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ample of methane activation by a closed-shell oxide cluster is
given in Equation (6). In contrast to the inertness of diatomic
[HfO]+ towards methane, the closed-shell, halogenated oxide
ions [XHfO]+ (X=F, Cl, Br) activate the H3CꢀH bond and form
the insertion products [XHf(OH)(CH3)]+. The presence of the
halogen ligand strengthens the formation of a HfꢀC bond and,
at the same time, weakens the p(HfꢀO) bond such that CꢀH
bond activation of CH4 can take place under ambient condi-
tions.[13]
½HTiOꢁþþCH4 ! ½ðCH3ÞTiOꢁþþH2
½TaO2ꢁþþCH4 ! ½TaðOHÞðOÞðCH3Þꢁþ
½XHfOꢁþþCH4 ! ½XHfðOHÞðCH3Þꢁþ
ð4Þ
ð5Þ
ð6Þ
Figure 1. Mass spectra showing the room-temperature reactivity of [OSiOH]+
with a) Ar, b) CH4, c) CD4, and d) CH2D2 at a pressure of 1.0ꢁ10ꢀ9 mbar and
reaction delay of 15 s; and e) the reactivity of [OSiOD]+ with CH4 at a pres-
sure of 2.0ꢁ10ꢀ9 mbar and reaction delay of 3 s. The signals labeled as B
and J are due to the reaction with background impurities, and the signal A
in e) is due to an H/D exchange between [OSiOD]+and background water.
Silicon oxide constitutes an abundant substance on earth
and serves as an important catalyst support widely used in
many large-scale chemical transformations.[14] This material is
usually considered to act as a catalytically innocent linker be-
tween the active metal oxide sites and the support.[15] For ex-
ample, high catalytic activities have been observed when SiO2
is used as a support for the activation of methane, such as in
the selective oxidation of methane to formaldehyde,[16] or the
direct conversion of methane to ethylene.[17] In recent years,
the concept of catalytic functionalization of CꢀH bonds using
metal-free catalysts have fascinated scientists and led to impor-
tant conceptual breakthroughs with regard to various aspects
of CꢀH bond activation.[18] Therefore, probing the reactivity of
silicon oxide clusters may not only deepen the mechanistic un-
derstanding of CꢀH bond activation by silica-supported sys-
tems, but may also shed light on the active site of a metal-free
catalyst. Here, we report our combined experimental/computa-
tional findings on the gas-phase reactivity of the silicon oxide
cluster [OSiOH]+ towards CH4, and a detailed comparison with
the closed-shell systems [OCOH]+/CH4 and [MgOH]+/CH4 is
made.
amounts of [SiOCHD2]+ are also observed. The ions
[SiOH]+:[SiOD]+ are generated in a ratio of 1:1.8, while in the
reaction with CH2D2, this ratio amounts to 4.2:1. We have also
reacted [OSiOD]+ with CH4 (Figure 1e); here, the ratio
[SiOH]+:[SiOD]+ amounts to 2.2:1. An interpretation of these
branching ratios will be given below together with the analysis
of the theoretical results. In the reactions of [OSiOH]+ with CH4
and CD4, no intermolecular kinetic isotope effect (KIE) could be
identified, that is, the intermolecular KIE is approximated to 1.0
within the error bars of the experiment.[20]
½OSiOHꢁþþCH4 ! ½SiOCH3ꢁþþH2O DHr ¼ ꢀ118 kJ molꢀ1
½OSiOHꢁþþCH4 ! ½SiOHꢁþþCH3OH DHr ¼ ꢀ69 kJ molꢀ1
ð7Þ
ð8Þ
Mechanistic insight into the details of the methane activa-
tion step by [OSiOH]+ has been derived from high-level quan-
tum chemical calculations. The most favorable pathways for
the reactions of the [OSiOH]+/CH4 couple were located on the
singlet potential energy surface (PES) as shown in Figure 2. An
encounter complex 1 is initially formed from the reactants; this
barrier-free step is exothermic by 104 kJmolꢀ1, thus indicating
a rather strong interaction between the positively charged sili-
con atom and methane (the charge on the silicon atom of
[OSiOH]+ amounts to 2.5jej based on a natural bond orbital
(NBO) analysis). Subsequently, one CꢀH bond of the incoming
hydrocarbon substrate is activated and a hydrogen atom is
transferred to the oxo-group of [OSiOH]+ via transition state
TS1/2 to form the rather stable silyl cation 2. In the latter, the
positive charge at the silicon atom amounts to 2.41jej, and
the formations of strong SiꢀC and OꢀH bonds account for the
high stability of 2; the calculated homolytic SiꢀC bond energy
in 2 is 428 kJmolꢀ1, close to the reported value of silicon car-
bide.[21] Next, rather than homolytically splitting the SiꢀC bond
of 2, the methyl group can migrate via TS2/3 to one of the hy-
Results and Discussion
The cluster ion [OSiOH]+ (m/z: 61) was generated in the reac-
tion of [SiO2]+· with water; see the Experimental and Computa-
tional Details section for further information. The Fourier trans-
form-ion cyclotron resonance (FT-ICR) mass spectra of the reac-
tions of mass-selected, thermalized [OSiOH]+ ions with meth-
ane are reproduced in Figure 1; the reactivity of [OSiOH]+
toward inert argon as well as background impurities have
been included as a reference spectrum (Figure 1a). When
[OSiOH]+ is exposed to methane (Figure 1b), two ionic prod-
ucts, [SiOH]+ (m/z: 45) and [SiOCH3]+ (m/z: 59) are formed with
a branching ratio of 35 and 65%, Eqs. (7) and (8), respectively.
The rate constant k([OSiOH]+/CH4) is determined to be 1.64ꢁ
10ꢀ10 cm3 sꢀ1 moleculeꢀ1, corresponding to an efficiency of
22%, relative to the collision rate.[19] Mechanistic insights are
provided by labeling experiments: reacting [OSiOH]+ with CD4
gives rise to the corresponding deuterated [SiOD]+ and
[SiOCD3]+ ions; quite unexpectedly, [SiOH]+ and sparse
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Chem. Eur. J. 2016, 22, 1 – 8
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