Self-assembly of a ferrocene-substituted porphyrin capable of electrochemically sensing neutral molecules via a tail on-tail off process

Article abstract of DOI:10.1039/b301177a

The supramolecular assembly of a ferrocene-porphyrin conjugate allowed ferrocene-based electrochemical sensing of the metalloporphyrin axial coordination via a tail on-tail off binding process.

Full text of DOI:10.1039/b301177a

Self-assembly of a ferrocene-substituted porphyrin capable of
electrochemically sensing neutral molecules via a “tail on–tail off” process
Christophe Bucher,* Charles H. Devillers, Jean-Claude Moutet,* Guy Royal and Eric Saint-Aman
Laboratoire d’Electrochimie Organique et de Photochimie Rédox, UMR CNRS 5630, Institut de Chimie
Moléculaire de Grenoble, FR CNRS 2607, Université Joseph Fourier, BP 53, 38041 Grenoble Cédex 9,
France
Received (in Cambridge, UK) 29th January 2003, Accepted 20th February 2003
First published as an Advance Article on the web 5th March 2003
The supramolecular assembly of a ferrocene–porphyrin
conjugate allowed ferrocene-based electrochemical sensing
of the metalloporphyrin axial coordination via a “tail on–tail
off” binding process.
The 1H NMR spectrum of 7, recorded in CDCl3 and assigned
using 1H–1H and 1H–13C correlation spectroscopies, evidenced
an interaction between the amine “tail” and the porphyrin metal
center (Fig. 1). In particular, five unresolved signals were
observed at high field, between 0 and 25 ppm, reflecting the
effect of the porphyrin ring current on the coordinated amine
fragment. Even the ferrocene protons were seen to be
significantly shifted and observed as a broad signal at 2.1 ppm.
The 1H NMR spectrum of 7 was found to be essentially
concentration independent over the concentration range of the
1H NMR measurements (Dd < 0.1 ppm for 7 3 1022 M 4 C
4 7 3 1025 M in CDCl3). These interactions were further
confirmed by UV-Vis spectroscopy measurements showing a
Soret band located at 429 nm (CH2Cl2, RT), as compared to 419
nm observed for square planar ZnTPP analogues.8 While the
spectra of 7 recorded in the concentration range of 1025 to 1023
M showed no change in the absorption wavelengths, further
dilution induced significant shifts, e.g. 424 nm at 5 3 1028 M.
These concentration dependent features brought us to consider
inter-molecular interactions leading to a (7)n-type supramo-
lecular assembly.9 Vapor pressure osmometry experiments
were performed to assess the self-assembled structure evi-
denced in solution. Measurements in toluene between concen-
trations of 5 3 1023 and 15 3 1023 M at 60 °C yielded an
effective molecular weight of 1957 ± 30, showing that 7 forms
a dimeric structure under these conditions.10 Insight into the
geometry of such species, obtained through molecular model-
ing,11 is depicted in Fig. 2.
Due to its unique structural and electronic properties, combined
with extensive chemical reactivity, the redox responsive unit
ferrocene has been incorporated into a large variety of substrate-
specific receptors.1 Ferrocene-appended porphyrins have in
particular received attention mainly because of their potential
applications in molecular electronic devices or as models for
multiple-electron transfer reactions.2 While numerous exam-
ples of ferrocene-appended porphyrins exist, directly connected
ferrocene–porphyrin conjugates are rare.3 We considered that a
direct linkage would promote communication between the
fragments and enable their use as electrochemical sensors. Here,
we report the synthesis and characterisation of such a ferro-
cene–porphyrin conjugate wherein the ferrocene is further
substituted by a hexylamine “tail” and show that the self-
assembly observed in solution allows unprecedented ferrocene-
based electrochemical sensing of neutral species via a metal-
loporphyrin-centred “tail on–tail off” binding process.4
Scheme 1 illustrates the synthesis of the ferrocene–porphyrin
conjugate 7. The sequence starts with the mono-protection of the
known diformyl ferrocene 15 according to a modified literature
procedure6 to give 2 in ca. 70% yield. The MacDonald type
condensation of the latter with tolyldipyrromethane in the
presence of TFA, followed by oxydation with chloranil, afforded
a mixture of ferrocene-substituted porphyrin derivatives from
which 3 was isolated in 8% yield. 4 was obtained from 3 by
removal of the cyclic dithioketal using a mixture of Ag+ ions and
N-chlorosuccinimide (NCS). A nitrogenous “tail” was in-
troduced onto the ferrocene moiety by reaction of n-hexylamine
with the regenerated formyl group to produce 5.7 This was
immediately reduced with an excess of NaBH4 to produce 6.
Finally, the zinc complex 7† was prepared quantitatively in a
dichloromethane–methanol mixture using the acetate salt of the
metal cation.
Electrochemical investigations carried out in CH2Cl2 con-
taining 0.1 M TBAP also evidenced an assembling phenome-
non. Using cyclic or differential pulsed voltammetry, five
different oxidation processes were observed (Fig. 3): a
reversible ferrocene-based oxidation at E1/2 = 60 mV (vs. Fc0/+
)
followed by four porphyrin-based oxidations at 280, 370, 630
and 810 mV. As has already been observed for polymeric
structures,12 the shoulder at 280 mV on the first porphyrin
oxidation wave arises from interactions between the porphyrin
rings in the supramolecular assembly. The irreversible oxy-
dation process at 630 mV can be attributed to the second
porphyrin oxidation to give an electrophilic species known to
irreversibly lead to isoporphyrins in the presence of nucleo-
philes.13 The last oxidation process, at 810 mV, thus result most
presumably from the oxidation of an isoporphyrin cation
wherein the nitrogenous “tail” has been incorporated into the
porphyrin framework.
Cleavage of the supramolecular structure was induced upon
addition of nitrogenous bases to a CH2Cl2 solution of 7 and
Scheme 1 Reagents and conditions: i (70%), CH2Cl2, Ar, SH(CH2)2SH
(1.05 eq.), BF3·OEt2, col. chromat. (SiO2, hexane–20% EtOAc); ii (8%),
CH2Cl2, Ar, tolyldipyrromethane (1 eq.), TFA (cat.), 30 min; chloranil, col.
chromat. (SiO2, CH2Cl2–50% hexane); iii (70%), THF/CH3CN/H2O,
AgNO3 (6 eq.), NCS (6 eq.), col. chromat. (SiO2, CH2Cl2–0.5% MeOH);
iv–v (80%), THF, mol. sieves, Ar, CH3(CH2)5NH2 (1.2 eq.), 16 h; diethyl
ether/EtOH, NaBH4 (5 eq.), 3 h, col. chromat. (SiO2, CH2Cl2–5% MeOH);
vi (100%), CH2Cl2, ZnOAc2.H2O/MeOH (excess), 10 h, col. chromat.
(SiO2, CH2Cl2–5% MeOH).
Fig. 1 Part of the 1H NMR spectrum of 7 (RT, CDCl3, 500 MHz).
888
CHEM. COMMUN., 2003, 888–889
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process, this result reveals the relevance of directly connected
ferrocene–porphyrin conjugates as electrochemical sensors and
will lead us to consider new and effective multipoint archi-
tecture for the electrochemical recognition of target mole-
cules.
We thank the CECIC (UJF Grenoble) and especially M.-L.
Dheu-Andries for assistance in molecular modeling.
Notes and references
† Spectroscopic data for 7: MS (MALDI-TOF): [M]+ = 939.2; 1H NMR
(CDCl3, 500 MHz, 293 K): d 25.18 (bs, 1H), 22.54 (bs, 2H), 21.98 (bs,
2H), 21.76 (bs, 2H), 20.86 (bs, 2H), 20.03 (bs, 2H), 0.43 (t, 3J = 7 Hz,
3H, –CH2CH3), 0.52 (m, 2H, –CH2CH3), 2.23 (bs, 2H, Fc), 2.72 (s, 3H, Ph–
CH3), 2.73 (s, 6H, Ph–CH3), 3.64 (bs, 2H, Fc), 4.24 (bs, 2H, Fc), 5.0 (bs,
2H, Fc), 7.53–7.60 (m, 6H), 8.07–8.15 (m, 6H), 8.87 (m, 6H), 9.82 (bs, 2H,
b-pyr). 13C NMR (CDCl3, 62.5 MHz, 293 K): 13.5, 21.5, 22.0, 23.9, 24.9,
30.2, 42.8, 42.9, 69.4, 69.5, 71.0, 78.2, 82.5, 90.7, 116.1, 120.2, 120.9,
127.1, 130.7, 131.4, 131.5, 134.3, 134.7, 136.7, 140.2, 140.4, 149,2, 149.8,
150.0, 150.2; lmax/nm (CH2Cl2) (log e) 429 (5.43), 580 (4.06), 631 (4.28);
Elemental analysis: calc. for C58H53FeN5Zn, C 74.00, H 5.67, N 7.44%;
obs. C 73.15, H 5.66, N 7.30%.
Fig. 2 Molecular modeling of (7)2 using InsightII-Discover 2000.
Fig. 3 (—) Cyclic voltammogram of a 5 3 1024 M CH2Cl2 (TBAP, 0.1 M)
solution of 7; (…) cyclic voltammogram corresponding to the ferrocenyl
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3 mm diameter carbon disk).
evidenced through the low-field shift of the 1H NMR signals, a
slight red shift in the absorption maximum observed in the UV-
Vis spectrum and, as expected, by means of electrochemical
techniques. Confirming the assumption that the shape of the
first porphyrin-based oxidation is inherent to intermolecular
assembling, addition of pyridine or imidazole causes the
disapearance of the shoulder observed at ca. 280 mV, along
with a marked irreversibility for all the porphyrin-based
oxidation processes.
On the other hand, the oxidation of the ferrocenyl moiety
remains reversible in the presence of coordinating bases and
allowed us to electrochemically monitor the axial coordination
of the metalloporphyrin via a “tail on–tail off” binding process.
The Fc/Fc+ half-wave potential negatively shifted by 110, 140
and 150 mV upon the addition of an excess pyridine, imidazole
and 2-methylimidazole respectively (Fig. 4). Compared to the
rare reported molecular receptors able to electrochemically
sense neutral guests,1,14 the DE values induced by the addition
of nitrogenous bases to the redox active supramolecular
assembly 7 are particularly high.15 In addition, the differences
between the potential decays, which reflect the individual
binding strengths,16 emphasize the efficient electronic commu-
nication between the p-aromatic systems of the porphyrin and
the ferrocene moieties.
10 A temperature increase from 295 to 330 K (C6D6) proved to greatly
1
enhance the resolution of most H NMR signals with only a low field
In conclusion, we synthesized a novel functionalized ferro-
cene-substituted porphyrin and showed that a self-assembly
phenomenon, coupled with efficient electronic communication
throughout the receptor, allows an unprecedented ferrocene-
based electrochemical sensing of neutral species via a metal-
loporphyrin-centred “tail on–tail off” binding process.17 While
current work is focused on studying in detail this dynamic
shift (0.4 to 0.5 ppm) for the f, g and e signals.
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without a nitrogen based tail induced only small potential shifts, e.g. DE
= 220 mV after addition of an excess of pyridine.
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17 Different indicator displacements methods can be found in the literature.
As an example of a colorimetric sensing ensemble see Z. Zhong and E.
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Fig. 4 Evolution of the ferrocene potential in 7 as a function of the addition
of nitrogenous bases. Same conditions as in Fig. 3.
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