A series of highly stable porphyrinic metal-organic frameworks based on iron-oxo chain clusters: design, synthesis and biomimetic catalysis

Article abstract of DOI:10.1039/d0ta02033h

Iron-based porphyrinic metal-organic frameworks (PMOFs) are desirable for biomimetic applications, due to the low toxicity and high abundance of Fe as well as the rich biomimetic functions of metalloporphyrins. Besides, the uniform dispersion of porphyrin centers in PMOFs can effectively protect them from self-dimerization. Nevertheless, it remains a big challenge to synthesize iron-based PMOFs. In this study, a series of Fe-oxo chain-based PMOFs incorporating different metals in porphyrinic centers (namely M-PMOF-3(Fe), M = Fe, Co, Ni, Cu) are synthesized directly from the reaction of metalloporphyrin and iron salts with an improved modulating strategy using a pair of monocarboxylic acids and water as the three-component modulator. The prepared materials of M-PMOF-3(Fe) possess high stability to resist a broad pH range (0-11) and even 2 M HCl in aqueous solutions for 2 days, and their frameworks can be maintained up to 350 °C. Catalytic tests show that M-PMOF-3(Fe) are effective in the aerobic oxidation of C-H bonds using oxygen from the air as the oxidant.

Full text of DOI:10.1039/d0ta02033h

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DOI: 10.1039/D0TA02033H
ARTICLE
A series of highly stable porphyrinic metal-organic frameworks
based on iron-oxo chain cluster: design, synthesis and biomimetic
catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Gang Liu, Hao Cui, Sujuan Wang, Li Zhang* and Cheng-Yong Su*
DOI: 10.1039/x0xx00000x
Iron-based porphyrinic metal-organic frameworks (PMOFs) are desirable for biomimetic applications, due to the low
toxicity and high abundance of Fe as well as the rich biomimetic functions of metalloporphyrins. Besides, the uniform
dispersion of porphyrin centers in PMOFs can effectively prevent them from self-dimerization. Nevertheless, it remains a
big challenge to synthesize iron-based PMOFs. In this study, a series of Fe-oxo chain-based PMOFs incorporating different
metals in porphyrinic centers (namely M-PMOF-3(Fe), M = Fe, Co, Ni, Cu) are synthesized directly from the reaction of
metalloporphyrin and iron salts with an improved modulating strategy using a pair of monocarboxylic acids and water as
the three-component modulator. The prepared materials of M-PMOF-3(Fe) possess high stability to resist a broad pH
range (0-11) and even 2 M HCl in aqueous solutions for 2 days, and their frameworks can be maintained up to 350 °C.
Catalytic tests show that M-PMOF-3(Fe) are effective for the aerobic oxidation of C-H bonds using the oxygen from the air
as the oxidant.
accelerates the leaving of the products; 3) the heterogeneous
nature of MOF endows the porphyrin-based catalysts with
recyclability.16-22
1. Introduction
Metalloporphyrins exist widely in biological systems such as
cytochrome, peroxidase, isomerase, dehalogenase and
transferase, and they can initiate a range of fundamentally and
practically important chemical transformations, including
epoxidation of olefins, hydroxylation of C–H bonds, and
oxidation of amines, sulfides, alcohols and aldehydes, even for
unreactive substrates hydrocarbons under mild conditions.1-3
Despite the well-demonstrated advantages, porphyrin
molecules are prone to dimerization and self-oxidation
deactivation in homogeneous catalytic system. In order to
solve this problem, a series of immobilization methods of
porphyrins have been developed, such as coating or
immobilization of porphyrin molecules on molecular sieves,
carbon materials, silica, etc.4-7 As an alternative strategy,
porphyrin compounds have been encapsulated in the cavities
of metal-organic frameworks (MOFs) or behave as the organic
linkers of MOFs.8-15 The resultant porphyrinic metal-organic
frameworks (PMOFs) display several advantages: 1) the
uniform dispersion of porphyrin centers effectively prevents it
from self-dimerization; 2) the porosity of MOFs makes each
porphyrin unit available for incoming substrates and
Nevertheless, the ideal biomimetic performances of PMOFs
are greatly limited by their stability, especially regarding to
hydrolysis. Conceptually, strong coordination bonds between
metal ions and ligands can assist in the formation of stable
MOFs.23 Based on this strategy, some stable PMOFs have been
successfully prepared by employing the combination of either
high valence metal ions (e.g. Ti4+, Zr4+, Hf4+, Al3+, In3+ and Fe3+)
with carboxylates24-33 or low valence metal ions with nitrogen-
containing ligands.34,35 Among them, Fe3+-based PMOFs are
the best adequate for biomimetic applications due to the low
toxicity in biology and high abundance in Earth, but they
remain difficult to synthesize.33,36,37 The few reported Fe-based
PMOFs are synthesized with two kinds of methods. The one is
to employ preassembled Fe-oxo clusters to react with
porphyrin ligands.38 A series of Fe-based PMOFs named after
PCN-600 have been prepared through the self-assembly of
preassembled
[Fe3O(OOCCH3)6]
clusters
and
metalloporphyrins.33 Due to the mesoporosity of PCN-600 with
a diameter of 3.1 nm, direct activation of PCN-600 is
unsuccessful. The activation needs to be carried out under
supercritical carbon dioxide, after applying a dilute acid
solution for pre-activation treatments. The other one is to use
an either inorganic or organic base to deprotonate the
carboxylic acid groups and thus facilitate the coordination of
the iron ion to the porphyrin.36,37 During the crystal growth,
the organic bases such as 4,4´-bipyridine and pyrazine can not
only deprotonate the carboxylic acid but also coordinate to the
metal centers in the porphyrin ring, the latter of which makes
MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of
Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou
510275, China. E-mail: zhli99@mail.sysu.edu.cn; cesscy@mail.sysu.edu.cn.
†Electronic Supplementary Information (ESI) available: Synthesis of
metalloporphyrin ligands and porphyrinic metal-organic frameworks (PMOFs),
crystal structure data, physical characterizations, stability study and catalytic
performances of PMOFs. CCDC number 1892975 and 1983557. See
DOI: 10.1039/x0xx00000x
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the metalloporphyrin inaccessible to the incoming substrates and acetone sequentially for 12 h, and then actVivieawtAerdticleuOnndliener
for the further transformations.
vacuum at 150 °C for 12 h. EneDrgOyI: 10d.i1s0p3e9/rDsi0vTeA02X0-3r3aHy
In this work, we employ an improved modulating synthetic spectroscopy (EDS) is recorded on a JSM-6330F field emission
strategy39 using a pair of monocarboxylic acids and water as scanning electron microscope combined with EDS. X-ray
the three-component modulator, and facilely prepare a series photoelectron spectroscopy (XPS) is performed on a ULVAC
of highly stable Fe-oxo chain-based porphyrinic MOFs (namely PHI Quantera microprobe. GC-MS data are recorded on an
M-PMOF-3(Fe), M = Fe, Co, Ni, Cu) (Figure 1). The three- Agilent 7890-5975C. Thermogravimetric (TG) analysis is carried
component modulator can effectively control crystal out on a Netzsch STA 449 F3 system from 30°C to 900 °C at a
nucleation and growth.40-42 The synthesis process is carried out rate of 5 °C min–1. GC-MS data are recorded on an Agilent
in one-pot, and the obtained crystals display high crystallinity 7890-5975C.
and purity. The frameworks of M-PMOF-3(Fe) with the fry-
topology are composed of metalloporphyrin linkers and
[Fe(OH)O4]n chains, which possess high stability, survive in a
broad pH range (0-11) and even in 2 M HCl for 2 days, and can
be maintained up to 350 °C. Biomimetic applications of M-
PMOF-3(Fe) towards the oxidation of C-H bonds in air have
been carried out thereafter.
2.2 Synthesis of M-PMOF-3(Fe)
The synthesis procedure of M-PMOF-3(Fe) (M = Fe, Co, Ni,
Cu) is using Fe-PMOF-3(Fe) as a typical example: A mixture of
[5,10,15,20-tetrakis(4-methoxycarbonylphenyl)porphyrinato]
Fe(III) chloride that is named after Fe-TCPP (8 mg, 9.45 µmol),
Fe(NO3)3·9H2O (9 mg, 22.3 µmol), benzoic acid (BA, 400 mg,
3.28 mmol), trifluoroacetic acid (TFA, 270 μL, 3.64 mmol) and
H2O (20 μL, 1.11 mmol) is ultrasonically dissolved in N,N-
dimethylacetamide (DMAC, 2 mL) in a 15 mL Teflon-lined
autoclave. The autoclave is afterwards placed in
a
programmable oven (DHG-9145A, Keelrein instrument co. ltd,
Shanghai, China) at 160 °C for 12 h with a heating rate of
1.5 °C/min and a cooling rate of 0.38 °C/min. After being
cooled down to room temperature, the dark red crystals of Fe-
PMOF-3(Fe) are obtained by filtration, and washed with DMF
for five times and with acetone for three times.
Anal. Calcd. for Fe-PMOF-3(Fe)·7BA·3TFA·DMF·5H2O: C,
53.41; H, 3.61; N, 2.94 Found: C, 53.23; H, 5.32; N, 2.37. FTIR
(cm-1, ATR): 3402 (w), 2570 (w), 1607 (w), 1514 (m), 1410 (s),
1180 (m), 995 (m), 773 (m), 711 (m), 543 (m), 509 (m), 439 (m).
Anal. Calcd. for Co-PMOF-3(Fe)·5BA·2.3TFA·0.2DMF·4H2O: C,
54.38; H, 3.38; N, 3.02. Found: C, 54.31; H, 6.24; N, 2.66. FTIR
(cm-1, ATR): 3390 (w), 1610 (w), 1578 (w), 1508 (m), 1411 (s),
1350 (m), 999 (m), 774 (m), 713 (m), 554 (w), 512 (m), 459 (m).
Anal. Calcd. for Ni-PMOF-3(Fe)·5BA·2.1TFA·4H2O: C, 54.82; H,
3.36; N, 2.93. Found: C, 54.38; H, 5.74; N, 2.43. FTIR (cm-1, ATR):
3431 (m), 2928 (w), 2350 (w), 1514 (s), 1414 (s), 1182 (w),
1001 (m), 773 (m), 712 (m), 556 (m), 516 (m), 463 (m).
Anal. Calcd. for Cu-PMOF-3(Fe)·1.8TFA·0.02DMF·3H2O: C,
55.83; H, 3.33; N, 3.02. Found: C, 55.55; H, 5.84; N, 2.57. FTIR
(cm-1, ATR): 3045 (w), 2928 (w), 1517 (s), 1411 (s), 1345 (s),
1182 (m), 998 (m), 773 (m), 713 (m), 605 (w) , 550 (m), 516 (m),
447 (m).
Figure 1. The syntheses of Fe-based porphyrinic metal-organic
frameworks in previous reports and this work (M = Fe, Co, Ni,
Cu. BA and TFA represent benzoic acid and trifluoroacetic acid,
respectively).
2. Experimental Section
2.3 Typical procedure for catalytic oxidation of hydrocarbons
In a typical reaction, an acetonitrile (2 mL) solution of Cu-
PMOF-3(Fe) (4 mg, 0.004 mmol) is placed into a round bottom
flask (10 mL) that is equipped with a reflux condenser, and
then stirred for 24 h at room temperature. Afterwards, N-
2.1 General
All chemicals used in this paper are purchased from
commercial sources without further treatment. The synthesis
of metalloporphyrin ligands is discussed in the Supporting
Information. The single crystal X-ray diffraction data are
collected using an Agilent Technologies SuperNova X-ray
diffractometer system equipped with Cu-Kα radiation (1.54178
Å). Powder X-ray diffraction (PXRD) patterns are measured on
a Japan Rigaku Miniflex 600. Nitrogen adsorption–desorption
isotherms are measured using a Micromeritics ASAP 2020
system at 77 K. Before the examinations, the as-synthesized
samples are washed with DMF for five times, soaked in DMF
hydroxyphthalimide (NHPI,
6
mg, 0.037 mmol) and
ethylbenzene (21.2 mg, 0.2 mmol) are added to the flask. The
reaction mixture is stirred at 60 ℃ in air for 24 h, which is then
analyzed by GC-MS. The yield (82%) of the product
acetophenone is determined by GC-MS.
After the reaction, Cu-PMOF-3(Fe) is separated by
centrifugation, washed with acetonitrile for 3 times and then
soaked in acetonitrile for 12 h. After that, Cu-PMOF-3(Fe) is
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separated by centrifugation, and collected for the recycling the structure of the M-PMOF-3(Fe). Ni-PMOF-3(Fe) possesses
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experiments.
a three-dimensional reticular structureDOwIi:t1h0.1a039fr/yD-0tToAp0o2l0o3g3yH,
which is composed of NiII-TCPP (TCPP = mesotetrakis(4-
carboxyphenyl) porphyrin) linkers and Fe-oxo chains (Figure 2).
Each porphyrin ligand connects four Fe-oxo chains via
carboxylate groups to construct the 3D network, featuring 1D
channel with the size of 13.6 × 7.2 Å2 running through the a
axis (Figure 2a). The porphyrin rings in Ni-PMOF-3(Fe) are
arranged in parallel, and the distance of the adjacent
porphyrin rings is 6.8 Å (Figure 2b). The Fe3+ ions in the Fe-oxo
chain are octahedrally coordinated by six oxygen atoms, and
four of these oxygen atoms in the equatorial plane are from
carboxylate groups of four different porphyrin linkers and the
remaining two oxygen atoms in the axial are from two μ2-OH
groups. In this way, these oxygen atoms bridge adjacent Fe(III)
ions to give rise to [Fe(OH)O4]n chain (Figure 2c). PLATON
calculations indicate that Ni-PMOF-3(Fe) display an effective
pore volume of 57.9%. The well-defined PXRD patterns of the
as-synthesized samples are consistent with the simulated
pattern, indicative of the high phase purity and crystallinity of
M-PMOF-3(Fe) (Figure S6).
3. Results and Discussion
3.1 Synthesis
Modulated synthesis has been reported to be an effective
method for the synthesis of highly crystalline MOFs, especially
the carboxylate-based MOFs.43-47 The monocarboxylic acid as
the modulator is added to the self-assembly system to
compete with the organic linker, and then control crystal
nucleation and growth. In the improved modulating synthesis,
a trace amount of water is added to promote the hydrolysis of
metal salts and accelerate the crystal nucleation. Nevertheless,
modulated synthesis hasn’t been successfully applied to the
synthesis of Fe-based PMOFs.
Through trials and errors, single crystals couldn’t be
obtained by using a single monocarboxylic acid (e.g. benzoic
acid (BA), trifluoroacetic acid (TFA) or acetic acid) as the
modulator yet (Figures S1 and S2). To our delight, by
introducing BA, TFA and H2O as the three-component
modulator, the desired crystals of the Fe-based PMOFs
(namely M-PMOF-3(Fe), M = Fe, Co, Ni, Cu) are obtained from
the self-assembly of metalloporphyrin and Fe(NO3)3·9H2O
(Figure 1). It is noted that the use of FeCl3·xH2O as the iron salt
can’t give rise to any crystals in this work, although this iron
salt has been usually used for the synthesis of Fe-based
MOFs.36,37,48,49
The effect of the amounts of BA and TFA on the crystal
growth has been studied. When other reagents except BA are
fixed (e.g. with the molar ratio of metalloporphyrin,
Fe(NO3)3·9H2O and TFA being 3.63 : 6.11 : 1000), the crystal
sizes increase with the amounts of BA increasing from 0 to 1.5
times of TFA. The crystals become wider horizontally and
shorter longitudinally, and finally approximately rectangular
(Figure S3). On the other hand, with the fixed amounts of
metalloporphyrin, Fe(NO3)3·9H2O and BA with the molar ratio
of 3.60 : 6.04 : 1000, the amount of TFA also takes an
important role on the crystal growth (Figure S4). When the
amount of TFA increases from 0.6 to 1.4 times of BA, the
crystal sizes increase gradually and their morphologies change
from spindle, cubic rod to octahedron.
The effect of the amount of water on the crystal growth is
further investigated (Figure S5). In the absence of water, a rod-
like crystal with uncertain structure is yielded. Meanwhile, the
increase of the water amount leads to a decrease of the crystal
size.
In short, the crystal nucleation and growth can be finely
tuned by the three-component modulator, and highly
crystalline M-PMOF-3(Fe) can be prepared by the improved
modulating synthesis.
Figure 2. Views of the 3D network of Ni-PMOF-3(Fe) along the
a (a) and c axes (b) (Ni2+ ion, violet; Fe3+ ion, green; N atom,
blue; O atom, red). The structure of [Fe(OH)O4]n chain is shown
in (c), in which the oxygen atom from μ2-OH is marked with
magenta for easy identification and the porphyrin ring is
represented by an orange ball. The hydrogen atoms are
omitted for clarity.
3.2 Structure
Single-crystal X-ray diffraction analyses disclose that the M-
PMOF-3(Fe) series are isostructural and crystallize in the
orthorhombic system with the space group of Cmmm (Table
S1). Ni-PMOF-3(Fe) is herein taken as an example to illustrate
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The surface area and porous structure of M-PMOF-3(Fe) an ultra-harsh condition (e.g. 2 M HCl), the PXRD patterns of
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DOI: 10.1039/D0TA02033H
series are evaluated by N2 adsorption–desorption isotherms at Ni-PMOF-3(Fe) and Cu-PMOF-3(Fe) are almost unchanged,
77 K (Figureꢀ3). They display type I isotherms, indicative of the whereas those of Fe-PMOF-3(Fe) become very weak. For
microporosity of the structure. The Brunauer–Emmett–Teller comparison, Al-PMOF31 and USTC-832, which are based on Al-
(BET) surface areas of M-PMOF-3(Fe) (M = Fe3+, Co2+, Ni2+ and oxo and In-oxo chains, respectively, are isomorphous to M-
Cu2+) are calculated to be 1241, 1318, 1237 and 1458 m2/g, PMOF-3(Fe), and are reported to be stable in a pH range of 5-8
respectively (Figures S7-10). According to the Density and 2-11, respectively.
Functional Theory (DFT) calculations, the pore sizes of M-
PMOF-3(Fe) are in the range of 7.4-7.8 Å (Table S2), which are
consistent with the single-crystal analysis (7.8 Å).
Figure 3. N2 adsorption-desorption isotherm of M-PMOF-3(Fe).
3.3 Characterizations and Stability
Energy dispersive X-ray spectroscopy (EDS) discloses that C,
N, O, and Fe elements exist in the M-PMOF-3(Fe) series
(Figures S11-14 and Tables S3-6). In addition, Co, Ni and Cu
elements occur in Co-PMOF-3(Fe), Ni-PMOF-3(Fe) and Cu-
PMOF-3(Fe), respectively. The exact contents of metallic
elements in M-PMOF-3(Fe) are determined by inductively
coupled plasma optical emission spectroscopy (ICP-OES),
which are almost close to the theoretical values (TableꢀS7).
High-resolution X-ray photoelectron spectroscopy (XPS) of M-
PMOF-3(Fe) indicate that the metal elements in the
metalloporphyrin of are Fe3+ (Fe 2p3/2, 712 eV; Fe 2p1/2, 725.7
eV), Co2+ (Co 2p3/2, 780.3 eV; Co 2p1/2, 795.7 eV), Ni2+ (Ni 2p3/2
Figure 4. Stability test of Cu-PMOF-3(Fe) in aqueous solutions
with the pH range of 0-12 and 2 M HCl for 48 h (a) and TG
curves of M-PMOF-3(Fe).
,
854.8 eV; Ni 2p1/2, 872.1 eV) and Cu2+ (Cu 2p3/2, 934.5 eV; Cu
2p1/2, 954.4 eV), respectively, and the Fe ion in Fe-oxo chain
([Fe(OH)O4]n) is +3 (Figures S15-18 and Tables S8-11).50-53
High structural stability, especially regarding to hydrolysis, is
critical for the biomimetic application of the prepared
materials. After the M-PMOF-3(Fe) series are soaked in
aqueous solutions with a pH range of 0-12 for 48 h, the
supernatant is almost colorless in the solution of pH 0-11, but
turns to dark red in the solution of pH 12. M-PMOF-3(Fe) can
resist in 2 M HCl, although the supernatant is of a slight yellow
color. The PXRD patterns of the recycled M-PMOF-3(Fe)
samples after the above stability tests in the aqueous solutions
with the pH range of 0-11 keep unchanged relative to the as-
synthesized samples, indicative of their high resistance
towards a broad pH range (Figures 4a and S19-21). Even under
Besides, after the fresh crystals of M-PMOF-3(Fe) are
immersed in a wide range of organic solvents, such as
dichloromethane (DCM), ethyl acetate (EA), diethyl ether
(Et2O), acetonitrile (MeCN), tetrahydrofuran (THF), methanol
(MeOH) and tolulene (PhCH3) for 7 days, the PXRD patterns of
the recycled samples retain the prominent peak profiles
(Figure S22-24). Thermogravimetric (TG) analyses further
demonstrate that the frameworks of M-PMOF-3(Fe) can be
maintained up to 350 °C, indicative of a relatively high thermal
stability (Figure 4b).
3.4 Catalysis
As for an oxidation reaction, oxygen (O2), especially from the
air, is obviously the most ideal oxidant, and then it is of high
interests to develop catalytic oxidation systems that use O2
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from the air as the oxidants. Inspired by nature, biomimetic respectively (Table S12). The catalytic performance of Cu-
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catalytic oxidation systems have been developed by simulating PMOF-3(Fe) is further investigated in thDeOsIo: 1lv0e.1n0t3-9f/rDe0eTAsy0s2t0e3m3H,
cytochrome P450 enzymes to realize the activation of O2 under and the conversion of ethylbenzene to acetophenon is 9% in
mild conditions and further oxidization of the inert C-H bond of the presence of 0.025 mol% of the catalyst in 7 days (Table
hydrocarbons.54 Considering that the M-PMOF-3(Fe) series S13). Moreover, the heterogeneous nature of Cu-PMOF-3(Fe)
contain highly dispersed porphyrin sites and are highly stable, is then studied, disclosing that Cu-PMOF-3(Fe) can be easily
they are applied for biomimetic aerobic oxidation of recovered by centrifugation and maintain its high catalytic
hydrocarbons in air.
activity after 11 continuous runs (Figure 5). After catalytic
reactions, the PXRD patterns of the recovered catalysts are
mainly remained (Figure S25). These results disclose of the
high stability of Cu-PMOF-3(Fe).
Table 1. Aerobic oxidation of ethylbenzenea
O
Catalyst/NHPI
air, 60 oC
Ketone
Selectivity (%)
Entry
Catalyst
Yield (%)
1
2
3
4
5
Fe-PMOF-3(Fe)
Co-PMOF-3(Fe)
Ni-PMOF-3(Fe)
Cu-PMOF-3(Fe)
No catalyst
61
26
97
88
8
79
82 (93)b
13
98 (98)b
79
aReaction conditions: A mixture of ethylbenzene (21.2 mg, 0.2
mmol), NHPI (6 mg, 0.037 mmol) and catalyst (0.004 mmol) is
stirred in acetonitrile (2 mL) at 60℃ under atmospheric
pressure for 24 h. The yield is determined by GC-MS. bThe
reaction time is extended to 35 h.
Figure 5. Recycling experiments of the catalytic oxidation of
ethylbenzene.
Finally, other hydrocarbons are also examined in the Cu-
PMOF-3(Fe)-catalyzed oxidation reactions (Table S14). The
oxidative conversions of tetralin and (methoxymethyl)benzene
to the corresponding ketones are 99 and 96%, respectively. For
comparison, 9H-fluorene is less efficient and the conversion is
58%. The C-H bond dissociation energies for tetralin,
(methoxymethyl)benzene and 9H-fluorene are 82.9, 85.8 and
80.1 kcal/mol, respectively.55,56 On the basis of these data, 9H-
fluorene should display higher efficiency than the other two
due to the relatively weaker C-H bond, but however this
conclusion is inconsistent with the experimental findings. On
the other hand, if the catalytic reactions were carried out in
the inner cavities of Cu-PMOF-Fe (13.6 × 7.2 Å2), the substrate
sizes should also play an important role on the reaction
The aerobic oxidation of ethylbenzene in air is used as a
model reaction to study the catalytic performances of M-
PMOF-3(Fe) combined with N-hydroxyphthalimide (NHPI) as
the auxiliary catalyst (Table 1). Among all of the tested
catalysts, Fe-PMOF-3(Fe) and Cu-PMOF-3(Fe) are much more
efficient and produce the oxidized product acetophenone in 61
and 82% yields in 24 h, respectively, and the ketone/alcohol
selectivity is up to 98:2 (entries 1 and 4). With the extension of
the reaction time, the yield of acetophenone is improved to 93%
in 35 h. It is reported that metalloporphyrin complexes based
on Fe, Co and Cu are desirable catalysts for the oxidation
reactions because they can form high-valent oxo complexes
with relatively high stability, which are thought to be the
intermediates.3 However, Co-PMOF-3 (Fe) doesn’t display as
high performances as Fe-PMOF-3(Fe) and Cu-PMOF-3(Fe) in
this work. The reason for the poor reactivity of Co-PMOF-3 (Fe)
with only 26% yield (entry 2) is unclear yet. The reaction in the
presence of Ni-PMOF-3(Fe) is even worse than the one using
NHPI alone (entries 3 and 5). In Ni-PMOF-3(Fe), Ni2+-porphyrin
is square planar geometry and with d8 configuration. The
energy of the empty dx2-y2 orbital of the Ni2+ ion is too high to
accept electrons from O2, and thus it is difficult for the
reaction to occur between Ni2+-porphyrin and O2.
efficiency.
The
molecular
sizes
of
tetralin,
(methoxymethyl)benzene and 9H-fluorene are 5.9  5.5, 7.7 
4.3 and 8.4  5.4 Å2 as disclosed by Chem 3D software with
energy minimization. Therefore, the lower efficiency of 9H-
fluorene might be due to its bigger molecule size.
A plausible reaction mechanism for the biomimetic catalytic
oxidation initiated by Cu-PMOF-3(Fe)/NHPI is proposed, which
is carried out under the synergistic catalysis of
metalloporphyrin and NHPI (Figure 6a, path a). Within the
framework, the metalloporphyrin unit reacts with O2 from the
air to form the dioxygen-coordinated metalloporphyrin
intermediates (A), which extract the hydrogen atom from NHPI
to form the hydroperoxo intermediate (B) followed by the
high-valent oxo-metal intermediate (C).57 It is reported that O2
On the basis of these results, Cu-PMOF-3(Fe) is selected for
the following study. The advantages of Cu-PMOF-3(Fe) prior to
the corresponding molecular catalyst Cu-TCPP are firstly
studied. The turn-over numbers (TON) in the presence of 0.1
mol% of Cu-PMOF-3(Fe) and Cu-TCPP are 330 and 180,
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could be coordinated to the metal center in the porphyrin ring 21821003, and 21890380), the Guangdong Natural Science
View Article Online
via an η1, end-on binding mode.58,59 The oxygen atom of the Funds for Distinguished Young Scholar (2019B151502017), the
DOI: 10.1039/D0TA02033H
oxygenated metalloporphyrin (C) is then inserted into the C–H Tip-top Youth Talents of Guangdong special support program
bond of ethylbenzene to realize aerobic oxidation.
(No. 20173100042150021), Science and Technology Planning
Meanwhile, after the loss of the hydrogen atom, NHPI is Project of Guangzhou (201707010168), and Local Innovative
transformed to the phthalimido-N-oxyl (PINO) radical, which and Research Teams Project of Guangdong Peal River Talents
then extracts the hydrogen atom from the benzyl C-H bond to Program (2017BT01C161).
generate an organic radical that reacts with O2 to afford the
oxygenated product (Figure 6b, path b).60,61 Considering that
the reaction yield is 82% in the presence of both Cu-PMOF-
Notes and references
3(Fe) and NHPI, whereas the yield is decreased to 13% when
NHPI is used alone (Table 1, entries 4 and 5), path b can’t
explain the high efficiency of Cu-PMOF-3(Fe)-catalyzed
oxidation.
1
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DOI: 10.1039/D0TA02033H
ARTICLE
TOC
A facile synthesis of a series of Fe-oxo chain-based porphyrinic MOFs (namely M-PMOF-3(Fe), M = Fe, Co, Ni, Cu) has been
reported.
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