Exploration of charge-transfer solids utilizing nucleobases: Nanoarchitectures by hydrogen-bonds in the ionic assemblies of guanine and TCNQ derivatives

Article abstract of DOI:10.1021/cg400059h

We studied formation and structural characteristics of charge-transfer solids of 9-n-butylguanine (BuG) with fluorinated tetracyanoquinodimethane derivatives (F_nTCNQ, n = 4, 2, and 1). Complex formation in a methanol (MeOH)-containing solvent generated two types of salts composed of either a methoxy-substituted anion or a fully ionic anion radical of F _nTCNQ. In all anion radical salts, BuG existed as a protonated or a hemiprotonated species, BuGH⁺ or (BuG)(BuGH⁺), respectively, and formed hydrogen-bonded (H-bonded) assemblies. In these BuGH⁺ assemblies, F_nTCNQ^?- molecules were fixed and aligned periodically, providing H-bonded polycationic templates. In (BuGH⁺)(F₄TCNQ^?-), BuGH⁺ dimers by complementary H-bonds formed a two-dimensional (2D) polycationic sheet. The F₄TCNQ^?- face-to-face dimers formed a one-dimensional (1D) segregated column aided by formation of H-bonds with BuGH⁺. In (BuGH⁺)(F₂TCNQ^?-)(MeOH), BuGH⁺ dimers by complementary double H-bonds formed a 1D polycationic ribbon supported by MeOH-mediated H-bonds. A 1D mixed stack column of (BuGH⁺) ₂ and (F₂TCNQ^?-)₂ dimers was formed owing to their complementary geometry and size. In (BuG)(BuGH ⁺)(F₁TCNQ^?-), a new type of BuG-BuGH ⁺ pair formed a 1D ribbon supported by complementary H-bonds, and F₁TCNQ^?- dimers were aligned by H-bonds with the BuG-BuGH⁺ ribbon.

Full text of DOI:10.1021/cg400059h

Article
pubs.acs.org/crystal
Exploration of Charge-Transfer Solids Utilizing Nucleobases:
Nanoarchitectures by Hydrogen-Bonds in the Ionic Assemblies of
Guanine and TCNQ Derivatives
Tsuyoshi Murata,*,† Gunzi Saito,*,†,‡ Kenji Nakamura,† Mitsuhiko Maesato,† Takaaki Hiramatsu,‡
and Yukihiro Yoshida†,‡
†Division of Chemistry, Graduate School of Science, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan
‡Faculty of Agriculture, Meijo University, Shiogamaguchi 1-501, Tempaku-ku, Nagoya 468-8502, Japan
S
* Supporting Information
ABSTRACT: We studied formation and structural character-
istics of charge-transfer solids of 9-n-butylguanine (BuG) with
fluorinated tetracyanoquinodimethane derivatives (FnTCNQ, n
= 4, 2, and 1). Complex formation in a methanol (MeOH)-
containing solvent generated two types of salts composed of
either a methoxy-substituted anion or a fully ionic anion radical
of FnTCNQ. In all anion radical salts, BuG existed as a
protonated or a hemiprotonated species, BuGH+ or (BuG)-
(BuGH+), respectively, and formed hydrogen-bonded (H-bonded) assemblies. In these BuGH+ assemblies, FnTCNQ•−
molecules were fixed and aligned periodically, providing H-bonded polycationic templates. In (BuGH+)(F4TCNQ•−),
BuGH+ dimers by complementary H-bonds formed a two-dimensional (2D) polycationic sheet. The F4TCNQ•− face-to-face
dimers formed a one-dimensional (1D) segregated column aided by formation of H-bonds with BuGH+. In (BuGH+)-
(F2TCNQ•−)(MeOH), BuGH+ dimers by complementary double H-bonds formed a 1D polycationic ribbon supported by
MeOH-mediated H-bonds. A 1D mixed stack column of (BuGH+)2 and (F2TCNQ•−)2 dimers was formed owing to their
complementary geometry and size. In (BuG)(BuGH+)(F1TCNQ•−), a new type of BuG−BuGH+ pair formed a 1D ribbon
supported by complementary H-bonds, and F1TCNQ•− dimers were aligned by H-bonds with the BuG−BuGH+ ribbon.
methoxy-substituted RTCNQ anions, (II) semiconducting fully
ionic RTCNQ radical anion salts, and (III) highly conductive
partially ionic or mixed-valence RTCNQ radical anion salts.
Also, we studied formation of CT solids between C species and
Ni(dmit)2 (Scheme 1) and obtained fully ionized insulators of
Ni(dmit)2•−, and semiconductive and metallic conductors of
INTRODUCTION
■
One-dimensional (1D) π-stacks of nucleobases are regarded as
the hole transport path of DNA, and many exciting,
controversial transport properties of DNA, such as metallic or
proximity superconducting electrical conduction, have been
reported in recent studies of biomolecule-based materials.1−4
Formation of such a path in DNA is one example of a
biosupramolecular architecture based on the self-assembling
ability of nucleobases by complementary hydrogen-bonds (H-
bonds).5 For the development of organic conductors, such
highly selective, directional, and robust H-bonds have been
utilized as an effective tool to control not only molecular
arrangements with a variety of dimensionality in real space6 but
also the electronic dimensionality and structures in k-space.7
Nucleobases have been utilized to control molecular arrange-
ments in organic conductors, and tetrathiafulvalene (TTF,
Scheme 1) derivatives having nucleobase skeletons8,9 have been
designed and synthesized. These donor molecules form diverse
supramolecular assemblies based on the complementary H-
bonds of nucleobases and have provided several conductive
charge-transfer (CT) solids.8,9 Previously, we investigated CT
solids prepared by mixing cytosine (C, Scheme 1) in methanol
(MeOH) and tetracyanoquinodimethane derivatives (RTCNQ,
Scheme 1) in acetonitrile (MeCN)10 and observed formation
of three types of ionic solids: (I) insulators composed of
11
0.5−
partially ionized Ni(dmit)2
.
As for the C species, in all
cases, C preferentially behaves as a proton-acceptor, but not an
electron-donor, and becomes a protonated (CH+) or a
hemiprotonated (CHC+, Scheme 1) cation whose H-bond-
mediated self-assembling ability enables construction of
uniform supramolecular frameworks. Furthermore, the H-
bonds between the C species and RTCNQ afforded a robust
3D architecture, in which a Mott insulator (CHC+)(TCNQ•−)
retained its uniform 1D framework down to 10 K despite the
fact that a typical 1D electronic lattice is very susceptible to
lattice distortion (spin-Peierls transition).10,12 Nucleobases
possess multiple H-bonding sites forming further diverse self-
assembled architectures.5 These foregoing studies suggest the
high potential of self-assembling ability of nucleobases as
structural templates for anion radical molecules, and further
Received: January 11, 2013
Revised: May 19, 2013
Published: May 30, 2013
© 2013 American Chemical Society
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Scheme 1. Molecular Structures of Guanine (G), Cytosine (C), Hemiprotonated Cytosine (CHC+), RTCNQ, TTF, and
a
Ni(dmit)2
a
(D) and (A) in G and C indicate the H-bond donating and accepting sites, respectively.
Scheme 2. H-Bonded 1D Ribbons Observed in Neutral G Derivatives (Ribbon-A (a) and Ribbon-B (b)),17 (CHC+)(TCNQ•−)
(c),10a and (CHC+)(2,5-diethyl-TCNQ•−) (d),10a,b Complementary Triple H-Bonds in Partially Deprotonated Guanine
(Left)21a and Hemiprotonated 7,9-Dimethylguanine Pairs (Right) (e),21b and H-Bonded 1D Ribbon of Hemiprotonated
a
Guanine Pairs (f)21d,e
a
Green arrows in a and b indicate the dipole. D and A in a−d indicate the H-bond donating and accepting sites, respectively.
investigations of CT solids of nucleobases are essential for the
development of organic conductors.
bonding patterns with different H-bond donating and accepting
sites have been reported (Ribbon-A and Ribbon-B in Schemes
2a and 2b, respectively). In Ribbon-A, a G−G pair is formed
with two H-bond donating sites on one G (N1 amide and N2
amino) and two H-bond accepting sites on the other G (O6
and N7), and the net dipole is retained. On the other hand, in
Ribbon-B, G−G pairs are formed with double N1−H···O6 and
N2−H···N3 H-bonds in each G unit, resulting in quenching of
the net dipole.
Similar H-bonded 1D ribbon motifs have been observed in
assemblies of neutral C18 and its derivatives19 as well as CH+
and CHC+.10,11,20 Furthermore, multiple H-bond connections
between neighboring ribbons modulate the relative positioning
of C, CH+, and CHC+ molecules to provide 1D H-bonded
ribbons, 2D H-bonded sheets, and 3D H-bonded net-
Guanine (G, Scheme 1) is a stronger electron-donor (Eox
=
+1.25 V vs SCE;13 ionization potential Ip = 7.77−7.85 eV14)
than C (+1.90 V,13 8.45−8.68 eV14), but still much weaker than
TTF (+0.34 V, 6.4 eV15). The nucleobase G has three H-bond
accepting sites (N7, N3, and O6; denoted as A in Scheme 1)
and two H-bond donating sites (N1 amide and N2 amino
group, denoted as D in Scheme 1), while C has two H-bond
donating and two accepting sites. Similar to C, G is a Brønsted
acid and a base (pKa = 3.3, 9.2, and 12.3 vs 4.45 and 12.2 for
C),16 and readily forms a protonated cation (GH+).
Neutral G forms various supramolecular H-bonded struc-
tures, such as dimers, G-quartets, G-quadruples, and 1D
ribbons.17 In the 1D ribbons of neutral G derivatives, two H-
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works.10,11,17−20 Schemes 2c and 2d illustrate two types of
CHC+ 1D ribbons observed in the RTCNQ CT solids. Each
CHC+ unit is connected to each other with double N(amino)−
H···O (Scheme 2c) or N(amido)−H···O (Scheme 2d) H-
bonds on the same side. Similar to CHC+, G derivatives form
hemideprotonated G pairs by triple H-bonds (Scheme 2e).21a−c
Furthermore, a 1D infinite ribbon is formed by the N7−H···N7
H-bond of a pair of GH+ and G species (Scheme 2f).21d,e
One of the most fascinating features of utilizing C in the
study of CT solids10,11 is that 1D CHC+ ribbons, shown in
Schemes 2c and 2d, afford polycationic templates that have
multiple periodic H-bonding sites to anion radicals, which have
exhibited unprecedented crystal structures and solid-state
transport and magnetic properties. Such templates are expected
to be common among the protonated nucleobases, including G
derivatives.
Table 1. Crystallographic Data of BuG and Charge-Transfer
Solids (1 and 3)
3, (BuGH+)
1, (BuGH+)
(F2TCNQ•−
(MeOH)
)
BuG
(F4TCNQ•−
)
formula
C9H13N5O1
207.24
C21H14N9F4O1
484.40
C22H20N9F2O2
480.46
formula weight
crystal system
space group
crystal size (mm3) 0.30 × 0.20 × 0.30 × 0.02 ×
orthorhombic
P212121
monoclinic
triclinic
I2/a
P1
̅
0.30 × 0.10 × 0.10
0.15
0.02
a (Å)
11.029(1)
10.759(2)
26.357(4)
21.835(2)
6.8446(3)
29.771(3)
10.219(4)
10.475(2)
12.735(4)
69.56(3)
85.46(3)
61.98(3)
1121.7(9)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
99.016(8)
It is difficult to dissolve G in conventional organic solvents,
preventing from the synthesis of CT solids with various
acceptor molecules. We prepared 9-n-butylguanine (BuG,
Scheme 3), in which an N9 H-bonding site was quenched by
3127.5(8)
12
4394.4(6)
8
Dcalcd (g cm−3
)
1.320
200
1.464
290
1.423
temperature (K)
μ (cm−1
290
c
d
c
)
0.093
1.047
0.109
Scheme 3. Molecular Structures of Neutral and Protonated
a
9-n-Butylguanine (BuG and BuGH+, Respectively)
no. of reflections 3507
19751
4016
5452
measured
no. of
independent
reflections
3507
5168
no. of reflections 2484
2381
316
2185
376
used
no. of refined
406
parameters
a
a
R1 (I > 2σ(I))
0.115
0.333
1.329
0.0613
0.1935
0.0704
0.2219
0.998
C5−N7−C8 bond angle is shown by θ (°). (D) and (A) indicate the
b
R2 (all data)
H-bond donating and accepting sites, respectively.
GOF
1.050
a
b
c
2
2
R1 = Σ(|F0| − |Fc|)/Σ|F0|. R2 = [Σw(F0 − Fc2)2/Σw(F0 )2]0.5. Mo
an n-butyl group to improve the solubility, and studied the
preparation and structural features of its CT solids with some
RTCNQs. Since BuG served as a proton-acceptor, not as an
electron-donor, to give a protonated cation, similar to C, we
elucidated the function of BuG as a source of polycationic
templates. A preliminary result with F1TCNQ CT solids was
reported.22
d
Kα. Cu Kα.
RESULTS AND DISCUSSION
■
Preparation of BuG and Its Suitability as an Electron-
Donor Molecule. BuG was prepared from 2-amino-6-
chloropurine in two steps as described in the Experimental
Section. BuG is soluble in alcohols, N,N-dimethylformamide,
dimethylsulfoxide, ethylene glycol, and pyridine, but insoluble
in tetrahydrofuran, ethyl acetate, acetone, MeCN, and
halogenated solvents. BuG is a slightly stronger electron-
peak
donor (E = +1.22 V vs SCE) than G, although its strength is
ox
peak
red
not sufficient to ionize even F4TCNQ (E
= +0.53 V vs
Figure 1. Molecular structures of BuG (A, B, and C) showing the
SCE). The redox potential difference (ΔE(DA) = Eox − Ered)
between BuG and RTCNQs does not meet the requirements
for a partial CT state: −0.02 ≤ ΔE(DA) ≤ 0.34 V.23
atom labeling scheme.
Crystal Structure of BuG. Table 1 summarizes the
crystallographic data of BuG. There are three crystallo-
graphically independent BuG molecules (BuG-A−C, Figure
1). BuG-A forms a 1D infinite ribbon (denoted as the A-
ribbon; width of 7.5 Å (except the n-butyl groups)) along the b
axis supported by H-bonds with the same H-bonding pattern
shown in Scheme 2a (Figure 2a, amino N2A−H···OC, 2.77
Å and amide N1A−H···imino N7A, 2.81 Å; H-bonds shorter
than the sum of van der Waals (vdW) radii24 are summarized in
Supporting Information Table S1). Also, a unique infinite 1D
ribbon is formed by BuG-B and BuG-C (denoted as the BC-
ribbon; width of 7.5 Å (except the n-butyl groups)), which are
alternatingly arranged along the a axis with the same type of H-
bonds as the A-ribbon (amino N2B−H···OC, 2.88 Å; and
amino N2C−H···OC 2.89 Å; amide N1B−H···imino N7C,
2.85 Å; amide N1C−H···imino N7B, 2.79 Å, Figure 2b). The G
skeletons in the A-ribbon are nearly parallel to each other and
have a small twist angle (2.6°), while the molecular planes of
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Figure 2. Crystal structure of neutral BuG. H-bonded infinite 1D ribbons composed of molecule A (a) and molecules B and C (b). In panel b, the π-
overlap of the dimerized BC-ribbons is shown by the half-colored molecules. Green arrows indicate the directions of the dipole of each molecule. (c
and d) Projections of the A-ribbon (a) and BC-ribbon (b), viewed along the ribbon directions, showing the twisting angles of adjacent molecules. (e)
Crystal structure viewed along the a axis showing two adjacent A-ribbons and the interactions between the A- and BC-ribbons. (f) Crystal structure
viewed along the b axis showing three neighboring dimerized BC-ribbons and interactions between the A- and BC-ribbons. H-bonds within the A-
ribbon and BC-ribbon, and those between the A- and BC-ribbons are indicated by red, green, and orange, and purple dotted lines, respectively, with
bond lengths (Å).
Table 2. H-Bonded Ribbons and Dipole Moments
BuG-B and BuG-C in the BC-ribbon twist by 9.0° with respect
to each other (Figures 2c and 2d).
The arrangement of dipole moments of component
molecules is important to the determination of relative
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molecular orientation, and is also essential for intriguing solid
state properties such as ferroelectricity and pyroelectricity in
the H-bonded crystals.25 Table 2 summarizes the nature of the
H-bonded ribbons and their dipoles observed in this study. As
shown by the green arrows in Figures 2a and 2b, both the A-
ribbon and BC-ribbon have substantial permanent dipoles;
however, the crystal does not have a net dipole for the
following reasons. Each infinite A-ribbon is aligned along the c
axis with adjacent ribbons running in the opposite direction,
canceling out the net dipole (Figure 2e). The BC-ribbon forms
a π-stacked pair with a small π−π overlap between BuG-B and
BuG-C (Figure 2b; interplanar distance, 3.33 Å and width, 9.3
Å). It is noticeable that dipole moments of each ribbon in a pair
are parallel to each other and that dimerization of the 1D
infinite BC-ribbons does not lead to cancellation of the dipoles.
However, the dimerized BC-ribbons are aligned along the c axis
with adjacent dimerized ribbons running in the opposite
direction, and this extended assembly does cancel out the net
dipole (Figure 2f).
One of the most characteristic features of neutral BuG
crystals is that the two types of ribbons run in perpendicular
directions and are connected to each other by H-bonds (purple
dotted lines, Figure 2e, 2.90 Å) between amino groups of BuG-
A and carbonyl groups of BuG-B (N2A−H···O6B), construct-
ing a H-bonded 3D network (Supporting Information Figure
S1). The n-butyl groups of both ribbons are aligned parallel to
the molecular plane to fill the 4−5 Å voids (Figures 2e and 2f).
Preparation and Classification of Products. Table 3 lists
the products obtained by mixing BuG and FnTCNQ (n = 4, 2,
and 1) in mixed solvents with MeOH. The weaker acceptor
TCNQ did not afford TCNQ•− salts as solid products,
although the solution turned blue, indicating the formation of
the TCNQ•− anion radicals.
solvent (MeOH) in open air. The ratios of BuG to FnTCNQ
species in 1 and 5 were determined as 1:1 and 2:1, respectively,
from elemental and crystal structure analyses (Table 3). For 3,
the ratio was determined by crystal structure analysis only.
Evaporation of the filtrate yielded a green powder (2 for n = 4,
4 for n = 2, and 6 for n = 1). A spectroscopic study of these
compounds indicated that the major components of the filtrates
were salts of the methoxy-substituted FnTCNQ anions
(FnTCNQ-OMe−, Group I salts), although further purification
to isolate the corresponding salts was not successful (vide
infra). As shown in the reaction between C and FnTCNQ in
MeOH and MeCN,26 nucleophilic addition of MeOH to
FnTCNQ produces an acidic adduct, which is easily
deprotonated by BuG to generate a salt of protonated BuG
(BuGH+, Scheme 3) and FnTCNQ-OMe− (i.e., a Group I salt
of (BuG)m(BuGH+)(FnTCNQ-OMe−)) (Scheme 4). One-
electron transfer from FnTCNQ-OMe− to neutral FnTCNQ
results in the formation of FnTCNQ•− (Scheme 4) to give the
Group II salt (BuG)m(BuGH+)(FnTCNQ•−).
Spectroscopic Features. IR spectra of 1 and 2 in the C
N stretching (Figure 3a, 2050−2300 cm−1) and CC and C
O stretching regions (Figure 3b, 1450−1800 cm−1) are shown
in Figure 3 and are compared with those of F4TCNQ,
K+F4TCNQ•−, and the C−F4TCNQ ((CHC+)(F4TCNQ•−)
and (CHC+)(F4TCNQ-OMe−)(H2O)).26 Salt 1 exhibits three
CN stretching peaks in the 2180−2210 cm−1 region, which
are ascribed to the F4TCNQ•− species. The peaks are shifted to
lower frequencies than K+F4TCNQ•− and are similar to those
observed for (CHC+)(F4TCNQ•−); the shape is more
complicated than those of these two salts. The difference in
the shape may originate from the difference in the stacking
modes of F4TCNQ•− in the crystal structures (vide infra). The
shape and location of the peaks for the CN stretching modes
of 2 (2154 and 2194 cm−1) are markedly different from those
of K+F4TCNQ•−, 1, and (CHC+)(F4TCNQ•−); however, they
are very similar to those observed for (CHC+)(F4TCNQ-
OMe−)(H2O) (2153 and 2193 cm−1), indicating formation of
the F4TCNQ-OMe− species in the reaction between BuG and
F4TCNQ in MeOH/MeCN (Scheme 4).
In the CC and CO stretching region (Figure 3b), the
bands above 1620 cm−1 are ascribable to either BuG or
BuGH+; both 1 and 2 show a broad band at 1700 cm−1 which
corresponds to BuGH+, and no band ascribable to neutral BuG
is detected. Salt 1 exhibits bands at 1540 and 1503 cm−1
corresponding to F4TCNQ•−, while 2 exhibits a broad band
around 1480−1500 cm−1 corresponding to F4TCNQ-OMe−.
Figure 4 compares the UV−vis-NIR spectra of 1 and 2 with
those of K+F4TCNQ•−, (CHC+)(F4TCNQ•−), and (CHC+)-
(F4TCNQ-OMe−)(H2O).26 The spectrum of 1 is very similar
to that of (CHC+)(F4TCNQ•−) and has peaks at 12 × 103, 15
× 103, and 27 × 103 cm−1, denoted as C, D, and E bands,
respectively, which are ascribable to the intramolecular
transitions of F4TCNQ•−. The broad band at 7.5 × 103 cm−1
(denoted as the B band) is originated from the intermolecular
transition between F4TCNQ•− species. The band appears at an
energy region higher than that observed for K+F4TCNQ•− (6.2
× 103 cm−1), indicating 1 has a loose 1D segregated column or
a dimerized nature. Compared with the rather sharp band in
K+F4TCNQ•−, the B band of 1 is markedly broad strongly
suggesting that it consists of both intra- and interdimer
transitions. The absence of any absorption bands below 5 × 103
cm−1, which originate from the intermolecular transition in the
mixed-valence state, confirms the fully ionic nature of 1.
Table 3. Appearance, Assignment, and Composition of the
Reaction Products between BuG and FnTCNQ (n = 4, 2, and
a
1)
TCNQ
solvent
no. appearance group
composition
F4TCNQ MeOH/MeCN
1
dark blue
rod
II
(BuGH+)
(F4TCNQ•−
)
2
pale green
powder
I
mixture of
F4TCNQ-
OMe− salt
F2TCNQ MeOH/1,2-
dichloroethane
3
4
dark blue
rod
II
I
(BuGH+)
(F2TCNQ•−
(MeOH)
)
green
powder
mixture of
F2TCNQ-
OMe− salt
F1TCNQ MeOH/MeCN
5
6
blue plate
II
I
(BuG)(BuGH+)
(F1TCNQ•−
)
green
powder
mixture of
F1TCNQ-
OMe− salt
a
Groups I and II salts are illustrated in Scheme 4.
Mixing a solution of BuG in MeOH with a solution of
FnTCNQ in MeCN (or 1,2-dichloroethane or benzonitrile)
yielded a dark blue to green solution, indicating the generation
of the FnTCNQ•− species. The blue crystals suitable for
structural analysis (Group II salts, 1 for n = 4, 3 for n = 2, and 5
for n = 1, Supporting Information Figure S2) were isolated by
recrystallization from a MeOH solution of the concentrated (or
dried) reaction mixture. The crystals of 3 gradually loose crystal
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Scheme 4. Formation Mechanism of FnTCNQ-OMe− and FnTCNQ•− in the Presence of BuG in MeOH
Figure 3. IR spectra of 1 and 2 together with those of F4TCNQ, K+F4TCNQ•−, (CHC+)(F4TCNQ•−), (CHC+)(F4TCNQ-OMe−)(H2O), BuG,
and BuGH+Cl−: (a) CN region and (b) CC and CO region. Red lines are guides to eyes.
found to be BuGH+ by structural analysis (vide infra); however,
the protonated BuG species in 2 could not be identified.
IR and UV−vis-NIR spectroscopic analyses identified 3 and
5 as group II salts of fully ionic FnTCNQ•− and 4 and 6 as
group I salts of FnTCNQ-OMe− contaminated with small
amount of FnTCNQ•− species (Supporting Information
Figures S3−S6). The lowest absorption bands of 3 and 5
appear at an energy region higher than those of K+FnTCNQ•−
(8.2 × 103 vs 6.3 × 103 cm−1 for n = 2, and 9.3 × 103 vs 7.0 ×
103 cm−1 for n = 1) and show marked broadening. These
observations indicate the dimerized nature of FnTCNQ•−
molecules in the segregated stack.
Crystal Structures of 1, 3, and 5: Protonated Site. It is
known that the bond angle C5−N7−C8 of 9-alkyl-substituted
G derivatives (θ in Scheme 3) is somewhat sensitive to
protonation of G at the N7 position in comparison with the
Figure 4. UV−vis-NIR spectra of 1 and 2 together with those of
other angles (Supporting Information Table S2).21e,27−29 Table
4 summarizes the θ values of BuG species in neutral,
FnTCNQ•− salts (1, 3, and 5), 9-ethylguanine (EtG),27 and
protonated EtG (EtGH+).28 Neutral EtG and BuG exhibit θ
values in the range of 101.8−106.0°, while EtGH+ exhibits θ
values in the range of 107.1−107.3° because of protonation.
The θ values for 1 and 3 are 107.8° and 108.2°, respectively,
clearly indicating protonation at the N7 position. There are two
kinds of different values for θ in 5; 104.0° and 107.7°; hence,
the former is assigned to the more neutral species and the latter
to the more protonated one at N7, in the hemiprotonated
(BuG)(BuGH+) species.
K+F4TCNQ•−, (CHC+)(F4TCNQ•−), and (CHC+)(F4TCNQ-
OMe−)(H2O). For bands B−F, see text.
The strong band of 2 around 29 × 103 cm−1 (denoted as the
F band) corresponds to the intramolecular transition of
F4TCNQ-OMe− as observed for (CHC+)(F4TCNQ-OMe−)-
(H2O). Salt 2 also shows weak bands around 10−15 × 103
cm−1 that are ascribable to F4TCNQ•− species. This fact
suggests that 2 is mainly composed of F4TCNQ-OMe− species
but contaminated with a small amount of F4TCNQ•− species. A
pure F4TCNQ-OMe− salt should be colorless as opposed to
the pale green color observed for 2. The cation species in 1 was
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Table 4. C5−N7−C8 Bond Angles (θ) of Neutral and
Protonated 9-Alkyl G Derivatives
Scheme 5. Bonds a−d in the FnTCNQ Molecule
compound
molecule
θ (deg)
ref
BuG
A
104.0(6)
101.8(6)
106.0(6)
107.8(2)
108.2(4)
104.0(2)
107.7(2)
104.8
this work
B
F1TCNQ•− are not accessible, the r value of 5 (0.498) being
close to 0.5 strongly indicates the fully ionized state of the
acceptor moiety.
C
1, (BuGH+)(F4TCNQ•−
3, (BuGH+)(F2TCNQ•−)(MeOH)
5, (BuG)(BuGH+)(F1TCNQ•−
)
BuGH+
this work
this work
this work
BuGH+
)
BuG
BuGH+
Salt 1 (BuGH+)(F4TCNQ•−). Figure 5 shows the molecular
structures of crystallographically independent one BuGH+ and
one F4TCNQ•− molecules with the atom labeling scheme.
EtG
A
B
A
B
27
28
104.0
(EtGH+)(HCOO−)
107.1(2)
107.3(2)
Ionicity of FnTCNQ. In Table 5, the C−C bond lengths of
the FnTCNQ molecule (a−d, Scheme 5) of salts 1, 3, and 5 are
compared with those of neutral and fully ionized FnTCNQ
species,30−33 although the data of fully ionized F1TCNQ•− has
not been reported. In salt 1, all a−d bond lengths except one of
the d bonds (1.430 Å) are consistent with those in the fully
ionized state,31 which is in good agreement with the
characterizations by IR and UV−vis-NIR spectra. Although
the variation between each of the a−d bond lengths even in
FnTCNQ molecules is large, a similar tendency can be observed
for the compounds where n = 2 and 1 (Table 5).32,33 The
charge of a TCNQ moiety (ρ) can be estimated from the C−C
bond lengths using Kistenmacher’s equation,34 ρ = −(r − r0)/
(r1 − r0), where r = c/(b + d), and r0 and r1 are the values
calculated for neutral and fully ionized TCNQ molecules,
respectively. The ρ values of 1 and 3 estimated by this method
are −0.95 and −1.17, respectively, which are similar to those for
(Bu4N+)(F4TCNQ•−) and (TTF•+)(F2TCNQ•−) (ρ = −1),
suggesting the fully ionized state of FnTCNQ moieties.
Although earlier studies of the crystal structures of fully ionized
Figure 5. Molecular structures of BuGH+ and F4TCNQ•− in 1 with
the atom labeling scheme.
Figure 6 illustrates the packing of BuGH+ molecules viewed
along the c axis, where six GH+ (GH+ is the π-segment of a
BuGH+ molecule) columns are depicted, and n-butyl groups
are allocated between columns 1 and 2, 3 and 4, and 5 and 6
with a period of 10.9 Å. Two BuGH+ molecules form a dimer
(red ellipses in Figure 6 except n-butyl groups) by the double
complementary H-bonds (amine N2−H···N3, 2.99 Å, red
dotted lines) having no net dipole, and each dimer is connected
to the neighboring four dimers by N5−H···O6 H-bonds (2.73
Å, red dotted lines; for example, dimer ① is linked to dimers
Table 5. Bond Lengths of FnTCNQ Molecules (a−d (Å))
a
b
compound
a (Å)
b (Å)
c (Å)
1.372
d (Å)
1.435
r
ρ
ref
F4TCNQ
1.334
1.436
0.478
0.499
0.498
0
30
31
(Bu4N+)(F4TCNQ•−
)
1.357
1.415
1.418
1.425
−1
1
1.354(4)
1.348(4)
1.414(4)
1.417(4)
1.418(3)
1.418(4)
1.443
1.411(4)
1.411(4)
1.411(4)
1.417(4)
1.418(4)
1.430(4)
1.437
−0.95
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(BuGH+)(F4TCNQ•−
)
F2TCNQ
1.328
1.331
1.334
1.376
1.414
1.411
0.478
0.496
0
32a
32b
(TTF•+)(F2TCNQ•−
)
1.417
1.414
−1
1.418
1.433
1.437
1.435
1.425
1.419
3
1.345(5)
1.347(5)
1.419(5)
1.421(5)
1.429(5)
1.416(5)
1.433
1.417(5)
1.420(5)
1.420(5)
1.417(5)
1.418(6)
1.421(6)
1.412
0.499
−1.17
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(BuGH+)(F2TCNQ•−)(MeOH)
c
F1TCNQ
1.336
1.370
0.480
0.498
0
33
1.429
1.430
5
1.353(4)
1.363(4)
1.428(4)
1.430(4)
1.405(4)
1.424(4)
1.409(4)
1.416(4)
1.423(5)
1.412(4)
1.410(5)
1.423(4)
−1
this work
(BuG)(BuGH+)(F1TCNQ•−
)
a
b
c
r = (b + d)/c. ρ = −(r − r0)/(r1 − r0), where r0 and r1 are the values calculated for neutral and fully ionized FnTCNQ. Owing to the disorder of F
atoms, the result was of poor quality (R ≈ 10%).
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Figure 6. Polycationic layer of 1 viewed along the c axis. BuGH+
columns 1−6 form a 2D polycationic layer. The red ellipse shows
BuGH+ dimers except n-butyl groups. Red dotted lines indicate H-
bonds between BuGH+ molecules with bond length (Å).
②−⑤ in Figures 6 and S7), forming a 2D polycationic layer in
the ab plane (GH+ layer = thickness of 4.2 Å) with n-butyl
groups protruding up and down.
Figure 8. (a) Crystal structure of 1 viewed approximately along the
[110] direction showing the relative orientation of the 2D polycationic
template of BuGH+ and F4TCNQ•− molecules. Red lines are guides to
eyes separating GH+ and F4TCNQ layers. Purple lines show the
molecular long axis of F4TCNQ•−. Green dotted lines represent H-
bonds between BuGH+ and F4TCNQ•− molecules, respectively.
Purple dotted lines and black arrow indicate the periodicity of N2 sites
of BuGH+ along the b axis. Red solid and dotted arrows represent
intra- and interdimer interactions of F4TCNQ•− dimers. (b and c)
Intra- and interdimer overlap patterns, respectively.
One CN group of F4TCNQ•− is attached to the cationic
template by a H-bond with amine N2−H (N−H···N16C,
2.93 Å), and molecular long axis of F4TCNQ•− molecule is
nearly parallel to the c axis (Figures 7 and 8). Thus, F4TCNQ•−
along the molecular long axis for intradimer stacking (Figure
8b) and along the molecular short axis for interdimer stacking
(Figure 8c). The intra- and interdimer face-to-face separations
are 3.16 and 3.27 Å, respectively (Figure 8a), and the overlap
integrals are 22.35 × 10−3 and 3.27 × 10−3, respectively.
Broadening of the intermolecular CT band around 7.5 × 103
cm−1 in the UV−vis-NIR spectrum (Figure 4) is consistent
with the dimerization feature. The fully ionic valence state and
strong dimerization feature of F4TCNQ•− give rise to a spin-
singlet insulator (room temperature conductivity <10−7
cm−1).
S
Salt 3 (BuGH+)(F2TCNQ•−)(MeOH). A crystallographically
independent unit includes one molecule each of BuGH+,
F2TCNQ•−, and MeOH (Figure 9a). The N7 atom of a BuG
molecule is protonated to give BuGH+, which then forms a H-
bonded dimer by double complementary H-bonds (red dotted
lines in Figure 9b; amine N2−H···N3, 3.06 Å), where the
dipoles of BuGH+ cancel out each other. Each dimerized G
skeleton (indicated by the red ellipse) forms an infinite 1D
ribbon with ∼8 Å in width (except the n-butyl groups)
established by formation of MeOH-mediated H-bonds (orange
dotted lines in Figure 9b; N7−H···O(MeOH), 2.77 Å, and
O(MeOH)−H···O6, 2.73 Å).
Figure 7. Crystal structure of 1 viewed along the b axis. Red and green
dotted lines indicate H-bonds between BuGH+ molecules and those
between BuGH+ and F4TCNQ•−, respectively. Red lines are guides to
eyes separating GH+ and F4TCNQ layers.
molecules form a segregated 1D column along the b axis
according to the periodic pattern of the H-bond donating site
(N2 atom, 6.8 Å intervals) in the polycationic template.
Although there is no short contact between the 1D F4TCNQ
columns along the a axis owing to the intrusion of n-butyl
groups, F4TCNQ molecules form a thick layer (thickness of
10.7 Å) in the ab plane, and the F4TCNQ•− and GH+ layers
alternate along the c axis (Figures 7 and 8). Within a column,
F4TCNQ molecules are dimerized, where the molecules slip
Figure 10a demonstrates that π-stacking dimers of
F2TCNQ•− (encircled by red dotted ellipses) with an
interplanar distance of 3.16 Å (overlap integral = −25.99 ×
10−3) and an H-bonded dimer unit of BuGH+ are alternately
aligned along the b axis. They form a 1D π-column in which
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Figure 9. (a) Molecular structures of 3 with the atom labeling scheme. (b) H-bonded 1D ribbon of BuGH+ in 3 parallel to the [101] direction (H-
̅
bonds are mediated by MeOH). The red ellipse shows a BuGH+ dimer except n-butyl groups (GH+)2 with complementary double H-bonds. Red
and orange dotted lines show H-bonds with bond lengths (Å). Green arrows indicate the dipole.
Figure 10. (a) Crystal structure of 3 viewed approximately along the c axis (z = 0) showing the stacking pattern in the (GH+)2(A•−)(A•−) column
and the intercolumnar H-bonds (A = F2TCNQ). F2TCNQ•− dimers are encircled by red dotted ellipses. (b) The stacking structure of 3 viewed
along the [101] direction showing the 1D F TCNQ•− dimer ribbon (yellow sheet) and 2D BuGH+−F2TCNQ•− sheet (green sheet). (c) Overlap
̅
2
pattern of the F2TCNQ•− dimer, and (d) overlap pattern of the H-bonded BuGH+ dimer and the F2TCNQ•− dimer. Green, blue, and red dotted
lines in panels a, b, and d indicate the H-bonds between BuGH+ and F2TCNQ•−, those between F2TCNQ•− molecules, and those within a BuGH+
dimer, respectively.
(GH+)2(A•−)(A•−) is a repeating π-unit (A = F2TCNQ) as
shown in Figures 10a and 10b. Figure 10c shows the eclipsed
overlap pattern within a F2TCNQ•− dimer that is sandwiched
between BuGH+ dimers along the b axis (Figure 10d)
indicating a suitable size and shape match between the π-
skeletons of the BuGH+ dimer and F2TCNQ molecule. The
interplanar distance between the π-moieties of the BuGH+
dimer and F2TCNQ•− is 3.37 Å.
Adjacent F2TCNQ dimers are linked through C21−
H···N27C H-bonds (2.86 Å, slightly longer than the sum
of vdW radii,24 overlap integral = −1.27 × 10−5) to form a 1D
structure along the [021] direction (blue dotted lines on yellow
sheet in Figures 10b and 11a). The MeOH-mediated BuGH+
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Figure 11. (a) Packing of F2TCNQ•− molecules of 3 viewed along the b axis showing the F2TCNQ•− dimer ribbons (yellow sheets) by C−H···N
C bonds (blue dotted lines) extending along the [021] direction. (b) 2D BuGH+−F2TCNQ•− sheet structure composed of MeOH mediated
BuGH+ ribbons (blue sheets) and dimers of F2TCNQ•− viewed along the b axis. Red, orange, and green dotted lines indicate H-bonds with bond
lengths (Å) within a BuGH+ dimer, between BuGH+ and MeOH molecules, and between BuGH+ and F2TCNQ•− molecules, respectively. Two red
lines are guides for indicating that the size of the F2TCNQ molecule approximately corresponds to the distance between two BuGH+ molecules
linked to MeOH molecules.
Figure 12. (a) Molecular structures of component molecules in (BuG)(BuGH+)(F1TCNQ•−) (5) showing the atom labeling scheme. F20 and F24
atoms are disordered in a 0.64:0.36 ratio. (b) H-bonded infinite 1D ribbon of (BuG)(BuGH+) in 5. Red dotted lines show the H-bonds with bond
lengths (Å). Green arrows indicate the dipole of BuG and BuGH+.
ribbons (blue sheets in Figure 11b) and F2TCNQ dimers are
connected by the N1−H···N16C H-bonds (2.98 Å) to form
a 2D sheet (green sheets and green dotted lines in Figures 10b
and 11b, respectively). This salt gradually loses crystal solvents,
and thus, the measurements of physical properties (magnetic
susceptibility and electrical conductivity) could not be
performed. However, it is anticipated that dimerization of
radical spins of F2TCNQ•− results in an electric insulating
nature.
(BuG)(BuGH+) (width except n-butyl groups, 8.5 Å) along the
a axis.
The H-bonded (BuG)(BuGH+) ribbons stack along the b
axis to form a 2D polycationic layer in the ab plane (blue sheet
in Figure 13), where two types of overlap patterns for the
ribbons (Types A and B, Supporting Information Figure S8)
with interplanar distances of 3.31 and 3.27 Å, respectively, are
observed. Each ribbon has an infinite dipole; however, the
neighboring ribbon in the π-stack has the dipole in the opposite
direction, canceling out individual dipoles in the crystal.
As shown in Figure 13, the 2D layer of π-moieties of the
(BuG)(BuGH+) ribbons (blue sheets; thickness, 5.4 Å) and
F1TCNQ•− layers (red sheets; thickness, 11.7 Å) alternately
stack along the c axis. The F1TCNQ•− layer includes n-butyl
groups of BuG and BuGH+ molecules. F1TCNQ forms a face-
to-face dimer with an interplanar distance of 3.23 Å (Figure
14a). The F1TCNQ dimers are connected to each other
through short C21−H···N27C H-bonds (2.67 Å, blue dotted
Salt 5 (BuG)(BuGH+)(F1TCNQ•−). Three types of mole-
cules, neutral BuG, protonated BuGH+, and F1TCNQ•−, are
crystallographically independent (Figure 12a). The F atom in
F1TCNQ is disordered, appearing at two positions in the ratio
of 0.64:0.36. BuG and BuGH+ are connected through
complementary double H-bonds, amine N2B−H···O6A (2.93
Å), and amide N1B−H···N7A (2.83 Å), N7B−H···O6A (2.69
Å) and N1A−H···O6B (2.80 Å) (red dotted lines in Figure
12b), establishing an infinite flat 1D ribbon of hemiprotonated
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Figure 13. Crystal structure of 5 viewed along the a axis. Blue sheets
include the π-moieties of a (BuG)(BuGH+) layer, while red sheets
include F1TCNQ and n-butyl groups. Green dotted lines are H-bonds
between (BuG)(BuGH+) and F1TCNQ•− molecules. Red lines are
guides to eyes separating a layer of G−GH+ from layers of F1TCNQ
and n-butyl group.
Figure 15. Crystal structure of 5 viewed along the b axis. Green and
red dotted lines show the H-bonds between BuGH+ and F1TCNQ•−
(amino N2B−H···N16C) and those between BuG and BuGH+,
respectively. Purple dotted lines and black arrow in (a) show the
periodicity of N2 sites of BuGH+ within a (BuG)(BuGH+) ribbon
along the a-axis. Red lines in panel b are guides to eyes for separating a
layer of G−GH+ from layers of F1TCNQ and n-butyl group. Red solid
and dotted arrows represent intra- and interdimer interactions of
F1TCNQ•− dimers, respectively.
Figure 14. (a) Overlap pattern of the F1TCNQ molecules of 5 in a
dimer. (b) 1D array of F1TCNQ dimers supported by C−H···NC
H-bonds (blue dotted lines) extending along the b axis.
lines in Figure 14b) to form a 1D ribbon along the b axis. The
adjacent ribbons of F1TCNQ dimers are nearly separated from
each other by n-butyl groups along the a axis (Figure 15). The
intradimer overlap integral of F1TCNQ•− is −18.98 × 10−3,
and the interdimer overlap integrals are calculated as −8.08 ×
10−5 and −2.79 × 10−6 along the side-by-side direction (b axis)
and face-to-face direction (a axis), respectively.
The F1TCNQ dimer ribbons are linked to the (BuG)-
(BuGH+) ribbons by N2B−H···N16C H-bonds (3.04 Å,
green dotted lines in Figures 13 and 15), constructing a 3D
architecture (Supporting Information Figure S9). Consistent
with the fully ionic state, the solid is insulating (room
temperature conductivity <10−8 S cm−1), and most spins are
magnetically quenched by the strong antiferromagnetic
coupling between F1TCNQ•− molecules within a dimer (only
∼9% of the total of noninteracting S = 1/2 spins are active at
300 K).22
H-Bonded Architectures of BuG and Their Role as
Template. In this section, the motifs representing the H-bond
connectivity and relative position of BuG and/or BuGH+
molecules in the crystals of neutral BuG, and salts 1, 3, and
5 are examined to determine how BuG species form
supramolecular architectures to establish templates for the
counter molecules.
Scheme 6 illustrates several reported H-bonding patterns of
G pairs of N9-substituted derivatives, by classifying the manner
of connectivity and relative positioning. Alternation of Type 1
and Type 2 H-bonds, in which the dipoles of each G molecule
are canceled out, forms a 1D ribbon structure (Ribbon-B in
Scheme 2b).35 Type 3 H-bonds are observed in the cyclic G-
quartet.36 In the case of Type 4 H-bonds, the dipoles are not
quenched completely, and the 1D ribbon of these G pairs
possesses a large dipole along the chain direction (Ribbon-A in
Scheme 2a).35,37
Neutral BuG. In the crystal structure of BuG, 1D ribbons of
Type 4 pairing are observed (A-ribbon and BC-ribbon in
Figures 2a and 2b, respectively). The N2A−H···O6B H-bonds
(purple dotted lines in Figures 2e−2f) between the A-ribbon
and π-dimer of BC-ribbons, which are perpendicular to each
other, generate a 3D network structure (Supporting Informa-
tion Figure S1). A similar 3D network has been observed in the
crystal structure of EtG in which both ribbons running
perpendicular to each other exhibit the π-dimerization feature
like the BC-ribbon in BuG.27
Protonated G. Protonation at the N7 atom increases the
variation of H-bonding motifs (Types 5−7 in Scheme 6). In
Type 5 paring, the dipoles of GH+ are canceled out.38
Alternation of Types 2 and 5 pairing establishes a 1D ribbon
structure.29 The coexistence of G and GH+ species gives the
Type 7 H-bond (N7−H···N7, Scheme 6), which links pairs of
Type 2 H-bonds forming a 1D ribbon as shown in Scheme
2f.21d,e In some cases, these GH+ ribbons stack to form a 2D
layer structure,39 in which the anion species are linked to the
cationic template by H-bonds and are arranged in a 2D motif
between the GH+ layers. Figure 16 shows redrawn motifs of the
H-bonded ribbon structures of 3 (a), 5 (b), and 1 (c) with H-
bonding connectivities. The positive site in a BuGH+ molecule
is represented by the symbol ⊕.
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Scheme 6. H-Bonding Patterns of G Pairs in Neutral and Protonated States
Figure 16. Motifs of H-bonded ribbons of (a) 3, (b) 5, and (c) 1 (only columns 2−5 in Figure 6 are depicted). Types 2, 4, and 6 are the H-bonding
patterns depicted in Scheme 6. The symbol ⊕ represents the postulated positive site in BuGH+ as a point charge. The n-butyl groups are omitted for
simplicity. Gray lines and purple lines in (c) represent the isosceles triangular cationic lattice formed by connecting the ⊕ symbols.
Salt 3. In salt 3, BuGH+ molecules form a dimer with the
Type 2 H-bond pairing, and the dimers are connected by
MeOH-mediated H-bonds (Figure 16a). Similar solvent-
mediated 1D ribbon structures have been observed in the
hydrate salts of protonated 9-methylguanine (MeGH+) and
EtGH+.40 The N1 sites of BuGH+ molecules of 3 fixed the
anion molecules by H-bonds (N1−H···N16C), in which
MeOH participation is the key feature responsible for adjusting
the separation between the N1 sites of the neighboring BuGH+
dimers (see the two red lines in Figure 11b) to the separation
observed between the N16 sites of the F2TCNQ•− dimers (9.6
Å). The H-bonded ribbons in 3 run parallel to the molecular
long axis of the anion molecule.
neutral BuG species as well. The N2 sites of BuGH+ species fix
F1TCNQ•− molecules by H-bonds, and the period of the
F1TCNQ dimers is 9.8 Å, corresponding to that of N2 sites of
BuGH+ molecules within the 1D ribbon (black arrow in Figure
15).
The F1TCNQ dimers are arranged via side-by-side C−
H···NC H-bonds and via π−π interactions along the a-axis,
forming a 2D layer in the ab plane, which is sandwiched by the
π-moieties of polycationic (BuG)(BuGH+) layers (Figure 15).
Therefore, the (BuG)(BuGH+) ribbon functions as both a
polycationic template and a wall forming a segregated layer
with the F1TCNQ•− molecules.
Salt 1. Only salt 1 has a 2D polycationic network by Type 2
H-bond pairing and N7−H···O6 H-bonds (Figure 16c). A 2D
sheet formed by similar H-bond motifs has also been observed
in MeGH+Br− salt.41 The N2 atoms of BuGH+ fixed the
F4TCNQ•− molecules by N2−H···NC H-bonds with a
Salt 5. In salt 5, BuG and BuGH+ molecules are alternately
linked through H-bonds of Types 4 and 6 pairings to form a 1D
ribbon structure (Figure 16b). One side of the ribbon is
composed of only BuGH+, and the opposite side includes
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period of GH+ pairs (6.8 Å, black arrow in Figure 8a). The
F4TCNQ•− 1D column is sandwiched by the 2D layer of
BuGH+ with the dihedral angle of 104° between the BuGH+
wall plane and the molecular long axis of F4TCNQ•− (Figure
8a). The n-butyl group array inhibits the side-by-side
interactions of the F4TCNQ•− column.
Different connectivity and packing manner of the poly-
cationic ribbons and anion molecules are noticed among salts 1,
3, and 5; namely, ribbons of salt 3 grow parallel to the
molecular long axis of the anion molecule (black arrows in
Figure 11b), while the ribbon of salts 1 and 5 is directed such
that it forms a large angle with the molecular long axis of the
anion molecule (Figures 8a and 15). Both salts 1 and 5 have
segregated columns of anion molecules, while in salt 3, a dimer
of anion molecules is sandwiched by BuGH+ (Figure 10).
Existence of segregated stacking of radical molecules is the first
essential requirement for high conductivity of a metallic band
structure or a Mott insulating state.
modification of G to control the arrangement of GH+ 1D
ribbons will provide a favorable template for not only
conductive but also magnetic CT solids.
EXPERIMENTAL SECTION
■
General. 2-Amino-6-chloropurine was used as purchased. FnTCNQ
derivatives were synthesized in our laboratory and were purified by
sublimation. MeOH, MeCN, and 1,2-dichloroethane used for complex
formation were distilled under an inert gas prior to use, and the
reactions were performed under an inert atmosphere.
Synthesis of BuG. BuG was prepared according to the modified
procedures in the literatures (Scheme 7).44−46 Into a suspension of 2-
Scheme 7. Synthetic Procedure of BuG
The cationic sites of the 2D sheet in Figure 16c form a
triangular lattice, which was achieved by connecting the ⊕
symbols. Each isosceles triangle is connected by sharing one
side, 7.21 Å in length, along the b axis. Such a H-bonded
BuGH+ assembly may serve as a template for the molecular
arrangement of various anion radical species to form a
triangular zigzag spin chain via Coulomb interactions, that
has become recently important as renewed spin frustrated
system as found for CaV2O4, Cu(ampy)Br2 (ampy = 2-(2-
aminomethyl)pyridine), and (VO)(SO4)(bpy) (bpy = 2,2′-
bipyridine).42 Considering the fact that the triangular zigzag
spin chain can transform to other geometrical spin systems such
as 1D spin chain and two-leg spin ladder43 by modifying the
interunit interactions, the self-assembling nature of BuGH+ is
promising for the systematic exploration of various spin system.
Especially, it is possible that the derivatization of guanine,
which modifies the self-assembling structure, allows the
modification of spin structures formed by anion radicals as
counter species.
amino-6-chloropurine a (26.6 g, 157 mmol) and K2CO3 (21.7 g, 157
mmol) in DMF (400 mL), 1-bromobutane (17.0 mL, 158 mmol) was
added, and the mixture was stirred at room temperature for 2 days.
The reaction mixture was diluted with brine, and then extracted with
ethyl acetate. The organic extract was concentrated under reduced
pressure. The resulting precipitate was collected by filtration, to give
the crude product as a pale yellow powder (30.9 g). The precipitate
(25.0 g) was subjected to silica gel column chromatography (hexane/
ethyl acetate, 1:5) to give the pure product of 2-amino-9-n-butyl-6-
chloropurine b (20.5 g, 87%) as a colorless solid (mp. 138−141 °C, lit.
138−140 °C44). Compound b (8.09 g, 36.0 mmol) was dissolved in 6
M aqueous HCl solution (360 mL), and the mixture was refluxed for 1
h. After cooling to room temperature, the solution was neutralized by
an aqueous NaOH solution (33 wt %, 240 g) and a saturated aqueous
Na2CO3 solution. The resulting precipitate was collected by filtration
to give crude BuG (7.04 g) as a white powder. After it was dissolved in
an aqueous NaOH solution (4 wt %, 85 mL), the crude product was
treated with charcoal under reflux, and then the charcoal was then
removed by filtration. The filtrate was neutralized by 3 M aqueous HCl
solution. The resulting precipitate was collected by filtration, to give
BuG (5.71 g, 77%) as a white powder. Further purification was
performed by recrystallization from MeOH (1.7 L) to give colorless
prisms (3.81 g, 51%). dec 321−326 °C (lit. mp. > 260 °C46); 1H NMR
(400 MHz, DMSO-d6) δ 0.87 (t, 3H, J = 7.3 Hz), 1.18−1.27 (m, 2H),
1.64−1.72 (m, 2H), 3.91 (t, 2H, J = 7.2 Hz), 6.40 (s, 2H), 7.66 (s,
1H), 10.49 (s, 1H); EI-MS, m/z 207 (M+, 29%); IR (KBr) 3478, 3292,
3153, 2958, 2931, 2874, 2822, 2710, 1693, 1630, 1606 cm−1. Anal.
Calcd. for C9H13N5O: C, 52.16; H, 6.32; N, 33.79; O, 7.72%. Found:
C, 51.79; H, 6.30; N, 33.92; O, 8.01%.
Preparation of FnTCNQ Salts. F4TCNQ Salt 1. In a 100-mL two-
necked-modified round-bottomed flask, BuG (62.4 mg, 0.30 mmol)
was dissolved in MeOH (50 mL) under reflux. To this solution, a
solution of F4TCNQ (150.4 mg, 0.55 mmol) in hot MeCN (40 mL)
was added, and the reaction solution immediately turned green. After
it was stirred for 5 min, the mixture was evaporated under reduced
pressure, and the residual solid was suspended in MeCN (30 mL). The
insoluble materials were removed by filtration, and the filtrate was
concentrated under reduced pressure to ∼3 mL. After the addition of
MeOH (2 drops), the solution was left standing at 5 °C to give 1 (16.0
mg, 11% from BuG) as dark blue crystals. dec 218−229 °C (gradually
decomposed). Anal. Calcd. for C21H14F4N9O: C, 52.07; H, 2.91; N,
26.02; F, 15.69%. Found: C, 52.09; H, 2.82; N, 26.25; F, 15.59%.
F2TCNQ Salt 3. In a 100-mL two-necked-modified round-bottomed
flask, BuG (30.0 mg, 0.15 mmol) was dissolved in MeOH (40 mL)
under reflux. To this solution, a solution of F2TCNQ (140.0 mg, 0.58
mmol) in hot 1,2-dichloroethane (50 mL) was added, and the reaction
solution gradually turned green. The mixture was evaporated under
reduced pressure, and the residual solid was dissolved in MeOH (5
CONCLUSION
■
Structural template effect of a nucleobase derivative, BuG, in
CT complexes was investigated in the BuG−FnTCNQ system.
The reaction between BuG and FnTCNQ derivatives in a
mixed solvent containing MeOH proceeded via generation of
FnTCNQ-OMe− to yield two types of salts, FnTCNQ-OMe−
(Group I) and fully ionic FnTCNQ•− (Group II) with
protonated BuGH+ species, as observed in C−TCNQ systems.9
Structural analyses of the BuG and Group II salts revealed that
BuG possesses a strong tendency to form 1D ribbon structures
supported by robust complementary H-bonds in the neutral,
hemiprotonated (5), and protonated states (3), and a H-
bonded 2D sheet structure was also observed in 1. The 1D
ribbons of hemiprotonated (BuG)(BuGH+) in 5 further
stacked owing to the π−π interaction to form a 2D layer.
Although 2D layers were not formed in the crystals of BuG and
salt 3, the 1D ribbons observed in these crystals have the
potential to form a 2D segregated layer, as shown in the
literatures.39−41 The fact that the H-bonded arrays in salts 1, 3,
and 5 were linked with FnTCNQ•− through N−H···NC H-
bonds indicates the high potential of BuGH+ assemblies as
structural templates for TCNQ•− derivatives to fix and arrange
anionic π-molecules periodically. Unfortunately, mixed-valence
and highly conductive salts (Group III) could not be obtained
in the BuG−FnTCNQ system; however, further chemical
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Crystal Growth & Design
Article
mL) at 50 °C. The solution was left standing at −18 °C for 2 days to
give 3 (1.4 mg) as dark blue microcrystals. Cooling the filtrate at −18
°C for 4 days afforded additional 3 (7.5 mg, total 8.9 mg, 11% from
BuG) as dark blue columnar crystals. Because of the efflorescence of 3
that caused decomposition of crystals, elemental analysis and
measurements of physical properties (magnetic susceptibility and
conductivity) could not be performed.
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Measurements. H NMR spectra were measured at 400 MHz on
a JEOL JNM-FX400 spectrometer using DMSO-d6 as the solvent and
tetramethylsilane as an internal standard. The electron impact mass
spectra (EI-MS) were measured with a Thermo Finnigan Trace DSQ.
Elemental analyses were performed at the Center for Organic
Elemental Microanalysis, Kyoto University. Melting points were
measured with a Yanaco MP-500D micro melting-point apparatus
and were not corrected. Measurements of absorption spectra were
performed with a KBr disk on a Perkin-Elmer PARAGON 1000 Series
FT-IR (resolution 2 or 4 cm−1) for IR and near-IR regions (400−7800
cm−1), and on a SHIMADZU UV-3100 spectrometer for near-IR,
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Electrical conductivity of the single crystal of 1 was measured with
Keithley 2400 by two probe method. The 10 μm gold wires were
attached to the crystal with gold paste as the electrodes.
Cyclic Voltammetry Measurement. Cyclic voltammetric meas-
urements were performed using a solution of 0.1 M Bu4NBF4 in
MeCN vs SCE on an ALS/chi Electrochemical Analyzer Model 650 A
at room temperature. The experiments employed a Pt plate working
electrode, a Pt wire counter electrode, and a SCE reference electrode.
X-ray Crystallography. The intensity data of the structural
analysis were collected using an oscillator-type X-ray imaging plate
(DIP-2020K) or a Rigaku Raxis-Rapid imaging plate using
monochromated Mo Kα (0.71070 Å) or Cu Kα (1.54187 Å)
radiation, respectively. The structures were solved by a direct method
using SHELXS-9747 or SIR-2004.48 Refinements of structures were
achieved by a full matrix least-squares method (SHELXL-97).49
Positions of hydrogen atoms were determined by assuming an sp2 or
sp3 configuration for each atom with an X−H (X = C, N, and O)
distance of 1.0 Å. Parameters were refined by adopting anisotropic and
isotropic temperature factors for non-hydrogen and hydrogen atoms,
respectively.
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̈
ASSOCIATED CONTENT
* Supporting Information
■
S
́
(c) Jerome, D.; Schulz, H. J. Adv. Phys. 1982, 31, 299−490.
UV−vis-NIR and IR spectra of products of F2TCNQ and
F1TCNQ, pictures of crystalline products, selected bond angles
of G molecules in BuG, salts 1, 3, and 5, and related
compounds, and X-ray crystallographic data for each structure
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
■
*Tel: (+81)-52-838-2542. Fax: (+81)-52-833-7200. E-mail:
tmurata0921@gmail.com (T.M.); gsaito@meijo-u.ac.jp (G.S.).
(19) Hulme, A. T.; Tocher, D. A. Cryst. Growth Des. 2006, 6, 481−
487 and references therein..
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Munno, G.; Rossi, R. New J. Chem. 2006, 30, 13−17. (d) García-
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by a Grant-in-Aid for Scientific
Research (S) (23225005) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan.
Teran
Roman
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