Nanoporous iron(III) porphyrin frameworks: an efficient catalyst for [4+2] cycloaddition reactions of unactivated aldehydes with a diene

Article abstract of DOI:10.1039/c5nj03592a

The one pot conversion of several unactivated aldehydes with a diene to their corresponding [4+2] cycloaddition products has been studied using newly synthesized nanoporous iron(iii) porphyrin frameworks as catalysts. The heterogeneous catalysts have been synthesized from 5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato iron(iii) chloride (PFe·X) with the help of Pinnar type synthesis. The catalytic activity of the monomeric catalyst (PFe·X) vs. that of the new nanoporous catalysts (1·X) where X = Cl^-, BF₄^-etc. in the reaction of dienes and aldehydes were then compared. The structure of the catalysts was investigated using different characterization techniques including Fourier transform infrared spectroscopy (FT-IR), CHN, atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM) and the BET surface area which was measured via a physisorption study of N₂ at 77 K. The reaction parameters determining yield, including the quantity of the catalyst and the solvent, were optimized to afford the hetero Diels-Alder product efficiently. It was observed that the nanoporous catalysts (1·X) were much superior to the monomeric (PFe·X) catalysts at giving selective products in high yields. Secondly, the new catalysts remained active for at least 8 cycles without any observable decay in their catalytic ability. Attempts are made to rationalize the results by considering the nature of X and the BET surface area of the 1·X catalysts.

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Nanoporous iron(III) porphyrin frameworks: An
efficient catalyst for [4+2] cycloaddition reactions of
unactivated aldehydes with a diene
Cite this: DOI: 10.1039/x0xx00000x
Manoj Kumar Singh and Debkumar Bandyopadhyay*
Received 00th January 2012,
Accepted 00th January 2012
The one pot conversion of several unactivated aldehydes with a diene to their corresponding
[4+2] cycloaddition products has been studied using newly synthesized nanoporous iron(III)
porphyrin frameworks as catalysts. The heterogeneous catalysts have been synthesized from
5,10,15,20-tetrakis(4-cyanophenyl) porphyrinato iron(III) chloride (PFe.X) with the help of
DOI: 10.1039/x0xx00000x
www.rsc.org/
pinnar type synthesis. The catalytic activity of the monomeric catalyst (PFe.X) vs that of
-
nanoporous new catalysts (
1
.X) where X = Cl-, BF4 etc. in the reactions of dienes and
aldehydes were then compared. The structure of the catalysts were investigated using different
characterization techniques including Fourier transform infrared spectroscopy (FT-IR), CHN,
atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM) and BET surface
area was measured by physisorption study of N2 at 77K. The reaction parameters determining
yield, including quantity of the catalyst and solvent, were optimized to afford efficiently
hetero Diels-Alder product. It has been observed that nanoporous catalysts (1.X) are much
superior to the monomeric (PFe.X) catalysts in giving selective products in high yields.
Secondly the new catalysts remained active for at least 8 cycles without any observable decay
in their catalytic ability. Attempts are made to rationalize the results with the nature of X and
BET surface area of
1
.X catalysts.
have used Suzuki coupling reaction and the heterogeneous
1. Introduction
catalyst is found very robust with respect to TPPFeCl.9
The synthesis of such robust catalysts (MOFs) and their catalytic
activity for [4+2] cycloaddition reactions for few activated
aldehydes with a diene are reported by Zhou et al. Reports of
only few more, to the best of our knowledge, for such [4+2]
cycloaddition reactions using monomeric iron(III) porphyrins
are also known where catalyst recyclability are not reported.10 In
the present work we have taken 5,10,15,20-tetrakis (4-
cyanophenyl) porphyrinato iron(III) chloride and have
covalently attached one to the other by Pinnar type reaction to
get tetrazine linked covalent metalloporphyrin frameworks
(CMFs) as heterogeneous catalyst (Scheme 2). In this material
the iron(III) bound anion is also varied to see their role on the
[4+2] cycloaddition reaction of unactivated aldehydes with a
simple diene. The physical properties of the materials are also
reported.
The metalloporphyrins are well-known model compounds for
cytochrome P450.1 The mechanistic pathways are well addressed
and it has been convincingly demonstrated that oxo-iron(IV)
porphyrin cation radical, oxo-iron(IV) porphyrin, catalyst-
oxidant adduct, and ROO• are the primary reactive intermediates
in these reactions.2 In these studies it has also been noted that
alkanes and alkenes are indeed converted to the selective
oxidized products in high yields but problem remained in the
survival of the catalysts in homogeneous reaction.3 It has been
clearly demonstrated that
5,10,15,20-tetrakis phenyl
porphyrinato iron(III) chloride, TPPFeCl gets degraded by the
formation of µ-oxo dimer.4 In later studies with electronegatively
substituted iron(III) porphyrins, the efficiency of catalysis were
indeed improved, but the catalyst degradation remained
unsolved.5 This has evolved the modification of TPPFe(III)Cl
catalyst
to
functionalized
metalloporphyrins.
These
functionalized materials were then either anchored over solid
surfaces or covalently attached to each other through appropriate
spacers to make heterogeneous catalysts, most of which are
amorphous in nature.6 Few such well known frameworks are
given in scheme 1.7-8 In our endeavour we have used 5,10,15,20-
tetrakis (4-bromophenyl) porphyrinato iron(III) chloride and
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condenser, and it was degased with argon gas for half an hour.
Then 0.5 ml of hydrazine monohydrate (3.57 mmol) was added
to the flask. The color change was observed from dark brown to
red and then it was refluxed for another half an hour. Then,
sulphur powder (4 mg) was added to the reaction mixture and
evolution of ammonia gas was observed. This reflux was
continued till the rate of ammonia evolution was slowed down.
After 48 h very little amount of solid iodine (~1mg) was added
into the reaction mixture and it was refluxed for another 24 h.
The mixture was poured into a beaker containing 300 ml of
saturated Na2S2O3 solution and the insoluble materials were
precipitated. It was then filtered by a glass crucible and was
washed with water 3 times then with methanol, dichloromethane,
chloroform, and acetone. The unreacted monomer was removed
by using
a
Soxhlet extractor using THF, methanol,
dichloromethane and acetone. The compound was finally dried
using vacuum. The yield was 85 mg (~80%).
2.3 Other catalyst preparation with variable anions
The 5,10,15,20-tetrakis(4-cyanophenyl) porphyrinato iron(III)
chloride (CN)4TPPFeCl (monomer) (100 mg, 0.122 mmol) and
AgBF4/ AgSbF6/AgClO4/AgPF6 (0.5mmol) were dissolved in
dry CH2Cl2 (10mL) and were stirred for 2 h under argon
atmosphere. The AgX formed were filtered and the filtrate was
dried.10 This monomeric iron(III) porphyrin was used without
further purification for the synthesis of compound
1 analogues
with variable X as described above. The starting material
(CN)4TPPFeCl catalyst was synthesized according to literature
procedures and details are given in supporting information.11
2.4 Catalyst characterization
1
The H NMR spectra were recorded on a Bruker spectrospin
DPX-300 NMR spectrometer at 300.13 and 75.47 MHz
Scheme 1: Examples of some metalloporphyrin based covalent respectively, using CDCl3 as a solvent at room temperature.
frameworks.7,8
Mass spectra (HRMS) were recorded in the electro-spray
ionization (ESI) mode (10eV, 180ºC source temperature and
sodium formate as calibrant) on a Bruker micrOTOF-QII, taking
the samples in acetonitrile/methanol. UV-Vis absorption spectra
were measured in CH2Cl2 in a quartz cell of 1-cm path length on
an Agilent Technologies 8453 model spectrometer and the
electronic solid state absorption spectra were recorded on a
PerkinElmer UV-Vis spectrometer Lambda Bio 20 model
spectrometer. Infrared spectra were recorded on an Agilent
Technologies Cary 660 model Fourier transform infrared
spectrophotometer. Elemental analysis of C, H, and N were
carried out on a Perkin Elmer instrument series II CHNS/O
Analyser 2400 model. Amount of Iron present in the various
polymer were measured by ICP-AAS method on a Perkin Elmer
instruments Analyst 100 model plasma spectrometer. Scanning
electron microscopy and energy dispersive X-ray spectroscopy
were performed on a ZEISS EVO Series model EVO50
microscope operating at an accelerating voltage of 5.0 kV.
Nitrogen sorption isotherms were measured with Quantachrome
NovaWin-Data Acquisition analyzer and reduction for NOVA
instruments ©1994-2007, Quantachrome Instruments version
10.01 at 77K. Before measurement, the samples were degassed
2. Experimental
2.1 Materials
The 1,2 dichlorobenzene was distilled under Ar before use. Other
organic solvents for reactions were distilled over appropriate
drying reagents under argon or obtained as dehydrated reagents
from Merck (India) Chemicals. Deuterated solvents for NMR,
pyrrole, p-cyanobenzaldehyde, Sulphur powder and hydrazine
monohydrate were obtained from Spectrochemicals. Ferrous
chloride tetrahydrate was obtained from Loba Chemie. Ag salts,
benzaldehyde and diene obtained from Sigma-Aldrich Chemical
Pvt. Ltd (U.S.A.). Ethyle acetate, hexane, ethanol and methanol
were obtained from Qualigen’s.
2.2
SYNTHESIS OF NANOPOROUS IRON(III) PORPHYRIN
COVALENT FRAMEWORKS (1)
In 25 ml of 1,2 dichlorobenzene 100 mg of 5,10,15,20-tetrakis
(4-cyanophenyl) porphyrinato iron(III) chloride (0.122 mmol)
was added in a 100 ml three necked flask, connected with a water
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in vacuum at 200 °C for more than 12 h. The Brunauer-Emmett- under argon atmosphere. We have standardized the reactions and
Teller (BET) method was utilized to calculate the specific found that 1:4 is the best ratio (details are given in table S3). The
surface areas and the nonlocal density function theory was vial was sealed and the contents were stirred magnetically for the
applied for the estimation of pore size, pore volume and pore specified time period indicated in the table and text. The reaction
distribution.
mixture was diluted with hexane, passed through a short silica
gel pad and was washed with hexane/ethyl acetate = 10/1, finally
it was concentrated at reduced pressure. The crude product was
2.5 Catalytic experiment
In a screw capped vial (4 mL) [
1
][BF4] (1 mol%), aldehyde (0.25 purified by flash column chromatography using basic Al2O3
mmol), the 2,3-dimethyl-1,3-butadiene (1 mmol) and dry (hexane/ethyl
acetate)
or
SiO2
(hexane/ethyl
toluene (1.5 mL) were taken and the reaction mixture was kept acetate/triethylamine) as stationary phase.
Scheme 2. The general strategy for the synthesis of nanoporous iron(III) porphyrin covalent frameworks.
Table 1: Porous characteristic features of compound
1
.X materials.
SLangmuir [m2/g]
SBET [m2/g]
Total Pore Volume
(cm3/g)
Average Pore Size (nm)
Materials
1.Cl
1.BF4
1.SbF6
1.ClO4
1.PF6
525
592
600
453
530
793
878
905
682
855
0.62
2.1
2.3
2.0
2.1
2.1
0.68
0.72
0.51
0.64
formula for an infinite 2D polymer {C48H28ClFeN12}n C 66.72%,
H 3.27%, N 19.45%). The amount of iron (4.72 wt%) was
estimated by ICP-AAS (Inductively coupled plasma atomic
absorption spectroscopy). Monomeric compound has calculated
3. Results and Discussion
Considering the known structural features of the
metalloporphyrin, the most probable structure of the proposed
materials is shown in Scheme 2. These materials are insoluble in
most of the organic solvents (table S1). These materials were
characterized by various spectroscopic techniques such as
elemental analysis and electron microscopy.
3.1 Catalyst Characterization
Infrared (IR) spectroscopy provided evidence for the presence of
tetrazine linkage in 1, showing the characteristic vibration bands
at 1604 cm-1, 1397 cm-1, and 1199 cm-1 for -N=N, -C=N and -C–
N units of tetrazine ring, respectively (Figure S1, Table S2). The
Fe–N stretching frequency of metalloporphyrin was observed at
999 cm-1. Elemental analysis showed the content of C, H, and N
to be 65.98%, 3.25% and 19.20%, respectively (theoretical
Fe 6.95 %. The value of the expected polymer (
have the lower value and it seems our result is along the line.
The solid-state electronic absorption spectra of is given in
1) is supposed to
1
figure S2. This shows the π-π* transitions for both porphyrin and
tetrazine moieties. The absorption bands at 440 nm attributed to
the Soret bands of iron(III) porphyrin unit. The band at 553 nm
appeared due to the tetrazine moeity. There is a 21 nm red shift
of the Soret band in
1 with respect to the corresponding
(CN)4TPPFeCl monomer (419 nm).This could be due to the
presence of extended π conjugation over the 2D sheet of the
COFs.
Scanning electron microscopy (SEM, Fig. 1a,b) revealed that
1
has rough surface with lengths of micrometres and have
agglomerates of particles ranging in size from about 500 to 900
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nm. The presence of iron in
1 was confirmed by elemental
analysis EDX (Energy dispersive X-ray, figure S3).
a
Figure 2. N2 gas absorption study for compound 1.BF4.
3.2 Catalytic activity
Now to see the catalytic activity of these materials we performed
the reaction of benzaldehyde with 2,3-dimethyl-1,3-butadiene in
2 µm
the presence of 1.X (1 mol %) in toluene at 90ºC for 12 h which
gives pyran derivative. Some selected reactions were also
conducted at room temperature, where product yield were low
even if the reactions was conducted for prolonged time (~48h)
(table S3, Entry No. 8-10). The yields varied with variable X
b
and the results are given in table 2. Thus 1.BF4 and 1.SbF6 gives
the best result. Results with other anions, blank reactions, with
monomers and with tetrabutyl ammonium salts are also
collected. It is noted that tetrabutyl ammonium salts of BF4,
SbF6, ClO4, and PF6 salts do not catalyse the reactions. Also
catalytic role of free Ag+ and Fe2+ ions are not observable.
Table 2. Iron-catalyzed cycloaddition of Ia with IIaa
2 µm
Entry
1
Catalyst
(CN)4TPPFe.BF4
1.BF4
Solvent
Toluene
Toluene
Benzene
MeCN
Yield (%)b
60
2
94
3
1.BF4
90
Figure1. (a, b) SEM images for compound 1.X (Mag. = 2.43KX;
4
1.BF4
<1
Scale 2µm).
5
(CN)4TPPFe.SbF6 Toluene
65
6
1.SbF6
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
98
In order to understand the 1D channel accessibility and porosity
7
(CN)4TPPFe.PF6
1.PF6
24
of the
measurements were conducted. They displayed typical type-IV
sorption isotherm curve for .BF4 shown in figure 2 and for other
1
.X
materials the nitrogen sorption isotherm
8
60
9
(CN)4TPPFe.ClO4
1.ClO4
40
1
10
11
12
13
14
15
16
17
18
19
72
anions in figure S4-7 which are the characteristic of the
mesoporous materials. The calculated Brunauer-Emmett-Teller
(BET) surface areas and the Langmuir surface areas were given
in table 1. The total pore volumes were estimated for all the
materials and the pore size distributions were evaluated by using
the nonlocal density functional theory (NLDFT) method. The
polymer has one main peak centred at 2.0-2.2 nm (figure S8),
which is close to their theoretical value ~2.2 nm (table 1). These
results indicated that these COFs materials may be the good hosts
to guest suitable substrates of appropriate size.
(CN)4TPPFeCl
1.Cl
<1
<1
FeCl2
<1
AgBF4
<1
-
<1
(n-Bu)4NBF4
(n-Bu)4NSbF6
(n-Bu)4NClO4
(n-Bu)4NPF6
<1
<1
<1
<1
aReaction were carried out using iron catalysts (1 mol%), aldehyde Ia (0.25
mmol), and Diene ( 1mmol, 4 equivalent) in 1.5 mL of solvent at 90ºC for 12
h. bIsolated yield based on aldehyde Ia. The characterization of IIIaa are
given in the supporting information.
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The catalytic reactions proceed in toluene and benzene, but not note that most of the compounds studied give successful
in acetonitrile. In order to understand the role of CH3CN we have reactions with our heterogeneous catalyst .BF4.
1
taken the IR of the catalyst in presence and in absence of CH3CN We now have used other substituted dienes to see the versatility
of our catalyst for the cyclization reactions. First we used 1,4
vapour and we observed that the νCN stretching frequency was
diphenyl-buta-1,3-diene and have reacted that with
shifted towards lower energy by ~12±2 cm-1 which indicates the
benzaldehyde under our catalytic conditions in presence of 1.BF4
probable coordination of CH3CN to Fe(III) centres of the
polymeric catalyst, blocking the catalytic site for substrate
binding. Selected reactions were also tested in EtOH, MeOH,
H2O and CH2Cl2 and we observed almost no product formation
in 48h (table S3, Entry No. 11-14).
catalyst and we could not detect any cyclized product (table 4, S.
No. 1). This is never tested in earlier studies too.10 Next we used
2-methyl-buta-1,3-diene and reacted with benzaldehyde to
compare our catalytic result with those reported earlier and we
observed 85% of the cyclic product to be found in 12h with
respect to 36 h by other (table 4, S. No. 2).10
Table 4. [1]BF4 porphyrin catalysed cycloaddition of Ia with dienes IIa
S.No.
Variable dienes
% Yield of pyran
derivativesb
1
2
1,4-diphenylbuta-1,3-diene (IIb)
2-methyl-buta-1,3-diene (IIc)
<1
85
aReactions were carried out using [1]BF4 (1 mol %), benzaldehyde Ia (0.25 mmol),
and variable dienes II (1.0 mmol, 4 equiv.) in 1 mL of toluene at 90 °C for 12 h,
unless otherwise noted. bIsolated yields based on aldehyde Ia are shown. The
characterization of III are given in the supporting materials.
Figure 3. IR spectra for acetonitrile solvent neat, the catalyst and impregnated
catalyst with acetonitrile solvent.
In order to evaluate the recyclability of the catalyst 1.BF4,
reaction 3 (table 3) was chosen and after one set of reaction was
over in 12 h the catalyst was recovered and reused for the next
set of reaction on another pot with fresh loading of 4-methyl
benzaldehyde. The result showed almost equal yield of the
product. This process was continued for 8 cycles and the product
yield remained in all the sets in the range of 90 ± 5 %. The details
of the yields are given in table 5.
With the optimized reaction conditions in hand, we evaluated the
reactivity of catalyst 1.BF4 for hetero-Diels-Alder-type reaction
of various substituted aldehydes (I) with diene (IIa). It has been
reported that monomeric catalysts are not efficient for the
reaction
of
2,3-dimethyl-1,3-butadiene
with
4-
nitrobenzaldehyde.10 With our heterogeneous catalyst we noted
that this reaction gives the cyclic desired product in 87% yields.
Now to see the effect of other substituted aldehydes we have
performed few reactions and the results are given in table 3. It is
interesting to
Table 5. Recyclability test of catalyst [1]BF4 using 4-methyl benzaldehyde
(4-MB)
Table 3. [1]BF4 porphyrin catalysed cycloaddition of aldehydes I with IIaa
Cycle
Isolated yield
(%)
1
95
2
93
3
91
4
91
5
90
6
88
7
87
8
85
S.No.
Variable aldehydes
% Yield of pyran
derivativesb
1
2
3
4
5
6
7
8
9
4-nitro benzaldehyde(Ib)
Benzaldehyde(Ia)
87
94
95
90
64
90
90
91
90
90
92
92
4-methyl benzaldehyde(Ic)
4-trifluoromethyl benzaldehyde(Id)
4- cyano benzaldehyde(Ie)
4-bromo benzaldehyde(If)
4-chloro benzaldehyde(Ig)
3-bromo benzaldehyde(Ih)
3-chloro benzaldehyde(Ii)
2-fluoro benzaldehyde(Ij)
2-chloro benzaldehyde(Ik)
2-bromo benzaldehyde(Il)
10
11
12
aReactions were carried out using [1]BF4 (1 mol %), aldehyde I (0.25 mmol), and
diene II (1.0 mmol, 4 equiv.) in 1 mL of toluene at 90 °C for 12 h, unless otherwise
noted. bIsolated yields based on aldehyde I are shown. The characterization of III
are given in supporting materials.
Figure 4. Plot for yield of pyran 4-MB derivative [a, d] and 2-FB derivative
[b, c] Vs. time (hrs.), respectively. [c] and [d] was plotted after removal of
the catalyst after 4hrs.
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could be partly responsible for this (table 1). We also anticipate
Further, we performed the leaching test for the catalyst 1.BF4.
-
-
that the axial ligands X= Cl- vs X = BF4 , SbF6 are also
After 50% of the reaction, the catalyst was filtered and the
reaction was continued and we found almost no product
formation from 4h to 12h. This supports no leaching. The
observed yields in various sets of reactions were measured at
different times and these are given in figure 4. In the plot c and
d are given for leaching experiments for substrate Ic and Ij in
table 3.
In order to understand the mechanistic aspects we further studied
the IR spectra for the catalyst with impregnated benzaldehyde
and compared that with neat benzaldehyde and the results are
given in figure 5. It was found that νc=o stretch of benzaldehyde
was indeed shifted from 1702 cm-1 (neat benzaldehyde) to 1683
cm-1 (benzaldehyde with catalyst). It is expected that if our
catalyst acts as a strong Lewis acid towards the benzaldehyde,
the νc=o stretch of benzaldehyde is supposed to be proportionately
down field shifted well before it interacts with the diene. Thus
we conclude that the catalysts are activating the benzaldehyde
first by Lewis acid-base interaction and then the coupling of
benzaldehyde with dienes are taking place to evolve the pyran
derivatives.
-
responsible. The Cl- are known to be stronger ligands than BF4
-
and SbF6 . Thus possibility of better Lewis acid-base interaction
-
is possible where X is either BF4- or SbF6 vs Cl-.
Acknowledgements
We thank the department of science and technology (DST, India)
(Project no. SR/S1/(IC-48/2010) for funding and UGC for the
fellowship to MKS. We also thank Prof. Gopinathan Sankar,
Department of chemistry, University college London for his
valuable suggestion during development of this manuscript.
Notes and references
Department of chemistry, Indian Institute of Technology Delhi, Hauz
Khas, New Delhi – 110016
*E-mail: dkbp@chemistry.iitd.ac.in
Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/c000000x/
1
J. T. Groves and Y. –Z. Han in Cytochrome P450: Structure,
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J. L. Mclain, J. Lee and J. T. Groves, in Biomimetic Oxidations
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Figure 5. IR spectra for benzaldehyde, catalyst 1.BF4 and impregnated catalyst
1.BF4 with benzaldehyde. The νc=o stretching frequency for neat benzaldehyde
was observed at 1702 cm-1 while impregnated one with catalyst 1.BF4 was
observed at 1683 cm-1.
4
5
C. Che, V. K. Lo, C. Zhou and J. Huang, Chem. Soc. Rev., 2011,
40, 1950-1975; D. H. Chin, G. N. L. Mar; A. L. Balch, J. Am.
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4. Conclusions
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-
-
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DOI: 10.1039/C5NJ03592A
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J. Name., 2012, 00, 1-3 | 7

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DOI: 10.1039/C5NJ03592A
TOC
Nanoporous iron(III) porphyrin frameworks: An efficient
catalyst for [4+2] cycloaddition reactions of unactivated aldehydes
with a diene
Manoj Kumar Singh and Debkumar Bandyopadhyay*
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-
110016, India
*Corresponding address: Tel. +91-11-2659 1509, Fax: +91-11- 2658 1102, Email:
dkbp@chemistry.iitd.ac.in
Pyran derivatives
>90 % yield
X = Cl, BF4, SbF6, PF6, ClO4
R = H, CH3, NO2, CN, F, Cl, Br, CF3
The one pot conversion of several unactivated aldehydes with a diene to their corresponding
[4+2] cycloaddition products has been studied using nanoporous iron(III) porphyrin
frameworks as catalysts.