Towards New Iron(III) Chelators: Synthesis and Complexing Ability of a Water-Soluble Tripodal Ligand Based on 2,2′-Dihydroxybiphenyl Subunits

Article abstract of DOI:10.1002/(SICI)1099-0682(199805)1998:5<613::AID-EJIC613>3.0.CO;2-L

A new water-soluble iron(III) sequestering agent has been designed. The tris-bidentate tripodal ligand consists of three 2,2′-dihydroxybiphenyl subunits connected via amide linkages at their meta (4-) positions to a framework of the "tren" type. The key step of the synthesis involves the coupling of suitably substituted monophenyl moieties in order to obtain the biphenyl precursor. The deprotonation constants of the ligand, and the formation and deprotonation constants of the Fe^III complex have been determined from potentiometric and spectrophotometric measurements. The results are compared with those of a previously described homologous ligand in which the chelating subunits are attached to the tren framework via the ortho (3-) position of the biphenyl rather than the 4-position.

Full text of DOI:10.1002/(SICI)1099-0682(199805)1998:5<613::AID-EJIC613>3.0.CO;2-L

FULL PAPER
Towards New Iron(III) Chelators: Synthesis and Complexing Ability of a
Water-Soluble Tripodal Ligand Based on 2,2Ј-Dihydroxybiphenyl Subunits
`
´
Paul Baret*, Vırginie Beaujolais, Claude Beguin, Didier Gaude, Jean-Louis Pierre, and Guy Serratrice*
´
´
Laboratoire de Chimie Biomimetique, LEDSS, UMR CNRS 5616, Universite J. Fourier,
BP 53, F-38041 Grenoble Cedex 9, France
Fax: (internat.) ϩ 33(0)4/76 51 48 36
E-mail: Paul.Baret@ujf-grenoble.fr; Guy.Serratrice@ujf-grenoble.fr
Received December 4, 1997
Keywords: Dihydroxybiphenyl / Iron chelation / Formation constants / Protonation constants
A new water-soluble iron(III) sequestering agent has been
designed. The tris-bidentate tripodal ligand consists of three
2,2Ј-dihydroxybiphenyl subunits connected via amide link-
ligand, and the formation and deprotonation constants of the
FeIII complex have been determined from potentiometric and
spectrophotometric measurements. The results are compared
ages at their meta (4-) positions to a framework of the “tren” with those of a previously described homologous ligand in
type. The key step of the synthesis involves the coupling of which the chelating subunits are attached to the tren frame-
suitably substituted monophenyl moieties in order to obtain work via the ortho (3-) position of the biphenyl rather than
the biphenyl precursor. The deprotonation constants of the
the 4-position.
Introduction
the tren framework via the 4-position, i.e. meta to the
hydroxy group, rather than via the 3-position (ortho to the
hydroxy group), should be more conducive to an octahedral
arrangement of the ligation sites around the metal center.
However, this new mode of connection renders impossible
the so-called salicylate-like binding of the metal[9] (which
generally occurs at low pH). On the other hand, the pres-
ence of an amide group at the position ortho to the hydroxy
group has been shown by Shanzer et al.[10] to result in hy-
drogen bonding in the complex between the amide and the
oxygen coordinated to FeIII. These interactions suggest a
preorganization of the ligand around the metal center that
improves its binding properties. In order to examine the ef-
fect of the position of the amide linkage (ortho or meta to
the phenolic group) on the complexing ability of the ligand,
we report herein on the synthesis and iron(III) complexing
properties of Lm, a tripodal ligand in which the biphenyl
subunits are attached via their 4-positions (Figure 1).
Iron overloading is a significant health problem, for
which iron chelation therapy is required[1]. A number of
limitations to the use of desferrioxamine, the only approved
drug for treatment of iron overload (parenteral adminis-
tration, prohibitive price, short plasma half-life), underline
the need for the development of a truly orally effective drug.
The search for effective ferric ion chelating agents was orig-
inally centered on mimics of natural siderophores [tris(cate-
cholate) or tris(hydroxamate) ligands][2]. Catechol-based li-
gands are the most powerful in the thermodynamic sense
[enterobactin, a tris(catecholate) siderophore produced by
E. Coli has been shown[3] to have a stability constant for
FeIII complexation of 1049], but these ligands are extremely
air-sensitive and they have been shown to promote bacterial
infections. Hydroxamate-based ligands seem to suffer limi-
tations inherent to all the hydroxamate chelating agents. Re-
cently, potential drugs for iron chelation therapy incorpo-
rating “non-natural” chelating subunits have been studied,
such as hydroxypyridinone[4], pyridoxal isonicotinoyl hy-
drazone[5], and 8-hydroxyquinoline[6]
.
Figure 1. Structural formulae of tris-2,2Ј-dihydroxybiphenyl
ligands
In a previous publication[7], we described tripodal ligands
based on three 2,2Ј-dihydroxybiphenyl subunits connected
to a tris(2-aminoethyl)amine (tren) framework via the ortho
(3-) positions of their phenolic groups (an example, Lo, is
depicted in Figure 1). The results obtained with these water-
soluble and air-stable ligands were promising[8]. Neverthe-
less, the complexing ability of these ligands was found to be
lower than that of tris(catecholates) or tris(hydroxamates)
on the basis of the respective pFe values. The interpretation
of molecular modelling studies suggested that connection
of the 2,2Ј-dihydroxybiphenyl chelating pendant arms to
Eur. J. Inorg. Chem. 1998, 613Ϫ619
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
1434Ϫ1948/98/0404Ϫ0613 $ 17.50ϩ.50/0
613

´
P. Baret, V. Beaujolais, C. Beguin, D. Gaude, J.-L. Pierre, G. Serratrice
FULL PAPER
Results and Discussion
Synthesis of Ligands
Figure 3. Synthesis of 2,2Ј-dimethoxy-1,1Ј-diphenyl-4-carboxylic
acid 9
The synthesis of tripodal ligands based on 2,2Ј-hydroxy-
1,1Ј-biphenyl subunits, attached either via the 3-position[7a]
or the 4-position (this work, Figure 2), is centered on the
formation of amide links from carboxylate moieties, which
are activated and then conjugated with the tetraamine,
tris(2-aminoethyl)amine (tren). Coupling of the protected
2,2Ј-dimethoxy-1,1Ј-diphenyl-4-carboxylic acid 9 with tren
was most conveniently accomplished using N,NЈ-carbonyl-
diimidazole (CDI).[18] For comparative complexation stud-
ies, simple bidentate ligands 14 and 15 were also prepared
(Figure 2).
Figure 2. Synthesis of the ligands
Figure 4. Potentiometric titration curves for (a) 0.5 mmol dmϪ3
ligand Lm; (b) ligand Lm ϩ Fe3ϩ 1:1 0.5 mmol dmϪ3; m ϭ moles
of base added per mol of ligand; all solutions were at 25°C and
I ϭ 0.1 mol dmϪ3 (NaClO4); the deprotonation constants of the
ligand and the complexation constants were calculated using the
SUPERQUAD program[15]
Sulfonation, using H2SO4/oleum, of tripod 11 and of bi-
dentate ligand 14 afforded the water-soluble products 12
and 15, respectively.
In the previously studied[7a] series of ligands, in which the
connection with the spacer tren was at the 3-position of the
biphenyl group, the starting material for the synthesis was
commercially available 2,2Ј-hydroxy-1,1Ј-biphenyl. Func-
tionalization at the position ortho to the phenolic hydroxy
group was readily and regiospecifically effected by various
methods. The synthesis of 2,2Ј-dimethoxy-1,1Ј-diphenyl-4-
carboxylic acid (9) involves heterocoupling[13][19][20] (Figure
3) of the iodo derivative 5 of methyl 2-methoxybenzoate (4)
and 2-methoxyphenylboronic acid (7) (Figure 3B). The iodo
derivative was obtained[12] from 5-methyl-2-nitroanisole (1)
(Figure 3A).
Ligand Deprotonation Constants
The ligand Lm possesses six hydroxy groups and one ter-
8Ϫ
tiary amine function and is denoted LH7 taking into ac-
count the negative charges of the nine sulfonate groups. The
The deprotonation constants of LH213Ϫ and LH14Ϫ spec-
deprotonation constants were determined by potentio- ies are too high to be accurately determined by direct po-
metric titration of the fully protonated ligand with NaOH tentiometry. Spectrophotometric titration was carried out
in 0.1  NaClO4 at 25°C (Figure 4a). Analysis of the ti- at pH ϭ 10.2Ϫ12.7 and revealed one buffer region. The
tration curve yielded only the following five pKa values: corresponding UV spectra exhibit isosbestic behavior, indi-
10.62 ± 0.07; 7.48 ± 0.11; 6.23 ± 0.12; 5.85 ± 0.13, and 4.88 cating the presence of only two absorbing species (Figure
± 0.13.
5). The data were analyzed using the LETAGROP-
614
Eur. J. Inorg. Chem. 1998, 613Ϫ619

Water-Soluble Tripodal Ligand Based on 2,2Ј-Dihydroxybiphenyl Subunits
FULL PAPER
SPEFO[16][17] program (absorbance values at 4 wave- charge transfer band was found to shift from 520 nm (ε ഠ
lengths). The model delineated involves a single-proton step 600 molϪ1 dm3 cmϪ1) to 470 nm (ε ഠ 1400 molϪ1 dm3
for the deprotonation equilibrium, yielding a pKa value of cmϪ1). The absorbance data were processed with the LET-
11.70 ± 0.03. The pKa values of the monomeric analogue 15 AGROP-SPEFO program. The best fit was obtained by
(5,3Ј,5Ј-trisulfonate derivative of compound 14) were also considering the formation of the [FeLH6]6Ϫ and [FeLH5]7Ϫ
[23]
determined: 5.50 ± 0.01 by potentiometry in the acidic species, generating the values log β116
ϭ 61.9 ± 0.1 and
range, and 11.68 ± 0.03 by UV/Vis spectrophotometry in log β115 ϭ 60.0 ± 0.1, with εmax ϭ 580 molϪ1 dm3 cmϪ1
the basic range. The value of 5.50 befits a hydroxy group (λ ϭ 520 nm) for [FeLH6]6Ϫ and 1510 molϪ1 dm3 cmϪ1
meta to the electron-withdrawing amide group.
(λ ϭ 480 nm) for [FeLH5]7Ϫ. Increasing the pH to 5.3 led
to an increase in the band intensity, which was ac-
companied by a shift to lower wavelength (Figure 6b). The
best fitting model for the interpretation of spectrophoto-
metric data was obtained by considering the equilibrium
between [FeLH5]7Ϫ and [FeLH3]9Ϫ. The calculated value of
log β113 is 51.8 ± 0.1, with ε ϭ 2600 molϪ1 dm3 cmϪ1 at
420 nm (shoulder). Only a marginal spectral change was
observed over the pH range 5.3Ϫ7.5. At pH > 7.5, ab-
sorbances in the range 400Ϫ550 nm were found to decrease.
Figure 5. UV/Vis absorption spectra of the Lm ligand as a function
of pH; 1: pH ϭ 10.2; 2: pH ϭ 12.4; [Lm] ϭ 0.05 mmol dmϪ3; I ϭ
0.1 mol dmϪ3 (NaClO4)
Figure 6. UV/Vis absorption spectra of Fe3ϩϪLm ligand as a func-
tion of pH; (a) 1: pH ϭ 0.7; 2: pH ϭ 2.45; (b) 1: pH ϭ 2.5; 2:
pH ϭ 5.3; [Fe3ϩ] ϭ [Lm] ϭ 0.25 mmol dmϪ3; I ϭ 0.1 mol dmϪ3
(NaClO4)
The three lower pKa values (4.88, 5.85, and 6.23) and the
two higher pKa values (10.62 and 11.70) were assigned to
hydroxy groups. The former reflect a roughly statistical sep-
aration (log 3 or 0.48), which is consistent with non-inter-
acting arms if one allows for a difference up to one pK unit,
as is encountered in several tripodal structures containing
2,2Ј-dihydroxybiphenyl[7b], catechol[21] or hydroxamate[22]
binding units. It should be noted that the average value of
these three pKaЈs, 5.65, is very close to the value of 5.50
determined for the monomeric analogue. A similar behavior
is expected for the pKaЈs of the three remaining hydroxy
groups, the value of 11.70 being very close to that of 11.68,
measured for the monomeric analogue. Furthermore, it is
assumed that the deprotonation of at least two OH groups,
each borne by two different pendant arms of the tripod,
induces a change in the UV spectrum. We can thus estimate
the third pKa in this range as having a value of 12.7 on the
basis of a statistical separation (see above). The pKa of 7.48
was attributed to the tertiary amine group by comparison
with the value of 7.1 determined for tripodal ligands con-
taining 2,2Ј-dihydroxybiphenyl subunits[7b]
.
Stability Constants of Ferric Complexes
Stability constants for the FeIII complexes were deter-
mined by means of spectrophotometric and potentiometric
Potentiometric titration on 1:1 solutions in ferric ion and
titrations. The spectrophotometric behavior of the ferric ligand was also carried out over the pH range 3Ϫ10 (Figure
complexes at a 1:1 metal-to-ligand molar ratio was investi- 4b). Data were analyzed using the SUPERQUAD program
gated as a function of pH. The spectrum changed signifi- by considering the [FeLH5]7Ϫ species at the beginning of
cantly over the pH range 0.7Ϫ5.3. The ligand reacts with the titration and using the value of β115 calculated from
FeIII in acidic media, as indicated by the absorption spectra the spectrophotometric titration. The model was best fitted
(Figure 6a). As the pH was increased from 1 to 2.5, the (σfit ϭ 2.9) with six FeIII complex species [FeLH4]8Ϫ
,
Eur. J. Inorg. Chem. 1998, 613Ϫ619
615

´
P. Baret, V. Beaujolais, C. Beguin, D. Gaude, J.-L. Pierre, G. Serratrice
FULL PAPER
[FeLH3]9Ϫ
,
[FeLH2]10Ϫ
,
[FeLH]11Ϫ
,
[FeL]12Ϫ
,
and dihydroxybiphenyl subunits, which are attached to a central
[FeLOH]13Ϫ giving the following values of log β11h: log “tren” framework via amide linkages at their 4-positions,
β114 ϭ 56.49 ± 0.02; log β113 ϭ 51.86 ± 0.03; log β112
46.26 ± 0.04; log β111 ϭ 39.11 ± 0.06; log β110 ϭ 30.03 ± work was to compare the complexing ability of Lm with
0.08; log β11Ϫ1 ϭ 20.26 ± 0.08. that of Lo, a previously described iron chelator in which the
ϭ
i.e. meta to the phenolic hydroxy groups. The aim of this
From these values, the following deprotonation constants chelating pendant arms are attached via their 3-positions,
can be deduced for the complexes: pKFeLH5 ϭ 3.51; i.e. ortho to the phenolic groups.
pKFeLH4 ϭ 4.63; pKFeLH3 ϭ 5.60; pKFeLH2 ϭ 7.15;
pKFeLH ϭ 9.08; pKFeL ϭ 9.77.
The ligand deprotonation constants (six hydroxy groups
and one tertiary amine function) and the stability constants
β11h of the ferric complexes (in a pH range of 1Ϫ10) have
It should be noted that the value of log β113 ϭ 51.86 is
in good agreement with that of 51.8 determined from the
spectrophotometric study, thus validating the proposed
model.
been determined. The pFe value of 19.7 calculated for Lm
[7b]
as compared to the value of 22.8 for Lo
reveals an inter-
esting structural effect. Since molecular models suggested
that the meta connection (Lm) should be more conducive to
the formation of an octahedral coordination cavity than the
ortho connection (Lo), the observed result was unexpected.
The structure of the ligand Lo favours an initial coordi-
nation of the salicylate type (with oxygen atoms of the car-
bonyl and hydroxy groups), which is assumed to “preor-
ganize” the ligand for a complexation of the metal ion by
the other arms. This is not possible for Lm, thus diminishing
its complexing ability towards FeIII. Furthermore, as the
coordination changes from a salicylate to a dihydroxylate
mode of bonding upon deprotonation of the complex, an
amide conformational change is assumed to occur for the
complexation of the metal center by Lo. Thus, hydrogen
bonding between the amide group and the coordinated oxy-
gen atom (six-membered ring) can occur, which is not pos-
sible in the complex of FeIII with Lm. The importance of the
preorganization of the ligand was emphazised by Shanzer et
al.[10], and seems to play a major role in determining the
complexing ability of a given ligand. Thus, for the design
of novel ligands, geometry of the octahedral coordination
sphere around the metal center is not the sole criterion for
the prediction of complexation stability.
Structural attributes of these complexes can be proposed
on the basis of their spectral characteristics. For [FeLH5]7Ϫ
,
the band at 470 nm (ε ϭ 1400 molϪ1 dm3 cmϪ1) is indica-
tive of a dihydroxylate mode of coordination involving one
arm of the ligand, as is apparent from comparison with the
system FeIII sodium 2,2Ј-dihydroxybiphenyl-3,5,3Ј,5Ј-tetra-
sulfonate[9]. Over the pH range 2.5Ϫ5.3, the observed in-
crease in absorbance and shift to shorter wavelength
(shoulder at λ ϭ 430 nm with ε ϭ 2600 molϪ1 dm3 cmϪ1
)
suggest a bis(dihydroxylate) coordination involving two
arms of the ligand in the [FeLH3]9Ϫ species. A region of
constant specific absorbance at pH ϭ 5.3Ϫ7.5 indicates no
increased degree of coordination to the dihydroxylate
groups. At pH > 7.5, a decrease in the absorption in the
range 400Ϫ550 nm may correspond to the formation of
hydroxo complexes. The successive deprotonation constants
of the complex [FeLH3]9Ϫ may thus be attributed to depro-
tonation of the tertiary amine group, of one hydroxy group
of the uncoordinated arm, and of the two water molecules
that complete the coordination sphere around FeIII. The
fact that only two arms of the ligand Lm are involved in
coordination of FeIII might be explained in terms of repul-
sive effects between the negative charges on the sulfonate
groups in the 3Ј-positions. It is assumed that such interac-
tions prevent the formation of a hexadentate coordination
cavity involving all three arms.
We are grateful to Dr. Hakim Boukhalfa (L.E.D.S.S., Grenoble)
for performing the UV/Vis spectrophotometric measurements.
Experimental Section
The pFe (Ϫlog [Fe3ϩ]) values have been calculated to pro-
vide a comparison of the relative ability of the ligands to
complex FeIII (under biologically reasonable conditions, i.e.
pH ϭ 7.4, [L]tot ϭ 10Ϫ5 mol dmϪ3, [Fe]tot ϭ 10Ϫ6 mol
dmϪ3). The calculated value for ligand Lm is 19.7. This
value shows that the ligand Lm is less effective than the
analogous tripodal Lo (pFe ϭ 22.8),[7b] in which the subun-
its are connected via their 3-positions, i.e. ortho to the
hydroxy groups. This lower affinity of Lm for FeIII can be
partly related to the available coordination modes. The
structure of the ligand Lo favours an initial coordination of
the salicylate type (with oxygen atoms of the carbonyl and
hydroxy groups). A preorganization of the ligand is thus
expected for a complexation of the metal ion by the other
arms. This cannot occur for Lm.
Materials and Equipment: Solvents were purified by standard
techniques; 5-methyl-2-nitroanisole and 2-bromoanisole are com-
mercially available (Aldrich). The amine “tren” was distilled from
sodium. All other compounds were of reagent grade and were used
without further purification. Iron(III) stock solutions were pre-
pared by dissolving appropriate amounts of ferric perchlorate hy-
drate (Aldrich) in standardized HClO4/NaClO4 solutions. The
solutions were standardized for ferric ion spectrophotometrically
by using a molar extinction coefficient of 4160 molϪ1 dm3 cmϪ1 at
240 nm[11]. Ϫ IR spectra were recorded with Perkin-Elmer 397 or
Nicolet Impact 400 spectrometers. UV/Vis absorption spectra were
recorded using 1.000-cm path length quartz cells with a Perkin-
Elmer Lambda 2 spectrometer connected to a COMPAQ Deskpro
386s microcomputer. Ϫ Mass spectra were recorded with a NER-
1
MAG R 10 1 C mass spectrometer. Ϫ H- and 13C-NMR spectra
were obtained using 5-mm tubes at 25°C with a Bruker AC 200 or
a Bruker AM 400 spectrometer. Ϫ Microanalyses were performed
by the Central Service of CNRS, Solaise (France). Ϫ Melting
points were determined with a Büchi apparatus and are not cor-
Conclusion
A multistep synthesis of the tris-bidentate tripodal ligand
Lm has been described. This ligand incorporates three 2,2Ј- rected.
616
Eur. J. Inorg. Chem. 1998, 613Ϫ619

Water-Soluble Tripodal Ligand Based on 2,2Ј-Dihydroxybiphenyl Subunits
3-Methoxy-4-nitrobenzoic Acid (2): A mixture of 5-methyl-2-
FULL PAPER
2-Methoxyphenylboronic Acid (7): At Ϫ78°C, nBuLi (80 ml, 1.6
nitroanisole (1) (25.0 g, 0.15 mol), potassium permanganate (97.5
 in hexane, 0.128 mol) was added to a solution of 2-bromoanisole
g, 0.62 mol), and sodium hydrogen carbonate (12.3 g, 0.12 mol) in (6) (13.3 ml, 0.107 mol) in diethyl ether (100 ml) and the mixture
1.5 l of water was refluxed for 4 h. Thereafter, the MnO2 formed
was filtered off. The filtrate was cooled to 0°C and acidified with
20% sulfuric acid to pH ϭ 1. The yellow solid thus produced was
washed with water, dried, and purified by recrystallization from
was stirred at this temperature for 30 min. With a protective nitro-
gen atmosphere, the lithio derivative was then cannulated into a
solution of triisopropyl borate (32 ml, 0.214 mol) in diethyl ether.
After stirring at Ϫ78°C for 30 min, then at room temperature, the
ethanol (white crystals, 21 g, 73%). Ϫ M.p. 226°C (ref.[12] mixture was poured into aqueous HCl (10%, 300 ml). The product
1
230Ϫ233°C). Ϫ H NMR (200 MHz, [D6]DMSO): δ ϭ 4.01 (s, 3 was extracted with diethyl ether, the combined extracts were
H, OCH3), 7.71 (dd, J1 ϭ 8.3, J2 ϭ 1.4 Hz, 1 H), 7.79 (d, J ϭ 1.4 washed with water (100 ml), and dried with Na2SO4. The oily,
Hz, 1 H), 7.83 (d, J ϭ 8.3 Hz, 1 H). Ϫ 13C NMR (50 MHz,
crude product was stored in a refrigerator for 24 h, giving white
[D6]DMSO): δ ϭ 56.8 (OCH3), 114.6 (CH), 121.3 (CH), 125.0 crystals. These somewhat unstable crystals were rapidly washed
(CH), 135.8, 141.9, 151.5 (CϪO), 165.8 (CϭO). Ϫ IR (KBr): ν˜ ϭ
with cold pentane and then dried in vacuo (11 g, 0.072 mol, 68%).
1
3250Ϫ2200 cmϪ1 [ν(OH)], 1680 [ν(CϭO)].
Ϫ M.p. 104Ϫ105°C (ref.[13] 105Ϫ106°C). Ϫ H NMR (200 MHz,
CDCl3): δ ϭ 3.85 (s, 3 H, OCH3), 6.85 (d, J ϭ 8.3 Hz, 1 H, ArH),
6.93Ϫ7.01 (m, 1 H, ArH), 7.34Ϫ7.43 (m, 1 H, ArH), 7.77 (dd, J1 ϭ
7.3, J2 ϭ 1.8 Hz, 1 H, ArH). Ϫ 13C NMR (50 MHz, CDCl3): δ ϭ
55.3 (OCH3), 109.9 (CH), 121.1 (CH), 132.7 (CH), 136.8 (CH),
164.4 (CϪO).
Methyl 3-Methoxy-4-nitrobenzoate (3): 3-Methoxy-4-nitroben-
zoic acid (2) (21.0 g, 0.11 mol) was dissolved in methanol (350 ml)
under nitrogen. BF3 (40 ml, 50% w/w in methanol) was added and
the mixture was stirred under reflux for 12 h. The solvent was then
removed, the residue was treated with water (300 ml), and the re-
sulting suspension was filtered. The collected yellow solid was
washed with water (500 ml) and then dried in vacuo (21.9 g, 0.10
mol, 91%). Ϫ M.p. 91°C (ref.[12] 90Ϫ91°C). Ϫ 1H NMR (200
MHz, CDCl3): δ ϭ 3.97 (s, 3 H, CO2CH3), 4.02 (s, 3 H, OCH3),
7.67 (dd, J1 ϭ 8.3, J2 ϭ 1.5 Hz, 1 H), 7.71 (d, J ϭ 1.5 Hz, 1 H),
7.84 (d, J ϭ 8.3 Hz, 1 H). Ϫ 13C NMR (50 MHz, CDCl3): δ ϭ
52.8 (CO2CH3), 56.7 (OCH3), 114.6 (CH), 121.4 (CH), 125.3 (CH),
134.8, 142.0, 152.4 (CϪO), 165.2 (CϭO). Ϫ IR (KBr): ν˜ ϭ 1740
cmϪ1 [ν(CϭO)].
Methyl 2,2Ј-Dimethoxy-1,1Ј-diphenyl-4-carboxylate (8): To
a
solution of 5 (1.0 g, 3.42 mmol) in freshly distilled DMF (15 ml)
under nitrogen were added 7 (78 mg, 5.13 mmol), Et3N (1.4 ml,
10.3 mmol), Pd(OAc)2 (23 mg, 0.103 mmol), and P(o-Tol)3 (65 mg,
0.212 mmol). The orange solution was heated at 100°C for 3 h,
whereupon it became black. The solvent was evaporated and the
residue was treated with dichloromethane and 10% ammonia. The
organic phase was washed with water and then dried with Na2SO4.
The orange oil was purified by column chromatography (silica gel;
pentane/diethyl ether, 9:1). 8 was obtained as a white solid (0.9 g,
3.3 mmol, 97%). Ϫ M.p. 112°C. Ϫ Rf ϭ 0.55 (pentane/diethyl ether,
Methyl 4-Amino-3-methoxybenzoate (4): Compound 3 (10.0 g,
47.4 mmol) was dissolved in a mixture of dichloromethane (50 ml)
and ethanol (300 ml). The resulting solution was stirred for 12 h
under hydrogen in the presence of Pd/charcoal (10% Pd, 923 mg).
The catalyst was subsequently removed by filtration and the solvent
was evaporated. The yellow solid thus obtained was purified by
recrystallization from ethanol (7.84 g, 43.3 mmol, 91%). Ϫ M.p.
1
2:1). Ϫ H NMR (200 MHz, CDCl3): δ ϭ 3.33 (s, 3 H, CO2CH3),
3.39 (s, 3 H, OCH3), 3.50 (s, 3 H, OCH3), 6.53Ϫ6.62 (m, 2 H,
ArH), 6.62Ϫ6.90 (m, 2 H, ArH), 6.87 (d, J ϭ 7.7 Hz, 1 H), 7.20
(d, J ϭ 1.4 Hz, 1 H), 7.25 (dd, J1 ϭ 7.7, J2 ϭ 1.4 Hz, 1 H). Ϫ 13C
NMR (50 MHz, [D6]DMSO): δ ϭ 52.2 (CO2CH3), 55.4 and 55.6
(OCH3), 111.3 (CH), 111.4 (CH), 120.2 (CH), 121.2 (CH), 126.3,
129.2 (CH), 130.7 (CH), 131.1, 131.4 (CH), 132.2, 156.6 (CϪO),
156.7 (CϪO), 167.2 (CϭO). Ϫ IR (KBr): ν˜ ϭ 1710 cmϪ1 [ν(Cϭ
O)]. Ϫ MS (EI); m/z: 272 [Mϩ], 241, 225, 213, 198. Ϫ C16H16O4:
calcd. C 70.58, H 5.29; found C 70.40, H 5.47.
1
128°C (ref.[12] 127Ϫ128°C). Ϫ H NMR (200 MHz, CDCl3): δ ϭ
3.86 (s, 3 H, OCH3), 3.89 (s, 3 H, OCH3), 4.22 (m, 2 H, NH2), 6.66
(d, J ϭ 8.0 Hz, 1 H), 7.45 (d, J ϭ 1.7 Hz, 1 H), 7.54 (dd, J1 ϭ 8.0,
J2 ϭ 1.7 Hz, 1 H). Ϫ 13C NMR (50 MHz, CDCl3): δ ϭ 51.6
(CO2CH3), 55.5 (OCH3), 111.1 (CH), 113.1 (CH), 119.5, 124.0
2,2Ј-Dimethoxy-1,1Ј-diphenyl-4-carboxylic Acid (9): Ester 8 (2.0
g, 7.34 mmol) was refluxed with ethanolic KOH (excess) for 3 h.
The mixture was then concentrated and 10% HCl was added. The
resulting precipitate was filtered off, washed with water, and dried
in vacuo to give 9 as a white solid (1.9 g, quant.). Ϫ M.p. 170°C.
(CH), 141.0, 146.1 (CϪO), 167.3 (CϭO). Ϫ IR (KBr): ν ϭ
˜
3450Ϫ3330 cmϪ1 [ν(NH2)], 1670 [ν(CϭO)].
Methyl 4-Iodo-3-methoxybenzoate (5): To a mixture of 4 (8.1 g,
44.7 mmol), water (110 ml) and 20% H2SO4 (24 ml), a solution of
sodium nitrite (3.47 g, 49.6 mmol) in water (20 ml) was slowly
added at 0°C. At the end of the reaction, the excess nitrous acid
was destroyed by the addition of urea (800 mg). Then, 20 ml of
aqueous KI (7.74 g, 46.6 mmol) was added dropwise and the mix-
ture was stirred for 2 h at 50°C. To the resulting brown mixture
was added an aqueous solution of Na2S2O5. The product was iso-
lated by extraction with diethyl ether, neutralization with 1 
NaOH (to pH ϭ 5), washing with brine, and drying with Na2SO4.
The solvent was evaporated and the iodo compound was purified
by column chromatography (alumina, pentane containing 3% ethyl
1
Ϫ H NMR (200 MHz, CDCl3): δ ϭ 3.78 (s, 3 H, OCH3), 3.85 (s,
3 H, OCH3), 6.98Ϫ7.07 (m, 2 H, ArH), 7.23Ϫ7.28 (m, 1 H, ArH),
7.33Ϫ7.41 (m, 1 H, ArH), 7.36 (d, J ϭ 7.8 Hz, 1 H), 7.77 (d, J ϭ
1.5 Hz, 1 H), 7.8 (dd, J ϭ 7.8, 1.5 Hz, 1 H). Ϫ 13C NMR (50 MHz,
[D6]DMSO): δ ϭ 55.4 and 55.5 (OCH3), 111.4 (CH), 111.5 (CH),
120.2 (CH), 121.4 (CH), 126.5, 129.1 (CH), 130.8 (CH), 131.1,
131.3 (CH), 132.2, 156.6 (CϪO), 156.7 (CϪO), 167.2 (CϭO). Ϫ
IR (KBr): ν˜ ϭ 3600Ϫ2300 cmϪ1 [ν(OH)], 1680 [ν(CϭO)]. Ϫ MS
(EI); m/z: 258 [Mϩ], 184.
N,NЈ,NЈЈ-(Nitrilotri-2,1-ethanediyl)tris(2,2Ј-dimethoxy-4-biphen-
acetate), giving a white solid (10.4 g, 33.6 mmol, 72%). Ϫ M.p. ylcarboxamide) (10): Under nitrogen, acid 9 (1.0 g, 3.87 mmol) was
1
50Ϫ52°C (ref.[12] 52Ϫ53°C). Ϫ H NMR (200 MHz, CDCl3): δ ϭ treated with CDI (690 mg, 4.26 mmol) in freshly distilled THF (50
3.92 (s, 3 H, OCH3), 3.94 (s, 3 H, OCH3), 7.36 (dd, J1 ϭ 8.1, J2 ϭ ml) and the mixture was stirred for 12 h; tren (160 µl, 1.06 mmol)
1.8 Hz, 1 H), 7.44 (d, J ϭ 1.8 Hz, 1 H), 7.84 (d, J ϭ 8.1 Hz, 1 H).
Ϫ
was added and the mixture was stirred overnight at room tempera-
13C NMR (50 MHz, CDCl3): δ ϭ 52.3 (CO2CH3), 56.5 (OCH3), ture. The solvent was then evaporated and the brown oil was puri-
92.6 (CI), 111.2 (CH), 123.3 (CH), 131.6, 139.5 (CH), 158.2 (CϪO), fied by column chromatography (silica gel, dichloromethane con-
166.5 (CϭO). Ϫ IR (KBr): ν˜ ϭ 1720 cmϪ1 [ν(CϭO)].
Eur. J. Inorg. Chem. 1998, 613Ϫ619
taining 2% isopropylamine) giving 10 as a white solid (1.0 g, 3.50
617

´
P. Baret, V. Beaujolais, C. Beguin, D. Gaude, J.-L. Pierre, G. Serratrice
FULL PAPER
mmol, 90%). Ϫ M.p. 116°C. Ϫ Rf ϭ 0.6 (CH2Cl2/MeOH, 9:1). Ϫ nitrogen, 13 (900 mg, 3.15 mmol) was added and the mixture was
1H NMR (200 MHz, CDCl3): δ ϭ 2.77 (br. s, 6 H, NCH2), 3.61 stirred for 12 h at room temperature. The mixture was then treated
(m, 24 H, NCH2, OCH3), 6.84Ϫ7.04 (m, 12 H, ArH), 7.24Ϫ7.44 with methanol and the solvents were evaporated. The residue was
(m, 9 H, ArH), 7.50 (br. s, 3 H, NH). Ϫ 13C NMR (50 MHz, redissolved in 20 ml of 10% aq. NaOH and subsequent addition
CDCl3): δ ϭ 38.4 (CH2), 54.7 (CH2), 55.3 and 55.4 (OCH3), 110.2
of 10% HCl at 0°C gave 14 as a white precipitate, which was dried
1
(CH), 110.9 (CH), 118.6 (CH), 120.1 (CH), 126.7, 128.7 (CH), in vacuo (800 mg, 3.10 mmol, 98%). Ϫ M.p. 186°C. Ϫ H NMR
131.0 (CH), 131.2 (CH), 134.0, 156.6 (CϪO), 156.9 (CϪO), 167.8 (200 MHz, [D6]DMSO): δ ϭ 2.96 (s, 6 H, NCH3), 6.77Ϫ6.90 (m,
(CϭO). Ϫ IR (KBr): ν˜ ϭ 1635 cmϪ1 [ν(CϭO)]. Ϫ MSM [FAB(ϩ), 4 H, ArH), 7.10Ϫ7.88 (m, 3 H, ArH), 9.23 (s, 1 H, OH), 9.43 (s, 1
NBA matrix]; m/z: 867 [M ϩ Hϩ], 596, 284, 257, 241, 213. Ϫ
H, OH). Ϫ 13C NMR (50 MHz, [D6]DMSO): δ ϭ 34.8 (NCH3),
C51H54N4O9 ·3H2O: calcd. C 66.64, H 6.36, N 6.10; found C 66.65, 114.4 (CH), 115.7 (CH), 117.3 (CH), 118.8 (CH), 125.2, 127.0,
H 6.04, N 6.15.
128.3 (CH), 131.3 (CH), 131.4 (CH), 136.2, 154.4 (CϪOH), 154.6
(CϪOH), 154.6 (CϭO). Ϫ IR (KBr): ν˜ ϭ 3500Ϫ2300 cmϪ1
[ν(OH)], 1600 [ν(CϭO)]. Ϫ MS (EI); m/z: 275 [M], 213.
N,NЈ,NЈЈ-(Nitrilotri-2,1-ethanediyl)tris(2,2Ј-dihydroxy-4-biphen-
ylcarboxamide) (11): A solution of BBr3 in dichloromethane (1 ,
20 ml) was cooled to 0°C under nitrogen. Podand 10 (860 mg, 0.99
mmol) was added and the mixture was stirred for 12 h at room
temperature, resulting in the deposition of a white precipitate. The
mixture was treated with methanol and the solvents were evapo-
rated. The residue was dissolved in 30 ml of 10% aq. NaOH. Sub-
sequent addition of 10% HCl at 0°C gave 11 as a white precipitate,
which was dried in vacuo (700 mg, 0.89 mmol, 90%). Ϫ M.p.
2,2Ј-Dihydroxy-4-(N,N-dimethylcarbamoyl)-3,3Ј,5Ј-trisulfo-1,1Ј-
biphenyl (15): Sulfonation of 14 was performed as for 12. Ϫ M.p.
> 286°C. Ϫ 1H NMR (200 MHz, CD3OD): δ ϭ 2.87 (s, 3 H, CH3),
3.03 (s, 3 H, CH3), 6.64 (s, 1 H), 7.7 (d, J ϭ 2.3 Hz, 1 H), 7.75 (s,
1 H), 8.16 (d, J ϭ 2.3 Hz, 1 H). Ϫ 13C NMR (50 MHz, CD3OD):
δ ϭ 35.3 (CH3), 39.8 (CH3), 114.5 (CH), 126.3, 126.5 (CH), 127.2,
129.9, 132.6 (CH), 132.7 (CH), 133.5, 136.3, 137.0, 154.3 (CϪN),
158.0 (CϪN), 172.9 (CϭO).
1
170°C. Ϫ H NMR (200 MHz, [D6]DMSO): δ ϭ 3.48 (br. s, 6 H,
CH2), 3.73 (br. s, 6 H, CH2), 6.76Ϫ6.92 (m, 6 H, ArH), 7.10Ϫ7.21
(m, 9 H, ArH), 7.34Ϫ7.41 (m, 6 H, ArH), 8.81 (br. s, 3 H, NH),
9.37 (br. s, 3 H, OH), 9.50 (br. s, 3 H, OH). Ϫ 13C NMR (50 MHz,
[D6]DMSO): δ ϭ 34.3 (CH2), 52.1 (CH2), 115.1, 115.8, 117.4,
118.8, 128.5, 129.3, 131.4, 133.8, 154.6 (CϪO), 166.9 (CϭO). Ϫ
IR (KBr): ν˜ ϭ 3650Ϫ2300 cmϪ1 [ν(OH)], 1640 [ν(CϭO)]. Ϫ MS
[FAB(ϩ), NBA matrix]; m/z: 783 [M ϩ Hϩ], 613, 540, 528. Ϫ
C45H42N4O9 ·HBr·H2O: calcd. C 61.35, H 5.15, N 6.36; found C
61.61, H 5.12, N 6.27.
Potentiometric Experiments: All measurements were made at
25°C. Solutions were prepared with deionized, doubly distilled
water. The ionic strength was fixed at 0.1  with sodium perchlo-
rate (PROLABO puriss). The pH measurements were performed
using a TACUSSEL Ionoprocesseur-II millivoltmeter equipped
with glass and calomel electrodes (TACUSSEL). The electrodes
were calibrated to read p[H] according to the classical method[14]
.
Potentiometric titrations employed a TACUSSEL Electroburex bu-
rette and a pH meter (Ionoprocesseur-II) connected to a Hewlett
Packard microcomputer. The titrations were automated using
software developed in our laboratory. The ligands and their
iron(III) complexes at concentrations of ca. 0.001  were titrated
with standardized 0.05  sodium hydroxide. The titration data were
refined by the nonlinear least-squares refinement program SUPER-
QUAD[15] in order to determine the equilibrium constants (pro-
tonation and complexation).
N,NЈ,NЉ-(Nitrilotri-2,1-ethanediyl)tris(2,2Ј-dihydroxy-5,3Ј,5Ј-
trisulfo-4-biphenylcarboxamide) (12): The hydroxylated tripod 11
(830 mg, 1.06 mmol) was dissolved in 60 ml of oleum (H2SO4 ϩ
15% SO3) at 0°C. The mixture was then stirred at 90°C for 3 h.
After cooling to room temperature, the brown solution was cau-
tiously poured onto crushed ice with stirring. With cooling, the
solution was brought to pH ϭ 4 by the slow addition of 10% aq.
NaOH. Sodium sulfate was eliminated by repeated precipitations
by adding methanol. Ϫ M.p. > 286°C. Ϫ 1H NMR (200 MHz,
D2O): δ ϭ 3.80 (m, 2 H, CH2), 4.00 (m, 2 H, CH2), 7.17 (s, 1 H),
7.91 (s, 1 H), 7.94 (d, J ϭ 2.3 Hz, 1 H), 8.22 (d, J ϭ 2.3 Hz, 1 H).
Spectrophotometric Experiments: The deprotonation constant of
one hydroxy group was determined in the UV region (the ligand
concentration in aqueous solution was ca. 5·10Ϫ5 ). The ferric
complexes were investigated by spectrophotometry: The UV/Vis
spectrum of a solution containing equal amounts of ligand and
FeIII (10Ϫ4 ) was monitored as a function of pH over the range
1Ϫ10 (adjusted with HClO4 or NaOH); an aliquot was taken from
the solution after each adjustment of the pH (which was measured
using a pH meter) and its spectrum was recorded. The ionic
strength was fixed at 0.1  with NaClO4/HClO4 for this second set
of experiments. The spectrophotometric data were fitted with the
LETAGROP-SPEFO program[16][17]. The program uses a nonlin-
ear least-squares method and calculates the thermodynamic con-
stants of the absorbing species and their corresponding electronic
spectra. The calculations were performed on the basis of ab-
sorbance data at about 6Ϫ8 wavelengths (between 400 and 600
nm).
Ϫ
13C NMR (50 MHz, CD3OD): δ ϭ 37.4 (CH2), 55.3 (CH2),
118.6 (CH), 128.4 (CH), 129.4, 131.4, 134.1 (CH), 134.5, 134.9
(CH), 137.1, 138.0, 156.4 (CϪO), 159.2 (CϪO), 174.2 (CϭO).
2,2Ј-Dimethoxy-4-(N,N-dimethylcarbamoyl)-1,1Ј-biphenyl (13):
Under nitrogen at room temperature, acid 9 (0.9 g, 3.48 mmol) was
treated with CDI (623 mg, 3.84 mmol) in THF (100 ml) for 12 h.
The mixture was then cooled to 0°C, dimethylamine (5 ml) was
added, and stirring was continued overnight at room temperature.
The solvent was then evaporated and the residual brown oil was
purified by column chromatography (silica gel, dichloromethane
containing 2% isopropylamine) to afford 13 as a white solid (800
mg, 2.80 mmol, 80%). Ϫ M.p. 123°C. Ϫ 1H NMR (200 MHz,
CDCl3): δ ϭ 3.04 (s, 6 H, NCH3), 3.69 (s, 3 H, OCH3), 3.71 (s, 3
H, OCH3), 6.89Ϫ6.98 (m, 4 H, ArH), 7.13Ϫ7.31 (m, 3 H, ArH).
13C NMR (50 MHz, CDCl3): δ ϭ 36.0 (NCH3), 40.0 (NCH3),
[1] [1a]
[1b]
Ϫ
P. S. Dobbin, R. C. Hider, Chem. Ber. 1990, 565. Ϫ
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Herschko, D. J. Weatherall, Crit. Rev. Clin. Lab. Sc. 1988, 26,
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R. J. Bergeron, G. M. Brittenham, The Development of Iron
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55.6 (OCH3), 55.7 (OCH3), 110.0 (CH), 111.1 (CH), 118.7 (CH),
120.3 (CH), 127.0, 128.8 (CH), 129.2, 131.1 (CH), 131.2 (CH),
136.3, 156.9 (CϪOMe), 157.0 (CϪOMe), 171.5 (CϭO). Ϫ MS (EI):
m/z: 285 [M], 257, 213.
[2]
[3]
[4]
2,2Ј-Dihydroxy-4-(N,N-dimethylcarbamoyl)-1,1Ј-biphenyl (14):
To a solution of BBr3 in dichloromethane (1 , 20 ml) at 0°C under
[5]
G. M. Brittenham, Semin. Hematol. 1990, 27, 112.
618
Eur. J. Inorg. Chem. 1998, 613Ϫ619

Water-Soluble Tripodal Ligand Based on 2,2Ј-Dihydroxybiphenyl Subunits
FULL PAPER
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[15]
[16]
[17]
[18]
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[20]
[21]
[22]
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[13]
[14]
β11n ϭ [FeLHn]/[Fe3ϩ] [L] [Hϩ]n is the equilibrium constant for
Fe3ϩ ϩ L ϩ nHϩ v FeLHn.
[97293]
Eur. J. Inorg. Chem. 1998, 613Ϫ619
619