Sequential methyl-fluorine exchange reactions of siloxide ions in the gas phase

Article abstract of DOI:10.1002/anie.201203970

Exchange Me for a fluorine: Trimethylsiloxide ions in the presence of NF3 in the gas?phase undergo an unusual and sequential metathesis-type reaction wherein methyl groups are exchanged for fluorine. Theoretical calculations suggest that the reaction proceeds by a three-step internal-nucleophilic-displacement mechanism which features a pentacoordinated siliconate species (see picture) as a transition state rather than as an intermediate.

Full text of DOI:10.1002/anie.201203970

Angewandte
Chemie
DOI: 10.1002/anie.201203970
Gas-Phase Reactions
Sequential Methyl–Fluorine Exchange Reactions of Siloxide Ions in the
Gas Phase**
Thiago C. Correra and Josꢀ M. Riveros*
Organosilanols have received increasing attention as useful
reagents in organic synthesis,[1] as precursor compounds for
materials,[2] and as intermediates in sol-gel processes that
involve the hydrolysis and polycondensation of alkoxysilanes,
thus leading to hybrid nanomaterials, siloxane-type oligo-
mers, and zeolite-type compounds.[3] Silanols are more acidic
than alcohols both in solution and in the gas phase,[4] and their
ꢁ
ꢀ ꢁ
conjugate bases, Si O species, play a crucial role in the
initial steps of polymerization reactions of alkoxysilanes in
basic media. As a result, a number of theoretical studies have
addressed the role of silanols and siloxide species to better
understand the mechanism of the initial steps of sol-gel
processes.[5] Likewise, several aspects of the fundamental
reactivity of alkoxysilanes in these processes have been
probed by gas-phase ion-chemistry techniques in an attempt
to characterize the intrinsic properties of these substrates.[6]
During our studies of the gas-phase reactivity of simple
R3MOꢁ (M = Si, Ge, and Ti) ions we discovered an unusual
gas-phase metathesis-type reaction for some of these anions
that is promoted by NF3 and other fluorinated species and
results in the exchange of the alkyl groups by fluorine. This is
a remarkable reaction and should attract much interest
Figure 1. Reaction kinetics of NF3 (2.12ꢀ10ꢁ8 Torr) reacting with
Me3SiOꢁ (m/z 89), which is generated from the reaction of Fꢁ and
Me3SiOEt (7.8ꢀ10ꢁ9 Torr). The partial pressures are nominal values
measured with an ionization gauge.
imately 2.1 ꢀ 10ꢁ8 Torr)[9] and Me3SiOEt (maintained at
nominal pressures of approximately 7.8 ꢀ 10ꢁ9 Torr). The
primary fluoride ion reactants were obtained by dissociative
electron attachment inside the cell of the FT-ICR spectrom-
eter. Because the gas-phase reaction of Fꢁ and Me3SiOEt
yields predominantly EtOꢁ by direct displacement, isolation
of the Me3SiOꢁ ions was achieved by continuous ejection of
the EtOꢁ ions and removal of other primary products by
a series of short radiofrequency pulses tuned to the cyclotron
frequencies of the unwanted ions. This method prevented
reactions involving EtOꢁ ions, thus proving effective for our
purposes.
because the presence of a fluorine atom directly bonded to
ꢁ
ꢀ
the silicon center of a SiO species can dramatically
influence the reactivity of this moiety.[7]
The kinetics data for the gas-phase reaction of Me3SiOꢁ
ions in the presence of NF3 is presented in graphical form in
Figure 1. In these experiments, Me3SiOꢁ ions were generated
and trapped in an FT-ICR spectrometer[8] from the reaction of
Fꢁ (generated from NF3 at nominal pressures of approx-
[*] T. C. Correra, J. M. Riveros
Instituto de Quıꢁmica, Universidade de S¼o Paulo
Caixa Postal 26077, S¼o Paulo, CEP 05599-970 (Brazil)
E-mail: jmrnigra@quim.iq.usp.br
The kinetics data (Figure 1) are consistent with a set of
reactions in which methyl groups are sequentially exchanged
with fluorine [Eq. (1)].
J. M. Riveros
Centro de CiÞncias Naturais e Humanas
Universidade Federal do ABC
Me3ꢁnFnSiOꢁ þ NF3 ! Me2ꢁn
F
nþ1SiOꢁ þ MeNF2 ðn ¼ 0 ꢁ 2Þ
ð1Þ
Rua Santa Adelia 166, Santo Andrꢂ, SP, CEP 09210-170 (Brazil)
[**] This project was supported by funding from the Air Force Office of
Scientific Research (AFOSR), the Brazilian Research Council
(CNPq), the S¼o Paulo Research Foundation (FAPESP), and the
National Institute of Science, Technology and Innovation of
Complex Functional Materials (INOMAT). We thank Mathias
Bickelhaupt for suggesting the halogen-attack pathway and Veron-
ica Bierbaum for discussions regarding our experimental results.
T.C.C. acknowledges CNPq for a graduate fellowship and J.M.R.
acknowledges the support of the Office for Higher Education of
Brazil (CAPES) through its National Senior Visiting Professorship
Program.
These sequential exchange reactions were calculated at
the DFT/B3LYP/6-311 + G(3df,2p) level of theory as being
highly exothermic: DH8 at 298 K for the exchange reaction of
Me3ꢁnFnSiOꢁ where n = 0, 1, and 2 was calculated as being
ꢁ358.0, ꢁ353.4, and ꢁ338.2 kJmolꢁ1, respectively. The rate
constant for the n = 0 reaction was estimated using the
calibrated pressure for NF3 and the general procedure
outlined in a previous publication[8] and was found to be
approximately 7 ꢀ 10ꢁ11 cm3 molecule sꢁ1.[8] The fact that
methyl groups are replaced sequentially in this transforma-
tion was further confirmed by studying the dependence of the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203970.
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kinetics on the partial pressure of NF3. In addition, the second
Me/F exchange reaction was independently characterized
through experiments in which Me2FSiOꢁ was generated
directly from a different precursor.
Considering that NF3 as a neutral substrate is relatively
inert in gas-phase negative-ion chemistry,[10] we explored
other neutral substrates to better understand these reactions.
For example, the use of SO2F2, both as a precursor of Fꢁ ions
and as a neutral substrate to react with Me3SiOꢁ, also resulted
in the same type of sequential reactions together with the
observation of other reaction products that are presently
under investigation.
Scheme 1. Proposed reaction mechanism for the Me/F exchange
reaction of Me3SiOꢁ mediated by NF3. Dashed line represents loose
bonds between fragments.
The apparent simplicity and the novelty of the Me3SiOꢁ/
NF3 reaction [Eq. (1)] motivated us to carefully consider
possible mechanisms. Previously, when the gas-phase reaction
between Me3SiOꢁ and SiF4 was studied using flowing-after-
glow techniques, it was revealed that F3SiOꢁ presumably
forms through a mechanism that involves nucleophilic attack
on SiF4 followed by nucleophilic internal return of the
nascent fluoride ion,[11] a mechanism, which is analogous to
that of the reaction promoted by alkoxide ions.[12] The
outcome of reaction between Me3SiOꢁ and NF3 is different
from what would be predicted based on these previous
examples. Therefore, the reaction was investigated using
model chemistry calculations, which is a valuable means for
studying reactions involving siloxide-type anions.[13] Calcula-
tions were carried out at the DFT/B3LYP/6-311 + G(3df,2p)
level of theory, after a preoptimization at the DFT/B3LYP/6-
31 + G(d) level of theory, with the Gaussian03 suite of
programs.[14a] Stationary points were characterized by vibra-
tional-frequency analyses using the same basis set. Connec-
tivity along the reaction coordinate was confirmed by IRC
calculations,[15] which were carried out with the Gaussian09
suite of programs.[14b] A scale factor of 0.964 was used for the
zero-point-energy (ZPE) corrections.[16]
Three possibilities for the first step of the reaction were
considered: a) nucleophilic attack of the siloxide ion at the
nitrogen center; b) nucleophilic attack of the siloxide ion at
a fluorine atom of the neutral NF3; and c) activation of
Me3SiOꢁ to release an incipient Meꢁ anion and silanone, that
is, induced dissociation of Me3SiOꢁ ions.[17] Only the first
mechanism was predicted to proceed through a nearly
barrierless process, which would be compatible with our
experimental rate constant (kexp > 10ꢁ2 kcollision). The other two
mechanisms were predicted to involve sizable activation
energies (in the range of 125 kJmolꢁ1 above the energy of the
reactants) and thus would not be expected to occur within the
time scale of our FT-ICR experiment in the absence of some
form of ion activation.
Figure 2. Energy diagram for the Me3SiOꢁ/NF3 reaction calculated at
the B3LYP/6-311+G(3df,2p) level of theory. The dashed line repre-
sents the hypothetical (and unobserved) pathway that would lead to
the formation of Me3SiF and F2NOꢁ via adduct Int1a.
through internal nucleophilic return of the nascent fluoride
ion, which becomes loosely attached to Me3SiONF2 to yield
an intermediate identified as Int1 [Fꢁ···Me3SiONF2], which is
predicted to be considerably more stable than the reactants.
Int1 then evolves further, via TS2, through a concerted attack
of the loosely attached fluoride ion on the silicon center and
displacement of a Meꢁ ion, which attacks the nitrogen center.
This concerted transformation, which proceeds via TS2,
leads to the loosely bound intermediate Int2
[Fꢁ···N(F)(Me)OSi(F)Me2]. Int2 can then further proceed
by an Fꢁ attack on the nitrogen center via TS3 resulting in the
ꢁ
breakage of the O N bond and formation of the product
complex (PC) [Me2SiFOꢁ···MeNF2]. This complex can then
dissociate into the final products of reaction (1; n = 0)
The calculated geometries for the intermediates and
transition states of Scheme 1 are given in Figure 3 (the
coordinates and energy data for all species can be found in the
Supporting Information). Several points related to the
structural data deserve highlighting:
Nucleophilic attack at a nitrogen center is not a very
common process and only a few facile gas-phase displacement
reactions at a nitrogen center have been experimentally
observed[18] and are supported by theoretical calculations.[19]
In our case, backside attack of the siloxide ion on the nitrogen
center is predicted to proceed initially through the formation
of a weakly bound reactant complex (RC), which then
proceeds further through a transition state (TS1; Scheme 1)
that is isoenergetic with the reactants at our level of theory
(Figure 2). The reaction is then calculated to proceed further
a) the geometry of TS1 resembles the transition states
calculated for SN2 reactions of halide ions at the nitrogen
center of haloamines (NH2X, X = F, Cl, Br, I);[20]
b) comparison of the geometries of Int1 and TS2 reveals that
while the approach of Fꢁ to the silicon center results in the
ꢁ
elongation of the Si Me bond (from 1.878 to 2.610 ꢁ), the
approach of the Me group to the nitrogen center causes
ꢁ
the synchronous elongation of the N F bond (from
approximately 1.5 to 1.944 ꢁ);
2
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Figure 3. Calculated geometries for reaction intermediates and transition states at the B3LYP/6-311+G(3df,2p) level of theory. Distances are
given in ꢃ and angles are given in degrees.
c) the Me2FSiO moiety is essentially formed in Int2 and
plays a secondary role in TS3 other than the contraction of
context of the transformation described herein, is currently
being explored.
ꢁ
the Si O bond that leads to the formation of the product
complex (PC).
Received: May 22, 2012
Published online: && &&, &&&&
There are other interesting features in this mechanism
that deserve further discussion. The internal nucleophilic
return of Fꢁ does not yield directly a pentacoordinated-
siliconate species, [F(Me)3SiONF2]ꢁ, which, by analogy with
the reaction of Me3SiOꢁ and SiF4 reported by Damrauer
et al., would be predicted to give NF2Oꢁ as the final ionic
product.[11] Interestingly enough, attempts to optimize the
pentacoordinated structure of the siliconate species result in
Keywords: exchange interactions · FT-ICR mass spectrometry ·
gas-phase reactions · reaction mechanisms · siloxides
.
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Chemie
Communications
Gas-Phase Reactions
Exchange Me for a fluorine: Trimethylsil-
oxide ions in the presence of NF3 in the
gas phase undergo an unusual and
sequential metathesis-type reaction
wherein methyl groups are exchanged for
fluorine. Theoretical calculations suggest
that the reaction proceeds by a three-step
internal-nucleophilic-displacement
mechanism which features a pentacoor-
dinated siliconate species (see picture) as
a transition state rather than as an
intermediate.
T. C. Correra,
J. M. Riveros*
&&&& — &&&&
Sequential Methyl–Fluorine Exchange
Reactions of Siloxide Ions in the
Gas Phase
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!