Angewandte
Chemie
DOI: 10.1002/anie.201209846
Methane Oxidation
Efficient Oxidation of Methane to Methanol by Dioxygen Mediated by
Tricopper Clusters**
Sunney I. Chan,* Yu-Jhang Lu, Penumaka Nagababu, Suman Maji, Mu-Cheng Hung,
Marianne M. Lee, I-Jui Hsu, Pham Dinh Minh, Jeff C.-H. Lai, Kok Yoah Ng,
Sridevi Ramalingam, Steve S.-F. Yu,* and Michael K. Chan*
Methane oxidation is extremely difficult chemistry to perform
that these model tricopper complexes can mediate efficient
catalytic oxidation of methane to methanol as well.
À
in the laboratory. The C H bond in CH4 has the highest bond
energy (104 kcalmolÀ1) amongst organic substrates. In nature,
the controlled oxidation of organic substrates is mediated by
an important class of enzymes known as monooxygenases and
dioxygenases,[1] and the methane monooxygenases are unique
in their capability to mediate the facile conversion of methane
to methanol.[2,3] With a turnover frequency approaching 1 sÀ1,
the particulate methane monooxygenase (pMMO) is the most
efficient methane oxidizer discovered to date. Given the
current interest in developing a laboratory catalyst suitable
for the conversion of methane to methanol on an industrial
scale, there is strong impetus to understand how pMMO
works and to develop functional biomimetics of this enzyme.
pMMO is a complex membrane protein consisting of three
subunits (PmoA, PmoB, and PmoC) and many copper
cofactors.[3] Inspired by the proposal that the catalytic site
might be a tricopper cluster, we have recently developed
a series of tricopper complexes that are capable of supporting
facile catalytic oxidation of hydrocarbons.[4,5] We show herein
The oxidation of CH4 mediated by the tricopper complex
[CuICuICuI(7-N-Etppz)]1+ in acetonitrile (ACN), where 7-N-
Etppz corresponds to the ligand 3,3’-(1,4-diazepane-1,4-
diyl)bis[1-(4-ethylpiperazine-1-yl)propan-2-ol], is summar-
ized in Figure 1A. A single turnover (turnover number;
TON = 0.92) is obtained when this CuICuICuI complex is
activated by excess dioxygen in the presence of excess CH4
(Figure 1B). The reaction is complete within ten minutes,
clearly indicating that the oxidation is very rapid. In
accordance with the single turnover, the kinetics of the
overall process is pseudo first-order with respect to the
concentration of the fully reduced tricopper complex with
a rate constant k1 = 0.065 minÀ1 (Figure 1B, inset). If we
assume that the kinetics is limited by the dioxygen activation
of the CuICuICuI cluster with the subsequent O-atom transfer
to the substrate molecule being rapid, then k1 = k2·[O2]0, and
from the solubility of oxygen in ACN at 258C (8.1 mm),[6] we
obtain the bimolecular rate constant k2 of 1.33 ꢀ 10À1mÀ1 sÀ1
for the dioxygen activation of the CuICuICuI cluster. This
second-order rate constant is similar to values that we have
previously determined for the dioxygen activation of other
model tricopper clusters at room temperature.[7,8]
[*] Prof. Dr. S. I. Chan, Y.-J. Lu, Dr. P. Nagababu, Dr. S. Maji,
M.-C. Hung, P. D. Minh, J. C.-H. Lai, K. Y. Ng, Prof. Dr. S. S.-F. Yu
Institute of Chemistry, Academia Sinica
Nankang, Taipei 11529 (Taiwan)
The process can be made catalytic by adding the
appropriate amounts of H2O2 to regenerate the spent catalyst
after O-atom transfer from the activated tricopper complex to
CH4. This multiple-turnover reaction is depicted in Figure 1C.
In these experiments, the [CuICuICuI(7-N-Etppz)]1+ catalyst
is activated by O2 as in the single-turnover experiment
described earlier, but the spent catalyst is regenerated by two-
electron reduction by a molecule of H2O2 (Figure 2A).
Because the effective turnover number (TON), or the total
equivalent of products formed over the time course of the
experiment, peaks at approximately six when the turnover is
initiated with 20 equivalents of H2O2, it is evident that
abortive cycling begins to kick in when the steady-state
concentration of the H2O2 concentration exceeds approx-
imately ten equivalents. When the steady-state H2O2 concen-
tration is above this level, reductive abortion of the activated
catalyst becomes competitive with the O-atom transfer to
methane to produce methanol. In this case, the rate of O-atom
transfer is limited by the relatively low solubility of CH4 in
ACN under ambient conditions of temperature and pressure
(Figure 2B).
E-mail: sunneychan@yahoo.com
M. M. Lee, S. Ramalingam, Prof. Dr. M. K. Chan
Department of Chemistry and Biochemistry, Ohio State University
Columbus, OH 43210 (USA)
E-mail: chan@chemistry.ohio-state.edu
Prof. Dr. I.-J. Hsu
Department of Molecular Science and Engineering
National Taipei University of Technology
Taipei 10608 (Taiwan)
[**] This work is supported by funds from Academia Sinica and Ohio
State University (OSU), and grants from the National Science
Council of the Republic of China (NSC 95-2113M-001-046, 97-
2113M-001-027, and 98-2113M-001-026 to S.I.C.; 96-2627M-001-
006 and 97-2113M-001-006-MY3 to S.S.F.Y.). S.R. was supported by
a summer research fellowship from the National Science Founda-
tion, USA (DBI 0750631). We acknowledge the assistance of Dr. Jyh-
Fu Lee and his staff at the National Synchrotron Radiation Research
Center, Hsinchu, Taiwan with the X-ray absorption measurements.
Supporting information for this article, including experimental
details on the synthesis and characterization of the tricopper-
peptide complexes, the biomimetic tricopper complexes, and the
TEL-SAM protein and studies of the methane oxidation, is available
The [CuICuICuI(7-N-Etppz)]1+ complex also mediates the
catalytic oxidation of normal C2–C6 alkanes (data not shown)
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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