Flavonol based ruthenium acetylides as fluorescent chemosensors for lead ions

Article abstract of DOI:10.1016/j.jorganchem.2007.10.046

Flavonol based alkynyl ruthenium complexes devoted to the detection of metal traces in solution are described. The sensitivity of both absorption and emission properties of the 3-hydroxyflavone unit as a receptor for the metal cations, and of an alkynyl ruthenium moiety as an extended π-conjugated system, provides an efficient molecular sensor for rapid, sensitive and selective detection of lead(II) cations.

Full text of DOI:10.1016/j.jorganchem.2007.10.046

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 228–234
www.elsevier.com/locate/jorganchem
Flavonol based ruthenium acetylides as fluorescent chemosensors
for lead ions
a,
a
a
a
*
`
Jean-Luc Fillaut , Julien Andries , Ripu Daman Marwaha , Pierre-Henri Lanoe¨ ,
a
b
c
Olivier Lohio , Loic Toupet , J.A. Gareth Williams
a
UMR 6226 CNRS – Universite´ Rennes 1, Sciences Chimiques de Rennes, Avenue du Ge´ne´ral Leclerc, 35042 Rennes cedex, France
´
b
`
´
UMR 6626 CNRS – Universite´ Rennes 1, Laboratoire de Physique de la Matiere Condensee, Universite de Rennes 1, 35042 Rennes cedex, France
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK
c
Received 8 October 2007; received in revised form 25 October 2007; accepted 25 October 2007
Available online 4 November 2007
Abstract
Flavonol based alkynyl ruthenium complexes devoted to the detection of metal traces in solution are described. The sensitivity of both
absorption and emission properties of the 3-hydroxyflavone unit as a receptor for the metal cations, and of an alkynyl ruthenium moiety
as an extended p-conjugated system, provides an efficient molecular sensor for rapid, sensitive and selective detection of lead(II) cations.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Ruthenium acetylides; Luminescent chemosensors; Flavonols; Heavy metal sensors
1. Introduction
metal alkynyl derivative. Our interest in flavonol-contain-
ing ruthenium alkynyl derivatives came from the idea that
the metal acetylide fragment could allow the absorption
and emission properties of the resulting complex to be dis-
placed beyond the limits of that of usual organic com-
pounds, into the useful visible region. Ruthenium alkynyl
derivatives have been extensively used, for example, in non-
linear optics [13–17] and as molecular wires [18–23]. These
properties arise as a result of overlap between the d-orbitals
of ruthenium and the conjugated p system and can thus be
modified systematically either through variation of the
organic p system or by fine-tuning the electron richness
of the alkynyl ruthenium moiety itself. We already esta-
blished that alkynyl ruthenium complexes bearing terminal
hydrogen-bond receptors can also act as efficient anion
sensors exhibiting large guest-induced colour changes and
show high selectivity to fluoride ions [24].
Whereas transition metal ions play an important role in
many fundamental physiological processes in organisms,
heavy metal ions can often have a profoundly detrimental
effect upon them. In particular, lead(II) ions can affect
almost every organ and system in the human body, causing
various symptoms such as anaemia, disorder of the blood,
muscle paralysis, memory loss, and mental retardation
[1–3]. Development of effective fluorescent chemosensors
[4,5] for discriminating lead ions from alkali, alkaline earth
metal and transition-metal ions [6–9] is therefore helpful
for measuring the amount of lead in contaminated sources
[10], including the human body, as well as for clarifying the
cellular role of the Pb2+ ions in vivo [11].
In a new development of our research on the use of
ruthenium alkynyl derivatives as chemosensors [12], we
report here on our discovery of a fluorescent sensor based
on the association of a flavonol moiety with a transition
Being sensitive to the perturbations of their environment,
3-hydroxyflavone and its derivatives attract current interest
as luminescent probes for analytical chemistry, bio-physics,
and cellular biology [25,26]. In particular, 3-hydroxyflavone
derivatives coordinate to cations of different charges, radii,
*
Corresponding author.
E-mail address: jean-luc.fillaut@univ-rennesl.fr (J.-L. Fillaut).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.10.046

J.-L. Fillaut et al. / Journal of Organometallic Chemistry 693 (2008) 228–234
229
and electronic properties [27–32]. In this contribution, we
discuss the preparation and photophysical properties of a
new class of luminescent alkynyl ruthenium complexes, in
which the presence of a 3-hydroxyflavone unit provides very
attractive properties as a receptor for heavy metal ions and
reveals high sensitivity and selectivity towards lead(II)
cations.
125.4 (CH), 124.7 (Cq), 124.3 (CH), 120.8 (Cq), 118.1
(CH), 115.6 (Cq), 30.7 (qt, |1JP–C + 3JP–C| = 23 Hz, CH2
dppe).31P {1H} NMR (CDCl3, 81 MHz, d ppm): 50.50 (s).
2.3. trans-[MeCNRu(dppe)2{C„C-p-phenyl-(3-hydroxy-
4H-chromen-4-one)]PF6 (7a)
6a (260 mg, 0.20 mmol), 25 mL of dichloromethane and
5 mL of acetonitrile (dry and degassed solvents) was intro-
duced under N2 atmosphere together with 1 mmol of
potassium hexafluorophosphate KPF6. The resulting mix-
ture was degassed, placed under N2 atmosphere, and stir-
red at room temperature for 7 h. The resulting orange
solution was washed several times with water and pumped
to dryness (without using any drying agent containing
metal cation). The obtained dark orange powder was
washed with n-pentane to afford 7a as an orange powder
(275 mg, 94%). Mass spectrum: m/z calc. for C69H57O3P4
102Ru; [MÀMeCN]+: 1159.2302; Found: 1159.2315.
2. Experimental
Experimental procedures and detailed characterization
data for all new compounds are described in the supple-
mentary material section. In this section, we will describe
the most significant synthesis (complexes 5a, 6a, 7a) as
illustrative examples.
2.1. trans-[RuCl(dppe)2{C@CH-p-phenyl-(3-hydroxy-4H-
chromen-4-one)}]TfO (5a)
In a Schlenk tube, [(dppe)2RuCl]TfO [33,34] (0.22 mmol,
240 mg) was added under argon to the solution of 4a
(58 mg, 0.22 mmol, 1.0 equiv.) in 30 mL of freshly distilled
and degassed dichloromethane. The resulting mixture was
stirred at room temperature overnight. The dark orange
solution formed was pumped to dryness to give a red–
m
max(KBr/Nujol)/cmÀ1 2275 (m) m(C„N) and 2070 (w)
1
m(C„C); H NMR (CDCl3, 200 MHz, d ppm): 8.30 (dd,
3
3
3
1H, JH–H = 8.0 Hz, JH–H = 1.3 Hz), 8.23 (d, 2H, JH–H
=
8.4 Hz), 8.1 (m, 8H, aromatics), 7.8–7.1 (m, 29H, aromat-
ics), 6.7 (m, 8H, aromatics), 6.0 (br s, 1H, OH), 2.75 (m,
8H, CH2 dppe), 1.39 (s, 3H, CH3CN). 13C {1H} NMR
(CDCl3, 75 MHz, d ppm): 173.2 (Cq), 155.4 (Cq), 145.3
(Cq), 138.1 (Cq), 134.8 (qt, |1JP–C + 3JP–C| = 11 Hz, Cq,
Cipso dppe phenyl), 134.4 (CH dppe phenyl), 133.7 (CH),
132.6 (qt, |1JP–C + 3JP–C| = 10 Hz, Cq, Cipso dppe phenyl),
132.2 (CH dppe phenyl), 130.7 (Cq), 130.6 and 130.0 (CH
dppe phenyl), 129.7 (CH) 128.9 (m, Cq), 128.4 and 128.0
(CH dppe phenyl), 127.7 (CH), 126.9 (Cq, CH3CN),
125.4 (CH), 124.6 (CH), 124.0 (Cq), 120.7 (Cq), 118.3
(CH), 117.9 (Cq), 30.3 (qt, |1JP–C + 3JP–C| = 24 Hz, CH2
dppe), 3.5 (CH3, CH3CN); 31P {1H} NMR (CDCl3,
81 MHz, d ppm): 50.59 (s), À144.12 (sept, 1JP–F = 712 Hz).
1
orange solid. H NMR (CDCl3, 200 MHz, d ppm): 8.23
3
3
(dd, 1H, JH–H = 7.9 Hz, JH–H = 1.4 Hz), 7.80–7.65 (m,
3
3H), 7.43 (d, 2H, JH–H = 8.6 Hz), 7.4–7.0 (m, 40H), 6.7
3
(br s, 1H, OH), 5.82 (d, 2H, JH–H = 8.6 Hz), 4.76 (m,
1H, Ru = C@CH), 2.96 (m, 8H, CH2 dppe). 31P {1H}
NMR (CDCl3, 75 MHz, d ppm): 37.09 (s).
2.2. trans-[RuCl(dppe)2{C”C-p-phenyl-(3-hydroxy-4H-
chromen-4-one)] (6a)
Triethylamine (0.24 mmol, 34 lL) was then added to
the solution of the crude vinylidene 5a in 50 mL of dis-
tilled and degassed dichloromethane and the mixture
was stirred at room temperature for 1 h. The resulting
orange solution was washed with water (5 · 50 mL) and
pumped to dryness (without using any drying agent con-
taining metal cation). The obtained red–orange powder
was washed with n-pentane to afford the pure compound
6a (225 mg, 82% calculated from [(dppe)2RuCl]TfO).
Mass spectrum: m/z calc. for M+C69H57ClO3P4Ru:
1194.19901; Found: 1194.19905; mmax(KBr/Nujol)/cmÀ1
3. Results and discussion
3.1. Synthesis
Scheme 1 shows the synthetic route to the flavonol-
containing ruthenium complexes 7a–c. The alkynes bearing
3-hydroxyflavone 4a or 3,6-dihydroxyflavone 4b were
obtained via a two step procedure starting from 4-ethynyl-
benzaldehyde 1 (Scheme 2). The first step consisted of
a Claisen–Schmidt reaction with 2-acetophenone 2a or
2,5-dihydroxyacetophenone 2b, and led to compounds
3a,b. The second step entailed an Agar–Flynn–Oyamada
reaction in basic medium [35–37], and gave rise to the
expected terminal alkynes in satisfactory yields (50% over-
all). 4c was obtained from 4a upon addition of methyl
iodide in the presence of potassium carbonate.
1
2055 (w) m(C„C); H NMR (CDCl3, 200 MHz, d ppm):
3
3
8.26 (dd, 1H, JH–H = 8.0 Hz, JH–H = 1.5 Hz), 8.06 (d,
3
2H, JH–H= 8.6 Hz), 7.75–6.95 (m, 44H, aromatics),
3
6.74 (d, 2H, JH–H = 8.6 Hz), 6.0 (br s, 1H, OH), 2.70
(m, 8H, CH2 dppe) 13C {1H} NMR (CDCl3, 75 MHz, d
ppm): 172.9 (Cq), 155.3 (Cq), 146.2 (Cq), 137.7 (Cq),
136.2 (qt, |1JP–C + 3JP–C| = 10 Hz, Cq, Cipso dppe phe-
nyl), 135.5 (qt, |1JP–C + 3JP–C| = 10 Hz, Cq, Cipso dppe
phenyl), 134.4 and 134.2 (CH dppe phenyl), 133.9 (m,
Cq), 133.2 (CH), 132.6 (Cq), 130.1 (CH), 129.0, 128.9
and 127.3 (CH dppe phenyl), 127.0 (CH dppe phenyl),
The flavonol based alkynyl ruthenium compounds 7a–c
were obtained starting from these terminal alkynes via a
three-step procedure (Scheme 2) [38]. The first step con-
sisted of the synthesis of vinylidene species 5, which were

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J.-L. Fillaut et al. / Journal of Organometallic Chemistry 693 (2008) 228–234
O
RO
R'
RO
O
Ph2P
Cl
PPh2
C C
O
i.
H
Ru+
TfO-
O
R'
H
Ph2P
PPh2
4a, R = H, R' = H
5a,b,c
4b, R = H, R' = OH
4c, R = CH3, R' = H
RO
O
Ph2P
PPh2
ii.
Cl
Ph2P
Ru
O
R'
PPh2
6a,b,c
RO
O
Ph2P
MeCN Ru+
PPh2
PPh2
iii.
-
PF6
R'
O
Ph2P
7a,b,c
Scheme 1. The synthesis of flavonol based complexes 5–7. (i) [(dppe)2RuCl]TfO; solvent = CH2Cl2 for 5a or THF – CH2Cl2 for 5b; RT, 16 h (dppe: 1,2-
bis(diphenylphosphino)ethane, TfO: trifluoromethanesulfonate). (ii) NEt3, CH2Cl2, RT, 1 h. (iii) KPF6, CH3CN, CH2Cl2, RT, 7 h.
Compounds 6 were obtained upon addition of triethyl-
O
O
OH
OH
amine to complexes 5. These compounds themselves are
not viable for testing as sensors for metal salts, as the metal
ions react with the chloro ruthenium unit leading to the
substitution of the chlorine atom by an acetonitrile solvent
molecule. In order to avoid the occurrence of such a compet-
ing reaction, the isomerically pure trans-[Ru(C”C–Ar)-
(CH3C”N)(dppe)2][PF6] complexes 7a–c (Ar represents
3-hydroxyflavone or 3,6-hydroxyflavone residues) were deli-
berately prepared from the chloro complexes 6 in the presence
of acetonitrile, KPF6 and NEt3 in methylene chloride [39].
O
i. (3a)
or ii. (3b)
H
+
H
R
R
3a,b
1
2a, R = H
2b, R = OH
HO
O
O
iii.
H
H
R
R
O
4a,b
H3CO
3.2. Electronic absorption and emission properties
iv.
O
Compounds 7 are characterised by strong charge trans-
fer absorption bands in the visible region (7a kmax
=
4c
405 nm in methylene chloride; emax = 2.9 · 104 dm3 molÀ1
cmÀ1; 7b kmax = 410 nm; emax = 3.0 · 104 dm3 molÀ1 cmÀ1),
in contrast to the corresponding organic terminal alkynes
for which all the transitions are confined to the UV region
(4a kmax = 350 nm in methylene chloride; emax = 1.8 · 104
dm3 molÀ1 cmÀ1; 4b kmax = 360 nm; emax = 1.9 · 104 dm3
molÀ1 cmÀ1). A broad absorption band is also observed
in the visible region for the chloro complexes 6 (e.g. at
Scheme 2. The synthesis of alkynes bearing 3-hydroxyflavone 4a or 3,6-
hydroxyflavone 4b. (i) BaOH, 8H2O, 60 ꢁC, 16 h, 80%. (ii) (a) 3,4-
dihydropyran, pyridinium p-toluene sulfonate (PPTS), CH2Cl2, 99% (b)
BaOH, 8H2O, 60 ꢁC, 16 h. (c) PPTS, THF. (iii) KOH, H2O2, DMSO,
0 ꢁC, 16 h, 4a: 60%; 4b: 67%. (iv) CH3I, K2CO3, CH3CN, 40 ꢁC, 16 h, 4c:
77%.
purified by washing with diethylether in order to remove
the excess of the terminal alkyne derivatives 4. These viny-
lidenic species were characterised by 1H and 31P NMR. The
X-ray structure determination of the vinylidene intermedi-
ate 5b indicates that the conjugation in the ligand is almost
preserved, even if a lack of coplanarity between the flavo-
nol and the interposed phenyl ring can be observed, and
reveals the absence of steric constraints around the termi-
nal organic receptor moiety being located far from the
envelope of the dppe ligands (Fig. 1). It is reasonable to
assume the same would be observed for the complete series
of the flavonol based alkynyl ruthenium complexes 5–7.
k
max = 415 nm for 6a).
Luminescence associated with the flavonol moiety is
observed from the complexes upon excitation into these
CT bands. For example, 7a emits at kmax = 560 nm upon
excitation at 400 nm, with a quantum yield of emission of
0.016 in acetonitrile at 298 K (Fig. 2). Interestingly, the
emission is short-lived, with a lifetime of 2.1 ns (see ESI),
that is unchanged upon degassing the solution. This sug-
gests that the emission is fluorescence, emanating from a
flavonol-based singlet excited state, rather than phosphores-
cence from a triplet state [40] as commonly observed in
many ruthenium(II) coordination compounds when the

J.-L. Fillaut et al. / Journal of Organometallic Chemistry 693 (2008) 228–234
231
Fig. 1. The X-ray crystal structure of 5b at 130 K, showing 50% probability ellipsoids (less relevant hydrogen atoms are omitted for clarity). Table 1 gives
˚
selected bond lengths [d/A] and angles [x/ꢁ] values in 5b.
fluorescence changes of 7a upon the addition of various
metal ions such as Li+, Na+, Ba2+, Ca2+, Cd2+, Mg2+
,
Co2+, Cr2+, Ni2+, Zn2+, and Pb2+ in acetonitrile. The qual-
itative estimation of the affinity of 7a towards various cat-
ions was first performed visually. Fig. 2 shows the
appearance of solutions of 7a in the presence of perchlorate
salts in daylight and upon irradiation with a broad-band
near-UV lamp (kmax 365 nm). It can be seen that it is not
possible to discriminate visually between the cations in day-
light (Fig. 2a), i.e., on the basis of absorption, although
Pb2+, Ni2+ and Zn2+ do lead to darker solutions. Con-
versely, upon irradiation in the near-UV and observation
of the fluorescence, there is a dramatic and clear-cut discrim-
ination between Pb2+ and the other cations, Mg2+, Cr2+
Co2+, and Ba2+ [31,32], and also Ni2+ and Zn2+ (Fig. 2b).
,
Quantitative titrations were performed by addition of
small amounts of metal perchlorate salts in MeCN
(1 · 10À2 mol/L) to 7a (5 · 10À5 mol/L in MeCN). The test
solutions were prepared by placing 5–50 lL of the probe
stock solution into a test tube, adding the appropriate ali-
quot of each metal stock. For the fluorescence measure-
ments, an excitation wavelength of 400 nm was
employed, with excitation and emission band passes of
Fig. 2. (a) Colour changes induced by the addition of various metallic
salts or Pb2+ cations (5 equiv.), to an acetonitrile solution of 7a. (b) The
light emitted by the same solutions upon irradiation at 365 nm. Cations
were added in the form of their perchlorate salts. Caution! Perchlorate
salts of metal complexes are potentially explosive with or without organic
ligands and should be handled with care.
5 nm. Under these conditions, the addition of Na+, Ba2+
,
Ca2+, Co2+, Mg2+ or Cr2+ did not result in significant
changes in either the absorption or emission spectra. In
contrast, interaction of 7a with Pb2+ cations resulted in
large bathochromic shifts of both absorption and emission
spectra (Fig. 3a and b). The presence of small amounts of
lead cations results in the disappearance of the initial
absorption band at kmax = 400 nm and in the appearance
of a new long-wavelength absorption band at 465 nm
(Fig. 3a). In the fluorescence spectrum, the emission band
is shifted by 40 nm to the red (from 565 to 605 nm), with
frontier orbitals incorporate significant contributions from
the metal orbitals. Apparently, in the excited state, the flavo-
nol unit is effectively decoupled from the ruthenium centre.
3.3. Optical sensing of metal ions
To obtain insight into the ability of 1 to selectively sense
metal ions, we first investigated colour (absorption) and

232
J.-L. Fillaut et al. / Journal of Organometallic Chemistry 693 (2008) 228–234
Fig. 3. Changes in the absorption (a) and fluorescence spectra (b) of 7a (5 · 10À5 mol/L) in air-equilibrated acetonitrile upon addition of Pb2+ cations in
the form of their perchlorate salt (1 · 10À2 mol/L). Inset: dependence of the emission intensity on Pb2+ concentration.
a reduction in the quantum yield to 0.0083. The lifetime of
the species formed is too short to measure reliably with our
instrumentation (<1 ns). Non-radiative deactivation of the
flavonol singlet excited state is probably promoted in the
complex by the lower energy of the excited state and also
by the heavy atom effect of the Pb2+ ion.
can compete with lead: association constants of
1.3 0.2 · 107 molÀ1 dm3 and 0.1 · 104 molÀ1 dm3
6
were calculated from absorption spectral data respectively
for the nickel and zinc salts. For Ni2+, the emission is
quenched without significant shifting, probably due to an
electron-transfer mechanism of deactivation by the high-
spin Ni2+ ion with its unpaired electrons. The behaviour
with Zn2+ (Fig. 4) is more complicated: an initial small
decline in the intensity, without shifting of kmax, (up to
0.5 equiv. of Zn2+ added) is followed at higher concentra-
tions of Zn2+ by the appearance of a shoulder to high-
energy of the starting band. However, the effect is less
pronounced than that induced by lead, and much higher
concentrations are required.
The studies of the dependence of absorption and emis-
sion spectra on Pb2+ concentration allow us to conclude
that the flavonol based alkynyl ruthenium sensor 7a exhib-
its 1:1 stoichiometry of binding with this cation. An associ-
ation constant Ka of
8
0.1 · 106 molÀ1 dm3 was
calculated from absorption spectral data, using iterative
non-linear least-squares methods analogous to those previ-
ously described [24].
Only two other metal ions have any significant influence
on the fluorescence, namely Ni2+ and Zn2+. These cations
Cornard and co-workers reported on spectroscopic
investigation of the complex formation of lead(II) with

J.-L. Fillaut et al. / Journal of Organometallic Chemistry 693 (2008) 228–234
233
3.4. Influence of water on metal ion binding and sensing
Practicable sensors normally need to be able to function
in the presence of at least some water. We found that 7a also
exhibits efficient binding of lead in acetonitrile-distilled
water (75:25 v/v). Under aqueous conditions, the OH group
is partially dissociated under neutral conditions, leading to
an anionic form, which inhibits to some extent the complex-
ation of ions from being observed. The optimum pH at
which 7a is mainly in the neutral form and allows the stron-
ger photophysical effects upon lead binding to be observed
was found to be 6. Under these conditions, the calculated
association constant was lowered (Ka = 13.5 0.4 · 103
molÀ1 dm3) but the selectivity retained. Meanwhile, it is of
interest to note that the flavonol-based fluorescence was
also maintained in the presence of water.
In conclusion, we have developed a new series of alkynyl
ruthenium sensors, destined for the detection of metal
traces in solution, in which the receptor units are 3-hydrox-
yflavone and 3,5-dihydroxyflavone. The sensitivity of both
absorption and emission properties of these flavonol based
alkynyl ruthenium sensors to the presence of metal cations
suggests their use as sensors for metal traces. As a prelimin-
ary result, we have shown that the combined use of the 3-
hydroxyflavone unit as a receptor for the metal cations,
and of an alkynyl ruthenium moiety as an extended p con-
jugated system, provides an efficient molecular sensor for
rapid, sensitive detection of lead(II) cations. Current stud-
ies are in progress in our group to improve the selectivity of
the complexation of lead using modified receptor units
derived from 3-hydroxyflavone.
Fig. 4. Changes in the fluorescence spectrum of 7a (5 · 10À5 mol/L) in air-
equilibrated acetonitrile upon addition of Zn2+ cations in the form of their
perchlorate salt (1 · 10À2 mol/L).
3-hydroxyflavone. These results were similar to ours in
main aspects. These authors concluded to the formation
of species of stoichiometry (1:1) with a complete deproto-
nation of the hydroxyl group [41]. Roshall [31,32] previ-
ously established, on the basis of the characteristic
spectroscopic changes, that flavonol complexes with metal
cations can be classified into two groups. In the complexes
of the first group, the proton of the flavonol hydroxylic
group is not substituted by a metal cation. The metal cat-
ion forms a donor–acceptor linkage with the electron lone
pair on the oxygen of the C@O group, and does not cause
essential changes in the flavonol electronic structure. The
properties of these complexes are similar to the properties
of flavonols without cations. We can suggest that the com-
plexes that 7a formed with Na+, Ba2+, Ca2+, Co2+, or
Mg2+ belong to this class, as in these cases the cation bind-
ing does not cause essential changes in the flavonol elec-
tronic structure, according to our experimental results. In
contrast, in the complexes of the second group [27], the fla-
vonol hydroxylic group participates in the formation of the
complexes. The interaction of metal cations with flavonols
then results in a cyclic arrangement of chelates with metal
cations. This stronger interaction results in significant
bathochromic shifts of both absorption and emission spec-
tra, as essential changes in the flavonol electronic structure
occur. Our result allows us to suggest that the interaction
of the Ni2+, Pb2+ and Zn2+ cations with the flavonol-based
alkynyl ruthenium sensor 7a leads to a chelating complex
involving the participation of the hydroxyl group. This is
also supported by the observation that 7c, which lacks
the OH group, does not display this behaviour, reflecting
the need for the 3-hydroxyl group to deprotonate and
allow the metal cations to bind.
Acknowledgement
´
We would like to thank la Region Bretagne for a PhD
studentship, and the French Ministry of Research, the
French Ministry for Foreign Affairs, the CNRS, and
E.U.COST D035-0010-05 for financial support.
Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2007.10.046.
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