The interplay of inverted redox potentials and aromaticity in the oxidized states of new π-electron donors: 9-(1,3-dithiol-2-ylidene)fluorene and 9-(1,3-dithiol-2-ylidene)thioxanthene derivatives

Article abstract of DOI:10.1002/chem.200501326

Derivatives of 9-(1,3-dithiol-2-ylidene)fluorene (9) and 9-(1,3-dithiol-2-ylidene)thioxanthene (10) have been synthesised using Horner-Wads-worth-Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with fluorenone and thioxanthen-9-one. X-ray cryst

Full text of DOI:10.1002/chem.200501326

FULL PAPER
DOI: 10.1002/chem.200501326
The Interplay of Inverted Redox Potentials and Aromaticity in the Oxidized
States of New p-Electron Donors: 9-(1,3-Dithiol-2-ylidene)fluorene and
9-(1,3-Dithiol-2-ylidene)thioxanthene Derivatives
Samia Amriou,[a] Changsheng Wang,[a] AndreiS. Batsanov, [a] Martin R. Bryce,*[a]
Dmitrii F. Perepichka,[a, b] Enrique Ortí,[c] Rafael Viruela,[c] JosØ Vidal-Gancedo,[d] and
Concepció Rovira[d]
Abstract: Derivatives of 9-(1,3-dithiol-
2-ylidene)fluorene (9) and 9-(1,3-di-
thiol-2-ylidene)thioxanthene (10) have
been synthesised using Horner–Wads-
worth–Emmons reactions of (1,3-di-
thiol-2-yl)phosphonate reagents with
fluorenone and thioxanthen-9-one. X-
ray crystallography, solution electro-
chemistry, optical spectroscopy, spec-
troelectrochemistry and simultaneous
electrochemistry and electron para-
magnetic resonance (SEEPR), com-
bined with theoretical calculations per-
formed at the B3P86/6-31G** level,
elucidate the interplay of the electronic
and structural properties in these mole-
cules. These compounds are strong
two-electron donors, and the oxidation
potentials depend on the electronic
structure of the oxidised state. Two,
single-electron oxidations (Eo1x <E2ox)
were observed for 9-(1,3-dithiol-2-yli-
dene)fluorene systems (9). In contrast,
derivatives of 9-(1,3-dithiol-2-ylidene)-
thioxanthene (10) display the unusual
phenomenon of inverted potentials
(Eo1x >Eo2x) resulting in a single, two-
electron oxidation process. The latter is
due to the aromatic structure of the
thioxanthenium cation (formed on the
loss of a second electron), which stabil-
ises the dication state (102+) compared
with the radical cation. This contrasts
with the nonaromatic structure of the
fluorenium cation of system 9. The
two-electron oxidation wave in the
thioxanthene derivatives is split into
two separate one-electron waves in the
corresponding sulfoxide and sulfone
derivatives 27–29 owing to destabilisa-
tion of the dication state.
Keywords: aromaticity
donors fluorene
chemistry · thioxanthene
·
electron
· redox
·
Introduction
[a] Dr. S. Amriou, Dr. C. Wang, Dr. A. S. Batsanov, Prof. M. R. Bryce,
Prof. D. F. Perepichka
Since the discovery of the remarkable properties of tetra-
thiafulvalene (TTF, 1; Scheme 1) and its charge-transfer
complexes,[1] the 1,3-dithiol-2-ylidene unit has been widely
used as a component of new p-electron donor systems due
to the high thermodynamic stability of derived cation radical
species that contain the heteroaromatic 6p-electron dithioli-
um cation.[2] In this context, there has been considerable in-
terest in bis(1,3-dithiole) derivatives with extended p-conju-
gation between the dithiole rings, notably the incorporation
of vinylogous[3] and quinoidal[4–12] spacer units which reduce
intramolecular Coulombic repulsion. For example, the 9,10-
bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene system (2)
undergoes a single, quasi-reversible, two-electron oxidation
wave to yield a thermodynamically stable dication at Eox ꢀ
ꢁ0.03 V (vs Fc/Fc+ for R=Me) in cyclic voltammetry (CV)
experiments (the oxidation potential varies with the differ-
ent substituents R).[5,6,9] X-ray crystal structures have shown
Department of Chemistry, University of Durham
Durham DH1 3LE (UK)
Fax : (+44)191-374-4737
E-mail: m.r.bryce@durham.ac.uk
[b] Prof. D. F. Perepichka
Chemistry Department, McGill University
Montreal H3A 2K6 (Canada)
[c] Dr. E. Ortí, Dr. R. Viruela
Insititut de CiØncia Molecular, Universitat de Val›ncia
46100 Burjassot (Valencia) (Spain)
[d] Dr. J. Vidal-Gancedo, Prof. C. Rovira
Institut de Ci›ncia de Materials de Barcelona
C.S.I.C., Campus de la UAB., 08193 Bellaterra (Spain)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author: crystal packing
diagrams for compounds 13b and 18; crystal structure data for 13b,
16, 18, 28; cyclic voltammogram of 23; absorption spectra for com-
pounds 23, 24 and 182+ in solution.
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culations show that the neutral
molecule 7 and its cation radi-
cal exhibit folded butterfly-
shaped structures (reminiscent
of 2), whereas the dication
comprises a planar polyacenic
cation with a perpendicular di-
thiolium cation. The third oxi-
dation process for
7 corre-
sponds to formation of the di-
cation of the acene unit. Com-
pound 8 undergoes two, single-
electron oxidations at low po-
tentials
(Eox =ꢁ0.07
and
+0.06 V vs Fc/Fc+). The neu-
tral form of 8 is nonplanar,
whereas the cation radical is
planar.[15]
Scheme 1. TTF (1), p-extended TTFs (2 and 4) and some related donors containing one dithiole ring.
that a major structural change accompanies oxidation of 2
to the dication: the central anthracenediylidene ring, which
is boat shaped in the neutral “butterfly” molecule becomes
planar, and a fully aromatic anthracene system with perpen-
dicular, 6p-electron 1,3-dithiolium cations is formed.[4] Theo-
retical calculations have established that the steric hindrance
introduced by benzoannulation of the central quinodi-
methane unit determines the saddle shape of the neutral
molecules.[6] The strong electron-donor ability of this ring
system has led to its use as a component of intermolecu-
lar[4,11] and intramolecular[7,8,11,12] charge-transfer systems.
Molecules bearing only one 1,3-dithiol-2-ylidene unit have
received much less attention. They are generally weaker
electron donors than the bis(1,3-dithiole) systems, but none-
theless they offer a range of interesting redox, spectroscopic
and structural properties. For example, compounds of gener-
al formula 3, bearing a hydrogen atom on the exocyclic
carbon, readily undergo electro-
Herein we report a detailed study of the synthesis, X-ray
crystal structures and electrochemical and spectroscopic
characterisation of a range of derivatives of the title systems
9 and 10, combined with theoretical calculations that shed
further light on the interplay of the electronic and structural
properties in 1,3-dithiole-derived p donors.
Results and Discussion
Synthesis: The synthesis of 9-(1,3-dithiol-2-ylidene)fluorene
derivatives is shown in Scheme 2. 2,7-Dibromo- and 2,7-di-
iodofluorenone (11a and 11b, respectively) reacted with the
carbanion generated from reagent 12 under standard condi-
tions[5] to afford the 1,3-dithiol-2-ylidene derivatives 13a
(70% yield) and 13b (82% yield), respectively. Reaction of
chemical or chemical oxidative
dimerisation to afford substitut-
ed vinylogous TTFs 4.[3d,e]
Other examples are provided
by the isomeric cyclopentadi-
thiophene systems
5 and 6,
which adopt planar structures
as determined by X-ray analysis
and exhibit irreversible oxida-
tion waves at approximately
0.4 V (vs Fc/Fc+).[13] 7-(1,3-Di-
thiol-2-ylidene)-7-hydrobenz-
[d,e]anthracene (7) has also
been studied in this context and
shown to exhibit multistage
redox behaviour in CV ex-
periments with sequential for-
mation of the radical-cation,
-dication and -trication spe-
cies (Eox =+0.15, +0.33 and
+0.91 V, respectively, vs Fc/Fc+
Scheme 2. Reagents and conditions: a) 12, LDA, THF, ꢁ788C, then either 11a, THF, 20!508C, or 11b, THF,
in CH2Cl2).[14] Theoretical cal- 208C; b) 11a, DMF, [Pd(PPh3)4], 14, Na2CO3 (aq), 808C; c) 12, LDA, THF, 788C, 15, 208C.
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Oxidised States of p-Electron Donors
FULL PAPER
11a with 4-(tert-butyl)phenylboronic acid (14) under stan-
dard Suzuki conditions using sodium carbonate as base and
tetrakis(triphenylphosphino)palladium as catalyst in N,N-di-
methylformamide (DMF) at 808C gave compound 15 in
89% yield. Subsequent reaction of 15 with reagent 12, as de-
scribed above, gave the expected product 16 in 92% yield.
This route to 16 was considerably more efficient than pro-
ceeding via 13a or 13b for which the analogous Suzuki
cross-couplings were less clean and product purification was
difficult.
oxidised to the sulfoxide derivative 25 (14% yield) and the
sulfone 26 (81% yield) by using hydrogen peroxide in hexa-
fluoropropan-2-ol[17] or acetic acid, respectively (Scheme 4).
Subsequent reactions of 25 and 26 with phosphonate ester
reagents 12 and 19 gave the products 27–29.
The synthesis of 9-(1,3-dithiol-2-ylidene)thioxanthene de-
rivatives is shown in Scheme 3. Compound 18[5] was ob-
tained in 50% yield from thioxanthen-9-one (17) and re-
agent 12, analogously to the preparation of 13. As a precur-
sor to more highly functionalised derivatives of this system,
the monomethyl analogue 20 was prepared (48% yield)
using reagent 19.[10] Deprotonation of 20 using lithium diiso-
propylamide (LDA) in THF at ꢁ788C, followed by in situ
trapping of the lithiated species with methyl chloroformate
gave the methyl ester derivative 21 (67% yield) which was
reduced with lithium aluminium hydride to the hydroxy-
methyl derivative 22 (79% yield). The suitability of com-
pound 22 for further elaboration was established by reaction
of its lithium alkoxide salt with 2,5,7-trinitro-4-chlorocarbo-
nylfluoren-9-one[16] to afford compound 23 in 20% yield.
The electron-acceptor ability of the fluorenone moiety in 23
was increased by conversion to the dicyanomethylene deriv-
ative 24 (51% yield) by treatment with malononitrile in
DMF. Compounds 23 and 24 assisted in the analysis of the
redox properties of the 9-(1,3-dithiol-2-ylidene)thioxanthene
system (see below).
Scheme 4. Reagents and conditions: a) H2O2 (35% aq), hexafluoro-2-
propanol, reflux; b) H2O2 (35% aq), AcOH, reflux; c) 12, LDA, THF,
ꢁ788C, then 25, THF, ꢁ78!208C; d) 12 or 19, LDA, THF, ꢁ788C, then
26, THF, ꢁ78!208C.
Electrochemical, spectroscopic and spectroelectrochemical
properties: The solution electrochemical properties of com-
pounds 13a, 13b, 16, 18, 20–24 and 27–29 were studied by
using cyclic voltammetry (CV) and the data are collated in
Table 1, together with literature values for compounds 2 and
5–8 for comparison. Com-
The sulfoxide 27 and sulfone derivatives 28 and 29 were
synthesised to examine the effect of sulfur oxidation on the
redox properties of the system. Thioxanthen-9-one (17) was
pounds 13a, 13b and 16 show
two, one-electron oxidation
waves, corresponding to the se-
quential formation of the radi-
cal-cation and dication species,
with the second wave being ir-
reversible.
The
additional
phenyl rings in 16 lower the ox-
idation potentials by approxi-
mately 100 mV and a third oxi-
dation wave was observed for
this compound (Figure 1). The
reversibility of the first oxida-
tion process for 13a, 13b and
16 is in contrast to the isoelec-
tronic analogues 5 and 6, al-
though the oxidation potential
of all the systems are similar
(Table 1).
Scheme 3. Reagents and conditions: a) 12, LDA, THF, ꢁ788C, then 17, THF, ꢁ78!208C; b) 19, LDA, THF,
ꢁ788C, then 17, THF, ꢁ78!208C; c) LDA, THF, ꢁ788C, then ClCO2Me, ꢁ78!208C; d) LiAlH4, THF, 208C;
e) 22, nBuLi, THF, ꢁ1008C, then 2,5,7-trinitro-4-chlorocarbonylfluoren-9-one, ꢁ100!208C; f) malononitrile,
DMF, 208C.
The
(1,3-dithiol-2-ylidene)-
thioxanthene derivatives 18 and
20–24 behave very differently.
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M. R. Bryce et al.
Table 1. Electronic absorption spectra and cyclic voltammetric data.[a]
the corresponding value is ap-
proximately 300 mV due to the
loss of aromaticity and the
large conformational change
that occurs on reduction of the
dication species 22+ back to the
neutral species. The behaviour
of 18, 20 and 21 was essentially
unchanged when different elec-
trolytes [tetrabutylammonium
Compound
lmax [nm]
Eo1x [V][b]
Eo2x [V]
Ered [V]
Solvent, ref. electrode
2: R=Me[5] 433, CH3CN
ꢁ0.03 (2e)
+0.45 (ir)[c]
+0.4 (ir)[c]
+0.15
CH3CN, Ag/AgCl
EtOH, SCE
CH2Cl2, SCE
CH2Cl2, SCE
5[13]
6[13]
7[14]
411, EtOH
420, CH2Cl2
438, CH2Cl2
+0.33
(Eo3x =0.91)
+0.06
8[15]
13a
13b
16
402, CH2Cl2
434, CHCl3
437, CHCl3
432, CHCl3
ꢁ0.07
+0.46
+0.44
+0.33
CH3CN, SCE
+1.20[d]
+1.11[d]
+1.05[d]
(Eo3x =1.25[d]
CH2Cl2, Ag/AgNO3
CH2Cl2, Ag/AgNO3
CH2Cl2, Ag/AgNO3
)
(TBA)BF4,
TBAPF6,
18
20
21
22
23
24
27
28
29
384, CHCl3
379, CHCl3
368, CHCl3
376, CHCl3
600 (br), CHCl3
800 (br), CHCl3
396, CHCl3
431, CHCl3
431, CHCl3
+0.23 (2e)
+0.25 (2e)
+0.38 (2e)
+0.25 (2e)
+0.28 (2e)
+0.25 (2e)
+0.56
CH3CN, Ag/AgCl
CH3CN, Ag/AgCl
CH3CN, Ag/AgCl
CH3CN, Ag/AgCl
TBAClO4], scan rates (10–
500 mVsꢁ1) or lowered temper-
ature (08C) were used, whereas
ꢁ0.67, ꢁ0.94, ꢁ1.70 CH2Cl2, Ag/AgNO3
ꢁ0.36, ꢁ0.89, ꢁ1.58 CH2Cl2, Ag/AgNO3
CH3CN, Ag/AgCl
a
significant increase in the
value of DE was observed in
system 2 at lower temperature
and higher scan rates.[6,19]
+0.90[d]
+1.11[d]
+1.12
+0.58
+0.59
CH3CN, Ag/AgCl
CH3CN, Ag/AgCl
The CV data for the donor–
acceptor diad compounds 23
and 24 clearly established that
the oxidation wave of the (1,3-
dithiol-2-ylidene)thioxanthene
system involves a two-electron
process, as twice the current is
[a] See ref. [20] for details of standard procedures and equipment used in these experiments. [b] The potentials
were measured versus the Ag/AgCl or Ag/AgNO3 electrodes and presented versus Fc/Fc+, which in our condi-
tions showed +0.45 V versus Ag/AgCl (in CH3CN), +0.39 versus SCE (in CH3CN), +0.49 V versus Ag/AgCl
(in CH2Cl2), +0.43 versus SCE (in CH2Cl2), +0.20 V versus Ag/AgNO3 (in CH2Cl2). [c] ir=irreversible.
[d] The potential of the anodic peak is given for the irreversible oxidation processes.
associated with this wave compared with the three, one-elec-
tron reductions of the acceptor moieties (Figure 1, and Fig-
ure S3 in the Supporting Information). As expected,[11,20] the
reduction potentials of the fluoren-9-dicyanomethylene unit
of 24 are anodically shifted (especially Er1ed) compared with
that of the fluorenone precursor 23. The former compound
(24) is a highly electrochemically amphoteric system, with
the difference between the oxidation and reduction poten-
tial being only 0.6 V.
We postulated that for the sulfoxide and sulfone deriva-
tives 27–29, in contrast to 18 and 20–24, the radical cation
should be observable because the electron-withdrawing
effect of SO and SO2 groups, combined with the lack of aro-
maticity of an S-oxidised thioxanthenium cation, would
make the removal of the second electron more difficult.
Indeed, two oxidation waves were observed for 27–29 in
THF. Coulometric analysis established that the first wave
was a one-electron process [0.85e and 0.95e (ꢂ0.05e) for 27
and 28, respectively, were deduced from the analysis] lead-
Figure 1. Cyclic voltammetric data for 16, 18, 24 and 28. For details of
conditions see Table 1.
A single, two-electron wave is observed in all cases at 0.23–
0.38 V (vs Fc/Fc+). A general trend in the data, which is
consistent with previous observations for derivatives of TTF
1[18] and the quinoidal-TTF 2,[10] is the lowering of the oxida-
tion potential on introducing the electron-donor methyl sub-
stituents (by 20 mV, compounds 20!18) and a reverse
effect due to the electron-withdrawing carbomethoxy sub-
stituent (by 130 mV, 20!21). The oxidation wave for 18 and
20–24 is a reversible process with the difference between the
potentials of the oxidation peak and the coupled reduction
ing to the radical cations, 27+ and 28+ , respectively; the
second wave (dication formation) was irreversible
(Figure 1). It is notable that the first oxidation potentials,
Eo1x, for both 27–29 are very similar (0.56–0.59 V) and are
positively shifted relative to the parent system 18. The
second wave, Eo2x, however, is 210–220 mV more positive for
the sulfone than for the sulfoxide (Table 1) indicating that
the second electron is subtracted mostly from the thioxan-
thenium fragment.
C
C
ox
pa
red
pc
peak DE=(E ꢁE ) at approximately 40 mV, which is less
than the 59 mV value characteristic for a single-electron
process, and thus confirms the two-electron oxidation. This
low anodic–cathodic peak gap contrasts with the quasi-re-
versible oxidation of quinoidal-TTF derivatives 2 for which
Simultaneous electrochemistry and electron paramagnetic
resonance (SEEPR) experiments performed in solution for
the representative compounds 18 and 29 are in agreement
with the CV data. No radical species was detected upon oxi-
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Oxidised States of p-Electron Donors
FULL PAPER
ꢁ
dation for compound 18, which is consistent with the single-
stage, 2e process (18!182+) observed electrochemically.
of compound 18 in acetonitrile containing 0.2m TBA+ PF6
as electrolyte is shown in Figure 3. Progressive oxidation of
the neutral species to the dication was accompanied by a de-
crease in the peak at lmax =384 nm and the appearance of
two new features at lmax =325 and ꢀ400 nm; the latter
being a weaker broad band extending to approximately
500 nm. Upon reversing the potential, the original spectrum
of the neutral species was cleanly obtained. The dication salt
The nondetectable low concentration of 18+ implies a very
C
high disproportionation constant for the radical cation, sug-
gesting reversed values for the oxidation potentials (Eo2x <
Eo1x),[11,21] that is, the second electron is removed more easily
than the first. Oxidation of 29 gave an intense, very symmet-
rical EPR signal, with extensive fine structure, assigned to
ꢁ
29+ (Figure 2). Simulation of the spectrum showed that the
182+·(ClO4 )2 was isolated by bulk electrolysis (electrolyte
C
ꢁ
TBA+ ClO4 ) as a red solid (lmax =325 and 408 nm in
CH3CN, see Figure S6 in the Supporting Information). This
salt was unstable in the presence of air, decomposing to
thioxanthene-9-one (17). We reported a similar oxidative de-
composition of the transient, photolytically generated,
cation radical of system 2 to yield the 10-(1,3-dithiol-2-ylid-
ene)anthracene-9(10H)one derivative.[23]
The long-wavelength, very weak (eꢀ10mꢁ1 cmꢁ1) absorp-
tion bands observed in the solution UV-visible spectra of
diads 23 and 24 were attributed to charge transfer (CT) be-
tween the electron-donor dithiolylidene-thioxanthene and
the electron-acceptor fluorene moieties. A redshift for the
CT transition was observed for 24 (Figure S5 in the Support-
ing Information; l=700–1000 nm) relative to that of 23 (l=
500–850 nm) owing to the dicyanomethylene group enhanc-
ing the electron-acceptor ability. The degree of charge trans-
fer to a dicyanomethylene acceptor unit can be monitored
by the lowering of the nitrile stretching frequency in the IR
spectrum, compared with that of a neutral 9-dicyanometh-
ylenefluorene analogue.[20] For compound 24 this value (n˜ =
2231 cmꢁ1) is very similar to that of neutral nitrofluorene-9-
dicyanomethylene derivatives, which suggests that there is
very little charge transfer in the ground state. This is in con-
trast to a related diad system possessing a stronger “extend-
ed-TTF” two-electron donor moiety (namely, a derivative of
2) which has complete charge (electron) transfer (dCT ꢀ1).[11]
Figure 2. EPR spectrum of in situ electrochemically generated 29+
.
C
spin is delocalised over the whole molecule. Species 29+
C
should, therefore, be viewed as a p-extended system. The
spin is coupled with the proton and the methyl group of the
1,3-dithiol-2-ylidene moiety and with four sets of two equiv-
alent protons in the aromatic rings showing the following
hyperfine coupling constants: aH =2.24 G (1H), aH =1.86 G
(3H), aH =1.46 G (2H), aH =1.38 G (2H), aH =1.03 G (2H),
aH =0.91 G (2H). The EPR parameters for 29+ (g factor
2.0059, line width 0.2 G) are very similar to those found for
C
[22]
TTF+ derivatives.
X-ray crystal structures of 13b, 16, 18 and 28: The single-
C
UV-visible absorption spectra reveal a strong absorption
from the neutral (1,3-dithiol-2-ylidene)thioxanthene moiety
at lmax ꢀ360–385 nm (Table 1). The spectroelectrochemistry
crystal structures of 13b, 16·CDCl3, 18 and 28·1= EtOAc
2
were characterised by means of X-ray diffraction analyses
(Figure 4). The dithiole rings are slightly folded along the
S···S vector, by 3, 10, 8 and 88, respectively. The exocyclic
C9=C14 bonds are slightly longer [1.369(3), 1.365(5),
1.360(2) and 1.369(2) Š, respectively] than the standard ole-
finic (R2C=CR2) bond length[24] of 1.33(1) Š and are typical
for 1,3-dithiol-2-ylidene moieties [average 1.36(1) Š for 115
structures in the November 2004 release of the Cambridge
Structural Database].[25] Molecule 13b is nearly planar. In
16 the 1,3-(dithiol-2-ylidene)fluorene system is only slightly
puckered, with a 3.88 twist around the C9=C14 bond and a
5.58 angle between benzene rings A and B, although the
twists between rings A and C (288) and between B and D
(478) are substantial. Similar planar conformations have
been found in 5[13] and in 4’,5’-diazafluorene-1,3-dithiol-2-yli-
dene ligands coordinated to Ag[26] or Pd.[27] By contrast, the
thioxanthene system in 18 and 28 adopts a butterfly confor-
mation resembling that of 9,10-bis(1,3-dithiol-2-ylidene)-
9,10-dihydroanthracene derivatives 2:[4,10,19] benzene rings A
Figure 3. Spectroelectrochemistry of 18: oxidation process in CH3CN
ꢁ
containing 0.2m TBA+ PF6 as electrolyte.
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M. R. Bryce et al.
to a fully planar C2v structure.
The equilibrium geometry cal-
culated is in very good agree-
ment with the X-ray structure
obtained for 13b, the maximum
difference between theory and
experiment being of only
0.006 Š for the bond lengths.
The shortest bond lengths cor-
respond to the carbon–carbon
(CC) bond of the dithiole ring
(1.344 Š) and to the exocyclic
ꢁ
C9 C14 bond (1.369 Š). The
benzene rings of the fluorene
unit preserve their aromaticity
since all the CC bonds forming
these rings have lengths of
1.40ꢂ0.01 Š. The presence of
bromine (13a) or iodine (13b)
atoms has no particular effect
on the molecular structure. For
molecule 16, calculations pre-
dict that the dithiole-fluorene
skeleton retains its planarity
and that the outer phenyl rings
Figure 4. Molecular structures of 13b, 16, 18 and 28, determined by X-ray crystallography, showing thermal el-
lipsoids at 50% probability.
and B form a dihedral angle of 1458 (18) and 1388 (28). In
both molecules, the dithiole ring is folded along the S2···S3
vector by 88, and there is a 108 twist around the C9=C14
bond in 18.
are twisted by 378. This rotational angle is intermediate be-
tween the X-ray values measured in the crystal (28 and 478;
Figure 4).
The minimum-energy conformation of 18 corresponds to
the butterfly-shaped Cs structure sketched in Figure 5a. Sim-
ilar conformations are predicted for compounds 27 and 28.
The warped conformations of 18 and 28 can be explained
by steric repulsion between the dithiole S and the peri H
atoms, which in the imaginary planar structures would be
absurdly short (ca. 2.0 Š). In the experimentally determined
structures these distances are increased to 2.60–2.68 Š, al-
though this is still less than the sum of the van der Waals
radii (2.91 Š).[28] In the fluorene derivatives, the peri CH
bonds are not parallel to the C9=C14 bond but are directed
more outward, thus the S···H distances in the actual, almost
planar, structures of 13b and 16 are increased to 2.44–
2.49 Š (all contacts calculated for the CH distance of
1.08 Š, as determined by neutron diffraction).[24] Such dis-
tances must still be sterically straining, but the fluorene
system (unlike dihydroanthracene or thioxanthene) has no
easy way of relieving the strain.
Theoretical calculations: To gain a deeper understanding of
the experimental data, the molecular structures and elec-
tronic properties of compounds 13a, 16, 18, 27 and 28, in
both neutral and oxidised states, were theoretically investi-
gated. Calculations were performed within the density func-
tional theory (DFT) approach at the B3P86/6-31G** level.
DFT calculations include electron correlation effects at a
relatively low computational cost and are known to provide
accurate equilibrium geometries.[29]
Figure 5. a) Minimum-energy conformation and b) optimised bond
lengths (in Š) calculated at the B3P86/6-31G** level for 18 (Cs symme-
try). Averaged X-ray values of the bond lengths (italics, underlined) are
included for comparison.
Neutral molecules: The molecular structure of 13a was opti-
mised under different symmetry restrictions and converged
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Oxidised States of p-Electron Donors
FULL PAPER
For 18, the Cs structure is computed to be 21.7 kcalmolꢁ1
more stable than the fully planar C2v conformation in which
short steric contacts (2.03 Š) between the dithiole sulfur
atoms and the peri hydrogen atoms are present. Upon relax-
ation to the butterfly conformation, the distance between
these atoms increases to 2.70 Š (18), 2.73 Š (27) and 2.68 Š
(28). As displayed in Figure 5b for 18, the calculated bond
lengths are in very good accord with the averaged X-ray
values and show that, as for the dithiole-fluorene deriva-
tives, the two lateral benzene rings retain their aromaticity.
Calculations slightly overestimate the folding of 18
(138.48, Figure 5a) compared with X-ray data (144.98) be-
cause they are performed for the isolated molecule; in the
crystal, there is a tendency to reduce the folding to achieve
the most compact packing. Slightly higher foldings are pre-
dicted for 28 (135.78), in agreement with the X-ray value
(138.08) and especially for 27 (130.38). The sulfoxide oxygen
approach, for the less-positive oxidation potential measured
for the thioxanthene derivative (see Table 1).
To investigate the nature of the electronic transitions that
give rise to the absorption bands observed in the experimen-
tal UV-visible spectra, the electronic excited states of 13a,
18, 27 and 28 were calculated by using the time-dependent
DFT (TDDFT) approach and the B3P86/6-31G** optimised
geometries. Calculations predict that the lowest-energy ab-
sorption band observed at l=384 nm for 18 (Table 1 and
Figure 3) is due to the excitation to the first electronic excit-
ed state that is calculated at 3.42 eV (l=362 nm) and corre-
sponds to the promotion of one electron from the HOMO
to the LUMO. As shown in Figure 7, the HOMO!LUMO
promotion implies some electron-density transfer from the
dithiole ring, on which the HOMO is mainly located, to the
thioxanthene unit. Calculations therefore suggest that the
absorption band implies some intramolecular charge trans-
fer. The absorption band has the same nature for com-
pounds 13a, 27 and 28.
in 27 actually produces
a
lengthening effect on the adja-
ꢁ
cent S C bonds (18: 1.770 Š;
27: 1.801 Š) that is larger than
the effect caused by the two
sulfone oxygen atoms in 28
(1.776 Š). As depicted in
Figure 6, the oxygen atom in 27
prefers to be pointing up to the
peri hydrogen atoms due to the
attractive electrostatic interac-
tion with these atoms. This in-
teraction produces a stabilisa-
Oxidised compounds: The equilibrium geometries of the
monocations and dications were computed to get a deeper
understanding of the oxidation process. As a representative
example, Figure 8 displays the minimum-energy conforma-
Figure 6. B3P86/6-31G** mini-
mum-energy conformation cal-
culated for 27 (Cs symmetry).
tion of 5.0 kcalmolꢁ1 with respect to the conformation in
which the oxygen atom points down. Calculations also pre-
dict the folding of the dithiole rings along the S2···S3 vector
(18: 10.68; 27: 8.18; 28: 7.08).
Figure 7 shows the atomic orbital (AO) composition of
the highest-occupied molecular orbital (HOMO) and the
lowest-unoccupied molecular orbital (LUMO) of 18. Similar
topologies are found for the fluorene derivatives. Whereas
the HOMO is mainly localised on the 1,3-dithiol-2-ylidene
unit, the LUMO extends over the carbon skeleton of the
thioxanthene or fluorene unit with small contributions from
the dithiole ring. Compared to 13a and 16, for which the
HOMO is calculated at ꢁ5.83 and ꢁ5.62 eV, the HOMO of
18 lies at ꢁ5.43 eV. This destabilisation accounts, in a first
Figure 8. B3P86/6-31G** minimum-energy conformations and optimised
2+
bond lengths (in Š) calculated for a) 18+ (Cs symmetry) and b) 18 (C2v
C
symmetry). B3P86/6-31G**-optimised bond lengths calculated for
Figure 7. Electron density contours (0.03 ebohrꢁ3
)
calculated for the
c) TMTTF+ (planar, D2h symmetry) and d) TMTTF2+ (planar, D2 sym-
C
HOMO and the LUMO of 18.
metry) are included for comparison.
Chem. Eur. J. 2006, 12, 3389 – 3400
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3395

M. R. Bryce et al.
tions and the optimised bond lengths calculated for 18+ and
14p electrons). There is, therefore, a substantial gain in aro-
C
182+. Oxidation affects the whole molecule by modifying
the lengths of the bonds in which the electron density in the
HOMO (that is, the orbital from which the electrons are re-
moved) is concentrated. In this way, the largest changes in
going to the dication correspond to (compare Figures 5b
maticity on going from 18+ to 182+, as is the case with 22+
C
(two 6p-electron dithiolium cations and neutral 14p-electron
anthracene). This gain of aromaticity accounts for the fact
that the first two oxidation processes of 18 coalesce under
the same oxidation wave as observed with 2. The thioxan-
thenium unit in 182+ (14p electrons) is more stable than the
acenium unit in 72+ (16p electrons) and, therefore, the ex-
traction of a third electron is not observed for 18.
ꢁ
and 8b) 1) the exocyclic C9 C14 bond, which lengthens
from 1.362 Š (18) to 1.491 Š (182+), 2) the dithiole S C14
ꢁ
bonds, which shorten from 1.769 to 1.687 Š, 3) the thioxan-
ꢁ
thene C S1 bonds, which shorten from 1.770 to 1.711 Š, and
The S-oxidised compounds 27 and 28 were calculated to
undergo the same structural evolution upon oxidation as
compound 18. However, the presence of the electron-with-
drawing SO and SO2 groups determines that two oxidation
waves are observed in the CVs of 27 and 28 instead of the
single, two-electron oxidation recorded for 18. To investigate
this effect, compounds 18, 27 and 28 were recalculated in so-
lution (CH3CN) to take into account solvent effects. The
energy required for the first ionisation process (neutral!
cation radical) was computed to be slightly higher for 27
(5.59 eV) and 28 (5.73 eV) than for 18 (5.35 eV). This is con-
sistent with the positively shifted Eo1x values measured for 27
and 28 (Table 1). In contrast, the energy required for the
second ionisation process (cation radical!dication) was
found to be significantly larger for 27 (6.62 eV) and 28
(6.84 eV) than for 18 (5.44 eV). This indicates that the re-
moval of the second electron is more difficult for 27 and 28
thus justifying the fact that the two ionisation processes
appear as well-separated, one-electron oxidation processes
in the CVs of 27 and 28. These data are consistent with the
SEEPR results, which showed the spin is delocalised over
both moieties.
ꢁ
4) the C C9 bonds, which are reduced from 1.476 to
1.416 Š. Similar changes are found for 27 and 28 (e.g., the
C9 C14 bond lengthens from 1.362 to 1.490 Š in 27 and
from 1.367 to 1.495 Š in 28).
ꢁ
The cation radical of 18 is predicted to retain the butterfly
structure of the neutral molecule (see Figure 8a), although is
slightly less folded (147.48). The charge distribution calculat-
ed for 18+ using the natural populations analysis (NPA) ap-
C
proach indicates that the charge is extracted from both the
dithiole ring, which accumulates a charge of +0.58e, and
the thioxanthene unit (+0.42e). The geometry of the dithio-
lium unit in 18+ is indeed closer to that obtained for the tet-
C
ramethyl-TTF cation (TMTTF+ ), where both rings bear a
C
charge of +0.5e, than to that found for TMTTF2+, where
both rings support a charge of +1e (compare Figure 8a, c
and d).
The removal of the second electron to form 182+ leads to
more marked effects on the molecular conformation. The
ꢁ
lengthening of the exocyclic C9 C14 bond allows rotation
of the dithiole ring to minimise the steric interactions of the
dithiole sulfur atoms with the peri hydrogen atoms. The
minimum-energy conformation thus corresponds to the C2v
structure depicted in Figure 8b, in which the dithiole ring
lies perpendicular to the fully planar thioxanthene unit. The
As found for 18, electrons are extracted from both the di-
thiole and the fluorene units in 13a. For the cation radical,
the dithiole ring accumulates a charge of +0.70e. This
butterfly Cs structure obtained for 18 and 18+ was also con-
charge is higher than that obtained for 18+ (+0.58e) show-
C
C
sidered for 182+, but it was found to be 9.5 kcalmolꢁ1 higher
in energy. The major structural changes found for the 18!
182+ oxidation process are similar to those reported for 2[4,6]
and 7.[14] For 182+, the dithiole ring and the thioxanthene
unit are predicted to support almost identical charges (+
0.96e and +1.04e, respectively).
ing that electrons are more difficult to remove from the
fluorene unit in 13a. For the dication, both the dithiole ring
and the fluorene unit support a charge of +1.00e. As shown
in Figure 9 for 13a2+, the planarity of the fluorene unit is
preserved upon oxidation and the dithiole ring twists out of
ꢁ
the molecular plane. The exocyclic C9 C14 bond lengthens
from 1.370 Š in neutral 13a to 1.405 Š in 13a+ and 1.444 Š
C
Theoretical results therefore indicate that both the first
and the second electrons are extracted from the dithiole and
the thioxanthene units of 18. The C2v conformation of 182+
in 13a2+. This is accompanied by an increase in the twisting
angle between the two p systems (14.68 for the cation radi-
cal and 38.58 for the dication). The twisting reduces the in-
teraction between the dithiole sulfur atoms and the peri hy-
drogen atoms, which increases in electrostatic character
upon oxidation. In the 13a2+ dication, the dithiole sulfur
atoms have an NPA net charge of +0.78e and are situated
at 2.67 Š from the peri hydrogen atoms, which support a
charge of +0.28e. In contrast to 182+, for which the most
stable conformation is fully orthogonal (see Figure 8b), the
twisted C2 structure depicted in Figure 9a for 13a2+ is calcu-
lated to be 4.4 kcalmolꢁ1 lower in energy than the fully per-
pendicular C2v structure.
was used as an alternative structure of 18+ , in which an aro-
C
matic dithiolium cation (6p electrons) is oriented perpendic-
ularly to the neutral, fully planar thioxanthene radical unit
(15p electrons). This structure assumes that the first electron
is removed exclusively from the dithiole ring and it was
found to be 11.0 kcalmolꢁ1 higher in energy that the butter-
fly-shaped Cs structure. This suggests that 18+ prefers to
C
adopt a charge-delocalised butterfly structure rather than a
perpendicular structure with the charge localised on the di-
thiolium ring. A similar conclusion was reached for the
cation radical of 7.[14]
In contrast to the monocation, the dication 182+ consists
of two singly charged, closed-shell aromatic units (6p and
The dication 13a2+ can therefore be visualised as an aro-
matic dithiolium cation (6p electrons) linked to a fluorenyl
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Chem. Eur. J. 2006, 12, 3389 – 3400

Oxidised States of p-Electron Donors
FULL PAPER
lations for 182+ assign the low-energy band to the two elec-
tronic transitions computed at 3.14 eV (l=395 nm) and
3.39 eV (l=366 nm), which only involve MOs localised on
the thioxanthene unit. In contrast, the band at l=325 nm is
due to an electronic transition calculated at 3.98 eV (l=
311 nm) which involves the HOMO and the LUMO of the
dithiolium cation. The intense band observed at approxi-
mately l=260 nm is again due to an electronic transition
(4.89 eV, 254 nm) that only concerns the thioxanthene MOs.
The electronic spectrum of 182+ thus results from the addi-
tion of the spectroscopic features of the dithiolium and thio-
xanthenium cations. The electronic interactions between the
two p subsystems are limited by the perpendicular disposi-
tion they adopt in 182+
.
Conclusion
Figure 9. a) Minimum-energy conformation and b) optimised bond
lengths (in Š) calculated at the B3P86/6-31G** level for 132+ (C2 symme-
try). c) B3P86/6-31G**-optimised bond lengths calculated for the fluoren-
yl cation.
We have prepared two series of novel p-electron donors, 9-
(1,3-dithiol-2-ylidene)fluorenes (9) and 9-(1,3-dithiol-2-yli-
dene)thioxanthenes (10), and studied in detail their electro-
chemical and spectroscopic properties as well as their mo-
lecular and electronic structures (by means of X-ray analysis
and DFT calculations, respectively). All these compounds
are strong two-electron donors, but the oxidation potentials
depend critically on the electronic structure of the oxidised
state. Whereas standard two, single-electron oxidations
(Eo1x <Eo2x) were observed for 9-(1,3-dithiol-2-ylidene)fluo-
rene systems, the unusual phenomenon of inverted poten-
tials (Eo1x >Eo2x) in 9-(1,3-dithiol-2-ylidene)thioxanthene de-
rivatives results in a single, two-electron oxidation process.
The latter is due to the aromatic structure of the thioxanthe-
nium cation (formed on the loss of a second electron),
which stabilises the dication state compared with the radical
cation and contrasts with the nonaromatic structure of the
fluorenium cation. The two-electron oxidation wave in thio-
xanthene derivatives can be split into two separate one-elec-
tron waves by destabilisation of the dication state in S-oxide
derivatives. An added attraction of systems 9 and 10 is that
they can be readily functionalised to provide new deriva-
tives for inter- and intramolecular charge-transfer studies.
cation (12p electrons). The structure for the fluorenyl
moiety in 13a2+ is very similar to that of the fluorenyl
cation (compare Figure 9b and c), and does not show the
bond alternation of a typical antiaromatic system, such as
the cyclopentadienyl cation (0.225 Š).[30] As discussed by
Schleyer et al.,[30] the term “antiaromatic” is not appropriate
for the fluorenyl cation, and these authors prefer the term
“nonaromatic”. This could explain the different electro-
chemical behaviour observed for 13a when compared with
18. As discussed above for 18, a gain in aromaticity is associ-
2+
ated with the 18+ !18 process (two aromatic units are
C
formed) and the first two oxidation processes coalesce
under the same oxidation wave. For 13a, the aryl unit is not
actually aromatised in going to the dication and the two oxi-
dation processes appear well separated in the CV (see
Table 1).
As shown in Figure 1, compound 16 presents a third irre-
versible oxidation process near the solvent limit and separat-
ed by only 0.20 V from the second oxidation process. Calcu-
lations indicate that oxidation in 16 takes place in a different
manner than in 13a. In the cation radical 16+ the dithiole
C
ring accumulates a charge of +0.47e, the fluorene unit
+0.22e and the two tert-butylphenyl units +0.31e. In pass-
ing to 162+, the net charges increase to +0.59, +0.53 and
+0.88e, respectively, thus indicating that the phenyl sub-
stituents are actively participating in the oxidation process.
This result explains the fact that the third oxidation process
Experimental Section
X-ray crystallography: X-ray diffraction experiments (see Table S1 in the
Supporting Information) were carried out on a SMART 3-circle diffrac-
tometer with a 1 K CCD area detector, using graphite-monochromated
MoKa radiation (l=0.71073 Š) and a Cryostream (Oxford Cryosystems)
open-flow N2 cryostat. Numerical absorption correction based on real
crystal shape was applied for 13b and 16. The structures were solved by
direct methods and refined by using full-matrix least squares against F2
of all data with SHELXTL 5.1 software (Bruker AXS, Madison, Wiscon-
sin, U.S.A., 1997). The solvent of crystallisation (deuteriochloroform in
16; ethyl acetate in 28) is disordered. CCDC-285998 (13b), -285999 (16),
-286000 (18) and -286001 (28) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
in 16 (162+!163+ ) takes place at a potential (+1.25 V) sim-
C
ilar to that measured for the second oxidation process in
13a (+1.20 V).
We finally discuss the evolution of the optical properties
of 18 upon oxidation. As shown in Figure 3, the intense
band observed at l=384 nm for the neutral system disap-
pears as oxidation takes place and two new features grow
up at l=325 nm and approximately 400 nm. TDDFT calcu-
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3397

M. R. Bryce et al.
UV/Vis spectroelectrochemistry: Spectroelectrochemical data were re-
corded in CH3CN (ca. 10ꢁ4 m solution) with TBA+ PF6ꢁ (0.2m) as the sup-
porting electrolyte on a Varian Cary 5 spectrophotometer at ambient
temperature. Spectra were corrected for background absorption arising
from the cell, the electrolyte and the working electrode. The OTTLE cell
(optical path 1 mm) used a Pt gauze working electrode, Pt wire counter
and pseudo-reference electrodes. The solutions were analysed in situ: the
working electrode was held at a potential at which no electrochemical
work was being done in the cell and the spectrum of the neutral com-
pound was recorded; this was identical to the spectrum recorded for
open-circuit conditions. The potential was then increased in 50–200 mV
increments and held until equilibrium had been obtained, as evidenced
by a sharp drop in the cell current.
found: C 39.58, H 2.13. A single crystal suitable for X-ray analysis was
obtained by crystallisation from chlorobenzene.
2,7-Bis[4-(tert-butyl)phenyl]fluorenone (15): [Pd(PPh3)4] (140 mg) was
added to the solution of 2,7-dibromofluorenone (11a) (0.676 g, 2 mmol)
in degassed DMF (30 mL), and the mixture was stirred at 208C for
5 min. 4-(tert-Butyl)benzeneboronic acid (0.89 g, 5 mmol) and 1.5m aque-
ous sodium carbonate solution (3 mL) were added and the mixture was
stirred at 808C for 22 h. Water (60 mL) was added to the reaction mix-
ture and the yellow precipitate was collected by suction filtration, washed
with water and purified by column chromatography (silica gel, chloro-
form/hexane 1:1, v/v) to afford 15 as golden yellow crystals (0.79 g,
89%). M.p. 319.1–319.78C; 1H NMR ([D2]tetrachloroethane, 300 MHz):
d=1.40 (s, 9H), 7.51 (d, J=8.4 Hz, 2H), 7.60 (d, J=7.8, 1H), 7.61 (d, J=
8.4 Hz, 2H), 7.77 (dd, J1,2 =7.8 Hz, J1,3 =1.8 Hz, 1H), 7.92 ppm (d, J=
1.8 Hz, 1H); 13C NMR ([D2]tetrachloroethane, 75 MHz) d=197.4, 154.6,
146.3, 145.2, 140.0, 138.6, 136.6, 129.8, 129.5, 126.1, 124.3, 38.1, 34.9 ppm;
UV/Vis (CHCl3): lmax (loge)=294 (5.30), 326 (4.76), 339 (4.68), 446 nm
(3.65); MS (EI): m/z (%): 444 (100) [M+]; elemental analysis calcd (%)
for C33H32O: C 89.15, H 7.25; found: C 88.62, H 7.23.
Simultaneous electrochemistry and EPR: Data were recorded in CH2Cl2
ꢁ
(ca. 10ꢁ4 m solution) with TBA+ PF6 (0.2m) as the supporting electrolyte
placed in a quartz homemade electrochemical cell equipped with a Pt
wire working electrode, Pt wire counter and pseudo-reference electrodes.
The cell was placed in the cavity of an X-band Bruker ESP 300E spectro-
photometer and spectra were taken after some minutes of electrolysis at
a constant potential of 1.04 V.
2,7-Bis[4-(tert-butyl)phenyl]-9-(4,5-dimethyl-1,3-dithiol-2-ylidene)fluo-
rene (16): Analogous to the synthesis of 13a and 13b, compound 12
(0.52 g, 2.18 mmol) was treated with LDA (2.20 mmol) in dry THF
(30 mL) at ꢁ788C. Compound 15 (0.614 g, 1.38 mmol) was added and the
mixture was stirred at 208C for 24 h. The product was purified on a silica
gel column (eluent CS2), to afford 16 as bright yellow crystals (0.71 g,
92%). M.p. >3508C; 1H NMR (CDCl3 +CS2, 300 MHz): d=1.42 (s, 9H),
2.24 (s, 3H), 7.46 (d, J=8.1 Hz, 3H), 7.60 (d, 2H, J=8.1 Hz), 7.79 (d, J=
7.8 Hz, 1H), 7.89 ppm (s, 1H); 13C NMR (CDCl3, 75 MHz): d=13.1, 31.5,
34.6, 119.7, 121.6, 123.7, 124.2, 125.7, 127.1, 136.4, 138.3, 139.4, 139.9,
150.1 ppm; UV/Vis (CHCl3): lmax (loge)=273 (4.71), 332 (5.51), 414
(4.31), 432 nm (4.35); MS (EI): m/z (%): 558 (100) [M+]; elemental anal-
ysis calcd (%) for C38H38S2: C 81.67, H 6.85; found: C 81.68, H 6.88. A
crystal for X-ray analysis was obtained by slow evaporation of a solution
of 16 in a mixture of CDCl3 and carbon disulfide.
Computational details: All theoretical calculations were carried out with
the DFT approach by using the C.02 revision of the Gaussian 03 program
package.[31] DFT calculations were performed using Beckeꢁs three-param-
eter B3P86 exchange-correlation functional[32] and the 6-31G** basis
set.[33] The B3P86 functional has been recognised as providing equilibri-
um geometries for sulfur-containing compounds in better accord with ex-
perimental data and ab initio post-Hartree–Fock (HF) calculations than
the more widely used B3LYP functional.[34] The radical cations were
treated as open-shell systems and were computed using spin-unrestricted
UB3P86 wavefunctions. Vertical electronic excitation energies were de-
termined by means of the TDDFT approach.[35] Numerous hitherto re-
ported applications indicate that TDDFT employing current exchange-
correlation functionals performs significantly better than HF-based
single-excitation theories for the low-lying valence excited states. Net
atomic charges were calculated by using the NPA analysis[36] included in
the natural bond orbital (NBO) algorithm proposed by Weinhold and co-
workers.[37] Solvent effects were considered with the SCRF (self-consis-
tent-reaction-field) theory using the polarised continuum model (PCM)
approach to model the interaction with the solvent.[38] The PCM model
considers the solvent as a continuous medium with a dielectric constant
(e), and represents the solute by means of a cavity built with a number of
interlaced spheres.[39]
9-(4,5-Dimethyl-1,3-dithiol-2-ylidene)thioxanthene (18) was prepared as
described previously.[5]
9-(4-Methyl-1,3-dithiol-2-ylidene)thioxanthene (20): Following the proce-
dure for compound 18, reagent 19[10] (3.75 g, 16.37 mmol), LDA (12 mL,
18.01 mmol) and 17 (3.47 g, 16.37 mmol) were stirred overnight. Chroma-
tography on silica gel (eluent, dichloromethane/hexane 1:1 v/v) and re-
crystallisation from ethyl acetate gave compound 20 as yellow crystals
1
(2.45 g, 48%). M.p. 140–1428C; H NMR (CDCl3): d=7.65 (t, J=7.6 Hz,
2H), 7.40 (dd, J=7.6 Hz, J=1.2, 2H), 7.28 (t, J=7.6 Hz, 2H), 7.19 (t, J=
7.6 Hz, 2H), 5.79 (d, J=1.5 Hz, 1H), 2.0 ppm (d, J=1.5 Hz, 3H);
13C NMR (CDCl3): d=139.1, 137.3, 136.9, 132.23, 132.20, 129.3, 126.95,
126.94, 126.86, 126.83, 126.47, 126.43, 125.9, 121.8, 110.9, 16.0 ppm; MS
(EI): m/z (%): 312 (100) [M+]; UV/Vis (CHCl3): lmax (loge)=378.8 nm
(0.365); elemental analysis calcd (%) for C17H12S3: C 65.34, H 3.87;
found: C 65.31, H 3.79.
2,7-Dibromo-9-(4,5-dimethyl-1,3-dithiol-2-ylidene)fluorene (13a): LDA
(5.1 mmol, 2.55 mL of 2m solution in THF/heptane, ACROS) was added
dropwise within 0.5 h to a stirred solution of phosphonate ester 12[5]
(1.22 g, 5.1 mmol) in dry THF (30 mL) at ꢁ788C. A solution of 2,7-dibro-
mofluorenone 11a (1.70 g, 5.03 mmol) in THF (30 mL, warmed to dis-
solve) was syringed into the flask through a septum. The cooling bath
was removed and the yellow suspension was stirred at 208C for 15 h fol-
lowed by heating to 508C for 15 min. Ethanol (50 mL) was added and
the solid was collected by suction filtration, then recrystallised from
chlorobenzene to yield compound 13a as yellow needles (1.60 g, 70%).
M.p. 295.0–295.88C; 1H NMR (CDCl3, 300 MHz): d=2.23 (s, 6H), 7.41
(d, J=8.1 Hz, 2H), 7.67 (d, J=8.0 Hz, 2H), 7.91 ppm (s, 2H); UV/Vis
(CHCl3): lmax (loge)=260 (4.88), 302 (4.21), 315 (4.29), 414 (4.52),
434 nm (4.59); MS (EI): m/z (%): 452 (100) [M+]; elemental analysis
calcd (%) for C18H12Br2S2: C 47.81, H 2.67; found: C 47.72, H 2.65.
9-(4-Methoxycarbonyl-5-methyl-1,3-dithiol-2-ylidene)thioxanthene (21):
LDA (5.64 mL, 8.46 mmol) was added to a stirred solution of compound
20 (2.40 g, 7.69 mmol) in dry THF (50 mL) under N2 at ꢁ788C. The reac-
tion mixture was stirred for 2 h. Methyl chloroformate (1.18 mL,
15.38 mmol) was added and the mixture was stirred and left to warm to
room temperature overnight. The solvents were removed in vacuo and
the residue was purified by chromatography on silica gel with dichloro-
methane as the eluent; recrystallisation from ethyl acetate gave com-
pound 21 as orange crystals (1.90 g, 67%). M.p. 206–2078C; 1H NMR
(CDCl3): d=7.60 (m, 2H), 7.44 (m, 2H), 7.32 (m, 2H), 7.24 (m, 2H),
3.76 (s, 3H), 2.368 ppm (s, 3H); 13C NMR (CDCl3): d=160.6, 145.4,
136.18, 136.16, 132.44, 132.37, 132.2, 127.23, 127.18, 127.0, 126.9, 126.6,
126.4, 126.1, 126.0, 123.0, 117.0, 52.2, 15.5 ppm; UV/Vis (CHCl3): lmax
(loge)=368.2 nm (0.462); MS (EI): m/z (%): 370 (100) [M+]; elemental
analysis calcd (%) for C19H14O2S3: C 61.59, H 3.81; found: C 61.31, H
3.74.
2,7-Diiodo-9-(4,5-dimethyl-1,3-dithiol-2-ylidene)fluorene (13b): 2,7-Diio-
dofluorenone 11b (1.81 g, 4.19 mmol) was synthesised in 78% yield
based on the reported method,[40] and was added in one portion to a solu-
tion of 12 (1.12 g, 4.69 mmol) treated with LDA (4.8 mmol) as above,
and the mixture was stirred at 208C for 36 h. Product 13b was obtained
as yellow needles after recrystallisation from chlorobenzene (1.88 g,
82%). M.p. ꢀ2708C (decomp); 1H NMR (CDCl3, 300 MHz): d=2.23 (s,
6H), 7.56 (d, J=8.1 Hz, 2H), 7.61 (dd, J1,2 =8.0 Hz, J1,3 =1.2 Hz, 2H),
8.10 ppm (d, J1,3 =1.2 Hz, 2H); UV/Vis (CHCl3): lmax (loge)=264 (5.05),
320 (4.58), 417 (4.66), 437 nm (4.73); MS (EI): m/z (%): 546 (12) [M+],
149 (100); elemental analysis calcd (%) for C18H12I2S2: C 39.58, H 2.21;
9-(4-Hydroxymethyl-5-methyl-1,3-dithiol-2-ylidene)thioxanthene
(22):
Lithium aluminium hydride (0.61 g, 16.19 mmol) was added to a stirred
solution of compound 21 (1.50 g, 4.05 mmol) in dry THF (50 mL) under
3398
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 3389 – 3400

Oxidised States of p-Electron Donors
FULL PAPER
N2 at ꢁ788C, and the mixture was stirred for 2 h at room temperature.
After adding wet sodium sulphate (to quench the unreacted LiAlH4)
dropwise, the reaction was stirred for 30 min. The reaction was filtered
through Celite, while washing with methanol. After evaporation of the
filtrate, the residue was recrystallised from ethyl acetate to give 22 as
yellow crystals (1.09 g, 79%). M.p. 209–2128C; 1H NMR ([D6]DMSO):
d=7.57 (t, J=7.6 Hz, 2H), 7.47 (d, J=7.6 Hz, 2H), 7.36 (m, 2H), 7.28 (t,
J=7.6 Hz, 2H), 5.34 (t, J=5.6 Hz, 1H), 4.17 (m, 2H), 1.93 ppm (s, 3H);
13C NMR ([D6]DMSO): 136.22, 136.8, 135.9, 131.02, 131.01, 128.0, 127.1,
126.9, 126.8, 126.3, 121.9, 120.6, 56.0, 12.7 ppm; UV/Vis (CHCl3): lmax
(loge)=376 nm (0.314); MS (EI): m/z (%): 342 (100) [M+]; elemental
analysis calcd (%) for C18H14OS3: C 63.12, H 4.12; found: C 63.07, H
4.20.
sulting mixture was put under reflux for 2 h, then cooled to 208C to
afford a precipitate which was filtered and washed with hexane to afford
compound 30 as yellow crystals (0.938 g, 81%). M.p. 179–1808C;
1H NMR (CDCl3): d=8.35 (dd, J=7.5 Hz, J=1.5 Hz, 2H), 8.19 (dd, J=
7.5 Hz, J=1.5 Hz, 2H), 7.88 (td, J=8 Hz, J=1.5 Hz, 2H), 7.78 ppm (td,
J=8 Hz, J=1.5 Hz, 2H); 13C NMR (CDCl3): d=178.6, 141.2, 134.9,
133.5, 130.9, 129.4, 123.8 ppm; UV/Vis (CHCl3): lmax (loge)=383 nm
(0.253); MS (EI): m/z (%): 244 (53) [M+], 196 (100); elemental analysis
calcd (%) for C13H8O3S: C 63.92, H 3.30; found: C 63.69, H 3.29.
9-(4,5-Dimethyl-1,3-dithiol-2-ylidene)thioxanthene-S-oxide (27): Follow-
ing the procedure for the preparation of 18, reagent 12 (0.155 g,
0.64 mmol), LDA (0.47 mL, 0.7 mmol) and compound 25 (0.146 g,
0.64 mmol) in THF (40 mL) were stirred overnight. After evaporation
and chromatography on silica gel (eluent, dichloromethane), recrystallisa-
tion from ethyl acetate gave compound 27 as yellow crystals (0.03 g,
14%). M.p. 235–2378C; 1H NMR (CDCl3): d=7.84 (d, J=8 Hz, 2H),
7.73 (d, J=8 Hz, 2H), 7.40 (m, 4H), 2.09 ppm (s, 6H); UV/Vis (CHCl3):
lmax (loge)=396 nm (0.209); HRMS: m/z calcd for C18H14OS3: 342.02067;
found: 342.0217.
9-[4-(2,5,7-Trinitrofluoren-9-one)carbonyloxymethyl-5-methyl-1,3-dithiol-
2-ylidene]thioxanthene (23): n-Butyllithium (0.91 mL, 1.45 mmol) in
THF was added to a stirred solution of compound 22 (0.50 g, 1.45 mmol)
in dry THF under N2 at ꢁ1008C, and the reaction mixture was stirred for
30 min. Then 2,5,7-trinitro-4-chlorocarbonylfluoren-9-one[16] (0.55 g,
1.45 mmol) was added very slowly and the solution was warmed to 208C
and stirred overnight. After evaporation, the residue was taken up in di-
chloromethane, and washed several times with water and then dried over
magnesium sulfate. The residue was purified by chromatography on silica
gel with chloroform as the eluent to afford compound 23 as a black solid
(0.20 g, 20%). M.p. 162–1638C; 1H NMR (CDCl3): d=8.92 (s, 1H), 8.85
(s, 1H), 8.79 (s, 1H), 8.75 (s, 1H), 7.58 (d, J=7.6 Hz, 2H), 7.39 (t, J=
7.6 Hz, 2H), 7.28 (dd, J=7 m, 2H), 7.20 (m, 2H), 5.06 (d, J=7 Hz, 1H),
5.00 (d, J=7 Hz, 1H), 2.13 ppm (s, 3H); 13C NMR (CDCl3): d=184.8,
164.0, 149.6, 149.3, 146.5, 143.4, 139.6, 138.7, 137.7, 136.5, 136.4, 134.0,
132.2, 131.6, 131.4, 130.9, 127.12, 127.09, 126.99, 126.5, 126.4, 126.02,
125.98, 125.3, 122.657, 122.53, 122.0, 118.6, 60.5, 13.6 ppm; UV/Vis
(CHCl3): lmax (loge)=361 nm (0.223); MS (ES): m/z (%): 682 (100) [M+
]; elemental analysis calcd (%) for C32H17N3O9S3: C 56.22, H 2.51, N
6.15; found: C 55.95, H 2.55, N 5.89.
9-(4,5-Dimethyl-1,3-dithiol-2-ylidene)thioxanthene-S,S-dioxide (28): Di-
rectly analogous to the preparation of 27, compound 26 gave compound
28 as yellow crystals (0.148 g, 10%). M.p. 243–2448C; 1H NMR (CDCl3):
d=8.06 (dd, J=8.0 Hz, J=1.2 Hz, 2H), 7.91 (d, J=8.0 Hz, J=1.2 Hz,
2H), 7.55 (td, J=8.0 Hz, J=1.2 Hz, 2H), 7.41 (td, J=8.0 Hz, J=1.2 Hz,
2H), 1.97 ppm (s, 6H); 13C NMR (CDCl3): d=144.7, 138.6, 134.9, 132.0,
127.2, 127.1, 123.9, 122.1, 112.8, 13.3 ppm; UV/Vis (CHCl3): lmax (loge)=
431 nm (0.282); MS (EI): m/z (%): 358 (100) [M+]; elemental analysis
calcd (%) for C18H14O2S3: C 60.30, H 3.94; found: C 60.35, H 3.92. A
crystal for X-ray analysis was obtained from ethyl acetate.
9-(4-Methyl-1,3-dithiol-2-ylidene)thioxanthene-S,S-dioxide (29): Follow-
ing the procedure for the preparation of 28, compound 26 and reagent 19
gave compound 29 as a yellow powder (0.45 g, 49%). M.p. 184–1868C;
1H NMR (CDCl3): d=8.06 (dd, J=8.0 Hz, J=1.3 Hz, 2H), 7.93 (d, J=
8.0 Hz, J=1.2 Hz, 2H), 7.55 (td, J=8.0 Hz, J=1.3 Hz, 2H), 7.42 (td, J=
8.0 Hz, J=1.3 Hz, 2H), 5.95 (s, 1H), 2.10 ppm (s, 3H); 13C NMR
(CDCl3): d=138.8, 138.3, 134.9, 132.1, 130.7, 127.2, 127.1, 123.9, 113.6,
110.0, 16.5 ppm; UV/Vis (CHCl3): lmax (loge)=431 nm (0.282); MS (EI):
m/z (%): 344 (100) [M+]; elemental analysis calcd (%) for C17H12O2S3: C
59.27, H 3.51; found: C 59.12, H 3.60.
9-[4-(2,5,7-Trinitrofluoren-9-dicyanomethylene)carbonyloxymethyl-1,3-di-
thiol-2-ylidene]thioxanthene (24): Malononitrile (14.5 mg, 0.21 mmol)
was added to a stirred solution of compound 23 (0.03 g, 0.043 mmol) dis-
solved in dry DMF (ca. 2 mL) under N2 at room temperature, and the so-
lution was stirred overnight at 208C. The solvent was removed in vacuo,
and methanol was added and the mixture was placed at 58C for 30 min.
The solid was filtered and washed with methanol to give compound 24 as
a green solid (16 mg, 51%). M.p. >3008C; 1H NMR (CDCl3): d=9.69
(d, J=2 Hz, 1H), 9.62 (d, J=2 Hz, 1H), 8.98 (d, J=2 Hz, 1H), 8.90 (d,
J=2 Hz, 1H), 7.58 (d, J=7.6 Hz, 2H), 7.40 (d, J=7.6 Hz, 2H), 7.15–7.35
(m, 4H), 5.07 (d, J=7 Hz, 1H), 5.00 (d, J=7 Hz, 1H), 2.13 ppm (s, 3H);
13C NMR (CDCl3): d=163.9, 153.4, 149.5, 149.0, 147.0, 140.6, 138.9,
137.9, 137.1, 136.8, 136.7, 132.5, 132.2, 131.8, 130.7, 127.4, 127.3, 127.2,
126.8, 126.7, 126.3, 124.9, 124.1, 123.4, 123.0, 118.7, 111.7, 111.6, 60.9,
13.9 ppm; UV/Vis (CHCl3): lmax (loge)=305 (0.46), 358 (0.38), 374 nm
Acknowledgements
We thank EPSRC for funding the work in Durham. D.F.P. thanks the
Royal Society of Chemistry for a Journals Grant for International Au-
thors for funding a visit to Durham. The work at Valencia was supported
by the MEC of Spain and by FEDER funds (Grant BQU2003-05111).
Funding from the Generalitat Valenciana (OCYT-GRUPOS03/173) is
also acknowledged.
(0.41); IR (KBr): n˜ =2231 (CN), 1741 (CO), 1636, 1540, 1458, 1344 cmꢁ1
;
MS (ES): m/z (%): 730.4 (100) [M+]; HRMS: calcd for C35H17N5O8S3:
731.0239; found: 731.0225.
Thioxanthen-9-one-S-oxide (25): Hydrogen peroxide (35% aqueous solu-
tion, 0.32 g, 9.43 mmol) was added to a stirred solution of thioxanthen-9-
one 17 (1.0 g, 4.71 mmol) in hexafluoro-2-propanol (5 mL) at 208C, and
the mixture was refluxed for 15 min. The hydrogen peroxide was
quenched by addition of a saturated aqueous solution of sodium sulfite.
Then water was added to the mixture; the precipitate was filtered and
subjected to chromatography on a silica gel column with hexane/diethyl
ether (1:1 v/v) as eluent, to afford compound 25 as yellow crystals
(0.146 g, 14%). M.p. 118–1208C; 1H NMR (CDCl3): d=8.39 (dd, J=
7.8 Hz, J=1.2 Hz, 2H), 8.19 (dd, J=7.8 Hz, J=1.2 Hz, 2H), 7.86 (t, J=
7.6 Hz, 2H), 7.74 ppm (t, J=7.6 Hz, 2H); 13C NMR (CDCl3): d=180.1,
145.2, 134.0, 131.7, 129.6, 128.8, 127.3 ppm; UV/Vis (CHCl3): lmax
(loge)=327 nm (0.914); MS (EI): m/z (%): 228 (100) [M+]; elemental
analysis calcd (%) for C13H8O2S: C 68.40, H 3.53; found: C 68.25, H 3.50.
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Received: October 24, 2005
Published online: February 2, 2006
3400
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