Vanadyl cationic complexes as catalysts in olefin oxidation

Article abstract of DOI:10.1039/c4dt03174a

Three new mononuclear oxovanadium(iv) complexes [VO(acac)(R-BIAN)]Cl (BIAN = 1,2-bis{(R-phenyl)imino}acenaphthene, R = H, 1; CH₃, 2; Cl, 3) were prepared and characterized. They promoted the catalytic oxidation of olefins such as cyclohexene, cis-cyclooctene, and styrene with both tbhp (tert-butylhydroperoxide) and H₂O₂, and of enantiopure olefins (S(-)- and R(+)-pinene, and S(-)- and R(+)-limonene) selectively to their epoxides, with tbhp as the oxidant. The TOFs for styrene epoxidation promoted by complex 3 with H₂O₂ (290 mol mol^-1_V h^-1) and for cis-cyclooctene epoxidation by 2 with tbhp (248 mol mol^-1_V h^-1) are particularly good. Conversions reached 90% for several systems with tbhp, and were lower with H₂O₂. A preference for the internal CC bond, rather than the terminal one, was found for limonene. Kinetic data indicate an associative process as the first step of the reaction and complex [VO(acac)(H-BIAN)]⁺ (1⁺) was isolated in an FTICR cell after adding tbhp to 1. EPR studies provide evidence for the presence of a V(iv) species in solution, until at least 48 hours after the addition of tbhp and cis-cyclooctene, and cyclic voltammetry studies revealed an oxidation potential above 1 V for complex 1. DFT calculations suggest that a [VO(H-BIAN)(MeOO)]⁺ complex is the likely active V(iv) species in the catalytic cycle from which two competitive mechanisms for the reaction proceed, an outer sphere path with an external attack of the olefin at the coordinated peroxide, and an inner sphere mechanism starting with a complex with the olefin coordinated to vanadium. This journal is

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Vanadyl cationic complexes as catalysts in olefin oxidation
Carla D. Nunes,a Pedro D. Vaz,a,1 Vítor Félix,b Luis F. Veiros,c Tânia Moniz,e Maria Rangel,d Sara
Realista, a Ana C. Mourato,a Maria José Calhordaa
aCentro de Química e Bioquímica, DQB, Faculdade de Ciências, Universidade de Lisboa, 1749-
016 Lisboa, Portugal
bDepartamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal
c Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico,
Universidade de Lisboa , Av. Rovisco Pais, 1049-001 Lisboa, Portugal
d REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do
Porto, 4169ꢀ007 Porto, Portugal
e REQUIMTE, Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, 4099ꢀ003
Porto, Portugal
Abstract
Three new mononuclear oxovanadium(IV) complexes [VO(acac)(R-BIAN)]Cl (BIAN = 1,2-
bis{(R-phenyl)imino}acenaphthene, R = H, 1; CH3, 2; Cl, 3) were prepared and characterized.
They promoted the catalytic oxidation of olefins such as cyclohexene, cisꢀcyclooctene, and styrene
with both tbhp (tertꢀbutylhydroperoxide) and H2O2, and of enantiopure olefins (S(-)- and R(+)-
pinene, and S(-)- and R(+)-limonene) selectively to their epoxides, with tbhp as oxidant.
Particularly good are the TOFs for styrene epoxidation promoted by complex 3 with H2O2 (290 mol
molꢀ1 hꢀ1) and for cisꢀcyclooctene epoxidation by 2 with tbhp (248 mol molꢀ1 hꢀ1). Conversions
V
V
reached 90% for several systems with tbhp, and were lower with H2O2. A preference for the internal
C=C bond, rather than the terminal one, was found for limonene. Kinetic data indicate an
associative process as the first step of the reaction and complex [VO(acac)(H-BIAN)]+ (1+) was
isolated in a FTICR cell after adding tbhp to 1. EPR studies provide evidence for the presence of a
V(IV) species in solution, until at least 48 hours after the addition of tbhp and cisꢀcyclooctene, and
cyclic voltammetry studies revealed an oxidation potential above 1 V for complex 1. DFT
calculations suggest that a [VO(H-BIAN)(MeOO)]+ complex is a likely active V(IV) species in the
catalytic cycle from which two competitive mechanisms for the reaction proceed, an outer sphere
1 Present address: ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire X11
0QX, United Kingdom
1

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path with external attack of the olefin at the coordinated peroxide, and an inner sphere mechanism
starting with a complex with the olefin coordinated to vanadium.
Keywords: Vanadium / Cationic complexes / Epoxidation / EPR / DFT
Introduction
Vanadium is one of the essential elements of living organisms, playing an important role in several
biological processes, namely in halide oxidation catalyzed by haloperoxidases, or in nitrogen
fixation carried out by vanadium containing nitrogenases.1 These aspects contributed to a renewed
interest in the coordination chemistry of vanadium,2 only helped by the applications of vanadium
complexes in the clinical treatment of diabetes 3 as well as their potential role as catalysts.4
The activity of vanadium enzymes in biological oxidation processes led to the development of a
rich oxidation chemistry aimed at the synthesis of valuable molecules. Vanadium complexes are
usually active in the presence of H2O2, a much more environment friendly oxidant than organic
peroxides, which generates water as a byꢀproduct. Oxovanadium(IV) complexes are easily
synthesized from available precursors, such as vanadyl salts or [VO(acac)2],5 and their catalytic
activity in a plethora of reactions, ranging from the oxidation of alkenes, to the oxidation of other
functional groups,6 has been widely studied in the presence of several oxidants, such as H2O2 or
alkylhydroperoxides, and including molecular oxygen,7 in variable experimental conditions. The
immobilization of vanadium species in solids has also led to establishing new active heterogeneous
catalysts.8
Although VO(IV) complexes are relatively stable toward oxidation, they may be easily oxidized to
V(V) in the presence of oxidants. Therefore, when trying to understand the mechanisms of the
catalytic reactions promoted by oxovanadium(IV) complexes, a major question concerns the nature
of the active catalytic species, in particular the formal oxidation state of vanadium. On the other
hand, it is accepted that the mechanism strongly depends on the oxidant, with, for instance, H2O2
leading to the formation of peroxide complexes, while in the presence of alkylhydroperoxides
(ROOH) the ROO- anion coordinates to the metal.9 The role of these complexes has been well
described in the oxidation chemistry promoted by molybdenum species, either Mo(VI) complexes,
or oxidation products of Mo(II) precursors.10 Several works have addressed these mechanistic
aspects during the last decade,8,11 but many questions remain open. While it is clear that at the end
of the catalysis V(V) species, often catalytically inactive, are present, it is not yet obvious whether
the active species is a VO(IV) complex resulting from the reaction of the precursor with the
oxidant, without oxidation of the metal. In a systematic mechanistic study,12 all the likely species
2

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that might be obtained from [VO(acac)2] and tbhp were calculated (DFT) and their relative energies
examined, in order to infer the most pausible mechanism. The possibility of V(IV) active species in
catalysis was not discarded and the authors detected their presence in the complex solution in the
initial stages of its reaction with tbhp.
In this work, new mononuclear oxovanadium(IV) complexes [VO(acac)(R-BIAN)]Cl (BIAN =
1,2-bis{(R-phenyl)imino}acenaphthene, R = H, CH3, Cl) have been prepared and characterized by
FTIR, 1H and 13C NMR, and elemental analysis. They were tested for their activity as catalyst
precursors in the epoxidation of olefins (cisꢀcyclooctene, ciclohexene, and styrene) and chiral
olefins [S(-)- and R(+)-pinene, and S(-)- and R(+)-limonene], in the presence of tꢀ
butylhydroperoxide (tbhp) or H2O2 as oxygen sources. Kinetic, EPR, cyclic voltammetry, High
Resolution Mass Spectrometry and DFT studies were performed to address the nature of the active
catalytic species and the reaction mechanism.
Results and Discussion
Chemical studies
The three bidentate dinitrogen ligands 1,2-bis[(phenyl)imino]acenaphthene (H-BIAN, L1), 1,2ꢀ
bis[(4-methylphenyl)imino]acenaphthene (Me-BIAN, L2), and 1,2-bis[(4-
chlorophenyl)imino]acenaphthene (Cl-BIAN, L3), were prepared according to Scheme 1 and
characterized by comparison with literature.13
Scheme 1
L1, L2, and L3 were allowed to react with a solution of [VO(acac)2] in MeOH at reflux, in a 1:1
ratio, to afford complexes [VO(acac)(R-BIAN)]Cl (RꢀBIAN=L1, 1; L=L2, 2; L=L3, 3), as
crystalline powders (Scheme 2). The formulation proposed is based on spectroscopic data and
elemental analysis (see Experimental). The molar conductivity of complexes 1-3 (~14.6 S cm2
molꢀ1) is lower than the typical for 1:1 salts,14 though ~5 times higher than that of the [VO(acac)2]
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precursor (Λm= 2.85 S cm2 molꢀ1). This suggests that the chloride is not coordinated to the metal,
forming an ionꢀpair in solution.
Scheme 2
The coordination of the nitrogen ligands was reflected in the shift of the characteristic νC=N
vibrational mode in the FTIR spectra, from 1653, 1657, and 1658 cmꢀ1, respectively, in the free
ligands L1, L2 and L3, to a strong band at 1566, 1560, and 1566 cmꢀ1 in the three complexes. The
bands observed at 1524, 1523, and 1524 cmꢀ1, for 1, 2, and 3, respectively, are assigned to the C=O,
and those at 1486, 1502, and 1485 cmꢀ1, to the C=C stretches of the coordinated acetylacetonate
ligand.15 The strong νV=O stretching mode appears as a strong band at 997, 995, and 978 cmꢀ1, for 1,
2, and 3, in agreement with the V=O stretching frequencies reported for other oxovanadium (IV)
complexes.16,17 In the [VO(acac)2] precursor complex this mode is observed at 995 cmꢀ1, suggesting
that this strong bond is not much affected by the other ligands around vanadium.
The signals of all the protons of the BIAN and acac ligands could be assigned in the 1H NMR
spectra (Experimental, see atom numbering in Scheme 1) of the vanadyl complexes despite the
paramagnetism.
The electronic spectra of the ligands L1-L3 present strong absorption at 304 nm, while those of the
complexes are observed at 302, 305 or 306 nm for 1, 2, and 3, respectively.
In an attempt to grow crystals of complex 2, crystals suitable for an X-ray single crystal analysis
were obtained by diffusion of a solution of diethyl ether in a solution of methanol. However, they
turned out to be the imide of naphthaleneꢀ1,8ꢀdicarboxylic acid (4) shown in Supplementary
Information.
Catalytic studies
The catalytic activity of the three vanadyl complexes [VO(acac)(R-BIAN)]Cl (R= H, 1; CH3, 2; Cl
3) in olefin oxidation was tested in the presence of hydrogen peroxide (H2O2) and t-
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butylhydroperoxide (tbhp), using cyclohexene (Cy6), cisꢀcyclooctene (Cy8), and styrene (Sty), and
in the presence of tbhp using the chiral olefins, S(-)- and R(+)-pinene, and S(-)- and R(+)-
limonene. Control experiments showed that no reaction took place in the absence of a vanadium
complex. Detection and identification of products was done by both GC and 1HꢀNMR experiments.
The reaction of complexes 1-3 with cyclohexene, cisꢀcyclooctene and styrene was carried out at
328 K using tbhp and H2O2 as oxidants (molar ratio of substrate:oxidant:catalyst = 100:200:1), and
yielded the corresponding epoxides as the only products. The solvent was dichloromethane with
tbhp, and acetonitrile with hydrogen peroxide. This last choice was prompted by the need to avoid a
biphasic system, as water started to form after loss of oxygen from H2O2, and no epoxide was
obtained when dichloromethane was used. The turnover frequencies (TOF) and the conversions of
the olefins are given in Table 1.
Table 1. Turnover frequencies and conversions in the epoxidation of cyclohexene, cisꢀcyclooctene,
and styrene using complexes [VO(acac)(R-BIAN)]Cl (1-3) as catalyst precursors, and tbhp or
H2O2 as oxidants, at 328 K.
tbhp
Conv.b
H2O2
Conv.b
Complex
Olefin
Cy6
Cy8
Sty
TOFa
222
191
212
222
248
218
235
187
216
TOFa
254
184
264
210
162
286
252
110
290
92
82
78
98
93
83
90
87
76
85
66
77
78
76
69
91
54
77
1
Cy6
Cy8
Sty
2
Cy6
Cy8
Sty
3
a calcd. at 0.25 h as mol molꢀ1[V] hꢀ1; b calcd. after 24 h
The three complexes 1-3 exhibit relatively similar initial catalytic activities for all substrates with
both oxidants, although there is a smaller dispersion with tbhp, with TOFs ranging between 187 to
248 mol molꢀ1 hꢀ1; when using H2O2 as oxidant, the TOFs can go down to 110 (Cy8) and up to 290
V
mol molꢀ1 hꢀ1 (Sty).
V
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With tbhp, the highest initial activity in the oxidation of cyclohexene is achieved when complex 3 is
the precursor (TOF of 235 mol molꢀ1V hꢀ1), with small differences from the other complexes, while
complex 2 is the best for cisꢀcyclooctene (248 mol molꢀ1V hꢀ1). All behave similarly toward styrene.
Conversions are usually higher with tbhp than with H2O2, the highest conversion being achieved in
the epoxidation of cyclohexene with any of the catalysts (90-98); complex 2 leads to higher
conversions independently of the substrate, and is thus less substrateꢀselective. When H2O2 is used,
the trends are less clear and conversions can be as low as 54 (Cy8 with complex 3). Although H2O2
might be considered a more environment friendly oxidant, as it leads to formation of H2O, the byꢀ
product of tbhp (tꢀbutanol) has many applications.
The kinetic profiles shown in Figure 1 for the oxidation of Cy6 using both oxidants and the three
catalyst precursors give a more detailed view of the reaction pattern. The reaction is initially fast in
all cases, reaching a conversion of ~70 % in about one hour, consistent with the formation of the
active species after addition of the oxidant, and a plateau after a few hours. It has been shown for
Mo(VI) catalysts that the formed tꢀBuOH (or H2O) competes with the oxidant in the reaction with
the metal center, 10,18 thus decreasing the conversion. A similar observation was reported for
vanadium catalysts.6a The final conversions of Cy6 are very close in the presence of complexes 1
and 3, but differ more for complex 3 (methyl substituent). The lower conversion in H2O2 may be
associated with lower solubility of 3.
Figure 1. Kinetic profiles for the epoxidation of Cy6 catalyzed by complexes [VO(acac)(L)]Cl (1-
3), in the presence of tbhp or H2O2 (molar ratio of substrate:oxidant:catalyst = 100:200:1), at 328 K.
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As a comparison, [VO(Saloph)] (Saloph = N,N’ꢀdisalicylideneꢀoꢀphenylenediaminate) converted
only 10.7 % of styrene after 2 hours at 363 K with tbhp, though with a higher TOF (726 mol molꢀ
1[V] hꢀ1). 8 VO(Schiff base) containing polymers also exhibited styrene conversions between 12 and
76 % at 348 K (tbhp, 6 hours) depending on substrate:catalyst:oxidant ratios.
The vanadium complexes [VO(acac)(R-BIAN)]Cl (1-3) were also tested in the catalytic
epoxidation of four enantiopure chiral olefins, S(-)- and R(+)-pinene, and S(-)- and R(+)-
limonene, at 328 K, in dichloromethane, using tbhp as the oxygen source (molar ratio of
substrate:oxidant:catalyst = 100:200:1). Only epoxides (Scheme 3) were formed, confirming the
high selectivity already observed in the experiments described above. The limonene substrates have
two competing olefin sites where epoxidation may occur, but NMR experiments demonstrated that
only the epoxides shown in Scheme 3 are formed, indicating that such catalytic systems are
chemospecific, and there is retention of configuration.
O
O
S(-)-pinene
R(+)-pinene
O
O
H
H
H
H
S(-)-limonene
R(+)-limonene
Scheme 3
The initial activities do not follow a specific trend, but vary within narrow intervals (177 to 243
mol·molꢀ1V ·hꢀ1), comparable to those found above (Table 1). Complex 2 leads to the highest TOFs
with S(-) olefins, while complex 3 displays a preference for R(+) molecules.
The conversions are found to be above 70%, with a maximum of 87% for the complex 2/ S(-)-
limonene system, again resembling the values obtained for the first set of substrates (Table 1).
Although the catalytic systems exhibit a high selectivity (only one epoxide, with retention of
configuration), their efficiency in terms of conversion is not outstanding.
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Table 2. Turnover frequencies and conversions in the epoxidation of S(-) and R(+)-pinene, and
S(-) and R(+)-limonene, using complexes [VO(acac)(R-BIAN)]Cl (1-3) as catalyst precursors,
and tbhp as oxidant.
Complex
Olefin
TOF[a] Conv.[b]
192
201
204
194
243
193
198
188
202
177
208
223
82
87
83
81
83
87
73
81
82
80
85
82
S(-)-pinene
S(-)-limonene
R(+)-pinene
R(+)-limonene
S(-)-pinene
S(-)-limonene
R(+)-pinene
R(+)-limonene
S(-)-pinene
S(-)-limonene
R(+)-pinene
R(+)-limonene
1
2
3
[a] calcd. at 0.25 h as mol molꢀ1[V] hꢀ1; [b] calcd. after 24 h
UV/Vis and Mass Spectrometry Kinetic Studies
Kinetic studies were carried out in the reaction of complexes [VO(acac)(R-BIAN)]Cl (1-3) with
each of the oxidants (tbhp and H2O2) in order to investigate the formation of the catalytically active
species, and evaluate the role of the H, CH3 and Cl ligand-substituents on the metal. Gould et al.6a
performed a similar study for the epoxidation of cyclohexene catalyzed by [VO(acac)2] and V(III)
complexes, after checking the absence of radical pathways.
The disappearance of the initial vanadyl complex was followed by UV/Vis spectroscopy. The
kinetics of the formation of the first intermediate shows a first order reaction in metal complex and
in oxidant. The observed rate constants were determined by fitting the absorbance vs time data to
the equation
At = A∞ (1− e−k t ) − offset − L*t
obs
(1)
where At and A∞ are the absorbances at time t and at the end of reaction, respectively, the offset is
the absorbance value at t = 0 s, and the L parameter takes into account a very small drift observed in
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the absorbance vs time data at the end of the reaction.19 An example of a typical curve is shown in
Figure 2, for complex 3 and tbhp, at 303 K, and with a molar ratio 1:100 (complex:tbhp).
Figure 2. Change in absorbance during the reaction between complex 3 and tbhp at 303 K
(experimental points and fit according to equation 1).
The observed pseudo-first order rate constants kobs, calculated varying the concentration of the
oxidant and keeping the concentration of the complex (1-3) constant (1×10ꢀ4 M) at 298 K, are
summarized in Table 3.
Table 3. Kinetic data (kobs) for the reaction between complexes [VO(acac)(R-BIAN)]Cl (1-3) and
at different concentrations of the oxidants (tbhp or H2O2), in acetonitrile, at 298 K.
kobs / sꢀ1
Oxidant
103 [Ox] / M
1
2
3
0.101
0.202
0.505
1.010
0.101
0.202
0.505
1.010
2.4 ± 0.2
2.8 ± 0.2
3.8 ± 0.2
7.2 ± 0.2
10.6 ± 0.1
21.3 ± 0.1
33.0 ± 0.1
53.9 ± 0.3
3.0 ± 0.2
3.7 ± 0.2
8.1 ± 0.2
12.5 ± 0.2
7.5 ± 0.1
11.2 ± 0.1
23.4 ± 0.7
37.2 ± 0.9
3.1 ± 0.1
4.1 ± 0.1
7.7 ± 0.1
14.5 ± 0.2
10.0 ± 0.1
16.0 ± 0.2
30.4 ± 0.2
54.7 ± 0.3
tbhp
H2O2
The fit of kobs values vs. oxygen donor concentration was done according to equation kobs=
k2[Oxidant], where kobs is the pseudo-first order rate constant and k2 is the second-order rate
constant. The fit shows, in the presence of excess of tbhp or H2O2, a clear firstꢀorder dependence on
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the concentration of the oxidant (tbhp or H2O2). The secondꢀorder rate constants (k2) for the reaction
of the three complexes [VO(acac)(R-BIAN)]Cl (1-3) with tbhp at several temperatures are given
in Table 4.
Table 4. Second-order rates (k2) for the reaction between complexes [VO(acac)(R-BIAN)]Cl (1-
3) and the oxidants (tbhp or H2O2), in acetonitrile, in the 298-333 K temperature range.
k2 / dm3 molꢀ1 sꢀ1
Oxidant T / K
1
2
3
298
303
313
328
333
298
303
313
328
333
5.2 ± 0.1
10.8 ± 0.2
14.6 ± 0.2
18.8 ± 0.5
32.6 ± 1.2
42.0 ± 0.7
32.7 ± 0.5
37.4 ± 0.5
61.4 ± 0.7
86.8 ± 0.8
12.6 ± 0.1
13.5 ± 0.2
14.7 ± 0.4
16.9 ± 0.5
17.6 ± 0.4
48.7 ± 0.2
67.4 ± 0.4
87.2 ± 0.4
120.8 ± 0.6
9.6 ± 0.2
tbhp
16.0 ± 0.4
23.6 ± 1.8
33.7 ± 1.8
45.0 ± 0.2
56.0 ± 0.3
93.6 ± 0.2
143.6 ± 0.4
165.0 ± 1.0
H2O2
113.0 ± 2.5 125.1 ± 0.75
The reaction with H2O2 is much faster than with tbhp, though the acceleration with increasing
temperature is not the same from one oxidant to the other and with different complexes. The
activation parameters ꢁS≠ and ꢁH≠ for catalyst formation were determined from the kinetic data
(Table 5) obtained in the range of 298–333 K, by fitting the second order rate constants with the
Eyring equation:
ꢁꢂ
ln ꢀ ꢄ =
ꢃ
−∆ꢅ‡
ꢆ
1
ꢃ
ꢁꢉ
+ ꢇꢈ ꢀ ꢄ +
ℎ
∆ꢊ‡
ꢆ
∙
where k2 is the secondꢀorder rate constant, T temperature (K), R, kB and h are the gas, Boltzmann,
and Planck constants. ꢁH≠ and ꢁS≠ are the activation enthalpy and entropy, respectively.
The effect of the substituent on the BIAN ligand in the rate constants and activation parameters is
clear. When the oxidant is tbhp, rate constants increase in the order 1 < 2
≈ 3; when H2O2 is used as
the oxygen source, the trend is 2 < 1 3 (Table 4).
≈
The large and negative ꢁS≠ values indicate an associative mechanism for the first step in the
activation of the vanadyl complexes 1-3. The ꢁH≠ values are relatively low in general, with a
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minimum for complex 3, [VO(acac)(Cl-BIAN)]Cl, in the reaction with tbhp. These values are
comparable, though lower, than ꢁH≠ 54 kJ molꢀ1obtained in the similar reaction between
≈
[VO(acac)2] and tbhp, in the earlier study of Gould et al,6a where cyclohexane was used as solvent
in the absence of alkene; the ꢁS≠ value was also very negative (–83 J molꢀ1 Kꢀ1). Although it is
expected that a similar reaction will take place, there are significant differences between
[VO(acac)2] and cationic complexes [VO(acac)(Cl-BIAN)]Cl (1-3) to justify the disparities. More
interesting is the comparison with several MoO2(VI)/tbhp systems for which the calculated
activation enthalpies (ꢁH≠
≈
50 kJ molꢀ1) are much closer.19 In these systems, there is no oxidation
of the metal center, and it is known that the first step consists of the activation of the O-H bond of
the alkylhydroperoxide, which protonates one oxide, while the ROO- anion binds the metal. On the
other hand, in the CH3ReO3/H2O2 system, the activation enthalpy is lower (ꢁH≠ = 29 kJ molꢀ1) and
the initial step is not the same (probably peroxide complexes are formed). 20
Table 5. Activation parameters ꢁS≠ and ꢁH≠ for the reaction between complexes [VO(acac)(R-
BIAN)]Cl (1-3) and the oxidants (tbhp or H2O2 in MeCN).
Oxidant Complex
ꢀ
H≠ / kJ.mol–1
36.4 ± 5.2
27.6 ± 1.8
5.1 ± 0.1
ꢀ
S≠ / J.mol–1 K–1
ꢀ107.4 ± 18.1
ꢀ132.2 ± 19.2
ꢀ206.6 ± 19.7
ꢀ118.9 ± 19.2
ꢀ128.8 ± 19.1
ꢀ149.0 ± 18.9
1
tbhp
2
3
1
2
3
28.0 ± 1.9
26.0 ± 2.0
18.7 ± 2.5
H2O2
The mechanism of the reaction between complexes [VO(acac)(R-BIAN)]Cl (1-3) and tbhp has an
associative nature and could start with a bond breaking process, probably the O–H bond in ROOH.
The ROO– ligand may then transfer the oxygen to the alkene substrate, forming epoxide and
butanol. The catalytic experiments show the decay in activity associated with formation of the
alcohol, which competes with the alkylhydroperoxide to yield the active species, a fact that has
been well demonstrated in Mo(VI) systems.19
Highꢀresolution mass spectrometry (HRMS) experiments were also carried out to study the
formation of vanadium species in the presence of the oxidant (tbhp) and to complement it the
detection of the relevant species produced in the reactions, which is possible under MS conditions.
The experiments were carried out adopting ESIꢀMS as the ionization technique and using collision
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induced dissociation (CID) on a FTICR spectrometer. This equipment returns the exact masses of
the observed fragments, and allows an accurate isolation of a given fragment. This can afterwards
react with another reactant introduced by means of an inlet valve. The first experiment was
accomplished by mixing complex 1 with tbhp (ratio 1:10) in acetonitrile (conditions similar to those
used in the UV/Vis experiments). Analysis of the resulting mixture (after addition of methanol for a
good quality spray) evidenced the presence of a single species containing vanadium detected at m/z
= 611.24, which can be formulated as [C33H40N2O6V]+. This formula corresponds to a species
[VO(H-BIAN)(tbhp)2(CH3OH)]+ , which can easily lose the methanol molecule to yield the
complex [VO(H-BIAN)(tbhp)2]+ (1x) detected at m/z = 579.21. This result shows that, in the
presence of tbhp, complex 1 loses easily the acac ligand, which is replaced by two tbhp molecules.
The species 1x can be considered as a rest mode of the catalyst prior to reaction with the olefin. To
confirm this hypothesis, kinetics were conducted with the MS spectrometer. Complex
[VO(acac)(H-BIAN)]+ (1+, detected at m/z = 498.11) was isolated in the ICR cell and allowed to
react with added tbhp with increasing time. The kinetic curve in Figure 3 evidences the reaction of
1+ with tbhp given by the decrease of the peak intensity. In a second experiment, complex 1x was
isolated and made to react with cisꢀcyclooctene to check if this could be the active species. In fact,
the decrease of the peak intensity of 1x with increasing reaction time, though at a slower rate than
the first step, supports this idea.
Figure 3. HRꢀMS kinetic experiments for the reactions: 1+ with tbhp yielding 1x (left); 1x with cisꢀ
cyclooctene (right).
The comparison between the two kinetic experiments shows that the second step can be defined as
the rate limiting step, at least in the early stages of the catalytic cycle. These results provide strong
evidence that the active species is based on a V(IV) complex, since the proposed V(IV) complexes
were observed at the right m/z ratios and no other V species were detected.
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EPR studies
The investigation of the oxidation state of the metal in the active catalytic species was carried out
by Electron Paramagnetic Resonance, by studying the reaction between the initial complex and
tbhp, and the reaction between complex 1, tbhp and substrate (cisꢀcyclooctene, Cy8).
At room temperature, complex [VO(acac)(H-BIAN)]Cl (1) exhibits a well resolved isotropic
signal, which exhibits anisotropy upon freezing the solution (Figure 4). The g and A values are
within the expected values for this type of V(IV) d1 complex and the computer simulation of the
experimental spectra reproduces well the observed features and provides the values of the Spinꢀ
Hamiltonian parameters.
Figure 4. EPR signal of complex [VO(acac)(H-BIAN)]Cl (1) obtained in dichloromethane at 296
K (left) and in frozen matrix at 100 K (right). Experimental (a) and simulated (b) spectra. Spinꢀ
Hamiltonian parameters obtained by simulation of the EPR spectra of complex 1: giso = 1.959,
Aiso= 97.0 x 10ꢀ4 cmꢀ1 at 296 K and g1 = 1.9785, A1= 60.5 x 10ꢀ4 cmꢀ1; g2 = 1.9660, A2= 64.0 x 10ꢀ4
cmꢀ1; g3 = 1.9476, A3= 167.5 x 10ꢀ4 cmꢀ1 at 100 K.
Tbhp was added to a dichloromethane solution of complex 1 and tbhp plus Cy8 were also added to
a solution of 1. Upon addition of the reagents, the EPR signal was not lost and its evolution along
time is shown in Figure 5 (see also Figure S3 in Supplementary Information). The signal of
complex 1 at 100 K (a) is easily identified by comparison with Figure 4. When tbhp is added, a less
intense signal remains (b). It is almost superimposable with (a), and characteristic of a V(IV)
complex. The signal obtained after addition of tbhp + Cy8 to complex 1 (c) is practically the same
and has not changed after 24 hours (d).These results indicate that despite the large excess of
oxidant, as in the catalytic conditions, a V(IV) species remains in solution and is still there 24 hours
later. The analysis of the spectra, registered along time, is indicative of the presence of the V(IV)
species and of a variation of the intensity of the signals, which is consistent with a variation in
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concentration of the paramagnetic species. These experiments prove the presence of a V(IV) species
in solution under catalytic conditions for at least 24 hours.
Figure 5. EPR spectra obtained at 100 K in frozen dichloromethane matrix of: complex 1
[VO(acac)(HꢀBIAN)]Cl (black line, a); 1 + tbhp (red line, b); 1 + cy8 + tbhp (blue line, c); 1 + cy8
+ tbhp, 24 hours later (green line, d). As the intensities are different, the less intense spectra (b, c
and d) were multiplied by a factor of 20. The integration areas, obtained from the corresponding
second derivative spectra, are (a.u.): 2.87 x 109 (a), 2.28 x 108(b), 2.33 x 108 (c), and 4.76 x 108 (d).
Cyclic voltammetry studies
1 mM solutions of the vanadium(IV) complexes [VO(acac)(H-BIAN)]Cl (1) and the precursor
[VO(acac)2] were prepared in supporting electrolyte (0.1 M TBA[PF6] in acetonitrile and
voltammograms of the Pt electrode were obtained at the sweep rate of 50 mV/s in the potential
range of ꢀ1.5 to 1.5 V. Both complexes display an irreversible oxidation process at high potentials
which is not present in the H-BIAN ligand, and can be assigned to the oxidation of V(IV) to V(V).
The oxidation potential Eox increases from 0.91 V in the neutral [VO(acac)2] to 1.04 V in the
cationic [VO(acac)(H-BIAN)]+ (1+) as expected, confirming that it is more difficult to oxidize this
latter species.
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DFT calculations
The mechanism of the epoxidation reaction was investigated by means of DFT calculations 21
(GAUSSIAN 0322, see Computational Details). As mentioned in the Introduction, literature data are
not very clear about the nature of the catalyst when the precursor is a vanadyl complex. It has been
shown that V(V) species can be recovered after the catalysis but they are inactive,11a,12 and we
demonstrated that the VO(IV) species remain in solution after adding tbhp (EPR). Mass
spectrometry studies led to its identification as [VO(acac)(H-BIAN)]+ (1+).
The first step in our mechanism proposal corresponds to addition of the oxidant
(methylhydroperoxide in the calculations) to the catalyst precursor, complex 1+. This is an
endergonic reaction with a calculated ꢁG of 30 kJ.molꢀ1 (Eq. 1).
[VO(acac)(H-BIAN)]+ (1+) + MeOOH → [VO(acac)(H-BIAN)(MeOOH)]+ (A) ꢁG = 30 kJ.molꢀ1
(Eq. 1)
In the resulting species, A, the methylhydroperoxide coordinates the metal by means of its terminal
Oꢀatom, occupying the sixth coordination position, trans to the vanadyl oxygen in a distorted
octahedral geometry.
From A, the formation of the catalytic active species follows a three step mechanism, in which the
acac ligand is protonated and lost as acetylacetone (Hacac). The corresponding free energy profile
is represented in Figure 6.
Figure 6. Free energy profile (kJ.molꢀ1) for the formation of the catalytic active species, [VO(H-
BIAN)(κ2ꢀO,OꢀMeOO)]+ (D), from [VO(acac)(H-BIAN)(MeOOH)]+ (A).
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In the first step of the mechanism, from A to B, the methylperoxide transfers a proton to the acac
ligand. MeOO- becomes a formally negative κ2ꢀOO ligand, while acac becomes acetylacetone
(Hacac). The second step involves a rearrangement of the hydrogen bonds in the two ligands,
MeOO- and Hacac. The O•••HꢀO hydrogen bonds between the OH group in acac and the
methylperoxide becomes an internal O•••HꢀO bond in acetylacetone. Finally, in the third step there
is loss of acetylacetone, leaving Hacac and [VO(H-BIAN)(κ2ꢀO,OꢀMeOO)]+ in D. Actually, this
last complex is similar to the catalyst precursor (1+) but with a methylperoxide ligand κ2ꢀOO
coordinated, substituting the acac ligand of the initial complex.
The path illustrated in Figure 6 presents a free energy barrier of 95 kJ.molꢀ1 being, thus, accessible
in the experimental conditions used for the reaction (see above).
From D to E, the Hacac is replaced by one ethylene molecule (the olefin substrate used in the
calculations). This is a practically energy neutral process (ꢁG = 2 kJ.molꢀ1) as, perhaps, might be
expected since the complex remains as [VO(H-BIAN)(κ2ꢀO,OꢀMeOO)]+, and Hacac and ethylene
are not coordinated. Epoxidation occurs in one single step (Figure 7) from E with ethylene attacking
the distal Oꢀatom of the methylperoxide, following the Sharpless type mechanism.23 This is a
clearly exergonic reaction (ꢁG = –231 kJ.molꢀ1) with a free energy barrier of only 72 kJ.molꢀ1, and
is quite feasible in the experimental conditions. In the product, F, both the recently formed epoxide
and methoxide bind to the metal through their Oꢀatom.
TSEF
72
0
E
231
F
Figure 7. Free energy profile (kJ.molꢀ1) for the epoxidation reaction based external attack of olefin
on intermediate E, [VO(H-BIAN)(κ2ꢀO,OꢀMeOO)]+ plus ethylene.
Although the epoxidation reaction is finished in F, as the product is already formed, a series of
ligand exchange reaction will still have to happen in order to close the catalytic cycle. The first one
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is the exchange between epoxide and the methylhydroperoxide, releasing the reaction product
(epoxide) and acquiring a new oxidant molecule to be used in the following reaction. This reaction
is represented in Eq. 2 and has a free energy balance of 18 kJ.molꢀ1.
[VO(H-BIAN)(MeO)(OCH2CH2)]+ (F) + MeOOH → [VO(H-BIAN)(MeO)(MeOOH)]+ (G) +
OCH2CH2 ꢁG = 18 kJ.molꢀ1 (Eq. 2)
The methylhydroperoxide ligand coordinates the vanadium in G by its distal oxygen, while the four
remaining coordination positions are occupied by the methoxide, the oxide and the two Nꢀatoms of
BIAN, all already present in F. The geometry of G is better described as a distorted trigonal
bipyramid with the axial positions defined by the methylhydroperoxide and one trans Nꢀatom of
BIAN (N–V–O = 171°).
The reaction proceeds by proton transfer from the methylhydroperoxide to the methoxide ligand,
through a very facile process with a free energy barrier of only 17 kJ.molꢀ1 and a ꢁG of –21 kJ.molꢀ1
(Figure 8) which converts G in H.
Figure 8. Free energy profile (kJ.molꢀ1) for the intramolecular proton exchange between [VO(H-
BIAN)(MeO)(MeOOH)]+ (G) and [VO(H-BIAN)(MeOH)(MeOO)]+ (H). The energy values are
referred to E.
Exchange of methanol, the byꢀproduct from the decomposition of the methylhydroperoxide, with a
fresh molecule of substrate completes the catalytic cycle (Scheme 4).
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Scheme 4
This may occur by regenerating E, where ethylene is not coordinated to [VO(H-BIAN)(κ2ꢀO,Oꢀ
MeOO)]+ (top of Scheme 4), and immediately closing the cycle, to repeat the mechanism previously
described. The corresponding transformation (H to E) is endergonic with a ꢁG of 40 kJ.molꢀ1. As
mentioned above, this mechanism based on E as active species follows the Sharpless pathway and
is an outer sphere path23 since it does not involves direct coordination of the substrate to the metal.
Alternatively, one may consider the possibility of coordinating the incoming ethylene molecule to
the metal (I, bottom of Scheme 4). The methylhydroperoxide ligand remains κ2ꢀOO coordinated
and the ethylene molecule occupies the adjacent coordination position (I). This reaction is only 14
kJ.molꢀ1 less favorable than the previous one (forming E) and a mechanism involving I as active
species is also possible. This will be an inner sphere process (Mimoun type mechanism),24 since the
olefin coordinates to vanadium, and the multiꢀstep profile calculated is represented in Figure 9.
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Figure 9. Free energy profile (kJ.molꢀ1) for the epoxidation reaction based on the ethylene complex
I. The energy values are referred to E.
The first step of the mechanism is a cycloaddition of the C=C bond of the olefin to the V–O bond,
from I to J, and resulting in a fourꢀmember metallacyle, V–O–C–C. This metallacycle is expanded
from fourꢀ to fiveꢀmembers (V–O–O–C–C) in the second step of the mechanism, from J to K, by
means of a coordination shift of vanadium from the distal to the proximal Oꢀatom of the
methylperoxide.
In the third step (K to L), there is a homolytic cleavage of the O–O bond forming an intermediate
(L) with two new ligands, one methoxide (MeO) and one alkyl with a dangling Oꢀatom
(CH2CH2O•), resulting from the two fragments of the metallacycle. In the following step, from L to
M, the coordination of the latter ligand shifts from Cꢀ to Oꢀ and it becomes an alkoxide with a
•
dangling carbon end, in M: OCH2CH2 . In the final step, the C–O bond in the coordinated epoxide
of complex F is formed. The overall process is clearly exergonic (ꢁG = –245 kJ.molꢀ1) and the
highest barrier is the one associated with the first step (TSIJ, 58 kJ.molꢀ1). Curiously, the energy of
the corresponding transition state, TSIJ, differs only 0.4 kJ/mol from the energy of the transition
state calculated for the epoxidation reaction via the outer sphere mechanism (TSEF, in Figure 7),
indicating that the two mechanisms are competitive and most probably occur simultaneously.
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Interestingly, the intermediates L and M, in the path represented in Figure 9 are formally V(V)
complexes with the unpaired electron located mostly in the dangling atom of the CH2CH2O ligand,
the oxygen atom in the case of intermediate L, and the carbon atom in the case of M. This is clearly
reflected in the spin density represented in Figure 10 for these two intermediates.
Figure 10. Spin density of intermediates L (top) and M (bottom).
Inhibition of the catalyst activity by alcohol can be explained by the exchange reaction depicted in
Scheme 4, representing the closing of the cycles through the replacement of methanol by one new
molecule of substrate. In fact, those are endergonic processes and the corresponding equilibria will
be shifted backwards by increasing concentrations of alcohol, since this is a reaction product.
The formation of the active species from the precursor complex, as well as the catalytic cycle are
shown in Scheme 5, emphasizing the relevant intermediates. It also stresses the role of the active
complex [VO(H-BIAN)(κ2ꢀO,OꢀMeOO)]+, which appears D at the end of the reaction MeOOH that
also releases Hacac. This molecule remains hydrogen bonded to the cation and is displaced by the
substrate, first weakly bound in E and giving the reaction product (F). The amounts of D and E will
depend on the concentration of the substrate.
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Scheme 5
Conclusions
The new oxovanadium(IV) complexes [VO(acac)(R-BIAN)]Cl (BIAN = 1,2-bis{(R-
phenyl)imino}acenaphthene, R = H, 1; CH3, 2; Cl, 3) were found to be active catalyst precursors for
the epoxidation of cyclohexene, cisꢀcyclooctene, and styrene using either tbhp or H2O2 as oxidants,
and for the epoxidation of enantiopure olefins with tbhp. All led to high selectivity toward epoxide
formation, and diols were never observed. In olefins such as limonene with two C=C bonds, the
secondary olefin was preferred to the terminal one.
Kinetic studies indicated a first order dependency on the oxidant (tbhp or H2O2) in all metal
complexes, with positive ꢀH≠ and very negative ꢀS≠ suggesting an associative mechanism. Mass
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spectrometry studies of the reaction of complex 1 and tbhp allowed the isolation and detection of a
V(IV) complex [VO(acac)(H-BIAN)]+. EPR experiments demonstrated that V(IV) persists in
solution after addition of tbhp or tbhp and Cy8, for at least 48 hours (weaker signal), even though
most of the initial complex has been oxidized suggesting that a V(IV) species may be involved in
catalysis. Cyclic voltammetry experiments showed that it is more difficult to oxidize the cationic
complex 1+ than [VO(acac)2], supporting the previous statement. Based on this evidence, the
possibility of a V(IV) catalyzed reaction mechanism was investigated by means of DFT
calculations. After the formation of the V(IV) active species in an associative step, the results
indicate two competitive processes that probably occur simultaneously: an outer sphere path with
external attack of the olefin at the coordinated peroxide, and an inner sphere mechanism based on a
complex with coordinated substrate. The largest barrier (95 kJ.molꢀ1) occurs in the formation of the
catalytic active species, [VO(H-BIAN)(κ2ꢀO,OꢀMeOO)]+ (D).
Experimental
All preparations and manipulations were performed using standard Schlenk techniques under
nitrogen. Commercial grade solvents were dried and deoxygenated by standard procedures (hexane
and diethyl ether over Na/benzophenone ketyl; CH3CN over CaH2), distilled under nitrogen, and
kept over 4 Å molecular sieves (3 Å for CH3CN). Tbhp, hydrogen peroxide, ciclooctene,
ciclohexene, styrene, S(-)-pinene, R(+)-pinene, S(-)-limonene and R(+)-limonene,
acenaphthenequinone, aniline, p-toluidine, and 4-chloroaniline were purchased from Aldrich and
used as received. [VO(acac)2] was prepared by a method described in the literature.25 The synthesis
of the vanadyl complexes was adapted from the literature.26
1H and 13C solution NMR spectra were obtained at 400.13 MHz and 100.62 MHz, respectively,
with a Bruker Avance 400 spectrometer using CDCl3 and CD3OD as solvents. Chemical shifts are
quoted in ppm from TMS.
FTIR spectra were measured with a Nicolet Nexus 6700 FTIR spectrometer as KBr pellets in
transmission mode, with 2 cmꢀ1 resolution. Microanalyses were performed at CACTI at the
University of Vigo. UVꢀVIS spectra were measured in a Shimadzu spectrometer in solution of
MeCN or CH2Cl2.
Mass Spectrometry experiments
All ESI–MS experiments were performed using an ApexQe FTICR Mass Spectrometer from Bruker
Daltonics (Billerica, MA, USA) equipped with an electrospray ion source and a 7 T actively
shielded superconducting magnet. The samples were introduced, by means of an infusion pump
from KD Scientific (Holliston, MA, USA), with a flow rate of 120 ꢂL·h–1. The vacuum was
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maintained by means of mechanical vacuum pumps followed by turbomolecular pumps in two
different regions: ion source (maintained ~6.0×10–6 mbar) and cell region (maintained ~4.0×10–10
mbar).The mass spectrometer was calibrated using a 2.8×10–6 mol·L–1 solution of polyethylenglycol
acquired from SigmaꢀAldrich (St. Louis, MO, USA) in HPLC grade methanol acquired from
Panreac (Barcelona, Spain) and acidified with 0.1% (v/v) of formic acid acquired from Fluka
(Seelze, Germany).
The mass spectra were acquired in the positive ion broadband mode, with an acquisition size of
512k, in the mass range of 50–500. The nebulizer gas flow rate was set to 2.5 L·min–1, the dry gas
flow rate was set to 4.0 L·min–1 at a temperature of 493 K. The capillary voltage was set to 4200 V
and the spray shield voltage was set to 3800 V. The ion source optics, collision cell and ICR cell
parameters were optimized to ensure the highest abundance possible for the ions of interest. All
mass spectra presented are the average of 32 mass spectra. The ions were accumulated in the
collision cell during 1.0 s prior to their transfer to the ICR cell. The MS2 experiments were
performed using Argon, acquired from Praxair (Danbury, CT, USA), as collision gas at a flow rate
of 0.30 L·s–1. The collision cell voltage was varied for each compound in order to find the
appropriate value.
The MS kinetic experiments were carried out by isolating the ion of interest in the ICR cell and by
introducing the reactants (tbhp or cisꢀcyclooctene) through a leak valve. The valve was opened until
the pressure in the cell achieved a stationary pressure of 10–8 mbar. Prior to the time delay of
interest the ions were thermalized to achieve consistent results.
Synthesis
Preparation of the ligands (L1-L3)
To a suspension of acenaphthenequinone (C12H6O2) (3.28 mmol, 0.6 g) in acetonitrile (MeCN) (30
mL) was added 5.3 mL of acetic acid. This mixture was stirred under reflux until all the
acenaphthenequinone was dissolved. A solution of 4ꢀchloroaniline, aniline or pꢀtoluidine (7.1
mmol) in acetonitrile was then added, and the new solution was stirred for 4 h 30 min under reflux.
The ligand precipitated after cooling the solution in the fridge, and was recrystallized from
acetonitrile.
1,2-bis[(phenyl)imino]acenaphthene (p-BIAN, L1)
Yield: 0.932 g, 86%. C24H16N2 (332.40) calcd: C 86.72, H 4.85, N 8.43, found: C 86.49, H 4.43, N
8.00. IR (ν cmꢀ1): 3082 (vw), 3054 (w), 3018 (vw), 1737 (vs), 1653 (vs), 1593 (vs), 1484 (s), 1423
(m), 1360 (vw), 1278 (m), 1230 (m), 1176 (w), 1072 (m), 1026 (m), 914 (m), 893 (m), 830 (s), 803
(w), 777 (vs), 765 (vs), 696 (vs).
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1H NMR (400.13 MHz, CDCl3, r.t., δ ppm): 7.01 (d, 4H, Ph (4)), 7.21 (d, 2H, Ph (6)), 7.34ꢀ7.42 (m,
4H, Ph (5)), 7.74 (t, 2H, BIAN (2)), 7.92 (d, 2H, BIAN (3)), 8.08 8.11 (dd, 2H, BIAN (1)). UVꢀVis
(MeCN solution): λmax = 304 nm (
= 1.833×105 dm2·molꢀ1).
–
ε
1,2-bis[(4-methylphenyl)imino]acenaphthene (mp-BIAN, L2)
Yield: 0.908 g, 77%. C26H20N2 (360.45) calcd: C 86.64, H 5.59, N 7.77, found: C 86.30, H 7.33, N
5.13. IR (ν cmꢀ1): 3052 (vw), 3022(vw), 2919 (w), 2863 (vw), 1731 (vs), 1657 (s), 1603 (m), 1589
(s), 1501 (vs), 1489 (s), 1419 (m), 1270 (s), 1237 (s), 1177 (m), 1107 (m), 1072 (m), 1031 (m),
1015 (m), 927 (w), 911 (m), 821 (vs), 779 (s).
1H NMR (400.13 MHz, CDCl3, r.t., δ ppm): 2.37 (s, 6H, CH3), 6.95 (t, 4H, Ph (4)), 7.20 (d, 4H, Ph
(5)), 7.74 (t, 2H, BIAN (2)), 7.80ꢀ7.94 (dd, 2H, BIAN (3)), 8.09 (dd, 2H, BIAN (1)). UVꢀVis
(MeCN solution): λmax = 303 nm ( ε
= 1.001×105 dm2·molꢀ1).
1,2-bis[(4-chlorophenyl)imino]acenaphthene (cp-BIAN, L3)
Yield: 1.150 g, 87%. C24H14N2Cl (401.28) calcd: C 71.83, H 3.52, N 6.98, found: C 71.63, H 3.16,
N 6.55. IR (ν cmꢀ1): 3056 (vw), 1731 (vs), 1658 (s), 1588 (m), 1479 (vs), 1420 (w), 1275 (m), 1228
(m), 1172 (w), 1090 (m), 1071 (w), 1032 (m), 1012 (m), 910 (w), 827 (vs), 776 (s), 721 (w), 648
(w), 589 (w), 529 (m).
1H NMR (400.13 MHz, CDCl3, r.t., δ ppm): 6.99 (t, 4H, Ph (4)), 7.39 (d, 4H, Ph (5)), 7.77 (t, 2H,
BIAN 2), 7.97 (d, 2H, BIAN 1), 8.13 (dd, 2H, BIAN (3)). UVꢀVis (solution MeCN): λmax = 304 nm
(ε
= 4.825×104 dm2·molꢀ1).
Preparation of the complexes (1-3)
A solution of [VO(acac)2] (1 mmol, 265 mg), in methanol (10 mL) was treated with a solution of
each of the ligands L1ꢀL3 (1 mmol) in methanol and 1 mmol of HCl, and stirred for 4 h under
reflux. The complex was obtained by evaporation of the solvent. It was washed several times with
hexane and diethyl ether, dried under vacuum, and recrystallized from methanol/diethyl ether.
[VO(acac)(R-BIAN)]Cl (R = H, 1)
Yield: 0.469 g, 88%. C29H23N2O3VCl (533.44) calcd: C 65.24, H 4.32, N 5.25, found: C 64.89, H
4.65, N 4.91. IR (ν cmꢀ1): 3056 (w), 2933 (w), 2831 (w), 1566 (vs), 1524 (s), 1486 (s), 1363 (vs),
1284 (s), 1187 (m), 1066 (m), 1027 (m), 1000 (s), 932 (m), 834 (m), 778 (s), 694 (m), 586 (m), 464
(w).
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1H NMR (400.13 MHz, CDCl3, r.t., δ ppm): 1.93ꢀ2.52 (d bd, 6H, acac–CH3), 6.42 (s bd, 1H, acac–
H), 6.83ꢀ8.27 (m, 16H, BIAN).
UVꢀVis (solution MeCN): λmax = 306 nm (ε
= 2.246×105 dm2·molꢀ1).
[VO(acac)(R-BIAN)]Cl (R = Me, 2)
Yield: 0.480 g, 86%. C31H27N2O3VCl (561.44) calcd: C 66.26, H 4.84, N 4.98, found: C 66.05, H
4.53, N 4.94. IR (ν cmꢀ1): 3042(vw), 2972 (vw), 2925 (vw), 1560 (vs), 1523 (s), 1502 (s), 1286 (vs),
1189 (m), 1019 (m), 995 (s), 937 (m), 834 (m), 784 (s), 488 (m).
1H NMR (400.13 MHz, CDCl3, r.t., δ ppm): 2.00 (t bd, 6H, acac–CH3), 2.33 (d, 6H, BIAN–CH3),
6.47 (s bd, 1H, acac–H), 6.93–6.99 (dd bd, 2H, BIANꢀ4), 7.17–7.25 (m, 4H, BIANꢀ5, BIANꢀ2),
7.20 (s, 1H, Ph 12), 7.54–7.78 (m, 4H, BIANꢀ1, BIANꢀ3). UVꢀVis (solution MeCN): λmax = 306 nm
(ε
= 1.582×105 dm2·molꢀ1).
[VO(acac)(R-BIAN)]Cl (R = Cl, 3)
Yield: 0.543 g, 90%. C29H21N2O3VCl3 (602.44) calcd: C 57.78, H 3.51, N 4.65, found: C 57.32, H
3.51, N 4.48. IR (ν cmꢀ1): 3057 (vw), 2931 (w), 2832 (w), 1566 (vs), 1524 (s), 1485 (s), 1363 (vs),
1283 (s), 1191 (s), 1090 (s), 1014 (s), 979 (s), 938 (m), 833 (m), 789 (vs), 746 (m), 680 (m), 589
(m), 462 (m).
1H NMR (400.13 MHz, CDCl3, r.t., δ ppm): 1.94ꢀ2.46 (d bd, 6H, acac–CH3), 6.48 (s bd, 1H, acac–
H), 7.03–8.24 (m, 14H, BIAN).
UVꢀVis (solution MeCN): λmax = 307 nm (ε
= 2.314×105 dm2·molꢀ1).
Catalytic epoxidation of cis-cyclooctene, cyclohexene, and styrene in the presence of 1-3 with
tbhp or H2O2
Cyclooctene (7.3 mmol), cyclohexene (9.7 mmol), or styrene (7.7 mmol) and t-butyl
hydroperoxide (200 mol %, 5.5 M solution in decane) or H2O2 (200 mol %, 30% solution) were
mixed in the presence of complexes 1-3 (1 mol %) in a glass reactor, at 328 K, and vigorously
stirred. Conversion was monitored by taking samples and analyzing them by gas chromatography
using a capillary column (HP-5) and a flame ionization detector. Cyclohexene, cisꢀcyclooctene,
and styrene epoxides were quantified using NMR spectroscopy.
25

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Catalytic epoxidation of
tbhp.
R(+), S(+)-limonene, R(-), S(-)-pinene in the presence of 1-3 with
S(-)-, R(+)-pinene, S(-)-,and R(+)-limonene (5.87 mmol), and t-butyl hydroperoxide (200 mol
%, 5.5 M solution in decane) were mixed in the presence of complexes 1-3 (1 mol %) in a glass
reactor, at 328 K, and vigorously stirred. Conversion was monitored by taking samples and
analyzing them by gas chromatography using a capillary column (HP-1) and a flame ionization
detector. The respective epoxides were quantified using NMR spectroscopy.
UV-vis Kinetic studies
All kinetic measurements were carried out using a large excess of tbhp (pseudo-first-order
conditions). Typically, an appropriate small quantity of concentrated tbhp in decane (5.5 M) was
added to a thermostated UV quartz cell, at 298, 303, 313, 323, 328 K, containing an appropriate
amount of a solution of the metal complex in CH3CN in order to obtain a total volume of 3 mL with
a final concentration of [VO(acac)(R-BIAN]Cl (1) equal to 1x10ꢀ4 M. The product formation was
monitored against time at regular time intervals by following absorbance changes at 302 nm on a
UV-VIS Shimadzu spectrometer with a variable temperature cell and stirring.
Electron paramagnetic resonance spectroscopy (EPR). Samples of the various experiments were
prepared by dissolution of the reagents in dichloromethane and placed in capillary tubes, which
were then introduced in the quartz EPR tubes. The samples to be recorded at room temperature
were prepared, placed in the spectrometer and analyzed. The samples to be recorded in frozen
solution were prepared, frozen in liquid nitrogen, and placed in the spectrometer at the desired
temperature. EPR spectra were recorded using a Bruker ELEXSYS E500 spectrometer operating at
9 GHz (Xꢀband) equipped with a variable temperature unit (ER 4B1 VT), available at Laboratório
de Ressonância Magnética Electrónica (Centro de Estudos de Materiais da Universidade do Porto).
EPR spectra were obtained in the following experimental conditions: microwave frequency of
(9.4505 GHz), modulation frequency of 100 kHz, microwave power of 6 mW, modulation
amplitude of 4 G and 60 dB of receiver gain. Room temperature spectra were recorded with
acquisition time of 5 s, 5 scans at (296 ± 1) K and spectra acquired at (100 ± 1) K were recorded
with acquisition time of 100 s, 5 scans. The spectra were simulated with the computer suite program
Bruker WinEPR/SimFonia.
Cyclic voltammetry studies. All electrochemical measurements were performed using a CHI
Electrochemical Analyserꢀ620A Model controlled by a computer at room temperature in an oneꢀ
compartment electrochemical Teflon cell. A polycrystalline platinum (Pt) working electrode (area A
26

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= 0.008 cm2), a platinum foil counter electrode (area 2.0 cm2) and a saturated calomel electrode
(SCE) as reference electrode were used. Before each experiment, a mirrorꢀfinishing platinum
surface was generated by handꢀpolishing the electrode in an aqueous suspension of successively
finer grades of alumina (down to 0.05 µm). All the solutions were deoxygenated directly in the
electrochemical cell with nitrogen gas (N2).
The electrochemical studies were performed at 0.050V/s sweep rate, in the potential range of ꢀ1.5 –
1.5 V. To minimize solution resistance and promote the flow of electrons, the electrolyte solution
used was 0.1 M tetrabutylammonium hexafluorophosphate (TBA[PF6]) in acetonitrile (ACN). 1
mM solutions of the vanadium (IV) complex (1) and precursor [VO(acac)2] were prepared in
TBAPF6/ ACN electrolyte.
Computational Details. All DFT21 calculations were performed using the GAUSSIAN 03 software
package,22 and the PBE1PBE functional, without symmetry constraints. That functional uses a
hybrid generalized gradient approximation (GGA), including 25% mixture of Hartree-Fock27
exchange with DFT exchangeꢀcorrelation, given by Perdew, Burke and Ernzerhof functional
(PBE).28 The optimized geometries were obtained with the LanL2DZ basis set29 augmented with an
fꢀpolarization function,30 for V, and a standard 6-31G(d,p)31 for the remaining elements (basis b1).
Transition state optimizations were performed with the Synchronous TransitꢀGuided QuasiꢀNewton
Method (STQN) developed by Schlegel et al,32 following extensive searches of the Potential Energy
Surface. Frequency calculations were performed to confirm the nature of the stationary points,
yielding one imaginary frequency for the transition states and none for the minima. Each transition
state was further confirmed by following its vibrational mode downhill on both sides and obtaining
the minima presented on the energy profile. The electronic energies (Eb1) obtained at the
PBE1PBE/b1 level of theory were converted to free energy at 298.15 K and 1 atm (Gb1) by using
zero point energy and thermal energy corrections based on structural and vibration frequency data
calculated at the same level.
Single point energy calculations were performed using a standard 6ꢀ311++G(d,p)33 basis set for all
the elements, and the geometries optimized at the PBE1PBE/b1 level. Solvent effects (CH2Cl2)
were considered in the PBE1PBE/6ꢀ311++G(d,p)//PBE1PBE/b1 energy calculations using the
Polarizable Continuum Model (PCM) initially devised by Tomasi and coworkers34 as implemented
on GAUSSIAN 03.35 The molecular cavity was based on the united atom topological model applied
on UAHF radii, optimized for the HF/6ꢀ31G(d) level.
The free energy values presented along the text (Gb2soln) were derived from the electronic energy
values obtained at the PBE1PBE/b2//PBE1PBE/b1 level, including solvent effects (Eb2soln),
according to the following expression:
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Gb2soln = Eb2soln + Gb1 – Eb1
Acknowledgements
We thank FCT, POCI, and FEDER for funding (projects POCI/QUI/58925/2004, PEstꢀ
OE/QUI/UI0100/2014, and PEstꢀOE/QUI/UI0612/2014). The authors also thank the Portuguese
National Mass Spectrometry Network (REDE/1501/REM/2005). The Bruker ELEXYS EPR
spectrometer was purchased under the framework of QREN, through project NORTEꢀ07ꢀ0162ꢀ
FEDERꢀ000048. To all financing sources the authors are greatly indebted.
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29