Intramolecular interactions in crystals of tris(2,6-diisopropylphenoxy) silanethiol and its sodium salts

Article abstract of DOI:10.1021/ic2013073

Hydrolytically stable silanethiol tris(2,6-diisopropylphenoxy)silanethiol (TDST) has been synthesized and reacted with sodium metal. In solid state TDST exhibits π-interactions between the S-H unit and the π-system of the arene, replaced by cation-π inter

Full text of DOI:10.1021/ic2013073

Article
pubs.acs.org/IC
Intramolecular Interactions in Crystals of Tris(2,6-
diisopropylphenoxy)silanethiol and Its Sodium Salts
,†
†
†
‡
́
Anna Dołega,* Wojciech Marynowski, Katarzyna Baranowska, Maciej Smiechowski,
̨
and Janusz Stangret‡
‡
†Department of Inorganic Chemistry and Department of Physical Chemistry, Gdansk University of Technology, Chemical Faculty,
́
ul. Narutowicza 11/12, 80-233 Gdansk, Poland
S
* Supporting Information
ABSTRACT: Hydrolytically stable silanethiol tris(2,6-diisopropylphenoxy)-
silanethiol (TDST) has been synthesized and reacted with sodium
metal. In solid state TDST exhibits π-interactions between the S−H unit
and the π-system of the arene, replaced by cation-π interactions in its
sodium salts. The interactions are documented by crystal structures and
FT-IR spectroscopy.
the structure of thiolates of group 1 elements were reviewed.17
1. INTRODUCTION
Because of the well recognized ability of thiolato ligands to
form bridges between metal ions,18 alkali metal thiolates exhibit
the varying extent of aggregation; they crystallize as rare,
mononuclear species,19 common, four-membered M2S2
rings,16,19b,20 ladders,15,20c,21 cubanes,8a,16b,21b or other coordi-
nation oligomers and polymers.19b,20c,d,21b,22 The aggregation
can be prevented by the increase of the steric hindrance of
thiolato ligand7,18,19a,20 and/or addition of coligands such as
tetrahydrofuran16b,19,20c,d or N,N,N′,N′-tetramethylethylenedia-
mine.16a,19b,20c,21a,22a It is possible to obtain crystalline com-
pounds with separate or contact thiolato anions and alkali metal
cations surrounded either by water13a or crown ethers.19b,20c,22b,23
Most siliconsulfur compounds are very susceptible to
hydrolysis.1−3 Yet the enclosure of the silicon−sulfur bond by
a hydrophobic environment slows down or almost entirely
prevents the reaction with water, hampering the access of
molecules of water to the positively charged atom of silicon.
Tri-tert-butoxysilanethiol (TBST) was the first silicon−sulfur
compound protected against hydrolysis in this way.2 TBST was
used to synthesize a large number of thiolate complexes and
salts with main group elements and transition metals, and the
reactions were often carried out in biphasic systems consisting
of water and organic solvent.4 Some of the described products
were air- and moisture-sensitive depending on the relative
stability of their metal−sulfur bonds.5 Nearly 100 metal tri-tert-
butoxysilanethiolates were structurally characterized;6 the
bioinorganic aspect of the structural studies was reviewed.7
Similar bulk tri-tert-butylsilyl (supersilyl) thiolates have been
used to construct transition-metal centers.8 Other voluminous
protection groups, such as 2,4,6-tris[bis(trimethylsilyl)methyl]-
phenyl and 2,4,6-trimethylphenyl have been described to
stabilize compounds with silanedithiolate9 and silanethione
functional groups.10
Silanethiolate complexes with less sterical hindrance about
the silicon atom were usually reported as the potential pre-
cursors to metal−sulfur clusters and particles. These included
trimethylsilanethiolates,11 triisopropylsilanethiolate,12 and tri-
phenylsilanethiolates.3,13 Mixed-ligand silanethiolate cadmium
complexes were probed as precursors of CdS thin films.14
Alkali metal thiolates and silanethiolates were often obtained
and described as byproducts15 or substrates for the further syn-
thesis of coordination compounds.16 Systematic studies under-
taken to analyze the influence of metal, ligand, and solvent on
−
Quite unusual ion pairs of different character, 2Li(TBST)2
24
Li2(D2O)62+ and Na(TBST)2− Na(DME)2+,25 were structurally
characterized by the researchers from our department.
Introduction of other functional groups into the molecule of
thiol greatly influences the structure of the resulting alkali
metal thiolate (e.g., ref 26), but this research is not reviewed
in detail here.
In this paper we present the continuation of our studies on
silicon−sulfur compounds stabilized by the presence of bulky
organoxy substituents on silicon atom. With the method
27
̨
developed by Wojnowski and Piekos
́
we have obtained new
aryloxysilanethiols. The crystal structure of tri(mesityloxy)-
silanethiol (TMST) was communicated in 2010.28 Herein we
describe the new member of the family, highly resistant toward
hydrolysis tris(2,6-diisopropylphenoxy)silanethiol, TDST. The
synthesis of sodium salt of TDST is reported with the prospect
Received: June 17, 2011
Published: December 22, 2011
© 2011 American Chemical Society
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2.1.2.1. {μ-Tris(2,6-diisopropylphenoxy)silanethiolato-κ2-
O:S:O′:S′}-sodium-{μ-tris(2,6-diisopropylphenoxy)silanethiolato-κ7-
η6C25,C26,C27,C28,C29,C30:S:η6C25′,C26′,C27′,C28′,C29′,C30′:S′}-
sodium n-Hexane Solvate (3). Slow crystallization of 2 from diluted
n-hexane solution at room temperature yielded few crystals of 3. which
were characterized by X-ray diffraction.
of further utilization in a synthetic route to transition metal-
chalcogenolates. The intra- and intermolecular interactions in
the title compounds are characterized by X-ray diffraction and
FT-IR spectroscopy.
2.1.2.2. Di-aqua-Bis{μ-tris(2,6-diisopropylphenoxy)silanethiolato-
κ8-S:S′:η6C25,C26,C27,C28, C29,C30}-disodium (4). Recrystallization
of 2 from commercial n-hexane at room temperature produced crystals
of 4 suitable for X-ray analysis. FT-IR (solid, cm−1) 3666s, 3407s,
3200w, 3067w, 3020w, 2968vs, 2935s,sh, 2870s, 2810w, 1619 m,
1587w, 1462s, 1440vs, 1382 m, 1361 m, 1331s, 1253s, 1203 m,sh,
1192s, 1181s, 1164w,sh, 1109 m, 1098 m, 1059w, 1044 m, 940s, 909s,
880 m, 800 m, 762vs, 734 m.
2.1.3. Synthesis of Tris(2,6-diisopropylphenoxy)silanethiolate of
Naphtho-15-crown-5 Complexed Sodium (5). 2 (0.01043 g, 1.696 ×
10−5 mol) and naphtho-15-crown-5 (0.01079 g, 3.393 × 10−5 mol)
were dissolved in a mixture of 1 mL of n-hexane and 1 mL of toluene.
The reaction mixture which was kept in the temperature of 255 K
yielded few colorless crystals of 5. FT-IR (solid, cm−1) 3565 m,vbr,
3235 m,br, 3058 m, 3024 m, 2959vs, 2938s,sh, 2921vs, 2864s, 1631 m,
1603 m, 1588w, 1512s, 1491s, 1461vs, 1452vs, 1418s, 1377 m, 1361 m,
1348 m, 1335s, 1292w, 1268vs, 1252s,sh, 1201s, 1183s, 1149 m,sh
1132vs, 1106s, 1080s, 1056vs, 1048vs, 945 m, 940 m, 914vs, 881w, 864 m,
852 m, 838w, 796 m, 756s, 740vs.
2.2. Physical Measurements. 2.2.1. X-ray Crystallographic
Analysis. Experimental diffraction data were collected on KM4CCD
kappa-geometry diffractometer, equipped with a Sapphire2 CCD
detector. Enhanced X-ray MoKα radiation source with a graphite
monochromator was used. Determination of the unit cells and data
collection were carried out at 120 K. Data reduction, absorption
correction, and space group determination were made using the
CrysAlis software package (Oxford Diffraction, 2008).31 Solutions and
refinements were carried out using the SHELX-97 program package.32
Structures of 1, 3, 4, 5 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 the isotropic model with Uiso(H) values fixed to
be 1.5 times Ueq of C atoms for CH3 or 1.2 times Ueq for CH2 and
CH groups. Hydrogen of SH bond in 1 and hydrogens of OH bonds
in 4 were positioned from differential electron-density maps and
refined freely. In crystals of 3 there is a disordered n-hexane molecule,
which lays in a special position on a 2-fold axis. It was refined with the
occupancy ratio 50:50. A summary of the crystallographic data for 1, 3,
4, 5 is listed in Table 1.
2. EXPERIMENTAL SECTION
2.1. Synthesis - General Remarks. n-Hexane was dried over
Na/K and distilled under nitrogen prior to use. Silicon disulfide (raw
material that contained 45% of pure SiS2 as confirmed by the analysis
of sulfides content) was obtained by the direct reaction of sulfur and
silicon. 2,6-Diisopropylphenol (Alfa-Aesar, 97%) and toluene were
used as purchased. Naphtho-15-crown-5 was synthesized as described
in ref 29 and purified as in ref 30.
Elemental analyses were performed on an Elemental Analyzer EA
1108 (Carlo Erba Instruments).
2.1.1. Synthesis of Tris(2,6-diisopropylphenoxy)silanethiol TDST
(1). TDST was obtained by phenolysis of SiS2 as described for
tris(2,6-dimethylphenoxy)silanethiol27c and tri(mesityloxy)silanethiol
(TMST)28 SiS2 (54.4 g) and 2,6-diisopropylphenol (257 g) were
stirred and heated for 80 h in 150−180 °C under the stream of
nitrogen. After that 250 mL of toluene was added to the reaction
mixture, and the unreacted SiS2 was removed by centrifugation.
Toluene and the excess phenol were removed by vacuum distillation.
The crude red-brownish solid (32 g, ∼20% yield with regard to SiS2)
was dissolved in n-hexane. Large colorless crystals of TDST (see
Supporting Information) were obtained at 0 °C. Melting point: 105.3 °C,
Anal. Calcd.: C-72.9; H-8.84; S-5.41; Anal. Found: C-72.8; H-8.75;
S-5.42, 1H NMR (500 MHz) in CDCl3: δ 0.50 (s, 1H), 1.12 (d, 36H),
3.53 (sept, 6H), 6.95 (m, 3H), 7.01 (d, 6H) ppm. 13C NMR (125
MHz) in CDCl3: δ 23.81 (CH(CH3)2), 27.58 (CH(CH3)2), 124.16
(aromatic ring 4-C), 124,20 (aromatic ring 3,5-C), 139.31 (aromatic
ring 2,6-C), 148.00 (aromatic ring C−O-Si). FT-IR (solid, cm−1) 3073 m,
3026 m, 2970vs, 2931vs, 2872s, 2552 m, 1930w,br, 1868w, 1853w,
1752w,br, 1718w, 1703w, 1659w, 1591w, 1465s, 1443vs, 1383 m,
1362 m, 1329s, 1257s, 1189vs, 1157w, 1115s, 1107 m,sh, 1046s,
973vs, 934s,sh, 884w, 795 m, 790w,sh, 755vs FT-IR (solution in
carbon tetrachloride, cm−1) 3141w, 3067w, 3026vw, 2967vs, 2930 m,
2906 m,sh, 2870 m, 2758w, 2691w, 2657w,sh, 2558w, 2531vw,sh,
1615w, 1588vw, 1465s, 1441vs, 1384 m, 1363 m, 1340 m,sh, 1330s,
1270 m, 1257s, 1187vs, 1163w,sh, 1148w, 1113 m, 1099 m, 1058w,
1047 m, 975vs, 935 m, 882vw, 790vs, 653 m, 644 m,sh, 624vw,
587w,sh, 576w,sh, 564 m, 534 m, 504w, 444vw, 422w.
2.1.2. Synthesis of Sodium Tris(2,6-diisopropylphenoxy)-
silanethiolate (2). TDST (0.500 g, 0.843 mmol) was dissolved in
n-hexane (20 mL). To the solution of TDST an excess of sodium
(0.046 g, 2 mmol) was added. The reaction mixture was stirred and
heated for 2 days under nitrogen. White powder of sodium tri(2,6-
diisopropylphenoxy)silanethiolate was separated with the reversing frit
(under nitrogen) and vacuum-dried. Yield of 2 approximately 60%
(the pieces of sodium were removed mechanically under the stream of
nitrogen and the remaining white powder was weighed in a nitrogen-
filled flask). Anal. Calcd. C72H102O6S2Si2Na2 (MW 1229.9) C-70.31;
2.2.2. FT-IR Spectra. FT-IR spectra of solids were measured with a
Momentum microscope (IR detector) attached to a Mattson Genesis II
Gold spectrometer (IR source). Spectra acquisition was controlled by
WINFIRST software package
FT-IR spectra of TBST solutions were recorded in ATR mode on a
Nicolet 8700 FT-IR spectrometer, using the Turbo mode of the
EverGlo infrared source. A total of 128 scans were made with a
selected resolution of 4 cm−1. Spectra acquisition was controlled by the
OMNIC 7.2a software package (Thermo Electron Corporation,
Madison, WI). Thereafter the spectra were transformed to GRAMS
file format and analyzed using GRAMS/32 software (Galactic
Industries Corporation, Salem, MA) and RAZOR program (Spectrum
Square Associates, Ithaca, NY) run under GRAMS/32. This analysis
was also carried out for FT-IR spectra of solid 4.
1
H-8.36; S-5.21. Anal. Found: C-70.08; H-8.74; S-4.93. H NMR (500
MHz) in C6D6: δ 1.21 (d, 72H), 3.81 (sept, 12H), 6.88 (t, 6H), 7.03
(d, 12H) ppm, the admixture of n-hexane visible at 0.89 ppm (triplet, 1
molecule per 6 molecules of thiolato residue). 13C NMR (125 MHz)
in C6D6: δ 23.98 (CH(CH3)2), 27.61 (CH(CH3)2), 122.80 (aromatic
ring 4-C), 123,80 (aromatic ring 3,5-C), 140.62 (aromatic ring 2,6-C),
150.05 (aromatic ring C−O-Si), the admixture of n-hexane visible as
three signals of small intensity at 14.31, 23.01, 31.92 ppm. Hydration
of sodium thiolate was observed in the FT-IR spectra obtained in the
air (very weak bands at 3413 and 3178 cm−1, the spectrum is
presented in the Supporting Information, Figure 5S. FT-IR (solid,
white precipitate, cm−1) 3413w,br, 3178w,br, 3073 m,sh, 3063w,
3025w, 2965vs, 2928vs, 2901s,sh, 2868s, 2751w, 2718w, 2707w,
1912w, 1851w, 1588w, 1466vs, 1451s, 1441s, 1381 m, 1360s, 1348w,
1334s, 1297w, 1258vs, 1199vs, 1158w, 1138 m,sh, 1111vs, 1060 m,
1047 m, 1018 m, 933vs, 902 m, 881 m, 864w,sh, 841w, 799 m, 758s,
734 m.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Molecular Structure. Similar to its
predecessor compounds tri(o-xylenoxy)silanethiol and tri-
(mesityloxy)silanethiol (TMST),27c,28 tris(2,6-diisopropylphenoxy)-
silanethiol (TDST) was isolated as the only product of the
reaction between SiS2 and 2,6-diisopropylphenol. The reaction
was carried out in a high temperature of 180 °C under nitrogen.
The high temperature favors the formation of silanethiol,27c and
the atmosphere of nitrogen prevents combustion of the phenol
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Table 1. Crystal Data and Structure Refinement Parameters for Compounds 1, 3, 4, 5
1
3
4
5
empirical formula
formula weight
temperature/K
wavelength (Mo Kα)/Å
crystal system
space group
a /Å
C36H52O3SSi
592.93
C78H116Na2O6S2Si2
1315.99
C72H106Na2O8S2Si2
1265.85
C86H111NaO13SSi
1435.89
120(2)
120(2)
120(2)
120(2)
0.71073
monoclinic
Cc
0.71073
0.71073
0.71073
orthorhombic
P21212
triclinic
triclinic
P1
̅
P1
̅
11.4061(8)
35.0837(9)
10.2841(4)
90
12.6124(7)
24.0235(9)
12.7530(4)
90
10.7839(5)
12.1344(6)
16.0385(8)
86.689(4)
72.849 (4)
66.292(5)
1831.72(15)
1
15.0557(6)
16.9264(8)
16.9715(7)
82.752(4)
87.179(4)
68.636(4)
3995.6(3)
2
b /Å
c /Å
α /deg
β /deg
122.677(4)
90
90
γ /deg
90
3
V /Å
3464.0(3)
4
3864.1(3)
2
Z
Dc /Mg m−3
μ /mm−1
F(000)
1.137
1.134
1.148
1.193
0.160
0.161
0.168
0.122
1288
1428
684
1544
Θ range/deg
Index ranges
2.32 to 25.5
−13 ≤ h ≤ 13
−38 ≤ k ≤ 42
−12 ≤ l ≤ 12
2.33 to 25.5
−15 ≤ h ≤ 15
−11 ≤ k ≤ 29
−8 ≤ l ≤ 14
2.16 to 25.5
−9 ≤ h ≤ 13
−9 ≤ k ≤ 14
−19 ≤ l ≤ 19
2.42 to 25.5
−14 ≤ h ≤ 18
−19 ≤ k ≤ 20
−20 ≤ l ≤ 20
reflections collected/unique 10968/5572 [R(int) = 0.0151] 7890/5871 [R(int) = 0.0235] 11349/6812 [R(int) = 0.0243] 25869/14837 [R(int) = 0.0433]
data/restraints/parameters
goodness of fit on F2
5572/2/382
1.057
5871/2/418
1.013
6812/0/408
0.949
14837/0/933
1.080
final R indices [I > 2σ(I)]
R1 = 0.0316
w R2 = 0.0778
R1 = 0.0323
w R2 = 0.0783
0.256, −0.214
830182
R1 = 0.0409
w R2 = 0.104
R1 = 0.0471
w R2 = 0.1067
0.765, −0.375
830184
R1 = 0.0404
w R2 = 0.0961
R1 = 0.0573
w R2 = 0.1008
0.509, −0.225
830183
R1 = 0.0686
w R2 = 0.1977
R1 = 0.0973
w R2 = 0.2063
1.143 −0.363
830185
R indices (all data)
−3
largest diff. peaks [e Å
CCDC no.
]
and reaction products. The raw, brownish product was
recrystallized from n-hexane. The solutions of TDST in hexane
showed no signs of decomposition (such as turbidity) even in
direct contact with water. TDST easily forms large and well-
shaped crystals having a very faint odor of burning rubber
(Supporting Information, Figure 1S). The molecular structure
of TDST is presented in Figure 1. Important bond lengths and
one exception, which is the position of the SH bond. In TDST
the SH hydrogen weakly interacts with one of the phenyl rings,
which is seen both in the crystal structure (Figure 2a) and FT-
IR spectrum, which will be discussed later. The interaction
influences the geometry of the molecule and forces the rotation
of one of the phenyl rings so that the weak hydrogen bond can
be created. In the structure of TMST the SH bond was initially
refined in the “opposite direction” (see Figure 2b).28 Now we
have decided the hydrogen position should rather be refined as
disordered between several states. Satisfactory results have been
obtained for two-position disorder with hydrogen occupancies
72/28. Position occupied in 28% is analogous to location of
H1a in TDST (Figure 2c) whereas in the position occupied in
72% the hydrogen interacts with the phenyl ring of the
neighboring molecule of TMST. In this model the isotropic
temperature factors of split hydrogen atom substantially de-
crease, and the overall R factor is lowered by 0.02%. Distances
between H1a and carbon atoms of the interacting ring are
shorter in TDST than in TMST (see Figure.2 caption). Both
the absence of disorder and the shortened distances are logically
connected with the growing, electron-releasing effect of alkyl
substituents. The corrected cif file and the pictures illustrating
different intermolecular interactions in solid TMST and TDST
are included in Supporting Information, Figures 2S and 3S.
Silicon atom in TDST is surrounded by hydrophobic alkyl
substituents and practically inaccessible to solvent molecules
which explains its increased resistance toward hydrolysis.
Sodium silanethiolates 2, 3, and 4 were obtained in the
reaction of sodium metal with TDST in n-hexane solution
Figure 1. Molecular structure of TDST (1). Hydrogen atoms (except
for H1a) have been removed for clarity of the picture.
angles are gathered in Table 2. The molecular structure of
TDST is similar to the structure of its congener TMST28 with
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a
Table 2. Crystallographic Data for 1−4
1
3
4
5
Bond Lengths [Å]
S1−H1
1.1074
Na1−Na1i/Na2
Na1−S1
3.791(2)
3.7625(14)
2.7582(9)
2.7631(8)
2.7474(13)
Na1i-S1
Na1−S1ii
2.7474(13)
2.7830(14)
2.833(3)
2.954(3)
3.010(3)
3.033(3)
2.965(3)
2.881(3)
2.596
Na2−S1
Na1i/Na2−C25
Na1i/Na2−C26
Na1i/Na2−C27
Na1i/Na2−C28
Na1i/Na2−C29
Na1i/Na2−C30
2.9896(19)
2.9567(19)
2.8951(19)
2.897(2)
2.959(2)
3.031(2)
2.606
Na1i/Na2−
centroid
Na1−O2
Na1−O4
Si1−S1
2.4327(17)
2.1145(6)
2.0289(9)
2.0248(7)
85.91(2)
2.0053(12)
Bond Angles [deg]
Si1−S1−H1
94.1
Na1−S1−Na1i/
86.54(4)
Na2
S1−Na1−S1i
Si1−S1−Na1
94.09(2)
113.71(3)
112.85(3)
80.82(3)
Si1−S1−Na1i/
103.81(3)
Na2
S1 Na1 S1ii
94.25(6)
92.68(6)
70.98(5)
112.41(6)
S1 Na2 S1ii
O4/O2 Na1 S1
95.29(5)
O4/O2 Na1
126.47(5)
S1i/ii
O2 Na1 O2ii
175.35(13)
132.24
centroid−Na2−
centroidii
centroid−Na1/
101.43
111.15
97.70
Na2−S1
centroid−Na1/
Na2−S1i
116.73
123.63
O4−Na1−
centroidi
a
3, ii (1−x, 1−y, z); 4, i (1−x, 1−y, −z).
similar to the sodium thiolates17 and silanethiolates25,33
described previously. The reaction in n-hexane was rather
slow, requiring continuous stirring and heating. The white,
powdery precipitate of sodium silanethiolate 2 was separated
from the rest of the solution by filtration and characterized by
NMR and elemental analysis (see Experimental Section). Two
types of crystals (compounds 3 and 4) that were characterized
by X-ray diffraction were obtained from the filtrate. Both of
them contained dimeric sodium thiolate.
Figure 2. Molecular structures of (a) TDST (1), marked distances
between and H1a and C1−C6 carbons are between 2.823 −4.444 Å;
(b) TMST;28 (c) TMST-split model, distances between and H1a and
C1−C6 carbons are between 3.127 −4.514 Å. Hydrogen atoms
(except for H1/H1a) and isopropyl (TDST) or methyl (TMST)
substituents have been removed for clarity of the picture.
Compound 3 is a dimeric sodium thiolate with the planar
Na2S2 core (Figure 3), but each atom of sodium in 3 exhibits
different coordination. Na1 is coordinated by two O atoms and
two S atoms in a distorted seesaw geometry. Na2 is also tetra-
coordinated by two S atoms and two aryl groups with the
geometry that is closer to the trigonal pyramidal than to
tetrahedral (τ4 = 0.82734). The two independent Na−S
distances in 3 are 2.7474(13) and 2.7830(14) and Na−O
distance is 2.4327(17) Å (Table 2) which is relatively long as
the bond constitutes a part of the strained deltoid ring. The
Na2-centroid distance of 2.596 Å is significantly shorter than in
sodium salts of HSC6H3-2,6-Trip2 (2.701 Å) described in 1996
by Niemeyer and Power.20b TDST and HSC6H3-2,6-Trip2 are
compared in Scheme 1. It is probably the result of greater
flexibility of phenyl rings in TDST compared to HSC6H3-2,6-
Trip2 which allows the aryl-cation interaction to strengthen.
The coordination manner of arene ligands is similar to that in
the sodium-arene complex described in 1990.35
The structure of 4 presented in Figure 4 resembles the struc-
tures of dimeric lithium and sodium salts of sterically hindered
(HSC6H3-2,6-Trip2).20b Compound 4 has a crystallographically
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Figure 4. Molecular structure of 4. Hydrogen atoms (except for water
molecule) and isopropyl substituents have been removed for clarity of
the picture. Parameters of the hydrogen bond O4−H4B···O1: D−H
0.85(3) Å; H···A 1.96(3) Å; D···A 2.800(2) Å; ∠ DHA 172(3) deg.
crystal between the other OH bond of water and the phenyl
ring of the adjacent molecule of 4 (Supporting Information,
Figure 4S). The situation is confirmed by the FT-IR spectrum
of solid 4 and will be discussed in the next section.
Herman and Chojnacki investigated the effect of deproto-
nation of the silanethiol on the Si−S bond length and found
that it results in a shortening of the Si−S bond length.36
Chojnacki analyzed the relationship between Si−S bond length
and the character of the S−M bond in silanethiolate metal
complexes and concluded that the increase in the ionic
character of S−M is accompanied by the decrease of Si−S
distance.37 We decided to test these predictions experimentally
and produce separate sodium-thiolate ion pairs applying the
approach described by Chadwick and co-workers.23 Crown
ethers can coordinate and separate metal cations from anions
and so that isolated anions are obtained in crystals. Using
several crown ethers and mixtures of solvents we produced one
such crystalline ion pair 5 in small amounts. The crystal
structure of 5 consists of silanethiolate anions and [Na-
(naphtho-15-crown-5)2]+ cations (Figure 5). The sulfur and
Figure 3. Molecular structure of 3. Hydrogen atoms (except for
solvating n-hexane moiety) and isopropyl substituents have been
removed for clarity of the picture.
Scheme 1
required center of symmetry and exhibits a planar Na2S2
core with two independent Na−S distances of 2.7582(9)
and 2.7631(8) Å (see Table 2 for important bond lengths and
angles). The internal Na2S2 ring angles are 85.91(2) and
94.09(2)°. Similar to 3 the sodium interacts with one of the
phenyl rings of TDST with the Na−C distances ranging from
2.8951(19) to 3.031(2) Å. Interestingly in both complexes 3
and 4 the sodium-to-centroid distances are definitely shorter
than the sodium-to-sulfur bond lengths (the difference is ca.
0.15 Å or more). In 4 sodium is additionally coordinated by the
oxygen atom of a single molecule of water. In the hydrophobic
environment formed by the silanethiolate ligands water forms
only one “classical” hydrogen bond with the adjacent O-atom
of phenoxy moiety (parameters of the hydrogen bond are given
in the Figure 4 caption). There is a weak interaction in the
Figure 5. Structure of 5. H atoms and isopropyl substituents of TDST
removed for clarity of the picture.
sodium atoms are separated by a distance of 7.124 Å, the
nearest distances between sulfur and adjacent atoms are those
to TDST oxygens (ca. 3.15 Å) and additionally: S1 C72 3.480 Å
(nearest distance to one of crown ether atoms), S1 C13
3.505 Å, S1 C31 3.677 Å, S1 C7 3.757 Å, S1 C25 3.771 Å,
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Inorganic Chemistry
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S1 C71 3.786 Å, S1 C33 3.804 Å, S1 C14 3.825 Å, S1 C19 3.843 Å,
S1 C8 3.943 Å. The remaining distances of sulfur-to-atom
exceed 4 Å. Sodium Na1 is ten-coordinated by oxygen atoms
from two crown ether molecules complexed in a sandwich
manner and the geometry of pentagonal antiprism.
We expected that sodium would rather fit the ligand cavity as
it usually does with crown ethers of this type and size,38 but we
underestimated the influence of the counterion. Earlier infrared
spectroscopic studies revealed that benzo-15-crown-5 ether
forms a similar 2:1 sandwich with Na+, if the anion is
tetraphenyl borate.39 It was explained that such anion could
not provide donor atoms for the Na+ which then coordinates
a second crown to complete the coordination sphere.40
Though this explanation is not entirely satisfactory, since
sodium can be in fact solvated by the phenyl rings, the
coordination of Na+ which is a hard Lewis acid with oxygen
must be preferred, and complex 5 is a structural evidence of
this. The geometry of sandwich-type cation [Na(naphtho-
15-crown-5)2]+ strongly resembles the geometry of analo-
gous complex [K(naphtho-15-crown-5)2]+ including the rota-
tion of crown ether molecules in the complex so that the angle
between the aromatic rings is approximately 30 deg.30,41
The distances Na−O vary in the wide range 2.597−3.012 Å
with an average Na−O distance equal to 2.784 Å, whereas
analogous potassium complexes exhibit distances K−O
between 2.784 and 3.096 Å with an average of approximately
2.903 Å.30,41
Figure 6. ATR FT-IR absorbance spectra of tri-tert-butoxysilanethiol
(TBST) and tri-tert-butoxysilanethiol-xylene mixtures in CCl4.
binding site composed of aromatic rings can compete with
aqueous solvation.46c The main features of the cation-π
interactions are well explained by classical electrostatic trends:
smaller ions with greater charge have larger affinities for
aromatic rings.46
e
Noncovalent interactions in ion-benzene-water clusters have
been studied by the infrared predissociation spectroscopy and
ab initio calculations for lithium and sodium clusters consisting
of metal cation, water, and benzene molecules: Li+(C6H6)x(H2O)y
and Na+(C6H6)x(H2O)y.47 The position of water W1 in the
argonated cluster Na+(C6H6)2(H2O)2Ar1 (Figure 7) corresponds
As predicted and explained by theoretical methods, the Si1−
S1 bond length in this definitely ionic compound is
considerably shorter than in 1 and also still significantly shorter
than in 3 and 4 (see Table 2).36,37
3.2. FT-IR Spectroscopy. The FT-IR spectrum of the solid
sample of TDST exhibits a relatively narrow band of SH
stretching vibration at 2552 cm−1, comparable to that of
TMST, which is located at 2662 cm−1.28 Such frequency is not
quite typical neither of (Si)−S-H nor of (C)−S-H stretching
mode, and the red-shift amounts to approximately 35−40 cm−1;
for example, tri-tert-butoxysilanethiol exhibits νSH at 2592 cm−1.42
On the basis of X-ray molecular structure we attributed this
difference, at least partly, to the formation of the intramolecular
SH-π interaction. To strengthen this hypothesis we analyzed
FT-IR ATR spectra of tri-tert-butoxysilanethiol in CCl4
and CCl4-xylene solutions. Stretching mode νSH of tri-tert-
butoxysilanethiol dissolved in xylene-CCl4 mixture shows the
band at 2572 cm−1, which is 20 cm−1 red-shifted in comparison
to the solution in CCl4 (Figure 6). Experimentally, very similar
shifts were observed upon the formation of the SH---π
interaction between molecules of organic thiols43 and for the
intermolecular interactions between hydrogen sulfide and
indole.44 Interestingly the preference for the formation of SH---π
over the formation of NH---S was observed in the latter case.
Computational studies indicate that the energy of the SH---π
interaction is either comparable with that of XH---π (where
X = O)45 or even the SH---π interaction is the strongest
among the studied XH---π (where X = C, N, and O).44 The
SH---π interaction is preferred over SH---S,45 which is con-
firmed by the crystal packing of both TMST and TDST (see
Figure.2 and Supporting Information, Figure 3S).
Figure 7. Cluster Na+(C6H6)2(H2O)2Ar1.47
to the environment of water in the compound 2. There were
four experimentally observed OH stretching frequencies for the
two molecules of water in this cluster. No detectable interaction
between the benzene and water W2 was reflected by the OH
symmetric and asymmetric stretches at 3649 and 3733 cm−1
(W2 in Figure 7). Stretching bands of water W1 were located
at 3712 (free) and 3435 cm−1 (water−water H-bond)47. In the
FT-IR spectrum of our compound 4 there are two strong bands
of the water molecule at 3666 and 3406 cm−1. The latter must
be certainly ascribed to the OH---O hydrogen-bonded to the
oxygen atom (Figures 4 and 8). However, there is a considerable
discrepancy of 50 cm−1 between the values of free OH in the
cluster Na+(C6H6)2(H2O)2Ar1 (3712 cm−147) and in com-
pound 4 (3666 cm−1). We analyzed the crystal packing of 4
and found out that there is an OH-π interaction between the
“free” OH of water and the phenyl ring of the adjacent
molecule of the sodium thiolate. The intermolecular distances
between the H atom of water and C atoms of the Ph ring
are between 2.860 and 4.880 Å (see Supporting Information,
In sodium salts of TDST SH---π interactions are replaced by
cation-π noncovalent forces. Cation-π bonds are relatively
strong interactions that are recognized as important for
molecular recognition and stability of protein structure.46 The
studies on synthetic receptors established that a hydrophobic
841
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail: anndoleg@pg.gda.pl.
■
ACKNOWLEDGMENTS
Financial support of Polish Ministry of Science and Higher
Education Grant N N204 511639 is acknowledged. We are very
■
grateful to Prof. Elzbieta Luboch from the Department of
̇
Chemical Technology, Gdansk University of Technology, for
the kind gift of several crown ethers and the reprints of her
papers where their syntheses and complexes with potassium
ions are described. A.D. is grateful to Dr. Rafał Grubba for
technical help with the preparation of NMR sample of 2.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic information files (the crystal structures of
compounds 1, 3, 4, and 5 have been also deposited at the
Cambridge Crystallographic Data Centre and allocated the
deposition numbers CCDC-830182, CCDC-830184, CCDC-
830183, and CCDC-830185). Figures 1S (crystal of TDST), 2S
(crystal packing of TDST), 3S (crystal packing of TMST), 4S
(crystal packing of 4), 5S (FT-IR spectrum of 2), and 6S (FT-
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