VIVO versus VIV complex formation by tridentate (O, Narom, O) ligands: Prediction of geometry, EPR 51V hyperfine coupling constants, and UV-Vis spectra

Article abstract of DOI:10.1021/ic400186x

Systems formed using the V^IVO²⁺ ion with tridentate ligands containing a (O, N_arom, O) donor set were described. Examined ligands were 3,5-bis(2-hydroxyphenyl)-1-phenyl-1H-1,2,4-triazole (H ₂hyph^Ph),

Full text of DOI:10.1021/ic400186x

Article
pubs.acs.org/IC
VIVO Versus VIV Complex Formation by Tridentate (O, Narom, O)
51
Ligands: Prediction of Geometry, EPR V Hyperfine Coupling
Constants, and UV−Vis Spectra
Luisa Pisano,† Katalin Varnagy,‡ Sarolta Timari,‡ Kaspar Hegetschweiler,§ Giovanni Micera,†,∥
́
́
and Eugenio Garribba*,†,∥
†Dipartimento di Chimica e Farmacia, Universita
̀
di Sassari, Via Vienna 2, I-07100 Sassari, Italy
‡Department of Inorganic and Analytical Chemistry, University of Debrecen, H-4010 Debrecen, Hungary
§Fachrichtung Chemie, Universitat des Saarlandes, Postfach 151150, D-66041 Saarbrucken, Germany
̈
̈
∥Centro Interdisciplinare per lo Sviluppo della Ricerca Biotecnologica e per lo Studio della Biodiversita
Sassari, Italy
̀
della Sardegna, I-07100
S
* Supporting Information
ABSTRACT: Systems formed using the VIVO2+ ion with tridentate ligands
containing a (O, Narom, O) donor set were described. Examined ligands
were 3,5-bis(2-hydroxyphenyl)-1-phenyl-1H-1,2,4-triazole (H2hyphPh), 4-
[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid (H3hyphC),
4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzenesulfonic acid
(H3hyphS), and 2,6-bis(2-hydroxyphenyl)pyridine (H2bhpp), with
H3hyphC being an orally active iron chelator that is commercially available
under the name Exjade (Novartis) for treatment of chronic iron overload
arising from blood transfusions. The systems were studied using EPR, UV−
Vis, and IR spectroscopies, pH potentiometry, and DFT methods. The
ligands bind vanadium with the two terminal deprotonated phenol groups
and the central aromatic nitrogen to give six-membered chelate rings. In
aqueous solution the main species were the mono- and bis-chelated VIVO
complexes, whereas in the solid state neutral non-oxido VIV compounds
were formed. [V(hyphPh)2] and [V(bhpp)2] are hexacoordinated, with a geometry close to the octahedral and a meridional
arrangement of the ligands. DFT calculations allow distinguishing VIVO and VIV species and predicting their structure, the 51V
hyperfine coupling constant tensor A, and the electronic absorption spectra. Finally, EPR spectra of several non-oxido VIV species
were compared using relevant geometrical parameters to demonstrate that in the case of tridentate ligands the 51V hyperfine
coupling constant is related to the geometric isomerism (meridional or facial) rather than the twist angle Φ, which measures the
distortion of the hexacoordinated structure toward a trigonal prism.
One exception is the ascidians and polychaete worms, in which
VIII is the predominant oxidation state.4,5 State +IV is
particularly important in the human organism because
vanadium is transported in the blood, almost independently
of its initial chemical form, as oxidovanadium(IV) or VIVO2+.10
In vivo blood circulation monitoring-electron paramagnetic
resonance (BCM-EPR) performed on rats showed that
approximately 90% or more of vanadium administered as
VOSO4 is present in the VIVO2+ form in nearly all organs.11
Furthermore, it is well accepted that inside the cell vanadium
exists mainly as VIVO2+ because of the presence of reductants,
such as GSH, NADH, and ascorbate, although the ratio among
VIV and VV remains unknown.10,12
INTRODUCTION
■
Vanadium is a ubiquitous trace element,1 and it is present in
two families of enzymes, i.e., vanadium-dependent haloperox-
idases2 and nitrogenases.3 In the sea it is accumulated in several
marine organisms such as ascidians4 and polychaete fan
worms,5 and it is found in very high concentrations in some
species of the mushroom genus Amanita.6 It is commonly
accepted that vanadium also has an essential role in humans,
and its inhibition of many phosphate-metabolizing enzymes,
such as phosphatases, ribonuclease, and ATPases, suggests that
it may be involved in regulation of phosphate metabolism.7
Vanadium compounds have been proved to have pharmaco-
logical activity, in particular, as antidiabetic agents,5b,8 and their
transport in blood serum was recently investigated.9
The only oxidation states relevant for biological systems are
+III, +IV, and +V.1,7 However, VIII is very susceptible to
oxidation and appears to be of less importance than VIV and VV.
Received: January 24, 2013
© XXXX American Chemical Society
A
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a
The chemistry of vanadium(IV) is dominated by the VIVO2+
ion. Formation of non-oxido or ‘bare’ hexacoordinated
vanadium(IV) complexes is rather difficult because the strong
VO bond must be broken and the oxido ligand must leave
the complex as a good leaving group. The number of non-oxido
vanadium(IV) complexes in the electronic Cambridge Struc-
tural Database remains scarce and is much lower than that of
oxidovanadium(IV) species.13 Nevertheless, non-oxido
vanadium(IV) centers have important biological functions, for
example, in amavadin, the complex isolated from Amanita
muscaria,14 and in the cofactor of vanadium nitrogenase.3 This
has stimulated the synthesis and investigation of new non-oxido
vanadium(IV) complexes over the last 10 years. Recently, new
models of amavadin have been synthesized with tetradentate
ligands.15
Scheme 1. Ligands
A bidentate ligand can form non-oxido species if it is
provided with two very strong donors with a high affinity for
vanadium(IV), for example, 2-mercaptophenol with 3 × (O, S)
coordination,16 catechol and enterobactins with 3 × (O, O)
coordination,17 and maleonitriledithiol, 2-thioxo-1,3-dithiole-
4,5-dithiol, and dithiolene with 3 × (S, S) coordination.18 On
the contrary, a tridentate ligand needs only two terminal strong
donors (essentially phenolate or alkoxide oxygens), and non-
oxido compounds have been isolated and characterized through
X-ray diffraction analysis with (O, N, O)19−21 and (O, X, O)
ligands (X = S, Se, P).22 Formation of a non-oxido
vanadium(IV) species is significantly favored by the preorga-
nization of the tridentate ligand, as in the case of cis-inositol
derivatives.23
a
Donor atoms are represented in red. H2hyphPh and H2bhpp are
poorly soluble in water, whereas H3hyphC and H3hyphS are soluble.
into two groups according to their solubility in water.
Furthermore, H3hyphC is also known as ICL670 or Deferasirox
and is an orally active iron chelator29 that is more effective than
the parental desferrioxamine (desferal, DFO) to reduce liver
iron concentrations in patients with nontransfusion-dependent
thalassemia (NTDT), and it is now commercially available as
Exjade (produced by Novartis). H3hyphC is also a good
complexing agent for other metal ions, such as CuII, ZnII, and
AlIII.30
This study was conducted with the combined application of
spectroscopic (EPR, UV−Vis, and IR), analytical (pH
potentiometry), and computational (DFT methods) techni-
ques. Data obtained will aid in understanding the experimental
conditions that favor formation of non-oxido complexes and
establishing the thermodynamic stability of these potential
hexacoordinated vanadium(IV) compounds. Finally, the results
may be used to shed light on the geometric and electronic
structure of these particular complexes as well as analogous
species found in biological systems, such as amavadin.
Depending on the specific ligand, various geometries
(octahedral and trigonal prismatic),24 ground state (dxy or
25
2
dz ), isomers (facial or meridional when the ligand is
tridentate),21,22 and spectroscopic responses are possible.26,27
In particular, EPR spectroscopy can be used to follow the
transformation of an oxido to a non-oxido vanadium(IV)
complex through examination of the isotropic (Aiso) and
anisotropic 51V hyperfine coupling constant along the z axis
(Az), which decrease from the normal values of 80−105 and
150−180 × 10−4 cm−1 (Aiso and Az, respectively, for VIVO
species) to 50−75 and 100−140 × 10−4 cm−1 (Aiso and Az,
respectively, for VIV species).27 Changes in the EPR parameters
2
are associated with the switch of the ground state from dxy to dz
when the geometry changes from octahedral to trigonal
prismatic.28
In this study, the behavior of systems containing the VIVO2+
ion with tridentate ligands potentially able to form non-oxido
species was examined in aqueous solution and the solid state.
The studied ligands were 3,5-bis(2-hydroxyphenyl)-1-phenyl-
1H-1,2,4-triazole (H2hyphPh), 4-[3,5-bis(2-hydroxyphenyl)-1H-
1,2,4-triazol-1-yl]benzoic acid (H3hyphC), 4-[3,5-bis(2-hydrox-
yphenyl)-1H-1,2,4-triazol-1-yl]benzenesulfonic acid (H3hyphS),
and 2,6-bis(2-hydroxyphenyl)pyridine (H2bhpp). These ligands
can bind a metal ion with simultaneous coordination of the two
terminal deprotonated phenol groups and central aromatic
nitrogen, and they can form two six-membered chelate rings
with donor sets (O−, Ntriaz, O−) or (O−, Npyr, O−) (Scheme 1).
To the best of our knowledge, only non-oxido vanadium(IV)
species formed by (O, N, O) ligands with imine (in most cases)
or amine nitrogens have been reported in the literature.19−21
The presence of carboxylic and sulfonic groups in H3hyphC and
H3hyphS renders these ligands soluble in water, whereas
H2hyphPh and H2bhpp are poorly soluble and can be used to
form solid complexes. Therefore, these ligands can be divided
EXPERIMENTAL AND COMPUTATIONAL SECTION
■
Chemicals. Water was deionized prior to use through a Millipore
Milli-Q Academic purification system. VIVO2+ solutions were prepared
from VOSO4·3H2O following methods in the literature.31
Ligands. H2hyphPh, H3hyphC, and H3hyphS were synthesized
according to the methods established in the literature.32
H2bhpp was prepared in a two-step synthesis. In the first step, a
mixture of 2,6-dibromopyridine (6.6 mmol), (2-methoxyphenyl)-
boronic acid (13.2 mmol, 2 equiv), Pd(OAc)2 (1.5 mol %), and
Na3PO4·12H2O (26.3 mmol) in 100 mL of iPrOH/H2O 1:1 was
stirred at 80 °C in air for 2 h. The mixture was added to brine (100
mL) and extracted four times with ethyl acetate (4 × 50 mL). The
combined organic phase was dried over CaCl2 and concentrated under
vacuum. The residue was purified by flash chromatography (petroleum
ether/AcOEt/Et3N 8:2:0.1) to afford 1.73 g (90%) of 2,6-bis(2-
methoxyphenyl)pyridine as a pale yellow powder and characterized by
comparison with spectroscopic data from the literature.33 Demethy-
lation of 2,6-bis(2-methoxyphenyl)pyridine was performed by reaction
with pyridinium hydrochloride, according to a general procedure
described in the literature,34 to afford the desired product in an almost
quantitative yield. 2,6-Bis(2-hydroxyphenyl)pyridine (H2bhpp) was
B
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characterized by comparison with the spectroscopic data reported in
the literature.34
Spectroscopic Measurements. Anisotropic EPR spectra were
recorded using an X-band (9.15 GHz) Varian E-9 spectrometer at 100
K. All operations were performed under a purified argon atmosphere
to prevent oxidation of the VIVO2+ ion. Spectra were simulated using
Bruker WinEPR SimFonia software.42 Electronic spectra were
recorded using a Perkin-Elmer Lambda 35 spectrophotometer in the
same concentration range used for potentiometry or by dissolving the
solid compounds [V(hyphPh)2] and [V(bhpp)2] in MeOH.
Potentiometric Measurements. The stability constants of
proton and VIVO2+ complexes were determined by pH potentiometric
titrations of 3 mL samples. The ligand to metal molar ratio was
between 1:1 and 10:1, and the VIVO2+ concentration was 0.0003−
0.004 M. Titrations were performed from pH 2.0 until precipitation or
very extensive hydrolysis by adding carbonate-free KOH of known
concentration (ca. 0.2 M KOH).35 pH was measured using a Metrohm
6.0234.110 combined electrode, calibrated for hydrogen ion
concentration by the method of Irving et al.36 Measurements were
obtained at 25.0 0.1 °C and at a constant ionic strength of 0.2 M
KCl using a MOLSPIN pH meter and a MOL-ACS microburet (0.50
mL) controlled by computer. Purified argon was bubbled through the
samples to ensure the absence of oxygen. The number of experimental
points was 50−70 for each titration curve, and the reproducibility of
the points included in the evaluation was within 0.005 pH units in the
measured pH range. The stability of the complexes was reported as the
logarithm of the overall formation constant βpqr = [(VO)pLqHr]/
[VO]p[L]q[H]r, where VO stands for VIVO2+ ion, L is the
deprotonated form of the ligand, and H is the proton, and it was
calculated using SUPERQUAD37 and HYPERQUAD software.38
Standard deviations were determined based on random errors.
Conventional notation was used, and negative indices for protons
denote hydroxido ligands. Hydroxido complexes of VIVO2+ were taken
into account, and the following species were assumed: [VO(OH)]+
(log β10−1 = −5.94), [(VO)2(OH)2]2+ (log β20−2 = −6.95), with
stability constants calculated from the data of Henry et al.39 and
corrected for different ionic strengths using the Davies equation,40
Infrared spectra (4000−600 cm−1) were obtained with a Jasco FT/
IR-480Plus spectrometer using KBr disks. Elemental analysis (C, H,
N) was carried out using a Perkin-Elmer 240 B elemental analyzer.
DFT Calculations. All of the geometries of the VIVO and VIV
complexes were optimized in the gas phase with Gaussian 09 (revision
C.01) software43 using the hybrid exchange-correlation functional
B3P8644 and the basis set 6-311g.45 The functional B3P86 ensures a
high degree of accuracy in the prediction of the structures of first-row
transition metal complexes,46 in particular, for vanadium com-
pounds.47
The 51V A tensor was calculated using the functional BHandHLYP
(as incorporated in Gaussian) and 6-311g(d,p) basis set with Gaussian
0943 or using the functional PBE048 and VTZ basis set using ORCA
software,49 according to the procedures published in the literature.50 It
is important to note that for a VIVO2+ species the Az value is usually
negative, but in the literature the absolute value is usually reported.
The 51V HFC tensor A has three contributions: the isotropic Fermi
contact (AFC), the anisotropic or dipolar hyperfine interaction (AD),
and one second-order term that arises from spin−orbit (SO) coupling
(ASO),49 i.e., A = AFC1 + AD + ASO, where 1 is the unit tensor. The
values of the 51V anisotropic hyperfine coupling constants along the x,
y, and z axes are Ax = AFC + AxD + AxSO, Ay = AFC + ADy + AySO, and Az =
AFC + ADz + ASzO. From these equations, the value of Aiso is Aiso = 1/3
(Ax + Ay + Az) = AFC + 1/3(AxSO + ASyO + ASzO) = AFC + APC, where the
[VO(OH)3]− (log β10−3 = −18.0), and [(VO)2(OH)5]− (log β20−5
=
−22.0).41 The uncertainty (σ values) of the stability constants is given
in parentheses in Table 1.
term 1/3(ASxO + ASyO + ASzO) is called the isotropic pseudocontact,
49
APC
.
Gaussian 09 neglects the effect of SO coupling, and the
Table 1. Protonation Constants (log β) of the Ligands and
VIVO2+ Stability Constants (log βpqr) at 25.0 0.1 °C and I =
equations above become Aiso = AFC, APC = 0, Ax = Aiso + AxD, Ay = Aiso
+
ADy , and Az = Aiso + ADz . The theoretical background was described in
a
0.20 M (KCl)
detail previously.50b The percent deviation from the absolute
experimental value, |Az|exptl, was calculated as 100 × [(|Az|calcd
−
species
H3hyphC
H3hyphS
|Az|exptl)/|Az|exptl] (see Table 5).
HL
10.61(2)
19.30(3)
b
10.49(4)
19.17(5)
<20.17
<1
The relative energy of the mono-chelated species formed in water
by H3hyphC, H3hyphS, and H2bhpp was calculated at the B3P86/6-
311g level of theory by computing the solvent effect using the SMD
model available in Gaussian 09 software.51 The SMD model is based
on the quantum mechanical charge density of a solute molecule
interacting with a continuum description of the solvent, which takes
into account the full solute electron density without defining partial
atomic charges and represents the solvent not explicitly but rather as a
dielectric medium with surface tension at the solute−solvent
boundary. This model has been demonstrated to give good results
H2L
H3L
pKa(COOH)/pKa(SO3H)
pKa(OH)1
pKa(OH)2
VOL
b
8.69
8.68
10.61
10.49
17.41(8)
7.68 (40)
33.04 (12)
16.70(5)
8.38 (70)
VOLH−1
VOL2H
33.06(12)
a
b
in the prediction of solvation free energy.51 The total value of ΔGtot
The uncertainties (σ values) are given in parentheses. Not
aq
measurable for the low solubility of the ligand until deprotonation
of the carboxylic group.
can be separated into the electronic plus nuclear repulsion energy
(ΔEele), the thermal contribution (ΔGtherm), and the solvation free
energy (Δ(ΔGsolv)): ΔGtot = ΔEele + ΔGtherm + Δ(ΔGsolv). The
aq
thermal contribution was estimated using the ideal gas model and the
calculated harmonic vibrational frequencies were employed to
determine the correction arising from zero-point energy and thermal
population of the vibrational levels.52
Time-dependent density functional theory (TD-DFT) calcula-
tions53 were used to predict the excited states of the VIVO and VIV
complexes and obtain the expected electronic absorption spectra.
Calculations were performed at the B3LYP/6-311g level of theory and
based on the geometry simulated in the gas phase. The electronic
absorption spectra of [VO(hyphC)(H2O)]− and [V(hyphPh)2]
reported in Figure 8 were generated using GaussView version 4.1
software.54
Synthesis of [V(hyphPh)2]. [V(hyphPh)2] was synthesized
following the procedure established by Chaudhuri and co-workers.21
Briefly, 2 × 10−4 mol of [VCl3(THF)3] and 4.0 × 10−4 mol of
H2hyphPh were dissolved in a minimum amount of MeOH. Et3N (0.5
mL) was then added to the solution, and the resulting solution was
refluxed under argon for 1 h. After cooling to room temperature, the
solution was opened to air and stirred for 10 min to favor oxidation of
V
III to VIV. The solution was then concentrated by passing argon over
the surface of the solution to yield a crystalline solid. Anal. Calcd for
[V(hyphPh)2]·2CH3OH, C42H34N6O6V (769.71): C, 65.54; H, 4.45;
N, 10.92. Found: C, 65.29; H, 4.59; N, 10.70.
Synthesis of [V(bhpp)2]. [V(bhpp)2] was prepared using a
procedure similar to that used for [V(hyphPh)2], i.e., by dissolving 1.5
× 10−4 mol of [V(acac)3] and 3.0 × 10−4 mol of H2bhpp in MeOH.
Anal. Calcd for [V(bhpp)2]·CH3OH, C35H26N2O5V (605.54): C,
69.42; H, 4.33; N, 4.63. Found: C, 69.22; H, 4.37; N, 4.62.
RESULTS AND DISCUSSION
■
1. Synthesis of 2,6-Bis(2-hydroxyphenyl)pyridine.
Because pyridine derivatives are ubiquitous in natural products,
C
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Scheme 2. Synthesis of 2,6-Bis(2-hydroxyphenyl)pyridine
pharmaceuticals, and functional materials, continued interest
has been generated in the synthesis of polydentate (O, N, O)
ligands, where N belongs to a pyridine ring. In this work, 2,6-
bis(2-hydroxyphenyl)pyridine was synthesized in a two-step
reaction (Scheme 2). Initially, 2,6-bis(2-methoxyphenyl)-
pyridine was prepared via a Suzuki−Miyaura protocol in
aqueous media under aerobic conditions.55 Demethylation of
this compound was subsequently performed according to a
procedure from the literature.34
2. Studies in Aqueous Solution. As mentioned above,
among the four examined ligands, only H3hyphC and H3hyphS
are soluble in water. The pH potentiometric titrations allowed
us to find the pKa values of the two ligands and the stability
constants of the complexes formed with VIVO2+ ion (Table 1).
H2bhpp is moderately soluble in water and has been studied
through EPR spectroscopy in a H2O/DMSO 50:50 v/v
mixture.
Figure 1. Species distribution diagram for the VIVO2+/H3hyphC (H3L)
system as a function of pH with a molar ratio of 1:10 and a VIVO2+
concentration of 1 mM.
In their fully protonated form, H3hyphC and H3hyphS have
three protons, two belonging to phenol −OH and another one
present on the carboxylic or sulfonic acid group. Deprotonation
of the −COOH and −SO3H groups cannot be measured
because in the first case the ligand dissolves in water only after
deprotonation of the carboxylic group and in the second case
the pKa is smaller than 1. pKa values of 8.69 and 8.68 were
assigned to deprotonation of one of the two phenolic −OH
groups (Table 1). The two larger values (10.61 and 10.49,
Table 1) belong to the other −OH group. If the pKa values of
the two ligands are compared with that of phenol (∼9.9), the
first is clearly lower and the second is higher, which could be
explained by assuming that deprotonation of the first −OH
group results in an intramolecular hydrogen bond that favors
deprotonation of the first phenolic and disfavors that of the
second phenolic group. Values of 8.80 and 10.61 (H3hyphC)
and 8.74 and 10.63 (H3hyphS) were previously reported in the
literature32 and are in good agreement with those found in the
present study.
The species distribution diagram for the system containing
H3hyphC is shown in Figure 1, and the anisotropic EPR spectra
recorded as a function of pH for the system with H3hyphS are
in Figures 2 and 3. The values of gz and Az are reported in
Table 2.
The stoichiometry and structure of the species found were in
good agreement with those reported in the literature for FeIII.32
At low pH values, a 1:1 complex with stoichiometry [VOL]−
is formed (I in Figure 2 and a in Scheme 3), which is the main
species in solution in the pH range 2−5. The ligand exists in
the fully deprotonated form and coordinates a VIVO2+ ion with
donor set (O−, Ntriaz, O−); H2O. The value of Az in this
environment is 166.7 for H3hyphC and 167.5 × 10−4 cm−1 for
H3hyphS.
Figure 2. High-field region of the X-band anisotropic EPR spectra
recorded from pH 2.5 to pH 8.5 in the VIVO2+/H3hyphS (H3L) system
with a molar ratio of 1:10 and a VIVO2+ concentration of 4 mM.
VIVO2+, I, IIa, and IIb indicate the resonances of [VO(H2O)5]2+,
[VOL]−, and the two isomers of [VOL2H]3−, respectively.
formed by transition metal ions.46 In particular, for complexes
of first-row transition metals, the functional B3P86, which is
composed of the exchange Becke hybrid (B3) and correlation
part P86, has proven to be the most effective in terms of the
mean deviation of the bond distances and angles from the
experimental values and corresponding standard deviation.47
The basis set influences the results less than the functional, and
a valence triple-ζ basis set such as 6-311g, which was developed
by Pople and co-workers, is sufficient to correctly simulate an
experimental structure. It therefore represents a reasonable
compromise between the accuracy and the speed of the
simulation.47
The geometry of the oxidovanadium(IV) complex formed by
H3hyphC, H3hyphS, and H2bhpp was optimized using Gaussian
09 software through DFT calculations that, as recently
demonstrated, give good results in optimization of structures
The stability can be evaluated by calculating the Gibbs free
energy in aqueous solution (ΔGtotaq) for formation of
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Scheme 3. Complexes Formed in Aqueous Solution by
H3hyphC (R = p-C6H4−COO−) and H3hyphS (R = p-C6H4−
a
−
SO3 )
Figure 3. High-field region of the X-band anisotropic EPR spectra
recorded from pH 8.5 to pH 12.3 in the VIVO2+/H3hyphS (H3L)
system with a molar ratio of 1:10 and a VIVO2+ concentration of 4 mM.
IIa, IIb, III, and IV indicate the resonances of the two isomers of
[VOL2H]3−, the non-oxido complex [VL2]2−, and [VOLH−1]2−,
respectively. Intensity of the spectrum recorded at pH 11.70 was
also amplified five times to reveal the resonances of the minor [VL2]2−
species.
Table 2. Anisotropic EPR Parameters and Donor Sets for
VIVO and VIV Complexes Formed in Aqueous Solution by
H3hyphC, H3hyphS, and H2bhpp
a
Dashed lines in structures b and c indicate the weak axial interaction.
(H2O)2]x−(ii). The difference in thermodynamic stability
between the pentacoordinated and hexacoordinated species
with one equatorial and one axial water ligand increases
significantly from the gas phase to aqueous solution because of
a favorable ΔGsolv. The three structures for H3hyphC are shown
in Figure 4.
a
ligand
species
gz
Az
donor set
H3hyphC
VOL
1.948
1.948
1.950
1.957
1.954
1.947
1.947
1.948
1.952
1.954
1.942
1.945
166.7 (O−, Ntriaz, O−); H2O
VOL2H
VOL2H
VOLH−1
VL2
164.5 (O−, Ntriaz, O−); (Ntriaz, O− ax
)
162.7 (O−, Ntriaz, O−); (Ntriazax, O−)
159.9 (O−, Ntriaz, O−); OH−
138.4 2 × (O−, Ntriaz, O−)
Above pH 6, the transformation of [VOL]− into [VOL2H]3−
is observed (Figures 1 and 2). EPR resonances are rather broad
and unsymmetrical, with an evident shoulder at high fields. This
behavior was clearly observed for the transition at the highest
field (MI = −7/2) between 3980 and 4030 G and suggests the
presence of two species in aqueous solution. The finding that
the position, shape, and intensity ratio between the two
absorptions do not change in the pH range 7−12 indicates that
two cis-octahedral isomers coexist in solution, as already
observed for 8-hydroxyquinoline and its derivatives.56 Because
we demonstrated above that in 1:1 complexes the ligands
occupy three equatorial positions, in these species the second
ligand should behave as a bidentate ligand with an equatorial−
axial arrangement and one of the two phenolic groups still
protonated. The two cis-octahedral complexes are indicated
with IIa and IIb in Figures 2 and 3 and by b and c in Scheme 3.
This deduction was supported by the pH potentiometric
studies, which predicted the presence of [VOL2H]3− as the only
species in solution between pH 7 and 11. Az values for this
species are 165.3 and 162.8 × 10−4 cm−1 in the case of H3hyphS
and 164.5 and 162.7 × 10−4 cm−1 in the case of H3hyphC. On
the basis of the “additivity rule”,57 for which a deprotonated
phenolic oxygen gives a lower contribution to the 51V hyperfine
coupling constant along the z axis than a triazole nitrogen, it
H3hyphS
VOL
167.5 (O−, Ntriaz, O−); H2O
VOL2H
VOL2H
VOLH−1
VL2
165.3 (O−, Ntriaz, O−); (Ntriaz, O− ax
162.8 (O−, Ntriaz, O−); (Ntriazax, O−)
160.3 (O−, Ntriaz, O−); OH−
139.1 2 × (O−, Ntriaz, O−)
)
b
H2bhpp
VOL
167.2 (O−, Npyr, O−); H2O
165.1 (O−, Npyr, O−); (Npyr, O−ax
)
VOL2H
a
b
Az measured in 10−4 cm−1. System measured in a mixture H2O/
DMSO 50:50 v/v.
[VOL(H2O)2]x− from [VOL(H2O)]x−, where L = hyphC,
hyphS, and bhpp and x = 1 with hyphC and hyphS and x = 0
with bhpp (reactions 1 and 2)
[VOL(H2O)]x− + H2O → [VOL(H2O)2]x−(i)
(1)
[VOL(H2O)]x− + H2O → [VOL(H2O)2]x−(ii)
(2)
where superscripts (i) and (ii) indicate the species with one
equatorial and one axial water and two equatorial waters,
respectively. Results are reported in Table 3 and show that for
all three ligands the stability order in the gas phase and aqueous
solution is [VOL(H2O)]x− ≫ [VOL(H2O)2]x−(i) ≫ [VOL-
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Table 3. ΔG Values in the Gas Phase and Aqueous Solution for Formation of the Hexacoordinated 1:1 Species of H3hyphC,
ab
,
H3hyphS, and H2bhpp from the Pentacoordinated Square Pyramidal Complex
c
d
reaction
ΔEele
ΔGtherm
ΔGtot
Δ(ΔGsolv
)
ΔGtot
aq
gas
e
f
[VO(hyphC)(H2O)]− + H2O → [VO(hyphC)(H2O)2]− (i)
[VO(hyphC)(H2O)]− + H2O → [VO(hyphC)(H2O)2]− (ii)
[VO(hyphS)(H2O)]− + H2O → [VO(hyphS)(H2O)2]− (i)
[VO(hyphS)(H2O)]− + H2O → [VO(hyphS)(H2O)2]− (ii)
[VO(bhpp)(H2O)] + H2O → [VO(bhpp)(H2O)2](i)
[VO(bhpp)(H2O)] + H2O → [VO(bhpp)(H2O)2](ii)
−46.0
−9.9
−44.6
−10.7
−23.0
10.8
48.6
47.2
50.2
47.0
39.6
44.7
2.6
37.3
5.6
32.2
28.3
31.2
29.1
21.6
6.1
34.8
65.6
36.8
65.4
38.2
61.6
e
f
36.3
16.6
55.5
e
f
a
b
c
Energies reported in kJ mol−1. Calculations performed at the B3P86/6-311g level of theory. Thermal contribution at 298 K with the zero-point
d
e
energy included in the calculations. SMD model used with water as solvent. Hexacoordinated species with one equatorial and one axial water
molecule. Hexacoordinated species with two equatorial water molecules.
f
Figure 4. Optimized structures with DFT methods at the B3P86/6-311g level of theory for the three possible 1:1 species formed by H3hyphC: (a)
hexacoordinated complex with two equatorial waters, (b) pentacoordinated complex with one equatorial water, and (c) hexacoordinated complex
with one equatorial and one axial water.
could be deduced that the higher concentration species has
coordination (O−, Ntriaz, O−); (Ntriaz ax, O−) (IIa in Figures 2
and 3 and b in Scheme 3), and the second complex is an isomer
with donor set (O−, Ntriaz, O−); (Ntriaz, O− ax) (IIb in Figures 2
and 3 and c in Scheme 3). A similar coordination mode, with
one phenolic oxygen protonated, was determined for 1:2
species of FeIII with H3hyphC and H3hyphS.32 This mode was
stabilized by an intramolecular hydrogen bond between the
phenolic −OH group and the noncoordinated nitrogen atom of
the triazole ring.
the non-oxido species with respect to the (O−, Nimine, O−)
ligands.21
At pH values higher than 11, another species appeared in
solution. This species was identified by potentiometry as
[VOLH−1]2− and by EPR spectroscopy based on the shift of
the spectral resonances MI = −5/2 and −7/2 to lower magnetic
field values (IV in Figure 3 and e in Scheme 3). The Az values
of 160.3 for H3hyphS and 159.9 × 10−4 cm−1 for H3hyphC,
which were not significantly different from those for VOL2H,
indicate that an OH− ion replaced the phenolate oxygen or
triazole nitrogen of the (equatorial−axial) ligand molecule to
yield a species with (O−, Ntriaz, O−); OH− coordination.
Because of the poor solubility of the ligand in water, the
VIVO2+/H2bhpp system was studied in a H2O/DMSO 50:50 v/
v mixture. EPR spectra as a function of pH are represented in
Figure 5. Species formed were similar to those found in the
systems with H3hyphS and H3hyphC, with the exception that
only one isomer of the bis-complex was observed. Complexes
[VOL] with coordination mode (O−, Npyr, O−); H2O (I in
Figure 5 and a in Scheme 4) and [VOL2H]− with coordination
(O−, Npyr, O−); (Npyr, O− ax) (II in Figure 5 and b in Scheme
4) were revealed. EPR parameters were comparable to those of
the analogous species formed by H3hyphS and H3hyphC and are
listed in Table 2.
The non-oxido VIV complex was formed in only a very small
amount in the pH range 11.5−12.0 (III in Figure 3 and d in
Scheme 3). Because the complex is formed at a low
concentration and in a very basic solution its existence could
not be confirmed by potentiometric measurements. The very
low value of Az (138.4 × 10−4 cm−1 for H3hyphC and 139.1 ×
10−4 cm−1 for H3hyphS, Table 2), however, clearly proves
formation of the non-oxido species. The reason for the lower
stability of hexacoordinated non-oxido complexes [VL2]2−
formed by H3hyphC and H3hyphS in aqueous solution relative
to complexes formed in the solid state is not fully clear at the
moment. Considering that formation of these structures
involves leaving of the oxido ligand of the VO bond as a
water molecule, it is possible that deprotonation of the second
phenolic −OH occurs at a value of pH so high that this process
could not be favored. In addition, the low basicity of the triazole
nitrogen of H3hyphC and H3hyphS may disfavor formation of
3. Studies in the Solid State. Rather surprisingly, few
papers have described the coordination chemistry of H2hyphPh,
H2bhpp, and their derivatives. Some recent studies established
F
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species with CuII and VV has been reported in the literature.32,61
Hancock noted that the size of the chelate ring had a significant
influence on metal ion size selectivity: ligands forming five-
membered rings such as iminodiacetic acid preferentially bind
large metal ions, whereas ligands forming six-membered rings
are more selective for small metal cations.62 For tridentate
ligands with a flexible ligand backbone a facial and a meridional
coordination mode are possible. For example, with iminodi-
acetic acid facial coordination is preferred,63 whereas the
reverse behavior was observed for methylamino-N,N-bis(2-
methylene-4,6-dimethylphenol).64 On the contrary, the high
rigidity of H2hyphPh and H2bhpp may favor meridional
coordination.
From steric considerations, H2hyphPh and H2bhpp, which
form six-membered chelate rings exclusively, should exhibit a
clear preference for small metal ions, such as high-spin FeIII
(ionic radius 0.645 Å) and AlIII (ionic radius 0.535 Å).65
Because the ionic radius of VIV (0.580 Å) is intermediate
between those of FeIII and AlIII, it seems plausible that stable
species with H2hyphPh and H2bhpp can be formed.
The basicity of the central atom, which is a pyridine or a
triazole nitrogen, is another factor that can influence the
stability of the complex. In the literature, it has been
demonstrated that a structure with two parallel phenyl rings
should have very short M−O bond distances, i.e., approx-
imately 1.6 Å for H2hyphPh and 1.2 Å for H2bhpp.60 These
distances can increase through out-of-plane-twisting of the two
phenyl rings (measured by the γ angle) with respect to the
value of 0°.21
In this study, solid complexes [V(hyphPh)2] and [V(bhpp)2]
were synthesized. The procedure proposed by Chaudhuri and
co-workers was used,21 based on oxidation of a VIII complex
through exposure to air and subsequent storage of the solution
under an argon stream to prevent further oxidation and
concentrate of the solution (Scheme 5).
Figure 5. High-field region of the X-band anisotropic EPR spectra
recorded in a H2O/DMSO 50:50 v/v mixture as a function of pH in
the VIVO2+/H2hbhpp (H2L) system with a molar ratio of 1:10 and a
VIVO2+ concentration of 4 mM. VIVO2+, I, and II indicate the
resonances of [VO(H2O)5]2+, [VOL], and [VOL2H]−, respectively.
Scheme 4. VIVO2+ Complexes Formed by H2bhpp in a H2O/
DMSO 50:50 v/v Mixture
Elemental analysis together with the dark color characteristic
of the non-oxido vanadium(IV) species with phenolate-O
donors as well as the absence of the stretching frequency of the
double bond VO, which usually falls in the range 950−1000
cm−1, confirmed the proposed stoichiometry.
A hexacoordinated structure formed by a tridentate ligand
can be described in terms of the meridional (mer), symmetric
facial (sym-fac), and unsymmetric facial (unsym-fac) isomers.
Because of the high rigidity of the ligands, only the meridional
arrangement was stable, as confirmed by DFT simulations. The
geometry of [V(hyphPh)2] and [V(bhpp)2] was optimized by
Gaussian 09 software (see DFT calculations in Experimental
and Computational Section) using the functional B3P86 and
the basis set 6-311g.47 Results are shown in Table 4, whereas in
Figure 6 the two optimized structures are presented. Structural
details calculated by DFT methods were compared with those
of solid complexes [FeIII(hyphPh)2]−, [AlIII(hyphPh)2]−,
[FeIII(bhpp)2]−, and [AlIII(bhpp)2]−. These structures can be
described on the basis of several geometrical parameters. The
first is the “twist angle” Φ, which is defined as the angle
between the two triangular faces of side s describing the
that ligands based on the 2,6-bis(2-hydroxyphenyl)pyridine
backbone bind very small Lewis acids such as BIII with high
selectivity.58 The X-ray single-crystal structure of [Cu(bhppMe)-
(py)2] has been published, where H2bhppMe is the ligand with
one of the two phenyl rings substituted by a methyl in position
5.59 Nevertheless, to the best of our knowledge, the only
homoleptic species synthesized are those containing FeIII and
AlIII.60 With H2hyphPh only formation of hexacoordinated
complexes with FeIII and AlIII and di-μ-phenolate dinuclear
a
Scheme 5. General Procedure for Synthesis of the Non-Oxido Vanadium(IV) Complexes (H2L = H2hyphPh and H2bhpp)
a
Et3N favors deprotonation of the H2L ligand and neutralizes the HCl formed in the reaction.
G
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Table 4. Geometrical Parameters Obtained by DFT Methods for [V(hyphPh)2] and [V(bhpp)2] and Comparison with Similar X-
a
ray Structures
b
c
c
b
d
d
complex
ionic
radius
[V(hyphPh)2]
[Fe(hyphPh)2]−
0.645 Å
[Al(hyphPh)2]−
0.535 Å
[V(bhpp)2]
0.580 Å
[Fe(bhpp)2]−
0.645 Å
[Al(bhpp)2]−
0.535 Å
0.580 Å
V−O
1.821, 1.861; 1.893,
1.931
1.958, 2.003; 1.958,
2.003
1.875, 1.891; 1.875,
1.891
1.872, 1.873; 1.874,
1.875
1.953, 1.975; 1.958,
1.977
1.855,1.876; 1.868,
1.882
V−N
2.030, 2.107
165.3, 173.8
177.6
2.091, 2.091
166.5, 166.5
172.8
1.972, 1.972
173.8, 173.8
177.6
2.124, 2.126
174.5, 174.5
179.8
2.187, 2.188
168.9, 169.3
177.6
2.088, 2.096
174.8, 175.1
178.5
O−V−O
N−V−N
e
Φ
Θ
Ω
55.0
51.6
56.1
59.6
57.3
57.3
f
172.2
168.6
175.0
176.4
171.9
176.1
g
169.5
166.5
173.8
174.6
169.1
174.9
h
Ψ
88.0
93.3
90.3
85.1
88.7
85.8
i
γ
18.4
29.0
25.9
47.8
52.6
50.1
j
s/h
1.233
1.259
1.226
1.219
1.283
1.286
k
s/a
1.417
1.427
1.415
1.412
1.435
1.437
k
b/a
1.375
1.341
1.387
1.413
1.372
1.411
a
b
c
Distance in Angstroms and angles in degrees. Structure simulated by DFT methods at the level of theory B3P86/6-311g. Experimental structure
d
e
f
reported in ref 32. Experimental structure reported in ref 60. Twist angle: 60° for octahedral and 0° for trigonal prismatic geometry. Angle formed
by donors in the trans position with vanadium (mean value): 180° for octahedral and 135° for trigonal prismatic geometry. Angle formed by the
two terminal donors of each ligand molecule with vanadium (mean value): 180° for mer and 90° for facial isomers. Angle formed by the two
terminal donors of each ligand molecule with the central one (mean value). Torsional angle between the two phenyl rings. 1.225 for octahedral and
g
h
i
j
k
1.000 for trigonal prismatic geometry. 1.414 for octahedral and 1.309 for trigonal prismatic geometry.
Scheme 6. Structure of the VIV Complexes Formed by
H2hyphPh and H2bhpp in the Solid State: (a) [V(hyphPh)2]
and (b) [V(bhpp)2]
Figure 6. Structure of the complexes [V(hyphPh)2] (a) and
[V(bhpp)2] (b) optimized by using the DFT methods at the
B3P86/6-311g level of theory.
coordination polyhedron and is 60° for an octahedron with the
two triangles staggered and 0° for a trigonal prism with the two
triangles eclipsed. Additional parameters include the ratios s/h,
b/a, and s/a, where h is the distance between the two triangles,
b is the ligand bite, and a is the mean metal−ligand bond length
(s/h, b/a, and s/a are 1.225, 1.414, and 1.414 for an octahedron
and 1.000, 1.309, and 1.309 for a trigonal prism). The third
parameter is the angle Θ formed by the donors trans to the
vanadium, which is 180° for octahedral and 135° for trigonal
prismatic geometry. Finally, the angles Ω (formed by the two
terminal donors of each ligand molecule with vanadium, which
is 180° for mer and 90° for fac isomers) and Ψ (formed by the
two external donors of each ligand molecule with the central
donor) also help to describe these complexes.
From Table 4, the bond lengths M−O and M−N follow the
size of the metal ions. Thus, for [Al(bhpp)2]− the mean values
Al−O and Al−N were 1.870 and 2.092 Å, for [V(bhpp)2] the
V−O and V−N values were 1.874 and 2.125 Å, and for
[Fe(bhpp)2]− the Fe−O and Fe−N values were 1.966 and
2.187 Å. For both [V(hyphPh)2] and [V(bhpp)2] the arrange-
ment of the two ligands is meridional and the structure is close
to the limit form represented by the octahedron, Scheme 6. On
the whole, the geometrical parameters of the complexes formed
by H2bhpp (Φ in the range 57.3−59.6°, Θ in 171.9−176.4°, Ω
in 169.1−174.9°, s/h in the range 1.219−1.286, s/a in 1.412−
1.437, and b/a in 1.372−1.413) were closer to the values
expected for an octahedron than those of the H2hyphPh species
(Φ in the range 51.6−56.1°, Θ in 168.6−175.0°, Ω in 166.5−
173.8°, s/h in the range 1.226−1.259, s/a in 1.415−1.427, and
b/a in 1.341−1.387). As expected for a meridional arrange-
ment, the values of Ω were close to 180°. Analogously to the
solid-state structures formed by FeIII and AlIII, complexes of VIV
show a twist of the phenyl rings with respect to a parallel
arrangement. The torsional angle γ expected for [V(bhpp)2]
(47.8°) was larger than that for [V(hyphPh)2] (18.4°).
4. Prediction of 51V Hyperfine Coupling Constants for
VIVO and VIV Species. The vanadium(IV) complexes were
characterized by EPR spectroscopy. The spectrum of [V-
(hyphPh)2] was simulated with gx = 1.967, gy = 1.974, gz = 1.943,
Ax = −55.8 × 10−4 cm−1, Ay = −52.4 × 10−4 cm−1, Az = −143.2
× 10−4 cm−1, and that of [V(bhpp)2] was simulated with gx =
1.962, gy = 1.982, gz = 1.939, Ax = −53.6 × 10−4 cm−1, Ay =
−45.8 × 10−4 cm−1, Az = −142.0 × 10−4 cm−1. The
experimental spectrum and comparison with the simulated
spectrum for [V(hyphPh)2] are shown in Figure 7. EPR spectra
of hexacoordinated vanadium(IV) complexes can be divided
into two groups: those with gz ≪ gx ≈ gy < 2.0023 and Az ≫ Ax
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underestimate Az;50d the percent deviation from |Az|exptl was
between −1.6% and 1.1%. Therefore, the Az values calculated
for [VOL(H2O)]x−, with L = hyphC, hyphS, and bhpp, allowed
us to reconfirm the characterization discussed above and
demonstrate, using another method, that the complexes were
pentacoordinated with only one equatorial water molecule
bound to vanadium. For VIV species formed by H2hyphPh and
H2bhpp, the prediction appeared to be significantly worse and
the percent deviation from |Az|exptl was from −6.9% to −14.9%.
Both ORCA and Gaussian underestimated |Az|, but ORCA
performed better than Gaussian. The possibility of ORCA
calculating the second-order spin−orbit contribution, which
represents more than 6% of Az, may account for this difference.
5. Prediction of the UV−Vis Spectra for VIVO and VIV
Species. Electronic absorption spectra of the oxido and non-
oxido vanadium(IV) species formed by H2hyphPh and H3hyphC
were simulated by TD-DFT methods according to procedures
reported in the literature.66 It has been recently shown that
such methods provide a qualitative description of an
experimental spectrum, and results can be useful in the
assignment of the transitions and prediction of the electronic
structure of a metal complex. Even if, as shown in Figure 8, a
Figure 7. X-band anisotropic EPR spectrum of [V(hyphPh)2]: (a)
experimental and (b) simulated.
≈ Ay, and those with gx ≈ gy ≪ gz ≈ 2.0023 and Az ≪ Ax ≈
Ay.28 The first situation is associated with a dxy ground state and
2
geometry close to the octahedral and the second with a dz
ground state and a trigonal prismatic structure. Therefore, both
[V(hyphPh)2] and [V(hyphPh)2] are in the first group, and a dxy
ground state is predicted. It must be mentioned that the
spectrum closely resembles that of a VIVO species (charac-
terized by an analogous dxy ground state), except for the
significantly lower value of |Az|.
DFT methods have made the prediction of the 51V hyperfine
coupling constant tensor A possible. In this work, we used
Gaussian 09 and ORCA software to predict 51V A values of
oxido and non-oxido vanadium(IV) complexes (see below).
Gaussian is advantageous because it has half-and-half hybrid
functionals, such as BHandHLYP and BHandH, available to
treat the critical core shell spin polarization, whereas ORCA
allows inclusion of the second-order spin−orbit effects (see
DFT calculations in the Experimental and Computational
Section). Results are displayed in Table 5. For VIVO species the
performances of Gaussian and ORCA in the prediction of Az
were comparable with a slight tendency for ORCA to
Figure 8. Experimental and simulated electronic absorption spectra of
[V(hyphPh)2] (black and red, respectively) and [VO(hyphC)(H2O)]−
(green and blue, respectively).
Table 5. 51V Hyperfine Coupling Constants Calculated Through DFT Methods (with ORCA and Gaussian 09 softwares) for
a
Oxido and Non-Oxido Vanadium(IV) Complexes
b
complex
Ax
Ay
Az
%|Az|
[VO(hyphC)(H2O)]−
[VO(hyphC)(H2O)]−
[VO(hyphC)(H2O)]−
[VO(hyphS)(H2O)]−
[VO(hyphS)(H2O)]−
[VO(hyphS)(H2O)]−
[VO(bhpp)(H2O)]
[VO(bhpp)(H2O)]
[VO(bhpp)(H2O)]
[V(hyphPh)2]
aqueous solution
aqueous solution
aqueous solution
aqueous solution
aqueous solution
aqueous solution
aqueous solution
aqueous solution
aqueous solution
solid state
exptl
−60.3
−62.0
−65.0
−60.7
−62.2
−65.1
−60.0
−60.4
−64.1
−55.8
−33.2
−26.7
−53.6
−26.4
−21.1
−60.3
−62.2
−66.2
−60.7
−62.6
−66.5
−62.0
−60.6
−65.3
−52.4
−39.6
−22.4
−45.8
−37.2
−20.2
−166.7
−164.8
−168.5
−167.5
−166.4
−169.4
−167.2
−164.6
−168.4
−143.2
−133.3
−127.5
−142.0
−129.9
−120.8
calcd (ORCA)
calcd (Gaussian)
exptl
−1.1
+1.1
calcd (ORCA)
calcd (Gaussian)
exptl
−0.7
+1.1
calcd (ORCA)
calcd (Gaussian)
exptl
−1.6
+0.7
[V(hyphPh)2]
solid state
calcd (ORCA)
calcd (Gaussian)
exptl
−6.9
−11.0
[V(hyphPh)2]
solid state
[V(bhpp)2]
solid state
[V(bhpp)2]
solid state
calcd (ORCA)
calcd (Gaussian)
−8.5
−14.9
[V(bhpp)2]
solid state
a
b
Values reported in 10−4 cm−1. Percent deviation from the experimental value calculated as 100 × (|Az|calcd − |Az|exptl)/|Az|exptl
.
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Table 6. Main Calculated and Experimental Electronic Transitions for [VO(hyphC)(H2O)]− and [V(hyphPh)2] up to 350 nm
f ×
a
b
c
bd
,
main transition
[VO(hyphC)(H2O)]− H(α) → L + 2(α)
character
λ
105
λ
exptl/εexptl
dxy → dxz
dxy → dyz
603.7
496.9
479.2
390.0
361.7
355.3
2687.2
1809.5
657.7
632.4
526.3
499.8
495.0
477.9
469.8
465.0
455.7
434.6
417.8
412.4
408.0
404.7
379.3
378.0
354.3
70
20
10
10
560/20
H(α) → L + 4(α)
H(α) → L + 3(α)/H(α) → L + 8(α)
H(α) → L + 1(α)
H(α) → L(α)
2
2
dxy → π* (Ph)/dxy → dx −y
dxy → π* (Ph)
dxy → π* (Ph)
1560
1170
5
H-3(α) → L(α)/H-1(β) → L(β)
H-1(α) → L(α)
π (Ph)→ π* (Ph)/π (Ph) → π* (Ph)
dxy → dyz
[V(hyphPh)2]
H-1(α) → L + 1(α)
H(α) → L(α)
H(α) → L + 1(α)
H-2(α) → L + 1(α)
H(β) → L(β)
dxy → dxz
60
π (Ph + Olp) → dyz
π (Ph + Olp) → dxz
π (Ph + Olp) → dxz
π (Ph + Olp) → dyz
π (Ph + Olp) → dyz
π (Ph + Olp) → dxz
π (Ph + Olp) → dxz
400
700/1300 (sh)
630/2180 (sh)
570/1890
2620
1310
1610
830
530/3350
H-3(α) → L(α)
H-3(α) → L + 1(α)
H(β) → L + 1(β)
1960
5400
3220
2290
1400
1740
1970
1300
620
470/3650
H-3(α) → L + 1(α) /H(β) → L + 1(β) π (Ph + Olp) → dxz/π (Ph + Olp) → dxz
H-4(α) → L + 1(α)
H-1(β) → L(β)
π (Ph + Olp) → dxz
π (Ph + Olp) → dyz
π (Ph + Olp) → dxz
H-1(β) → L + 1(β)
lp
2
2
H-1(α) → L + 10(α)/H-1(β) → L + 1(β) dxy → dx −y /π (Ph + O ) → dxz
π (Ph + Olp) → dx −y + π* (Ph)/π (Ph + Olp) → dxz
2
2
H(β) → L + 2(β)/H-1(β) → L + 1(β)
H-1(α) → L + 6(α)/H(β) → L + 2(β)
H-2(β) → L+1(β)
lp
2
2
2
dxy → dz /π (Ph + O ) → dx −y + π* (Ph)
π (Ph + Olp) → dxz
3280
1250
430
H-5(α) → L + 1(α)/H-2(β) → L + 1(β) π (Ph) → dxz/π (Ph + Olp) → dxz
H-1(α) → L + 3(α)/H-1(β) → L + 2(β) dxy → π* (Ph + Tr)/π (Ph + Olp) → dx −y + π* (Ph)
2
2
a
b
c
d
lp indicates the lone pair on the O atom, and Ph and Tr indicate the phenyl and triazolyl rings. λ values measured in nm. Oscillator strength. ε
measured in M−1 cm−1.
quantitative agreement with the experiments cannot be
obtained, TD-DFT calculations can be used to distinguish
VIVO from VIV species rather easily. For the oxido complex
[VO(hyphC)(H2O)]−, the bands observed in the visible range
(up to 400 nm) are mainly pure d−d transitions with very low
oscillator strength, corresponding to a molar absorption
coefficient lower than 200 M−1 cm−1. The energy of the dxy
→ dxz, dxy → dyz, and dxy → dx −y excitations reflects the order
of the energy levels of the V-d based molecular orbitals (MO)
for a square pyramidal geometry.67
Figure 8 shows that a qualitative prediction of the spectrum
is possible. For [VO(hyphC)(H2O)]− only a broad absorption
in the visible range centered around 590 nm (560 nm
experimental) arising from superimposition of the dxy → dxz,
dxy → dyz, and dxy → π*/dxy → dx −y transitions is expected,
whereas for [V(hyphPh)2] the calculated absorption at
approximately 450 nm must be compared with the
experimentally measured band at 470 nm.
6. Correlation between the Geometrical Parameters
and 51V Az of VIV Species. Prediction of the 51V hyperfine
coupling constant along the z axis (Az) for the “usual” VIVO
species is possible through application of the “additivity rule”,
formulated by Chasteen and re-examined by Pecoraro and co-
workers, which calculates Az from the sum of the contributions
of each equatorial donor57,68
2
2
2
2
For the non-oxido species [V(hyphPh)2] the behavior is
completely different, and the d−d excitations are expected at
very low energy in agreement with a geometry close to
octahedral (Table 6). Absorptions in the 400−800 nm region
arise from the contribution of a large number of excitations and
are mainly ligand-to-metal charge transfer (LMCT) transitions,
particularly those from the MOs formed by the π backbone and
the p orbital of phenolate oxygen perpendicular to the aromatic
4
Az =
∑
Az(donor i)
i=1
2
2
plane toward the V orbitals dxz, dyz, and dx −y . This finding
confirms what was proposed by Raymond and co-workers, i.e.,
for non-oxido vanadium(IV) species all transitions have to be
considered LMCT in which the metal orbitals involved are dxy,
= Az(donor 1) + Az(donor 2) + Az(donor 3)
+ Az(donor 4)
(4)
dxz, dyz, and dx −y (see Table 6).26 The oscillator strength for
excitations in the visible range was generally more than 3 orders
of magnitude larger than that for the VIVO complexes (ε is
>4000 M−1 cm−1 at 400 nm). For [V(hyphPh)2] the broad and
asymmetrical bands in the visible range arise from electronic
transitions between highly delocalized molecular orbitals
resulting from the mixing of excited configurations. In these
cases, deconvolution of the experimental spectrum is necessary
to resolve the several absorptions.
2
2
Such an empirical rule (valid in cases when the geometrical
distortion can be neglected68,69) allows correlation of Az to the
number and type of ligands in the equatorial plane of a VIVO
ion and prediction of Az with deviations generally smaller than
∼3 × 10−4 cm−1 with respect to the experimental value.
Conversely, prediction of the hyperfine coupling constants of
a non-oxido VIV species is not possible a priori. In the literature,
A values have been placed in a relationship with the trigonal
prismatic distortion of the hexacoordinated structure.28 For
J
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Article
non-oxido complexes with intermediate distortion formed by
catecholate and pyridinonate derivatives, the situation Az ≪ Ax
≈ Ay is found, with Ax ≈ 120 × 10−4 cm−1, Ay ≈ 100 × 10−4
cm−1, and Az < 20 × 10−4 cm−1.70 However, a correlation
between the twist angle Φ and the values of A could not be
found; for example, the two tris-chelated species with VS6
coordination [V(dithiolene)3] and [V(bdt)3]2−, with bdt 1,2-
benzenedithiolate, were characterized by Ax ≈ Ay ≈ 85 × 10−4
cm−1 but showed a significant difference in the value of Φ (0°
and 37°, respectively).18c
Another type of spectroscopic behavior was shown by the
vanadium compound isolated in A. muscaria, i.e., amavadin, for
which Az ≫ Ax ≈ Ay (Az = 152.9 × 10−4 cm−1, Ax = Ay = 45 ×
10−4 cm−1).71,72 In this study we examined [V(hyphPh)2] and
[V(bhpp)2] and four non-oxido VIV complexes formed by
tridentate bisphenol ligands with (O, N, O), (O, S, O), (O, P,
O), and (O, Se, O) donor sets (indicated with LN, LS, LP, and
LSe, respectively). The first ligand forms a meridional species
Figure 10. Az values for non-oxido vanadium(IV) complexes as a
function of the angle Ω (see Table 4). Blue rhombi indicate
[V(hyphPh)2] and [V(bhpp)2], and pink squares indicate [VLN2],
[VLS ], [VLP ], and [VLSe2]. Dotted gray line represents the best linear
[VLN ], and the other three form the facial compounds [VLS ],
2
[VLP 2], and [VLSe2].21,22 The six complexes share the order Az
2
2
2
fitting of the six points.
≫ Ax ≈ Ay of the 51V hyperfine coupling constants, which
indicates a dxy ground state and a geometry close to octahedral.
The values of Az were plotted as a function of the Φ, Θ, Ω, and
Ψ angles, as defined in Table 4. Results are presented in Figures
9 and 10 and Figures S1 and S2 of the Supporting Information.
It is noteworthy that the experimentally measured Az and the
EPR spectral pattern for a mer isomer may be incorrectly
attributed to an oxidovanadium(IV) species (see Figure 7). In
this case, careful analysis must be conducted, and the value of
Az of non-oxido VIV must be compared with that of the VIVO
species obtainable from the “additivity rule”. For example, Az
values of 157.4 and 159.2 × 10−4 cm−1 expected on the basis of
the “additivity rule” for the two possible isomers of the bis-
chelated VIVO complex formed by H2bhpp were not
compatible with 142.0 × 10−4 cm−1 experimentally measured
for [V(bhpp)2]. Furthermore, other pieces of evidence, such as
an absorption molar coefficient significantly larger than 200
M−1 cm−1 in the visible region and the lack of a strong
stretching vibration attributable to a VO bond in the range of
950−1000 cm−1, demonstrated formation of a VIV rather than a
VIVO complex.
CONCLUSIONS AND OUTLOOK
■
This study describes the complexing capability of four ligands
containing a (O, Narom, O) donor set. These ligands are strong
chelating agents, and one of them, 4-[3,5-bis(2-hydroxyphen-
yl)-1H-1,2,4-triazol-1-yl]benzoic acid (known also as ICL670
or Deferasirox and commercially available as Exjade from
Novartis) is an effective iron chelator used for treatment of
chronic iron overload arising from blood transfusions.
Figure 9. Az values for non-oxido vanadium(IV) complexes as a
function of the twist angle Φ (see Table 4). Blue rhombi indicate
[V(hyphPh)2] and [V(bhpp)2], and pink squares indicate [VLN2],
[VLS ], [VLP ], and [VLSe2]. Dotted gray line represents the best linear
2
2
fitting of the six points.
The ligands form square pyramidal mono-chelated and
octahedral bis-chelated VIVO species in aqueous solution, and
the hexacoordinated complex [VOL2H]3− was the main species
at physiological pH. The rare non-oxido vanadium(IV)
compounds, very small amounts of which are present in
aqueous solution, however, were isolated in the solid state. 3,5-
Bis(2-hydroxyphenyl)-1-phenyl-1H-1,2,4-triazole and 2,6-bis(2-
hydroxyphenyl)pyridine formed neutral [VL2], as confirmed by
EPR, electronic absorption, and IR spectroscopy, with a
meridional arrangement of the two ligand molecules.
Geometry can be described by many structural parameters,
such as the twist angle Φ, the angle Θ formed by the donors in
trans to the vanadium, the angle Ω formed by the two terminal
O donors of each ligand molecule with vanadium, and the angle
Ψ formed by the two terminal O donors with the central Narom
for each ligand. The values of such parameters suggest that the
Figures 9 shows that the twist angle Φ, which measures the
distortion of an octahedron toward the trigonal prism, could
not be related to Az. Rather, a correlation was found when Az
was plotted as a function of Ω (Figure 10) and Ψ (Figure S2,
Supporting Information), whose values depend on the type of
isomer formed (i.e., meridional or facial). The three mer
isomers and the three fac isomers could be divided into two
groups, with Az larger than 140 × 10−4 cm−1 and slightly
smaller than 130 × 10−4 cm−1, respectively. Therefore, it is clear
that the geometric arrangement of the ligands (mer or fac)
significantly influences the 51V hyperfine coupling constant. On
the basis of these data, this factor seems to be more effective
than the twist angle Φ for determining the electronic structure
of the complexes. This aspect is actually under examination in
our research group.
K
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Inorganic Chemistry
Article
geometries of [V(hyphPh)2] and [V(bhpp)2] are close to
octahedral. DFT calculations were able to predict 51V hyperfine
coupling constants and electronic transitions of VIVO and VIV
species, and they allowed us to distinguish among these species
on the basis of Az in the EPR and ε values in the electronic
absorption spectra.
A correlation of Az values with the most important structural
angles revealed that VIV compounds could be divided into
meridional and facial isomers. For [VL2] species formed by
tridentate ligands the geometric arrangement (mer or fac)
rather than the twist angle Φ influences the electronic structure
of the complexes and, thus, the 51V hyperfine coupling constant
along the z axis, Az. The geometrical parameters which can be
put in relationship with Az are the angles Ω and Ψ (see above).
In particular, high values of Ω and Ψ result in high values of Az.
The value of Az for amavadin (152.9 × 10−4 cm−1) well
correlates with Ω and Ψ experimentally measured (155.2° and
104.6°, respectively).14c In other words, the meridional
arrangement of the pro-ligand N-hydroxyimino-2,2′-diisopro-
pionic acid (or (S,S)-H3hidpa) in its triply deprotonated form
stabilizes a ground state based mainly on the V dxy orbital.
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ASSOCIATED CONTENT
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: garribba@uniss.it.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Hungarian Scientific Research
Fund (K 72956) and TAMOP (4.2.2.B-10/1-2010-0024).
■
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