A luminescent metalloreceptor exhibiting remarkably high selectivity for Mg2+ over Ca2+ [1]

Full text of DOI:10.1021/ja010931q

8402
J. Am. Chem. Soc. 2001, 123, 8402-8403
Scheme 1a
A Luminescent Metalloreceptor Exhibiting
Remarkably High Selectivity for Mg2+ over Ca2+
Shigeru Watanabe,* Shinsuke Ikishima, Takeshi Matsuo, and
Katsuhira Yoshida
Department of Material Science
Faculty of Science, Kochi UniVersity
Akebono-cho, Kochi 780-8520, Japan
ReceiVed April 11, 2001
Fluorescence spectroscopy is the most powerful research tool
for visualizing the structure and dynamics of living systems at
the molecular level.1-4 During the last two decades, extensive
efforts have been exerted to develop fluorescent probes for
biologically relevant ions and molecules.5-7 Effective intracellular
probes for alkali and alkali earth metal ions are commercially
available at present.8 However, there is still a need for probes
with an improved specific response, in particular, in binding
selectivity for Mg2+ over Ca2+; under high calcium concentrations,
it is often difficult to discriminate the effects of Mg2+ and Ca2+
because almost all of the current Mg2+ probes are a variant of
the Ca2+ probes, which respond to these two cations with identical
spectral changes.9 Numerous approaches to enhance the Mg2+
/
Ca2+ selectivity have predominantly exploited differences in the
size of these cations, the so-called “size selectivity”. To find
a i, 4,4′-bis(chlorocarbonyl)-2,2′-bipyridine,10 Et3N, DMAC, rt, 12 h;
ii, N-methylmorpholine, 20% DMAC/MeOH, 70 °C, 7 days; iii,
Ru(bpy)2Cl2, 50% EtOH, reflux, NH4PF6; iv, 10% aq Et4NOH, EtOH.
another possible way to discriminate between Mg2+ and Ca2+
,
we have designed an induced-fit type of cation chelator in which,
upon approach of a desired cation, the binding site can be
selectively organized against the electrostatic repulsion of two
negative ligands. Here we report the synthesis and optical ion-
sensing properties of a metalloreceptor 6 (Figure 2), in which
the induced-fit chelator is incorporated into a luminescent tris-
(bipyridyl) ruthenium complex.
The synthesis of 6 is summarized in Scheme 1. The starting
aniline 1 was prepared by reaction of m-nitrobenzyl bromide and
trimethyl phosphite at 100 °C for 2 h, followed by reduction with
NaBH4/NiCl2 in 77% overall yield. Coupling of 1 and 4,4′-bis-
(chlorocarbonyl)-2,2′-bipyridine10 in N,N′-dimethylacetamide
(DMAC) gave 2 in 65% yield. Demethylation of 2 with
N-methylmorpholine gave 3 in 98% yield. Condensation of 3 with
11
Ru(bpy)2Cl2 (bpy ) 2,2′-bipyridine) followed by addition of
Figure 1. Change in the UV-vis absorption and luminescence spectra
of 6 upon addition of Mg(ClO4)2 in DMSO at 293K: [6] ) 2.0 × 10-5
M, [Mg2+] ) 0-6.0 × 10-4 M. Excitation was at 489 nm.
NH4PF6 gave 5 in 92% yield. The acid form 5 was converted to
a deprotonated form by treating with 10% aqueous Et4NOH, and
then its counterions were removed by washing with organic
solvents to give free zwitterionic metalloreceptor 6 in 91% yield.
A metalloreceptor 4, which does not have negatively charged
ligands, was also prepared in analogy to 5, starting from 2.12
Absorption and luminescence titration of 6 with alkali (Li+,
Na+, K+) and alkali earth (Mg2+, Ca2+, Ba2+) metal ions were
preliminarily performed in DMSO at 20 °C. Upon addition of
these cations except for Mg2+, 6 showed no change in absorption
and luminescence spectra. In contrast, the addition of Mg2+ led
to a blue-shift of the MLCT absorption band with clear isosbestic
points at 489, 422, and 368 nm (Figure 1a). The luminescence of
6 was also affected by addition of Mg2+, significantly increasing
the MLCT emission band with a slight blue-shift (5 nm) as shown
in Figure 1b. Thus, the optical response of 6 for cation binding
is highly selective for Mg2+. Titration of 4 with the cations was
also carried out under the same conditions. The absorption and
luminescence spectra of 4 showed no change upon addition of
any cations. Electrostatic attraction between Mg2+ and negative
phosphinates is responsible for the cation binding observed in 6.
It is interesting to note that, despite the flexible binding site, 6
exhibits perfect selectivity for Mg2+ over Ca2+. The complexation
process was further studied by 1H NMR spectroscopy. When 0-5
equiv of Mg2+ was added to a 3 × 10-3 M solution of 6 in
DMSO-d6, the most pronounced upfield shift (>0.5 ppm) occurred
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1999.
(8) Handbook of Fluorescent Probes and Research Chemicals; Spence, M.
T. Z., Ed.; Molecular Probes: Eugene, OR, 1996.
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C300.
(10) Whittle, C. P. J. Heterocycl. Chem. 1977, 14, 191.
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3334.
(12) All new compounds have been characterized by IR, 1H NMR, and
mass spectrometry, and give satisfactory elemental analyses. The experimental
details will be reported elsewhere.
10.1021/ja010931q CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/07/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8403
Ca2+ (0.24) < Li+ (0.33) , Mg2+ (0.75). Magnesium cation has
the highest charge density in a series of metal ions, which is most
suitable to organize the binding pocket in the face of the
electrostatic repulsion of the negative phosphinates.
To explore solvent effects on the high Mg2+ selectivity of 6,
binding and optical sensing of metal ions were examined in
different ratios of H2O/DMSO. In percentages between 0 and 15,
6 responded strongly and selectively to the presence of Mg2+
.
As H2O percentages approached and exceeded 15, 1:1 and 2:1
complexes of 6 and Mg2+were dominantly formed. At 15% H2O/
DMSO, binding constants K1 and K2 for Mg2+ were calculated
from the titration data using Hyperquad2000 computer program14
to be 19800 ( 3400 and 1800 ( 760 M-1, respectively. An 80%
increase in the emission intensity was observed over the 0-0.5
mM Mg2+ concentration range (see Supporting Information S4
and S7). At 20-25% H2O, the luminescence enhancement became
negligible (<5%). The complexes were too weak to estimate the
accurate value of Ka’s. However, the high selectivity for Mg2+
was still observed even in 25% H2O/DMSO. The enhanced
emission intensity can be explained by the induced-fit complex-
ation that decreases the high conformational flexibility of 6, and
thereby nonradiative decay processes of vibrational and rotational
relaxation are inhibited.
Figure 2. Proposed structure for the 1:1 and 2:1 complexes formed
between 6 and Mg2+
.
Although the binding nature of 6 is still not completely
elucidated, it is clear from the present study that “size selectivity”
is not only the way to distinguish between Mg2+ and Ca2+. The
highly selective optical sensing of Mg2+ would be attainable by
using an appropriate induced-fit receptor of phosphinate-lumi-
nophore conjugate. We are continuing our work on this topic.
in the P-OCH3 protons. The significant downshift was also
observed in the P-CH2- protons. These spectral changes indicate
the coordination of Mg2+ to the phosphinate groups. In addition,
the phenyl and 3,3′-bipyridyl protons showed distinct chemical
shift changes. There was a marked tendency for the resonances
to saturate before 1 equiv of Mg2+ was added to the solution.
These results suggest multiple equilibria with the possible binding
of Mg2+ with two and more metalloreceptors 6. Figure 2 shows
possible binding geometries for the 1:1 and 2:1 complexes. The
complex formation is accompanied with a dramatic structural
rearrangement of the binding site. Absorption and luminescence
titrations described above clearly indicate that Mg2+ can selec-
tively organize the binding site to the optimal binding geometry.
As the highly flexible binding site of 6 can accommodate all of
the cations tested, the high preference for Mg2+ cannot be
explained by “size selectivity.” As shown in Figure 2, the negative
phosphinates need to come close to the coordination position upon
complexation. The charge densities 13 of the metal ions increase
in following order: K+ (0.05) < Na+ (0.10) < Ba2+ (0.13) <
Acknowledgment. S.W. thanks the Ministry of Education, Science,
Sports and Culture in Japan for financial support by a Grant-in-Aid for
Exploratory Research.
1
Supporting Information Available: Binding details and H NMR,
UV-vis, and luminescence titration data of 6 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA010931Q
(13) The charge density (F) is defined as the amount of electric charge per
unit volume. It is here given by F ) q/(4/3πr3) where q is the formal charge
(+1 or +2) and r is Shannon ionic radius. The following values of r (Å)
were used: Li+ (0.90), Na+ (1.32), K+ (1.65), Mg2+ (0.86), Ca2+ (1.26), Ba2+
(1.56). Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(14) Gans, P.: Sabatini, A.: Vacca, A. Talanta 1996, 43, 1739.