Iron(III) chelation: Tuning of the pH dependence by mixed ligands

Article abstract of DOI:10.1002/ejic.200200643

The iron(III) chelating properties of two heteropodands with 8-hydroxyquinoline and catechol binding groups were examined and compared to those of the corresponding homopodal analogues, O-TRENSOX and TRENCAMS. Like the parent homopodands, the two heteropo

Full text of DOI:10.1002/ejic.200200643

FULL PAPER
Iron(III) Chelation: Tuning of the pH Dependence by Mixed Ligands
Anne-Marie Albrecht-Gary,*[a] Sylvie Blanc,[a] Frederic Biaso,[b] Fabrice Thomas,[b]
Paul Baret,[b] Gisele Gellon,[b] Jean-Louis Pierre,[b] and Guy Serratrice*[b]
Keywords: Tripodal ligands / Iron / Thermodynamics / UV/Vis spectroscopy / Siderophores
The iron(III) chelating properties of two heteropodands with
tentiometric and spectrophotometric methods in combination
8-hydroxyquinoline and catechol binding groups were exam- with 1H NMR spectroscopy. The respective pFeIII values
ined and compared to those of the corresponding homopodal
analogues, O-TRENSOX and TRENCAMS. Like the parent
homopodands, the two heteropodands are based on the
TREN scaffold and the chelating units are connected by
showed a cooperative effect of the mixed chelating units.
Moreover, the pFeIII dependence on pH showed that the
mixed ligands exhibit a higher complexing ability than the
parent ligands over the pH range 5−9 which is of biological
amide groups, TRENSOX2CAMS having two 8-hydroxyqui- relevance. This result could be of great interest for medical
noline and one catechol arms and TRENSOXCAMS2 one 8-
hydroxyquinoline and two catechol moieties. The aqueous
applications.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
coordination chemistry of these ligands was examined by po- Germany, 2003)
Introduction
Siderophores are iron chelating agents that are excreted pH, constitutes an interesting challenge for chemists. We
by microorganisms to render iron soluble in the environ- previously reported the dependence of pFeIII on pH for two
ment and favour the uptake of this metal. Iron chelation by homopodands based on catechol and on 8-hydroxyquino-
some natural or synthetic chelators can be applied to hu- line bidentate units.[2,3] We showed that the tris(catechol)
man diseases characterised by iron overload. Although the ligand TRENCAMS is a stronger ferric chelator than the
coordination chemistry of a great number of water-soluble tris(hydroxyquinolinate) analogue O-TRENSOX at pH val-
synthetic iron chelators has been described,[1] the pH de- ues greater than 7 and that the opposite sequence is ob-
pendence of the coordination abilities has not been thor- served at pH values lower than 7. In order to obtain an
oughly investigated. Most of the comparative data in the efficient chelator of iron() over a large range of pH, we
literature are allowed by the use of the pFeIII values (calcu- have synthesised mixed tripodal siderophores with both cat-
lated for [FeIII
]
ϭ 1 µ, [L]tot ϭ 10 µ at pH ϭ 7.4) and, echol and 8-hydroxyquinoline moieties. Recently, the syn-
tot
of course, pFeIII reflects only the complexing ability of a thesis of mixed tripodal ligands based on (i) two hydroxy-
given ligand at physiological pH. Owing to the fact that pH piridinone and one catechol moieties, and (ii) two hydroxy-
can vary in a relatively large range between the different piridinone and one 2-hydroxyisophtalamide groups has
biological compartments, an understanding of the pH de- been published.[4] We report in this paper the synthesis,
pendence of the pFeIII value may be a decisive criterion for acid-base properties, and ferric complexation of two mono-
the understanding of the in vivo behaviour of a given iron topic tripodal mixed ligands TRENSOX2CAMS (L1) and
chelator. On the other hand, the design and the synthesis TRENSOXCAMS2 (L2; Figure 1). These molecules belong
of iron chelators with high pFeIII values in a large range of to a family of tripods with a tris(2-aminoethyl)amine group
anchoring arms that bear bidentate coordination subunits
[a]
Laboratoire de Physico-Chimie Bioinorganique, UMR CNRS (namely either a 5-sulfo-8-hydroxyquinoline-7-carbamoyl
7509, European School of Chemistry, 25 rue Becquerel, 67200
group or 5-sulfo-2,3-dihydroxybenzoyl unit). The ligands
Strasbourg, France,
differ by the number of catecholylamide groups, none for
ligand L0 (O-TRENSOX), one for ligand L1, two for ligand
L2 and three for ligand L3 (TRENCAMS; Figure 1). Com-
bining potentiometric and absorption spectrophotometric
titrations with 1H NMR spectroscopy we were able to
characterise in water the protonated and ferric species
formed with ligands L1 and L2 over a large pH range.
Fax: (internat.) ϩ33-(0)3/90 24 26 39
E-mail: amalbre@chimie.u-strasbg.fr
[b]
´
Laboratoire de Chimie Biomimetique, LEDSS, UMR CNRS
´
5616, Universite Joseph Fourier,
BP 53, 38041 Grenoble Cedex 9, France
Fax: (internat.) ϩ33-(0)4/76 51 48 36
E-mail: guy.serratrice@ujf-grenoble.fr
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200200643
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Iron(III) Chelation: Tuning of the pH Dependence by Mixed Ligands
FULL PAPER
Results and Discussion
Synthesis
The synthesis of the heterotripodal hexadentate ligands
L1 and L2 is presented in Figure 2. The two catecholate su-
bunits of ligand L2 were connected to the tripodal scaffold
by direct condensation[5] of two equivalents of 2,3-dimeth-
oxybenzoic chloride with one equivalent of tris(2-aminoe-
thyl)amine (TREN) [path (aЈ)], then the oxinate subunit was
grafted onto the free primary amine group by coupling with
activated (CDI) 7-carboxy-8-hydroxyquinoline. The syn-
thesis of L1 required protection of one arm of TREN with
a trityl group [paths (a, b, c)]. Phenol groups were protected
with benzyl groups (oxine) and methyl groups (catechol),
respectively; deprotections were performed with BBr3. Fi-
nally, regiospecific sulfonation in position 5 of both oxinate
and catecholate subunits afforded the hydrosoluble ligands
L1 and L2.
Figure 1. Chemical formulae of the tripodal ligands: (a) L0 (O-
TRENSOX), (b) L1 (TRENSOX2CAMS), (c) L2 (TRENSOX-
CAMS2), (d) L3 (TRENCAMS)
Acid-Base Properties of Ligands
The protonation constants of L1 and L2 were determined
by combination of potentiometric and spectrophotometric
Figure 2. Synthesis of the heterotripodal ligands L1 and L2
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A.-M. Albrecht-Gary, G. Serratrice et al.
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titrations (see details in Exp. Sect.). Both ligands possess a hydroxyquinoline-5-sulfonic acid (log KH ϭ 2.01).[11] The
total of ten protonable functions. However, protonation of hydroxyl protonation constant of 8-hydroxyquinoline-5-sul-
the three sulfo groups that are moderately strong acids fonic acid[10] is equal to 8.42 and those of L0 vary between
(log KH Ͻ 2)[6] was not considered likely under our con- 7.44 and 8.62.[2] We attribute the values 8.78 and 7.85 for
ditions. Seven stepwise protonation constants were deter- L1 and 7.93 for L2 to the protonation of the 8-hydroxy-
mined for each ligand. The corresponding values and stat- quinoline OH groups.
istical errors are given in Table 1. Speciation diagrams are
presented in Figures S1 and S2 of the supporting infor- the same absorption spectra, suggesting that protonation
Based on the fact that the species L2H3 and L2H4 have
mation.
does not occur on a chromophore, the protonation constant
log K5H ϭ 6.56 was attributed to the tertiary amine nitrogen
Table 1. Assignment of the successive protonation constants of of L2 anchor. This was confirmed by the 1H NMR titration
various heterotripodal ligands with 8-hydroxyquinoline and/or cat-
of L2 which showed an inflection point at pD ഠ 6 and a
echol subunits; solvent: water, I ϭ 0.10  (NaClO4), T ϭ (25.0 Ϯ
significant upfield shift of the methylene protons, character-
0.2) °C; numbers in parentheses correspond to statistical errors (3σ)
istic of the protonation of its closest acid-base site, i.e. the
on the last digit
tertiary amine (Figure 3). Similarly for L1, the protonation
constant log K4H ϭ 6.96 was attributed to the tertiary amine.
The values of log KH of the tertiary amine anchor deter-
Function
L0 [a]
L1
L2
L3[b]
mined for L1 and L2 are close to that of L0 (log KH
ϭ
NH pyridinium
1.8(1)
2.55(8) 2.89(15)
3.01(4)
7.44(4) 7.85(9)
8.18(5) 8.78(3)
8.62(4)
1.8(2)
2.34(6)
6.36).[2] They are, however, much lower than that of the
tris(catechol) ligand L3 (log KH ϭ 8.01).[3] It has already
been reported[12] that the protonation of this site is sensitive
to the ability of the tertiary amine to form hydrogen bonds
with substituents in the β-position of the TREN scaffold.
Formation of a hydrogen bond between the lone electron
pair of the tertiary amine nitrogen and the amide hydrogen
OH quinoline
7.93(2)
OH catechol (ortho)
5.62(6)
5.55(4)
7.35(2)
5.57(5)
6.32(4)
7.05(4)
OH catechol (meta)
11.5(2)
11.41(8)[c]
12.5(3)[c]
6.56(3)
12.05(10)[d]
8.01(2)
NH ammonium
6.36(3) 6.96(6)
[a]
[b]
[c]
Ref.[2]
Ref.[3]
Spectrophotometric determinations under
[d]
basic conditions. Average value for the three meta catechol OH
groups.
The protonation constants of L1 were determined by po-
tentiometry except for log K1H ϭ 11.5 and log K7H ϭ 1.8,
which were determined spectrophotometrically. As for L2,
the values in Table 1 represent averages of values deter-
mined by both methods, except for log K5H ϭ 6.56 (deter-
mined by potentiometry) and log K1H ϭ 12.5 (determined
by spectrophotometry).
The protonation constants were attributed to L1 and L2
by comparison with the log KH values of the ligand L0,
bearing three hydroxyquinoline groups,[2] and with those of
the tris(catecholate) ligand L3 [3] (Table 1). The highest pro-
tonation constants (11.5 for L1; 12.5 and 11.41 for L2) can
be assigned to the meta hydroxyl groups of the sulfocatech-
olyl moieties. These values are in agreement with those re-
ported under similar conditions for some sulfocatechol de-
rivatives such as 1,2-dihydroxybenzene-4-sulfonic acid (log
KH ϭ 12.16),[7] 1,2-dihydroxybenzene-3,5-disulfonic acid
(Tiron; log KH ϭ 12.5)[8] or N,N-dimethyl-2,3-dihydroxy-5-
sulfonatobenzamide (DMBS; log KH ϭ 11.5).[9] The values
of the lowest protonation constants (1.8 and 2.89 for L1;
2.34 for L2) correspond to the pyridine nitrogen of the sul-
fohydroxyquinoline moieties. Compared to 8-hydroxy-
quinoline-5-sulfonic acid (log KH ϭ 3.93),[10] the pro-
tonation constants determined for L1 and L2 are two orders
of magnitude lower. This could be explained by the elec-
tron-withdrawing effect of the carbonyl linker in position
Figure 3. 1H NMR chemical shift variation as a function of pD
for the methylene (a) and aromatic (b) protons of L2; solvent: D2O,
7, which is similar to that of the nitro group in 7-nitro-8- [L2] ϭ 5 m
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Iron(III) Chelation: Tuning of the pH Dependence by Mixed Ligands
FULL PAPER
decreases strongly its basicity, providing a sufficient stabilis-
ation of the deprotonated form of the amine, as observed
with the tripodal ligands bearing 8-hydroxyquinoline
groups.
The interactions of the amide linkers with the tertiary
amine nitrogen can be further affected by the protonation
state of the ortho-hydroxyl groups either of the 8-hydroxy-
quinoline or the catechol subunits. Indeed, the protonation
of these sites could induce a different geometry around the
TREN anchor, as suggested by the shoulder at pD ϭ 7Ϫ8.5
1
in the curve Hb during the H NMR titration of ligand L2
(Figure 3). The isomer shift of the Hc as well as Hd and He
(Figure 3) observed for L2 increases from pD ϭ 6.5 to 9
with an inflection point at pD ϭ 7.5Ϫ8 in relation to the
protonation of hydroxyl groups (log KH ϭ 7.35 and 7.93).
This indicates that such a protonation is responsible for a
conformational change around the amide-catechol axis,
which induces a shift of the proton resonance.
The remaining protonation constants (7.35 and 5.55 for
L2 and 5.62 for L1) are attributed to the catechol hydroxyl
groups ortho with respect to the amide linker. These values
are in agreement with those of other tripodal catecholylam-
ide-type ligands.[13]
The calculated electronic spectra of protonated species of
L1 and L2 are given in Figure 4. The spectral properties of
L1 and L2 are qualitatively in agreement with those of other
8-hydroxyquinoline and/or catechol based ligands. Catechol
derivatives show a π Ǟ π* transitions in the UV region
(300Ϫ400 nm) with the maximum of the absorption band
shifting to longer wavelengths and the molar absorption co-
efficient increasing as the hydroxyl groups are depro-
tonated.[14] Similarly, 8-hydroxyquinoline-5-sulfonic acid
displays spectral variations in this region upon pro-
tonation.[15] It should be noted that molar absorption coef-
ficients of the heterotripodal ligands (ഠ2 ϫ 104 Ϫ1·cmϪ1
at around 340 nm and ഠ5Ϫ6 ϫ 104 Ϫ1·cmϪ1 at around
270 nm for totally deprotonated L1 and L2) are higher than
the value obtained by addition of molar absorption coef-
ficients of simple 8-hydroxyquinolinate or catecholate li-
gands.[15,16]
Figure 4. Calculated electronic spectra of the protonated species of
heterotripodal ligands; solvent: water, I ϭ 0.10  (NaClO4), T ϭ
(25.0 Ϯ 0.2) °C: (a) ligand L1: (1) L1H7, (2) L1H6, (3) L1H, (4) L1;
(b) ligand L2: (1) L2, (2) L2 H, (3) L2H2, (4) L2H3 ϵ L2H4, (5)
L2H5, (6) L2H6, (7) L2H7; charges are omitted for the sake of clarity
nation of the stability constant log βFeL. All the results are
summarised in Table 2.
The electronic spectra of de-, mono- and polyprotonated
Characterisation of the Ferric Complexes
Both ligands are hexadentate and form only 1:1 ferric ferric complexes of L1 and L2 calculated from the refine-
complexes. The potentiometric titration curves recorded for ment of absorbance data are shown in Figure 6; speciation
a 1:1 metal-to-ligand ratio (Figure 5) show a large pH jump diagrams are presented in Figure 7.
at a ϭ 7, indicating that all the ligand protons are released
The ferric complexes of catechol derivatives generally
when the ferric ion is bound. Since both L1 and L2 form show a strong ligand-to-metal charge transfer (LMCT)
ferric complexes totally at pH values less than 3 the global band with a maximum at about 700 nm for mono(catechol-
stability constant log βFeL could not be determined from ate) species, at about 560 nm for bis(catecholate) species
the potentiometric data.
and finally at about 480 nm for tris(catecholate) complexes.
The spectra obtained during the spectrophotometric ti- The charge-transfer bands observed are due to electronic
trations of ferric complexes of L1 and L2 are shown in Fig- transitions from the highest valence orbitals of the ligand
ures S3 and S4. The absorbance data were refined with the to the 3d orbitals of iron().[23] As the maximum moves to
Letagrop-Spefo[17,18] and Specfit[19Ϫ22] programs and pro- shorter wavelengths with increasing number of coordinating
vided values of global stability constants of ferric complexes ligands the molar absorption coefficient increases. Electron-
of both L1 and L2. The spectrophotometric data obtained withdrawing substituents on the aromatic rings of the cat-
from the competition for ferric ion between L2 and EDTA echol moieties increase the molar absorption coefficient but
were treated with the Specfit program allowing the determi- have little effect on the wavelength of the maximum. Ferric
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A.-M. Albrecht-Gary, G. Serratrice et al.
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Table 2. Cumulative stability constants βFeLHn, pFeIII values and
UV/Vis spectral characteristics of ferric complexes formed with the
tripodal ligands; solvent: water, I ϭ 0.10  (NaClO4), T ϭ (25.0 Ϯ
0.2) °C; numbers in parentheses correspond to statistical errors (3σ)
on the last digit; an incertitude of 5% is estimated for the molar
absorption coefficients
[a]
Ligand Species
log βFeLH
pFeIII[b] λmax
(nm)[c] (Ϫ1·cmϪ1
εmax
n
)
L0 [2]
FeL0H5
FeL0H
FeL0
42.2(1)
36.5(2)
30.9(1)
48.6(2)
29.5
31.6
32.3
29.6
435
525*
443
595
443
595
440*
540*
560
580
560
550
409
520*
573
542
525
525
495
520
500
488
8200
3500
5200
5200
5400
5400
6900
3700
4000
3900
4500
4500
4700
3300
3500
4500
4800
4800
4300
4800
5000
5300
L1
FeL1H5
FeL1H3
FeL1H2
FeL1H
FeL1
46.2(4)
44.3(3)
41.6(2)
36.8(4)
55.91(3)
L2
FeL2H5
FeL2H4
FeL2H2
53.7(1)
47.7(3)
FeL2H[d] 44.7(6)
FeL2 [d]
FeL3H5
FeL3H3
FeL3H
FeL3
41.3(6)
64.05(7)
57.4(1)
49.1(1)
43.6(1)
L3 [3]
[a]
[b]
[c]
βFeLHn ϭ [FeLHn]/([FeIII ϫ [L] ϫ [Hϩ]n).
tot
Species have identical electronic spectra.
Calculated for
* ϭ shoulder.
[FeIII
]
ϭ 10Ϫ6 , [L]tot ϭ 10Ϫ5 , pH ϭ 7.4.
[d]
and are reported in Table 3 together with the values from
tris(catecholate) and tris(8-hydroxyquinolinate) complexes
for comparison. The deprotonation constants of ferric L1
and L2 complexes have been attributed by considering that
the deprotonation of an 8-hydroxyquinoline group involved
in the ferric coordination occurs at a pH much lower than
that of the deprotonation of the catechol group. This is sup-
ported by the pKFeLH values recently determined for trip-
odal ferric complexesn containing a C-pivot scaffold with
Figure 5. Potentiometric titration curves for the heterotripodal li-
gands and their ferric complexes; solvent: water, I ϭ 0.10  (Na-
ClO4), T ϭ (25.0 Ϯ 0.2) °C: (a) titration of L1 (1 m): (1) free
ligand, (2) 1:1 ferric complex; (b) titration of L2 (1 m): (1) free
ligand, (2) 1:1 ferric complex
complexes of 8-hydroxyquinoline-5-sulfonic acid are either an 8-hydroxyquinoline (ligand COX)[24] or a catechol
characterised by two absorption maxima in the visible re- subunit (ligand CacCAM).[25] In particular, the values of
gion. The maximum at 443 nm is assigned to an LMCT pKFeLH (2.12 for Fe-COX and 6.59 for Fe-CacCAM) have
band belonging to the phenolic oxygen, while the band at been unambiguously attributed to the coordination of the
595 nm is due to charge transfer from the pyridine nitrogen third chelating arm.
to the metal cation.[2] It should be noted that in common
The electronic spectrum of FeL1H5, which exhibits two
with L0 and L3, both L1 and L2 form protonated ferric com- shoulders at 440 nm (ε ϭ 6940 Ϫ1·cmϪ1) and 540 nm (ε ϭ
plexes, indicating that the coordination sites of the com- 3680 Ϫ1·cmϪ1), is similar to that of the bis(salicylate) com-
plexes can be protonated without losing the ferric ion. This plex of L0 [2] involving a coordination with two 8-hydroxy-
behaviour can be attributed to a change of catecholate or quinoline arms of the ligand through the oxygen atoms of
oxinate bonding mode to the salicylate mode (coordination the carbonyl and hydroxyl groups. The spectrum of the
with carbonyl and o-hydroxyl oxygens).
FeL1H3 complex exhibits an absorption band at λmax
ϭ
It is of interest to consider the different multiprotonated 560 nm (ε ϭ 3970 Ϫ1·cmϪ1), in agreement with the bis(ox-
ferric complexes of L1 and L2 by examining their electronic inate) coordination. The oxygen and pyridine nitrogen
spectra (Table 2) as this can provide information on the co- atoms of two 8-hydroxyquinoline arms are involved in the
ordination modes and their successive deprotonation con- coordination. The value pKFeL H ϭ 2.4 (FeL1H5/FeL1H3
1
5
stants pKFeLH (defined in Table 3). These values are calcu- equilibrium) corresponds to the deprotonation of both 8-
n
lated from the difference between the values of log βFeLH
hydroxyquinoline nitrogens in a two-proton step. The small
n
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Iron(III) Chelation: Tuning of the pH Dependence by Mixed Ligands
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Figure 7. Speciation diagrams for (a) ferric-L1 complexes, (b) ferric-
L2 complexes
Table 3. Successive deprotonation constants of ferric complexes;[a]-
solvent: water, I ϭ 0.10  (NaClO4), T ϭ (25.0 Ϯ 0.2) °C
Figure 6. Calculated electronic spectra of the ferric complexes
formed with the heterotripodal ligands; solvent: water, I ϭ 0.10 
(NaClO4), T ϭ (25.0 Ϯ 0.2) °C: (a) ferric L1 species: (1) FeL1, (2)
FeL1H, (3) FeL1H2, (4) FeL1H3, (5) FeL1H5; (b) ferric L2 species:
(1) FeL2 ϵ FeL2 H, (2) FeL2H2, (3) FeL2H4, (4) FeL2H5; charges
are omitted for the sake of clarity
Ligand[a][b]
pKFeLH pKFeLH pKFeLH pKFeLH pKFeLH
5
4
3
2
L0 [2]
5.7[c]
2.4[d]
2.2
5.6
4.8
3.4
5.5
4.95
5.74
2.12
6.59
L1
1.9
2.7
3.0
L2
6.0[d]
3.5
red-shift of λmax to 580 nm for the formation of FeL1H2
without significant change of ε (3930 Ϫ1·cmϪ1) might be
attributed to the deprotonation of the ammonium nitrogen.
L3 [3]
3.1
8.34[d]
2.5
3.46
Enterobactin[26]
3.52
4.10
[9]
MECAMS
COX
CacCAM
4.8[e]
[24]
1
The value pKFeL H ϭ 1.9 shows a strong stabilisation of
[25]
the deprotonated te3rtiary amine nitrogen by intramolecular
hydrogen bonds. The increase of ε to 4490 Ϫ1·cmϪ1 with
the concomitant blue shift of λmax to 560 nm for the forma-
tion of FeL1H is in agreement with the coordination of the
catechol arm in a salicylate mode. The last deprotonation
equilibrium is accompanied with a spectral change (λmax ϭ
550 nm, ε ϭ 4560 Ϫ1·cmϪ1) that is indicative of a shift to
a catecholate coordination and formation of the FeL1 spec-
ies. It should be pointed out that the values of pKFeLH2 and
5.15
[a]
[b]
KFeLHn ϭ ([FeLHnϪ1] ϫ [H])/[FeLHn].
MECAMS: 1,3,5-
tris{[(2,3-dihydroxy-5-sulfobenzoyl)amino]methyl}benzene; COX:
1,1,1-tris[3-(8-hydroxyquinoline-7-carboxamido)propyl]-(poly-
ethylene glycolϽ43Ͼmethyl ether]; CacCAM: 2,2,2-tris[3-(2,3-di-
[c]
hydroxybenzamido)propyl]acetic acid.
KFeLHn ϭ ([FeLH] ϫ
[H]4)/[FeLH5].
KFeLHn ϭ ([FeLHnϪ2] ϫ
[H]2)/[FeLHn].
[d]
[e]
KFeLHn ϭ ([FeLH] ϫ [H]3)/[FeLH4].
The spectral properties of the FeL2H5 species (ε ϭ 4700
pKFeLH (2.7 and 4.8) are significantly lower than those ob- Ϫ1·cmϪ1 at λmax ϭ 409 nm and a shoulder at 520 nm with
served for tris(catecholate) complexes: 8.34 (two-proton ε ϭ 3260 Ϫ1·cmϪ1) are intermediate between the bis(sal-
step) and 5.5 for L3, and 4.1 and 5.74 for MECAMS[9] icylate) coordination of ferric L3 (two catechols)[3] and fer-
(Table 3), while the pKa’s of the free ligand are of the same ric L0 (two 8-hydroxyquinolines).[2] This confirms that L2
order of magnitude.
forms a bis(salicylate) complex with one catechol and one
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A.-M. Albrecht-Gary, G. Serratrice et al.
FULL PAPER
8-hydroxyquinoline coordinated to the ferric ion. For the mixed ligands lead to enhanced affinity for the ferric ion at
formation of FeL2H4 the λmax is red-shifted to 573 nm, con- pH above 5 (L1) or 6 (L2) relative to ligands with three
sistent with the change of coordination from a salicylate to identical arms. This is related to the different basicities of
an oxinate mode for the 8-hydroxyquinoline arm. There is the binding subunits. Catechol is more basic than 8-
only a slight change in ε (3530 Ϫ1·cmϪ1), indicating that hydroxyquinoline, which leads to a stronger competition of
2
iron is still coordinated by two arms. The value pKFeL H5 ϭ protons with ferric ions for catechol ligands at low pH.
2.2 of the FeL2H5/ FeL2H4 equilibrium corresponds well to Hence, 8-hydroxyquinoline is a better chelating agent at low
the pyridinium nitrogen deprotonation. The formation of pH, while catechol is a stronger chelator at high pH as ob-
the FeL2H2 and FeL2H species is accompanied by spectral served for L0 [tris(8-hydroxyquinoline) ligand] and L3 [tris-
shifts of λmax from 573 nm to 542 nm and 525 nm, respec- (catechol) ligand]. Our results with L1 (one catechol group)
tively, and an increase of ε to 4530 Ϫ1·cmϪ1 and 4820 and L2 (two catechol groups) show that a greater pKa differ-
Ϫ1·cmϪ1 is observed. The equilibrium between FeL2H4 ence between the proton dissociation of the catechol in the
and FeL2H2 (pKFeL H ϭ 6.0) is attributed to the coordi- free ligand and that of the same group in the complex is
2
4
nation of the third arm (catechol) in a salicylate mode and observed in comparison to other tripodal ligands contain-
the change of coordination from salicylate to catecholate ing only catechol groups (see Table 3). This suggests that
for the coordinated catechol arm. The equilibrium between the initial coordination of the 8-hydroxyquinoline arm with
FeL2H2 and FeL2H corresponds to the shift of bonding FeIII favours proton loss of the free catechol and thus in-
mode from salicylate to catecholate of the third catechol creases the affinity of this group for FeIII. This can explain
arm (pKFeL H ϭ 3.0). As for FeL1 complexes, the depro- that the highest values for pFeIII are obtained with L1 and
2
tonation of th2e catechol groups upon complexation occurs L2 over the pH range 5Ϫ9, which is biologically relevant,
at lower pH than for tris(catechol) ligands cited above. The as shown by the plot in Figure 8. We have also reported in
last equilibrium leading to FeL2 species does not induce any Table 2 the pFeIII values calculated under standard con-
spectral changes. It is likely that the deprotonation of the ditions (pH ϭ 7.4, [L]tot ϭ 10Ϫ5 , [FeIII
]
ϭ 10Ϫ6 ) for
tot
2
tertiary amine nitrogen is involved. The value of pKFeL H ϭ L1 (31.6) and for L2 (32.3). These values are higher than
3.4 is higher than the corresponding constant for ferric L1 those determined for L0 (29.5) and for L3 (29.6) and among
1
complex (pKFeL H ϭ 1.9).
the highest values determined for an iron sequestering ag-
3
ent.
pFeIII Values
Since the ligands are weak acids, proton competition oc-
curs depending on their protonation constants and the pH.
The pFeIII value (Ϫlog [FeIII]) is thus a better measure of
the relative efficiency of the ligands under given conditions
Conclusion
Iron() complexation by two heterotripodal ligands L1
and L2 has been characterised. The results are compared to
the parent homotripodal ligands allowing the study of the
variation of the ligand properties from tris(8-hydroxyquino-
line) to tris(catechol) ligand. The thermodynamic and spec-
troscopic study reveals that in the acidic medium the com-
plex formation starts with two arms coordinating the ferric
ion in the salicylate coordination mode. Both 8-hydroxy-
quinoline subunits are involved in the case of L1, and one
8-hydroxyquinoline and one catecholate subunit in the case
of L2. This indicates that 8-hydroxyquinoline is a better li-
gand than catechol at low pH values. Despite the fact that
catechol oxygens are better donors than the oxygen and ni-
trogen atoms of 8-hydroxyquinoline, the competition be-
tween Hϩ and the metal cation ‘‘balances’’ the com-
plexation efficiency. The striking feature of the mixed li-
gands is their higher complexing ability than that of the
parent ligands over a range of biologically relevant pH val-
ues, and especially at neutral pH. This can be explained by
the finely tuned complexation power of 8-hydroxyquinoline
and catechol groups. It should be noted that the com-
plexation efficiency of the tris(8-hydroxyquinoline) ligand
L0 is less sensitive to pH than the tris(catechol) ligand L3
over the pH range 2Ϫ10. The results described in this work
illustrate the great interest of the mixed ligands approach,
which, via a synergistic action, leads to a more efficient iron
complexing agent than the homopodate ligands.
of pH, [FeIII
]
and [L]tot. The pFeIII values have been cal-
tot
culated for pH values over the range 2Ϫ10 and are reported
as the plot of pFeIII vs. pH presented in Figure 8, together
with the plots calculated for L0 and L3.
The important result, as seen from Figure 8, is that the
mixed ligands L1 and L2 are stronger iron chelating agents
than L0 and L3 over the pH range 5Ϫ9. This shows that
Figure 8. Plot of pFe versus pH for the tripodal ligands; calculated
for [L]tot ϭ 10Ϫ5  and [FeIII
]
ϭ 10Ϫ6

tot
2602
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2003, 2596Ϫ2605

Iron(III) Chelation: Tuning of the pH Dependence by Mixed Ligands
FULL PAPER
(dd, 3JAB(H-H) ϭ 1.7, 4.1 Hz, 2 H, Hquin), 7.97 (d, 3JH,H ϭ 8.6 Hz,
Experimental Section
2 H, Hquin), 7.5Ϫ7.3 (m, 14 H, ArH and Hquin), 5.5 (s, 4 H, CH2
3
benzyl), 3.25 (dt, JH,H ϭ 5.6, 6.4 Hz, 4 H, CH2), 3.2 (br. t, 2 H,
Materials and Equipment: Solvents were purified by standard tech-
niques. The amine TREN was distilled from over CaH2. All other
compounds were of reagent grade and were used without further
purification. Mass spectra were obtained on NERMAG R 10 10C
or Thermo Finnigan Polaris Q or Bruker Esquire-LC 1.6n (electro-
spray) mass spectrometers. 1H and 13C NMR spectra were recorded
in 5 mm tubes at 25 °C with Bruker AC 200 or WM 250 or Avance
300 spectrometers (δ ppm, TMS reference). Microanalyses were
performed by the Central Service of CNRS, Solaise (France). Melt-
ing points were determined with a Büchi apparatus and are not cor-
rected.
3
NH2), 2.59 (m, 2 H, CH2), 2.50 (m, 2 H, CH2), 2.36 (t, (H-H) ϭ
6.4 Hz, 4 H, CH2) ppm. 13C NMR (300 MHz, CDCl3): δ ϭ 165.5
(CO), 153.7 (Cq), 149.6 (CH), 142.4 (Cq), 136.7 (Cq), 136.1 (CH),
131.3 (Cq), 128.9 (CH), 128.8 (CH), 128.7 (CH), 127.3 (CH), 125.3
(Cq), 123.2 (CH), 122.3 (CH), 78.5 (CH2), 55.1 (CH2), 53.5 (CH2),
38.9(CH2), 37.9(CH2) ppm. MS (DCI, NH3/isobutane): m/z ϭ 669
[M ϩ H]ϩ.
Compound 5a: 2,3-Dimethoxybenzoic acid (0.62 g, 3.4 mmol) was
dissolved in SOCl2 (15 mL) and the mixture was stirred overnight
under argon. The solution was then evaporated to dryness to give
2,3-dimethoxybenzoic chloride (0.685 g). Compound 4a (2.27 g,
3.4 mmol), triethylamine (0.35 g, 3.5 mmol) and dry CH2Cl2
(150 mL) were added and the mixture was stirred at room tempera-
ture under argon. The mixture was treated successively with 25%
NaOH and brine and then dried and the solvent was evaporated
off. Column chromatography (silica, CH2Cl2 then CH2Cl2/meth-
anol 1 Ǟ 5%) gave 5a as a white foam (1.665 g, 2 mmol, yield ϭ
N1,N1-bis(2-aminoethyl)-N2-(trityl)-1,2-ethanediamine (2): Under
argon at room temperature, triphenylchloromethane (2.65 g,
9.5 mmol) in dry CH2Cl2 (150 mL) was added dropwise to a solu-
tion of tris(2-aminoethyl)amine (1; 5.56 g, 38 mmol) in CH2Cl2
(100 mL). The mixture was stirred overnight and then successively
washed with 10% NaOH (ca. 100 mL) and brine. The organic phase
was dried over sodium sulfate and concentrated under vacuum to
give 2 as a yellowish solid (3.7 g, yield: 9.5 mmol, 100%). M.p.
3
59%). 1H NMR (250 MHz, CD Cl3): δ ϭ 8.94 (dd, JH,H ϭ 1.6,
1
4.0 Hz, 2 H, Hquin), 8.2Ϫ8.1 (m, 7 H, Hquin ϩ NH amide),
63Ϫ65 °C. H NMR (200 MHz, CDCl3): δ ϭ 7.5 (m, 6 H, ArH),
3
3
7.6Ϫ7.3 (m, 15 H, ArH), 7.01 (t, JH,H ϭ 7.9 Hz, 1 H, H catech.),
7.3Ϫ7.1 (m, 9 H, ArH), 2.9 (br. s, 5 H, NH), 2.7 (t, JH,H ϭ 6 Hz,
3
3
3
6.89 (dd, JH,H ϭ 1.6, 6.4 Hz, 1 H, H catech.), 5.55 (s, 4 H, CH2
4 H, CH2), 2.6 (t, JH,H ϭ 6 Hz, 2 H, CH2), 2.4 (t, JH,H ϭ 6 Hz,
3
4 H, CH2), 2.2 (t, JH,H ϭ 6 Hz, 2 H, CH2) ppm. 13C NMR
benzyl), 3.80 (s, 3 H, OCH3), 3.75 (s, 3 H, OCH3), 3.41Ϫ3.25 (m,
6 H, CH2), 2.6 (m, 2 H, CH2), 2.4 (m, 4 H, CH2) ppm. 13C NMR
(250 MHz, CDCl3): δ ϭ 165.2 (CO), 165.0 (CO), 153.8(Cq), 152.3
(Cq), 149.5 (CH), 147.4 (Cq), 142.4 (Cq), 136.7 (Cq), 135.9 (CH),
131.2 (Cq), 128.6 (unresolved CH’s), 127.4 (CH), 126.8 (Cq), 125.0
(Cq), 123.9 (CH), 123.0 (CH), 122.4 (CH), 122.2 (CH), 114.9 CH),
78.3 (CH2), 61.1 (CH3), 55.7 (CH3), 52.7 (CH2), 52.4 (CH2), 37.5
(CH2), 37.2 (CH2) ppm. MS (FAB, NBA matrix): m/z ϭ 833 [M
ϩ H]ϩ, 541, 305 [C19H17N2O2]ϩ. C49H48N6O7 (832.9): calcd. C
70.66, H 5.81, N 10.09; found C 70.21, H 5.83, N 9.96.
(250 MHz, CDCl3): δ ϭ 146.1 (Cq), 128.5 (CH), 127.7 (CH), 126.1
(CH), 70.6 (Cq), 57.4 (CH2), 55.0 (CH2), 40.8 (CH2), 39.9 (CH2)
ppm. MS (DCI, NH3/isobutane): m/z ϭ 389 [M ϩ H]ϩ, 243
[C19H15]ϩ.
Compound 3a: Under a stream of argon in order to remove CO2 as
and when it is produced, a solution of 8-benzyloxy-7-carboxyquin-
oline (3.65 g, 13 mmol) in dry THF (150 mL) was treated with a
solution of carbonyldiimidazole (CDI; 2.2 g, 13.6 mmol) in THF
(40 mL), under reflux for 2 h. Tripod 2 (2.535 g, 6.5 mmol) in THF
(50 mL) was then added dropwise and the mixture was stirred over-
night under reflux. The solvent was evaporated off and CH2Cl2 was
added to the residue. The solution was washed successively with
saturated NH4Cl and brine and then dried. Concentration afforded
a dark oil which was chromatographed (silica, cyclohexane/CH2Cl2
gradient, then CH2Cl2/methanol 1 Ǟ 2%) to give 3a as a yellow
foam (3.7 g, 4 mmol, 62%). 1H NMR (250 MHz, CDCl3): δ ϭ 8.95
(dd, 3JH,H ϭ 1.6, 4.0 Hz, 2 H, ArH), 8.14 (m, 4 H, ArH), 8.07 (br.
Compound 6a: Under argon at 0 °C, BBr3 (5.3 g, 21 mmol) in
CH2Cl2 (30 mL) was added dropwise to a solution of 5a (1.52 g,
1.8 mmol) in CH2Cl2 (250 mL). After stirring overnight at room
temperature, the mixture was cooled to 0 °C and treated with meth-
anol (50 mL). After 4 h the solution was concentrated and then
repeatedly evaporated with methanol to remove the borate. The
residue was treated with 4  NaOH and extracted with CH2Cl2 to
remove the impurities. The aqueous solution was acidified with 4
 HCl to give a beige precipitate which was washed with water and
then dried under vacuum to give pure 6a (0.937 g, 1.5 mmol, 83%).
1H NMR (300 MHz, DMSO): δ ϭ 9.0Ϫ8.7 (m, 5 H, NH ϩ
ArH),8.3Ϫ8.1 (m, 2 H, ArH), 8.0 Ϫ7.8 (m, 2 H, ArH), 7.6Ϫ7.5
(m, 2 H, ArH), 7.3Ϫ7.1 (m, 4 H, ArH), 6.9Ϫ6.8 (m, 1 H, ArH),
3.5Ϫ3.3 (m, 6 H, CH2), 2.8Ϫ2.7 (m, 6 H, CH2) ppm. 13C NMR
(250 MHz, CDCl3): δ ϭ 169.6 (CO), 167.7 (CO), 156.7 (Cq), 151.2
(Cq), 149.4(Cq), 148.7 (CH), 146.2 (Cq), 139.1 (Cq), 135.9 (CH),
130.4 (Cq), 125.2 (CH), 118.5 (CH), 117.7 (CH), 117.6 (CH), 116.6
(CH), 112.8 (Cq), 52.9 (CH2), 52.8 (CH2), 37.3 (CH2), 37.2 (CH2)
ppm. MS (DCI, NH3/isobutane): m/z ϭ 625 [M ϩ H]ϩ.
3
t, JH,H ϭ 4.8 Hz, 2 H, NH amide), 7.6Ϫ7.0 (m, 29 H, ArH), 5.5
3
(s, 4 H, CH2 benzyl), 3.2 (dt, JH,H ϭ 6.3, 4.8 Hz, 4 H, CH2), 2.47
3
3
(t, JH,H ϭ 5.5 Hz, 2 H, CH2), 2.21 (t, JH,H ϭ 6.3, 4.8 Hz, 4 H,
CH2), 2.13 (t, 3JH,H ϭ 5.3 Hz, 2 H, CH2), 2.0 (br. s, 1 H, NH) ppm.
13C NMR (250 MHz, CDCl3): δ ϭ 165.1 (CO), 153.8 (Cq), 149.5
(CH), 146.1 (Cq), 142.6 (Cq), 136.7 (Cq), 136.0 (CH), 131.3 (Cq),
128.8Ϫ128.5 (unresolved CH’s), 127.6 (unresolved CH’s), 126.0
(CH), 125.3 (Cq), 123.1 (CH), 122.3 (CH), 78.3 (CH2), 70.7 (Cq),
53.9 (CH2), 52.5 (CH2), 40.5.(CH2) ppm. MS (DCI, NH3/
isobutane): m/z
ϭ
911 [MH]ϩ, 243 [C19H15]ϩ; 91 [C7H7]ϩ.
C59H54N6O4 (911.1): calcd. C 77.78, H 5.97, N 9.22; found C 77.40,
H 5.92, N 9.31.
Compound L1: In portions, 6a (0.7 g, 1.1 mmol) was added to
Compound 4a: Under argon at room temperature, a solution of 3a
oleum (SO3/H2SO4; 15 mL) while stirring vigorously. After stirring
(3.61 g, 4 mmol) in CH2Cl2 (100 mL) was stirred with CF3CO2H overnight at room temperature, the mixture was carefully poured
(3.7 g, 32 mmol) for 12 h. The mixture was treated with 10%
NaOH, then washed with brine, dried and concentrated. Column
onto ice to give a beige precipitate. Filtration and washing with
cold water gave the product, which was recrystallised from a mini-
mum amount of water. The pure product (acidic form) was thor-
chromatography (silica, CH2Cl2 then CH2Cl2/methanol 5%/isopro-
1
pylamine 1%) afforded 4a as a beige foam (2.15 g, 81%). H NMR oughly dried under vacuum at 30 °C. A yellow powder was ob-
(300 MHz, CDCl3, 22 °C, TMS): δ ϭ 8.95 (dd, 3JH,H ϭ 1.7, 4.1 Hz, tained (0.347 g, 0.4 mmol, 36.5%). 1H NMR (300 MHz, D2O/
3
3
2 H, Hquin), 8.22 (br. t, JH,H ϭ 5.6 Hz, 2 H, NH amide), 8.11
NaOD): δ ϭ 8.65 (dd, JH,H ϭ 1.6, 8.6 Hz, 2 H, Hquin), 8.56 (dd,
Eur. J. Inorg. Chem. 2003, 2596Ϫ2605
www.eurjic.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2603

A.-M. Albrecht-Gary, G. Serratrice et al.
FULL PAPER
3
3
3JH,H ϭ 1.5, 4.1 Hz, 2 H, Hquin), 8.40 (s, 2 H, Hquin), 7.63 (d, H, Hquin.), 7.20 (d, JH,H ϭ 7.4 Hz, 2 H, Hcat.), 6.84 (d, JH,H
ϭ
3
3
3JH,H ϭ 2.5 Hz, 1 H, Hcatech.), 7.47 (dd, JH,H ϭ 4.1, 8.6 Hz, 2
7.4 Hz, 2 H, Hcat.), 6.54 (t, JH,H ϭ 7.4 Hz, 2 H, Hcat.), 3.42 (m,
H, Hquin), 7.03 (d, JH,H ϭ 2.6 Hz, 1 H, Hcatech.), 3.54 (m, 6 H, 6 H, CH2), 2.77 (m, 6 H, CH2) ppm. 13C NMR (250 MHz,
3
CH2), 2.85 (m, 6 H, CH2) ppm. 13C NMR (300 MHz, D2O- DMSO): δ ϭ 169.7 (CO), 167.9 (CO), 156.4 (Cq), 149.4 (Cq), 149.0
NaOD): δ ϭ 171.3 (Cq), 170.7 (Cq), 170.2 (Cq), 161.3 (Cq), 148.9
(Cq), 147.4 (CH), 145.1 (Cq), 134.6 (CH), 129.1 (CH), 127.8 (Cq),
125.9 (Cq), 123.9 (CH), 119.6 (Cq), 119.0 (CH), 116.1 (Cq), 111.4
(CH), 146.1 (Cq), 139.0 (Cq), 136.1 (CH), 130.5 (Cq), 125.3 (CH),
123.4 (CH), 118.8 (CH), 117.9 (CH), 117.3 (CH), 116.8 (CH), 115.1
(Cq), 112.7 (Cq), 52.6 (CH2), 36.8 (CH2) ppm. MS (DCI, NH3/
(Cq), 111.0 (CH), 53.2 (CH2), 36.9 (CH2) ppm. MS (Electrospray, isobutane): m/z ϭ 590 [M ϩ H].
methanol/water, ϩve mode): m/z 865 [M
C33H32N6O16S3·3.5H2O: calcd. C 42.72, H 4.24, N 9.06, S 10.37;
found C 42.51, H 3.85, N 8.49, S 9.62.
ϭ
ϩ
H]ϩ.
Compound L2: In portions, 6b (1.49 g, 2.5 mmol) was added to
oleum (SO3/H2SO4) (20 mL) while stirring vigorously. After stirring
at room temperature for 12 h, the mixture was carefully poured
onto ice to give a brown precipitate. Filtration and washing with
cold water afforded the product, which was recrystallised from a
minimum amount of water. The pure product (acidic form) was
dried under vacuum at 30 °C. A yellowish powder was obtained
Compound 4b: Under argon, a mixture of 2,3-dimethoxybenzoic
acid (5.28 g, 29 mmol) and CDI (5.16 g, 31.9 mmol) in dry CH2Cl2
(350 mL) was refluxed for 1.5 h. A solution of TREN (2.11 g,
14.5 mmol) in CH2Cl2 (40 mL) was added dropwise and reflux was
maintained overnight. The solution was filtered and washed with
4  NaOH then brine. After drying over anhydrous Na2SO4, the
solvent was eliminated to afford an oil. Chromatography (silica,
CH2Cl2/methanol 1 Ǟ 8% and iPrNH2 1%) gave 4b as a pale yellow
oil (6.53 g, 13.7 mmol, 95%) pure enough for the following steps.
1
(0.672 g, 0.8 mmol, 32%). H NMR (250 MHz, D2O/NaOD): δ ϭ
8.48 (m, 1 H, Hquin), 8.48 (m, 1 H, Hquin), 8.23 (s, 1 H, Hquin),
7.30 (m, 1 H, Hquin), 7.14 (m, 2 H, Hcatech), 6.49 (m, 2 H, Hca-
tech), 3.30 (m, 6 H, CH2), 2.62 (m, 6 H, CH2) ppm. 13C NMR
(250 MHz, D2O-NaOD): δ ϭ 174.3 (Cq), 174.0 (Cq), 173.1 (Cq),
170.2 (Cq), 163.2 (Cq), 150.1 (CH), 147.8 (Cq), 137.0 (CH), 131.6
(CH), 130.3 (Cq), 128.6 (CH), 126.5 (Cq), 122.1 (Cq), 117.6 (Cq),
115.5 (CH), 115.4 (CH), 113.8 (Cq), 55.9 (CH2), 39.3 (CH2), 39.2
(CH2) ppm. MS (Electrospray, methanol/water, ϩve mode): m/z ϭ
830 [M ϩ H]ϩ, 750 [M Ϫ SO3]. C30H31N5O17S3 5H2O: calcd. C
39.17, H 4.49; N 7.61; S 10.46; found C 39.29, H 4.59, N 7.64,
S 9.65.
3
1H NMR (200 MHz, CDCl3): δ ϭ 8.22 (br. t, JH,H ϭ 5 Hz, 2 H,
NH), 7.61 (dd, 3JH,H ϭ 1.6, 8 Hz, 2 H, ArH), 7.11 (t, 3JH,H ϭ 8 Hz,
3
2 H, ArH), 7.01 (dd, JH,H ϭ 1.6, 8 Hz, 4 H, ArH), 3.88 (s, 6 H,
3
OCH3), 3.87 (s, 6 H, OCH3), 3.60 (dt, JH,H ϭ 5.8, 6.2 Hz, 4 H,
CH2), 2.75 (m, 8 H, CH2), 2.64 (m, 2 H, CH2), 2.2 (br. s, 2 H,
NH2) ppm. 13C NMR (200 MHz, CDCl3): δ ϭ 165.3 (CO), 152.3
(Cq), 147.2 (CH), 126.7 (Cq), 124.1 (CH), 122.3 (CH), 115.1 (CH),
61.1 (OCH3), 57.0 (CH2), 55.87 (OCH3), 53.5 (CH2), 39.7 (CH2),
37.6 (CH2) ppm. MS (DCI, NH3/isobutane): m/z ϭ 475 [M ϩ H],
432, 280, 208, 182.
Potentiometric and Spectrophotometric Measurements: The solu-
tions were prepared with boiled deionised water, de-oxygenated and
flushed continuously with argon (purified by a Sigma Oxiclear car-
tridge) in order to exclude CO2 and O2. The ionic strength was
maintained at 0.1  with sodium perchlorate (Prolabo, Puriss or
Merck, p.a.) and all measurements were carried out at (25.0 Ϯ 0.2)
°C. Perchloric acid (70%, Fluka; approx. 0.1 ) and sodium hy-
droxide (Normex, Carlo Erba; approx. 0.1 ) concentrations were
determined respectively by titration of sodium tetraborate solution
(purris. p. a., ACS, Fluka, approx. 0.1 ) in the presence of methyl
red as indicator (Pointet Girard, France) and potassium hydrogen-
phthalate (purris. p. a., Fluka, approx. 0.1 ), with phenolphthalein
as indicator (Prolabo, France). The ligands L1 and L2 were dis-
solved in aqueous 0.1  NaClO4 medium and their concentrations
were calculated by weight. Stock iron() solutions
[Fe(ClO4)3·9H2O, pract., Fluka or Aldrich; approx. 0.01 ], pre-
pared under acidic conditions (0.1  HClO4), were back titrated
with Th(NO3)4·5H2O (Merck, approx. 0.1 ) in the presence of
excess EDTA (Titrisol, Merck, approx. 0.1 ) and xylenol orange
(Merck) as end-point indicator.[27] The concentration was also con-
trolled spectrophotometrically by using the molar extinction coef-
ficient[28] of 4160 Ϫ1·cmϪ1 at 240 nm in 0.1  HClO4. The acid in
excess was evaluated by a potentiometric titration with standard-
ised 0.1  NaOH solution using quantitative formation of a com-
Compound 5b: Under argon a solution of 7-carboxy-8-hydroxy-
quinoline (2.30 g, 12 mmol) and CDI (2.27 g, 14 mmol) in dry THF
(200 mL) was refluxed for 3 h. Compound 4b (6 g, 12 mmol) in
THF (80 mL) was added dropwise and the reflux was maintained
during 12 h. The THF was evaporated off and the residue was dis-
solved CH2Cl2 and treated with NH4Cl and brine. Drying and
evaporation of the solvent afforded an orange foam which was
chromatographed (silica, iPrNH2 1%, CH2Cl2, MeOH 1 Ǟ 5%) to
give 4b (5.8 g, 9 mmol, 75%) pure enough for the following step.
3
1H NMR (20 MHz, CD Cl3): δ ϭ 8.82 (dd, JH,H ϭ 1.6, 4.2 Hz, 1
H, Hquin), 8.19 (br. t,
3 H, NH), 8.11Ϫ8.02 (m, 1 H,
Hquin),7.56Ϫ7.44 (m, 3 H, ArH ϩ Hquin),7.27Ϫ6.86 (m, 6 H,
ArH ϩ Hquin), 3.83 (s, 6 H, 0 CH3), 3.76 (s, 6 H, 0 CH3),
3.65Ϫ3.56 (m, 6 H, CH2), 2.94Ϫ2.82 (m, 6 H, CH2) ppm. 13C
NMR (250 MHz, CDCl3): δ ϭ 167.2 (CO), 165.5 (CO), 154.4 (Cq),
152.2 (Cq), 148.1 (CH), 147.1 (Cq), 138.9 (Cq), 135.6 (Cq), 130.1
(Cq), 126.7 (Cq), 126.1 (CH), 124.0 (CH),122.8 (CH), 122.2 (CH),
116.7 (CH), 114.9 (CH), 112.8 (Cq), 61.1 (COCH3), 55.7 (COCH3),
53.7 (COH2), 53.2 (CH2), 37.8 (CH2), 37.7 (CH2) ppm.
Compound 6b: Under argon at 0 °C, BBr3 (9.04 g, 36 mmol) in
CH2Cl2 (30 mL) was added dropwise to a solution of 5b (1.94 g,
3 mmol) in CH2Cl2 (300 mL). After stirring overnight at room tem- plex with maltol (99%, Aldrich).[29] L1 and L2 ferric complexes were
perature, the mixture was treated with methanol (50 mL) at 0 °C.
The solution was concentrated and then repeatedly evaporated with
methanol. The residue was treated with 4  NaOH and extracted
prepared by mixing adequate volumes of stock ferric perchlorate
and stock ligand solutions. The free hydrogen concentrations were
measured with a glass-Ag/AgCl combined electrode (Metrohm or
with CH2Cl2. The aqueous solution was then acidified to pH 7.6 Tacussel High Alkalinity, filled with 0.1  NaCl and saturated with
with 4  HCl to give a beige precipitate which was washed with
water and dried under vacuum to give 6b pure enough for the fol-
AgCl). The electrode was calibrated in order to read the pH accord-
ing to the classical method (neutralisation of 0.1  HClO4 by 0.1
lowing step (1.65 g, 2.8 mmol, 93%). 1H NMR (200 MHz, DMSO):  NaOH).[30]
3
δ ϭ 8.9 (m, 1 H, Hquin), 8.7 (m, 3 H, NH), 8.32 (d, JH,H
ϭ
The potentiometric titrations were performed into a jacketed cell,
thermostatted by the water flow of a Haake thermostat, using
3
8.2 Hz, 1 H, Hquin), 7.94 (d, JH,H ϭ 8.0 Hz, 1 H, Hquin), 7Ϫ6.3
3
3
(dd, JH,H ϭ 4.80, 4 Hz, 1 H, Hquin), 7.37 (d, JH,H ϭ 28.2 Hz, 1 either a digital millivoltmeter (Tacussel Isis 20,000) with a piston-
2604  2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjic.org
Eur. J. Inorg. Chem. 2003, 2596Ϫ2605

Iron(III) Chelation: Tuning of the pH Dependence by Mixed Ligands
FULL PAPER
[1]
fitted microburette (Gilmont) or an automatic titrator system (Me-
trohm, DMS Titrino 716) connected to an IBM Aptiva microcom-
puter. The titrations of the ligands ([L] ഠ 10Ϫ3 , 2.5 Ͻ pH Ͻ 10.5
for L1 and 3.9 Ͻ pH Ͻ 9.8 for L2) and of their iron() complexes
A. M. Albrecht-Gary, A. L. Crumbliss, Iron Transport and
Storage in Microorganisms, Plants and Animals, in Metal Ions
in Biological Systems (Eds.: A. Sigel and H. Sigel), M. Dekker
1998; vol. 35, pp. 239Ϫ327.
[2]
[3]
[4]
[5]
([FeIII
]
ϭ [L]tot ഠ 10Ϫ3 , 2.2 Ͻ pH Ͻ 11.0 for L1 and 2.3 ϽpH
tot
´
G. Serratrice, H. Boukhalfa, C. Beguin, P. Baret, C. Caris, J.
L. Pierre, Inorg. Chem. 1997, 36, 3898Ϫ3910.
Ͻ 8.9 for L2) were carried out by addition of known volumes of
standardised sodium hydroxide. The data obtained during the
potentiometric titrations were fitted with the Superquad[31] or Hyp-
erquad[32] software packages.
´
F. Thomas, C. Beguin, J. L. Pierre, G. Serratrice, Inorg. Chim.
Acta 1999, 291, 148Ϫ157.
S. M. Cohen, B. O’Sullivan, K. N. Raymond, Inorg. Chem.
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Spectrophotometric measurements were carried out on double-
beam UV/Vis spectrophotometers, either Uvikon 941 (Kontron) or
PerkinϪElmer (Lambda 2), equipped with a thermostatting cell
holder (PerkinϪElmer PTP-1) and connected to an IBM PC 340
microcomputer for data acquisition (UV Winlab software,
[6]
[7]
[8]
[9]
PerkinϪElmer). Absorption spectra of the free ligands ([L]tot
10Ϫ4 , 240 nm Ͻ λ Ͻ 400 nm, 2.5 Ͻ pH Ͻ 10.5 for L1 and 2.6 Ͻ
pH Ͻ 12.5 for L2) and of their iron() species ([L]tot ϭ [FeIII
ഠ
]
tot
ഠ
10Ϫ4 , 400 nm Ͻ λ Ͻ 800 nm, for L1 2.5 Ͻ pH Ͻ 10.5 and 1.0 Ͻ
pH Ͻ 9.0 for L2) were measured at different pH values using quartz
spectrophotometric cells of 0.2 or 1 cm optical path length
(Hellma) against 0.1  aqueous NaClO4 solution as a reference.
Absorption spectra recorded as a function of pH for ferric com-
plexes of L1 and L2 are given in the supporting information (Fig-
ures S3 and S4).
[10]
[11]
[12]
[13]
[14]
[15]
[16]
S. J. Rodgers, L. Chi-Woo, Y. N. Chiu, K. N. Raymond, Inorg.
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1384Ϫ1390.
A batch competition titration between the ferric L2 complex and
EDTA (99.5%, A.C.S. reagent, Aldrich) was carried out under
conditions of [L2]tot ϭ 5.4 ϫ 10Ϫ5 , [FeIII
]
ϭ 4.5 ϫ 10Ϫ5 ,
tot
V. L. Pecoraro, F. L. Weitl, K. N. Raymond, J. Am. Chem. Soc.
1981, 103, 5133Ϫ5140.
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[EDTA]tot ϭ 9.21 ϫ 10Ϫ4  and the pH values adjusted with per-
chloric acid over the range 3.9 to 5.5 (in steps of 0.1 pH unit). The
solution was equilibrated for two days and then the absorption
spectra were recorded (400Ϫ800 nm).
[17]
[18]
[19]
The spectrophotometric data using absorbance values from 20
wavelengths (between 400 and 800 nm) were processed with both
the Specfit[19Ϫ22] and Letagrop-Spefo[17,18] programs in order to cal-
culate the thermodynamic constants of the absorbing species and
their corresponding electronic spectra. The range of values for the
residual-squares sum [Σ(Aexp Ϫ Acalcd.)2] of the fits was over the
range 10Ϫ2 to 10Ϫ3. The calculated electronic spectra are presented
in Figure 6.
[20]
[21]
[22]
[23]
[24]
NMR Titration: The proton NMR titration was carried out for L2
over the pD range 3.5Ϫ9.2 in order to attribute protonation con-
stants to different protonable sites of the ligand. The ligand was
dissolved in D2O. The pD was adjusted with DCl or NaOD solu-
tions. pH measurements were performed with a Tacussel PHN 850
apparatus equipped with a microelectrode Radiometer XC61. pD
values were calculated according to pD ϭ pHmeas ϩ0.4.[33]
[25]
[26]
[27]
[28]
[29]
[30]
D. Imbert, F. Thomas, P. Baret, G. Serratrice, D. Gaude, J. L.
`
Pierre, J. P. Laulhere, New J. Chem. 2000, 24, 281Ϫ288.
L. D. Loomis, K. N. Raymond, Inorg. Chem. 1991, 30,
906Ϫ911.
´
´
Methodes d’analyses complexometriques avec les Titriplex,
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Received November 25, 2002
Acknowledgments
This work has been supported by the Centre National de la Recher-
che Scientifique (UMR, 7509 and 5616, PCV program), the Faculty
of Chemistry, University Louis Pasteur, Strasbourg (France) and
the University Joseph Fourier, Grenoble (France). The authors
[31]
[32]
[33]
´
thank Professor C. Beguin (LEDSS) for his interest in these studies.
Eur. J. Inorg. Chem. 2003, 2596Ϫ2605
www.eurjic.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2605