Structural characterization, solution dynamics, and reactivity of palladium complexes with benzimidazolin- 2-ylidene N-heterocyclic carbene ligands

Article abstract of DOI:10.1002/ejic.201301339

Mononuclear and halide-bridged dinuclear palladium(II) complexes with symmetrically (1 and 3) and nonsymmetrically (2, 4-6) substituted benzimidazolin-2-ylidene N-heterocyclic carbene ligands were synthesized and characterized by ¹H and ¹³C NMR spectroscopy, elemental analysis, and X-ray crystallography. The mononuclear complexes exist as a mixture of cis and trans isomers, and the identity of these was ascertained by NMR spectroscopy and structural characterization. The dinuclear complexes 4-6 with nonsymmetrically substituted benzimidazolin-2-ylidene ligands form rotamers (cis/anti, cis/syn, trans/anti, and trans/syn) at room temperature, which can be transformed to a single rotamer at higher temperatures as evidenced by ¹H NMR spectroscopy. Crystal structure analyses of representative examples show that the Pd^II centers are bound in a distorted square-planar environment by halides and carbene C ligands, and the carbene ligands are perpendicular to the Pd-halide plane. All of the complexes were tested for their efficiency as (pre)catalysts in the Suzuki-Miyaura cross-coupling reaction, as well as in hydrodehalogenation reactions, and they show good activity. The Suzuki-Miyaura coupling reactions can be performed under air and in water as an environmentally benign solvent. The most active catalyst for the Suzuki-Miyaura coupling was also used in a sequence together with a "click reaction" to synthesize valuable sterically demanding tripodal triazole ligands.

Full text of DOI:10.1002/ejic.201301339

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DOI:10.1002/ejic.201301339
Structural Characterization, Solution Dynamics, and
Reactivity of Palladium Complexes with Benzimidazolin-
2-ylidene N-Heterocyclic Carbene Ligands
Stephan Hohloch,[a] Naina Deibel,[b] David Schweinfurth,[a]
Wolfgang Frey,[c] and Biprajit Sarkar*[a]
Keywords: Carbene ligands / Structure elucidation / Palladium / Click chemistry / Cross coupling
Mononuclear and halide-bridged dinuclear palladium(II)
complexes with symmetrically (1 and 3) and nonsymmetri-
cally (2, 4–6) substituted benzimidazolin-2-ylidene N-hetero-
cyclic carbene ligands were synthesized and characterized
by 1H and 13C NMR spectroscopy, elemental analysis, and
X-ray crystallography. The mononuclear complexes exist as
a mixture of cis and trans isomers, and the identity of these
was ascertained by NMR spectroscopy and structural charac-
terization. The dinuclear complexes 4–6 with nonsymmetri-
Crystal structure analyses of representative examples show
that the PdII centers are bound in a distorted square-planar
environment by halides and carbene C ligands, and the carb-
ene ligands are perpendicular to the Pd–halide plane. All of
the complexes were tested for their efficiency as (pre)cata-
lysts in the Suzuki–Miyaura cross-coupling reaction, as well
as in hydrodehalogenation reactions, and they show good ac-
tivity. The Suzuki–Miyaura coupling reactions can be per-
formed under air and in water as an environmentally benign
cally substituted benzimidazolin-2-ylidene ligands form rota- solvent. The most active catalyst for the Suzuki–Miyaura cou-
mers (cis/anti, cis/syn, trans/anti, and trans/syn) at room tem- pling was also used in a sequence together with a “click re-
perature, which can be transformed to a single rotamer at
higher temperatures as evidenced by H NMR spectroscopy.
action” to synthesize valuable sterically demanding tripodal
triazole ligands.
1
gands and their metal complexes drastically.[4] Metal com-
plexes of NHC ligands continue to be a lively research
field,[1] and new concepts such as abnormal carbenes[5] and
the redox-active nature of NHCs[6] continually add to its
flavor.
Introduction
N-Heterocyclic carbenes (NHCs) have established them-
selves as a privileged class of ligands in organometallic
chemistry.[1] The reasons for this are the relatively easy tun-
ing of the steric and electronic properties of these ligands
by simple variation of the substituents at the nitrogen
atoms. Such a tunable nature is extremely useful when these
compounds are used as ligands in homogeneous catalysis.[2]
Thus, metal complexes of NHCs have been one of the most
extensively studied classes of homogeneous catalysts in re-
cent decades.[1] Although NHCs based on imidazolin-2-
ylidenes have certainly been the most popular class of such
ligands,[3] particularly since the seminal isolation and char-
acterization by Arduengo et al. of such compounds,[3c] the
ones based on benzimidazolin-2-ylidenes have been investi-
gated by several groups in recent years (Figure 1).[4] The
introduction of a six-membered aromatic ring to the imid-
azolin-2-ylidene backbone changes the properties of the li-
Figure 1. Selected examples of NHC ligands.
We have recently reported CuI complexes of substituted
benzimidazolin-2-ylidenes and have shown these to be ef-
ficient catalysts for the click-type cycloaddition reactions
between alkynes and azides.[7] Furthermore, we have also
reported on PdII complexes of substituted benzimidazolin-
2-ylidenes and have proven that these classes of ligands are
redox-noninnocent.[6e] During the course of those studies,
we became interested in comparing the properties of PdII
complexes of symmetrically and nonsymmetrically substi-
tuted benzimidazolin-2-ylidenes in terms of their structures,
dynamics, and NMR characteristics. PdII complexes of
[a] Institut für Chemie und Biochemie, Freie Universität Berlin,
Fabeckstraße 34–36, 14195 Berlin, Germany
E-mail: biprajit.sarkar@fu-berlin.de
www.bcp.fu-berlin.de/en/chemie/sarkar
[b] Institut für Anorganische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, Germany
[c] Institut für Organische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301339.
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symmetrically substituted benzimidazolin-2-ylidenes have
been extensively studied by Huynh et al. in recent
years.[4g,4i] Furthermore, we were interested in the catalytic
properties of such complexes in cross-coupling reactions
with a special focus on post-functionalization of certain
tripodal ligands that can be synthesized by the CuI-cata-
lyzed click reactions.[8] In the following, we present the syn-
thesis and characterization of cis-diiodo-bis(1,3-di-n-butyl-
benzimidazolin-2-ylidene)palladium(II) (cis-1), trans-di-
iodo-bis(1,3-di-n-butylbenzimidazolin-2-ylidene)palladi-
um(II) (trans-1), and di-μ-iodo-bis(1,3-di-n-butylbenzimid-
azolin-2-ylidene)diiododipalladium(II) (3) with symmetri-
cally substituted benzimidazolin-2-ylidenes as well as cis-
diiodo-bis(1-benzyl-3-n-butylbenzimidazolin-2-ylidene)pal-
ladium(II) (cis-2), trans-diiodo-bis(1-benzyl-3-n-butylbenz-
imidazolin-2-ylidene)palladium(II) (trans-2), di-μ-iodo-bis-
(1-benzyl-3-n-butylbenzimidazolin-2-ylidene)diiododipalla-
dium(II) (4), di-μ-bromo-bis(1-benzyl-3-isopropylbenzimid-
azolin-2-ylidene)dibromodipalladium(II) (5), and di-μ-
bromo-bis(1-butyl-3-isopropylbenzimidazolin-2-ylidene)d-
ibromodipalladium(II) (6) with nonsymmetrically substi-
tuted benzimidazolin-2-ylidenes. The dynamic behavior of
such complexes in solution with respect to the existence of
Scheme 1. Mononuclear complexes presented in this work (top)
and the synthesis of dinuclear complexes (bottom).
cis/trans isomers and rotamers was investigated by NMR proved by the absence of the low-field-shifted C–H proton,
spectroscopy. The identity of the isomers was determined which is characteristic of the benzimidazolium salt. Ad-
by using structural characterization by single-crystal X-ray ditionally, the carbene C atom was observed at δ =
diffraction. The complexes were tested for their activity in 179.8 ppm in the 13C NMR spectrum of cis-1. The 1H
the Suzuki–Miyaura cross-coupling and hydrodehalogena- NMR spectrum of cis-1 shows a clearly resolved triplet for
tion reactions. The cross-coupling reactions were performed the methyl protons of the n-butyl groups at δ = 1.06 ppm.
in air with water as an environmentally benign solvent. The Similarly, the CH2 protons (NCH2) of the n-butyl groups
best catalyst of this series was used for multiple C–C bond- appear as a clear triplet at δ = 4.97 ppm (Figures 2 and S1,
formation reactions to generate a new kind of tripodal li- top). The other two CH2 protons of the n-butyl groups ap-
gand.
pear as multiplets at δ = 2.49–2.32 and 1.70 ppm. The
chemical shifts of the protons and the multiplicities rule out
any kind of additional interactions between the protons of
the n-butyl substituents and the PdII center in the cis com-
plex cis-1 (Figures 2 and S1, see Exp. Section). The identity
of cis-1 as the cis-isomer was eventually established by
Results and Discussion
Synthesis and NMR Spectroscopy
The symmetrically substituted benzimidazolin-2-ylidenes structure determination through single-crystal X-ray dif-
were synthesized by one-pot alkylations of benzimidazole, fraction (see below).
and the nonsymmetric ones were synthesized by stepwise
syntheses of benzimidazoles with two different alkyl halides at ambient temperature for 10 months, a quantitative con-
by following reported procedures.[4a–4c]
version of cis-1 to trans-1 was observed. Heating a mixture
When cis-1 was left in N,N-dimethylformamide (DMF)
The PdII complexes were synthesized by reacting of cis-1 to force a faster conversion to trans-1 was not suc-
Pd(OAc)2 and the corresponding benzimidazolium salts in cessful. Structure determination through single-crystal X-
dimethyl sulfoxide (DMSO) or tetrahydrofuran (THF). ray diffraction established the identity of trans-1 as the trans
Such reactions resulted in the mononuclear complexes 1 isomer (vide infra). In comparison with that of cis-1, the
and 2. The addition of sodium halides to the reaction mix- 1H NMR spectrum of the trans-1 is markedly different
ture led to the formation of the halide-bridged dinuclear (Figures 2 and S1, bottom). The signal of the
complexes 3–6 (Scheme 1).
The complexes were characterized by H and 13C NMR into two and appears as multiplets at δ = 4.69–4.81 and
spectroscopy, elemental analyses, and in select cases by 5.11–5.23 ppm. Similarly, the signals of the
mass spectrometry. NCH2CH2CH2CH3 protons of the n-butyl group are also
NCH2CH2CH2CH3 protons of the n-butyl groups is split
1
For the mononuclear complexes cis-1 and trans-1 with split into two signals, which appear as multiplets at δ =
the symmetrically substituted benzimidazolin-2-ylidene li- 2.04–2.26 and 2.28–2.51 ppm (Figure 2 and S1, bottom).
gand, the initial product formed directly after synthesis was The splitting of these signals, and their appearance as mul-
1
isomer-pure cis-1 as evidenced by H NMR spectroscopy. tiplets points to the inequivalence of the respective protons,
The binding of the carbene C atom to the PdII center was which could be a result of their interaction with the PdII
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1
Figure 2. H NMR spectra of cis-1 (top) and trans-1 (bottom) in deuterated DMF at 295 K. The central spectrum shows a mixture of
cis-1 and trans-1.
center. Additionally, the large downfield shift of the NCH2 isolated single crystals helped us to identify the signals of
protons of the n-butyl substituents from the benzimidazol- the respective isomers (Figure S2).
ium salt to the PdII complex is also an indication of such
For the halide-bridged dipalladium complex with the
interactions. Similar interactions have recently been ob- symmetrically substituted (n-butyl, 3) ligand, an isomer-
served for d8 metal complexes with benzimidazolylidene li- pure product was obtained after purification. The identity
gands.[4c,4i] However, in those cases, only drastic changes in of this product as the trans isomer was established through
the chemical shift were reported, not changes in the multi- structure determination. In contrast to the synthesis 3, the
1
plicities of the signals in the H NMR spectra. These inter- synthesis of the dinuclear complexes 4–6 with the nonsym-
actions have been labeled as “anagostic” interactions.[4c,4i] metrically substituted benzimidazolylidene ligands deliv-
1
However, halide···H interactions are also possible, as has ered mixtures of various isomers, as indicated by H NMR
been reported for PdII complexes with chelating imidazole- spectroscopy (Exp. Section and Figure S3). For example,
2-ylidene ligands.[3d] The signals corresponding to the rest the 1H NMR spectra of 5 and 6 show huge downfield shifts
of the protons of the n-butyl groups and those of the phenyl of the iPr CH proton (ca. 1 ppm shift with respect to the
backbone of the benzimidazolylidene ring have similar pat- signals of the free benzimidazolium salts). The dinuclear
terns for both cis-1 and trans-1 (Figures 2 and S1). The 13
C
natures of 4–6 were confirmed by mass spectrometry experi-
NMR signal of the carbene C atom for trans-1 appears at ments. For each of these complexes, a peak corresponding
δ = 176.1 ppm.
to [M – halide]+ was observed in their mass spectrum. Di-
The synthesis of the mononuclear complex with a non- nuclear complexes with such nonsymmetrically substituted
symmetrically substituted benzimidazolylidene ligand deliv- ligands can exist as mixtures of various isomers (cis/trans,
ered a mixture of cis-2 and trans-2 in a ratio of 1:1 syn/anti, and various combinations of these).[9] The 1H
(Scheme 1, see Exp. Section). The 13C NMR signal corre- NMR spectra of these complexes displayed relatively broad
sponding to the carbene C atom in these complexes is ob- signals at room temperature (a representative spectrum is
served at δ = 181 ppm. The interactions between the substit- shown in Figure S3 for 4). The broadness of these peaks is
uents on the nitrogen atoms and the PdII center as observed an indication of dynamic phenomena in solution, which can
1
in the H NMR spectrum of trans-1 were not detected in be attributed to the interconversion between the various iso-
the spectra of cis-2 or trans-2. The identity of the trans iso- mers mentioned above. Progressive heating of the 1H NMR
mer was determined by a single-crystal X-ray diffraction samples of these complexes in [D6]DMSO resulted in the
1
study of trans-2 (see below). The H NMR spectrum of the narrowing of the signals, and the final conversion to one
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isomer at 363 K. In keeping with literature reports on the atoms within the molecule is clearly seen, and the cis form
isomer stability for such complexes, and in view of steric of the mononuclear complex cis-1 can be unequivocally es-
arguments, we tentatively assign the isomer formed at tablished from this structure (Figure S4).
363 K to the trans/anti form.[5i,9] The 13C NMR signals for
the carbene C atoms were observed at δ = 156.9 and
159.6 ppm for 4 and 5, respectively. For 6, the signal for the
carbene C atom was unfortunately not resolved.
The PdII centers in all of the complexes are in distorted
square-planar environments and are coordinated by the
carbene C atom and the iodo ligands. The structures of the
mononuclear complexes trans-1 and trans-2 clearly establish
these as the trans and trans/anti isomer, respectively (Fig-
ure 3). Complex trans-2, which contains a nonsymmetri-
cally substituted benzimidazolylidene ligand, preferentially
crystallizes from a mixture of cis- and trans-2. In trans-2,
the benzimidazolylidene ligands are trans to each other, and
the substituents on the nitrogen atoms are anti to each
other. Such an observation has been made for related NiII
complexes.[4c] The dinuclear complex 3 contains two cis
iodo ligands and one trans to the benzimidazolylidene li-
gands (Figure 3).
Crystal Structures
Single crystals suitable for X-ray diffraction studies were
grown for cis-1, trans-1, trans-2, and 3 (see Exp. Section).
The crystallographic details are provided in Table S1. Com-
¯
plexes cis-1 and 3 crystallize in the triclinic P1 space group,
and trans-1 and trans-2 crystallize in the monoclinic P21/n
space group. The quality of the structural data for cis-1 is
not good enough for a discussion of individual bond
lengths (Figure S4). Attempts to improve the quality of the
The C–C and C–N bond lengths within the imidazolylid-
crystals were unsuccessful. However, the connectivity of the ene rings are in the range expected for a delocalized system
Figure 3. ORTEP plots of trans-1 (top), trans-2 (bottom right), and 3 (bottom left). Ellipsoids are drawn at 50% probability. Hydrogen
atoms have been omitted for clarity.
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(Table 1).[4] The Pd–C(carbene) distances in the mononu- tances are usually an indication of weak interactions be-
clear complexes trans-1 and trans-2 are in the expected tween the C–H groups and the PdII centers. In recent litera-
range, as are the Pd–I distances.[4i] The Pd–C(carbene) dis- ture, such interactions have been labeled as anagostic inter-
tance of 1.968(3) Å for the dinuclear complex 3 is shorter actions.[4i] We would like to note that for the complex trans-
than the Pd–C(carbene) distances of 2.015(5) and 1, such interactions seem to exist in solution as well, as seen
1
2.009(5) Å, respectively, for the trans mononuclear com- from its H NMR spectrum (Figure 2).
plexes trans-1 and trans-2. This observation is related to the
superior trans influence of the benzimidazolylidene ligands
Table 2. C–H···PdII bond lengths [Å] and angles [°].
trans-1
trans-2
3
as compared to that of the iodo ligands. In trans-1 and
trans-2, the benzimidazolylidene ligands are trans to each
other and, hence, mutually weaken their bonds to the PdII
center. The Pd–I(bridging) bond that is trans to the carbene
C atom is longer than the Pd–I bond in 3. In all three com-
plexes, the benzimidazolylidene rings are almost perpendic-
ular to the plane containing the Pd and iodo ligands with
dihedral angles of 80.5(1) and 80.8(1), 89.9(1), 78.8(2) and
74.0(1)°, respectively, for trans-1, 3, and trans-2. The dihe-
dral angles between the two benzimidazolylidene planes are
2.0(2) and 27.0(2)°, respectively, for trans-1 and trans-2.
Pd–H
Pd–H
Pd–H
Pd–H
2.9907(5)
2.9481(5)
2.9443(5)
2.9405(5)
2.7968(4)
2.8950(4)
2.9372(4)
2.9754(4)
2.8229(3)
2.9057(2)
C–H–Pd
C–H–Pd
C–H–Pd
C–H–Pd
111.6(3)
108.9(3)
110.9(3)
111.3(3)
109.5(6)
117.6(4)
115.6(3)
116.8(3)
114.6(2)
114.4(2)
Catalytic Cross-Coupling, Hydrodehalogenation, and Click
Reactions
Table 1. Selected bond lengths [Å] and angles [°] for trans-1, trans-
2, and 3.
PdII complexes of NHC ligands are potent (pre)catalysts
for the Suzuki–Miyaura[10] cross-coupling reaction between
aryl halides and arylboronic acids.[11] Such reactions have
also been catalyzed by PdII complexes of benzimidazolylid-
ene ligands.[4i] Furthermore, in recent years, Pd–NHC com-
plexes have been shown to be extremely effective catalysts
for the hydrodehalogenation of haloarenes.[2b] In the back-
drop of these literature reports, we were interested in testing
the efficiency of the complexes reported here as (pre)cata-
lysts for both these transformations. Inspired by our recent
success in performing the Suzuki–Miyaura cross-coupling
reaction with abnormal carbene complexes of PdII in
water,[5i] we were interested in probing if the complexes pre-
sented here would also act as (pre)catalysts in water. All of
the complexes were tested for their activity in the formation
of the biphenyl compound shown in Scheme 2 with 0.5 mol-
% catalyst loading. At room temperature, the mononuclear
complex cis-1 with the symmetrically substituted benzimid-
azolylidene ligand and its dinuclear counterpart 3 were least
active (Table 3). However, at 90 °C, both of these complexes
delivered the desired product in more than 95% yield. All
of the other complexes delivered the desired product in 80–
95% yield at room temperature.
trans-1
trans-2
3
Pd–C1A
Pd–C1
Pd–I1
2.028(6)
2.015(5)
2.606(1)
–
–
2.009(5)
2.612(1)
1.968(3)
2.664(1) or
2.609(1)
2.590(1)
1.355(4)
1.353(4)
1.459(4)
1.460(4)
1.401(4)
1.399(4)
1.390(5)
107.6(3)
Pd–I2
2.599(1)
1.355(7)
1.355(7)
1.459(7)
1.466(6)
1.390(7)
1.382(6)
1.386(8)
2.593(1)
1.346(6)
1.355(6)
1.468(7)
1.466(6)
1.393(6)
1.388(6)
1.394(7)
106.0(4)
C1–N1
C1–N2
N1–C8
N2–C12
N1–C7
N2–C2
C2–C7
N1–C1–N2 105.9(5)
The C–H groups from the alkyl substituents on the nitro-
gen atoms of the ligands possess relatively short distances
to the PdII centers (Figures 4, S5, and S6). The relevant
distances and angles are shown in Table 2. Such short dis-
Scheme 2. Suzuki–Miyaura cross-coupling reaction with the condi-
tions stated in Table 3.
Thus, from this limited screening, it appears that the
complexes that contain nonsymmetrically substituted benzi-
midazolylidene ligands are the most potent (pre)catalysts
for the Suzuki–Miyaura cross-coupling reaction under the
conditions described here. Even though we are not in a po-
Figure 4. C–H···PdII interaction in the solid state for trans-1.
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Table 3. Catalytic results for the reaction shown in Scheme 2.[a]
Table 4. Catalytic results for the reaction shown in Scheme 3.[a]
Catalyst
Temperature
Yield [%]
Catalyst
Temp.
[°C]
Time
[h]
Conv.
[%]
Time
[h]
Conv.
[%]
cis-1
cis-1
trans-1
2 (cis + trans)
3
3
4
5
6
r.t.
90 °C
r.t.
r.t.
r.t.
90 °C
r.t.
r.t.
20
99
95
95
11
98
99
90
99
cis-1
trans-1
2 (cis + trans)
3
4
5
6
70
70
70
70
70
70
70
5
5
5
5
5
5
5
Ͼ99
*
15
Ͼ99
95
Ͼ99
Ͼ99
–
24
24
–
24
–
–
Ͼ99
Ͼ99
–
Ͼ99
–
–
–
r.t.
[a] Reactions were performed with 0.002 mol-% catalyst in 2-prop-
anol. Reactions were stirred under argon for 5 or 24 h and analyzed
by GC or HPLC; * conversion for the 5 h reaction was not mea-
sured.
[a] Reactions were performed for 5 h in H2O with 0.5 mol-% of the
catalyst and analyzed by H NMR spectroscopy.
1
sition to comment on the exact nature of the active catalyst,
blind runs with PdII–acetate and PdCl2 salts under identical
conditions resulted in poor yields of the desired products,
which emphasizes the need for the benzimidazolylidene li-
gands to form highly active catalytic species. On the basis
of literature reports, it can be assumed that the complexes
first undergo ligand dissociation and reduction prior to be-
coming catalytically active.[11] The formation of the bi-
phenyl product from the Suzuki–Miyaura coupling was fol-
lowed as a function of time with catalyst 4. As can be seen
from Figure S7, there is an initial induction period for this
reaction, which is probably related to the initial formation
of the catalytically active species through ligand dissoci-
ation and reduction of the PdII center. Approximately 80%
conversion is achieved after 2 h. However, full conversion
to the products requires a reaction time of 5 h.
All of the complexes were also tested for their activity in
the hydrodehalogenation reaction of 1,4-dibromobenzene in
2-propanol in the presence of KOtBu as a base (Scheme 3).
As can be seen from Table 4, all of the complexes presented
here are highly active catalysts for the hydrodehalogenation
reaction, and most of them delivered quantitative product
formation. It was possible to perform the hydrodehalogena-
tion reaction with a catalyst loading of only 0.002 mol-%.
As can be seen from the time versus conversion plot in Fig-
ure S8 with catalyst 6, the amount of both bromobenzene
and benzene increase initially at the expense of 1,4-dibro-
mobenzene. Eventually, the amount of benzene increases at
the expense of bromobenzene.
Finally, we were interested in using the complexes pre-
sented here as (pre)catalysts for the post-functionalization
of click-based ligands to generate sterically bulky tripodal
ligands. To do this, we first synthesized the tris(bromo-
phenyl)-substituted tripodal ligand (Scheme 4) in reason-
able yield by using a standard click protocol. We then used
one of the most potent catalysts presented above (4) to per-
form post-functionalization of the bromo groups through
a Suzuki–Miyaura cross-coupling reaction. To do this, the
bromo-substituted tripodal triazole ligand was reacted with
phenylboronic acid and K2CO3 in toluene under reflux.
Complex 4 (0.05 mol-%) was used as a precatalyst for this
reaction (Scheme 4 and Exp. Section). This reaction led to
the formation of the desired tris(biphenyl)-substituted li-
gand in reasonable yield. Thus, in this reaction, three bro-
moaryl groups are converted into their biphenyl counter-
part. Such biphenyl-substituted sterically bulky ligands can
provide a useful platform for small molecule activation at
transition metal centers.
Conclusions
We have presented here eight new PdII complexes by
using one symmetrically substituted benzimidazol-2-ylidene
ligand and three nonsymmetrically substituted benzimida-
zol-2-ylidene ligands. Four of these complexes have been
characterized by single-crystal X-ray diffraction, and these
Scheme 3. Hydrodehalogenation reactions under the conditions
stated in Table 4.
Scheme 4. Sequential click and Suzuki–Miyaura cross-coupling reactions for the generation of new bulky tripodal ligands.
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After 10 months in DMF solution at ambient temperature, quanti-
tative conversion of cis-1 to trans-2 occurred, as monitored by H
studies establish the identity of cis-1 as mononuclear, trans-
1 as mononuclear, trans-2 as anti-mononuclear, and 3 as
an iodo-bridged dinuclear complex. 1H NMR spectroscopic
studies on trans-1 show that there are strong C–H···PdII in-
teractions in solution. For the dinuclear complexes 4–6 with
nonsymmetrically substituted benzimidazolylidene ligands,
the formation of rotamers (cis/anti, cis/syn, trans/anti, and
trans/syn) and their slow interconversion have been detected
on the NMR timescale at room temperature. Complexes 4–
1
NMR spectroscopy. Heating of the solution unfortunately did not
1
result in the enhancement of the rate of this reaction. trans-1: H
NMR (250 MHz, [D7]DMF): δ = 1.17 (t, J = 7.33 Hz, 12 H,
N C H 2 C H 2 C H 2 C H 3 ) , 1 . 7 2 ( s e x t , J = 7 . 4 5 H z , 8 H ,
NCH2CH2CH2CH3), 2.04–2.26 (m, 4 H, NCH2CHHCH2CH3),
2.28–2.51 (m, 4 H, NCH2CHHCH2CH3), 4.69–4.81 (m, 4 H,
NCHHCH2CH2CH3), 5.11–5.23 (m, 4 H, NCHHCH2CH2CH3),
7.47–7.53 (m, 4 H, aryl-H), 7.86–7.94 (m, 4 H, aryl-H) ppm. 13C
6 converted to single isomers at higher temperatures, and NMR (CDCl3, 125 MHz): δ = 14.0, 20.8, 30.4, 49.4, 111.1, 123.2,
134.4, 176.1 (carbene C) ppm. C30H44I2N4Pd (820.07): calcd. C
43.89, H 5.40, N 6.82; found C 43.51, H 5.40, N 6.60.
these isomers have been tentatively assigned as the
trans/anti isomers. All complexes presented here are active
(pre)catalysts for the Suzuki–Miyaura cross-coupling reac-
tion. Most complexes catalyzed these C–C bond-forming
reactions at room temperature in water as an environmen-
tally benign solvent. All of the complexes are also active
cis/trans-Diiodo-bis(1-benzyl-3-n-butylbenzimidazolin-2-ylidene)-
palladium(II) (cis-2 and trans-2): The corresponding benzimidazol-
ium salt (79 mg, 0.2 mmol; 2 equiv.) was mixed with palladium(II)
acetate (22 mg, 0.1 mmol; 1 equiv.) in THF (10 mL, c =
catalysts for the hydrodehalogenation reaction of 1,4-di- 0.01 molL–1), and the mixture was stirred overnight. The reaction
mixture was then diluted with dichloromethane (50 mL) and
washed twice with water (100 mL). The organic layer was dried
with sodium sulfate and filtered, and the solvents were evaporated.
The remaining yellow solid was washed twice with diethyl ether
(15 mL). The product was obtained as yellow crystals as a 1:1 (by
1H NMR spectroscopy) cis/trans mixture in a yield of 92% (82 mg,
0.092 mmol). 1H NMR (250 MHz, CDCl3): δ = 0.79 (t, J = 7.5 Hz,
6 H, CH3), 1.06 (t, J = 7.5 Hz, 6 H, CH3), 1.21–1.36 (m, 8 H,
CH2), 2.05–2.31 (m, 8 H, CH2), 4.64 (t, J = 8 Hz, 4 H, CH2, trans),
4.75 (t, J = 8 Hz, 4 H, CH2, cis), 5.84 (s, 4 H, CH2, cis), 6.03 (s, 4
H, CH2, trans), 6.93–7.22 (m, 17 H, aryl-H), 7.28–7.44 (m, 15 H,
aryl-H), 7.54–7.61 (m, 4 H, aryl-H) ppm. 13C NMR (60 MHz,
CDCl3): δ = 13.89, 13.51 (CH3-butyl), 20.6, 20.3 (CH2-butyl), 31.2,
30.9 (CH2-butyl), 48.9, 48.8 (CH2-butyl), 53.8, 53.44 (CH2-benzyl),
110.2, 110.3, 111.4, 111.5, 122.5, 122.5, 122.6, 127.7, 127.8, 127.9,
128.0, 128.1, 128.5, 128.7, 129.0, 134.7, 134.9, 135.1, 135.2, 135.2,
135.3 (all aryl-C), 181.0 (carbene C) ppm. C36H40I2N4Pd (888.95):
calcd. C 48.64, H 4.54, N 6.30; found C 47.81, H 4.45, N 6.20.
bromobenzene in 2-propanol with KOtBu as base. Further-
more, one of the most potent catalysts was used in a se-
quential reaction together with the click method to generate
a sterically bulky, tripodal ligand. In that reaction, the PdII–
NHC catalysts presented here are capable of performing
three C–C bond formation reactions. Metal complexes of
such bulky tripodal ligands can display a variety of fascinat-
ing properties, as we have recently shown.[12] Further work
in that direction is being pursued in our laboratories.
Experimental Section
General: All solvents were dried and distilled by using common
techniques unless otherwise mentioned. All ligands used in this
work[4a–4c] and 2-azidophenyl bromide[13] were synthesized by fol-
lowing literature procedures. 1H and 13C NMR spectra were re-
corded at 250.13 MHz with a Bruker AC250 instrument and at
500 MHz with a Bruker Avance500 instrument. Elemental analyses
were performed by using a Perkin–Elmer 240 analyzer. Mass spec-
trometry measurements were performed by using an Agilent 6210
ESI-TOF instrument.
General Procedure for the Synthesis of the Dipalladium Complexes:
All of the dipalladium complexes were synthesized by a general
procedure as described below.
The corresponding benzimidazolium salt, palladium(II) acetate,
and the corresponding sodium halide were dissolved in dimethyl
sulfoxide in a capped flask. The reaction mixture was then heated
to 90 °C overnight. After cooling, dichloromethane and water were
added to the mixture, and the product was extracted. The aqueous
phase was washed five times with dichloromethane. The combined
organic layers were dried with sodium sulfate and filtered, and the
solvent was evaporated. The solids were then dissolved in dichloro-
methane/hexane (1:1) to obtain the clean microcrystalline products
after one night at –20 °C.
GC analysis was performed with a Varian Saturn 2100C GC instru-
ment (column: Varian factory four capillary column VF-5ms,
method: 35 to 280 °C at a heating rate of 10 K/min) with hexa-
decane as an internal standard.
cis/trans-Diiodo-bis(1,3-di-n-butylbenzimidazolin-2-ylidene)pallad-
ium(II) (cis-1 and trans-1): The product was obtained by heating
the corresponding benzimidazolium salt (72 mg, 0.2 mmol;
2 equiv.) with palladium(II) acetate (22 mg, 0.1 mmol; 1 equiv.) in
DMSO (10 mL, c = 0.01 molL–1) at 90 °C for 12 h. The resultant
suspension was cooled to ambient temperature and filtered, and
the solid was washed with diethyl ether (20 mL). Upon drying in
vacuo, the cis product was obtained as a yellow crystalline solid
Di-μ-iodo-bis(1,3-di-n-butylbenzimidazolin-2-ylidene)diiododipallad-
ium(II) (3): The product was obtained by following the general pro-
cedure with palladium(II) acetate (44 mg, 0.2 mmol;1 equiv.), the
1
in a yield of 80% (73 mg, 0.09 mmol). cis-1: H NMR (250 MHz,
CDCl3): δ = 1.06 (t, J = 7.28 Hz, 12 H, CH3), 1.54 (m, 8 H, CH2), corresponding benzimidazolium salt (78 mg, 0.2 mmol; 1 equiv.),
2.11–2.30 (m, 8 H, CH2), 4.71 (t, J = 7.94 Hz, 8 H, CH2), 7.22–
7.31 (m, 4 H, aryl-H), 7.35–7.43 (m, 4 H, aryl-H) ppm. H NMR
and sodium iodide (120 mg, 0.8 mmol; 4 equiv.) in DMSO (4 mL; c
1
= 0.05 molL–1). The product was obtained as an orange solid in a
1
(250 MHz, [D7]DMF): δ = 1.06 (t, J = 7.37 Hz, 12 H, CH3), 1.70
(sext, J = 7.60 Hz, 8 H, CH2), 2.32–2.49 (m, 8 H, CH2), 4.97 (t, J
= 7.77 Hz, 8 H, CH2), 7.49–7.57 (m, 4 H, aryl-H), 7.89–7.99 (m, 4
yield of 84% (108 mg, 0.084 mmol). H NMR (250 MHz, CDCl3):
δ = 1.13 (t, J = 7.31 Hz, 12 H, CH3), 1.59 (m, 8 H, CH2), 2.20 (m,
8 H, CH2), 4.81 (t, J = 7.82 Hz, 8 H, CH2), 7.50–7.57 (m, 4 H,
H, aryl-H) ppm. 13C NMR (CDCl3, 60 MHz): δ = 13.9 (CH3), 20.6 aryl-H), 7.66–7.74 (m, 4 H, aryl-H) ppm. 13C NMR (CDCl3,
(CH2), 31.1 (CH2), 48.7 (CH2), 110.3, 122.4, 135.0 (all aryl-C),
179.8 (carbene C) ppm.
60 MHz): δ = 13.7 (CH3), 20.2 (CH2), 29.5 (CH2), 49.1 (CH2),
110.4, 123.0, 134.6 (all aryl-C) ppm; the carbene signal was not
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detected. C30H44I4N4Pd2·1.5hexane: calcd. 35.75, H 5.00, N 4.28; enediaminetetraacetic acid (EDTA; 1 %) dissolved in a concen-
found C 35.81, H 4.72, N 4.15.
trated ammonia solution (3ϫ 50 mL) to remove traces of copper.
Afterwards, the organic phase was washed with water (3ϫ 50 mL)
and dried with Na2SO4, and the solvent was evaporated. The prod-
uct was purified by column chromatography over silica (DCM/
Di-μ-iodo-bis(1-benzyl-3-n-butylbenzimidazolin-2-ylidene)diiodo-
dipalladium(II) (4): The product was obtained by following
the general procedure with palladium(II) acetate (44 mg,
0.2 mmol;1 equiv.), the corresponding benzimidazolium salt
(78 mg, 0.2 mmol; 1 equiv.), and sodium iodide (120 mg, 0.8 mol;
4 equiv.) in DMSO (4 mL; c = 0.05 molL–1). The product was ob-
tained as a red solid in 99 % yield (122 mg, 0.099 mmol). 1H
([D6]DMSO, 250 MHz, 363 K): δ = 1.03 (t, J = 7.5 Hz, 6 H, CH3),
1.51 (sex, J = 7.5 Hz, 4 H, CH2), 2.17 (p, J = 7.5 Hz, 4 H, CH2),
4.68 (t, J = 7.75 Hz, 4 H, CH2), 5.93 (s, 4 H, CH2), 7.09–7.37 (m,
12 H, aryl-H), 7.55–7.68 (m, 6 H, aryl-H) ppm. 13C NMR ([D6]-
DMSO, 60 MHz, 298 K): δ = 13.6 (CH3), 19.5 (CH2), 29.8 (CH2),
48.5 (CH2), 53.0 (CH2), 111.1, 111.5, 123.0, 123.1, 125.2, 127.9,
128.1, 128.4, 128.8, 133.6, 134.4, 134.7 (all aryl-C), 156.9 (carbene
C) ppm. C36H40I4N4Pd2 (1249.16): calcd. C 34.61, H 3.23, N 4.49;
found C 34.79, H 3.52, N 4.33. HRMS (ESI): calcd. for
C36H40N4I3Pd2 [6 – I]+ 1122.8461; found 1122.8566.
MeOH 98:2) to yield a white solid (2.30 g, 60 %). 1H NMR
3
(250 MHz, CDCl3): δ = 4.03 (s, 6 H, NCH2), 7.38 (t, JH,H
=
7.9 Hz, 3 H, aromatic), 7.48 (t, 3JH,H = 7.4 Hz, 3 H, aromatic), 7.57
(d, 3JH,H = 7.8 Hz, 3 H, aryl-H), 7.76 (d, 3JH,H = 7.9 Hz, 3 H, aryl-
H), 8.20 (s, 3 H, triazole-5-H) ppm. 13C NMR (62.5 MHz, CDCl3):
δ = 47.5, 119.0, 126.1, 128.3, 128.6, 131.2, 134.0, 136.8, 143.9 ppm.
C27H21Br3N10 (725.24): calcd. C 44.72, H 2.92, N 19.31; found C
44.62, H 2.93, 19.21. HRMS (ESI): calcd. for C27H22Br3N10 [M +
H]+ 722.9574; found 722.9564.
Tris{[1-(2-biphenyl)-1H-1,2,3-triazole-4-yl]methyl}amine: Tris{[1-(2-
bromophenyl)-1H-1,2,3-triazole-4-yl]methyl}amine (116 mg,
0.16 mmol), phenylboronic acid (98 mg, 0.80 mmol), K2CO3
(132 mg, 0.96 mmol), and 4 (10 mg, 0.008 mmol) were dissolved in
degassed toluene. The reaction mixture was heated to reflux for
3 d. The organic phase was extracted with water (3ϫ 10 mL) and
dried with Na2SO4. The solvent was evaporated, and the crude
mixture was purified by column chromatography over silica (DCM/
MeOH 95:5) to yield the product (75 mg, 65 %). 1H NMR
(250 MHz, CDCl3): δ = 3.32 (s, 6 H, NCH2), 7.03–7.15 (m, 18 H,
aryl-H), 7.49–7.63 (m, 12 H, aryl-H) ppm. 13C NMR (62.5 MHz,
CDCl3): δ = 47.0, 125.7, 126.8, 128.0, 128.6, 128.7, 128.7, 130.0,
131.1, 135.4, 137.6, 137.6, 143.9 ppm. C45H36N10·2MeOH (780.36):
calcd. C 72.29, H 5.68, N 17.94; found C 71.69, H 5.39, 17.97.
HRMS (ESI): calcd. for C45H37N10 [M + H]+ 717.3197; found
717.3202.
Di-μ-bromo-bis(1-benzyl-3-isopropylbenzimidazolin-2-ylidene)di-
bromodipalladium(II) (5): The product was obtained by following
the general procedure with palladium(II) acetate (44 mg, 0.2 mmol;
1 equiv.), the corresponding benzimidazolium salt (66 mg,
0.2 mmol; 1 equiv.), and sodium bromide (81 mg, 0.8 mmol;
4 equiv.) in DMSO (4 mL; c = 0.05 molL–1). The product was ob-
tained as a yellow solid in 98% yield (102 mg, 0.098 mmol). 1H
NMR ([D6]DMSO, 250 MHz, 363 K): δ = 1.76 (d, J = 7.0 Hz, 12
H, CH3), 6.06–6.23 (m, 6 H, CH2-benzyl and CH-isopropyl), 7.17–
7.38 (m, 12 H, aryl-H), 7.59–7.63 (m, 4 H, aryl-H), 7.80–7.83 (m,
2 H, aryl-H) ppm. 13C NMR ([D6]DMSO, 60 MHz, 298 K): δ =
20.8 (CH3), 52.9 (CH), 55.4 (CH2), 112.7, 113.7, 124.1, 129.0,
129.4, 129.4, 132.7, 135.2, 135.9 (all aryl-C), 159.6 (carbene C)
ppm. C34H36Br4N4Pd2 (1033.10): calcd. C 39.53, H 3.51, N 5.42;
found 39.56, H 3.68, N 5.30. HRMS (ESI): calcd. for
C34H36N4Br3Pd2 [7 – Br]+ 952.8543; found 952.8694.
X-ray Crystallography: Single crystals of cis-1, trans-1, and trans-2
were grown by layering concentrated dichloromethane solutions
with n-hexane at 8 °C. Single crystals of 3 were obtained by slow
diffusion of diethyl ether into a concentrated solution of dichloro-
methane at 8 °C. The crystal data were recorded with a Bruker
Kappa ApexII duo diffractometer or a Kappa CCD Nonius instru-
ment using “omega–phi” scan or “CCD scan” techniques. The
structures were solved by direct methods (SHELXS-97) and refined
by full-matrix least-squares procedures (based on F2, SHELXL-
97).[14] For complex trans-2, four protons belonging to the n-butyl
substituents are heavily disordered and, hence, it was not possible
to refine them.
Di-μ-bromo-bis(1-butyl-3-isopropylbenzimidazolin-2-ylidene)di-
bromodipalladium(II) (6): The product was obtained by following
the general procedure with palladium(II) acetate (44 mg, 0.2 mmol;
1 equiv.), the corresponding benzimidazolium salt (60 mg,
0.2 mmol; 1 equiv.), and sodium bromide (81 mg, 0.8 mmol;
4 equiv.) in DMSO (4 mL; c = 0.05 molL–1). The product was ob-
tained as a yellow solid in 99 % yield (96 mg, 0.099 mmol). 1H
(250 MHz, [D6]DMSO, 363 K): δ = 1.02 (t, J = 7.5 Hz, 6 H, CH3),
1.51 (sex, J = 7.5 Hz, 4 H, CH2), 1.71 (d, J = 7.0 Hz, 12 H, CH3),
2.13 (q, J = 7.5 Hz, 4 H, CH2), 4.72 (t, J = 7.5 Hz, 4 H, CH2), 6.08
(hept, J = 7.5 Hz, 2 H, CH), 7.30–7.36 (m, 4 H, aryl-H), 7.49–7.69
(m,2 H, aryl-H), 7.77–7.80 (m, 2 H, aryl-H) ppm. 13C ([D6]DMSO,
60 MHz, 298 K): δ = 13.6 (CH3), 19.5 (CH3), 19.9 (CH2), 30.3
(CH2), 47.5 (CH), 54.3 (CH2), 111.2, 112.5, 122.9, 123.1, 131.3,
133.8, 134.8 (all aryl-C) ppm (carbene signal not detected).
C28H40Br4N4Pd2 (965.07): calcd. C 34.85, H 4.18, N 5.81; found C
35.05, H 4.29, N 5.84. HRMS (ESI): calcd. for C28H40N4Br3Pd2
[8 – Br]+ 884.8856; found 884.8792.
CCDC-805490 (for 3), -838652 (for trans-2), -838653 (for cis-1), and
-838654 (for trans-1) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Catalytic Experiments
Suzuki–Miyaura Cross-coupling Reactions: 0.5 mol-% of the cata-
lyst, 4-bromobenzaldehyde (1 equiv., 0.1 mmol, 185 mg), phenyl-
boronic acid (1.2 equiv., 1.2 mmol, 146 mg), and potassium carb-
onate (1.5 equiv., 1.5 mmol, 207 mg) were dissolved in water
(3 mL), and the mixture was stirred for 5 h at room temperature
(or 90 °C). Afterwards, the reaction mixture was diluted with water,
and the organic compounds were extracted with dichloromethane
(3 times). The combined organic layers were dried with sodium
sulfate and filtered, and the solvents were evaporated. The conver-
sions were determined by 1H NMR spectroscopy. For time-depend-
ent profiles of the reaction, small aliquots were taken and analyzed
Tris{[1-(2-bromophenyl)-1H-1,2,3-triazole-4-yl]methyl}amine: 2-
Azidophenyl bromide (4.75 g, 24.0 mmol) and tripropargylamine
(699 mg, 5.33 mmol) were dissolved in a mixture of CH2Cl2/H2O/
tert-butanol (25/25/50 mL). CuSO4·5H2O (200 mg, 0.8 mmol), so-
dium ascorbate (633 mg, 3.20 mmol), and tris(benzyltriazolyl-
methyl)amine (TBTA; 42.5 mg, 0.08 mmol) were added, and the
solution was stirred for 5 d at 60 °C. The reaction mixture was
poured into water (100 mL) and extracted with CH2Cl2 (3. 50 mL).
The combined organic layers were washed several times with ethyl-
1
by H NMR spectroscopy after a small workup (see above).
Hydrodehalogenation Reactions: 1,4-Dibrombenzene (1 equiv.,
1 mmol, 236 mg), the catalyst (0.002 mol-%), and potassium tert-
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Supporting Information (see footnote on the first page of this arti-
cle): Table of crystallographic details; 1H NMR spectra of cis-1,
trans-1, and 4; ORTEP plot of cis-1; and figure showing H bonding
in 3 and trans-2.
Acknowledgments
The authors are indebted to the Baden-Württemberg-Stiftung for
the financial support of this work. Dr. F. Lissner is kindly acknowl-
edged for the measurements of the crystals of cis-1 and 3.
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Received: October 15, 2013
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I31339
/KAP1
Date: 12-03-14 17:03:19
Pages: 10
www.eurjic.org
FULL PAPER
N-Heterocyclic Carbenes
S. Hohloch, N. Deibel, D. Schweinfurth,
W. Frey, B. Sarkar* ........................ 1–10
PdII complexes with symmetrically and
nonsymmetrically substituted benzimidaz-
olylidene ligands are presented. The com-
plexes exist as various isomers and rota-
mers in solution and exhibit C–H···Pd in-
teractions in the solid state and in solution.
All of the complexes are potent (pre)cata-
lysts for the Suzuki–Miyaura cross-cou-
pling reaction in water and the hydrode-
halogenation reactions of haloarenes.
Structural Characterization, Solution Dy-
namics, and Reactivity of Palladium Com-
plexes with Benzimidazolin-2-ylidene N-
Heterocyclic Carbene Ligands
Keywords: Carbene ligands
/ Structure
elucidation / Palladium / Click chemistry /
Cross coupling
Eur. J. Inorg. Chem. 0000, 0–0
10
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim