A novel conjugated [2]rotaxane with an Ru-containing axle constructed from a carboxy-functionalized bis-terpyridyl ruthenium complex and β-cyclodextrin: Synthesis, characterization, and properties

Article abstract of DOI:10.1016/j.jorganchem.2009.10.047

A novel Ru-containing conjugated [2]rotaxane, which utilizes a carboxy-functionalized bis-terpyridyl ruthenium complex as an end group, has been prepared by Suzuki coupling in the presence of β-cyclodextrin (β-CD). The structure has been characterized by NMR spectroscopy and ESI mass spectrometry. Its photophysical properties have shown that insulation by the cyclodextrin reduces the fluorescence intensity compared to that of the free dumbbell.

Full text of DOI:10.1016/j.jorganchem.2009.10.047

Journal of Organometallic Chemistry 695 (2010) 323–326
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A novel conjugated [2]rotaxane with an Ru-containing axle constructed from
a carboxy-functionalized bis-terpyridyl ruthenium complex and b-cyclodextrin:
Synthesis, characterization, and properties
*
Gang Chen, Jianlong Xia, Ziyong Li, Guang-Ao Yu, Shan Jin, Sheng Hua Liu
Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2009
Received in revised form 24 October 2009
Accepted 26 October 2009
Available online 1 November 2009
A novel Ru-containing conjugated [2]rotaxane, which utilizes a carboxy-functionalized bis-terpyridyl
ruthenium complex as an end group, has been prepared by Suzuki coupling in the presence of b-cyclo-
dextrin (b-CD). The structure has been characterized by NMR spectroscopy and ESI mass spectrometry.
Its photophysical properties have shown that insulation by the cyclodextrin reduces the fluorescence
intensity compared to that of the free dumbbell.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
[2]Rotaxane
Bis-terpyridyl ruthenium complex
b-Cyclodextrin
Synthesis
1. Introduction
2. Results and discussion
Recently, some interlocked molecules, such as rotaxanes and
catenanes, have attracted growing attention by virtue of their un-
ique structures and potential applications in molecular devices [1].
Rotaxanes have been prepared by threading an axle into a ring,
then blocking the ends using bulky groups [2,3]. Various organic
functional groups have been introduced as end groups, while tran-
sition metals with ligands have also been employed as component
molecules [4] and in kinetically stabilized rotaxanes [5]. However,
few of the hitherto reported rotaxanes with metal-containing axles
have contained conjugated axles. Bis-terpyridyl metal complexes
possess interesting photophysical properties [6] and play impor-
tant roles in building conjugated functional supramolecular de-
vices [7]. Considering bis-terpyridyl metal complexes as
precursors, coupling reactions can be envisaged as providing a pos-
sible strategy for the construction of conjugated rotaxanes with
metal-containing axles. In this paper, we describe the synthesis
and characterization of a novel Ru-containing conjugated [2]rotax-
ane, which utilizes a carboxy-functionalized bis-terpyridyl ruthe-
nium complex as the stopper and b-cyclodextrin (b-CD) as the
macrocycle.
2.1. Synthesis and characterization
Suzuki coupling in aqueous solution may be used to efficiently
synthesize conjugated rotaxanes, a protocol developed by Ander-
son and co-workers [8]. The [2]rotaxane 9 ꢀ b-CD was readily ob-
tained by Suzuki coupling of diboronic acid 8 and carboxy-
functionalized bis-terpyridyl ruthenium complex 7 in the presence
of b-cyclodextrin and palladium(II) acetate in aqueous potassium
carbonate solution under nitrogen at 50 °C (Scheme 1) [8,9].
9 ꢀ b-CD was isolated in 35% yield by preparative thin-layer chro-
matography (eluent: methanol/butanol/ammonia, 5:5:2). The free
dumbbell 9 was also synthesized by using the same conditions as
those for the synthesis of 9 ꢀ b-CD but in the absence of b-
cyclodextrin.
Both 9 ꢀ b-CD and the dumbbell 9 are poorly soluble in the
usual solvents (CH3CN, DMF, CH3OH and DMSO) for bis-terpyridyl
ruthenium complexes, such that their maximum concentrations
are insufficient for NMR spectroscopy. However, these compounds
dissolve well in CH3OH or DMSO containing a little acid. The poor
solubility of these compounds may result from loss of the elements
of HPF6, and the addition of acid will inhibit this process [10].
The structure of the present rotaxane was confirmed by NMR
spectroscopy and ESI mass spectrometry. In the mass spectrum
of 9 ꢀ b-CD ammonium salt, the molecular ion corresponds to
the tetraanion formed by the loss of four ammonium cations
(Fig. S4). NMR spectra of 9 ꢀ b-CD and dumbbell 9 were obtained
* Corresponding author.
E-mail address: chshliu@mail.ccnu.edu.cn (S.H. Liu).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.10.047

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G. Chen et al. / Journal of Organometallic Chemistry 695 (2010) 323–326
Scheme 1. Synthesis of 9 ꢀ b-CD.
from samples dissolved in d6-DMSO containing a small quantity of
d-TFA. As shown in Fig. 1a, the aromatic protons of dumbbell 9
gave rise to simple peaks, indicating the formation of a symmetric
structure. However, the aromatic protons of 9 ꢀ b-CD showed
splitting and peak shifts owing to the asymmetric structure in-
cluded in the b-CD cavity (Fig. 1b). For example, the signals of
the aromatic protons B and C in dumbbell 9 are seen as single
peaks, but the signals of the corresponding aromatic protons in
9 ꢀ b-CD split into two separate resonances due to B/B0 and C/C0.
The signals of protons D/D0, E/E0, and F/F0 of the axle in 9 ꢀ b-CD
are shifted to higher field with respect to those of free dumbbell 9.
Significant differences in the resonance frequencies of the free
and interlocked b-CD are also evident when comparing the corre-
sponding 1H NMR spectra (Fig. 2). The signals of protons H3/H5/
H6/H60 and H2/H4 of interlocked b-CD (Fig. 2a) are resolved into
discrete peaks compared with those of free b-CD (Fig. 2b) [11]. This
can be attributed to the aromatic ring current of the threaded
guest. The above results confirm the formation of a [2]rotaxane
with interlocked b-CD.
A 2D NOESY experiment provided further evidence for the for-
mation of 9 ꢀ b-CD (Fig. 3). NOEs between the two components
Fig. 1. 1H NMR (600 MHz) spectra (in d6-DMSO with a trace of d-TFA, 25 °C): (a)
dumbbell 9; (b) 9 ꢀ b-CD.
Fig. 2. 1H NMR (600 MHz) spectra (in d6-DMSO with a trace of d-TFA, 25 °C): (a) 9 ꢀ b-CD; (b) dumbbell 9 and an equimolar amount of free b-CD.

G. Chen et al. / Journal of Organometallic Chemistry 695 (2010) 323–326
325
Fig. 3. 1H NOESY NMR (600 MHz) spectrum of 9 ꢀ b-CD (in d6-DMSO with a trace of d-TFA, 25 °C).
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
a
b
1400
1200
1000
800
600
400
200
0
550
600
650
700
750
800
850
300
400
500
600
700
Wavelength / nm
Wavelength / nm
Fig. 4. (a) Absorption spectra of dumbbell 9 (pink line) and 9 ꢀ b-CD (green line), each at 10
lm in methanol solution; (b) fluorescence spectra of dumbbell 9 (pink line) and
M in ethanol/methanol (4:1, v/v) glass (kex = 510 nm). (For interpretation of the references to colour in this figure legend, the reader is
9 ꢀ b-CD (green line) at 77 K, at 10
l
referred to the web version of this article.)
showed that the cyclodextrin was mainly located on the central
axle. As shown in Fig. 3, protons H3 of the wider 2,3-rim and H5
of the narrower 5,6-rim of the b-cyclodextrin in 9 ꢀ b-CD show
NOEs to the axle protons B, C, D, E, F, F0, E0, D0, C0, and B0. Protons
H6 and H60 of the b-cyclodextrin show NOEs to proton A0 of the
bis-terpyridyl ruthenium component.
The redox behaviors of 9 ꢀ b-CD and dumbbell 9 (1 mm in
CH3CN) have been investigated by cyclic voltammetry and
square-wave voltammetry techniques with 0.1 M n-Bu4NPF6 as
the supporting electrolyte. TFA (50 mM in CH3CN) was added to in-
crease the solubility. 9 ꢀ b-CD and dumbbell 9 exhibit quasi-
reversible oxidation waves at 1.126 and 1.128 V, respectively,
which can be ascribed to a one-step two-electron process, corre-
sponding to RuI2I;II=Ru2III;III oxidation. There is no electronic commu-
nication between the two metal centers in either 9 ꢀ b-CD or
dumbbell 9 because of the long distance between them.
The thermal stability of 9 ꢀ b-CD has been examined by heating
at 80 °C in d6-DMSO for three days. 1H NMR spectra recorded be-
fore and after the heating did not show any noticeable differences,
indicating that unthreading did not take place. Even after one day
at 100 °C in d6-DMSO, no sign of unthreading was detected.
2.2. Physical properties
The UV/Visible absorption and fluorescence spectra of 9 ꢀ b-CD
have been compared with those of free dumbbell 9 (Fig. 4). Both
9 ꢀ b-CD and the dumbbell 9 display intense absorption bands in
the visible region due to metal-to-ligand charge-transfer (1MLCT)
transitions, as well as broad intense bands in the UV region attrib-
utable to ligand-centered (1LC) transitions (Fig. 4a) [12]. The two
complexes display very weak fluorescence at room temperature
because of the short fluorescence lifetimes. In a frozen glass at
77 K, however, intense fluorescence spectra can be obtained [13].
When 9 ꢀ b-CD and the dumbbell are excited at 510 nm, 9 ꢀ b-
CD emits with kmax = 641.6 nm while dumbbell 9 emits with
kmax = 643.4 nm, but the former band is of somewhat lower inten-
sity than the latter (Fig. 4b). This result is in agreement with that
obtained by Anderson [14] for a symmetrical heptamethine chro-
3. Conclusion
In summary, we have reported an effective synthesis of a novel
conjugated [2]rotaxane with an Ru-containing axle that utilizes
carboxy-functionalized bis-terpyridyl ruthenium complexes as
end groups. The photophysical properties of the [2]rotaxane have
shown that insulation by the cyclodextrin reduces the fluorescence
mophore threaded through a-cyclodextrin.

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G. Chen et al. / Journal of Organometallic Chemistry 695 (2010) 323–326
intensity compared to that of the free dumbbell. This synthetic
strategy offers a potential method for the construction of homo-
dyad (same metal ions) and heterodyad (different metal ions) me-
tal-containing conjugated rotaxanes bearing bis-terpyridyl metal
(Fe, Ru, Os, Ir, etc.) complexes as end groups.
7.64 (d, J = 5.4 Hz, 8H), 7.59 (d, J = 6.6 Hz, 4H), 2.54 (s, 6H). ESI-
MS m/z calcd for C106H78N16O16Ru2 2034.4; found, [Mꢁ4NH4]4ꢁ
489.7.
Acknowledgments
The authors acknowledge financial support from National Nat-
ural Science Foundation of China (Nos. 20572029, 20772039, and
20931006).
4. Experimental
4.1. General
Unless otherwise noted, all starting materials were purchased
from commercial sources and were used without further purifica-
tion. Preparative thin-layer chromatography (TLC) was performed
using glass plates precoated with GF 254. 1H NMR and 13C NMR
spectra were recorded on Varian Mercury Plus-400 and Unityino-
va-600 with 13C{1H} cryoprobe. NOESY was performed on VARIAN
UNITYINOVA-600. Electron impact ionization (EI) mass spectra
were carried on Trace MS 2000, respectively electrospray ioniza-
tion (ESI) mass spectra were carried on Finnigan LCQ. Elementary
analysis was obtained on Vario EL III. UV–Vis spectra were per-
formed on Hitachi U-3310. Fluorescence data were recorded on
Hitachi Model F-4500. The syntheses of intermediates 2, 5 and 8
have been reported [15]. The synthetical procedures of compounds
1, 3, 4, 6 and 7 were shown in Supplementary material.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.10.047.
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