DOI: 10.1002/chem.201000310
Competitive Hydrogen-Atom Abstraction versus Oxygen-Atom and Electron
+
Transfers in Gas-Phase Reactions of [X O ]C (X=P, V) with C H
4
10
2
4
[
a]
[b]
[a]
Nicolas Dietl, Marianne Engeser, and Helmut Schwarz*
In memoriam Professor Herbert Schumann
+
[10]
+
[11]
+
[12]
Despite enormous efforts and considerable research activ-
ities conducted over the past decades, oxidative coupling of
methane (OCM) remains a challenge for contemporary cat-
[OsO ]C , [V O ]C , and [(Al O ) ]C (x=3, 4, 5), as
4 4 10 2 3 x
+
well as the recently reported metal-free oxides [SO ]C and
2
+
[13,14]
the polynuclear oxide cluster [P O ]C .
The latter brings
about H-atom transfer at 298 K with a rate constant of k=
4
10
[1]
alysis. In the context of catalytic CꢀH bond activation of
saturated and unsaturated hydrocarbons, as being realized
in large-scale chemical processes, a detailed mechanistic de-
scription of the elementary processes and a complete under-
standing of the functional and structural properties of the
reactive sites of, for example, solid-state catalysts are still
ꢀ
10
3
ꢀ1
ꢀ1
[14]
6.4ꢀ10 cm s molecule [Eq. (1)].
þ
þ
½P
O
10
ꢁ
C þ CH ! ½P O ðOHÞꢁ þ CH C
ð1Þ
4
4
4
9
3
Not entirely unexpected and in line with observations
[15]
[2]
with other gas-phase bond-activation processes, the che-
moseletivity of these reactions varies considerably for differ-
ent couples of oxide ions and hydrocarbon substrates. For
example, Castleman and co-workers observed in the reac-
tion of [V O ]C with C H (n=4, 6) the oxidation of the
limited.
Over the last few years the investigation of small cluster
systems, in particular metal and metal oxide species, using
advanced mass-spectrometric methods in conjunction with
theoretical studies proved useful in elucidating mechanistic
+
4
10
2
n
[16]
[3]
hydrocarbons due to oxygen-atom transfer [Eq. (2)].
aspects. Based on these and other studies there is now con-
sensus that in the context of CꢀH bond activation the en-
þ
þ
½
V O ꢁC þ C H ! ½V O ꢁC þ C H O
ð2Þ
hanced reactivity of metal-oxide based catalysts is due to
4
10
2
n
4
9
2
n
[4]
the presence of oxygen-centered radicals. In the case of
OCM, these oxygen-centered radicals bring about efficient
As to the chemical nature of the oxygenated products, ac-
cording to DFT calculations, C H gives rise to the forma-
hydrogen-atom abstraction from CH to generate CH C, and
4
3
2
4
this step is considered to be decisive for the oxidative dehy-
tion of acetaldehyde (CH CHO) rather than ethylene oxide
3
[1,5]
drogenation and dimerization of methane.
Hydrogen-
in the O-atom transfer; thus, hydrogen migration is involved
[17]
atom abstraction has also been observed in well-defined
gas-phase experiments, and various mechanistic variants
were unraveled in combination with quantum-chemical cal-
culations. In fact, numerous examples for the effective
room-temperature activation of methane by a structurally
rather diverse set of metal oxides have been studied, includ-
in the reaction. Similar results have been reported for the
+
[17]
+
[18]
+
polynuclear clusters [V O ] , [Ag O] , [Zr O ] and
C
C
C
2
5
2
2
4
+
[19]
ꢀ [20,21]
[
Zr O ] , as well as for the anionic cluster [Zr O ] .
C
C
4
8
2
5
For the isostructural tetranuclear phosphorus oxide clus-
+
[11,14]
ter [P O ] ,
C
O-atom transfer to C H does not take
2 6
4
10
place; rather, hydrogen-atom abstraction dominates, thus
+
[6]
+
[7]
+
[8]
+ [9]
ing [MgO]C ,
[FeO]C ,
[MoO ]C ,
[ReO (OH)]C ,
+
3
3
producing the cationic closed-shell species [P O (OH)]
4
9
[22]
[
Eq. (3)].
[
a] Dipl.-Chem. N. Dietl, Prof. Dr. H. Schwarz
þ
þ
½P O ꢁC þ C H ! ½P O ðOHÞꢁ þ C H C
ð3Þ
Institut fꢁr Chemie, Technische Universitꢂt Berlin
Strasse des 17. Juni 135, 10623 Berlin (Germany)
Fax : (+49)30-314-21102
4
10
2
6
4
9
2
5
+
Here we describe the gas-phase reaction of [P O ]C with
C H , and the findings reveal an unexpected reactivity in
4
10
E-mail: Helmut.Schwarz@mail.chem.tu-berlin.de
2
4
[
b] Dr. M. Engeser
+
comparison
with
the
well-studied
[V O ]C /C H
4 10 2 4
Kekulꢃ-Institut fꢁr Organische Chemie und Biochemie
Universitꢂt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
[16,17]
+
system.
react by oxygen-atom transfer to the unsaturated hydrocar-
For example, the [P O ]C /C H couple does not
4
10
2
4
4452
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 4452 – 4456