Unprecedented 1,3-diaza[3]ferrocenophane scaffold as molecular probe for anions

Article abstract of DOI:10.1021/ic102314r

The guanidine unit in the guise of 2-aminoimidazole in the new structural motif 2-arylamino-1,3-diaza[3]ferrocenophane 4 acts as a binding site for anions. The electrochemical behavior of this compound has been studied by cyclic voltammetry (CV) and diffe

Full text of DOI:10.1021/ic102314r

ARTICLE
pubs.acs.org/IC
Unprecedented 1,3-Diaza[3]ferrocenophane Scaffold as Molecular
Probe for Anions†
‡
§
||
||
^
ꢀ
Antonia Sola, Raꢀul A. Orenes, María Angeles García, Rosa M. Claramunt, Ibon Alkorta,
Josꢀe Elguero,^ Alberto Tꢀarraga,*,‡ and Pedro Molina*,‡
‡Departamento de Química Orgꢀanica, Facultad de Química, and §Servicio de Apoyo a la Investigaciꢀon (SAI),
Campus de Espinardo, Universidad de Murcia, E-30100 Murcia, Spain
Departamento de Química Orgꢀanica y Bio-Orgꢀanica, Facultad de Ciencias, UNED, Senda del Rey 9, E-28040 Madrid, Spain, and
^Instituto de Química Mꢀedica (CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain
S Supporting Information
b
ABSTRACT: The guanidine unit in the guise of 2-aminoimi-
dazole in the new structural motif 2-arylamino-1,3-diaza-
[3]ferrocenophane 4 acts as a binding site for anions. The
electrochemical behavior of this compound has been studied by
cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) and was found to exhibit a quasi reversible oxidation
peak, associated to the Fe(II)/Fe(III) redox couple (Ep =440mV),
and a non-reversible oxidation wave (Ep = 817 mV), probably
associated to the oxidation of the CdN unit present in the
guanidine bridge. Recognition of AcO-, PhCO2-, F-, Cl-, and
Br- anions by the free receptor and the less basic anions Br-, Cl-, and NO3- by its monoprotonated form takes place by unusual
redox-ratiometric measurements and spectroscopic (1H NMR and UV-vis) changes.
’ INTRODUCTION
Recently, some papers have reported functionalized ferrocene
derivatives bearing chromogenic or fluorogenic moieties, which
display multichannel signaling for anions.9 Despite the rich
chemistry of guanidines, as the binding site, and ferrocene, as
the redox signaling unit, only two examples of guanidinyl-
ferrocene ligands have been described.10 In this work, we
combine in a highly preorganized system the redox activity of
the ferrocene group with the chromogenic behavior of the p-
nitrophenyl ring and the binding ability of the guanidine group.
In the benzimidazole-like receptor 4, 2-arylamino-1,3-diaza-
[3]ferrocenophane, the redox reporter (ferrocene) is integrated
within the ferrocenophanic unit while the chromogenic fragment
(p-nitrophenyl ring) is linked to the guest guanidine (in the guise
of 2-aminoimidazole) anion binding site, which is embedded into
the framework of the “ferrimidazole” system. Despite its simple
nature, this system may be altered following receptor-anion
interaction thus providing redox, colorimetric, and spectral
sensing of the recognition event.
The recognition and sensingof anions hasemerged recentlyasa
key research area within the generalized field of supramolecular
chemistry for the important role placed by anions in biological,
industrial, and environmental processes.1 In particular, designing
receptors capable of anion binding by hydrogen bonding con-
tinues to be an area of active research.2 The guanidine function,
because of its amphoteric nature, has a rich history in biological,3
and bioinspired molecular recognition.4 The guanidinium group
within a variety of molecular architectures forms strong noncova-
lent interactions with anionic groups through hydrogen-bonding
and charge-pairing interactions. In addition, deprotonated guani-
dines (guanidinates) have the potential to develop into valuable
ancillary ligands in coordination and organometallic chemistry,5
although the straightforward coordination of neutral guanidines to
metal centers remains comparatively underdeveloped,6 and metal-
guanidinyl complexes are barely known and unexplored.7
Ferrocene has largely proved to be a simple and remarkably
robust building block for the preparation of derivatives which
have been considered as prototype chemosensor molecules
displaying interesting electrochemical-sensing properties. In ferro-
cene-based ligands, cation binding at an adjacent receptor site
induces a positive shift in the redox potential of the ferrocene-
ferrocenium couple by through-space electrostatic communica-
tion, and the complexing ability of the ligand can be switched on
and off by varying the applied electrochemical potential.8
’ EXPERIMENTAL SECTION
General Comments. Melting point was determined on a hot-plate
melting point apparatus and are uncorrected. Routine 1H- and 13C NMR
spectra were recorded at 200 and 50 MHz, respectively. Chemical shifts
Received: November 19, 2010
Published: February 14, 2011
r
2011 American Chemical Society
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1
refer to signals of tetramethylsilane (TMS) in the case of H and 13C
spectra. The external standard used in the NMR titrations was also TMS.
The following abbreviations are used to represent the multiplicity of the
signals: st (pseudotriplet), d (doublet), and q (quaternary carbon atom).
UV-vis spectra were carried out in a UV-vis-NIR spectrophot-
ometer using a dissolution cell of 10 mm path. The samples were solved
in CH3CN (c = 1 ꢀ 10-4 M), and the spectra were recorded with the
spectra background corrected before and after of the sequential addi-
tions of aliquots of 0.5 equiv of anions in CH3CN (c = 2.5 ꢀ 10-2 M).
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
techniques were performed with a conventional three-electrode config-
uration consisting of platinum working and auxiliary electrodes and a SCE
These optimized geometries were further optimized at the B3LYP/6-
311þþG(d,p) level.13 Absolute shieldings and NICS(1)14 were calcu-
lated within the GIAO approximation15 on the last optimized geome-
tries [GIAO/B3LYP/6-311þþG(d,p)]. These calculations were
carried using the facilities of the Gaussian 03 software.16 The electron
density of the systems has been analyzed with the Atoms in Molecules
(AIM) methodology17 and the MORPHY program.18 The absolute
shieldings were transformed into chemical shifts (δ, ppm) using three
empirical equations we have established previously:19
δ13C ¼ 175:7 - 0:963σ13C
δ15N ¼ - 152:0 - 0:946σ15N
δ1H ¼ 31:0 - 0:97σ1H
reference electrode. The experiments were carried out with a 10-3
M
solution of sample in CH3CN containing 0.1 M [(n-C4H9)4N]PF6
(TBAPF6) as supporting electrolyte. All the potential values reported
are relative to the decamethylferrocene (DMFc) couple at room tempera-
ture. Deoxygenation of the solutionswasachievedbybubbling nitrogenfor
at least 10 min, and the working electrode was cleaned after each run. The
cyclic voltammograms were recorded with a scan rate increasing from 0.05
to 1.00 V s-1, while the DPV were recorded at a scan rate of 100 mV s-1
with a pulse high of 10 mV and a step time of 50 ms. Typically, receptor
(1 ꢀ 10-3 M) was dissolved in CH3CN (5 mL) and TBAPF6 (base
electrolyte) (0.170 g) added. The guest under investigation was then
addedasa 0.1M solutioninappropriate solvent using a microsyringe while
the cyclic voltammetric properties of the solution were monitored.
Decamethylferrocene (DMFc) was used as an external reference both
for potential calibration and for reversibility criteria. Under similar condi-
tions the DMFc has E1/2 =-0.07 V versus SCE and E1/2 = -0.46 V versus
ferrocene, and the anodic peak-cathodic peak separation is 67 mV.
Suitable single crystals of 1 for X-ray structural analysis were obtained
by slow evaporation of a solution of the solid in acetone. The diffraction
data were collected with a Bruker Smart APEX diffractometer using a
monochromated Mo KR radiation (λ= 0.71073 Å) in ω-scan mode. The
structure was solved by direct methods, and all non-hydrogen atoms
refined anisotropically on F2 using the program SHELXL-97. All H
atoms were initially located in a difference Fourier map and refined using
a riding model, except for the NH hydrogens, which were refined freely.
The ordered methyl groups of the acetone were refined by using rigid
groups.
Preparation of 2-(4-Nitrophenyl)amino-1,3-diazaferroce-
nophane 4. To a solution of p-nitrophenylisocyanate in dry CH2Cl2
(40 mL) a solution of 1,10-bis(triphenylphosphoranylidenamino) ferro-
cene 1 (0.2 g, 0.27 mmol) in the same solvent (10 mL) was added
dropwise, and the solution was stirred for 1 h at room temperature. The
solvent was removed under vacuum, and the resulting residue was
chromatographed on a silica-gel column using ethyl acetate/n-hexane
(6/4) as eluent to give 4 which was crystallized from CH2Cl2/Et2O
(1:2). Yield: 74 mg (75%); mp 174-176 ꢀC (d). 1H NMR (200 MHz,
acetone-d6): δ 3.84 (st, 2H), 4.09 (st, 2H), 7.91 (d, 1H, J = 9.4 Hz), 8.16
(d, 2H, J = 9.4 Hz). 13C NMR (50 MHz, acetone-d6): δ 67.7 (2ꢀCH),
69.4 (2ꢀCH), 118.3 (CH), 125.2 (CH), 141.6 (q), 148.9 (q), 153.0 (q).
EIMS, m/z (relative intensity): 362 (Mþ, 100), 316 (42), 224 (55).
Anal. Calcd for C17H14FeN4O2: C, 56.38; H, 3.90; N, 15.47. Found: C,
56.63; H, 3.68; N, 15.58.
’ RESULTS AND DISCUSSION
Synthesis, Structure, and Tautomeric Studies. The aza-
Wittig protocol20 was the method of choice for building up the
2-aminoimidazole architecture in 4, so that the bis(iminophos-
phorane) 1, readily available from 1,10-diazidoferrocene and
triphenylphosphine21 was used as starting material. Receptor 4
was obtained in 75% yield from the aza-Wittig reaction between
the bis(iminophosphorane) 1 and p-nitrophenylisocyanate in dry
dichloromethane at room temperature. Formation of compound
4 can be explained by an initial aza-Wittig-type reaction
between one iminophosphorane group of compound 1 and
1 equiv of the isocyanate to give the carbodiimide 2, which
undergoes cyclization by nucleophilic attack of the nitrogen
atom of the remaining iminophosphorane moiety on the
central carbon atom of the carbodiimide functionality to give
the zwitterionic compound 3. This compound undergoes
hydrolytic cleavage during the workup to give 4 (Scheme 1).
This mechanism is in accordance with previous results ob-
tained from the aza-Wittig reaction of aryl, vinyl bis-
(iminophosphoranes) with aryl isocyanates substituted with
an electron-withdrawing group.22
Crystals suitable for X-ray diffraction analysis were obtained
which unambiguously demonstrate the proposed structure for
compound 4. A summary of crystallographic data collection
parameters and refinement parameters for 4 are compiled in
the Supporting Information (see Table 1 in Supporting In-
formation). Compound 4 crystallizes in the orthorhombic space
group Pbca. Figure 1 shows the ORTEP drawing of the molecule
structure of 4 with the atomic numbering scheme. In the
ferrocenyl moiety, the two cyclopentadienyl (Cp) rings are
Solid state 13C (100.73 MHz) and 15N (40.60 MHz) CPMAS NMR
spectra were obtained on a Bruker WB 400 spectrometer at 298 K using a
4 mm DVT probehead. Samples were carefully packed in 4-mm diameter
cylindrical zirconia rotors with Kel-F end-caps. Operating conditions
involved 3.2 μs 90ꢀ 1H pulses and decoupling field strength of 78.1 kHz
by TPPM sequence. The NQS (Non Quaternary Suppression) techni-
que to observe only the quaternary C-atoms was employed. 13C spectra
were originally referenced toa glycine sampleand then thechemical shifts
were recalculated to theMe4Si [for thecarbonyl atom δ (glycine) = 176.1
ppm] and 15N spectra to 15NH4Cl and then converted to nitromethane
scale using the relationship: δ15N (nitromethane) = δ15N(ammonium
chloride) - 338.1 ppm. The typical acquisition parameters for 13C
CPMAS were as follows: spectral width, 40 kHz; recycle delay, 5 s;
acquisition time, 30 ms;contact time, 2 ms; andspin rate, 12 kHz. And for
15N CPMAS they were as follows: spectral width, 40 kHz; recycle delay,
5 s; acquisition time, 35 ms; contact time, 7 ms; and spin rate, 6 kHz.
Solution NMR spectra were recorded on a Bruker DRX 400 (9.4 T,
400.13 MHz for 1H) spectrometer with a 5-mm QNP-probe equipped
with a z-gradient coil, at 298 K. Chemical shifts (δ in ppm) are given
1
1
from internal solvent, CDCl3 7.26 for H. Typical parameters for H
NMR spectra were spectral width 4800 Hz and pulse width 9.25 μs at an
attenuation level of -3 dB.
Computational Details. All the molecules were optimized at the
B3LYP/6-31G(d) level11 where frequencies12 were calculated to verify
that all of them were minima (number of imaginary frequencies = 0).
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Scheme 1. Synthesis of the Ferrocene-Imidazole 4a
Table 1. Hydrogen Bonds [Å and deg]a
D-H
A
d(D-H) d(H A) d(D A) —(DHA)
3 3 3
3 3 3
3 3 3
N(2)-H(02) O(3)#1 0.82(3)
2.33(3)
2.09(3)
2.58(3)
2.51
3.033(2)
2.892(2)
3.181(2)
3.361(3)
144(2)
165(2)
131(2)
149.8
3 3 3
N(3)-H(03) O(3)#1 0.82(3)
3 3 3
N(2)-H(02) O(1)#2 0.82(3)
3 3 3
C(7)-H(7) O(2)#3
0.95
3 3 3
a Symmetry transformations used to generate equivalent atoms: #1 -x
þ 1/2, -y, z þ 1/2 #2 -x þ 1/2, y -1/2, z #3 x þ 1, y, z .
Chart 1. Acid-Base Equilibria and Tautomeric Forms of
Receptor 4 and 4-
a Conditions: (a) Dry CH2Cl2, r.t. 1 h; (b) work up.
Figure 1. ORTEP view of the molecular structure of compound 4
showing the hydrogen bonds formed with the molecule of acetone.
Thermal ellipsoids are drawn at 50% probability level.
Table 2. Energies at the B3LYP/6-311þþG(d,p) Levela of
Neutral, Anionic, and Cationic Forms
structure
SCF energy
Erel
perfectly planar but deviate from being parallel, that is, the angle
between the planes is 15.7ꢀ and the rings are completely eclipsed.
The organic ligand attached to the ferrocenyl moiety is almost
planar, the deviation from the main plane including the nitro
group being 0.0633 Å. The planarity is induced by the presence of
acetone retained in the lattice.23 The oxygen atom of the acetone
4anti
-2289.58519
-2289.58564
-2289.58630
-2289.05566
-2289.05230
-2289.97252
2.9
1.7
0.0
0.0
8.8
4syn
4imino-
4-anti/syn
4-sym
4 Hþ
forms two intermolecular N-H O hydrogen bonds with the
3 3 3
3
hydrogens of the amino groups of the organic moiety in a way
that fix its conformation preventing the free rotation of N3-C21
bond. The C24-N4 bond is also hindered from rotation by the
presence of some hydrogen bonds involving the oxygen atoms
O1 and O2. The organic ligand adopts a nearly perpendicular
orientation with respect to the Cp rings, the angle between mean
planes being 85.9ꢀ (C1-ring) and 85.8ꢀ (C6-ring).
a Absolute values, hartree; relative values, kJ mol-1
.
experiments using different deuterated solvents, as well as through
the calculated energies for these forms at the B3LYP/6-311þþG-
(d,p) level (Supporting Information). These results demonstrate
that, in CDCl3 solution, 4 is as a mixture in equilibrium of syn and
imino forms in similar abundance (the two most stable according
to the theoretical calculations reported in Table 2. In acetone-d6,
receptor 4 exists exclusively in the syn form, independently of the
temperature, because of the formation of a bifurcated hydrogen
bond with the solvent. Similarly, in the solid state (CPMAS NMR
measurements, see Supporting Information), 4 also exists in the
syn form because of the presence of acetone in the crystals.
Analogous NMR studies carried out on the 4- species demon-
strate that this guanidinate anion exists exclusively in the anti/syn
form (Supporting Information).
Regarding the crystal structure of 4, molecules are interlinked
by N2-H02 O1 and C7-H7 O2 hydrogen bonds (Table 1),
3 3 3
3 3 3
forming layers perpendicular to the c axis (see crystal packing diagram
of 4 in Supporting Information).
Compound 4 could exist in three tautomeric forms: 4anti,
4syn, and 4imino (Chart 1). Thus, an in depth study about the
tautomerism of both the neutral and the charged forms of 4 have
been carried out through the comparison of the calculated and
experimental chemical shifts obtained by 1H, 13C, and 15N NMR
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Chart 2. Tautomeric Forms of 4 and Related Structures for the NICS Calculations
Table 3. Isotropic NICS Values (ppm)
structure
NICS(0)
NICS(þ1)
NICS(-1)
4anti
1.8
2.0
1.8
1.5
0.9
8.7
1.8
0.3
1.2
0.8
8.6
1.9
4syn
4imino-
imidazole
imidazoline
2.5
11.6
4.2
The nucleus-independent chemical shift (NICS) is a compu-
tational method that calculates the chemical shifts of a hypothe-
tical lithium ion positioned directly inside the ring or, more
recently, 1 Å above (þ1) or below (-1) the ring.24 In the original
Schleyer definition negative NICS values indicate aromaticity
and positive values antiaromaticity, but in the Gaussian imple-
mentation, the signs are reversed. Then, by using the Gaussian
convention25 nucleus-independent chemical shift calculations
have been carried out for the three tautomers of 4, yielding
values between 0 and 2 ppm [both NICS(0) and NICS(1)]
proving that the “ferrimidazole” ring is not aromatic and resem-
bles more imidazolines [NICS(1) = 1.8 ppm] than imidazoles
[NICS(1) = 8.7 ppm] (Chart 2 and Table 3).
Anion Recognition Studies. The recognition properties of
receptor 4, having a particularly effective neutral hydrogen
bonding motif, toward various anions which possess spherical
(F-, Cl-, Br-), trigonal planar (AcO-, PhCO2-, NO3-) or
tetrahedral (HSO4-, H2PO4-, HP2O73-) geometries have been
investigated by electrochemistry, UV-vis spectroscopic mea-
surements, and 1H NMR spectroscopy.
Figure 2. DPV, from 0.0 to 1.1 V of compound 4 (1 mM) in CH3CN
using [(n-C4H9)4N]PF6 (0.1 M) as supporting electrolyte. Inset: CV
corresponding to receptor 4, from 0.0 to 0.8 V.
The recognition capability of the receptor 4 toward the above-
mentioned set of anions, in the form of their corresponding
tetrabutylammonium salts, was evaluated by DPV.28 In general,
the results obtained demonstrate that while addition of AcO-,
PhCO2-, NO3-, F-, Cl-, and Br- anions promotes remarkable
responses, addition of HSO4-, H2PO4-, and HP2O73- anionic
species had no effect on the CV or DPV of this receptor, even
when present in a large excess. Nevertheless, the results obtained
on the stepwise addition of substoichiometric amounts of AcO-,
PhCO2-, NO3-, F-, Cl-, or Br- anions revealed two different
electrochemical behaviors. Thus, addition of increasing amounts
of the basic AcO-, PhCO2-, and F- anions to the free ligand
promotes the complete disappearance of the oxidation peak at
Ep = 817 mV. Moreover, a typical “two wave behavior”was observed
for the evolution of the peak at Ep = 440 mV, which consists in
the progressive appearance of a second oxidation peak at more
negative potentials (ΔEp = -182 mV for AcO-, ΔEp = -152
mV for PhCO2-, and ΔEp = -128 mV for F-) together with the
corresponding to the free receptor, which completely disap-
peared when addition of 1 equiv of anion was achieved (Figure 3a
and Supporting Information). This kind of electrochemical
behavior has also been observed in biferrocene systems bearing
guanidine bridges as anion binding sites.10b,c However, after
addition of NO3-, Cl-, and Br- anions the two oxidation
peaks are still observed, although negatively shifted compared
to the free receptor (“shifting behavior”).29 Nevertheless, while
the first oxidation peak was slightly cathodically shifted the
second one was cathodically shifted in a larger values30
(Figure 3b and Supporting Information). The difference be-
tween the potentials of the two peaks observed (ΔEp = 2Ep -
1Ep = 377 mV), is decreased when the complexes between the
receptor and the less basic NO3-, Cl-, and Br- anions are
As it was expected, receptor 4 shows electroactivity because of
the Fe(II)/Fe(III) redox couple. In fact, CV of an electrochemical
solution of 4, (c = 10-3 M in MeCN), containing 0.1 M TBAHP as
supporting electrolyte gave rise to a quasi reversible oxidation wave
at Ep = 440 mV versus decamethylferrocene (DCMF) when the
oxidation was carried out in the range from 0 to 800 mV
(Figure 2a). Working under the same electrochemical conditions
but from 0 to 1100 mV, receptor 4 displays an additional non-
2
reversible oxidation peak at Ep = 817 mV versus DCMF which
could be assigned to the oxidation of the CdN group present in the
phane bridge, as has previously reported for related moieties such as
formamidine26 or 2-aza-1,3-butadiene bridges27 Likewise, the DPV
also exhibits two oxidation peaks at the same potentials (Figure 2b).
The electrochemical behavior of receptor 4 acidic media was
also studied by addition of acetic acid to the electrochemical
solution. While addition of acetic acid did not cause any effect on
the OSWV of this species, giving rise to the two characteristic
oxidation peaks at 1Ep = 440 mV and 2Ep = 817 mV. By contrast,
addition of a stronger acid such as HBF4 promotes the appear-
ance of only one oxidation peak at Ep = 860 mV assigned to the
monooxidation of the ferrocene unit within the guanidinium
species [4 Hþ] formed (Supporting Information)
3
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Figure 3. Evolution of the DPV of 4 (1 mM in CH3CN), scanned at 0.1
V s-1 with [(n-Bu)4N]ClO4 as supporting electrolyte, upon addition of
0 to 3 equiv of: (a) AcO- anion and (b) Cl- anion.
Figure 4. (a) Variation of the UV/vis in CH3CN of ligand 4 (c = 10-4 M)
upon addition of increasing amounts of AcO- anions, from 0 to 3 equiv.
Arrows indicate the absorptions that increase (up) and decrease (down)
during the experiments. (b): Job’s plot evaluated from the absorption
spectra of the titration solution exhibiting the inflection point at 0.5
(formation of a 1:1 complex): the total [4]þ [AcO-] = 10-4 M.
formed: ΔEp = 265 mV for Cl-, ΔEp = 242 mV for Br-, and
-
ΔEp = 258 mV for NO3
.
To get insight into the roles that the anions play when they are
added to the free receptor 4, two different titration experiments
were carried out. On the one hand, upon titration with a strong
base, such as Bu4NOH, which can only promote the deprotona-
tion of the free ligand, disappearance of the second oxidation peak
and a considerable cathodic shift of the first oxidation peak were
observed. (ΔEp = -228) (Supporting Information). On the other
hand, titration experiments in the presence of AcOH (20 equiv)
were also carried out toavoid any deprotonationeffect. Theresults
derived from these experiments, show that addition of AcO-,
PhCO2-, and F- still gave rise to significant oxidation potential
shifts although in a slightly smaller magnitude than those obtained
from the free ligand, (ΔEp = -116 mV for AcO-, ΔEp = -105
mV for PhCO2- and ΔEp = -62 mV for F-) anions (Supporting
Information). The results obtained upon addition of AcO-,
PhCO2-, and F- anions seems to suggest that in the absence of
an acidic medium a recognition process should take place, the
differences in the values of the oxidation waves being attributable
to the changes promoted in the electrochemical solutions by the
presence of the AcOH. Deprotonation processes should be ruled
out because of the absence of any wave which could be attributable
to the deprotonated species which should appear at higher
oxidation potential values than those observed upon addition of
these anions, even when they are added in a large excess.
region. The UV-vis spectrum of receptor 4 in CH3CN (c = 1 ꢀ
10-4 M) exhibits strong absorption bands at λ = 226 nm (ε =
16550 M-1 cm-1) and 350 (ε = 14710 M-1 cm-1) nm. Figure 4
shows the family of spectra taken in the course of the titration
receptor 4, with the Y-shaped anion AcO-, in a MeCN solution.
Upon addition of AcO-, the band at 350 nm (ε = 14710 M-1
cm-1) progressively decreases, while a new band at 374 nm (ε =
16770 M-1 cm-1) (Δλ = 24 nm) is formed and developed
(Table 3). The presence of two sharp isosbestic points at 276 and
357 nm indicates that only two species coexist in the equilibrium.
Analogous investigations were also carried out by using
PhCO2-, F-, Cl-, Br-, NO3-, HSO4-, H2PO4-, and
HP2O73-. For PhCO2 and F- anions, spectral features are the
-
same as those observed for AcO- anion, whereas no spectral
changes were observed for NO3-, HSO4-, H2PO4-, and
3-
HP2O7 anions, and a small red-shift (Δλ = 6 nm) of the
absorption band was observed for Br-, Cl- anions (Supporting
Information). Addition of Bu4NOH, however, induced the appear-
ance of an additional low-energy band at 481 nm (Δλ = 11 nm)
attributable to the deprotonated species, which is responsible for the
development of a deep orange-red color in the solution. Thus,
deprotonation seems to be signaled by the appearance of a new
absorption band at a longer wavelength, taking only place in the case
of adding hydroxide anion (Supporting Information).
Interestingly, addition of AcO- anion to an electrochemical
solution containing triethylamine induces a shift in the first
oxidation peak of ΔEp = -70 mV, indicating that the recognition
process is still taking place even under these conditions. By
contrast, Cl-, Br-, and NO3- anions do not apparently promote
any perturbation, which is indicative that the interaction between
receptor 4 and these anions mainly involve hydrogen-bonded
complex formation (Supporting Information).
Previous studies on ferrocene-based ligands have shown that
their characteristic low energy (LE) bands in the absorption
spectra are perturbed upon complexation.31 Moreover, the chro-
mogenic p-nitrophenyl group also provides a further advantage,
allowing one to monitor complex formation through definite
color changes and spectral modifications. Therefore, the occur-
rence of the receptor-anion interaction was also studied by the
distinctive changes of the receptor’s absorbance in the UV-vis
For AcO-, and PhCO2- anions, binding assays using titration
isotherms suggest a 1:1 binding model. The corresponding
association constants weredetermined by treatmentof the spectro-
photometric titration data,32 the values obtained being those
shown in Table 4. Moreover, the calculated detection limits33
were 1.0 ꢀ 10-5 M for both AcO- and PhCO2- anions, 2.8 ꢀ 10-5
M for F-, 2.2 ꢀ 10-5 M for Cl-, and 3.4 ꢀ 10-5 M for Br-.
For the reported constants to be taken with confidence, we
have proved the reversibility of the complexation processes. If the
sensing system is reversible, depletion of the anion that coordi-
nates 4 must produce a change of the absorption spectrum,
causing it to revert to the original spectrum. Formation of the
complexes 4 AcO- or 4 PhCO2- and the subsequent decom-
3
3
plexation by extraction of the anion with H2O were carried out
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Inorganic Chemistry
ARTICLE
Table 4. Relevant Data of the Ligand/Anion Complexes
Formed
b
c
d
Δλa
Kas
ΔHΑ
ΔHΒ
4 AcO-
24
24
3.54 ꢀ 103 ((0.39)
3.64 ꢀ 103((0.36)
7.3 ꢀ 104 ((1.20)
4.8 ꢀ 104 ((1.28)
6.3 ꢀ 103 ((1.56)
þ0.27
þ0.37
þ0.16
þ0.17
þ0.19
-0.18
-0.13
-0.05
-0.11
-0.09
-0.05
-0.05
-0.06
-0.05
þ0.24
-0.09
-0.02
-0.03
3
-
4 PhCO2
3
4 F-
3
4 Cl-
6
3
4 Br-
6
3
[4 Hþ]
-58
3
[4 Hþ] Cl- e
8
8
8
4.0 ꢀ 105 ((0.18)
2.8 ꢀ 104 ((0.11)
1.6 ꢀ 104 ((0.07)
3
3
[4 Hþ] Br- e
3
3
[4 Hþ] NO3
- e
3
3
a Shift, in nm, of the LE band upon complexation Δλ = λ(complexed)
-
λ(free ligand). b Association constant, in M-1, determined, from the UV-
vis titration data, by Specfict/32 Global Analysis System. c ΔHΑ
=
HΑ(complexed) - HΑ(free ligand), in ppm. d ΔHΒ = HΒ(complexed)
-
e
HΒ(free ligand) in ppm. For the calculation of these data, the values of
the variables corresponding to the [4 Hþ] species were taken as
3
reference.
over several cycles. The optical spectra were recorded after each
step and found to be fully recovered on completion of the step,
thus demonstrating the high degree of reversibility of the sensing
process (Supporting Information).
1
Figure 5. Evolution of the H NMR spectra of 4 in acetone-d6 upon
addition of AcO-, from (a) 0 to (d) 1 equiv.
With the view to shedding some light on the structure of
1
complex formation of this receptor 4, we carried out H NMR
promote an upfield shift; (ii) the polarization of the C-H bonds,
induced by a through-space effect, in particular the partial positive
charge created onto the proton, causes a deshielding effect and
promotes a downfield shift. The latter effect is expected to vanish at
higher distances and should therefore operate only on the C-HΑ
bond. In fact, in the case of the C-HA proton, the electrostatic
effect dominates, and a progressive downfield shift (Δδ = 0.27
ppm) is observed. On the other hand, C-HB protons feel neither
the anion-induced electrostatic effect nor the C-HΑ polarization
effect, thus being affected only by the through-bond effect, which
induces a moderate upfield shift (Δδ = -0.09 ppm)34 (Table 4).
On the other hand, some interesting facts are also detected in the
ferrocene region: while the broad singlet corresponding to the Hβ
titration experiments upon addition of increasing amounts of a
solution of the aforementioned anions to an acetone-d6 solution
of 4. CD3CN was chosen as a solvent of the anions to afford
a concentration suitable for 1H NMR spectroscopic studies (c =
2.5 ꢀ 10-2 M). The free receptor shows two characteristic broad
singlets, at δ = 3.94 ppm and δ = 4.09 ppm, integrating 4H each,
corresponding to the HRþHR and HβþHβ protons, respec-
tively, within the two monosubstituted cyclopentadienyl (Cp)
units present in the ferrocene moiety. Additionally, in the
aromatic region, it exhibits the typical AB pattern of signals
corresponding to the HA and HB protons present in the nitro-
phenyl substituent. It is worth mentioning that in the free ligand
the signals corresponding to the guanidine protons do not appear
whenthe spectrum was carried outat room temperature, probably
because the tautomeric equilibrium is fast on the NMR time scale.
However, when the 1H NMR spectrum was obtained at -55 ꢀC
two significant features were observed: (i) the appearance of two
broad singlets in the ferrocene region at δ = 4.11 ppm and δ =
3.85 ppm, integrating 6H and 2H, respectively; and (ii) the
presence of two additional broad singlets at δ = 7.03 ppm and δ =
9.11 ppm, attributable to the guanidine NH protons.
0
0
0
and Hβ protonsremained almost unalteredthe onecorresponding
0
to the HR and HR is now split into two new broad singlets, one
appearing at a chemical shift (δ = 3.91 ppm), almost equivalent to
that observed in the free ligand (δ = 3.94 ppm), and another one
(δ = 3.69 ppm) upfield shifted by Δδ = 0.10 ppm. Moreover,
addition of AcO- promotes the appearance of two new singlets at
δ = 11.05 ppm and δ = 12.47 ppm which could be associated to the
guanidine NH protons. The last observations are in agreement
with the results obtained when the 1H NMR spectrum of the free
ligand 4 was carried out at -55 ꢀC.
1H NMR titrations of 4 with the above-mentioned set of anions
clearly evidenced that only the addition of AcO-, PhCO2-, F-,
Cl-, and Br- promoted significant perturbations on the proton
signalsof thisreceptor. Two common features, in the regions of the
ferrocene and aromatic protons, were observed during the titration
experiments. These features are illustrated in Figure 5 correspond-
ing to the spectra taken over the course of the titration with AcO-.
On the one hand, it illustrates the spectral shifts of the HA and HB,
aromaticprotonspresentin the phenyl ring linked to the guanidine
moiety indicating the formation of a discrete H-bond complex. In
fact, two effects are expected to derive from the hydrogen bond
formation between the guanidine subunit and the anion: (i) the
increase of electron density in the phenyl rings, with a through-
bond propagation; this fact causes a shielding effect and should
1
This anion-induced chemical shift changes in the H NMR
spectra of 4, suggesting that both NH guanidine protons are
involved in the complexation of this anion which prevent, to
some extent, both the tautomeric effect and the free rotation of
the aryl group linked to the C2 of the ferrocenophane framework
thus making the Cp rings of the ferrocene units non-equivalent
and, as a consequence, justifying the appearance of two different
0
signals for the HR and HR protons of the ferrocene unit
(Chart 3).
Completely analogous results were also obtained when the
-
PhCO2 anion was used as analyte (Table 4 and Supporting
Information), also indicating that this anion is interacting with
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Inorganic Chemistry
ARTICLE
1
Figure 6. Evolution of the H NMR spectra of 4 in acetone-d6 upon
addition of F-, from (a) 0 to (f) 1 equiv.
Figure 7. Evolution of the DPV of 4 Hþ (1 mM in CH3CN), (red line)
3
scanned at 0.1 V s-1 with [(n-Bu)4N]ClO4 as supporting electrolyte,
upon addition of (a) 1 equiv of AcO- (blue line) and (b) 3 equiv of Cl-
anion (blue line). Black lines in (a) and (b) correspond to the DPV of
the free ligand 4.
Chart 3. Plausible Binding Mode in the Complex [4 AcO-]
3
the free receptor via possible bonding between the anion and the
guanidine moiety.
1H NMR titration experiments with F- (HΒ Δδ = -0.05; HΑ
Δδ = 0.16 ppm), Cl- (HΒ Δδ = -0.06; HΑ Δδ = 0.17 ppm),
Br- (HΒ Δδ = -0.05; HΑ Δδ = 0.19 ppm) showed also similar
patterns than those observed with AcO- anion (Table 4),
although in these cases the two signals due to the aromatic
protons are almost overlapped and no signals corresponding to
the guanidine protons were observed. Moreover, the HR protons
within the ferrocene moiety appeared as a very broad singlet with
very low intensity (Figure 6).
Because it is very well-known that the guanidinium group is an
extremely effective functional unit in the binding of anionic
species we decided to transform the guanidine moiety within the
receptor 4 into the corresponding guanidinium specie [4 H]þ,
3
by treatment with 1 equiv of HBF4 in MeCN.
Figure 8. Evolution of the 1H NMR spectra of [4 Hþ] in acetone-d6
3
The electrochemical response of 4 after addition of HBF4 was
previously studied by using CV and DPV and a 0.1 M solution of
TBAHP as supporting electrolyte (Supporting Information). As
expected, the protonation process resulted in the appearance, at
Ep = 860 mV, of a new oxidation peak anodically shifted (ΔEp =
420 mV) with reference to the free receptor.
upon addition of Cl- anion, from (a) 0 to (d) 1 equiv.
and Supporting Information). In such cases the redox peak is
cathodically shifted: ΔEp = -170 mV for Cl- and NO3- anions
and ΔEp = -200 mV for Br-.
1
The H NMR spectrum of the receptor 4 is also strongly
The [4 H]þ guanidinium receptor has also been used to
perturbed upon protonation with HBF4 in deuterated MeCN.
3
investigate its ability for sensing the anion binding by using the
previously mentioned set of anions: F-, Cl-, Br-, AcO-,
PhCO2-, NO3-, HSO4-, H2PO4-, and HP2O73-. While the
tetrahedral oxoanions used do not promote any changes in the
oxidation potential of the ferrocene unit, addition of AcO-,
PhCO2-, and F- anions induced the appearance of a new
oxidation peak, which is cathodically shifted. The evolving peak,
appeared at virtually the same potential than that of the free
receptor 4, indicating that a deprotonation has taken place
Then, the 1H NMR spectrum of the resulting complex [4 Hþ]
3
displays two singlets at 11.40 and 9.61 ppm due to the N-H
protons. The aromatic HB protons are downfield shifted by 1.8
ppm and the HA protons are upfield shifted in almost the same
extension. Moreover, both pseudotriplets due to the protons
within the cyclopentadienyl rings are downfield shifted by 0.56
and 0.46 ppm, respectively. Addition of 1 equiv of AcO-,
PhCO2-, and F- anions to a solution of [4 Hþ] induces a
3
deprotonation effect and, as a consequence, the spectrum of the free
receptor 4 is recovered (Supporting Information). However, in the
presence of Cl- (Figure8) (Δδ= 0.97 ppm), Br- (Δδ= 0.44 ppm),
-
(Figure 7a). However, the addition of Cl-, Br-, and NO3
anions elicited a different electrochemical response (Figure 7b
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Inorganic Chemistry
ARTICLE
fashion and spectroscopic (1H NMR and UV-vis) measure-
ments. Its monoprotonated form [4 Hþ] is able to selectively
3
sense the less basic Cl-, Br-, and NO3- anions: the oxidation
redox peak is cathodically shifted (170-200 mV), and the low-
energy band of the absorption spectrum is red-shifted (Δλ = 8 nm)
upon complexation. In addition, the signals due to the NH
1
protons in the H NMR spectrum are remarkably downfield
shifted (0.44-0.97 ppm).
’ ASSOCIATED CONTENT
1
Supporting Information. Electrochemical. H and 13C
S
b
NMR of 4. Electrochemical and spectroscopic titration data.
CPMAS spectra. Theoretical calculations. X-ray data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 9. Variation of the UV/vis in CH3CN of ligand 4 Hþ (c = 10-4
3
M) upon addition of (a) 1 equiv of AcO- and (b) 1 equiv of Br- anions.
Arrows indicate the absorptions that increase (up) and decrease (down)
during the experiments.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: pmolina@um.es; (P.M.), atarraga@um.es (A.T.).
and NO3- (Δδ = 0.61 ppm) anions, the signals correspond-
ing to the N-H protons are remarkably downfield shifted,
while the remaining signals are not apparently perturbed
(Supporting Information).
’ ACKNOWLEDGMENT
We gratefully acknowledge the financial support from MI-
CINN-Spain, Project CTQ2008-01402, CTQ2009-13129-C02-
02, CTQ2010-16122 and Fundaciꢀon Sꢀeneca (Agencia de Ciencia
y Tecnología de la Regiꢀon de Murcia) project 04509/GERM/06
(Programa de Ayudas a Grupos de Excelencia de la Regiꢀon de
Murcia, Plan Regional de Ciencia y Tecnología 2007/2010) and
the Comunidad Autꢀonoma de Madrid (Project MADRISO-
LAR2, ref S2009/PPQ-1533). We deeply acknowledge the
CTI (CSIC) for an allocation of computer time.
Salient features of UV-vis spectrum of the complex [4 Hþ]
3
when compared to that of the free receptor is the appearance of a
low-energy band at 292 nm, which is blue-shifted by Δλ = 52 nm.
However, upon addition of 1 equiv of AcO-, PhCO2-, and F-
anions the original absorption spectrum of the free receptor 4 is
recovered (Figure 9a). By contrast, in the presence of Cl-, Br-,
and NO3- anions the low-energy band is red-shifted by Δλ = 8
nm (Figure 9b). In this case, the presence of two isosbestic points
around 252 and 292 nm indicates that only two species coexist at
the equilibrium.
DEDICATION
†A tribute in memory of the Organic Chemistry Professor Dr.
Rafael Suau Suarez (University of Mꢀalaga, Spain).
The binding isotherms were fitted to a 1:1 binding stoichi-
ometry, and the binding constants were found to be Ka = 4.0 ꢀ
105 ((0.18) M-1 for Cl-, Ka = 2.8 ꢀ 104 ((0.11) M-1 for Br-,
-
and Ka = 1.6 ꢀ 104 ((0.07) M-1 for NO3 anion. As it was
expected, these values are higher than those found for the free
receptor toward the same anions. The 1:1 stoichiometry of the
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3
and these anions was also confirmed by electrospray ionization
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3
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3
’ CONCLUSION
We have designed a new guanidinoferrocene-based receptor 4
which exists in the syn form in the solid state. In CDCl3 solution
appears as a mixture of syn and imino forms in similar abundance,
whereas in acetone exclusively exists in the syn form.
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-
F-, Cl-, and Br- anions through an unusual redox ratiometric
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ARTICLE
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