2588
J. Am. Chem. Soc. 1997, 119, 2588-2589
Scheme 1
The Dendritic Effect in Molecular Recognition:
Ferrocene Dendrimers and Their Use as
Supramolecular Redox Sensors for the Recognition
of Small Inorganic Anions
Christine Vale´rio,† Jean-Luc Fillaut,† Jaime Ruiz,†
Joe¨lle Guittard,‡ Jean-Claude Blais,‡ and Didier Astruc*,†,§
Table 1. Titration of the FcDs by Various n-Bu4N+ Salts
Monitored by the Variation ∆E° (mV for 1 Equivalent of Anion per
Branch) of the Standard Redox Potential E° of the Redox Couple in
Cyclic Voltammetrya
Laboratoire de Chimie Organique et Organome´tallique
URA CNRS No. 35, UniVersite´ Bordeaux I, 351
Cours de la Libe´ration, 33405 Talence Ce´dex, France
Laboratoire de Chimie Structurale Organique et
Biologique, EP 103, UniVersite´ Paris VI, Place Jussieu
75252 Paris Ce´dex 05, France
1-Fc
3-Fc
9-Fc
18-Fc
-
H2PO4
45
ꢀ
ꢀ
110
30
ꢀ
220
65
20
ꢀ
315
130
45
-
ReceiVed December 2, 1996
HSO4
Cl-
NO3
Dendrimers1-6 could serve as supramolecular exoreceptors
for molecular recognition,4 viewing the fractality of their surface
which resembles that of viruses, cancer cells, and biomolecules.2
Other supramolecular aspects of dendrimers such as transport5
and self-assembly6 have been exemplified, but molecular
recognition using specific sensors based on dendrimers is not
known. Various sensors have been reported with endorecep-
tors4,7,8 inter alia for the recognition of anions,7-9 in particular
redox sensors with macrocycles and tripods.8 A few cores with
ferrocene and other redox active units are known.8,10-12 We
report here the synthesis of amido-ferrocene dendrimers (FcDs)
and the remarkable dendritic effect found in the recognition of
small anions (Figure 1).
-
ꢀ
ꢀ
30
a In the case of HSO4-, the variation ∆E° along the titration is
represented in Figure 1 for the various dendrimers. The uncertainties
of the ∆E° values are estimated to be 20 mV. Thus, lower shifts are
indicated by ꢀ.
as well-known in dendrimer chemistry,1 surface steric saturation
was reached between the 18-Fc and 36-Fc.
Titrations of the FcDs were effected by n-Bu4N+ salts of
H2PO4-, HSO4-, Cl-, Br-, and NO3- and monitored by cyclic
voltammetry (CV) and 1H NMR (200 MHz) which are
complementary techniques.
The CV of all the ferrocene dendrimers show a single anodic
reversible wave at E° ) 0.69 V vs SCE indicating that all the
FeII/FeIII redox centers are independent and equivalent. Titration
by n-Bu4NH2PO4 provokes the progressive appearance of a new
wave at a less positive potential and the progressive disappear-
ance of the initial wave. The replacement of the initial wave
by the new one is complete after addition of 1 equiv of n-Bu4-
NH2PO4. This is rationalized by Scheme 1 where K(+) is
several orders of magnitude larger than K(0) because of the
electrostatic interaction between the cationic ferricinium branch
and the H2PO4- anion. Equation 1 applies, K(+) and K(0) being
the apparent association constants of the anion with the
ferricinium- and ferrocene dendrimers, respectively:15
The orange-red powdery polyamidoferrocene dendrimers 9-Fc
and 18-Fc (Chart 1) were synthesized by reaction of 9- and 18-
amine dendrimers13 with FcCOCl (Fc ) ferrocenyl) at 20 °C:14
Dendri-NH2 + FcCOCl + NEt3 f
Dendri-NHCOFc + NEt3H+Cl-
Reactions of the 36- and 72-amine with FcCOCl gave
materials which were insoluble in all solvents, indicating that,
† Universite´ Bordeaux I. Part of the Ph.D. Thesis of C.V.
‡ Universite´ Paris VI (mass spectral analysis).
§ E-mail: d.astruc@lcoo.u-bordeaux.fr.
(1) (a) Tomalia, D. A.; Dupont Durst, H. Top. Curr. Chem. 1993, 165,
193. (b) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic
Molecules: Concepts, Syntheses and PerspectiVes; VCH: New York, 1996.
(c) Fre´chet, J. M. J. Science 1994, 263, 1710.
Ef°ree - Eb°ound ) ∆E°(V) ) 0.059 log(K(+)/K(0)) at 25 °C (1)
(2) For a recent review on dendrimers in molecular biology, see: Astruc,
D. C. R. Acad. Sci. Paris, 1996, 322, 757.
Measurement of the ∆E° values (Table 1 and Figure 1) leads
to K(+)/K(0), and eventually to K(+) if K(0) is accessible by
1H NMR,16,17 Vide infra.
(3) For comprehensive reviews on dendrimers, see refs 1, 2, and Ardoin,
N.; Astruc, D. Bull. Soc. Chim. Fr. 1995, 132, 875.
(4) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes;
Titration of the ferrocene dendrimers by HSO4-, Cl-, and
VCH: Weinheim, 1995.
(5) Jansen, J. F. G. A.; de Brabander-van den Berg, E. M. M.; Meijer,
E. W. Science 1994, 266, 1226.
-
NO3 salts did not give rise to a new wave but only to a
progressive cathodic shift of the initial wave. This means that
K(0) is small (<1), and eq 2 allows the calculation of K(+)
(6) (a) Zimmerman, S. C.; Zeng, F.; Reichert, D. E. C.; Kolotuchin, S.
V. Science 1996, 272, 1095. (b) Bell, T. W. Science 1996, 271, 1077.
(7) (a) Czarnik, A. W. Acc. Chem. Res. 1994, 27, 302. (b) Atwood, J.
L.; Holman, K. T.; Steed, J. W. Chem. Commun. 1996, 1401.
(8) Beer, P. D. Chem. Commun. 1996, 689; AdV. Inorg. Chem. 1992,
39, 79.
(14) A mixture of NEt3 (1.1 mL, 8 mmol), FcCOCl (0.4 g, 1.62 mmol)
in 5 mL CH2Cl2, and the 9-amine (0.15 g, 0.13 mmol), resp. 18-amine
(0.10 g, 0.047 mmol) was stirred at room temperature for a day; then the
medium became clear and orange-red. After removing the solvent under
vacuum, 50 mL of CH2Cl2 was added, the organic layer was washed with
water (4 × 50 mL) and dried over Na2SO4, and the solvent was removed
under vacuum. The orange-red oily residue was washed with 6 × 30 mL
ether and dried under vacuum, which gave 9-Fc (0.32 g, 80% yield), resp.
18-Fc (0.19 g, 68% yield) of orange-red powders. Anal. for 9-Fc: Calc. C,
62.15; H, 6.50; N, 4.00. Found: C, 61.92; H, 6.99; N, 3.58. MALDI TOF:
m/z: 3066 (MH+). Anal. for 18-Fc: Calc. C, 55.21; H, 5.97; N, 5.52.
Found: C, 54.95; H, 6.38; N, 5.00. MALDI TOF: m/z: 6024 (MNa+);
NMR, IR and other mass spectral data: see Supporting Information.
(15) Miller, S. R.; Gustowski, D. A.; Chen, Z.-h.; Gokel, G. W.;
Echegoyen, L.; Kaifer, A. E. Anal. Chem. 1988, 60, 2021.
(9) Hosseini, M. W.; Lehn, J.-M. HelV. Chim. Acta 1986, 69, 587.
(10) Astruc, D. Electron Transfer and Radical Processes in Transition-
Metal Chemistry; VCH: New York, 1995; Chapter 2.
(11) (a) Cuadrado, I.; Mora´n, M.; Casado, C. M.; Alonso, B.; Lobete,
F.; Garc´ıa, B.; Ibisate, M.; Losada, J. Organometallics 1996, 15, 5278.
Fillaut, J.-L.; Linare`s, J.; Astruc, D. Angew. Chem., Int. Ed. Engl. 1994,
33, 2460. Bard, A. J.; Nature 1995, 374, 13.
(12) Campagna, S.; Denti, G.; Serroni, S.; Juris, A.; Venturi, M.;
Ricevuto, V.; Balzani, V. Chem. Eur. J. 1995, 1, 211. Herring, A. M.;
Steffey, B. D.; Miedaner, A.; Wander, S. A.; Dubois, D. L. Inorg. Chem.
1994, 33, 5482; 1995, 34, 1100.
(13) The new 9-amine dendrimer was synthesized by FeCp+ induced
nonaallylation of mesitylene, removal of FeCp+ by visible photolysis,
hydroboration followed by oxidation to the nonol, Michael addition of
acrylonitrile, and reduction of the nonanitrile to the nonaamine using BH3-
Me2S. The 18-amine dendrimer was made from the above nonaamine using
a variation of Vo¨gtle’strategy,1 Michael addition of two acrylonitrile units
per branch, and reduction of the 18-nitrile using BH3-Me2S.
(16) Hynes, M. J.; J. Chem. Soc., Dalton Trans. 1993, 311. We thank
Dr Hynes for providing and discussing his EQ NMR program for the
1
determination of K values by H NMR.
(17) “Nonstoichiometric” NMR data resulting from these weak interac-
tions will be discussed in a full paper using molecular modeling and data
obtained with additional metallocene dendrimers and anions.
S0002-7863(96)04127-3 CCC: $14.00 © 1997 American Chemical Society