4-[3,5-bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]-benzoic acid: A novel efficient and selective iron(III) complexing agent

Article abstract of DOI:10.1002/(SICI)1521-3773(19990903)38:17<2568::AID-ANIE2568>3.0.CO;2-C

An exceptionally stable 1:2 complex [FeL₂]^3- is formed from the ligand H₃L and Fe(III). In contrast, the affinity of this ligand for other biometals is relatively small. These properties make H₃L a highly promising candidate for medical applications (e.g. for the treatment of iron overload).

Full text of DOI:10.1002/(SICI)1521-3773(19990903)38:17<2568::AID-ANIE2568>3.0.CO;2-C

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[4] A. P. Humphries, S. A. R. Knox, J. Chem. Soc. Dalton Trans. 1975, 1710.
respect to other biologically important metals of such com-
pounds will be of central importance.
Å
[5] Crystal data for C66H42BF24OPRuS3 (4a): triclinic, P1, a ˆ 13.2861(3),
b ˆ 15.4088(2), c ˆ 17.4920(3) Š, a ˆ 103.8463(2), b ˆ 94.7346(7), g ˆ
We herein report on the metal complexing properties of a
substituted 3,5-bis(ortho-hydroxyphenyl)-1,2,4-triazole in sol-
ution. Ryabukhin had already discovered that compounds of
this class tend to form complexes with divalent transition
metal ions in the form of insoluble polymers.[4] Our studies
have shown that the corresponding benzoic acid H3L[5]
exhibits ideal properties for use in the treatment of iron
overload. Owing to the good
oral bioavailability, tolerance
in animal studies, and efficient
excretion of iron, this ligand is
a possible successor to Desfer-
al.[6]
110.1334(8)8, Vˆ 3306.33(12) Š3, Z ˆ 2, Tˆ 198(2) K, 1calcd
ˆ
1.553 gcm 3, R(F) ˆ 0.0514 for 9428 observed independent reflections
(48 ꢀ 2q ꢀ 578). Further details on the crystal structure investigation
may be obtained from the Fachinformationszentrum Karlsruhe,
D-76344 Eggenstein-Leopoldshafen, Germany (fax: (‡49)7247-808-
666); e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository
number CSD-410356.
[6] Ligand priority order: Ru (C5H5 > thiophene > phosphane > CO); S
(Ru, C-thiophene, C CH3, lone pair), see K. Stanley, M. C. Baird, J.
Am. Chem. Soc. 1975, 97, 6598.
[7] a) D. D. Graf, K. R. Mann, Inorg. Chem. 1997, 36, 150; b) D. D. Graf,
N. C. Day, K. R. Mann, Inorg. Chem. 1995, 34, 1562. For reviews of
thiophene coordination: c) T. B. Rauchfuss, Prog. Inorg. Chem. 1991,
39, 259 ± 329; d) R. J. Angelici, Coord. Chem. Rev. 1990, 105, 61.
Free H3L reacts in solution
as a weak, triprotic acid, and in
the neutral pH range it is
present as the carboxylate ion
H2L . The nitrogen atoms of the five-membered heterocycle
cannot be protonated above pH 2. With FeIII this ligand forms
a dark violet 1:1 complex and a deep red 1:2 complex.[7] The
peripheral carboxylate group does not participate in the
metal-complex formation; [FeL] can therefore be protonated
once, and [FeL2]3 can be protonated twice. In accord with the
negative charge [FeL2]3 is the slightly stronger base. Thus L3
acts as a tridentate meridionally coordinating ligand,[8] where
binding of the metal center occurs through the two phenolate
groups and one of the nitrogen atoms of the heterocycle.
Consequently, the bis complex [FeL2]3 adopts a trans-N2O4
coordination. With AlIII complex formation is so slow that
separate signals for the free ligand, the 1:1 complex, and the
4-[3,5-Bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]-
benzoic Acid: A Novel Efficient and
Selective Iron(iii) Complexing Agent**
Uwe Heinz, Kaspar Hegetschweiler,* Pierre Acklin,
Â
Bernard Faller, Rene Lattmann, and
Hans Peter Schnebli
Clinical medicine recognizes several diseases which are due
to defects in iron homeostasis.[1] Iron deficiency anaemia is
rather common but can be treated easily. By comparison, iron
overload is relatively rare but is associated with more severe
morbidity. Since man is unable to actively excrete iron, excess
iron, once taken up, deposits in tissues in the form of solid
FeOOH which then can lead to organ damage and ultimately
death. Frequent blood transfusions, as are necessary in the
treatment of haematological defects such as b-thalassaemia
major (more than 500000 patients world-wide), are a well
recognized cause of iron overload.[2] Therapeutically useful
chelators can complex excess tissue FeIII, thus transforming it
into a soluble, excretable form. Currently, the siderophore
desferrioxamine-B (Desferal) is by far the clinically most
widely used chelator. However, this compound has a very
short biological half-life and, in addition, cannot be admin-
istered orally. Because of this, there has evolved an intensive
search for new iron chelators in recent years.[3] Clearly, in
addition to low toxicity, appropriate tissue distribution and
pharmacokinetics, the metal-binding properties, for example
high stability of the FeIII complexes, and high selectivity with
1
1:2 complex are observable in the H NMR spectrum.[9] The
individual protonation equilibria, however, exhibit time-
averaged signals with pD-dependent shifts. According to
these NMR spectroscopic studies, free H3L is exclusively
‡
present below pD 2. In the range 2 < pD < 4 [Al(HL)] is
formed, which is subsequently deprotonated to [AlL]. The
formation of 1:2 complexes begins above pD 5.5, and at higher
pH the 1:1 complex disappears completely. The NMR spectra
for the 1:2 complexes show only ten signals for the aromatic
protons. Apparently the
two ligands are symmetry-
COO–
equivalent (C2), which is
consistent with the struc-
ture proposed for [ML2]3 .
In strongly alkaline solu-
tions the complex decom-
poses to form [Al(OH)4] ,
3–
N
N
N
3
and in 5 moldm NaOD
O
[*] Prof. Dr. K. Hegetschweiler, Dipl.-Chem. U. Heinz
Universität des Saarlandes, Anorganische Chemie
Postfach 151150, D-66041 Saarbrücken (Germany)
Fax : (‡ 49)681-302-2663
O
M
N
only the signals of free,
deprotonated L3 are ob-
served.
O
O
E-mail: hegetsch@rz.uni-sb.de
The various protonation
and complex formation
equilibria could be eluci-
dated quantitatively by ex-
tensive potentiometric and
spectrophotometric meas-
N
N
Dr. P. Acklin, Dr. B. Faller, Dr. R. Lattmann, Dr. H. P. Schnebli
Novartis Pharma AG, CH-4002 Basel (Switzerland)
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
We thank Martin Kiefer, Prof. Dr. Michael Zeppezauer, Jürgen
Sander, Bernd Morgenstern, and Dirk Kuppert for their valuable
support.
–OOC
[ML2]3– (M = Al, Fe)
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Angew. Chem. Int. Ed. 1999, 38, No. 17

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urements. To achieve sufficient solubility over the entire pH
range, an H2O/DMSO mixture with a mole fraction of
0.20 DMSO was generally used.[10] In this medium the pKa
values of H3L were 4.62(2), 10.13(1), and 12.09(1).[11] When
Fe(NO3)3 and H3L were combined in concentrations of 5 Â
molecule in dilute solution is not significant. The data thus
show that, in view of stability and selectivity, H3L exhibits
ideal conditions for use in the therapy of iron overload. The
promising results of the in vivo studies[6] are thus clearly
confirmed: In the presence of other biorelevant metals, 4-[3,5-
bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]benzoic acid is a high-
ly selective complexing agent for FeIII. Owing to the high
affinity for AlIII, the compound could also be of interest for a
selective sequestration of aluminum.
4
3
10 and 10 3 moldm , an acidic solution was obtained in
‡
which [Fe(HL)] was already formed to an extent of more
than 95%. In such a solution the formation constants of the
products [FeL], [Fe(HL)2] , [FeL(HL)]2 , and [FeL2]3 can be
determined by potentiometric titration.[11±13] However, the
‡
stability of [Fe(HL)] is too high to be measured by using pH-
Received: February, 12, 1999
Revised version: May 27, 1999 [Z13027IE]
German version: Angew. Chem. 1999, 111, 2733 ± 2736
metric methods. This determination was achieved with a
series of spectrophotometric measurements.[14] The kinetics of
complex formation was also investigated spectrophotometri-
cally.[15] In the acidic range, the kinetics for the formation of
Keywords: aluminum ´ chelates ´ iron
‡
‡
[Fe(HL)] follow the rate law d[Fe(HL) ]/dt ˆ k[Fe3‡][H3L],
where k is strongly dependent on the pH value: At pH 3.5 the
value for k is 7 Â 102 s 1 mol 1 dm3, at pH 2 it decreases to
30 s 1 mol 1 dm3. This result is in good agreement with the
concept that the ligand coordinates upon deprotonation. The
[1] R. R. Crichton, Inorganic Biochemistry of Iron Metabolism, Ellis
Horwood, London, 1991 (Series in Inorganic Chemistry).
[2] a) S. Singh, Chem. Ind. 1994, 452; b) A. E. Martell, R. D. Hancock,
Metal Complexes in Aqueous Solutions, Plenum, New York, 1996.
[3] a) R. C. Hider, S. Singh, J. B. Porter, Proc. R. Soc. Edinburgh Sect. B
1992, 99, 137 ± 168; b) R. C. Hider, A. D. Hall, Prog. Med. Chem. 1991,
28, 40 ± 173.
[4] Yu. I. Ryabukhin, N. V. Shibaeva, A. S. Kuzharov, V. G. Korobkova,
A. V. Khokhlov, A. D. Garnovskii, Koord. Khim. 1987, 13, 869 ± 874;
Yu. I. Ryabukhin, N. V. Shibaeva, A. S. Kuzharov, V. G. Korobkova,
A. V. Khokhlov, A. D. Garnovskii, Sov. J. Coord. Chem. (Engl.
Transl.) 1987, 13, 493 ± 499.
[5] Prepared by the heating of equimolar amounts of 2-(2-hydroxyphe-
nyl)benz[e]-1,3-oxazin-4-one and 4-hydrazinobenzoic acid in ethanol
(R. Lattmann, P. Acklin (Novartis AG), WO-A 9749395A1 1997
[Chem. Abstr. 1998, 128, 114953e]). 1H NMR (500 MHz, [D6]DMSO,
TMS): d ˆ 6.88 (d, 1H), 7.01 (m, 3H), 7.38 (m, 2H), 7.55 (m, 3H), 8.00
(d, 2H), 8.06 (d, 1H), 10.02 (s, 1H), 10.78 (s, 1H), 13.15 (br, 1H);
13C NMR (125 MHz, [D6]DMSO, TMS): 19 signals in the range d ˆ
113.6 ± 166.4. The product gave correct elemental analyses for
C21H15N3O4.
‡
formation of [Al(HL)] proceeds approximately 200 times
slower (k ˆ 3.2 s 1 mol 1 dm3 at pH 3.5).[16] The formation
constants of all the AlIII complexes were determined potentio-
metrically.[11] Corresponding studies were also performed for
MgII, CaII, CuII, and ZnII. The determination of the formation
constants of the CuII and ZnII complexes proved, however,
difficult since the formation of solid phases was observed
during the titration.[17] In contrast such precipitation did not
occur for MgII and CaII.
The results of the equilibria studies are summarized in
Table 1. The exceptionally high affinity of L3 for FeIII is
demonstrated by the value of 38.6 for lgb(FeL2). However,
the high stability of [AlL2]3 is a further remarkable feature.
[b]
Table 1. Overall formation constants[a] lgbxy for the complexes [MLxHy]
[6] H. P. Schnebli, Brit. J. Haematol. 1998, 102, 280 (Abstr. Pap. ISH-
EHA Combined Haematology Congress, Amsterdam); a comprehen-
sive report of the biochemical investigations is in preparation.
(L ˆ 4-[3,5-bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]benzoic acid) in H2O/
DMSO.[c]
‡
[7] UV/Vis data: [FeHL] : lmax ˆ 512 nm (e ˆ 3.1  103 dm3 mol 1 cm 1);
MgII
7.6
CaII
5.5
CuII
18.8
ZnII
13.3
AlIII
FeIII
[FeL2]3 : lmax ˆ 403 nm (sh, e ˆ 5.0  103 dm3 mol 1 cm 1), 467 nm (sh,
e ˆ 3.0  103 dm3 mol 1 cm 1). The stoichiometry of the two complexes
can be shown unambiguously with Job diagrams. [FeL2]3 was also
isolated (from MeOH/H2O by addition of EtOH) as the sodium salt in
solid form. Elemental analysis (%) calcd for C42H24N6O8FeNa3 ´
10.75H2O: C 47.63, H 4.33, N 7.93, O 28.32, Na 6.51, Fe 5.27; found:
C 47.79, H 4.25, N 7.84, O 28.38, Na 6.34, Fe 5.31.
[ML]
[M(HL)]
[ML2]
[ML(HL)]
[M(HL)2]
19.8
24.1
34.0
39.4
44.7
23.3
27.5
38.6
44.4
48.7
23.9
17.5
[a] bxy ˆ [MLxHy][M] 1 [L] x [H] y. [b] The estimated standard deviations
are 0.1 or less. [c] Mole fraction for DMSO: 0.20. For the refinement of the
formation constants, the pKa values of H3L (4.62, 10.13, 12.09) as well as the
total concentration of the reactants were kept constant.
[8] According to molecular mechanics calculations a facial coordination is
clearly of higher energy due to significant strain: program Macro-
Model V4.0 (F. Mohamadi, N. G. J. Richards, W. C. Guida, R.
Liskamp, M. Lipton, C. Caufield, G. Chang, T. Hendrickson, W. C.
Still, J. Comput. Chem. 1990, 11, 440), AMBER ‡ -force field (S. J.
Weiner, P. A. Kollman, D. A. Case, U. C. Singh, C. Ghio, G. Alagona,
S. Profeta, Jr., P. Weiner, J. Am. Chem. Soc. 1984, 106, 765; S. J.
Weiner, P. A. Kollman, D. T. Nguyen, D. A. Case, J. Comput. Chem.
1986, 7, 230), modified for the calculation of FeIII complexes.
The highly oxophilic AlIII is generally believed to have a low
affinity for nitrogen donors. The unexpectedly high stability of
the AlIII complex could be a result of steric factors. The rigid
orientation of the donor set having an ideal preorientation,
along with the fact that six-membered chelate rings are
exclusively formed, obviously favors complex formation with
the small AlIII cation in particular.[18] In contrast, the affinity
for CuII and ZnII is relatively low, although a preference for
nitrogen donors is well established for these metal centers.[19]
A rather low affinity for this ligand is also observed with MgII
and CaII. Particularly, the coordination of a second ligand
[9] Bruker Avance DRX 500 spectrometer (resonance frequency:
500.13 MHz for 1H), [D6]DMSO/D2O (2/8), total concentration
3
3
[mol dm 3]: 2 Â 10 for L and 0.86 Â 10 for Al.
3
[10] Solubility of H3L at pH 7.4 in H2O: 0.4 gdm
.
[11] All solutions contain the same proportion of DMSO as well as
0.1 moldm 3 KNO3. The measurements were performed at 25.0 Æ
0.18C. Because of the constant ionic strength the equilibrium
constants are listed as concentration quotients, and the pH value is
‡
defined as lg[H3O ]. The calibration of the pH electrode (titration
of a 2 Â 10 3 moldm 3 HNO3 solution with KOH) resulted in a value of
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‡
lgKw ˆ 15.56 for Kw ˆ [OH ][H3O ]. The equipment has already
been described previously (K. Hegetschweiler, T. Kradolfer, V.
Gramlich, R. D. Hancock, Chem. Eur. J. 1995, 1, 74 ± 88). The
computer programs SUPERQUAD (P. Gans, A. Sabatini, A. Vacca,
J. Chem. Soc. Dalton Trans. 1985, 1195) and BEST (R. J. Motekaitis,
A. E. Martell, Can. J. Chem. 1982, 60, 2403) were used for evaluation.
The complex formation generally took place at such low pH values
that hydrolysis of the aqua ions could be neglected.
Supramolecular Organometallic Polymer
Chemistry: Self-Assembly of a Novel
Poly(ferrocene)-b-polysiloxane-b-poly-
(ferrocene) Triblock Copolymer in Solution**
Rui Resendes, Jason A. Massey, Hendrik Dorn,
K. Nicole Power, Mitchell A. Winnik,* and
Ian Manners*
[12] Spectrophotometric methods were less suited for the evaluation of
these subsequent reactions since the spectra of the various protonated
complexes differed only slightly. An evaluation with the program
SPECFIT[14] in the range 3.4 < pH < 6.4 yielded the following overall
formation constants: lgb ˆ 23.5(2) ([FeL]), 38.2(2) ([FeL2]3 ), 44.2(2)
([FeL(HL)]2 ). The formation of [Fe(HL)2] , which according to
potentiometric measurements (Figure 1) is present to an extent of
approximately 10% at pH 5, could not be verified with this method.
The immiscible segments of a diblock copolymer are known
to facilitate self-assembly into a variety of morphologies in the
solid state.[1, 2] In solution, the formation of spherical micelles
in a block-selective solvent with a core of the insoluble block
surrounded by a corona of the soluble block is well known.
However, until recently other morphologies have been rare,
and even now the number of systems leading to nonspherical
morphologies is limited, and our understanding of them
remains poor.[3, 4] These phenomena provide an attractive and
potentially powerful route to nanostructured materials, as
illustrated by recent studies of block copolymer films and
solution micelles containing metal or semiconductor nano-
particles in desired domains.[5]
As part of our research on novel poly(ferrocene) block
copolymers,[6, 7] we recently reported studies of the self-
assembly of the organometallic ± inorganic block copolymer
poly(ferrocenyldimethylsilane)-b-poly(dimethylsiloxane)
(PFS-b-PDMS, 1; PFS:PDMS block ratio 1:6, Mn ˆ 3.51  104,
polymer dispersity index (PDI) ˆ 1.10) in the solid state and in
solution.[8] Thin films of this material self-assemble to form a
Figure 1. Species distribution (component x [%]) for a complex solution
([Fe]t ˆ 5  10 4 moldm 3, [L]t ˆ 10 3 moldm 3 in H2O/DMSO (8/2)). Only
the Fe-containing species are shown. The formation constants and the pKa
values from Table 1 were used for the calculations.
[13] A species of the composition Fe:L:H ˆ 1:1:0 can also be interpreted in
terms of a binuclear complex [(HL)Fe-(m-OH)2-Fe(HL)] or [(HL)Fe-
(m-O)-Fe(HL)] (D. M. Kurtz, Jr., Chem. Rev. 1990, 90, 585 ± 606). The
evaluation of the titration curve according to this model, however, led
to less satisfactory agreement (spH ˆ [Sw(pHfound pHcalcd)2/Sw]1/2
ˆ
0.0067 for [(HL)2Fe2(OH)2], spH ˆ 0.0023 for [FeL]).
‡
[14] The determination was performed with solutions of 0.01 < [H ] <
0.1 moldm 3, [Fe]t ˆ [L]t ˆ 1.6  10 4 moldm 3, and m ˆ 0.1 (KNO3)
using a diode array spectrophotometer TIDAS-UVNIR/100-1 from
J&M with a HELLMA immersion probe and least squares calcula-
tions for evaluation (R. A. Binstead, A. D. Zuberbühler, SPECFIT
version2.10, Spectrum Software Associates, Chapel Hill, NC 27515-
4494 (USA); see also H. Gampp, M. Maeder, C. J. Meyer, A. D.
Zuberbühler, Talanta 1985, 32, 257 ± 264).
hexagonal array of PFS cylinders within a PDMS matrix,
whereas in n-hexane, a solvent selective for the PDMS block,
cylindrical micelles are formed with a core of PFS surrounded
by a sheath of PDMS. As the poly(ferrocene) homopolymer
becomes semiconducting on oxidative doping and forms
[15] The measurements were performed at 258C on a DX17MV instru-
ment (Applied Photophysics).
[16] The average residence time of a water ligand in [Al(OH2)6]3‡ is
approximately 150 times higher than for [Fe(OH2)6]3‡ (D. T. Richens,
The Chemistry of Aqua Ions, Wiley, Chichester, 1997).
[*] Prof. M. A. Winnik, Prof. I. Manners, R. Resendes, J. A. Massey,
Dr. H. Dorn, K. N. Power
[17] By analogy with the compounds reported by Ryabukhin,[4] these are
probably polymeric complexes. The C,H,N analysis of the Cu complex
is in agreement with the formulation Cu(HL) ´ 2H2O. To prevent the
formation of such solid phases, a competition method was used in
which the metal cation M2‡ was applied in the presence of H3L as
either the nitrilotriacetato (NTA) or iminodiacetato (IDA) complex
and converted into the 1:2 complex [ML2]4 by addition of KOH. All
the necessary pKa values and stability constants of H3NTA and H2IDA
for the H2O/DMSO system were determined separately.
Department of Chemistry
University of Toronto
80 St. George Street
Toronto, Ontario, M5S 3H6 (Canada)
Fax: (‡1)416-978-6157
E-mail: imanners@alchemy.chem.utoronto.ca
[**] This research was supported by the Natural Science and Engineering
Research Council of Canada (NSERC). R.R. is grateful for an
Ontario Graduate Scholarship, H.D. is grateful to the DFG for a
Postdoctoral Fellowship, and K.N.P. is grateful to NSERC for a
Postgraduate Scholarship. In addition, I.M. is grateful to the Alfred P.
[18] R. D. Hancock, J. Chem. Educ. 1992, 69, 615 ± 621.
[19] M. Ghisletta, L. Hausherr-Primo, K. Gajda-Schrantz, G. Machula, L.
Nagy, H. W. Schmalle, G. Rihs, F. Endres, K. Hegetschweiler, Inorg.
Chem. 1998, 37, 997 ± 1008.
Sloan Foundation for
a Research Fellowship (1994 ± 1998), the
NSERC for a Steacie Fellowship (1997 ± 1999), and the University
of Toronto for a McLean Fellowship (1997 ± 2003).
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