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Low-Temperature Activation of Methane It also Works Without a Transition Metal.

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Angewandte
Chemie
Methane Activation
DOI: 10.1002/anie.200601273
Low-Temperature Activation of Methane: It also
Works Without a Transition Metal**
Detlef Schr
der* and Jana Roithov*
Dedicated to Professor Antonn Holý
on the occasion of his 70th birthday
The activation of methane at ambient conditions and its
conversion into more valuable feedstocks is a challenge for
future, because it would permit a much more efficient use of
methane from either geological deposits or biogenic sources.[1] Most low-temperature routes for the activation of
methane are based on transition-metal catalysts, and particularly promising approaches involve the functionalization of
methane with the aid of platinum complexes.[2] While these
systems achieve the activation of methane under comparably
mild conditions, they are far removed from possible future
applications in large-scale processes because of the expensive
catalysts or because of the nature of the products.[3]
A promising catalyst for the low-temperature activation
of methane is Li+ ion doped magnesium oxide, although this
still needs considerable improvement before large-scale
applications would be feasible.[1, 4] This improvement is the
point where basic research can complement applied science
by means of providing a more detailed understanding of the
underlying elementary steps at a molecular level. Accordingly, the reactions of diatomic MgO as well as several model
clusters have been studied extensively by means of theory.[5–9]
Theory strongly suggests that the cationic species [MgO]+
should effectively activate methane,[10, 11] and also for neutral
MgO the computed activation barrier is rather low.[9] Accordingly, the elementary steps in the chemistry of magnesium and
its oxides have received considerable attention in several
experimental studies,[12–19] yet a decisive route for the
generation of ionic species such as [MgO]+ in amounts
[*] Dr. D. Schrder, Dr. J. Roithov
Institute of Organic Chemistry and Biochemistry
Academy of Science of the Czech Republic
Flemingovo n m. 2, 16610 Prague 6 (Czech Republic)
Fax: (+ 420) 220-183-583
E-mail: detlef.schroeder@uochb.cas.cz
jana.roithova@uochb.cas.cz
Dr. D. Schrder
Institut f:r Chemie
Technische Universit=t Berlin
Strase des 17. Juni 135, 10623 Berlin (Germany)
Fax: (+ 49) 303-142-1102
[**] This work was supported by the Grant Agency of the Academy of
Sciences of the Czech Republic (Nr. KJB4040302) and the Deutsche
Forschungsgemeinschaft (Sfb 546). The authors thank Pavel Jungwirth for inspiring discussions. D.S. very much appreciates the
continuous and grateful support of his research by Prof. Helmut
Schwarz (TU-Berlin, Berlin (Germany)).
Angew. Chem. Int. Ed. 2006, 45, 5705 –5708
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5705
Communications
sufficient for subsequent reactivity studies has not been
achieved to date.
Herein, we describe a new method for the generation of
gaseous [MgO]+ ions by means of electrospray ionization
(ESI)[20] of aqueous magnesium(II) nitrate under enforced
ionization conditions and address their gas-phase reactivity
for the very first time.[21–23] Under mild conditions, ESI of
Mg(NO3)2 affords cations of the type [Mg(NO3)(H2O)n]+ with
n up to 4 as the leading mononuclear species.[24] Increasingly
harsher ionization conditions lead to consecutive losses of the
water ligands to give bare [Mg(NO3)]+ ions, which—upon
further activation—undergo subsequent fragmentation to
afford [MgO]+ concomitant with neutral NO2 as the major
products.[25, 26] Thereby, ESI provides a straightforward route
for the generation of gaseous [MgO]+ in yields fully sufficient
for reactivity studies.
When mass-selected [MgO]+ is allowed to interact with
methane (Figure 1), the reaction in Equation (1) occurs,
½MgOþ þ CH4 ! ½MgOHþ þ CH3
ð1Þ
which proceeds with a rate constant of k = (3.9 1.3) :
1010 cm3 s1, corresponding to about 40 % of the gas-kinetic
collision rate. Likewise, [MgOD]+ is formed in the presence of
the deuterated methanes CH2D2 and CD4, and a neutral-gain
spectrum (Figure 1 B)[27, 28] reveals the expected isotope
pattern for [MgO]+ as the only ion present which is able to
activate methane. From the ratio of the products [MgOH]+
and [MgOD]+ in the reaction with CH2D2, an intramolecular
kinetic isotope effect (KIE) of 2.1 0.1 can be derived. A
value of similar magnitude has been observed in the reaction
of CH2D2 with [MoO3]+ (KIE = 2.0),[29] whereas in the
[V4O10]+/CH2D2 system—which can be regarded as a prototype of a simple hydrogen-atom abstraction mechanism—the
KIE amounts to only 1.35,[30] and is thus very similar to a KIE
of about 1.3 for the H(D)-atom abstraction from methane by
Figure 1. A) Ion/molecule reactions of mass-selected [MgO]+ with
a) CH4, b) CH2D2, and c) CD4. The major reaction corresponds to
H(D)-atom abstraction to afford [MgOH]+ and [MgOD]+, respectively.
B) a neutral-gain spectrum with Dm = 2 in the presence of CH2D2,
displaying the natural isotope pattern of [MgO]+, as expected. C) The
dependences of the [MgO]+ conversion (X) on the interaction energy
Elab (top) and the methane pressure (bottom). The small signal of
[MgOH]+ in the reaction with CD4 (Figure 1 A c) is due to hydrogenabstraction from water present in the background.
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free OH radicals.[31] Not surprisingly, however, the intermolecular KIE in the reaction of [MgO]+ with CH4 or CD4 is
significantly lower (KIE = 1.3 0.2).[32]
Additional mechanistic evidence concerns the rapid
decrease of the conversion at elevated collision energies as
well as the linear pressure dependence, which are both fully
consistent with the occurrence of a thermal ion/molecule
reaction (Figure 1 C). In addition to hydrogen-atom abstraction from methane, a small amount of oxygen-atom transfer is
observed [Eq. (2)] with a branching ratio below 2 % for all
½MgOþ þ CH4 ! ½Mgþ þ CH3 OH
ð2Þ
methane isotopologs. A trace amount of [MgCH2]+ + H2O is
also observed, but this channel is close to the detection limit
and therefore not pursued any further. Summarizing the
experimental findings, it is clear that bare [MgO]+ ions can
efficiently activate methane with H-atom abstraction as the
major pathway and O-atom transfer as only a minor route.
Further insight is gained by theoretical calculations of the
[MgCH4O]+ system. While previous theoretical work
exist,[10, 11] the experimental findings reported herein,
prompted us to reconsider the [MgCH4O]+ surface using
still limited, but yet somewhat more elaborate theoretical
methods. According to our theoretical calculations,[33] [MgO]+
ion forms a remarkably strong encounter complex in which
the methane is coordinated to the positively charged metal
center. Note that the spin is almost exclusively located at the
oxygen, such that the binding situation might be described as
CH4···Mg2+···OC, thereby mimicking the character of an
oxygen-centered radical which is considered to be crucial in
this type of methane activation.[1–4] The CH bond activation
of methane can then proceed via a transition structure (TS)
which is energetically close to the entrance channel
([MgO]+ + CH4). Furthermore, this TS also is the highest
point along the reaction coordinate of the reactions in
Equations (1) as well as (2), and it thus represents the ratedetermining step en route to product formation (Figure 2).
From the TS, the reaction proceeds to an intermediate,
which is best described as the complex of a methyl radical
with an [MgOH]+ ion; fully consistent with this description,
the spin is entirely located on the methyl group. The
intermediate CH3···[MgOH]+ also serves as the branch point
for product formation in that either the methyl radical is lost
directly or—in a long-range migration—the methyl group
undergoes a rebound with the hydroxy group located at
magnesium to afford the methanol/Mg+ complex as the global
minimum of the [MgCH4O]+ surface; this complex can
eventually dissociate into Mg+ + CH3OH. Even though
oxygen-atom transfer, Equation (2), is overall considerably
more exothermic than hydrogen abstraction, Equation (1),
the latter can compete very efficiently because the direct
dissociation of intermediary CH3···[MgOH]+ is entropically
preferred, once the rate-determining TS is surpassed. The
theoretical calculations thereby fully confirm present and
previous experimental observations:
1) The occurrence of a thermal ion/molecule reaction with a
rate on the order of magnitude of the gas-kinetic collision
rate, but yet below unit efficiency, is in harmony with the
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 5705 –5708
Angewandte
Chemie
simple cluster unit of [MgO]+.[36] Herein, we thus report
preliminary results about the gas-phase reactivity of this small
cluster ion which will the basis of forthcoming studies. Not
unexpectedly, [Mg2O2]+ turns out to be less reactive than
[MgO]+ itself and does not activate methane. Nevertheless,
[Mg2O2]+ still is able to activate small hydrocarbons, such as
propane and butane [Eq. (3) and (4) with n = 3 and 4,
Figure 2. Potential-energy surface (in eV) for the reaction of [MgO]+
with CH4 calculated at the MP2/6-311 + G(2d,2p) level of theory;
selected bond lengths [I]. The encircled structures depict the rearrangements occurring along the reaction coordinate. Note that calculations using the B3LYP hybrid method lead to qualitatively similar
results.
predicted rate-determining TS being situated only about
0.1 eV below the entrance channel of separated [MgO]+ +
CH4.
2) The observed distribution between the reactions in
Equations (1) and (2) is fully consistent with the computed
surface.
3) Last but not least, the calculated exothermicities for the
reactions in Equations (1) and (2) are in reasonable
agreement with literature data.[12, 13, 17, 34, 35]
The most important result of the theoretical investigations
is, however, that the CH bond activation cannot just be
described in terms of a simple hydrogen-atom abstraction
process. Instead, throughout the entire reaction coordinate
from the encounter complex to the CH3···[MgOH]+ intermediate a significant interaction exists between the magnesium center and the carbon atom, which favorably assists C
H bond activation. In contrast, the transition structure for a
hydrogen-atom abstraction by the oxygen atom in direct endon attack of the CH bond is only 0.01 eV below the entrance
channel and thus cannot compete with the mechanism shown
in Figure 2 in which the process is assisted by the MgC
interaction. Consequently, the consideration of Li+ ion doped
magnesium oxide as a mere hydrogen-atom abstractor may be
regarded as an oversimplification. The assisted mechanism
proposed herein offers a strategy to eventually facilitate the
activation of methane by deliberate incorporation of Lewis
acidic sites.
In a more general perspective, the question arises as to
whether the gas-phase studies of diatomic [MgO]+ described
herein can serve as models for the reactions occurring in
methane activation by heterogeneous catalysts. Fortunately,
the ESI conditions described above also permit the formation
of the cluster ion [Mg2O2]+, which can be regarded as the most
Angew. Chem. Int. Ed. 2006, 45, 5705 –5708
½Mg2 O2 þ þ Cn H2nþ2 ! ½Mg2 O2 Hþ þ Cn H2nþ1
ð3Þ
½Mg2 O2 þ þ Cn H2nþ2 ! ½Mg2 O2 H2 þ þ Cn H2n
ð4Þ
respectively], though with a rate about one order of
magnitude lower than for diatomic [MgO]+.
The ability of [Mg2O2]+ to activate CH bonds in general
and the occurrence of hydrogen-atom abstraction in particular [Eq. (3)] suggest that the radical character of the oxygen
atom also is maintained in the dinuclear cluster. While
clustering is associated with some decrease of reactivity, the
overall reactivity pattern is thus maintained. The cluster ion
[Mg2O2]+ is less reactive than the monomer, which can be
ascribed to the lower positive charge of the magnesium
centers in the cluster, resulting in a decreased energy gain
upon coordination of methane and hence an increased barrier
to CH activation. Therefore, future strategies for the lowtemperature activation of methane should not only focus on
the generation of particularly reactive oxygen-centered
radicals capable of hydrogen-atom abstraction, but also
consider the additional energy benefit which can be gained
by metal–carbon interactions.
Received: March 31, 2006
Published online: July 21, 2006
.
Keywords: ab initio calculations · hydrogen abstraction ·
magnesium · mass spectrometry · methane activation
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Angew. Chem. Int. Ed. 2006, 45, 5705 –5708
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