Electronic communication and negative binding cooperativity in diborylated bithiophenes

Article abstract of DOI:10.1021/ja064396b

The bifunctional conjugated organoboranes Ar₂B-bt-BAr ₂, which contain 2,2′-bithiophene (bt) linkers and different aryl substituents on boron (3: Ar = p-^tBuC₆H₄; 4: Ar = C₆F₅; 5: Ar = C₆F₅, Fc; Fc = ferrocenyl), have been synthesized. The electronic communication between the boron centers and cooperativity effects in the binding of pyridine have been investigated by a comprehensive study using X-ray crystallography, DFT calculations, cyclic voltammetry, ¹H and ¹⁹F NMR, and UV visible absorption and emission spectroscopy. A comparison of the single-crystal X-ray structures of 4 and 4Py₂ revealed a strongly diminished bond alternation in the thiophene rings for 4, indicative of a high degree of electronic delocalization. DFT calculations are in good agreement with the structural features determined from the X-ray analysis and, consistent with the experimental absorption and emission data, predict a smaller HOMO-LUMO gap for green luminescent 4 in comparison to blue luminescent 3. The complexation of pyridine to the two boron centers was further investigated by ₁H and ₁₉F NMR for 4 and by ₁H NMR and UV-visible absorption spectroscopy for 3. We found that binding of the first pyridine molecule to one of the boryl groups significantly lowers the Lewis acidity of the other boryl group. For 3, the interaction parameter a, which provides a measure of communication between the boron sites, was determined to be a = 0.23 by UV-visible titration and 0.21 by ₁H NMR spectroscopy. Further enhanced electronic communication was observed for the more highly Lewis acidic fluorinated derivative 4, for which a = 0.025 according to ₁₉F and ₁H NMR spectroscopy.

Full text of DOI:10.1021/ja064396b

Published on Web 12/07/2006
Electronic Communication and Negative Binding
Cooperativity in Diborylated Bithiophenes
Anand Sundararaman,† Krishnan Venkatasubbaiah,† Maria Victor,†
Lev N. Zakharov,‡,§ Arnold L. Rheingold,‡ and Frieder Ja¨kle*,†
Contribution from the Department of Chemistry, Rutgers UniVersity-Newark, 73 Warren Street,
Newark, New Jersey 07102, and Department of Chemistry and Biochemistry, UniVersity of
California, San Diego, La Jolla, California 92093
Received June 29, 2006; E-mail: fjaekle@rutgers.edu
Abstract: The bifunctional conjugated organoboranes Ar2BsbtsBAr2, which contain 2,2′-bithiophene (bt)
linkers and different aryl substituents on boron (3: Ar ) p-tBuC6H4; 4: Ar ) C6F5; 5: Ar ) C6F5, Fc; Fc )
ferrocenyl), have been synthesized. The electronic communication between the boron centers and
cooperativity effects in the binding of pyridine have been investigated by a comprehensive study using
1
X-ray crystallography, DFT calculations, cyclic voltammetry, H and 19F NMR, and UV visible absorption
and emission spectroscopy. A comparison of the single-crystal X-ray structures of 4 and 4Py2 revealed a
strongly diminished bond alternation in the thiophene rings for 4, indicative of a high degree of electronic
delocalization. DFT calculations are in good agreement with the structural features determined from the
X-ray analysis and, consistent with the experimental absorption and emission data, predict a smaller HOMO-
LUMO gap for green luminescent 4 in comparison to blue luminescent 3. The complexation of pyridine to
1
1
the two boron centers was further investigated by H and 19F NMR for 4 and by H NMR and UV-visible
absorption spectroscopy for 3. We found that binding of the first pyridine molecule to one of the boryl
groups significantly lowers the Lewis acidity of the other boryl group. For 3, the interaction parameter a,
which provides a measure of communication between the boron sites, was determined to be a ) 0.23 by
1
UV-visible titration and 0.21 by H NMR spectroscopy. Further enhanced electronic communication was
observed for the more highly Lewis acidic fluorinated derivative 4, for which a ) 0.025 according to 19F
1
and H NMR spectroscopy.
Introduction
following the observation that overlap between the empty
p-orbital on boron and the extended π-system frequently leads
Conjugated organic oligomers and polymers have been
established as an important class of new materials with many
exciting applications, for example, in electroluminescent devices,
organic field effect transistors, and smart electrochromic
windows.1 Much research has also been devoted to the design
and development of related hybrid systems, in which the
properties of organic π-systems are combined with the unique
features of semimetals or metal complexes leading to a wide
range of new materials with intriguing properties.2 In this regard,
the incorporation of electron-deficient boron centers into organic
structures with extended π-conjugation has attracted attention
to unusual optical and electronic properties.3,4 One of the most
appealing aspects is that the presence of highly Lewis acidic
centers provides an opportunity to design new sensors for Lewis
basic substrates.5,6 For instance, Yamaguchi et al. reported on
a series of tri-9-anthrylborane derivatives and their effective use
as fluoride sensor materials;5c many other borane derivatives
have since been investigated. Chujo and co-workers described
the use of a phenylene-vinylene-borane type p-π conjugated
organoboron polymer as a fluorescent fluoride sensor and
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† Rutgers University Newark.
‡ University of California, San Diego.
§ Current address: University of Oregon, Department of Chemistry,
Eugene, OR, 97403-1253.
(1) (a) Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds. Handbook
of Conducting Polymers, 2nd ed.; Marcel Dekker, Inc.: New York, 1997.
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Weinheim, 1999. (c) Hadziioannou, G., van Hutten, P. F., Eds. Semicon-
ducting Polymers; Wiley-VCH: Weinheim, 2000. (d) Nalwa, H. S., Ed.
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9
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J. AM. CHEM. SOC. 2006, 128, 16554-16565
10.1021/ja064396b CCC: $33.50 © 2006 American Chemical Society

Electronic Communication in Diborylated Bithiophene
A R T I C L E S
reported a considerable signal amplification effect.6a We recently
developed a new family of luminescent organoboron polymers,
in which Lewis acidic boron sites are embedded into the main
chain of polythiophenes.7 We observed highly efficient fluo-
rescence quenching upon coordination of very small amounts
of pyridine with an amplification factor of ca. 12 relative to a
molecular model. One of the key questions with regard to the
sensing behavior of such polymers is whether individual
recognition sites along the polymer backbone act independently
or in a cooperative fashion. The latter may provide important
information on the origin of the observed sensor signal
amplification.8
In the present study, bifunctional organoboranes with a
bithiophene linker have been chosen as a model system that
represents a fragment of a conjugated organoboron polymer and
allows us to carefully study the cooperativity between adjacent
Lewis acid sites. Previous electrochemical studies by Kaim and
co-workers on phenylene-bridged diboranes of the type Mes2Bs
(C6H4)nsBMes2 (n ) 1,2; Mes ) 2,4,6-trimethylphenyl)
indicated a considerable degree of electronic communication
between the two boron centers as reflected in two distinct
reduction waves that are separated by 0.69 and 0.25 V,
respectively, in the presence of a phenylene or 4,4′-biphenylene
bridge.9 A significant contribution of the quinoid resonance
structure in the doubly reduced species was postulated. More
recently, Shirota and co-workers have shown that the thiophene
analogue 5,5′-bis(dimesitylboryl)-2,2′-bithiophene also displays
two sequential cathodic waves with a considerable redox
splitting.10 However, interactions between the boron centers in
these and other diboron species5b,11 have in the past been studied
almost exclusively by cyclic voltammetry. Sequential fluoride
binding to arylborane oligomers has been investigated by
Yamaguchi et al. for a system containing four boron centers in
two different environments.5c Noteworthy is also some very
recent work by Li and Fang and co-workers and by Baumgartner
and co-workers, who reported on the use of diborylated aromatic
species as luminescent fluoride sensors while these studies have
been under way.5q,r,12 Li and Fang showed that the diborylated
species Mes2BsThsHCdCHsPhsHCdCHsThsBMes2 acts
as an efficient sensor based on two-photon excited fluorescence.5q
They found that fluoride binds sequentially in THF with binding
constants of log K1 ) 5.51 and log K2 ) 5.08, which indicate
moderate cooperativity (K2/K1 ) 0.37). Baumgartner and co-
workers reported on a highly stable dithienophosphole oxide
system with two Bpin (pin ) pinacolato) substituents that can
be effectively used as a ratiometric probe.5r The diboryldithieno-
phosphole oxide sequentially binds fluoride with log K1 ) 4.02
and log K2 ) 3.79, corresponding to the even smaller cooper-
ativity of K2/K1 ) 0.59.
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We describe here the preparation, structural, and photophysi-
cal characterization of highly Lewis acidic bifunctional conju-
gated organoboranes Ar2BsbtsBAr2 (3: Ar ) p-tBuC6H4; 4:
Ar ) C6F5; 5: Ar ) C6F5, Fc; bt ) 2,2′-bithiophene, Fc )
ferrocenyl). We further discuss the electronic communication
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A R T I C L E S
Sundararaman, et al.
Scheme 1. Synthesis of the Diborylated Bithiophene Derivatives 3, 4, and 5
crystallography, DFT calculations, cyclic voltammetry, 1H and
19F NMR, and UV visible absorption and emission spectroscopy.
compounds 3, 4, and 5 show resonances in the expected region
for triarylboranes at 59, 56, and 51 ppm, respectively. A
common feature in the 1H NMR spectra is the presence of two
doublets for the bithiophene linker in the region from 8.0 to
7.0 ppm. Consistent with the π-acceptor properties of tricoor-
dinate boron, these signals are significantly downfield shifted
with respect to the silylated precursor (1a; δ ) 7.23, 7.13). The
largest downfield shift is observed for 4 (δ ) 7.93, 7.73),
indicative of the very high Lewis acidity of the boryl groups.
For compound 5 additional peaks corresponding to the expected
pattern of a monoborylated ferrocene with two pseudotriplets
and a singlet are observed in the range 5.0 to 4.2 ppm. The
attachment of the pentafluorophenyl groups to 4 and 5 is
reflected in the 19F NMR spectra, both of which show a set of
three resonances. It has been previously reported that the
chemical shift difference ∆δm,p between the ortho-F and para-F
resonances in pentafluorophenylboranes provides an estimate
of the Lewis acidity of the boron centers.15 A comparison of
∆δm,p for 4 and 5 further demonstrates that the attachment of
two C6F5 groups to boron in 4 leads to a significantly higher
Lewis acidity (∆δm,p ) 11.8) relative to the mixed arylborane
5 that contains ferrocenyl and C6F5 groups (∆δm,p ) 7.9).
Pyridine Binding: Complexation of the boryl groups of 3,
4, and 5 upon pyridine addition is evident from a strong upfield
shift of the 11B NMR signal to +2.7 ppm (3: 59 ppm), -2.6
ppm (4: 56 ppm), and -3.1 ppm (5: 51 ppm), respectively,
which is indicative of tetra-coordination at boron. The boryl
groups are thus readily accessible for nucleophile binding, and
the respective Lewis acid-base complexes were isolated in high
yields. Complex 3Py2 precipitates as a white powder from
toluene, complex 4Py2 gives colorless crystals from CH2Cl2/
hexanes mixtures, and the ferrocene complex 5Py2 was obtained
as an orange microcrystalline material upon cooling of a hot
toluene solution. The isolated Lewis acid-base complexes were
also completely characterized by NMR spectroscopy. Notewor-
Results and Discussion
Synthesis of Diborylated Bithiophenes: Two different routes
to the diborylated bithiophene derivatives 3, 4, and 5 were
applied (Scheme 1). The dibromoboryl-functionalized bithiophene
2 was obtained readily via silicon-boron exchange from 2,2′-
bis(trimethylsilyl)-5,5′-bithiophene (1a) upon reaction with
BBr3. Species 2 served as a versatile precursor to the triarylbo-
rane derivatives 4 and 5, which were prepared through metath-
esis reactions with organotin and organocopper reagents (Scheme
1). Compound 4 was obtained in 71% yield by treatment of 2
with 4 equiv of pentafluorophenyl copper.13 Reaction of 2 with
FcSnMe3 very selectively led to transfer of one ferrocenyl
moiety to each boron center. Subsequent addition of 2 equiv of
pentafluorophenyl copper gave compound 5 as a dark red solid
in 77% yield. The attachment of two phenyl groups through
analogous tin-boron exchange reactions is complicated by a
methyl group transfer that is competing with the cleavage of
the Sn-aryl bonds in PhSnMe3 derivatives.14 However, the
phenyl-substituted derivative 3 was readily prepared in good
yield (74%) through reaction of 2,2′-bis(trimethylstannyl)-5,5′-
bithiophene (1b) with (tBuC6H4)2BBr. The bifunctional tri-
arylboranes 3-5 were fully characterized by multinuclear NMR
spectroscopy and high-resolution mass spectrometry.
High-resolution MALDI-TOF-TOF spectra that were ac-
quired in negative (3) or positive ion mode (5) using benzo[a]-
pyrene as the matrix showed the molecular ion peaks with the
expected isotope patterns, thus confirming the chemical com-
position of this class of compounds. The 11B NMR spectra of
(13) For the use of arylcopper species as aryl transfer reagents, see: Sundarara-
man, A.; Ja¨kle, F. J. Organomet. Chem. 2003, 681, 134-142.
(14) (a) Eisch, J. J.; Kotowicz, B. W. Eur. J. Inorg. Chem. 1998, 761-769. (b)
Cheng, X.; Slebodnick, C.; Deck, P. A.; Billodeaux, D. R.; Fronczek, F.
R. Inorg. Chem. 2000, 39, 4921-4926. (c) Gamboa, J. A.; Sundararaman,
A.; Kakalis, L.; Lough, A. J.; Ja¨kle, F. Organometallics 2002, 21, 4169-
4181. (d) Boshra, R.; Sundararaman, A.; Zakharov, L. N.; Incarvito, C.
D.; Rheingold, A. L.; Ja¨kle, F. Chem.sEur. J. 2005, 11, 2810-2824.
(15) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1-76.
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Electronic Communication in Diborylated Bithiophene
A R T I C L E S
Table 1. Selected Bond Lengths [Å], Interatomic Distances [Å],
and Angles [deg] for 4 and 4Py2
4
4Py2
B(1)-C(4)
B(1)-C(11)
B(1)-C(5)
1.507(3) C(4)-B(1)
1.576(3) B(1)-C(11)
1.578(3) B(1)-C(5)
B(1)-N(1)
1.703(2) S(1)-C(1)
1.732(2) S(1)-C(4)
1.372(3) C(1)-C(2)
1.390(3) C(2)-C(3)
1.383(3) C(3)-C(4)
1.445(4) C(1)-C(1A)
122.5(2) C(4)-B(1)-C(11)
118.6(2) C(4)-B(1)-C(5)
118.9(2) C(11)-B(1)-C(5)
128.3(3) C(2)-C(1)-C(1A)
1.617(3)
1.648(3)
1.645(3)
1.624(3)
1.730(2)
1.740(2)
1.363(3)
1.417(3)
1.359(3)
1.448(3)
105.4(1)
110.0(2)
116.5(2)
129.5(2)
61.5(2)
S(1)-C(1)
S(1)-C(4)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(1)-C(1A)
C(4)-B(1)-C(11)
C(4)-B(1)-C(5)
C(11)-B(1)-C(5)
C(2)-C(1)-C(1A)
C(11)-B(1)-C(4)-S(1) -171.5(2) C(11)-B(1)-C(4)-S(1)
C(5)-B(1)-C(4)-S(1)
C(1-4)S(1) // C(5-10)
C(1-4)S(1) // C(11-16) 44.0(1) C(1-4)S(1) // C(11-16)
C(5-10) // C(11-16) 89.0(1) C(5-10) // C(11-16)
9.4(3)
C(5)-B(1)-C(4)-S(1)
-172.1(1)
61.07(6)
84.58(5)
56.35(6)
69.4(1) C(1-4)S(1) // C(5-10)
C(1-4)S(1) // N1C(17-21) 52.96(6)
C6F5 groups are B1-C11 ) 1.576(3) Å and B1-C5 ) 1.578(3)
Å, respectively. In comparison, the B1-C4 distance to the more
electron-rich bithiophene moiety of 1.507(3) Å is significantly
shorter, indicating a stronger π-bonding of the trivalent boron
centers to the bithiophene rings than to the C6F5 groups. Upon
binding of pyridine, all of the B-C bonds become significantly
longer as the hybridization on boron changes to sp3. While the
B-C bonds to the bithiophene moiety (1.617(3) Å) in 4Py2
remain shorter than those to the C6F5 groups (1.648(3) and
1.645(3) Å), the difference is smaller than that for the free acid
4. A strong effect of the Lewis acidic boron centers on the
carbon-carbon distances within the thiophene rings is note-
worthy. In contrast to most other bithiophene derivatives, for
the free acid 4 the C-C distances within the thiophene rings
are quite similar to one another (all within 1.372(3) to 1.390(3)
Å). However, a distinct bond alternation is observed for the
pyridine complex 4Py2 with two short distances (C(1)-C(2) )
1.363(3) Å and C(3)-C(4) ) 1.359(3) Å) and one longer
distance (C(2)-C(3) ) 1.417(3) Å), which is typical of
bithiophenes. The carbon-carbon distance between the two
central thiophene rings of 1.445(4) Å in 4 is similar to that of
1.448(3) Å in 4Py2.17
Figure 1. Molecular structures of 4 and 4Py2 with thermal ellipsoids at
the 50% probability level; hydrogen atoms, cocrystallized toluene (4), and
cocrystallized mesitylene (4py2) are omitted for clarity.
thy is that the chemical shift differences ∆δm,p in the 19F NMR
spectra of 4Py2 (∆δm,p ) 6.4 ppm) and 5Py2 (∆δm,p ) 5.5 ppm)
are considerably smaller than those for the free acids.
Treatment of the complexes 4Py2 and 5Py2 with the stronger
Lewis acid B(C6F5)3 in CH2Cl2 solution led to decomplexation
and reformation of 4 and 5, respectively, according to 19F NMR
spectroscopy. The reversibility of the pyridine binding also
manifests itself in the FAB mass spectral data. FAB-MS analysis
of 4Py2 using 2-nitrophenyl octyl ether (NPOE) as the matrix
revealed the presence of molecular ion peaks corresponding to
the bis-adduct 4Py2 (20%), the monoadduct 4Py (25%), and
the free acid 4 (42%). Interestingly, molecular ion peaks
corresponding to the dimers 42Py4 (0.15%) and 42Py3 (0.11%)
were also observed, albeit in very small amounts. For 3Py2 and
5Py2 only peaks corresponding to loss of pyridine could be
detected, which may indicate weaker binding of pyridine in these
complexes.
Structural Studies: The effect of pyridine binding on the
structural parameters of diborylated bithiophene has been studied
for compound 4. Yellow needlelike single crystals of 4 were
grown by recrystallization from toluene at -38 °C, and colorless
rodlike crystals of the pyridine adduct 4Py2 were obtained by
recrystallization from hot mesitylene. The crystals of 4 contain
three disordered cocrystallized toluenes, and those of 4Py2, two
mesitylenes for each bithiophene molecule. The structures of 4
and 4Py2 are displayed in Figure 1, and selected geometric
parameters are summarized in Table 1. For both 4 and 4Py2,
the molecule lies on a center of symmetry between the thiophene
rings of the 2,2′-bithiophene moiety, which consequently adopt
a coplanar configuration (Figure 1). The two C6F5 substituents
are arranged in a propeller-like fashion in the free acid (4), a
feature that is commonly observed especially for triarylboranes
with bulkier groups.16 The boron-carbon bond lengths to the
It is also interesting to note that the packing diagram of 4
reveals π-stacking interactions between one of the cocrystallized
toluene solvent molecules and the bithiophene moiety, leading
to extended stacks in the direction of the crystallographic a-axis
(Figure 2). The distance between the average planes of the
aromatic rings is ca. 3.47 Å, indicative of strong arene-arene
interactions.18,19 Another cocrystallized toluene solvent molecule
also forms π-stacking interactions, but with the aromatic rings
(17) A Cambridge Structure Database search gave a mean C-C distance of
1.449 Å for other bithiophene derivatives.
(18) Alternating stacks of electron-deficient fluorinated arenes with arenes are
more common; for example, see: (a) Williams, J. H. Acc. Chem. Res. 1993,
26, 593-598. (b) Pratt Brock, C.; Naae, D. G.; Goodhand, N.; Hamor, T.
A. Acta Crystallogr. 1978, B34, 3691-3696. (c) Coates, G. W.; Dunn, A.
R.; Henling, L. M.; Dougherty, D. A.; Grubbs, R. H. Angew. Chem., Int.
Ed. Engl. 1997, 36, 248-251. (d) Bartholomew, G. P.; Bu, X.; Bazan, G.
C. Chem. Mater. 2000, 12, 2311-2318. (e) Collings, J. C.; Batsanov, A.
S.; Howard, J. A. K.; Marder, T. B. Acta Crystallogr. 2001, C57, 870-
872. (f) Collings, J. C.; Roscoe, K. P.; Thomas, R. L.; Batsanov, A. S.;
Stimson, L. M.; Howard, J. A. K.; Marder, T. B. New J. Chem. 2001, 25,
1410-1417. (g) Collings, J. C.; Roscoe, K. P.; Robins, E. G.; Batsanov,
A. S.; Stimson, L. M.; Howard, J. A. K.; Clark, S. J.; Marder, T. B. New
J. Chem. 2002, 26, 1740-1746.
(16) (a) Blount, J. F.; Finocchiaro, P.; Gust, D.; Mislow, K. J. Am. Chem. Soc.
1973, 95, 7019-7029. (b) Toyota, S.; Asakura, M.; Oki, M.; Toda, F. Bull.
Chem. Soc. Jpn. 2000, 73, 2357-2362. (c) See ref 3p.
9
J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006 16557

A R T I C L E S
Sundararaman, et al.
Table 2. UV-visible Absorption and Emission Data
diborylated
bithiophene
3
4
5
λ
abs,max [nm],
CH2Cl2
306, 396, 411a
287, 413, 437a
276, 387, 406a, 524b
log ꢀ [M-1 L-1], 4.57, 4.68, 4.63 4.09, 4.50, 4.34 4.26, 4.38, 4.36, 3.97
CH2Cl2
λ
em,max [nm],
436, 461
472, 485a
0.79
476, 495a
525, 547a
0.03
c
c
c
d
CH2Cl2
λ
em,max [nm],
solidd
Φe, CH2Cl2
a Shoulder. b d-d transition. c Emission quenched due to iron center of
ferrocene. d Excited at λmax e Measured relative to anthracene. Concentra-
.
Figure 2. Space filling diagram of 4 showing the formation of stacks along
the crystallographic a-axis through π-interactions between the bithiophene
moiety and one of the toluene molecules. The other toluene molecules are
omitted for clarity.
tions were 5.57 × 10-7 M for 3, 5.85 × 10-5 M for 4, and 9.44 × 10-5
for 5.
M
Figure 3. Absorption spectra of 3, 4, and 5 in CH2Cl2 solution.
of C6F5 groups (see Supporting Information). Both toluene
solvent molecules are disordered over two positions, and only
the one that forms stacks with the bithiophene moiety is shown
in Figure 2.
Figure 4. Emission spectra of 3 and 4 in CH2Cl2 solution and in the solid
Photophysical Properties and Quantum Chemical Calcu-
lations: Compounds 3, 4, and 5 display interesting photophysical
properties in solution and the solid state. While 3 is almost
colorless, compound 4 is yellow and the ferrocene derivative 5
is dark red colored (Figure 3). The spectra of the diborylated
bithiophenes in dichloromethane display strong absorption bands
in the UV, which reach into the visible region of the spectrum.
These bands are observed for 3 at λmax ) 396 nm (ꢀ ) 48 290),
for 4 at λmax ) 413 nm (ꢀ ) 31 400), and for 5 at λmax ) 387
nm (ꢀ ) 23 890) (Table 2). Interestingly, the absorption maxima
seem to experience a bathochromic shift with enhanced Lewis
acidity, which based on the electronic nature of the substituents
is anticipated to increase in the order 5 < 3 < 4. A red-shifted
shoulder is another common feature in the spectra for these
compounds. In addition, compound 5 shows a relatively strong
band at 524 nm, which is typical of d-d transitions with
considerable charge-transfer character in ferrocenylboranes.14c,20
For 3 and 4, a trend similar to that for the absorption maxima
is also observed for the emission spectra (Figure 4). A strong
blue emission with maxima at λem,max ) 436 and 461 nm is
evident upon excitation of a CH2Cl2 solution of 3 (quantum
efficiency Φ ) 0.79). These data are comparable to those
reported by Shirota for the mesityl derivative Mes2B-bt-BMes2
state.
with fluorescence maxima at λmax ) 446, 442 nm.10a,b In
contrast, the emission maximum of compound 4 is shifted by
ca. 40 nm to λem,max ) 476 nm and the quantum efficiency is
much lower with Φ ) 0.03.
In comparison to the solution maxima, the emissions of 3
and 4 experience a bathochromic shift in the solid state of ca.
35 and 50 nm, respectively (Table 2, Figure 4). Similarly, the
solid-state excitation edge of 3 and 4 is red-shifted by 60 to 70
nm in comparison to the solution absorption edge. This
phenomenon is consistent with the observation that the central
bithiophene unit adopts a coplanar structure in the solid state,
while in solution other ring-twisted conformations also con-
tribute. However, intermolecular π-π interactions, as deduced
from the single-crystal X-ray structure of 4, likely also impact
the photophysical properties in the solid state.21
The considerable bathochromic shift for the perfluorinated
analogue, both in the absorption and fluorescence spectra, can
be attributed to the strongly electron-withdrawing nature of the
C6F5 substituents, which leads to a higher Lewis acidity of the
boron centers and thus to stronger p-π interactions with the
bithiophene moiety in 4. The latter is expected to lead to
stabilization of the quinoid resonance structure and hence a
lowering of the HOMO-LUMO gap. DFT calculation have
(19) For an example of the use of mesitylene to fill structural voids and thus
aid in the crystallization of compound Mes2B-(anthracenediyl)-BMes2,
see: Yuan, Z.; Taylor, N. J.; Ramachandran, R.; Marder, T. B. Appl.
Organomet. Chem. 1996, 10, 305-316.
(20) Carpenter, B. E.; Piers, W. E.; Parvez, M.; Yap, G. P. A.; Rettig, S. J.
Can. J. Chem. 2001, 79, 857-867.
(21) For a detailed account on stacking effects in a related diborylated
dithienothiophene derivative, see ref 10f.
9
16558 J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006

Electronic Communication in Diborylated Bithiophene
A R T I C L E S
Table 3. Comparison of Geometric Parameters from X-ray
Analysis and DFT Calculations of 3, 3Py, 3Py2, 4, 4Py, and 4Py2
(1.5501 Å). Another important observation is that the bond
alternation in the thiophene rings is less pronounced for 4 than
in the case of 3, indicative of enhanced electronic delocalization.
Figure 5 shows the computed frontier orbitals for 3 and 4.
Pronounced overlap of the bithiophene π-system with the empty
p-orbital on boron is evident for the LUMOs, while the
contribution to the HOMO levels is small, though somewhat
more apparent in 4 than in 3. Both the HOMO and LUMO
orbitals for the fluorinated derivative 4 are stabilized relative
to those of 3 (see Table S1 in the Supporting Information). The
effect is slightly stronger for the LUMO, which effectively leads
to a smaller HOMO-LUMO gap for 4 (3.186 eV) relative to 3
(3.348 eV).
3Py (DFT)
3 (DFT)
uncomplexed
complexed
3Py2 (DFT)
B-C(Ph) (Å)
1.5684
1.5674
1.5501
1.4472
1.3865
1.4101
1.3898
1.7447
1.7596
1.5706
1.5690
1.5447
1.6347
1.6308
1.6254
1.6355
1.6309
1.6219
1.4490
1.3787
1.4218
1.3783
1.7558
1.7658
1.6912
B-C4(Th) (Å)
C1-C1* (Å)
C1-C2 (Å)
C2-C3 (Å)
C3-C4 (Å)
C1-S (Å)
1.4468
1.3872
1.4098
1.3901
1.7456
1.7636
1.3799
1.4201
1.3800
1.7544
1.7612
1.6875
We next performed calculations for the monoadducts and bis-
adducts of 3 and 4 with pyridine (Table 3). The results for 4Py2
compare favorably with the crystal structure data. The most
significant deviation from the X-ray data is that the B-N
distance to the pyridine rings is slightly overestimated (DFT:
1.6615 Å; X-ray: 1.624(3) Å). Similar limitations have been
reported previously for DFT calculations of other related Lewis
acid-base complexes.23 Nonetheless, all other parameters are
very well reproduced by the calculations. Both the X-ray data
and calculations show a lengthening of the B-C bond distances
as a result of the tetracoordination of boron and a more
pronounced bond alternation of the thiophene rings. Thus, the
C2-C3 distances are ca. 0.04 to 0.05 Å longer than the C1-
C2 and C3-C4 bonds (for the free acids: 0.02 Å for 3 and
0.01 Å for 4). Only small differences are observed for the
monocomplexed species 3Py and 4Py relative to the free acids
and bis-adducts, respectively. However a consistent trend is
found especially for 4Py, for which the B-C(Th) bond of the
uncomplexed borane is shortened relative to that of the free
acid 4 and the bond alternation within the adjacent thiophene
ring is nearly completely absent. This indicates that coordination
of pyridine to one of the boron centers leads to enhanced overlap
of the bithiophene π-system with the remaining tricoordinate
boron moiety. Since this effect is much more evident for 4Py
than 3Py stronger communication between the two binding sites
should be expected for the more electron-deficient 4 (see vide
infra).
C4-S (Å)
B-N(Py) (Å)
4 (X-ray/DFT)
4Py (DFT)
uncomplexed complexed
4Py2 (X-ray/DFT)
X-ray
DFT
X-ray
DFT
B-C(C6F5) (Å) 1.576(3) 1.5775
1.578(3) 1.5795
B-C4(Th) (Å) 1.507(3) 1.5244
1.5825
1.5811
1.5150
1.6544 1.648(3) 1.6464
1.6480 1.645(3) 1.6517
1.6261 1.617(3) 1.6146
1.448(3) 1.4475
1.3806 1.363(3) 1.3775
1.3173 1.417(3) 1.4213
1.3806 1.359(3) 1.3759
1.7516 1.730(2) 1.7564
1.7584 1.740(2) 1.7652
1.6667 1.624(3) 1.6615
C1-C1* (Å)
C1-C2 (Å)
C2-C3 (Å)
C3-C4 (Å)
C1-S (Å)
1.445(4) 1.4458
1.372(3) 1.3914
1.390(3) 1.4034
1.383(3) 1.3934
1.703(2) 1.7387
1.732(2) 1.7552
1.4442
1.3932
1.4019
1.3948
1.7394
1.7610
C4-S (Å)
B-N(Py) (Å)
Table 4. Calculated Electronic Transitions for 3, 3Py, 3Py2, 4,
4Py, and 4Py2 from TD-DFT (B3LYP) Calculationsa
oscillator
energy gap
eV (nm)
strength
f
compound
transition
MO contributions
3
S0 f S1
S0 f S2
HOMO f LUMO
HOMO f LUMO + 1
2.97 (417) 1.3343
3.05 (407) 0.9751
3.65 (339) 0.9548
2.88 (430) 1.2544
3.01 (411) 1.0192
3.50 (354) 0.7582
3Py
3Py2
4
4Py
4Py2
S0 f S15 HOMO f LUMO + 4
S0 f S1
S0 f S2
S0 f S7
HOMO f LUMO
HOMO f LUMO
HOMO - 3 f LUMO
HOMO - 2 f LUMO + 1
HOMO f LUMO + 4
Upon complexation of the boryl groups with pyridine, the
nature of the HOMO and LUMO orbitals changes dramatically.
Thus, for 3Py2 and 4Py2 the HOMO levels are significantly
higher in energy and primarily centered on the bithiophene
moiety without any contribution from the boron atoms (see
Supporting Information). While the LUMO levels show con-
tributions from the pyridine ligands, the LUMO + 4 orbitals
are bithiophene-centered, which are 3.969 eV (3Py2) and 3.861
eV (4Py2) above the HOMO, respectively; the energy differ-
ences between these orbitals are thus much larger than the
HOMO-LUMO gaps in the free acids (3, 3.348 eV; 4, 3.186
eV). In the case of the monoadducts, 3Py and 4Py, the HOMO
and LUMO (4Py)/LUMO + 1 (3Py) orbitals show overlap
between the bithiophene moiety and the tricoordinate but not
the tetracoordinate boryl groups. The HOMO and LUMO
orbitals are higher in energy than those in the free acids, but
the HOMO-LUMO gaps are nearly unchanged.
a Only the most intense low-energy transitions are shown; for a complete
list see the Supporting Information.
been performed using Gaussian03 to gain further insight into a
possible correlation between the Lewis acidity of the boron
centers in 3 and 4 and their optical properties (Tables 3 and
4).22
The calculations reproduce the structural parameters for 4
determined by single-crystal X-ray diffraction very well. A
comparison of the structural features derived for 4 with those
calculated for the non-fluorinated analogue 3 is of particular
interest (Table 3). The most striking difference is that the B-C4
bonds to the bithiophene moiety are significantly shorter for
the fluorinated derivative 4 (1.5244 Å) in comparison to 3
(22) DFT calculations were performed with the Gaussian03 program (Frisch,
M. J., et al. Gaussian03, revision C.02; Gaussian Inc.: Wallingford CT,
2004). Geometries and electronic properties were calculated by means of
the hybrid density functional B3LYP with the basis set of 6-31.G(d). The
input files were generated using Gaussview except for 4Py2 (input file from
X-ray data). The orbital representations were generated using Gaussview.
(23) Scheibitz, M.; Bolte, M.; Bats, J. W.; Lerner, H.-W.; Nowik, I.; Herber, R.
H.; Krapp, A.; Lein, M.; Holthausen, M. C.; Wagner, M. Chem. Eur. J.
2005, 11, 584-603.
9
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A R T I C L E S
Sundararaman, et al.
Figure 5. HOMO and LUMO diagrams of 3 and 4. Scaling radii of 0.75 and an isovalue of 0.02 were used.
We carried out TD-DFT calculations to gain further insight
into the origin of the electronic transitions for 3, 4, and their
respective pyridine complexes (Table 4). The calculations
revealed that, for both 3 and 4, the lowest energy transition
corresponds to promotion of electrons from the HOMO to the
LUMO levels. The calculated excitations of 417 nm for 3 and
430 nm for 4 correlate well with the experimentally observed
values for the absorption maxima λmax (3: 411 nm; 4: 437 nm).
Importantly, the calculations nicely reproduce the red shift for
the fluorinated derivative 4.
For the bis-adducts the lowest energy transitions with high
oscillator strength are the S0fS15 excitation corresponding to
a HOMOfLUMO + 4 transition for 3Py2 and the S0fS7
excitation for 4Py2 with multiple orbital contributions, the
primary of which is the HOMOfLUMO + 4 component. The
energy for these transitions of 3.65 eV (339 nm) for 3Py2 and
of 3.50 eV (354 nm) for 4Py2, respectively, is far higher than
that for the free acid (2.97 eV for 3; 2.88 eV for 4). Strongly
blue-shifted absorption bands are therefore expected for the bis-
adducts. In contrast, the monoadducts display transitions that
are only slightly higher in energy and lower in intensity than
those of the free acids.
Electrochemical Studies: Cyclic voltammetry was used to
investigate the electronic communication between the two
ferrocene moieties in 5. In CH2Cl2 with Bu4N[(B(C6F5)4)] as
the supporting electrolyte, compound 5 shows a reversible one-
electron oxidation at -11 mV and a quasi-reversible second
oxidation wave at +207 mV relative to the ferrocene/ferroce-
nium couple (Figure 6a). The peak separation of 218 mV can
be attributed to a moderate degree of metal-metal interaction
through the diborylated bithiophene linker.24 The cyclic volta-
mmogram of 5Py2 on the other hand showed significantly lower
redox potentials of -123 mV and -15 mV with a considerably
Figure 6. Cyclic voltammograms of (a) 5 and (b) 5Py2 with 0.1 M Bu4N-
[B(C6F5)]4 in CH2Cl2 as the supporting electrolyte (scan rate 100 mV/s).
The scans are referenced relative to the Fc/Fc+ couple.
smaller separation of ∆E ) 108 mV (Figure 6b). This indicates
that complexation of the boron centers leads to a breakdown of
conjugation25 and, thus, a decrease in the communication
through the diborylated bithiophene linker.
Pyridine Titration Studies: We performed titration studies
with pyridine to further investigate the communication between
(24) Related diferrocene species that feature R,ω-dialkynylbithiophene linkers
show only a single redox wave with [Bu4N]ClO4 in CH2Cl2 as the
supporting electrolyte: Thomas, K. R. J.; Lin, J. T.; Wen, Y. S.
Organometallics 2000, 19, 1008-1012.
(25) See also: Scheibitz, M.; Heilmann, J. B.; Winter, R. F.; Bolte, M.; Bats,
J. W.; Wagner, M. Dalton Trans. 2005, 159-170.
9
16560 J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006

Electronic Communication in Diborylated Bithiophene
A R T I C L E S
Figure 7. 19F NMR plot for the titration of 4 with pyridine (Pf ) C6F5).
the boron centers. The binding of pyridine was monitored
through (a) NMR spectroscopy and (b) UV-visible and
fluorescence spectroscopy.
Figure 8. (a) UV-vis absorption and (b) emission plots for the titration
of 3 (CH2Cl2, 51.9 µM) with pyridine (CH2Cl2, 1.49 mM).
(a) NMR Spectroscopy. Titration of compound 4 with
pyridine was followed by 1H and 19F NMR spectroscopy. Figure
7 shows the room temperature 19F NMR spectra for the free
acid 4, the mixture obtained upon addition of 1 equiv of
pyridine, and the fully complexed species 4Py2 that is formed
in the presence of 2 equiv of pyridine. Addition of 1 equiv of
pyridine resulted in the appearance of two new sets of C6F5
signals, which are upfield shifted in comparison to the signals
of the free acid 4. The most upfield shifted new peaks can be
assigned to the C6F5 groups on the boron atom that is
coordinated by pyridine, while the signals closer to those of
the starting material 4 are assigned to the C6F5 groups attached
to the tricoordinate boron moiety. The amount of monocom-
plexed diborane 4Py was estimated by NMR integration to ca.
86% with the remainder consisting of ca. 7% of starting material
and 7% of the fully complexed species 4Py2. The appearance
of small amounts of 4 and 4Py2 upon addition of 1 equiv of
pyridine is a result of dissociation of the intermediate 4Py into
the free acid 4 and fully complexed 4Py2. In the absence of
any interaction between the two boron centers one would expect
a 25:50:25 statistical ratio for [4]:[4Py]:[4Py2], which corre-
sponds to an interaction parameter of a ) 1, and therefore K11
) 4K12 holds for the relation between the stepwise binding
constants (see eqs 1 and 2).26 For the two Lewis acid sites in 4,
the interaction parameter a, which is related to the equilibrium
constant for the disproportionation process (2 4Py a 4 + 4Py2),
was estimated by 19F NMR to be a ) 0.025, indicative of strong
negative cooperativity. In line with this result is also the
observed upfield shift of the C6F5 signals of the uncomplexed
boron center of 4Py relative to those of 4. Similar analysis of
the corresponding 1H NMR spectra corroborated the value
were performed at -30 °C where the slow exchange regime is
reached, gave a significantly lower value of a ) 0.21, consistent
with a less pronounced binding cooperativity.
[LA Py2]
[LA Py]
K11
)
K12
)
(1)
(2)
[LA][py]
[LA Py][py]
K
[LA Py2][LA]
a ) 4 12 ) 4
K11
[LA Py][LA Py]
(b) UV-visible Absorption and Fluorescence Spectros-
copy. To independently confirm the negative binding cooper-
ativity, the pyridine binding process for 3 was further examined
by titration studies, which were monitored by UV-vis absorp-
tion and fluorescence spectroscopy as illustrated in Figure 8.
Sequential addition of pyridine to 3 resulted in a gradual
decrease of the absorption bands at 396 nm (ꢀ ) 48 290) and
306 nm (ꢀ ) 37 160). A new high-energy absorption band
developed at 333 nm (ꢀ ) 19 800) (Figure 8a). This process is
reflected in a color change of the solution from pale yellow to
colorless, which is consistent with the computational results and
can be related to the interruption of the extended p-π
conjugation through the two boron centers upon pyridine
coordination. The emission spectra, which were acquired
simultaneously, show a concomitant decrease in emission
intensity indicative of static fluorescence quenching (Figure 8b).
Closer examination of the UV-visible titration data revealed
that there are different regimes with separate isosbestic points,
indicating that two distinct cooperative binding processes are
encountered. Refinement according to a two-site binding model
that considers the Lewis acid groups as two tight binding sites
1
determined for a. In contrast, H NMR studies for 3, which
(26) Connors, K. A. Binding Constants; Wiley-Interscience: London, 1987.
9
J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006 16561

A R T I C L E S
Sundararaman, et al.
(trimethylstannyl)-5,5′-bithiophene (1b)33 were prepared according
to literature procedures for the same or closely related compounds.
All reactions and manipulations were carried out under an atmosphere
of prepurified nitrogen using either Schlenk techniques or an inert-
atmosphere glove box (Innovative Technologies). Ether solvents
were distilled from Na/benzophenone prior to use. Hydrocarbon
and chlorinated solvents were purified using a solvent purification
system (Innovative Technologies; alumina/copper columns for
hydrocarbon solvents); the chlorinated solvents were subsequently
distilled from CaH2 and degassed via several freeze-pump-thaw
cycles.
was performed using the program Hyperquad. A stable refine-
ment was obtained with binding constants log K11 ) 6.64 (
0.04 and log K12 ) 5.40 ( 0.04. The corresponding microscopic
binding constants KXX′ ) 0.5K11 ) 2.2 × 106 M-1 and KX′X′
)
2K12 ) 5.0 × 105 M-1 for the first and second binding process,
respectively, are in the range expected for triarylborane pyridine
complexes.27 Moreover, the cooperativity parameter a ) 0.23
1
is in very good agreement with that determined by H NMR
spectroscopy (a ) 0.21).
Summary and Conclusion
1
All 499.893 MHz H, 125.7 MHz 13C, 160.3 MHz 11B, and 470.2
MHz 19F NMR spectra were recorded on a Varian INOVA NMR
spectrometer operating at 499.91 MHz. The Varian INOVA 500
spectrometer was equipped with a boron-free probe, and boron-free
In conclusion, we have used a comprehensive set of tools
that encompass X-ray crystallography, DFT calculations, cyclic
voltammetry, 1H and 19F NMR, and UV visible absorption and
emission spectroscopy to study the electronic communication
and binding cooperativity of two boron centers that are separated
by a conjugated bithiophene bridge. These studies show that
electronic delocalization is most pronounced for the most highly
Lewis acidic fluorinated derivative 4. For instance, with
enhanced Lewis acidity of boron, the B-C(Th) bonds become
shorter and the bond alternation in the thiophene rings is less
pronounced. Moreover, the HOMO and LUMO levels are
considerably lowered, with a stronger effect on the LUMO level
that leads to a narrowing of the HOMO-LUMO gap, which is
also reflected in a bathochromic shift of the absorption and
emission bands. Concurrent with the enhanced electronic
delocalization in 4 goes a strong increase in the negative binding
cooperativity between the two Lewis acid groups. Binding of
pyridine to one of the boron centers significantly lowers the
affinity of the other boron center for pyridine. This effect can
be attributed to effective communication through the bithiophene
linker and is much more pronounced for the more highly Lewis
acidic 4 in comparison to 3.
The results described in here thus provide new insight into
the electronic communication in bifunctional boranes, which
have important implications on the use of these systems in
sensing applications and in the assembly of supramolecular
structures through donor-acceptor bonding. Moreover, the
consequences of electronic communication between Lewis acidic
boron sites are of key importance for a complete understanding
of recently developed conjugated organoboron polymers,7 their
behavior as so-called “molecular wires”, and their potential use
as highly sensitive sensor materials.
1
quartz NMR tubes were used. The 400 MHz H, 100.5 MHz 13C, and
79.45 MHz 29Si (DEPT mode) NMR spectra were recorded on a Varian
1
VXR-S spectrometer. All H and 13C NMR spectra were referenced
internally to solvent signals. 11B NMR spectra were referenced
externally to BF3‚Et2O in C6D6 (δ ) 0), 29Si NMR spectra, to Me4Si
in C6D6 (δ ) 0), and 19F NMR spectra, to R,R′,R′′-trifluorotoluene
(0.05% in C6D6; δ ) -63.73). All NMR spectra were recorded at
ambient temperature unless noted otherwise.
UV-visible absorption data were acquired on a Varian Cary 500
UV-vis/NIR spectrophotometer. Solutions were prepared using a
microbalance ((0.1 mg) and volumetric glassware and then charged
into quartz cuvettes with sealing screw caps (Starna) inside the glove
box. The fluorescence data were measured on a Varian Cary Eclipse
Fluorescence spectrophotometer with the same solutions as those used
in the UV-visible measurements. Anthracene was used as the standard
for determination of the quantum yields (Φ). The quantum yield of
anthracene was adopted from the Handbook of Photochemistry,34 and
the concentration of anthracene in THF was 6.86 × 10-6 M. Titration
experiments were performed by successive addition of predetermined
amounts of a pyridine solution into the sample quartz cuvette with a
sealing screw cap (Starna) using a microliter syringe inside the glove
box. After each measurement, the quartz cuvette was returned to the
glove box followed by addition of another predetermined amount of
the pyridine solution. Binding constants were determined with the
program Hyperquad.35
Cyclic voltammetry was carried out on a CV-50W analyzer from
BAS. The three-electrode system consisted of an Au disk as the working
electrode, a Pt wire as the secondary electrode, and a Ag wire as the
reference electrode. The voltammograms were recorded with ca. 1.8
× 10-3 M solution in dichloromethane containing Bu4N[B(C6F5)4] (0.1
M) as the supporting electrolyte. The scans were referenced after the
addition of a small amount of decamethylferrocene as the internal
standard. The potentials are reported relative to the ferrocene/ferroce-
nium couple (E1/2 ) -610 mV for decamethylferrocene with respect
to ferrocene in dichloromethane).
Experimental Section
Materials and General Methods. n-BuLi, Me3SiCl, and pyridine
(anhydrous grade) were purchased from Acros; BBr3 was purchased
from Aldrich; and C6F5Br was purchased from Fluorochem. BBr3 and
C6F5Br were distilled under a vacuum prior to use. Pentafluorophe-
nylcopper,28 trimethylstannylferrocene,29 p-tBuC6H4SnMe3,30 p-tBuC6H4-
BBr2,31 2,2′-bis(trimethylsilyl)-5,5′-bithiophene (1a),32 and 2,2′-bis-
GC-MS spectra were acquired on a Hewlett-Packard HP 6890 Series
GC system equipped with a series 5973 mass selective detector and a
series 7683 injector. A temperature profile with a heating rate of 20
°C/min from 50 °C to 280 °C was used. Mass spectral data in FAB
positive ion mode with NPOE (2-nitrophenyl octyl ether) or NBA (4-
nitrobenzylalcohol) as matrix were obtained at the Michigan State
University Mass Spectrometry Facility, which is supported, in part, by
a grant (DRR-00480) from the Biotechnology Research Technology
Program, National Center for Research Resources, National Institutes
of Health. MALDI-TOF-TOF measurements were performed on an
Applied Biosystems 4700 Proteomics Analyzer in reflectron mode with
(27) Brown, H. C.; Barbaras, G. K. J. Am. Chem. Soc. 1947, 69, 1137-1144.
(28) (a) Cairncross, A.; Sheppard, W. A.; Wonchoba, E. Org. Synth. 1980, 59,
122-131. (b) Sundararaman, A.; Lalancette, R. A.; Zakharov, L. N.;
Rheingold, A. L.; Ja¨kle, F. Organometallics 2003, 22, 3526-3532.
(29) Guillaneux, D.; Kagan, H. B. J. Org. Chem. 1995, 60, 2502-2505.
(30) Qin, Y.; Shah, S.; Kiburu, I.; Ja¨kle, F. Polym. Prepr. 2005, 46 (2), 1026-
1027.
(31) Qin, Y.; Cheng, G.; Achara, O.; Parab, K.; Ja¨kle, F. Macromolecules 2004,
37, 7123-7131.
(32) Herrema, J. K.; Wildeman, J.; Gill, R. E.; Wieringa, R. H.; van Hutten, P.
F.; Hadziioannou, G. Macromolecules 1995, 28, 8102-8116.
(33) Kamal, M. R.; Al-Taweel, S. A.; El-Abadelah, M. M.; Ajaj, K. M. A.
Phosphorus, Sulfur Silicon Relat. Elem. 1997, 126, 65-74.
(34) Murov, S. L., Carmichael, I., Hug, G. L., Eds. Handbook of Photochemistry,
2nd ed.; Marcel Dekker Inc.: New York, 1993.
(35) Gans, P.; Sabatini, A.; Vacca, A. Talanta 1996, 43, 1739-1753.
9
16562 J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006

Electronic Communication in Diborylated Bithiophene
A R T I C L E S
Table 5. Experimental Data for the X-ray Diffraction Studies of 4 and 4Py2
compound
4
4Py2
empirical formula
M
C32H4B2F20S2‚3 C7H8
1130.49
C42H14B2F20N2S2‚2 C9H12
1252.66
T, K
218(2)
100(2)
wavelength, Å
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
0.71073
triclinic
P1h
7.6711(8)
11.9741(12)
13.8819(13)
93.157(2)
105.488(2)
93.159(2)
1.54178
triclinic
P1h
9.07220(10)
12.84770(10)
13.12490(10)
105.5110(10)
100.7860(10)
105.9210(10)
1360.37(2)
1223.7(2)
Z
F
1
1
calc, g cm-3
1.534
0.223
568
0.25 × 0.12 × 0.10
-9 e h e 9
-13 e k e 14
-16 e l e 16
1.53 to 25.00
7447
1.529
1.893
634
0.17 × 0.14 × 0.09
-9 e h e 10
-14 e k e 14
-14 e l e 14
3.64 to 59.99
9585
µ, mm-1
F(000)
crystal size, mm3
limiting indices
θ range, deg
reflns collected
independent reflns
absorption correction
refinement method
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]a
4294 [R(int) ) 0.0226]
semiempirical from equivalents
full-matrix least-squares on F2
4294/18/389
1.059
R1 ) 0.0459
wR2 ) 0.1352
R1 ) 0.0575
wR2 ) 0.1427
0.337 and -0.196
3865 [R(int) ) 0.0176]
semiempirical from equivalents
full-matrix least-squares on F2
3865/0/392
1.029
R1 ) 0.0314
wR2 ) 0.0825
R1 ) 0.0352
wR2 ) 0.0854
0.236 and -0.207
R indices (all data)a
peak/hole (eÅ-3
)
2
2
2
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|; wR2 ) {Σ[w(Fo - Fc )2]/Σ[w(Fo )2]}1/2
.
delayed extraction. Benzo[a]pyrene was used as the matrix (10 mg/
mL toluene). Samples were prepared in toluene (10 mg/mL), mixed
with the matrix in a 1:10 ratio, and then spotted on the wells of a
sample plate inside a glove box. The samples were observed in either
positive or negative ion mode as specified. Peptides were used for
external calibration (Des-Arg-Bradykinin (904.4681), Angiotensin I
(1296.6853), Glu-Fibrinopeptide B (1570.6774), ACTH (clip 1-17)
(2093.0867), ACTH (clip 18-39) (2465.1989), ACTH (clip 7-38)
(3657.9294) with R-hydroxy-4-cyanocinnamic acid as the matrix).
Elemental analyses were performed by Quantitative Technologies Inc.,
Whitehouse, NJ.
X-ray diffraction intensities were collected on a Bruker SMART
APEX CCD diffractometer at T ) 218(2) (4) and 100(2) K (4Py2)
using MoKR (λ ) 0.71073 Å) (4) and CuKR (1.54178 Å) (4Py2)
radiations. Crystallographic data for 4 and 4Py2 and details of X-ray
diffraction experiments and crystal structure refinements are given in
Table 5. SADABS36 absorption corrections were applied in both cases.
Structures were solved using direct methods and completed by
subsequent difference Fourier syntheses and refined by full matrix least-
squares procedures on reflection intensities (F2). In the crystal of 4
there are two solvent toluene molecules, which are disordered over
two positions around an inversion center (in ratio 1:1) and in a
general position (in ratio 4:1). All non-hydrogen atoms were refined
with anisotropic displacement coefficients. The H atoms were placed
at calculated positions and were refined as riding atoms. The H atoms
of the disordered solvent molecules in 4 were not taken into
consideration. The disordered solvent toluene molecules in 4 were
refined with restrictions; the typical average values of C-C bond
lengths were used in the refinement as targets for corresponding C-C
bonds. All software and source scattering factors are contained in the
SHELXTL (5.10) program package.37 Crystallographic data for the
structures of 4 and 4Py2 have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication no. CCDC-
622334 and CCDC-622335, respectively. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; email: deposit@
ccdc.cam.ac.uk).
DFT calculations were performed with the Gaussian03 program as
described in ref 22. Excitation data were determined using TD-DFT
(B3LYP) calculations.
Caution! BBr3 is toxic and highly corrosive and should be handled
appropriately with great care. Fluorinated grease was used for ground
glass joints in reactions involving BBr3.
Synthesis of (p-tBuC6H4)2BBr: A solution of p-tBuC6H4SnMe3 (0.97
g, 3.29 mmol) in CH2Cl2 (5 mL) was added slowly under stirring to a
solution of p-tBuC6H4BBr2 (1.00 g, 3.29 mmol) in CH2Cl2 (5 mL) at
-78 °C. The reaction mixture was allowed to warm up to room
temperature in 1 h. All volatile components were removed under a
high vacuum. 1H NMR indicated that the product was more than 95%
1
pure. Isolated yield: 0.96 g (81%). H NMR (CDCl3, 499.893 MHz):
3
3
δ ) 7.99 (d, J ) 8.0 Hz, 4H, Ph-Ho), 7.53 (d, J ) 8.0 Hz, 4H,
Ph-Hm), 1.39 (s, 18H, Me). 13C NMR (CDCl3, 125.7 MHz): δ ) 156.9
(Ph-Cp), 137.9 (Ph-Co), 125.1 (Ph-Cm), 35.4 (CMe3), 31.3 (Me), not
observed Ph-Ci. 11B NMR (CDCl3, 160.3 MHz): δ ) 64.8 (w1/2
)
3000 Hz).
Reaction of 1b with (p-tBuC6H4)2BBr: Synthesis of 3. A solution
of (tBuC6H4)2BBr (0.508 g, 1.42 mmol) in CH2Cl2 (10 mL) was added
slowly under stirring to a solution of 1b (0.35 g, 0.71 mmol) in CH2-
Cl2 (10 mL). The reaction mixture was left stirring overnight at room
temperature. All volatile components were removed under a high
(36) Sheldrick, G.M. SADABS (2.01), Bruker/Siemens Area Detector Absorption
Correction Program; Bruker AXS: Madison, WI, 1998.
(37) Sheldrick, G. SHELXTL (5.10); Bruker XRD: Madison, WI.
9
J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006 16563

A R T I C L E S
Sundararaman, et al.
vacuum. The crude product was redissolved in hot CH2Cl2, and the
Cl2 (4 mL) the color changed from dark yellow to colorless. The mixture
1
product was precipitated at -35 °C. Isolated yield: 0.38 g (74%). H
was layered with hexanes (10 mL) and kept at -35 °C to obtain
3
1
NMR (CDCl3, 499.893 MHz): δ ) 7.72 (d, J ) 8.0 Hz, 8H, Ph-
colorless crystals. Isolated yield: 211 mg (89%). H NMR (CDCl3,
3
3
3
3
Ho), 7.61 (d, J ) 4.0 Hz, 2H, Th-H4, 4′), 7.48 (d, J ) 4.0 Hz, 2H,
499.893 MHz): δ ) 8.70 (d, J ) 6.0 Hz, 4H, Py-Ho), 8.06 (t, J )
Th-H3,3′), 7.45 (d, 3J ) 8.0 Hz, 8H, Ph-Hm), 1.35 (s, 36H, Me). 13
C
7.5 Hz, 2H, Py-Hp), 7.56 (pst, J ) 7.5 Hz, 4H, Py-Hm), 7.02 (d, J
3
3
3
NMR (CDCl3, 125.7 MHz): δ ) 154.8 (Ph-Cp), 149.2 (Th-C2,2′),
146.6 (Th-C5,5′), 144.2 (Th-C4,4′), 140.8 (Ph-Ci), 138.5 (Ph-Co),
127.2 (Th-C3,3′), 125.0 (Ph-Cm), 35.4 (CMe3), 31.7 (Me). 11B NMR
(CDCl3, 160.3 MHz): δ ) 59.3 (w1/2 ) 1500 Hz). UV-vis (CH2Cl2,
5.57 × 10-7 M): λmax ) 411 nm (ꢀ ) 42 740), 396 nm (ꢀ ) 48 290).
Fluorescence (CH2Cl2, 5.19 × 10-7 M): λem,max ) 436, 461 nm (λexc
) 394 nm), Φ ) 0.79. Solid-state fluorescence: λem,max ) 472 nm
(λexc ) 400 nm), excitation edge λ ) 454 nm (λem ) 470 nm). MALDI-
TOF-TOF MS (benzo[a]pyrene): calcd 718.96, observed (negative
mode) 718.54. Elemental analysis for C48H56B2S2 (718.71) calcd: C,
80.22; H, 7.85. Found: C, 80.52; H, 8.10.
) 3.5 Hz, 2H, Th-H4,4′), 6.79 (d, J ) 3.5 Hz, 2H, Th-H3,3′). 19F
NMR (470.2 MHz, CDCl3, 20 °C): δ ) -135.5 (m, 8F, Pf-Fo),
3
-161.7 (t, JFF ) 20 Hz, 4F, Pf-Fp), -168.1 (m, 8F, Pf-Fm). 13C
1
NMR (CDCl3, 125.7 MHz): δ ) 148.2 (dm, JCF ) 249 Hz, Pf-Co),
147.2 (Py-Co), 142.3 (Th-C4,4′), 140.3 (Th-C2,2′), 140.2 (dm, 1JCF
1
) 255 Hz, Pf-Cp), 137.5 (dm, JCF ) 248 Hz, Pf-Cm), 133.6 (Py-
Cp), 125.9 (Py-Cm), 124.1 (Th-C3,3′), 120.3 (b, Py-Ci). 11B NMR
(CD2Cl2, 160.3 MHz): δ ) -2.6 (w1/2 ) 400 Hz). UV-vis (CH2Cl2,
2.55 × 10-5 M): λmax ) 325 nm (ꢀ ) 17 400). FAB-MS (low-res)
with NPOE as matrix: m/z (%) ) 1012.1 [M+] (20), 933.1 [M+ - Py]
+
(25), 854.0 [M+ - 2Py] (42); also observed: m/z (%) ) 2024.4 [M2
(0.15), 1946.1 [M2 - Py] (0.11).
]
+
Reaction of 3 with 2 equiv of Pyridine: A solution of pyridine
(2.9 mg, 0.037 mmol) in toluene (5 mL) was added dropwise to a
solution of 3 (13.4 mg, 0.0186 mmol) in toluene (5 mL). The reaction
mixture was kept stirring overnight, which led to formation of a white
precipitate. The solid was redissolved in hot toluene, and the product
was obtained as a white powdery material at -35 °C. Isolated yield:
Reaction of 2 with 2 equiv of FcSnMe3 and then 2 equiv of
[C6F5Cu]: Synthesis of 5. A solution of 2 (3.62 g, 7.15 mmol) in
toluene (150 mL) was added slowly under stirring to a solution of
FcSnMe3 (4.90 g, 14.02 mmol) in toluene (100 mL) at -35 °C. The
mixture was allowed to warm up slowly and left stirring at room
temperature overnight. All volatile components were removed under a
high vacuum. The spectroscopic purity of the solid residue was >95%
by 1H NMR. The product was used without further purification for the
1
3
13.9 mg (85%). H NMR (CDCl3, 499.893 MHz): δ ) 8.59 (d, J )
3
3
7.0 Hz, 4H, Py-Ho), 8.01 (t, J ) 7.0 Hz, 2H, Py-Hp), 7.52 (pst, J
3
) 6.5 Hz, 4H, Py-Hm), 7.26 (d, J ) 8.0 Hz, 8H, tBu-Ph), 7.15 (d,
3J ) 8.0 Hz, 8H, tBu-Ph), 7.06, 6.80 (d, 3J ) 3.5 Hz, 2H, Th-H3,3′
1
next step. Isolated yield: 4.90 g (96%). H NMR (CDCl3, 499.893
and d, J ) 3.5 Hz, 2H, Th-H4,4′), 1.31 (s, 36H, Me). 11B NMR
3
3
3
MHz): δ ) 8.01 (d, J ) 4.5 Hz, 2H, Th-H4,4′), 7.53 (d, J ) 4.5
Hz, 2H, Th-H3,3′), 4.92, 4.87 (br, 4H, Cp-H2,5 and br, 4H, Cp-
H3,4), 4.20 (s, 10H, C5H5). 13C NMR (CDCl3, 125.7 MHz): δ ) 146.9
(Th-C2,2′), 142.6 (br, Th-C5,5′), 142.0 (Th-C4,4′), 126.7 (Th-
C3,3′), 77.6, 77.2 (Cp-C2,5 and Cp-C3,4 overlap with CDCl3 signals),
71.0 (C5H5). 11B NMR (CDCl3, 160.3 MHz): δ ) 50.5 (w1/2 ) 1100
Hz). A solution of [C6F5Cu] (1.09 g, 4.73 mmol) in toluene (20 mL)
was added dropwise at -35 °C to a portion of the bromoborane (1.70
g, 2.38 mmol) in toluene (20 mL) under stirring. The mixture was
allowed to warm up, left stirring for 3 h, and then filtered through a
fritted glass funnel. All volatile components were removed under a
high vacuum. The crude product was purified by recrystallization from
a toluene/hexane mixture (2:1) to give a dark red powdery material.
(CDCl3, 160.3 MHz): δ ) 2.7 (w1/2 ) 960 Hz). UV-vis (CH2Cl2,
5.19 × 10-5 M): λmax ) 336 nm (ꢀ ) 23 570). FAB-MS (low-res)
with NPOE as matrix: m/z (%) ) 718.4 [M+ - 2Py] (100).
Reaction of 1a with BBr3: Synthesis of 2. BBr3 (4.3 mL, 0.045
mol) was added neat to a solution of 1a (5.60 g, 0.018 mol) in toluene
(200 mL) at -78 °C. The reaction mixture was slowly warmed up to
room temperature and heated to 80 °C for 12 h. All volatile components
were removed under a high vacuum, and the crude product was
recrystallized from toluene to give pale brown crystals. Isolated yield:
5.29 g (57%). 1H NMR (CDCl3, 400 MHz): δ ) 7.99 (d, 3J ) 4.0 Hz,
2H, Th-H4,4′), 7.59 (d, 3J ) 4.0 Hz, 2H, Th-H3,3′). 13C NMR
(CDCl3, 125.7 MHz): δ ) 151.2 (Th-C2,2′), 145.0 (broad, Th-C5,5′),
144.7 (Th-C4,4′), 128.6 (Th-C3,3′). 11B NMR (CDCl3, 160.37
MHz): δ ) 47.1 (w1/2 ) 480 Hz).
1
Isolated yield: 1.63 g (77%). H NMR (CDCl3, 499.893 MHz): δ )
3
3
7.48, 7.47 (d, J ) 3.5 Hz, 2H, Th-H3,3′ and d, J ) 3.5 Hz, 2H,
Th-H4,4′), 4.96 (br, 4H, Cp-H3,4), 4.57 (br, 4H, Cp-H2,5), 4.28 (s,
10H, C5H5). 19F NMR (470.2 MHz, CDCl3, 20 °C): δ ) -130.4 (m,
4F, Pf-Fo), -155.2 (t, 3JFF ) 20 Hz, 2F, Pf-Fp), -163.1 (m, 4F, Pf-
Fm). 13C NMR (CDCl3, 125.7 MHz): δ ) 147.3 (Th-C2,2′), 146.1
Reaction of 2 with 4 equiv of [C6F5Cu]: Synthesis of 4. A solution
of compound 2 (2.19 g, 4.36 mmol) in toluene (40 mL) was cooled to
-78 °C. A solution of [C6F5Cu] (3.92 g, 17.0 mmol) in toluene (40
mL) was cooled to -78 °C and then added dropwise over a period of
15 min. The reaction mixture was stirred at -78 °C for 1 h, slowly
allowed to warm up to room temperature, and left stirring for 3 h. The
reaction mixture was filtered through a fritted glass funnel. All volatile
components were removed from the filtrate under a high vacuum, and
the remaining yellow solid was recrystallized from a toluene/hexane
(9/1) mixture to give the product as bright yellow crystals. Isolated
yield: 2.62 g (71%). 1H NMR (CD2Cl2, 499.893 MHz): δ ) 7.93 (d,
1
(dm, JCF ) 242 Hz, Pf-Co), 143.4 (Th-C5,5′), 142.5 (Th-C4,4′),
1
1
141.3 (dm, JCF ) 254 Hz, Pf-Cp), 137.5 (dm, JCF ) 252 Hz, Pf-
2
Cm), 126.7 (Th-C5,5′), 116.1 (t, JCF ) 32 Hz, Pf-Ci), 77.4 (Cp-
C2,3,4,5), 73.5 (br, Cp-Ci), 70.4 (C5H5). 11B NMR (CDCl3, 160.3
MHz): δ ) 50.5 (w1/2 ) 1700 Hz). UV-vis (CH2Cl2, 9.44 × 10-5
M): λmax ) 387 nm (ꢀ ) 23 890), 524 nm (ꢀ ) 9350). MALDI-TOF-
TOF MS (benzo[a]pyrene): calcd 889.99, observed (negative mode)
890.02. CV vs Fc/Fc+ (CH2Cl2, 1.8 × 10-3 M): E1/2 ) -11 mV (∆Ep
) 87 mV); E1/2 ) 207 mV (∆Ep ) 158 mV).
3
3J ) 4.0 Hz, 2H, Th-H4,4′), 7.73 (d, J ) 4.0 Hz, 2H, Th-H3,3′).
19F NMR (470.2 MHz, CD2Cl2, 20 °C): δ ) -129.1 (m, 8F, Pf-Fo),
3
-149.1 (t, JFF ) 20 Hz, 4F, Pf-Fp), -160.9 (m, 8F, Pf-Fm). 13C
Reaction of 5 with Pyridine: A solution of pyridine (78 mg, 0.99
mmol) in toluene (10 mL) was added dropwise to a solution of 5 (200
mg, 0.225 mmol). The reaction mixture was kept stirring overnight,
which led to formation of an orange precipitate. The solid was
redissolved in hot toluene, and the product was obtained as an orange
powdery material at -35 °C. Isolated yield: 216 mg (92%). 1H NMR
(CDCl3, 499.893 MHz): δ ) 8.32 (d, 3J ) 7.5 Hz, 4H, Py-Ho), 7.46,
NMR (CD2Cl2, 125.7 MHz): δ ) 154.2 (Th-C2,2′), 148.4 (Th-C4,4′),
147.2 (dm, 1JCF ) 245 Hz, Pf-Co), 145.5 (Th-C5,5′), 143.7 (dm, 1JCF
1
) 256 Hz, Pf-Cp), 138.3 (dm, JCF ) 253 Hz, Pf-Cm), 130.2 (Th-
3
C3,3′), 114.4 (t, JFF ) 30 Hz, Pf-Ci). 11B NMR (CD2Cl2, 160.3
MHz): δ ) 56.4 (w1/2 ) 1300 Hz). UV-vis (CH2Cl2, 5.85 × 10-5
M): λmax ) 437 nm (ꢀ ) 21 850), 413 nm (ꢀ ) 31 400). Fluorescence
(CH2Cl2, 5.85 × 10-5 M): λem,max ) 476 nm, 495 nm (sh) (λexc ) 413
3
3
7.28 (d, J ) 3.5 Hz, 2H, Th-H3,3′ and d, J ) 3.5 Hz, 2H, Th-
H4,4′), 6.59 (t, 3J ) 7.5 Hz, 2H, Py-Hp), 6.20 (pst, 3J ) 7.5 Hz, 4H,
Py-Hm), 4.28 (br, 4H, Cp-H3,4), 4.14 (br, 4H, Cp-H2,5), 4.06 (s,
10H, C5H5). 19F NMR (470.2 MHz, CDCl3, 20 °C): δ ) -129.6 (m,
4F, Pf-Fo), -159.3 (b, 2F, Pf-Fp), -164.8 (m, 4F, Pf-Fm). 11B NMR
nm), Φ ) 0.03. Solid-state fluorescence: λem,max ) 525 nm (λexc
415 nm), excitation edge 503 nm (λem ) 525 nm).
)
Reaction of 4 with 2 equiv of Pyridine: Upon addition of pyridine
(37.8 µL, 0.47 mmol) to a solution of 4 (200 mg, 0.23 mmol) in CH2-
9
16564 J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006

Electronic Communication in Diborylated Bithiophene
A R T I C L E S
(CDCl3, 160.3 MHz): δ ) -3.1 (w1/2 ) 800 Hz). FAB-MS (low-res)
with NBA as matrix: m/z (%) ) 890.0 [M+ - 2Py] (100). UV-vis
(CH2Cl2, 3.70 × 10-5 M): λmax ) 329 nm (ꢀ ) 13 200), 393 nm (ꢀ )
3400). CV vs Fc/Fc+ (CH2Cl2, 1.8 × 10-3 M): E1/2 ) -123 mV (∆Ep
) 135 mV); E1/2 ) -15 mV (∆Ep ) 145 mV).
F.J. is an Alfred P. Sloan research fellow. We thank Dr.
Lourderaj and Dr. Balamurugan for helpful discussions.
Supporting Information Available: Crystallographic data for
4 and 4Py2, a plot of the extended structure of 4, pyridine
titration plots for 3, and detailed results of the DFT calculations.
Complete ref 22. This material is available free of charge via
the Internet at http://pubs.acs.org.
Acknowledgment. Acknowledgment is made to the National
Science Foundation (NSF CAREER Award CHE-0346828 to
F.J. and NSF-CRIF 0443538), to the donors of the Petroleum
Research Fund, administered by the American Chemical Society,
and to the Rutgers Research Council for support of this research.
JA064396B
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J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006 16565