Cage Encapsulated Gold Nanoparticles as Heterogeneous Photocatalyst for Facile and Selective Reduction of Nitroarenes to Azo Compounds

Article abstract of DOI:10.1021/jacs.8b07767

A discrete nanoscopic organic cage (OC1^R) has been synthesized from a phenothiazine based trialdehyde treating with chiral 1,2-cyclohexanediamine building block via dynamic imine bond formation followed by reductive amination. The cage compound has been characterized by several spectroscopic methods, which advocate that OC1^R has trigonal prismatic shape formed via [2 + 3] self-assembled imine condensation followed by imine reduction. This newly designed cage has aromatic walls and porous interior decorated with two cyclic thioether and three vicinal diamine moieties suitable for binding gold ions to engineer the controlled nucleation and stabilization of ultrafine gold nanoparticles (AuNPs). The functionalized confined pocket of the cage has been used for the controlled synthesis of AuNPs with narrow size distribution via encapsulation of Au(III) ions. Inductively coupled plasma mass spectrometric (ICP-MS) analysis revealed that the composite Au@OC1^R has very high (?68 wt %) gold loading. In distinction, reduction of gold salts in absence of the cage yielded structureless agglomerates. The fine-dispersed cage anchored AuNPs (Au@OC1^R) have been finally used as potential heterogeneous photocatalyst for very facile and selective conversion of nitroarenes to respective azo compounds at ambient temperature in just 2 h reaction time. Exceptional chemical stability and reusability without any agglomeration of AuNPs even after several cycles of use are the potential features of this material. The composite Au@OC1^R represents the first example of organic cage supported gold nanoparticles as photocatalyst.

Full text of DOI:10.1021/jacs.8b07767

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Article
Cage Encapsulated Gold Nanoparticles as Heterogeneous Photocatalyst
for Facile and Selective Reduction of Nitroarenes to Azo compounds
Bijnaneswar Mondal, and Partha Sarathi Mukherjee
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07767 • Publication Date (Web): 10 Sep 2018
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Cage Encapsulated Gold Nanoparticles as Heterogeneous
Photocatalyst for Facile and Selective Reduction of Ni-
troarenes to Azo compounds
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Bijnaneswar Mondal and Partha Sarathi Mukherjee*
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Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India.
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Supporting Information
ABSTRACT: A discrete nanoscopic organic cage (OC1R) has been synthesized from a phenothiazine based trialdehyde
treating with chiral 1,2-cyclohexanediamine building block via dynamic imine bond formation followed by reductive ami-
nation. The cage compound has been characterized by several spectroscopic methods which advocate that OC1R has trig-
onal prismatic shape formed via [2+3] self-assembled imine condensation followed by imine reduction. This newly de-
signed cage has aromatic walls and porous interior decorated with two cyclic thioether and three vicinal diamine moieties
suitable for binding gold ions to engineer the controlled nucleation and stabilization of ultrafine gold nanoparticles
(AuNPs). The functionalized confined pocket of the cage has been used for the controlled synthesis of AuNPs with narrow
size distribution via encapsulation of Au(III) ions. Inductively coupled plasma mass spectrometric (ICP-MS) analysis re-
vealed that the composite Au@OC1R has very high (~68 wt%) gold loading. In distinction, reduction of gold salts in ab-
sence of the cage yielded structure less agglomerates. The fine-dispersed cage anchored AuNPs (Au@OC1R) have been
finally used as potential heterogeneous photocatalyst for very facile and selective conversion of nitroarenes to respective
azo compounds at ambient temperature in just 2 h reaction time. Exceptional chemical stability and reusability without
any agglomeration of AuNPs even after several cycles of use are the potential features of this material. The composite
Au@OC1R represents the first example of organic cage supported gold nanoparticles as photocatalyst.
organic cage supported AuNPs as very efficient photo-
catalyst in organic transformation.
INTRODUCTION
Heterogeneous photocatalysts1 seize a great attention in
In recent times by articulating different organic spacers
the past decade over conventional catalysts in terms of
with suitable metal centers, many shape-persistent dis-
converting solar energy into clean hydrogen energy by
crete assemblies have been developed.9,10 Furthermore,
splitting water, and decomposing harmful organ-
dynamic covalent chemistry11 has emerged as an effective
ic/inorganic pollutants. Semiconductors based gold na-
approach for the easy construction of organic architec-
noparticles2 (AuNPs) such as Au/TiO2 were extensively
tures having shapes and sizes analogous to coordination
used in various organic transformations as photocatalysts
assemblies. To this end, well-defined pore structures of
in green chemistry approach. It is well-known that AuNPs
such materials have been exploited as template to synthe-
are chemically very stable in nano-levels with exceptional
size metal nano-particles (MNPs) of different shapes and
properties such as the localised surface plasmon reso-
sizes. Compared to bulk metal counterparts, MNPs hav-
nance (LSPR) effect.3 LSPR denotes as a collective oscilla-
ing high surface-to-volume ratio with many active catalyt-
tion of conduction-band (CB) electrons which are in res-
ic sites per unit area make them better candidates for
onance with the oscillating electromagnetic field of inci-
superior catalytic performance.12 But such tiny MNPs hav-
dent light. The decay of excited LSPR can produce hot
ing high surface energy leads to the formation of aggre-
electrons and holes at the proximity of AuNPs surfaces to
gates during reactions leading to poor catalytic activity.13f
initiate chemical reactions.4 Moreover, AuNPs exhibit
To overcome this problem, several supports like metal
strong ultraviolet (UV) absorption that leads to an inter-
oxides,14 metal-organic frameworks,15 dendrimers,13 cova-
band excitation of electrons from 5d to 6sp.5 The photo-
lent organic frameworks16 have been used in last few
excitation of such AuNPs can be used to endorse electron
years. Despite of considerable advancement in this area,
transfer that would not be thermodynamically feasible in
development of new templates for accurate control over
the ground electronic state.6 Therefore, such kind of pro-
the size distribution and stability of the MNPs is still a
cess can produce unstable intermediates under mild con-
great challenge. To address the above-mentioned issues,
ditions7 that would have been forbidden in a thermal re-
discrete organic cages have attracted tremendous atten-
action at high pressure. It is established that AuNPs size
tion owing to their structural tunability and chemi-
and surface properties of the support are pivotal in de-
cal/thermal stability for homogeneous/heterogeneous
termining photocatalytic activity widely.8 However, our
catalysis.17
present work demonstrates the first time use of a discrete
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Synthesis and Characterization of the Organic Cage:
Molecular trigonal-prismatic cage OC1R was synthesized
(~ 83% yield) via the reaction of phenothiazine based
trialdehyde P3 with a chiral diamine 4 [(S, S)-1, 2-
cyclohexanediamine] (Scheme 1). The trialdehyde build-
ing block P3 was synthesized by a multistep synthetic
procedure starting from phenothiazine (Scheme S1). By
treating trialdehyde P3 with diamine 4 in 2:3 stoichio-
metric ratio in chloroform at room temperature for 48 h
yielded imine based organic cage OC1 which was isolated
as a yellow powder. Molecular cage with [2 + 3] assembly
of the aldehyde and amine is enthalpically favored owing
to the least angle strain; and entropically favored as it
comprises of minimum number of building blocks com-
pared to all the possible 3D architectures.11b Imine bonds
are sensitive to water and strong acids/bases. Hence, the
imine cage (OC1) was converted to stable amine cage
OC1R by treating with sodium borohydride which reduced
the imine bonds. As synthesized OC1R was well character-
ized by multinuclear NMR (1H, 13C), 1H-1H COSY, 1H
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Figure 1. Schematic representation of conventional azo
synthesis and photocatalytic routes for azo synthesis us-
ing Au@OC1R as photocatalyst.
1
Aromatic azo compounds have diverse applications in
academic and industrial research, such as in organic
dyes,18 food additives,19 pigments,20a colorants,20c thera-
peutic agents,20b,d and as precursors of natural products
synthesis.21 Conventional approaches for the development
of aromatic azo compounds involve the use of stoichio-
metric amounts of nitrite salts (NaNO2) or toxic oxidants
NOESY, H DOSY, FT-IR and ESI-MS analyses (Figures
S18-S23). The formation of [2+3] assembled molecular
cage was unambiguously established by the peak at m/z =
1269.6366 corresponding to [M+H]+ in ESI-MS spectrum
(Figure S21).
22
via diazonium salts or nitroso-benzene intermediates.23
Several catalytic protocols have been developed recently
for the synthesis of azo derivatives but the existing ap-
proaches24 have numerous drawbacks, like tedious work-
up process, lower yields, undesirable toxic side products,
poor catalyst reusability, limited functional-group com-
patibility with poor selectivity (Figure 1). Formation of
azoxy, amine derivatives and other byproducts is the
common drawback in the synthesis of azo compounds
using traditional approach. Thus, finding a more gentle
and economical heterogeneous protocol which is more
concise, more versatile and more chemo-selective for the
reduction of nitroarenes under mild conditions is highly
desirable.
Scheme 1. (a) Synthetic Routes for the Synthesis of Mo-
lecular Trigonal Prism OC1R, (b) Gas-Phase DFT
(B3LYP/6-31G)-Optimized Structure of the Organic Cage
OC1R.
Numerous efforts of crystallization of OC1R have so far
been unsuccessful. Yet, gas phase DFT (B3LYP/6-31G)
calculation exposes that top and bottom trigonal faces of
the cage are in eclipse conformation with slight parallel
shift in lowest energy conformer of OC1R. The distance
between the sulphur atoms of the phenothiazine cores is
~ 0.94 nm while the distance between the two utmost
points inside the cage is ~ 1.67 nm with an outer distance
of ~2.31 nm (Figure S24).
Herein, we report the size controlled synthesis of AuNPs
of very narrow size distribution (~ 2 nm) using a multi-
functionalized nanoscopic organic cage as template. Large
confined pocket with interior decoration employing cyclic
thioether/vicinal diamine helped nucleation and stabiliza-
tion of gold nanoparticles with controlled size distribu-
tion. Furthermore, such cage anchored AuNPs have been
utilized as heterogeneous photocatalyst for very selective
and facile conversion of nitroarenes to corresponding
azo-compounds at ambient temperature in very high
yield in just 2 h reaction time. The newly developed com-
posite Au@OC1R represents the first example of cage
supported gold nanoparticles used as photocatalyst.
Synthesis of Cage Infused Gold Nanoparticles
(AuNPs): The cage OC1R having two cyclic thioether
moieties along with three vicinal diamine clefts in its
backbone make it a suitable candidate for the complexa-
RESULTS AND DISCUSSIONS
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tion with gold(III) salts. Consequently, it is expected that
84.0 eV corresponding to two distinct spin-orbit pairs 4f5/2
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two thioether and two chelating diamine moieties will
strongly interact with Au3+ ions17 and promote the con-
trolled nucleation of AuNPs along with the stabilization
by the furnished aromatic backbones. We also anticipated
that the cage template can offer a protecting shell with
least coverage and superior AuNPs surface accessibility
due to the presence of large open-windows of the cage.
and 4f7/2, respectively (Figure 3c).
Bright field transmission electron microscopy (BF-TEM)
of Au@OC1R portrays the homogeneously dispersed tiny
AuNPs with a narrow size distribution, having mean par-
ticle size of 1.99 ± 0.18 nm (Figures 3a and 3b). Further
calculation based on the optimized structure of OC1R il-
lustrates that the estimated internal cavity size is between
1.67 to 1.82 nm. This result is in close agreement with
most of the AuNPs are anchored within the organic cage
(Figure S24, for more details see the Supporting Infor-
mation). To validate that the AuNPs were anchored inside
the cage cavity and to exclude the possibility of AuNPs
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bound by multiple cages in their intermolecular space, H
diffusion-ordered spectroscopy (DOSY) NMR was per-
formed on both free cage OC1R and Au@OC1R at the
same temperature in CDCl317a (Figures S26 and S28). As
anticipated, we found similar diffusion coefficient values
for OC1R and Au@OC1R, -9.78±0.01 and -9.80±0.02 re-
spectively, which indicate the comparable size and shape
of free cage OC1R and Au@OC1R. These results echo that
the AuNPs formation is an outcome of a single cage en-
1
capsulation instead of inter-cage interactions. In the H
NMR spectrum of the Au@OC1R complex, a significant
broadening and shifting of not only the protons of alkyl
chains (-CH2/-CH) but also all aromatic protons of the
cage structure were observed (Figure S30). This suggests
that the AuNPs are wrapped by the cage skeleton and the
skeletal protons experience restricted mobility with fast
Figure 2. Schematic illustration of two-step formation of
Au@OC1R (inset images are shown for different materi-
als).
Treating one equivalent of OC1R with fourteen equiva-
lents of AuCl3 showed a sharp color change from yellow to
brown at room temperature (Figure 2) due to the binding
1
spin relaxation12e,17a, d which leads to line broadening in H
1
of gold ions with the cage compound as confirmed by H,
NMR spectrum. Similar kind of MNPs encapsulated or-
ganic cage was also reported by Zhang and co-workers
previously.17a
1H 2D NMR and XPS (Figures S27, S30-S32). Subsequently,
a methanolic solution of four equivalents sodium borohy-
dride was added to this reaction mixture and immediately
brown solution was turned into black without any precip-
itation. This result indicates reduction of Au3+ ions into
Auo. After centrifugation of the resulting mixture and
washing with methanol/THF, the Au@OC1R was obtained
as a black powder (Figure S29). Remarkably, this material
was insoluble in common organic solvents except in
CHCl3 without any agglomeration even after several
months. Experiments with lower equivalents of AuCl3
with respect to the cage yielded similar AuNPs size (Fig-
ure S37) but with less gold loading. Moreover, beyond
fourteen equivalents of AuCl3 yielded structureless ag-
glomerates as confirmed by TEM (Figure S38). So, 1:14 is
the optimum OC1R:AuCl3 ratio for obtaining minimum
size tiny AuNPs (~2 nm) with maximum loading (~68
wt%) of gold.
Figure 3. (a) TEM, (b) particle size distribution, (c) XPS
spectrum and (d) absorption spectrum of Au@OC1R.
Powder X-ray diffraction (PXRD) analysis of as-
synthesized Au@OC1R displayed peaks at 2θ ≈ 38.3º, 44.5º,
64.7º, 77.6º and 81.8º along with peaks for OC1R. These
peaks could be attributed to (111), (200), (220), (311) and
(222) lattice planes which are characteristic signatures for
Au with a face centered cubic (fcc) structure (Figure S33).
Furthermore, X-ray photoelectron spectroscopy (XPS)
advocates that Au is in zero oxidation state as designated
by its characteristic binding energy values of 87.7 eV and
The scanning electron microscopy (SEM) image portrays
an uneven morphology (Figure S34a) composed of granu-
lar size particles of the material. Energy-dispersive X-ray
spectroscopic (EDS) image of Au@OC1R displayed (Figure
S35) the presence Au along with other organic cage ele-
ments (C, H, N, S). The elemental mapping (Figure S34b)
revealed the well dispersed nature of AuNPs at the surface
during SEM experiments. The absorption peak at nearly
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523 nm in the UV/Vis spectrum (Figure 3d) of the
As it is already reported in literature that 2-propanol is a
3e,7a
14e
Au@OC1R can be ascribed to a SPR absorption
by the
hydrogen donor source7a,
as well as solvent, we have
AuNPs. Finally, the gold loading estimated by inductively
coupled plasma mass spectrometry (ICP-MS) was found
to be ~ 68 wt %, which is highest gold loading achieved in
any porous organic cage compound till date. Typically, by
the wet chemical approach beyond 20 wt% of NPs loading
leads to agglomeration of metal nanoparticles (MNPs) as
conveyed in preceding document.15f
tested other alcohols for checking their capability as a
hydrogen donor without changing other parameters. The
experimental results convey that none of solvents could
deliver azo compound completely (Table 1, entries 6-8)
(Figure S42). Also, no azo- formation was observed when
the same reaction was performed either in absence of
AuNPs or in presence of only cage OC1R under identical
conditions (entries 9, 11). Moreover, phenothiazine was
used as catalyst which is known as a photosensitizer in
the literature. But negligible amount (<1%) of azo for-
mation was observed under both UV and visible light (en-
try 13) (Figure S44). It was thus evident that the reaction
was driven by AuNPs; and the free cage has no catalytic
role other than stabilizing the AuNPs (Figure S43).
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To recognize the role of structural features (size and
shape) of organic cages on the evolution of nanoparticles
and their size control, we performed a controlled experi-
ment. In that instance AuCl3 in methanol was treated
with four equivalents of methanolic NaBH4 but in the
absence of cage compound (OC1R). TEM analysis of the
resulting black precipitate (usually named as gold bulk
i.e. Au@bulk) displayed the formation of structure-less
bumpy agglomerates (Figure S36a). Such observation is
indisputably due to the absence of nucleation sites (i.e.,
thio-/diamine- moieties) and confined pocket of cage for
stabilization and controlling size. Similar agglomerates
formation was observed when only phenothazine was
used instead of cage (Figure S36b). Such a marked differ-
ence in the AuNPs size/morphology in presence and ab-
sence of cage indicates the sheer role of structural fea-
tures of the organic cage on the nucleation of AuNPs in-
cluding their size tuning and stabilization.
It is well-known that the catalytic activity of NPs signifi-
cantly depends on their size whereas increase in the size
leads to decrease in catalytic activity. The same logic was
applicable when the model reaction was done in presence
of Au@bulk under identical reaction conditions (Table 1,
entry 10). Fascinatingly, the photocatalytic reduction of
model system was highly influenced by UV/visible light
probably due to inter-band excitation of electrons (5d to
6sp)/ LSPR effect whereas only negligible amount (1%) of
azo product was detected in dark in presence of AuNPs
under identical reaction conditions (Table 1, entry 12)
(Figure S43).
Reduction of Nitroarenes: An industry-friendly devel-
opment towards one-step nitro- to azo- conversion has
fundamental as well as practical interest in organic syn-
thesis.25 In 2008, Corma and co-workers reported that
aromatic azo compounds could be synthesized from the
corresponding nitroarenes through a two-step, one-pot
reaction with catalysts embracing AuNPs on TiO2 or CeO2
at 100oC or higher.26 In this thermal hydrogenation, azo-
benzene was formed as an intermediate but, it was unsta-
ble under such reaction conditions (high pressure, high
temperature) and rapidly reduced to aniline.27 As photo-
catalytic reactions are typically conducted at ambient
temperature and atmospheric pressure,28 several interme-
diates are moderately stable under such conditions.
Hence, by using a photocatalytic process, the synthesis of
aromatic azo compounds would be a much more simple,
controlled, and greener process.
Table 1. Optimization of reaction parameters for the reduc-
tion of 1-chloro-4-nitrobenzenea
entry
solvent
time
(mins)
mol%
Au
conversionb
(1b) [%]
UV
32
Vis
15
29
31
40
40
11
1
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
n-propanol
n-butanol
ethanol
60
60
0.25
0.5
0.25
0.5
0.5
0.5
0.5
0.5
2
3
4
5
6
7
8
60
68
>99
>99
30
To evaluate the catalytic activity of the cage supported
gold nanoparticles (Au@OC1R) as photocatalyst, the re-
duction of 1-chloro-4-nitrobenzene has been selected as
the model reaction. The reaction conditions were opti-
mized by examining the influence/impact of catalyst load-
ing, time and solvent in a series of experiments and the
results are depicted in Table 1. The best result was ob-
tained with 0.5 mol% of Au@OC1R in 2-propanol (hetero-
geneous way) under UV radiation for 2 h in inert atmos-
phere which gave selectively corresponding azo com-
pound with >99% conversion (Table 1, entry 4) (Figure
S41, S45). A profound effect was observed on the conver-
sion of the desired product by altering reaction parame-
ters (Table 1, entries 1-3).
120
120
150
120
120
120
17
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2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
120
120
120
120
120
0
0
9
0
1
0
4
3
92
89
9200
8900
0.5c
0.5d
0.5e
0.5f
4
0
12
13
-
5
93
90
9300
9000
<1
<1
aReaction conditions: under N2 atmosphere in heterogene-
ous way, 1-chloro-4-nitrobenzene (6.35 mmol), NaOH (1.27
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6
mmol). bConversions are based on H NMR analysis of the
1
c
d
crude product. using Au@bulk as catalyst. using free cage
OC1R as catalyst. using Au@OC1R as catalyst in dark condi-
e
7
8
9
92
91
9200
9100
8800
tions. fusing phenothiazine as catalyst.
To validate the general applicability of this novel hetero-
geneous catalyst and the scope of the procedure, we ex-
tended our synthetic strategy for a wide range of ni-
troarenes to produce their corresponding azo-compounds
(Table 2). We were delighted to find that a numerous
functionalized nitroarenes, specially electron-deficient
substrates, are converted smoothly and selectively into
the desired products with good to excellent yields (Table
2, entries 1–6, 13-15). Also, the electron-donating such as
88
10
11
85
8500
9000
8700
8800
8700
methyl-substituted
nitrobenzenes
and
methoxy-
substituted nitrobenzene were fully converted to the cor-
responding azo compounds in good yields (Table 2, en-
tries 7–11).
90
87
88
87
It is notable that halogenated nitroarenes employed in
this catalysis model lead to the formation of correspond-
ing fluoro-, chloro-, bromo- or iodo- azo compounds
without any dehalogenation (Table 2, entries 1–6). By the
other procedures24e of azo-compounds synthesis from
nitro-compounds, byproducts formation including cata-
lytic hydrogenation is a common problem. In addition to
halogen substituents, delicate reducible functional groups
like ketone, nitrile, and even ester moieties were finely
tolerated without undergoing reduction to any substan-
tial extent during the photocatalytic reduction of nitro-
compounds to corresponding azo-derivatives (Table 2,
entries 13–15) using our newly developed Au@OC1R.
12
13
14
15
16
84
83
8400
8300
Table 2. Au@OC1R catalyzed selective reduction of various
nitroarenesa
entry
1
product
yieldb
[%]
TOFc
[h-1]
17
80
8000
91
9100
aReaction conditions: under N2 atmosphere in heteroge-
neous way, Nitroarenes (1 equv.), NaOH (0.2 equv.),
Au@OC1R (0.5 mol% Au), 15 mL 2-Propanol, time 120 mins,
2
94
9400
temperature 30oC. Isolated yield of pure products after col-
b
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umn chromatography. cTurn over frequency calculated based
on isolated yield of pure product.
pendent 1H NMR and ESI-HRMS (Figures S48, S49) analy-
1
2
sis of the reaction, we propose the following reaction
steps that are involved in this photocatalytic process (Fig-
ure 4). Frist, NaOH enhances the abstraction of a hydro-
gen atom from 2-propanol7a,14e leading to the formation of
sodium isopropoxide. Now, in presence of UV/Vis light,
AuNPs generate hot electrons and holes at surfaces which
further generate AuNPs-H species by abstracting hydride
from sodium isopropoxide and leaving acetone as oxi-
dized product. This Au—H bond is relatively stable at the
AuNPs surface12f which transfers hydride to nitroarene (K)
and regenerate the catalyst. Further, two hydrides trans-
fer leads to the formation of nitroso-arene (L) by elimi-
nating water. The nitroso-arene is the key intermediate in
this catalytic system which is finally converted to
azoarene (P) by two probable pathways. In path A, nitro-
so-arene converted to amino-arene (N) by similar hydride
transfer via hydroxyamino-arene (M) formation. Further,
reaction between nitroso- and amino-arene leads to the
formation of azoarene by removal of water. In path B,
azoxy-arene (O) is possible to form by the reaction be-
tween nitroso- and hydroxyamino-arene. In the subse-
quent step, azoxy-arene is likely to be converted to azo-
arene by simple hydride transfer with eliminating water.
In this entire heterogeneous catalysis, formation of
AuNPs-H species under UV light is probably faster than
visible light which clearly portrayed by product formation
in Table 1.
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AuNPs have been widely used as catalyst in the reduction
of 4-nitrophenl to 4-aminophenol.16e Such reduction of
nitroarenes to amines is quite straightforward. However,
using the new catalyst Au@OC1R, we have successfully
isolated its corresponding azo compound in good yield
without any degradation due its sensitive -OH groups
(Table 2, entry 12). Furthermore, this catalytic system has
great potential to alter other polyaromatic nitroarenes
like biphenyl and naphthyl to corresponding azo-
derivatives with high yield, which is usually hard to
achieve using traditional organic synthesis (Table 2, en-
tries 16–17). These experimental results establish the ver-
satility of the current methodology for selective and very
facile synthesis of azo compounds with high yields.
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Steric properties of the substituents also affect the reduc-
tive coupling significantly. In this fact, the substitution on
phenyl ring on different positions i.e. ortho, meta and
para was also explored in this catalytic reaction. Role of
both the electron withdrawing (-Cl) as well as electron
donating (-Me) groups was investigated to check the cata-
lytic performance (Table 2, entries 2–4, 8–10). In both the
cases selectivity and yields were in the order of pa-
ra>meta>ortho. We believe these results were solely due
the steric effect on phenyl ring around the AuNPs surface
which always favors the para- substituent with higher
yield rather than its ortho- substituent.
Figure 4. A plausible reaction pathway for the reduction
of nitroarene in this photocatalytic process.
Figure 5. (a) Reusability of Au@OC1R (conversions are based
on 1H NMR analysis of the crude product), (b) PXRD of recy-
cled Au@OC1R after 5th cycles.
Although the exact routes by which the reduction occurs
are not yet fully understood but based on the time de-
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For practical and industrial application of any heteroge-
EXPERIMENTAL SECTION
2
neous catalyst, reusability is the crucial subject which
makes it more beneficial over its homogeneous counter-
part. Unfortunately, during catalytic reactions such small
metal nanoparticles (MNPs) lose their catalytic activity
because of aggregation owing to their high surface energy.
Therefore, to address this key issue, the same model cata-
lytic system was tested (reduction of 1-chloro-4-
nitrobenzene) up to five consecutive cycles using
Au@OC1R as catalyst. As portrayed in the Figure 5a, a
minor change in catalytic activity was observed after sev-
eral cycles of reuse of the catalyst (Figure S40). PXRD
analysis of Au@OC1R was carried out after 5th cycle, which
shows no significant change as compared with as-
synthesized material (Figure 5b). Moreover, TEM images
of the recovered material reveal same size distribution
pattern with no alteration in morphology (Figures S39a,
b) after the catalytic cycles. Moreover, XPS and UV-Vis
spectra show a clear proof of its oxidation state (0) and
characteristic absorption band with no alteration (Figures
S39c, d) after catalysis. These outcomes further approve
that the finely dispersed AuNPs are strongly attached to
the porous architecture of organic cage and stable enough
for several cycles in presence of UV light.
Materials and Methods: All the chemicals and solvents
were purchased from available sources and used without
further purification. The NMR spectra were recorded on a
Bruker 400 MHz instrument. The chemical shifts (δ) in
3
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7
8
9
1
13
the H, C NMR spectra are accounted in ppm relative to
TMS (Me4Si) as an internal standard (0.0 ppm) in CDCl3
or proton resonance resulting from incomplete deuter-
ation of the solvents. High resolution mass spectra were
recorded on a Q-TOF instrument by electrospray ioniza-
tion (ESI) technique using standard spectroscopic grade
solvents. IR spectra were recorded on a Bruker ALPHA
FTIR spectrometer. Electronic absorption was recorded
using a Perkin-Elmer LAMBDA 750 UV-visible spectro-
photometer. Powder X-ray diffraction (PXRD) patterns
were recorded on a Phillips PANalytical diffractometer.
Scanning Electron Microscopy (SEM) was performed on a
Carl-Zeiss Ultra 55 at an operating voltage of 3-20 kV.
Transmission electron microscopy (TEM) was performed
on a JEOL 2100F operating at 200 kV. X-ray photoelectron
spectroscopy (XPS) was carried out on an Axis ultra-
instrument. Inductively coupled plasma mass spectrome-
try (ICP-MS) was carried out on a Thermo-iCAP 6000
series instrument. A Philips made 60W lamp was used for
UV radiation (254 and 365 nm) and visible light source
was a Philips 60W LED lamp (400-700 nm) with white
light. Analab µ-ThermoCal10 instrument was used for
melting point range determination.
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CONCLUSIONS
In summary, we have designed and synthesized a shape-
obstinate organic molecular cage utilizing dynamic cova-
lent chemistry. The organic cage has large interior pocket
and stable aromatic backbone with cyclic thioether and
vicinal diamine moieties to foster the control synthesis of
AuNPs. The strong interaction between Au3+ with the
thioether and vicinal diamine moieties of the cage is a
principal factor during nucleation of the AuNPs while
aromatic walls protect the newly blossomed NPs from
agglomeration. The present outcomes establish the dis-
crete organic cage compound as a novel platform for the
size controlled synthesis of AuNPs using proper function-
alization. As-synthesized cage supported gold nanoparti-
cles are found to act as efficient heterogeneous photo-
catalyst. It offers a superior catalytic activity with selective
photo-reduction of nitroarenes to form their correspond-
ing azo compounds at ambient temperature in very short
reaction time. The gifted features of these catalyst em-
brace with easy separation, wide range of functional
group tolerance and high yield of the products. This facile
and selective nitro reduction approach via a robust and
recyclable cage supported AuNPs has numerous benefits
over the known approaches for the conversion of ni-
troarenes to corresponding azo compounds as the tradi-
tional organic synthesis approaches give mixture of prod-
ucts containing azoxy-, amine and other byproducts. The
newly developed composite establishes an innovative
footstep for the advancement of new generation hetero-
geneous photocatalyst by tuning of structure and proper
decoration of covalent cages with required functional
groups.
Synthesis of P1: Compound P1 was prepared according
to the literature procedure.29a 1H NMR (CDCl3, 400 MHz):
δ 9.89 (s, 1H), 7.77 (d, 2H), 7.44 (d, 2H), 7.31–7.26 (m, 4H),
7.20-7.16 (m, 4H). 13C NMR (CDCl3, 100 MHz): δ 190.1,
150.6, 141.6, 132.7, 131.7, 130.7, 128.9, 127.4, 126.1, 125.4, 117.1.
ESI-HRMS (CHCl3-CH3CN): m/z for C19H13NOS: [M+H]+
304.0775 (calcd 304.0796). FTIR (cm-1) ν: 1686 (CH=O),
1580, 1463, 1281, 1222, 1167, 1112, 816, 730, 637, 533.
Synthesis of P2: Compound P1 (1 g, 3.3 mmol) was dis-
solved in 60 mL of dichloromethane (DCM) in a 100 mL
round bottom flask at room temperature. 1.3 g of NBS
(7.25 mmol, 2.2 equv.) was added to it portion wise and
refluxed with stirring for 12 h. The reaction was cooled to
room temperature and washed with water several times.
The combined organic phase was dried with anhydrous
Na2SO4. The filtered organic solvent was dried over re-
duced pressure and the residue was purified by the col-
umn chromatography using silica gel and dichloro-
methane to obtain white compound P2. Isolated yield:
75.4% (1148 mg, 2.49 mmol). Melting point range: (143-145
1
oC); H NMR (CDCl3, 400 MHz): δ 9.87 (s, 1H), 8.01 (d,
2H), 7.77 (d, 2H), 7.62 (d, 2H), 7.53 (s, 2H), 7.39 (d, 2H).
13C NMR (CDCl3, 100 MHz): δ 191.3, 150.5, 141.5, 138.9, 136.1,
133.0, 132.0, 130.3, 127.9, 125.7, 117.4. ESI-HRMS (CHCl3-
CH3CN): m/z for C19H11NOSBr2: [M+H]+ 461.8993 (calcd
461.8986). FTIR (cm-1) ν: 1686 (CH=O), 1573, 1457, 1278,
1215, 1160, 1104, 829, 739, 691, 643, 522.
Synthesis of P3: In a 250 mL double-neck round-bottom
flask, 2 g (4.34 mmol) of P2 and 1.95 g (13 mmol) of 4-
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formylphenylboronic acid (3) were taken in 100 mL THF
and into that 20 mL aqueous solution of 2.4 g (17.36
mmol) K2CO3 was added. The resulting mixture was
stirred under nitrogen atmosphere at room temperature
for 10 minutes followed by addition of 250 mg (5 Mol%) of
Pd(PPh3)4 and heated to 70°C for 48h. After completion of
the reaction THF was removed and the crude part was
extracted with dichloromethane (100 mL × 3). Organic
part was then dried over anhydrous Na2SO4 under re-
duced pressure to obtain crude solid. The crude solid was
purified by silica gel column chromatography in di-
chloromethane/hexane mixture to get yellow solid pow-
der. Isolated yield: 80% (1.78 g, 3.48 mmol). Melting point
range: (163-165 C); H NMR (CDCl3, 400MHz): δ 9.87 (s,
1H), 9.77 (s, 2H), 8.15 (d, 2H), 8.01 (d, 4H), 7.93 (d, 2H),
7.77 (d, 4H), 7.59 (d, 2H), 7.51 (s, 2H), 7.35 (d, 2H). 13C
NMR (100 MHz, CDCl3): δ 191.2, 190.4, 150.4, 148.1, 141.3,
139.0, 137.0, 135.6, 134.1, 132.8, 130.4, 128.0, 126.5, 125.1, 122.0,
117.6. ESI-HRMS (CHCl3-CH3CN): m/z for C33H21NO3S:
[M+H]+ 512.1327 (calcd 512.1320). FTIR (cm-1) ν: 1692
(CH=O), 1594, 1463, 1298, 1208, 1167, 829, 739, 691, 526,
471.
for C84H84N8S2: [M+H]+ 1269.6366 (calcd 1269.6339). FTIR
(cm-1) ν: 3296, 2887, 2807, 1556, 1450, 1326, 1201, 1050, 864,
815, 774, 685, 629.
Synthesis of Au@OC1R Catalyst: In a typical synthetic
protocol, 20 mg (0.015 mmol) of OC1R dissolved in 8 mL
CHCl3 (yellow colored) was treated with 4 mL CH3OH
solution of 66.94 mg (0.22 mmol) of AuCl3 (yellow col-
ored) and stirred for 1 h at 298K. Into this reaction mix-
ture (brown colored), 2 mL methanolic solution of NaBH4
(33.44 mg, 0.88 mmol) was added dropwise at 298K and
stirred for 1h (black colored). After this said time, the so-
lution was centrifuged to precipitate down. The product
was washed several times with methanol and THF fol-
lowed by drying under vacuum for overnight to obtain
Au@OC1R as a black solid powder.
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o
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Sample Preparation for ICP-MS Analysis: In a typical
stock solution preparation, 4.63 mg of Au@OC1R was
added to a 10 mL concentrated nitric acid and stirred for
overnight at room temperature. After complete dissolu-
tion of the catalyst, the solution was filtered to remove
any undissolved materials. The filtrate was then diluted
with deionized water to make a 100 mL aqueous stock
solution. A series of standard aqueous solutions was pre-
pared by dissolving AuCl3 in deionized water. The exper-
Synthesis of OC1: In a 250 mL round bottom flask 60 mL
CHCl3 solution of diamine 4 (33.51 mg, 0.293 mmol) was
added slowly dropwise to a stirring solution of aldehyde
P3 (100 mg, 0.195 mmol) dissolved in 120 mL CHCl3. The
resulting reaction mixture was stirred at room tempera-
ture (298K) for 48 h. After completion of the reaction
solvent was removed and the obtained yellow solid was
washed with CH3OH several times. Isolated yield: 92%
imental result shows ̴
68 Wt% of Au in Au@OC1R.
Sample Preparation for Solution State UV-Vis Spec-
troscopy: To prepare a stock solution, 5.26 mg of
Au@OC1R was taken in 10 mL CHCl3 and stirred for 2
minutes to obtain a solution at 298K. In a quartz cuvette,
1980 µL of CHCl3 and 20 µL of stock solution was added to
get a 2 mL of solution. This solution was used for UV-Vis
experiment.
o
(113 mg, 0.089 mmol). Melting point range: (182-184 C);
1H NMR (CDCl3, 400MHz): δ 8.87 (s, 2H), 7.78 (s, 4H),
8.15 (d, 4H), 8.01 (d, 8H), 7.93 (d, 4H), 7.77 (d, 8H), 7.60
(d, 4H), 7.52 (s, 4H), 7.36 (d, 4H), 3.37 (d, 6H), 2.03-1.78
(m, 24H). 13C NMR (100 MHz, CDCl3): δ 162.2, 160.5, 150.1,
148.0, 141.1, 139.1, 137.0, 135.6, 134.2, 132.9, 130.3, 128.1, 126.4,
125.2, 122.2, 117.5, 74.2, 33.8, 25.5. ESI-HRMS (CHCl3-
CH3CN): m/z for C84H72N8S2: [M+H]+ 1257.5377 (calcd
1257.5400), [M+2H]2+ 629.2769 (calcd 629.2739). FTIR
(cm-1) ν: 3431, 2869, 2800, 1639 (CH=N), 1567, 1421, 1332,
1257, 1015, 884, 671, 553.
General Experimental Procedure for Reduction of
Nitroarenes: Typical procedure for the reduction of 1-
chloro-4-nitrobenzene with Au@OC1R: In a flame dried
25 mL quartz tube, 1 g (6.35 mmol) of 1-chloro-4-
nitrobenzene, 50.8 mg (1.27 mmol) of NaOH and 9.12 mg
(0.032 mmol, 0.5 mol% of Au) of Au@OC1R were taken in
15 mL dry 2-propanol. The resulting reaction mixture was
degassed and stirred under nitrogen atmosphere for 2 h
under UV light. After completion, reaction mixture was
filtered to separate the solid catalyst. The solid catalyst
was washed with water and ethyl acetate properly and dry
it in vacuum for next cycle practice. Then the filtrate was
extracted with ethyl acetate (100 mL × 3). Organic part
was washed with water and dried over anhydrous Na2SO4
followed by complete removal of solvent. The resulting
crude mass was purified by silica gel column chromatog-
raphy in petroleum ether/ethyl acetate (9:1, v/v) to afford
pure E-1,2-bis(4-chlorophenyl)diazene as orange crystal-
line solid.
Synthesis of OC1R: In a 250 mL round bottom flask 100
mg (0.08 mmol) of OC1 was taken in 120 mL CHCl3-
MeOH (1:1, v/v) binary solvent mixture. Into above reac-
tion mixture, 36.3 mg (0.95 mmol) of NaBH4 was added
portion wise at room temperature (298K) and stirred for
48 h. After completion of the reaction, solvent was com-
pletely removed and the product was extracted in CHCl3.
Organic part was washed several times with water and
dried over Na2SO4 followed by removal of solvent to get
pale yellow solid. Isolated yield: 83.2% (84 mg, 0.066
mmol). Melting point range: (222-224 oC); 1H NMR
(CDCl3, 400MHz): δ 8.06 (d, 4H), 7.97 (d, 8H), 7.85 (d,
4H), 7.72 (d, 8H), 7.59 (d, 4H), 7.51 (s, 4H), 7.35 (d, 4H),
4.09 (d, 4H), 3.84 (d, 8H), 2.35 (d, 6H), 1.36-1.07 (m, 24H).
13C NMR (100 MHz, CDCl3): δ 149.0, 147.7, 141.5, 139.7,
137.3, 135.7, 134.4, 133.0, 130.4, 128.4, 126.8, 125.4, 122.8, 117.3,
62.8, 51.6, 50.1, 32.1, 25.8. ESI-HRMS (CHCl3-CH3CN): m/z
o
1
94% yield (1.51 g); orange solid; mp 186-188 C. H NMR
(CDCl3, 400MHz): δ 7.86 (d, 4H), 7.49 (d, 4H). 13C NMR
(100 MHz, CDCl3): δ 150.9, 137.3, 129.5, 124.3. The physical
data were identical in all respect to those of previously
reported data.29b
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E-1,2-bis(3-chlorophenyl)diazene: 92% yield (1.47 g);
400MHz): δ 8.11 (d, 4H), 8.00 (d, 4H), 2.68 (s, 6H). 13C
NMR (100 MHz, CDCl3): δ 197.5, 154.5, 139.9, 129.7, 123.2,
26.9.29b
o
orange solid; mp 99-101 C. 1H NMR (CDCl3, 400MHz): δ
7.90-7.87 (m, 2H), 7.85-7.82 (m, 2H), 7.49-7.45 (m, 4H).
13C NMR (100 MHz, CDCl3): δ 153.1, 139.43, 131.4, 130.3,
122.7, 121.8. The physical data were identical in all respect
to those previously reported.29d
E-1,2-bis(4-methylcarboxylatephenyl)diazene:
84%
yield (1.39 g); orange solid; mp 224-226 oC. 1H NMR
(CDCl3, 400MHz): δ 8.22 (d, 4H), 8.00 (d, 4H), 3.99 (s,
6H). 13C NMR (100 MHz, CDCl3): δ 166.4, 155.1, 132.5, 130.5,
123.1, 52.2.29b
E-1,2-bis(2-chlorophenyl)diazene: 89% yield (1.43 g);
red orange solid; mp 138-140 oC. 1H NMR (CDCl3,
400MHz): δ 7.78-7.76 (m, 2H), 7.56-7.54 (m, 2H), 7.40-
7.32 (m, 4H). 13C NMR (100 MHz, CDCl3): δ 148.3, 135.8,
132.2, 130.5, 127.2, 118.3.29b
9
E-1,2-bis([1,1’-biphenyl]-3-yl)diazene: 83% yield (1.40 g);
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bright orange solid; mp 65.0-66.0 oC. H NMR (CDCl3,
400MHz): δ 8.20 (s, 2H), 7.94-7.92 (d, 2H), 7.75-7.70 (m,
6H), 7.63-7.59 (m, 2H), 7.51-7.47 (m, 4H), 7.42-7.38 (m,
2H). 13C NMR (100 MHz, CDCl3): δ 153.7, 142.8, 140.6, 130.2,
129.9, 129.4, 128.2, 127.7, 122.2.15d
E-1,2-bis(4-fluorophenyl)diazene: 91% yield (1.41 g); a
yellow-orange solid; mp 100-102 oC. 1H NMR (CDCl3,
400MHz): δ 7.17-7.22 (m, 4H), 7.89-7.95 (m, 4H). 13C NMR
(100 MHz, CDCl3): δ 165.7, 148.9, 124.2, 116.2.29e
E-1,2-bis(1,1’-naphthyl)diazene: 80% yield (1.31 g); light
o
E-1,2-bis(4-bromophenyl)diazene: 93% yield (1.57 g);
deep orange solid; mp 205-207 oC. 1H NMR (CDCl3,
400MHz): δ 7.79 (d, 4H), 7.65 (d, 4H). 13C NMR (100 MHz,
orange solid; mp 188-190 C. 1H NMR (CDCl3, 400MHz): δ
9.07 (d, 2H), 8.06-7.95 (d, 6H), 7.72-7.60 (d, 6H). 13C NMR
(100 MHz, CDCl3): δ 152.8, 142.9, 138.7, 129.5, 128.7, 127.4.
124.1, 123.1.29f
CDCl3): δ 151.3, 132.5, 125.9, 124.5.29b
.
E-1,2-bis(4-iodophenyl)diazene: 90% yield (1.57 g); a
Computational Methodology: Full geometry optimiza-
tions were performed using Gaussian 09 package.30 The
hybrid B3LYP functional was used in all calculations as
implemented in Gaussian 09 package, mixing the exact
Hartree-Fock-type exchange with Becke’s expression for
the exchange functional31 that was proposed by Lee-Yang-
Parr for the correlation contribution.32 The 6-31G basis set
was used for all calculations. Frequency calculations were
carried on the optimized structures that confirmed the
absence of any imaginary frequencies.
1
brownish-orange solid; mp 214-216 oC. H NMR (CDCl3,
400MHz): δ 7.90 (d, 4H), 7.66 (d, 4H). 13C NMR (100 MHz,
CDCl3): δ 151.6, 138.3, 124.4, 97.75.29c
.
E-1,2-bisphenyldiazene: 92% yield (1.36 g); deep orange
o
solid; mp 65-67 C. 1H NMR (CDCl3, 400MHz): δ 7.91 (d,
4H), 7.51-7.45 (m, 6H). 13C NMR (100 MHz, CDCl3): δ 152.8,
131.3, 129.3, 122.8.29b
E-1,2-bis(4-methylphenyl)diazene: 91% yield (1.40 g); a
red orange solid; mp 144-146 oC. 1H NMR (CDCl3,
400MHz): δ 7.81 (d, 4H), 7.31 (d, 4H), 2.44 (s, 6H). 13C
NMR (100 MHz, CDCl3): δ 150.9, 141.2, 129.9, 122.4, 21.5.29b
ASSOCIATED CONTENT
Syntheses and characterization data (NMR, FTIR, ESI-MS,
TEM, SEM, PXRD). This material is available free of charge
via the Internet at http://pubs.acs.org.
E-1,2-bis(3-methylphenyl)diazene: 88% yield (1.35 g);
deep orange solid; mp 51-53 oC. 1H NMR (CDCl3, 400MHz):
δ 7.74-7.71 (m, 4H), 7.40-7.38 (m, 2H), 7.30-7.27 (m, 2H),
2.47 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 152.9, 139.1,
131.5, 129.3, 123.4, 120.4, 21.4.29b
AUTHOR INFORMATION
Corresponding Author
E-1,2-bis(2-methylphenyl)diazene: 85% yield (1.31 g);
o
1
psm@iisc.ac.in.
orange solid; mp 53-55 C. H NMR (CDCl3, 400MHz): δ
7.62-7.60 (m, 2H), 7.35-7.32 (m, 4H), 7.27-7.25 (m, 2H),
2.75 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 151.3, 138.2,
131.4, 130.4, 126.5, 115.9, 17.7.29b
Notes
The authors declare no competing financial interest.
E-1,2-bis(4-methoxylphenyl)diazene: 90% yield (1.43
g); orange solid; mp 150-152 oC. 1H NMR (CDCl3,
400MHz): δ 7.81 (d, 4H), 7.00 (d, 4H), 3.91 (s, 6H). 13C
NMR (100 MHz, CDCl3): δ 161.7, 147.1, 124.2, 114.2, 55.8.29b
ACKNOWLEDGMENT
P. S. M. is grateful to SERB-New Delhi (India) for financial
support. B. M. acknowledges the CSIR, New Delhi for the
research fellowship. We are thankful to Mr. Jayanta Ghosh
for theoretical calculations and Mr. Manoj Kumar for Powder
XRD data collection.
E-1,2-bis(4-hydroxyphenyl)diazene: 87% yield (1.34 g);
pale orange solid; mp 174-176 oC. 1H NMR (CD3OD,
400MHz): δ 7.97 (d, 4H), 6.50 (d, 4H). 13C NMR (100 MHz,
CD3OD): δ 161.3, 147.5, 125.4, 116.7.15d
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Cage Encapsulated Gold Nanoparticles as Heterogeneous
Photocatalyst for Facile and Selective Reduction of Ni-
troarenes to Azo Compounds
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Bijnaneswar Mondal and Partha Sarathi Mukherjee*
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