Linking ion and neutral chemistry in C-H bond electrophilic activation: Generation and detection of HO2. reactive radicals in the gas phase

Article abstract of DOI:10.1002/anie.201107224

The flip side: Both the charged and uncharged products formed by C-H bond electrophilic activation have been experimentally detected in the gas phase. The HO₂^. radical is formed by a process involving the prototypical oxygen-centered radical cation O₂^.+ and the methane derivative CH₂F₂. Copyright

Full text of DOI:10.1002/anie.201107224

Angewandte
Chemie
DOI: 10.1002/anie.201107224
ꢀ
C H Activation
ꢀ
Linking Ion and Neutral Chemistry in C H Bond Electrophilic
Activation: Generation and Detection of HO2C Reactive Radicals in the
Gas Phase**
Giulia de Petris,* Giancarlo Angelini, Ornella Ursini, Marzio Rosi, and Anna Troiani
ꢀ
The activation of inert C H bonds by charged electrophiles is
the subject of extensive investigation mainly motivated by
one of the most widely debated questions, that is, the
conversion of alkanes into more valuable compounds. Studies
of ion-molecule reactions in the idealized gaseous state have
provided insightful information on this elementary step, thus
lending models and concepts which span from the molecular
level to the borders of the nanoscale size.[1] These studies have
shown the enhanced ability of oxygen-centered radical
fate of the radicals formed are central to the evolution of the
environment or microenvironment where they are generated,
a concept that can easily be extended to biological and
atmospheric systems.
+
In this light, the reactivity of XOC radical cations, so far
investigated with alkanes and also alkenes, indicates the
formation of many radicals and neutral species in addition to
CH3C: C2H5C,[2i,3c] C2H3C,[2i,k,3d] P4O9(OH)C,[3c] C2H4,[2i,3e]
C2H4O,[1c,2d,k] CH3OH.[2b,j] While a number of mechanistic
details have been elucidated by mass spectrometric experi-
ments, the nature of the uncharged products has never been
experimentally probed.[8] This lack becomes a significant
[2]
+
ꢀ
cations to activate C H bonds; the inventory of XOC
reactants now available includes metal oxides and oligomeric
cluster ions, mixed metal/nonmetal cluster ions,[2] and also
effective metal-free oxide ions.[3]
As a result of the H abstraction, the charge and spin of
XOC are separated in the products [Eqs. (1) and (2)], that is,
ꢀ
limitation when the C H bond is activated by the two-
electron channel [Eq. (2)], as the signature of the H transfer
rests on the radical product of the reaction.
+
depending on whether one or two electrons are formally
transferred in the process, the XOH+/RC or XOHC/R+ products
are formed, respectively (RH = alkane).[4]
In this work, we provide direct experimental evidence for
ꢀ
the radical produced by an ion-molecule C H activation
+
reaction. The reaction between O2C and CH2F2 was chosen as
a model of a two-electron process leading to an oxygen-
centered radical. O2C is the prototype of metal-free oxygen-
centered radical cations, CH2F2 (HFC-32; HFC = hydrofluo-
+
Cþ
C
XO þ RH ! XOHþ þ R
ð1Þ
ð2Þ
Cþ
C
þ
ꢀ
rocarbon) is a methane derivative having a C H bond
XO þ RH ! XOH þ R
strength (102.7 kcalmolꢀ1) quite close to that of CH4,[9] and
a global warming potential approximately 30 times higher for
which it is covered by the Kyoto protocol.[10,11] The radical
detected is the hydroperoxy radical HO2C that can dimerize to
H2O2. HO2C is a known important player in atmospheric ozone
cycles,[10] biochemical systems,[12] solution chemistry,[13] and
also suggested to be involved in catalyzed oxidation of
A very important implication of these thermal ionic
reactions is therefore the formation of reactive radicals. The
most striking example is the production of methyl radical
CH3C,[2a–c,e–j,3a,b] which can dimerize to give ethane. Likewise,
processes of the type shown in Eq. (2) produce oxygen-
centered radicals, which can undergo oxyfunctionalization in
condensed and gas phases.[1l,5,6] It is worth noting that surface-
generated CH3C and HOC radicals have been detected in the
catalyzed oxidation of methane.[7] Accordingly, the nature and
+
alkanes.[7d–e] The reactions of O2C with some HFCs have
previously been studied,[14] whereas to the best of our
knowledge no data is available for CH2F2. Among the oxide
ions, only the vanadium oxide cluster cations have been
ꢀ
assayed with CH2F2; in these experiments O transfer and C F
[*] Prof. Dr. G. de Petris, Dr. A. Troiani
Dipartimento di Chimica e Tecnologie del Farmaco
“Sapienza” Universitꢀ di Roma
activation products were observed.[15]
In our experiments performed under the pressure of
+
P. le Aldo Moro 5, 00185 Roma (Italy)
E-mail: giulia.depetris@uniroma1.it
approximately 10ꢀ8 Torr, the reaction between isolated O2C
+
ions and CH2F2 gives CHF2 as the only charged product of
ꢀ
Dr. G. Angelini, Dr. O. Ursini
Istituto di Metodologie Chimiche
the C H bond activation.
Area della Ricerca di Roma del CNR
CP 10, 00016 Monterotondo Stazione (RM) (Italy)
Cþ
C
þ
O2 þ CH2F2 ! CHF2 þ HO2
ð3Þ
Prof. Dr. M. Rosi
Dipartimento di Ingegneria Civile e Ambientale
Universitꢀ di Perugia and ISTM-CNR
Via Elce di Sotto 8, 06123 Perugia (Italy)
The other product observed, the CH2F+ ion, cannot be
+
ꢀ
traced to C F activation by O2C . The kinetics exhibits the
time profile typical of a consecutive reaction (Figure 1), thus
[**] This work is supported by the Italian Government and the
“Sapienza” University of Rome. The authors thank Stefania Recaldin
for editorial assistance.
showing that the primary product CHF2+ subsequently reacts
ꢀ
activating the C F bond of CH2F2.
Angew. Chem. Int. Ed. 2012, 51, 1455 –1458
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1455

.
Angewandte
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Figure 1. Time profiles and best-fit lines of the O2C+ ( , R =0.999),
CHF2 ( , R =0.994) and CH2F ( , R =0.997) ions from the
reaction of O2C+ with CH2F2. P CH2F2 =2.4ꢁ10ꢀ8 Torr.
2
&
+
2
+
2
~
Figure 2. High-resolution CAD spectrum (a) and +NfR+ spectrum (b)
of [C, ꢀH2, F2, 18O2]C+ ions. The inset shows the +NfRꢀ spectrum with the
H18O2 peak at m/z 37. Unresolved peaks at m/z 50 (CF2C+), 33
(CH2F+), 32 (CHFC+), 31 (CF+), 19 (F+, H18O+), and 18 (18OC+). The
+NfR+ spectrum of unlabeled [C, H2, F2, O2]C+ ions display an intense
*
CHF2þ þ CH2F2 ! CH2Fþ þ CHF3
ð4Þ
signal corresponding to the HO2 ion superimposed to CH2F+ (m/z
+
33).
+
ꢀ
Accordingly, O2C selectively activates the C H bond
of CH2F2 with
a
rate constant k3 = 3.3 ꢀ 10ꢀ10 (ꢁ
30%) cm3 sꢀ1 moleculeꢀ1, which corresponds to the efficiency
k/kcoll = 17% (kcoll = collision rate). The rate constant of the
consecutive reaction [Eq. (4)], k4, amounts to 2.2 ꢀ 10ꢀ10
(ꢁ 30%) cm3 sꢀ1 moleculeꢀ1, which is in excellent agreement
of Figure 2b). Accordingly, the positive detection of H18O2
+
and H18O2 unambiguously demonstrates that the ionic
ꢀ
population contains a detectable amount of reaction inter-
mediate that dissociates into CHF2+ and H18O2C.
with the value obtained by experiments independently
Quantum chemical calculations performed at the
CCSD(T)/6-311 ++ G(3df,3pd)//B3LYP/6-311 + G(d) level
of theory are in very good agreement with the experimental
evidence. The reaction shown in Eq. (3) is computed to be
exothermic by 25.5 kcalmolꢀ1, a value quite close to that
evaluated from experimental available data (DH8 = ꢀ24 kcal
molꢀ1).[9] According to the energy profile picture in Figure 3,
ions, k = 2.1 ꢀ 10ꢀ10
+
performed with isolated CHF2
(ꢁ 30%) cm3 sꢀ1 moleculeꢀ1.
The formation of HO2C has been directly probed by
experiments, performed at high pressure, that allow detection
of the intermediate of the reaction shown in Eq. (3). The
stabilization and observation of these intermediates are very
rare; in a few cases reported only the ionic products have been
characterized.[3a,16] Here, the intermediate formally denoted
+
the reactants generate an encounter complex [O2-CH2F2]C
(A) where O2C interacts at a distance of 2.345 ꢁ with the
+
+
+
as [C, H2, F2, O2]C has been obtained by ionization of O2/
fluorine atom of CH2F2. The binding energy of [O2-CH2F2]C is
CH2F2 mixtures, using 18O2 to avoid mixing of the isobaric O2
and CHF groups. The high-resolution collisionally activated
13.3 kcalmolꢀ1.
dissociation (CAD) spectrum of isolated [C, H2, F2, 18O2]C
+
+
+
ions shows two main peaks (Figure 2a): CHF2 and 18O2C .
This pattern would indicate the presence of two complexes,
[H18O2C-CHF2 ] and [18O2C -CH2F2]. Alternatively, the latter
+
+
complex could account for both the CHF2 and 18O2C
+
+
fragments, considering the quite close ionization energies of
+
O2 and CH2F2 and the easy dissociation of CH2F2C into
+
CHF2 + HC (DH8 = 11.8 kcalmolꢀ1).[9] The latter is actually
+
consistent with the very low CH2F2C signal observed in the
chemical ionization (CI) spectrum. The analysis of the neutral
species coming from the dissociation of the reaction inter-
mediate clarifies the issue, showing the nature of the radical
formed in the reaction.
To this end, all neutral fragments have been separated
from all charged species and reionized by the neutral
fragments reionization (NfR) technique.[17] The spectrum of
Figure 2b shows an intense peak at m/z 37 corresponding to
the H18O2+ ion, that can only be formed by reionization of the
H18O2C radical. Notably, the failure to detect CH2F2C+, from
Figure 3. Energy profile of the reaction between O2C+ and CH2F2. DH8
values at 298 K in kcalmolꢀ1, bond lengths in ꢂ.
The encounter complex A easily isomerizes to [HO2-
CHF2]C+ (B), where the unpaired electron is entirely located
on the newly formed HO2C moiety interacting through the
+
reionization of the neutral counterpart of 18O2C , confirms its
easy dissociation into CHF2+. An intense peak at m/z 37 has
oxygen atom with the carbon atom of CHF2 . The structures
+
also been obtained by negative reionization to H18O2 (inset
of the two intermediates are consistent with the experimental
ꢀ
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1455 –1458

Angewandte
Chemie
observations; in particular, the deep well of B, 59.1 kcalmolꢀ1
using the 6-311 ++ G(3df,3pd) basis set and the CCSD(T) energies
were corrected to 298.15 K by adding the zero point energy correction
and the thermal correction computed using the scaled harmonic
vibrational frequencies evaluated at B3LYP/6-311 + G(d) level. All
calculations were performed using Gaussian 03.[27]
below the reactants, accounts for the experimental detection
of the [HO2-CHF2]C complex in sufficient amounts to identify
+
its dissociation radical product. Along the double-well path,
the encounter complex A is expected to undergo back
dissociation in competition with the isomerization, which is
consistent with the moderate efficiency of the reaction at
room temperature. The rate constant is likely to be sensitive
to temperature, as observed for instance with the analogue
Received: October 12, 2011
Published online: December 23, 2011
ꢀ
Keywords: C H activation · density functional calculations ·
gas-phase reactions · mass spectrometry · radicals
.
+
reaction of O2C with CH4.[18]
In conclusion, in this study we have given direct exper-
imental evidence for an oxygen-centered radical formed by
ꢀ
electrophilic C H bond activation in the gas phase. This is the
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first experimental detection of both the ion and radical
formed in this elementary step. The model reaction also
shows that two-electron processes can generate species
potentially relevant to functionalization or other chemical
cycles. The ensuing chemistry strictly depends on the ther-
modynamic and kinetic constraints of the reaction pathways;
nonetheless this result brings into sharp focus the intimate
link existing between ion and neutral chemistry.
´
ˇ ´
2010, 110, 1170; c) G. E. Johnson, R. Mitric, V. Bonacic-
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Experimental Section
The NfR experiments were performed using a modified ZABSpec oa-
TOF instrument (VG Micromass) of EBE-TOF configuration
described elsewhere.[19] High-resolution CI spectra were recorded at
25000 full width at half maximum (fwhm) at the first detector, to rule
out isobaric contaminations and assign the elemental composition [C,
H2, F2, 18O2]C+. High-resolution CAD spectra of mass- and energy-
selected ions were recorded in a gas cell located in the TOF sector
(0.8 keV, collision gas: Helium, transmittance T= 80%). The NfR
spectra were recorded in a pair of cells located after the magnet: i) the
mass-selected ions underwent high-energy (8 keV) CAD in the first
cell by collision with He (T= 80%); ii) the charged fragments were
removed at the exit of the cell by a pair of high-voltage deflecting
electrodes; iii) a beam containing only neutral fragments entered the
second cell, where they were reionized by collision either with O2
(+NfR+) or with Xe (+NfRꢀ) (T= 80%). No signal was observed by
switching the deflector on in the absence of the reionizing gas.
The kinetics of the ion-molecule reactions was studied by an
[2] a) S. Feyel, J. Dçbler, D. Schrçder, J. Sauer, H. Schwarz, Angew.
Chem. 2006, 118, 4797; Angew. Chem. Int. Ed. 2006, 45, 4681;
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Hçckendorf, M. K. Beyer, J. Sauer, H. Schwarz, Angew. Chem.
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´
ˇ ´
´
Johnson, R. Mitric, E. C. Tyo, V. Bonacic-Koutecky, A. W.
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ˇ
´
Bozovic, D. K. Bçhme, Phys. Chem. Chem. Phys. 2009, 11, 5940;
f) Z.-C. Wang, X.-N. Wu, Y.-X. Zhao, J.-B. Ma, X.-L. Ding, S.-G.
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+
EXTREL FTMS 2001 double-cell mass spectrometer. The O2C ions
were generated in the “source cell” by electron impact (EI) (50 ms,
30 eV) (P O2 = 1.0 ꢀ 10ꢀ7 Torr). After a cooling period of 4.1 seconds,
they were isolated and transferred to the “analyzer cell” containing
CH2F2 (P CH2F2 = 2.3–9.1 ꢀ 10ꢀ8 Torr). The pressure calibration was
carried out using the known rate constant of the reference reaction
CH4C+
+
CH4
!
CH5
+
CH3 (k = 1.1 ꢀ 10ꢀ9 (ꢁ
+
15%) cm3 sꢀ1 moleculeꢀ1),[20] and the reading was further corrected
for the response factor of CH2F2.[21] The O2C+, CHF2 and CH2F+
+
intensities fit the equations derived by consecutive kinetics, where
+
O2C fits a pseudo-first order kinectics.[22] In independent experiments,
+
the CHF2 ions were generated in the analyzer cell by EI (50 ms,
18 eV) of CH2F2 (P CH2F2 = 2.1–11 ꢀ 10ꢀ8 Torr), isolated after a
cooling time of 1.5 seconds and allowed to react. The bimolecular rate
constants k (cm3 sꢀ1 moleculeꢀ1) were obtained by the pseudo-unimo-
lecular rate constants k(obs) (relative square deviation 5%) and the
neutral reactant density. The reaction efficiency k/kcoll (kcoll = collision
rate constant) was calculated according to the ADO theory.[23]
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points at the B3LYP[24] level of theory with the 6-311 + G(d) basis set.
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[4] One- and two-electron processes strictly correspond to HC and
Hꢀ transfer, respectively; however they may not occur by pure
ꢀ
homolytic and heterolytic rupture of the C H bond when inner-
sphere electron transfer is involved (see Ref. [1l]).
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