Synthesis and structure of palladium 1,2,3-triazol-5-ylidene mesoionic carbene PEPPSI complexes and their catalytic applications in the mizoroki-heck reaction

Article abstract of DOI:10.1021/om3005228

New mono- and bimetallic 1,2,3-triazol-5-ylidene mesoionic carbene (MIC) complexes of Pd have been synthesized, isolated, and characterized. We have described the synthesis of a bis(MIC) complex via transmetalation from a Ag-MIC complex and two PEPPSI-type complexes which are directly available from their respective triazolium salts by treatment with PdCl₂ in pyridine. The X-ray structures are reported for all complexes described herein. Interestingly, each complex described exhibits various secondary noncovalent interactions in the solid state of the general type C-H...Cl, which appear to be important for the stabilization of the solid-state structure of the complexes. We further demonstrated the utility of the new PEPPSI complexes in the Mizoroki-Heck reaction. In the case of aryl iodides and electron-deficient bromides, high conversion is observed with methyl acrylate. Hg poisoning tests suggest that, even with an easily dissociated ligand, the reaction likely proceeds via Pd nanoparticles.

Full text of DOI:10.1021/om3005228

Article
pubs.acs.org/Organometallics
Synthesis and Structure of Palladium 1,2,3-Triazol-5-ylidene
Mesoionic Carbene PEPPSI Complexes and Their Catalytic
Applications in the Mizoroki−Heck Reaction
Eric C. Keske, Olena V. Zenkina, Ruiyao Wang, and Cathleen M. Crudden*
Department of Chemistry, Queen’s University, Chernoff Hall, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6
S
* Supporting Information
ABSTRACT: New mono- and bimetallic 1,2,3-triazol-5-ylidene mesoionic carbene (MIC)
complexes of Pd have been synthesized, isolated, and characterized. We have described the synthesis
of a bis(MIC) complex via transmetalation from a Ag-MIC complex and two PEPPSI-type complexes
which are directly available from their respective triazolium salts by treatment with PdCl2 in pyridine.
The X-ray structures are reported for all complexes described herein. Interestingly, each complex
described exhibits various secondary noncovalent interactions in the solid state of the general type
C−H···Cl, which appear to be important for the stabilization of the solid-state structure of the
complexes. We further demonstrated the utility of the new PEPPSI complexes in the Mizoroki−Heck
reaction. In the case of aryl iodides and electron-deficient bromides, high conversion is observed with
methyl acrylate. Hg poisoning tests suggest that, even with an easily dissociated ligand, the reaction
likely proceeds via Pd nanoparticles.
transformations.34−41 In particular, Pd-MIC complexes have
received significant interest; however, the diversity of the
INTRODUCTION
■
The development of N-heterocyclic carbenes (NHCs) for use
in organic synthesis has witnessed impressive growth in recent
complexes reported (Figure 1) is significantly less than that for
analogous NHC complexes.42−48
years.1−7 The high steric bulk and strongly σ-donating
properties of these ligands have made them particularly well
suited for use in cross-coupling reactions.8−11 In many cases,
the Pd-NHC complex is formed in situ, which can result in
difficulties with reproducibility and controlling the nature of the
Pd complex formed. As a result, many researchers have instead
moved toward well-defined preformed PdII NHC complexes,
which are often air and moisture stable and can be used without
rigorous anaerobic conditions.12−15 The reduction of these
complexes can then be performed in situ to generate the active
Pd0 species, although there can also be difficulties with this
process.16
Figure 1. Selected Pd-MIC and Pd-NHC complexes relevant to this
study.13,42,44,49
Heterocyclic carbenes that are only stabilized by one
heteroatom have also attracted significant interest, as they
generally act as stronger donors than the more common
NHCs.17−20 These are often termed abnormal carbenes or
mesoionic carbenes (MICs), and ligands of this type have been
utilized as catalysts in organic synthesis.21−26 An important
development in the chemistry of MICs was reported by
Albrecht and co-workers, who demonstrated the feasibility of
employing alkylated 1,2,3-triazolium salts as MIC precursors.27
The facile synthesis of these species, via copper-catalyzed “click
reactions”,28 has resulted in the rapid development of these
ligands,29−33 which are effective for a variety of catalytic
During the preparation of this paper, Albrecht and Trzeciak
reported the synthesis of a series of triazole MIC-derived
PEPPSI13 (pyridine enhanced precatalyst preparation stabiliza-
tion and initiation) complexes and investigated their use in the
Suzuki−Miyaura reaction.49 Interestingly, these complexes
appear to be less effective than the analogous NHC derivatives
Received: June 11, 2012
© XXXX American Chemical Society
A
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reported by Organ.50,51 Most importantly, through a detailed
analysis, it was shown that these complexes readily decomposed
to Pd nanoparticles at elevated temperatures, suggesting
significant differences in Pd binding between NHCs and MICs.
We have also been interested in developing triazole MIC
PEPPSI-type complexes for C−C bond forming reactions, in
the hopes that the increased electron donor capabilities of these
ligands could lead to increased reactivity versus the standard
NHCs. Herein we report the synthesis and characterization of
mono- and bimetallic PEPPSI-type complexes bearing triazole
MIC ligands and an investigation of their catalytic activity in the
Mizoroki−Heck reaction of methyl acrylate with aryl iodides
and bromides. Similarly to previous reports, mercury poisoning
tests suggest that these complexes may readily form Pd
nanoparticles at only moderately elevated temperatures.49 The
PEPPSI complexes described herein are conveniently synthe-
sized by a one-step route directly from PdCl2 in the presence of
a weak base in good yield, without requiring preactivation with
Ag2O. All palladium complexes have been characterized by X-
presence of only one MIC-derived Pd complex. The 13C{1H}
NMR spectrum of 3 displayed the Pd−carbene resonance at
162.9 ppm, which is consistent with bis(MIC) complexes
described in the literature.42,43,47 X-ray-quality crystals were
grown via a slow diffusion of hexanes into a concentrated
solution of CH2Cl2. The crystal structure of complex 3 is
displayed in Figure 2, and selected geometrical parameters are
reported in Table S1 (Supporting Information).
Fukuzawa and co-workers demonstrated that similar bis-
(MIC) complexes required very high temperatures (150 °C) to
efficiently catalyze the Mizoroki−Heck reaction.43 Computa-
tional analysis by Rosch and co-workers had previously
̈
demonstrated the likely benefits of a catalyst which consists
of only one NHC ligand, as well as a second labile ligand to help
stabilize the complex but provides open coordination sites for
catalysis to occur.53 This concept was exploited by Herrmann
in his synthesis of heteroleptic NHC phosphine catalysts.54−56
These results inspired us to investigate the reactivity of
PEPPSI-type catalysts13 derived from triazole MIC ligands in
the Mizoroki−Heck reaction. Surprisingly, although there have
been several reports of PEPPSI-type catalysts utilized in a
variety of C−C and C−N bond forming reactions,14,50,57
examples of these complexes in the Mizoroki−Heck reaction
are rare.58,59
1
ray crystallography, as well as H and 13C{1H} NMR.
RESULTS/DISCUSSION
■
Synthesis of Complexes. We recently reported the
synthesis of mono- and bimetallic rhodium complexes
coordinated to triazole ligands derived from MIC precursors
1 and 2 (1, MIC1; 2, MIC2, Schemes 1 and 2).52 The synthesis
Related triazole MIC PEPPSI complexes described by
Albrecht and Trzeciak were synthesized by activation first
onto Ag2O, followed by transmetalation to PdCl2 conducted in
3-chloropyridine as the solvent at 100 °C.49 In our case, we
were able to synthesize complexes 4 and 5 bearing Dipp
substituents by the reaction of a slight excess of triazolium salts
MIC1 and MIC2 directly with PdCl2 in pyridine in the presence
of K2CO3. This resulted in the isolation of the desired
compounds in 82% and 77% yields, respectively, after
purification by column chromatography (Scheme 2). Although
Organ has demonstrated the benefits of 3-chloropyridine as the
throwaway ligand,60 the syntheses of these complexes proved to
be problematic, due to significant insolubility of the product. In
contrast, both complexes 4 and 5 are soluble in many organic
solvents and are air and moisture stable.
Scheme 1. Synthesis of Bis(MIC) Complex 3
Scheme 2. Synthesis of Complexes 4 and 5
The formation of complex 4 was evident by the loss of the
singlet resonance at 8.98 ppm corresponding to the C−H
triazole peak in the 1H NMR spectrum, as well as the presence
of new aromatic peaks corresponding to the coordinated
pyridine. Only one trz-CH3 peak was present at 4.14 ppm,
indicating the presence of only one MIC-derived complex. The
formation of complex 5 was also characterized by the loss of the
singlet resonance corresponding to the C−H triazole peaks of
1
the triazolium salt in its H NMR spectrum at 8.84 ppm. The
highly symmetrical nature of the 1H NMR spectrum was
retained, indicating the likely formation of a bimetallic complex.
Similar to our previous results,52 the formation of syn and anti
diastereomers did not seem to be an issue.61 No cyclo-
metalation was observed, since a broad resonance correspond-
ing to the o-C−H of the central benzene was still observed at
8.52 ppm. Interestingly, the 13C{1H} NMR spectra of
complexes 4 and 5 display CMIC−Pd resonances that are
dramatically more upfield than that of complex 3 by 16.1 and
20.8 ppm, respectively. This was also observed for the related
PEPPSI MIC complexes previously reported,49 which may
indicate relatively weak Pd−MIC bonds, which in turn may
have important ramifications for catalysis (vide infra).
of these complexes required transmetalation from Ag-MIC
complexes. In order to first investigate their coordination
chemistry with Pd, ligand MIC1 was treated with Ag2O and KBr
in MeCN and then transmetalated to [Pd(MeCN)2Cl2] by a
procedure similar to the one we reported previously, resulting
in the bis(MIC) complex 3 in overall 75% yield (Scheme 1).52
Formation of the desired complex was confirmed by 1H NMR,
which was characterized by the disappearance of the triazole
C−H peak. In addition, only one resonance at 3.98 ppm was
observed for the triazole methyl group (trz-CH3), indicating the
X-ray-quality single crystals of 4 were grown by a slow
diffusion of hexanes into a concentrated CH2Cl2−THF solution
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Figure 2. (A) Crystallographically determined structure of [Pd(MIC1)2(Cl)2] (3) displaying thermal ellipsoids drawn at the 50% confidence level.
Hydrogen atoms and solvent molecules (CH2Cl2) are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Pd(1)−C(1),
2.022(6); Pd(1)−Cl(1), 2.3512(15); N(1)−C(1), 1.382(7); N(2)−N(1), 1.344(7); N(2)−N(3), 1.306(7); C(1)−Pd−C(1A), 180.000(1); C(1)−
Pd(1)−Cl(1), 90.35(17); Cl(1)−Pd(1)−Cl(2), 180.0. (B) 2D structure of [Pd(MIC1)2(Cl)2]·2CH2Cl2 showing the hydrogen C−H···Cl bonds.
Hydrogen atoms and solvent molecules are omitted for clarity, with the exception of those involved in hydrogen bonding.
of 4. The X-ray crystal structure of complex 4 is displayed in
Figure 3a, and selected geometrical parameters are reported in
Table S1. X-ray-quality single crystals of 5 were grown by a
slow diffusion of hexanes into a concentrated CH2Cl2 solution
of 5. The X-ray crystal structure of 5 is displayed in Figure 3b,
and selected geometrical parameters are reported in Table S1.
Upon detailed inspection of the X-ray structures of complexes
3-5, it appears that all complexes are characterized by multiple
weak noncovalent interactions that influence the crystal
organization (vide infra). Complexes 4 and 5 were additionally
characterized by high resolution mass spectrometry and
elemental analysis.
Crystallography. Complex 3 exhibits a square-planar
geometry around the palladium metal center with the two
MIC ligands oriented trans to one another (Figure 2). The Pd
center is lying on a crystallographic inversion center. The
triazole rings of the carbene ligands are planar with negligible
deviation from planarity, which is represented by the torsion
angle (C2N3N2N1) = 1.15°. In addition, the triazole rings of
the carbenes are coplanar. The two palladium−MIC bonds are
2.022(6) Å, which is consistent with structurally related Pd
bis(NHC) complexes.22,62 The triazole rings of the MICs are
tilted with respect to their corresponding Dipp wingtip groups
with a dihedral angle of 46.16°.
Since complex 3 was crystallized from a CH2Cl2/THF
mixture, we observed CH2Cl2 incorporated into the crystal
lattice. Interestingly, multiple weak noncovalent interactions
were observed between the CH2Cl2 molecules and Pd complex
3, likely playing an important role in crystal packing (see Figure
2B). These contacts may reasonably be considered to be C−
H···Cl hydrogen bonds, since they are less than the sum of the
van der Waals radii (2.95 Å) and are charge assisted by the
opposite charges of anion and cation.63−67 Strong interactions
exist between the terminal chlorine atoms of complex 3 and the
hydrogen atoms from a CH2Cl2 solvent molecule with H···Cl
bond lengths of 2.558(2) and 2.650(9) Å (see Table S2 in the
Supporting Information for details). Complex 4 crystallized
with two crystallographically independent molecules per
asymmetric unit. Complex 4 displays a distorted-square-planar
geometry about the Pd atom with the pyridine bound trans to
the MIC ligand. The angles between the carbene carbons and
nitrogens of the pyridines are of 172.02 and 177.11°. The
triazole rings of the carbene ligands in complex 4 are planar,
with negligible deviations from planarity represented by the
torsion angles (C2N3N2N1) of 0.41° and (C28N8N7N6) of
0.62°. The Pd−CMIC bond lengths of 1.977(5) and 1.973(5) Å
are slightly shorter than in complexes 3 and 5. The plane of the
heterocyclic ring of the carbene ligand is tilted with respect to
the phenyl group with a dihedral angle of 46.37° and is almost
perpendicular to the corresponding Dipp wingtip group with a
dihedral angle of 78.07°, probably due to the steric constraints
of the isopropyl groups.
Although complex 4 was crystallized from a CH2Cl2/THF/
hexanes mixture, CH2Cl2 was not observed within the crystal
structure; however, several weak noncovalent C−H···Cl
interactions were observed. These interactions occur between
one of the Pd-bound chlorine atoms and the meta hydrogen of
the pyridine of a neighboring molecule (H···Cl bond length
2.744(1), 2.879(1), and 2.86(4) Å, Figure 3B). A second
interaction is also observed, as the other Pd-bound chlorine
atom is interacting with a hydrogen atom of the methyl group
of the triazole MIC ring of a neighboring molecule (H···Cl
bond length 2.84(3) Å (see Table S3 in the Supporting
Information for details).
The X-ray structure of complex 5 clearly displays the
bimetallic nature of the complex. The two palladium centers
have nearly identical environments, with each of the MICs
oriented trans to a pyridine. The heterocyclic rings of the
carbene ligand are planar with negligible deviation from
planarity, represented by the torsion angles (C23N3N2N1)
of 1.37° and (C13N4N5N6) of 0.67°. The Pd−CMIC bond
lengths for Pd1 and Pd2 are 2.000(13) and 2.004(13) Å,
respectively, which are quite similar to those of structurally
related complexes.49,60 The triazole rings of the carbene ligand
are almost perpendicular to their corresponding Dipp wingtip
groups, with dihedral angles of 78.62 and 80.75° for Pd1 and
Pd2, respectively. The triazole rings are tilted with respect to
the central phenyl ring with the angles of 50.22 and 50.34°.
C
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Figure 3. Crystallographically determined structures of (A) [Pd(MIC1)(Pyr)(Cl)2] (4) and (B) [(Pd)2(MIC2)(Pyr)2(Cl)4] (5), displaying thermal
ellipsoids drawn at the 50% confidence level. There are two independent molecules of 4 per asymmetric unit. The second crystallographically
independent molecule of 4 has a very similar geometry and was omitted for clarity. (C) 2D structure of [Pd(MIC1)(Pyr)(Cl)2], (4) showing the
hydrogen C−H···Cl bonds. Non-hydrogen-bonding H atoms are omitted for clarity. (D) 2D structure of [(Pd)2(MIC2)(Pyr)2(Cl)4] (5) showing
weak C−H···Cl interactions. Hydrogen atoms and solvent molecules are omitted for clarity, with the exception of those involved in hydrogen
bonding. Selected interatomic distances (Å) and angles (deg) are as follows. For 4: Pd(1)−C(1), 1.977(5); Pd(1)−Cl(1), 2.3066(13); Pd(1)−
Cl(2), 2.3003(13); N(1)−C(1), 1.367(6); N(2)−N(1), 1.316(6); N(2)−N(3), 1.306(7); Pd(1)−N(5), 2.116(4); C(1)−Pd−N(5), 176.70(17);
C(1)−Pd(1)−Cl(1), 89.15(15); C(1)−Pd(1)−Cl(2), 87.84(15); Cl(1)−Pd(1)−Cl(2), 172.02(5). For 5: Pd(1)−C(23), 2.004(13), Pd(2)−C(13),
2.000(13); Pd(2)−Cl(1), 2.322(4); Pd(2)−Cl(2), 2.324(4); Pd(1)−Cl(3), 2.299(4); Pd(1)−Cl(4), 2.297(4); N(6)−C(13), 1.358(15); N(1)−
C(23), 1.349(14); N(2)−N(1), 1.349(13); N(6)−N(5), 1.350(13); N(2)−N(3), 1.300(12); N(4)−N(5), 1.304(12); Pd(2)−N(7), 2.145(11);
Pd(1)−N(8), 2.103(11); C(13)−Pd(2)−N(7), 178.8(5); C(23)−Pd(1)−N(8), 178.5(5); C(13)−Pd(2)−Cl(1), 88.3(3); C(13)−Pd(2)−Cl(2),
87.5(3); C(23)−Pd(1)−Cl(4), 89.7(4); C(23)−Pd(1)−Cl(3), 88.5(4); Cl(3)−Pd(1)−Cl(4), 173.34(15); Cl(1)−Pd(2)−Cl(2), 168.63(16).
Complex 5 exhibits a distorted-square-planar geometry around
the metal centers with visible distortions from planarity with
the corresponding angles Cl(1)−Pd(2)−Cl(2) = 168.63(16)°
and Cl(3)−Pd(1)−Cl(4) =173.34(15)°. This is likely due to
the multiple weak intermolecular noncovalent C−H···Cl
interactions (vide infra) that appear to have a significant effect
on the solid-state structure of complex 5.
Similarly to complex 4, weak intermolecular noncovalent
interactions in complex 5 exist between the chlorine atoms and
one of the pyridine hydrogens (H···Cl bond length 2.87(4) Å),
Figure 3D) and a second type of interaction is also observed
between one of the Pd-bound chlorine atoms and one of the
hydrogen atoms of the methyl group of the triazole carbene in a
neighboring molecule (H···Cl bond length 2.91(9) Å) (see
Table S4 in the Supporting Information for details).
Catalysis. The Mizoroki−Heck reaction68 has been
extensively studied with a wide variety of Pd-NHC com-
plexes.22,69−71 Although this reaction is commonly performed
with phosphine-ligated catalysts,72 Pd-NHC complexes benefit
from increased thermal stability, which make them ideally
suited for the often harsh conditions required in this
reaction.69,73 Some of the most common precatalysts
investigated are PdII bis(NHC) complexes,43 which are reduced
in situ to the catalytically active Pd0 complexes. Several groups,
however, have demonstrated the beneficial effect of Pd-NHC
complexes sporting hemilabile ligands,53,56,70,74,75 whose role
would be to open up coordinate sites on the Pd center and
facilitate binding of the substrate. This prompted us to study
the catalytic activity of complexes 4 and 5. It was our hope that
the increased donating capabilities of the ligands 4 and 5 would
D
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make them effective catalysts for this reaction under relatively
mild conditions.76
We began by examining the coupling of p-iodoacetophenone
and methyl acrylate utilizing complexes 4 and 5 as catalysts
(Table 1). Both complexes proved to be competent catalysts
Table 1. Optimization Results in the Mizoroki−Heck
Reaction
c
d
entry
cat.
cat. loading (mol %)
time (h)
yield (%)
TON
a
1
5
5
5
5
5
4
3
1
2
3
96
97
40
81
61
96
99
48
97
a
2
0.5
0.25
0.25
0.125
2
Figure 4. Comparison of catalytic activity for catalysts 4 and 5 in the
Mizoroki−Heck reaction at 2% catalyst loading.
a
3
6
80
b
4
24
48
2
162
244
48
°C;49 however, to definitively show that heterogeneous Pd is
the sole active species, significantly more studies would be
required.79−81
Higher temperatures (125 °C instead of 80 °C) and longer
reaction times were required in order for electron-deficient aryl
bromides to react (Table 2). In all of the experiments
b
5
a
6
a
7
2
2
49
a
Conditions: p-iodoacetophenone (0.20 mmol), methyl acrylate (0.28
b
mmol), NEt3 (0.5 mL), DMF (2.5 mL). Conditions: p-
iodoacetophenone (0.40 mmol), methyl acrylate (0.56 mmol), NEt3
(1.0 mL), DMF (5.0 mL). HCO2Na (5 equiv/[Pd]) as reducing
agent. Isolated yields.
c
d
Table 2. Mizoroki−Heck Reaction of Aryl Iodides and
a
Bromides
for the coupling of aryl iodides using sodium formate as the
reducing agent.77,78 Complete conversion was obtained in 2 h
at 80 °C using 1 mol % of complex 5 (2 mol % Pd). In the
absence of sodium formate, minimal conversion (<5%) was
observed in the same time period, as determined by GC
analysis. Decreasing the catalyst loading slowed down the
reaction, but good conversion could still be obtained at
increased reaction times (entries 2−5). Encouragingly, no Pd
black was observed at this temperature, in contrast to the
performance of related catalysts in the Suzuki−Miyaura
reaction at the same temperature.49 However, once the catalyst
loading was decreased to 0.25 mol %, decomposition become
evident, as complete conversion could not be observed even at
extended times (entries 3−5). In all cases, the E isomer was the
only product observed.
Catalyst 4 also proved to be competent in this trans-
formation (entry 6), giving yields comparable to those for
catalyst 5. A direct comparison between complexes 4 and 5 was
performed by GC analysis, which revealed nearly identical
catalytic behavior for the two complexes (Figure 4). A short
induction period was present for 5 relative to 4, which may be
related to the in situ reduction to the active Pd0 species. TOF
values at 50% conversion were calculated as 50 and 37 h−1 for
complexes 4 and 5, respectively.
Since Trzeciak and Albrecht clearly demonstrated that the
Pd-MIC catalysts used in their detailed study of the Suzuki−
Miyaura reaction decomposed to Pd nanoparticles,49 we also
examined our catalyst system for the presence of heterogeneous
Pd. Thus, we performed a mercury poisoning test by exposing
the reaction solution to a large excess of elemental mercury
after the reaction had reached 12% conversion as determined
by GC analysis. Under these conditions, subsequent conversion
to product was inhibited, suggesting that heterogeneous Pd
nanoparticles are the active catalyst, despite the fact that Pd
black was not visually detected when the reaction was run at 80
temperature
time
(h)
yield
(%)
entry
1
substrate
(°C)
catalyst
b
X = I, R = CH3
80
80
2
4
5
4
5
4
5
4
5
4
5
3
90
b
93
b
2
3
4
5
X = I, R = OCH3
X = I, R = C(O)CH3
X = Br, R = NO2
X = Br, R = CH3
2
95
b
98
b
125
125
125
5
98
b
95
b
5
92
b
97
c
16
trace
c
5
c
trace
a
General conditions: aryl halide (0.20 mmol), methyl acrylate (0.28
mmol), HCO2Na (0.02 mmol), catalyst (0.004 mmol Pd), NEt3 (0.5
b
c
mL), DMF (2.5 mL). Isolated yields. Determined by GC.
performed at 125 °C, Pd black was observed within minutes
of the reaction. Regardless, at 125 °C, full conversion of
electron-deficient aryl bromides was observed in 5 h. Electron-
rich aryl bromides failed to react under these conditions (entry
5), and even after higher temperatures and extended reaction
times (150 °C, 24 h), only minimal conversion (<5%) was
observed. These results likely indicate significant thermal
instabilities of the catalysts, which limits the applicability of
the catalyst to less reactive substrates. The precise mode of
deactivation of the Pd catalyst is not apparent at this time, but
this result is consistent with previous reports highlighting the
differences in metal bonding with triazole MIC and NHC
complexes and the resulting instability of these species under
catalytic conditions.49
E
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CONCLUSIONS
■
(13) Valente, C.; Çalimsiz, S.; Hoi, K. H.; Mallik, D.; Sayah, M.;
In conclusion, we have described the synthesis and character-
ization of several Pd 1,2,3-triazol-5-ylidene MIC complexes. All
complexes were characterized by X-ray crystallography and
displayed multiple weak noncovalent interactions within the
solid structure. We further illustrated the use of complexes 4
and 5 as catalysts in the Mizoroki−Heck reaction. Complexes 4
and 5 behave essentially identically and successfully couple aryl
iodides with methyl acrylate in 2 h at 80 °C using 2 mol % Pd.
Mercury poisoning tests were performed which suggest the
formation of Pd nanoparticles as the active catalyst. Higher
temperatures resulted in the decomposition of the catalysts and
the obvious formation of Pd black, which limits the utility of
these complexes for this transformation. Current work in our
laboratory is dedicated to increasing the catalytic activity and
thermal stability of these complexes, in order to successfully
couple more difficult substrates such as aryl chlorides.
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ASSOCIATED CONTENT
* Supporting Information
Text giving experimental procedures for the synthesis of the
Pd-MIC complexes and their use in catalysis tables and CIF
files giving crystal data for the complexes described herein. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: cathleen.crudden@chem.queensu.ca.
■
(29) Crowley, J. D.; Lee, A.-L.; Kilpin, K. J. Aust. J. Chem. 2011, 64,
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Notes
The authors declare no competing financial interest.
(31) Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand, G.
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ACKNOWLEDGMENTS
■
The Natural Sciences and Engineering Research Council of
Canada (NSERC) is thanked for support of this work through a
Discovery Research Grant, a discovery accelerator supplement
to C.M.C., and RTI grants to the Department of Chemistry.
NSERC and the Ontario government are acknowledged for
postgraduate fellowships to E.C.K., and Queen’s University is
thanked for graduate awards to E.C.K.
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