI(2)
(0.41 mmol), and dimethylacetamide (2 mL). The quantities of other
reagents and the temperature are given in the table.
=
174.18(3), C(5)ꢀPd(1)ꢀC(21) = 173.3(3), C(5)ꢀPd(1)ꢀI(1) =
90.89(16), C(5)ꢀPd(1)ꢀI(2) = 90.50(17), C(21)ꢀPd(1)ꢀI(1) =
Table 4. Substrate Scope for Heck Reaction between n-Butyl
Acrylate and Aryl Halides Using 3a as the Catalyst
89.45(17), C(21)ꢀPd(1)ꢀI(2) = 88.52(18).
a
entry
substrate
time (h)
yield (%)
Table 2. Results for the Heck Reaction between n-Butyl
Acrylate and 4-Bromoacetophenone Using Complexes 3aꢀd
as the Catalyst
1
2
3
4
5
6
7
8
9
4-bromoacetophenone
4-bromonitrobenzene
bromobenzene
4
4
100
100
30
50
14
100
100
99
88
94
0
8
a
b
cat. (amt (mol %))
time (h)
yield (%)
TOF
4-bromotoluene
4-bromoanisole
8
8
3
3
3
3
3
3
a (0.1)
a (0.01)
a (0.01)
b (0.1)
c (0.1)
d (0.1)
1
1
67
6.5
71
59
80
72
672
650
323
600
792
720
4-iodoacetophenone
4-iodonitrobenzene
iodobenzene
<3
3
22
1
4
4-iodotoluene
4
1
1
0
4-iodoanisole
4
1
a
1
11
4-chloroacetophenone
4-chlorotoluene
8
The average of three runs determined by H NMR using an internal
1
2
8
0
standard; only the trans isomer was observed. No increase in yield was
a
1
observed if the reaction was left for longer times, except where specified.
The average of two runs determined by H NMR using an internal
b
Turnover frequency (TOF) = (mol of product)/((mol of Pd) h))
standard; only the trans isomer was observed. No increase in yield was
observed if the reaction was left for longer times. Conditions: all reactions
used 0.1 mol % 3a, Na CO (0.46 mmol), substrate (0.41 mmol), 2 equiv
Conditions: NaOAc (0.46 mmol), tetra-n-butylammonium bromide
(0.041 mmol), 4-bromoacetophenone (0.41 mmol), n-butyl acrylate
2
3
(0.57 mmol), dimethylacetamide (2 mL), 140 °C.
of n-butyl acrylate, and dimethylacetamide (2 mL) at 140 °C.
1
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Organometallics
ARTICLE
Table 5. Substrate Scope for Suzuki Reaction between Phe-
nylboronic Acid and Aryl Halides Using 3a as the Catalyst
MeCN) was used as an external standard and to reference the potential
to the NHE. A basal plane carbon electrode was used as the working
electrode to reduce background oxidation. The electrode consisted of a
brass cylinder, sheathed in a Teflon tube. At the tip of the brass, a two-
part Ag conducting epoxy (Alfa Aesar) was used to firmly attach the basal
plane carbon electrode surface to the brass. Finally, the tip was sealed
with the organic solvent resistant, electrically insulating, two-part epoxy
Tra-bond 2151 (Emerson and Cuming, Canton, MA). Immediately
prior to experiments, the working electrode was polished with 1 μm
alumina paste, washed with copious amounts of water, and allowed to
dry completely. Then, the surface of the working electrode was
resurfaced with tape to restore the gray, basal surface.
a
entry
substrate
time (h)
yield (%)
1
2
3
4
5
6
7
8
9
4-bromoacetophenone
4-bromonitrobenzene
bromobenzene
12
6
100
84
70
20
100
98
89
92
0
3
4-bromoanisole
24
1
4-iodoacetophenone
4-iodonitrobenzene
iodobenzene
5
3
DFT Calculations. Density functional calculations were carried out
4-iodoanisole
17
8
5
0
using Gaussian 09 Revision A.01. Calculations were performed using
the B3LYP functional and the LANL2DZ basis set for Pd and the
4-chloroacetophenone
4-chlorotoluene
1
0
8
0
6-31G** basis set for all other atoms. The coordinates from X-ray
a
1
The average of two runs determined by H NMR using an internal
standard. No increase in yield was observed if the reaction was left for
longer times. Conditions: all reactions used 1 mol % 3a, Cs CO (0.60
mmol), substrate (0.20 mmol), phenylboronic acid (0.30 mmol), and
,4-dioxane (2 mL) at 100 °C.
structures were used where possible to generate the initial structure for
optimization. In cases where no X-ray structures were available, several
different initial structures were used to ensure that the lowest energy
structure represented an overall minimum and not a local minimum. All
optimized structures were verified using frequency calculations to check
that they did not contain any imaginary frequencies.
2
3
1
’
CONCLUSIONS
X-ray Crystallography. The diffraction experiments were carried
out on either a Bruker AXS SMART CCD three-circle diffractometer
with a sealed tube using graphite-monochromated Mo KR radiation (λ =
0.710 73 Å) or a Rigaku Spider diffractometer equipped with a Rigaku
RAXIS RAPID imaging plate area detector with graphite-monochro-
mated Cu KR radiation (λ = 1.541 87 Å). The software used were
SMART for collecting frames of data, indexing reflections, and deter-
mining lattice parameters, SAINT for integration of intensity of reflec-
tions and scaling, SADABS for empirical absorption correction, and
SHELXTL for space group determination, structure solution, and least-
We have generated one of the first families of bidentate trans-
chelating NHC ligands and only the second series which contains
9
,15,16
a flexible linker.
The ligands are straightforward to synthe-
size, and both the sterics and electronics can be easily modified.
The ligands preferentially appear to form monomeric species,
and Pd complexes supported by the new ligands are catalytically
active for the Heck reaction and Suzuki reactions. Electrochemi-
cal and chemical studies suggest the ligands may be able to
support high-oxidation-state Pd species, and future work will
focus on isolating complexes of this type.
2
squares refinements on |F| . The crystals were mounted at the end of
glass fibers and used for the diffraction experiments. Anisotropic thermal
parameters were refined for the rest of the non-hydrogen atoms. The
hydrogen atoms were placed in their ideal positions. Details of the crystal
and refinement data for complexes 2d, 3a, 3c, and 5b’. are given in
Table S1 in the Supporting Information.
’
EXPERIMENTAL SECTION
General Methods. Experiments were performed under a dinitro-
gen atmosphere using standard Schlenk techniques, unless otherwise
noted. Solvents were dried by passage through a column of activated
alumina followed by storage under dinitrogen. All commercial chemicals
were used as received, except where noted. Ethyl acrylate, benzylamine,
LiAlH
n-butyl acrylate, 4-bromoacetophenone, and PdCl
purchased from Aldrich. 1-tert-Butylimidazole was synthesized using a
literature procedure. Deuterated solvents were obtained from Cam-
bridge Isotope Laboratories. CD Cl and CDCl were dried using P
and vacuum-transferred prior to use. NMR spectra were recorded on
Bruker AMX-400 and -500 spectrometers at ambient probe tempera-
tures unless noted. Chemical shifts are reported in ppm with respect to
Synthesis and Characterization of Compounds. BnN-
(
2 2 2
CH CH C(O)OEt) (a). This compound was prepared following a
5
1
modified literature procedure. Ethyl acrylate (21.9 g, 218.3 mmol)
was added to a methanol solution (30 mL) of benzylamine (10.0 g, 93.3
mmol). The reaction mixture was purged with nitrogen, covered in
aluminum foil, and stirred for 5 days. The solvent was removed in vacuo
to give a as a colorless liquid. The product was stored under vacuum.
Yield: 28.6 g, 99%. H NMR data were consistent with those previously
reported.
BnN(CH CH CH OH) (b). This compound was prepared following a
4
, 1-methylimidazole, 1-benzylimidazole, 1-methylbenzimidazole,
2
(MeCN) were
2
49
1
2
2
3
2 5
O
5
2
2
2
2
2
5
3
modified literature procedure. Under an atmosphere of nitrogen,
BnN(CH CH C(O)OEt) (a; 49.3 g, 160 mmol) was added dropwise
over a period of 1 h to a solution of LiAlH (7.61 g, 201 mmol) in dry
THF at 0 °C. The reaction mixture was stirred at room temperature for
4 h. Methanol (15 mL) was then carefully added, followed by a 15% by
weight NaOH(aq) solution (10 mL) and water (10 mL) (Fieser and
1
13
1
residual internal protio solvent for H and C{ H} NMR spectra;
J values are given in Hz. All assignments are based on two-dimensional
2
2
2
4
1
13
H, C-HMQC and HMBC experiments. HRMS were recorded
2
at The Mass Spectrometry (MS) and Proteomics Resource of the
W. M. Keck Foundation Biotechnology Resource Laboratory at Yale
University. Robertson Microlit Laboratories, Inc., performed the ele-
mental analyses.
Electrochemistry. Electrochemical experiments were carried out
with a Princeton Applied Research Versastat 4-400 potentiostat/galva-
nostat using a standard three-electrode configuration. Experiments were
carried out in still-dried acetonitrile solution containing 0.1 M tetra-
butylammonium perchlorate (Fluka, electrochemical grade) as the
supporting electrolyte. A Pt wire and a Ag wire were used as the counter
electrode and pseudo reference electrode, respectively. The reversible
5
4
Fieser method for quenching LiAlH reactions ). The reaction mixture
4
was stirred until the precipitate settled at the bottom of the flask. The
solution was then filtered through a Buchner funnel and dried under
1
vacuum to give b as a light yellow oil. Yield: 30.2 g, 85%. H NMR data
5
3
were consistent with those previously reported.
BnN(CH CH CH Cl) (c). A solution of SOCl (20 mL) in dichlor-
2
2
2
2
2
omethane (30 mL) was added dropwise to a solution of BnN-
(CH CH CH OH) (b; 29.7 g, 133 mmol) in dichloromethane
2
2
2
2
(200 mL) with stirring. The reaction mixture was stirred for 40 min
and then purged with nitrogen to liberate the HCl gas. Water (6 mL) was
þ
ferrocene/ferrocenium redox couple (Fc/Fc : þ0.690 V vs NHE in
1
825
dx.doi.org/10.1021/om100890q |Organometallics 2011, 30, 1818–1829

Organometallics
ARTICLE
3
then added along with K
2
CO
3
(20 g, 0.115 mol) to generate the free
3 2 2 2
ImCH ), 4.74 (4 H, t, BnNCH CH CH N, JHꢀH = 15.4), 7.60 (13, Ar),
1
3
1
amine. BnN(CH CH CH Cl) (c) was isolated as a light yellow oil by
11.40(2 H, s, NCHN). C{ H} NMR (101 MHz, D O):25.3, 33.3, 45.5,
49.9, 57.6, 113.4, 113.6, 127.4, 127.4, 127.8, 128.6, 129.6, 131.4, 132.4,
137.3, 141.2. HR FT-ICR MS: found (calcd for C H Cl N ): m/z (M
2
2
2
2
2
1
vacuum distillation at 150 °C (0.1 Torr). Yield: 31.9 g, 92%. H NMR
51
data were consistent with those previously reported.
2
9
35
2 5
Me
Me
[
C(H)N(Bn)C(H) ] 2[Cl] (1a). 1-Methylimidazole (0.91 g, 11.1
ꢀ Cl) 488.2555 (488.2270).
3
Me
Me
Me
Me
mmol) and BnN(CH CH CH Cl) (c; 1.02 g, 3.9 mmol) were mixed
[( CN(Bn)C )Ag] [AgCl ] (2a). [ C(H)N(Bn)C(H) ] 2[Cl]
(1a; 0.51 g, 1.19 mmol) was dissolved in dichloromethane (20 mL), and
Ag O (0.43 g, 1.86 mmol) was added. The reaction vessel was covered in
aluminum foil and the mixture stirred for 24 h. The reaction mixture was
then filtered through Celite and the filtrate dried under reduced pressure
to give 2a as a white powder. Yield: 0.70 g, 93%.
H NMR (300 MHz, CD Cl ): 2.14 (4H, app p, NCH CH CH N,
= 7.3), 2.52 (4H, t, CH CH CH NBn, J
NCH Ar), 3.98 (6H, s, NCH ), 4.40 (4H, t, NCNCH CH CH , J
HꢀH
= 1.8), 7.01 (2H, d, HCdCH, J
1.8), 7.35 (5H, m, C H ). C{ H} NMR (126 MHz, CDCl ): 30.0, 38.9,
49.8, 51.4, 59.2, 121.6, 122.6, 127.3, 128.5, 129.1, 139.1, 180.2. Anal.
Found (calcd) for C H Ag Cl N : C, 39.79 (39.53); H, 4.59 (4.58); N,
2
2
2
2
2
3
3
with dioxane (3 mL). The reaction mixture was heated at reflux under
nitrogen for 3 h. The solvent was then removed in vacuo and the product
dissolved in water and washed with toluene (3 ꢁ 15 mL). The water was
removed by heating the crude reaction mixture for 12 h at 60 °C under
2
reduced pressure to give 1a as a yellow-orange glass. Yield: 1.35 g, 84%.
1
1
H NMR (400 MHz, CD
2
Cl
2
): 1.99 (4H, app p, NCH
2
CH
2
CH
2
N,
2
2
2
2
2
3
3
J
= 7.0), 2.49 (4H, t, CH CH CH NBn, J
= 6.6), 3.55 (2H, m,
J
= 6.4), 3.54 (2H, m,
HꢀH
2
2
2
HꢀH
HꢀH
2
2
2
HꢀH
3
3
NCH
2
Ar), 3.80 (6H, s, NCH
3
), 4.13 (4H, t, NCNCH
2
CH
2
CH
2
, JHꢀH
=
=
=
2
3
2
2
2
HꢀH
HꢀH
3
7
1
4
.4), 7.32 (5H, m, C ), 7.43 (2H, s, HCdCH), 7.78 (2H, s, HCdCH),
0.69 (2H, s, NCHN). C{ H} NMR (126 MHz, CDCl ): 25.8, 35.7,
7.5, 49.2, 57.3, 122.0, 123.6, 127.7, 128.5, 129.8, 135.8, 137.3. HR FT-
6
H
5
7.7), 6.97 (2H, d, HCdCH, J
13
1
13
1
3
6
5
3
ICR MS: found (calcd for C21
2 5
H33Cl N ): m/z (M ꢀ Cl) 388.2263
2
1
29
2
2 5
(
388.2263).
11.09 (10.97).
tBu
tBu
tBu tBu
tBu
tBu
[
C(H)N(Bn)C(H) ] 2[Cl] (1b). 1-tert-Butylimidazole (1.27 g, 7.70
[( CN(Bn)C )Ag] [AgCl ] (2b). [ C(H)N(Bn)C(H) ] 2[Cl]
2
(1b; 250 mg, 0.492 mmol) was dissolved in dichloromethane (25 mL),
3
3
3
mmol) and BnN(CH
2
CH
2
CH
2
Cl)
2
(c; 1.00 g, 3.84 mmol) were mixed
with dioxane (1 mL). The reaction mixture was heated at reflux under
nitrogen for 24 h. The solvent was then removed in vacuo and the
product dissolved in water and washed with toluene (2 ꢁ 20 mL) and
pentane (20 mL). The water was removed by heating the crude reaction
mixture for 12 h at 60 °C under reduced pressure to give 1b as a yellow
and Ag O (285 mg, 1.23 mmol) was added. The reaction vessel was
covered in aluminum foil and the mixture stirred for 24 h. The reaction
mixture was then filtered through Celite and the filtrate dried under
2
reduced pressure to give 2b as a white powder. Yield: 297 mg, 98%.
1
H NMR (400 MHz, CD Cl ): 1.71 (18 H, s, C(CH ) ), 2.03 (4 H,
2
2
3 3
3
glass. Yield: 1.81 g, 94%.
app p, CH CH CH , J
6.5), 3.57 (2H, s, PhCH N), 4.21 (4 H, t, CH CH CH , J
7.02 (2H, d, HCdCH, J
2 3
7.33 (5 H, ArCH N). C NMR (126 MHz, CDCl ): 29.9, 30.2, 31.1,
= 7.0), 2.51 (4H, t, CH CH CH
J
HꢀH
=
2
2
2
HꢀH
2
2
2
1
3
H NMR (400.0 MHz, D O): 1.58 (18H, s, NCCH ), 2.05 (4H, app
= 6.5),
= 1.9),
2
3
2
2
2
2
HꢀH
p, NCH
2
CH
2
CH
2
N, JHꢀH = 7.0), 2.45 (4H, t, CH
Ar), 4.16 (4H, t, NCNCH
= 6.8), 7.15 (2H, m, HCdCH), 7.24 (2H, m, HCdCH), 7.26
2
CH
2
CH
2
NBn,
HꢀH HꢀH
= 1.9), 7.16 (2H, d, HCdCH, J
3
13
J
J
HꢀH = 7.8), 3.59 (2H, s, NCH
HꢀH
2
2
CH
2
CH
2
,
3
32.0, 51.0, 51.5, 57.8, 59.3, 119.1, 120.0, 127.3, 128.7, 129.2, 139.0, 177.3.
HR FT-ICR MS: found (calcd for C27 Cl
42.2409 (542.2402). Anal. Found (calcd) for C27
5.11 (44.83); H, 5.74 (5.85); N, 9.71 (9.68).
(1H, m, C
6
H
5
CH
2
N), 7.33 (2H, app t, C
6
H
5
CH
N, JHꢀH = 1.9) 8.80 (2H, s, NCHN). C{ H}
O): 26.2, 29.2, 48.0, 49.7, 57.7, 60.2, 120.6, 122.7,
2
N, JHꢀH = 1.8), 7.54
H
41Ag
2
2
N
5
): m/z (M ꢀ AgCl
2
)
N : C,
2 5
13 1
(2H, app t, C
6
H
5
CH
2
5
4
H
41Ag
2
Cl
NMR (126.0 MHz, D
2
13
1
Bn
Bn
Bn
Bn
1
2
1
28.2, 129.0, 130.1, 133.8, 137.4. C{ H} NMR (101 MHz, D O):
5.3, 33.3, 45.5, 49.9, 57.6, 113.4, 113.6, 127.4, 127.4, 127.8, 128.6, 129.6,
31.4, 132.4, 137.3, 141.2 HR FT-ICR MS: found (calcd for
2
2
[( CN(Bn)C )Ag] [AgCl ] (2c). [ C(H)N(Bn)C(H) ] 2[Cl]
3
3
(
1c; 130 mg, 23.7 mmol) was dissolved in dichloromethane (25 mL),
and Ag O (110 mg, 47.5 mmol) was added. The reaction vessel was
2
C H Cl N ): m/z (M ꢀ Cl) 472.3208 (472.2896).
2
7
43
2
5
covered in aluminum foil and the mixture stirred for 24 h. The reaction
mixture was then filtered through Celite and the filtrate dried under
reduced pressure to give 2c as a white powder. Yield: 151 mg, 83%.
Bn
Bn
C(H)N(Bn)C(H) ] 2[Cl] (1c). 1-Benzylimidazole (0.66 g, 4.2
[
3
2 2 2 2
mmol) and BnN(CH CH CH Cl) (c; 0.50 g, 1.9 mmol) were mixed
1
with dioxane (2 mL). The reaction mixture was heated at reflux under
nitrogen for 3 h. The solvent was removed in vacuo, and the product was
dissolved in water and washed with toluene (2 ꢁ 10 mL) and pentane
3 2 2 2
H NMR (400 MHz, CDCl ):2.02(4H, appp, NCH CH CH N, JHꢀH
3
= 7.2), 2.51 (4H, t, CH CH CH NBn, J
2
2
2
HꢀH
= 6.5), 3.56 (2H, m,
HꢀH
NCH Ar), 4.16 (4H, t, NCNCH CH CH , J = 7.4), 5.28 (4H, s,
3
2
2
2
2
(10 mL). The water was removed by heating the crude reaction mixture
NCNCH Ar), 6.89 (2H, s, HCdCH), 7.05 (2H, s, HCdCH),
2
13
1
for 12 h at 60 °C under reduced pressure to give 1c as a yellow oil. Yield:
7.18ꢀ7ꢀ38 (15H, m, C H ). C{ H} NMR (101 MHz, CDCl ): 29.9,
6
5
3
0
.88 g, 90%.
50.3, 51.1, 55.8, 59.2, 121.3, 122.0, 127.4, 128.0, 128.7, 128.7, 129.2, 135.8,
1
H NMR (400 MHz, CDCl ): 2.17 (4H, app p, NCH CH CH N,
139.0, 179.7. HR FT-ICR MS: found (calcd for C H Ag Cl N ):m/z(M
3
2
2
2
33 37
2
2 5
3
J
HꢀH = 7.1), 2.57 (4H, t, CH
2
CH
2
CH
CH
2
NBn, JHꢀH = 6.4), 3.54 (2H, m,
ꢀ 2AgCl þ H) 504.3099 (504.3127). Anal. Found (calcd) for
3
NCH
NCNCH
7
2
Ar), 4.44 (4H, t, NCNCH
2
2
CH
2
, JHꢀH = 7.7), 5.51 (4H, s,
),
.61 (2H, s, HCdCH), 10.71 (2H, s, NCHN). C{ H} NMR (101
C H Ag Cl N : C, 50.55 (50.13); H, 4.85 (4.72); N, 8.94 (8.86).
33
37
Me
2
2 5
Me
Me
Me
2
Ar), 7.12 (2H, s, HCdCH), 7.28ꢀ7ꢀ48 (15H, m, C
6
H
5
[( BzCN(Bn)CBz )Ag] [AgCl ] (2d). [ BzC(H)N(Bn)C(H)Bz
2
]
3
13
1
2[Cl] (1d; 250 mg, 0.477 mmol) was dissolved in dichloromethane
3
MHz, D
2
O): 25.6, 47.6, 49.0, 52.8, 57.2, 122.4, 122.6, 127.6, 128.5,
(25 mL), and Ag O (276 mg, 1.19 mmol) was added. The reaction vessel
2
129.3, 129.3, 129.3, 129.7, 133.4, 133.6, 136.9. HR FT-ICR MS: found
was covered in aluminum foil and the mixture stirred for 24 h. The reaction
(
calcd for C H Cl N ): m/z (M ꢀ Cl) 540.2850 (540.2889).
mixture was then filtered through Celite and the solvent removed under
3
3
39
2 5
Me
Me
BzC(H)N(Bn)C(H)Bz ] 2[Cl] (1d). 1-Methylbenzimidazole (1.02
[
reduced pressure to give 2d as a white powder. Yield: 247 mg, 70%.
3
1
g, 7.72 mmol) and BnN(CH
2
CH
2
CH
2
Cl)
2
(c; 1.00 g, 3.84 mmol) were
H NMR (400 MHz, CD Cl ): 2.12 (4 H, app p, NCH CH CH N,
2
2
2
2
2
3
mixed with dioxane (1 mL). The reaction mixture was heated at reflux
under nitrogen for 24 h. The solvent was removed in vacuo, and the
product was dissolved in water and washed with toluene (2 ꢁ 20 mL)
and pentane (20 mL). The water was removed by heating the crude
reaction mixture for 12 h at 60 °C under reduced pressure to give 1d as a
yellow oil. Yield: 1.62 g, 82%.
J
= 7.0), 2.64 (4H, t, NCH CH CH N, J
HꢀH
= 6.5), 3.61 (2 H, s,
HꢀH
2
2
2
3
2
3
2
2
2
HꢀH
ArCH N), 4.08 (6 H, s, NCH ), 4.52 (4 H, t, NCH CH CH N J =
7.0), 7.21ꢀ7.26 (5H, m, C H ), 7.31ꢀ7.35 (4H, m, C H ), 7.36ꢀ7.42
6
5
6 5
1
3
(4H, m, C H ). C NMR (126 MHz, CDCl ): 28.1, 35.8, 47.3, 51.5,
6
5
3
58.8, 111.2, 123.9, 127.0, 128.3, 128.8, 133.4, 134.4, 138.8, 189.8. HR
FT-ICR MS: found (calcd for C29 Cl
): m/z (M ꢀ AgCl
558.1780 (558.1776). Anal. Found (calcd) for C H Ag Cl N : C,
H
33Ag
2
2
N
5
2
)
1
H NMR (400.0 MHz, CDCl ): 2.38 (4H, app t, NCH CH CH N),
3
2
2
2
29 33
2
2 5
2
.77 (4H, s br, NCH CH CH NCN), 3.63 (2H, s, ArCH ), 4.27 (6H, s,
46.52 (47.18); H, 4.36 (4.51); N, 9.37 (9.49).
2
2
2
2
1
826
dx.doi.org/10.1021/om100890q |Organometallics 2011, 30, 1818–1829

Organometallics
ARTICLE
Me
Me
CN(Bn)C }PdCl
{
2
(3a). PdCl
2
(MeCN)
2
(21 mg, 0.080 mmol)
chloroform (2.5 mL). The solution was heated at 50 °C for 50 h.
The solvent was then removed under reduced pressure and the
crude product dissolved in a minimum amount of dichloromethane.
Pentane was then added to precipitate the product, which was collected
by filtration and dried under reduced pressure to give 5b as a yellow
solid. Yield: 48 mg, 70%. X-ray diffraction quality crystals were grown
by slow evaporation from a saturated chloroform solution at room
Me
Me
and [( CN(Bn)C )Ag] [AgCl ] (2a; 51 mg, 0.080 mmol) were
3
2
dissolved in dichloromethane (50 mL) and stirred for 3 h. The reaction
mixture was filtered through Celite and the solvent removed under
reduced pressure to give 3a as a white powder. Yield: 45 mg, 90%. X-ray
diffraction quality crystals were grown from a ternary solution of
dichloromethane, acetonitrile, and methyl isobutyl ketone.
H NMR (400 MHz, CDCl
t, 4H, J
1
3
): 2.25 (app p, 4H, CH
2
CH
2
CH
2
), 2.91
temperature.
3
1
(
(
(
= 5.6 Hz, CH CH CH NMe), 3.66 (s, 2 H, CH Ar), 4.11
), 6.63ꢀ4.67 (t, 4H, JHꢀH =6.8 Hz, NCNCH
H NMR (400 MHz, CDCl ): 1.92 (18H, s, C(CH ) ), 2.24 (4H, s
3 3 3
HꢀH
2
2
3
2
2
t
3
s, 6H, NCH
3
2
CH
2
CH
2
), 6.78
br, NCH
3.58 (2H, s, ArCH
2
CH
2
CH
2
N), 2.73 (4H, t, NCH
2
CH
2
CH
CH
2
N Bu, JHꢀH = 5.7),
CH N), 6.85 (2H, d,
1
3
s, 4H, HCdCH),7.31ꢀ7.36 (m, 3H, C H ), 7.43ꢀ7.45 (m, 2H, C H ). C
2
), 4.88 (4 H, s br, BnNCH
2
2
2
6
5
6 5
NMR (126 MHz, CDCl
28.6, 128.8, 140.7, 169.6. HR FT-ICR MS: found (calcd for C21
m/z (M þ H) 530.0902 (530.0917). Anal. Found (calcd) for
Pd: C, 48.05 (47.70); H, 5.20 (5.53); N, 13.10 (13.24).
3
): 31.0, 37.7, 48.3, 53.6, 60.6, 121.0, 122.0, 127.1,
HCdCH, JHꢀH = 2.0), 7.07 (2H, d, HCdCH, JHꢀH = 2.0), 7.22(1H, br,
3
C H ), 7.32 (2 H, t, C H , J
= 7.3), 7.39 (2H, d, C H , J
3
): 28.6, 32.6, 51.9, 53.3, 38.2, 61.0, 120.3,
= 7.3).
HꢀH
1
H
29Cl
2
N
5
Pd):
6
5
6
5
HꢀH
6
5
1
3
C NMR (126 MHz, CDCl
120.8, 127.3, 128.7, 129.2, 140.0, 161.4. HR FT-ICR MS: found (calcd
21 2 5
C H29Cl N
tBu
tBu
for C H Cl N Pd): m/z (M ꢀ Cl) 688.144 (688.145).
{
CN(Bn)C }PdCl (3b). PdCl (MeCN) (22 mg, 0.082 mmol)
27 41
2 5
2
2
2
tBu
tBu
When this reaction was performed in an NMR tube, an intermediate
2
and [( CN(Bn)C )Ag] [AgCl ] (2b; 58 mg, 0.082 mmol) were
3
tBu
tBu
was observed, which we believe is { CN(Bn)C }PdClI (4b). As
dissolved in dichloromethane (50 mL) and stirred for 3 h. The
reaction mixture was filtered through Celite and the solvent removed
under reduced pressure to give 3b as a white powder. Yield: 40
mentioned in the text, this product could not be isolated. The chemical
1
shifts for the H NMR resonances of 4b are listed below, and the
1
actual H NMR spectrum from the reaction is given in the Supporting
mg, 95%.
4
0
1
Information. Where possible, shifts were assigned using 2D NMR
3 3 3
H NMR (400 MHz, CDCl ): 2.00 (18H, s, C(CH ) ), 2.33 (4H, s
br, NCH CH CH N), 2.77 (4H, t, NCH CH CH N Bu, J = 5.9),
.62 (2H, s, ArCH ), 5.04 (4 H, s br, BnNCH CH CH N), 6.77 (2H, d,
t
3
experiments.
2
2
2
2
2
2
HꢀH
1
H NMR (400 MHz, CDCl ): 2.01 (9H, s, C(CH ) ), 2.04 (9H, s,
3
3
3 3
2
2
2
2
C(CH
3
)
3
), 2.28 (2H, br, NCH
2
CH
2
CH
2
N, overlapping with 5b), 2.37
HCdCH, JHꢀH = 2.0), 6.97 (2H, d, HCdCH, JHꢀH = 2.0), 7.32 (5 H,
t
13
(
(
2H, br, NCH CH CH
2H, t, NCH CH CH N Bu, br, overlapping with 5b), 3.65 (1H, s,
2
2
2
N), 2.63 (2H, br, NCH
2
CH
2
CH
2
N Bu), 2.79
C
1
(
6
H
5
). C NMR (126 MHz, CDCl
3
): 31.1, 32.2, 51.0, 54.2, 58.4, 62.1,
t
19.1, 120.1, 127.3, 128.6, 129.1, 140.2, 166.0. HR FT-ICR MS: found
2
2
2
ArCH ), 3.66 (1H, s, ArCH ), 4.64 (1H, s br, BnNCH CH CH N),
calcd for C27
Found (calcd) for C27
1.36 (11.43).
H
41Cl
2
N
5
Pd): m/z (M ꢀ Cl) 576.2051 (576.2080). Anal.
Pd: C, 52.71 (52.90); H, 6.78 (6.74); N,
2
2
2
2
2
2 2 2
5.04 (2H, s br, BnNCH CH CH N, overlapping with 5b), 5.68 (1H, s
H
2 5
41Cl N
br, BnNCH
d, HCdCH, J
2
CH
2
CH
2
N), 6.81 (1H, d, HCdCH, JHꢀH = 2.0), 6.84 (1H,
1
Bn
Bn
= 2.0), 7.01 (1H, d, HCdCH, J = 2.0), 7.05 (1H,
{
CN(Bn)C }PdCl (3c). PdCl (MeCN) (21 mg, 0.080 mmol and
HꢀH
HꢀH
2
2
2
Bn
Bn
6 5
d, HCdCH, JHꢀH = 2.0), 7.36 (3H, m, C H , overlapping with 5b), 7.44
[
2
( CN(Bn)C )Ag] [AgCl ] (2c; 63 mg, 0.080 mmol) were dissolved
3
6 5
(3H, d, C H , overlapping with 5b).
in dichloromethane (50 mL) and stirred for 2 h. The reaction mixture
was then filtered through Celite and the filtrate dried under vacuum to
give 3c as a white powder. Yield: 45 mg, 82%. X-ray diffraction quality
crystals were grown from a ternary solution of dichloromethane,
Representative Procedures for Catalysis. Heck Reaction.
acetonitrile, and methyl isobutyl ketone.
1
H NMR (500 MHz, CD Cl ): 2.32 (m, 4H, NCH CH CH N, J
2
2
2
2
2
HꢀH
=
(
7.2), 2.89 (m, 4H, CH
2
CH
2
CH
2
NBn), 3.69 (m, 2H, NCH
Ar), 6.73 (s, 2H,
),
.42ꢀ7.55 (m, 6H, C H ). C{ H} NMR (126 MHz, CD Cl ): 31.5,
2
Ar), 4.67
m, 4H, NCNCH CH CH ), 5.65 (s, 4H, NCNCH
2
2 2 2
This procedure was modified from the literature method of
HCdCH), 6.85 (s, 2H, HCdCH), 7.23ꢀ7.40 (m, 9H, C
6 5
H
6
13
1
Douthwaite et al. Tetra-n-butylammonium bromide (13.0 mg, 0.041
7
4
1
6
5
2
2
2 3
mmol) and Na CO (48.7 mg, 0.46 mmol) were added to a dimethy-
9.0, 61.0, 120.7, 122.4, 127.4, 128.5, 128.8, 129.2, 129.2, 129.4, 137.3,
1
13
lacetamide (DMA; 1 mL) solution containing 0.1 mol % (relative
to the aryl halide) of the Pd catalyst. This was heated under nitrogen
with stirring until the temperature reached 140 °C. Subsequently
a DMA solution (1 mL) of bromoacetophenone (81.6 mg, 0.41 mmol),
n-butyl acrylate (73.1 mg, 0.57 mmol), and the internal standard
trimethoxybenzene (69.0 mg, 0.41 mmol) were added. The catalytic
mixture was stirred under nitrogen at 140 °C for the specified time. After
2 2
41.1, 170.9 (two peaks under CD Cl as determined using H, C-
HSQC). HR FT-ICR MS: found (calcd for C H Cl N Pd): m/z (M
33
37
2 5
þ H) 682.1514 (682.1543). Anal. Found (calcd) for C H Cl N Pd:
33
37
2 5
C, 58.08 (58.18); H, 5.40 (5.48); N, 9.90 (10.29).
Me
Me
{
2 2 2
BzCN(Bn)CBz }PdCl (3d). PdCl (MeCN) (21 mg, 0.080
Me
Me
mmol) and [( BzCN(Bn)CBz )Ag] [AgCl ] (2d; 53 mg, 0.080
3
2
mmol) were dissolved in dichloromethane (50 mL) and stirred for 8 h.
The reaction mixture was then filtered through Celite and the solvent
removed under reduced pressure to give 3d as a white powder. Yield:
1
it was cooled, a H NMR spectrum was taken of the reaction mixture in
3
CDCl .
Suzuki Reaction.
4
1 mg, 96%.
H NMR (400 MHz, CDCl
1
3
): 2.28 (app t, 4 H, NCH
2
CH
2
CH
), 4.37
, JHꢀH = 4.7), 7.26ꢀ7.33
2
N),
3
2
.99 (t, 4H, CH
s, 6H, NCH ), 5.07 (t, 4H, NCNCH
6H, C ), 7.48ꢀ7.54 (2H, C
), 7.33ꢀ7.43 (5H, C
126 MHz, CDCl ): 27.4, 33.5, 43.9, 52.4, 58.7, 109.1, 109.2, 121.8, 121.9,
2
CH
CH
2 2
NBn, JHꢀH = 5.6), 3.74 (s, 2H, ArCH
2
3
(
(
(
3
2 2 2
CH CH
1
3
H
6 5
H
6 5
6 5
H ). C NMR
3
This procedure was modified from the literature method of Smith
1
7
1
(
26.0, 127.5, 129.9, 133.3, 134.0, 139.5, 180.1. HR FT-ICR MS: found
calcd for C29
Pd): m/z (M ꢀ Cl) 592.1445 (592.1454). Anal.
Found (calcd) for C29 Pd: C, 54.40 (55.38); H, 5.13 (5.29); N,
2 3
et al. Cs CO (196 mg, 0.60 mmol), phenylboronic acid (36.6 mg,
H
33Cl
2
N
H
5
0.30 mmol), bromoacetophenone (39.8 mg, 0.20 mmol), 1 mol %
(relative to the aryl halide) of the Pd catalyst, and the internal standard
trimethoxybenzene (33.6 mg, 0.20 mmol) were added to a 10 mL
round-bottom flask equipped with a stir bar. 1,4-Dioxane (1.4 mL)
was then added to the reaction flask and the solution heated, with
2 5
33Cl N
1
1.00 (11.14).
tBu tBu
tBu
tBu
{
CN(Bn)C }PdI2 (5b). { CN(Bn)C }PdCl2 (3b; 50 mg,
0.081 mmol) and MeI (115 mg, 0.814 mmol) were dissolved in
1
827
dx.doi.org/10.1021/om100890q |Organometallics 2011, 30, 1818–1829

Organometallics
ARTICLE
stirring, at 100 °C under nitrogen for the desired period of time. After
(24) Chianese, A. R.; Mo, A.; Lampland, N. L.; Swartz, R. L.; Bremer,
P. T. Organometallics 2010, 29, 3019.
1
it was cooled, a H NMR spectrum was taken of the reaction mixture
in CDCl
3
.
(25) Peris, E.; Mata, J.; Loch, J. A.; Crabtree, R. H. Chem. Commun.
001, 201.
2
(
26) Tulloch, A. A. D.; Danopoulos, A. A.; Tizzard, G. J.; Coles, S. J.;
’
ASSOCIATED CONTENT
Hursthouse, M. B.; Hay-Motherwell, R. S.; Motherwell, W. B. Chem.
Commun. 2001, 1270.
S
Supporting Information. Text, tables, figures, and CIF
b
(27) Miecznikowski, J. R.; Gr €u ndemann, S.; Albrecht, M.; M ꢀe gret,
files giving X-ray data for 2d, 3a, 3c, and 5b’. and further details
about electrochemical, NMR, and computational experiments.
This material is available free of charge via the Internet at http://
pubs.acs.org.
C.; Clot, E.; Faller, J. W.; Eisenstein, O.; Crabtree, R. H. Dalton Trans.
003, 831.
2
(28) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2007,
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6, 150.
(
29) Cross, W. B.; Daly, C. B.; Ackerman, R. L.; George, I. R.; Singh,
K. Dalton Trans. 2011, 40, 495.
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’
AUTHOR INFORMATION
(
Corresponding Author
252, 1755.
*E-mail: nilay.hazari@yale.edu.
(31) Douthwaite, R. E.; Houghton, J.; Kariuki, B. M. Chem. Commun.
004, 698.
2
(
32) Lee, C. K.; Vasam, C. S.; Huang, T. W.; Wang, H. M. J.; Yang,
’
ACKNOWLEDGMENT
R. Y.; Lee, C. S.; Lin, I. J. B. Organometallics 2006, 25, 3768.
(
(
33) Bondi, A. J. Phys. Chem. 1964, 68, 441.
34) Ramírez, J.; Corber ꢀa n, R.; Sana ꢀu , M.; Peris, E.; Fernandez, E.
This work was partially (J.D.B.) supported by the ANSER
EFRC under US DoE Award DE-PS02-08ER15944.
Chem. Commun. 2005, 3056.
(
35) Lee, C. K.; Lee, K. M.; Lin, I. J. B. Organometallics 2002, 21, 10.
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