Synthesis and characterization of di-, tri- and tetraboronic acids based on phenyl- and thienylsilane cores

Article abstract of DOI:10.1016/j.jorganchem.2015.01.024

The synthesis of a series of di- tri- and tetraboronic acids based on respective phenyl- and thienylsilane cores is described. The optimal protocols involved lithiation of respective arylsilane precursors using either deprotonative lithiation or halogen/lithium exchange with n-BuLi followed by treatment of resultant intermediates with B(Oi-Pr)₃ and subsequent hydrolysis, which afforded final products in good yields. X-ray crystal structures of selected diboronic derivatives were determined showing that hydrogen-bonding interactions of B(OH)₂ groups are the main factor governing the supramolecular assembly.

Full text of DOI:10.1016/j.jorganchem.2015.01.024

Journal of Organometallic Chemistry 783 (2015) 1e9
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and characterization of di-, tri- and tetraboronic acids based
on phenyl- and thienylsilane cores
,
*
a
a, *
Krzysztof Gontarczyk a, Krzysztof Durka a , Piotr Klimkowski , Sergiusz Lulinski
Janusz Serwatowski , Krzysztof Wozniak
,
ꢀ
a
b
ꢀ
a Physical Chemistry Department, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
b Laboratory of Crystallochemistry, Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a series of di- tri- and tetraboronic acids based on respective phenyl- and thienylsilane
cores is described. The optimal protocols involved lithiation of respective arylsilane precursors using
either deprotonative lithiation or halogen/lithium exchange with n-BuLi followed by treatment of
resultant intermediates with B(Oi-Pr)3 and subsequent hydrolysis, which afforded final products in good
yields. X-ray crystal structures of selected diboronic derivatives were determined showing that
hydrogen-bonding interactions of B(OH)2 groups are the main factor governing the supramolecular
assembly.
Received 8 December 2014
Received in revised form
23 January 2015
Accepted 26 January 2015
Available online 7 February 2015
Keywords:
Boronic acids
Arylsilanes
© 2015 Elsevier B.V. All rights reserved.
Lithiation
X-ray structural analysis
Hydrogen-bonding
Introduction
very low in some cases which is highly advantageous in view of
their potential use as gas nanocontainers.
Boronic acids are extensively used in current synthetic organic
chemistry and a range of other fields. Apart from the use as re-
agents, compounds possessing more than one boronic acid group
were recognized as perspective starting materials for diverse ap-
plications, specifically related to molecular recognition and supra-
molecular chemistry. For example, the presence of two or more
boronic groups enables the formation of extended H-bonded as-
semblies in the solid crystalline state [1,2]. In some cases they may
further incorporate additional guest molecules. This was amply
demonstrated by Wuest et al. through the synthesis and structural
characterization of tetraboronic acids based on tetraphenyl-
methane and tetraphenylsilane cores [2]. More recently, various
oligoboronic acids were employed for the construction of poly-
meric systems possessing microporous structure and hence clas-
sified as Covalent Organic Frameworks (COFs). Their formation is
based on the ability of boronic acids to form trimeric anhydrides
(boroxines) or cyclic esters with cis-1,2-diols [3]. Due to presence of
large voids in their structures they exhibit significant adsorption of
gases such as N2, H2, CO2 and CH4. The density of these materials is
Oligoboronic acids based on thiophene cores are relatively rare
[4]. An exception is commercially available 2,5-thiophenediboronic
acid, which was used recently for the synthesis of COF material by
the polycondensation with 2,3,6,7,10,11-hexahydroxytriphenylene
(HHTP) [5]. Some diboronic acids derived from fused thiophene-
ring systems were also obtained and employed for the construc-
tion of respective COFs [5]. In this paper, we report on the synthesis
of a series of novel boronic acids based on various oligophenyl- and
oligothienylsilane cores (Chart 1). We believe that compounds
described here could be useful alternative for those reported pre-
viously. Other applications, for instance, as starting materials in
Suzuki-Miyaura coupling reactions leading to various systems
bearing silicon atom(s) with attached phenyl or thienyl rings can
also be envisioned. This is due to strong interest in such com-
pounds, which is related to their diverse applications in optoelec-
tronics [6].
Results and discussion
Synthesis of arylsilanes
* Corresponding authors. Tel./fax: þ48 22 234 7273.
Phenylsilanes 1a-1i were prepared by a standard route involving
E-mail addresses: kdurka@gmail.com (K. Durka), serek@ch.pw.edu.pl
ꢀ
(S. Lulinski).
Br/Li reactions of starting aryl bromides (1,3-Br2C6H4, 1,4-Br2C6H4
http://dx.doi.org/10.1016/j.jorganchem.2015.01.024
0022-328X/© 2015 Elsevier B.V. All rights reserved.

2
K. Gontarczyk et al. / Journal of Organometallic Chemistry 783 (2015) 1e9
Chart 1. General structures of obtained oligoboronic acids.
or difluoro derivatives 3,5-F2C6H3Br or 2,4-F2C6H3Br) followed by
the treatment with the appropriate chloromethylsilane MexSiCl4ꢀx
isomerisation in THF to give thermodynamically more stable 2-
thienyllithium, the use of Et2O as the solvent for the preparation
of 3d-f is mandatory [11]. It should be noted that all arylsilanes
were obtained on a multigram scale.
,
x ¼ 0e2 (Scheme 1) [7]. All products were obtained in good yields.
The synthesis of Me2Si(2-Th)2 (3a) and MeSi(2-Th)3 (3b) [8]
involved deprotonative lithiation of thiophene with n-BuLi in
THF followed by the metathesis of resultant 2-thienyllithium 2-
ThLi with Me2SiCl2 and MeSiCl3, respectively (Scheme 2). A
slight (ca. 10%) excess of 2-ThLi was used to ensure that all SieCl
bonds will be substituted with 2-thienyl nucleophiles. Tetrakis(2-
thienyl)silane (3c) was obtained according to literature method
involving tetramethyl orthosilicate Si(OMe)4 as the silicon source
[9]. Isomeric (3-thienyl)silanes Me2Si(3-Th)2 (3d) [10], MeSi(3-
Th)3 (3e), and Si(3-Th)4 (3f) were prepared using methods,
which were analogous to those used for their respective 2-thienyl
counterparts. Since 3-thienyllithium (prepared by bromi-
neelithium exchange from 3-bromothiophene) is prone to rapid
Synthesis of oligoboronic acids
Various synthetic approaches were tested for bis(4-
dihydroxyborylphenyl)dimethylsilane (2a) as the model target
compound (Scheme 3). The first method involved the classical
double Br/Li exchange in bis(4-bromophenyl)dimethylsilane
(1a) and subsequent reaction with B(Oi-Pr)3 followed by hy-
drolysis. In a simplified version of this protocol, the silane 1a
was not taken as an isolated pure compound but it was gener-
ated in situ as described above and converted directly into the
desired diboronic acid. In this case, however, the purity of the
Scheme 1. The synthesis of arylsilanes and respective oligoboronic acids.

K. Gontarczyk et al. / Journal of Organometallic Chemistry 783 (2015) 1e9
3
Scheme 2. The synthesis of thienylsilanes and respective oligoboronic acids.
Scheme 3. Testing different approaches for the synthesis of 2a.

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K. Gontarczyk et al. / Journal of Organometallic Chemistry 783 (2015) 1e9
product and the reaction yield was lower. Alternatively, we have
checked lithiated aryl boronates [12] in combination with
Me2SiCl2 as we previously found that these reagents are
compatible with the related silicon-based electrophile Me3SiCl
giving rise to a number of silylated arylboronic acids. The
anionic lithium 4-lithiophenyl(trialkoxy)borate was generated in
situ from 1,4-dihalobenzene (Hal ¼ Br, I) using Hal/Li exchange
followed by boronation with B(Oi-Pr)3 and the second Hal/Li
exchange with the next equivalent of alkyllithium [13]. The
neutral 2-(4’-lithiophenyl)-6-butyl[1,3,6,2]dioxazaborocane was
obtained by the Br/Li exchange in its 4-bromophenyl precursor
[14]. The quench of obtained lithiumeboron intermediates with
Me2SiCl2 gave 2a after hydrolysis but again the product was less
pure than that obtained from 1a and the reaction yield was
unsatisfactory. Thus, we have decided to prepare other boronic
acids using a procedure involving the formation and isolation of
appropriate arylsilanes in the first step (Scheme 1). Then, an
efficient and selective lithiation/boronation could be performed
resulting in the selective high-yield formation of pure oligo-
boronic acids 2a-2i.
The classical lithiation/boronation approach was also used for
the conversion of thienylsilanes into their corresponding boronated
derivatives 4a-4e (Scheme 2). Initially, n-BuLi was used as the
lithiating agent. However, we have found that it is not optimal. In
general, it is advisable to use it in excess to ensure the complete
deprotonation of all active, i.e. sufficiently acidic, positions. Unfor-
tunately, trapping of unreacted n-BuLi results in the formation of
butylboronic acid byproduct after hydrolysis. This implies that the
final product should be purified to remove n-BuB(OH)2 impurity.
These problems were overcome when a combination of LDA-
mediated lithiation/B(OiPr)3 boronation was used. The complete
conversion to desired products was achieved. It is known that LDA
effectively co-exists with B(Oi-Pr)3 in solution [15]. Hence, it is
plausible that the reaction occurs in a stepwise manner: the
initially formed lithiated intermediate is trapped with the added
boron reagent. The resultant boronated species is then lithiated
with unreacted LDA still remaining in the reaction mixture fol-
lowed by trapping with the next equivalent of electrophile. The
the silicon atom and the midpoint between two aromatic ring
centroids. The boronic groups in both compounds are rotated
around the BeC bonds by torsion angles ranging from 14.0(1)ꢁ to
86.6(1)ꢁ (Fig. 1). Conformational flexibility of boronic group de-
pends on intramolecular contacts with atoms located at the ortho-
positions to a boronic group as well as it is influenced by inter-
molecular interactions with adjacent molecules [1e,17]. According
to our latest results, the rotation barrier for a boronic group in 2,6-
difluoro-substituted phenylboronic acids is low (ca. 5 kJ molꢀ1
)
[1e], which allows the occurrence of different conformers in the
structure.
Despite the fact, that B2 boronic group in molecule 2g_A is
almost perpendicular to the aromatic ring plane, both boronic
groups in this molecule have the same relative orientation as that
observed in 2g_B. Thus, regular hydrogen-bonded chains with
alternating sequence of molecules 2g_A and 2g_B are formed
(Fig. 2a). Such a scheme of hydrogen interactions is commonly
observed for crystal structures of diboronic acids [1]. The geomet-
rical parameters of hydrogen bonds are given in Table S1 in the
Supporting Information. The adjacent chains associate further by
lateral hydrogen bonds formed between two neighboured mole-
cules 2g_A and between molecules 2g_A and 2g_B, which leads to
the formation of a molecular layer parallel to (001) crystal plane. In
addition, the molecule 2g_B participates in a lateral H-bond inter-
action formed between an anti-oriented O(5)eH group and THF
molecule. It is noticeable that O(8)eH group is only involved in one
H-bond within a molecular chain, and does not participate in lateral
hydrogen interactions. As a result, the structure 2g possesses voids
(see Fig. 2b) available for guest inclusions. Calculations of the void
surface with the Spackman approach [18] show that the pore vol-
ume is equal to 403 Å3 (per unit cell), which constitutes 20% of the
total unit-cell volume.
The crystal structure of isomeric 2i presents quite similar picture
of supramolecular assembly. Molecules form hydrogen-bonded
chains, which propagate further via lateral hydrogen bonds to
produce a 2-dimensional sheet (Fig. 3). However, in this case, all
boronic groups are involved in lateral hydrogen bonds and hence
the structure is closely packed. An interesting feature of this
structure is the occurrence of auxiliary OeH…F and CeH…F
interactions.
reactions stops when all active positions (
a to sulphur atom in
thiophene ring) are boronated. The additional advantage of the use
of LDA is the fact that its excess can be readily removed during
aqueous acidic workup of a reaction mixture. Unfortunately, at-
tempts to a conversion of 3f into a respective tetraboronic acid
eventually proved unsuccessful. Presumably, this is due to the weak
solubility of 3f, which hampers its effective deprotonation. Even the
attempted boronation of 3f with B2Pin2 in refluxing THF in the
Conclusions
In conclusion, a simple and general protocol for the multigram-
scale synthesis of a series of oligoboronic acids based on phenyl
and thienylsilane cores was developed. Selected products were
obtained as crystalline solids which may incorporate solvent
molecules in their structure. This is apparently due to the domi-
nation of H-bonding interactions which are strongly directional
and thus, the possibility for the existence of voids available for
guest molecules is opened. This is in line with previous studies on
related tetraboronic acids derived from tetraphenylmethane and
tetraphenylsilane [2]. Further studies on the application of ob-
tained boronic acids and their derivatives for the preparation of
porous materials are currently in progress and the results will be
given in due course.
presence of [Ir(cod)(m-OMe)]2 catalyst [16] did not give the targeted
tetraboronated product.
Crystal structures of 2g and 2i
The monocrystals suitable for X-ray diffraction analysis were
grown for fluorinated boronic acids 2g and 2i. The studied com-
pounds crystallize in triclinic P-1 and orthorhombic Pnna space
groups of symmetry, respectively. The molecular structures are
shown in Fig. 1. Central silicon atoms in both compounds exhibit
slightly distorted tetrahedral geometry. In the case of 2g the
asymmetric part of the unit cell contains two molecules of dibor-
onic acid (2g_A and 2g_B) accompanied by one molecule of THF,
which seems to be disordered. The two aromatic ring planes in
2g_B creates V-shaped molecule with approximate C2-symmetry.
In turn, in 2g_A one of the aromatic ring is significantly twisted
around SieC bond (the interplanar angle between mean-squared
aromatic ring planes equals to 63.8(1)ꢁ). For compound 2i, the ar-
omatic moieties are related by the C2 rotation axis passing through
Experimental section
General comments
All reactions involving air- and moisture-sensitive reagents
were carried out under an argon atmosphere. Key reagents
including n-BuLi (10 M solution in hexanes), trialkyl borate, N-
butyldiethanolamine, diisopropylamine, chloromethylsilanes,

K. Gontarczyk et al. / Journal of Organometallic Chemistry 783 (2015) 1e9
5
Fig. 1. Labelling of atoms and the atomic thermal motion estimation as ADPs for (a) 2g and (b) 2i (only symmetrically unequivalent atoms). H-bonding interactions are shown as red
dashed lines (2g). Selected bond lengths (Å) and angles (ꢁ) for 2g: B1eC1 1.587(8), B1eO1 1.349(7), B1eO2 1.36(1), B2eC7 1.600(8), B2eO3 1.356(9), B2eO4 1.352(8), B3eC15
1.586(8), B3eO5 1.367(8), B3eO6 1.35(1), B4eC21 1.578(8), B4eO7 1.35(1), B4eO8 1.354(8), C4eSi1eC10 107.4(3), C18eSi2eC24 107.3(3), C2eC1eB1eO1 22.4(1), C8eC7eB2eO3
86.6(1), C16eC15eB3eO5 28.8(1), C22eC21eB4eO7 14.0(1). Selected bond lengths (Å) and angles (ꢁ) for 2i: B1eC3 1.578(2), B1eO1 1.341(2), B1eO2 1.351(2), C1eSi1eC1(x,1/2ꢀy,1/
104.0 (1), C2eC3eB1eO2 40.1 (1), C2eC1eC1(x,1/2ꢀy,1/2ꢀz)eC6(x,1/2ꢀy,1/2ꢀz) 34.1(1).
2ꢀz)
Si(OMe)4 were received from Aldrich and used without addi-
tional purification. Bis(bromophenyl)dimethyl- (1a, 1d), tris(-
bromophenyl)methyl- (1b, 1e) and tetrakis(bromophenyl)lsilanes
(1e, 1f), and (2-thienyl)silanes (3a-c) were obtained according to
the literature procedures [7e10]. Solvents used for reactions
were dried by refluxing with sodium/benzophenone and distilled
under argon atmosphere. The NMR chemical shifts are given
relative to TMS using known chemical shifts of residual proton
(1H) or carbon (13C) solvent resonances. For some compounds the
1H NMR resonances of B(OH)2 groups were not observed, pre-
sumably due to rapid proton exchange processes involving water
molecules introduced with a sample or formed upon a partial
dehydration of boronic acids in solution. In the 13C NMR spectra
the resonances of boron-bound carbon atoms were not observed
in most cases due to their broadening caused by partially relaxed
BeC couplings. 11B and 19F NMR chemical shifts are given relative
to Et2O$BF3 and CFCl3, respectively.
Description of syntheses and compound characterization
Bis(4-bromophenyl)dimethylsilane (1a) [7a]: Yield: 86%, m.p.
74e75 ꢁC (lit. 78 ꢁC).
Fig. 2. (a) H-bonded chains and their arrangement into 2D layers in 2g. (b) Unit cell packing diagram with void surface generated in CrystalExplorer (
and aliphatic hydrogen atoms are omitted for clarity.
r
¼ 0.0003 au) [18]. Aromatic

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K. Gontarczyk et al. / Journal of Organometallic Chemistry 783 (2015) 1e9
J ¼ 30.0, 4.0 Hz), 111.4 (dd, J ¼ 20.0, 3.5 Hz), 103.5 (dd, J ¼ 30.0,
24.0 Hz), ꢀ2.2 ppm. 19F NMR (CDCl3, 360.6 MHz):
d
¼ ꢀ96.06 (dd,
J ¼ 16.5, 8.5 Hz), ꢀ107.7 (m) ppm. C14H12F4Si (284.32) calcd. C 59.14,
H 4.25; found C 58.98, H 3.86.
Bis(4-dihydroxyborylphenyl)dimethylsilane (2a): n-BuLi (10 M in
hexane; 1.45 mL, 0.0145 mmol) was added to THF (20 mL)
at ꢀ78 ꢁC. Then a solution of 1a (11.5 mL, 0.073 mol) in THF (5 mL)
was added while the temperature was kept below ꢀ75 ꢁC. The
resulting mixture was stirred for 1 h, then B(Oi-Pr)3 (3.4 mL,
0.0145 mol) was added. The resulting mixture was stirred for
30 min and hydrolyzed with H2SO4 (1.5 m aq.; 20 mL) at ꢀ15 ꢁC. The
organic phase was separated, and the solvents were evaporated
under reduced pressure to give the title compound (1.28 g, 59%) as
white crystals, m.p. > 300 ꢁC. 1H NMR (acetone-d6, 400 MHz):
d
¼ 7.88 (d, J ¼ 8.0 Hz, 4H, Ph), 7.88 (d, J ¼ 8.0 Hz, 4H, Ph), 7.21
(broad, 4H, B(OH)2), 0.56 (s, 6H, CH3) ppm. 11B NMR (acetone-d6,
64.2 MHz):
d
d
¼ 31 ppm. 13C NMR (acetone-d6, 100.6 MHz):
¼ 140.4, 133.3, 133.2, ꢀ3.3 ppm. C14H18B2O4Si (300.12) calcd. C
Fig. 3. Molecular layer formed via OeH…O bonds (red dashed lines) in 2i. Aromatic
and aliphatic H atoms are omitted for clarity. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
56.05, H 6.05; found C 56.14, H 6.23.
Tris(4-dihydroxyborylphenyl)methylsilane (2b): This compound
was prepared as described for 2a starting with 1b (5.1 g, 0.01 mol)
and using 6 equivalents of n-BuLi (10 M, 6.0 mL, 0.06 mol). Yield:
Tris(4-bromophenyl)methylsilane (1b) [7a]: Yield: 82%, m.p.
110e112 ꢁC (lit. 106e112 ꢁC).
40% (2.06 g), m.p. > 300 ꢁC. 1H NMR (acetone-d6, 400 MHz):
d
¼ 7.81
(d, J ¼ 8.0 Hz, 6H, Ph), 7.45 (d, J ¼ 8.4 Hz, 6H, Ph), 0.79 (s, 3H, CH3)
Tetrakis(4-bromophenyl)silane (1c) [7a]: Yield: 69%, m.p.
ppm. 11B NMR (acetone-d6, 64.2 MHz):
(acetone-d6, 100.6 MHz):
d
¼ 31 ppm. 13C NMR
239e241 ꢁC (lit. 235 ꢁC).
d
¼ 140.4, 133.3, 133.2, ꢀ3.3 ppm.
Bis(3-bromophenyl)dimethylsilane (1d) [7b]: Yield: 75%, B.p.
90e95 ꢁC (1 Tr).
C19H21B3O6Si (406.14) calcd. C 56.22, H 5.21; found 56.52, H 5.41.
Tetrakis(4-dihydroxyborylphenyl)silane (2c): This compound was
Tris(3-bromophenyl)methylsilane (1e) [7b]: Yield: 60% (oil).
Tetrakis(3-bromophenyl)silane (1f) [7e]: Yield: 90%, m.p.
174e175 ꢁC.
prepared according to the literature procedure [2]. Yield: 60%,
m.p. > 300 ꢁC (lit. m.p. > 300 ꢁC).
Bis(3-dihydroxyborylphenyl)dimethylsilane (2d): This compound
was prepared as described for 2a using 1d (3.70 g, 0.01 mol) as
starting material. The product (2.64 g, 88%) was isolated as a white
powder, m.p. ¼ 258e260 ꢁC. 1H NMR (acetone-d6, 400 MHz):
Bis(3,5-difluorophenyl)dimethylsilane
(1g):
1-bromo-3,5-
difluorobenzene (11.5 mL, 0.10 mol) was added to a stirred solu-
tion of n-BuLi (10 M, 10.1 mL, 0.101 mol) in Et2O (100 mL) at ꢀ78 ꢁC.
It was stirred for 1 h followed by the addition of Me2SiCl2 (5.9 mL,
0.05 mol). The mixture was allowed to warm to ꢀ10 ꢁC, then hy-
drolyzed with aqueous H2SO4 (1.5 M) and warmed to the room
temperature. The water phase was separated followed by the
extraction with Et2O (2 ꢂ 10 mL). The extracts were added to the
organic phase, which was concentrated under reduced pressure.
Distillation under reduced pressure (b.p. 65e66 ꢁC, 1 Tr) afforded
1g as a colourless oil. Yield: 60% (8.52 g). 1H NMR (CDCl3, 400 MHz):
d
¼ 8.16 (s, 2H, Ph), 7.90 (d, J ¼ 9.0 Hz, 2H, Ph), 7.59 (d, J ¼ 9.0 Hz, 2H,
Ph), 7.35 (t, 2H, Ph), 7.24 (s, 4H, B(OH)2), 0.57 (s, 6H, CH3) ppm. 11
B
NMR (acetone-d6, 64.2 MHz):
100.6 MHz):
C
d
¼ 29.0 ppm. 13C NMR (acetone-d6,
d
¼ 139.9, 137.1, 136.1, 134.9, 126.9, ꢀ3.0 ppm.
14H18B2O4Si (300.12) calcd. C 56.05, H 6.05; found 56.52, H 6.08.
Tris(3-dihydroxyborylphenyl)methylsilane (2e): This compound
was prepared as described for 2b using 1e (5.0 g, 0.01 mol) as
starting material. The product (69%) was isolated as a white pow-
d
¼ 6.98 (m, 4H, Ph), 6.82 (tt, J ¼ 9.0, 2.5 Hz, 2H, Ph), 0.58 (s, 6H,
der, m.p. > 300 ꢁC. 1H NMR (acetone-d6, 400 MHz):
d
¼ 8.13 (s, 3H,
CH3). 13C NMR (CDCl3, 100.6 MHz):
d
¼ 162.9 (dd, J ¼ 252.0,10.5 Hz),
Ph), 7.84 (d, J ¼ 8.5 Hz, 3H, Ph), 7.63 (d, J ¼ 8.5 Hz, 3H, Ph), 7.30 (t,
141.6 (t, J ¼ 5.0 Hz), 116.1 (dd, J ¼ 16.5, 5.5 Hz), 105.0 (t,
J ¼ 8.5 Hz, 3H, Ph), 0.57 (s, 3H, CH3) ppm. 11B NMR (acetone-d6,
J ¼ 25.0 Hz), ꢀ3.0 ppm. 19F NMR (CDCl3, 360.6 MHz)
d
¼ ꢀ109.7
64.2 MHz):
d
¼ 29 ppm. 13C NMR (acetone-d6, 100.6 MHz):
(ddd, J ¼ 8.5, 6.0, 1.5 Hz) ppm. C14H12F4Si (284.32) calcd. C 59.14, H
d
¼ 139.7, 137.3, 135.8, 135.2, 126.8, ꢀ1.6 ppm. C19H21B3O6Si$2H2O
4.25; found C 58.77, H 3.98.
(441.92) calcd. C 51.64, H 5.70; found C 52.18, H 5.34.
Tri(3,5-difluorophenyl)methylsilane (1h): This compound was
prepared as described for 1g using MeSiCl3 (5.9 mL, 0.05 mol), 1-
bromo-3,5-difluorobenzene (38.7 g, 0.20 mol) and n-BuLi (10 M,
21 mL, 0.21 mol). Yield: 60%, m.p. 127e131 ꢁC. 1H NMR (CDCl3,
Tetrakis(3-dihydroxyborylphenyl)silane (2f): This compound was
prepared as described for 2a starting with 1b (6.52 g, 0.01 mol) and
using 8 equivalents of n-BuLi (10 M, 8.0 mL, 0.08 mol). Resulting
product was purified by recrystallization from acetone/hexane
(1:1) to give title compound (1.79 g, 35%) as a white powder.
400 MHz):
d
¼ 7.00 (m, 9H, Ph), 0.95 (s, 3H, CH3) ppm. 13C NMR
(CDCl3, 100.6 MHz)
d
¼ 160.3 (dd, J ¼ 245.0, 11 Hz), 140.0 (t,
M.p. > 300 ꢁC. 1H NMR (acetone-d6, 400 MHz):
d
¼ 8.24 (s, 4H, Ph),
J
¼
4.5 Hz), 118.9 (dd,
J
¼
17.0, 4.5 Hz), 104.0 (t,
7.62 (m, 4H, Ph), 7.42 (t, J ¼ 7.5 Hz, 4H, Ph), 7.26 (d, J ¼ 7.5 Hz, 4H,
J ¼ 24.0 Hz), ꢀ2.7 ppm. C19H12F6Si (382.37) calcd. C 59.68, H 3.16;
Ph) ppm. 11B NMR (acetone-d6, 64.2 MHz):
(acetone-d6, 100.6 MHz):
C
d
¼ 29 ppm. 13C NMR
found C 59.24, H 2.89.
d
¼ 142.2, 138.3, 136.3, 135.4, 127.1 ppm.
Bis(2,4-difluorophenyl)dimethylsilane (1i): This compound was
prepared as described for 1g using Me2SiCl2 (5.9 mL, 0.05 mol), 1-
bromo-2,4-difluorobenzene (11.3 mL, 0.10 mol) and n-BuLi (10 M,
10.5 mL, 0.105 mol). Yield: 81%, b.p. 81e83 ꢁC (1 Tr). 1H NMR (CDCl3,
24H24B4O8Si (512.16) calcd. C 56.33, H 4.73; found C 56.11, H 4.99.
Bis(4-dihydroxyboryl-3,5-difluorophenyl)dimethylsilane (2g):
A
solution of 1g (2.81 g, 0.01 mol) in THF (5 mL) was added dropwise
to a freshly prepared solution of LDA [diisopropylamine (3.3 mL,
0.022 mol) and n-BuLi (10 M; 2.1 mL, 0.021 mol) in THF (20 mL)]
at ꢀ75 ꢁC. The resulting white slurry was stirred for ca 30 min
at ꢀ75 ꢁC, and then B(OEt)3 (3.8 mL, 0.022 mol) was added drop-
wise. The mixture was stirred for 1 h, and then it was hydrolyzed
400 MHz):
2.5 Hz, 2H, Ph), 6.77 (td, J ¼ 9.0, 2.5 Hz, 2H, Ph), 0.64 (s, 6H, CH3)
ppm. 13C NMR (CDCl3, 100.6 MHz)
¼ 167.5 (dd, J ¼ 280.0, 12.0 Hz),
165.0 (dd, J ¼ 287.0, 12.0 Hz), 136.8 (dd, J ¼ 13.0, 9.5 Hz), 118.9 (dd,
d
¼ 7.33 (dd, J ¼ 15.5, 7.0 Hz, 2H, Ph), 6.89 (td, J ¼ 8.5,
d

K. Gontarczyk et al. / Journal of Organometallic Chemistry 783 (2015) 1e9
7
with H2SO4 (1.5 M aq., 20 mL). The aqueous phase was separated
and then extracted with Et2O (2 ꢂ 10 mL). The extracts were added
to the organic phase, which was concentrated under reduced
pressure. The solid residue was filtered and washed consecutively
with hexane (5 mL) and toluene (5 mL). Drying in vacuo gave the
title compound (2.64 g, 72%) as a white powder, m.p. 110e116 ꢁC. 1H
217e219 ꢁC, yield 22.2 g (61%). 1H NMR (DMSO-d6, 400 MHz):
d
¼ 7.70 (dd, J ¼ 5.0, 3.0 Hz, 4H, Th), 7.66 (dd, J ¼ 3.0, 1.0 Hz, 4H, Th),
7.21 (dd, J ¼ 5.0, 1.0 Hz, 4H, Th) ppm. 13C NMR (DMSO-d6,
100.6 MHz):
d
¼ 135.5, 135.1, 132.4, 127.2 ppm. C16H12S4Si (359.96):
calcd. C 53.29, H 3.35, S 35.57; found: C 53.58, H 3.87, S 35.39.
Bis(5-dihydroxyborylthiophen-2-yl)dimethylsilane (4a): Diiso-
propylamine (8.4 mL, 0.06 mol) was added to a solution of n-BuLi
(10 M, 6.3 mL, 0.063 mol) in THF (150 mL) at ꢀ70 ꢁC. The mixture
was stirred for 30 min at ꢀ78 ꢁC followed by dropwise addition of
3a (6.72 g, 0.03 mol) in THF (40 mL). After 30 min the reaction
mixture was allowed to warm to ꢀ10 ꢁC and then cooled again
to ꢀ78 ꢁC. Then triisopropyl borate (13.9 mL, 0.06 mol) was added
dropwise. The mixture was stirred for 30 min and allowed to warm
to the room temperature. The resulting pale yellow solution was
hydrolyzed with aq. 1.5 M H2SO4 (70 mL) followed by the addition
of Et2O (100 mL). The water phase was extracted with Et2O (50 mL).
The collected organic solutions were dried with anhydrous MgSO4
and concentrated to give a pale yellow solid. It was stirred with
toluene-hexane mixture (1:1, 50 mL) and filtered to give the title
compound as a white solid, m.p. 104e105 ꢁC, yield 6.41 g (69%). 1H
NMR (acetone-d6, 400 MHz):
d
¼ 7.81 (broad, 4H, B(OH)2), 7.06 (dd,
J ¼ 6.5, 2.0 Hz, 4H, Ph), 0.64 (s, 6H, CH3) ppm. 11B NMR (acetone-d6,
64.2 MHz):
d
¼ 28 ppm. 13C NMR (acetone-d6,100.6 MHz):
¼ 164.8
d
(dd, J ¼ 247.5, 13.0 Hz), 128.5 (d, J ¼ 73.0 Hz), 115.53 (d,
J ¼ 26.0 Hz), ꢀ4.1 ppm. 19F NMR (acetone-d6, 360.6 MHz):
d
¼ ꢀ104.08 (dd, J ¼ 6.0, 2.0 Hz) ppm. C14H14B2F4O4Si (372.08)
calcd. C 45.21, H 3.79; found C 45.07, H 3.71.
Tris(4-dihydroxyboryl-3,5-difluorophenyl)methylsilane (2h): This
compound was prepared as described for 2g starting with 1h (7.64 g,
0.02 mol) and using 20% excess of LDA (0.072 mol). Yield: 72% (7.4 g),
m.p. 133e137 ꢁC. 1H NMR (acetone-d6, 400 MHz):
d
¼ 7.03 (d,
J ¼ 8.0 Hz, 6H, Ph), 0.97 (s, 3H, CH3) ppm. 11B NMR (acetone-d6,
64.2 MHz):
d
¼ 29 ppm. 13C NMR (acetone-d6, 100.6 MHz):
¼ 163.0
d
(dd, J ¼ 251.5, 11.0 Hz), 117.3 (dd, J ¼ 17.0, 6.0 Hz), 105.5 (t,
J ¼ 25.5 Hz), ꢀ5.2 ppm. 19F NMR (acetone-d6, 360.6 MHz):
NMR (acetone-d6, 400 MHz):
d
¼ 7.75 (d, J ¼ 3.0 Hz, 2H, Th), 7.42 (d,
d
¼ ꢀ110.36 (ddd, J ¼ 9.0, 6.0, 2.0 Hz) ppm. C19H15B3F6O6Si (514.08)
J ¼ 3.0 Hz, 2H, Th), 7.35 (s, 4H, (B(OH)2)), 0.64 (s, 6H, CH3) ppm. 13
C
B
calcd. C 45.41, H 2.94; found C 45.55, H 2.78.
NMR (acetone-d6, 100.6 MHz):
d
¼ 143.7, 136.6, 136.1, ꢀ0.9 ppm. 11
Bis(3-dihydroxyboryl-2,4-difluorophenyl)dimethylsilane (2i): This
compound was prepared as described for 2g starting with 1i. Yield:
63% (11.7 g), m.p. 127e129 ꢁC. 1H NMR (acetone-d6, 400 MHz):
NMR (acetone-d6, 96 MHz):
d
¼ 26.2 ppm. C10H14B2O4S2Si (312.03):
calcd. C 38.49, H 4.52, S 20.55; found C 38.13, H 4.92, S 20.83.
Tris(5-dihydroxyborylthiophen-2-yl)methylsilane (4b): This com-
pound was obtained using a procedure described for 4a using 3b
(2.92 g, 0.01 mol) as the starting material. White solid,
m.p. > 400 ꢁC; yield 2.67 g (63%). 1H NMR (acetone-d6, 300 MHz):
d
¼ 7.81 (broad, 4H, B(OH)2), 7.41 (dd, J ¼ 15.5, 8.0 Hz, 2H, Ph), 6.92
(t, J ¼ 8.0 Hz, 2H, Ph), 0.58 (s, 6H, CH3) ppm. 13C NMR (acetone-d6,
100.6 MHz)
d
¼ 167.5 (dd, J ¼ 280, 12.0 Hz), 165.0 (dd, J ¼ 287,
12.0 Hz), 136.8 (dd, J ¼ 13.0, 9.5 Hz),118.9 (dd, J ¼ 30.0, 4.0 Hz), 111.4
d
¼ 7.76 (d, J ¼ 3.5 Hz, 3H, Th), 7.45 (d, J ¼ 3.5 Hz, 3H, Th), 0.93 (s, 3H,
CH3) ppm. 13C NMR (acetone-d6, 100.6 MHz):
¼ 142.3, 138.4, 137.6,
0.2 ppm. 11B NMR (acetone-d6, 96 MHz):
26.7 ppm.
13H15B3O6S3Si (424.01): calcd. C 36.83, H 3.57, S 22.69; found C
37.01, H 3.78, S 22.57.
(dd, J ¼ 20.0, 3.5 Hz), 103.5 (dd, J ¼ 30.0, 24.0 Hz), ꢀ2.2 ppm. 19
F
d
NMR (acetone-d6, 360.6 MHz):
d
¼ ꢀ89.5 (t, J ¼ 8.0 Hz), ꢀ102.0 (dd,
d
¼
J ¼ 16.5, 8.5 Hz) ppm. C4H14B2F4O4Si (372.08) calcd. C 45.21, H 3.79;
C
found C 45.30, H 3.64.
Bis(2-thienyl)dimethylsilane (3a) [8a]: Yield: 78%, b.p.131e134 ꢁC
Tetrakis(5-dihydroxyborylthiophen-2-yl)silane (4c): This com-
pound was obtained using a procedure described for 4a using 3c
(3.60 g, 0.01 mol) as the starting material. White solid, m.p.
157e160 ꢁC; yield 3.8 g (67%). 1H NMR (acetone-d6, 300 MHz):
(2 Torr).
Tris(2-thienyl)methylsilane (3b) [8b]: Yield: 75%, b.p. 203e206 ꢁC
(2 Torr).
Tetrakis(2-thienyl)silane (3c) [9]: Yield: 70%, m.p. 128e131 ꢁC (lit.
d
¼ 7.82 (d, J ¼ 3.5 Hz, 4H, Th), 7.54 (d, J ¼ 3.5 Hz, 4H, Th), 7.49 (s, 8H,
135 ꢁC).
B(OH)2) ppm. 13C NMR (acetone-d6, 100.6 MHz):
d
d
¼ 140.4, 139.7,
¼ 27.3 ppm.
Bis(3-thienyl)dimethylsilane (3d) [10]: Yield: 83%, b.p.
137.5 ppm. 11B NMR (acetone-d6, 96 MHz):
C
153e155 ꢁC (2 Torr).
16H16B4O8S4Si $2H2O (571.91): calcd. C 33.60, H 3.52, S 22.43;
Tris(3-thienyl)methylsilane
(3e):
A
solution
of
3-
found C 33.73, H 3.91, S 22.63.
bromothiophene (48.9 g, 0.30 mol) in Et2O (50 mL) was added
dropwise at ꢀ65 ꢁC to a solution of n-BuLi (10 M, 31 mL, 0.31 mol) in
Et2O (300 mL). The resulting white suspension was stirred
at ꢀ78 ꢁC for 30 min followed by dropwise addition of MeSiCl3
(15.0 g, 11.8 mL, 0.10 mol) in Et2O (40 mL) while maintaining the
temperature below ꢀ50 ꢁC, then it was allowed to warm to room
temperature. The resultant white suspension was quenched with
aq. H2SO4 (1.5 M aq., 100 mL). Et2O (100 mL) was added to the
mixture and the organic phase was separated. The aqueous phase
was extracted with Et2O (2 ꢂ 50 mL). Collected organic phases were
dried over anhydrous MgSO4 and concentrated. The crude product
was crystallized from cold (ca. ꢀ20 ꢁC) hexane (70 mL) to give a
pale yellow crystalline solid, m.p. 70e73 ꢁC, yield 17.3 g (59%). 1H
Bis(4-dihydroxyborylthiophen-2-yl)dimethylsilane (4d): This
compound was obtained using a procedure described for 4a using
3d (2.24 g, 0.01 mol) as the starting material. White solid, m.p.
119e122 ꢁC, yield 2.88 g (68%). 1H NMR (acetone-d6, 300 MHz):
d
¼ 7.84 (d, J ¼ 1.0 Hz, 2H, Th), 7.78 (d, J ¼ 1.0 Hz, 2H, Th), 0.52 (s, 6H,
CH3) ppm. 13C NMR (acetone-d6, 100.6 MHz):
d
¼ 141.7, 141.1,
¼ 26.93 ppm.
139.6, ꢀ1.0 ppm. 11B NMR (acetone-d6, 96 MHz):
d
C
10H14B2O4S2Si (312.03): calcd. C 38.49, H 4.52, S 20.55; found C
38.76, H 4.60, S 20.93.
Tris(4-dihydroxyborylthiophen-2-yl)methylsilane (4e): Diisopro-
pylamine (4.62 mL, 0.033 mol) was added to a solution of n-BuLi
(10 M, 3.3 mL, 0.033 mol) in THF (50 mL) at ꢀ70 ꢁC. The mixture was
stirred for 30 min at ꢀ78 ꢁC followed by dropwise addition of a
solution of 3e (2.92 g, 0.01 mol) in THF (20 mL). After 30 min the
reaction mixture was warmed slowly to ꢀ10 ꢁC and then cooled
again to ꢀ78 ꢁC. Then triisopropyl borate (6.92 mL, 0.03 mol) was
added to a colourless solution. After 30 min the mixture was left to
warm to the room temperature and hydrolyzed with water (25 mL)
and 1.5 M H2SO4 (15 mL) (pH ca.7e8) followed by the addition of
Et2O (50 mL). The resultant white slurry was filtered. The obtained
white solid was stirred with 1.5 M H2SO4 (10 mL), water (25 mL)
and diethyl ether (50 mL). The product was precipitated from
NMR (acetone-d6, 300 MHz):
7.55 (dd, J ¼ 5.0, 2.5 Hz, 3H, Th), 7.26 (dd, J ¼ 5.0,1.0 Hz, 3H, Th), 0.82
(s, 3H, CH3) ppm. 13C NMR (acetone-d6,100.6 MHz):
d
¼ 7.63 (dd, J ¼ 2.5, 1.0 Hz, 3H, Th),
d
¼ 138.0,134.7,
132.8, 126.9, ꢀ1.7 ppm. C13H12S3Si (359.96): calcd. C 53.38, H 4.13, S
32.82; found C 53.01, H 4.12, S 32.32.
Tetrakis(3-thienyl)silane (3f): The title compound was obtained
from 3-thienylithium (generated as described for 3e, 0.4 mol) and
Si(OMe)4 (15.2 g, 0.1 mol). It was obtained as a white solid by
recrystallization of
a crude product from hot toluene, m.p.

8
K. Gontarczyk et al. / Journal of Organometallic Chemistry 783 (2015) 1e9
organic phase with hexane (150 mL) followed by filtration and
drying in vacuo to give a white solid, m.p. 166e169 ꢁC, yield 1.70 g
Appendix A. Supplementary data
(40%). 1H NMR (acetone-d6, 400 MHz):
d
¼ 7.86 (d, J ¼ 0.5 Hz, 3H,
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.01.024.
Th), 7.81 (d, J ¼ 0.5 Hz, 3H, Th), 7.36 (s, 6H, CH3), 0.80 (s, 3H) ppm.
13C NMR (acetone-d6, 100.6 MHz):
11B NMR (acetone-d6, 96 MHz):
d
¼ 142.1, 140.8, 139.6, ꢀ1.3 ppm.
¼ 27.2 ppm. C13H15B3O6S3Si
d
References
(424.01): calcd. C 36.83, H 3.57, S 22.69; found C 37.17, H 3.72, S
23.07.
€
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(e) K. Durka, K.N. Jarzembska, R. Kaminski, S. Lulinski, J. Serwatowski,
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K. Wozniak, Cryst. Growth Des. 12 (2012) 3720e3734;
ꢀ
ꢀ
ation and equipped with an Oxford Cryosystems nitrogen gas-flow
apparatus. Data reduction and analysis were carried out with the
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methods using SHELXS-97 and refined using SHELXL-2013 [20]. All
non-hydrogen atoms were refined anisotropically and all of the CH
(phenyl, methylene) hydrogen atoms were placed in calculated
positions with CeH distances of 0.95 Å (phenyl) and 0.99 Å
(methylene). The positions of all OH hydrogen atoms were refined
freely constraining their Uiso parameters. Hydrogen atoms were
included in the refinement in riding-motion approximation with
(f) K. Durka, K.N. Jarzembska, R. Kaminski, S. Lulinski, J. Serwatowski,
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K. Wozniak, Cryst. Growth Des. 13 (2013) 4181e4185;
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U
iso(phenyl H) ¼ 1.2$Ueq(C), and Uiso(methyl H) ¼ 1.5$Ueq(C). The
ꢀ
ꢀ
positions of OH hydrogen atoms were refined with Uiso(hydroxyl
H) ¼ 1.5$Ueq(O). In all cases except of the disordered THF molecule
in the compound 2g, hydrogen atoms were clearly visible on the
residual density maps. A large peaks of residual electron densities
(1.0e2.5$e$Åꢀ3) found near THF molecule in 2g indicate presence of
disorder. It is also possible that other guest molecules (for example
acetone) partially incorporated during the crystal growth. However,
we were unable to propose any reasonable model for this case. The
unrefined density results in relatively high R and wR parameter
values and also, in consequence, many CHECKCIF's B and C alerts
appeared. Crystal data for 2g: 2(C14H14B2F4O4Si)$C4H8O,
Mr ¼ 816.03 a.u.; T ¼ 100 K; triclinic; space group: P-1, a ¼ 11.372
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(1) Å, b ¼ 14.212 (1) Å, c ¼ 14.377 (1) Å,
a
¼ 67.67 (1)ꢁ,
b
¼ 70.99
(1)ꢁ,
m
g
¼ 67.21 (1)ꢁ, V ¼ 1937.5 (3) Å3; dcalc ¼ 1.399 g cmꢀ3
;
¼ 0.18 mmꢀ1; Z ¼ 2; F(000) ¼ 840; number of collected/unique
reflections
(I ꢃ 3
(Rint
¼
8.9%)
¼
28,547/8925,
R[F]/wR[F]
¼ þ2.28/
max=min
res
s
(I)) ¼ 9.3%/31.7%, GoF ¼ 1.052, D9
ꢀ0.93$e$Åꢀ3. Crystal data for 2i: C14H14B2F4O4Si, Mr ¼ 371.96 a.u.;
T ¼ 100 K; orthorhombic; space group: Pnna, a ¼ 12.286 (1) Å,
b
¼
18.087 (1) Å,
c
¼
7.335 (1) Å,
V
¼
1629.9 (1) Å3;
dcalc ¼ 1.516 g cmꢀ3
;
m
¼ 0.02 mmꢀ1; Z ¼ 4; F(000) ¼ 760; number
of collected/unique reflections (Rint ¼ 3.8%) ¼ 25,930/2811, R[F]/wR
max=min
res
[F] (I ꢃ 3
s
(I)) ¼ 4.5%/12.8%, GoF ¼ 1.042, D9
¼ þ0.63/
ꢀ0.73$e$Åꢀ3
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Acknowledgements
This work was supported by the Narodowe Centrum Nauki
(National Science Centre) in the framework of project DEC-2011/
03/B/ST5/02755. Support from the Aldrich Chemical Co., Milwau-
kee, WI, U.S.A., through continuous donation of chemicals and
equipment is gratefully acknowledged.
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Appendix A. Supplementary data
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