Evaluating the electronic properties of ditopic and hetero-ditopic ligands derived from benzimidazole and pyrazole by 13C NMR spectroscopy

Article abstract of DOI:10.1016/j.jorganchem.2020.121409

Huynh's electronic parameter (HEP) was applied to distinguish the electronic properties of benzimidazole and pyrazole donors in N^N ditopic and N^N′ hetero-ditopic, dinucleating ligands by ¹³C NMR spectroscopy of their dipalladium complexes bea

Full text of DOI:10.1016/j.jorganchem.2020.121409

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Evaluating the electronic properties of ditopic and hetero-ditopic ligands derived from
13
benzimidazole and pyrazole by C NMR spectroscopy
Qiaoqiao Teng, Yuchen Lu, Rongmei Zhu, Han Vinh Huynh, Qi Meng
PII:
S0022-328X(20)30311-9
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121409
JOM 121409
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 13 April 2020
Revised Date: 2 June 2020
Accepted Date: 15 June 2020
Please cite this article as: Q. Teng, Y. Lu, R. Zhu, H.V. Huynh, Q. Meng, Evaluating the electronic
13
properties of ditopic and hetero-ditopic ligands derived from benzimidazole and pyrazole by
NMR spectroscopy, Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/
j.jorganchem.2020.121409.
C
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Evaluating the electronic properties of ditopic and
hetero-ditopic ligands derived from benzimidazole and
pyrazole by 13C NMR spectroscopy
Qiaoqiao Teng,a Yuchen Lu,a Rongmei Zhu,c Han Vinh Huynh,*,b Qi Meng*,a
a Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Jiangsu
Key Laboratory of Fine Petrochemical Engineering, School of Petrochemical
Engineering, Changzhou University, Changzhou 213164, P. R. China. E-mail:
cczumengqi@163.com
b
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
117543 Singapore, Republic of Singapore. E-mail: chmhhv@nus.edu.sg
c School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou
225009, P. R. China
Abstract. Huynh’s electronic parameter (HEP) was applied to distinguish the
electronic properties of benzimidazole and pyrazole donors in N^N ditopic and N^N’
hetero-ditopic, dinucleating ligands by 13C NMR spectroscopy of their dipalladium
complexes bearing terminal iPr2-bimy reporter ligands. The 13Ccarbene NMR resonances
i
of the Pr2-bimy ligand (HEPs) indicate stronger donation of the symmetrical
dibenzimidazole compared to that of the dipyrazole with a ∆HEP value of 1.54 ppm.
Based on this benchmark value, the effect of spacer groups on the electron donation
of the mixed benzimidazole-pyrazole ligands was investigated for the first time. The
donicity gap between the benzimidazole and pyrazole donors increases with alkylene
1

spacers, but shrinks with the 1,4-phenylene linker, suggesting electronic
communication between the two different heterocycles via π-electron conjugation.
Furthermore, these complex probes could also be used to catalyze the direct arylation
of pentafluorobenzene, which showed that the unsymmetrically bridged, dinuclear
complexes with alkylene linkers are more reactive.
13
Keywords: ditopic ligands; electronic properties; N-Heterocyclic carbene; C NMR
spectroscopy; direct arylation of pentafluorobenzene.
1. Introduction
The specific properties and reactivities of complexes are intrinsically linked to the
Lewis acidities of their metal centers [1], which in turn are largely determined by the
electron donation of the surrounding ligands. Therefore, the knowledge of a ligand’s
ability to donate electrons is of great importance for the tailored design of complexes
for specific purposes [2]. Among various experimental methods that have been
proposed for ligand donor strength evaluation, there are three that have received wider
acceptance. The first and most popular is Tolman’s electronic parameter (TEP), which
determines the amount of nickel-to-carbonyl backdonation in [Ni(CO)3L] complexes
using IR spectroscopy [3]. By measuring the A1 carbonyl stretching frequencies in
these complexes, the relative donor/acceptor strength of ligands L can be compared
[4]. The second method is the Lever’s electronic parameter (LEP), which ranks
ligands’ influences on the redox potential of their complexes, e.g. RuII/RuIII in
analogous [RuL6]2+ complexes [5]. By assuming ligand additivity, the contribution of
2

each ligand was derived as its ligand electrochemical parameter (EL), which is
somewhat related to its donor strength [6]. The more recent Huynh’s electronic
13
parameter (HEP) ranks the donor strengths of ligands L according to the
NMR chemical shifts of
[PdBr2(iPr2-bimy)L]
C
carbene
(iPr2-bimy
=
1,3-diisopropylbenzimidazolin-2-ylidene) complexes [7]. A stronger donating ligand
was found to decrease the Lewis acidity of the PdII center leading to a downfield shift
i
of the carbene carbon resonance of the Pr2-bimy ligand (HEP value). In comparison
with the former two methods, the HEP has the advantages of an easy preparation of
less toxic, but more stable complex probes, a higher sensitivity and accuracy based on
13
the C NMR spectroscopy combined with a convenient recoverability of the probes
for further applications and reactivity studies.
To date, HEP has been successfully applied to the donor strengths determination of
monodentate and bidentate ligands that could be charge-neutral or anionic[8], while
the measurements of bridging ligands is still limited [9]. In particular, unsymmetrical
bridging ligands have not been tackled.
Herein, we report the extension of HEP for the measurement of symmetrical N^N
ditopic and unsymmetrical N^N’ hetero-ditopic, dinucleating ligands derived from
benzimidazole and/or pyrazole. The possible electronic communication between the
metal centers through the N-donors, which has been observed for N^N bridged
dinuclear complexes [10], is also investigated.
2. Results and Discussion
3

2.1 Synthesis of probe complexes
Five N^N and N^N’ dinucleating ligands were studied of their electron donating
abilities, including the two symmetrical (ditopic) dibenzimidazole (I) and dipyrazole
(II) as well as three unsymmetrical (hetero-ditopic) ligands containing both
benzimidazole and pyrazole donors and linked with 1,3-propylenen (III), 1,2-ethylene
(IV) and 1,4-phenylene (V) groups (Chart 1).
Chart 1. Selected N^N and N^N’ dinucleating ligands.
The HEP complex probes of the formula trans-[Pd2Br4(iPr2-bimy)2(N^N)] (2, 3)
or trans-[Pd2Br4(iPr2-bimy)2(N^N’)] (4-6) were synthesized by mixing the above
ligands with one equivalent of the parent dimeric complex [PdBr2(iPr2-bimy)]2 (1)
(Scheme 1) [9]. A prompt and distinct color change from orange to yellow indicated
the successful cleavage and replacement of the µ-bromido bridges.
4

Scheme 1. Synthesis of the dinuclear trans-[Pd2Br4(iPr2-bimy)2(N^N)] complexes
2-6.
Complexes 2-5 bearing the alkylene bridged diazole ligands were isolated in
quantitative yields by simple evaporation of the solvents. They are well soluble in
aprotic polar organic solvents such as CHCl3, CH2Cl2, CH3CN and DMSO, but
sparingly soluble in hexane, diethyl ether and MeOH. The 1H NMR resonances of the
iPr2-bimy and diazole ligands are in a 2:1 ratio, confirming the coordination of both
azole moieties to the two palladium centers, respectively. Due to the symmetry of
1
complexes 2 and 3, only one H and 13C signal was observed for each NCH2 and
NCH(CH3)2 substituents. The NCH2 triplet is more downfield for complex 3
indicating stronger negative inductive effect (-I) of the NN moiety in the pyrazole ring
compared to that of the NC moiety in the benzimidazole. In the unsymmetrical
complexes 4 and 5, four signals are observed for these N-substituents. However, the
chemical shifts closely resemble those in the respective symmetrical complexes,
allowing for easy assignment of each function.
Scheme 2. Reaction of complex 1 with ligand V.
5

The reaction of [PdBr2(iPr2-bimy)]2 with the 1,4-phenylene-bridged
benzimidazole-pyrazole ligand V was not so straightforward. Here, a mixture of the
targeted dinuclear complex 6 was obtained along with unreacted the starting material
1 and a third component (Scheme 2). The latter was identified as the monopalladium
complex 7, where only the benzimidazole donor coordinated, while the pyrazole part
remains pendent. The ratio of complexes 6, 1 and 7 was determined to be 16.2:1:2
1
based on the integration of H NMR signals for the respective complexes suggesting
an 89% yield of the targeted complex 6.
The dinuclear complex 6 was separated via column chromatography and
recrystallization. However, dissolution of each isolated material led to reformation of
a mixture containing all three complexes. It was therefore hypothesized that complex
6 is in equilibrium with 1 and 7 in solution. When complex 1 and V were mixed,
seemingly the benzimidazole moiety coordinated first due to its stronger electron
donating ability (vide infra) leading to a quantitative conversion of ligand V into the
mononuclear complex 7. This consumes half-equivalent of the dimeric complex 1,
and the other half could be subsequently cleaved by the pendent pyrazole moiety of
complex 7. However, its electron donation is significantly weakened leading to a
maximum conversion of 89% despite prolonged stirring and elevated temperature.
This is in sharp contrast to the reactivity observed for the alkylene bridged counterpart
III and IV, where full conversions to the dinuclear complexes were observed. As the
desired dinuclear complex 6 is the major component in the mixture, the 13C NMR
spectrum was obtained from the mixture for HEP determination.
6

By increasing the diazole loading to two equivalents, the benzimidazole moiety
in ligand III and V selectively coordinated to the palladium center affording the
mononuclear complexes 8 and 7 in quantitative yields. Their formation was confirmed
by NMR spectroscopy, in which signals for the iPr2-bimy and diazole ligands are in a
i
1:1 integral ratio. Moreover, only one set of signals was observed for the Pr2-bimy
ligand. Treatment of complexes 7 and 8 with 0.5 equivalent of complex 1 lead to
different outcomes. Complex 8 was fully converted to the dinuclear complex 4 in a
short time, while complex 7 could still be observed after overnight stirring.
Scheme 3. Synthesis of single side coordinated mononuclear complexes 7 and 8.
Single crystals of complexes 3 and 7 were obtained via slow evaporation of their
saturated solutions in CHCl3/hexane and analyzed by X-ray diffraction analysis. There
are two independent molecules in the asymmetric unit of the complex 7. The
structural parameters are essentially identical within 3σ, therefore only one of them is
shown in Figure 1. The molecular structures confirm their compositions. The
palladium centers are all coordinated by one carbene, two bromido and one azole
nitrogen in trans-configurations. The carbene planes are oriented perpendicularly to
the square planar coordination planes with dihedral angles ranging from 78.2-86.2°.
7

The Pd-C bond lengths are around ~1.95 Å, which are within literature values [7].
Figure 1. Molecular structures of 3 CHCl3 and 7 showing 50% probability ellipsoids;
hydrogen atoms and solvent molecules are omitted for clarity. Selected bond length
[Å] and bond angles[°]: 3, Pd1-C10 1.952(6), Pd1-Br1 2.4121(8), Pd1-Br2 2.4245(8),
Pd1-N3 2.092(5), Pd2-C23 1.955(5), Pd2-Br3 2.4078(8), Pd2-Br4 2.4375(8),
Pd2-N6 2.090(4); C10-Pd1-Br1 87.86(17), C10-Pd1-Br2 90.76(17), N3-Pd1-Br1
92.13(13), N3-Pd1-Br2 89.22(13), C23-Pd2-Br3 87.59(15), C23-Pd2-Br4
88.47(15), N6-Pd2-Br3 91.10(13), N6-Pd2-Br4 92.86(13). PdCNBr2/NHC dihedral
angles: 85.8° and 86.2° for Pd1 and Pd2, respectively. 7, Pd1-C10 1.962(6), Pd1-Br1
2.4295(9), Pd1-Br2 2.4256(10), Pd1-N3 2.103(5); C10-Pd1-Br1 87.67(18),
C10-Pd1-Br2 88.03(18), N3-Pd1-Br1 92.62(15), N3-Pd1-Br2 91.83(15).
PdCNBr2/NHC dihedral angle: 78.2°.
2.2 Electron donating strengths determination
Due to the symmetry, the dibenzimidazole and dipyrazole bridged complexes 2
13
and 3 show one
C
NMR signal (HEP) at 162.05 and 160.51 ppm [11],
carbene
respectively (Table 1), indicating the equivalence of the two palladium centers. It is
worth mentioning that a standard deviation of σ ~ 0.01 ppm has been estimated for
typical 13C NMR signals [7]. Therefore, the difference of ∆HEP = 1.54 ppm is
significant, indicating absolute weaker electron donation of the dipyrazole compared
to that of the dibenzimidazole ligand. The electronic difference between the two
8

heterocycles is due to a more pronounced negative inductive effect (-I effect) of the
adjacent nitrogen atom to the donor atom in the pyrazole, making it weaker donating.
Thus, the respective HEP signal is found at higher field.
Table 1. HEP values for complexes 2-8a
Complex
HEPbenz (ppm)
162.05
-
HEPpyr (ppm)
∆HEP (ppm)
-
2
3
4
5
6
7
8
b
1.54
160.51
160.12
160.01
159.99
-
162.11
162.52
161.36
161.78
162.26
1.99
2.51
1.37
-
-
-
aMeasured in CDCl3 and internally referenced to the solvent signal at 77.7 ppm
relative to tetramethylsilane (TMS). bHEP difference between 2 and 3.
For the unsymmetrical benzimidazole-pyrazole complexes, two distinct HEP
signals are observed, which are referred to as HEPbenz and HEPpyr for the
benzimidazole and pyrazole donors, respectively (Table 1). In the propylene-linked
complex 4, the two carbene NMR signals shift in reversed directions as compared to
those of the symmetrically bonded complexes 2 and 3. The HEPbenz shifts downfield
to 162.11 ppm, while the HEPpyr is found upfield at 160.12 ppm, overall leading to an
increase of ∆HEP to 1.99 ppm.
Complexes 2-4 contain propylene linkers, which not only allows for a detailed
comparison between the two heterocycles, but also their influences toward each other
(Figure 2). The slight downfield shift of HEPbenz going from the ditopic I to
hetero-ditopic ligand III indicates an increase of electron density at the benzimidazole
nitrogen. The opposite observation was made for the pyrazole N-donor going from
9

ditopic II to hetero-ditopic ligand III. This indicates a greater group electronegativity
of the benzimidazole versus the pyrazole heterocycle possibly as a result of
benzannulation. The benzimidazole thus withdraws more electron density from the
pyrazole, making itself a better donor and concurrently weakening the latter. This
inductive interplay between the two metal centers is even more obvious for the shorter
1,2-ethylene-linked hetero-ditopic ligand IV in complex 5 with HEPbenz and HEPpyr
values of 162.52 and 160.01 ppm, respectively, leading to a magnified ∆HEP of 2.51
ppm.
13
Figure 2. Effect of the linkers on the
C
NMR resonances (HEPs) in the
carbene
dinuclear complexes 2-6.
Compared to the alkylene spacers, the phenylene linker in ligand V is electron
withdrawing, and both heterocycles in this hetero-ditopic ligand are weaker donors.
10

Therefore, their HEP values of 161.36 (benz) and 159.99 ppm (pyr), respectively,
determined using complex 6 are found at relatively higher field. The highest field of
HEPpyr in this complex confirms the rather weak donating ability of the pyrazole in
ligand V, and explains its decreased reactivity towards complex 1 (vide supra).
Although, the benzimidazole group is still more electron-rich, the ∆HEP of 1.37 ppm
is significantly smaller than for all alkylene-bridged ligands. We believe that the
increased electronic similarity of the two heterocyclic moieties is a result of greater
π-delocalization across the aromatic spacer leading to a mixing of mesomeric
(resonance) and inductive effects. Electronic communication between the two
different N-donors becomes possible.
Figure 3. Comparison of HEPbenz values of mono- and dinuclear complexes as
indicated by the benzimidazole ligands.
11

With inclusion of the HEPbenz values determined using monopalladium complexes
7 and 8, the donicity of four benzimidazoles bearing different N-substituents can also
be compared, and the result is shown in Figure 3. The dinuclear complexes 4 and 6
can be formally considered as HEP complex probes of benzimidazole-metalloligands
containing the entire alkyl- and phenylpyrazole-palladium complex as an
N-substituent. Compared to the alkyl- and phenylpyrazole substituted benzimidazole
ligands in 8 and 7, coordination of the pyrazole units to [PdBr2(iPr2-bimy)] complex
fragments in complexes 4 and 6 leads to reduced electron donating abilities of the
benzimidazole moieties. Again, this effect is much greater for the phenylene-linked
ligand, where a larger ∆HEP of 0.42 ppm is noted when compared to its aliphatic
counterpart with a significantly lower ∆HEP of 0.15 ppm. The electronic
communication across the aromatic linker (vide supra) leads to a more efficient flow
of electron density away from the benzimidazole unit to the pyrazole-bound
palladium Lewis acid, which results in a much weaker benzimidazole donor. A similar
electron flow is not observed for the propylene-linked system due to lack of
conjugation.
2.3 Catalytic study
In a preliminary study, all complexes 2-8 were tested as catalyst precursors in
the challenging direct arylation of pentafluorobenzene with bromobenzene under
identical conditions to reveal possible structure-activity relationships (Table 2) [12].
12

Table 2. Catalyst screening for the direct arylation of pentafluorobenzene.a
Entry
Complex
Catalyst loading
Yield (%)b
1
2
3
4
5
6
7
1
1
1
1
1
2
2
62
64
79
76
54
40
9
2
3
4
5
6
7
8
aReaction conditions: pentafluorobenzene (0.75 mmol),
bromobenzene (0.5 mmol), K2CO3 (0.75 mmol), 2 mol% [Pd]
b
precatalyst, DMA (2 mL), 120 °C, 18 h, N2. GC yields for an
average of two runs with n-dodecane as the internal standard.
The dinuclear complexes clearly show better performances compared to the
mononuclear complexes at the same [Pd] loading of 2 mol%. The yields obtained are
comparable with those of other Pd-NHC complexes [13,8b]. The most similar
performance was observed for dinuclear complex 6 (entry 5) and mononuclear
complex 7 (entry 7). This can be ascribed to the easy dissociation of the Pd-complex
motif from the pyrazole donor in 6, which then gives rise to complex 7 (vide supra).
Among the dinuclear complexes, 4 and 5 bearing the alkylene-bridged, ditopic
benzimidazole-pyrazole ligands give the best yields of 79% and 76%, respectively
(entries 3 and 4), while the phenylene-linked counterpart 6 is much less active (entry
5). Seemingly, the dinuclear complexes with greater Lewis acidity difference between
13

the two metal centers, which is reflected by the differences between the HEP probes
(vide supra) [14], are more favorable in this coupling reaction. However, the trend is
not strictly followed as the symmetrical dipalladium complexes have intermediate
activities (entries 1 and 2). Other factors including the inherent Lewis acidity
magnitude, metal-metal distance [15] or configurational flexibility [16] may also
contribute and leading to the current results.
To improve the catalytic activity of complex 4 further, different bases, solvents,
and reaction temperatures were screened. However, all alterations gave inferior or
comparable yields (see supporting information Table S1). Therefore, the initial
reaction condition was used for subsequent small substrate scope study.
Aryl bromides with electron-donating substituents (Table 3, entries 1-3) are
more reactive under such conditions than electron deficient aryl bromides (entries
6-8). However, ortho-substituted aryl bromides cannot be coupled due to steric
hindrance (entries 4 and 5).
Table 3. Substrate scope study with the complex 4.a
Entry
1
Product
F
Yield (%)b
F
F
F
O
82
F
2
81
14

3
4
5
77
0
0
6
7
8
62
29
65
aReaction conditions: pentafluorobenzene (0.75 mmol), aryl bromide (0.5 mmol),
K2CO3 (0.75 mmol), 1 mol% complex 4, DMA (2 mL), 120 °C, 18 h, N2. bIsolated
yields for an average of two runs.
3. Conclusion
We have prepared five dinuclear palladium complexes as HEP complex probes for
ditopic and hetero-ditopic diazole ligands derived from benzimidazole and pyrazole
with varying linkers. Initial, 13C NMR spectroscopic analyses reveal stronger electron
donation of the symmetrical dibenzimidazole over the dipyrazole counterpart. This
can be exploited to prepare two mononuclear complexes with hetero-ditopic ligands
15

containing both benzimidazole and pyrazole donors. In these cases, stoichiometry
control leads to selective benzimidazole coordination to afford complexes with
pendant pyrazole moiety. The comparison between the HEP values obtained for the
mononuclear and dinuclear complexes suggests weaker electron donation of the
benzimidazole nitrogen upon remote pyrazole coordination, which is nine bonds away
from the carbene carbon, highlighting the sensitivity of the HEP method. Moreover,
the donor strength differences (ΔHEP) between the benzimidazole and pyrazole
donors in the mixed ligands was found to be larger with ethylene and propylene
groups, but smaller with phenylene spacer as compared to that between the parent
dibenzimidazole and dipyrazole. The detailed analyses of these differences lead to the
conclusion that benzimidazole has a greater group electronegativity possibly due to
benzannulation. In addition, HEP allows detection of electronic communication across
the conjugated phenylene spacer between the two different heterocycles. Finally, the
complex probes could be recovered after HEP analyses and applied to catalyze the
direct arylation of pentafluorobenzene with arylbromides. In this reaction, the
unsymmetrical dinuclear complexes with alkylene spacers perform better than the
symmetrical or the phenylene-linked dipalladium complex. The sensitivity of the HEP
method observed in this study is currently being exploited to study electronic
communication in hetero-bimetallic complexes. Further extension of the HEP method
beyond donor strength determination will be reported in due course.
4. Experimental Section
16

General Considerations. All operations were performed without taking precautions
to exclude air or moisture unless otherwise stated. All solvents and chemicals were
used as received. 1H and 13C NMR spectra were recorded at 298 K on a Bruker
AVANCE spectrometer, and the chemical shifts (δ) were internally referenced to the
residual solvent signals relative to tetramethylsilane. HRMS analyses were performed
on a Thermo Scientific LTQ Orbitrap spectrometer. Elemental analyses were
performed on a Vario EL cube elemental analyzer. The N^N bridging ligands I [17], II
[18], IV [19], V [20] and complex 1 [21] were prepared according to literature and
their formation was confirmed by 1H NMR spectroscopy.
3-pyrazolylpropylbenzimidazole (III). A mixture of pyrazole (204 mg, 3.0 mmol, in
excess) and NaOH (480 µL, 6.25 M) in THF (5 mL) was stirred for 1 h before
N-(3-bromopropyl)-benzimidazole (338 mg, 1.5 mmol) was added. The resulting
mixture was reacted for 12 h. The solvent was removed by rotary evaporation and the
residue was suspended in CH2Cl2 (50 mL) before extraction with H2O (3 × 20 mL).
Drying of the organic phase over Na2SO4 followed by removal of the solvent afforded
1
the product as a yellow oil. Yield: 329 mg, 1.5 mmol, 97%. H NMR (500 MHz,
3
CDCl3): δ 7.89 (s, 1H, NCHN), 7.81-7.79 (m, 1H, Ar-H), 7.57 (d, 1H, J = 2 Hz,
pyr-H), 7.33-7.26 (m, 4H, Ar-H & pyr-H), 6.28 (t, 1H, 3J = 2 Hz, pyr-H), 4.16 (t, 2H,
3
3J = 7 Hz, pyr-NCH2), 4.08 (t, 2H, J = 6 Hz, benz-NCH2), 2.46-2.41 (m, 2H, CH2).
13C{1H} NMR (125 MHz, CDCl3): δ 144.5, 143.8, 140.4, 134.1, 130.1, 123.6, 122.9,
121.1, 110.2, 106.4 (10 Ar-C), 48.9 (pyr-NCH2), 42.4 (benz-NCH2), 30.7 (CH2).
+
HRMS (ESI): m/z calcd. for C13H15N4 [M + H]+ 227.1297, found 227.1288.
17

General procedure for the preparation of palladium complexes. The respective
nitrogen heterocycle (1.0/2.0 equiv.) was added to suspension of
a
di-µ-bromo-bis(1,3-diisopropylbenzimidazolin-2-ylidene)-dibromo-dipalladium(II)
complex 1 (28 mg, 0.03 mmol, 1.0 equiv.) in dichloromethane (4 mL) and the
resulting mixture was stirred at ambient temperature for 1 h. The solvent was removed
under reduced pressure and the remaining solid was dried in vacuo to afford the
product as yellow solids.
1
trans-[Pd2Br4(iPr2-bimy)2(I)] (2). Yield: 36 mg, 0.03 mmol, 99%. H NMR (500
MHz, CDCl3): δ 8.56 (d, 2H, 3J = 8 Hz, Ar-H), 8.45 (s, 2H, NCHN), 7.61-7.59 (m, 4H,
Ar-H), 7.45-7.41 (m, 2H, Ar-H), 7.39-7.32 (m, 4H, Ar-H), 7.23-7.22 (m, 4H, Ar-H),
6.51-6.46 (m, 4H, NCH), 4.19 (t, 4H, 3J = 7 Hz, NCH2), 2.53-2.47 (m, 2H, CH2), 1.84
3
(d, 24H, J = 7 Hz, CH3). 13C{1H} NMR (125 MHz, CDCl3): δ 162.0 (Ccarbene), 145.2
(NCHN), 141.6, 134.2, 133.2, 125.2, 124.3, 122.9, 122.5, 113.3, 110.7 (Ar-C), 55.2
(NCH), 43.1 (NCH2), 29.9 (CH2), 21.4 (CH3). Anal. Calcd. for C43H52Br4N8Pd2: C
42.56, H 4.32, N 9.23, found: C 42.28, H 4.49, N 9.00. HRMS (ESI): m/z calcd. for
+
C43H52Br3N8Pd2 [M - Br]+ 1132.9918, found 1132.9924.
trans-[Pd2Br4(iPr2-bimy)2(II)] (3). Yield: 33 mg, 0.03 mmol, >99%. The single
crystals were obtained by slow evaporation of a saturated solution in CHCl3/hexane.
3
1H NMR (300 MHz, CDCl3): δ 7.86 (dd, 2H, J = 2 Hz, pyr-H), 7.62-7.58 (m, 6H,
pyr-H & Ar-H), 7.24-7.21 (m, 4H, Ar-H), 6.40-6.31 (m, 4H, NCH), 6.29 (t, 2H, 3J = 2
Hz, pyr-H), 4.90 (t, 4H, 3J = 7 Hz, NCH2), 3.70-3.61 (m, 2H, CH2), 1.81 (d, 24H, 3J =
7 Hz, CH3). 13C{1H} NMR (125 MHz, CDCl3): δ 160.5 (Ccarbene), 143.0, 134.1, 133.5,
18

123.1, 113.4, 107.7 (Ar-C), 55.4 (NCH), 50.8 (NCH2), 30.9 (CH2), 21.3 (CH3). Anal.
Calcd. for C35H48Br4N8Pd2: C 37.76, H 4.35, N 10.07, found: C 38.10, H 4.61, N
+
10.06. HRMS (ESI): m/z calcd. for C35H48Br3N8Pd2 [M - Br]+ 1032.9605, found
1032.9591.
1
trans-[Pd2Br4(iPr2-bimy)2(III)] (4). Yield: 35 mg, 0.03 mmol, >99%. H NMR (300
MHz, CDCl3): δ 8.54-8.52 (m, 1H, Ar-H), 8.46 (s, 1H, NCHN), 7.95 (d, 1H, 3J = 2 Hz,
pyr-H), 7.61-7.58 (m, 4H, Ar-H), 7.52-7.49 (m, 2H, Ar-H & pyr-H), 7.44-7.33 (m, 2H,
Ar-H), 7.24-7.21 (m, 4H, Ar-H), 6.53-6.44 (m, 2H, NCH), 6.39-6.30 (m, 3H, NCH&
3
3
pyr-H), 4.84 (t, 2H, J = 7 Hz, pyr-NCH2), 4.23 (t, 2H, J = 7 Hz, benz-NCH2),
3.13-3.04 (m, 2H, CH2), 1.83 (d, 12H, 3J = 7 Hz, CH3), 1.81 (d, 12H, 3J = 7 Hz, CH3).
13C{1H} NMR (75 MHz, CDCl3): δ 162.1, 160.1 (Ccarbene), 145.2 (NCHN), 143.6,
141.6, 134.2, 134.0, 133.7, 133.3, 125.0, 124.1, 123.1, 122.8, 122.3, 113.3, 113.2,
111.1, 107.9 (Ar-C), 55.4, 55.1 (NCH), 50.1, 43.5 (NCH2), 30.1 (CH2), 21.3 (CH3).
Anal. Calcd. for C39H50Br4N8Pd2: C 40.27, H 4.33, N 9.63, found: C 40.32, H 4.23, N
9.24. HRMS (ESI): m/z calcd. for C39H50Br3N8Pd2 [M - Br]+ 1082.9762, found
+
1082.9756.
trans-[Pd2Br4(iPr2-bimy)2(IV)] (5). Yield: 34 mg, 0.03 mmol, 99%. 1H NMR (300
MHz, CDCl3): δ 8.47-8.44 (m, 1H, Ar-H), 8.29 (s, 1H, NCHN), 7.94 (d, 1H, 3J = 2 Hz,
pyr-H), 7.63-7.58 (m, 5H, Ar-H), 7.42-7.37 (m, 2H, Ar-H), 7.28-7.20 (m, 4H, Ar-H),
3
6.76 (d, 1H, J = 2 Hz, pyr-H), 6.52-6.43 (m, 2H, NCH), 6.41-6.31 (m, 2H, NCH),
6.19 (t, 1H, 3J = 2 Hz, pyr-H), 5.21 (br-s, 4H, NCH2), 1.85 (d, 12H, 3J = 7 Hz, CH3),
3
1.84 (d, 12H, J = 7 Hz, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 162.5, 160.0
19

(Ccarbene), 145.0, 144.2, 141.1, 134.2, 134.1, 133.2, 125.2, 124.2, 123.2, 122.8, 122.1,
113.6, 113.4, 113.2, 110.7, 108.9 (Ar-C), 55.5, 55.1 (NCH), 53.2, 45.8 (NCH2), 21.4
(CH3). Anal. Calcd. for C38H48Br4N8Pd2: C 39.71, H 4.21, N 9.75, found: C 39.33, H
+
4.28, N 9.51. HRMS (ESI): m/z calcd. for C38H48Br3N8Pd2 [M - Br]+ 1068.9605,
found 1068.9583.
trans-[Pd2Br4(iPr2-bimy)2(V)] (6). Compound 6 was in equilibrium with complex 1
1
and 8, and therefore was not isolatable. Yield: 89% (calculated based on H NMR
1
3
spectrum). H NMR (300 MHz, CDCl3): δ 8.81 (s, 1H, NCHN), 8.68 (d, J = 8 Hz,
1H, Ar-H), 8.28 (d, 3J = 8 Hz, 2H, Ar-H), 8.14 (d, 3J = 2 Hz, 1H, pyr-H), 7.85 (d, 1H,
3J = 2 Hz, pyr-H), 7.82-7.79 (m, 2H, Ar-H), 7.64-7.40 (m, 7H, Ar-H), 7.27-7.23 (m,
2H, Ar-H), 7.18-7.15 (m, 2H, Ar-H), 6.66 (t, 3J = 2 Hz, 1H, pyr-H), 6.56-6.47 (m, 2H,
3
NCH), 6.13-6.04 (m, 2H, NCH), 1.86 (d, 12H, J = 7 Hz, CH3), 1.67 (d, 12H, 3J = 7
Hz, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 161.4, 160.0 (Ccarbene), 145.1, 144.6,
141.6, 140.9, 136.3, 134.2, 134.0, 133.6, 133.5, 128.6, 125.9, 125.7, 124.7, 122.9,
122.8, 113.3, 113.2, 111.2, 109.7 (Ar-C, two are coincident), 55.2, 55.1 (NCH), 21.4,
+
21.3 (CH3). HRMS (ESI): m/z calcd. for C42H48Br3N8Pd2 [M - Br]+ 1116.9605, found
1116.9598.
trans-[PdBr2(iPr2-bimy)(V)] (7). Yield: 43 mg, 0.06 mmol, 99%. The single crystals
were obtained by slow evaporation of a saturated solution in CHCl3/hexane. 1H NMR
(300 MHz, CDCl3): δ 8.68 (s, 1H, NCHN), 8.63-8.60 (m, 1H, Ar-H), 8.02 (d, 1H, 3J =
2 Hz, pyr-H), 7.95-7.92 (m, 2H, Ar-H), 7.79 (d, 1H, 3J = 2 Hz, pyr-H), 7.62-7.59 (m,
4H, Ar-H), 7.49-7.36 (m, 4H, Ar-H), 7.24-7.21 (m, 2H, Ar-H), 6.54-6.49 (m, 3H,
20

13
NCH & pyr-CH), 1.85 (d, 12H, 3J = 7 Hz, CH3) . C{1H} NMR (75 MHz, CDCl3): δ
161.8 (Ccarbene), 144.8, 142.5, 141.5, 141.0, 134.2, 133.7, 133.6, 127.5, 126.2, 125.7,
125.6, 124.4, 122.9, 122.6, 121.1, 121.0, 113.2, 111.3, 109.1 (Ar-C), 55.2 (NCH),
21.3 (CH3). Anal. Calcd. for C29H30Br2N6Pd: C 47.79, H 4.15, N 11.53, Found: C
47.49, H 4.14, N 11.27. HRMS (ESI): m/z calcd. for C29H31Br2N6Pd+ [M + H]+
728.9986, found 728.9972.
1
trans-[PdBr2(iPr2-bimy)(IV)] (8). Yield: 42 mg, 0.06 mmol, > 99%. H NMR (300
MHz, CDCl3): δ 8.53 (d, 3J = 7 Hz, 1H, Ar-H), 8.40 (s, 1H, NCHN), 7.62-7.59 (m, 3H,
pyr-H & Ar-H), 7.48 (d, 1H, 3J = 2 Hz, pyr-H), 7.43-7.33 (m, 3H, Ar-H), 7.24-7.21 (m,
2H, Ar-H), 6.53-6.44 (m, 2H, NCH), 6.32 (t, 1H, 3J = 2 Hz, pyr-H), 4.22-4.13 (m, 4H,
3
13
NCH2), 2.55-2.46 (m, 2H, CH2), 1.84 (d, 12H, J = 7 Hz, CH3). C{1H} NMR (75
MHz, CDCl3): δ 162.3 (Ccarbene), 145.6, 141.7, 140.8, 134.2, 133.4, 130.7, 124.9,
124.0, 122.9, 122.4, 113.3, 110.7, 106.6 (Ar-C), 55.2 (NCH), 48.8, 43.3 (NCH2), 30.4
(CH2), 21.4 (CH3). Anal. Calcd. for C26H32Br2N6Pd: C 44.95, H 4.64, N 12.10, Found:
C 44.69, H 4.93, N 12.28. HRMS (ESI): m/z calcd. for C26H33Br2N6Pd+ [M + H]+
695.0142, found 695.0140.
General procedure for the direct arylation of pentafluorobenzene. In a typical run,
a Schlenk tube was charged with the precatalyst, base (0.75 mmol, 1.5 equiv.), and
aryl bromide (0.50 mmol, 1.0 equiv.) if it is a solid. The vessel was evacuated and
backfilled with nitrogen for three times before solvent (2 mL) and pentafluorobenzene
(0.75 mmol, 1.5 equiv.), aryl bromide (0.50 mmol, 1.0 equiv.) if it is a liquid, were
added. The reaction was placed in a preheated oil bath for 18 h. After cooling to
21

ambient temperature, ethyl acetate (10 mL) was added before extraction with H2O (3
× 20 mL). Drying of the organic phase over Na2SO4 followed by removal of the
solvent afford the crude product, which was purified by flash chromatography (SiO2,
PE).
Declaration of competing interest
The authors declare no competing interests or personal relationships that could have
appeared to influence the work reported in this paper
Acknowledgement
The authors thank the Natural Science Research of Jiangsu Higher Education
Institutions of China (18KJB150002), Applied Basic Research of Changzhou Science
and Technology Bureau (CJ20190036), Jiangsu Key Laboratory of Fine
Petrochemical Engineering (KF1903) and Postgraduate Research & Practice
Innovation Program of Jiangsu Province (KYCX19_1743) for financial support.
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[ 11 ] A standard deviation of σ ~ 0.01 ppm is estimated by the
29