Synthesis, characterization and catalytic activity of two novel cis-dioxovanadium(V) complexes: [VO2(L)] and [VO2(HLox)]

Article abstract of DOI:10.1590/S0103-50532011000400008

Two novel complexes, [VO₂(L)] (1) and [VO₂(HLox)] (2), were synthesized and characterized by IV, UV-Vis and NMR spectroscopy, cyclic voltammetry, elemental analysis and X-ray diffraction. The synthesis of a new ligand, H2Lox, is also

Full text of DOI:10.1590/S0103-50532011000400008

J. Braz. Chem. Soc., Vol. 22, No. 4, 660-668, 2011.
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0103 - 5053 $6.00+0.00
Article
A
Synthesis, Characterization and Catalytic Activity of Two Novel
cis-Dioxovanadium(V) Complexes: [VO2(L)] and [VO2(HLox)]
Natália M. L. Silva,a Carlos B. Pinheiro,b Eluzir P. Chacon,a Jackson A. L. C. Resende,a
José Walkimar de M. Carneiro,a Tatiana L. Fernández,c Marciela Scarpellinic and
Mauricio Lanznaster*,a
aInstituto de Química, Universidade Federal Fluminense, Outeiro S. João Batista S/N,
24020-141 Niterói-RJ, Brazil
bDepartamento de Física, Universidade Federal de Minas Gerais, Av. Antonio Carlos 6627,
Pampulha, 31270-901 Belo Horizonte-MG, Brazil
cInstituto de Química, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos 149,
Bl. A, 21941-909 Rio de Janeiro-RJ, Brazil
Dois novos complexos [VO2(L)] e [VO2(HLox)] foram sintetizados e caracterizados por
espectroscopias no IV, UV-Vis e RMN, voltametria cíclica, análise elementar e difração de
raios X. A síntese do ligante inédito H2Lox também é descrita. Os complexos 1 e 2 foram obtidos
pela reação de [VO(acac)2] com os respectivos ligantes HL e H2Lox. Alternativamente, 2 foi
preparado a partir da reação de HL com [VO(acac)2] na presença de hidroxilamina, e através da
reação de 1 com hidroxilamina. Dados cristalográficos mostram que 1 e 2 apresentam estruturas
moleculares similares, onde o centro de vanádio(V) cis-dioxo encontra-se coordenado em um
ambiente octaédrico distorcido formado pelos ligantes L- e HLox-, respectivamente. A atividade
catalítica destes compostos foi avaliada na oxidação do cicloexano, utilizando H2O2 e t-BuOOH
como oxidantes. Ambos apresentam seletividade > 70% para formação de cicloexilidroperóxido.
Cálculos B3LYP/6-31G(d) foram empregados na otimização da geometria e para auxiliar na
atribuição do espectro eletrônico.
Two novel complexes, [VO2(L)] (1) and [VO2(HLox)] (2), were synthesized and characterized
by IV, UV-Vis and NMR spectroscopy, cyclic voltammetry, elemental analysis and X-ray diffraction.
The synthesis of a new ligand, H2Lox, is also described. Complexes 1 and 2 were obtained by the
reaction of [VO(acac)2] with the ligands HL and H2Lox, respectively. Alternatively, 2 was also
obtained by the reaction of HL with [VO(acac)2] in the presence of hydroxylamine, and by the
reaction of 1 with hydroxylamine. Crystallographic data show that complexes 1 and 2 have similar
molecular structures, in which the cis-dioxovanadium(V) center is coordinated to L- or HLox-,
respectively, in a distorted octahedral environment. The catalytic activity of these compounds
towards cyclohexane oxidation was evaluated using H2O2 and t-BuOOH as oxidants. Both
complexes presented > 70% selectivity for cyclohexylhydroperoxide formation. B3LYP/6-31G(d)
calculations were used to confirm the geometry and to help assign the electronic spectra.
Keywords: oxovanadium(V), bioinspired catalysts, peroxide activation
Introduction
haloperoxidases (VHPOs), isolated from the marine algae
Ascophyllum nodosum.1,2 Later, these enzymes were
isolated from different sources such as seaweed, lichens
and other algae and characterized as iodoperoxidases,
bromoperoxidases and chloroperoxidases, depending on
the most electronegative halogen the enzyme is capable
of oxidizing.3 These enzymes are responsible for the
two-electron oxidation catalyses of chloride, bromide
Vanadium coordination chemistry has received
attention in recent years due to its presence in several
biological systems and its applications in catalysis. The first
example of vanadium-dependent enzyme are the vanadium
*e-mail: mlanz@vm.uff.br

Vol. 22, No. 4, 2011
Silva et al.
661
and iodide by hydrogen peroxide and are involved in the
halogenation of a large variety of organic substrates in vivo.4
The discovery of its structure initiated the investigation of
vanadium complexes as functional and structural models
for these enzymes.5 Special attention was given by Pecoraro
and co-workers6 who proposed the catalytic cycle for the
vanadium haloperoxidases based on the behavior of several
compounds.
As oxovanadium(IV) and (V) complexes are strong
Lewis acids, due to their high charge to radius ratio, they
can activate peroxide reagents and then be used as catalysts
in several oxidation reactions of organic compounds,
such as oxidation of sulfides to sulfoxides and sulfones,
alkene epoxidation and hydroxylation, aromatic, alkane
and alcohol oxidations, and inorganic compounds such as
halide and SO2 oxidation.7-13
Among the hydrocarbon oxidation reactions, catalytic
oxidation of alkanes is especially interesting because
this kind of reaction leads to products which have
considerable application in the pharmaceutical and plastic
industry. The cyclohexane oxidation into cyclohexanol
and cyclohexanone is a very important industrial reaction
for the manufacture of Nylon-6 and Nylon-6,6, which is
achieved with high temperature and pressure.14,15 To obtain
these oxidized products, the actual industrial process
requires temperature and pressure over 150 °C and 115 psi,
respectively; and only about 3-8% of overall conversion
is attained.16 Thus, the development of bioinspired
catalysts, which have low activation energy and require
friendly oxidants, embrace important advantages from
the environmental standpoint.16-18 In this way, a number of
model complexes for vanadium containing enzymes and
other vanadium complexes had their catalytic properties
evaluated towards the peroxidative oxidation of alkenes
and aromatic hydrocarbons.6,19-23
Inthiswork,wepresentthesynthesisandcharacterization
of two novel vanadium(V) cis-dioxo complexes of
general formula [VO2(L)] and [VO2(HLox)], where HL =
(3-[(bis-pyridin-2-yl-methylamino)-methyl]-2-hydroxy-5-
methyl-benzaldehyde) and H2Lox = (3-[(bis-pyridin-2-yl-
methylamino)-methyl]-2-hydroxy-5-methyl-benzaldehyde
oxime), and preliminary reactivity studies on the catalytic
oxidation of cyclohexane by these complexes employing
H2O2 or t-BuOOH as oxidants.
and cyclohexanone were purchased from Merck and used
as received from freshly opened bottle. In addition, their
purity was checked out by gas chromatography (GC). The
oxidation agents H2O2 (30%) and t-BuOOH (70%) were,
respectively, purchased from Vetec and Across, and their
real concentrations were determined by iodometric titrations
([H2O2] = 11.8 mol L-1 and [t-BuOOH] = 7.5 mol L-1).
Microanalysis were performed using a Perkin-Elmer
CHN 2400 micro analyzer at the Central Analítica do
Instituto de Química, Universidade de São Paulo, SP,
Brazil. UV-Visible absorption spectra were recorded
from 1.0 × 10-4 mol L-1 dimethyl sulfoxide (HPLC grade,
Tedia) solutions on a Varian Cary 50 spectrophotometer,
using 1 cm optical length quartz cuvettes. IR spectra
(KBr pellets) were recorded on a FT-IR Spectrum One
(Perkin Elmer) spectrophotometer. 1H NMR spectra were
recorded on a Varian Unit Plus 300 MHz spectrometer in
DMSO-d6; chemical shifts (d) are reported in parts per
million (ppm) relative to the internal standard Me4Si. The
hydrogen signals were attributed based on the coupling
constant values (J) and 2D COSY experiments. Cyclic
voltammetry experiments were carried out in a BAS Epsilon
potentiostat-galvanostat system, using a conventional
electrochemical cell with three electrodes: Ag/AgCl for
organic media as reference, platinum wire as auxiliary,
and glassy carbon as working electrode. Electrochemical
grade tetrabutylammonium hexafluorophosphate (TBAPF6,
SigmaAldrich) 0.1 mol L-1 in DMSO (TediaAnhydrosolv)
was used as supporting electrolyte, and the whole cell was
deoxygenated using ultrapure argon. Ferrocene was used
as an internal reference (E1/2 = 0.40 V vs. NHE).24
Syntheses of HL and HLox
The synthesis of HL was previously reported.25 The
ligand HLox was prepared according to Scheme 1. An
ethanol solution of HL (1.25 g, 3.60 mmol), hydroxylamine
hydrochloride (0.50 g, 3.60 mmol) and sodium carbonate
(0.38 g, 3.60 mmol) was stirred under reflux during 24 h.
The solvent was removed under vacuum, and the resulting
oil was dissolved in dichloromethane and washed with
water. The organic layer was dried over MgSO4 and the
solvent was removed under vacuum. The product was
obtained as a greenish viscous oil (1.04 g, 80%); 1H NMR
(300 MHz, CDCl3, 25°C), d 8.53 (ddd, J 4.98, 1.68,
0.80 Hz, 2H, Ha), 8.40 (s, 1H, Hg), 7.65 (td, J 7.69, 7.69,
1.78 Hz, 2H, Hc), 7.49 (d, J 7.84 Hz, 2 H, Hd), 7.16 (ddd,
J 7.47, 4.99, 1.10 Hz, 2H, Hb), 7.12 (d, J 1.74 Hz, 2H, He
and Hf), 3.99 (s, 4 H, N-CH2-py), 3.88 (s, 2 H, N-CH2-Ar),
2.24 (s, 3H, CH3); IR nmax/cm-1 (KBr) 1596, 1473, 1440
n(C=C, C=N), 1231 n(C-O), 763 d(C-H).
Experimental
Materials and methods
Reagents and solvents were used without further
purification. High purity (99%) cyclohexane, cyclohexanol

662
Synthesis, Characterization and Catalytic Activity of Two Novel cis-Dioxovanadium(V) Complexes
J. Braz. Chem. Soc.
undisturbed for few days (44 mg, 40%). Found: C, 55.63;
H, 4.73; N, 12.30%. Calc. for C21H21N4O4V: C, 56.76; H,
4.76; N, 12.61%. H NMR (300 MHz, DMSO-d6, 25°C)
N
N
N
N
NH2OH.HCl
CaCO3
O
N
N
N
OH
1
OH
OH
d 9.20 (d, J 5.34 Hz, 1H, Ha), 8.65 (d, J 5.07 Hz, 1H, Ha’),
8.00 (s, 1H, Hg), 7.97 (d, J 7.62 Hz, 1H, Hc’), 7.60 (d,
J 7.85 Hz, 2H, Hd’), 7.58 (d, J 7.61 Hz, 2H, Hc), 7.45 (t,
J 6.90, 6.20 Hz, 1H, Hb’), 7.23 (t, J 6.17, 6.10 Hz, 1H; Hb),
7.04 (d, J 1.91 Hz, 1H, Hf), 6.92 (d, J 2.07 Hz, 1H, He), 6.88
(d, J 8.05 Hz, 1H, Hd), 5.11 (d, J 12.14 Hz, 1H, CH2-N-Ar),
4.83 (d, J 14.83 Hz, 1H, CH2-N-Ar), 4.33 (d, J 14.93 Hz,
1H, CH2-N-Ar), 4.09 (d, J 17.63 Hz, 1H, CH2-N-Ar), 3.88
(d, J 12.18 Hz, 1H, CH2-N-Ar), 3.75 (d, J 17.56 Hz, 1H,
CH2-N-Ar), 2.11 (s, 3H, CH3); IR nmax/cm-1 (KBr) n(C=C,
C=N) 1596, 1473 and 1440, n(C-O) 1231, n(O=V=O) 895
and 854, n(C-H) 763.
HL
HLox
OH
O
N
O
V
O
V
O
O
O
O
N
N
N
(iv)
N
NH2OH.HCl
Et3N
N
N
[VO2(L)]
[VO2(Lox)]
Scheme 1. Synthesis of HL, HLox, [VO2(L)] (1) and [VO2(Lox)] (2).
Synthesis of [VO2(L)] (1)
(iii). To a solution of HL (174 mg, 0.50 mmol) in
methanol (30 mL), [VO(acac)2] (132 mg, 0.50 mmol),
hydroxylamine hydrochloride (52 mg, 0.75 mmol) and
triethylamine (0.14 mL, 1.0 mmol) were added, and the
mixture heated to 80 °C under stirring for 1 h and then
cooled to room temperature. Brown crystals were collected
after leaving the solution rest undisturbed for few days
(111 mg, 50%).
(i). To a solution of HL (174 mg, 0.5 mmol) in methanol
(30 mL) was added [VO(acac)2] (132 mg, 0.5 mmol) and
the mixture heated at 80 °C under stirring for 30 min.
After cooling to room temperature, this solution was
left undisturbed for few days, given yellow crystals that
were isolated (94 mg, 43%). Found: C, 56.44; H, 4.72; N,
•
9.44%. Calc. for (C21H20N3O4V)2 H2O: C, 57.54; H, 4.83;
N, 9.59%. 1H NMR (300 MHz, DMSO-d6, 25°C) d 9.22 (d,
J 5.10 Hz, 1H, Ha), 8.65 (d, J 4.50 Hz, 1H, Ha’), 8.05 (td,
J 7.50, 1.80 Hz, 1H, Hc’), 7.67 (d, J 7.80 Hz, 1H, Hd’), 7.56
(td, J 7.50, 1.80 Hz, 1H, Hc), 7.50 (t, J 6.60, 6.30 Hz, Hb’),
7.26-7.22 (m, 2H, Hb and Hf), 6.98 (d, J 2.10 Hz, 1H, He),
6.83 (d, J 7.80 Hz, Hd), 5.07 (d, J 12.00 Hz, 1H, CH2-N-Ar),
4.89 (d, J 15.00 Hz, 1H, CH2-N-Ar), 4.37 (d, J 15.00 Hz,
CH2-N-Ar), 4.06 (d, J 17.40 Hz, 1H, CH2-N-Ar), 3.94
(d, J 12.00 Hz, CH2-N-Ar), 3.75 (d, J 17.40 Hz, 1H,
CH2-N-Ar), 2.12 (s, 3H, CH3). IR nmax/cm-1 (KBr) n(C=O)
1660, n(C=C, C=N) 1610, 1568, 1471, 1456 and 1431,
n(C-O) 1262, n(O=V=O) 895 and 875, n(C-H) 760.
(iv). To a solution of 1 (50 mg, 0.12 mmol) in methanol
(30 mL), hydroxylamine hydrochloride (12 mg, 0.18 mmol)
and triethylamine (34 mL, 0.24 mmol) were added, and
the mixture heated to 80 °C under stirring for 1 h and then
cooled to room temperature. Brown crystals were collected
after leaving the solution resting undisturbed for few days.
Crystal structure determination
X-ray diffraction data collection for both complexes
were performed on a Bruker-Kappa-CCD diffractometer
(LdrX-UFF) using graphite-monochromatized MoKa
radiation (l = 0.71073 Å) at room temperature.26 Final
unit cell parameters were based on the refinement of
randomly selected reflections positions (DIRAX programs)
during PHICHI scanning.27,28 Data integration and scaling
of the reflections were performed with the EVALCCD
suite.28,29 Empirical multiscan absorption corrections using
equivalent reflections were performed with the program
SADABS.30 The molecular structures were solved by direct
methods using the SHELXS-97 program.31 The positions of
all atoms could be unambiguously assigned on consecutive
difference Fourier maps. Refinements were performed
using SHELXL-97 based on F2 through full-matrix least
square routine.32 All but hydrogen atoms were refined with
anisotropic atomic displacement parameters. Hydrogen
Synthesis of [VO2(Lox)] (2)
The complex [VO2(Lox)] was prepared by three
different ways, according to Scheme 1, from the reaction
of [VO(acac)2] and the ligand HLox (ii); of [VO(acac)2],
HL and hydroxylamine (iii); and of [VO2(L)] with
hydroxylamine (iv).
(ii). To a solution of HLox (90 mg, 0.25 mmol) in
methanol (30 mL), [VO(acac)2] (66 mg, 0.25 mmol) was
added. The mixture was heated at 80 °C under stirring
for 30 min and then cooled to room temperature. Brown
crystals were collected after leaving the solution resting

Vol. 22, No. 4, 2011
Silva et al.
663
atoms were added geometrically in the structure and further
refined according to the riding model.33 Crystal structure and
refinement data for compounds 1 and 2 are summarized in
Table S1. ORTEP of the structures are depicted in Figures 3
and 4.34 Relevant distances and angles are indicated in
Table 2.Atomic coordinates and complete crystal structure
results are given as supplementary material.
hydroxylamine, as a greenish viscous oil with 80% yield.
The conversion of the aldehyde of HL to the oxime in
H2Lox was confirmed by IR and H NMR spectroscopy.
1
The most relevant information from IR data is the presence
of an intense peak at 1678 cm-1 in the spectrum of HL,
assigned to the C=O stretching of benzaldehyde, which is
absent in the spectrum of H2Lox (Figure 1). The 1H NMR
spectra show that the signal of Hg in HL shifts from d 10.43
to 8.40 ppm in the spectrum of HLox, due to the conversion
of HC(R)=O into HC(R)=NOH. The full assignment of HL
and HLox NMR spectra is shown in Figure 2.
Theoretical calculations
Density functional calculations were carried out using
the Gaussian03W molecular orbital package.35 Geometries
were fully optimized (starting from the structure obtained
after the X-ray diffraction data refinements) using the
B3LYP functional with the standard 6-31G(d) basis set.26,37
NMR spectra for the optimized geometry were calculated
using B3LYP/6-31G(d). Electronic spectra were calculated
using the TD (time dependent) methodology available in
Gaussian with the B3LYP/6-31+G(d) method.
Reactivity studies
The cyclohexane oxidation experiments followed typical
proceduresreportedpreviously.15 Reactionswerecarriedoutin
acetonitrile, usingeitherH2O2 ort-BuOOHasoxidantandthe
complexes1and2ascatalysts.Thecatalyst:substrate:oxidant
ratio was of 1:1000:1000, were the catalyst concentration
was kept at 7 × 10-4 mol L-1. The reactions were carried out
under inert atmosphere (argon) at room temperature in a
sealed round-bottomed flask. The catalyst was dissolved in
acetonitrile, the solution was deoxygenated with argon and
the oxidant was added with a syringe. Catalytic reactions
were triggered by the addition of cyclohexane under stirring.
After24hitwasquenchedbyadditionofanaqueoussolution
of Na2SO4 (0.4 mol L-1) and extracted with 10 mL of diethyl
ether.Theorganiclayerswerecombined,driedoveranhydrous
Na2SO4 and analyzed by GC (gas chromatography). Control
experiments were carried out in the same conditions in the
absenceofcatalyst.Theproductsweredeterminedbyseparated
injection of standard grade cyclohexanol and cyclohexanone,
and confirmed by GC-MS. Cyclohexylhydroperoxide was
identifiedastheproductwithhigherretentiontime(14.0min)
and confirmed by GC-MS.
Figure 1. Selected IR spectra (KBr, transmittance / % vs. frequency / cm-1)
of HL (a), H2Lox (b), 1 (c) and 2 (d).
The reaction of HL with [VO(acac)2] in methanol
produced 1 as yellow single crystals after slow evaporation
of the mother solution. Complex 2 was prepared by
three different procedures, according to Scheme 1. In
all cases, brown single crystals were obtained after slow
evaporation of the mother solution. Besides the same color
Results and Discussion
Syntheses
1
The ligand HL was prepared following a procedure
and morphology, IR and H NMR analyses confirmed
1
reported previously, and characterized by H NMR.
the same composition for the solids obtained from the
three procedures. Figures 1 and 2 show the main spectral
The H2Lox was obtained by a reaction between HL and

664
Synthesis, Characterization and Catalytic Activity of Two Novel cis-Dioxovanadium(V) Complexes
J. Braz. Chem. Soc.
the cis-dioxo vanadium center. The three nitrogen atoms,
from the two pyridines and the tertiary amine, are arranged
in a facial configuration, in such a way that pyridine-N1
is trans to phenol-O1, pyridine-N2 is trans to oxo-O2 and
the tertiary-N3 is trans to oxo-O3. In both complexes, the
aldehyde and oxime groups remain uncoordinated. The
asymmetric unit of 1 contains two molecules of complex
and one water molecule, connected by hydrogen bonds
(Figure 3). The aldehyde-O4B atom of molecule B binds
one of the H atoms of the water, while the other water
hydrogen atom binds the oxo-O2A atom of moleculeA. For
complex 2, the crystal packing is assured by intramolecular
hydrogen bonds, in which the hydrogen atom of the oxime
of one molecule binds the oxo-O2 atom of the next one
(Figure 4). These hydrogen bonds direct the crystalline
packing towards [100] direction. Table 1 summarizes the
hydrogen-bonds for 1 and 2.
Figure 2. 1H RMN spectra of HL and H2Lox in CDCl3, and of 1 and 2 in
DMSO-d6 (from the bottom to the top, respectively).
remarks for complexes 1 and 2 compared with their
precursor ligands HL and HLox. Although changes in
frequencies of almost all peaks were observed in IR spectra
after complexation, the general feature of ligands spectra
remained in their respective complexes. The most drastic
change in the ligands spectra, however, was observed
1
by H NMR after coordination. For the free ligands, the
hydrogen atoms Ha−Hd are equivalent to Ha’−Hd’, as the
two pyridine rings have similar chemical environments. The
same happens with the –CH2− hydrogen atoms, observed
as two singlet peaks between d 3.5 and 4.0 ppm. For the
complexes, each hydrogen atom of the ligand, aliphatic
and aromatic, can be associated with an individual peak in
the spectra. This fact indicates that after complexation, one
single conformation of the ligand is arranged around the
metal center.As electron density is transferred to the metal
through the N,O-donor atoms, the chemical environment
of each H atom is now different. The full assignment of
the spectra shown in Figure 2 was performed according to
2D COSY experiments of all compounds and calculation
of the 1H NMR spectrum for complex 1.
Figure 3. ORTEP32 (top) and frontier orbitals HOMO (bottom, left) and
LUMO (bottom, right) of complex 1.
Figure 4. ORTEP32 of complex 2.
X-ray structures
The molecular structures of complexes 1 and 2 consist
of identical distorted octahedral environments, in which the
ligands L- and Lox-, respectively, are coordinated around
As can be observed in Table 2, the bond lengths and
angles around the metal center are very similar for both
complexes. According to information retrieved from the

Vol. 22, No. 4, 2011
Silva et al.
665
Table 1. Hydrogen-bond geometry for 1 and 2 (Å, °)
spectra typical of diamagnetic species (Figure 2), ligand
field transitions are not expected. Therefore, the observed
bands should be related to LMCT and intra-ligand charge
transfer transitions. To get a better insight into the UV-Vis
spectra, the time dependent (TD) approach available
in Gaussian was employed to calculate the electronic
transitions for [VO2(L)]. The calculated spectra at the
B3LYP/6-31+G(d) level revealed two intense bands at 386
and 356 nm, which may be superimposed in the observed
absorption maxima at 370 nm. The first band, at 386 nm,
is assigned to the HOMO à LUMO transition, which
correspond to a LMCT process involving a pp orbital
on the aromatic phenolate ring (the HOMO) and one
empty 3d orbital on the metal (the LUMO). The second
band, at 356 nm, is attributed to an intraligand transition
involving the HOMO and the antibonding orbital of the
carbonyl group. Frontier orbitals for 1 are depicted in
Figure 3. In the region between 259 and 275 nm several
bands of medium to high intensity are predicted, most
of them involving contributions of several transitions,
mainly LMCT. Kwiatkowski et al.8 reported a series of
amino/imino-phenolate oxovanadium(V) complexes with
similar spectral features. For these complexes, the lowest
energy bands, in the 349-404 nm range, were assigned to
LMCT transitions involving the pp orbital of phenolate
and the empty 3d orbital of the metal. The highest energy
bands, observed in approximately 325 nm, were assigned
to intraligand p−p*transitions.
D—H···A
D—H
H···A
D···A
D—H···A
[VO2(L)]
O1W—H1W2···O4B
O1W—H1W1···O2A
[VO2(Lox)]
0.86
0.86
2.06
1.95
2.864(5)
2.809(5)
155
170
O4—HOx···O2
0.82
2.00
2.794(3)
163
Table 2. Bond lengths (Å) and angles (°) for 1 and 2
[VO2(L)] (1)
[VO2(Lox)]
Complex
(2)
Molec. A
1.949(3)
1.654(3)
1.632(3)
2.150(4)
2.306(3)
2.312(3)
106.97(2)
156.69(1)
72.79(1)
89.99(1)
72.31(1)
Molec. B
1.947(3)
1.642(3)
1.639(3)
2.135(4)
2.325(3)
2.315(3)
106.59(2)
156.44(2)
72.99(1)
88.84(1)
72.29(1)
B3LYP
1.908
1.609
1.607
2.191
2.373
2.421
108.99
153.46
71.62
90.26
70.09
V1-O1
1.932(2)
1.668(6)
1.631(6)
2.151(2)
2.286(2)
2.304(7)
105.97(9)
156.85(7)
72.99(7)
88.65(8)
73.07(7)
V1-O2
V1-O3
V1-N1
V1-N2
V1-N3
O2-V1-O3
O1-V1-N1
N1-V1-N3
O3-V1-N2
N3-V1-N2
Cambridge Structural Database (CSD, Conquest),38 for a
vanadium cis-dioxo complex in an octahedral geometry,
the average angle O=V=O is 105.8(9)°, and the average
distance forV-Npyridine is 2.22(9)Å, forV-Ophenol is 1.94(5)Å,
for V=O is 1.64(2) Å and for V-Namine is 2.26(9)Å. These
values are in a good agreement with those found for
complexes 1 and 2 (Table 2). It should be noted, however,
that in both complexes the V1-N1 bond is about 0.17 Å
shorter than the V1-N2 bond, despite the fact that both
are V-Npyridine bonds. This difference can be explained by
the influence of the ligand trans to the pyridine nitrogen
atoms. The N1 atom has one phenolic oxygen in the trans
position, while N2 is trans to an oxo group. As the V=O
bond is much shorter that the V-Ophenol, the bond distance
of the ligand trans to the former (V-N2) is longer than
the other (V-N1). These results are reinforced by the
B3LYP/6-31G(d) calculations for [VO2(L)].
Redox properties
Cyclic voltammograms (CV) of complexes 1 and
2, in DMSO, show two reduction processes, as can be
observed in Figure 5. All potentials are reported versus
the ferrocene/ferrocinium (Fc/Fc+) redox couple, used as
internal reference.
The first process is an irreversible wave with peak
potential at -1.89 V for 1 and -1.96 V for 2. The second
process, with half wave potentials at -2.19V (DE = 0.11V)
for 1 and -2.23V (DE = 0.16V) for 2, is better described as
quasi-reversible. It was also observed for 1 a low intensity
irreversible wave at -0.68 V, which seems to be related to
the reduction peak at -1.89 V, as it does not appear when
the scan direction is inverted, starting from -1.40 V (see
Figure S2). This wave is absent in the CV of complex 2,
which has, however, an intense irreversible peak at +0.58V
that is not observed for 1. Cyclic voltammograms of the
ligands were also measured under similar conditions
(Figure S3). While H2Lox did not present any redox
response in the negative segments of the CV, an irreversible
peak at -2.14V was observed for HL. Based on these data,
Electronic spectra
UV-Visible spectra, in DMSO, show the presence of
absorptions with maxima at 370 nm (e = 9 × 103 mol L-1 cm-1)
for 1 and 333 nm (e = 9 × 103 mol L-1 cm-1) for 2.A shoulder
in 260-270 nm was also observed for both compounds.
As the vanadium center is in a +5 oxidation state (3d0
1
configuration), what is in agreement with the H NMR

666
Synthesis, Characterization and Catalytic Activity of Two Novel cis-Dioxovanadium(V) Complexes
J. Braz. Chem. Soc.
OH
O
OOX
catalyst
oxidant
+
+
Cy-OH
Cy=O
Cy-OOX
Scheme 2. Cyclohexane oxidation products (X = H or t-butyl).
Table 3. Product distribution for the cyclohexane oxidation after 24 h
Overall
conversion
Catalyst Oxidant
H2O2
Product
Turnover Selectivity
13%
Cy-OH
Cy=O
12%
137
14%
73%
0%
1
Cy-OOH
Cy-OH
t-BuOOH
Figure 5. Cyclic voltammograms of complexes 1 (bottom) and 2 (top)
in a DMSO/TBAPF6 0.1 mol L-1 solution, scan rate = 0.1 V s-1, working
electrode = glassy carbon, reference = Ag/AgCl (DMSO/TBAPF6
0.1 mol L-1), auxiliary = platinum wire and ferrocene as internal reference.
Cy=O
1.4%
5.7%
2.1%
15
14%
86%
17%
12%
71%
0%
Cy-OO-t-Bu
Cy-OH
H2O2
Cy=O
62
2
Cy-OOH
Cy-OH
the first irreversible reduction wave observed for complexes
1 and 2 is tentatively assigned to be centered in the ligand,
while the more negative quasi-reversible reduction process
is tentatively assigned to the VV/VIV couple. The slightly
more negative reduction potentials of complex 2 could be
related to the presence of the oxime group, which has a
weaker electron withdrawing capability compared with the
aldehyde carbonyl present in complex 1.39 CV experiments
using dichloromethane as solvent were also carried out
for 1, with identical results to those obtained in DMSO.
However, experiments in this solvent were not possible for
complex 2 due to its low solubility.
t-BuOOH
Cy=O
23
42%
58%
Cy-OO-t-Bu
ratio catalyst:substrate:oxidant as 1:1000:1000. In fact, this
same behavior has been observed before and may be one
of the reasons of the overall conversion values using H2O2
to be about nine times larger compared with t-BuOOH,
for complex 1.16,18,40-42 However, it cannot be excluded that
acetonitrile plays an important role in this process because
of a competition between the solvent and the hydrocarbon
by an oxidizing species, which can be generated with
both oxidants.16,43 Although the overall conversion values
for both complexes are low, they showed to be selective
to the intermediates cyclohexylhydroperoxide (Cy-OOH)
or cyclohexyl-tert-butylhydroperoxide (Cy-OO-t-Bu),
depending if the oxidant was H2O2 or t-BuOOH. For
complex 1, the intermediate was selectively formed with
yields of 73 and 86% when the oxidants were, respectively,
H2O2 and t-BuOOH. For complex 2, the yields were 73%
when the oxidant was H2O2, and 58% when the oxidant was
t-BuOOH. The turnover number for 1 is about twice the
value observed for 2, while both complexes present similar
turnover numbers when t-BuOOH is used. These results
are comparable with those previously reported for a VIVO
complex in a N3O3 environment and for a dinuclear CuII
complex.16,18 Despite the low overall conversion presented
by complexes 1 and 2, it is higher than the corresponding
uncatalyzed reactions which lead to less than 0.5% of overall
conversion and selectivity to cyclohexylhydroperoxide or
cyclohexyl-tert-butylhidroperoxide. Considering that these
experiments were carried out at 1 atm pressure, room
Catalytic oxidation of cyclohexane
In order to investigate the potential use of complexes 1
and 2 as catalysts in oxidative processes, their reactivities
were preliminary investigated through the oxidation
of cyclohexane (Scheme 2). In a typical experiment,
cyclohexane was oxidized in the presence of the desired
complex and H2O2 or t-BuOOH. After 24 h of reaction, the
products cyclohexanol (Cy-OH), cyclohexanone (Cy=O),
cyclohexylhydroperoxide (Cy-OOH) and/or cyclohexyl-
tert-buthylhydroperoxide (Cy-OO-t-Bu) were detected,
and the results are summarized in Table 3.
Complex 1 is the most active catalyst, presenting an
overall conversion of 12% when the oxidant is H2O2 and
only 5.7% employing t-BuOOH. For complex 2, the overall
conversion values are 5.7 and 2.1%, respectively, for the
same oxidants.According toTable 3, the oxidation reactions
exhibited better results when the oxidant was H2O2,
probably because is a stronger oxidant than t-BuOOH, since
their initial concentrations were kept the same to assure the

Vol. 22, No. 4, 2011
Silva et al.
667
temperature, with no co-catalysts, and that the intermediates
selectively formed are easily converted to cyclohexanol,
cyclohexanone and adipic acid upon reduction, which are
very important to the production of Nylon-6 and Nylon-6,6,
the complexes can be considered as quite efficient catalysts.
6. Hamstra, B. J.; Colpas, G. J.; PecoraroV. L.; Inorg. Chem. 1998,
37, 949.
7. Sun, J.; Zhu, C.; Dai, Z.; Yang, M.; Pan, Y.; Hu, H.; J. Org.
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9. Mimoun, H.; Mignard, M.; Brechot, P.; Saussine, L.; J. Am.
Chem. Soc. 1986, 108, 3711.
Conclusions
A novel tripodal ligand presenting an oxime pendant
group (HLox) was successfully synthesized from the
precursor ligand containing an aldehyde function (HL).
Both HLox and HL were employed in the syntheses of two
novelVV complexes, 1 and 2, which were also characterized
as cis-dioxoVV systems. It was demonstrated that complex
2 can be prepared by three different pathways: from HLox,
from HL and from complex 1. Complexes 1 and 2 presented
overall conversions towards the cyclohexane oxidation
between 5.7 and 12% using the friendly oxidant H2O2, with
high selectivity (up to 85%) to the cyclohexylhydroperoxide
intermediate in mild conditions (1 atm and room
temperature). The overall conversion, turnover number and
selectivity obtained for complexes 1 and 2 are in the range
of those values found for other oxovanadium complexes
previously reported.10,16,19,22
10. Mimoun, H.; Saussine, L.; Daire, E.; Postel, M.; Fischer, J.;
Weiss, R.; J. Am. Chem. Soc. 1983, 105, 3101.
11. Shulpin, G. B.; Attanasio, D.; Suber, L.; J. Catal. 1993, 142,
147; Bonchio, M.; Conte, V.; Di Furia, F.; Modena, G.; J. Org.
Chem. 1989, 54, 4368; Conte, V.; Di Furia, F.; Modena, G.;
J. Org. Chem. 1988, 53, 1665.
12. Secco, F.; Inorg. Chem. 1980, 19, 2722.
13. Bhattacharjee, M. N.; Chaudhuri, M. K.; Islam, N. S.; Inorg.
Chem. 1989, 28, 2420.
14. Saji, P.V.; Ratnasamy, C.; Gopinathan, S.; US pat 6,392,093B1,
2002.
15. Yuan, Y.; Ji, H.; Chen, Y.; Han, Y.; Song, X.; She, Y.; Zhong,
R.; Org. Process Res. Dev. 2004, 8, 418.
16. Fernández, T. L.; Souza, E. T.; Visentin, L. C.; Santos, J. V.;
Mangrich, A. S.; Faria, R. B.; Antunes, O. A. C.; Scarpellini,
M.; J. Inorg. Biochem. 2009, 103, 474.
Supplementary Information
17. Silva, A. C.; Fernández, T. L.; Carvalho, M. F. N.; Herbst, M.
H.; Bordinhão, J.; Horn Jr, A.; Wardell, J. L.; Oestreicher, E.
G.; Antunes, O. A. C.; Appl. Catal., A 2007, 317, 154.
18. Martins, L. R.; Souza, E. T.; Fernández, T. L.; Souza, B.;
Rachinski, S.; Pinheiro, C. B.; Faria, R. B.; Casellato, A.;
Machado, S. P.; Mangrich,A. S.; Scarpellini, M.; J. Braz. Chem.
Soc. 2010, 21, 1218.
Crystallographic data have been deposited at the
Cambridge Crystallographic Data Centre (deposition
numbers CCDC 772836 (1) and CCDC 772836 (2).
Copies of available material can be obtained by request
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax 44-1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).
19. Silva, T. F.; Luzyanin, K. V.; Kirillova, M. V.; Silva, M. F. G.;
Martins, L. M. D. R. S.; Pombeiro, A. J. L.; Adv. Synth. Catal.
2010, 352, 171.
Acknowledgments
20. Kirillova, M. V.; Kuznetsov, M. L.; Romakh, V. B.; Shul’pina,
L. S.; Silva, J. J. R. F.; Pombeiro, A. J. L.; Shulpin, G. B.;
J. Catal. 2009, 267, 140.
We thank the financial support conceived from
CNPq, FAPERJ, FINEP, PIBIC-UFF, and the X-Ray
Diffraction Laboratory at Universidade Federal Fluminense
(LdrX-UFF) for the data collection.
21. Baran, E. J.; J. Inorg. Biochem. 2000, 80, 1.
22. Reis, P. M.; Silva, J. A. L.; Silva, J. J. R. F.; Pombeiro, A. J. L.;
Chem. Commun. 2000, 1845.
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Submitted: April 10, 2010
Published online: December 2, 2010

J. Braz. Chem. Soc., Vol. 22, No. 4, S1-S3, 2011.
Printed in Brazil - ©2011 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Supplementary Information
SI
Synthesis, Characterization and Catalytic Activity of Two Novel
cis-Dioxovanadium(V) Complexes: [VO2(L)] and [VO2(HLox)]
Natália M. L. Silva,a Carlos B. Pinheiro,b Eluzir P. Chacon,a Jackson A. L. C. Resende,a
José Walkimar de M. Carneiro,a Tatiana L. Fernández,c Marciela Scarpellinic and
Mauricio Lanznaster*,a
a Instituto de Química, Universidade Federal Fluminense, Outeiro S. João Batista S/N,
24020-141 Niterói-RJ, Brazil
b Departamento de Física, Universidade Federal de Minas Gerais, Av. Antonio Carlos 6627,
Pampulha, 31270-901 Belo Horizonte-MG, Brazil
c Instituto de Química, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos 149,
Bl. A, 21941-909 Rio de Janeiro-RJ, Brazil
Figure S1. UV-Vis spectra for 1 and 2 recorded in DMSO solutions.
*e-mail: mlanz@vm.uff.br

S2
Synthesis, Characterization and Catalytic Activity of Two Novel
J. Braz. Chem. Soc.
Figure S2. Cyclic voltammograms of complexes 1 in a DMSO/TBAPF6 0.1 mol L−1 solution, scan rate = 0.1 V s−1, working electrode = glassy carbon,
reference = Ag/AgCl (DMSO/TBAPF6 0.1 mol L−1), auxiliary = platinum wire and ferrocene as internal reference. The arrows indicate the scan directions.
This figure illustrates the dependence of the process at −0.68 V with the reduction peak at −1.89 V.
Figure S3. Cyclic voltammograms of the ligands HL (top) and H2Lox (bottom) in a DMSO/TBAPF6 0.1 mol L−1 solution, scan rate = 0.1 V s−1, working
electrode = glassy carbon, reference = Ag/AgCl (DMSO/TBAPF6 0.1 mol L−1), auxiliary = platinum wire and ferrocene as internal reference. The arrows
indicate the scan direction.

Vol. 22, No. 4, 2011
Silva et al.
S3
Table S1. Summary of the crystal structure data collection and refinement for 1 and 2
Identification code
[VO2(L)] (1)
[VO2(Lox)] (2)
Empirical formula
Formula weight
Temperature
C21H21N4O4V
444.36
(C21H20N3O4V)2
876.7
⋅
H2O
293(2) K
293(2) K
0.71073 Å
monoclinic
P21/c
Wavelength
0.71073 Å
monoclinic
P 21/c
Crystal system
Space group
a = 13.316(3) Å
b = 14.293(3) Å
c = 22.902(7) Å
b = 113.70(2)°
a = 10.126(2) Å
b = 15.044(3) Å
c = 13.143(3) Å
b = 97.61(3)°
Unit cell dimensions
Volume
3991.2(17) Å3
4
1984.6(7) Å3
4
Z
Density (calculated)
Absorption coefficient
F(000)
1.459 mg m−3
0.533 mm−1
1816
1.487 mg m−3
0.537 mm−1
920
Crystal size
0.26 × 0.25 × 0.13 mm3
5.16 to 25.03°
0.47 × 0.22 × 0.14 mm3
3.61 to 26.00°
θ range for data collection
−15 ≤ h ≤ 15, −17 ≤ k ≤ 17,
−27 ≤ l ≤ 27
−12 ≤ h ≤ 12, −18 ≤ k ≤ 17,
−16 ≤ l ≤ 16
Index ranges
Reflections collected
26034
6986 [R(int) = 0.0431]
99.1%
23865
3893 [R(int) = 0.0351]
99.6%
Independent reflections
Completeness to θ = 26.37°
Absorption correction
Max. and min. transmission
Refinement method
empirical
empirical
0.948 and 0.897
Full-matrix least-squares on F2
0.931 and 0.785
Full-matrix least-squares on F2
Data / restraints / parameters
Goodness-of-fit on F2
6986 / 2 / 540
3893 / 0 / 271
1.169
1.088
Final R indices [I > 2s (I)]
R indices (all data)
R1 = 0.0578, wR2 = 0.1291
R1 = 0.0872, wR2 = 0.1386
0.487 and −0.364 e.Å−3
R1 = 0.0379, wR2 = 0.0966
R1 = 0.0502, wR2 = 0.1043
0.488 and −0.308 e.Å−3
Largest diff. peak and hole