Oxidovanadium complexes with tridentate aroylhydrazone as catalyst precursors for solvent-free microwave-assisted oxidation of alcohols

Article abstract of DOI:10.1016/j.apcata.2015.01.005

Aroylhydrazone oxidovanadium compounds, viz. the oxidoethoxidovanadium(V) [VO(OEt)L] (1) (H₂L = salicylaldehyde-2-hydroxybenzoylhydrazone), the salt like dioxidovanadium(V) (NH₃CH₂CH₂OH)⁺[VO₂L]^- (2), the mixed-ligand oxidovanadium(V) [VO(hq)L] (Hhq = 8-hydroxyquinoline) (3) and the vanadium(IV) [VO(phen)L] (phen = 1,10-phenanthroline) (4) complexes (3 and 4 obtained by the first time), have been tested as catalysts for solvent-free microwave-assisted oxidation of aromatic and alicyclic secondary alcohols with tert-butylhydroperoxide. A facile, efficient and selective solvent-free synthesis of ketones was achieved with yields up to 99% (TON = 497, TOF = 993 h^-1 for 3) and 58% (TON = 291, TOF = 581 h^-1 for 2) for acetophenone and cyclohexanone, respectively, after 30 min under low power (25 W) microwave irradiation.

Full text of DOI:10.1016/j.apcata.2015.01.005

Applied Catalysis A: General 493 (2015) 50–57
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Oxidovanadium complexes with tridentate aroylhydrazone as catalyst
precursors for solvent-free microwave-assisted oxidation of alcohols
Manas Sutradhara, Luísa M.D.R.S. Martinsa,b, M. Fátima C. Guedes da Silvaa,
Armando J.L. Pombeiroa,∗
a Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
b Chemical Engineering Department, ISEL, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Aroylhydrazone oxidovanadium compounds, viz. the oxidoethoxidovanadium(V) [VO(OEt)L] (1) (H2L
= salicylaldehyde-2-hydroxybenzoylhydrazone), the salt like dioxidovanadium(V) (NH3CH2CH2OH)+
[VO2L]− (2), the mixed-ligand oxidovanadium(V) [VO(hq)L] (Hhq = 8-hydroxyquinoline) (3) and the vana-
dium(IV) [VO(phen)L] (phen = 1,10-phenanthroline) (4) complexes (3 and 4 obtained by the first time),
have been tested as catalysts for solvent-free microwave-assisted oxidation of aromatic and alicyclic
secondary alcohols with tert-butylhydroperoxide. A facile, efficient and selective solvent-free synthe-
sis of ketones was achieved with yields up to 99% (TON = 497, TOF = 993 h−1 for 3) and 58% (TON = 291,
TOF = 581 h−1 for 2) for acetophenone and cyclohexanone, respectively, after 30 min under low power
(25 W) microwave irradiation.
Received 26 August 2014
Received in revised form
16 December 2014
Accepted 5 January 2015
Available online 13 January 2015
Keywords:
Oxidovanadium complexes
Aroylhydrazone
© 2015 Elsevier B.V. All rights reserved.
X-ray structure
Microwave-assisted oxidation
1. Introduction
yields and high selectivities, either in liquid or supported sys-
tems. The use of polyoxidovanadates has also been reported for the
Functionalized oxygenated products, namely ketones, are on
the basis of important industrial synthetic strategies, as sol-
vents, polymer precursors and substrates for the syntheses, e.g., of
pharmaceuticals, agrochemicals and fragrances [1–4]. Within the
vast ketone synthetic methods, recent efforts focus on energy- and
atom-efficiency, elimination of the use of hazardous substances,
and reduction of time and of the generation of toxic (e.g. organic sol-
vents) and heavy-metal wastes [5–9]. Hence, aerobic [10–13] and
peroxidative [14–16] selective oxidations of secondary alcohols are
regardedassimpleandveryusefulsyntheticmethodsfortheprepa-
ration of ketones, this conversion being of a pivotal significance in
organic synthesis. Moreover, it is known that microwave (MW) irra-
diation can provide a much more efficient synthetic method than
conventional heating, allowing to achieve similar (or higher) yields
in a shorter time and/or to improve the selectivity [17–27].
efficient oxidation of benzylic alcohols to the corresponding car-
bonyl compounds with p-toluenesulfonic acid [40]. However, the
vanadium catalyzed peroxidative oxidation of secondary alcohols is
comparatively scarce [35–39,41–43]. Some efficient systems con-
cern the use of tert-butylhydroperoxide (TBHP) in the presence of
silica [41] or graphene [42] supported oxidovanadium Schiff bases,
or of hydrogen peroxide in a homogeneous mixture composed of
vanadate, acid and TEMPO functionalized ionic liquids [43]. To our
knowledge there is no report for solvent-free MW-assisted oxida-
tion of secondary alcohols catalyzed by vanadium complexes.
On the other hand, aroylhydrazones are known to form stable
complexes with vanadium in +4 or +5 oxidation states [44–48].
Therefore, the main objective of the present study was to establish
the viability of oxidovanadium complexes based on an aroylhydra-
zone ligand as possible catalyst precursors for alcohols oxidation
under the above conditions (in the absence of added solvent and
under MW irradiation). We thus report herein the preparation
and characterization of the novel complexes [VO(hq)L] (H2L =
salicylaldehyde-2-hydroxybenzoylhydrazone, Hhq = 8-hydroxy-
quinoline) (3) and [VO(phen)L] (phen = 1,10-phenanthroline)
(4), and their use, as well as of the known related compounds
[VO(OEt)L] (1) and (NH3CH2CH2OH)+[VO2L]− (2), as catalysts for
the peroxidative oxidation of 1-phenylethanol and cyclohexanol
under green and mild reaction conditions. Those model reactions
There is also a strong interest in the design of oxidation catalysts
based on earth-abundant metals, such as vanadium. Oxidovana-
dium complexes are good candidates as catalysts for oxidation
reactions, e.g., of alkanes [28–34]. They also catalyze [35–39] (or
co-catalyze) the aerobic oxidation of secondary alcohols with good
∗
Corresponding author. Tel.: +351 218419237; fax: +351 218464455.
E-mail address: pombeiro@tecnico.ulisboa.pt (A.J.L. Pombeiro).
http://dx.doi.org/10.1016/j.apcata.2015.01.005
0926-860X/© 2015 Elsevier B.V. All rights reserved.

M. Sutradhar et al. / Applied Catalysis A: General 493 (2015) 50–57
51
Table 1
Spectroscopic characterization data for 3 and 4.
Catalyst precursor
IR (KBr, cm−1
)
1H NMR
51V NMR
ESI-MS(+)
ꢀmax
m/z
(CH3CN, nm (ε,
L M−1 cm−1))
(DMSO-d6, ı)
3
4
1599 ␯(C N), 1257
␯(C O)enolic, 1103
␯(N N), 955 ␯(V O)
11.12 (s, 1H,OH), 8.59
(s, 1H, CH N),
8.26–6.78 (m, 14H,
C6H4 and C9H6NO)
–
−478
466 [3+H]+
(100%)
552 (4480), 331
(13,440), 273 (21,280),
240 (34,720).
1603 ␯(C N), 1252
␯(C O)enolic, 1056
␯(N N), 961 ␯(V O)
–
502 [4+H]+
(100%)
701 (41.2), 412
(15,600), 351 (18,540),
269 (45,320).
mode (capillary voltage = 80–105 V). The 1H spectra were recorded
at room temperature on a Bruker Avance II+300 (UltraShieldTM
Magnet) spectrometer operating at 300.130 MHz for proton. The
chemical shifts are reported in ppm using tetramethylsilane as the
internal reference. 51V NMR spectra were recorded on a Bruker 400
UltraShield spectrometer at ambient temperature (297 K) in DMSO-
d6. The vanadium chemical shifts are quoted relative to external
[VOCl3]. The UV–Vis absorption spectra of acetonitrile solutions of
3 (1.12 × 10−5 M) and 4 (1.03 × 10−5 M) in 1.00 cm quartz cells were
recorded at room temperature on a Lambda 35 UV–Vis spectropho-
tometer (Perkin–Elmer) by scanning the 200–1000 nm region at a
are justified by their importance in organic synthesis and chemical
industry [3,49].
2. Experimental
2.1. Preparation of catalyst precursors
The
Schiff
base
pro-ligand
salicylaldehyde-2-
hydroxybenzoylhydrazone (H2L) was prepared by a reported
method [50,51] from condensation of 2-hydroxybenzoylhydrazide
with salicylaldehyde. The catalyst precursors [VO(OEt)L] (1) and
(NH3CH2CH2OH)+[VO2L]− (2) were synthesized according to lit-
erature methods [50–52]. The syntheses of the other two catalyst
precursors, the oxidovanadium(V) complex [VO(hq)L] (3) and the
oxidovanadium(IV) complex [VO(phen)L] (4), are described below.
Oxidovanadium(V) complex [VO(hq)L] (3) – To a 30 mL acetoni-
trile suspension of H2L (0.256 g, 1.00 mmol), 0.265 g (1.00 mmol) of
[VO(acac)2] was added and the reaction mixture was refluxed for
1 h in an oil bath, in open air. To this mixture 0.145 g (1.00 mmol) of
8-hydroxyquinoline (Hhq) was added and the reflux was continued
for another hour. The resultant dark violet solution was filtered and
the filtrate was kept in air. After ca. 2 d, X-ray quality dark violet
single crystals were isolated, washed 3 times with cold acetonitrile
and dried in open air. Yield 76% (0.353 g, based on vanadium). Anal.
Calcd for C23H16N3O5V: C, 59.37; H, 3.47; N, 9.03. Found: 59.28; H,
3.39; N, 8.91. Spectroscopic and ESI-MS data are given in Table 1.
Oxidovanadium(IV) complex [VO(phen)L] (4) – This complex was
isolated as orange red crystals using the procedure adopted in the
preparation of 3 but with 1,10-phenanthroline (phen) instead of 8-
hydroxyquinoline (Hhq). Yield 72% (0.360 g, based on vanadium).
Anal. Calcd for C26H18N4O4V: C, 62.28; H, 3.62; N, 11.17. Found:
61.96; H, 3.56; N, 11.09. Spectroscopic and ESI-MS data are given
in Table 1.
rate of 240 nm min−1
.
2.2.2. X-ray measurements
X-ray single crystals of complexes 3 and 4 were immersed in
cryo-oil, mounted in Nylon loops and measured at a tempera-
ture of 150 (3) or 296 K (4). Crystals of 4 were of bad quality
and very weakly diffracting (see Supplementary Information file).
Intensity data were collected using a Bruker AXS-KAPPA APEX II
or Bruker APEX-II PHOTON 100 with graphite monochromated
Mo-K␣ (ꢀ 0.71073) radiation. Data were collected using omega
scans of 0.5◦ per frame and full sphere of data were obtained.
Cell parameters were retrieved using Bruker SMART [53] software
and refined using Bruker SAINT [53] on all the observed reflec-
tions. Absorption corrections were applied using SADABS [53].
Structures were solved by direct methods by using the SHELXS-
97 package [54] and refined with SHELXL-97 [54]. Calculations
were performed using the WinGX System-Version 1.80.03 [55].
The hydrogen atoms were inserted in calculated positions. Least
square refinements with anisotropic thermal motion parameters
for all the non-hydrogen atoms and isotropic for the remaining
atoms were employed. There were disordered solvents present
in the structures of both compounds. Since no obvious major site
occupations were found for those molecules, it was not possible to
model them. PLATON/SQUEEZE [56] was used to correct the data
and potential volume of 119 (3) and 221 (4) Å3 were found with,
respectively, 30 and 66 electrons per unit cells worth of scatter-
ing. These were removed from the model, but not included in the
empirical formula. Crystallographic data are summarized in Table 2
and selected bond distances and angles are presented in Table 3.
CCDC 1005764 (3) and 1005765 (4) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
2.2. Characterization of catalyst precursors
2.2.1. General materials and procedures
All synthetic work was performed in air. The reagents and sol-
vents were obtained from commercial sources and used as received,
i.e., without further purification or drying. Complexes 1 and 2
were synthesized according to the reported procedure [50–52].
[VO(acac)2] was used as the metal source for the synthesis of com-
plexes 3 and 4. C, H, and N elemental analyses were carried out
by the Microanalytical Service of the Instituto Superior Técnico.
Infrared spectra (4000–400 cm−1) were recorded on a BRUKER VER-
TEX 70 or Jasco FT/IR-430 instrument in KBr pellets, wavenumbers
are in cm−1. Mass spectra were run in a Varian 500-MS LC Ion Trap
Mass Spectrometer equipped with an electrospray (ESI) ion source.
For electrospray ionization, the drying gas and flow rate were opti-
mized according to the particular sample with 35 p.s.i. nebulizer
pressure. Scanning was performed from m/z 100 to 1200 in ace-
tonitrile solution. The compounds were observed in the positive
2.3. Catalytic tests
2.3.1. Solvent-free microwave-assisted oxidation of secondary
alcohols
The catalytic tests under MW irradiation were performed in a
focused microwave Anton Paar Monowave 300 discover reactor
(25 W), using a 10 mL capacity reaction tube with a 13 mm internal

52
M. Sutradhar et al. / Applied Catalysis A: General 493 (2015) 50–57
Table 2
(1–100 ␮mol, 0.02–2 mol% vs. substrate) and a 70% aqueous solu-
tion of ButOOH or H2O2 (10 mmol) were introduced in the tube.
This was then placed in the microwave reactor and the system
was stirred and irradiated (25 W) for 0.25–1 h at 80 ◦C. After the
reaction, the mixture was allowed to cool down to room tem-
perature. 300 ␮L of benzaldehyde (internal standard) and 5 mL of
NCMe (to extract the substrate and the organic products from the
reaction mixture) were added. The obtained mixture was stirred
during 10 min and then a sample (1 ␮L) was taken from the organic
phase and analyzed by GC using the internal standard method.
Blank tests indicate that only traces (<0.7%) of acetophenone (or
cyclohexanone) are generated in a V-free system.
Crystal data and structure refinement details for complexes 3 and 4.
3
4
Empirical formula
Formula Weight
Crystal system
Space group
C23H16N3O5V
465.33
Triclinic
C26H18N4O4V
501.38
Triclinic
P-1
P-1
a/Å
b/Å
c/Å
␣/◦
ˇ/◦
8.1264 (10)
11.8080 (15)
12.5868 (15)
112.746 (5)
101.275 (8)
96.266 (5)◦
1069.3 (2)
2
12.3188 (12)
12.5684 (12)
16.4642 (15)
85.024 (5)
71.596 (4)
88.915 (4)
2409.5(4)
4
␥/◦
V (Å3)
Z
Dcalc (g cm−3
)
1.445
0.504
1.382
0.451
3. Results and discussion
ꢁ(Mo K˛) (mm−1
)
Rfls. collected/unique/observed
Rint
19,619/3892/2841
0.0673
0.0679, 0.1834
1.037
28,101/8122/2228
0.2054
0.1299, 0.3363
0.927
3.1. Synthesis
Final R1a, wR2b (I ≥ 2ꢂ)
Goodness-of-fit on F2
To examine the possible catalytic activity of oxidovanadium
complexes with an aroylhydrazone ligand towards solvent-free
MW-assisted oxidation of secondary alcohols under mild reaction
conditions, we have used the vanadium complexes (1–4). Two of
the complexes (1 and 2) were already known and were synthe-
sized by a reported method [50–62], whereas the other two (3 and
4) were newly synthesized and are now reported (Scheme 1).
Complexes [VO(hq)L] (3) and [VO(phen)L] (4) are pre-
pared by reaction of [VO(acac)2] (acac− = acetyleacetonate)
in refluxing CH3CN with the Schiff base salicylaldehyde-
2-hydroxybenzoylhydrazone (H2L), in the presence of
8-hydroxyquinoline (Hhq) or 1,10-phenanthroline (phen), respec-
tively. They were isolated as dark violet (3) or orange red (4)
crystalline solids, and characterized by IR, NMR (in the case of 3)
and UV/Vis spectroscopies, elemental analysis, ESI-MS and single
crystal X-ray diffraction (see below). [VO(hq)L] (3) and [VO(phen)L]
(4) are hexacoordinate mixed-ligand oxidovanadium(V) and oxi-
dovanadium(IV) complexes, respectively, whereas the known
[VO(OEt)L] (1) and (NH3CH2CH2OH)+[VO2L]– (2) [59–61] are pen-
tacoordinate oxidoethoxidovanadium(V) compounds, where the
sixth coordination position is available for further coordination.
It is well known that [VO(acac)2] undergoes aerial oxidation in
solution (preferably in alcohol medium) in the presence of hydra-
zone Schiff bases [48,57,58]. Sometimes, the presence of a neutral
bidentate N,N donor auxiliary ligand in CH3CN helps to stabilize
the oxidovanadium(IV) centre, which can be isolated as a sta-
ble solid product [59,60]. Herein, we used two different bidentate
auxiliary ligands to stabilize oxidovanadium centres in 3 and 4:
the mono negative 8-hydroxyquinolinate (hq−) and the neutral
1,10-phenanthroline (phen) which stabilize the vanadium(V) and
the vanadium(IV) centre, respectively, with the tridentate hydra-
zone Schiff base [48,59,60]. The four complexes of this study (1–4)
present different coordination environments and have been used
for a comparative study of their catalytic activities towards solvent-
free MW-assisted oxidation of secondary alcohols.
a
R = ꢃ||Fo|-|Fc||/ꢃ|Fo|.
b
2
2
4
wR(F2) = [ꢃw(|Fo| - |Fc| )2/ꢃw|Fo| ]1/2
.
Table 3
Selected bond distances (Å) and angles (◦) in complex 3.
N1 V1
N3 V1
O1 V1
O2 V1
O4 V1
O10 V1
2.090(3)
2.385(3)
1.852(3)
1.948(3)
1.840(3)
1.580(3)
O10 V1 O4
O10 V1 O1
O4 V1 O1
O10 V1 O2
O4 V1 O2
O1 V1 O2
O10 V1 N1
O4 V1 N1
98.68(14)
100.17(15)
96.55(12)
95.99(14)
98.74(12)
155.76(13)
102.94(14)
158.05(13)
O1 V1 N1
O2 V1 N1
O10 V1 N3
O4 V1 N3
O1 V1 N3
O2 V1 N3
N1 V1 N3
83.44(12)
75.38(12)
171.66(14)
75.44(11)
86.51(13)
79.30(12)
82.65(12)
diameter, fitted with a rotational system and an IR temperature
detector. Gas chromatographic (GC) measurements were carried
out using a FISONS Instruments GC 8000 series gas chromatograph
with a DB-624 (J&W) capillary column (FID detector) and the Jasco-
Borwin v.1.50 software. The temperature of injection was 240 ◦C.
The initial temperature was maintained at 120 ◦C for 1 min, then
raised 10 ◦C/min to 200 ◦C and held at this temperature for 1 min.
Helium was used as the carrier gas. GC–MS analyses were per-
formed using a Perkin–Elmer Clarus 600 C instrument (He as the
carrier gas). The ionization voltage was 70 eV. Gas chromatogra-
phy was conducted in the temperature-programming mode, using
a SGE BPX5 column (30 m × 0.25 mm × 0.25 ␮m). Reaction products
were identified by comparison of their retention times with known
reference compounds, and by comparing their mass spectra to frag-
mentation patterns obtained from the NIST spectral library stored
in the computer software of the mass spectrometer.
3.2. Spectroscopic characterization of the complexes
The IR spectra of complexes 3 and 4 (Table 1) contain all the char-
acteristic bands of the coordinated tridentate anionic ligand L2− in
the enol form, viz., 1599, 1257 and 1103 cm−1 for 3 and 1603, 1252
and 1056 cm−1for 4. In addition, the ␯(V O) bands are observed at
955 and 961 cm−1for 3 and 4, respectively.
2.3.2. Typical procedures for the catalytic oxidation of alcohols
and product analysis
Oxidation reactions of the alcohols were carried out in sealed
cylindrical Pyrex tubes under focused MW irradiation as fol-
lows: the alcohol substrate (5 mmol), catalyst precursor 1–4
The 1H NMR spectrum of 3 in DMSO-d6 (Table 1) shows the
characteristic resonance of the ligand [50,51]: the proton of the
azomethine nitrogen at 8.59 ppm, the aromatic protons as multi-
plets in the range of 8.26–6.78 ppm, and the phenolic OH proton at
11.2 ppm.

M. Sutradhar et al. / Applied Catalysis A: General 493 (2015) 50–57
53
Scheme 1. Synthesis of vanadium complexes.
the auxiliary ligand (hq−) makes an angle of 87.76◦ with plane A.
The V1 N3 distance of 2.385(3) Å, with that nitrogen lying trans
to the Ooxido atom, is much longer than the V NSchiff bond length
[V1 N1 2.090(3) Å] what is due not only to the trans influence
of the Ooxido atom but also to 5-membered ring constraints. The
short V Ooxido distance [V1 O10 1.580(3) Å] is consistent with a
vanadium–oxygen double bond (V O) as it is commonly found in
five- and six-coordinate vanadium (IV) and (V) complexes [48]; the
two V Ophenolate lengths, V1 O1 from L2− and the V1 O4 from
hq− [1.852(3) and 1.840(3) Å, in this order], slightly differ and
are shorter than the V Oenolate [V1 O2 1.948(3) Å]. As previously
found [48,59], the four vanadium oxygen bond lengths in 3 follow
the order: oxido < phenolate < enolate.
Complex 3 was also identified by the 51V NMR spectroscopy
(Table 1). The spectrum shows characteristic peak at −478 ppm
corresponds to oxidiovanadium(V) centre [48].
The electronic spectral data of the oxidovanadium complexes
3 and 4 are given in Table 1. The electronic spectrum of the oxi-
dovanadium(V) complex 3 in acetonitrile solution displays a strong
absorption at 552 nm assigned to the PhO− → V(d ) LMCT transi-
␲
tion and three other intense absorption band in the 331–240 nm
region allocated to intraligand (␲ → ␲*) transitions [59]. The oxi-
dovanadium(IV) complex 4 exhibits a low intensity band at 701 nm
due to ligand field dxz, dyz → dxy transitions of the VO2+ centre [60].
The strong band observed at 412 nm is due to the Oenolate → V(d )
␲
LMCT transition. Bands centred at 350 and 269 nm are assigned to
intraligand transitions [60].
The ESI-MS spectra of 3 and 4 in acetonitrile solution (Table 1)
display the parent peaks at m/z = 466 [3+H]+ (100%) and at m/z = 502
[4+H]+ (100%) respectively, which also support the formulations.
3.3. X-ray structural characterization
The molecular structures of complexes 3 and 4 are depicted in
Figs. 1 and 2, respectively, whereas crystal data are presented in
Table 2 and selected bond distances and angles for 3 are given in
Table 3. A discussion of the low quality structure of 4 is given in the
Supplementary Information file whereas Table S1 shows distances
and angles for this compound. The dianionic Schiff base ligand
(L2−) in 3 coordinates to vanadium(V) in the enol form (Fig. 1).
The vanadium ion exhibits a distorted octahedral O4N2 coordina-
tion (quadratic elongation of 1.048 and angle variance of 104.79◦
2) in which the basal plane is made of the phenolic (O1), the eno-
lic (O2) oxygen and the imine (N1) nitrogen atoms from the L2−
ligand together with a phenolic (O4) oxygen of the auxiliary quino-
late ligand. The apical positions are occupied by the oxido (O10) and
the pyridine nitrogen (N3) atom of the quinolate. The L2− ligand
binds the vanadium ion generating six-[V1 O1 C1 C6 C7 N1]
and five-membered [V1 O2 C8 N2 N1] chelate rings.
The L2− ligand in 3 deviates slightly from planarity. Indeed, rel-
ative to the least-square plane formed by the coordinating atoms
N1, O1 and O2 (plane A), the least-square planes of the pheno-
late rings C1 > C6 and C9 > C10 make angles of 10.09 and 15.30◦,
˚
respectively. The metal cation is situated 0.230 A away from plane
Fig. 1. Ellipsoid plot for complex 3 (drawn at the 30% probability level) with atom
A and shifted towards the oxido atom; the least-square plane of
labelling scheme.

54
M. Sutradhar et al. / Applied Catalysis A: General 493 (2015) 50–57
3.4. Oxidation of secondary alcohols
Complexes 1–4 were tested as catalyst precursors for the
homogeneous oxidation of aromatic and alicyclic secondary
alcohols (1-phenylethanol and cyclohexanol were chosen as
model substrates) to the respective ketones using aqueous tert-
butylhydroperoxide (ButOOH, TBHP) as oxidizing agent, under
typical conditions of 80 ◦C, low power (25 W) microwave irradia-
tion (MW), 30 min reaction time and in a solvent- and additive-free
medium (Scheme 2).
Other C6 linear secondary alcohols (Table 5) were also tested as
substrates, and the ketones are the only detected oxidation prod-
ucts. The effects of various reaction parameters, such as, the type
and amount of oxidant, time, temperature, amount of catalyst pre-
cursor and presence of additives, were investigated and the results
are summarized in Tables 4 and 5 and Figs. 3–6.
Under typical solvent- and additive-free conditions (Table 4) all
catalyst precursors 1–4 exhibit a high activity [TON (TOF) values up
to 1.8 × 103 (3.6 × 103 h−1) (3)], leading to yields (based on the alco-
hol) of acetophenone and cyclohexanone in the ranges of 77–94%
and 30–38%, respectively. A high selectivity towards the formation
of ketones was displayed by these MW-assisted transformations,
since no traces of by-products were detected by GC–MS analysis of
the final reaction mixtures (only the unreacted alcohol was found,
apart from the ketone).
Fig. 2. Ellipsoid representation of one of the molecules of complex 4 (drawn at the
30% probability level) with atom labelling scheme.
Table 4
MW-assisted solvent-free oxidation of selected aromatic and alicyclic secondary alcohols using 1–4 as catalyst precursors.a
c
Entry
Catalyst
Substrate
Yieldb (%)
TOF (h−1
)
Selectivity (%)d
precursor
1
2
1
2
1-Phenylethanol
Cyclohexanol
94.3
36.1
943
361
98
90
3
4
1-Phenylethanol
Cyclohexanol
77.1
37.6
771
376
94
92
5
92.6
36.3
90.9
30.3
12.3
30.1
926
97
93
86
91
83
72
6e
7f
8
1-Phenylethanol
Cyclohexanol
3.64 × 103
3
4
91
303
9e
10f
1.2 × 103
30
11
12
1-Phenylethanol
Cyclohexanol
86.4
35.4
863
354
92
87
Reaction conditions unless stated otherwise: 5 mmol of substrate, 10 ␮mol (0.2 mol% vs. substrate) of 1–4, 10 mmol of TBHP (2 eq., 70% in H2O), 80 ◦C, 30 min of MW
a
irradiation (25 W power).
b
Moles of ketone product per 100 moles of alcohol.
TOF = number of moles of product per mol of catalyst precursor (TON) per hour.
Moles of ketone per mole of converted substrate.
1 ␮mol (0.02 mol% vs. substrate) of 3.
c
d
e
f
100 ␮mol (2 mol% vs. substrate) of 3.
Table 5
MW-assisted solvent-free oxidation of selected linear secondary alcohols using 1–4 as catalyst precursorsa.
c
Entry
Substrate
Catalyst precursor
Yieldb (%)
TOF (h−1
)
Selectivity (%)d
1
7
8
9
1
2
3
4
29.4
40.2
35.6
31.9
59
80
71
64
79
87
82
67
2-Hexanol
11
13
19
20
1
2
3
4
32.3
32.8
28.9
28.1
65
66
58
56
84
79
73
52
3-Hexanol
a
Reaction conditions unless stated otherwise: 5 mmol of substrate, 10 ␮mol (0.2 mol% vs. substrate) of 1–4, 10 mmol of TBHP (2 eq., 70% in H2O), 80 ◦C, 2 h 30 min of MW
irradiation (25 W power).
b
Moles of ketone product per 100 moles of alcohol.
TOF = number of moles of product per mol of catalyst precursor (TON) per hour.
Moles of ketone per mole of converted substrate.
c
d

M. Sutradhar et al. / Applied Catalysis A: General 493 (2015) 50–57
55
Fig. 5. Influence of the temperature on the yield of acetophenone or cyclohexanone
obtained by MW-assisted oxidation of 1-phenylethanol in the presence of 3 (
or cyclohexanol in the presence of 4 ( ) (0.5 h reaction time).
)
Scheme 2. Solvent-free homogeneous oxidation of 1-phenylethanol and cyclo-
hexanol to acetophenone and cyclohexanone, respectively, by the 1–4/TBHP/MW
system.
Fig. 3. Influence of different additives (TEMPO, K2CO3, HNO3, radical traps) on the
Fig. 6. Influence of the type of the oxidant on the yield of acetophenone (
)
yield of acetophenone ( ) or of cyclohexanone ( ) obtained from MW-assisted
peroxidative oxidation of 1-phenylethanol in the presence of 1 or of cyclohexanol
in the presence of 2.
or cyclohexanone (
) obtained by MW-assisted oxidation of the corresponding
alcohol in the presence of 1–4.
As expected, the aliphatic cyclohexanol is less reactive than
1-phenylethanol leading, under the same reaction conditions, to
moderate yields (Table 4), as reported in other cases [22,23,61]. In
contrast with the comparable yields obtained for cyclohexanone
by MW-assisted peroxidative oxidation of cyclohexanol with the
various catalyst precursors 1–4 (entries 2, 4, 8 and 11, respectively,
Table 4), the oxidation of 1-phenylethanol, under the same condi-
tions, appears to be more sensitive to the metal catalyst precursor.
The best yields were obtained in the presence of the oxidovana-
dium(V) complexes 1 (94%, entry 1, Table 4) and 3 (93%, entry 5,
Table 4). They were followed by the oxidovanadium(IV) 4 (86%,
entry 11, Table 4) and then by the anionic dioxido-V(V) 2 (77%,
entry 3, Table 4). To achieve considerably better conversions of
2- or 3-hexanol, although still moderate, to the corresponding
ketones (selectivities of 52–87%) an extended reaction time (2.5 h)
is required (see Table 5 and Fig. 4b).
Addition of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) rad-
ical, an efficient mediator for the aerobic oxidation of alcohols
[23,24,61–65], although scarcely used for the peroxidative oxida-
tion [35,43,66,67] of those substrates, provided almost quantitative
formation of acetophenone (Fig. 3) and a significant increase in
cyclohexanone yield (from 38 to 58% (2), Fig. 3). It should be noted
Fig. 4. Yield of ketones (obtained from MW-assisted peroxidative oxidation of the corresponding alcohols) along the time: (a) acetophenone in the presence of 1(
cyclohexanone in the presence of 2 ( ); (b) 2-hexanone in the presence of 3 ( ) and 3-hexanone in the presence of 4 ( ).
) and

56
M. Sutradhar et al. / Applied Catalysis A: General 493 (2015) 50–57
that reactions carried out with TEMPO, but in the absence of 1–4,
led to very low ketone yields (<4%, see, e.g., entries 4 and 8, Table
S2).
The peroxidative oxidation of the tested secondary alcohols is
believed to proceed mainly via a radical mechanism which involves
both carbon- and oxygen-centred radicals [76–78], in view of the
strong inhibition effect observed when it is carried out in the pres-
ence of either a carbon-radical trap such as CBrCl3 (Fig. 3) or an
oxygen-radical trap such as Ph2NH (Fig. 3). It may involve e.g.,
tBuO• and tBuOO• radicals produced in the V promoted decom-
position of TBHP [79,80] according to the Eqs. (1)–(6), where
VOn+ represent arylhydrazone derived oxido-vanadium species.
The oxidoperoxido species [VO(OO)L]−, conceivably involved in the
catalytic process, was detected by ESI-MS (see SI).
To our knowledge, no MW-assisted TBHP/oxidovanadium cat-
alytic system has been previously reported and this study
demonstrates its viability for the peroxidative oxidation of alco-
hols. A favourable effect of MW irradiation was observed, even
applying the low power of 25 W, as already reported for other sys-
tems [27]. For example, only 12% of product was obtained under
the same conditions of those adopted for 1 (Table 4, entry 1) but
using conventional heating (an oil-bath). Moreover, an increase
of the acetophenone yield to 89% required 15 h of reaction with
conventional heating, in the presence of 1.
The present catalytic system presents several advantages over
those using conventional heating. For example, it allows the fast
(30 min.) quantitative formation of acetophenone (Fig. 4) whereas,
e.g., the 3 h oxidation of 1-phenylethanol with TBHP (5 M in decane)
in the presence of a silica supported oxidovanadium Schiff base
[41] leads to a maximum 90% yield of acetophenone [41]. How-
ever, the latter system leads to higher yields (93%) of cyclohexanone
from cyclohexanol [41]. Aerobic (by O2) conversions (up to 90%)
of cyclohexanol to cyclohexanone were previously obtained in the
presence of N-hydroxyphthalimide as a radical producing agent
and the co-catalyst [VO(acac)2] [38], although requiring at least
18 h reaction time.
A noteworthy feature of 1–4 concerns the relatively low loading
(0.2 mol% vs. substrate) necessary to reach high yields of ace-
tophenone (i.e. up to 99% (1 or 3) in the presence of TEMPO)
with substantial TOF values of 987 and 993 h−1, respectively. The
effect of the amount of catalyst precursor 3 was studied for the
1-phenylethanol (entries 5–7, Table 4) and cyclohexanol (entries
8–10, Table 4) oxidations. Its increase from 1 ␮mol (0.02 mol% vs.
substrate) to 10 ␮mol (0.2 mol% vs. substrate) results in a yield
enhancement from 36 to 93% (acetophenone) or from 12 to 30%
(cyclohexanone). However, beyond 10 ␮mol of catalyst, the yield
remained almost unchanged, leading to the expected TON lowering
(compare e.g. entries 5 and 7 or 8 and 10, Table 4).
Blank tests (in the absence of any catalyst precursor) were
performed under common reaction conditions and no significant
conversion was observed (<0.7%, see, e.g., entries 1 and 5, Table
S2). Moreover, the use of [VO(acac)2] (the starting complex for the
catalytic precursors) or of V2O5, instead of our vanadium catalysts,
resulted in much lower yields (e.g., 47% or 13% for 1-phenylethanol,
and 24% or 9% for cyclohexanol, respectively, Table S2, entries 9–10,
versus 94% for 1-phenylethanol and 36% for cyclohexanol in the case
of 1, Table 4, entry 1).
t
t
VO3+ + BuOOH → VO2+ + BuOO• + H+
(1)
(2)
(3)
(4)
(5)
(6)
t
t
VO2+ + BuOOH → VO3+ OH + BuO•
VO3+ OH + BuOOH → VO3+ OO tBu + H2O
tBuO• + R2CHOH → BuOH + R2C• OH
tBuOO• + R2CHOH → BuOOH + R2C• OH
VO3+ OO Bu + R2C• OH → R2C O + BuOOH + VO2+
t
t
t
t
t
It may also proceed via the coordination of the alcohol substrate
to an active site of the catalyst, and its deprotonation to form the
alkoxide ligand, followed by a metal-centred (and TEMPO assisted)
dehydrogenation [68,71,72].
Other important features of the 1–4/TBHP/MW system concern
the use of weak MW irradiation (25 W) and of solvent- and additive-
free oxidation conditions, which contrast with the common use
of organic solvents or costly ionic liquids in many state-of-the-art
methods for the oxidation of alcohols [1–4,17–19,25,26].
4. Conclusions
In this study, we have successfully explored the catalytic
activity of four different types of vanadium complexes, i.e., an
oxidoethoxidovanadium(V) (1), a salt like dioxidovanadium (2),
a mixed-ligand oxidovanadium(V) (3) and a mixed-ligand oxi-
dovanadium(IV) complex (4) towards MW-assisted homogeneous
oxidation of secondary alcohols. 1–4 act as efficient and selective
catalyst precursors for the mild MW-assisted oxidation of sec-
ondary alcohols (1-phenylethanol and cyclohexanol) in solvent-
and additive-free systems. A comparative study of their catalytic
efficiency has been drawn. Cyclohexanone is obtained in compa-
rable yields in the presence of 1–4, while the oxidovanadium(V)
complexes 1 and 3 are the most efficient ones for the peroxidative
oxidation of 1-phenylethanol.
The optimal reaction temperature is 80 ◦C as depicted in Fig. 5 for
the MW-assisted oxidations of 1-phenylethanol and cyclohexanol,
with 3 or 4, respectively. Attempts to perform the oxidation at room
temperature failed and the minimum required temperature is ca.
45 ◦C. Moreover, temperatures above 80 ◦C do not lead to higher
ketone yields. The overall temperature coefficient of the oxidations
is ca. 2.2 (temperature range of 50–80 ◦C).
Experiments with the cheaper and environmentally friendly
hydrogen peroxide (30% aqueous solution) as oxidant are less effec-
tive, as attested by the marked yield lowering, e.g. from 94% to 26%
(1) (Fig. 6), in accord with the expected decomposition of H2O2
under the used reaction conditions (80 ◦C). Moreover, the use of
higher amounts of oxidant does not lead to a better conversion.
The previously recognized promoting effect of basic additives,
which facilitate the deprotonation of the alcohol [61,62,69–75], was
not detected for the present systems. In contrast, a strong inhibitor
effect of the catalytic activity (Fig. 3) was observed for the reac-
tions carried out in the presence of 1 M K2CO3 solution. Moreover,
the presence of HNO3 also exhibited an inhibitory effect on the
acetophenone yield (Fig. 3).
The application, for the first time, of
a MW-assisted
TBHP/oxidovanadium complex catalytic system for the oxidation of
secondary alcohols widens the scope of peroxidative catalytic sys-
tems suitable for MW-assisted oxidative transformations of such
substrates. Moreover, a cooperative action of the radical TEMPO
with 1–4 towards the peroxidative oxidation of the tested alcohols
was found. This study should open a new window for the catalytic
application of vanadium complexes in solvent- and additive-free
systems.
Supporting information
Description of the crystal structure of compound 4 is given
as electronic supplementary information. CCDC 1005764 (3) and
1005765 (4) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data request/cif.

M. Sutradhar et al. / Applied Catalysis A: General 493 (2015) 50–57
57
Acknowledgments
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We are grateful for the financial support received from the
Fundac¸ ão para a Ciência e a Tecnologia (FCT), Portugal, and its
projects PTDC/EQU-EQU/122025/2010 and PEst-OE/QUI/UI0100/
2013. M.S. acknowledges the FCT, Portugal, for a postdoctoral fel-
lowship (SFRH/BPD/86067/2012).
Appendix A. Supplementary data
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