Hetero- A nd Homobimetallic Complexes Bridged by a Bis(NHC) Ligand: Synthesis via Selective Sequential Metalation and Catalytic Applications in Tandem Organic Transformations

Rajeev C. Nishad, Shashi Kumar, and Arnab Rit*

Article

Hetero- and Homobimetallic Complexes Bridged by a Bis(NHC)

Ligand: Synthesis via Selective Sequential Metalation and Catalytic

Applications in Tandem Organic Transformations

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sı Supporting Information

ABSTRACT: A (bis)azolium salt [L1-H ]Br₂(5), synthesized

following multistep procedures, was realized to be a suitable platform

III

for accessing the bis(NHC) ligand supported heterobimetallic Ir -M

(

M = Pd /Au ) complexes via a sequential metalation strategy for

their potential catalytic applications in one-pot tandem organic

transformations. First, the reaction of 5 with 0.5 equiv of

III

[

Ir(Cp*)Cl ] selectively yielded a monometallic Ir complex 6,

which was further metalated using Pd(OAc) /NaOAc to aﬀord the

heterobimetallic Ir -Pd complex 7. On the other hand, complex 6

III

was reacted with Ag O, followed by transmetalation with [Au-

III

(

SMe )Cl] in a one-pot manner, to yield the Ir -Au complex 8.

Further, the related homobimetallic Ir and Pd complexes 9 and 10,

respectively, have also been synthesized directly from [L1-H ]Br . All

III

the homo/heterobimetallic complexes have been well-characterized by multinuclear NMR spectroscopy, ESI-mass spectrometry, and

III

via single-crystal X-ray diﬀraction studies of the complexes 7, 8, and 10. The heterobimetallic Ir -Pd complex 7 has been tested as a

catalyst for three one-pot tandem catalytic reactions: (a) Suzuki−Miyaura coupling and transfer hydrogenation of ketones, (b)

hydrodeﬂuorination and transfer hydrogenation of ketones, and (c) hydrodehalogenation and transfer hydrogenation of imines.

Importantly, the catalytic activity of heterobimetallic complex 7 in the above-mentioned reactions was found to be better than the

mixture of their corresponding homobimetallic counterparts 9 and 10, keeping the concentration of the metal centers constant.

These observations aﬃrm some sort of cooperativity between the two metal centers (Ir and Pd) connected via a single ligand frame

in 7 when catalytic activity is concerned, which thus constitutes a superior catalytic system than that of the cases where two separate

metal complexes (hence, the two metal centers are not connected by a single ligand framework) are used.

INTRODUCTION

Bimetallic catalysts, especially the heterobimetallic ones,

supported by a single ligand framework, are excellent

candidates for orthogonal tandem catalysis. In these catalytic

systems, two diﬀerent metals can catalyze two or more distinct

types of reactions with some synergism between the two metals

leading to overall improved reactivity and selectivity compared

to the equivalent catalyst systems comprising their homo-

■

Mother nature has always been a huge source of information

that oﬀers many lessons for those who seek knowledge and

take the time to reﬂect and discover. From discovering drugs to

designing an eﬃcient catalytic system, nature has always been a

fundamental source of motivation. Inspired by nature, tandem

catalysis has opened up a new dimension of eﬃcient catalytic

systems, transcending the conventional catalytic methods for

organic synthesis by linking two or more fundamentally

diﬀerent reactions in one pot, which substantially reduces the

amount of solvents, catalysts, workup procedures, and reaction

bimetallic or monometallic counterparts. Although it may

sound very straightforward, synthesis of these heterobimetallic

complexes supported by a single ligand frame endures an

important threshold of compatibility; e.g., both the catalytic

centers must sustain the same reaction conditions (such as

solvent, additives, reaction temperature, and reaction time),

a,b

duration.

Apart from that, the growing demand for

environmentally benign and economical synthetic procedures

has also compelled us to develop one-pot multiple catalytic

transformations to obtain the desired product in the most

eﬃcient ways. Orthogonal tandem catalysis is a subset of one-

pot catalysis involving sequential processes functioning

through two or more distinct and non-interfering catalytic

Received: January 12, 2021

Published: April 1, 2021

cycles.

2021 American Chemical Society

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Figure 1. Previously reported heterobimetallic complexes based on the NHC ligand system.

byproducts, catalyst lifetimes, etc., over the whole course of a

catalytic transformation. Ultimately, the stability and adapt-

ability appear to be important factors in synthesizing these

bimetallic catalysts for which the nature of the supporting

homobimetallic counterparts 9 and 10, keeping the concen-

tration of the metal centers constant.

RESULTS AND DISCUSSION

Synthesis of Ligand. The bis(azolium) salt [L1-H ]Br

■

2a,d

ligand plays a crucial role.

N-Heterocyclic carbene (NHC)

based ligand systems are well-known to yield stable metal

(5) was synthesized following the multistep procedure as

3a,b

complexes because of their strong σ-donating nature,

and

shown in Scheme 1a. Reduction of the nitro group in 4-

thus, these ligand systems might be the answer. Presently,

NHCs have established themselves as one of the most suitable

types of ligands in designing new catalysts by providing a large

variety of topological modiﬁcations and coordination capability

Scheme 1. (a) Synthesis of Bis(azolium) Salt [L1-H ]Br .

(b) Ellipsoid Representation of the Molecular Structure of

[L1-H₂]²⁺Cation

to diverse metals in diﬀerent oxidation states. For the

synthesis of heterobimetallic complexes, a variety of NHC

based ligand systems such as ditriazolylidene (ditz), Y-

shaped tris-NHC, phenylene-bridged bis(NHC)/tris-

NHC), etc., have been reported (Figure 1).

(

All the heterobimetallic complexes synthesized from the

above-mentioned ligand systems followed the deprotonation/

metalation strategy; however, an alternate method to obtain

heterobimetallic complexes from the bis-NHC precursor which

follows two chemically diﬀerent reactions (deprotonation/

metalation and oxidative addition) has recently been reported

Counterions and the hydrogen atoms except N-CH-N are removed

by Hahn et al. The precarbene centers (azolium moieties) in

for clarity. N-Ethyl groups are shown in capped stick. Selected bond

distances (Å) and bond angles (deg): N1−C1 1.325(11), N2−C1

1.316(11), N3−C10 1.331(11), N4−C10 1.316(12), N1−C1−N2

the aforementioned ligands are identical and thus possess

almost the same acidities of the N-CH-N protons which make

it diﬃcult to selectively deprotonate/metalate at one site,

leaving the other precarbene moiety free for a diﬀerent metal.

Formation of the corresponding homobimetallic complexes

during monometalation often obscures the synthesis of

heterobimetallic complexes and thus, reduces the yield of the

desired complex. Keeping all these in mind, we have

11.9(9), N3−C10−N4 109.1(9).

bromo-2-nitroaniline using Fe/NH Cl, followed by Phillips

condensation of the generated ortho-diamine with formic

acid in isopropanol at 80 °C, provided the 5/6-bromo-1H-

benzimidazole (2). Ethylation of 2 by using ethyl bromide and

synthesized a (bis)azolium salt [L1-H ]Br₂featuring two

diﬀerent precarbene centers with dissimilar acidities of the N-

CH-N protons to facilitate the selective metalation in a

stepwise manner. Compound 5 proved to provide a suitable

5% aqueous NaOH in acetonitrile provided 4/5-bromo-1-

ethyl-benzimidazole (3) in 80% yield as a mixture of

regioisomers due to 1,3-shift of the NH hydrogen in the

III

platform for accessing the heterobimetallic Ir -Pd /Au

imidazole ring of compound 2 as evident from the H NMR

spectrum (Figure S3). The imidazole moiety was then installed

over the benzimidazole core of 3 following the standard

Ullmann coupling protocol to produce 4. Further quaterni-

zation of the free imine moieties in 4 by reacting with ethyl

bromide in DMF results in the formation of a hygroscopic

bis(azolium) salt [L1-H ]Br , 5, in 87% yield. The H NMR

spectrum of compound 5 in CD CN showing the two highly

downﬁeld shifted signature signals at 10.58 and 10.08 ppm for

the imidazolium and benzimidazolium N-CH-N protons

(Figure S9), respectively, along with the N-Et resonances in

the regions of 4.37−4.76 and 1.61−1.68 ppm conﬁrms the

quaternizations of all the free imine groups.

complexes 7/8, respectively, supported by a single ligand

framework. The heterobimetallic complex 7 was applied to

three diﬀerent tandem reactions: (a) Suzuki−Miyaura (SM)

coupling and transfer hydrogenation (TH) of ketones, (b)

hydrodeﬂuorination (HDF) and transfer hydrogenation (TH)

of ketones, and (c) hydrodehalogenation (HDH) and transfer

hydrogenation (TH) of imines, keeping in mind that the Ir

center is suitable for (transfer)hydrogenation, whereas the Pd

center is suitable for C-C coupling/hydrodehalogenation

reaction. The catalytic outcome of the above-mentioned

reactions clearly demonstrates that heterobimetallic complexes

are advantageous over the combination of their corresponding

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1.319(9), N1 −C1 1.357(9), N2 −C1 1.366(9), N1 −C1 −N2 105.8(5), N3 −C10 −N4 108.8(6), Br1 −Ir −Br2 86.07(3).

Article

Scheme 2. (a) Synthesis of the Monometallic Complex 6. (b) Ellipsoid Representation of the Molecular Structure of Complex

Counterion and the hydrogen atoms except N-CH-N were removed for clarity. N-Ethyl and Cp* groups are shown in capped stick. Selected bond

distance (Å) and bond angle (deg): C1−Ir 2.047(7), C10−H10 0.93, Ir−Br1 2.5467(8), Ir−Br2 2.5530(8), N3−C10 1.337(9), N4−C10

Figure 2. Section of the H NMR of [L1-H ]Br (bottom) and the complex 6 (top) in CD CN, showing the absence of the benzimidazolium N-

CH-N proton and upﬁeld shift of the imidazolium N-CH-N proton.

Further, the ESI-HRMS (positive ions) displaying peaks at

benzimidazolium unit (Scheme 1b), as deduced by NMR and

mass spectrometric analyses.

m/z = 453.0089 (for [M + Na] ion) and m/z = 349.1031 (for

Synthesis of Metal Complexes of the Ligand [L1-

H ]Br . After having the well-characterized ligand [L1-H ]Br

[

M − Br] ion) also supports the composition of compound 5.

However, suitable crystals of compound 5 for X-ray diﬀraction

analysis could not be obtained. Thus, for structure

determination, the bromide counterions were replaced by

non-coordinating PF₆⁻ions to yield [L1-H ](PF ) , 5a

in our hand, we focused on the synthesis of heterobimetallic

complexes via sequential selective metalations. Accordingly, a

III

monometallic Ir complex [(L1-H)Ir(Cp*)Br ]Br, 6, was

6 2

synthesized by reacting [L1-H ]Br with [Ir(Cp*)Cl ] (0.5

2 2

(

Scheme 1a), and appropriate crystals for X-ray diﬀraction

equiv) in the presence of NaOAc as base in acetonitrile for 3 h

studies were obtained via slow evaporation of a methanolic

solution of the compound 5a. The structural analysis of a

crystal unambiguously conﬁrmed the composition of the

ligand, in which the imidazolium moiety is connected to the

(

Scheme 2a). Excess KBr was added to prevent the scrambling

III

of halide ions. The monometallic Ir complex 6 was isolated as

a hygroscopic solid, possibly due to the presence of an

unreacted imidazolium unit. Formation of complex 6 was

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preliminarily established by ESI-mass spectrometry which

shows the peaks at m/z = 757.0918 and 675.1677 (calculated

m/z = 757.0910 and 675.1668) corresponding to the

Scheme 3. Synthesis of Heterobimetallic Complexes 7 and 8

from the Complex 6

molecular ions [M − Br] and [M − 2Br − H] , respectively,

with isotopic distributions of each fragment matching closely

with the calculated patterns.

In the H NMR spectrum of 6, only one downﬁeld shifted

N-CH-N proton signal (out of two observed in [L1-H ]Br ),

which was established to be the imidazolium proton via the

H− H COSY NMR spectrum (Figure S12 ), was present,

conﬁrming the deprotonation of the C2-carbon atom of the

benzimidazolium unit (Figure 2). It is worth mentioning that,

although the benzimidazolium proton is more upﬁeld shifted

than the imidazolium one, it was selectively deprotonated/

metalated to exclusively yield the complex 6 and the reason for

selective metalation at the benzimidazolium site could be due

to the steric diﬀerence between the azolium moieties. Further,

the imidazolium N-CH-N proton is highly upﬁeld shifted to

ambient temperature in a one-pot manner. Importantly, both

III

the heterobimetallic complexes Ir -Pd (7) and Ir -Au (8)

were isolated as air stable material, which is beneﬁcial for their

catalytic applications.

.74 ppm from the equivalent resonance in compound 5 at

0.58 ppm (Figure 2), which clearly manifests the impact of

ﬁrst metalation at the benzimidazole site and thus, expected to

inﬂuence the electronics of the second metal centers at the

imidazole site in the corresponding heterobimetallic com-

plexes.

The complexes 7 and 8 were well-characterized by NMR

spectroscopy, ESI-mass spectrometry, and single-crystal X-ray

diﬀraction studies. The absence of the imidazolium N-CH-N

proton, present in metalloligand 6 at 9.74 ppm, in the H

NMR spectrum of 7 and 8 conﬁrms its deprotonation, and

Coordination of iridium to the generated benzimidazolin-2-

ylidene center was conﬁrmed by the characteristic Ir -bound

further coordination of the generated imidazolin-2-ylidene

III

center to Pd was concluded by their characteristic Pd -C

NHC

C_N_H_Csignal at 173.1 ppm, in the previously reported range for

C{ H} NMR signal at 154.5 ppm. Along with this, the

III

the Ir -C

signal, and the imidazolium carbon signal at

presence of the Ir -C

signal at 170.7 ppm clearly shows

NHC

III

36.5 ppm. Finally, the molecular structure of complex 6 was

unequivocally established by X-ray diﬀraction studies of single

crystals obtained via slow evaporation of a saturated solution of

in chloroform. As concluded from the NMR studies, the X-

ray structure analysis conﬁrmed that the iridium metal is

indeed coordinated to the benzimidazol-2-ylidene center,

leaving the imidazolium unit unaltered (Scheme 2b), which

provides the suitable platform for further metalation to

produce our target heterobimetallic complexes. Importantly,

that the Ir center is still connected to the benzimidazolin-2-

III

ylidine site, supporting the formation of heterobimetallic Ir -

III

Pd complex (7). The Ir -C

signal in 7 (170.7 ppm) is

NHC

upﬁeld shifted than the corresponding value (173.1 ppm) in

complex 6, which certainly suggests the presence of some

electronic interaction between the metal centers through the

ligand framework. The presence of pyridine on the Pd center

was established by the H NMR spectrum in which three sets

of signals in the intensity of 2:2:1 were observed in the usual

metal-bound pyridine region. Further, the composition of

complex 7 was substantiated by the appearance of the highest

single metalation of [L1-H ]Br₂gave us the unique

opportunity to directly compare the metric parameters of the

NHC precursors (azolium salts) and the NHC-metal

complexes. After the metalation at the benzimidazol-2-ylidene

position, the bond lengths of N1−C1 and N2−C1 are

observed to be elongated to 1.357(9) and 1.366(9) Å,

respectively, and the angle N1−C1−N2 is reduced to

intensity peak corresponding to the [M − Br − py] ion at m/z

= 942.9023 (calculated m/z = 942.9051) along with the peaks

at m/z = 1021.9417 (calculated m/z = 1021.9474) and m/z =

860.9759 (calculated m/z = 860.9800) for the [M − Br] and

[M − 2Br − py − H] ions, respectively, in the ESI-MS

III

05.8(5)° as compared to the corresponding parameters in

spectrum. Finally, the structure of Ir -Pd complex 7 was

undoubtedly conﬁrmed by the single-crystal X-ray diﬀraction

the parent ligand 5a [N1−C1/N2−C1 1.325(11)/1.316(11)

Å and N1−C1−N2 111.9(9)°]. However, the metric

parameters of the imidazolium unit remained unaltered as

presumed from the monometalation at the benzimidazolium

III

studies which clearly show that the Ir center is connected to

benzimidazolin-2-ylidene, whereas the Pd center is attached

to the imidazolin-2-ylidene and a pyridine donor in a trans-

fashion (Figure 3). As observed for complex 6, the bond

parameters of the imidazole moiety altered signiﬁcantly after

the metalation. The N3−C10/N4−C10 bond length has

increased to 1.37(3)/1.35(3) Å (1.337(9)/1.319(9) Å in 6),

and the N4−C10−N3 angle has shortened to 104.0(2)° from

108.8(6)° in 6.

site.

Monometallic Ir^I^I^Icomplex, 6, turned out to be a useful

III

synthon for accessing heterobimetallic Ir /M complexes as the

remaining imidazolium N-CH-N proton can be activated by

using a suitable base to generate a free carbene center for

further metalation. Indeed, complex 6 was stable enough to

undergo deprotonation cum metalation to yield the corre-

III

Similarly, coordination of both Au and Ir centers to the

III

sponding heterobimetallic Ir /M complexes 7 and 8 (M = Pd

bis(carbene) L1 was well-established by the characteristic Ir -

III

and Au , Scheme 3). The Ir -Pd complex 7 was synthesized

by the reaction of complex 6 with an equimolar amount of

signal at 172.5 ppm and the Au -C

in the C{ H} spectrum of complex 8, and the

signal at 174.4

NHC

ppm

NaOAc and [Pd(OAc) ] at 80 °C in the solvent mixture of

formation of this complex was also supported by ESI-MS

studies displaying the highest intensity peak corresponding to

the monopositive fragment [M]⁺at m/z = 898.1342

(calculated m/z = 898.1203). X-ray diﬀraction studies of the

pyridine and acetonitrile. Complex 8 was prepared via a well-

known Ag O method in which the complex 6 was reacted with

Ag O, followed by transmetalation with [Au(SMe )Cl] at

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III

−

H − 2Br] validates the formation of a binuclear Ir -NHC

complex 9. Similarly, the dipalladium complex 10 was obtained

by reacting [L1-H ]Br with [Pd(CH CN) Cl ] using K CO

as base in pyridine at 80 °C and was isolated as an air stable

yellow solid in a good yield of 80%. As expected, the H NMR

spectrum clearly shows the absence of azolium protons, which

suggests their complete deprotonation and in situ generation of

Figure 3. Ellipsoid representations of the molecular structures of

compounds 7 (left) and 8 (right). Hydrogen atoms were removed for

clarity. N-Ethyl groups are shown in capped stick. Selected bond

distances (Å) and bond angles (deg) in 7: C1−Ir1 2.04(2), C1−N1

free carbenes whose coordination to the Pd center was

conﬁrmed by the presence of characteristic C{ H} NMR

signals of Pd -bound benzimidazolin-2-ylidene and imidazoli-

at 164.4 and 149.5 ppm,

respectively, as conﬁrmed by 2D NMR analysis. It is worth

4,16a

din-2-ylidene carbons

1.40(3), C1−N2 1.37(3), Ir1−Br 2.552(4)−2.553(7), C10−Pd1

1.94(2), C10−N4 1.35(3), C10−N3 1.37(3), Pd1−Br 2.424(4)−

2.432(4), Pd1−N5 2.128(19), N1−C1−N2 105.0(17), Br1−Ir1−Br2

91.6(2), N4−C10−N3 104.0(2), Br3−Pd1−Br4 176.47(15), C10−

mentioning that the C{ H} NMR chemical shift of the Pd -

connected imidazolidin-2-ylidene carbon in 10 at 149.5 ppm is

shifted upﬁeld when compared to the corresponding signal for

complex 7 at 154.5 ppm, which certainly suggests that the

electronics of one metal center is aﬀected by the second metal

fragment present in this type of bimetallic systems, in line with

our purpose of synthesizing the heterobimetallic complexes for

application in tandem catalytic processes. Observation of a

Pd1−N5 175.3(9). Selected bond distances (Å) and bond angles

deg) in 8: C1−Ir1 2.042(10), C1−N1 1.355(13), C1−N2

.357(12), Ir1−Cl 2.417(3)−2.435(3), C10−Au1 1.987(11), C10−

N4 1.345(13), C10−N3 1.353(12), Au1−Cl 2.284(3), N1−C1−N2

(

06.7(8), Cl1−Ir1−Cl2 84.95(11), N4−C10−N3 105.4(9), C10−

Au1−Cl3 177.3(3).

peak at m/z = 840.7987 (calculated m/z = 840.7985) for the

[

M + CH CN − Br − pyridine] ion in the ESI-MS spectrum

crystals, obtained by vapor diﬀusion of n-pentane into a

saturated solution of complex 8 in chloroform, conﬁrmed the

further supports the formation of dipalladium complex 10, and

the structure was ﬁnally conﬁrmed by the single-crystal X-ray

diﬀraction studies of the crystals obtained by slow evaporation

of its saturated methanol solution (Scheme 4b). The molecular

III

formation of heterobimetallic Ir -Au bis(NHC) complex with

III

the Ir center coordinated by benzimidazolin-2-ylidene while

the Au center by the imidazolin-2-ylidene donor (Figure 3).

Here also, the increase of the C10−N3/C10−N4 (of the

imidazole fragment) bond lengths to 1.353(12)/1.345 (13) Å

and the decrease in the N3−C10−N4 bond angle to 105.4(9)°

III

Scheme 4. (a) Synthesis of Homobimetallic Ir - and Pd -

NHC Complexes. (b) Ellipsoid Representation of the

Molecular Structure of Compound 10

from that in 6 were observed after metalation to Au .

After having the well-characterized heterobimetallic com-

plexes 7 and 8 in hand, we focused on their catalytic activity in

various one-pot multiple reactions which are tandem

combinations of mechanistically distinct transformations, the

main aim of this paper. Further, we were also interested to ﬁnd

out whether these heterobimetallic complexes supported by a

single-ligand frame impart some synergistic cooperation

between the metal centers for these tandem catalytic

transformations or not. To learn if any synergism exists

between the diﬀerent metal centers in heterobimetallic

catalysts, one way would be to compare their catalytic

performances with the equivalent catalytic systems derived

Hydrogen atoms are removed for clarity. Selected bond distances

from their homobimetallic counterparts. Hence, we have also

III

(Å) and bond angles (deg): C1−Pd1 1.958(8), C1−N1 1.343(10),

synthesized the corresponding homobimetallic Ir and Pd

C1−N2 1.351(10), Pd1−Br 2.4208(11)−2.4254(11), Pd1−N3

complexes 9 and 10, respectively, directly from the ligand [L1-

.104(7), C10−Pd2 1.983(8), C10−N4 1.344(10), C10−N5

.339(10), Pd2−Br 2.4233(12)−2.4135(12), Pd2−N6 2.098(6),

H ]Br .

Complex 9 was prepared by heating a mixture of [L1-

H ]Br , [Ir(Cp*)Cl ] , and KBr at 65 °C for 24 h using

N1−C1−N2 107.5(7), Br1−Pd1−Br2 176.25(5), N4−C10−N5

104.6(7), Br3−Pd2−Br4 176.25(5).

2 2

Cs CO as a base and isolated in 75% yield as an air stable

yellowish-orange solid. The H NMR spectrum of the complex

shows the absence of both acidic N-CH-N azolium protons,

structure reveals that both the Pd^I^Icenters have the same

geometry around them with the only diﬀerence being the

NHC donors (benzimidazolin-2-ylidene vs imidazolin-2-

ylidene), and thus, bond parameters are observed to be similar.

Catalytic Applications of the Synthesized Complexes.

The heterobimetallic Ir -Pd complex 7 was chosen as a

catalyst for one-pot tandem catalytic transformations, and

initially, a basic reaction which is a combination of Suzuki−

Miyaura cross-coupling and transfer hydrogenation of ketone

was examined as a proof of concept because of the presence of

Ir (suitable for hydrogenation) and Pd (suited for C-C

indicating their deprotonation and, thus, generation of carbene

donors, and additionally, one aromatic proton of the

benzimidazole backbone was also missing, which suggests

III

orthometalation of one Ir center with the aromatic ring

III

which was conﬁrmed by the characteristic C{ H} NMR

13a

signal of the orthometalated carbon at 142.4 ppm. Along

III

with this, complex 9 shows two characteristic Ir -C

NHC

2a,15

signals

at 165.0 and 163.8 ppm which are upﬁeld shifted

III

as compared to the monoiridium complex 6 (Ir -C_N_H_C, 173.1

ppm). Further, the ESI-mass spectrum displaying the

maximum intensity peak at m/z = 1001.2306 (calculated m/

z = 1001.2308) corresponding to the molecular fragment [M

coupling) centers. To aﬀord the best catalytic outcome,

catalyst loading, base, and reaction time have been altered

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Table 1. Tandem Suzuki−Miyaura Coupling and Transfer Hydrogenation of p-Bromoacetophenone

conversion (%)

entry

cat (mol %)

7 (2)

7 (1)

9 (1)

10 (1)

9 + 10 (0.5 + 0.5)

time (h)

base

Cs CO₃

K CO₃

KOH

KO Bu

7 (1)

KO Bu

Reaction conditions: p-bromoacetophenone (0.05 mmol), phenylboronic acid (0.065 mmol), base (0.15 mmol), catalyst (x mol %), isopropanol

(2 mL), reﬂux. Conversions were determined by GC−MS using mesitylene as internal standard. 1-(4-Bromophenyl)ethan-1-ol was obtained in

0%. p-Bromoacetophenone and 1-(4-bromophenyl)ethan-1-ol were observed in 54% and 45%, respectively. The reaction was performed in the

presence of excess mercury (300 times of the catalyst).

Table 2. Tandem Hydrodeﬂuorination and Transfer Hydrogenation of p-Fluoroacetophenone

conversion (%)

entry

cat (mol %)

7 (2)

7 (1)

7 (2)

9 + 10 (1 + 1)

7 (2)

time (h)

temp (°C)

base

(R = Me)

(R = Et)

(R = Ph)

120

Cs CO

KO Bu

7 (2)

KO Bu

Reaction conditions: p-ﬂuoroacetophenone (0.05 mmol), base (0.15 mmol), catalyst (x mol %), isopropanol (2 mL). Conversions were

determined by GC−MS using mesitylene as internal standard.

(

Table 1). The reaction using KO Bu as base and 1 mol % of 7

corresponding homobimetallic complexes 9 and 10 in half

equivalent (each in 0.5 mol % to maintain the same

equivalence of the metal centers as of the complex 7) of the

heterobimetallic case has been carried out (entry 8).

Gratifyingly, the heterobimetallic complex 7 was found to be

a better catalyst than that of a mixture of 9 and 10 (75% of the

desired product A and 19% of 1-phenylethanol, C). These

observations undoubtedly conﬁrm that some sort of cooper-

ativity exists between the two metal centers (Ir and Pd)

connected via a single ligand framework in 7 when catalytic

activity is concerned. The complex 7 constitutes a superior

catalytic system than that of the cases where two separate

metal complexes are used and hence, the two metal centers are

not connected by a single ligand framework. The present

in isopropanol for 12 h were found to be the best conditions

entry 5) as the desired compound A (generated via C-C

(

coupling as well as transfer hydrogenation reaction in one-pot)

was obtained in 87% yield along with 8% of the only C-C

coupled ketone (B) and 5% of the 1-phenylethanol (C),

generated via hydrodehalogenation. Isopropanol in this

reaction acts as both solvent and hydride source for the

transfer hydrogenation process. Longer reaction time has an

adverse eﬀect on the formation of the desired product (A) as it

increases the amount of dehalogenation which yields C (entry

To ﬁnd out the advantages of installing two diﬀerent metal

centers in a single frame (complex 7) during catalysis, if

III

present, a comparative study with a mixture of two

catalyst system exhibited better activity than the related Rh -

Organometallics 2021, 40, 915−926

Organometallics

Article

Pd complex developed by Hahn et al. which, in the presence

of a higher catalyst loading of 2 mol % and longer reaction

duration, yielded only 53% of A along with 46% of the only

coupled product B. Furthermore, as anticipated, the

III

homobimetallic Ir complex 9 is only suitable for transfer

hydrogenation as established by the generation of only transfer

hydrogenated product 1-(4-bromophenyl)ethan-1-ol in 70%

and the desired product in 8% (entry 6). Moreover, when the

reaction was performed in the presence of excess elemental

mercury, no considerable change in the catalytic activity and

the selectivity of the process was observed (entry 10 vs 5). This

ﬁnding discards the possibility of a nanoparticle catalyzed

heterogeneous process.

Motivated by the success in tandem Suzuki−Miyaura

coupling/transfer hydrogenation reactions, especially with 2

mol % catalyst loading for 20 h data, which resulted in the

formation of 34% of debrominated product C (Table 1, entry

), which indicates that the catalyst 7 may also be suitable for

Figure 4. Time-dependent reaction proﬁle of the hydrodeﬂuorina-

tion/transfer hydrogenation of p-ﬂuoroacetophenone in isopropanol

using catalyst 7. Reaction conditions: p-ﬂuoroacetophenone (0.05

dehalogenation reaction, we thought of expanding the

utilization of this catalytic system toward the activation of

the very strong C−F bond (bond dissociation energy = 513.8

mmol), KO Bu (0.15 mmol), isopropanol (2 mL), catalyst 7 (2 mol

10.0 kJ/mol). The functionalization of the C−F bond is

), 120 °C, 20 h. Reaction progress was monitored by GC−MS.

important due to its potential applications in pharmaceuticals,

17b,c

especially as bioisosteres of C−H bonds and many more.

higher than that of E. Further, hydrodeﬂourination is much

slower than transfer hydrogenation as manifested by the

presence of a lesser amount of acetophenone (F) at any time

of the reaction, which possibly indicates that the hydro-

Further interest in the C−F bond activation emerged from the

rising public concern over the impact of long-lived

anthropogenic “super-greenhouse gases” like CFCs, HFCs,

PFCs (lifetime > 2000 years), etc., in the upper atmosphere.

deﬂuorination is the rate limiting step of the overall reaction.

Accordingly, we planned a tandem reaction of hydro-

deﬂuorination/transfer hydrogenation of p-ﬂuoroacetophe-

none to yield 1-phenylethanol which has never been performed

before with any catalyst systems, to the extent of our

knowledge. After altering all the reaction parameters (temper-

ature, time, base, and catalyst loading), the best outcome

As soon as it is produced, transfer hydrogenation of the ketone

functionality generates the ﬁnal 1-phenylethanol (E) which is

also endorsed by the presence of 1-(4-ﬂuorophenyl)ethanol

(G) in a decent amount at the beginning of the reaction itself.

Amines constitute a valuable class of compounds as they are

important building blocks in organic synthesis mainly for

(

formation of the desired product E in 92% along with 8% of

pharmaceuticals, agrochemicals, and fragrance molecules.

the acetophenone F) was achieved when the reaction was

performed with 2 mol % of 7 at 120 °C for 20 h in isopropanol

Selective synthesis of an amine via (transfer)hydrogenation of

an imine is a desirable approach, although this conversion is

considered to be more diﬃcult than that of aldehydes and

ketones. Given the easy access to diverse halogenated imines

via simple condensation of the corresponding aniline and

aldehyde derivatives, a tandem combination of dehalogenation

and transfer hydrogenation would possibly be a reasonable

approach for the selective synthesis of functionalized amine

derivatives. Accordingly, a combination of hydrodehalogena-

tion and transfer hydrogenation of 4-halo-N-benzylideneaniline

derivatives to yield N-benzylaniline was selected.

(

Table 2; entry 3).

Although the reaction temperature of 80 °C is enough for

transfer hydrogenation (100%, entry 5), higher temperature

appears to be necessary for hydrodeﬂuorination (entry 3 vs 5).

To uncover whether the catalytic outcome beneﬁts from the

presence of two diﬀerent metal centers connected by a single

ligand framework (as observed before), comparative studies

with a mixture of the two corresponding homobimetallic

complexes (9 and 10; entry 6) have been conducted. The

results show an obvious advantage of the heterobimetallic

catalyst over a mixture of homobimetallic catalyst systems in

the tandem catalytic reaction and therefore, validate our

postulate. Further, to explore the general applicability of the

present catalytic system, p-ﬂuoropropiophenone and p-

ﬂuorobenzophenone were also screened under the optimized

conditions, and the desired products were obtained in 97% and

III

To study the scope of our Ir -Pd catalyst 7, N-(4-

bromophenyl)-1-phenylmethanimine was chosen as model

substrate, and the best catalytic outcome was aﬀorded by

utilizing NaO Bu as base, 2 mol % of 7, under reﬂux condition

for 12 h, which provided the desired compound N-benzylani-

line in near quantitative yield (96%; Table 3, entry 3) with a

trace amount of decomposed aniline and benzyl alcohol. As

observed for the previous two reactions, here also, complex 7

provides superior activity than the comparable catalytic

systems derived from a mixture of the corresponding

homobimetallic complexes 9 and 10 (entry 3 vs 4). Further,

6% yield, respectively, along with 3−4% of the corresponding

deﬂuorinated ketones (entries 7 and 8).

To get some insight into the reaction pathway, we studied

the kinetics of this reaction, by investigating the formation of

1-phenylethanol from p-ﬂuoroacetophenone as a function of

a blank experiment with NaO Bu resulted mostly in the

time under optimized reaction conditions (Figure 4). From the

reaction proﬁle, it is evident that at the initial stage, p-

ﬂuoroacetophenone was rapidly consumed and converted

simultaneously to 1-(4-ﬂuorophenyl)ethanol (G) and 1-

phenylethanol (E), although the rate of formation of G is

decomposition of the imine substrate (entry 5). On the basis of

our initial screening, we proceeded with catalyst 7 and the

scope of the reaction was explored by applying a variety of

aromatic aldimines featuring variations on both C-phenyl and

N-phenyl rings (Table 4, 11a−m).

Organometallics 2021, 40, 915−926

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Article

Table 3. Optimization of Tandem Transfer-Hydrogenation and Hydrodebromination of N-(4-Bromophenyl)-1-

phenylmethanimine to N-Benzylaniline by Using Catalyst 7

conversion (%)

entry

cat (mol %)

7 (2)

9 + 10 (1 + 1)

time (h)

base

KO Bu

KOH

NaO Bu

Reaction conditions: N-(4-bromophenyl)-1-phenylmethanimine (0.05 mmol), base (0.1 mmol), isopropanol (2 mL), catalyst 7 (2 mol %), 4 Å

molecular sieve. Conversions were determined by GC−MS using mesitylene as internal standard. 4-Phenyl-butan-2-ol was obtained in 38%. 8%

unidentiﬁed product was observed. N-(4-Bromophenyl)-1-phenylmethanimine (13%) and 4-bromoaniline (60%) were also observed.

Table 4. Substrate Scope for Tandem

Hydrodehalogenation/Transfer Hydrogenation Reactions

yield the N-benzylideneaniline (12k) in 95%, making it a really

eﬃcient catalyst for this transformation.

CONCLUSION

■

III

In conclusion, the heterobimetallic Ir -M (M = Pd /Au )

complexes 7 and 8, connected via a mixed donor bis(NHC)

ligand 5, have been synthesized following a sequential selective

III

metalation strategy. The heterobimetallic Ir -Pd complex 7

has been tested as a catalyst for three one-pot tandem catalytic

reactions which are Suzuki−Miyaura coupling/hydrodeﬂuori-

nation (HDF) and transfer hydrogenation (TH) of ketones,

and hydrodehalogenation (HDH) and transfer hydrogenation

of imines. To our delight, along with the Suzuki−Miyaura

coupling/TH, the heterobimetallic catalyst 7 was observed to

be highly active for the HDF reaction, and for the ﬁrst time, a

tandem combination of HDF and TH of ketone was

successfully catalyzed (up to 98% yield) by a heterobimetallic

catalyst. Further, catalyst 7 was also found to be eﬃcient for

the HDH/TH of imines to obtain amine derivatives in near

quantitative yields. Importantly, it is evident from the catalytic

studies of all three aforementioned reactions that hetero-

bimetallic catalyst 7 shows a clear advantage over the

equivalent catalytic systems generated by combining the

General conditions: imine (0.05 mmol), NaO Bu (0.1 mmol),

isopropanol (2 mL), catalyst 7 (2 mol %), 4 Å molecular sieve.

Conversions were determined by GC−MS using mesitylene as

internal standard.

III

corresponding homobimetallic Ir and Pd catalysts 9 and

0, respectively. These ﬁndings support our postulate that

some sort of cooperativity exists between two diﬀerent metal

centers (Ir and Pd) bridged by a single frame ligand which

beneﬁts the catalytic activity of the heterobimetallic complexes

in complex organic transformations. However, at this moment,

the detailed reaction mechanism and mode of cooperativity

between the metal centers are unclear, which need further

research in this direction. Further, catalytic applications of the

heterobimetallic Ir /M (M = Pd /Au ) catalysts 7 and 8 in

other tandem organic transformations are underway in our

laboratory.

Methyl and methoxy substituents on either C- or N-phenyl

rings provided essentially full conversions, and the correspond-

ing amines were attained in 97−100% yields (Table 4, 12b−e).

Further, bromide on the N-phenyl ring also works well (12f).

After the success with bromo-substituted N-benzylideneaniline

derivatives, we tried our venture toward the chloro analogues,

keeping in mind that C−Cl bonds (BDE = 394.9 ± 13.4 kJ/

mol) are stronger than C−Br bonds (BDE = 318.0 ± 8.4 kJ/

mol). To our delight, an excellent catalytic outcome was

also achieved with catalyst 7 for the tandem hydrodechlori-

nation and transfer hydrogenation reactions. Under the

optimized conditions, N-(4-chlorophenyl)-1-phenylmethani-

mine was almost quantitatively converted to N-benzylaniline

III

EXPERIMENTAL SECTION

General Considerations. All reactions were carried out under an

inert atmosphere using standard Schlenk line and/or Glove box

techniques unless stated otherwise. Glassware was oven-dried at 130

■

(

Table 4, 12g). Further, other (methyl, methoxy) substituents

on either phenyl rings of the imine moiety were well-tolerated

C prior to use. All the solvents were distilled by standard procedures

to provide the ﬁnal product amines in excellent yields (97−

prior to use. H and C{ H} NMR spectra were recorded on Bruker

400 and 500 MHz FT-NMR spectrometers. Chemical shifts (δ) are

expressed in ppm downﬁeld from tetramethylsilane (TMS) using the

00%; 12i,j and 12l,m). Importantly, double hydrodechlori-

nation in addition to imine reduction (11k) is also possible to

Organometallics 2021, 40, 915−926

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Article

residual protonated solvent as an internal standard ( H: CDCl 7.26

tube, 3 mL of dry DMF was added, and the reaction mixture was

heated to 90 °C for 24 h. During the reaction, a white colored solid

started precipitating which was isolated by ﬁltration, washed with

diethyl ether (2 × 10 mL), and dried in vacuo to get [L1-H ]Br as an

ppm, CD CN 1.94 ppm, and DMSO-d 2.50 ppm; C{ H}: CDCl

7.1 ppm, CD CN 1.32 and 118.26 ppm, and DMSO-d 39.52 ppm).

Coupling constants are expressed in hertz (Hz). High resolution ESI-

mass spectra were recorded with an Agilent 6545A Q-TOF. EtBr,

imidazole, iron powder, CuO, NH Cl, Ag O, Cs CO , and NH PF

oﬀ-white hygroscopic solid. Yield: 1.4 g (3.28 mmol, 87%). H NMR

(400 MHz, CD CN): δ 10.58 (s, 1H), 10.08 (s, 1H), 9.03 (s, 1H),

were purchased from commercial sources and used as received

8.30 (s, 1H), 8.14 (bs, 2H), 7.74 (s, 1H), 4.73 (q, J = 7.5 Hz, 2H),

4.58 (q, J = 7.5 Hz, 2H), 4.40 (q, J = 7.5 Hz, 2H), 1.69−1.59 (m, 9H)

without further puriﬁcation.

Synthesis of 4/5-Bromobenzimidazole (2).

A 100 mL

ppm. C{ H} NMR (126 MHz DMSO-d ): δ 144.2 (N-CH-N),

Schlenk tube was charged with 4-bromo-2-nitroaniline (1.0 g, 4.6

135.9 (N-CH-N), 132.9, 131.3, 131.1, 123.2, 121.6, 120.4, 115.5,

108.0, 44.9, 42.7, 42.5 (N-CH ), 14.8, 14.3, 14.1 (N-CH CH ) ppm.

MS (ESI, positive ions): m/z 453.0089 (calculated for [M + Na]

mmol), Fe powder (3.09 g, 55.2 mmol), and NH Cl (2.95 g, 55.2

mmol). Then isopropanol (40 mL) and conc. formic acid (18 mL)

were added to it, and the reaction mixture was reﬂuxed for 12 h at 80

453.0083).

°C. After the completion, the reaction mixture was cooled and the

Synthesis of [(L1-H)Ir(Cp*)Br

]Br (6). A mixture of [Ir(Cp*)-

]Br (100 mg, 0.232 mmol),

greenish solution was ﬁltered through a Buchner funnel. The ﬁltrate

was then concentrated by rotary evaporation, and the obtained

residue was dissolved in 15 mL of DCM followed by washing with

saturated sodium bicarbonate solution to neutralize the unreacted

formic acid. After separation, the organic phase was dried over

]

(93 mg, 0.116 mmol), [L1-H

NaOAc (21.0 mg, 0.255 mmol), and an excess of KBr in acetonitrile

(5 mL) was stirred at 85 °C for 3 h in a pressure tube. After the

reaction, the obtained solution was ﬁltered through a pad of Celite

and all the volatiles from the ﬁltrate were removed in high vacuo. The

residue was further puriﬁed by silica-gel column chromatography

(DCM:MeOH = 95:5 v/v) to provide compound 6 as an orange

anhydrous MgSO and concentrated in vacuo to provide a yellow

solid which was then puriﬁed via silica-gel column chromatography by

using DCM:MeOH (98:2 v/v) as eluent. Yield: 0.770 g (3.91 mmol,

hygroscopic solid. Yield: 130 mg (0.155 mmol, 67%). H NMR (500

5%). H NMR (400 MHz, CDCl ): δ 8.07 (s, 1H, benzimidazole N-

MHz, CD CN): δ 9.74 (t, J = 1.8 Hz, 1H), 8.04 (d, J = 2.1 Hz, 1H),

CH-N proton), 7.82 (s, 1H), 7.53 (d, J = 8.6 Hz, 1H), 7.41 (dd, J =

7.92 (t, J = 1.8 Hz, 1H), 7.77 (d, J = 8.7 Hz, 1H), 7.67 (t, J = 1.8 Hz,

1H), 7.61 (dd, J = 8.7, 2.1 Hz, 1H), 5.12−5.07 (m, 2H), 4.37 (m,

.6, 1.7 Hz, 1H) ppm. C{ H} NMR (101 MHz, CDCl ) δ 141.7,

39.1, 136.8, 126.4, 118.6, 116.8, 116.2 ppm. MS (ESI, positive ions):

4H), 1.60−1.54 (m, 25H) ppm. C{ H} NMR (126 MHz, CD CN):

III

m/z 196.9711 (calculated for [M + H] 196.9714).

δ 173.1 (Ir -CNHC), 136.5, 136.4, 136.3, 131.3, 123.7, 123.0, 113.5,

107.4, 91.5 (Cp*), 46.3, 46.1, 46.0, 17.0, 16.9, 15.5, 9.5 (Cp*) ppm.

Synthesis of 4/5-Bromo-1-ethyl-1H-benzimidazole (3). In a

round-bottom ﬂask, 4/5-bromobenzimidazole (0.5 g, 2.53 mmol),

MS (ESI, positive ions): 757.0918 (calculated for [M − Br]

5% NaOH aq. soln. (2 mL), and 20 mL of acetonitrile were taken.

757.0910), 675.1677 (calculated for [M − 2Br − H] 675.1668),

Then ethyl bromide (0.226 mL, 3.03 mmol) was added dropwise to

this suspension, which was stirred for 24 h at room temperature. After

the reaction time, all the volatiles were removed by a rotary

evaporator, diluted with 20 mL of DCM, and washed with water.

After separation, the organic phases were combined and dried over

338.0898 (calculated for [M − 2Br] 338.0873).

Synthesis of Complex (7). A mixture of 6 (50 mg, 0.06 mmol),

Pd(OAc) (13.5 mg, 0.06 mmol), NaOAc (5 mg, 0.06 mmol), and an

excess of KBr in acetonitrile (2 mL) and pyridine (3 mL) solvent

mixture was heated at 80 °C overnight in a Schlenk tube. After the

completion of the reaction, the resultant solution was ﬁltered through

a pad of Celite and all the volatiles were removed in high vacuo. The

obtained residue was further puriﬁed by silica-gel column chromatog-

raphy (DCM:MeOH = 95:5 v/v), followed by crystallization, to yield

MgSO , and removal of all the volatiles under reduced pressure

yielded a mixture of regioisomers of (4/5)-bromo-1-ethyl-1H-

benzimidazole as a yellow viscous liquid. The obtained viscous liquid

was further puriﬁed via silica-gel column chromatography

the compound 7 as a yellow solid. Yield: 34.0 mg (0.031 mmol, 52%).

(

DCM:MeOH = 99:1 v/v). Yield: 0.427 g (1.89 mmol, 75%). H

NMR (500 MHz, CDCl ) δ 7.94 (d, J = 1.7 Hz, 1H), 7.89 (m, 2H),

H NMR (400 MHz, CDCl ) δ 8.82−8.81 (m, 2H), 8.59 (s, 1H),

.66 (d, J = 8.6 Hz, 1H), 7.56 (d, J = 1.7 Hz, 1H), 7.41−7.36 (m,

7.71−7.67 (m, 1H), 7.58−7.52 (m, 2H), 7.28 (s, 1H), 7.26−7.21 (m,

2H), 7.17 (d, 1H), 5.24−5.06 (m, 2H), 4.85−4.70 (m, 2H), 4.34−

H), 7.27 (d, J = 8.6 Hz, 1H), 4.22−4.16 (m, 4H), 1.55−1.51 (m,

H) ppm. ¹³C{ H} NMR (101 MHz, CDCl ): δ 145.2, 143.3, 143.0,

4.26 (m, 2H), 1.73−166 (24H) ppm. C{ H} NMR (126 MHz,

III

42.9, 134.7, 132.6, 125.9, 125.4, 123.2, 121.7, 116.2, 115.2, 112.7,

CDCl ): δ 170.7 (Ir -CNHC), 154.5 (Pd -CNHC), 152.6, 149.5, 138.0,

10.8, 40.1, 40.0, 15.2 (broad singlet) ppm. MS (ESI, positive ions):

135.1, 135.07, 135.0, 125.2, 124.7, 123.9, 121.8, 120.9, 111.3, 110.6,

m/z 225.0029 (calculated for [M + H] 225.0027).

90.3 (Cp*), 46.8, 45.8, 45.6, 16.7, 16.6, 15.6, 9.6 (Cp*) ppm. MS

Synthesis of 1-Ethyl-5/6-(1H-imidazol-1-yl)-1H-benzimida-

zole (4). N-Ethyl-4/5-bromobenzimidazole (0.774 g, 3.43 mmol),

imidazole (0.9 g, 13.76 mmol), K CO (1.9 g, 13.76 mmol), and CuO

(ESI, positive ions): 1021.9417 (calculated for [M − Br]

1021.9474), 942.9023 (calculated for [M-Br-pyridine] 942.9051),

860.9759 (calculated for [M − 2Br − H − pyridine] 860.9800).

(0.068 g, 0.858 mmol) were dissolved in dry DMSO (10 mL). Then

Synthesis of Complex (8). A mixture of 6 (50 mg, 0.06 mmol)

the reaction mixture was stirred at 150 °C for 48 h, after which the

reaction mixture was partitioned between water and DCM. The

organic layer was separated, and the aqueous layer was extracted with

DCM. The combined organic phases were washed with brine and

and Ag O (8.5 mg, 0.035 mmol) in 5 mL of DCM was stirred

overnight at room temperature in a Schlenk tube under the exclusion

of light. Then [Au(SMe )Cl] (19.5 mg, 0.065 mmol) was added to

the reaction mixture, which was again stirred for an additional 12 h.

After the completion of reaction, the resultant solution was ﬁltered

through a pad of Celite and all the volatiles from the ﬁltrate were

removed in high vacuo. The obtained product was washed with

diethyl ether (2 × 5 mL) and dried under high vacuum to yield a

yellow-orange solid which was crystallized from a chloroform solution

dried over MgSO , and all solvents were removed in vacuo. The crude

product was loaded on a silica-gel column and eluted with

DCM:MeOH (95:5 v/v) to provide compound 4 (as a mixture of

regioisomers) as a light brownish liquid. Yield: 0.58 g (2.75 mmol,

0%). H NMR (500 MHz, CDCl ): δ 8.02 (d, J = 4.9 Hz, 2H), 7.89

(

d, J = 8.4 Hz, 3H), 7.82 (d, J = 1.7 Hz, 1H), 7.50 (d, J = 8.4 Hz, 1H),

via slow diﬀusion of Et

compound 8. The presence of chloride instead of bromide at the

iridium center in 8 possibly happened either during the Ag O reaction

in DCM or during the crystallization from chloroform, which is not

O at ambient temperature to yield the

.41 (d, J = 1.7 Hz, 1H), 7.36 (d, J = 1.9 Hz, 1H), 7.35−7.31 (m,

H), 7.23 (d, J = 4.6 Hz, 2H), 4.29 (p, J = 7.3 Hz, 4H), 1.59 (td, J =

.3, 0.9 Hz, 6H) ppm. C{ H} NMR (126 MHz, CDCl ) δ 144.5,

20b

44.2, 143.9, 143.3, 136.4, 134.1, 133.2, 133.1, 132.7, 130.4, 130.2,

21.6, 119.4, 117.8, 1172.2, 114.0, 110.6, 103.6, 41.1, 40.3, 40.2, 15.4,

clear yet. Yield: 28 mg (0.028 mmol, 47%). H NMR (400 MHz,

CDCl ): δ 7.79 (s, 1H), 7.51 (d, J = 8.6 Hz, 1H), 7.44 (d, J = 8.6 Hz,

5.37 (broad singlet) ppm. MS (ESI, positive ions): m/z 237.1116

1H), 7.29 (d, J = 1.9 Hz, 1H), 7.20 (d, J = 1.9 Hz, 1H), 5.12−4.98

(m, 2H), 4.38 (q, J = 7.0 Hz, 4H), 1.74−1.63 (m, 24H) ppm.

(

calculated for [M + Na] 237.1104).

III

Synthesis of [L1-H ]Br (5). To a mixture of compound 4 (0.8 g,

C{ H} NMR (126 MHz, CDCl ): δ 174.4 (Au -CNHC), 172.5 (Ir -

3.77 mmol) and ethyl bromide (1.46 mL, 19.5 mmol) in a Schlenk

C_N_H_C), 135.3, 135.2, 134.6, 122.3, 120.7, 119.9, 111.8, 108.5, 90.6

Organometallics 2021, 40, 915−926

Organometallics

The Supporting Information is available free of charge at

Article

(

Cp*), 47.1, 45.8, 45.7, 16.6, 16.4, 9.7 (Cp*) ppm. MS (ESI, positive

used for the data collection and unit cell determination, respectively.

Processing of the raw frame data was performed using SAINT/

ions): 898.1342 (calculated for [M] 898.1203).

21b,c

Synthesis of Complex (9). A mixture of [L1-H ]Br (50 mg,

XPREP or Bruker SAINT.

SHELXT-2014/4 or SHELXS-97 methods and reﬁned against F

using all reﬂections with the SHELXL-2014/7 or SHELXL-2018/3

The structures were solved by

21d

.116 mmol), [Ir(Cp*)Cl ] (93 mg, 0.116 mmol), Cs CO (190 mg,

2 2 2 3

.581 mmol), and an excess of KBr in 5 mL of acetonitrile was heated

21e,f

to 65 °C for 12 h in a Schlenk tube. After the completion of reaction,

the resultant solution was ﬁltered through a pad of Celite and all the

volatiles from the ﬁltrate were removed in high vacuo. The obtained

residue was further puriﬁed by silica-gel column chromatography

(WinGX) program.

Non-hydrogen atoms were reﬁned anisotropi-

cally. All the hydrogen atoms were placed in calculated positions. The

crystal data (CCDC Nos. 2053897 −2053901 ) and reﬁnement details

are summarized in Table S1 of the Supporting Information. The

graphical representations were performed using the program

(DCM:MeOH = 95:3 v/v) to yield the compound 9 as an orange

21g

solid. Yield: 81 mg (0.069 mmol, 60%). H NMR (400 MHz, CDCl )

Mercury.

δ 7.72 (s, 1H), 7.44 (s, 1H), 7.16 (s, 1H), 7.09 (s, 1H), 5.13−4.92

(

m, 2H), 4.49−4.17 (m, 4H), 1.81−1.62 (m, 42H) ppm. C{ H}

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.organomet.1c00019.

■

III

NMR (126 MHz, CDCl ): δ 165.0 (Ir -C ), 163.8 (Ir -CNHC⁾^,

NHC

III

42.4 (orthometalated Ir -C ), 136.1, 133.9, 131.4, 119.5, 118.4,

15.2, 93.6 (Cp*), 91.3(Cp*), 89.5, 45.2, 44.9, 44.0, 16.8, 16.44,

6.37, 9.9, 9.4 ppm. MS (ESI, positive ions): 1001.2306 (calculated

for [M − 2Br − H] 1001.2308).

Full characterization data for the compounds 1 −10, the

NMR and mass spectra of the compounds (PDF )

Synthesis of Complex (10). A mixture of [L1-H ]Br (50 mg,

.116 mmol), [Pd(CH CN) Cl ] (60.5 mg, 0.232 mmol), K CO

3 2 2 2 3

Accession Codes

(

80.1 mg, 0.58 mmol), and an excess of KBr in 5 mL of pyridine was

CCDC 2053897−2053901 contain the supplementary crys-

tallographic 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.

heated at 80 °C for 12 h in a Schlenk tube. After the completion of

reaction, the resultant solution was ﬁltered through a pad of Celite

and all the volatiles from the ﬁltrate were removed in high vacuo. The

obtained residue was further puriﬁed by silica-gel column chromatog-

raphy (DCM:MeOH = 98:2 v/v) to yield the compound 10 as a

yellow solid. Yield: 88 mg (0.081 mmol, 70%). H NMR (500 MHz,

CDCl ): δ 9.10 (d, J = 5.5 Hz, 2H), 8.82 (d, J = 5.0 Hz, 2H), 8.44 (s,

H), 7.80 (t, J = 7.5 Hz, 1H), 7.69 (t, J = 7.5 Hz, 1H), 7.66−7.62 (m,

H), 7.55 (d, J = 8.6 Hz, 1H), 7.42−7.37 (m, 2H), 7.29−7.24 (m,

H), 7.20 (d, J = 2.1 Hz, 1H), 7.18 (d, J = 2.1 Hz, 1H), 5.02−4.94

AUTHOR INFORMATION

Corresponding Author

■

Arnab Rit − Department of Chemistry, Indian Institute of

Technology Madras, Chennai 600036, India; orcid.org/

0000-0003-0224-8387 ; Email: arnabrit@iitm.ac.in

(

m, 4H), 4.76 (q, J = 7.3 Hz, 2H), 1.79 (t, J = 7.3 Hz, 3H), 1.75−1.70

m, 6H) ppm. ¹³C{ H} NMR (126 MHz, CDCl ): δ 164.5 (Pd -

C_N_H_C), 152.8, 152.6, 149.5 (Pd -C_N_H_C), 138.2, 138.0, 135.2, 134.3,

34.1, 124.8, 124.7, 124.1, 121.8, 121.5, 110.8, 110.4, 46.8, 44.5, 44.3,

Authors

5.6, 14.6, 14.58 ppm. MS (ESI, positive ions): 840.8010 (calculated

Rajeev C. Nishad − Department of Chemistry, Indian Institute

of Technology Madras, Chennai 600036, India

Shashi Kumar − Department of Chemistry, Indian Institute of

Technology Madras, Chennai 600036, India

for [M − Br − pyridine] 840.7917).

General Procedure for the Tandem Suzuki−Miyaura

Coupling and Transfer Hydrogenation of p-Bromoacetophe-

none. A pressure tube was charged with potassium tert-butoxide

(0.15 mmol), p-bromoacetophenone (0.05 mmol), phenylboronic

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.1c00019

acid (0.065 mmol), and catalyst 7 (0.0005 mmol). After the addition

of 2-propanol (2 mL), the reaction mixture was stirred at 100 °C for

the speciﬁed time. Upon completion of the reaction, a 50 μL aliquot

was taken out, diluted with methanol, and ﬁltered through neutral

alumina for GC−MS analysis.

General Procedure for the Tandem Hydrodeﬂuorination/

Transfer Hydrogenation of Fluorophenones. A pressure tube

was charged with potassium tert-butoxide (0.15 mmol), p-

ﬂuorophenones (0.05 mmol), and catalyst 7 (0.001 mmol). After

the addition of 2-propanol (2 mL), the reaction mixture was stirred at

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

We sincerely acknowledge SERB, India (Project No. ECR/

016/001272), for the ﬁnancial support. R.C.N. and S.K. thank

■

20 °C for the speciﬁed time. Upon completion of the reaction, a 50

the IIT Madras and CSIR, respectively, for the research

fellowship. We acknowledge the NMR and ESI-MS (DST-

FIST funded) facilities at the Department of Chemistry, IIT

Madras, and the single-crystal X-ray diﬀraction facility at the

Department of Chemistry and SAIF, IIT Madras. We thank

Mr. V. Ramkumar for helping with the crystal structure

solution.

μL aliquot was taken out, diluted with methanol, and ﬁltered through

neutral alumina for GC−MS analysis.

General Procedure for the Tandem Hydrodehalogenation/

Transfer Hydrogenation of Imines. To an oven-dried pressure

tube with 4 Å molecular sieves, imine (0.05 mmol), sodium tert-

butoxide (0.10 mmol), and catalyst 7 (0.001 mmol) were added. After

the addition of 2-propanol (2 mL), the reaction mixture was stirred at

00 °C for the speciﬁed time. Upon completion of the reaction, a 50

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■

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