Radiochemical study of the gas phase reaction of nucleogenic diethylsilylium ions with methanol and butanol

Article abstract of DOI:10.1016/S0022-328X(02)01625-X

The gas phase ion-molecule reactions between alcohols (methanol and n-butanol) and nucleogenic diethylsilylium ions generated by the β-decay of the tritiated diethylsilane were studied by the radiochromatographic method. Among the labelled neutral product

Full text of DOI:10.1016/S0022-328X(02)01625-X

Journal of Organometallic Chemistry 656 (2002) 258ꢀ261
/
www.elsevier.com/locate/jorganchem
Radiochemical study of the gas phase reaction of nucleogenic
diethylsilylium ions with methanol and butanol
T.A. Kochina a, D.V. Vrazhnov a, I.S. Ignatyev b
a Institute for Silicate Chemistry, Russian Academy of Sciences, Odoevskogo 24, c.2, 199155 St. Petersburg, Russia
b Radiochemistry Laboratory, Department of Chemistry, St. Petersburg State University, St. Petersburg 199034, Russia
Received 25 November 2001; accepted 8 April 2002
Abstract
The gas phase ionꢀ
/
molecule reactions between alcohols (methanol and n-butanol) and nucleogenic diethylsilylium ions generated
by the b-decay of the tritiated diethylsilane were studied by the radiochromatographic method. Among the labelled neutral products
of the silylation of alcohols diethylsilylalkoxysilanes exhibit the major yield. Diethyl-butoxysilane contains not only n-butyl groups
as in the substrate (21%), but also small amounts of s-butyl (5%) and t-butyl (7%) groups. In other alkoxysilanes the diethylsilyl
group of the nascent nucleogenic cation is rearranged and isomerized. Products with the similar distribution of the silylium group
isomers were observed earlier in the reaction of benzene, however the yield of the products with rearranged silylium groups was
substantially greater for benzene. This difference in the yield of the products with rearranged silylium groups between benzene and
alcohols is rationalized taking into account the more acidic character of the proton in the adduct oxonium ions. # 2002 Elsevier
Science B.V. All rights reserved.
Keywords: Radiochemistry; Silylium cations; Ionꢀmolecule; Radiochromatography
/
1. Introduction
alcohols. To the best of our knowledge there are only
two works devoted to the study of the silylium ion
Generation of free silylium ions was a subject of
numerous studies but their existence in condensed
reactions with alcohols [13,14]. In the ICR studies of
(CH3)3Siꢁ interacting with ROH (Rꢀ
/
Me, Et, n-Pr, t-
Bu) it was shown that the first step is the formation of
the adduct
phases is still a matter of discussion [1ꢀ10]. This
/
elusiveness of silylium ions in condensed phases is
usually ascribed to the extraordinarily high ability of
silicon to react with nucleophiles and to expand its
(Me3)3Siꢁ ꢁROH 0 [Me3SiO (H)R]ꢁ
(1)
coordination sphere [8ꢀ10]. However, little is known
/
Further transformations of the adduct may go
through elimination of methane in the case of methanol
and alkenes for higher alcohols [13,14]. In contrast to
mass-spectrometric studies our method reveals neutral
products of the reaction. The important feature of ths
method is that it allows to identify different isomers
which may appear in the course of the reaction due to
the substantial excess energy accumulated in the en-
counter complex. This energy originates from the
‘deformation’ energy of the nucleogenic cation [11] as
well as from the complexation energy. In our previous
study of the reaction between the diethylsilylium cation
and benzene we observed different isomers of the
silylium group in the final labelled products [12]. In
this communication, we report the nuclear chemical
about their reactions with particular nucleophiles. The
tritium decay method succesfully used previously for the
generation of carbocations [11] is especially suitable for
the study of silylium ion reactions, since it may produce
these ions both in the gas and condensed phases.
Previously we reported the studies of the reaction
between the diethyl-silylium ion and benzene [12]. This
communication deals with the reaction of diethylsily-
lium ions generated by the nuclear decay technique with
* Corresponding author. Fax: ꢁ
/
7-812-328-5401
E-mail address: igor@isi.usr.pu.ru (I.S. Ignatyev).
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 6 2 5 - X

T.A. Kochina et al. / Journal of Organometallic Chemistry 656 (2002) 258ꢀ
/
261
259
study of nucleogenic diethylsilylium ions with methanol
and n-butanol. For the latter case we may observe
isomerization of both silylium ions and the alkyl group
of alcohol, provided our statement that transformations
of the diethylsilylium group occur not in the free cation
but rather in the excited complex [12] is true.
The structure of the diethylsilylium ionꢀ
/
methanol
adduct optimized at the B3LYP/6-31(d,p) theory level is
depicted in the Fig. 1. It is characterized by the nearly
planar SiOHC moiety with the SiO bond length of 1.872
˚
A. The formation of the SiO bond weakens SiC bonds;
˚
their length increases from 1.845 A in the free cation to
˚
1.865 A in the adduct.
Although this equilibrium structure shows substan-
tially strong covalent bonds of the trivalent oxygen, the
real adduct has a significant amount of the excess
energy. Complexation energy of the adduct estimated
at the same level of theory is 49.8 kcal molꢂ1. More-
over, the nucleogenic silylium ion may possess addi-
tional energy due to the fact that its nascent geometry is
characterized by the tetrahedral arrangement of atoms
around silicon (as in the neutral precursor), while the
planar arrangement is inherent to the equilibrium
structure of the cation [12]. Our theoretical estimate of
this ‘deformation’ energy, calculated as the energy
difference betweeen the ‘wrong’ tetrahedral geometry
and the equilibrium one, is 21.6 kcal molꢂ1. Due to the
excess energy the adduct is vibrationally excited. Ac-
2. Experimental
The tritiated Et2SiTꢁ ions were generated by the b-
decay of tritium atoms in the tritiated diethylsilane.
bꢂ
ꢁ
3
(C2H5)2SiT2 0[(C2H5)2SiTꢁ He]
ꢁ
3
0 (C2H5)2SiT ꢁ He
(2)
Reactions of labelled diethylsilyl ions with substrate
yield radioactive products conveniently analyzed by the
radiochromatographic technique [11,15ꢀ17]. The tri-
tiated diethylsilane was prepared and purified according
to techniques described earlier [17]. Commercial alco-
hols were distilled and their purity was analyzed by gas
chromatography.
/
cording to the concept of the ionꢀ
/
molecule complex
[23ꢀ25] the constituent moieties of this complex (in our
/
Reaction mixtures were prepared by introducing
1mCi of the tritiated diethylsilane (diluted by cyclohex-
ane to a specific activity of 5 Ci molꢂ1) and alcohols at a
pressure of 10 Torr into the evacuated and carefully
outgased Mo-glass ampoules. The mixtures were stored
in the dark at room temperature for 2 months. After the
storage period, the ampoules were opened and their
content was analyzed using the ‘Tsvet’ gas chromato-
graph equipped with a running proportional counter.
The tritiated products of reactions were identified by
comparing retention times with those of authentic
reference compounds under identical chromatographic
condi-tions. The 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 the products identified.
case diethylsilylium and butyl groups) may exist in the
comparatively free state of the analogous ions. During
the lifetime of the excited adduct these moieties may
isomerize and/or eliminate stable fragments.
In our case the diethylsilylium ion incorporated in the
ionꢀmolecule complex may isomerize into the more
/
stable form. This behaviour of the diethylsilylium ion
was observed in the study of its reaction with benzene
[12]. The same is true for the butyl group of the alcohol.
This vibrationally excited complex may be deactivated
by the interaction with the substrate molecule. However,
substrate may also deactivate the excited adduct by the
proton transfer. This bimolecular reaction produces
neutral alkoxysilanes.
[Et2TSiO(H)R]ꢁ ꢁROH 0 EtTSiORꢁROH2ꢁ
(4)
The B3LYP method [18,19] as implemented in
GAUSSIAN-94 program [20] was used for the quantum-
chemical calculations. This method was used in our
previous studies of silylium cations [12,21,22].
Table 1
The relative yields (%) for the tritiated neutral products of the
Et2SiꢁTꢁROH (RꢀMe, n-Bu) reaction in the gas phase
Number Tritiated products
RꢃMe RꢃBu
3. Results and discussion
1
2
3
4
5
6
7
8
9
Et2HSiOR
Et3SiH
35
31
12
9
33 (n*
29
9
/
21, s*
/
5, t*7)
/
Relative yields of the tritiated products of reactions of
nucleogenic diethylsilylium ions are shown in the Table
1. The appearance and relative yields of the observed
products may be explained by the following mechanism.
The first step of the reaction is the formation of the
adduct.
Me2HSiOR
EtH2SiOR
ROH
11
4
7
(Et2HSi)2O
C2H4
EtSiH3
4
9
2
1
1
1
Et2MeSiH
1
1
Et2SiTꢁ ꢁROH 0 [Et2TSiO(H)R]ꢁ
(3)

260
T.A. Kochina et al. / Journal of Organometallic Chemistry 656 (2002) 258ꢀ261
/
The last product may be produced by dealkylation of
the adduct with formation of silanols which immediately
condense.
[Et2TSiO(H)R]ꢁ 0 Et2TSiOH
2Et2TSiOH 0 (Et2THSi)2OꢁH2O
(6)
(7)
Alkoxysilanes observed in the study have different
silyl and butyl groups. In addition to the major product,
i.e. diethylalkoxysilane, which possesses the unchanged
diethylsilyl group corresponding to the reactant cation,
two other alkoxysilanes (with the number of carbons in
the silylium group diminished by two) are observed.
This decrease in the carbon contents may be either due
to the elimination of ethene from the diethylsilyl group
of the adduct
[Et2TSiO (H)R]ꢁ 0 [EtTHSiO(H)R]ꢁ ꢁC2H4
(8)
Fig. 1. The structure of the complex between diethylsilylium cation
and methanol optimized at the B3LYP/6-31G(d,p) level of theory. The
or originate from the reactions of ethylsilylium cation
produced by (5).
equilibrium geometry parameters of the heavy atom skeleton: Siꢁ
/
˚
˚
˚
˚
1.542 A,
C1ꢃ
OꢁHꢃ
117.88, Siꢁ
/
1.865 A, SiOꢃ
/
1.872 A, C5ꢁ
OꢁSiꢃ127.98, Oꢁ
116.18.
/
Oꢃ
/
1.481 A, C1ꢁ
/
C2ꢃ
/
The presence of the tritiated ethene observed with
very low yields (2% for MeOH and 1% for BuOH) may
indicate that the first mechanism (8) is partly operative.
However, this mechanism presupposes that the ex-
change between T and ethyl hydrogens in the excited
complex may occur before the ethene elimination. The
other observed alkoxysilane with two carbons in the
silylium group is dimethylalkoxysilane. It may be
produced by the rearrangement of the ethylsilylium
ion incorporated in the adduct into its most stable
form, i.e. the dimethylsilylium ion [21].
˚
/
/
0.971 A, C5ꢁ
/
/
/
/
Siꢁ
/
C1ꢃ106.58, C1ꢁ
/
/
Siꢁ
/
C3ꢃ
/
/
C1ꢁC2ꢃ
/
/
The proton withdrawl usually provides the major part
of neutral labeled products identified in the radio-
chromatographic studies of the nucleogenic cation
reactions.
Indeed alkoxysilanes are main products of reactions
of the diethylsilylium cation with both methanol and
butanol. Their relative yields are 56% for methanol and
53% for butanol (the sum of the yields of products 1,3,4
in the Table 1). The other major product is triethylsilane
which originates from the reaction between nucleogenic
cations and their mother molecule, i.e. diethylsilane, as
described in [12].
[C2H5HTSiO(H)R]ꢁ 0 [(CH3)2TSiO (H)R]ꢁ
(9)
This process in the free SiC2 H7ꢁ system is 22.7 kcal
molꢂ1 exothermic and has a barrier of 42.8 kcal molꢂ1
as shown by B3LYP/6-31(d,p) calculations [21]. This
barrier height may be easily overcome by the excess
complexation energy.
Et2TSiꢁ ꢁEt2SiT2 0 Et3SiTꢁEtT2Siꢁ
(5)
Comparing the present results with those obtained in
our previous study of benzene reactions one may notice
that the similar products of the decay of the excited
adduct were observed in both studies, that are Et2HSiX,
It was also observed as a main product in our
previous study of the reaction between diethylsilylium
ion and benzene [12] in which it has the relative yield of
49%. In the present study we observe the reduced yield
of this product (31% for methanol, 29% for butanol).
This decrease may be rationalized taking into account
the greater energy of the formation of the complex with
alcohols and therefore the higher rate of this process
EtH2SiX, and Me2SiHX (XꢃPh, OR). However, their
/
relative yields are different. While in the diethylsilylium
ion-benzene system [12] the product with the unchanged
diethylsilylium group, i.e. Et2HSiX, had the lowest yield
among those three compounds (6%), in the diethylsily-
lium ion-alcohol systems the yield of this product is the
highest one (35%). Correspondingly, the yields of
compounds with rearranged silylium groups for
MeOH (9 and 12%) and BuOH (11 and 9%) are
substantially lower than those for benzene (15 and
26%). Note that no products of isomerization of Et2HSi
which competes with the silylium ion*silane interac-
/
tion. Two other products with minor yields, i.e. ethylsi-
lane and diethylmethylsilane (8,9 in the Table 1) also
originate from the interaction of diethylsilylium ions
with diethylsilane. Both groups of products (alkoxysi-
lanes and products of interaction with silane) comprise
ca. 87% of the labelled products. The remaining
products (with the 13% total yield) are: tritiated
substrate (alcohol), ethene, and tetraethyldisiloxane.
ꢁ
into the most stable form, i.e. EtMe2Siꢁ were
observed in both cases. This is in accord with our
theoretical predictions that the EtH2Siꢁꢁ
C2H4 disso-
/

T.A. Kochina et al. / Journal of Organometallic Chemistry 656 (2002) 258ꢀ
/
261
261
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height for isomerization into EtMe2Siꢁ [12].
The reason of the difference in the yield of products
with rearranged silylium groups between reactions with
benzene and alcohol may be explained by the analysis of
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bonded Wheland intermediates with silylium ions,
however they are characterized by a rather weak
bonding and significant contribution of a p-bonded
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complex is 0.2 that differs not much from other
hydrogen atoms of the cycle, while in the methanol
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33% contain not only n-butyl groups as in the substrate
(21%), but also small amounts of s-butyl (5%) and t-
butyl (7%) groups.
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