Radiochemical study of the reaction between the diethylsilylium cation and benzene in gas and liquid phases: Experimental and theoretical evidence for rearrangements of the diethylsilylium cation

Article abstract of DOI:10.1016/S0022-328X(99)00286-7

The ion-molecule reaction between the nucleogenic diethylsilylium cation generated by the β-decay of the tritiated diethylsilane and benzene was studied by the radiochromatographic method in the gas and liquid phases. Diethylphenylsilane possessing the diethylsilyl group of the nascent cation has the lowest yield in the gas phase, although in the liquid phase the yield increases. Two other silylphenylsilanes are products of rearrangement of the diethylsilylium cation. The stationary points on the potential energy surface of the SiC₄H₁₁⁺ system were located by the ab initio correlated methods and the plausible rearrangements within this system are discussed.

Full text of DOI:10.1016/S0022-328X(99)00286-7

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Journal of Organometallic Chemistry 586 (1999) 241–246
Radiochemical study of the reaction between the diethylsilylium
cation and benzene in gas and liquid phases: experimental and
theoretical evidence for rearrangements of the diethylsilylium
cation
T.A. Kochina, D.A. Vrazhnov, I.S. Ignatyev *
Institute for Silicate Chemistry, Russian Academy of Sciences, Odoe6skogo 24, c.2 199155 St. Petersburg, Russia
Received 9 March 1999; received in revised form 12 May 1999
Abstract
The ion–molecule reaction between the nucleogenic diethylsilylium cation generated by the b-decay of the tritiated diethylsilane
and benzene was studied by the radiochromatographic method in the gas and liquid phases. Diethylphenylsilane possessing the
diethylsilyl group of the nascent cation has the lowest yield in the gas phase, although in the liquid phase the yield increases. Two
other silylphenylsilanes are products of rearrangement of the diethylsilylium cation. The stationary points on the potential energy
surface of the SiC4H1+1 system were located by the ab initio correlated methods and the plausible rearrangements within this
system are discussed. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Diethylsilylium cation; Radiochemical; Rearrangements
comparative study of the silylium cation reactivity in
gas and condensed phases, since the nucleogenic ionic
species are generated without the counterion: the b-par-
ticle, which balances the cation charge, has significant
kinetic energy and moves far away from the decay ions
by the time of nuclear motions. For this reason, even in
condensed phase, the decay cation is initially unsol-
vated (more precisely, it is in the same solvation state as
its neutral precursor) and in many cases may react
before the solvent shell has time to assemble. These
studies may shed some light on the reasons for the
elusiveness of free silylium cations for detection in the
condensed phase.
1. Introduction
The trivalent organosilicon cation species, i.e.
silylium (silyl, silicenium, silylenium), have been objects
of experimental and theoretical studies for more than
50 years [1–17]. Silylium ions are the isoelectronic
analogs of carbocations, but in condensed phases, elu-
siveness of silyl cations stands in remarkable contrast to
the behavior of carbocations [11–14]. All reactions and
techniques successfully used for generation and isola-
tion of stable carbenium ions failed to produce stable
silylium cations in condensed phase [10]. Only in a
recent work have Lambert and Zhao [17] succeeded in
the search for the free silylium cations by using bulky
(mesityl) substituents.
Formation of silylated adducts from the exothermic
reaction
The main difference between silicon and carbon in
their compounds is the tendency of silicon to react with
nucleophiles of any kind with expansion of its coordi-
nation sphere [12–16]. The decay technique for genera-
tion of cations [18,19] seems to be a suitable tool for the
Me3Si+ +PhH“PhHSiMe3+
(1)
was detected in mass-spectrometry studies carried out
at 3–5 torr [20,21]. However the character of bonding
in the adduct of silylium cations with arenes was a
point for discussion. First it was proposed that this
adduct is a p- rather than a s-complex [20,21]. How-
ever the ab initio study of the silyl cation–arene com-
* Corresponding author.
E-mail address: lvs@isc.nw.ru (I.S. Ignatyev)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00286-7

242
T.A. Kochina et al. / Journal of Organometallic Chemistry 586 (1999) 241–246
plexes shows that R3Siꢀarene+ species were s-com-
plexes with a sufficiently strong SiꢀC s-bond [7]. The
recent experimental studies [22,23] also indicate the
formation of a s-complex as an adduct of the reaction
between silylium cations and arenes.
One of the advantages of the radiochromatographic
method for the study of ion–molecule reactions is the
possibility to detect the presence of the different iso-
mers among the observed products [24–26]. The ability
of the silylium cations to undergo isomerization is well
known [27–32] and thus the radiochemical study of
reactions between silylium cations and benzene may
give new information about the mechanism of this
reaction.
tritiated products of the reactions were identified by
comparing retention times with those of authentic refer-
ence compounds under identical chromatographic con-
ditions. A 3 m long column (2 mm in diameter) packed
with 15% SE-30 on Chezasorb N-AW was used.
The relative yields of the products were determined
as the ratio of the activity of each product to the
combined activity of all products identified.
In addition to the Hartree–Fock (HF) method the
hybrid HF/DFT method known as B3LYP was used.
This method combines the Becke’s three parameter
exchange function [34] with the LYP correlation func-
tion [35]. The B3LYP method was found to be very
reliable in predictions of the reaction energies [36] and
it is computationally much less expensive than other
correlated methods. The B3LYP method was used as
realized in the GAUSSIAN 94 program [37]. The basis
sets employed were the standard 6-31G* and 6-31G**
sets. For each stationary point found the vibrational
frequencies were calculated.
In this work we report the generation of the Et2SiH+
cation from tritiated diethylsilane and radiochromato-
graphic characterisation of the products of its reaction
with benzene in both the gas and liquid phase. Since
knowledge of the potential energy surface (PES) for the
SiC4H1+1 cation is essential for the interpretation of the
experimental data and since this system was not theo-
retically studied before, we also report the results of the
scanning of the PES for the SiC4H1+1 system by ab initio
methods.
3. Results and discussion
The observed tritiated products of the reaction and
their relative yields are presented in Table 1. The most
abundant product in both phases appeared to be the
tritiated triethylsilane. It is obviously the product of the
reaction between the silylium cations (generated by
decay of the tritium atom in diethylsilane) and the
diethylsilane molecules.
2. Experimental and theoretical methods
The tritiated Et2SiH+ cations were generated by
b-decay of tritium atoms in the tritiated diethylsilane:
i−
(C2H5)2SiT2 “(C2H5)2SiT+ +He
(2)
Et2TSi+ +Et2SiT2“Et3SiT+EtT2Si+
(3)
Reactions of labelled diethylsilylium cations with
substrate yield radioactive products conveniently ana-
lyzed by the radiochromatographic technique [19,24–
26]. The tritiated diethylsilane was prepared and
purified according to techniques described earlier [33].
Commercial benzene was distilled and its purity was
analyzed by gas chromatography. The reaction mix-
tures for the liquid phase study were prepared by
introducing 1 mCi (0.005 ml) of tritiated diethylsilane,
diluted by cyclohexane to a specific activity of 5 Ci
mol−1, and 0.1 ml of benzene into vessels of spherical
form with capillars. These vessels were sealed in such a
way that the gas phase over the liquid phase was
minimal.
For the gas-phase study the reaction mixtures were
prepared by introducing 1 mCi of tritiated diethylsilane,
diluted by cyclohexane to a specific activity of 5 Ci
mol−1, and benzene at a pressure of 60 Torr into the
evacuated and carefully outgased 20 ml Pyrex vessels.
The mixtures (liquid and gas) were stored in the dark at
room temperature for 4–6 months. After the storage
period, the ampules were opened and their contents was
analyzed using the ‘Tsvet’- gas chromatograph
equipped with a running proportional counter. The
It is well known that the silylium cations rapidly
react with alkanes and silanes with the withdrawal of
alkyl groups from the neutral molecules [13].
Three other observed tritiated compounds are pro-
duced by the reaction between silylium cations and
benzene. These are products of the dissociation of the
silylium cation–benzene adducts. However, only one of
these labelled products, i.e. PhEt2SiH, may be ascribed
to the dissociation of the adduct of the originally
generated diethylsilylium cation and benzene. To ratio-
Table 1
The relative yields for the tritiated neutral products of the reaction
Et2SiT++C6H6
Tritiated products
Gas (%)
Liquid (%)
Et3SiH
49.094.0
26.0913.0
15.2911.0
6.391.0
1.990.3
0.990.1
0.770.1
57.3910.8
9.690.8
17.390.2
13.590.1
–
PhMe2SiH
PhEtSiH2
PhEt2SiH
EtSiH3
C6H6
Et2MeSiH
2.390.1
–

T.A. Kochina et al. / Journal of Organometallic Chemistry 586 (1999) 241–246
243
Table 2
Total energies, DH (0 K, 298 K) of the most stable isomer (a.u.) and relative thermochemical parameters (kcal mol−1) of other stationary points
at the SiC4H+11 PES
Stationary points
SCF
3-21G
DEe
B3LYP
6-31G**
DEe
6-31G*
DEe
DH0
0.17483
DH298
(C2H5)(CH3)2Si+
1
−444.19500
20.3
−446.56103
13.7
−448.31325
14.1
0.16475
13.1
15.0
18.1
18.9
20.3
28.6
20.8
62.4
68.5
36.4
25.1
40.7
63.2
59.4
(CH3)2HSi+ · C2H4
5
13.4
15.4
18.5
19.2
20.6
29.1
21.8
63.8
69.4
37.5
26.3
40.2
62.9
59.2
(CH3)2HSi+ · C3H6 5p
(n-C3H7)(CH3)HSi+ 3n
(i-C3H7)(CH3)HSi+ 3i
27.2
21.9
22.3
23.6
40.0
38.6
97.3
102.7
57.8
46.5
46.8
71.6
69.6
15.2
16.1
18.0
18.1
27.7
28.4
89.9
94.7
42.5
31.2
37.2
60.7
17.0
18.4
19.4
20.6
29.9
22.7
65.9
71.6
38.8
28.0
44.4
67.1
(C2H5)HSi+
2
(C2H5)HSi+ · C2H4 5e
TS1
TSp
TS1e
TS2
TS3
(CH3)2HSi++C2H4
(C2H5)H2Si++C2H4
(CH3)H2Si++C3H6
57.2
63.8
nalize the appearance of the two other products with a
lower number of carbon atoms in the silylium moiety
than that in the nascent silylium cation we shall first
discuss the results of the theoretical prediction of the
stationary points on the potential energy surface of the
SiC4H1+1 system.
geometric relaxation [38] and association energy for
more simple systems [7] allow an estimation for the
contribution of each factor of about 5 kcal mol−1
.
Thus, one may expect that the diethylsilylium cation (2)
has enough excess energy to rearrange to the most
stable dimethylethylsilylium form 1.
As in SiC2H7+ and SiC3H9+ systems [30–32] the
lowest-energy isomer is the one which has the maxi-
mum number of the alkyl groups attached to silicon. In
our case it is the dimethylethylsilylium cation 1 (Table
2, Fig. 1). But in contrast to previously studied systems
the second most stable isomer in SiC4H1+1 appears not
to be the secondary cation, but rather a complex be-
tween the dimethylsilylium cation and ethene (5). It is
followed by the other complex between the methylsi-
lylium cation and propene (5b). Such complexes were
found to have low energies in previously studied sys-
tems [30,31], but only in SiC4H1+1 are they thermody-
namically more stable than the secondary cations.
Among the three plausible secondary cations, the low-
est are n- and i-propylmethylsilylium cations (3n, 3i)
and the diethylsilylium cation (2). The latter is the
cation generated in our experiment by the decay of
tritium atoms in (C2H5)2SiT2. It is the least thermody-
namically stable of the group of low-lying isomers in
the SiC4H1+1 system.
However, tritiated dimethylethylphenylsilane was not
observed in the experiment. The reasons for this may be
elucidated by analyzing the predicted barrier heights for
the isomerization paths in the SiC4H1+1 system. As in
the previously studied systems with two and three car-
bon atoms [30,31], the rearrangement of the most stable
isomer 1 into 2 goes through the synchronous transfer
of two hydrogens from adjacent methyl groups to the
silicon atom (Fig. 1). The corresponding transition state
(TS1e) is about twice as high in energy as those (TS1,
TS2) which involve the shift of only one hydrogen [39].
The critical parameter for the occurrence of the isomer
interconversion is the relative height of this barrier and
the dissociation limits (with the ethene elimination). In
the SiC2H7+ system the barrier is below the dissociation
limit and the interconversion of isomers may proceed
without dissociation [30]. In the SiC3H9+ [31] and
SiC4H1+1 systems this barrier is above the dissociation
limit and cations dissociate before isomerization to the
most stable isomer (Fig. 2). Thus, the diethylsilylium
cation (2) cannot rearrange to the most stable
dimethylethylsilylium form 1, but rather dissociates into
EtH2Si+ +C2H4. However, since the highest barrier for
isomerization in the SiC2H+ system is below the disso-
ciation limit, the EtH2Si7+ ion can isomerize into the
most stable form, i.e. Me2HSi+, provided the ion has a
sufficient excess energy.
The diethylsilylium cation becomes vibrationally ex-
cited at the time of its generation (the excess energy is
due to the fact that the nascent ion has the geometry of
its precursor [19]) and additional energy may be ac-
quired through association with the benzene molecule.
There are no theoretical estimates for the system dis-
cussed but theoretical predictions for the energy of the

244
T.A. Kochina et al. / Journal of Organometallic Chemistry 586 (1999) 241–246
The experimentally observed products are in keeping
ethene elimination provided that sufficient excess en-
ergy is saved inside the EtH2Si group. The probability
of quenching of this excess vibrational energy in the
liquid phase is greater than in the gas phase and
therefore one may expect a decrease in the yield of this
product in the liquid phase. Indeed the yield of
dimethylphenylsilane drops substantially in going from
the gas to liquid phase (Table 1). In contrast to
with this interpretation. In the gas phase the most
abundant product among three alkylphenylsilanes is
dimethylphenylsilane. It contains the SiHMe2 group,
i.e. the product of the two steps of rearrangement of the
original diethylsilylium cation. The first step is ethene
elimination with formation of the EtH2Si+ cation. This
cation may isomerize into the most stable form without
Fig. 1. The equilibrium structures of stationary points on the SiC4H1+1 PES predicted by the B3LYP/6-31G** method.

T.A. Kochina et al. / Journal of Organometallic Chemistry 586 (1999) 241–246
245
There are several products with minor yields which
can be assigned to the tritiated diethylmethylsilane,
ethylsilane and benzene. The tritiated diethylmethylsi-
lane is produced obviously by the methide ion trans-
fer from the initial diethylsilane to the diethylsilylium
cation. The tritiated ethylsilane is probably the
product of the reaction of the ethylsilylium cation
(such a cation may be produced by the dissociation
of the diethylsilylium cation or by its reaction with
initial diethylsilane as it was mentioned above) with
diethylsilane by hydride ion transfer. The labelled
benzene is presumably formed by H/T exchange be-
tween the diethylsilylium cation and benzene in s-
complex.
Acknowledgements
Computations were carried on in the Computing
Center of the Institute of Organic Chemistry of the
Russian Academy of Sciences and were supported by
the RFFI Grant 98-07-9020. The work was also sup-
ported by the RFFI Grant 9603-33254.
Fig. 2. The scheme of energy levels (kcal mol−1) for the SiC4H1+1
isomer interconversion.
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that it cannot be quenched by collision with the sol-
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