A self-assembled supramolecular optical sensor for Ni(II), Cd(II), and Cr(III)

Article abstract of DOI:10.1021/ol035587b

A new chromogenic supramolecular sensor for transition metals is reported. It is based on a newly designed phenanthroline-containing molecule that self-assembles via an organometallic "clip" into a supramolecular optical sensor for metals.

Full text of DOI:10.1021/ol035587b

ORGANIC
LETTERS
2004
Vol. 6, No. 5
651-653
A Self-Assembled Supramolecular
Optical Sensor for Ni(II), Cd(II), and
Cr(III)
,†
Marino J. E. Resendiz, Juan C. Noveron,* Hendrick Disteldorf,
Sonja Fischer, and Peter J. Stang*
Department of Chemistry, UniVersity of Utah, 315 S. 1400 E.,
Salt Lake City, Utah 84112, and Department of Chemistry,
UniVersity of Texas at El Paso, El Paso, Texas 79912
stang@chem.utah.edu
Received August 22, 2003
ABSTRACT
A new chromogenic supramolecular sensor for transition metals is reported. It is based on a newly designed phenanthroline-containing
molecule that self-assembles via an organometallic “clip” into a supramolecular optical sensor for metals.
In recent years, chemical sensing has attracted the interest
of many research groups.1 One of the most important areas
in contemporary sensor development is the design of
functionalized molecules that trigger a measurable response
upon exposure to specific analytes.2 While the technology
of electrical transducers in analytical instrumentation is
relatively advanced, well characterized chemoselective ma-
terials are still needed.1
One of the more attractive approaches in this field involves
the construction of optical sensor devises due to their
potential for the rapid analysis of substrates.3 Development
of optical sensors for hazardous trace metals are of great
interest in this area and will have applications in environ-
mental and the biomedical fields.4 Herein we report the
design, synthesis, and evaluation of a supramolecular chro-
matic sensor that is sensitive to micromolar concentrations
of Ni2+, Cd2+, and Cr3+.
Supramolecular metal-organic assemblies could have a
significant impact in the areas of molecular sensing, as they
can combine the properties of their organic and inorganic
components in order to design receptor sites for chemical
sensing. Several examples using this approach have been
previously reported.5
Recently, we reported that a 1,8-platinum-functionalized
anthracene unit, which acts as a “molecular clip,” allows for
the preparation of complex supramolecular metallacyclo-
phanes.6 Herein we built upon this design and extend that
† University of Texas at El Paso.
(1) (a) Gopel, W. Microelectron. Eng. 1996, 32, 75. (b) Takagi, M. In
Cation Binding by Macrocycles; Inoue, Y., Gokel, W. G., Ed.; Marcel
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Chemistry (Supramolecular Chemistry I); Weber, E., Ed.; Springer: Berlin,
1993; Vol. 165. (d) Lohr, H.-G.; Vogtle, F. Acc. Chem. Res. 1985, 18, 65.
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10.1021/ol035587b CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/05/2004

Scheme 1. Synthesis and Self-Assembly of Rectangular Chemosensor 6
work by synthesizing a new functionalized ligand, namely,
[1,10]phenanthroline 2 (88%). This species was then treated
with K2CO3 in methanol to give 3,8- diethynyl[1,10]phenan-
throline 3, as reported in the literature8 (95%), which in turn,
was coupled with 2.3 equiv of 4-iodopyridine in the presence
of Pd(0)/CuI catalysts to provide the required ditopic pyridine
linker 4 (85%) (Scheme 1).
The self-assembly of the rectangular chemosensor 6 is
based upon a 1:1 stoichiometric combination of 4 and 1,8-
bis(trans-Pt(PEt3)2(NO3))anthracene 56b in an acetone/H2O
3,8-bis-pyridin-4-ylethynyl[1,10]-phenanthroline (BPP), that
upon self-assembly with the molecular clip generates a
supramolecular rectangle that exhibits optical sensing func-
tions toward various hazardous metals.
The ditopic pyridine linker BPP was prepared as shown
in Scheme 1. The 3,8-dibromo-1,10-phenanthroline 17 was
coupled with (trimethylsilyl)acetylene in the presence of
Pd(0)/CuI catalysts to give 3,8-bis-trimethylsilanylethynyl-
Figure 1. (a) Simulated isotope distribution for species [6 + 2H2O]2+ {m/z ) 1439.0}. (b) Experimental isotope distribution for [6 +
1
2H2O]2+ {m/z ) 1439.3}. (c) Molecular structure of 6. (d) H NMR spectra of 6 (e) 31P{1H} of 6.
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Org. Lett., Vol. 6, No. 5, 2004

mixture and was isolated by precipitation with KPF6 (yield
95%) (Scheme 1). Of particular diagnostic value for the
1
structure of 6 are the 31P{1H} and H NMR spectra. In the
former, the formation of a single, highly symmetrical species
is evident from the appearance of a sharp singlet peak with
concomitant 195Pt satellites, shifted 5.7 ppm upfield (-∆δ)
1
relative to 5 (∆1JPPt ) -2660 Hz). For the H NMR of 6,
two additional doublets appeared for the R- and â-pyridyl
protons that also exhibit a downfield shift (0.2-0.6 ppm)
due to the loss of electron density upon coordination, Figure
1. Further proof of complex 6 was obtained using elementary
analysis9 and electrospray ionization mass spectrometry (ESI/
MS). In the mass spectrum of 6, peaks (isotopically
unresolved) attributable to the consecutive loss of hexa-
fluorophosphate counterions [6-2PF6]2+{m/z ) 1566.5},
[6-3PF6]3+{m/z ) 995.6}, and [6-4PF6]4+{m/z ) 710.5} were
detected with good signal-to-noise ratio.
Figure 2. Titration of 6 in MeOH with Ni(NO3)2‚6H2O.
The supramolecular chemosensor 6 was also characterized
with UV-vis absorption spectroscopy. Methanolic solutions
of 6 exhibit several absorbance bands in the UV region, of
which the most prominent are 350 nm (ꢀ ) 3200), 280 nm
(5200), 230 nm (5900). These bands are diagnostic for
changes in the electronic structure of the macromolecule.
The binding constants for 7-9 are shown in Table 1. The
observed equilibrium constants for complexes 7-9 follow
the same trend of affinity as those observed for free 1,10-
phenanthroline with the same ions (i.e., for Ni2+ K1 ) 8 ×
107, Cd2+ K1 ) 5 × 105, Cr3+ NA).10 However, complex 6
provides a supramolecular complexity that endows the
phenanthroline moiety with a preorganized architecture for
metal binding as well as new optical properties.
The addition of Ni(II)(NO3)2‚6H2O to a solution of 6 in
methanol induced a dramatic change in the UV-vis spectrum
(Figure 2). This optical response arises from the 1:1
complexation of 6 with the Ni2+ ions to generate the Ni
complex 7. Evidence from the job’s method and ESI/MS
support this conclusion. For example, a peak corresponding
to [7-3PF6]3+{m/z ) 1084.3} is readily observed, see
Supporting Information. The generation of an isosbestic
point at 300 nm indicates that the complexation is clean and
takes place without disturbing the supramolecular structure
of 6. Titration of 6 with other transition metal ions of
environmental concern such as Cd2+ and Cr3+ showed a
similar effect to that of Ni2+ and generated the correspond-
ing Cd complex 8 and Cr complex 9, see Supporting
Information.
Table 1. Binding Constantsa for 6 with Ni2+, Cd+2, and Cr3+
Ni complex 7
Cd complex 8
Cr complex 9
K ) 2.01 ( 0.05 × 107
K ) 3.39 ( 0.5 × 104
K ) 7.53 ( 0.4 × 103
a Equilibrium constants were obtained at 25 °C and 0.2 mM ionic strength
in methanol.
In summary, we have prepared the functionalized ligand
5 that undergoes self-assembly to form the supramolecular
rectangle 6 that allows for the rapid optical sensing of Ni(II),
Cd(II), and Cr(III) ions in solution.
(5) (a) Pugh, V. J.; Hu, Q. S.; Zuo, X.; Lewis, F. D.; Pu, L. J. Org.
Chem. 2001, 66, 6136. (b) Rudzinski, C. M.; Young, A. M.; Nocera, D. G.
J. Am. Chem. Soc. 2002, 124, 1723. (c) Mallouk, T. E.; Gavin, J. A. Acc.
Chem. Res. 1998, 31, 209. (d) Mortellaro, M. A.; Nocera, D. G. J. Am.
Chem. Soc. 1996, 118, 7414. (e) Hiraoka, S.; Fujita, M. J. Am. Chem. Soc.
1999, 121, 10239. (f) Yu, S.-Y.; Kusukawa, T.; Biradha, K.; Fujita, M. J.
Am. Chem. Soc. 2000, 122, 2665. (g) Ibukuro, F.; Kusukawa, T.; Fujita,
M. J. Am. Chem. Soc. 1998, 120, 8561. (h) Biradha, K.; Aoyagi, M.; Fujita,
M. J. Am. Chem. Soc. 2000, 122, 2397.
(6) (a) Kuehl, C.; Huang, S. D.; Stang, P. J. J. Am. Chem. Soc. 2001,
123, 9634. (b) Kuehl, C.; Mayne, C. L.; Arif, A. M.; Stang, P. J. Org. Lett.
2000, 2, 3727.
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37, 1121.
Acknowledgment. Financial support by the NSF (CHE
0303720), the NIH (5R01GM57052), and the NIH postdoc-
toral fellowship (GM 66504-01) for J.C.N. is gratefully
acknowledged.
Supporting Information Available: Experimental data
for the binding of Cd2+ and Cr3+ with 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL035587B
(8) Pelter, A.; Jones, E. D. J. Chem. Soc., Perkin Trans. 1 2000, 1,
2289.
(9) Elemental analysis of 6: C, 53.93; H, 6.08; N, 3.93. Found: C, 54.13;
H, 6.19; N, 3.84.
(10) Sillen, D. L.; Martell, A. E. Stability Constants of Metal-ion
Complexes; The Chemical Society: London, 1971; Spec. Pub. No. 17.
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