Dimeric palladium 1,2,3-triazol-5-ylidene complexes-synthesis, structure, reactivity and catalytic properties in Suzuki coupling

Article abstract of DOI:10.1039/c8nj00981c

New dimeric palladium 1,2,3-triazol-5-ylidene complexes of the general formula [{Pd(μ-Cl)Cl(tzNHC)}₂] (tzNHC = 1-mesityl-3-methyl-4-phenyl-1H-1,2,3-triazol-5-ylidene; 1-hexyl-3-methyl-4-phenyl-1H-1,2,3-triazol-5-ylidene and 1-butyl-3-methyl-4-phenyl-1H-1,2,3-triazol-5-ylidene) were synthesized and characterized by spectroscopic methods, X-ray analysis and DFT calculations. For the mesityl substituted complex, the crystallographic structures of both the cisoidal and transoidal isomer were obtained. In the presence of an excess of 4-tolylboronic acid, the loss of the tzNHC ligand from the palladium coordination sphere as a 5-tolyltriazolium salt was observed. The complexes are precursors of palladium species catalytically active in the coupling of arylboronic acids with aryl bromides.

Full text of DOI:10.1039/c8nj00981c

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Dimeric palladium 1,2,3-triazol-5-ylidene complexes – synthesis,
structure, reactivity and catalytic properties in Suzuki coupling
Jan Lorkowskia, Patrycja Żaka, Maciej Kubickia, Cezary Pietraszuka,*,
Dawid Jędrzkiewiczb, Jolanta Ejflerb,*
Abstract: New dimeric palladium 1,2,3-triazol-5-ylidene complexes
of the general formula [{Pd(µ–Cl)Cl(tzNHC)}2] (tzNHC = 1-mesityl-3-
methyl-4-phenyl-1H-1,2,3-triazol-5-ylidene;
1-hexyl-3-methyl-4-
phenyl-1H-1,2,3-triazol-5-ylidene and 1-butyl-3-methyl-4-phenyl-1H-
1,2,3-triazol-5-ylidene) were synthesized and characterized by
spectroscopic methods, X-ray analysis and DFT calculations. For the
mesityl substituted complex, crystallographic structures of both the
cisoidal and transoidal isomer were obtained. In the presence of an
excess of 4-tolylboronic acid, the loss of tzNHC ligand from the
palladium coordination sphere as 5-tolyltriazolium salt was observed.
The complexes are precursors of palladium species catalytically
active in coupling of arylboronic acids with aryl bromides.
Figure 1. Selected examples of 1,2,3-triazol-5-ylidene palladium complexes.
Palladium 1,2,3-triazol-5-ylidene complexes exhibit catalytic
activity in such C-C bond forming reactions as alpha arylation23,
Introduction
direct arylation27, hydroarylation of alkynes28, Heck29,30
,
Sonogashira29 and Hiyama31 coupling as well as conversion of
internal alkynes to 1,2-diketones.35 The largest number of
literature reports concern the use of these complexes in
catalysis of the Suzuki-Miyaura reaction.16-26 Despite numerous
reports, some aspects of the Suzuki – Miyaura coupling,
especially the nature of the active species still need to be
clarified. Albrecht and Trzeciak indicated that under the
conditions of Suzuki coupling, the triazolylidene complexes
generate palladium nanoparticles that serve as a palladium
source for generation of molecular, catalytically active species.18
Moreover, authors suggest that less bulky substituents at N1
and C4 position of tzNHC induce better catalytic activity than the
bulkier ones. In contrast, [PdCl(allyl)(tzNHC] complex (where:
N-heterocyclic carbenes (NHC)1 have become ligands of
choice for many processes catalyzed by transition metal
complexes2 thanks to their stereoelectronic properties superior
to those of traditional phosphine systems1,2. The attractiveness
of these ligands in catalysis follows from a combination of steric
bulkiness and strong σ-donating properties of NHC.1,2 A high
demand for new ligands with potentially better properties has
prompted much effort aimed at the synthesis of new classes of
NHC such as imidazol-5-ylidenes3,4, cyclic alkylamino carbenes5,
1,2,4-triazol-5-ylidenes6, 1,2,3-triazol-5-ylidenes7-9 and other
bearing diverse atoms architectures.10 In 2008, Albrecht reported
the transition metal complexes bearing 1,2,3-triazol-5-ylidenes
(tzNHC) as ligands.7 These readily accessible11–14 and easy to
modify ligands are characterized by higher Tolman electronic
parameter (TEP)15 than other abnormal carbenes7 and ligands,
relative to imidazole derived NHCs, which indicates their
stronger σ-donating properties. These features make tzNHC
potentially useful ligands in homogeneous catalysis. Relatively
broad spectrum of palladium 1,2,3-triazol-5-ylidene complexes
bearing both mono- (Fig. 1a-c)7,16,18,22-35 and bis substituted
tzNHC metal center (Fig. 1d)20,21 have been reported. The group
tzNHC
= 1,4-bis(2,6-diisopropylphenyl)-1,2,3-triazol-5-ylidene)
reported by Fukuzawa25 and PEPPSI-type complex described by
Hong22 bearing the same, bulky tzNHC ligand remain highly
active in Suzuki coupling of challenging ortho-substituted aryl
chlorides with boronic acids, already at room temperature, which
can suggest a different mechanism of the reaction.
Herein, in order to contribute to the understanding of
structural and catalytic properties of 1,2,3-triazol-5-ylidene
palladium complexes, we disclose the synthesis of a series of
compounds of the general formula [{Pd(µ–Cl)Cl(tzNHC)}2] (3a-c)
bearing new 1,2,3-triazol-5-ylidene ligands, discuss their
structures in solid state and in solution, demonstrate the lability
of 1,2,3-triazol-5-ylidene ligands and evaluate the catalytic
properties of complexes in Suzuki coupling.
of mono-substituted complexes includes dimeric species (Fig.
7,22,24,32-35
1a),
PEPPSI – type structural family (Fig. 1b),
16,18,19,20,22,30,31
and allyl complexes (Fig. 1c).23-25 The other
reported palladium tzNHC complexes include acetato-bridged
dimers28, palladacycles,32,33 polymeric34 imidazolium triazolium17
or triazolium phosphine ones31 and recently reported
palladium(II) cationic complexes.35
Results and Discussion
aAdam Mickiewicz University in Poznań, Faculty of Chemistry
Umultowska 89b, 61-614 Poznań; E-mail: pietrasz@amu.edu.pl
bFaculty of Chemistry, University of Wrocław, 14 F. Joliot – Curie,
50-383 Wrocław; E-mail: jolanta.ejfler@chem.uni.wroc.pl
Synthesis of complexes 3a-c
The synthesis of 1,4-disubstituted 1,2,3-triazoles (1) was
performed from the appropriate azide and phenylacethylene by
1,3-dipolar Huisgen cycloaddition catalyzed by copper(I)
ions.36,37 Subsequently, the obtained triazoles were subjected to
Supporting information for this article is given via a link at the end of
the document.

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substitution reaction with MeI according to literature
procedures.38 Dimeric palladium complexes (3a-c) were then
obtained via subsequent metalation/transmetalation sequence
according to a modified literature method.22 The appropriate
triazolium salt was treated with silver oxide in the dark in CH2Cl2
complexes are in trans disposition, with a few in a skewed one,
with improper torsion angles around 120º - as in 3a (transoidal).
1
at 35 °C overnight, until the H NMR spectrum of the reaction
mixture indicated the loss of the signal assigned to proton at C5.
The afforded silver carbene complexes were then treated,
without prior isolation, with [PdCl2(PhCN)2] to form palladium
dimers (3a-c) (Scheme 1). Successful palladation was indicated
spectroscopically by the absence of the low-field triazolium
proton resonance. The synthesized complexes (3a-c) were
1
purified by column chromatography and characterized by H and
13C NMR spectroscopy and mass spectrometry. Moreover,
single crystals suitable for X-ray diffraction analysis were
obtained by slow diffusion of hexane to saturated
dichloromethane solution of complexes in a closed vessel at
5 °C.
Figure 2. A perspective view of complex 3a (cisoidal); ellipsoids are drawn at
the 30% probability level, hydrogen atoms are shown as spheres of arbitrary
radii.
Scheme 1. Synthesis of palladium complexes 3a-c.
Crystallographic studies of complexes 3a-c
The perspective views of the complexes are shown in Figures 2
– 5. Selected geometrical data are listed in Table S2 in ESI. For
complex 3a, the structures of both cisoidal (Fig. 2) and
transoidal form (Fig. 3) were obtained. All complexes are dimeric,
bis-square planar, with the central Pd2Cl2 ring exactly planar - by
virtue of the Ci-symmetry of 3b and 3c, almost planar – in 3a
(cisoidal) the dihedral angle between two PdCl2 planes (roof
angle) is 9.3(2)°, or significantly tilted, as in C2-symmetrical
complex 3b (transoidal) (roof angle 38.55(3)°). The square
planar coordination of all Pd atoms (CCl3) is quite regular, with
maximum deviation from the mean-square plane calculated for
four coordinating atoms not larger than 0.050(3)Å. Molecules 3b
and 3c are very similar (cf. Figs. 4 and 5), with the only
important difference in the conformation of the aliphatic side
chain, which is gt in 3c while tgtt in 3b (relevant torsion angles
are listed in Table S2). Complex 3a (cisoidal) differs from the
other ones in the lack of inversion symmetry and disposition of
the ligand. Similar coordination centers (C/C, terminal
Cl/terminal Cl) are disposed pseudo-trans in 3b and 3c, with
respect to the Pd···Pd line, while they are pseudo-cis in 3a
(cisoidal). Improper torsion angles C-Pd···Pd-C and Cl-Pd···Pd-
Cl are 12.8 and 4.2°, respectively, while in 3b and 3c the
appropriate values are 180°. It should be noted that in the
Cambridge Structural Database39 there are 29 Pd complexes
which have similar bis(2-chloro)-dichloro-di(C-substituent)
fragment, but only in a very simple one , i.e. [{Pd(-Cl)Cl(CO)}2],
the cis disposition is observed.40 The majority of the known
Figure 3. A perspective view of complex 3a (tranoidal); ellipsoids are drawn at
the 30% probability level, hydrogen atoms are shown as spheres of arbitrary
radii.
Figure 4. A perspective view of complex 3b; ellipsoids are drawn at the 30%
probability level, hydrogen atoms are shown as spheres of arbitrary radii.

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calculated theoretical diffusion coefficients for monomeric and
dimeric forms generated in a DFT study according to the
procedure described in our previous paper.41
Figure 5. A perspective view of complex 3c; ellipsoids are drawn at the 30%
probability level, hydrogen atoms are shown as spheres of arbitrary radii.
Figure 6. 1H DOSY NMR spectrum of 3b (CDCl3)
The geometry of all four molecules does not differ significantly
on the level of bonds lengths, bond angles etc. The difference
can be spotted while comparing overall shapes of the molecules.
It seems that the access from outside to metal centers is easier
for transoidal form (Fig. 3) than for cisoidal one (Fig. 2). Also
both exactly-transoidal forms, 3b and 3c, have more hidden
metal atoms than 3a (transoidal). Figure S3 shows a comparison
of van der Waals spheres representation of the three molecules.
Solvent – dichloromethane molecules are included in the crystal
structures of 3a (cisoidal) and 3c, however, they are bound
mainly by weak, C-H···Cl and other van der Waals interactions.
It can be observed that complexes 3b and 3c are packed in very
similar ways in the crystal structure (Fig. 5); roughly speaking,
the space taken by longer chain in 3b is occupied by the solvent
molecules in 3c.
The results of the optimization are presented in Tables S3-8
while geometrically optimized structures for dimers (transoidal
and cisoidal) and their monomers are shown in Fig 7 for 3b and
Fig S15 for 3a. The higher Df values calculated for monomers for
both van der Waals and Connolly surfaces indicate that
monomers [PdCl2(tzNHCPh/Hex)] are absent in the solution (Table
1). The Df values for transoidal and cisoidal dimers [{Pd(-
Cl)Cl(tzNHCPh/Hex)}2] calculated in the DFT study and estimated
via NMR are comparable. These results suggest that the solid
state dimeric structures are retained in the solution.
Table 1. Van der Waals volumes VvdW, Connolly volumes VSES, theoretical
diffusion coefficients DvdW, DSES and theoretical diffusion coefficients corrected
by shape factor DfvdW, DfSES for DFT optimized structures of Pd species.
Structure of complexes in solution
exp
DvdW
VSES [Å3] [10-10
m2/s]
DSES
[10-10
m2/s]
DfvdW
[10-10
m2/s]
DfSES
[10-10
m2/s]
Df
Structures of complexes in solution were studied by NMR
VvdW
[10-10
m2/s]
1
[Å3]
spectroscopy. H NMR spectra of complexes 3a-c recorded in
CDCl3 at room temperature are characterized by the presence of
broad signals in a range characteristic of alkyl and aromatic
protons (Figs S4, S6, S8, S10 and S12, ESI). The spectrum of
3a recorded at -50 C showed two distinct groups of sharp
singlets of unequal intensities assigned to methyl protons of
trans-3a
cis-3a
4
631.61 794.09
631.95 790.36
6.20
6.20
6.68
6.69
7.17
5.99
6.02
7.04
6.45
6.50
7.16
6.66
7.63
571.86 668.63 7.05
1
mesitylene group (Fig. S5). The H NMR spectra of 3b recorded
at -50 C show not fully suppressed conformational changes of
alkyl group and the splitting of the signal assigned to the methyl
group at N3 into two signals of slightly different intensity ( =
3.89 ppm and 3.90 ppm, respectively) (Fig. S7).
hypothetical
monomer
319.16 368.26 11.43
9.33
6.96
6.87
11.13
6.57
6.43
9.08
6.94
6.71
trans-3b
cis-3b
586.26 717.56
586.48 741.12
6.59
6.59
6.89
On the basis of NMR spectroscopic analysis it was
impossible
to
discern
whether
the
intramolecular
hypothetical
monomer
rearrangements in complexes 3a and 3b or the monomer/dimer
equilibrium (or both) are involved. Therefore the DOSY NMR
measurements were conducted to explore the nuclearity of
palladium complexes in solution. The DOSY NMR data indicate
that palladium complexes remain as dimeric structure in the
solution (see Fig. 6 for 3b and Fig. S14 for 3a). In order to verify
that transformation of these dimeric palladium complexes in
solution to monomeric species is rather impossible, we
296.67 339.48 12.34
9.69
11.58
9.09
In order to gain a more detailed picture of the potential monomer
and/or dimer existence in the solution, the relative energies
ΔEZPE for optimized palladium complexes are shown in Figure 8.

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Stability of 3a in solution
Complex 3a was found to be unstable in solution. 1H DOSY
NMR analysis of the solution of 3a in CDCl3 stored for one
month in NMR tube in a glove-box at 22 C showed formation of
a new complex.
Figure 9. 1H DOSY NMR spectrum of 3a (CDCl3) 1 month after dissolution
(sample stored in glove-box at 22 C).
Calculation of diffusion coefficients using the DFT method
allowed us to propose a structure of the complex formed (Table
S10). For the trans-[PdCl2(tzNHC)2] complex (4) (tzNHC = 1-
mesityl-3-methyl-4-phenyl-1H-1,2,3-triazol-5-ylidene),
consistency of the calculated diffusion coefficient with that
obtained experimentally was achieved (Table 1).
good
Disproportionation of 3a leading to formation of complex 4
requires formal carbene transfer and indicates lability of
Figure 7. Structures of Pd species bearing tzNHCPh/Hex ligand (schematic on
the left, DFT optimized on the right).
PdtzNHC bond. The complex
4
was obtained by an
as bright-yellow
independent literature
method42
a
microcrystalline solid, hardly soluble in organic solvents,
irrespective of the polarity. Its mass spectrum (Figure S41)
clearly confirms the structure. Moreover, mass spectroscopic
studies confirm formation of 4 via disproportionation of 3a. In
subsequent experiments, it was observed that the
transformation of 3a to 4 proceeds efficiently in alcohols. In
ethanol-d6 at 22 C the conversion of 3a was 32 % after 1 h of
the reaction course and 90 % after 24 h. Although the efficiency
of this transformation may be related to the low solubility of
complex 4 that precipitates from the alcoholic solution, the
experiment demonstrates the substantial lability of tzNHC ligand
in palladium(II) complexes.
Reactivity of 3a in the presence of boronic acid
Preliminary studies of the catalytic properties of complexes 3a-c
revealed their activity in coupling of 4-tolylboronic acid with
bromobenzene and permitted optimization of the reaction
conditions (Scheme 2, Table S11). Under the conditions of
Suzuki reaction, even at room temperature, a quick precipitation
of black deposit of palladium nanoparticles was observed.
Figure 8. The calculated relative energies in the gas-phase for the
geometrically optimized dimers and monomers for 3a-c complexes.
The lowest energy values found for dimeric palladium
complexes additionally demonstrate the preference of dimeric
form of complexes over the monomeric form.

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in the studied reaction, the mercury poisoning experiment44 was
performed. Accordingly, bromobenzene was treated with 4-
tolylboronic acid at 22 C for five minutes followed by the
addition of a 1000 equiv. of mercury to the reaction system.
Experiment shows total inhibition of the reaction by mercury
(Figure 10).
Scheme 2. Suzuki-Miyaura coupling of 4-tolylboronic acid with bromobenzene.
To learn more about the origin of the formation of palladium(0),
complex 3a was treated with an excess of 1000 equivalents of 4-
tolylboronic acid in EtOH at 22 C. ESI-MS spectrum of the
reaction mixture shows an intensive peak at m/z=368 (Figure
S17). In analogous experiment, the treatment of 3b with an
excess of 6 led to the formation of the product characterized by
1
an intensive peak at m/z=334 (Fig. S18). H NMR investigation
of the reaction of 3a with 2 equiv. of 4-tolylboronic acid in CDCl3
revealed the appearance of a singlet at δ = 4.45 ppm in the
spectral range characteristic of methyl groups at N3 in triazolium
salts. At the same time, no signal that could be assigned to the
proton at C5 in triazolium salt, was detected (Figures S19–S21).
These results suggest the formation of triazolium salt substituted
at C5 atom with 4-tolyl group which indicates the loss of tzNHC
ligand from the palladium coordination sphere under the applied
conditions of Suzuki coupling. To confirm the above
observations, complex 3a in ethanol was treated with four
equivalents of 4-tolylboronic acid in the presence of KOH (4
equiv) at 22 C for 18 h. Tedious column chromatography of the
reaction mixture gave triazolium salt 8 (Scheme 3) in 23% yield.
Salt 8 was characterized by 1H and 13C NMR spectroscopy (Figs
S22-S23) and ESI HRMS (Fig. S43). Further experiments
showed practically quantitative transformation in 1000-fold
excess of boronic acid within 5 min.
Figure 10. Mercury poisoning experiment. Profiles of the reactions of
bromobenzene with tolylboronic acid performed in the presence of 3a.
Reaction conditions: [ArBr]:[ArB(OH)2]:[KOH] = 1:1.05:1.1, air; 22 C, Hg
added after 5 min of the reaction course.
The results indicate the catalytically active role of palladium
nanoparticles or “naked” molecular Pd(0) species44. Fine solid
formed upon the coupling of bromobenzene with 4-tolylboronic
acid
(EtOH,
22
°C,
[PhBr]:[ArB(OH)2]:[KOH]:[Pd]
=
1:1.05:1:1.1:1×10-2) was isolated and characterized by
transmission electron microscopy (TEM) after 35 minutes and 2
hours of the reaction course (Fig. 11). The experiment revealed
formation of palladium nanoparticles and the increase in their
average diameter in the course of the reaction from 2-5 nm
(after 35 min) to 5-10 nm (after 2 h). The nanoparticles isolated
from the post-reaction mixture were found inactive in Suzuki
coupling of bromobenzene with 4-tolylboronic acid.
Scheme 3. Reaction of 3a with an excess of 4-tolylboronic acid leading to
formation of 8.
Reductive elimination from aryl–palladium–NHC complexes
have been described for a number of NHC ligands.43 However,
to the best of our knowledge, such a decomposition has not
been described for 1,2,3-triazol-5-ylidene complexes.
Catalytic applications
Elimination of triazolylidene ligand from the coordination sphere
of palladium as triazolium salt, which proceeds efficiently in the
presence of a large excess of boronic acid, generates tzNHC-
free Pd(0) complexes. However, it cannot be excluded that
some amount of the complex remains unreacted (thanks to
equilibrium that can be achieved in the presence of salt 8 in the
reaction mixture). Preliminary tests show catalytic activity of the
of complexes 3a-c in Suzuki coupling of bromobenzene with 4-
tolylboronic acid. To learn more on the nature of active species
Figure 11. TEM micrographs showing the palladium nanoparticles formed in
the reaction mixture after 35 min (left) and 2 h (right) of the reaction course.
See also Figs S24 and S25.

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Observations regarding the catalysis of Suzuki coupling in the
presence of 3a-c are consistent with the model proposed by
Trzeciak and Albrecht18, assuming heterogenization of the
molecular precatalyst to palladium nanoparticles and
subsequent dissolution of palladium atoms from these
nanoparticles. However, in the presence of 3a-c the ESI-MS
analysis of the post reaction mixture from SM coupling of
bromobenzene with 4-tolylboronic acid did not show the
formation of dimeric or trimeric complexes, as observed by
Albrecht and Trzeciak.18
effects of the starting aryl halides influence the conversion.
Phenyl and aryl bromides substituted by activating groups in
para position undergo efficient conversion at low catalyst loading
(0.1 mol%) and at room temperature. High conversion of
heteroaryl bromides requires elevated reaction temperature (78
C). Sterically hindered reagents undergo coupling with
moderate yield. In this case, an increase in reaction temperature
leads to undesirable homocoupling of boronic acid.
In view of the observed transformation of dimer 3a to complex
[PdCl2(tzNHC)2] (4) the catalytic activity of the latter was
examined. Despite the low solubility of the complex, the coupling
of bromobenzene with 4-tolylboronic acid carried out in EtOH at
22 C ([ArB(OH)2]: [KOH] = 1: 1.1, cat. 0.1 mol%) permitted
reaching the conversion of 51%. Mercury poisoning experiment
performed for the reactions catalyzed by complex 4, resulted in
complete inhibition of the reaction by addition of mercury. In the
experiment mercury was added after 2 hours of the reaction
course when the conversion reached 32 %. The results of
mercury poisoning test should be interpreted with caution.44
However, it seems that in the presence of complex 4 reaction is
catalyzed by palladium nanoparticles or “naked” Pd(0) species.
According to the model proposed by Trzeciak and Albrecht, the
differences in activities of 3a-c in the Suzuki coupling, reflects
the differences in the effectiveness of palladium nanoparticles
formation. In the system studied, the key step leading to
formation of palladium nanoparticles is the decomposition of
palladium dimers 3a-c taking place as in scheme 3. Comparison
of catalytic activities of complexes 3a-c revealed insignificant
differences, indicating a small impact of the character of
substituent (aryl vs alkyl) at N1 on stability of palladium complex
under Suzuki coupling conditions (Fig. 12).
Scheme 4. Suzuki coupling of 4-tolylboronic acids with aryl bromides in the
presence of 3a. Reaction conditions: 3a (0.1 mol%); 24 h, EtOH, KOH,
a)
[ArCl]:[ArB(OH)2]:[KOH] = 1:1.05:1.1, 22 C, air;
78 C; figures denote
isolated yields.
Conclusions
New dimeric palladium(II) complexes of the general formula
[{Pd(µ–Cl)Cl(tzNHC)}2] bearing 1-aryl- and 1-alkyl-3-methyl-4-
phenyl-1H-1,2,3-triazol-5-ylidene ligands were synthesized via
the silver-carbene transfer route as a mixture of isomers. Crystal
structures of cisoidal and transoidal forms of 3a were described.
Cisoidal form of 3a is the first reported example of such a
specific arrangement of NHC ligands in palladium dimers. In
solution 3a tends to transform to [PdCl2(tzNHC)2] via formal
carbene transfer, indicating a significant lability of the tzNHC
ligand in the palladium(II) complexes. In excess of arylboronic
acid complex 3a undergoes room temperature decomposition,
leading to the formation of 5-arylotriazolium salt. Complexes 3a-
c act as efficient precursors of palladium(0) species catalytically
active at room temperature in efficient Suzuki coupling of
boronic acids with aryl bromides.
Experimental Section
Figure 12. Reaction profiles. SM coupling of bromobenzene with 4-
tolylboronic acid catalyzed by complexes 3a-c. Reaction conditions: 22 C, 24
h, [ArBr]:[ArB(OH)2]:[KOH] = 1:1.05:1.1, air; conversion calculated on the basis
of GC analysis with dodecane as an internal standard.
1. General methods and chemicals
1H and 13C NMR spectra were recorded on a Varian 400 or 300 at 25 ˚C.
Chemical shifts (δ in ppm and coupling constants J in Hz) were
referenced to residual solvent resonances. 1H NMR spectra of the
reaction of 3a with tolylboronic acid and low temperature spectra were
recorded on Bruker Avance DRX 600, operating at frequencies of 600.13
MHz. GC/MS analyses were performed on a Varian Saturn 2100T
equipped with (DB-5, 30 m capillary column) and Ion Trap Detector. GC
The most catalytically active complex 3a was chosen to evaluate
the catalytic activity in Suzuki-Miyaura coupling of selected aryl
bromides. The results (Scheme 4) indicated that the electronic

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analyses were performed using Varian CP-3800 (column: RTX-5 30 m ID
0.53 mm), equipped with TCD. Mass spectrometry analyses of
complexes were performed using Synapt G2-S mass spectrometer
(Waters) equipped with the ASAP (Atmospheric Solids Analysis Probe)
ion source and quadrupole-time-of-flight mass analyser. Small amounts
of samples were applied directly onto the glass probe and the excess of
the sample was removed using a paper tissue. The measurements were
performed in gradient temperature rate from room temperature to 650 ˚C
in 6 minutes with the desolvation gas flow 850 L/h and corona current set
to 12 µA. ESI-MS spectra were performed using Waters Micromass ZQ
2000 spectrometer. The chemicals were obtained from Aldrich.
Potassium hydroxide, ethanol 99.8 %, hexane and ethyl acetate were
purchased from POCH S.A BASIC (Poland). Dry dichloromethane was
received by distillation of HPLC grade solvent (Aldrich) over the calcium
hydride (72 h) and kept in oven-dried glassware under an inert
atmosphere over the 4 Å molecular sieves (Aldrich) prior to use. Unless
mentioned otherwise, all reactions were carried out in aerobic conditions
with chemicals and solvents used as received from supplier.
37.6, 31.6, 19.8, 13.6; HRMS: m/z calcd. for C26H34N6Cl3Pd2 [M–Cl]+
764.9980; found 764.9980.
Representative procedure for the catalytic test
A 2 mL glass reactor equipped with a condenser and a magnetic stirring
bar was charged in the air with boronic acid (2.5 × 10-4 mol, 1.05 equiv)
and 1 mL of ethanol (99.8 %). The reaction mixture was stirred at 22 °C
for 5 min. until complete acid dissolution. Then, aryl halide (1 equiv),
dodecane (internal standard), the appropriate amount of palladium
complex and KOH (1.1 equiv) were added. The mixture was stirred at
22 °C or at reflux for 24 h. The reaction course was monitored by gas
chromatography and GC/MS.
Acknowledgements
The research was co-financed by the National Centre for
Research and Development (NCBR) under the Project
ORGANOMET No: PBS2/A5/40/2014.
2. Synthesis of palladium complexes.
Palladium dimers were synthesized via a modified literature method.22
Under an argon atmosphere a 25 mL Schlenk tube, equipped with a
magnetic stirring bar and a stopper, was charged with triazolium iodide (1
equiv), silver oxide (1 equiv) and dry dichloromethane. Then, the reaction
mixture was stirred at 35 °C for 16 h in the dark and afterwards it was
cannulated to another 25 mL Schlenk tube. Bis(benzonitrile)palladium(II)
chloride (1 equiv) was added in one portion and the mixture was stirred
for additional 16 h at 35 °C. The residue was filtered through a pad of
Celite and after evaporation of the solvent the pure product was isolated
by column chromatography (hexane→DCM→DCM/methanol, 10/1, v/v).
Single crystals suitable for X-ray diffraction analysis were obtained by
slow diffusion of hexane to saturated solution of complexes at 5 °C.
Keywords: 1,2,3-triazol-5-ylidenes • Palladium • Suzuki-Miyaura
coupling
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FULL PAPER
Palladium Catalysis
Jan Lorkowski, Patrycja Żak, Maciej
Kubicki, Cezary Pietraszuk*, Dawid
Jędrzkiewicz, Jolanta Ejfler*
Page No. – Page No.
Palladium 1,2,3-triazol-5-ylidene dimers transform to bis(carbene) species and in
Dimeric palladium 1,2,3-triazol-5-ylidene
Suzuki coupling conditions lose carbene via reductive elimination of 5-aryltriazolium
salt.
complexes
reactivity and catalytic properties in
Suzuki coupling.
–
synthesis, structure,

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Palladium 1,2,3-triazol-5-ylidene dimers transform to bis(carbene) species and in Suzuki
coupling conditions lose carbene via reductive elimination of 5-aryltriazolium salt.

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