A new binuclear oxovanadium(v) complex as a catalyst in combination with pyrazinecarboxylic acid (PCA) for efficient alkane oxygenation by H 2O2

Article abstract of DOI:10.1039/c3dt50584g

A new binuclear oxovanadium(v) complex [{VO(OEt)(EtOH)}₂L] (1) where H₄L is bis(2-hydroxybenzylidene)terephthalohydrazide has been synthesized and fully characterized. The combination of 1 with pyrazine-2-carboxylic acid (PCA; a cocatalyst) affords a catalytic system for the efficient oxidation of saturated hydrocarbons, RH, with hydrogen peroxide and air in acetonitrile solution at 50°C to produce alkyl hydroperoxides, ROOH, as the main primary products. Very high turnover numbers (TONs) have been attained in this reaction: for example, after 2220 min, TON = 44 000 and initial TOF (turnover frequency) = 3300 h^-1 per molecule of complex 1. The estimated activation energy of the cyclohexane oxygenation in the presence of 1/PCA is E_a = 16 ± 2 kcal mol^-1. This value is identical to that obtained for the cyclohexane oxidation with H ₂O₂ catalyzed by the (n-Bu₄N)[VO ₃]/PCA combination (17 ± 2 kcal mol^-1). The dependences of initial oxidation rates W₀ on the initial concentrations of all components of the reaction mixture have been determined. Based on these kinetic data and on the regio- and bond-selectivity parameters measured in the oxidation of linear and branched alkanes a mechanism of the oxidation has been proposed which includes the generation of hydroxyl radicals in the crucial stage. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3dt50584g

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New binuclear oxovanadium(V) complex as a catalyst
in combination with pyrazinecarboxylic acid (PCA) for
efficient alkane oxygenation by H2O2
Manas Sutradhar,a Nikita V. Shvydkiy,a,b,с tima C. Guedes da Silva,a,d
Marina V. Kirillova,a Yuriy N. Kozlov, b Armando J. L. Pombeiro,*a
and Georgiy B Shul’pin *b
___________________________________________________________________________
a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Technical University of Lisbon,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: pombeiro@ist.utl.pt
b Semenov Institute of Chemical Physics, Russian Academy of Sciences, ulitsa Kosygina,
dom 4, Moscow 119991, Russia. E-mail: Shulpin@chph.ras.ru
c Chemistry Department, Moscow State University, Leninskie gory, dom 1, Moscow 119991, Russia
d
fona de Humanidades e Tecnologias, Campo Grande 376, 1749-024 Lisboa,
Portugal
† Electronic supplementary information (ESI) available: details of kinetic study. For ESI and
crystallographic data in CIF or other electronic format see DOI: xxxxxxxx
Abstract:
A new binuclear oxovanadium(V) complex [{VO(OEt)(EtOH)}2L] (1) where H4L is bis(2-
hydroxybenzylidene)terephthalohydrazide) has been synthesized and fully characterized. The
combination of 1 with pyrazine-2-carboxylic acid (PCA; a cocatalyst) affords a catalytic system for the
efficient oxidation of saturated hydrocarbons, RH, with hydrogen peroxide and air in acetonitrile solution
at 50 ºC to produce alkyl hydroperoxides, ROOH, as main primary products. Very high turnover numbers
(TONs) have been attained in this reaction: for example, after 2220 min, TON = 44 000 and initial TOF
(turnover frequency) = 3300 h–1 per molecule of complex 1. Estimated activation energy of the
cyclohexane oxygenation in the presence of 1/PCA is Ea = 16 ± 2 kcal mol–1. This value is identical to
that obtained for the cyclohexane oxidation with H2O2 catalyzed by the (n-Bu4N)[VO3]/PCA combination
(17 ± 2 kcal mol–1). The dependences of intitial oxidation rates W0 on the initial concentrations of all
components of the reaction mixture have been determined. Based on these kinetic data and on the
regio- and bond-selectivity parameters measured in the oxidation of linear and branched alkanes a
mechanism of the oxidation has been proposed which includes the generation of hydroxyl radicals in the
crucial stage.
1

Introduction
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Vanadium is known to play an important role in living nature.1 For example, some enzymes containing
vanadium ions are capable of oxidizing C–H bond in organic compounds: vanadium-containing
haloperoxidases 2 catalyse the halogenation of organic compounds, RH, in the presence of hydrogen
peroxide. Soluble and heterogeneous vanadium derivatives are efficient reagents and catalysts in
oxygenation reactions of various organic compounds,3 particularly, hydrocarbons.3j,4 Mono- and
polynuclear vanadium complexes bearing chelating ligands, for example, Schiff bases 5 ,
aminopolyalcohols 6a, N-hydroxyimino-dicarboxylates 6a, polyoxometalates 6b-f containing one or many
vanadium ions can be considered as models of natural vanadium-based catalysts in hydrogen peroxide
oxygenation reactions.
In 1993, Shul’pin and co-workers 7 discovered that, in the case if pyrazine-2-carboxylic acid
(PCA) is added in low concentration to the reaction mixture, the vanadate salt (n-Bu4N)[VO3] catalyzes
efficient oxidation of organic compounds (alkanes, arenes, alcohols) with hydrogen peroxide under mild
conditions (20–60 °C, in acetonitrile solution). The PCA/vanadate ratio was between ca. 4 and 10. It has
been shown later that methane and light alkanes 8 and olefins 9 can be easily oxidized by the
H2O2/vanadate/PCA reagent. The oxidation of isoeugenol by this reagent was found to be a convenient
route to vanilline.9b In addition to monovanadate (the most simple and efficient catalyst), many other
vanadium derivatives have been later proposed as catalysts. Thus, mononuclear salts and complexes
VOSO4, VCl3, [VO(acac)2],10a oxovanadium(V) triethanolaminate (vanadatrane),6a,10b and
[VCl2(Py2S2)]Cl (Py2S2 is a tetradentate ligand)10c were demonstrated to catalyze the oxygenation of
alkanes. Mono- and pentavanadium oxometalates [PMo11VO40]4− and [PMo6V5O39]12− (Ref. 10d) and
hexanuclear complexes [(p-MeC6H4-iPr)4Ru4V6O19] (Ref. 10a) and [V3O3(OEt)L2(-OEt)]2 (L is N,O-
donor ligand) 10e also played the role of oxidation catalysts. In all these cases the efficiency of the
oxidation was much lower in the absence of PCA. It was found that PCA is typically a much more
efficient cocatalyst in comparison with other related acids (for a review, see ref. 10f).
A possible mechanism of the oxidation by the H2O2/vanadium derivative/PCA reagent was
discussed in a few publications.10a,b,f,11 On the basis of a kinetic study of the alkane oxidation, a
mechanism of the alkane oxygenation was proposed 10a,11b-e,h,i which involves two crucial steps:
decomposition of a transient peroxovanadium(V) complex to produce a peroxyl radical HOO• and a
V(IV) derivative (eq. i) and further interaction of this V(IV) complex with another hydrogen peroxide
molecule to afford hydroxyl radical (equation ii) which attacks the hydrocarbon.
“VV–OOH” → “VIV” + HOO•
(i)
“VIV” + H2O2 → “VV” + HO– + HO•
(ii)
Reaction (i) was assumed to be the rate-limiting step.10a In the present paper, we use PCA to name
pyrazine-2-carboxylic acid. In some cases to depict the acidic nature of this compound we use the formula
2

Hpca where pca– is pyrazine-2-carboxylate and H+ is the proton. It has been assumed 10a that PCA
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coordinated to the vanadium centre in the form of the pca– ligand (PCA = H+pca–) facilitates the proton
transfer between molecules of hydrogen peroxide or water on one hand and oxo or hydroxy ligands of the
vanadium complex on the other hand, for example:
…
“H2O2 O=V” → “HOO–V–OH”
(iii)
Such a “robot’s arm mechanism” 10a may have analogies in enzyme catalysis (vanadium is known to
promote hydroxyl radical formation in living organisms; see, for example, Ref. 3a). It is found that a
vanadium complex with one pca (or pca– or PCA) fragment and one H2O2 ligand is the precursor to the
species responsible for the radical HOO• generation. Studies of the mechanism of radicals generation and
particularly of the “robot’s arm mechanism” by the DFT method led Khaliullin, Bell and Head-Gordon 12
to the conclusion that “the generation of HOO• radicals cannot occur via cleavage of a V–OOH bond in
the complex formed directly from the precursors” because the activation barrier for this process is too
high. Instead, peroxyl radicals are formed via a sequence of additional steps and diperoxo complexes are
involved into this transformation. Besides, the conversion of the precursors requires hydrogen transfer
from H2O2 to a vanadyl group, and the calculations by Bell and co-workers showed “that direct transfer
has a higher barrier than pca-assisted indirect transfer. Indirect transfer occurs by migration of hydrogen
from coordinated H2O2 to the oxygen of a pca ligand connected to the vanadium atom” (eq. iv). For Bell’s
variant of our mechanism, the hydrogen transfer from a coordinated hydrogen peroxide molecule to a
vanadyl group becomes the rate-determining step. The experimentally measured 10a activation energy
(63–80 kJ/mol) agreed very well with calculated 12 values (67–81 kJ/mol). The theoretical calculation
suggested that it might not be possible to find a better cocatalyst than PCA.
3

An alternative pathway for proton migration in the vanadium complex has been proposed more
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recently by Kuznetsov and co-workers 10b on the basis of DFT calculations. The proposed “water-assisted
mechanism” includes the molecule H2O as a part of a six-membered transition state. A water-assisted
mechanism of proton transfer turned out to be even more effective than the robot’s arm mechanism. In the
water-assisted mechanism the rate limiting is the H-transfer from ligated H2O2 to an oxygen-containing
ligand. The pca ligand plays in this case a key role as a stabilizer of transition state V(IV) species
involved into the rate-limiting stage.10b It should be noted that earlier Sen and co-workers 13a described an
experimentally supported water-assisted mechanism in the V-catalyzed benzene hydroxylation. Related
mechanisms have been discussed for Fe-catalyzed olefin cis-dihydroxylation 13b and for cyclopentadienyl-
Mo catalysed olefin epoxidation, 13e and, for the first time in alkane oxidation, by Kuznetsov and
Pombeiro 13d, for radical formation in the [MeReO3]–H2O2 system. A route to 18O incorporation involves
an equilibrium with the vanadium peroxo species, as shown in eq. (v).13a
Soluble and supported vanadium complexes have been reported to catalyze the oxidation of
saturated hydrocarbons with molecular oxygen instead of H2O2 at relatively high temperatures.14a-e The
activity of the catalysts in cyclohexane oxidation was often enhanced in the presence of PCA. It should be
noted that a great rate-enhancing power of PCA additive10f has been recently found in the hydrocarbon
oxidation reactions with H2O2 catalyzed by compounds of other metals, namely iron, 14f-o manganese, 14p,q
copper 14r and rhenium.14s
In the present work, we studied kinetics and selectivity of alkanes oxygenation by the
1/PCA/H2O2 system where 1 is a novel binuclear vanadium(V) complex. The mechanism of the oxidation
was proposed on the basis of this study.
Results and discussion
Synthesis and structure of complex 1
4

Condensation of two equivalent of salicylaldehyde with one equivalent of terephthalolhydrazide produces
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the Schiff base pro-ligand bis(2-hydroxybenzylidene)terephthalohydrazide (H4L), which has two potential
tridentate coordination pockets and is capable to bind two metal centres simultaneously. In the presence
of a metal ion in solution, H4L undergoes enolization and deprotonation. The active ligand L4– forms
stable bis chelates with two metal ions. When [VO(acac)2] is treated with H4L in ethanol, the binuclear
oxovanadium(V) complex 1 is obtained (Scheme 1). It is known from the literature that the
oxovanadium(IV) species undergoes aerial oxidation on treatment with acid hydrazide Schiff base ligands
10e,15 in alcohol medium. Compound 1 is a bis chelate oxoethoxovanadium(V) complex where both
vanadium centres are hexacoordinated and bear solvent ethanol molecules. This coordinated ethanol
molecule trans to the V=O bond is highly labile and can be easily replaced by other substrates. Thus, such
coordination positions could be easily accessible during a catalytic process.
Scheme 1 Synthesis of complex 1.
Fig. 1 displays a perspective view of the binuclear oxovanadium(V) complex 1. Crystallographic
data are summarized in Table 1 and selected dimensions are presented in the caption of Fig. 1.
5

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Fig. 1 Molecular structure of complex 1 with atom numbering scheme and hydrogen bond interactions. Thermal
ellipsoids are drawn at the 30% probability level. Selected bond distances (Å) and angles (º): N1—V1 2.1317 (17);
O1—V1 1.8441 (16); O2—V1 1.9430 (14); O10—V1 1.5971 (17); O20—V12.3795 (16); O30—V1 1.7738 (15);
O10—V1—O30 101.00 (8); O10—V1—O1 100.10 (8); O30—V1—O1 101.90 (7); O10—V1—O2 98.04 (8); O30—
V1—O2 94.83 (6); O1—V1—O2; 152.40 (7); O10—V1—N1 95.19 (7); O30—V1—N1 161.57 (7); O1—V1—N1
83.70 (7); O2—V1—N1 74.06 (6); O10—V1—O20 174.61 (7); O30—V1—O20 82.83 (7); O1—V1—O20 82.68 (7);
O2—V1—O20 77.76 (6); N1—V1—O20 80.47 (6). Hydrogen bond [d(D···A) Å, (DHA) º]: 2.907(3),
166(3).Symmetry operations to generate equivalent atoms: i) x,y,1z; ii) 1x,y,1z; iii) 1+x,y,z.
Table 1 Crystal data and structure refinement details for complex 1.
1
Empirical formula
C30H36N4O10V2
714.51
Formula Weight
Crystal system
Triclinic
Space group
a/Å
P-1
8.8519(5)
9.8955(6)
10.3659(6)
78.910(3)
68.375(2)
65.880(3)
769.45(8)
b/Å
c/Å

/

V (Å3)
6

Z
1
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Dcalc (g cm–3)
1.542
(Mo K) (mm–1)
Rfls. collected/unique/observed
Rint
0.672
13491/ 2794/ 2582
0.0361
Final R1 a, wR2 b (I  2σ)
Goodness-of-fit on F2
0.0382, 0.1042
1.081
a R = ||Fo|–|Fc||/|Fo|; b wR(F2) = [w(|Fo|2 – |Fc|2)2/w|Fo|4]½
The tetraanionic ligand L4 derived from the Schiff-base bis(2-
hydroxybenzylidene)terephthalohydrazide (H4L) acts as an hexadentate (N1O2)2 chelating species to two
vanadium cations. The asymmetric unit of 1 contains half of the complex molecule, with a
crystallographically imposed inversion centre in the middle of the terephthalo ring. Each vanadium cation
displays a distorted N1O5 octahedral environment in which the basal plane is made of the phenolic (O1)
and the enolic (O2) oxygens and the imine nitrogen (N1) atoms from the ligand together with an ethoxide
(O30) oxygen. The axial positions are occupied by the apical oxygen (O10) and the coordinated ethanol
oxygen (O20). The deviations from a regular octahedral geometry are considerable. Thus, the trans
O1V1O2 angle is of 152.42(8) º and that of trans N1V1O30 is of 161.59(8) º; moreover, consistent
with the five- and six-membered metallacycles in which the vanadium atom is involved, the N1V1O2
and the O1V1N1 angles are of 74.06(7) and 83.70(7) º, respectively. Although the ligand skeleton is
essentially planar, coordination to the metal leads to a significant twisting, as revealed by the
V1O1C1C2 torsion angle of 147.4(2) º in comparison with that of V1O2C11C12 [177.8(1) º].
Accordingly, the vanadium atom is displaced 0.292 Å above the coordination basal plane towards the oxo
atom. The metaloxygen bond distances range from 2.3797(18) to 1.5975(19) Å the former belonging to
the VOethanol and the latter pertaining to a V=O bond, in accord with those found in five- and six-
coordinate octahedral complexes of vanadium(IV) and (V).15 The various vanadium-oxygen bond lengths
found in 1 follow the order VOoxo < VOethoxide < VOphenoxide < VOenolate < VOethanol, which is not
uncommon for V(V) complexes of related ligands. 15
The molecules of 1 are gathered by means of intermolecular OHN hydrogen bonds (see Fig. 1
legend) and layered by means of  interactions involving the central terephthalo and the terminal
phenolate rings [centroidcentroid distance of 3.5625(15) Å]. Such interactions expand the structure
along the crystallographic a axis.
7

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Oxygenation of alkanes
We studied in detail the oxygenation of cyclic, linear and branched alkanes with the 1/PCA/H2O2 system
in acetonitrile solution under mild conditions (typical temperature of 50 °C). Kinetic curves of
accumulation of oxygenates at different concentrations of complex 1 in the presence of in the absence of
PCA are shown in Fig. 2. It should be noted that it has been shown earlier 7–12 the combination of various
other vanadium complexes as catalysts and PCA as cocatalyst is a very efficient catalytic system if H2O2
is used as oxidant. Other cocatalysts such as H2SO4 and TFA also promote the activity of 1 (see Fig. 3),
albeit with an inferior efficiency (we have shown previously that addition of H2SO4, TFA or oxalic acid
to the simple vanadate leads to the formation polyvanadates which also catalyze the oxidation with
hydrogen peroxide 6b,c).The curves of oxygenate accumulation with time at relatively low concentrations
of complex 1 are depicted in Fig. 2, Graph B. The highest TON = 44 000 (after 2220 min) and initial TOF
= 3300 h–1 have been attained at [1]0 = 1 ×10–6 M (Fig. 2, curve 3). When the concentration of the catalyst
was lower ([1]0 = 5 × 10–7 M) the corresponding values were TON = 33 000 (after 14 h) and initial TOF =
1420 h–1 (Fig. 2, curve 4).
Fig. 2 here
Fig. 3 here
Complex 1 + PCA
0.14
A
1a
0.12
1
0.08
Complex 1
2a
0.04
2
0
1
3
4
0
2
Time (h)
3
0.04
B
0.03
0.02
0.01
0
3a
4a
4
400
1200
1600
0
800
Time (min)
8

Fig. 2 Oxygenation of cyclohexane with the H2O2/1/PCA/CH3CN–H2O (closed red circles, curve 1) system. Kinetic
curve of accumulation of oxygenates (predominantly cyclohexyl hydroperoxide) in the absence of PCA (open pink
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squares, curve 2) are also shown. Conditions for Graph A: [catalyst 1]0 = 2.0 × 10–5 M (initial amount of 1 = 1 × 10–6
mol); [PCA]0 = 0.005 M; [H2O2]0 = 2.2 M (50% aqueous), [H2O]added = 0 M; [H2O]total = 4.2 M; [cyclohexane]0 = 0.46
M; MeCN up to total volume 5 mL; 50 °C. Conditions for Graph B: [catalyst 1]0 = 1.0 × 10–6 M (closed blue circles,
curve 3) or 5.0 × 10–7 M (closed red squares, curve 4); [PCA]0 = 6 × 10–4 M; [H2O2]0 = 0.4 M (50% aqueous),
[H2O]added = 0 M; [H2O]total = 0.76 M; [cyclohexane]0 = 0.4 M; MeCN up to total 5 mL volume; 50 °C. In the
experiment shown by curve 3, after 2220 min, TON = 44 000 and initial TOF = 3300 h–1. Each point corresponds to
the sum of concentrations of cyclohexanol and cyclohexanone measured after reduction of the reaction sample
with PPh3. Maximum initial rates W0 were determined from the slopes of tangents (two examples are depicted with
straight dotted lines 1a, 2a, 3a and 4a) to the kinetic curves in the intervals where the rate attains maximum (in
these cases in the beginning of the oxidation reaction).
0.08
0.06
0.04
0.02
0
PCA
H2SO4
TFA
1
3
4
0
2
Time (h)
Fig. 3 Kinetic curves of oxygenate accumulation (cyclohexanone + cyclohexanol after treating by PPh3) in the
presence of pyrazine-2-carboxylic acid (PCA), sulphuric acid and trifluoroacetic acid (TFA) in the cyclohexane
oxidation catalyzed by compound 1. Conditions: [1]0 = 2.0 × 10–5 M; [acid]0 = 4 × 10–4 M; [H2O2]0 = 0.4 M (50%
aqueous), [H2O]added = 0 M; [H2O]total = 0.76 M; [cyclohexane]0 = 0.40 M; MeCN up to total volume 5 mL; 50 °C.
In the kinetic study of alkane oxidation by the H2O2/1/PCA/CH3CN–H2O system, we measured
the dependences of initial rate of oxygenate accumulation W0 (determined from the slope of tangent to the
kinetic curve in the interval where the rate attains maximum; four examples are presented by the dotted
lines in Fig. 2) on initial concentrations of each reactant at fixed concentrations of all other reactants. At
the relatively small initial catalyst amount m of 1 < 1 × 10–7 mol (and respectively low concentration of
the complex [1]0 < 2 × 10–5 M) the dependence of W0 on the initial amount of 1 gives in Fig. 4 a straight
line, that is the initial oxidation rate W0 is proportional to m. At amount m of 1 > 1 × 10–6 mol ([1]0 > 2 ×
10–5 M) the rate W0 does not depend on the catalyst amount (for the original kinetic curves, see Fig. S1).
Such mode of behaviour is due to the poor solubility of complex 1 in the reaction solution. Under the
conditions of studied experiments at m0 = 1 × 10–7 mol ([1]0 > 2 × 10–5 M) the solubility limit for 1 is
attained. At higher amounts of 1 introduced into the reaction solution only m0 of 1 is dissolved whereas
m–m0 of the catalyst is present in the solid phase. This heterogeneous form of the catalyst does not exhibit
noticeable catalytic activity. As a support to our assumption about existence of 1 in two (soluble and
insoluble) forms at its relatively large amount, we noticed the formation of a precipitate adding some
amount (ca. 1 × 10–7 mol) of the catalyst to the homogeneous reaction solution.
9

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Fig. 4 here
20
15
10
5
0
2
6
8
10
12
0
4
Amount of 1  107 (mol)
Fig. 4 Dependence of the initial rate of oxygenate accumulation W0 in the cyclohexane oxidation with H2O2
catalyzed by 1 in acetonitrile on initial amount of 1. Conditions: [PCA]0 = 6 × 10–4 M; [H2O2]0 = 0.40 M; [H2O]added
0 M; [H2O]total = 0.76 M; [cyclohexane]0 = 0.40 M; 50 °C; MeCN up to total volume 5 mL. Concentrations of
=
oxygenates (sum of cyclohexanol and cyclohexanone) were measured after reduction with PPh3. Maximum initial
rates W0 were determined from the slopes of tangents (four examples are depicted with straight dotted lines in Fig.
2) to the kinetic curves in the intervals where the rate attains maximum (for the original kinetic curves, see Fig. S1).
Fig. 5 here
1
8
6
1a
4
2
0
2
6
8
0
4
Concentration [PCA]0  104 (M)
Fig. 5 Dependence of the initial oxidation rate W0 on initial concentration of PCA (curve 1) in the cyclohexane
oxidation in acetonitrile. Conditions: [1]0 = 2.0·× 10–5 M; [H2O2]0 = 0.40 M; [H2O]added = 0 M; [H2O]total = 0.76 M;
[cyclohexane]0 = 0.40 M; 50 °C. Concentrations of oxygenates (sum of cyclohexanol and cyclohexanone) were
measured after reduction with PPh3. The dotted curve 1a corresponds to the calculated dependences in
accordance with equation (8).
The dependence of W0 on the concentration of added PCA is displayed in Fig. 5 (curve 1). It can
be seen that at low concentrations of PCA the initial rate W0 is proportional to the concentration of PCA
which is introduced into the reaction solution. This is in agreement with our assumption that an adduct of
compound 1 with one PCA molecule is a catalytically active species. Fig. 5 shows that when [PCA]0
further rises, the rate W0 attains it maximum value and then decreases. Taking into account this
dependence we can assume that an adduct containing two PCA molecules in the coordination sphere does
10

not possess catalytic activity. Based on these data we can propose the following kinetic scheme for the
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formation of an oxidizing species X which induces the alkane oxidation:
K1
1.(PCA)
1 + PCA
(1)
K2
1.(PCA) + PCA
1.(PCA) + H2O2
1.(PCA)2
(2)
(3)
k3
X
In this scheme, reactions (1) and (2) are the equilibrium stages of adduct formation between compound 1
and one or two PCA molecules, respectively. Reaction (3) is a rate-limiting stage in the sequence of
transformations which are induced by the interaction of 1.(PCA) and H2O2. These transformations lead to
the generation of species X and regeneration of the catalyst active form. The rate W(X) of the X
generation in reaction (3) is equal to
W(X) = k3[1.(PCA)][H2O2]
(4)
Let us analyze the sequence of equilibrium transformations (1) and (2) assuming that the
concentrations [1.(PCA)] and [1.(PCA)2] are in quasi-equilibrium, i.e., the rate of reaction (3) is much
lower than the rate of the reversed stage of reaction (1). Taking into account that [1]0 is much lower than
[PCA]0 we obtain the following expression for the quasi-equilibrium concentration of the catalytically
active adduct 1.(PCA):
K1[1]0[PCA]0
[1.(PCA)] =
(5)
2
1 + K1[PCA]0 + K1K2[PCA]0
In this case the initial rate W0 of the species X generation in accord with (4) will be:
k3K1[1]0[PCA]0[H2O2]0
(6)
W0(X) =
2
1 + K1[PCA]0 + K1K2[PCA]0
The initial rate W0 of the cyclohexane oxidation is
W0 = W0(X) (7)
Here  is the fraction of species X of the total amount generated in reaction (3) which directly induces the
transformation of cyclohexane (CyH) into cyclohexyl hydroperoxide (CyOOH). Later in our analysis of
the experimental data we will see that parameter  does not depend on [PCA]0. Due to this, in order to
describe the dependence of W0 on [PCA]0 shown in Fig. 5 we can use the formula which contains in a
pronounced form only the dependence of W0 on [PCA]0:
[PCA]0
(8)
W0 =
2
1 + [PCA]0 + [PCA]0
Here we have the following values:
 = k3K1[1]0[H2O2]0 (s–1) for [1]0 = 2.0·× 10–5 M and [H2O2]0 = 0.4 M,
11

 = K1 (M–1),
View Article Online
 = K1K2 (M–2).
The analysis of accordance of data shown in Fig. 5 with equation (8) by the least-square method
taking into account the condition that  cannot be a negative value gave us:
 = 5.3 × 10–2 s–1,
 = 0,
 = 1.1 × 107 M–2.
The dependence of W0 on [PCA]0 calculated using equation (8) and the values for the coefficients
,  and  given above is presented in Fig. 5 by a dotted curve 1a. The mean-square deviation of the
experimental points from the simulated curve is 2.3 × 10–7 M s–1, which is less than 5% from the absolute
rate. Thus, we can conclude that we have a very good agreement between simulated dependence and the
experimental data. The calculated parameter  ≈ 0 means that the K1 value is low in such extent that the
linear relative [PCA]0 term in equation (8) is sufficiently less than unity at low [PCA]0. This term is also
sufficiently lower than the quadratic term in the denominator of equation (8) at high values of [PCA]0.
Values of K1 and consequently also of K2 can be very roughly estimated as K1 < 103 M–1, K2 > 104 M–1
whereas K1K2 = 1.1 × 107 M–2. It is necessary to note that the obtained values of parameters ,  and  as
well as the estimated effective constants K1 and K2 are right only for the experimental conditions shown
in Fig. 5.
Fig. 6 (Graph A) shows that in the dependence of W0 on the initial concentration of cyclohexane,
the rate W0 is approaching to a plateau at relatively high concentrations of cyclohexane. The selectivity of
alkane oxygenation reactions with the participation of the oxidizing species X corresponds to the
selectivity typical for the reactions with the participation of hydroxyl radicals (see below, Table 2). Thus,
we can write X = HO• for our case. The kinetic data shown in Fig. 6 also support our assumption about
the participation of hydroxyl radicals. The dependence presented in Fig. 6 reflects the competition
between cyclohexane and some other component (which concentration is constant in all experiments
shown in Fig. 6) for the oxidizing species (hydroxyl radical). Such “other” component could be PCA,
H2O2 and acetonitrile. An estimation of rate constants of the pseudo-first order reactions of HO• with
PCA, H2O2 and acetonitrile based on the parameters summarized in Refs. 10a,14d,16 assuming that
k(HO• + PCA) = 1010 M–1 s–1 demonstrates that acetonitrile is the main competitor for cyclohexane (CyH)
in their interaction with hydroxyl radical. In this case for treating the data shown in Fig. 6 it is reasonable
to consider the following kinetic scheme (3),(9),(10):
k3
1.(PCA) + H2O2
k9
X
(3)
(9)
...
HO + CyH
CyOOH
12

k10
products
(10)
HO + CH3CN
View Article Online
Here (3) is the reaction of hydroxyl radical generation with the rate W(HO•) = k3[1.(PCA)][H2O2]; (9) is
the process of CyH transformation into CyOOH with a bimolecular interaction between HO• and CyH as
the rate-limiting stage; (10) is the process of acetonitrile oxidation with the participation of hydroxyl
radicals (a bimolecular interaction between HO• and CH3CN as the rate-limiting stage). In the quasi-
stationary approximation, it follows from scheme (3)(9)(10) that within the total amount of hydroxyl
radicals generated in reaction (3) the fraction of HO• radicals  which induce the oxidation of the alkane
is
1
(11)
=
k10[CH3CN]
k9[CyH]
1 +
Taking into account the expression (11) for parameter  as well as equations (7) and (6) we can
obtain the general formula for the initial oxidation rate:
k3K1[1]0[PCA]0[H2O2]0

W0 =
2
1 + K1[PCA]0 + K1K2[PCA]0
1
(12)

k10[CH3CN]
k9[CyH]
1 +
The analysis of experimental data given in Fig. 5 demonstrated that there is a good agreement of these
data with eq. (12) because a straight line corresponds to the linear dependence of W0–1 on [CyH]0–1 (Fig.
6, Graph B). It follows from the linear dependence that
1
= 0.15 M
k10[CH3CN]
1 +
k9
As [CH3CN] = 18 M we obtain the ratio k10/k9 = 8.3·× 10–3. The value of this parameter is in agreement
with the assumption about the participation of hydroxyl radicals.10a,14h,16 For the conditions of
experiments presented in Fig. 6, parameter  = 0.73. Using the determined above values of  and
concentrations of 1 and H2O2 we can calculate the combination k3K1 = 900 M–2 s–1. As K1 < 10 M–1 it is
easy to estimate k3 > 1 M–1 s–1.
In conclusion, the data obtained in this work on kinetics of cyclohexane oxidation support our
assumption (see also the above discussion of the PCA role in the process and below the selectivity
parameters) about the participation of hydroxyl radicals as the key species in the oxidation catalyzed by
compound 1.
Fig. 6 here
13

10
View Article Online
A
8
6
4
0.18
0.14
0.10
B
2
0
0
2
6
4
[cyclohexane]01 (M1
)
0.2
0.6
0.8
0
0.4
Concentration [cyclohexane]0 (M)
Fig. 6 Graph A: Dependence of the initial oxidation rate W0 on initial concentration of cyclohexane in the
cyclohexane oxidation in acetonitrile. Conditions: [1]0 = 2.0·× 10–5 M; [PCA]0 = 6 × 10–4 M; [H2O2]0 = 0.2 M (50%
aqueous), [H2O]added = 0 M; [H2O]total = 0.38 M; 50 °C. Concentrations of oxygenates (sum of cyclohexanol and
cyclohexanone) were measured after reduction with PPh3. Graph B: linear anamorphosis of curve shown in Graph
–1
A in the coordinates [cyclohexane]0–1–W0
.
A linear dependence of W0 on the initial concentration of hydrogen peroxide at the constant total
concentration of water was found (Fig. 7). The oxidation rate decreases with increase of added water (Fig.
8), at least for water total concentrations above 0.4 M. The estimated activation energy is Ea = 16 ± 2 kcal
mol–1 (Fig. 9). This value is very close to that obtained for the cyclohexane oxidation with H2O2 catalyzed
by the (n-Bu4N)[VO3]/PCA combination (17 ± 2 kcal mol–1).10a,12
Fig. 7 here
Fig. 8 here
Fig. 9 here
8
6
4
2
0
0.1
0.2
0.4
0.3
0
Concentration [H2O2]0 (M)
14

Fig. 7 Dependence of the initial oxidation rate W0 on initial concentration of hydrogen peroxide in the cyclohexane
oxidation in acetonitrile. Conditions: [1]0 = 2.0·× 10–5 M; [PCA]0 = 6 × 10–4 M; [H2O]total = const =1.5 M;
View Article Online
[cyclohexane]0 = 0.15 M; 50 °C.
8
6
4
2
0
0.6 0.8
1.2
1.4 1.6
0.4
1.0
Concentration [H2O]total (M)
Fig. 8 Dependence of the initial oxidation rate W0 on total concentration of water in the cyclohexane oxidation in
acetonitrile. Conditions: [1]0 = 2.0·× 10–5 M; [PCA]0 = 6 × 10–4 M; [H2O2]0 = 0.2 M (50% aqueous), [cyclohexane]0 =
0.15 M; 50 °C.
11.5
12.0
12.5
13.0
13.5
14.0
3.1
3.2
3.3
3.0
1000/T (K1)
Fig. 9 Dependence of the initial oxidation rate W0 on the temperature of the reaction in the cyclohexane oxidation
in acetonitrile. Conditions: [1]0 = 2.0·× 10–5 M; [PCA]0 = 6 × 10–4 M; [H2O2]0 = 0.2 M (50% aqueous), [H2O]added = 0
M; [H2O]total = 0.38 M; [cyclohexane]0 = 0.15 M. Estimated activation energy is Ea = 16 ± 2 kcal mol–1.
In order to get a further mechanistic understanding of this process, we studied the oxidation of
some assay alkanes. The results are summarized in Table 2 (for the selectivity parameters, see Refs.
10a,14h,16,17; for selectivity definition, see Ref. 17g). The selectivity parameters for n-heptane are low,
C(1):C(2):C(3):C(4) ≈ 1:5:4:3. To compare these data (Entry 1), the corresponding parameters for some
other systems are also given. Thus, it is believed that the oxidation by hydrogen peroxide in air under
UV-irradiation (Entry 3) or in the presence of Fe(II) salt (the Fenton reagent; Entry 4) proceeds via the
15

formation of hydroxyl radicals which attack C–H bonds of the alkane. The selectivity parameters for the
View Article Online
–
oxidation by the H2O2/VO3 /PCA system (Entry 2) are also low. Other systems, which oxidize with the
participation of hydroxyl radicals are given in Entries 5–7. Unlike all these systems, alkane oxygenation
reactions by H2O2/[(TMTACN)MnIV(-O)3MnIV(TMTACN)]2+/CH3COOH system (Entry 8, compare
also Entry 9) apparently involve the interaction of the C–H bonds with MnV=O species. Selectivity
parameters for oxidation with Oxone and m-CPBA (Entries 10 and 11, respectively) are noticeably higher
than that for the 1/H2O2/PCA system.
In summary, it follows from Table 2 that the oxidation by the 1/H2O2/PCA system exhibits very
low selectivities for n-heptane and a branched alkane and the reaction with isomers of 1,2-
dimethylcyclohexane is not stereoselective. These data testify clearly that alkane oxygenation with the
1/H2O2/PCA system proceeds mainly with the participation of free hydroxyl radicals. It should be noted
that the data shown in Fig. 6 also support the assumption regarding the participation of hydroxyl radicals
in alkane oxidation by the system under discussion.
Table 2 Selectivity parameters obtained in the oxidation of linear and branched alkanes in acetonitrile with H2O2 catalyzed by complex 1
and (for comparison) some other systems. a,b
_________________________________________________________________________________________________________________
C(1):C(2):C(3):C(4)
________________
1º:2º:3º
___________
trans:cis
_________________________
System
n-Heptane
MCH
c-1,2-DMCH
t-1,2-DMCH
_________________________________________________________________________________________________________________
1
2
1/PCA/H2O2 (this work)
1:5:4:3
1:5:14
1:6:18
0.72
0.75
0.90
1.3
0.64
0.80
(n-Bu4N)[VO3]/PCA/H2O2 (Ref. 10a)
h/H2O2 (Ref. 10a,16)
1:9:7:7
3
1:7:6:7
4
FeSO4/H2O2 (Ref. 10a)
1:5:5:4.5
1:7:7:6
1:3:6
1.2
0.80
0.85
0.8
4.1
13
5
[Cp2Fe]/PCA/H2O2 (Ref. 14o)
[Os3(CO)12]/py/H2O2 (Ref. 17a)
Al(NO3)3/H2O2 (Ref. 17b)
[Mn2L2O3]2+/MeCO2H/H2O2 (Ref. 17c)
2/oxalic acid/H2O2 (Ref. 17d)
[Mn2L2O3]2+/oxalic acid/Oxone (Ref. 17e)
FeCl3/L3/m-CPBA (Ref. 17f)
1:10:33
1:5:11
1:6:23
1:26:200
0.80
6
1:4:4:4
7
1:5:5:5
0.8
8
1:42:37:34
1:91:99:68
1:30:28:30
1:29:30:27
0.34
0.31
0.5
9
10
11
1:12:150
0.2
3.0
1:21:211
0.25
a All parameters were measured after reduction of the reaction mixtures with triphenylphosphine before GC analysis and calculated based on
the ratios of isomeric alcohols. Parameter C(1):C(2):C(3):C(4) is the relative normalized (taking into account the number of hydrogen atoms
at each carbon) reactivities of hydrogen atoms at carbons 1, 2, 3 and 4 of the chain of n-heptane. Parameter 1º:2º:3º is the relative normalized
reactivities of hydrogen atoms at primary, secondary and tertiary carbons of methylcyclohexane (MCH). Parameter trans/cis is the ratio of
isomers of tert-alcohols with mutual trans- and cis-orientation of the methyl groups formed in the oxidation of cis- and trans-1,2-
dimethylcyclohexane (DMCH).
b Abbreviations. Term h means UV irradiation [Cp2Fe] is ferrocene. Complex [Mn2L2O3]2+ is the binuclear manganese derivative [LMn(-
O)3MnL]2+, where L = 1,4,7-trimethyl-1,4,7-triazacyclononane (TMTACN). Complex 2 is [Mn2(R-LMe2R)2(-O)2]3+ where R-LMe2R = (R)-1-
(2-hydroxypropyl)-4,7-dimethyl-1,4,7-triazacyclononane. Oxone is 2KHSO5.KHSO4.K2SO4. L3 is tetradentate amine N,N_-bis(2-
pyridylmethylene)-1,4-diaminodiphenyl ether. m-CPBA is metachloroperoxybenzoic acid.
16

By decreasing the concentration of catalyst 1, we were able to significantly increase VTieOw ANrtiscle(Oi.nelin.e,
turnover numbers; mols of products per one mol of the catalyst; see Ref. 18) and initial TOFs (i.e.,
turnover frequencies; mols of products per one mol of the catalyst obtained during the first hour of the
reaction) (see Fig. 2B). Thus, in the experiment shown by curve 3, after 2220 min, TON = 44 000, and
initial TOF = 3300 h–1. These values are comparable with the corresponding parameters obtained earlier
by some of us for the cyclohexane oxidation with the [Os3(CO)12]/pyridine/H2O2 system (60 000 and 24
000 h–1; Ref. 17a) and the [Cp*2Os]/pyridine/H2O2 system (51 000 and 6000 h–1; Ref. 17f).
The activity of complex 1 attained in the present work is two times higher than the activity of
mononuclear complexes: vanadate anion (n-Bu4N)[VO3] and vanadatrane.7–12 Fig. 10 shows the kinetic
curves at two different concentrations of the three abovementioned catalysts. If we recalculate the yields
of products per one vanadium ion we clearly see that the activities of one vanadium ion in the three
complexes are approximately equal. Thus, TON values per one vanadium ion are 5350, 5000 and 5700
for complexes 1, (n-Bu4N)[VO3] and oxovanadium(V) triethanolaminate (vanadatrane), respectively,
after 360 min at a catalyst concentration of 1.0 × 10–5 M (Fig. 10A). At a lower concentration of each
catalyst (1.0·× 10–6 M) the corresponding TONs per one vanadium ion for the same complexes are 19
000, 20 000 and 19 000 after 1000 min (Fig. 10B). In the last case, for the catalysis by (n-Bu4N)[VO3] the
initial TOF = 2100 h–1.
Fig. 10 here
17

N
V
View Article Online
A
O
O
0.10
0.08
O
O
vanadatrane
Complex 1
0.06
0.04
0.02
0
V
O
O
O
vanadate
100
300 400
200
0
Time (min)
B
Complex 1
0.05
0.04
vanadate
0.03
0.02
0.01
0
vanadatrane
400 600 800 1000 1200
Time (min)
0
200
Fig. 10 Kinetic curves of product accumulation in the cyclohexane oxidation catalyzed by various vanadium-
containing systems: complex 1, vanadate anion (n-Bu4N)[VO3] and oxovanadium(V) triethanolaminate
(vanadatrane). Conditions: [catalyst]0 = 1.0·× 10–5 (Graph A) or 1.0·× 10–6 M (Graph B); [PCA] = 6 × 10–4 M; [H2O2]0
= 0.4 M (50% aqueous), [H2O]added = 0 M; [H2O]total = 0.76 M; [cyclohexane]0 = 0.40 M; MeCN up to 5 mL total
volume; 50 °C.
18

We have also found that the 1/PCA/H2O2 reagent oxidizes efficiently light alkanes in acetonitrile
View Article Online
solution at a relatively low temperature. Main results are summarized in Tables 3 and 4. The great
difference of measured concentrations of alcohols and ketones (aldehydes) indicates 11c,d,i,17a that the
corresponding alkyl hydroperoxides are formed in substantial concentrations (see Table 3).
Table 3 Oxidation of light alkanes with the 1/PCA/H2O2 system at relatively high amount of 1. a
______________________________________________________________________________________________________
Concentration (M)
_______________________
Alkane
(atm; mmol)
Total
yield (%)
Total
TON
Time (h) Product
Before PPh3
After PPh3
______________________________________________________________________________________________________
CH4 (10; 2.9)
C2H6 (10; 2.9)
2
6
CH3OH
0
0.0012
0.3
10
25
CH3CH2OH
CH3CHO
CH3COOH
0.0061
0.0163
0.0039
0.0438
0.0086
0.0036
1100
C3H8 (8; 2.3)
6
6
(CH3)2CHOH
(CH3)2CO
CH3CH2CH2OH
CH3CH2CHO
0.0019
0.0323
0.0040
0.0061
0.0865
0.0378
0.0269
0.0035
34
21
3000
230
n-C4H10 (1.0; 0.3)
CH3CH2CH(OH)CH3
CH3CH2COCH3
CH3CH2CH2CH2OH
CH3CH2CH2CHO
0.0003
0.0094
0.0009
0.0003
0.0011
0.0087
0.0018
0.0002
a Reaction conditions: [1]0 = 5 × 10–5 M, [PCA]0 = 4 × 10–4 M; [H2O2]0 = 1.28 M, [H2O]total = 2.4 M,
MeCN up to total volume 5 mL, 50 °C. Volume of the autoclave was 13 mL. Accumulation of products
with time is shown in Fig. S2.
Table 4 Oxidation of light alkanes with the 1/PCA/H2O2 system at
relatively low concentration of 1. a
________________________________________________________
Alkane
Product
Total
Total
(atm; mmol)
(M, after PPh3)
yield (%) TON
________________________________________________________
C2H6 (10; 2.9)
C3H8 (8, 2.3)
CH3CH2OH (0.0480)
CH3CHO (0.0142)
15
6
6220
2840
(CH3)2CHOH (0.0151)
(CH3)2CO (0.0027)
CH3CH2CH2OH (0.0106)
n-C4H10 (1.0; 0.3) CH3CH2CH(OH)CH3 (0.0023) 12
730
CH3CH2COCH3 (0.0050)
a Conditions: [1]0 = 5 × 10–5 M, [PCA]0 = 8 × 10–5 M; [H2O2]0 = 1.28
M, [H2O]total = 2.4 M, 50 °C, 23 h. Total volume of the reaction
solution was 5 mL. Volume of the autoclave was 13 mL.
19

Experimental section
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General procedures
All synthetic work was performed in air. The reagents and solvents were obtained from commercial
sources and used as received, i.e., without further purification or drying. The salt VOSO4·3H2O was used
as a precursor for the synthesis of complex 1. The C, H, and N elemental analyses were carried out by the
Microanalytical Service of the Instituto Superior Técnico. Melting points were determined with a Leica
Gallen III instrument. Infrared spectra (4000–400 cm–1) were recorded on a BIO-RAD FTS 3000MX or
Jasco FT/IR-430 instrument in KBr pellets; wavenumbers are in cm–1. 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.
Synthesis of the pro-ligand bis(2-hydroxybenzylidene)terephthalohydrazide (H4L)
The Schiff base pro-ligand bis(2-hydroxybenzylidene)terephthalohydrazide (H4L), was prepared
by condensation of two equivalent of salicylaldehyde with one equivalent of terephthalolhydrazide by a
reported method as described in the literature.15a
Yield: 91.2%. Anal. calc. for H4L(C22H18N4O4): C, 65.66; H, 4.51; N, 13.92; found: C, 65.58; H,
5.19; N, 13.85%. IR (KBr pellet, cm1): 3212 (OH), 3055(NH), 1653 (C=O). 1H NMR (DMSO-d6, ):
12.09 (s, 2H, OH), 11.18 (s, 2H, NH), 8.09 (s, 2H, -CH=N),7.58-6.94 (m, 12H, C6H4).
Synthesis of the binuclear oxovanadium(V) complex [{VO(OEt)(EtOH)}2L] (1) (see Scheme 1)
To a 50 mL ethanolic suspension of H4L [0.402 g (1.00 mmol)], 0.530 g (2.00 mmol) of
[VO(acac)2] was added and the reaction mixture was refluxed for 2 h in an oil bath, in open air. The
resultant reddish brown solution was filtered and the filtrate was kept in air. After ca. 3-4 d, a crystalline
product was isolated, washed 3 times with cold ethanol and dried in open air. X-ray quality single crystals
were isolated from a mixture of acetonitrile-ethanol solution.
Yield 72%. Anal. Calcd for C30H36N4O10V2: C, 50.43; H, 5.08; N, 7.84. Found: 50.31; H, 5.02; N,
7.73. IR (KBr; cm1): 1603 (C=N), 1281 (C–O)enolic, 1087 ( N–N), 970 (V=O).
1H NMR (DMSO-d6, ): 8.19 (s, 2H, -CH=N),7.87-7.15 (m, 12H, C6H4), 5.48 (q, 4H, CH2, J =
7.0 OEt,), 1.71(t, 6H, CH3, J = 6.98, OEt). 51V NMR (DMSO-d6; /ppm) -542. ESI-MS(+): m/z 825
[1+2EtOH+H2O+H]+ (100%).
20

X-ray measurements
View Article Online
The X-ray quality single crystal of complex 1 was immersed in cryo-oil, mounted in a Nylon loop and
measured at the temperature of 150 K. Intensity data were collected using a Bruker AXS-KAPPA APEX
II diffractometer with graphite monochromated Mo-Kα ( 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 SMART18a software and refined using Bruker SAINT 19a on all the observed reflections.
Absorption corrections were applied using SADABS.19a Structures were solved by direct methods by
using the SHELXS–97 package19b and refined with SHELXL–97.19b Calculations were performed using
the WinGX System–Version 1.80.03.19c 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. Crystallographic data are summarized in Table 1 and
selected bond distances and angles are presented in the caption to Fig. 1. CCDC 926559 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.
Alkane oxygenation reactions
Catalyst 1 and co-catalyst (PCA) were used in the form of stock solutions in MeCN. Aliquots of these
solutions were added to the reaction mixtures in alkane oxidation. The reactions of alkanes with H2O2
(50% aqueous) were typically carried out in air in thermostated Pyrex cylindrical vessels with vigorous
stirring (CAUTION: the combination of air or molecular oxygen and H2O2 with organic compounds at
elevated temperatures may be explosive!). The reactions were stopped by cooling, and typically analyzed
twice, i.e., before and after the addition of an excess of solid PPh3. This method was developed previously
by Shul’pin 11c,d,i,17a for the analysis of reaction mixtures obtained from various alkane oxidations. In the
case of the formation of the corresponding alkyl hydroperoxides as the main products, if an excess of
solid PPh3 is added to a sample of the reaction solution, ca. 10 min before GC analysis, the alkyl
hydroperoxide present is completely reduced to the corresponding alcohol. As a result, the chromatogram
differs from that of a sample not subjected to the reduction: the alcohol peak rises, while the intensity of
the ketone peak decreases. In our kinetic studies for precise determination of oxygenate concentrations
only data obtained after reduction of the reaction sample with PPh3 were used. A Fisons Instruments GC
8000 series gas chromatograph with a capillary column 30 m × 0.22 mm × 25 m, BP20 (SGE) (helium
was the carrier gas; the internal standard was nitromethane) was used. Typical chromatograms obtained
for the mixtures of products from n-heptane and methylcyclohexane oxidations are shown in Figs. S3 and
S4, respectively.
21

The oxidations of gaseous alkanes with H2O2 were carried out in a 13 mL stainless steel autoclave,
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equipped with a Teflon-coated magnetic stirring bar. In a typical experiment, after additions of all liquid
reagents in the form of stock solutions in acetonitrile, the autoclave was closed and pressurized with
gaseous alkane. The reaction mixture was stirred 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.
Conclusions
A synthesized in this work binuclear oxovanadium(V) complex 1 exhibits high catalytic activity in alkane
oxidation with H2O2 when PCA is used as a co-catalyst. Detailed kinetic and selectivity studies on the
oxidation of linear and branched alkanes indicate that the mechanism of the reaction involves the
hydroxyl radical as the key alkane oxidant species.
A synergy between the two vanadium metal centres is not observed in the current catalytic system
since the activity of 1 per each V ion is similar to those of some active mononuclear vanadium(V)
complexes, such as vanadate or vanadatrane. The terephthalohydrazide-type bridge in 1 is too long to
allow a cooperative effect of the vanadium ions, in contrast to what we have reported 6c for divanadates
(or oligovanadates), with oxo-bridges that allow proton-transfer steps from H2O2 ligating one of the V
ions to an O-ligand at the other V ion. Such H+-transfer, involving a divanadium 6-membered transition
state, is then more effective than that assisted by water. It is noteworthy to mention that even when PCA
can be replaced by another H+-shift promoter, its presence can be favourable by stabilizing involved
transition states. 13d
Although PCA appears as quite an affective and general co-catalyst for alkane oxidation with
H2O2, by promoting H+-shifts on the way to the formation of the hydroxyl radical, one should not expect
such a role when the mechanism does not involve HO. In fact, quite efficient vanadium catalysts have
been reported by Pombeiro and co-workers3j,20 to operate, without requiring PCA, in alkane
functionalization (carboxylation). Relevant cases concern the natural V-complex amavadin, its model and
vanadatrane, for the carboxylation of alkanes (RH, including methane and ethane) with CO and K2S2O8 in
trifluoroacetic acid (TFA), at 80 ºC, to yield the corresponding carboxylic acids (RCOOH) with very high
yields and TONs. In these reactions the sulphate radical, instead of the hydroxyl, is the alkane oxidant,
and the use of PCA is not required.
Acknowledgements
The authors thank the Russian Foundation for Basic Research, grant 12-03-00084-a, and the Fundação
para a Ciência e a Tecnologia (FCT), projects PTDC/QUI-QUI/102150/2008 and PEst-
22

OE/QUI/UI0100/2011, for support. G. B. S. expresses his gratitude to the FCT for making it possible for
View Article Online
him to stay at the Instituto Superior Técnico, TU Lisbon, as an invited Professor and to perform a part of
the present work.
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DT-ART-03-2013-050584-GraphAbstr-REVISED
Graphical Abstract:
O
EtOH
O
OEt
O
N N
V
N N
V
O
OOH
H
O
catalyst:
EtO
O EtOH
COOH
N
N
conditions:
H2O2, MeCN,
TON = 44 000
TOF = 3300 h1
cocatalyst:
air, 50 o
C
(PCA)
Text for Graphical Abstract:
The combination of a new binuclear oxovanadium(V) complex with pyrazine-2-carboxylic acid (PCA; a cocatalyst)
affords a catalytic system for the efficient oxidation of saturated hydrocarbons with hydrogen peroxide to produce
alkyl hydroperoxides.
28