Synthesis and characterization of novel oxovanadium(IV) complexes with 4-acyl-5-pyrazolone donor ligands: Evaluation of their catalytic activity for the oxidation of styrene derivatives

Article abstract of DOI:10.1016/j.apcata.2010.02.022

Reaction of VOSO₄·5H₂O with a methanol solution of the HQ proligand (HQ = 1-R¹-4-R²(C{double bond, long}O)pyrazol-5-one in general, in detail: HQ^nPe, R¹ = phenyl, R² = neopentyl; HQ^Me,Me, R¹ = R² = methyl; HQ^Me,naph, R¹ = methyl, R² = naphthalen-1-yl; HQ^naph, R¹ = phenyl, R² = naphthalen-1-yl; HQ^Ph, R¹ = R² = phenyl; HQ^CF3, R¹ = phenyl, R² = trifluoromethyl; HQ^py,CF3, R¹ = pyridin-2-yl, R² = trifluoromethyl) gave seven novel VO(Q)₂(H₂O) complexes which have been characterized by elemental analyses, IR, ESI-MS, electronic spectroscopy and magnetic susceptibility measurements and, in the case of derivative VO(Q^CF3)₂, also by EPR spectroscopy. The X-ray diffraction study, carried out on derivative VO(Q^nPe)₂(H₂O), evidenced a distorted octahedral environment with the two pyrazolonates in anti configuration and the vanadium atom 0.2914(7) A? away from the least-squares plane defined by the four equatorial oxygen atoms. The catalytic activity of these new oxovanadium(IV) complexes has been exhaustively tested for the oxidation of styrene, α-methylstyrene and cis-β-methylstyrene, in the presence of H₂O₂ as primary oxidant. The effects of oxidant to substrate molar ratio, catalyst amount, solvent and temperature have been studied. Overall, the vanadium complexes showed high activity and high to moderate selectivity toward the benzaldehyde (acetophenone) formation, depending from both the type of starting substrate and experimental reaction times.

Full text of DOI:10.1016/j.apcata.2010.02.022

Applied Catalysis A: General 378 (2010) 211–220
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis and characterization of novel oxovanadium(IV) complexes with
4
-acyl-5-pyrazolone donor ligands: Evaluation of their catalytic activity for the
oxidation of styrene derivatives
a,∗
a
a
a
Fabio Marchetti , Claudio Pettinari , Corrado Di Nicola , Riccardo Pettinari ,
b
c,∗∗
, Andrea Di Giuseppec
Alessandra Crispini , Marcello Crucianelli
Dipartimento di Scienze Chimiche, Università degli Studi di Camerino, Via S. Agostino 1, I-62032 Camerino, MC, Italy
a
b
Centro di Eccellenza CEMIF.CAL-LASCAMM, CR-INSTM (Unità della Calabria, Dipartimento di Chimica), Dipartimento di Scienze Farmaceutiche, Edificio Polifunzionale,
Università della Calabria, I-87030 Arcavacata di Rende, CS, Italy
c
Dipartimento di Chimica, Ingegneria Chimica e Materiali, Università dell’Aquila, via Vetoio, I-67100 Coppito, AQ, Italy
a r t i c l e i n f o
a b s t r a c t
1
2
Article history:
Reaction of VOSO ·5H O with a methanol solution of the HQ proligand (HQ = 1-R -4-R (C O)pyrazol-
nPe 1 2 Me,Me 1 2
4
2
Received 20 November 2009
Received in revised form 13 February 2010
Accepted 15 February 2010
Available online 23 February 2010
, R = R = methyl; HQMe,naph,
, R = phenyl, R = naphthalen-1-yl; HQ , R = R = phenyl;
, R = pyridin-2-yl, R = trifluoromethyl) gave seven novel
VO(Q)2(H2O) complexes which have been characterized by elemental analyses, IR, ESI-MS, electronic
5
-one in general, in detail: HQ , R = phenyl, R = neopentyl; HQ
1
2
naph
1
2
Ph
1
2
R = methyl, R = naphthalen-1-yl; HQ
CF3
1
2
py,CF3
1
2
HQ , R = phenyl, R = trifluoromethyl; HQ
CF3
spectroscopy and magnetic susceptibility measurements and, in the case of derivative VO(Q )2, also
Keywords:
Oxovanadium complexes
nPe
by EPR spectroscopy. The X-ray diffraction study, carried out on derivative VO(Q )2(H2O), evidenced
a distorted octahedral environment with the two pyrazolonates in anti configuration and the vanadium
atom 0.2914(7) Å away from the least-squares plane defined by the four equatorial oxygen atoms. The
catalytic activity of these new oxovanadium(IV) complexes has been exhaustively tested for the oxida-
tion of styrene, ␣-methylstyrene and cis-␤-methylstyrene, in the presence of H2O2 as primary oxidant.
The effects of oxidant to substrate molar ratio, catalyst amount, solvent and temperature have been
studied. Overall, the vanadium complexes showed high activity and high to moderate selectivity toward
the benzaldehyde (acetophenone) formation, depending from both the type of starting substrate and
experimental reaction times.
␤
-Diketonates
Catalytic oxidation
Hydrogen peroxide
Olefin
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
of the most important properties of the vanadium complexes,
which considerable effort has been devoted to, is that they possess
versatile oxidation states and unusual redox ability. Commonly,
vanadium can exist in +3, +4 and +5 oxidation states. It has been
documented that in +4 and +5 oxidation states vanadium has
strong redox ability which can lead to some important reactions
[20–22]. Currently, there is a growing interest in the study of cat-
alytic applications of high-valent vanadium complexes, also due
to the presence of vanadium in several metalloenzymes [23,24].
Soluble and supported vanadium complexes showed to efficiently
catalyze various oxidative transformation of organic compounds
as olefins and allylic alcohols, alkylaromatic compounds, alkanes,
sulfides and alcohols, being able to activate environment friendly
oxidants as dioxygen or hydrogen peroxide (see some reviews
The family of vanadium compounds is receiving increasing
interest due to its rich applications in many fields [1–7]. In fact,
it plays an important role in many enzymatic reactions such as
halogenation of organic substrate and fixation of nitrogen [8–10].
Moreover, oxovanadium(IV) compounds with O,O-donor ligands
are very effective insulin-like agents, as they show insulin-mimetic
response which can simulate the uptake and metabolism of glu-
cose through alternative signalling pathways [11–17]. In addition,
a variety of enzymatic activities have been observed by vana-
dium complexes able to mimic, among others, the peroxidase and
catalase activities [13], as showed in the case of different oxo-
vanadium(IV) complexes in recently published papers [18,19]. One
[
25,26] and recent selected original papers [27–35]). H O2 is an
2
attractive oxidant not solely because it is more active than O2,
but rather because it reacts affording H O as the only by-product.
Moreover, in certain circumstances, it is better than dioxygen
2
∗
Corresponding author. Fax: +39 0737 637345.
Corresponding author. Fax: +39 0862 433753.
E-mail addresses: fabio.marchetti@unicam.it (F. Marchetti),
∗
∗
insofar as O /organic mixtures can sometimes spontaneously
2
marcello.crucianelli@univaq.it (M. Crucianelli).
ignite.
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.02.022

2
12
F. Marchetti et al. / Applied Catalysis A: General 378 (2010) 211–220
phenyl-3-methyl-4-trifluoroacetyl-5-pyrazolone), and HQpy,CF3
[1-(2-pyridyl)-3-methyl-4-trifluoroacetyl-5-pyrazolone], were
Oxo and peroxo derivatives of vanadium complexes play
an important role in such catalytic oxidations, acting as oxo-
transfer agents [36,37]. Many different papers describing the
synthesis, characterization and catalytic applications of peroxo-
vanadium complexes, may be found in literature (for selected
examples see [38–43]). A very interesting study devoted to
elucidate the mechanism of oxidation with H O , of either
synthesized as previously reported [46–49]. The samples for
◦
microanalyses were dried in vacuo to constant weight (20 C, ca.
0.1 Torr). Elemental analyses (C, H, N, S) were performed in-house
with a Fisons Instruments 1108 CHNS-O Elemental Analyzer.
−
1
2
2
IR spectra were recorded from 4000 to 600 cm
with a Perkin
Elmer Spectrum 100 FT-IR instrument. 1H and F NMR spectra of
19
isopropanol or cyclohexane, promoted by oxovanadium(V)
triethanolaminate/pyrazine-2-carboxylic acid system, has been
recently published [44].
proligands HQ were recorded on a VXR-300 Varian spectrometer
1
operating at room temperature (300 MHz for H and 282.2 MHz
19
Herein we report the synthesis and full characterization of new
oxovanadium(IV) acylpyrazolonate complexes, where acylpyra-
zolonate ligands are a new version of ␤-diketonates containing a
pyrazole moiety fused to the chelating ring [45–49]. The catalytic
properties of the new mononuclear complexes have also been thor-
oughly studied, in the hydrogen peroxide promoted oxidation of
styrene derivatives, under homogeneous conditions. Interestingly,
working with optimized experimental conditions, high to moder-
ate selectivity toward benzaldehyde formation, has been observed,
during oxidation of either styrene or cis-␤-methylstyrene, irrespec-
tive from the nature of used catalyst. Benzaldehyde is an attractive
target because of it is an important intermediate in the production
of derivatives for perfumery, pharmaceutical, dyestuff and agro-
chemical industries.
for F). The electrical conductances of the acetonitrile solutions
were measured with a Crison CDTM 522 conductimeter at room
temperature. The magnetic susceptibilities were measured at room
◦
temperature (20 C) by the Gouy method, with a Sherwood Scien-
tific Magnetic Balance MSB-Auto, using HgCo(NCS)4 as calibrant
and correcting for diamagnetism with the appropriate Pascal con-
stants. The magnetic moments (in BM) were calculated from the
corr
−1/2
corr
equation ꢀeff = 2.8428 (ꢁm T)
(ꢁ
is the molar susceptibility
m
corrected for diamagnetism and T is the temperature in kelvin
nPe
CF3
degrees). The electronic spectra of the ligands HQ
and HQ
and
of the corresponding derivatives I and VI (see onward Scheme 1)
were recorded in ethanol (or acetonitrile) solution with a HP 8452A
Diode Array spectrophotometer. The positive and negative electro-
spray mass spectra were obtained with a Series 1100 MSI detector
HP spectrometer, using an acetonitrile or methanol or ethanol
mobile phase. Solutions (3 mg/mL) for electrospray ionization
mass spectrometry (ESI-MS) were prepared using reagent grade
acetonitrile or methanol or ethanol. For the ESI-MS data, mass
and intensities were compared to those calculated using IsoPro
Isotopic Abundance Simulator, version 2.1 [50]. Peaks containing
vanadyl(IV) and vanadium(III) ions were identified as the highest
peak of an isotopic cluster. TGA-DTA spectra were obtained with
a STA 6000 Simultaneous Thermal Analyzer Perkin-Elmer. The
2
. Experimental
2
.1. Instruments and reagents
All reagents and solvents were purchased from Alfa (Karlsruhe)
and Aldrich (Milwaukee) in the highest purity grade available and
were used as such. A 35% aqueous solution of hydrogen peroxide
was used as primary oxidant. The acylpyrazolone proligands
HQ, namely HQnPe (1-phenyl-3-methyl-4-tert-butylacetyl-5-
9
.5 GHz EPR X-band spectrometer was a conventional assembly
of Bruker units (Bruker Spectrospin, Milano, Italy), namely: a
magnet B-M8, a resonant cavity 4108 TMH/9101, and a microwave
bridge ER040XR, equipped with the field controller BH15. Gas
chromatography–mass spectrometry (GC–MS) analyses of the
pyrazolone),
HQMe,Me
(1,3-dimethyl-4acetyl-5-pyrazolone),
naph
HQ
HQ
[1-phenyl-3-methyl-4-(1-naphthoyl)-5-pyrazolone],
Me,naph
Ph
[1,3-dimethyl-4-(1-naphthoyl)-5-pyrazolone], HQ
CF3
(
1-phenyl-3-methyl-4-benzoyl-5-pyrazolone),
HQ
(1-
Scheme 1. General procedure for the preparation of oxovanadium(IV) complexes I–VII.

F. Marchetti et al. / Applied Catalysis A: General 378 (2010) 211–220
213
Table 1
Yields and elemental analyses obtained for the novel oxovanadium(IV) complexes I–VII.
Compound
Yield (%)
Formula
Calculated (found)
%
C
%H
%N
8.93 (9.31)
14.32 (14.39)
9.10 (8.89)
7.57 (7.37)
9.01 (9.21)
8.63 (8.84)
13.55 (13.84)
VO(QnPe)2(H2O) I
78
88
84
86
80
83
77
C32H40N4O6V
C14H20N4O6V
C32H28N4O6V
C42H32N4O6V
C34H26N4O5V
C24H16F6N4O5V
C22H14F6 N6O5V
61.24 (61.56)
42.98 (43.34)
62.44 (62.06)
68.20 (67.83)
65.70 (66.02)
45.75 (45.49)
44.50 (43.51)
6.42 (6.60)
5.15 (4.93)
4.59 (4.57)
4.36 (4.31)
4.22 (4.30)
2.43 (2.54)
2.25 (2.32)
Me,Me
VO(Q
VO(Q
VO(Q
)2(H2O) II
)2(H2O) III
)2(H2O) IV
Me,naph
naph
Ph
VO(Q )2 V
CF3
VO(Q )2 VI
py,CF3
VO(Q
)2 VII
VO(QMe,naph) (H O) (III): soluble in acetone, acetonitrile, DMSO
and chlorinated solvents. ꢀeff: 1.85 BM. IR (nujol, cm ): 3350br
ꢂ(O–H), 3054w ꢂ(Carom–H), 1660sh ı(O–H), 1580vs, 1570vs, 1506m
ꢂ(C O + C N + C C), 967vs ꢂ(V O). ESI-MS (MeOH) (+): m/z
oxidative reaction products were performed by means of a Varian
000 GC-MAS instrument, using a 30 m × 0.32 mm × 0.25 ␮m film
2
2
−
1
2
thickness (crosslinked 5% phenylmethylsiloxane) column and
chromatography grade helium, as carrier gas. In GC calculations,
all peaks amounting to at least 0.5% of the total products were
taken into account. When necessary, 1H and C NMR analyses of
products were performed after flash-chromatographic purification
on columns packed with silica gel (230–400 mesh), and compared
with authentical sample. Mass spectra were recorded with an
electron beam of 70 eV.
Me,naph
+
Me,naph
+
(%) = 289 (30) [(HQ
)Na] , 598 (30) [{VO(Q
) }H] , 620
2
13
Me,naph
+
Me,naph
+
(20) [{VO(Q
) }Na] , 1196 (10) [{VO(Q
) } H] , 1218
2 2
2
Me,naph
+
Me,naph
+
(100) [{VO(Q
) } Na] , 1234 (10) [{VO(Q
) } K] . TGA-
2 2
2
2
◦
◦
DTA (mg% vs. C): heating from 35 to 600 C with a speed of
◦
◦
7 C/min; from 60 to 110 C loss of one water molecule (weight loss
◦
found 3.12%, calcd 2.92%); from 230 to 600 C progressive decom-
position, with a final black residual of 50% weight.
naph
2
2
.2. Catalysts preparation
VO(Q
) (H O) (IV): soluble in DMSO and slightly soluble in
2 2
−
1
acetonitrile. ꢀeff: 1.80 BM. IR (nujol, cm ): 3390br ꢂ(O–H), 3053w
ꢂ(Carom–H), 1665sh ı(O–H), 1600s ꢂ(C O), 1563vs, 1528m ꢂ(C
N + C C), 971s ꢂ(V O). ESI-MS (MeCN) (+): m/z (%) = 351 (100)
.2.1. General procedure for the preparation of VO(QnPe) (H O)
2
2
(
I)
naph
+
naph
+
naph
+
A water solution (10 mL) of VOSO ·5H O (0.253 g, 1.0 mmol)
[(HQ
)Na] , 367 (50) [(HQ
)K] , 722 (10) [{VO(Q
) }H] ,
4
2
2
naph
+
naph
+
was added to a methanol solution (20 mL) containing the proli-
gand HQ
744 (20) [{VO(Q
) }Na] , 760 (25) [{VO(Q
) }K] . TGA-DTA
2
2
nPe
◦
◦
◦
(0.544 g, 2 mmol) and NaOCH3 (0.108 g, 2 mmol), and
(mg% vs. C): heating from 35 to 600 C with a speed of 7 C/min;
◦
the reaction mixture was refluxed by stirring for 4 h. A pale green
precipitate slowly afforded which was filtered off, washed with
methanol (10 mL) and dried under vacuum. It is soluble in acetone,
acetonitrile, dimethylsulfoxide (DMSO) and chlorinated solvents.
Compounds II–VII were prepared following the same procedure as
that reported for I, by using VOSO ·5H O, the corresponding proli-
from 140 to 200 C loss of one water molecule (weight loss found
2.62%, calcd 2.43%); from 230 to 600 C progressive decomposition,
◦
with a final black residual of 55% weight.
Ph
VO(Q ) (V): soluble in acetone, acetonitrile, DMSO and chlori-
2
−
1
nated solvents. ꢀeff: 1.85 BM. IR (nujol, cm ): 3051w ꢂ(Carom–H),
1601s, 1591s ꢂ(C O), 1567vs, 1536m ꢂ(C N + C C), 900vs ꢂ(V
4
2
Me,Me
Me,naph
naph
Ph
Ph
+
gands (i.e. HQ
V, HQ
for II, HQ
for III, HQ
for IV, HQ for
O). ESI-MS (MeCN) (+): m/z (%) = 644 (60) [{VO(Q ) }Na] , 660
2
CF3
for VI, HQpy,CF3 for VII, respectively) and NaOCH3.
Ph + Ph +
(95) [{VO(Q ) }K] , 1265 (100) [{VO(Q ) } Na] . TGA-DTA (mg%
2
2 2
◦
◦
◦
◦
vs. C): heating from 35 to 700 C with a speed of 7 C/min; at 325 C
◦
2.3. Catalyst characterization
an onset fusion (ꢃHfusion = 51.15 kJ/mol); from 327 to 700 C pro-
gressive decomposition, with a final black residual of 44% weight.
CF3
Yields and elemental analyses obtained for oxovanadium(IV)
complexes I–VII, are shown in Table 1.
VO(Q )2 (VI): soluble in acetone, acetonitrile, DMSO and chlo-
−
1
rinated solvents. ꢀeff: 1.76 BM. IR (nujol, cm ): 3067w ꢂ(Carom–H),
1618sbr ꢂ(C O), 1587sbr, 1543m ꢂ(C C + C N), 896vs ꢂ(V O),
519s, 480s, 400s ꢂ(V–OQ). UV–vis (CH3CN): 242 nm (n–␲*), 296 nm
(␲–␲*), 627 nm broad, 745 nm very broad (d–d). UV–vis of
the proligand HQ
nPe
−1
VO(Q ) (H O) (I): ꢀeff: 1.82 BM. IR (nujol, cm ): 3325br
2
2
ꢂ(O–H), 3070w ꢂ(Carom–H), 1658sh ꢂ(O–H), 1600m, 1592s ꢂ(C
O), 1575vs, 1532m ꢂ(C N + C C), 977vs ꢂ(V O), 506sh, 494vs,
CF3
4
71s ꢂ(V–O ). UV–vis (CH CN): 255 nm very broad (n–␲* and
(CH3CN): 231 nm (n–␲*), 271 nm (␲–␲*).
Q
3
CF3
+
␲
–␲*), 580 nm very broad, 660 nm broad (d–d). UV–vis of the
ESI-MS (CH3CN) (+): m/z (%) = 293 (95) [(HQ )Na] , 606 (35)
nPe
CF3
+
CF3
+
proligand HQ
(CH CN): 233 nm (n–␲*), 268 nm (␲–␲*). ESI-
[{VO(Q )2}H] , 628 (100) [{VO(Q )2}Na] . ESI-MS (EtOH) (+):
3
nPe
+
CF3 + CF3 +
MS (MeCN) (+): m/z (%) = 610 (35) [{VO(Q ) }H] , 632 (45)
m/z (%) = 293 (82) [(HQ )Na] , 428 (100) [{VO(Q )(EtOH)2}] ,
CF3 + CF3 +
2
nPe
+
nPe
+
[
{VO(Q ) }Na] , 648 (100) [{VO(Q ) }K] . TGA-DTA (mg%
606 (70) [{VO(Q )2}H] , 628 (38) [{VO(Q )2}Na] , 652 (25)
CF3 + CF3 +
2
2
◦
◦
◦
vs. C): heating from 35 to 500 C with a speed of 7 C/min;
[{VO(Q )2(EtOH)}H] , 674 (15) [{VO(Q )2(EtOH)}Na] , ESI-MS
CF3 CF3
◦
−
−
from 98 to 120 C loss of one water molecule (weight loss found
(EtOH) (−): m/z (%) = 269 (80) [(Q )] , 561 (100) [{Na(Q )2}] ,
CF3 CF3
◦
−
−
2
.15%, calcd 2.86%; ꢃH = 14.90 kJ/mol); an onset fusion at 238 C
650 (40) [{VO(Q )2(EtO)}] , 718 (17) [{VO(Q )2(EtO)2Na}] ,
CF3 CF3
−
−
(
ꢃHfusion = 16.87 kJ/mol).
874 (20) [VO(Q )3] , 1227 (15) [{VO(Q )2}2OH] . TGA-DTA
VO(QMe,Me) (H O) (II): scarcely soluble in all common solvents.
◦
◦
◦
(mg% vs. C): heating from 35 to 600 C with a speed of 7 C/min;
2
2
−1
◦
ꢀeff: 1.95 BM. IR (nujol, cm ): 3300br ꢂ(O–H), 1665sh ı(O–H),
from 250 to 355 C sublimation, confirmed by the IR spectrum of
1
595vs ꢂ(C O), 1535s, 1503vs ꢂ(C N + C C), 890vs ꢂ(V O). ESI-
the sublimed powder (ꢃHsublimation = 33.42 kJ/mol).
Me,Me
+
py,CF3
MS (MeCN) (+): m/z (%) = 374 (100) [{VO(Q
) }H] , 412 (80)
VO(Q
)2 (VII): soluble in acetone, acetonitrile, DMSO and
2
Me,Me
Me,Me
+
Me,Me
+
−1
[
[
5
{VO(Q
) }K] , 437 (40) [{VO(Q
) (MeCN)}Na] , 769 (75)
chlorinated solvents. ꢀeff: 1.85 BM. IR (nujol, cm ): 3148w
2
2
◦
+
{VO(Q
) } Na] . TGA-DTA (mg% vs. C): heating from 35 to
ꢂ(Carom–H), 1675s ꢂ(C
O), 1611m, 1581s, 1558s, 1538m
2
2
◦
◦
◦
00 C with a speed of 7 C/min; from 110 to 130 C loss of one water
ꢂ(C C + C N), 919vs ꢂ(V O), 518s, 475s, 436s ꢂ(V–NQ). ESI-
py,CF3
+
molecule (weight loss found 4.50%, calcd 4.60%, ꢃH = 39.56 kJ/mol);
from 280 to 350 C sublimation, confirmed by the IR spectrum of
the sublimed powder (ꢃHsublimation = 134.04 kJ/mol).
MS (MeCN) (+): m/z (%) = 294 (100) [(HQ
)Na] , 608 (50)
◦
py,CF3
+
py,CF3
+
[{VO(Q
)2}H] , 629.8 (100) [{VO(Q
)2}Na] . TGA-DTA
◦
◦
◦
(mg% vs. C): heating from 35 to 700 C with a speed of 7 C/min;

2
14
F. Marchetti et al. / Applied Catalysis A: General 378 (2010) 211–220
Table 2
acetonitrile. Then, 1.140 mmol of H O2 (3.0 equiv.) solution were
added followed, after 2 min, by 0.380 mmol of selected olefin. The
2
Crystal data and structure refinement for complex I.
Compound
I
mixture was allowed to stir at chosen temperature, for selected
times. The reaction products were monitored at periodic time
intervals using GC–MS analyses. At the end of reaction, mixture
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C32H40N4O6V
627.62
293(2)
0.71073
Triclinic
P-1
was quenched with 5.0 mg of MnO , and after 15 min, filtered and
2
evaporated under reduced pressure. The oxidation products were
identified by comparison of their GC retention times with those of
authentic samples.
Unit cell dimensions
a (Å)
b (Å)
9.5785(6)
13.3511(8)
13.7135(8)
68.560(2)
76.160(3)
77.988(3)
2
3. Results and discussion
c (Å)
◦
˛
( )
3.1. Synthesis and characterization of oxovanadium(IV)
◦
ˇ ( )
◦
complexes
ꢄ ( )
Z
3
Dcalc (g/cm )
1.327
0.365
The vanadyl(IV) derivatives here reported, having the general
composition VO(Q) (H O) (I–IV) and VO(Q) (V–VII) respectively,
Absorption coefficient (mm−1)
2
2
2
Index ranges
−11 ≤ h ≤ 11
have been synthesized by a general procedure based on the mixing
−
16 ≤ k ≤ 16
17 ≤ l ≤ 17
of an aqueous solution of VOSO ·5H O with a methanol solution of
4
2
−
the HQ proligand and of sodium methoxide as base, in a 1:2:2 molar
ratio, and isolation of final precipitate by filtration (Scheme 1). They
are all stable to air and moisture, without any kind of decomposition
also after several months, however derivatives V–VII absorb water
when left in a moist environment for a prolonged time, thus assum-
ing a final VO(Q)2(H2O) composition, as for derivatives I–IV. All
derivatives are insoluble in water, but quite soluble in chlorinated
solvents, alcohols, acetonitrile and DMSO. Acetonitrile solutions of
complexes VI and VII showed to be stable at least for 72 h, at r.t., as
confirmed by UV–vis and EPR analyses.
Reflections collected
Independent reflections [Rint]
Data/restraints/parameters
28056
6278 [0.0420]
6278/3/395
1.073
R1 = 0.0686, wR2 = 0.2099
R1 = 0.0828, wR2 = 0.2209
1.082 and −0.357
2
Goodness-of-fit on F
a,b
Final R indices [I > 2ꢅ(I)]
R indices (all data)
Largest diff. HLR3 and hole (e Å
−3
)
a
R1 = ꢆ(|Fo| − |Fc|)/ꢆ|Fo|.
wR2 = [˙w(Fo − F ) ˙w(F ) ]
1
/2
2
2
b
2
2
2
.
c
o
◦
from 300 to 700 C progressive decomposition, with a final black
residual of 32% weight.
3.2. IR spectral studies
The IR spectra of I–VII show strong absorption bands between
500 and 1675 cm , due to the stretching modes ꢂ(C O), ꢂ(C C)
2
.3.1. X-ray crystallography
X-ray data for complex I were collected at room temperature
−
1
1
and ꢂ(C N) of the acylpyrazolonate ligands [45]. In particular, in
the spectra of I–VI, the higher frequency band due to ꢂ(C O) is
shifted to lower frequency upon coordination. In the IR spectrum
on a Bruker-Nonius X8 Apex CCD area detector equipped with
graphite monochromator and Mo K␣ radiation (ꢇ = 0.71073 Å). Data
were processed through the SAINT [51] reduction and SADABS
−
1
of derivative VII the highest ꢂ(C O) band falls at 1673 cm , a value
[
52] absorption software. The structure was solved by standard
analogous to that found in the previously reported silver com-
Patterson methods through the SHELXTL-NT [53] structure deter-
mination package and refined by full-matrix least-squares based
on F . All non-hydrogen atoms were refined anisotropically and
hydrogen atoms were included as idealized atoms riding on the
respective carbon atoms with C–H bond lengths appropriate to the
carbon atom hybridization. Water hydrogen atoms were included
in the refinement with fixed bond lengths. Details of the crystal
data collection are listed in Table 2.
py,CF3
plex [Ag(Q
) (Meim)] (Meim = 1-methylimidazole) containing
2
py,CF3
2
a N,N-chelating Q
ligand, as confirmed by X-ray crystal data
[
46–49]. On this basis we have hypothesized for derivative VII the
structure depicted in Scheme 1. However, we cannot exclude the
existence of weak intermolecular interactions involving the C–O
arms of the ligands, as previously observed either in the polynu-
py,CF3
py,CF3
clear [Ag (Q
) (MeCN]∞ or in the same [Ag(Q
)2(Meim)]
2
2
complexes [46–49]. An additional information from IR spectra
arises from the strong band observed in the range 880–1000 cm
−
1
,
2.3.2. EPR experiments
assignable to ꢂ(V O) fragment [21]. For example, such stretching
Under typical conditions, the spectrometer operated at a cen-
−
1
band was found at 996 cm
in the IR of [VO(acac) ] [54]. The
2
tral magnetic field of 355 mT (3550 G), scan range 110 mT (1100 G),
sweep time 165 s, time constant 100 ms, modulation frequency
1
The effective 28 mW microwave power on the samples was gen-
erated by a 280 mW microwave power source (10 dB attenuation).
The analyzed samples were prepared by dissolving 1.0 mg of VI
in 1.0 mL of acetonitrile, and spectra were recorded at room tem-
perature (298 K). After addition of single drop portions of a 35%
hydrate derivatives I–IV exhibit this band at frequencies higher
−
1
than 960 cm , whereas in the IR spectra of the anhydrous V–VII
00 kHz, modulation amplitude 0.1 mT (1 G), at room temperature.
−
1
this band falls in the range 890–920 cm . The lowering of the
ꢂ(V O) value may arise from the presence of a bridging vanadyl
group [46–49] which coordinates the vanadium atom of a neigh-
bour molecule. Finally, IR spectra of derivatives I–IV show a broad
−
1
band in the region 3100–3300 cm , due to the –OH group of the
coordinated H O molecule in the complex, whereas this band is
2
aqueous solution of H O2 to the previous solution, the successive
EPR spectra were recorded, at the same temperature, after 5 min.
2
absent in the IR spectra of V–VII.
2.4. Catalytic tests
3.3. Magnetic and EPR measurements
Catalytic experiments were carried out in a 5.0 mL two-necked
glass flask, fitted with a water condenser. In a typical experiment,
a 0.5 mol% of catalyst (0.0019 mmol) were dissolved in 1.5 mL of
The room-temperature magnetic susceptibility data for I–VII
show paramagnetic behaviour due to the d1 system. The effective
magnetic moments obtained for the oxovanadium(IV) complexes

F. Marchetti et al. / Applied Catalysis A: General 378 (2010) 211–220
215
Table 3
◦
Selected bond distances (Å) and angles ( ) for complex I.
V–O(1)
V–O(2)
V–O(3)
V–O(4)
V–O(5)
V–O(6)
O(1)–V–O(2)
O(1)–V–O(3)
O(1)–V–O(4)
2.025(3)
1.992(2)
1.989(2)
2.013(3)
1.584(3)
2.262(3)
89.1(1)
O(1)–V–O(5)
O(1)–V–O(6)
O(2)–V–O(3)
O(3)–V–O(4)
O(3)–V–O(5)
O(3)–V–O(6)
O(4)–V–O(5)
O(4)–V–O(6)
O(5)–V–O(6)
98.2(2)
81.1(1)
163.3(1)
88.4(1)
99.3(1)
81.4(1)
98.7(2)
82.1(1)
179.0(1)
87.9(1)
163.2(1)
tively [46–49,59]. Because of the presence of a strong axial field,
the energy levels associated with these two structures do not differ
considerably [60]. Moreover, in ethanol solution it is not unlikely
the coordination of an EtOH molecule in the sixth coordination site
trans to the oxygen of V O fragment in derivative VI, to give a
tetragonally distorted octahedral complex as for I [61–63]. A distor-
tion from the square pyramidal geometry removes the degeneracy
between the dxz and dyz orbitals. The e level will then split into two,
Fig. 1. EPR spectrum of oxovanadium complex VI in acetonitrile at 298 K.
b1(dxz) and b (dyz), and accordingly four transitions are expected
2
in the case of a complex with a distorted square pyramidal or a
tetragonally distorted octahedral geometry [46–49,64–66]. In the
electronic spectra of our complexes we were not able to precisely
assign such transitions due to broadness of the bands, that however
exhibit a general pattern similar to that observed for vanadyl(IV)
(
1.76–1.95 BM) are within the normal range for the spin only con-
1
tribution of a d system when the orbital contribution is completely
quenched [55].
In order to confirm, by X band EPR analyses, the fast oxida-
tion of metal from +4 to +5 oxidation state, a well known process
occurring for the oxovanadium(IV) complexes after H O addition,
we focused our attention on complex VI being it the most soluble
in acetonitrile. As expected, EPR spectrum of VI in acetonitrile, at
2
␤-diketonate derivatives [46–49,67].
2
2
3.5. ESI-mass spectrometry analyses
The ESI-MS studies of I–VII have been carried in acetonitrile,
apart for derivative III that is almost insoluble in this solvent,
98 K, exhibited a hyperfine structure (HFS) derived from the inter-
1
action of free electron (3d ) with the magnetic nuclear moment of
5
1
and for which a methanol solution was used. The positive spectra
V (I = 7/2, 99.76% of natural abundance), so confirming the pres-
+
IV
always show at least a peak due to the [VO(Q) E] species (where
ence of isolated V species as pertain to a mononuclear complex. In
this case, the EPR signal splits into eight-fold lines of all anisotropic
2
+
E = H, Na or K), together with absorptions due to [(HQ)Na] species
nPe
+
in the spectra of III, IV, VI and VII and to [VO(Q ) (MeCN)Na] in
components (Fig. 1) with values of giso = 1.98 and A = 109 G. This
2
iso
values could correspond to vanadyl ions (VO2+) in octahedral dis-
the spectrum of I. Additionally, in the spectra of derivatives II, III
+
and V, some dinuclear charged species of the type [{VO(Q) } E]
torted symmetry [56]. Moreover, the g-value lies in the 1.9–1.99
range and agrees with the fact that when the d-shell is less than
half filled, the spin–orbit effect reduces the g-value in comparison
with that of a free electron (2.0023) [57]. The same measurements
were carried out on the same sample after dropwise titration with
a 35% aqueous solution of H O , in order to mimic catalytic reac-
tion medium. The new EPR spectra showed the disappearance of
previous well defined HFS, as expected when the oxidation to oxo-
vanadium(V) complex derivatives, occurred.
2 2
(
E = H, Na or K) have been identified, which further confirms the
ability of V O systems to self-aggregate. The negative spectrum of
CF3
−
CF3
−
derivative VI exhibits peaks attributable to (Q ) , [(Q ) Na] ,
2
CF3
−
VO(Q ) (OEt)] , [VO(Q ) ] and also to a dinuclear species of
2
CF3
−
[
3
CF3
−
composition [{VO(Q ) } (OH)] . Finally, the positive and neg-
2
2
2
2
ative ESI-MS spectra of VI in ethanol have been recorded for
comparative purposes. It is worth to note that two new peaks at
6
52 and 675 m/z have been found, which could be assigned to
CF3
+
CF3
+
[
{VO(Q ) (EtOH)}H] and [{VO(Q ) (EtOH)}Na] , respectively,
2
2
thus confirming the coordination of solvent molecules to vanadium
in ethanol solution, as previously hypothesized.
3
.4. Electronic spectral studies
The electronic spectra of I and VI and the corresponding proli-
nPe
CF3
3.6. Thermogravimetric analyses
gands HQ
and HQ
have also been performed in ethanol. For
the ligands two main bands have been observed below 300 nm, in
nPe
Thermal studies have been performed for all derivatives I–VII.
particular at 233 and 268 nm for HQ
and at 231 and 271 nm for
CF3
The data indicate that they are stable to decomposition up to
HQ , those at lower frequency being due to the ␲–␲* transition of
the phenyl group while the others at higher frequency are assigned
to the n–␲* transition of the carbonyl group in the acyl fragment.
Upon coordination these absorption bands are red shifted to form
a broad and unresolved band at 255 nm (derivative I) and to 242
and 296 (derivative VI), respectively, indicating the involvement of
the carbonyl group in the complex formation with the metal ion, a
behaviour similar to that of a series of oxovanadium(IV) 4-acyl-5-
pyrazolone complexes, previously reported [58]. Moreover, in the
region 500–900 nm there are overlapping absorptions correspond-
ing to d–d transitions of these d1 complexes. Oxovanadium(IV)
compounds usually show five and six coordination numbers giving
tetragonal pyramidal and distorted octahedral structures respec-
◦
2
50–300 C. Moreover, derivatives I–IV loose thecoordinated water
◦
within the range 60–200 C, as expected [68]. During the thermal
run of derivatives II and VI, we have observed the sublimation of
their anhydrous form [VO(Q) ] without any decomposition in the
2
◦
range 280–350 C.
3.7. X-ray structure determination of [VO(QnPe) (H O)] (I)
2
2
Crystals of complex I suitable for X-ray analysis were obtained
from acetonitrile. A view of the structure of I is shown in Fig. 2.
Table 3 lists relevant bond lengths and angles. The two O,O-
chelating acylpyrazolonates occupy the equatorial plane of the

2
16
F. Marchetti et al. / Applied Catalysis A: General 378 (2010) 211–220
Scheme 2. Main products observed during the catalytic oxidation of styrene deriva-
tives 1–3.
Fig. 2. Perspective view of complex I with atomic numbering scheme (ellipsoids at
the 40% level).
oxovanadium(IV) complex in an anti configuration to each other
and the distorted octahedral environment is reached through the
coordination to V centre of a water molecule trans to the oxo group.
The geometry of the equatorial coordinating donor atoms is slightly
pyramidal, with the vanadium atom 0.2914(7) Å away from the
least-squares plane defined by the four equatorial oxygen atoms.
As reported in Table 3, the V–O bond distances of the two
chelated acylpyrazolonates are in the range 1.99–2.02 Å; the V
distance of 1.584(3) Å as well as the average “bite” angles of
O
◦ ◦
8
9.1(1) and 88.4(1) , are comparable with those found searching
the CSD(CambridgeStructuralDatabase, Version 5.30)foroxovana-
dium(IV) compounds with O,O-donor ligands. The rotationally free
phenyl rings of the two acylpyrazolonate ligands show different
Fig. 4. Effect of H O concentration on styrene oxidation with catalyst VI. Reaction
conditions: styrene (0.3 mmol), catalyst (0.5 mol%), acetonitrile (1.5 mL), r.t.
2
2
◦
◦
values of the tilt angle around the Npy–C bond, of 24.1 and 5.7 ,
respectively, indicating the different intermolecular environment
experienced by the two moieties. Indeed, only the less tilted phenyl
ring is involved in both an intermolecular hydrogen bond of the
C–H· · ·O type with the oxygen atom of the oxo group of a sym-
3.8. Catalytic activity studies
In order to screen and evaluate the catalytic oxidative activity
of the synthesized complexes, several of them (I, II, IV–VII) were
applied in the oxidation of styrene 1 and of its derivatives namely
i
metrically related molecule [C(28)· · ·O(5) 3.184(5) Å, H(28)· · ·O(5)
␣
-methylstyrene 2 and cis-␤-methylstyrene 3, with H O2 as pri-
2
◦
2
.56 Å, C(28)–H(28)· · ·O(5) 125(3) , i = −x + 2, −y + 2, −z + 1] and an
mary oxidant. The formation of different products is outlined in
Scheme 2. In the absence of the catalyst, conversion of substrates
lower than 2% was observed.
associated ␲–␲ stacking contact of 3.6 Å (measured stacking inter-
planar distance) (Fig. 3a).
Moreover, each molecule of complex I is involved in the forma-
tion of four intermolecular hydrogen bonds of the Owater–H· · ·Npy
type, working at the same time as acceptor, with both nitrogen
atoms N(1) and N(3) of the acylpyrazolonates and donor, with
the coordinated water molecule, in the formation of a tight
In search of suitable reaction conditions to achieve the best
results in terms of both maximum conversion of reagents and prod-
ucts selectivity, the effect of oxidant concentration (moles of H O2
2
per moles of olefins), catalyst ratio (referred as mol% per mole of
olefin), nature of solvent and temperature of the reaction, were
studied. The effect of oxidant concentration on the oxidation of sub-
strate 1, is illustrated in Fig. 4. Different H O /styrene molar ratios
i
supramolecular aggregation [O(6)· · ·N(1) 2.908(4) Å, H(6w)· · ·N(1)
◦
2
.06 Å, O(6)–H(6w)· · ·N(1) 176(3) , i = −x + 2, −y + 1, −z + 2;
2
2
ii
O(6)· · ·N(3) 2.948(4) Å, H(7w)· · ·N(3) 2.12 Å, O(6)–H(7w)· · ·N(3)
(1:1, 3:1 and 5:1) were evaluated, while keeping the fixed amount
◦
1
67(3) , ii = −x + 2, −y + 2, −z + 1] (Fig. 3b).
of styrene (0.3 mmol) and catalyst (VI, 0.5 mol%) in 1.5 mL of ace-
Fig. 3. Packing diagram showing the formation of intermolecular hydrogen bonds of (a) C–H· · ·O and (b) Owater–H· · ·Npy type (view down the b axis) in complex I.

F. Marchetti et al. / Applied Catalysis A: General 378 (2010) 211–220
217
Fig. 5. Effect of catalyst amount on styrene oxidation with catalyst VI. Reaction
conditions: styrene (0.3 mmol), H2O2 (1:1), acetonitrile (1.5 mL), r.t.
Fig. 6. Effect of temperature on styrene oxidation with catalyst VI. Reaction condi-
tions: styrene (0.3 mmol), catalyst (0.5 mol%), H2O2 (1:1), acetonitrile (1.5 mL).
tonitrile, at r.t., running for 18 h. The progress of the reaction was
regularly monitored at different intervals of time, by GC–MS analy-
sis. Increasing the H O /styrene ratio from 1:1 to 5:1 increases the
gen peroxide, has also been observed during the evaluation of the
role exerted by the temperature. As can be seen in Fig. 6, a lower
temperature (r.t.) is necessary even if working with a stoichiometric
amount of oxidant (1:1), in order to optimize substrate conversions.
We then selected the best experimental conditions to carry out
further oxidative catalytic studies with the remaining oxovana-
dium(IV) complexes, as I–II, IV–V and VII, as follows: substrate
2
2
conversion until about 98%, even if larger conversion values are not
accompanied by a corresponding increase in the yield of product
having more added value as epoxide (1c–3c, Scheme 2). For this
reason and with the aim to reduce the oxidant consumption, we
decided to work with a 3:1 excess of oxidant. As can be observed in
Fig. 4, the best conversion values were reached after only one-third
of the overall running reaction time (18 h). The reaction comes to
almost steady state within 6 h, and then after only minor changes
in the percent conversion were observed.
A deep evaluation devoted to select the best solvent for
our catalytic reactions, has been effected by comparing the
results obtained working in 1.5 mL of four different solvents as
dichloromethane, acetonitrile, THF and ethanol, while keeping
fixed the amount of olefins (1–3, 0.3 mmol) and catalysts (V or VI,
(0.38 mmol), catalyst (0.5 mol%), H O2 (3:1), acetonitrile (1.5 mL),
2
r.t., 18 h. The results we have obtained in the oxidation of styrene
derivatives 1–3, are described in Table 4, which provides details on
conversion values, turnover numbers (TON), selectivities and yields
for the various products obtained. Firstly, it can be observed that the
selectivity toward the formation of the most commercially valuable
product, the epoxide, is generally low (13–30%) being this product
detectable only in the oxidation of cis-␤-methylstyrene 3 (Table 4,
product 3c). A similar trend was previously observed by some of
us, during the hydrogen peroxide promoted oxidation of styrene
derivatives by means of methyltrioxorhenium based catalysts [69].
Probably, after its initial formation the epoxide is transformed
into other products. Moreover, the selectivity toward the forma-
tion of benzaldehyde 1a (or the corresponding acetophenone 2a) is
generally higher for olefin 1 (or 2), than for cis-␤-methylstyrene 3.
The hypothesis that benzaldehyde formation (during oxidation of
olefin 1) could be due to the nucleophilic attack of H O on the less
0.5 mol%), at r.t., running for 18 h. While dichloromethane acted as
the worst solvent showing very low conversion values, also when
used in 1:1 mixtures with acetonitrile, THF on the contrary afforded
complex mixtures of products most of them attributable to sol-
vent oxidation. Ethanol worked well as did acetonitrile, in terms
of conversion of substrate but, differently from the latter, showed
to interact widely with the formed oxidation products affording, as
an example in the case of styrene 1 oxidation, ethyl benzoate and
2
-ethoxy-2-phenylethanol (derived from the nucleophilic styrene
2
2
epoxide opening), in mixture with other products. So, in order to
reduce solvent interactions and better evaluate the selectivity dur-
ing catalytic oxidations, we decided to use acetonitrile as reaction
solvent.
substituted carbon of the initially formed epoxide ring, followed by
cleavage of the intermediate hydroxyl-hydroperoxystyrene, cannot
be excluded [70]. In the case of substrate 3 the opening of epoxide
ring should be less favourable due to the steric hindrance exerted
by the presence of a methyl group as substituent in the ␤-carbon
of vinyl residue, so the formation of a discrete amount of epox-
ide 3c, is favoured. In addition, benzaldehyde (1a or 3a, Scheme 2)
or acetophenone (2a, Scheme 2) formation could also be facili-
tated by direct oxidative cleavage of the vinyl double bond, via a
radical mechanism [70–72]. The formation of benzoic acid (1b or
3b, Scheme 2) in consequence of benzaldehyde oxidation is quite
likely. A plausible mechanism involving the hydrolysis of epoxide
by water present in H O solution to the corresponding diol deriva-
Similarly, four different amounts of catalyst i.e. 0.5, 1.0, 1.5 and
2
.0 mol% with respect to substrate, were considered to evaluate the
effect of catalyst amount on the rate of the reaction, while keep-
ing fixed the amount of styrene (0.3 mmol) and H O2 (1:1), at r.t.,
2
running for 18 h. As shown in Fig. 5, an increase in the amount of cat-
alyst hadadverse effectsand conversionvalues regularly decreased,
the large differences being observed between 0.5 and 1.0 mol%. The
decrement in the substrate conversion at higher catalyst concen-
trations may be due to the faster decomposition of H O , in the
2
2
2
2
presence of a larger number of available metal active catalytic sites,
even at r.t.
This trend, i.e. the importance to prevent the negative effect on
the substrate conversion due to the fast decomposition of hydro-
tives (1d or 2d, Scheme 2), may be rationally invoked. Fig. 7 shows
a typical plot of benzaldehyde 1a, benzoic acid 1b, epoxystyrene
1c and diol 1d formation vs. time during styrene 1 oxidation by
oxovanadium(IV) complex VI, in acetonitrile (Table 4, entry 13).

2
18
F. Marchetti et al. / Applied Catalysis A: General 378 (2010) 211–220
Table 4
a
Oxidation of olefins 1–3 in CH3CN with H2O2 and catalysts I–II, IV–VII .
Entry
Olefinb
Catalyst
Conv. (%)
TONc
Productsd (% selectivity) [% yield]e
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
I
I
I
II
II
II
IV
IV
IV
V
88
82
89
48
76
64
35
46
32
84
81
86
90
98
94
85
94
84
176
164
178
96
152
128
70
1a (62) [55], 1b (23) [20], 1d (15) [13]
2a (83) [68], 2d (17) [14]
3a (63) [56], 3b (11) [10], 3c (26) [23]
1a (86) [41], 1b (4) [2], 1c (6) [3], 1d (4) [2]
2a (100) [76]
3a (70) [45], 3c (30) [19]
1a (100) [35]
2a (100) [46]
3a (72) [23], 3c (28) [9]
1a (70) [59], 1b (19) [16], 1d (11) [9]
2a (80) [65], 2d (20) [16]
3a (63) [54], 3b (13) [11], 3c (24) [21]
1a (67) [60], 1b (21) [19], 1d (12) [11]
2a (78) [77], 2d (22) [21]
3a (73) [69], 3b (12) [11], 3c (15) [14]
1a (71) [60], 1b (23) [20], 1d (6) [5]
2a (88) [83], 2d (12) [11]
f
4
5
6
7
8
9
f
f
f
f
92
64
f
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
168
162
172
180
196
188
170
188
168
V
V
VI
VI
VI
VII
VII
VII
3a (74) [62], 3b (13) [11], 3c (13) [11]
a
b
c
d
e
f
The reactions were performed in CH3CN, with 3 equiv. of H2O2, 0.5 mol% of catalyst, at r.t., running for 18 h.
Olefins: 1 = styrene; 2 = ␣-methylstyrene; 3 = cis-␤-methylstyrene.
TON: (turnover number) moles of substrate converted per mole of catalyst.
Products: 1a = 3a = benzaldehyde; 1b = 3b = benzoic acid; 1c = epoxide; 1d = glycol; 2a = acetophenone; 2d = glycol; 3c = epoxide.
Yields are calculated on substrate conversion.
Running time = 4 h.
The epoxide 1c was mainly formed at the beginning of the reac-
tion, but progressively disappeared with time, at the expense of
the oxidative cleavage compounds. A similar behaviour has been
previously observed in the oxidation of 2-methyl-2-pentene with
the vanadium(V) peroxo complex [VO(O )(Pic)·H O] [43].
catalyst selectivities are shown in Figs. 8 and 9, respectively. In
general, the catalytic activity of complexes I, V–VII showed to be
mutually comparable (Fig. 8), with TON values within the range
of 162–196, according with others oxovanadium complexes previ-
CF3
ously described in literature [27,73], being the complex VO(Q )2
2
2
Shorter reaction times afforded higher selectivities as observed
in the oxidation of olefins 1–2 with catalysts II and IV (Table 4,
entries 5, 7–8), even if with lower conversion values. This confirms
that the increasing conversion of starting materials is accompa-
nied by a further reactivity of initially formed oxidation products
(VI) the most active one. In terms of selectivity observed for oxi-
dation of styrene, undoubtedly the highest selectivity (in the range
62–100%) was observed for the formation of benzaldehyde, irre-
spective to the nature of the complex used as catalytic system
(Fig. 9).
(
i.e. benzaldehyde to benzoic acid, and epoxide to diol), and conse-
This result confirms the accepted hypothesis that, under these
experimental conditions, an oxovanadium(V)monoperoxo com-
plex (␣) or, more likely, an oxovanadium(V)bisperoxo anionic
complex (␤) (the latter lacking of any potential ligand contribution
to the reaction outcome) can be invoked as the real active catalytic
species (Fig. 10). On the other hand, the formation of the anionic
complex (␤) prone to participate in oxidation reactions, by transfer-
ing one of its oxygens to substrate, in the H O promoted oxidations
quently by a decreasing of selectivity.
For what concerns a comparison in terms of activity and selec-
tivity showed by the different oxovanadium(IV) complexes on
catalytic oxidations of styrene derivatives 1–3, we have focused
our attention on complexes I, V–VII, because of their larger solu-
bility in acetonitrile. The results in terms of catalyst activities and
2
2
of various olefins, has been already largely demonstrated [36–44].
Moreover, the relevant occurrence of oxidation products deriving
from an oxidative cleavage mechanism (Scheme 2, products a–b)
Fig. 8. Comparison between the activities showed by the oxovanadium(IV) com-
plexes I, V–VII, in the oxidation of olefines 1–3. Reaction conditions: substrate
(0.3 mmol), catalyst (0.5 mol%), H2O2 (3:1), acetonitrile (1.5 mL), r.t., 18 h.
Fig. 7. Oxidation of styrene 1 by complex VI. Reaction conditions: styrene (0.3 mmol),
catalyst (0.5 mol%), H2O2 (3:1), acetonitrile (1.5 mL), r.t.

F. Marchetti et al. / Applied Catalysis A: General 378 (2010) 211–220
219
Fig. 9. Comparison between the selectivities showed by oxovanadium(IV) com-
plexes I, V–VII, in the oxidation of styrene. Reaction conditions: styrene (0.3 mmol),
catalyst (0.5 mol%), H2O2 (3:1), acetonitrile (1.5 mL), r.t., 18 h.
Fig. 11. Electronic spectra of HQCF3 and VO(QCF3)2 (VI). The reactivity of VI with
H2O2, is also showed. Spectra were recorded after the successive addition (2.0 ␮L)
of 0.05 M H2O2 aqueous solution to 10 mL of acetonitrile solution of VI (1.0 mg).
implies that, in this study, an alternative but otherwise important
role for catalytically active radical complex species, cannot be ruled
out.
4. Conclusions
A series of new oxovanadium(IV) complexes containing var-
iously substituted 4-acyl-5-pyrazolone donor ligands have been
synthesized and fully characterized. In the case of complex I,
3
.9. The fate of complex VI in the presence of H O2
2
nPe
In order to evaluate intermediate species formed during cat-
namely VO(Q )2(H2O), its crystal and molecular structures have
alytic oxidation, the acetonitrile solution of neat complex VI was
been resolved by single crystal X-rays diffraction, showing a dis-
torted octahedral environment reached through the coordination
of two O,O-chelating acylpyrazolonate ligands occupying the equa-
torial plane in anti configuration to each other, and a water
molecule trans to the oxo group. The oxovanadium(IV) complexes
I–II, IV–VII have been used as catalysts for oxidation of aromatic
olefins like styrene 1 and its derivatives 2–3, with H2O2 as pri-
mary oxidant. Various parameters as solvent, temperature, catalyst
amount and oxidant concentration have been tested in order to
optimize both activity and selectivity of the oxidative process.
Under optimized conditions, high values of conversion of substrates
within the range 98–81% have been observed, with TON values
ranging from 196 to 162. Generally, shorter reaction times (4 h)
afforded higher selectivity, even if with lower conversion values
ranging from 32% to 76% (TON from 64 to 152). Among the dif-
ferent oxidized products formed from aromatic olefins 1–3, the
selectivity for benzaldehyde is highest, regardless the type of cat-
alyst employed. The best selectivity toward epoxide formation has
been solely observed during oxidation of cis-␤-methylstyrene 3.
The catalytic activity of selected oxovanadium complexes I, V–VII in
the oxidation of olefins 1–3 showed to be reciprocally comparable,
being the complex VI the most active one. Further studies devoted
to gain more mechanistic details on the mechanism active with this
family of catalysts or with similar derivatives (including the effects,
if any, exerted by molecular dioxygen) and to extend their applica-
bility toward catalytic synthetic oxidations, will be planned in our
laboratories.
treated with a 35% aqueous solution of H O , and the reaction was
2
2
monitored by UV–vis spectroscopy (Fig. 11). The results showed
that the successive additions of 2.0 ␮L of a 0.05 M H O2 aqueous
2
solution to 10 mL of acetonitrile solution of VI (1.0 mg), resulted in
the reduction of the intensity of n–␲* transition band at 296 nm.
At the same time this band marginally shifts toward higher wave-
lengths along with broadening, while in the region 500-900 nm
the overlapping absorptions corresponding to d–d transitions (not
shown) slowly become weaker and finally disappears. These results
are in accord with the above suggested hypothesis of formation
of peroxovanadium(V) species [73,74]. It should be noted that
the UV–vis spectra of titrated solution of complex VI, remained
unchanged after 24 h.
Acknowledgements
“Valle Esina SPA”, “COOPERAZIONE UNIVERSITARIA INTER-
NAZIONALE 2008” and Universities of, respectively, Camerino,
L’Aquila and Calabria are greatly acknowledged for financial sup-
ports.
Fig. 10. Probable oxoperoxovanadium(V) complex derivatives, formed during H2O2
oxidation.

2
20
F. Marchetti et al. / Applied Catalysis A: General 378 (2010) 211–220
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