Selective mono-N-methylation of nitroarenes with methanol catalyzed by atomically dispersed NHC-Ir solid assemblies

Article abstract of DOI:10.1016/j.jcat.2020.06.004

A series of N-heterocyclic carbene-iridium (NHC-Ir) coordination assemblies based on bis-pyrenoimidazolium salts are prepared, and shown to function as efficient solid molecular catalysts in selective mono-N-methylation of nitroarenes with methanol under mild conditions. The atomically dispersed active Ir(I) centers and the large π-conjugation rings endow the solid catalysts with an exceptionally high activity and selectivity for a broad substrate scope. Such solid NHC-Ir coordination assemblies are robust, which can be easily recovered and reused more than 10 runs without significant loss of their catalytic activity and selectivity. When combined with a subsequent formylation using the same solid catalysts under ambient conditions, this novel protocol can afford diverse formamides in excellent yields, further highlighting the applicability of the present solid catalysts for an efficient diversification of nitroarenes to a broad number of functional amines.

Full text of DOI:10.1016/j.jcat.2020.06.004

Journal Pre-proofs
Selective Mono-N-methylation of Nitroarenes with Methanol Catalyzed by
Atomically Dispersed NHC-Ir Solid Assemblies
Jiaquan Wang, Jiajie Wu, Zhe-Ning Chen, Daheng Wen, Jiangbo Chen,
Qingshu Zheng, Xin Xu, Tao Tu
PII:
S0021-9517(20)30234-7
https://doi.org/10.1016/j.jcat.2020.06.004
YJCAT 13775
DOI:
Reference:
To appear in:
Journal of Catalysis
Received Date:
Revised Date:
Accepted Date:
17 March 2020
31 May 2020
2 June 2020
Please cite this article as: J. Wang, J. Wu, Z-N. Chen, D. Wen, J. Chen, Q. Zheng, X. Xu, T. Tu, Selective Mono-
N-methylation of Nitroarenes with Methanol Catalyzed by Atomically Dispersed NHC-Ir Solid Assemblies,
Journal of Catalysis (2020), doi: https://doi.org/10.1016/j.jcat.2020.06.004
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Selective Mono-N-methylation of Nitroarenes with
Methanol Catalyzed by Atomically Dispersed NHC-
Ir Solid Assemblies
Jiaquan Wanga, Jiajie Wua, Zhe-Ning Chenc, Daheng Wena, Jiangbo Chena, Qingshu Zhenga, Xin
Xuc,*, Tao Tua,b,*
a Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan
University, 2005 Songhu Road, Shanghai 200438, China
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China
c Collaborative Innovation Center of Chemistry for Energy Materials, MOE Laboratory for Computational Physical
Science, Fudan University, 2005 Songhu Road, Shanghai 200438, China
* Corresponding author.
E-mail addresses: xxchem@fudan.edu.cn (X. Xu), taotu@fudan.edu.cn (T. Tu).
ARTICLE INFO
keywords: coordination assemblies, mono-N-methylation, methanol, NHC-Ir complex, self-
supported catalyst
ABSTRACT
A series of N-heterocyclic carbene-iridium (NHC-Ir) coordination assemblies based on bis-
pyrenoimidazolium salts are prepared, and shown to function as efficient solid molecular catalysts
in selective mono-N-methylation of nitroarenes with methanol under mild conditions. The
atomically dispersed active Ir(I) centers and the large π-conjugation rings endow the solid catalysts
with an exceptionally high activity and selectivity for a broad substrate scope. Such solid NHC-Ir
coordination assemblies are robust, which can be easily recovered and reused more than 10 runs
without significant loss of their catalytic activity and selectivity. When combined with a
1

subsequent formylation using the same solid catalysts under ambient conditions, this novel
protocol can afford diverse formamides in excellent yields, further highlighting the applicability
of the present solid catalysts for an efficient diversification of nitroarenes to a broad number of
functional amines.
1. Introduction
As one of the most important building blocks for the synthesis of dyes, surfactants,
agrochemicals, pharmaceuticals and natural products [1,2], N-methylamines have attracted a broad
interest from both academic community and industry. In contrast to the conventional synthetic
methodologies involving stoichiometric toxic reagents (e.g. methyl halides, Me2SO4, or Me2CO3)
[3], N-methylation of primary amines with green and readily available feedstocks, such as
methanol, is more appealing [4-6]. However, it is challenging to selectively synthesize mono-N-
methylamines via this route, as the mono-N-methylamine products are usually more reactive than
the starting primary amines and can readily be transformed into N,N-dimethylated derivatives. It
is only up to the last few years that several homogeneous and heterogeneous catalysts based on Ag
[7] Ru [8-10], Re [11], Ir [6,12,13], Mn [14-16], Fe [17-19] or Co [20] have been reported for
selective mono-N-methylation of primary amines with methanol.
Meanwhile, nitroarenes are also the common N source in the various transformations [21-23].
For example, anilines are generally produced by the reduction of nitroarenes under a high pressure
of hydrogen or with stoichiometric metal-hydrides, which can then undergo N-methylation to N-
methylanilines (Fig. 1) [24-26]. However, from an environmental point of view, straightforward
N-methylation of nitroarenes with methanol is considered as a much more efficient and attractive
protocol for the synthesis of diverse N-methylanilines.
2

Fig. 1. Direct and selective catalytic mono-N-methylation of nitroarenes with methanol: (a)
reported homogenous catalysts; (b) our newly designed recyclable NHC-Ir solid molecular catalyst
and its applicability in further transformation explored in this work.
Nonetheless, to the best of our knowledge, there are only a limited number of examples on the
N-methylation of nitroarenes with methanol up to date [27-34], which mainly focused on the N,N-
dimethylation of nitroarenes [27,28]. However, the selective mono-N-methylation of nitroarenes
is more chanllenging and is thus of great interest. The recent advances of the latter are compiled
in Fig.1a. By using the “hydrogen borrowing” strategy [35,36], Kundu and co-workers realized
the first tandem reductive amination of nitro compounds with methanol to various N-methylamines
[33] in the presence of a RuII-N^N^N pincer-type catalyst [37-39]. Very recently, Beller and co-
workers reported a Pd-catalyzed N-methylation of nitroarenes with methanol using a imidazole-
substituted monophosphine as the ligand [34]. Despite these notable advances, the use of
nonrecyclable homogeneous catalysts and the tedious nature of product purification processes may
impede further industrial application of these protocols.
Recently, N-heterocyclic carbene-iridium (NHC-Ir) complexes have been shown to display an
excellent catalytic activity toward dehydrogenation [40], hydrogenation [41], reductive amination,
3

formylation and C-H functionalization [42,43]. Meanwhile, by using the “self-supporting” strategy
[44-47], we have developed a series of NHC-metal coordination assemblies [48-50], which have
been successfully applied as solid molecular catalysts in various transformations [51-55]. Among
them, NHC-Ir coordination assembly 1 was shown to exhibit excellent activity and recyclability
toward selective mono-N-methylation of amines [52]. In this work, we sought to design new NHC-
Ir coordination assemblies as catalysts in direct and more challenging tandem mono-N-methylation
of nitroarenes with methanol (Fig. 1b). Considering that the strong σ-donating ability of the NHC
ligands can enhance the catalytic activity in the homogenous catalytic systems [56-62], we
envisioned that NHC-Ir coordination assemblies 2 with NHCs containing large -extended rings
may also increase their σ-donating ability to further inprove their cataltic activity. Such a design
has been barely studied in the heterogenous system. Furthermore, in light of the observation that,
for complexes including NHC-Ir 6b, inactive species are readily formed due to the possible di-
/polymerization or cluster formation under the reaction conditions [51-55,63,64], we presumed
that bulky NHCs may prevent the possible di-/polymerization due to the isolation ability of their
bulkiness. Pleasingly, we show here that NHC-Ir coordination assemblies designed accordingly
indeed display excellent catalytic activity, selectivity and recyclability towards direct mono-N-
methylation of nitroarenes under mild reaction conditions, they also exhibit excellent activity in
the formylation reaction with CO2, phenylsilane and the resultant mono-N-methylanilines,
affording diverse formamides in good to excellent yields with high selectivity. The present
protocol opens a viable way for future production of mono-N-methylanilines of various
functionalities.
2. Experimental
2.1. Chemicals
All commercial reagents were used directly without further purification, unless otherwise
stated.
2.2. Synthesis of catalysts
4

tBu
tBu
tBu
tBu
tBu
X
LiHMDS
H
N
NaH
MeI
L
Ir
X
N
N
N
N
N
N
N
N
[Ir]
N
DMF
DMF
X
N
H
N
tBu
n
4
2a:
L = (CO)2, X = BF4, 97%;
L = (CO)2, X = I, 99%;
L = (CO)2, X = PF6, 97%;
L = COD, X = BF4, 88%
3a:
X = I
6
BF
3
2b:
2c:
2d:
3b:
3c:
X = BF4
X = PF6
KPF
EtO
Fig. 2. Syntheses of NHC-Ir assemblies 2a-d.
With our previous experience [51-55], a series of pyrene analogues 2a-d were prepared (Fig.
2). 2,7-di-tert-butylpyrene imidazole and the bis-pyrenoimidazolium iodide 3a were synthesized
−
according to the literature report [65]. The 3a was readily converted to the corresponding BF4 and
−
PF6 salts (3b and 3c) in good yields by anion-exchange. Treatment of these Janus type ditopic
NHC precursors with lithium bis(trimethylsilyl)amide (LiHMDS) led to the formation of bis-NHC
species, with two carbene sites geometrically well-constrained and opposite to each other in
orientation, which helped to further generate structurally defined coordination assemblies 2a-d in
N,N-Dimethylformamide (DMF) with selected iridium salts in very good yields (88-99%).
3b: A Schlenk tube was charged with salt 3a (184 mg, 0.26 mmol), triethyloxonium
tetrafluoroborate (117 mg, 1.04 mmol), dry CH2Cl2 (5 mL), and stirred for 12 hours at ambient
temperature, methanol (3 mL) was then added. The resulting reaction mixture was stirred for an
additional hour and then poured into excess diethyl ether (50 mL). After precipitation completed,
the solids were collected via filtration and dried under reduced pressure to afford the desired salts
as a white powder.
3c: To a solution of salt 3a (184 mg, 0.26 mmol) in 5 mL H2O was added KPF6 (191 mg, 1.04
mmol) and reaction mixture was stirred for 3 hours at ambient temperature. After precipitation
completed, the solids were collected via filtration and dried under reduced pressure to afford the
desired salts as a white powder.
2a-d: salts 3 (0.5 mmol) and corresponding iridium precursor (0.5 mmol) were dissolved in
DMF under N2 at room temperature, LiHMDS (1 mmol) solution in THF was added dropwise.
5

o
The resulting mixture was stirred at 80 C for 12 h. The dark brown solids were isolated after
filtration and washed with deionized water for three times. The solids were then dried over
vacuum.
2.3.
Catalysts characterization
1H, 13C and 19F NMR spectra were recorded on Jeol ECA-400 and Bruker 400 DRX
spectrometers. GC-MS spectra were recorded on Agilent technologies 1890A GC system and
5975C inert MSD with Triple-Axis Detector. ESI-MS spectra were recorded on a Bruker
microTOF II instrument. IR spectra were recorded on AVATAR FT-IR 360 instrument. Powder
XRD studies were performed on a Bruker AXS D8. SEM experiments were carried out on a Philips
XL30 microscope operated at 20 kV. TEM experiments were carried out on a JEOL JEM-2010
transmission electron microscope. Gel permeation chromatography (GPC) system was equipped
with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of
Waters Styragel columns (HR3, HR4, and HR5, 7.8 mm × 300 mm) and calibrated with linear
polystyrene standards.
2.4.
General procedure for the mono-N-methylation of nitroarenes with methanol
Nitroarenes (1.0 mmol), 2a (7.9 mg, 1 mol %), KOtBu (112 mg, 1.0 equiv.) were charged in a
15 mL pressure tube with magnetic bar. Then, 3.5 mL of methanol was added into the mixture.
The reaction tube was closed with the Teflon stopper and was heated to the desired temperature
(130 oC) under stirring. After the desired reaction time, the reaction mixture was allowed to cool
to room temperature. The solvent was removed under vacuum and then the product was purified
by column chromatography and the product was analyzed by NMR spectroscopy.
2.5.
General procedure for the formylation of mono-N-methylanilines
Under an inert atmosphere (N2), a 50 mL Schlenk flask equipped with a stir bar was charged
successively with N-methylanilines (0.5 mmol), phenylsilane (54.1 mg, 1.0 equiv.), 2a (3.9 mg, 1
mol %) and CH3CN (2 mL). Then, the reaction mixture was stirred at room temperature for 6 h
under 1 atmosphere of CO2 (99.999%, balloon). After 6 h, the reaction was concentrated and
purified by column chromatography and the product was analyzed by NMR spectroscopy.
6

3. Results and discussion
3.1. Morphological and structural characterization
Morphologies of solid assemblies 2a-d were investigated by scanning electron microscopy
(SEM), transmission electron microscopy (TEM), as well as high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM). Typical SEM and TEM images, as
shown in Fig. 3a and 3b for solid 2a, revealed that these solid assemblies were nanosized irregular
particles. The powder X-ray diffraction (XRD) patterns further confirmed their amorphous nature,
and no obvious diffraction peaks were discernible (Figs. S11 and S12). Energy dispersive X-ray
(EDX) mapping indicated that the iridium centers and other elements including C, N, O, B and F
were uniformly dispersed in the solid matrixes of 2a, whereas, no obvious iridium nanoparticles,
aggregates or clusters were found (Fig. 3c). Moreover, magnified HAADF-STEM further
confirmed that the iridium sites were atomically dispersed in the matrix of solid 2a (Fig. 3g).
Similar morphologies and spectra were also observed for solids 2a-d (Figs. S1-S5, S8, S9, S11
and S12).
In light of their virtual insolubility in all common solvents (including water), the coordination
assemblies were further characterized by solid-state 13C-NMR spectroscopy, infrared spectroscopy
(FT-IR), and elemental analysis (Figs. S14-S18), etc. In the solid-state 13C NMR spectra of solid
2a, broad signals at 182 and 170 ppm were assigned to CNHC and CCO, respectively, similar to those
observed in the spectra of NHC-Ir complex 4 (180 and 172 ppm, respectively, Fig. 3j), further
suggesting their similar coordination environments. In the FT-IR spectrum of solid 2a, the
absorption band at 1955 cm -1 corresponding to the auxiliary CO ligand attached to the iridium
center [66] was found, which is also similar to that of complex 4 (Fig. S15 vs. S20). In addition,
using X-ray photoelectron spectroscopy (XPS), peaks (at binding energies of 65.3 and 62.3 eV,
assigned to Ir(I) 4f7/2 and 4f5/2) were observed (Fig. 3h), which are also consistent with those of
molecular complex 4. All these outcomes clearly indicated, after the recursive assembly of the
geometrically well-constrained Janus type NHC ditopic ligands with the iridium centers via NHC-
Ir coordination bonds, the resultant NHC-Ir coordination assemblies 2a-d have a well defined
structure as illustrated in Fig. 2.
7

Fig. 3. (a) SEM, (b) TEM, (c) EDX, (g) HAADF-STEM images and (h) XPS spectrum for the
freshly prepared NHC-Ir coordination assembly of solid 2a; (d) SEM, (e) TEM, (f) EDX and (i)
XPS spectrum of the recovered solid 2a after the 12th run; (j) Solid 13C NMR spectrum of solid 2a
and NHC-Ir complex 4 (* indicates the C signal of DMF trapped in the matrix of coordination
assembly of solid 2a) [67].
Furthermore, in order to probe the possible porosities of solid 2a, nitrogen adsorption
measurements were carried out, which showed barely adsorption of N2 with a Brunauer-Emmett-
Teller (BET) surface area of 45 m2/g (Fig. S23). This result is also consistent with the desired one-
dimensional linear structure of solid 2a [51]. Thermo-gravimetric analysis revealed a good thermal
stability (Fig. S21) with a decomposition temperature of around 200 °C. Due to its good thermal
stability and insensitive towards air and moisture, the self-supported catalyst has been stored under
ambient conditions for more than one year with maintained properties.
8

3.2. Catalytic performance
As a preliminary assessment of catalytic activities of solid assemblies 2a-d, mono-N-
methylation of nitrobenzene with methanol to amine 5 was selected as a benchmark reaction. For
comparison, their analogues 1, 4, 6a-b and pincer complexes 7a-b were also evaluated (Fig. 4a).
A careful survey (Table S1) indicated that at 1 mol %, solid 2a exhibited the highest catalytic
activity. When the reaction was carried out at 1 mmol scale with 1 equiv. KOtBu and 3.5 mL
MeOH at 130 oC for 24 h, amine 5 was isolated in 96% yield, no other possible by-products like
nitrosobenzene, aniline, azobenzene or azoxybenzene were detected, highlighting its excellent
selectivity. By contrast, a slightly lower yield (89%) of product 5 was observed with solid analogue
1 [51-55], which further supported our concept that increasing the σ-donating ability of the NHC
ligands could efficiently enhance the activity of the resulting catalyst [56-62], even in the
heterogeneous system. The influence of counter ions and ancillary ligands, e.g., carbon monoxide
(CO) or 1,5-cyclooctadiene (COD), were also investigated. Only small differences on the yields
−
were observed for solid molecular catalysts 2a-c with different counterions, BF4 (96%) > I− (93%)
−
> PF6 (92%). Hence, even though these counterions have different structures and
neocleophilicities[68], the chemcial environment of the active Ir center is only slightly affected in
−
our cases. While solid 2d bearing the COD ligand and the BF4 counter ion, gave an inferior yield
as compared to that of solid 2a (96% vs 67%, Fig. 4a), indicating the significant impact of the
ancillary ligand. Subsequently, NHC-Ir complexes 4 and 6a-b were also examined. Even with the
−
same compositions of counter ion (BF4 ) and ancillary ligand (CO) as those in 2a, obvious
decreases in yields (10~30% lower, Fig. 4a) were observed. These results were probably caused
by the possible di-/polymerization of molecular NHC-Ir species under the reaction conditions
[63,64], further confirming the advantageous isolation effects of bulky ligands in solid 2a. In
addition, pincer complexes 7a-b, which exhibited excellent activity in various transformations
[69], also resulted in only moderate yields (70% and 60% respectively, Fig. 4a). When the catalyst
loading of solid 2a was further decreased to 0.1 mmol %, turnover numbers (TONs) of 650 were
achieved.
9

Fig. 4. (a) Catalytic activities of NHC-Ir coordination assemblies 1, 2a-d and their molecular
analogues 4 and 6a-b, as well as pincer complexes 7a-b towards mono-N-methylation of
nitrobenzene. (b) Recyclability and reusability of NHC-Ir coordination assembly 2a in the catalytic
mono-N-methylation of 3-nitrobiphenyl. (Reactions were carried out in 1 mmol scale with 1 mol%
solid 2a with 1 equiv. KOtBu in 3.5 mL methanol at 130 oC for 24 h).
After catalyst screening, the scope and the limitation of the protocol were then explored (Table
1). To our delight, a variety of nitroarenes bearing electron-donating (8a-c and 9), electron-
withdrawing (10a and 12) substituents or sterically hindered ones (13) were all well tolerated, and
moderate to excellent yields with good selectivity were obtained (up to 93%, Table 1). The position
of substituents on the nitroarenes exhibited obvious impacts on the transformation; a meta-
substituted substrate 8b produced a higher yield than its ortho- and para- isomers (8a and 8c).
Therefore, inferior yields were also observed with ortho-substituted and bulky substances (8a and
13). For the reactions with meta-halo-nitroarenes (10a-d), good to excellent yields were obtained
by prolonging reaction times (48 h), in which cholo-nitroarene gave out the best isolated yield
(90%). Moreover, C=C double bonds (14) could also be well tolerated (83%), indicating the high
chemoselectively of the protocol.
10

Table 1. Mono-N-methylation of Nitroarenesa.
H
N
t
NO2
2a
, KO Bu
Me
H
+
R
H
R
MeOH
130 °C, 24 h
8-31b
R
R
CF3
F
Me
N
H
N
Me
N
N
N
N
N
Me
H
Me
Me
Me
Me
H
H
H
c
10a
: 40%/75% (F);
c
8a
8b
8c
: 72% (o-Me);
: 93% (m-Me);
: 91% (p-Me)
10b
10c
10d
: 70%/90% (Cl);
c
c
c
11
12
13
: 48%
: 51%/71%
: 70%
14
: 83%
9
: 74%
: 56%/86% (Br);
c
: 43%/81% (I)
Ph
H
N Me
H
N Me
H
N Me
H
N Me
H
N Me
F
F
O
S
O
H
N
Me
Cl
F3CO
R
16a
16b
: 98% (3,5-diMe);
: 99% (2,6-diMe)
15
17
18
19
20
: 83%
: 99%
: 88%
: 83%
: 99%
H
N Me
H
N Me
H
H
H
N Me
H
N Me
N Me
HO
N Me
MeX
O
X
n
O
N
d
23
24
: 91% (n=0)
: 88% (n= 1)
21a
21b
22
25
26
27a
27b
: 98% (X = O);
: 72% (X = S)
: 91%
: 98%
: 94%
: 96% (X = O);
: 93% (X = S)
Me
H
N
H
N Me
N
O
N
Me
Me
N
N
MeO
N
N
H
H
H
N
H
H
O
Me
N
N
e
e
e
28
29
30
31
: 97%
: 99%
: 94%
: 99%
Me
Me
a Reactions were generally carried out on 1 mmol of the nitroarenes with 1 mol % solid catalyst
2a and 1 equiv. KOtBu in 3.5 mL methanol at 130 oC for 24 h; b isolated yield; c After 48 h; d With
2 equiv. KOtBu; e 0.5 mmol scale, 48 h.
Presumably, owing to the presence of the strongly electron-withdrawing nitro group, several
ortho-, para-substituted substrates (Scheme S3) tended to undergo the SNAr reactions [70] rather
than N-methylation, which led to low conversions in some cases. Therefore, a series of biphenyl
substrates were explored, in which electron-donating or electron-withdrawing and heterocyclic
substituents were all well compatible. Good to excellent yields with high selectivity were obtained
(15-28, 72-99%, Table 1). Pleasingly, when a methyl-sulfonyl substituted substance was applied,
mono-N-methylaniline with unchanged sulfone group was produced in good yield (20, 83%, Table
11

1). For the substrates bearing free hydroxyl groups, the desired products were also isolated in good
yields (91% and 88% for 23 and 24, respectively), while the hydroxyl groups remained untouched,
further confirming the selectivity of the present protocol. In light of the crucial role of heterocyclic
N-methylamines in pharmaceutical and material sciences [1,2], heterocyclic substrates containing
O, S, N atoms were also tested and excellent yields were obtained (27a,b and 28, 93-99%, Table
1). Unfortunately, aliphatic nitro compounds were incompatible to this protocol.
Remarkably, dinitro and trinitro-arenes were also found to be suitable substrates for mono-N-
methylation. Under the otherwise identical conditions, the multiple-nitiro groups were all mono-
N-methylated, and no N,N-dimethylated products were detected, despite that the corresponding
secondary anilines were more nucleophilic. By extending reaction time to 48 h, di-mono-N-
methylaniline 29 was obtained in 94% yield, and almost quantitative yield was observed for tri-
mono-N-methylaniline 30. The protocol was further extended to synthesize mono-N-methylated
rhodamine derivative 31 and an excellent yield was obtained (97%, Table1). The rhodamine core
structure was not affected during the methylation, highlighting the potential applications for
modification of drugs and structurally elaborated molecules.
3.3. recyclable performance
In consideration of its insolubility of solid 2a in methanol, the solid catalyst was readily
recovered when the reaction mixture was simply centrifuged and washed with fresh methanol. The
recovered solid 2a was applied as the catalyst directly in the next cycle without additional
activation or treatment. Taking account of low boiling point of mono-N-methylaniline, we thus
selected 3-nitrobiphenyl as the substrate for the catalyst-recycling test. To our delight, catalyst 2a
could be reused for more than 10 runs while maintaining quantitative yields (Fig. 4b). Inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) was applied to monitor the potential
iridium leaching in each run (Table S2). Only trace amount of leaching (< 0.09 mg/L) was observed
with the initial three runs, which might be the unreactive iridium precursors or low-molecular
weight polymers trapped in the solid matrix of 2a during the coordination assembly. By using Gel
permeation chromatography (GPC), a NHC-Ir dimer 2a’ could be detected with molecular weight
of 1090 (Fig. S24), which supported that the possible leaching may be aroused by the trace amount
of the low-molecular weight assemblies.
12

For the recovered solid 2a after the 12th run, similar SEM and TEM images as those freshly
prepared amorphous samples were observed (Figs. 3d and 3e). EDX mapping indicated that the
iridium sites were still uniformly dispersed in the solid matrix without the formation of
nanoparticles (Figs. 3f, S6 and S7). The unchanged XPS pattern (Fig. 3i) further supported the
structural robustness of the NHC-Ir(I) assembly of solid 2a. As shown in Fig. 5a, by using complex
6a as a catalyst model, density functional theory (DFT) calculations [71-73] showed a strong
binding strength between the NHC ligand and the Ir center, in which the dissociation free energy
is as high as 36.4 kcal/mol. Whereas, the dissociation of the CO ligand is much more feasible, with
an observably smaller dissociation free energy of 14.2 kcal/mol. These findings demonstrated that
the NHC ligands are firmly bonded to the Ir center, supporting the structural stability of the solid
assembiles observed in experiments. Instead, the dissociation of the CO ligand is a viable route to
provide the reactive site for the catalytic reactions.
3.4. Catalytic mechanism
To probe the plausible reaction mechanism, several control experiments were carried out
(Scheme S5). The reaction between 3-nitrobipenyl and CD3OD under the standard conditions
produced deuterated mono-N-methylaniline 15a in 75% yield (Scheme S5a), suggesting that
methanol served as both a hydrogen source and a methylation reagent. Further reaction with
CD3OH gave a mixture of N-methylated products 15b (61%) and 15b-d2 (39%) in an overall yield
of 90% (Scheme S5a). The slightly lower yield with CD3OD than that with CD3OH suggests that
the activation of the O-H bond in methanol can have an impact on the transformation. When
formaldehyde was used instead of methanol in tert-butanol, no product was observed (Scheme
S5b), revealing that the dehydrogenation of methanol and the produced hydrogen were crucial for
the reduction of nitroarenes. In addition, when the possible intermediates, including
nitrosobenzene, azobenzene and aniline were subjected to the reaction, respectively, under the
standard conditions, the N-methylaniline was obtained in moderate to good yields in each case
(Scheme S5c). Meanwhile, we also successfully detected the formation of azobenzene in the
reaction. Whereas, azobenzene was isolated as a side-product in Beller’s system[34] but not found
by Kundu and coworkers [33]. Consequently, azobenzene or its derivatives were thought to be
hardly converted into the corresponding N-methyl anilines in the previous reports. Therefore, all
the information may indicate a completly new reducution pathway for N-methylation of
13

nitroarenes catalyzed by solid 2a, in which the direct transfer hydrogenation of nitrobenzene to
aniline could be excluded [39,74]. Additionally, to investigate the excellent selectivity in the
mono-N-methylation, N-methylaniline was subjected to the standard conditions at 120, 130 and
160 oC respectively. Only 9% yield of N,N-dimethylaniline was detected at 160 oC, indicating the
effectiveness of this catalytic system to the mono-N-methylation of nitroarenes, while showing
that further methylation of the secondary amines could be kinetically prevented under the standard
conditions. (Scheme S5d).
Fig. 5. (a) The DFT calculated free energy changes for the ligand detachment from NHC-Ir
complex 6a. (b) Proposed reaction mechanism. The DFT calculated value for dehydrogenation
transition state represents the relative free energy with respect to NHC-Ir complex 6a and
methanol.
Based on aforementioned results and our previous work [51-55], a plausible mechanism is
proposed in Fig. 5b. Firstly, oxidative dehydrogenation of methanol is mediated by the NHC-Ir
catalyst and the base, giving formaldehyde with the simultaneous generation of a NHC-Ir-H
species. Nitroarenes are reduced by the latter to anilines via an azoarene intermediate.
Condensation of anilines with formaldehyde gives rise to N-methylated imines accompanied by
water liberation. The resulting imines are hydrogenated by the NHC-Ir-H species and tBuO-H to
produce the mono-N-methylated product, and regenerate the catalyst. In the middle of Fig. 5b, we
illustrate the structure of the dehydrogenation transition state. DFT calculations confirmed that the
14

NHC-Ir catalyzed dehydrogenation reaction of methanol occurs after a CO ligand dissociation
from the Ir center, with the participation of the solvated base (tBuO-), to generate formaldehyde.
Intriguingly, such a dehydrogenation step proposed here is a redox-neutral Ir(I)/Ir(I) process
without changing the apparent valence of the Ir center. This process is indeed feasible with an
apparent activation free energy of 17.1 kcal/mol only.
3.5. Further N-formylation with the same catalyst
Table 2. Further formylation of N-methylanilines with the same catalysta.
With the same catalyst
CHO
2a
1 mol %
H
N
2a
1 mol %
N
NO
2 KOtBu, MeOH
130 oC, 24 h
PhSiH3, CO2
Me
Me
R
R
R
CH3CN, rt, 6h
32-39b
Me
N
Me
N
Me
N
Me
N
CHO
CHO
CHO
CHO
32
33
: 86%
: 90%
34
: 82%
35
: 97%
Cl
OCF3
Me
N
Me
N
Me
N
CHO
Me
N
O
N
CHO
CHO
OHC
N
N
O
N
S
OMe
38
36
37
39
: 75%
: 89%
: 92%
: 86%
a Reactions were carried out in 0.5 mmol scale with 1 mol % solid catalyst 2a and 1.0 equiv. PhSiH3
in 2 mL CH3CN under 1 atm CO2 (balloon) at room temperature for 6h; b isolated yield.
As is well known, formamides are versatile chemicals and important building blocks, which
can be prepared by formylation of amines [75,76]. In order to extend the applicability of our newly
developed NHC-Ir coordination assembly catalysts, we explored the feasibility of N-methylamines
in further transformation. We used the direct formylation to demonstrate this concept, in which the
selected products obtained by mono-N-methylation of nitroarenes were readily functionalized to
the corresponding formamides using CO2 and phenylsilane in the presence of 1 mol % solid 2a at
15

room temperature within 6 h (Table 2). Broad substrate scope was observed with this new catalytic
formylation approach. Substrates bearing electron-donating and electro-withdrawing substituents,
as well as heterocyclic (37, 38) substrates, were all compatible. Good to excellent yields were
achieved. Moreover, rhodamine derivative 39 was produced in 75% isolated yield, further attesting
the broad catalytic applicability of the NHC-Ir coordination assemblies.
4. Conclusions
A series of robust NHC-Ir coordination assemblies based on bis-pyrenoimidazolium salts have
been prepared, which can function as solid single-site catalysts with atomically dispersed iridium
centers, having a similar coordination environment as their molecular analogues. These catalysts
have been shown to exhibit a high catalytic activity and selectivity in the mono-N-methylation of
nitroarenes with methanol. Besides a broad substrate scope, the robust NHC-Ir assemblies have
been found to be readily recovered and reused for more than 10 runs without loss of their catalytic
activity and selectivity. DFT calculations have disclosed a redox-neutral Ir(I)/Ir(I) process with a
synergetic participation of the solvated base in support of the observed robustness, reusability, but
extremely high catalytic activity of the catalytic system. In addition, this protocol has been shown
here to be readily extended with further formylation using the same solid molecular catalyst under
the ambient reaction conditions. The direct and selective mono-N-methylation of nitroarenes to
amines using our recyclable solid molecular catalysts may pave the way to future industrial
production of various bioactive or fluorescent mono-N-methylanilines.
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgments
Financial support from the National Key R&D Program of China (No. 2016YFA0202902 and
2018YFA0208600), the National Natural Science Foundation of China (Nos. 21572036,
16

21871059, 21861132002, and 21688102), and the Department of Chemistry at Fudan University
is gratefully acknowledged.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the online version, at doi: XXXXX
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 Highly efficient and recyclable catalysts with atomically dispersed catalytic sites.
 Selective and effective mono-N-methylation of nitroarenes via one pot tandem reaction.
 Using methanol as green carbon source and water is the only byproduct.
 Avoiding the usage of toxic reagents.
26

Graphical Abstracts
27