Mono- and dinuclear palladium(II) complexes incorporating 1,2,3-triazole-derived mesoionic carbenes: syntheses, solid-state structures and catalytic applications

Article abstract of DOI:10.1007/s11243-018-0267-8

Two 1,2,3-triazole-derived monocationic salts 3 and 4 bearing N-aryl wingtips were prepared using copper-catalyzed “click” reactions followed by alkylations with iodomethane. Employing a silver–carbene transfer method, two dinuclear palladium(II) complexes of triazolin-5-ylidenes (5/6) were obtained, the former of which has been reported previously. Bridge-cleavage reaction of 5 as a representative with PPh₃ yielded cis-configured mesoionic carbene/phosphine hybrid complex cis-7 and homoleptic bis(phosphine) complex 8, suggesting the presence of a ligand exchange process. In contrast, bridge breakages of 5/6 with pyridine cleanly afforded PEPPSI-type complexes 9 and 10 in near quantitative yields. Finally, all complexes were exploited to catalyze Mizoroki–Heck coupling reactions with aryl bromides as the substrates, and PEPPSI-type complex 9 was found to be the best performer generally giving good to excellent yields.

Full text of DOI:10.1007/s11243-018-0267-8

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0267-8
Mono‑ and dinuclear palladium(II) complexes incorporating
1,2,3‑triazole‑derived mesoionic carbenes: syntheses, solid‑state
structures and catalytic applications
Xin Zhang1 · Xuechao Yan1 · Bo Zhang1 · Ran Wang1 · Shuai Guo1 · Shiyong Peng2
Received: 6 June 2018 / Accepted: 21 July 2018
© Springer Nature Switzerland AG 2018
Abstract
Two 1,2,3-triazole-derived monocationic salts 3 and 4 bearing N-aryl wingtips were prepared using copper-catalyzed “click”
reactions followed by alkylations with iodomethane. Employing a silver–carbene transfer method, two dinuclear palladium(II)
complexes of triazolin-5-ylidenes (5/6) were obtained, the former of which has been reported previously. Bridge-cleavage
reaction of 5 as a representative with PPh3 yielded cis-confgured mesoionic carbene/phosphine hybrid complex cis-7 and
homoleptic bis(phosphine) complex 8, suggesting the presence of a ligand exchange process. In contrast, bridge breakages of
5/6 with pyridine cleanly aforded PEPPSI-type complexes 9 and 10 in near quantitative yields. Finally, all complexes were
exploited to catalyze Mizoroki–Heck coupling reactions with aryl bromides as the substrates, and PEPPSI-type complex 9
was found to be the best performer generally giving good to excellent yields.
Introduction
exhibited remarkable catalytic performances in various
organic transformations.3
Palladium chemistry of NHCs (NHCs = N-heterocyclic
carbenes) has gained rapidly growing interest in the past
decade, which has found manifold applications, especially
in transition-metal-mediated catalysis.1 Up to date, a large
number of well-defined palladium(II) NHCs have been
reported, and most of them bear imidazolin-2-ylidenes or
benzimidazolin-2- ylidenes as the supporting ligands (A,
Fig. 1). In this regard, trans- or cis-oriented heteroleptic
complexes of the type I (Fig. 1) are quite promising motifs,2
since they combine the structural and stereoelectronic fea-
tures of NHCs and a second class of ancillary ligands L.
For example, PEPPSI (PEPPSI = Pyridine-Enhanced Pre-
catalyst Preparation Stabilization and Initiation) complexes,
generally bearing NHCs and trans-standing pyridines, have
In contrast to classical Arduengo-type4 NHCs of the type
A, MICs (MICs=mesoionic carbenes, B–E, Fig. 1), which
require additional charges in all their canonical resonance
structures and are thus also named as abnormal NHCs, have
been much less investigated.5 Generally, MICs are stronger
donors compared to normal NHCs according to the Tol-
man’s6 and Huynh’s [15]7 electronic parameters. In this
context, 1,2,3-triazolin-5-ylidenes (E) have gained increas-
ing attention in recent years due to the intriguing reactivities
and remarkable catalytic behaviors of their transition-metal
complexes.8 In addition, 1,2,3-triazoles, which typically
serve as the ligand precursors of E, can be regioselectively
and facilely constructed through copper-catalyzed “click”
1
For selected reviews, see [1–10].
2
For selected examples of the type I with imdazolin-2-ylidenes as
the supporting ligands, see [11–13]. For selected examples of the type
I with benzimdazolin-2-ylidenes as the supporting ligands, see [14,
15].
Xin Zhang and Xuechao Yan are contributed equally to this work.
3
For selected reviews on PEPPSI-type complexes, see [16, 17].
See [18].
For selected reviews on mesoionic carbenes, see [19–24].
See [25–27].
* Shuai Guo
4
guoshuai@cnu.edu.cn
5
1
6
Department of Chemistry, Capital Normal University,
7
Beijing 100048, People’s Republic of China
See [28].
2
8
School of Chemical and Environmental Engineering,
For the paper providing the frst example of 1,2,3-triazolin-
Wuyi University, Jiangmen 529020, Guangdong Province,
People’s Republic of China
5-ylidene complex, see [29]. For selected reviews on 1,2,3-triazolin-
5-ylidenes and their complexes, see [30, 31].
Vol.:(0123456789)
1 3

Transition Metal Chemistry
Fig. 1 General structure of
NHCs (A), hybrid complexes
of the type I and some hitherto
known examples of mesoionic
carbenes (B–E)
Scheme 1 Syntheses of triazole-based ligand precursors 3 and 4
reactions.9 Various substituents R can be incorporated ren-
dering ligands of the type E electronically and sterically
easily tunable.
comparative catalytic studies in Mizoroki–Heck coupling
reactions were also carried out employing all newly synthe-
sized palladium MICs as precatalysts.
Notably, previous studies predominantly focused on
hybrid complexes I bearing normal NHCs (vide supra)
[11–15]. In recent years, this type of motif has been fur-
ther extended to 1,2,3-triazolin-5-ylidenes (E) by Albrecht
and others,10 and the structural diversities and reactivity
studies of the corresponding complexes remain largely
unexplored. As our contribution to this area and in line
with our continuous interest in mesoionic carbene chem-
istry, herein we report the syntheses and reactivities of a
series of new palladium(II) triazolin-5-ylidene complexes
with phosphines/pyridines as ancillary ligands. In addition,
Results and discussion
Carbene precursors
Monocationic triazole-derived salts 3 and 4, which can serve
as ligand precursors in the present work, were synthesized
via a two-step procedure (Scheme 1).
In the first step, azidations of aromatic amines
a/b followed by CuAACs (CuAACs = copper-cat-
alyzed azide-alkyne cycloadditions) afforded the
desired triazoles 1 and 2 in high yields of ≥ 90%. For
this reaction, a copper(I)–NHC complex [CuCl(IPr)]
9
For selected reviews on copper-catalyzed click reactions, see [32–
34].
10
For selected examples of hybrid complexes bearing MICs in anal-
ogy to I, see [35–39].
1 3

Transition Metal Chemistry
Scheme 2 Syntheses of dimeric
complexes 5 and 6
Fig. 2 Isotopic pattern of the [6 – I]+ species in the positive ESI mass spectrometry (left) and simulated pattern (right)
(IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-
2-ylidene)11 was utilized instead of a copper(II) salts/reduc-
ing agent system, the latter of which was typically used for
such “click” reactions [32–34]. Subsequent alkylations with
iodomethane successfully produced triazolium salts 3 and 4
in good and moderate yields, respectively. Compounds 1 and
3 have been previously synthesized,12 and acetyl-function-
alized triazole 2 and its derivative 4 are reported for the frst
Dinuclear complexes of palladium(II)
By employing an Ag–carbene transfer protocol, two dinu-
clear palladium complexes 5 and 6 were successfully pre-
pared (Scheme 2). It should be noted that the former has
been reported by Albrecht et al. [43]. The newly synthesized
complex 6 was fully characterized by 1H, 13C NMR spectros-
copy, ESI mass spectrometry and elemental analysis.
Positive-mode ESI mass spectrometric analysis of com-
plex 6 provided the frst evidence for its formation. In the
mass spectrum, a base peak at m/z 1109 assigned to the
monocationic fragment [6 – I]+ was observed, which implies
that the dinuclear structure can be retained in the gas phase
surviving from bridge cleavages. The observed and simu-
lated isotopic patterns are shown in Fig. 2.
1
time. In the H NMR spectrum of compound 4 in CDCl3,
the C–H proton of the triazole ring gives a characteristic
signal at 9.67 ppm, which shows a signifcant downfeld shift
compared to that (cf. 7.97 ppm) for precursor 2. A singlet at
2.66 ppm is observed which can be assigned to the methyl
group of the acetyl moiety. The ESI mass spectrum of 4
shows the expected peak at m/z 258 corresponding to the
monocationic species [4 – I]+.
In the 1H NMR spectrum of 6, the signal characteristic for
the C–H proton of the triazole ring observed for precursor
4 is absent, which is indicative of successful palladation.
The most downfeld singlet13 at 9.03 ppm can be assigned
11
13
Complex [CuCl(IPr)] has been previously employed to cata-
The aryl C–Ha proton expectedly give a doublet with a small cou-
lyze azide-alkyne cycloaddition reactions. For selected papers and
pling constant of~2–3 Hz, due to (i) the absence of protons attached
to the Cortho atoms and (ii) the splitting by C–H protons of the Cmeta
atoms. In our case, a singlet was observed probably due to the low
resolution of NMR spectrometer.
reviews, see [40–42].
12
For earlier reports on the syntheses and characterizations of tria-
zole 1 and salt 3, see [43–45].
1 3

Transition Metal Chemistry
Scheme 3 Attempt to synthesize a MIC/phosphine hybrid complex of palladium(II)
to the aryl C–Ha proton (Scheme 2). Interestingly, this reso-
nance shows a signifcant downfeld shift (ΔδΗ =0.53 ppm)
compared to that (cf. 8.50 ppm) for precursor 4. This spec-
troscopic feature may indicate a C–Ha···Pd anagostic interac-
tion, which is known to be evidenced by a downfeld shift of
C–H protons upon ligand coordination.14 A similar interac-
tion was also observed in the pyridine-ligated derivative of
complex 6, which was additionally characterized by X-ray
difraction analysis (vide infra). In the 13C NMR spectrum
of 6, the carbonyl carbon atom of 6 gives a characteristic sin-
glet at 196.8 ppm, and the 13Ccarbene signal was not detected.
transphobia efect16 induced by the electron-releasing car-
bene ligand. Similar orientations have been also observed
for some analogue complexes of the type [PdX2(tazy)(PR3)]
(X = halido ligand, tazy = triazolin-5-ylidene) [38].17 The
Pd1–I2 bond distance amounting to 2.6488(4) Å is found
to be shorter than that (2.6699(4) Å) of the Pd1–I1 bond,
although the former is excepted to be longer due to a
stronger structural trans efect of carbene versus phosphine
ligands [25–28]. The triazole ring is almost perpendicular
to the [PdI2CP] coordination plane with a dihedral angle of
ca. 79°. The Pd–Ccarbene bond length of 2.000(4) Å is found
to be comparable to that observed in the parent complex 5
[35]. In addition, as shown in the fgure (the lower one), two
molecules of complex cis-7 are found to aggregate through
a π–π interaction of N1-phenyl groups. The two participat-
ing phenyl rings are found to be parallel as clearly indicated
by a dihedral angle of 0°. The centroid–centroid separation
measured as 3.619 Å falls in the range typically observed for
those molecules featuring π–π stackings.18
Bridge‑cleavage reactions with phosphines
Halido-bridged dimeric complexes 5/6 can be employed
as potential precursors to yield triazolin-5-ylidene hybrid
complexes of the type I (vide supra) upon bridge cleavage
by incoming donors. Phosphines are ubiquitous ancillary
ligands in organometallic chemistry and palladium-mediated
catalysis. Hence, dinuclear complex 5 was chosen as a repre-
sentative and treated with two equivalents of PPh3 in aiming
to prepare a MIC/phosphine mixed complex as an initial
attempt (Scheme 3).
The formation of bis(phosphine) complex 8 is probably
due to the ligand redistribution of cis-7. Similar processes
have been observed for palladium(II) NHC/phosphine hybrid
complexes reported by Hermann et al.19 Several attempts to
isolate analytically pure compound cis-7 were to no avail.
Slow evaporation from a solution of the crude product in
a mixture of CH2Cl2 and n-hexane gives two sorts of X-ray
quality single crystals, which were both structurally charac-
terized, and proved to be a heteroleptic complex cis-7 and
homoleptic bis (phosphine) complex 8. The molecular struc-
ture of cis-7 and its intermolecular stacking are depicted in
Fig. 3. Selected important crystallographic data are sum-
marized in Table 1. The crystal data of 8 were previously
reported15 and thus not discussed herein.
Bridge‑cleavage reactions with pyridines
When pyridine was utilized as an electron donor to break
the iodido bridges, the reactions proceeded smoothly and
cleanly with formation of the PEPPSI-type complexes 9
and 10 in near quantitative yields (Scheme 4). Compounds
9/10 were isolated as red brownish powders, which are
fairly stable and readily soluble in common polar organic
solvents such as chlorinated solvents and DMSO. Finally,
As expected, the complex adopts an essentially square-
planar geometry. The triazole-derived MIC ligand is cis to
the phosphine donor, which is expected owing to the strong
16
The term “transphobia efect” was frst coined by Vicente and
Jones’s groups. For this paper, see [49].
17
14
For selected examples, see [50, 51].
For a paper on the defnition of π–π stacking, see [52].
See [53].
For a review on agostic and anagostic interactions, see [46].
For selected publications reporting the crystal data of trans-
18
15
19
[PdI2(PPh3)2], see [47, 48].
1 3

Transition Metal Chemistry
Fig. 3 Molecular structure
and stacked packing (sym-
metry operator: 2−x, 1−y,
1−z) of cis-7 showing 50%
probability ellipsoids. Hydro-
gen atoms, solvent molecules
and the disordered atoms are
omitted for clarity. Selected
bond (Å) and angles (deg):
Pd1–I1 2.6699(4), Pd1–I2
2.6488(4), Pd1–P1 2.2789(10),
Pd1–C26 2.000(4); I2–Pd1–
I1 92.633(14), P1–Pd1–I1
178.57(3), P1–Pd1–I2 88.73(3),
C26–Pd1–I1 86.80(11), C26–
Pd1–I2 176.96(11), C26–Pd1–
P1 91.82(11)
in contrast to phosphine/MIC mixed complex cis-7, no
ligand redistribution product was found for the PEPPSI-
type complexes.
The formations of 9 and 10 were further verified by
their positive ESI mass spectra. In the gas phase, losses of
pyridine as well as iodido ligands and further captures of
CH3CN molecules from the solvents were observed giving
rise to the monocationic species [M – Pyr – I+CH3CN]+,
as indicated by the dominant peaks at m/z 488 (for 9) and
531 (for 10).
1
The H NMR spectra of complexes 9/10 essentially
resemble those of 5/6 except for the presence of the signals
from pyridine moieties. The aryl C–Ha proton of 10 (vide
supra) gives a triplet at 9.21 ppm with an expected coupling
constant of ~ 2 Hz. Again, this resonance shows a marked
downfeld shift compared to that (cf. 8.50 ppm) observed
for the ligand precursor 4, which is also likely owing to a
C–Ha···Pd anagostic interaction. Additional evidences can
be provided by the bond parameters obtained by X-ray dif-
fraction analysis of complex 10 (vide infra). The 13Ccarbene
signals were not observed despite overnight accumulation
of the 13C NMR signals.
The single crystals of complex 10 were grown via slow
evaporation of a saturated solution in CH2Cl2. The molecu-
lar structure and its intermolecular stacking are depicted
in Fig. 4, and selected important crystallographic data are
summarized in Table 1. The palladium metal center is coor-
dinated by a triazolin-5-ylidene donor, a trans-standing
pyridine and two anionic iodido co-ligands. The Pd–C and
Pd–N bond lengths are comparable to those observed for
1 3

Transition Metal Chemistry
Table 1 Selected X-ray
crystallographic data for
compounds cis-7 and 10
Comp.
cis-7
10
Formula
Formula weight
Color, habit
Crystal size (mm)
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C33H36Cl4I2N3PPd
1007.62
Yellow, block
0.43×0.40×0.18
105.3
Triclinic
P-1
10.3150 (7)
10.9619 (7)
18.5705 (7)
86.879 (4)
84.429 (4)
62.426 (7)
1852.41 (18)
2
C20H24I2N4OPd
696.63
Orange, block
0.15×0.1×0.05
100.00 (10)
Triclinic
P-1
8.0922 (4)
10.0607 (7)
15.2554 (10)
73.695 (6)
86.100 (5)
82.258 (5)
1180.58 (13)
2
1.960
γ (°)
V (Å3)
Z
Dc (g·cm−3
)
1.807
Radiation used
Mo-Kα (λ=0.7107)
2.527
Cu-Kα (λ=1.54184)
27.005
µ (mm−1
)
θ range (°)
6.12‒52
7275
R1 =0.0337, wR2 =0.0692
R1 =0.0363, wR2 =0.0705
1.142
4.613‒73.343
8020
R1 =0.0445, wR2 =0.1153
R1 =0.0477, wR2 =0.1204
1.059
Refections collected
Final R indices (I>2σ(I))
R indices (all data)
goodness of ft on F2
Peak/hole (e·Å−3
)
1.742/−1.791
1.48/−2.38
Catalytic studies
All isolated mesoionic carbene complexes reported herein
were employed as catalyst precursors to catalyze Mizo-
roki–Heck couplings, and the results are summarized in
Table 2. In a model reaction, 4-bromobenzaldehyde and
tert-butyl acrylate were chosen as the substrates, and
1 mol % of mononuclear palladium complex (or 0.5 mol %
of dinuclear palladium complex) was employed as the
precatalyst. All complexes (5, 6, 9 and 10) proved to be
catalytically active in this cross-coupling reaction, giving
good to excellent yields. Complexes 6 and 10 are found
to be inferior performers compared to their counterparts 5
and 9 (entries 1/3 vs. entries 2/4), highlighting the infu-
ence of N-wingtips in the MIC supporting ligands. Thus,
the catalytic behaviors of complexes 5 and 9 were further
compared. When a less activated aryl halide (4′-bromoace-
tophenone) was used as the substrate, sharp decreases in
yields were observed (entries 5–6). The yields were sig-
nifcantly improved when TBAB (TBAB = tetra-n-butyl-
ammonium bromide) was utilized as an additive (entries
7–8). Finally, with complex 9 as the precatalyst, a decent
yield was obtained even when a deactivated aryl bromide
was employed (entry 9).
Scheme 4 Syntheses of PEPPSI-type complexes 9/10
earlier reported analogues [35–37, 39]. The C6–N2–C7–C8
torsion angle measured as ca. 21° indicates that the N–aryl
ring is far from perpendicular to the carbene backbone. This
is interesting since such twisting is expected to enhance the
steric repulsion between the N-aryl moiety and the [PdI2CN]
coordination plane. The C–H···Pd angle and Pd1···H8 dis-
tance amounting to ca. 129° and ca. 2.58 Å, respectively, fall
in the range of structural parameters characteristic for ana-
gostic interactions [46]. The molecules are found to dimer-
ize in the solid state through a π–π interaction [52] of two
essentially parallel pyridine rings, and the centroid–centroid
separation is measured as 3.651 Å.
1 3

Transition Metal Chemistry
carbenes. The reactivities, ligand redistributions, and cata-
lytic behaviors unraveled herein have obvious implications
for the syntheses and applications of triazolin-5-ylidene
complexes as well as related MIC motifs. Further expan-
sion of complex scope and detailed elucidations of ligand
efect on catalytic behaviors are currently under way in our
laboratory.
Experimental section
General considerations
All the manipulations were carried out without taking pre-
cautions to exclude air and moisture. All chemicals were
used as received without further purifcation if not men-
tioned otherwise. All reagents (AR) were purchased from
Alfa Aesar Chemical Co. Ltd., Adamas Reagent Co. Ltd. and
Accela ChemBio Co. Ltd. Solvents (AR) were purchased
1
from Beijing Chemical Works. H and 13C NMR spectra
were recorded on a Varian 600 MHz spectrometer, and the
chemical shifts (δ) were internally referenced to the resid-
ual solvent signals relative to (CH3)4Si (1H,13C). ESI mass
spectra were measured using an Agilent 6540 Q-TOF mass
spectrometer. X-ray single-crystal difraction studies were
done using a SuperNova, Dual, Cu at zero and AtlasS2 dif-
fractometer. Elemental analyses were obtained on a vario
EL analyzer of Elementar Analysensysteme GmbH at the
Analytical Instrumentation Center of Peking University.
Fig. 4 Molecular structure and stacked packing (symmetry opera-
tor: −x, 1−y, 2−z) of complex 10 showing 50% probability ellip-
soids. Hydrogen atoms are omitted for clarity. Selected bond (Å) and
angles (deg): Pd1–I1 2.6016(4), Pd1–I2 2.6007(4), Pd1–C6 1.969(5),
Pd1–N1 2.091(4); C6–Pd1–I1 88.35(14), C6–Pd1–I2 88.57(14),N1–
Pd1–I1 91.76(12), N1–Pd1–I2 91.28(12); C6–Pd1–N1 178.94(17),
I1–Pd1–I2 176.294(16)
Synthesis of compound 2
A 100-mL round-bottom fask was charged with a hydro-
chloric acid solution (3.2 mL, 6 M), and 3-aminoaceto-
phenone (676 mg, 5 mmol) was added. This mixture was
cooled to 0 °C with an ice bath, and a solution of NaNO2
(350 mg, 5.13 mmol) in H2O (1.8 mL) was added. The
reaction mixture was stirred for 10 min, and a solution of
NaN3 (330 mg, 5.1 mmol) in H2O (1.8 mL) was added drop-
wise. The ice bath was removed, and the reaction mixture
was stirred at ambient temperature for 2 h. Diethyl ether
(10 mL×3) was added, and the organic phase was dried in
vacuo afording the azide intermediate as a dark brown liq-
uid. To the fask charged with organic azide, 1-hexyne (555
μL, 5 mmol), complex [CuCl(IPr)] as the precatalyst (25 mg,
0.05 mmol), MeOH (2 mL) and H2O (1 mL) were succes-
sively added. The reaction mixture was stirred and heated at
60 °C for 24 h. After cooling down to ambient temperature,
the resulting precipitate was collected and dried in vacuo
afording the product as a dark brown liquid 2 (1.024 g,
Conclusion
In summary, we have reported a series of new heteroleptic
palladium(II) complexes bearing 1,2,3-triazole-based mes-
oionic carbenes and phosphine/pyridine ancillary ligands.
These complexes can be conveniently accessed via bridge-
cleavage reactions with dimeric triazolin-5-ylidene com-
plexes as precursors. In the case of phosphine/MIC mixed
complex cis-7, a ligand redistribution process occurred lead-
ing to the formation of homoleptic bis(phosphine) complex
8. For cis-7 and PEPPSI-type complex 10, the solid-state
structures were determined revealing π–π interactions
in both cases. Finally, in a preliminary catalytic study, a
remarkable infuence of N-wingtips of carbenes on catalytic
performance was observed.
1
3.2 mmol, 95%). H NMR (600 MHz, CDCl3): δ 7.97 (s,
This report provides additional examples of heterolep-
tic palladium complexes bearing less studied mesoionic
1H, Ar–H), 7.77 (s, 1H, Ar–H), 7.68–7.62 (m, 2H, Ar–H),
7.27–7.25 (m, Ar–H, 1H), 2.45 (br s, 2H, CH2CH2CH2CH3),
1 3

Transition Metal Chemistry
Table 2 Mizoroki–Heck reactions catalyzed by palladium complexes 5, 6, 9 and 10
Entry
Aryl Halide
Precatalyst
Additive
Yield (%)a
1
2
3
4
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4′-bromoacetophenone
4′-bromoacetophenone
4′-bromoacetophenone
4′-bromoacetophenone
4-bromoanisole
5
6
–
–
–
–
–
–
>99
72
>99
58
26
17
87
>99
78
9
10
5
5
6
9
7b
8b
9b
5
TBAB
TBAB
TBAB
9
9
Reaction conditions generally not optimized. Reaction conditions: 1.0 mmol aryl halide, 1.5 mmol of tert-butyl acrylate, 1 mol % of mononu-
clear palladium complex (or 0.5 mol % of dinuclear palladium complex), 1.5 mmol of NaOAc, DMF (3 mL), 24 h, 120 °C
aYields were determined by 1H NMR spectroscopy. bWith addition of 0.2 equiv. of TBAB
2.31 (s, 3H, COCH3), 1.38 (t, 2H, CH2CH2CH2CH3,
3JH,H =6 Hz), 1.09–1.08 (m, 2H, CH2CH2CH2CH3), 0.60 (t,
3H, CH2CH2CH2CH3, 3JH,H =6 Hz). 13C NMR (150 MHz,
CDCl3): δ 196.6 (CO), 149.3, 138.3, 137.5, 130.1, 130.0,
127.9, 124.2, 119.3 (Ar–C), 31.4 (CH2CH2CH2CH3), 26.6
(COCH3), 25.3 (CH2CH2CH2CH3), 22.2 (CH2CH2CH2CH3),
13.8 (CH2CH2CH2CH3). MS (ESI): m/z 244 [M+H]+.
Synthesis of complex 6
A round-bottom fask was charged with salt 4 (462 mg,
1.2 mmol), Ag2O (139 mg, 0.6 mmol) and CH2Cl2 (10 mL).
The reaction mixture was stirred in the dark at ambient tem-
perature for 24 h. A solution of [PdCl2(CH3CN)2] (212.6 mg,
1.2 mmol), which was prepared by heating PdCl2 (212 mg,
1.2 mmol) in CH3CN (5 mL) at 80 °C, was added. The
reaction mixture was stirred for 16 h, and a solution of NaI
(1079.2 mg, 7.2 mmol) in acetone was added. The resulting
suspension was stirred for another 4 h. All the volatiles were
removed in vacuo. The crude product was extracted with
CH2Cl2 (10 mL×3), and purifed using column chromatog-
raphy (SiO2, CH2Cl2) afording the product as a brown red
Synthesis of compound 4
A 25-mL Schlenk tube was charged with compound 2
(932.2 mmol, 3.8 mmol), CH3I (38 mmol, 2.4 mL) and
CH3CN (1 mL). The reaction mixture was stirred and
heated at 90 °C for 3 d. After cooling down to ambient
temperature, all the volatiles were removed in vacuo. The
residue was washed with diethyl ether (20 mL × 3) and
dried in vacuo afording the product as a yellow powder
1
powder (278 mg, 0.22 mmol, 37%). H NMR (600 MHz,
d6–DMSO): δ 9.03 (s, 2H, Ar–H), 8.38 (d, 2H, Ar–H,
3JH,H =7.8 Hz), 8.16 (d, 2H, Ar–H, 3JH,H =7.2 Hz), 7.83 (t,
1
3
(777.4 mg, 2 mmol, 53%). H NMR (600 MHz, CDCl3):
2H, Ar–H, JH,H = 7.8 Hz), 4.21 (s, 6H, N–CH3), 2.99 (t,
4H, CH2CH2CH2CH3, 3JH,H =7.8 Hz), 2.68 (s, 6H, COCH3),
δ 9.67 (s, 1H, Ar–H), 8.50 (s, 1H, Ar–H), 8.28 (d, 1H,
Ar–H, 3JH,H =8 Hz), 8.07 (d, 1H, Ar–H, 3JH,H =8 Hz), 7.66
3
1.94 (t, 4H, CH2CH2CH2CH3, JH,H = 7.5 Hz), 1.45–1.42
3
(t, 1H, Ar–H, JH,H = 8 Hz), 4.40 (s, 3H, N–CH3), 2.98 (t,
(m, 4H, CH2CH2CH2CH3), 0.97 (t, 6H, CH2CH2CH2CH3,
3JH,H =7.2 Hz). 13C NMR (150 MHz, d6–DMSO): δ 196.8
(CO), 144.5, 139.3, 137.1, 129.8, 129.5, 128.3, 123.8
(Ar–C), 37.2 (N–CH3), 29.0 (CH2CH2CH2CH3), 27.1
2H, CH2CH2CH2CH3, 3JH,H =8 Hz), 2.66 (s, 3H, COCH3),
3
1.83 (t, 2H, CH2CH2CH2CH3, JH,H = 7 Hz), 1.43–1.39
(m, 2H, CH2CH2CH2CH3), 0.88 (t, 3H, CH2CH2CH2CH3,
3JH,H =7 Hz). 13C NMR (150 MHz, CDCl3): δ 196.8 (CO),
146.7, 139.1, 135.5, 131.6, 131.4, 128.4, 126.4, 121.8
(Ar–C), 40.1 (N–CH3), 29.5 (CH2CH2CH2CH3), 28.2
(COCH3), 24.4 (CH2CH2CH2CH3), 22.7 (CH2CH2CH2CH3),
14.1 (CH2CH2CH2CH3). MS (ESI): m/z 258 [M – I]+.
(COCH3), 24.7 (CH2CH2CH2CH3), 21.9 (CH2CH2CH2CH3),
13
13.6 (CH2CH2CH2CH3). The
C
signal was not
carbene
observed. MS (ESI): m/z 1109 [M – I]+. Anal. Calcd. for
C30H38I4N6O2Pd2: C, 29.17; H, 3.10; N, 6.80%; Found: C,
29.83; H, 3.57; N, 6.65%.
1 3

Transition Metal Chemistry
washed with dichloromethane (3 × 3 mL). All the organic
phase was collected and dried over Na2SO4. The solvent was
removed under vacuum to give a crude product, which was
analyzed by 1H NMR spectroscopy.
Synthesis of complex 9
Pyridine (22 μL, 0.25 mmol) was added to a stirred solution
of 5 (135.4 mg, 0.1 mmol) in CH2Cl2 (5 mL). The mixture
was stirred at ambient temperature for 5 min. All the vola-
tiles were removed in vacuo, and then the resulting mix-
ture was washed with diethyl ether (10 mL). The residue
was dried under vacuum to aford the product as a brown
X‑ray difraction studies
For complex cis-7, the data were collected on an Agilent
Xcalibur Eos Gemini difractometer using Mo-Kα radiation.
For complex 10, the data were collected on a SuperNova,
Dual, Cu at zero, AtlasS2 difractometer using Cu-Kα radi-
ation. Structural solution was carried out with the Olex2
program.20 All H-atoms were put at calculated positions.
A summary of the most important crystallographic data is
given in Table 1. CCDC 1555284 and 1847545 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
1
powder in a near quantitative yield. H NMR (600 MHz,
CDCl3): δ (ppm) 8.91 (d, 2H, Ar–H, 3JH,H =6 Hz), 8.36 (d,
2H, Ar–H, 3JH,H =6 Hz), 7.67 (t, 1H, Ar–H, 3JH,H =6 Hz),
7.62–7.59 (m, 2H, Ar–H), 7.56–7.53 (m, 1H, Ar–H),
7.27–7.24 (m, 2H, Ar–H), 4.06 (s, 3H, NCH3), 3.11 (t,
3
2H, CH2CH2CH2CH3, JH,H = 12 Hz), 2.12–2.09 (m, 2H,
CH2CH2CH2CH3), 1.59–1.55 (m, 2H, CH2CH2CH2CH3),
1.08 (t, 3H, CH2CH2CH2CH3, JH,H = 6 Hz). 13C NMR
3
(150 MHz, CDCl3): δ (ppm) 154.5, 145.4, 140.5, 138.0,
133.8, 130.6, 129.5, 126.1, 125.5, 125.0 (Ar–C), 37.3
(NCH3), 30.6 (CH2CH2CH2CH3), 26.6 (CH2CH2CH2CH3),
23.4 (CH2CH2CH2CH3), 14.5 (CH2CH2CH2CH3). MS
(ESI): m/z 488 [M – I – Pyr + CH3CN]+. Anal. Calcd. for
C18H22I2N4Pd: C, 33.03%, H, 3.39%, N, 8.56%; Found: C,
33.46%, H, 3.68%, N, 8.28%.
Acknowledgements This work is fnancially supported by Capacity
Building for Sci-Tech Innovation-Fundamental Scientifc Research
Funds (Grant No. 025185305000/208) and Department of Education of
Guangdong Province (Grant No. 2016KCXTD005, 2017KQNCX204).
Synthesis of complex 10
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