Palladium Complexes with Phenoxy- And Amidate-Functionalized N-Heterocyclic Carbene Ligands Based on 3-Phenylimidazo[1,5- a]pyridine: Synthesis and Catalytic Application in Mizoroki-Heck Coupling Reactions with Ortho-Substituted Aryl Chlorides

Cheng-Hau Hung, Wei-Yuan Zheng, and Hon Man Lee *

Article

Palladium Complexes with Phenoxy- and Amidate-Functionalized

N‑Heterocyclic Carbene Ligands Based on

‑Phenylimidazo[1,5‑a]pyridine: Synthesis and Catalytic Application

in Mizoroki−Heck Coupling Reactions with Ortho-Substituted Aryl

Chlorides

†

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sı Supporting Information

ABSTRACT: Mononuclear and tetranuclear palladium complexes with

functionalized “abnormal” N-heterocyclic carbene (aNHC) ligands based on

-phenylimidazo[1,5-a]pyridine were synthesized. All of the new complexes

were structurally characterized by single-crystal X-ray diﬀraction studies. The

new complexes were applied in the Mizoroki−Heck coupling reaction of aryl

chlorides with alkenes in neat n-tetrabutylammonium bromide (TBAB). The

mononuclear palladium complex with a tridentate phenoxy- and amidate-

functionalized aNHC ligand displayed activity superior to that of the palladium

complex with a bidentate amidate-functionalized aNHC ligand. The new

tetranuclear complex with the tridentate ligand displayed the best activities,

capable of the activation of deactivated aryl chlorides as substrates with a low

Pd atom loading. Even challenging sterically demanding ortho-substituted aryl

chlorides were successfully utilized as substrates. The studies revealed that the

robustness of the catalyst precursor is crucial in delivering high catalytic

activities. Also, the promising use of tetranuclear palladium complexes with functionalized aNHC ligands as the catalyst precursors in

the Mizoroki−Heck coupling reaction in neat TBAB was demonstrated.

INTRODUCTION

and palladium complexes with aNHC ligands based on

imidazo[1,2-a]pyridine were reported by us recently. On

the other hand, the isomeric imidazo[1,5-a]pyridine has been

■

Currently, N-heterocyclic carbene (NHC) ligands are widely

used in synthetic organic chemistry because of their distinctive

shown to be a versatile architecture for a stable N-heterocyclic

catalytic properties. Among the diﬀerent NHC ligands,

carbene. While a vast number of metal complexes with

imidazole-based NHC ligands have been highly popular.

Crabtree et al. had shown that imidazole-based NHCs can

have an alternative binding mode via the C4/5 atom, instead of

nNHCs based on imidazo[1,5-a]pyridine have been reported,

aNHC complexes based on the heterocyclic compound are

7b,8,10

the commonly observed C2-bound counterpart. These

rarer in the literature.

In a previous work, we had shown

variants of NHC ligands, commonly termed “abnormal

NHCs (aNHCs)”, feature less heteroatom stabilization of

that a tetranuclear palladium complex with an amidate- and

phenoxy-functionalized imidazole-based aNHC ligand (A) was

highly versatile in catalyzing the Mizoroki−Heck coupling

reaction, capable of utilizing deactivated aryl chlorides as

substrates (Figure 1). In this work, we intend to prepare

mononuclear and tetranuclear palladium complexes bearing

mutidentate ligands with aNHC moieties based on imidazo-

the carbenic carbon and have been shown to be more electron-

rich than their normal NHCs (nNHCs) and thus improved

catalytic activities of their transition-metal complexes have

b−d,5

been reported.

As a matter of fact, imidazole-based

aNHCs are a subclass of mesoionic carbenes that contain no

resonance form having all-neutral formal charges. Mesoionic

and related less heteroatom-stabilized NHC complexes are

attracting much attention, and a range of heterocyclic

compounds have been successfully utilized in the construction

of mesoionic NHC ligands such as 1,2,3-triazolylidene, 4-

Received: December 20, 2020

Published: March 3, 2021

imidazolylidene, pyrazolylidene, oxazolylidene, etc.

We have been interested in benzannulated derivatives of

imidazol-2-ylidenes (Figure S1 in the Supporting Information),

2021 American Chemical Society

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Figure 1. Tetranuclear palladium complexes bearing a functionalized aNHC derived from imidazole and imidazo[1,5-a]pyridine.

[

1,5-a]pyridine and investigate their catalytic potentials in the

ca. 10 ppm. According to the HMBC spectrum, the most

downﬁeld, relatively sharp signal at 10.34 ppm was attributable

to the NCHN on the imidazole ring. This proton is

signiﬁcantly downﬁeld in comparison with those in 1a,b

because the acidic proton is situated between two electron

negative atoms, instead of only one in the cases of 1a,b. The

other two broad signals at 10.13 and 10.17 ppm were

attributed to the NH and OH protons, respectively.

Mizoroki−Heck coupling reaction. Their catalytic activities

would be compared with those catalyzed by complex A and a

similar palladium complex with a benzimidazole-based nNHC

ligand. To our delight, the new tetranuclear palladium complex

was also highly active in catalyzing the Mizoroki−Heck

coupling reaction. Remarkably, even sterically hindered aryl

chlorides were successfully applied as substrates.

Preparation of Palladium Complexes. The preparation

of new palladium carbene complexes is shown in Scheme 2.

RESULTS AND DISCUSSION

■

Preparation of Ligand Precursors. The preparation of

the ligand precursors is shown in Scheme 1. The abnormal

The reaction of ligand precursor 1a, Pd(OAc) , and pyridine in

the presence of NaOAc as a base in DMF at 50 °C produced

pure palladium complex 2a with a bidentate aNHC/amidate

ligand in a good yield of 77%. The disappearance of the

downﬁeld NH proton signal suggested the successful

coordination of the ligand around the palladium center.

Interestingly, the use of the amidate-functionalized imidazo-

Scheme 1. Synthesis of Ligand Precursors

[

1,5-a]pyridine precursor 1a led to the formation of the

palladium aNHC complex 2a, but a similar amidate-function-

alized imidazo[1,2-a]pyridine precursor reacted with

palladium(II) ion, resulting in the deprotonation of the

methylene and NH protons and the formation of a

palladalactam instead.

Under reaction conditions similar to those for 2a, ligand

precursor 1b reacted with Pd(OAc) to produce the palladium

aNHC complex 2b with a tridentate CNO ligand scaﬀold in a

similar yield of 80%. The disappearance of the two downﬁeld

OH and NH signals suggested the successful chelation of the

tridentate ligand. Its isomeric palladium complex 2b′ with a

normal NHC ligand was obtained in 74% yield from the ligand

precursor 1b′ using pyridine as the solvent. Again, the

disappearance of the three downﬁeld signals due to NH,

OH, and NCHN protons at ca. 10 ppm unequivocally proved

the successful chelation of the tridentate ligand. Notably, the

use of palladium acetate as the metal precursor was essential

NHC ligand precursors 1a,b were derived from 3-

phenylimidazo[1,5-a]pyridine, whereas the normal NHC

ligand precursor 1b′ was prepared from its isomeric analogue,

-phenyl-1H-benzo[d]imidazole. The ionic products 1 were

prepared readily by a quanternization reaction of the

heterocyclic compound with N-aryl 2-chloroacetamide in

DMF at 100 °C, producing the products in good yields

(

72−77%). The downﬁeld signal at 10.94 ppm was assignable

for the formation of pure compounds; the use of PdCl

gave a complex mixture with an appreciable amount of trans-

PdCl (py) as a side product.

instead

to the NH proton in 1a. The singlet at 8.53 ppm was attributed

to the proton on the imidazole ring. According to the HMBC

spectrum of 1b, the two downﬁeld signals at 9.83 and 10.09

ppm are due to the NH and OH protons, respectively. The

proton on the imidazole ring was observed at 8.50 ppm, which

It has been shown that when a lone pair donor, such as

pyridine, is absent in the complexation reaction between a

tridentate CNO ligand and palladium(II) ion, the phenoxyl

oxygen can switch to a bridging coordination mode, delivering

a lone pair to an adjacent palladium center and thus forming a

was similar to that observed for 1a. In the H spectrum of

NHC precursor 1b′, three downﬁeld signals were observed at

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Scheme 2. Synthesis of Palladium Complexes

tetranuclear palladium complex. In the preparation of 2b,

pyridine was added, which ligated around the palladium center.

In the absence of pyridine, compound 1b reacted with PdCl₂

in DMF, aﬀording the tetranuclear palladium aNHC complex

were obtained. The coordinated carbon signals for the

palladium aNHC complexes 2a,b were observed at 126.9

and 131.8 ppm, whereas the corresponding signal for the

palladium NHC signal in 2b′ was much more downﬁeld at

169.4 ppm. The carbenic signal for the tetranuclear palladium

aNHC complex was identiﬁed at 129.5 ppm, which was

comparable to those in mononuclear complexes 2a,b.

bt, featuring a tridentate CNO ligand (see Scheme 2b).

Interestingly, the two hydrogen atoms on the methylene

group in 2a,b and 2b′ appeared as singlets at ca. 5.15 ppm,

indicating that a fast ﬂipping of the six-membered chelate ring

occurred in solution such that the two methylene protons

became equivalent. In sharp contrast, the structure of 2bt is

rigid in structure, as evidenced by the stereochemical

nonequivalence of the two methylene protons, which led to

two distinct doublets at 4.77 and 5.28 ppm with a geminal

coupling constant of 21 Hz. The rigidity of 2bt can be

explained by the energetically unfavorable ring ﬂipping in the

closely spaced tridentate ligands in the tetranuclear complex

Structural Analysis. All of the new complexes have been

successfully analyzed by single-crystal X-ray diﬀraction studies

(Figures 2−5). The Pd atom in 2a adopts an almost ideal

square coordination geometry with the four bond angles

around the palladium center close to the ideal 90° (87.5−

92.6°). The τ and τ′ parameters that describe the extent of

distortion around a four-coordinate complex (τ = 0 for a

perfect square-pyramidal geometry and τ = 1 for a perfect

tetrahedral geometry) are indeed very close to zero (0.02 and

0.01, respectively). The pyridyl nitrogen is trans to the carbenic

carbon, and the amidate nitrogen is trans to the Cl atom. In

contrast, the coordination environment around the Pd atoms

(

see Figure 5).

To unambiguously locate the carbenic carbon signals in the

C NMR spectra of the new complexes, their HMBC spectra

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9.18(6); N4 −Pd1 −N2, 90.79(8); N2 −Pd1 −C5, 87.47(9).

Article

b,b′, respectively. The τ and τ′ parameters for 2b,b′ are

larger. The values for 2b are 0.10 and 0.09, respectively,

whereas those for 2b′ are 0.12 and 0.11, respectively. Also due

to the tridentate chelation of the ligand in 2b,b′, the pyridine

ligand is trans to the amidate nitrogen, instead of trans to the

carbene moiety in 2a. The Pd−C bonds in 2a,b are almost

identical (ca. 1.964 Å). However, both the Pd−N(amidate)

and Pd−N(pyridine) bonds in 2a (2.027(2) and 2.1083(19)

Å, respectively) are longer than those in 2b (1.9983(13) and

2.0399(13) Å, respectively). Despite the generally accepted

higher electron-donating property of the aNHC moiety, the

Pd−carbene bond of 1.9637(15) Å in the aNHC complex 2b

is marginally longer than that in its isomeric nNHC complex

Figure 2. Molecular structure of 2a with 50% probability ellipsoids.

Hydrogen atoms are omitted for clarity. Selected bond distances (Å)

and angles (deg): Pd1−C5, 1.964(2); Pd1−Cl1, 2.3151(6); Pd1−N4,

b′, which has a bond length of 1.958(4) Å. The other bond

distances around the Pd atoms are also similar in the two

isomeric complexes.

.1083(19); Pd1−N2, 2.027(2); C5−Pd1−N4, 178.09(9); N2−

Pd1−Cl1, 179.57(6); C5−Pd1−Cl1, 92.57(7); Cl1−Pd1−N4,

Complex 2bt is a tetranuclear palladium complex, as

conﬁrmed by the X-ray analysis. In the complex, each

palladium center is chelated by a tridentate CNO ligand.

The coordinating oxygen atom forms a μ-bridge linking with

an adjacent palladium center. The fourth coordination site is

completed by another μ-oxygen atom from another palladium

center. The Pd atoms adopt a slightly distorted square planar

coordination environment with τ and τ′ parameters in the

ranges of 0.07−0.11 and 0.06−0.10, respectively. The Pd−

carbene bonds in the tetranuclear complex (1.951−1.964 Å)

are comparable to those in the mononuclear complexes

(

1.958−1.964 Å). The Pd−C bonds in 2bt are longer than

those observed in the previously reported tetranuclear

palladium aNHC complex A (ca. 1.932 Å). Nevertheless,

these Pd−carbene bonds from amidate-functionalized aNHC

ligands are short in comparison with the average Pd−carbene

bond distance of 1.99(3) Å in reported Pd−aNHC complex-

Figure 3. Molecular structure of 2b with 50% probability ellipsoids.

Hydrogen atoms are omitted for clarity. Selected bond distances (Å)

and angles (deg): Pd1−C2, 1.9637(15); Pd1−N4, 2.0399(13); Pd1−

O2, 2.0443(11); Pd1−N1, 1.9983(13); C2−Pd1−O2, 170.57(5);

N1−Pd1−N4, 174.77(5); C2−Pd1−N4, 92.23(6); N4−Pd1−O2,

es. In the tetranuclear complex, the Pd1 and Pd2 atoms make

a close contact with a nonbonding distance of 3.2122(18) Å,

which is smaller than the sum of the van der Waals radii of Pd

atoms (3.26 Å). This nonbonding distance was, however,

92.44(5); O2 −Pd1 −N1, 83.61(5); N1 −Pd1 −C2, 92.18(6).

signiﬁcantly longer than that in the reported tetranuclear

complex A (3.1759(11) Å). In contrast, the Pd(3) and Pd(4)

do not interact with each other, with a longer noninteracting

distance of 3.275 Å.

Catalytic Reactions. The catalytic activities of the new

palladium complexes in Mizoroki−Heck coupling reaction in

neat tetrabutylammonium bromide (TBAB) were investigated.

The reaction between 4-chloroacetophenone and styrene was

chosen as the benchmark reaction, and the reported reaction

conditions were followed. Initially, the new palladium

complexes were screened (Table 1). Entries 1 and 2 showed

that under the same monopalladium loading of 0.2 mol % (i.e.,

a catalyst loading of 0.05 mol % for 2bt since it is composed of

four Pd atoms), the tridentate complex 2b delivered a

quantitative yield of the coupled product, in comparison to

the yield of 88% from the bidentate complex 2a. The

tetranuclear complex 2bt also delivered a quantitative yield

of coupled product. Notably, the mononuclear nNHC complex

2b′ delivered a much inferior yield of 46% (entry 4); the lower

activity may be related to a poorer electron donating nature of

nNHC in comparison with that of aNHC moieties such that

the rate-determining C−Cl bond activation in 2b′ was much

slower. The monopalladium loading was then reduced to 0.1

mol %, and under such conditions, the tetranuclear complex

2bt still delivered an excellent yield of 96% (entry 6), while the

mononuclear complex 2b delivered a lower yield of 88% (entry

Figure 4. Molecular structure of 2b′ with 50% probability ellipsoids.

Hydrogen atoms are omitted for clarity. Selected bond distances (Å)

and angles (deg): Pd1−C1, 1.958(4); Pd1−N4, 2.043(3); Pd1−O1,

.047(3); Pd1−N3, 1.994(3); C1−Pd1−O1, 168.17(13); N3−Pd1−

N4, 174.24(13); C1−Pd1−N4, 94.35(14); N4−Pd1−O1, 93.82(12);

O1−Pd1−N3, 83.23(12); N3−Pd1−C1, 89.33(14).

in the isomeric pair 2b,b′ are more distorted due to the ﬁve-

membered chelate rings from the tridentate ligands, resulting

in small N−Pd−O bite angles of 83.61(5) and 83.23(12)° in

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Figure 5. (a) Molecular structure of 2bt with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Pd1−C1, 1.951(19); Pd1−N3,

.985(13); Pd1−O2, 2.041(11); Pd1−O8, 2.042(11); C1−Pd1−O2, 177.6(6); N3−Pd1−O8, 171.8(6); C1−Pd1−O8, 93.4(6); O8−Pd1−O2,

8.6(5); O2−Pd1−N3, 83.5(5); N3−Pd1−C1, 94.6(7); Pd2−C23, 1.958(19); Pd2−N6, 1.984(15); Pd2−O4, 2.054(12); Pd2−O6, 2.072(11);

C23−Pd2−O4, 174.4(6); N6−Pd2−O6, 170.4(6); C23−Pd2−O6, 95.9(7); O6−Pd2−O4, 88.6(4); O4−Pd2−N6, 82.0(6); N6−Pd2−C23,

3.6(8); Pd3−C44, 1.956(17); Pd3−N9, 1.989(14); Pd3−O6, 2.053(11); Pd3−O2, 2.051(11); C44−Pd3−O6, 177.2(6); N9−Pd3−O2,

72.6(5); C44−Pd3−O2, 93.5(6); O2−Pd3−O6, 88.9(4); O6−Pd3−N9, 83.9(5); N9−Pd3−C44, 93.8(6); Pd4−C65, 1.964(19); Pd4−N12,

.010(14); Pd4−O8, 2.069(12); Pd4−O4, 2.044(11); C65−Pd4−O8, 178.5(6); N12−Pd4−O4, 171.6(6); C65−Pd4−O4, 92.7(6); O4−Pd4−

O8, 88.7(5); O8−Pd4−N12, 83.0(6); N12−Pd4−C65, 95.7(7); Pd(1)···Pd(2), 3.2122(18).

Table 1. Screening of Catalysts and Conditions

entry

complex

Pd atom (mol %)

temp (°C)

solvent

TBAB

yield (%)

2bt

2b′

0.2

0.1

0.2

0.1

0.2

140

130

140

88 (97:3)

>99 (96:4)

>99 (95:5)

46 (98:2)

88 (96:4)

96 (95:5)

76 (97:7)

26 (92:8)

TBAB

toluene

DMF

2bt

Pd(OAc)₂

PdCl (IMes)(3-Clpy)

2bt

THF

1,4-dioxane

TBAB

72 (92:8)

>99 (93:7)

Reaction conditions unless speciﬁed otherwise: 1.4 mmol of styrene, 1 mmol of aryl halide, 1.1 mmol of NaOAc, 2 g of TBAB, 0.1−0.2 mol % of

monopalladium loading, 140 °C. NMR yield using 1,3,5-trimethoxybenzene as an internal standard. 1 mmol of styrene. Preheating of the catalytic

solution for 2 h prior to the addition of substrates.

), reﬂecting the desirable use of multinuclear palladium

complexes as catalyst precursors. Also, the yield aﬀorded by

bt was superior to those by ligandless Pd(OAc) and the

toluene, DMF, THF, and 1,4-dioxance (entries 10−13). The

fact that the catalytic reaction only proceeds in TBAB, which is

an ionic liquid and is a general stabilizer for a colloidal Pd

system, strongly indicates the involvement of Pd nanoparticles

in the catalytic pathway (vide infra). The use of ionic liquids as

solvents in Pd-catalyzed cross-coupling reactions has been

broadly used, commercially available PEPPSI complex

PdCl (IMes)(3-Clpy) (entriy 3 vs entries 7 and 8).

After catalyst screening, the reaction conditions were further

investigated. The optimal reaction temperature was at 140 °C;

performing the reaction at a lower temperature led to a drastic

reduction in the product yield (Table 1, entry 6 vs entry 9).

Neat TBAB was proven to be the best solvent, as the catalytic

reaction did not proceed at all in other organic solvents such as

well-documented, and most recently, tunable aryl alkyl ionic

liquids containing diﬀerent palladate counteranions have also

been developed. The use of a slight excess of styrene was

essential to give a quantitative yield, as the use of 1 equiv

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Table 2. Mizoroki−Heck Coupling Reactions of Aryl Halides and Styrene Derivatives

Reaction conditions: 1.4 mmol of styrene, 1 mmol of aryl halide, 1.1 mmol of NaOAc, 2 g of TBAB, 0.05 mol % of 2bt (i.e., 0.2 mol % Pd atom

loading), 140 °C. Isolated yields are given.

aﬀorded only a lower product yield of 72% instead (entry 3 vs

entry 14).

contrast, the product yields were reduced signiﬁcantly when

the activated aryl chloride was replaced by chlorobenzene, 4-

chlorotoluene, or 4-chloroanisole (entries 16−18). Entry 16

showed that 4-methoxystilbene (4h) was obtained in a poor

yield of 32% from the reaction between chlorobenzene and 4-

methoxystyrene. This compound was best obtained from the

reaction between 4-chloroanisole and styrene, which doubled

the product yield (see entry 8). Similarly, to obtain 1-methoxy-

4-[2-(4-methylphenyl)ethenyl] benzene (4m), the best

synthetic route was via the coupling reaction between 4-

chloroanisole and the less electron-releasing 4-methylstyrene,

giving a product yield of 60% (entry 14), instead of using the

highly electron releasing 4-methoxystyrene, which gave 4m in

an inferior 45% yield (entry 18). For n-butyl acrylate, a good

yield of 88% was obtained with 4-chlorobenzaldehyde (entry

19).

Then we tested the applicability of the catalyst system in

using sterically hindered substrates (Table 3). Electron-

withdrawing groups at the meta positions were eﬀectively

utilized as substrates, delivering the coupled products with

quantitative yields in 2 h (entries 1 and 2). An electron-

donating group, however, reduced the product yield

signiﬁcantly (entry 3). Most importantly, the catalyst system

based on the tetranuclear complex can tolerate a high degree of

steric bulkiness in the substrates with ortho substitutions

After the identiﬁcation of the best palladium complex and

reaction conditions for the catalytic reaction, the substrate

scope was then thoroughly investigated (Table 2). A range of

styrene derivatives could be used as substrates for the reaction.

Entries 1−4 showed that a wide range of electron-withdrawing

aryl chlorides was able to couple eﬀectively with styrene in 2−6

h, delivering quantitative yields of coupled products. However,

a slightly lower product yield of 87% was obtained when the

electron-poor 4- chlorobenzaldehyde was used (entry 5).

Electron-neutral chlorobenzene reacted with styrene in 12 h to

deliver a good product yield of 91% (entry 6). However, only

mediocre yields were obtained when electron-donating 4-

chlorotoluene and 4-chloroanisole were used as substrates

(

entries 7−8). When 1,4-dichlorobenzene was used, the

monocoupled product, (E)-4-chlorostilbene, was obtained in

2% yield (entry 9). The catalyst system also allowed the use

of various substituted styrenes as substrates (entries 10−18).

-Methylstilbene (4g) was obtained from the reaction between

-methylstyrene and chlorobenzene, in a yield of 70% (entry

3), which is higher than that of 52% from the reaction

between 4-chlorotoluene and styrene (see entry 7). A

quantitative yield was obtained in the reaction between 4-

chloroacetophenone and 4-methoxystyrene (entry 15). In

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Table 3. Mizoroki−Heck Coupling Reactions of Sterically

supported zinc oxide nanoparticle, aﬀording a yield of 95% in

Hindered Aryl Halides

15 h.

Next, we compare the catalytic performance of 2bt and the

reported complex A. In general, complex A showed a better

catalytic activity with unhindered substrates. For example, the

coupling reaction between styrene and the challenging

deactivated substrate 4-chloroanisole A aﬀorded a quantitative

yield, whereas 2bt gave a yield of 64% under the same Pd

atom loading and reaction conditions (see entry 8, Table 2).

Also, in the reaction between 4-methystyrene and 4-

chloroanisole, complex A produced a quantitative yield of

product, instead of the 60% obtained from 2bt (see entry 14,

Table 2). In contrast, 2bt was much more reactive with

sterically hindered substrates. As an example, under identical

conditions, complex 2bt aﬀorded a good 93% yield of the

coupled product with 2-chlorobenzonitrile (see Table 3, entry

6) in 2 h, whereas A gave a yield of 36% in 12 h. These

diﬀerences in catalytic properties can be attributed to the

diﬀerences in steric and electronic properties between the two

tetranuclear complexes. Complex A featuresan imidazole-based

aNHC moiety and a ﬂanking group on the imidazole ring,

whereas 2bt contains an aNHC moiety based on imidazo[1,5-

a]pyridine without a ﬂanking group. From the structural data,

complex A possesss shorter Pd−carbene bonds in comparison

with those in 2bt. Also, the nonbonding Pd···Pd distance in A

is shorter than that in 2bt. In solution, the overall robustness of

A may lead to its better catalytic activities at high temperature

in comparison to thos of 2bt. In contrast, the lack of a ﬂanking

group rendered 2bt less sterically congested and thus more

favorable for sterically hindered substrates.

Finally, to demonstrate the practicality of complex 2bt in

synthetic organic chemistry, we carried out an upscaled

reaction between sterically hindered 2-chlorobenzonitrile (10

mmol) and styrene (14 mmol). The reaction successfully

aﬀorded the coupled product 4c′′ in an excellent yield of 1.97

g (96%) (Scheme 3).

Reaction conditions: 1.4 mmol of styrene, 1 mmol of aryl halide, 1.1

mmol of NaOAc, 2 g of TBAB, 0.05 mol % of 2bt (i.e., 0.2 mol % Pd

atom loading), 140 °C. Isolated yields are given.

Scheme 3. Mizoroki−Heck Reaction of 2-

Chlorobenzonitrile and Styrene on a Preparative Scale

(

entries 4−7). In the literature, there has been only a single

report describing the coupling reaction of 2-chloroacetopheone

with styrene using a palladium/magnesium−lanthanum mixed

oxide catalyst, giving 70% of the product in 20 h with a Pd

loading of 1.5 mol %. In our case, a good 77% yield of

product was obtained in 2 h with a 0.2 mol % of Pd loading

Mechanistic Aspects. It is worth noting that palladium

nanoparticles (NPs) in TBAB have been successfully used in

the Mizoroki−Heck coupling reaction with aryl chloride

(

entry 4). In the literature, various catalyst systems based on

palladium nanoparticles and palladium complexes allowed the

production of 1-nitro-2-[(E)-2- phenylethenyl]benzene (4b″)

from 1-chloro-2-nitrobenzene and styrene with good yields

using Pd loadings of 0.1−1.0 mol %. However, a prolonged

heating time of 20−24 h was needed in all of these cases. In

sharp contrast, entry 5 shows that 1-chloro-2-nitrobenzene

reacted smoothly with styrene to give 4b″ in a yield of 84% in

just 2 h. 2-Chlorobenzonitrile also coupled eﬀectively with

styrene to give an excellent product yield of 93% in 2 h (entry

substrates. For the catalytic systems based on 2bt and A,

in addition to homogeneous catalytic centers, the plausible

involvement of heterogeneous species cannot be excluded. In

this regard, complex 2bt may undergo a cleavage reaction

under the catalytic conditions to generate ligand-disconnected

Pd nanoparticles. To understand the stability of 2bt, a DMF

solution of 2bt was heated at 140 °C for 2 h. No distinct peaks

appeared in the H NMR spectrum, showing that the

). The product was prepared in a similar yield but using a 2.5

compound was quite thermally robust. However, when the

same solution was heated under the same conditions in the

presence of NaOAc, 16% of 2bt was converted to new species.

Also, the solution became dark with the formation of palladium

black being observed (Figure S2). This result reﬂects the

possible formation of palladium nanoparticles under basic

conditions. The fact that the catalytic reaction only occurred in

times higher Pd loading of (1,3-dimesitylimidazol-2-ylidene)-

(

benzoquinone)palladium(0) in TBAB with a reaction time of

4 h. Finally, the aryl chloride with an o-triﬂuoromethyl

group was also successfully applied as a substrate, aﬀording a

good 89% yield in 6 h (entry 7). The only reported catalyst

system describing the same reaction was based on a palladium-

Organometallics 2021, 40, 702−713

Organometallics

Article

TBAB, which is a general stabilizer for palladium nanoparticles,

is also consistent with such a claim (see Table 1). In fact, a

EXPERIMENTAL SECTION

■

General Information. All manipulations were performed under a

dry nitrogen atmosphere using standard Schlenk techniques. Solvents

were dried with standard procedures. Starting chemicals were

mercury drop test was performed on the benchmark reaction

between 4-chloroacetophenone and styrene catalyzed by 2bt.

In the presence of a drop of Hg (ca. 50 mg), the product yield

was markedly reduced from 99 to 11%, suggesting a

predominantly heterogeneous pathway. To unambiguously

conﬁrm the presence of palladium nanoparticles, a TEM image

from the benchmark catalytic solution was obtained after 2 h,

indeed showing the presence of palladium nanoparticles with

sizes of ca. 2−3 nm (Figure S3). Importantly, it should be

noted that the catalytic systems based on 2bt and A showed

strong ligand eﬀects, allowing the activation of aryl chlorides

and even sterically hindered aryl chloride substrates (see

Tables 2 and 3). In this regard, it is generally accepted that

NHC-ligated active centers, which are molecular complexes,

purchased from commercial sources and used as received. H and

C{ H} NMR spectra were recorded at 300.13 and 75.47 MHz,

respectively, on a Bruker AV-300 spectrometer. Elemental analyses

were performed on a Thermo Flash 2000 CHN-O elemental analyzer.

ESI-MS was carried out on a Finnigan/Thermo Quest MAT 95XL

mass spectrometer at National Chung Hsing University (Taiwan).

TEM images were obtained from a Jeol JEM-2010 200 kV

transmission electron microscope. 3-Phenylimidazo[1,5-a]pyridine

and 1-phenyl-1H-benzo[d]imidazole were prepared according to

literature procedures.

Synthesis of 2-(2-Oxo-2-(phenylamino)ethyl)-3-

phenylimidazo[1,5-a]pyridin-2-ium Chloride (1a). A mixture

of 3-phenylimidazo[1,5-a]pyridine (1.00 g, 5.15 mmol) and 2-chloro-

N-phenylacetamide (0.87 g, 5.15 mmol) was placed in a 50 mL

Schlenk ﬂask. DMF (20 mL) was added via a syringe. The mixture

was stirred at 100 °C for 72 h under nitrogen. The solvent was then

removed under vacuum. The residue was washed with THF and

diethyl ether several times. The gray solid was then collected on a frit

24b

nanoclusters, or nanoparticles, should be involved.

conﬁrm the involvement of ligand-connected species, a

preliminary preheating experiment on the benchmark reaction

was also conducted (Table 1, entry 15). The catalytic solution

was preheated for 2 h prior to the addition of substrates, and

such preheating should lead to a preliminary decomposition of

precatalyst 2bt, favoring the formation of palladium nano-

and dried under vacuum. Yield: 1.36 g, 72%. Mp: 213.2−213.8 °C. H

NMR (DMSO-d ): δ 5.40 (s, 2H, CH ), 7.06−7.16 (m, 2H, Ar H),

.29−7.31 (m, 3H, Ar H), 7.50 (d J = 6.0 Hz, 2H, Ar H), 7.73 (br

s, 5H, Ar H), 7.99 (d, J = 9.0 Hz, 1H, Ar H), 8.10 (d, J = 6.0

particles.

Markedly, the same quantitative yield as that

Hz, 1H, Ar H), 8.53 (s, 1H, imi H), 10.94 (s, 1H, NH). C{ H}

NMR (DMSO-d ): δ 52.5 (CH ), 116.5, 119.3, 119.4, 120.2, 121.1

quaternary C), 123.2, 124.9, 125.8, 129.5 (quaternary C), 129.7,

without preheating was obtained (Table 1, entries 15 vs 3). It

should be noted that this product yield from the preheating

^e^x^p^e^rⁱ^m^eⁿ^t^w^a^s^hⁱ^g^h^e^r^t^h^aⁿ^t^h^a^t^gⁱ^v^eⁿ^b^y^lⁱ^g^aⁿ^d^l^e^s^s^P^d⁽^O^A^c⁾2^,

(

30.8, 131.5, 133.5, 135.4 (quaternary C), 138.7 (quaternary C),

63.4 (CO). HRMS (ESI): m/z calcd for C H N O [M − Cl]

which was a precursor for ligandless palladium nanoparticles

28.1444, found 328.1440.

(Table 1, entry 15 vs entry 7). All of these results suggest the

Synthesis of 2-(2-((2-Hydroxyphenyl)amino)-2-oxoethyl)-3-

possible involvement of ligand-connected palladium nano-

phenylimidazo[1,5-a]pyridin-2-ium Chloride (1b). The com-

pound was prepared following a procedure similar to that for 1a. 2-

Chloro-N-(2-hydroxyphenyl)acetamide (0.96 g, 5.15 mmol) was

used. A gray solid was obtained. Yield: 1.46 g, 77%. Mp: 201.1−201.3

24b

particles in the catalytic pathway.

In fact, the precursor

complex 2bt bears a strong chelating aNHC ligand which

should be much more stable to decoordination in comparison

to common monodentate NHC ligands. Also, 2bt is a

tetranuclear complex, and after its in situ reduction of Pd(II)

to Pd(0), it should form ligated-NHC palladium nanoclusters

or nanoparticles more readily in comparison to a mononuclear

palladium complex. Notably, the more robust complex A may

be a better precursor for a slow release of Pd NPs in

comparison to complex 2bt.

C. H NMR (DMSO-d ): δ 5.42 (s, 2H, CH ), 6.73 (t, J = 6.0

6 2 HH

Hz, 1H, Ar H), 6.89−6.97 (m, 2H, Ar H), 7.16 (t, J = 9.0 Hz, 1H,

Ar H), 7.35 (t, J = 6.0 Hz, 1H, Ar H), 7.70−7.77 (m, 6H, Ar H),

.02 (d, J = 9.0 Hz, 1H, Ar H), 8.13 (d, J = 6.0 Hz, 1H, Ar H),

HH HH

8.50 (s, 1H, imi H), 9.83 (s, 1H, NH), 10.09 (s, 1H, OH). C{ H}

NMR (DMSO-d ): δ 52.3 (CH ), 116.0, 116.4, 118.9, 119.2, 119.3,

21.0 (quaternary C), 122.8, 123.0, 125.5, 125.6 (quaternary C),

25.7, 129.2, 130.5, 131.4, 133.2, 135.1 (quaternary C), 148.8

(

quaternary C), 164.0 (CO). HRMS (ESI): m/z calcd for

C H N O [M − Cl] 344.1394, found 344.1390.

CONCLUSIONS

Synthesis of 3-(2-((2-Hydroxyphenyl)amino)-2-oxoethyl)-1-

■

phenyl-1H-benzo[d]imidazol-3-ium (1b′). The compound was

prepared following a procedure similar to that for 1a. 2-Chloro-N-(2-

hydroxyphenyl)acetamide (0.96 g, 5.15 mmol) was used. A pale

orange solid was obtained. Yield: 1.46 g,75%. Mp: 217.6−217.9 °C.

We successfully prepared new mononuclear and tetranuclear

palladium complexes with functionalized aNHC ligands based

on 3-phenylimidazo[1,5-a]pyridine. Theses complexes were

eﬀective catalyst precursors in the Mizoroki−Heck coupling

reaction of aryl chlorides with alkenes in TBAB. The palladium

complex with a tridentate amidate- and phenoxy-functionalized

aNHC ligand displayed an activity superior to that of the

palladium complex with a bidentate amidate-functionalized

aNHC ligand. The new tetranuclear complex with the

tridentate ligand displayed the best activities, capable of the

activation of deactivated aryl chlorides as substrates with a low

Pd atom loading of 0.2 mol %. Most importantly, the new

catalyst system developed also allowed the use of challenging

sterically hindered aryl chlorides to couple eﬀectively with

styrene. The studies again demonstrated the promising use of

tetranuclear palladium complexes with chelating aNHC ligands

in Mizoroki−Heck coupling reactions conducted in an ionic

liquid.

H NMR (DMSO-d ): δ 5.37 (s, 2H, CH ), 6.72−6.78 (m, 1H, Ar

H), 6.97 (d, JHH = 3.0 Hz, 2H, Ar H), 7.71−7.81 (m, 5H, Ar H),

(

.84−7.90 (m, 4H, Ar H), 8.17 (d, J = 9.0 Hz, 1H, Ar H), 10.13

13 1

s,1H, NH), 10.17 (s, 1H, OH), 10.34 (s, 1H, NCHN). C{ H}

NMR (DMSO-d ): δ 50.1 (CH ), 114.1, 114.9, 116.1, 119.4, 122.6,

(

25.7, 125.7, 126.0 (quaternary C), 127.8, 128.1, 131.1, 131.2, 132.4

quaternary C), 133.6 (quaternary C), 144.3, 148.7 (quaternary C),

63.9 (CO). HRMS (ESI): m/z calcd for C H N O [M − Cl]

344.1394, found 344.1395.

Synthesis of Palladium Complex 2a. In a 20 mL Schlenk tube

were placed 1a (0.1 g, 0.27 mmol), Pd(OAc) , (0.062 g, 0.27 mmol),

and NaOAc (0.045 g, 0.54 mmol). Pyridine (22.14 μL, 0.27 mmol)

and DMF (8 mL) were then added via a syringe. The mixture was

then heated at 50 °C for 12 h. After the mixture was cooled, the

solvent was removed completely under vacuum. Dichloromethane

and water were added. The product was extracted into the organic

phase, which was separated and dried with anhydrous MgSO . The

Organometallics 2021, 40, 702−713

Organometallics

The Supporting Information is available free of charge at

Article

solvent was removed under vacuum. The yellow solid that formed was

³J_H_H= 9.0 Hz, 1H, Ar H), 8.31 (d, ³J_H_H= 9.0 Hz, 2H, Py H). ¹³C{ H}

washed thoroughly with diethyl ether, ﬁltered, and dried under

NMR (CDCl ): δ 53.4 (CH ), 111.1, 111.4, 114.7, 115.7, 123.7,

vacuum. Yield: 77%. Mp: 198.2−198.6 °C. H NMR (CDCl ): δ 5.17

123.9, 124.4, 124.9, 127.3 (Py C), 129.1, 129.3 (quaternary C), 129.6,

133.1 (quaternary C), 134.7, 136.3 (quaternary C), 137.6 (Py C),

141.7 (quaternary C), 152.1(Py C), 163.6 (CO or C−O), 163.7

(

s, 2H, CH ), 6.71−6.79 (m, 2H, Ar H), 6.82−6.88 (m, 1H, Ar H),

.94 (t, J = 9.0 Hz, 2H, Ar H), 7.06 (t, J = 9.0 Hz, 2H, Ar H),

HH HH

.41 (d, J = 9.0 Hz, 2H, Ar H), 7.49−7.53 (m, 3H, Ar H, Py H),

(

CO or C−O), 169.4 (Pd−C). Anal. Calcd for C H O N Pd: C,

26 20

.61−7.65 (m, 3H, Ar H, Py H), 7.74 (d, J = 6.0 Hz, 1H, Ar H),

9.27; H, 3.83; N, 10.63. Found: C, 59.37; H, 4.01; N, 10.71. Crystals

.48 (d, J = 9.0 Hz, 1H, Ar H), 8.54 (d, J = 6.0 Hz, 2H, Py H).

suitable for single-crystal X-ray analyses were obtained by slow

evaporation of a solution of 2b′ in acetone/hexane.

C{ H} NMR (DMSO-d ): δ 57.8 (CH ), 118.1, 119.2, 120.3, 112.5,

23.2, 124.2, 125.0, 126.0, 126.7, 126.9 (Pd−C), 127.5, 130.2, 130.3,

General Procedure for Mizoroki−Heck Reactions. In a typical

reaction, a mixture of aryl halide (1 mmol), styrene (1.4 mmol), base

31.7, 132.0, 132.5, 137.2, 150.7, 167.1 (CO). Anal. Calc. for

C H ON ClPd: C, 57.05; H, 3.87; N, 10.24; found: C, 57.22; H,

(1.1 equiv), palladium catalyst (0.1−0.2 mol %), and TBAB (2 g) was

.25; N, 10.30%. Crystals suitable for single-crystal X-ray analyses

stirred at 140 °C for an appropriate period of time (2−12 h). After

the reaction mixture was cooled, the product was extracted with ethyl

ether (3 × 10 mL) and the organic layer was dried with anhydrous

were obtained by vapor diﬀusion of diethyl ether into a solution of

bt in dichloromethane.

Synthesis of Palladium Complex 2b. The complex was

MgSO . The solvent was removed completely under high vacuum.

prepared following a procedure similar to that for 2a. Compound

b (0.1 g, 0.26 mmol), Pd(OAc) (0.059 g, 0.26 mmol), and NaOAc

The NMR yield was determined by using 1,3,5-trimethoxybenzene as

an internal standard. The crude product was puriﬁed by column

chromatography.

(

0.065 g, 0.78 mmol) were used. A pale yellow solid was obtained.

Yield: 0.11 g, 80%. Mp: 202.6−202.9 °C. H NMR (CDCl ): δ 5.11

(

s, 2H, CH ), 6.17 (d, J = 9.0 Hz, 1H, Ar H), 6.41−6.46 (m, 2H,

Single-Crystal X-ray Diﬀraction. Samples of 2b and 2b′ were

collected on a Bruker D8 VENTURE instrument equipped with a

PHOTON 100 CMOS detector. Samples of 2a and 2bt were

collected on a Bruker SMART APEX II instrument equipped with a

CCD area detector. Data collections were performed at 150(2) K

utilizing Mo Kα radiation (λ = 0.71073 Å). The unit cell parameters

were obtained by least-squares reﬁnement. Data collection and

reduction were performed using the Bruker APEX and SAINT

Ar H), 6.63 (t, J = 6.0 Hz, 1H, Ar H), 6.72−6.79 (m, 2H, Ar H),

.44 (t, J = 6.0 Hz, 2H, Ar H), 7.50−7.54 (m, 2H, Ar H), 7.65−

.72 (m, 4H, Py H, Ar H), 7.90 (t, J = 9.0 Hz, 1H, Py H), 7.99 (d,

J_H_H= 6.0 Hz, 1H, Ar H), 8.94 (d, J = 6.0 Hz, 2H, Py H). C{ H}

NMR (CDCl ): δ 57.5 (CH ), 114.1, 116.0, 117.6, 119.1, 120.9,

22.0, 122.5, 123.3, 123.7, 125.6, 128.7, 129.9, 130.2, 130.3, 131.8

Pd−C), 132.0, 138.3, 141.8, 153.0, 163.6 (CO), 164.0 (C−O).

Anal. Calcd for C H O N Pd: C, 59.27; H, 3.83; N, 10.63. Found:

(

software. Absorption corrections were performed using the

C, 59.44; H, 3.93; N, 10.26. Crystals suitable for single-crystal X-ray

analyses were obtained by slow evaporation of a solution of 2b in

chloroform.

SADABS program. All of the structures were solved by direct

methods and reﬁned by full-matrix least-squares methods against F

with the SHELXL software package. All non-H atoms were reﬁned

Synthesis of Tetranuclear Palladium Complex 2bt. In a 20

anisotropically. All H atoms were ﬁxed at calculated positions and

reﬁned with the use of a riding model. The structure of complex 2bt

contains a large solvent-accessible volume that was occupied by

mL Schlenk tube were placed 1b (0.1 g, 0.26 mmol), PdCl (0.047 g,

.26 mmol), and K CO (0.15 g, 1.04 mmol). DMF (8 mL) was

2 3

added via a syringe. The mixture was then heated at 30 °C for 12 h.

After the mixture was cooled, the solvent was removed completely

under vacuum. Dichloromethane and water were added. The product

was extracted into the organic phase, which was separated and dried

toluene molecules. The total accessible volume was calculated to be

183 Å (34% of the cell volume). In the asymmetric unit, 3.5

molecules of toluene were successfully modeled and these molecules

occupied a total volume of 2713 Å , leaving a void space of 470 Å .

with anhydrous MgSO . The solvent was removed under vacuum.

Each toluene molecule occupied about 2713/(3.5 × Z) = 193 Å (Z =

The yellow solid that formed was washed thoroughly with methanol

and diethyl ether, ﬁltered, and dried under vacuum. Yield: 72%. Mp:

4). Thus, the void space contained about 470/193 ≈ 2 strongly

disordered toluene molecules (i.e., half of a molecule in the

asymmetric unit), which were not modeled successfully. Also, a

twin matrix was applied in the re ﬁ nement of 2bt . CCDC ﬁles

2015837 (2a), 2015846 (2b), 2023606 (2bt), and 2015845 (2b′)

contain supplementary crystallographic data for this paper.

80.1−280.6 °C. H NMR (CDCl ): δ 4.77 (d, J = 21.0 Hz, 4H,

CH H ), 5.28 (d, J = 21.0 Hz, 4H, CH H ), 5.94−6.03 (m, 8H, Ar

H), 6.33−6.38 (m, 4H, Ar H), 6.49 (t, J = 6.0 Hz, 4H, Ar H), 7.31

(

d, J = 9.0 Hz, 4H, Ar H), 7.52−7.63 (m, 12H, Ar H), 7.73 (d,

J_H_H= 9.0 Hz, 8H, Ar H), 7.81 (d, J = 9.0 Hz, 4H, Ar H), 7.99−

.01 (m, 4H, Ar H), 8.54 (d, J = 9.0 Hz, 4H, Ar H). C{ H}

NMR (CDCl ): δ 56.5 (CH ), 115.4, 117.5, 118.0, 119.3, 120.8,

ASSOCIATED CONTENT

21.4, 122.6, 122.7, 123.8, 125.9, 129.5 (Pd−C), 129.6, 130.7, 131.3,

■

31.5, 142.0, 159.0 (C−O), 165.1 (CO). Anal. Calcf for

sı

Supporting Information

https://pubs.acs.org/doi/10.1021/acs.organomet.0c00791.

C H O N Pd : C, 56.32; H, 3.37; N, 9.38. Found: C, 56.17; H,

.79; N, 9.14. Crystals suitable for single-crystal X-ray analyses were

obtained by slow evaporation of a solution of 2b′ in acetone/toluene.

Synthesis of Palladium Complex 2b′. In a 20 mL Schlenk tube

Additional drawings, crystallographic data, H NMR

were placed 1b′ (0.1 g, 0.26 mmol), Pd(OAc) , (0.059 g, 0.26 mmol),

assignment of coupled products, NMR spectra of ligand

precursors, palladium complexes, and coupled products,

and ESI-MS spectra of ligand precursors (PDF )

and NaOAc (0.065 g, 0.78 mmol). Pyridine (8 mL) was added via a

syringe. The mixture was then heated at 50 °C for 12 h. After the

mixture was cooled, the solvent was removed completely under

vacuum. Dichloromethane and water were added. The product was

extracted into the organic phase, which was separated and dried with

Accession Codes

anhydrous MgSO . The solvent was removed under vacuum. The pale

yellow solid that formed was washed thoroughly with diethyl ether,

CCDC 2015837, 2015845 −2015846 , and 2023606 contain the

supplementary crystallographic data for this paper. These data

can be obtained free of charge via www.ccdc.cam.ac.uk/

data_request/cif , or by emailing data_request@ccdc.cam.ac.

uk , or by contacting The Cambridge Crystallographic Data

Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44

1223 336033.

ﬁltered, and dried under vacuum. Yield: 74%. Mp: 196.1−196.4 °C.

H NMR (CDCl ): δ 5.20 (s, 2H, CH ), 6.52 (t, J = 6.0 Hz, 1H,

Ar H), 6.65 (d, J = 6.0 Hz, 1H, Ar H), 6.76 (t, J = 6.0 Hz, 1H,

Ar H), 6.99 (t, J = 6.0 Hz, 2H, Ar H), 7.12 (d, J = 9.0 Hz, 1H,

Ar H), 7.19−7.21 (m, 3H, Ar H), 7.28 (t, J = 9.0 Hz, Ar H), 7.35−

.45 (m, 3H, Ar H, Py H), 7.50−7.59 (m, 2H, Ar H, Py H), 8.05 (d,

Organometallics 2021, 40, 702−713

Organometallics

Taiwan; orcid.org/0000-0002-9557-3914;

Article

Reactivity and Selectivity for Cross-Coupling Applications. Acc. Chem.

Res. 2017, 50, 2244−2253.

AUTHOR INFORMATION

Corresponding Author

Hon Man Lee − Department of Chemistry, National

Changhua University of Education, Changhua 50058,

Email: leehm@cc.ncue.edu.tw

■

(

2) (a) Grundemann, S.; Kovacevic, A.; Albrecht, M.; Faller Robert,

J. W.; Crabtree, H. Abnormal binding in a carbene complex formed

from an imidazolium salt and a metal hydride complex. Chem.

Commun. 2001, 0, 2274−2275. (b) Grundemann, S.; Kovacevic, A.;

Albrecht, M.; Faller, J. W.; Crabtree, R. H. Abnormal Ligand Binding

and Reversible Ring Hydrogenation in the Reaction of Imidazolium

Salts with IrH (PPh ) . J. Am. Chem. Soc. 2002, 124, 10473−10481.

Authors

3 2

Cheng-Hau Hung − Department of Chemistry, National

Changhua University of Education, Changhua 50058,

Taiwan

Wei-Yuan Zheng − Department of Chemistry, National

Changhua University of Education, Changhua 50058,

Taiwan

Miecznikowski, J. R.; Macchioni, A.; Clot, E.; Eisenstein, O.; Crabtree,

R. H. An Anion-Dependent Switch in Selectivity Results from a

Change of C-H Activation Mechanism in the Reaction of an

Imidazolium Salt with IrH

6299−16311.

3) (a) Bacciu, D.; Cavell, K. J.; Fallis, I. A.; Ooi, L.-l. Platinum-

(PPh ) . J. Am. Chem. Soc. 2005, 127,

3 2

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.0c00791

Mediated Oxidative Addition and Reductive Elimination of

Imidazolium Salts at C4 and C5. Angew. Chem., Int. Ed. 2005 , 44 ,

282 −5284. (b) Poulain, A.; Iglesias, M.; Albrecht, M. Abnormal

Author Contributions

C.-H.H. and W.-Y.Z. contributed equally.

Notes

NHC Palladium Complexes: Synthesis, Structure, and Reactivity.

Curr. Org. Chem. 2011, 15, 3325−3336.

(

†

4) (a) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.;

Crabtree, R. H. Abnormal C5-Bound N-Heterocyclic Carbenes:

Extremely Strong Electron Donor Ligands and Their Iridium(I) and

Iridium(III) Complexes. Organometallics 2004, 23, 2461 −2468.

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

We are grateful to the Ministry of Science and Technology of

Taiwan for ﬁnancial support of this work (108-2113-M-018-

■

(b) Heckenroth, M.; Kluser, E.; Neels, A.; Albrecht, M. Neutral

Ligands with Exceptional Donor Ability for Palladium-Catalyzed

Alkene Hydrogenation. Angew. Chem., Int. Ed. 2007, 46 , 6293 −6296.

01).

Complexes Containing C4-Bound N -Heterocyclic Carbenes: Syn-

thesis, Coordination Chemistry, and Catalytic Activity in Transfer

Hydrogenation. Organometallics 2008 , 27 , 3161 −3171. (d) Iglesias,

M.; Albrecht, M. Expanding the family of mesoionic complexes:

donor properties and catalytic impact of palladated isoxazolylidenes.

Dalton Trans. 2010, 39, 5213−5215.

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