Syntheses, spectroscopic and structural properties of phenoxysilyl compounds: X-ray structures, FT-IR and DFT calculations

Article abstract of DOI:10.1016/j.molstruc.2013.09.058

The reaction of silicon disulfide with alkylphenols yields tetraphenoxysilane, cyclodisilthiane and silanethiol. The outcome of the reaction depends on the presence of the steric hindrance in the ortho position on the reacting phenol. New products of the

Full text of DOI:10.1016/j.molstruc.2013.09.058

Journal of Molecular Structure 1054–1055 (2013) 359–366
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Syntheses, spectroscopic and structural properties of phenoxysilyl
compounds: X-ray structures, FT-IR and DFT calculations
a
a
b
a
a,
⇑
´
Agnieszka Jabłonska , Łukasz Ponikiewski , Krzysztof Ejsmont , Aleksander Herman , Anna Dołe˛ga
a Department of Inorganic Chemistry, Chemical Faculty, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdan´sk, Poland
b Faculty of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
New products of the reaction of silicon disulfide with phenols: phenoxysilane, cyclodisilthiane and sila-
nethiol have been characterized by FT-IR, NMR and X-ray diffraction. The outcome of the reaction
depends on the presence of the steric hindrance in the ortho position of the reacting phenol. The intra-
molecular interactions in the compounds are mainly XH—pi (X = C, S) whereas the intermolecular inter-
ꢀ
ꢀ
ꢀ
ꢀ
X-ray molecular structures of three
phenoxysilyl compounds are
reported.
Vibrational spectra of two
polymorphs of phenoxysilanethiol are
analyzed.
actions are very weak CH—p/CH—O contacts as in aryloxysilane or electrostatic dipole-dipole attractions
in cyclodisilthiane and silanethiol. The energy of intramolecular S–H—
p interaction is estimated with the
use of DFT/GGA BLYP-D XC potentials.
The energy of intramolecular SH—
interaction is estimated from DFT
calculations.
p
DFT calculations are in good
agreement with experimental results.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2013
Received in revised form 4 September 2013
Accepted 27 September 2013
Available online 11 October 2013
The reaction of silicon disulfide with alkylphenols yields tetraphenoxysilane, cyclodisilthiane and sila-
nethiol. The outcome of the reaction depends on the presence of the steric hindrance in the ortho position
on the reacting phenol. New products of the reaction of silicon disulfide with phenols are characterized
by FT-IR, NMR, X-ray diffraction and DFT calculations. The intramolecular interactions in the compounds
are mainly XH—
contacts found in aryloxysilane or electrostatic dipole–dipole attraction in cyclodisilthiane and silaneth-
iol. The S–H— interactions in the obtained silanethiol are analyzed with the use of DFT/GGA BLYP-D XC
potentials. The energy of intramolecular S–H— interaction is estimated.
Ó 2013 Elsevier B.V. All rights reserved.
p (X = C, S) whereas the intermolecular interactions are either very weak CH—p/CH—O
Keywords:
p
Silicon disulfide
Tetraphenoxysilane
Silanethiol
p
Cyclodisilthiane
Crystal structure
DFT calculations
⇑
Corresponding author. Tel.: +48 583472983.
E-mail address: anndoleg@pg.gda.pl (A. Dołe˛ga).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.09.058

´
A. Jabłonska et al. / Journal of Molecular Structure 1054–1055 (2013) 359–366
360
obtained by the direct reaction of sulfur and silicon. 2,6-Diisopro-
1. Introduction
pylphenol (Alfa-Aesar, 97%), 2-isopropylphenol (Alfa-Aesar, 98%)
and toluene were used as purchased.
Alkoxysilanes are produced for diverse purposes such as sol–gel
processes, surface modification, protection of hydroxyl groups and
others. The laboratory and industrial method of synthesis is based
on the reaction of a chlorosilane with an alcohol [1–3]. Several
other methods were proposed in the literature such as the prepa-
ration of (EtO)4Si from Ca3(SiO4)O, Ca2SiO4, and portland cement
[4] or the use of cyclic [5] and unsymmetrical [3] ethers instead
of alcohols. The latter one described quite recently was proposed
with the purpose to avoid generation of toxic and corrosive hydro-
gen chloride during the production process [1]. Even elemental sil-
icon can react with primary alcohols [6–9] and phenols [10] to
afford triorganoxysilanes and tetraorganoxysilanes. With the use
of copper(I) chloride as a catalyst, trialkoxysilanes and triphenox-
ysilane were selectively obtained by the reactions of silicon with
primary alcohols or phenol respectively [8–10]. Direct syntheses
of organodichlorosilanes from silicon, hydrogen chloride and al-
kanes or alkenes [11] and generation of alkylalkoxysilanes from sil-
icon and methanol [12] or alcohol/alkene mixtures [13] were also
developed by the same group of Japanese scientists. These reac-
tions are likewise catalyzed by CuCl.
2.1.1. Tetra(2-isopropylphenoxy)silane (1)
The compound was obtained by phenolysis of SiS2 as described
for tris(2,6-dimethylphenoxy)silanethiol [17], tri(mesityloxy)sila-
nethiol (TMST) [18] and tris(2,6-diisopropylphenoxy)silanethiol
[19]. In the first synthesis SiS2 (60%, 23.5 g) and 2-isopropylphenol
(125 g) were stirred and heated for approx. 30 h in 150–180 °C un-
der the stream of nitrogen. After that 250 ml toluene was added to
the reaction mixture and the unreacted SiS2 was removed by cen-
trifugation. Toluene and the excess phenol were removed by vac-
uum distillation. The crude solid (46.8 g, ꢁ53% yield with regard
to pure SiS2) was dissolved in n-hexane. Colourless crystals of 1
were obtained at À18 °C. M. p. 56 °C, Anal. Calcd.: C-76.01; H-
7.80; S-0.00; Anal. Found: C-75.71; H-7.83; S-0.01 (indicates the
presence of traces of silanethiol or other sulfur-containing com-
3
pounds), 1H NMR (300 MHz) in CDCl3: 1.16 (d, 24H, JHH 6.9 Hz),
3.27 (sept, 4H, 3JHH, 6.9 Hz), 7.05–7.16 (m, 12H), 7.26–7.32 (m, pro-
ton in position 3 of the aromatic ring, 4H) ppm. 13C NMR
(125 MHz) in CDCl3: 22.9 (CH(CH3)2), 26.96 (CH(CH3)2), 119.2 (aro-
matic ring 6-C), 123.37 (aromatic ring 4-C), 126,67 (aromatic ring
5-C), 126,72 (aromatic ring 3-C), 138.91 (aromatic ring 2-C),
150.15 (aromatic ring C-O-Si) ppm. 29Si NMR (59.6 MHz) in CDCl3:
In our department a different method of synthesis of organoxy-
silicon compounds was elaborated i.e. the reaction of silicon disul-
fide with alcohols and phenols. The reactions of silicon disulfide
with primary, secondary and tertiary alcohols were studied in detail
À101.6 ppm. FT-IR (solid, cmÀ1
) 3070 m,br, 3033 m, 2967vs,
´
by Wojnowski, Pie˛kos and Wojnowska [14–16]. The reaction of SiS2
2931vs, 2912s,sh, 2885 m,sh, 2869s, 1944w, 1938w,sh, 1904w,
1873vw, 1830vw, 1794w, 1752w,br, 1718vw, 1703w, 1697vw,br,
1649vw, 1640vw, 1602 m, 1582s, 1493vs, 1450vs, 1384 m,
1363 m, 1348s, 1283s, 1245vs, 1234vs,sh 1195vs, 1159 m,
1156 m,sh, 1114 m, 1088vs, 1038s, 990vs, 968s, 939 m, 929w,
895 m, 859 m, 783vs, 758vs, 754s, 711s.
with 2,6-dimethylphenol and its products: aryloxysilanethiol and
aryloxycyclodisilthiane were first described in 1973 [17]. The most
important advantage of the method is the absence of strong acid
among the reaction products. Gaseous hydrogen sulfide that
evolves during the reaction is burnt outside the reaction vessel. It
allows to obtain pure substances without traces of acid catalyst that
could initiate the sol–gel processes and decomposition of the prod-
ucts. For us it is important since we apply the obtained products
such as silanethiols and silanols for further syntheses of metal com-
plexes e.g. [18–26]. Application of technical SiS2 without the need
for its purification is another favorable feature of the method.
Since 2010 we have re-investigated the reaction of SiS2 with
phenols and added new members to the aryloxysilanethiols family
[18,19]. In this paper we describe the continuation of the studies
and report the synthesis and crystal structure of tetra(2-isopropyl-
phenoxy)silane and tetra(2,6-diisopropylphenoxy)cyclodisilthiane.
The influence of the location of the alkyl substituents in phenol
substrate on the outcome of the reactions between silicon disulfide
and 2- or 2,6-substituted phenols is discussed.
The synthesis was repeated in lower temperatures within the
range 100–120 °C with other conditions unchanged and the same
product was obtained with similar yield. In 60 °C the reaction pro-
ceeded very slowly and the experiment was aborted.
2.1.2. Tetrakis(2,6-diisopropylphenoxy)cyclodisilthiane (2)
The compound was obtained as a side product during the syn-
thesis of tris(2,6-diisopropylphenoxy)silanethiol [19]. Almost all
conditions of the synthesis were the same as in [19] in spite of
the temperature that was lowered to 150–160 °C. SiS2 (40.8 g)
and 2,6-diisopropylphenol (257 g) were stirred and heated for ap-
prox. 30 h in 150–160 °C under the stream of nitrogen. After that
250 ml toluene was added to the reaction mixture and the unre-
acted SiS2 was removed by centrifugation. Toluene and the excess
phenol were removed by vacuum distillation. The crude product
was suspended in n-hexane but partly it did not dissolve thus
the suspension was filtered. The white precipitate obtained as a re-
sult of the filtration was dried and weighed (3.1 g, 2.8% yield, the
yield is given with regard to pure SiS2). Colourless blocks suitable
for X-ray diffraction measurements were obtained from toluene
at 0 °C. Melting point: 191–193 °C, Anal. Calcd.: C-69.51; H-8.26;
S-7.73; Anal. Found: C-69.46; H-7.83; S-7.43, 1H NMR (300 MHz)
Recently a great deal of attention has been sacrificed to various
types of weak interactions involving aromatic rings [27] that in-
clude: anion—p interaction [28], cation—p [29] as well as X–H—p
interactions where X = N, O, S, C e.g. [30]. It has been acknowledged
that such non-covalent forces are responsible for chemical (crystal
packing, supramolecular assembly) and biological (protein folding)
events. Since the participation of sulfur in hydrophobic S–H—
p
interactions is relatively little investigated [31,32] we present the
crystal structures of the obtained compounds as experimental
3
3
in CDCl3: 1.08 (d, 48H, JHH, 6.8 Hz), 3.26 (sept, 8H, JHH, 6.9 Hz),
6.99–7.11 (m, 12H) ppm. 13C NMR (125 MHz) in CDCl3: 23.4
(CH(CH3)2), 28.05 (CH(CH3)2), 123.86 (aromatic ring 4-C), 123,91
(aromatic ring 3,5-C), 139.03 (aromatic ring 2,6-C), 147.3 (aromatic
ring C-O-Si) ppm. 29Si NMR (59.6 MHz) in CDCl3 À58.1 ppm. FT-IR
examples of weak X–H—p interactions (X = C, S) and analyze the
S–H— bonding present in the polymorphic forms of tris(2,6-diiso-
p
propylphenoxy)silanethiol by means of FT-IR and DFT calculations
with the use of GGA BLYP-D XC potential at the TZP level.
2. Experimental section
(solid, crystalline sample cmÀ1
) 3081w,sh, 3063 m, 3030 m,
2969vs, 2931vs, 2907s,sh, 2887m,sh 2869s, 1923w, 1857w,
1796vw, 1730vw, 1712vw, 1695w, 1651w, 1598w,sh, 1588w,
1463s, 1445vs, 1384 m, 1362 m, 1330s, 1256s, 1185vs, 1148w,
1115s, 1107 m,sh, 1099s, 1056 m, 1045s, 968s, 934s, 905vw,
884w, 796s, 773s, 759vs.
2.1. Synthesis of studied compounds
Technical silicon disulfide (material that contained ꢁ60% of
pure SiS2 as confirmed by the analysis of sulfides content) was

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A. Jabłonska et al. / Journal of Molecular Structure 1054–1055 (2013) 359–366
2.1.3. Tris(2,6-diisopropylphenoxy)silanethiol (3 or TDST)
the SHELX-97 program package [34]. A summary of the crystallo-
graphic data is listed in Table 1.
3 with was obtained along with 2 (for synthetic details see
2.1.2) and recrystallized from n-hexane (24.1 g of recrystallized
product, 15.3% yield with regard to pure SiS2). The synthesis of
TDST was described previously [19], however here we have ob-
tained and therefore describe a different polymorphic form of TDST
with a different space group (see Table 1) and distinct FT-IR spec-
trum. Melting poit: 101–103 °C; 29Si NMR (59.6 MHz) in CDCl3-
71.9 ppm. FT-IR (solid, cmÀ1) 3066 m, 3027 m, 2970vs, 2931vs,
2910s, 2872s, 2586w, 2555m, 1923w, 1858vw, 1791vw,
1715vw,br, 1694vw, 1588w, 1552vw, 1465s, 1443vs, 1383 m,
1362 m, 1329s, 1312w,sh, 1255vs, 1202s, 1183vs, 1146w, 1113s,
1099s, 1047s, 970vs, 938m,sh, 931m,sh, 890w, 794m, 790w,sh,
752vs.
2.3. Spectroscopic and thermal measurements, elemental analysis
FT-IR spectra of solids were measured with Momentum micro-
scope (IR detector) attached to Mattson Genesis II Gold spectro-
metr (IR source). Spectra acquisition was controlled by WINFIRST
software package. NMR spectra were measured with Bruker
Avance 300 MHz (1H, 29Si) or Varian Unity Plus 500 MHz (13C)
spectrometer. Thermal measurements were performed in a flow
of argon using Netzsch termobalance TG 209. Samples were heated
in aluminum crucibles.
Elemental analyses were performed on an Carlo Erba Elemental
Analyser EA 1108.
2.4. Quantum–mechanical calculations
2.2. X-ray crystal structures analysis
Based on the solid state geometry, the molecular structures of
1–3 were optimized with the use of ADF implemented functionals:
and FT-IR spectra of 3 were calculated. The calculations have been
performed using ADF release 2013.01 of theoretical chemistry
package developed at Vrije Universiteit, Amsterdam, The Nether-
lands [38–40]. The starting geometries have been taken from
experimental crystallographic data obtained in this work. The
GGA BLYP-D XC potential in SCF and final energy evaluation with
Becke integration scheme at TZP Slater basis set with large frozen
core constrains have been used. Delocalized coordinates have been
used for structure optimization and IR frequencies calculation.
The molecular structures of 1, 2 and 3 were also optimized by
using the B3LYP hybrid functional [35–37] with the 6-
311++G(d,2p) level of theory for 1 and its reference compound,
and 6-31+G(d) for 2 and 3 and their reference compounds. These
calculations were performed using the GAUSSIAN09 program
package [41] and used for the assessment of aromaticity of the
studied compounds (see Supplementary materials).
The single crystals of 1, 2 and 3 were used for data collection at
293(2) K on a four-circle Oxford Diffraction Xcalibur diffractometer
equipped with a two-dimensional CCD detector with graphite
monochromatised Mo K radiation (k = 0.71073 Å) and the
x-scan
technique. Integration of the intensities and correction for Lorenz
and polarisation effects were performed using CrysAlis RED soft-
ware [33].
Structures of 1, 2 and 3 were solved by direct methods and all
non-hydrogen atoms were refined with anisotropic thermal
parameters by full-matrix least squares procedure based on F2.
Hydrogen atoms were usually refined using isotropic model with
U
iso(H) values fixed to be 1.5 times Ueq of C atoms for ÀCH3 or
1.2 times Ueq for ÀCH2 and ÀCH groups. Hydrogen of SH bond in
3 was positioned from differential electron-density maps and re-
fined as disordered between two positions with the occupancy ra-
tio 50:50. Such model is in agreement with the FT-IR spectrum of 3
in the solid state. Crystal structures were solved and refined using
Table 1
3. Results and discussion
Crystal data and structure refinement parameters for 1–3.
Symbol
1
2
3
3.1. Synthesis
Empirical formula
Formula weight
Temperature (K)
Wavelength (Mo K
Crystal system
Space group
a (Å)
C
36H44O4Si
C48H68O4S2Si2 C36H52O3SiS
829.32
298
0.71073
Monoclinic
C2/c
23.5053(14)
9.0129(5)
24.4715(16)
90
105.870(7)
90
4986.7(5)
4
1.105
0.193
568.8
150
592.93
298
0.71073
Monoclinic
P21/n
17.901(4)
12.073(2)
18.797(4)
90
116.69(3)
90
3629.4(14)
4
1.085
0.153
1288
Formulas of the obtained compounds are presented in
Scheme 1.
a
) (Å)
0.71073
Monoclinic
P21/n
16.1798(3)
8.52100(10)
23.3860(4)
90
95.827(2)
90
3207.52(9)
4
1.178
0.110
1224
2.88–25
À19 < h < 19
À10 < k < 7
À27 < l < 27
19472/5621
Silicon disulfide is a fibrous solid containing chains of edge-
sharing SiS4 tetrahedra. The SiS bond lengths within the chains
are 2.133(1) Å and the S–Si–S and Si–S–Si bond angles in the planar
four-membered Si2S2 rings are 98.8(1) and 81.2(1)° respectively
[42]. Oxygenophilic silicon in silicon disulfide reacts readily with
water and primary alcohols [14,43]. Tertiary alcohols react slowly
and so do 2,6-substituted phenols [15–19]. In the reactions of 2,6-
substituted phenols with silicon disulfide silanethiols [17–19] and
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dc (Mg mÀ3
)
tetrakis(2,6-diisopropylphenoxy)cyclodisilthiane
obtained.
[17]
were
l
(mmÀ1
)
F(000)
h range (°)
Index ranges
1792
The reaction of silicon disulfide with mono substituted 2-iso-
propylphenol has led to the formation of tetra(2-isopropylphen-
oxy)silane (1). The reaction of 2-isopropylphenol with SiS2 was
initially carried out in 180 °C. On finding out that the only product
of the reaction is 1 we decided to lower the temperature to 120 °C
with the intention of isolating new silanethiol. However at this
lower temperature 1 was again the only product. Thus we have
concluded that the steric hindrance in both ortho positions of the
phenol molecule is an important factor that allows to stop the
reaction at the stage of silanethiol [17–19] and to prevent the fur-
ther progress that leads to the formation of tetraaryloxysilane
derivative (see Scheme 2).
2.43–26
À17 < h < 28
À9 < k < 11
2.55–26
À22 < h < 13
À14 < k < 14
À30 < l < À29 À23 < l < 23
Reflections collected/
unique
9044/4897
13518/7127
Data/restraints/parameters
5621/0/370
1.026
R1 = 0.0311
wR2 = 0.0806
R1 = 0.0448
wR2 = 0.0832
4897/0/262
1.023
R1 = 0.0538
wR2 = 0.1305
R1 = 0.0848
wR2 = 0.1565
7127/2/378
1.018
R1 = 0.0671
wR2 = 0.1575
R1 = 0.1174
wR2 = 0.1947
Goodness of fit on F2
Final R indices [I > 2
R indices (all data)
CCDC numbers
r
(I)]
CCDC 917842 CCDC 917841 CCDC 917843

´
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A. Jabłonska et al. / Journal of Molecular Structure 1054–1055 (2013) 359–366
Scheme 1.
Scheme 2.
Tetrakis(2,6-diisopropylphenoxy)cyclodisilthiane
2
was ob-
structures were determined [2,45–52] there are only two aryloxy-
silanes [2,50]. Some of those acyclic organoxysilanes were obtained
and studied as parts of more complicated metal complexes [50,51].
Tetra(2-isopropylphenoxy)silane 1 crystallizes at À18 °C from
n-hexane as colourless prisms with monoclinic symmetry. The
molecular structure of 1 is presented in Fig. 1. The compound
exhibits a typical tetrahedral coordination sphere at the Si-atom.
The small distortion of the tetrahedron, with angles ranging from
107.78(5)° to 111.59(5)°, is introduced by the intramolecular
tained as a minor by-product in the reaction of silicon disulfide
with 2,6-diisopropylphenol carried out in the temperature range
150–160 °C. The major product of the reaction was silanethiol 3
(TDST) as described before [19]. The molar ratio of cyclodisilthiane
to silanethiol was close to 1:8. Compared to the conditions de-
scribed in [19] the temperature of the synthesis was slightly low-
ered and the decrease of the temperature must have slowed
down the final step of the reaction presented in Scheme 2. Similar
observations were made by Wojnowski who performed the reac-
tion of SiS2 with 2,6-dimethylphenol either in 150 °C (without sol-
vent) [17] or with the addition of benzene in the temperature up to
110 °C [17,44]. Since 2 is practically insoluble in n-hexane (con-
trary to the respective silanethiol) it was possible to separate and
purify the obtained compounds.
C–H—p interactions between the aromatic rings and the alkyl
substituents of their interacting partners. As the result of these
interactions two pairs of aryloxy groups in the nearly parallel
conformation can be distinguished within the molecule. These
intramolecular C–H—p contacts are listed in Table 3 and illustrated
in Fig. 1S. The Si–O bond lengths are in the expected range for
tetracoordinate silicon compounds [3,46–53]. Compound 1 forms
typical, molecular crystals of low melting temperature (56 °C)
which are built due to weak, intermolecular attraction forces
3.2. Crystal and molecular structures: X-ray measurements and DFT
calculations
including C–H—O, C–H—C and C–H—
p interactions. The closest
intermolecular contacts are indicated in Figs. 2S and 3S.
The crystal data, description of the diffraction measurements
and details of structure refinements are presented in Table 1 (see
Section 2.2 X-ray Crystal Structures Analysis). Selected interatomic
distances and bond and torsion angles for the studied compounds,
obtained by X-ray diffraction analysis and calculated by DFT/GGA
BLYP-D XC/TZP are given in Table 2.
The crystal structures of plain alkoxysilanes are very rare since
they are usually liquid in a wide range of temperatures [45–47].
Among several acyclic Si(OR)4 monomers for which crystal
The molecular structure of tetrakis(2,6-diisopropylphen-
oxy)cyclodisilthiane (2) is shown in Fig. 2. The centrosymmetric
molecule can be compared to the related compounds described
earlier: tetrakis(tert-butoxy)cyclodisilthiane [53], di-l-sulfo-bis-
1,3-di-tert-butyl-2,3-dihydro-1H-1,3,2-diazasilol-2-ylidene [54],
and tetrakis(trimethylsilyl)cyclodisilthiane [55]. Similar to its pre-
decessors, 2 features planar, four-membered ring Si2S2 with small
angle at sulfur Si–S–Si (80.95(4)°) resulting in a short transannular

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A. Jabłonska et al. / Journal of Molecular Structure 1054–1055 (2013) 359–366
Table 2
Important bond lengths and angles in 1–3 (Å, °) from X-ray measurements and DFT calculations.
Parameter
1
2
3
X-ray
DFTb
X-ray
DFTb
X-ray
DFTb,c
DFTb,d
Si1–O1
Si1–O2
Si1–O3
Si1–O4
Si1–S1
Si1–S2
Si1–Sia
1.615 (1)
1.619(1)
1.615(1)
1.616(1)
–
–
–
1.648
1.648
1.648
1.648
–
1.627(2)
1.636(2)
–
1.656
1.658
–
1.628(2)
1.627(2)
1.630(2)
–
2.129(1)
–
–
1.649
1.648
1.656
–
2.148
–
1.653
1.644
1.657
–
–
2.133(1)
2.133(1)
2.769(1)
2.158
2.172
2.792
2.161
–
–
–
O1–Si1–O2
O1–Si1–O3
O1–Si1–O4
O2–Si1–O3
O2–Si1–O4
O3– Si1–O4
Si1–S1–Si1a
Si1–S2–Si1a
O1–Si1–S1
O1–Si1–S2
O2–Si1–S1
O2–Si1–S2
O3–Si1–S1
111.46(5)
107.78(5)
109.56(5)
108.57(5)
107.91(5)
111.59(5)
–
–
–
–
–
–
–
112.30
107.94
108.27
108.04
107.98
112.37
–
–
–
–
–
–
–
106.88(9)
–
–
–
–
–
80.95(4)
80.94(4)
106.89(7)
116.47(7)
116.79(6)
110.97(6)
–
107.06
–
–
–
–
104.70(10)
109.59(11)
–
112.37(10)
–
–
–
104.83
109.93
106.46
106.71
112.67
–
–
–
108.48
–
80.64
80.00
106.28
116.85
116.70
110.50
–
–
–
115.75(8)
–
107.33(9)
–
107.20(8)
115.74
–
108.86
–
113.70
111.78
109.46
104.74
a
b
c
Àx, y, Àz + ½.
GGA BLYP-D XC/TZP.
Intramolecular SH—pi interaction.
No intramolecular SH—pi interaction.
d
The energy of C–H—
strongly depends on the orientation of C–H bonds in relation to
p
forces is rather low (ꢁ4–10 kJ/mol) but it
the plane of aromatic ring [57–59]. The intramolecular C–H—
p
interactions in 2 are shown in Fig. 4S. Crystal packing of 2 is pre-
sented in Fig. 3. Contrary to 1 there is no directional interactions
between the molecules of 2 in crystal and it initially seems that
the packing of molecules is dependent exclusively on the van der
Waals interactions between the aliphatic substituents. There is
however large difference in melting points of 1 and 2 which are
56 °C and 190 °C respectively. This difference cannot be explained
by the presence of weak interactions since the molecular
structures and molecular weights of 1 and 2 are similar. Closer
examination of the crystal packing of 2 reveals the asymmetry of
its molecule viewed along the a axis (see Fig. 3). In order to
evaluate the influence of this asymmetry onto the intermolecular
interactions and melting points of the studied compounds we have
optimized the molecular geometries of 1 and 2 using quantum–
mechanical density functional theory (DFT) calculations. There
are no significant deviations between the values of the bond
lengths and angles of the studied compounds in the solid state
and those found for the calculated molecular structures in vacuum
(Table 2). The molecule of 2 retains the asymmetrical character
during the geometry optimization. The dipole moment associated
Fig. 1. Molecular structure of 1 with the labeling scheme (hydrogen atoms are
omited). Thermal ellipsoids drawn at 30% probability level.
Si–Si distance of 2.769 Å. Similar structural feature was described
for two simple cyclodisilthianes: Si2S2Cl4 and Si2S2Br4 [56]. The
conformation of phenyl rings in the molecule of 2 is partly deter-
mined by the presence of intramolecular C–H—
p
interactions,
Table 3
Shortest intramolecular C–H—
p contacts in 1 and 2.
Compound
Contact
HÁ Á ÁA (Å)
DÁ Á ÁA (Å)
\DHA (°)
X-ray
DFTb
X-ray
DFTb
X-ray
DFTb
1
C7–H7Á Á ÁC10
2.88
2.94
2.83
3.13
3.18
3.04
2.644
2.660
2.637c
2.669c
2.726
2.564
3.7323(20)
3.6811(24)
3.6966(22)
3.9222(20)
4.0141(59)
3.8967(39)
3.612
3.634
3.606c
3.640c
3.589
3.647
144.9
135
151.2
138.6
146.2
146.7
146.6
147.5
146.8c
147c
135.1
171.1
C9–H9CÁ Á ÁCl2
C27–H27AÁ Á ÁC32
C34–H34Á Á ÁC24
C9–H9AÁ Á ÁC16a
C22–H22AÁ Á ÁC1
2
a
b
c
(Àx, y, Àz + 0.5).
GGA BLYP-D XC/TZP.
Different contacts than in the crystal structures.

´
A. Jabłonska et al. / Journal of Molecular Structure 1054–1055 (2013) 359–366
364
SiS2. FT-IR spectroscopy and X-ray diffraction measurements re-
vealed that an arrangement of molecules in the crystals of 3 was
different than the one described before [19]. We suppose that TDST
crystallized as a different polymorph due to the contamination of
the n-hexane solution with small amounts of 2. The polymorphic
form of TDST 3 features disordered SH proton (contrary to the
polymorph described in [19]). According to the crystallographic
model of 3 both proton sites are approximately equally occupied.
The geometry of 3 has also been optimized with DFT method.
The calculations have been performed separately for the molecule
featuring the S–H proton interacting with the phenyl ring (Struc-
ture A) and for the ‘‘non-interacting’’ form (Structure B). The dipole
moments of Structure A (1.08 D) and Structure B (0.82 D) are inter-
mediate between those obtained for 1 and 2 and the same can be
stated about the melting point of 3 which is 103 °C. The crystal
packings of two polymorphic forms: Cc [19] and P21/n (this work)
are presented in Supplementary materials in Figs. 6S and 7S. The
molecules of TDST in the form Cc adopt two positions in crystal
and form well-ordered chains parallel to z axis (Fig. 6S). Form
P21/n is less regular and the molecules of 3 adopt four different
positions in crystal (Fig. 7S). The melting point of TDST in the form
Cc was sharply 105 °C [19]. The difference in melting points of both
polymorphic forms is small but consistent with the differences of
dipole moments. In both polymorphic forms the most important
interactions between the molecules are of electrostatic dipole–di-
Fig. 2. Molecular structure of 2 with the labeling scheme (hydrogen atoms and
isopropyl groups are omitted). Thermal ellipsoids drawn at 30% probability level.
i
Àx, y, Àz + 1/2.
pole character as there are no close intermolecular C–H—
p or S–
H— contacts.
p
3.3. Vibrational spectra of 3 – analysis of the
mode
SH
Interestingly, the disorder observed in the molecular structure
of 3 (see Section 3.2) is reflected in the IR spectra of crystalline
compound. In the experimental FT-IR spectrum of 3 two bands of
S–H stretching vibrations are observed:
– 2554 cmÀ1 – S–H bond interacting with the phenyl ring and.
– 2585 cmÀ1 – non-interacting S–H – the location of the band
very similar to the aliphatic silanethiol derivatives such as tri-
methylsilanethiol (2584 cmÀ1) [60] or tri-tert-butoxysilanethiol
(2592 cmÀ1) [61].
Fig. 3. Crystal packing of 2. A view along the
a axis.
with this asymmetry is surprisingly large and equal to 1.45 D. In
Fig. 3 the head-to-tail manner of packing of these dipoles in the
crystal lattice is easily seen. Thus, quite counterintuitively, there
is a substantial electrostatic contribution to the interactions be-
tween the molecules of 2 in the crystal lattice. It nicely explains
high melting temperature and low solubility of 2 in hexane. Com-
pared to 1, the thermal resistance of 2 is also enhanced (see Sup-
plementary materials Figs. 8S–11S). Unlike in 2 the calculated
dipole moment of 1 is merely 0.04 D. These differences in polarity
of 1 and 2 are reflected in their melting points and in their solubil-
ity in hexane: 1 is soluble in hexane and 2 is not. Additionally the
results of the calculations support the existence of the intramolec-
ular C–H—p interactions in 1 and 2. The distances between the
interacting atoms within the molecules are shorter in the opti-
mized structures than in the crystal structures. It is a consequence
of the lack of intermolecular interactions in the systems being opti-
mized (single molecules in vacuum). The X-ray experimental and
calculated C–H—p contacts are compared in Table 3. The overlays
of experimental and calculated structures of 1 and 2 are presented
as Fig. 5S in Supplementary materials.
Tris(2,6-diisopropylphenoxy)silanethiol TDST 3 was obtained as
the main product in the reaction of 2,6-diisopropylphenol with
Fig. 4. Experimental (lower part of the figure) and calculated (upper part) IR spectra
of TDST in the SH streching region, Letter A denotes optimized structure and
calculated spectrum of the molecule with the intramolecular SH—
P interaction.
Letter B stands for the calculated structure and spectrum of the molecule, in which
SH group does not interact with the phenyl ring.

´
365
A. Jabłonska et al. / Journal of Molecular Structure 1054–1055 (2013) 359–366
The polymorphic form described previously exhibited only one
band at 2552 cmÀ1 [19]. In Fig. 4 the FT-IR spectra of both polymor-
phic forms are compared.
for providing computational facilities (GAUSSIAN09 program
package; Grant ID G33-17). AD is very grateful to Dr. Hebanowska
for express TG/DTG measurements and to Dr. Grubba for determi-
nation of 29Si NMR shifts.
In order to confirm the above assignment of the spectral bands,
we simulated the FT-IR spectrum of 3 using DFT calculations (see
experimental). The GGA BLYP-D XC potential implemented in
ADF produced the closest match to the experimental data. Initially
we applied LDA functional but the obtained S–H bands were
strongly shifted towards lower energy (over 120 cmÀ1). The simu-
lated spectra are presented in Fig. 4 along with the experimental
ones. Capital letter A denotes calculated spectrum and molecular
Appendix A. Supplementary material
CCDC 917841 (compound 2), CCDC 917842 (compound 1) and
CCDC 917843 (compound 3) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary materials
include additional figures illustrating crystal packings and specific
intra- and intermolecular interactions, TG and DTG curves of 1–3,
and the discussion on the aromaticity of phenyl substituents in
the studied compounds. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.molstruc.2013.09.058.
structure of the molecule of TDST in which the S–H—
was present whereas spectrum and structure B were calculated for
the molecule of TDST without intramolecular S–H— bond. Opti-
p interaction
p
mized structures A and B represent local minima of energy that
produce SH modes with different frequencies:
– 2520 cmÀ1 – S–H interacting with the phenyl ring (optimized
structure A).
– 2564 cmÀ1 – non-interacting S–H (optimized structure B).
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