A ternary charge-transfer complex composed of cyclotriveratrylene (CTV) and a polyoxometalate (POM) with quinone as an electronic modulator

Article abstract of DOI:10.1002/asia.201200057

Triple stack: A facile synthesis of charge-transfer-type crystals composed of CTV and POM was demonstrated. The crystals further accommodated 1,4-benzoquinone as a guest to form a ternary crystal with a donor-acceptor-acceptor arrangement. The electronic spectra of the crystals suggested that the incorporated quinone served as an electronic modulator to adjust the charge-transfer property. Copyright

Full text of DOI:10.1002/asia.201200057

DOI: 10.1002/asia.201200057
A Ternary Charge-Transfer Complex composed of Cyclotriveratrylene (CTV)
and a Polyoxometalate (POM) with Quinone as an Electronic Modulator
Shohei Tashiro, Shizuka Hashida, and Mitsuhiko Shionoya*[a]
Discrete metal oxide clusters, or polyoxometalates
(POMs), have attracted a great deal of attention in view of
various applications such as catalysis, magnetism, photonics,
and supramolecular building blocks based on their structural
diversity and electronic characteristics.[1] In particular, their
reversible electron-accepting property is key to the explora-
tion of the fundamental electronic functions with regard to
charge-transfer reactions of POMs.[2] So far, hybridization of
POMs and various donor molecules has been examined to
explore charge-transfer complexes. For example, POM-
based charge-transfer complexes as crystalline materials
with an electron-rich organic component, such as large aro-
matic compounds[3] (e.g. from the tetrathiafulvalene (TTF)
family[4] or ferrocene derivatives)[5] and nitrogen-containing
compounds such as amides[6] or amines,[7] have been report-
ed, and their conductivity,[4c,d] magnetism,[5a,b,7b] and photo-
nics[3b,6,7d] were also evaluated.
ternary charge-transfer crystalline complexes composed of
CTV (1) and an a-Keggin-type POM, Na3PMo12O40·nH2O
(Na3·2·nH2O, Figure 1). In addition, the resulting binary
complex has proven to further accommodate 1,4-benzoqui-
none (5) as an electronic modulator in the cavity of CTV to
form a ternary complex with a donor–acceptor–acceptor
(D–A–A) arrangement.[13] The modulation of the charge-
transfer properties of the resulting crystal through the third
component was also studied.
In the design of charge-transfer complexes as crystalline
materials, it is obvious that the combination and arrange-
ment of donor and acceptor molecules are important factors.
With the recent development of crystal engineering,[8] incor-
poration of a third (or higher) component as a modulator
into charge-transfer complexes has shown great promise for
coordinating their charge-transfer effects and concomitant
functions.[9] Herein, a conventional cyclic host molecule, cy-
clotriveratrylene (CTV, 1), with a bowl-shaped cavity sur-
rounded by three electron-rich veratrole moieties was used
as an electron donor against the counterpart POMs. If the
cavity of CTV is applicable to the binding of one or more
electron-accepting guest molecules, binary or ternary CTV-
based charge-transfer complexes are expected to be generat-
ed. While CTV and its derivatives have been extensively
studied as host molecules for some spherical guests such as
fullerenes, o-carborane, and organometallic salts in the crys-
talline state[10] and as building blocks for coordination net-
works with metal ions,[11] to the best of our knowledge there
are no reports about the combination of CTV and POMs.[12]
Herein, we report a facile synthetic method of binary and
Figure 1. Schematic representation of the formation and pictures of a) 3,
b) 4, and c) 6, from a mixture of CTV (1) and POM (2) with or without
5.
A crystalline material 3 composed of CTV (1) and
Na3PMo12O40·nH2O (Na3·2·nH2O) was obtained from ace-
tone. The colorless 1 (30 mg, 67 mmol) and the yellow-col-
ored Na3·2·nH2O (70 mg, 34 mmol) were dissolved in acetone
(34 mL), and diethyl ether was then slowly diffused into the
acetone solution at room temperature.[14] After six days,
a green-brown, fine-needle crystalline product Na3·(1)2·2·
ACHUTNRGENUN(G acetone)3·ACHTUNGTRNEN(UGN H2O)7 (3) was obtained (53 mg, 17 mmol, 51%;
Figure 1a). Several trials of the preliminary single-crystal X-
ray diffraction experiments of 3 indicated that the resulting
product 3 contains a mixture of at least two kinds of poly-
morphic crystals. However, due to their low crystallinity, the
final structures of 3 could not be defined.
On the other hand, another crystalline material 4 having
quite a different color and a shape from 3 was obtained
from a CH3CN solution of CTV (1) and POM (2). A
CH3CN solution (1 mL) of 1 (20 mg, 44 mmol) was gently
put on a CH3CN solution (0.5 mL) of Na3·2·nH2O (138 mg,
67 mmol), and then brown block crystals of Na3·(1)2·2·
[a] S. Tashiro, S. Hashida, M. Shionoya
Department of Chemistry
Graduate School of Science, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)3-5841-8061
E-mail: shionoya@chem.s.u-tokyo.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200057.
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COMMUNICATION
A
E
As for the relative position of CTV (1) and POM (2), the
terminal oxygen atoms of 2 and the convex aromatic faces
of 1 are close to each other (3.3–3.4 ꢁ), possibly producing
charge-transfer interactions between donor 1 and acceptor 2
over two days (Figure 1b).[15] For this procedure, excess
POM (2) was used to improve the yield of 4. In the crystal
structure of the binary complex 4, a chainlike infinite struc-
ture [Na3·12]n was found in which all veratrole units of 1 che-
late Na+ ions. In the structure of [Na3·12]n, two Na+ ions
and two facing molecules of 1 form a unit Na2·12, with the
respective characteristic D and L configurations at the two
hexacoordinate Na+ centers. The Na2·12 units are further
connected nearly perpendicularly to each other by one Na+
ion each to form a chainlike structure, [Na3·12]n (Figure 2a).
The parallel packing of [Na3·12]n chains resulted in the inter-
3À
(Figure 2d). Remarkably, one PMo12O40 ion is loosely
sandwiched by two molecules of 1 to form 12·2, in which dis-
ordered CH3CN molecules are included in the bowl-shaped
cavity of 1, and therefore this structure is not regarded as
a well-defined host–guest complex between 1 and 2, unlike
some other examples in literatures[10] (Figure 2e). Then we
envisioned that, if these solvent molecules can be replaced
by alternatives, a ternary charge-transfer complex such as
a D–A–A complex should be formed.
stitial void formation to arrange PMo12O40 ions along
a and b-c axes at 13.202 and 13.199 ꢁ intervals, respectively
(Figure 2b,c).
To construct a ternary charge-transfer system in 4, some
acceptor or donor molecules were added to the crystalliza-
tion
solution.
Specifically,
a mixed CH3CN solution of
CTV (1; 44 mm) and excess ac-
ceptor or donor was gently put
on a CH3CN solution of POM
(2; 66 mm). The resulting crys-
tals were collected and washed
with CH3CN and dissolved in
[D6]acetone to measure their
1H NMR spectra (Figure S3–S9,
see the Supporting Informa-
tion). Interestingly, some mem-
bers of the quinone family, 1,4-
benzoquinone (5) and 1,4-naph-
thoquinone, were efficiently ac-
commodated in the resulting
crystals with the ratios of 1/
guestꢀ2 as estimated from the
integral ratios.[16] Notably, the
electron-accepting ability of the
quinone acceptor was essential
to form ternary complexes with
1 and 2, because hydroquinone,
possessing almost identical
structure to 5, was not incorpo-
rated at all into the crystals. In
addition to the electronic char-
acter, their steric compatibility
with the cavity of 1 was also an
important factor. Indeed, bulky
quinone derivatives, such as
2,5-dibromo-1,4-benzoquinone,
2,3,5,6-tetrachloro-1,4-benzo-
quinone (chloranil), and 9,10-
phenanthrenequinone,
and
other-electron deficient mole-
cules such as nitrobenzene, ben-
zonitrile, and I2 did not serve as
an additional acceptor.
In the case of quinone 5,
good crystals were efficiently
obtained. A mixed CH3CN so-
Figure 2. Crystal structure of 4. a) Chainlike infinite [Na3·12]n found as a partial structure of 4. b,c) Packing of
[Na3·12]n and 2 along the a axis (b) and the b-c axis (c). d) Structure of neighboring 1 and 2. e) A loose capsu-
le-like structure of 12·2 with disordered CH3CN molecules. Mo pink, P orange, Na purple, O red, N blue,
C gray, H white.
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A Ternary Charge-Transfer Complex
lution (1 mL) of 1 (20 mg, 44 mmol) and 5 (95 mg, 880 mmol)
Na+ ion, unlike 4 (Figure 3d).[18] As the result, the whole
packing structure of 6 is also different from 4.
We next evaluated the electronic states of resulting crys-
tals of 3, 4, and 6 from their diffuse reflectance UV/Vis/NIR
spectra (Figure 4). Whereas the starting materials 1 and 2
did not show any absorption features around 350 and
500 nm, respectively, all co-crystals, 3, 4, and 6, showed
a lower-energy absorption feature in the long-wavelength
region over 500 nm. For example, the spectrum of 3 showed
a broad absorption feature in the region from 500 to
1500 nm, which can be roughly ascribed to the combination
of at least two broad absorptions around 600 and 1000 nm.
The lower-energy absorption
was gently put on
Na3·2·nH2O (138 mg, 67 mmol) to afford red-brown platelike
crystals of Na3·(1)2·2·(5)1.28 A(CH3CN)5.5·A(H2O)4.5 (6, 53 mg,
a CH3CN solution (0.5 mL) of
·
C
CHTUNGTRENNUNG
16 mmol, 73%) after four days (Figure 1c).[15,17] The X-ray
diffraction analysis of the single-crystal of 6 revealed that
the quinone 5 was tightly trapped between 1 and 2 to form
a D–A–A arrangement composed of 1, 5, and 2, in which
anion–p interactions between the terminal oxygen atom of
À
POM (2) and quinone
5 (neighboring O C distance
3.033 ꢁ) and p–p interactions between 1 and 5 (neighboring
À
C C distance 3.401 ꢁ) were observed (Figure 3a–c). This
around 1000 nm appears to be
due to the intervalence transi-
tion from Mo5+ to Mo6+ within
the cluster, suggesting that the
reduction of 2 took place in
3.[7b,19] As for the other higher-
energy
absorption
around
600 nm, it should arise from
charge-transfer interactions be-
tween donor 1 and acceptor 2
or from intervalence or d–d
transition of reduced 2.[5c,7c] In
contrast, spectra of 4 and 6 dis-
played a higher-energy absorp-
tion feature around 600 nm
only, indicating that only
charge-transfer
transition
occurs between 1 and 2 in their
crystals.
The absorption band around
600 nm of the binary complex 4
can be assigned to charge-trans-
fer transition between 1 and 2,
as described above. The absorp-
tion band around 540 nm found
for the ternary complex 6 possi-
bly arises from charge-transfer
transition between 1 and 2, or
1 and 5. However, we ascribed
this absorption to the transition
between 1 and 2, in light of the
higher-energy charge-transfer
absorption around 370 nm of
Figure 3. Crystal structure of 6. a) The loose capsule-like structure of 12·2 with 5, disordered CH3CN molecules,
and Na+ ions. b) An anion–p interaction between 2 and 5. c) A p–p interaction between 1 and 5. d) The chain-
like structure [Na3·12]n with 5. Mo pink, P orange, Na purple, O red, N blue, C except for 5 gray, C of 5 lime,
H white.
phenomenon is also regarded as replacement of some disor-
dered CH3CN molecules by 5 in the loose capsule-like struc-
ture of 12·2, suggesting that quinone 5 well fits to the steric
and electronic characters of the cavity of 1. The crystal
structure of the ternary complex 6 showed a slight differ-
ence in the packing mode from 4. For example, complex 6
has a chainlike structure composed of 1 and Na+ ions simi-
lar to 4, in which POMs (2) are arranged in the interstitial
voids formed among the chainlike structures in the ratio of
1:2=2:1. However, in the chainlike structure of 6, each
Na2·12 unit is connected in parallel with each other by one
the co-crystals of CTV (1) and quinone (5), 12·5·ACTHUNGTERNNU(G H2O)2 (Fig-
ure S10–S12, see the Supporting Information).[15] This result
also indicates that the energy of the charge-transfer transi-
tion between 1 and 2 is adjustable by incorporation of qui-
none 5. In other words, coexistence of 5 induced a blue shift
of the charge-transfer band. This phenomenon is explained
by the decreased level of the HOMO energy of 1 through
charge-transfer interaction with the LUMO of 5.
To obtain more information about the amount and va-
lence of the oxidant and reductant in 3, 31P MAS NMR
measurement was carried out using green-brown crystals
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COMMUNICATION
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Figure 4. Diffuse reflectance UV/Vis/NIR spectra (room temperature) of
the crystals of 3, 4, and 6 dispersed BaSO4 powder.
prepared by slow evaporation of an acetone solution con-
taining 1 and 2.[14] An intense signal at d=À2.96 ppm and
a small shoulder signal at d=À4.85 ppm were observed in
an integral ratio of approximately 9:1 (Figure S13, see the
Supporting Information). Compared with the intense signal
3À
of original PMo12O40 ion, the small high-field shifted signal
at d=À4.85 ppm can be assigned to two-electron-reduced
5À [20]
PMo12O40
.
Therefore, the results of electronic and 31P
MAS NMR spectra suggest that crystal 3 contains a small
amount of two-electron-reduced POM 2.[21]
In summary, a facile synthesis[22] of crystalline charge-
transfer complexes from commercially available compounds
CTV (1) and POM (2), was established. The combination of
donor 1 and acceptor 2 resulted in the formation of crystal-
line charge-transfer complexes. Furthermore, 1,4-benzoqui-
none (5) was incorporated as an electronic modulator into
the co-crystal to form a ternary D–A–A-type charge-transfer
complex. Incorporation of quinone 5 induced a blue shift of
the charge-transfer band. Notably, the structures and proper-
ties of the co-crystals depend on the crystallization solvent.
This way of modulating charge-transfer transition would be
further advanced by chemical modification of 1,[23] replace-
ment of 2 with other POMs, or selection of additional ac-
ceptor or donor molecules.
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Acknowledgements
We are grateful to Prof. S. Kobayashi and Ms. K. Maehata for their kind
help in the 31P MAS NMR measurements. This research was supported
by KAKENHI, the Japan Society for the Promotion of Science and
MEXT, Japan.
Keywords: charge transfer · cyclotriveratrylene · molecular
recognition · polyoxometalates · ternary complexes
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A Ternary Charge-Transfer Complex
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[16] Due to the poor quality of the resulting crystals formed from
a mixed solution of 1, 2, and 1,4-naphthoquione in acetonitrile,
single-crystal X-ray analysis of the crystal could not be conducted.
[17] The 1H NMR analysis of digested 6 and the elemental analysis of 6
suggest the ratio of 1:5=2:1.28. This result is not consistent with the
result of single-crystal X-ray analysis (1:5=2:1). There are two pos-
sibilities explaining this difference: 1) homogeneous crystals of 6
contained disordered 5 in the interstitial voids of crystal packing,
and 2) a small amount of other crystals with the ratio of 1:5=1:1
concomitantly formed.
[18] In the chainlike structure of 6, the occupancy of Na1 is 0.5. Instead
of the Na+ ion, a water molecule with the occupancy of 0.5 exists
near Na1 to connect two molecules of 1 through hydrogen bonds.
Remaining Na4 with the occupancy of 0.25 is positioned around the
cavities of 1 and bridges two ionic 2.
¯
3997–3999; d) R. Harada, Y. Matsuda, H. Okawa, T. Kojima,
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Soc. Dalton Trans. 1999, 3639–3642.
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arrangements: a) L.-Y. Lin, Y.-H. Chen, Z.-Y. Huang, H.-W. Lin, S.-
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[14] We used diethyl ether to obtain crystals of 3 suitable for X-ray anal-
ysis. However, even without diethyl ether, green-brown fine-needle
crystals could be obtained from a solution of a mixture of 1 and 2 in
acetone only.
[15] Crystal data for 4: C70H89Mo12N8Na3O54.5P, Mr =3165.72, crystal di-
[19] C. Sanchez, J. Livage, J. P. Launay, M. Fournier, Y. Jeannin, J. Am.
Chem. Soc. 1982, 104, 3194–3202.
mensions=0.1ꢈ0.1ꢈ0.1 mm3, triclinic, space group P1, a=
¯
13.2016(5), b=17.2227(6), c=25.8373(8) ꢁ, a=72.5210(9), b=
[20] a) V. Artero, A. Proust, Eur. J. Inorg. Chem. 2000, 2393–2400; b) E.
Ishikawa, T. Yamase, Bull. Chem. Soc. Jpn. 2000, 73, 641–649.
84.3774(10), g=68.5483(9)8, V=5214.7(4) ꢁ3, Z=2, 1calcd
=
2.016 gcmÀ3, m=1.514 mmÀ1, l
G
=
[21] It was indicated from the 31P MAS NMR measurement that a more
3À
55.08, reflections measured/unique 48579/22853, Rint =0.0818, R=
0.0993 (I>2s(I)), wR=0.3193 (for all data), max/min residual elec-
probable formula of 3 appears to be (Na+)3·(X)2y·(1)2·(PMo12O40
)
1-
y·(PMo12O405À) · (H2O)7 (X=Na+ or H+, y=ca. 0.1). In
y ACHTNUTRGNENUG(acetone)3·AHCUTNGTRNENGUN
tron density 2.900 and À2.920 eꢁÀ3
.
CCDC reference number
contrast, any redox reactions did not occur with 4 and 6 as judged
from the electronic spectra.
861942. Crystal data for 6: C73H87.5Mo12N6.5Na3O56P, Mr =3203.23,
crystal dimensions=0.25ꢈ0.25ꢈ0.1 mm3, monoclinic, space group
[22] We found a facile and instant synthesis of the binary complex 4
within 5 min. Upon the addition of a CH3CN solution (0.25 mL) of
1 (10 mg, 22 mmol) to a CH3CN solution (0.25 mL) of Na3·2·nH2O
(69 mg, 34 mmol), brown fine crystals (23 mg, 7.3 mmol, 66%) were
obtained after shaking for 5 min. The powder X-ray diffraction pat-
tern of the crystals was identical to that of 4. Their diffuse reflec-
tance UV/Vis/NIR spectra were also shown to be identical with
each other. Complex 6 was also obtained quickly in a similar way
(Figure S14–S17, see the Supporting Information).
C2/m,
96.4542(7)8, V=10524.4(4) ꢁ3, Z=4, 1calcd =2.021 gcmÀ3
1.502 mmÀ1, l
a=30.1400(7),
b=17.4232(4),
c=20.1692(4) ꢁ,
b=
m=
,
(MoKa)=0.71075 ꢁ, T=95 K, 2qmax =55.08, reflections
measured/unique 50541/12400, Rint =0.0405, R=0.0814 (I>2s(I)),
wR=0.1855 (for all data), max/min residual electron density 1.840
and À2.020 eꢁÀ3. CCDC reference number 861943. Crystal data for
12·5·ACHTUNGTRENNUNG(H2O)2: C60H68O16, Mr =1045.19, crystal dimensions=0.2ꢈ0.1ꢈ
3
¯
0.02 mm , triclinic, space group P1, a=10.665(2), b=10.6911(17),
c=12.988(3) ꢁ, a=100.228(6), b=107.240(7), g=109.655(5)8, V=
[23] a) A. Collet, Tetrahedron 1987, 43, 5725–5759; b) M. J. Hardie,
Chem. Soc. Rev. 2010, 39, 516–527.
1267.1(5) ꢁ3, Z=1, 1calcd =1.370 gcmÀ3, m=0.099 mmÀ1, l
ACTHNUGTRNEUGN(MoKa)=
0.71075 ꢁ, T=93 K, 2qmax =49.48, reflections measured/unique 9664/
4291, Rint =0.0764, R=0.0865 (I>2s(I)), wR=0.3069 (for all data),
max/min residual electron density 0.45 and À0.56 eꢁÀ3. CCDC ref-
Received: January 17, 2012
Published online: && &&, 0000
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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COMMUNICATION
Charge-Transfer Complexes
Triple stack: A facile synthesis of
charge-transfer-type crystals composed
of CTV and POM was demonstrated.
The crystals further accommodated
1,4-benzoquinone as a guest to form
a ternary crystal with a donor–
acceptor–acceptor arrangement. The
electronic spectra of the crystals sug-
gested that the incorporated quinone
served as an electronic modulator to
adjust the charge-transfer property.
Shohei Tashiro, Shizuka Hashida,
Mitsuhiko Shionoya*
&&&& — &&&&
A Ternary Charge-Transfer Complex
composed of Cyclotriveratrylene
(CTV) and a Polyoxometalate (POM)
with Quinone as an Electronic Modu-
lator
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!