Download Iron(III)- and copper(II) complexes of an asymmetric, pentadentate

JOURNAL OF
Inorganic
Biochemistry
Journal of Inorganic Biochemistry 99 (2005) 1480–1489
www.elsevier.com/locate/jinorgbio
Iron(III)- and copper(II) complexes of an asymmetric, pentadentate
salen-like ligand bearing a pendant carboxylate group
Attila Jancsó
a,1
, Zoltán Paksi b, Satu Mikkola a, Antal Rockenbauer c, Tamás Gajda
b,*
a
b
Department of Chemistry, University of Turku, Vatselankatu 2. FIN-20014 Turku, Finland
Department of Inorganic and Analytical Chemistry, University of Szeged, H-6701 Szeged, P.O. Box 440, Hungary
c
Chemical Research Center, Institute of Chemistry, H-1525 Budapest, P.O. Box 17, Hungary
Received 16 November 2004; received in revised form 7 April 2005; accepted 7 April 2005
Available online 31 May 2005
Abstract
The equilibrium and solution structural properties of the iron(III) and copper(II) complexes of an asymmetric salen-like ligand
(N,N 0 -bis(2-hydroxybenzyl)-2,3-diamino-propionic acid, H3bhbdpa) bearing a pendant carboxylate group were characterized in
aqueous solution by potentiometric, pH-dependent electron paramagnetic resonance (EPR) and UV–Vis (UV–Visible) measurements. In the equimolar systems the pentadentate ligand forms very stable, differently protonated mononuclear complexes with both
metal ions. In the presence of iron(III) {NH, PhO, COO}, {2NH, 2PhO, COO} and {2NH, 2PhO, COO, OH} coordinated
complexes are dominant. The EPR titrations reflected the presence of microscopic complex formation pathways, leading to the formation of binding isomers in case of Cu(H2bhbdpa)+, Cu(Hbhbdpa) and Cu(bhbdpa). The {2NH, 2PhO + COO/H2O} coordinated Cu(bhbdpa) is the only species between pH 6–11. At twofold excess of metal ion dinuclear complexes were detected with
both iron(III) and copper(II). In presence of iron(III) a l-carboxylato-l-hydroxo-bridged dinuclear complex (Fe2(bhbdpa)(OH)3)
is formed from Fe(H2bhbdpa)2+ through overlapping proton release processes, providing one of the rare examples for the stabilization of an endogenous carboxylate bridged diiron core in aqueous solution. The complex Cu2(bhbdpa)+ detected in the presence
of copper(II) is a paramagnetic (S = 1) species with relatively weakly coupled metal ions.
2005 Elsevier Inc. All rights reserved.
Keywords: Salen-like ligands; Iron(III) complexes; Copper(II) complexes; Equilibrium and solution structural studies
1. Introduction
Metal complexes of salen (N,N 0 -ethylenebis(salicylideneimine)) and its derivatives have been extensively
studied as model complexes of the active sites of mononuclear dioxygen activating enzymes, such as some irondependent dioxygenases [1–3] or galactose oxidase [4–6].
Besides, a number of complexes formed with salen-like
ligands were used as catalysts for diverse organic redox
*
Corresponding author. Tel.: +36 62544054; fax: +36 62420505.
E-mail addresses: [email protected], gajda@chem.
u-szeged.hu (T. Gajda).
1
Present address: Department of Inorganic and Analytical Chemistry, University of Szeged, P.O. Box 440, H-6701 Szeged, Hungary.
0162-0134/$ - see front matter 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jinorgbio.2005.04.006
reactions [7–9]. The lack of conjugation in the reduced
salen derivatives results in more flexible metal binding
properties and resistance to hydrolysis in aqueous solutions. Nevertheless, limited data are available on the
iron(III) [10] and copper(II) [11,12] complexes of N,N 0 bis(2-hydroxybenzyl)-1,2-diaminoethane ([H4]salen) or
its alkyl-substituted derivatives [13,14]. Since the structural mimicking of the above mentioned metalloenzymes would require non-planar coordination of the
ligand, mostly tripodal compounds were used for this
purpose. However, salen-like compounds are known to
easily adopt the non-planar cis-b configuration [1–3],
and this is also proved for the [H4]salen derivatives
[10]. Some hexadentate [H4]salen-like compounds with
two additional carboxylate groups are widely studied
A. Jancsó et al. / Journal of Inorganic Biochemistry 99 (2005) 1480–1489
O
HO
+
+
NH2
NH2
OH
HO
Scheme 1. Schematic structure of the ligand H5bhbdpa2+.
as strong metal ion chelators [15–17], however, to our
knowledge, no data are available on pentadentate
[H4]salen derivatives.
Carboxylate bridged dimetallic cores are very common structural motifs in the active center of metalloenzymes. The design of their low molecular weight models
with endogenous carboxylate is a great challenge in
bioinorganic chemistry. In most cases an alternative approach, using non-carboxylato (e.g. alkoxo or phenoxo)
bridging units have been utilised to construct homo- and
heterodinuclear complexes [18–21]. Up to date, only a
few examples are known for the bridging coordination
of an endogenous carboxylate [22–25].
In this work the solution chemical properties of
the iron(III) and copper(II) complexes formed with a
new, asymmetric pendadentate [H4]salen derivative
(N,N 0 -bis(2-hydroxybenzyl)-2,3-diamino-propionic acid,
H3bhbdpa, Scheme 1) are described. The additional carboxylate group may notably alter the structure and stability of the formed complexes as compared to the
corresponding species of salen or [H4]salen, and create
a possibility for the formation of dinuclear units. The
mononuclear complexes of bhbdpa may be of relevance
to structural mimicking of the active center of protocatechuate 3,4-dioxygenase [26] and galactose oxydase
[27], while the dinuclear complexes of bhbdpa provide
new examples for carboxylate bridged dimetallic cores.
2. Experimental
2.1. Materials
Copper(II) and iron(III) chloride (Fluka) solutions
were standardized complexometrically. NaOH (Fluka)
standard solutions were used for the titrations. D,L-2,3diaminopropionic acid hydrochloride (Sigma), salicylaldehyde (Aldrich), N-ethyldiiso-propylamine (Fluka) and
sodium borohydride (Fluka) were used without further
purification.
2.2. Preparation of the ligand – (N,N 0 -bis(2hydroxybenzyl)-2,3-diamino-propionic acid (H3bhbdpa)
D,L-2,3-diaminopropionic acid hydrochloride (dpa)
(5 g, 35.6 mmol) was stirred in 100 mL methanol for
1481
0.5 h, meanwhile 9.6 g (74.3 mmol) N-ethyldiisopropylamine was added for neutralization. To this mixture
8.87 g (72.6 mmol) salicylaldehyde, dissolved in methanol, was added in a few portions. The colour of the solution continuously changed to yellow, and the
undissolved dpa completely disappeared after 6 h, as a
result of Schiff-base formation. During rapid stirring sodium borohydride (6 g, 159 mmol) was very carefully
added to the yellow solution being cooled on an ice
bath. The addition of reductant resulted in a white,
dense precipitate, which was filtered off. The crude product was suspended into 100 mL methanol, stirred for 3 h
then filtered off again and recrystallized from 1500 mL
methanol. Yield: 6.9 g, 61.3%. The structure and purity
was confirmed by nuclear magnetic resonance (NMR)
spectroscopy and potentiometry. 1H NMR (in D2O, d
(ppm), tentative assignment): d (range for the 2 · 4 aromatic protons) = 7.35–7.23 (m, 1H + 1H and 1H + 1H,
ArH), 6.95–6.89 (m, 1H + 1H and 1H + 1H, ArH),
4.254 (s, 2H, Ar–CH2–), 4.272 and 4.246 (d and d,
3
J = 13.2 Hz,
1H + 1H,
Ar–CH2–),
3.857
(t,
3
J = 7.5 Hz, 1H, –CH–CH2), 3.392 (d, 3J = 7.5 Hz,
2H, CH–CH2–NH). No other signal was detected.
2.3. pH-metric measurements
The protonation and complex formation equilibria
were investigated by potentiometric titration in aqueous
solution (I = 0.1 M NaCl, and T = 298 ± 0.1 K) using
an automatic titration set including a Dosimat 665
(Metrohm) autoburette, an Orion 710A precision digital
pH-meter and an IBM-compatible PC. The Orion
8103BN semimicro pH glass electrode was calibrated
[28] via the modified Nernst equation Eq. (1):
E ¼ E0 þ K log½Hþ þ J H ½Hþ þ
J OH K w
½Hþ ð1Þ
where JH and JOH are fitting parameters in acidic and
alkaline media for the correction of experimental errors,
mainly due to the liquid junction and to the alkaline and
acidic errors of the glass electrode; Kw = 1013.75 M2 is
the autoprotolysis constant of water [29]. The parameters were calculated by a non-linear least squares method. The complex formation was characterized by the
following general equilibrium process Eq. (2):
bMp Lq Hr
pM þ qL þ rH $ Mp Lq Hr
bMp Lq Hr ¼
½Mp Lq Hr ½Mp ½Lq ½Hr
ð2Þ
ð3Þ
where M denotes the metal ion and L the fully deprotonated ligand molecule. All complexes in this report are
suggested to have octahedral geometry, where the free
coordination sites are occupied by water molecules.
The neutral form of the ligand can be described with
1482
A. Jancsó et al. / Journal of Inorganic Biochemistry 99 (2005) 1480–1489
the composition of LH3. The corresponding formation
constants ðbMp Lq Hr bpqr Þ were calculated using the
computer program PSEQUAD [30].
The protonation and the formation constants were
determined from 4 to 10 independent titrations (50–70
data points per titration). The metal-to-ligand ratios
varied between 1:2 and 2:1, with the metal ion concentration ranged from 4.2 · 104 to 3.1 · 103 M, and
from 2.1 · 103 to 4.1 · 103 M in the case of iron(III)
and copper(II), respectively.
2.4. Electronic absorption measurements
UV–Vis (UV–Visible) spectra were measured either
on a Hewlett Packard 8452A diode array or a UNICAM
HELIOSa spectrophotometer. The individual spectra of
the metal complexes were calculated by the previously
mentioned PSEQUAD computer program [30].
2.5. Electron paramagnetic resonance measurements
A 12 cm3 stock solution was titrated under argon
atmosphere and the bubbling inert gas was also used
for mixing the sample. The initial concentration of copper(II) was 2.15 · 103 and 1.87 · 103 M in the 1:1 and
1:2 ligand-to-metal systems, respectively. A Masterflex
CL peristaltic pump ensured the circulation
(14 cm3 min1) of the solution through the capillary
tube in the cavity. The EPR spectra were taken after
equilibration/circulation for 3 min at a chosen pH at
room temperature (T = 298 K) on an upgraded JEOLJES-FE3X spectrometer with 100 kHz field modulation,
using a manganese(II)-doped magnesium oxide powder
for the calibration of g. The series of EPR spectra (16
spectra) were evaluated by a recently developed two
dimensional simulation method able to adjust the formation constants of the various species together with
the magnetic parameters of the component EPR spectra
[31]. Structural isomers of certain complexes were considered in the calculation. The exclusion of any of these
isomers decreased the otherwise very high regression
parameter (R = 0.99843) by a factor that was in any case
over a magnitude larger than the significance level of a
species (DRsign. = 1.1 · 105). Further details of the
measurements and the detailed evaluation procedure
were described previously [31,32].
2.6. NMR measurements
1
H NMR experiments were performed on a Bruker
Avance DRX 500 spectrometer. The chemical shifts d
were measured with respect to dioxane as internal reference and converted relative to SiMe4, using ddioxane =
3.70 ppm. The ligand concentration was 0.005 M. Measurements were made in H2O–D2O (95:5).
3. Results and discussion
3.1. Protonation equilibrium of bhbdpa
The protonation constants and pK values derived
from the potentiometric titrations are listed in Table 1.
The pK of the carboxyl group is too low to be determined,
which is due to the strong electron withdrawing effect of
the neighbouring protonated amino groups. The tetraprotonated LHþ
4 is the only species in the solution between pH 2–4. Its deprotonation (pK = 5.90) is well
Table 1
Formation constants (logb) of the proton, iron(III) and copper(II) complexes of the ligand (with the estimated errors in parentheses (last digit))
together with some derived data (I = 0.1 M NaCl, T = 298 K)
Species
logbpqr
014
013
012
011
35.76(2)
29.86(1)
21.39(1)
11.23(6)
Iron(III)
113
112
111
110
11–1
210
21–2
21–3
Exp. points
Fitting parameter (cm3)
pK
pK1
pK2
pK3
pK4
pK5
34.93(1)
28.63(2)
21.47(3)
Copper(II)
pKpqra
34.96(5)
31.71(2)
27.12(3)
21.58(3)
pK113
pK112
pK111
pK110
pK212
<1.5
5.90
8.47
10.16
11.23
Iron(III)
–
7.16
3.06
Copper(II)
3.25
4.59
5.54
–
24.91(8)
26.36(4)
23.30(2)
638
0.012
315
0.005
Formation constants for the monochloro-complex of iron(III) and for three hydrolysis species (extrapolated for 0.1 M ionic strength) have been used
for the calculations: log KFeCl2þ ¼ 0.86 [36,37], logb101 = 2.19, logb102 = 6.20, logb202 = 2.84 [34,35].
a
pKpqr = logbpqrlogbpq(r1).
A. Jancsó et al. / Journal of Inorganic Biochemistry 99 (2005) 1480–1489
+
6
-
H4L
H2L
3-
L
2-
HL
-
3
+
H4L
0
220
H3L
240
H2L
3-
L
260
280
λ / nm
300
320
Fig. 1. Molar UV absorption spectra of the differently protonated
bhbdpa molecules (I = 0.1 M NaCl, T = 298 K).
separated from the following three consecutive deproto2
nation steps leading to LH
and L3. Considering
2 , LH
the two identical 2-hydroxy-benzylamino moieties of the
ligand, the above mentioned significant separation of pKs
suggests the formation of strong hydrogen bonds between the phenolic hydroxyl groups and the unprotonated amino group(s). Strong intramolecular hydrogen
bonds have been also suggested for the closely related reduced [H4]salen [11]. The presently determined pK2 is
lower, while pK3, pK4 and pK5 are higher than the corresponding values of [H4]salen. This indicates a somewhat
even stronger hydrogen bonding network within the molecule, probably due to the participation of the additional
carboxylate group. The two highest pK of [H4]salen [11]
and those of some related ligands [15,16] were assigned
to the deprotonation of the phenolic oxygens. In order
to check the validity of this assignation in our case, the
equilibrium processes were followed by UV spectroscopy, since the deprotonation of a phenolic hydroxyl
group results in significant changes on its electronic spectrum. The individual molar spectra of the differently protonated species are depicted in Fig. 1 (see also Figure S1
in the Supplementary Material). According to our data,
þ
the process LHþ
4 ¼ LH3 þ H can be assigned, indeed,
to the deperotonation of an amino group, since practically no spectral change occured during this step (Fig.
1). However, the further deprotonation processes are followed by a continuous increase of the two absorption
bands (at 238 and 292 nm) attributed to the well-known
intramolecular transitions of the phenolate rings [33].
Consequently, the three consecutive deprotonations of
LH3 are better characterised by the term of micro-deprotonation processes related to the second amino group and
the two phenolic –OH groups.
3.2. Iron(III) complexes of bhbdpa
The complex formation constants derived from the
potentiometic titrations performed at different Fe(III)/
bhbdpa ratios, together with some calculated data, are
100
110
11-1
80
112
60
III
9
% Fe
2-
HL
40
100
21-2
20
21-3
100Cl
0
(a)
2
4
6
pH
8
10
100
21-3
80
60
III
ε x 10-3 / M-1 cm-1
12
summarized in Table 1. In the evaluation of potentiometric data, the presence of three hydroxo [34,35] and
the monochloro complex of iron(III) [36,37] have been
taken into calculation (see Table 1). Due to the high
affinity of bhbdpa to iron(III), which is reflected by
the very high logb values in Table 1, the above mentioned complexes are only minor species in the whole
pH range. At equimolar ratio of bhbdpa and iron(III)
mainly mononuclear complexes are dominant in the
solution (Fig. 2(a)). Between pH 2 and 3 the species
Fe(H2bhbdpa)2+ is formed which is transformed into
Fe(bhbdpa) by losing two protons in a strongly cooperative manner. The complex Fe(bhbdpa) is dominant in a
wide pH range (3.5–7). The hydrolysis of this species
takes place with a pK of 7.16, leading to
Fe(bhbdpa)(OH). These processes are followed by significant UV–Vis spectral changes, therefore combined
pH-metric/spectrophotometric titrations were also performed to assist the interpretation of the pH-metric
data and to obtain information about the coordination
% Fe
3-
L
15
1483
40 100
112
21-2
20
100Cl
0
(b)
110
2
3
4
pH
5
6
Fig. 2. Species distribution curves of the bhbdpa–iron(III) 1:1 (a) and
1:2 (b) systems ([FeIII] = 7.5 · 104 (a) and 1.5 · 103 M (b), I = 0.1 M
NaCl, T = 298 K). The complexes are marked by their pqr numbers:
100 = Fe3+, 112 = Fe(H2bhbdpa)2+, 110 = Fe(bhbdpa), 11–1 =
21–2 = Fe2(bhbdpa)(OH)2+,
21–3 = Fe2
Fe(bhbdpa)(OH),
(bhbdpa)(OH)3. The chlorocomplex FeCl2+ is depicted as 100Cl.
The dashed lines stand for the sum of the hydrolysis products of
iron(III).
1484
A. Jancsó et al. / Journal of Inorganic Biochemistry 99 (2005) 1480–1489
environment of the metal ion in the different complexes.
The pH-dependent electronic spectra of the equimolar
system are depicted on Fig. 3(a). The spectral characteristics of the complexes are rather similar in the UVrange, all species possess an absorption band around
270–280 nm and a low energy shoulder at 300–315 nm
which are tentatively assigned to the blue-shifted p–p*
transition of the phenolate rings and to a pp–dr* phenolate to iron(III) charge transfer (CT) transition [33],
respectively. The high energy intraligand transition band
observed for the free, fully deprotonated ligand
(kmax = 238 nm) is also shifted to 220–225 nm as a result of metal ion coordination. The UV–Vis spectra of
the complexes also exhibit an intensive band in the range
of 400–600 nm. Since high spin octahedral iron(III)
complexes have very weak, spin forbidden d–d transitions, the observed intensive band can be assigned to a
pp–dp* phenolate to iron(III) charge transfer transition
[33]. The spectral characteristics of the individual species
are summarized in Table 2. These data indicate the coor-
1.5
1.2
pH
A
0.9
0.6
pH=11.0
pH=5.0
0.3
0.0
(a)
300
400
500
λ / nm
600
700
dination of a phenolate oxygen to iron(III) in the complex Fe(H2bhbdpa)2+. Although several possibilities
can be enumerated, the tridentate {NH, PhO, COO}
type coordination is the most probable one, resulting in
the formation of fused (6,5)-membered chelate rings
(Scheme 2). Indeed, the kmax value of the CT transition,
the
corresponding
molar
absorbance
(e538 1812 M1 cm1, Table 2) and the basicity corrected formation constant determined for Fe(H2bhbdpa)2+ are
similar to those observed for the related mono complex
of the Na-salicyl-L-alanin [38], possessing identical coordination mode. The following cooperative deprotonations leading to the complex Fe(bhbdpa) result in
important spectral changes (Figs. 3(a) and 4(a)). The
55 nm hypsochromic shift of the p–dp* ligand to
metal charge transfer (LMCT) band, the significantly
increased intensity together with the high stability of this
species (Table 1) refer to a rearrangement of the coordination environment of iron(III). The increasing number
of phenolate oxygens bound to iron(III) induces important hypsochromic effect but also intensity increase of
the pp–dp* LMCT band [33]. Accordingly, the presented potentiometric and spectrophotometric data support the coordination of both phenolate and both amino
groups of bhbdpa to iron(III). However, the question
may arise concerning the coordination of the carboxylate group. The high affinity of iron(III) towards oxygen
donors, the fact that only a single hydrolytic process has
been observed up to pH 11.2, as well as the lower stability of the Fe(salen) complex (logK = 25.85 in 80 w/w%
dmso–water [39]) all support the pentadentate binding
of the ligand. It is worth to note that the pentadentate
{N2O3} type coordination in the Fe(bhbdpa) complex
is related to the active center of some mononuclear
non-heme iron(III) dioxygenases, such as protocatechuate 3,4-dioxygenase [26].
1.5
Table 2
UV–Vis spectral data of bhbdpa and its Iron(III) and Copper(II)
complexes (I = 0.1 M NaCl, T = 298 K)
1.2
pH
A
0.9
0.6
pH=5.3
0.3
0.0
(b)
300
400
500
λ / nm
600
700
Fig. 3. UV–VIS absorption spectra measured in the bhbdpa–iron(III)
1:1 (a) and 1:2 (b) systems ([FeIII] = 7.9104 (a) and 1.4 · 103 M (b),
I = 0.1 M NaCl, T = 298 K). The spectral change by increasing pH is
marked by arrows. The pH values belonging to the spectra are as
follows: (a) 1.93, 2.13, 2.33, 2.53, 2.79, 3.05, 3.29, 3.53, 3.93, 4.81, 5.86,
6.33, 6.65, 7.08, 7.24, 7.64, 8.13, 8.93, 9.69, 10.34, 10.99 and (b) 1.90,
2.14, 2.32, 2.54, 2.77, 2.96, 3.16, 3.38, 3.77, 4.43, 5.34.
Species
kmax/nm (e/M1 cm1)
H4L
H3L
H2L
HL
L
–
–
238 (4438)a
238 (10901)
238 (15424)
276
276
278
276
–
iron(III)
Copper(II)
–
406 (396)
396 (621)
385 (1129)
M(H3bhbdpa)
M(H2bhbdpa)
M(Hbhbdpa)
M(bhbdpa)
M(bhbdpa)(OH)
M2(bhbdpa)(OH)2
M2(bhbdpa)(OH)3
a
b
538 (1812)
482
422
475
476
(2492)
(2848)
(1680)b
(2952)
(4166)
(3823)
(3055)
(3334)a
Shoulder.
Data should be considered as rough estimates.
–
–
292 (1839)a
292 (4822)
292 (7045)
717
700
640
596
(45)
(87)
(130)
(221)
A. Jancsó et al. / Journal of Inorganic Biochemistry 99 (2005) 1480–1489
1485
Scheme 2. Proposed schematic structures for the major species formed in the bhbdpa–iron(III) and bhpdpa–copper(II) systems.
Iron(III) complexes having unsaturated coordination
environment tend to form mixed hydroxo complexes
[40]. In our system a single hydrolytic process has been
observed around pH 7, which is followed by a characteristic hypsochromic shift (Fig. 3(a)). The proton release
from a metal bound water molecule raises the electron
density on iron(III), thus increases the energy of the
CT transition. The hydrolysis of the FeL parent complex resulted in the formation of a l-oxo-bridged dimer
in the case of salen (in 80 w/w % dmso–water [41]), a ldihydroxo-bridged dimer with [H4]salen in solid state
[10], while a monomer hydroxo complex (FeL(OH))
3.0
21-3
11-1
110
ε x 10-3 / M-1 cm-1
2.5
2.0
112
1.5
1.0
0.5
0.0
400
450
500
550
600
λ / nm
650
700
750
Fig. 4. Individual VIS absorption spectra of the main complexes
formed in the bhbdpa–iron(III) system (I = 0.1 M NaCl, T = 298 K).
The complexes are marked by their pqr numbers (for the notations see
Fig. 2).
has been detected with a closely related pentadentate ligand [40]. In our case the potentiometric data and the
observed spectral changes strongly suggest the formation of the monomeric Fe(bhbdpa)(OH) complex, too.
At twofold excess of metal ion, no precipitate formation has been observed up to pH 6. Beside the already
mentioned mononuclear complexes, two dinuclear species are formed under this condition. The electronic spectra recorded between pH 2 and 3 clearly show the presence
of the species Fe(H2bhbdpa)2+, similarly to the 1:1 system
(Fig. 3(b)). Further increase of pH results in a continuous
and significant shift of the phenolate–iron(III) CT transition towards the higher energies (kmax 478 nm), with
increasing band intensities. The observed changes are
rather similar to those appeared in the 1:1 system roughly
in the same pH-range, however, the pH-metric data refer
to fundamentally different processes in the equimolar and
1:2 ligand-metal systems (see the titration curves in Figure
S2). After the formation of Fe(H2bhbdpa)2+, five equivalents of protons are released in a rather narrow, strongly
buffered pH region (pH 2.7–4). The evaluation of the
pH-metric data proved the presence of two dinuclear speþ
cies Fe2 ðbhbdpaÞðOHÞ2 and Fe2(bhbdpa)(OH)3, the latter one being dominant above pH 3.5 (Fig. 2(b)). In the
complex Fe(H2bhbdpa)2+ one binding site of the ligand
is occupied by iron(III), but the other ‘‘arm’’ of the molecule may bind to another metal ion in the case of metal excess. According to the pH-metric data, the formation of
the dinuclear complexes also requires proton releases
from the coordinated water molecules. The spectral
changes are in good coherence with these processes. The
increased molar absorbance of Fe2(bhbdpa)(OH)3 as
A. Jancsó et al. / Journal of Inorganic Biochemistry 99 (2005) 1480–1489
100
100
II
% Cu
111
112
60
40
20
0
113
2
4
6
8
10
pH
(a)
100
100
80
60
40
112
210
20
113
111
110
0
2.0
2.5
3.0
3.5
4.0
4.5
5.0
pH
(b)
Fig. 5. Species distribution curves of the bhbdpa–copper(II) 1:1 (a) and
1:2 (b) systems ([CuII] = 2.0 · 103 (a) and 4.0 · 103 M (b), I = 0.1 M
NaCl, T = 298 K). Species are marked by their pqr numbers:
100 = Cu2+, 113 = Cu(H3bhbdpa)2+, 112 = Cu(H2bhbdpa)+, 111 =
Cu(Hbhbdpa), 110 = Cu(bhbdpa), 210 = Cu2(bhbdpa)+.
0.25
110
110
ε x 10-3 / M-1 cm-1
1.0
0.20
0.8
0.6
0.4
0.2
0.0
0.15
111
111
0.10
112
112
113
ε x 10-3 / M-1cm-1
1.2
3.3. Copper(II) complexes of bhbdpa
Due to the weaker Lewis-acidity of copper(II) compared to iron(III), the complex formation processes start
at higher pH (Fig. 5). The successive deprotonations of
Cu(H3bhbdpa)2+, formed between pH 2 and 3, yield the
complexes Cu(H2bhbdpa)+, Cu(Hbhbdpa) and finally
Cu(bhbdpa). The latter one is the only species above
pH 7. The formation constants of above complexes are
listed in Table 1. The equilibrium processes have been
also monitored by combined spectrophotometric/
pH-metric (Table 2, Fig. 6) and pH-dependent EPR
titrations (Table 3 and Fig. 7) to obtain detailed pHdependent information on the solution structure of the
species. In addition, the EPR titrations allowed us to
determine the formation constants, independently from
the pH-metric data.
In the pH domains, where the Cu(H2bhbdpa)+,
Cu(Hbhbdpa) and Cu(bhbdpa) species have the largest
concentrations, the fit of the EPR spectra, characterized
110
80
II
compared to Fe(H2bhbdpa)2+ (Table 2) indicates the
presence of an additional chromophore (a phenolate
bound iron(III)), while the blue shift of the LMCT transition is due to the deprotonation of the coordinated water
molecules. The structure of the two dinuclear species presumably differs only in the protonation state of one coordinated water molecule. The role of the carboxylate group
in the formation of the dinuclear complexes is again an issue to be clarified. In spite of the numerous studies performed, dinuclear iron(III) complexes of tetradentate
salen-like ligands are not reported in the literature. In
the present case, probably the bridging coordination of
the carboxylate group stabilizes the dimetallic core. Due
to geometric reasons, the carboxylate group of bhbdpa
may form only l-1,1-type bridge between the two iron(III) centers. Furthermore, the coordinated hydroxo
group(s) may also form bridge(s). Taking into account
the composition of Fe2(bhbdpa)(OH)3, one l-1,1-bridged
and two terminally coordinated hydroxide ions is the
most plausible assumption (Scheme 2). One has to mention that the formation of a dimer of above dinuclear complex, with a composition of Fe4(bhbdpa)2(OH)6, can not
be ruled out based on our data. However, the formation
of l-oxo-bridges would result in a significantly increased
CT transition around 500 nm [42–44]. The molar extinction coefficient determined for Fe2(bhbdpa)(OH)3 is only
2952 M1 cm1, which falls in the range expected for two
iron(III)-phenolate bonds [33].
The formation of several dimer iron(III) species have
been observed earlier with tetra- or pentadentate ligands
[42–46], generally in non-aqueous medium. However, to
our knowledge, the bhbdpa–iron(III) system is the first
example when a pentadentate ligand is able to hold
two iron(III) in aqueous solution by forming a dinuclear
structure.
% Cu
1486
0.05
113
400
500
600
700
λ / nm
800
0.00
900
Fig. 6. Individual UV–VIS absorption spectra of the complexes
formed in the bhbdpa–copper(II) system (I = 0.1 M NaCl,
T = 298 K). Species are marked by their pqr numbers (for the
notations see Fig. 5).
by the R regression parameter, was not satisfactory.
This indicated that additional species have to be taken
into account. Since the presence of further complexes
A. Jancsó et al. / Journal of Inorganic Biochemistry 99 (2005) 1480–1489
1487
Table 3
EPR parameters and formation constants (logb) of the complexes formed in the bhbdpa–copper(II) system, calculated from the pH-dependent EPR
experimentsa (I = 0.1 M NaCl, T = 298 K, the estimated errors for the log b, g0, A0, AN values and for the relaxation parameters are ±0.1, ±0.0006,
±0.4G, ±1.0G and 0.6G, respectively)
Species
g0
A0 b
AN0 b
AN00 b
ab
bb
cb
{donor set}c
logbid
logbe
Xif
Cu2þ
aq.
2.199
2.158
2.140
2.142
2.126
2.111
2.115
2.110
2.159
33.5
54.9
62.3
74.5
63.9
57.7
80.3
78.1
35.3i
–
10.0
10.5
12.7
11.4
12.7
10.7
10.4
25.9j
–
–
2.9g
8.8
8.2
0.0
8.9
10.4
25.9j
52.0
41.5
30.0
37.9
25.5
57.4
20.9
27.4
49.1
2.1
15.2
15.3
20.0
14.4
12.1
13.0
12.3
0.6
0.3
1.8
1.6
3.3
3.5
7.9
3.8
0.2
2.5
–
{NH, COO, 2H2O}
{NH, PhO,2O}
{2NH, 2O}
{2NH, PhO,O}
{NH,2PhO,O}
{2NH, 2PhO}h
{2NH, 2PhO}
{NH, PhO,2O}
–
–
31.94
31.38
27.19
26.89
21.54
21.47
–
–
35.42
32.05
–
–
0.78
0.22
0.67
0.34
0.54
0.46
–
Cu(H3bhbdpa)2+
Cu(H2bhbdpa)+(a)
Cu(H2bhbdpa)+(b)
Cu(Hbhbdpa) (a)
Cu(Hbhbdpa) (b)
Cu(bhbdpa) (a)
Cu(bhbdpa) (b)
Cu2(bhbdpa)+
a
b
c
d
e
f
g
h
i
j
27.37
21.80
24.65
The details of the calculation are described in Ref. [31].
Parameters are given in Gauss.
Proposed donor set in the equatorial plain of copper(II), PhO= phenolate oxygen, O = carboxylate or water oxygen.
Individual formation constants of the isomers.
Macroscopic formation constants.
Mole fraction of the isomers (Xi = bi/b).
The value of this AN’’ parameter may be considered as zero.
Axial coordination of the carboxylate group.
As the J-exhange is much stronger than the hyperfine coupling, the obtained A0 is a half value.
The unusually large AN0 AN00 values might originate from the broad hf lines.
113
ment of the fit. We used a criterium of normalized
regression Rn defined as
1 Rn ¼ N ð1 RÞ;
112
(b)
112
(a)
111
(b)
111
(a)
110
(b)
110
(a)
210
(b)
3000
3200
3400
3600
Magnetic field / G
Fig. 7. Individual EPR-spectra of the species formed in the bhbdpa–
copper(II) system (I = 0.1 M NaCl, T = 298 K). Species are marked by
their pqr numbers (for the notations see Fig. 5).
with different compositions were supported by neither
the pH-metric nor the EPR data, binding isomers of
above complexes were considered. In order to justify
the necessity of new species, we analysed the improve-
which indicates if the fit is improving faster than the N
number of adjusted parameters, when a new species is
included. For the complete equilibrium model including
9 species and 67 parameters Rn was found to be 0.895,
while in the 8 species models containing 58–59 parameters, where one of the isomers among the Cu(H2bhbdpa)+, Cu(Hbhbdpa) and Cu(bhbdpa) pairs was
omitted, the respective Rn values were all significantly
smaller: 0.881, 0.869 and 0.873.
Table 3 summarizes the microscopic and macroscopic
formation constants, the EPR parameters and the suggested coordination environment of the formed complexes. The macroscopic formation constants
calculated by the two independent methods (pH-metry
and EPR) agree reasonably well with each other (see Tables 1 and 3).
The pH-metric and spectroscopic data are in favour
[47,48] of the amino-acid, i.e. {NH,COO} type coordination of the metal ion in Cu(H3bhbdpa)2+. The subsequent deprotonations are followed by profound changes
both on the UV–Vis (Fig. 6 and S3) and the EPR spectra
(Fig. 7 and S4). The appearance of a new electronic
absorption band at 406 nm, which is assigned to a
d–p* copper(II) to phenolate charge transfer transition
[12], indicates phenolate binding in Cu(H2bhbdpa)+
and in the succeeding complexes. It is noteworthy, that
the intensity of this CT transition continuously increases
during the three subsequent deprotonations (Fig. 6).
This indicates that microscopic equilibria are operating
1488
A. Jancsó et al. / Journal of Inorganic Biochemistry 99 (2005) 1480–1489
during the complex formation processes, confirming the
results obtained by the EPR titrations. The g0 values
and ACu coupling constants determined for the two
binding isomers of Cu(H2bhbdpa)+ suggest {NH,
PhO, 2O} and {2NH, 2O} equatorial coordination
environment (where O denotes either water or carboxylate oxygen) in the (a) and (b) isomers, respectively
(Scheme 2, Table 3). The spectral parameters indicate
further increase of the ligand field strength for the species Cu(Hbhbdpa). The evaluation of EPR data suggests
again the presence of two isomers (Table 3). Although
the EPR parameters of the isomer (b) have higher uncertainty due to the broad spectrum, its low A0 value and
the notably increased intensity of the CT transition at
400 nm suggest {NH, 2PhO, O} type coordination in
this isomer. Its high a relaxation parameter, which results in an unusual shape of the spectrum (Fig. 7), indicates a bulky species with rather loose structure, which
can be explained by the formation of a macrochelate between the two phenolate oxygens. The combination of
the pH-metric, UV–VIS and EPR data refers to
{2NH,PhO, O} binding mode in the (a) isomer.
The next deprotonation (pK = 5.54) leads to the complex Cu(bhbdpa). The high stability and the strongly
blue shifted d-d transition (Dkmax 45 nm compared to
Cu(Hbhbdpa), Fig. 6) of this species indicate the equatorial coordination of two amino and two phenolate donors
around the metal ion. The evaluation of the pH-dependent EPR spectra suggests again the presence of two isomers. Their EPR parameters are only slightly different
and they confirm the aforementioned {2NH, 2PhO}
type coordination in both isomers. The presence of isomers is probably related to the additional coordination
of the carboxylate group (Scheme 2). Indeed, the macroscopic formation constant of Cu(bhbdpa) is nearly identical with the logb110 value determined for the CuL
complexes of N,N 0 -bis(2-hydroxybenzyl)ethylenediamine-N,N 0 -diacetic acid [15] and its diamide derivative
[49] but one log unit higher than that obtained earlier
for the copper(II) complex of [H4]salen [11]. The slight increase of the stability as compared to Cu[H4]salen may
indicate that the carboxylate group is bound in one isomer
while not coordinated in the other one. EPR spectroscopy
is not sensitive for the axial coordination of copper(II) in
case of distorted octahedral geometry, which explains the
small differences between the EPR parameters of the two
isomers. The somewhat smaller a value suggests a slightly
more compact structure for the (a) isomer, which can be
expected in case of the pentacoordinated species.
Potentiometric titrations have been also performed at
twofold excess of metal ion. Beside the differently protonated mononuclear complexes the formation of a
dinuclear Cu2(bhbdpa)+ has been detected, but above
pH 5 precipitate formation prevented the further studies
(the formation of Cu(OH)+ and Cu2(OH)22+ was also
considered [34], but these species are present only above
pH 6). The formation of Cu2(bhbdpa)+ was also proved
by EPR measurements, since above pH 4 a singlet spectrum, belonging to a spin-coupled dimer with S = 1, has
been detected. The DM = 1 EPR-transitions of the triplet isomer (S = 1) have 8/3 times stronger intensity compared to the S = 1/2 species, which was taken into
account in the calculations. The unusual AN values
determined for the EPR-active complex is probably an
artifact which may originate from the broad hyperfine
lines. Considering the structure of the ligand and the relatively weak spin–spin coupling, probably no carboxylate bridge is present between the two {NH, PhO}
coordinated metal ions in this dinuclear species.
4. Conclusions
Solution chemical studies on the iron(III) and copper(II) complexes of a new pentadentate, asymmetric
salen-like ligand have been performed. Combined
potentiometric/solution structural experiments (pHmetric/UV–Vis and pH-metric/EPR) provided information on the coordinated donor groups and the geometry
of the formed complexes. In the equimolar iron(III)bhbdpa system {NH, PhO, COO}, {2NH, 2PhO,
COO} and {2NH, 2PhO, COO, OH} coordinated
complexes are formed. The pentadentate ligand has
rather versatile coordination properties, as reflected by
the presence of microscopic complex formation pathways, and thus binding isomers in the copper(II)bhbdpa system. At twofold excess of metal ion dinuclear
complexes were detected with both iron(III) and copper(II). In presence of iron(III) a l-carboxylato-lhydroxo-bridged dinuclear complex formed through
overlapping proton release processes, providing a unique example for the stabilization of a carboxylate
bridged diiron core in aqueous solution by a pentadentate ligand. In the paramagnetic Cu2(bhbdpa)+ species
relatively weak spin–spin coupling was detected between
the metal centers.
Acknowledgments
This research has been supported by a Marie Curie
Fellowship of the European Community program
‘‘Improving the Human Research Potential and the Socio-Economic Knowledge Base’’ (contract number
‘‘HPMF-CT-2002-01860’’), and by the Hungarian Research Foundation (OTKA T037385 and T046953).
Appendix A. Supplementary data
Supplementary information for this article is available:
http://www.staff.u-szeged.hu/~gajda/SuppMat_
A. Jancsó et al. / Journal of Inorganic Biochemistry 99 (2005) 1480–1489
JIB2005A.pdf. Supplementary data associated with this
article can be found, in the online version at
doi:10.1016/j.jinorgbio.2005.04.006.
References
[1] R.B. Lauffer, R.H. Heistand II, L. Que Jr., Inorg. Chem. 22
(1983) 50–55.
[2] L. Que Jr., R.C. Kolanczyk, L.S. White, J. Am. Chem. Soc. 103
(1987) 5373–5380.
[3] P. Mialane, E. Anxholabéhère-Mallart, G. Blondin, A. Nivorojkine, J. Guilhem, L. Tchertanova, M. Cesario, N. Ravi, E.
Bominaar, J.-J. Girerd, E. Münck, Inorg. Chim. Acta 263 (1997)
367–378.
[4] Y. Wang, T.D.P. Stack, J. Am. Chem. Soc. 118 (1996) 13097–
13098.
[5] Y. Wang, J.L. DuBois, B. Hedman, K.O. Hodgson, T.D.P.
Stack, Science 279 (1998) 537–540.
[6] P. Chaudhuri, M. Hess, J. Müller, K. Hildenbrand, E. Bill, T.
Weyhermüller, K. Wieghardt, J. Am. Chem. Soc. 121 (1999)
9599–9610.
[7] R.H. Holm, Chem. Rev. 87 (1987) 1401–1449.
[8] T. Katsuki, Coord. Chem. Rev. 140 (1995) 189–214.
[9] L. Canali, D.C. Sherrington, Chem. Soc. Rev. 28 (1999) 85–93.
[10] L. Borer, L. Thalken, C. Ceccarelli, M. Glick, J.H. Zhang, W.M.
Reiff, Inorg. Chem. 22 (1983) 1719–1724.
[11] D.W. Gruenwedel, Inorg. Chem. 7 (1968) 495–501.
[12] A.R. Amundsen, J. Whelan, B. Bosnich, J. Am. Chem. Soc. 99
(1977) 6730–6739.
[13] A. Böttcher, H. Elias, E.-G. Jäger, H. Langfelderova, M. Mazur,
L. Müller, H. Paulus, P. Pelikan, M. Rudolph, M. Valko, Inorg.
Chem. 32 (1993) 4131–4138.
[14] R. Klement, F. Stock, H. Elias, H. Paulus, P. Pelikán, M. Valko,
M. Mazúr, Polyhedron 18 (1999) 3617–3628.
[15] F. LÕEplattener, I. Murase, A.E. Martell, J. Am. Chem. Soc. 89
(1967) 837–843.
[16] R.J. Motekaitis, A.E. Martell, M.J. Welch, Inorg. Chem. 29
(1990) 1463–1467.
[17] R.J. Bergeron, J. Wiegand, G.M. Brittenham, Blood 99 (2002)
3019–3026.
[18] S. Albedyhl, D. Schnieders, A. Jancsó, T. Gajda, B. Krebs, Eur.
J. Inorg. Chem. (2002) 1400–1409.
[19] C. Belle, I. Gautier-Luneau, L. Karmazin, J.L. Pierre, S. Albedyhl, B. Krebs, M. Bonin, Eur. J. Inorg. Chem. (2002) 3087–3090.
[20] T. Gajda, A. Jancsó, S. Mikkola, H. Lönnberg, H. Sirges, J.
Chem. Soc., Dalton Trans. (2002) 1757–1763.
[21] N.S. Gonçalves, L.M. Rossi, L.K. Noda, P.S. Santos, A.J.
Bortoluzzi, A. Neves, I. Vencato, Inorg. Chim. Acta 329 (2002)
141–146.
[22] C. Hemmert, M. Verelst, J.-P. Tuchagues, Chem. Commun.
(1996) 617–618.
[23] J.W. Yun, T. Tanase, S.J. Lippard, Inorg. Chem. 35 (1996) 7590–
7600.
1489
[24] V.M. Trukhan, A.A. Shteinman, Russ. Chem. Bull. 46 (1997)
202–203.
[25] T.J. Mizoguchi, S.J. Lippard, J. Am. Chem. Soc. 120 (1998)
11022–11023.
[26] D.H. Ohlendorf, J.D. Lipscomb, P.C. Weber, Nature 366 (1988)
403–405.
[27] N. Ito, S.E.V. Phillips, K.D.S. Yadav, P.F. Knowles, J. Mol.
Biol. 238 (1994) 794–814.
[28] F.J.C. Rosotti, H. Rosotti, The Determination of Stability
Constants, McGraw-Hill Book Co, New York, 1962, p. 149.
[29] E. Högfeldt, Stability Constants of Metal–ion Complexes, Part A.
Inorganic Ligands, Pergamon, New York, 1982, p. 32.
[30] L. Zékány, I. Nagypál, in: D.J. Leggett (Ed.), Computational
Methods for the Determination of Formation Constants, Plenum,
New York, 1991.
[31] A. Rockenbauer, T. Szabó-Plánka, Zs. Árkosi, L. Korecz, J. Am.
Chem. Soc. 123 (2001) 7646–7654.
[32] T. Szabó-Plánka, A. Rockenbauer, L. Korecz, D. Nagy, Polyhedron 19 (2000) 1123–1131.
[33] B.P. Gaber, V. Miskowski, T.G. Spiro, J. Am. Chem. Soc. 96
(1974) 6868–6873.
[34] C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, Wiley, New
York, 1976.
[35] R.H. Byrne, Y.-R. Luo, R.W. Young, Mar. Chem. 70 (2000) 23–
35.
[36] E. Rabinowitch, W.H. Stockmayer, J. Am. Chem. Soc. 64 (1942)
335–347.
[37] J.M. White, P. Kelly, N.C. Li, J. Inorg. Nucl. Chem. 16 (1961)
337–344.
[38] T. Ozawa, K. Iwai, K. Jitsukawa, H. Masuda, H. Einaga,
Polyhedron 13 (1994) 1523–1538.
[39] F. Lloret, J. Moratal, J. Faus, J. Chem. Soc., Dalton Trans.
(1983) 1743–1748.
[40] C.J. Carrano, K. Spartalian, G.V.N. Appa Rao, V.L.
Pecoraro, M. Sundaralingam, J. Am. Chem. Soc. 107
(1985) 1651–1658.
[41] F. Lloret, J. Moratal, J. Faus, J. Chem. Soc. Dalton Trans. (1983)
1749–1753.
[42] S. Yan, L. Que Jr., J. Am. Chem. Soc. 110 (1988) 5222–5224.
[43] R.E. Norman, S. Yan, L. Que Jr., G. Backes, J. Ling, J. SandersLoehr, J.H. Zhang, C.J. OÕConnor, J. Am. Chem. Soc. 112 (1990)
1554–1562.
[44] S. Yan, X. Pan, L.F. Taylor, J.H. Zhang, C.J. OÕConnor, D.
Britton, O.P. Anderson, L. Que Jr., Inorg. Chim. Acta 243 (1996)
1–8.
[45] A. Hazell, K.B. Jensen, C.J. McKenzie, H. Toftlund, J. Chem.
Soc. Dalton Trans. (1993) 3249–3257.
[46] A. Neves, L.M. Rossi, I. Vencato, W. Haase, R. Werner, J.
Chem. Soc. Dalton Trans. (2000) 707–712.
[47] E. Prenesti, P.G. Daniele, M. Prencipe, G. Ostacoli, Polyhedron
18 (1999) 3233–3241.
[48] S. Bruni, F. Cariati, P.G. Daniele, E. Prenesti, Spectrochim. Acta
A 56 (2000) 815–827.
[49] A.E. Martell, R.J. Motekaitis, E.T. Clarke, J.J. Harrison, Can. J.
Chem. 64 (1986) 449–456.