Electrochemical properties of arylsilanes

Article abstract of DOI:10.1016/j.elecom.2019.03.010

In the past, the electrochemical properties of organosilicon compounds were investigated for both fundamental reasons and synthesis purposes. Little is, however, known about the electrochemical behaviour of hydrogen-bearing arylsilanes. Here, we throw light on the electrochemical properties of 11 arylsilanes compounds, 2 of them synthesized for the first time. The oxidation potentials are found to depend on both the nature and number of the aryl groups. Based on these findings it was possible to establish some variation trends that match the expected structure–property correlations. Furthermore, we present first insights into the electrochemical reaction kinetics behind and identify several soluble electrochemical oxidation products.

Full text of DOI:10.1016/j.elecom.2019.03.010

Accepted Manuscript
Electrochemical properties of arylsilanes
Judith Biedermann, H. Martin R. Wilkening, Frank Uhlig, Ilie
Hanzu
PII:
S1388-2481(19)30074-8
DOI:
Reference:
https://doi.org/10.1016/j.elecom.2019.03.010
ELECOM 6431
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Electrochemistry Communications
Received date:
Revised date:
Accepted date:
1 January 2019
15 March 2019
15 March 2019
Please cite this article as: J. Biedermann, H.M.R. Wilkening, F. Uhlig, et al.,
Electrochemical properties of arylsilanes, Electrochemistry Communications,
https://doi.org/10.1016/j.elecom.2019.03.010
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ACCEPTED MANUSCRIPT
Electrochemical properties of arylsilanes

b)
a)
b)
a)†
Judith Biedermann , H. Martin R. Wilkening , Frank Uhlig and Ilie Hanzu
a) Institute for Inorganic Chemistry (NAWI Graz), Stremayrgasse 9, A-8010 Graz, Austria
b) Institute for Chemistry and Technology of Materials (NAWI Graz), Christian Doppler Laboratory for Lithium
Batteries, Stremayrgasse 9, A-8010 Graz, Austria
Abstract
In the past, the electrochemical properties of organosilicon compounds were investigated for both
fundamental reasons and synthesis purposes. Little is, however, known about the electrochemical
behaviour of hydrogen-bearing arylsilanes. Here, we throw light on the electrochemical properties of
11 arylsilanes compounds, 2 of them synthesized for the first time. The oxidation potentials are found
to depend on both the nature and number of the aryl groups. Based on these findings it was possible
to establish some variation trends that match the expected structure–property correlations.
Furthermore, we present first insights into the electrochemical reaction kinetics behind and identify
several soluble electrochemical oxidation products.
Keywords: arylsilanes, synthesis, oxidation potential, thermodynamics, kinetics, fluorination
1. Introduction
The interest in the electrochemistry of arylsilicon compounds stems from several intriguing electronic
structure properties of these molecules. When an aromatic system is substituted with a silyl group in
α-position, the HOMO energy level of π-electron systems is elevated whereas the LUMO energy level
is lowered. In principle, this should result in a lowering of the energy band-gap of the compound.
Consequently, the difference between the electrochemical oxidation and reduction potentials is
expected to decrease. For instance a lower LUMO would result in a cathodic reaction (reduction)
occurring at a more positive potential [1]. Furthermore, from a mechanistic point of view, silyl-
substituents are good leaving groups acting as “super protons”, especially when cathodic
intermediates are formed in β-position [2]. Another interesting point is related to the easier formation
and stabilization of α-anions [3].
During the past decades, the electrochemical synthesis of organosilicon compounds was investigated,
albeit the amount of information available varies with the nature of the compounds and their
technological relevance. For instance, one can find extensive electrochemical data on organosilicon
halides and polysilanes [4-10]. Also, the electrochemical reduction potentials of
triorganohalosilanes [11] are known. Far less information is available on hydrogen-bearing arylsilanes
and electrochemical studies of these compounds remain extremely rare [12, 13]. Even when some data
is available, not everything is elucidated. For instance, the influence of size and nature of the aryl
substituents is hitherto unknown [14]. Since hydrogen-rich aryl silanes are promising candidates for
†
see also for correspondence: frank.uhlig@tugraz.at
 corresponding author: hanzu@tugraz.at
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various practical applications including precursors for composite materials such as ceramics,
protecting groups in organic chemistry, materials for surface modification etc. [15-18] a more detailed
knowledge and understanding of electrochemical properties of this class of compounds would
certainly be required.
Herein, we report for the first time the oxidation potentials of a series of two new and nine previously
known mono-, di- and triarylsilanes bearing hydrogen. In addition, we assess the influence of the
nature and size of the aryl residues on the electrochemical oxidation reaction. In order to shed some
light onto the electrochemical reaction, the products formed during electrochemical reactions are
characterized by GC-MS as well as DI-MS measurements in an attempt to establish the chemical
speciation of the resulting compounds.
Scheme 1 The different arylsilanes synthesized and investigated in this work and the corresponding numeration
scheme used.
R1
R1
R2
R1
R2
R3
phenyl
2,5-xylyl
1-naphthyl
phenyl
phenyl
2,5-xylyl
phenyl
1-naphthyl
phenyl
phenyl
phenyl
phenyl
1*
4*
8*
[19]
†
[20, 21]
2,5-xylyl
1-naphthyl
1-naphthyl
1-naphthyl phenyl
2
3
5
9
[22]
[23]
†
1-naphthyl 1-naphthyl phenyl
6
10
11
[24]
[22, 25, 26]
1-naphthyl 1-naphthyl 1-naphthyl
7
* commercially available; † new compound
2. Experimental
2.1.Materials and methods
The substrate aryl(hydrido)silanes were prepared by a reduction reaction with lithium aluminium
hydride (LiAlH4) of the corresponding chlorosilanes. The chlorosilanes were prepared via Grignard or
lithiation reactions according to literature procedures [24].
Figure 1. A summary of the synthesis methods of RnSiZ4-n compounds (R = phenyl, 1-naphthyl, 2,5-xylyl; Z = Cl,
H).
2.1.1. Aryl-Substituted Chlorosilanes (RnSiCl4–n)
RSiCl3 Aryl trichlorosilanes were prepared via reaction of the respective aryl Grignard reagent with
SiCl4 in THF (tetrahydrofuran), as reported in literature [27]. In a previous study it was established
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that using a large excess of SiCl4 (8 equivalents) could reduce the formation of by-products [22].
However, in the present work it was found that also a smaller excess, i.e. 4 - 6 equivalents, leads to the
desired effect, yielding 76% of 2,5-xylylSiCl3 and 90% of 1-naphthylSiCl3. Purification of the products
by distillation resulted in pure crystals suitable for single crystal X-ray diffraction.
R2SiCl2 For the synthesised diaryl dichlorosilanes a lithiation reaction was preferred over the
Grignard route due to comparatively less by-products. In a typical reaction the aryl bromide was
lithiated at –78 °C in Et2O (diethyl ether) and subsequently transferred onto a solution of 0.5
equivalents SiCl4 at 0 °C (Figure 1, route 2a). After stirring at room temperature, filtration of the
formed salts and evaporation of the solvent, the products were purified by distillation or
recrystallization from THF, toluene or ethyl acetate. The compounds 2,5-xylyl2SiCl2 and 1-
naphthyl2SiCl2 could be obtained in satisfactory yields of 65%. In the case of the mixed substituent
compound 1-naphthyl(phenyl)SiCl2 the procedure was optimised by transferring the lithiated aryl
species onto a solution of 2.0 equivalents phenylSiCl3 at –20 °C (Figure 1, route 2b). Thus, the
formation of side-products was minimised and the yield of the desired product reached 62% while the
standard procedure yields only approximately 40%.
R3SiCl Triaryl chlorosilanes were prepared according to literature procedures [22, 25] via lithiation of
the respective aryl bromide with n-butyl lithium (n-BuLi) in Et2O (diethyl ether) at –78 °C and
subsequent reaction with a chlorosilane at 0°C. While 2,5-xylyl3SiCl and 1-naphthyl3SiCl were
prepared with 0.33 equivalents of SiCl4 each (Figure 1, route 2c), phenylated chlorosilanes were
employed for the formation of 1-naphthyl/phenyl mixed substituent monosilanes, namely 1.0
equivalent phenyl2SiCl2 (Figure 1, route 2e) for 1-naphthyl(phenyl)2SiCl and 0.5 equivalents
phenylSiCl3 (Figure 1, route 2d) for 1-naphthyl2(phenyl)SiCl. The solid products were purified by
recrystallization in THF, toluene or ethyl acetate resulting in pure crystals suitable for single crystal X-
ray diffraction studies.
2.1.2. Aryl-Substituted Silanes (RnSiH4–n)
The preparation of all hereby-investigated aryl silanes was based on a reaction of the precursor aryl
chlorosilanes with lithium aluminium hydride (LiAlH4) in Et2O at 0 °C. Phase separation with diluted,
degassed sulphuric acid, removal of salts over aqueous, degassed sodium tartrate solution and
evaporation of the solvent resulted in the colourless hydride species.
RSiH3 (1-3) The primary silanes phenylSiH3 (1), 2,5-xylylSiH3 (2) and 1-naphthylSiH3 (3) were purified
by distillation resulting in air- and moisture sensitive colourless oils.
R2SiH2 (4-7) The secondary silanes phenyl2SiH2 (4) and 1-naphthyl(phenyl)SiH2 (6) were purified by
distillation resulting in colourless oils. Compounds 2,5-xylyl2SiH2 (5) and 1-naphthyl2SiH2 (7) were
purified by recrystallization from toluene or THF, resulting in pure crystals suitable for single crystal
X-ray diffraction.
R3SiH (8-11) Purification of the tertiary silanes phenyl3SiH (8), 1-naphthyl(phenyl)2SiH (9),
1-naphthyl2(phenyl)SiH (10) and 1-naphthyl3SiH (11) was carried out by recrystallization from THF,
toluene or ethyl acetate. For compounds 9-11 the hydrogenation reaction was performed in a mixture
of Et2O and THF due to the decreasing solubility caused by the introduction of the sterically
demanding 1-naphthyl groups.
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1
13
29
The purity of the silanes was checked by H, C and Si NMR, GC-MS (Gas Chromatography – Mass
Spectrometry), elemental analysis and, whenever possible, single crystal X-ray diffraction. The purity
of all 11 silane compounds used in electrochemical investigations was at least 99.5 % or better.
All the solvents used were dried either using a solvent drying system (Innovative Technology Inc.) or
distilled over phosphorous pentoxide and subsequent storage over molar sieves. Tetra-n-
butylammonium hexafluorophosphate (TBAF) was purchased from Sigma-Aldrich and was
recrystallized from anhydrous ethanol twice before use. All other chemicals from commercial sources
were used without further purification. GC-MS measurements were performed on an Agilent
Technologies GC system with a HP-SMS column and a mass-selective detector Type Agilent 5975C
using EI at 70 eV.
2.2.Electrochemical Characterization
Cyclic voltammetry and chronoamperometry experiments were carried out in a purpose-built cell.
®
The cell body consisted of a Swagelok PFA-620-3 tube fitting with a diameter of ⅜ inch, allowing for
sample volumes as small as 0.5 mL. A Pt microelectrode with a diameter of 20 µm (Metrohm) in a
PTFE (poly-tetrafluoroethylene) sleeve acted as the working electrode, while a glassy carbon disk
electrode with a diameter of 3 mm was the counter electrode. The PTFE sleeve was used to achieve an
airtight seal of the 20 µm Pt microelectrode insulator body (glass, 7 mm in diameter) into the ⅜ inch
Swagelok tube fitting. The reference electrode was a non-aqueous Ag/AgCl system. It consisted of an
AgCl-coated silver wire in contact with solid AgCl in acetonitrile (MeCN) as solvent. The reference
electrode filling solution was saturated with both AgCl and TBACl (tetrabutylammonium chloride).
The reference electrode potential should be at -0.042 V vs. NHE (Normal Hydrogen Electrode) or at
-0.264 V vs. aqueous Ag/AgCl electrode [28, 29]. The supporting electrolyte was 0.1 M TBAF
(tetrabutylammonium hexafluorphosphate) in MeCN. Experiments were carried out within the
practical electrochemical stability window of the solvent and the supporting electrolyte that was
previously determined by linear sweep voltammetry. Thus the 0.1 M TBAF in MeCN was found to be
stable between –2.0 and 2.5 V vs. Ag/AgCl (in MeCN).
3. Results and Discussion
Cyclic voltammetry (CV) on Pt microelectrodes was carried out with 2 mM solutions of the respective
–1
aryl(hydrido)silanes in 0.1 M TBAF/MeCN at a sweep rate of 100 mV s . The resulting curves are
characteristic to solely oxidation responses, which is in agreement with data reported for comparable
aryl(chloro)silanes that also show a totally irreversible oxidation behaviour [4-7, 30-32]. The onset
potentials and formal potentials are summarized in Table 1. Also listed in Table 1 are the open circuit
potentials (OCP) measured versus the same reference electrode before the start of the cyclic
voltammetry experiment. While not having an obvious thermodynamic significance, we believe the
OCP might be useful for ensuring the reproducibility of electrode surface and thus of the whole CV
experiment.
In all cases, no electrochemical activity was found for phenyl-substituted hydridosilanes within the
stability window of the electrolyte. The responses for differently aryl-substituted silanes RSiH3 show
clear differences in both the onset as well as the formal potentials. 1-naphthylSiH3 (3), for instance
exhibits a lower onset potential at 1.83 V vs. Ag/AgCl (MeCN) than 2,5-xylylSiH3 (2) at 2.05 V (Figure
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2, a)). Similarly shifted onset potentials are found for hydridosilanes with two or three aryl residues
(Figure 2 b) and c), Table 1).
Table 1. Oxidation onset potentials (Eonset), formal potentials (E0’)
and open circuit potentials (OCP) of compounds 1-11. All
potentials are expressed vs. Ag/AgCl (MeCN) reference
electrode.
0
Compound
1 phenylSiH3
2 2,5-xylylSiH3
3 1-naphthylSiH3
4 phenyl2SiH2
Eonset / V
–
2.05
1.83
–
2.15
1.90
1.87
–
1.90
1.92
1.89
E ′ / V OCP / V
–
0.09
–0.06
0.21
0.27
–0.10
0.60
0.46
0.47
0.08
0.36
0.17
2.16
1.91
–
5 2,5-xylyl2SiH2
–
6 1-naphthyl(phenyl)SiH2
7 1-naphthyl2SiH2
8 phenyl3SiH
9 1-naphthyl(phenyl)2SiH
10 1-naphthyl2(phenyl)SiH
11 1-naphthyl3SiH
1.94
1.93
–
1.96
1.94
1.94
However, the onset potentials for hydridosilanes with mixed 1-naphthyl/phenyl substituents (6, 9, 10)
correspond directly to the purely 1-naphthyl substituted silanes indicating a stronger effect caused by
this residue compared to the smaller phenyl. Obviously, compounds containing 1-napthyl
substituents are more readily undergoing an oxidation charge transfer. The potential-shifting
behaviour along with different residues was described by Becker et al. [12, 13] for silylarenes R(SiH3)n.
Regarding xylyl residues, their oxidation potential shifts towards lower values, with respect to the
phenyl residues. This was ascribed to the positive inductive effects exerted by the methyl groups. The
presence of these electron-donating groups will increase the electron density on the aromatic ring thus
lowering the oxidation potential. While silyl group also exerts a positive inductive effect, this proves
insufficient to lower the oxidation potential of phenyl residue below the upper potential limit of the
cyclic voltammetry experiment. This is expected since silicon has a lower (Pauling) electronegativity
than carbon. Formal potentials of 1-naphthyl containing hydridosilanes are significantly lower than
their phenyl analogues. We can understand this by noting that, on Pt, the electrochemical oxidation
potential of naphthalene is by almost 0.8 V lower than the oxidation potential of benzene [33]. Thus
for identical substituents (i.e. silyl) the 1-naphthyl will also show a lower oxidation potential than
phenyl. In case of 1-naphthylSiH3 (3) a second electrochemical process occurs at the edge of the
stability window of the supporting electrolyte – this has already been described for pure
naphthalene.[34] Furthermore, the mentioned inductive effects of Si are reduced by the presence of
more than one residue. This reduction is most pronounced upon introduction of a second residue,
whereas only minor changes occur when comparing arylhydridosilanes with two or three residues, i.e.
compare the oxidation onset potentials of compound 3 with 7 and 11.
While inductive effects will significantly influence thermodynamic parameters, steric effects will most
likely play also on the kinetics of electrochemical oxidation. Indeed, compared to the single aryl-
substituted hydridosilanes, responses for di- and triaryl hydridosilanes appear significantly
5

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broadened, indicating much slower reaction kinetics. A broadened response can actually be the result
of a polymer film formation on the surface of the electrode that lowers the apparent kinetics of an
electrochemical reaction. However, because the electrode is initially clean and some products are
soluble (see Table 3, film formation cannot be the only explanation. Also, the slower kinetics are
further indicated by the significantly lower cyclic voltammetry currents recorded for di- and triaryl
hydridosilanes. This behaviour may be explained by the sterically more demanding electrochemical
reaction environment. In principle, the more aryl substituents are present, the better the shielding of
the silicon centre and hydrogen functionalities would be, thus reducing their reactivity. Although we
cannot establish at this point an accurate reaction mechanism, it is clear that the electrochemical
oxidation is accompanied by fluorination of the silane, i.e. a Si-H bond is replaced by Si-F bond at least
in some products (see Table 3), with the fluoride source being the hexafluorophospate anion from the
supporting electrolyte. While it is not clear that the cationic intermediate presents hyperconjugation
between Si and the aromatic residue, it is obvious that the single aryl-substituted silanes are more
exposed to attack on the Si centre. Thus, assuming that the intermediary cation state has a limited, yet
similar, lifetime for all compounds (before reducing back to its initial state), there is higher probability
of a reaction on the Si centre for single aryl-substituted silanes.
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Figure 2. a) Cyclic voltammetry of 2 mM RSiH3 (R = phenyl (red), 1-naphthyl (black), 2,5-xylyl (blue)) in 0.1 M
TBAF/MeCN solution at 100 mV s–1. b) Cyclic voltammetry of 2 mM R2SiH2 (R = phenyl (red), 1-naphthyl (black),
phenyl/1-naphthyl mixed (green), 2,5-xylyl (blue)) in 0.1 M TBAF/MeCN solution at 100 mV s–1. c) Cyclic
voltammetry of 2 mM R2SiH2 (R = phenyl (red), 1-naphthyl (black), phenyl/1-naphthyl mixed (green/orange)) in
0.1 M TBAF/MeCN solution at 100 mV s–1. Working electrode: 20 µm Pt micro electrode, counter electrode: 3 mm
GC disk electrode, reference electrode: Ag/AgCl in MeCN.
To shed more light onto the electrochemical oxidation reaction kinetics, Tafel analysis (Figure 3) for
the respective polarization curves was carried out. Thus, the Butler-Volmer model of electrode kinetics
was used in order to determine the exchange current density (j0) and either the cathodic transfer
coefficient () or the number of exchanged electrons (n). Since no values for n and  for the electrode
reactions are available, either n = 1 or a symmetric reaction barrier ( = 0.5) were assumed,
respectively. The determined values are summarized in Table 2. The CV data allowed for these
determinations only in the cases of compounds 2, 3, 6, 7, 9-11. In case of phenyl(hydrido)silanes (1, 4,
8) and 2,5-xylyl2SiH2 (5), the electrochemical oxidation responses are situated outside of the
electrochemical stability window of the electrolyte solution and it was thus not possible to analyze
them by this technique.
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Figure 3. Tafel plots of a) 2,5-xylylSiH3 (2) and 1-naphthylSiH3 (3), b) 1-naphthyl2SiH2 (7) and 1-naphthyl3SiH (11).
Straight lines represent fits of the linear Tafel regions.
The values determined for n and  correlate nicely with the assumed values with  lying mostly close
to 0.5 and n at close to 1. The main exception is compound 3, whose reaction barrier appears shifted
towards the product side (i.e.  = 0.1) when only one electron is considered to be exchanged in the
charge transfer step. While having such a small value of  cannot be altogether excluded, we estimate
that this is a rather unlikely occurrence. Alternately, it may be that more electrons are exchanged.
Indeed, if we assume a symmetric potential energy reaction barrier ( = 0.5) we find n = 1.8, a number
indicating two rather than one electrons exchanged, very likely in close consecutive charge transfer
steps. Generally, a trend of  and n is evident showing a correlation of the steric and aromatic effects
of the residues. Thus, enlarging the steric bulk around the silicon centre by means of either the
substituent sizes or numbers of residues leads to a shift of the reaction barrier towards the educts ( >
0.5). These values correlate well with the exchange current density (j0) values also presented in Table 2.
We see for instance that for the single substituted 1-naphthylSiH3 compound (3) j0 is more than one
order of magnitude higher than for di- and tri-aryl substituted silanes (7, 11). This clearly
demonstrates faster oxidation kinetics of 3 with respect to 7 and 11. Also, we note that for di-and tri-
aryl compounds, as soon as at least 1-naphthyl substituent is present, the oxidation kinetics are fairly
similar regardless of the other substituents. This may be understood in terms of steric hindrances and
it quantitatively confirms our initial qualitative assessment of electrode kinetics (vide supra).
Table 2. Determined values of exchange current densities (j0) cathodic charge transfer coefficients () and
number of exchanged electrons (n) corresponding to the electrochemical oxidation of aryl(hydrido)silanes
on platinum.
j0
00
n
–2
mA cm
(assumed n = 1)
(assumed  = 0.5)
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2 2,5-xylylSiH3
3 1-naphthylSiH3
1.58
119.4
2.92
4.05
2.84
2.72
3.67
0.45
0.10 / 0.55*
0.68
1.09
1.80
0.64
0.79
0.91
0.93
1.28
6 1-naphthyl(phenyl)SiH2
7 1-naphthyl2SiH2
9 1-naphthyl(phenyl)2SiH
10 1-naphthyl2(phenyl)SiH
11 1-naphthyl3SiH
0.61
0.54
0.54
0.36
* assumed n = 2
The electrochemical response was also largely reduced after only few cycles due to deposition and
film formation on the working electrode. In order to investigate the nature of the deposits and the
products, chronoamperometry experiments (2.3 V for 15 min, 3 mm disk Pt WE) were conducted. The
initially colourless sample solutions changed color towards yellow, orange or dark brown during the
experiments. The colour changes varied depending on the number of aryl substituents: the most
intense colours (and thus formed products) were seen for RSiH3, while the samples R3SiH changed
only slightly or not at all. The soluble components were analysed by GC-MS and determined to
consist mostly of the starting arylsilanes compounds (educts) as well as fluorinated arylsilanes. The
residual insoluble solids were subjected to DI-MS measurements which confirmed the formation of
fluorinated (aryl)di-, tri- or tetrasilanes. However, due to the small amounts of sample used, a full
characterization of the structures remains subject of future work.
Table 3 Selected examples of electrochemically formed soluble products determined by GC-MS. The percentages
(%) are given with respect to the overall content of detected silanes. m/z – mass/charge ratio; r.t. – retention time.
Educts
r.t. / min Products
r.t. / min
13.58
17.45
17.97
9.59
21.02
14.40
24.60
%
29
14
26
<1
7
m/z
m/z
6 1-naphthyl(phenyl)SiH2
7 1-naphthyl2SiH2
9 1-naphthyl(phenyl)2SiH
234.1
284.1
310.1
14.14
18.11
18.16
1-naphthyl(phenyl)SiF2
270.1
320.1
328.1
220.1
378.2
278.1
428.2
1-naphthyl2SiF2
1-naphthyl(phenyl)2SiF
phenyl2SiF2
10 1-naphthyl2(phenyl)SiH
11 1-naphthyl3SiH
360.2
410.2
21.26
25.10
1-naphthyl2(phenyl)SiF
phenyl3SiF
1-naphthyl3SiF
<1
2
The formation of the fluorinated products would require two consecutive distinct reactions. The silane
is electrochemically oxidized and subsequently the formed cation is fluorinated with the readily
available fluorine ions stemming from the supporting electrolyte salt. However, by featuring a
delocalized electron structure, the hexafluorophosphate ion is a weak nucleophile reagent and it is not
clear at this point how the fluorination step actually takes place. As expected, this reaction is probably
facilitated for silane samples with the least sterically encumbered environment, i.e. RSiH3.
As a final observation, the initial charge transfer step necessary for these reactions is most likely
occurring through the aromatic system instead of directly to the silicon atom. There are
thermodynamic and kinetic arguments supporting this fact. First, for single substituted arylsilanes we
see large differences in oxidation onset and formal potentials as a function of the substituent. Thus,
1-naphthylSiH3 (3) has the lowest oxidation potential, followed by 2,5-xylylSiH3 (2) while phenylSiH3
(1) has the highest oxidation potential. This order matches the electrochemical oxidation potential
order of naphthalene, p-xylene, and benzene. Second, positive inductive effects of the silyl group
occur and slightly shift the oxidation potential as a function of the number and nature of aromatic
residues. Compare for instance 3 with 6, 10 and 11. If the charge transfer would not occur with
9

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electrons from the aromatic system, then the inductive effects would very likely not lead to
measurable changes in oxidation potentials as a function of the number of substituents. The more
substituents are present, the weaker the positive inductive effects are, and thus slightly higher
oxidation potentials are measured. Third, with the exception of 3, (see above for an explanation) the
exchange current densities are fairly similar for all compounds containing at least 1-naphthyl residues
(i.e. 6, 7, 9, 10, 11). This indicates a very similar charge transfer mechanism occurring on the substrate
in a locus with very similar chemical vicinity. Had the elementary charge transfer step occurred on the
silicon centre, one could expect more variability – for each compound the Si environment is quite
different, there are a variable number of weaker Si-H bonds, various substituents etc.
4. Conclusions
Cyclic voltammetry experiments conducted on a series of mono-, di- and triaryl hydridosilanes show
a good correlation of the onset and formal electrochemical oxidation potentials with the number and
nature of the aryl residue. Generally, the electrochemical oxidation potentials are lowered in the order
phenyl, 2,5-xylyl, 1-naphthyl. Positive inductive effects of silyl group are found to change the
oxidation potentials slightly. Electrochemical oxidation kinetic parameters show a significantly more
pronounced dependence on steric effects. The electrochemical oxidation of all samples results in the
formation of fluorinated arylsilanes. The initial elementary charge transfer step occurring during the
electrochemical oxidation of arylsilanes very likely occurs through the aromatic system rather than
directly to the silicon atom.
Acknowledgements:
This work was funded by the Austrian Research Promotion Agency (FFG) through the Research
Studio Austria project AIMS (Grant No. 4338751).
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Highlights



Electrochemical properties of single, double and triple substituted arylsilanes
Thermodynamics and kinetics of anodic oxidation
The influence of the aryl residue and the oxidation potential variation trends
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