BEDT-TTF Salts Formed with Tetrahedrally Coordinated Zinc(II) Complex Anions

Article abstract of DOI:10.1021/acs.cgd.6b01294

Twelve kinds of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) cation radical salts with tetrahedrally coordinated zinc(II) complex anions were obtained by electrocrystallization; most of them were produced via the additional reaction of Lewis-basic cyano-containing anions [N(CN)₂^-, C(CN)₃^-, and Au(CN)₂^-] with ZnX₂ (X = Cl^-, Br^-, and SCN^-) that occurred during electrocrystallization. On the basis of the charge and arrangement of ET molecules, these salts were predominantly categorized into four groups: (A) isolated ET^?+ dimers or tetramers, (B) infinite ET^?+ ribbon, (C) infinite layers either with a peculiar ET^0.5+ arrangement of ET molecules with partial charges other than +0.5, and (D) infinite layers of ET^0.5+ molecules. In A, zinc(II) complex anions including a unidentate-coordinated C(CN)₃ or Au(CN)₂ group spatially interrupt the infinite arrangement of ET molecules. The ET^?+ ribbon in B runs parallel to the pseudopolymeric chain of disordered Zn[C(CN)₃]₂Br₂^2- ions. In C, two salts are semiconductive, whereas the remaining two salts behave as metallic at room temperature and undergo a metal-insulator transition on cooling. The ET molecules in D are arranged in a θ₄₂₊₄₀-like packing motif, due to the nonplanar anionic layers. All salts are semiconductive and exhibit a gradual evolution in their superstructures with cooling associated with charge disproportionation.

Full text of DOI:10.1021/acs.cgd.6b01294

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Article
BEDT-TTF Salts Formed with Tetrahedrally
Coordinated Zinc(II) Complex Anions
Yukihiro Yoshida, Hiroshi Ito, Yuto Nakamura, Manabu Ishikawa, Akihiro Otsuka,
Hiromi Hayama, Mitsuhiko Maesato, Hideki Yamochi, Hideo Kishida, and Gunzi Saito
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01294 • Publication Date (Web): 27 Sep 2016
Downloaded from http://pubs.acs.org on October 5, 2016
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Cover Page
Title: BEDTꢀTTF Salts Formed with Tetrahedrally Coordinated Zinc(II) Complex Anions
Authors: Yukihiro Yoshida,* Hiroshi Ito, Yuto Nakamura, Manabu Ishikawa, Akihiro Otsuka,
Hiromi Hayama, Mitsuhiko Maesato, Hideki Yamochi, Hideo Kishida, and Gunzi Saito
Affiliations: Faculty of Agriculture, Meijo University, Tempakuꢀku, Nagoya 468ꢀ8502,
Japan, Department of Applied Physics, Nagoya University, Chikusaꢀku, Nagoya 464ꢀ8603,
Japan, Division of Chemistry, Graduate School of Science, Kyoto University, Sakyoꢀku,
Kyoto 606ꢀ8502, Japan, Research Center for Low Temperature and Materials Sciences,
Kyoto University, Sakyoꢀku, Kyoto 606ꢀ8501, Japan, Toyota Physical and Chemical
Research Institute, Nagakute 480ꢀ1192, Japan
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Abstract: 12 kinds of bis(ethylenedithio)tetrathiafulvalene (BEDTꢀTTF or ET) cation salts
with tetrahedrally coordinated zinc(II) complex anions were obtained by
electrocrystallization; most of them were produced via the additional reaction of Lewisꢀbasic
–
2
–
3
–
2
–
–
cyanoꢀcontaining anions [N(CN) , C(CN) , and Au(CN) ] with ZnX (X = Cl , Br , and
2
–
SCN ) occurred during electrocrystallization. Based on the charge and arrangement of ET
●
+
molecules, these salts were predominantly categorized into four groups: (A) isolated ET
●
+
0.5+
dimers or tetramers, (B) infinite ET ribbon, (C) infinite layers either with a peculiar ET
arrangement or of ET molecules with partial charges other than +0.5, and (D) infinite layers
0.5+
of ET
molecules. In A, zinc(II) complex anions including a unidentateꢀcoordinated
C(CN) or Au(CN) group spatially interrupt the infinite arrangement of ET molecules. The
3
2
●
+
2–
3 2 2
ET ribbon in B runs parallel to the pseudoꢀpolymeric chain of disordered Zn[C(CN) ] Br
ions. In C, two salts are semiconductive, whereas the remaining two salts behave metallic at
room temperature and undergo a metalꢀinsulator transition on cooling. The ET molecules in
D are arranged in a θ42+40ꢀlike packing motif, due to the nonꢀplanar anionic layers. All salts
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are semiconductive and exhibit a gradual evolution in their superstructures with cooling,
associated with charge disproportionation.
Corresponding Author: Yukihiro Yoshida (Meijo University, Tel: +81ꢀ52ꢀ838ꢀ2552, Fax:
+
81ꢀ52ꢀ833ꢀ7200, Eꢀmail: yyoshida@meijoꢀu.ac.jp)
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BEDTꢀTTF Salts Formed with Tetrahedrally
Coordinated Zinc(II) Complex Anions
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,†
‡
‡
§,#
§,#
Yukihiro Yoshida,* Hiroshi Ito, Yuto Nakamura, Manabu Ishikawa, Akihiro Otsuka,
†
§
§,#
‡
†,||
Hiromi Hayama, Mitsuhiko Maesato, Hideki Yamochi, Hideo Kishida, and Gunzi Saito
†
Faculty of Agriculture, Meijo University, Tempakuꢀku, Nagoya 468ꢀ8502, Japan
‡
Department of Applied Physics, Nagoya University, Chikusaꢀku, Nagoya 464ꢀ8603, Japan
Division of Chemistry, Graduate School of Science, Kyoto University, Sakyoꢀku, Kyoto
§
606ꢀ8502, Japan
#
Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyoꢀku,
Kyoto 606ꢀ8501, Japan
||
Toyota Physical and Chemical Research Institute, Nagakute 480ꢀ1192, Japan
Abstract: 12 kinds of bis(ethylenedithio)tetrathiafulvalene (BEDTꢀTTF or ET) cation salts
with tetrahedrally coordinated zinc(II) complex anions were obtained by
electrocrystallization; most of them were produced via the additional reaction of Lewisꢀbasic
–
–
–
–
–
2 3 2 2
cyanoꢀcontaining anions [N(CN) , C(CN) , and Au(CN) ] with ZnX (X = Cl , Br , and
–
SCN ) occurred during electrocrystallization. Based on the charge and arrangement of ET
●
+
molecules, these salts were predominantly categorized into four groups: (A) isolated ET
●
+
0.5+
dimers or tetramers, (B) infinite ET ribbon, (C) infinite layers either with a peculiar ET
arrangement or of ET molecules with partial charges other than +0.5, and (D) infinite layers
0.5+
of ET
molecules. In A, zinc(II) complex anions including a unidentateꢀcoordinated
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C(CN)
3
or Au(CN)
2
group spatially interrupt the infinite arrangement of ET molecules. The
●
+
2–
3 2 2
ET ribbon in B runs parallel to the pseudoꢀpolymeric chain of disordered Zn[C(CN) ] Br
ions. In C, two salts are semiconductive, whereas the remaining two salts behave metallic at
room temperature and undergo a metalꢀinsulator transition on cooling. The ET molecules in
D are arranged in a θ42+40ꢀlike packing motif, due to the nonꢀplanar anionic layers. All salts
are semiconductive and exhibit a gradual evolution in their superstructures with cooling,
associated with charge disproportionation.
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1. Introduction
Organic cation radical salts, which are primarily composed of electron donor molecules and
1
–8
inorganic anions, play a primary role in the exploration of molecular conductors. In these
salts, anions not only serve to compensate the charge of the (partially) oxidized electron
donors but also contribute to structural and physical properties of salts depending on the
shape, size, and charge state of the anions. Bis(ethylenedithio)tetrathiafulvalene (BEDTꢀTTF
or ET) cation radical salts are one of the most extensively studied and wellꢀestablished
families, primarily attributed to (i) the facile manner in which their single crystals are
obtained, (ii) a wide variety of electronic properties, including antiferromagnetic (AF)
insulators, quantum spin liquids, correlated metals, and unconventional superconductors
closely associated to molecular packing manner, and (iii) a drastic change in the electronic
properties associated with chemical or physical structural modification while maintaining the
10
basic packing manner. In particular, planar polymeric anions involving a d metal ion Cu(I)
–
9,10
–
11–16
and (pseudo)halide ligands such as [Cu(NCS)
2
∞
] ,
{Cu[N(CN)
2
]X }
∞
(X: halides),
and
–
17–19
2 3 ∞
[Cu (CN) ]
have played a key role as counter anions in novel electronic materials.
Recently, our group has reported some intriguing ET cation radical salts formed with planar
–
polymeric anions involving another coinage metal ion Ag(I), such as {Ag(CN)[N(CN) ] }
2
∞
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(weakly correlated ambientꢀpressure superconductor) and [Ag
2
3 ∞
(CN) ] (quantum spin
2
1,22
liquid at ambient pressure and superconductor under pressure).
We now extend our
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material exploration to the ET salts combined with another d metal ion Zn(II). Thus far, ET
salts formed with zinc(II) complex anions have been explored to a significantly lesser extent
despite the fact that coordination bonds can be readily formed with zinc(II), affording new
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complex anions, attributed to the lability of zinc(II) species. To the best of our knowledge,
2–
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zinc(II) complex anions combined with ET have been limited to discrete ZnX
4
(X = Cl
26,27
2– 28,29
–
and Br
M = Rb, Cs, and Tl).
(pseudo)halides ZnX via nucleophilic addition, affording divalent ZnX Y Y
B
), dimeric Zn
2
(SCN)
6
,
and oneꢀdimensional (1D) polymeric [MZn(SCN)
4
]
∞
2
8,30,31
–
–
(
Lewis basic monoanions (YA and Y ) readily react with zinc(II)
B
2–
ions in a
2
2
A
3
hybrid sp state in anhydrous media, attributed to the Lewis acidity of ZnX
2
. We expect that,
in contrast to the planar trigonal coordination for copper(I)ꢀ or silver(I)ꢀcontaining polymeric
anions, tetrahedral coordination results in a nonꢀplanar anionic surface, which would
significantly affect the molecular arrangement in the ET layers.
In this study, 12 kinds of new ET cation salts, formed with zinc(II) complex anions
coordinated by halides, (thio)cyano, and aqua groups, were obtained by the electrochemical
–
–
–
2
oxidation of ET in the presence of ZnX (X = Cl , Br , and SCN ). Typically, the following
chemical reactions occur during electrocrystallization:
Nucleophilic addition
–
A
–
2–
ZnX
2
+ Y
+ Y
B
2 A B
ꢀ ZnX Y Y
Oxidation
x ET + ZnX
–
(2/x)e
ꢀ [ET
2
–
(2/x)+
2–
2
Y
A
Y
B
x 2 A B
] (ZnX Y Y )
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Cyanoꢀcontaining zinc(II) complex anions produced during the process include monomeric
2
–
2–
Zn[C(CN)
3
3
]X
3
[X = Cl (1) and Br (2, 7)], Zn[Au(CN)
2
]X
3
[X = Cl (3) and Br (4)],
2–
2–
2–
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Zn[C(CN)
]
2
Br
2
(5), Zn(NCS)
2
X
2
[X = Cl (10) and Br (11)], Zn[N(CN)
2
]Cl
(12) and
–
2
2–
2
polymeric {Zn[N(CN) ]Cl } (6). Figure 1 shows the molecular structures of Zn(NCS) Cl ,
2
∞
2
0
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2–
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Zn[N(CN) ]Cl , Zn[C(CN) ]Cl , and Zn[Au(CN) ]Cl , which were crystallographically
2
3
2
characterized herein. Zinc(II) ions in these anions are tetrahedrally coordinated by nitrile
–
–
–
–
2 3 2
nitrogen(s), in addition to halides; namely, the NCS , N(CN) , C(CN) , and Au(CN)
groups in monomeric anions (1–5, 7, and 10–12) serve as unidentate ligands, whereas the
–
N(CN) group in polymeric anion (6) serves as a ꢀ ꢀbidentate bridging ligand, affording the
2
1,5
–
2
1D polymeric {Zn[N(CN) ]Cl } ions. In addition, new ET salts were obtained by
2
∞
–
2–
4
combining with zinc(II) halides, i.e. ZnCl
3
(H
2
O) (8) and ZnBr
(9). Their unique molecular
arrangement derived from the tetrahedral coordination of zinc(II), as well as
structureꢀproperty relationship, with focus on intermolecular interactions and charge states on
ET molecules is discussed.
********** Figure 1 **********
2
. Results and Discussion
2.1 Electrocrystallization
All salts were obtained by the electrochemical oxidation of ET (see Experimental section for
details), and the synthetic conditions such as reagents and solvents used are summarized in
Table 1. Typically, ET was added to an anodic compartment, while the other reagents were
added to the cathodic compartment of an Hꢀshaped glass cell, which was assembled in a
glove box filled with argon gas (H O, O < 1 ppm). After being stirred in a solvent such as
2
2
1
,1,2ꢀtrichloroethane (TCE) or a TCE/alcohol mixture at room temperature (RT), a constant
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current (1.0 ꢁA) was passed between the two platinum electrodes over approximately two
weeks at a constant temperature (15 or 20 °C). Several phases were simultaneously harvested,
which were separated under a microscope and identified by Xꢀray diffraction measurements.
At present, the formation of zinc(II) complex anions including other cyanoꢀcontaining groups
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–
–
such as Ag(CN) or B(CN) has not been successful.
2
4
********** Table 1 **********
The crystallographic data of 1–12 are summarized in Table 2. Crystal structures of the salts,
except for 8, were determined at 100 K. Although salt 8 undergoes a metalꢀinsulator
1 1
transition, accompanied by the symmetry reduction (from P2 /n to P2 ) at 210 K, structural
refinement at 100 K was not sufficient, possibly due to incomplete phase transition. In
addition, crystallographic studies at 298 K were performed for 4 and 8–12, because they
exhibit a sign of a structural or electronic phase transition below RT. In the following section,
we will describe the structural and physical properties of the salts in four groups: isolated
●+
●+
ET dimers or tetramers (Group A), infinite ET ribbon (Group B), infinite layers either
0.5+
with a peculiar ET
arrangement or of ET molecules with partial charges other than +0.5
0.5+
(
Group C), and infinite layers of ET molecules (Group D).
********** Table 2 **********
2.2 Group A
2.2.1 (ET)
2
Zn[C(CN) ]Cl (1): Salt 1 crystallizes in a monoclinic lattice with space group
3 3
P2 /c at 100 K, and the asymmetric unit contains two ET molecules (A and B) and one
1
2–
Zn[C(CN) ]Cl
ion (Fig. 1c). This composition indicates the completely oxidized ET
3
3
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molecules; however, ET molecules appear to be chargeꢀdisproportionated on the basis of
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bond distance analysis (+0.72(4) for A and +1.07(4) for B). This salt has a layered structure
within the ab plane (Fig. 2a). In the layer, ET molecules are arranged along the 2a + b
direction, affording a tetrameric subunit with a BAAB arrangement pattern (Fig. 2b),
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9
0
2
3
–
separated by a pair of Zn[C(CN) ]Cl . In a tetramer located on an inversion center, inner
3
(A) and outer ET (B) molecules appear to be chargeꢀpoor and chargeꢀrich, respectively, and
the tetramer composed of the two A–B pairs possibly has +4 charge. Overlap integrals
between the HOMOs of ET molecules within the tetramer were calculated on the basis of the
3
3
–3
extended Hückel method, where p1 = –28.2 × 10 for an A–B pair with a
–
3
ringꢀoverꢀbondꢀtype overlap (Fig. 2c) and p2 = –4.8 × 10 for an A–A pair with a
ringꢀoverꢀatom overlap (Fig. 2d).
********** Figure 2 **********
2
3
–
In Zn[C(CN) ]Cl ion, a unidentateꢀcoordinated C(CN) moiety exhibits an almost flat
3
3
geometry, as can be seen in Fig. 1c, which penetrates the ET layer. Such a penetration of a
34–38
part of anions has been observed for several ET salts.
3
Each C(CN) moiety approaches
another C(CN) moiety from the other side, related by an inversion center, and lies in the
3
same plane. On the other hand, the sterically bulky ZnCl moiety remains outside the ET
3
layer and connects with four ET molecules in the adjacent layer through S···Cl (3.41 Å vs.
39
sum of the van der Waals radii: 3.55 Å ) and C–H···Cl (2.71–2.94 Å vs. sum of the van der
39
Waals radii: 2.95 Å ) interactions.
2
.2.2 (ET) Zn[C(CN) ]Br (2): In this study, three kinds of ET salts were obtained by
2 3 3
combining with zinc(II) complex anions including bromides and unidentateꢀcoordinated
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C(CN)
3
group(s) (2, 5, and 7). In most cases, these crystals were simultaneously grown but
readily separated by the difference in their shape.
1
Salt 2 belongs to the monoclinic system with space group P2 /n at 100 K. Two ET
2–
molecules (A and B) and one Zn[C(CN) ]Br ion are crystallographically independent. The
3
3
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anionic structure and composition are identical with those of 1. Within the ab plane (Fig. 3a),
ET molecules are aligned along the 2a + b direction, affording a BAABꢀtype tetrameric
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subunit as a part of β’ꢀtype infinite stacking (Fig. 3b) unlike the case in 1, which is
2–
3 3
separated by a pair of Zn[C(CN) ]Br . Bond distance analysis estimates the charges of ET
32
molecules to be +0.69(4) for A and +1.09(4) for B; namely, the tetramer composed of the
two A–B pairs possibly has +4 charge as expected from the composition. Overlap integrals
–
3
between ET molecules within the tetramer were calculated to be p1 = –29.0 × 10 for an A–
–3
B pair with a ringꢀoverꢀbondꢀtype overlap (Fig. 3c) and p2 = –0.2 × 10 for an A–A pair
with a ringꢀoverꢀatom overlap (Fig. 3d).
********** Figure 3 **********
2
–
Zn[C(CN)
3
]Br
3
ion includes an almost flat C(CN)
3
moiety as a unidentate ligand, which
moiety approaches another C(CN)
penetrates the ET layer as in the case of 1. Each C(CN)
3
3
moiety from the other side, related by an inversion center, and is present in the same plane.
On the other hand, the sterically bulky ZnBr moiety remains outside the ET layer and
3
connects with four ET molecules in the adjacent layer through the S···Br (3.44 Å vs. sum of
39
the van der Waals radii: 3.65 Å ) and C–H···Br (2.86–2.98 Å vs. sum of the van der Waals
3
9
radii: 3.05 Å ) interactions.
2
.2.3 (ET) Zn[Au(CN) ]Cl (3) and (ET) Zn[Au(CN) ]Br (EtOH) (4): Salt 3 crystallizes
2 2 3 2 2 3 0.5
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2
2
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2
3
3
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3
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3
3
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3
4
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5
5
5
5
5
5
5
6
in the monoclinic system with space group P2
1
/c at 100 K, and an asymmetric unit contains
2
3
–
two ET molecules and one Zn[Au(CN)
2
]Cl
ion. The composition results in a nominal
charge of +1 on each ET, which was confirmed by the estimation of the charge of the ET
3
2
molecules on the basis of bond distance analysis (+0.99(3) and +0.91(3)). As illustrated in
Fig. 4a, ET molecules are assembled in a twoꢀdimensional (2D) layer within the bc plane.
Two crystallographically independent ET molecules form a faceꢀtoꢀface twisted dimer with a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
41
torsion angle around the central C=C bonds of 10.0° (Fig. 4b; called as α’ꢀ or δꢀtype; the
former involves the ET molecules twisted relative to the adjacent molecules along the
stacking direction, whereas the latter involves the parallel ET dimers twisted relative to the
adjacent dimers along the stacking direction), through S···S heteroatomic contacts (3.38–3.59
39
Å vs. sum of the van der Waals radii: 3.60 Å ) and C–H···S hydrogen bonds (H···S distance:
39
2.91–2.97 Å vs. sum of the van der Waals radii: 3.00 Å ). In fact, the intraꢀdimer overlap
–
3
integral between ET molecules (b1 = 41.4 × 10 ) is significantly greater than the interꢀdimer
–3
ones (< 21.4 × 10 ). Each dimer is almost perpendicular to the adjacent dimer along the c
axis; namely, the κꢀlike packing motif (orthogonal arrangement of faceꢀtoꢀface ET dimers)
2+
42
based on diamagnetic (ET)
2
dimers (Fig. 4c).
********** Figure 4 **********
As can be seen in Figs. 1d and 4a, each Au(CN) moiety has an almost linear geometry (C–
2
Au–C: 178.60(9)°) and penetrates the ET layer inclined at approximately 16° with respect to
the a axis, where half of them are aligned along the +a direction, while the remaining half run
along the –a direction. Within the ET layer, the terminal cyano group of Au(CN)
2
moieties
interacts with two neighboring ET molecules through C≡N···π (C21···C11: 3.36 Å,
43
39
N1···C12: 3.13 Å) and S···N (3.26–3.34 Å vs. sum of the van der Waals radii: 3.35 Å )
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contacts. Three chlorides are connected with six ET molecules, including three in the layer
and three in the adjacent layer, through S···Cl (3.22–3.48 Å) and C–H···Cl (2.73–2.94 Å)
contacts.
Here, the structural geometry of ET molecules is discussed on the basis of the interꢀdimer
interactions for comparison with related ET salts. The transfer integral (t) along the c axis (t1
10
11
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15
16
17
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34
35
36
37
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47
48
49
50
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55
56
57
58
59
60
|t
10 eV), is comparable to that along the b axis (tb2/2 = 107 meV and (2|tb3| + |tb4|)/2 = 85.6
meV result in an average value t = 96.3 meV). On the other hand, the transfer integral along
the oblique direction (t = (|t | + |t |)/2 = 1.7 meV) is significantly smaller relative to them,
c
= |/2 = 73.5 meV), estimated on the basis of the assumption t = Es (s: overlap integral, E =
–
2
3
p
q
2–
mainly due to the spatial interruption of the Zn[Au(CN) ]Cl ions (right panel in Fig. 4c).
2
3
As a result, these values provide the anisotropy of an isosceles triangular lattice t
3
1 2
/[(t +t )/2]
=
0.02. Therefore, the κꢀlike arrangement in 3 can be regarded as a square lattice as in the
4
0
case of β’ꢀ(ET)
2
X salts (0.16 for X = ICl
2
and 0.19 for X = AuCl
2
2
), whereas the κꢀ(ET) X
●
+
1–8,42
salts based on spinꢀ1/2 (ET)2 dimers exhibits more or less triangular form.
The present
assembly possibly provides an important clue for the preferential production of ET cation
2+
radical salts with the square spin lattice, although the (ET)
Notably, among the ET family, the β’ꢀ(ET)
typical strongly correlated system, and indeed, some of them exhibit longꢀrange AF ordering
2
dimers in 3 are diamagnetic.
●
2
+
2
X salts based on spinꢀ1/2 (ET)
dimers are a
44,45
and superconductivity under applied pressures.
Salt 4 appears to exhibit some similarities with 3, in addition to striking differences. The
molecular arrangement is very similar to that of 3, but the space group at low temperature is
1
different (monoclinic P2 /c at 298 K and triclinic P 1¯ at 100 K). In addition, salt 4 includes
orientationally disordered EtOH molecules on an inversion center between the ET layers.
Within the dimer, an ET molecule slides with respect to another molecule along the molecular
long axis by 1.3 Å (Fig. 4d) and 0.8 Å (Fig. 4e) for two crystallographically independent
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2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2 6
dimers. Such overlap patterns are reminiscent of those in monocationic salts, (ET) M O19 (M
VI
VI 46
47
=
Mo and W ) and (ET)
2
Pd(dto)
2
(H
2
dto: dithiooxalic acid) with similar molecular
2+
2
arrangements. Transfer integrals between (ET)
dimers, t
1
= 74.6 meV, t
2
= 94.8 meV, and t
3
=
3.4 meV, result in the anisotropy of an isosceles triangular lattice t /[(t + t )/2] = 0.04,
3 1 2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
40
which is also less than those of β’ꢀ(ET) X.
2
2
.3 Group B
.3.1 (ET) Zn[C(CN)
2
2
3
]
2
Br
2
(5): The isostructural chloride salt (ET)
2
Zn[C(CN)
3
]
Cl
2 2
was
obtained as a byꢀproduct of 1. However, structural refinement is not sufficient to discuss the
structural features in detail, due to its inferior crystallinity (R ~ 0.08); hence, the bromide salt
1
5
is focused in this paper.
Salt 5 is triclinic with space group P 1¯ , and an asymmetric unit contains one ET molecule
2–
and half Zn[C(CN)
3
]
2
Br
2
ion (Fig. 5a). In the crystal, ET molecules form a 1D array
through short S···S heteroatomic contacts (3.42–3.56 Å) along the sideꢀbyꢀside direction or
the crystallographic a axis (Fig. 5b). The two parallel ET arrays are connected to each other,
affording an infinite double array centered at z = 0.5 (Fig. 5c). On the basis of intramolecular
32
bond lengths, the charge of the ET molecule was estimated to be +0.78(7), deviated from a
nominal charge of +1. However, the salt exhibits semiconducting behavior with a RT
–1
conductivity (σ ) of 1.2 S cm and an activation energy (E ) of 0.078 eV (Fig. 6a),
RT
a
supporting the fully ionic state of ET. The optical absorption spectrum exhibits no remarkable
3
–1
6
band below 5 × 10 cm attributed to the partially oxidized state.
********** Figure 5 **********
********** Figure 6 **********
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2
–
Each Zn[C(CN)
3
]
Br
2 2
ion is positionally disordered over two sites with an equivalent
occupancy of 0.5, resulting in the formation of a pseudoꢀpolymeric chain in parallel to the ET
arrays along the a axis. Each zinc(II) ion is tetrahedrally coordinated by two C(CN) groups
3
and two bromides, and the average structure with respect to disorder is regarded such that
these ligands serve as a bidentate bridging ligand to link zinc(II) ions between adjacent zinc
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46
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48
49
50
51
52
53
54
55
56
57
58
59
60
‘
pseudoꢀdimers’ (Fig. 5d). Notably, bromides, protruding toward the ET layers on both sides,
are located at the vacancy sites in the ET layers, as shown in Fig. 5e; namely, the geometry of
2
2
–
the pseudoꢀpolymeric chain of Zn[C(CN)
in ET layers.
3
]
2
Br
ions significantly affects the packing motif
3
3
Based on the extended Hückel calculations, transfer integrals within the double array
●
+
a p q
were determined as t = 216, t = 54.9, and t = 62.2 meV; therefore, the ET array is
regarded as a zigꢀzag chain with a greater interaction along the chain direction (Fig. 5b).
Recently, 1D triangleꢀbased materials composed of cornerꢀsharingꢀtriangular (or ꢂꢀchain; Fig.
48
49,50
51,52
6b), sawtooth (Fig. 6c), zigꢀzag (Fig. 6d),
diamond (Fig. 6e),
or orthogonalꢀdimer
53
(
Fig. 6f) chains are attracting considerable attention as potentially benign spinꢀfrustrated
systems, apart from 2D (e.g. triangular and kagome) and 3D (e.g. hyperkagome) ones. In the
zigꢀzag chain system, the ratio J’/J, where J and J’ represent the nearestꢀ and
nextꢀnearestꢀneighbor exchange interactions, is a measure of spin frustration. A theoretical
study has predicted that the S = 1/2 zigꢀzag chain has a collinear gapless phase for J’/J <
4
9,50
II
54,55
0
.241,
as suggested for Cu [2ꢀ(2ꢀaminomethyl)pyridine]Br (J’/J = 0.2, J/k = 8.5 K)
2
B
IV
56,57
and (V O)(ꢀ
3
ꢀMoO
4
)(2,2’ꢀbipyridine) (J’/J = 0.2, J/k
B
= 51 K).
Figure 6a shows the temperature dependence of static magnetic susceptibility (χ) of the
2
polycrystalline sample. As the J’/J value (0.073) expressed as [(|t
p
| + |t
q
|)/2|t
a
|] is apparently
less than the critical value, the S = 1/2 Heisenberg AF linear chain studied by Bonner and
58,59
●+
Fisher (χBF)
is utilized, indicative of a uniform ET chain with negligible interchain
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2
2
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5
5
5
6
interactions. The χ value can be numerically fitted by the sum of χBF and a Curie component
C
(χ ) with a fraction f as follows;
χ = f χBF + (1–f)χC
(1)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
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0
1
2
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0
1
2
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5
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8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
2
Ng ꢀ
A + Bx + Cx
B
χ
BF
=
(x = J / k
B
T)
(1’)
2
3
k T 1+ Dx + Ex + Fx
B
χ
C
= C ’/T
(1’’)
(
N: Avogadro number, g: Landé gꢀfactor, ꢀ : Bohr magneton, k : Boltzmann constant, C ’:
B B
Curie constant). In the case of an equally spaced spin chain, coefficients are given by A =
0.25, B = 0.14995, C = 0.30094, D = 1.98654, E = 0.68854, and F = 6.0626. The bestꢀfitted
–3
–1
B
parameters are J/k = 250 K and C’ = 3 × 10 emu K mol and the former is expected to be
well tuned by the replacement of the protruded bromides with other halides.
2.4 Group C
2 2 2
2.4.1 (ET) Zn[N(CN) ]Cl (TCE)0.5 (6): Salt 6 was obtained by electrocrystallization under
the same conditions as those utilized for 12, and has a crystal symmetry (triclinic P 1¯ ) less
than that of 12. Crystallographically independent unit contains one full (A) and two half (B
–
2
and C) ET, one Zn[N(CN) ]Cl unit, and half TCE. As can be seen in Fig. 7a, ET molecules
2
form a layered structure within the ab plane at z ~ 0, in which parallel and coplanar adjacent
molecules exist along the sideꢀbyꢀside direction (a axis) for each molecule. In the layers, A is
located at a general position, whereas B and C are located on an inversion center. A and B
form a domain of parallelꢀoriented ET molecules, which is separated by C twisted with
respect to A, with a torsion angle of 31.0° around the central C=C bond. Such a twisted
41
structure reminds us of the α’ꢀ and δꢀtype packing pattern. To our best knowledge, salt 6 has
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an unprecedented packing pattern of ET molecules. The charges of A, B, and C were
estimated to be +0.36(8), +0.82(10), and +0.83(11), respectively, strongly implying the
presence of charge disproportionation. The chargeꢀpoor (A) and chargeꢀrich (B or C) ET
molecules are alternately arranged along the b axis, i.e. each of chargeꢀpoor or chargeꢀrich ET
10
11
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13
14
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16
17
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19
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21
22
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44
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47
48
49
50
51
52
53
54
55
56
57
58
59
60
6
0,61
molecules are arranged along the a axis (horizontal stripe).
*
********* Figure 7 **********
–
2
Within the ab plane at z ~ 0.5, two chlorides and two N(CN) coordinate to zinc(II) ion,
–
and the N(CN) ligands serve as a ꢀ ꢀbidentate bridging ligand to link the adjacent zinc(II)
2
1,5
ions, resulting in a polymeric anionic chain running along the a axis (Fig. 7b). Notably, the
–
2
coordination environment in {Zn[N(CN)
2
]Cl
Fig.
in which copper(I) is trigonally coordinated to one halide and two
N(CN) groups through a nitrile nitrogen. Besides the coordination number to central metal
}
∞
(Fig. 7c) is entirely different from that in
–
{
Cu[N(CN)
2
]X }
∞
(X:
halides;
7d) in wellꢀknown superconductors
11–16
κꢀ(ET) Cu[N(CN) ]X,
2
2
2
–
2
–
ions, different orientations of N(CN)
2 2 ∞
, i.e. ‘inꢀphase’ orientation for {Zn[N(CN) ]Cl } and
–
‘outꢀofꢀphase’ orientation for {Cu[N(CN)
2
]X }
∞
were observed. The assembly of the
polymeric chains affords an anionic layer within the ab plane, in which disordered TCE
molecules are accommodated in the cavities formed between the polymers.
–
2
As shown in Fig. 7e, chargeꢀrich B and C are located near the {Zn[N(CN) ]Cl } chains
2
∞
through the S···Cl (3.31 Å) and C–H···Cl (2.72–2.94 Å) interactions for B and the S···Cl
3.46 Å) and C–H···Cl (2.71 Å) interactions for C. Thus, the intermolecular Coulomb
(
interactions are most likely a dominant factor for determining the pattern of charge
disproportionation in 6.
–1
The salt exhibits semiconducting behavior with σRT = 0.4 S cm and E = 0.074 eV (Fig.
a
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1
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2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
8a), mainly because of the chargeꢀdisproportionated state. Figure 8b shows the temperature
–4
–1
dependence of χ of the polycrystalline sample. The χ value is 9.0 × 10 emu mol at RT,
–4
–1 6
which exceeds that for typical organic metals (3–5 × 10 emu mol ). It gradually increases
with decreasing temperature and exhibits a rounded maximum at around 20 K, attributed to
AF fluctuation. Notably, in the whole temperature range, the χ value (50–60% of free spin) is
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
8,59
significantly less than those expected from the S = 1/2 Heisenberg AF linear chain
with
62
J/k
B
= 15 K (Eq. 1’) and square lattice with J/k
B
= 10 K. Although the factors that suppress
the magnetic moments remain very unclear, the present results may indicate the dimerization
of some chargeꢀrich ET molecules along the sideꢀbyꢀside direction (// a axis) that cannot be
detected by Xꢀray diffraction measurements. An upturn below the temperature appears to be
●
x
+
attributed to crystal defect, Curie impurity, and/or isolated (ET)
species. The localized
nature apparently conflicts with the band structure calculated by assuming uniform charge
distribution in the ET layer (i.e. +0.50e on each ET), which shows the finite density of states
(DOS) at the Fermi level (Fig. S1 in in Supporting Information). It is possible that this
discrepancy arises from the occurrence of charge disproportionation as mentioned above.
********** Figure 8 **********
2.4.2 (ET) {Zn[C(CN) ]Br } (7): Salt 7 has a lower crystal symmetry (triclinic P 1¯ ) than
5
3
3 2
2–
3
that of 2, and includes ET and Zn[C(CN) ]Br in a ratio of 5:2; namely, the ET molecules
3
have a nominal charge of +0.8. The asymmetric unit contains four full (A, C, E, and F) and
2–
3
two half (B and D) ET molecules and two Zn[C(CN)
3
]Br
ions. In the crystal, two kinds of
ET layers (Layerꢀ1 and Layerꢀ2; Fig. 9a) are present, both of which are expanded within the
ab plane. Layerꢀ1 at z ~ 0 consists only of ET molecules, which are arranged in a θ ꢀlike
3
1
42
packing motif (Fig. 9b). The charges of ET molecules (A–D) were estimated to be
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+0.12(10), +0.39(9), +0.72(13), and +0.69(12), respectively, on the basis of bond distance
3
2
analysis; these values strongly indicate the presence of charge disproportionation with a
60,61
vertical stripe pattern,
in which each of chargeꢀpoor or chargeꢀrich ET molecules are
arranged along the b axis and the total charge in the unit lattice is +2.76(28). Reflecting the
chargeꢀdisproportionated state, the salt also exhibits semiconducting behavior with σRT = 1.1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
–3
–1
×
a
10 S cm and E = 0.225 eV (Fig. 8a).
********** Figure 9 **********
2–
3
In Layerꢀ2 at z ~ 0.5, on the other hand, ET and Zn[C(CN) ]Br are included in equimolar
3
2
–
amounts, and only the C(CN)
3
moieties of Zn[C(CN)
moieties facing each other are not in the same plane
and are not parallel, but form a dihedral angle of ~3.6°, in contrast to 2. The C(CN) moieties
3
]Br
3
ions penetrate the layer (Fig. 9c)
as in the case of 2. However, the C(CN)
3
3
are connected to surrounding ET molecules within the Layerꢀ2 through S···C (3.33–3.49 Å)
and S···N (3.18–3.19 Å) interactions, whereas the bromides are connected to ET molecules
within the adjacent Layerꢀ1 through C–H···Br (2.52–3.04 Å) hydrogen bonds. Bond distance
32
analysis has revealed that the ET molecules (E and F) are in a fully charged state (+1.18(10)
for E and +1.16(10) for F); thus, the faceꢀtoꢀface dimer composed of E and F with a
ringꢀoverꢀbondꢀtype overlap as A–B pair in 2 (Fig. 3c) appears to be nonmagnetic, with an
–3
intraꢀdimer overlap integral calculated to be –31.1 × 10 . Because the total charge of Layerꢀ2
in the unit lattice is –3.32(20), the negatively charged Layerꢀ2 is alternately arranged with the
positively charged Layerꢀ1 along the c axis.
2
.4.3 (ET) [ZnCl (H O)] (H O) (8): Salt 8 was obtained by electrooxidation of ET in the
3 3 2 2 2 2
presence of ZnCl , KCl, and 18ꢀcrownꢀ6 (18C6). Water molecules possibly come from ZnCl
2
2
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8
9
1
1
1
1
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1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
which has high hygroscopicity. A crystallographic study has revealed that this salt belongs to
1
a monoclinic lattice P2 /n at 298 K and undergoes a structural phase transition to a lower
symmetry state (P2
1
at 100 K). At 298 K, the asymmetric unit contains one (A) and a half (B)
–
of the ET molecules, one zinc(II) aquachloride ZnCl (H O) ion with pseudotetrahedral
3
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
32
geometry, and one H O molecule. On the basis of the bond distance analysis, charges of ET
2
molecules have been estimated to be +0.57(8) for A and +0.75(11) for B; charge
disproportionation is less significant as compared with that in 7, and the charge of +1.89(19)
per ET triad is consistent with the value expected from this composition (+2). ET molecules
40,63
are arranged in a β’’ ꢀtype packing motif as in the case of many 3:2 salts,
in which the
311
stacking column extends along the [1 0 –1] direction with an AAB repeating unit (Fig. 10a).
In the column, the A–B pair consists of a ringꢀoverꢀatom overlap motif with lateral slipping
(Fig. 10b), whereas the A–A pair consists of a ringꢀoverꢀatom overlap motif with oblique
slipping (Fig. 10c). Within the ac plane, the columns constitute the layered structure, which is
–
separated by an anionic layer composed of ZnCl (H O) and H O.
3
2
2
********** Figure 10 **********
–
3 2
The ZnCl (H O) species (Fig. 10d) has been reported for salts formed with various cations,
+
64
2+ 65
66
such as K , Zn(H O)
,
6
2ꢀ(2ꢀthioxoꢀ1,3ꢀthiazolidinꢀ3ꢀyl)ꢀ4,5ꢀdihydroꢀ1,3ꢀthiazolium,
2
6
7
68
2
ꢀcarboxyꢀ6ꢀmethylpyridinium, and benzyltriethylammonium. The Zn–Cl bond lengths at
an average of 2.24 Å fall well within the range of distances observed for
ZnCl (H O)ꢀcontaining salts (2.21–2.28 Å), and the zinc–aqua bond length (2.001 Å) is also
3
2
comparable to those observed for the aforementioned salts (1.98–2.04 Å). One of the
–
hydrogen atoms of the aqua ligand is linked to one chloride of neighboring ZnCl (H O) ion
3
2
through an OH···Cl (2.41 Å) hydrogen bond, whereas the other links to one oxygen atom of
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8
9
the solvent water molecule through an OH···O (1.76 Å vs. sum of the van der Waals radii:
39
2
.72 Å ) hydrogen bond. These intermolecular interactions result in the formation of an
infinite 2D network linked by hydrogen bonds.
The band structure, DOS, and Fermi surface of the HOMO band of ET molecules at 298 K
were calculated by assuming uniform charge distribution in the ET layer (i.e. +0.67e on each
ET). As can be seen in Fig. 10e, the highest branch is apparently more dispersive along the a
axis than along the c axis, possibly because of the significant transfer integrals along this
direction (tq2 and tq4). The Fermi surface consists of an electron pocket centered at the Z point;
hence, salt 8 is regarded as a 2D electronic system.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 8a includes the temperature dependence of resistivity (ρ) of 8, which has a high σRT
–1
value of 55 S cm and maintains metallic behavior down to 210 K where a sharp
69,70
metalꢀinsulator transition occurs. Its isostructural salts, β’’311ꢀ(ET)
3
(HSO
4
)
2
and
7
1
β’’311ꢀ(ET)
3
(ClO
4
2
) ,
undergo a metalꢀinsulator transition at TMI = 126 and 170 K,
72
respectively, and the chargeꢀordered state emerges below these temperatures. The transition
73
of the ClO salt is steadily suppressed by applying pressure, mainly related to the
4
stabilization of the metallic state. The observed pressure dependence appears to be consistent
3 2
with the lower TMI for β’’311ꢀ(ET) X salts with a smaller unit area of the ET layer (X =
–
69,70
– 71,72
– 70,74
– 75
– 76,77
HSO
4
,
ClO
4
,
BF
4
,
BrO
4
,
and Br
). The fact that salt 8 follows this
relation is an explicit demonstration of the emergence of charge ordering at low temperatures.
The structural refinement at 100 K, which is far below T , is apparently indicative of the
MI
32
lowered crystal symmetry to P2
1
. Bond length analysis revealed that the charges of three
crystallographically independent ET molecules are +0.51(19), +0.80(19), and +1.01(15),
respectively, which may be indicative of the development of charge disproportionation,
despite having large standard deviations, probably due to incomplete phase transition.
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8
9
1
1
1
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1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
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4
4
5
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5
5
5
5
5
6
2.4.4 (ET)
7
(ZnBr
4
)
2
(TCE)
2
(EtOH) (9): Salt 9 has been frequently obtained by the
electrooxidation of ET in the presence of ZnBr
crystallizes in a monoclinic lattice P2 /c at both 100 and 298 K. Three and half ET molecules,
one ZnBr4 ion, one TCE, and half EtOH are crystallographically independent. As illustrated
2
in a TCE/EtOH mixed solution (Table 1). It
1
2–
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
42
in Fig. 11a, ET molecules are arranged in a θꢀlike packing motif within the bc plane at x ~ 0,
and each column running along the c axis includes seven ET molecules in a unit cell; five of
them (A, B, and C) are almost uniformly stacked, while the other two (D), which are related
by an inversion center, are dislocated in a direction mutually opposite to each other along the
a axis (Fig. 11b). The charges of the ET molecules were estimated to be +0.37(7), +0.76(8),
+0.35(10), and +0.58(7) at 100 K and +0.65(7), +0.96(7), +0.34(9), and +0.75(7) at 298 K for
A–D, respectively, which may be indicative of the emergence of charge disproportionation
even at RT.
********** Figure 11 **********
2–
The other species, i.e. ZnBr
4
, TCE, and EtOH, are located within the bc plane at x ~ 0.5.
In the layer, each orientationally disordered EtOH molecule on an inversion center is
2–
connected with two ZnBr
4
through OH···Br (2.66 Å vs. sum of the van der Waals radii: 3.05
3
9
Å ) hydrogen bonds and with two TCE through CH···Cl (2.80 Å) hydrogen bonds (Fig. 11c).
2
–
In addition, short OH···Br (2.91 Å) hydrogen bonds are formed between ZnBr4 and TCE,
resulting in the formation of an infinite 2D network linked by hydrogen bonds. Notably, there
2
–
is an anionic hole surrounded by two ZnBr
4
, two TCE, and two EtOH centered at (1/2, 0,
1/2), as can be seen in Fig. 11d. The holes accommodate the disclosed ET molecules (D) in
the adjacent layers for stabilizing the peculiar ET arrangement.
–1
Salt 9 exhibits a high σRT value of 26 S cm and the ρ value slightly decreases with
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9
decreasing temperature at around RT as shown in Fig. 8a; namely, irrespective of the
conduction pathway, salt 9 behaves metallic at around RT despite possible charge
disproportionation. The ρ value passes through a broad minimum at around 270 K and then
increases with decreasing temperature. Transport properties are consistent with the band
structures calculated by assuming uniform charge distribution in the ET layer (i.e. +0.57e on
each ET); namely, this salt consists of 1D electronꢀlike and 2D holeꢀlike Fermi surfaces at
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
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46
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48
49
50
51
52
53
54
55
56
57
58
59
60
298 K (Fig. 11e), whereas the surface at 100 K consists only of a tiny hole pocket centered at
the Γ point (Fig. 11f). Notably, the Fermi level falls near the narrow energy gap region (37
meV) at 100 K. Although the salt exhibits some degree of charge disproportionation even at
RT, it is likely that the resistivity minimum observed at around 270 K is attributed to the
metalꢀtoꢀbandꢀinsulator transition.
2.5 Group D
Salts 10–12 are isostructural at 298 K (space group C2/m) but behave differently at low
temperatures; salt 10 exhibits a moderate growth of the c*/2 superstructure along the
sideꢀbyꢀside direction but undergoes no distinct structural phase transition down to 100 K,
whereas salts 11 and 12 undergo a structural phase transition to a lower symmetry state (C2/c
with a doubled c lattice for 11 and P2
structural comparison is made by considering the crystallographic data at 298 K.
.5.1 θ42+40-(ET) Zn(NCS) X (TCE) [X = Cl (10) and Br (11)]: Both salts were typically
1
/n with a practically unchanged unit cell for 12). Thus,
2
4
2
2
obtained as a sole singleꢀcrystalline compound. At 298 K, these salts with isomorphous
structures consist of two crystallographically independent ET molecules (A and B), half
2
–
2–
Zn(NCS)
2
X
2
ion (Fig. 1a), and half TCE molecule; namely, Zn(NCS)
2
X
2
ion and TCE are
located on the mirror plane at y = 0 and 0.5. In the following section, a discussion focusing on
the structural feature of 10 is present.
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9
1
1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
In the crystal, each of the crystallographically independent ET molecules forms a separate
column extending along the a axis: Columns 1 for A and 2 for B in Fig. 12a. In the 2D layer,
42
ET molecules are arranged in a θ42+40ꢀlike packing motif (see below) in the ac plane at y ~
2–
0
.25 and 0.75, which are separated by an anionic layer composed of Zn(NCS) Cl2 and TCE
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Fig. 12b). The composition results in a nominal charge of +0.5 on each ET, which appears to
be supported by the estimation of the charge of the ET molecules on the basis of bond
distance analysis (+0.65(7) for A and +0.65(8) for B at 298 K and +0.58(7) for A and
32
+
0.60(8) for B at 100 K). Notably, ET molecules in 11 also have an equivalent charge at
2
98 K (+0.59(6) for A and +0.58(7) for B) but exhibit a possible sign of charge
disproportionation in Column 1 at 100 K (+0.49(8) and +0.63(9); cf. +0.40(8) and +0.44(9) in
Column 2).
********** Figure 12 **********
For a deeper understanding of the molecular packing in the ET layer, the molecular
arrangement in the anionic layer at y ~ 0 and 0.5 is considered. As can be seen in Fig. 12b,
2
–
Zn(NCS)
of Zn(NCS)
H···Cl hydrogen bonds with an H···Cl distance of 2.69–2.83 Å. The anionic layer involves a
2
Cl
2
ions are dimerized in a faceꢀtoꢀface manner on an inversion center, and each
2–
2
Cl
2
ion is connected to a neighboring disordered TCE molecule through C–
2
2
78
large cavity (41.8 Å for 10 and 41.5 Å for 11; red ellipse in Fig. 12b) at the center of the
ac plane. Figures 12c and 12d show the structural relationship between the ET and anionic
layers for Columns 1 and 2, respectively. The ET molecules in Column 1 are arranged in a
θ
42ꢀtype zigꢀzag manner, which is in contrast to the rather uniform θ40ꢀtype array in Column 2.
This observation is attributed to the fact that half of ET molecules shift along the molecular
long axis to fall into the cavity in the anionic layer. It is important to note that the cavity areas
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9
2 2 3
are significantly greater than those for quantum spin liquid candidates, κꢀ(ET) M (CN) (19.5
2
2
22
Å for M = Cu(I) and 15.8 Å for M = Ag(I)), in which ET molecules are arranged with
respect to the anionic cavities but have no significant shift along the molecular long axis due
to the insufficient cavity size. Our recent studies have suggested that such a molecular shift
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
–
79
– 80,81
affects the degree of spin frustration in κꢀ(ET) X salts (X = B(CN)
and CF SO
3
).
2
4
3
The ‘multiple’ θꢀtype ET layer, including different stacking manners in 10 and 11, can be
designated as a θ42+40ꢀstructure (according to Mori’s classification in ref. 42); it has been
2
4
–
82
4– 83,84
reported for ET salts formed with M(CN)
(M = Ni(II) and Pt(II)), SiW12
O
40
,
6– 85
4– 84
3– 86
6– 87
PMnW O , PMo O , SMo O , and CoW O .
12 40
11 39
12 40
12 40
The band structure, DOS, and the Fermi surface of the HOMO band of ET in 10 and 11 at
98 K were calculated by assuming uniform charge distribution in the ET layer (i.e. +0.50e
2
on each ET). As can be seen in Fig. 12e, the branches are substantially more dispersive along
the sideꢀbyꢀside direction (c axis) than along the stacking direction (a axis), possibly because
of the significant transfer integrals along the interꢀstack direction (t –t ) relative to those
c1 c4
within the column (t –t ). The alternation of transfer integrals within the columns is not
a1 a4
large enough to make an energy gap associated with the dimerization in both Columns 1 and
2
. The Fermi surface is composed of a hole pocket centered at the Γ point and a quasiꢀ1D
electron surface along the k direction. The width of the band composed of eight branches
c
was estimated to be 0.876 and 0.864 eV for 10 and 11, respectively (Table 3). The slightly
larger bandwidth in 10 may be associated with the smaller unit area of the ET layer (acsinβ =
2
2
2–
1
90.3 Å for 10 and 191.2 Å for 11), attributed to the replacement of Zn(NCS)
2
Br
2
with a
2–
2 2
smaller isomorphic anion, Zn(NCS) Cl .
********** Table 3 **********
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
–
1
As can be seen in Fig. 13a, both salts have a rather high σRT value (2.7 S cm for 10 and
–1
1
a
.5 S cm for 11) but exhibit semiconducting behavior at around RT (E = 0.19 eV for 10 at
T > 250 K and 0.15 eV for 11 at T > 240 K). However, the salts behave differently upon
cooling; namely, salt 11 exhibits a marked anomaly at around 200 K, whereas salt 10 exhibits
only a gradual change in the resistivity slope at around 210 K. The critical temperature (T0),
defined as the temperature at which the slope of the curve in Fig. 13a becomes the maximum,
was estimated to be 210 K for 10 and 200 K for 11. The values are comparable to those
estimated from the temperature dependence of the intensity (I) at the superlattice Bragg
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
/2
position defined as I ~ (T – T) , which reflects a secondꢀorder transition (224 K for 10 and
0
189 K for 11; Fig. S2 in Supporting Information).
********** Figure 13 **********
–1
Figure 13b shows the Raman spectra of 10 in the range of 1300–1600 cm at 67 and 250
88
K, where salt 11 exhibits a very similar spectrum. At 250 K, two distinct bands were
–1
observed at around 1470 and 1540 cm , each of which are split into two components
–1
–1
centered at 1452 and 1491 cm for the former and 1541 and 1553 cm for the latter at 67 K.
This spectral feature is reminiscent of that of θꢀ(ET) Cu (CN)[N(CN) , which exhibits a
pronounced change in the resistivity associated with charge disproportionation at 220 K.
2
2
2 2
]
89,90
91
Of particular note is that the chargeꢀsensitive ν mode of θꢀ(ET) Cu (CN)[N(CN) ] splits
2
2
2
2 2
90
even at RT and continuously changes down to 220 K. According to ref. 90, dynamic charge
disproportionation occurring even at RT is responsible for the spectral feature. On the other
–1
–1
hand, salt 10 exhibits abrupt band splitting at ~1540 cm into 1541 and 1553 cm at around
T0, supporting the emergence of the chargeꢀdisproportionated state below the temperature.
–1
Notably, the assignment of the band at ~1470 cm , namely either the inclusion of the ν and
2
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ν
3
modes or the split ν
3
mode, remains a matter of debate. For discriminating between these
1
3
two possibilities, further spectroscopic studies using Cꢀenriched ET molecules, in which
13
central doubleꢀbonded carbon sites were selectively enriched with the C isotope, are in
8
8
progress.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
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29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 14 shows the temperature dependence of χ of polycrystalline 10 and 11 between 1.9
and 300 K, where the salts have two S = 1/2 spins per formula unit. The χ value at 300 K is
–3
–1
–4
–1
–3
estimated to be 1.92 × 10 emu mol (i.e. 9.6 × 10 emu spin ) for 10 and 1.75 × 10 emu
–1
–4
–1
mol (i.e. 8.8 × 10 emu spin ) for 11, which exceed that for typical organic metals (3–5 ×
–
4
–1 6
10
emu spin ). The χ value, in addition to the semiconducting transport behavior, is a
characteristic typical of dimerꢀtype Mott insulators. Upon cooling, the χ value gradually
increases down to ca. 20 K followed by a rapid upturn caused by Curie contribution. No
anomaly was observed in the measured temperature range, which includes resistivity anomaly.
A similar situation has been observed for several chargeꢀordered system such as
89,90
30,31
82
θꢀ(ET) Cu (CN)[N(CN) ] ,
θꢀ(ET) RbZn(SCN) ,
θ42+40ꢀ(ET) Pt(CN) ,
and
2
2
2 2
2
4
4
4
87
θ42+40ꢀ(ET) CoW O (H O) . At present, the dimerization pattern is unclear because of
8
12 40
2
5.5
comparable intermolecular transfer integrals for the stacking and sideꢀbyꢀside directions.
●
2
+
However, provided that charge disproportionation is concluded within an (ET)
dimer,
charge disproportionation would result in the loss of a significant charge degree of freedom,
but would be not accompanied by loss in the spin degree of freedom. The temperature
5
8,59
dependence is well described by the sum of the Heisenberg AF linear chain (χBF)
Curie (χ ) models with a fraction f as in the case of 5 (Eq. 1). The best fit parameters were J =
4.7 K and f = 0.880 for 10 and J = 85.4 K and f = 0.835 for 11. A significant fraction of the
Curie component is believed to rationalize charge disproportionation occurring at low
and
C
5
9
2
temperatures, because it resembles the case of θꢀ(ET) CsZn(SCN)
and rapidly cooled
2
4
93
94–96
θꢀ(ET) RbZn(SCN) in a chargeꢀglass state.
In these salts, most of the electrons appear
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1
1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
to be paired, affording a spinꢀsinglet state, albeit a small fraction of unpaired electrons is
maintained down to low temperatures so as to serve as localized paramagnetic spins.
********** Figure 14 **********
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
.5.2 θ42+40-(ET)
4
2 3
Zn[N(CN) ]Cl (TCE) (12): At 298 K, salt 12 is monoclinic with space
group C2/m as salts 10 and 11 at the same temperature, and an asymmetric unit contains two
2
3
–
ET molecules (A and B), half Zn[N(CN)
2
]Cl
ion with a unidentateꢀcoordinated N(CN)
2
2
3
–
moiety (Fig. 1b), and half TCE molecule; namely, Zn[N(CN) ]Cl and TCE are located on
2
the mirror plane at y = 0 and 0.5.
Figures 15a and 15b show the molecular arrangement in the ET and anionic layers within
the ac plane at y ~ 0.25 and 0, respectively. In the former layer, each crystallographically
independent ET molecule, which has a charge of +0.78(5) for A and +0.61(5) for B,
constructs the stacking column extending along the a axis (Column 1 for A and 2 for B).
Similar to 10 and 11, ET molecules in Columns 1 and 2 are arranged in a zigꢀzag (Fig. 15c)
and an almost uniform (Fig. 15d) manners, respectively, for constructing θ42+40ꢀtype
molecular arrangement. Notably, the zigꢀzag arrangement appeared in Column 1 is attributed
2
3
–
to the steric hindrance of chlorides in Zn[N(CN)
2
]Cl
and TCE, connected through C–
H···Cl hydrogen bonds with an H···Cl distance of 2.66–2.79 Å, in the anionic layer.
********** Figure 15 **********
1
At 100 K, salt 12 has the structure with a lower symmetry space group (P2 /n). Although
high refinement factors (R (for I > 2σ(I)) = 0.0755) and high standard deviations on atomic
1
coordinates are present, the estimated charges on ET molecules on the basis of intramolecular
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bond lengths (+0.54(13) and +0.40(13) in Column 1 and +0.87(14) and +0.23(13) in Column
2) are indicative of the appearance of the chargeꢀdisproportionated state at least in Column 2.
The estimated charges are somewhat different from those in 10 and 11, although the Raman
88
spectra of 12 are very similar to those of 10 and 11, as well as those of
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
9
0
θꢀ(ET) Cu (CN)[N(CN) ] . This discrepancy may be associated with the possible
2
2
2 2
dynamical fluctuation of the charges in the ET layers in these salts. The temperature
dependence of the intensity at the superlattice Bragg position follows the equation I ~ (T
0
–
1
/2
T) for a secondꢀorder transition, where the T
0
value was estimated to be 253 K (Fig. S2 in
Supporting Information).
Figure 15e displays the band structure, DOS, and Fermi surface of the HOMO band of ET
in 12 at 298 K, which is reminiscent of those of isostructural salts 10 and 11. The bandwidth
of the HOMO band was estimated to be 0.947 eV, significantly greater than those of 10
(
0.876 eV) and 11 (0.864 eV). This result is mainly attributed to the shrinkage of ET layers in
2
2
12 relative to 10 and 11; namely, the acsinβ value changes from 190.3 Å for 10 and 191.2 Å
2
for 11 to 189.3 Å for 12. Such lattice shrinkage results in a decreased dihedral angle between
the ET planes in adjacent columns (124.9° for 12 vs. 127.6° for 10 and 128.1° for 11).
The temperature dependence of ρ of 12 is shown in Figure 13a. As in the case in 10 and 11,
–1
salt 12 has a rather high σRT value (4.8 S cm ) and exhibits semiconducting behavior with E
a
= 0.11 eV at around RT. On cooling, the ρ value exhibits a slope maximum at around 230 K,
comparable to that estimated from the intensity of superlattice reflection (T = 253 K). As can
0
0
be seen in the inset of Fig. 13a, the T values steadily decrease with increasing acsinβ,
possibly attributed to the stabilization of the lowꢀtemperature chargeꢀdisproportionated phase
by the shrinkage of ET layers. The magnetic susceptibility of 12 indicates a similar
temperature dependence to those of 10 and 11, with J = 47.2 K and f = 0.842 (Fig. 14c) and
no noticeable anomaly at T . Considering the larger W and lower E values, the smaller J
0
a
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2
3
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
value appears to arise from the more itinerant nature of the carriers in the ET layers of 12.
3. Conclusion
In this study, a series of ET salts composed of various zinc(II) complex anions, with
properties ranging from metallic behavior to band and chargeꢀordered insulators, were
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
prepared. The key issue is the chemical instability of ZnX species, which allows the
additional reactions to proceed during electrocrystallization. The resultant complex anions
possess a tetrahedrally coordinated structure, unlike planar trigonal coordination for copper(I)
and silver(I) complex anions; hence, sterically bulky structures inevitably affect the
molecular arrangement in adjacent ET layers. For examples, the large cavities in the anionic
layer result in the extrusion of neighboring ET molecules, affording a specific ET packing
motif, whereas the penetration of a part of the sterically bulky anions in the ET layer results
in the isolation of ET dimers or tetramers. As the present results have significant implications
for the diverse production of zinc(II) complex anions by the coordination of various
(pseudo)halide ligands, such zinc(II) complex anions may serve as a template for a new
strategy for the molecular arrangement of cation radical species, which are responsible for
electronic functionalities. Even though the ligand species are the same, one can significantly
promote the further exploration of unprecedented cation radical salts by utilizing ligands with
multiꢀcoordination sites. For example, the unidentate coordination of the N(CN) moiety
2
2–
3
affords a discrete dianion Zn[N(CN) ]Cl (in 12), which has stoichiometry, geometry, and
2
–
2 2 ∞ 2
charge different from those of a polyanion {Zn[N(CN) ]Cl } (in 6), in which N(CN)
moieties serve as a ꢀ1,5ꢀbidentate bridging ligand, affording a 1D polymeric structure. In
addition, the sterically bulky complex anions appear to have an ability to include solvent
molecules in the crystals; therefore, the change in solvent is also effective for controlling the
arrangement in the radical cations.
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9
4
. Experiments
General. 1,1,2ꢀtrichloroethane was used as received, whereas MeOH, EtOH, and MeCN
were distilled prior to use. Zinc salts (ZnCl , ZnBr , and ZnSCN), KCl, KBr, KB(CN) , and
2
2
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(Ph P)Cl were used as received. KSCN and KC(CN) were recrystallized from EtOH,
4
3
whereas 18C6 was recrystallized from MeCN. (Bu
4
2
N)Au(CN) was prepared by the
metathesis of (Bu
4
N)I with KAu(CN)
2
in water, followed by recrystallization from EtOH.
P)C(CN) ] were prepared by the metathesis
Ph P salts [(Ph P)SCN, (Ph
4
4
4
P)N(CN) , and (Ph
2
4
3
of (Ph P)Cl with appropriate alkali metal salts in water, followed by recrystallization from
4
EtOH.
Electrocrystallization. Single crystals of ET salts were obtained by the electrochemical
oxidation of ET in an Hꢀshaped glass cell, which was assembled in a glove box filled with
2 2
argon gas (H O, O < 1 ppm; Table 1). Typically, 0.02 mmol of ET and 0.08 mmol of other
reagents were added to the anodic and cathodic compartments, respectively, separated by a
glass frit. After the cell was dried under vacuum, 18 mL of appropriate solvent(s) was added
to the cell, and the mixture was stirred at RT for 1–2 days. A constant current (1.0 ꢁA) was
passed between the two platinum electrodes at a constant temperature of 15 or 20 °C. Black
crystals were grown on the anodic electrode over approximately two weeks. The obtained
crystals were washed with MeOH and dried in air. In some cases, crystals of a known ET
9
7
cation radical salt δꢀ(ET) C(CN) , which is free of zinc(II), were obtained.
2
3
Crystal structure determination. Xꢀray diffraction experiments were performed on a
CCDꢀtype diffractometer (Bruker SMART APEX II) with graphiteꢀmonochromated Mo Kα
radiation (50 kV, 30 mA) at a controlled temperature (100–298 K). A crystal was mounted on
a glass capillary and cooled by a stream of cooled nitrogen gas. Crystal structures were
98
solved using a direct method using the SIR2004 program and were refined by a fullꢀmatrix
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