Bio-inspired oxidation of methane in water catalyzed by N-bridged diiron phthalocyanine complex

Article abstract of DOI:10.1039/b804405h

A stable μ-nitrido diiron phthalocyanine activates H₂O ₂ to oxidize CH₄ in water at 25-60 °C to methanol, formaldehyde and formic acid as evidenced using ¹³C and ¹⁸O labelling. The Royal Society of Chemistry.

Full text of DOI:10.1039/b804405h

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Bio-inspired oxidation of methane in water catalyzed by N-bridged diiron
phthalocyanine complexw
Alexander B. Sorokin,*a Evgeny V. Kudrika and Denis Bouchub
Received (in Cambridge, UK) 14th March 2008, Accepted 15th April 2008
First published as an Advance Article on the web 2nd May 2008
DOI: 10.1039/b804405h
A stable l-nitrido diiron phthalocyanine activates H2O2 to
oxidize CH4 in water at 25–60 1C to methanol, formaldehyde
and formic acid as evidenced using 13C and 18O labelling.
dimeric form (Fe–O–Fe fragment) exhibited better catalytic
properties than monomer FePc.13g This finding prompted us
to study binuclear MPc as oxidation catalysts.
m-Nitrido iron phthalocyanine complex (FePc)2N contains
two equivalent iron centres with a formal +3.5 oxidation state
bridged via nitrogen (Fig. 1).14 (FePc)2N is insoluble in
organic solvents which makes its purification and use in
catalysis difficult. To overcome this drawback we prepared
iron m-nitrido tetra-tert-butylphthalocyanine, (FePctBu4)2N.15
Stable and inert m-nitrido bridged iron complexes have never
been evoked as oxidation catalysts although the first m-nitrido
iron porphyrin complex was described in 1976.16
A selective low temperature oxidation of methane is a long
standing challenge.1 In Nature, methane monooxygenase en-
zymes (MMO) transform CH4 to CH3OH in water under
physiological conditions.2–4 Considerable research has been
dedicated to modelling of MMO. Complexes mimicking a
structural organisation and spectral features of MMO have
been described,5,6 but real functional chemical models capable
of oxidizing CH4 are still to be created. Along with biomimetic
studies, a low temperature oxidation of CH4 has been inspired
by the work of Shilov and co-workers on CH4 activation on
Pt.7,8 Perspectives and challenges of C–H bond activation at
metal centres have been reviewed.9 Salts of Hg, Tl, Pd and Au
promoted oxidation of CH4 in oleum or H2SO4.10 Stabiliza-
tion of Pt or Pd by appropriate ligands and using oleum as the
terminal oxidant allowed a catalytic process.11 The CH4
oxidation in oleum containing Pt(II) bipyrimidine complex at
220 1C provided the selective formation of methyl bisulfate.11a
The combination of this complex with polyoxometallate
allowed oxidation of CH4 to CH3OH and CH3CHO with
TON B30.12 Oxidative condensation of CH4 to CH3COOH in
a PdSO4–H2SO4 system at 180 1C provided TON of 5–18.11b
Harsh conditions (strong concentrated acids, high tempera-
ture) are often applied to oxidize CH4.
We have observed that (FePctBu4)2N interacts with biologi-
cally and ecologically relevant H2O2. The addition of H2O2 to a
solution of (FePctBu4)2N changed UV–Vis and EPR spectra.15
(FePctBu4)2N exhibited a signal at g = 1.99 indicative of one
unpaired electron and a low spin Fe(III) state. After addition of
H2O2 at 25 1C we detected a strong signal at g = 4.25 and a
feature at g = 8.16 along with an enlarged signal at g = 2.07 and
a small signal at g = 1.99 of the (FePctBu4)2N left.15 EPR
parameters of new species are close to those of high and low spin
Fe(III) peroxo complexes.17 The diiron structure was kept intact
as evidenced by positive electrospray ionization mass spectro-
metry (ESI-MS). ESI-MS spectra of (FePctBu4)2N in MeCN
containing H2O2 were remarkably clean.15 Along with a signal at
m/z = 1599.8 of the parent (FePctBu4)2N, a signal at m/z =
1615.9 corresponding to 16O-(FePctBu4)2N was detected. When
H218O2 was used instead of H216O2 the spectrum had a signal at
m/z = 1617.9 due to 18O-(FePctBu4)2N (Fig. 2). Isotopic patterns
of the molecular peak obtained with H216O2 and H218O2 were
identical to theoretical ones. When cyclohexene was added to the
solution containing (FePctBu4)2N and H2O2 the signal of
O-(FePctBu4)2N practically disappeared in favour of initial
(FePctBu4)2N.15 This result strongly suggests that oxygen origi-
nating from H2O2 is located at Fe, thus oxidizing cyclohexene,
and not at a phthalocyanine moiety. No signal of monomer
In contrast with organometallic CH4 activation, MMO
proceeds via a different mechanism, by creating a very strong
oxidizing diiron species able to attack a C–H bond in CH4. An
essential feature of MMO is an active site containing two iron
centres. However, for diiron synthetic models of MMO oxida-
tion of CH4 have yet to be achieved despite considerable
efforts which were concentrated on non-heme complexes.5
Notably, the use of binuclear porphyrin-like complexes for
CH4 oxidation has so far been completely neglected. Our
studies on metallophthalocyanines (MPc) as catalysts for clean
oxidation processes13 showed that FePc supported in m-oxo
a Institut de Recherches sur la Catalyse et l’Environnement de Lyon
´
(IRCELYON), UMR 5256, CNRS-Universite Lyon, 2, av. A.
Einstein, 69626 Villeurbanne Cedex, France. E-mail:
alexander.sorokin@ircelyon.univ-lyon1.fr; Fax: +33 47244539;
Tel: +33 472445337
´
Universite Lyon 1, UMR 5246, Centre Commun de Spectroscopie de
Masse, 43, bd du 11 Novembre, 69622 Villeurbanne Cedex, France
b
Fig. 1 m-Nitrido-bridged iron phthalocyanine complexes. R = H or
C(CH3)3 for m-nitrido complex of iron phthalocyanine (FePc)2N and
iron tetra-tert-butylphthalocyanine (FePctBu4)2N, respectively.
w Electronic supplementary information (ESI) available: Syntheses
and characterization of complexes, analytical data. See DOI:
10.1039/b804405h
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Fig. 3 Oxidation of methane using 13C and 18O labelling.
H13COOH corresponding to the isotopic composition of
CH4.15 Methanol was also detected and contained 13CH3OH
and CH3OH in 1 : 1 ratio. Acetic acid and acetone didn’t
contain 13C label, suggesting that they were not derived from
CH4 and were probably formed from the oxidation of the
catalyst. The stability of CH3OH under the reaction condi-
tions was tested using 0.01 M CH3OH solution in water. The
conversion of CH3OH was 82% after 1 h and the HCOOH
yield was 80%. CH3OH is the primary product of oxidation
which is converted to CH2O and HCOOH under reaction
conditions (Fig. 3).
Fig. 2 ESI-MS spectra of oxo-(FePctBu4)2N prepared in the reaction
between (FePctBu4)2N and H216O2 or H218O2 (MeCN, 25 1C): iso-
tope distribution patterns of the molecular peak cluster for
16O-(FePctBu4)2N (A) and 18O-(FePctBu4)2N (B), and simulated iso-
tope distribution patterns for these species (C and D, respectively).
FePctBu4 was detected during ESI-MS experiments, indicating
the stability of the Fe–N–Fe structure in the presence of H2O2.
The first catalytic tests showed very strong oxidizing proper-
ties of the (FePctBu4)2N–H2O2 system and prompted us to try
the oxidation of CH4. Initial tests with CH4 were performed in
MeCN containing 0.1 mM (FePctBu4)2N and 67 mM H2O2.
Formic acid was the main product along with traces of acetic
acid, methanol, acetone and MeCN oxidation products.
To distinguish between the oxidation of CH4 and the possible
oxidation of CH3CN, the reaction was performed in CD3CN.
GC–MS analysis of formic acid obtained in 6 mM concentration
showed 68% of HCOOH and 32% of DCOOH, thus indicating
parallel oxidation of CH4 and CD3CN in 2 : 1 ratio.15 Kinetic
analysis shows that this ratio is not compatible with a free radical
mechanism of oxidation.15 Reaction constants of OHꢁ with CH4
and MeCN are 3.80 Â 106 and 1.14 Â 107 MÀ1 sÀ1, respectively.
If OHꢁ were involved, the ratio products derived from CH4/
products derived from MeCN would be equal to 3.80 Â 106 Â 0.02
M/1.14 Â 107 Â 19.17 M = 1 : 2880 where 0.02 and 19.17 M are
the concentrations of CH4 and MeCN, respectively. The catalytic
oxidation of CH4 occurred with a high turnover number of 40.8.
However, the situation is complicated by the oxidation of the
organic solvent which is difficult to avoid in the presence of active
species strong enough to oxidize CH4.
When the reaction was performed in 97% H218O, CH3OH
didn’t contain 18O, indicating that its oxygen originated from
H216O2.15 Formic acid was composed of 18% of HC16O2H,
59% of HC16O18OH and 23% of HC18O2H. The exchange of
intermediate hydrated formaldehyde and formic acid with
H218O could result in introduction of 18O in HCOOH (Fig. 3).
The oxidation of CH4 was tested at different temperatures.
Remarkably, the oxidation of CH4 was still efficient even at
25 1C, providing HCOOH with TON = 13 (Table 1, run 1). At
40–50 1C HCOOH and CH2O were obtained in B2 : 1 ratio
(runs 2 and 3). At higher temperature the amount of CH2O
diminished in favour of HCOOH. The activity of the catalyst
was similar between 40 and 80 1C, providing 26–32 turnovers.
Based on the results obtained by UV–Vis, EPR, ESI-MS
techniques and in labelling experiments, we propose a tenta-
tive mechanism of the oxidation of CH4 (Fig. 4). In the first
step, (FePctBu4)2N coordinates H2O2 to form hydroperoxo
complex FeIVNFeIIIOOH which is probably in equilibrium
with the deprotonated form FeIVNFeIIIOOÀ. (FePctBu4)2N
oxo complex could be formed from FeIVNFeIIIOOH via
heterolytic cleavage of the O–O bond. Recent DFT calcula-
tions showed that water favoured the formation of non-heme
HO–FeVQO species via water-assisted O–OH bond clea-
vage.18 Recently, chemical and spectroscopic evidence for an
FeV–oxo complex was obtained.19 Otherwise, a homolytic
cleavage of the O–O bond in FeIVNFeIIIOOH would form
FeIVNFeIVQO with formation of OH radical. However,
this pathway is not compatible with a high stability of the
(FePctBu4)2N core in the presence of H2O2 and with observed
rates of CH4 vs. MeCN oxidation. If OHꢁ were to be formed,
(FePctBu4)2N should be rapidly destroyed. High concentra-
tions of HCOOH obtained in CH4 oxidations and the relative
reactivity of CH4 and MeCN found in homogeneous condi-
tions are also not compatible with free radical chemistry.15
The heterolytic cleavage of the O–O bond in FeIVNFeIIIOOH
complex and the formation of putative very strong oxidizing
FeIVNFeVQO species should be favoured in the presence of acid
by the protonation of peroxide oxygen. Indeed, a significant
The best solution to avoid uncertainties due to solvent
oxidation is to use water, an inert, green and biologically
relevant solvent. With this aim, m-nitrido complex was sup-
ported onto silica.15 Heterogeneous oxidations of CH4 were
performed in pure H2O. The CH4 oxidation was unambigu-
ously evidenced using labelled 13CH4 in D2O at 60 1C. When
the mixture of 53% of CH4 and 47% of 13CH4 was used for
oxidation, the 13C NMR spectrum of the final reaction mixture
showed two signals at 81.6 and 165.5 ppm attributed to
hydrated formaldehyde 13CH2(OH)2 and H13COOH, respecti-
vely.w Neither 13CH3OH (49 ppm) nor 13CO2 (125 ppm) signals
were found in this spectrum. CH2O and HCOOH were there-
fore the main products of CH4 oxidation. GC–MS analysis of
the reaction using the same CH4/13CH4 ratio (53/47)
run in H2O (to avoid an artefact due to possible H/D exchange
in HCOOH with D2O) indicated 56% of HCOOH and 44% of
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Table 1 Oxidation of methane by H2O2 in water catalyzed by
supported (FePctBu4)2N complexa
similar structural feature, binuclear iron sites. However, the
coordination spheres of these sites are different. In MMO, the
diiron centre contains four glutamate (carboxyl ligand) and
two histidine (N-ligand) residues and two iron ions are con-
nected via the carboxylate of glutamate and water or hydro-
xide fragments. Iron ions in m-nitrido dimer are coordinated
with four nitrogen atoms of planar exogenous phthalocyanine
ligands which are nevertheless structurally related to endogen-
ous porphyrin. Further research is necessary to provide deeper
insights into this novel fascinating chemistry.
[HCOOH]/
Run T/1C mM
[CH2O]/
TONHCOOH mM
Total
TONCH O TONb
2
1
2
3
4
5
6
7c
25
40
50
60
70
80
60
6.0
8.6
9.2
10.5
11.7
12.8
69.0
(34.1)
13.0
18.6
21.0
22.8
25.2
27.3
134.6
(72.8)
0
0
39.0
76.6
84.4
74.8
79.0
84.1
436.8
4.8
4.7
1.5
0.8
0.5
7.6
10.4
10.7
3.2
1.7
1.1
16.5
a
Conditions: 32 bars CH4; 2 mL H2O; catalyst, 0.925 mmol (0.875
mmol for run 3); 678 mmol H2O2; reaction time 20 h (48 h for
Notes and references
b
1 R. H. Crabtree, Chem. Rev., 1995, 95, 987.
2 M. Merkx, D. A. Kopp, M. H. Sazinsky, J. L. Blazyk, J. Muller
¨
run1). Total TON was calculated as 3 Â HCOOH/catalyst + 2 Â
c
CH2(OH)2/catalyst. In 0.1 M H2SO4; 678 mmol H2O2 were added at
reaction times 0 and 16 h. Values in parentheses were measured before
the second addition of H2O2.
and S. J. Lippard, Angew. Chem., Int. Ed., 2001, 40, 2782.
3 M.-H. Baik, M. Newcomb, R. A. Friesner and S. J. Lippard,
Chem. Rev., 2003, 103, 2385; E. G. Kovaleva, M. B. Neibergall,
S. Chakrabarty and J. D. Lipscomb, Acc. Chem. Res., 2007, 40,
475.
improvement of the catalytic activity was observed in the
presence of 0.1 M H2SO4. The TONHCOOH was increased by
more than a factor of 3 to attain 72.8. After completion of the
first reaction, a new portion of H2O2 was added directly to the
reaction mixture. Remarkably, the catalytic system retained
practically the same catalytic activity in the second cycle (Table 1,
run 7), indicating a high stability of the catalyst and even a
possibility of recycling. The catalyst exhibits a very high perfor-
mance: more than 150 moles CH4 per mole of catalyst were
oxidized to useful products. This activity is far higher than that of
most published systems operating via CH4 activation7–10,11b,12
and approaches that of the most efficient so far, Periana’s system
based on Pt(II) bipyrimidine complex in oleum at 220 1C.11a Total
TON = 437 and 30–50% product yields on H2O2 were attained
using (FePctBu4)2N. This catalytic system shows several attrac-
tive features: the clean oxidant (H2O2) and reaction medium
(H2O), and the fact that solid catalyst can easily be separated by
filtration. In contrast to the much more expensive porphyrin and
non-heme complexes, phthalocyanines can be accessible in bulk
quantities.20
It should be noted that the Fe–N–Fe unit of (FePctBu4)2N is
essential for the catalytic activity. Terminal nitrido ligand
stabilizes FeV and even FeVI states in mononuclear cyclam
complexes.21 One can suggest that m-nitrido ligand in a diiron
complex could also stabilize ultra high valent Fe states.
Mononuclear FePctBu4 and its diiron m-oxo (Fe–O–Fe) and
m-carbido (FeQCQFe) complexes15 showed no oxidation of
CH4. m-Nitrido diiron complex and soluble MMO share a
4 L. Shu, J. C. Nesheim, K. Kauffmann, E. Munck, J. D. Lipscomb
¨
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8 A. E. Shilov, Activation of Saturated Hydrocarbons by Transition
Metal Complexes, Reidel, Dordrecht, 1984.
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Fig. 4 Proposed mechanism of the formation of active species in the
(FePctBu4)2N–H2O2
system
and
oxidation
of
methane.
FeIV–N–FeVQO stands for species having 2 redox equivalents above
the FeIIIFeIV state.
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2564 | Chem. Commun., 2008, 2562–2564