Stereoselective optical sensing of dicarboxylate anions by an induced- fit type Ru(II) receptor

Article abstract of DOI:10.1016/S0040-4039(00)00635-3

An induced-fit type Ru(II) receptor 1 has been synthesized in which, upon approach of dicarboxylates, a highly flexible tetraamide binding site is organized to complement their chemical structures. Stereoselective optical sensing has been demonstrated by virtue of the luminescence response of 1 to the binding of cis/ trans-1,4-cyclohexanedicarboxylates. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00635-3

Tetrahedron Letters 41 (2000) 4583±4586
Stereoselective optical sensing of dicarboxylate anions by an
induced-®t type Ru(II) receptor
Shigeru Watanabe,* Natsuka Higashi, Michiko Kobayashi, Kaori Hamanaka,
Yoshiyuki Takata and Katsuhira Yoshida
Department of Material Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan
Received 24 January 2000; revised 12 April 2000; accepted 14 April 2000
Abstract
An induced-®t type Ru(II) receptor 1 has been synthesized in which, upon approach of dicarboxylates, a
highly ¯exible tetraamide binding site is organized to complement their chemical structures. Stereoselective
optical sensing has been demonstrated by virtue of the luminescence response of 1 to the binding of cis/
trans-1,4-cyclohexanedicarboxylates. # 2000 Elsevier Science Ltd. All rights reserved.
Optical and electrochemical sensing of anionic species in aqueous and nonaqueous media is an
area of great interest in current research.1 4 Beer recently described photo- and redox-responsive
anion receptors, based on Lewis acidic centres and amide binding sites.5 In particular, macro-
cyclic Ru(II) receptors showed electrochemical and optical sensing of anions with high binding
selectivity.6 As part of our own e€orts to develop new photoresponsive anion receptors,7 we have
designed an induced-®t type Ru(II) receptor 1 in which, upon approach of dicarboxylates,
conformational changes occur to organize a binding site for achieving optimal complexation.
Here we report the synthesis and binding properties of the Ru(II) receptor 1, and present stereo-
selective optical sensing of dicarboxylate anions.
Mono-N-acetylation of 4,40-methylenedianiline, followed by coupling to 4,40-bis(chlorocarbonyl)-
2,20-bipyridine8 at room temperature in the presence of Et3N in N,N0-dimethylacetamide (DMAC),
9
gave tetraamide bipyridine in 48% yield. Tetraamide bipyridine reacted with Ru(bpy)2Cl2
(bpy=2,20-bipyridine) at 100ꢀC in 90% DMAC for 24 h to give 1 in 35% yield.10
The UV±vis titrations with aromatic and aliphatic dicarboxylates in DMSO showed clear
structure dependence. In a series of 1,x-(CO2 )2C6H4 (x=2±4) and 2OC(CH2)nCO2 (n=1±8),
isophthalate (x=3), terephthalate (x=4), adipate (n=4) and pimelate (n=5) caused a slight blue
shift of the MLCT band with distinct isosbestic points, whereas the other dicarboxylates provided
* Corresponding author. Tel: +81 88 844 8301; fax: +81 88 844 8360; e-mail: watanabe@cc.kochi-u.ac.jp
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00635-3

4584
no isosbestic points. Alicyclic dicarboxylates such as 1,3-adamantane- and 1,4-cyclohexane-
dicarboxylates (n=3, 4) caused the most pronounced changes in the MLCT band with isosbestic
points.
Optical sensing of dicarboxylates was more dramatically demonstrated by virtue of the lumi-
nescence response to the complexation (Table 1). Addition of iso- or tere-phthalate to a solution
of 1 in DMSO resulted in a slight blue shift and a marked increased intensity of the MLCT
emission band. A similar spectral change was observed upon addition of p-toluene but it was
much less pronounced. Interestingly, the luminescence spectrum of 1 decreased by 3±5% until 1
equiv. of adipate or pimelate was added to the DMSO solution, and then increased by 16±19%.
There was no increase but a signi®cant decrease of emission intensity (ÁI=±15%) upon addition
of 1,3-adamantanedicarboxylate. Interestingly, the emission intensity of the MLCT emission
band of 1 was enhanced in the presence of trans-1,4-cyclohexanedicarboxylate but diminished by
the cis-isomer. The magnitude and the direction of the emission intensity change was strikingly
guest geometry-dependent. Such optical properties have not hitherto been observed for Ru(II)
receptors with a preorganized amide binding site to complement the size and shape of anions.
Table 1
Binding constants and luminescence changes of 1 with carboxylate anions in DMSO at 293 Ka
1H NMR titrations were carried out with the dicarboxylates which produced absorption spectral
changes going through isosbestic points on the UV±vis titrations. Addition of the dicarboxylate
to a solution of 1 in DMSO-d6 led to chemical shift changes in all of the tetraamide bipyridine
protons. The inner amide NH and the 3,30-bpy CH shifted down®eld by 2.03±2.77 and 0.22±0.56
ppm, respectively. The important down®eld shift of 0.41±0.63 ppm was also observed in the outer
amide NH. There are two possible complexation processes 1!2!4 and 1!2!5 to explain these
changes (Scheme 1). However, the outer amide NH shifted down®eld only by 0.13 ppm when
p-toluate, incapable of forming the complex 4, was added to a solution of 1 in DMSO-d6. These
results suggest that the induced-®t type complex 4 could be formed upon binding of dicarboxylate
anions. Additional changes were observed in the 1H NMR titrations. Both chemical shift changes
of the inner amide NH and the 3,30-bpy CH reached a plateau near 1±2 equiv. of dicarboxylates,
whereas no saturation characteristics were observed in the chemical shift changes of the outer

4585
amide NH, re¯ecting the complex 4 as being in equilibrium with the complex 2. Monitoring the
chemical shift changes of the outer amide NH as a function of dicarboxylate concentration led to
the sigmoidal titration curves. This indicates that the induced-®t type complex 4 undergoes
further conformational changes to form the 1:3 complex 5 on increasing the concentration of
dicarboxylates. Binding to alicyclic dicarboxylates formed a 2:1 receptor to anion complexes 3
under receptor excess conditions. In fact, the 3,30-bpy CH initially shifted down®eld until 0.5
equiv. of the alicyclic dicarboxylate was added to the solution; in¯ection occurred and then
moved up®eld.
Scheme 1.
All titration curves were analyzed by using nonlinear curve-®tting procedures11 and the calculated
binding constants were corrected in Table 1. Dicarboxylates form complexes of much higher
stability with 1 than monocarboxylates, re¯ecting a major role of electrostatic interactions in
anion binding. A variety of aromatic and aliphatic dicarboxylates are bound with considerable

4586
stability (Ka>104 M). The high adaptability of 1 to guest structures is also re¯ected in the poor
binding selectivity. Despite their closely similar binding constants, the Ru(II) receptor 1 is capable
of optically discriminating between the geometrical isomers of 1,4-cyclohexanedicarboxylate.
Under the luminescence titration conditions, the formation of the complex 3 was negligible
because of the low binding constants (K2<100 M^1). Neither complexes 2 or 5 are likely to
produce di€erent luminescence signals for these geometrical isomers. The stereoselective optical
sensing of 1,4-cyclohexanedicarboxylates would result from the formation of the induced-®t type
complex 4. The induced-®t complexation rigidifying the receptor structure and thereby inhibiting
non-radiation decay processes can induce intensity enhancement of the MLCT emission band of
1. However, receptor conformations induced by 1,3-adamantane- and cis-1,4-cyclohexane-
dicarboxylates might open non-radiation decay channels via vibrational relaxation and excitation,
which compete with the enhanced radiation decaying processes.12
In conclusion, we have shown that an induced-®t binding site incorporated in the photoactive
Ru(II) centre can produce the stereoselectivity in optical sensing of dicarboxylate anions. Further
investigation is in progress to clarify the e€ect of guest-induced conformational changes on the
optical sensing mechanism.
Acknowledgements
We thank Professor Michael J. Hynes for access to and assistance with his software for binding
constant analysis. S. Watanabe also thanks the Ministry of Education, Science, Sports and Culture
in Japan for ®nancial support by a Grant-in-Aid for Exploratory Research.
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1
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