Aryl-substituted unsymmetrical benzothiadiazoles: Synthesis, structure, and properties

Article abstract of DOI:10.1021/jo402111q

A family of unsymmetrical donor-acceptor, ferrocenyl-substituted benzothiadiazoles of types D₁-π-A-π-D₂, D ₁-π-A₁-π-A₂, D₁-A-π-D ₂, and D₁-A₁-A₂-D₂, bearing a variety of electron-donating and electron-withdrawing groups, were designed and synthesized. Their photophysical, electrochemical, and computational properties were explored, which show strong donor-acceptor interaction. The presence of electron-rich units anthracene (6f) and triphenylamine (6h), and an electron-deficient unit 1,1,4,4-tetracyanobuta-1,3- diene (TCBD) (9b) results in lowering of the band gap, which leads to a red shift of the absorption spectrum in these benzothiadiazole systems. The single crystal structures of 6c, 6g, 7a, and 7b are reported, which show marvelous supramolecular interactions.

Full text of DOI:10.1021/jo402111q

Article
pubs.acs.org/joc
Aryl-Substituted Unsymmetrical Benzothiadiazoles: Synthesis,
Structure, and Properties
Rajneesh Misra,* Prabhat Gautam, and Shaikh M. Mobin
Department of Chemistry, Indian Institute of Technology Indore, Indore 452 017, India
S
* Supporting Information
ABSTRACT: A family of unsymmetrical donor−acceptor,
ferrocenyl-substituted benzothiadiazoles of types D1−π−
A−π−D2, D1−π−A1−π−A2, D1−A−π−D2, and D1−A1−A2−
D2, bearing a variety of electron-donating and electron-
withdrawing groups, were designed and synthesized. Their
photophysical, electrochemical, and computational properties
were explored, which show strong donor−acceptor interaction.
The presence of electron-rich units anthracene (6f) and
triphenylamine (6h), and an electron-deficient unit 1,1,4,4-
tetracyanobuta-1,3-diene (TCBD) (9b) results in lowering of
the band gap, which leads to a red shift of the absorption
spectrum in these benzothiadiazole systems. The single crystal
structures of 6c, 6g, 7a, and 7b are reported, which show
marvelous supramolecular interactions.
designed and synthesized by the Sonogashira, Suzuki, and
INTRODUCTION
■
Ullmann coupling reactions, respectively. These mono-bromo-
benzothiadiazoles (3a, 4a, and 5a) were further subjected to the
Sonogashira cross−coupling reaction, which resulted in unsym-
metrical BTDs. The unsymmetrical benzothiadiazoles 7a and 8a
with a monoethyne spacer were subjected to [2 + 2] cyclo-
addition reaction with tetracyanoethene (TCNE), followed by
ring-opening, which resulted in 1,1,4,4-tetracyanobuta-1,3-diene
(TCBD) bridged unsymmetrical benzothiadiazoles 9a and 9b.
The design and synthesis of organic π-conjugated donor−
acceptor systems continues to create a great amount of interest
because of their application in organic photovoltaics (OPV) and
nonlinear optics (NLO).1 The properties of the donor−acceptor
system can be modulated either by increasing the donor and
acceptor strength or by varying the π-linker between the donor
and the acceptor.2 A wide variety of donors and acceptors have
been used for the synthesis of D−π−A type of systems.3 2,1,3-
Benzothiadiazole (BTD) is a strong acceptor due to its high
electron affinity.4 Ferrocene is a strong electron donor and highly
stable. Our group is interested in the design and synthesis of
ferrocene-based donor−acceptor molecular systems for a variety
of photonic applications.5,6 Recently we have reported the
synthesis of ferrocene-substituted symmetrical and unsym-
metrical BTD systems of types D−π−A−π−D, D−π1−A−π2−
D, and D−π1−A−π2−A−π1−D.7,8 In continuation of this work,
we were further interested in the design and synthesis of
unsymmetrical BTDs having different donors/acceptors of
varying strengths and to see their effect on the donor−acceptor
interaction. In this contribution, we wish to report unsym-
metrical donor−acceptor systems of the types D1−π−A−π−D2,
D1−π−A1−π−A2, D1−A−π−D2, and D1−A1−A2−D2. Three
sets of mono-bromobenzothiadiazoles 3a, 4a, and 5a were
RESULT AND DISCUSSION
■
The synthesis of unsymmetrical benzothiadiazoles 6a−6h,
7a−7c, 8a−8c, 9a, and 9b are outlined in Schemes 2−4. The
dibromo-BTD 2 was synthesized by the bromination reaction of
the BTD 1.9 The Pd-catalyzed Sonogashira coupling reaction of
ethynylferrocene and dibromo-BTD 2 resulted in 4-bromo-7-
ferrocenylethynylbenzo[1,2,5]thiadiazole (3a) in 60% yield
(Scheme 1).8 The Ullmann coupling reaction of carbazole and
dibromo-BTD 2 resulted in 4-(9-carbazolyl)-7-bromo-2,1,3-
benzothiadiazole (4a) in 40% yield (Scheme 1).10 The Pd-
catalyzed Suzuki coupling reaction of the dibromo-BTD 2 and
Received: September 24, 2013
Published: November 26, 2013
© 2013 American Chemical Society
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Scheme 1. Synthetic Route for Mono-bromobenzothiadiazoles 3a, 4a, and 5a
4-(N,N-diphenylamino)-1-phenylboronic acid resulted in [4-(7-
bromo-benzo[1,2,5]thiadiazol-4-yl)phenyl]diphenyl-amine
(5a) in 50% yield (Scheme 1).11
resulted in compounds 7a−7c, and 8a−8c in 60−70% yield,
respectively (Scheme 3).
The 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) bridged un-
symmetrical BTDs 9a and 9b were synthesized via [2 + 2]
cycloaddition reaction of the ferrocenyl-substituted BTDs 7a and 8a
with tetracyanoethene (12) to form an intermediate cyclobutene
that subsequently undergoes ring-opening (Scheme 4).12 The
purification of the unsymmetrical BTDs 6a−6h, 7a−7c, 8a−8c, 9a,
and 9b was achieved by column chromatography. All the
The precursors 3a, 4a, and 5a were further subjected to
Sonogashira coupling reaction, which resulted in unsymmetrical
BTDs. To study the effect of the aryl substituents with enhanced
conjugation on the unsymmetrical ferrocenyl-BTD, a series of
aromatic terminal alkynes were selected with one (10a−10c),
two (10d and 10e), and three (10f−10h) aromatic rings. The
Sonogashira cross−coupling reaction of the bromo-BTD 3a with
the respective aryl-acetylenes (10a−10h) resulted in BTDs 6a−
6h in 40−70% yield (Scheme 2).
To explore the effect of variation of the π-bridge, a set of
ethynyl-substituted ferrocenes, namely, ethynylferrocene (11a),
4-ferrocenylphenylacetylene (11b), and 4-ethynyl-phenylethynyl-
ferrocene (11c), were synthesized and subjected to Sonogashira
cross-coupling reaction with 4-(9-carbazolyl)-7-bromo-2,1,3-
benzothiadiazole (4a), and [4-(7-bromo-benzo[1,2,5]thiadiazol-
4-yl)phenyl]diphenyl-amine (5a). The Sonogashira coupling of
the BTDs 4a and 5a with the respective ferrocenyl-acetylenes
1
unsymmetrical BTDs were well-characterized by H and 13C
NMR and HRMS techniques. The BTDs 6c, 6g, 7a, and 7b were
also characterized by single-crystal X-ray diffraction.
Thermogravimetric Analysis. The thermal properties of
the unsymmetrical benzothiadiazoles 6a−6h, 7a−7c, 8a−8c, 9a,
and 9b were investigated by the thermogravimetric analysis
(TGA) at a heating rate of 10 °C min−1, under a nitrogen
atmosphere (Figure 1). The decomposition temperatures for
10% weight loss in the BTDs 6b−6h was above 450 °C, whereas
the BTD 6a with the phenyl substituent shows the 10% weight
loss at 249 °C. This reflects that the heteroaryl, biaryl, and the
polycyclic aromatic substituted BTDs show higher thermal
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Scheme 2. Synthesis of Unsymmetrical Benzothiadiazoles 6a−6h
Scheme 3. Synthesis of Benzothiadiazoles 7a−7c and 8a−8c
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Scheme 4. Synthesis of Benzothiadiazoles 9a and 9b
Figure 1. TGA plots of unsymmetrical BTDs 6a−6h, 7a−7c, 8a−8c, 9a, and 9b at a heating rate of 10 °C min−1, under a nitrogen atmosphere.
Figure 2. Normalized electronic absorption spectra of (i) benzothiadiazoles 6a−6h and (ii) benzothiadiazoles 7a−7c, 8a−8c, 9a, and 9b in
dichloromethane at 1.0 × 10−6 M concentration.
stability. The BTDs 7a−7c and 8a−8c exhibit the 10% weight
loss above 300 °C. The trend observed in the decomposition
temperature follows the order 8c > 7c > 8b > 7b > 8a > 7a. The
extension of the π-bridge between the ferrocene and the BTD
unit results in increased thermal stability. The incorporation of a
1,1,4,4-tetracyanobuta-1,3-diene (TCBD) bridge results in
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Table 1. Photophysical and Thermal Properties of the Benzothiadiazoles 6a−6h, 7a−7c, 8a−8c, 9a, and 9b
a
b
a
b
photophysical data
Td (°C)
photophysical data
Td (°C)
compound
λabs (nm)
ε (M−1 cm−1
)
compound
λabs (nm)
ε (M−1 cm−1
)
ferrocene
-
-
6a
406
509
399
511
402
515
413
512
421
521
465
423
514
460
32 791
9324
249
535
506
646
471
7a
7b
7c
8a
8b
443
520
453
35 138
sh
328
411
604
359
449
6b
6c
6d
6e
85 328
22 404
78 216
20 963
55 533
14 750
54 400
sh
47 855
454
458
466
62 777
37 842
51 474
6f
67 550
58 074
sh
611
563
8c
9a
467
502
84 788
46 178
625
549
6g
6h
58 729
462
9b
554
61 482
543
a
Absorbance measured in dichloromethane at 1 × 10−6 M concentration. sh: shoulder; λabs: absorption wavelength; ε: extinction coefficient.
b
Decomposition temperatures for 10% weight loss at a heating rate of 10 °C min−1, under a nitrogen atmosphere.
greater thermal stability in BTDs 9a and 9b compared to the
ethyne bridged BTDs 7a and 8a. The decomposition temper-
atures for 10% weight loss is above 500 °C for TCBD-bridged
BTDs 9a and 9b.
Photophysical Properties. The UV−vis absorption spectra
of the unsymmetrical benzothiadiazoles were recorded in
dichloromethane at room temperature (Figure 2), and the data
are compiled in Table 1. The unsymmetrical BTDs show the
characteristic absorption band between 300−340 nm due to the
BTD unit.13
BTDs 7a and 8a. The trend in the bathochromic shift follows the
order 9b > 9a > 8a > 7a. This reveals strong donor−acceptor
interaction in theTCBD π-bridge in BTDs9a and 9b. The effect of
systematic variation of π-conjugation through aryl substitution and
the π-bridge is also reflected from the colored solution of these
unsymmetrical BTDs 6a−6h, 7a−7c, 8a−8c, 9a, and 9b (Figure 3).
The BTDs 6a−6h show the presence of a lower-energy
absorption band between 399 and 465 nm, corresponding to the
π → π* transition, and between 509 and 521 nm attributed to the
charge-transfer (CT) transitions.7,8,14The red shift in the
absorption maxima for the lower-energy π → π* absorption
band follows the order 6f > 6h > 6g > 6e > 6d > 6a > 6c > 6b. The
magnitude in red shift observed for BTDs 6a−6h is a function of
the aryl substituent attached to the BTD core. A comparison
of the absorption data reveals that the increase in the number
of aromatic rings from one (6a−6c), two (6d, 6e), and three
(6f−6h) leads to a regular increase in the red shift of the
absorption maxima due to enhancement of π-conjugation. The
mode of conjugation in the aryl substituents also affects the
absorption bands. The BTDs with naphthalene 6e and
anthracene 6f substituents exhibit a red shift compared to the
biphenyl 6d and phenanthrene 6g substituted BTDs, respec-
tively. The BTDs 6a−6d exhibit distinct a CT band between
509 and 515 nm, whereas 6e and 6g show in the form of a shoulder
between 512 and 521 nm. The lower-energy electronic absorption
maxima are broad in BTDs 6f and 6h due to the overlap with the
charge-transfer absorption.8,15 The BTDs 7a−7c, 8a−8c, 9a, and
9b show the lower-energy absorption band between 443 and 554
nm. The BTD 7a exhibits a CT band as a shoulder at 520 nm. The
BTDs 7b, 7c, 8a−8c, 9a, and 9b exhibit broadening of the lower-
energy electronic absorption band, which can be attributed to the
overlap of the lower-energy π → π* transition with the CT
band. The lower-energy electronic absorption band shows a
bathochromic shift with the enhancement of conjugation length.
The incorporation of a 1,1,4,4-tetracyanobuta-1,3-diene (TCBD)
π-bridge in BTDs 9a and 9b results in substantial bathochromic
shift of the lower-energy electronic absorption band compared to
Figure 3. Unsymmetrical benzothiadiazoles 6a−6h, 7a−7c, 8a−8c, 9a,
and 9b at 1 × 10−4 M concentration in dichloromethane.
The emission studies of the unsymmetrical BTDs show complete
quenching of the fluorescence.
Theoretical Calculations. DFT calculations were per-
formed on the unsymmetrical benzothiadiazoles 6a−6h, 7a−
7c, 8a−8c, 9a, and 9b to explore the effect of the aryl
substituents on the electronic structure of the unsymmetrical
ferrocenyl-BTDs. The contours of the HOMO, and LUMO
of BTDs 6a−6h, 7a−7c, 8a−8c, 9a, and 9b are shown in
Figures 4 and 5.
The HOMO orbitals are localized over the ferrocene, the aryl
substituent, and the hydrocarbon portion of the BTD, whereas
the LUMO orbitals are mainly concentrated on the BTD unit.8,16
The comparison of the HOMOs of BTDs 6d and 6e reveals
greater delocalization of the HOMO orbital on the naphthalene
unit compared to the biphenyl unit. Similarly, the anthracene unit
in BTD 6f shows better delocalization in the HOMO orbital,
compared to the phenanthrene-substituted BTD 6g. The
presence of electron-rich substituents anthracene and triphenyl-
amine on the BTDs 6f and 6g lowers the contribution of
ferrocene in the HOMO. The band gap in BTDs 6a−6h follows
the order 6b > 6c ≅ 6a > 6d > 6e > 6g > 6h > 6f, which is
reflected in their electronic absorption. The band gaps in BTDs
7a−7c and 8a−8c were found to be inversely proportional to the
conjugation length. In the HOMO of BTDs 8a−8c, the
contribution of the ferrocene unit is reduced with the extension
of the π-bridge. The band gap in BTDs 7a−7c and 8a−8c follows
the order 7a > 7b > 7c and 8a > 8b > 8c. The observed trend
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Figure 4. HOMO and LUMO frontier orbitals of unsymmetrical benzothiadiazoles 6a−6h at the B3LYP/6-31G** level for C, N, S, and H, and the
Lanl2DZ level for Fe. (a) Determined from the onset wavelength of the UV−vis absorption.
supports the electronic absorption behavior of BTDs 7a−7c and
BTDs 8a−8c.
The 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) π-bridge in 9a
and 9b lowers the band gap, which results in a bathochromic shift
of the electronic absorption spectra. The trend in the energy gap
follows the order 7a > 8a > 9a > 9b.
Electrochemical Properties. The electrochemical behavior
of the BTDs 6a−6h, 7a−7c, 8a−8c, 9a, and 9b were explored by
the cyclic voltammetric (CV) analysis in dry dichloromethane
(DCM) solution at room temperature using tetrabutylammo-
niumhexafluorophosphate (TBAPF6) as a supporting electrolyte.
The electrochemical data are listed in Table 2, and the
representative cyclic voltammograms are shown in Figure 6,
and Figure S2 (Supporting Information). In general, the
unsymmetrical ferrocenyl-substituted BTDs show one reversible
oxidation wave in the region of 0.04−0.42 V, corresponding to
the oxidation of ferrocene to the ferrocenium ion. The BTDs
6a−6h, 7a−7c, 8a−8c, 9a, and 9b exhibit one reversible
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Figure 5. HOMO and LUMO frontier orbitals of unsymmetrical benzothiadiazoles 7a−7c, 8a−8c, 9a, and 9b at the B3LYP/6-31G** level for C, N, S,
and H, and the Lanl2DZ level for Fe. (a) Determined from the onset wavelength of the UV−vis absorption.
reduction wave in the region of −1.61 to −2.15 V attributed to
the acceptor benzothiadiazole unit.7,8,12
The trend in the oxidation potential of the ferrocenyl moiety in
the BTDs 6a−6h follows the order 6h > 6c > 6b > 6d > 6a > 6g >
6e > 6f, whereas the BTDs 7a−7c, 8a−8c, 9a, and 9b follow the
order 9b > 9a > 7a > 7c > 7b > 8a > 8c > 8b. The ferrocenyl
moieties in the BTDs exhibit higher oxidation potentials compared
to free ferrocene (E° = 0.00 V, as recommended by IUPAC),
confirming the strong electronic communication between the
ferrocene unit and the BTD core.8,17 1,1,4,4-Tetracyanobuta-1,3-
diene (TCBD) is a strong electron-withdrawing group, which
results in harder oxidation of the ferrocene unit.18
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Table 2. Electrochemical Data of Unsymmetrical BTDs 6a−6h, 7a−7c, 8a−8c, 9a, and 9b
a
a
electrochemical data
electrochemical data
-
b
b
b
b
compound
Eox (V)
Ered (V)
compound
Eox (V)
0.00
Ered (V)
ferrocene
0.00
0.12
0.13
0.14
0.13
0.11
0.11
0.12
-
ferrocene
6a
6b
6c
6d
6e
6f
−1.69
−1.61
−1.62
−1.67
−1.66
−1.64
−1.66
−1.69
7a
7b
7c
8a
8b
8c
9a
9b
0.10, 0.60, 0.82
0.04, 0.58, 0.80
0.10, 0.59, 0.82
0.13, 0.54
−1.73
−1.69
−1.68
−1.82
−1.78
−1.76
0.02, 0.52
0.11, 0.51
6g
0.42, 0.62, 0.82
0.41, 0.58
−0.83, −1.19, −2.08
−0.85, −1.29, −2.15
6h
0.52, 0.95
a
b
Recorded by cyclic voltammetry, in 0.1 M solution of TBAPF6 in DCM at 100 mV s−1 scan rate, vs Fc/Fc+ at 25 °C. For the irreversible redox
process, the peak potential is quoted.
Figure 6. Cyclic voltammograms of unsymmetrical benzothiadiazoles 6a−6h, 7a−7c, 8a−8c, 9a, and 9b representing the reduction wave (i−iii), and the
oxidation wave (iv−vi) at 0.01 M concentration in 0.1 M TBAPF6 in dichloromethane recorded at a scan rate of 100 mV s−1.
In addition to the ferrocene oxidation wave, a quasi-
reversible oxidation wave due to the triphenylamine unit in
BTDs 6h, 8a−8c, and 9b is observed in the region of 0.51−
0.58 V.19 The BTDs 7a−7c and 9a exhibit two irreversible
oxidation waves in the region of 0.58−1.82 V attributed to the
carbazole unit.20
The reduction potential for the BTDs 6a−6h, 7a−7c, and 8a−
8c shows less negative values compared to 2,1,3-benzothiadiazole
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1 (−1.98 V vs Fc/Fc+ in DCM), indicating that the BTD rings in
these ferrocenyl-substituted compounds are easier to reduce than
unsubstituted BTD.7,8,21 The BTDs 9a and 9b exhibit two
reversible reduction waves in the region of −0.83 to −1.29 V
attributed to the successive one-electron reductions of the
dicyanovinyl (DCV) groups of the TCBD π-linker.22 The third
reductionwave in BTDs9a and 9b is assigned to the BTD unit and
was observed at −2.08 and −2.15 V, respectively.
X-ray Analysis. The single crystal of the unsymmetrical
BTDs 6c, 6g, 7a, and 7b were obtained via slow diffusion of
ethanol into the chloroform solution at room temperature. The
BTDs 6c and 7a crystallize in the monoclinic space group P21/c,
whereas the BTD 6g crystallizes in the monoclinic space group
The packing diagram of BTD 6c exhibits short S1···N2
(3.031(2) Å), and N2···N2 (2.969(5) Å) interheteroatom
contacts between the BTD rings, which leads to the formation
of dimers in head-to-head fashion.4a,8,23 The dimer units are
interlinked through C−H···π interaction C4H4···C21−C25
(3.315(2) Å) to form a 2D zigzag chain (Figure 8).
The packing diagram of BTD 6g exhibits short S1···N2
(3.124(5) Å) interheteroatom contacts between the BTD rings,
resulting in the formation of dimers in head-to-head fash-
ion.4a,8,23 The dimer units are interlinked through C−H···π
interaction C21H21···C30−C34 (3.011(3) Å) to form a 2D
zigzag chain (Figure 9).
The packing diagram of 7a shows π···π stacking interaction
C7−C12···C1−C6 (3.817(2) Å) between the carbazole and the
BTD units, which leads to the formation of a 1D polymeric chain.
These chains are interlinked in anti fashion via intermolecular
C−H···π interaction C15H15···C7−C12 (3.410(3) Å) to form a
2D sheetlike structure (Figure 10).
P2 /n. The BTD 7b crystallizes in the triclinic space group P1.
̅
1
Figure 7 shows the single-crystal X-ray structure of 6c, 6g, 7a, and 7b.
The packing diagram of BTD 7b exhibits short S2···N2
(3.263(5) Å) interheteroatom contacts between the BTD rings,
which leads to the formation of dimers in head-to-head
fashion.4a,8,23 These dimers show π···π stacking interaction
C21−C26···C21−C26 (3.500(2) Å) between two adjacent
carbazole units of the dimer. The dimer units are interlinked
through C−H···π interaction C32H32···C15−C20 (2.893(4) Å)
to form a 1D polymeric chain (Figure 11).
CONCLUSION
■
In summary, a series of aryl-substituted unsymmetrical
benzothiadiazole derivatives were designed and synthesized by
the Pd-catalyzed Sonogashira cross-coupling reaction. The
photophysical and electrochemical properties show strong
electronic communication. The enhancement of conjugation
via a π-bridge resulted in the red shift of the absorption bands in
BTDs 7a−7c, 8a−8c, 9a, and 9b. The incorporation of the
1,1,4,4-tetracyanobuta-1,3-diene group in benzothiadiazoles
results in enhanced thermal stability. Our results provide the
rationale to design low-band-gap materials for various optoelec-
tronic applications. The detailed nonlinear optical character-
izations of these ferrocenyl-substituted unsymmetrical BTDs are
currently ongoing in our laboratory.
Figure 7. Single-crystal X-ray structure of ferrocenyl BTDs 6c, 6g, 7a,
and 7b: (i) front view and (ii) side view. Solvent molecule (chloroform)
is omitted from 7a for clarity.
The BTD core shows a planar structure in the unsymmetrical
BTDs 6c, 6g, 7a, and 7b. The cyclopentadienyl rings of the
ferrocenyl moiety shows an eclipsed skew conformation in BTD
6c, and an eclipsed conformation in BTDs 6g, 7a, and 7b. The
dihedral angle between the planes containing the BTD core and
the cyclopentadienyl ring of ferrocene units was found to be
9.44° in 6c, 41.06° in 6g, 70.66° in 7a, and 24.35° in 7b. The
important bond lengths and bond angles are listed in Table S2
(see the Supporting Information for details).
EXPERIMENTAL DETAILS
■
Chemicals were used as received unless otherwise indicated. All oxygen-
or moisture-sensitive reactions were performed under a nitrogen/argon
Figure 8. Packing diagram of ferrocenyl BTD 6c forming a 2D zigzag chain along the a axis.
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Figure 9. Packing diagram of ferrocenyl BTD 6g forming a 2D zigzag chain along the a axis.
spectrometer. Chemical shifts are reported in delta (δ) units, expressed
in parts per million (ppm) downfield from tetramethylsilane using the
solvent as an internal standard (CDCl3, 77.0 ppm; (CD3)2CO, 29.8;
DMSO-d6, 39.5 ppm). The 1H NMR splitting patterns have been
described as “s, singlet; bs, broad singlet; d, doublet; t, triplet; dd,
doublet of doublets; dt, doublet of triplets; and m, multiplet”.
UV−visible absorption spectra of all compounds were recorded in
DCM. Cyclic voltamograms (CVs) were recorded on an electro-
chemical analyzer using glassy carbon as the working electrode, Pt wire
as the counter electrode, and a saturated calomel electrode (SCE) as the
reference electrode. The scan rate was 100 mV s−1 for CV. A solution of
tetrabutylammoniumhexafluorophosphate (TBAPF6) in CH2Cl2
(0.1 M) was employed as the supporting electrolyte. DCM was freshly
distilled from CaH2 prior to use. All potentials were experimentally
referenced against the saturated calomel electrode couple but were then
manipulated to be referenced against Fc/Fc+ as recommended by
IUPAC.24 Under our conditions, the Fc/Fc+ couple exhibited E° = 0.38 V
versus SCE. HRMS was recorded on a TOF-Q mass spectrometer. The
density functional theory (DFT) calculations were carried out at the
B3LYP/6-31G** level for C, N, S, and H, and the Lanl2DZ level for Fe in
the Gaussian 09 program.25
Figure 10. Packing diagram of ferrocenyl BTD 7a forming a 2D sheet
along the a axis.
General Procedure for the Preparation of BTDs 6a−6c by
Sonogashira Coupling Reaction. To a stirred solution of the
respective aryl alkyne (10a−10h) (0.37 mmol), and ferrocenyl bromo-
BTD 3a (0.34 mmol) in THF, and TEA (1:1, v/v) were added
PdCl2(PPh3)2 (10 mg, 0.014 mmol) and CuI (2 mg, 0.01 mmol) under
an argon flow at room temperature. The reaction mixture was stirred for
15−24 h at 60 °C, and then cooled to room temperature. The solvent
was then evaporated under reduced pressure, and the mixture was
purified by SiO2 chromatography with DCM/hexane (2:3, v/v),
atmosphere using standard Schlenk methods. Triethylamine (TEA) was
received from a commercial source and distilled on KOH prior to use.
1H NMR spectra were recorded using a 400 MHz spectrometer.
Chemical shifts are reported in delta (δ) units, expressed in parts per
million (ppm) downfield from tetramethylsilane using residual
protonated solvent as an internal standard (CDCl3, 7.26 ppm;
(CD3)2CO, 2.05). 13C NMR spectra were recorded using a 100 MHz
Figure 11. Packing diagram of ferrocenyl BTD 7b forming a 2D sheet along the b axis.
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Article
followed by recrystallization in chloroform:ethanol (1:1) to obtain 6a−
6h as colored solids.
146.5, 132.4, 131.6, 131.4, 129.0, 124.8, 123.4, 121.2, 116.9, 116.3, 114.6,
97.4, 97.0, 84.6, 81.5, 71.5, 69.9, 69.1, 63.9; HRMS (ESI-TOF) m/z
calcd for C38H25FeN3S 611.1114 [M]+, found 611.1115 [M]+; UV/vis
(DCM) λmax (ε [M−1 cm−1]) 460 (58729).
Compound 6a: Deep-red solid (73 mg, Yield: 48%): mp 200.0−
201.5 °C; 1H NMR (400 MHz, (CD3)2CO, δ in ppm) 7.89 (d, 1H, J =
7.3 Hz), 7.84 (d, 1H, J = 7.3 Hz), 4.65 (t, 2H, J = 1.8 Hz), 4.40 (t, 2H, J =
1.8 Hz), 4.32 (s, 5H); 13C NMR (100 MHz, DMSO-d6, δ in ppm) 153.7,
134.3, 132.8, 131.8, 131.5, 131.4, 128.7, 128.5, 121.7, 117.1, 115.2, 96.3,
85.6, 81.8, 71.4, 69.9, 69.5, 63.6; HRMS (ESI-TOF) m/z calcd for
C26H16FeN2S 444.0378 [M]+, found 444.0376 [M]+; UV/vis (DCM)
λmax (ε [M−1 cm−1]) 406 (32791), 509 (9324).
Compound 6b: Deep-red solid (76 mg, Yield: 50%): mp 225.5−
226.5 °C; 1H NMR (400 MHz, (CD3)2CO, δ in ppm) 7.97 (d, 1H, J =
7.5 Hz), 7.92−7.86 (m, 3H), 7.73(dt, 1H, J = 7.8 Hz, J = 1 Hz), 7.46−
7.42 (m, 1H), 4.65 (t, 2H, J = 2.0 Hz), 4.41 (t, 2H, J = 2.0 Hz), 4.32 (s,
5H); 13C NMR (100 MHz, DMSO-d6, δ in ppm) 150.4, 145.9, 139.0,
136.9, 132.1, 131.9, 127.6, 124.2, 111.1, 108.4, 106.2, 101.9, 98.0, 92.7,
82.3, 71.5, 70.0, 69.7, 63.3; HRMS (ESI-TOF) m/z calcd for
C25H15FeN3S 446.0409 [M+H]+, found 446.0407 [M + H]+; UV/vis
(DCM) λmax (ε [M−1 cm−1]) 399 (85328), 511 (22404).
Compound 6c: Deep-red solid (91 mg, Yield: 60%): mp > 300 °C;
1H NMR (400 MHz, CDCl3, δ in ppm) 8.90 (bs, 1H),8.62−8.60 (m,
1H), 7.95 (dt, 1H, J = 8 Hz; J = 1.8 Hz), 7.80 (d, 1H, J = 7.6 Hz), 7.75
(d, 1H, J = 7.3), 4.66 (t, 2H, J = 1.8 Hz), 4.34 (t, 2H, J = 2.0 Hz), 4.31
(s, 5H); 13C NMR (100 MHz, DMSO-d6, δ in ppm) 153.6, 138.6, 135.9,
133.4, 133.1, 132.0, 131.8, 117.6, 114.4, 113.7, 113.2, 108.0, 104.5, 100.8,
95.3, 71.4, 69.9, 63.2; HRMS (ESI-TOF) m/z calcd for C25H15FeN3S
446.0363 [M + H]+, found 446.0360 [M + H]+; UV/vis (DCM) λmax
(ε [M−1 cm−1]) 402 (85328), 515 (20963).
Compound 6d: Deep-red solid (124 mg, Yield: 70%): mp 191.5−
192.5 °C; 1H NMR (400 MHz, (CD3)2CO, δ in ppm) 7.92 (d; 1H; J =
7.5 Hz), 7.85 (d, 1H, J = 7.5 Hz), 7.81−7.73 (m, 6H), 7.52−7.48 (m,
2H), 7.42−7.39 (m, 1H), 4.65 (t, 2H, J = 2.0 Hz), 4.40 (t, 2H, J = 1.8
Hz), 4.32 (s, 5H); 13C NMR (100 MHz, CDCl3, δ in ppm) 154.25,
154.22, 141.5, 140.0,132.4, 132.2, 131.7, 128.7, 127.6, 126.9, 121.3,
117.7, 116.2, 97.8, 97.0, 86.0, 81.9, 72.8, 71.6, 70.9, 65.8; HRMS (ESI-TOF)
m/z calcd for C32H20FeN2S 520.0692 [M]+, found 520.0692 [M]+; UV/vis
(DCM) λmax (ε [M−1 cm−1]) 413 (55533), 512 (14750).
Compound 6e: Deep-red solid (117 mg, Yield: 70%): mp 199.0−
200.5 °C; 1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.76 (d, 1H, J =
8.3 Hz), 8.06−8.02 (m, 3H), 7.94−7.88 (m, 2H), 7.77−7.73 (m, 1H),
7.67−7.58 (m, 2H), 4.66 (t, 2H, J = 1.8 Hz), 4.41 (t, 2H, J = 1.8 Hz), 4.33
(s, 5H); 13C NMR (100 MHz, CDCl3, δ in ppm) 154.0, 153.7, 132.7,
132.6, 131.7, 131.2, 130.1, 129.0, 127.7, 126.6, 126.0, 125.7, 124.7, 119.6,
117.4, 115.7, 97.2, 94.7, 90.0, 81.4, 71.5, 69.9, 69.4, 63.9; HRMS (ESI-
TOF) m/z calcd for C30H18FeN2S 494.0535 [M]+, found 494.0533
[M]+; UV/vis (DCM) λmax (ε [M−1 cm−1]) 421 (54400), 521 (sh).
Compound 6f: Deep-red solid (120 mg, Yield: 65%): mp 192.5−
193.8 °C; 1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.99−8.96 (m,
2H), 8.72 (s, 1H), 8.21−8.17 (m, 2H), 7.94 (d, 1H, J = 7.5 Hz), 7.79−
7.75 (m, 2H), 7.66−7.62 (m, 2H), 4.68 (t, 2H, J = 1.8 Hz), 4.42 (t, 2H,
J = 1.8 Hz), 4.34 (s, 5H); 13C NMR (100 MHz, CDCl3, δ in ppm) 154.4,
154.2, 132.6, 131.7, 131.6, 130.9, 128.5, 127.3, 127.2, 126.8, 126.6, 125.6,
123.0, 117.6, 116.4, 116.3, 97.6, 96.8, 94.2, 81.9, 71.9, 70.3, 69.7, 63.8;
HRMS (ESI-TOF) m/z calcd for C34H20FeN2S 544.0692 [M]+, found
544.0694 [M]+; UV/vis (DCM) λmax (ε [M−1 cm−1]) 465 (67550).
Compound 6g: Deep-red solid (74 mg, Yield: 40%): mp 201.5−
202.4 °C; 1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.93−8.86 (m,
3H), 8.32 (s, 1H), 8.10−8.05 (m, 2H), 7.91 (d, 1H, J = 7.3), 7.88−7.76
(m, 3H), 7.74−7.71 (m, 1H), 4.67 (t, 2H, J = 2.0 Hz), 4.42 (t, 2H, J = 2.0
Hz), 4.34 (s, 5H); 13C NMR (100 MHz, CDCl3, δ in ppm) 154.3, 154.1,
140.0, 132.2, 132.1, 131.5, 130.7, 130.6, 130.2, 129.7, 128.4, 127.6, 127.1,
127.0, 126.8, 122.5, 122.3, 118.8, 117.7, 115.9, 97.6, 95.2, 89.8, 81.7,
71.9, 70.3, 69.6, 63.5; HRMS (ESI-TOF) m/z calcd for C34H20FeN2S
544.0692 [M]+, found 544.0708 [M]+; UV/vis (DCM) λmax (ε [M−1
cm−1]) 423 (58074), 514 (sh).
General Procedure for the Preparation of BTDs 7a−7c and
8a−8c by Sonogashira Coupling Reaction. To a stirred solution of
the respective alkynyl ferrocene (0.37 mmol), and bromo-BTDs 4a/5a
(0.34 mmol) in THF, and TEA (1:1, v/v) were added PdCl2(PPh3)2
(10 mg, 0.014 mmol) and CuI (2 mg, 0.01 mmol) under an argon flow at
room temperature. The reaction mixture was stirred for 12 h at 60 °C,
and then cooled to room temperature. The solvent was then evaporated
under reduced pressure, and the mixture was purified by SiO2
chromatography with DCM/hexane (2:3, v/v), followed by recrystal-
lization in chlorofom:ethanol (1:1) to obtain 7a−7c and 8a−8c as
colored solids.
Compound 7a: Red solid (121 mg, Yield: 70%): mp 185.5−186.5 °C;
1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.26−8.24 (m, 2H), 8.09(d,
1H, J = 7.5 Hz), 7.99 (d, 1H, J = 7.5 Hz), 7.41−7.37 (m, 2H), 7.33−7.28
(m, 4H), 4.69(t, 2H, J = 1.8 Hz), 4.42 (t, 2H, J = 2.0 Hz), 4.35 (s, 5H); 13C
NMR (100 MHz, CDCl3, δ in ppm) 155.9, 151.2, 140.9, 131.9, 129.4, 127.7,
125.9, 123.9, 120.6, 120.4, 117.5, 110.3, 97.2, 81.46, 72.7, 71.3, 70.6, 65.3;
HRMS (ESI-TOF) m/z calcd for C30H19FeN3S 509.0644 [M]+, found
509.0666 [M]+;UV/vis(DCM)λmax (ε[M−1 cm−1]) 443 (35138), 520 (sh).
Compound 7b: Orange solid (129 mg, Yield: 65%): mp 198.0−199.5 °C;
1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.27−8.24 (m, 2H), 8.18 (d,
1H, J = 7.5 Hz), 8.04 (d, 1H, J = 7.5 Hz), 7.70−7.68 (m, 2H), 7.64−7.62
(m, 2H), 7.42−7.37 (m, 2H), 7.34−7.30 (m, 4H), 4.86
(t, 2H, J = 1.8 Hz), 4.42 (t, 2H, J = 1.8 Hz), 4.07 (s, 5H); 13C NMR
(100 MHz, DMSO-d6, δ in ppm) 155.4, 150.7, 141.1, 140.5, 133.1,
131.7, 129.2, 128.2, 126.1, 123.1, 120.5, 120.4, 118.7, 115.8, 110.8, 96.6,
85.6, 83.3, 69.6, 69.64, 66.6; HRMS (ESI-TOF) m/z calcd for
C36H23FeN3S 585.0957 [M]+, found 585.0952 [M]+; UV/vis (DCM)
λmax (ε [M−1 cm−1]) 453 (47855).
Compound 7c: Orangesolid(124mg, Yield:60%):mp220.5−221.2 °C;
1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.27−8.26 (m, 2H), 8.20 (d,
2H, J = 7.5 Hz), 8.06 (d, 2H, J = 7.5 Hz), 7.73−7.71 (m, 2H), 7.61−7.59
(m, 2H), 7.42−7.38 (m, 2H), 7.34−7.30 (m, 2H), 4.56
(t, 2H, J = 1.8 Hz), 4.34 (t, 2H, J = 2.0 Hz), 4.28 (s, 5H); 13C NMR
(100 MHz, DMSO-d6, δ in ppm) 155.2, 150.5, 140.3, 133.1, 131.6,
131.2, 129.4, 127.9, 125.9, 124.0, 123.0, 120.9, 120.3, 120.1, 115.3, 110.5,
95.5, 91.4, 86.9, 84.9, 71.1, 69.7, 69.0, 63.8; HRMS (ESI-TOF) m/z
calcd for C38H23FeN3S 609.0957 [M]+, found 609.0956 [M]+; UV/vis
(DCM) λmax (ε [M−1 cm−1]) 454 (62777).
Compound 8a: Red solid (120 mg, Yield: 68%): mp 180.5−181.6 °C;
1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.03−8.01 (m, 2H), 7.89
(d, 1H, J = 7.5 Hz), 7.84 (d, 1H, J = 7.5 Hz), 7.38−7.34 (m, 4H), 7.18−
7.10 (m, 8H), 4.63 (t, 2H, J = 1.8 Hz), 4.38 (t, 2H, J = 1.8 Hz), 4.32 (s,
5H); 13C NMR (100 MHz, CDCl3, δ in ppm) 155.3, 153.1, 148.3, 147.2,
133.2, 132.4, 130.3, 129.8, 129.3, 126.7, 124.9, 123.3, 122.6, 115.7, 95.5,
81.9, 71.9, 70.3, 69.4, 64.8; HRMS (ESI-TOF) m/z calcd for
C36H25FeN3S 587.1114 [M]+, found 587.1114 [M]+; UV/vis (DCM)
λmax (ε [M−1 cm−1]) 458 (37842).
Compound 8b: Orange solid (140 mg, Yield: 62%): mp 202.0−203.5 °C;
1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.04 (d, 2H, J = 9.0 Hz), 7.98
(d, 1H, J = 7.5 Hz), 7.89 (d, 1H, J = 7.5 Hz), 7.65 (d, 2H, J = 8.5 Hz), 7.58
(d, 2H, J = 8.0 Hz), 7.38−7.34 (m, 4H), 7.18−7.10 (m, 8H), 4.84 (t, 2H,
J = 2.0 Hz), 4.41 (t, 2H, J = 2.0 Hz), 4.06 (s, 5H); 13C NMR (100 MHz,
CDCl3, δ in ppm) 155.4, 153.1, 148.3, 147.3, 140.5, 133.9, 132.9, 131.8,
130.2, 130.0, 129.3, 126.6, 125.9, 125.0, 123.4, 122.6, 119.9, 115.2, 96.2,
85.8, 70.6, 70.2, 67.1; HRMS (ESI-TOF) m/z calcd for C42H29FeN3S
663.1427 [M]+, found 663.1426 [M]+; UV/vis (DCM) λmax
(ε [M−1 cm−1]) 466 (51474).
Compound 8c: Orangesolid(152mg, Yield:65%):mp189.5−190.5 °C;
1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.04 (d, 2H, J = 9 Hz), 8.00
(d, 1H, J = 7.28 Hz), 7.90 (d, 1H, J = 7.28 Hz), 7.67 (d, 2H, J = 8.80 Hz), 7.57
(d, 2H, J = 8.52 Hz), 7.39−7.35 (m, 4H), 7.18−7.11(m, 8H), 4.55 (t, 2H, J =
1.8 Hz), 4.33 (t, 2H, J = 2.0 Hz), 4.27 (s, 5H); 13C NMR (100 MHz, CDCl3,
δ in ppm) 154.9, 152.5, 147.9, 146.7, 145.3, 141.8, 133.8, 132.8, 131.3, 130.7,
129.5, 128.9, 126.0, 124.4, 123.0, 121.9, 114.0, 109.7, 96.3, 94.3, 93.2, 87.1,
Compound 6h: Deep-red solid (114 mg, Yield: 55%): mp 182.0−
1
183.0 °C; H NMR (400 MHz, (CD3)2CO, δ in ppm) 7.85−7.80
(m, 2H), 7.55−7.51 (m, 2H), 7.39−7.35 (m, 2H), 7.17−7.13 (m, 2H),
7.03−7.00 (m, 2H), 4.64 (t, 2H, J = 1.8 Hz), 4.39 (t, 2H, J = 2.0 Hz), 4.32
(s, 5H); 13C NMR (100 MHz, CDCl3, δ in ppm) 154.0, 153.9, 148.1,
12450
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73.4, 70.8, 69.4, 64.9; HRMS (ESI-TOF) m/z calcd for C44H29FeN3S
687.1427 [M]+, found 687.1426 [M]+; UV/vis (DCM) λmax (ε [M−1 cm−1])
467 (84788).
Ishi-i, T.; Mataka, S. Chem.Eur. J. 2006, 12, 2303−2317. (i) Kobayashi,
N.; Inagaki, S.; Nemykin, V. N.; Nonomura, T. Angew. Chem., Int. Ed.
2001, 40, 2710−2712. (j) Lindner, B. D.; Engelhart, J. U.; Marken, M.;
̈
General Procedure for the Preparation of BTDs 9a and 9b.
TCNE (77 mg, 0.60 mmol) was added to a solution of 7a/8a
(0.30 mmol) in DCM (60 mL) at room temperature. The mixture was
refluxed at 40 °C for 15 h. The solvent was removed in vacuo, and the
product was purified by SiO2 chromatography with DCM as the eluent
to yield 9a/9b as a dark colored solid.
Compound 9a: Purple solid (134 mg, Yield: 70%): mp 279.5−280.5 °C;
1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.76 (d, 1H, J = 7.5 Hz),
8.31 (d, 2H, J = 7.3 Hz), 7.40−7.29 (m, 4H), 7.24−7.12 (m, 2H), 5.46−
5.45 (m, 1H), 5.07−5.05 (m, 1H), 5.01−5.00 (m, 1H), 4.92−4.91 (m,
1H), 4.39 (s, 5H); 13C NMR (100 MHz, CDCl3, δ in ppm) 171.0, 160.9,
152.1, 150.5, 140.0, 135.2, 131.9, 126.0, 125.5, 124.3, 124.2, 122.8, 121.3,
120.2, 113.8, 111.4, 110.7, 110.5, 90.5, 79.3, 75.9, 74.8, 74.2, 73.0, 72.4,
70.7, 65.5; HRMS (ESI-TOF) m/z calcd for C36H19FeN7S 637.0767
[M]+, found 637.0774 [M]+; UV/vis (DCM) λmax (ε [M−1 cm−1]) 502
(46178).
Compound 9b: Deep-green solid (172 mg, Yield: 80%): mp 222.5−
223.5 °C; 1H NMR (400 MHz, (CD3)2CO, δ in ppm) 8.52 (d, 1H, J =
7.8 Hz), 8.10−8.07 (m, 3H), 7.40−7.36 (m, 4H), 7.19−7.10 (m, 8H),
5.40−5.38 (m, 1H), 5.00−4.99 (m, 1H), 4.89−4.87 (m, 1H), 4.85−4.83
(m, 1H), 4.35 (s, 5H); 13C NMR (100 MHz, CDCl3, δ in ppm) 171.3,
164.8, 161.5, 152.9, 151.4, 149.3, 146.4, 139.3, 132.0, 130.3, 129.2, 127.8,
125.3, 125.2, 125.1, 123.9, 121.8, 121.1, 111.7, 101.3, 89.0, 82.1, 79.2,
75.5, 74.5, 74.2, 72.7, 72.1, 70.6, 64.6; HRMS (ESI-TOF) m/z calcd for
C42H25FeN7S 715.1237 [M]+, found 715.1240 [M]+; UV/vis (DCM)
λmax (ε [M−1 cm−1]) 554 (61482).
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ASSOCIATED CONTENT
* Supporting Information
■
S
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2012, 2, 12105−12107. (b) Dhokale, B.; Gautam, P.; Mobin, S. M.;
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Characterization data for all the new compounds; copies of 1H,
13C NMR, and HRMS spectra of new compounds; crystallo-
graphic information files (CIFs) for compounds 6c (CCDC:
959150), 6g (CCDC: 959151), 7a (CCDC: 959148), and 7b
(CCDC: 959149); the DFT calculation data of all the new
compounds; and the electrochemical data of compounds 9a and
9b. This material is available free of charge via the Internet at
http://pubs.acs.org.
(8) Misra, R.; Gautam, P.; Jadhav, T.; Mobin, S. M. J. Org. Chem. 2013,
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AUTHOR INFORMATION
Corresponding Author
*E-mail: rajneeshmisra@iiti.ac.in.
■
(11) Velusamy, M.; Justin Thomas, K. R.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-
C. Org. Lett. 2005, 7, 1899−1902.
Notes
(12) Chen, S.; Li, Y.; Liu, C.; Yang, W.; Li, Y. Eur. J. Org. Chem. 2011,
32, 6445−6451.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
R.M. thanks CSIR, and DST, New Delhi, for financial support.
We are grateful to the Sophisticated Instrumentation Centre
(SIC) Single Crystal X-Ray diffraction Facility, IIT Indore.
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