Vanadium(IV) and-(V) complexes of reduced Schiff bases derived from aromatic o-hydroxyaldehydes and tyrosine derivatives

Article abstract of DOI:10.1002/ejic.201000948

The reduced Schiff bases of salicylaldehyde and pyridoxal (and o-vaniline) with L-tyrosine (Tyr) and D,L-o-tyrosine (o-Tyr), designated as sal-Tyr (1), sal-o-Tyr (2), pyr-Tyr (3), pyr-o-Tyr (4), and o-van-L-Tyr (5), as well as the oxidovanadium(IV) comple

Full text of DOI:10.1002/ejic.201000948

FULL PAPER
DOI: 10.1002/ejic.201000948
Vanadium(IV) and -(V) Complexes of Reduced Schiff Bases Derived from
Aromatic o-Hydroxyaldehydes and Tyrosine Derivatives
Isabel Correia,*[a] Susana Marcão,[a] Kamila Koci,[a][‡] Isabel Tomaz,[a][‡‡] Pedro Adão,[a]
Tamás Kiss,*[b] Tamás Jakusch,[c] Fernando Avecilla,[d] and João Costa Pessoa*[a]
Keywords: N,O ligands / Vanadium / Structure elucidation / Coordination modes / Potentiometry
The reduced Schiff bases of salicylaldehyde and pyridoxal
(and o-vaniline) with -tyrosine (Tyr) and -o-tyrosine (o-
Tyr), designated as sal-Tyr (1), sal-o-Tyr (2), pyr-Tyr (3), pyr-
o-Tyr (4), and o-van- -Tyr (5), as well as the oxidovanadi-
um(IV) complex VO(sal-o-Tyr) (6) have been prepared. The
compounds have been characterized in the solid state and in
solution. The structure of 3 has been determined by X-ray
diffraction. Complexation of these ligands with vanadium in
aqueous solution has been studied by pH potentiometry, UV/
have been determined by pH potentiometry (25 °C, I = 0.2 M
L
D,L
KCl) 1:1 complexes are formed in most systems with variable
protonation states VOLH2 (only with 3 and 4), VOLH, VOL,
and VOLH–1. Dinuclear species (VOL)2H and (VOL)2 were
identified only in the case of 3. Spectroscopic data provided
information about the most probable binding modes for each
stoichiometry. The VIVO complexes formed with the o-Tyr-
L
derived ligands are more stable than those with L-Tyr, the
more adequate coordination position of the phenolate in the
o-Tyr ligands shows a more significant stabilizing effect com-
pared with 3 and 4.
Vis, and circular dichroism (for the L-Tyr derivatives), as well
as by EPR for the VIVO systems and 51V NMR for the VVO2
systems. Stoichiometries and complex formation constants
The mechanisms by which vanadium compounds medi-
ate antidiabetic effects in vivo are poorly described and
understood. It is known that vanadate is a very potent in-
hibitor for phosphatases and other phosphorylases,[3,15] and
key events appear to involve inhibition of protein tyrosine
phosphatases and tyrosine kinases.[3,16–18] Several vanadium
complexes have been found to have better insulin enhancing
activities than inorganic vanadate(V) or oxidovanadi-
um(IV) salts, but most likely this improved efficacy relates
to bioavailability rather than to increased potency at the
phosphatase enzyme active sites.[16]
The binding of VIVO2+ and vanadate to the tyrosine resi-
dues at the C- and N-terminal ends of transferrin is well
documented,[19–23] as well as the direct binding of vanadate
to tyrosine in tyrosyl-DNA phosphodiesterase.[24] However,
the amino acid itself is quite ineffective in coordination to
vanadium. Dipeptides containing Tyr are more effective
binders but the main complexes exclude Tyr from direct
binding.[25] The presence of an anchor group in the ligand
may enhance the binding ability of the ligand. In this sense
Introduction
The presence of vanadium in biological systems, its pres-
ence in vanadium-dependent haloperoxidases[1] and nitro-
genases,[2] its possible physiological roles,[3] its insulin-en-
hancing action,[4–10] and anticancer activity[11–14] have
driven a considerable amount of research. Particular atten-
tion has been paid to the study of the potential benefits
of vanadium compounds as oral insulin substitutes for the
treatment of diabetes. Coordinated ligands should be able
to improve the absorption and possibly the transport and
uptake of vanadium into the cells, reducing the dose neces-
sary for efficacy and lowering metal ion toxicity.
[a] Centro Química Estrutural, Instituto Superior Técnico,
TU Lisbon,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal
[b] Department of Inorganic and Analytical Chemistry University
of Szeged,
P. O. Box 440, 6701 Szeged, Hungary
[c] Bioinorganic Chemistry Research Group of the Hungarian
Academy of Sciences, Department of Inorganic and Analytical
Chemistry, University of Szeged,
P. O. Box 440, 6701 Szeged, Hungary
[d] Departamento de Química Fundamental, Universidaded de N-salicylidene amino acidato-type complexes have been ex-
tensively studied[26–30] and, in many cases, they may be eas-
Coruña,
Campus de A Zapateira s/n, 15071 A Coruña, Spain
ily prepared by the condensation of an aromatic o-hydroxy-
aldehyde and an amino acid. However, these Schiff bases
(SBs) may hydrolyse in solution, and in many cases it is not
possible to characterize either the ligands or their com-
plexes in the solid state. This instability can sometimes be
overcome by reduction of the SB at the imine function to
give an amine.[31–35] A few reduced SBs of salicylaldehyde
[‡] Present address: Mass Spectrometry Laboratory, ITQB – UNL,
Av. República, EAN,
Apartado 127, 2781-901 Oeiras, Portugal
[‡‡]Present address: Centro de Ciências Moleculares e Materiais,
Faculdade de Ciências da Universidade de Lisboa,
Ed. C8, Campo Grande, Campo Grande, 1749-016 Lisboa,
Portugal
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000948.
694
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Vanadium(IV) and (V) Complexes of Reduced Schiff Bases
with amino acids have been isolated,[32,33] and their plasma or other physiological fluids and intracellular me-
[VVOL(hq)] complexes (Hhq = 8-hydroxyquinoline) pre- dia. Therefore the knowledge of the distribution and chemi-
pared and characterized.[33]
cal speciation of vanadium compounds in aqueous solution
Most of the biologically important reactions of vana- is of utmost importance. We report the preparation of the
dium occur in water-based environments such as blood reduced SBs of salicylaldehyde and pyridoxal (and o-vanil-
lin) with l-Tyr and d,l-o-tyrosine. Compounds sal-Tyr (1),
sal-o-Tyr (2), pyr-Tyr (3), pyr-o-Tyr (4), and o-van-l-Tyr (5)
have been prepared (their molecular formulae are depicted
in Scheme 1) and are characterized in the solid state and in
+
solution. Their complex formation with VIVO2+ and VVO2
in aqueous solution in the pH range 2–11 is also studied by
potentiometric and spectroscopic methods (except 5). For
comparison a few solution studies with the amino acid d,l-
o-tyrosine are also presented.
Results and Discussion
Synthesis and Characterisation
The reduced SBs 1–5, depicted in Scheme 1, were pre-
pared by the condensation of one equivalent of the appro-
priate aldehyde with one equivalent of amino acid. Treat-
ment of these condensation products with sodium boro-
hydride resulted in the reduction of the imine bonds, yield-
ing the reduced SBs. The compounds were characterised by
the usual spectroscopic techniques, mass spectrometry, and
elemental analysis.
Crystals of 3 suitable for X-ray diffraction studies were
obtained and details on the crystal structure are given in
the Supporting Information (SI). An ORTEP diagram with
the molecular structure is depicted in Figure 1. The l-Tyr
moiety is present in the zwitterionic form with the amine
nitrogen atom protonated and the carboxylate group depro-
tonated. It is interesting to note that in the pyridoxal moiety
the phenolate atom O(2) is deprotonated and the pyridinic
Scheme 1. Molecular formulae of the four reduced SB compounds
prepared. The totally protonated compounds 1, 2, and 5 corre-
spond to H4L+, and 3 and 4 to H5L2+ and by the main procedure
used were obtained in the solid state in the zwitterionic form. For
compound 5 a different synthetic procedure was used and it was
obtained in its protonated form H4L+·Cl–, with protons at the
amine and carboxylic groups. Its protonation/deprotonation prop-
erties were not studied. When designating these compounds with-
out specific attention to their protonation state, we will omit the
protonation state. Note that in this work Tyr corresponds to l-Tyr
and o-Tyr to the racemic mixture d,l-o-Tyr.
Figure 1. ORTEP diagram of 3 with thermal ellipsoids of the non-
hydrogen atoms drawn at 30% probability level.
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I. Correia, T. Kiss, J. Costa Pessoa et al.
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atom N(1) is protonated. The bond lengths found for 3 are a strong band at 957 cm–1, suggesting a square pyramidal
within the expected ranges and selected bond lengths and structure in the solid state possibly with apical interactions
angles are included in Table 1. In SBs the C=N distances between neighbouring molecules that decrease the strength
are typically 1.24–1.28 Å,[26–31] and the C(8)–N(2) and of the V=O bond.[39]
C(9)–N(2) bond lengths of 1.498(3) and 1.493(3) Å, respec-
tively, are consistent with the formation of single
bonds.[31,34,37]
Solution Studies
The protonation constants for the ligands and formation
constants for their VIVO complexes are defined according
to Equation (1), where charges are omitted for the products
for simplicity [Equation (1)].
Table 1. Selected bond lengths [Å] and angles (°) for 3.
Bond lengths (Å)
Angles (°)
N(1)–C(3)
N(1)–C(4)
O(1A)–C(1A) 1.394(8)
C(1A)–C(2) 1.546(7)
O(1B)–C(1B) 1.329(12)
C(1B)–C(2)
N(2)–C(9)
N(2)–C(8)
O(2)–C(6)
O(3)–C(10)
O(4)–C(10)
C(4)–C(5B)
C(4)–C(5A)
O(5)–C(15)
1.319(4)
1.342(3)
C(3)–N(1)–C(4)
O(1A)–C(1A)–C(2)
O(1B)–C(1B)–C(2)
C(9)–N(2)–C(8)
C(3)–C(2)–C(1B)
C(7)–C(2)–C(1B)
C(3)–C(2)–C(1A)
C(7)–C(2)–C(1A)
N(1)–C(3)–C(2)
N(1)–C(4)–C(6)
N(1)–C(4)–C(5B)
C(6)–C(4)–C(5B)
N(1)–C(4)–C(5A)
C(6)–C(4)–C(5A)
O(2)–C(6)–C(7)
O(2)–C(6)–C(4)
N(2)–C(8)–C(7)
N(2)–C(9)–C(10)
N(2)–C(9)–C(11)
O(3)–C(10)–O(4)
O(3)–C(10)–C(9)
O(4)–C(10)–C(9)
124.8(2)
116.3(5)
107.3(9)
117.59(17)
113.7(8)
126.1(9)
122.1(5)
119.0(5)
120.0(2)
118.8(2)
118.0(3)
122.0(3)
117.7(3)
121.1(3)
121.5(2)
121.9(2)
111.99(18)
113.01(16)
108.33(17)
125.76(19)
119.15(19)
115.04(18)
p VIVO2+ + q L3– + r H+ p (VIVO)p(L)q(H)r
(1)
A similar expression is valid for VVO2 complexes: (VVO2)p-
(L)q(H)r. In the particular case of the protonation constants
of ligands, the stoichiometric index p is equal to zero and
the species are represented as HrL instead of LHr. The com-
plexes corresponding to stoichiometries (VIVO)p(L)q(H)r
and (VVO2)p(L)q(H)r are written as (VIVO)pLqHr and
(VVO2)pLqHr, respectively.
1.456(13)
1.493(3)
1.498(3)
1.287(3)
1.234(3)
1.264(3)
1.492(7)
1.541(7)
1.373(3)
Ligands
Compounds 1–4 are stable in solution in a wide pH
range. Protonation constants and pKa values obtained from
pH potentiometric titrations are listed in Table 2. The
values for d,l-o-Tyr determined by pH potentiometry are
also included for comparison. The fully protonated com-
pounds 1 and 2 correspond to H4L+, 3 and 4 to H5L2+ and
The configuration of 3 is determined by medium/strong d,l-o-Tyr and l-Tyr to H3L+. The pKa of the carboxylic
intramolecular and intermolecular H-bonds: the –NH group is relatively low, nevertheless it was determined by
groups of the pyridoxyl rings are involved in an intermo- pH metric measurements for all ligands and spectophoto-
lecular hydrogen bond with the oxygen atom O(3) of a carb- metric titrations confirmed these values. UV spectra and
oxylate group. The disordered –CH2OH groups are in- details on the calculations are included in the Supporting
volved in an intermolecular H-bond with the phenolate Information.
O(2). The –NH2 groups present two types of H-bonds,
pH Potentiometry alone does not provide information
intermolecular and intramolecular, with the oxygen atoms on the sequence of protonation. The assignment of each
O(4) of the carboxylate group and O(2) of the phenolate, protonation step to each basic group may be done through
respectively. The oxygen atoms O(5) of the phenolic rings the pH dependency of the chemical shift (δ) of the neigh-
1
are involved in an intermolecular H-bond with the carb- bouring atoms of the ionisable groups. H NMR titration
oxylate group O(4).
studies [δ (ppm) vs. pH] were carried out for 1–4 in D2O.
The only neat VIVO complex characterized in the solid Compounds 1 and 2 precipitated in the pH range 4–9, and
state is VIVO(sal-o-Tyr) (6). Although vanadium-containing only limited information was obtained for these two li-
complexes were isolated for the other systems, no satisfac- gands. Figure 2 depicts the titration curves obtained for 4
tory formulation was found for these solids. Some selected (ligand 3 is included in the Supporting Information), and
IR data for the ligands and vanadium complex are shown in the assignment of the proton peaks. The protonation se-
the Supporting Information. All compounds present broad quence can be deduced from the relative values of the pro-
bands in the range 2400–3500 cm–1 corresponding to H- tonation shifts and by comparison with studies reported for
bonded (symmetrical and antisymmetrical) O–H, N–H and related compounds, however, the existence of microequilib-
overtone bands.[38] For all ligands sharp bands emerge from ria and/or isomers with same stoichiometry but protonated
this broad band, assigned to ν(N–H). The νas(COO) appear at different sites cannot be excluded.
at 1615–1650 cm–1, whereas the νs(COO) are assigned to the
The highest pKa value for ligands 1–4 is above 10, and,
bands observed at 1456–1464 cm–1. No bands are observed by comparison with d,l-o-Tyr, can be assigned to the phen-
in the range 1640–1750 cm–1 indicating the zwitterionic na- olic OH group of the Tyr side chain. This is consistent with
ture of compounds 1–4. The strong bands at 1250– the highest Δδ observed for the aromatic protons of the Tyr
1270 cm–1 are assigned to the ν(C–Ophenolate). In the IR side chain, which is between pH 9 and 12. The aromatic
spectrum of solid 6 the characteristic ν(V=O) appears as proton in the pyridinic ring H(10) is expected to be sensitive
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Vanadium(IV) and (V) Complexes of Reduced Schiff Bases
Table 2. Protonation constants for the ligands and formation constants for the VIVO and VVO2 complexes, as defined in Equation 1,
calculated with the PSEQUAD computer program;[57] T = 25 °C and I = 0.2 m KCl. The charges of the species are omitted.
Stoichiometry d,l-o-Tyr
logβpqr
sal-l-Tyr (1)
sal-o-Tyr (2)
logβpqr
pyr-l-Tyr (3)
pyr-o-Tyr (4)
logβpqr
pKa
logβpqr
pKa
pKa
logβpqr
pKa
pKa
Ligands
HL
10.96(5)
19.56(7)
21.81(7)
10.96
8.59
2.25
10.93(5)
20.69(7)
28.79(8)
10.93
9.76
8.10
12.32(27)
22.06(9)
30.08(10)
32.19(10)
12.32
9.75
10.50(3)
20.15(3)
27.82(4)
30.92(6)
32.53(9)
10.50
9.65
7.67
11.80(7)
11.80
9.32
7.68
3.19
H2L
H3L
H4L
H5L
21.12(10)
28.80(10)
31.99(11)
33.47(12)
8.02
30.51(10) 1.72[a]
2.12[a]
3.10
1.61[a]
1.49
[a]
VIV Complexes
VOLH2
VOLH
VOL
21.31(17)
17.14(16)
–
–
4.17
–
29.50(2)
23.04(5)
–
–
41.89(10) 8.50
33.39(26)
6.46
30.74(2)
25.61(4)
18.34(5)
8.86(7)
5.13
7.27
9.48
23.54(5)
16.43(9)
7.1(3)
7.11
9.3
24.73(3)
6.06
18.67(7)[b] 9.23
9.44(24)
VOLH–1
(VO)2L2H
(VO)2L2
VV Complexes
VO2LH2
VO2LH
VO2L
31.41(4)
25.48(5)
17.64(6)
5.93
7.84
[a] These pKa values were confirmed by spectrophotometric titrations (range ca. 200ϽλϽ360 nm), which gave the following values:
1.75Ϯ0.02 (1); 2.10Ϯ0.11 (2); 1.57Ϯ0.02 (3); 1.51Ϯ 0.02 (4). [b] When VOL was not included in the equilibrium model the dinuclear
species (VO)2L2 was refined with a logβ = 35.87Ϯ0.06, but the fitting parameter was much worse. When both species were included,
(VO)2L2 was rejected. This dinuclear species was also rejected in the refinement of the speciation model of the VO–1 system.
1
Figure 2. H NMR titration of 4 and labelling of the peaks. The aromatic protons H(6), H(7), H(8), and H(9) appear as doublets, and
are assigned in the Figure as HAr.
to the protonation of the adjacent Npyridine group. The signed to the second pKa value (ca. 3.1). This is also sug-
major shift in δ for this proton is observed between gested by the Δδ observed for H(1) in this pH region (δ =
2ϽpHϽ4, and the protonation of this group can be as- 0.23 ppm), however, as the Ophenolate is connected to the
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Scheme 2. Deprotonation sequence proposed for ligands 2 (above) and 4 (below).
aromatic ring, its protonation may have the same effect. The between the phenolate oxygen in ortho position and the de-
crystal structure of the ligand suggests that the deproton- protonated carboxylate or the NH group must be responsi-
ation of the phenolate group occurs first. Moreover, earlier ble for the higher basicity of both groups in this compound.
studies with pyridoxal derivatives showed the same depro-
tonation order.[31,41]
To assign the pKa to the Namine and Npyridine proton-
ations we can analyse the behavior of protons H(4) and
VIVO Complexes
H(3) near the Namine group. Proton H(4) shows high Δδ
Compounds 1–4 are able to coordinate to VIV, and com-
between 1ϽpHϽ4 and 7ϽpHϽ9; H(3) also shows a con- plex formation was monitored using potentiometry and
siderable shift between pH 6.0 and 9.0. Thus, we can tenta- spectroscopy (UV/Vis, CD and EPR). Although slow equil-
tively assign the pKa value of 7.68 to the deprotonation of ibration or precipitation of VIVO(OH)2 occurred in some
the Namine group and accordingly the pKa of 9.67 is as- cases for pHϾ4, carefully chosen experimental titration
signed to the Npyridine
.
conditions varying the total oxidovanadium(IV) concentra-
Taking into account the analysis of the NMR titration tion and ligand to metal ratios (L:M) allowed the solution
data for 3, we propose the following protonation sequence characterization of all systems. The complex formation con-
for both ligands: COO–, Ophenolate, Namine, Npyridinic, and the stants, defined by Equation (1), were calculated from
Tyr side chain Ophenolate, which is depicted in Scheme 2 for potentiometric titrations carried out at different vanadium/
ligand 4. Nevertheless, this protonation sequence cannot be ligand ratios and are included in Table 2. Concentration
simply extrapolated for 1 and 2 since the absence of the N distribution diagrams are presented in Figure 3.
atom and the aromatic substituents should not decrease the
Compound d,l-o-Tyr forms stable complexes with
Ophenolate pKa value that much. For 1 and 2 it is reasonable VIVO2+, with stoichiometries VOLH2 and VOLH in the pH
to accept that the carboxylic moiety deprotonates first and range 2–4, but the hydrolytic products of VIVO2+, namely
the side chain phenolic OH group last. The possibility of H [(VIVO)2(OH)2]2+, have a much higher relative importance
bond formation between Ophenolate and Namine makes the in the pH range of formation of species VOLH in the VIVO-
discussion of the order of deprotonation between these o-Tyr system than for the other studied VIVO systems (see
groups almost meaningless. Analysis of the statistical data below). Thus, o-Tyr is a weak VIVO binder as when the L:M
shows a low standard error for all pKa values except for ratio is 2:1 or lower it is unable to prevent hydrolysis of the
the side chain Ophenolate. This is due to the higher value metal ion and precipitation of VO(OH)2 at pH ≈ 4.5 (see
determined for this pKa in 2. The possibility of H- bonding Figure 3, e). Ligands 1–4 contain a suitable Ophenolate donor
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Vanadium(IV) and (V) Complexes of Reduced Schiff Bases
IV
V
Figure 3. Concentration distribution diagrams for the vanadium-containing systems. Conditions: CV
= 3 mm, L:M = 2:1, 25 °C
or V
+
and I = 0.2 m KCl: a) VIVO2+ + 1; b) VIVO2+ + 2; c) VIVO2+ + 3; d) VIVO2+ + 4; e) VIVO2+ + o-Tyr, f) VVO2 + 4. Dotted lines indicate
the pH range where the model is less accurate (due to hydrolysis and/or slow equilibria). The Ophenolate anchor from the aromatic aldehyde
in ligands 1–4 can stabilize vanadium in solution being able to efficiently extend the binding ability of Tyr (e) to a wider pH range,
especially if present in the ortho position (b, d, f).
which allows the formation of a (6+5) chelation system that
EPR spectra were measured in frozen solution (77 K) to
can stabilize the metal ion in solution in a wider pH range. confirm the speciation model and to elucidate the binding
The interaction of VIVO2+ with 1 results in the formation modes for the various stoichiometries. EPR spectra in the
of the neutral species VOLH, which predominates between region corresponding to MI = 5/2 and 7/2 obtained for the
pH 4 and 7. Further successive deprotonations yield VOL VIVO systems are depicted in the Supporting Information.
(pKa = 7.11) and VOLH–1 (pKa ≈ 9.3). The same behavior Table 3 shows the spin Hamiltonian parameters obtained
and stoichiometries are observed in the VIVO system with by simulation of the experimental spectra.[40] Wüthrich[42]
2. The pKa values of the VIVO complexes with 1 and 2 differ and Chasteen[43] developed an additivity rule to estimate
est
est
in more than one pH unit and the proton displacement con- the hyperfine coupling constant Az [Az = Σ Az,i (i = 1–
stants (K*) characteristic to the formation equilibrium: 4)] for VIVO systems based on the contributions Az,i of each
VIVO2+ + H3L+ pVIVOL + 3H+ have the following logK* of the four equatorial donor groups. The estimated accu-
est
values: –12.36 and –11.41 for 1 and 2, respectively. There- racy of Az is Ϯ1.5ϫ10–4 cm–1.
fore, 2, with a Tyr-Ophenolate donor atom in the ortho posi-
The Az,i values used in this work are as follows: Az,COO–
tion, has higher affinity for VIVO.
= 42.1ϫ10–4 cm–1,[44] Az,phenolate = 38.9ϫ10–4, Az,amine
=
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Table 3. Spin Hamiltonian parameters obtained by simulation of the EPR spectra. Plausible binding modes are also indicated for each
species formed. Hyperfine coupling constants (Az, AЌ) are given in ϫ104 cm–1. The Azest presented do not take into account any influence
from axially bound donors.[48]
est
System and stoichiometry
gЌ
gz
AЌ
Az
Az
Binding modes (Ophe = Ophenolate
)
VO–sal-Tyr
II – VOL
III – VOLH–1
[b]
[c]
1.977 1.946 59.0
1.976 1.949 56.5
1.978 1.949 52.4
168.6 166.6
165.4 165.3
159.1 159.5
(Namine, COO–, OH2, OH–)eq (O–
)
phe ax
(O–phe, COO–, OH2, OH–)eq (Namine ax
)
[b]
(Namine, COO–, 2ϫ OH–)eq (O–
)
phe ax
VO–sal-o-Tyr
II – VOLH
III – VOL
1.974 1.946 57.2
1.974 1.946 57.2
167.5 166.7
167.5 166.7
165.6
168.6
167.1 166.6
165.4
(O–phe, Namine, COO–, OH2)eq (OH2 or OHphe ax
)
(O–phe, Namine, COO–, OH2)eq (OH2 or O–
)
phe ax
(2ϫ O–phe, COO–, OH2)eq (Namine or OH2)ax
(2ϫ O–phe, 2ϫ OH2)eq (Namine or OH2 or COO–)ax
IV – VOLH–1
1.974 1.957 56.5
(Namine, COO–, OH2, OH–)eq (O–
)
phe ax
(O–phe, COO–, OH2, OH–)eq (Namine ax
)
(also possible)
VO–pyr-Tyr
I – VO
VOLH3
1.977 1.936 68.5
1.978 1.942 61.2
1.978 1.942 58.6
1.978 1.952 58.6
1.978 1.942 58.6
1.978 1.952 58.6
1.981 1.940 55.8
1.978 1.955 56.5
1.981 1.940 55.8
1.978 1.955 56.5
181.5 182.6
172.3 173.5
170.4 172.3
165.9 166.7
170.4 172.3
165.9 166.7
167.6 166.7
165.2 165.5
167.6 166.7
165.2 165.5
(4ϫ OH2)eq (OH2)ax
[b]
(COO–, Namine, 2ϫ OH2)eq (OHphe or OH2)ax
[a]
[b]
II – VOLH2
(O–phe, COO–, 2ϫ OH2)eq (Namine ax
)
[b]
(O–phe, Namine, COO–, OH2)eq (OH2)ax
III – VOLH[a]
(O–phe, COO–, 2ϫ OH2)eq (Namine ax
)
[b][d]
[b][d]
(O–phe, Namine, COO–, OH2)eq (OH2)ax
IV – (VOL)2H[a]
(Namine, COO–, OH2, O–
(O–phe, COO–, OH2, O–
)
(O–
)
[g]
phe eq
phe ax
[g]
)
(Namine)ax
phe eq
[g]
[a]
V – (VOL)2
(Namine, COO–, OH2, O–
)
(O–
)
phe eq
[g]
phe ax
(O–phe, COO–, OH2, O–
)
(Namine)ax
phe eq
VO–pyr-o-Tyr
I – VO
1.977 1.936 68.7
1.978 1.942 60.2
1.978 1.949 58.3
181.4 182.6
172.7 173.5
166.6 166.7
165.6
168.7 166.7
165.6
(4ϫ OH2)eq (OH2)ax
II – VOLH2
(COO–, Namine, 2ϫ OH2)eq (O– or OH2)ax
[a]
phe
[e]
(O–phe, Namine, COO–, OH2)eq (OHphe or OH2)ax
(2ϫ O–phe,COO–, OH2)eq (OH2)ax
[f]
(O–phe, Namine, COO–, OH2)eq (OHphe or OH2)ax
[d]
III – VOLH
IV – VOL
1.976 1.943 58.3
1.974 1.946 57.6
(2ϫ O–phe, COO–, OH2)eq (Namine ax
)
167.9 166.7
165.5
(O–phe, Namine, COO–, OH2)eq (OH2 or O–
)
phe ax
(2ϫ O–phe, COO–, OH2)eq (Namine or OH2)ax
168.6
163.5
(2ϫ O–phe, 2ϫOH2)eq (Namine or OH2 or COO–)ax
(O–phe, Namine, COO–, OH2)eq (O– or OH2)ax
phe
V – VOLH–1
VI – VOLH–2
1.974 1.946 56.6
167.8 165.3
≈ 159 158.3
(O–phe, COO–, OH2, OH–)eq (O–
)
phe ax
)
≈ 1.952
(O–phe, COO–, 2ϫ OH–)eq (Namine ax
(O–phe, Namine, 2ϫ OH–)eq (COO–)ax
159.5 (minor)
[a] Probably at least two isomeric species are formed. [b] Phenolate of side chain is protonated. [c] Phenolate of side chain is deprotonated.
[d] Npyr-deprotonated. [e] Npyr-protonated (NpyrH+). [f] Namine-protonated (NamineH+). [g] Deprotonated phenolate from the other mole-
cule of the dinuclear species.
40.1ϫ10–4, Az,H2O
=
45.7ϫ10–4 and Az,OH–
=
of the Tyr moiety usually deprotonates in the pH range 9–
38.7ϫ10–4 cm–1. These can be used to establish the most 10).[49–51] Due to the low solubility of the neutral VOLH,
probable binding mode of the complexes formed, but care no spectroscopic data was obtained for this species. How-
must be taken as the contributions of the donor groups to ever, the EPR spectra show the presence of only one species
the hyperfine coupling may depend on their orienta- at pH 7.00. According to the spin Hamiltonian parameters
tion,[45,46] on the charge of the ligand,[47] and the presence obtained for stoichiometry VOL (Az = 168.6ϫ10–4 cm–1)
of axial donor groups.[48] Moreover, since several species we can assign the following equatorial binding set to this
may coexist in solution and the contributions of each of species: (Namine, COO–, H2O, OH–). Above pH 9.3 the spe-
the donors are not much different, the additivity rule alone cies VOLH–1 is observed in which either the deprotonation
cannot provide proof for the binding mode set.
The anisotropic absorption study for 6 shows that be- other hydroxido group. As the CD data shows a totally dif-
tween pH 6.0 and 8.5, where the equilibrium VOLHpVOL ferent spectral pattern at pH 9.9 and Az
of the Tyr-OH moiety occurred, or the coordination of an-
=
takes place, CD spectra (see below) are very similar. There- 159.1ϫ10–4 cm–1, we consider the latter case as that pre-
fore, the ligand coordination sphere should be similar in dominating. Above pH 10, where the quantitatively less
both species. This is expected since in sal-Tyr the side chain clearly described hydroxido vanadium oligomers are
phenolate group is not in a coordinating position, thus this formed, the spectra lose intensity (Vis, CD, and EPR), con-
deprotonation step must involve a water molecule coordi- sistent with extensive hydrolysis and the formation of EPR
nated to the vanadium center, and not the phenolate of the silent vanadium species. Between pH 9.9 and 10.3 the re-
Tyr residue (in the ligand the pKa is 10.93; if the rest of the maining EPR spectra show the presence of two species,
ligand is coordinated to a metal ion the phenolic OH group which can be assigned to binding isomers, since their ratio
700
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Vanadium(IV) and (V) Complexes of Reduced Schiff Bases
does not change (see SI). Table 3 shows the binding modes
proposed for each stoichiometry, based on the EPR spectra
and spin Hamiltonian parameters obtained.
In 6 coordination of the phenolate of the Tyr residue is
possible and several binding modes may compete. The li-
gand, a racemate, is CD silent. EPR spectra measured for
this system (Figure 4) show species with two sets of param-
2+
eters: one set accounts for VIVO(OH2)5 (I) and another
accounts for VOLH, VOL, and VOLH–1 (II–IV). The simu-
lation of the EPR spectra at pH 4.52, where VOLH is the
major species, suggests tridentate coordination of 2: (Ophen-
est
olate, Namine, COO–, H2O)eq with Az = 166.7ϫ10–4 cm–1.
In VOL the o-Tyr phenolate group can deprotonate and
coordinate, and we propose several plausible binding modes
for this stoichiometry with similar Az values (see Table 3).
The coordination of the o-Tyr phenolate group equatorially
is supported by the stability difference between the VIVO2+
complexes of 1 and 2, therefore the most probable binding
mode is (2ϫOphenolate, COO–, H2O)eq(Namine)ax. The for-
mation of VOLH–1 is accompanied by a slight decrease in
the hyperfine parameter Az and in absorbance values above
pH 9.0. Assuming axially coordinating groups do not affect
Az values, the measured Az, 166.6ϫ10–4 cm–1, is relatively
high and requires a water molecule to remain in one of the
equatorial positions. The data is compatible with binding
modes: (Namine, COO–, H2O, OH–)eq(Ophenolate ax
)
for Az =
Figure 4. High field region of the EPR spectra of frozen solutions
(77 K) containing VIVO2+ and sal-o-Tyr at several pH values (indi-
cated), with CVO ca. 3 mm and L:M = 2:1. Species labeled I–IV are
assigned in Table 3.
166.6ϫ10–4 cm–1 and (Ophenolate, COO–, H2O, OH–)eq-
(Namine ax
for Az = 165.1ϫ 10–4 cm–1.
)
For the VIVO systems with 3 and 4 the complex forma-
tion starts with VOLH2 below pH 2.0 in both cases. Above
pH 3.0 the speciation shows the presence of complexes with
stoichiometries VOLH, (VOL)2H, and (VOL)2 in the case also starts to appear. In the pH range of about 3.0–6.0 these
of ligand 3, and VOLH, VOL, and VOLH–1, in the case of two EPR components coexist, corresponding to Az values
4. The logK* values (VIVO2+ + H4L+ pVIVOLH2 + 2H+) of 170.4ϫ10–4 and 165.9ϫ10–4 cm–1, respectively. Their
of –1.42 and –1.25, for ligands 3 and 4, respectively, show relative intensity is always the same so they must corre-
small differences between the two ligands and indicate sim- spond to isomeric structures of the same stoichiometry. In
ilar binding modes. This also indirectly suggests that in the Table 3 two binding sets are proposed. In these experimen-
deprotonation of pyr-l-Tyr the NH+
deprotonates tal conditions VOLH forms in low amount. We assume it
amine
(pKa3 = 7.67) before the NH+
(pKa4 = 9.65).
has the same spin-Hamiltonian parameters and the same
binding mode as VOLH2, and that deprotonation involves
pyridine
The logK* values of the process VIVO2+
+
H3L+ pVIVOLH + 2H+ of –4.78 and –3.19, for 3 and 4, the NpyrH+ group. At pH 6.12 a new species of lower Az
respectively, show the higher stabilily of the o-Tyr derivative appears and this should correspond to one of the metal ions
in VOLH and indicates that for this stoichiometry the in the (VOL)2H species, supposing the deprotonation and
Ophenolate (o-Tyr) binds to the metal center.
coordination of one of the Tyr-O– groups to the other com-
It is clear from the speciation diagrams that all ligands plex occurs. The dinuclear species has a weak EPR signal,
form quite stable complexes with VIVO2+ as its hydrolytic which implies that either the two VIVO centers are far from
products are not formed in measurable concentration at le- each other and/or the interaction between their unpaired
ast up to pH 8. The l-Tyr compounds show lower stability electrons is not strong. Tyrosyl-dipeptides have been shown
than the o-Tyr derivatives, and the hydrolytic products of to form phenolate-bridged dinuclear complexes with CuII,
–
–
oxidovanadium(IV), [(VIVO)2(OH)5 ]n and VIVO(OH)3 , which were stabilized by hydrophobic stacking interactions
have a much higher relative importance in the pH range between the aromatic rings.[51] A similar kind of dinuclear
typical of the formation of these species.
species might occur in these systems. On increasing the pH
Globally the EPR spectra (see Supporting Information) further this dinuclear species loses one more proton, which
of the VIVO–pyr-Tyr system agree well with the speciation does not dramatically change the EPR spectrum. Above
diagram depicted in Figure 3. Namely, at pH 2.01, besides pH ≈ 11, EPR-silent hydrolytic products become important,
2+
those arising from the aqua complex [VIVO(H2O)5 ϵ VO and the spectra lose intensity.
ϵ I], a signal, probably corresponding to VOLH2 (II), can
Although no differences are evident in the EPR behavior
already be detected. However, at pH 2.50 a new signal (III) of 3 and 4, the same donor atom arrangements should be
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I. Correia, T. Kiss, J. Costa Pessoa et al.
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assumed in both cases. However, while in the VIVO–pyr-Tyr reaches its maximum at pH 7, where VOLH is at maximum
system dinuclear complexes are formed, in the VIVO–pyr-o- concentration. The lower energy band then changes further
Tyr system our data only indicate the formation of mono- and its maximum intensity is observed at pH 7.89, where
meric species. This implies that for the VIVO–pyr-o-Tyr sys- (VOL)2H is at maximum concentration. From this point
tem the Ophenolate from the o-Tyr must be involved in bind- forward the intensity of this band decreases; however, at ca.
ing for the VOLH and VOL stoichiometries.
535 nm the CD intensity increases and reaches a maximum
The CD spectra depicted in Figure 5 show increasing in- at pH ≈ 9.12, where (VOL)2 is the major species. The in-
tensity and an isodichroic point at 850 nm between pH 1.65 crease in the CD signal in Figure 5 (E) at pH 8–10 is com-
and 2.50, indicating the formation of a VOLH3 species, patible with binding sets that include a second Ophenolate
which was not detected by pH potentiometry. As the pro- donor bound to the metal center. Upon further increasing
cess VOLH3 pVOLH2 develops, the band at 560 nm the pH all bands lose intensity and become approximately
reaches a maximum, that at 770 nm decreases, and that at zero at pH ≈ 11.8. Table 3 includes the proposed binding
935 nm increases in intensity and shifts to higher energy. modes for the different species based on the spectroscopic
These changes are in agreement with different binding data.
modes for the two stoichiometries and with an increase in
In the VO–pyr-o-Tyr system complex formation starts
the ligand field. As the processes VOLH2 pVOLHp below pH 2.0. At pH 1.76 the EPR spectrum shows the
2+
(VOL)2Hp(VOL)2 take place as the pH is increased the presence of the aqua complex [VIVO(H2O)5 ϵ VO ϵ I]
spectra increase in intensity, and the lower energy band[52] and a minor species, assigned to VOLH2 for which we pro-
is blue shifted (λmax = 810 nm at pH 7.89). Furthermore, pose the binding mode: (COO–, Namine
, 2ϫOH2)eq-
relevant modifications take place in the bands observed at (O–pyr–phe)ax. This coordination mode is present up to pH ≈
500–600 nm, a positive band at 600 nm is formed and 5.5, together with an isomer with an Az value of
Figure 5. CD spectra recorded for the VIVO2+–pyr-l-Tyr system at CVO ca. 3 mm and L:M 4:1. The pH values are indicated. The EPR
and Vis spectra for this system are included in the SI. The spectra shown in A involve mostly stoichiometries VOLH3 p VOLH2, those
of B and C mainly VOLH2 p VOLH p VOL and those of D mostly VOL p VOLH–1. E and F show how the CD data at 545 nm (E)
and 805 nm (F) fit the speciation (CVO = 3 mm and L:M = 4:1). The increase in the CD signal in E at pH 8–10 is compatible with binding
sets including a second Ophenolate donor bound to the metal center.
702
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Vanadium(IV) and (V) Complexes of Reduced Schiff Bases
Scheme 3. Az values (ϫ104 cm–1) for several of the binding modes proposed. It is assumed that axially-bound donor groups do not
est
affect the Az,i of the equatorial groups (this may not be entirely valid).[48]
166.7ϫ10–4 cm–1 (COO–, Namine, O–pyr–phe, OH2)eq. Above VVO2 Complexes
pH ≈ 4.0 two processes are most probably happening in
parallel (as with VV, vide infra): the deprotonation of the
To establish the ability of our ligands to interact with
+
pyridinic nitrogen (which eliminates one of the binding iso- VVO2
,
51V NMR spectra were measured with solutions
mers), and the deprotonation and coordination of the o- containing ca. 2 mm of vanadate and a L:M of 3:1 at pH ≈
Tyr-OH group. At pH 4.06 a new peak appears with an 7.5. To allow the solutions to fully attain equilibrium, spec-
Az value of 168.7ϫ10–4 cm–1. Several binding modes are tra were recorded 24 h after the preparation of the samples.
possible (see Table 3), but we suggest the same binding The formation of decavanadates was avoided by mixing
mode for this stoichiometry: (2ϫ O–phe, Namine, COO–, neutral vanadate solutions (pH ≈ 7.5) with neutral ligand
OH2)eq (Namine)ax. As the pH is increased only a slight de- solutions. All ligands showed the ability to form complexes
crease of the hyperfine coupling occurs with the formation with VVO2, and Table 4 presents the chemical shift [δ (ppm)]
of VOLH–1, which must involve the deprotonation of a co- and the amount of complexes formed. It is clear from the
ordinated water molecule. We assign the following binding 51V NMR experiments that the pyridoxal derivatives have
mode to this stoichiometry: (O–phe, COO–, OH2, OH–)eq a higher affinity for VVO2 than the salicylaldehyde ligands.
(O–phe or OH2)ax. At pH 11.44 a new species appears corre- For 1 and 2 the chemical shift is found at –526 ppm, in
sponding probably to VOLH–2, for which we propose the agreement with the formation of VVO2 complexes with a
binding of two OH– molecules to the vanadium center. Sev- binding mode involving the tridentate coordination of the
eral binding modes may have similar Hamiltonian param- ligand through the phenolate, amine, and carboxylate
eters, and thus we cannot clearly assign one to this species. groups.[53] At pH 7.5 the degree of complex formation is
Some possibilities are included in Table 3 and Scheme 3 de- low and there is no evidence for the binding of a second
picts several of the binding modes proposed.
Ophenolate donor in the case of 2.
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703

I. Correia, T. Kiss, J. Costa Pessoa et al.
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Table 4. 51V NMR spectroscopic data for the systems studied. The cies at pH 7.9, corresponds to VVO2LH and VVO2L. In the
spectra were measured in H2O containing 5% D2O at L:M = 3:1
or 6:2 mm 24 h after mixing.
latter species the ligand probably binds through two phe-
nolates, the amine, and the carboxylate groups. Both reso-
System
pH L/M
δ(51V) [ppm]
% Complex
nances shift as the pH is changed due to the change
VVO2LHǞVVO2L.
VVO2–sal-l-Tyr
VVO2–sal-o-Tyr
VVO2–pyr-l-Tyr
VVO2–pyr-o-Tyr
7.50 3:1 mm –526
7.50 6:2 mm –526
7.49 6:2 mm –522, –532
7.09 6:2 mm –490, –534
10[a]
15[a]
10, 90
70, 30
Conclusions
[a] The other complexes present are inorganic decavanadates. Its
presence at these pH values is thermodynamically unfavorable and
occurred due to slow equilibration of these solutions.
The reduced Schiff bases of salicylaldehyde, pyridoxal,
and o-vanillin with l-tyrosine and d,l-o-tyrosine, as well as
VIVO(sal-o-Tyr) were prepared and characterized in the so-
lid state and in solution. The reduced Schiff base ligands are
much less susceptible to hydrolysis than their corresponding
Schiff base compounds, which we were not able to isolate
in a pure form. They also form much more stable complexes
than the corresponding Schiff bases. As expected for a po-
tentially tetradentate ligand as compared to tridentate, the
VIVO complexes formed with o-Tyr-derived ligands are
more stable than those of l-Tyr. In fact, in aqueous solu-
tions containing 1:1 ligand to metal stoichiometry, no
VIVO-hydrolytic products were detected up to pH 10 in the
pyr-o-Tyr system, while for the pyr-Tyr system significant
For 3 and 4 resonances appear at slightly higher field (δ
≈ –533 ppm). Ligand 4 shows an additional peak, at
–490 ppm, which is the major species at pH ≈ 7, and we
propose that in this species there is one more group coordi-
nated, which is the side chain o-phenolate. Ligand 3 also
presents a very small peak at –522 ppm that we assign to
an isomeric complex.
Since pyr-o-Tyr showed high affinity for vanadate,
potentiometric titrations were carried at different L:VVO2
ratios to determine the complex formation constants, which
are also included in Table 2. For this system, the distribu-
tion diagram (Figure 3) includes stoichiometries (VVO2)-
LH2, (VVO2)LH, and (VVO2)L. A 51V NMR titration was
also performed to confirm the potentiometric model and a
few of the recorded spectra are included in Figure 6.
–
amounts of [(VIVO)2(OH)5 ]n form above pH 6.
Mainly 1:1 complexes are formed, the main species hav-
ing the composition VOLH2 (only for 3 and 4), VOLH,
VOL, and VOLH–1. The stability difference between the Tyr
and o-Tyr derivatives appears only after the binding of the
Ophenolate donor from the o-Tyr moiety, VOL for the Sal
and VOLH for the Pyr derivatives. Coordination of the side
chain ortho phenolic OH occurs to a high extent mainly in
the pyr-o-tyr complexes, and to a lower extent in the sal-o-
tyr complexes. A possible explanation for this is the dif-
ferent electronic charge of the ligands, due to the presence
of the pyridynic ring in the former ligand, which will in-
crease its solubility/stability in water, particularly in the case
of 4.
Experimental Section
Synthesis of Compounds
sal-L-Tyr (1): To a suspension of l-Tyr (0.90 g, 5 mmol) in water
(25 mL) was added NaOH (4 m) to dissolve the amino acid. To this
mixture were added small portions of a solution of salicylaldehyde
in MeOH (0.53 mL, 5 mmol in 5 mL); the mixture became yellow,
and this color became stronger after the mixing was completed.
The solution was cooled with an ice bath, and sodium borohydride
(0.37 g, 10 mmol) dissolved in 5 mL of water was slowly added. The
solution became colorless and after addition of HCl (until pH ≈
7.0 was reached) a white solid precipitated, which was collected by
filtration, washed with water and MeOH, and dried under vacuum;
yield 0.9 g, 60%. C16H17NO4·1.5H2O (314.34): calcd. C 61.14, H
6.41, N 4.46; found C 61.2, H 6.4, N 4.3. 1H NMR (500 MHz,
D2O, pD = 9.60): 2.9 [m, 2 H, Ar–CH2–CH], 3.4 [m, 1 H, CH–
Figure 6. 51V NMR spectra obtained at 131.404 MHz and at
25.0Ϯ0.5 °C of solutions containing VV and pyr-o-Tyr, CV = 4 mm
and L:M = 2:1 (see text). The pH values are indicated. V1 refers to
HVO4 and/or H2VO4 , the relative amount of HVO4 increasing
with pH.
2–
–
2–
The 51V NMR peak at ca. –533 ppm (C1 between
–538 ppm and –526 ppm), detected between pH ≈ 4.8 and
8.6 should correspond to two stochiometries VVO2LH2 and COOH], 3.9 [dd, 2H Ar–CH2–NH], 6.7 [m, 4 H, CHaromatic], 7.1
[m, 4 H, CHaromatic]. ESI-MS (MeOH) m/z: 288.1 (L+, 100%).
sal- -Tyr·HCl: The compound was also obtained as the hydrochlo-
ride salt: after the reduction reaction the pH was set to 1–2 by
VVO2LH, the ligand being probably tridentate and bound
by the phenolate, amine, and carboxylate donors. The other
peak which appears at ca. –490 ppm (C2 between –484 ppm
L
and –491 ppm) above pH 6.0, and becomes the major spe- addition of small amounts of concd. aqueous HCl. Then the mix-
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Vanadium(IV) and (V) Complexes of Reduced Schiff Bases
ture was evaporated until a dry white residue remained. This resi-
due was dissolved in a minimum amount of 2-propanol. The re-
sulting solution was filtered and diethyl ether was added to the
filtrate till a white solid precipitated. This was quickly collected by
filtration and washed with diethyl ether. Alternatively, the 2-prop-
anol solution was evaporated again and the dry residue was tritu-
rated with diethyl ether, collected by filtration, and washed with
diethyl ether. The reaction and isolation conditions may allow the
formation of the methyl and isopropyl esters of the desired product.
The amount of water found in the several syntheses varied from 0
to 2, and the values reported are only an example of some of those
ethyl ether was added to the filtrate until a solid precipitated. The
resulting cream colored solid was quickly collected by filtration and
washed with diethyl ether; yield 1.4 g, 80%. C17H20NO5Cl·1.5H2O
(380.82): calcd. C 53.6, H 6.1, N 3.7; found C 53.6, H 6.0, N 3.8.
In some preparations the solid that precipitated was contaminated
1
with 2-propanol. H NMR (400 MHz, [D4]MeOH) 3.18 (dd, 1 H,
Ja = 14.44 Hz, J1 = 6.69Hz, ArL-tyrCHaHbCH), 3.24 (dd, 1 H, Jb
= 14.44 Hz, J1 = 6.16Hz, ArL-tyrCHaHbCH), 3.84 (s, 3 H,
CH3OAro-van), 4.1 (t, 1 H, J = 6.5 Hz, ArL-tyrCH2CH), 4.25 (s, 2
H, Aro-vanCHHNH2), 6.78, 6.8 (d, 2 H, J = 8.47 Hz, ArL-tyr), 6.81,
6.83, 6.85, 6.87, 6.89 (m, 2 H, Aro-van), 6.96, 6.98 (dd, 1 H, J1
=
found. C16H18NO4Cl·1MeOH (355.82): calcd. C 57.4, H 6.2, N 3.9;
1.54 Hz, J2 = 7.8 Hz, Aro-van), 7.08, 7.11 (d, 2 H, J = 8.5 Hz,
1
found C 57.1, H 6.2, N 3.7. H NMR (400 MHz, [D6]DMSO): δ = ArL-tyr). [1H]13C NMR (APT) [D4]MeOD; 35.8 (ArL-tyrCH2), 47.17
3
3.02 [dd, 1 H, 2JHH = 14 Hz, J = 8.1 Hz, ArL-tyrCHAHBCH], 3.24 (Aro-vanCH2), 56.61 (CH3OAro-van), 61.29 (ArL-tyrCH2CHCOOH),
2
3
[dd, 1 H, JHH = 14, J = 4.8 Hz, ArL-tyrCHAHBCH], 3.91 [dd, 1 113.9 (Aro-van, CHCHCOMe), 116.7 [ArL-tyr, (CH2)COH], 117.7
H, J1 = 8.1, 3J2 = 4.8 Hz, ArL-tyrCHAHBCH], 4.1[s, 2 H,
[Aro-van
COMe], 124.1 [Aro-van, C(CH2)CHCHCHCOMe], 125.5 [ArL-tyr
(CH2)CCH2], 131.4 [ArL-tyr (CH2)CCH2], 146.7 [Aro-van
CHC(CH2)COH], 148.8 (Aro-van, CHCHCOMe), 158.1 [ArL-tyr
, CHC(CH2)COH], 120.9 [Aro-van, C(CH2)CHCHCH-
3
ArsalCH2NH2], 6.70, 6.72 [d, 2 H, J = 8.4 Hz, ArL-tyr], 6.81 [t, 1
,
3
3
H, J = 7.5 Hz, Arsal], 6.98 [d, 1 H, J = 8 Hz, Arsal], 7.02, 7.04 [d,
,
,
3
3
2 H, J = 8.4 Hz, ArL-tyr], 7.2 [t, 1 H, J = 8.5 Hz, Arsal], 7.38, 7.4
,
[d, 1 H, 3J = 7.5 Hz, Arsal]. 13C NMR (APT) [D6]DMSO: δ = 34.3 (CH2)COH], 170.4 (ArL-tyrCH2CHCOOH).
[ArL-tyrCH2], 44.4 [ArsalCH2], 60 [ArL-tyrCH2CHCOOH], 115.4
VO(sal-o-Tyr) (6): The synthesis was carried at 40 °C under N2. To
a solution of 2 (0.29 g, 1 mmol) in water (30 mL) was added NaOH
(1 m) until pH 9.5–10 was reached. VIVOSO4 (0.22 g, 0.85 mmol)
dissolved in water (10 mL) and sodium acetate trihydrate (0.27 g,
2 mmol) were slowly added. The solution became blue and precipi-
tation of a blue grey solid began. The solution was kept in the
refrigerator overnight and then the solid was collected by filtration,
washed with water, ethanol, and diethyl ether, and dried under vac-
uum; yield 0.2 g, 60%. C16H15NO5V·1.7H2O (382.86): calcd. C
50.19, H 4.84, N 3.66; found C 50.3, H 4.8, N 3.3.
[ArL-tyr
CHC(CH2)COH], 119.1 [Arsal,CH], 124.8 [ArL-tyr, (CH)2CCH2],
130.5 [ArL-tyr (CH)2CCH2], 130.6 [Arsal,CH], 132.1 [Arsal
,
(CH)2COH], 115.5 [Arsal,COHCH], 117.6 [Arsal,
,
,
CHC(CH2)], 156.4 [Arsal, CHC(CH2)COH], 156.7 [ArL-tyr, (CH2)-
COH], 169.6 [ArL-tyrCH2CHCOOH].
sal-o-Tyr (2): The synthesis was similar to the procedure described
for 1 but instead of l-Tyr d,l-o-Tyr was used as the amino acid
source; yield 0.8 g, 55%. C16H17NO4·0.2H2O (290.92): calcd. C
66.06, H 6.03, N 4.81; found C 66.0, H 5.9, N 4.8. 1H NMR
(500 MHz, D2O, pD = 10.2): δ = 3.1 [m, 2 H, Ar–CH2–CH], 3.6
[m, 1 H, CH–COOH], 4.0 [dd, 2H Ar–CH2–NH], 6.8 [m, 4 H,
CHaromatic], 7.1 [m, 4 H, CHaromatic].
A few other vanadium-containing complexes were isolated for the
other systems, but no satisfactory formulation could be found for
these solids.
pyr-L-Tyr (3): The synthesis was similar to the procedure described
pH Metric Measurements: All measurements were made in aqueous
solution. The purity of the ligands was checked pH potentio-
metrically and the exact concentration of solutions were deter-
mined by the Gran method.[54] A stock solution of VIVO was pre-
pared and standardized as reported earlier.[55,56] The H3O+ concen-
tration in the stock solutions was determined by pH potentiometry.
The VV stock solution was prepared by dissolving KVO3 (Sigma–
Aldrich) in an accurately measured volume of a KOH solution of
known molarity (ca. 0.20 m), and its OH– concentration was calcu-
lated taking into account the total volume of the VV stock solution
prepared.
for 1 but instead of salicylaldehyde pyridoxal hydrochloride was
used as the aldehyde source; yield 1.2 g, 70%. C17H20N2O5·0.8H2O
(341.36): calcd. C 58.88, H 6.28, N 8.08; found C 59.0, H 6.3, N
1
8.0. H NMR (500 MHz, D2O, pD = 9.4): δ = 2.2 [s, 3 H, CH3],
3.0 [m, 2 H, Ar–CH2–CH], 3.5 [m, 1 H, CH–COOH], 4.1 [dd, 2H
Ar–CH2–NH], 4.5 [s, 2H, CH2OH] 6.9 [m, 4H, CHaromatic], 7.6 [s,
1 H, CHpyridinic].
pyr-o-Tyr (4): The synthesis was again similar to the procedure de-
scribed for 1 but instead of salicylaldehyde pyridoxal hydrochloride
was used as the aldehyde source and d,l-o-Tyr as the amino acid
source; yield 1.3 g, 80%. C17H20N2O5·1.1H2O (352.17): calcd. C
57.98, H 6.35, N 7.95; found C 58.0, H 6.3, N 7.8. 1H NMR
(500 MHz, D2O, pD = 9.5): δ = 2.3 [s, 3 H, CH3], 3.1 [m, 2 H, Ar–
CH2–CH], 3.7 [m, 1 H, CH–COOH], 4.1 [dd, 2H Ar–CH2–NH],
4.5 [s, 2 H, CH2OH] 6.9 [m, 4 H, CHaromatic], 7.6 [s, 1 H,
CHpyridinic].
All solutions were manipulated in an inert atmosphere (high purity
N2 or purified argon). The ionic strength was adjusted to 0.20 m
KCl and the temperature was 25.0Ϯ0.1 °C. The pH was measured
with an Orion 710A precision digital pH meter equipped with an
Orion Ross 8103BN type combined glass electrode, calibrated for
hydrogen ion concentration.[55] The ionic product of water was pKw
= 13.76.
o-van-L-Tyr (5): l-tyrosine (5.00 g, 27.5 mmol) was suspended in
methanol (200 mL) and KOH (1.55 g) was added. Stirring was
maintained until the solid dissolved, and the resulting mixture was
filtered. To the filtrate was added o-vanillin (o-van, 4.19 g,
27.5 mmol). A yellow color developed and the mixture was left
stirring for 24 h. The large amount of yellow precipitate that
formed was collected by filtration, dissolved in methanol and the
resulting solution was filtered. NaBH4 was added to the filtrate
until the yellow color completely disappeared. The pH was adjusted
to ca. 2 with HCl and the mixture was evaporated till a dry brown-
ish residue remained. This residue was dissolved in the minimum
volume of 2-propanol. The resulting mixture was filtered and di-
The protonation constants of the ligands were determined from
four titration curves of 4 or 8 mL samples with initial concentra-
tions in the range 0.01 to 0.04 m. Stability constants of the VIVO–
ligand systems were determined by pH metric titrations of 5, 10, or
20 mL samples. The metal concentrations were in the range 0.0004–
0.004 m, and the L:M ratio from 0.5:1 to 8:1. Titrations were nor-
mally carried out with KOH solution of known concentration (ca.
0.2 m) under a purified argon atmosphere, from pH 2.0 up to 11.0,
unless very extensive hydrolysis, precipitation, or very slow equili-
bration was detected. Precipitation occurred between pH 4.0 to 9.0
for 1 when the ligand concentration was ca. 4 mm. For ligand 3
Eur. J. Inorg. Chem. 2011, 694–708
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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705

I. Correia, T. Kiss, J. Costa Pessoa et al.
FULL PAPER
slow equilibration or precipitation occurred above pH 6.0. The re-
producibility of the titration points included in the evaluation was
within 0.005 pH units in the pH range 5–11.
nate in solution. For the VIVO systems we used the additivity rule
est [3,42–44]
to estimate the hyperfine coupling constant Az
.
1H and 51V NMR Spectroscopy: All samples were prepared at room
temperature immediately before acquisition of the NMR spectra.
Ligand solutions for the H NMR pH titrations were prepared in
For the determination of the protonation constants corresponding
to pKa1 (pKa of the carboxylic acid group), several sets of UV ab-
sorption spectra (205–360 nm) with concentrations of ca. 8–
9ϫ10–4 m were measured in the pH range ca. 0.8 to 12. An individ-
ual calibration curve EMF vs. [H+] was established for pH values
less than ca. 2, valid for the medium and electrodes. For this pur-
pose a set of HCl solutions was prepared with known H+ concen-
tration, from pH ≈ 0.8 to 2.0, all with an ionic strength of 0.20 m
KCl.
1
D2O (99.995%D) weighing the ligand to obtain the desired concen-
tration. The pD values of these solutions were adjusted with DCl
and CO2-free NaOD solutions, and measured either using a Crison
MicropH 2002 pH-meter with an Ingold 405-M5 combined elec-
trode or a Thermo Orion 420A+ pH meter with a Mettler Toledo
U402-M3-S7/200 combined microelectrode (all calibrated at
20Ϯ1 °C with standard aqueous buffers at pH 4.0 and 7.0). The
final values of pD were determined from pD = pH* + 0.40,[61]
where pH* corresponds to the reading of the pH meter.
To obtain 51V NMR spectra, the solutions containing the VV com-
plexes were normally prepared by adding the appropriate amounts
of the ligand to aqueous sodium vanadate solutions of known con-
centration and at selected pH values (ca. 8.0) in order to have the
desired L:M ratio of 3:1. The initial VV concentration in the sam-
ples was 2 to 5 mm.
For the 1H NMR titration of 1, an 8 mm solution in D2O was
prepared at pD = 0.96 by addition of DCl. Spectra were recorded
up to pD = 11.5 by addition of NaOD or DCl. For 2, an 18 mm
solution was prepared in D2O by addition of NaOD and it was
used to measure the spectra by lowering the pD with DCl, but
precipitation occurred between pD 9.0 and 4.0. For 3 the concen-
tration was 5 mm and the compound was dissolved at low pD, but
even with this concentration precipitation occurred between pH 3.2
and 9.4 and therefore no spectra were measured in this range. A
5 mm solution of 4 was also prepared at pD ≈ 1 in D2O by addition
of DCl. The measurements were performed up to pD = 11.1 by
addition of NaOD or DCl.
The concentration stability constants βpqr = [MpLqHr]/[M]p[L]q[H]r,
where M = VIVO2+ or VVO2+, were calculated using the PSEQUAD
computer program.[57] The formation of the following VIVO–hy-
droxido complexes was taken into account: [VIVO(OH)]+, [(VIVO)2-
(OH)2]2+, [(VIVO)2(OH)5]–, and [VIVO(OH)3]–.[52,58,59] As is dis-
cussed in ref.[52] the logβ of [(VIVO)2(OH)5]– and [VIVO(OH)3]– are
not known accurately; therefore, the calculations for pHϾ4 may
not be entirely reliable, particularly for systems and experimental
conditions where these VIVO–hydroxido complexes have significant
concentrations. For the VV systems the stability constants were
similarly defined, where M refers to VVO2+. The speciation of vana-
date into monomeric, dimeric, tetrameric, pentameric, and decam-
eric species was taken into account.[60]
Physical and Spectroscopic Studies: IR spectra were recorded with
a Jasco FTIR 430 spectrometer. Visible spectra were recorded
either with a Hitachi U-2000 or a Perkin–Elmer Lambda 9 UV/
Vis/NIR spectrophotometer. The CD spectra were recorded with a
JASCO 720 spectropolarimeter, either with a red-sensitive photo-
multiplier (EXEL-308) suitable for the 400–1000 nm range or with
the photomultiplier suitable for the 200–700 nm range. The EPR
spectra were recorded at 77 K (on glasses made by freezing solu-
tions in liquid nitrogen) with a Bruker ESP 300E X-band spectrom-
1
The H and 51V NMR chemical shifts were referenced relative to
sodium 3-(trimethylsilyl)-[D4]propionate at 0 ppm and to external
neat VOCl3 at 0 ppm, respectively. 51V NMR acquisition param-
eters were: 33 kHz spectral width, 30 μs pulse width, 1 s acquisition
time, and 10 Hz line broadening. The signal intensities of the NMR
resonances were obtained using the line-fitting routine supplied
with the NUTSTM PC-based NMR spectral analysis program.[62]
1
eter. The H and 51 V NMR spectra were obtained with a Varian
Unity-500 NMR spectrometer operating at 499.824 and
131.404 MHz, respectively, using a 5 mm broad band probe and a
controlled temperature unit set at 25Ϯ1 °C. ESI-MS of methanol
or DMSO solutions of the compounds were recorded with a 500-
MS Varian Ion Trap Mass Spectrometer in the positive mode (cap-
illary voltage: 80 V; needle voltage: 5 kV; nebulizer gas: nitrogen;
nebulizer pressure: 35 psi; drying gas temperature: 350 °C; drying
gas pressure: 10 psi; m/z range recorded: 100–1000).
X-ray Crystal Structure Determination of 3: Three-dimensional X-
ray data for 3 was collected on a Siemens Smart 1000 CCD dif-
fractometer by the φ–ω scan method. Data was collected at room
temperature. Reflections were measured from a hemisphere of data
collected of frames each covering 0.3 ° in ω. Of the 10960 reflec-
tions measured, all of which were corrected for Lorentz and polar-
ization effects and for absorption by multiscan methods based on
symmetry-equivalent and repeated reflections, 3128 independent
reflections exceeded the significance level (|F|/σ|F|) Ͼ4.0. Complex
scattering factors were taken from the program package
SHELXTL.[63] The structures were solved by direct methods and
refined by full-matrix least-squares methods on F2. Non-hydrogen
atoms were refined with anisotropic thermal parameters in all
cases. Hydrogen atoms were included in calculated positions and
refined by using a riding mode for all the atoms except for N(1),
N(2), C(3), C(8), C(9), C(11), C(13), C(14), C(16), C(17), and O(5),
which were freely refined. Refinement was performed with allow-
ance for thermal anisotropy of all non-hydrogen atoms. The crystal
presents two disorders in C(1) and O(1) on the CH2OH group and
in C(5) on the CH3 group. These disorders have been solved and
two atomic sites for each atom have been observed and refined with
anisotropic atomic displacement parameters. The site occupancy
factors were 0.65905 for C(1A) and O(1A), and 0.46519 for C(5A).
Spectrophotometric Measurements
UV/Vis and CD Spectroscopy: All measurements were made on
water solutions. The temperature was kept at 25.0Ϯ0.3 °C with
circulating water. Unless otherwise stated, by visible or CD spectra
we mean a representation of εm or Δεm values vs. λ [εm = absorp-
tion/(bCM) and Δεm = differential absorption/(bCM) where b = op-
tical path and CM = total VIV concentration]. The spectral range
covered was normally 350–900 (Vis) and either 240–600 or 400–
1000 nm (CD). All measurements and operations of the spectropol-
arimeter were computer controlled. Normally the spectra were re-
corded changing the pH with approximately fixed total vanadium
and ligand concentrations.
EPR Spectroscopy: In the absence of ethylene glycol or DMSO a
relatively broad background may be present in the frozen solution
EPR spectra, therefore for the measurements with 1 the spectra
were run with aqueous solutions containing 5% of DMSO. The
VIVO EPR spectra were simulated using a program from Rocken-
bauer.[40] The EPR spectra help to elucidate which groups coordi-
706
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 694–708

Vanadium(IV) and (V) Complexes of Reduced Schiff Bases
tions (Eds.: A. S. Tracey, D. C. Crans), ACS, Washington, 1998,
p. 157–167.
The crystal used for analysis was a merohedral twin, as indicated
the Flack parameter of 0.7(13) (1252 Friedel pairs).[64,65] A final
difference Fourier map showed no residual density outside: 0.405,
–0.262 eÅ–3. Crystal data and structure refinement parameters are
collected in Table 5. The structure was solved using the SHELXS[66]
Program for Crystal Structure Determination and refined with
SHELXL[67] Program for Crystal Structure Refinement.
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Table 5. Crystal data and structure refinement parameters for 3.[a]
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[7]
[8]
Formula
Mr
C17H20N2O5
332.35
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T [K]
Crystal system
Space group
a [Å]
298(2)
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orthorhombic
P212121
8.6613(5)
12.2976(7)
15.9747(10)
1701.52(17)
704
[10]
b [Å]
[11]
[12]
c [Å]
V [Å3]
F(000)
[13]
Z
4
Dc [gcm–3]
1.297
0.096
μ [mm–1]
[14]
[15]
[16]
Rint
0.0265
10960
3128
1.024
0.0482
0.1214
Reflections measured
Independent reflections[b]
Goodness-of-fit on F2
[c]
R1
wR2 (all data)[c]
[a] The structures were solved using the SHELXS Program for
Crystal Structure Determination and refined with SHELXL (pro-
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=
Σ||Fo| – |Fc||/Σ|Fo|, wR2 = {Σ[w(||Fo|2 – |Fc|2|)2]/Σ[w(Fo4)]}1/2
.
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CCDC-791752 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[19]
[20]
Supporting Information (see footnote on the first page of this arti-
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figure SI2: IR spectra of the compounds, figure SI3: study of the
protonation processes of the compounds, figure SI4: spectroscopic
studies of the VIVO systems, figure SI5: deprotonation sequences
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The authors are grateful to the Hungarian National Research
Fund, Országos Tudományos Kutatási Alapprogramok (OTKA)
( (NI77833), the Hungarian Academy of Sciences, the Fundo
Europeu para o Desenvolvimento Regional (FEDER), the Fun-
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Mass Spectroscopy Network (RNEM, IST-UTL Center) for pro-
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Received: September 7, 2010
Published Online: January 10, 2011
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