Bimetallic Bis-NHC-Ir(III) Complex Bearing 2-Arylbenzo[d]oxazolyl Ligand: Synthesis, Catalysis, and Bimetallic Effects

Ligand: Synthesis, Catalysis, and Bimetallic E ﬀ ects

Article

Bimetallic Bis-NHC-Ir(III) Complex Bearing 2‑Arylbenzo[d]oxazolyl

Shuang Huang, Xi Hong, He-Zhen Cui, Bing Zhan, Zhi-Ming Li,* and Xiu-Feng Hou*

Cite This: https://dx.doi.org/10.1021/acs.organomet.0c00464

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sı Supporting Information

ABSTRACT: Herein, an unprecedented bimetallic bis-NHC Cp*Ir

complex 1 bearing 2-arylbenzo[d]oxazolyl and NHC ligands is reported.

A signiﬁcant increase in activity was observed for N-methylation of amines

and reduction of aldehydes with MeOH catalyzed by 1 compared to the

monometallic analogues (2−11). Under the optimal conditions, it showed

to be highly eﬀective in N-methylation of nitroarenes with MeOH as both

C1 and H source. Substrates, including aromatic amines, ketones, and nitro

compounds with various functional groups, can be well-tolerated.

Mechanistic studies and DFT calculation highlight the signiﬁcance of

bimetallic centers cooperativity.

INTRODUCTION

ligands. More recently, it is reported that monometallic

Cp*Ir complexes bearing 2-arylbenzo[d]oxazolyl ligands

showed superior catalytic performance in the N-methylation

■

Recently, bimetallic complexes as homogeneous catalysts have

attracted extensive attention in enhancement of reactivity via a

synergistic or cooperative eﬀects. They were often more

of aromatic amines by our group. Inspired by the above, we

sought to obtain a novel bimetallic bis-NHC Ir complex

connected by a phenyl scaﬀold, allowing for interaction

between two Ir centers, which bears benzo[d]oxazolyl and

NHC ligands to improve the catalytic eﬀect. Three reaction

types for catalysis were tested: N-methylation of amines,

reduction of aldehydes, and N-methylation of nitroarenes with

eﬃcient than monometallic counterparts in spite of their

similar structure. Their superior performance is derived from

interactions between the two metals and the substrates. In

some cases, when the distance of two metal centers is close to

.5−6.0 Å, it maybe results in increasing activity of the

substrate and stability of the reaction center. However, the

activity of a newly designed bimetallic catalyst is hard to

predict. It generally depends on discrete monometallic

complexes or ligands attached and the relationship between

two metal centers and substrates. In order to enhance catalytic

activity, it is crucial for conﬁguration of two metal centers and

scaﬀolds of ligands.

methanol as both C1 and H source. For comparison, its

monometallic analogues with diﬀerent coordinated modes,

electronic eﬀect, and steric hindrance have been prepared.

Taking the N-methylation of amines or nitroarenes as an

example, kinetic experiments and DFT calculations provide

insights into the bimetallic centers cooperativity. The research

will provide the information for the application of bimetallic

bis-NHC catalysts in borrowing-hydrogen catalysis.

N-Heterocyclic carbene (NHC), due to its strong σ-donor,

can form strong metal−ligand bonds, which has high

thermodynamic stability and low kinetic lability. It beneﬁts

RESULTS AND DISCUSSION

Synthesis and Characterization of Bimetallic Bis-NHC

Ir Complex. The synthesis of bimetallic bis-NHC Ir complex

was initiated from the preparation of 2-(2,4-diﬂuorophenyl)-

benzo[d]oxazole 1a from 2-aminophenol and 2,4-diﬂuoroben-

zaldehyde in our previous report, as depicted in Scheme 1.

■

for stabilizing reactive metal centers or intermediates in

catalysis. NHC is ideally suitable as a bimetallic unit bridging

III

the two metal fragments. For examples, bimetallic Rh /Au

III

and Ir /Au complexes using the bis-NHC ligand as

precursors, have superior performance in the coupling of

10c

nitrobenzene with benzylic alcohol. Bimetallic NHC Rh(I)

complexes were high-eﬃciency in the reaction of hydro-

Received: July 7, 2020

elementation of alkynes. Hence, NHC ligands are helpful for

improving catalytic reactivity of bimetallic complexes.

2-Arylbenzoxazole, with good electronic characteristics and

coordinated functionality, is an interesting tool used as a

scaﬀold of ligands. Our group has previously reported the

synthesis of 2-arylbenzo[d]oxazoles and application of NHC

XXXX American Chemical Society

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Article

Scheme 1. Synthesis of Bimetallic Bis-NHC Ir Complex 1

Through subsequent nucleophilic aromatic substitution (S Ar)

10e

with benzimidazole in the presence of Cs CO ,

1a was

transformed to bis-benzimidazole-substituted analogue 1b in

8% yield. Successive addition of methyl iodide (CH I) gave

Figure 1. Molecular structure of complex 1·DMSO. Ellipsoids at the

0% probability level. Hydrogen atoms, noncoordinated counterion

bisimidazolium salt (bis-NHC precursor) 1c in almost

quantitative yield. Treatment of 1c with 1.1 equiv of

−

ion Cl and PF₆anions omitted for clarity. Selected bond lengths

(Å) and angles (deg) for molecule 1·DMSO: Ir(1)−C(14) 2.032(5),

Ir(1)−N(1) 2.116(4), Ir(1)−Cl(1) 2.4117(14), N(2)−C(14)

[

Cp*IrCl ] and NaOAc as base at 35 °C overnight yielded

2 2

bimetallic bis-NHC complex 1 in 60% yield in the presence of

NH PF . The addition of more equivalents of [Cp*IrCl ]

1.377(7), N(2)−C(13) 1.436(6), C(8)−C(13) 1.394(7), C(7)−

2 2

C(8) 1.448(7), N(1)−C(7) 1.314(6), Ir(2)−C(21) 2.023(5), Ir(2)−

C(10) 2.038(5), N(4)−C(21) 1.366(7), N(5)−C(21) 1.328(7),

C(10)−C(11) 1.420(7), C(14)−Ir(1)−N(1) 88.45(18), C(21)−

Ir(2)−C(10) 77.2(2), C(7)−N(1)−Ir(1) 126.4(3), C(14)−N(2)−

C(13) 128.3(4), C(21)−N(4)−C(11) 115.7(4), N(3)−C(14)−

N(2) 105.2(4), N(5)−C(21)−N(4) 107.7(4).

would beneﬁt for metalation of the NHC donors in order to

avoid generating excess mononuclear complexes. Finally, air

stable complex 1 was fully characterized by NMR spectrosco-

py, high-resolution mass spectrometry (HRMS), Fourier

transform infrared (FT-IR), and X-ray crystallography.

The H NMR spectrum of 1 in DMSO-d₆features

resonances for methyl hydrogen atoms (δ = 4.21 ppm) that

are shifted upﬁeld slightly in comparison to those for the

parent bisimidazolium salt 1c (δ = 4.25−4.35 ppm). The N-

CH-N proton signal of the corresponding 1c at δ = 10.28−

observed for the ﬁve-membered chelate ring (Ir(2)−C(21)

.023(5) Å). These phenomena suggest that the Ir(1) center

may be easier and more probable than the Ir(2) center to react

with substrates.

0.41 ppm was no longer detected. The peaks at 1.37−1.90

ppm were assigned to CH of the Cp* group. The C NMR

Catalytic Studies. In general, when the distance of two

metal centers in bimetallic complexes is close to 3.5−6.0 Å, it is

suitable for electronic communication between metal centers

spectra showed the characteristic resonance for the C_N_H_C

atoms at δ = 180.18 ppm, shifted downﬁeld signiﬁcantly

from the corresponding C resonance of 1c (δ = 157.70

and substrates. In the bimetallic bis-NHC complex 1, the

ppm). The HRMS (positive ion) of 1 shows peaks at m/z =

distance between Ir(1) and Ir(2) centers (7.5827 Å) is long.

182.2807 (calcd for 1182.2835).

Suitable crystals of 1 were obtained by slow vapor diﬀusion

Considering there are some cases observed for bimetallic

11b,15

eﬀects at longer distances,

three kinds of reactions of N-

of the solvents from a concentrated ether/dimethyl sulfoxide

solution of 1 at ambient temperature. An X-ray diﬀraction

study with the crystal conﬁrmed the formation of the bimetallic

methylation of amines, reduction of aldehydes, and N-

methylation of nitroarenes with methanol as both C1 and H₂

source were used to evaluate the catalytic performance of 1.

In order to investigate the bimetallic cooperativity of the

−

iridium(III) complex 1·DMSO with Cl and PF₆as anions,

which is depicted in Figure 1. The cation [1·DMSO] consists

of two classic three-legged piano stool structures. It is

interesting to note that the benzo[d]oxazole moiety

coordinates to one iridium center Ir(1) in a bidentate mode

via a benzo[d]oxazolyl nitrogen and NHC carbon, forming a

seven-membered chelate ring. In the meantime, another NHC

carbon and aryl carbon coordinate to another iridium center

Ir(2), which constitutes a ﬁve-membered iridacycle. The

distance of Ir···Ir is 7.5827 Å. The bite angle of

complex 1, we prepared the related monometallic complexes

and 3 (Scheme 2). It is worth noting that the monometallic

complexes 2 and 3 can be regarded as being the left and right

half part of the bimetallic complex 1, respectively.

Scheme 2. Monometallic Complexes 2 and 3

^CNHC∧C

⁽^aⁿ^g^l^e^CNHC−Ir1−N

benzo[d]oxazole

8.45(18)°) is a little larger than that of C ∧C (angle

NHC

C_N_H_C−Ir2−C 77.2(2)°). A larger torsion angle was observed

for the seven-membered iridacycle (C8−C13−N2−C14

0.94°) in comparison to the ﬁve-membered metallocycle

(

C10−C11−N4−C21 4.53°). The Ir(1)−C

Ir(1)−C(14) 2.032(5) Å) is slightly longer than that

bond length

NHC

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Article

For comparative purposes, we also prepared the other

monometallic Ir complexes 4−11 with similar individual

stereoelectronic properties (Scheme 3). The monometallic

within 4 h (entry 1). In contrast, the analogues 2 and 3,

containing a half structure of 1, only give 70% and 51% yield

under the same conditions, respectively (entries 2 and 3).

Lengthening the reaction time to 12 h makes the reaction

catalyzed by 2 in 98% yield (entry 2). The mixture of

complexes 2 and 3 obtains only 65% yield (entry 4), indicating

that the combination of monometallic complexes is less

eﬀective than the well-deﬁned bimetallic complex. The

monometallic complexes coordinated with N∧P, N∧O, N∧N,

and N∧C bidentate modes gave moderate yields (42−65%)

Scheme 3. Monometallic Ir Complexes 4−11

(

Ph)

(

entries 5−8). For N∧C

chelated monometallic Ir(III)

(

Carbene)

complexes 8−11, 48−63% yields were obtained. Complex 11

with the small steric hindrance beneﬁts the catalytic reactivity,

which is consistent with our previous report. No better

activity than the bimetallic complex 1 was observed (entries

−12).

Next, the scope of N-methylation of amines using MeOH

catalyzed by 1 was explored (Table 2). The para-substituted

Table 2. Scope of N-Methylation of Amines with MeOH

a b

catalyzed by 1

N∧P (4), N∧O (5), N∧N (6), and N∧C₍

(7) bidentate

Phenyl)

Cp*Ir(III) complexes based on 2-arylbenzo[d]oxazole back-

bone were obtained. In addition, the monometallic complexes

−11 bearing benzo[d]oxazolyl with tBu and NHC with nBu

or o-Me-Ph on the imidazole or benzimidazole ring were

synthesized to rule out the factors of electronic eﬀects or steric

hindrance.

To evaluate the catalytic performance of these complexes, N-

methylation of amines was ﬁrst investigated and reaction of

aniline with MeOH was chosen as the benchmark using the

optimized conditions for the mononuclear catalyst similar to

Considering that the bimetallic complex 1 has two Ir

atoms, we used the same mole % of Ir-catalyst (Ir: 0.5 mol %)

to compare the activity between the bimetallic and

monometallic Ir complexes. As can be seen from Table 1,

complex 1 gives the best result in almost quantitative yield

Table 1. Comparison of Catalytic Activities of 1−11 with N-

Methylation of Aniline

Reaction conditions: anilines (0.5 mmol), 1 (0.25 mol %), KO Bu (1

equiv), MeOH (1 mL), 130 °C, 4 h. Isolated yield.

a b

entry

catalyst

yield (%)

aromatic anilines with an array of functional groups, such as

>99

70 (98 )

methoxyl (1a2), cyano (1a3), and diﬀerent halides (1a4−

a7), reacted well and provided the respective products in high

yield. The meta-substituted anilines with electron-donating m-

OMe (1b1) or withdrawing groups such as Cl, Br, and I

2 + 3

(

1b2−1b4) can be eﬀectively converted into N-methylated

products as well, aﬀording excellent yields. The sterically

encumbering o-OMe (1c1), o-Br (1c2), and o-I (1c3) slightly

diminished the reactivity relative to the above para- or meta-

substituted substrates. Even functionalities of CH OH (1c4),

SCH (1c5), and OPh (1c6) on the ortho- position of phenyl

ring resulted in great yields of N-methylated products.

Similarly, m-phenylenediamine also tolerated the reaction

condition well, and the dimethylated product (1d) was

obtained in 83% yield. Notably, heteroaryl aromatic anilines

such as aminoquinoline (1e), 1-naphthalamine (1f), pyridine

Reaction conditions: aniline (0.5 mmol), catalyst (Ir: 0.5 mol %),

KO Bu (1 equiv), MeOH (1 mL), 130 °C, 4 h. With 1,3,5-

trimethoxybenzene as the internal standard. 12 h.

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Article

(

1g), benzothiazole (1h), and sulfonamide (1i−1j) were also

Table 4. Scope of Reduction of Aldehydes with MeOH

a b

eﬃciently transformed to desired amines in 56−97% yields.

Thus, it can be concluded that the reaction time catalyzed by 1

was sharply shortened in comparison to its monometallic

counterparts, suggesting great improvement of catalytic

activity from bimetallic eﬀects.

Catalyzed by 1

With the results obtained in hand for the N-methylation of

amines, we next explored the performance of the bimetallic

complex 1 in the reduction of aldehydes. Reduction of

carbonyl compounds plays an important role in the ﬁelds of

pharmaceuticals and chemistry. Also, it is a promising

pathway for reduction of aldehydes with MeOH as the

hydrogen donor. The reduction of benzaldehyde with

MeOH was chosen as the model reaction; the catalytic activity

of 1−11 (Ir: 0.5 mol %) was tested in the presence of Cs CO

as base at 130 °C (Table 3). Almost full conversion (97%

Table 3. Comparison of Catalytic Activities of 1−11 with

Reduction of Benzaldehyde

Reaction conditions: aldehydes (0.5 mmol), 1 (0.25 mol %),

Cs CO (0.5 equiv), MeOH (1 mL), 130 °C, 4 h. Isolated yield.

entry

catalyst

yield (%)

with MeOH as both C1 and H source (Table 5). Nitroarenes

were reduced to amines with MeOH as H source, and then N-

2 + 3

Table 5. Scope of N-Methylation of Nitroarenes with MeOH

a b

Catalyzed by 1

Reaction conditions: benzaldehyde (0.5 mmol), catalyst (Ir: 0.5 mol

), Cs CO (0.5 equiv), MeOH (1 mL), 130 °C, 4 h. With 1,3,5-

2 3

trimethoxybenzene as the internal standard.

NMR yield) was observed within 4 h (entry 1). Catalyzed by

the monometallic complexes 2 and 3, benzyl alcohol was

obtained in 52% and 75% yield, respectively (entries 2 and 3).

The mixture of catalysts 2 and 3 produced the reductive

products in 78% yield (entry 4). Other monometallic

complexes 4−11 could catalyze the reaction as well, aﬀording

Reaction conditions: nitroarenes (0.5 mmol), 1 (0.25 mol %),

1−68% yields (entries 5−12). These results indicated that

KO Bu (1 equiv), MeOH (1 mL), 130 °C, 10 h. Isolated yield. 4 h.

bimetallic complex 1 was also signiﬁcantly more active than the

monometallic analogues in the reduction of benzaldehyde.

Inspired by the excellent results from reduction of

benzaldehyde, we tested other aldehydes in Table 4. The

good to excellent yields (68−99%) were obtained while

introducing electron-rich or -withdrawing substituents at para-,

meta-, and ortho- positions on the benzaldehyde moiety.

Functional groups such as methoxyl, ter-butyl, halo, cyano, and

nitro were well-tolerated (products 2a1−2a7, 2b1−2b5, 2c1−

methylation of amines occurred with MeOH as C1 source.

Amines are generally prepared by reduction of cheap and

available nitro compounds, which consumes metal catalysts

and energy to maintain the conditions for hydrogenation.

Herein, it is extremely attractive that replacing anilines with

nitroarenes in the N-methylation reaction leads to a one-step

19−22

N-methylation of nitroarenes with MeOH.

c2). We were delighted that heterocycle-containing alde-

Now, the N-methylation of nitroarenes with MeOH

catalyzed by the bimetallic complex 1 was investigated. With

the optimal conditions of N-methylation of amines, gratify-

ingly, N-methylaniline (1a1) was aﬀorded in 61% yield for 4 h.

Simultaneously, reductive aniline was detected in 22% yield. By

prolonging the reaction time to 10 h, N-methylaniline was

successfully isolated in excellent 92% yield. Meanwhile highly

hydes, such as thiophene aldehyde, or picolinaldehyde,

naphthaldehyde, and 6-bromobenzo[d][1,3]dioxole-5-carbal-

dehyde could also be transformed into corresponding products

in high yield up to 99% (products 2d−2g).

Encouraged by the above results, we further applied

bimetallic complex 1 for the N-methylation of nitroarenes

Organometallics XXXX, XXX, XXX−XXX

Organometallics

conducted under the optimal conditions (Figure 2 ). The

Article

electron-donating p-OMe (1a2), and weak electron-with-

drawing p-Cl (1a5), p-Br (1a6), and p-I (1a7) nitroarenes

were converted to the targeted products in 71−96% yield. In

addition, substrates containing a methoxy group (1b1) or

halide (1b2−1b3) substituent in the meta- position were N-

methylated leanly with high eﬃciency. Encouragingly, hetero-

cycle-containing functionalities, such as 1-nitronaphthalene

Scheme 4. Deuterium-Labeling Experiments

(1f) and nitropyridine (1k), were also compatible with the

catalyst, aﬀording in 80% and 85% yields, respectively.

Notably, 1,4-dinitrobenzene smoothly transformed to the N-

monomethylated product (1l), leaving one nitro group

unchanged (62% yield). In essence, a variety of functional

groups on the nitroarenes can be well-tolerated, further

emphasizing the high catalytic eﬃciency of the bimetallic bis-

NHC catalyst 1.

Mechanistic Experiments. To further assess the diﬀerent

performance between bimetallic and monometallic catalysts,

we took the N-methylation from aniline with MeOH as an

example. Kinetic experiments catalyzed by 1−3 were

using PhNH and aniline-d (1:1), we obtained a low KIE (k /

k_D= 0.27) (Scheme 4b). Thus, the reversibility of dehydrogen-

ation of MeOH and the formation of a Schiﬀ base from aniline

and formaldehyde were not involved in the rate-determining

step of this reaction.

The kinetic proﬁles on N-methylation of nitrobenzene

catalyzed by 1 were performed by NMR monitoring (Figure

3 ). At the initial 1 h, the amount of nitrobenzene exhibited a

Figure 3. NMR monitoring of N-methylation of nitrobenzene

catalyzed by 1. Reaction conditions: 1 (0.25 mol %), nitrobenzene

(

0.5 mmol), KO Bu (1 equiv), MeOH (1 mL), 130 °C.

Figure 2. NMR monitoring of N-methylation of aniline catalyzed by

−3. Reaction conditions: catalyst (Ir: 0.5 mol %), aniline (0.5

rapid decrease to be converted into aniline, simultaneously

accompanying the formation of N-methylaniline. Then,

nitrobenzene was continuously consumed, and the amount

of N-methylaniline kept increasing, whereas generation of

aniline began to reduce. After the ﬁrst 1 h, the process of

reduction and N-methylation nearly simultaneously happened.

No N-phenylmethanimine was detected. The results were in

agreement with the reaction proﬁle of N-methylation of

nitrobenzene with methanol catalyzed by monometallic Ru

catalyst reported by Yang’s group. We speculated that the

catalytic performance may be from two active metal centers of

1 working independently, or only an active metal center under

the electronic inﬂuence of the second one.

mmol), KO Bu (1 equiv), MeOH (1 mL), 130 °C.

results showed that the catalytic eﬃciency of N-methylation of

aniline by bimetallic complex 1 was much higher than those of

monometallic complexes 2, 3, or the equal molar mixture of 2

and 3. Besides, it is noted that the mixture 2 + 3 likely could

surpass the activity of 3 at reaction times longer than 4 h, but

we are not sure that it is related to a bimetallic eﬀect.

Kinetic isotope eﬀect (KIE) was studied to probe the rate-

determining step of N-methylation of aniline, by comparing

the reaction rates using undeuterated CH OH/PhNH and

deuterated CD OD/aniline-d . For the low conversion in 20

min, the concentrations of undeuterated and deuterated

product are approximately proportional to the corresponding

DFT Calculations. To gain deep insight into the superior

catalytic performance of the bimetallic complex 1, density

functional theory (DFT) B3LYP calculations were carried out

using the Gaussian 09 software package. N-Methylation of

aniline with MeOH was selected as the model reaction. In

consideration of the complicated structures of the Ir catalyst,

the corresponding structures were optimized using a combined

basis set. That is, 6-31+G(d,p) for the weakly coordinated

imine fragment (except for the remote three skeleton carbon

rate constants k_Hand k , respectively. Under the optimal

conditions, measuring the initial rates of N-methylation with

aniline using CH OH and CD OD (1:1) revealed a primary

KIE (k /k_D= 2.13) (Scheme 4a). The results indicated

dehydrogenation of MeOH was not the rate-limiting step,

which was consistent with the results from a previous report by

the Ke group about the monometallic Mn catalyst. When

Organometallics XXXX, XXX, XXX−XXX

Organometallics

modes are shown in Figure 4 . Among them, 2-D and 3-D refer

Article

atoms on the phenyl ring and the linked hydrogen atoms), the

coordinated atoms with the Ir atom and the atoms connected

directly, the skeleton carbon atoms of the benzene ring

connected with the benzimidazole ring and NHC ring,

skeleton carbon atoms of the Cp* ring, SDD for the Ir

processes in the title reaction. This is in good agreement with

the experimental facts.

Meanwhile, the Ir−H bond analysis in C intermediates gives

the similar conclusion (Figure S2). Similarly, 2-C and 3-C refer

to the C intermediates when 2 and 3 are used as catalysts,

respectively. And 1-Ir1-C, 1-Ir2-C, and 1-2C correspond to

the C intermediates when Ir1, Ir2, and both Ir centers are

catalytic centers, respectively. The Ir−H bond distanes in 2-C,

3-C from mononuclear catalysts 2 and 3 are 1.580, 1.564 Å,

while the Ir−H bond distanes in 1-Ir1-C, 1-Ir2-C, and 1-2C,

which are derived from bimetallic catalyst 1, are 1.587, 1.605,

1.586, and 1.600 Å, respectively. All Ir−H bond lengths in 1-

Ir1-C, 1-Ir2-C, and 1-2C are longer than that of monometallic

2-C or 3-C.

In addition, natural atomic orbital (2s) occupancies of N

atoms in amido species D were selected to evaluate lone pair

availabilities of species D. The more natural atomic orbital (2s)

occupancy, the less lone pair available for coordination with

the Ir center in D, the easier the Ir−N bond can be broken.

The NBO analysis results shows that lone pair availabilities in

2-D, 3-D, 1-Ir1-D, 1-Ir2-D, and 1-2D are 1.27, 1.27, 1.28,

1.30, 1.28, and 1.30 (for Ir1 and Ir2 in 1-2D, respectively)

atomic units. The lone pair availabilities in 1-Ir1-D, 1-Ir2-D,

and 1-2D are more than those in 2-D and 3-D derived from

mononuclear catalysts 2 and 3, meaning Ir−N bonds of these

amido species D are more reactive, and easier to be broken,

which can be another proof as well for the bimetallic eﬀect

between two Ir centers. These results are also in good

agreement with our bond length analysis of Ir−H and Ir−N

bonds in species C and D.

atom, and the 3-21G basis set for all the other atoms. Truhlar

and co-workers’ SMD solvation model was employed to

consider the solvent eﬀect (methanol). The other calculation

details are provided in the Supporting Information.

According to the generally accepted three-step reaction

12,27

pathway for N-methylation of aniline with MeOH

and the

relative enthalpies and free energies of B, C , and D (Table S4),

23,28

C and D are important intermediates (Figure S1 ).

Ir−H

and Ir−N bonds in C and D are among the reaction center,

and will be broken in the catalytic cycle. Therefore, the bond

lengths of the two bonds can be indexed for the catalytic

reactivity. The longer the bond length, the weaker the bond

and the higher the catalytic activity. In this part, the Ir−N

bond in D was selected as the index, since Ir−N bonds are

longer than the Ir−H bond and varied in a wider range.

The corresponding intermediate structures 2-D, 3-D, 1-Ir1-

D, 1-Ir2-D, and 1-2D for catalysts 2, 3, and 1 and catalytic

In view that the interaction between catalyst and substrates

through π stacking can improve catalytic eﬀects, an extended

π-conjugated system may be a beneﬁt for catalytic activity.

Therefore, we anticipated ﬁrst that the bigger conjugation

system in 1 may account for the higher eﬃciency and activity

than that of the monometallic analogues 2 or 3. To verify our

idea, 1-T1 catalyst and its intermediate 1-T1-D were designed

(

Scheme 5). However, the computation result shows that the

Scheme 5. Supposed Conjugation System

Figure 4. Optimized structures of 2-D, 3-D, 1-Ir1-D, 1-Ir2-D, and 1-

2D; the numbers represent the Ir−N and Ir−Ir distances in Å.

to the D intermediates when 2 and 3 are used as catalysts,

respectively. As there are two Ir centers in catalyst 1, one is

coordinated with the NHC and benzoxazole rings (Ir1), and

the other with NHC and benzene rings (Ir2); thus, 1-Ir1-D, 1-

Ir2-D, and 1-2D correspond to the D intermediates when Ir1,

Ir2, and both Ir centers are catalytic centers, respectively. The

computational results indicate that the Ir−N distances in 1-

Ir1-D (2.183 Å), 1-Ir2-D (2.192 Å), and 1-2D (2.187 and

similar sized conjugation system 1-T1 catalyst with only one Ir

center gives a very short Ir−N distance (1.995 Å), which

means the 1-T1 catalyst with a bigger conjugation system can

not show high catalytic activity. Therefore, the larger

conjugation eﬀect cannot explain the high catalytic activity of

.194 Å) intermediates, which are derived from bimetallic

catalyst 1, are lengthened compared to those in 2-D and 3-D

2.177 and 2.153 Å) (see Figure 4), which are from

(

On the other hand, since all the Ir−N bond lengths in 1-Ir1-

D, 1-Ir2-D, and 1-2D are longer than that of monometallic 2-

D or 3-D, maybe the higher catalytic activity is due to the

bimetallic synergistic eﬀect between two Ir metal centers. To

better understand the synergistic eﬀect, the second-order

mononuclear catalysts 2 and 3. Especially, Ir2−N distances

in 1-Ir2-D and 1-2D are lengthened by 0.039 and 0.041 Å

compared to the one in 2-D, respectively. Since these bonds

are longer, they should be easier to be broken, indicating

higher catalytic reactivity and lower activation barrier of

bimetallic catalyst 1, whether monocentral or bicentral catalytic

perturbation theory analysis was carried out. The result

indicates that there are second-order perturbations between

Organometallics XXXX, XXX, XXX−XXX

Organometallics

shown in Figure 5 . The interaction between BD(Ir2 -N124 )

Article

bonding orbital Ir-N and single excited orbitals of Ir1 in 1-Ir2-

D(BD(Ir2-N108)) and 1-2D(BD(Ir2-N124)). The interac-

tion energies are 13.0 and 10.2 kcal/mol, respectively, though

the distance Ir1−Ir2 is long up to 7.637 and 7.728 Å (Figure

centers and excluded the contribution of the large conjugation

system of the bimetallic catalyst. Both two metal centers were a

highly indispensable part for the superior catalytic activity.

Further research for reaction mechanism and application of

bimetallic catalysts is on going in our laboratory.

). All the relevant NBO orbitals, orbital energies, and

interaction energies (E(2)) are listed in Table S5. Among

them, the interaction between BD(Ir2-N108) and single

excited orbital RY*(6)Ir1 is the largest with an interaction

energy of 4.9 kcal/mol. The two interacting NBO orbitals are

EXPERIMENTAL SECTION

General Procedures. All commercially available compounds

Acros, Aldrich, Fluka, Merck, etc.) were used without puriﬁcation.

Dry solvents (dichloromethane) were collected from a solvent

dispenser system. All reaction vials were purchased from Beijing

Synthware Glass. Analytical thin-layer chromatography was performed

on GF 254 plates. Flash chromatography was performed on silica gel

■

(

200−300 mesh) by standard technical eluting with solvents as

indicated. Reaction temperatures are reported as the temperature of

the bath surrounding the vessel unless otherwise stated. All starting

materials were purchased from Energy Chemical, Macklin, and

Aladdin and used as received unless otherwise stated.

H and C NMR spectra were recorded on a Bruker Avance III

00 MHz NMR Spectrometer at 295 K in CDCl or DMSO-d .

HRMS(ESI) analyses were performed at the EPSRC UK National

Mass Spectrometry Facility (NMSF), Swansea. GC−MS analysis was

conducted on an HP 5973 GCD system using an HP5MS column (30

m × 0.25 mm). Fourier transform infrared (FT-IR) spectra were

performed on a Nicolet AVATAR-360 IR using KBr discs in the range

−

of 4000−400 cm .

10d

Complexes (2, 5, 8−11) and 4 were synthesized according to

our published report. The procedures for preparation of complexes 3,

10b

, and 7 are provided in the Supporting Information.

The synthesis of complex 1 was as follows.

10e

Figure 5. Two interacting NBO orbitals: BD(Ir2-N108) and single

excited orbital RY*(6)Ir1.

1a was synthesized according to our published procedure. 79%

Isolated yield (white solid): H NMR (400 MHz, CDCl ) δ 8.26 (dd,

J = 15.4, 7.5 Hz, 1H), 7.88−7.79 (m, 1H), 7.62 (d, J = 4.7 Hz, 1H),

.40 (dd, J = 4.4, 2.7 Hz, 2H), 7.05 (dd, J = 20.1, 10.3 Hz, 2H) ppm.

and single excited orbital RY*(11) of Ir1 in 1-2D is shown in

Figure S3.These considerable interactions can be proofs for

bimetallic synergistic eﬀects. The bicentral Ir system in C and

D intermediates derived from catalyst 1 makes the Ir−H and

Ir−N bonds weaker, compared to the mononuclear Ir catalysts

C NMR (101 MHz, CDCl ) δ 165.96 (d, J = 11.9 Hz), 131.77 (d, J

7.9 Hz), 125.41 (s), 124.66 (s), 120.21 (s), 112.27 (s), 112.01 (s),

10.56 (s), 105.63 (s), 105.43−105.42 (m), 105.25 (d, J = 25.3 Hz)

ppm. F NMR (377 MHz, CDCl ) δ −103.64 (m), −105.31 (m)

ppm. HRMS (ESI) calcd for C H F NO [M + H ] 232.0574, found:

7 2

and 3. Thus, a conclusion can be reached that bimetallic

232.0581.

10c

eﬀects are crucial for the extraordinary catalytic activity of

catalyst 1. Therefore, the bimetallic eﬀect between two Ir

centers should be the source of higher catalytic activity. Both Ir

elements were highly indispensable parts for the excellent

catalytic activity.

1b was prepared according to our published method. Into a glass

tube were added 1a (0.5 mmol), Cs CO (1 mmol, 2.0 equiv),

benzimidazole (0.75 mmol, 1.5 equiv), and DMF (3 mL); it was then

sealed by a rubber stopper. The glass tube was stirred under a

preheated 120 °C oil bath for 12 h. After ﬁnished, the reaction

solution was diluted by NH Cl solution (20 mL), extracted by ethyl

acetate (EA) (3 × 10 mL), combined the organic phase, washed by

brine (2 × 10 mL), dried by anhydrous sodium sulfate, and

concentrated under reduced pressure, and the residue was applied on

CONCLUSION

■

In summary, a novel well-deﬁned bimetallic bis-NHC Cp*Ir

complex 1 bearing 2-arylbenzo[d]oxazolyl ligands has been

successfully synthesized and fully characterized. The bimetallic

complex consists of a ﬁve-membered iridacycle and a seven-

membered iridacycle, sharing a benzene ring. The distance of

Ir···Ir is 7.5827 Å. The monometallic complexes 2−3 with a

half structure of 1 and the monometallic N∧P (4), N∧O (5),

silica gel chromatography (dichloromethane/methanol). 88% Isolated

yield (white solid): H NMR (400 MHz, CDCl ) δ 8.70 (d, J = 8.5

Hz, 1H), 8.26 (s, 1H), 8.16 (s, 1H), 7.97−7.87 (m, 3H), 7.82 (d, J =

.1 Hz, 1H), 7.72−7.61 (m, 2H), 7.42 (dt, J = 5.5, 3.7 Hz, 2H), 7.28

(s, 4H), 7.25 (d, J = 3.9 Hz, 2H) ppm. C NMR (101 MHz, CDCl )

δ 158.72 (s), 150.35 (s), 144.25 (s), 143.29 (d, J = 14.8 Hz), 141.27

d, J = 38.1 Hz), 139.48 (s), 136.11 (s), 134.73 (s), 133.35 (s),

(

N∧N (6), N∧C

(7), and N∧C

(8−11) bidentate

(Phenyl)

(Carbene)

132.65 (s), 125.92 (s), 124.83 (s), 124.48 (s), 123.94 (s), 123.65 (t, J

= 6.0 Hz), 123.25 (s), 122.81 (s), 121.08 (s), 120.45 (d, J = 10.1 Hz),

Cp*Ir(III) complexes based on a 2-arylbenzo[d]oxazole

backbone were prepared for control experiments. These

complexes 1−11 have been evaluated as catalysts for N-

methylation of amines and nitroarenes as well as reduction of

aldehydes with MeOH. The results showed that the bimetallic

complex 1 exhibited signiﬁcantly higher activity than the

monometallic analogues. A diverse range of amines, aldehydes,

and nitroarenes with electron-donating or -withdrawing

functional groups were well-tolerated. Both kinetic experiments

results and DFT calculations provided evidence for the

presence of a bimetallic synergistic eﬀect between two Ir

110.58 (s), 110.16 (s), 109.50 (s) ppm. HRMS (ESI) calcd for

O [M + H ] 428.1511, found: 428.1537.

Synthesis of 1c: Into a 15 × 150 mm tube were added 1b (1

mmol), methyl iodide (3 mmol), and acetonitrile (2 mL), and it was

then sealed by a rubber stopper. The mixture was stirred under an 82

C oil bath for 12 h. After ﬁnished, the reaction solution was

concentrated under under vacuum, and the residue was washed three

times by ethyl acetate (EA) (3 × 10 mL), which aﬀorded the

corresponding products. 97% Isolated yield (yellow solid): H NMR

(400 MHz, DMSO) δ 10.41 (s, 1H), 10.28 (s, 1H), 8.94 (d, J = 8.6

Hz, 1H), 8.63 (d, J = 2.0 Hz, 1H), 8.50 (dd, J = 8.6, 2.1 Hz, 1H), 8.27

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Complete contact information is available at:

Article

(

d, J = 8.3 Hz, 1H), 8.22 (d, J = 7.6 Hz, 1H), 8.15 (d, J = 7.8 Hz, 1H),

Accession Codes

.88−7.81 (m, 3H), 7.79 (d, J = 3.1 Hz, 1H), 7.77−7.75 (m, 1H),

CCDC 1972543 contains 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 Cam-

bridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

.71−7.66 (m, 1H), 7.63 (d, J = 7.9 Hz, 1H), 7.51−7.46 (m, 1H),

.40 (t, J = 7.6 Hz, 1H), 4.36 (s, 3H), 4.26 (s, 3H) ppm. C NMR

(

101 MHz, DMSO) δ 187.79 (s), 157.70 (s), 149.95 (s), 145.17 (s),

43.92 (s), 140.96 (s), 136.59 (s), 133.11 (s), 132.34 (s), 132.31−

32.29 (m), 131.96 (d, J = 24.2 Hz), 130.70 (s), 128.59 (s), 128.11

1.6 Hz), 125.27−125.09 (m), 120.82 (s), 114.58 (s), 114.36 (s),

AUTHOR INFORMATION

Corresponding Authors

14.24−114.12 (m), 113.97 (s), 113.61 (s), 111.49 (s), 34.17 (s)

■

−

ppm. HRMS (ESI) calcd for C H I N O [M − 2I ] 457.1903,

23 2

found: 457.1923.

Synthesis of 1: Precursor 1c (0.1 mmol) with [Cp*IrCl ] (88 mg,

Zhi-Ming Li − Department of Chemistry, Fudan University,

Shanghai 200433, China; Email: zmli@fudan.edu.cn

Xiu-Feng Hou − Department of Chemistry, Fudan University,

Shanghai 200433, China; Key Laboratory for Green Processing

of Chemical Engineering of Xinjiang Bingtuan, Shihezi 832003,

China; orcid.org/0000-0001-7513-4650; Email: xfhou@

fudan.edu.cn

.11 mmol), NaOAc (82 mg, 1.0 mmol) were added to a 25 mL

Schlenk ﬂask. It was then added into 2 mL of anhydrous

dichloromethane. The reaction mixture was warmed to 35 °C for 8

h. Then ammonium hexaﬂuorophosphate (NH PF ) (48.9 mg, 0.3

mmol) was added. By centrifugal separation, complex 1 was obtained.

0% Isolated yield (yellow solid): FT-IR (KBr):2919 (w), 1573 (w),

475 (s), 1455 (s), 1377 (m), 1225 (w), 1094 (w), 842 (s), 745 (m),

Authors

57 (m). H NMR (400 MHz, DMSO) δ 8.35 (d, J = 18.1 Hz, 2H),

Shuang Huang − Department of Chemistry, Fudan University,

Shanghai 200433, China

Xi Hong − Department of Chemistry, Fudan University,

Shanghai 200433, China

He-Zhen Cui − Department of Chemistry, Fudan University,

Shanghai 200433, China

Bing Zhan − Department of Chemistry, Fudan University,

Shanghai 200433, China

.23 (s, 1H), 8.13 (s, 1H), 8.00 (s, 1H), 7.84 (d, J = 25.3 Hz, 2H),

.47 (d, J = 43.9 Hz, 7H), 4.22 (s, 6H), 1.91 (s, 15H), 1.38 (s, 15H)

ppm. C NMR (101 MHz, DMSO) δ 180.18 (s), 172.41 (s), 164.39

(

s), 152.07 (s), 150.68 (s), 146.00 (s), 139.89−139.41 (m), 139.18

d, J = 33.1 Hz), 136.89−135.97 (m), 131.30 (s), 131.12 (d, J = 32.2

Hz), 127.40 (s), 125.31 (s), 124.89 (s), 124.88−124.82 (m), 124.88−

23.51 (m), 122.01 (s), 116.85 (s), 112.27 (dd, J = 71.6, 44.2 Hz),

11.64 (s), 93.45 (s), 91.82 (s), 36.53 (s), 35.09 (s), 9.75 (s), 9.00 (s)

ppm. F NMR (377 MHz, DMSO) δ −69.19 (s), −71.08 (s) ppm.

P NMR (162 MHz, DMSO) δ −144.19 (hept.) ppm. HRMS (ESI)

https://pubs.acs.org/10.1021/acs.organomet.0c00464

−

calcd for C H Cl F Ir N OP [M − PF ] 1182.2807, found:

182.2835.

General Procedure for the N-Methylation of Amines or

Nitroarenes. A mixture of amines or nitroarenes (0.5 mmol), catalyst

Notes

The authors declare no competing ﬁnancial interest.

(

Ir: 0.5 mol %), KO Bu (56 mg, 1.0 equiv), and MeOH (1 mL), in a

5 mL pressure tube with a magnetic bar, was stirred at the desired

ACKNOWLEDGMENTS

temperature for the desired reaction time. After cooling to room

temperature, the solvents were removed under vacuum, and the

residue was puriﬁed by column chromatography (petroleum ether/

ethyl acetate (PE/EtOAc) 50:1−1:1). The product was analyzed by

NMR spectroscopy.

■

We are grateful for the ﬁnancial support from the National Key

R&D Program of China (no. 2018YFF0215400) and the

Shanxi Science and Technology Department, China (project

no. MH2014-07).

General Procedure for the Reduction of Aldehydes. A

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