Palladium(II) Complexes of a Neutral CCC-Tris(N-heterocyclic carbene) Pincer Ligand: Synthesis and Catalytic Applications

Article abstract of DOI:10.1021/acs.organomet.1c00324

Treatment of tris-azolium precursor 1 with palladium acetate under thermal conditions provided a CCC-pincer palladium(II) complex (2) bearing three NHCs (one imidazolylidene and two triazolylidenes) and one iodide ligand. Further treatment of complex 2 with an excess of AgSbF6 generates tris(carbene) dicationic palladium complex 3 in which the iodine ligands are exchanged with SbF6 anions and the metal center is stabilized by one acetonitrile ligand. Complexes 2 and 3 were tested in several cross coupling reactions showing high conversions under low catalyst loadings and mild reaction conditions. Additionally, complexes 2 and 3 performed well in the hydrosilylation of terminal alkynes with good selectivity toward the E-isomer.

Full text of DOI:10.1021/acs.organomet.1c00324

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Article
Palladium(II) Complexes of a Neutral CCC-Tris(N-heterocyclic
carbene) Pincer Ligand: Synthesis and Catalytic Applications
David Rendón-Nava, Deyanira Angeles-Beltrán, Arnold L. Rheingold, and Daniel Mendoza-Espinosa*
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ABSTRACT: Treatment of tris-azolium precursor 1 with
palladium acetate under thermal conditions provided a CCC-
pincer palladium(II) complex (2) bearing three NHCs (one
imidazolylidene and two triazolylidenes) and one iodide ligand.
Further treatment of complex 2 with an excess of AgSbF6 generates
tris(carbene) dicationic palladium complex 3 in which the iodine
ligands are exchanged with SbF6 anions and the metal center is
stabilized by one acetonitrile ligand. Complexes 2 and 3 were
tested in several cross coupling reactions showing high conversions
under low catalyst loadings and mild reaction conditions.
Additionally, complexes 2 and 3 performed well in the hydrosilylation of terminal alkynes with good selectivity toward the E-isomer.
ether fragment,15 and phenolate-derived OCO pincers16 have
also been reported recently.
In line with our research focused on carbon-based ligands,
transformations.1 Because of their unique design, their high
we were interested in the exploration of CCC-pincer type
INTRODUCTION
Pincer complexes of transition metals have attracted a great
deal of attention as they have been used in a myriad of organic
■
ligands. The literature shows that frameworks A−C are the
most common examples of CCC-pincer type ligands stabilized
by NHC17 and MIC fragments.18 Usually, these CCC-pincer
ligands are monoanionic and possess the general carbene−
aryl−carbene framework (Figure 1), and because of the
flexibility of their backbone, they can present planar (A or C)
or twisted in C2 conformations (B) if methylene linkers are
present in the framework.
thermal stability, and the meridional tridentate nature of pincer
ligands, they are privileged platforms for stabilization of metal
complexes with usually enhanced catalytic performances
compared to those of monodentate ligands.2
The broad applications of N-heterocyclic carbenes (NHCs)
as organocatalysts and exceptional ligands in coordination
chemistry and catalysis have made them relevant tools in
modern chemistry.3 In particular, pincer architectures featuring
terminal NHC donors are becoming a prominent design motif,
combining the strong σ-donor characteristics of NHCs with
the favorable thermal stability and reaction control possible
with a mer-tridentate geometry.4 To date, a vast array of pincer
ligands bearing NHCs have been prepared, including those
based on neutral CNC,5 CNN,6 PCP,7 SCS,8 and CSC,9
anionic CNC,10 and dianionic OCO11 and SCS8b architec-
tures.
For several years, our group has been interested in the
synthesis and applications of a second generation of carbenes,
namely, mesoionic carbenes (MICs), which are even stronger
donors compared to classical NHC ligands.12 The exploration
of MIC-based pincer ligands has attracted a great deal of
attention in the past decade, leading to the isolation of several
interesting pincer platforms. For instance, a series of anionic
CNC tridentate ligands composed of carbazole and two MIC
carbenes efficiently coordinate with transition metals with the
respective complexes displaying remarkable catalytic and
photophysical applications.13 Moreover, the coordination
chemistry and applications of MIC-based CNC pincer ligands
featuring central pyridine rings,14 COC ligands with a central
Figure 1. Known NHC-based CCC-pincer ligands (A−C) that
presented in this work (D).
Received: June 2, 2021
Published: June 24, 2021
https://doi.org/10.1021/acs.organomet.1c00324
© 2021 American Chemical Society
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Interestingly, mentions of neutral (tris)carbene-based CCC-
pincer ligands in the literature are very scarce. To the best of
our knowledge, only two examples of monomeric metal
complexes (PtII and RuII) bearing a tris(carbene)-based pincer
ligand have been reported in the literature.19 Considering that
carbenes have a trans effect that is much stronger than that of
N or P donors and are more tightly bound to the metal, they
should provide pincer derivatives that exhibit usefully different
properties. With this in mind, we report herein the preparation
of an unprecedented CCC-pincer palladium(II) complex 2
stabilized by one imidazolylidene and two mesoionic
triazolylidene ligands. Exchange of the iodine atoms by the
reaction of complex 2 with an excess of AgSbF6 provides a
dicationic Pd(II) CCC-pincer complex 3 that is stabilized by
one acetonitrile ligand. The catalytic performance of complexes
2 and 3 in a series of cross coupling processes (formation of
biaryls, amides, and ketones) and the hydrosilylation of
terminal alkynes will be discussed.
unsuccessful, showing large amounts of unreacted starting
material. We found that the reaction of stoichiometric amounts
of palladium acetate with salt 1 in acetonitrile at 100 °C
produces after workup and purification complex 2 as a red
solid in 54% yield (Scheme 2). The palladation of salt 1 was
Scheme 2. Synthesis of Complex 2
spectroscopically confirmed by the disappearance of the low-
field signal above 9 ppm corresponding to the azolium proton
1
in the H NMR spectrum and the slight upfield shift of the
resonance corresponding to the N-CH3 group at 4.52 ppm. In
the 13C NMR spectrum, the new metal−carbene resonances
displaying a 2:1 ratio are located at 157.3 ppm (MIC-Pd) and
165.5 ppm (NHC-Pd), consistent with full deprotonation of
the tris-azolium precursor and the formation of the tris(NHC)-
palladium complex. Complex 2 is air stable and displays good
solubility in halogenated solvents and toluene.
Single crystals of complex 2 were obtained by hexane vapor
diffusion into a concentrated acetonitrile/dichloromethane
solution, and the molecular structure is depicted in Figure 2.
RESULTS AND DISCUSSION
■
The construction of tricationic salt 1 followed a slightly
modified procedure as reported previously by Elsevier and
Ku
̈
hn.19b,c The full synthetic methodology uses established
routes, including the copper alkyne−azide cycloaddition of
mesityl azide with bis-propargylated imidazolium A and the
subsequent triazole (B) N-alkylation using a large excess of
methyl iodide. To avoid the mixture of halogen counterions in
the final product, potassium iodide was added during the final
step of the reaction, producing salt 1 in 82% yield after
purification (Scheme 1).
Scheme 1. Synthesis of Tri-cationic Salt 1
Figure 2. Molecular structure of the cationic fragment of complex 2.
Ellipsoids are shown at 40% of probability. Iodine counterions and
hydrogen atoms have been omitted for the sake of clarity. Selected
bond distances in angstroms: C1−Pd1, 2.000(6); C6−Pd1, 2.037(5);
C19−Pd1, 2.053(5); I1−Pd1, 2.6131(7); C30−Pd2, 2.010(6); C35−
Pd2, 2.043(5); C48−Pd2, 2.047(5); I2−Pd2, 2.6228(6).
̅
Complex 2 crystallizes in triclinic space group P1 with the
asymmetric unit displaying two independent molecules in
which the meridional pincer type coordination of the
tris(carbene) ligands toward the metal center is confirmed.
In general, the two molecules display very similar parameters,
including the palladium centers residing in a slightly distorted
square planar geometry by coordination to the three carbene
centers and the iodine ligand. The NHC−Pd−I angles are
close to 180°, while the MIC−Pd−MIC angles deviate more
from linearity with angles of ∼171°. The coordination of
palladium to the two triazolylidenes results in the formation of
two six-membered rings that reside in a quasiplanar
conformation with only slight deviations at the methylene
linker (torsion angles of 2.37−5.4°). The mesityl substituents
are oriented perpendicular with respect to the triazolylidene
five-membered ring plane, providing steric protection at the
metal center.
The successful preparation of salt 1 was clearly indicated via
1H NMR spectroscopy by the presence of the singlet
corresponding to the N-CH3 groups (4.51 ppm) and the
two different low-field proton resonances located at 9.76 and
9.33 ppm (1:2 ratio), corresponding to the imidazolium and
triazolium fragments, respectively. Elemental analysis also
confirmed the presence of the three iodine counterions in
the structure of 1.
Pincer complexes of palladium have attracted a significant
amount of attention owing to their wide range of applications
in homogeneous catalysis, including cross coupling and aldol
condensation reactions, Michael additions, allylation of
aldehydes, etc.20 Thus, with the aim of promoting the pincer
type coordination of palladium to salt 1, we initially reacted the
tris-azolium precursor with an excess of KHMDS (3.5 equiv)
in the presence of stoichiometric amounts of PdCl2 or
PdCl2(MeCN)2. Unfortunately, both trials proved to be
With the premise that mesoionic triazolylidenes (MICs) are
significantly better donors than imidazole-2-ylidenes
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(NHCs),21 we compared the σ-donation capacity of these
1
ligands by measuring the JCH coupling constants obtained
1
from H NMR spectra of the imidazolium and triazolium
fragments integrated in salt 1.22 The constant values found by
this method were 221.85 and 213.78 Hz for the imidazolium
and triazolium, respectively. These data, consistent with a
stronger σ-donation of the MIC ligand, suggested that shorter
MIC−metal bond distances (compared to NHC−metal
distances) should be found in the solid-state structure of 2.
Interestingly, in complex 2, the Pd−NHC bond distances
range from 2.000(6) to 2.010(6) Å while the MIC−Pd bond
distances are slightly longer [2.037(5)−2.053(5) Å]. The
elongation of the MIC−Pd bond distances may be related to
the flexibility of the methylene linker and the trans effect
between the two triazolylidene ligands.
Cationic complexes of palladium of the type [Pd-
(L)n(solvent)n]2+ have been studied over the past decade as
electrophilic or Lewis acidic catalysts for a variety of
transformations.23 These complexes are characterized by
defined square planar geometries often with adjacent
coordination sites occupied by labile solvent molecules of
poorly coordinating ligands. To test the capability of the
tris(carbene) pincer ligand to stabilize a dicationic palladium
center, we performed the metathesis reaction of complex 2
with an excess of AgSbF6 in acetonitrile (Scheme 3). After
Figure 3. Molecular structure of the dicationic fragment of complex 3.
Ellipsoids are shown at 40% of probability. SbF6 counterions and
noncoordinating solvents have been omitted for the sake of clarity.
Selected bond distances in angstroms: C1−Pd1, 1.980(6); C6−Pd1,
2.036(0); C19−Pd1, 2.047(5); N9−Pd1, 2.042(6).
and preparation of palladium pincer complexes bearing NHC
ligands as they have demonstrated great catalytic performances
and thermal stabilities.23,24
The biphenyl motif is highly important in organic synthesis
as it is often present in natural products and fine chemicals.
Typically, aryl bromides or iodides are employed in the Suzuki
coupling with boronic acids to render biphenyl products in the
presence of metal catalysts. However, the use of “unreactive”
aryl chlorides as coupling partners in this process is in constant
development. With this in mind, we decided to test the
catalytic applicability of complexes 2 and 3 in the formation of
ortho-substituted biphenyls. Initially, 2-methylchlorobenzene
and 2-methylboronic acid were chosen as model substrates to
optimize the reaction conditions for palladium precatalysts 2
and 3. The best catalytic system for both complexes included a
1 mol % catalyst loading with NaOtBu in dioxane at room
temperature (Table 1).
With the optimized conditions in hand, and as observed in
Table 2, complexes 2 and 3 were effective in the Suzuki
coupling of several aryl chlorides with ortho-substituted aryls.
As expected, the less hindered coupling partners render the
highest yields (93−97%) while the more challenging tri-ortho-
substituted products reach maximum yields of 90%. In general,
the catalytic performance of complexes 2 and 3 in the synthesis
of tetra-ortho-substituted biaryls compares well with that of the
[Pd(iPr*)(cinnamyl)Cl] and Pd(PEPPSI)(IPent) complexes
reported by Nolan25 and Organ,26 in terms of catalyst loading,
conversion, time, and reaction temperature. However, the
performance of 2 and 3 is still behind that of the palladium
complex supported by N,N-bis(2,4,6-trimethylphenyl)ethane-
1,2-diamine (BEDA) ligands reported by Liu,27 in which a
catalyst loading of 0.01 mol % is required for assessing biaryls
in yields of >90%.
Scheme 3. Synthesis of Complex 3
purification, air stable complex 3 was isolated as a white solid
in 84% yield and characterized by NMR spectroscopy and
elemental analysis. As expected, after removal of the iodine
atom, a new signal corresponding to the coordinated
acetonitrile ligand is located at 2.14 ppm (singlet) while the
1
other H and 13C NMR patterns are very similar to that of
former complex 2.
Suitable single crystals of complex 3 were obtained by
hexane diffusion into a concentrated acetonitrile/dichloro-
methane solution, and the molecular structure is depicted in
Figure 3. Complex 3 crystallizes in monoclinic space group
P21/c and exhibits a distorted square planar geometry with the
NHC and MIC ligands coordinated in a meridional fashion
and completing the coordination sphere with an acetonitrile
ligand. As observed in former complex 2, the palladium-
triazolylidene Pd−C6 [2.036(6) Å] and Pd1−C19 [2.047(6)
Å] bond distances are longer than the palladium imidazoly-
lidene Pd1−C1 [1.980(6) Å] distance, which is likely related
to the flexibility of the methylene linker in the pincer ligand.
The NHC−Pd−MeCN angle is close to 180°, while the MIC−
Pd−MIC angles deviate more from linearity with an angle of
∼172°.
With the increased popularity of procedures aiming to form
new carbon−nitrogen bonds, we were interested in testing
complexes 2 and 3 in the recently reported use of esters as
coupling partners in the catalytic formation of amides.28 Thus,
following reaction conditions similar to those reported by
Newman and co-workers,28a we illustrate in Table 3 the
comparative results between complexes 2 and 3 using a 3 mol
% catalyst loading, potassium carbonate as the base, and 80 °C
in toluene. The scope of the ester and substituted anilines is
Cross coupling reactions represent one of the most
important classes of catalytic carbon−carbon bond-forming
reactions. The most widely used catalysts in this field are based
on transition metal complexes from groups 8−10, and those
containing palladium are particularly efficient in a variety of
processes. Recently, efforts have been focused on the design
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Table 1. Cross Coupling Reaction of 2-Methyl
Table 2. Preparation of Di- and Tri-ortho-Substituted
a
Phenylboronic Acid with 2-Methyl-chlorobenzene Using
Biaryls
a
Precatalysts 2 and 3
b
entry catalyst [catalyst] (mol %)
solvent
base
yield (%)
1
2
3
4
5
6
7
8
2
3
2
3
2
3
2
3
2
3
2
3
2
2
2
2
5
5
5
5
5
5
3
3
1
1
0.5
0.5
1
1
1
1
THF
THF
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
K2CO3
73
70
97
95
68
75
97
95
92
96
78
71
56
42
68
89
dioxane
dioxane
toluene
toluene
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
9
10
11
12
13
14
15
16
K3PO4
KOH
Cs2CO3
a
Reaction conditions: 2-methyl-chlorobenzne (1.0 mmol), 2-methyl
b
phenylboronic acid (1.2 mmol), solvent, 3 mL, 2 h. Isolated yields as
the average of two runs.
broad and tolerates sterically hindered, electron rich, and
electron-withdrawing substituents. In general, both complexes
display very similar conversions with a slightly better
performance of complex 3 reflected in the yield of the more
substituted amide (entry 6). The catalytic performance of
complex 3 in amide formation is similar in terms of catalyst
loading and temperatures to that reported by Newman28a using
Pd(IPr)(cinnamyl)Cl and the Pd-PEPPSI type precata-
lysts28c−g employed by Szostak; however, its activity is lower
than that of the (η3-1-tBu-indenyl)Pd(IPr)(Cl) complex
reported by Hazari,28b which uses only 1 mol % catalyst at
room temperature to obtain the desired amides in high yields.
To further compare the performances of complexes 2 and 3,
the catalytic profiles for the reaction of phenyl benzoate with
aniline under catalytic 2 and 3 were determined using the
reaction conditions presented in Table 3 (entry 1). As
illustrated in Figure 4, complex 3 reaches conversions of
>70% after reaction for only 6 h while its maximum
conversions are observed in 8 h. Conversion to product is
slower in case of complex 2, where the maximum conversions
achieved after reaction for 14 h remained unchanged after this
time. The catalytic profile results show that complex 2 is slower
than 3; however, the conversions after 16 h are almost the
same.
Pd-NHC complexes have been widely used for the coupling
of challenging alkyl and aryl electrophiles, and there have been
several reports of their use as catalysts for the coupling of
amides by C−N bond activation.29 Interested in challenging
the catalytic potential of complexes 2 and 3, we decided to test
them in this recently reported cross coupling reaction. Thus,
following reaction conditions similar to those reported by
Szostak and co-workers,29c we compared complexes 2 and 3 in
terms of the coupling of substituted N-glutarimides and various
boronic acids. According to Table 4, the scope of boronic acid
and substituted N-glutarimide is broad and tolerates sterically
a
Reaction conditions: aryl chloride (1.0 mmol), boronic acid (1.2
b
mmol), NaOtBu (1.5 mmol), dioxane, 2 mL, 2 h. Isolated yields as
the average of two runs.
hindered, electron rich, and electron-withdrawing substituents.
Interestingly, the yields obtained with complexes 2 and 3 are
very similar and comparable to that of the commercially
available IPr-Pd PEPPSI-based complex.30
The use of vinyl silanes as versatile building blocks in
organic synthesis has attracted a great deal of attention in
recent years.31 Within the various methods for the generation
of new C−Si bonds, the hydrosilylation of alkynes is by far the
most atom economical and straightforward approach. Despite
major advances over the past several years, the development of
regio- and stereoselective processes along the H−Si addition is
still a major challenge as the hydrosilylation of alkynes
produces up to three isomers, namely, b-(Z), b-(E), and a
isomers.32 Motivated by the good results obtained in the cross
coupling reactions, we decided to test the activities of
complexes 2 and 3 in the hydrosilylation of alkynes. As an
initial screening test for the hydrosilylation process, we carried
out the reaction of phenylacetylene with triethylsilane at 110
°C in toluene-d8 at catalyst loadings of 5 mol % (Table 5). The
1
conversions were monitored by H NMR spectroscopy by
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Table 3. Cross Coupling Reaction of Esters with Substituted
Anilines Using Precatalysts 2 and 3
Table 4. Suzuki−Miyaura Cross Coupling of Amides and
b
a
Boronic Acids Using Complexes 2 and 3
b
entry
R1
Ph
Ph
Ph
Ph
R2
catalyst: yield (%)
1
2
3
4
5
6
7
8
H
2: 90
2: 94
2: 93
2: 95
2: 91
2: 92
2: 92
2: 88
3: 92
2-CH3
4-CH3
4-OMe
4-CO2Me
H
3: 96
3: 92
3: 94
3: 91
3: 90
3: 96
3: 86
Ph
3-OMe
3-Me
4-CN
H
H
a
Reaction conditions: boronic acid (2.0 mmol), amide (1.0 mmol),
b
K2CO3 (3.0 mmol), catalyst (3 mol %), THF, 5 mL, 12 h. Isolated
yields as the average of two runs.
Table 5. Comparison of the Catalytic Performances of
Complexes 2 and 3 in the Hydrosylation of
a
Phenylacetylene
b
entry catalyst [catalyst] (mol %) temp (°C) yield (%)
E/Z/a
1
2
3
4
5
6
7
8
2
3
2
3
2
3
2
3
2
3
2
3
5
5
3
3
2
2
1
1
2
2
2
2
110
110
110
110
110
110
110
110
90
98
98
98
98
97
98
91
88
86
82
71
70
88/6/4
86/8/4
85/9/6
82/9/9
87/9/4
88/8/2
82/12/6
80/9/11
90/7/4
86/4/10
88/10/2
87/7/6
a
b
Isolated yields as the average of two runs. Reaction conditions:
ester (1.0 mmol), aniline (1.2 mmol), toluene, 3 mL, 16 h.
9
10
11
12
90
60
60
a
Reaction conditions: phenylacetylene (0.11 mmol), triethylsilane
b
(0.12 mmol), toluene-d8 (0.6 mL). Selectivity (E/Z/α) at 12 h.
After setting the optimized reaction conditions and finding a
similar performance between 2 and 3, we then explored the
scope of the hydrosylation process treating a variety of alkynes
with triethylsilane using complex 3 as a catalyst. As shown in
Table 6, the formation of the vinyl silanes is successful with a
variety of substituted phenylacetylenes, featuring either
electron-donating or electron-withdrawing groups (entries 1−
6), reaching conversions of >98%, and displaying a good
selectivity for the E-isomer in all cases. Gratifyingly, the
challenging additions of hydrosilane to propargyl alcohol and
ethynyltrimethylsilane present good conversions with a strong
preference for the E-isomer.
Although complexes 2 and 3 are not as active as the known
Pd2(dba)3/P(Cy)3 combination33 or the iminophosphine−
Pd(II) complex reported by Schmidt34 that achieve hydro-
silylation of electron-deficient alkynes at room temperature
with low catalysts loadings (0.5−2.0 mol %), the performance
of complexes 2 and 3 compares well (in terms of catalyst
loading, conversion, and selectivity) to those of the few
Figure 4. Cross coupling of phenyl benzoate with aniline under
catalytic 2 and 3. Reactions carried out with toluene-d8 at 80 °C and 3
1
mol % catalyst. Conversions determined by H NMR spectroscopy
based on the amount of aniline remaining in solution.
analyzing product formation at different times over 12 h. As
shown in Table 5, the performance of complexes 2 and 3 is
very similar, reaching conversions of 98%. With regard to the
selectivity, complexes 2 and 3 are highly E-selective, displaying
values of ≤90%. Optimization of the reaction conditions
demonstrates that the loading of complexes 2 and 3 can be
reduced to 2 mol % without significant losses of conversion or
selectivity.
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Table 6. Scope of the Hydrosylation of Various Alkynes
a
with Triethylsilane Catalyzed by Complex 3
Figure 5. Mercury poisoning tests for the coupling reaction of 2-
methyl phenylboronic acid with 2-methyl-chlorobenzene catalyzed by
2 and 3.
Figure 6. DCT poisoning tests for the coupling of 2-methyl
phenylboronic acid with 2-methyl-chlorobenzene catalyzed by 2 and
3.
a
Reaction conditions: alkyne (0.10 mmol), triethylsilane (0.12
benzylamide (carried out at 80 °C), in the presence of 2 and
3. Thus, the formation process of benzophenone was disturbed
by the addition of Hg(0) at different reaction times (0−6 h).
Under catalytic conditions, the conversion percentages when
the poisoning metal addition occurred between 0 and 4 h
decreased to values of 68−85% (Figure S17). Complementary
cross-check DCT poisoning tests show conversions of 17% and
25% for complexes 2 and 3, respectively (Figure S18). Both
data sets are consistent with catalytic activities mainly due to
molecular species.
Mercury poisoning tests for benzylamide preparation under
catalytic 2 and 3 were also performed at several reaction times.
At first sight, we noticed that the addition of Hg(0) at the
beginning of the process (t = 0 h) substantially decreased the
product conversions to a range of 26−30% (Figure 7),
indicating that at 80 °C the catalysis is due principally to
heterogeneous palladium. If the addition of mercury is
performed at 6−8 h, a steady increase in the conversion
percentages is observed (58−68%) as expected from the
presence of an active heterogeneous catalyst. Additionally,
Crabtree’s homogeneous catalyst poisoning tests for 2 and 3
show conversion percentages between 72% and 77% (Figure
S19), confirming that heterogeneous palladium is active under
the catalytic reaction conditions.
mmol), toluene (3 mL). Yields and E/Z/α ratios were determined
by H NMR spectroscopy.
1
reported palladium nanoparticle systems supported by porous
carbon,35 polymeric resins,36 and imidazolium salts.37
As different reaction temperatures were employed in the
catalytic transformations reported in this work, we performed
different tests to obtain deeper insight into the nature of the
catalysts. Taking advantage of the ability of Hg(0) to poison
metal−particle heterogeneous catalysts,38 we initially studied
the coupling reaction of 2-methyl phenylboronic acid with 2-
methyl-chlorobenzene catalyzed by 2 and 3 (at room
temperature) in the presence of an excess of elemental
mercury. According to GC analysis during the reaction time,
no significant changes in the product yields were observed
[85−90% (Figure 5)]. Further analysis of the nature of
complexes 2 and 3 was carried out following Crabtree’s test for
the selective homogeneous catalyst poisoning by the addition
of dibenzo[a,e]cyclooctatriene (DCT).39 In this case, the
activity of 2 and 3 is quickly deterred as time progresses,
affording a drastic decrease in the conversions to products to
18−23% (Figure 6). The latter conversion data complement
well the mercury poisoning tests, suggesting that at room
temperature, the catalytic conversions are performed essen-
tially by molecular species.
With the similar catalytic activities observed for complexes 2
and 3 and the temperature used in the hydrosilylation
procedures (110 °C), we explored the mercury poisoning
test in the hydrosilylation of phenylacetylene (Table 5, entry
6) in the presence of complex 3. The addition of an excess of
Hg(0) at different reaction times (t = 0, 2, 4, and 8 h) was
The possibility of heterogeneous palladium formation as the
catalytic temperature increases prompted us to perform
mercury poisoning tests in the catalytic formation of
benzophenone (carried out at 60 °C) and N-phenyl
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Figure 9. DCT poisoning tests (t = 0 h) for the hydrosilylation of
phenylacetylene catalyzed by complex 3.
significant thermal instabilities of the precatalysts at 80 and 120
°C, as previously reported for similar MIC-Pd systems.12
CONCLUSIONS
■
In summary, we have reported the synthesis of a rare CCC-
pincer palladium(II) complex (2) supported by three NHCs
(one imidazolylidene and two triazolylidenes) and one iodide
ligand. Exchange of the iodine atoms by the reaction of
complex 2 with an excess of AgSbF6 provides a dicationic
Pd(II) CCC-pincer complex 3 that completes its coordination
sphere by one acetonitrile ligand. Both complexes 2 and 3 are
air and moisture stable and were fully characterized by NMR
spectroscopy, X-ray diffraction, and elemental analysis. The
application of complexes 2 and 3 in the coupling of aryl
chlorides/amides with boronic acids under mild conditions
was investigated. Using standard Suzuki−Miyaura cross
coupling conditions, complexes 2 and 3 display a high
efficiency in the preparation of di- and tri-ortho-substituted
biaryls using a 1 mol % catalyst loading and room temperature.
Complexes 2 and 3 are also efficient in the recently reported
coupling of esters with anilines to produce amides under mild
reaction conditions. Finally, the efficiency of 2 and 3 was
compared in the coupling of substituted N-glutarimides and
various boronic acids providing similar yields in all cases.
Additionally, complexes 2 and 3 showed high performances in
the hydrosilylation of terminal alkynes with good selectivity
toward the E-isomer. The reaction scope described in this
study demonstrates the broad applicability of the new CCC-
pincer palladium(II) complexes in the catalyzed synthesis of a
variety of biaryls, amides, and vinyl silanes, providing a new
alternative for these important organic transformations.
Figure 7. Mercury poisoning tests at different reaction times for the
preparation of benzylamide catalyzed by complexes 2 (top) and 3
(bottom).
carried out, and as depicted in Figure 8, the conversion
percentages of 3 when mercury is added at 0 and 2 h decreased
Figure 8. Mercury poisoning tests at different reaction times for the
hydrosilylation of phenylacetylene catalyzed by complex 3.
EXPERIMENTAL SECTION
■
General Considerations. Commercially available reagents and
solvents were used as received. 1,3-Dipropargylimidazolium bro-
mide40 and mesityl azide41 were synthesized as reported in the
literature. Synthesis of all metal complexes was performed under an
atmosphere of dry nitrogen using standard Schlenk techniques.
Solvents were dried by standard methods and distilled under nitrogen.
Melting points were determined on a Fisher-Johns apparatus and are
uncorrected. NMR spectra were recorded with a Bruker Ascend (400
MHz) spectrometer. Elemental analyses were obtained with a
Thermo Finnegan CHNSO-1112 apparatus and a PerkinElmer Series
II CHNS/O 2400 instrument. X-ray diffraction analyses were
performed in an Agilent Gemini Diffractometer using Mo Kα
radiation (λ = 0.71073 Å). Data were integrated, scaled, sorted, and
averaged using the CrysAlisPro software package. We determined the
structures using direct methods, using SHELX 2014, and refined by
full matrix least squares against F2.42 All non-hydrogen atoms were
refined anisotropically. The positions of the hydrogen atoms were
to minima of 17% and 26%, respectively (Figure 8). These
numbers strongly suggest that the catalytic process is mainly
due to heterogeneous palladium. Additionally, if the poisoning
process is delayed from times between 4 and 8 h, a steady
increase in the percentages is observed (55−86%). Cross-
checking DCT poisoning results are consistent with the
presence of heterogeneous catalytic active species at 110 °C, as
maximum conversions of 74% are obtained (Figure 9).
Although it is very difficult to rule out completely the use of
a small amount of catalyst in the different conversions reported
in this work, the full series of data strongly suggests that as the
temperature of the reaction mixture increases, the presence of a
heterogeneous palladium catalyst plays an important role in the
organic conversions. Additionally, the results likely indicate
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Article
kept fixed with common isotropic display parameters. The crystallo-
graphic data and some details of the data collection and refinement
are listed in Table 7.
CHtriazolium), 8.07 (d, 2H, J = 1.3 Hz, CHimidazole), 7.20 (s, 4H,
CHmes), 6.04 (s, 4H, CH2), 4.51 (s, 6H, NCH3), 2.35 (s, 6H, CH3),
2.05 (s, 12H, CH3); 13C NMR (100 MHz, DMSO-d6) δ 142.0, 138.6,
138.3, 134.3, 133.0, 131.0, 129.5, 123.5, 41.4, 20.7, 17.2. Found: C,
39.78; H, 3.99; N, 12.91. Calcd for C29H37I3N8: C, 39.65; H, 4.25; N,
12.76.
Table 7. Crystallographic Data, Summary of Data
Collection, and Structure Refinement for 2 and 3
Synthesis of Palladium Pincer Complexes 2 and 3. Complex
2. Tricationic salt 1 (50 mg, 0.057 mmol) and palladium acetate (13
mg, 0.057 mmol) were added to a Schlenk flask and dissolved in 3 mL
of acetonitrile. The mixture was stirred and heated at 100 °C for 18 h
using a thick-walled Schlenk tube. After the mixture had reached
room temperature, the red solution was concentrated and purified by
column chromatography using a dichloromethane/methanol mixture
(95/5 → 9/1) as the eluent, providing the title product as an orange
solid in 54% yield (26 mg, 0.031 mmol). Single crystals were obtained
by diffusion of hexane vapor into a saturated solution of 2 in an
2
3
formula
formula weight
crystal system
space group
T (K)
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
C60H71I4N17Pd2
1750.73
triclinic
C35H43F12N11PdSb2
1195.70
monoclinic
P21/c
293(2)
15.1794(7)
8.5731(4)
35.6684(19)
90
92.905(5)
90
4635.7(4)
4
P1
̅
293(2)
12.2192(4)
16.4898(7)
20.4091(8)
68.231(4)
86.384(3)
83.399(3)
3792.8(3)
2
1
acetonitrile/dichloromethane mixture: mp = 158−160 °C; H NMR
(400 MHz, CDCl3) δ 8.37 (s, 2H, CHimidazole), 6.83 (s, 4H, CHmes),
5.97 (s, 4H, CH2), 4.52 (s, 6H, NCH3), 2.26 (s, 6H, CH3), 1.94 (s,
12H, CH3); 13C NMR (100 MHz, CDCl3) δ 165.7, 157.4, 140.1,
139.3, 136.0, 134.9, 129.1, 121.4, 45.0, 38.7, 21.3, 19.1. Found: C,
40.55; H, 4.23; N, 13.32. Calcd for C29H34I2N8Pd: C, 40.74; H, 4.01;
N, 13.11.
Z
dcalc (g cm−3
)
1.534
1.713
Complex 3. Complex 2 (60 mg, 0.127 mmol) and AgSbF6 (98 mg,
0.260 mmol) were added to a Schlenk flask and dissolved in 5 mL of
distilled acetonitrile. The mixture was stirred and heated at 60 °C for
18 h. After the mixture had reached room temperature, the white
solution was filtered, concentrated, and precipitated with ethyl ether
(3 × 7 mL), providing the title product as a white solid in 84% yield
(0.107 mmol). Single crystals were obtained by diffusion of hexane
vapor into a saturated solution of 3 in an acetonitrile/dichloro-
methane mixture: mp 201−203 °C dec; 1H NMR (400 MHz,
CD3CN) δ 7.45 (s, 2H, CHimidazole), 7.06 (d, 4H, CHmes), 5.45 (s, 4H,
CH2), 4.22 (s, 6H, NCH3), 2.31 (s, 6H, CH3), 2.14 (s, 3H, CH3CN),
1.98 (s, 12H, CH3); 13C NMR (100 MHz, CD3CN) δ 153.8, 153.4,
142.1, 141.2, 135.5, 135.4, 130.4, 123.9, 45.8, 38.1, 21.0, 17.8, 1.7.
Found: C, 34.27; H, 3.71; N, 11.51. Calcd for C32H40F12N9PdSb2: C,
34.05; H, 3.57; N, 11.17.
μ (mm−1
no. of reflections collected
min/Tmax
)
2.147
86018
0.912
1.628
80903
0.945
T
Nmeasd
19465
12103
Rint
0.0583
0.0684
R [I > 2σ(I)]
R (all data)
Rw [I > 2σ(I)]
Rw (all data)
goodness of fit
0.0504
0.0979
0.1202
0.1466
0.0696
0.0980
0.1649
0.1804
1.016
1.043
Synthesis of Triazole B and Tricationic Salt 1. Triazole B.
Cu(OAc)2·H2O (115 mg, 0.57 mmol), 1,10-phenanthroline mono-
hydrate (103 mg, 0.57 mmol), and sodium L-ascorbate (2.28 g, 4.0
mmol) were charged into a 100 mL round-bottom flask equipped
with a magnetic stirrer. After the addition of 15 mL of a DMF/H2O
mixture [3/1 (v/v)], the resulting suspension was stirred for 10 min
at room temperature. Subsequently, 1,3-dipropargylimidazolium
bromide (1.3 g, 5.77 mmol) and mesityl azide (2.23 mL, 13.8
mmol) were added to the reaction mixture and stirred for 48 h at
room temperature. The crude product was extracted with dichloro-
methane and washed with water, and the organic phase dried on
anhydrous sodium sulfate. The solvent was reduced to 1/3 of the
original volume, and diethyl ether was added until a precipitate was
formed. The solid was collected by filtration and washed thoroughly
with cold diethyl ether. After the solvent had been removed under
reduced pressure, the pure product is obtained as a brown solid in
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00324.
1
Sample H and 13C NMR spectra for new compounds
(PDF)
Accession Codes
CCDC 2055596−2055597 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
87% yield (2.76 g, 5.02 mmol): mp 188−190 °C; H NMR (400
MHz, DMSO-d6) δ 9.51 (s, 1H, CH+imid), 8.48 (s, 2H, CHtz), 7.87 (s,
2H, CHimid), 7.10 (s, 4H, CHmes), 5.70 (s, 4H, CH2), 2.32 (s, 6H,
CH3), 1.87 (CH3); 13C NMR (100 MHz, DMSO-d6) δ 140.5, 139.8,
136.6, 134.4, 133.0, 129.0, 126.5, 123.0, 43.9, 20.6, 16.8. Found: C,
59.50; H, 5.89; N, 20.73. Calcd for C27H31BrN8: C, 59.23; H, 5.71; N,
20.47.
AUTHOR INFORMATION
■
Corresponding Author
Salt 1. Methyl iodide (1.42 mL, 22.8 mmol) and potassium iodide
(1.0 mmol) were added to a 7 mL acetonitrile solution of triazole B
(250 mg, 0.46 mmol), and the resulting solution was refluxed for 48 h
at 100 °C using a thick-walled Schlenk tube. After the mixture had
reached room temperature, the solvent was reduced to 1/3 of the
original volume and diethyl ether was added until a precipitate was
formed. The solid was collected by filtration and washed thoroughly
with cold diethyl ether. After the solvent had been removed under
reduced pressure, the pure product is obtained as a brown solid in
́
Daniel Mendoza-Espinosa − Area Académica de Química,
Universidad Autónoma del Estado de Hidalgo, Mineral de la
Reforma, Hidalgo, Mexico 42090; orcid.org/0000-0001-
9170-588X; Email: daniel_mendoza@uaeh.edu.mx
Authors
́
David Rendón-Nava − Area Académica de Química,
Universidad Autónoma del Estado de Hidalgo, Mineral de la
Reforma, Hidalgo, Mexico 42090
1
82% yield (250 mg, 0.37 mmol): mp 223−225 °C; H NMR (400
MHz, DMSO-d6) δ 9.76 (s, 1H, CHimidazolium), 9.33 (s, 2H,
2173
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Deyanira Angeles-Beltrán − Departamento de Ciencias
Básicas, Universidad Autónoma Metropolitana-Azcapotzalco,
Ciudad de México, Mexico 02200
Arnold L. Rheingold − Department of Chemistry and
Biochemistry, University of California at San Diego, La Jolla,
California 92093, United States; orcid.org/0000-0003-
4472-8127
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00324
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors are grateful to CONACYT (Project A1-S-8892)
for financial support.
■
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