Highly efficient divanadium(V) pre-catalyst for mild oxidation of liquid and gaseous alkanes

Article abstract of DOI:10.1016/j.apcata.2013.03.034

A new binuclear oxovanadium(V) complex bearing an NO₂-donor Schiff base ligand, [{VO(EtO)(EtOH)}₂(1κ²O,κN: 2κ²O,κN-L)]·2H₂O (H₄L = bis(2-hydroxybenzylidene)oxalohydrazonic acid) was prepared and fully characterized by IR, ¹H NMR and electronic spectroscopies, elemental analysis and single crystal X-ray diffraction. In the presence of 2-pyrazinecarboxylic acid (PCA) or another acid promoter, this compound acts as a highly efficient pre-catalyst towards the oxidation of gaseous and liquid alkanes by aqueous H₂O₂ under mild conditions, in aqueous MeCN. Total yields of oxygenates up to 45% and overall turnover numbers up to 7.8 × 10³ are achieved.

Full text of DOI:10.1016/j.apcata.2013.03.034

Accepted Manuscript
Title: Highly efficient divanadium(V) pre-catalyst for mild
oxidation of liquid and gaseous alkanes
Author: <ce:author id="aut0005"> Samik Gupta<ce:author
id="aut0010"> Marina V. Kirillova<ce:author id="aut0015">
´
M. Fatima Guedes da Silva<ce:author id="aut0020">
Armando J.L. Pombeiro<ce:footnote
id="fn0005"><ce:note-para id="npar0055">Fax: +351 21 846
4455</ce:note-para></ce:footnote>
PII:
S0926-860X(13)00165-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2013.03.034
APCATA 14156
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
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1-12-2012
13-3-2013
26-3-2013
Please cite this article as: S. Gupta, M.V. Kirillova, M.F.G. Silva, A.J.L.
Pombeiro, Highly efficient divanadium(V) pre-catalyst for mild oxidation
of liquid and gaseous alkanes, Applied Catalysis A, General (2013),
http://dx.doi.org/10.1016/j.apcata.2013.03.034
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Highly efficient divanadium(V) pre-catalyst for mild oxidation of
liquid and gaseous alkanes
Samik Gupta,a Marina V. Kirillova,a* M. Fátima Guedes da Silva,a,b*
Armando J. L. Pombeiroa*
a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Technical University of Lisbon,
Av. Rovisco Pais, 1049-001 Lisbon, Portugal;
b
Universidade Lusófona de Humanidades e Tecnologias, ULHT Lisbon, Av. do Campo Grande, 376,
1749-024, Lisbon, Portugal.
FOOTNOTE:
* Corresponding authors. Fax: (+351 21 846 4455 (M.V. Kirillova, M.F.C. Guedes da Silva, A.J.L.
Pombeiro).
E-mail addresses: kirillova@ist.utl.pt (M.V. Kirillova); fatima.guedes@ist.utl.pt (M.F.C. Guedes da
Silva), pombeiro@ist.utl.pt (A.J.L. Pombeiro).
Highlights
New binuclear V(V) complex bearing an NO2-donor Schiff base ligand is synthesized
This compound is highly active catalyst precursor in the oxidation of alkanes by H2O2
2-Pyrazinecarboxylic acid acts as an efficient co-catalyst
High yields (up to 45% based on alkanes) and TONs (up to 7800) are achieved
Oxidation proceeds via radical mechanism with participation of hydroxyl radicals
Abstract
A new binuclear oxovanadium(V) complex bearing an NO2-donor Schiff base ligand,
[{VO(EtO)(EtOH)}2(12O,N:22O,N-L)]·2H2O
(H4L
=
bis(2-
hydroxybenzylidene)oxalohydrazonic acid) was prepared and fully characterized by IR,
1H NMR and electronic spectroscopies, elemental analysis and single crystal X-ray
diffraction. In the presence of 2-pyrazinecarboxylic acid (PCA) or another acid
promoter, this compound acts as a highly efficient pre-catalyst towards the oxidation of
gaseous and liquid alkanes by aqueous H2O2 under mild conditions, in aqueous MeCN.
Total yields of oxygenates up to 45% and overall turnover numbers up to 7.8 × 103 are
achieved.
Page 1 of 26

Keywords: C−H activation, Schiff bases, Hydrogen peroxide, Hydroxyl radical, 2-
Pyrazinecarboxylic acid
1.
Introduction
The interest on vanadium chemistry has grown in recent years due to the
relevance of this metal in biology [1–5] and catalysis [6–14]. Being involved in many
different biological and synthetic processes, vanadium may exist in a variety of
oxidation states, what explains the wide application of its compounds as efficient
catalysts in different organic reactions [6–38]. For example, vanadium containing
compounds are well known to efficiently catalyze oxidative (by H2O2 and O2)
transformations of alkanes to alkyl hydroperoxides, alcohols and ketones [11–30], and
2−
carboxylations (by the CO/S2O8 /TFA system) of hydrocarbons, including the highly
inert methane, to carboxylic acids [30–37]. Such transformations are of industrial
significance, since they contribute to the development of synthetic protocols based on
alkanes as primary carbon sources [38–40].
Following our interest on alkane functionalization, we have recently reported a
number of effective vanadium based catalytic systems towards the carboxylation [30–
37] and oxidation [18–30] of alkanes. In the latter reaction, the activity of V catalysts
can be dramatically enhanced by addition of a small amount of an acid promoter, such
as nitric [18,19], sulfuric [20], oxalic [20] or 2-pyrazinecarboxylic acid (PCA)
[21,22,29]. In particular, the use of PCA as an effective promoter for oxidations with
H2O2-vanadium catalytic systems was discovered by Shul’pin et al. [41]. Further studies
−
have shown that PCA in combination with VO3 anion provides one of the most
efficient and versatile catalytic systems for the mild oxidation of both gaseous and
liquid alkanes [12–14,17,21,42]. The promoting effect of PCA in alkane oxidations by
H2O2 catalyzed by different vanadium derivatives, as well as by other metals (Fe, Mn,
−
Cu and Re), is now well recognized [43]. The role of PCA in VO3 /H2O2 assisted alkane
oxidations has been disclosed on the basis of DFT calculations [21], consisting in
stabilizing the transition state of H-transfer in the ‘‘water-assisted” mechanism of free
radical generation. Moreover, the acidity of the reaction solution leads to the formation
of polyvanadates, which exhibit higher catalytic activity in comparison with
monovanadium species [22].
In this context, we report here a new binuclear oxovanadium(V) complex
bearing a NO2-donor Schiff base ligand, [{VO(EtO)(EtOH)}2(12O,N:22O,N-
Page 2 of 26

L)]·2H2O (1), which is easily synthesized from [VO(acac)2] and bis(2-
hydroxybenzylidene)oxalohydrazonic acid (H4L). The obtained product 1 was fully
characterized by standard methods and by single-crystal X-ray diffraction analysis.
Compound 1 was found to efficiently catalyze, in the presence of PCA co-catalyst, the
oxidation, by aqueous H2O2 under mild conditions, of different alkanes mainly to alkyl
hydroperoxides, alcohols and ketones, apart from other minor oxidation products. The
effects of catalyst concentration, type of acid promoter and its amount on the reaction
rate of cyclohexane oxidation were studied in detail. In order to get some mechanistic
information, the KIE (kinetic isotope effect) as well as various selectivity parameters
were determined in the oxidations of linear, branched and cyclic alkanes. The catalytic
activity of the newly synthesized complex was compared with that of a simple vanadate
salt under identical experimental conditions.
2. Experimental
2.1. Materials and general methods
Diethyl oxalate, Hydrazine hydrate (80%, aqueous solution), salicylaldehyde and
vanadyl acetylacetonate were all purchased from Aldrich and used as received. Solvents
were also used without further purification. The compound H4L, bis(salicylhydrazone)
of oxalyl hydrazide, was prepared by modification of a previously reported method [44].
Infrared spectra (4000–400 cm-1) were recorded on a Nicolet Impact 400D or a BIO-
RAD FTS 3000 MX spectrophotometer instrument in KBr pellets; wavenumbers are in
cm-1; abbreviations: vs, very strong; s, strong; m, medium. UV-Vis spectra were
recorded in 10–4 M EtOH solutions of the complexes with a Perkin Elmer instrument
(Lambda 35). Electrospray mass spectra (ESI-MS) were run with an ion-trap instrument
(Varian 500-MS LC Ion Trap Mass Spectrometer) equipped with an electrospray ion
source. For electrospray ionization, the drying gas and flow rate were optimized
according to the particular sample with 35 p.s.i. nebulizer pressure. Scanning was
performed from m/z 50 to 1000 in EtOH or CH3CN (in the catalytic studies) solution.
The compound was observed in both positive and negative modes (capillary voltage =
80–105 V). EPR spectra were recorded with a Bruker ESP 300E X-band spectrometer.
2.2. Synthesis of the H4L pro-ligand
Page 3 of 26

Synthesis of the oxalyl dihydrazide was carried out from diethyl oxalate and
hydrazine hydrate according to a literature method [44]. A modification was made in the
preparation of the Schiff base with salicylaldehyde. To an aqueous solution of 236 mg
(2 mmol) of oxalyl dihydrazide, 488 mg salicylaldehyde (4 mmol) were added; the
obtained suspension was stirred for about 4 h at room temperature. Within this time, a
yellowish white precipitate of the Schiff base H4L appeared. This was filtered off,
washed with ethanol and ether and left for slow evaporation at r.t.
1
Molecular Formula: C16N4O4H12, Yield 554.2 mg (85% ); mp 321–322 °C; H NMR
[300.13 MHz, (CD3)2SO], δ: 6.79–7.57 (m, 8H), 8.81 (s, 2H), 11.00 (s, 2H), 12.63 (s,
2H).
2.3. Synthesis of complex 1
40.75 mg (0.125 mmol) of H4L were added to a round bottom flask containing 20
mL of ethanol. Then, 66.25 mg (0.25 mmol) of [VO(acac)2] were added and the system
was left with vigorous stirring for 24 h at 50˚C. Within this time a brown transparent
solution was obtained, which was filtered off, and left for slow evaporation at r.t. Within
a week brown block like crystals appeared which were filtered off, washed with ethanol
and dried over fused calcium chloride to give 1.
Complex 1: [{VO(EtO)(EtOH)}2(12O,N:22O,N-L)]·2H2O: Yield 63.0 mg
(75%). Brown solid soluble in EtOH, CHCl3, CH2Cl2, etc. Anal. Calc. for C24H36
N4O12V2 (F.W. = 674.45): C, 42.70; H, 5.33; N, 8.30. Found: C, 42.27; H, 5.06; N, 8.27.
MS (ESI): m/z: 519.0 [{V2O2L(OEt)}2+H2O] 2+ IR (KBr, selected bands, cm-1): 3201 vs
1
ν(OH) (coordinated), 1548 s ν(C=C), 1605 vs ν(C=N), 754 w (Ar), 980 s ν(V=O), H
NMR (300.13 MHz, CDCl3), δ: 1.25(t, 12H, CH3CH2OH and CH3CH2O), 1.80 (broad,
H2O H-bonded and CH3CH2OH) 3.7(q, 4H, CH3CH2OH,), 5.65(q, 4H, CH3CH2O),
7.20–7.60 (m, 8H, Ar-H) , 9.22(s, 2H, C(H)=N ), UV-vis (EtOH, λmax, nm (ε, L M-1
cm-1): 419(18600), 316(41000).
2.4. Crystallographic measurements
The X-ray quality single crystal of complex 1 was mounted in a Nylon loop and
measured at a temperature of 293 K (Table 1). Intensity data were collected using a
Bruker AXS-KAPPA APEX II diffractometer with graphite monochromated Mo-Kα (
Page 4 of 26

0.71073) radiation. Data were collected using omega scans of 0.5º per frame and full
sphere of data were obtained. Cell parameters were retrieved using Bruker SMART
software and refined using Bruker SAINT [45] on all the observed reflections.
Absorption corrections were applied using SADABS [45]. Structure was solved by
direct methods by using the SHELXS–97 package [46] and refined with SHELXL–97
[47]. Calculations were performed using the WinGX System–Version 1.80.03 [48]. All
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.
CCDC-913164 contains 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.
2.5. Peroxidative oxidation of alkanes
The alkane oxidations were typically carried out in air in thermostated (50 ºC)
Pyrex cylindrical vessels or round bottom flasks with vigorous stirring and using MeCN
as solvent (up to 5.0 mL total volume). Typically, the catalyst precursor 1 was
introduced into the reaction mixture in the form of a stock solution in acetonitrile (2.5 ×
10−4 M). Then PCA (optional) was added as a solid or in the form of a stock solution in
MeCN (0.44 M). The alkane substrate, typically cyclohexane (0.25 mL, 2.3 mmol) was
then introduced, and the reaction started when hydrogen peroxide (50% in H2O, 0.68
mL, 11 mmol) was added in one portion. The final concentrations of the reactants in the
reaction mixture were as follows: catalyst precursor 1 (5 × 10−6 − 5 × 10−4 M), PCA
(0−0.005 M), substrate (0.46 M) and H2O2 (2.2 M). The samples were analyzed by GC
using nitromethane (0.05 mL) as an internal standard. Attribution of peaks was made by
comparison with chromatograms of authentic samples. Chromatographic analyses were
undertaken by using a Fisons Instruments GC 8000 series gas chromatograph (He as
carrier gas) with a BP20/SGE (30 m × 0.22 mm × 0.25 μm) capillary column (FID
detector) and the Jasco-Borwin v.1.50 software.
Since the oxygenation of alkanes often gives rise to the formation of the
corresponding alkyl hydroperoxides as the main primary products, the quantification
was performed by a method developed by Shul'pin [12−14]. For precise determination
of oxygenate concentrations only data obtained after reductions of the reaction sample
Page 5 of 26

with PPh3 were usually used, taking into account that the original reaction mixture
typically contained the three products: alkyl hydroperoxide (as the primary product),
ketone and alcohol.
The oxidations of gaseous alkanes were carried out in a 13 mL stainless steel
autoclave, equipped with a Teflon-coated magnetic stirring bar. In a typical experiment,
after additions of all liquid (in the form of stock solutions in acetonitrile) and solid
reagents, the autoclave was closed and pressurized with 0.7–20.0 atm of gaseous alkane
(typically 20.0, 6.0, or 0.7 atm of CH4 and C2H6, C3H8, or n-C4H10, respectively). The
reaction mixture was stirred for 4 h at 50 °C using a magnetic stirrer and an oil bath,
whereupon it was cooled in an ice bath, degassed, opened and transferred to a flask for
GC analysis. In the oxidations of gaseous alkanes, traces of acetic acid were also
detected due to partial oxidation of the MeCN solvent.
The kinetic isotope effect (KIE) was calculated as a ratio between the reaction
rates W0 of C6H12 and C6D12 oxidations under the following reaction conditions:
[C6H12]0 or [C6D12]0 = 0.46 M, [1]0 = 5 × 10-5 M, [PCA]0 = 5 × 10-3 M, [H2O2]0 = 1.1 M
in CH3CN (up to 5 mL of the total reaction volume) at 50°C.
3.
Results and discussion
3.1. Synthesis and spectroscopic characterization
The H4L pro-ligand was prepared (Scheme 1) by a modification of a method
described in the literature [44]. For this Schiff base condensation reaction we have used
water as the reaction medium, therefore breaking the general procedure of using
methanol or ethanol. Moreover, the preparation of complex 1 has been carried out using
[VO(acac)2] as the metal precursor, water and ethanol as solvents, and mild reaction
conditions (50 ºC). The molecular structure of complex 1 has been established by X-ray
diffraction analysis (see below) and in this section we report its spectroscopic
characterization.
Scheme 1 here
The IR spectrum of the complex shows a single strong peak for the V=O
stretching at 980 cm-1 [49–53]. The presence of H-bonded water is evident by a broad
peak at 3201 cm-1. ν(C=N) appears at 1605 cm-1, a little higher in energy than that of the
Page 6 of 26

free ligand [44], whereas the aromatic ν(C=C) at 1548 cm-1 is unperturbed in the
complex as compared to the free ligand. The pro-ligand H4L loses hydrogen from both
NH and OH functionalities upon coordination to the VO3+ centre, in accord with the
disappearance in the complex of the broad ν(NH) band at 3405 cm-1.
The 1H NMR spectrum of 1 in CDCl3 shows the characteristic resonances of the
ligand: the proton of the azomethine nitrogen at δ 9.2 [54]; the aromatic protons as
multiplets in the δ range of 7.2–7.6 [55]; the CH2 protons of the coordinated ethoxide at
δ 5.65 [56], while those of the coordinated ethanol occur at δ 3.7; the CH3 protons of
both the coordinated ethoxide and ethanol at δ 1.25 [57]. The H-bonded OH proton of
the coordinated ethanol and the protons of the water molecule appear as a broad peak at
δ 1.80.
The electronic spectrum of 1 in EtOH shows two bands at 419 and 316 nm
assigned to L(pπ)–V(dπ) LMCT charge transfer bands [58]. Other bands in the high
energy region are due to intra-ligand charge transfer processes [58]. An ethanol solution
of the complex 1 was found to be EPR silent, as expected on the basis of the +5 metal
oxidation state. Mass spectral analysis by ESI-MS shows an ion peak at m/z 519.0 in the
positive mode assigned to [{V2O2L(OEt)} + H2O]+.
3.2. Description of the structure
The X-ray structural analysis of 1 confirmed that the complex is binuclear in the
solid state (Figure 1) and contains a crystallographically imposed centre of inversion in
the middle of the C8C8i bond. The complex crystallizes in the centrosymmetric
triclinic space group P-1 with half a molecule in the asymmetric unit and one molecule
of water. The tetranegative L4- ligand binds the two oxovanadium(V) species through
the two tridentate binegative O2N donor sites; the other two coordination positions of
the metal cation are occupied by one ethoxide ion and an ethanol molecule. The
coordination sphere of the 6-coordinate vanadium is distorted octahedral with the
vanadium atom 0.303 Å above the basal least-square plane defined by O1-N1-O2-O20,
and shifted towards the apical position which is occupied by the O3-oxo atom. This
deviation is comparable to the data concerning related vanadium complexes [55,56].
The vanadium cation belongs to two fused five and six-membered metallacycles, V1-O1-
C1-C6-C7-N1 and V1-N1-N2-C8-O2. Accordingly, the O1-V1-N1 and the N1-V1-O2
angles are of 83.50(11) and 74.07(10)º, respectively, and represent the major deviations
Page 7 of 26

from the octahedral geometry around the metal. The bond distances in the coordination
sphere of vanadium are comparable to those of related monoxo alkoxo V(V) complexes
[59]. However, the V=O bond length of 1.579(3) Å is shorter than that previously
reported for the dioxovanadium(V) complex with the same ligand [1.616(4) and
1.630(3) Å] [60], what also occurs with the other metal-L ligand bonds. The shortest V-
O bonds in the coordination sphere of vanadium pertain to those involving the O20
ethoxide oxygen atoms, indicating a stronger binding of this group as compared to that
of phenolate or ethanol oxygens.
The crystal packing diagram of 1 (Fig. 2) is organized forming parallel chains of
the dimetallic moieties, with crystallization water molecules hosted between the parallel
chains of the complexes and involved in strong intermolecular contacts (Fig. 1) leading
to the assembly of the vanadium units into 1D chains (running along the
crystallographic a axis). The shortest intra- and interchain metal∙∙∙metal separations are
of 6.820 and 8.316 Å, respectively, which correspond, in this order, to the unit cell
parameters a and b (Table 1), whereas the intramolecular V∙∙∙V separation is equivalent
to 7.023 Å.
Figure 1 here
Figure 2 here
Table 1 here
3.3. Oxidation of liquid alkanes
In the present work, we have found that 1 in the presence of PCA efficiently
catalyzes the oxidation of alkanes by aqueous H2O2. In the absence of co-catalyst, the
oxidation of cyclohexane to the mixture of cyclohexanol and cyclohexanone (Scheme 2)
catalyzed by 1 (5 × 10−5 M) is slow (Fig. 3, curve 1) affording, after 4 h of reaction, the
overall 2.4% yield of oxygenates (TON 224) with an equal content of alcohol and
ketone in the reaction mixture after its treatment by PPh3. The presence of PCA (0.005
M) noticeably increases the oxidation rate, leading to 29% total yield and TON of 2.64
× 103 after 4 h (Fig. 3, curve 3).
Scheme 2 here
Figure 3 here
Page 8 of 26

In the cyclohexane oxidation catalyzed by 1 (5 × 10-4 M) in the presence of PCA
(0.005 M), the detailed analysis of kinetic curves (Fig. 4) of product accumulation
shows that in the beginning of the reaction cyclohexanol is predominantly formed,
although cyclohexanone is also present in the solution. After 30 min of the reaction, the
concentration of alcohol falls slightly, but that of ketone continues to grow, while the
total amount of oxygenate products remains constant. This observation, together with
the fact that a difference in the alcohol/ketone molar ratio in the reaction solution was
observed before and after its treatment by PPh3 (in order to reduce ROOH to ROH),
points out the presence of cyclohexyl hydroperoxide in the reaction mixture as a
primary product of cyclohexane oxidation [12–14] which, however, decomposes to the
corresponding alcohol and ketone. After 1 h of the reaction, the concentrations of
alcohol and ketone become close and practically no difference in the product
composition was observed by GC before and after the treatment with PPh3 (i.e.,
cyclohexyl hydroperoxides are not present any more in the solution).
Figure 4 here
The addition of another strong acid co-catalyst such as trifluoroacetic acid (TFA,
0.005 M) also improves the catalytic performance of 1 (Fig. 3), but the oxidation of
cyclohexane proceeds slower and less efficiently than that in the presence of PCA,
leading to 12% total yield (alcohol/ketone molar ratio = 2) and TON of 1.12 × 103.
The increase of the PCA amount results in the growth of the catalytic activity of
1 (Fig. 5). In the absence of PCA, the reaction rate of the cyclohexane oxidation with 1
is rather low. The maximum reaction rate Wmax increases up to the PCA concentration
of 25 × 10–4 M (Fig. 3 B), thus testifying the direct involvement of PCA in the
formation of catalytically active vanadium containing species. The Wmax does not
depend on the content of PCA upon further addition beyond 25 × 10–4 M, indicating that
at these conditions the rate of the formation of the intermediate active PCA containing
species is constant.
Figure 5 here
The rate of cyclohexane oxidation is directly proportional to the concentration of
complex 1 in the 5 × 10–6 M – 5 × 10–4 M range, in the presence of a constant amount of
PCA (0.005 M) (Fig. 6). Moreover, monomeric species conceivably are involved in the
Page 9 of 26

reaction mechanism, as suggested by ESI-MS analysis of the acetonitrile solution
containing complex 1 (3 × 10–6 M), PCA (2.7 × 10–5 M) and H2O2 (1 mL). The negative
scan showed a single peak in the mass range of 50–500 at m/z 345.1. This corresponds
to a monoperoxo vanadium species with two coordinated pca ligands [VO(O-O)(pca)2]–
(pca = monodeprotonated form(1−) of PCA). Such a complex was earlier synthesized
[61] and fully described [17], and its formation was also observed in other PCA-assisted
V-based catalytic systems [21].
At the concentration of complex 1 of 5 × 10-4 M, the reaction is fast (Fig. 6) and
the maximum yield of ca. 28% with the corresponding TON of 254 was achieved
already after 45 min (Table 2). Further decrease of the catalyst amount to 5 × 10-5 M
results in some deceleration of the oxidation, but leading to the identical 29% yield after
4 h, with a TON of 2.64 × 103. The reaction rate is reduced at a lower concentration of 1
(5 × 10−6 M) leading to 8.5% of oxygenates after 8 h of the reaction, although with a
remarkably high turnover number of 7.83 × 103 (Table 2).
Although the role of the NO2-donor Schiff base ligand could not be fully
established, we propose that it can stabilize a peroxo vanadium species formed during
the catalytic cycle (as suggested by theoretical studies performed on other vanadium
catalysts [21]). Besides, the bi or multinuclearity can be a favourable feature for
oxovanadium catalysts, as recently demonstrated by us [22,29]. In accord, oxidation of
cyclohexane catalyzed by 1/PCA proceeds faster (maximum initial reaction rate W0 =
4.5×10-6 M/s) and more efficiently than that in the presence of (Bu4N)VO3/PCA (W0 =
3.3×10-6 M/s) under identical reaction conditions, that were found to be optimal for
vanadates: ([catalyst]0 = 10−5 M), [PCA]0 = 6 × 10−4 M, [C6H12]0 = [H2O2]0 = 0.40 M,
50 ºC), leading to the higher TOF of 1612 h−1 vs. 805 h−1 for the vanadate/PCA system.
Figure 6 here
Table 2 here
In order to get some information about the nature of the mechanism, we have
performed the oxidation of other alkanes apart from cyclohexane (Table 3) and
measured various selectivity parameters. Hence, in the oxidation of linear n-heptane and
branched methylcyclohexane (MCH) by the 1/PCA/H2O2 system, the regio-
[C(1):C(2):C(3):C(4) = 1:6:6:6] and bond selectivity [1°:2°:3° = 1:8:19] parameters are
rather low, being close to those (Table 3) previously determined for systems that are
Page 10 of 26

known to oxidize alkanes with participation of hydroxyl radicals, e.g., the well studied
¯
VO3 /PCA/H2O2 system. In addition, a low normalized 3°/2° C–H selectivity (3) was
observed in the oxidation of adamantane, being again consistent with the presence of
nonselective oxidizing species [62]. However, the oxidations of cis- and trans-
dimethylcyclohexanes (cis- and trans-DMCH) proceed with some stereoselectivity,
resulting in the trans/cis ratio of isomeric alcohols of 0.38 and 0.34, respectively.
The competitive oxidation of cyclohexane / cyclohexane-d12 catalyzed by 1/PCA
in the presence of H2O2 reveals a low kinetic isotope effect (KIE = 1.3), which is
indicative of a powerful and rather indiscriminate oxidizing species, such as the
hydroxyl radical [62].
Table 3 here
Summarizing the abovementioned kinetic and selectivity data, we can conclude
that the PCA co-catalyst reacts with vanadium complex 1 to form catalytically active
species that participate in the generation of HO• radicals from H2O2, possibly via the
formation of a V-peroxo complex containing PCA. The HO• abstracts hydrogen from
alkane with formation of R•, which then reacts with O2 (e.g. from air) affording ROO•
and leading to the formation of ROOH. However, alkyl hydroperoxide ROOH rapidly
decomposes (conceivably by V-catalyzed processes) [12–14,19–22] to the
corresponding alcohol and ketone.
3.4. Oxidation of gaseous alkanes
We have also found that complex 1 in the presence of PCA efficiently catalyses
the oxidation of gaseous alkanes by aqueous H2O2 at 50 °C into the corresponding
carbonyl compounds (Table 4). Hence, the oxidation of ethane leads after 4 h to the
formation of acetaldehyde (main product after PPh3) and ethanol, in addition to a
smaller amount of acetic acid presumably generated via the oxidation of the formed
acetaldehyde. The total yield of the products based on ethane reaches 8.1%,
corresponding to the total TON of 1.98 × 103. In contrast to ethane, propane is more
reactive due to the presence of the secondary carbon atom, leading to the 35.7% total
yield of oxygenates (TON 2.67 × 103). Among them, acetone and isopropanol are the
main products although some minor amounts of propanal, propanol and propionic acid
have also been detected. The oxidation of n-butane by the 1/PCA/H2O2 system results in
Page 11 of 26

the formation of butanone-2 and butanol-2 as major products, whereas butanal, 1-
butanol and butyric acid are formed in minor amounts. In the n-C4H10 oxidation, a high
total yield of oxygenates (45.3% based on n-butane) has been obtained, with the
corresponding TON of 562. In contrast, methane oxidation undergoes to a much lower
extent (TON 63, 0.3% yield of MeOH, Table 4).
Table 4 here
4.
Conclusion
The present study reported the synthesis, full characterization and X-ray crystal
structure of the new divanadium(V) Schiff-base complex 1. It was found that this
compound, in the presence of 2-pyrazinecarboxylic acid co-catalyst, efficiently
catalyzes the oxidation of liquid and gaseous alkanes by aqueous H2O2 into the
corresponding alkyl hydroperoxides, alcohols and ketones. Thus, the oxidation of
cyclohexane to the mixture of cyclohexanol and cyclohexanone proceeds with a high
yield of ca. 30% and turnover numbers of ca. 8.0 × 103. The oxidation of light gaseous
alkanes leads to 45% total products yield (n-butane oxidation) and to the TON of ca. 2.7
× 103 (ethane oxidation). The oxidations appear to proceed with participation of
hydroxyl radicals, which are generated from H2O2 upon catalytic action of vanadium
species containing PCA. Among the reported vanadium and other transition metal
catalysts applied for alkane oxidations by H2O2, the achieved herein efficiency of the
1/PCA system is remarkably high, both in terms of product yields and TONs, especially
concerning the oxidations of inert gaseous alkanes under rather mild conditions in
aqueous MeCN.
Acknowledgments
This work was partially supported by the Fundação para a Ciência e a Tecnologia
(FCT)
(project
numbers
PTDC/QUI-QUI/102150/2008
and
PEst-
OE/QUI/UI0100/2011). SG and MK acknowledge the FCT for post-doctoral grants.
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Page 15 of 26

Figure / scheme captions
Fig. 1. Representative diagram of the molecular structure of complex 1 with atom
labeling scheme. Ellipsoids are drawn at 30 % probability. H-bond interactions are
indicated as light blue dashed lines. Selected bond distances (Ǻ) and angles (º): V1-O1
1.850(3), V1-O2 1.971(2), V1-O3 1.579(3), V1-O10 2.316(3), V1-O20 1.771(3), V1-
N1 2.119(3), N1-N2 1.402(4); O1-V1-O2 150.67(12), O1-V1-O3 100.25(14), O1-V1-
O10 80.39(11), O1-V1-O20 104.73(14), O1-V1-N1 83.50(12), O2 -V1-O3 99.66(13),
O2-V1-O10 78.17(9), O2-V1-O20 91.90(12), O2 -V1-N1 74.07(10), O3-V1-O10
175.54(15), O3-V1-O20 101.96(15), O3 -V1-N1 93.24(14), O10 -V1-O20 82.08(11),
O10-V1-N1 82.43(10), O20-V1-N1 160.94(12). Selected hydrogen bonds [d(H···A) Å,
(DHA) º]: O10H10CO30 1.813, 164.93(19); O30H30AN2 2.263, 153;
O30H30AO2 2.414, 121.9(2); O30H30BO20 2.619, 157.4(2). Symmetry
operations to generate equivalent atoms: i) -x,1-y,1-z; ii) -1+x,y,z; iii) 1+x,y,z: iv) 1-
x,1-y,1-z.
Fig. 2. Perspective views of the crystal packing of 1 (arbitrary views) generated through
H-bond interactions, showing the parallel chains (spread along the crystallographic a
axis) with intercalated water molecules (shown as red balls). Hydrogen atoms are
omitted for clarity; V atoms are represented as grey balls.
Fig. 3. Effect of acid co-catalyst: accumulation of oxygenates (total of cyclohexanol and
cyclohexanone, mM) with time in the oxidation of cyclohexane (0.46 M) by H2O2 (2.2
M) at 50 ºC in CH3CN catalyzed by complex 1 (5 × 10−5 M) in the absence of acid co-
catalyst (curve 1), or in the presence of TFA (0.005 M, curve 2) or PCA (0.005 M,
curve 3). Maximum concentrations of oxygenates correspond to the total yields (based
on alkane) and TONs of 28.7% and 2.64 × 103 (in the presence of PCA), 12.2% and
1119 (in the presence of TFA), 2.4% and 224 (without any co-catalyst), respectively.
Fig. 4. Accumulation of oxygenates (mM): cyclohexanone (curve 1), cyclohexanol
(curve 2), and total (curve 3) with time in the oxidation of cyclohexane (0.46 M) by
H2O2 (2.2 M) at 50 ºC in CH3CN, catalyzed by complex 1 (5 × 10−4 M) in the presence
of PCA (5 × 10−3 M). Maximum concentration of oxygenates corresponds to the total
yield (based on alkane) of 30% and TON of 270.
Fig. 5. Effect of PCA amount: accumulation of oxygenates (total of cyclohexanol and
cyclohexanone, mM) with time (Graph A) in the oxidation of cyclohexane (0.46 M) by
H2O2 (2.2 M) at 50 ºC in CH3CN, catalyzed by complex 1 (5 × 10-5 M) in the presence
of different concentration of PCA: 0 M (curve 1), 5 × 10−4 M (curve 2), 25 × 10−4 M
(curve 3), 50 × 10−4 M (curve 4), and dependence (Graph B, obtained from data of
graph A) of the maximum reaction rate Wmax (0–25 × 10−6 M·s-1 ordinate scale) on PCA
amount (0–60 × 10−4 M abscissa scale).
Fig. 6. Effect of catalyst amount. Accumulation of oxygenates (total of cyclohexanol
and cyclohexanone, mM) with time (Graph A) in the oxidation of cyclohexane (0.46 M)
by H2O2 (2.2 M) at 50 ºC in CH3CN, in the presence of PCA (0.005 M) catalyzed by
complex 1: 5 × 10−6 M (curve 1), 5 × 10−5 M (curve 2), 5 × 10−4 M (curve 3), and
dependence (Graph B, obtained from data of graph A) of the maximum reaction rate
Wmax (0–120 × 10−6 M·s-1 ordinate scale) on the concentration of complex 1 (0–600 ×
10−6 M abscissa scale).
Page 16 of 26

Scheme 1. Synthetic procedure for the preparation of complex 1.
Scheme 2. Peroxidative oxidation of cyclohexane (model substrate) to cyclohexyl
hydroperoxide (main primary product), cyclohexanol and cyclohexanone (major final
products).
Page 17 of 26

Table 1
Crystallographic data and refinement parameters for complex 1.
1∙H2O
Molecular Formula
Formula Weight
Crystal System
Space group
a (Å)
C24H32N4O10V2,2(H2O)
674.45
Triclinic
P-1
6.8197(6)
8.3164(7)
14.9479(14)
97.756(3)
92.413(2)
110.001(3)
785.80(12)
1
b (Å)
c (Å)
α (º)
β (º)
γ (º)
V (Å3)
Z
calc (Mg/m3)
1.425
μ(Mo Kα) (mm–1)
F(000)
0.658
350
Refls collected
Refls unique/obs.
Rint
6963
2820/1927
0.0508
R1a (I ≤ 2σ)
wR2b (I I ≤ 2σ)
GOF
0.0498, 0.0858
0.1136, 0.1316
1.024
a R1 = ∑||Fo| – |Fc||/∑|Fo|.
b wR2 = [∑[w(Fo2 – Fc2)2]/∑[w(Fo2)2]]1/2
Page 18 of 26

Table 2
Oxidation of cyclohexane by H2O2 catalyzed by 1 in the presence of PCA.a
Reaction
time
[min]
45
Yield [%]b
Catalyst
Entry
TONc
amount [M]
Cyclohexanol Cyclohexanone
Total
1
2
3
5 × 10−4
5 × 10−5
5 × 10−6
20.6
19.4
3.9
7.0
9.3
4.6
27.6
28.7
8.5
254
240
2.64 × 103
7.83 × 103
500
a Reaction conditions: cyclohexane (0.46 M), PCA (0.005 M), H2O2 (50% aqueous, 2.2 M), CH3CN (up to
5 mL total volume), 50ºC.
b Based on cyclohexane, calculated from GC analysis after treatment of the reaction mixture with PPh3.
c Total TON corresponds to moles of products (cyclohexanol and cyclohexanone) per mol of catalyst.
Page 19 of 26

Table 3
Selectivity parameters in the oxidation of linear and branched cyclic alkanes.a
Catalytic system
Regioselectivity
Bond
Stereoselectivity
(n-C7H16)
selectivity trans/cis
C(1):C(2):C(3):C(4) (MCH)
1°:2°:3°
cis-
trans-
DMCH
0.34
0.80
0.75
–
DMCH
0.38
0.75
0.70
0.9
1/PCA/H2O2/CH3CN
1:6:6:6
1:9:7:7
1:8:19
1:6:18
1:4:10
−
–
VO3 /PCA/H2O2/CH3CN (40°)b
[VO{N(C2H4O)3}]/PCA/H2O2/CH3CN (40°)b 1:5.5:6.0:5.1
hν/H2O2/CH3CN (20°)b
1:7:6:7
a Reaction conditions: catalyst 1 (5 × 10−5), PCA (0.005 M), substrate (0.46 M), H2O2 (50% aqueous, 2.2
M), MeCN (up to 5 mL total volume), 50 °C. All parameters were measured after reduction of the
reaction mixtures with PPh3 before GC analysis and calculated based on the ratios of isomeric alcohols.
The calculated parameters were normalized, i.e., recalculated taking into account the number of H atoms
at each carbon atom. Parameters C(1):C(2):C(3):C(4) are relative reactivities of hydrogen atoms at
carbons 1, 2, 3 and 4 of the n-heptane chain. Parameters 1º : 2º : 3º are relative normalized reactivities of
the hydrogen atoms at primary, secondary and tertiary carbons of methylcyclohexane. Parameter trans/cis
is determined as the ratio of the formed tertiary alcohol isomers with mutual trans and cis orientation of
the methyl groups. Abbreviations: MCH is methylcyclohexane; cis-DMCH and trans-DMCH are isomers
of 1,2-dimethylcyclohexane.
b Taken from reference [21].
Page 20 of 26

Table 4
Oxidation of gaseous alkanes by the 1/PCA/H2O2 system.a
Alkane
Products (M)b
Total TONc Total
(pressure, atm; mmol)
Ethane (20; 6.23)
Yield [%]d
acetaldehyde (0.069), ethanol 1.98 × 103
(0.0225), acetic acid (0.0075)[e]
acetone (0.059), isopropanol 2.67 × 103
(0.028), propanal (0.028), n-
propanol (0.015), propionic acid
(0.0037)
8.1
Propane (6; 1.87)
35.7
45.3
0.3
Butane (1; 0.31)
2-butanone (0.0176), 2-butanol
(0.0059), butanal (0.0029), 1-
butanol (0.001), butyric acid
(0.0007)
562
Methane (20; 6.23) MeOH (0.0031 M)
63
a
Reaction conditions: complex 1 (5 × 10–5 M), PCA (0.005 M), H2O2 (50% aqueous, 2.2 M), MeCN (up
to 5 mL total volume), 50 °C, 4 h in a 13 mL stainless steel autoclave.
b Determined by GC (after reduction of the reaction mixtures with PPh3).
c Total moles of products / mol of catalyst.
d Yield (%) (moles of products / 100 moles of alkane).
e After deduction of the acetic acid amount (0.002 M) generated from MeCN solvent in a blank test.
Page 21 of 26

Figures
Fig. 1. Representative diagram of the molecular structure of complex 1 with atom labeling scheme.
Ellipsoids are drawn at 30 % probability. H-bond interactions are indicated as light blue dashed lines.
Selected bond distances (Ǻ) and angles (º): V1-O1 1.850(3), V1-O2 1.971(2), V1-O3 1.579(3), V1-O10
2.316(3), V1-O20 1.771(3), V1-N1 2.119(3), N1-N2 1.402(4); O1-V1-O2 150.67(12), O1-V1-O3
100.25(14), O1-V1-O10 80.39(11), O1-V1-O20 104.73(14), O1-V1-N1 83.50(12), O2 -V1-O3 99.66(13),
O2-V1-O10 78.17(9), O2-V1-O20 91.90(12), O2 -V1-N1 74.07(10), O3-V1-O10 175.54(15), O3-V1-
O20 101.96(15), O3 -V1-N1 93.24(14), O10 -V1-O20 82.08(11), O10-V1-N1 82.43(10), O20-V1-N1
160.94(12). Selected hydrogen bonds [d(H···A) Å, (DHA) º]: O10H10CO30 1.813, 164.93(19);
O30H30AN2 2.263, 153; O30H30AO2 2.414, 121.9(2); O30H30BO20 2.619, 157.4(2).
Symmetry operations to generate equivalent atoms: i) -x,1-y,1-z; ii) -1+x,y,z; iii) 1+x,y,z: iv) 1-x,1-y,1-z.
Fig. 2. Perspective views of the crystal packing of 1 (arbitrary views) generated through H-bond
interactions, showing the parallel chains (spread along the crystallographic a axis) with intercalated water
molecules (shown as red balls). Hydrogen atoms are omitted for clarity; V atoms are represented as grey
balls.
Page 22 of 26

Fig. 3. Effect of acid co-catalyst: accumulation of oxygenates (total of cyclohexanol and cyclohexanone,
mM) with time in the oxidation of cyclohexane (0.46 M) by H2O2 (2.2 M) at 50 ºC in CH3CN catalyzed
by complex 1 (5 × 10−5 M) in the absence of acid co-catalyst (curve 1), or in the presence of TFA (0.005
M, curve 2) or PCA (0.005 M, curve 3). Maximum concentrations of oxygenates correspond to the total
yields (based on alkane) and TONs of 28.7% and 2.64 × 103 (in the presence of PCA), 12.2% and 1119
(in the presence of TFA), 2.4% and 224 (without any co-catalyst), respectively.
Fig. 4. Accumulation of oxygenates (mM): cyclohexanone (curve 1), cyclohexanol (curve 2), and total
(curve 3) with time in the oxidation of cyclohexane (0.46 M) by H2O2 (2.2 M) at 50 ºC in CH3CN,
catalyzed by complex 1 (5 × 10−4 M) in the presence of PCA (5 × 10−3 M). Maximum concentration of
oxygenates corresponds to the total yield (based on alkane) of 30% and TON of 270.
Page 23 of 26

Fig. 5. Effect of PCA amount: accumulation of oxygenates (total of cyclohexanol and cyclohexanone,
mM) with time (Graph A) in the oxidation of cyclohexane (0.46 M) by H2O2 (2.2 M) at 50 ºC in CH3CN,
catalyzed by complex 1 (5 × 10-5 M) in the presence of different concentration of PCA: 0 M (curve 1), 5 ×
10−4 M (curve 2), 25 × 10−4 M (curve 3), 50 × 10−4 M (curve 4), and dependence (Graph B, obtained from
data of graph A) of the maximum reaction rate Wmax (0–25 × 10−6 M·s-1 ordinate scale) on PCA amount
(0–60 × 10−4 M abscissa scale).
Fig. 6. Effect of catalyst amount. Accumulation of oxygenates (total of cyclohexanol and cyclohexanone,
mM) with time (Graph A) in the oxidation of cyclohexane (0.46 M) by H2O2 (2.2 M) at 50 ºC in CH3CN,
in the presence of PCA (0.005 M) catalyzed by complex 1: 5 × 10−6 M (curve 1), 5 × 10−5 M (curve 2), 5
× 10−4 M (curve 3), and dependence (Graph B, obtained from data of graph A) of the maximum reaction
rate Wmax (0–120 × 10−6 M·s-1 ordinate scale) on the concentration of complex 1 (0–600 × 10−6 M abscissa
scale).
Page 24 of 26

Schemes
O
O
O
O
N2H4, H2O
EtOH, 0oC
H5C2
H2N HN
NH NH2
C2H5
O
O
OH
HO
O
O
O
O
OH
CHO
N
N
H2O, RT
H2N HN
NH NH2
HN
NH
H4L
H
H
OC2H5
O
O
V
N
OC2H5
H5C2O
O
H5C2O
O
V
N
EtOH
O
H4L
[VO(acac)2]
O
+
50 o
C
N
N
1
Scheme 1. Synthetic procedure for the preparation of complex 1.
OOH
OH
O
Vcatalyst / PCA
H2O2, CH3CN
+
Scheme 2. Peroxidative oxidation of cyclohexane (model substrate) to cyclohexyl hydroperoxide (main
primary product), cyclohexanol and cyclohexanone (major final products).
Page 25 of 26

Graphical Abstract
Page 26 of 26