Homo-and heterodinuclear complexes of the tris(catecholamide) derivative of a tetraazamacrocycle with Fe3+, Cu2+ and Zn2+ metal ions

Article abstract of DOI:10.1039/b712916e

The new ditopic catecholamide 3,7,11-tris-{N-[3,4-(dihydroxybenzoyl)- aminopropyl]} derivative of a 14-membered tetraazamacrocycle containing pyridine (H₆L¹) has been synthesized. The protonation constants of (L¹)^6-

Full text of DOI:10.1039/b712916e

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Homo-and heterodinuclear complexes of the tris(catecholamide) derivative of
a tetraazamacrocycle with Fe3+, Cu2+ and Zn2+ metal ions†
Krassimira P. Guerraa and Rita Delgado*a,b
Received 22nd August 2007, Accepted 26th October 2007
First published as an Advance Article on the web 12th November 2007
DOI: 10.1039/b712916e
The new ditopic catecholamide 3,7,11-tris-{N-[3,4-(dihydroxybenzoyl)-aminopropyl]} derivative of a
14-membered tetraazamacrocycle containing pyridine (H6L1) has been synthesized. The protonation
constants of (L1)6− and the stability constants of its mono-, homo- and hetero-dinuclear complexes with
Fe3+, Cu2+ and Zn2+ metal ions were determined at 298.2 K and ionic strength 0.10 mol dm−3 in KNO3.
The large overall basicity of the ligand was ascribed to the very high protonation constants of the
catecholate groups, and its acid–base behaviour was correlated with the presence of tertiary nitrogen
atoms and secondary amide functions. The UV-vis spectrum of the red solution of [FeL1]3− complex
exhibits the LMCT band of catecholate to iron(III), and its EPR spectrum revealed a typical isotropic
signal of a rhombic distorted ferric centre in a high-spin state and E/D ≈ 0.31, both characteristic of a
tris-catecholate octahedral environment. The ligand forms with copper(II) and zinc(II) ions mono- and
dinuclear protonated complexes and their stability constants were determined, except for the [ML1]4−
complexes as the last proton is released at very high pH. Electronic spectroscopic studies of the copper
complexes revealed the involvement of catecholate groups in the coordination to the metal centre in the
mono- and dinuclear copper(II) complexes. This information together with the determined stability
constants indicated that the copper(II) ion can be involved in both types of coordination site of the
ligand with comparable binding affinity. The EPR spectrum of [Cu2L1]2− showed a well resolved
seven-line hyperfine pattern of copper(II) dinuclear species typical of a paramagnetic triplet spin state
with weak coupling between the two metal centres. Thermodynamically stable heterodinuclear
complexes, [CuFeHhL1]h−1 (h = 0–3) and [CuZnHhL1]h−2 (h = 0–4), were formed as expected from a
ditopic ligand having two dissimilar coordination sites. At physiological pH, the [CuFeL1]− complex is
formed at ≈100%. The formation of the [CuFeHhL1]h−1 complexes in solution was supported by
electronic spectroscopic measurements. The data indicated the specific coordination of each metal
centre at the dissimilar sites of the ligand, the iron(III) bound to the oxygen donors of the catecholate
arms and the copper(II) coordinated to the amine donors of the macrocyclic ring. The two metal centres
are weakly coupled, due to the fairly large distance between them.
of such enzymes.3–6 The coordination versatility of ditopic ligands
Introduction
extends their applications to catalysis, redox processes, mixed-
valence chemistry or DNA interaction.3–7 The ditopic ligands still
have applications in the removal of toxic metals or radioactive
isotopes.8
Over the last twenty years a continuous interest in the search
for synthetic ditopic ligands has been carried out. These ligands
should provide donor atoms, coordination number and geometric
dissimilarity in the two different coordination environments, and
they should be capable of binding two different metal ions.1 The
recognition of homo- and heterodinuclear cores at the active sites
of metalloenzymes, responsible for specific biological functions,2
has enhanced attention to these ligands able to be used as models
Ditopic asymmetric macrocyclic compounds, with or without
pendant arms, may supply well-defined specific environments for
the coordination of two metal ions. Additionally it may be possible
to tune their physicochemical properties by control of the length
and type of bridging groups between the two specific coordination
sites.9,10
aInstituto de Tecnologia Qu´ımica e Biolo´gica, UNL, Apartado 127, 2781-
901, Oeiras, Portugal. E-mail: delgado@itqb.unl.pt; Fax: +351-214 41 12
77; Tel: +351-214 46 97 37/8
With these applications in mind we have synthesized the new
ditopic macrocyclic ligand H6L1 {3,7,11-tris-[N-(3,4-dihydroxyl-
benzoyl)-aminopropyl]-3,7,11,17-tetraazabicyclo[11.3.1]hepta-
deca-1(17),13,15-triene}, see Scheme 1. This ligand offers the
amine donors of the 14-membered tetraazamacrocycle for the
coordination of copper(II) or zinc(II) and three catecholate units
specific for the coordination of iron(III) ions, both sites separated
by propyl spacers. The acid–base behaviour of this ligand and
its mono-, homo- and heterodinuclear complexes of iron(III),
copper(II) and zinc(II) have been studied by several techniques.
bInstituto Superior Te´cnico, Av. Rovisco Pais, 1049-001, Lisboa, Portugal
† Electronic supplementary information (ESI) available: Spectroscopic
UV-vis data for the solutions of 1 : 1 and 2 : 1 Cu2+ ion to H6L1 ratios
at different pH values; spectroscopic UV-vis data for Fe3+/H6L1 1 : 1
ratio solutions at different pH regions; UV-vis spectroscopic data of
Cu2+/Fe3+/H6L1 1 : 1 : 1 ratio solutions at different pH; species distribution
curves calculated for the Cu2+/H6L1 system in aqueous solution; X-band
EPR signal of the Fe3+/H6L1 1 : 1 ratio solution at pH 10.2. See DOI:
10.1039/b712916e
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In Fig. 1 is shown the species distribution diagram starting
from the protonated form of the ligand (H9L1)3+, obtained with
the help of the Hyss program.16 The compound has ten basic sites,
six from the phenolate oxygen atoms of the catecholate groups of
the pendant arms and four from the macrocyclic nitrogen atoms.
Only six protonation constants could be accurately determined
under our experimental conditions. Indeed, the three first pro-
tonation constants corresponding to the first deprotonation of
each catechol group (very high values) and the last protonation
of the macrocyclic ring (very low value) cannot be determined by
potentiometry.
Scheme 1 Molecular structure of H6L1 and L1p.
Results and discussions
Synthesis of the macrocyclic compounds
The ligand H6L1 was synthesized according to the procedure
described by Raymond et al.11,12 for related compounds. Its
methyl protected derivative (L1p) was prepared using the Schotten–
Baumann technique by addition of 3,4-dimethoxybenzoyl chloride
to a mixture of L12 (ref. 13) and sodium hydroxide. The selective
deprotection of L1p was achieved with BBr3 in CH2Cl2. Upon
purification the pure H6L1 compound was obtained in good yield
as a light beige crystalline powder.
Fig. 1 Species distribution curves calculated for the ligand (H9L1)3+ in
aqueous solution from the protonation constants of Table 1. CL = 1.0 ×
10−3 mol dm−3. L = L1.
The 2,3-dihydroxybenzene has a very high value for the first
protonation constant and a large difference between the intrinsic
acidity of the two dissociable protons of the phenolic oxygen
atoms (log K1 ≈ 13 and log K2 = 9.22).17 This is explained by
electronic effects and intramolecular hydrogen bonding formation
between neighbouring protonated and deprotonated phenolic
oxygen atoms.18–21
The direct determination of the first three protonation constants
of (L1)6− in aqueous solution, corresponding to the first protona-
tion of each catecholate group, is not possible, and was not tried
in other solvents owing to solubility reasons. This determination
Acid–base behaviour
The protonation constants of the completely deprotonated ligand
(L1)6− were determined from potentiometric measurements in
aqueous solution at 298.2 K and ionic strength 0.10 mol dm−3 in
KNO3. The determined log KiH values (i = 1–9) are listed in Table 1
together with the values of related ligands12,14,15 (see Scheme 2 for
the corresponding structures).
Table 1 Protonation constants (log KiH) of (L1)6− and of other related compounds for comparison reasons. T = 298.2 K and I = 0.10 mol dm−3
in KNO3
a
b
c
d
c
Reaction Equilibrium
(L1)6−
(L2)6−
(L3)6−
(L4)6−
(L5)6−
(L1)6− + H+ ꢀ (HL)5−
(HL)5− + H+ ꢀ (H2L)4−
(H2L)4− + H+ ꢀ (H3L)3−
(H3L)3− + H+ ꢀ (H4L)2−
(H4L)2− + H+ ꢀ (H5L)−
(H5L)− + H+ ꢀ H6L
(H7L)+ + H+ ꢀ (H8L)2+
(H8L)2+ + H+ ꢀ (H9L)3+
(L1)6− + 6 H+ ꢀ (H6L)
(L1)6− + 7 H+ ꢀ (H7L)+
(L1)6− + 9 H+ ꢀ (H9L)3+
12.9e
12.1e
11.3e
9.76(5)
9.65(4)
9.05(3)
6.95(5)
6.19(5)
4.70(5)
12.9e
12.1e
11.26
8.75
8.61
6.71
5.88
—
12.9e
12.1e
11.3e
8.55
7.5
6.0
—
—
—
12.9e
12.1e
11.3e
9.26
8.65
7.86
—
—
—
62.1
—
12.9e
12.1e
11.3e
8.4
7.4
5.9
—
—
—
58.0
—
H6L + H+ ꢀ (H7L)+
—
64.8
60.3
66.2
—
58.4
—
71.7
82.6
—
—
—
7.40f
7.49g
7.36g
8.59g
7.2g
a This work. b Ref. 12. c I = 0.1 mol dm−3 KCl, 5% CH3OH, ref. 14. d Ref. 15. e Estimated values. f Average log Ki of the three more acidic catecholamide
H
ꢀ
ꢀ
protonation constants: (log K6 + log K7 + log K8)/3. g See footnote (f), but the corresponding constants are K4, K5 and K6: (log K4 + log K5 + log
K6)/3.
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Scheme 2 Molecular structure of related compounds.
H
H
is not only difficult because of the very high values, but also due
to the possible oxidation of the ligand at pH ≈ 12, as observed for
other catecholamide derivatives.22 These values are around 13 for
the tris- and bis-catecholate compounds.14,15,19,20 Taking this into
account and the ineluctable statistical factor, and according to
usual practice12,19 we have estimated for these constants the values
The quite high values of K4 and K5 of the ligand when
compared with those of the parent macrocycle can be explained by
electronic effects due to the formation of the zwitterionic species
and/or additional stabilization by hydrogen bonds. The values
of K6H–K8 are ascribed to the three consecutive protonations
H
of the less basic oxygen atoms of the catecholamide dianions
closer to the amide carbonyl function. The average value for
these three constants is comparable to the corresponding value for
trencam,12 enterobacin,14 and for other catecholamide ligands20
H
H
H
of log K1 = 12.9, log K2 = 12.1 and log K3 = 11.3, see Table 1.
The titration curve of (H9L1)3+ contains three buffer regions.
The first region at pH 4 to 5 corresponds to the titration of
one proton of one ammonium centre of the macrocycle,23 the
second one at pH 6 to 9 is a well defined region of two protons of
catecholate groups, and the last one above pH 9 corresponds to
the titration of the remaining six protons. At pH 8 about 90% of
the ligand is in the zwitterionic form H6L1 (see Fig. 1), having the
two nitrogen atoms contiguous to the pyridine of the macrocycle
protonated and the catecholate arms bound to these ammonium
groups with only one proton, each one with one negative charge,
and the remaining arm completely protonated. The sequence of
protonation is straightforward taking into account the acid–base
behaviour of the parent macrocycle (L8),23 and ligands containing
or not tertiary amines coupled with catecholamide groups, such
as mecam,14 and trencam.12
H
(see Table 1), in good agreement with the value of log K2
=
7.34 for H2L6.24 On the other hand, the average value of the three
more acidic protonation constants of 3,3,4-cycam15 is 8.59, which
is similar to that of H2L7,25 (see Table 1 and Scheme 2). In fact,
theoretical calculations, together with the crystal structures and
experimental potentiometric data for a series of catecholamide
derivatives established that the presence of amide nitrogen atoms
on the catecholamide molecule increases the first protonation
constant of the nearby catecholate by about one log unit.21 Finally,
it can be seen that the difference between consecutive log K values
of (L1)6− are larger than the expected statistical separation (log
3 or 0.48), which points to interactions between the catecholate
oxygen atoms in the pendant arms.20
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Metal complex studies
from the ligand in the form (H9L1)3+, however the ligand will be
indicated in the following text by the neutral form H6L1.
Mono- and homodinuclear complexes
Mononuclear and dinuclear complexes were found under our
experimental conditions. Hydroxocomplexes, [M2H−hL1]−h (h =
1–3), were only observed for M = Fe3+.
The stability constants of complexes of (H9L1)3+ with Fe3+, Cu2+
and Zn2+ metal ions were determined under the experimental
conditions indicated above, and the values are listed in Table 2.
The constants were determined for the entire pH range starting
The potentiometric titration curves of the metal ion to H6L1 1 : 1
ratio solutions with strong base display a plateau in the pH 4–5
region and then a gradual increase of pH, indicating the successive
deprotonation of complexes. Along the titration, in the case of the
Fe3+ ion, the colour of the solution changes from blue to blue green
(pH 2–4), light to dark purple (pH 4.5–8.5) and red wine to dark
red (pH > 11), while the Cu2+ solutions change from light blue to
greenish blue (pH 3.1–6.5) and greenish yellow to bright yellow
(pH 7 to >10.5). In the curves of the 2 : 1 solutions the plateau
region extends for 3 more equiv. of base.
m Hh L1
m Hh L1
) stability constants
Table 2 Overall (log bM
) and stepwise (log KM
of complexes of H6L1 with M = Fe3+, Cu2+ and Zn2+a
Fe3+
Cu2+
Zn2+
Equilibrium reactionb
m Hh L1
log bM
M +L1 ꢀ ML1
43.38(9)
—
—
M + H + L1 ꢀ MHL1
53.11(8) 39.19(9) 27.7(1)
61.54(7) 49.59(9) 38.84(7)
68.48(5) 59.16(8) 48.82(6)
73.46(4) 67.18(8) 57.44(5)
76.77(2) 73.38(3) 64.34(2)
79.40(5) 77.82(1) 69.75(6)
65.25(6) 47.83(6)
69.78(5) 57.04(4) 38.50(7)
—
—
Six different mono [FeHh−1L1]h−3 and one dinuclear protonated
[Fe2HL1]+ species can be formed, but only dinuclear complexes
exist for the 2 : 1 ratio, as shown in the distribution curves of Fig. 2.
At pH 6–7, the [Fe2L1] complex exists at 90% of the total metal
concentration, and then at higher pH values hydroxo complexes
are formed. The latter species derive from the hydrolysis of water
molecules directly bound to the metal, these water molecules
should complete the coordination sphere of the metal coordinated
to the macrocyclic amines. The dinuclear complexes may contain
one metal coordinated to the amine donors of the macrocycle and
the other one coordinated to the catecholate oxygen donors of the
arms. In fact, due to the 3,4-dihydroxybenzoyl position of the arms
in the ligand, the iron(III) coordination sphere is not of the “salicy-
late” type,26 the metal coordination to the carbonyl oxygen atoms
is prevented due to conformational limitation of the ligand.19,27
The deprotonation of the last two protons from [MH2L1]2− (M =
Cu2+ or Zn2+) complexes, is only achieved at very high pH, and
the corresponding constants were not determined by the reasons
already discussed for the first three protonation constants of the
ligand. This fact indicates that these protons are located at the
catecholate arms, the corresponding constants should be only
slightly lower than the first two protonation constants of the free
ligand due to the presence of the metal centre, suggesting that these
metal ions prefer the macrocyclic environment when only 1 equiv.
of metal ion is added, or that one of the arms is not involved in
the coordination of the second metal ion.
M + 2 H + L1 ꢀ MH2L1
M + 3 H + L1 ꢀ MH3L1
M + 4 H + L1 ꢀ MH4L1
M + 5 H + L1 ꢀ MH5L1
M + 6 H + L1 ꢀ MH6L1
2 M + L1 ꢀ M2L1
—
2 M + H + L1 ꢀ M2HL1
2 M + 2 H + L1 ꢀ M2H2L1
2 M + 3 H + L1 ꢀ M2H3L1
2 M + L1 + H2O ꢀ M2L1(OH) + H
62.73(3) 47.62(3)
67.26(6) 54.88(3)
57.79(8)
—
—
—
—
—
—
2 M + L1 + 2 H2O ꢀ M2L1(OH)2 + 2 H 49.00(8)
2 M + L1 + 3 H2O ꢀ M2L1(OH)3 + 3 H 38.90(9)
m Hh L1
log KM
M +L1 ꢀ ML1
43.38
9.73
8.43
6.94
4.98
3.31
2.63
21.87
4.53
—
—
—
—
—
ML1 + H ꢀ MHL1
MHL1 + H ꢀ MH2L1
MH2L1 + H ꢀ MH3L1
MH3L1 + H ꢀ MH4L1
MH4L1 + H ꢀ MH5L1
MH5L1 + H ꢀ MH6L1
ML1 + M ꢀ M2L1
10.40
9.57
8.02
6.20
4.44
—
9.21
5.69
4.53
—
11.15
9.98
8.62
6.90
5.41
—
M2L1 + H ꢀ M2HL1
M2HL1 + H ꢀ M2H2L1
M2H2L1 + H ꢀ M2H3L1
M2L1(OH) + H ꢀ M2L1
M2L1(OH)2 + H ꢀ M2L1(OH)
M2L1(OH)3 + H ꢀ M2L1(OH)2
—
9.12
7.26
—
—
7.46
8.79
10.10
—
—
—
—
a T = 298.2 K and I = 0.10 mol dm−3 KNO3. b The charges of species were
omitted for simplicity reasons, due to the fact that metal ions of different
charges are considered.
The distribution diagram curves of Cu2+/H6L1 2 : 1 ratio (see
Fig. S1 in ESI†) show successively the formation of [Cu2H2L1]
Fig. 2 Species distribution curves calculated for the Fe3+/H6L1 system in aqueous solution. (a) CFe = CL = 1.0 × 10−3 mol dm−3 and (b) CFe = 2 × CL
=
2.0 × 10−3 mol dm−3. L = L1.
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Table 3 The pFe values for Fe3+ complexes of H6L1 and related ligands
at pH = 7.4a
pM values for H6L1 and other related ligands for M = Cu2+ or
Zn2+ ions at pH 7.4.23,28–31 The pCu value at physiological pH for
H6L1 is very similar to that for other N-derivatives of the parent
macrocyclic ligand, such as H4L9,28 and H2L10.30 However, the pCu
values are also of the same order as those for typical catecholamide
ligands, such as H6L13 (trencams)31 and H6L14 (mecams),29 see
Table 4. Copper(II) complexed with H2L10 exhibits a distorted
octahedral environment with the four macrocyclic nitrogen atoms
forming the equatorial plane and two oxygen atoms from the
methylcarboxylate arms in axial positions,30 while in the complex
with the tris-catecholate ligand H6L13 (trencams) the copper is
coordinated in bis-catecholate fashion.31 These results indicate
that the two types of coordination of copper(II) afford comparable
binding affinities at physiological pH, and therefore we cannot
determine whether the metal is coordinated at the arms or at the
macrocycle, and probably an equilibrium between both types of
coordination should exist. On the other hand, the relatively low
pZn value for H6L1/Zn2+ indicates that the structure of H6L1 does
not favour the effective coordination of Zn2+.
Chelatorb
pFe
Ref.
H6L1
Enterobactin
mecam
27.34
35.5
29.1
27.8
26.6
23.6
23.0
This work
19
15
12
25
25
15
trencam
Desferrioxamine B
Transferrin
3,3,4-cycam
a CFe = 10−6 mol dm−3, CL = 10−5 mol dm−3
.
b H6L1 is written in the neutral
form while the other chelators are designated by their traditional names.
(≈60% at pH 6), [Cu2HL1]− (100% at pH 8.5) and then starts to
form [Cu2L1]2− complexes. At pH < 6 mononuclear protonated
species also exist.
The binding affinity of H6L1 for Zn2+ is much lower than
for Cu2+, the complexes start to form at pH ≈ 5, and for 2 : 1
ratio solutions Zn(OH)2 is formed at pH ≈ 10 precluding the
determination of the stability constant of the [Zn2L1]2− complex.
The stability constant of [FeL1]3− and the overall basicity of the
ligand are both very large. In order to compare the iron(III) binding
properties of H6L1 with other iron(III) chelators, the different
proton competitions should be taken into account, for instance by
the calculation of pFe (= −log [Fe3+]) at a given pH.16 In Table 3,
the pFe values determined for solutions of iron(III) complexes of
several chelators at pH 7.4 were compiled. At this pH, the pFe
value of the ferric complex of H6L1 is slightly lower than that
of enterobactin,19 but it is still among the values of those of the
most effective catecholate iron(III) chelators, such as mecam15 and
trencam.12 On the other hand, the pFe for H6L1 is higher than that
of the plasma-iron transport protein, transferrin,25 indicating that
H6L1 may take the iron from transferrin and facilitate its excretion
from the body. Additionally, the pFe value for our ligand is also
higher than those of 3,3,4-cycam15 or desferrioxamine B.25
The H6L1 ligand was not designated for the specific coordination
of iron(III), but instead for the study of heterodinuclear complexes,
even so the results have shown that it is one of the best chelators for
this metal ion. In fact the chelator should adopt an arrangement
with the three long aminopropyl arms located at the same side
of the macrocyclic plane13 forming a tripodal configuration and
the amide functions located out of the catecholate oxygen atom’s
network. This preorganized arrangement should favour metal
complexation.
Cu2+/Fe3+ and Cu2+/Zn2+ heterodinuclear complexes
The formation of heterodinuclear complexes is expected to be
due to the presence of two different coordination sites in the
molecule of H6L1 and the observed tendency of this chelator
to form dinuclear complexes. The stability constants of the
heterodinuclear complexes of H6L1 with Cu2+/Fe3+, and Cu2+/Zn2+
were determined, and the values are listed in Table 5.
The potentiometric titration curves of Cu2+ : Fe3+ : H6L1 (or
Cu2+ : Zn2+ : H6L1) 1 : 1 : 1 molar ratio show the break upon ad-
dition of nine (or eight) equivalents of base. Eight equivalents for
the second system because the deprotonation of the last proton
of [CuZnHL1]− occurs at pH ≈ 9.5, see Table 5. The colour of
the Cu2+/Fe3+/H6L1 (1 : 1 : 1) solution changes during the titration
from intense blue-green at low pH to red-purple (pH 4.0–8.0), and
dark red purple at high pH, significantly different from the colour
of solutions with only one metal ion.
The species distribution diagram for the Cu2+/Fe3+/H6L1 system
is shown in Fig. 3. Mononuclear [FeHhL1]h−3 (h = 5, 6) species are
formed simultaneously with [CuFeHhL1]h−1 (h = 2, 3) complexes
at pH 2–4. At pH > 4 only heterodinuclear (CuFeHhL1)h−1
(h = 0, 1) complexes exist in solution. At physiological pH the
[CuFeL1]− complex exists at more than 95% of the total ligand
Studies of chelators with catecholate substituents with cop-
per(II) or zinc(II) are very rare in the literature. Table 4 lists the
Table 4 The pM values for M = Cu2+ and Zn2+ complexes of H6L1 and
related ligands at pH = 7.4a
Chelator
pCu
pZn
Ref.
H6L1
18.15
16.21
16.9
17.00
18.85
18.9
8.44
14.34
11.3
13.34
13.13
11.7
This work
H4L9
28
29
23
30
31
H6L14 (mecams)
L8
H2L10
H6L13 (trencams)
Fig. 3 Species distribution curves calculated for the heterodinuclear
Cu2+/Fe3+/H6L1 systems in aqueous solution (CCu = CFe = CL = 1.0 ×
10−3 mol dm−3). L = L1.
a CM = 10−6 mol dm−3, CL = 10−5 mol dm−3
.
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Table 5 Overall (log bMM H L) and stepwise (log KMM H L) stability constants of heterodinuclear complexes of H6L1 with M/Mꢀ = Cu2+/Fe3+and Cu2+/Zn2+
.
ꢀ
ꢀ
h
h
T = 298.2 K and I = 0.10 mol dm−3 KNO3
ꢀ
log b
MM Hh
L
Reaction equilibrium
M = Cu2+; Mꢀ = Fe3+
M = Cu2+; Mꢀ = Zn2+
M + Mꢀ + L1 ꢀ MMꢀL1
63.40(7)
69.26(4)
72.71(3)
75.74(2)
—
41.6(1)
51.22(9)
58.86(7)
65.26(9)
71.3(1)
—
M + Mꢀ + H + L1 ꢀ MMꢀHL1
M + Mꢀ + 2 H + L1 ꢀ MMꢀH2L1
M + Mꢀ + 3 H + L1 ꢀ MMꢀH3L1
M + Mꢀ + 4 H + L1 ꢀ MMꢀH4L1
M + Mꢀ + L1 ꢀ MMꢀL1(OH) + H
54.40(9)
ꢀ
log K
MM Hh
L
ML1 + Mꢀ ꢀ MMꢀL1
36.2a
20.02
5.86
3.45
3.03
—
—
—
9.59
7.64
6.40
6.0
—
MꢀL1 + M ꢀ MMꢀL1
MMꢀL1 + H ꢀ MMꢀHL1
MMꢀHL1 + H ꢀ MMꢀH2L1
MMꢀH2L1 + H ꢀ MMꢀH3L1
MMꢀH3L1 + H ꢀ MMꢀH4L1
MMꢀL1(OH) + H ꢀ MMꢀL1
9.00
a Determined taking the value of log KCuL = 27.19 obtained by estimation of log KCuHL = 12.0, which is a very realistic value, see text.
1
1
concentration and at pH ≈ 7.5 the [CuFeL1(OH)]2− complex
starts to form. The formation of protonated mononuclear iron(III)
complexes at very low pH reveals that the iron(III) coordinates to
the catecholate oxygen atoms, being the most thermodynamically
stable species formed and then the copper(II) ion coordinates to
the remaining sites of the ligand, which are the donor atoms of the
macrocyclic ring. The formation of [CuFeL1(OH)]2− at high pH
values implies the presence of one water molecule in one of the
metal coordination spheres. The pCu and pFe values determined
at pH 7.4 for this system are 18.10 and 27.30, respectively. These
values are very close to the corresponding ones for each metal ion
determined independently (see Tables 3 and 4), which attest to the
weak interaction between both centres.
The distribution curves diagrams for a 1 : 1 : 1 ratio of
Cu2+/Zn2+/H6L1 shows a similar behaviour, but in this case it
is not possible to assign a specific site to each metal ion. At
low pH values (< 4) the copper(II) complexes are formed while
the catecholate groups are still protonated and then the zinc(II)
coordinates to some of the free binding sites of the ligand.
At pH 10 the [CuZnL1]2− complex is formed at 80% of the
total ligand concentration, and the [CuZnL1(OH)]− complex is
formed at high pH values. It is also interesting to note that the
stability constants determined are consistent with formation of
heterodinuclear complexes and that homodinuclear complexes are
not formed over the entire pH range.
of the macrocyclic cavity. Therefore a geometric arrangement
is necessary for the final coordination of the copper ion. This
geometric reorganization appeared to be thermodynamically
favoured. For the [CuZnL1]2− complex, in spite of the non-specific
tendency of Zn2+ to catechol coordination, the formation of the
heterodinuclear species is also favoured, and in this case again
homodinuclear species are not formed (Fig. 4). In conclusion the
ligand H6L1 is able to coordinate simultaneously two different
metal ions at its different binding sites forming [CuFeL1]− and
[CuZnL1]2− heterodinuclear complexes, which are thermodynam-
ically stable species at physiological pH. Accordingly, this ligand
can be explored for mimetic models of heterodinuclear protein
sites of cytochrome a oxidase32 and Cu/Zn SOD enzymes, which
catalyse the dismutation of the superoxide anion and play an
important role in the protection of cells from oxidative damage.33
The rationalization of the coordination behaviour of H6L1 with
the studied metal ions based only on the potentiometric data are
not completely certain, but some comments can be advanced. The
ditopic ligand H6L1 strongly binds Fe3+ and Cu2+ simultaneously,
each metal in a different coordination site of the ligand forming
the heterodinuclear complex. The tendency to form other types of
coordination, means that the Fe3+ coordinated in the macrocyclic
nitrogen atoms and the Cu2+ in the bis-catecholate mode, should
also favour homodinuclear species formation, and this did not
occur. It is known that iron(III) has a very high affinity for the
negative catecholate groups. However, the coordination of iron(III)
to the arms of the ligand probably imposes a special arrangement
Fig. 4 Species distribution curves calculated for the heterodinuclear
Cu2+/Zn2+/H6L1 systems in aqueous solution (CCu = CZn = CL = 1.0 ×
10−3 mol dm−3). L = L1.
UV-vis and EPR spectroscopic studies
Copper(II) complexes
UV-vis spectra of Cu2+ to H6L1 solutions, 1 : 1 and 2 : 1 ratios,
were recorded in the pH 5.4–10 range. The data are listed in ESI
544 | Dalton Trans., 2008, 539–550
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Table S1† and the UV region of some spectra is shown in Fig. 5.
All the spectra show a broad asymmetric band at 626 nm with
shoulders at 430, 550 and 740 nm due to the copper d–d transition.
The molar absorptivities (e/dm3 mol−1 cm−1) of all of them steadily
increase with an increase in pH. In the UV region the bands of the
ligand at 256 and 290 nm shift to 250 and 316 nm (with a shoulder
at 326 nm) with copper(II) complex formation in the pH 3–7 range.
Although the ligand bands shift to 275 and 305 nm at pH > 8, the
bands of the complex keep the same position but with increasing
intensities (see Table S1†). The 2 : 1 ratio solutions present the
same bands but larger molar absorptivities compared to the 1 : 1
ratio solutions, having almost double intensities at pH 9.1. The
shoulders at 430 and 550 nm are assigned to catecholate to copper
charge transfer bands, and they are indicative of the binding of
the catecholate oxygen atoms of H6L1 to the copper(II) centre,
together with the bands in the UV region. The same features were
also found in copper complexes of other catecholate ligands.4,34,35
Fig. 6 X-Band EPR spectra of Cu2+/H6L1 solutions: (a) 1 : 1 ratio at
pH = 5.6 (dot line) and 10 (solid line), recorded at 13 and 5 K, respectively,
microwave power of 2.4 mW, and (b) 2 : 1 ratio at pH = 12, recorded at
5 K, microwave power of 2.35 mW. All spectra acquired in water–DMSO
(1 : 1 v/v), modulation amplitude of 1 mT and frequency of 9.644 GHz.
Fig. 5 Absorption spectra of the H6L1 free ligand (pH 9.7, black line) and
its 1 : 1 (pH 10.0, dot line) and 2 : 1 (pH 9.1, gray line) copper complexes
in the UV region. The Y-axis is multiplied by the factor 10−3
.
The X-band EPR spectra of the Cu2+/H6L1 1 : 1 ratio aqueous
solutions at pH 5.6 and 10, taken at 13 and 5 K, respectively,
are very similar, exhibiting three well resolved lines of the four
expected at low field arising from coupling of the unpaired electron
spin with the copper nucleus and no superhyperfine splitting was
due to coupling with nitrogen atoms. The fourth copper line is
partially overlapped by the strong and not resolved line of the
high field part of the spectra, see Fig. 6(a). The hyperfine coupling
constants (A) and g values obtained by the simulation of the
spectra36 are compiled in Table 6 together with those of other
copper(II) complexes having the same macrocyclic framework but
different arms.30,37,38
2
2
and dx
ground state.30,37,39,40 The gi and Ai (i = x, y and z)
−y
values should be related to the strength of the axial donor and the
displacement of the copper(II) ion from the donor atom plane as
derived from ligand-field theory.39,40
In agreement, the coordination of axial ligands to the square-
planar geometry of [CuL8]2+38,40 has the effect of decreasing Az
and increasing gz with a simultaneous red shift in kmax, as observed
in [CuL11Cl]+37 and [CuL10]30 where the metal centres adopt a dis-
torted square pyramidal and an octahedral geometry, respectively,
the macrocyclic nitrogen atoms forming the equatorial plane.
Therefore the EPR parameters of the copper(II) complexes of
Cu2+/H6L1 1 : 1 ratio solutions are consistent with distorted square
pyramidal (at pH 5.6) and, probably, octahedral (at pH 10.0)
The simulations of the EPR spectra of both 1 : 1 ratio Cu2+/H6L1
solutions led to three different g values with gz > (gx + gy)/2, which
is typical of the tetragonal copper(II) ion in rhombic symmetry
Table 6 X-Band EPR data for Cu2+/H6L1 1 : 1 ratio solutions at pH 5.6 and 10, and for other related complexes
EPR parameters (104 Ai/cm−1); gz/Az/cm
Copper(II) complexes
kmax/nm
gx
gy
gz
Ax
Ay
Az
gz/Az
Ref.
Cu2+/H6L1 (pH 5.6)
Cu2+/H6L1 (pH 10)
[CuL11Cl]+
632
632
600
614
560
2.031
2.040
2.032
2.034
2.034
2.086
2.074
2.074
2.08
2.198
2.202
2.199
2.209
2.188
6.4
24.7
0.8
14.3
0.5
11.3
22.2
17.2
8.5
173.6
166.3
170.3
167.1
192.9
126
132
129
132
113
This work
This work
37
30
38
[CuL10]
[CuL8]2+
2.060
3.4
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geometries for the copper(II) centre. The empirical value gz/Az,
which is almost equal for all the complexes in Table 6, except for
[CuL8]2+, is also indicative of their structural similarity.41
In the UV region the absorption maxima of the free H6L1
(pH 10) at 275 and 305 nm shift to 250 and 316 nm, respectively,
which is indicative of the formation of the iron(III) tris-catecholate
complex.51 Similar bands were observed with the biological
substrate of 1,2-catechol dehydrogenase, an intradiol enzyme that
coordinates iron(III) as its catecholate dianion.52
The EPR spectrum of the ferric/H6L1 1 : 1 ratio aqueous
solution was recorded at 4.7 K and pH 10.2. At this pH value the
[FeL1]3− complexis present in90% ofthe total metal concentration.
The spectrum shows an intense isotropic signal at g ≈ 4.27 of the
transition between the middle Kramer’s doublet, and a broad peak
at g ≈ 9.6 from the ground state doublet of a rhombic distorted
iron(III) ion in high-spin state (S = 5/2) with E/D ≈ 0.31, Fig. S3,
in ESI.† The signal is sharp (line width is 10 G) and no splitting
was observed, which is indicative of homogenous environment
around the metal centre. These features are typical of ferric centres
in a tris-catecholate environment.53,54 The high intensity of the
signal is characteristic of the tris-catecholate mode of bonding,
corresponding to the formation of a trigonal distorted octahedral
geometry.54
The EPR spectrum of the Cu2+/H6L1 2 : 1 ratio solutions at
pH 12, where the [Cu2L1]2− complex is formed at 100%, is indicative
of dinuclear copper(II) species in a paramagnetic triplet spin
state, see Fig. 6(b). The parallel hyperfine pattern shows seven
regular well resolved lines in the DMs = 1 transition.42–45 If
the unpaired electrons of the two copper nuclei with individual
S = 1/2 are coupled (ICu = 3/2), each of the two resonances
will exhibit seven-line copper hyperfine patterns, (2In + 1).45
Therefore it is expected that the two septets shift in respect to each
other by zero-field splitting, 2D.42,43 In the [Cu2L1]2− spectrum,
the well defined seven-lines have a coupling constant Aꢁ of 70.7
G, a value that is about one-half of the equivalent value for
mononuclear copper(II) complexes, indicating also that a metal–
metal interaction occurs.42,46 No superhyperfine splitting due to the
coupling with the nitrogen atoms of the macrocycle was observed,
which is common in copper(II) macrocyclic complexes.28,30,37,38 The
DMs = 2 transition signal was not observed. When the coupling
between the metal centres is weak the signal in the half-field can
be indiscernible, which have been observed for varied copper(II)
dinuclear complexes.47–49
[CuFeHhL1]h−1 complexes
From the D value it is possible to determine the mean distance
between the two copper centres, when the zero-field splitting
parameter is due to a pure dipole–dipole interaction.42–44,47 In
this case the effective dipolar zero-field splitting is proportional
to 1/r3 (r being the distance between the two copper(II) centres)
by the equation: Dꢁ = 0.65(gꢁ)2/r3.42,44,48 The calculated values
The spectroscopic UV-vis data of Cu2+/Fe3+/H6L1 1 : 1 : 1 ratio
aqueous solutions recorded at pH 7.0 and 10.0 are listed in
Table S3, ESI,† and in Fig. 8 are shown the spectrum at pH 10.0
together with those of Fe3+/H6L1 1 : 1 and Cu2+/H6L1 solutions at
pH 11.4 and 10.0, respectively. Following the speciation diagram
of Fig. 3, the [CuFeL1]− and [CuFeL1(OH)]2− complexes are
presented at about 100% of the total ligand amount at pH ≈ 7
and 10, respectively.
The band at 474 nm of [FeL1]3− shift to 454 nm for the
heterodinuclear species and the corresponding e value is about half
of that for the LMCT catecholate to iron(III) band of the [FeL1]3−
complex. The copper d–d transition band of the heterodinuclear
complex is hidden by the intense iron(III)/copper(II) catecholate
charge transfer band. The UV spectra of Cu2+/Fe3+/H6L1 1 : 1 : 1
ratio solutions also exhibit shoulders at 406 nm (pH 7.0) or 370 nm
(pH 10).
for [Cu2L1]2− complex are: Dꢁ = 0.02062 cm−1 and r = 4.87 A.
˚
These values should only be considered as tentative,42,46,50 but they
are indicative of a weak interaction between the two copper(II)
centres.
Fe3+ complexes
The UV-vis spectra of the aqueous solutions of Fe3+/H6L1 1 : 1
ratio were recorded at different pH and are shown in Fig. 7, and
the respective data are presented in Table S2.†
Two or more species coexists at each pH value till 11, see Fig. 2
(a): [FeH4L1]+ and [FeH3L1] at pH 5.0; [FeH3L1] and [FeH2L1]− at
pH 6.9, respectively. At pH 11.4 the [FeL1]3− complex is the unique
species in solution, which exhibits a characteristic red colour and
intense band at 474 nm (e = 4800 dm3 mol−1 cm−1), which arise
from ligand to metal d transitions.51
In conclusion, the observed bands of the heterodinuclear
complex are shifted relatively to the corresponding ligand bands,
and on the other hand, also move compared to the bands of
mononuclear iron(III) and copper(II) species, see Table S1–S3 in
ESI† and Fig. 5, 7 and 8.
Fig. 7 Absorption spectra of the UV (a) and visible (b) regions of Fe3+/H6L1 1 : 1 ratio aqueous solutions at different pH: 5.0 (grey line), 6.9 (black line),
11.4 (dot line). The Y-axis is multiplied by the factor 10−3
.
546 | Dalton Trans., 2008, 539–550
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Fig. 8 UV (a) and vis (b) spectra of Cu2+/Fe3+/H6L1 1 : 1 : 1 ratio at pH 10.0 (grey line), Fe3+/H6L1 1 : 1 ratio at pH 11.4 (dot line), and Cu2+/H6L1 1 : 1
at pH 10.0 (black line) aqueous solutions. The Y-axis is multiplied by a factor 10−3
.
The EPR spectra of the Cu2+/Fe3+/H6L1 1 : 1 : 1 ratio aqueous
solutions at 5 K and at pH 4.6 and 10.2, are shown in Fig. 9.
These solutions contain 90% of [CuFeHL1] and about 100% of
[CuFeL1]− complexes of the total amount of ligand, respectively
(Fig. 3). Both spectra exhibit two types of signal, one at lower field
assigned to a rhombic distorted iron(III) complex in high-spin state
d5 state (S = 5/2), and the other of the DMs = 1 transition (g ≈
2) showed a hyperfine pattern comparable to that observed for the
[Cu2L1]2− complex at pH 12, Fig. 6b.
Using the spin-Hamiltonian formalism for the high-spin d5
state of iron(III),55 the lower field resonances of the spectra of
the solution at pH 4.6 can be assigned to two iron(III) species
with rhombic distortion E/D of 0.126 and 0.278, respectively. The
solution at pH = 10.2 can be attributed to a species with different
rhombic distortion of 0.288.53,54 The effective g values expected
for each Kramers doublet are indicated in Table 7, and the signals
observed in the spectra are in italics.
Table 7 Rhombic distortion (E/D) and effective g values expected from
the high-spin species of Cu2+/Fe3+/H6L1 1 : 1 : 1 ratio solutions at pH 4.6
and 10.2. The values observed in the experimental spectra are in italics
pH
4.6
E/D
Doublet
g1
g2
g3
0.126
|
|
|
|
|
|
|
|
|
5/2>
3/2>
1/2>
5/2>
3/2>
1/2>
5/2>
3/2>
1/2>
9.963
5.534
1.57
9.791
4.597
0.805
9.773
4.539
0.766
0.99
0.113
2.752
3.135
0.585
4.200
1.216
0.630
4.229
1.141
2.495
8.395
0.440
3.955
9.516
0.469
4.019
9.551
0.278
0.288
10.2
the peak to peak line width for the signal at 1570 G is 20 G (pH 4.6)
and 30 G (pH 10.2) while the value for the mononuclear complex
is about 10 G (at pH 10).
The presence of two types of high-spin species of different E/D
at pH 4.6 arises from the possible formation of a bis-catecholate
together with a small percentage of a tris-catecholate environment
around the iron(III) centre. Indeed the E/D value increases with
increasing number of catecholate oxygen atoms in the iron(III)
coordination sphere, the existence of single species, as found at
pH 10.2, with E/D ≈ 0.3 is predicted for the trigonally distorted
octahedral geometry in the tris-catecholate binding mode.54 The
signals at g ≈ 4.2 of both spectra of the heterodinuclear species
exhibit more asymmetric features than those of the iron(III)
mononuclear complex (see Fig. 7), which can be attributed to
the presence of more than one metal centre in solution.53,54 Indeed
The broader hyperfine structure of the spectrum at pH 10.2
in the DMs = 1 transition with an average coupling constant
of 75.16 G indicates the presence of the second metal ion. The
hyperfine pattern is normally more complicated when the coupling
between the two metal centres is weak.56 The four (a–d in Fig. 9a)
and five (1–5 in Fig. 9b) resonances clearly observed at g =
1.849, 1.887, 1.926, 1.968 (pH 4.6) and 1.828, 1.865, 1.904, 1.944,
1.986 (pH 10.2) are associated with the presence of coupled
metal centres, see Fig. 9. Therefore the EPR spectra clearly
testify the formation of the copper(II)/iron(III) heterodinuclear
complexes of H6L1 with two metal centres, which are weakly
coupled.56–58
Fig. 9 EPR spectrum of the Cu2+/Fe3+/H6L1 1 : 1 : 1 ratio solutions at 4.6 (a) and 10.2 (b) pH values in water–DMSO (1 : 1 v/v), recorded at
5 K. Microwave power of 2.35 mW, modulation amplitude of 1 mT and frequency of 9.64 GHz.
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Syntheses
Conclusions
3,7,11-Tris[N-(3,4-dimethoxybenzoyl)-(3-aminopropyl)]-3,7,11,
17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene, L1p. 3,4-
Dimethoxybenzoyl chloride (1.54 g, 7.7 mmol) dissolved in dry
CH2Cl2 (15 cm3) was added dropwise to the stirred solution
of L12 (1.04 g, 2.6 mmol) in water–CH2Cl2 (35 cm3) simultaneously
with 50 cm3 of 0.1 mol dm−3 sodium hydroxide (50 cm3) during
about 30 min. Then the mixture was stirred for 2 h, and the
layers were separated. The aqueous layer was extracted with
4 × 20 cm3 of CH2Cl2. The combined CH2Cl2 solutions were
dried over Na2SO4 and evaporated to dryness. Recrystallization
from ethyl acetate–cyclohexane afforded white-yellow flakes of the
methyl catecholate protected L1p. Yield: 85%. Rf (10% CH3OH–
CH2Cl2) 0.37. 1H NMR (300 MHz, CDCl3) d 1.503 (q, 4 H,
N3,11CH2CH2CH2N7), 1.583 (q, 2 H, N7CH2CH2CH2NH), 1.795
(q, 4 H, N3,11CH2CH2CH2NH), 2.385 (br, 6 H, N3,11CH2CH2,
N7CH2CH2), 2.486 (t, 4 H, J = 6 Hz, N3,11CH2CH2), 2.624
(t, 4 H, J = 6 Hz, N7CH2CH2), 3.230 (t, 2 H J = 5.4 Hz,
CH2CH2NH), 3.522 (t, 4 H, J = 5.7 Hz, CH2CH2NH), 3.567
(s, 4 H, N3,11CH2CH), 3.779, 3.836 and 3.856 [s, 3 × 6 H, OCH3],
6.712 (d, 2 H, J = 8.4 Hz, CHbz), 6.766 (d, 1 H, J = 8.4 Hz, CHbz),
7.077 (d, 2 H, J = 7.5 Hz, CHpy), 7.171 (d, 2 H, J = 1.8 Hz, CHbz),
7.353 (d, 1 H, J = 1.8 Hz, CHbz), 7.383 (d, 2 H, J = 1.8 Hz, CHbz),
7.435 (d, 1 H, J = 2.1 Hz, CHbz), 7.704 (t, 1 H, CHpy). 13C NMR
(300 MHz, CDCl3) d 24.03 (N7CH2CH2CH2N3,11) 25.83, 26.67,
38.74, 39.26, 51.97, 52.09, 52.55, 54.20, 56.12 and 56.19 (OCH3),
60.18 (N7,11CH2CH), 110.3, 110.4, 110.9, 111.0 and 119.8 (CHbz),
119.9 (CHpy), 122.8 (CHbz), 127.4 and 127.7 (Cbz), 137.1 (CHpy),
Spectroscopic and/or potentiometric studies have shown that the
new ditopic macrocyclic ligand with tris-catecholate arms, H6L1,
forms heterodinuclear Cu2+/Fe3+ and Cu2+/Zn2+ complexes due to
the presence of two distinct and distant types of donor atom in the
H6L1 molecule.
The acid–base behaviour of the ligand is characteristic of the two
types of basic centre in the molecule, the tetraaza macrocycle and
the catecholate moieties. It was shown that the H6L1 ligand is very
selective for the Fe3+ ion, forming mono-and dinuclear complexes.
The high stability constant of the [FeL1] complex is explained
by the strong coordination of the tris-catecholate to the iron in
a distorted octahedral geometry. As expected for a catecholate
ligand, the mononuclear copper(II) and zinc(II) complexes are
formed only at very high pH, which precluded the determination
of the stability constants of these species. The H6L1 ligand did not
show special affinity for the Zn2+ ion, but a much more significant
one for the Cu2+ ion. The electronic data of Cu2+/H6L1 solutions
suggested the equilibrium between complexes with the copper
coordinated to the macrocycle and in the catecholate arms. The
EPR spectrum of the [Cu2L1]2− complex showed typical features
for a dinuclear complex with weak interactions between the two
copper(II) centres.
The study of Cu2+/Fe3+/H6L1 1 : 1 : 1 ratio aqueous solutions
revealed that mononuclear protonated iron(III) complexes are
formed at very low pH values and then protonated heterodinuclear
species are formed. At pH 7.5 the non-protonated heterodinuclear
[CuFeL1]− complex is formed in ≈ 95%. UV-vis and EPR
spectroscopic studies revealed that the copper is coordinated to
the amine donors of the macrocyclic moiety and the iron to the
oxygen donors from the catecholate of the arms. The interaction
between both paramagnetic centres is weak, as expected due to
the relative length of the arms.
148.9, 149.0, 151.6 and 151.7 (Cbz), 158.4 (Cpy) 167.2 and 167.59
−1
=
(C O). IR (KBr, cm ): 3429, 2962, 2929, 2838 (OCH3), 1705 and
=
1636 (C O), 1604 (C–C ring), 1583, 1550 and 1509 (N–H), 1463
(C –C O), 1441, 1383, 1339, 1262, 1229, 1180, 1129, 1101, 1021,
bz
=
874, 800, 764, 668 and 631. Found: C, 63.22; N, 10.53; H, 8.08.
The heterodinuclear [CuFeL1]− and [CuZnL1]2− complexes may
be explored as simple models of the active sites of cytochrome
c oxidase and bovine erythrocyte superoxide dismutase and the
[Cu2L1]2− complex as a model of dicopper tyrosinases.
Calc. for C49H67N7O9·2H2O: C, 63.02; N, 10.53; H, 7.60%.
3,7,11-Tris[N-(3,4-dihydroxybenzoyl)-(3-aminopropyl)]-3,7,11,
17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene,
H6L1.
BBr3 (3.5 cm3, 29 mmol) was added dropwise through a syringe
to a stirring solution of L1p (0.155 g, 0.23 mmol) in dry CH2Cl2
(4 cm3), at 0 ◦C and under N2. The resulting white-yellow
suspension was allowed to stir during 2 d at r.t. Then to the
reaction mixture, CH3OH (10 cm3) was added at 0 ◦C and the
clear solution was stirred at r.t during 4 h. Upon repeated addition
and evaporation of CH3OH (10 × 10 cm3) to remove the borate
ester, the product was dissolved in CH3OH, precipitated with
Et2O and collected by filtration. The final product, which is a
light beige, crystalline and very hygroscopic powder, was dried
Experimental
General
Microanalyses were carried out by the ITQB Microanalytical
service, and the IR spectra were recorded from KBr pellets on
a UNICAM Mattson 7000 spectrometer.
Reagents
1
under vacuum. Yield: 56%. H NMR (300 MHz, DMSO-d6) d:
3,4-Dimethoxybenzoyl chloride (98% purity) and boron tribro-
mide (99% of purity) were obtained from Aldrich. All the
chemicals were of reagent grade and used as supplied with-
out further purification. 3,7,11-Tris(3-aminopropyl)-3,7,11,17-
tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L12) was syn-
thesized and characterized as reported.13 The organic solvents
were purified by standard methods.59 The reference used for the
1H NMR measurements in D2O was 3-(trimethylsilyl)-propanoic
acid-d4-sodium salt and in CDCl3 and Me2SO-d6 the solvent
itself.
1.648 (br, 2 H, N7CH2CH2CH2NH), 1.920 (br, 8 H, CH2), 2.724
(br, 2 H, CH2), 2.888 (br, 6 H, CH2), 3.022 (br, 10 H, CH2), 3.369
(br, 2 H), 4.294 (s, 4 H, N3,11CH2CH), 6.762 (d, 3 H, J = 8.1 Hz,
CHbz), 7.171 (d, 3 H, J = 7.8 Hz, CHbz), 7.246 (d, 3 H, CHbz),
7.486 (d, 2 H, J = 7.8 Hz, CHpy), 7.841, (t, 1 H, J = 8.1 Hz,
CHpy), 8.201, (t, 1 H, J = 5.4 Hz, NH), 8.266, (t, 2 H, J = 6.9 Hz,
NH), 9.082 and 9.507 (s, OH). 13C NMR (300 MHz, D2O) d
21.93, 26.82, 29.71, 37.68, 38.48, 50.74, 51.28, 51.64, 53.62, 59.65,
110.1, 110.3, 110.8, 112.5, (CHbz), 119.9, (CHpy), 120.2 and 122.9,
(CHbz), 123.3 and 127.3, (Cbz), 137.1 (CHpy), 148.3 and 148.8
548 | Dalton Trans., 2008, 539–550
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bz
bz
py
=
(C ), 151.6, 151.8 (C ), and 157.9 (C ), 167.4 and 171.7 (C O).
of the overall stability constants given directly by the program for
the input data, which include all the experimental points of all the
titration curves.
−1
=
IR (KBr, cm ): 3378 (OH), 1633 (C O), 1597, 1538, 1515 (N–H),
1457 (C –C O), 1440, 1384, 1314, 1290, 1251, 1198, 1141, 1112,
1060, 952, 877, 826, 784, 760, 633. Found: C, 44.76; N, 8.37; H,
4.89. Calc. for C43H59Br4N7O9.H2O: C, 44.70; N, 8.40; H, 4.95%.
bz
=
Spectroscopic studies
All the NMR measurements were performed on a Bruker CXP-
300 or a Bruker DRX-500 spectrometer. Electronic spectra
were recorded on a UNICAM model UV-4 or a Shimadzu
model UV-3100 spectrometer. Aqueous solutions of 2 × 10−4–
2.0 × 10−5 mol dm−3 were used to record spectra in the UV
region and ꢂ 1.0 × 10−3 mol dm−3 in the other regions. EPR
spectroscopic measurements were recorded with a Bruker ESP 380
spectrometer equipped with continuous-flow cryostats for liquid
helium or liquid nitrogen, operating at X-band. The complexes
were prepared at 4.0 × 10−3–1 × 10−2 mol dm−3 in aqueous or
DMSO–H2O (1 : 1 v/v) solutions. The spectra of all complexes
were recorded at 5 to 13 K.
Potentiometric measurements
Reagents and solutions. Metal ion solutions were prepared
at 0.025–0.050 mol dm−3 from the nitrate salts of the metals of
analytical grade with demineralized water (from a Millipore/Milli-
Q system). The solutions were standardized by titration with
Na2H2edta.60 The carbonate free solution of the titrant, KOH,
was prepared and discarded as described previously.13,28 For the
back titrations a 0.100 mol dm−3 solution of HNO3 was used.
Equipment and work conditions. The equipment used was
described previously.13,28 The temperature was kept at 298.2
0.2 K and atmospheric CO2 was excluded from the cell during
the titration by passing purified nitrogen across the top of the
experimental solution in the reaction cell. The ionic strength of
the solutions was kept at 0.10 mol dm−3 with KNO3.
Acknowledgements
The authors acknowledge H. Pereira-Matias for help with the 2D
NMR experiments, M. Pereira for the acquisition of EPR spectra,
and M. C. Almeida for the elemental analysis data. K. P. Guerra
also acknowledges FCT for a grant (SFRH/BD/6492/2001).
Measurements. The [H+] of the solutions was determined◦by
the measurement of the electromotive force of the cell, E = Eꢀ
+
◦
Q log [H+] + Ej, Eꢀ , Q, Ej and Kw = ([H+][OH−]) were obtained
as described previously.13,28 The value of Kw was found to be equal
to 10−13.80 mol2 dm−6.
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550 | Dalton Trans., 2008, 539–550
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©