Novel approach to stabilize unstable molecular wires by simultaneous rotaxane formation - Synthesis of inclusion complexes of oligocarbynes with cyclic host molecules

Article abstract of DOI:10.1002/ejoc.200700630

An effective method to stabilize oligocarbynes as a model for polycarbyne by the simultaneous complexation with α-cyclodextrin (α-CD) is described. 1,12-Bis(9-phenanthryl)-1,3,5,7,9,11-dodecahexayne possessing one or two α-CD molecules was prepared by the

Full text of DOI:10.1002/ejoc.200700630

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200700630
Novel Approach to Stabilize Unstable Molecular Wires by Simultaneous
Rotaxane Formation – Synthesis of Inclusion Complexes of Oligocarbynes with
Cyclic Host Molecules
[a]
[a]
Jiro Sugiyama and Ikuyoshi Tomita*
Keywords: Rotaxane / Cyclodextrin / Inclusion compounds / Carbynes
An effective method to stabilize oligocarbynes as a model
for polycarbyne by the simultaneous complexation with
α-cyclodextrin (α-CD) is described. 1,12-Bis(9-phenanthryl)-
of the corresponding triyne in the presence of α-CD. The re-
sulting rotaxanes proved to be stable enough in comparison
with the corresponding free oligoynes.
1
,3,5,7,9,11-dodecahexayne possessing one or two α-CD
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
molecules was prepared by the oxidative coupling reaction
Introduction
tallic fragments often serve as good end groups for the syn-
thesis of molecules having longer carbyne chains. For ex-
ample, Gladysz and co-workers reported the synthesis of
oligoynes having (pentafluorophenyl)bis(phosphane)plati-
num end groups, in which oligoynes having up to 16 acetyl-
ene units could be successfully isolated and fully characteri-
Among polymers possessing π-conjugated repeating
units, polycarbyne has the simplest one-dimentional struc-
ture, in whitch sp-hybridized carbon atoms are linked
through σ- and π-bonds. Despite its potential applications
to unique functional materials exhibiting optical, electrical,
and mechanical properties, its instability and poor solubil-
ity made it difficult to perform the related studies.
One possible approach to improve its stability and poor
solubility is to introduce appropriate end groups on both
ends of the carbyne chains. Whiting et al. have described
the synthesis and characterization of a series of oligoynes
having phenyl end groups with up to eight triple bonds.
UV/Vis spectra of the series of oligoynes suggested the ef-
fective π-conjugation along the acetylene units, because the
wavelength of each absorption band shows a clear bathoch-
[1e]
zed.
However, the stabilization effect of end groups becomes
not sufficient if the chain length is increased. In order to
obtain much longer carbyne chains as stable molecules, a
spatial protection might be required to insulate the carbyne
chains form other moecules. Gladysz et al. reported a novel
method to insulate the carbyne chains possessing platinum
moieties on both ends by use of bis(phosphanyl)alkanes in
which alkane chains are supposed to insulate each carbyne
chain by the formation of a double-helix conformation
[1f]
around the carbyne chain. Nevertheless, this method may
[
1a]
romic shift by the elongation of the acetylene units.
almost linear alignment of the carbyne chain has been sup-
ported by the single-crystal X-ray structural investigation
An
also require sequential synthetic steps, which would limit
the length of the carbyne chain by the instability of “naked”
intermediates.
As an alternative method, we are trying to stabilize and
to solubilize the carbyne chains by simultaneous formation
of oligoynes such as 1,10-diphenyl-1,3,5,7,9-decapentay-
ne[
1b]
and 1,12-diphenyl-1,3,5,7,9,11-dodecahexayne.[1c]
Hirsh and co-workers prepared oligoynes having dendritic
end groups, whose stability increased as a function of the
bulkiness of the end groups. For example, the third-genera-
tion dendrimer-capped molecule having ten acetylene units
of pseudo-rotaxanes and rotaxanes with cyclodextrin
[2]
(
CD). Herein, we wish to show this possibility using oli-
goynes as preliminary models of polycarbyne.
[
1d]
is stable enough to be isolated and characterized.
Not
only the sterically demanding groups, but also organome-
Results and Discussion
As oligoynes for the present study, we employed
Chemistry, Interdisciplinary Graduate School of Science and 1,3,5,7,9,11-dodecahexaynes (C12) having two aromatic
[
a] Tokyo Institute of Technology, Department of Electronic
Engineering
substituents on α- and ω-positions [i.e., 1,12-diphenyl- (P-
4
259-G1-9 Nagatsuta-cho, Midori-ku, Yokohama, 226-8502,
Japan
C12) and 1,12-bis(9-phenanthryl)- (PH-C12), Figure 1], be-
cause their moderate instability both in bulk and solution
at ambient conditions is suitable to demonstrate the stabili-
zation effect.
Fax: +81-45-924-5489
E-mail: tomita@echem.titech.ac.jp
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2007, 4651–4653
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4651

J. Sugiyama, I. Tomita
SHORT COMMUNICATION
Figure 1. Structure of oligoynes [P-C12, PH-C12, and PH-C12-
n
(TBDMS-α-CD) ].
The UV/Vis absorption spectra of P-C12 taken in a di-
lute solution of N,N-dimethylformamide (DMF) ([P-C12]
0
–
5
=
1
1.0ϫ10 ) exhibits a gradual irreversible change after
h, indicating that intermolecular reactions occur under
[5]
the examined conditions (Figure 2a). On the other hand,
the corresponding change could be suppressed significantly
in the presence of an excess amount of α-CD ([α-CD] =
0
–
3
1
.0ϫ10 ) (Figure 2b). The result can be taken to mean
Figure 2. Change in the UV/Vis absorption spectra of P-C12 in
that the formation of a pseudo-rotaxane of P-C12 and
α-CD takes place simply by mixing the two compounds
in DMF, which is effective to stabilize P-C12 by preventing 1.0ϫ10  (b), and 1.0ϫ10  (c)}.
it from intermolecular crosslinking reactions. In this case,
DMF without (a) and with (b, c) α-CD after 1 h at ambient tem-
–5
0 0
perature {conditions: [P-C12] = 1.0ϫ10 , [α-CD] = 0  (a),
–
3
–5
however, an excess amount of α-CD is required to stabilize
P-C12 (e.g., stoichiometric conditions; Figure 2c), because ride/imidazole to improve their solubility. Consequently, ro-
the complex of P-C12 and α-CD, a pseudo-rotaxane, is in taxanes of PH-C12 possessing one and two α-CD molecules
an equilibrium with the dissociated form.
[PH-C12-(TBDMS-α-CD) and PH-C12-(TBDMS-α-CD) ]
1
2
The complexation also takes place by bringing in contact were produced in 12% and 17% yields, respectively. Under
the crystals of P-C12 with an aqueous solution of α-CD the examined reaction conditions, about 61% and 51% of
(0.15 ) under ultrasonication conditions. From the UV/Vis the hydroxy groups of these rotaxanes could be silylated,
spectra of the water-soluble part obtained by filtration of and the resulting rotaxanes [PH-C12-(TBDMS-α-CD) and
1
the emulsified product, the characteristic absorption bands PH-C12-(TBDMS-α-CD) , respectively] exhibited good sol-
2
of P-C12 were observed.[ Hexagonal crystals of the ubility in common organic solvents such as chloroform and
6]
pseudo-rotaxane of P-C12 and α-CD were obtained by THF.
recrystallization of an aqueous solution. The crystals exhib-
A remarkable stabilization effect of α-CD is clearly ob-
ited higher thermal stability than the “naked” P-C12 judg- servable from the time-course UV/Vis absorption spectra of
ing from the measurements with an optical microscope PH-C12-(TBDMS-α-CD)1 and PH-C12-(TBDMS-α-CD)2
equipped with a hot stage. That is, the crystals did not show in DMF: the spectra of both PH-C12-(TBDMS-α-CD)1
a color darkening below 270 °C (i.e., the melting point of and PH-C12-(TBDMS-α-CD)2 exhibit no significant
α-CD), while the free P-C12 exhibited a noticeable color change on keeping at ambient temperature for 1 h (Fig-
change above 130 °C and the resulting dark sample was in- ure 3b and Figure 3c), while the “naked” PH-C12 is some-
soluble in organic solvents.
what unstable under the same conditions (Figure 3a). It is
In order to stabilize oligocarbynes more effectively, the also of note that the bulky phenanthryl groups prohibit the
simultaneous synthesis of rotaxane molecules was per- unthreading of α-CD, leading to a much more effective sta-
formed by locking α-CD with much sterically demanding bilization of the oligocarbyne in comparison with the case
end groups. That is, a hexayne having phenanthryl substitu- of the pseudo-rotaxane of P-C12.
ents on both ends (PH-C12) was prepared by oxidative
In summary, we reported a method to stabilize oligoynes
coupling reaction of a 1:1 pseudo-rotaxane of 1-phenan- not by direct structural modification but by “simultaneous
thryl-1,3,5-hexatriyne (PH-C6-H)/α-CD in a mixed solvent formation of an inclusion complex with α-CD”. Since the
[
7]
of DMF/H O (1:2, v/v) containing α-CD (3 equiv.). The complexation with α-CD is confirmed to be a quite effective
2
rotaxanes are insoluble in water and many organic solvents method to improve the stability of oligocarbynes, we are
except that they could be dissolved slightly in DMF. Thus, currently trying to extend the chain length of the carbyne
the chemical modification of the hydroxy groups of the system and elucidate the nature of this series of unique
α-CD units was performed using tert-butyldimethylsilyl chlo- functional materials.
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Eur. J. Org. Chem. 2007, 4651–4653

Novel Approach to Stabilize Unstable Molecular Wires
R. McDonald, F. A. Hegmann, R. R. Tykwinski, Org. Lett.
004, 7, 51–54; d) A. Gibtner, F. Hampel, J. Gisselbrecht, A.
2
Hirsh, Chem. Eur. J. 2002, 8, 408–432; e) W. Mohr, J. Stahl, F.
Hampel, J. A. Gladysz, Chem. Eur. J. 2003, 9, 3324–3340; f) J.
Stahl, J. C. Bohling, E. B. Bauer, T. B. Peters, W. Mohr, J. M.
Alvarez, F. Hampel, J. A. Gladysz, Angew. Chem. Int. Ed. Engl.
2
002, 41, 1871–1876.
[3]
[
[
2] There are preceding works on stabilization and improvement
of solubility[ of guest molecules by formation of inclusion
complexes with cyclic host molecules.
4]
3] Some unstable molecules have been reported to be stabilized
by the formation of inclusion complexes with cyclic host mole-
cules, see a) J. Buston, J. Young, H. Anderson, Chem. Commun.
2
000, 905–906; b) T. Oku, Y. Furusho, T. Takata, Org. Lett.
2
003, 5, 4923–4925.
[
4] Improvement of the solubility by the formation of inclusion
complexes has also been reported, see: a) H. Dodziuk, A. Ejch-
art, W. Anczewski, H. Ueda, E. Krinichnaya, G. Dolgonos,
W. Kutner, Chem. Commun. 2003, 986–987; b) A. Harada, H.
Okumura, M. Okada, S. Suzuki, M. Kamachi, Chem. Lett.
2
000, 548–549.
[
5] From the MS measurement of a sample kept in DMF for 1 h,
degraded products attributable to dimers were observed.
Therefore, the change observed in the UV/Vis spectra of P-C12
is at least partly due to oligomerization reactions.
[
6] The crystals of P-C12 (0.030 g, 0.10 mmol) were brought in
contact with an aqueous solution of α-CD (0.15 , 5.0 mL)
under ultrasonication for 6 h. On the basis of the molar extinc-
tion coefficient of the UV/Vis spectrum of P-C12 in DMF ([P-
Figure 3. UV/Vis absorption spectra of PH-C12 (a), PH-C12-
(
TBDMS-α-CD)
before (solid line) and after 1 h (dotted line) {conditions:
PH-C12] = [PH-C12-(TBDMS-α-CD) = [PH-C12-(TBDMS-α-
CD) = 1.0ϫ10 , at ambient temperature}.
1 2
(b), and PH-C12-(TBDMS-α-CD) (c) in DMF
–
5
5
[
0
1
]
0
C12]
in the aqueous solution obtained by filtration of this mixture
0
= 1.0ϫ10 , ε = 1.1ϫ10 ), the concentration of P-C12
–5
2 0
]
–
6
was estimated to be 1.5ϫ10 .
[
7] If the oxidative homo-coupling reaction of the free PH-C6-H
was performed in the presence of α-CD (i.e., without prior
formation of pseudo-rotaxane), the rotaxanes were obtained
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, 1H NMR spectra of TBDMS-α-CD,
photographs of crystals of the pseudorotaxanes.
in lower yields [PH-C12-(TBDMS-α-CD) : 7%; PH-C12-
1
2
(TBDMS-α-CD) : 5%]. Also, if the homo-coupling reaction of
the pseudo-rotaxane of PH-C6-H and α-CD was carried out in
the absence of an excess amount of α-CD, the rotaxanes were
[
1] a) J. B. Armitage, N. Entwisle, E. R. H. Jones, M. C. Whiting,
J. Chem. Soc. 1954, 147; b) Y. Rubin, S. S. Lin, C. B. Knobler,
J. Anthony, A. M. Boldi, F. Diederich, J. Am. Chem. Soc. 1991,
produced in lower yields [PH-C12-(TBDMS-α-CD)
C12-(TBDMS-α-CD) : 4%].
1
: 5%; PH-
2
Received: July 9, 2007
Published Online: August 22, 2007
113, 6943–6949; c) T. Luu, E. Elliott, A. D. Slepkov, S. Eisler,
Eur. J. Org. Chem. 2007, 4651–4653
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4653