Iridium Tetra(4-carboxyphenyl) Porphyrin, Calix[4]pyrrole and Tetraphenyl Porphyrin Complexes as Potential Hydrogenation Catalysts

Article abstract of DOI:10.1002/ejic.201900762

Here we report the preparation of first examples of iridium-based organometallic macromolecules, viz., iridium-tetra (4-carboxyphenyl)porphyrin (IrTCPP), iridium-calix[4]pyrrole (IrCP) iridium-tetraphenylporphyrin (IrTPP), which are effective catalysts for hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) and 1,4-pentanediol under base free conditions. The turnover number in the range of 1220–2850 is evident for the chosen reaction using iridium macromolecule based catalysts. The heterogenization of homogeneous IrTCPP, IrCP, and IrTPP result in stable reactivity of the catalysts for several runs.

Full text of DOI:10.1002/ejic.201900762

DOI: 10.1002/ejic.201900762
Full Paper
Biomass Building Blocks
Iridium Tetra(4-carboxyphenyl) Porphyrin, Calix[4]pyrrole and
Tetraphenyl Porphyrin Complexes as Potential Hydrogenation
Catalysts
Kaiprathu Anjali,[a] Manammuzhangiyil Sivadasan Aswini,[a] Peringayi Aswin,[a]
Venkatachalam Ganesh,[b] and Ayyamperumal Sakthivel*[a]
Abstract: Here we report the preparation of first examples of tions. The turnover number in the range of 1220–2850 is evi-
iridium-based organometallic macromolecules, viz., iridium- dent for the chosen reaction using iridium macromolecule
tetra (4-carboxyphenyl)porphyrin (IrTCPP), iridium-calix[4]pyrr- based catalysts. The heterogenization of homogeneous IrTCPP,
ole (IrCP) iridium-tetraphenylporphyrin (IrTPP), which are IrCP, and IrTPP result in stable reactivity of the catalysts for
effective catalysts for hydrogenation of levulinic acid (LA) to γ- several runs.
valerolactone (GVL) and 1,4-pentanediol under base free condi-
Introduction
for biological, photophysical and chemical activities.[20,21] In
other words, in recent years, great attention has been focused
on the conversion of biomass into valuable chemicals, in which
group VIII metal complexes play a vital role.[16,17,22–24] In partic-
ular, hydrogenation of levulinic acid to γ-valerolactone and 1,4-
pentanediol has a significant role as a green solvent, as well as
an important intermediate in several fine chemical synthe-
ses.[25,26] Herein, we report the preparation of novel iridium-
based macromolecules [iridium-tetra(4-carboxyphenyl)porphy-
rin (IrTCPP), iridium-calix[4]pyrrole (IrCP), iridium terta-phenyl
porphyrin (IrTPP) and subsequently, heterogenized on a func-
tionalized molecular sieves support. The homogeneous (IrTCPP,
IrTPP, IrCP) and heterogenized iridium complexes (SBA-AM-
IrTCPP/IrTPP/IrCP) are explored as catalysts for the conversion of
levulinic acid under phosphane, and base-free conditions with
moderate temperature and hydrogen pressure.
Iridium-based organometallic complexes have a similar chemis-
try as rhodium; they possesses unique properties and are ideal
catalysts for oxidative addition, hydrogenation, bond breaking,
and bond making reactions in various chemical processes.[1–5,6–9]
Several iridium based organometallic complexes (iridium-olefin,
arene, allyl, alkyl, and carbene complexes) have been exten-
sively studied for hydrogenation, C–H activation, and oxidative
addition.[10,11] Vaska's catalyst[12] and Crabtree's catalyst[13–15]
are the well-known examples of important iridium-based ho-
mogeneous catalysts used for oxidative addition and hydrogen-
ation reactions. Recently, Bayram et al. reported the complete
hydrogenation of benzene at room temperature using zero-va-
lent iridium nanoparticles.[8] Similarly, iridium hydride based
complexes possessing various coordinating ligands have been
shown to be potential catalysts for hydrogenation of ketones
and alcohols.[10] Also, iridium-containing pincer complex[16,17]
was shown as to be a promising catalyst for the conversion of
biomass derivatives. Cyclo-metallated iridium complexes have
also attracted great interest owing to their unique photophysi-
cal properties.[18,19] At this juncture, to the best of our knowl-
edge there are no reports on iridium-based macromolecules
(Scheme 1). The development of such iridium complexes may
broaden the scope as catalysts and may act as model molecules
[a] Inorganic Materials & Heterogeneous Catalysis Laboratory, Department of
Chemistry, School of Physical Sciences, Central University of Kerala,
Kasaragod 671316, Kerala, India
E-mail: sakthiveldu@gmail.com
Scheme 1. Representative examples of iridium based macromolecules.
sakthivelcuk@cukerala.ac.in
[b] Electrodics & Electrocatalysis Division, CSIR-Central Electrochemical
Research Institute (CSIR-CECRI),
Results and Discussion
Karaikudi 630003, Tamil Nadu, India
All of the macromolecules were prepared using the Schlenk
technique by treating nucleophile (pyrrole) and electrophile
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900762.
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(benzaldehyde, 4-carboxy benzaldehyde, and acetone).[27–33] iridium ion incorporation, the Soret band shows a slight red
The incorporation of iridium into the center of the macromole- shift (from 230 to 271 nm) which is characteristic of the forma-
cules was carried out by using iridium trichloride (IrCl3) as the tion of IrCP complex.
iridium source.[27–31] The structure of the ligands (TCPP, TPP, and
CP), as well as iridium complexes (IrTCPP, IrTPP, and IrCP), was
thoroughly investigated by various spectroscopic techniques,
including FT-IR, UV/Vis, NMR, CHNS and mass analysis. Thor-
oughly characterized homogeneous iridium complexes were
heterogenized on the surface of amino-silane modified SBA-15
materials and subsequently, both homogeneous and heteroge-
neous catalysts were explored for hydrogenation of levulinic
acid.
FT-IR spectra of ligands (TCPP, TPP and CP) and their corre-
sponding iridium complexes (IrTCCP, IrTPP and IrCP), as well as
surface grafted heterogenized complexes are displayed in Fig-
ure 1. All these samples show that the major bands in the re-
gion of 450–1000 cm–1 are due to the in-plane bending, out-
of-plane bending, and the ring rotation and torsion modes of
the CP, TPP, and TCPP skeleton.[27–31] The vibrational bands de-
tected in the region of 1000–1600 cm–1 are typical of symmetric
and asymmetric stretching of C=C, C–N, and C–C bonds-related
to skeletal pyrrole molecules. The bands appearing at 1102 and
1180 cm–1 correspond to C–H asymmetric deformation modes
in the pyrrole ring. CP, TPP, and TCPP showed obvious vibra-
tional bands in the range of 3316–3446 cm–1, which correspond
to N–H stretching modes derived from the pyrrole unit present
in the ligands, which are absent in IrCP, IrTPP, and IrTCPP. These
findings support the metalation of macromolecules (i.e., direct
bonding of iridium ions to the pyrrole nitrogen in the ligands).
The vibrational bands correspond to aromatic C=C (1496 cm–1),
C–N (1231 cm–1) stretching yielded a red shift after iridium ion
incorporation. In TCPP after iridium incorporation, a new peak
was observed at 609 cm–1 that corresponds to the Ir–N bond.
Furthermore, the vibration stretching bands characteristics of
the meso-benzoic group [e.g., C=O (1700 cm–1; carboxyl func-
tionality), C=C (1600 cm–1; aromatic), and C–O (1385 cm–1)] also
shows a red shift towards 1694, 1500, and 1358 cm–1 after the
iridium ion incorporation (IrTCPP). Similarly, in IrTPP, an extra
shoulder peak was observed at 667 cm–1 and in IrCP at
Figure 1. FT-IR spectra of ligands, iridium complexes and heterogenized sam-
ples.
672 cm–1, which corresponds to the Ir–N bond. The UV/Visible
spectrum (Figure 2) of TCPP shows peaks at 426 (Soret band),
1
514, 550, 593, and 649 nm (Q bands). The values are in good
agreement with literature reports.[31] Upon iridium ion incorpo-
ration, the pyrrole nitrogen of TCPP is coordinated to the irid-
ium ion to form an IrTCPP complex. As a result, the intensity of
Q bands is decreased, and the Soret band shows a slight blue
shift (from 426 to 421 nm) that is characteristic of the formation
of a metal-TCPP (Ir-TCPP) complex. This could be due to the
improved structural symmetry (D4h point group). UV/Visible
spectra studies once again support the formation of the iridium
macromolecular complex. Similarly, TPP shows a peak at
424 nm (Soret band), 517 nm, 550 nm, 591 nm, and 649 nm (Q
bands). Upon iridium ion incorporation, the pyrrole nitrogen of
TPP is coordinated to the iridium ion. Consequently, the intensi-
ties of the Q bands at 517 and 550 nm decreased, and those at
The H NMR spectra of the IrTCPP complex (See ESI Figure
S1) showed peaks (δ) at 8.86 (S, Hpyrrolic), 8.40 (d, Ho-phenyl ring),
8.36(d, Hm-phenyl ring) respectively. These results confirm the for-
mation of IrTCPP complex. The IrTCPP complexes display the
absence of a peak in the highly shielded region at –2.74 ppm,
which is characteristic of N–H protons of TCPP[25] confirmed
that iridium ion is bonded to the nitrogen center of TCPP. Simi-
larly, after the iridium ion incorporation into the calix[4]-pyrrole
cavities (See ESI* Figure S2), the absence of a broad N–H proton
peak at 7.1 ppm confirms the coordination of iridium ions
through the pyrrole nitrogen of the CP ligand. The other peaks
characteristic of macromolecules (TCPP, TPP and CP) are identi-
cal for both ligands and iridium macromolecules (See ESI* Fig-
ure S1, Figure S2, Figure S3). IrCP shows very weak signals ap-
591 and 649 nm disappeared. On the other hand, calix[4]pyrrole pear at 7.0 and 8.3 ppm which are due to the presence of trace
shows peaks at 230 nm with a weak shoulder at 416 nm, upon amount of poly-pyrrolic impurities. Similarly the peak obtained
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Figure 2. UV/Visible spectra of TCPP, IrTCPP, TPP, IrTPP, CP, IrCP.
Table 1. CHNS analysis result of TCPP, IrTCPP, CP, IrCP, TPP, IrTPP.
Material
% of carbon
Theoretical
% of nitrogen
Theoretical
% of hydrogen
Theoretical
Experimental
value
Experimental
value
Experimental
value
value
value
value
TCPP
Ir TCPP
CP
IrCP
TPP
72.90
57.39
78.46
51.56
85.97
62.88
71.25
58.38
78.02
50.16
85.40
61.09
7.09
5.51
8.74
8.59
9.11
6.67
6.21
5.79
8.39
8.90
9.80
6.59
3.82
2.78
13.07
4.95
4.92
3.36
4.60
3.23
13.32
4.44
4.10
3.17
IrTPP
at 2.089 ppm on IrTCPP is derived from acetone proton (which observed at 2θ around 0.80 corresponds to (100) diffraction
is used for washing).
planes, accompanying the small peaks assigned to (110) and
(200) diffraction planes characteristic of hexagonal SBA-15
structure. For the macromolecule encapsulated SBA-15 material,
the intensity of the diffraction peak decreased drastically, which
is due to the presence of bulk IrTCPP/IrTPP/IrCP inside the chan-
nel of SBA-15. However, the diffraction peaks correspond to
(100) plane remain intact with broadening supports that the
material did not collapse after the grafting of bulk metal com-
plex on the amino-functionalized SBA-15 (Figure 3).
Similarly, the CHNS analysis of macromolecular ligands (CP,
TPP, and TCPP) and the corresponding metallo-macromolecules
are summarized in Table 1. The percentage of C, H, and N in
both the ligands and iridium complexes, show that the calcu-
lated values are well-match with the theoretical values (Table 1).
The formation of iridium complexes were further confirmed by
mass spectral analysis, where parent ion peak appeared at
1034.37 [C44H28IrN4Cl(H2O), Calculated m/z = 1033.43], 669.49
[C28H32N4IrCl(H2O), Calculated m/z
= 670.20] and 840.05
[C44H28N4IrCl calculated m/z= 840] respectively for IrTCPP, IrCP
and IrTPP.
Thoroughly characterized, homogeneous catalysts (IrTCPP,
IrCP, IrTPP) were grafted on the aminosilane functionalized SBA-
15 (SBA-AM) materials. The presence of organo amine and met-
allo-macromolecules are evident from FT-IR spectra (Figure 1),
which show that, vibrational bands appear around at 2920,
1557, and 1492 cm–1, which are characteristics of C–H stretch-
ing, N–H bending, and NH2 wagging modes of vibration of the
organic and amine functionality present on the SBA-15 surface.
A broad vibrational band appeared at 1660 cm–1 in SBA-AM-
IrTCPP, which confirms the formation of amide linkage between
the meso-carboxybenzyl group of the porphyrin group and NH2
of the functionalized SBA-15. The presence of all vibrational
bands characteristic of IrTCPP, IrCP, and IrTPP (Figure 1) support
the notion that the iridium molecular structure remains intact
after the grafting on the functionalized SBA-15 surface. The low
Figure 3. Low angle powder XRD patterns of SBA-15, SBA-AM-IrTCPP, SBA-
AM-IrCP and SBA-AM-IrTPP.
Adsorption isotherm of SBA-15 materials exhibit (Figure 4) a
angle powder X-ray diffraction patterns of SBA-15 and SBA-AM- sharp uptake in the relative pressure (p/p0) range between 0.6
IrTCPP, SBA-AM-IrTPP, and SBA-AM-IrTPP were shown in Fig- and 0.8. due to multilayer capillary adsorption with capillary
ure 3. It can be seen from the figure that the intense peak condensation of nitrogen and is characteristic of type IV iso-
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therm. A well-defined hysteresis loop of type H1 was evident TGA profile of all the heterogeneous catalyst did not show any
according to the IUPAC classification, which is associated with appreciable weight loss in the temperature range of 25 –
the capillary condensation in open-ended cylindrical meso- 200 °C, which suggest that the heterogenized samples are sta-
pores.[34]
ble up to 200 °C. The observed weight loss in the temperature
range of 200–600 °C corresponds to the decomposition of
macromolecules (porphyrin and calix[4]pyrrole) and the linker
organo-amine present in the heterogenized catalysts. Based on
TGA studies, the catalytic studies on heterogeneous catalysts,
the reaction temperature set below its decomposition tempera-
ture.
To understand the nature of the active species, present on
the surface, XPS has been carried out, and the Ir 4f core level
spectra of SBA-AM-IrTCPP, SBA-AM-IrCP and SBA-AM-IrTPP sam-
ples are shown in Figure 6. The XPS spectrum of both the
IrTCPP and IrCP grafted samples show two prominent peaks
due to spin-orbital splitting of Ir 4f7/2 and Ir 4f5/2 with binding
energies at 61.33 eV and 64.30 eV indicating that iridium is
exclusively present with Ir (III) oxidation state on the surface
and might act as the centre of hydrogenation reaction.[23]
Similarly SBA-AM-IrTPP shows peaks at 60.09 eV and 64.24 eV
characteristic of iridium in trivalent oxidation state. The signal
observed at 399 eV in the N 1S XPS spectra is characteristic of
N-ligand present in the complexes coordinated with Ir3+, con-
firming the existence of Ir (III) centred TCPP, CP, TPP units in
SBA-AM-IrTCPP, SBA-AM-IrCP, SBA-AM-IrTPP respectively[23] (Fig-
ure 7). Further the XPS spectra of C 1s, Cl 2p also supports the
existence of IrTCPP, IrCP, IrTPP in the heterogeneous grafted
samples (See ESI* Figure S4, S5).
Figure 4. N2 sorption isotherm of (A) SBA-15 (B) SBA-AM-IrTPP, (C) SBA-AM-
IrCP and (D) SBA-AM-IrTCPP.
Furthermore, compared to the parent sample (SBA-AM), SBA-
AM-Ir TCPP, SBA-AM-IrTPP, and SBA-AM-IrCP samples showed a
drastic decrease in surface area and pore volume. The decrease
in pore volume and pore size confirms that the iridium-based
macromolecules are well encapsulated on the surface of
SBA-15-AM (Table 2).
Table 2. Textural properties of SBA-15, SBA-AM, SBA-AM-IrTCPP, SBA-AMIrCP,
and SBA-AM-IrTPP.
BET surface
area (m2 g–1
BJH Pore
volume
BJH Pore
size
[nm]
Sample code
)
(cm3 g–1
)
SBA-15
SBA-15 AM
SBA-15-AM-IrTCPP
SBA-AM-IrCP
SBA-AM-IrTPP
757.6
243
66.7
69.2
164.0
0.57
0.44
0.13
0.15
0.30
7.2
6.7
5.7
5.0
5.1
The iridium content present in the heterogeneous catalysts
were calculated using the ICP-AES technique, and it shows
about 1.62, 4.67 and 0.85 wt.-% of Ir in SBA-AM-IrCP, SBA-AM-
IrTCPP, SBA-AM-IrTPP, respectively (Table S1 ESI*).
Further to understand the thermal stability of the heterogen-
ized catalysts (SBA-AM-IrCP/TCPP/TPP) thermo-gravimetric anal-
ysis was carried out and the results are displayed in Figure 5.
Figure 6. XPS of Ir 4f in SBA-AM-IrTCPP, SBA-AM-IrCP, SBA-AM-IrTPP.
Figure 5. TGA profile of fresh and used iridium complexes grafted heterogeneous catalysts.
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gated for catalytic hydrogenation of levulinic acid to γ-valero-
lactone and 1, 4 pentane-diol (Scheme 2).
Scheme 2. Hydrogenation of levulinic acid on iridium based macromolecular
catalysts.
The reaction was carried out in an autoclave reactor with
the required amount of catalyst (0.01 g Homogeneous catalyst,
0.100 g Heterogeneous catalyst), substrate (5 mmol), and sol-
vent (5 mL) under 10 bar hydrogen pressure for 12 h. The prod-
uct was confirmed by GC and FT-IR spectroscopic methods. All
homogeneous catalysts were studied for several cycles by the
introduction of 5 mmol of levulinic acid after each run (Table 3).
γ-valerolactone and 1, 4 pentanediol were obtained as the ma-
jor products. The formation of γ-valerolactone was further con-
firmed by FT-IR spectral studies (Figure S6 ESI*), which showed
a shoulder peak at 1721 cm–1, which is characteristic of
C-O present in the lactone group. The homogeneous catalysts
(IrTCPP, IrTPP, and IrCP) showed conversion levels of 90–100 %
even after several cycles.
Figure 7. N 1s XPS spectra of SBA-AM-IrCP, SBA-AM-IrTCPP and SBA-AM-IrTPP.
HR-TEM micrograph (Figure 8) of a representative heterogen-
ized catalyst viz., SBA-AM-Ir-CP is shown in Figure 8. The TEM
images of SBA-AM-Ir-CP is providing evidence that the hexago-
nal honeycomb type mesoporous structure of SBA-15 retains
long range ordering even after grafting of bulk macrocyclic irid-
ium complex.
In case of IrTCPP and IrTPP, during the initial run, γ-valero-
lactone was obtained, and in subsequent runs, the formation
of diol was predominant, which is due to the presence of water
molecule obtained as the by-product. The presence of the carb-
oxylate group on IrTCPP makes the system relatively hydro-
philic, and thus the by-product formed might facilitate more
diols in the fourth and fifth cycles. In other words, the case of
IrTPP and IrCP complexes, although the water was formed as a
by-product, the more hydrophobic environment might facilitate
better selectivity of γ-valerolactone. To improve the catalytic
stability, the homogeneous IrTCPP, IrTPP, and IrCP catalysts were
heterogenized on the surface of amino-functionalized SBA-15,
Figure 8. High resolution transmission electron microscopy (HRTEM) micro-
graphs of SBA-AM-Ir-CP.
The developed iridium-based homogeneous macromolec- and the heterogeneous SBA-AM-IrTCPP, SBA-AM-IrCP, and SBA-
ular catalysts (IrCP, IrTCPP, IrTPP) and their heterogenized SBA- AM-IrTPP catalysts were studied for the chosen reaction. Table 4
AM-IrTCPP, SBA-AM-IrTPP, SBA-AM-IrCP samples were investi- summarizes the catalytic activity of heterogenized catalysts, for
Table 3. Hydrogenation of leuvlinic acid over various iridium macro-molecules.[a]
Catalyst
No. of cycles
Conv. [%]
Selectivity
TON
γ-valero-lactone
1,4-pentane-diol
IrTCPP
1
2
3
4
5
88.5
100
98.8
98.1
98.5
91.5
30
28.5
23.7
19
8.5
70
71.5
76.3
81
434
924
1408
1889
2850
IrTPP
IrCP
1
2
3
4
100
100
0
403
787
1187
1574
95.1
99.3
95.9
77.4
75.2
65.9
22.6
28.8
34.1
1
2
3
4
100
72.5
76.6
74.6
92.1
27.5
23.4
25.4
7.9
312
621
926
1227
98.8
97.6
96.4
[a] Reaction conditions: Levulinic acid 5 mmol, IrTCPP= 0.0102 mmol (10 mg), IrTPP= 0.0124 mmol (10 mg), IrCP=0.016 mmol (10 mg), temperature 100 °C,
DMF 5 mL, H2 pressure 10 bar.
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Table 4. Hydrogenation of levulinic acid using heterogenous catalyst and their Recyclability.[a]
Catalyst
No. of cycles
Conv. [%]
Selectivity
γ-valerolactone
1,4-pentanediol
SBA-AM-IrTCPP
1
2
3
4
100
99
98
6
3
3
29
94
97
97
71
62
SBA-AM-IrTPP
SBA-AM-IrCP
1
2
3
4
100
89
82.8
79.2
7.9
92.1
70.4
81.3
84.9
29.5
18.7
15.4
1
2
3
4
100
100
100
100
3.8
0.9
3.9
1.8
96.2
99.1
96.1
98.2
[a] Reaction conditions: Levulinic acid 5 mmol, Catalyst=100 mg, temperature 120 °C, DMF 5 mL, H2 pressure 10 bar.
which we found complete conversion of levulinic acid with ex- Conclusions
clusive formation of 1,4-pentane diol in all cases. For the hetero-
In summary, this is the first report on preparation of iridium
containing calix-[4]-pyrrole and porphyrin based complexes.
The formation of IrTCCP, IrTPP and IrCP evident from proton
NMR, Mass spectra and CHNS studies. The presence of iridium
in the +3 oxidation state was confirmed from XPS studies. The
studies revealed that both the homogeneous organometallic
complexes as well as heterogeneous catalysts show promising
activity for the levulinic acid hydrogenation with the formation
of γ-valerolactone and 1,4-pentane-diol as the major product.
Both homogeneous and heterogeneous catalysts retain their
activity even after several runs.
geneous catalysts SBA-AM-IrTCPP and SBA-AM-IrTPP, the con-
version was decreased considerably after the third cycle. This
loss in activity might be due to the chemisorption of reactant
molecules on the surface of mesoporous materials, which block
the active sites. The SBA-AM-IrCP catalyst showed the complete
conversion of levulinic acid even after several cycles and retain
its activity.
In order to understand the decrease in catalytic activity after
third cycle, thermo-gravimetric analysis of the used heterogene-
ous catalysts was studied and the results are displayed in Fig-
ure 5. The used SBA-AM-Ir-TCPP and SBA-AM-Ir-TPP showed an
increase in weight loss compared to fresh catalysts in the tem-
°
perature range of 200–600 C, which supports the chemisorp-
Experimental Section
tion of reactant molecules on the surface of mesoporous cata-
lysts (Figure 5). The chemisorption of organic molecules on the
surface of mesoporous molecule may leads to block the accessi-
bility of reactant molecule on active sites of the heterogeneous
catalysts and hence results decrease in catalytic activity after
third cycles. Further, the nature of the used heterogeneous cat-
alysts were followed by HRTEM, FT-IR and ICP AES analysis. The
HR TEM of a representative used catalyst viz., SBA-AM-Ir-CP cat-
alysts is shown in Figure S7, which showed the uniform meso-
porous with long range ordering retained even after the cata-
lytic reaction. Further the ICP AES analysis shows that the irid-
ium content in the used catalysts showed iridium content of
3.41, 1.43 and 0.6 respectively for U-SBA-AM-IrTCPP, U-SBA-AM-
IrCP, and U-SBA-AM-IrTPP respectively (Table S1 ESI*). The de-
crease in iridium content in SBA-AM-IrTCPP, and the presence
of relatively less iridium content on SBA-AM-IrTPP results de-
crease in activity after 4th cycle. The intactness of iridium con-
tent on the surface of SBA-AM-IrCP facilitates the retention of
activity even after several cycles. Further, the filtrate obtained
from reaction mixture after 1 h of reaction using SBA-AM-IrCP
did not show any appreciable conversion of levulinic acid fur-
ther confirm that there is no leaching of iridium complex on
SAB-AM-IrCP. In addition, the FT-IR spectra of the used catalysts
(Figure S8 ESI*) also supports that even after the catalytic reac-
tion, the nature of heterogeneous catalysts are retaining its
structure without any significant changes.
Preparation of the Catalyst: All experiments were conducted using
the Schlenk technique. Before the experiments, all the solvents and
starting materials were dried by standard procedure.[27]
Synthesis of Ir-Meso-Tetra-(4-carboxyphenyl)phenylporphyrin Com-
plex (IrTCPP): Meso-tetra-(carboxyphenyl) porphyrin (TCPP) was pre-
pared by dissolving freshly dried pyrrole (2.3 mmol) and 4-formyl
benzoic acid (2.33 mmol) in 100 mL propionic acid and the mixture
was refluxed for 4 hours.[27–32] The initial mixture was yellow in
colour, which turn to green and subsequently it became brownish
black in colour. The product solution was cooled to room tempera-
ture and introduced methanol (50 mL) in an ice bath with stirring.
The deep purple crystals were precipitated, which was filtered and
washed using methanol. The resulting solid was re-crystallised and
purified (yield 18 %). The iridium incorporated meso-tetra-(4-car-
boxyphenyl) porphyrin (IrTCPP) was prepared by dissolving the
1 mmol Iridium chloride (IrCl3·6H2O) in acetone followed by the
addition of 1 mmol TCPP under inert atmosphere and refluxed for
24 hours. The precipitate was collected and thoroughly washed
with acetone. Deep green coloured shiny crystals are formed. Yield:
82 %. HR-MS m/z= 1034.37 [C44H28IrN4Cl(H2O), Calculated m/z=
1033.43], 1H NMR (400 MHz, [D6]DMSO): 8.86 (S, Hpyrrolic), 8.40 (d,
HO-phenyl ring), 8.36 (d, Hm-phenyl ring) respectively.
Preparation of Iridium calix[4]pyrrole (IrCP): The synthesis of metal
incorporated calix[4]pyrrole was carried out in an inert nitrogen
atmosphere. Meso-octamethylcalix[4]pyrrole (CP) was prepared as
per the described procedure earlier.[29–32] About 3 mL of pyrrole
and 15 mL of acetone was added to round-bottomed flask and
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allowed to stir for few minutes in an ice bath. Then 2.5 mL 37 % HCl
was added to the above mixture and the solution became colorless,
subsequently change into milky yellow. After completion of the re-
action the brownish precipitate was formed which was separated
and dried. The precipitate was further washed using acetone results
white fine powder. The Iridium (Ir) incorporated meso-octamethyl-
calix[4]pyrrole (IrCP) was prepared by the following method. About
0.5 g (1 mmol) of CP was dissolved in 15 mL acetone and kept for
stirring. Then 1 mmol of iridium salt (0.298 g, IrCl3. XH2O) was dis-
solved in 5 mL acetone and added into the RB flask containing
calix[4]pyrrole. The reaction mixture was allowed to reflux for 12
hours. After completion of the reaction the solvent was removed,
the product was washed with acetone, dried and collected which
is brown in colour (yield: 86 %). HR-MS m/z= 669.49
[C28H32N4IrCl(H2O), Calculated m/z = 670.20], 1H NMR (400 MHz,
DMSO): δ 5.65(d, Hpyrrolic), 1.51(s, Hmeso CH3).
uct was vacuum-dried. Grafted on materials were represented as
SBA-AMIrTCPP, SBA-AM-IrCP, SBA-AM-IrTPP.
Catalytic Studies: In a typical reaction, 5 mmol Levulinic acid
(0.5 mL) and 5 mL DMF (Solvent) was introduced into a 50 mL
autoclave; 0.01 g of homogeneous catalysts [IrTCPP (0.0102 mmol),
IrCP (0.0160 mmol), IrTPP (0.0124 mmol)] was added in to the auto-
clave. Subsequently, the autoclave was flushed with H2 several
times. The reaction was charged with the required H2 pressure of
10 bar. The reaction was performed at 100 °C, for 12 h. In the case
of heterogeneous catalyst, the catalyst was activated for 2 hours at
80 °C and vacuum dried. About 0.1 g of the catalyst (SBA-AM-IrTCPP,
SBA-AM-IrCP, and SBA-AM-IrTPP) was added to the autoclave con-
taining 5 mmol Levulinic acid and 5 mL DMF and flushed with H2.
The reaction was charged with the required H2 pressure of 10 bar.
The reaction was performed at 120 °C, for 12 h. After completion
of reaction, the reactor was cooled to room temperature, and the
excess H2 was vented out. The results were analysed quantitatively
by gas chromatography using an FID detector equipped with an
HP 88 column (Mayora Analytical Model 2100). The products were
further confirmed by GC, GC- MS and FT-IR data.
Synthesis of Ir-Tetraphenylporphyrin Complex (IrTPP): Meso-tetra-
phenylporphyrin (TPP) was prepared by refluxing freshly dried pyrr-
ole (0.027 mol) and benzaldehyde (0.028 mol) in 100 mL of pro-
pionic acid, for 2 h.[27–33] After the stipulated period, the reaction
°
Characterization: FT-IR spectra of all the materials were recorded on
Perkin-Elmer spectrum-2 FTIR in the range of 400–4000 cm–1 using
KBr method and were collected with 4 cm–1 resolutions and 120
scans. UV/Vis spectra were recorded on Perkin Elmer Lambda 35
spectrometer. Elemental analyses were performed on an Elementar-
Vario Micro Cube by grinding the sample in to fine powder and
mixture was cooled to 25 C, filtered, and washed thoroughly with
methanol. The resulting purple crystals were air-dried and washed
with dichloromethane (yield 12 %).[25,29] The Iiridium-incorporated
meso-tetraphenylporphyrin (IrTPP) complex was prepared by dis-
solving metal salt (IrCl3·XH2O; 1 mmol) in 5 mL of dry acetone,
followed by introducing the solution containing 1 mmol of TPP in
45 mL of dry acetone. The final mixture was refluxed under N2
atmosphere for 24 h. After completion of the reaction, the solvent
was removed, and washed several time with pentane to remove
impurities, after which the deep purple coloured product was iso-
lated (yield: 73 %). HR-MS m/z=840.05 [C44H28N4IrCl calculated
m/z = 840]. 1H NMR (400 MHz, CDCl3): δ 8.91(s, Hpyrrolic), 8.28 (m,
Ho), 7.8(m, Hm,p).
1
packed in aluminum foil. The H NMR spectra of the complex were
recorded using CDCl3/DMSO as solvent and TMS as the standard
on a Bruker AVANCEIII 400 instrument. ESI-TOF mass spectra were
recorded on a BrückermaXis mass spectrometer using methanol as
the solvent. Powder X-ray diffraction patterns of all the heterogen-
ized materials were collected on a Bruker-D8 high resolution X-ray
diffractometer with Cu-Kα radiation (λ=1.5418), between 2θ range
of 0.5–10 ° with a scan speed and step size of 0.5 °/min and 0.02 °
respectively. The textural properties (BET surface area, BJH average
pore volume) of the heterogenized samples were followed by N2
sorption measurements at –196 °C using an automatic micropore
physisorption analyser (Micrometrics ASAP 2020, USA). The samples
were degassed at 110 °C for 10 h. under 10–3 Torr pressure prior
to each run. Thermo gravimetric analyses (TGA) performed using a
TGA instrument (Perkin Elmer STA 6000), under a nitrogen atmos-
phere with a heating rate of 10 °C/min, ranging from 40 to 850 °C.
X-ray photoelectron spectra (XPS) of the catalysts were recorded by
using a custom-built ambient pressure photoelectron spectrometer
(Prevac, Poland) that was equipped with a VG Scienta′s R3000HP
analyser and a MX650 monochromator.[27] A monochromatic Al-Ka
X-ray was generated at 450 W and used for measuring the XPS data
of the above samples. Base pressure in the analysis chamber was
maintained in the range of 2 × 10–10 Torr. The energy resolution of
the spectrometer was set at 0.7 eV at a pass energy of 50 eV. The
binding energy (BE) was calibrated with respect to the Au 4f7/2 core
level at 84.0 eV. The error in the reported BE values is within 0.1 eV.
Elemental composition present in the final materials was deter-
mined using ICP-AES. The morphology of the materials were ana-
lysed using a High Resolution Transmission Electron Microscope
(HR-TEM) Model of FEI –TECNAI G2–20 TWIN with LaB6 filament
operated at 200 kV.
Synthesis of SBA-15: In a typical procedure, about 4.0 g of the
EOnPO70EOn copolymer was dissolved in 180 g of 1.6 M HCl solu-
tion. To this solution 8.50 g of TEOS (tetraethyl orthosilicate) was
added and the resulting mixture was allowed to stir continuously
at 35–40 °C for 24 hours and then additionally for 24 hours at 100 °C
under static conditions. The products were filtered, dried, and cal-
cined at 550 °C. The calcined SBA 15 was used as a solid support
material.
Synthesis of Functionalised SBA 15 (SBA-AM): The calcined SBA-15
support (6.5 g) was pre-activated at 200 °C in an air oven, followed
by drying in a vacuum, in a round-bottomed flask to remove the
physisorbed molecules. Subsequently, a pre-determined amount
(3 mmol/g of solid support) of (3-aminopropyl) trimethoxy silane
was introduced to the activated SBA-15 support material.[34] The
mixture was refluxed in dry toluene (30 mL/g of support) for 24 h
under N2 atmosphere. This procedure was repeated thrice to com-
pletely convert the surface silanol groups into organofunctional sil-
anes. The resultant mixture was cooled to room temperature,
washed with dichloromethane, and then dried in vacuo to get the
final product, which was denoted as SBA-AM.
Synthesis of SBA-AM-IrTCPP/IrCP/IrTPP: The grafting of the iridium
complexes on the functionalised material (SBA-AM) was achieved
by introducing 0.5 mmol iridium complex (IrTCPP/IrCP/IrTPP) dis-
solved in 3 mL DMF, in to 1 g of support material suspended in
50 mL dry methanol, followed by refluxing for 24 h under N2 atmos-
phere. Subsequently, the metalloporphyrin (IrTCPP/IrCP/IrCP)-
grafted materials were collected by separating the solvent and
Acknowledgments
Authors thank CSIR-India (01(2921/18EMR-II) for the financial
support. Anjali is grateful to DST-Inspire (IF160353) for the fel-
washing with methanol and dichloromethane; afterward, the prod- lowship.
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Keywords: Calix[4]pyrrole · Hydrogenation · Iridium ·
Levulinic acid. Macrocylic ligands · meso-Tetra-(4-carboxyphenyl)-
porphyrin · Tetraphenyl porphyrin
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Received: July 11, 2019
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