Platinum(II) 1,2,4-Triazolin-5-ylidene Complexes: Stereoelectronic Influences on Their Catalytic Activity in Hydroelementation Reactions

Article abstract of DOI:10.1021/acs.organomet.0c00260

Five platinum(II) 1,2,4-triazolin-5-ylidene (tazy) complexes of the general formula [PtCl2(DMSO)(R-tazy)] (1-5), bearing different N4 substituents (R = Dipp (1), Mes (2), Ph (3), Nap (4), and Bn (5)), have been successfully synthesized. The compounds have

Full text of DOI:10.1021/acs.organomet.0c00260

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Platinum(II) 1,2,4-Triazolin-5-ylidene Complexes: Stereoelectronic
Influences on Their Catalytic Activity in Hydroelementation
Reactions
Van Ha Nguyen,* Thanh Tuan Dang, Hung Huy Nguyen, and Han Vinh Huynh*
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ABSTRACT: Five platinum(II) 1,2,4-triazolin-5-ylidene (tazy) complexes of the
general formula [PtCl2(DMSO)(R-tazy)] (1−5), bearing different N4 substituents
(R = Dipp (1), Mes (2), Ph (3), Nap (4), and Bn (5)), have been successfully
synthesized. The compounds have been fully characterized by means of ESI-MS
mass spectrometry, NMR spectroscopy, elemental analysis, and X-ray diffraction
analysis. The presence of two rotamers in complex 4 was elucidated using
multinuclear magnetic resonance spectroscopy (1H, 13C NMR) spectroscopy and
theoretical calculations. The five complexes were tested for their catalytic activities in
the hydration and hydrosilylation of phenylacetylene. Significant differences in
catalytic performance were observed. In the hydration of phenylacetylene, only
Markovnikov products were formed with yields ranging from poor to moderate with
compound 3 performing the best. In the hydrosilylation reaction, both Markovnikov
(α) and anti-Markovnikov (β-E) products were obtained. While complex 1 was not
very active, reactions catalyzed by complex 5 show complete conversions. A detailed study on the steric and electronic properties of
the five triazolinylidenes have been carried out to rationalize the different catalytic performances of their complexes.
Among the four classical NHCs, imidazole and 1,2,4-
triazole-derived carbenes are most alike as the latter is formed
by the isolobal replacement of a CH group by a N atom. In
addition, these two types of NHCs were the first to be isolated
as the free base by Arduengo et al.27and Enders et al.,28
respectively. Moreover, Enders et al.’s 1,3,4-triphenyltriazolin-
5-ylidene was even the first free commercially available
carbene.28 Nevertheless, catalytic applications of 1,2,4-
triazolin-5-ylidene complexes are much less explored in
comparison to those of imidazolin-2-ylidenes. Hardly any
PtII-triazolin-5-ylidenes have been catalytically tested with the
exception of [Pt(NHC)(py)I2] (NHC = 1,4-di(n-butyl)-1,2,4-
triazolin-5-ylidene; py = pyridine), which was active in the
diboration of cyclic alkenes.29
As part of our current interest in exploring the coordination
chemistry and catalytic applications of 1,2,4-triazole-derived
carbenes, we herein report the syntheses and characterizations
of five platinum(II) 1,2,4-triazolin-5-ylidene complexes. More-
over, their catalytic activities in the hydration and hydro-
silylation of phenylacetylene is reported. A detailed study on
INTRODUCTION
■
N-heterocyclic carbene (NHC) complexes of late transition
metals have attracted great interest due to their potential
applications in the development of bioactive compounds,1,2
light emitting materials,3−5 and especially homogeneous
catalysis.6−10 Their popularity can be attributed not only to
the ease of their preparation but more importantly to the
excellent tunability in sterics and electronics of NHC ligands in
general.8,11,12 Many of the complexes have proven to be
effective catalysts for various organic transformations. Prom-
inent examples include Ru−NHC complexes in olefin
metathesis,13,14 Ir/Ru−NHCs in hydrogenation and hydrogen
transfers,15−17 Rh−NHCs in hydrosilylation,18 and Au−NHCs
in the activation of π-bonds.19 This list would be incomplete
without mentioning the importance of group 10 metals,
especially palladium. A huge collection of palladium(II) NHC
complexes have been synthesized and tested in homogeneous
catalysis, where they have shown to affect a wide range of
reactions such as C−C cross-coupling reactions, Hartwig−
Buchwald aminations, and olefin polymerizations.6,7,20−23 In
contrast to the wide popularity of Pd−NHCs, applications of
their heavier homologues in catalysis are much less established,
although a notable number of platinum(II)−NHC complexes
have been successfully employed as catalysts in hydro-
elementation reactions of alkenes or alkynes and in cyclo-
isomerizations.24−26
Received: April 14, 2020
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the stereoelectronic properties of these carbenes have also
been conducted in an attempt to rationalize the observed
differences in the catalytic performances of their complexes.
of expected complexes were first suggested by ESI mass
spectrometry, where base peaks assignable for [M + H2O]+
cations were observed at m/z of 681 (1), 639 (2), 597 (3), 647
(4), and 611 (5), respectively. In addition, less intense peaks
for [M − Cl]+ cations were also observed at m/z of 628, 586,
544, 594, and 558 in the spectra of complexes 1−5.
RESULTS AND DISCUSSION
■
Synthesis of Platinum(II) 1,2,4-Triazolin-5-ylidene
(tazy) Complexes. Five 4-substituted 1,2,4-triazoles bearing
2,6-diisopropylphenyl (Dipp), mesityl (Mes), phenyl (Ph),
naphthyl (Nap), and benzyl (Bn) substituents were prepared
following a reported procedure.30 Nucleophilic substitution
reactions between the triazoles and benzyl bromide afforded
the respective triazolium salts A−E in excellent yields (Scheme
1). The formation of the salts were evidenced by characteristic
1
Their H NMR spectra show signals due to the NHC and
DMSO ligands in a 1:1 ratio. Notably, the two methyl groups
of the DMSO molecule become inequivalent upon coordina-
tion and give rise to two distinct signals. This observation
suggests a hindered rotation around the Pt−S bonds in the
complexes. Finally, signals for the carbene donors were
observed in their 13C NMR spectra at 152.5 (1), 151.8 (2),
151.1 (3), 152.9/152.2 (4), and 151.2 (5) ppm, respectively.
The NMR spectra of complex 4 bearing the 1-benzyl-4-
naphtyl-1,2,4-triazolin-5-ylidene ligand are particularly interest-
ing and show two sets of signals suggesting the presence of two
inseparable isomers. Considering the unsymmetrical nature of
the N-naphthyl substituent, two rotamers 4a and 4b can be
proposed, which differ in the orientation of this aryl group with
respect to the DMSO ligand (Figure 1). It is interesting to note
Scheme 1. Synthesis of Platinum(II) Triazolin-5-ylidene
Complexes
downfield signals at 12.57 ppm (A), 12.49 ppm (B), and 12.70
ppm (C) in their 1H NMR spectra. These signals are assigned
to the acidic C5−H proton of the heterocyclic ring. In
addition, their ESI mass spectra show base peaks assignable to
the molecular [M − Br]+ cations further corroborating the
identities of the triazolium salts. It is noted that, even in the
presence of excess benzyl bromide, the alkylation of the third
nitrogen of the heterocycle was not observed. In fact, such a
reaction would only occur with much stronger electrophiles,
e.g., trialkyl oxonium salts.31
1
Figure 1. Aliphatic region of H NMR spectrum of 4, showing the
presence of two rotamers 4a and 4b in a 2:1 ratio.
The syntheses of the cis-[PtCl2(DMSO)(NHC)]-type
complexes from the triazolium salts were carried out via the
silver carbene transfer pathway.32 The triazolium salts were
first stirred with silver(I) oxide to form silver carbene
complexes, which were then transferred into a suspension of
K2PtCl4 (1 equiv) in dimethyl sulfoxide followed by stirring at
ambient temperature for 12 h. The products were purified by
silica gel chromatography with dichloromethane as eluent and
isolated as white solids in good yields (77−86%). All the five
complexes are stable in air at ambient temperature and show
good solubilities in dichloromethane, chloroform, acetone, and
dimethyl sulfoxide. They are slightly soluble in methanol and
ethanol but insoluble in water or solvents with a low polarity
such as tetrahydrofuran, hexane, or diethyl ether.
that the naphthyl ring can act as a NMR probe allowing for the
convenient assignment of proton resonances to the two
rotamers. In 4a, the naphthyl group is closer to the DMSO
ligand. Since the two methyl groups are pointing in different
directions, only one of them will be exposed to the anisotropic
(shielding) effect of the naphthyl group, and a greater chemical
shift difference is observed. On the other hand, the naphthyl
group is oriented away from the DMSO ligand in 4b. Here,
neither of the two methyl groups is strongly affected by the
anisotropy of the aromatic substituent, and the signal
separation is less pronounced (Figure 1). On the basis of
this, a 4a/4b ratio of 2:1 was established for a solution of 4 in
CDCl3 at room temperature.
To estimate the energy barrier for the rotameric
interconversion, DFT calculations were carried out. Several
geometries of complex 4 were optimized with various fixed
dihedral angles (from 0° with 30° incremental steps) between
the naphthyl and triazolinylidene planes in order to build the
The identity of all the complexes were fully established by
means of multinuclear magnetic resonance spectroscopy (1H,
13C NMR), mass spectrometry (ESI-MS), elemental analysis,
and single crystal X-ray analysis (for 3 and 4). The formation
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energy profile for 4 for the rotation of the naphthyl group
around the CN bond (Figure S1). Two minima are associated
with this rotation, and the first minimum is achieved at a
dihedral angle of 120°, which corresponds to the geometry of
rotamer 4a. The second minimum, assignable to rotamer 4b, is
established at a dihedral angle of 270°. The two minima are
separated by a 47 kJ/mol rational barrier. In addition, the
calculations also show that 4a is 2.5 kJ/mol lower in energy in
comparison to 4b. This energy difference translates into an
equilibrium constant (Keq) of 0.4, which agrees well with the
experimentally observed ratio for 4b/4a of 0.5 (vide supra).
Notably, the solid-state molecular structure of 4 obtained from
single-crystal X-ray diffraction analysis (vide infra) resembles
that of the more stable form 4a.
safer alternatives, and various complexes of Ru, Au, Pt, Pd, and
Ir have been tested as catalysts for such transformations.36−39
Beside gold(I) and gold(III),36,40 platinum(II)-based com-
pounds are considered the better choice for catalyst develop-
ments. Chatt et al. first reported the hydration of alkynes in
ethanolic solutions in the presence of Na2PtCl4·H2O.41
Thereafter, platinum(II) complexes ligated by olefins,
phosphines, or NHCs including Zeise’s [PtCl2(C2H4)]2
dimer,42 [PtCl2(PAr3)2],43 [PtCl2(DMSO)(imidazolin-2-yli-
dene)], and [PtCl2(bis-imidazolin-2-ylidene)]26 were demon-
strated to be active as well.
Encouraged by this, our five complexes 1−5 were evaluated
for their activity in the catalytic hydration of phenylacetylene
as a standard test reaction. To find suitable reaction conditions,
several experiments were carried out using complex 5 as a
chosen catalyst precursor due to its easy preparation. The
results of these optimization experiments are summarized in
Table 1.
Molecular Structures. Single crystals of 3 and 4 were
obtained by slow evaporation of their respective solutions in
chloroform (for 3) or chloroform/hexane (for 4). Their solid-
state molecular structures are presented in Figure 2. Both
a
Table 1. Optimization of Reaction Conditions
c
entry
solvent
cat. mol % T (°C) time (h) yield
1
2
3
4
5
6
7
H2O
H2O
1
2
2
2
2
2
3
60
80
80
80
80
6
12
12
12
12
12
12
trace
11
12
25
26
b
dioxane/H2O (2:1)
THF/H2O (2:1)
acetone/H2O (2:1)
EtOH/H2O (2:1)
EtOH/H2O (2:1)
80
100
36
50
a
Reaction conditions: 0.5 mmol of phenylacetylene, 1.5 mL of
b
c
Figure 2. Molecular structures of 3 and 4. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (deg). 3:
Pt−C1 1.980(7), Pt−Cl1 2.327(2), Pt−Cl2 2.348(2), and Pt−S1
2.211(2) and C1−Pt−Cl2 177.2(2), C1−Pt−Cl1 88.9(2), C1−Pt−
S1 91.1(2), and S1−Pt−Cl1 177.28(8); 4: Pt−C1 1.987(7), Pt−Cl1
2.325(2), Pt−Cl2 2.355(2), and Pt−S1 2.205(2) and C1−Pt−Cl2
176.7(2), C1−Pt−Cl1 87.1(2), C1−Pt−S1 91.8(2), and S1−Pt−Cl1
178.25(7).
solvent. v/v ratio. Yields were determined by NMR spectroscopy.
In the initial test, negligible conversion of phenylacetylene
was obtained for 6 h when only water was used as reactant/
solvent at 70 °C in the presence of complex 5 (1 mol %, entry
1). At higher catalyst loading (2 mol %) and increased
temperature (80 °C), the formation of acetophenone in 11%
yield was observed after 12 h (entry 2). The yield did not
change significantly in dioxane/H2O (entry 3). When THF/
H2O or acetone/H2O mixtures were used as solvents, the
yields improved but remain unsatisfactory (entries 4 and 5).
An improvement was achieved when the reaction was
conducted in ethanol/H2O (entry 6). Finally, a modest yield
of 50% can be reached at a 3 mol % catalyst loading, 100 °C,
and a reaction time of 12 h. The addition of water follows
Markovnikov’s selection rule, and the anti-Markovnikov
product (phenylacetaldehyde) was only observed in trace
amounts. Using these conditions, the catalytic efficiencies of all
five complexes in the hydration of phenylacetylene were
compared and summarized in Table 2.
The results show that complexes 1 and 2 show poor
reactivities giving <10% yields (entries 1 and 2). A slightly
better yield was obtained with 4 (entry 4), but complex 5
remains superior with a moderate yield of 50%. Complex 3, on
the other hand, performed better giving a significantly better
yield of 84%. Using this best precatalyst, complete conversion
of phenylacetylene into acetophenone was achieved when the
reaction time was extended to 24 h (entry 6). Notably, all
complexes are outperformed by cis-[PtCl2(DMSO)(NHC)]
complexes bearing functionalized imidazolin-2-ylidenes, which
structures are consistent with the formula cis-[PtCl2(DMSO-
κS)(tazy)] with platinum(II) centers in square-planar geo-
metries. The metal in each complex is coordinated by a NHC,
a sulfur-bound DMSO, and two chlorido ligands in a cis-
configuration. The Pt−Ccarbene bond distances are essentially
identical within an error margin {1.980(7) Å in 3; 1.987(7) Å
in 4} and in the range reported for other Pt−Ccarbene distances
for similar [PtCl2(DMSO)(NHC)] complexes.33,34 The Pt−
Cl2 distances {2.348(2) Å in 3; 2.355(2) Å in 4} are slightly
longer than the Pt−Cl1 ones {2.327(2) Å in 3; 2.325(2) Å in
4}, which is in line with the stronger trans influence exerted by
the carbenes. The heterocycles are almost perpendicular to the
coordination planes with dihedral angles of 75° (3) and 78°
(4). In both cases, the N-aromatic substituents (phenyl in 3
and naphthyl in 4) are twisted from the carbene plane with
angles of 60° and 74°, respectively.
Catalytic Hydration of Phenylacetylene. The hydration
of terminal alkynes to form ketones or aldehydes is arguably
one of the most important C−O coupling reactions in organic
synthesis. Traditionally, such reactions can be achieved using
mercury-based catalysts.35,36 Due to the potential toxicity of
mercury compounds, great efforts has been devoted to develop
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do not show any preference, leading to essentially equimolar
α/β(E) ratios. On the other hand, reactions catalyzed by 5
favor the anti-Markovnikov β(E) product with α/β(E) ratios of
close to 1:2. Notably, the performance of complex 5 is superior
to other reported PtII−NHC complexes in terms of turnover
number including cis-[PtBr2(DMSO)(NHC)] (NHC =
benzimidazole/imidazole-derived carbene), cis-
[PtI2(imidazolin-2-ylidene)2], cis-[PtX2(diimidazolin-2-yli-
dene)] (X = Br, I),44 and the [PtCl2(Im-Oz)] complex,
which features a bidentate oxazole-functionalized imidazolin-2-
ylidene.45
Stereoelectronic Profiling of the Triazole-Derived
Carbenes. Although the five complexes 1−5 have very similar
structural features and merely differ in the N4 substituent, their
catalytic performances in the two addition reactions discussed
above vary dramatically. In order to understand the origin of
these catalytic differences, the stereoelectronic profiles of the
five NHCs were investigated. The steric bulk of the carbenes
was quantified using the calculated percentage volume buried
measure (%Vbur),46,47 while Huynh’s electronic parameter
(HEP)48,49 was utilized to evaluate their electronic properties.
In order to estimate %Vbur values of the five NHCs in their
PtII complexes, gas-phased optimized geometries of 1−5 were
first obtained using DFT calculations. The optimized geo-
metries are presented in Figure 3 (see xyz file in the supporting
Table 2. Performances of Complexes 1−5 in the Catalytic
a
Hydration of Phenylacetylene
b
entry
catalyst
time [h]
yield
1
2
3
4
5
6
cis-[PtCl2(DMSO)(Dipp-tazy)] (1)
cis-[PtCl2(DMSO)(Mes-tazy)] (2)
cis-[PtCl2(DMSO)(Ph-tazy)] (3)
cis-[PtCl2(DMSO)(Nap-tazy)] (4)
cis-[PtCl2(DMSO)(Bn-tazy)] (5)
cis-[PtCl2(DMSO)(Ph-tazy)] (3)
12
12
12
12
12
24
7
7
84
19
50
100
a
Reaction conditions: 0.5 mmol of phenylacetylene, 1 mL of EtOH,
b
0.5 mL of H2O. Yields determined by NMR spectroscopy for an
average of two runs.
offered better yields at lower catalyst loadings and shorter
reaction times.26
Catalytic Hydrosilylation of Phenylacetylene. The
hydrosilylation of alkynes is one of the most important
pathways to produce organosilicon compounds. Therefore, the
five new complexes were also tested in this reaction. The
relatively inert bis(trimethylsiloxy)methylsilane was used as
silylation reagent for phenylacetylene, and the results are listed
in Table 3.
Table 3. Performances of Complexes 1−5 in the Catalytic
a
Hydrosilylation of Phenylacetylene
b
entry cat. loading (%) T (°C) time (h) conversion (%) a/b ratio
1
2
3
4
5
6
7
8
5
1
2
3
4
1
2
3
4
5
5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
80
80
80
80
80
110
110
110
110
100
110
2
2
2
2
2
2
2
2
2
2
1
80
1:1.8
trace
trace
trace
trace
38
91
63
54
100
100
1:1.1
1:1.0
1:1.0
1:1.1
1:1.8
1:1.6
9
10
11
Figure 3. Gas-phased optimized geometries of complexes 1−5.
a
Reaction conditions: 1 mmol of phenylacetylene, 1.1 mmol of
b
information). Selected bond lengths and angles are listed in
Table S1. The results show that all the complexes adopt
slightly distorted square planar geometries. The Pt−C and Pt−
S bond lengths are in the range of 1.990−2.002 and 2.312−
2.319 Å, respectively. The elongation of the Pt−Cl bonds trans
to the NHC (2.431−2.433 Å) compared to the other Pt−Cl
bonds (2.379−2.384 Å) is in line with the stronger trans
influence exerted by the NHC as compared to the DMSO
ligand. The calculations accurately predict the Pt−C distance
{1.980 Å (for both 3 and 4) compared to experimental values
of 1.980(7) Å (3) and 1.987(7) Å (4)} but slightly
overestimate the other metal−ligand bond distances. For
example, Pt−Cl1 distances are 2.3409 and 2.3423 Å compared
to the experimental values of 2.327(2) and 2.325(2) Å for 3
and 4, respectively. In the optimized geometries, dihedral
angles formed between phenyl and naphthyl planes with the
heterocycles are 54° and 64° compared with experimental
angles of 60° and 74° for 3 and 4, respectively. Overall, the gas-
(Me3SiO)2MeSiH, 1 mL of toluene. Yields were determined by
NMR spectroscopy.
The initial hydrosilylation of phenylacetylene was tested
using complex 5. At 0.5 mol % catalyst loading, a conversion of
80% was observed after 2 h of stirring at 80 °C (entry 1).
Surprisingly, complexes 1−4 are inactive under these
conditions and only gave trace amounts of products (entries
2−5). As the temperature is increased to 110 °C, a poor
conversion of 38% was observed for precatalyst 1 (entry 6),
while better conversions of 63% and 54% were afforded using
catalysts 3 and 4, respectively (entries 8 and 9). On the other
hand, an excellent conversion (91%) was achieved with
complex 2 (entry 7). Complex 5 still stood out as the most
active precatalyst leading to 100% conversion at 100 °C for 2 h
(entry 10) or at 110 °C for 1 h (entry 11). All the
hydrosilylation reactions only gave Markovnikov α and anti-
Markovnikov β(E) silylation products. The precatalysts 1−4
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Table 4. Calculated %Vbur Values for the Five NHCs
R
%Vbur
Dipp-tazy
32.5
Mes-tazy
30.2
Ph-tazy
29.9 (28.4)
Nap-tazy
29.3 (4a)/30.3 (4b) (29.0)
Bn-tazy
27.5
a
a
a
Calculated using molecular structures determined by single crystal X-ray diffraction.
phased optimized geometries resemble the experimentally
determined solid-state molecular structures.
Using the optimized geometries of 1−5 as input, %Vbur
values of the five NHCs were calculated using the SambVca
webtool47 and are listed in Table 4. Consistent with chemical
intuition, the highest %Vbur of 32.5% was observed for Dipp-
tazy, the carbene with the bulkiest N4-substituent, followed by
Mes-tazy (30.2%), Ph-tazy (29.9%), and Nap-tazy (29.3%/
30.3%). The least bulky NHC is the dibenzylated species, Bn-
tazy, with a %Vbur value of 27.5%.
The electronic properties of the five triazole-derived
carbenes were also examined using Huynh’s methodology. In
this method, the donor strength of a monodentate ligand L is
13
Figure 4. Donor strength comparison of the five triazolin-5-ylidenes
on the HEP scale.
evaluated using the
C
NMR chemical shift of the 1,3-
carbene
diisopropylbenzimidazolin-2-ylidene (iPr2-bimy) reporter in
complexes of the type trans-[PdBr2(iPr2-bimy)(L)].48,49
A
stronger donor L would lead to a relatively more downfield
shifted HEP signal, whereas a more upfield signal is obtained
for a weaker donor. With the exception for Bn-tazy, whose
donating ability has previously been reported,48 the donor
strengths of the remaining NHCs were evaluated using the new
trans-[PdBr2((iPr2-bimy)(tazy)] complex probes {tazy = Dipp-
tazy (6), Mes-tazy (7), Ph-tazy (8), and Nap-tazy (9)}. Their
syntheses according to Scheme 2 are rather straightforward and
In general, HEP values for the five triazolin-5-ylidenes fall in
the range from 176.1 to 176.6 ppm, which are generally more
upfield compared to reported values for imidazoline-,
imidazole- and benzimidazole-derived carbenes, suggesting
their weaker donating nature. This is not surprising, since
triazolin-5-ylidenes contain an additional electronegative
nitrogen atom in their heterocycle.
A closer examination reveals that the donor strength of the
five carbenes are influenced by the N4-substituent following
the sequence Mes-tazy (176.09 ppm) < Ph-tazy (176.21 ppm)
< Dipp-tazy (176.41 ppm) < Bn-tazy (176.60 ppm) = Nap-tazy
(176.64 ppm) (Figure 4). Consistent with chemical intuition,
Bn-tazy is among the stronger donors since it has an alkyl
group (Bn) instead of less electron-releasing aryl groups as the
N4-substituent. Our results also show that Dipp-tazy is more
donating than Mes-tazy. However, it must be noted that
differentiating these two aryl groups is difficult, because they
differ in the number and nature of alkyl substituents (i.e., two
isopropyl vs three methyl groups).
Scheme 2. Preparation of HEP Complex Probes for Donor
Strength Determinations
Moreover, we have previously reported that aromatic
wingtips can have varying degrees of mesomeric (resonance)
contributions dependent on the dihedral angle with respect to
the NHC plane. For example, a 4-phenyltriazolin-5-ylidene can
be a superior donor compared to mesityl- or 2,6-
diisopropylphenyl-substituted analogues due to a smaller
dihedral angle, which allows π-overlap with the NHC
plane.51 Mesityl and 2,6-diisopropylphenyl groups are
preferably perpendicular to the NHC plane, while the sterically
less encumbered phenyl ring has greater rotational freedom
along the N−C bond. Better alignment of the triazolinylidene
and phenyl plane (θ → 0) has little impact on the σtazy orbital
and the lone pair of the carbene but allows significant
interactions between the two energy-matched πtazy and πPh
orbitals (Figure 5). This enhances the +M effect of the phenyl
ring, and a more electron-rich carbene is obtained.51 This
could explain the slightly increased donicity of Ph-tazy
compared to Mes-tazy.
involve one-pot reactions between the parent [PdBr2(iPr2-
bimy)]2 dimer,50 silver(I) oxide, and the respective triazolium
salts (A−D·HBr). The desired trans-[PdBr2((iPr2-bimy)-
(tazy)] complexes (6−9) were obtained in good yields and
purified by silica gel column chromatography using dichloro-
methane as eluent. The formation of the complexes were
confirmed by 1H, 13C NMR spectroscopy, ESI mass
spectrometry, and elemental analysis (see the Supporting
Information).
In each of the complexes, there are two carbene carbon
resonances, which belong to triazolinylidene and benzimida-
zolinylidene, respectively. These are separated by only 0.8−1.7
ppm, and an unambiguous assignment of the HEP signals (i.e.,
i
for Pr2-bimy) were made with reference to reported trans-
[PdBr2(iPr2-bimy)(tazy)] complexes51 and by two-dimensional
heteronuclear multiple bond correlation (HMBC) NMR
spectroscopy. The HEP values for the five triazolin-5-ylidenes
in this study are compared in Figure 4.
In addition, it is notable that Nap-tazy is among the
strongest donors in this series. While there are hundreds of
studies on the electronic properties of various carbenes using
TEP (Tolman's electronic parameter), LEP (Lever's ligand
E
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a favorable energy match between σtazy and πNap, which is much
higher in energy compared to πPh. The facile interaction
between the πNap and σtazy would lead to an increase in donor
strength of the Nap-tazy ligand. In summary, the calculations
suggest that donicity enhancement of this ligand occurs via
direct πNap/σtazy and πNap/πtazy interactions.
Figure 5. Donor strength of Ph-tazy is enhanced by πtazy−πPh
interactions.
It is interesting to note that the relative donor strengths of
the five carbenes are well differentiated by HEP in agreement
with DFT calculations. Recently, it has been suggested that the
electrochemical parameter), and HEP scales,49 the effects of
polyaromatic N-substituents on the donor/acceptor properties
of carbenes remain largely unexplored. Although TEP values
have been reported for naphthyl-substituted imidazolidin-2-
ylidenes, there was no elaboration on the influence of such
polyaromatic wingtips.52 The structural similarity between
phenyl and naphthyl substituents would suggest a comparable
impact influenced by the rotational freedom of naphthyl along
the C−N bond. To gain insight into this hypothesis, DFT
calculations were carried out with a specific focus on the
changes of the Nap-tazy’s frontier orbitals as the aromatic
group undergoes C−N bond rotation (see the Experimental
Section for more details). To simplify the calculation, the
flexible N1-benzyl substituent was replaced by a smaller methyl
group.
1
donicity of NHCs can also be evaluated using the JC−H
coupling constants of their azolium salts.53,54 However, only
marginal differences in 1JC−H coupling constants of <1 Hz were
observed for our triazolium salts. Following this approach, the
donor strengths of the five triazolinylidenes would increase in
the order of Nap-tazy ∼ Dipp-tazy (228 Hz) < Mes-tazy ∼ Ph-
tazy (227 Hz) < Bn-tazy (226 Hz). Notably, the pairs Nap-
tazy/Dipp-tazy and Mes-tazy/Ph-tazy cannot be differentiated.
Moreover, a range of only 2 Hz is obtained with all triazolium
salts, which could be well within the error margin, implying
1
that JC−H coupling constants are not suitable for the
unambiguously differentiation of the NHCs in this study.
Some limitations of this methodology have been noted
previously.49,55,56
Overall, the five complexes perform very differently in the
catalyzed hydration and hydrosilylation of phenylacetylene. In
the hydration reactions, it appears that compounds with less
bulky and not too electron donating carbenes give rise to the
best catalyst. On the other hand, the best catalyst for
hydrosilylation reactions is the one bearing the least bulky
and most donating carbene.
The key results are summarized in Figure 6, and the change
in energy and nature of the frontier orbitals are presented in
CONCLUSION
■
A series of 1,2,4-triazolin-5-ylidene complexes of platinum(II)
with the general formula of [PtCl2(R-tazy)(DMSO)] bearing
different carbene wingtips R (R = Dipp (1), Mes (2), Ph (3),
Nap (4), and Bn (5)) has been successfully synthesized. Their
structures have been fully characterized by means of ESI mass
spectrometry, 1H, 13C NMR spectroscopy, and elemental
analysis. For 3 and 4, their structures were unambiguously
confirmed by X-ray diffraction analysis.
Figure 6. Simplified model showing the changes of frontier orbitals of
Nap-tazy due to rotation.
The complexes can catalyze the hydration of phenyl-
acetylene giving only Markovnikov’s product, however, with
poor to moderate yields. In this reaction, complex 3 gives rise
to the most active catalyst. The complexes also demonstrated
catalytic activities in the hydrosilylation of phenylacetylene.
Here, complex 5 was found to be the most active. A detailed
analysis on the stereoelectronic properties of the five triazole-
derived carbenes has been carried out using calculated
percentage volume buried (%Vbur) values, Huynh’s electronic
parameter (HEP), and DFT calculations to rationalize the
different catalytic performances of complexes 1−5. The results
suggest that complex 3, which bears a less bulky and less
electron donating carbene, affords the most active catalyst for
the hydration of phenylacetylene. On the other hand, the least
bulky and most donating carbene in complex 5 gives rise to the
best catalyst for the hydrosilylation reactions.
Figures S2 and S3. When the naphthyl and triazole rings are
near or exactly perpendicular (θ ∼ 70−120°), the pictures of
the frontier orbitals are rather simple (Figure 6a). The highest
occupied molecular orbital (HOMO) of the Nap-tazy ligand is
of sigma symmetry and essentially corresponds to the lone pair
on the carbene carbon (σtazy). The HOMO-1 and HOMO-4
orbitals represent the π-conjugation of the naphtyl ring (πNap),
and HOMO-5 is the π-system of the triazolinylidene moiety
(πtazy). A negligible interaction between πNap with πtazy is
observed due to their orthogonality (Figure 6a). As the
naphthyl and triazolinylidene planes align (θ ∼ 0°), significant
mixing between two π-systems occur, leading to a HOMO of
π-symmetry (Figure 6c). Similar mixing of πtazy and πPh in Ph-
tazy was also observed. The πHOMO, however, does not surpass
σtazy (as in Nap-tazy) due to a much lower lying πPh compared
to πtazy (Figure S4).
In addition, DFT calculations reveal an uncommon closed-
shell π−σ interaction between the aromatic π-system of the N-
substituent with the carbene’s σ orbital, leading to an
enhancement of donicity. Work to further explore the effect
of the polyaromatic substituent on the donor strength of the
carbenes is being pursued in our laboratory.
More interesting is the case when the dihedral angle is in
between the two extremes. For the Nap-tazy model, we
observe a substantial mixing between the πNap and σtazy (Figure
6b), which is in stark contrast to Ph-tazy. Such mixing indicates
F
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240 μL, 2 mmol). Yield: 310 mg (0.95 mmol, 95%). Spectroscopic
data closely resemble the reported value.48
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
received. 13C{1H} NMR spectra of all complex probes were measured
in CDCl3 and internally referenced to the CDCl3 solvent resonance
signal at 77.7 ppm relative to tetramethylsilane.
Synthesis of cis-[PtCl2(DMSO)(Dipp-tazy)] (1). A mixture of A
(200 mg, 0.5 mmol) and silver(I) oxide (65 mg, 0.28 mmol, 0.55
equiv) was suspended in dichloromethane (10 mL) and stirred for 10
h shielded from light. In another flask, potassium tetrachloroplatinate-
(II) (207 mg, 0.5 mmol, 1 equiv) was stirred with dimethylsulfoxide
(1 mL) for 3 h; then, the silver carbene complex was added by
filtration over a plug of Celite. The obtained suspension was further
stirred for 10 h at ambient temperature. After the reaction, the volatile
was removed under reduced pressure to obtain a solution of the
complex in DMSO. Water (20 mL) was then added to precipitate out
the complex, which was collected by filtration. The crude product was
purified using silica gel chromatography with dichloromethane as
Synthesis of 1-Benzyl-4-(2,6-diisopropylphenyl)-1,2,4-triazolium
Bromide (A). A mixture of (2,6-diisopropylphenyl)-1,2,4-triazole (230
mg, 1 mmol) and benzyl bromide (340 mg, 240 μL, 2 mmol) in
acetonitrile (5 mL) was stirred at 80 °C for 5 h. Complete conversion
of the triazole was monitored by thin layer chromatography (TLC).
Upon completion, the solvent was removed under reduced pressure.
The residual was subsequently washed with diethyl ether (3 × 5 mL)
to obtain the pure product as a white solid. Yield: 385 mg (0.96
mmol, 96%). 1H NMR (CDCl3, 500 MHz): δ = 12.57 (s, 1 H,
NCHN), 8.25 (s, 1 H, NCHN), 7.75−7.29 (m, 2 H, Ar−H), 7.58 (t,
1 H, Ar−H), 7.44−7.40 (m, 3 H, Ar−H), 7.33 (d, 2 H, Ar−H), 6.22
(s, 2 H, NCH2), 2.18 (sep, 3JH−H = 6.7 Hz, 2 H, CH(CH3)2), 1.24 (d,
1
eluent. Yield: 275 mg (0.41 mmol, 82%). H NMR (CDCl3, 500
MHz): δ = 8.06 (s, 1 H, NCHN), 7.55 (t, 1 H, Ar−H), 7.48 (d, 2 H,
Ar−H), 7.44 (d, 1 H, Ar−H), 7.41−7.38 (m, 2 H, Ar−H), 7.35−7.30
(m, 2 H, Ar−H), 6.30 (d, 2JH−H = 15 Hz, 1 H, CHH), 5.57 (d, 2JH−H
3
= 15 Hz, 1 H, CHH), 3.34 (sept., JH−H = 6.8 Hz, 1 H, CH(CH3)2),
3.02 (s, 3 H, CH3), 2.51 (s, 3 H, CH3), 2.44 (sept., 3JH−H = 6.8 Hz, 1
3JH−H = 6.7 Hz, 6 H, CH3), 1.12 (d, 3JH−H = 6.7 Hz, 6 H, CH3). 13
C
3
H, CH(CH3)2), 1.49 (d, JH−H = 6.8 Hz, 3 H, CH(CH3)2), 1.22 (d,
3
3JH−H = 6.8 Hz, 3 H, CH(CH3)2), 1.12 (d, JH−H = 6.8 Hz, 3 H,
NMR (CDCl3, 126.7 MHz): δ = 145.9, 144.5 (NCHN), 133.5, 133.1,
130.4, 130.3, 129.9, 127.4, 125.8 (Ar−C), 57.2 (NCH2), 29.68
(CH(CH3)2), 24.9 (CH3), 24.6 (CH3). Anal. Calcd for C21H26BrN3:
C, 63.00; H, 6.55; N, 10.50; found: C, 62.47; H, 6.35; N, 10.72.57 MS
(ESI, m/z): calcd for [M − Br]+, C21H26BrN3, m/z 320; found, m/z
320.
3
CH(CH3)2), 0.93 (d, JH−H = 6.8 Hz, 3 H, CH(CH3)2). 13C NMR
(CDCl3, 126.7 MHz): δ = 152.5 (NCN), 148.6 (NCN), 147.1, 144.5,
135.3, 131.9, 130.5, 129.4, 129.2, 128.9, 125.8, 124.8 (Ar−C), 57.5
(NCH2), 46.0, 44.8 (CH(CH3)2), 29.3, 29.0, 28.2, 26.8, 23.6, 23.2
(CH3). Anal. Calcd for C23H31Cl2N3OPtS: C, 41.63; H, 4.71; N, 6.33;
found: C, 42.07; H, 4.74; N, 6.76. MS (ESI, m/z): calcd for [M −
Cl]+, C23H31ClN3OPtS, m/z 628; found, m/z 628 (10%); calcd for
[M + H2O]+, C23H33Cl2N3O2PtS, m/z 681; found, m/z 681 (100%).
Synthesis of cis-[PtCl2(DMSO)(Mes-tazy)] (2). The compound was
prepared in a manner similar to 1 from B (180 mg, 0.5 mmol),
silver(I) oxide (65 mg, 0.28 mmol, 0.55 equiv), and potassium
tetrachloroplatinate(II) (207 mg, 0.5 mmol, 1 equiv). The product
was purified using silica gel chromatography and isolated as a white
solid. Yield: 240 mg (0.38 mmol, 77%). 1H NMR (CDCl3, 500
MHz): δ = 7.99 (s, 1 H, NCHN), 7.48 (d, 3JH−H = 7.4 Hz, 2 H, Ar−
Synthesis of 1-Benzyl-4-mesityl-1,2,4-triazolium Bromide (B).
The compound was prepared in a manner similar to that for B from 4-
mesityl-1,2,4-triazole (190 mg, 1 mmol) and benzyl bromide (340 mg,
240 μL, 2 mmol). The product was obtained as an off-white solid.
1
Yield: 330 mg (0.92 mmol, 92%). H NMR (CDCl3, 500 MHz): δ =
12.49 (s, 1 H, NCHN), 8.20 (s, 1 H, NCHN), 7.72 (d, 2 H, Ar−H),
7.47−7.39 (m, 3 H, Ar−H), 7.04 (s, 2 H, Ar−H), 6.14 (s, 2 H,
NCH2), 2.36 (s, 3 H, CH3), 2.09 (s, 6 H, CH3). 13C NMR (CDCl3,
126.7 MHz): δ = 143.9, 143.2 (NCHN), 134.7, 132.9, 131.0, 130.4,
130.3, 129.9, 127.9 (Ar−C), 57.3 (NCH2), 21.8, 18.63 (CH3). Anal.
Calcd for C18H20BrN3: C, 60.34; H, 5.63; N, 11.73; found: C, 60.01;
H, 5.48; N, 11.96. MS (ESI, m/z): calcd for [M − Br]+, C18H20N3, m/
z 278; found, m/z 278.
Synthesis of 1-Benzyl-4-phenyl-1,2,4-triazolium Bromide (C).
The compound was prepared in a manner similar to that for A from 4-
phenyl-1,2,4-triazole (145 mg, 1 mmol) and benzyl bromide (340 mg,
240 μL, 2 mmol). The product was obtained as a white solid. Yield:
310 mg (0.98 mmol, 98%). 1H NMR (CDCl3, 500 MHz): δ = 12.7 (s,
1 H, NCHN), 8.90 (s, 1 H, NCHN), 7.84 (d, 2 H, Ar−H), 7.71 (dd,
2 H, Ar−H), 7.59−7.53 (m, 3 H, Ar−H), 7.39−7.38 (m, 3 H, Ar−H),
5.91 (s, 2 H, NCH2). 13C NMR (CDCl3, 126.7 MHz): δ = 142.5
(NCHN), 141.7 (NCHN), 132.5, 132.1, 132.0, 131.6, 130.6, 130.5,
129.9, 122.9 (Ar−C), 57.3 (NCH2). Anal. Calcd for C15H14BrN3: C,
56.98; H, 4.46; N, 13.29; found: C, 54.72; H, 4.40; N, 12.99.57 MS
(ESI, m/z): calcd for [M − Br]+, C15H14N3, m/z 236; found, m/z
236.
3
H), 7.39 (t, 2 H, Ar−H), 7.34 (t, JH−H = 7.4 Hz, 1 H, Ar−H), 7.09
2
(s, 1 H, Ar−H), 6.98 (s, 1 H, Ar−H), 6.22 (d, JH−H = 15 Hz, 1 H,
CHH), 5.56 (d, 2JH−H = 15 Hz, 1 H, CHH), 3.0 (s, 3 H, CH3), 2.60
(s, 3 H, CH3), 2.42 (s, 3 H, CH3), 2.35 (s, 3 H, CH3), 2.05 (s, 3 H,
CH3). 13C NMR (CDCl3, 126.7 MHz): δ = 151.8, 144.2 (NCN),
141.1, 137.3, 136.2, 135.6, 131.1, 130.9, 129.9, 129.5, 129.2, 128.9
(Ar−C), 57.4 (NCH2), 45.9, 45.3, 21.6, 20.8, 19.3 (CH3). Anal. Calcd
for C20H25Cl2N3OPtS·2H2O: C, 36.53; H, 4.45; N, 6.39; found: C,
36.13; H, 3.85; N, 6.21.57 MS (ESI, m/z): calcd for [M − Cl]+,
C20H25ClN3OPtS, m/z 586; found, m/z 586 (15%). MS (ESI, m/z):
calcd for [M + H2O]+, C20H27Cl2N3O2PtS, m/z 639; found, m/z 639
(100%).
Synthesis of cis-[PtCl2(DMSO)(Ph-tazy)] (3). The compound was
prepared in a manner similar to 1 from C (160 mg, 0.5 mmol),
silver(I) oxide (65 mg, 0.28 mmol, 0.55 equiv), and potassium
tetrachloroplatinate(II) (207 mg, 0.5 mmol, 1 equiv). Yield: 250 mg
1
Synthesis of 4-(1-Naphthyl)-1,2,4-triazolium Bromide (D). The
compound was prepared in a manner similar to that for A from 4-
phenyl-1,2,4-triazole (195 mg, 1 mmol) and benzyl bromide (340 mg,
240 μL, 2 mmol). The product was obtained as a white solid. Yield:
(0.43 mmol, 86%). H NMR (CDCl3, 500 MHz): δ = 8.23 (s, 1 H,
NCHN), 7.86 (dd, 2 H, Ar−H), 7.63−7.53 (m, 3 H, Ar−H), 7.47 (d,
2 H, 3JH−H = 7.3 Hz, Ar−H), 7.38 (t, 2 H, Ar−H), 7.32 (t, 3JH−H = 7.3
Hz, 1 H, Ar−H), 6.08 (d, 2JH−H = 15 Hz, 1 H, CHH), 5.51 (d, 2JH−H
= 15 Hz, 1 H, CHH), 2.76 (s, 3 H, CH3), 2.48 (s, 3 H, CH3). 13C
NMR (CDCl3, 126.7 MHz): δ = 151.1, 142.5 (NCN), 135.8, 135.4,
130.7, 130.3, 129.4, 129.2, 129.0, 126.9 (Ar−C), 56.9 (NCH2), 45.16,
45.03 (CH3). Anal. Calcd for C17H19Cl2N3OPtS·0.5H2O: C, 34.70;
H, 3.43; N, 7.14; found: C, 34.70; H, 3.21; N, 7.27. MS (ESI, m/z):
calcd for [M − Cl]+, C17H19ClN3OPtS, m/z 544; found, m/z 544;
calcd for [M + H2O]+, C17H21Cl2N3O2PtS, m/z 597; found, m/z 597.
Synthesis of cis-[PtCl2(DMSO)(Nap-tazy)] (4). The compound was
prepared in a manner similar to 1 from D (185 mg, 0.5 mmol),
silver(I) oxide (65 mg, 0.28 mmol, 0.55 equiv), and potassium
tetrachloroplatinate(II) (207 mg, 0.5 mmol, 1 equiv). Yield: 250 mg
1
330 mg (0.90 mmol, 90%). H NMR (CDCl3, 500 MHz): δ = 12.18
(s, 1 H, NCHN), 8.55 (s, 1 H, NCHN), 8.07 (d, 1 H, Ar−H), 8.00−
7.97 (m, 2 H, Ar−H), 7.78−7.76 (m, 2 H, Ar−H), 7.65−7.61 (m, 2
H, Ar−H), 7.55 (t, 1 H, Ar−H), 7.48−7.47 (m, 1 H, Ar−H), 7.43−
7.41 (m, 3 H, Ar−H), 6.07 (s, 2 H, NCH2). 13C NMR (CDCl3, 126.7
MHz): δ = 144.6, 144.1, 134.8, 133.1, 132.6, 130.8, 130.4, 129.9,
129.7, 128.6, 127.8, 126.1, 120.8). Anal. Calcd for C19H16BrN3: C,
62.31; H, 4.40; N, 11.47; found: C, 62.02; H, 4.20; N, 11.53. MS
(ESI, m/z): calcd for [M − Br]+, C19H16N3, m/z 286; found, m/z
286.
Synthesis of 1,4-Dibenzyl-1,2,4-triazolium Bromide (E). The
compound was prepared in a manner similar to that for A from 4-
benzyl-1,2,4-triazole (159 mg, 1 mmol) and benzyl bromide (340 mg,
1
(0.40 mmol, 80%). H NMR (CDCl3, 500 MHz): δ = 8.33 (d, 1 H,
Ar−H), 8.28 (s, 1 H, NCHN), 8.10 (d, 1 H, Ar−H), 8.01−7.98 (m, 1
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H, Ar−H), 7.76−7.73 (m, 1 H, Ar−H), 7.67−7.58 (m, 4 H, Ar−H),
7.57−7.55 (m, 1 H, Ar−H), 7.46−7.43 (m, 2 H, Ar−H), 7.39−7.36
21.1, 20.3 (CH3). Anal. Calcd for C31H37Br2N5Pd: C, 49.92; H, 5.00;
N, 9.39; found: C, 50.89; H, 5.30; N, 9.55.57 MS (ESI, m/z): calcd for
[M − Br]+, C31H37BrN5Pd, m/z 666; found, m/z 666.
2
(m, 1 H, Ar−H), 6.12 (d, JH−H = 15 Hz, 1 H, NCHH), 5.72 (d,
Synthesis of trans-[PdBr2(iPr2-bimy)(Ph-tazy)] (8). The compound
was prepared in a manner similar to 6 with C (160 mg, 0.5 mmol),
silver(I) oxide (130 mg, 0.55 mmol), and [PdBr2(iPr2-bimy)] (235
mg, 0.25 mmol). Yield: 290 mg (0.41 mmol, 82%). 1H NMR (CDCl3,
2JH−H = 15 Hz, 1 H, NCHH), 2.87 (s, 3 H, CH3), 2.23 (s, 3 H, CH3).
Second isomer: 8.15−8.13 (m, 2 H, Ar−H, NCHN), 8.01−7.98 (m, 1
H, Ar−H), 7.76−7.73 (m, 1 H, Ar−H), 7.70 (d, 1 H, Ar−H), 7.67−
7.58 (m, 4 H, Ar−H), 7.57−7.55 (m, 1 H, Ar−H), 7.46−7.43 (m, 2
H, Ar−H), 7.39−7.36 (m, 1 H, Ar−H), 6.28 (d, 2JH−H = 15 Hz, 1 H,
3
500 MHz): δ = 8.15 (s, 1 H, NCHN), 7.97 (d, JH−H = 7.8 Hz, 2 H,
3
2
Ar−H), 7.76 (d, JH−H = 7.4 Hz, 2 H, Ar−H), 7.64−7.57 (m, 3 H,
NCHH), 5.66 (d, JH−H = 15 Hz, 1 H, NCHH), 2.83 (s, 3 H, CH3),
3
2.68 (s, 3 H, CH3). 13C NMR (CDCl3, 126.7 MHz): δ = (152.9,
152.2) (Ccarbene), (144.5, 144.1) (NCNH), 132.5, 131.8, 131.7, 131.4,
130.0, 129.7 (2 C), 129.6 (2 C), 129.5, 129.3 (2 C), 129.2 (2 C),
128.8, 128.7, 128.5 (2 C), 128.0 (2 C), 126.8, 126.4, 124.7 (2 C),
124.4, 124.2, 123.0 (2 C) (Ar−C), 57.3 (2 C, NCH2), 45.6, 45.4,
45.3, 45.2 (CH3). Anal. Calcd for C21H21Cl2N3OPtS: C, 40.07; H,
3.36; N, 6.68; found: C, 40.31; H, 3.10; N, 6.38. MS (ESI, m/z): calcd
for [M − Cl]+, C21H21ClN3OPtS, m/z 594; found, m/z 594; calcd for
[M + H2O]+, C21H23Cl2N3O2PtS, m/z 647; found, m/z 647.
Ar−H), 7.53−7.49 (m, 2 H, Ar−H), 7.45 (t, JH−H = 7.8 Hz, 2 H,
3
Ar−H), 7.39 (t, JH−H = 7.4 Hz, 1 H, Ar−H), 7.18−7.16 (m, 2 H,
Ar−H), 6.02−5.94 (m, 3 H, CH(CH3)2 and NCH2), 5.74 (sept,
3JH−H = 7.1 Hz, 1 H, CH(CH3)2), 1.73 (d, 3JH−H = 7.1 Hz, 6 H, CH3),
1.55 (d, 3JH−H = 7.1 Hz, 6 H, CH3). 13C NMR (CDCl3, 126.7 MHz):
δ = 177.0, 176.2 (Cprobe), 142.9, 137.2, 135.5, 134.2, 134.0, 129.9,
129.9, 129.8, 129.4, 129.1, 127.30, 122.7, 113.2 (Ar−C), 57.4
(NCH2), 54.5, 54.3 (CH(CH3)2), 21.7, 21.3 (CH3). Anal. Calcd for
C28H31Br2N5Pd·2H2O: C, 45.46; H, 4.77; N, 9.47; found: C, 45.81;
H, 4.26; N, 9.93.57 MS (ESI, m/z): calcd for [M − Br]+,
C28H31BrN5Pd, m/z 624; found, m/z 624. HR-MS (ESI, m/z):
calcd for [M − Br]+, C28H31BrN5Pd, m/z 624.0719; found, m/z
624.0709.
Synthesis of cis-[PtCl2(DMSO)(Bn-tazy)] (5). The compound was
prepared in a manner similar to 1 from E (165 mg, 0.5 mmol),
silver(I) oxide (65 mg, 0.28 mmol, 0.55 equiv), and potassium
tetrachloroplatinate(II) (207 mg, 0.5 mmol, 1 equiv). Yield: 250 mg
1
(0.42 mmol, 84%). H NMR (CDCl3, 500 MHz): δ = 7.88 (s, 1 H,
Synthesis of trans-[PdBr2(iPr2-bimy)(Nap-tazy)] (9). The com-
pound was prepared in a manner similar to 6 with D (180 mg, 0.5
mmol), silver(I) oxide (130 mg, 0.55 mmol), and [PdBr2(iPr2-bimy)]
2
NCHN), 7.44−7.33 (m, 10 H, Ar−H), 5.77 (d, JH−H = 15 Hz, 1 H,
CHH), 5.68 (d, 2JH−H = 15 Hz, 1 H, CHH), 5.62 (d, 2JH−H = 15 Hz, 1
2
1
H, CHH), 5.56 (d, JH−H = 15 Hz, 1 H, CHH), 3.13 (s, 3 H, CH3),
(235 mg, 0.25 mmol). Yield: 295 mg (0.39 mmol, 78%). H NMR
3.00 (s, 3 H, CH3). 13C NMR (CDCl3, 126.7 MHz): δ = 151.2
(NCN), 143.3 (NCN), 135.2, 134.1, 130.0, 129.8, 129.6, 129.3,
129.2, 129.1 (Ar−C), 56.9 (NCH2), 53.1 (NCH2), 46.1 (CH3), 45.9
(CH3). Anal. Calcd for C18H21Cl2N3OPtS·H2O: C, 35.36; H, 3.79; N,
6.87; found: C, 35.70; H, 3.46; N, 7.18. MS (ESI, m/z): calcd for [M
− Cl]+, C18H21ClN3OPtS, m/z 558; found, m/z 558 (15%); calcd for
[M + H2O]+, C18H23Cl2N3O2PtS, m/z 611; found, m/z 611 (100%).
Synthesis of trans-[PdBr2(iPr2-bimy)(Dipp-tazy)] (6). A mixture of
A (200 mg, 0.5 mmol), silver(I) oxide (130 mg, 0.55 mmol), and
[PdBr2(iPr2-bimy)] (235 mg, 0.25 mmol) was suspended in
dichloromethane (10 mL) and stirred at ambient temperature for 5
h shielded from light. The precipitate was then removed by filtration
over a short plug of Celite. After removing the solvent, the solid
obtained was subjected to purification using silica gel column
chromatography and DCM as the eluent. The product was isolated
(CDCl3, 500 MHz): δ = 7.17 (s, 1 H, NCHN), 8.11 (d, 1 H, Ar−H),
8.04−8.02 (m, 1 H, Ar−H), 7.97 (d, 1 H, Ar−H), 7.89−7.87 (m, 1 H,
Ar−H), 7.82 (m, 2 H, Ar−H), 7.69−7.63 (m, 3 H, Ar−H), 7.49−7.46
(m, 3 H, Ar−H), 7.41 (t, 1 H, Ar−H), 7.35−7.33 (m, 1 H, Ar−H),
7.14−7.08 (m, 2 H, Ar−H), 6.22 (brd. d, 1 H, NCH2), 5.92−5.86 (m,
3
2 H, NCH2 and CH(CH3)2), 4.92 (sept., JH−H = 7.0 Hz, 1 H,
3
CH(CH3)2), 1.70 (d, JH−H = 7.0 Hz, H, CH3), 1.18 (brd. s, 3 H,
CH3), 1.06 (brd. s, 3 H, CH3). 13C NMR (CDCl3, 126.7 MHz): δ =
177.8, 176.6 (Cprobe), 144.4, 135.8, 134.7, 134.1, 133.9, 133.0, 130.8,
130.0, 129.9 (2 C), 129.5 (2 C), 129.1, 128.5, 128.4, 127.9, 127.8,
125.5, 124.9, 122.5, 113.1, 113.0 (Ar−C), 57.5 (NCH2), 54.4, 53.9
(CH(CH3)2), 21.6 (CH3). Anal. Calcd for C32H33Br2N5Pd: C, 50.98;
H, 4.41; N, 9.29; found: C, 50.90; H, 4.90; N, 9.00.57 MS (ESI, m/z):
calcd for [M − Br]+, C32H33BrN5Pd, m/z 674; found, m/z 674. HR-
MS (ESI, m/z): calcd for [M − Br]+, C32H33BrN5Pd, m/z 674.0875;
found, m/z 674.0877.
1
as a pale yellow solid. Yield: 350 mg (0.44 mmol, 88%). H NMR
3
(CDCl3, 500 MHz): δ = 7.98 (s, 1 H, NCHN), 7.77 (d, JH−H = 8.0
Hydration of Phenylacetylene Reaction. In a typical run, a
Schlenk tube was charged with a mixture of 0.5 mmol of
phenylacetylene, an appropriate amount of the catalyst, and solvent
(1.5 mL). The mixture was then stirred at the appropriate
temperature for the required time. The product was then extracted
with dichloromethane (3 × 2 mL). The combined extracts were
Hz, 2 H, Ar−H), 7.57 (t, 3JH−H = 8.0 Hz, 1 H, Ar−H), 7.51−7.45 (m,
3 H, Ar−H), 7.43−7.36 (m, 4 H, Ar−H), 7.15−7.10 (m, 2 H, Ar−H),
6.07 (s, 2 H, NCH2), 5.97 (brd. s, 1 H, CH(CH3)2), 5.39 (brd. s, 1 H,
CH(CH3)2), 2.99 (sept, 3JH−H = 6.8 Hz, 2 H, CH(CH3)2), 1.71 (brd.
3
s, 6 H, CH3), 1.39 (d, 3JH−H = 6.8 Hz, 12 H, CH3), 1.01 (d, JH−H
=
6.8 Hz, 6 H, CH3). 13C NMR (CDCl3, 126.7 MHz): δ = 178.1, 176.4
(Cprobe), 147.9, 145.1, 135.8, 132.4, 131.2, 129.9 (2 C), 129.5, 129.0,
124.5, 122.5, 112.9 (Ar−C), 57.7 (NCH2), 30.4, 29.3 (CH(CH3)2),
27.3, 23.1, 21.4 (CH3). Anal. Calcd for C34H43Br2N5Pd: C, 51.83; H,
5.50; N, 8.89; found: C, 52.37; H, 5.53; N, 8.69.57 MS (ESI, m/z):
calcd for [M − Br]+, C34H43BrN5Pd, m/z 708; found, m/z 708. HR-
MS (ESI, m/z) calcd for [M − Br]+, C34H43BrN5Pd, m/z 708.1658;
found, m/z 708.1667.
subjected to H NMR to determine the reaction yields.
1
Hydrosilylation of Phenylacetylene Reaction. In a typical run,
a Schlenk tube was charged with a mixture of 2 mmol of
phenylacetylene, 2.2 mmol of bis(trimethylsiloxy)methylsilane, an
appropriate amount of catalyst, and 1 mL of toluene as solvent. The
mixture was then stirred at the appropriate temperature for the
required time. The solvent was then removed under reduced pressure,
1
and the crude products were then subjected to H NMR analysis to
Synthesis of trans-[PdBr2(iPr2-bimy)(Mes-tazy)] (7). The com-
pound was prepared in a manner similar to 6 with B (180 mg, 0.5
mmol), silver(I) oxide (130 mg, 0.55 mmol), and [PdBr2(iPr2-bimy)]
determine the reaction yields.
X-ray Crystallography. Single-crystal X-ray diffraction were
collected on a Bruker D8 QUEST instrument at 298 K with Mo
Kα radiation (λ = 0.71073 Å) using a TRIUMPH monochromator.
Standard procedures were applied for data reduction and absorption
correction.58,59 Structure solution and refinement were performed
with the SHELXT and SHELXL 2014/7 programs.60,61 Hydrogen
atoms were calculated for idealized positions and treated with the
“riding model” option of SHELXL. The representation of molecular
structures was done using the program OLEX2-1.2.62
Computation. Gas-phase structures of all complexes were
optimized by DFT methods using the B3PW91 hybrid functional63,64
with the 6-31G(d)65 basis set applied for light atoms, whereas the
heavy atom Pt was described by the Stuttgart-Dresden (SDD)
1
(235 mg, 0.25 mmol). Yield: 300 mg (0.4 mmol, 80%). H NMR
3
(CDCl3, 500 MHz): δ = 7.92 (s, 1 H, NCHN), 7.72 (d, JH−H = 7.4
Hz, 2 H, Ar−H), 7.52−7.50 (m, 1 H, Ar−H), 7.47−7.43 (m, 3 H,
3
Ar−H), 7.38 (t, JH−H = 7.4 Hz, 1 H, Ar−H), 7.16−7.14 (m, 2 H,
Ar−H), 7.09 (s, 2 H, Ar−H), 6.00 (s, 2 H, NCH2), 5.87 (sept, 3JH−H
3
= 7.0 Hz, 1 H, CH(CH3)2), 5.58 (sept, JH−H = 7.0 Hz, 1 H,
CH(CH3)2), 2.43 (s, 3 H, CH3), 2.33 (s, 6 H, CH3), 1.72 (d, 3JH−H
=
3
7.0 Hz, 6 H, CH3), 1.47 (d, JH−H = 7.0 Hz, 6 H, CH3). 13C NMR
(CDCl3, 126.7 MHz): δ = 177.5, 176.1 (Cprobe), 144.1, 140.2, 137.1,
136.1, 134.2, 134.1, 132.8, 129.7, 129.6, 129.4, 128.9, 122.5, 113.19,
112.9 (Ar−C), 57.5 (NCH2), 54.5, 53.9 (CH(CH3)2), 21.8, 21.6,
H
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Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
relativistic effective core potential and associated basis sets.66 The
imaginary frequency was checked to ensure the stationary point was a
minimum. To investigate the influence of Ph and Nap rotation along
C−N bonds, the respective carbenes (Ph-tazy and Nap-tazy) were
optimized at several different fixed Aryl-tazy dihedral angles. To
simplify the calculation, the flexible benzyl group was replaced by a
methyl group. For a better description of the carbene electronic
structures, the B3PW91 hybrid functional in combination with a
larger basis set (6-311+G(d))67,68 was employed.
Containing N-Heterocyclic Carbene Ligands with Potent in Vitro and
in Vivo Anti-Cancer Properties Accumulate in Cytoplasmic Structures
of Cancer Cells. Chem. Sci. 2011, 2 (4), 728−736.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00260.
■
sı
Characterization of the complex probes (6−9);
structural parameters for gas-phased optimized geo-
metries of 1−5; crystal data for 3 and 4; shape and
energy change for frontier orbitals of Ph-tazy and Nap-
1
tazy; H and 13C{1H} NMR spectra of the salts and
complexes (PDF)
Cartesian coordinates for the optimized structures
(XYZ)
(7) Marion, N.; Nolan, S. P. Well-Defined N-Heterocyclic
Carbenes−Palladium(II) Precatalysts for Cross-Coupling Reactions.
Acc. Chem. Res. 2008, 41, 1440−1449.
Accession Codes
(8) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. An
overview of N-heterocyclic carbenes. Nature 2014, 510, 485−496.
CCDC 1966117 and 1966118 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.
́
(9) Díez-Gonzalez, S.; Marion, N.; Nolan, S. P. N-Heterocyclic
Carbenes in Late Transition Metal Catalysis. Chem. Rev. 2009, 109,
3612−3676.
(10) Peris, E.; Crabtree, R. H. Recent Homogeneous Catalytic
Applications of Chelate and Pincer N-Heterocyclic Carbenes. Coord.
Chem. Rev. 2004, 248 (21), 2239−2246.
AUTHOR INFORMATION
Corresponding Authors
Van Ha Nguyen − Faculty of Chemistry, University of Science,
Vietnam National University, Hanoi, Hanoi 110403, Vietnam;
Email: hanv@hus.edu.vn
(11) Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. Steric and
electronic effects in the bonding of N-heterocyclic ligands to
transition metals. J. Organomet. Chem. 2005, 690, 5407−5413.
(12) Clavier, H.; Nolan, S. P. Percent buried volume for phosphine
and N-heterocyclic carbene ligands: steric properties in organo-
metallic chemistry. Chem. Commun. 2010, 46, 841−861.
(13) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H.
Increased Ring Closing Metathesis Activity of Ruthenium-Based
Olefin Metathesis Catalysts Coordinated with Imidazolin-2-Ylidene
Ligands. Tetrahedron Lett. 1999, 40 (12), 2247−2250.
■
Han Vinh Huynh − Department of Chemistry, National
University of Singapore, Singapore 117543, Republic of
Singapore; orcid.org/0000-0003-4460-6066;
Email: chmhhv@nus.edu.sg
(14) Vougioukalakis, G. C.; Grubbs, R. H. Ruthenium-Based
Heterocyclic Carbene-Coordinated Olefin Metathesis Catalysts.
Chem. Rev. 2010, 110 (3), 1746−1787.
Authors
Thanh Tuan Dang − Faculty of Chemistry, University of
Science, Vietnam National University, Hanoi, Hanoi 110403,
Vietnam
Hung Huy Nguyen − Faculty of Chemistry, University of
Science, Vietnam National University, Hanoi, Hanoi 110403,
Vietnam
(15) Hey, D. A.; Reich, R. M.; Baratta, W.; Kuhn, F. E. Current
̈
Advances on Ruthenium(II) N-Heterocyclic Carbenes in Hydro-
genation Reactions. Coord. Chem. Rev. 2018, 374, 114−132.
(16) Kaufhold, S.; Petermann, L.; Staehle, R.; Rau, S. Transition
Metal Complexes with N-Heterocyclic Carbene Ligands: From
Organometallic Hydrogenation Reactions toward Water Splitting.
Coord. Chem. Rev. 2015, 304−305, 73−87.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00260
(17) Azua, A.; Mata, J. A.; Peris, E.; Lamaty, F.; Martinez, J.;
Colacino, E. Alternative Energy Input for Transfer Hydrogenation
Using Iridium NHC Based Catalysts in Glycerol as Hydrogen Donor
and Solvent. Organometallics 2012, 31 (10), 3911−3919.
Notes
The authors declare no competing financial interest.
̈
̈
(18) Riener, K.; Hogerl, M. P.; Gigler, P.; Kuhn, F. E. Rhodium-
Catalyzed Hydrosilylation of Ketones: Catalyst Development and
Mechanistic Insights. ACS Catal. 2012, 2 (4), 613−621.
(19) Marion, N.; Nolan, S. P. N-Heterocyclic Carbenes in Gold
Catalysis. Chem. Soc. Rev. 2008, 37 (9), 1776−1782.
ACKNOWLEDGMENTS
■
This research is funded by Vietnam National Foundation for
Science and Technology Development (NAFOSTED) under
grant number 104.03-2017.14. This paper is dedicated to
Professor F. Ekkehardt Hahn on the occasion of his 65th
birthday.
(20) Hillier, A. C.; Grasa, G. A.; Viciu, M. S.; Lee, H. M.; Yang, C.;
Nolan, S. P. Catalytic Cross-Coupling Reactions Mediated by
Palladium/Nucleophilic Carbene Systems. J. Organomet. Chem.
2002, 653 (1), 69−82.
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Applications of Palladium Complexes Bearing N-Heterocyclic
Carbene Ligands. Curr. Org. Chem. 2011, 15 (18), 3309−3324.
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Organometallics XXXX, XXX, XXX−XXX