PEPPSI-type palladium 1,2,3-triazolin-5-ylidene complexes – Synthesis, structure and catalytic properties in Suzuki-Miyaura coupling

Article abstract of DOI:10.1016/j.ica.2018.06.028

Palladium complexes of the general formula [PdCl₂(tzNHC)(pyridine)] bearing a series 1,2,3-triazolin-5-ylidene (tzNHC) ligands were synthesized and characterized by spectroscopic methods and X-ray analysis. Complexes exhibit catalytic activity in the Suzuki-Miyaura coupling of boronic acids with aryl bromides.

Full text of DOI:10.1016/j.ica.2018.06.028

Accepted Manuscript
Research paper
PEPPSI-type palladium 1,2,3-triazolin-5-ylidene complexes – synthesis, struc-
ture and catalytic properties in Suzuki–Miyaura coupling
Sylwia Ostrowska, Maciej Kubicki, Cezary Pietraszuk
PII:
S0020-1693(17)31917-5
https://doi.org/10.1016/j.ica.2018.06.028
ICA 18320
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
20 December 2017
18 June 2018
19 June 2018
Please cite this article as: S. Ostrowska, M. Kubicki, C. Pietraszuk, PEPPSI-type palladium 1,2,3-triazolin-5-ylidene
complexes – synthesis, structure and catalytic properties in Suzuki–Miyaura coupling, Inorganica Chimica Acta
(2018), doi: https://doi.org/10.1016/j.ica.2018.06.028
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

PEPPSI-type palladium 1,2,3-triazolin-5-ylidene complexes – synthesis, structure
and catalytic properties in Suzuki–Miyaura coupling
Sylwia Ostrowska, Maciej Kubicki and Cezary Pietraszuk*
Adam Mickiewicz University in Poznań, Faculty of Chemistry,
Umultowska 89b, 61-614 Poznań, Poland
* Corresponding author
Phone: 48 61 8291699, E-mail address: pietrasz@amu.edu.pl
1

Abstract: Palladium complexes of the general formula [PdCl2(tzNHC)(pyridine)] bearing a
series 1,2,3-triazolin-5-ylidene (tzNHC) ligands were synthesized and characterized by
spectroscopic methods and X-ray analysis. Complexes exhibit catalytic activity in the Suzuki–
Miyaura coupling of boronic acids with aryl bromides.
Keywords: 1,2,3-triazolin-5-ylidene complexes; palladium; Suzuki–Miyaura coupling
1. Introduction
N-heterocyclic carbenes (NHC) have become valuable ligands for many processes catalyzed
by transition metal complexes thanks to a combination of their steric bulkiness, strong σ-
donating properties and high stability of metal-carbene bond [1,2]. Rapid development of
NHC resulted in the synthesis of new classes of NHC ligands beyond imidazolin-2-ylidenes
[3,4]. In 2008 Albrecht reported 1,2,3-triazolin-5-ylidene (tzNHC) metal complexes [5].
These readily accessible [6–9] and easy to modify ligands are characterized by higher values
of the Tolman electronic parameter [10] than ligands relative to imidazole derived NHCs.
These desirable features make tzNHC potentially useful in homogeneous catalysis. Relatively
broad spectrum of palladium 1,2,3-triazolin-5-ylidene complexes have been reported and their
catalytic activity has been studied in a number of processes, mainly in Suzuki–Miyaura
coupling [12–22] but also in alpha arylation [20], direct arylation [23], hydroarylation of
alkynes [24], Heck [25,26], Sonogashira [25] and Hiyama [27] coupling. Recently, a
comprehensive review has appeared discussing different aspects of metal complexes of
triazole-based ligands [28]. Although the catalytic activity of palladium 1,2,3-triazolin-5-
ylidene complexes in the Suzuki–Miyaura reaction has been studied by many research groups,
some aspects of the reaction still need to be clarified. Albrecht and Trzeciak shows that less
bulky substituents induce higher catalytic activity than bulkier IMes-type analogues. This
phenomenon was proposed to originate from a different mode of action of the triazolylidene
complexes. Authors emphasized the significance of underligated species generated from
palladium nanoparticles in the presence of boronic acids and under conditions of Suzuki–
Miyaura coupling [12]. In contrast, Hong [13] and Fukuzawa [14] reported high activity of
palladium complexes bearing bulky 1,2,3-triazolin-5-ylidene ligand in the coupling of boronic
acids with aryl bromides and chlorides already at room temperature. It may indicate different
nature of catalytically active species in the latter systems.
Herein, we report on the synthesis and characterization of palladium complexes of the
general formula [PdCl2(tzNHC)(pyridine)] bearing new 1,2,3-triazolin-5-ylidene ligands,
describe their catalytic properties in the Suzuki–Miyaura coupling and study the nature of
active species.
2. Experimental
2.1. 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.
GC/MS analyses were performed on a Varian Saturn 2100T equipped with (DB-5, 30 m
capillary column) and Ion Trap Detector. GC analyses were performed using Varian CP-3800
2

(column: RTX-5 30 m ID 0,53 mm), equipped with TCD. Mass spectrometric 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
analyzer. Small amounts of samples were applied directly onto the glass probe and the excess
of the sample were 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 the following
sources: palladium (II) chloride, potassium carbonate, tetrabutylammonium hydroxide, aryl
halides, boronic acids, pyridine, toluene, dichloromethane, chloroform-d, benzene-d6 were
purchased from Aldrich. Potassium hydroxide, ethanol (99.8 %), hexane and ethyl acetate
were purchased from POCH S.A. BASIC (Poland).
2.2. General synthetic procedure and characterization of complexes (2a-g)
Palladium complexes (2a-g) were synthesized according to a modified literature procedure
[12]. A solution of the appropriate triazole salt (1equiv.) in CH2Cl2 (2 mL) and Ag2O (0.5
equiv.) was added to a Schlenk vessel equipped with a magnetic stirrer. The mixture was
stirred in the dark at room temperature for 6 h. Then PdCl2 (1.5 equiv.) and pyridine (1 mL)
were added and the mixture was stirred at 100 °C for 12 hours. The reaction mixture was
cooled to room temperature, diluted with CH2Cl2 (7 mL) and passed through a column of
SiO2 with a layer of celite. After elution of the product with CH2Cl2 and subsequently MeOH,
all the volatiles were evaporated under reduced pressure. The residue was dissolved in a
minimum amount of CH2Cl2 and precipitated with Et2O (50 mL). The precipitate was filtered
and the filtrate was evaporated to dryness to give a pure palladium complex. Monocrystals
were obtained by crystallization from CH2Cl2/hexane.
Complex 2a. Yellow solid, isolated yield 87%. 1H NMR (300 MHz, CDCl3) δ: 8.74–8.67 (m,
2H CHAr py), 8.20–8.10 (m, 2H CHAr Ph), 7.67–7.51 (m, 3H CHAr Ph ), 7.23–7.13 (m, 3H
CHAr py), 7.06 (s, 2H CHAr Mes), 4.14 (s, 3H N-CH3), 2.39 (s, 3H p-CH3 Mes), 2.31 (s, 6H o-
13
CH3 Mes). C NMR (75 MHz, CDCl3) δ: 151.42, 140.32 (Pd-C), 137.44, 135.92, 130.62,
130.06, 129.27, 128.84, 124.01, 37.66, 21.27, 18.80. HRMS (ESI-TOF, m/z): 497.1245
C23H24ClN4Pd [M-Cl], calculated: 497.1240.
Complex 2b [24]. Yellow solid, isolated yield = 82%.1H NMR (403 MHz, CDCl3) δ: 8.73–
8.69 (m, 2H, CHAr py), 8.23–8.16 (m, 2H, CHAr Ph ), 7.64–7.52 (m, 5H, CHAr Ph), 7.38 (d, J
= 7.8 Hz, 1H, m-IPr-H ), 7.18–7.14 (m, 2H, CHAr py), 4.13 (s, 3H, N-CH3), 2.89–2.73 (m, 2H,
13
CH-i-Pr), 1.44 (d, J = 6.8 Hz, 6H, CH3-i-Pr), 1.09 (d, J = 6.8 Hz, 6H, CH3-i-Pr). C NMR
(101 MHz, CDCl3) δ: 151.33, 146.57(Pd-C), 143.77, 142.17, 137.39, 134.91, 131.14, 130.66,
130.04, 128.79, 126.96, 124.05, 123.89, 37.67, 28.86, 26.12, 22.86. HRMS (ESI-TOF, m/z):
539.1196 C26H30ClN4Pd [M-Cl], calculated: 539.1194.
Complex 2c. Dark-yellow solid, isolated yield = 58%). 1H NMR (403 MHz, CDCl3) δ: 8.94–
8.90 (m, 2H CHAr py), 8.02–7.96 (m, 2H CHAr Ph), 7.75–7.68 (m, 1H CHAr py), 7.60–7.49
(m, 2H CHAr Ph), 7.32–7.27 (m, 2H CHAr py), 4.99–4.93 (m, 2H N-CH2), 3.99 (s, 3H N-CH3),
3

13
2.38 (dt, J = 15.2, 7.6 Hz, 2H CH2), 1.50–1,35 (m, 6H CH2), 0.97–0.86 (m, 3H CH3). C
NMR (101 MHz, CDCl3) δ: 151.28, 143.56 (Pd-C), 137.71, 130.37, 129.92, 128.85, 126.85,
124.26, 55.00, 37.34, 31.20, 29.62, 26.33, 22.49, 14.04. HRMS (ESI-TOF, m/z): 463.0860
C20H26ClN4Pd [M-Cl], calculated: 463.0881.
1
Complex 2d. Light yellow solid, isolated yield 51%. H NMR (300 MHz, CDCl3) δ: 8.98–
8.92 (m, 2H CHAr py), 8.07 (d, J = 8.6 Hz, 4H CHAr Ph), 7.73–7.63 (m, 2H CHAr Ph), 7.54–
7.39 (m, 2H CHAr Ph), 7.30–7.26 (m, 1H CHAr py), 5.00–4.93 (m, 2H N-CH2), 4.03 (s, 3H N-
CH3), 2.39 (dt, J = 15.3, 7.5 Hz, 2H CH2), 1.70 (s, 2H), 1.58–1.31 (m, 6H CH2), 0.91 (t, J =
7.1 Hz, 3H CH3).13C NMR (75 MHz, CDCl3) δ: 151.31, 137.70, 130.76, 129.08, 128.88,
127.53, 127.20, 124.26, 55.06, 37.41, 31.24, 29.59, 26.34, 22.47, 14.01. HRMS (ESI-TOF,
m/z): 539.0878 C26H30ClN4Pd [M-Cl], calculated: 539.0880.
1
Complex 2e. Yellow solid, isolated yield 69%. H NMR (403 MHz, CDCl3) δ: 8.96–8.88 (m,
1H CHAr py), 7.96–7.91 (m, 2H CHAr Ph), 7.72–7.63 (m, 3H CHAr Ph ), 7.60–7.50 (m, 3H
CHAr py), 4.85–4.79 (m, 2H N-CH2), 3.96 (s, 3H N-CH3 ), 2.37–2.26 (m, 4H CH2), 1.08 (t, J
= 7.4 Hz, 2H CH3).13C NMR (75 MHz, CDCl3) δ: 151.18, 143,48 (Pd-C), 137.68, 137.59,
130.29, 129.85, 128.76, 124.21, 54.65, 37.30, 31.53, 19.82, 13.57. HRMS (ESI-TOF, m/z):
435.0554 C18H22ClN4Pd [M-Cl], calculated: 435.0568.
1
Complex 2f. Yellow solid, isolated yield 82%. H NMR (300 MHz, CDCl3) δ: 8.94–8.89 (m,
2H CHAr py), 8.01–7.95 (m, 1H CHAr Ph), 7.82–7.68 (m, 2H CHAr Ph), 7.59–7.51 (m, 2H
CHAr Ph), 7.43–7.26 (m, 5H CHAr Ph), 6.20 (s, 2H N-CH2), 3.95 (s, 3H N-CH3).13C NMR (75
MHz, CDCl3) δ: 151.30, 143.80 (Pd-C), 138.57, 137.73, 133.90, 130.41, 129.99, 129.76,
128.83, 128.79, 126.72, 124.28, 58.64, 37.50. HRMS (ESI-TOF, m/z): 469.0401
C21H20ClN4Pd [M-Cl], calculated: 469.0411.
1
Complex 2g. Light yellow solid, isolated yield 62%. H NMR (403 MHz, CDCl3) δ: 8.95–
8.91 (m, 2H CHAr py), 8.09–8.05 (m, 2H CHAr Ph), 7.65 (dq, J = 2.5, 1.7 Hz, 4H CHAr Ph),
7.51–7.38 (m, 6H CHAr Ph), 7.33–7.27 (m, 2H CHAr py), 6.21 (s, 2H N-CH2), 4.00 (s, 3H N-
CH3). 13C NMR (101 MHz, CDCl3) δ: 151.30, 142.67(Pd-C), 137.75, 133.87, 130.76, 129.77,
128.87, 128.79, 127.82, 127.52, 127.19, 124.22, 58.67, 37.58. HRMS (ESI-TOF, m/z):
545.0720 C27H24ClN4Pd [M-Cl], calculated: 545.0724.
2.3. General Procedure for Suzuki–Miyaura coupling
A boronic acid (2.5  10-4 moles, 1.05 equiv.) and 1 mL of ethanol (99.8%) were placed in a 2
mL glass reactor equipped with a reflux condenser and magnetic stirrer. The reaction mixture
was stirred at 22 °C until complete dissolution of the acid (ca 5 minutes). Then aryl halide (1
equiv.), dodecane (internal standard), a suitable amount of palladium complex and KOH (1.1
equiv.) were added. The reaction mixture was stirred at 22 °C or at 78 °C for 24 hours. The
course of the reaction was monitored by gas chromatography and GC / MS. After completion
of the reaction, 2 mL of distilled water and 2 mL of hexane were added to the reaction
mixture. The organic phase was washed with water (2  2 mL). The product was isolated
4

from the organic phase by column chromatography (SiO2, hexane). For details see
supplementary information.
3. Results and Discussion
3.1. Synthesis of complexes
The synthesis of 1,4-disubstituted 1,2,3-triazoles was performed by Huisgen 1,3-dipolar
cycloaddition catalyzed by copper(I) ions according to the literature procedures [32,33]. The
obtained triazoles were treated with MeI to form the respective triazolium salts (1) [24].
Palladium complexes (2a-g) were then obtained via a modified literature method (Scheme 1)
[13]. The appropriate triazolium salt was treated in CH2Cl2 at 22 °C with silver oxide for 6 h.
1
After that time the H NMR spectrum of the reaction mixture does not show the signal
assigned to proton at C5. Silver carbene complexes were then treated without prior isolation
with PdCl2 and pyridine at 100 °C for 12 h to form palladium complexes 2a-g. The
synthesized complexes were purified by column chromatography and characterized by 1H and
13C NMR spectroscopy and mass spectrometry.
Scheme 1. Synthesis of PEPPSI-type 1,2,3-triazolin-5-ylidene complexes 2a-g
3.2. Crystallographic studies
Structures of complexes 2a, 2c, 2e, 2f and 2g were confirmed by X-ray diffraction analysis.
The perspective views of the complexes are shown in Figures 1 - 5. Some geometrical data
are listed in Table S2. The main geometrical features, i.e. bond lengths and angles, of the
studied complexes are similar. In all complexes Pd ion is four coordinated, in a very regular
square-planar fashion. Both Pd-Cl bonds are equivalent, within an experimental error. In the
triazole rings the two N-N and two C-N bonds are quite similar in pairs – suggesting
significant delocalization throughout this ring. Interestingly, complex 2c crystallizes with two
symmetry-independent molecules in the asymmetric part of the unit cell, and these molecules
exhibit different conformations of the hexane side chain; it is trans-trans-trans-trans in the
molecule A while (+)gauche-trans-trans-(-)gauche in molecule B (Fig. 2). Significant
differences are observed in the conformation of molecules, which in this case might be
defined by the dihedral angles between the least-square planes of planar fragments: pyridine
ring (A), coordination plane (B), triazole ring (C), and additionally – although less important
– the phenyl ring at C14. Table S2 lists the appropriate values. The above differences lead to
the differences in long-distance contacts within the molecules. The crystal structures are built
mainly be weak, unidirectional van der Waals forces; some secondary C-H···Cl contacts (or
interactions) are also observed.
5

Figure 1. A perspective view of complex 2a; ellipsoids are drawn at the 50% probability
level, hydrogen atoms are shown as spheres of arbitrary radii.
Figure 2. Perspective view of the molecule A of complex 2c (left); ellipsoids are drawn at the
50 % probability level, hydrogen atoms are shown as spheres of arbitrary radii. A comparison
of the two symmetry-independent molecules of 2c (right).
Figure 3. Perspective view of the complex 2e; ellipsoids are drawn at the 50 % probability
level, hydrogen atoms are shown as spheres of arbitrary radii.
6

Figure 4. Perspective views of the complex 2f; ellipsoids are drawn at the 50 % probability
level, hydrogen atoms are shown as spheres of arbitrary radii.
Figure 5. Perspective view of the complex 2g; ellipsoids are drawn at the 50 % probability
level, hydrogen atoms are shown as spheres of arbitrary radii. Only one of the disordered
fragments (C15 – C20A) is shown
3.3. Catalytic applications
Synthesized palladium complex 2a was tested as precatalyst in the Suzuki–Miyaura coupling
of 4-tolylboronic acid with bromobenzene (Scheme 2). The results of studies aimed at
optimizing the reaction conditions are shown in Table 1.
Scheme 2. Suzuki–Miyaura coupling of 4-tolylboronic acid with bromobenzene.
7

Table 1. Coupling of 4-tolylboronic acid with bromobenzene in the presence of 2a.
Entry
1
Base
Solvent
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
iPrOH
MeOH
dioxane
EtOH
EtOH
EtOH
EtOH
2a (mol%)
Yielda [%]
52
TBAF
KOtBu
H3COONa
Cs2CO3
NaOH
KOH
1
2
1
81
3
1
traces
79
4
1
5
1
47
6
1
>99
76
7
KOH
1
8
KOH
1
88
9
KOH
1
14
10
11
12
13
KOH
0.1
0.1
0.05
0.01
98
70b
KOH
KOH
88
KOH
61
Reaction conditions: 22 C, 24 h, [ArBr]:[ArB(OH)2]:[KOH] = 1:1.05:1.1; air;
a) GC yield; b) argon
The tests indicated that high yields were obtained using ethanol as a solvent and KOH as a
base. The reaction is effective at room temperature. Under such conditions, the quantitative
yield of the coupling product was obtained in the presence of 0.1 mol% of catalyst. Further
lowering of the catalyst loading resulted in a decrease in reaction yield.
To compare the catalytic activity, the coupling reaction of 4-tolylboronic acid with
bromobenzene (Scheme 3) was investigated in the presence of each of the synthesized
complexes.
Scheme 3. Suzuki–Miyaura coupling of 4-tolylboronic acid with phenyl halides
Relevant reaction profiles were show in Figure 6. The highest activity was observed for
complex 2g, which allows for quantitative conversion after only 10 minutes of reaction
course.
8

Figure 6. Reaction profiles. Suzuki–Miyaura coupling of bromobenzene with 4-tolylboronic
acid. Reaction conditions: cat. = 2a-g (0.1 mol%), [ArBr]:[ArB(OH)2]:[KOH] = 1:1.05:1.1;
EtOH, 22 C, air.
High activity was observed for complexes 2a, i 2d-f, in the presence of which complete
conversion was observed after 1 h of the reaction course. The study of the coupling of 4-
tolylboronic acid with chlorobenzene (Scheme 3) revealed no reaction at room temperature.
Effective coupling was found to require higher reaction temperature (78 C), increased
loading of Pd complex (1 mol%) and prolonged reaction time (Table 2). The highest yields
were obtained in the presence of complexes 2a and 2g. Increasing the concentration of
catalyst to 2 mol% allows moderate yield of the product, i.e. 62% for the catalyst 3a, and 77%
for the complex 3g.
Table 2. Coupling of 4-tolylboronic acid with bromobenzene and chlorobenzene in the
presence of complexes 2a-g
Entry
Cat. (mol %)
2a (1)
2a (2)
2b (1)
2c (1)
2d (1)
2e (1)
2f (1)
2g (1)
2g (2)
Yielda [%]
X
Temp [C]
1
2
3
4
5
6
7
8
9
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
78
78
78
78
78
78
78
78
78
61
62
29
34
55
44
55
71
77
Reaction conditions: [ArCl]:[ArB(OH)2]:[KOH] = 1:1.05:1.1; EtOH, 24 h, air; a) GC yield;
9

Optimization tests (Table 1) permitted specification of the conditions for efficient progress of
the reaction in the presence of catalyst 2a. In the optimized reaction conditions a series of aryl
bromides were tested in the reaction with 4-tolylboronic acid in the presence of catalyst 2a to
determine the versatility of the method. The results are shown in Figure 7.
O
5a: 95 %
5d: 97 %
5b: 98 %
5c: 98 %
5f: 85 %
Cl
MeO
NC
5e: 74 %
N
N
5i: 84a %
5h: 73a %
5g: 64 %
S
S
5j: 94 %
5k: 98%
Figure 7. Suzuki–Miyaura coupling of 4-tolylboronic acid with aryl bromides in the presence
of complex 2a. Specified values refer to isolated yields. Reaction conditions: EtOH, 22 C, 24
h, [ArBr]:[ArB(OH)2]:[KOH] = 1:1.05:1.1; [Pd] = 0.1 mol %, air; a) 78 C
High yields were obtained for nearly all tested compounds. Bromobenzene and aryl bromide
with a substituent in the para position were effectively converted at room temperature at a
low catalyst loading (0.1 mol%). It has been shown that complex 2a tolerates the presence of
a variety of functional groups. For high conversion of heteroaryl bromides, the reaction
temperature should be raised to 78 °C. The coupling of aryl bromide sterically hindered in the
ortho position proceeds with a moderate yield. An attempt to raise the reaction temperature in
this case led to the formation of an undesired product of boronic acid homocoupling.
To evaluate the scope of the reaction we performed a number coupling by using 4-
bromotoluene as electrophile and selected boronic acids (Scheme 4, Figure 8).
2a
Br
(HO)2B R
4b-g
R
+
EtOH, KOH
3c
5l-r
NMe2
g
R =b
; c
; d
; e
; f
;
S
NO2
O
Scheme 4. Suzuki–Miyaura coupling of 4-bromotoluene with selected boronic acids
10

Figure 8. Suzuki–Miyaura coupling of 4-bromotoluene with selected boronic acids in the
presence of complex 2a. Specified values refer to isolated yields. Reaction conditions: EtOH,
22 C, 24 h, [ArBr]:[ArB(OH)2]:[KOH] = 1:1.05:1.1; [Pd] = 0.1 mol %, air
High isolated yields were obtained for all reagents tested. Nearly quantitative yields were
obtained in reaction with phenylboronic acids containing substituents differing in electronic
properties. Under the conditions used, an efficient conversion of the thiophenyloboronic acid
and selected alkenylboronic acid derivatives was also observed. These results indicate high
scope of the proposed procedures.
3.4. Mechanistic insights
The Suzuki–Miyaura coupling initiated by complexes 2a-g was accompanied by the
formation of fine black solid. Therefore, the possibility of the reaction to be catalyzed by
palladium nanoparticles was examined by using the mercury poisoning experiment [34].
Accordingly, bromobenzene was treated with 4-tolylboronic acid at 22 C for one minute
followed by the addition of a high excess (1000 equivalents) of mercury to the reaction
system. The reaction was carried out upon vigorous stirring for 24 hours. Total inhibition of
the reaction was observed by adding mercury excess (Figure 9).
Figure 9. Mercury poisoning experiment. Profiles of the reactions of bromobenzene with 4-
tolylboronic acid performed in the presence of 2g (0.1 mol%) with and without the addition of
mercury excess. Reaction conditions: [ArBr]:[ArB(OH)2]:[KOH] = 1:1.05:1.1, air; 22 C, Hg
was added after 1 min. of the reaction course.
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
scanning and transmission electron microscopy after 5 minutes or 2 hours of the reaction
11

course (Figures 10 and 11). The experiment revealed formation of palladium nanoparticles
and the increase in their average diameter in the course of the reaction from 1.4 nm (after 5
min) to 3.6 nm (after 2 h).
Figure 10. SEM micrographs showing the palladium nanoparticles formed in the reaction
mixture after 5 min (a) and 2 h (b) of the reaction course in the presence of complex 2a.
12

Figure 11. TEM micrographs showing the palladium nanoparticles formed in the reaction
mixture after 5 min (a) and 2 h (b) of the reaction course in the presence of complex 2a.
The nanoparticles isolated from the post-reaction mixture were found inactive in the
Suzuki–Miyaura coupling of bromobenzene with 4-tolylboronic acid. Our attempts to isolate
catalytically active species have failed. It is very likely that the particles of strictly defined
size are catalytically active. During the reaction they grow or dissolve due to Oswald
ripening, resulting in the loss of catalytic activity.
ESI-MS analysis of the post reaction mixture from the Suzuki–Miyaura coupling of
bromobenzene with 4-tolylboronic acid did not show the formation of dimeric or trimeric
complexes, as observed by Albrecht and Trzeciak [12].
13

4. Conclusions
PEPPSI-type palladium(II) 1,2,3-triazolin-5-ylidene complexes bearing sterically diverse
ligands can be easily synthesized via metalation/transmetalation sequence. The complexes are
efficient precatalyst in the Suzuki–Miyaura coupling of boronic acids with aryl bromides and
exhibit moderate activity in the coupling with of boronic acids with aryl chlorides. Mercury
poisoning experiment indicates the participation of nanoparticles or underligated palladium
complexes in the catalytic process and 1,2,3-triazolin-5-ylidene palladium complexes act as
source of catalytically active species. Despite the catalytic activity observed, synthesized
1,2,3-triazolin-5-ylidene palladium complexes 2a-g does not bring the expected advantages as
catalysts of Suzuki–Miyaura coupling.
Acknowledgment
The research was co-financed by the National Science Centre (Poland), project UMO-
2013/11/N/ST5/01612.
References
[1] a) Han Vinh Huynh, The Organometallic Chemistry of N‐heterocyclic Carbenes, Wiley,
2017; b) M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 510 (2014) 485–
496.
[2] a) S. P. Nolan, N-Heterocyclic Carbenes: Effective Tools for Organometallic Synthesis,
Wiley-VCH, 2014; b) C. S. J. Cazin, N-Heterocyclic Carbenes in Transition Metal
Catalysis and Organocatalysis, Springer, 2011; c) F. Glorius, N-Heterocyclic Carbenes
in Transition Metal Catalysis, Springer, 2007; d) S. P. Nolan, N-Heterocyclic Carbenes
in Synthesis, Wiley-VCH, Weinheim, 2006.
[3] S. Gründemann, A. Kovacevic, M. Albrecht, J. W. Faller R. H. Crabtree, J. Am. Chem.
Soc. 124 (2002) 10473–10481.
[4] S. Gründemann, A. Kovacevic, M. Albrecht, J. W. Faller R. H. Crabtree, Chem.
Commun. 4 (2001) 2274–2275.
[5] P. Mathew, A. Neels, M. Albrecht, J. Am. Chem. Soc. 130 (2008) 13534–13535.
[6] H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem. Int. Ed. 40 (2001) 2004–2021.
[7] R. Huisgen, Angew. Chem. Int. Ed. 2 (1963) 565–598.
[8] M. Meldal, C. Tomøe, W. Chem. Rev. 108 (2008) 2952–3015.
[9] M. G. Finn, V. V. Fokin, Chem. Soc. Rev. 39 (2010) 1231–1232.
[10] R. Tonner, G. Frenking, Organometallics 28 (2009) 3901–3905.
[11] T. Mitsui, M. Sugihara, Y. Tokoro S. I. Fukuzawa, Tetrahedron 71 (2015) 1509–1514.
[12] D. Canseco-Gonzalez, A. Gniewek, M. Szulmanowicz, H. Möller-Bunz, A. M.
Trzeciak, M. Albrecht, Chem. Eur. J. 18 (2012) 6055–6062.
[13] J. Huang, J. T. Hong, S. H. Hong, Eur. J. Org. Chem. 3 (2012) 6630–6635.
[14] T. Terashima, S. Inomata, K. Ogata S. I. Fukuzawa, Eur. J. Inorg. Chem. (2012) 1387–
1393.
14

[15] J. B. Shaik, V. Ramkumar, B. Varghese, S. Sankararaman, Beilstein J. Org. Chem. 9
(2013) 698–704.
[16] L. A. Schaper, L. Graser, X. Wei, R. Zhong, K. Öfele, A. Pöthig, M. Cokoja, B.
Bechlars, W. A. Herrmann, F. E. Kühn, Inorg. Chem. 52 (2013) 6142–6152.
[17] S. Hohloch, W. Frey, C.-Y. Su, B. Sarkar, Dalton Trans. 42 (2013) 11355–11358.
[18] B. Sureshbabu, V. Ramkumar S. Sankararaman, J. Organomet. Chem. 799–800,
(2015) 232–238.
[19] A. Mohan, V. Ramkumar, S. Sankararaman, J. Organomet. Chem. 799–800 (2015)
115–121.
[20] D. Mendoza-Espinosa, R. González-Olvera, G. E. Negrón-Silva, D. Angeles-Beltrán,
O. R. Suárez-Castillo, A. Álvarez-Hernández, R. Santillan, Organometallics 34 (2015)
4529–4542.
[21] L. Hettmanczyk, B. Schmid, S. Hohloch, B. Sarkar, Molecules 21 (2016) 1561–1574.
[22] R. Maity, A. Verma, M. van der Meer, S. Hohloch, B. Sarkar, Eur. J. Inorg. Chem.
(2016) 111–117.
[23] D. Yuan, H. V. Huynh, Organometallics 31 (2012) 405−412.
[24] R. Saravanakumar, V. Ramkumar, S. Sankararaman, Organometallics 30 (2011)
1689–1694.
[25] a) S. Inomata, H. Hiroki, T. Terashima, K. Ogata, S. I. Fukuzawa, Tetrahedron 67
(2011) 7263–7267; b) M. Gazvoda, M. Virant, A. Pevec, D. Urankar, A. Bolje, M.
Kocevar, J. Kosmrlj, Chem. Commun. 52 (2016) 1571–1574.
[26] E. C. Keske, O. V. Zenkina, R. Wang, C. M. Crudden, Organometallics 31 (2012)
6215–6221.
[27] S. Modak, M. K. Gangwar, M. Nageswar Rao, M. Madasu, A. C. Kalita, V. Dorcet,
M. A. Shejale, R. J. Butcher P. Ghosh, Dalton Trans. 44 (2015) 17617–17628.
[28] D. Schweinfurth, L. Hettmanczyk, L. Suntrup, B. Sarkar, Z. Anorg. Allg. Chem. 643
(2017) 554–584.
[29] A. Poulain, D. Canseco-Gonzalez, R. Hynes-Roche, H. Möller-Bunz, O. Schuster, H.
Stoeckli-Evans, A. Neels, M. Albrecht, Organometallics 30 (2011) 1021–1029.
[30] K. F. Donnelly, R. Lalrempuia, H. Möller-Bunz, M. Albrecht, Organometallics 31
(2012) 8414–8419.
[31] S. Guchhait, K. Ghosh, B. Sureshbabu, V. Ramkumar, S. Sankararaman, J.
Organomet. Chem. 768 (2014) 68–74.
[32] K. J. Kilpin, S. Crot, T. Riedel, J. Kitchen, P. J. Dyson, Dalton Trans. 43 (2014) 1443–
1448.
[33] K. Barral, A. D. Moorhouse, J. E. Moses, Org. Lett. 9 (2007) 1809–1811.
[34] N. T. S. Phan, M. Van Der Sluys, C. W. Jones, Adv. Synth. Catal. 348 (2006) 609–
679.
15

1,2,3-triazolin-5-ylidene palladium complexes catalyze effective Suzuki coupling
Triazolin-5-ylidene palladium complexes decompose when treated with boronic acid
Mercury poisoning test indicates contribution of nanoparticles in catalysis
16

17