Rhodium-calix[4]pyrrole and rhodium-tetraphenyl porphyrin: preparation, surface grafting and their catalytic application in nitro-benzene reduction

Article abstract of DOI:10.1039/C8DT02151A

Rhodium containing macromolecules calix[4]pyrrole (RhCP) and tetraphenyl porphyrin (RhTPP) were prepared and grafted on the surface of functionalised molecular sieve materials. ¹H and ¹³C NMR, CHNS and mass spectral analyses were used for the structural elucidation of homogeneous organometallic complexes.¹H NMR and CHNS analyses confirm the formation of calix[4]pyrrole (CP) and tetraphenyl porphyrin (TPP). The introduction of rhodium ions into the macromolecule is well evident from the disappearance of the ¹H NMR signal characteristic of N-H proton (7.1 ppm in CP and ?2.74 ppm in TPP). Furthermore the formation of RhCP and RhTPP complexes is confirmed using CHNS and mass spectral analysis; the data are in line with theoretically calculated values. The grafting of RhCP and RhTPP on a diamino functionalised SBA-15 support was confirmed through low angle XRD, ¹³C MAS-NMR and SEM-EDAX analysis. Both homogeneous and heterogeneous catalysts were utilized for nitrobenzene reduction. RhCP and RhTPP heterogenized on SBA-15-F showed complete conversion of nitrobenzene with exclusive formation of aniline as a product. The catalytic activities were retained in both the systems even after several runs.

Full text of DOI:10.1039/C8DT02151A

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Rhodium‐calix[4]pyrrole and Rhodium‐tetraphenyl porphyrin:
Preparation, surface grafting and its catalytic application on nitro‐
benzene reduction
Received 00th January 20xx,
Accepted 00th January 20xx
K. Anjalia, M. Ahmed b, J. Christopherc and A. Sakthivela*
DOI: 10.1039/x0xx00000x
Abstract: Rhodium containing macro molecules Calix[4]pyrrole (RhCP) and tetrapenyl Porphyrin
(RhTPP) were prepared and grafted on the surface of functionalised molecular sieve materials. 1H
and 13C NMR, CHN analysis and Mass spectral analysis was used for structural elucidation of
homogeneous organometallic complexes.1H NMR and CHNS analysis confirms the formation of
Calix [4] pyrrole (CP) and Tetraphenyl porphyrin (TPP). The introduction of rhodium ion into the
macromolecule is well evident from the disappearance of 1H NMR signal characteristic of N-H bond
(7.1ppm in CP and -2.74 ppm in TPP). Further the formation of RhCP and RhTPP complex is
confirmed by CHNS and Mass spectral analysis, the data are in line with theoretically calculated
value. The grafting of RhCP and RhTPP on diamino functionalised SBA-15 support was conformed
through low angle XRD, 13C MAS-NMR and SEM-EDAX analysis. Both homogeneous and
heterogeneous catalyst were utilized for nitrobenzene reduction. The RhCP and RhTPP
heterogenized on SBA-15-F showed complete conversion of nitrobenzene with exclusive formation
of aniline as a product. The catalytic activities were retained in both the systems even after several
runs.
www.rsc.org/
heterogenization of homogeneous catalysts.6‐11 Particularly,
mesoporous silica‐based materials having uniform channels,
large pore sizes, high surface areas, and thermal stability3c offer
Introduction
Catalysis is the backbone of chemical processes involved in
natural metabolic pathways and many industrial processes
involved in day‐to‐day life.1 About 80–90% of the chemical
produced in industries are manufactured using catalysts, in
which homogenous catalysts such as organometallic complexes
containing transition metal ions play a major role.2‐5 The major
problems associated with homogenous catalysts are difficulty in
catalyst recovery from reaction mixtures, product isolation,
expensive precursor materials, and tedious procedures. Despite
their limitations, industries prefer homogeneous catalysts
owing to their high reactivity and selectivity. Development of
heterogeneous catalysts or heterogenization of homogeneous
catalysts overcomes the limitations of homogeneous catalysts
and offers the advantages of both. This involves various
approaches such as surface grafting through covalently
connected organic moieties on a support, direct immobilization
of homogenous catalysts on inorganic support, and ship‐in‐
bottle methods.2 Solid supports including microporous and
mesoporous silica‐based materials such as zeolites, MCM 41,
SBA‐16, SBA‐15, and polymers have proved promising for
great opportunities for grafting and encapsulation of various
organometallic homogeneous catalysts. variety of
A
homogeneous catalysts such as Schiff‐bases, cyclopentadienyl,
and phosphine‐based complexes have been heterogenized and
demonstrated potential activity for various organic
transformations.4 A lot of work has been carried out on
homogeneous organometallic complexes derived from
macromolecular
calix[4]pyrrole (CP), and calix‐arenes play an important role in
catalysis and bio‐catalysis. For example, macrocyclic
ligands
such
as
protoporphyrin’s,
a
compound comprising four pyrrole subunits that are connected
through α‐carbon analogues to a heme unit can encapsulate
various transition metal ions.5 In particular, lot of work has been
focused on development of metallo‐porphyrin based molecules
and its applications on various fields such as hydration of
alkenes and alkynes, oxidation of alcohols and oxygen
reductions in fuel cell.6 The early work on these systems known
as potential enzyme catalysts for several biological processes7
and was reported by Alder and Longo in 1964, Besides
porphyrin, calix[4]pyrrole CP is an excellent macrocyclic
molecule first synthesized by Baeyer in 1886 by acid
condensation of pyrrole and acetone.8 These macrocyclic
molecules play a vital role in catalysis, as well as medicinal, and
analytical sciences, and in chemical sensors.9 The catalytic
activities and stabilities of these macrocyclic materials are
enhanced by encapsulating in inorganic solid supports. In 1983,
a. Department of Chemistry, Central University of Kerala, Kasaragod, kerala, India
b. Department of Chemistry, University of Delhi, Delhi, India.
c. Indian Oil Corporation Ltd., R & D Centre, Faridabad, India
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Grove’s et al.10 demonstrated the use of a synthetic iron using Pluronic 123 triblock copolymer (poly(ethylene oxide)‐
porphyrin as a catalyst for the epoxidation of alkenes and poly(propylene oxide)‐poly(ethylene oxide)EOnPO70EOn as
hydroxylation of alkanes by iodosylbenzene. Similarly, Nakagi et surfactant. In a typical procedure, about 4.0g of the EOnPO70EOn
al.11 reported the incorporation of metalloporphyrines into
porous vycor glass and zeolites and studied them for
cyclohexane and cyclohexene oxidation. Transition metal ion
(e.g., Co and Fe)‐containing macromolecules are often studied
for oxidation of various organics and have shown excellent
activities both in homogeneous and heterogeneous forms.5, 12
However, limited reports exist on hydrogenation of organics
using heterogenized homogeneous catalysts. Rhodium‐based
organometallic complexes are promising catalysts for
hydrogenation and hydrogen transfer reduction processes.
Schwarze et al. reported the hydrogenation of C=C double
bonds using rhodium‐based complexes.13 Peris and co‐workers
first synthesized rhodium(III) carbene complexes and studied
their catalytic activities for hydrogen transfer from alcohols to
ketones and imines.14a Historically nitro group was one of the
first functional groups to be reduced and this reduction is
carried out using the catalysts such as palladium on carbon,
nickel based catalyst, platinum oxide etc.,.14 But the major
drawback of these catalysts was the ability to react with a wide
variety of other functionalities present on the substrate.
Leeumen and Roobeek reported hydroformylation of un‐
reactive olefins under mild conditions15 and hydroformylation
of higher alkenes16a,b using rhodium‐based complexes. Further,
nitrobenzene reduction, hydrogenation and oxidative
amidation with very high conversion and selectivity is achieved
by blending the inorganic complexes on the surface of inorganic
matrix.16d,e,f For example Cobalt based nano‐catalyst is used for
the reduction of polyaromatics with good conversion and
selectivity at ambient conditions.16d These catalysts are often
homogeneous and limited approaches are known for the
copolymer was dissolved in 180 g of 1.6M HCl solution.
Scheme 1. Pictorial representation of grafting of Rh macromolecules
on SBA‐15‐F materials.
To this solution 8.50g of TEOS (tetraethyl orthosilicate) was added
and the resulting mixture was allowed to stir continuously at 35‐40
0C. The mixture was placed in an oven for 24 hours and then
additionally for 24 hours at 1000C under static conditions. The
products were filtered, dried, and calcined at 5500C. The calcined SBA
15 was used as a solid support material. Then the calcined SBA‐15
support (6.5 g) was pre‐activated at 200 °C in an air oven followed by
drying under vacuum in a round bottom flask to remove the
physisorbed molecules. Then, a pre‐determined amount (3 mmol per
gram of solid support) of N‐[3‐(trimethoxysilyl)‐propyl]‐
ethylenediamine was introduced to the activated SBA‐15 support
material after it had cooled to room temperature. The mixture was
refluxed in dry toluene (30 mL g–1 of support) for 24 h in a N2
atmosphere. This procedure was repeated in thrice in order to
completely convert surface silanol groups into N‐[3‐(trimethoxysilyl)‐
propyl]‐ethylenediamine functionality. The resultant mixture was
cooled to room temperature, washed with dichloromethane, and
then dried under vacuum to give the final product SBA‐15‐F. Calix [4]
pyrrole(CP) was prepared by dissolving distilled pyrrole (1mmol) in
15ml dry acetone. The mixture was maintained at 00C for 10 minutes.
Subsequently, 2.5 mL of conc. HCl (37 %) was introduced drop by
drop with continues stirring in ice bath. After completion of the
reaction, the solvent was removed by filtration followed by washing
several times with ethanol in order to remove excess reactants and
other impurity, and the white precipitate was isolated.
Rhodium containing Calix[4]pyrrole (Rh‐CP) was prepared by the
following procedure. First 1mmol of metal salt (RhCl3.XH2O) was
dissolved in 5ml dry acetone. Another solution was prepared using
about 1 mmol of calix[4]pyrrole in 45 ml dry acetone, which was
introduced into the above prepared metal salt solution. The resultant
mixture was allowed to reflux in inert atmosphere for 24 hours. After
completion of reaction, the solvent was removed by rota‐vapour and
the solid product was separated and washed with pentane in order
to remove any organic impurities. Meso tetra phenyl porphyrin (TPP)
was prepared by refluxing the freshly dried pyrrole (0.027mol) and
benzaldehyde (0.0.28mol) in 100 mL propionic acid for 2 hours. After
the stipulated period, the reaction solution was cooled to room
preparation
and
application
of
rhodium‐based
macromolecules. There is no report on preparation of rhodium
containing calix [4]pyrrole based macro‐molecules and its
applications. Further, there are no clear reports regarding
regeneration and recyclability of encapsulated macro‐molecule
based catalysts. Thus, it will be interesting to introduce rhodium
ions into the cavities of macromolecules such as calix [4]pyrrole,
phenyl Porphyrin, followed by heterogenization on a support
and subsequent utilization as catalysts for hydrogenation
reaction.
This is the first report on the preparation of rhodium‐
calix[4]pyrrole (Rh‐CP) and rhodium‐tetra phenyl‐porphyrin
(Rh‐TPP) complexes and their subsequent encapsulation into
organo‐amine‐functionalized SBA‐15 (SBA‐15‐F; see Scheme 1).
The resultant homogeneous and heterogenized homogeneous
catalysts demonstrated as potential for catalyzing
hydrogenation of organics.
Experimental
Preparation of the catalyst:
All experiments were conducted using a Schlenk line. Before the
experiment, all solvents and pyrrole were dried by standard
procedure. SBA‐15 was prepared as per the reported procedure
2 | J. Name., 2012, 00, 1‐3
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temperature, filtered and washed thoroughly with methanol. The measurements at ‐196oC using an automatic micropore
resulting purple crystals was air dried. The rhodium incorporated physisorption analyser (Micromeritics ASAP 2020, USA). The samples
Meso teraphenylporphyrin (RhTPP) was prepared by dissolving metal were degassed at 110oC for 10 hours under 10‐3 Torr pressure prior
salt (RhCl3.XH2O; 1mmol) in 5ml dry acetone followed by introducing to each run.
the solution containing 1 mmol of TPP in 45 mL dry acetone. The final Catalytic studies: The reaction is started by introducing
mixture was allowed to reflux in presence of nitrogen atmosphere stoichiometric amount of hydrazine hydrate into the round bottom
for 24 hours. After completion of the reaction, the solvent was flask having appropriate amount of nitrobenzene and catalysts. The
removed, washed several times with pentane in order to remove reaction was monitored by thin layer chromatography and GC.
impurities and the product was isolated. The preparation of ligands
(CP and TPP) under normal atmospheric conditions using solvents as
Results and Discussion
received form facilitate the formation of polymeric species, which is
The FT‐IR spectra of CP, TPP, (Fig.1) and their rhodium complexes
evident from the 1H‐NMR studies (Figure S2 ESI*). Thus, all the
(Rh‐CP and Rh‐TPP) show characteristic bands in the region of
reaction was carried out with inert atmosphere and dry solvents. In
4000450 cm−1. The FT‐IR spectra of CP and Rh‐CP are similar. The
the second step the formation of metallo‐macromolecules by the use
stretching bands appearing around 523 and 657 cm−1 are
of hydrated salts doesn’t influence the stability of the ligand once it
has been formed.
a
b
Grafting of Rh‐CP and Rh‐TPP macromolecules on SBA‐15‐F is
achieved by the following method. About 1 g of SBA‐15‐F was
activated by treating at 1000C for 2 hours and then kept in
vacuum and dried for 1 hour. 0.5 mmol Rh‐CP / Rh‐TPP is
dissolved in 50 ml dry methanol. Then the above mixture was
added to the round bottom flask containing pre‐activated SBA‐
15‐F and allowed to reflux for 24 hour under nitrogen
atmosphere. Then the precipitate was collected by separating
the solvent and the product was vacuum dried.

N-H

c
M-N
d


C=C
C=N
e
f

M-N

N-H
g

M-X


M-O-S
CN
Characterization

The 1H and 13C NMR spectra of all of the ligands and homogeneous
macro‐molecules were recorded in CDCl3. 1H NMR spectra were
recorded on a Brücker AVANCEIII 400 instrument. ESI‐TOF mass
spectra were recorded on a Brücker maXis mass spectrometer.
Elemental analyses were performed on a Elementar Vario Micro
Cube. Solid state MAS NMR spectra were recorded on a Brücker
Si-O-Si (assy)
2000
1500
1000
500
and C=N vibrational
-1
Wave number(cm )
characteristic of C=C
modes. Bands in the
Figure 1. FT‐IR spectra of (a) TPP, (b) CP, (c) RhTPP, (d) RhCP, (e) SBA‐
15‐F, (f) SBA‐15‐F‐RhTPP, and (g) SBA‐15‐F‐RhCP.
AVANCE 400 wide bore spectrometer equipped with
a
superconducting magnet with a field of 7.1 T using 4 mm double
resonance magic angle spinning (MAS) probe operating at resonating
at frequencies of 79.4 MHz was employed for 29Si MAS NMR. In each
case samples were packed in 4 mm zirconia rotors and subjected to
a spinning speed of 10 kHz single pulse experiment with pulse
duration of 4.5 μs and a relaxation delay time of 6 s were used for
recording all 29Si, and13C MAS NMR patterns. All chemical shift values
are expressed with respect to TMS for the 29Si nucleus, and glycine
for 13C nucleus. Morphology and particle size of materials were
obtained by scanning electron microscopy (SEM) images on
EVOMA15 Zeiss operated at 10‐20 kV. Finely crushed powder
samples were ‘dusted’ on a double‐sided carbon tape, mounted over
the sample holder. Gold sputtering of the materials were carried out
over gold sputter coater JEC 300, to avoid charging and make the
surface conducting. Finally, the specimens were analysed in SEI
(secondary electron imaging) mode at different magnifications and
desired voltages. FT‐IR spectra were recorded on Perkin‐Elmer
spectrum‐2 FTIR using KBr pellets. Spectra were collected with 4cm‐
region of 1000–1460 cm−1 are typical of symmetric and asymmetric
C=C and C‐C bonds‐related skeletal pyrrole vibration of CP and TPP.
Evident bands at 1173 and 1158 cm−1 correspond to C‐H asymmetric
deformation modes in the pyrrole ring. A strong and sharp N‐H
stretching band was observed at 3443 cm−1 in the spectrum of CP,
which disappeared after metal incorporation. In addition to this, an
obvious vibrational band at 957cm−1 corresponds to N‐H bending
which is absent in Rh‐CP. This indicates that the rhodium metal ions
is directly bonded to pyrrole nitrogen in CP ligand.12a, 17 In TPP, most
of the bands appear in the region of 456–979 cm−1, due to the in‐
plane bending, out‐of‐plane bending, ring rotation, and ring torsion
modes of the TPP skeleton. In addition, the sharp vibrational bands
observed at 3318 and 988 cm−1 correspond to N‐H stretching and
bending modes, respectively. The incorporation of metal (Rh3+) ions
into TPP is supported by the disappearance of the sharp band at 3318
cm−1. Instead, the broadened band indicates that intra‐molecular
hydrogen bond in the TPP no longer exists after metal complexation.
In addition, the Rh‐TPP sample has no spectral band at around 988
cm−1, indicating the absence of N‐H bending in the metallo‐
porphyrin. This is clear evidence that the metal is covalently bonded
to the nitrogen present in TPP. After metal incorporation, a new
1
resolution and 120 scans in the mid IR (400‐4000 cm 1) region. UV‐
-
Vis spectra of homogeneous and DR‐UV‐Vis spectra of solid samples
were recorded on Perkin Elmer Lambda 35 spectrometer. The
textural properties (BET surface area, BJH average pore volume) of
the heterogenized samples were followed by N2 sorption
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shoulder peak was observed at 672 cm−1 that corresponds to the M‐ The CHNS analysis of CP, RhCP, TPP and RhTPP were
N bond. There are corresponding slight shifts in the vibrational bands studied using Elementar Vario Micro Cube and the results are
of C‐H bending and C=N stretching in TPP from 738, 701, and 1158 depicted in Table 2 and the original data reproduced in Table S1. The
cm−1 for TPP to 722, 694, and 1152 cm−1 for Rh‐TPP after metal ion elemental analysis gives strong evident for the formation of the
incorporation,16respectively. This further supports the formation of complex CP, RhCP, TPP, RhTPP. The experiment values are
Rh‐TPP.
well match and agreement with the theoretical values, which
1H NMR studies also support the formation of Rh‐CP and Rh‐TPP strongly support the formation of CP, TPP, RhTPP and RhCP
1
(Table 1). the H NMR spectrum of CP showed a broad singlet complexes. The formation of Rh‐TPP and Rh‐CP was further
peak at 7.1 ppm, which is assigned to the four N‐H protons from confirmed from mass spectrometry analysis. Rh‐CP showed a parent
pyrrole rings.19a,b Importantly, after the rhodium ions were peak at 545 with the fragment peak at 428 correspond to CP+ moiety
incorporated to the calix[4]‐pyrrole cavities in Rh‐CP, the confirming the successful preparation of novel macromolecules
absence of N‐H proton peak at 7.1ppm confirms the containing the rhodium ion. Similarly for Rh‐TPP, the parent ion peak
coordination of rhodium ions through the pyrrole nitrogen CP appeared at 715.6 and also has two fragment peaks at 615.2 and
ligand. Other characteristic peaks for calix‐[4]‐pyrrole are 537.2 that correspond to [C44H30N4+1]+and [C38H22N4]+ by losing
retained for CP and Rh‐CP (Table 1). These results are consistent C6H5.18
with the FT‐IR studies. The peak at ‐2.74 ppm for TPP is
characteristic of the highly shielded region that corresponds to
the N‐H bond, which is absent in Rh‐TPP. Hence, in the latter,
the rhodium is bonded to the nitrogen centre of TPP. The other
peaks, being characteristic of the phenyl porphyrin framework,
remain identical for both TPP ligand and Rh‐TPP.
1
Table 1. H‐NMR of macromolecular ligands CP (Calix[4]pyrrole) &
TPP (Tetra phenyl porphyrin) and its rhodium complexes RhCP
1H NMR
Sample
Code
Chemical Shift (ppm) for ligand and complexes
Pyrrole
5.8
N‐H
7.1
Meso
1.50
1.58
CP
RhCP
5.9
‐‐‐
Pyrrole
8.87
N‐H ortho
meta
7.75
7.76
Para
Figure
RhTPP.
2. UV‐Visible spectra of (a) CP, (b) RhCP, (c) TPP and (d)
TPP
‐2.74
‐‐‐
8.25
8.80
7.78
7.78
The UV‐Visible spectra (Fig.2) of CP shows the absorbance
maxima at 232 nm, which corresponds to soret band. Upon
incorporation of rhodium ions, this maxima was blue shifted to
204 nm,19 once again supporting the formation of the macro‐
molecular complex. Similarly UV‐Vis spectrum of TPP shows
peaks at 405 (Soret band), 513, 547, 590, and 645nm (Q bands).
Upon rhodium ion incorporation, the pyrrole nitrogen’s of TPP
are coordinated to the rhodium ion to form Rh‐TPP complex.19
As a result, the intensity of Q bands is decreased, and the Soret
band shows slight red shift (from 405 to 413nm) that is
characteristic of the formation of Rh‐TPP complex. It could be
due to the improved structural symmetry (D4h point group) and
lowered energy gap of the Rh‐TPP complex compared to TPP
(D2h point group).19 The solubility of the Rh‐TPP and RhCP were
thoroughly investigated and subsequently attempt were made
to get single crystal. Both TPP and Rh‐TPP are found soluble in
solvents such as carbon tetra chloride, chloroform, acetonitrile,
toluene, dimethyl formamaide and tetrahydrofuran. Prolong
withstanding RhTPP in coordinating solvent facilitate isolation
of TPP crystal and may be metal is coordinating with the
solvents used for crystallization. As in the case of CP is soluble
in solvents such as dichloromethane, acetone, chloroform and
RhTPP
8.86
(Rhodium containg Calix[4]pyrrole) and RhTPP (Rhodium containing
Tetraphenyl porphyrin) .
Table 2. CHNS of macromolecular ligands CP (Calix[4]pyrrole) &
TPP (Tetra phenyl porphyrin) and its rhodium complexes RhCP
(Rhodium containing Calix[4]pyrrole) and RhTPP (Rhodium
containing Tetraphenyl porphyrin) .
% of C
% Of H
% of N
Sample
code
Expt Theor. Expt Theor. Expt Theor
CP
RhCP
TPP
78.83 78.46 8.49
59.95 59.74 5.80
85.35 85.90 4.98
8.47
5.73
4.88
3.76
13.03 13.07
9.95
9.17
7.50
9.95
9.11
7.46
RhTPP 70.67 70.36 4.10
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tetrahydrofuran and Rh‐CP is soluble only in chloroform, DMF
and DMSO and in other solvents it is insoluble. Nevertheless
there is no single crystal able to get for CP, RhCP and RhTPP.
After thorough characterization, the homogeneous catalysts
were grafted into the diamine functionalized SBA‐15. The
corresponding heterogenized samples are denoted as SBA‐15‐
F‐RhCP and SBA‐15‐F‐RhTPP. The FT‐IR spectrum of SBA‐15‐F
shows (Fig. 1) a vibration band at 2933 cm‐1 (C‐H stretching),
while spectral peaks at around 1643, 1468, and 691 cm‐1
correspond to NH bending and NH2 wagging modes of the
diamine group on the SBA‐15‐F surface, in agreement with the
literature values.20,21a,b The spectra of SBA‐15‐F‐Rh‐CP and SBA‐
15‐F‐RhTPP samples clearly show the vibrational band
characteristic of CP and TPP ligands, respectively, as well as a
band at around 672 cm‐1 characteristic of the Rh‐N bond. Hence,
the molecular structures were intact after immobilization.
N2 sorption isotherms of SBA‐15, SBA‐15‐F, and the grafted
samples SBA‐15‐F‐RhCP and SBA‐15‐F‐RhTPP (Fig. 3) shows the
typical type IV isotherm as per the IUPAC classification. A well‐
defined sharp inflection was observed between relative
pressures of 0.6 and 0.8, due to capillary condensation of
nitrogen inside the primary mesoporous sample. Further,
compared to the parent sample (SBA‐15‐F), SBA‐15‐F‐RhCP and
SBA‐15‐F‐RhTPP have drastic decrease in surface area and pore
volume (Table 3), thus confirming that RhCP and RhTPP are
encapsulated in the channels of the mesopores
a
Adsorption
Desorption

b
c
d
0.00
0.25
0.50
0.75
1.00
Relative Pressure (P/Po)
Figure 3. N2 sorption isotherm (a) SBA‐15, (b) SBA‐15‐F, (c) SBA‐
15‐F‐RhTPP and (d) SBA‐15‐F‐RhCP.
Table 3. Textural properties of SBA‐15, SBA‐15‐F, SBA‐15‐F‐RhCP and
Sample
code
BET
BJH
BJH
Surface area Pore volume Pore size
(nm)
(m2g‐1)
824.2
170.5
28.4
(cm3g‐1)
1.05
SBA‐15
SBA‐15‐F
7.2
7.6
5.1
6.2
Figure 4. 13C‐MAS‐NMR spectra of (a) SBA‐15‐F‐RhTPP and (b) SBA‐
15‐F‐RhCP.
0.35
Q4
SBA‐15‐F‐RhCP
0.12
Q
3
Q
2
SBA‐15‐F‐RhTPP
SBA‐15‐F‐RhTPP.
124.2
0.18
T
T
3
2
c
b
13C and 29Si MAS‐NMR spectra were used to confirm the covalent
grafting in SBA‐15‐F‐RhCP and SBA‐15‐F‐RhTPP. 13C NMR spectra
(Fig. 4) of all the samples showed peaks at 8.3 (C1), 19.4 (C2), 36.7
(C3), 45.9 (C5), and 50 ppm(C6), which correspond to aliphatic carbon
functionalities arising from the diaminosilane linker.21 SBA‐15‐F‐
RhCP shows peaks corresponding to CP17 at around 18.7, 33.5, 101,
and 138 ppm, which once again confirm the successful encapsulation
of RhCP in SBA‐15‐F. Similarly SBA‐15‐F‐RhTPP showed peaks at 150,
137, 140, 133, 125, 128, and 122ppm22 that are characteristic of TPP
a
0
-50
-100
-150
-200
(Chemical Shift (ppm)
Figure 5. 29Si‐MAS‐NMR spectra of (a) SBA‐15, (b) SBA‐15‐F‐RhTPP
and (c) SBA‐15‐F‐RhCP.
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The grafting of Rh‐CP and Rh‐TPP on SBA‐15‐F was further
confirmed through SEM‐EDX analysis (Fig. 6). The SEM images
of SBA‐15‐F‐RhCP and SBA‐15‐F‐RhTPP shows that the typical
hexagonal morphology of SBA‐15 materials was retained.20b The
surface roughness in the SEM images may be due to the
presence of organometallic complexes (Rh‐CP and Rh‐TPP)
grafted on the surface. Overall, the SEM results indicate that the
SBA‐15 morphology remained intact after functionalization and
incorporation of organometallic complexes. Importantly, the Rh
contents on SBA‐15‐F‐RhCP and SBA‐15‐F‐RhTPP from EDX
analysis are in good agreement with the loading of RhCP and
RhTPP complexes.
The characterized RhCP, Rh‐TPP, SBA‐15‐F‐RhCP, and SBA‐15‐F‐
RhTPP were tested for the hydrogenation of nitrobenzene
Figure 6. SEM image of rhodium‐containing macromolecules loaded
SBA‐15‐F and the corresponding EDX pattern of (a) SBA‐15‐F‐RhTPP
and (b) SBA‐15‐F‐RhCP.
14b
using hydrazine as hydrogen donor, and the resulting catalytic
activity is summarized in Table 4. The homogeneous Rh‐CP and
Rh‐TPP showed very low conversion (2 and 7 %, respectively)
with exclusive formation of aniline. In contrast, SBA‐15‐F‐RhCP
and SBA‐15‐F‐RhTPP showed excellent conversion of
nitrobenzene with the formation of both aniline and
azoxybenzene as products. In particular, the catalytic activity of
SBA‐15‐F‐RhCP and SBA‐15‐F‐RhTPP remained constant for
several cycles, as shown in table 5 and the rhodium ions on the
surface were intact. The above fact was further confirmed
through leaching experiments, which is carried out by
separating catalyst from the reaction mixture (after 4 h) and
subsequently follow the reaction in the filtrate solution. The
filtrate solution doesn’t improve any further conversion of the
reactants, supports the absence of Rh ions in the filtrate. This
support the rhodium ion remain intact on the surface. The CHN
analysis data of used heterogeneous catalyst is matching with
fresh heterogeneous catalyst (Table S2) further support that the
catalyst remain intact. This is the first report on the preparation
and successful grafting of rhodium‐based macromolecules as
potential catalysts for redox reactions. Further these systems
were studied for series of nitroarene reduction and results are
summarized in Table 6. These catalysts showed excellent
conversion of various nitroarenes to corresponding aniline as
only product.
These observations confirm that the Rh‐TPP complex is successfully
immobilized on the functionalized SBA‐15 materials. In the 29Si NMR
(Fig. 5) spectrum of pure SBA‐15 displays three broad resonances
with the chemical shifts between ‐110 to ‐97 ppm, which can be
assigned to the Q4, Q3, and Q2 species of the silica framework of
Qn=Si(OSi)n(OH)4‐n 21 The 29Si NMR spectra (Fig. S1) of both SBA‐15‐F‐
.
RhTPP and SBA‐15‐F‐RhCP show two peaks at around ‐67 and ‐110
ppm, which correspond to the T and Q sites of silicon. The stronger
peak at ‐110 ppm corresponds to Q4 sites, indicating the complete
condensation of organosilane with the surface silanol groups. The
peak at around ‐67ppm corresponds to T3 sites, which are due to
covalent bonding of the organosilane on the SBA‐15 surface.20,21 The
29Si NMR spectrum of SBA–15‐F is similar to those of SBA‐15‐F‐RhTPP
and SBA‐15‐F‐RhCP, because the chemical environment of silicon
was unchanged upon the metallo‐complex grafting.
Table. 4. Catalytic activity of rhodium macromolecular based
catalysts RhCP, SBA‐15‐RhCP, SBA‐15‐RhCP‐U, U* (Catalyst after
3 cycle), RhTPP, SBA‐15‐F‐RhTPP, SBA‐15‐RhTPP‐U
Selectivity (%)
Conversion
Catalysts
(mol %)
Aniline Azoxy‐benzene
Table 5. Catalytic recyclability of rhodium macromolecular
based catalysts.
RhCP
2
100
100
82.4
100
95.3
100
‐‐
‐‐
SBA‐15‐F‐RhCP
SBA‐15‐F‐RhCP‐U*
RhTPP
100
100
7.2
SBA‐15‐F‐Rh‐CP
SBA‐15‐F‐RhTPP
Selectivity
%
Cy
cle
17.6
‐‐
Selectivity
%
Conve
rsion
conve
rsion
Azoxyb
enzne
Azoxyb
enzene
Aniline
Aniline
SBA‐15‐F‐RhTPP
SBA‐15‐F‐RhTPP‐U*
100
93.78
4.7
‐‐
1
2
3
100
100
100
64.50
100
35.50
‐‐‐
100
95.30
100
4.65
‐‐‐
96.78
98.80
Reaction conditions: Catalyst = 0.1 g; Hydrazine : substate‐2:1;
temperature = 80 0C; time = 24h; solvent = iso‐propyl alcohol (15mL).
100
‐‐‐
100
‐‐‐
Reaction conditions: Catalyst = 0.1 g; Hydrazine : substate‐2:1;
temperature = 80 0C; time = 24h; solvent = iso‐propyl alcohol (15mL).
6 | J. Name., 2012, 00, 1‐3
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Table 6. Various nitroarene conversion using heterogenized withdrawing group present in the aromatic ring facilitate the
RhCP and RhTPP catalysts.
formation of azo and azoxy derivate as minor bi‐product using
SBA‐15‐F‐Rh‐TPP as catalyst. However, the use of SBA‐15‐F‐Rh‐
CP as catalyst yield 100 % aniline derivative in all cases. The
catalytic activity was further compared with some of the
catalyst studied in recent literature and results are summarised
in the Table S3 (ESI*).23 Overall conversion and selectivity of the
SBA‐15‐F‐RhCP and SBA‐15‐F‐RhTPP shows comparable to that
of literature results for the reduction of nitro arene and its
derivatives. Further, most of the literature the reaction was
studied under high pressure or high temperature. Thus, the
heterogenized rhodium based macro‐molecule catalyst used in
the present studies found to be potential for nitro‐arene
reduction at ambient conditions with good conversion and
selectivity.
Yield of aniline (%)
Sl.
Substrate
No.
Product
SBA‐15‐
F‐Rh‐TPP
SBA‐15‐
F‐Rh‐CP
O
NH2
N+
-O
1
100
100
100
100
H2N
NH2
2
3
100
100
100
100
Conclusion
This is the first report on the encapsulation of rhodium‐
containing calix[4]pyrrole and meso‐tetraphenylporphyrin on
functionalised SBA 15. The formation of RhCP and RhTPP was
evident from 1H NMR, Mass, FT‐IR, CHNS, UV‐Vis analysis. The
presence of Rh‐N bond is evident from the absence of N‐H
bending and presence of new band at 672cm‐1 which
corresponds to M‐N bond, supports the formation of RhCP and
RhTPP. The mass spectra shows parent peak at 545 and 715
respectively for RhCP and RhTPP conforming the successful
formation of rhodium containing macromolecule. The
incorporation of bulk RhCP and RhTPP were evident from the
low angle XRD as well as decrease in surface area and pore
volume which is observed in N2 sorption studies. The prepared
macromolecules are highly stable and active for the
hydrogenation of nitrobenzene. In particular the heterogenized
homogenous catalyst viz SBA‐15‐F‐RhCP and SBA‐15‐F‐RhTPP
shows complete conversion of nitrobenzene with exclusive
formation of aniline as the product and catalytic activity was
improved drastically upon grafting these macromolecules on
functionalized SBA‐15 support. The activity remained intact
after several catalytic runs.
4
5
6
7
8
100
95.8
94.7
100
100
100
100
100
9
98
100
Conflicts of interest
“There are no conflicts to declare”.
10
11
100
90
100
100
Acknowledgements
Authors thank to CSIRHRD Project No: 01(2921/18EMR‐II) for
the financial support. Anjali is grateful to DST‐Inspire (IF160353)
and CUK for the fellowship and Lab facilities. Zeswa Catalyst Pvt.
Ltd for their kind support on providing noble metal salts.
Reaction conditions: Catalyst‐100mg; Hydrazine: substate‐2:1;
temperature‐800C; time‐24h; solvent‐isopropyl alcohol(15ml).
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