μ-Nitrido-bridged iron phthalocyanine dimer bearing eight peripheral 12-crown-4 units and its methane oxidation activity

Article abstract of DOI:10.1039/d0nj04601a

A novel μ-nitrido-bridged iron phthalocyanine dimer with eight peripheral 12-crown-4 units as an electron-donating substituent was synthesized and characterized. Examination of its methane oxidation activity in the presence of H2O2 in an acidic aqueous solution suggested that the high-valent iron oxo species generated in situ was unstable and the transiently generated decomposed species showed methane oxidation activity via Fenton-type reaction. This journal is

Full text of DOI:10.1039/d0nj04601a

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l-Nitrido-bridged iron phthalocyanine dimer
bearing eight peripheral 12-crown-4 units
Cite this:DOI: 10.1039/d0nj04601a
and its methane oxidation activity†
abc
a
Yasuyuki Yamada,
*
Jyunichi Kura,b Yuka Toyodab and Kentaro Tanaka
*
A novel m-nitrido-bridged iron phthalocyanine dimer with eight peripheral 12-crown-4 units as an electron-
donating substituent was synthesized and characterized. Examination of its methane oxidation activity in the
presence of H2O2 in an acidic aqueous solution suggested that the high-valent iron oxo species generated
in situ was unstable and the transiently generated decomposed species showed methane oxidation activity
via Fenton-type reaction.
Received 15th September 2020,
Accepted 27th October 2020
DOI: 10.1039/d0nj04601a
rsc.li/njc
sulfonate substituent decreased it.16 Therefore, it would be
interesting to know whether other strong electron-donating
Introduction
Methane is a potential feedstock for next-generation chemicals substituents can increase the reaction rate of methane oxida-
because it is abundant in nature as natural gas or methane tion. However, the synthesis and electronic structure of the
hydrate.1,2 However, because of its high chemical stability, it is m-nitrido-bridged iron phthalocyanine dimer with strong electron
difficult to convert it into more valuable chemical raw materials donating substituents such as alkoxy groups and its methane
under mild reaction conditions.3,4 Therefore, the development oxidation activities have not been investigated so far. Since the
of novel catalysts for efficient and easier chemical conversion of introduction of peripheral substituents to the phthalocyanine
methane has been intensively studied. Although some potent ring would strongly influence both the electronic structure
catalysts based on metal oxides,5 zeolites,6–9 or inorganic and reactivity of m-nitrido-bridged iron phthalocyanine dimers,
salts with light irradiation10,11 have recently been developed, more investigation should help advance the chemistry of
the number of molecular catalysts that can efficiently convert m-nitrido-bridged iron phthalocyanine dimers.
methane under mild reaction conditions is still limited.12–17
In this study, we designed a novel monocationic m-nitrido-
m-Nitrido-bridged iron phthalocyanine dimer (1) is one of bridged iron phthalocyanine dimer bearing eight 12-crown-4
the most potent molecular catalysts for methane conversion moieties (2+ in Fig. 1c). The 12-crown-4 units act as electron-
under mild reaction conditions.13–16 Both neutral species with donating groups. Moreover, it is promising that introduction
Fe(III) and Fe(IV) ions (1) and a monocationic species containing of flexible 12-crown-4 units should increase the solubility of the
two Fe(IV) ions (1+) have been reported to oxidize methane m-nitrido-bridged iron phthalocyanine dimer core in various
to afford methanol, formaldehyde, and formic acid in an acidic organic solvents. This would be helpful for purification of 2+.
aqueous solution in the presence of H2O2 below 100 1C It is also expected that the 12-crown-4 units would not interfere
(Fig. 1a). The actual reactive species of 1 is its high-valent with the oxidation reaction involving the high-valent iron-oxo
iron-oxo species (1oxo) generated in situ (Fig. 1b).13,15 Moreover, species of 2+ (2oxo) since the 12-crown-4 units of 2oxo would not
it has been reported that the introduction of the electron- reach the iron-oxo center, as revealed by molecular modeling
donating tert-butyl substituent to the phthalocyanine ring of (Fig. S1 in the ESI†). We recently demonstrated that catalytic
m-nitrido-bridged iron phthalocyanine dimer increased the methane and ethane oxidation reactions proceeded success-
methane oxidation rate, whereas the electron-withdrawing fully by using a high-valent iron-oxo species of a m-nitrido-
bridged dinuclear iron complex of a supramolecular cofacial
a Department of Chemistry, Graduate School of Science, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan. E-mail: yy@chem.nagoya-u.ac.jp,
kentaro@chem.nagoya-u.ac.jp
assembly of a porphyrin and a phthalocyanine connected through
four 24-crown-8-dialkylammonium rotaxanes (Fig. 1d).17,18 These
suggested that peripheral crown ether units of the catalyst
would not interfere the reaction of high-valent iron-oxo species
as long as they can not reach the reaction center. Based on
these considerations, we decided to investigate the electronic
structure of 2+ as well as its catalytic methane oxidation activity
b Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8602, Japan
c JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj04601a
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Fig. 1 (a) Stepwise methane oxidation catalyzed by m-nitrido-bridged iron
phthalocyanine dimers (1 or 1+). (b) Generation of high-valent iron oxo
species from 1 or 1+. (c) Structure of a monocationic m-nitrido-bridged
iron phthalocyanine dimer with eight peripheral 12-crown-4 units (2+).
(d) Catalytic oxidation reactions of methane and ethane by using a
m-nitrido-bridged dinuclear iron complex of a supramolecular cofacial
assembly of a porphyrin and a phthalocyanine connected through four 24-
crown-8-dialkylammonium rotaxanes.
Fig. 2 (a) Synthesis of 2+ꢀIꢁ. (b) 1H-NMR spectrum of 2+ꢀIꢁ in pyridine-d5.
of a m-nitrido-bridged iron phthalocyanine dimer containing Fe(III)
1
and Fe(IV) centers exhibits a significant broadening of H-NMR
signals with paramagnetic shifts due to the S = 1/2 spin in the iron
center.21 In contrast, a monocationic species with two Fe(IV)
centers shows sharp 1H-NMR signals.17,18,21–23 Therefore, we
concluded that 2+ꢀIꢁ contains two Fe(IV) centers interacting with
each other in an antiferromagnetic fashion. The other character-
istic feature of the 1H-NMR spectrum of 2+ꢀIꢁ is that the two
germinal protons of each CH2 group of 12-crown-4 units were
under mild reaction condition in the presence of H2O2 as an
oxidant.
Results and discussion
The synthetic procedure for 2+ is summarized in Fig. 2a. 4,5- observed as two different signals. This is because the chemical
Dicyanobenzo-12-crown-419 was heated in the presence of Fe(OAc)2 environments of these two germinal protons became different in
in N,N-dimethylaminoethanol to obtain Fe(II)-phthalocyanine 3 a cofacial assemblage of two iron phthalocyanines connected to
bearing four 12-crown-4 units in 63% yield. The formation of each other through a nitrogen atom. As for the UV-Vis spectrum
m-nitrido-bridged iron phthalocyanine dimer was performed as of 2+ꢀIꢁ, peaks corresponding to Q-bands of the phthalocyanine
described in our previous report on the synthesis of the rings were observed at 588, 645, and 683 nm (Fig. S4, ESI†).
phthalocyanine dimer with no peripheral substituent.20 First,
A cyclic voltammogram of 2+ꢀIꢁ was measured in a pyꢁridine
3 was heated at 280 1C in 1-chloronaphthalene in the presence solution (200 mM) containing 100 mM of nBu4N+ꢀPF6 and
of excess NaN3 under air. After successive oxidation by iodine compared with that of a monocationic m-nitrido-bridged iron
followed by purification by silica-gel column chromatography phthalocyanine dimer with no peripheral substituents (1+ꢀIꢁ),
and gel permeation chromatography, a monocationic species of as shown in Fig. 3a. Five quasi-reversible 1eꢁ redox waves were
the desired m-nitrido-bridged iron phthalocyanine dimer 2+ꢀIꢁ observed at ꢁ2.07, ꢁ1.81, ꢁ1.57, ꢁ0.66, and 0.22 V vs. Fc/Fc+ in
was obtained in 20% isolated yield as a bluish-green solid.
the potential range from ꢁ2.5 to 0.5 V. Four reductions at
1H-NMR, MALDI-TOF MS, and UV-Vis spectroscopies ꢁ2.07, ꢁ1.81, ꢁ1.57, and ꢁ0.66 V can be assigned as those of
and elemental analysis were performed to characterize 2+ꢀIꢁ. the two iron centers based on a previous report by C. Ercolani
MALDI-TOF MS showed signals at around m/z = 2318.7, et al. on the redox mechanism of 1+.24 One oxidation wave at
which correspond to those of the calculated isotope pattern 0.22 V should be the redox of the phthalocyanine ring. Impor-
1
for 2+ (Fig. S3, ESI†). In the H-NMR spectrum (Fig. 2b), sharp tantly, a comparison of the voltammograms of 1+ꢀIꢁ and 2+ꢀIꢁ
signals were assigned as C4-symmetrical species. A neutral species revealed that all five redox signals showed negative shifts,21 as
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Significant amounts of methanol, formaldehyde, and formic
acid were detected during the course of the oxidation for both
of 2+ꢀIꢁ/SiO2 and 1+ꢀIꢁ/SiO2 as shown in Table 1 and Fig. S5
(ESI†). The fact that the TTNeff was increased suggests that
methane was actually oxidized by these catalysts in this reac-
tion condition. The TTNeff of 2+ꢀIꢁ/SiO2 after 4 h of oxidation
was found to be larger than that of 1+ꢀIꢁ/SiO2 under the same
reaction conditions (Table 1, entries 2 and 6).
However, apparent bleaching of the color of the phthalocya-
nine was observed especially after 6 h only in the case of 2+ꢀIꢁ/
SiO2, indicating that catalyst 2+ gradually decomposed during
the reaction. Actually, the amount of the oxidized products by
2+ꢀIꢁ/SiO2 was also considerably larger than those for 1+ꢀIꢁ/SiO2
even in the absence of methane (under N2) (Fig. S5 and Table
S1, ESI†). This suggests that these oxidized products were also
generated from the catalyst 2+. We attribute that the oxidized
products observed in the reaction by 1+ꢀIꢁ/SiO2 under N2 condi-
tion were originated from the adsorbed organic solvents on SiO2
surface, since it is known that 1+ core is enough stable even in
this reaction condition.15–17 We also performed the oxidation
reaction of 13C-labeled methane by 2+ꢀIꢁ/SiO2 in order to confirm
the origin of the oxidized products. As shown in Fig. S6 (ESI†),
isotopic patterns of the mass spectra of the oxidized products
were clearly changed from those observed in the reactions using
unlabeled methane, indicating that part of these products were
actually originated from methane. Considering the fact that
TTNeff for 2+ꢀIꢁ/SiO2 began to increase after 2 h and reached a
maximum at approximately 6 h (Fig. 4a), the transiently generated
species should have shown methane oxidation activity.
Fig. 3 (a) Cyclic voltammogram of 2+ꢀIꢁ (200 mM) in a pyridine solution
containing 100 mM of nBu4N+ꢀPF6 at room temperature. [scan rate] =
ꢁ
100 mV sꢁ1. (b) Comparison of the redox potentials (V vs. Fc/Fc+) of 1+ꢀIꢁ
(ref. 20) and 2+ꢀIꢁ.
shown in Fig. 3b, clearly indicating the electron-donating
character of the peripheral 12-crown-4 units.
Heterogeneous methane oxidation reactions using silica-
supported catalyst 2+ꢀIꢁ/SiO2 and 1+ꢀIꢁ/SiO2 were performed
in an aqueous solution containing excess H2O2 (189 mM) and
TFA (51 mM) at 60 1C. It is known that 1+ affords high-valent
oxo species (1oxo) in this reaction condition.15–17 The amounts
of the oxidized products (methanol, formaldehyde, and formic
acid) were determined by GC-MS analyses. To evaluate the
methane oxidation activity of the catalysts, the effective total
turnover numbers (TTNeff) defined by equations (i) and (ii) were
calculated. TTNeff is based on the idea that methane is oxidized
in a stepwise manner, as shown in Fig. 1a.
To confirm the possibility of the Fenton-type reaction, in
which the hydroxyl radical (ꢃOH) acts as a reactive species
instead of a high-valent iron-oxo species, we performed an
oxidation reaction in the presence of excess Na2SO3, a radical
scavenger (Table 1, entry 5 and Table S1, entry 5, ESI†).9 As
shown in entry 5 of Table 1, the TTNeff was significantly lowered
in the presence of Na2SO3, indicating that methane oxidation
by the transiently generated species should have proceeded via
a Fenton-type reaction mechanism. In addition, the bleaching of
the color of the catalyst was observed even in this case, implying
that the decomposition of the catalyst center was not suppressed.
Therefore, it is considered that the initially generated iron-oxo
species of 2oxo is unstable and prone to decomposition. Taking these
facts into consideration, we suggest a reaction mechanism of
methane oxidation by 2+ꢀIꢁ/SiO2 as summarized in Fig. 4b. It is
TTNeff = TTN(CH4) ꢁ TTN(N2),
(i)
TTN(CH4) or TTN(N2) = (CMethanol + 2 ꢂ CFormaldehyde
+ 3 ꢂ CFormic acid)/CCat
(ii)
Table 1 Oxidation of methane by m-nitrido-bridged iron phthalocyanine dimer-based catalysts on silica supports
Entry
Catalyst
Reaction Time/h
Gas
Additive
[CH3OH]/mM
[HCHO]/mM
[HCOOH]/mM
TTNeff
1
2
3
4
5
6
2+ꢀIꢁ/SiO2
2+ꢀIꢁ/SiO2
2+ꢀIꢁ/SiO2
2+ꢀIꢁ/SiO2
2+ꢀIꢁ/SiO2
1+ꢀIꢁ/SiO2
2
4
6
8
6
4
CH4 (1.0 MPa)
CH4 (1.0 MPa)
CH4 (1.0 MPa)
CH4 (1.0 MPa)
CH4 (1.0 MPa)
CH4 (1.0 MPa)
—
—
—
—
0.03
0.07
0.15
0.14
0.11
0.04
0
0
0
6
27
26
4
0.11
0.33
0.24
0.05
0
0.03
0.22
0.27
0
100 mM Na2SO3
—
0.07
5
All reactions were performed in the presence of methane (1.0 MPa), H2O2 (189 mM), and TFA (51 mM) in H2O (3.0 mL) containing a silica-
supported catalyst (55 mM as 2+ or 1+). Concentrations of the oxidized products are the mean values of three different reactions. The TTNeff values
were calculated using equations (i) and (ii).
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Fig. 4 (a) Time dependence of TTNeff for methane oxidation by 2+ꢀIꢁ/SiO2. (b) Proposed mechanism of methane oxidation using 2+ꢀIꢁ/SiO2 in an acidic
aqueous solution containing H2O2.
interesting that apparent bleaching was not observed in the catalytic phthalocyanine bearing four 12-crown-4 units (4) is described
methane oxidation reaction using a m-nitrido-bridged dinuclear in the ESI.† A monocationic m-nitrido-bridged iron phthalocya-
iron complex of a supramolecular cofacial assembly of a por- nine dimer with no peripheral substituents (1+ꢀIꢁ) was prepared
phyrin and a phthalocyanine connected through four 24-crown- according to our previous report.20 1H NMR spectra were
1
8-dialkylammonium rotaxanes (S5+ꢀ5Clꢁ in Fig. 1d), suggesting a recorded on a JEOL JNM-ECS400-A (400 MHz for H) spectro-
high-valent iron-oxo species acted stably as a reactive species for meter at a constant temperature of 298 K. Elemental analysis
methane oxidation in this case. We suppose that this difference in was performed on a Yanaco MT-6 analyzer. The absorption
the reaction mechanism might be originated from the difference in spectrum was recorded with a Hitachi U-4100 spectrophoto-
the structures of porphyrinoids contained in the catalysts.
meter in pyridine solutions at 20 ꢄ 0.1 1C in 1.0 cm quartz cells.
MALDI-TOF MS was performed on Bruker Daltonics ultra-
Xtreme using a-CHCA as a matrix.
Conclusions
Synthesis of 2+ꢀIꢁ
We synthesized a novel monocationic m-nitrido-bridged iron phtha-
locyanine dimer with eight 12-crown-4 units (2+ꢀIꢁ). 1H-NMR
studies indicated that 2+ꢀIꢁ contains two iron(IV) ions interacting
with each other in an antiferromagnetic manner because 2+ꢀIꢁ was
observed as a diamagnetic species. The strong electron-donating
ability of the peripheral eight 12-crown-4 units of 2+ was confirmed
by the fact that five redox signals in the cyclic voltammogram of
2+ꢀIꢁ shifted significantly to lower potentials compared to those of
2+ꢀIꢁ. Although we first expected that the high-valent iron-oxo
species of 2+ possesses strong methane oxidation activity, a series
of our methane oxidation experiments suggested that 2+ is prone
to decomposition during the reaction to afford transient species
possessing methane oxidation activity. The introduction of
12-crown-4 units might have destabilized the high-valent iron-
oxo species generated in situ under these reaction conditions.
This result provides valuable information for designing a
potent methane oxidation catalyst based on m-nitrido-bridged
iron phthalocyanine dimers.
4 (156 mg, 135 mmol), NaN3 (86.0 mg, 1.32 mmol), and
1-chloronaphthalene (5.2 mL) were placed in a 50 mL flask.
The mixture was refluxed under air at the bath temperature of
280 1C for 30 minutes. The reaction mixture was diluted
with hexane (50 mL) and filtered. The resulting blackish green
solid was washed with hexane (50 mL) and H2O (100 mL).
After dried under vacuum at 60 1C, the crude product was
dissolved in pyridine (500 mL). Iodine (122 mg, 0.962 mmol)
was added. The resulting solution was stirred for 14 hours
at room temperature, followed by evaporation of the volatile
compounds. The residue was washed with Et2O (1 L) to remove
the remaining iodine, and then, dried under reduced pressure.
The crude product was purified by silica gel column chromato-
graphy (4.0 cmf ꢂ 10.5 cm, hexane : CHCl3 : pyridine = 1 : 2 : 3)
to give crude solid (88.7 mg), which was further purified by
GPC (eluent: pyridine) to give the desired compound 6 as a
dark green solid (35 mg, 15 mmol, 20%). 1H-NMR (400 MHz,
pyridine-d5/TMS): d = 9.17 (s, 16H), 5.13–5.10 (m, 16H),
4.72–4.68 (m, 16H), 4.34–4.29 (m, 16H), 4.20–4.17 (m, 32H),
4.02–3.99 (m, 16H). MALDI-TOF MS: m/z = 2318.64: calcd for
Experimental
General
C
112H112Fe2N17O32 ([2]+) found: 2318.69. Anal. calcd for
C112H114Fe2IN17O33 (2+ꢀIꢁꢀH2O): C; 54.58, H; 4.66, N; 9.66,
All reagents and solvents were purchased at the highest com- found: C; 54.45, H; 4.65, N; 9.74 (0.13% error).
mercial quality available and used without further purification,
Preparation of silica-supported catalyst (2+ꢀIꢁ/SiO2)
unless otherwise stated. 4,5-Dicyano-benzo-12-crown-4 (3) was
prepared by slight modification of the procedure reported by C. 2+ꢀIꢁ (41.6 mg, 16.9 mmol) was dissolved in 30 mL of pyridine.
Ma et al. as described in the ESI.† 25 Synthesis of an iron After the addition of silica gel (2.84 g) to the solution, the mixture
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containing 100 mM TBAPF6 in a standard one-component cell
under an N2 atmosphere equipped with a 3 mm-O.D. glassy carbon
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the reaction mixture was filtrated through a disposable
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Agilent 7890A equipped with JEOL JMS-T100GCV, detection: EI,
column: Agilent DB-WAX UI, external standard: isovaleric
acid (5 mM), temperature conditions: initial: 70 1C to 220 1C
(10 1C minꢁ1) – hold (5 min)). The yields of methanol and
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was financially supported by a JSPS KAKENHI Grant-
in-Aid for Scientific Research (A) (Number 19H00902) to KT, a
JSPS KAKENHI Grant-in-Aid for Scientific Research (B) (Number
19H02787), JST PRESTO (Number JK114b) for YY.
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