Iron-Decorated, Guanidine Functionalized Metal-Organic Framework as a Non-heme Iron-Based Enzyme Mimic System for Catalytic Oxidation of Organic Substrates

Article abstract of DOI:10.1007/s10562-019-02691-0

A novel porous functionalized metal-organic framework (MOF) as a non-heme iron-based enzyme mimic system was achieved via two-step post-synthetic modification of the MIL-101(Cr)-NH₂, and characterized by FT-IR, PXRD, TGA, SEM, EDS, CHN, BET surface area, and ICP-OES analyses. This new modified MOF (MIL-101(Cr)-guanidine-Fe) has been demonstrated to be a highly efficient, active, and reusable catalyst for oxidation of various organic substrates, including alcohols, alkenes and alkyl arenes at room temperature using H₂O₂ as an oxidant. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02691-0

Catalysis Letters
https://doi.org/10.1007/s10562-019-02691-0
Iron-Decorated, Guanidine Functionalized Metal-Organic Framework
as a Non-heme Iron-Based Enzyme Mimic System for Catalytic
Oxidation of Organic Substrates
Ahmad Shaabani1 · Reza Mohammadian1 · Hassan Farhid1 · Masoumeh Karimi Alavijeh1
Mostafa M. Amini1
·
Received: 3 October 2018 / Accepted: 26 January 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
A novel porous functionalized metal-organic framework (MOF) as a non-heme iron-based enzyme mimic system was
achieved via two-step post-synthetic modification of the MIL-101(Cr)-NH , and characterized by FT-IR, PXRD, TGA,
2
SEM, EDS, CHN, BET surface area, and ICP-OES analyses. This new modified MOF (MIL-101(Cr)-guanidine-Fe) has
been demonstrated to be a highly efficient, active, and reusable catalyst for oxidation of various organic substrates, including
alcohols, alkenes and alkyl arenes at room temperature using H O as an oxidant.
2
2
Graphical Abstract
Keywords Metal-organic framework · Enzyme mimic system · Post-synthetic modification · Oxidation reactions ·
Heterogeneous catalyst
1
Introduction
synthetic strategy design and total syntheses [1–9]. Differ-
ent types of iron-enzymes such as non-heme iron oxyge-
nases and iron-binding-glycopeptides have been evolved by
nature and play a vital role in a number of metabolically
important transformations (Fig. 1) [3, 10, 11]. For exam-
ple, soluble methane monooxygenase (sMMO) promotes
the oxidation of methane to methanol [12], ribonucleotide
reductase (RNR) is a natural enzyme with the potency of
formation of deoxyribonucleotides from ribonucleotides
In recent years, the development of novel iron-based cat-
alysts as simulated chemical models of iron-containing
enzymes for oxidation of various organic compounds
has attracted a significant amount of attention in modern
*
*
Ahmad Shaabani
a-shaabani@sbu.ac.ir
[
13, 14], Rieske dioxygenases stereospecifically oxidizes
Mostafa M. Amini
arenes [15, 16] and iron-bleomycin as an antitumor drug
catalyzes oxidative DNA cleavage and oxidation of a vari-
ety of organic compounds [17, 18]. It has been found that
in the most of these iron-enzymes catalyzed oxidation
m-pouramini@sbu.ac.ir
1
Faculty of Chemistry, Shahid Beheshti University, G.C,
Tehran, 19396-4716, Iran
Vol.:(0
1
123456789)
3

A. Shaabani et al.
Fig. 1 Representative examples
His
O
of non-heme iron oxygenases
HN
N
His
HN
N
Fe
O
OH2
Fe
O
OH2
Fe
NHis
NHis
NHis
O
NHis
NHis
O
OH
N
2
His
Glu
O
O
Asp
O
HO
Taurine -Ketoglutarate Dioxygenase
a
Apocarotenoid Oxygenase
Naphthalene Dioxygenase
His = Histidine
=
Asp Aspartate
Glu = Glutamic acid
reactions, the key-species to promote the reaction is Fe-
mentioned for the oxidation of organic substrates by a non-
heme iron-containing enzyme and also in continuation of our
ongoing interest in developing catalytic systems for synthetic
methodologies [46–53], herein, we report a new strategy to
construct organo-functionalized metal-organic frameworks;
based on the post-synthetic modification (PSM) of MOFs
by grafting the guanidine moieties onto organic linkers.
Subsequently, this can be used as N-containing ligands to
coordination with iron and to create low molecular weight
quasi-enzyme scaffolds for oxidation of various organic sub-
strates, including alcohols, alkenes and alkyl arenes using
H O as a green oxidant (Scheme 1).
III
peroxo intermediates specially Fe –OOH species that are
produced during the oxidation process [19]. Deprotonation
III
of H O following by ligation to the Fe centers produces
2
2
III
Fe –OOH species as the precursor for high-valent Fe-oxo
intermediates, which is responsible for many organic sub-
strate oxidation [20]. Thus, from this perspective, construct-
ing low molecular weight quasi-enzyme scaffolds, which are
capable of binding to iron metal, can provide structural and
functional properties of natural non-heme iron-dependent
enzymes. However, the resulting material does not have
some of the important shortcomings of the enzymes such as
temperature and chemical sensitivity, non-recyclable nature
and expensive cost. In addition, achieving the more envi-
ronmentally benign methods for oxidation reactions using
green oxidants such as H O or O is particularly in demand
2
2
2 Results and Discussion
2
2
2
[
21–27]. Therefore, improving novel catalytic protocol con-
Recently, Kong and co-workers have synthesized amine-
functionalized MIL-101 based on chromium and 2-amino-
terephthalic acid as an organic linker [54]. The presence of
amine moieties and high chemical stability of this type of
sidering green chemistry and advantages of iron-containing
enzyme catalytic systems for oxidation of organic substrates
can be beneficial.
Within the last decade, enormous research efforts have
been made in the investigation of MOFs as potential hetero-
geneous catalysts [28–32]. Although, metal-organic frame-
works can directly act as a heterogeneous catalyst without
any manipulation in a variety of reactions such as oxidation
2
PhCH R3
OH
R1
R2
R3
R4
[
33–37], hydrogenation [38], Knoevenagel condensation
39], Suzuki reaction [40] and so on [39], emergence of new
MIL-101-
guanidine-Fe
H
[
applications which emphasize the requirement for new types
of materials is one of the constant challenges in the area
of material science [41]. In this regard, chemical function-
alization of MOFs by a post-modification technique which
converts MOFs to a series of new materials with the basic
stable structure bearing different functionalities, provide the
possibility of creating new structures and improving the per-
formance of the MOFs by following their specific applica-
tions [42–45]. Catalytic behavior of the resulting modified
material is entirely different from the parent MOF and can
be used as a novel heterogeneous catalyst in chemical trans-
formations. In this context and concerning the advantages
2O
2
r.t.
O
O
R1
R2
R3
R4
O
R1
R2
=
=
R3
R4
=
=
aryl, alkyl
aryl, alkyl,
aryl, alkyl
H
aryl, alkyl,
H
Ph
R3
R3
= aryl, alkyl,
H
Scheme 1 MIL-101(Cr)-guanidine-Fe-catalyzed oxidation of various
organic substrates
1
3

Iron-Decorated, Guanidine Functionalized Metal-Organic Framework as a Non-heme Iron-Based…
Cl
Cl
H O Fe Cl
2
H
N
H
N
N
NH
NH
N
NH2
NH2
NH2
NH2
HO
O
O
NH2
NH2
NH2
DCC
NH
2
FeCl3.6H
O
2
NH2
Cr(III)
+
HN
HN
N
N
NH
Cl
Fe
OH
Cl
Cl
2
OH
HN
MIL-101-NH2
MIL-101-guanidine
MIL-101-guanidine-Fe
Scheme 2 Schematic illustration of heterogeneous MIL-101(Cr)-guanidine-Fe catalyst preparation
Fig. 2 FTIR spectra of MIL-101(Cr)-NH (A), MIL-101(Cr)-guani-
2
Fig. 3 XRD patterns of MIL-101(Cr)-NH (A), MIL-101(Cr)-giani-
dine (B), and MIL-101(Cr)-gianidine-Fe (C)
2
dine (B) and MIL-101(Cr)-gianidine-Fe (C)
MOF provide a good possibility for ligand PSM. Therefore,
functionalization of MOF can be established by grafting the
guanidine moieties onto organic linkers via nucleophilic
attack of NH groups on the reasonably positive carbon atom
2
of N,N′-dicyclohexylcarbodiimide (DCC) producing N-con-
taining ligands with the ability of coordination to iron(III)
chloride (Scheme 2).
The synthesized catalyst was characterized by FTIR,
XRD, TGA, SEM, CHN, EDS and ICP-OES analyses. The
formation of guanidine moieties onto organic linkers was
supported by FTIR spectroscopy based on the appearing
−
1
two strong stretching bands at 2847 and 2924 cm which
associated with the cyclohexyl groups present in the MIL-
1
01(Cr)-guanidine structure. Also, in confirming the immo-
bilization of the guanidine functionality, the disappearance
−
1
of the stretching band at 2100 cm , related to N=C=N moi-
ety present in DCC, and enhancement of the stretching bands
−
1
at 1608 and 1152 cm in agreement with the formation of
imine (C=N) groups and C–N bonds, respectively (Fig. 2).
Fig. 4 TGA curves of MIL-101(Cr)-NH (A) and MIL-101(Cr)-gia-
2
nidine-Fe (B)
1
3

A. Shaabani et al.
Powder X-ray diffraction (PXRD) patterns of MIL-
1
01(Cr)-NH , MIL-101(Cr)-guanidine and MIL-101(Cr)-
2
guanidine-Fe were recorded and used to investigate the
structure of the synthesized catalyst. As shown in Fig. 3,
the characteristic diffractions of MIL-101(Cr)-NH appeared
2
at 2θ values of 3.3°, 5.1°, 8.9° and 16.6°. From retaining
almost all the characteristic diffractions of MIL-101(Cr)-
NH in the final catalyst, it can be concluded that even after
2
the post-synthetic reaction, the structure of the material is
not changed during the functionalization process.
Thermal gravimetric analysis (TGA) was performed on
MIL-101(Cr)-NH , and MIL-101-(Cr)guanidine-Fe in the
2
air to investigate the thermal behavior of pure MIL-101(Cr)-
NH and MIL-101(Cr)-guanidine-Fe and results are illus-
2
trated in Fig. 4. The thermogram of MIL-101(Cr)-NH and
2
Fig. 5 BET isotherms of MIL-101(Cr)-NH (A), MIL-101(Cr)-guani-
2
MIL-101(Cr)-guanidine-Fe shows an initial mass loss due
to the desorption of water and volatile materials around the
temperature ranges of 82–210 and 84–240 °C, respectively.
At a temperature higher than 700 °C, a mass loss of about
dine (B), and MIL-101(Cr)-guanidine-Fe at 77 K
Table 1 Textural parameters of MIL-101(Cr)-NH , MIL-101(Cr)-
71% and 75% have been observed for MIL-101(Cr)-NH
2
2
guanidine and MIL-101(Cr)-guanidine-Fe
and MIL-101(Cr)-guanidine-Fe, respectively. In the case
of MIL-101(Cr)-guanidine-Fe, the additional 4% mass loss
can be attributed to the decomposition of the guanidine
functionality which is present in the structure of the cata-
lyst. Also, the results showed that the functionalization of
Samples
BET surface area Total pore
2
(
m /g)
volume
3
(
cm /g)
MIL-101(Cr)-NH2
MIL-101(Cr)-guanidine
MIL-101(Cr)-guanidine-Fe
2498
1656
1478
2.10
1.61
1.44
MIL-101(Cr)-NH by grafting the guanidine moieties onto
2
organic linkers caused thermal stability of the final material
to be enhanced.
Fig. 6 SEM images of MIL-101(Cr)-NH (a) and MIL-101(Cr)-guanidine-Fe (b)
2
1
3

Iron-Decorated, Guanidine Functionalized Metal-Organic Framework as a Non-heme Iron-Based…
Fig. 7 EDS result for MIL-
1
01(Cr)-guanidine-Fe
Nitrogen adsorption–desorption at 77 K was used to
decreased in both MIL-101(Cr)-guanidine, and MIL-
101(Cr)-guanidine-Fe, due to the presence of guanidine
groups projecting into the pores. The related textural param-
prove the porous structure and determine BET surface area
of the MIL-101(Cr)-NH , MIL-101(Cr)-guanidine and
2
MIL-101(Cr)-guanidine-Fe [55, 56]. Figure 5 shows the N
eters which were measured by N adsorption–desorption iso-
2
2
adsorption–desorption isotherms of the three samples, and
as can be seen type I adsorption behavior occurred [55], with
small hysteresis due to the interstitial voids formed by the
therms are listed in Table 1.
A typical scanning electron microscopy (SEM) analy-
ses were carried out to study the morphology and structure
of the obtained catalyst. The polyhedral crystals and good
small crystal particles. Comparing with MIL-101(Cr)-NH ,
2
BET surface areas, and total pore volumes are substantially
monodispersity and crystallinity of the MIL-101(Cr)-NH
2
Table 2 Optimization of
reaction conditions for the
oxidation of benzyl alcohol
Time (h) Yield (%)a
Entry Catalyst (mg)
Oxidant
Solvent Tem-
perature
o
(
C)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
–
H O (2 eq)
CH CN 80
24
8
8
8
8
24
24
24
8
8
8
8
8
8
8
8
6
13
42
66
84
95
44
51
23
6
78
93
16
91
12
26
95
97
2
2
3
MIL-101(Cr)-guanidine-Fe (5)
MIL-101(Cr)-guanidine-Fe (10)
MIL-101(Cr)-guanidine-Fe (20)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-NH (25)
MIL-101(Cr)-guanidine (25)
FeCl .6H O (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
H O (2 eq)
CH CN r.t
2
2
3
H O (2 eq)
CH CN r.t
2
2
3
H O (2 eq)
CH CN r.t
2
2
3
H O (2 eq)
CH CN r.t
2
2
3
H O (2 eq)
CH CN r.t
2
2
2
3
H O (2 eq)
CH CN r.t
2
2
3
H O (2 eq)
CH CN r.t
3
2
2
2
3
–
CH CN r.t
3
0
1
2
3
4
5
6
7
H O (1 eq)
CH CN r.t
2
2
3
H O (1.5 eq)
CH CN r.t
2
2
3
O (1 bar)
CH CN r.t
2
3
t
BuOOH (1.5 eq) CH CN r.t
3
H O (1.5 eq)
Water
r.t
2
2
H O (1.5 eq)
Toluene r.t
2
2
H O (1.5 eq)
CH CN 50
2
2
3
H O (1.5 eq)
CH CN 80
2
2
3
Reaction conditions: benzyl alcohol (1.00 mmol), solvent (5 ml), catalyst and oxidant
a
Determined by GC
1
3

A. Shaabani et al.
are visible from Fig. 6 a. As Fig. 6 b shows, the loading of
the guanidine functionality did not significantly affect the
nanomorphology of parent MOF and only led to a slight
deformation. We have also performed CHN analysis for
solvents showed that acetonitrile is the most appropriate
medium for this catalytic system. Finally, screening differ-
ent temperatures revealed that by increasing the temperature,
the reaction time could be decreased to 6 h and the yield can
be increased to 97%, but from the energy-saving viewpoint
this is not economical.
MIL-101(Cr)-NH and MIL-101(Cr)-guanidine in which the
2
C:H:N ratio in MIL-101–NH is ~37.84:2.67:5.48 whereas
2
for MIL-101(Cr)-guanidine C:H:N ratio is ~40.80:4.26:6.34
After optimizing the reaction conditions, the scope of the
reaction for the oxidation of various primary and secondary
aliphatic and aromatic alcohols to corresponding aldehydes
and ketones, respectively, was explored. All products were
obtained in high yields, and it was observed that obtained
aldehydes did not undergo subsequent oxidation to carbox-
ylic acids (Table 3).
indicating the functionalization of MIL-101(Cr)-NH with
2
guanidine scaffolds. Finally, to determine the chemical com-
position of MIL-101(Cr)-guanidine-Fe, energy dispersive
X-ray spectroscopy (EDS) analysis was used, and results
revealed that chromium and iron metals are present in the
catalyst (Fig. 7). Furthermore, inductively coupled plasma-
optical emission spectroscopy (ICP-OES) analysis of MIL-
To investigate the generality and utility of this catalytic
system, we further examine the efficiency of using of MIL-
101(Cr)-guanidine-Fe catalyst for the epoxidation of sty-
rene utilizing H O as an oxidant (Table 4). The best reac-
1
01(Cr)-guanidine-Fe showed 0.21 mmol.g−1 iron loading
in the final catalyst.
2
2
2.1 Catalytic Activity of MIL‑101(Cr)‑Guanidine‑Fe
tion conditions were 25 mg catalyst in acetonitrile at room
temperature.
The catalytic activity of MIL-101(Cr)-guanidine-Fe in the
oxidation of primary and secondary alcohols to desired alde-
hydes and ketones was evaluated. For optimization of the
reaction conditions, oxidation of benzyl alcohol was chosen
as a model reaction and carried out in various conditions
such as solvent, amount of catalyst and oxidant (Table 2).
At the outset, acetonitrile was used as a reaction medium,
and the model reaction was explored in various amounts
of catalyst. In the absence of MIL-101(Cr)-guanidine-Fe at
With the optimized conditions established, the oxida-
tion of various alkenes was examined in the presence of
MIL-101(Cr)-guanidine-Fe as a catalyst under optimized
conditions (Table 5). As shown in Table 4 all of the starting
materials were oxidized to the corresponding epoxides with
high conversion and selectivity.
Inspired by the success of reactions mentioned above,
we decided to extend the study to oxidation of alkyl arenes
using the MIL-101(Cr)-guanidine-Fe catalyst. For this
purpose, oxidation of diphenylmethane was performed as
a model reaction, and the reaction was carried out in vari-
ous conditions. As illustrated in Table 6 (Entry 4), the opti-
mized conditions were obtained in the presence of 25 mg of
catalyst and 2 equivalents of H O in acetonitrile at room
8
0 °C for 24 h, only 13% yield was achieved, and most of
starting materials were recovered (Table 2, entry 1). When
the reaction was explored in the presence of MIL-101(Cr)-
guanidine-Fe (5 mg), the yield was obtained in 42% within
8
h (Table 2, entry 2). The amount of MIL-101(Cr)-guani-
2
2
dine-Fe was optimized to be 25 mg due to higher yield and
shorter reaction time than the other amount of catalyst load-
ing (Table 2, entry 5). The model reaction was also exam-
ined in the presence of other catalysts such as FeCl .6H O,
temperature for 18 h.
To show the generality of the MIL-101(Cr)-guanidine-
Fe catalyst, a variety of alkyl arenes to the corresponding
carbonyl compounds were subjected to the optimal condi-
tions. It can be seen that all the alkyl arene derivatives gave
products in good yields (Table 7).
3
2
MIL-101(Cr)-NH , and MIL-101(Cr)-guanidine individu-
2
ally and the results were compared with the catalytic activity
of the MIL-101(Cr)-guanidine-Fe to confirm the catalytic
efficiency of it. The results of the comparisons revealed
that MIL-101(Cr)-guanidine-Fe has superior catalytic effi-
ciency than the three other catalysts in the model reaction
The heterogeneous nature of the catalysis was proved
using a filtration test and atomic absorption spectroscopy.
A filtration test was performed after ~50% completion of the
reaction by oxidizing benzyl alcohol as the model oxidation
reaction to determine whether the catalyst is functioning in
a heterogeneous manner or catalyst is merely a reservoir
for more active soluble iron species. The filtrates were then
transferred to another flask under the same initial reaction
conditions. Upon further stirring of the catalyst-free solution
(
Table 2, entries 6–8). In the next step, oxidation of benzyl
alcohol was tested in the absence and presence of various
oxidants (Table 2, entries 9–13). Considering results, the
most appropriate oxidant is H O with the optimal amount of
2
2
1
.5 equivalents. Exploring the reaction progress in various
1
3

Iron-Decorated, Guanidine Functionalized Metal-Organic Framework as a Non-heme Iron-Based…
Table 3 Oxidation of primary and secondary aliphatic and aromatic alcohols
OH
O
H O
MIL-101-guanidine-Fe,
2
2
R1 R2
R1 R2
CH CN, r.t.
3
a
Entry
1
Alcohol
OH
Product
Time (h)
Yield (%)
O
8
93
95
95
OH
O
2
8
8
Cl
Cl
OH
OH
O
3
Cl
Cl
Cl
Cl
O
4
8
94
Br
F
Br
F
OH
OH
O
O
5
8
97
6
8
98
O N
O N
2
2
OH
O
7
8
98
OH
O
8
9
8
8
99
93
93
OH
O
OH
O
1
0
12
1
1
1
2
OH
O
12
12
91
94
OH
O
OH
O
1
1
3
4
8
99
O
O
OH
O
12
90
Reaction conditions: Alkene (1.00 mmol), MIL-101(Cr)-guanidine-Fe (25 mg), H O (1.5 eq), acetonitrile (5 ml), room temperature
2
2
a
Yields determined by GC analysis
1
3

A. Shaabani et al.
Table 4 Optimization of reaction conditions for the epoxidation of styrene
o
Yield (%)a
Entry
Catalyst (mg)
Oxidant
Solvent
Temperature ( C)
Time (h)
1
2
–
H O (1.5 eq)
CH CN
r.t
r.t
8
8
Trace
40
2
2
3
MIL-101(Cr)-guanidine- H O (1.5 eq)
CH CN
2
2
3
Fe (5)
MIL-101(Cr)-guanidine- H O (1.5 eq)
3
4
5
6
7
8
CH CN
r.t
8
8
8
8
8
8
56
2
2
3
Fe (10)
MIL-101(Cr)-guanidine- H O (1.5 eq)
CH CN
r.t
73
2
2
3
Fe (20)
MIL-101(Cr)-guanidine- H O (1.5 eq)
CH CN
r.t
81
2
2
3
Fe (25)
MIL-101(Cr)-guanidine- H O (1 eq)
CH CN
r.t
67
2
2
3
Fe (25)
MIL-101(Cr)-guanidine- H O (1.5 eq)
Water
Water
r.t
Trace
7
2
2
Fe (25)
MIL-101(Cr)-guanidine- H O (1.5 eq)
100
2
2
Fe (25)
Reaction conditions: styrene (1.00 mmol), solvent (5 ml), catalyst and oxidant
a
Determined by GC
for 24 h, no considerable progress (~7% by GC analysis) was
observed. Moreover, using atomic absorption spectroscopy
of the same reaction solution at the midpoint of completion
indicated that no significant quantities of iron are lost to the
reaction liquors during the process.
3 Conclusions
We have successfully developed a straightforward approach
to construct guanidine moieties on MIL-101(Cr)-NH metal-
2
organic framework via post-synthetic modification which
can play the role of N-containing ligands to coordination
with iron and create low molecular weight quasi-iron con-
taining enzyme scaffolds. The new nanoscale MIL-101(Cr)-
guanidine-Fe showed high activity and selectivity in the oxi-
dation of various organic substrates such as alcohols, alkenes
and alkyl arenes at room temperature using H O as an oxi-
The reusability and activity of MIL-101(Cr)-guanidine-Fe
were examined in four consecutive runs of the oxidation of
benzyl alcohol, styrene and diphenylmethane. After comple-
tion of the reaction, the catalyst was separated by filtration,
washed with chloroform, ethanol, and acetone, dried and
reused in the subsequent run. The recycled catalyst was effi-
ciently used in the next three consecutive runs with only an
insignificant loss of activity of the catalyst as shown in Fig. 8.
Powder X-ray diffraction (PXRD) pattern of recovered
MIL-101(Cr)-guanidine-Fe was recorded and compared
with the XRD pattern of fresh catalyst. Result confirmed
the stability of the corresponding catalyst during the oxida-
tion reaction. As Fig. 9 shows, no significant changes are
visible, and MIL-101(Cr)-guanidine-Fe is remained intact
during the reaction. Some trivial differences between the
reflection intensities of the fresh and recovered catalyst can
be attributed to the variation in the orientation of powder
samples during PXRD patterns recording.
2
2
dant. This novel heterogeneous catalyst overcomes essential
shortcomings of the utilization of iron-based enzymes for
oxidation reactions such as temperature and chemical sen-
sitivity, non-recyclable nature and high cost.
4 Materials and Methods
All reagents were purchased from Sigma-Aldrich and used
without further purification. The X-ray powder diffraction
(XRD) patterns were recorded on an STOE diffractometer
with Cu-Kα radiation (λ = 1.5418 Å). The FT-IR spectra
were recorded on a Bomem MB-Series FT-IR spectrom-
eter. Identification and quantification were carried out on
a Varian model 3600 gas chromatograph (GC) (Varian
Iberica, Madrid, Spain) equipped with a split/splitless cap-
illary injection port and flame ionization detector (FID).
Thermogravimetric analysis (TGA) was carried out using
The catalytic activity of this catalyst in comparison with
other reported heterogeneous catalysts [57–61] is listed in
Table 8. The results show that in the case of MIL-101(Cr)-
guanidine-Fe, the reaction yield is increased at optimal in
environmentally-friendly reaction conditions. Moreover, a
small amount of catalyst is sufficient enough to promote the
reaction, and the catalyst, unlike metal salts, can be recycled
up to four runs with easy separation and work-up.
1
STA 1500 instrument at a heating rate of 10 °C min in
the air. The concentration of iron was measured using an
1
3

Iron-Decorated, Guanidine Functionalized Metal-Organic Framework as a Non-heme Iron-Based…
Table 5 Epoxidation of alkenes
H O
2
O
MIL-101-guanidine-Fe,
r.t.
2
R1
R2
CH3
CN,
R
2
R1
a
Entry
1
Alkene
Product
Time (h)
Yield (%)
O
8
81
83
F
F
O
2
3
8
O
8
85
4
5
O
8
8
69
82
MeO
Me
MeO
Me
O
6
7
O
8
8
79
66
O
Reaction conditions: Alcohol (1.00 mmol), MIL-101(Cr)-guanidine-Fe (25 mg), H O (1.5 eq), acetonitrile (5 ml), room temperature
2
2
a
Yields determined by GC analysis
Table 6 Optimization of reaction conditions for the oxidation of diphenylmethane
o
Yield (%)a
Entry
Catalyst (mg)
Oxidant
Solvent
Temperature ( C) Time (h)
1
2
3
4
5
6
7
–
H O (1.5 eq)
CH CN
r.t
r.t
r.t
r.t
r.t
r.t
100
18
18
18
18
12
24
24
–
2
2
3
MIL-101(Cr)-guanidine-Fe (10)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
MIL-101(Cr)-guanidine-Fe (25)
H O (1.5 eq)
CH CN
21
58
72
59
Trace
5
2
2
3
H O (1.5 eq)
CH CN
2
2
3
H O (2 eq)
CH CN
2
2
3
H O (2 eq)
CH CN
2
2
3
H O (2 eq)
water
water
2
2
H O (2 eq)
2
2
Reaction conditions: diphenylmethane (1.00 mmol), solvent (5 ml), catalyst and oxidant
a
Determined by GC
1
3

A. Shaabani et al.
Table 7 Oxidation of alkyl arenes
O
H O
MIL-101-guanidine-Fe,
2
2
PhCH R
2
CH CN, r.t.
Ph
R
3
a
Entry
1
Alkyl arene
Product
Time (h)
Yield (%)
O
18
72
O
2
3
4
18
18
18
63
67
71
O
O
O
O
5
18
73
O
O
O
6
O
18
18
68
73
O
7
O
8
9
18
18
18
69
57
67
O
O
1
1
0
1
O
O
18
74
O
Reaction conditions: Alkyl arene (1.00 mmol), MIL-101(Cr)-guanidine-Fe (25 mg), H O (2 eq), acetonitrile (5 ml), room temperature
2
2
a
Yields determined by GC analysis
1
3

Iron-Decorated, Guanidine Functionalized Metal-Organic Framework as a Non-heme Iron-Based…
of the samples were calculated by the Brunauer-Emmett-
Teller method in the P/P0 range of 0.05–0.1. The total pore
volume was obtained at the relative pressure (p/p ) of 0.99 in
0
the isotherm. Scanning electron microscopy (SEM) observa-
tions were carried out with a Philips XL-30 ESEM electron
microscope. All samples were sputtered with gold before
observation. Elemental analysis is performed by an Elemen-
tar Analysensysteme GmbH VarioEL.
4.1 Preparation of MIL‑101(Cr)‑NH2
The MIL-101(Cr)-NH was synthesized according to a
2
previously reported method [54]. Chromic nitrate hydrate
Fig. 8 Recycle of the MIL-101(Cr)-guanidin-Fe nanocatalyst for the
(
2.40 g, 6.00 mmol), 2-aminoterephthalic acid (1.08 mg,
oxidation of benzyl alcohol, styrene and diphenylmethane
6
.00 mmol) and sodium hydroxide (0.59 g, 15.00 mmol)
were sonicated in deionized water (45 ml) and thoroughly
stirred. The suspension was transferred in an autoclave and
kept at 150 °C for 12 h. After the autoclave was cooled, the
green precipitate was filtered off and washed with DMF to
remove unreacted ligands. Finally, the solid was further puri-
fied by alcohol at 100 °C for 24 h in an autoclave, filtered
off, and dried at 80 °C in the air.
4.2 Preparation of MIL‑101(Cr)‑Guanidine
A mixture of the prepared MIL-101(Cr)-NH (1.0 g) and
2
N,N′-dicyclohexylcarbodiimide (0.4 g) was added to a 25-ml
flask, and the reaction mixture was stirred in the preheated
oil bath at 50 °C for 24 h under solvent-free conditions. After
the reaction was cooled to room temperature, the reaction
mixture was eluted with ethanol, and the catalyst was sepa-
rated by filtration and washed with chloroform, acetone, and
ethanol three times. Finally, the collected solid was dried in
vacuum at 80 °C for 24 h.
Fig. 9 PXRD patterns of MIL-101(Cr)-guanidin-Fe before (A), after
using (B)
inductively coupled plasma optical emission spectrometer
4.3 Preparation of MIL‑101(Cr)‑Guanidine‑Fe
(
ICP-OES; Varian Vista PRO Radial) and Shimadzu AA-680
flame atomic absorption spectrophotometer. Adsorption/
desorption isotherm measurement was performed at 77 K
In a 25-ml flask, the above-prepared MIL-101(Cr)-guanidine
(0.50 g) and FeCl .6H O (0.05 g) were dispersed in absolute
3
2
(
BELSORP-mini II, Japanese). The specific surface areas
ethanol (15 ml). Then, the flask was sealed, and the mixture
Table 8 Comparison performance of this work catalyst with the heterogeneous catalysts found in the literature for the oxidation of benzyl alco-
hol
o
Entry
1
Catalyst
Temperature ( C) Time (h) Solvent
Oxidant
H O
Additive
–
Yield (%)
93
MIL-101(Cr)-guanidine- r.t
Fe (This work)
8
Acetonitrile
2
2
2
2
3
4
5
6
PdO/CuO-Zeolite-Y
Fe O @P4VP@FeCl
60
60
60
r.t
1
12
24
3
Acetonitrile
Acetonitrile
Acetonitrile
H O
–
70
99
99
99
99
2
O (1 atm)
TEMPO,KNO2
TEMPO,NaHCO3
TEMPO
3
4
3
2
UiO-66-Sal-CuCl2
O (1 atm)
2
CuNPs/nanocelloluse
GO/Fe O /HPW
H O
air
2
70
3
Neat
H O
–
3
4
2
2
1
3

A. Shaabani et al.
was stirred in the preheated oil bath at 50 °C for 24 h. After
the reaction mixture was cooled to room temperature, the
resulting precipitate was collected by filtration, washed with
ethanol followed by drying under vacuum at 80 °C for 24 h
to afford MIL-101(Cr)-guanidine-Fe powder.
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Alcohols, alkenes or alkyl arenes (1.00 mmol) was added to
a round-bottomed flask containing H O (1.50–2.00 mmol,
1
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2
2
3
0%) and MIL-101(Cr)-guanidine-Fe (0.025 g) in acetoni-
trile (10 ml) with a magnetic stirrer. The reaction mixture
was stirred at room temperature for indicated times as given
in Tables 3, 5 and 7, respectively, while the progress of the
reaction was followed by thin layer chromatography (TLC).
Upon compilation the reaction, the reaction mixture was fil-
tered and the filtrate analyzed by GC method. The conver-
sions were determined from the integrals of the GC analysis.
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Acknowledgements We gratefully acknowledge financial support from
Shahid Beheshti University.
1
9. Pap JS, Cranswick MA, Balogh-Hergovich É et al (2013) An iron
Compliance with Ethical Standards
(
II)[1, 3-bis (2′-pyridylimino) isoindoline] complex as a catalyst
for substrate oxidation with H O –evidence for a transient peroxi-
2
2
Conflict of interest All authors declare no conflicts of interest.
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