Synthesis and properties of catechol-fused tetrathiafulvalene derivatives and their hydrogen-bonded conductive charge-transfer salts

Article abstract of DOI:10.1016/j.tetlet.2012.06.020

Catechol-fused tetrathiafulvalene (TTF) derivatives have been designed and synthesized as a new type of π-electron donor molecules having two phenolic hydroxyl groups. Cyclic voltammetry measurements and quantum chemical calculations demonstrated the electronic effect of the direct fusion of the catechol unit to the TTF π-skeleton. In the charge-transfer (CT) salts with bromide or chloride anions, a one-dimensional hydrogen-bonded chain was formed by the intermolecular OH?X network between the catechol moieties and the halide anions. The slight dissimilarity of the hydrogen-bond distances for the two CT salts gave rise to the significant differences in their overall molecular arrangements and intermolecular interactions as well as the electrical resistivities.

Full text of DOI:10.1016/j.tetlet.2012.06.020

Tetrahedron Letters 53 (2012) 4385–4388
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and properties of catechol-fused tetrathiafulvalene derivatives
and their hydrogen-bonded conductive charge-transfer salts
Hiromichi Kamo a, , Akira Ueda a, , Takayuki Isono a, Kazuyuki Takahashi b, Hatsumi Mori a,
⇑
⇑
⇑
a The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
b Department of Chemistry, Graduate School of Science, Kobe University, Kobe, Hyogo 657-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Catechol-fused tetrathiafulvalene (TTF) derivatives have been designed and synthesized as a new type of
-electron donor molecules having two phenolic hydroxyl groups. Cyclic voltammetry measurements
and quantum chemical calculations demonstrated the electronic effect of the direct fusion of the catechol
unit to the TTF -skeleton. In the charge-transfer (CT) salts with bromide or chloride anions, a one-
dimensional hydrogen-bonded chain was formed by the intermolecular OHꢀ ꢀ ꢀX network between the cat-
echol moieties and the halide anions. The slight dissimilarity of the hydrogen-bond distances for the two
CT salts gave rise to the significant differences in their overall molecular arrangements and intermolec-
ular interactions as well as the electrical resistivities.
Received 10 May 2012
Revised 4 June 2012
Accepted 6 June 2012
Available online 13 June 2012
p
p
Keywords:
Tetrathiafulvalene
Catechol
Charge-transfer salt
Hydrogen bond
Electrical resistivity
Ó 2012 Elsevier Ltd. All rights reserved.
Tetrathiafulvalene (TTF), a well-known strong organic electron
donor molecule with a planar -conjugated skeleton, and its deriv-
(Fig. 1). By directly fusing the catechol unit to the TTF skeleton,
we especially expect that the electronic state of the TTF -skeleton
p
p
atives have been extensively studied as a promising component of
functional solids and materials that show (super)conductivity,
magnetism, dielectric and optical properties, etc.1 Chemists have
and the overall bulk physical properties of the 1-based molecular
system can be significantly changed depending on the hydroxyl-
group environment of the catechol moiety. In contrast to a lot of
studies on the TTF derivatives having a hydroxyl group linked by
devoted a lot of efforts to functionalize TTF-based
p-electronic
molecules to explore unique structural and electronic features
and physical properties attributable to combination effects of the
parent TTF system with the introduced functional groups or
substituents.1–4 For example, a number of TTF derivatives bearing
hydrogen-bonding functionalities, such as alcohols,3a–c (thio)ami-
des,3d–g nucleobases,3h–j and heterocycles,3k–m have been designed
and synthesized so far. In their charge-transfer (CT) salts and
complexes, such hydrogen-bonding functionalities made a signifi-
cant contribution to control the electronic state, molecular
arrangements, and solid-state physical properties.3,4
a r p-conjugated system have rarely
-bond,3a–c,4 those linked by a
been reported.7 Here we report the synthesis and properties of the
first catechol-fused TTF derivatives 1 and their hydrogen-bonded
charge transfer salts.
The synthesis of catechol-fused TTF derivatives 1a–e and dise-
lenadithiafulvalene (STF) derivative 1f was achieved by the five-
step synthetic protocol from 1,2-bis(benzyloxy)-4,5-dibromoben-
zene8 (Scheme 1). The Stille-type cross-coupling reaction of the
dibromobenzene derivative with 3-[(tributylstannyl)thio]propane-
nitrile9 quantitatively gave the bis(2-cyanoethylthio) derivative 2.
Catechol is a well-known aromatic molecule that has two phe-
nolic hydroxyl groups at the neighboring positions of a benzene
ring. Because of this structural feature, this system can act not only
as chelating ligands, such as semiquinone and catecholate, in the
metal complexes,5 but also as an anion binder or receptor with
an OHꢀ ꢀ ꢀX hydrogen bond.6 Therefore we have designed cate-
OH
S
S
TTF
S
S
fusion
OH
catechol
hydrogen bonding
metal binding
electron donor
promising component
of functional materials
chol-fused TTF derivatives 1 as new type of
p-electron donor
molecules having hydrogen bonding and metal binding abilities
R
OH
OH
S
S
⇑
S
S
1
Corresponding authors. Tel./fax: +81 4 7136 3410 (H.K., A.U.); tel./fax: +81 4
7136 3444 (H.M.)
R
E-mail addresses: a-ueda@issp.u-tokyo.ac.jp (A. Ueda), hmori@issp.u-tokyo.ac.jp
(H. Mori).
Figure 1. Molecular design of catechol-fused TTF derivatives 1.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.020

4386
H. Kamo et al. / Tetrahedron Letters 53 (2012) 4385–4388
Br
Br
OBn
OBn
S
S
OBn
OBn
OR
S
(a)
(b)
NC
NC
S
S
OR
2
3
4
: R = Bn
: R = TBDMS
(c)
1, 5, 6
X
R'
a
b
c
d
e
f
S
S
S
S
S
SCH3
R'
R'
R'
R'
OR
OR
X
X
(d)
X
S
-SCH2S-
+ 4
O
X
S
-S(CH2)2S-
-S(CH2)3S-
-O(CH2)2O-
5a-f
6a-f
1a-f
: R = TBDMS
(e)
: R = H
Se -S(CH2)2S-
Figure 2. (a) HOMO pictures and (b) HOMO energy levels of EDT-TTF, Benzo-EDT-
TTF, and 1c calculated at the B3LYP/6-31G⁄ level of theory.
Scheme 1. Synthesis of catechol-fused TTF derivatives 1a–f. Reagents and condi-
tions: (a) nBu3SnS(CH2)2CN, Pd(PPh3)4, toluene, reflux, 95%: (b) (i) NaOMe, ZnCl2,
Bu4NBr, MeOH, CH2Cl2, room temperature, (ii) 1,10-thiocarbonyldiimidazole, AcOH,
THF, ꢁ78 °C–room temperature, 60%: (c) (i) BF3ꢀEt2O, nBuSH, CHCl3, 60 °C, (ii) AcOH,
MeOH, 60 °C, (iii) TBDMSCl, Et3N, N,N-dimethyl-4-aminopyridine, CH2Cl2, room
temperature, 53%: (d) P(OEt)3, 100–110 °C, 14–56%: (e) Bu4NF, AcOH, THF, room
temperature, 36–79%.
and EDT-TTF (+0.42 V) are almost the same but significantly smal-
ler than that of Benzo-EDT-TTF (+0.57 V). This result suggests that
the HOMO energy level decreases by the benzo-annelation to the
parent EDT-TTF skeleton, and then it increases by the introduction
of two electron-donating hydroxyl groups into the benzene ring to
form the catechol structure. Furthermore, we found that 1c has a
The treatment of 2 with NaOMe and ZnCl2 followed by the reaction
with 1,10-thiocarbonyldiimidazole under an acidic condition
yielded the 1,3-benzodithiole-2-thione derivative 3. The benzyl
protecting groups of 3 were removed by BF3ꢀEt2O and nBuSH10
and the subsequent reprotection by TBDMS groups afforded 4.11 Fi-
nally the target compounds 1a–f were obtained12 by the cross-cou-
pling reaction between 4 and the corresponding ketone 5a–f in
P(OEt)3 and the following deprotection of the TBDMS groups. These
donors 1a–f are stable in the solid state in air at ꢁ20 °C for a few
months, whereas a solution of 1a–f gradually decomposes under
an aerobic condition. This synthetic scheme can be applicable to
explore further various kinds of asymmetric TTF derivatives with
smaller
DE value (0.25 V) than both EDT-TTF (0.32 V) and Benzo-
EDT-TTF (0.30 V), indicating that the direct fusion of the catechol
skeleton to the TTF system significantly contributes to an exten-
sion of the
p-conjugated system, to lead to the decrease in DE, that
is, the on-site Coulomb repulsion energy.
These electronic effects induced by the direct fusion of the cat-
echol moiety were also illustrated by the density functional theory
(DFT) calculations ( Fig. 2).14 The HOMO coefficients of 1c are
widely distributed not only on the TTF skeleton but also on the cat-
echol moiety including the two oxygen atoms (Fig. 2a). In contrast,
there are much smaller coefficients on the benzene ring in Benzo-
EDT-TTF. Thus the higher delocalization of HOMO in 1c probably
a benzo-fused p-electronic structure by using an appropriate 1,2-
dibromobenzene derivative as the starting material.
contributes to the decrease of the observed
DE value. Furthermore
To evaluate electrochemical properties of this system, we mea-
sured cyclic voltammograms of 1a–f in a CH3CN solution. Similar
to typical TTF derivatives, all the donor molecules 1a–f exhibited
two reversible oxidation waves. As listed in Table 1, the first and
second oxidation potentials (E11/2, E21/2) of 1a–f vary to some ex-
tent (+0.35 to +0.48 V and +0.62 to +0.76 V, respectively), indicat-
ing that the HOMO energy level changes depending on the
substituents R0 at the TTF skeleton. In contrast, differences between
the calculated HOMO energy levels of EDT-TTF (ꢁ4.71 eV), Benzo-
EDT-TTF (ꢁ4.82 eV), and 1c (ꢁ4.72 eV) (Fig. 2b) qualitatively repro-
duce the difference of their observed oxidation potentials (Table 1).
All these experimental and calculated results demonstrate that a
robust p-electronic communication exists between the TTF skele-
ton and the catechol moiety, as we expected.
In order to examine how the two phenolic hydroxyl groups of
the introduced catechol moiety in 1 affect the intermolecular inter-
action and conductivity in the solid state, we attempted to prepare
the charge transfer (CT) salts of 1a–f with various inorganic anions
by the electrochemical oxidation. As a result, two kinds of CT salts
composed of the ethylenedithio derivative 1c and chloride or bro-
mide ion were successfully obtained as black plate crystals and
black block crystals which are suitable for X-ray crystal analyses,
respectively. Both salts crystallize in a space group of P-1 (triclinic)
where two inversion-symmetric donor molecules 1c and one halide
ion exist in the unit cell.15 Thus the compositional formulae of these
salts were determined as (1c)2Cl and (1c)2Br, respectively, indicat-
ing that the charge of the donor 1c is +0.5 with a 1/4-filled band
structure. This donor-anion ratio was also confirmed by the bond
length analyses of the donor skeletons (see the Supplementary
data). In each crystal, both two OH groups of the catechol unit par-
ticipate in the OHꢀ ꢀ ꢀX (X = Cl, Br) intermolecular hydrogen bonds
with the corresponding counter halide ions, respectively
(Fig. 3a,b). The Oꢀ ꢀ ꢀX distances are found to be O1-Cl = 3.23 Å, O2-
Cl = 3.06 Å, O1-Br = 3.28 Å, and O2-Br = 3.31 Å, respectively, which
all are shorter than the sum of their van der Waals radius (O-Cl:
3.27 Å, O-Br: 3.37 Å). Consequently the halide ions are surrounded
by four hydroxyl groups in both crystals, to give an infinite one-
dimensional (1D) chain structure along the a-axis. Although the
hydrogen-bonding manners in (1c)2Cl and (1c)2Br are almost simi-
lar, their overall molecular arrangements are different (Fig. 3c,d).
Namely, (1c)2Cl and (1c)2Br are categorized as b’’-type and b-type,
two oxidation potentials
D
E (=E21/2ꢁE11/2) are similar to each other
(0.24–0.28 V). This result means that the on-site Coulomb repul-
sion energy in 1a–f is almost unchanged in spite of the difference
in R0. Namely the on-site Coulomb repulsion energy of this system
is mostly governed by the catechol-fused TTF p-conjugated struc-
ture itself. Furthermore, to investigate an electronic effect of the
fused catechol moiety on the TTF system, we compared the E1
1/2
and
DE values of the ethylenedithio derivative 1c with those of
EDT-TTF13a and Benzo-EDT-TTF13b (Table 1, see also Fig. 2 for their
chemical structures). The first oxidation potentials of 1c (+0.40 V)
Table 1
The first and second oxidation potentials (E1
,
E2
)
1/2
and their differences
DE
1/2
(=E21/2ꢁE1
) for 1a–f obtained by CV measurements (V vs SCE, glassy carbon
1/2
working electrode with 0.1 M Bu4NClO4 in CH3CN). The literature values for EDT-
TTF13a and Benzo-EDT-TTF13b were also listed as reference
Compound
E11/2/V
E21/2/V
DE/V
1a
1b
1c
1d
1e
1f
+0.39
+0.41
+0.40
+0.43
+0.35
+0.48
+0.42
+0.57
+0.64
+0.65
+0.65
+0.71
+0.62
+0.76
+0.74
+0.87
0.25
0.24
0.25
0.28
0.27
0.28
0.32
0.30
EDT-TTF13a
Benzo-EDT-TTF13b

H. Kamo et al. / Tetrahedron Letters 53 (2012) 4385–4388
4387
Figure 3. X-ray crystal structures of (1c)2Cl and (1c)2Br. Hydrogen bonding motif along the b axis in (a) (1c)2Cl (O1-Cl = 3.23 Å and O2-Cl = 3.06 Å) and in (b) (1c)2Br (O1-
Br = 3.28 Å and O2-Br = 3.31 Å). Molecular arrangement with the calculated overlap integrals of (c) (1c)2Cl and (d) (1c)2Br. Black, yellow, red, pink, green, and brown spheres
represent carbon, sulfur, oxygen, hydrogen, chlorine, and bromine atoms, respectively.
respectively.16 This difference is probably originating from the sub-
tle difference in the hydrogen bond distances depending on the size
of the halide ion. Then, to quantitatively estimate intermolecular
interactions in the crystal, the overlap integrals between two donor
skeletons were calculated by using its HOMO obtained by the ex-
tended Hückel method.17,18 As a result, it turns out that (1c)2Cl pro-
vides its largest value (ꢁ14.3 ꢂ 10ꢁ3) in the q1 direction, in which
the oxidized donors 1c are connected by the hydrogen bonds
through the Cl anion. In contrast, (1c)2Br shows a very small overlap
significant differences in their overall molecular arrangement (b00-
and b-structures) and intermolecular interactions as well as the
electrical resistivity. These results demonstrated that the structure
and physical properties of this system can be readily changed
depending on the hydroxyl-group environment of the catechol
unit. In order to further investigate structural and electronic fea-
tures of this system attributable to the robust intramolecular elec-
tronic interaction and the flexible hydroxyl-group environment
including protonation/deprotonation behaviors, we are now pre-
paring another type of hydrogen-bonded molecular conductors
based on 1a–f as well as the o-semiquinone or catecholate-metal
complexes. Furthermore the synthesis of a tetraselenafulvalene
(TSF) analogue of 1 is of interest and is currently underway.
integral between the hydrogen-bonded donors (p1 = 2.3 ꢂ 10ꢁ3
)
and the largest value is found in the a2 direction (22.9 ꢂ 10ꢁ3). Thus
(1c)2Br has a strong dimerization (a2/a1 = 3.8) in the b-direction in
contrast to no dimerization (q1 = ꢁ14.3 ꢂ 10ꢁ3) for (1c)2Cl.
These dissimilar intermolecular interactions also gave a signifi-
cantly different conducting behavior of two salts (1c)2Cl and
(1c)2Br. The electrical resistivity measurements of the single crys-
tals (see the Supplementary data) indicate that two salts show
semiconducting behaviors with different resistivity by over two
orders of magnitude at room temperature for (1c)2Cl
Acknowledgments
This work was partially supported by Grant-in Aid for Scientific
Research (B) (No. 20340087) and Grant-in Aid for Scientific Re-
search on Innovative Areas of Molecular Degrees of Freedom (No.
20110007) from the Ministry of Education, Culture, Sports, Science,
and Technology, Japan. We also thank Professor H. Yoshizawa (The
University of Tokyo) to utilize PPMS (Quantum Design) for the
measurement of electrical resistivity and Dr. Y. Okano (the Instru-
ment Center in the Institute for Molecular Science) to use the X-ray
diffractometer with Rigaku Mercury CCD.
(qRT = 0.14 X qRT = 50 X cm). The activation en-
cm)19 and (1c)2Br (
ergy of (1c)2Cl (Ea = 0.04 eV) is also smaller than that of (1c)2Br
(Ea = 0.10 eV).19 These results indicate that the large overlap inte-
gral in the q1 direction in (1c)2Cl worked as an anisotropic conduc-
tion path, whereas the dimeric character of (1c)2Br in the a2
direction enhanced hole localization in the dimer, to form the di-
mer-Mott state with a relatively high resistivity.
In summary, we have successfully synthesized and character-
ized novel TTF-based functional electron donor systems with a
Supplementary data
catechol-fused extended
p-electronic structure 1a–f and their
Crystallographic data (excluding structure factors) for the struc-
tures in this paper have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publication CCDC
876709–876711. Copies of the data can be obtained, free of charge,
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK,
(fax: +44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).
Supplementary data (experimental procedures and character-
ization data for all new compounds, detailed crystallographic data,
electrical resistivity, charge estimation, and band structures) asso-
charge-transfer salts (1c)2Cl and (1c)2Br. The electronic effects in-
duced by the direct fusion of the catechol moiety to the TTF system
were evaluated in terms of the redox properties obtained from the
CV measurements with the help of the DFT calculations. In both CT
salts, two hydroxyl groups in the catechol unit formed intermolec-
ular hydrogen bonds with halide ions, to construct a 1D hydrogen-
bonded chain structure. Interestingly the very subtle differences in
the hydrogen-bond distances in (1c)2Cl and (1c)2Br gave rise to the

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X cm
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observed probably due to the strong one-dimensionality of the band structure.
For detail, see the Supplementary data.