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Molecular Recognition of Carboxylate Ions Based on the MetalЦLigand Interaction and Signaled through Fluorescence Quenching.

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[3] BlNAP = ?.~'~bis(diplienylphosphano)-l
.l'-binaphthyl: R. Noyori. H. Takaya.
A r Chi~m.R c 1990. 33, 345
[4] a) P. von Matt. A. Pfaltz. Aiigeii.. Chrri?.1993. 105, 614: Ailgeu.. Chen?.Int. E d
EiigI. 1993. 32, 566. P. von Matt, 0. Loiseleur. G. Koch. A. Pfaltz. C. Lefeber.
T. Feucht. G Helmchen. Terruhedroii: A.?ynmierry 1994, 5, 573; b) G. Koch,
G. C. Lloyd-Jones, 0 . Loiseleur, A Pfaltz, R. PretBt, S. Schaffner, P Schnider.
1995, 114. 206.
P. von Matt, R e d Trm. Chmi. Puy~~-Bos
151 See also: a) J. Spi-inz. M. Kiefer, G. Helmchen, M. Reggelin, G. Huttner. 0.
Walter. L. Zsolnai, Terruherlron Lett. 1994.35. 1523; P. Sennhenn, B. Gabler. G .
Helmchen. ihid. 1994. 35,8595; b) G . J. Dawson. C. G. Frost. J. M. J. Williams,
S. J. Coote, ?hid. 1993, 34. 3149; G. J. Dawson, J. M. J. Williams. ihid. 1995. 36.
461; I C. Baldwin. J. M . J. Williams, R. P. Beckett, Tetruhedron: A.\wiinetry
1995. 6. 679.
[6j G . C. Lloyd-Jones, A. Pfaltz, Angeu.. Chem. 1995. 107. 534: Angel$..Chern. In/.
Ed. Engl. 1995. 34, 462.
171 [Pd,(dba),.dba]: Y. Takahashi. T. Ito, S. Sakai, Y Ishii. J. Chml. Soc Chein.
Commun. 1970,1065: T. Ukai, H. Kawazura, Y. Ishii, J. J. Bonnet, J. A. Ibers. J.
Orgunomel. Chem. 1974. 65, 253.
[XI For the effect of halide ions in Heck reactions, see for example ref. [Id]. Facile
oxidative addition of chloroform to [(diphosphane)(dba)Pd'] complexes has
been reported: W A. Herrmann. W R. Thiel. C. Brossmer. K. Ofele. T. Priermeier. W. Scherer. J Orgunorner. Chern. 1993, 461, 51.
[9] Configurational assignments: 2 and (R)-(-)-2-(l-cyclohexenyl)2.3-dihydrofurdn [2b] were converted to (R)-( +)-2-(l-cyclohexenyl)tetrdhydrofuran. (R)(+)-3:ref.[2b].(R)-(+)-4:CDcomparisonwith(R)-(
+)-2.(R)-(+)-S: ref.[2d].
(R)-(+)-6: H. B Hopps, Diss. Absrr. 1962. 23, 439.
Molecular Recognition of Carboxylate Ions
Based on the Metal-Ligand Interaction and
Signaled through Fluorescence Quenching**
Giancarlo De Santis, Luigi Fabbrizzi,*
Maurizio Licchelli, Antonio Poggi, and Angelo Taglietti
Molecular recognition of a given substrate can be signaled in
a number of ways: the change in the the potential of an electrode, the shift of an NMR signal, an abrupt color change.
However, the most spectacular and easily detectable signaling
effect is probably a change in fluorescence. Fluorescent sensing
procedures have been developed over the last decade for several
metal ions.['] Usually the recognition of the metal center is communicated to the outside through the quenching (for transition
metal ions)r21o r the revival (for alkali and alkaline earth metal
cations)[31of the fluorescence of a light-emitting unit linked to
the receptor. In any case, the variation of the fluorescence emission is related to an intramolecular electron transfer process,
which directly involves the metal center. On the other hand,
fluorescence sensing of anions is a much less developed field,
which may be due to the low energy of the receptor-substrate
interactions (hydrogen bonding, electrostatic effects). A successful case is the binding of hydrogen phosphate to a polyammonium unit, to which an anthracene subunit is appended
as a fluorescent group.r41
We considered that the metal - ligand interaction, which can
be significantly stronger than hydrogen bonding and other van
['I
[**I
202
Prof. Dr. L. Fabbrizzi, Dr. G. De Santis. Dr. M. Licchelli. Prof. Dr. A. Poggi.
Dr. A. Tdglietti
Dipartimento di Chimica Generale. Universita di Pavia
Via Taramelli 12, 1-27100 Pavia (Italy)
Fax: Int. code + (382)528544
This work was supported by the Italian National Council of Research (CNR:
Progetto Strategico: Tecnologie Chimiche Innovative) and by the European
Union (HCM program: Network Contract no. ERBCHRXCT940492).
0 VCH
Verlrigsge,sellsrlinfr~t
nibH, 0-69451 Weinheirn. 1996
der Waals interactions, could be conveniently used for anion
recognition. In this connection, we first appended a 9-anthrdcenyl group to a peripheral nitrogen atom of tris(2-aminoethyl)amine (tren). The resulting anthrylamine 1 was reacted
with Zn" to give the corresponding complex. In the two-component system [Zn"(l)]'+, the four-coordinate metal center has a
vacant site for coordination of a solvent molecule or for an
anion to give a trigonal-bipyramidal arrangement. The proximate luminescent unit is in a favorable position to signal the
occurrence of the anion recognition. In methanolic solutions
[Zn'1(l)]2f shows a strong affinity towards anions bearing a
carboxylate group. In particular, a stable 1 :1 adduct is formed
with the benzoate ion, as shown by spectrophotometric titrations. However, parallel titration experiments carried out in the
spectrofluorimetric cuvette indicate that anion binding does not
interfere with the photophysical activity of the proximate anthracene group, whose emission spectrum was not altered even
after the addition of several equivalents of the anion. Thus, in
this case the anthracene subunit is only a "silent witness" of the
anion binding to the Zn" center.
However, when a solution of [Zn"(1)]2f was titrated with the
4-N,N-dimethylaminobenzoateion (X-) , the fluorescence intensity progressively decreased until complete quenching
(Fig. 1). The plot of fluorescence intensity (IF) versus equiva-
I
100 -
% Q W V
V
Q
V
b
0
v
0
2
4
6
0
v
8
1
0
n Fig. 1. Spectrofluorimetric titrations of the [Zn"(l)]z+ receptor in methanolic soluM) with standard methanolic solutions of N,N-dimethylaminobenzoate
tion
(T). acetate (v); N,N-dimethylaminobenzoate t acetate ( 0 ) : n = equivalents of
anion/equivalents of complex.
lents of anion added indicates that the 1 :1 adduct [Zn"(l)X]+ is
formed, and least-squares treatment of the curve gave a IgK
value of 5.45+0.03 for the equilibrium [Zn"(l)]*'
X[Zn"(l)X] .[sl It is suggested that fluorescence quenching is due
to an intramolecular electron transfer (ET) process within the
[Zn"( 1)X]+ adduct, in particular from the Zn"-bound dimethylaminobenzoate subunit D to the photo-excited anthracene unit
*An. The electron donor tendencies of N,N-dimethylaniline
+
+
+
057o-0833/9613So2-0202 $10.00 + ,2510
Angew. Chem. Int. Ed. Engl. 1996, 35. No. 2
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~~
-
and its derivatives are well recognized.[61 Moreover, the
D *An ET process is thermodynamically feasible, as
indicated by the distinctly negative value of AC:T =
E+, - AG,, . I, AC,,,,,
= (- 3.1 + 0.3
2.4) eV = - 0.4 eV.
The ET nature of *An quenching has been confirmed by spectrofluorimetric studies at 77 I(.It is known that immobilization
of the solvent molecules in a frozen matrix prevents charge
separation and strongly disfavors the ET process, thus restoring
fluorescence.['] Indeed, when an ethanolic solution containing
[Zn"( l)]' and an excess of 4-N,N-dimethylaminobenzoate,
which does not emit at room temperature, was frozen at 77 K,
fluorescence o f the anthracene subunit was restored. demonstrating the ET nature of the quenching at room temperature.
To verify the selectivity of the [Zn"(l)]2+ receptor, spectrofluorimetric titration experiments were carried out with the following inorganic anions: NO;, NCS-, and CI-. The anthracene emission was not altered or showed an intensity decrease of
less than than 5 Yo (el-).Competitive titration experiments
were also conducted. A 5 x
M solution of [Zn"(l)]"
in
methanol was titrated with a solution containing equimolecular
amounts of 4-N,N-dimethylaminobenzoate and the particular
anion. The I,. titration curves obtained with NO; and NCSwere identical to that with 4-N,N-dimethylaminobenzoate
alone, indicating no competition for the binding to the metal
center. However, in the presence of CI-, Z, was not fully
quenched. but decreased to about 80% of the value observed in
the titration with 4-N,N-dimethylaminobenzoatealone. This
indicates that the chloride ions compete successfully with the
X - ions, binding about 20% of the available [Zn"(l)]'+ complex cations. The acetate ion behaved in a similar way. The
acetate alone does not exert any quenching effect on the anthracene unit o f the [ZnT'(1)J2+receptor; however, in a competitive titration experiment (CH,COO-, X-). fluorescence
quenching was not complete, but corresponded to about 85 YOof
that observed in the titration with 4-N,N-dimethylaminobenzoate alone (see Fig. 1 ) . This would imply a ratio of the equilibrium constants for the adduct formation KbenroaiejKacelalr
z 6.
The distinctly higher value of the binding constant observed for
the 4-N.N-dimethylaminobenzoate anion may suggest that a
7c--71 interaction between the Zn"-bound benzoate group and the
anthracene subunit provides an additional contribution to the
solution stability of the [Zn"(l)X]+ adduct.
In this context. it should be noted that the occurrence of the
intramolecular ET process in the [Zn"(l)X] supramolecular
system requires that the fluorophore and the acceptor are close
enough for the appropriate orbitals to overlap. Stacking of benzoate and anthracene fragments may favor electron transfer and
induce the nonradiative deactivation of the *An state. A pictorial representation of the intramolecular ET process is sketched in
Figure 2. A similar stereochemical arrangement has been proposed for the 1 : 1 adduct between the benzoate anion and a
Zn" -cyclen complex with an acridine pendant.[*' Noticeably,
interaction ofthe thymine anion with the Zn" center of the same
complex induced a distinct decrease of the intensity of acridine
fluorescence emission .I9]
In any case. fluorescence quenching of the excited anthracene
unit of the [Zn"(l)]'+ receptor should not be limited to 4-N,Ndimethylaminobenzoate, but should be feasible for any benzoate anion bearing either a donor o r an acceptor group, provided t h a t the thermodynamic requirements for the transfer of
an electron to or from the photo-excited fluorophore are
fulfilled. I n fact. titration of [Zn"(l)]'+ with 4-nitrobenzoate
ion in methanolic solution induced the decrease and quenching
of the fluorescence. according to a profile analogous to that
displayed in Figure 1. Nitrobenzene and its derivatives are
+
~
e-
+-
+
0
+
+
Fig. 2. Representation of the electron transfer mechanism responsible for the fluorescence quenching of the excited anthracene unit *An, which signals the binding of
a carboxylate anion by the Zn" center.
classical acceptors. Thus, fluorescence quenching has to be ascribed to an intramolecular ET process from *An to the
metal-bound 4-nitrobenzoate ion A. The negative value of
AC&[=Eo_,+AGAi,- - A C A n T i A n = ( - 3 . 1 +1.3 +0.8)eV=
- 1.0 eV] accounts for its thermodynamic feasibility. Quite interestingly, decrease and almost complete quenching of the fluorescence was observed also when [Zn"(l)]'+ was titrated with 9-anthracenoate ion, which itself is fluorescent. Fluorescence
quenching should arise from the "disproportionation" process
involving an electron transfer between the two anthracene subunits of the supramolecular adduct: *An An
AnAn+.
Both the anthracenoate-to-anthracene and anthracene-to-anthracenoate electron transfer processes exhibit a slightly negative
ACiT value (~ 0.2 eV). In any case, the observed revival of the
fluorescence emission in an ethanolic solution frozen at 77 K
demonstrates also for the present system that electron transfer is
responsible for the quenching of the photo-excited aromatic subunit.
The [Zn"(l)]' system is able to signal the binding of carboxylate ions other than benzoates, provided that they have definite
donor or acceptor properties in a photophysical sense. For instance, fluorescence quenching is observed also on titration
of [Zn"(l)12+ with 1 -ferrocenecarboxylate (Fcc-). Ferrocene
(Fc) and its derivatives are quite strong donors, and the
Fcc- + *An electron transfer is thermodynamically favored:
AC:T =
AG,4,,,A,.- AGr,,,Fcc. = (- 3.1 + 2.4 0.3) eV
= - 0.4 eV. In this case, too, the electron transfer nature of the
quenching process was confirmed by the fluorescence revival observed at 77 K.
The Zn" ion was deliberately chosen as a Lewis acid for carboxylate binding owing in part to its photophysical innocuosness.
Transition metal ions bind to the tren framework to form even
more stable adducts with inorganic and organic anions, but tend
to complicate the photophysical response. This is the case with the
[Cu"(l)]'+ system, which, even in absence of any coordinating
anion, displays a much less intense emission spectrum (about
15% of that observed for [Zn"(l)]'+). This partial fluorescence
quenching has to be ascribed to a direct interaction between the
Cu" center and the *An fragment. We note that freezing an
ethanolic solution of [Cu"(l)]'+ at 77 K did not induce any
+
+
+
+
+
+
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fluorescence revival. This may suggest that quenching takes
place through an *An-to-Cu" energy transfer process, although
the occurrence of a very fast electron transfer process cannot be
excluded in principle. The Cu" ion (d9) is well inclined to undergo an energy transfer process, as it has a half-tilled d level of low
energy (s2- y 2 ) , which is available for a double electron exchange process (Dexter mechanism) .[''] In any case, titration
of [ C U " ( ~ ) ] ~with
+ 4-nitrobenzoate o r 4-N~V-dimethylaminobenzoate caused complete quenching of the residual fluorescence emission.
This work has demonstrated that metal -1igand interactions
can be conveniently used for anion binding and recognition. The
coordination environment provided by the tren hgdnd leaves
one binding site of the metal center free for the approach of
anions of various sizes and shapes: the selectivity is essentially
determined by the energy of the metal-anion coordinative interaction. Then, the appended fluorescent unit signals the occurrence of the recognition. A further element of selectivity is introduced by the signal transduction mechanism: only the
interactions with anions displaying distinctive electron donor or
electron acceptor tendencies will be signaled and sensed. The
Zn"-containing system described is well suited for carboxylate
anions. Following a similar approach, more sophisticated systems can be designed for the recognition and sensing of anions
of various types and complexities.
Experimental Procedure
1: Anthracene-9-carbdldehyde (2.0mmol) was allowed to react with an excess of
tren (6.0 mmol) in 30 mL of a 1 : 1 Et0H;MeCN mixture at room temperature for
36 h. The solvent and most of the excess tren were distilled off at reduced pressure.
The remaining oil was dissolved in 30 mL of EtOH and the resulting solution was
treated with 0.7 g of NaBH,. in small portions, then heated at 50 'C for 4 h. On
concentration at reduced pressure. a sticky solid was obtained. to which 25 mL of
water was added. The suspension that formed was extracted with three 25 mL
portions of CH,CI,. The organic layer was dried over MgSO, and the solvent
removed on a rotary evaporator to give 1 as a bright orange oil. Yield 7 5 % MS
(ESI): mi; (%) 337 (100) ( M i + H). The corresponding Zn"complex was obtained
as the salt [Zn"(l)](ClO,), in the form of a pale yellow solid, through reaction of
equimolar amounts of 1 and Zn(CI04),.6H,0 in ethanol. The salt gave a satisfactory C,H.N analysis.
Emission spectra were recorded on a Perkin-Elmer LS-50 luminescence spectrometer and were not corrected for instrument response. Emission spectra at 77 K were
measured in dry ethanol by using quartz sample tubes and the same luminescence
spectrometer equipped with a low-temperature luminescence accessory (PerkinElmer). Pertinent electrode potentials (of the A/A- and D'/D redox couples) for
the relevant acceptor and donor units (Cnitrobenzoate. 4-N,N-dimethylaminobenzoate, 9-anthrdcenecdrboxykate.and 1-ferrocenecarboxylate as sodium salts) were
determined through differential pulse voltammetry experiments (vs. the F c + / F c
internal reference couple, MeCN, 0.1 M Bu,NCIO,). A platinum microsphere was
used as the working electrode. The calculated AG& values refer to the MeCN
values ( = - FE,,,,,,) were obtained in MeCN solutions. Cormedium, as AG&
responding dG& vdlues in MeOH solution should be even more negative, and the
ET process more favorable. due to the higher solvent polarity.
Received: July 26. 1995
Revised version: September 25. 1995 [Z8259IE]
German version: Angcw.. Clieni. 1996. /OR. 224-227
Keywords: anion recognition . anthracene derivatives . fluorescent sensors . supramolecular chemistry . zinc compounds
[5] The HYPERQUAD program was used- A. Sabatini, A. Vacca, P. Gans, Coord.
Cliwn. Rrv. 1992. 1211, 389-405.
[6] G . J. Kavarnos, Fundurnentuls of Pliotoinducrd Electron Transfer. VCH, New
York, 1993. p. 40
[7] M. R. Wasielewski. G . L. Gaines 111, M. P. O'Neil, M. P. Niemczyk, W. A.
Svec in Supruriiolec.ulur Cherni$try (Eds.: V. Balzani, L. De Cola), Kluwer,
Dordrecht, 1992, p. 202.
[8] E. Kimura, T. Ikeda, M. Shionoya. M. Shiro. 4ngeir. Chrm. 1995. 107. 711713: Angris Cliern. Int. Ed. Engl. 1995, 34, 663-664.
191 M. Shionoya, T. Koike, E. Kimura, M. Shiro. J. A m . Cliem. Soc. 1994. 116,
3848-3859.
[lo] V. Balzani, F. Scandola. Suprumnliwlur- Plrotochrmiitr~,Ellis Horwood. London. 1991, p . 71.
[S(NtBu),]' --A Cap-Shaped Dianion,
Isoelectronic with the Sulfite Ion and
Oxidizable to a Stable Radical Anion""
Roland Fleischer, Stefanie Freitag, F r a n k P a u e r , and
Dietmar Stalke*
Dedicated to Professor Herbert W Roesky
on the occasion of his 60th birthday
Recently we have been interested in the synthesis of new ligands with main group elements in low oxidation states as central
atoms. These tripodal ligands, as a result of the lone pair on the
central atom, have potentially Lewis base character; metal
atoms are coordinated by nitrogen atoms in the substituents on
the ligands and not by the central atom. Tripodal ligands of this
kind include, in particular, compounds with structural units
such as E(NR); (E = Ge, Sn, Pb),"] as well as polypyrazolylgermanates and -stannates;['] trislithium tris(trimethylsily1R = Me, tBu, Ph;I3]
amino)silanes [(LiNSiMe,),SiR],,
R = HC4]and [{(thf)LiNSiMe,tBu},SiPh] 15] are described as
trianionic tripodal ligands.
Our aim was to extend this range of mono- and trianionic
ligands by synthesizing a dianionic ligand system, to make homoleptic complexes of low-coordinate bivalent metals accessible. Here we report on the tris(tert-buty1)triazasulfite ligand,
which is formally derived from the dianionic SO:- ion in that
the three oxygen atoms were substituted by isoeiectronic tBuN
groups. Together with bivalent metals this ligand could lead to
interesting organometallic precursors for IIjVI semiconducting
materials.
In analogy to the addition of organo alkali metal compounds
to sulfurdiimines, which leads to aminoiminosulfinates,[61 we
have added alkali metal amides to sulfurdiimides. Although
equimolar amounts of the starting materials were used, the
equimolar addition of lithium terr-butylamide to di(tertbuty1)sulfurdiimide did not occur, but two equivalents of the
sulfurdiimide react with four equivalents of lithium tert-butylamide to give the dimeric bislithium tris(tert-buty1)triazasulfite
1 [Eq. (all.
2S(NfBu),
["I
[l] Flirorescent Cliemosensors ,fi)r Ion and M o k v uli, Recognition (Ed.. A. W.
Czarnik) ( A C S S>.n?p.Ser. 1993, 538).
[2] L. Fabbrizzi. M. Licchelli, P. Pallavicini, A. Perotti. D. Sacchi, A n ~ e i r Chem.
.
1994, 106, 2051 -2053; Angeu.. Cliern. Inr. Ed. EngI. 1994, 33. 1975-1977.
[31 R. A. Bissell, A. P. de Silva, H. Q. N. Gunaratne. P. L.M. Lynch. G . E. M
Maguire. C. P. McCoy. K . R. A. S. Sandanayake, Top. Curr. C/imi. 1993. 168,
223-264.
I41 M. E. Huston. E. U. Akkaya, A. W Czarnik, J. A m . Chern. Soc. 1989. 111,
8735- 8737.
204
c>VCH Verlugsgc~seNschqfrnihH. 0.69451
Weinlieiin. 1996
['I
[**I
+ 4LiNHtBu
toluene
[Li,(NtBu),S],
2
+ 2H,NtBu
(a)
Priv.-Doz. Dr. D. Stalke."' R. Fleischer,"' Dr. S. Freitag, Dr. F. Pauer
Institut fur Anorganische Chemie der Universitit
Tammannstrasse 4, D-37077 Gottingen (Germany)
Fax: Int. code +(551)392582
New address:
Institut fur Anorganische Chemie der Universitit
Am Hubland. D-97074 Wurzburg (Germany)
This work was supported by the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie. We thank Dr. D. Marsh, Max-Planck-Institut fur Biophysikalische Chemie. Gottingen, for recording the ESR spectra.
O570-ON33iY6:35U2-0204 3 111.00+ 2511)
Angriv Cliern I n / . Ed. EngI. 1996. 35, No. 2
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