Hexathiophenalenyliums cations: Syntheses, structures, and redox chemistry

Article abstract of DOI:10.1021/ol400452f

Preparations of two hexathiophenalenylium compounds as stable salts from the reaction of 3,4,6,7-tetrathio-9-hydroxyphenalenone with Lawesson's reagent have been reported. The presence of three disulfide linkages on the periphery of the core phenalenyl unit is confirmed by X-ray crystallographic characterizations. Electrochemical cell potentials are lower than those of related dithio- and tetrathio-bridged phenalenyl radicals, and the hexathiophenalenyl radical shows a strong electron paramagnetic resonance (EPR) signal in the solid state.

Full text of DOI:10.1021/ol400452f

ORGANIC
LETTERS
2013
Vol. 15, No. 6
1198–1201
Hexathiophenalenyliums Cations:
Syntheses, Structures, and Redox Chemistry
Pradip Bag, Fook S. Tham, Bruno Donnadieu,† and Robert C. Haddon*
Departments of Chemistry and Chemical & Environmental Engineering,
University of California, Riverside, California 92521-0403, United States
haddon@ucr.edu
Received January 10, 2013
ABSTRACT
Preparations of two hexathiophenalenylium compounds as stable salts from the reaction of 3,4,6,7-tetrathio-9-hydroxyphenalenone with
Lawesson’s reagent have been reported. The presence of three disulfide linkages on the periphery of the core phenalenyl unit is confirmed by
X-ray crystallographic characterizations. Electrochemical cell potentials are lower than those of related dithio- and tetrathio-bridged phenalenyl
radicals, and the hexathiophenalenyl radical shows a strong electron paramagnetic resonance (EPR) signal in the solid state.
The past few decades have witnessed the continuing
interest in organic conductors based on charge transfer
salts.1ꢀ4 There are a number of alternative approaches,5
one of which makes use of neutral radicals possessing an
unpaired electron that could function as organic conduc-
tors in analogy with the classical monatomic metals and
superconductors.6ꢀ8 There are, however, several difficul-
ties in pursuing the neutral radical conductor model: (1)
dimerization of the radical molecules either by σ- or by
π-association can compete with the metallic ground state;
(2) the half filled band, coupled witha low value oftheratio
of the bandwidth (W) to the Coulomb repulsion energy (U),
can lead to Mott insulators. Thus, the design of materials with
a higher ratio of W/U is crucial to improve the conductivities.
Considerable progress has been made in this field by using
heterocyclic and phenalenyl (PLY) based radicals.9ꢀ40
The phenalenyl radical41,42 and related alkyl deriv-
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σ-association,43ꢀ46 but recent progress in phenalenyl
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† Present address: Institut Charles Gerhardt UMR 5233, Universite
Montpellier 2, Montpellier Cedex 5, France.
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10.1021/ol400452f
Published on Web 03/06/2013
2013 American Chemical Society

chemistry19,20,22,25,27ꢀ29,47,48 has led to the isolation of the
radical in the crystalline state through the introduction of
bulky substituents.22,25,26,29 To obtain stable neutral radi-
cals with less steric hindrance and further delocalization
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both chalcogen and halogen substitution at the periphery
of the PLY unit (Figure 1).22ꢀ24,49,50 Chalcogens have been
terminate their valences and they are often associated with
strong intermolecular interactions.51,52 Furthermore, they
seem to be able to stabilize multiple oxidation states and to
effectively delocalize spin density.49
In principle three dithio-bridged derivatives of PLY
are possible; dithiophenalenyl (DTPLY, 3),23,49 tetra-
thiophenalenyl (TTPLY, 4),24 and hexathiophenalenyl
(HTPLY, 5). DTPLY was prepared in 1978, and solution
EPR and electrochemical measurements were reported.49
Subsequent studies led to the crystallization of the 1,9-
dithiophenalenyl radical (3) which gave rise to a π-dimer,
and this was the first example of a radical based on a single
phenalenyl unit that has been stabilized against σ-dimer-
ization in the solid state by electronic effects rather than by
the presence of sterically bulky substituents.23,49 Recently,
we made use of the cationic species 4þSbF6 to prepare
ꢀ
and characterize the tetrathiophenalenyl radical (4).24 The
synthesis of the TTPLY framework is a further develop-
ment of the chemistry used to produce DTPLY.49 The
crystals of TTPLY are diamagnetic in the solid state and
the X-ray structure confirmed the sulfur bridged dimer
structure, although dissolution of the crystals in toluene
affords a well resolved EPR spectrum corresponding to the
TTPLY radical. In principle, HTPLY, which has yet to be
prepared, should have a threefold symmetrical molecular
skeleton although a sulfur bridged dimeric structure may
prevail.24
Figure 1. Mono-PLY radicals.
popular atoms for incorporation into the periphery of the
molecular building blocks for organic conductors because
these atoms do not require additional functionality to
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radical (4) leads to the lowest disproportionation potential
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this respect sulfur substitution is much more successful
than the introduction of other heteroatoms in reducing
34
the value of ΔE2ꢀ1
,
and thus we pursued the synthesis
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(5þTFABꢀ, TFABꢀ = tetrakis(pentafluorophenyl)borate)
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of the disulfide unit,23,49 in which 3,4,6,7-tetrathio-9-
hydroxyphenalenone serves as the starting material. The
HTPLY radical (5) was characterized by solid state EPR
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Org. Lett., Vol. 15, No. 6, 2013
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treatment with triflic acid (HOTf), affords [HTPLY]þOTfꢀ
in pure form; metathesis with KþTFABꢀ and recrystalliza-
tion from dichloromethane/hexane yield [HTPLY]þTFABꢀ
as black crystals. The tetraphenylboron salt of nitro-
HTPLY (6þBPh4ꢀ) was obtained by the use of nitric acid
instead of triflic acid, followed by a metathesis reaction
using sodium tetraphenylborate. The yields of the reac-
tions for the oxidative introduction of the third disulfide
bridge into the tetrathiophenalenone moiety were quite
low (3ꢀ10%).
Scheme 1. Synthesis of TTPLY Salt
The synthesis of the TTPLY (4) framework is related to
that developed for DTPLY (3); reaction of 4,9-dimethoxy-
3-hydroxyphenalenone with P2S5, followed by treatment
with HCl, affords [TTPLY]þClꢀ (Scheme 1), but this
oxidative strategy is not useful in the present connection.
The replacement of the alkoxy groups of 3-hydroxy-
4,6,7,9-tetramethoxyphenalenone (II) by sulfur substitu-
tionproved to be difficult(Scheme 1). Hence we turnedour
attention to a route involving the stepwise introduction
of the disulfide linkages, and thus we focused on the
tetrathio derivative of 9-hydroxyphenalenone as our initial
goal. Recently, we developed a versatile synthetic ap-
proach for the introduction of chalcogenide substituents
into the phenalenyl ring system which involved the intro-
duction of better leaving groups at the active positions of
9-hydroxyphenalenone in order to facilitate their replace-
ment by nucleophiles.34 The synthesis of the precursor
salts to radicals 5 and 6 began with the addition of the
ethyl malonyl group to 1,3,6,8-tetramethoxynaphthalene
(Scheme 2).26 The product of this reaction was cyclized
with concentrated hydrochloric acid and then demethyl-
The cyclic voltammograms for the salts (5þOTfꢀ and
6þBPh4ꢀ) in 1,2-dichlorobenzene showed two reversible
waves (Figure 2) corresponding to the expected triad of
oxidation states: a reversible (0/þ1) wave and a reversible
(ꢀ1/0) wave. The reduction potentials and disproportio-
nation potentials (ΔE2ꢀ1) are given in Table 1. Addition-
ally, we carried out cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) experiments for
5 using DMF as solvent (Figure S1, Supporting
Information), and the reduction potentials are tabulated
in Table 1. The estimated cell potentials are substantially
smaller than those found for other reported mono-PLY
systems (1ꢀ4), a conclusion that argues for a low onsite
Coulomb potential in solids based on these radicals. Pre-
liminary results indicate that the cationic salt [5þTFABꢀ]
can be reduced to the radical, as evidenced by the strong
solid state EPR observed for the HTPY (5) radical with
g = 2.0055 (Figure S4, Supporting Information).
ated using pyridine hydrochloride.34 We introduced the
3
mesyl groups (ꢀOMs) into the active positions of the
9-hydroxyphenalenone unit, and this increased the reac-
tivity of these positions toward nucleophilic substitution
by potassium thioacetate. By carrying out the reaction in
the atmosphere, 3,4,6,7-tetramesylate-9-hydroxyphenale-
none reacts smoothly with potassium thioacetate to
produce the corresponding disulfide bridged 3,4,6,7-
tetrathio-9-hydroxyphenalenone as the only isolable
product (Scheme 2). Reaction of 3,4,6,7-tetrathio-9-
hydroxyphenalenone with Lawesson’s reagent, followed by
Figure 2. Cyclic voltammograms in 1,2-dichlorobenzene for
5þOTfꢀ (left) and 6þBPh4ꢀ (right), with half wave E1/2 poten-
tials. The internal reference Fc/Fcþ (0.56 V) is not shown.
Scheme 2. Syntheses of Hexathiophenalenyliums
Table 1. Half-Wave Potentials and Disproportionation
Potentials (volts vs SCE) of Monophenalenyl Salts
1/2
ΔE2ꢀ1 = E2
E11/2 (V)
ꢀ
compd
E11/2 (V) E21/2 (V)
ꢀ a
PLY (1þBF4
)
0.7
ꢀ0.9
ꢀ1.6
PCPLY (2þGaCl4
)
1.14
ꢀ0.08
ꢀ0.80
ꢀ0.71
ꢀ0.60
ꢀ0.62
ꢀ0.55
ꢀ1.22
ꢀ0.57
ꢀ0.33
ꢀ0.18
ꢀ0.28
ꢀ0.22
ꢀ b
DTPLY (3þOTfꢀ)a
ꢀ0.23
ꢀ0.34
ꢀ0.42
ꢀ0.34
ꢀ0.33
TTPLY (4þSbF6
)
ꢀ b
HTPLY (5þOTfꢀ)c
HTPLY (5þOTfꢀ))d
Nitro-HTPLY (6þBPh4
ꢀ d
)
a Acetonitrile. b 1,2-Dichloroethane. c N,N-Dimethylformamide.
d 1,2-Dichlorobenzene.
1200
Org. Lett., Vol. 15, No. 6, 2013

Figure 3. X-ray crystal structures of the salts, 5þTFABꢀ and
6þBPh4ꢀ. The asymmetric units contain a single (a) hexathio-
phenalenyl cation unit and a (b) nitro-hexathiophanalenyl
cation unit.
ꢀ
Figure 5. Dimeric pairs of cations for 5þTFABꢀ and 6þBPh4
.
position of nitration on the phenalenyl ring in the nitro-
hexathiophenalenyl framework. X-ray crystal structures
were obtained for 5þTFABꢀ and 6þBPh4ꢀ, and the crystal
data are provided in Table S1 (Supporting Information).
The asymmetric unit of 5þTFABꢀ is composed of a
single cation/anion pair within the monoclinic, P2(1)/c
space group, whereas, for 6þBPh4ꢀ, there are two dichloro-
methane molecules as solvent of crystallization in addi-
tion to a single cation/anion pair within the monoclinic
P2(1)/n space group. Packing diagrams of the cations
(5þ and 6þ) are given in Figure 4; the HTPLYþ units for
both compounds occur as dimers in the lattice which are
oriented along the b axis and the molecules in the dimer of
6þBPh4 are rotated with respect to each other. Perpen-
ꢀ
dicular views of the dimer pairs are given in Figure 5. The
interplanar separation between the two PLYꢀrings within
þ
˚
˚
each dimer is 3.42 A for compound 5 TFAB and 3.39 A
for compound 6þBPh4
.
ꢀ
In summary, we have developed a synthetic strategy
which allows the isolation and characterization of the
precursor salts for hexathiophenalenyl radicals, and pre-
liminary studies suggest the formation of the HTPLY
radical by chemical reductions. Finally, the enhancement
in spin delocalization of phenalenyl radicals, occasioned
by the attachment of successive disulfide substituents and
the concomitant improvement in their electrochemical cell
potentials, suggests that these materials may be important
for exploration of new organic materials having interesting
electrical and magnetic properties.
Acknowledgment. This work was supported by the
Office of Basic Energy Sciences, U.S. Department of
Energy under Grant No. DE-FG02-04ER46138.
Figure 4. Pacꢀking of the cationic molecules of 5þTFABꢀ (top)
and 6þBPh4 (bottom) (anions and solvent molecules are
omitted for clarity).
Supporting Information Available. Experimental proce-
dures, CV and DPVs, 1H and 13C NMR spectra, an EPR
spectrum of 5, crystal information, and CIF files for com-
pounds 5þTFABꢀ and 6þBPh4ꢀ. This material is available
free of charge via the Internet at http://pubs.acs.org.
Syntheses and characterizations of the radicals (5 and 6)
in the solid state are underway. However, we were able to
establish the crystal structures of the two hexathiophena-
lenyl salts (Figure 3). These determinations confirmed the
presence of the three dithio-bridged functional groups in
both compounds and also established unequivocally the
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 6, 2013
1201