1,2,3-Triazol-5-ylidene bearing a pyridine auxiliary based bidentate PEPPSI type palladium(II) complex: Synthesis, structure, and application in the direct arylation of pyridine N-oxide

Article abstract of DOI:10.1016/j.catcom.2019.105889

A novel PEPPSI (pyridine enhanced precatalyst preparation stabilization and initiation) type palladium(II) complex has been synthesized and fully characterized by NMR, HR-MS, as well as elemental analysis. The molecular structure was finally confirmed by single crystal X-ray diffraction, and it revealed that the pyridyl-armed 1,2,3-triazole ligand is coordinated to the palladium in bidentate fashion. This air and moisture insensitive complex has proven to be a suitable catalyst for the arylation of pyridine N-oxides and 2-bromopyrroles with a broad functional group tolerance, demonstrating that the robustness and high efficiency of our complex.

Full text of DOI:10.1016/j.catcom.2019.105889

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1,2,3-Triazol-5-ylidene bearing a pyridine auxiliary based
bidentate PEPPSI type palladium(II) complex: Synthesis,
structure, and application in the direct arylation of pyridine N-
oxide
Lin-Yu Jiao, Xiao-Mei Yin, Shanshan Liu, Ze Zhang, Ming Sun,
Xiao-Xun Ma
PII:
S1566-7367(19)30350-4
DOI:
https://doi.org/10.1016/j.catcom.2019.105889
CATCOM 105889
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
7 September 2019
1 November 2019
8 November 2019
Please cite this article as: L.-Y. Jiao, X.-M. Yin, S. Liu, et al., 1,2,3-Triazol-5-ylidene
bearing a pyridine auxiliary based bidentate PEPPSI type palladium(II) complex:
Synthesis, structure, and application in the direct arylation of pyridine N-oxide, Catalysis
Communications (2019), https://doi.org/10.1016/j.catcom.2019.105889
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1,2,3-Triazol-5-ylidene bearing a pyridine auxiliary based bidentate
PEPPSI type palladium(II) complex: Synthesis, structure, and
application in the direct arylation of pyridine N-oxide
Lin-Yu Jiaoa,* lyjiao@nwu.edu.cn, Xiao-Mei Yina, Shanshan Liub, Ze Zhanga, Ming Suna,
Xiao-Xun Maa,* maxym@nwu.edu.cn
aSchool of Chemical Engineering, International Scientific and Technological Cooperation Base for Clean Utilization of
Hydrocarbon Resources, Chemical Engineering Research Center of the Ministry of Education for Advance Use
Technology of Shanbei Energy, Shaanxi Research Center of Engineering Technology for Clean Coal Conversion,
Collaborative Innovation Center for Development of Energy and Chemical Industry in Northern Shaanxi, Northwest
University, Xi'an, Shaanxi, 710069, P. R. China
bShaanxi Key Laboratory of Chemical Additives for Industry, College of Chemistry and Chemical Engineering, Shaanxi
University of Science and Technology, Xi’an, Shaanxi, 710021, P. R. China
*Corresponding authors.
Abstract
A novel PEPPSI (pyridine enhanced precatalyst preparation stabilization and initiation)
type palladium(II) complex has been synthesized and fully characterized by NMR,
HR-MS, as well as elemental analysis. The molecular structure was finally confirmed
by single crystal X-ray diffraction, and it revealed that the pyridyl-armed 1,2,3-triazole
ligand is coordinated to the palladium in bidentate fashion. This air and moisture
insensitive complex has proven to be a suitable catalyst for the arylation of pyridine
N-oxides and 2-bromopyrroles with a broad functional group tolerance, demonstrating
that the robustness and high efficiency of our complex.
Keywords:
Mesoionic
carbene,
PEPPSI,
N-heterocyclic
carbene,
1,2,3-Triazol-5-ylidene, Bidentate palladium(II) complex, Arylation

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1. Introduction
Transition-metal-catalyzed cross coupling has been recognized as one of the most
powerful carbon–carbon bond formation processes [1–3]. The development of
catalysts, especially the evolution of ligands is critical for homogeneous catalysis. In
this area, phosphine-based ligands have been predominant for a long time until
N-heterocyclic carbene–palladium (NHC–Pd) type precatalysts exhibited their
effectiveness and versatilities [4]. Since then, a broad variety of complexes containing
imidazole-, triazole-, or tetrazole-based carbene have been introduced [5,6].
Fig. 1. Representative NHC–palladium complexes.
The mesoionic carbenes (MICs) [7], also known as abnormal carbenes (aNHCs), are
similar to normal NHCs except that the canonical resonance structures cannot be
depicted without the introduction of extra charges [8]. Carbenes based on
1,2,3-triazoles which could be readily accessed through Click reaction [7,9,10] are
well studied. The first typically trisubstituted triazolylidenes with alkyls in the N1 and
N3 positions and aryls in the C4 or C5 position were reported by Albrecht in 2008 [11]
(Fig. 1, left). The presence of triazole unit offers an interesting alternative to the
traditional framework and has been found to render interesting variation in catalysis.
Importantly, these ligands usually exhibit strong σ-donating properties, easily tunable
steric properties, excellent air and moisture stability, as well as high dissociation
energies [12].
Recently, chelated MICs assembled with a donor group are readily established
themselves as privileged ligands in homogeneous catalysis [13–17]. As a
consequence, the discovery of PEPPSI complexes is an important breakthrough in
cross coupling chemistry (Fig. 1, center left) [18,19]. This combined electronic effect
renders the metal more electron rich [20,21], allowing for more favorable oxidative

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insertion. In this class of tightly bonded complexes, the pyridyl group readily departs in
situ [22] to generate a vacant site to promote the coupling process [23].
It has been demonstrated that triazolylidene-based palladium complexes show
enhanced thermal stability (above 100 °C), and documented that catalysts bearing
both NHCs and auxiliary ligands such as N- [24], (Fig. 1, center right) P- [25], and
other donors [26] exhibit enhanced catalytic activities in several coupling reactions
[27]. In order to enhance the structure rigidity and overcome the drawbacks of free
pyridine donors, we envisioned that a 1,2,3-triazole backbone with an additional
pyridyl arm (Fig. 1, right) could coordinate with palladium in bidentate fashion to
provide both bulky steric hindrance and strong electronic donation, and therefore a
more efficient cross coupling reaction could be conducted on this basis. Accordingly,
we reported herein a preparation of a 1,2,3-triazole based PEPPSI type palladium
complex and its application in cross coupling reaction.
2. Results and discussion
Scheme 1 Sequential procedure for the synthesis of palladium complex 5.
2.1 Synthesis and characterization of palladium complex 5
The complex 5 was synthesized through a four-step procedure in good overall yield
(Scheme 1). The protocol was started with the reaction between

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4-(ethoxycarbonyl)pyridine 1-oxide (1) and NaN3 in the presence of stoichiometric
amount of toluenesulfonyl chloride (TsCl) as internal activator at elevated temperature,
tetrazole 2 was generated as the majority form in the tetrazole/azide equilibrium. The
mixture was then subjected to a copper catalyzed Click reaction with phenylacetylene
to deliver the triazole 3 in 85% isolated yield. The following step of the reaction
sequence was a quaternization of a triazole nitrogen atom with trimethyloxonium
tetrafluoroborate ([Me3O]BF4) under ice cold to room temperature conditions in DCM
to give triazolium tetrafluoroborate 4, which was isolated as a white powder. Next,
treatment of 4 with Ag2O under dark yields AgI(carbene)2 complex, which served as a
carbene transfer agent, reacted with palladium precursor in a appropriate molar ratio.
It has proved that Pd(CH3CN)2Cl2 was the choice of reagent. In this step the initial
brown mixture gradually turned yellow, indicating the completion of the reaction.
Finally, 1,2,3-triazol-5-ylidene based PEPPSI type palladium complex 5 was
recrystallized in 68% yield as a yellow solid, exhibiting stable properties towards air
and moisture.
The complex was characterized thoroughly by various spectroscopic methods. High
resolution mass spectrometry (HR-MS) of the complex 5 showed single ions with a
m/z value corresponding to the [(pyridyl-armed-1,2,3-triazol-5-ylidene)PdCl]+ ion,
indicating the NHC−Pd complex composition. Likewise, elemental analysis for 5 was
in agreement with a [(pyridyl-armed-carbene)PdCl2]n formulation which again ruled
out the possibility of a non-chelating complex. The structural assignment was
ultimately confirmed by the X-ray crystallographic structure determination (vide infra).

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2.2 X-ray crystallographic analyses of the complex 5
Fig. 2 The molecular structure of the complex 5, hydrogen atoms are omitted for clarity.
Single crystal suitable for the X-ray diffraction studies was obtained by slow diffusion
of diethyl ether into a chloroform solution at room temperature. An oak ridge thermal
ellipsoid plot (ORTEP) drawing of 5 is depicted in Fig. 2. Most of NHC−palladium
complexes possess the square-planar coordination around the central palladium atom
with either trans or cis arrangements of the X groups (e.g., chloride or acetate).
Among the monocarbene complexes, the trans isomers are the most prevalent with
the donor group usually completing the coordination of the metal center [28]. The cis
arrangement is less populated and is likely to occur when the additional donor group
is directly bonded to the NHC fragment to form a four- to eight-membered
palladacycle [15,29]. As expected, the geometry of complex 5 around palladium is
square planar sphere with the triazole ligand and the attached pyridyl donor disposed
at a cis position and the other two chlorine atoms also placed in cis fashion,
respectively. As a consequence, a five-membered chelate ring is formed through the
functionalized pyridine donor to the metal centre (Figure 1). For the classic PEPPSI
type complexes with free pyridyl donors, the triazolylidene rings is usually nearly
perpendicular to the plane containing a palladium atom, X groups, carbene carbon
and pyridine nitrogen [30]. In this complex, however, the triazolylidene ring, pyridine
ring, the palladium, as well as two chlorine atoms are almost coplanar.

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Table 1 Selected bond lengths (nm) and elected bond angles (°) of complex 5.
Pd1‒Cl1
Pd1‒C8
0.2353(2)
0.1986(8)
0.2039(6)
0.1335(9)
0.1316(9)
91.57(8)
94.06(19)
175.3(2)
93.1(2)
C14‒N1
C10‒N2
0.1362(10)
0.1424(10)
0.1355(10)
0.1472(10)
0.1385(11)
124.1(7)
123.0(7)
121.0(6)
115.9(6)
Pd1‒N1
C8‒N2
N2‒N3
C9‒N4
N3‒N4
C8‒C7
Cl1‒Pd1‒Cl2
Cl1‒Pd1‒N1
Cl1‒Pd1‒C8
Cl2‒Pd1‒C8
Cl2‒Pd1‒N1
C8‒Pd1‒N1
Pd1‒N1‒C10
Pd1‒N1‒C14
N1‒C10‒N2
N1‒C10‒C11
N2‒C10‒C11
C10‒N2‒C8
C10‒N2‒N3
C8‒N2‒N3
N2‒C8‒C7
N2‒C8‒Pd1
C7‒C8‒Pd1
C8‒C7‒N4
C7‒N4‒C3
N2‒N3‒N4
174.31(19)
81.3(3)
103.4(7)
110.3(5)
115.7(5)
129.1(6)
109.4(7)
126.4(7)
146.2(6)
104.5(7)
115.4(6)
100.8(6)
Selected principal bond lengths and interbond angles are shown in Table 1. The
Pd−Ctrz (trz = triazolylidene) bond length is 1.986(8) Å, which is in line with those of
related triazolylidene–palladium complexes (Table 1) [31]. The Pd–Cl bond distance
trans to the triazole carbene Pd1–Cl1, is 2.353(2) Å, 0.087 Å longer than the other
Pd–Cl distances trans to the pyridyl nitrogen atom, Pd1–Cl2. This bond elongation
phenomenon is usually observed and could be attribute to the trans effect of the NHC
ligands [15,29]. Compared with other carbenes, the Pd1‒N1 bond length, 2.039(6) Å,
is 0.01–0.04 Å shorter than nitrogen containing groups chelated complexes [15,29],
and 0.06–0.09 Å shorter than complexes with free pyridyl donors [32], indicating the
stability of the resulting coordination. The bite angle of C(7)–Pd(1)–N(1) is 81.3(3)°,
similar with previous reports [32]. The palladium and atoms bonded around are
exactly in a plane and there is no out-of-plane distortion, with the summation of the
four angles around the palladium central being 360.03 °. Other structural parameters
are unexceptional.

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2.3 Direct arylation of pyridine N-oxide
Table 2 Identification reaction conditions for the palladium complex 5 catalyzed cross coupling
reaction pyridine N-oxide and 2-bromopyrrolea
Entry
1
R
x
1
Base
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
K2CO3
K2CO3
Cs2CO3
tBuOK
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Solvent
DMF
T (oC)
100
100
100
100
100
100
100
100
100
100
100
100
80
t (h)
18
18
18
18
18
18
18
12
18
18
12
12
24
12
12
12
18
18
Conversion (%)b
Yield (%)c
NDd
58
H (7a)
<5
70
72
76
92
<5
>99
97
92
88
72
60
85
33
42
47
95
83
2
Me (7b)
Ph (7c)
Bn (7d)
Ts (7e)
NMe2 (7f)
Bn (7d)
Bn (7d)
Bn (7d)
Bn (7d)
Bn (7d)
Bn (7d)
Bn (7d)
Bn (7d)
Bn (7d)
Bn (7d)
Bn (7d)
Bn (7d)
1
DMF
3
1
DMF
55
4
1
DMF
70
5
1
DMF
tracee
NDd
91
6
1
DMF
7
1
DMF
8
1
DMF
87
9
1
DMF
83
10
11
12
13
14
15
16
17
18
1
DMF
82
1
dioxane
toluene
DMF
64
1
55
1
77
28f
40g
44h
88
−
DMF
100
100
100
100
100
−
DMF
−
DMF
0.5
0.2
DMF
DMF
80
a Reaction conditions: pyridine N-oxide 6a (0.40 mmol, 1.0 equiv.), 2-bromopyrrole 7a−7f (0.48 mmol, 1.2
equiv.), base (0.60 mmol, 1.5 equiv.), indicated amount of catalyst 5, tetracosane (15 mg) in solvent (2.0
mL, 0.2 M) under an inert atmosphere at the indicated temperature for the indicated reaction time.
b Determined by GLC analysis with tetracosane as the internal standard.
c Isolated yield after purification by flash column chromatography on silica gel.
d ND = Not determined.
e Complicate.reaction system with up to 10 compounds formed in situ.
f Pd(OAc)2 (10 mol%) was used as catalyst instead of catalyst 5.
g
Pd(OAc)2 (10 mol%) was used as catalyst instead of catalyst 5, PPh3 (20 mol%) was introduced as
ligand.
h
Pd(OAc)2 (10 mol%) was used as catalyst instead of catalyst 5, 1,4-bis(diphenylphosphino)butane
(DPPB, 20 mol%) was introduced as ligand.
Palladium catalyzed cross coupling reaction has established itself as a benchmark

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process for the formation of biaryl compounds [33]. The steric and electronic features
of ligands bound to palladium modify the activity of the metal center and their
structures play a central role in catalysis. Consequently, catalytic performance studies
of the palladium complex 5 with pyridyl-functionalized triazole ligand have been
focused on this type of transformation. Pyridine N-oxide 6a and protected
2-bromopyrroles 7a−7f were employed as model substrates to furnish biaryl product
8aa−8af. As illustrated in Table 2, initial protecting group screening (entries 1−6) at
100 oC in the presence of 1 mol% catalyst loading indeed provided desired products,
in which benzyl was the best choice of accessory to give biaryl product 8ad in 70%
yield (entry 4). However, tosyl (Ts, 7e), dimethylamino (7f), as well as unprotected 7a
substrates remained almost unreacted (entries 1 and 6) or resulted in a mess mixture
(entry 5). To our delight, both conversions and yields, as determined by GLC analysis
with the introduction of tetracosane as internal standard, increased dramatically
immediately when other bases such as potassium carbonate (K2CO3) or cesium
carbonate (Cs2CO3) were employed instead (entries 7 and 9) even with a shortened
reaction time (entry 8). Other solvents such as dioxane and toluene gave rise to
moderate conversions and yield under the identical reaction conditions (entries 11 and
12). The detrimental effect was observed when the reaction temperature was lowered
o
to 80 C, reactivity disparities could not bridged even with a prolonged reaction time
(entry 13). In order to clearly determine the real influence of the complex 5 compared
to other Pd sources, we introduced Pd(OAc)2 as catalyst instead. As illustrated in
entries 14−16, poor to moderate yield were obtained. It is worthy to mention that
phosphorus containing ligands were crucial to promote the transformation, in which
1,4-bis(diphenylphosphino)butane (DPPB) showed better reactivity than PPh3. Our
palladium complex indeed showed better reactivity towards the desired cross coupling
over ordinary palladium salts. Furthermore, reducing the catalyst loading by half did
not lead to much different, however, continuously decreasing would generate poor
results noticeable (entries 17−18). From the viewpoint of reactivity and economic, we
chose entry 14 as the optimal conditions for further investigation. It is worthy
mentioning that the undesired side reactions such as homocoupling of

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2-bromopyrroles, deoxygenation, and debromination of the substrates were almost
suppressed.
Table 3 Variation of the coupling partners in the palladium complex 5 catalyzed biaryl
formation reactiona
a
Except for other statement, all reactions were proceeded under the following reaction conditions:
pyridine N-oxide (0.40 mmol, 1.0 equiv.), 2-bromopyrrole (0.48 mmol, 1.2 equiv.), K2CO3 (0.60 mmol, 1.5
equiv.), catalyst 5 (0.002 mmol, 0.5 mol%), and tetracosane (15 mg) in DMF (2.0 mL) under N2
atmosphere at 100 oC for 18 h; isolated yield reported.
b Reactions proceeded for 24 h.
Encouraged by these results, we extended our investigations to the other substrates
derivatives to broaden the application of our catalyst. A series of coupling partners

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were examined under the optimized conditions and the results were presented in
Table 3. It can be concluded that the electronic and steric profiles of pyridine N-oxides
didn’t exhibit significant influences on the reactivities and efficiency, good to excellent
results were achieved accordingly (8ad−8jd). It was worth mentioning that quinoline
N-oxide and pyridine N-oxide performed equally well (6a → 8ad vs 6k → 8kd).
Additionally, however, pyrroles bearing other substituent normally exhibited slightly
lower reactivities (8ag−8ai).
2.4 Mercury-poisoning test
The mercury drop test is a widely used protocol to confirm the formation of
palladium(0) nanoparticles resulting from decomposition of homogeneous palladium
complexes, since mercury(0) is able to poison the “naked” metal particles [34,35] The
direct arylation of pyridine N-oxide initiated by complex 5 was accompanied by the
formation of fine black solid in situ. The coordination between the bulky armed ligand
and palladium atom would loosen during the catalysis process. Therefore, the
possibility of formation of highly active palladium species as the active catalyst
prompted us to perform mercury poisoning tests. Based on an identical condition with
entry 9 of Table 2, we applied the experimental set-up with the addition of an excess
amount of (200 equiv.) mercury. The most noticeable decrease in conversion (full
conversion to 38%) was observed when mercury is added at the very beginning of the
catalysis. Otherwise, the addition of mercury after 3 h resulted in small variations in
the yields (97% to 91%), suggesting that the molecular nature of the active catalyst
species. We isolated this solid material at relative lower reaction temperature (Table 2,
entry 13, at 50 oC for 2 h) and characterized with transmission electron microscope
(TEM), fine palladium nanoparticles were observed (refer to the Supplementary
material in detail).
3. Conclusions
To sum up, we developed a novel and stable triazolylidene-based PEPPSI complex
with a five-membered palladacycle. The complex was prepared by convenient method

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and full characterized. It was confirmed by the crystal structure that the existence of a
coplaner containing the triazole ring, pyridine ring, palladium central, as well as
chlorides. The resulting complex exhibited very high reactivity towards the cross
coupling between pyridine N-oxide and 2-bromopyrrole with very low catalyst loading.
The rigid backbone based complex exhibited an air stabilization effect on the catalytic
process, and the reaction could be performed in the presence of air and moisture. The
cross coupling reaction was applicable for a broad substrate scope with high
functional group tolerance under mild conditions, and a series of biaryl compounds
were prepared in high yields. Mercury poisoning experiment, together with TEM
characterization were established to preliminary proof the in situ generation of
palladium(0) nanoparticles from the decomposition of the complex, which was
recognized as highly catalytically active intermediate. Further investigation of the
catalytic potential and structure modifications of the palladium complex is currently
undertaking in our laboratory.
Acknowledgments
We are grateful to the National Natural Science Foundation of China (NSFC:
21606182, 21901150) for financial support.
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships
that could have appeared to influence the work reported in this paper.
Appendix A. Supplementary material
Supplementary data to this article can be found in the online version, at: XXX.
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Highlights
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Design and synthesis of a PEPPSI type bidentate palladium complex.
The complex was applied to the arylation of pyridine N-oxides and bromopyrroles.
Mercury poisoning test was performed.
Pd(0) nanoparticles were generated in situ as highly catalytically active species.
Graphical abstract