Postmodification Approach to Charge-Tagged 1,2,4-Triazole-Derived NHC Palladium(II) Complexes and Their Applications

Article abstract of DOI:10.1021/acs.organomet.7b00329

Charge-tagged bis(1,2,4-triazolin-5-ylidene)palladium(II) complexes have been successfully synthesized via a postmodification strategy. Reacting PdBr₂ with bromo-functionalized 1,2,4-triazolium salts A·HBr and B·HBr in the presence of silver oxide afforded the bis(carbene)palladium(II) complexes trans-[PdBr₂(A)₂] (1a) and trans-[PdBr₂(B)₂] (1b), which contain tethered bromoalkyl chains. Subsequent postcoordinative nucleophilic substitution converted the bromo into ammonium groups, producing the water-soluble complexes trans-[PdBr₂(C)₂]Br₂ (2a) and trans-[PdBr₂(D)₂]Br₂ (2b), while attempts to prepare ammonium-functionalized triazolium salts for direct metalation were futile. All four complexes were fully characterized by means of multinuclear NMR spectroscopy, ESI mass spectrometry, elemental analysis, and X-ray diffraction analysis. The presence of trans-anti and trans-syn rotameric complexes in solution was elucidated by ¹H and ¹³C NMR spectroscopy and theoretical calculations. Additionally, the two charge-tagged complexes, 2a,b, were found to be highly active precatalysts for the Suzuki-Miyaura and Mizoroki-Heck reactions in ⁱPrOH/H₂O and molten TBAB as an ionic liquid.

Full text of DOI:10.1021/acs.organomet.7b00329

Article
pubs.acs.org/Organometallics
Postmodification Approach to Charge-Tagged 1,2,4-Triazole-Derived
NHC Palladium(II) Complexes and Their Applications
Van Ha Nguyen,† Mansur B. Ibrahim, Waseem W. Mansour, Bassam M. El Ali,*
‡
‡
,‡
,
†
and Han Vinh Huynh*
†Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore
‡Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
*
S Supporting Information
ABSTRACT: Charge-tagged bis(1,2,4-triazolin-5-ylidene)palladium(II) com-
plexes have been successfully synthesized via a postmodification strategy.
Reacting PdBr with bromo-functionalized 1,2,4-triazolium salts A·HBr and B·
2
HBr in the presence of silver oxide afforded the bis(carbene)palladium(II)
complexes trans-[PdBr (A) ] (1a) and trans-[PdBr (B) ] (1b), which contain
2
2
2
2
tethered bromoalkyl chains. Subsequent postcoordinative nucleophilic
substitution converted the bromo into ammonium groups, producing the
water-soluble complexes trans-[PdBr (C) ]Br (2a) and trans-[PdBr (D) ]Br
2
2
2
2
2
2
(
2b), while attempts to prepare ammonium-functionalized triazolium salts for
direct metalation were futile. All four complexes were fully characterized by
means of multinuclear NMR spectroscopy, ESI mass spectrometry, elemental
analysis, and X-ray diffraction analysis. The presence of trans-anti and trans-syn rotameric complexes in solution was elucidated
1
13
by H and C NMR spectroscopy and theoretical calculations. Additionally, the two charge-tagged complexes, 2a,b, were found
i
to be highly active precatalysts for the Suzuki−Miyaura and Mizoroki-Heck reactions in PrOH/H O and molten TBAB as an
2
ionic liquid.
INTRODUCTION
less established and pales in comparison to their imidazolin-2-
ylidene cousins. Most reported 1,2,4-triazolin-5-ylidene com-
plexes feature carbenes with simple N substituents, such as alkyl
■
The past few decades have witnessed remarkable advances in
the development of N-heterocyclic carbenes (NHCs) for
organometallic catalysis and coordination chemistry. The
success of NHCs is largely attributed to the extraordinary
tunability of their stereoelectronic properties achievable by
7
1
or aryl groups, but functionalized triazole-derived NHC
8
complexes are very rare.
As a contribution to a better understanding of such species
and to explore their potential applications, we herein report on
the syntheses of the first ammonium-functionalized 1,2,4-
triazolin-5-ylidene complexes of palladium(II) via a post-
functionalization approach. The postcoordinative ligand mod-
ification, which involves functional group transformations on
suitable complexes, can provide efficient and versatile access to
1
d,2
wingtip and backbone modifications.
In parallel, the development of charge-tagged ligands as
valuable tools in organometallic chemistry has also received
3
increasing interest, where they have found applications in
4
5
immobilization and aqueous catalysis. Generally, carboxylate
and sulfonate or tetraalkylammonium groups can be used as
anionic or cationic charge tags, respectively. However, only the
latter are essentially pH independent and retain their charge
regardless of reaction conditions. In addition to imparting
increased solubility in polar solvents and phase-transfer
capabilities, a tetraalkylammonium group may also exert a
positive effect on the catalytic performance (Jeffery conditions)
and act as a mass spectrometric tag.
9
a wide range of functionalized-NHC complexes. This strategy
is especially advantageous when metalation conditions are
10
incompatible with the desired functional group.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Bromo-Function-
II
alized Pd Bis(carbene) Complexes. Triazolium bromides
A·HBr and B·HBr, were prepared in good yields by direct N-
alkylation of two 4-aryl-1,2,4-triazoles with the bulky and
popular 2,4,6-trimethylphenyl (Mes) and 2,6-diisopropylphenyl
Among the classical NHCs, imidazolin-2-ylidenes and 1,2,4-
triazolin-5-ylidenes were the first to be isolated in stable free
form. These two types are also most comparable in terms of
reactivity and spectroscopic and structural features, since they
are solely differentiated by an isolobal substitution of a CH with
an N group. Moreover, Enders’ 1,2,4-triazole-derived NHC was
(
Dipp) wingtips in neat 1,3-dibromopropane (Scheme 1).
Formation of the triazolium salts was evidenced by the
presence of downfield signals at 12.05 ppm (A·HBr) and
6
the first commercially available free NHC. Despite this fact
and their similarities, it is surprising to note that the
organometallic chemistry of 1,2,4-triazolin-5-ylidenes is far
Received: April 28, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00329
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13
Scheme 1. Synthesis of Bromo-Functionalized Triazolium
two isomers. The C
carbene
signals are marginally separated and
Salts A·HBr and B·HBr
appear at 172.8/172.7 ppm (1a) and 173.6/173.2 ppm (1b),
suggestive of a trans orientation of the two triazolin-5-ylidenes.
A more upfield shift is expected when the carbenes are trans to
12
the weaker donating bromido ligands. Considering the
unsymmetrical nature of the two NHC ligands, the two sets
of signals can be assigned to trans-anti and trans-syn rotamers.
1
Notably, the H NMR resonances of the bromopropyl side
chains of the two isomers are well separated with one set
distinctively shifted more to high field. This set of signals is
assignable to the trans-anti rotamer, in which the bromopropyl
tethers are exposed to the magnetic anisotropic shielding from
the aromatic rings (Figure 1). In contrast, the bromopropyl
1
1
2.16 ppm (B·HBr) in their H NMR spectra, which are
assigned to the acidic C5 protons of the heterocyclic rings.
Furthermore, base peaks for the molecular [M − Br] cations
+
9b,13
chains in the syn isomer are free from such shielding.
were observed in their ESI mass spectra at m/z 308 (A·HBr)
and m/z 350 (B·HBr), respectively, confirming the identity of
the salts. The alkylations also produced propylene-bridged
ditriazolium salts as minor side products. Nevertheless, this side
reaction is minimized when 10 equiv of 1,3-dibromopropane is
used. Excess 1,3-dibromopropane was recovered by distillation
at 80 °C and 50 mbar. It is noted that, even with an excess of
1
,3-dibromopropane, only one of the two cyclic imine nitrogen
atoms was alkylated and no 1,2-bis(3-bromopropyl)-1,2,4-
triazolium dibromides were observed. In fact, to alkylate the
third ring nitrogen, much stronger electrophiles, e.g. trialkyl
11
oxonium salts, are required.
Initial attempts to install an ammonium function on the
carbene precursors as a cationic tag via direct nucleophilic
substitution reactions between A·HBr or B·HBr and triethyl-
amine were futile and only led to decomposition. This is
probably due to the interference of the rather acidic C5 proton
of the triazolium salts with the amine base. To circumvent this
problem, a postmodification strategy was pursued as an elegant
1
Figure 1. H NMR spectrum of 1a in CD
CN from ∼2.3−5.1 ppm.
3
9a,b
alternative instead.
Accordingly, bromopropyl-substituted
complexes trans-[PdBr (A) ] (1a) and trans-[PdBr (B) ] (1b)
2
2
2
2
The differences in resonances induced by the aromatic rings
as “built-in sensors” allow for an unambiguous spectral
assignment and easy determination of the relative anti:syn
were obtained in good yields by reacting PdBr with the
2
respective salts A·HBr and B·HBr and Ag O at ambient
2
temperature (AT) in acetonitrile (Scheme 2) for subsequent
ratio in solution. For example, in CDCl solutions, the ratios
3
amount to 0.9:1 and 0.8:1 for 1a,b, respectively. The ratios
were found to vary from one solvent to another. For example,
Scheme 2. Synthesis of Complexes 1a,b
the anti:syn ratio for 1a changes from 0.9:1 in CDCl to 0.6:1
3
in CD CN and 0.5:1 in DMSO-d . A similar trend is also
3
6
observed for 1b, for which anti:syn ratios of 0.6:1 and 0.4:1
were found in CD CN and DMSO-d , respectively. The
3
6
predominance of the trans-syn isomers for both 1a and 1b
suggests that they are more stable in CDCl than their trans-
3
anti counterparts, and more polar solvents (i.e., CD CN,
3
DMSO) stabilize them even further.
To rationalize the above observation, theoretical calculations
were carried out to assess the relative energy of trans-anti and
trans-syn rotamers for 1a as a representative. The structures of
both anti and syn rotamers were first optimized in the gas
phase, and then single-point calculations were performed with
solvation described by an implicit solvent effect model (see the
Supporting Information for more calculation details). Opti-
mized structures of the two rotamers are presented in Figure S1
in the Supporting Information. Structural parameters of the
optimized geometries of the anti rotamer agree well with the
molecular structure determined by single-crystal X-ray
diffraction studies (vide infra).
postcoordinative modification. The two complexes were
purified by column chromatography (SiO ) using dichloro-
2
methane as eluent and isolated as pale yellow solids. They are
soluble in common polar organic solvents, such as dichloro-
methane, chloroform, and acetonitrile, but insoluble in hexane
or diethyl ether. In their ESI mass spectra, base peaks at m/z
+
8
03 (1a) and m/z 887 (1b) were observed for the [M − Br]
fragments, corroborating the formation of the desired
complexes. Furthermore, the absence of downfield signals in
1
their H NMR spectra characteristic for the acidic C5 protons
in their salt precursors supports their successful palladation.
The calculation results show that trans-anti-1a is 4.5 kJ/mol
lower in energy in comparison to trans-syn-1a in the gas phase,
likely due to steric repulsion of the two mesityl substituents.
1
Moreover, two sets of signals were observed in the H and
C NMR spectra of both 1a and 1b, indicating the presence of
1
3
B
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However, the symmetrical trans-anti isomer has an overall zero
dipole moment, whereas the trans-syn isomer possesses a
dipole moment of 8.2 D. The larger dipole moment leads to a
significant stabilization of the trans-syn isomer in polar solvents,
where it can become the major species. In fact, the syn form is
lower in energy than the anti counterpart by 1.8, 3.7, and 3.8
kJ/mol in chloroform, acetonitrile, and dimethyl sulfoxide,
respectively. These energy differences translate into the anti:syn
ratios of 0.48:1, 0.22:1, and 0.21:1 for their respective solutions
in chloroform, acetonitrile, and dimethyl sulfoxide. Although
the calculations underestimate the anti:syn ratio, they do
predict and rationalize the experimentally observed stabilization
of the syn isomer in polar solvents.
respectively. All of these peaks exhibit isotopic distribution
patterns identical with the theoretical ones, confirming the
identities of the two complexes. A representative comparison
2+
for the [PdBr (D) ] cation of 2b is depicted in Figure 2.
2
2
Synthesis and Characterization of Charge-Tagged
Complexes via Postfunctionalization Approach. Since
suitably ammonium functionalized carbene precursors were not
accessible, a postfunctionalization approach had to be explored
for the preparation of the charge-tagged NHC complexes trans-
Figure 2. Experimental (a) and theoretical (b) isotopic patterns of the
[
PdBr (C) ]Br (2a; C = 1-(3-triethylammonium)propyl-4-
2 2 2
2+
[
PdBr (D) ] cation of 2b.
2 2
mesityl-1,2,4-triazolin-5-ylidene) and trans-[PdBr (D) ]Br
2
2
2
(
2b; D = 1-(3-triethylammonium)propyl-4-(2,6-diisopropyl)-
phenyl-1,2,4-triazolin-5-ylidene). Indeed, these were success-
fully prepared with ease by nucleophilic substitution reactions
between the bromo-functionalized complexes 1a,b and triethyl-
amine (Scheme 3). The reactions were complete after stirring
1
13
Similar to the case for 1a,b, the H and C NMR spectra of
a,b also show two sets of signals assignable to the trans-anti
2
and trans-syn rotamers of the palladium(II) bis(carbene)
complexes. The Ccarbene signals remain largely unaffected by
13
the remote postfunctionalization. Again, unambiguous signal
assignment was made convenient by the anisotropic effect
exerted by the aromatic ring as “built-in sensors”, which causes
Scheme 3. Synthesis of Complexes 2a,b
1
the propylene H NMR signals of the trans-anti isomers to shift
distinctly upfield in comparison to those of the trans-syn forms.
The relative ratios of anti:syn rotamers were determined to be
0
.3:1 (2a) and 0.5:1 (2b) in acetonitrile, suggesting a better
stabilization of the syn rotamers in comparison to their anti
counterparts, which was also observed for 1a,b and attributed
to their larger dipole moments in line with theoretical
calculations (vide supra). The same preference for the syn
forms for 2a,b is somewhat surprising, since one would expect
intramolecular charge repulsion between the ammonium
9b
at 70 °C for 15 h, affording white solids of 2a,b in very good
yields. Both complexes are insoluble in nonpolar solvents, such
as hexane and diethyl ether, but they display good solubilities in
dichloromethane, methanol, acetonitrile, and dimethyl sulf-
oxide. Notably, both complexes also dissolve in water with
solubilities of 1.3 mg/mL (1.2 mmol/L, 2a) and 1.7 mg/mL
moieties. On the other hand, π−π stacking interactions
between the aromatic substituents could contribute to a
stabilization of the trans-syn rotamers. The only compound
that crystallized in the trans-syn conformation was complex 2a
(vide infra). In its solid-state structure, the shortest interplanar
C−C distance between two aromatic mesityl rings is 3.51 Å.
This distance is slightly smaller than the sum of the van der
(
1.5 mmol/L, 2b).
Generally, mass spectroscopic analysis of neutral bis-
carbene) complexes of the type [PdX (NHC) ] is not
15
Waals radius (1.77 Å) of two carbon atoms and is well within
(
the optimal distance for a significant π−π stacking interaction
2
2
16
straightforward and often only shows base peaks for azolium
(3.3−3.8 Å). This could indeed point to the contributing
factor of π−π stacking interactions.
cations from NHC decomposition and minor patterns for
+
complex fragments, such as the [PdX(NHC) ] cation (vide
Additionally, in order to probe the syn preference more
deeply, we set out to examine the molecular dipole moments of
the complexes. However, a routine geometry optimization for
2a,b in the gas phase would be computationally inefficient, as
both molecules possess multiple flexible alkyl chains and mobile
bromide anions. Alternatively, their dipole moments were
assessed using solid-state molecular structures determined by
X-ray diffraction (vide infra). Using these for calculations,
dipole moments of 34 and 0 D were found for trans-syn-2a and
trans-anti-2b, respectively. It is noted that the predominant
species in acetonitrile are syn isomers, which possess much
larger dipole moments. In addition, the syn preference in
solution could also be attributed to favorable electrostatic
attractions. The syn arrangement of the two tethered
2
+
supra). Desirable molecular cations such as [M + H] are rarely
9
b,14
detected,
and thus evidence for the full square-planar
coordination sphere around a metal center cannot be directly
obtained. Complexes 2a,b are decorated with ammonium
functions, and harsh ionization processes in the gas phase are
not required, which makes them much more ESI-MS
responsive. This should allow for an easy detection of the
intact molecular complex cation. Indeed, the ESI mass spectra
of 2a,b show base peaks for their molecular cations
+
+
[
PdBr (C) ] and [PdBr (D) ] at m/z 1005 (2a) and m/z
3 2 3 2
1
089 (2b), respectively. In addition, peaks for the intact
2
+
2+
dications [PdBr (C) ] and [PdBr (D) ] at m/z 462 (2a)
2
2
2
2
and m/z 504 (2b) are also observed at 50% and 40%,
C
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Article
ammonium cations can be further supported by electrostatic
attraction to a small bromide anion in the interstitial space. If
this electrostatic interaction is one of the contributing factors,
replacement of bromide with a larger and more charge diffused
anion, such as hexafluorophosphate, would reduce the
favoritism for the syn arrangement. To test this notion, an
anion exchange was carried out, which gave trans-[PdBr (C) ]-
Table 1. Selected Bond Lengths (Å) and Bond Angles (deg)
in Structures of 1a,b, 2a·CH Cl , and 2b·2CHCl
2
2
3
param
Pd1−C1
Pd1−Br1
Pd1−C3
Pd1−Br2
C1−Pd1−Br1
C1−Pd1−Br1A
C1−Pd1−C1A
Br1−Pd1−Br1A
C1−Pd1−Br2
C1−Pd1−C3
Br1−Pd1−Br2
1a
1b
2a·CH Cl2 2b·2CHCl3
2
2.027(6)
2.021(6)
2.04(1)
2.448(2)
2.00(1)
2.446(2)
89.3(3)
2.016(3)
2.4452(5)
2.4235(7)
2.4424(3)
2
2
1
(
PF ) (2a′) and [PdBr (D) ](PF ) (2b′). The H NMR
6
2
2
2
6 2
90.9(1)
89.1(1)
180.00
91.7(2)
88.3(2)
180.0
90.43(9)
89.57(9)
180.0(1)
180.0
spectra of 2a′,b′ also show two sets of signals, indicating the
presence of two rotamers. The protons on alkyl chains of the
two isomers are well separated, and therefore, the anti:syn
ratios can be determined with ease. Indeed, their respective
anti:syn ratios in acetonitrile solutions have increased from
180.00(3)
180.0
90.3(3)
177.3(4)
172.25(3)
0
.3:1 (2a) to 0.7:1 (2a′) and from 0.5:1 (2b) to 1.2:1 (2b′),
respectively, supporting the aforementioned notion. The
3
1
1
19
1
P{ H} and F{ H} spectra of 2a′,b′, on the other hand,
show only one set of signal for hexafluorophosphate counter-
anions, suggesting the absence of any significant interactions
between the counteranions with the complex cations in
solution.
essentially square planar coordination geometry. All of the
complexes adopt the trans configuration, which is in line with
conclusions drawn from solution NMR data. The molecular
structures of 1a,b and 2b show a trans-anti arrangement for the
two unsymmetrical carbene ligands, whereas a trans-syn
orientation is displayed by 2a. Notably, a bromide anion is
located in the space between the two ammonium groups of
trans-syn-2a (Figure 3), which is in line with the notion given
Molecular Structures. Single crystals of complexes 1a,b,
2
a·CH Cl , and 2b·2CHCl were grown by slow evaporation of
2 2 3
concentrated solutions in CHCl /hexane (1a,b), chloroform
3
(2a), or dichloromethane (2b). The molecular structures of the
four complexes are shown in Figure 3, and selected bond
lengths and bond angles are given in Table 1.
In the crystals of 1a and 2a,b, one independent molecule was
found, whereas two independent molecules were found in
crystals of 1b. In all of the structures, the palladium(II) center is
coordinated by two carbene and two bromido ligands in an
above. In addition, the Pd−C
bond distances are identical
carbene
within 3σ, ranging from 2.00(1) to 2.027(6) Å, which are close
to the range reported for Pd−C distances in trans-
carbene
bis(triazolin-5-ylidene)palladium(II) complexes (2.017(5)−
17
2.034(5) Å). In complexes 1a,b and 2b, the two NHC
rings are coplanar but they are twisted from the coordination
plane, with dihedral angles of 67° (1a), 87° and 79° (1b), and
6
8° (2b). On the other hand, the NHC rings of trans-syn-2a are
twisted by 34° from each other due to steric repulsion of the
ammonium moieties. The two NHC ring planes are nearly
perpendicular to the coordination plane with dihedral angles of
7
2 and 74°.
Catalytic Applications. N-heterocyclic carbene complexes
of palladium(II) have been shown to be efficient catalysts for a
wide range of organic reactions, especially C−C and C−
1
e,f,2
c
heteroatom couplings.
Among the lesser explored
palladium(II) triazolin-5-ylidene complexes, several demon-
7
b
strated good catalytic activities in Mizoroki−Heck coupling,
8a
7c
Sonogashira /cyclic hydroalkoxylation reactions, and hydro-
7d
amination of olefins. Notably, these reactions were performed
in water-free media. Thus, the water-soluble complexes 2a,b
were tested for their catalytic activities in the Suzuki−Miyaura
and Mizoroki−Heck coupling reactions using water as a
cosolvent and in molten TBAB as an ionic liquid.
Suzuki−Miyaura Coupling Reactions. In order to deter-
mine the optimal reaction conditions, several experiments were
performed using 4-bromoacetophenone or 4-bromoanisole and
phenylboronic acid, as model substrates, at a low catalyst
loading of 0.03 mol %. The optimization results are
summarized in Table 2. Initially, no conversion was observed
when the reaction was conducted in DMF/H O (1/1 v/v) at
2
room temperature using catalyst 2a (entry 1). However, a yield
of 60% was achieved when the reaction temperature was
increased to 80 °C (entry 2). A sharp decrease in the yield was
noted when either DMF or water was used as the sole solvent
(
entries 3 and 4). Notably, excellent yields were observed when
i
the reactions were carried out in an PrOH/H O (1/1 v/v)
Figure 3. Molecular structures of 1a,b, 2a·CH Cl , and 2b·2CHCl .
Thermal ellipsoids are plotted at 50% probability. Hydrogen atoms
and solvent molecules are omitted for clarity.
2
2
2
3
mixture at 80 °C (entry 7). A decrease in reaction temperature
from 80 to 60 °C or room temperature was accompanied by a
D
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Article
a
Table 2. Optimization of Suzuki−Miyaura Coupling
Table 3. Suzuki−Miyaura Coupling of Aryl Halides
a
Reaction Conditions
R1
X
R2
yield (%)
b
TON
b
entry
entry
R
solvents
T (°C)
catalyst yield (%)
1
2
3
4
5
6
7
8
9
COCH3
COCH3
COCH3
NO2
H
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
H
96
96
95
92
95
94
92
96
94
95
95
93
95
96
72
74
69
3200
3200
3170
3070
3170
3130
3070
3200
3130
3170
3170
3100
95
1
2
3
4
5
6
7
8
9
COCH3
COCH3
COCH3
COCH3
COCH3
COCH3
COCH3
COCH3
OCH3
DMF/H O
room temp
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
traces
60
2
CH3
OH
H
DMF/H O
80
80
80
r.t.
60
80
80
80
80
2
DMF
traces
traces
25
H O
2
i
H
PrOH/H O
2
H
OH
i
PrOH/H O
51
2
H
CH3
i
PrOH/H O
96
2
OCH3
NH2
CHO
CN
H
iPrOH
traces
96
H
i
PrOH/H O
2
1
1
0
1
H
i
1
0
OCH3
PrOH/H O
95
2
H
a
Reaction conditions: 8.0 mmol of aryl bromide, 10 mmol (1.25
equiv) of arylboronic acid, 0.03 mol % of precatalyst 2a/2b, 12 mmol
1.5 equiv) of K CO , solvent (24 mL), H O (24 mL), 80 °C, 2 h.
12
13
14
CH3
CHO
COCH3
H
H
c
H
(
c
2
3
2
H
96
b
Isolated yield for an average of two runs.
d
d
d
15
16
17
H
72
H
OCH3
OCH3
74
OCH3
69
significant drop in yield (entries 5 and 6). Moreover, only trace
amounts of the product were obtained when the reaction was
conducted in only isopropyl alcohol (entry 8). Therefore,
subsequent reactions were carried out in PrOH/H O at 80 °C.
Under these conditions, phenylboronic acid reacted smoothly
with both activated (entry 7) and deactivated (entry 9) aryl
bromides to give the products in excellent yields.
a
Reaction conditions unless specified otherwise: 8.0 mmol of aryl
halide, 10 mmol (1.25 equiv) of arylboronic acid, 0.03 mol % of 2a, 12
i
mmol (1.5 equiv) of K CO , PrOH (24 mL), H O (24 mL), 80 °C, 2
2
3
2
i
b
c
2
h. Isolated yield for an average of two runs. Catalyst loading 1.0 mol
d
%
of 2a, 120 °C, 24 h. Catalyst loading 1.0 mol % of 2a, 140 °C, 24 h.
Complex 2b was also tested as catalyst against 2a for the
coupling reaction between less reactive 4-bromoanisole and
phenylboronic acid under the same reaction conditions. The
reaction went smoothly, giving the desired product in equally
high yield (95%; Table 2, entry 12) in comparison to 2a.
Since 2b was not distinctively superior in performance,
complex 2a was chosen for a substrate scope study. Various
arylboronic acids were reacted with a diverse array of aryl
bromides and aryl chlorides using 2a as catalyst under the
optimized conditions (Table 3). Activated as well as deactivated
aryl bromides reacted smoothly, affording the respective
substituted biphenyl in excellent yields (92−96%) (entries 1−
2a,b were also tested in the Mizoroki−Heck reaction, which
usually requires harsher conditions. The coupling of 4-
bromoacetophenone and styrene was chosen as the standard
reaction for optimization (Table 4). The reaction was initially
tested at 100 °C in the presence of catalyst 2a using biphasic
toluene/H O (entry 1) or DMF/H O (entry 2) mixtures.
2
2
These two preliminary reactions gave no coupling product after
20 h.
However, a sharp increase in the product yield to 71% was
observed when reaction temperature was increased to 120 °C
i
(entry 3). Again, the best conversion was achieved in PrOH/
H O (1/1 v/v) where a high isolated yield of 92% was obtained
2
i
12). The coupling reactions can tolerate various functional
(entry 5). Using these conditions ( PrOH/H O, 120 °C, 20 h),
2
groups on either the aryl halide or arylboronic acid. The
coupling of activated aryl chlorides was also successful, forming
products in excellent yields (95−96%), although higher catalyst
loading and more forcing conditions (120 °C, 24 h) were
required (entries 13 and 14). Notably, decent yields were still
observed for coupling reactions between phenylboronic acid
and unactivated (chlorobenzene) or deactivated aryl chlorides
the coupling with the less reactive 4-bromoanisole was tested,
and a decent yield (80%) was obtained when K CO was used
2
3
as a base (entry 6). Our previous study of the Mizoroki−Heck
II
coupling reaction catalyzed by Pd -bis(oxazoline) complexes
has demonstrated that KOH could be a more efficient base in
19
comparison to K CO . In this regard, KOH was then tested
2
3
under otherwise identical conditions, and the reaction gave an
excellent yield of 93% after 12 h at 120 °C (entry 8). Notably,
the reaction did not initiate at 100 °C (entry 7). A more
detailed examination shows that the reaction was essentially
complete after 4 h (entries 9 and 10) with KOH. On the other
hand, the coupling using K CO only gave a 32% yield after 4 h
(
4-chloroanisole) (entries 14−17). The high catalytic activity of
the charge-tagged complex 2a was found to be comparable to
those of reported sulfonate-tethered imidazolin-2-ylidene
palladium(II) complexes under very similar reaction con-
1
8
ditions. However, it is interesting to note that these required
the addition of excess tetrabutylammonium bromide (1.5
equiv), while complexes 2a,b contain built-in and only “catalytic
amounts” of tetraalkylammonium functions. In addition, their
preparation by the postmodification approach avoided harsh
experimental conditions, such as prolonged heating at high
2
3
i
(entry 11). The set of reaction conditions ( PrOH/H O, KOH,
2
120 °C, 4 h) were then considered as optimal conditions, under
which a good yield was also achieved with catalyst precursor 2b
(entry 13).
Catalyst 2a (1 mol %) was further tested in a substrate scope
study using the optimal conditions (Table 5). Nonactivated,
activated, and deactivated aryl bromides reacted smoothly with
styrene, affording the cross-coupling products in good to
excellent yields (89−93%). Various functional groups were
18
temperatures in DMSO.
Mizoroki−Heck Coupling Reactions between Aryl Bro-
mides and Substituted Styrenes. In addition to Suzuki−
Miyaura coupling reactions, catalytic activities of the complexes
E
DOI: 10.1021/acs.organomet.7b00329
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 4. Optimization of Reaction Conditions for the
However, the coupling reaction was successful when it was
carried out at 140 °C in tetrabutylammonium bromide (TBAB)
as an ionic liquid (IL), using cesium carbonate as a base, and
run for 20 h. It is anticipated that the ammonium tethers of the
complex allow for efficient anchoring onto the IL and
facilitating formation of ion pairs. These modified conditions
gave the desired product in 72% yield (entry 4). Under similar
reaction conditions, a minimally lower yield was obtained when
Mizoroki−Heck Coupling Reactions of Aryl Bromides with
a
Styrene
yield
R3
base
solvent
T (°C) t (h) cat.
(%)
b
entry
1
2b was used as a catalyst (entry 5). Additionally, 2a shows good
COCH3
K CO
2
toluene/
100
100
120
100
120
120
100
120
120
120
120
120
20
20
20
20
20
20
20
20
12
4
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
3
3
3
3
3
3
H O
catalytic performance for the coupling of several activated and
deactivated aryl chlorides (entries 6−10).
2
2
3
4
5
6
7
8
9
COCH3
COCH3
COCH3
COCH3
OCH3
K CO
2
DMF/
H O
2
The catalytic performance of 2a in Mizoroki−Heck coupling
reactions between aryl bromides and styrene is comparable to
that reported for nonfunctionalized Pd-bis(NHC) complexes
containing imidazolin-2-ylidenes and mesoionic 1,2,3-triazolin-
K CO
2
DMF/
71
H O
2
i
K CO
2
PrOH/
H O
2
7
b,9b,20
i
5-ylidenes in pure organic solvents.
However, 2a
K CO
2
PrOH/
H O
2
92
outperforms these in the coupling of more challenging aryl
i
20b,c
K CO
2
PrOH/
H O
2
80
chlorides.
i
OCH3
KOH
KOH
KOH
KOH
PrOH/
H O
2
traces
93
CONCLUSION
■
i
OCH3
PrOH/
H O
2
A series of four functionalized bis(1,2,4-triazolin-5-ylidene)-
palladium(II) complexes (1a,b and 2a,b) have been successfully
synthesized and fully characterized by various spectroscopic and
spectrometric means, including single-crystal X-ray diffraction.
Complexes 1a,b, containing tethered bromoalkyl chains, were
conveniently transformed into ammonium-functionalized com-
plexes 2a,b, respectively, via a postmodification approach.
Notably, the respective ammonium-functionalized triazolium
salts are still elusive, making a direct palladation impossible. All
four complexes are rare examples of functionalized 1,2,4-
triazole-derived NHC complexes, and their rotameric ratios in
i
OCH3
PrOH/
H O
2
92
i
1
1
1
0
1
3
OCH3
OCH3
OCH3
PrOH/
H O
2
90
i
K CO
2
PrOH/
H O
2
4
32
3
i
KOH
PrOH/
4
87
H O
2
a
Reaction conditions: 1.0 mmol of aryl bromide, 1.5 mmol (1.5 equiv)
of styrene, 1.0 mol % of catalyst 2a/2b, 2.0 mmol (2 equiv) of base,
b
solvent/H O (3.0 mL/3.0 mL). Isolated yield for an average of two
1
2
solution have been obtained by H NMR spectroscopy. The
runs.
preference for the syn rotamers in all cases has been
rationalized by DFT calculations and can be traced back to
their larger dipole moments. Moreover, complexes 2a,b contain
cationic tethers, which can serve as (i) ESI MS-responsive tags
for the mild mass-spectrometric detection of complete complex
spheres, (ii) built-in phase-transfer catalysts, and (iii) built-in
Jeffery-type cocatalysts for palladium catalysis. Thus, their
catalytic activities in the Suzuki−Miyaura and Mizoroki−Heck
couplings of aryl bromides and chlorides have been examined as
well. Overall, excellent yields were obtained for all the aryl
bromide substrates, while average to good yields were observed
for aryl chlorides. These results show that the postmodification
approach allows the preparation of useful complexes that are
otherwise not obtainable using conventional methods. Current
research in our laboratory is concerned with the extension of
this approach to complexes with more elaborate multidentate
ligands.
Table 5. Mizoroki−Heck Coupling Reactions of Aryl
a
Bromides with Substituted Styrenes
b
entry
R4
R5
yield (%)
TON
1
2
3
4
5
6
7
8
9
H
H
H
90
93
93
90
92
90
92
89
92
90
93
93
90
92
90
92
89
92
COCH3
COCH3
OCH3
COCH3
Cl
CN
H
H
OCH3
H
OCH3
H
H
Cl
C(CH3)3
a
Reaction conditions: 1.0 mmol of aryl bromide, 1.5 mmol (1.5 equiv)
of alkene, 1.0 mol % of catalyst 2a, 2.0 mmol (2 equiv) of KOH,
i
b
EXPERIMENTAL SECTION
PrOH/H O (3.0 mL/3.0 mL), 120 °C, 4 h. Isolated yield for an
■
2
average of two runs.
General Considerations. All syntheses were carried out without
precautions to exclude air and moisture unless otherwise stated.
Solvents used for syntheses and spectroscopic measurements were
2
1
used as received. 4-Mesityl-1,2,4-triazole and 4-(2,6-diisopropyl-
tolerated, and the effect of varying the substituents on both the
aryl bromides and the styrene derivatives was found to be
insignificant.
Mizoroki−Heck Coupling Reactions between Aryl Chlor-
ides and Substituted Styrenes. The catalytic activity of 2a was
also tested for less reactive aryl chloride substrates (Table 6).
Unfortunately, no coupling product was obtained under the
optimized reaction conditions for aryl bromides (entries 1−3).
2
1
phenyl)-1,2,4-triazole were synthesized following reported proce-
1
13
1
dures. H and C{ H} NMR spectra were recorded on a Bruker ACF
00, Advance III, or AMX 500 spectrometer, and the chemical shifts
δ) were internally referenced to the residual solvent signals relative to
3
(
tetramethylsilane. Electrospray ionization mass spectra (ESI-MS) were
obtained using a Finnigan LCQ spectrometer. Elemental analyses were
carried out at the Elemental Analysis Laboratory in the Department of
Chemistry, National University of Singapore.
F
DOI: 10.1021/acs.organomet.7b00329
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 6. Mizoroki−Heck Coupling Reactions of Aryl Chlorides and Substituted Styrene
R6
R7
base
solvent
T (°C)
cat.
yield (%)
b
TON
entry
i
i
i
1
2
3
CHO
COCH3
H
H
H
H
H
H
H
H
H
KOH
KOH
KOH
PrOH/H O
120
120
120
140
140
140
140
140
140
140
2a
2a
2a
2a
2b
2a
2a
2a
2a
2a
0
0
0
0
2
PrOH/H O
2
PrOH/H O
0
0
2
c
4
H
Cs CO
2
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
72
70
90
89
70
74
72
72
70
90
89
70
74
72
3
3
3
3
3
3
3
c
5
H
Cs CO
2
c
6
COCH3
CHO
OCH3
H
Cs CO
2
c
7
c
Cs CO
2
8
Cs CO
2
c
9
OCH3
Cl
Cs CO
2
c
10
H
Cs CO
2
a
Reaction conditions unless specified otherwise: 1.0 mmol of aryl chloride, 1.5 mmol (1.5 equiv) of styrene, 1.0 mol % of catalyst, 2.0 mmol (2
b
c
equiv) of base, solvent/H O (3.0 mL/3.0 mL), 20 h. Isolated yield for an average of two runs. TBAB (2.0 g).
2
4
-Mesityl-1-(3-bromopropyl)-1,2,4-triazolium Bromide (A·
HBr). A mixture of 4-mesityl-1,2,4-triazole (374 mg, 2 mmol) and
,3-dibromopropane (2.1 mL, 20 mmol) was heated at 70 °C for 4 h.
(NCH N), 140.2, 136.5, 132.5, 130.1 (Ar-C), 52.4 (NCH ), 33.9
2 2
(CH Br), 31.5 (CH CH CH ), 21.3 (o-CH ), 19.5 (p-CH ). Anal.
2
2
2
2
3
3
1
Calcd for C H Br N Pd: C, 38.10; H, 4.11; N, 9.52. Found: C,
2
8
36
4
6
+
The reaction mixture was cooled to ambient temperature, and diethyl
ether (20 mL) was added to precipitate the triazolium salt. The
precipitate was then filtered and washed with diethyl ether (3 × 5 mL)
to give the product as an off-white powder (740 mg, 1.9 mmol, 95%).
H NMR (300 MHz, CDCl ): δ 12.05 (s, 1 H, NCHN), 8.37 (s, 1 H,
3
8.15; H, 4.15; N, 9.61. MS (ESI): calcd for [M − Br] ,
23
C H Br N Pd, m/z 803; found, m/z 803.
28
36
3
6
trans-[PdBr (B) ] (1b). Complex 1b was synthesized in analogy to
2
2
1
a, from B·HBr (431 mg, 1 mmol), Ag O (128 mg, 0.55 mmol), and
2
1
3
PdBr (133 mg, 0.5 mmol). The product was obtained as a yellow
2
NCHN), 7.05 (s, 2 H, Ar-H), 5.12 (t, 2 H, NCH ), 3.61 (t, 2 H,
2
solid containing a mixture of rotamers trans-anti-1b and trans-syn-1b
CH Br), 2.72 (m, 2 H), 2.35 (s, 3 H, CH ), 2.13 (s, 6 H, CH ).
1
2
3
3
(425 mg, 0.44 mmol, 88%). Data for trans-anti-1b are as follows. H
1
3
1
C{ H} NMR (75.47 MHz, CDCl ): δ 145.2 (NCHN), 143.6
NCHN), 142.6, 134.3, 130.5, 127.4 (Ar-C), 52.3, 31.7, 29.1, 21.3,
3
3
NMR (400 MHz, CD CN): δ 8.29 (s, 1 H, NCHN), 7.57 (t, J(H−H)
3
(
=
7.8 Hz, 1 H, p-Ar-H), 7.40 (d, 2 H, m-Ar-H), 4.47 (t, 2 H,
1
8.2. Anal. Calcd for C H Br N : C, 43.21; H, 4.92; N, 10.80.
14 19 2 3
22
NCH CH ), 3.15 (t, 2 H, CH Br), 2.73−2.84 (m, 2 H, CH(CH ) ),
2
2
2
3 2
+
Found: C, 43.17; H, 5.47; N, 11.13. MS (ESI): calcd for [M − Br] ,
2
.33 (m, 2 H, CH CH CH ), 1.30 (d, 6 H, CH(CH ) ), 0.95 (d, 6 H,
2 2 2 3 2
C H BrN , m/z 308; found, m/z 308.
13
1
1
4
4
19
3
CH(CH ) ). C{ H} NMR (75.47 MHz, CD CN): δ 173.6 (Ccarbene),
3
2
3
-(2,6-Diisopropylphenyl)-1-(3-bromopropyl)-1,2,4-triazo-
1
2
47.3, 144.3, 131.0, 130.6, 128.9 (Ar-C), 51.9 (NCH ), 32.3, 30.2,
2
lium Bromide (B·HBr). The triazolium salt B·HBr was prepared in a
manner similar to that for A·HBr from 4-(2,6-diisopropylphenyl)-
1
8.7, 26.6, 22.9. Data for trans-syn-1b are as follows. H NMR (400
3
MHz, CD CN): δ 8.19 (s, 1 H, NCHN), 7.44 (t, J(H−H) = 7.8 Hz, 1
3
1
,2,4-triazole (458 mg, 2 mmol) and 1,3-dibromopropane (2.1 mL, 20
mmol). The product was obtained as a pale yellow solid (707 mg, 1.64
mmol, 82%). H NMR (500 MHz, CDCl ): δ 12.16 (s, 1 H, NCHN),
3
H, p-Ar-H), 7.14 (d, J(H−H) = 7.8 Hz, 2 H, m-Ar-H), 4.98 (t, 2 H,
NCH ), 3.67 (t, 2 H, CH Br), 2.72−2.84 (2 H, CH(CH ) ), 2.53 (m,
1
2
2
3 2
3
3
2 H, CH
2
CH
2
CH
2
), 0.87 (d, 6 H, CH(CH ) ), 0.86 (d, 6 H,
3 2
8
.43 (s, 1 H, NCHN), 7.58 (t, J(H−H) = 8.0 Hz, 1 H, Ar-H), 7.33 (d,
13
1
3
CH(CH
46.6, 144.5, 131.5, 131.0, 124.2, 52.3, 32.6, 30.6, 28.3, 26.3, 22.8. Anal.
Calcd for C34 Pd: C, 42.24; H, 5.00; N, 8.69. Found: C,
) ). C{ H} NMR (75.47 MHz, CD CN): δ 173.2 (Ccarbene),
3 2 3
J(H−H) = 8.0 Hz, 2 H, Ar-H), 5.21 (t, 2 H, NCH ), 3.64 (t, 2 H,
2
1
3
CH Br), 2.73 (m, 2 H, CH CH CH ), 2.27 (m, J(H−H) = 6.8 Hz, 2
2
2
2
2
3
H48Br N
4 6
H, CH(CH ) ), 1.24 (d, J(H−H) = 6.8 Hz, 6 H, CH ), 1.14 (d,
3
2
3
+
3
13
1
42.23; H, 4.82; N, 8.95. MS (ESI): calcd for [M − Br] ,
C H Br N Pd, m/z 887; found, m/z 887.
J(H−H) = 6.8 Hz, 6 H, CH ). C{ H} NMR (125.76 MHz, CDCl ):
23
3
3
34
48
3
6
δ 145.4 (NCHN), 145.1 (NCHN), 144.2, 132.9, 126.7, 125.2 (Ar-C),
trans-[PdBr (C) ]Br (2a). A Schlenk tube was charged with 1a
2
2
2
5
5
2.3, 31.6, 29.1, 24.3, 24.2. Anal. Calcd for C H Br N : C, 47.35; H,
17 25 2 3
22
(310 mg, 0.35 mmol), triethylamine (480 μL, 3.5 mmol), and CH CN
3
.84; N, 9.74. Found: C, 49.39; H, 6.58; N, 10.26. MS (ESI): calcd
+
(5 mL). The mixture was then heated at 70 °C for 5 h, and then
another portion of triethylamine (480 μL, 3.5 mmol) was added. The
reaction was further stirred at 70 °C for 10 h. The volatiles were then
removed under reduced pressure to give a white solid, which was
washed with tetrahydrofuran (3 × 5 mL) and dichloromethane (3 × 1
for [M − Br] , C H BrN , m/z 350; found, m/z 350.
17
25
3
trans-[PdBr (A) ] (1a). A mixture of A·HBr (390 mg, 1 mmol),
2
2
Ag O (128 mg, 0.55 mmol), and PdBr (133 mg, 0.5 mmol) was
2
2
stirred in CH Cl at ambient temperature in the dark for 14 h. The
2
2
suspension was then filtered over Celite, and the residue was washed
with CH Cl (3 × 1 mL). The solvent of the filtrate was evaporated to
obtain the crude product, which was then purified by silica gel
chromatography using CH Cl as eluent. The product was obtained as
a pale yellow powder containing the rotamers trans-anti-1a and trans-
syn-1a (353 mg, 0.4 mmol, 80%). Data for trans-anti-1a are as follows.
H NMR (400 MHz, CD CN): δ 8.14 (s, 1 H, NCHN), 7.07 (s, 2 H,
Ar-H), 4.84 (t, 2 H, NCH ), 3.23 (t, 2 H, CH Br), 2.39 (m, 2 H,
mL) to give 2a (360 mg, 0.33 mmol, 95%). Data for trans-anti-2a are
2
2
1
as follows. H NMR (400 MHz, CD CN): δ 8.32 (s, 1 H, NCHN),
3
7
.13 (s, 2 H, Ar-H), 4.56 (t, 2 H, NCH CH ), 3.52 (m, 6 H,
2 2
2
2
CH CH ), 3.26 (m, 2 H, CH Br), 2.99 (m, 2 H, CH CH CH ), 2.38
2
3
2
2
2
2
(
s, 3 H, p-CH ), 2.20 (s, 6 H, o-CH ), 1.14 (t, 9 H, CH CH ).
C{ H} NMR (75.47 MHz, CD CN): δ 173.2 (Ccarbene), 146.4
3
3 3 2 3
1
3
1
1
3
(
NCH N), 141.0, 137.3, 132.9, 130.2, 55.1, 54.0, 50.6, 22.8, 21.4, 19.9,
2
2
2
1
13
1
8
.0. Data for trans-syn-2a are as follows. H NMR (400 MHz,
CD CN): δ 8.22 (s, 1 H, NCHN), 6.91 (s, 2 H, Ar-H), 4.89 (t, 2 H,
NCH ), 3.25 (m, 6 H, CH CH ), 3.10 (m, 2 H, CH Br), 2.61 (m, 2 H,
CH CH CH ), 2.47 (s, 3 H, p-CH ), 1.87 (s, 6 H, o-CH ), 1.22 (t, 9
H, CH CH ). C{ H} NMR (75.47 MHz, CD CN): δ 173.2
N), 140.4, 136.4, 132.3, 130.2, 55.7, 54.3,
51.0, 23.1, 21.4, 19.5, 8.2. Anal. Calcd for C H Br N Pd: C, 44.28; H,
CH CH CH ), 2.37 (s, 3 H, p-CH ), 2.17 (s, 6 H, o-CH ). C{ H}
2
2
2
3
3
NMR (125.76 MHz, CD CN): δ 172.76 (C
40.9, 137.2, 132.9, 130.1 (Ar-C), 52.5 (NCH ), 33.6 (CH Br), 30.9
CH CH CH ), 21.2 (o-CH ), 19.8 (p-CH ). Data for trans-syn-1a are
as follows. H NMR (400 MHz, CD CN): δ 8.08 (s, 1 H, NCHN),
.92 (s, 2 H, Ar-H), 4.85 (t, 2 H, NCH ), 3.60 (t, 2 H, CH Br), 2.72
m, 2 H, CH CH CH ), 2.46 (s, 3 H, p-CH ), 1.87 (s, 6 H, o-CH ).
C{ H} NMR (125.76 MHz, CD CN): 172.8 (Ccarbene), 145.6
), 145.5 (NCH N),
3
3
carbene
2
1
2
2
3
2
2
2
(
2
2
2
3
3
2
2
2
3
3
1
13
1
3
2
3
3
6
(Ccarbene), 146.4 (NCH
2
2
2
(
2 2 2 3 3
40 66
4
8
1
3
1
6.13; N, 10.33. Found: C, 44.35; H, 6.21; N, 10.40. MS (ESI): calcd
3
G
DOI: 10.1021/acs.organomet.7b00329
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
+
23
for [M − Br] , C H Br N Pd, m/z 1005; found, m/z 1005; calcd
removed using a rotary evaporator. The residue was subjected to
40
66
3
8
2+
for [M − 2Br] , C H Br N Pd, m/z 462; found, m/z 462.
column chromatography (SiO ) using a hexane/ethyl acetate (4/1 v/
40
66
2
8
2
trans-[PdBr (C) ](PF ) (2a′). A solution of 2a (200 mg, 0.18
v) mixture as eluent.
2
2
6 2
mmol) in 2 mL of methanol was added dropwise to a solution of KPF6
680 mg, 3.7 mmol) in 100 mL of water. The white precipitate that
Mizoroki−Heck Catalysis. A 25 mL pressure tube was charged
with aryl halide (1.0 mmol), alkene (1.2 mmol), an appropriate
amount of the catalyst, base (2.0 mmol), and solvents (3 mL of
(
formed was filtered through a plug of Celite. The solid was washed
with water (3 × 0.5 mL) and collected using methanol. Removal of the
volatile solvent gave the product as white solids (210 mg, 0.17 mmol,
isopropyl alcohol and 3 mL of H O). The mixture was then allowed to
2
react with stirring for an appropriate amount of time. After the mixture
1
9
4%). H NMR (400 MHz, CD CN, anti:syn = 0.7:1): trans-anti-2a′,
was cooled, the product was extracted with ethyl acetate (3 × 5 mL).
3
δ 8.24 (s, 1 H, NCHN), 7.12 (s, 2 H, Ar-H), 4.53 (t, 2 H, NCH ), 3.06
The combined organic extract was dried over MgSO , and the volatile
2
4
3
(
q, J(H−H) = 7.1 Hz, 6 H, NCH CH ), 2.93−2.89 (m, 2 H, CH Br),
solvent was then removed using a rotary evaporator. The expected
2
3
2
2
.37 (s, 3 H, p-CH ), 2.15−2.25 (2 H, CH CH CH ), 2.19 (s, 6 H, o-
product was isolated by column chromatography (SiO ) using a
3
2
2
2
2
3
CH ), 1.12 (t, J(H−H) = 7.1 Hz, 9 H, NCH CH ); trans-syn-2a′, δ
hexane/ethyl acetate (4/1 v/v) mixture as eluent.
3
2
3
8
3
2
.19 (s, 1 H, NCHN), 6.92 (s, 2 H, Ar-H), 4.84 (t, 2 H, NCH ), 3.23−
X-ray Crystallography. Single-crystal X-ray diffraction was carried
out on a Bruker AXS SMART APEX diffractometer using graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). The software used
was as follows: SMART for collecting frames of data, indexing
2
3
.03 (m, 2 H, CH Br), 3.22 (q, J(H−H) = 7.1 Hz, 6 H, NCH CH ),
2
2
3
.43−2.52 (2 H, CH CH CH ), 2.19 (s, 3 H, p-CH ), 1.87 (s, 6 H, o-
2
2
2
3
3
19
1
CH ), 1.12 (t, J(H−H) = 7.1 Hz, 9 H, NCH CH ). F{ H} NMR
3
2
3
1
−
24
(
376.29 MHz, CD CN): δ −72.8 (d, J(P−F) = 707 Hz, PF ).
reflections, and determining lattice parameters; SAINT for
3
6
3
1
1
1
25
P{ H} NMR (161.92 MHz, CD CN): δ −144.6 (sep, J(P−F) = 707
integration of intensity of reflections and scaling; SADABS for
3
2
6
−
Hz, PF ). Anal. Calcd for C H Br F N P Pd: C, 39.54; H, 5.47; N,
empirical absorption correction; SHELXTL for space group
6
40 66
2
12
8 2
9
.22. Found: C, 39.71; H, 5.51; N, 9.32.
determination, structure solution, and least-squares refinements on |
2
27
trans-[PdBr (D) ]Br (2b). Complex 2b was synthesized in analogy
to 2a using 1b (386 mg, 0.4 mmol) and triethylamine (2 × 560 μL, 2
4 mmol). The product was obtained as a white powder, which was
hygroscopic (400 mg, 0.34 mmol, 86%). Data for trans-anti-2b are as
follows. H NMR (400 MHz, CD CN): δ 8.51 (s, 1 H, NCHN), 7.66
F| . Anisotropic thermal parameters were refined for the rest of the
non-hydrogen atoms. The hydrogen atoms were placed in their ideal
positions.
2
2
2
×
Computational Details. Gas-phase structures of all complexes
1
were optimized by the DFT method using the B3PW91 hybrid
3
2
8
29
(
3
2
t, 1 H, p-Ar-H), 7.46−7.52 (2 H, m-Ar-H), 4.63 (t, 2 H, NCH CH ),
functional. The 6-31G(d) basis set was used for H, C, and N; 6-
30
2
2
3
.05 (q, J(H−H) = 7.2 Hz, 6 H, NCH CH ), 2.87 (m, 2 H, CH N),
311G(d) was used for Br. The heavy Pd atoms were described by a
2
3
2
.81 (m, 2 H, CH(CH ) ), 2.11 (m, 2 H, CH CH CH ), 1.34 (d, 6 H,
Stuttgart−Dresden (SDD) relativistic effective core potential and
3
2
2
2
2
3
1
32
3
CH(CH ) ), 1.12 (t, J(H−H) = 7.2 Hz, 9 H, CH CH ), 0.99 (d, 6 H,
CH(CH ) ). C{ H} NMR (125.76 MHz, CD CN): δ 171.9
associated basis sets. The solvent effect CPCM model was utilized
for single-point calculations for 1a in solution.
3
2
2
3
1
3
1
3
2
3
(
2
Ccarbene), 146.7, 145.5, 131.0, 130.5, 123.9, 53.0, 52.2, 49.1, 28.3,
5.8, 22.4, 21.5, 7.1. Data for trans-syn-2b are as follows. H NMR
1
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00329.
■
(
400 MHz, CD CN): δ 8.36 (s, 1 H, NCHN), 7.46−7.52 (1 H, p−Ar-
3
*
S
H), 7.18 (d, 2 H, m-Ar-H), 5.01 (t, 2 H, NCH CH ), 3.57 (m, 2 H,
2
2
3
CH N), 3.36 (q, J(H−H) = 7.2 Hz, 6 H, NCH CH ), 2.74 (m, 2 H,
2
2
3
3
CH(CH ) ), 2.51 (m, 2 H, CH CH CH ), 1.26 (t, J(H−H) = 7.2 Hz,
3
2
2
2
2
13
1
9
H, CH CH ), 0.88−0.91 (12 H, CH(CH ) ). C{ H} NMR
2
3
3 2
1
(
125.76 MHz, CD CN): δ 171.9 (Ccarbene), 146.4, 145.5, 131.3, 130.6,
3
DFT-optimized geometries for rotamers of 1a and H,
1
23.8, 53.8, 52.4, 49.7, 27.8, 25.5, 22.4, 21.7, 7.2. Anal. Calcd for
13
1
19
1
31
1
C{ H}, F{ H}, P{ H} NMR spectra of the salts and
C H Br N Pd: C, 47.25; H, 6.72; N, 9.58. Found: C, 44.35; H, 6.21;
4
6
78
4
8
2
2
+
complexes (PDF)
N, 10.40. MS (ESI): calcd for [M − Br] , C H Br N Pd, m/z
1
5
4
6
78
3
8
23
2+
089; found, m/z 1089; calcd for [M − 2Br] , C H Br N Pd, m/z
4
6
78
2
8
Accession Codes
04; found, m/z 504.
trans-[PdBr (D) ](PF ) (2b′). 2b′ was synthesized in a manner
CCDC 1548181−1548182 and 1548184−1548185 contain the
supplementary crystallographic 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.
2
2
6 2
similar to that for 2a′, using 150 mg of 2b (0.13 mmol) and 480 mg
(
2.6 mmol) of KPF . The product was obtained as white solids (140
6
1
mg, 0.11 mmol, 85%). H NMR (400 MHz, CD CN, anti:syn =
3
1
7
6
.2:1): trans-anti-2b′, δ 8.42 (s, 1 H, NCHN), 7.62 (t, 1 H, Ar-H),
3
.45 (d, 2 H, Ar-H), 4.59 (t, 2 H, NCH ), 2.99 (q, J(H−H) = 7.2 Hz,
2
H, NCH CH ), 2.83−2.86 (m, 2 H, CH Br), 2.79 (m, 2 H,
2
3
2
CH(CH ) ), 2.03−2.11 (2 H, CH CH CH ), 1.30 (d, 6 H,
CH(CH ) ), 1.08 (t, J(H−H) = 7.2 Hz, 9 H, NCH CH ), 0.96 (d,
3
2
2
2
2
3
3
2
2
3
AUTHOR INFORMATION
Corresponding Authors
*E-mail for B.M.E.A.: belali@kfupm.edu.sa.
E-mail for H.V.H.: chmhhv@nus.edu.sg.
■
6
H, CH(CH ) ); trans-syn-2b′, δ 8.31 (s, 1 H, NCHN), 7.48 (t, 1 H,
3
2
Ar-H), 7.16 (d, 2 H, Ar-H), 4.95 (t, 2 H, NCH ), 3.31−3.35 (m, 2 H,
2
3
CH Br), 3.25 (q, J(H−H) = 7.2 Hz, 6 H, NCH CH ), 2.52−2.61 (m,
2
2
3
3
*
2
7
H, CH CH CH ), 2.47 (m, 2 H, CH(CH ) ), 1.21 (t, J(H−H) =
.2 Hz, 9 H, NCH CH ), 1.21 (d, 6 H, CH(CH ) ), 0.86−0.90 (m, 6
2
2
2
3 2
2
3
3
2
ORCID
1
9
1
H, CH(CH ) ). F{ H} NMR (376.29 MHz, CD CN): δ −72.9 (d,
3
2
3
Van Ha Nguyen: 0000-0002-6426-4385
Han Vinh Huynh: 0000-0003-4460-6066
Notes
1
−
31
1
J(P−F) = 707 Hz, PF ). P{ H} NMR (161.92 MHz, CD CN): δ
6
3
1
−
−
146.8 (sep, J(P−F) = 707 Hz, PF6 ). Anal. Calcd for
C H Br F N P Pd: C, 42.52; H, 6.05; N, 8.62. Found: C, 42.73;
4
6
78
2
12
8 2
2
2
H, 5.49; N, 8.85.
The authors declare no competing financial interest.
Suzuki−Miyaura Catalysis. In a typical run, a pressure tube was
charged with a mixture of aryl halide (8 mmol), arylboronic acid (10
mmol), potassium carbonate (16 mmol), an appropriate amount of the
catalyst, isopropyl alcohol (24 mL), and H O (24 mL). The mixture
was stirred at the appropriate temperature for the required time. The
product was then extracted with ethyl acetate (3 × 40 mL). The
combined extracts were dried over MgSO , and volatile solvent was
ACKNOWLEDGMENTS
■
The work reported in this manuscript was funded by the
KFUPM/NUS collaboration (R-143-005-617-597). We are
grateful for technical support from the CMMAC staff at the
Department of Chemistry, National University of Singapore.
2
4
H
DOI: 10.1021/acs.organomet.7b00329
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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(
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DOI: 10.1021/acs.organomet.7b00329
Organometallics XXXX, XXX, XXX−XXX