Stereoelectronic Flexibility of Ammonium-Functionalized Triazole-Derived Carbenes: Palladation and Catalytic Activities in Water

Article abstract of DOI:10.1021/acs.organomet.8b00347

The postmodification approach allows convenient access to charge-tagged ammonium-functionalized bis(1,2,4-triazolin-5-ylidene)palladium(II) complexes even when the respective azolium salts are elusive. Bromo-functionalized 1,2,4-triazolium salts were first metalated to form the respective bromo-functionalized bis(NHC) complexes trans-[PdBr₂(R-tazy-Br)₂] (R = Ph, Cy, 1a,b). Subsequent postcoordinative nucleophilic substitutions converted the bromo into ammonium functions, leading to the water-soluble, charge-tagged complexes trans-[PdBr₂(R-tazy-NEt₃)₂]Br₂ (R = Ph, Cy, 2a,b). The catalytic activities of 2a,b in the aqueous Suzuki-Miyaura reaction were compared to those of their analogues 2c,d bearing more bulky mesityl and diisopropylphenyl substituents, and a detailed stereoelectronic profiling of the NHCs using %V_bur, HEP, and DFT calculations was conducted to rationalize their catalytic differences. Although all complexes are active, the more donating and less bulky complexes 2a,b performed significantly better than 2c,d at a very low catalyst loading of 0.001 mol %. 2a was found to be highly active for various aryl and heteroaryl bromides and some aryl chlorides. More importantly, this study discloses that NHCs with N-phenyl and N-cyclohexyl groups exhibit stereoelectronic flexibility, which could be the cause for the greater activities of their complexes.

Full text of DOI:10.1021/acs.organomet.8b00347

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Stereoelectronic Flexibility of Ammonium-Functionalized Triazole-
Derived Carbenes: Palladation and Catalytic Activities in Water
Van Ha Nguyen,† Bassam M. El Ali,§ and Han Vinh Huynh*
,‡
,†
†
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore
Faculty of Chemistry, VNU University of Science, 19 Le Thanh Tong, Hanoi 110403, Vietnam
‡
§
Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
*
S Supporting Information
ABSTRACT: The postmodification approach allows convenient access to charge-
tagged ammonium-functionalized bis(1,2,4-triazolin-5-ylidene)palladium(II) complexes
even when the respective azolium salts are elusive. Bromo-functionalized 1,2,4-triazolium
salts were first metalated to form the respective bromo-functionalized bis(NHC)
complexes trans-[PdBr (R-tazy-Br) ] (R = Ph, Cy, 1a,b). Subsequent postcoordinative
2
2
nucleophilic substitutions converted the bromo into ammonium functions, leading to the
water-soluble, charge-tagged complexes trans-[PdBr (R-tazy-NEt ) ]Br (R = Ph, Cy,
2
3 2
2
2
a,b). The catalytic activities of 2a,b in the aqueous Suzuki−Miyaura reaction were
compared to those of their analogues 2c,d bearing more bulky mesityl and
diisopropylphenyl substituents, and a detailed stereoelectronic profiling of the NHCs
using %V , HEP, and DFT calculations was conducted to rationalize their catalytic
bur
differences. Although all complexes are active, the more donating and less bulky
complexes 2a,b performed significantly better than 2c,d at a very low catalyst loading of 0.001 mol %. 2a was found to be highly
active for various aryl and heteroaryl bromides and some aryl chlorides. More importantly, this study discloses that NHCs with
N-phenyl and N-cyclohexyl groups exhibit stereoelectronic flexibility, which could be the cause for the greater activities of their
complexes.
INTRODUCTION
1,2,4-triazole-derived carbenes and their catalytic applications,
we herein report on an in-depth stereoelectronic profiling of
such palladium(II) 1,2,4-triazolin-5-ylidene complexes in an
attempt to rationalize their catalytic activities in aqueous
■
N-heterocyclic carbenes (NHCs) are unarguably one of the
most powerful ligands available for organometallic chemistry
1
−5
and catalysis.
The success of this ligand class can be
1
6−20
Suzuki−Miyaura coupling reactions.
Moreover, the
attributed to their ease of preparation and, more importantly,
the excellent modularity of their stereoelectronic properties
controllable by judicious wingtip and backbone variations.
The backbone derived from 1,2,4-triazole closely resembles
that derived from imidazole, since they are related by a simple
isolobal replacement of a CH group with a nitrogen atom.
Moreover, the first two types of stable NHCs reported were
synthetic pathways applied herein highlight the advantage of
postmodification strategies as an important complementary
tool to access NHC complexes, since the direct carbene
precursors are elusive.
6
−8
RESULTS AND DISCUSSION
■
9
Synthesis and Characterization of Ammonium-
Arduengo’s imidazolin-2-ylidene and Enders’ 1,2,4-triazolin-5-
II
1
0
Functionalized Pd Bis(carbene) Complexes. 4-Cyclo-
ylidene. Despite their similarities, it is surprising that the
chemistry of 1,2,4-triazolin-5-ylidene complexes has received
far less attention. The few reported 1,2,4-triazolin-5-ylidene
metal complexes feature carbenes bearing simple wingtip
substituents, such as alkyl or aryl, and functionalized 1,2,4-
hexyl-1,2,4-triazole and 4-phenyl-1,2,4-triazole were synthe-
21
sized by following the reported procedures. Nucleophilic
substitution reactions between the triazoles and 1,3-dibromo-
propane afforded the respective bromopropylene-tethered
triazolium salts A and B (Scheme 1). Their identities were
11−14
triazole-derived carbene complexes are very rare,
although
1
13
confirmed by H and C NMR spectroscopy, mass
spectrometry, and elemental analysis.
additional functional groups would allow for a more diverse
reactivity. Recently, we reported that complexes of ammo-
nium-functionalized 1,2,4-triazolin-5-ylidenes of the general
formula [PdBr (tazy) ]Br (tazy = 4-mesityl-1-(3-
Attempts to prepare the respective ammonium-function-
alized triazolium salts via nucleophilic substitution reactions
between A and B with triethylamine did not give the expected
products, which we attribute to the incompatibility of the
2
2
2
triethylammonium)propyl-1,2,4-triazolin-5-ylidene or 4-diiso-
propylphenyl-1-(3-triethylammonium)propyl-1,2,4-triazolin-5-
ylidene) gave rise to highly active precatalysts for Suzuki−
1
5
Miyaura and Heck coupling reactions in polar solvents. In
Received: May 23, 2018
our efforts to further explore the coordination chemistry of
©
XXXX American Chemical Society
A
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Scheme 1. Synthesis of Palladium(II)-Bis(NHC) Complexes
and 2a have a significant shielding effect on the trans ligand
and can act as “built-in NMR probes” for an easy spectral
assignment and determination of anti:syn rotameric ratios. The
1
a,b and 2a,b
1
H NMR spectrum of 1a is shown in Figure 1 as an example.
rather acidic C proton of the triazolium salt with the basic
triethylamine nucleophile (Scheme 1).
5
1
Figure 1. H NMR spectrum of 1a in CDCl from ∼2.1−5.3 ppm.
3
As the ammonium-functionalized 1,2,4-triazolium salts are
elusive, a postcoordinative ligand modification was employed
as an alternative pathway to access the targeted ammonium-
Using this approach, an anti:syn ratio of 3.8:1 was determined
for 1a in chloroform-d, while 2a gave rise to a ratio of 3.4:1 in
acetonitrile-d . Both complexes show the preference for the
trans-anti form.
Further corroborations for all complexes were obtained by
ESI-MS mass spectrometry. In the mass spectra of 1a,b, base
peaks assignable for [M − Br] cations were observed at m/z
719 (1a) and 731 (1b). Complexes 2a,b are decorated with
charge tags and do not require ionization steps, which allows
for the detection of the intact coordination spheres with ease.
Thus, the ESI mass spectrum of 2a shows intense peaks
II
functionalized Pd −NHC complexes. This approach commen-
ces with the palladation of the triazolium salt by a silver
3
22
carbene transfer method or by directly reacting A and B with
palladium(II) bromide in the presence of silver(I) oxide to
produce the bromoalkyl-functionalized bis(NHC) complexes
trans-[PdBr (Ph-tazy-Br) ] (1a) and trans-[PdBr (Cy-tazy-
+
2
2
2
Br) ] (1b), which were purified by column chromatography.
2
Subsequently, postcoordinative ligand modification con-
verted the tethered bromo into an ammonium function via
nucleophilic substitution reactions with triethylamine to
produce the final complexes trans-[PdBr (Ph-tazy-NEt ) ]Br
+
assignable to [PdBr (Ph-tazy-NEt ) ]Br at m/z 921 and
2
3 2
2
2
3 2
2
+
(
2a) and trans-[PdBr (Cy-tazy-NEt ) ]Br (2b). The SN2
[PdBr (Ph-tazy-NEt ) ] at m/z 420. Similarly, cationic peaks
2
3 2
2
2 3 2
+
reactions proceeded smoothly without any interference from
the coordination sphere, giving the products in excellent yields
of 92% (2a) and 96% (2b), respectively. Complex 2b was
found to be particularly hydroscopic, absorbing moisture
rapidly upon storage, and thus was notably more difficult to
handle than complex 2a.
for [PdBr (Cy-tazy-NEt ) ]Br at m/z 933 and [PdBr (Cy-
2 3 2 2
2
+
tazy-NEt ) ] at m/z 427 were found for 2b.
3
2
Molecular Structures. Single crystals of all complexes
were grown by slow evaporation of concentrated solutions in
chloroform/hexane (1a,b and 2b) or dichloromethane/hexane
(2a). The solid-state molecular structures of the complexes are
shown in Figure 2, and selected bond lengths and angles are
given in Table S1 in the Supporting Information. In all cases,
1
The identities of all complexes have been confirmed by H
1
3
and C NMR spectroscopy, mass spectrometry, elemental
1
13
II
analysis, and single-crystal X-ray diffraction. The H and
NMR spectra of all complexes display two sets of signals,
C
the Pd center is in an essentially square planar geometry,
coordinated by two bromido and two carbene ligands. The two
NHCs are trans to each other, which is in line with results from
solution NMR analysis. Moreover, 1a,b and 2a show a trans-
anti orientation of the two unsymmetrical NHC ligands,
whereas a trans-syn orientation is adopted by 2b. In all trans-
anti structures, the two carbenes are coplanar and are twisted
from the coordination plane with dihedral angles ranging from
73 to 83° (Figure 2).
Previously, we reported the related species [PdBr (Mes-tazy-
NEt ) ]Br (2c) and [PdBr (Dipp-tazy-NEt ) ]Br (2d) with
N-mesityl (Mes) and N-diisopropylphenyl substituents
(Dipp), respectively. The bulky aromatic rings in their
solid-state structures are almost perpendicular to the
triazolinylidene plane, giving rise to rigid structures (Figure 2).
On the other hand, the N-phenyl and N-cyclohexyl rings in
the structures of 1a,b and 2a,b are expected to be more flexible
1
3
suggesting the presence of two isomers. The Ccarbene
resonances are marginally separated, appearing at 172.6/
1
72.0 ppm (1a), 171.9/171.8 ppm (1b), 170.5/169.9 ppm
(2a), and 172.5/172.3 ppm (2b). These differences are
insufficient to support cis/trans isomers, where larger
separations are usually observed. However, the chemical shifts
are close to those reported for trans-[PdBr (tazy) ] complexes
2
2
15,23
(
169.3−173.6 ppm),
which suggests the presence of trans-
2
anti and trans-syn rotamers arising from the unsymmetrical
NHC ligands. Trans−cis isomerization was not observed for
any complexes.
3
2
2
2
3 2
2
15
1
Unambiguous assignment of H NMR resonances to specific
rotamers of 1b and 2b containing N-cyclohexyl substituents
was not possible, as the two sets of signals are heavily
overlapping. On the other hand, the N-phenyl wingtips in 1a
B
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Figure 3. Comparison of dihedral angles θ and associated rotational
freedom in phenyl, cyclohexyl, mesityl, and diisopropylphenyl N
substituents.
Initially, standard test reactions between phenylboronic acid
and 4-bromobenzaldehyde in the presence of potassium
carbonate were studied in order to differentiate and identify
the best among the bis(NHC) palladium(II) complexes (Table
1
).
Table 1. Suzuki−Miyaura Coupling Reactions Catalyzed by
a
2
a−d
b
entry [Pd] cat. loading (mol %) T (°C) time (h) yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
90
90
90
90
90
25
25
25
25
90
90
90
90
70
70
70
70
90
90
1.5
4
4
4
4
4
4
4
4
1.5
1.5
1.5
1.5
2
2
2
2
3
0
2a
2b
2c
2d
2a
2b
2c
2d
2a
2b
2c
2d
2a
2b
2c
2d
2a
2b
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.01
0.01
0.01
0.01
0.005
0.005
0.005
0.005
0.001
0.001
100
100
100
100
trace
trace
0
Figure 2. Molecular structures of 1a,b, 2a·CH Cl , 2b, and the
2
2
24
previously reported 2c,d.
0
0
1
2
3
4
5
6
7
8
9
100
100
100
100
89
97
29
35
90
in terms of rotation about the N−C bond, which allows them
to adopt various orientations with respect to the carbene plane.
Indeed, the phenyl ring twists from the triazolinylidene plane
with dihedral angles of only θ = 42° (1a) and θ = 53° (2a).
Similarly, the plane dissecting the cyclohexyl ring adopts
different dihedral angles θ of 45° (1b) and 14° (2b) relative to
the carbene plane (Figure 3). The certain degree of rotational
freedom could lead to complexes with a more flexible
stereoelectronic profile (vide infra).
3
86
a
Reaction condition: 1.0 mmol of 4-bromobenzaldehyde, 1.2 mmol of
phenylboronic acid (1.2 equiv), 1.5 mmol of K CO (1.5 equiv), 2
mL of water as solvent. NMR yield for the average of two runs.
Catalytic Applications. NHC complexes of palladium
have proven to be efficient catalyst precursors for various
carbon−carbon and carbon−heteroatom coupling reac-
2
3
b
1
,25
tions,
including those that have become standard tools in
26
total synthesis, such as the Suzuki−Miyaura reaction.
Although relatively less explored, several palladium(II) bis-
At the onset of the study, a blank experiment was conducted
in the absence of any catalyst, and as anticipated no product
formation was observed (Table 1, entry 1). In the first few
trials at 0.1 mol % catalyst loading, complexes 2a−d all gave
quantitative yields after 4 h at 90 °C (entries 2−5). Elevated
temperatures are a requirement, as reactions carried out at 25
°C gave either no or only trace amounts of coupling products
(entries 6−9). At a lower loading of 0.01 mol % and reactions
carried out at 90 °C, the NMR yields were still quantitative,
which did not allow for a differentiation among the four
(
1,2,4-triazolin-5-ylidene) complexes have demonstrated good
27,28
catalytic activities in this reaction.
Notably, most of the
reactions were carried out in water-free media. Since all four
complexes 2a−d are water-soluble with solubilities of 1 mg/
mL (1 mmol/L, 2a), 1.5 mg/mL (1.5 mmol/L, 2b), 1.3 mg/
mL (1.2 mmol/L, 2c), and 1.7 mg/mL (1.5 mmol/L, 2d), they
were tested for their catalytic activity in pure water as a low-
cost and environmentally benign solvent.
C
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precatalysts (entries 10−13). Finally, discrimination of
catalytic activities was observed at 0.005 mol % for reactions
carried out at 70 °C in 2 h (entries 14−17). The results show
that the new complexes 2a,b were superior in performance in
comparison to 2c,d. Moreover, when the catalyst loading is
further reduced to 0.001 mol %, both complexes were still
highly active, giving excellent yields at 90 °C for 3 h (entries 18
and 19).
For a broader substrate scope study, we chose complex 2a,
since its synthesis is overall more convenient. Moreover, its
performance is not significantly inferior to 2b, and it is not
hygroscopic, which simplifies storage and handling. Thus,
various aryl/heteroaryl halides were coupled with phenyl-
boronic acid in the presence of precatalyst 2a, and the results
are summarized in Table 2.
from the double coupling reaction between 1,3-dibromoben-
zene and phenylboronic acid. Notably, the coupling also
proceeded well with various heteroaryl bromides, although
higher catalyst loadings and extended reaction time were
required (entries 7−10). Indeed, phenylboronic acid reacted
with 2-bromothiophene (entry 7), 3-bromopyridine (entry 8),
and 2-bromopyridine (entry 9) to give the respective products
in good to excellent yields. Quantitative diarylation of 1,6-
dibromopyridine can also be achieved at 0.1 mol % loading
(entry 10). Such heterodiaryl products are useful cyclo-
metalating ligands for various applications, especially in
luminescent materials. In addition, the couplings of 4-
chloroacetophenone and 4-chlorobenzaldehyde were also
successful, giving moderate product yields (entry 11, 12).
However, 2-chloropyridine as a highly challenging substrate
cannot be coupled, which highlights the limitation of this
catalyst system (entry 13).
At 0.001 mol % catalyst loading, activated aryl bromides
react smoothly with phenylboronic acid to give substituted
biphenyl products in excellent yields at 90 °C for 10 h (Table
While most of the palladium(II) triazolinylidene complexes
require higher catalyst loading for the Suzuki−Miyaura
2
, entries 1 and 2). Moderate to good yields are also obtained
26,27
reaction,
the precatalysts 2a can give good yields at even
for deactivated aryl bromides under the same reaction
conditions (entries 3−5). A decent yield of 83% resulted
0
.001 mol % catalyst loading in pure water. A similarly good
performance was also reported for palladium(II) complexes
with sulfonate-functionalized imidazolin-2-ylidenes in the
coupling reaction between acetophenone bromide and phenyl-
boronic acid under the same conditions. However, the
preparation of these catalysts required harsher conditions
Table 2. Suzuki−Miyaura Coupling of (Hetero)aryl
a
Halides
29
(
prolonged heating at high temperature). In addition, higher
catalyst loadings (i.e., 1 mol %) were required for a broader
substrate scope study, and the catalytic activities in coupling of
heteroaryl halides have not been demonstrated.
Stereoelectronic Profiling of the Functionalized
,2,4-Triazolin-5-ylidenes. The stereoelectronic property
1
of a NHC ligand is believed to be the key factor that governs
the catalytic activity of its complexes. In an attempt to
rationalize the significant catalytic differences between the
charge-tagged complexes 2a−d, which differ solely in one N
substituent, we compared the steric bulk and donor strength of
the respective carbenes R-tazy-NEt (R = Ph, Cy, Mes, Dipp)
3
3
0
using the percentage volume buried (%V ) and Huynh’s
bur
3
1−34
electronic parameters (HEP).
Their direct complex
precursors 1a−d were also included to study the any
stereoelectronic effect of bromo versus ammonium functions.
The availability of solid-state molecular structures of all
functionalized triazolinylidene complexes 1a−d and 2a−d
allow for a direct comparison of estimated steric effect exerted
by these carbenes. The calculated data are summarized along
with the conformer used. Consistent with chemical intuition,
the %Vbur value is largest for the ammonium-functionalized
NHC with the bulky diisopropylphenyl substituent (%Vbur
=
32.4%), followed by the NHCs with mesityl (%Vbur = 31.0%)
and phenyl (%Vbur = 29.7%) substituents (Table 3). The
carbene with cyclohexyl as an N substituent is the least bulky
ligand with %Vbur = 28.1%.
For the bromo-functionalized NHCs a very similar trend was
observed. It is also plausible to assume that the remote
Table 3. Calculated %Vbur for R-tazy-X Ligands
a
Reaction conditions unless specified otherwise: 0.1 mmol of aryl
R
halide, 0.12 mmol of phenylboronic acid (1.2 equiv), 1.5 mmol of
X
Ph
Cy
Mes
Dipp
b
K CO (1.5 equiv), 1 mL of H O as solvent. NMR yield for average
2
3
2
c
d
e
NEt3
Br
29.7 (anti)
29.5 (anti)
28.1 (syn)
29.5 (anti)
31.0 (syn)
30.7 (anti)
32.4 (anti)
31.3 (anti)
of two runs. Reaction time 10 h. Reaction time 24 h. 1.5 equiv of
NBu Br was added.
4
D
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substitution of the bromo with a tetraalkylammonium function
leads to a small increase in steric bulk. Nevertheless, these are
only rough estimations, since some structures used for %Vbur
calculations differ in the orientation of the NHCs: i.e., anti vs
syn.
Scheme 2. Preparation of Complex Probes 3a−d and 4a−d
for HEP Determination
The fact that the phenyl (in 1a/2a) and cyclohexyl rings (in
b/2b) are more likely to rotate around the N−C bond (vide
1
supra) suggests that they can exert a range of steric bulk to the
coordination sphere. To evaluate the steric flexibility of the Ph-
tazy-NEt3 and Cy-tazy-NEt3 ligands, %Vbur values for
complexes 2a,b were calculated using modeled hypothetical
molecular structures, where the dihedral angles θ vary from 0
to 180°. The results demonstrate the dependence of the steric
bulk on the dihedral angle θ (Figure 4). Rotation of the phenyl
and single-crystal X-ray diffraction (for 3c,d and 4d). Synthetic
procedures and characterization details are given in the
Supporting Information.
Most important are the 13Ccarbene resonances of these
1
3
complexes. In their C NMR spectra, two carbene carbon
signals are observed, which are separated by 0.7−4.2 ppm.
i
Those for Pr -bimy reporter ligands (i.e., HEP values) were
2
unambiguously assigned using heteronuclear multiple bond
correlation (HMBC) 2D NMR experiments. The HEP values
of the bromo- and ammonium-functionalized 1,2,4-triazolin-5-
ylidenes are given in Table 4.
Table 4. Electronic Parameter (HEP) of the Triazole-
Derived NHCs of the Type R-tazy-X Ligands under Study
R (X = Br)
HEP (ppm)
R (X = NEt3)
HEP (ppm)
Ph
Cy
Mes
Dipp
176.3
177.8
176.2
176.4
Ph
Cy
Mes
Dipp
175.8
176.5
174.7
174.9
Figure 4. Change in %Vbur of Ph-tazy-NEt and Cy-tazy-NEt as the
3
3
phenyl/cyclohexyl ring rotates around the N−C bond.
Comparison of these HEPs with previously determined
values indicates that 1,2,4-triazolin-5-ylidenes are more weakly
ring around the N−C bond induces %V values in the range
bur
donating than classical imidazole-, imidazoline-, and benzimi-
of 28.4−31.5% for the Ph-tazy-NEt ligand. Similarly, rotation
30−32
3
dazole-derived carbenes.
This is reasonable because 1,2,4-
of the cyclohexyl ring leads to a potentially larger %V range
bur
triazolin-5-ylidenes contain an additional electronegative ring
nitrogen atom.
A closer inspection reveals that the donor strength of the
of 27.7−34.0% for the Cy-tazy-NEt carbene. For hypothetical
3
structures, where the cyclohexyl adopts a boat conformation, a
similar range of %Vbur 27.7−50.1% (Figure S3 in the
Supporting Information) can be obtained by rotation of the
cyclohexyl group.
ammonium-functionalized carbenes R-tazy-NEt increases in
3
the R substituent order Mes (174.7 ppm) < Dipp (174.9 ppm)
<
Ph (175.8 ppm) < Cy (176.5 ppm) (Figure 5). In line with
In parallel, the donor strengths of the carbenes were
evaluated using our parameter (HEP) with complex probes of
chemical intuition, Cy-tazy-NEt is the strongest donor due to
3
the +I effect exerted by the cyclohexyl substituent. Previous
work has also shown that imidazolin-2-ylidenes with 2,6-
diisopropylphenyl substituents (e.g., IPr) are slightly stronger
i
i
the general formula trans-[PdBr ( Pr -bimy)(NHC)] ( Pr -
2
2
2
bimy = 1,3-diisopropylbenzimidazolin-2-ylidene). Since the
NHCs in 2a−d contain salt moieties, complex probes of the
i
formula trans-[PdBr ( Pr -bimy)(NHC)]Br will result.
2
2
The syntheses of the complex probes are summarized in
Scheme 2. Again, since the ammonium-functionalized
triazolium salts are not accessible, a postmodification approach
i
was employed. Complexes containing the Pr -bimy reporter
2
ligand and bromo-functionalized 1,2,4-triazolin-5-ylidenes
i
(
3a−d) were first prepared by reacting the [PdBr ( Pr -
2
2
3
5
bimy)] dimer with the respective triazolium salts in the
2
presence of silver(I) oxide. In the subsequent step, these trans-
i
[
PdBr ( Pr -bimy)(R-tazy-Br)] complexes underwent nucleo-
2
2
philic substitution reactions with triethylamine to form trans-
i
[
PdBr ( Pr -bimy)(R-tazy-NEt )]Br complex probes 4a−d for
2
2
3
donor strength determination of the R-tazy-NEt ligands (R =
3
Figure 5. 13C NMR chemical shift of carbene probes (HEP) as an
indication of the carbene’s relative donor strengths.
Ph, Cy, Mes, Dipp). Formation of all complexes was confirmed
by H and C NMR spectroscopy, ESI mass spectrometry,
1
13
E
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donors in comparison to those with mesityl substituents (e.g.,
IMes), which agrees with the trend observed here. However,
respective 1,2,4-triazolin-5-ylidene ligands, single-point calcu-
lations were then performed. Plots of the highest occupied
orbitals of σ and π symmetry with respect to the carbene plane
are presented in Figure 6. As expected, the HOMOs of the
three carbenes show σ symmetry with respect to the carbene
plane and correspond to the lone pair on the carbene atom.
The shapes and spatial distributions are approximately the
same. On the other hand, significant differences can be noted
for the π-HOMO orbitals. While the π-HOMOs of the Mes-
and Dipp-substituted species are primarily localized on the
carbene cores, the π-HOMO of the Ph-substituted NHC
shows significant overlap and conjugation between the π
systems of the phenyl and the carbene core. This clearly
indicates that (i) the two π systems have the right energy and
symmetry to interact and (ii) such interaction leads to an
increased +M effect exerted by the phenyl ring to the carbene
ring, making the respective NHC more electron rich.
3
0
it is less intuitive that Ph-tazy-NEt appears to be a stronger
3
donor than Mes-tazy-NEt and Dipp-tazy-NEt , which contain
3
3
additional alkyl groups on the aryl system.
It has already been noted that the bulky Mes and Dipp
substituents are preferably perpendicular to the triazole plane
(
e.g., a dihedral angle of 85° in 3c, 87° in 3d, and 85° in 4d),
while the unsubstituted phenyl ring is more flexible and can
rotate around the N−C bond (vide supra). In addition to
altering the sterics, such a rotation could potentially allow
partial overlapping between the π systems of the phenyl and
triazolinylidene rings. This would enhance the +M effect from
the phenyl ring and make the NHC more electron rich. In
order to probe such a notion, DFT calculations were carried
out to gain insights into the nature of frontier orbitals of
related phenyl-, mesityl-, and diisopropylphenyl-substituted
triazolin-5-ylidenes.
Moreover, an interesting trend was also observed when the
HOMO and π-HOMO energies of this NHC were examined
in dependence of the dihedral angle θ (partial optimization
with fixed θ, Figure 7). As the phenyl ring changes its
The flexible (3-triethylammonium)propylene group of the
NHCs was replaced by a small methyl group to facilitate
calculation. Gas-phase optimized structures of the modeled
analogues of 3a,c,d and 4a,c,d and the respective free NHCs
are shown in Figure 6. All complexes show slightly distorted
Figure 7. Correlation between HOMO (blue) and π-HOMO
(purple) energies of Ph-tazy-Me and dihedral angle.
orientation from coplanar (θ = 0°) to perpendicular (θ = 90°),
the HOMO gets slightly destabilized. On the other hand, the
π-HOMO energy is significantly increased as the two planes
align (θ = 90 → 0°). The change in the energies of the HOMO
and π-HOMO as well as %Vbur suggests the flexible
stereoelectronic property of N-phenyl-substituted NHC
ligands.
Overall, it is interesting to note that the less active
precatalysts trans-[PdBr (Mes-tazy-NEt ) ]Br (2c) and
2
3
2
2
Figure 6. Optimized molecular structures of the model complexes and
plots of HOMOs and π-HOMOs for the respective triazolin-5-
ylidenes.
trans-[PdBr (Dipp-tazy-NEt ) ]Br (2d) contain more weakly
2
3 2
2
donating and more bulky 1,2,4-triazolin-5-ylidene ligands
(
Table 3). On the other hand, the complexes trans-
[
PdBr (Ph-tazy-NEt ) ]Br (2a) and trans-[PdBr (Cy-tazy-
2
3 2
2
2
square planar geometries with two coplanar NHCs as observed
in the solid-state molecular structures determined by X-ray
diffraction (vide supra). The calculated Pd−Ctazy bond
distances increases in the order 2.045 Å (Ph) < 2.055 Å
NEt ) ]Br (2b) with more strongly donating and less bulky
3 2 2
carbenes displayed better catalytic activity under the given
conditions. The superior performance of 2a,b may also be
attributed to the flexible steric and electronic properties of
their carbene ligands.
(
Dipp) < 2.058 Å (Mes).
More important is that the optimized structures feature
CONCLUSION
A series of functionalized 1,2,4-triazolin-5-ylidene complexes of
the form trans-[PdBr (R-tazy-Br) ] (1a,b) and trans-[PdBr (R-
nearly perpendicular orientations of the mesityl and
diisopropylphenyl plane with respect to the carbene plane.
On the other hand, the dihedral angle between the phenyl and
carbene rings is merely 58°. Using the geometries of the
■
2
2
2
tazy-NEt ) ]Br (2a,b) differing in their wingtips (R = Ph, Cy)
3
2
2
F
DOI: 10.1021/acs.organomet.8b00347
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
have been synthesized and fully characterized by means of
multinuclear NMR spectroscopy, mass spectrometry, and
elemental analysis and in most cases by single-crystal X-ray
diffraction. Despite the elusiveness of ammonium-function-
alized triazolium salts, the respectively functionalized NHC
complexes 2a,b could be obtained by a postmodification
NMR (CDCl
3
, 125.7 MHz): δ 143.4 (NCHN), 141.9 (NCHN),
1
32.2, 131.9, 131.5, 123.3 (Ar-C), 52.4 (NCH ), 32.0 (CH Br), 29.9
2 2
(
CH ). Anal. Calcd for C H Br N : C, 38.07; H, 3.78; N, 12.11.
2 11 13 2 3
36
+
Found: C, 38.80; H, 3.90; N, 12.33. MS (ESI): calcd for [M + H] ,
C H BrN m/z 266; found, m/z 266.
1
1
13
3
4
-Cyclohexyl-1-(3-bromopropyl)-1,2,4-triazolium Bromide
(
B). Compound B was prepared in a manner similar to that for A
approach via S 2 reactions between the bromo-functionalized
N
from 4-cyclohexyl-1,2,4-triazole (905 mg, 6 mmol) and 1,3-
dibromopropane (3.0 mL, 30 mmol). The product was obtained as
a while powder (2.0 g, 5.7 mmol, 95%). H NMR (400 MHz,
complexes 1a,b with triethylamine. The catalytic activities of
1
2
a,b in aqueous Suzuki−Miyaura reactions at very low loading
CDCl ): δ 11.75 (s, 1 H, NCHN), 8.90 (s, 1 H, NCHN), 4.81 (t,
were found to be superior to those of their previously reported
mesityl (2c)- and diisopropylphenyl-substituted (2d) ana-
logues.
3
3
3
JH−H = 6.8 Hz, 2 H, NCH ), 4.63 (m, 1 H, NCH), 3.54 (t, J
=
2
H−H
6
.3 Hz, 2 H, CH Br), 2.62 (quint, 2 H, CH CH CH ), 2.33 (m, 2 H,
2 2 2 2
Cy-H), 1.94 (m, 2 H, Cy-H), 1.85−1.72 (3 H, Cy-H), 1.47 (m, 2 H,
Cy-H), 1.31 (m, 1 H, Cy-H). C{ H} NMR (125.7 MHz, CDCl ): δ
Detailed stereoelectronic profiling of the respective 1,2,4-
triazolin-5-ylidenes using percentage volume buried (%Vbur),
Huynh’s electronic parameter (HEP), and DFT methods were
conducted to rationalize the different reactivities of complexes
1
3
1
3
1
43.4 (NCHN), 142.7 (NCHN), 60.2 (NCH), 51.9 (NCH ), 33.9
2
(CH Br), 32.1, 29.7, 25.3, 25.0 (CH ). Anal. Calcd for C H Br N :
2 2 11 19 2 3
C, 37.42; H, 5.42; N, 11.90. Found: C, 37.31; H, 5.80; N, 12.22. MS
+
2
a−d. The results suggest that complexes 2a,b with more
(
ESI): calcd for [M − Br] , C H BrN m/z 272; found, m/z 272.
1
1
19
3
strongly donating and less bulky NHCs give rise to more active
precatalysts. More importantly, this study reveals that NHCs
with N-phenyl or N-cyclohexyl wingtips are stereoelectroni-
cally flexible, which may help in the future design of tailor-
made NHC complexes for various applications.
trans-[PdBr (Ph-tazy-Br) ] (1a). A mixture of A (347 mg, 1
2 2
mmol), Ag O (128 mg, 0.55 mmol), and PdBr (133 mg, 0.5 mmol)
2
2
was stirred in dichloromethane at ambient temperature in the dark for
4 h. The suspension was then filtered over Celite, and the residue
1
was washed with dichloromethane (3 × 1 mL). The solvent of the
filtrate was evaporated to obtain the crude product, which was then
purified by silica gel chromatography using dichloromethane as
eluent. The product was obtained as a pale yellow powder containing
trans-syn-1a and trans-anti-1a rotamers (385 mg, 0.48 mmol, 96%).
Data for trans-syn-1a are as follows. H NMR (400 MHz, CDCl ): δ
EXPERIMENTAL SECTION
■
General Considerations. All syntheses were carried out without
precautions to exclude air and moisture unless otherwise stated.
Solvents for syntheses and spectroscopic measurements were used as
1
3
8
4
.05 (s, 1 H, NCHN), 7.63−7.59 (4 H, Ar-H), 7.47 (t, 1 H, Ar-H),
36
3
3
.98 (t, J
= 6.9 Hz, 2 H, NCH ), 3.63 (t, J
= 6.3 Hz, 2 H,
H−H
received. N,N-Dimethylformamide azine dihydrochloride and 4-
H−H
2
2
1
13
1
CH Br), 2.79 (quint, 2 H, CH CH CH ). C{ H} NMR (125.7
phenyl-1,2,4-triazole, were synthesized by following reported
procedures. Synthetic procedures for 1,2,4-triazolium salts C and D
and complexes trans-[PdBr (Mes-tazy-NEt ) ]Br and trans-
2
2
2
2
MHz, CDCl ): δ 172.6 (Ccarbene), 143.1 (NCHN), 136.6, 130.5,
3
1
29.7, 126.3 (Ar-C), 52.5 (NCH ), 33.1 (CH Br), 31.1
2 2
2
3
2
2
1
(CH CH CH ). Data for trans-anti-1a are as follows. H NMR
[
PdBr (Dipp-tazy-NEt ) ]Br have been reported in our recent
2 2 2
2
3
2
2
1
5
13
1
work.
C{ H} NMR spectra of all complex probes were measured
(500 MHz, CDCl
7.63−7.59 (2 H, m-Ar-H), 7.33 (t, 1 H, p-Ar-H), 4.56 (t, JH−H = 6.8
): δ 8.10 (s, 1 H, NCHN), 7.89 (d, 2 H, o-Ar-H),
3
3
in CDCl and internally referenced to the CDCl solvent resonance
3
3
3
signal at 77.7 ppm relative to tetramethylsilane.
Hz, 2 H, NCH
H, CH CH CH
(Ccarbene), 142.7 (NCHN), 136.9, 130.4, 130.2, 127.1 (Ar-C), 51.9
(NCH ), 32.9 (CH Br), 30.7 (CH CH CH ). Anal. Calcd for
C H24Br N Pd: C, 33.09; H, 3.03; N, 10.52. Found: C, 33.38; H,
22 4 6
2
), 3.32 (t, JH−H = 6.5 Hz, 2 H, CH
2
2
Br), 2.46 (quint, 2
1
3
1
4
-Cyclohexyl-1,2,4-trizaole. A mixture of N,N-dimethylforma-
mide azine dihydrochloride (2.15 g, 10 mmol) and cyclohexylamine
0.96 mL, 8.3 mmol) was heated at 120 °C under neat conditions for
4 h. The mixture was cooled to ambient temperature to obtain a
2
2
). C{ H} NMR (125.7 MHz, CDCl ): δ 173.1
3
(
2
2
2
2
2
1
+
solid, which was then suspended in H O (50 mL) and neutralized
3.37; N, 10.48. MS (ESI): calcd for [M − Br] , C22
719; found, m/z 719.
H24Br N Pd m/z
3 6
2
using a dilute potassium hydroxide solution. The resulting mixture
was extracted with dichloromethane (3 × 50 mL). The organic phase
was washed with water (3 × 50 mL) and dried over sodium sulfate.
Volatiles were removed under reduced pressure to give a brown solid,
which was then recrystallized using diethyl ether to obtain the product
trans-[PdBr
analogy to 1a, from B (355 mg, 1 mmol), Ag
and PdBr (133 mg, 0.5 mmol). The product was obtained as a white
solid (355 mg, 0.44 mmol, 88%). H NMR (400 MHz, CDCl
7.96 (s, 1 H, NCHN), 5.16−5.05 (1 H, Cy-H), (4.82, 4.78) (t, 2 H,
NCH ), (3.56, 3.53) (t, 2 H, CH Br), 2.75−2.67 (m, 2 H,
CH CH CH ), 2.54−2.48 (br s, 2 H, Cy-H), 2.01−1.98 (br s, 2 H,
Cy-H), 1.85−1.81 (br s, 1 H, Cy-H), 1.67−1.48 (4 H, Cy-H), 1.35−
(Cy-tazy-Br)
] (1b). Complex 1b was synthesized in
2
2
O (128 mg, 0.55 mmol),
2
2
1
): δ
3
1
as a white powder (0.66 g, 4.4 mmol, 53%). H NMR (300 MHz,
CDCl ): δ 8.19 (s, 2 H, NCHN), 4.07−3.97 (m, 1 H, Cy-H), 2.18−
2
2
3
2
.15 (m, 2 H, Cy-H), 1.96−1.89 (m, 2 H, Cy-H), 1.80−1.74 (m, 2 H,
2
2
2
Cy-H), 1.71−1.57 (m, 2 H, Cy-H), 1.51−1.36 (m, 1 H, Cy-H), 1.34−
1
3
1
13
1
1
.19 (m, 1 H, Cy-H). C{ H}-NMR (75.4 MHz, CDCl ): δ 141.7
1.23 (m, 1 H, Cy-H). C{ H}-NMR (75.4 MHz, CDCl
172.4) (Ccarbene), 140.2 (NCHN), 59.7 (Cy-C), (52.1, 51.9) (NCH
34.5, 33.2, (31.5, 31.1), (26.4, 26.3), 25.8. Anal. Calcd for
Pd: C, 32.60; H, 4.48; N, 10.37. Found: C, 32.44; H,
3
): δ (172.5,
3
(
NCHN), 56.2 (NCH), 34.7, 25.6, 25.5 (CH ). Anal. Calcd for
2
),
2
37
C H N : C, 63.54; H, 8.67; N, 27.79. Found: C, 63.29; H, 7.78; N,
2
1
8
13
3
+
7.45. MS (ESI): calcd for [M + H] , C H N m/z 152; found, m/z
C H36Br N
22 4 6
8
14
3
+
52.
-Phenyl-1-(3-bromopropyl)-1,2,4-triazolium Bromide (A).
4.73; N, 10.66. MS (ESI): calcd for [M − Br] , C22
731; found, m/z 731.
trans-[PdBr (Ph-tazy-NEt ) ]Br (2a). A Schlenk tube was
2 3 2 2
charged with 1a (200 mg, 0.25 mmol), triethylamine (350 μL, 2.5
mmol), and acetonitrile (2 mL). The mixture was heated at 80 °C for
5 h, and then another portion of triethylamine (350 μL, 2.5 mmol)
was added. The mixture was further stirred at 80 °C for 10 h. The
volatiles were then removed under reduced pressure to obtain a white
solid, which was washed with tetrahydrofuran (3 × 5 mL).
Recrystallization from its solution in acetonitrile with diethyl ether
H36Br N Pd m/z
3 6
4
The triazolium salt was synthesized in a manner similar to that
reported previously. 4-Phenyl-1,2,4-triazole (1.45 g, 10 mmol) was
placed in small portions in a round-bottom flask containing 1,3-
dibromopropane (5 mL, 50 mmol) at 70 °C. The mixture was then
stirred at 70 °C for 3 h. After the mixture was cooled to ambient
temperature, diethyl ether (20 mL) was added to form a suspension.
The solid was then collected on a sintered-glass funnel and washed
with diethyl ether (3 × 5 mL) to obtain the product as an off-white
1
solid (3.40 g, 9.8 mmol, 98%). H NMR (CDCl , 500 MHz): δ 12.25
afforded pure 2a (230 mg, 0.23 mmol, 92%). Data for trans-syn-2a are
3
1
(
7
s, 1 H, NCHN), 8.96 (s, 1 H, NCHN), 7.94−7.79 (m, 2 H, Ar-H),
as follows. H NMR (500 MHz, CD CN): δ 8.50 (s, 1 H, NCHN),
3
3
3
.65−7.52 (m, 3 H, Ar-H), 4.92 (t, J
= 6.8 Hz, 2 H, NCH ), 3.57
7.75−7.70 (2 H, Ar-H), 7.55 (t, 1 H, p-Ar-H), 7.44 (t, J
2 H, Ar-H), 4.95 (t, J
= 7.9 Hz,
H−H
2
H−H
3
13
3
(
t, J
= 6.2 Hz, 2 H, CH Br), 2.69 (quint, 2 H, CH CH CH ).
C
= 5.9 Hz, 2 H, NCH ), 3.48 (m, 2 H,
H−H
2
2
2
2
H−H
2
G
DOI: 10.1021/acs.organomet.8b00347
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
CH N), 3.27 (q, J
= 7.2 Hz, 6 H, NCH CH ), 2.64 (br s, 2 H,
C H Br N Pd: C, 38.92; H, 4.90; N, 9.46. Found: C, 38.91; H, 4.90;
24 36 3 5
2
H−H
2
3
3
13
1
+
CH CH CH ), 1.21 (t, J
125.7 MHz, DMSO-d ): δ 169.9 (Ccarbene), 144.4 (NCH N), 135.5,
29.4, 129.0, 124.6 (Ar-C), 53.6, 52.2, 49.7, 21.5, 7.3. Data for trans-
anti-2a are as follows. H NMR (500 MHz, CD CN): δ 8.59 (s, 1 H,
NCHN), 7.98 (d, J
H), 7.66 (t, 1 H, p-Ar-H), 4.60 (t, J
= 7.2 Hz, 9 H, CH3). C{ H} NMR
N, 9.60. MS (ESI): calcd for [M − Br] , C H Br N Pd: m/z 660;
2
2
2
H−H
24 36
2
5
(
1
found, m/z 660.
6
2
i
trans-[PdBr ( Pr -bimy)(Mes-tazy-Br)] (3c). Complex 3c was
2 2
1
synthesized in analogy to 3a using salt C (389 mg, 1 mmol), silver(I)
3
3
i
= 7.9 Hz, 2 H, Ar-H), 7.70−7.75 (2 H, Ar-
oxide (128 mg, 0.55 mmol), and [PdBr ( Pr -bimy)] (469 mg, 0.5
H−H
2
2
2
3
= 6.0 Hz, 2 H, NCH ), 3.09
mmol). The product was purified by column chromatography (SiO2,
dichloromethane), and the complex was isolated as a pale yellow solid
H−H
2
3
(
q, J
= 7.1 Hz, 6 H, NCH CH ), 2.97 (m, 2 H, CH N), 2.25
H−H
2
3
2
3
1
(
broad, 2 H, CH CH CH ), 1.09 (t, J
= 7.1 Hz, 9 H, CH3).
(746 mg. 0.96 mmol, 96%). H NMR (500 MHz, CDCl ): δ 7.93 (s,
2
2
2
H−H
3
1
3
1
3
4
C{ H} NMR (125.7 MHz, DMSO-d ): δ 170.5 (Ccarbene), 144.2
NCH N), 136.1, 129.6, 129.5, 125.3 (Ar-C), 53.2, 52.2, 48.9, 21.5,
1 H, NCHN), 7.52 (dd, J
= 6.7 Hz, J
= 1.9 Hz, 1 H, Ar-H),
6
H−H
H−H
3
4
(
7.42 (dd, JH−H = 6.7 Hz, JH−H = 1.9 Hz, 2 H, Ar-H), 7.17−7.13 (m,
2 H, Ar-H), 7.07 (s, 2 H, Ar-H), 6.01 (sept, JH−H = 7.0 Hz, 1 H,
2
3
7
1
.1. Anal. Calcd for C H Br N Pd·CH Cl : C, 38.72; H, 5.20; N,
3
4
54
4
8
2
2
3
0.32. Found: C, 38.71; H, 5.42; N, 10.40. MS (ESI): calcd for [M −
CH(CH
J
3
)
2
), 5.49 (sept, JH−H = 7.0 Hz, 1 H, CH(CH
)
3
2
), 4.94 (t,
), 3.65 (t, JH−H = 6.1 Hz, 2 H, CH Br),
2.85−2.80 (m, 2 H, CH CH CH ), 2.42 (s, 3 H, CH ), 2.28 (s, 6 H,
+
3
3
Br] , C H Br N Pd m/z 921; found, m/z 921; calcd for [M −
H−H = 6.7 Hz, 2 H, NCH
2
2
34
54
3
8
2+
2
Br] , C H Br N Pd m/z 420; found, m/z 420.
34 54 2 8
2
2
2
3
3
3
trans-[PdBr (Cy-tazy-NEt ) ]Br (2b). Compound 2b was
CH ), 1.75 (d, J
= 7.0 Hz, 6 H, CH(CH ) ), 1.42 (d, J
= 7.0
2
3 2
2
3
H−H
3
2
H−H
1
3
1
synthesized in analogy to 2a using 1b (400 mg, 0.49 mmol) and
triethylamine (2 × 695 μL, 2 × 5 mmol). The product was obtained
as a white powder (480 mg, 0.47 mmol, 96%). H NMR (400 MHz,
Hz, 6 H, CH(CH ) ). C{ H} NMR (125.7 MHz, CDCl ): δ 177.5
3
2
3
(NCNcarbene), 176.18 (NCNprobe), 143.8 (NCHN), 140.3, 137.2,
134.2, 134.1, 132.7, 129.8, 122.6, 113.3, 112.9 (Ar-C), 54.7
(CH(CH ) ), 53.9 (CH(CH ) ), 52.26 (NCH ), 33.4 (CH Br),
1
CD CN): δ (9.08, 9.07) (1 H, NCHN), 5.02−4.96 (1 H, Cy-H), 4.66
3
3
2
3
2
2
2
(
br t, 2 H, NCH ), 3.39−3.35 (2 H, NCH ), 3.25−3.23 (6 H,
31.2 (CH CH CH ), 21.7 (CH ), 21.6 (CH ), 21.1 (CH ), 20.3
2
2
2 2 2 3 3 3
CH CH ), 2.38 (br s, 2 H, CH CH CH ), 2.21−2.13 (2 H, Cy-H),
(CH ). Anal. Calcd for C H Br N Pd: C, 41.75; H, 4.67; N, 9.02.
3 27 36 2 5
2
3
2
2
2
+
1
.98−1.93 (2 H, Cy-H), 1.86−1.71 (3 H, Cy-H), 1.47−1.36 (2 H,
Found: C, 41.75; H, 4.67; N, 8.84. MS (ESI): calcd for [M − Br] ,
1
3
1
Cy-H), 1.32−1.24 (1 H, Cy-H), 1.15−1.08 (9 H, CH CH ). C{ H}
C H Br N Pd, m/z 696; found, m/z 696.
2
3
24 30
2
5
i
NMR (125.7 MHz, DMSO-d ): δ 170.4 (C
), (142.4, 142.3),
trans-[PdBr ( Pr -bimy)(Dipp-tazy-Br)] (3d). Complex 3d was
6
carbene
2 2
8.8, (53.5, 53.4), (52.3, 53.2), 48.9, (32.7, 32.6), (25.5, 25.2), (24.6,
4.5), (21.8, 21.7), (7.19, 7.11). Anal. Calcd for C H Br N Pd: C,
synthesized in analogy to 3a using salt D (430 mg, 1 mmol), silver(I)
i
oxide (128 mg, 0.55 mmol), and [PdBr ( Pr-bimy)] (469 mg, 0.5
3
4
66
4
8
2
2
0.31; H, 6.57; N, 11.06. Found: C, 40.10; H, 6.40; N, 11.34. MS
mmol). The product was purified by column chromatography (SiO2,
dichloromethane/hexane (2/1)) to obtain compound 3d as a pale
+
3
4
66
3
8
2+
1
yellow solid (760 mg, 0.93 mmol, 93%). H NMR (500 MHz,
3
4
66
2
8
3
CDCl ): δ 8.02 (s, 1 H, NCHN), 7.59 (t, 1 H, J
= 7.8 Hz, Ar-H),
3
H−H
i
7.54 (br, d, 1 H, Ar-H), 7.43−7.41 (m, 3 H, Ar-H), 7.15 (m, 2 H, Ar-
2
2
H), 6.09 (br s, 1 H, NCH(CH ) ), 5.34 (br s, 1 H, NCH(CH ) ),
3
2
3 2
i
3
3
5.03 (t, J
= 7.1 Hz, 2 H, NCH ), 3.71 (t, J
= 6.2 Hz, 2 H,
2
2
2
H−H
2
H−H
mL) was stirred in the dark for 12 h. The precipitate was filtered
through a plug of Celite, and the solvent of the filtrate was removed
under reduced pressure. The crude product was purified by column
CH Br), 2.94−2.88 (m, 4 H, CH CH CH and CCH(CH ) ), 1.78
2
2
2
2
3 2
3
3
(br s, 6 H, CH ), 1.37 (d, J
= 7.1 Hz, 12 H, CH ), 1.04 (d, J
3
H−H
3 H−H
1
3
1
= 6.8 Hz, 6 H, CH3). C{ H} NMR (125.7 MHz, CDCl ):δ 178.06
3
chromatography (SiO , dichloromethane) to obtain the pure
(NCN), 176.40 (NCNprobe), 147.9, 144.7, 134.2, 133.9, 32.3, 131.3,
2
1
compound 3a as a pale yellow solid (690 mg, 0.94 mmol, 94%). H
124.4, 122.5, 113.2, 112.9 (Ar-C), 54.7 (NCH(CH ) ), 53.7
3
2
3
NMR (400 MHz, CDCl ): δ 8.16 (s, 1 H, NCHN), 7.95 (dd, J
=
(NCH(CH ) ), 52.5 (NCH ), 33.2 (CH Br), 31.3 (CH(CH ) ),
3
H−H
3 2 2 2 3 2
4
8
.0 Hz, J
= 1.3 Hz, 2 H, Ar-H), 7.64−7.58 (m, 3 H, Ar-H), 7.54
29.2 (CH ), 27.3, 23.1, 21.6, 21.4 (CH ). Anal. Calcd for
2 3
H−H
(
(
m, 1 H, Ar-H), 7.48 (m, 1 H, Ar-H), 7.19−7.14 (m, 2 H, Ar-H), 6.09
C H Br N Pd: C, 44.01; H, 5.17; N, 8.55. Found: C, 44.06; H,
30 42 3 5
3
3
+
sept, J
= 7.0 Hz, 1 H, CH(CH ) ), 5.65 (sept, J
= 8.0 Hz, 1
5.25; N, 8.70. MS (ESI): calcd for [M − Br] , C H Br N Pd, m/z
738; found, m/z 738.
trans-[PdBr ( Pr -bimy)(Ph-tazy-NEt )]Br (4a). A solution of
H−H
3
2
H−H
30 42
2
5
3
3
H, CH(CH ) ), 4.93 (t, J
= 7.1 Hz, 2 H, NCH ), 3.67 (t, J
=
=
3
2
H−H
2
H−H
3
i
6
7
.1 Hz, 2 H, CH Br), 2.83 (m, 2 H, CH CH CH ), 1.77 (d, J
.1 Hz, 6 H, CH ), 1.51 (d, J
2
2
2
2
H−H
2 2 3
3
13
1
= 7.1 Hz, 6 H, CH3). C{ H}
compound 3a (368 mg, 0.5 mmol) in acetonitrile (10 mL) was heated
under reflux, and triethylamine (3 × 210 μL, 3 × 1.5 mmol) was
added in portions over 36 h. The volatiles were then removed under
reduced pressure to obtain a brown solid, which was then washed
with water (3 × 5 mL) and tetrahydrofuran (3 × 2 mL).
Recrystallization from its solution in acetonitrile with diethyl ether
3
H−H
NMR (CDCl , 126.7 MHz): δ 176.3 (NCNprobe), 175.6 (NCNNHC),
3
41.9 (NCN), 136.5, 133.5, 133.4, 129.2, 126.7, 121.9, 112.6, 112.5
3
2
3
2
2
2
2
2
2
3
3
for C H Br N Pd: C, 39.24; H, 4.12; N, 9.53. Found: C, 39.67; H,
2
4
30
3
5
+
4
6
.53; N, 9.03. MS (ESI): calcd for [M − Br] , C H Br N Pd m/z
54; found, m/z 654.
trans-[PdBr ( Pr -bimy)(Cy-tazy-Br)] (3b). Complex 3b was
afforded the pure complex as a pale yellow solid (375 mg, 0.45 mmol,
2
4
30
2
5
1
90%). H NMR (500 MHz, CDCl ): δ 8.35 (s, 1 H, NCHN), 7.92
3
i
(m, 2 H, Ar-H), 7.61−7.60 (m, 3 H, Ar-H), 7.54 (dd, 1 H, Ar-H),
2
2
3
synthesized in analogy to 3a, using salt B (353 mg, 1 mmol),
7.48 (dd, 1 H, Ar-H), 7.20−7.16 (m, 2 H, Ar-H), 5.92 (sept, J
=
H−H
i
3
silver(I) oxide (128 mg, 0.55 mmol), and [PdBr ( Pr -bimy)] (469
7.1 Hz, 1 H, CH(CH ) ), 5.59 (sept,
J
= 7.1 Hz, 1 H,
2
2
2
3
2
H−H
mg, 0.5 mmol). The product was purified by column chromatography
SiO , dichloromethane/hexane (3/1)) and isolated as a pale yellow
CH(CH ) ), 4.96 (t, 2 H, NCH ), 3.62−3.52 (m, 8 H, NCH ), 2.81
3
2
2
2
3
(
(br s, 2 H, CH CH CH ), 1.81 (d, J
= 7.1 Hz, 6 H, CH(CH ) ),
2
2
2
2
H−H 3 2
1
3
3
solid (665 mg, 0.9 mmol, 90%). H NMR (500 MHz, CDCl ): δ 7.97
1.49 (d, J
= 7.1 Hz, 6 H, CH(CH ) ), 1.34 (t, J
= 7.1 Hz, 9
3
H−H
3
2
H−H
1
3
1
(
s, 1 H, NCHN), 7.60−7.55 (m, 2 H, Ar-H), 7.21 (dd, 2 H, Ar-H),
H, CH CH ). C{ H} NMR (125.7 MHz, CDCl ): δ 177.1
2
3
3
3
3
6
.24 (sept, 1 H, J
= 7.0 Hz, CH(CH ) ), 6.17 (sept, J = 7.0
(NCNcarbene), 175.82 (NCNprobe), 143.7 (NCN), 136.8, 134.2,
133.9, 130.3, 130.0, 127.4, 122.9, 113.4, 113.3 (Ar-C), 56.0, 54.8,
54.5, 54.3, 50.5 (NCH), 22.9, (CH CH CH ), 21.9 (CH(CH ) ),
H−H
3
2
H−H
3
Hz, 1 H, CH(CH ) ), 5.21 (m, 1 H, NCH), 4.81 (t, J = 7.0 Hz, 2
3
2
H−H
3
H, NCH ), 3.61 (t, J
= 6.1 Hz, 2 H, CH Br), 2.75−2.70 (m, 2 H,
2
H−H
2
2
2
2
3 2
CH CH CH ), 2.60 (d, 2 H, CH ), 2.03 (d, 2 H, CH ), 1.83−1.80
21.2 (CH(CH ) ), 8.8 (CH CH ). Anal. Calcd for C H Br N Pd:
2
2
2
2
2
3 2 2 3 30 45 3 6
(
m, 12 H, CH ), 1.69−1.62 (m, 2 H, CH ), 1.58−1.48 (m, 2 H,
C, 43.11; H, 5.43; N, 10.05. Found: C, 42.79; H, 5.86; N, 9.85. MS
3
2
1
3
1
+
CH ), 1.34−1.27 (m, 1 H, CH ), 0.87 (m, 1 H, CH ). C{ H} NMR
(
(ESI): calcd for [M − Br] , C H Br N Pd, m/z 755; found, m/z
2
2
2
30 45
2
6
125.7 MHz, CDCl ): δ 177.8 (NCN ), 174.90 (NCNNHC), 140.1
755.
3
probe
i
(
NCHN), 134.2, 134.1, 122.8, 113.4, 113.2 (Ar-C), 59.8 (NCH),
trans-[PdBr ( Pr -bimy)(Cy-tazy-NEt )]Br (4b). Compound 4b
2 2 3
5
3
4.8 (CH(CH ) ), 54.6 (CH(CH ) ), 52.1 (NCH ), 34.4 (CH Br),
was synthesized following the procedure used for 4a using 3b (370
mg, 0.5 mmol) and triethylamine (3 × 210 μL, 3 × 1.5 mmol). The
3
2
3
2
2
2
3.3, 31.2, 26.4, 25.9 (CH ), 21.8 (CH ), 21.6 (CH ). Anal. Calcd for
2
3
3
H
DOI: 10.1021/acs.organomet.8b00347
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2
41
product was isolated as a pale yellow solid (390 mg, 0.46 mmol, 92%).
F| . Anisotropic thermal parameters were refined for the rest of the
non-hydrogen atoms. The hydrogen atoms were placed in their ideal
positions.
1
H NMR (500 MHz, CDCl ): δ 8.20 (s, 1 H, NCHN), 7.60−7.56 (m,
3
3
2
H, Ar-H), 7.24−7.22 (m, 2 H, Ar-H), 6.22 (sept, J
H, CH(CH ) ), 5.98 (sept, J
= 7.2 Hz, 1
3 2
H−H
3
= 7.2 Hz, 1 H, CH(CH ) ), 5.20
Computational Details. Gas-phase structures of all complexes
were optimized by DFT methods using the B3PW91 hybrid
3
2
H−H
3
(
m, 1 H, NCH ), 4.84 (t, J
= 7.1 Hz, 2 H, NCH ), 3.55−3.46 (m,
2
H−H
2
42,43
8
H, NCH ), 2.69 (br, 2 H, CH CH CH ), 2.53 (m, 2 H, CH ), 2.04
functional.
The aug-cc-pvtz basis set was used for H, C, and
The heavy atoms Pd and Br were described by Stuttgart−
Dresden (SDD) relativistic effective core potential and associated
2
2
2
2
2
3
44,45
(
(
m, 2 H, CH ), 1.85 (d, J
= 7.2 Hz, 6 H, CH(CH ) ), 1.82−1.77
N.
2
H−H
3 2
m, 8 H, CH(CH ) and CH ), 1.70 (m, 2 H, CH ), 1.50 (m, 2 H,
CH ), 1.38−1.25 (m, 9 H, CH CH and CH ). C{ H} NMR (125.7
MHz, CDCl ): δ 176.54 (NCNprobe), 175.75 (NCN), 141.2
3
2
2
2
46
1
3
1
basis sets. The imaginary frequency was checked to ensure the
stationary point was a minimum.
2
2
3
2
3
(
5
NCHN), 134.2, 134.1, 123.0, 113.5, 113.3 (Ar-C), 60.1 (NCH),
5.8 (NCH ), 54.9 (NCH ), 54.7 (CH(CH ) ), 54.4 (CH(CH ) ),
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00347.
2
2
3
2
3 2
■
5
0.2 (NCH ), 34.4 (CH ), 26.5 (CH ), 25.8 (CH ), 22.9
2
2
2
2
*
S
(
(
9
CH CH CH ), 21.9 (CH(CH ) ), 21.6 (CH(CH ) ), 8.7
CH CH ). Anal. Calcd for C H Br N Pd: C, 42.80; H, 6.11; N,
.98. Found: C, 40.83; H, 6.54; N, 9.32. MS (ESI): calcd for [M −
2 2 2 3 2 3 2
2
3
30 51
3
6
+
Br] , C H Br N Pd, m/z 761; found, m/z 761.
30
45
2
6
i
trans-[PdBr ( Pr -bimy)(Mes-tazy-NEt )]Br (4c). Compound 4c
2
2
3
1H and
13
1
C{ H} NMR spectra of the salts and
was synthesized in analogy to 4a using 3c (390 mg, 0.5 mmol) and
triethylamine (3 × 210 μL, 3 × 1.5 mmol). The product was isolated
as a pale yellow solid (370 mg, 0.42 mmol, 84%). H NMR (500
complexes, crystallographic data, and solid-state struc-
tures (PDF)
1
3
MHz, CDCl ): δ 8.05 (s, 1 H, NCHN), 7.52 (dd, J
= 6.8 Hz,
= 2.0 Hz,
H−H
3
H−H
4
3
4
JH−H = 2.0 Hz, 1 H, Ar-H), 7.42 (dd, J
= 6.8 Hz, J
Accession Codes
H−H
1
H, Ar-H), 7.18−7.14 (m, 2 H, Ar-H), 7.06 (s, 2 H, Ar-H), 5.80
CCDC 1838402−1838406, 1838406, and 1838408 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
3
3
(
sept, J
= 7.0 Hz, 1 H, CH(CH ) ), 5.40 (sept, J
= 7.0 Hz, 1
H−H
3
2
H−H
3
H, CH(CH ) ), 4.95 (t, J
= 7.1 Hz, 2 H, NCH ), 3.61−3.54 (8
3
2
H−H
2
H, CH CH and NCH ), 2.73 (br s, 2 H, CH CH CH ), 2.40 (s, 3 H,
CH ), 2.28 (s, 6 H, CH ), 1.75 (d, J
2
3
2
2
2
2
3
= 7.0 Hz, 6 H, CH(CH ) ),
3
3
H−H
3 2
3
3
1
.37 (d, J
= 7.0 Hz, 6 H, CH(CH ) , 1.33 (t, J
= 7.1 Hz, 9 H,
H−H
H−H
3
2
1
3
1
CH CH ). C{ H} NMR (125.7 MHz, CDCl ): δ 178.29 (NCN),
2
3
3
1223 336033.
1
1
5
2
74.74 (NCNprobe), 144.8 (NCHN), 140.6, 136.8, 134.1, 133.9, 132.3,
29.9, 122.7, 113.3, 113.0 (Ar-C), 55.9 (NCH ), 54.7 (CH(CH3)2),
2
4.4 (NCH ), 54.0 (CH(CH ) ), 50.4 (NCH ), 23.0 (CH CH CH ),
AUTHOR INFORMATION
Corresponding Author
E-mail for H.V.H.: chmhhv@nus.edu.sg.
2
3
2
2
2
2
2
■
1.8 (CH ), 21.7 (CH(CH ) ), 21.0 (CH(CH ) ), 20.2 (CH ), 8.70
3
3
2
3
2
3
(
CH ). Anal. Calcd for C H Br N Pd·2H O: C, 43.37; H, 6.07; N,
.20. Found: C, 43.95; H, 5.84; N, 9.06. MS (ESI): calcd for [M −
+
3 33 51 3 6 2
*
9
Br] , C H Br N Pd, m/z 797; found, m/z 797.
ORCID
3
3
51
2
6
i
trans-[PdBr ( Pr -bimy)(Dipp-tazy-NEt )]Br (4d). Compound
2
2
3
Van Ha Nguyen: 0000-0002-6426-4385
Bassam M. El Ali: 0000-0003-4005-6386
Han Vinh Huynh: 0000-0003-4460-6066
4
d was synthesized in analogy to 4a using 3d (410 mg, 0.5 mmol) and
triethylamine (3 × 210 μL, 3 × 1.5 mmol). The product was isolated
as a pale yellow solid (380 mg, 0.41 mmol, 82%). H NMR (500
1
MHz, CDCl ): δ 8.07 (s, 1 H, NCHN), 7.58 (t, 1 H, Ar-H), 7.53 (br
Notes
3
s, 1 H, Ar-H), 7.41−7.38 (m, 3 H, Ar-H), 7.16 (br s, 2 H, Ar-H), 5.87
The authors declare no competing financial interest.
(
br s, 1 H, CH(CH ) ), 5.24 (br s, 1 H, CH(CH ) ), 5.02 (t, 2 H,
3 2 3 2
NCH ), 3.66−3.58 (m, 8 H, NCH and NCH CH ), 2.85−2.79 (m, 4
2
2
2
3
ACKNOWLEDGMENTS
■
H, CH CH CH and CCH(CH ) ), 1.79 (s, 9 H, CH ), 1.36−1.31
2
2
2
3
2
3
3
13
1
(
(
m, 18 H, CH ), 0.99 (d, J
= 6.8 Hz, 6 H, CH ). C{ H}-NMR
The work reported in this paper 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.
3
H−H
3
CDCl , 125.7 MHz): δ 179.1 (NCN), 174.9 (NCNprobe), 174.7,
3
1
5
4
45.6, 131.9, 131.6, 124.7, 122.8, 115.5, 113.1 (Ar-C), 55.9 (NCH2),
4.9 (CH(CH ) ), 54.5 (NCH ), 53.8 (CH(CH ) ), 50.5 (NCH ),
3
2
2
3
2
2
6.7, 29.4 (CH(CH ) ), 27.2 (CH(CH ) ), 23.0, 22.9, 21.7, 21.4, 8.6
3
2
3 2
(
CH ). Anal. Calcd for C H Br N Pd: C, 47.00; H, 6.25; N, 9.13.
3 36 57 3 6
REFERENCES
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Found: C, 46.91; H, 6.56; N, 8.96. MS (ESI): calcd for [M − Br] ,
C H Br N Pd, m/z 839; found, m/z 839.
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DOI: 10.1021/acs.organomet.8b00347
Organometallics XXXX, XXX, XXX−XXX