Jordan R.
Lopez
a,
Lee
Martin
*a,
John D.
Wallis
a,
Hiroki
Akutsu
b,
Yasuhiro
Nakazawa
b,
Jun-ichi
Yamada
c,
Tomofumi
Kadoya
c,
Simon J.
Coles
d and
Claire
Wilson
d
aSchool of Science and Technology, Nottingham Trent University, Clifton Lane, Clifton, Nottingham, NG11 8NS, UK. E-mail: lee.martin@ntu.ac.uk; Tel: +44 (0)1158483128
bDepartment of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
cGraduate School of Material Science, University of Hyogo, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan
dSchool of Chemistry, Faculty of Natural and Environmental Sciences, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
First published on 10th May 2016
We have synthesized the first examples of radical-cation salts of BEDT-TTF with chiral borate anions, [B(malate)2]−, prepared from either enantiopure or racemic bidentate malate ligands. In the former case only one of two diastereoisomers of the borate anion is incorporated, while for the hydrated racemic salt one racemic pair of borate anions containing a R and a S malate ligand is incorporated. Their conducting and magnetic properties are reported. The tight-binding band calculation indicates that the chiral salt has an effective half-filled flat band, which is likely to be caused by the chiral structural feature.
There are three routes through which chirality can be introduced into radical-cation salts: a chiral donor molecule, chiral anion, or a chiral electrolyte. There have been a large number of enantiopure TTF-based donor molecules synthesised since the first example, tetramethyl-(S,S,S,S)-BEDT-TTF, in 1986.6 Recent promise of the use of chiral donor molecules to produce bulk chiral conductors is evidenced by the aforementioned work of Pop et al.5 Recently a semiconducting TTF-type donor has shown lower activation energy for its racemic salt with BF4 compared to the isostructural analogues for both individual enantiomers.7
Other examples of salts from chiral donor molecules include hydroxyalkyl-BEDT-TTFs,8 bis(oxazoline)-TTFs,9 iodo-TTFs,10 pyrrolo-TTFs,11 and TTF-sulphoxides which have chiral sulphur atoms.12
Racemic and chiral anions have been used to produce radical-cation salts with BEDT-TTF including Fe(croconate)3,13 Cr(2,2′-bipy)(oxalate)2,14 Sb2(L-tartrate)2,15 TRISPHAT16 and Fe(C6O4Cl2)3.17a Fe(C6O4Cl2)3 has also been combined in salts with TM-BEDT-TTF17b in a rare case where a chiral donor and chiral anion have been combined, although the anion crystallised as a racemic mixture. An enantiopure bis-hydroxy-substituted TTF donor has also produced a 4:
1 salt with the meso stereoisomer of the dinuclear [Fe2(oxalate)5]4− anion.8
The most extensive family of BEDT-TTF salts with chiral anions contain tris(oxalato)metallate and these have provided a large number of materials which combine magnetism with conductivity in the same lattice.18 These salts contain a 50:
50 mixture of the Δ and Λ enantiomers of [MIII(oxalate)3]3− to give an overall racemic lattice.19 However, the spatial distribution of these two enantiomers within the anion layers determines the donor packing arrangement and thus the conducting properties.
The inclusion of a guest solvent molecule within the lattice of these tris(oxalato)metallate salts introduces the possibility of using a chiral solvent for crystal growth. Crystals containing either enantiopure or racemic sec-phenethyl alcohol molecules show a difference in their conducting properties owing to the disorder of the enantiomers of the guest solvent molecule in the anion layer of the racemic salt.20 Recently, the first examples containing a single enantiomer of tris(oxalato)metallate in the lattice have been produced by using chiral induction through electrocrystallisation from the chiral electrolyte (R)-carvone containing racemic Cr(C2O4)3 or Al(C2O4)3.21
We report here two new BEDT-TTF radical-cation salts incorporating the anion B(malate)2− prepared from either racemic malic acid or enantiopure D-(+)-malic acid. The synthesis of such bis-chelated borate anions offers the prospect of creating complexes with more than one stereogenic centre. In the case of the B(malate)2− reported here the chirality of the bidentate chelated malate ligand is retained but diastereomers are produced through two possible stereochemical configurations at the boron centre which is labile in solution. Therefore, when using enantiopure D-(+)-malic acid ((R)-hydroxybutanedioic acid), a mixture of diastereoisomeric anions will be produced in solution: BSRR with an S boron centre and two R malate ligands, and BRRR which differs only in having an R boron centre (Scheme 1). 1H NMR in deuterio-acetonitrile shows the anions exist in a 2:
1 ratio with very similar NMR spectra. When using racemic DL-malic acid, four further diastereomers will be produced: BSSS and BRSS (the mirror images of the anions shown in Scheme 1) and BSRS and BRRS (Scheme 2). 1H NMR in deuterio-acetonitrile is complex but consistent with the presence of these diastereomers. BSRR, BRRR and BSSS, BRSS are racemic pairs and therefore only four signals are expected.
We report here a chiral radical-cation salt of BEDT-TTF, synthesised electrochemically in the presence of chiral BSRR and BRRR anions, in which just the BSRR anion is incorporated into the structure. When repeating this synthesis using a mixture of the six possible diastereomeric borate anions prepared from racemic malic acid a racemic radical-cation salt is obtained but only two of the diastereomeric anions are present in the crystal (BSRS and BRRS).
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Fig. 1 ORTEP diagram of the asymmetric unit of I showing atomic labelling, hydrogens are omitted for clarity. |
The terminal ethylene carbons of each donor show half chair conformations with each sp3 carbon equidistant from the plane of the TTF unit. The two donors each have thirteen side-to-side S⋯S contacts below the sum of van der Waals radii (Table 1). These contacts are all side-to-side between neighbouring stacks of crystallographically independent donors. Donor A has two short contacts with two of the H2O molecules (S7–O13, 3.285 Å and S2–O11, 3.142 Å) and donor B has one short contact with a borate anion (S10–O4, 3.145 Å) (Fig. 4).
S⋯S | Donor A⋯B |
---|---|
Contact/Å | |
S1⋯S10 | 3.53(1) |
S1⋯S12 | 3.58(1) |
S5⋯S16 | 3.47(1) |
S7⋯S16 | 3.58(1) |
S2⋯S9 | 3.48(1) |
S4⋯S9 | 3.56(1) |
S8⋯S13 | 3.55(1) |
S8⋯S15 | 3.49(1) |
S8⋯S10 | 3.48(1) |
S8⋯S12 | 3.57(1) |
S1⋯S13 | 3.47(1) |
S1⋯S15 | 3.46(1) |
S7⋯S9 | 3.56(1) |
The molecular formula suggests that the two independent donors have a cumulative charge of +1 to balance the charge of the BR/S[(R/S)-malate]2− anion. Using the method of Guionneau et al.22 for estimating oxidation state from molecular geometry the two donors are calculated each to carry a charge of 0.5+ (Table 2). The central CC bond lengths are 1.366 Å in both A and B donors.
Salt | Donor | a/Å | b/Å | c/Å | d/Å | δ | Q | Charge |
---|---|---|---|---|---|---|---|---|
I at 150 K | A | 1.366 | 1.743 | 1.750 | 1.358 | 0.769 | 0.60+ | |
B | 1.366 | 1.742 | 1.752 | 1.354 | 0.771 | 0.59+ | 1.19 ± 0.2 = 1 | |
I at 250 K | A | 1.366 | 1.740 | 1.7525 | 1.3495 | 0.777 | 0.55+ | |
B | 1.366 | 1.742 | 1.7505 | 1.3535 | 0.773 | 0.58+ | 1.13 ± 0.2 = 1 | |
II | A | 1.357 | 1.762 | 1.756 | 1.357 | 0.804 | 0.37+ | |
B | 1.379 | 1.731 | 1.746 | 1.373 | 0.725 | 0.94+ | 1.31 ± 0.3 = 1 |
The resistivity of I shows exponential temperature dependence only below ∼250 K with an activation energy of 0.050 eV (ρRT = 0.00163 Ω cm) (Fig. 5).
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Fig. 8 ORTEP diagram of the asymmetric unit of II showing atomic labelling, hydrogens are omitted for clarity. |
Donor A⋯B | |
---|---|
Contact/Å | |
S4⋯S7A | 3.48(1) |
S8⋯S1A | 3.34(1) |
S8⋯S3A | 3.44(1) |
S7⋯S2A | 3.45(1) |
S7⋯S4A | 3.49(1) |
Donor A⋯A | |
---|---|
Contact/Å | |
S2⋯S7 | 3.45(1) |
S4⋯S7 | 3.48(1) |
S3⋯S8 | 3.57(1) |
Donor B⋯B | |
---|---|
Contact/Å | |
S8A⋯S3A | 3.54(8) |
The formula (BEDT-TTF)2[BS(R-malate)2] suggests that the donors have a cumulative charge of +1 to balance the charge of the borate anion. Using the method of Guionneau et al.22 the two crystallographically independent donors carry charges of 0.94+ and 0.37+, and central CC bond lengths are 1.357 and 1.379 Å. This indicates charge localisation with a 1+ charge on one donor and the other donor being neutral.
Salt II shows low resistivity at room temperature (ρRT = 15.015 Ω cm) and semiconducting behaviour is observed upon cooling from 300 K to 165 K. Hysteresis is also present on heating back up to 300 K giving a less efficient conductor as we observe an increase in Ea (Ea cooling = 0.082 eV, Ea heating 0.099 eV) (Fig. 10).
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Fig. 11 End of the donor layers of I (a) and II (b). Green and pink coloured molecules are molecules A and B, respectively. |
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Fig. 12 Band dispersions and Fermi surfaces of salts I (a: 150 K; b: 250 K) and II (c). Transfer integrals (×10−3 eV, see also Fig. 11) are p1 = −3.07, p2 = −7.13, q1 = −5.04, q2 = −1.67, c = −7.41, d = −12.21, e = +8.32 and f = +6.58 for salt I at 150 K, p1 = −3.21, p2 = −6.42, q1 = −4.62, q2 = −1.97, c = −7.25, d = −11.42, e = +7.93 and f = +6.59 for salt I at 250 K and p1 = +5.07, p2 = −1.38, a = +5.57, b = +10.64, c = +16.11 and d = +1.10 for salt II. |
As mentioned above, although salt II has a similar α-type packing motif to salt I, the arrangement of the green and pink molecules is different (as shown in Fig. 11b). The band dispersions and Fermi surfaces are shown in Fig. 12c, which is very different to that of salt I. The band dispersion has a mid-gap at the centre of the band dispersion, a so-called Mott gap, indicating that the band is effective half filled. So the salt can become a Mott insulator. However, the salt selects a charge-ordered state, in which the charge of the green molecule (A) is almost zero and the charge of the pink molecule (B) is almost one as shown in Table 2. This charge-ordered pattern is the so-called “horizontal stripe”, which is commonly observed in the α-type BEDT-TTF salts. An α-type packing motif can theoretically have many types of charge disproportionation states.26 However, the horizontal stripe type is the most common, which was observed in salt II, and a vertical type, which salt I may have if the salt becomes a charge-ordered state, is not common. Although, salt II has the common horizontal stripe type charge order for α-type BEDT-TTF salts, the band structure is very different from usual α-type BEDT-TTF salts. As shown in Fig. 12c, an effectively half-filled flat band is observed from Y to V, on which the Fermi level is located. The resultant Fermi surfaces are straight line like a 1D band located on the edge of the Brillouin zone. The band structure of the usual α-type BEDT-TTF salts consists of quasi-1D and 2D Fermi surfaces, similar to the κ- or λ-type Fermi surface. The comparison of the transfer integrals indicates that the b and c (see Fig. 11b) values are almost twice as large as those of normal α-type BEDT-TTF salts. This indicates that salt II has strong 1D nature along the side-by-side, b-axis direction. If we change the value of the transfer integrals of b and c into the half values, the calculation provides the common α-type Fermi surfaces. Magnetic susceptibility measurement using 2.70 mg of powder sample of II (Fig. 13) suggests that the magnetic curve obeys a 2D Heisenberg model with J = −71.1 K but with a spin concentration of only 25%. A residual temperature independent magnetic susceptibility of 5.7 × 10−4 emu mol−1 was observed, suggesting that some fraction of the electrons are still itinerant. The strong side-by-side interaction may make some percentage (75%?) of spin freedom. The observation of the separation of the charge and spin freedom may be due to the polar nature of the crystal (see below). In addition, materials having a half-filled flat band can become ferromagnetic, which is so called Flat-Band Ferromagnetism.28 Salt II may be a candidate for a Flat-Band Ferromagnet. Now the sample has a band gap in the middle of the effective upper band caused by charge ordering. Applying static or uniaxial pressure to the sample can make the band gap become null, and the sample may become a ferromagnet. Moreover, a flat band material may have such a large density of state that it may have a large Seebeck coefficient.29 It is therefore a candidate for a thermoelectric transducer. Further research is now in progress.
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Fig. 13 Magnetic susceptibility of II after subtracting a Curie tail of 0.80% of S = 1/2 spin. The solid line is calculated on the basis of a 2D Heisenberg model.27 |
We believe that the unique electronic structure of salt II is owing to its chiral and polar anion. We calculated the dipole moment of the chiral anion using Winmostar Ver.4.10430 (MOPAC AM1) using atomic coordinates of the crystal structure. A dipole moment of 5.104 Debye is calculated, which directs to approximately the crystallographic c axis (Fig. S2a†). However, there are two anions in the anion layer, both of which appear to be arranged to cancel both dipole moments. Therefore the calculation of the dipole moment of both anions was performed. A dipole moment of 3.103 Debye was calculated, which was still large. The dipole vector directs to approximately the -a direction (Fig. S2b†). For the cancellation of the dipole moment along the a axis, the donor layer appears to go into the 0101 charge-ordered state along the a axis. Finally we calculated the dipole moment of four anions that surround a donor layer. A residual dipole moment of 1.431 Debye was calculated, which directs to the –b direction. The dipole moment can be converted into a voltage using the equation, ΔΦ = μNcos
θ/εε0 (ref. 31) = 0.14 eV, where ε ≈ 2 and θ = 0.0° were assumed.
The dipole moment caused by the anion layer should be cancelled, for which the donor layers should also be polarized. The donor layers would be in a unique charge disproportionate state to produce dipole moments also along the b-axis. In the circumstance, we speculate that the strong 1D interaction of salt II along the b axis, which provides the unique flat band, is owing to the residual dipole moment, whose origin is the chirality of the borate anion, because a crystal including a chiral molecule cannot possess any inversion centres or mirror and glide planes and become chiral and polar. Thus we believe that chirality provides a significant effect upon the electronic structure.
Racemic Salt; δH (400 MHz, CD3CN): four diastereomers, 4.44–4.51 (2H, m) (2 × OCH), 2.65–2.74 (2H, m) (2 × CHαHβCO2H), 2.46–2.55 (2H, m) (2 × CHαHβCO2H); δC (100 MHz, CD3CN): 178.3 & 178.4 (2 × CO2B), 171.8 (2 × CO2H), 72.2 & 72.4 (2 × OCH), 38.87, 38.91, 39.11 & 39.14 (2 × CH2CO2H). CHN: K[B(C4O5H4)2]·H2O; expected: C 28.95%, H 3.01%; found: C 28.94%, H 3.04%.
Chiral Salt; δH (400 MHz, CD3CN): two diastereomers in ratio 2:
1, 4.47 (2H, dd, J = 7.9, 4.3 Hz, major) & 4.49 (2H, dd, J = 8.6, 4.2 Hz, minor) (2 × OCH), 2.67 (2H, dd, J = 15.6, 4.2 Hz, minor) & 2.71 (2H, dd, J = 15.8, 4.3 Hz, major) (2 × CHαHβCO2H), 2.49 (2H, dd, J = 15.6, 8.6 Hz, minor) & 2.52 (2H, dd, J = 15.8, 7.9 Hz, major) (2 × CHαHβCO2H); δC (100 MHz, CD3CN): 178.2 & 178.3 (2 × CO2B), 171.8 (2 × CO2H), 72.1 & 72.3 (2 × OCH), 38.8 & 39.0 (2 × CH2CO2H). CHN: K[B(C4O5H4)2]; expected: C 30.6%, H 2.57%; found: C 30.32%, H 2.72%.
Platinum electrodes were cleaned by applying a voltage across the electrodes in 1 M H2SO4 in each direction to produce H2 and O2 at the electrodes, then washed with distilled water and thoroughly dried. K[B(malate)2] (100 mg) and 18-crown-6 (200 mg) were dissolved in 1,1,2-trichloroethane (30 ml) with stirring overnight before filtering into an H-cell containing BEDT-TTF (10 mg) in the anode compartment. Salt I was prepared using K[B(malate)2] synthesized from racemic malic acid. Salt II was prepared using K[B(malate)2] synthesized from D-(+)-malic acid. H-cells were placed in a dark box on a vibration-free bench at a constant current of 0.2 μA and after 21 days a large number of black hexagonal crystals were harvested from the anode of the H-cell containing the racemic anion. A large number of black plates were collected from the H-cell containing the enantiopure anion. Attempts were also made to grow crystals using K[B(malate)2] prepared from L-(−)-malic acid, but despite repeated attempts using a variety of conditions it was not possible to obtain single crystals.
We have synthesised a variety of borate anions with numerous chiral ligands and are continuing to synthesise further salts with BEDT-TTF to closely examine the effect of chirality upon the physical properties. We are also performing experiments using chiral hydroxylalkyl-BEDT-TTF derivatives with the aim of transmitting the chirality between anion and conducting donor layers through hydrogen-bonding interactions.
Footnotes |
† Electronic supplementary information (ESI) available. CCDC 1468076–1468078. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt01038e |
‡ Crystal data: I at 150 K: C28H29.44O12.72S16B1, M = 1093.26, black plate, a = 8.6620(4), b = 11.9323(5), c = 21.7062(7) Å, α = 76.855(5), β = 84.526(6), γ = 70.436(5)°, U = 2058.16(16) Å3, T = 150 K, space group P![]() ![]() ![]() Crystal data: I at 250 K: C28H29.43O12.72S16B1, M = 1093.25, black plate, a = 8.7239(3), b = 12.0104(3), c = 21.7699(6) Å, α = 76.913(5), β = 84.377(6), γ = 70.239(5)°, U = 2090.34(14) Å3, T = 250 K, space group P Crystal data: II: C28H24O10S16B1, M = 1044.24, black plate, a = 9.1696(8), b = 10.2343(9), c = 41.196(4) Å, α = β = γ = 90°, U = 3866.0(6) Å3, T = 100 K, space group P212121, Z = 4, μ = 0.950 mm−1, reflections collected = 44 Data for I were collected on a Rigaku R-AXIS VII imaging plate system with FR-E SuperBright High-Brilliance Rotating Anode Generator with confocal monochromated MoKα radiation, using a Rapid Auto software for control and processing. Data for II were collected on a Rigaku AFC12 diffractometer with Mo rotating anode, using standard control and processing software. All structures were solved and refined with programs from the SHELX family of computer programs. CCDC 1468076–1468078 contains supplementary X-ray crystallographic data for I and II. |
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