Symmetrical and Non-symmetrical Pd (II) Pincer Complexes Bearing Mesoionic N-heterocyclic Thiones: Synthesis, Characterizations and Catalytic Properties

Article abstract of DOI:10.1002/aoc.5885

In contrast to well-established symmetrical pincer complexes, non-symmetrical metal pincers with the loss of C_2v symmetry are much less studied. In this work, mesoionic NHTs (NHTs = N-heterocyclic thiones), which can be viewed as the sulfur adducts of mesoionic carbenes, are incorporated into pincer complexes for the first time. Two symmetrical and non-symmetrical phenylene-bridged bis (NHT) compounds 3a/3b were synthesized as proligands via a “cycloaddition-alkylation-thionation” reaction sequence. In a case to access bis (NHT) compound 3c, N-dealkylation reactions occurred. The carbene NMR signals of NHTs are only partially correlated to the π-accepting abilities of carbenes, which is different from Bertrand's carbene-phosphinidene system. The structural analysis of 3a/3b indicates that they possess C-S partial double bonds. 3a and 3b served as the precursors to access two aryl anion-linked [SCS/S′] pincer complexes 6a/6b. An external base proved to be essential for this cyclopalladation process. The catalytic properties of 6a/6b in the cycloisomerizations of alkynoic acids have been examined. Finally, non-symmetrical complex 6a shows superior catalytic performance in such transformations contrasting to its symmetrical counterpart 6b.

Full text of DOI:10.1002/aoc.5885

Received: 29 April 2020
DOI: 10.1002/aoc.5885
Revised: 28 May 2020
Accepted: 30 May 2020
F U L L P A P E R
Symmetrical and Non-symmetrical Pd (II) Pincer
Complexes Bearing Mesoionic N-heterocyclic Thiones:
Synthesis, Characterizations and Catalytic Properties
Xuechao Yan | Bo Zhang | Xin Zhang | Haiying Wang | Yu-Ai Duan |
Shuai Guo
Department of Chemistry, Capital Normal
In contrast to well-established symmetrical pincer complexes, non-
symmetrical metal pincers with the loss of C2v symmetry are much less stud-
ied. In this work, mesoionic NHTs (NHTs = N-heterocyclic thiones), which
can be viewed as the sulfur adducts of mesoionic carbenes, are incorporated
into pincer complexes for the first time. Two symmetrical and non-
symmetrical phenylene-bridged bis (NHT) compounds 3a/3b were synthesized
as proligands via a “cycloaddition-alkylation-thionation” reaction sequence. In
a case to access bis (NHT) compound 3c, N-dealkylation reactions occurred.
The carbene NMR signals of NHTs are only partially correlated to the
π-accepting abilities of carbenes, which is different from Bertrand's carbene-
phosphinidene system. The structural analysis of 3a/3b indicates that they pos-
sess C-S partial double bonds. 3a and 3b served as the precursors to access two
aryl anion-linked [SCS/S0] pincer complexes 6a/6b. An external base proved to
be essential for this cyclopalladation process. The catalytic properties of 6a/6b
in the cycloisomerizations of alkynoic acids have been examined. Finally, non-
symmetrical complex 6a shows superior catalytic performance in such trans-
formations contrasting to its symmetrical counterpart 6b.
University, Beijing, 100048, China
Correspondence
Shuai Guo, Department of Chemistry,
Capital Normal University, Beijing,
China, 100048.
Email: guoshuai@cnu.edu.cn
Funding information
Support Project of High-level Teachers in
Beijing Municipal Universities in the
Period of 13th Five-year Plan, Grant/
Award Number: CIT&TCD201904087;
General Project of Scientific Research
Program of Beijing Education
Commission, Grant/Award Number:
KM201810028007; Natural Science
Foundation of Beijing Municipality,
Grant/Award Number: 2192012
K E Y W O R D S
Mesoionic, N-heterocyclic thione, non-symmetrical, pincer
1 | INTRODUCTION
manifold applications spanning from catalytic organic
transformations to photoluminescence.
The past 40 years have witnessed the rapid development
of transition metal pincer chemistry.[] Notably, compared
to [PXP]- and [NXN]-type analogues, sulfur-based [SXS]
However, it should be additionally noted that most of
the reported [SXS] pincers are C2v-symmetric, and non-
symmetric [SXS']-type pincer frameworks with two differ-
ent S-based functionalities remain largely unexplored.[6]
The desymmetrization of pincer platforms undoubtedly
further expands the structural diversity of pincer family.
In addition, employing two arm-donors with different elec-
tron-releasing capacities can make one donor hemilabile
pincer frameworks are relatively less investigated.[2]
A
number of S-donors such as thioethers/thiolates,[3] phos-
phine sulfides[4] and thioamides[5] have been employed
as arm donors to access various sulfur-containing pincer
complexes (e.g. I, II, III, Figure 1), and they have found
Xuechao Yan and Bo Zhang, contribute equally to this work.
Appl Organomet Chem. 2020;e5885.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5885

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YAN ET AL.
are targeted (Figure 2). In addition to non-symmetrical
SCS' pincer system A bearing two different mesoionic
NHTs, two symmetrical counterparts (B and C) have also
been accessed to make a systematic comparison. The
preparation, reactivity and catalytic performance of their
proligands as well as palladium (II) pincers are reported.
FIGURE 1 Some examples (I/II/III) of [SXS] pincer complexes
2 | RESULTS AND DISCUSSION
and help to generate a vacant coordination site. Such con-
cept has been employed in a few non-symmetric pincers
such as Milstein's PNN and PNS systems and proved to be
successful.[1j,7]
2.1 | Synthesis of non-symmetrical
proligands
NHTs (NHTs = N-heterocyclic thiones) such as
imidazolin-2-thiones can be viewed as the sulfur adducts
of NHCs (NHCs = N-heterocyclic carbenes) and have
been extensively used as S-donors.[8] The strong interest
in NHTs partially roots in the fact that they can be conve-
niently accessed by reacting well-established NHCs[9]
with elemental sulfur. As such, odorous and toxic
organosulfur compounds (e.g. thiols), which are typically
used as sulfur sources to access S-based pincers, can be
avoided. Furthermore, the fine steroelectronic tunability
and convenient functionalization of NHCs also render
their NHT derivatives highly appealing.
In contrast to classical NHTs, mesoionic NHTs (e.g. i,
Figure 2) derived from mesoionic carbenes[10] are much
less investigated. For mesoionic compounds, a totally
uncharged canonical resonance structure cannot be
represented,[11] and this unique type of molecules as well
as their complexes have gained increasing interest in
recent years.[12] For 1,2,3-triazole-derived mesoionic
NHT i, a number of limiting resonance forms (i.e. thiolate
or thione forms) can be deduced. Surprisingly, although
the synthesis of i dates back to Begtrup's work almost
50 years ago,[13] its metal complexes and further catalytic
applications have been rarely reported.[14]
In aiming to access the non-symmetrical target ligand A,
the corresponding dicationic precursor 2a was synthe-
sized via a sequence shown in Scheme 1. The copper-
mediated click reaction using 3-aminophenyl-acetylene
and benzyl azide as the starting materials gave the mono-
triazole a. For this step, a well-defined copper carbene
complex [CuCl (IPr)] (IPr = 1,3-bis-(2,6-diisopropylphenyl)
imidazolin-2-ylidene) was used as the catalyst. Without iso-
lation, compound a underwent diazotization, azidation
followed by [3 + 2] click reaction successfully affording
the non-symmetrical bis (triazoles) 1a in good yield. The
subsequent alkylation using Meerwein's salt (CH3)3OBF4
yielded the desired dicationic proligand salt 2a in moderate
1
yield. In the H NMR spectrum of 2a in d6-DMSO, two
downfield singlets at 9.45 and 9.33 ppm were observed,
which can be assigned to the two non-symmetrical Ctriazole
-
H protons. Symmetrical 1,2,3-triazolium salts 2b and 2c,
which have been previously reported by others and our
group respectively,[15] are used herein as precursors of
ligands B and C (see Supporting Information, Scheme S1).
2.2 | Mesoionic carbene-sulfur adducts
Herein, we report the first examples of mesoionic
NHT-based pincer complexes, and three pincer platforms
Then, 2a-2c were used as precursors to access the target
S-donors. The reactions of 2a/2b with elemental sulfur in
FIGURE 2 Resonance forms of
mesoionic NHT i and three targeted
ligand platforms (A-C)

YAN ET AL.
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of 2c,[15c] and has also been observed in earlier work of
Bertrand and others.[12a,16]
2.3 | Spectroscopic and structural features
of 3a/3b
Compounds 3a/3b are air-stable and readily soluble in
EtOAc and CH2Cl2 but less soluble in Et2O. Their forma-
tion was first corroborated by positive mode ESI mass
spectrometric analysis. Two base peaks at m/z 451 and
485, which can be assigned to the corresponding proton-
ated species [M + H]+, were observed. In addition, in the
1H NMR spectra of 3a/3b, the downfield signals assigned
to Ctriazole-H protons disappeared, which indicate that the
deprotonation processes have occurred. As expected, the
two N-methyl groups of 3b gave only one singlet
(4.14 ppm), while two different NCH3 signals (4.16 ppm
and 3.99 ppm) were observed in the case of 3a owing to
the loss of C2v symmetry.
SCHEME 1 Synthesis of 1a and 2a. Condition (i): [CuCl
(IPr)], EtOH/H2O; Condition (ii): NaNO2, HCl, and then NaN3,
and then 1-hexyne, [CuCl (IPr)], EtOH/H2O, 82% (two-step
combined yield); Condition (iii): (CH3)3OBF4, C2H4Cl2, 56%
the presence of an external base were examined. KOtBu
and an even weaker base K2CO3 proved to be effective.
The desired non-symmetrical mesoionic bis (NHT) com-
pound 3a and the symmetrical analogue 3b can be iso-
lated as yellow solid in good yields (Scheme 2).
In sharp contrast, the reaction of 2c with elemental
sulfur in the presence of an external base produced a very
complicated mixture (Scheme 3). ESI-MS analysis of the
crude product suggested the formation of bis (NHT) com-
pound 3c as well as a number of demethylated products
1c, 4c and 5c (Figure 3). Similar dealkylation process has
been reported to occur during the course of the palladation
In the 13C NMR spectrum of 3a, two downfield sin-
glets at 157.2 ppm and 157.0 ppm were found, which can
be assigned to the sulfur-attached Ccarbene atoms. Only
one carbene signal was observed at 157.0 ppm in the case
of 3b. These signals are in agreement with that observed
for monodentate mesoionic NHT i (Figure 4),[14] upfield
than the related ‘normal’ thiones reported by Kühn
et al,[18] and slightly downfield compared to those for
SCHEME 2 Synthesis of mesoionic carbene-sulfur
adducts 3a and 3b
SCHEME 3 Attempt to synthesize
3c

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FIGURE 3 ESI-MS spectrum of the crude product for the attempt to synthesize 3c
FIGURE 4 Comparison of 13Ccarbene NMR signals of mesoionic NHT i with other representative structurally relevant molecules. The
data is from reference 14 (for i), reference 17 (for ii/iii/iv), reference 12a (for v). CDCl3 was used as the solvent except iii (using d6-DMSO)
mesoionic oxides (v, Figure 4) previously documented by
Albrecht's group.[12a]
The mesoionic NHTs studied herein are highly
related to Bertrand's carbene-phosphinidene system and
abilities of carbenes? Previously, Frenking and Dutton
et al. have used DFT calculations to reveal a trend of
carbene π-accepting abilities, iii > iv > ii > i.[20] The ben-
zannulated carbene in iii is expected to be the strongest
π-acceptor, while the mesoionic carbene in i is expected
to be the least π-accepting. However, this carbene
π-accepting trend is only partially correlated to the
carbene signals of i-iv. For example, mesoionic NHT i,
bearing the least π-acidic carbene, indeed gives the most
upfield carbene resonance. However, compound iv,
although possessing the second strongest π-accepting
carbene in i-iv, gives the most downfield carbene signal.
These observations implicate the difference of spectro-
scopic features of NHT system versus Bertrand's carbene-
phosphinidene system.
Ganter's carbene-selenium system (Figure 5). In Ber-
13
trand's case,[9a, 19a] they have found that the
C
carbene
NMR and 31P NMR signals are both correlated to the
π-accepting capacities of carbenes: C (XY). Generally, a
more downfield shift was observed when a more
π-accepting carbene was used in their system. In contrast,
[9a, 19b, c]
in Ganter's case,
the π-accepting abilities can
only be correlated to the 77Se NMR resonances rather
13
than the
C
NMR signals.
carbene
The carbene signals of mesoionic NHT i was com-
pared with a few representative non-mesoionic NHTs (ii-
iv),[17] which is shown in Figure 4. The question that
Several attempts to grow single crystals of 3a/3b were
to no avail, and thus DFT calculations were carried out
to provide further insight into their structural features.
arises here, is: Is there a correlation between the
13
C
carbene
signals of NHTs i-iv and the π-accepting
FIGURE 5 Bertrand and
Ganter's systems to determine
the π-accepting capacities of
carbenes

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TABLE 1 Comparison of C-S bond lengths of 3a/3b versus
Btb
The optimized molecular structures are depicted in
Figure 6, and the calculated C-S bond lengths are listed
in Table 1.
C-S
For both cases of 3a and 3b, the C-S moieties adopted
an anti-configuration relative to the central phenylene
group. In contrast, Hollis and Jia et al. have reported
a series of bis (imidazolin-2-thione) analogues Btb
bond
length
(Å)
Compd.
1.6888b
1.6912b
n
(Table 1),[21] and a syn orientation was reported for Bu-
Btb.[21a] The calculated C-S bond distances up to
1.6975 Å is close to the typical C-S partial double bond
distance (1.72 Å),[22] and thus the thiocarbonyl moieties
of 3a/3b are better described as partial double bonds. In
addition, the C-S bond lengths of 3a/3b are slightly lon-
n
ger than those for Bu-Btb indicating a less pronounced
C-S double bond character in the former case.
Finally, it should be noted that for Albrecht's
mesoionic oxides v (Figure 4), the C-O bond length of
1.243 Å clearly indicates that the carbonyl moiety pos-
sesses a highly characteristic C-O double bond feature.
The diminished carbon-chalcogen bond order of 3a/3b
(vs. v) can be rationalized by the poor overlap of C
(2p) and S(3p) orbitals.[8] This also accounts for the
higher reactivity of thioketones compared to ketones in
general.
1.6975b
1.6943b
2.4 | Cyclopalladations to access [SCS] and
[SCS']-type complexes
R = nBu
1.6758c
1.6842c
—
Compounds 3a and 3b were then employed as ligand pre-
cursors to access cyclopalladated complexes bearing non-
symmetrical [SCS'] or symmetrical [SCS] pincer ligands,
respectively (Scheme 4).
R = Et
R = Allyl
—
ꢀ
Heating 3a/3b with PdCl2 at 90 C in the presence of
NaOAc gave the desired complexes 6a and 6b, and the
products can be isolated as yellow solid. Compared to 3a/
3b, complexes 6a/6b show much decreased solubility in
common organic solvents such as CH2Cl2.
aBtb = 1,3-bis(30-butylimidazolyl-20-thione)benzene.
bDFT calculated.
cThe data is from reference 21a.
In the ESI mass spectra of 6a and 6b, two base peaks
at m/z 555 and 589 were observed respectively. These
peaks can be assigned to the mono-cationic fragments
[M – Cl]+, which indicates the loss of Cl ligands and the
retainment of metal pincer frameworks in the ESI-MS
compared to those for the proligands 3a/3b, owing to
the greater shielding of the anionic aryl donors in the
former cases.
Previously, Hollis, Jia et al. have reported the
cyclopalladations of Btb (Scheme 5), in which cases
1
condition. In the H NMR spectra of 6a/6b, the signals
of pincer central C-donors all show an upfield shift
FIGURE 6 Optimized molecular
structures of 3a/3b based on DFT
calculations. Atoms with yellow color
(S), atoms with blue color (N), atoms
with gray color (C)

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NaOAc was not utilized.[21] As such, 3a was used as a
representative, and under Hollis's condition, the metal-
ation reactivity in the absence of NaOAc was examined.
1H NMR spectrum of the as-formed product shows a very
downfield singlet at 12.17 ppm, which is tentatively
assigned to the Caryl-Ha proton of non-cyclopalladated
complex 7a. The formation of 7a can be further verified
by ESI mass spectrometric analysis revealing a dominant
peak at m/z 591 corresponding to [7a – Cl]+. Further
treating 7a with NaOAc can afford the desired pincer
complex 6a. These observations indicate that in the cur-
rent case OAc− plays an essential role in promoting the
Caryl-H activation and cyclopalladation.
SCHEME 4 Synthesis of Pd bis (NHT) pincer complexes 6a
and 6b
SCHE ME 5 Cyclopalladation reactivity
comparison of previously reported non-
mesoionic bis (NHT) and mesoionic bis (NHT)
3a
TABLE 2 Catalytic condition screening of cycloisomerization of 4-pentynoic acid 8a
Entry
Cat.
−
Cat. Loading
Et3N (mol%)
t (hr)
12
Yield(%)b
1
2
3
4
5
6
−
−
−
−
−
2
3
38
6a
6a
6b
6a
6b
0.1%
0.1%
0.1%
0.1%
0.1%
12
2
6
12
37
12
>99
57
2
12
aReaction condition: 0.2 mmol of 4-pentynoic acid, 0.5 ml of CDCl3, 0.1 mol% of [Pd] (for entries 2–6), 2 mol% of Et3N (for entries 5–6),
50 ꢀC.
bYields were determined using 1H NMR spectroscopy.

YAN ET AL.
7 of 11
2.5 | Catalytic study
In the model reaction, 4-pentynoic acid 8 was used as
the substrate. Without the addition of any Pd catalyst, a
very low yield was obtained even after a reaction time of
12 hr (Entry 1). The yield of the enol lactone product
9 can be increased to 38% in the presence of only 0.1 mol
% of catalyst 6a (Entry 2). Shortening the reaction time to
2 hr gave a sharp decrease of yield (Entry 2 vs. Entry 3).
Essentially the same yield was obtained using 6b as the
catalyst (Entry 4). These observations suggest that an
external base was not necessary to drive this transforma-
tion. However, this reaction can be accelerated using
Et3N, and a nearly full conversion of 8 to 9 can be
achieved under otherwise identical conditions. In con-
trast, when symmetrical complex 6b was employed as
the catalyst, only a moderate yield was obtained clearly
Intramolecular cycloisomerization of alkynoic acids is an
important reaction of high atom economy to construct
enol lactones. Such transformations have been known to
be mediated by a few transition metals including Pd.[23]
Particularly for structurally related [SCS] pincer systems,
Bourissou et al. have documented a few symmetrical
phosphine sulfide-based Pd pincer complexes of the type
II (vide supra, Figure 1), and reported their catalytic
cycloisomerization of alkynoic acids at a 5 mol% catalyst
loading.[24] In a preliminary study and as a comparison,
the newly synthesized pincer complexes 6a/6b were
tested for their catalytic performances in the same reac-
tion (Table 2).
TABLE 3 Expanding the substrate scope of intramolecular cycloisomerization of alkynoic acids catalyzed by 6aa
Entry
Substrate
Product
T (ꢀC)
t (hr)
Yield(%)b
1
50
12
>99%
2
3
50
50
12
12
>99%
47
4
5
50
90
12
12
<1
<1
aReaction condition: 0.2 mmol of alkynoic acid, 0.5 ml of CDCl3, 0.1 mol% of [Pd], 2 mol% of Et3N.
bYields were determined using 1H NMR spectroscopy.

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indicating the superiority of non-symmetrical pincer
catalyst 6a.
Elemental analyses were done on a vario EL cube ele-
mental analyzer.
Under the optimized condition, non-symmetrical
complex 6a served as the catalyst, and with a catalyst
loading of 0.1 mol%, the cycloisomerization of a small
selection of alkynoic acids were then tested (Table 3).
2-Ethynylbenzoic acid can undergo a complete conver-
sion yielding the benzannulated enol lactone (Table 3,
Entry 2). A moderate yield was obtained when a sub-
strate with a long aliphatic tail was used (Entry 3). The
cycloisomerization to construct a 6-membered enol lac-
tone proved to be very sluggish even at an elevated tem-
perature of 90 ꢀC (Entries 4 and 5).
4.2 | Synthesis of a
Benzyl bromide (595 μL, 5 mmol), NaI (37.5 mg,
0.25 mmol) and DMF (5 ml) were mixed in a 25 ml round
bottom flask. The reaction mixture was stirred at the ambi-
ent temperature for 20 min. Sodium azide (358 mg,
5.5 mmol) was added, and then the reaction mixture
was stirred at the ambient temperature for another 2 hr.
Diethyl ether and deionized H2O were added, and the
organic phase was dried in vacuo affording benzyl azide as
a yellow liquid. Then, 3-ethynylaniline (570 μL, 4.5 mmol),
CH3CH2OH (15 ml), H2O (5 ml) and the catalyst [CuCl
(IPr)] (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-
2-ylidene, 25 mg, 0.05 mmol) were added. The mixture
was stirred at 60 ꢀC for 24 hr. The resulting precipitate was
collected and dried in vacuo affording the mono-triazole
product a as a dark solid, which was directly used for the
next step without further purification.
3 | CONCLUSION
In summary, the multi-step synthesis and characteriza-
tions of symmetrical or non-symmetrical bis (mesoionic
NHT) compounds 3a-3b as well as their complexes 6a/6b
has been described. In the case of 3c, N-dealkylations
were observed during the course of thionation. In con-
trast to Bertrand's carbene-phosphinidene system, the
carbene signals of NHT system are not fully correlated to
the π–accepting abilities of carbenes. DFT-assisted struc-
tural analysis of 3a/3b indicates a diminished C-S double
bond order. The comparative catalytic study of 6a/6b in
cycloisomerizations of alkynoic acids revealed the superi-
ority of non-symmetrical 6a over symmetrical 6b in such
transformations.
To the best of our knowledge, this report provides the
first examples of mesoionic NHT-based pincer complexes,
and also enriches the less studied non-symmetrical pincer
chemistry. The synthetic, spectroscopic and catalytic
study explored in the current work will facilitate the
revisit of “old” mesoionic NHTs. Currently, our lab is
engaged in further expanding the catalytic applications of
the as-synthesized non-symmetrical pincer complexes.
4.3 | Synthesis of 1a
Compound a (ca. 5 mmol) was mixed with HCl (3.2 ml,
ꢀ
6 M in H2O) at 0 C in a flask charged with a magnetic
stirring bar, and a solution of NaNO2 (350 mg, 5.13 mmol)
in deionized H2O (1.8 ml) was added. After stirring for
10 min, a solution of NaN3 (330 mg, 5.1 mmol) in
deionized H2O (1.8 ml) was added dropwise. The reaction
mixture was stirred at the ambient temperature for 2 hr.
Diethyl ether (10 mL × 3) was added, the organic phase
was collected and dried in vacuo affording the azide inter-
mediate as an off-white solid. Then 1-hexyne (555 μL,
5 mmol), CH3CH2OH (15 ml), H2O (5 mL) and the cata-
lyst [CuCl (IPr)] (25 mg,ꢀ 0.05 mmol) were added. The
mixture was stirred at 60 C for 24 hr. The resulting pre-
cipitate was collected and dried in vacuo affording 1a as a
dark solid (4.1 mmol, 1.03 g, 82%).
4 | EXPERIMENTAL SECTION
4.1 | General considerations
1H NMR (400 MHz, d6-DMSO): δ 8.79 (s, 1H, Ar − H),
8.64 (s, 1H, Ar − H), 8.35 (s, 1H, Ar − H), 7.95 (d, 1H,
3
Ar − H, JH,H = 7.6 Hz), 7.84 (dd, 1H, Ar − H,
3
3JH,H = 8 Hz, JH,H = 1.2 Hz), 7.64 (t, 1H, Ar − H,
If not mentioned otherwise, all reagents and solvents
3JH,H = 8 Hz), 7.42–7.33 (m, 5H, Ar − H), 5.69 (s, 2H,
1
3
were used as received without further purification. H,
CH2), 2.71 (t, 2H, CH2CH2CH2CH3, JH,H = 7.8 Hz),
13C and 31P NMR spectra were recorded on a Varian
600 MHz spectrometer or Bruker 400 MHz spectrometer.
The chemical shifts (δ) were internally referenced to the
residual solvent signals. ESI mass spectra were measured
using an Agilent 6,540 Q-TOF mass spectrometer.
1.70–1.62 (m, 2H, CH2CH2CH2CH3), 1.42–1.33 (m, 2H,
CH2CH2CH2CH3), 0.92 (t, 3H, CH2CH2CH2CH3,
3JH,H = 7.2 Hz). 13C NMR (100 MHz, d6-DMSO): δ 148.2,
145.7, 137.4, 135.9, 132.3, 130.5, 128.8, 128.2, 128.0, 124.7,
122.4, 120.1, 119.0, 116.2 (Ar − C), 53.2 (N − CH2),

YAN ET AL.
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30.9 (CH2CH2CH2CH3), 24.7 (CH2CH2CH2CH3), 21.7
(CH2CH2CH2CH3), 13.7 (CH2CH2CH2CH3).
CH2CH2CH2CH3), 1.42–1.38 (m, 2H, CH2CH2CH2CH3),
3
0.91 (t, 3H, CH2CH2CH2CH3, JH,H = 7.8 Hz). 13C NMR
(150 MHz, CDCl3): δ 157.2, 157.0 (C − S), 139.7, 136.4,
135.6, 134.5, 131.1, 129.7, 129.2, 128.9, 128.6, 126.5, 125.9,
125.2 (Ar − C), 51.4 (N − CH2), 39.5, 37.3 (N − CH3),
29.0 (CH2CH2CH2CH3), 24.3 (CH2CH2CH2CH3), 22.7
(CH2CH2CH2CH3), 13.9 (CH2CH2CH2CH3). MS (ESI)
m/z: 451 [M + H]+.
4.4 | Synthesis of 2a
In a Schlenk tube, a mixture of compound 1a (359 mg,
1 mmol), trimethyloxonium tetrafluoroborate (592 mg,
4 mmol) and 1,2-dichloroethane (1 mL) were stirred at
90 ꢀC for 24 hr. After the mixture was cooled to the ambi-
ent temperature, CH3OH (4 mL) was added and the
resulting mixture was stirred at the ambient temperature
for 30 min in order to remove the excessive oxonium salt.
All the volatiles were removed in vacuo. The residue was
washed with diethyl ether (3 × 10 mL), then dried under
vacuum to give the product as an off-white solid
(0.56 mmol, 350 mg, 56%).
1
3b. Yield: 72%. H NMR (600 MHz, CDCl3): δ 8.16 (t,
3
1H, Ar − H, JH,H = 1.8 Hz), 7.81–7.80 (m, 2H, Ar − H),
7.64–7.62 (m, 1H, Ar − H), 7.57–7.55 (m, 4H, Ar − H),
7.37–7.32 (m, 6H, Ar − H), 5.73 (s, 4H, PhCH2), 4.14 (s,
6H, NCH3). 13C NMR (150 MHz, CDCl3): δ 157.0 (C − S),
136.0, 134.4, 131.4, 130.4, 129.3, 129.2, 128.9, 128.6, 126.7
(Ar − C), 51.5 (N − CH2), 39.2 (N − CH3). MS (ESI) m/z
485 [M + H]+.
1H NMR (600 MHz, d6-DMSO): δ 9.45 (s, 1H,
Htriazole), 9.33 (s, 1H, Htriazole), 8.35 (s, 1H, Ar − H), 8.29
(d, 1H, Ar − H, 3JH,H = 7.8 Hz), 8.06–8.01 (m, 2H, Ar − H),
4.6 | General procedure for the synthesis
of 6a and 6b
3
7.57 (d, 2H, Ar − H, JH,H = 7.2 Hz), 7.50–7.46 (m, 3H,
Ar − H), 5.99 (s, 2H, CH2), 4.35 (s, 3H, N − CH3),
4.32 (s, 3H, N − CH3), 2.95 (t, 2H, CH2CH2CH2CH3,
3JH,H = 6 Hz), 1.78–1.73 (m, 2H, CH2CH2CH2CH3),
1.51–1.45 (m, 2H, CH2CH2CH2CH3), 0.98 (t, 3H,
In a 25 ml Schlenk tube, compounds 3a or 3b (0.5 mmol),
PdCl2 (0.5 mmol), NaOAc (1.5 mmol) and 2 ml of CH3CN
were added. The reaction mixture was stirred and heated
ꢀ
at 90 C for 24 hr. After cooling down to the ambient
3
CH2CH2CH2CH3, JH,H = 7.8 Hz).13C NMR (150 MHz,
temperature, all the volatiles were removed in vacuo.
CH2Cl2 (5 ml) was added to extract the crude product.
All the insolubles were filtered off, and the organic phase
was dried and further purified using column chromatog-
raphy (SiO2, CH2Cl2/CH3OH (V:V) = 10: 1) affording the
product as a yellow solid.
d6-DMSO): δ 145.4, 140.9, 135.3, 132.8, 132.5, 131.5,
129.7, 129.4, 129.1, 129.0, 126.8, 124.6, 124.2, 122.5
(Ar − C), 56.4 (N − CH2), 39.0, 37.9 (N − CH3), 28.1
(CH2CH2CH2CH3),
(CH2CH2CH2CH3), 13.6 (CH2CH2CH2CH3). MS (ESI):
m/z 194 [M – 2BF4]2+
22.3
(CH2CH2CH2CH3),
21.5
1
.
6a: Yield 26%. H NMR (600 MHz, d6-DMSO): δ 7.81
(br s, 1H, Ar − H), 7.52–7.51 (m, 3H, Ar − H), 7.43–7.35
(m, 4H, Ar − H), 5.70 (s, 2H, CH2), 4.33 (s, 3H, NCH3),
4.24 (s, 3H, NCH3), 2.83 (t, 2H, CH2CH2CH2CH3,
3JH,H = 6.6 Hz), 1.66–1.61 (m, 2H, CH2CH2CH2CH3),
1.41–1.37 (m, 2H, CH2CH2CH2CH3), 0.93 (t, 3H,
CH2CH2CH2CH3, 3JH,H = 7.2 Hz). Due to the low solubil-
ity of this compound in common organic solvents, a satis-
fying 13C NMR spectrum cannot be obtained even after
long-time accumulation of signals. MS (ESI) m/z:
555 [M − Cl]+. Anal. Calcd. for C23H25ClN6PdS2: C,
46.70; H, 4.26; N, 14.21; S, 10.84%; Found: C, 46.35; H,
4.46; N, 14.58; S, 11.21%.
4.5 | General procedure for the synthesis
of 3a and 3b
A 25 ml Schlenk tube was charged with compound 2a/2b
(0.1 mmol), K2CO3 (28 mg, 0.2 mmol), S (6.4 mg,
0.2 mmol) and CH3CN (2 ml). The tube was sealed and
ꢀ
the reaction mixture was stirred and heated at 90 C for
20 hr. All the volatiles were removed in vacuo. To the res-
idue, CH2Cl2 (5 ml) was added, then the crude product
was purified using column chromatography (SiO2,
CH2Cl2/CH3OH (V:V) = 40: 1) affording the product as
an yellow solid.
6b: Yield 31%. 1H NMR (600 MHz, CDCl3): δ
7.55–7.54 (m, 4H, Ar − H), 7.33–7.30 (m, 8H, Ar − H),
7.14 (t, 1H, Ar − H, 3JH,H = 7.2 Hz), 5.62 (s, 4H, PhCH2),
4.18 (s, 6H, NCH3). Due to the low solubility of this com-
pound in common organic solvents, a satisfying 13C NMR
spectrum cannot be obtained even after long-time accu-
mulation of signals. MS (ESI) m/z: 589 [M − Cl]+. Anal.
Calcd. for C26H23ClN6PdS2: C, 49.93; H, 3.71; N, 13.44; S,
10.25%; Found: C, 49.58; H, 3.89; N, 13.16; S, 10.56%.
1
3a. Yield: 76%. H NMR (600 MHz, CDCl3): δ 8.49 (s,
3
1H, Ar − H), 8.16 (d, 1H, Ar − H, JH,H = 7.8 Hz), 7.80
3
(d, 1H, Ar − H, JH,H = 8.4 Hz), 7.61 (t, 1H, Ar − H,
3
3JH,H = 7.8 Hz), 7.52 (d, 2H, Ar − H, JH,H = 7.8 Hz),
7.32–7.27 (m, 3H, Ar − H), 5.69 (s, 2H, N − CH2), 4.16 (s,
3H, N − CH3), 3.99 (s, 3H, N − CH3), 2.80 (t, 2H,
3
CH2CH2CH2CH3, JH,H = 7.8 Hz), 1.66–1.61 (m, 2H,

10 of 11
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1
jected to H NMR spectroscopic analysis and compared
1
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4.8 | DFT calculation
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ACKNOWLEDGMENTS
The authors thank Natural Science Foundation of Beijing
Municipality (Grant No. 2192012), “General Project of
Scientific Research Program of Beijing Education Com-
mission” (Grant No. KM201810028007) and Support
Project of High-level Teachers in Beijing Municipal
Universities in the Period of 13th Five-year Plan (Grant
No. CIT&TCD201904087) for financial support.
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CONFLICT OF INTEREST
There are no conflicts to declare.
ORCID
Shuai Guo https://orcid.org/0000-0001-7729-0980
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How to cite this article: Yan X, Zhang B,
Zhang X, Wang H, Duan Y-A, Guo S. Symmetrical
and Non-symmetrical Pd (II) Pincer Complexes
Bearing Mesoionic N-heterocyclic Thiones:
Synthesis, Characterizations and Catalytic
Properties. Appl Organomet Chem. 2020;e5885.
https://doi.org/10.1002/aoc.5885