Charge transfer complexes and radical cation salts of chiral methylated organosulfur donors

Article abstract of DOI:10.1039/c3ce42539h

The single crystal X-ray structure of the all-axial conformer of the (R,R,R,R) enantiomer of the chiral donor tetramethyl-BEDT-TTF (TM-BEDT-TTF) was described and compared to the all-equatorial conformer. (S,S,S,S)-Tetramethyl- BEDT-TTF formed crystalline

Full text of DOI:10.1039/c3ce42539h

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Charge transfer complexes and radical cation
salts of chiral methylated organosulfur donors†
Cite this: CrystEngComm, 2014, 16,
Songjie Yang,a Flavia Pop,b Caroline Melan,b Andrew C. Brooks,ac Lee Martin,a
3906
d
e
f
b
*
Peter Horton, Pascale Auban-Senzier, Geert L. J. A. Rikken, Narcis Avarvari
a
*
and John D. Wallis
The single crystal X-ray structure of the all-axial conformer of the (R,R,R,R) enantiomer of the chiral
donor tetramethyl-BEDT-TTF (TM-BEDT-TTF) was described and compared to the all-equatorial
conformer. (S,S,S,S)-Tetramethyl-BEDT-TTF formed crystalline 1 : 1 complexes with TCNQ and TCNQ-F4,
as well as a THF solvate of the TCNQ complex. Donors bis((2S,4S)-pentane-2,4-dithio)tetrathiafulvalene
and (ethylenedithio)((2S,4S)-pentane-2,4-dithio)tetrathiafulvalene, which contain seven-membered rings
bearing chirally oriented methyl groups, only formed complexes with TCNQ-F4. The TCNQ-F4 complexes
contain planar organosulfur systems, in contrast to the TCNQ complexes in which there is minimal charge
transfer. A variety of crystal packing modes were observed. Electrocrystallization experiments with both
enantiomers and the racemic form of tetramethyl-BEDT-TTF afforded mixed valence radical cation salts
with the AsF6 and SbF6 anions formulated as (TM-BEDT-TTF)2XF6 (X = As, Sb). Electrical conductivity was
only found in one charge transfer complex, while the radical cation salts are all semiconducting.
Received 12th December 2013,
Accepted 16th February 2014
DOI: 10.1039/c3ce42539h
www.rsc.org/crystengcomm
models for the design of new electron donors and acceptors
and have even been associated in the same molecule.4
Introduction
One of the first discoveries of conductivity in organic systems
was in the molecular complex of tetrathiafulvalene (TTF) 1
with TCNQ 2, which forms separate stacks with ca. 59%
electron transfer from the TTF stack to the TCNQ stack.1
Conducting materials have been prepared by grinding the
two components together,2 and more recently, conductivity
has been observed at the interface between crystals of these
two components.3 These two components have been used as
The reduction potential of TCNQ can be modified by the
introduction of substituents, e.g. fluorine, culminating in
the much more oxidizing tetrafluoro-TCNQ 3 for which the
reduction potential is ca. 0.53 V to 0.17 V for TCNQ relative
to SCE.
a School of Science and Technology, Nottingham Trent University, Clifton Lane,
Nottingham NG11 8NS, UK. E-mail: john.wallis@ntu.ac.uk;
Tel: +44 (0)115 848 8053
b Laboratoire MOLTECH-Anjou, Université d'Angers, CNRS, UMR 6200, UFR Sciences,
Bât. K, 2 Bd. Lavoisier, 49045 Angers, France. E-mail: narcis.avarvari@univ-angers.fr;
Fax: +33 02 41 73 54 05; Tel: +33 02 41 73 50 84
c Department of Chemistry, University College London, 20 Gordon Street,
London WC1H 0AJ, UK
d National Crystallography Service, School of Chemistry, University of Southampton,
Highfield Campus, Southampton, SO17 1BJ, UK
e Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, Bât. 510,
91405 Orsay, France
f Laboratoire National des Champs Magnétiques Intenses, UPR3228 CNRS/INSA/UJF/UPS,
Toulouse & Grenoble, France
BEDT-TTF 4, with an oxidation potential ca. 0.2 V higher
than TTF, has been reported to form three 1 : 1 molecular
complexes with TCNQ.5 A monoclinic phase, composed of
† Electronic supplementary information (ESI) available. CCDC 976361–976367,
976560–976564. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ce42539h
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stacks of alternating BEDT-TTF and TCNQ molecules, has a
high resistivity of ca. 106 ohm cm but is semiconducting
with the charge transfer estimated to be 20–30%.5c Two tri-
clinic phases have segregated stacks, with the donor and
acceptor planes roughly parallel in one case or perpendicular
in the other.5b,d The former is metallic with a room tempera-
ture resistivity of 10−1 to 10−2 ohm cm and the charge transfer
estimated to be ca. 74%. In contrast, the 1 : 1 complex of
BEDT-TTF with TCNQ-F4 is a magnetic insulator with full
charge transfer to the acceptors, which behaves as isolated
spins and undergoes antiferromagnetic ordering at TN of
14 K.6 Although methyl substituted BEDT-TTF derivatives
possessing stereogenic centers, such as the enantiopure
tetramethyl-BEDT-TTF (S,S,S,S)-5, abbreviated henceforth as
(S)-5, have been known for more than 25 years,7 to date there
has been, surprisingly, no example of a charge transfer com-
plex with TCNQ or TCNQ-F4. It was reported, however, that
donor (S)-5 forms 2 : 1 and 3 : 2 radical cation salts, in which
the donor is only partially oxidized and the dithiin rings
adopt approximate envelope conformations with methyl
groups organized equatorially,8 as well as a 1 : 1 complete fam-
ily of salts with triiodide, comprising both (S) and (R) enantio-
mers and the racemic form, in which the methyl groups
adopt also equatorial positions.9 Moreover, the same situation
was observed in the solid state structure of neutral (S)-5 and
(R)-5, although theoretical calculations suggest that the axial
conformer is slightly more stable.9 The use of chiral TTF pre-
cursors, such as 5, in charge transfer complexes or radical cat-
ion salts in which a combination of chirality and conductivity
might allow the observation of a synergistic effect referred to as
the electrical magneto-chiral anisotropy (eMChA) effect10 when the
transport is measured under an applied parallel magnetic field,
presents a great opportunity to search for this synergy in bulk
conductors. It is therefore of crucial importance to devote much
effort towards the synthesis of chiral TTFs and derived mate-
rials, in which the chiral information is addressed in different
ways, e.g. stereogenic centers, axial chirality, helical chirality,
supramolecular chirality, or chirality on the anions.11 Moreover,
the presence of stereogenic centers favours the occurrence of
original crystal structures when compared to the achiral precur-
sors and materials. The analogue of (S)-5 with external seven-
membered rings, namely S,S,S,S-bis(pentane-2,4-dithio)TTF 6,
and a hybrid donor containing one dimethylated seven-
membered ring and one unsubstituted six membered ring 7
have been described more recently.12 Donor 7 forms a 1 : 1 salt
with triiodide in which the seven-membered ring adopts
a chair conformation with one axial methyl group and one
equatorial methyl group12 as observed in the neutral donors
6 and 7. Other enantiopure families of donors include TTF-
oxazolines,13 TTF-bis(oxazolines),14 diversely substituted BEDT-
TTFs,15 TTF-sulfoxides,16 bis(pyrrolo)-TTFs,17 TTF-binaphthyls,18
tris(TTFs) showing supramolecular chirality,19 halogen contain-
ing derivatives,20 TTF-amides,21 TTF-helicenes,22 DM-EDT-TTF,23
and so on. However, complete series of conducting salts,
comprising both enantiomers and the racemic form, are still
very rare.9,13c,e,23
As a continuation of our effort in the field of chiral TTFs
and derived molecular conductors, we report herein our
investigations into TCNQ and TCNQ-F4 charge transfer com-
plexes with the chiral donors 5, 6 and 7, together with two
complete series of semiconducting salts based on the donor
−
−
5 and the anions AsF6 and SbF6 . Note that both enantio-
pure salts [(S)-5]2XF6 (X = As, Sb) have been mentioned in a
previous report, but no structural analysis was provided.8 The
conducting properties of the charge transfer complexes and
radical cation salts are described. Moreover, the solid state
structure of the axial conformer of (R)-5 is reported, together
with a comparison to that of the equatorial conformer which
was recently reported.9 The conformational issue related to
the disposition of the methyl groups and understanding of
the factors governing the occurrence of one form or the other
are certainly of great importance since they have a massive
impact on the packing of the donors, which ultimately influ-
ences the physical properties.
Results and discussion
The donors 5,7,9 610 and 710 have been prepared according
to the published procedures. The solid state structures of
(S)- and (R)-5 as equatorial (eq) conformers were previously
described by some of us as a triclinic P1 phase with two
independent molecules in the unit cell.9 In the same report,
DFT calculations have shown the slightly higher stability of
the axial conformer, also supported by the experimental CD
spectra in solution. In the present work we have succeeded
in growing single crystals of the axial (ax) conformer of
(R)-5 by slow diffusion of cyclohexane into a solution of
compound in carbon disulfide. While crystals of 5-eq were
orange needles,9 those of 5-ax are deep red prismatic
blocks. As a matter of fact, a polycrystalline powder of 5
clearly shows the presence of two types of solid phases, an
orange one and a pink-red one, of which the proportion in
between depends on the solvent used in the mother liquor.
For example, evaporation of toluene solutions yields a mix-
ture of the orange and pink-red solids in roughly equal pro-
portions, while the solid that resulted from carbon disulfide
solutions consists almost exclusively of the pink-red phase.
This clearly proves that both forms, i.e. equatorial and axial,
coexist in solution and depending on the crystallization con-
ditions, either of them crystallizes. The donor (R)-5-ax crystal-
lizes in the monoclinic system, non-centrosymmetric space
group P21, with one independent molecule in the asymmetric
unit (Fig. 1).
The six-membered rings adopt sofa type conformations,
with opposite displacements of the sp3 carbon atoms of the
rings (Table 1). Note the quasi-planarity of the dithiole rings,
with dihedral angles of 2.4–4.5° along the internal S⋯S axes,
in sharp contrast with the strong distortions observed in the
structure of (R)-5-eq ranging between 15.8 and 27.2°.9
Crystalline 1 : 1 charge transfer (CT) complexes with
TCNQ-F4 were obtained from all three donors of all (S) config-
urations, i.e. (S)-5, 6 and 7, but only donor 5 gave crystalline
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Fig. 1 Molecular structures of (R)-5-eq (top) and (R)-5-ax (bottom).
complexes with TCNQ: a 1 : 1 complex and a 1 : 1 + 0.5THF
complex. Crystal structures of all five complexes were measured
at 120 K. In all of the complexes, the donors and acceptors
pack together to form layers where the longest axes of the
donor and the acceptor lie perpendicular to the layers; how-
ever, the ways in which the molecules are organized within
the layer vary considerably.
Complexes of (S)-5
The reaction of donor (S)-5 with TCNQ in 1,1,2-trichloroethane
or THF gave 1 : 1 complexes whose structures differ by the
latter having a molecule of THF per every two donor and two
acceptor molecules. The crystal structures are closely related;
they are both triclinic systems with space group P1 with very
similar unit cell dimensions apart from the solvate having a
longer c axis. In both complexes, molecules of donor 5 and
TCNQ are stacked alternately along the a axis, with stacks
lying side by side in the −b and b directions to complete the
layers in the ab plane. In the solvated complex, the THF
Fig. 2 Structures of (S)-5·TCNQ (above) and its solvate with THF
(below) showing stacking along the a axis, and the location of THF
molecules in the solvate phase.
molecules are sandwiched between the layers (Fig. 2–3). In
the stacks of both complexes, the central CC bond of the
donor lies between the six-membered ring of a TCNQ above
and an exocyclic CC bond of a TCNQ below.
Table 1 Orientations of methyl groups and displacements of methine carbon atoms with respect to the mean plane formed by the four other
atoms of the six-membered rings in 5 and its CT complexes
Compound
Orientation of the methyl group
Displacements of CH atoms/Å
(R)-5-ax
Axial
Axial
+0.563, −0.331
+0.285, −0.593
+0.195, +0.909
−0.697, +0.133
+0.083, −0.682
−1.028, −0.029
+0.517, −0.332
+1.028, +0.364
+0.469, −0.415
+1.138, +0.523
+0.463, −0.407
+1.028, +0.376
+0.503, −0.370
+1.096, +0.441
+0.681, −0.055
+0.529, −0.341
+0.695, −0.172
+0.529, −0.340
(R)-5-eq
Equatorial
Equatorial
Equatorial
Equatorial
Axial
Equatorial
Axial
Equatorial
Axial
TMET·TCNQ
TMET·TCNQ·0.5THF
TMET·TCNQ-F4
Equatorial
Axial
Equatorial
Equatorial
Equatorial
Equatorial
Equatorial
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arranged pseudo- and exactly centrosymmetrically, respec-
tively, in the unit cell. The donor and acceptor molecules
pack together with their best planes roughly parallel to form
layers whose composition is shown in Fig. 4.
Within the layer there are lines composed of donor and
TCNQ-F4 molecules packed alternately side by side, and the
relative orientation of successive lines in the layer leads to
zigzag stacks of donors or acceptors running in the perpen-
dicular direction through the layer. There is interpenetration
between layers by the chiral hydrocarbon termini of the
donors whose major axes are considerably longer than that
of TCNQ-F4 (Fig. 5). All four methyl groups of the donors
lie in pseudo-equatorial positions, and there is a range of
conformations for the dithiin ring which all lie between an
envelope and a half chair. Along the zigzag stacks of donors
there is minimal face-to-face overlap with edge-to-edge S⋯S
Fig. 3 Packing for (S)-5·TCNQ·0.5THF showing the relative alignment
of molecular components between stacks.
The conformations of the donor molecules in the two crys-
tal structures are very similar. The donors adopt gently
bowed structures with flexing about the S⋯S vectors across
each dithiole ring of 11.4–14.3°, a feature observed in the
crystal structure of the neutral donor (R)-5-eq though with a
wider range of angles (15.8–27.2°),9 while in (R)-5-ax both
dithioles are practically planar (vide supra). The donor's
methyl groups take equatorial orientations on one ethylene
bridge, but axial orientations on the other ethylene bridge. A
similar situation was encountered in 1 : 1 cycloadducts
between 5 and tetrachlorocatecholate, for which both confor-
mations were observed in the same molecule.24 The dithiin
rings bearing axially oriented methyl groups adopt half
chair conformations, while in contrast, all of the dithiin rings
bearing equatorial methyl groups take a conformation with
both sp3 ring carbon atoms displaced to the same side of the
mean plane of the four other ring atoms, with one displace-
ment being at least double the other (Table 1). Using the
empirical correlation between molecular geometry and
oxidation state for BEDT-TTF developed by Guionneau
et al.,25 the averaged molecular geometry for the two crystal-
lographically unique donors in each complex predicts net
charges of 0.0 and +0.14 for the unsolvated and THF solvated
complexes, respectively, suggesting little net charge transfer
in the ground state.
Fig. 4 Packing of (S)-5·TCNQ-F4 showing the zigzag stacking of donor
In this respect, a useful comparative analysis can be done
between (S)-5·TCNQ or (S)-5·TCNQ·0.5THF, both showing
practically no charge transfer, and the monoclinic phase of
BEDT-TTF·TCNQ which also has mixed donor–acceptor
stacks.5c However, a charge transfer of approximately 20–30%
is estimated in the latter. This is consistent with the observed
planar geometry of the donor, while (S)-5 in both crystalline
phases we describe herein is bent about the S⋯S axes. Since
the oxidation potentials of BEDT-TTF and TM-BEDT-TTF are
essentially the same, the difference in charge transfer is very
likely due to the less favourable contacts or packing provoked
by the additional four methyl groups.
cations and acceptor anions.
The reaction of donor (S)-5 with the more oxidizing
acceptor TCNQ-F4 3 produced a 1 : 1 complex which has quite
different structural characteristics from the two TCNQ com-
plexes. The crystal structure is triclinic with space group P1,
with two donor molecules and two acceptor molecules
Fig. 5 View down the stacking axis in (S)-5·TCNQ-F4.
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contacts in the range 3.43–3.79 Å. In contrast, for the accep-
tors, there are pairs which partially overlap face-to-face,
so that a F and a N atom make short contacts to the central
ring of the other (F⋯C, 3.38; N⋯C, 3.47 Å); however there
is no such overlap between pairs for which the shortest con-
tacts are between edges (F⋯CF, 3.32; F⋯CN, 3.41; and F⋯N,
3.37 Å). Bond length data for the two donors predict a charge
of +0.97 in each case suggesting a complete charge transfer
in this salt, and, in support of this, the organosulfur cores of
the donor molecules are almost planar in contrast to those of
the TCNQ complexes of 5.
Complexes of donors 6 and 7 with TCNQ-F4
Fig. 7 Crystal packing of 6·TCNQ-F4 viewed with the b axis vertical
and c axis horizontal.
Donors 6 and 7 both form complexes with TCNQ-F4, though
no crystalline complexes could be isolated by reaction with
unsubstituted TCNQ. This may be related to the significant
non-planarity of the neutral donors (Fig. 6), which are not
compatible with the planar TCNQ, which would not be
expected to oxidize donors 6 or 7. The first oxidation poten-
tials of donors 6 and 7 (0.50–0.51 V vs. Ag/AgCl)10 are similar
to that of TM-BEDT-TTF (+0.49 V vs. SCE).9
The crystal structure of the 1 : 1 molecular complex of the
tetramethylated donor 6 with TCNQ-F4 is particularly interest-
ing since it shows separate stacks for the donors and the
acceptors (Fig. 7).
major conformation is twisted and unsymmetrical with the
three ring sp3 carbon atoms displaced to the same side of the
organosulfur plane – with the methylene carbon displaced
the most (2.190 Å) and the two methine carbon atoms
displaced by quite different amounts (1.747 Å and 1.242 Å)
(Fig. 8). The minor conformation is a chair, but displaced to
the opposite side of the organosulfur plane with respect to
the other end of the molecule.
According to the correlation between charge and bond
lengths in the TTF portion of the donor, in this complex
the donor carries a charge of +1.04, and so there has been
full charge transfer. Thus, the crystal structure contains
stacks of donor cations 6+ and acceptor anions 3−. Indeed the
organosulfur region of the donor has undergone a major
structural change upon oxidation from a bowed structure to
an almost planar one (Fig. 6). The planes of the cations and
The crystal system is monoclinic and the space group is
P21 with stacking organized along the b axis. In contrast to
the neutral donor, the organosulfur portion of donor 6 is
planar (Fig. 6), strongly suggesting a change in the oxidation
state. One seven-membered ring adopts a well ordered chair
conformation with one axial and one equatorial methyl
group, and the two methine carbons and the methylene carbon
displaced by similar amounts from the organosulfur plane
(1.433–1.510 Å). There is disorder in the other seven-membered
ring between two conformations in the ratio ca. 3 : 1. The
Fig. 6 Structures of the donor cation in 6·TCNQ-F4 (above) and the
neutral donor 6 (ref. 10) (below).
Fig. 8 Molecular geometries of the cations 6 (above) and 7 (below) in
their TCNQ-F4 complexes.
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anions lie at 23.2° to each other, and each stack of cations is
surrounded on four sides by stacks of acceptor anions.
Within the donor cation stacks the main axis of the
organosulfur plane lies at 46° to the stacking axis with six
S⋯S contacts in the range 3.61–3.86 Å. In the acceptor anion
stacks, the main axes of the acceptors lie at 41° to the stack-
ing axis, such that two exocyclic “double bonds” lie opposite
to each other but oriented anti, so cyano groups of one mole-
cule lie over the edge of the six-membered ring of the other
and there are six C⋯C contacts in the range 3.33–3.44 Å.
In contrast, the crystal structure of the complex of the
unsymmetrical dimethylated donor 7 adopts stacks of alter-
nating donors and acceptors (Fig. 9).
c axis. Between donors the shortest S⋯S contacts are 3.51
and 3.54 Å. The overlap between acceptors produces two
fluorine/nitrile interactions (F⋯C: 3.22 & 3.35 Å; F⋯N: 3.19 &
3.41 Å) and two C–F bonds overlapping with C⋯F contacts of
3.34 and 3.37 Å. The seven-membered ring does not adopt
the chair conformation but rather the two methine C atoms
are displaced to opposite sides of the organosulfur plane
(Fig. 8). The organosulfur portion of the donor is planar,
in contrast to the neutral donor, and correlation of the
molecular geometry of the TTF portion with the oxidation
state suggests that it bears a charge of +0.92, and thus is in
the monocation form.
However, each species is tilted so that the shorter axis of
the molecular plane lies at ca. 50° to the stacking axis, which
leads to a lateral overlap between donor molecules (and
between acceptor molecules) in adjacent stacks (Fig. 10). The
crystal system is monoclinic with space group P21, with stack-
ing along the b axis, and the lateral overlap of donors or
acceptors in the a direction forms layers perpendicular to the
Radical cation salts (5)2XF6 (X = As, Sb)
Electrocrystallization of both enantiomers and the racemic
mixture (obtained by mixing equimolar amounts of (S) and
(R)) of TM-BEDT-TTF 5 in the presence of [NBu4]AsF6 and
[NBu4]SbF6 as supporting electrolytes gave the corresponding
2 : 1 salts, (5)2AsF6 and (5)2SbF6. The (S) enantiomeric salts
have been previously reported, although their structures
were not detailed since they were isostructural to the
[(R)-5]2PF6 salt described in more detail.8 [(R)-5]2AsF6 and
[(R)-5]2SbF6 salts are isostructural and crystallize in the triclinic
system, space group P1, with two independent donor units for
one molecule of anion. The racemic salts [(rac)-5]2AsF6 and
[(rac)-5]2SbF6 crystallize also in the triclinic system, centrosym-
¯
metric space group P1, with the anions situated on the inver-
sion centre. The cell parameters are the same throughout the
respective AsF6 and SbF6 series, except for the space groups,
¯
i.e. P1 for the enantiopure forms and P1 for the racemic ones.
A slight increase of the cell volume (≈1.2%) is observed when
passing from AsF6 to SbF6, in agreement with the larger size
of the latter.
In all cases the methyl groups adopt pseudo-equatorial
positions, probably as a means to maximize the inter-
molecular interactions. The dimethylethylene bridge is disor-
dered at 70 : 30 in the structure of the racemic salt (Fig. 11),
such that both enantiomers are present on the same crystal-
lographic site, a feature already encountered with (rac)-5.9
The values of the central CC and internal C–S bonds are
indicative of the mixed valence state of the donor molecules
(Table 2).
Fig. 9 Alternate stacking of donor cations and acceptor anions along
the b axis in 7·TCNQ-F4.
Fig. 11 Donor molecule in the structure of [(rac)-5]2SbF6 together
with the atom numbering scheme. The disorder model of the donor
involves common positions for the methyl groups and disordered
methine carbon atoms at s.o.f. 0.7 : 0.3.
Fig. 10 Side to side overlap between donors and between acceptors
in 7·TCNQ-F4 viewed down the c axis.
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Table 2 Selected bond lengths (Å) for the salts (5)2XF6 (X = As, Sb)
Compound
[(S)-5]2AsF6
[(R)-5]2AsF6
[(rac)-5]2AsF6
a
b
1.353(11)
1.744(9)
1.747(9)
1.744(9)
1.746(9)
1.765(8)
1.780(9)
1.769(9)
1.748(9)
1.338(11)
1.338(12)
0.820
1.370(11)
1.743(10)
1.735(9)
1.736(9)
1.740(10)
1.744(9)
1.735(9)
1.725(9)
1.738(9)
1.352(12)
1.353(12)
0.752
1.352(9)
1.732(8)
1.753(6)
1.743(6)
1.754(8)
1.748(6)
1.754(7)
1.756(7)
1.753(6)
1.367(8)
1.347(9)
0.789
1.366(9)
1.727(7)
1.740(6)
1.741(7)
1.749(8)
1.742(7)
1.734(7)
1.742(7)
1.755(6)
1.361(9)
1.332(8)
0.770
1.363(4)
1.737(3)
1.740(3)
1.741(3)
1.741(3)
1.743(3)
1.748(3)
1.746(3)
1.749(3)
1.349(4)
1.347(4)
0.775
c
d
δ
Q
0.249
0.738
0.456
0.600
0.561
[(S)-5]2SbF6
a
b
[(R)-5]2SbF6
1.355(7)
1.731(5)
1.738(4)
1.742(5)
1.755(5)
1.736(5)
1.745(5)
1.746(5)
1.752(5)
1.360(7)
1.342(7)
0.78
[(rac)-5]2SbF6
1.360(4)
1.741(3)
1.741(2)
1.741(2)
1.741(3)
1.743(2)
1.746(2)
1.747(3)
1.750(2)
1.354(3)
1.350(3)
0.760
1.378(10)
1.753(7)
1.726(8)
1.733(9)
1.729(7)
1.738(7)
1.744(7)
1.715(7)
1.735(8)
1.339(10)
1.383(11)
0.729
1.364(10)
1.753(7)
1.726(8)
1.733(9)
1.729(7)
1.738(7)
1.744(7)
1.715(7)
1.735(8)
1.339(10)
1.383(11)
0.743
1.370(7)
1.724(5)
1.736(5)
1.737(5)
1.753(5)
1.741(4)
1.756(6)
1.755(5)
1.757(5)
1.347(7)
1.338(7)
0.777
δ = (b + c) − (a + d)
Q = 6.347 − 7.4638δ
c
d
δ
Q
0.904
0.800
0.5230
0.546
0.559
Conducting properties
Measurements of resistivity for the charge transfer complexes
show that all of them except 7·TCNQ-F4 have a room temper-
ature resistance too high to be measured (>1.2 Mohm limit
of the machine). For comparison, the monoclinic form of
BEDT-TTF·TCNQ which has mixed stacks similar to the
TMET·TCNQ complexes, but with 20–30% charge transfer,
shows a resistivity ca. 106 ohm cm.5c 7·TCNQ-F4 was found to
be a semiconductor but with very low conductivity. Upon
cooling from room temperature to just 290 K the resistance
quickly increased but below 280 K the resistance was above
the measurement limit. The activation energy is estimated
to be 0.025 eV.
In a previous report the [(S)-5]XF6 (X = P, As, Sb) series of
salts was found to be semiconducting.8 With the complete
series of AsF6 and SbF6 in our hands we proceeded to the
measurement of their conducting properties. In spite of the
structural disorder present in the racemic salts, which in gen-
eral disfavors the transport properties,9 the enantiopure and
racemic compounds within the same family, i.e. AsF6 or
SbF6, show practically the same semiconducting behavior,
with activation energies ranging between 920 and 1340 K,
and similar room temperature conductivity values of around
0.5–1.0 S cm−1 (Fig. 13).
Transport property measurements under an applied paral-
lel magnetic field for both series of salts did not allow the
detection of the electrical magneto-chiral anisotropy effect,11
probably because the chiral information is only poorly
expressed at the crystal level.
Fig. 12 Packing diagram of [(S)-5]2SbF6 with emphasis on the S⋯S short
contacts. SbF6 anions and H atoms were omitted.
The donors form parallel columns reminiscent of a β phase,
with short intra- and inter-stack S⋯S contacts (Fig. 12 and
ESI†), with a classical organic–inorganic segregation (ESI†).26
Bond length data for the donors, reported in Table 2,
clearly indicate a mixed valence state for all of the salts, with
a mean charge not far from +0.5. The higher discrepancy for
the (S) salts arises from the somewhat lower quality of the
crystal data set measurements.
3912 | CrystEngComm, 2014, 16, 3906–3916
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Fig. 13 Temperature dependence of the electrical resistivity ρ for single crystals of [5]2AsF6 (left) and [5]2SbF6 (right).
deep red prismatic blocks while those of the all-equatorial
one are orange needles.
Conclusions
Three chiral donors have been investigated in this work,
namely the tetramethyl-BEDT-TTF 5 as enantiopure and
racemic forms and the related seven-membered ring ana-
logues 6 and 7 as the (S) enantiomers. The conformation of
the enantiomerically pure donor 5 adopts all-axial or all-
equatorial methyl groups in two crystalline polymorphs. Its
salts with TCNQ and TCNQ-F4 show very little or complete
charge transfer, respectively, and do not have any transport
properties, in comparison to BEDT-TTF which forms three
conducting salts with TCNQ. The radical cation salts of both
Preparation of TCNQ complexes
Donor (S)-5 + TCNQ. (a) A hot solution of donor 5 (10 mg)
in 1,1,2-trichloroethane (2 ml) was added to a hot solution
of TCNQ (5 mg) in 1,1,2-trichloroethane (5 ml) and the
mixture was heated at 90 °C for 2 h. Cooling gave black
flat oblong crystals.
(b) A hot solution of donor 5 (10 mg) in THF (2 ml) was
added to a hot solution of TCNQ (5 mg) in THF (4 ml) and
the mixture was heated at 65 °C for 1 h. Cooling gave crystals
as long black plates.
−
enantiomers and the racemate of TM-BEDT-TTF with XF6
(X = As, Sb) all adopt very similar crystal structures, and,
consistent with this, there was little difference between the
conductivities of the racemate and enantiomers in the two
families. The methyl groups adopted equatorial positions in
all of these materials except in the TCNQ complexes where
one pair was oriented axially. The donors 6 and 7 containing
two or one seven-membered ring are significantly non-planar
in their outer rings, which appears not compatible to the for-
mation of TCNQ salts, but both form salts with TCNQ-F4, and
that with 7 showed weak semiconducting properties. BEDT-
TTF with only small hydrogen substituents is particularly well
suited to forming tightly packed radical cation salts, but inclu-
sion of other groups at both ends of the molecule makes such
close packing more difficult. Nevertheless, interesting electro-
active materials are likely to be produced if those groups bring
attractive intermolecular interactions, more ordered crystal
structures, and other physical properties resulting from the
combination of chirality with conductivity and magnetism.
Donor (S)-5 + TCNQ-F4. A hot solution of donor 5 (6 mg)
in THF (2 ml) was added to a hot solution of TCNQ-F4 (4 mg)
in THF (4 ml) and the mixture was heated at 65 °C for 1 h.
Cooling gave black crystals.
Donor 6 + TCNQ-F4. A hot solution of donor 6 (10 mg) in
1,2-dichloroethane (2 ml) was added to a hot solution of
TCNQ-F4 (6 mg) in DCM (3 ml) and the mixture was heated
at 65 °C for 1 h. Cooling gave black crystals.
Donor 7 + TCNQ-F4. A solution of donor 7 (12 mg) in
chloroform (3 ml) was slowly diffused into a solution of
TCNQ-F4 (8 mg) in acetonitrile (3 ml) to give black crystals.
Electrocrystallization of donor 5 with AsF6 and SbF6 anions
[(S)-5]2AsF6. 25 mg of [NBu4]AsF6 were dissolved in CHCl3
(12 ml) and half of the solution was poured in the cathodic
compartment of the electrocrystallization cell. The anodic
chamber was filled with 5 mg of (S)-5 dissolved in 6 mL
[NBu4]AsF6–CHCl3 solution. Single crystals of the salt were
grown at 20 °C over a period of 3 days on a platinum wire
electrode, by applying a constant current of 0.5 μA. Solid
black plates were collected on the electrode.
[(R)-5]2AsF6. Same conditions and amounts as for [(S)-5]2AsF6
were employed.
[(rac)-5]2AsF6. 25 mg of [NBu4]AsF6 were dissolved in
CHCl3 (12 ml) and half of the solution was poured in
the cathodic compartment of the electrocrystallization cell.
The anodic chamber was filled with 5 mg of (rac)-5 dissolved
Experimental
Donors 5,7,9 610 and 710 have been prepared following the
literature procedures.
(R)-5-ax
Suitable single crystals for X-ray diffraction were obtained by
slow diffusion of cyclohexane into a solution of compound in
carbon disulfide. The crystals of the all-axial conformer are
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in 6 mL [NBu4]AsF6–CHCl3 solution. Single crystals of the salt
were grown in solution at 20 °C over a period of 5 days on a
platinum wire electrode by applying a constant current of
0.5 μA. Solid black plates were collected on the electrode.
[(S)-5]2SbF6. 27 mg of [NBu4]SbF6 were dissolved in CHCl3
(12 ml) and half of the solution was poured in the cathodic
compartment of the electrocrystallization cell. The anodic
chamber was filled with 5 mg of (S)-5 dissolved in 6 mL
[NBu4]SbF6–CHCl3 solution. Single crystals of the salt were
grown at 20 °C over a period of 4 days on a platinum wire
electrode by applying a constant current of 0.5 μA. Solid
black plates were collected on the electrode.
(S)-5·TCNQ·THF solvate. Crystal data for (S)-5·TCNQ·0.5THF:
2C14H16S8·2C12H4N4·C4H8O, Mr = 1361.98, triclinic, a = 7.31800(10),
b = 7.83250(10), c = 28.2325(6) Å, α = 94.2870(10), β = 95.5350(10),
γ = 110.9930(10)°, V = 1493.34(4) Å3, Z = 1, P1, Dc = 1.51 g cm−3,
μ(MoKα) = 0.63 mm−1, T = 120(2) K, 12 286 unique reflections,
10 819 with F > 4σ(F), Flack parameter: 0.08(6), R = 0.040,
wR = 0.083.
(S)-5·TCNQ-F4. Crystal data for (S)-5·TCNQ-F4: C14H16S8·C12N4F4,
Mr = 716.91, triclinic, a = 8.4889(2), b = 13.7156(2), c = 14.0914(3) Å,
α
=
=
110.8440(10),
β
=
Z
100.8070(10),
2, P1, Dc
γ
=
=
107.0480(10)°,
1.72 ,
cm−3
V
1385.52(5) Å3,
=
g
μ(MoKα) = 0.70 mm−1, T = 120(2) K, 11 840 unique reflections,
10 784 with F > 4σ(F), Flack parameter: 0.14(6), R = 0.037,
wR = 0.083.
[(R)-5]2SbF6. Same conditions and amounts as for [(S)-5]2SbF6
were employed.
[(R)-5]2SbF6. 27 mg of [NBu4]SbF6 were dissolved in CHCl3
(12 ml) and half of the solution was poured in the cathodic
compartment of the electrocrystallization cell. The anodic
chamber was filled with 5 mg of (rac)-5 dissolved in 6 mL
[NBu4]SbF6–CHCl3 solution. Single crystals of the salt were
grown in solution at 20 °C over a period of 3 days on a
platinum wire electrode by applying a constant current of
0.5 μA. Solid black plates were collected on the electrode.
6·TCNQ-F4. Crystal data for 6·TCNQ-F4: C16H20S8·C12N4F4,
Mr = 744.96, monoclinic, a = 13.4068(4), b = 5.14840(10),
c = 22.4161(6) Å, β = 101.1600(10)°, V = 1517.98(7) Å3,
Z = 2, P21, Dc = 1.63 g cm−3, μ(MoKα) = 0.64 mm−1
,
T = 120(2) K, 6920 unique reflections, 6164 with F > 4σ(F),
Flack parameter: 0.02(8), R = 0.047, wR = 0.11.
7·TCNQ-F4. Crystal data for 7·TCNQ-F4: C13H14S8·C12N4F4,
Mr = 702.88, monoclinic, a = 5.6084(4), b = 8.6087(7),
c = 27.868(2) Å, β = 90.489(4)°, V = 1345.47(18) Å3, Z = 2, P21,
Dc = 1.74 g cm−3, μ(MoKα) = 0.72 mm−1, T = 120(2) K, 5387
unique reflections, 4262 with F > 4σ(F), Flack parameter:
0.06(9), R = 0.050, wR = 0.12.
X-ray crystallography
Crystal structures for the charge transfer complexes with
TCNQ and TCNQ-F4 were measured using MoKα radiation at
120 K on a Nonius Kappa CCD area-detector diffractometer
located at the window of a Nonius FR591 rotating-anode X-ray
generator, and solved and refined using the SHELX program
suite.27 X-ray diffraction measurements for (R)-5-ax and the
salts (5)2XF6 (X = As, Sb) were performed on a Bruker Kappa
CCD diffractometer using graphite-monochromated MoKα
radiation (λ = 0.71073 Å) at room temperature. The structures
were solved by direct methods and refined by full-matrix least
squares techniques based on F2. The non-H atoms were refined
with anisotropic displacement parameters. Calculations were
performed using the SHELX-97 crystallographic software pack-
age. CCDC reference numbers: CCDC 976560 ((S)-5·TCNQ),
CCDC 976561 ((S)-5·TCNQ·0.5THF), CCDC 976562 ((S)-5·TCNQ-F4),
CCDC 976563 (6·TCNQ-F4), CCDC 976564 (7·TCNQ-F4), CCDC
976361 ([(S)-5]2AsF6), CCDC 976362 ([(R)-5]2AsF6), CCDC 976363
([(rac)-5]2AsF6), CCDC 976364 ([(S)-5]2SbF6), CCDC 976365
([(R)-5]2SbF6), CCDC 976366 ([(rac)-5]2SbF6), CCDC 976367 ((R)-5-ax).
(R)-5-ax. Crystal data for (R)-5-ax: C14H16S8, Mr = 440.75,
monoclinic, a = 6.4428(3), b = 13.612(2), c = 11.8759(12) Å,
α = 90.000, β = 91.722(5), γ = 90.000°, V = 1041.02(19) Å3, Z = 2,
P21, Dc = 1.406 g cm−3, μ(MoKα) = 0.851 mm−1, T = 293(2) K,
4230 unique reflections, 3048 with F > 4σ(F), Flack parameter:
0.1(2), R = 0.040, wR = 0.092.
[(S)-5]2AsF6. Crystal data for [(S)-5]2AsF6: C28H32AsS16F6,
Mr = 1070.42, triclinic, a = 6.91660(10), b = 8.1526(5),
c = 19.4288(11) Å, α = 83.250(5), β = 84.511(3), γ = 71.107(3)°,
V
= Z = 1, P1, Dc = 1.730 g
1027.41(9) Å3, cm−3,
μ(MoKα) = 1.692 mm−1, T = 293(2) K, 5469 unique reflections,
4552 with F > 4σ(F), Flack parameter: 0.033(17), R = 0.058,
wR = 0.102.
[(R)-5]2AsF6. Crystal data for [(R)-5]2AsF6: C28H32AsS16F6,
Mr
= 1070.42, triclinic, a = 6.9129(5), b = 8.1497(3),
c = 19.4274(9) Å, α = 83.261(5), β = 84.544(5), γ = 71.107(4)°,
V
= Z = 1, P1, Dc = 1.732 g ,
1026.48(10) Å3, cm−3
μ(MoKα) = 1.693 mm−1, T = 293(2) K, 6157 unique reflections,
5430 with F > 4σ(F), Flack parameter: 0.030(13), R = 0.046,
wR = 0.096.
[(rac)-5]2AsF6. Crystal data for [(rac)-5]2AsF6: C28H32AsS16F6,
Mr
= 1070.42, triclinic, a = 6.8912(3), b = 8.1537(4),
c = 19.3920(6) Å, α = 83.680(3), β = 84.452(4), γ = 70.835(3)°,
¯
V
=
1020.84(7) Å3,
Z
=
1, P1, Dc
=
1.741
g
cm−3,
μ(MoKα) = 1.703 mm−1, T = 293(2) K, 3702 unique reflections,
3025 with F > 4σ(F), R = 0.052, wR = 0.101.
[(S)-5]2SbF6. Crystal data for [(S)-5]2SbF6: C28H32SbS16F6,
Mr
= 1117.25, triclinic, a = 6.8834(5), b = 8.2494(4),
c = 19.4622(13) Å, α = 84.137(6), β = 84.642(6), γ = 71.382(5)°,
V
= Z = 1, P1, Dc = 1.784 g ,
1039.66(11) Å3, cm−3
(S)-5·TCNQ. Crystal data for (S)-5·TCNQ: C14H16S8·C12H4N4,
Mr = 644.94, triclinic, a = 7.3290(6), b = 7.7825(6), c = 26.4496(13),
α = 85.246(5), β = 84.518(5), γ = 68.414(7)°, V = 1394.54(17) Å3,
Z = 2, P1, Dc = 1.54 g cm−3, μ(MoKα) = 0.67 mm−1, T = 150(2) K,
7607 unique reflections, 6995 with F > 4σ(F), Flack parameter:
−0.06(9), R = 0.049, wR = 0.13.
μ(MoKα) = 1.518 mm−1, T = 293(2) K, 6734 unique reflections,
5794 with F > 4σ(F), Flack parameter: −0.05(2), R = 0.050,
wR = 0.094.
[(R)-5]2SbF6. Crystal data for [(R)-5]2SbF6: C28H32SbS16F6,
Mr
= 1117.25, triclinic, a = 6.8792(2), b = 8.2487(4),
c = 19.4530(12) Å, α = 84.123(4), β = 84.581(5), γ = 71.299(3)°,
3914 | CrystEngComm, 2014, 16, 3906–3916
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V
=
1037.85(9) Å3,
Z
=
1, P1, Dc
=
1.788
g
cm−3,
6 T. Hasegawa, K. Inukai, S. Kagoshima, T. Sugawara,
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and N. Avarvari, Tetrahedron, 2005, 61, 10935–10942; (c)
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μ(MoKα) = 1.521 mm−1, T = 293(2) K, 6904 unique reflections,
6441 with F > 4σ(F), Flack parameter: 0.006(19), R = 0.037,
wR = 0.088.
[(rac)-5]2SbF6. Crystal data for [(rac)-5]2SbF6: C28H32SbS16F6,
Mr
= 1117.25, triclinic, a = 6.8720(6), b = 8.2521(4),
c = 19.4369(12) Å, α = 84.441(4), β = 84.474(5), γ = 71.056(5)°,
V
=
1035.17(12) Å3,
Z
=
1, P1, Dc
=
1.792
g
cm−3,
¯
μ(MoKα) = 1.525 mm−1, T = 293(2) K, 4363 unique reflections,
3839 with F > 4σ(F), R = 0.040, wR = 0.090.
Resistivity measurements
For the charge transfer salts, two-probe DC transport mea-
surements were made on several crystals of each salt using a
HUSO HECS 994C multi-channel conductometer. Gold wires
(15 μm diameter) were attached to the crystal, and the attached
wires were connected to an eight-pin integrated circuit plug
with gold conductive cement. The machine had an upper limit
for resistance measurement of 1.2 MOhm.
For the radical cation salts [5]2XF6 (X = As, Sb), four-probe
transport measurements were performed on crystals of each
salt using an AC technique with an applied current Iac = 1 μA
and low-frequency lock-in detection. Annular contacts were
made by gold evaporation on which gold wires were attached
with silver paste. Low temperature was achieved using a
cryocooler equipment.
Acknowledgements
We thank the EPSRC for grant EP/C510488/1 and for a stu-
dentship, the EPSRC National Crystallography Service for
datasets, and the EPSRC Mass Spectrometry Service for mea-
surements. The work has benefited from support from the
ESF COST action D35. This work was supported in France by
the National Agency for Research (ANR, Project 09-BLAN-
0045-01), the CNRS, the University of Angers and the Ministry
of Education and Research (grant to C.M.).
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Commun., 2008, 220–222; (b) M. Chas, F. Riobé, R. Sancho,
C. Minguíllon and N. Avarvari, Chirality, 2009, 21, 818–825.
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