Electroactive thiazole derivatives capped with ferrocenyl units showing charge-transfer transition and selective ion-sensing properties: A combined experimental and theoretical study

Article abstract of DOI:10.1021/ic061803b

The synthesis, optical and electrochemical properties, and X-ray characterization of two thiazole derivatives capped by ferrocenyl groups (5 and 7) and their model compounds with one ferrocenyl, either at 2 or 5 position of the mono- or bis-thiazolyl rings (3, 9, 11, and 14), are presented. Bisferrocenyl thiazole 5 forms the mixed-valence species 5^?+ by partial oxidation which, interestingly, shows an intramolecular electron-transfer phenomenon. Moreover, the reported heteroaromatic compounds show selective ion-sensing properties. Thus, ferrocenylthiazoles linked across the 5 position of the heteroaromatic ring are selective chemosensors for Hg ²⁺ and Pb²⁺ metal ions; 5-ferrocenylthiazole 3 operates through two channels, optical and redox, for Hg²⁺ and only optical for Pb²⁺, whereas 1,1′-bis(thiazolyl)ferrocene 14 is only an optical sensor for both metal ions. Moreover, complex 3 behaves as an electrochemically induced switchable chemosensor because of the low metal-ion affinity of the oxidized 3^?+ species. On the other hand, ferrocenylthiazole 9, in which the heterocyclic ring and the ferrocene group are linked across the 2 position, is a selective redox sensor for Hg²⁺ metal ions, and it responds optically, as does bis(thiazolyl)-ferrocene 11, to a narrow range of cations (Zn²⁺, Cd²⁺, Hg²⁺, Ni²⁺, and Pb²⁺). Finally, bis(ferrocenyl)thiazole 5 is a dual optical and redox sensor for Zn²⁺, Cd²⁺, Hg ²⁺, Ni²⁺, and Pb²⁺, whereas bis(ferrocenyl) compound 7, bearing a bis(thiazole) unit as a bridge, is only a chromogenic sensor for Zn²⁺, Cd²⁺, Hg²⁺, Ni²⁺, and Pb²⁺. The experimental data and conclusions about both the electronic and ion-sensing properties are supported by DFT calculations which show, in addition, an unprecedented intramolecular electron-transfer reorganization after the first one-electron oxidation of compound 5.

Full text of DOI:10.1021/ic061803b

Inorg. Chem. 2007, 46, 825−838
Electroactive Thiazole Derivatives Capped with Ferrocenyl Units
Showing Charge-Transfer Transition and Selective Ion-Sensing
Properties: A Combined Experimental and Theoretical Study
Antonio Caballero,† Vega Lloveras,‡ David Curiel,† Alberto Ta´rraga,† Arturo Espinosa,†
Rafaela Garc´ıa,† Jose´ Vidal-Gancedo,‡ Concepcio´ Rovira,‡ Klaus Wurst,§ Pedro Molina,*,† and
Jaume Veciana*,‡
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, Campus de Espinardo, UniVersidad de
Murcia, E-30100 Murcia, Spain, Institut de Cie`ncia de Materials de Barcelona (CSIC), Campus
UniVersitari de Bellaterra, E-08193 Cerdanyola, Spain, and Institut fu¨r Allgemeine Anorganische
und Teoretische Chemie, UniVersita¨t Innsbru¨ck, Innrain 52a, A-6020 Innsbruck, Austria
Received September 22, 2006
The synthesis, optical and electrochemical properties, and X-ray characterization of two thiazole derivatives capped
by ferrocenyl groups (5 and 7) and their model compounds with one ferrocenyl, either at 2 or 5 position of the
mono- or bis-thiazolyl rings (3, 9, 11, and 14), are presented. Bisferrocenyl thiazole 5 forms the mixed-valence
species 5•+ by partial oxidation which, interestingly, shows an intramolecular electron-transfer phenomenon. Moreover,
the reported heteroaromatic compounds show selective ion-sensing properties. Thus, ferrocenylthiazoles linked
+
+
across the 5 position of the heteroaromatic ring are selective chemosensors for Hg2 and Pb2 metal ions;
5-ferrocenylthiazole 3 operates through two channels, optical and redox, for Hg2+ and only optical for Pb2+, whereas
1,1′-bis(thiazolyl)ferrocene 14 is only an optical sensor for both metal ions. Moreover, complex 3 behaves as an
electrochemically induced switchable chemosensor because of the low metal-ion affinity of the oxidized 3•+ species.
On the other hand, ferrocenylthiazole 9, in which the heterocyclic ring and the ferrocene group are linked across
+
the 2 position, is a selective redox sensor for Hg2 metal ions, and it responds optically, as does bis(thiazolyl)-
ferrocene 11, to a narrow range of cations (Zn2+, Cd2+, Hg2+, Ni2+, and Pb2+). Finally, bis(ferrocenyl)thiazole 5 is
a dual optical and redox sensor for Zn2+, Cd2+, Hg2+, Ni2+, and Pb2+, whereas bis(ferrocenyl) compound 7, bearing
a bis(thiazole) unit as a bridge, is only a chromogenic sensor for Zn2+, Cd2+, Hg2+, Ni2+, and Pb2+. The experimental
data and conclusions about both the electronic and ion-sensing properties are supported by DFT calculations
which show, in addition, an unprecedented intramolecular electron-transfer reorganization after the first one-electron
oxidation of compound 5.
Introduction
electron transfer,3 has been of considerable interest to both
chemists and biologists. Complexes featuring equivalent
redox-active ferrocene groups as end-caps in a molecular
array represent one of the most widely investigated families
of MV systems. Generally, IVCT transitions are observed
when the two ferrocenes are joined either by linear π-con-
jugated carbon-chain bridges,4 or metal-containing frag-
ments,5 although weak IVCT transitions have also been
observed in a few saturated systems.6 However, heteroaro-
Mixed-valence (MV) compounds can be regarded as the
simplest models for testing electron-transfer systems.1,2
Analysis of their intervalence charge-transfer (IVCT) transi-
tions by the Hush theory, which links optically induced
electron transfer with the Marcus theory of thermally induced
* To whom correspondence should be addressed. E-mail: vecianaj@
icmab.es (J.V.), pmolina@um.es (P.M.).
† Universidad de Murcia.
‡ Institut de Cie`ncia de Materials de Barcelona (CSIC).
§ Universita¨t Innsbru¨ck.
(3) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391-444. (b) Hush, N.
S. Coord. Chem. ReV. 1985, 64, 135-157.
(4) Barlow, S.; O’Hare, D. Chem. ReV. 1997, 97, 637-669 and references
therein.
(1) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247-
422.
(2) Allen, G. C.; Hush, N. S. Prog. Inorg. Chem. 1967, 8, 357-389.
10.1021/ic061803b CCC: $37.00
Published on Web 01/11/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 3, 2007 825

Caballero et al.
matic ring systems7 capped by ferrocenyl groups are rare,
and moreover, in all the reported cases the heterocyclic
spacers do not allow significant electronic communication
between the two ferrocene units.
Experimental Section
General Procedures. All reactions were carried out using
solvents which were dried by routine procedures. All melting points
were determined on a hot-plate melting point apparatus and are
uncorrected. IR spectra were determined as Nujol emulsions or
films. 1H and 13C NMR spectra were recorded at 200 or 300 MHz.
The following abbreviations for stating the multiplicity of the signals
have been used: s (singlet), bs (broad singlet), d (doublet), t (triplet),
bt (broad triplet), st (pseudotriplet), m (multiplet), and q (quaternary
carbon atom). Chemical shifts refer to signals of tetramethylsilane
in the case of 1H and 13C NMR spectra. EPR spectra were obtained
with a Bruker 300 spectrometer equipped with a TE102 microwave
cavity, a variable temperature unit, and a field frequency lock
system, and line positions were determined with a NMR gaussmeter.
The modulation amplitude was kept well below the line width, and
the microwave power was well below saturation. Crystallographic
measurements were made at 233(2) K on a diffractometer with the
area detector positioned at the window of a rotating anode generator
using Mo KR radiation (0.71073 Å). The cyclic voltammetric
measurements were performed on a potentiostat/galvanostat con-
trolled by a personal computer and driven by dedicated software
with a conventional three-electrode configuration consisting of
platinum working and auxiliary electrodes and an SCE reference
electrode. The experiments were carried out with a 10-3 M solution
of sample in dry CH2Cl2 containing 0.1 M [(n-Bu)4N]ClO4
(Warning: Caution! Potential formation of highly explosiVe
perchlorate salts of organic deriVatiVes) as supporting electrolyte.
Deoxygenation of the solutions was achieved by bubbling nitrogen
for at least 10 min, and the working electrode was cleaned after
each run. The cyclic voltammograms were recorded with a scan
rate between 0.05 and 0.5 V s-1. The DPV voltammograms were
recorded before and after the addition of aliquots of 0.1 equiv of
2.5 × 10-2 M solutions of HBF4‚Et2O in CH3CN. The following
We have recently reported8 that bishomometallic com-
plexes containing two ferrocene units linked by the electro-
active 2-aza-1,3-butadiene bridge allow the study of the
influence of this disymmetric bridge on the intramolecular
electron-transfer (IET) phenomenon, demonstrating that this
oxidizable bridge promotes the IET between the two metallic
centers through two different pathways. On the basis of this
body of work, we have decided to study the electrochemical
and optical behavior of ferrocene derivatives linked to an
electron-accepting thiazole ring as a core π-electron system,
which can be viewed structurally as a 2-aza-1,3-butadiene
closed by a sulfur atom. The π-bond order, calculated either
by HMO or by CNDO/2 methods, is indicative of a thiazole
molecule situated between an aromatic ring and a diene
system, and therefore, a significant electronic communication
was expected for such a system. On the other hand, in this
kind of molecule, the nitrogen and sulfur atoms in the
thiazole ring add another interesting and useful function such
as the ability to act as metal-ion ligands. Moreover, the
reversibility of the ferrocene/ferrocenium redox couple and
the ability of the thiazole ring to act as a ligand toward metal
ions may operate cooperatively within the molecule. This
synergistic relation may create a molecular switch which
would allow the complexing ability of the thiazole subcom-
ponent to be turned off when a positive charge within the
ferrocene moiety is generated. Upon reduction, complexing
ability would be restored, and consequently, the combination
of ferrocenes and thiazole rings could be of interest for the
construction of heterobimetallic systems which can behave
not only as suitable models to study the intramolecular
charge-transfer across this heteroaromatic ring system but
also as redox-switching receptors, via electrochemical and/
or optical methodologies, with the capability of selectively
sensing metal-ion guests.9
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826 Inorganic Chemistry, Vol. 46, No. 3, 2007

ElectroactiWe Thiazole DeriWatiWes
settings were used: pulse amplitude, 50 mV; pulse width, 50 ms;
scan rate, 100 mV/s; sample width, 17 ms; pulse period, 200 ms.
Decamethylferrocene (DMFe) (-0.07 V vs SCE) was used as an
internal reference both for potential calibration and for reversibility
criteria. Oxidations were performed by electrolysis in a three-
electrode cell under argon using dry CH2Cl2 as a solvent and 0.15
M [(n-Bu)4N]PF6 as supporting electrolyte. The progress of the
oxidation was followed coulombimetrically (or chronoampero-
metrically) by 263A of EG&PAR potentiostat-galvanostat. The
reference electrode and the counter electrode were separately
immersed in the solvent containing the supporting electrolyte and
isolated from the bulk solution by a glass frit. The working electrode
was a platinum grid. UV-vis-near-IR absorption spectra were
regularly recorded by transferring a small aliquot of the solution
contained in the electrochemical cell into a UV quartz cell for
different average number of removed electrons.
X-ray Measurement and Structure Determination. Data
collection was performed on a Nonius Kappa CCD equipped with
graphite-monochromatized Mo KR radiation (λ ) 0.71073 Å) and
nominal crystals to area detector distance of 36 mm. Intensities
were integrated using DENZO and scaled with SCALEPACK.
Several scans in φ and ω direction were made to increase the
number of redundant reflections, which were averaged in the
refinement cycles. This procedure replaces in a good approximation
an empirical absorption correction. The structures were solved with
direct methods SHELXS86 and refined against F2 SHELX97. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atoms attached to carbon atoms were
calculated and refined with isotropic displacement parameters 1.2
times higher than the value of their carbon atoms. A 1:1 disorder
occurs for 5 at position N1 and C2, whereas each position is
occupied by half nitrogen and carbon atom. Hydrogen atoms,
attached to these positions, were refined with occupancy of 0.5.
The disorder can be recognized by the abnormal behavior of the
thermal ellipsoids. Refinement of each disordered position alternate
with a complete nitrogen atom or C-H group increase the R1-
value from 0.0268 to 0.0294 or 0.0296, and the thermal ellipsoids
for the nitrogen atoms are around 1.5 times higher in each direction
as for three ordered atoms of the five-membered ring. Alternatively,
the disorder can be seen by refinement of the disordered model
without the hydrogen atoms at the disordered positions. The rest
of the electron density shows two strong peaks with nearly the same
electron density at the former hydrogen positions. All the crystal
structures described in this article have been deposited at the
Cambridge Crystallographic Data Center and allocated the deposi-
tion numbers CCDC 612848-612851. These data can be obtained
free of charge from The Cambridge Crystallographic Data Center
via www.ccdc.cam.ac.uk/data_request/cif.
energies are uncorrected for the ZPVE (zero point vibrational
energy) and computed in the gas phase. Atomic charges were
obtained from SP calculations at the B3LYP/6-311G** level in
terms of the natural atomic orbital (NAO) basis set, by using the
natural bond orbital (NBO) method. All complexation studies were
carried out with all structures being fully optimized using the
B3LYP functional and two different basis sets, 6-31+G* for all
atoms, and 6-311G* for Mg2+, Ni2+, and Zn2+, Stuttgart RSC-ECP14
for Cd2+ and Hg2+, and aug-cc-pV5Z-PP-ECP14 for Pb (method
A); 6-31G* for all atoms, but including diffuse functions for N, S,
and Fe (denoted as aug-6-31G*), and using Lanl2DZ-ECP for Mg2+
,
Ni2+, Zn2+, Cd2+, Hg2+, and Pb2+ (method B). Complexation
energies were computed from SPE calculations at the B3LYP/6-
311+G** level except for the divalent metal cations for which the
same basis sets used in the optimization step were employed.
Solvent (dichloromethane) effects were obtained by reaction field
SP calculations on the gas-phase optimized geometries, using Cossi
and Barone’s CPCM (conductor-like polarizable continuum model)
formalism15 and correcting the radius for Ni2+, Zn2+, Cd2+, Hg2+
,
and Pb2+ to 1.406, 1.523, 1.720, 1.732, and 1.709 Å (i.e., increasing
the implemented radius of magnesium, by the rcovM/rcov ratio),
Mg
respectively.
General Procedure for the Preparation of â-Ketoamides. To
a suspension of the desired R-aminocarbonyl hydrochloride com-
pound (1.5 mmol), in dry THF (15 mL), at room temperature and
under nitrogen atmosphere, was added dry triethylamine (3.5 mmol).
After the mixture was stirred for 5 min, the appropriate amount of
the adequate acid chloride in dry THF (5 mL) was added, and the
resulting solution was stirred at room temperature for 4-6 h. The
solvent was removed by rotary evaporation, and the residue was
purified by column chromatography on silica gel, eluting with the
appropriate solvent to afford the corresponding â-ketoamide which
was crystallized from CH2Cl2/Et2O (1/1).
N-(Ferrocenecarbonylmethyl)benzamide (2).16 Yield 68%; mp
1
145-147 °C; Rf ) 0.58 (CH2Cl2/AcOEt/n-Hex, 1/1/1). H NMR
(300 MHz, CDCl3): δ 4.25 (s, 5H), 4.61 (s, 2H), 4.67 (d, 2H,
J(H,H) ) 4.05 Hz), 4.91 (s, 2H), 7.26 (bs, 1H), 7.46-7.50 (m,
3H), 7.87-7.91 (m, 2H). Anal. Calcd for C19H17FeNO2: C, 65.73;
H, 4.94; N, 4.03. Found: C, 65.95; H, 4.68; N 4.09.
N-(Ferrocenecarbonylmethyl)ferrocenecarboxamide (4).16 Yield
49%; mp 211-214 °C (d); Rf ) 0.50 (CH2Cl2/AcOEt/n-Hex, 1/1/
1
1). H NMR (300 MHz, CDCl3): δ 4.25 (s, 5H), 4.26 (s, 5H),
4.38 (st, 2H), 4.59-4.62 (m, 4H), 4.77 (st, 2H), 4.90 (st, 2H), 6.72
(bt, 1H). Anal. Calcd for C23H21Fe2NO2: C, 60.70; H, 4.65; N,
3.08. Found: C, 60.48; H, 4.78; N, 3.23.
N,N′-Bis(ferrocenecarbonylmethyl)oxalamide (6). Yield 47%;
1
mp 198-201 °C; Rf ) 0.63 (AcOEt/n-Hex, 4/1). H NMR (300
MHz, CDCl3): δ 4.18 (s, 5H), 4.46-4.53 (m, 4H), 4.79 (bs, 2H),
8.1 (bt, 1H). Anal. Calcd for C26H24Fe2N2O4: C, 57.81; H, 4.48;
N, 5.19. Found: C, 57.56; H, 4.20; N, 5.41.
Computational Procedures. Calculated geometries of com-
pounds 3, 5, 7, 9, and 15, and their oxidized counterparts, were
fully optimized with the Gaussian 0310 package at the DFT level
of theory by using the B3LYP functional11 (Becke’s three param-
eters hybrid functional12 with the Lee-Yang-Parr correlation
functional13) and the 6-31G* basis set, with tight convergence
criteria. Energy data were computed as SP (single-point) calcula-
tions at the B3LYP/6-311G** level. Reported total electronic
(14) The Stuttgart relativistic small core basis sets were obtained from the
Extensible Computational Chemistry Environment Basis Set Database,
Version 10/29/02, as developed and distributed by the Molecular
Science Computing Facility, Environmental and Molecular
Sciences Laboratory, which is part of the Pacific Northwest Laboratory,
P.O. Box 999, Richland, Washington 99352, and funded by the
U.S. Department of Energy. The Pacific Northwest Laboratory
is a multiprogram laboratory operated by Battelle Memorial Institute
for the U.S. Department of Energy under contract DE-AC06-76RLO
1830.
(10) Gaussian 03, Revision B.03, Frisch, M. J. et al. Gaussian, Inc.,
Pittsburgh PA, 2003. For more related references see Supporting
Information.
(11) Bartolottiand, L. J.; Fluchick, K. In ReViews in Computational
Chemistry; Lipkowitz, K. B.; Boyd, B. D., Eds.; VCH: New York,
1996; Vol. 7, pp 187-216.
(12) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(13) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(15) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995-2001.
(b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem.
2003, 24, 669-681.
(16) Ta´rraga, A.; Molina, P.; Curiel, D.; Velasco, M. D. Tetrahedron Lett.
2002, 43, 8453-8457.
Inorganic Chemistry, Vol. 46, No. 3, 2007 827

Caballero et al.
N-(Phenacyl)ferrocenecarboxamide (8). Yield 76%; mp 140-
143 °C; Rf ) 0.39 (CH2Cl2/AcOEt/n-Hex, 1/1/1). H NMR (300
was filtered over a Celite pad which was washed with acetic acid
(3 × 15 mL). To the combined filtrates a stream of dry HCl was
bubbled for 2 h, and then, the solvent was evaporated under reduced
pressure. The resulting oil was triturated with diethyl ether (20 mL)
to afford the title compound, in 90% yield, which was used without
further purification. IR (Nujol; cm-1): 3405, 2925, 2864, 2614,
1
MHz, CDCl3): δ 4.25 (s, 5H), 4.39 (s, 2H), 4.78 (s, 2H), 4.90 (d,
2H, J(H,H) ) 4.3 Hz), 6.80 (bs, 1H), 7.48-7.65 (m, 3H), 8.04
ppm (d, 2H, J(H,H) ) 7.2 Hz). Anal. Calcd for C19H17FeNO2: C,
65.73; H, 4.94; N, 4.03. Found: C, 65.63; H, 4.88; N, 4.12.
N,N′-Bis(phenacyl)-1,1′-ferrocenedicarboxamide (10). Yield
53%; mp 179-180 °C; Rf ) 0.46 (AcOEt CH2Cl2, 7/3). 1H NMR
(300 MHz, CDCl): δ 4.49 (s, 4H), 4.88 (d, 2H, J(H,H) ) 5.9 Hz),
4.92 (s, 4H), 7.48-7.65 (m, 2H), 8.00 (d, 4H, J(H,H) ) 7.3 Hz),
8.27 (t, 2H, J(H,H) ) 5.9 Hz). Anal. Calcd for C28H24FeN2O4: C,
66.16; H, 4.76; N, 5.51. Found: C, 66.38; H, 4.60; N, 5.48.
1,1′-Bis(benzoylaminoacetyl)ferrocene (13). Yield 79%; mp
1
1694, 1675, 1588, 1516, 1270, 1079, 880. H NMR (300 MHz,
CDCl3): δ 4.17 (bs, 4H), 4.84 (bs, 4H), 5.08 (bs, 4H), 8.45 (bs,
4H). 13C NMR (300 MHz, CDCl3): δ 47.8 (2 × CH2), 73.9 (4 ×
CH, Cp), 78.5 (4 × CH, Cp), 200.5 (2 × CdO). EIMS (70 eV)
m/z (relative intensity): 300 (100) [M+ - 2HCl], 271 (98), 254
(44), 178 (54), 165 (70), 121 (70).
Results and Discussion
1
222-225 °C; Rf ) 0.37 (CH2Cl2/AcOEt/n-Hex, 1/1/1). H NMR
(300 MHz, CDCl3): δ 4.60 (d, 4H, J(H,H) ) 4.8 Hz), 4.65 (st,
4H), 4.98 (st, 4H), 7.24 (bt, 2H), 7.39-7.54 (m, 6H), 7.88 (d, 4H,
J(H,H) ) 7.2 Hz). Anal. Calcd for C28H24FeN2O4: C, 66.16; H,
4.76; N, 5.51. Found: C, 65.93; H, 4.52; N, 5.39.
Synthesis. The general synthetic route for the 2,5-
disubstituted thiazole derivatives is based on a two-step
sequence. First, acylation of the suitable R-aminocarbonyl
compound with the proper acyl chloride in the presence of
Et3N provides the corresponding â-ketoamide intermediate
which, in a second step, undergoes a heterocyclization
reaction, promoted by the action of the Lawesson’s reagent
[4-MeOC6H4P(S)S]2 (LR), to afford the suitable disubstituted
thiazole ring. In this approach, the substituent at position 5
of the thiazole ring arises from the R-aminocarbonyl
compound, whereas the substituent at position 2 comes from
the acyl chloride component. Using this methodology,
compounds 3, 5, and 7 were prepared in 31%, 62%, and
30% yields, respectively, starting from of R-aminoacetylferro-
cene7e 1 and using benzoyl chloride, chlorocarbonyl fer-
rocene, or oxalyl choride, respectively, as acylating agents
(Scheme 1).
General Procedure for the Preparation of 2,5-Disubstituted
Thiazole Derivatives. To a suspension of the appropriate mono-
(0.8 mmol) or bis-â-ketoamide (0.6 mmol) in dry THF (15 mL),
Lawesson’s reagent (1.6 mmol, for mono- or 2.4 mmol, for bis-
â-ketoamides, respectively), was added in one portion. The suspen-
sion was heated at reflux under nitrogen for 5 h, and then the solvent
was removed under reduced pressure to give the corresponding
thiazole derivatives, as crude products, which were column chro-
matographed on silica gel, using EtOAc/n-hexane (7/3) as eluent,
and afterward crystallized from CH2Cl2/ Et2O (1/3).
5-Ferrocenyl-2-phenylthiazole (3).16 Yield 31%; mp 91-
1
94 °C; Rf ) 0.85 (CH2Cl2/AcOEt/n-Hex, 1/1/1). H NMR (300
MHz, CDCl3): δ 4.13 (s, 5H), 4.34 (st, 2H), 4.60 (st, 2H), 7.41-
7.47 (m, 3H), 7.71 (s, 1H), 7.91-7.97 (m, 2H). Anal. Calcd for
C19H15FeNS: C, 66.10; H, 4.38; N, 4.06. Found: C, 65.96; H, 4.50;
N 4.24.
2,5-Bis(ferrocenyl)thiazole (5).16 Yield 62%; mp 221-222 °C;16
Rf ) 0.72 (CH2Cl2/AcOEt/n-Hex, 1/1/1).
Likewise, starting from phenacylamine and using chloro-
carbonyl ferrocene or 1,1′-dichlorocarbonyl ferrocene as
acylating reagents, the monoferrocenyl-thiazole 9 and 1,1′-
bisthiazolyl ferrocene, 11, were obtained in 74% and 71%
yields, respectively (Scheme 2).
5,5′-Bis(ferrocenyl)-2,2′-bis(thiazole) (7). Yield 30%; mp 112-
1
114 °C; Rf ) 0.74 (CH2Cl2/AcOEt/n-Hex, 1/1/1). H NMR (300
MHz): δ 4.14 (s, 10H), 4.39 (st, 4H), 4.64 (st, 4H), 7.72 (s, 2H).
Anal. Calcd for C26H20Fe2N2S2: C, 58.23; H, 3.76; N, 5.22.
Found: C, 58.06; H, 3.89; N, 5.04.
On the other hand, 1,1′-bis(2-phenylthiazol-5-yl)ferrocene
14 was also obtained, in 43% yield, from the reaction of
benzoyl chloride with the previously unreported 1,1′-bis(R-
aminoacetyl)ferrocene 12, synthesized by catalytic hydro-
genation of 1,1′-bis(R-azidoacetyl)ferrocene7d (Scheme 3).
Spectral (MS and 1H and 13C NMR) and elemental analysis
data of both the newly synthesized â-ketoamide intermediates
and thiazole derivatives are consistent with the proposed
structures.
Molecular Structures. Single crystals of compounds 3,
5, 11, and 14, suitable for X-ray structure determination, were
grown from diffusion of n-hexane into a solution of the
compound in CH2Cl2 or by slow evaporation of the solvent.
Unfortunately, all attempts to grow good quality single
crystals of compounds 7 and 9 failed.
Compound 3 crystallizes in the monoclinic space group
P21/n with four molecules in the unit cell. Figure 1a shows
the ORTEP drawing of the molecular structure of 3 with
the atomic numbering scheme. The two Cp rings have a small
tilt angle between them, and the angle between the mean
planes of the bridged Cp and the attached thiazole ring is
2.5°. Moreover, the angle between the thiazole plane and
the attached phenyl ring, linked at the 2 position of the
2-Ferrocenyl-5-phenylthiazole (9).16 Yield 74%; mp 156-
1
158 °C; Rf ) 0.67 (CH2Cl2/AcOEt/n-Hex, 1/1/1). H NMR (300
MHz, CDCl3): δ 4.15 (s, 5H), 4.43 (st, 2H), 4.89 (st, 2H), 7.25-
7.44 (m, 3H), 7.57(m, 2H), 7.83 (s, 1H). Anal. Calcd for C19H15-
FeNS: C, 66.10; H, 4.38; N, 4.06. Found: C, 66.32; H, 4.51; N
3.91.
1,1′-Bis[(5-phenyl)-1,3-thiazol-2-yl]ferrocene (11). Yield 71%;
1
mp 205-206 °C; Rf ) 0.41 (AcOEt CH2Cl2, 7/3). H NMR (300
MHz, CDCl3): δ 4.42 (st, 4H), 4.92 (st, 4H), 7.23-7.27 (m, 6H),
7.32-7.37 (m, 4H), 7.65 (s, 2H). Anal. Calcd for C28H20FeN2S2:
C, 66.67; H, 4.00; N, 5.55. Found: C, 66.90; H, 4.13; N 5.77.
1,1′-Bis[(2-phenyl)-1,3-thiazol-5-yl]ferrocene (14). Yield 43%;
1
mp 165-167 °C; Rf ) 0.58 (AcOEt CH2Cl2, 7/3). H NMR (300
MHz, CDCl3): δ 4.32 (st, 4H), 4.53 (st, 4H), 7.25-7.32 (m, 6H),
7.64-7.76 (m, 6H). Anal. Calcd for C28H20FeN2S2: C, 66.67; H,
4.00; N, 5.55. Found: C, 66.48; H, 4.19; N 5.38.
Preparation of 1,1′-Bis(r-aminoacetyl)ferrocene Dihydro-
chloride (12). To a suspension of Pd/C (10%) (0.2 g) in glacial
acetic acid (25 mL) was added dropwise a solution of 1,1′-bis(R-
azidoacetyl)ferrocene (0.6 g, 1.70 mmol) in CH2Cl2 (5 mL), and
the reaction mixture was stirred at room temperature for 3 h, while
a stream of H2 was bubbled through the solution. Then, the solution
828 Inorganic Chemistry, Vol. 46, No. 3, 2007

ElectroactiWe Thiazole DeriWatiWes
Scheme 1. Synthesis of Compounds 3, 5, and 7
Scheme 2. Synthesis of Compounds 9 and 11
Scheme 3. Synthesis of Compound 14
bridge, is 3.4°. Thus, all the atoms of the thiazole bridge
and the two attached rings are disposed in a planar
conformation indicating a large degree of π conjugation in
this compound. Molecules of 3 pack in the crystal (see Figure
S1 in Supporting Information) in groups of two molecules
in a head-to-tail arrangement, and these groups are stacked
along the b-axis in a herringbone fashion.
Compound 5 crystallizes in the orthorhombic space group
Pccn with eight molecules in the unit cell. Figure 1b shows
the ORTEP drawing of the molecular structure of 5 with
the atomic numbering scheme. The molecular structure
reveals a 1:1 occupation disorder of the N1(dC2A):C2(d
N1A) unit which is probably originated by the absence of
significant directional intermolecular interactions that fix the
relative position of N atoms, a situation that arises from the
predominant role played by the ferrocene groups in the
crystal packing. Cp rings in each independent ferrocene unit
have very small tilt angles between them. The two Cp rings
of ferrocene 2 are essentially eclipsed while those in
ferrocene 1 are less eclipsed. The angles between the mean
planes of the bridged Cp ring of ferrocenes 1 and 2 and the
attached thiazole ring are 3.2° and 2.6°, respectively. Thus,
all the atoms of the thiazole bridge and the two attached
rings are disposed in an almost completely planar conforma-
tion indicating also a large degree of π conjugation in this
compound. Molecules of 5 pack in the crystal (see Figure
S2 in Supporting Information) in groups of four molecules
adopting a rhombic clusters when viewed along the c-axes
and such rhombic molecular clusters are packed parallel
along the ab-plane.
Inorganic Chemistry, Vol. 46, No. 3, 2007 829

Caballero et al.
Table 1. UV-Vis Absorption and Electrochemical Data of the Studied
Compounds
compd
λmaxa (10-3 ꢀ)
E1/2b [L]
E1/2b,c [LHn+
]
3
5
7
9
11
14
327 (14.9), 449 (1.2)
327 (11.6), 457 (1.7)
369 (10.6), 486 (3.8)
326 (13.7), 459 (0.99)
320 (29.8), 469 (1.6)
321 (23.5), 459 (1.6)
0.52
0.49, 0.63
0.55
0.57
0.67
0.59
0.63
0.72
0.58
a λmax, in nm; ꢀ, in dm3 mol-1 cm-1. Spectra were made in CH2Cl2
solutions (c ) 1 × 10-4M). b Volts vs SCE, Pt working electrode, CH2Cl2
containing 1 M [(n-Bu)4N]ClO4, 20 °C. c n ) 1 for 3, 5, and 9; n ) 2 for
7, 11, and 14.
Molecules of 11 pack in the crystal (see Figure S3 in
Supporting Information) in a herringbone arrangement along
the b-axes forming chains which are disposed perpendicularly
to each other along the c-axes. Molecules of 14 pack in the
crystal (see Figure S4 in Supporting Information) forming
layers in the ac-plane in which the molecules adopt a
herringbone distribution along the a-axes. Molecules belong-
ing to neighboring layers are inverted 180°.
Atomic distances and bond angles of all four compounds
are close to those previously observed for ferrocene17 and
other substituted ferrocene compounds.6c,8a,c
Redox and Optical Properties. Reversibility and relative
potentials of redox processes in the compounds were
determined by cyclic (CV) and differential pulse (DPV)
voltammetry18 in 10-3 M acetonitrile/dichloromethane (3/2)
solutions containing 0.1 M [(n-Bu)4N]ClO4. Oxidation
potentials of compounds 3, 9, 11, and 14 were used to
determine the influence of the substitution pattern between
the ferrocene and thiazole groups in the electronic interaction
between the two units in the isomeric derivatives (Table 1).
Likewise, CV and DPV were also used to evaluate the
strength of the interaction between the metal centers in the
homobimetallic complexes 5 and 7 (Table 1).
The cyclic voltammograms of the monoferrocenyl deriva-
tives 3 and 9, which display a single anodic process with
features of chemical and electrochemical reversibility, showed
that 5-ferrocenylthiazole 3 is more easily oxidized by 50 mV
than the isomeric 2-ferrocenylthiazole 9. This is in agreement
with the predictions from the MO calculations of varying
degrees of sophistication of the thiazole ring, which predict
a slightly higher electron density at position C(5) than C(2).19
The CV data of the isomeric 1,1′-bis(thiazolyl)ferrocenes
11 and 14, which display a reversible redox wave, showed
that compound 14, bearing 5-substituted thiazole rings, is
more easily oxidized by 90 mV than 11, with 2-substituted
thiazole rings, indicating not only that the introduction of a
Figure 1. ORTEP view of the molecular structure of compounds 3 (a), 5
(b), 11 (c), and 14 (d) showing the atom labeling in the asymmetric unit.
Thermal ellipsoids are drawn at 50% probability level.
Compounds 11 and 14 are well ordered, but both are
racemic twins with an accidentally similar twin ratio of 0.52:
0.48. Compound 11 crystallizes in the monoclinic space
group P21 with two molecules in the unit cell while
compound 14 crystallizes in the orthorhombic space group
P212121. Figure 1c,d shows the ORTEP drawings of the
molecular structures of 11 and 14 with the corresponding
atomic numbering schemes. The two Cp rings in the two
compounds have small tilt angles between them and are
nearly eclipsed in 11 and less eclipsed in 14. The angles
between the mean planes of the Cp rings and the attached
thiazole rings are 11.8° and 11.5° in 11 and 2.8° and 10.9°
in 14. Moreover, the angles between the thiazole planes and
the attached phenyl rings are 3.0° and 3.4° in 11 and 7.5°
and 16.2° in 14. Thus, while in compound 11 the planes of
the two thiazole bridges and the attached phenyl rings are
almost parallel to each other, those in compound 14 are
somewhat less coplanar.
(17) Se´ller, P.; Dunitz, J. D. Acta Crystallogr., Sect. B 1982, 38, 1741-
1745.
(18) All processes observed were reversible, according to the criteria of
(i) separation of 60 mV between cathodic and anodic peaks, (ii) close-
to-unity ratio of the intensities of the cathodic and anodic currents,
and (iii) constancy of the peak potential on changing sweep rate in
the CVs. The same halfwave potential values have been obtained from
the DPV peaks and from an average of the cathodic and anodic cyclic
voltammetric peak.
(19) Metzger J. V. In ComprehensiVe Heterocyclic Chemistry; Katritzky,
A. R., Rees, C. W., Eds.; Pergamon: Oxford, U.K., 1984; Vol. 6,
Chapter 4.19, pp 226-331.
830 Inorganic Chemistry, Vol. 46, No. 3, 2007

ElectroactiWe Thiazole DeriWatiWes
as 1,4-bis(ferrocenyl)-1,3-butadiene22 (∆E1/2 ) 129 mV) and
1,4-bis(ferrocenyl)butadiyne23 (∆E1/2 ) 120 mV) indicating
the presence of a certain strength in the metal-metal
coupling.
Finally, the voltammograms of compound 7, containing
two ferrocenyl groups linked through a 2,2′-bis(thiazole) unit,
show only a reversible two-electron oxidation wave, indicat-
ing that there is little or a null direct interaction between the
metal centers over the large conjugated bis-heterocyclic
bridge.
The UV-vis data obtained in CH2Cl2 for the mono- and
bis-thiazolyl ferrocenes are collected in Table 1; they are
consistent with most ferrocenyl chromophores in that they
exhibit two charge-transfer bands in the visible region.24
These spectra contain a prominent absorption band with a
maximum between 320 nm, for 11, and 369 nm, for 7, which
can safely be ascribed to ligand-centered π-π* electronic
transitions (L-π*) (HE band) (Table 1). In addition to this
band, another weaker absorption is detected between 449
nm, for 3, and 486 nm, for 7, which is assigned to another
localized excitation with a lower energy produced either by
two nearly degenerate transitions: a Fe(II) d-d transition25
or by a metal-ligand charge transfer (MLCT) process (dπ-
π*) (LE band). This assignment is in accordance with the
latest theoretical treatment (model III) reported by Barlow
et al.26
Spectroelectrochemical Studies. The generation of the
oxidized species derived from 3, 9, 11, and 14 was performed
electrochemically and monitored by absorption spectroscopy.
Stepwise Coulommetric titrations were performed on ca. 3.5
× 10-3 mol‚L-1 solutions of these complexes in CH2Cl2,
with [(n-Bu)4N] PF6 (0.15 M) as supporting electrolyte, and
the absorption spectra were regularly recorded for different
average number (0 e n e 1) of removed electrons. UV-
vis-near-IR spectral data of 3•+, 9•+, 11•+, and 14•+ are
collected in Table 2.
Spectra of 3•+, 9•+, 11•+, and 14•+ show three main groups
of absorption bands. The high-energy absorptions in the UV
appear at similar wavelengths and show comparable intensi-
ties to the HE bands exhibited by the corresponding neutral
complexes 3, 9, 11, and 14. The absorption bands in the NIR
region, which are not observed in the neutral complexes, are
assigned to a thiazole f FeIII LMCT transition. This
assignment is based on the following considerations: (i)
during the course of the oxidations of complexes 3, 9, 11,
and 14, these low-energy bands increase continuously in
intensity, achieving their maximum when the oxidation
Figure 2. DPV (upper) and CV (lower) of compound 5 (1 mM), in CH2-
Cl2/CH3CN (3/2) using [(n-Bu)4N]ClO4 as supporting electrolyte and
scanned at 100 mV s-1 from 0 to 1 V.
second thiazole ring in the unsubstituted cyclopentadienyl
ring of 3 and 9 gives rise to addition effects but also that
there is electrochemical evidence of the stronger electron-
withdrawing character of the 2 position of the thiazole ring
than the 5 position.
The CV of 5 (Figure 2) shows two closely spaced
reversible one-electron oxidations for the ferrocenyl groups.
Likewise, the DPV voltammogram (Figure 2) also exhibits
two well resolved oxidation waves of the same area. It is
worth noting that in compound 5 the ferrocene subunit at 5
position is more easily oxidized by 30 mV than the ferrocene
group in model compound 3, whereas the ferrocene subunit
at 2 position is more difficult to oxidize by 60 mV than in
model compound 9. These results could be explained by the
fact that the phenyl group is a weaker electron-donating
group than ferrocenyl unit and the ferrocenium group is a
stronger electron-withdrawing group than phenyl ring.
The comproportionation constant Kc for 5•+ was calcu-
lated,20 from the observed wave splitting, ∆E1/2 ) 140 mV,
resulting in a value of Kc ) 2.32 × 102 from which a free
energy of comproportionation, ∆Gc°, of 1128 cm-1 is
obtained. The ∆E1/2 (and ∆Gc°) value may be used for the
assessment of the extent of ferrocenyl-ferrocenyl coupling
although it only provides an approximate value of such an
interaction because this magnitude has been shown to be
extremely medium-dependent21 and contains additional con-
tributions that are difficult to estimate for delocalized
1
systems, such as the statistical contribution (1/2RT ln /4),
the electrostatic interaction arising from the repulsion of two
similarly charged metal centers, and the synergistic factor
due to metal-ligand backbonding interaction. Nevertheless,
comparison of the ∆E1/2 (and ∆Gc°) value of 5 with those
shown by other similar bisferrocenyl compounds provides a
qualitative estimation of the actual metal-metal electronic
(22) Ribou, A. C.; Launay, J. P.; Sachtleben, M. L.; Li, M.; Spangler, C.
W. Inorg. Chem. 1996, 35, 3735-3740.
(23) LeVanda, C.; Bechgaard, K.; Cowan, D. O. J. Org. Chem. 1976, 41,
2700-2704.
(24) Farrel, T.; Meyer-Friedrichsen, T.; Malessa, M.; Haase, D.; Saak, W.;
Asselberghs, I.; Wostyn, K.; Clays, K.; Persoons, A.; Heck, J.;
Manning, A. R. J. Chem. Soc., Dalton Trans. 2001, 29-36 and
references therein.
(25) (a) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry;
Academic Press: New York, 1979. (b) Sohn, Y. S.; Hendrickson, D.
N.; Gray, H. B. J. Am. Chem. Soc. 1971, 93, 3603-3612.
(26) Barlow, S.; Bunting, H. E.; Ringham, C.; Green, J. C.; Bublitz, G.
U.; Boxer, S. G.; Perry, J. W.; Marder, S. R. J. Am. Chem. Soc. 1999,
121, 3715-3723.
coupling. Thus, the wave splitting in compound 5 (∆E1/2
)
140 mV) is smaller than those for 1,4-bis(ferrocenyl)-2-aza-
1,3-butadiene8a (∆E1/2 ) 210 mV), whereas the value is
slightly larger than those observed for most bis-ferrocenyl
compounds with carbon-chain π-conjugated bridges, such
(20) Richarson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278-1285.
(21) Barriere, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff, U. T.;
Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262-7263.
Inorganic Chemistry, Vol. 46, No. 3, 2007 831

Caballero et al.
Table 2. UV-Vis-Near-IR Absorption Data of the Oxidized
Compounds
λ
max (10-3 ꢀ)a
compd
3+
323(>14.0),415(5.2),478(2.4),
591(0.28)
316(10.8),352(8.0),409(4.6),
535(2.5)
350(8.1),398(sh),470(sh),
556(0.99)
818 (0.46), 932 (0.43)
810 (0.25), 1355 (0.32)
791 (0.75)
5+
52+
72+
9+
376(17.5), 435
(sh),564(sh)
834 (0.95)
349(> 8.0), 481(2.5), 570(sh)
334(> 13.0),405(6.9), 495(3.1)
319(26.6), 387(8.2), 487(2.8)
816 (0.39), 927 (0.28)
855 (0.49), 1018 (0.58)
867 (0.62), 1019 (0.77)
11+
14+
a λmax, in nm; ꢀ, in dm3 mol-1 cm-1. Spectra were made in CH2Cl2 with
[(n-Bu)4N]PF6 (0.15 M).
process is completed (see Figure S12 in Supporting Informa-
tion). (ii) Similar low-energy transitions have been observed
in the NIR region in other FeIII and RuIII complexes with
conjugated oligothiophene ligands,27,28 having -CH2-N(R)-
CH2- and -CH-N-CH-CH- (2-aza-1,3-butadiene) as
bridges.8a,29 (iii) A thiazole ring can structurally be viewed
as a 2-aza-1,3-butadiene closed by a sulfur atom having
hereby a similar electronic structure. Therefore, the absorp-
tion bands in the NIR exhibited by 3•+, 9•+, 11•+, and 14•+
are assigned to an optically induced electron transfer from
the thiazole to the FeIII center.
Oxidized species derived from 5 were also generated and
studied in the same way as the other four compounds, and
absorption spectra were regularly recorded for different
numbers (0 e n e 2) of removed electrons. Figure 3 exhibits
the evolution of the spectra during the oxidation of 5 in the
0 e n e 1 range showing an increase of the Cp f FeIII
LMCT bands appearing at 409 and 535 nm, which is charac-
teristic of a ferrocenium ion. Besides, a decrease of the band
at 457 nm is observed, which corresponds to a localized
excitation of a ferrocene group. Along with the changes of
these bands, the appearance and maintenance of three well-
defined isosbestic points at 234, 321, and 347 nm were
observed. Nevertheless, the most interesting feature detected
is that during the oxidation of 5 two new bands appeared:
one which is weak and centered at 810 nm, and another one,
at 1355 nm, which is broad. Their intensities continuously
grow until one electron is removed, i.e., when the formation
of 5•+ is completed. On removing more electrons (1 e n e
2), the intensity of the first band continues increasing while
the band at 1355 nm decreases until it disappears when the
dication 52+ is completely formed. Table 2 collects all the
spectral data for oxidized complexes 5•+ and 52+.
Figure 3. Top: Evolution of vis-near-IR spectra during the course of
the oxidation of compound 5. Inset: near-IR region enlargement. The
average number of removed electrons is given on each spectrum. Bottom:
Deconvolution of NIR spectrum of 5•+PF6- salt. The experimental spectrum
was deconvoluted by means of three Gaussian functions (---). The sum of
the dashed lines (___) closely matches the experimental spectrum (s).
of the position, width, and intensity of the two observed CT
bands, which are gathered in Table 3.30 These spectral
parameters are relevant for the characterization of intramo-
lecular electron-transfer phenomena occurring in 5•+.31
The presence of several NIR bands in the spectrum of a
mixed-valence complex is not uncommon, and their occur-
rence is generally explained by means of three different
possible causes.32,33 The first one could be the presence of a
strong spin-orbit coupling effect, which becomes important
only for complexes composed by third-row transition met-
als.32 A second origin for these bands could be the presence
of a double-exchange mechanism,33 a mechanism that
becomes more likely as the bridge length and the level of
π-orbitals increase. Finally, such multiple NIR bands might
be caused by the presence of a bridge with accessible
electronic state levels or with redox active bridges, as has
been recently proposed to explain the rich absorption spectra
of certain mixed-valence compounds with redox active
bridges.34 On the basis of the behavior of 3•+, 9•+, 11•+, and
(30) Gould, I. R.; Noukakis, D.; Go´mez-Jahn, L.; Young, R. H.; Goodman,
J. L.; Farid, S. Chem. Phys. 1993, 176, 439-456.
(31) The proportion of mixed valence species at half-oxidation (P), where
P ) Kc1/2/(2 + Kc1/2), is generally used to compute the intensity of
corrected spectra. The comproportionation constant Kc, which is related
to the thermodynamic stability of the mixed-valence compound, of
the equilibrium of comproportionation, [FeII-L-FeII] + [FeIII-L-
FeIII] / 2[FeII-L-FeIII], was calculated for 5 from the electrochemical
data, resulting a value of Kc ) 226 which yields values of P about 90
The deconvolution of the experimental spectrum of 5•+
in the NIR region, assuming Gaussian shapes in the wave-
number domain (Figure 3), allows an accurate determination
% and ꢀcorr ) 315 M-1 cm-1
.
(32) (a) Kober, E. M.; Goldsby, K. A.; Narayane, D. N. S.; Meyer, T. J. J.
Am. Chem. Soc. 1983, 105, 4303-4309. (b) Lay, P. A.; Magnuson,
R. H.; Taube, H. Inorg. Chem. 1988, 27, 2364-2371. (c) Laidlaw,
W. M.; Denning, R. G. J. Chem. Soc., Dalton Trans. 1994, 1987-
1994.
(33) (a) Richardson, D. E.; Taube, H. J. Am. Chem. Soc. 1983, 105, 40-
51. (b) Halpern, J.; Orgel, L. E. Discuss. Faraday Soc. 1960, 29, 32-
41.
(27) (a) Zu, Y. B.; Wolff, M. O. Chem. Mater. 1999, 11, 2995-3001. (b)
Zu, Y. B.; Millet, D. B.; Wolf, M. O.; Rettig, S. J. Organometallics
1999, 18, 1930-1938.
(28) Zu, Y. B.; Wolf, M. O. J. Am. Chem. Soc. 2000, 122, 10121-10125.
(29) (a) Yamaguchi, I.; Sakano, T.; Ishii, H.; Osakada, K.; Yamamoto, T.
J. Organomet. Chem. 1999, 584, 213-216. (b) Horie, M.; Sakano,
T.; Osakada, K.; Nakao, H. Organometallics 2004, 23, 18-20. (c)
Horie, M.; Suzaki, Y.; Osakada, K. J. Am. Chem. Soc. 2004, 126,
3684-3685.
(34) Nelsen, S. F.; Ismagilov, R. F.; Powell, D. F. J. Am. Chem. Soc. 1998,
120, 1924-1925.
832 Inorganic Chemistry, Vol. 46, No. 3, 2007

ElectroactiWe Thiazole DeriWatiWes
Table 3. IVCT Band Parameters (hνmax, ꢀmax, and ∆ν1/2°) Obtained from the Spectral Deconvolution and Calculated IET Parameters (Vab) for Radical
Cation 5+•
IVCT band
hνmax/cm-1
ꢀmax/M-1 cm-1
∆ν1/2°/cm-1
f,a 10+3
db/Å
Vab/cm-1
LMCT
MMCT
12346
7380
250
315
2570
2700
2.96
3.90
5.175
8.407
354 (VLM
193 (VMM
)
)
b
a Total oscillator strength obtained by f ) 4.6 × 10-9ꢀmax∆ν1/2
between Fe-Fe′ and Fe-N atoms.
.
Effective electron-transfer distances between electroactive sites taken as distances
14•+, the latter case seems to be the most feasible for the
two NIR bands of 5•+. Accordingly, also on the basis of the
spectral evolution observed during the oxidation of 5, the
band centered at 810 nm is assigned to a thiazole f FeIII
LMCT transition. This result probably implies a photoin-
duced electron transfer from the N atom of the thiazole bridge
to the FeIII site linked at the 5 position of the bridge. On the
other hand, the band centered at 1355 nm is attributed to
the bridge-mediated superexchange between the two coupled
iron sites (M and M′) or an IET between the two iron sites;
i.e., to an MM′CT transition.
determined from the experimental and predicted MM′CT
bandwidths, provides a useful criterion for determining
whether a particular system is weakly coupled, moderately
coupled, at the class II-III transition, or strongly coupled
(class III).40 Thus, the resulting value of Γ ) 0.29 indicates
that 5•+ is a moderately coupled class II system.
On the other hand, the calculated value of VMM′ is
somewhat smaller than that obtained for 1,4-bis(ferrocenyl)-
2-aza-1,3-butadiene,8a showing that, as expected for a het-
eroaromatic bridge,7 the electronic communication in the
closed structure (ring) is a little bit worse than in the open
chain bridge.
The application of the two-state Hush1,3a,35 model to the
lowest energy MM′CT band observed in the spectrum of
the mixed-valence compound 5•+ predicts a half-height
bandwidth, ∆ν1/2, of 3800 cm-1; as calculated from ∆ν1/2
(2310λ)1/2, where λ is the total reorganization energy of
Marcus theory which was determined from the observed
energy of the MM′CT, hνmax, and the free energy of
comproportionation in order to take into account the in-
equivalent redox sites of the system.36 Such a predicted ∆ν1/2
value is larger than the value determined experimentally,
∆ν1/2° ) 2700 cm-1, by the spectral deconvolution assuming
Gaussian profiles. This result suggests a considerable degree
of electronic delocalization in the ground state of 5•+. The
availability of the energy of the MM′CT transition, hνmax
also enables the estimation of the electronic coupling
parameter between the two metal sites, VMM′, of 193 cm-1
(adiabatic electronic coupling element), which can be used
to gauge the degree of the ground-state delocalization, given
by the coefficient R2 ) 0.026.37 Since the Hush model can
only be rigorously applied to weakly coupled mixed-valence
species, the electrochemical, intermetallic charge transfer
study, and the overestimation of the bandwidth compared
with the theoretical value,38 suggest that the 5•+ system lies
between the class II and the class II-III transition classifica-
tions for mixed-valence compounds.39 The parameter Γ,
Oxidized species derived from 7 were also generated
electrochemically, and their formation followed by absorption
spectroscopy, under the same conditions used for compound
)
5. However, in this case no intervalence band was observed,
which implies no intramolecular electron transfer. Presum-
ably, the length of the bis-heteroaromatic bridge in 5
precludes electron coupling, as also indicated by its CV,
which has a single reversible oxidation wave only.
Finally, compounds 3•+, 9•+, 11•+, 14•+, 5•+, 52+, and 72+
were characterized by EPR presenting the typical pattern
signals corresponding to a low-spin FeIII species (see the EPR
data in Supporting Information).
,
Ion Sensing Properties. One of the most interesting
attributes of the new ferrocene-thiazole compounds reported
here is the presence of at least one metal-ion binding site on
the thiazole ring which is in proximity to a ferrocene redox-
active moiety. Due to this structural feature, the detailed study
of their protonation41 and metal-recognition properties was
of interest. At first, the electrochemical behavior of these
new ferrocenyl thiazole ligands in the presence of variable
concentrations of HBF4 was investigated. With addition of
stoichiometric quantities of HBF4 in CH3CN to a solution
of compounds 3 and 9 in CH3CN/CH2Cl2 (3:2), the redox
potential of the ferrocene nucleus was shifted anodically. The
protonation-induced redox potential shift was higher for
compound 9 (∆E1/2 ) 152 mV) than for compound 3 (∆E1/2
) 72 mV). These values are in good agreement with the
linear relationship between the inverse iron-nitrogen separa-
(35) (a) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1-73. (b) Nelsen, S. F.
Chem. Eur. J. 2000, 6, 581-588.
(36) Hush theory indicates that, for a mixed-valence systems with two
inequivalent redox centers, the energy of the IVCT transition is given
by hνmax ) λ + ∆Gc˚, where λ is the reorganization energy of Marcus,
and theory and ∆Gc˚ can be estimated from the electrochemical data.
For an elegant application see: Barlow, S. Inorg. Chem. 2001, 40,
7047-7053.
(39) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. ReV. 2001,
(37) Vab ) 2.05 × 10-2 (ꢀmax∆ν1/2νmax
)
1/2/r is the Hush equation for the
101, 2655-2685.
calculation of the electronic interaction in mixed-valence complexes
and R ) (Vab/hνmax) was used to estimate the degree of electronic
delocalization.
(40) The magnitude of the Γ parameter, Γ ) 1- (∆ν1/2/∆ν°1/2), distinguishes
the class of mixed-valence system: 0< Γ < 0.1 for weakly coupled
class II system; 0.1< Γ < 0.5 for moderately-coupled class II systems;
Γ ) 0.5 at the transition between class II and class III; and Γ > 0.5
for class III. See: (a) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem.
Soc. ReV. 2002, 31, 168-184. (b) D’Alessandro, D. M.; Keene, F. K.
Chem. Soc. ReV. 2006, 35, 424-440.
(38) The origin of this discrepancy is not immediately clear, although
differences between theoretical and experimental values of ∆ν1/2 are
not uncommon. They often arise from system nonidealities with respect
to the model. See: (a) Elliot, C. M.; Derr, D. L.; Matyushov, D. V.;
Newton, M. D. J. Am. Chem. Soc. 1998, 120, 11714-11726. (b) Curtis,
J. C.; Meyer, T. J. Inorg. Chem. 1982, 21, 1562-1571.
(41) Previous studies related to the protonation of compounds 3, 5, and 9
have already been published in a short communication. See ref 16.
Inorganic Chemistry, Vol. 46, No. 3, 2007 833

Caballero et al.
that the CV and DPV of the adduct 5‚H+, recorded after
base addition, is identical to that obtained before the addition
of acid.
A similar electrochemical study carried out on receptor 7
reveals that addition of HBF4 to an electrochemical solution
of the ligand in CH3CN/CH2Cl2 (3:2) does not cause any
change in the redox potential of the ferrocene moiety.
Having proven the ability of ferrocenylthiazoles to offer
an electrochemical response to a variation in the concentra-
tion of H+, the electrochemical behavior of these receptors
was also investigated in the presence of several divalent metal
ions, such as Ca2+, Mg2+, Zn2+, Cd2+, Mn2+, Co2+, Ni2+,
Hg2+, and Pb2+ as their perchlorate salts (Table 4). In this
context, DPV43 studies of ferrocenyl thiazoles 3 and 9
indicate that both receptors behave as selective electrochemi-
cal sensors for Hg2+, eliciting the same response: the
appearance of a new redox peak, anodically shifted from
the original ferrocene potential in the free ligand. However,
the magnitudes of these redox shifts were different depending
on the position of the ring to which the ferrocene unit is
bonded. The magnitude of ∆E1/2 is 100 mV for 3, and 140
mV for 9, which bears the ferrocene moiety appended at 5
and 2 positions of the thiazole ring, respectively. In contrast,
addition of Ca2+, Mg2+, Zn2+, Cd2+, Mn2+, Co2+, Ni2+, and
Pb2+ ions to these ligands do not promote any significant
change in the corresponding electrochemical responses.
Similar voltammetric titrations were also carried out by
adding the above-mentioned cations to a solution of ligands
7, 11, and 14. Unfortunately, a dramatic decrease in the
sensing properties of these receptors was evidenced, and no
useful responses could be obtained.
Interestingly, ligand 5 possessing two ferrocene units
linked to the 2 and 5 positions of the thiazole ring gives rise
to noticeable metal ion binding processes upon addition of
Zn2+, Cd2+, Ni2+, and Pb2+ cations. Whereas no perturbation
of the voltammograms was observed upon addition of Ca2+,
Mg2+, Mn2+, and Co2 even in a large excess, a significant
modification was observed for the first oxidation peak upon
addition of Zn2+, Cd2+, Ni2+, and Pb2+ metal ions. In contrast,
the second oxidation peak was apparently not perturbed on
the addition of these cations, as happened for the protonation
studies. Again, this suggests that the complex is disrupted
after the first monoelectronic oxidation of the complex and
the second oxidation really takes place on the uncomplexed
mono-oxidized species 5•+ (Figure 5).
Figure 4. Cyclic voltammogram of compound 5 (1 mM), in CH2Cl2/CH3-
CN (3/2) using [(n-Bu)4N]ClO4 as supporting electrolyte scanned at 100
mV s-1 from 0 to 1 V (black), after addition of 0.5 equiv of HBF4 (blue)
and 1 equiv of HBF4 (red). Inset: DPV of compound 5 (1 mM), in CH2-
Cl2/CH3CN (3/2) using [(n-Bu)4N]ClO4 as supporting electrolyte scanned
at 100 mV s-1 from 0 to 1 V (black) and after addition of 1 equiv of HBF4
(red).
tion and the shifts of the potentials found upon protonation
for several kinds of aza-substituted ferrocenes.42
Unfortunately, it was not possible to investigate the effect
of the addition of HBF4 on the redox chemistry of ligand 11
because H+ promoted the formation of the corresponding
very insoluble thiazolium salt immediately. Nevertheless, for
ligand 14, a new redox wave evolved on addition of 2 equiv
of HBF4, which was shifted 134 mV anodically from the
original ferrocene redox potential in the neutral ligand.
Compound 14, however, showed a more complex perturba-
tion of the redox wave on addition of substoichiometric
amounts of HBF4. The original wave was shifted and
resolved progressively as the proton addition was increased
until 2 equiv of acid was added.
Curiously, the cyclic voltammogram of compound 5, upon
protonation under the same conditions, showed a clear
evolution of the first wave from E1/2 ) 0.49 to 0.63 V
whereas there was no effect on the second one. Remarkably,
the current intensity of the cathodic peak of the second wave
increases, while that of the first one decreases with a linear
dependence on the equivalents of the added acid. In
particular, the second wave reaches the maximum current
intensity value at 1.0 equiv of added acid, and at this point
the first wave disappears (Figure 4). The occurrence of this
wave at the same potential as that for unprotonated 5 suggests
that, after oxidation of the ferrocene unit linked at 5-position,
the adduct is deprotonated and a subsequent oxidation takes
place on the partially oxidized 5•+. In other words, these
results show that the 5‚H+ adduct undergoes reversible
electrochemically induced deprotonation/reprotonation pro-
cesses on a time scale faster than that of the electrochemical
experiments, at 100 mV s-1. It is noteworthy that the
reversibility of the chemical process is confirmed by the fact
Previous studies on ferrocene-based ligands have shown
that their characteristic low-energy (LE) bands in the
absorption spectra are perturbed by complexation.44 There-
fore, the metal recognition properties of both mono- and bis-
thiazolyl ferrocene ligands toward Ca2+, Mg2+, Zn2+, Cd2+,
(43) DPV technique has been employed to obtain well-resolved potential
information, while the individual redox process are poorly resolved
in the CV experiments in individual E1/2 potential can not be accurately
extracted easily from this data. See: Serr, B. R.; Andersen, K. A.;
Elliot, C. M.; Anderson, O. P. Inorg. Chem. 1988, 27, 4499-4504.
(44) (a) Marder, S. R.; Perry, J. W.; Tiemann, B. G. Organometallics 1991,
10, 1896-1901. (b) Coe, B. J.; Jones, C. J.; McCleverty, J. A.; Bloor,
D.; Cross, G. J. J. Organomet. Chem. 1994, 464, 225-232. (c) Mu¨ller,
T. J.; Netz, A.; Ansorge, M. Organometallics 1999, 18, 5066-5074.
(42) Plenio, H.; Yang, J.; Diodone, R.; Huinze, J. Inorg. Chem. 1994, 33,
4098-4104.
834 Inorganic Chemistry, Vol. 46, No. 3, 2007

ElectroactiWe Thiazole DeriWatiWes
Table 4. Relevant Physicochemical Data of the Ligand/Metal Complexes Formed
compd
Cd2+
Zn2+
Hg2+
Ni2+
Pb2+
3
λa
337 (13320), 514 (1930)
65
3.58 ( 0.11
1/1
337 (13200), 514 (1920)
65
3.58 ( 0.13
1/1
∆λb
log Kas
L/Md
Epe
c
0.62
5
7
λa
337 (11564), 523 (3236) 336 (11484), 518 (2989) 339 (12024), 525 (3983) 338 (12109), 524 (3483) 340 (11970) 525 (3950)
∆λb
log Kas
L/Md
Epe
66
61
68
67
68
c
3.58 ( 0.11
1/1
0.63
6.43 ( 0.54
2/1
0.63
6.24 ( 0.57
2/1
0.67
6.28 ( 0.55
2/1
0.63
6.50 ( 0.40
2/1
0.65
λa
393 (10790), 565 (4250) 395 (10680), 571 (3958) 401 (10040), 514 (2720), 393 (10856), 571 (4256) 400 (10120) 513 (2730),
595 (2740) 595 (2660)
109 109
∆λb
79
85
85
c
c
c
c
log Kas
L/Md
Epe
6.33 ( 0.43
2/1
6.21 ( 0.39
2/1
5.29 ( 0.32
1/1
6.40 ( 0.45
2/1
5.00 ( 0.27
1/1
9
11
14
λa
333 (12320), 515 (1400) 342 (13440), 525 (2380) 343 (13380), 526 (2460) 333 (12870), 515 (1520) 340 (13400) 523 (2470)
∆λb
56
66
67
66
64
log Kas
L/Md
Epe
7.22 ( 0.34
2/1
6.23 ( 0.35
1/1
7.38 ( 0.37
2/1
0.71
7.11 ( 0.35
2/1
7.30 ( 0.30
2/1
λa
329 (30420), 476 (2120) 323 (28350), 489 (1750) 332 (23730), 516 (1620) 322 (28810), 508 (1710) 333 (23900) 514 (1590)
∆λb
7
20
47
39
45
log Kas
L/Md
Epe
3.65 ( 0.11
1/2
4.56 ( 0.35
1/1
4.47 ( 0.41
1/2
4.93 ( 0.37
1/1
4.30 ( 0.36
1/2
λa
330 (21740), 485 (2820)
26
3.25 ( 0.13
1/2
331 (21500), 485 (2790)
26
3.10 ( 0.09
1/2
∆λb
log Kas
L/Md
Epe
a λmax /nm (ꢀ [dm3 mol-1 cm-1]). b Red-shift of the LE band upon metal complexation (∆λ ) λcomplexed - λfreeligand). c Association constant determined by
Specfit/32 Global Analysis System. d Stoichiometry of the complex formed (ligand/metal). e Volts vs SCE, Pt working electrode, CH2Cl2 containing 1 M
[(n-Bu)4N]ClO4, 20 °C.
affected by the position of the heterocyclic ring at which
the ferrocene nucleus is linked, indicating that shorter
distances between the nitrogen donor atom within the
heterocyclic ring and the ferrocene unit correspond to smaller
selectivity in the recognition properties of the ligand.
For compounds 3 and 14, in which the ferrocene unit is
linked at the 5 position of the thiazole ring, the presence of
Hg2+ and Pb2+ ions induced a red-shift of the LE bands from
449 and 459 nm to 514 and 485 nm, respectively (Table 4),
whereas no changes in the UV-vis spectra of these ligands
were observed upon addition of an excess of Ca2+, Mg2+,
Zn2+, Cd2+, Mn2+, Co2+, and Ni2+ ions. In contrast, the UV-
vis data obtained by addition of Ca2+, Mg2+, Mn2+, Co2+,
Ni2+, Hg2+, and Pb2+ ions to the mono- (5 and 9) and bis-
Figure 5. (a) Evolution of cyclic voltammogram of free ligand 5 (1 mM)
(red) until the formation of the complex 5‚Cd2+ (blue) in CH2Cl2/CH3CN
thiazolyl (11) ligands, bearing the ferrocene nucleus attached
to the 2 position, revealed that while no changes were
detected in the case of Ca2+, Mg2+, Mn2+, and Co2+ ions, a
red-shift of their LE bands upon addition of Zn2+, Cd2+,
Hg2+, Ni2+, and Pb2+ ions was observed (Table 4).
Regarding bidentate compound 7, the addition of Zn2+,
Cd2+, and Ni2+ metal ions induced a red shift of the LE band
by 79-85 nm, whereas the Hg2+ and Pb2+ metal ions
(3/2) using [(n-Bu)4N]ClO4 as supporting electrolyte scanned at 100 mV
s-1. (b, inset) Evolution of DPV of free ligand 5 (1 mM) (red) until the
formation of the complex 5‚Cd2+ (blue) in CH2Cl2/CH3CN (3/2) using [(n-
Bu)4N]ClO4 as supporting electrolyte scanned at 100 mV s-1
.
Mn2+, Co2+, Ni2+, Hg2+, and Pb2+ ions were also evaluated
by UV-vis spectroscopy (Table 4). Titration experiments
for dichloromethane solutions of ligand (c ) 1 × 10-4 M)
and the corresponding ions were performed and analyzed
quantitatively.45 It is worth mentioning that no changes were
observed in the UV-vis spectra of any of these ligands upon
addition of 1 equiv of Ca2+, Mg2+, Mn2+, and Co2+ metal
ions. However, from the spectrophotometric behavior of these
derivatives upon addition of Zn2+, Cd2+, Ni2+, Hg2+, and
Pb2+ ions, it is clear that the binding events are strongly
(45) All titration experiments were analyzed using the computer program
Specfit/32 Global Analysis System, 1999-2004, Spectrum Software
Associates (SpecSoft@compuserve.com). The equation to be adjusted
by nonlinear regression, using the above mentioned software, was the
following: ∆A/b ) {K11∆ꢀHG[H]tot[G]}/{1 + K11[G]}, where H )
host, G ) guest, HG ) complex, ∆A ) variation in the absorption,
b ) cell width, K11 ) association constant for a 1:1 model, and
∆ꢀHG ) variation of molar absorptivity.
Inorganic Chemistry, Vol. 46, No. 3, 2007 835

Caballero et al.
promoted a higher red shift by 109 nm. As it happened for
all the series of ferrocenylthiazoles, no significant response
was obtained when the divalent cations added were Ca2+,
Mg2+, Mn2+, or Co2+.
In those cases where an optical response could be detected,
the observation of isosbestic points in the evolution of the
UV-vis spectra clearly indicates that clean complexation
equilibria take place. It is also worth mentioning that addition
of cations (not shown) beyond the point of equivalence for
each complex did not cause a significant change in the ab-
sorption spectra. Titration analysis data gave the stability con-
stants displayed in Table 4 and confirmed the formation in
solution of the reported ligand/metal complex stoichiometries.
A further exciting property of these ligands is that they
are not only able to monitor binding but they are also able
to behave as actuators through the progressive electrochemi-
cal release of the metal cation; that is, the binding constant
upon electrochemical oxidation is decreased.
In this context, a spectroelectrochemical study of complex
3‚Hg2+, as a model complex, was carried out. Thus, 1 equiv
of Hg(ClO4)2 was added to a solution of 3 in CH2Cl2, with
[(n-Bu)4N]PF6 (0.15 M) as supporting electrolyte, in order
to obtain the complexed 3‚Hg2+ species that exhibits an
intense purple color. The complex was oxidized at +0.9 V
versus Ag/AgNO3 until the complete oxidation was reached
and the color of the solution changed from purple to yellow.
Afterward, the solution was completely reduced with a
potential of -0.1 V versus Ag/AgNO3, and the initial
spectrum was fully recovered as well as its purple color.
Thus, the free oxidized ligand 3•+ is reduced to 3 which has
a huge cation Hg2+ binding affinity leading the initial
complex 3‚Hg2+. Oxidation of the complex 3‚Hg2+ and its
subsequent reduction were carried out over several cycles
Figure 6. Cyclic stepwise oxidation and reduction of 3‚Hg2+ carried out
in CH2Cl2 with a chronoamperometric technique following the changes by
UV-vis-near-IR spectroscopy. Upper: changes observed at a wavelength
of 513 nm (reduction) and 863 nm (oxidation). Lower: fixed potentials
used in the different steps of cyclic redox experiments.
in
a chronoamperometric experiment, as shown in
Figure 6. After each step, the optical spectrum was recorded,
and it was found to be recovered, demonstrating the
high degree of reversibility of the complexation/decomplex-
ation process, although in the reduction steps the intensity
of the optical spectra recorded decrease slowly, as the
working electrode is passivated. This result shows that
complex 3 is an electrochemically induced switchable
chemosensor.
From the combined voltammetric and optical studies,
several trends can be extracted from this study. First,
ferrocenylthiazoles linked accross the 5 position of the
heteroaromatic ring are selective chemosensors for Hg2+ and
Pb2+ metal ions with the 5-ferrocenylthiazole 3 operating
through two channels: optical and redox (Figure 7), whereas
1,1′-bis(thiazolyl)ferrocene 14 only operates as an optical
sensor. Second, ferrocenylthiazole 9, with the ferrocenyl unit
linked to the 2 position of the heterocyclic ring, is a selective
redox sensor for Hg2+ metal ions, and it responds optically
to a narrow range of cations, comprising Zn2+, Cd2+, Hg2+,
Ni2+, and Pb2+, as the bis(thiazolyl)ferrocene 11 does. Third,
bis(ferrocenyl)thiazole 5 is a dual optical and redox sensor
for Zn2+, Cd2+, Hg2+, Ni2+, and Pb2+, whereas bis(ferrocenyl)
compound 7, bearing a bis(thiazole) as a bridge, is only a
chromogenic sensor for Zn2+, Cd2+, Ni2+ (color changes from
Figure 7. (a) Changes in the absorption spectra of 3 upon addition of
increasing amounts of Hg2+. The initial spectrum (black) is that of the free
ligand, and the final spectrum (purple) corresponds to the addition of 1
equiv of Hg2+. (b, inset) Cyclic voltammogram of compound 3 (1 mM), in
CH2Cl2/CH3CN (3/2) using [(n-Bu)4N]ClO4 as supporting electrolyte
scanned at 100 mV s-1 from -0.1 to 0.95 V (red), after addition of 0.5
equiv of Hg2+ (blue) and 1 equiv of Hg2+ (black).
orange to blue), and Hg2+ (color changes from orange to
green) (Figure 8). This study also revealed the ability of these
ligands to control the trapping and the expulsion of these
metal ions by application of an external electrochemical
stimulus.
Theoretical Calculations. Quantum chemical calculations
at the DFT level provided insight concerning the complex-
ation abilities (both in their neutral or oxidized states) toward
divalent metal ions of the compounds presented here as well
as their electronic structures. Calculations were performed
in bisferrocenylthiazoles 5 and 7 and in monoferrocenylthi-
azoles 3 and 9, as well as on the recently reported “open-
chain” 1,4-bis(ferrocenyl)-2-aza-1,3-butadiene.8a The com-
836 Inorganic Chemistry, Vol. 46, No. 3, 2007

ElectroactiWe Thiazole DeriWatiWes
Figure 8. Changes of colors observed upon addition to the free ligand 7
(left), Cd2+, Zn2+, or Ni2+ ions (center), and Hg2+ (right).
Figure 9. Calculated (method A) structures for the most stable complexes
formed between Hg2+ and 2-ferrocenylthiazole (left) and 5-ferrocenylthi-
azole (right).
prehensive theoretical study carried out is shown completely
in the Supporting Information.
Upon one-electron oxidation, bisferrocenyl thiazole 5
behaves in an unprecedented way, inasmuch as both fer-
rocene units lose nearly equal amounts of electron density
initially, but the unstable radical cation generated undergoes
geometrical relaxation with IET from the most oxidized Fc1
unit (the one closer to the N atom) to the less positively
charged Fc2 unit. This results in a highly positively charged
“fully oxidized” Fc1 unit with an almost “unoxidized” Fc2
moiety. The second one-electron oxidation takes place again
by removing the electron density from Fc2.
The ion binding properties of ferrocenylthiazole derivatives
have also been studied by means of DFT quantum chemical
calculations using model systems. A divalent metal perchlo-
rate was complexed with the model ligand L (where L stands
for 2-ferrocenylthiazole or 5-ferrocenylthiazole) to give [L‚
M]2+ and two perchlorate anions, with every species being
considered as noninteracting, and their energies were com-
puted in dichloromethane as solvent (see the Computational
Procedures subsection).
From the studies carried out, several conclusions can be
extracted. First, the ferrocenyl substitution at the 2 position
of thiazole leads to more stable complexes (involving Fe and
N as donor atoms) than at the 5 position with all studied
divalent metal ions. Except for the complexation of Cd2+,
the Fe,N and Fe,S binding modes are the best combinations
of donor sites for 2- and 5-ferrocenylthiazoles, respectively.
Thus, in bisferrocenyl thiazole 5 metal complexation is
expected to occur involving Fe1 and the heterocyclic N atom.
Second, with both model ligands the most stable complexes
were found with mercury (see Figure 9). Indeed, the
complexation of Hg2+ by 5-ferrocenyl thiazole is the only
thermodynamically favored reaction at the working level of
theory. The relative difference in complexation energy
with other ions (31.24, 43.86, 74.53, 78.09, and 91.20 kcal/
mol for Cd2+, Zn2+ Ni2+, Mg2+, and Pb2+, respectively) is
large enough to account for a high selectivity. These
differences are much lower in the case of 2-ferrocenylthiazole
ligand (16.53, 22.27, 60.82, 55.66, and 81.12 kcal/mol for
Cd2+, Zn2+ Ni2+, Mg2+, and Pb2+, respectively), indicating
therefore a lower selectivity. Third, minima corresponding
to the Fe,S binding mode were also found for 2-ferroce-
nylthiazole (except with Ni2+ and Pb2+ which evolves to the
Fe,N complexation mode), with similar energies to those of
5-ferrocenylthiazole. Additional minima involve the outer
side of Cp1 as η5 donor site (except for Hg2+), correspon-
ding to the most stable complexes of Cd2+ for the former
ligand.
Summary
The synthesis, optical and electrochemical properties, and
X-ray characterization of two thiazole ring systems capped
with two ferrocenyl groups (5 and 7) and their model
compounds with only one ferrocenyl, either at 2 or 5 position
of the mono- or bis-thiazolyl ligands (3, 9, 11, and 14), are
presented. These ferrocenyl heteroaromatic ligands could
serve as important models for the investigation of intramo-
lecular electron transfer and for metal recognition processes.
Bisferrocenyl thiazole 5 forms the mixed-valence species 5•+
by partial oxidation which, interestingly, shows an IET
phenomenon because of the electronic coupling between the
two ferrocene units which is rather unusual for heteroaro-
matic ring systems capped by ferrocenyl groups. Monofer-
rocenyl thiazoles in their oxidized forms display absorption
bands assigned to thiazole f FeIII LMCT transitions. On
the other hand, the compounds studied exhibit interesting
ion-sensing properties which show high selectivity for
divalent metal ions giving responses through one or two
channels. Thus, ferrocenylthiazoles linked accross the 5
position of the heteroaromatic ring are selective chemosen-
sors for Hg2+ and Pb2+ metal ions with 5-ferrocenylthiazole
3 operating through two optical and redox channels whereas
the 1,1′-bis(thiazolyl)ferrocene 14 operates as an optical
sensor only. Compound 3 also exhibited a selective Hg2+
induced complexation/decomplexation type of signaling
pattern that can be used for the construction of more elaborate
supramolecular switching systems. Moreover, ferrocenylthi-
azole 9, in which the heterocyclic ring and the ferrocene
group are linked across the 2 position, is a selective redox
sensor for Hg2+ metal ions, and it responds optically, as bis-
(thiazolyl)ferrocene 11 does, to a narrow range of cations,
comprising Zn2+, Cd2+, Mn2+, Co2+, Hg2+, Ni2+, and Pb2+.
Finally, bis(ferrocenyl)thiazole 5 is a dual optical and redox
sensor for Zn2+, Cd2+, Mn2+, Co2+, Hg2+, Ni2+, and Pb2+,
whereas bis(ferrocenyl) compound 7, bearing a bis(thiazole)
as a bridge, is only a chromogenic sensor for Zn2+, Cd2+,
Mn2+, Co2+, and Ni2+, promoting a color change from orange
to blue, and Hg2+ and Pb2+, where a change of color from
Inorganic Chemistry, Vol. 46, No. 3, 2007 837

Caballero et al.
orange to green is observed. The experimental data and
conclusions about both the electronic and ion-sensing proper-
ties are supported by DFT computations and show, in
addition, an unprecedented intramolecular electron-transfer
reorganization after the first one-electron oxidation of
compound 5.
David B. Amabilino (ICMAB, CSIC) for his corrections and
suggestions for this work.
Supporting Information Available: Relevant spectroscopic
data for the new synthesized compounds; crystal data and details
of data collection and structure refinement of compounds 3, 5, 11,
and 14; crystal packing of neutral compounds 3, 5, 11, and 14; CV
and DPV of compounds 3, 5, 9, and 14 before and after addition
of metal cations; evolution of the UV-near-IR data during the
course of the oxidation of compound 3; variation of the UV-vis
data of 3, 5, 7, 9, 11, and 14 upon addition of different metal cations;
EPR data of some compounds; comprehensive theoretical study of
the oxidative event in ferrocenylthiazoles and their complexating
behavior. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. We gratefully acknowledge the grants
from Ministerio de Educacio´n y Ciencia (MEC), Spain,
CTQ2004-02201 and MAT2003-04699, and “Eje C-Con-
solider” EMOCIONa (CTQ2006-06333/BQU), from Gen-
eralitat de Catalunya 2005 SGR-00591 and from Fundacio´n
Se´neca (CARM) 02970/PI/05. A.C. also thanks Ministerio
de Eucacio´n y Ciencia for a predoctoral grant. D.C. is grateful
to Ministerio de Educacio´n y Ciencia and Universidad de
Murcia for a Ramo´n y Cajal contract. The authors also thank
IC061803B
838 Inorganic Chemistry, Vol. 46, No. 3, 2007