Assembly of dendrimers with redox-active [{CpFe(μ3-CO)} 4] clusters at the periphery and their application to oxo-anion and adenosine-5′-triphosphate sensing

Article abstract of DOI:10.1002/anie.200502291

9, 16, or 27 [CpFe(μ₃-CO)]₄ clusters contain the title assemblies (see picture). The one-electron oxidation Fe ₄→Fe₄⁺of all Fe₄ units appears as a single reversible cyclic voltammetry wave and was used in solution and with dendrimer-modified electrodes for oxo-anion recognition. ATP^2- is selectively recognized and better than H₂PO₄^-. The larger the dendrimer, the easier is the re-use of the modified electrode sensor. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200502291

Communications
+
ꢀ
ꢀ
Dendrimers
{ S(CH2)11NH3 Cl }2, and the N-succinimidyl ester 5 by
reaction of 3 with N-hydroxysuccinimide (Scheme 1).
Complex 3 reacts with 9-branched amino dendrimer 7 in
DOI: 10.1002/anie.200502291
the presence ofNEt to give the expected 9-branched amido
3
Assembly of Dendrimers with Redox-Active
[{CpFe(m3-CO)}4] Clusters at the Periphery and
Their Application to Oxo-Anion and
cluster dendrimer 8 [Eq. (1)]. However, this is not the case for
commercial (DSM) third-generation 16-branched amino
dendrimer 9,[11] probably for steric reasons. Compound 9 did
not give the expected dendritic amide complex in the
Adenosine-5’-Triphosphate Sensing**
presence ofNEt , but unexpected formation of the diethyl-
3
amido cluster 6 was observed, presumably resulting from
electron transfer from NEt3 to 3 to generate the acyl radical
Jaime Ruiz Aranzaes, Colette Belin, and Didier Astruc*
Metallodendrimers[1–8] are well-established nanoma-
terials that have applications as electronic devices,[2]
sensors,[3] and catalysts.[3,4] However, only a few
examples are known with transition-metal clusters at
the dendrimer periphery.[5] A star decorated with four
polyoxometallate groups active in oxidation catalysis
was reported,[6] and other redox-active transition-
metal groups surrounding dendrimers at the periph-
ery include metallocenes[7] and ruthenium polypyr-
idine complexes.[2a] These molecular assemblies show
promise as electronic devices and sensors which can
be used by electrode modification.[8] Herein we report
1) functionalization of the long-known tetrairon clus-
ter [{CpFe(m3-CO)}4] (1),[9,10] the prototype ofredox-
rich organometallic clusters; 2) derivatization of9-,
16-, and 27-branched dendrimers therewith; 3) redox
behavior and redox robustness ofthese new metal-
lodendrimers and formation of modified electrodes
on which the cluster dendrimers are more strongly
adsorbed with increasing size; 4) their application as
selective sensors for oxo anions including adenosine-
5’-triphosphate (ATP2ꢀ); and 5) dendritic and struc-
tural effects on anion recognition, in particular the
selectivity ofrecognition ofATP 2ꢀ in the presence of
other anions and, for the first time, better recognition
ofATP 2ꢀ than H2PO4
.
ꢀ
Cluster 1 and its rich redox chemistry have been
known for a long time.[9] We have synthesized its acyl
Scheme 1. i) a) LiNiPr2, ꢀ408C, 1 h, THF; b) CO2, ꢀ40!208C; c) aq. 1n HCl,
208C (30% yield); ii) (COCl)2, CH2Cl2, 08C, 12 h (100% yield); iii) N-hydroxysuc-
chloride derivative
[Fe4Cp3(h5-C5H4CO2H)] (2)[9d] with (COCl)2, the
disulfide by reaction of with
3 by reaction ofthe acid
cinimide, NEt3, CH2Cl2, 208C, 12 h (85% yield); iv) { SCH2)11NH3+Clꢀ}2, NEt3,
ꢀ
4
3
CH2Cl2, 208C, 12 h (63% yield); v) NEt3, CH2Cl2, 208C, 12 h (65% yield).
[*] Dr. J. R. Aranzaes, Prof. D. Astruc
Nanosciences and Catalysis Group
LCOO, UMR CNRS No 5802
UniversitØ Bordeaux I
[Fe4(m3-CO)4Cp3(h5-C5H4COC)], which further reacts with
+
NEt3C or NEt2C. Successful functionalization of dendrimer 9
was performed by using N-succinimidyl ester 5[12] to give 16-
branched amido cluster dendrimer 10 [Eq. (2)].
33405 Talence Cedex (France)
Fax: (+33)540-006-646
E-mail: d.astruc@lcoo.u-bordeaux1.fr
The reaction of 5 with the fourth-generation 32-branched
amino dendrimer (DSM, following that of third-generation 9)
gave a dark green powder that was insoluble in all solvents,
obviously due to excess steric bulk at the dendrimer
periphery. However, the functionalization reaction worked
smoothly with the new 27-NH2 dendrimer 11[13] to give 27-Fe4
dendrimer 12 [Eq. (3)], because the interior and peripheral
tethers are longer in 11 than in the diaminobutane dendrimers
from DSM. The new metallodendrimers 8, 10,and 12 are air-
stable, forest-green powders. They were characterized by
Dr. C. Belin
LPCM, UMR CNRS No 5803
UniversitØ Bordeaux I
33405 Talence Cedex (France)
[**] Financialsupport from the Institut Universitaire de France (IUF,
DA), the UniversitØ Bordeaux I, and the CNRS is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
132
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Angew. Chem. Int. Ed. 2006, 45, 132 –136

Angewandte
Chemie
standard spectroscopic and elemental analyses, and atomic
force microscopy (AFM) on 10 and 12 (Figure 1 and
Supporting Information) showed monolayers of aggregated
metallodendrimers are formed on the mica surface. Indeed,
the heights of1.5 nm for 10 and 2.4 nm for 12 are reproducibly
obtained by AFM and correspond to the dimensions of
slightly flattened molecular models.[14]
ditions that avoid adsorption. This gave a result of27 ꢁ 3
electrons in CH2Cl2 and DMF with [FeCp*2] (Cp* = h5-
C5Me5) as internal reference. This stoichiometry could be
confirmed by titration of 12 in CH2Cl2 with 27 equiv ofgreen
[CpFe(h5-C5H4COCH3)]PF6, which generates [CpFe(h5-
C5H4COCH3)] and a dark green precipitate of[ 12](PF6)27,
characterized by the CO IR band at n˜CO = 1690 cmꢀ1, 55 cmꢀ1
The cyclovoltammograms (CVs) ofdendrimer clusters 8,
10,and 12 in CH2Cl2 (Pt, 0.1m nBu4NPF6) resemble that ofthe
monomeric cluster 1,[9c] that is, the clusters are sufficiently
remote from one another in the dendrimers to render the
electrostatic factor almost nil. Therefore all the redox sites
higher than n˜CO of 12 (1635 cmꢀ1). The color ofthe CH Cl2
2
solution changes from dark green (12) to red (acetylferro-
cene) at the equivalence point.[9]
This property offers the possibility of recognizing anions,
an area pioneered and deeply studied by Beer et al.,[16] then
also by Moutet et al.,[17] with endoreceptors functionalized
with various redox active species, although clusters have not
yet been used for such sensing. We are dealing here with
dendritic exoreceptors, a family that also proved successful
for sensing, but only with metallocene units.[3b]
+
corresponding to the redox change Fe4!Fe4 appear in a
+
2+
single reversible wave (the other waves Fe4 !Fe4 and
0
ꢀ
Fe4 !Fe4 are also reversible, see Supporting Informa-
tion).[15] For this wave, the Bard–Anson equation[15a] was
applied to determine the number ofelectrons under con-
Angew. Chem. Int. Ed. 2006, 45, 132 –136
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www.angewandte.org
133

Communications
value ofthe apparent association constant was K(+) = 412 ꢁ 70
(Supporting Information).
Contrary to the case of( nBu4N)H2PO4, addition of
(nBu4N)2(ATP) to the electrochemical cell containing the
solution ofthe host in CH Cl2 provoked the appearance ofa
2
new CV wave, although with very different features for each
dendrimer. In addition, the potential shifts are larger than
those observed with (nBu4N)H2PO4 (Figure 2, Table 1), which
contrasts with the behavior found for all metallocenyl
dendrimers. The titration diagrams were recorded by using
the decrease in intensity ofthe initial wave and increase in
intensity ofthe new wave ofr both 10 and 12. They show
equivalence points for 0.5–0.6 equiv (nBu4N)2(ATP) per
branch due to the double negative charges, which seemingly
2ꢀ
means that each phosphate monoanion unit ofATP
Figure 1. AFM pictures of 27-Fe4 cluster dendrimer 12 on a mica
surface.
interacts with one Fe4 cluster branch. Various other stoichio-
metries have been reported in such titrations.[17]
The addition of( nBu4N)HSO4 to dendrimer 10 provokes a
shift of the initial wave. This shift reaches 110 mV at
saturation, which leads to an apparent association constant
of K+ = (55 ꢁ 5) ” 103 Lmolꢀ1. The titration diagram shows an
equivalence point at 0.75 equiv (nBu4N)HSO4 per Fe4 cluster
branch, although saturation is obtained at 1 equiv
(nBu4N)HSO4 per branch (see Supporting Information). In
this case the interaction is ofthe weak type, loose, and not
selective.
The addition ofequimolar amounts of( nBu4N)2(ATP),
(nBu4N)HSO4, and (nBu4N)Cl to the electrochemical cell
containing dendrimer 10 leads to a shift of the initial wave by
0.1 V. The equivalence point is reached at 0.7 equiv (nBu4N)2
(ATP) (see Supporting Information), although no new CV
wave is observed. This wave shift instead of the appearance of
a new wave, when only (nBu4N)2(ATP) is added, and the
slight increase in stoichiometry, can tentatively be taken into
account by a dynamic equilibrium among the various ions of
Recognition ofthe oxo anions HSO ꢀ, H2PO4ꢀ, and
4
adenosine-5’-triphosphate (ATP2ꢀ) as their n-tetrabutylam-
monium salts by exoreceptors 8, 10, and 12 was investigated
by adding their nBu4N+ salts to electrochemical cells con-
taining a solution ofthe exoreceptor in CH 2Cl2 at a Pt anode.
Interestingly, for comparison, addition of these salts to a
solution ofthe monomeric amido cluster
6
or
[Fe4(CO)4Cp3(C5H4CONHnPr)] did not provoke any change
in the CV ofthe monomeric cluster. On the other hand,
addition of( nBu4N)2(ATP) to one ofthese three dendritic
clusters (even in the presence ofHSO and Clꢀ, vide infra)
ꢀ
4
gave recognition features that were very different from one
another and different from those of previous dendritic
metallocenyl exoreceptors.[3b]
With H2PO4ꢀ, a progressive wave shift was observed on
titration, which was approximated according to the weak-
interaction case in the Echegoyen–Kaifer model,[18] and the
134
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Angewandte
Chemie
positive potential. After disappearance of the initial wave, the
final chemically reversible wave has an EpaꢀEpc value of70 or
100 mV (Table 1), which signifies that electron transfer at the
electrode surface is slow. This is due to structural reorganiza-
tion ofthe dendritic host–guest supramolecular assembly that
involves, as in solution, formation versus disruption of large
ion pairs in synergy with double hydrogen bonding between
the oxo anion and the amido group.[16] The shape ofthis wave
is a fingerprint of the oxo anion. The salt (nBu4N)2(ATP) can
be washed away with CH2Cl2 to leave the modified electrode,
which now only shows the initial wave ofthe dendritic cluster,
although its current intensity is lower than initially. Subse-
quently, this washed, modified electrode can be used again.
The addition ofequimolar amounts of( nBu4N)2(ATP),
(nBu4N)HSO4, and (nBu4N)Cl to the electrochemical cell
containing dendrimer 12 leads to the appearance ofa new
wave, but the initial wave does not completely disappear after
equivalence, consistent with the suggested hypothesis ofa
dynamic equilibrium (see below and Supporting Informa-
tion).
In conclusion, we have successfully functionalized the
cluster [CpFe(m3-CO)]4 for covalent attachment through a Cp
ligand to the periphery of9-, 16-, and 27-branched dendrimers
and characterized the resulting cluster dendrimers inter alia
by AFM on mica, which showed their flattening. Their cyclic
voltammograms show a single reversible wave for the redox
+
change Fe4!Fe4 , and this CV wave can be used for redox
recognition and titration ofoxo anions and in particular
(nBu4N)2(ATP) in CH2Cl2 solution. The larger cathodic wave
shift with 10 than with 12 on ATP2ꢀ addition is presumably
due to the shorter distance between two clusters in 10 (12
bonds) than in 12 (16 bonds). Remarkably, and for the first
Figure 2. Titration of ATP2ꢀ with 8, 10, and 12 (6”10ꢀ5 m) in CH2Cl2.
Top: CVs before (bottom), during (middle), and after (top) addition of
(nBu4N)2(ATP) (x axis: voltage [V] vs [FeCp*2]). Bottom: Decrease of
&
the intensity of the initialCV wave ( ) and increase of the intensity of
*
the new CV wave ( ) versus the number n of equivalents of (nBu4N)2
time,
(nBu4N)2(ATP)
is
better
recognized
than
(ATP) added per cluster branch of 10.
(nBu4N)H2PO4, whereas the oppo-
site holds with metallocenyl den-
drimers. This specificity is certainly
Table 1: Cyclic voltammetry data for 8, 12 and 10 before, during, and after titration of (nBu4N)2(ATP).
due to the mutual nanosize ofthe
[a]
[b]
[c]
[d]
Fe4 clusters and ATP2ꢀ, which facil-
E1/2 (EpaꢀEpc)
E1/2 freeꢀE1/2new
E1/2bound (EPaꢀEPc)
K+/K0
itates their interaction, whereas the
smaller ferrocenyl groups do not
exhibit this property. Dendritic
effects (dendritic structure and com-
pacity) are dramatic as is the
replacement ofmetallocenes ofr
cluster redox sensors. A Pt electrode
modified with the 16-Fe4 or 27-Fe4
monomer
8
12
10
modified Pt electrode with 12
modified Pt electrode with 10
0.630 (60)
0.580 (30)
0.620 (30)
0.590 (30)
0.600 (10)
0.590 (30)
0.610 (100)
0.465 (100)
0.500 (100)
0.385 (200)
0.495 (70)
0.500 (100)
0.040
0.110
0.165
0.095
0.070
5
700
43
16
[a] E1/2 [V] vs [FeCp*2] (internalreference) convertibel into the value vs [FeCp 2] by subtracting 0.545 V;[19]
electrolyte, (nBu4N)PF6; working and counterelectrodes, Pt; solvent, CH2Cl2. The EpaꢀEpc values are
indicated in parentheses. [b] Difference of E1/2 value [V] between the free wave and the new wave at half
titration in order to observe and compare both waves (see Figure 1). [c] E1/2 after addition of 0.5–
0.6 equiv ATP2ꢀ (equivalence point). [d] Ratio between the apparent association constants of the
dendrimer provides selective ATP
recognition. It is possible to wash
the dendrimer with CH2Cl2 for
cationic (K+) and neutralform ( K0) with ATP2ꢀ
.
recycling, and the quality ofthe
modified electrode is optimum
the ion pairs and complexation kinetics that are different in
the presence and absence ofa mixture ofanions.
with the larger 27-Fe4 dendrimer 12 due to better adsorption.
Finally, given the known properties of 1 as a selective
hydrogenation catalyst for various functional groups,[9h] this
family of metallodendritic catalysts should also find use as
recyclable catalysts.[4,20]
Modification of a Pt electrode with dendrimers[6,7] such as
10 and 12 is possible (although not cleanly with the mono-
cluster thiol derivative 4); the best results were obtained with
12 due to its larger size (EpaꢀEpc = 10 mV, see Supporting
Information). Recognition of ATP2ꢀ also then proceeds with
the replacement ofthe initial wave by the new wave at less
Angew. Chem. Int. Ed. 2006, 45, 132 –136
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135

Communications
Westmeyer, M. A. Massa, T. B. Rauchfuss, S. R. Wilson, J. Am.
Chem. Soc. 1998, 120, 114; f) W.-Y. Yeh, C.-Y. Wu, L.-W. Chiou,
Organometallics 1999, 18, 3547; g) J. Ruiz, F. Ogliaro, J.-Y.
Saillard, J.-F. Halet, F. Varret, D. Astruc, J. Am. Chem. Soc. 1998,
120, 1163; h) E. Alonso, J. Ruiz, D. Astruc, C. R. Acad. Sci. Ser.
IIc 2001, 3, 189; i) cluster 1 is also a selective hydrogenation
catalyst: see for instance j) C. U. Pittman, R. C. Ryan, J. McGee,
J. P. OꢀConnor, J. Organomet. Chem. 1979, 178, C43.
Experimental Section
27-cluster dendrimer 12: The 27-NH2 dendrimer 11 (0.020 g, 3.8 ”
10ꢀ3 mmol) and triethylamine (0.029 mL, 0.2 mmol) were dissolved in
dry CH2Cl2. Complex 5 (0.112 g, 0.152 mmol) in CH2Cl2 (10 mL) was
added to this solution. The green solution was stirred at room
temperature for 7 d under positive nitrogen pressure. The solution
was then washed twice with a saturated sodium carbonate solution
and twice with water, and the green organic solution was dried over
sodium sulfate. The volume was reduced to 5 mL, and 25 mL of dry
diethyl ether was added, which gave a green powder. This precipitate
[10] C. ValØrio, J.-L. Fillaut, J. Ruiz, J. Guittard, J.-C. Blais, D. Astruc,
J. Am. Chem. Soc. 1997, 119, 2588.
[11] E. M. M. de Brabander-van den Berg, E. W. Meijer, Angew.
Chem. 1993, 105, 1370; Angew. Chem. Int. Ed. Engl. 1993, 32,
1308; for examples of seminal applications of diaminobutane-
based polyamine dendrimers using an amido linkage, see:
J. F. G. A. Jansen, E. M. M. de Brabander-van der Berg, E. W.
Meijer, Science 1994, 266, 1226.
[12] For previous syntheses ofamides rfom N-succinimidyl esters,
see: P. D. Beer, C. Harlewood, D. Hesek, J. Hodacova, S. E.
Stokes, J. Chem. Soc. Dalton Trans. 1993, 1327.
was dissolved in 5 mL ofCH Cl2, and the solution was poured over
2
25 mL ofdry diethyl ether with stirring, which yielded a forest-green
precipitate. The powdery 27-Fe4 dendrimer 12 was finally dried under
vacuum (0.030 g, 30% yield); dendrimer 10 was synthesized in the
same way (see data in Supporting Information).
See the Supporting Information for the syntheses of all the
clusters and dendrimers, AFM, and CVs including redox recognition
and titration data.
[13] J. Ruiz, G. Lafuente, S. Marcen, C. Ornelas, S. Lazarre, E.
Cloutet, J.-C. Blais, D. Astruc, J. Am. Chem. Soc. 2003, 125, 7250.
[14] a) A. Hierlemann, J. K. Campbell, L. A. Baker, R. M. Crooks,
A. J. Ricco, J. Am. Chem. Soc. 1998, 120, 5323; b) J. Li, L. T.
Piehler, D. Qin, J. R. Baker, Jr., D. A. Tomalia, D. J. Meier,
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[15] a) J. B. Flanagan, S. Margel, A. J. Bard, F. C. Anson, J. Am.
Chem. Soc. 1978, 100, 4248; b) A. J. Bard, R. L. Faulkner,
Electrochemical Methods, Wiley, New York, 1980.
Received: June 30, 2005
Revised: September 28, 2005
Published online: November 22, 2005
Keywords: anions · cluster compounds · dendrimers · iron ·
.
sensors
[16] P. D. Beer, Adv. Inorg. Chem. 1993, 39, 79; P. D. Beer, Chem.
Commun. 1996, 689 (feature article); P. D. Beer, Acc. Chem. Res.
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Collison, Chem. Soc. Rev. 2002, 31, 147; P. D. Beer, E. J. Hayes,
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[17] O. Reynes, J.-C. Moutet, J. Pecaut, G. Royal, E. Saint-Aman,
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