Synthesis, characterization and DFT studies of a new unsymmetrical dinuclear Vanadium(IV) complex with a bipodal N2O-donor ligand

Article abstract of DOI:10.1016/j.molstruc.2019.04.098

A new dinuclear vanadium(IV) complex [(VO)₂(μ-HBPA)₂(μ-SO₄)].4.75H₂O was synthesized by the reaction between a bipodal N₂O-donor ligand N-(2-hydroxybenzyl)-N-(pyridin-2-ylmethyl)amine (HBPA) and vanad

Full text of DOI:10.1016/j.molstruc.2019.04.098

Accepted Manuscript
Synthesis, Characterization and DFT studies of a New Unsymmetrical Dinuclear
Vanadium(IV) Complex with a bipodal N O-donor Ligand
2
Paula M.A. Machado, Rafael A. Allão Cassaro, Vagner M. de Assis, Sérgio de P.
Machado, Adolfo Horn, Elizabeth Roditi Lachter
PII:
S0022-2860(19)30507-1
10.1016/j.molstruc.2019.04.098
MOLSTR 26470
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Accepted Date:
20 December 2018
24 April 2019
Please cite this article as: Paula M.A. Machado, Rafael A. Allão Cassaro, Vagner M. de Assis,
Sérgio de P. Machado, Adolfo Horn, Elizabeth Roditi Lachter, Synthesis, Characterization and DFT
studies of a New Unsymmetrical Dinuclear Vanadium(IV) Complex with a bipodal N O-donor
2
Ligand, Journal of Molecular Structure (2019), doi: 10.1016/j.molstruc.2019.04.098
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ACCEPTED MANUSCRIPT
Synthesis, Characterization and DFT studies of a New Unsymmetrical
Dinuclear Vanadium(IV) Complex with a bipodal N2O-donor Ligand
Paula M. A. Machado1, Rafael A. Allão Cassaro2, Vagner M. de Assis2, Sérgio
de P. Machado2, Adolfo Horn Jr.3, Elizabeth Roditi Lachter1*.
1Universidade Federal do Rio de Janeiro, Departamento de Química Orgânica,
Rio de Janeiro, Brazil
2Universidade Federal do Rio de Janeiro, Departamento de Química
Inorgânica, Rio de Janeiro, Brazil
3Universidade Estadual do Norte Fluminense Darcy Ribeiro, Laboratório de
Ciências Químicas, Campos dos Goytacazes, Brazil
*lachter@iq.ufrj.br
Abstract
A new dinuclear vanadium(IV) complex [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O was
synthesized by the reaction between a bipodal a N2O-donor ligand N-(2-
hydroxybenzyl)-N-(pyridin-2-ylmethyl)amine (HBPA) and vanadyl sulfate. The
complex was characterized by elemental analysis, thermogravimetric,
electrochemical analysis, ESI-MS, FT-IR and UV-visible. The structure was
determined by single crystal X-ray diffraction. The two non-equivalent
vanadium(IV) ions are hexacoordinated being bridge-coordinated by the oxygen
atoms from of HBPA and sulfate group. The complex was studied by DFT
calculations and showed high polarity.
Keywords: Vanadium complex, binuclear, crystal structure, DFT calculation.
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Graphical Abstract
Highligths
 A new binuclear vanadium(IV) complex was synthesized.
 DFT studies show a very high polarity for the complex.
 Thermogravimetric analysis indicates a high thermal stability of the
dessolvated complex.
1.
Introduction
Vanadium-based compounds have received attention in the last years
due to its structural particularities, versatility at applications in biological
activities [1] and as catalyst for many processes, mainly oxidation reactions [2,
3, 4]. Vanadium complexes have been extensively applied in medicine studies,
as insulin mimetic in vivo [5] and in vitro tests (1), in the inhibition of cancer cells
proliferation [7, 8], as anti-inflammatory, antibiotic [9], antifungal [10], antioxidant
agent [11] and emerged as a potential agent for the neglected parasitic
diseases such as leishmaniasis and trypanosomiasis [12, 13]. Vanadium has an
important role in biological systems, as in some enzymes present in brown
seaweed, bacteria, fungi and others organisms [14]. The activity of these
enzymes containing vanadium ions in oxidizing halide ions and biosynthesizing
brominated compounds has inspired the study of this metal as catalyst in
chlorination and bromination reactions [15, 16]. Some vanadium-based
coordination compounds catalyzed oxygen transfer reactions [17] to substrates
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containing sulfides [18], alkenes [19, 20] as well as epoxidation reactions [21].
Recently, mononuclear and weakly magnetic vanadyl complexes have been
considered potential for application as spin qubits for quantum information
processing [22, 23, 24].
In view of the great multifunctionality of vanadium complexes, there is a
large contribution of the literature in the synthesis, characterization and
application of these compounds, mainly about vanadium-based coordination
compounds containing N,O donor atoms ligands [25, 26]. Schiff base ligands,
containing phenol and imine, as well ligands with amines and pyridines
electronic donors produce different structural complexes, as mononuclear
vanadium (IV) metal center with penta and six-coordination [25, 26] or binuclear
oxovanadium complexes [27, 28]. In particular, ligands containing phenol
moieties have been shown ability to form phenoxide bridges leading to
multinuclear complexes, including with vanadium ions [28, 29].
In this work, we describe a new dinuclear vanadium(IV) coordination
compound containing the ligand N2O-donor called HBPA (with amine, pyridine
and phenol donor groups) and coordinated sulfate. To the best of our
knowledge it is the first example of dinuclear vanadium complex containing the
metal ions with fac- and mer-coordination environments. The structure of the
complex was determined by single-crystal X-ray. DFT studies shown high
polarity and stability for the molecule.
2.
Experimental
2.1. Materials
All chemicals and solvents were used without further purifications. The
chemicals 2-(aminomethyl)pyridine, salicylaldehyde, vanadyl sulfate,
dichloromethane and 2-propanol were purchased from Sigma-Aldrich, USA.
Anhydrous magnesium sulfate and sodium borohydride were obtained from
VETEC, Brazil, while acetonitrile and dimethylsulfoxide were purchased from
ISOFAR, Brazil, and Grupo Química, Brazil, respectively.
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2.2. Characterization
FTIR analyses were carried out on a Fourier Nicolet Magna-IR 760 using
CsI pellets in the range 4000-150 cm-1. Ultraviolet-visible spectra were recorded
in the range from 200 to 900 nm, using a Shimadzu UV-1800 spectrometer.
1
Solution H NMR was recorded on a Bruker AV500 spectrometer. Elemental
analyses (CHN) were carried out on a Perkin Elmer 2400 series II. Thermal
analysis was recorded on a TGA-50 Shimadzu, performed under nitrogen flux of
50 mL min-1. The sample was heated from 25 °C to 700 °C using a 2 °C min-1
rate and a 10 °C min-1 rate from 700 °C to 1000 °C. Cyclic voltametry was
performed on a potentiostat PGSTAT100, from Autolab, equipped with a carbon
glass as work electrode and Ag/AgCl (saturated) as reference electrode. The
complex was analyzed in 1.4x10-3 mol L-1 in methanol solution, using
tetrabutylammonium hexafluorophosphate in 0.10 mol L-1 as transport
electrolyte. Electrospray ionization mass spectra were obtained on a Bruker
MicroTOF spectrometer [ESI-(+)-MS], prepared with a collision cell RF in 650.0
Vpp and the capillary in 4500 V with the nebulizer in 0.4 bar, the temperature of
dry heater was 180 °C, and the dry gas flow was 4.0 L min−1, employing
methanol as solvent.
2.3.
X-ray crystallography
Single crystal X-ray data were collected on a Bruker D8 Venture
diffractometer using graphite monochromatic MoK radiation ( = 0.71069 Å) at
room temperature. Data collection and cell refinement were performed with
Bruker Instrument Service and APEX2 software, respectively. Data reduction
was carried out using Bruker SAINT V8.37A software [30]. Empirical multiscan
absorption correction using equivalent reflections was performed using the
SADABS program [31]. The structure solution was performed using SHELXS-97
while the refinement was performed using SHELXL-2014/7 based on F2 through
the full-matrix least-squares routine [32]. All non-hydrogen atoms were refined
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with anisotropic displacement parameters. Hydrogen atoms were located in the
difference Fourier map and were also placed in calculated positions and refined
isotropically using a riding model [33]. Large thermal displacement parameters
were found for lattice water molecules due to disorder. EADP, ISOR and DELU
commands were used in the refinement of the disordered solvent. Summary of
data collection and structure refinement is shown in Table 1.
Table 1. Summary of data collection and structure refinement.
Identification
Formula
Fw (g mol-1)
T (K)
[(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O
C26H37N4O13.5SV2
732.45
289(2)
0.71073
Monoclinic
C2/c
 (Å)
Crystal system
Space group
a (Å)
26.8357(13)
18.1154(9)
13.6568(6)
90
b (Å)
c (Å)
(º)
98.131(2)
90
(º)
(º)
Volume (Å3)
6572.4(5)
8
Z
calc (Mg m-3)
1.480

 (mm-1)
0.699
F(000)
2992
2.25-26.40
-33 ≤ h ≤ 33
-22≤ k ≤ 22
-17≤ l ≤ 17
52806
 range(°)
Index ranges
Data collected
Independent reflections
6736
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Rint
0.0443
Refinement method
Data / restraints / parameters
GOF on F2
Full-matrix least-squares on F2
6736 / 18 / 428
1.086
0.0517, 0.1499
0.0696, 0.1675
0.885, -0.45
R1, wR2 [I>2(I)]
R1, wR2 (all )
max, min (e·Å-3)
2.4.
Density functional theory (DFT) calculations
The DFT calculations were performed using B3LYP level and LANL2DZ
basis set in Gaussian 09 software [34]. The ground state geometries were fully
optimized considering the complex in the gas phase, starting from the crystal
structure omitting the lattice water molecules. Theoretical studies were
performed in order to obtain information about hardness, charge distribution and
the frontier orbitals, Highest Occupied Molecular Orbital (HOMO) and Lowest
Unoccupied Molecular Orbital (LUMO), which are directly related to the
reactivity and hardness of the molecule.
2.5.
Synthesis of the ligand N-(2-hydroxybenzyl)-N-(pyridin-2-ylmethyl)amine
The ligand HBPA (N-(2-hydroxybenzyl)-N-(pyridin-2-ylmethyl)amine) was
synthesized according to a previously described procedure [35]. The ligand was
obtained by condensation of 2-(aminomethyl)pyridine (6.3 g, 58 mmol) and
salicylaldehyde (7.1 g, 58 mmol) in 50 mL of methanol at room temperature
under stirring for 1 hour, followed by subsequent slowly addition of NaBH4 (2.2
g, 58 mmol) in ice bath. Then, the solvent was evaporated and the product was
dissolved in 50 mL of dichloromethane, and washed with distilled water. The
organic phase was dried with anhydrous MgSO4, which was filtered off. Some
hours later, the colorless crystalline product was obtained and washed with cold
1
2-propanol. M.p.= 62–63°C, yield = 69 %, 8.6 g. H NMR (ppm) in CDCl3: 8.57
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(dd, 1 H), 7.66 (dt, 1 H), 7.20 (m, 3 H), 7.00 (d, 1 H), 6.89 (d, 1 H), 6.77 (dt, 1
H), 4.02 (s, 2 H), 3.95 (s, 2 H).
2.6. Synthesis of the complex [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O
The complex [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O was obtained by the
reaction between HBPA ligand (0.214 g, 1.0 mmol) and VOSO4.2H2O (0.163 g;
1.0 mmol) dissolved in 30 mL of 1:1 acetonitrile and methanol solution, under
constant stirring for 2 hours at 75 °C, resulting in a dark violet solution. Dark
purple single crystals were obtained after keeping the solution at room
temperature for 72 hours. Crystal yield = 0,37 g, 49%. Elemental analysis
calculated for [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O, C, 42.09, H, 4.82, N, 7.55%
found C, 42.99, H, 4.61, N, 7.48%. The crystals can show loss of water
molecules after many days at room temperature. ESI-(+)-MS, m/z: 298.1, 608.1,
657.0, 795.5, 871.1, 936.1. Calculated for C26H26N4O8SV2, ([(VO)2(µ-HBPA)2(µ-
SO4)]), M+ 656.5 g found 657.
3.
Results and discussion
3.1. Crystal structure
The complex [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O crystallizes in a
monoclinic system. The asymmetric unit consists in a dinuclear V(IV) complex,
where each metal ion has a N2O4 coordination environment and 4.75 lattice
water molecules. Each vanadium(IV) ion is in a distorted octahedral geometry
being coordinated by one oxygen atom and two nitrogen atoms from phenoxide,
amine and pyridine moieties of HBPA ligand, respectively. The other positions
are occupied by two oxygen atoms from sulfate and oxo groups. The phenoxide
moiety and sulfate groups are bridge-coordinated to the metal ions (Fig. 1).
Selected bond lengths and bond angles are gathered on Table 2. Both
vanadium(IV) ions are coordinated by the same donor atoms, however the bond
lengths, bond angles and distortion on the deprotonated HBPA ligand make the
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coordination environment of them quite different. The shortest bond length is
vanadium-oxygen atom from oxo group (V1─O3 and V2─O4), while the longest
ones are V1─O1 (phenoxide moiety) and V2─O5 (sulfate group). The tridentate
deprotonated HBPA coordinated to V2 adopts a meridional conformation while
a facial one is adopted when this ligand is coordinated to V1. To the best of our
knowledge it is the first example of dinuclear vanadium complex containing the
metal ions with fac- and mer-coordination environments. The intramolecular
distance between vanadium(IV) ions is 3.2234(8) Å, while the shortest
intermolecular distance between these ions (V1…V1ii =1.5-x; 0.5-y, 1-z) is
6.1096(8) Å.
Fig. 1. ORTEP view of the asymmetric unit of [(VO)2(µ-HBPA)2(µ-
SO4)].4.75H2O with thermal ellipsoids at 30% of probability. Hydrogen atoms
and oxygen of water molecules were omitted for the sake of clarity. Color code:
carbon, nitrogen, oxygen, sulfur atoms and vanadium(IV) ion stands for black,
blue, red, yellow and cyan, respectively.
Table 2. Bond lengths and bond angles for [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O.
Bond lengths (Å)
V1—O1
V1—O2
V1—O3
2.242(2)
2.014(2)
1.592(3)
V2—O1
V2—O2
V2—O4
2.017(2)
2.013(2)
1.594(2)
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V1—O6
V1—N1
V1—N2
1.987(3)
2.122(3)
2.116(3)
V2—O5
2.193(2)
2.121(3)
2.123(3)
V2—N3
V2—N4
Bond angles (°)
74.67(8)
O2—V1—O1
O3—V1—O1
O6—V1—O1
O6—V1—O2
O3—V1—O2
O3—V1—O6
N1—V1—O1
O2—V1—N1
O3—V1—N1
O6—V1—N1
O2—V1—N2
O3—V1—N2
O6—V1—N2
N2—V1—N1
N2—V1—O1
O2—V2—O1
O4—V2—O1
O1—V2—O5
O2—V2—O5
O4—V2—O2
O4—V2—O5
O1—V2—N3
O2—V2—N3
O4—V2—N3
O1—V2—N4
O2—V2—N4
O4—V2—N4
N3—V2—O5
N4—V2—O5
N3—V2—N4
79.87(9)
173.24(13)
84.23(10)
91.00(10)
101.77(13)
101.76(14)
78.21(9)
101.32(11)
81.03(9)
82.46(9)
100.24(11)
176.66(11)
106.07(10)
162.20(11)
95.06(12)
161.15(10)
91.59(10)
96.71(12)
81.97(9)
95.27(10)
96.58(12)
158.44(12)
160.64(10)
97.35(13)
87.03(11)
79.43(11)
86.00(10)
81.19(10)
77.53(11)
Many dinuclear vanadium(IV) complexes have been reported in the
literature. For all complexes, the ligands are coordinated in facial or meridional
conformation [36, 37]. In particular, complexes containing NNO-tridentated and
sulfate
ligands
brigde-coordinated
to
oxovanadium(IV)
such
as
[(L1−)2(VIVO)2(SO4)]·½CH2Cl2(6·½CH2Cl2), where
L
is (E)-2-((quinolin-8-
ylimino)methyl)phenol were described [36]. In this complex both tridentated L
ligands adopt the mer conformation in contrast to observed in the complex
described in this work. A dinuclear oxovanadium(VI) complex, [{VO(sal-
aembz)}2SO4].2H2O, where sal-aembz was obtained from the reaction between
from salicylaldehyde and 2-aminoethylbenzimidazol, also contains one sulfate
and two phenoxide moieties of the mer-sal-aembz bridge-coordinated to the
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metal [36] having the V=O bonds oriented anti-coplanar, different than observed
for [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O.
Short contacts between the non-coordinated oxygen atoms of sulfate
group and two oxygen atoms of lattice water molecules are observed with
distances O7…O10 and O8…O9 of 2.791(8) and 2.698(7) Å, respectively for
[(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O. Other intermolecular short contacts were
also observed between the lattice water molecules. These water molecules are
located in the space between [(VO)2(µ-HBPA)2(µ-SO4)] molecules, mainly along
the 1D channels parallel to the crystallographic c axis (Fig.2). These
intermolecular interactions contribute to stabilize the crystal packing of the
complex.
Fig. 2. View of the crystal packing of [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O along
the crystallographic c axis. Hydrogen atoms were omitted for the sake of clarity.
3.2. Density functional theory (DFT) studies
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The structure of the complex was optimized in a triplet state by means of
the DFT/B3LYP method along the LAN2DZ basis set (Fig. 3). The absence of
negative normal modes in frequency calculations emphasizes that the optimized
structures are in minimum energies. The optimized structure is similar to the
structure obtained by X-ray diffraction, the bonding lengths of the metal centers
differ by a maximum of 5% and the angles by 7%, which indicates that the
methodology used for the calculations was efficient. The computed values of
the electronic chemical potential (μ) and global hardness are -0.14463 eV and
(η) of 0.06766 eV, respectively. The global hardness (η) measures the stability
of a system in terms of resistance to electron transfer and the chemical potential
(μ) characterizes the escaping tendency of electrons from the equilibrium
system [38]. The dipole moment calculated for the optimized structure is
12.6800 D, indicating a high polarity.
Fig. 3. Structure optimized by DFT using B3LYP functional and LANL2DZ basis
set without hydrogen atoms. Colors: gray for C, red for O, cyan for V, yellow for
S and blue for N.
Frontier molecular orbitals have an effective role for providing an insight
into the chemical activities and some of the physical properties of the molecule
[39]. The frontier molecular orbitals of the complex are shown in Fig. 4. Singly
Occupied Molecular Orbital (SOMO) of α-spin in the complex have 21% V
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character, 16% sulfate character along with 60% of ligand orbitals contribution
(phenol, amine and pyridine donors). Similarly, LUMO of α-spin have 22% V
character, 14% sulfate character along with 57% of ligand orbitals contribution.
In both SOMO and LUMO formation, it was observed a similar and smaller
contribution of oxo orbitals, about 3% and 7%, respectively.
Fig. 4. Frontier molecular orbitals of [(VO)2(µ-HBPA)2(µ-SO4)]: SOMO (top) and
LUMO (bottom).
The calculated Mulliken charges for complex are shown in Table 3. The
Mulliken charge of vanadium atom is considerably lower than the formal charge
of 4+ which confirms a significant charge donation from the ligands to the metal
core. Furthermore, the Mulliken charge on the terminal oxygen atom (oxo
group) is significantly less negative than 2-. Also, the Mulliken charge on
oxygen atoms from the ligand are significantly less negative than 1- which can
be attributed to electron density delocalization from these oxygen atoms
towards the vanadium centers. The terminal oxygen atoms from oxo group are
less negative in comparison with the oxygen atoms from the ligand which is
related to higher electron density delocalization from the terminal oxo towards
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the vanadium centers and agrees with the observed differences in bond lengths
between each vanadium center and the oxygens from phenol groups (O1 and
O2) and from oxo groups (O3 and O4), shown in Table 2.
Table 3. Selected calculated atomic Mulliken charges for [(VO)2(µ-HBPA)2(µ-
SO4)].
Atom
V1
Mulliken charges
+0.979
+0.814
-0.638
V2
O1(phenol)
O2(phenol)
N2
-0.702
-0.476
N4
-0.465
O5(sulfate)
O6(sulfate)
O3(oxo)
O4(oxo)
-0.685
-0.671
-0.362
-0.398
3.3. Characterization
3.3.1. Infrared vibrational spectroscopy (FTIR)
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Fig. 5. FTIR spectra for HPBA ligand (top) and [(VO)2(µ-HBPA)2(µ-
SO4)].4.75H2O (bottom).
The spectrum of HBPA (Fig. 5) shows a sharp band at 3263 cm-1 is
attributed to N-H bond stretching from secondary amine. The weak band
observed in 3039 cm-1 was attributed to aromatic C-H while symmetric and
asymmetric stretching of C-H bonds from methylene group are attributed to the
bands at 2931 cm-1 and 2859 cm-1, respectively. The bands at 1593, 1456 and
1429 cm-1 corresponds to the C=N and C=C from pyridine and phenol of the
ligand. The band at 1356 cm-1 is related to the angular deformation of O-H bond
from phenol group [35].
The IR spectrum of [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O (Fig. 5) shows a
broad band at 3454 cm-1 that refers to the stretching of O-H bonds attributed to
lattice water molecules involved in hydrogen bondings. As observed for the
HBPA, the sharp band in 3265 cm-1 is attributed to N-H bond indicating that
the amine was not deprotonated after the coordination of the ligand to
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vanadium(IV) ions. The aromatic C-H was assigned to the bands at 3076,
3039 cm-1 and the aliphatic C-H of methylene group were observed at 2933
cm-1 (symmetric) and 2860 cm-1 (asymmetric). The bands attributed to C=N or
C=C at 1614, 1485, 1452 cm-1 were shifted to higher wavenumbers when
compared to the ligand itself, indicating the coordination through the donor
atoms of pyridine and phenoxide moieties of the ligand to the vanadium ion. An
evidence of phenoxide formation is the disappearance of the band related to
δO-H. The vanadyl (V=O) stretching is observed at 968 cm-1, suggesting the
presence of vanadium(IV) ions. The V-O stretching is assigned to the band at
405 cm-1 while the V-N can be attributed to the band at 471 cm-1 [40, 41]. The
IR data are summarized in Table 4.
Table 4. Mainly absorptions (cm-1) attributed to the stretching in the vibration
spectra of the HBPA ligand and [(VO)2(µ-HBPA)2(µ-SO4)].5.5H2O.
HBPA
Complex
3454
Group vibration
O-H (water)
-
3263
3265
N-H (secondary amine)
C-H (aromatic)
3039
3076, 3039
2933
2931
C-H (aliphatic, symmetrical)
C-H (aliphatic, unsymmetrical)
C=C, C=N
2859
2860
1593, 1456,
1614, 1485, 1452
1429
(double bonds in aromatics)
δO-H (phenol)
1356
-
-
-
-
1028
968
471
S=O (sulfate ion)
V=O (vanadyl)
N-V
3.3.2. Ultraviolet visible electronic spectroscopy
The electronic spectrum of [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O dissolved
in methanol (Fig. 6) shows strong bands at 208 nm, 260 nm, and a shoulder at
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282 nm, attributed to a intraligand transitions π-π*, as usually observed in
aromatic compounds, such as phenol and pyridine rings. At 383 nm, it is
observed a weak broad band that may be assigned to the phenoxide to
vanadium charge transfer (LMCT p-π orbital), and the two bands (550 nm and
769 nm) observed at lower energy are attributed to the d-d transitions of
vanadium ions [40, 21].
Fig. 6. Electronic spectra of complex [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O at
1.75x10-4 mol L-1 (straight line) and 7.00x10-3 mol L-1 (dashed line). Inset: zoom.
3.3.3. [ESI-(+)-MS]
ESI-(+)-MS spectrum of complex [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O (Fig.
7) was obtained in methanol and showed the presence of eight peaks. The
most stable fragment corresponded the protonated HBPA, [C13H15N2O]+, at m/z
215.1. The peak at m/z 298.1 is the mononuclear complex [(VO)(OH)(HBPA)]+.
The peak at m/z = 608.1 corresponds to a fragment composed by a
mononuclear vanadium(IV) complex containing two units of HBPA ligand and
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one sulfate ion, [(VO)(HBPA)2(SO4)]+. The most important fragment observed,
m/z = 657.0, is a dinuclear oxovanadium(IV) with two units of the HBPA ligands,
connected by a sulfate bridge, corresponding to the complex [(VO)2(HBPA)2(µ-
SO4)]+ as observed in the structure solved by single crystal X ray study.
Fragments with higher mass can be probably formed in the gaseous phase as
clusters obtained by reaction between fragments with lower mass, as we
propose for the peak at m/z = 871 and 936.1. The first one corresponds to a
dinuclear species containing a fragment of m/z 657 plus one unit of HBPA
ligand. The second one, at m/z 936, is possibly formed for three species of
mononuclear oxovanadium(IV), with structure [(VO)(HBPA)]+ added to one
sulfate ion.
Fig. 7. Mass spectrum of [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O in methanol
solution.
3.3.4. Thermogravimetric analysis
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Thermogravimetric analysis (TGA) was used to investigate the thermal
stability of the complex. The TGA curve shows two defined weight losses
(Fig.8). The first one is attributed to the loss of 8.8% of mass that is associated
to the loss of 3.75 lattice water molecules (calc. 9.1%) in temperatures smaller
than 72 ºC. It is in agreement with other compounds reported in the literature
that shows loss water lattice molecules usually below 100ºC [42]. A smooth loss
of 1.8% of mass occurs in the temperature range 72-290ºC that can be
attributed to the loss of one more water molecule (calc 2.4%). A second loss of
mass started after 290ºC, and the total weight loss of 79.7% is observed up to
990ºC leading probably to vanadium(IV) dioxide (calc. 77.8%) [43].
Fig. 2. TGA curve for [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O. The straight line
represents the mass in percentage.
3.3.5. Electrochemical studies
The redox behavior of [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O was
investigated by cyclic voltametry. The cyclic voltammogram obtained in
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methanol solution shows two irreversible electrochemical processes at anodic
region (Fig. 9), the first one at +0.73 V and second one at +0.99V. This
electrochemical behavior is common in dinuclear vanadium(IV) complexes at
the anodic region [44] and it is attributed to the oxidation of both vanadium (IV)
centers in two steps. The first one electron oxidation of one vanadium(IV) center
from VIVVIV to form the mixed valence complex VVVIV, followed by a second
electron oxidation of vanadium(IV) center, producing the dinuclear pair VVVV
[45]. The variation of the potential scan rate in the electrochemical experiment
produced the linear increasing of the peaks related to the oxidation processes
(Fig. 9), which supports that the electron oxidation processes in both vanadium
(IV) centers are controlled by diffusion mass transport in solution [46]. Then, it is
observed a reduction at 0.11 V, which can be assigning as a partial reduction of
the vanadium(V) pairs after oxidation.
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Fig. 3. Cyclic Voltammogram for [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O at 100 mV/s
scan rate (top), cyclic voltammograms at scan rates of 25, 50, 75, 100, 150,
200, 250, 300 and 350 mV.s-1 (middle) and linearity between current and scan
rate (S1/2) (bottom).
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4. Conclusions
A
new
dinuclear
vanadium(IV)
complex
[(VO)2(µ-HBPA)2(µ-
SO4)].4.75H2O was synthesized. The structure was determined by single crystal
X-ray diffraction, and the two vanadium ions are bridge coordinated moiety of
HBPA ligand and sulfate ion. Although these two ions are coordinated by the
same donor atoms, these ions have a quite different coordination environment
being V1 and V2 coordinated by fac-HBPA and mer-HBPA conformations,
respectively. This kind of coordination, having both fac- and mer- coordination
environment is new for dinuclear vanadium(IV) complexes. The DFT studies for
the complex, in agreement with the crystal structure, showed a high polarity.
The stability can be confirmed by thermogravimetric analysis, where the crystal
starts to degrade at temperature up to 300 °C, referred to the ligand loss.
Electrochemical analysis was important to observe two irreversible
electrochemical processes related to the oxidation of each vanadium(IV) ions.
Acknowledgements
We are thankful to LABEE (Universidade Federal do Rio de Janeiro) for
support in electrochemical analysis, LCQUI (Universidade Estadual do Norte
Fluminense) for ESI-MS analysis and LDRX-UFF (Universidade Federal
Fluminense) for using its crystallographic facilities. RAAC acknowledges CNPq-
Universal for financial support.
Supplementary Material
X-ray crystallographic data for [(VO)2(µ-HBPA)2(µ-SO4)].4.75H2O has
been deposited as a CIF file at the Cambridge Crystallographic Data Center
(CCDC code 1884251).
This file can be obtained freely at
http://www.ccdc.cam.ac.uk/structures.
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