Synthesis and electropolymerization of a series of 2,2′-(ortho-carboranyl)bisthiophenes

Article abstract of DOI:10.1016/j.jorganchem.2016.11.029

A series of 5,5′-functionalized 2,2′-(ortho-carboranyl)bisthiophene with bromo, vinyl, trimethylsilylethyne, N-methylpyrrole or thienyl groups was synthesized and all compounds were characterized by NMR, MS, and X-ray analysis. Electrochemical characterizations of 12a,b and 13a,b were conducted, and the di(thienyl-N-methylpyrrole)-o-carborane 12a and di(bisthienyl)-o-carborane 12b were successfully electropolymerized on a suitable anode to produce the corresponding conducting polymers. DFT calculations are consistent with the electrochemical data.

Full text of DOI:10.1016/j.jorganchem.2016.11.029

Accepted Manuscript
Synthesis and electropolymerization of a series of 2,2′-(ortho-
carboranyl)bisthiophenes
Ning Zhao, Bruno Fabre, Petia Bobadova-Parvanova, Frank R. Fronczek, M. Graça
H. Vicente
PII:
S0022-328X(16)30533-2
10.1016/j.jorganchem.2016.11.029
JOM 19714
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 20 October 2016
Revised Date: 18 November 2016
Accepted Date: 19 November 2016
Please cite this article as: N. Zhao, B. Fabre, P. Bobadova-Parvanova, F.R. Fronczek, M.G.H. Vicente,
Synthesis and electropolymerization of a series of 2,2′-(ortho-carboranyl)bisthiophenes, Journal of
Organometallic Chemistry (2016), doi: 10.1016/j.jorganchem.2016.11.029.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis and Electropolymerization of a series of 2,2’-(ortho-
Carboranyl)bisthiophenes
Ning Zhao,a Bruno Fabre,b Petia Bobadova-Parvanova,c Frank R. Fronczek,a and M. Graça H. Vicentea,*
a.Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, USA.
b.Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Matière Condensée et Systèmes Electroactifs (MaCSE),
Université de Rennes 1, 35042 Rennes Cedex, France.
c.Rockhurst University, Department of Chemistry, Kansas City, MO 64110, USA.
Abstract
A
series of 5,5’-functionalized 2,2’-(ortho-carboranyl)bisthiophene with bromo, vinyl,
trimethylsilylethyne, N-methylpyrrole or thienyl groups was synthesized and all compounds were
characterized by NMR, MS, and X-ray analysis. Electrochemical characterizations of 12a,b and 13a,b
were conducted, and the di(thienyl-N-methylpyrrole)-o-carborane 12a and di(bisthienyl)-o-carborane 12b
were successfully electropolymerized on a suitable anode to produce the corresponding conducting
polymers. DFT calculations are consistent with the electrochemical data.
Introduction
Carboranes[1] belong to a class of boron clusters that contain carbon and boron atoms.
The neutral isomeric carboranes ortho-, meta-, and para-C2B10H12 and the negatively charged
-
-
nido-C2B9H12 and closo-CB11H12 carboranes (1-5, respectively, shown in Figure 1), possess
unique steric, electronic and physicochemical properties, including high hydrophobicity and
remarkably high thermal and chemical stabilities, which has led to their wide application in
diverse areas, for example in cancer therapy, molecular recognition, catalysis, and in
electronics.[1-3] These unique clusters with delocalized electrons, are readily synthesized and
functionalized, and the carbon atoms in these clusters make convenient sites for attachment of
organic molecules. In particular the synthesis of carborane-containing polymers[4-6] and their
investigation for applications as pre-ceramic,[7] thermally robust,[8, 9] conducting,[10] and
luminescent[11, 12] materials, have been an active area of research in recent years.
Figure 1: Neutral, anionic nido and closo- carboranes.
Since conducting polymers were first reported by Heeger, MacDiarmid and Shirakawa in
1977,[13, 14] a large body of research has been dedicated to the synthesis and investigation of
conducting polymers, including the polypyrrole (PPy), polythiophene (PTh), and polyaniline

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(PANI) types.[15] However, there are several challenges that remain associated with these
systems, including their poor thermal and oxidative stabilities in the environment, and short
lifetimes during the charge-discharge process, that have hampered the commerciality of
conducting polymers.[16] Due to their special properties as mentioned above, carboranes are
excellent candidates to be used for the functionalization of both monomers and polymers.
Figure 2: Structures of compounds 7-9.
Several strategies for incorporation of carboranes into polymers have been reported: (
[Co(C2B9H11)2]- (
) was used as doping agent for the electropolymerization of monomers;[17-20]
ii) pyrrole and thiophene monomers were modified by functionalization at the 2-position[21] and
the N-position (e.i. )[22] with anion ( ), followed by electropolymerization to form the self-
i)
6
(
7
6
doped polymers under suitable conditions; (iii) monomers were modified by using neutral
carboranes (o-, m-, p-) at the 2-position[23, 24] and at the 3 and/or 4-positions (e.g. 8) [25-28]
followed by polymerization; and (iv) copolymerization between carborane-containing monomers
with α-free monomers (pyrroles or thiophenes) using both chemical[29, 30] and
electrochemical[22, 31] methods, for the introduction of carborane groups into the main chain
(Figure 2). These carborane groups entrust to the monomers and relevant conducting polymers
several desirable properties, including enhanced oxidation potentials, which can help to overcome
the “polythiophene paradox”,[32] and high thermal and chemical stabilities. However, several
challenges in carborane-functionalized organic polymers remain. Among them, decreased
conductivity (e.g. 10-2 S cm-1 and 20 S cm-1 for poly(
8)[26] and poly(9)[23], respectively,
compared with values of up to 200 S cm-1 and 100 S cm-1 measured for traditional PPy and PTh,
respectively[33]), and blue-shifted π - π* transition bands from UV-vis spectroscopy analysis, are
often observed. These might be due to their less conjugated backbone caused by the electron-
withdrawing effect and steric hindrance induced by the carborane groups. Furthermore, some
carborane-containing molecules even fail to be successfully electropolymerized when the
carborane groups are too close to the aromatic rings.[24, 25, 27, 28]
On the other hand, a wide range of structurally-modified π-conjugated oligomers have
been designed, synthesized and polymerized to afford materials with specific properties (e.g.
photovoltaic properties and controlled length of the main chain).[34, 35] For example, in the
process of electropolymerization of thiophene monomers, oligomers with oxidation potential
higher than that of polymers are usually generated.[32] This may lead to a significant degradation
of the mechanical and chemical properties of the materials. However, electropolymerization of
short-chain oligomers (such as bis- and tetra-thiophenes) can overcome the “polythiophene
paradox” and produce better defined polymers, although these might show decreased
conductivity.[36-38] Another advantage of using oligomers rather than monomers is the
versatility they bring to polymer synthesis by taking advantage of the number and ratio of co-

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polymers used.[35] In the last two decades, a large number of thiophene oligomers[35] and
pyrrole oligomers[39-41] have been reported.
In that context, we have been interested in the design and synthesis of new electroactive
carborane-functionalized oligomers for electropolymerization. Based on our previous work, we
observed that poly(
enhanced oxidation resistance.[23] We now report a new series of derivatives of ortho
carboranylbisthiophene bearing vinyl, thienyl or pyrrole groups at the 5- or 5,5’-positions. We
9) showed high conductivity and lifetime, with high thermal stability and
-
9
anticipated that such modifications could lead to more aerated polymer structures and more
conjugated main chains due to decreased steric hindrance and electronic effect induced by the
carborane cluste. On the other hand, it has been reported that pyrrolic groups in the oligomers and
polymers can promote and stabilize the radical cations formed during electropolymerization,
which further promotes the π-π intermolecular dimerizations.[42, 43] Such phenomena may lead
to supramolecular π-dimers with enhanced conductivity.[43-45] Therefore, electron-rich
N-
methylpyrrole units were introduced to the 5,5’-positions (12a) and this compound was
anticipated to be easier to polymerize to form a more conductive material, relative to analogues
12b and 13a
.
Materials and methods
Synthesis and characterization
General
All the reagents and solvents were purchased from Fisher or Sigma-Aldrich without further
purification. All the reactions were monitored by Sorbent TLC using 0.2 mm silica gel or neutral
alumina with UV- 254/366 nm. Silica gel (230×400 mesh) and neutral alumina (50-200 µm, Act.
I) for column chromatography were purchased from Sorbent. All NMR spectra were collected
using a Bruker AV-400 MHz, AV-500 MHz or DPX-400 MHz NMR spectrometers in CDCl3 at
1
room temperature. The chemical shifts (
δ
) are reported in ppm with CDCl3 (
δ
= 7.27 ppm for H
13
NMR and 77.0 ppm for C NMR) as reference. All mass spectra were collected using an Agilent
6210 ESI-TOF MS, Varian Saturn 2000 Ion Trap with CP 3800 GC MS and Applied Biosystems
QSTAR for MALDI.
Synthesis
2,2’-o-Carboranyl-bisthiophene
(9) was synthesized using a slightly modified published
procedure.[23]
5,5’-Dibromo-2,2’-o-Carboranyl-bisthiophene (10a).
To a 50 mL flask equipped with a magnetic stirrer were added
9 (630 mg, 2.04 mmol) and NBS
(2.18 g, 12.24 mmol). The flask was evacuated and refilled with argon 3 times. Chloroform (15
mL) and acetic acid (10 mL) were added into the flask. The mixture was stirred and refluxed
overnight under Ar. The reaction was stopped when starting material disappeared, according to
TLC. After cooling to room temperature, the mixture was poured into water. Dichloromethane (30
mL × 3) was used to extract the organic components. The organic layers were combined and
washed with saturated aqueous NaHCO3 and brine. The organic layer was dried using dry sodium
sulfate then the solvents were removed under reduced pressure. The residue was purified by silica
gel column chromatography using hexane as the eluent, to give 10a as a white solid product

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o
1
(722.9 mg, 76% yield). mp 99-100 C. H NMR (400 MHz, CDCl3) δ 6.94-6.95 (d,
1H), 6.84-6.85 (d,
= 4.0 Hz, 1H), 1.60-3.50 (br, 10 H); 13C NMR (400 MHz, CDCl3): 135.6,
132.8, 130.3, 116.8, 80.0; MS (GC-MS) m/z [M]+ 466.0, calculated for C10H14B10Br2S2 466.0.
5-Bromo-2,2’-o-Carboranyl-bisthiophene (10b)
A similar procedure as that described above was used, starting from
J = 4.0 Hz,
J
.
9
(257 mg, 0.83 mmol), NBS
(443 mg, 2.49 mmol), chloroform (4 mL), and acetic acid (4 mL). The mixture was stirred and
refluxed overnight under Ar. After cooling to room temperature, the mixture was poured into
water, extracted with dichloromethane (10 mL × 3) and the organic layers washed with saturated
aqueous NaHCO3 and brine. The resulting residue was purified by silica gel column
chromatography using hexane as the eluent to give 10b as a white solid (160.8 mg, 50%). mp
107-108 oC. 1H NMR (400 MHz, CDCl3) δ 7.23-7.25 (dd,
3.7, 1.2 Hz, 1H), 6.92-6.93 (d, = 4.0 Hz, 1H), 6.85-6.87 (dd,
= 4.0 Hz, 1H), 1.60-3.50 (br,10H); C NMR (CDCl3, 400Hz): 135.9, 134.5, 132.7, 132.6, 130.0,
129.7, 127.2, 116.4, 80.8, 80.0; MS (GC-MS) m/z [M]+ 387.3, calculated for C10H15B10BrS2 387.1.
2,2’-o-Carboranyldi(2-trimethylsilylethynylthiophene) (11)
J
= 5.2, 1.2 Hz, 1H), 7.19-7.20 (dd,
J
=
J
J
J
= 5.1, 3.8 Hz, 1H), 6.79-6.80 (d,
13
To a 25 mL flask equipped with a magnetic stirrer were added 10a (92.3 mg, 0.2 mmol),
Pd(PPh3)2Cl2 (27 mg, 6 mol%) and CuI (14 mg, 10 mol%). The flask was evacuated and refilled
with argon 3 times. Dry diethylamine (DEA) (5 mL) and ethynyltrimethylsilane (58.9 mg, 0.6
o
mmol) were added into the flask. The mixture was stirred at 60 C under argon for 2 h. The
reaction was stopped when all the starting material disappeared according to TLC. After cooling
to room temperature, the solvent was removed under reduced pressure. The catalysts were
removed by filtration through a pad of silica gel and washed with dichloromethane. The organic
solvents were removed under reduced pressure and the resulting residue was purified by silica gel
column chromatography using hexane as the eluent, to give 11 as a white solid (132.6 mg, 80%).
mp 140-141 oC. 1H NMR (400 MHz, CDCl3) δ 7.00 (d,
J = 3.9 Hz, 1H), 6.92 (d, J = 3.9 Hz, 1H),
1.60-3.50 (10H, br), 0.23 (s, 18H); 13C NMR (400 MHz, CDCl3): 134.9, 132.3, 132.1, 127.4,
101.7, 96.0, 80.3, -0.3; MS (GC-MS) m/z [M]+ 500.8, calculated for C20H32B10Si2S2 501.2.
General procedure for Stille reactions
Into a 100 ml flask equipped with a magnetic stirrer were added 10a or 10b (0.1 mmol) and
Pd(PPh3)4 (12 mg, 10 mol%). The flask was evacuated and refilled with argon 3 times. Toluene
(10 ml) and stannane reagent (3 equiv for the synthesis of 12a,b and 13a, or 1.5 equiv for
synthesis of 13b) were added into the flask. The mixture was stirred and refluxed for 2-4 h under
nitrogen. The reaction was stopped when starting material disappeared according to TLC. The
catalysts were removed by filtration through a pad of celite and washed with dichloromethane.
The solvents were removed under reduced pressure. The residue was purified by silica gel column
chromatography using hexane/dichloromethane as the eluents to give the desired products.
o
1
Di(thienyl-
MHz, CDCl3) δ 7.14 (d,
1.7 Hz, 2H), 6.11 (dd,
N
-methylpyrrole)-o-carborane (12a): 44.3 mg
= 3.9 Hz, 2H), 6.73 (d, = 3.9 Hz, 2H), 6.67 (m, 2H), 6.26 (dd,
= 3.6, 2.8 Hz, 2H), 3.60 (s, 6H), 1.60-3.50 (10H, br); C NMR (CDCl3,
,
95%; mp 152-154 C; H NMR (400
J
J
J
= 3.7,
13
J
100Hz): 139.3, 133.0, 132.7, 125.8, 125.1, 124.0, 110.8, 108.2, 81.6, 35.3; HRMS (ESI-TOF) m/z
[M+H]+ 468.2595, calculated for C20H27N2S2B10 468.2600.
o
1
Di(bisthienyl)-o-carborane (12b): 43.5 mg, 92%; mp 172-174 C; H NMR (400 MHz, CDCl3)
δ 7.22–7.24 (m, 2H), 7.12-7.13 (m, 2H), 7.09-7.10 (d, = 3.9 Hz, 2H), 6.98-7.00 (dd, = 5.0, 3.7
Hz, 2H), 6.88 (d,
= 3.9 Hz, 2H), 1.60-3.50 (br, 10H); 13C NMR (126 MHz, CDCl3) δ 141.5,
J
J
J

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135.8, 133.2, 132.8, 128.0, 125.7, 124.8, 123.3, 81.4; MALDI-TOF m/z [M]+ 473.138 , calculated
for C18H20S4B10 473.143.
o
1
5,5’-Divinyl-2,2’-Carboranyldithiophene (13a): 24.5 mg, 68%; mp 144-147 C; H NMR (400
MHz, CDCl3) δ 7.02-7.03 (d, = 3.9 Hz, 2H), 6.68-6.69 (d, = 3.8 Hz, 2H), 6.56-6.63 (dd, J =
J
J
17.4, 10.9 Hz, 2H), 5.51-5.56 (d,
J = 17.4 Hz, 2H), 5.17-5.20 (d, J = 10.9 Hz, 2H), 1.60-3.50 (br,
10H); 13C NMR (126 MHz, CDCl3) δ 146.7, 132.9, 132.7, 129.0, 125.4, 115.5, 81.3. MS (GC-
MS) m/z [M]+ 360.5, calculated for C14H20B10S2 360.2.
5-Vinyl-2,2’-Carboranyldithiophene (13b): 25.1 mg, 75%; mp 91-93 C; 1H NMR (CDCl3, 400
o
Hz) δ 7.19–7.22 (m, 2H), 7.01-7.02 (d,
6.67 (d, = 3.8 Hz, 1H), 6.55-6.62 (dd,
5.16-5.19 (d,
J
= 3.8 Hz, 1H), 6.83-6.85 (dd,
J
= 5.0, 3.9 Hz, 1H), 6.66-
= 17.4 Hz, 1H),
J
J
= 17.4, 10.9 Hz, 1H), 5.50-5.54 (d, J
J
= 10.9 Hz, 1H), 1.60-3.50 (br, 10H); 13C NMR (CDCl3, 100 Hz) δ 146.6, 134.8,
133.0, 132.7, 132.6, 129.5, 129.0, 127.1, 125.3, 115.5, 81.0; MS (GC-MS) m/z [M]+ 334.3,
calculated for C12H18B10S2 334.2.
X-Ray Diffraction
Diffraction data for 10a
Apex-II DUO diffractometer equipped with MoK
monochromator (CuK radiation from a microfocus source for 13b). For 12b, data were from a
Nonius KappaCCD diffractometer with MoK radiation at T=90K. In 10a, there are four
,
10b
,
11
,
12a
,
13a and 13b were collected at T=100K on a Brüker Kappa
α
radiation and a Triumph curved
α
α
independent molecules, and for two of them, disorder exists in the conformation of one of the
bromothiophenes. A similar disorder exists for the nonbrominated thiophene of 10b, and for the
terminal thiophenes in 12b. In 11, the molecule lies on a crystallographic twofold axis. CIFs are
provided as supplementary material. CCDC deposition numbers are 1012320-1012322 for 10a
10b 11, and 1484163-1484166 for 12a 12b 13a, and 13b, respectively.
,
,
,
,
Electrochemical Characterizations
Tetra- -butylammonium hexafluorophosphate Bu4NPF6 was purchased from Fluka (puriss,
n
electrochemical grade). Anhydrous dichloromethane (less than 50 ppm water from Merck) and
acetonitrile (anhydrous, >99.8%, from Sigma-Aldrich) were used as received. The electrolytic
medium was dried in situ over activated, neutral alumina from Aldrich. Alumina was previously
activated at 450°C under vacuum for several hours. Linear potential cyclic voltammetry
experiments were performed with an Autolab PGSTAT 30 potentiostat from Eco Chemie B.V.,
equipped with General Purpose Electrochemical System GPES software. The working electrode
was a 1 mm-diameter platinum disk (area: 0.8 mm2) and the counter electrode was a glassy carbon
rod. Potentials were relative to the system 10-2 M Ag+ | Ag in acetonitrile and the
ferrocene/ferrocenium couple in CH2Cl2 + 0.2 M Bu4NPF6 was observed at
E°’ = 0.19 V vs this
reference. All electrochemical measurements were carried out inside a home-made Faraday cage
at room temperature (20±2°C) and under a constant flow of argon.
Computer Modelling
The geometries of all structures were optimized without symmetry constraints using B3LYP/6-
31+G(d,p) level calculations. The hybrid Becke’s Three Parameter DFT Functional was used.[46,
47] The solvent effects were taken into account using the Polarized Continuum Model (PCM).[48,
49] All calculations were performed using the Gaussian 09 program package.[50]

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Results and Discussion
Synthesis of ortho-Carboranylbisthiophenes
The key intermediate 2,2’- -carboranyldithiophene
o
9 was obtained using a slightly
modified procedure from that previously published by us,[23, 24] in 30% overall yield. The
Sonogashira cross-coupling reaction of commercially available 2-iodothiophene and 2-
[(trimethylsilyl)ethynyl]-thiophene, deprotected in situ in the presence of 1,8-diazabicycloundec-
7-ene (DBU), using bis(triphenylphosphine)-palladium(II) dichloride and copper(I) iodide as
catalysts, produced 2,2’-ethylenedithiophene in 87% yield.[51] Reaction of 2,2’-
ethylenedithiophene with decaborane and diethyl sulfide gave the key precursor 2,2’-o-
carboranyldithiophene
carboranyldithiophene
9
9
in 35% yield.[23, 24] Bromination of the two α-positions of 2,2’-
was accomplished using 6 equiv of N-bromosuccinimide (NBS) in
o
chloroform and acetic acid (1:1), at 130 C overnight, producing the dibrominated product 10a in
76% yield,[52] as shown in Scheme 1. The two α-positions of 2,2’-carboranyldithiophene are
9
deactivated toward bromination due to the electron-withdrawing character of the ortho-carborane
group, therefore requiring high temperature to proceed. At lower reaction temperature, or using
THF, DMF, CCl4 or pure CHCl3 as the solvent, no bromination took place. On the other hand,
using pure acetic acid as the solvent at 110 oC, produced multiple brominated products in low
yields,[52] while using different ratios of chloroform/acetic acid (e.g. 2:1) also decreased the yield
of the target product 10a. The mono- brominated product 10b was obtained as the main product in
o
50% yield, along with 15% of 10a, using only 3 equiv of NBS at lower temperature (110 C,
rather than 130 oC).
Scheme 1. Synthesis of 5-bromo- and 5,5’-dibromo-o-carboranyl-bisthiophenes 10a,b.
The reactivity of the newly generated o-carboranylbisthiophenes 10a,b were investigated
under Sonogashira and Stille cross-coupling reactions, as shown in Scheme 2. The Sonogashira
coupling reaction between 10a and ethynyltrimethylsilane, using bis(triphenylphosphine)-
palladium(II) dichloride and copper(I) iodide as the catalysts, produced the corresponding di-
ethynyl product 11, in 80% yield. On the other hand, 10a reacted with 3 equiv of 2-
(tributylstannyl)thiophene, or with 1-methyl-2-(tributylstannyl)pyrrole or with tributyl(vinyl)tin,
in the presence of tetrakis(triphenylphosphine)palladium(0) as the catalyst, in refluxing toluene
for 2-4 h, to provide di(thienyl-N-methylpyrrole)-o-carborane 12a, di(bisthienyl)-o-carborane 12b
and di(2’-vinylthienyl)-o-carborane 13a, respectively, in good to excellent yields (68%-95%), as
shown in Scheme 2. In addition, the Stille cross-coupling reaction between 10b and
tributyl(vinyl)tin produced the corresponding mono-vinyl product 13b in 75% yield. The
structures of all new carboranylthiophenes were confirmed by 1H-NMR, 13C-NMR, MS (GC, ESI-
TOF or MALDI-TOF), and by X-ray analysis (Figure 3).

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Scheme 2. Reactivity of 5-bromo- and 5,5’-dibromo-o-carboranyl-bisthiophenes 10a,b.under Pd(0)-
catalyzed cross coupling reactions.
X-Ray Analysis
By slow evaporation of hexanes or from hexane/dichloromethane mixed solvents, crystals of o-
carboranyl-bisthiophene products 10a-b, 11, 12a-b, and 13a-b suitable for X-ray analysis were obtained,
and their structures are shown in Figure 3. Compound 10a has four independent molecules, two of which
have the thiophene S atoms relatively syn (approximate Cs symmetry). The dihedral angles formed by the
thiophene planes in these two molecules are 63.8 and 72.3o. The other two independent molecules each
have one thiophene disordered into both syn and anti conformations, with the anti (approximate
symmetry C2) being the dominant conformer. In the disordered molecules, the two thiophene planes form
dihedral angles of 63.5 and 66.2o. The four carborane C-C distances are in the range 1.732(4)-1.734(4) Å.
Compound 10b has disorder in its unsubstituted thiophene, the dihedral angle formed by the substituted
and unsubstituted thiophenes is 54.1o, and the carborane C-C distance is 1.737(3) Å. 11 lies on a
crystallographic two-fold axis, thus the thiophenes are anti, forming a dihedral angle of 54.2o. The C-C
distance in the carborane is 1.7338(13) Å, and the alkyne triple bond distance is 1.2130(10) Å. Compound
12a has approximate C2 symmetry, with the thiophene planes forming a dihedral angle of 52.6o. The
thiophene and N-methylpyrrole rings have their S and N atoms anti, with S-C-C-N torsion angles of
151.81(12) and 164.04(11)o. The carborane C-C distance is 1.753(2) Å. Compound 12b also has
approximate C2 symmetry, with the carborane-bound thiophene planes forming a dihedral angle of 61.6o.
Both terminal thiophenes are disordered, such that both syn and anti bisthiophenes are present. The

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carborane C-C distance is 1.730(11) Å. Compound 13a has approximate C2 symmetry, with the
thiophene planes forming a dihedral angle of 63.3o and the vinyl groups syn to thiophene S. The
carborane C-C distance is 1.7364(14) Å. In 13b, the thiophene rings are anti and form a 58.6o dihedral
angle, the vinyl group is syn to the thiophene S, and the carborane C-C distance is 1.740(2) Å.
Figure 3. Crystal structures of monomers 10a-b, 11, 12a-b, and 13a-b, with 50% ellipsoids. Only the
major conformer is shown for disordered thiophene rings.

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Electrochemistry of the Monomers and their Corresponding Conducting Polymer Films
The cyclic voltammetry characterization of the
o-carboranes derivatives at 5 mM in
thoroughly dried CH2Cl2 + 2 x 10-1 M Bu4NPF6 (except for 12a examined in CH3CN + 0.1 M
Bu4NPF6) revealed two irreversible oxidation peaks when cycled between 0.0 and 2.2 V vs
Ag/Ag+ 10-2 M (Figure 4a and Table 1). Since unsubstituted carboranes are not oxidized under
these electrolytic conditions, these can be assigned to the oxidation of the aromatic rings into
radical cation species. Furthermore, the anodic potentials of these systems were found to increase
in the order 12a
<
12b
<
13a
≈
13b, in perfect line with the trend observed for the electrochemical
-pyrrole[53,
oxidation of the aromatic rings substituting the carborane, namely 2-(2-thienyl)-1
H
54] < bithiophene[55] < 2-vinylthiophene.[56] For 12a, both oxidation processes at 0.74 and 1.30
V can be assigned to the oxidation of the pyrrole and thiophene rings, respectively.
Figure 4. (a) Cyclic voltammograms at 0.1 V s-1 of different o-carborane derivatives at 5 mM in CH2Cl2
+ 0.2 M Bu4NPF6 (for 12b, 13a and 13b) or CH3CN + 0.1 M Bu4NPF6 (for 12a). (b) Corresponding time
semi-derivative voltammograms for the cathodic region.
Table 1. Cyclic voltammetry data of the o-carboranyl derivatives at 5 mM in CH2Cl2 or CH3CN medium.
Potential scan rate: 0.1 V s-1. All potentials are reported vs. 10-2M Ag+/Ag.
Monomer
Polymer
Eox/ Va
Ered / Vb
Eox / Vc
Ered / Vd
g
g
13be
13ae
12af
12be
1.58; 1.96
1.56; 1.93
0.74; 1.30
1.32; 1.68
—
—
-1.43
g
g
-1.40
-1.42
-1.39
—
—
0.40; 0.75
0.95; 1.30
-1.50
-1.36
aIrreversible anodic peak potentials corresponding to the monomer oxidation.
Formal potential
b
corresponding to the quasi-reversible reduction step. c Formal potential corresponding to the reversible p-
doping/undoping of the electrogenerated polymer (average of anodic and cathodic peak potentials).d

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Formal potential corresponding to the reduction of the carborane (average of anodic and cathodic peak
e
f
g
potentials). in CH2Cl2 + 0.2 M Bu4NPF6. in CH3CN + 0.1 M Bu4NPF6. No conducting polymer film
was electrogenerated; electrode passivation was observed.
We now turn towards the electrochemical behavior of these systems when the potential is
scanned towards negative potentials. As shown in Figure 4, their reduction gives rise to two quasi-
reversible closely spaced systems around -1.40 V which can be clearly evidenced on the backward
scans of the corresponding time semi-derivative curves (Figure 4b). These are ascribed to the
formation of radical anion and dianion species of carborane,[57, 58] respectively. Studies with
analogous diaryl-substituted ortho-carborane systems have suggested that the electronic charge
was located within the carborane cage.[58] Moreover, the fact that this cathodic process is
observed at approximately the same potential for the four molecules is not surprising in so far as
both the geometry of these molecules and the nature of the substituents in closeproximity to the
carborane are similar.Among these carboranes, only the electrochemical oxidation of 12a and 12b
led to the formation of a conducting polymer deposit on the electrode surface. Such films could be
electrogenerated either potentiodynamically or potentiostatically with no significant effect of the
electropolymerization method on their respective electrochemical responses. Representative
cyclic voltammograms corresponding to the potentiodynamical electropolymerization of these
compounds are shown in Figure 5. As can be seen, the progressive increase of both anodic and
cathodic currents with the number of scans indicates the regular growth of a conducting deposit
onto the electrode surface. In contrast to the facile electropolymerization of 12a and 12b, 13a and
13b did not yield a conducting polymer deposit under various tested experimental conditions
(monomer concentration, oxidation potential, solvent and electrochemical method). Instead, a
poorly electroactive film was electrogenerated yielding the progressive passivation of the
electrode surface. Such a result can be explained by the inhibition of the electropolymerization
process owing to the substitution of the α-position of the thiophene rings by vinyl units.
Figure 5. Successive cyclic voltammograms of 12a (a) and 12b (b) at 5 mM in CH3CN + 0.1 M Bu4NPF6
(a) or CH2Cl2 + 0.2 M Bu4NPF6 (b). Potential scan rate: 0.1 V s-1.
Following their electrosynthesis, the polymer films, namely poly(12a) and poly(12b), were
examined in a monomer-free electrolytic medium (Figure 6). In the anodic range, their electrochemical
response was characterized by two broad reversible redox processes corresponding to the p-

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doping/undoping of the expected conjugated segments (Table 1), namely 2,2'-bi-1H-pyrrole, 1,1'-
dimethyl-1,6-di-2-thienyl and quaterthienyl units for poly(12a) and poly(12b), respectively. As expected
for surface-immobilized electroactive species,[59] the anodic peak current intensities corresponding to
1
2
these two systems (Ipa and Ipa ) were found to vary linearly with the potential scan rate v. Moreover, the
doping level [60] of poly(12a) and poly(12b) were estimated respectively at 0.10-0.15 and 0.15-0.20
δ
positive charge per monomer unit. Comparedwith unsubstituted poly(2-(2-thienyl)-1H-pyrrole)[53] and
polythiophene,[38] the oxidation level of both polymers is lower, which demonstrates the prominent role
of the incorporated carborane on the ion transport in these films.In the cathodic potential range, poly(12a)
and poly(12b) showed a reversible system at -1.50 and -1.36 V, respectively, corresponding to the
reduction of the carborane incorporated between two conjugated segments of the polymer (Figure 6). As
for the polymer redox process, the peak currents ascribed to this system varied linearly with v.
Nevertheless, upon increasing the film thickness from the anodic charge consumed during the
electropolymerization reaction, the redox signature of the carborane became less and less visible. Such
intriguing effects of the film thickness on the electroactivity of the surface-confined reducible couple have
already been reported by us with polythiophenes containing in-chain cobaltabisdicarbollide centers.[21] It
must be kept in mind that the cathodic process occurs within a potential range where the polymer is in its
neutral reduced state, i.e. in its electronically insulating state. Consequently, the reduction of the
carborane in the insulating polymer matrix is expected to occur by electron hopping until the electrode
surface and therefore involves cation migration through the film in order to ensure the electroneutrality of
the material. For thick films, it is very likely that a large amount of carborane within the film is not
electrochemically addressable because electrolyte cations can not migrate until them.
Figure 6. Electrochemical responses of the electrogenerated poly(12a) (a) and poly(12b) (b) films at 0.1
V s-1 with the corresponding Ipa - v plots for the two anodic processes (insets). Electrolyte: CH3CN + 0.1
M Bu4NPF6 (a) or CH2Cl2 + 0.2 M Bu4NPF6 (b). The consumed electropolymerization charges are 20 (a)
and 64 mC cm-2 (b).
Computer Modelling
To gain insights into their properties, the monomers and the dimers of carboranyl-bisthiophenes
12a,b and 13a,b were modeled computationally. Figure 7 shows the shape and the energies of their
frontier molecular orbitals. The HOMO and LUMO of 2,2’-carboranyldithiophene 9 are also given for
comparison purposes. In general, the orbitals look similar for all compounds: both HOMO and LUMO
show almost no contribution from the carborane cage. The HOMO has C-C π-bonding character and

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mostly non-bonding character for the remaining of the atoms in the backbone, while the LUMO shows
bonding character between the carborane cage and the adjacent thiophene units. Substitution with N-
methylpyrrole, thienyl, and vinyl groups at the 5- or 5,5’-positions destabilizes the HOMO by 0.5 to 1.4
eV when compared to 9. The effect is greatest for the N-methylpyrrole, followed by thienyl and then
vinyl. This trend is consistent with the experimentally observed trend in the anodic potentials of these
systems. The effect on LUMO is very small, only 0.1-0.4 eV. Thus, the resulting band gaps follow the
trend of HOMO destabilization. As can be seen from Table 2, all 5,5’-substituted carboranyl-
bisthiophenes exhibit smaller band gaps than 9 with the effect being most pronounced for 12a: about 1.4
eV.
Figure 7. Frontier orbitals of carboranyl-bisthiophenes 9, 12a,b, and 13a,b. Energies in a.u.
Table 2. Band gap energies, syn-anti difference energies, and energies of dimerization and
intracyclization for carboranyl-bisthiophenes 9, 12a-b and 13a-b.
Compound
Band Gap (eV)
∆Esyn-anti (kcal/mol)
∆Edimer(kcal/mol)
∆Eintracycl(kcal/mol)
5.11
3.68
3.78
4.11
4.30
0.22
0.27
0.39
0.39
0.25
8.36
7.81
8.30
1.53
5.01
7.34
8.05
7.93
7.61
7.50
9
12a
12b
13a
13b

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Table 2 also shows the energy difference between the anti and syn comformers. In agreement
with the X-ray Analysis (Figure 3), the syn conformer is more stable but the energy difference is very
small: less than 0.5 kcal/mol. This finding is also consistent with previously performed calculations on
compound 9 and its meta, and para analogues.[24]
The last two columns of Table 2 list the energies required for formation of the dimer, ∆Edimer, and
the energy required for intracyclization, ∆Eintracycl. The energies required for dimerization of 12a and 12b
are similar to that of 9 and are around 8 kcal/mol. The vinyl substitution has more pronounced effect on
the dimerization energy: it appears that each vinyl substitution reduces the required energy and makes the
dimer easier to form. In view of these facts, the reason why 13a and 13b fail to polymerize is unlikely to
be in the difficulty of the formation of the dimer. Another possible mechanism for the polymerization is
the previously proposed intramolecular cyclization.[24] The energies required for the intramolecular
cyclization are given in the last column of Table 2. All energies are between 7 and 8 kcal/mol without
significant differences when pyrrole, thienyl, vinyl, or no substitution is used. All structures are planar,
which is in agreement with previously reported data for 9.[24]
Conclusions
A new series of 2,2’-(ortho-carboranyl)-bisthiophenes bearing N-methylpyrrole, vinyl,
trimethylsilylethyne, or thienyl groups at the 5,5’-positions were synthesized in good yields via
-
bromination of 2,2’-carboranyldithiophene 9 followed by palladium(0)-catalyzed cross-coupling reactions
of the Sonogashira and Stille types. Seven X-ray structures were obtained to confirm the regioselectivity
of the reactions, and the solid state characteristics of the new compounds. The electrochemical
characterization of these systems showed that their anodic potential increased in the order 12a < 12b <
13a ≈ 13b, which is consistent with the trend observed from DFT calculations. In addition, poly(12a) and
poly(12b) could be electro-generated from new compounds 12a and 12b under the suitable conditions.
The lower doping levels determined for both electro-generated polymers compared with the parent
conducting polymers, suggest the prominent effect of the incorporated carborane clusters. Furthermore,
DFT calculations also suggest that low energy is required for the intramolecular cyclization of compounds
12a and 12b, which may provide more organized structures for the generated polymers. Therefore,
poly(12a) and poly(12b) are promising candidates for the development of carborane-functionalized
conducting materials with enhanced chemical and thermal stabilities.
Acknowledgements
This research was supported by the US National Science Foundation, grant number CHE 1362641. BF
acknowledges CNRS and the Université de Rennes for support. The authors thank the Louisiana Optical
Network Initiative (www.loni.org) for the use of the computer facilities.
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Synthesis and Electropolymerization of a series of 2,2’-(ortho-Carboranyl)bisthiophenes
Ning Zhao, Bruno Fabre, Petia Bobadova-Parvanova, Frank R. Fronczek, M. Graça H. Vicente
•
•
•
A series of 2,2’-(ortho-carboranyl)bisthiophenes has been synthesized.
Seven X-ray structures of 2,2’-(ortho-carboranyl)bisthiophenes were obtained.
Electrochemical characterization and DFT calculations were conducted to study the properties of
the new compounds.
•
Di(thienyl-N-methylpyrrole)-ortho-carborane and di(bisthienyl)-ortho-carborane were
successfully electropolymerized to generate conducting materials.