Synthesis, structure and catalysis of a NHC-Pd(II) complex based on a tetradentate mixed ligand

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Synthesis, Structure and Catalysis of NHC-Pd(II) Complex Based on

Tetradentate Mixed Ligand

Qing-Xiang Liu*, Kang-Qing Cai, Zhi-Xiang Zhao

Tianjin Key Laboratory of Structure and Performance for Functional Molecules; Key

Laboratory of Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of

Education; College of Chemistry, Tianjin Normal University, Tianjin 300387, China.

* Corresponding author, E-mail: tjnulqx@163.com

Bis-benzimidazolium

salt

(S)-2,2’-bis[2’’-(N-picolyl-benzimidazoliumyl)ethoxy]-1,1’-binaphthyl

hexafluorophosphate [(S)-LH₂]·(PF₆)₂and its NHC palladium(II) complex

[(S)-LPd](PF₆)₂(1) have been prepared and characterized. Complex 1 is formed by

one tetradentate mixed ligand (S)-L and one Pd(II) ion, in which one 15-membered

ring and two 6-membered rings are contained. In this complex, the intramolecular π-π

interactions between naphthalene rings and benzimidazole rings are observed.

Additionally, the catalytic activity of complex 1 in three types of C-C coupling

reactions (Suzuki-Miyaura, Heck-Mizoroki and Sonogashira reactions) was

investigated. The results show that complex 1 is an effective catalyst in these coupling

reactions.

Keywords: N-heterocyclic carbene, palladium(II) complex, structure, catalysis

Introduction

N-Heterocyclic carbenes (NHCs) have become a type of important ligands of

organometallic chemistry and catalysis.¹NHC ligands can form strong carbon-metal

bonds with transition metals due to the strong σ-donating ability of carbene carbon.²

So far, numerous successful methods have been developed to synthesize NHC

complexes with different metal centers. Among these NHC-metal complexes, NHC

palladium(II) complexes play a very important role in the N-heterocyclic carbene

1

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chemistry, ^{3, 4}and they have turned out to be efﬁcient catalysts for some C-C coupling

reactions, such as Suzuki-Miyaura, Heck-Mizoroki and Sonogashira reactions.⁵

Compared with palladium(II) complexes based on phosphine ligands, NHC

palladium(II) complexes have better stability to air and moisture and catalytic

activity.⁶Some NHC palladium(II) complexes bearing monodentate or polydentate

ligands have been prepared and studied, and they show good catalytic activity in the

construction of C-C, C-N and other C-heteroatom bonds.^{4, 7-9}In order to improve

further catalytic activity of NHC palladium(II) complexes, some heteroatoms (P,¹⁰

N,^{10a, 11}or O¹²) were introduced into NHC ligands. The research results show that the

catalytic activity of palladium(II) complexes bearing mixed ligands of NHC and

heteroatoms can be improved remarkably, and the reason may be that strong

coordinating NHC and weak coordinating heteroatoms easily form catalytically active

species with vacant coordination sites. Therefore, these NHC palladium(II) complexes

bearing mixed ligands may have potential applications in organic reactions.¹³

We are interested in NHC palladium(II) complexes with mixed ligands. Herein, we

report the preparation of bis-benzimidazolium salt (S)-2,2’-bis[2’’-(N-

picolyl-benzimidazoliumyl)ethoxy]-1,1’-binaphthyl

hexafluorophosphate

[(S)-LH₂]·(PF₆)₂, as well as the preparation and structure of N-heterocyclic carbene

palladium(II) complex [(S)-LPd](PF₆)₂(1). Additionally, the catalytic activity of

complex 1 in three types of C-C coupling reactions (Suzuki-Miyaura, Heck-Mizoroki

and Sonogashira reactions) was investigated.

Results and discussion

Synthesis and characterizations of precursor [(S)-LH₂]·(PF₆)₂. As shown in

Scheme 1, the (S)-2,2’-dihydroxy-1,1’-binaphthyl as starting material was reacted

with 2-chloroethanol to afford (S)-2,2’-di(2’’-hydroxyethoxy)-1,1’-binaphthyl,

followed by chlorination of hydroxyl groups with thionyl chloride to form

(S)-2,2’-di(2’’-chloroethoxy)-1,1’-binaphthyl,

which

was

reacted

with

N-picolyl-benzimidazole in toluene to afford the bis-benzimidazolium salt

[(S)-LH₂]·Cl₂, and subsequent anion exchange with ammonium hexafluorophosphate

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in

methanol

was

carried

out

to

give

(S)-2,2’-bis[2’’-(N-picolyl-benzimidazoliumyl)ethoxy]-1,1’-binaphthyl

hexafluorophosphate ([(S)-LH₂]·(PF₆)₂). This precursor is stable toward air and

moisture, and soluble in organic solvents, such as CH₂Cl₂, CH₃CN and DMSO,

1

however, the solubility is poor in water, petroleum ether and diethyl ether. In the H

NMR spectra of [(S)-LH₂]·(PF₆)₂, the proton signals (NCHN) of benzimidazolium

appear at δ = 9.43 ppm, which is consistent with the chemical shifts of reported

benzimidazolium salts.¹⁴

OH

O

OH

Cl

O

SOCl₂

Cl OH

N

(1)

(2)

N

O

N

-

Pd(OAc)₂

K₂CO₃

2PF₆

Pd

NH₄PF₆

N

-

2PF₆

[(S)-LH₂]·(PF₆)₂

1

Scheme 1 Preparation of precursor [(S)-LH₂]·(PF₆)₂and complex 1

Synthesis, characterization and structure of complex 1. As shown in Scheme 1,

complex [(S)-LPd](PF₆)₂(1) was prepared via the reaction of [(S)-LH₂]·(PF₆)₂with

Pd(OAc)₂in the presence of K₂CO₃in acetonitrile/ethylene glycol solution. The

single crystals of complex 1 suitable for X-ray diffraction were grown by slow

diffusion of diethyl ether into its solution. Complex 1 is stable to heat, air and

moisture, soluble in CH₃CN and DMSO, but scarcely soluble in diethyl ether and

1

hydrocarbon solvents. In the H NMR spectra of 1, the disappearance of the

resonances for the benzimidazolium protons (NCHN) shows the formation of the

expected metal carbene complexes, and the chemical shifts of other hydrogen atoms

are similar to those of corresponding precursor. In ¹³C NMR spectra, the signals for

the carbene carbon of 1 appear at δ = 179.2 ppm, which are similar to known metal

carbene complexes.¹⁵

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Complex 1 is formed by one tetradentate mixed ligand (S)-L and one Pd(II) ion, in

which one 15-membered ring and two 6-membered rings are contained (Fig. 1). The

Pd(II) ion is tetra-coordinated with two carbene carbon atoms and two nitrogen atoms

from two pyridine rings to adopt a square-planar geometry. The bond distances of

Pd-C and Pd-N are 1.966(3)-1.976(3) Å and 2.095(3)-2.098(3) Å, respectively. The

bond angles of C(7)-Pd(1)-C(38) and N(1)-Pd(1)-N(6) are 96.0(1)˚ and 94.4(1)˚,

respectively. The bond angles of C-Pd-N are from 84.9(1)˚ to 175.0(1)˚. These values

are similar to those of reported NHC-Pd(II) complexes.¹⁶The axial chirality of

complex 1 is consistent with corresponding precursor because its chiral binaphthyl

unit (S configuration) was not changed during the preparation of metal complex. In

this complex, two naphthalene rings form the dihedral angle of 69.2(3)˚, and the

dihedral angle between two benzimidazole rings is 87.2(2)˚. The dihedral angle

between benzimidazole ring and adjacent pyridine ring is 71.5(3)˚, and the dihedral

angle between two pyridine rings is 50.7(4)˚. The internal ring angles (N-C-N) at the

carbene centers are 107.1(3)˚ and 107.6(3)˚, and these values are similar to those of

known NHC-metal complexes.¹⁵Besides, the intramolecular π-π interactions between

the naphthalene rings and the benzimidazole rings are observed¹⁷with the face-to-face

distance of 3.405(4) Å and the center-to-center distance of 3.858(3) Å.

In the crystal packing of 1, 2D supramolecular layer is formed through three types

of π···π stacking interactions as shown in Fig. S1. In three types of π···π stacking

interactions, one is from intermolecular pyridine rings with the face-to-face distance

of 3.761(7) Å and the center-to-center distance of 3.773(2) Å, and another is from

intermolecular benzimidazole rings with the face-to-face distance of 3.654(5) Å and

the center-to-center distance of 3.659(3) Å, and third is from naphthalene rings and

pyridine rings with the face-to-face distance of 3.534(5) Å and the center-to-center

distance of 3.901(3) Å.

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Fig. 1. Perspective view of 1. All hydrogen atoms were omitted for clarity. Selected

bond lengths (Å) and angles (˚): C(38)-Pd(1) 1.966(3), C(7)-Pd(1) 1.976(3),

N(6)-Pd(1) 2.095(3), N(1)-Pd(1) 2.098(3), O(1)-C(15) 1.409(4), O(2)-C(36) 1.432(4);

N(4)-C(38)-N(5) 107.6(3), N(2)-C(7)-N(3) 107.1(3), C(38)-Pd(1)-C(7) 96.0(1),

C(38)-Pd(1)-N(6) 84.9(1), N(1)-Pd(1)-N(6) 94.4(1), C(7)-Pd(1)-N(1) 85.2(1),

C(7)-Pd(1)-N(6) 172.3(1), C(38)-Pd(1)-N(1) 175.0(1).

Catalytic activity of NHC-Pd(II) complex 1 in the Suzuki-Miyaura reaction. The

Suzuki-Miyaura reaction, a cross-coupling process between an organoboron reagent

and an aryl halide, has become a successful and useful tools for forming C-C bonds.¹⁸

In order to examine the catalytic activity of NHC-Pd(II) complex 1 in the

Suzuki-Miyaura reaction, we chose the cross-coupling reaction of 4-bromotoluene

with phenylboronic acid as a model reaction in the air to test the effects of solvent and

base (Table S1 in Supplementary Information). Use of K₂CO₃as base and water as

solvent gave 97% coupling yield at 40 ˚C over the course of 12 h in the presence of

air (Table S1, Entry 1), whereas use of CH₃OH/H₂O (1:1) as solvent gave 98% yield

in 6 h (Table S1, Entry 2). Use of K₂CO₃as base and other solvents (such as MeOH,

C₂H₅OH, 1,4-dioxane, THF or C₂H₅OH/H₂O (1:1)) gave yields of 7-80% (Table S1,

Entries 3-7). Additionally, use of water as solvent and other common bases (such as

^tBuOK, K₃PO₄·3H₂O and NaOAc) gave yields of 52-74% (Table S1, Entries 8-10).

Under conditions similar to those of entry 1 (but the time being shorten from 12 h to 6

h), 10 mol% of TBAB were added to afford 99% yield (Table S1, Entry 11). The

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result indicates that the reaction could be effectively improved in the presence of

TBAB. It has been known that the addition of TBAB as a co-catalyst can enhance the

activity of many catalytic systems. Besides, TBAB is able to activate and stabilize

palladium(0) species generated in situ, which is believed to be the real catalytically

active species.¹⁹Based on this extensive screening, water and K₂CO₃were found to

be the efficient solvent and base in the presence of 10 mol% TBAB with a catalyst

loading of 0.1 mol% of complex 1 at 40 ˚C in the air, and these conditions proved

ideal. Reactions were monitored by GC at appropriate time intervals. Althouth the

good yield can be also obtained using CH₃OH/H₂O (1:1) as solvent, we chose water

as solvent for the purpose of environmental benignancy.

We attempted cross-coupling reactions of various aryl halides with phenylboronic

acids under the optimal reaction conditions. The catalyst is highly efficient toward the

coupling of aryl bromides (or aryl dibromides) with phenylboronic acid to afford

excellent yields of 90-98% (Table 1, 2a, 2b, 2c(1)-2e(1) and 2f-2i, 2n(2)). The

catalyst is also effective toward the coupling of aryl chlorides bearing

electronic-donating or -withdrawing substituents with phenylboronic acid to give

moderate to high yields of 64-89% (Table 1, 2c(2)-2e(2) and 2j-2m). Coupling

reaction of chlorobenzene with phenylboronic acid gave relatively poor yields of 43%

(Table 1, 2n(1)). The coupling of iodobenzene with phenylboronic acid gave

quantitative yield (Table 1, 2n(3)).

In order to further study the influence of the ligand on catalysis, the control

experiment was performed using 4-bromotoluene and phenylboronic acid as

substrates, in which Pd(OAc)₂(or PdCl₂) and TBAB were added in the absence of the

ligand with the optimized conditions. Only 39% for Pd(OAc)₂and 32% for PdCl₂

desired coupling product were obtained, and these values are smaller than 99% yield

in the case of complex 1 as catalyst. This result highlights the preponderant catalytic

role of the ligand in these aqueous and aerobic coupling reactions.

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Table 1 Suzuki-Miyaura Reaction of aryl halides with phenylboronic acid catalyzed

by complex 1^[a]

0.1 mol% cat.

K₂CO₃, TBAB

Ar

(HO)₂B

Ar

X

H₂O, 40

2a-2n

X = Cl, Br, I

H₃CO

H₂N

H₃CO

2c

2a

2b

X = Br, 10 h, 90%

(1) X = Br, 6 h, 98%

(2) X = Cl, 8 h, 81%

X = Br, 10 h, 92%

O₂N

H₃COC

2d

2e

(1) X = Br, 6 h, 91%

(2) X = Cl, 6 h, 85%

(1) X = Br, 6 h, 96%

(2) X = Cl, 8 h, 89%

2f

X = Br, 6 h, 95%

2g

2h

2i

X = Br, 8 h, 93%

X = Br, 10 h, 95%

X = Br, 12 h, 92%

O₂N

NO₂

CHO

2j

2k

2l

X = Cl, 8 h, 81%

X = Cl, 10 h, 74%

X = Cl, 8 h, 83%

CH₃

2n

2m

(1) X = Cl, 8 h, 43%

(2) X = Br, 8 h, 94%

(3) X = I, 8 h, 100%

X = Cl, 6 h, 64%

[a] Reaction conditions: Aryl halide (0.5 mmol), phenylboronic acid (0.6 mmol), K₂CO₃(1.2

mmol), complex 1 (0.1 mol %), TBAB (10 mol %), H₂O (6 mL) at 40 ˚C in the air.

[b] Reactions were monitored by TLC.

[c] Isolated yield using aryl halide as a standard.

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The above results show that complex 1 has high stability to both air and water,

while also demonstrating excellent tolerance of various functional groups.

Additionally, complex 1 can resolve partly in aqueous medium due to the existence of

hydrophilic groups (such as oxygen atom and nitrogen atom). These qualities make 1

a valuable precatalyst for thermally sensitive substrates. This provides an effective

phosphine-free catalyst system for facile coupling of aryl halides with aryl boronic

acids in water under mild and aerobic conditions. These favorable reaction conditions

of Suzuki-Miyaura reaction are important toward ensuring environmental

responsibility while also reducing costs for fine chemical and pharmaceutical

industries.

Catalytic activity of NHC-Pd(II) complex 1 in the Heck-Mizoroki reaction. The

Heck reaction has evolved signiﬁcantly from its original mode as the arylation of

oleﬁns with aryl iodides, and it is a powerful tool for the coupling of less reactive

bromides, chlorides and pseudo-halides such as triﬂates, tosylates, mesylates, and aryl

diazonium salts.²⁰We chose bromobenzene and styrene as the coupling partners to

test the solvent and base effects in the air (Table S2). Using 1,4-dioxane as solvent

and NaOAc as base gave 92% coupling yields at 120 ˚C over the course of 24 h in the

presence of air (Table S2, Entry 1), whereas using other bases (such as K₂CO₃,

t

K₃PO₄·3H₂O, Et₃N, BuOK, KOH, NaHCO₃and Cs₂CO₃) gave the yields of 15-83%

(Table S2, Entries 2-8). Using NaOAc as base and other solvents (such as H₂O, DMF,

CH₃CN, THF, DME or DMSO) gave the yields from trace to 61% (Table S2, Entries

9-15). Under conditions similar to those of entry 1 (but temperature being dropped

from 120 ˚C to 100 ˚C), 10 mol% of PEG-400 was added to give coupling yields of

95% (Table S2, Entry 16). This result shows that the reaction could be dramatically

improved in the presence of PEG-400. It has been known that the addition of

PEG-400 as a co-catalyst can enhance the activity of many catalytic systems.²¹Based

on this extensive screening, 1,4-dioxane and NaOAc were found to be the efficient

solvent and base in the presence of 10 mol% of PEG-400 as co-catalyst with a catalyst

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loading of 0.1 mol% of complex 1 at 100 ˚C in the air, thus giving the optimal

coupling result.

We attempted cross-coupling reactions of various aryl halides with styrene under

the optimal reaction conditions. The cross-coupling of various aryl bromides with

styrene gave good to excellent yields of 80-98% (Table 2，3a, 3b, 3c(1)-3e(1) and

3f-3i). The coupling of 4-chloronitrobenzene or 2-chloronitrobenzene with styrene

gave good yields of 85% or 80%, respectively (Table 2, 3d(2) and 3j). The coupling

of

4-chloroaniline,

4-chloroacetophenone,

2,4-dinitrochlorobenzene,

2-chlorobenzaldehyde or chlorobenzene with styrene gave relatively poor yields of

20-56% (Table 2, 3c(2), 3e(2), 3k, 3l and 3n(1)). And 2-chlorotoluene even gave trace

coupling yield (Table 3: 3m). The catalyst is highly efficient in the coupling of

iodobenzene with styrene to give almost quantitative yield (Table 2, 3n(3)). Notably,

there is high selectivity for the trans product in this reaction system. Hardly any cis

products could be detected after the reaction. The trans isomer appears to be the

dominant conformation assumed during the course of these reactions.

Considering all these facts, complex 1 is an effective catalyst for Heck-Mizoroki

reaction. In order to further study the influence of the ligand on catalysis, the control

experiment was performed using bromobenzene and styrene as substrates, in which

Pd(OAc)₂(or PdCl₂) and PEG-400 were added in the absence of the ligand with the

optimized conditions. Only 26% for Pd(OAc)₂and 21% for PdCl₂coupling product

were obtained, and these values are smaller than 95% yield in the case of complex 1

as catalyst. This result highlights the preponderant catalytic role of the ligand in the

coupling reactions.

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Table 2 Heck-Mizoroki reaction of aryl halides with styrene catalyzed by complex 1^[a]

0.1 mol% cat.

NaOAc, PEG-400

Ar

X

Ar

1,4-dioxane, 100

3a-3n

X = Cl, Br, I

H₃CO

H₂N

H₃CO

3c

3a

3b

(1) X = Br, 24 h, 85%

(2) X = Cl, 24 h, 20%

X = Br, 20 h, 98%

X = Br, 24 h, 91%

O₂N

H₃COC

3d

3e

3f

(1) X = Br, 24 h, 93%

(2) X = Cl, 24 h, 85%

(1) X = Br, 24 h, 95%

(2) X = Cl, 24 h, 38%

X = Br, 24 h, 82%

3g

3h

3i

X = Br, 24 h, 80%

X = Br, 24 h, 85%

X = Br, 20 h, 86%

NO₂

CHO

NO₂

O₂N

3j

3k

3l

X = Cl, 24 h, 80%

X = Cl, 24 h, 68%

X = Cl, 24 h, 56%

CH₃

3m

3n

X = Cl, 24 h, trace

(1) X = Cl, 24 h, 39%

(2) X = Br, 24 h, 95%

(3) X = I, 24 h, 99%

[a] Reaction conditions: aryl halide (0.5 mmol), styrene (0.6 mmol), K₂CO₃(1.2 mmol), complex

1 (0.1 mol%), PEG-400 (10 mol%), 1,4-dioxane (6 mL), 100 ˚C in air.

[b] Monitored by TLC.

[c] Isolated yield using aryl halide as a standard.

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Catalytic activity of NHC-Pd(II) complex 1 in the Sonogashira reaction. In order

to prove that complex 1 would be a good precatalyst for the Sonogashira reaction, we

chose the cross-coupling reaction of 4-bromoanisole with phenylacetylene as a model

reaction to test the solvent, base and catalyst activity in the air.²²As illustrated in

Table S3, using K₃PO₄·3H₂O as base and 1,4-dioxane/H₂O (1:1) as solvent gave 79%

yield at 100 ˚C over the course of 24 h in the presence of air (Table S3, entry 1). In the

same conditions, when 10 mol% of TBAB was added, the yield rose to 93% (Table S3,

entry 2). Using Cs₂CO₃as base and other solvents (such as 1,4-dioxane, DMF, DMSO,

CH₃CN, H₂O, THF, DMF/H₂O (1:1) or 1,4-dioxane/H₂O (1:1)) gave yields from trace

to 69% (Table S3, Entries 3-10). Using 1,4-dioxane/H₂O (1:1) as solvent in the

presence of 10 mol% of PPh₃or TBAB, the different bases (such as K₂CO₃, KOH,

NaOAc, Et₃N or NaHCO₃) were investigated to give the yields of 70-91% (Table S3,

Entries 11-17). Based on this extensive screening, 1,4-dioxane/H₂O (1:1) and

K₃PO₄·3H₂O were proved to be the efficient solvent and base in the presence of 10

mol% of TBAB with a catalyst loading of 0.1 mol% of complex 1, thus giving the

optimal coupling result.

We attempted the cross-coupling reactions of various aryl halides with

phenylacetylene under the optimized conditions. Various aryl bromides could be

easily coupled to phenylacetylene to give the good to excellent yields of 86-96%

(Table 3, 4a, 4b, 4c(1)-4e(1), 4f-4i and 4n(2)). Coupling reactions of 4-chloroaniline,

4-chloronitrobenzene or 2-chloronitrobenzene with phenylacetylene gave good yields

of 76-83% (Table 3, 4c(2), 4d(2) and 4j). While the couplings of

4-chloroacetophenone,

2,4-dinitrochlorobenzene,

2-chlorobenzaldehyde,

2-chlorotoluene or chlorobenzene with phenylboronic acid gave poor yields of

10-45% (Table 3, 4e(2), 4k-4m and 4n(1)). The catalyst was highly efficient in the

coupling of iodobenzene with phenylacetylene to give almost quantitative yield (Table

3, 4n(3)). The above results show complex 1 is an effective precatalyst for

Sonogashira reaction. Additionally, a control experiment was also performed using

4-bromoanisole with phenylacetylene as substrates, in which Pd(OAc)₂(or PdCl₂) and

TBAB were added without the ligand with the optimized conditions. Only 31% for

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Pd(OAc)₂and 29% for PdCl₂coupling product were obtained, and these values are

smaller than 93% yield in the case of complex 1 as catalyst. This result highlights the

preponderant catalytic role in the coupling reaction.

Table 3 Sonogashira reaction of aryl halides with phenylacetylene catalyzed by

complex 1^[a]

0.1 mol% cat.

K₃PO₄·3H₂O, TBAB

Ar

X = Cl, Br, I

H₃CO

X

Ar

1,4-dioxane/H₂O (1:1), 100

4a-4n

H₂N

H₃CO

4c

4a

4b

(1) X = Br, 24 h, 96%

(2) X = Cl, 30 h, 78%

X = Br, 24 h, 93%

X = Br, 24 h, 90%

O₂N

H₃COC

4d

4e

(1) X = Br, 24 h, 89%

(2) X = Cl, 30 h, 83%

(1) X = Br, 24 h, 95%

(2) X = Cl, 30 h, 34%

4f

X = Br, 24 h, 91%

4i

4g

4h

X = Br, 24 h, 92%

X = Br, 30 h, 86%

X = Br, 24 h, 93%

O₂N

NO₂

CHO

4j

4k

4l

X = Cl, 30 h, 76%

X = Cl, 30 h, 45%

X = Cl, 30 h, 10%

CH₃

4n

4m

(1) X = Cl, 24 h, 43%

(2) X = Br, 24 h, 87%

(3) X = I, 24 h, 99%

X = Cl, 30 h, 11%

[a] Reaction conditions: aryl halide (0.5 mmol), phenylacetylene (0.6 mmol), base (1.2 mmol),

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complex 1 (0.1 mol%), TBAB (10 mol%), 1,4-dioxane/H₂O (6 mL), 100 ˚C in air.

[b] Monitored by TLC.

[c] Isolated yield using aryl halide as a standard.

It is noteworthy that the good catalytic activity of NHC-Pd(II) complex 1 can be

ascribed to the strong donating ability of NHC and the weak coordination of nitrogen.

The weak coordinating abilities of nitrogen atoms allow them to readily dissociate

from metal centers thus creating catalytically active species with vacant coordination

sites. The strong Pd-C bond presumably facilitates oxidative addition of aryl halogens

to the metal. The dynamic dissociation and association of nitrogen atoms allows the

approach of substrate to the metal center, and the substituents on the benzimidazole

rings favor stabilization of the active metal species after reductive elimination of the

products.

Additionally, the reaction conditions (catalyst loading, temperature, solvents and

bases) between the present paper and literatures are compared as shown in Table S4.

The catalyst loading of complex 1 is all 0.1% in three types of C-C coupling reactions,

which is same as the most case of reported data in Suzuki-Miyaura^{22(b), 23}and

Heck-Mizoroki reactions,^{22(b), 23(k), 23(l), 24}whereas this value is smaller than reported

data (1-4%) in Sonogashira reaction.^{22, 23m, 25}The reaction temperature for these

reactions in the present paper is in the middle of reported data (in three types of C-C

coupling reactions: 40 ˚C, 100 ˚C and 100 ˚C, respectively, in the present paper;

25-140 ˚C, 80-150 ˚C and 80-120 ˚C, respectively, in literatures). In the present paper,

pure water, organic solvent and water/organic solvent are used, respectively, in three

types of C-C coupling reactions, and the similar cases can be also found in reported

results. In the use of bases, we selected inexpensive K₂CO₃, NaOAc and K₃PO₄·3H₂O,

respectively, in three types of C-C coupling reactions, and the similar bases are also

used in literatures.

On the whole, the results obtained using tetradentate Pd(II) complex 1 as catalyst in

most Suzuki-Miyaura, Heck-Mizoroki and Sonogashira reactions are similar to or

slightly better than those of related NHC Pd(II) complexes. By the analysis and

comparison of complex 1 with other NHC metallic catalysts published,^26-28we can

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know that the catalytic activity of a catalyst is related to a number of factors such as

the steric hindrance and electronic effects of different substituents around Pd, as well

as the stability and solubility of complexes. Besides, the reaction conditions are also

unneglected factors.

Conclusion

In summary, bis-benzimidazolium salt [(S)-LH₂]·(PF₆)₂and its NHC palladium(II)

complex 1 have been prepared and characterized. In the crystal structure of complex 1,

one 15-membered ring and two 6-membered rings are contained. Additionally, the

catalytic activity of the complex 1 in three types of C-C coupling reactions

(Suzuki-Miyaura, Heck-Mizoroki and Sonogashira reactions) was studied. The results

show that complex 1 can smoothly catalyze most Suzuki-Miyaura reactions of various

aryl halides (iodide, bromide and chloride) with arylboronic acids using water as

solvent and K₂CO₃as base in the air at 40 ˚C to afford moderate to high yields.

Likewise, most Heck-Mizoroki or Sonogashira reactions can also be performed

efficiently at 100 ˚C in the air.

Compared with reported NHC Pd(II) complexes, complex 1 shows similar or

slightly better catalytic activity in these coupling reactions. Complex 1 may be a

valuable precatalyst for the preparation of diaryl derivatives, styrene derivatives or

phenylacetylene derivatives. Further studies of new organometallic complexes from

precursor [(S)-LH₂]·(PF₆)₂as well as analogous ligands are underway.

Experimental section

General procedures. N-Picolyl-benzimidazole was prepared according to the

literature method.²⁹All the reagents for synthesis and analyses were of analytical

grade and used without further purification. Melting points were determined on an

1

Digital Vision MP Instrument. H and ¹³C NMR spectra were recorded at 400 MHz

and 100 MHz, respectively. Chemical shifts, δ, are reported in ppm relative to the

1

internal standard TMS for both H and ¹³C NMR. J values are given in Hz. The

elemental analyses of all compounds were obtained from the powder compounds

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recrystallised. GC analyses were carried out with a capillary column (CBPī or CBP-5,

0.25 mm i.d × 40 m).

Preparation of (S)-2,2’-di(2’’-hydroxyethoxy)-1,1’-binaphthyl. A DMF (100 mL)

suspension of (S)-2,2’-dihydroxy-1,1’-binaphthyl (5.000 g, 17.4 mmol) and potassium

carbonate (9.646 g, 69.9 mmol) was stirred for 1 h at 50 ˚C. Then 2-chloroethanol

(5.628 g, 69.9 mmol) was added dropwise, and stirred continually for 24 h under

refluxing. The solvent was removed under vacuum, and 500 mL water was added.

The solution was extracted with CH₂Cl₂(3 × 30 mL) and the extracting solution was

dried over anhydrous MgSO₄. After removing CH₂Cl₂, a pale yellow oil was obtained,

and the crude product was recrystallized with MeOH to give

(S)-2,2’-di(2’’-hydroxyethoxy)-1,1’-binaphthyl as a white powder. Yield: 6.008 g

(92%). M.p.: 112-114 ˚C. Anal. Calcd for C₂₄H₂₂O₄: C, 76.98; H, 5.92%. Found: C,

1

76.63; H, 5.88%. H NMR (400 MHz, CDCl₃): δ 8.02 (d, J = 8.8 Hz, 2H, ArH), 7.93

(d, J = 8.0 Hz, 2H, ArH), 7.48 (d, J = 9.2 Hz, 2H, ArH), 7.41 (t, J = 7.4 Hz, 2H, ArH),

7.30 (t, J = 7.4 Hz, 2H, ArH), 7.17 (d, J = 8.4 Hz, 2H, ArH), 4.28 (m, 2H, CH₂), 4.08

13

(m, 2H, CH₂), 3.66 (m, 4H, CH₂), 2.31 (s, 2H, OH). C NMR (100 MHz, CDCl₃): δ

153.5 (PhC), 133.8 (PhC), 129.8 (PhC), 129.6 (PhC), 128.1 (PhC), 126.7 (PhC), 125.2

(PhC), 124.2 (PhC), 120.3 (PhC), 115.9 (PhC), 71.7 (OCH₂CH₂), 61.2 (OCH₂CH₂).

Preparation of (S)-2,2’-di(2’’-chloroethoxy)-1,1’-binaphthyl. To a chloroform (100

mL) solution of (S)-2,2’-di(2’’-hydroxyethoxy)-1,1’-binaphthyl (5.000 g, 13.4 mmol)

and pyridine (5.288 g, 66.8 mmol), thionyl chloride (7.947 g, 66.8 mmol) was added

dropwise at room temperature within 1 h. Then the solution was stirred continually for

12 h at 70 ˚C. The mixture was cooled and washed with water (3 × 100 mL). The

organic layer was dried over anhydrous MgSO₄. After removing chloroform,

(S)-2,2’-di(2’’-chloroethoxy)-1,1’-binaphthyl was obtained as a pale yellow powder.

Yield: 3.571 g (65%). M.p.: 101-103 ˚C. Anal. Calcd for C₂₄H₂₀O₂Cl₂: C, 70.08; H,

4.90%. Found: C, 70.37; H, 5.14%. ¹H NMR (400 MHz, CDCl₃): δ 7.97 (d, J = 8.8 Hz,

2H, ArH), 7.88 (d, J = 8.4 Hz, 2H, ArH), 7.43 (d, J = 8.8 Hz, 2H, ArH), 7.37 (t, J =

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7.4 Hz, 2H, ArH), 7.25 (t, J = 7.6 Hz, 2H, ArH), 7.13 (d, J = 8.4 Hz, 2H, ArH),

4.22(m, 4H, CH₂), 3.40 (m, 4H, CH₂). ¹³C NMR (100 MHz, CDCl₃): δ 153.6 (PhC),

147.9 (PhC), 134.0 (PhC), 129.6 (PhC), 127.9 (PhC), 126.5 (PhC), 125.4 (PhC), 124.1

(PhC), 121.0 (PhC), 116.3 (PhC), 70.1 (OCH₂CH₂), 41.7 (OCH₂CH₂).

Preparation of [(S)-LH₂]·(PF₆)₂.

A

toluene (35 mL) solution of

g, 6.1 mmol) and

(S)-2,2’-di(2’’-chloroethoxy)-1,1’-binaphthyl (1.000 g, 2.4 mmol) was stirred for 7

days under refluxing, and brown oil of

N-picolyl-benzimidazole

(1.274

a

(S)-2,2’-bis[2’’-(N-picolyl-benzimidazoliumyl)ethoxy]-1,1’-binaphthyl chloride was

obtained after the solvent was removed via a rotary evaporator. Then NH₄PF₆(0.938 g,

5.7 mmol) was added to

a

methanol solution (100 ml) of

chloride

(S)-2,2’-bis[2’’-(N-picolyl-benzimidazoliumyl)ethoxy]-1,1’-binaphthyl

(2.016 g, 2.4 mmol) with stirring for 3 days, and a white precipitate was formed. The

precipitate was collected by filtration and washed with a small portion of methanol to

give [(S)-LH₂]·(PF₆)₂. Yield: 1.734 g (68%). M.p.: 148-150 ˚C. Anal. Calcd for

C₅₀H₄₂F₁₂N₆O₂P₂: C, 57.25; H, 4.03; N, 8.01%. Found: C, 57.62; H, 4.41; N,

8.35%.¹H NMR (400 MHz, DMSO-d₆): δ 9.43 (s, 2H, 2-bimiH), 8.43 (d, J = 4.4 Hz,

2H, ArH), 7.98 (d, J = 8.8 Hz, 2H, ArH), 7.90 (q, J = 2.6 Hz, 4H, ArH), 7.77 (d, J =

8.4 Hz, 2H, ArH), 7.63 (d, J = 8.0 Hz, 2H, ArH), 7.50 (t, J = 7.8 Hz, 2H, ArH), 7.43

(d, J = 9.6 Hz, 2H, ArH), 7.37 (q, J = 4.0 Hz, 2H, ArH), 7.28 (t, J = 8.4 Hz, 4H, ArH),

7.21 (t, J = 7.8 Hz, 2H, ArH), 6.99 (m, J = 5.0 Hz, 2H, ArH), 6.58 (d, J = 8.4 Hz, 2H,

ArH), 5.68 (m, 3H, CH₂), 4,63 (t, J = 4.6 Hz, 3H, CH₂), 4.38 (t, J = 4.4 Hz, 3H, CH₂),

4.15 (q, J = 5.2 Hz, 3H, CH₂). ¹³C NMR (100 MHz, DMSO-d₆): δ 153.1 (ArC), 149.9

(ArC), 142.9 (ArC), 138.0 (ArC), 132.9 (bimi-NCN), 131.1 (ArC), 130.9 (ArC),

130.0 (ArC), 129.3 (ArC), 128.4 (ArC), 126.8 (ArC), 126.7 (ArC), 126.6 (ArC), 126.4

(ArC), 125.5 (ArC), 125.0 (ArC), 124.2 (ArC), 123.2 (ArC), 119.6 (ArC), 116.3

(ArC), 114.0 (ArC), 113.0 (ArC), 67.8 (OCH₂CH₂), 51.2 (OCH₂CH₂), 49.0 (CH₂),

47.2 (CH₂) (bimi = benzimidazole).

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Prepartion of [(S)-LPd](PF₆)₂(1). An acetonitrile/ethylene glycol (20 mL, v:v = 1:1)

suspension of K₂CO₃(0.179 g, 1.3 mmol), [(S)-LH₂]·(PF₆)₂(0.209 g, 0.2 mmol) and

Pd(OAc)₂(0.042 g, 0.2 mmol) was stirred for 24 h under 60 ˚C. The resulting solution

was filtered and concentrated to 10 mL, and Et₂O (5 mL) was added to precipitate 1

as a yellow powder. Yield: 0.057 g (25%). M.p.: 280-282 ˚C. Anal. Calcd for

C₅₀H₄₀F₁₂N₆O₂P₂Pd: C, 52.07; H, 3.49; N, 7.29%. Found: C, 52.34; H, 3.63; N,

1

7.52%. H NMR (400 MHz, DMSO-d₆): δ 8.24 (m, 7H, PhH), 7.98 (m, 4H, PhH),

7.80 (d, J = 8.4 Hz, 2H, PhH), 7.65 (t, J = 6.6 Hz, 2H, PhH), 7.44 (m, 7H, PhH), 4.90

(m, 2H, CH₂), 4.52 (m, 2H, CH₂), 4.24 (m, 2H, CH₂), 2.89 (s, 3H, CH₂), 2.73 (s, 3H,

CH₂). ¹³C NMR (100 MHz, DMSO-d₆): δ 179.2 (bimi-NCN), 154.5 (PhC), 153.8

(PhC), 153.2 (PhC), 147.5 (PhC), 135.2 (PhC), 133.6 (PhC), 133.0 (PhC), 129.6

(PhC), 129.5 (PhC), 126.6 (PhC), 126.4 (PhC), 125.8 (PhC), 125.2 (PhC), 124.7

(PhC), 124.6 (CH₃), 112.4 (PhC), 111.7 (PhC), 68.9 (CH₂), 63.3 (CH₂), 62.9 (CH₂).

General procedure for the Suzuki-Miyaura reaction. In a typical reaction, aryl

halide (0.5 mmol), arylboronic acid (0.6 mmol), base (1.2 mmol), NHC-Pd(II)

complex 1 (0.1 mol%) and TBAB (10 mol%) in solvent (6 mL) were stirred at 40 ˚C

in air. After the desired reaction time, the reaction was stopped, and water (20 mL)

was added to the reaction mixture. The mixture was extracted by n-hexane (10 mL ×

3), and the organic layer was washed with water (10 mL × 3) and dried over

anhydrous MgSO₄. For the determination of GC-MS yields, the above solution was

filtered through a thin pad of silica gel (elution with CH₂Cl₂(100 mL)) and

concentrated to 5 mL. n-Tridecane (92 mg) was added as internal standard. The

isolated products were purified by column chromatography on silica gel using a

mixture of n-hexane and dichloromethane (from pure n-hexane to

n-hexane/dichloromethane = 10/1) as eluant. The isolated yields were determined

using aryl halides as standards.

General procedure for the Heck-Mizoroki reaction. A mixture of aryl halide (0.5

mmol), styrene (0.6 mmol), base (1.2 mmol), complex 1 (0.1 mol%) and PEG-400 (10

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mol%) in solvent (6 mL) was stirred at 100 ˚C or 120 ˚C in air. After the desired

reaction time, the reaction was stopped, and water (20 mL) was added to the reaction

mixture. The mixture was extracted by ethyl ether (10 mL × 3), and the organic layer

was washed with water (10 mL × 3) and dried over anhydrous MgSO₄. Then the

solution was filtered and concentrated to about 2 mL. The isolated products were

purified by column chromatography on silica gel using a mixture of n-hexane and

dichloromethane (from pure n-hexane to n-hexane/dichloromethane = 10/1) as eluant.

The isolated yields were determined using aryl halides as standards.

General procedure for the Sonogashira reaction. A mixture of aryl halide (0.5

mmol), phenylacetylene (0.6 mmol), base (1.2 mmol), complex 1 (0.1 mol%) and

TBAB (10 mol%) in solvent (6 mL) was stirred at 100 ˚C in air. After the desired

reaction time, the reaction was stopped, and water (20 mL) was added to the reaction

mixture. The mixture was extracted by diethyl ether (10 mL × 3), and the organic

layer was washed with water (10 mL × 3) and dried over anhydrous MgSO₄. Then the

solution was filtered and concentrated to about 2 mL. The isolated products were

purified by column chromatography on silica gel using a mixture of n-hexane and

dichloromethane (from pure n-hexane to n-hexane/dichloromethane = 10/1) as eluant.

The isolated yields were determined using aryl halides as standards

X-ray data collection and structure determinations. X-ray single-crystal

diffraction data for complexs were collected by using a Bruker Apex II CCD

diffractometer at 173(2) K for 1 with Mo-Ka radiation (λ = 0.71073 Å) by ω scan

mode. There was no evidence of crystal decay during data collection in all cases.

Semiempirical absorption corrections were applied by using SADABS and the

program SAINT was used for integration of the diffraction profiles.³⁰All structures

were solved by direct methods by using the SHELXS program of the SHELXTL

package and refined with SHELXL³¹by the full-matrix least-squares methods with

anisotropic thermal parameters for all non-hydrogen atoms on F². Hydrogen atoms

bonded to C atoms were placed geometrically and presumably solvent H atoms were

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first located in difference Fourier maps and then fixed in the calculated sites. Further

details for crystallographic data and structural analysis are listed in Table 4. Figures

were generated by using Crystal-Maker.³²

Table 4 Summary of crystallographic data complex 1

1·0.5C₂H₆O₂

Chemical

formula

Formula weight

Cryst syst

Space group

a, Å

C₅₀H₄₀F₁₂N₆O₂P₂Pd

·0.5C₂H₆O₂

1184.25

Triclinic

Pī

F(000)

1198

Cryst size, mm

θ_min, θ_max, deg

T, K

0.18 × 0.17 × 0.15

1.36, 25.01

173(2)

12.526(1)

14.488(1)

15.330(1)

98.5(1)

No. of data collected

No. of unique data

No. of refined params

Goodness-of-fit on F^{2 a}

Final R indices^b[I > 2σ(I)]

R₁

12770

b, Å

8625

c, Å

677

α, deg

1.016

β, deg

95.0(1)

γ, deg

V, Å³

115.0(1)

2456.2(3)

2

0.0397

0.1012

wR₂

Z

R indices (all data)

R₁

D_calcd, Mg m^-3

Abs coeff, mm^-1

1.601

0.0469

0.1077

0.540

wR₂

2

^aGOF = [Σw(F_o- F_c²)²/(n-p)]^1/2, where n is the number of reflection and p is the

b

2

number of parameters refined. R₁= Σ(||F_o| - |F_c||)/Σ|F_o|; wR₂= [Σ[w(F_o- F_c²)²]/

2 2 1/2

Σw(F_o) ] .

Acknowledgements

This work was financially supported by the National Natural Science Foundation of

China (No.21172172), Tianjin Natural Science Foundation (No.11JCZDJC22000) and

The Program for Innovative Research Team in University of Tianjin (TD12-5038).

Supplementary information

1

Tables and figures for complex 1, and H NMR and ¹³C NMR spectra for precursor,

complex and all coupling products in Suzuki-Miyaura, Heck-Mizoroki and

Sonogashira reactions. CCDC 1023636 contains the supplementary crystallographic

data for complex 1. These data can be obtained free of charge via

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http//www.ccdc.cam.ac.uk/conts/retrieving.html,

or

from

the

Cambridge

Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK; fax: (+44)

1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

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A table of contents entry:

NHC-Pd(II) complex 1 has been prepared. The catalytic activity of 1 in C-C coupling

reactions was studied.

Software of Graphics:

Scheme 1: Chem Draw 8.0

Fig. 1: Diamand 3.0

26

Article Doi

Synthesis, structure and catalysis of a NHC-Pd(II) complex based on a tetradentate mixed ligand

DOI: 10.1039/c5ra11089k

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c5ra11089k

Full text of DOI:10.1039/c5ra11089k

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