Phenanthroline complexes bearing fused dipyrrolylquinoxaline anion recognition sites: Efficient fluoride anion receptors

Article abstract of DOI:10.1021/ja017298t

Novel anion recognition host molecules, tris-1,10-phenanthroline cobalt(III) and bis-2,2-bipyridine mono-1,10-phenanthroline ruthenium(II) complexes bearing fused dipyrrolylquinoxaline moieties have been synthesized. As determined by UV-vis spectroscopic and electrochemical studies, these metal complexes bind fluoride with high affinity in polar media both in absolute terms and relative to the metal-free phenanthroline dipyrrolylquinozaline precursor from which they are derived (fluoride is bound to the tris-1,10-phenanthroline cobalt(III) dipyrrolylquinoxaline system with a 1:1 binding constant of 54000 M^-1 in DMSO). The large observed binding constants are ascribed to two factors, (i) the presence of a phenanthroline-coordinated cationic charge that decreases the electron density on the pyrrole NH protons and (ii) pure electrostatic effects. Copyright

Full text of DOI:10.1021/ja017298t

Published on Web 01/25/2002
Phenanthroline Complexes Bearing Fused Dipyrrolylquinoxaline Anion
Recognition Sites: Efficient Fluoride Anion Receptors.
Toshihisa Mizuno, Wen-Hao Wei, Leah Renee Eller, and Jonathan L. Sessler*
Department of Chemistry and Biochemistry, The UniVersity of Texas, Austin, Texas, 78712-1167
Received October 13, 2001
The development of receptors capable of recognizing neutral and
anionic species continues to provide a challenge for supramolecular
chemists.1 For some time now, we have been interested in devel-
oping receptors for simple, water-soluble anions such as fluoride,
chloride, and phosphate. Such anions play critical roles in a range
of biological processes and are implicated in a number of disease
states, ranging from fluorosis to cystic fibrosis and are thus con-
sidered important targets in terms of receptor design.2 Prior efforts
in our group have focused on the use of pyrrole-based recognition
motifs and have led to the recent development of functionalized
calixpyrrole (CP) and dipyrrolylquinoxaline (DPQ) receptors.3-5
The latter systems have the advantage of possessing a built-in chro-
mophore and being readily accessible in two steps from commer-
cially available materials. While the parent DPQ system 1 was found
to bind fluoride strongly in dichloromethane-d2 (Ka) 18 200 ( 1820
M-1), yet higher affinities are considered desirable for many ap-
plications. In an effort to produce a DPQ system that binds fluoride
anion more strongly, we have recently introduced a tetrafluorina-
ted DPQ derivative, 2.6 In this contribution, we report a new and
Figure 1. UV-vis spectral changes of Co(III) complex 5 upon the addition
of fluoride anion; [5] ) 1.0 × 10-5 M, [TBAF] ) 0-5 × 10-4 M, DMSO,
25 °C. The inset shows the fit of the experimental data to a 1:1 binding profile.
Figure 2. Color changes seen for DMSO solutions of complex 5 upon the
addition of anions as TBA salts.
Scheme 1
4 and 5 were obtained in ca. 90 and 80% yield, respectively, by
reacting 3 with bis-bipyridine ruthenium(II) dichloride monohydrate
and bis-phenanthroline cobalt(III) trichloride,10 as appropriate.
Subsequently, treatment of the complexes with NaClO4 afforded
complex 4 and 5 in the form of their perchlorate salts.
The rationale for making 4 and 5 was that they would contain
electron-withdrawing moieties that would render the pyrrole N-H
protons more acidic, thereby promoting the key anion-to-DPQ
interactions, just as in 2. In the case of complexes 4 and 5, these
electron-withdrawing effects were expected to take place efficiently
through the quinoxaline backbone and lead to enhanced anion
binding affinities.
As a test of the above hypothesis, the fluoride, chloride, and
dihydrogen phosphate anion binding properties of 4 and 5 were
studied by UV-Vis spectroscopy (Figure 1) in DMSO using the tetra-
n-butylammonium (TBA) salts of the anions in question. Figure 1
shows the spectroscopic changes observed when Co(III) complex
potentially more generalizable strategy that is based on metal co-
ordination. Specifically, we present the synthesis and fluoride anion
recognition properties of the fused dipyrrolylquinoxaline phenan-
throline derivative, 3, and its ruthenium(II) and cobalt (III) com-
plexes, 4 and 5. The latter species, containing as they do electron-
withdrawing metal centers, display fluoride anion affinities that are
greatly enhanced compared to those of the neutral systems 1 and
3.7
The phenanthroline derivative 3, which contains a phenanthroline
metal coordination site covalently attached to a DPQ-type anion
recognition site, was synthesized according to Scheme 1. Briefly,
dipyrrolediketone 6 was reacted with 1,2-diamino-4,5-dinitroben-
zene8 in AcOH5 to give 7, which was then reduced to the
corresponding diamine (8). Condensation with 1,10-phenanthroline-
5,6-diketone9 produced ligand 3 in 65%. The target metal complexes
9
1134 VOL. 124, NO. 7, 2002 J. AM. CHEM. SOC.
10.1021/ja017298t CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Association Constants (M-1) of 1-5 with Various Anions
in DMSOa
1
2
3
4
5
F-
<100b
59000c
440
12000
10
40
54000
20
50
Cl-
NDd
NDd
NDd
NDd
NDd
5000
-
H2PO4
a Anions used in this assay were in the form of their tetrabutylammonium
(TBA) salts. b Estimate; clean binding profiles were not observed, presumably
as the result of competitions involving solvent. c These values have been
previously measured in dichloromethane-d2, wherein values for the binding of
F- and H2PO4- to 2 of 61 000 M-1 and 17 300 M-1, respectively, were observed.6
d ND ) not determined.
Figure 3. Changes in the DPV (vs SHE) spectrum of anion complex 5
observed opon the addition of fluoride. The two curves are those seen in
the absence of fluoride and in the presence of 5 equiv. The voltage scale is
plotted vs SHE.
Table 2. Electrochemical Characteristics of Complex 5 in the
Presence and Absence of Anions in DMSOa
b
anions
E/mV
∆E/mV
none
F-
160
c
-
c
The fact that the CoIII/CoII wave may be restored upon the
addition of water indicates that the complexation between 5 and
F- is not irreversible. As such, the results presented here provide
a hint that complex 5 and its congeners could prove useful in the
fabrication of electrochemical sensors. The present results also
provide support for the notion that modifying the electronic features
of DPQs through metal complexation might allow for a convenient
and general approach to enhancing the anion affinities and, possibly,
modulating the selectivites of DPQ-type anion receptors. Further
studies of this theme are ongoing in our group.
Cl-
320
90
160
70
-
H2PO4
a Potential measured vs Ag/AgCl (satd KCl) but are tabulated here vs SHE,
using a correction factor of 199 mV. b ∆E refers to change in redox potential
observed opon the addition of the anion in question. c No CoIII/CoII reduction
peak is seen between -500 mV and 500 mV.
5 is treated with increasing quantities of TBAF. In this case, the
peaks at 323 and 525 nm decreased upon the addition of TBAF,
and a new peak at 652 nm appeared (Figure 1), with saturation
being observed after the addition of ca. 3 equiv (cf. Supporting
Information for corresponding observations). Furthermore, the color
of the solution was seen to change from red-pink to pale purple
(Figure 2).
From Job-plot analyses, these spectral changes are ascribed to
the formation of 1:1 complexes between the metal complex and
fluoride anion. Standard curve-fitting procedures were then used
to derive binding constants.11 The resulting values are collected in
Table 1, along with those for 1-3.
Acknowledgment. This work was supported by the National
Instititues of Health (GM 58907) and the Robert A. Welch
Foundation (F-1018). Postdoctoral support for T.M. was also
provided in part by the Japanese Society for the Promotion of
Science. L.E. was supported in part by an NSF IGERT fellowship.
We thank Professor Pavel Anzenbacher (Bowling Green State
University) for a sample of 2.
Inspection of Table 1 provides support for the proposal that
appending cationic charges to a DPQ binding entity can indeed be
used to increase anion affinities. For instance, while the free
phenanthroline 3 displayed a rather low F- affinity, presumably as
a result of the additional electron density donated to the DPQ NH
anion binding functionality from the nitrogen-rich phenanthroline
moiety, the Ru(II) complex 4 displayed a fluoride anion affinity
that is higher than DPQ 1 (i.e., ca. 30 times higher than 3).
Moreover, the Co(III) complex 5, with its incrementally greater
charge, displayed an affinity that was even higher.
In an effort to characterize 5 more thoroughly, it was studied by
both cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) in DMSO (Table 2). It is known that bis-phenanthroline
dipyridophenadine Co(III) displays a reversible CoIII/CoII redox
wave at approximately 400 mV (vs SHE).12 In the case of 5, a
clearly reversible redox signal was observed at 160 mV (vs SHE)
in the CV in the absence of anions that we likewise assign to a
CoIII/CoII reduction wave. A sharp reduction peak was also observed
when 5 was studied by DPV. In the DPV studies, the addition of
F- led to a complete disappearance of the CoIII/CoII reduction signal
(or at least its displacement from the observable electrochemical
window).
These changes, illustrated in Figure 3, are interpreted in terms
of the complexes formed between 5 and fluoride being very strong
and, perhaps as a consequence, redox inactive. Consistent with this
hypothesis, the addition of a small amount of H2O, thought to
dissociate the anion-DPQ complex, leads to a restoration of the
CoIII/CoII wave. Likewise, the addition of other anions, notably
chloride and dihydrogen phosphate, lead to observable shifts in the
CoIII/CoII reduction potential, but not to a complete loss of the
associated wave (Table 2).
Supporting Information Available: Experimental procedures,
characterization data, and titration results (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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