Palladium(II) complexes of 1,2,4-triazole-based N-heterocyclic carbenes: Synthesis, structure, and catalytic activity

Article abstract of DOI:10.1021/om500342z

Six palladium(II) complexes bearing three different triazole-based N-heterocyclic carbene (NHC) ligands, [1-tert-butyl-4-{2-[(N,N-dimethylamino) methyl]phenyl}-3-phenyl-1H-1,2,4-triazol-4-ium-5-ide, 1-tert-butyl-4-(2- methoxyphenyl)-3-phenyl-1H-1,2,4-triazol-4-ium-5-ide, and 1-tert-butyl-4-(4- methylphenyl)-3-phenyl-1H-1,2,4-triazol-4-ium-5-ide], were synthesized and fully characterized. NMR spectroscopy and X-ray diffraction analysis revealed that the amino-group-substituted NHC ligand is coordinated in bidentate fashion, forming a monocarbene chelate complex with an additional intramolecular Pd ← N bond with the nitrogen donor atom. The 4-methylphenyl- and 2-methoxyphenyl-substituted NHC ligands coordinate as C-monodentate donors, forming simple biscarbene Pd(II) complexes. The evaluation of the catalytic performance in the Suzuki-Miyaura cross-coupling reaction revealed very promising performance of the intramolecularly coordinated monocarbene complexes under relatively mild conditions even in direct comparison with the commercially available PEPPSI catalyst. In contrast, the biscarbene complexes proved inactive in this catalytic process. According to theoretical calculations (EDA and NOCV analysis), functionalization of the 1,2,4-triazole-based NHC with the 2-[(N,N-dimethylamino)methyl]phenyl group has a significant effect on the stability of the NHC-metal bond.

Full text of DOI:10.1021/om500342z

Article
pubs.acs.org/Organometallics
Palladium(II) Complexes of 1,2,4-Triazole-Based N‑Heterocyclic
Carbenes: Synthesis, Structure, and Catalytic Activity
†
‡
§
§
∥
†
̌
Jan Turek, Illia Panov, Miloslav Semler, Petr Stepnicka, Frank De Proft, Zdenka Padelkova,
̌
̌
̌
̌
́
and Ales Ruzicka*,†
̌
̌
̌
̊
†Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska
10 Pardubice, Czech Republic
́
573, CZ-532
‡Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentska
10 Pardubice, Czech Republic
́
573, CZ-532
§Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, CZ-128 40 Prague, Czech
Republic
∥Eenheid Algemene Chemie (ALGC) and QCMM VUB−UGent Alliance Research Group, Vrije Universiteit Brussel, Pleinlaan 2,
B-1050 Brussels, Belgium
S
* Supporting Information
ABSTRACT: Six palladium(II) complexes bearing three
different triazole-based N-heterocyclic carbene (NHC) ligands,
[1-tert-butyl-4-{2-[(N,N-dimethylamino)methyl]phenyl}-3-
phenyl-1H-1,2,4-triazol-4-ium-5-ide, 1-tert-butyl-4-(2-methoxy-
phenyl)-3-phenyl-1H-1,2,4-triazol-4-ium-5-ide, and 1-tert-
butyl-4-(4-methylphenyl)-3-phenyl-1H-1,2,4-triazol-4-ium-5-
ide], were synthesized and fully characterized. NMR
spectroscopy and X-ray diffraction analysis revealed that the
amino-group-substituted NHC ligand is coordinated in
bidentate fashion, forming a monocarbene chelate complex with an additional intramolecular Pd ← N bond with the nitrogen
donor atom. The 4-methylphenyl- and 2-methoxyphenyl-substituted NHC ligands coordinate as C-monodentate donors,
forming simple biscarbene Pd(II) complexes. The evaluation of the catalytic performance in the Suzuki−Miyaura cross-coupling
reaction revealed very promising performance of the intramolecularly coordinated monocarbene complexes under relatively mild
conditions even in direct comparison with the commercially available PEPPSI catalyst. In contrast, the biscarbene complexes
proved inactive in this catalytic process. According to theoretical calculations (EDA and NOCV analysis), functionalization of the
1,2,4-triazole-based NHC with the 2-[(N,N-dimethylamino)methyl]phenyl group has a significant effect on the stability of the
NHC−metal bond.
Chart 1. Palladium Complexes with Various Carbene
Ligands: (A) Imidazole-Based,6 (B) Triazole-Based,7 (C)
Tetrazole-Based,8 (D) Mesoionic,9 and (E) CAAC10
INTRODUCTION
■
The rapid developments in the field of palladium-catalyzed
carbon−carbon and carbon−heteroatom coupling reactions are
reflected by an enormous increase of interest in this topic
during the last two decades.1 The phosphine-based ligands
originally employed in this field2 have gradually been replaced
by various N-heterocyclic carbene (NHC) ligands,3−5 which
have attracted exceptional attention from both academia and
industry mainly because of their versatility and unique σ-
donating abilities. Nowadays, the chemistry of NHC−
palladium(II) complexes is rather rich with a high number of
examples of highly active catalysts based on various types of
such donors. A broad variety of complexes containing
imidazole-, triazole-, or tetrazole-based carbene, mesoionic
carbene, or cyclic alkylamino carbene (CAAC) ligands (Chart
1) have been introduced. Among them, the imidazole-based
ones are widely used in catalysis as they are thermally robust
and reasonably stable (see, e.g., the commercially available
PEPPSI catalyst6), which is the major advantage compared with
the phosphine-derived and other Pd catalysts.
Received: March 31, 2014
© XXXX American Chemical Society
A
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From the structural point of view, NHC-substituted PdX2
complexes, where X is an anionic ligand (e.g., chloride or
acetate) and NHC is in most cases a five-membered
heterocycle or, rarely, a chain containing the carbene carbon
atom, can be generally divided into two classes: monocarbene-
type compounds (NHC)Pd(D)X2 (D = donor group) and
biscarbene complexes (NHC)2PdX2. All of these compounds
possess the expected square-planar coordination sphere around
the central palladium atom with both cis and trans arrangements
of the X groups. Among the monocarbene complexes, the trans
isomers are the most prevalent (Chart 2A),9a,11 with the donor
pyridine-,19 phosphine-,20 or ether-based21 donor moieties,
although applications of aliphatic amino groups, as successfully
implemented in the chemistry of pincer compounds,22 still
remain rare.
In this paper, we introduce a 2-[(dimethylamino)methyl]-
phenyl-substituted triazole-based NHC ligand that has already
been successfully applied in rhodium(I) complexes23 into the
palladium(II) chemistry. Furthermore, two analogous NHC
ligands substituted with 2-methoxyphenyl and 4-methylphenyl
groups were synthesized for a comparison of the bonding
properties in similar complexes with the palladium(II) ion. The
catalytic activities of the resulting complexes in model Suzuki−
Miyaura reactions were evaluated and are discussed in
comparison with the activity of the commercial PEPPSI
catalyst.
Chart 2. Possible Bonding Modes and Structural Motifs in
a
NHC−Pd Complexes
RESULTS AND DISCUSSION
■
Three carbene precursors, 1-tert-butyl-4-{2-[(dimethylamino)-
methyl]phenyl}-3-phenyl-4H-1,2,4-triazol-1-ium perchlorate
(2a), 1-tert-butyl-4-(2-methoxyphenyl)-3-phenyl-4H-1,2,4-tria-
zol-1-ium perchlorate (2b), and 1-tert-butyl-4-(4-methylphen-
yl)-3-phenyl-4H-1,2,4-triazol-1-ium perchlorate (2c), were
synthesized in two steps in moderate (2a) to high (2b and
2c) overall yields according to the procedure reported in our
recent work (2a)23 or by means of the general procedures
published earlier (2b and 2c).24 Triazoles 1 were formed via the
recyclization25 reaction of 2-phenyl-1,3,4-oxadiazole with the
respective anilines in 1,2-dichlorobenzene (o-DCB) at elevated
temperature. The next step of the reaction sequence was a
quaternization with the 2-chloro-2-methylpropane (tBuCl)
under reflux conditions in acetic acid to give triazolium
perchlorates 2, which were finally crystallized and isolated as
white powders. The formation of 2 was clearly manifested in
a
D = donor group; X = an electronegative substituent; Y = C, N, P, or
Si.
group usually completing the coordination of the metal center.
The cis arrangement is less populated and is most likely to
occur when the additional donor group is directly bonded to
the NHC fragment, thus enabling the formation of four- to
eight-membered palladacycles (Chart 2B).12 Another example
is the group of mixed NHC/triphenylphosphane Pd dibromide
or diiodide complexes, where exclusive formation of the cis
arrangement was observed (Chart 2C).13 The Pd−X bonds in
the trans arrangement are almost equidistant (lengths in an
approximate range of 2.29−2.31 Å)14 but in the case of cis
isomers, the Pd−X bond located trans to the carbene carbon is
usually elongated as a result of the trans influence of the NHC
ligand (lengthening by ca. 0.05 Å).14 The cis arrangement is
also possible for the biscarbene-type compounds with either
two separate NHC ligands or one bidentate ligand in which two
NHCs are connected via a ring or a chain (Chart 2D).15 When
the bridge between the two carbene units is long enough, trans
carbene complexes can be isolated as well.16 Nonetheless, the
trans isomers are more likely to be formed when two
independent carbenes of the same or different type are present
in the complex.17 It has also been found very recently that weak
CH−π and CF−π interactions can drive the conversion of the
trans isomer to the cis-anti or cis-syn isomer of (NHC)2PdX2,
depending on the weak interactions provided by the substituent
groups of the NHC.18 For the trans isomers, the lengths of both
the Pd−X and Pd−C bonds vary only marginally. It is typically
found that the Pd−C bonds (ca. 2.05 Å)14 are slightly
elongated and the Pd−X bonds (ca. 2.01 Å for Pd−O in
acetates and 2.32 Å for Pd−Cl)14 are slightly shortened in the
trans isomers in comparison with the cis arrangement (ca. 1.96
Å for Pd−C, 2.07 Å for Pd−O and 2.36 Å for Pd−Cl).14
The use of a carbene with an adjacent donor group with
potential hemilabile coordination to the metal center is quite
frequent nowadays. A plethora of different strategies have been
applied to the design of such NHC donors, including the use of
1
the H and 13C NMR spectra, where the characteristic peaks
due to N−CH−N moieties were observed at 10.58 (2a), 10.75
(2b), and 10.57 (2c) ppm and at 143.3 (2a), 153.2 (2b), and
1
142.6 (2c) ppm in the H and 13C NMR spectra, respectively,
similar to those reported for the related N-alkyl(aryl)triazolium
salts.26 The final two steps leading to the formation of NHC−
Pd complexes were the generation of the free carbenes from the
respective triazolium salts with a strong base (potassium tert-
butoxide) and the subsequent in situ complexation with Pd(II)
precursors in the appropriate molar ratios (Scheme 1).
Scheme 1. Synthesis of the Carbene Precursors, Generation
of the Free Carbenes, and Subsequent Formation of the Pd
a
Complexes
a
X = Cl for 4a−c, OAc for 4a′−c′.
B
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On the basis of the previous results obtained for the Rh(I)
complexes,23 the formation of a very rare (with only two
previously published examples12d) seven-membered diazame-
tallacycle via strong intramolecular coordination of the pendant
amino group was expected for the products 4a and 4a′. This
assumption was at first confirmed by 1H and 13C NMR
spectroscopy. The generation of the free carbene and the
subsequent formation of the NHC−Pd complex were
1
unambiguously confirmed by the absence of the H NMR
resonance due to the imidazolium proton (N−CH−N) and by
the appearance of the low-field resonance of the Pd-bound
carbene carbon (157.6 ppm for 4a, 159.0 ppm for 4a′) in the
13C NMR spectrum. The coordination of the amino nitrogen
atom was indicated by the 1H NMR spectra, in which the NMe2
protons appeared as two singlets and the NCH2 protons gave
rise to an AB spin pattern. The same trend was noticed in the
13C NMR spectra, where two resonances were present for the
NMe2 carbons. These NMR-based conclusions were unequivo-
cally corroborated by X-ray structure determination in both
cases.
Figure 2. Molecular structure (ORTEP, 30% probability level) of 4a′·
H2O. H atoms and the water molecule have been omitted for clarity.
Selected interatomic distances [Å] and angles [deg]: Pd1−C10
1.968(3), Pd1−O3 2.033(2), Pd1−O1 2.063(2), Pd1−N1 2.085(3),
C10−N4 1.325(4), C10−N2 1.371(4), C10−Pd1−O3 90.18(11),
O3−Pd1−O1 84.41(10), C10−Pd1−N1 91.24(12), O1−Pd1−N1
94.52(10), N4−C10−N2 104.3(3), N4−C10−Pd1 134.6(2), N2−
C10−Pd1 120.1(2).
The geometry of complex 4a (Figure 1) is essentially square-
planar with an C1−Pd1−N4 bite angle of almost ideally 90°,
between the triazole ring and the plane determined by the
Pd atom and its four ligating atoms. The Pd−C bond in 4a′ is
similar in length to that in 4a, whereas the Pd−N distance
involving the amino-N donor atom is shorter than in 4a,
suggesting stronger coordination of the pendant arm in the
acetate complex. Again, a lengthening of the Pd−O1 bond
located trans to the carbene unit with respect to the Pd−O3
bond trans to the amino-N donor group [2.063(2) vs 2.033(2)
Å] due to the larger trans influence of the carbene ligand is
observed. This is in contrast to structural features found in the
previously published NHC-substituted palladium acetates with
either an internal28 or an external29 phosphane donor group,
where the trans influence exerted by the electron-donating
phosphane group overpowers that of the carbene.
In order to operate with a broader set of compounds for the
sake of comparison of their structural properties and possible
catalytic activities, two new simplified NHC ligands were
synthesized, and similar reactions with Pd(II) precursors were
successfully performed to generate the complexes 4b/4b′ and
4c/4c′ (see Scheme 1). The two complexes possessing the 2-
methoxyphenyl-substituted NHC ligands (4b and 4b′) were
prepared in order to distinguish between the formation of
either biscarbene or monocarbene complexes. The formation of
Figure 1. Molecular structure (ORTEP, 30% probability level) of 4a.
H atoms have been omitted for clarity. Selected interatomic distances
[Å] and angles [deg]: Pd1−C1 1.968(4), Pd1−N4 2.123(3), Pd1−Cl1
2.3034(9), Pd1−Cl2 2.3509(11), C1−N3 1.335(4), C1−N1 1.360(5),
C1−Pd1−N4 90.38(14), C1−Pd1−Cl1 85.40(10), N4−Pd1−Cl2
93.93(9), Cl1−Pd1−Cl2 90.44(4), N3−C1−N1 103.6(3), N3−C1−
Pd1 134.8(3), N1−C1−Pd1 121.1(3).
the carbene complexes was established by means of 1H and 13
C
thus proving the flexibility of the aliphatic amino group
compared with the rigid geometry of the oxazoline-based NHC
ligands.12a On the other hand, the C1−Pd1−Cl1 and N4−
Pd1−Cl2 angles are slightly more distorted. The elongation of
the Pd1−Cl2 bond trans to the carbene ligand compared with
the Pd−Cl1 bond trans to the amino-N donor group
[2.3509(11) vs 2.3034(9) Å] indicates a greater trans influence
of the NHC ligand. This is consistent with previous structural
studies.12a,27 The interplanar angle between the plane defined
by the triazole ring and the plane of the square-planar
environment of the Pd atom is 72.94(13)°. The values of the
remaining Pd−C [1.968(4) Å] and Pd−N [2.123(3) Å]
distances lie within the expected ranges.23
NMR spectroscopy in the same way as in the case of 4a and
1
4a′. The H NMR spectra of 4b and 4b′ lacked the resonance
due to the imidazolium proton (N−CH−N), while the low-
field-shifted resonance of the carbene carbon appeared in the
13C NMR spectra (172.0 ppm for 4b, 171.9 ppm for 4b′).
Unambiguous proof of the formation of trans biscarbene
complexes came from the X-ray structure determinations
(Figure 3 for 4b; Figure S1 in the Supporting Information
for 4b′). When the 1H and 13C NMR spectra of samples of 4b
and 4b′ dissolved in CDCl3 were measured after a few days at
room temperature, another set of signals with a similar spectral
pattern appeared in each NMR spectrum. This is in line with
the very recent work of Hong18 describing the possible trans to
cis conversion of analogous biscarbene complexes in CDCl3. To
The molecular structure of 4a′ (Figure 2) resembles that of
4a, having the same conformation and interplanar angle
C
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confirm this assumption, the spectra of 4b in CDCl3 were
measured immediately after preparation and then again after 24
h of heating at 40 °C. The spectra of the former sample showed
only one set of signals attributed to the trans isomer, while the
spectra of the latter contained signals of both isomers in an
approximate ratio of 1:1 (for comparison, see Figures S10−S13
in the Supporting Information).
Compound 4b is a square-planar Pd(II) complex showing a
trans geometry and a coplanar arrangement of the NHC ligands
in anti positions, with the Pd atom residing at the inversion
center (Figure 3). The methoxy group of the NHC ligand is
Figure 4. Molecular structure (ORTEP, 40% probability level) of 4c·
2CHCl3. H atoms and the solvent molecules have been omitted for
clarity. Selected interatomic distances [Å] and angles [deg]: Pd1−C20
2.023(5), Pd1−C1 2.033(5), Pd1−Cl2 2.3249(14), Pd1−Cl1
2.3328(13), N1−C1 1.361(6), C1−N3 1.329(6), C20−N6 1.338(6),
C20−N4 1.373(6), C20−Pd1−Cl2 88.26(14), C1−Pd1−Cl2
89.95(13), C20−Pd1−Cl1 91.38(14), C1−Pd1−Cl1 90.42(13),
N3−C1−N1 104.1(4), N3−C1−Pd1 132.3(4), N1−C1−Pd1
123.5(3), N6−C20−N4 103.8(4), N6−C20−Pd1 132.5(4), N4−
C20−Pd1 123.4(3).
Figure 3. Molecular structure (ORTEP, 40% probability level) of 4b·
CHCl3. H atoms and the chloroform molecule have been omitted for
clarity. Selected interatomic distances [Å] and angles [deg]: Pd1−C1
2.025(4), Pd1−Cl1 2.3145(11), C1−N3 1.336(5), C1−N1 1.366(5),
C1−Pd1−Cl1 88.89(11) [91.12(11)], N3−C1−N1 103.1(3), N3−
C1−Pd1 134.8(3), N1−C1−Pd1 122.1(3).
directed away from the metal center and does not take part in
intra- or intermolecular coordination. This is in good
agreement with the results published earlier for similar O-
functionalized NHCs.30 The lengths of the Pd−C and Pd−Cl
bonds are comparable to those in other related structurally
characterized NHC−Pd complexes with chloride auxiliary
ligands.30,31
Figure 5. Molecular structure (ORTEP 30%, probability level) of 4c′.
H atoms have been omitted for clarity. Selected interatomic distances
[Å] and angles [deg]: Pd1−O1A 2.007(3), Pd1−C1 2.024(5), C1−
N1 1.455(15), N1−C3 1.431(8), O1A−Pd1−C1 85.74(18)
[93.58(17)], N3−C1−N1 101.2(5), N3−C1−Pd1 139.3(9), N1−
C1−Pd1 119.2(7).
The third and most simplified version of the NHC ligand
having only a p-tolyl substituent was finally synthesized and
employed in the same manner as the previous two ligands,
completing the set of studied compounds. A comparison with
the above-mentioned methoxy-substituted ligand left no doubt
about the formation of the biscarbene complexes 4c and 4c′.
1
The H and 13C NMR spectra provided clear signs supporting
the formation of these products. The 13C NMR chemical shifts
for the carbene carbon atoms in 4c and 4c′ were similar to
those in 4b and 4b′ (171.4 ppm for 4c, 171.5 ppm for 4c′). In
both cases, single crystals suitable for X-ray structure
determination were obtained, and the complexes were
structurally characterized. The X-ray structure of 4c (Figure
4) has an almost identical trans-anti arrangement as in the case
of 4b with bond distances and angles within the expected
ranges. On the other hand, the X-ray structure of 4c′ has quite
interesting features (Figure 5). First of all, there are very few
known examples of palladium complexes with two mono-
dentate-bonded acetate ligands with a trans-anti-type con-
formation of coplanar NHC ligands.32 The square-planar
geometry of 4c′ is slightly distorted with a deviation of about
0.127 Å, which does not have any significant effect on the Pd−
C bond distances compared with the previous complexes. The
most surprising feature is seen in the mutual twisting of the
carbene ligands. The interplanar angle of 59.48(11)° is, to the
best of our knowledge, unprecedented among complexes of this
kind.
Cross-coupling reactions, including the Suzuki−Miyaura
reaction, have become some of the most important and widely
used methods in fine chemical synthesis during the last
decades.1 Therefore, the catalytic activity of carbene complexes
4 was evaluated in the Suzuki−Miyaura cross-coupling of aryl
halides with phenylboronic acid to give the corresponding
biphenyls as the model reaction (Scheme 2). In the tested
series, only the N-chelated carbene complexes 4a and 4a′
D
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were achieved in azeotropic ethanol. The reaction in this
solvent, which is the cheapest and least environmentally
demanding among the tested solvents, proceeded very rapidly,
giving the coupling product in essentially quantitative yield
(>95%) within 30 min at 80 °C for both catalysts 4a and 4a′
(0.5 mol %). This solvent was therefore applied in all of the
subsequent reaction tests.
Scheme 2. Suzuki−Miyaura Cross-Coupling of Aryl Halides
with Phenylboronic Acid
showed some catalytic activity, whereas the biscarbene
complexes 4b/4b′ and 4c/4c′ were totally inactive in this
reaction. This contrasts with the previously published
results,30,31b,c,33 among which the most surprising is the good
catalytic activity of [1-(2-methoxybenzyl)-3-tert-butylimidazol-
2-ylidene]2PdCl2,30 which features a structural motif very
similar to that in 4b.
The dependence of the yield of 4-methylbiphenyl on the
reaction temperature is illustrated in Figure 8, which
Figures 6 and 7 show the kinetic profiles for the coupling
reactions of 4-bromotoluene with phenylboronic acid per-
Figure 8. Dependence of the yield of the coupling product (4-
methylbiphenyl) on the reaction temperature after 1 h (red bars) and
6 h (blue bars) using (left) 4a and (right) 4a′ as the catalyst.
Conditions: 4-bromotoluene (1.0 mmol), phenylboronic acid (1.2
mmol), K2CO3 (1.2 mmol), and 4 (0.5 mol %) in ethanol (5 mL).
summarizes the results achieved with catalysts 4a and 4a′ in
ethanol. The conversions increased with the reaction tempera-
ture as expected and were rather similar for the two complexes.
Furthermore, we studied the possible influence of the base
additive. For this purpose, the model coupling reaction was
carried out with 0.5 mol % 4a in the presence of various bases
(1.2 molar equiv. with respect to the aryl halide) in 96%
ethanol at room temperature for 1 h. In the reactions
performed with anhydrous alkali-metal carbonates, the yield
of the coupling product increased practically linearly with the
square of the ionic radius of the alkali-metal cation34 (Figure 9).
In other words, salts with larger, more polarizable cations that
Figure 6. Kinetic profiles for the Suzuki−Miyaura cross-coupling of 4-
bromotoluene with phenylboronic acid in the presence of K2CO3 and
0.5 mol % 4a in various solvents.
Figure 7. Kinetic profiles for the Suzuki−Miyaura cross-coupling of 4-
bromotoluene with phenylboronic acid in the presence of K2CO3 and
0.5 mol % 4a′ in various solvents.
formed in the presence of 0.5 mol % 4a and 4a′, respectively,
and K2CO3 as the base at 80 °C in various solvents. In dioxane
and MeCN, the complexes performed practically identically,
affording the coupling product in yields of ca. 30% and 55%,
respectively, after 8 h. In contrast, the reactions in DMF
proceeded considerably faster and also differentiated the
catalysts (conversions of 77% for 4a and 91% for 4a′ after 8
h). A similar difference was observed in N,N-dimethylacetamide
(DMA), where the yields of the biphenyl were 52% and 71%
after 8 h for 4a and 4a′, respectively. Finally, the best results
Figure 9. Dependence of the yield of 4-methylbiphenyl in the model
coupling reaction on the square of the ionic radius of the alkali-metal
cation (M) in M2CO3 used as the base. Conditions: 4-bromotoluene
(1.0 mmol), phenylboronic acid (1.2 mmol), M2CO3 (1.2 mmol), and
0.5 mol % 4a in ethanol (5 mL) at 25 °C for 1 h.
E
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are more soluble in the reaction medium showed better results,
which is indeed in line with previous observations.35 Variation
of the anion was then performed for a small series of sodium
salts. However, with the exception of Na3PO4 (23%
conversion), common sodium salts (NaF, CH3CO2Na)
afforded only poor yields (≲10%; see Table S1 in the
Supporting Information).
The results of the “preparative” reaction tests summarized in
Table 1 confirmed that the coupling reactions with aryl
a
Table 1. Summary of Preparative Catalytic Experiments
substrate
temp./time
yield of biphenyl
4-bromoacetophenone
4-bromoacetophenone
4-bromoacetophenone
4-bromoanisole
1 h/25 °C
24 h/25 °C
1 h/80 °C
1 h/25 °C
24 h/25 °C
1 h/80 °C
1 h/25 °C
1 h/25 °C
1 h/25 °C
3 h/80 °C
3 h/80 °C
3 h/80 °C
3 h/80 °C
3 h/80 °C
65%
84%
100%
37%
Figure 10. Comparison of the catalytic activity of 4a (red circles) with
that of the commercial catalyst PEPPSI (black circles). Conditions: 4-
chlorotoluene (1.0 mmol), phenylboronic acid (1.2 mmol), Cs2CO3
(1.2 mmol), and catalyst (0.5 mol %) in azeotropic ethanol (5 mL) at
80 °C. The lines connecting the experimental points are shown only as
guides to the eye.
4-bromoanisole
35%
4-bromoanisole
100%
100%
1-bromo-4-(trifluoromethyl)benzene
4-bromo-1-nitrobenzene
4-bromobenzonitrile
4-chloroacetophenone
4-chloroanisole
b
82%
100%
24%
3%
1-chloro-4-(trifluoromethyl)benzene
4-chloro-1-nitrobenzene
4-chlorobenzonitrile
75%
ca. 73%
ca. 25%
a
Conditions: Substrate (1.0 mmol), phenylboronic acid (1.2 mmol),
base (1.2 mmol; K2CO3 for aryl bromides, Cs2CO3 for aryl chlorides),
and catalyst 4a (0.5 mol %) in azeotropic ethanol (5 mL).
b
Nitrobenzene, a dehalogenation product, was formed in 18% yield.
bromides mediated by catalyst 4a (0.5 mol %) can be
performed well at room temperature. The activated substrates
were fully converted to the biphenyl products within 1 h, while
the deactivated ones achieved only partial conversions.
Extending the reaction time to 24 h improved the yields only
partially. Eventually, complete conversions were obtained
within 1 h upon heating to 80 °C. The corresponding aryl
chlorides afforded considerably lower yields of the coupling
products. The catalytic performance of complex 4a thus
appears to be slightly worse than that of the commercial
catalyst PEPPSI, which is also a chloro−carbene Pd(II)
complex.6 This was indeed confirmed by a series of comparative
experiments. For instance, the reaction of phenylboronic acid
with 4-chlorotoluene as a deactivated aryl chloride afforded 4-
methylbiphenyl in 54% yield when performed with 1 mol % 4a.
A similar reaction in the presence of PEPPSI-IPr afforded the
coupling product in 69% yield (for the kinetic profiles, see
Figure 10). On the other hand, catalyst 4a maintained its
catalytic activity for a longer time. Whereas the reaction with
the PEPPSI-IPr catalyst practically stopped within ca. 30 min,
compound 4a maintained its activity for ca. 3 h.36
In another series of experiments (Figure 11), we focused on
the nature of the actual catalytic species.37 Thus, the reaction of
4-bromotoluene with phenylboronic acid was allowed to
proceed in the presence of catalyst 4a (0.5 mol %) at 80 °C
for 1 h, and then the reaction mixture was filtered while hot
through a PTFE syringe filter (pore size 0.45 μm). The
filtration did not apparently affect the reaction, which
proceeded similarly in the filtrate and the nonfiltered part of
the reaction mixture. However, when a small amount of
mercury (0.1 mL) was added after reaction for 1 h, the reaction
Figure 11. Comparison of the kinetic profiles for a standard reaction
mixture (black circles), a reaction mixture that was filtered after 1 h
(blue circles), and a reaction mixture to which mercury metal was
added after 1 h (red circles). Conditions: 4-bromotoluene (1.0 mmol),
phenylboronic acid (1.2 mmol), K2CO3 (1.2 mmol), and 4a (0.5 mol
%) in azeotropic ethanol (5 mL) at 80 °C. The lines connecting the
black and blue experimental points are shown only as guides to the
eye.
stopped as a result of the metal scavenging effect of the mercury
metal, which suggests the formation of fine palladium(0)
particles38 or a heterogeneous nature for the catalyst formed in
the reaction mixture from complex 4a. This observation is
indeed in line with those made earlier for similar, PEPPSI-like
catalysts.9 In order to rule out possible degradation of complex
4a under the given reaction conditions, neat 4a was heated in
ethanol at 80 °C for 4 h. This stability test, which mimicked the
most severe reaction conditions used in any of catalytic
experiments of this study, showed no signs of the
decomposition of 4a.
In order to gain more detailed insight into the nature of the
bonding situation in the complexes reported in this paper, a
theoretical survey was performed. The first part of this study
was conducted on compounds 4a and 4b, starting from their
structures determined by X-ray diffraction analysis. Next, the
F
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donating ability of the different types of NHC ligands was
explored in the model PEPPSI-type6 complex. A similar
approach as in our study on coinage metal−NHC complexes
was adopted,39 using energy decomposition and natural orbital
for chemical valence (NOCV) analyses40 of the optimized
structures (for details, see the Experimental Section). For
calculation purposes, the tert-butyl and phenyl substituents on
the carbene ligand were replaced by methyl groups, as these
groups exhibit only a minor influence on the metal−carbene
bonding.
The formation of the metal−carbene bond in the complex is
associated with an electron density reorganization. The
eigenvectors that diagonalize this deformation density are the
NOCVs, which can be shown to occur in pairs, each pair
describing a contribution to the total charge transfer upon
bonding. For each of the NOCV pairs, one can also compute
the energetic contribution to the charge transfer component of
bonding (or orbital interaction contribution) and the number
of electrons transferred. We found that for the analogues of
compounds 4a and 4b (Figure 12) the density depletion (red)/
accumulation (blue) patterns of the two most dominant NOCV
pairs similarly describe the strong σ bonding (NOCV-1) and
the weak π back-bonding (NOCV-2) of the carbene with the
palladium dichloride fragment. In the case of 4a, we found an
additional important NOCV pair (NOCV-3) showing very
clearly a strong contribution of the coordinated amino group to
the overall bond formation. For the exact determination of the
donating ability of the nitrogen atom itself, the fact that a minor
part of this contribution is also expressed in NOCV-1 must be
taken into account (for the whole set of results of these
calculations, see the Supporting Information). The conclusion
from both studies is that even though the triazole-based
carbene itself has the second-lowest stabilizing effect in the
series of six different NHCs, the presence of the amino group
with the extra donating ability significantly improves the
performance of the whole carbene ligand.
CONCLUSION
■
Six novel palladium complexes containing three different
triazole-based NHCs were synthesized and structurally
characterized. Among these compounds, only PdCl2 and
Pd(OAc)2 derivatives bearing the 2-[(dimethylamino)methyl]-
phenyl-substituted NHC ligand revealed a configuration
involving a monocarbene-type complex with a strong intra-
molecular (i.e., chelating) coordination of the pendant amino
group, resulting in the formation of an unusual seven-
membered diazapalladacycle. On the other hand, the other
two ligands bearing a 2-methoxyphenyl or 4-methylphenyl
substituent form the usual biscarbene complexes. The catalytic
activity evaluation revealed that only the monocarbene
complexes 4a and 4a′ are active in the Suzuki−Miyaura
cross-coupling of aryl halides with phenylboronic acid.
However, in comparison with other systems, the catalytic
performance of 4a and 4a′ can be achieved under “greener”
conditions (lower temperature, shorter time, azeotropic ethanol
as a solvent). Nonetheless, the catalytic performance of 4a was
slightly worse than that of the commercially available catalyst
PEPPSI-IPr. Theoretical calculations revealed that the
functionalization of the triazole backbone with the amino
group has a significant stabilizing effect on the NHC−metal
bonding and changes the overall coordinating ability of the
modified NHC ligand.
EXPERIMENTAL SECTION
■
General Comments. All of the reactions leading to free carbenes
and their Pd(II) complexes were performed under an argon
atmosphere using standard Schlenk techniques. A solvent purification
system was used to dry the solvents, which were then degassed and
stored under an argon atmosphere.
Materials. N,N-Dimethyl-1-[2-(3-phenyl-4H-1,2,4-triazol-4-yl)-
phenyl]methanamine (1a) and 1-tert-butyl-4-{2-[(dimethylamino)-
methyl]phenyl}-3-phenyl-4H-1,2,4-triazol-1-ium perchlorate (2a)
were prepared according to the previously published procedures.23
NMR Spectroscopy. Samples for NMR measurements were
obtained by dissolving ca. 40 mg of the analyzed compound in ca. 0.7
mL of a suitable deuterated solvent (DMSO-d6, CDCl3, or CD2Cl2).
1
The H chemical shifts are given relative to internal tetramethylsilane
[δ(1H) = 0.00 ppm] or to residual solvent signals [DMSO-d6, δ(1H) =
2.50 ppm; CDCl3, δ(1H) = 7.26 ppm; CD2Cl2, δ(1H) = 5.32 ppm].
The 13C chemical shifts were calibrated similarly [DMSO-d6, δ(13C) =
39.52 ppm; CDCl3, δ(13C) = 77.16 ppm; CD2Cl2, δ(13C) = 53.84
ppm]. All of the 13C NMR spectra were measured using the standard
proton-decoupled experiment, and CH and CH3 versus C and CH2
groups were distinguished by the APT method.41
Figure 12. Relevant NOCV pairs with the corresponding density
changes, numbers of electrons transferred, and contributions to the
total orbital interaction energy component of the binding energy for
the simplified models of 4a and 4b.
Elemental Analysis. Elemental compositions were determined
under an argon atmosphere using an automatic combustion analyzer.
G
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Analysis could not be performed for compound 2 because of the
possibly explosive nature of the perchlorate salts.
Pd(OAc)2 for 4a′−c′), which was mixed with dry THF (ca. 10 mL).
The solution of the freshly prepared free carbene 3 dissolved in 30 mL
of dry THF was added dropwise to a stirred suspension of the Pd(II)
complex at room temperature (3:Pd(II) complex molar ratio = 1:0.9
for 4a/4a′ and 1:1.9 for 4b/4b′ and 4c/4c′). After the reaction
mixture was stirred overnight, the solid residue was filtered off, and the
filtrate was evaporated to dryness. The crude product was washed with
hexane to give pure 4 as a yellow to yellow-brown powder. Single
crystals suitable for X-ray diffraction were obtained by gas-phase
diffusion of hexane into a solution of 4 in dichloromethane.
X-ray Crystallography. The diffraction data (see Table S2 in the
Supporting Information) were obtained at 150(2) K on a Nonius
KappaCCD diffractometer with Mo Kα radiation (λ = 0.71073 Å)
using a graphite monochromator and the ϕ and χ scan mode. Data
reductions were performed with DENZO-SMN.42 The data were
corrected for absorption by integration methods.43 The structures
were solved by direct methods (SIR92)44 and refined by full matrix
least-squares based on F2 (SHELXL97).45 Hydrogen atoms could be
mostly localized on the difference Fourier maps. However, to ensure
uniformity of the treatment of the crystal structures, all of the
hydrogen atoms were recalculated into their idealized positions (riding
model) and assigned temperature factors of Uiso(H) = 1.2Ueq (pivot
atom) or 1.5Ueq (methyl moiety), with C−H distances of 0.96, 0.97,
and 0.93 Å for methyl, methylene, and aromatic ring or allyl moieties,
respectively.
(1-tert-Butyl-4-{2-[(N,N-dimethylamino)methyl]phenyl}-3-phe-
nyl-1H-1,2,4-triazol-4-ium-5-ide)palladium(II) Dichloride (4a). Yield:
32%. 1H NMR (500 MHz, CDCl3): δ 7.61−7.57 (m, 2H, ArH), 7.46−
3
3
7.43 (m, 2H, ArH), 7.31 (t, 2H, J = 8.0 Hz, ArH), 7.18 (d, 1H, J =
3
8.0 Hz, ArH), 7.08 (d, 2H, J = 7.0 Hz, ArH), 3.39, 3.31 (AB spin
system, Δδ = 0.08 ppm = 38 Hz, 2J = 12 Hz, 2H, NCH2), 2.95 (s, 3H,
NCH3), 2.89 (s, 3H, NCH3), 2.16 (s, 9H, C(CH3)3). 13C NMR (126
MHz, CDCl3): δ 158.0 (NCcarbN), 152.2 (NCN), 135.8 (ArC), 133.3
(ArC), 131.2 (ArC), 130.9 (ArC), 130.6 (ArC), 130.0 (ArC), 128.9
(ArC), 127.3 (ArC), 124.2 (ArC), 67.7 (NCH2), 64.2 (C(CH3)3), 54.5
(NCH3), 52.5 (NCH3), 31.1 (C(CH3)3). Anal. Calcd (%) for
C21H26Cl2N4Pd: C, 49.28; H, 5.12; N, 10.95; Cl, 13.85. Found: C,
49.22; H, 5.02; N, 10.87; Cl, 13.94. Mp: 230 °C (dec.).
The crystallographic data have been deposited at the Cambridge
Crystallographic Data Centre (CCDC) under CCDC deposition
numbers 921181−921186. Copies of this information may be obtained
free of charge from The Director, CCDC, 12 Union Road, Cambridge
CB2 1EY, U.K. (fax: +44−1223−336033; e-mail: deposit@ccdc.cam.
ac.uk; Web: http://www.ccdc.cam.ac.uk).
Preparation of 1 (Recyclization). Compounds 1b and 1c were
(1-tert-Butyl-4-{2-[(N,N-dimethylamino)methyl]phenyl}-3-phe-
nyl-1H-1,2,4-triazol-4-ium-5-ide)palladium(II) Diacetate (4a′).
prepared according to the published general procedure.24a
1
3
Yield: 48%. H NMR (500 MHz, CD2Cl2): δ 7.59 (t, 1H, J = 7.5
4-(2-Methoxyphenyl)-3-phenyl-4H-1,2,4-triazole (1b). Yield: 68%.
1H NMR (400 MHz, DMSO-d6): δ 8.75 (s, 1H, CH), 7.52 (td, 1H, 3J
= 7.8, 4J = 1.6 Hz, ArH), 7.44−7.33 (m, 6H, ArH), 7.22 (dd, 1H, 3J =
3
Hz, ArH), 7.53−7.49 (m, 2H, ArH), 7.44 (t, 1H, J = 7.5 Hz, ArH),
7.33 (t, 2H, 3J = 8.0 Hz, ArH), 7.27 (d, 1H, 3J = 7.5 Hz, ArH), 7.20 (d,
3
4
3
4
1H, J = 7.5 Hz, ArH), 3.34, 3.19 (AB spin system, Δδ = 0.15 ppm =
8.4, J = 0.8 Hz, ArH), 7.08 (td, 1H, J = 7.6, J = 0.8 Hz, ArH), 3.58
(s, 3H, OCH3). 13C NMR (100 MHz, DMSO-d6): δ 153.9 (NCN),
152.9 (NCHN), 146.0 (ArC), 131.4 (ArC), 129.7 (ArC), 128.6 (ArC),
128.4 (ArC), 127.4 (ArC), 127.2 (ArC), 123.2 (ArC), 121.1 (ArC),
113.1 (ArC), 55.8 (OCH3). Anal. Calcd (%) for C15H13N3O: C, 71.70;
H, 5.20; N, 17.72. Found: C, 71.78; H, 5.23; N, 17.69. Mp: 107−110
°C.
2
76 Hz, J = 12.5 Hz, 2H, NCH2), 2.99 (s, 3H, NCH3), 2.43 (s, 3H,
NCH3), 2.07 (s, 9H, C(CH3)3), 1.78 (s, 3H, CH3COO−), 1.67 (s, 3H,
CH3COO−). 13C NMR (126 MHz, CD2Cl2): δ 177.2 (CH3COO−),
175.6 (CH3COO−), 159.6 (NCcarbN), 152.1 (NCN), 137.3 (ArC),
132.4 (ArC), 131.1 (ArC), 130.6 (ArC), 130.5 (ArC), 129.7 (ArC),
129.1 (ArC), 129.0 (ArC), 128.9 (ArC), 125.1 (ArC), 67.3 (NCH2),
63.5 (C(CH3)3), 55.1 (NCH3), 50.3 (NCH3), 30.5 (C(CH3)3), 24.3
(CH3COO−), 22.7 (CH3COO−). Anal. Calcd (%) for C25H32N4O4Pd:
C, 53.72; H, 5.77; N, 10.02. Found: C, 53.75; H, 5.80; N, 10.11. Mp:
196−197 °C (dec.).
4-(4-Methylphenyl)-3-phenyl-4H-1,2,4-triazole (1c). Yield: 73%.
1H NMR (400 MHz, DMSO-d6): δ 8.84 (s, 1H, CH), 7.43−7.35 (m,
5H, ArH), 7.32−7.30 (m, 2H, ArH), 7.28−7.25 (m, 2H, ArH), 2.35 (s,
3H, CH3). 13C NMR (100 MHz, DMSO-d6): δ 152.3 (NCN), 145.7
(NCHN), 138.97 (ArC), 132.07 (ArC), 130.27 (ArC), 129.77 (ArC),
128.77 (ArC), 128.47 (ArC), 126.87 (ArC), 125.97 (ArC), 20.77
(CH3). Anal. Calcd (%) for C15H13N3: C, 76.57; H, 5.57; N, 17.86.
Found: C, 76.49; H, 5.62; N, 17.88. Mp: 144−146 °C.
trans-Bis[1-tert-butyl-4-(2-methoxyphenyl)-3-phenyl-1H-1,2,4-tri-
azol-4-ium-5-ide]palladium(II) Dichloride (4b). Yield: 62%. 1H NMR
(400 MHz, CDCl3): δ 7.94 (d, 2H, 3J = 7.6 Hz, ArH), 7.49 (t, 2H, 3J =
3
7.8 Hz, ArH), 7.31−7.14 (m, 12H, ArH), 6.94 (d, 2H, J = 8.4 Hz,
ArH), 3.52 (s, 6H, OCH3), 1.67 (s, 18H, C(CH3)3). 13C NMR (101
MHz, CDCl3): δ 172.0 (NCcarbN), 155.6 (NCN), 152.4 (ArC), 132.0
(ArC), 131.1 (ArC), 129.9 (ArC), 128.3 (ArC), 128.2 (ArC), 126.6
(ArC), 121.2 (ArC), 112.8 (ArC), 62.0 (C(CH3)3), 56.1 (OCH3), 31.0
(C(CH3)3). Anal. Calcd (%) for C38H42Cl2N6O2Pd: C, 57.62; H, 5.34;
N, 10.61; Cl, 8.95. Found: C, 57.63; H, 5.40; N, 10.68; Cl, 9.06. Mp:
295−297 °C (dec.).
Preparation of 2 (Quaternization). Compounds 2b and 2c were
prepared according to the published general procedure.24b
1-tert-Butyl-4-(2-methoxyphenyl)-3-phenyl-4H-1,2,4-triazol-1-
ium Perchlorate (2b). Yield: 84%. 1H NMR (400 MHz, DMSO-d6): δ
10.75 (s, 1H, CH), 7.81 (dd, 1H, 3J = 7.9, 4J = 1.6 Hz, ArH), 7.66 (td,
3
4
1H, J = 7.8, J = 1.6 Hz, ArH), 7.61−7.57 (m, 1H, ArH), 7.52−7.44
(m, 4H, ArH), 7.29−7.23 (m, 2H, ArH), 3.56 (s, 3H, OCH3), 1.76 (s,
9H, C(CH3)3). 13C NMR (100 MHz, DMSO-d6): δ 153.4 (NCN),
153.2 (NCHN), 143.1 (ArC), 133.3 (ArC), 132.1 (ArC), 129.1 (ArC),
128.4 (ArC), 128.1 (ArC), 123.4 (ArC), 121.4 (ArC), 120.6 (ArC),
113.4 (ArC), 63.8 (C(CH3)3), 56.1 (OCH3), 28.1 (C(CH3)3).
1-tert-Butyl-4-(4-methylphenyl)-3-phenyl-4H-1,2,4-triazol-1-ium
Perchlorate (2c). Yield: 87%. 1H NMR (400 MHz, DMSO-d6): δ
10.57 (s, 1H, CH), 7.59−7.55 (m, 1H, ArH), 7.53−7.51 (m, 2H,
ArH), 7.48−7.46 (m, 4H, ArH), 7.42−7.40 (m, 2H, ArH), 2.39 (s, 3H,
CH3), 1.75 (s, 9H, C(CH3)3). 13C NMR (100 MHz, DMSO-d6): δ
152.6 (NCN), 142.6 (NCHN), 141.1 (ArC), 131.9 (ArC), 130.4
(ArC), 129.9 (ArC), 129.3 (ArC), 129.1 (ArC), 126.5 (ArC), 123.1
(ArC), 63.4 (C(CH3)3), 28.1 (C(CH3)3), 20.8 (CH3).
trans-Bis[1-tert-butyl-4-(2-methoxyphenyl)-3-phenyl-1H-1,2,4-tri-
azol-4-ium-5-ide]palladium(II) Diacetate (4b′). Yield: 67%. 1H NMR
4
(400 MHz, CDCl3): δ 8.64 (dd, 2H, 3J = 8.0, J = 1.6 Hz, ArH), 7.53
3
4
(td, 2H, J = 7.7, J = 1.6 Hz, ArH), 7.36−7.23 (m, 12H, ArH), 6.88
(dd, 2H, 3J = 8.4, 4J = 0.8 Hz, ArH), 3.42 (s, 6H, OCH3), 1.76 (s, 18H,
C(CH3)3), 1.69 (s, 6H, CH3COO−). 13C NMR (126 MHz, CD2Cl2):
δ 175.5 (CH3COO−), 171.9 (NCcarbN), 155.2 (NCN), 152.3 (ArC),
132.0 (ArC), 131.0 (ArC), 130.0 (ArC), 128.4 (ArC), 128.3 (ArC),
127.0 (ArC), 121.7 (ArC), 112.1 (ArC), 62.1 (C(CH3)3), 55.8
(OCH3), 30.4 (C(CH3)3), 23.1 (CH3COO−). Anal. Calcd (%) for
C42H48N6O6Pd: C, 60.10; H, 5.76; N, 10.01. Found: C, 60.03; H, 5.83;
N, 10.15. Mp: 255 °C (dec.).
trans-Bis[1-tert-butyl-4-(4-methylphenyl)-3-phenyl-1H-1,2,4-tria-
General Procedure for the Generation of Free Carbenes
(3a−c) and the Preparation of (NHC)nPdX2 Complexes (4a−c
and 4a′−c′). Free carbene 3 was generated at room temperature from
the reaction of 2 with t-BuOK (2:t-BuOK molar ratio = 1.05:1) in dry
toluene (ca. 30 mL). After 1 h of stirring, the resulting precipitate was
filtered off, and the colorless to light-yellow filtrate was evaporated to
dryness to afford free carbene 3. Another Schlenk tube was charged
with the precursor Pd(II) complex ([PdCl2(CH3CN)2] for 4a−c or
1
zol-4-ium-5-ide]palladium(II) Dichloride (4c). Yield: 65%. H NMR
(400 MHz, CDCl3): δ 7.51−7.49 (m, 4H, ArH), 7.33−7.29 (m, 6H,
ArH), 7.23−7.22 (m, 8H, ArH), 2.50 (s, 6H, CH3), 1.62 (s, 18H,
C(CH3)3). 13C NMR (101 MHz, CDCl3): δ 171.4 (NCcarbN), 151.7
(NCN), 139.4 (ArC), 134.8 (ArC), 130.2 (ArC), 129.9 (ArC), 129.6
(ArC), 128.6 (ArC), 128.3 (ArC), 125.7 (ArC), 61.8 (C(CH3)3), 30.7
(C(CH3)3), 21.3 (CH3). Anal. Calcd (%) for C38H42Cl2N6Pd: C,
H
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60.05; H, 5.57; N, 11.06; Cl, 9.33. Found: C, 59.98; H, 5.52; N, 11.21;
Cl, 9.40. Mp: 299−301 °C (dec.).
513. (c) Suzuki, A. Acc. Chem. Res. 1982, 15, 178. (d) Miyaura, N.;
Suzuki, A. Chem. Rev. 1995, 95, 2457. (e) Maes, B. U. W.; R’Kyek, O.;
Kosmrlj, J.; Lemiere, G. L. F.; Esmans, E.; Rozenski, J.; Dommisse, R.
̀
trans-Bis[1-tert-butyl-4-(4-methylphenyl)-3-phenyl-1H-1,2,4-tria-
1
zol-4-ium-5-ide]palladium(II) Diacetate (4c′). Yield: 69%. H NMR
A.; Haemers, A. Tetrahedron 2001, 57, 1323. (f) Cross-Coupling
ReactionsA Practical Guide; Miyaura, N., Ed.; Springer: Berlin, 2002.
(g) Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.; de Meijere, A.,
Diederich, F., Eds.; Wiley-VCH: Weinheim, Germany, 2004.
(h) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem. 2005,
117, 4516; Angew. Chem., Int. Ed. 2005, 44, 4442. (i) Brasholz, M.;
Luan, X.; Reissig, H.-U. Synthesis 2005, 3571. (j) Tang, Q.;
Gianatassio, R. Tetrahedron Lett. 2010, 51, 3473. (k) Wu, X.-F.;
Anbarasan, P.; Neumann, H.; Beller, M. Angew. Chem., Int. Ed. 2010,
49, 9047. (l) Matos, M. J.; Vazquez-Rodriguez, S.; Borges, F.; Santana,
L.; Uriarte, E. Tetrahedron Lett. 2011, 52, 1225.
(400 MHz, CDCl3): δ 7.55−7.53 (m, 4H, ArH), 7.31−7.26 (m, 10H,
ArH), 7.22−7.19 (m, 4H, ArH), 2.49 (s, 6H, CH3), 1.73 (s, 6H,
CH3COO−), 1.63 (s, 18H, C(CH3)3). 13C NMR (100 MHz, CDCl3):
δ 175.5 (CH3COO−), 171.5 (NCcarbN), 151.2 (NCN), 138.9 (ArC),
135.0 (ArC), 129.7 (ArC), 129.6 (ArC), 129.5 (ArC), 128.8 (ArC),
128.1 (ArC), 126.1 (ArC), 61.5 (C(CH3)3), 30.2 (C(CH3)3), 23.5
(CH3COO−), 21.4 (CH3). Anal. Calcd (%) for C42H48N6O4Pd: C,
62.49; H, 5.99; N, 10.41. Found: C, 62.51; H, 6.03; N, 10.37. Mp:
340−345 °C (dec.).
Catalytic Tests. A Schlenk tube was charged with the respective
aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol), the base (1.2
mmol), and the Pd catalyst, flushed with nitrogen, and sealed. The
solvent was introduced, and the reaction vessel was transferred to a
parallel batch reactor. The progress of the coupling reactions was
monitored by gas chromatography.
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The reaction mixtures resulting from the “preparative” experiments
(Table 1) were diluted with water and extracted into dichloromethane.
The organic layer was dried over MgSO4 and passed through a short
silica gel column. The eluate was evaporated and analyzed by 1H and/
or 19F NMR spectroscopy (CDCl3 was used as the solvent).
Computational Details. Density functional theory (DFT)46
geometry optimizations of the molecules were performed with the
Gaussian 09 program47 using the B3LYP hybrid functional48 with the
Dunning-type basis set cc-pVDZ49 for carbon, hydrogen, nitrogen, and
chlorine and the cc-pVDZ-pp50 basis set for palladium. Frequency
calculations were carried out to confirm that the obtained structures
corresponded to minima on the potential energy surface. Next, the
Ziegler−Rauk-type energy decomposition51 and NOCV analyses were
carried out on the B3LYP/cc-pVDZ(-pp)-optimized structures using
the PBE52/TZ2P (small-core) functional/basis set combination as
implemented in ADF2012.02.53 Relativistic effects as well as dispersion
corrections were taken into account using the zeroth-order regular
approximation (ZORA)54 and Grimme’s revised DFT-D3 method.55
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̈
ASSOCIATED CONTENT
* Supporting Information
■
18, 6055. (b) Keske, E. C.; Zenkina, O. V.; Wang, R.; Crudden, C. M.
Organometallics 2012, 31, 6215. (c) Huang, J.; Hong, J.-T.; Hong, S. H.
Eur. J. Org. Chem. 2012, 6630.
S
Molecular structure of complex 4b′, additional results from
catalytic tests, crystallographic tables and details (CIF) for
compounds 4a−c and 4a′-c′, an extended discussion of the
computational survey, NMR spectra for compounds 4a−c and
4a′-c′, and a text file containing all of the computed molecule
Cartesian coordinates in a format for convenient visualization.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(10) Lavallo, V.; Canac, Y.; Prasang, C.; Donnadieu, B.; Bertrand, G.
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Glorius, F. Synthesis 2008, 2221. (d) Nasielski, J.; Hadei, N.;
Achonduch, G.; Kantchev, E. A. B.; O’Brien, C. J.; Lough, A.;
Organ, M. C. Chem.Eur. J. 2010, 16, 10844.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ales.ruzicka@upce.cz.
Notes
H. Organometallics 2002, 21, 5204. (b) Bonnet, L. G.; Douthwaite, R.
E.; Kariuki, B. M. Organometallics 2003, 22, 4187. (c) Bonnet, L. G.;
Douthwaite, R. E.; Hodgson, R.; Houghton, J.; Kariuki, B. M.;
Simonovic, S. Dalton Trans. 2004, 3528. (d) Peng, H. M.; Song, G.; Li,
Y.; Li, X. Inorg. Chem. 2008, 47, 8031. (e) Netland, K. A.; Krivokapic,
A.; Schroder, M.; Boldt, K.; Lundvall, F.; Tilset, M. J. Organomet.
Chem. 2008, 693, 3703.
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
J.T., Z.P., and A.R. thank the Czech Science Foundation for
financial support (Project P207/12/0223). M.S. and P.S.
(13) For example, see: (a) Batey, R. A.; Shen, M.; Lough, A. J. Org.
■
Lett. 2002, 4, 1411. (b) Turkmen, H.; Pape, T.; Hahn, F. E.;
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Çetinkaya, B. Eur. J. Inorg. Chem. 2009, 285. (c) Baker, M. V.; Brown,
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2009, 1977. (d) Jothibasu, R.; Huynh, H. V. Chem. Commun. 2010, 46,
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T.; Rominger, F.; Kunz, D. Eur. J. Inorg. Chem. 2012, 1423.
(14) As found in the Cambridge Crystallographic Data Centre in
September 2013.
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acknowledge support from the Czech Science Foundation
(Project 104/09/0561). F.D.P. acknowledges the Free
University of Brussels (VUB) and the Research Foundation
Flanders (FWO) for continuous support of his group.
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