Synthesis and properties of functionalized oligo(arylene) molecular wires with thiolated termini: Competing thiol-au and nitro-au assembly

Article abstract of DOI:10.1021/jo902205p

(Chemical Presented) We report the synthesis by Suzuki cross-coupling methodology of oligo(arylene) molecular wires with protected thiolates at both termini and a central electron-acceptor unit (3,5-dinitrofluorenone, compounds 10-12) or an electron-donor unit [9-(1,3-dithiol-2-ylidene)fluorene, compounds 14-17] in the backbone. Core reagents are 2,7-dibromo-3,5-dinitrofluorenone 7 (obtained by nitration of 2,7-dibromofluorenone) and 2,7-dibromo-9-(4,5- dimethyl-1,3-dithiol-2-ylidene)fluorene 13. The solution electrochemical redox properties of these oligo(arylene) derivatives have been studied. The reduction CVs of the dinitrofluorenone-containing molecules display three reversible/quasireversible couples yielding, sequentially, radical anion, dianion, and radical trianion species, e.g., for 11 E^1red -1.02 V (vs. Ag/Ag⁺ in THF). The 1,3-dithiol-2-ylidene unit imparts good electron donor properties tomolecules 14, 15, and 16 with radical cation formation observed at E^ox ca. 0.65 V (vs. Ag/Ag⁺ in DCM). We also report studies on the assembly of 11 and 15 on gold substrates. Current-voltage (I-V) characteristics and X-ray photoelectron spectra of the monolayers reveal that 11 assembles via competing S-Au and NO₂-Au interactions. This unusual phenomenon is ascribed to the very electron deficient dinitrofluorenone core of 11 weakening the S-Au interaction. An important conclusion is that thiolated molecules which possess strongly electron-withdrawing core units, especially those containing nitro groups, may not be suitable for controlled assembly in junctions. In contrast, 15 assembles via conventional S-Au interactions.

Full text of DOI:10.1021/jo902205p

pubs.acs.org/joc
Synthesis and Properties of Functionalized Oligo(arylene) Molecular
Wires with Thiolated Termini: Competing Thiol-Au and
Nitro-Au Assembly
Xianshun Zeng,† Changsheng Wang,† Andrei S. Batsanov,† Martin R. Bryce,*,†
Joanna Gigon,‡ Barbara Urasinska-Wojcik,‡ and Geoffrey J. Ashwell‡
†Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom and
‡The Nanomaterials Group, College of Physical and Applied Sciences, Bangor University, Bangor,
Gwynedd LL57 2UW, United Kingdom
m.r.bryce@durham.ac.uk
Received October 14, 2009
We report the synthesis by Suzuki cross-coupling methodology of oligo(arylene) molecular wires with
protected thiolates at both termini and a central electron-acceptor unit (3,5-dinitrofluorenone, com-
pounds 10-12) or an electron-donor unit [9-(1,3-dithiol-2-ylidene)fluorene, compounds 14-17] in the
backbone. Core reagents are 2,7-dibromo-3,5-dinitrofluorenone 7 (obtained by nitration of 2,7-
dibromofluorenone) and 2,7-dibromo-9-(4,5-dimethyl-1,3-dithiol-2-ylidene)fluorene 13. The solution
electrochemical redox properties of these oligo(arylene) derivatives have been studied. The reduction CVs
of the dinitrofluorenone-containing molecules display three reversible/quasireversible couples yielding,
sequentially, radical anion, dianion, and radical trianion species, e.g., for 11 E1red -1.02 V (vs. Ag/Agþ
in THF). The 1,3-dithiol-2-ylidene unit imparts good electron donor properties to molecules 14, 15, and 16
with radical cation formation observed at Eox ca. 0.65 V (vs. Ag/Agþ in DCM). We also report studies on
the assembly of 11 and 15 on gold substrates. Current-voltage (I-V) characteristics and X-ray
photoelectron spectra of the monolayers reveal that 11 assembles via competing S-Au and NO2-Au
interactions. This unusual phenomenon is ascribed to the very electron deficient dinitrofluorenone core of
11 weakening the S-Au interaction. An important conclusion is that thiolated molecules which possess
strongly electron-withdrawing core units, especially those containing nitro groups, may not be suitable
for controlled assembly in junctions. In contrast, 15 assembles via conventional S-Au interactions.
The synthesis and electronic behavior of organic conjugated
molecules is a rapidly expanding field of study. A long-term aim
is to utilize an individual molecule, or small assemblies of
molecules, as active components in molecular electronic devices1
to overcome the impending miniaturization threshold of silicon
devices.2 Currently, the most promising way to interconnect an
organic molecule into electronic circuitry is by thiol-gold
bonds.3 For this purpose the organic molecule is functionalized
with terminal thiol groups, or protected thiols which hydrolyze
during the assembly protocol to liberate thiol functionalities.4
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Published on Web 12/04/2009
DOI: 10.1021/jo902205p
r
2009 American Chemical Society

Zeng et al.
JOCArticle
Alternative terminal groups such as amines5 and isocyanides6
are also promising candidates. For electrical characterization of
the resulting hybrid nanostructures, scanning probe microscopy
(SPM), mechanically controllable break-junction (MCBJ), and
electromigration techniques have been developed.7
To test the scope and limitations of these contacting
techniques and to probe basic structure-property relation-
ships in molecular electronics, there is a requirement for new
thiol-terminated organic molecules, e.g., molecules which
possess tailored conformations, conjugation lengths, and/or
tunable carrier transport properties at nanometer length
scales. In this regard, the incorporation of redox-active
groups into molecular wires can impart a basic electrical
function to the system. However, integrating such mole-
cules, with their added structural complexity, into electrode
|molecule|electrode architectures can be experimentally very
demanding, especially at the single-molecule level. None-
theless, for a few systems this has been achieved.8
The rationale behind the present work is to study new
oligo(arylene)s with thiolated groups at both termini de-
signed to bridge a gap between two gold surfaces and thereby
create an electrode|molecule|electrode junction. Redox-
active units are incorporated into the backbone. Self-
assembled molecules which can be reduced or oxidized
are expected to play a key role in molecular electronics.
A major goal is to modulate conductance by an electro-
chemical or chemical process involving the molecules
within a molecular junction, thereby providing molecular
switches or sensors at the level of single molecules or small
ensembles.8 Oligo(arylenes)s are an important family of
compounds within the field of molecular electronics. For
example, 1,4-benzenedithiol continues to be a benchmark
compound for single-molecule devices9 and recent experi-
mental and theoretical studies have focused on 4,40-biphe-
nyldithiol and its derivatives.5,10 Thiol-terminated oligo-
(thiophenes)11 and benzene-furan co-oligoaryls with up to
nine aryl rings and terminal -C6H4-CH2-S units have been
synthesized recently.12 In this article we describe synthetic
protocols for new thiol-terminated oligo(arylene) wires with
redox-active building blocks in the backbone. The synthesis
of compounds 10-12 and 14-17 and the solution electro-
chemical properties of 11, 12, and 14-16 are presented.
The key molecular subunits are the following: (i) for
10-12, 3,5-dinitrofluorenone, which has enhanced elec-
tron-acceptor properties (compared to fluorenone),13,14
and (ii) for 14-17, 9-(1,3-dithiol-2-ylidene)fluorene, which
has good electron-donor properties.15 The assembly of 11
and 15 on gold substrates is described. The monolayers are
characterized by XPS data and scanning tunneling micro-
scopy (STM) with a gold probe to create Au|molecule|Au
junctions.
Results and Discussion
Synthesis. For the synthesis of thiol-terminated oligo-
(arylenes) we required the boronic acid derivatives 2 and 5,
whose syntheses are shown in Scheme 1. 4-Bromo(tert-butyl-
sulfanyl)benzene 1 was converted into the corresponding
boronic acid derivative 2 by lithium-halogen exchange,
followed by addition of trimethylborate and an aqueous acid
workup. Reaction of 2,5-bis(2-ethylhexyloxy)-1,4-dibromo-
benzene 316 with 1 equiv of 2 under standard Suzuki-
Miyaura cross-coupling conditions17 gave the bromobiphe-
nyl derivative 4 in 85% yield, which was converted to the
biphenyl boronic acid derivative 5 in high yield, by analogy
with the synthesis of 2. The alkoxy substituents in 5 were
present to ensure good solubility of the subsequent oligo-
(arylene) products.
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SCHEME 1. Synthesis of Reagents 2 and 5
SCHEME 2. Synthesis of 11 and 12
acid gave a dinitro product in 61% yield, which had only
limited solubility. Initially its structure could not be assigned
unambiguously from NMR data. Reaction of the product
with reagent 814b (2.2 equiv) under Sonogashira conditions19
gave an interesting result. A monosubstituted derivative
was obtained as the major product (62% yield). X-ray crys-
tal analysis revealed structure 9 (see the Supporting
Information), which, in turn, established that the 3,5-dinitro
isomer 7 had been produced in the nitration of 6. Nitration at
the 3,5-positions is unusual and contrasts with the nitra-
tion of 2,7-diX-fluorenone (X = NO2, CO2H), which yields
initially 2,4,7 (mononitration)- and then 2,4,5,7 (dinitration)-
substitution patterns.13 The formation of compound 7, in
preference to the 4,5-isomer, can be explained as follows. The
first nitration occurs at C(4) of compound 6 as with other
fluorenone derivatives (i.e., the meta directing effect of the
carbonyl group plus the ortho directing effect of the other
phenyl ring dominate over the ortho/para directing effect of
bromine). Electronic and steric factors then combine to favor
the second nitration ortho to the other bromine atom to give
product 7. Steric hindrance is thereby avoided between the two
nitro groups. The increased reactivity of the 2-bromo substitu-
ent of 7 (compared to the 7-bromo substituent) is due to an
enhanced activating effect of the o-nitro substituent at C(3).20
In contrast to the Sonogashira reaction, both the 2- and 7-
bromo substituents of 7 were readily replaced upon Suzuki-
Miyaura reaction with 2 or 5 (2.0-2.1 equivs) to afford the
disubstituted oligo(arylene) derivatives 10 and 12, respec-
tively. The t-Bu group is a useful protecting group for
thiols because it increases solubility and t-Bu-S-Ar species
are resistant to both strongly basic and acidic conditions.21
(20) Attempts to replace both the 2- and 7-bromo substituents in 7 by
using an increased molar ratio of 8 and triethylamine and/or elevating the
reaction temperature led to an inseparable mixture of the 2,7-disub-
stituted product and the butadiyne derivative obtained by oxidative self-
coupling of 8.
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J. Org. Chem. 2003, 68, 1275–1282. (b) Stuhr-Hansen, N.; Sorensen, J. K.;
Moth-Poulsen, K.; Christensen, J. B.; Bjornholm, T.; Nielsen, M. B. Tetra-
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SCHEME 3. Synthesis of 15 and 17
Conversion of the t-BuS groups of 10 into the more labile
acetylthio groups was achieved by a bromine-catalyzed depro-
tection protocol.22 We have found that 2-(ethyl)hexyloxy side
chains are cleaved under these conditions (see Scheme 3), so the
comparable reaction was not attempted for 12.
To incorporate an electron donor unit into the backbones,
the 9-(1,3-dithiol-2-ylidene)fluorene reagent 1315 was treated
with compounds 2 and 5, as above, to give the oligo(arylene)s
14 and 16, respectively. Compound 14 was cleanly converted
into the bis(acetylthio) analogue 15. The comparable reac-
tion of 16 resulted in concomitant removal of the 2-(ethyl)-
hexyl chains to give the bis(acetylthio) derivative 17 (Scheme 3).
Nonetheless, compounds 12 and 16 may be useful for molecular
electronics applications as t-BuS-terminated wires can be phy-
sisorbed onto electrodes by a weak van der Waals contact.23 The
X-ray crystal structure of 16 is reported in the Supporting
Information.
FIGURE 1. Cyclic voltammograms of compounds 11 and 12 in
THF solution containing Bu4NPF6 (0.1 M) as supporting electrolyte,
scan rate 100 mV s-1. The peak potentials are shown for the redox
waves.
Solution Electrochemical Properties. Cyclic votammetry
(CV) studies have probed the solution redox properties of the
oligo(arylene) wire molecules. The reduction CVs of the
dinitrofluorenone-containing molecules 11 and 12, shown
in Figure 1, display three reversible/quasireversible couples
yielding, sequentially, radical anion, dianion, and radical
trianion species (peak potentials are given in Figure 1). The
electron-donating dialkoxyphenyl rings in 12 lead to a
negative shift (i.e., reduced electron affinity) for all three
waves compared with 11.
The 1,3-dithiol-2-ylidene unit imparts good electron do-
nor properties to molecules 14, 15, and 16 with radical cation
formation observed at Eox ca. 0.65 V (vs. Ag/Agþ). The CVs
are shown in Figure 2. No oxidative features were observed
for 12 (scanning to þ2.0 V); no reductive features were
observed for 16 (scanning to -2.0 V).
of 11 and 15 and performed STM experiments on the resulting
structures. Gold-coated substrates were plasma cleaned prior
to use and then immersed in dilute THF solutions of the
molecules (0.1 mg cm-3, 20 cm3) and in solutions to which
ammonium hydroxide solution (0.1 cm3) was added to facili-
tate cleavage of the acetyl groups. They were repeatedly
immersed for 20 min intervals and washed with THF to
remove physisorbed material. The process was monitored
from the frequency change following assembly on the gold
electrodes of 10 MHz quartz crystals and the chemisorption
was verified by X-ray photoelectron spectroscopy (XPS).
Self-assembly of 15 occurs in the absence of the deprotect-
ing agent and a Sauerbrey analysis24 of the frequency data
yielded a limiting area of 0.72-0.76 nm2 molecule-1 after ca.
600 min compared with 200 min when self-assembled in the
presence of NH4OH. X-ray photoelectron spectra (XPS)
confirm chemisorption and show an S 2p peak at 162 eV
characteristic of the Au-S link and another at 164 eV that
relates to the 1,3-dithiol-2-ylidene unit and unbound term-
inal group.
Self-Assembly, XPS, and Electrical Characterization. We
have investigated self-assembled monolayer (SAM) formation
(22) Blaszczyk, A.; Elbing, M.; Mayor, M. Org. Biomol. Chem. 2004, 2,
2722–2724.
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(23) Kubatkin, S.; Danilov, A.; Hjort, M.; Cornil, J.; Bredas, J.-L.; Stuhr-
Hansen, N.; Hedegard, P.; Bjornholm, T. Nature 2003, 425, 698–701.
(24) Sauerbrey, G. Z. Phys. 1959, 155, 206–222.
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FIGURE 2. Cyclic voltammograms of compounds 14, 15, and 16 in
dichloromethane solution containing Bu4NPF6 (0.1 M) as support-
ing electrolyte, scan rate 100 mV s-1
.
In contrast, 11 exhibits a limiting area of 1.0-1.1 nm2
molecule-1 after ca. 150 min when assembled both with and
without the deprotecting agent. The larger area of this less
bulky analogue may be explained by an alternative type of
packing that involves assembly of some of the molecules of
11 via one, or both, of the nitro substituents. This explana-
tion is consistent with a report that electron-withdrawing
substituents on aromatic thiols decrease the rate of conven-
tional adsorption25 and thereby weaken the interaction
between sulfur and gold.26 XPS data obtained for 11 also
suggest that the gold-thiolate link is not necessarily the
dominant adsorption in the SAM. The sulfur core-level
spectra, fitted with a Gaussian-Lorentzian function, exhibit
the characteristic Au-S doublet for an assembled aromatic
thiol27 at 161.9 (S 2p3/2) and 163.1 eV (S 2p1/2), as well as a
second, considerably stronger doublet at 163.4 and 164.6 eV
(cf. 163.7 and 164.9 eV for a powdered sample of 11) that
corresponds to the unbound sulfur-containing groups.
Quantitative analysis of the areas under the curves suggests
that only ca. 40% of molecules assemble via an Au-S link
(thus, ca. 60% via NO2) in the absence of a deprotecting
agent (Figure 3). The relative areas for SAMs formed under
basic conditions suggest that 40-65% of molecules assemble
via Au-S links with the remainder attached via their nitro
substituents (Figure 4).28
SAMs of 11 formed in the presence and absence of the
deprotecting agent exhibit similar N 1s core-level spectra
(Figure 5). A single peak at 405.6 eV is characteristic of the
NO2 groups and the ratio of areas under the N 1s and S 2p
peaks, when corrected for atomic sensitivity factors, is ca. 1:1
and consistent with the molecular formula. The XPS data
provide no evidence of the chemical instability reported by
Stapleton et al.29 These authors reported that nitro-substi-
tuted oligo(phenyleneethynylene) thiolate molecules as-
sembled in the presence of NH4OH reveal the expected
NO2 peak at 405.6 eV as well as others at 402.1 and 399.3 eV
FIGURE 3. S 2p3/2 and S 2p1/2 spectra of a SAM of 11 prepared
without the deprotecting agent where the red doublet corresponds
to the Au-S link and the blue doublet to S-C(O)CH3. The area
ratio, corrected for atomic sensitivity factors, is ca. 1:4, whereas if
self-assembled only via Au-S, this ratio should be 1:1.
FIGURE 4. Diagrammatic representation of the assembly of 11 on
a gold substrate showing competing nitro and thiol group attach-
ment.
whichwereassignedtoNH(OH) andNH2 arising from thiolate-
nitro redox processes. We observed a broadened N 1s peak in
the same range but only from some monolayer samples. More-
over, its occurrence each time coincided with a higher than
expected N:S ratio (as high as 2:1) whereas the ratio calculated
with the area of the NO2 peak at 405.6 eV was 1:1. Thus, from
this study, we attribute the anomalous lower energy peak, when
observed (see the Supporting Information, Figure S3), to a
contaminant derived from NH4OH and note that its range is
consistent with the binding energies of NH3 (399 eV)30 and
NH4þ (399-402 eV).31-33
Current-voltage (I-V) curves of self-assembled mono-
layers (SAMs) formed from 11 and 15 on gold-coated highly
oriented pyrolytic graphite were obtained by scanning tun-
neling spectroscopy. The gold probe was landed at several
locations across each of the SAMs: I-V data were recorded
for different set-point currents and voltages and averaged
each time for ten scans at each of the sites. Typical I-V
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occurred only after the thiol unit had initially assembled on the bottom
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Minachev, K. M. J. Electron Spectrosc. Relat. Phemon. 1986, 40, 187–192.
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134 J. Org. Chem. Vol. 75, No. 1, 2010

Zeng et al.
JOCArticle
FIGURE 7. I-V characteristics of SAMs of 15 formed under basic
conditions on gold contacted by a gold probe. The data were
obtained by averaging 10 scans on the same site for a set point
current of 200 pA and bias of þ0.2 V where the polarity is defined by
the sign of the substrate electrode.
FIGURE 5. N 1s spectrumofa SAM of 11 wherethepeakat405.6eV
is characteristic of NO2.
Conclusions
New oligo(arylene) molecular wires endowed with func-
tional groups at their cores and thiolated anchor groups at
both terminal positions have been obtained via efficient
synthetic transformations. 4-(tert-Butylsulfanyl)phenylboronic
acid 2, 2,7-dibromo-3,5-dinitrofluorenone 7, and 2,7-dibromo-
9-(1,3-dithiol-2-ylidene)fluorene 13 serve as key reagents in
these procedures. The solution redox properties of the oligo-
(arylene)s have been studied by cyclic voltammetry, which
reveals reduction waves for the dinitrofluorenone compounds
11 and 12, and oxidation processes for the 9-(1,3-dithiol-
2-ylidene)fluorene derivatives 15 and 16. Compounds 11 and
15 form self-assembled monolayers on gold substrates and their
current-voltage (I-V) characteristics have been probed by a
gold STM tip. Electrical studies and XPS data reveal very
different modes of assembly of 11 and 15. Evidence has been
presented that 11 forms a disordered monolayer due to assembly
via competing S-Au and NO2-Au interactions as a conse-
quence of the very electron deficient dinitrofluorenone core,
which is in conjugation with the thiolate group, thereby weak-
ening the S-Au interaction. This is consistent with previous
evidence that electron-withdrawing substituents decrease the
rate of conventional S-Au adsorption.25 In contrast, 15 assem-
bles via conventional S-Au interactions. One of the goals of
molecular electronics is to understand the factors that govern
the assembly of specifically functionalized molecules. The work
described in this paper is informative in probing functional
group compatibility in electroactive molecular wires and mole-
cular electronic circuitry. An important conclusion is that
thiolated molecules which possess strongly electron-withdraw-
ing core units, especially those containing nitro groups in
conjugation with the thiolate group, may not be suitable for
controlled assembly in junctions.
FIGURE 6. I-V characteristics of SAMs of 11 formed under basic
conditions on gold contacted by a gold probe. The data were
obtained by averaging 10 scans on the same site for a set point
current of 200 pA and bias of þ0.2 V where the polarity is defined by
the sign of the substrate electrode.
characteristics are shown in Figures 6 (11) and 7 (15) and, for
the most part, their profiles are unaffected by the set point
conditions and the method of assembly (i.e., with or without
the deprotecting reagent). Both systems exhibit slight elec-
trical asymmetry. This is more pronounced for 11 where the
higher negative-quadrant current corresponds to electron
flow from the gold substrate (cathode) to gold probe (anode)
and this behavior can be explained by some of the molecules
being aligned parallel to the substrate and the electron-
accepting nitro groups of the central core being adsorbed
to gold. The considerably weaker asymmetry exhibited by 15
with the same preferred direction of electron flow can be
explained by an upright alignment with all molecules as-
sembled via Au-S links and with the electron-donating 1,3-
dithiol-2-ylidene unit in this case being tilted toward the gold
probe. We further note that an analogue without the redox-
active dithiole unit (namely, a 9,9-dimethylfluorene core)10e
exhibits symmetrical I-V characteristics.
Experimental Section
4-(tert-Butylsulfanyl)phenylboronic Acid (2). A solution of
4-tert-(butylsulfanyl)bromobenzene (12.26 g, 50 mmol) in THF
(200 mL) was cooled to -78 °C and n-BuLi (2.5 M in hexane,
25 mL, 62.5 mmol) was added. The solution was stirred for 5 h at
-78 °C and then trimethylborate (20 mL, excess) was added slowly.
The mixture was stirred for 30 min at -78 °C and then slowly
warmed to room temperature and stirred for 12 h at room
temperature. The mixture was concentrated in vacuo and the
These data establish that 11 forms a disordered assembly
on gold, which means the compound is unlikely to find
an application in molecular electronics. However, 15 forms
an ordered monolayer and functions as a molecular wire
in an electrode|molecule|electrode junction.
J. Org. Chem. Vol. 75, No. 1, 2010 135

Zeng et al.
JOCArticle
residue was dissolved in dichloromethane (50 mL) and stirred with
HCl (5% aq, 200 mL) for 2 h. The organic phase was separated and
the aqueous phase was extracted with dichloromethane. The
combined organic phases were dried over MgSO4. The solvent
was removed under reduced pressure. The residue was triturated
with hexane (30 mL) and the solid product was filtered, washed
with hexane, and dried to give 2 as a white powder (8.26 g, 78%
ES-MS 598.3 (Mþ); 1H NMR (400 MHz, CDCl3) δ 8.53 (s, 1H),
8.45 (d, J = 2.0 Hz, 1H), 8.26 (d, J = 2.0 Hz, 1H), 7.88 (s, 1H),
7.68 (d, J = 8.0 Hz, 2H), 7.64 (d, J = 8.0 Hz, 2H), 7.61 (d, J =
8.0 Hz, 2H), 7.33 (d, J=8.0 Hz, 2H), 1.34 (s, 9H), 1.33 (s, 9H); 13
C
NMR (100 MHz, CDCl3) δ 189.0, 153.5, 145.0, 144.1, 140.0, 138.2,
138.0, 137.8, 137.1, 136.8, 136.3, 135.7, 135.3, 134.8, 133.7, 128.7,
127.6, 127.5, 127.1, 126.8, 122.0, 46.7, 46.6, 31.1, 30.9. Anal. Calcd
for C33H30N2O5S2: C, 66.20; H, 5.05; N, 4.68. Found: C, 66.18; H,
5.10; N, 4.56.
1
yield): mp 172-174 °C; H NMR (400 MHz, CDCl3) δ 8.15 (d,
J=8.0 Hz, 2H), 7.67 (d, J=8.0 Hz, 2H), 1.35 (s, 9H); 13C NMR
(100 MHz, CDCl3) δ 138.7, 136.7, 135.7, 46.9, 31.3. Anal. Calcd for
C10H15BO2S: C, 57.17; H, 7.20. Found: C, 57.28; H, 7.16.
Compound 11. A suspension of 10 (120 mg, 0.20 mmol),
dichloromethane (15 mL), and acetyl chloride (4 mL) was
cooled to 0 °C and then BBr3 (1.8 mL, 1.8 mmol) solution
in dichloromethane (10 mL) was added in one portion.
The resulting clear orange solution was stirred for 20 min at
0 °C then poured into ice. The organic layer was separated,
dried over MgSO4, and concentrated, then the residue was
chromatographed (silica, eluent DCM) to give 11 as a yellow
powder (87 mg, 76% yield): mp 205-206 °C; ES-MS: 570.1
(Mþ); 1H NMR (400 MHz, CDCl3) δ 8.56 (s, 1H), 8.45 (d, J=
1.2 Hz, 1H), 8.26 (d, J=1.6 Hz, 1H), 7.90 (s, 1H), 7.73 (d, J=
8.4 Hz, 2H), 7.58 (d, J = 8.4 Hz, 2H), 7.52 (d, J = 8.4 Hz,
2H), 7.41 (d, J = 8.4 Hz, 2H), 2.48 (s, 3H), 2.46 (s, 3H);
13C NMR (100 MHz, CDCl3) δ 193.1, 193.0, 188.8, 153.4,
145.0, 143.8, 140.1, 137.7, 137.4, 137.1, 136.39, 136.36, 135.2,
134.8, 133.8, 130.1, 129.8, 128.8, 128.4, 128.3, 128.0, 127.54,
127.49, 127.1, 122.0, 30.38, 30.35. Anal. Calcd for
2,7-Dibromo-3,5-dinitro-9-fluorenone (7). 2,7-Dibromo-9-fluo-
renone 634 (10.14 g, 30 mmol) was dissolved in sulfuric acid
(200 mL) at room temperature with stirring and sonication. The
viscous solution was cooled to -10 °C followed by the dropwise
addition of fuming nitric acid (100%, 150 mL) with cooling and
stirring. During the addition, the temperature was maintained
between -15 and -10 °C to afford a yellow viscous suspension.
The cooling bath was removed and the mixture was stirred for 12 h
at room temperature then poured onto crushed ice (∼500 g). A
yellow solid was obtained by suction filtration and washed with a
large volume of water. Crystallization of the solid from DMF
yielded 7 as yellow crystals (7.78 g, 61% yield): 1H NMR (200 MHz,
DMSO-d6) δ 8.47 (d, J = 1.8 Hz, 1H), 8.30 (s, 1H), 8.21 (d, J =
1.8 Hz, 1H), 8.18 (s, 1H); 13C NMR (100 Hz, DMSO-d6) δ 186.7,
153.3, 144.7, 139.3, 136.8, 136.4, 133.0, 131.6, 131.4, 129.9, 123.9,
122.2, 116.3; MS (EI) (m/z, %) 427.7 (Mþ, 86), 149 (100).
Compound 10. To a mixtureof compound 7 (428 mg, 1.0 mmol),
2 (441 mg, 2.1 mmol), THF (30 mL), toluene (30 mL), and aqueous
sodium carbonate (1 M, 3.0 mL, 3 mmol) was added Pd(PPh3)4
(46 mg, 0.04 mmol) in one portion, then the mixture was heated at
reflux for 24 h. The solvent was removed under reduced pressure
and the residue was dissolved in dichloromethane (40 mL) and
washed with water (40 mL). The red organic phase was separated
and dried over MgSO4. The solvent was removed and the residue
was purified by column chromatography (SiO2, eluent DCM) to
give 10 as a yellow powder (525 mg, 88% yield): mp 241-242 °C dec;
C29H18N2O7S2: C, 61.04; H, 3.18; N, 4.91. Found: C, 61.12;
H, 3.13; N, 4.87.
Acknowledgment. We thank the EPSRC (U.K.) and EC
FP7 ITN “FUNMOLS” project no. 212942 for funding this
work.
Supporting Information Available: Additional experimental
details, including synthesis, characterization, electrical data,
X-ray crystal structures of 9 and 16, and copies of NMR spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(34) Gallos, J.; Varvoglis, A. J. Chem. Res. (S) 1982, 150–151.
136 J. Org. Chem. Vol. 75, No. 1, 2010