Vanadium complexes of different nuclearities in the catalytic oxidation of cyclohexane and cyclohexanol-an experimental and theoretical investigation

Article abstract of DOI:10.1039/c9nj00348g

The mono-, bi-and tri-nuclear oxidovanadium(v) complexes [VO(OEt)(L¹)] (1), [{VO(OEt)(EtOH)}₂(L²)] (2) and [{VO(OMe)(H₂O)}₃(L³)]·2H₂O (3) (L¹ = 2-hydroxy-(2-hydroxybenzylidene)benzohydrazide, L² = bis(2-hydroxybenzylidene)terephthalohydrazide and L³ = tris(2-hydroxybenzylidene)benzene-1,3,5-tricarbohydrazide) (3 was synthesized for the first time) were used to investigate the role of vanadium nuclearity in the catalytic oxidation of cyclohexane and cyclohexanol. They are active homogeneous catalysts for the microwave-assisted neat peroxidative oxidation of cyclohexane (with aq. H₂O₂ to cyclohexanol and cyclohexanone) and cyclohexanol (with aq. ^tBuOOH to cyclohexanone). A mechanism of the HO radical generation-the rate limiting step of the cyclohexane oxidation by H₂O₂-was investigated in detail by theoretical DFT methods. The mechanism is different from those usually accepted for the Fenton or Fenton-like systems and it includes (i) coordination of H₂O₂ to the catalyst molecule, (ii) proton transfer from ligated H₂O₂ to the methoxy ligand and elimination of the formed methanol molecule and (iii) coordination of the second H₂O₂ molecule followed by HO-OH bond cleavage to give HO. The activation of the ligated H₂O₂ towards this cleavage is associated with the redox active nature of the ligand L in the catalyst molecule, which acts (instead of the metal) as the reducing agent of the H₂O₂ ligand.

Full text of DOI:10.1039/c9nj00348g

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: M. Sutradhar, L.
M. Martins, T. Roy Barman, M. L. Kuznetsov, M. F. C. Guedes da Silva and A. J. L. Pombeiro, New J. Chem.,
2019, DOI: 10.1039/C9NJ00348G.
This is an Accepted Manuscript, which has been through the
Volume 40 Number
1 January 2016 Pages 1–846
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
Accepted Manuscripts are published online shortly after
New Journal of Chemistry
www.rsc.org/njc
A journal for new directions in chemistry
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Page 1 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
Vanadium complexes of different nuclearities in the catalytic
oxidation of cyclohexane and cyclohexanol ‒ An experimental
and theoretical investigation
9
Manas Sutradhar,* Luísa M.D.R.S. Martins,* Tannistha Roy Barman, Maxim L. Kuznetsov,* M.
Fátima C. Guedes da Silva and Armando J. L. Pombeiro*
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco
Pais, 1049-001 Lisboa, Portugal
Email: manas@tecnico.ulisboa.pt (MS); luisamargaridamartins@tecnico.ulisboa.pt (LMDRSM);
max@mail.ist.utl.pt (MLK); pombeiro@tecnico.ulisboa.pt (AJLP)
Abstract
The mono-, bi- and tri-nuclear oxidovanadium(V) complexes [VO(OEt)(L1)] (1),
[{VO(OEt)(EtOH)}2(L2)] (2) and [{VO(OMe)(H2O)}3(L3)]·2H2O (3) (L1 = 2-hydroxy-(2-
hydroxybenzylidene)benzohydrazide, L2 = bis(2-hydroxybenzylidene)terephthalohydrazide and L3
= tris(2-hydroxybenzylidene)benzene-1,3,5-tricarbohydrazide) (3 was synthesized for the first time)
were used to investigate the role of vanadium nuclearity in the catalytic oxidation of cyclohexane
and cyclohexanol. They are active homogeneous catalysts for the microwave-assisted neat
peroxidative oxidation of cyclohexane (with aq. H2O2 to cyclohexanol and cyclohexanone) and
t
cyclohexanol (with aq. BuOOH to cyclohexanone). A mechanism of the HO• radical generation –
the rate limiting step of the cyclohexane oxidation by H2O2 – was investigated in detail by
theoretical DFT methods. The mechanism is different from those usually accepted for the Fenton or
Fenton-like systems and it includes (i) coordination of H2O2 to the catalyst molecule, (ii) proton
transfer from ligated H2O2 to the methoxy ligand and elimination of the formed methanol molecule
and (iii) coordination of the second H2O2 molecule followed by HO–OH bond cleavage to give HO•.
The activation of the ligated H2O2 towards this cleavage is associated with the redox active nature of
the ligand L in the catalyst molecule, which acts (instead of the metal) as the reducing agent of the
H2O2 ligand.
Keywords: Oxidovanadium(V) complexes; aroylhydrazone; microwave-assisted oxidation; DFT
calculations; Mechanism.
1

New Journal of Chemistry
Page 2 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
Introduction
The oxidation of alkanes is receiving special attention as hydrocarbons of this type are cheap
and abundant in nature and can be important as potential precursors for the synthesis of useful
products.1-18 However, the oxidation of alkanes is highly challenging due to their inertness, and their
activation for such processes is rather difficult. Also, the selective oxidation of alcohols to afford
carbonyl compounds is recognized as a key reaction in synthetic organic chemistry.19-23 In view of
economic and environmental reasons, the development of efficient and selective catalysts for such
oxidation reactions under sustainable conditions is challenging. The use of microwave irradiation to
assist such reactions can contribute to that aim, on account of its simplicity, accelerating power and
energy saving.22-24
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The role of vanadium in the development of oxidation catalysts has been object of current
and growing interest from both biological and chemical perspectives.1-8,25,26 They catalyze various
C-H oxidation reactions of organic substrates, e.g., saturated, aromatic, and olefinic hydrocarbons
by H2O2 and some other organic peroxides (for example, tert-butyl hydroperoxide, TBHP).27
Vanadium complexes have also shown potential as active catalysts for oxidation of alcohols in the
presence of suitable stoichiometric oxidants like H2O2, TBHP or dioxygen.1-8
In continuation of our interest in designing metal complexes derived from aroyhydrazones to
catalyze oxidation reactions28-37 herein we report the catalytic activities of three oxidovanadium(V)
complexes with three different nuclearities (mono, bi and tri) having a similar type of azine based
ligands towards the microwave assisted oxidation of alkanes and alcohols under mild conditions.
The mono and binuclear complexes [VO(OEt)(L1)] (1) and [{VO(OEt)(EtOH)}2(L2)] (2) have been
synthesized according to the literature.28,38,39 The synthesis of multidentate N,O donor azine based
ligand
having
three
coordination
and of
pockets,
its
tris(2-hydroxybenzylidene)benzene-1,3,5-
trinuclear oxidovanadium(V) complex
tricarbohydrazide
(H6L3)
[{VO(OMe)(H2O)}3(L3)]·H2O (3) are reported for the first time in this study. The catalytic activities
of these complexes having three different nuclearities towards the solvent-free microwave-assisted
oxidation of cyclohexane and cyclohexanol are explored by experimental and theoretical DFT
studies, to propose a possible mechanistic pathway of the catalytic peroxidative oxidation of
cyclohexane.
2

Page 3 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
Results and discussion
4
5
6
7
8
9
Three aroylhydrazone ligands having similar coordinating moieties but differing in their
number have been used to synthesize the oxidovanadium(V) complexes 1-3 with different
nuclearities. Complexes 1 and 2 have been synthesized according to a reported method.28,38,39 To
prepare a trinuclear oxidovanadium(V) complex having a tris-tridentate aroylhdrazone moiety the
pro-ligand tris(2-hydroxybenzylidene)benzene-1,3,5-tricarbohydrazide (H6L3) has been synthesized
(see experimental) and used. This tris(aroylhydrazone) behaves as a potentially nonadentate ligand.
It undergoes enolization in solution can act as a tris(di-negative) chelator (Scheme 1) being able to
simultaneously encapsulate three metal ions. Its reaction with VOSO4 in methanol led to the
formation of the trinuclear oxidomethoxidovanadium(V) complex [{VO(OMe)(H2O)}3(L3)]·2H2O
(3) (Scheme 1). The oxidation of the oxidovanadium(IV) moiety to the oxidovanadium(V) species
in solution under aerobic conditions is well known.4,28,29,32,33,38-41
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The characterization of complex 3 has been carried out by elemental analysis, IR and NMR
(1H, 51V) spectroscopy, ESI-MS and single crystal X-ray diffraction. The IR spectrum of complex 3
displays the (O-H) bands of coordinated and non-coordinated water molecules at 4218 and 4061
cm-1, the (C=N) band at 1602, the enolic (C–O) band at 1250 and (N–N) band at 1036 cm-1.
Three characteristic (V=O) bands are observed at 1007, 986 and 914 cm-1, which are the signature
of oxidovanadium(IV) and (V) compounds.4,28,29,32,33,38-41 In the ESI-MS spectrum, the m/z value
indicates the presence of the trinuclear species with the loss of two water molecules (see
1
Experimental). The H NMR spectrum shows the aromatic protons in the range  7.65-6.94, the
benzylic protons at  8.62 and a broad signal of methyl protons of the methoxido groups at  3.72.
51
The presence of three oxidovanadium centres is observed at -529, -541 and -579 ppm in the V
NMR spectrum. The UV-Vis spectra of 1-3 have been recorded in acetonitrile and also under
catalytic reaction conditions (Fig. S1, supporting information). From these spectra it can be assumed
that the complexes form new species in the presence of H2O2, therefore being better considered as
catalytic precursors than catalysts. The speciation and reactivity of metal complexes in solution42-44
is therefore important to identify reacting species, what concerns a future work. The X-ray crystal
structure of 3 is described below.
3

New Journal of Chemistry
Page 4 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
HO
HO
N
N
N
7
8
NH
O
HO
9
H
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N
N
N
N
N
N
N
N
O
O
OH
Enol
OH
HO
HO
OH
OH
Keto
VOSO4
MeOH
Reflux, 2h
O
O
V
O
N
N
O
H2O
2H2O
N
O
N
N
N
O
O
O
V
V
OH2
O
OH2
O
O
O
3
(a)
O
V
O
EtO
O
OEt
N
O
N
N
O
O
EtOH
HO
N
V
V
O
O
EtOH
N
N
O
EtO
1
2
(b)
Scheme 1 (a) Synthesis of [{VO(OMe)(H2O)}3(L3)]·2H2O (3); (b)Structures of 1 and 2.
4

Page 5 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
Description of Crystal Structure
8
9
X-ray quality crystals of 3 were obtained upon slow evaporation of a methanol solution of
the compound, at room temperature. Fig. 1 displays a perspective view of the molecule,
crystallographic data are summarized in Table 1 and selected dimensions are presented in Table 2.
The hexaanionic ligand (L3)6 acts as tris(tridentate) N3O6 chelating species towards three
vanadium cations. Each metal displays distorted N1O5 octahedral environment with the basal plane
engaged with the O-phenolic, the O-enolic and the N-imine atoms of the chelating ligand and with
an O-methoxide. The axial positions are occupied by the O-oxido and the O-water molecule.
Considering the least square (l.s.) planes constructed with the equatorial binding atoms, the metal
cations are situated 0.309 (V1), 0.314 (V2) and 0.274 Å (V3) towards the corresponding oxido
atom.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Various structural parameters in 3 are in agreement with those already reported.4,28,29,32,33,38-
45 Thus, the Ooxido-V-Omethoxide bond angles assume values of 173.0(3)º, 174.1(3)º and 176.1(3)º, but
those involving the chelator O-atoms adopt values of 150.8(3)º, 151.6(3)º and 152.6(2)º because of
chelation constrains and justify the observed octahedral distortion. The average N-V-O angles in the
five- and six-membered metallacycles are of 74.3(2) and 83.6(2)º, respectively. The V=O and
VOwater bond distances average 1.555(6) and 2.303(7) Å, respectively. The various vanadium-
oxygen bond lengths in 3 follow the order V-Ooxido < V-Omethoxido < V-Ophenoxido < V-Oenolate < V-
Owater
.
Considering the least-square plane that contains the vanadium cations (the V-plane), the
coordinated water molecules are all on the same side of that plane, together with the central
aromatic ring of (L3)6 whose l.s. plane is twisted, relatively to the V-plane, by 3.36º. The phenolate
rings bound to V2 and V3 also incline to that side of the V-plane, their l.s. planes making angles of
7.95º and 10.66º, in this order, with the latter. The phenolate ring bound to V1, however, is
displaced to the opposite side by 22.13º.
In view of the number of water molecules in the structure, the molecules of 3 are involved in
several medium intense H-bond interactions that give rise to an infinite 2D network with base
vectors [010] and [101]. The minimum intramolecular V···V distance is of 9.417 Å, but the
intermolecular one is considerably shorter, 5.970 Å, and comprises the V2 and V3 cations.
5

New Journal of Chemistry
Page 6 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 1 Molecular structure of [{VO(OMe)(H2O)}3(L3)]·2H2O (3) with partial atom numbering
scheme. Solvated water molecule is omitted for clarity.
6

Page 7 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
Table 1. Crystal data and structure refinement details for complex 3.
4
5
6
7
8
Empirical formula
C33H33N6O15V3·2H2O
942.49
Formula Weight
9
Crystal system
Triclinic
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Space group
P-1
a/Å
11.6728 (12)
13.1012 (12)
13.9688 (12)
98.888 (5)
111.434 (4)
92.418 (5)
1953.3 (3)
2
b/Å
c/Å

/

V (Å3)
Z
Dcalc (g cm-3)
1.572
(Mo K) (mm-1)
Rfls. collected/unique/observed
Rint
0.782
21572/6755/3185
0.0793
Final R1 a, wR2 b (I  2σ)
Goodness-of-fit on F2
0.0880, 0.1849
1.044
a R = ||Fo|-|Fc||/|Fo| ; b wR(F2) = [w(|Fo|2 - |Fc|2)2/w|Fo|4]1/2
7

New Journal of Chemistry
Page 8 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
Table 2. Selected parameters, bond distances (Å) and angles (º) in complex 3.
4
5
6
7
8
9
O1—V1
O1V—V1
O2—V1
O2V—V2
O3—V2
1.849(6)
1.557(6)
1.932(5)
1.583(6)
1.834(6)
O4—V2
O3V—V3
O5—V3
O6—V3
O7—V1
1.934 (5)
1.612(6)
1.814(6)
1.921(5)
2.292(6)
O8—V1
O9—V2
O10—V2
O11—V3
O12—V3
1.762(6)
2.343(7)
1.760(6)
2.336(8)
1.759(6)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O1V—V1—O8
O1V—V1—O1
O8—V1—O1
O1V—V1—O2
O8—V1—O2
O1—V1—O2
O1V—V1—N1
O8—V1—N1
O1—V1—N1
O2—V1—N1
O1V—V1—O7
O8—V1—O7
O1—V1—O7
O2—V1—O7
N1—V1—O7
104.0 (3)
O2V—V2—O10
O2V—V2—O3
O10—V2—O3
O2V—V2—O4
O10—V2—O4
O3—V2—O4
O2V—V2—N3
O10—V2—N3
O3—V2—N3
O4—V2—N3
O2V—V2—O9
O10—V2—O9
O3—V2—O9
O4—V2—O9
N3—V2—O9
102.3 (3)
101.7 (3)
100.6 (3)
98.5 (3)
95.4 (3)
150.8 (3)
94.2 (3)
161.7 (3)
83.6 (2)
74.1 (2)
173.0 (3)
82.9 (3)
81.7 (3)
76.2 (2)
80.0 (2)
O3V—V3—O12
O3V—V3—O5
O12—V3—O5
O3V—V3—O6
O12—V3—O6
O5—V3—O6
99.3 (3)
99.8 (4)
103.2 (3)
99.6 (3)
93.8 (3)
151.6 (3)
94.2 (3)
163.5 (3)
83.7 (3)
74.4 (2)
176.1 (3)
84.1 (3)
81.1 (3)
78.2 (3)
82.1 (3)
99.4 (3)
103.5 (3)
98.1 (3)
92.4 (3)
152.6 (2)
94.1 (3)
159.0 (3)
83.6 (3)
74.3 (2)
174.1 (3)
81.8 (3)
79.6 (3)
80.8 (2)
80.0 (2)
O3V—V3—N5
O12—V3—N5
O5—V3—N5
O6—V3—N5
O3V—V3—O11
O12—V3—O11
O5—V3—O11
O6—V3—O11
N5—V3—O11
Catalysis
Following our attempt28,29,32 of using oxidovanadium(V) complexes to catalyze the oxidation
of cyclohexane with aqueous hydrogen peroxide usually in NCMe at room temperature, herein
compounds 1‒3 were successfully tested as homogeneous catalysts for the microwave-assisted neat
oxidation (without added solvent) of cyclohexane with aq. hydrogen peroxide to cyclohexanol and
cyclohexanone. Cyclohexyl hydroperoxide (CyOOH) is formed as a primary product and evolves in
a mixture of cyclohexanol and cyclohexanone (final products, Scheme 2). The formation of CyOOH
was proved by using Shul’pin’s method8,46-48 (see experimental): the addition of PPh3 prior to the
GC analysis of the products resulted in a marked increase of the amount of cyclohexanol (due to the
reduction of CyOOH by PPh3 and formation of phosphine oxide) and decrease in the amount of
cyclohexanone relatively to those of the sample not subjected to the reduction (see Table 3, entry 14
8

Page 9 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
versus entry 12). The product yields given in Table 3 (except if indicated otherwise) were obtained
upon addition of PPh3, where A concerns the total of cyclohexanol + CyOOH at the end of the
catalytic reaction.
6
7
8
9
O
OOH
OH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
V catalyst
aq. H2O2
+
MW, 60 ºC
Scheme 2 MW-assisted neat oxidation of cyclohexane to cyclohexyl hydroperoxide, cyclohexanol
and cyclohexanone with aqueous hydrogen peroxide catalysed by 1‒3.
Under the optimized eco-friendly conditions of these solvent- and additive-free systems (60
ºC and 1.5 h of low power (25 W) MW irradiation), yields up to 39% (for the trinuclear complex 3,
entry 12 of Table 3) of the oxygenated products are obtained (Scheme 2, Fig. 2) using a low
vanadium catalyst load (0.2% molar ratio relatively to cyclohexane). This yield value is ca. 5 times
higher than the best ones (8%) reported49,50 for the industrial aerobic process to assure a good
selectivity. Cyclohexanol and cyclohexanone were the only products detected by GC-MS analysis
under these assayed conditions, thus revealing a very selective oxidation system. The reaction
proceeds very quickly, in accord with a high activity of our V catalysts. In fact, after only 15 min of
MW irradiation 15 (1), 23 (2) and 26% (3) yield of oxygenates are achieved (Table 3, entries 1, 5
and 9, respectively).
9

New Journal of Chemistry
Page 10 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 2 Effect of the reaction time on the total yield of cyclohexanol and cyclohexanone obtained by
microwave-assisted neat oxidation of cyclohexane with H2O2 catalyzed by complexes 1‒3.
Table 3. Selected dataa for the MW-assisted oxidation of cyclohexane with H2O2 catalysed by 1 - 3.
Yield / % b
Total
Catalyst
A/Kc
Time /
h
Total
Cyclohexanol +
CyOOH
Entry
TOF
Cyclohexanone
(K)
Total
TON d
/ h-1 e
(A)
1
2
0.25
0.5
1.0
1.5
0.25
0.5
1.0
1.5
0.25
0.5
1.0
1.5
1.5
1.5
1.5
10.1
17.4
16.9
15.8
17.8
24.3
24.9
24.2
19.2
27.6
32.2
32.8
0.6
4.9
6.6
7.1
8.2
5.2
6.7
6.3
8.8
6.8
6.4
5.8
6.2
0.5
12.2
4.6
15
24
24
24
23
31
31
33
26
34
38
39
1
2.1
2.6
2.4
1.9
3.4
3.6
4.0
2.8
2.8
4.3
5.6
4.8
1.2
1.2
5.8
75
300
220
120
80
1
110
120
120
115
155
155
165
130
170
190
190
6
3
4
5
460
310
155
110
520
340
190
127
4
6
2
3
7
8
9
10
11
12
13 f
14 g
15h
14.8
26.4
27
31
135
21
90
14
16 i
-
1.5
1.5
0.1
2.1
5.8
0.0
0.9
1.2
0.1
3
-
-
-
17
VOSO4
[VO(acac)2]
2.3
4.8
15
35
10
23
1.5
18
7
10

Page 11 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
aReaction conditions (unless stated otherwise): CyH (2.5 mmol), aq. H2O2 (5.0 mmol), 5.0 µmol of 1 - 3 (0.2
mol% vs. CyH), 60 ºC, 0.25 – 1.5 h of MW irradiation. Yields and TONs determined by GC analysis (upon treatment
with PPh3). bPercentage molar yield (moles of product per mole of cyclohexane); values obtained upon addition of PPh3
5
6
7
8
9
c
d
(see experimental). Ratio between the molar concentrations of A and K. Total turnover number = moles of A and K
per mol of catalyst. eTOF=TON per hour. In the presence of NHPh2. Without PPh3 treatment. hUnder N2 atmosphere.
f
g
iWithout catalyst.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The effect of MW radiation (that generates heat evenly throughout the reactor) on the
catalytic performance of 3 was evaluated by comparison with the use of the conventional heating
(oil bath) and is shown in Fig. 3. For the same reaction conditions (including reaction time), the
catalytic activity was always strongly enhanced by MW irradiation, while selectivity for the KA oil
was preserved until 1.5 h reaction time. For longer reaction periods, under MW irradiation, small
amounts of byproducts such as 1,2- and 1,4-cyclohexanediols were detected by GC-MS analysis in
the final reaction mixtures with the concomitant slight decrease of KA oil yield (Fig. 3).
MW irradiation
45
Oil bath
40
35
30
25
20
15
10
5
0
0
1
2
4
5
6
Tim3e / h
Fig. 3 Effect of the heating mode (MW irradiation or oil bath) on the total yield of cyclohexanol and
cyclohexanone obtained by neat oxidation of cyclohexane with aq. H2O2 catalyzed by complex 3.
Thus, the use of MW radiation as alternative energy source is appealing for the present
catalytic system, enhancing product yield as well as being more energy efficient and economical in
comparison to the conventional oil bath heating method. The catalytic performance found for the
present V-systems is much better than those previously observed32 for the oxidovanadium(V)
11

New Journal of Chemistry
Page 12 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
aroylhydrazone complexes [VOL1(OEt)]·[VOL1(OEt)(EtOH)], [VOL2(OEt)], [Et3NH][VO2L1],
[VO2(HL2)]·2EtOH, [(VOL1)2(µ-O)] or [(VOL2)2(µ-O)] (0.6 - 1.7% KA oil yield).
5
6
7
8
9
Addition of a radical trap (e.g., Ph2NH) to the reaction mixture results in almost suppression
of the catalytic activity of 3 (Table 3, entry 13). This behaviour, along with the formation of
cyclohexyl hydroperoxide (typical intermediate product in radical-type reactions) supports the
hypothesis of a free-radical mechanism for the cyclohexane oxidation carried out in this study,
similarly to the reported for other vanadium-catalyzed oxidations of cyclohexane by hydrogen
peroxide.28,29,51-53 The V-assisted decomposition of H2O2 leading to the oxygen-centred radicals
HOO• and HO• (reactions 1 and 2) can be proposed (see Scheme 3). Cyclohexyl radical (Cy•) is then
formed upon H-abstraction from cyclohexane (CyH) by HO• (reaction 3). Reaction of Cy• with O2
leads to CyOO• (reaction 4), and CyOOH can then be formed e.g., upon H-abstraction from H2O2 by
CyOO• (reaction 5). Vanadium complex-assisted decomposition of CyOOH to CyO• and CyOO•
(reactions 6 and 7) would then lead to cyclohexanol (CyOH) and cyclohexanone (Cy-H=O) products
(reactions 8 and 9).51-55
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
V(V) + H2O2 → HOO• + H+ + V(IV)
V(IV) + H2O2 → HO• + V(V) + HO‒
HO• + CyH → H2O + Cy•
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
Cy•+ O2 → CyOO•
CyOO• + H2O2 → CyOOH + HOO•
CyOOH + V(IV) → CyO• + V(V) + HO−
CyOOH + V(V) → CyOO• + H+ + V(IV)
CyO• + CyH → CyOH + Cy•
2CyOO• → CyOH + Cy-H=O + O2
Scheme 3 free-radical mechanism for the cyclohexane oxidation
Oxidation of cyclohexanol
In order to test the catalytic activity of complexes 1 - 3 towards the homogeneous MW-
assisted oxidation of cyclohexanol to cyclohexanone, the same operation conditions of the oxidation
of cyclohexane were applied (see experimental part) except that aq. tert-butyl hydroperoxide
12

Page 13 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
(TBHP) was used as oxidizing agent, typically 1.5 h reaction time at 60 ºC and in a solvent- and
additive-free medium under MW radiation (Scheme 4).
5
6
7
8
O
OH
V catalyst
aq. TBHP
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
MW, 60 ºC
Scheme 4 MW-assisted neat oxidation of cyclohexanol to cyclohexanone with tert-butyl
hydroperoxide catalysed by 1‒3.
Under the adopted conditions, complexes 1 - 3 are able to catalyse the oxidation of
cyclohexanol leading to yields of cyclohexanone in the range of 36 - 79% (Table 4, entries 1, 3 and
5). However, by performing the reactions at higher temperatures, considerably higher yields of
cyclohexanone (up to 94% for 3 at 100 ºC, entry 8, Table 4) are achieved in shorter reaction times
(0.5 h, Table 4). Decomposition of H2O2 hampered its use at temperatures higher than 60 ° C and
therefore it was replaced by TBHP. The optimal reaction temperature for catalysts 1 - 3 was found
to be 100 ºC (see Fig. 4). A high selectivity towards the formation of the ketone (>99%) was
displayed by these MW-assisted reactions, since only the unreacted alcohol (apart from
cyclohexanone) was detected by GC–MS analysis of the final reaction mixtures.
Table 4. MW-assisted solvent-free oxidation of cyclohexanol using 1 - 3 as catalysts.a
Entry
Catalyst
Reaction Temperatu Yieldb
TOF /h-1 c
time /h
1.5
re /ºC
60
/%
36
1
1
120
2
3
4
5
8
0.5
1.5
0.5
1.5
0.5
0.5
0.5
0.5
0.5
100
60
100
60
100
100
100
100
100
61
73
87
79
94
6
2
20
37
2
3
243
870
263
940
60
20
100
185
9 d
10 e
11
12
-
VOSO4
[VO(acac)2]
aReaction conditions unless stated otherwise: 2.5 mmol of cyclohexanol, 5.0 mol (0.2 mol% vs. substrate) of 1 - 3, 5.0
b
c
mmol of TBHP (70% in H2O) under MW radiation. Moles of cyclohexanone per 100 moles of cyclohexanol. TOF =
number of moles of cyclohexanone per mol of catalyst (TON) per hour. d In the presence of NHPh2. eWithout catalyst.
13

New Journal of Chemistry
Page 14 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 4 Influence of the temperature and time on the yield of cyclohexanone obtained by
MW-assisted oxidation of cyclohexanol with TBHP.
The present catalytic system displays a high activity. For example, it allows the fast (30 min.)
almost quantitative formation of cyclohexanone (Fig. 4). 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],56 although
requiring at least 18 h reaction time.
The peroxidative oxidation of cyclohexanol is believed to proceed mainly via a radical
mechanism which involves both carbon- and oxygen-centred radicals,57-59 in view of the strong
inhibition effect observed when it is carried out in the presence of an oxygen-radical trap such as
Ph2NH (Table 4, entry 9). It may involve e.g., tBuO˙ and tBuOO˙ radicals produced in the V
promoted decomposition of TBHP60,61 according to the equations 10 – 15.
V(V) + tBuOOH  V(IV) + tBuOO˙ + H+
V(IV) + tBuOOH  V(V)–OH + tBuO˙
V(V)–OH + tBuOOH  V(V)–OO–tBu + H2O
tBuO˙ + CyOH  tBuOH + Cy˙–OH
(10)
(11)
(12)
(13)
14

Page 15 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
tBuOO˙ + CyOH  tBuOOH + Cy˙–OH
V(V)–OO–tBu + Cy˙–OH  C-H =O + tBuOOH + V(IV)
(14)
(15)
5
6
7
8
9
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 dehydrogenation.62-64
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Theoretical calculations
Aiming to elucidate the plausible mechanism of the alkane oxidation with peroxides
catalysed by complexes 1–3, quantum chemical (DFT) calculations have been carried out. In accord
with the experimental data (see above) and previous experimental and theoretical studies of the
related V catalytic systems,51-53 the global mechanism is of a radical type involving the formation of
the radical species RO• (or HO• if H2O2 is used as an oxidant). This radical then oxidises the alkane
molecule R–H abstracting a hydrogen atom to give the corresponding alkyl radical R•. The latter
species reacts with molecular oxygen yielding finally alkyl hydroperoxide ROOH detected
experimentally. The rate limiting step of the whole process is the generation of the RO• (HO•)
radical.
The first mechanistic proposal for the formation of HO• radicals from H2O2 was formulated
by Haber and Weiss65 for the Fenton system (Fe(II) + H2O2).66 In this mechanism, HO• radicals are
directly formed upon the homolytic HO–OH bond cleavage in the hydrogen peroxide molecule. As
a result, one of the generated HO• radicals is reduced by Fe(II) to the OH– anion, and complex
[FeIII(H2O)5(OH)]2+ is formed (reaction 16). At the second step, another H2O2 molecule reacts with
the Fe(III) hydroxo complex regenerating the initial catalyst and producing the HOO• radical
(reaction 17).
[FeII(H2O)6]2+ + H2O2  [FeIII(H2O)5(OH)]2+ + HO• + H2O
[FeIII(H2O)5(OH)]2+ + H2O2  [FeII(H2O)6]2+ + HOO•
(16)
(17)
For the catalytic systems with the metal in its highest oxidation state [e.g., V(V) or Re(VII)],
a modified mechanistic scheme was proposed.51-53,67,68 In this case, the sequence of the main
reaction steps is inversed, i.e., firstly, reduction of the metal centre in the catalyst by a peroxide
(H2O2, ROOH) occurs (reaction 18, n is the oxidation state of the metal) and then the second
peroxide molecule oxidizes the {M(n–1)}H intermediate generating the HO• (RO•) radical (reaction
15

New Journal of Chemistry
Page 16 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
19). Such general mechanism of the HO• (RO•) generation was previously proposed on the basis of
experimental and theoretical studies for several related V(V) catalysts.51-53
5
6
7
8
9
{Mn} + H2O2 (ROOH)  {M(n–1)}H + HOO• (ROO•)
{M(n–1)}H + H2O2 (ROOH)  {Mn} + HO• (RO•) + H2O
(18)
(19)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
It is important to note that in both mentioned mechanisms, the metal changes its oxidation
state during the reaction and it plays the role of reductant of H2O2 with O‒O bond cleavage to form
the HO• radical. Such a reduction significantly stabilizes the reaction system and accounts for the
relatively low O–O bond dissociation energy in a peroxide when it is coordinated to a metal centre.
Conventional Fenton-like mechanism (Mechanism I). In this work, firstly, we calculated
the mechanism summarized by reactions 18 and 19 which is conventional for the V(V) catalytic
systems [H2O2 was considered as an oxidant and the model complex [VV(=O)(L)(MeO)] 1’ (H2L is
3-hydroxy-N'(3-hydroxyallylidene)acrylohydrazonic acid) was used as catalyst]. The first step of
this mechanism is coordination of H2O2 to the metal centre to give the hydrogen peroxide complex
[VV(=O)(L)(MeO)(H2O2)] (4) (Scheme 5). The formation of the H2O2 adduct 4 is slightly
endoergonic (by 2.4 kcal/mol, in terms of Gs).
O
MeO
O
N
H
O
V
O
V
=
O
V
V
MeO
MeO
O
O
MeO
H2O2
H2O
O
H
O
N
O
N
O
N
N
O
H2O
O
HO
H
H
O2H2
(2.4)
OOH
OOH
5 (12.4)
1'(0.0)
4
TS1
(18.4)
HO
O
O
O
V
O
N
–HOO•
TS2
(38.2)
–HO•
(32.5)
TS3
V
N
O
{H2O}
–MeOH
O
H
H
O
V
O
N
O
OOH
O2H2
MeO
H2O2
O
MeO
V
O
9 (28.1)
(29.1)
8
V
O
N
O
N
O
OH
(17.4)
H
6
(24.4)
O2H2
H
OH
10
O
V
MeO
O
OH
7
(29.6)
{H2O} MeO
{H2O}
TS4 (44.6)
TS6
(42.6)
V
N
O
–HO•
–MeOH
TS5(54.0)
N
O
V – VV
V – VIV
OOH
12
OOH
11 (38.6)
(33.5)
Scheme 5. Conventional mechanism of the HO• formation from H2O2 with the model catalyst 1’
(Mechanism I). The Gs values are indicated in kcal/mol relative to 1’.
16

Page 17 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
9
At the next step, a proton migration from the coordinated H2O2 to the methoxy ligand occurs
via transition state TS1 to give complex [VV(=O)(L)(OOH)(MeOH)] (5) with the activation barrier
of 16.0 kcal/mol. Transition state TS1 has a 6-membered cyclic structure of the reaction centre and
it includes one H2O molecule which plays the role of the proton shuttle. It was found51-53,67,68 that
such a water assisted proton transfer is more favorable than the direct H+ shift via a four-membered
cyclic TS without involvement of any proton shuttle. The proton transfer from H2O2 to the ligand L
or to the oxo-ligand is less favorable.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Complex 5 can lose the HOO• radical to give intermediate [VIV(=O)(L)(MeOH)] 6. This
process corresponds to the reduction of V(V) to V(IV) by H2O2 and it is significantly endoergonic
(by 12.0 kcal/mol). Following addition of a second H2O2 molecule to 6 gives complex
[VIV(=O)(L)(H2O2)(MeOH)] (7). Then, reaction can follow two possible pathways. The first one
includes the elimination of the methanol molecule in 7 to give [VIV(=O)(L)(H2O2)] (8) followed by
the water assisted proton transfer from H2O2 to the oxo ligand leading to complex
[VIV(L)(OH)(OOH)] (9) via TS2. Finally, the O–OH bond cleavage upon reduction by V(IV) with
generation of HO• can occur in 9 via TS3 with rather low activation barrier (Gs = 10.1 kcal/mol).
Another pathway includes two consecutive proton transfers in 7 from H2O2 to the ligand L
and then to the oxo-ligand to give finally complex [VIV(L)(OH)(OOH)(MeOH)] (12). The formation
of the HO• radical occurs upon the O–OH bond cleavage in 12 upon reduction by V(IV) via TS6
(Gs = 9.1 kcal/mol) with the simultaneous extrusion of the methanol molecule. The first pathway
(7 – MeOH  8  9  10 + HO•) is significantly (by 15.8 kcal/mol) more favorable than the
second one (7  11  12  10 + HO• + MeOH).
The rate limiting transition state of the whole process of the HO• generation for this
mechanism is TS3. The overall activation barrier is 38.2 kcal/mol. This value seems to be too high
to permit the efficient realization of the reaction even under MW conditions. Thus, the mechanism
of the radical formation from peroxides usually accepted for the V(V) catalysts does not seem to be
feasible for the particular catalyst 1.
Mechanism based on the simple coordination of H2O2 (Mechanism II). Meanwhile, the
calculations indicated that the energy of the homolytic O–O bond cleavage in complex 4 is
surprisingly low being only 22.3 kcal/mol (compared with the 47.5 kcal/mol in free H2O2). Hence, a
simple coordination of H2O2 to the metal centre of the catalyst activates the peroxide towards the
HO• radical generation by 25.2 kcal/mol. The reason of such a tremendous activation of H2O2
becomes clear from the analysis of the electronic structure of complex [V(=O)(L)(OMe)(OH)] (13)
17

New Journal of Chemistry
Page 18 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
9
– the product of the O–O bond cleavage in 4. As shown in Fig. 5, the spin electron density in 13 is
delocalized among the atoms of the ligand L. Therefore, as a result of the O–O bond rupture in 4,
the HO• ligand bound to V is reduced to OH– by the ligand L which is oxidized to L•+. At the same
time, the oxidation state of vanadium is not changed remaining to be +V. Such an intramolecular
redox process significantly stabilizes one of the O–O bond cleavage products accounting for the
significant decrease of the O–O bond dissociation energy. It is important to mention that the ligand
L in complex 4 being redox active plays the same role as a transition metal does in the classical
Fenton or Fenton-like mechanisms, i.e. it reduces one of the HO• radicals to OH–.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
O
V
O
MeO
V
N
MeO
N
+
OH
O
HO
O
OH
OH
4
(2.4)
(24.7)
13
Fig. 5 The HO–OH bond cleavage and intramolecular electron transfer in complex 4 and spin
density distribution in 13. The Gs values are indicated in kcal/mol relative to 1’.
The O–O bond cleavage energy in the hydrogen peroxide complexes is slightly higher for
the model bi and trinuclear catalysts 2’ and 3’ compared to 1’ (22.3, 24.4 and 25.5 kcal/mol for 1’,
2’ and 3’, respectively, Scheme 7). This qualitatively correlates with the experimental data revealing
the lower product yields per metal unit for the catalysts 2 and 3 (Table 1).
18

Page 19 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
9
O2H2
O
OH
MeO
O
O
N
MeO
O
MeO
O
N
V
V
H2O2
V
N
O
O
O
–HO•
O
N
N
N
HO
HO
HO
(0.0)
1'
4 (2.4)
(24.7)
13
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
V
O2H2
OMe
V
OH
Me
O
MeO
Me
N
O
N
OMe
O
N
OH
H2O2
–MeOH
O
V
OMe
O
N
N
–HO•
O
N
V
N
O
OH
O
N
O
O
O
(25.4)
15
(0.0)
2'
(1.0)
14
O
O
N
V
MeO
H2O
N
O
O2H2
OMe
OH
OMe
H2O
O
N
O
N
H2O2
OMe
O
O
V
V
V
N
–HO•
O
N
O
–H2O
N
O
N
N
O
O
O
N
(28.9)
17
(3.4)
16
O
V
OH2
OMe
O
(0.0)
3'
Scheme 6. Mechanism based on the simple coordination of H2O2 (Mechanism II) for the model
catalysts 1’–3’. The Gs values are indicated in kcal/mol relative to initial catalyst.
Mechanism II involving two H2O2 molecules. In the previous works, some of us
demonstrated that two H2O2 molecules participate in the HO• radical generation catalyzed by the
aqua complexes [M(H2O)n]p+ (M = Al, Ga, In, Sc, Y, La, Be, Zn, Cd, Bi).69-71 In this mechanism,
one H2O2 molecule is a direct source of HO• whereas another H2O2 molecule is a precursor of the
redox active OOH– ligand. The possibility of a similar mechanism for the V catalysts under study
was also analyzed.
This mechanism includes the coordination of H2O2 to 1’, proton migration to the methoxy
ligand (these two steps were discussed above), liberation of the methanol and coordination of the
19

New Journal of Chemistry
Page 20 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
second H2O2 molecules to give complex [VV(=O)(L)(OOH)(H2O2)] (18) (Scheme 7). The HO–OH
bond cleavage in 18 affords the HO• radical and complex [V(=O)(L)(OOH)(OH)] (19). The
hydrogen peroxide molecule in 18 is even more activated than that in 4, the HO–OH bond cleavage
energy being only 12.6 kcal/mol. The spin density in 19 is also delocalized along the ligand L (Fig.
6). Thus, the joint presence of both L and OOH– ligands in the complex molecule activates H2O2
most efficiently despite the OOH– ligand is not redox active in this case. The reduction of the ligated
H2O2 (with O–O bond cleavage) to HO• and hydroxide ligand is carried out by the L2– ligand (to
form L•–) with preservation of the metal oxidation state (V) and of the OOH– ligand. A related type
of behaviour was suggested by some of us for a carbohydrazone ligand (on the basis of
electrochemical studies)72 and for a quinolin-8-ate ligand.73
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
V
H
MeO
O
N
MeO
O
V
O
O
V
O
MeO
O
HOO
HOO
HOO
OH
N
O
H2O2
O
{H2O}
H2O2
V
V
O
O
N
O
N
N
O
–HO•
N
O
O
TS1 (18.4)
–MeOH
HO
O2H2
(2.4)
O2H2
(3.6)
(0.0)
1'
4
(12.4)
5
(16.2)
19
18
Scheme 7. Mechanism II involving two H2O2 molecules. The Gs values are indicated in kcal/mol
relative to 1’.
Fig. 6 Spin density distribution in 19.
Due to the change of the system spin state as a result of the HO–OH bond cleavage in 18, the
straightforward estimate of the activation energy for this step is not a simple task. Therefore, the
relaxed scan of the triplet and singlet potential energy surfaces (PESs) for the HO–OH bond
cleavage in 18 was carried out (Fig. 7). On one hand, the increase of the HO–OH bond distance in
18 at the singlet close shell PES (which corresponds to the heterolytic HO–OH bond rupture) results
in the monotonous enhancement of the system energy until the HO–OH distance of 2.12 Å. On the
20

Page 21 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
9
other hand, the excitation of 18 to the triplet spin state requires only 12.1 kcal/mol. The scan energy
curve for the triplet PES has a maximum at the HO–OH distance of 1.82 Å. Curves of both close
shell and triplet energy scans intersect at d(HO–OH)  1.92 Å. A singlet biradical structure 20 was
optimized using the broken symmetry approach starting from the intersection point with the fixed
HO–OH distance and all other internal coordinates being relaxed. The energy of 20 determines the
highest limit of the activation energy. The thus estimated activation energy limit of the homolytic
HO–OH bond cleavage in 18 is 29.5 kcal/mol and the overall activation energy limit for the whole
reaction of the HO• generation (i.e. that relative to the level of initial reactants) is 33.1 kcal/mol
(both values are in terms of Gibbs free energy in solution). The last value is rather high indicating
that under thermal conditions the catalytic activity of the V complexes under study should be rather
low. However, under the MW conditions, the reaction should be efficient due to strong and rapid
local overheating, pressure enhancement and possible nonthermal effects on the preexponential
factor in the Arrhenius equation. All this correlates with the experimental observations
demonstrating a crucial role of MW in the reaction.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 7 Energy scan for the HO–OH bond cleavage in 18. The HO–OH distance was fixed while all
other internal coordinates were relaxed.
Experimental
General Materials and Procedures
21

New Journal of Chemistry
Page 22 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
9
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. [VO(OEt)(L1)]
(1)38,39 and [{VO(OEt)(EtOH)}2(L2)] (2)28 were synthesized as reported. VOSO4·3H2O was used as
the metal source for the synthesis of complex 3. 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 VERTEX 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 optimized according to the particular sample with 35 p.s.i. nebulizer
pressure. Scanning was performed from m/z 100 to 1200 in acetonitrile solution. The compounds
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
were observed in the positive mode (capillary voltage = 80–105 V). The H NMR spectra were
recorded at room temperature on a Bruker Avance II + 300 (UltraShield™ Magnet) spectrometer
operating at 300.130 MHz. 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 were quoted relative to
external [VOCl3]. The catalytic investigations under microwave irradiation were performed in a
focused Anton Paar Monowave 300 microwave reactor. The UV-vis absorption spectra in the 200 –
700 nm region were recorded with a scan rate of 240 nm∙min−1 by using a Lambda 35 UV-vis
spectrophotometer (Perkin-Elmer) in 1.00 cm quartz cells at room temperature, with a concentration
of 1-3 of 1.0 x10−6 mol L−1 in CH3CN and under catalytic conditions.
Synthetic procedures
Synthesis of H6L3
The Schiff base pro-ligand tris(2-hydroxybenzylidene)benzene-1,3,5-tricarbohydrazide (H6L3) was
prepared by a reported method38 upon condensation of benzene-1,3,5-tricarbohydrazide with 3
equivalents of 2-hydroxybenzaldehyde in methanol (Scheme 1).
H6L3: Yield 86%. Anal. Calcd for C30H24N6O6 (H6L3): C, 63.82; H, 4.28; N, 14.89. Found: C,
63.76; H, 4.26; N, 14.84. IR (KBr; cm1): 3423 (OH). 1607 (C=N), 1253 (C–O) enolic and 1071
( N–N). 1H NMR (DMSO-d6, ): 12.44 (s, 1H, OH), 11.17 (s, 1H, OH), 9.01 (s, 1H, OH), 8.73 (s,
22

Page 23 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
3H, -CH=N), 7.63-6.92 (m, 15H, C6H4). IR (KBr; cm1): 3328 (OH), 2957 (NH), 1658 (C=O),
1590 (C=N).
5
6
7
8
9
HO
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
NH2
N
NH
O
Methanol
O
NH
+ 3
NH2
O
HO
Reflux, 4h
H
H
N
N
H
H
H2N
N
N
N
O
O
N
O
O
HO
OH
Scheme 8 Synthesis of H6L3.
Synthesis of [{VO(OMe)(H2O)}3(L3)].2H2O (3)
To 30 mL of methanol solution of H6L (0.565 g, 1.00 mmol), 0.651 g (3.00 mmol) of VOSO4.3H2O
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 3 d, single crystals of 3
(Scheme 1, Fig. 1) were isolated, washed 3 times with cold methanol and dried in open air.
Yield 67%. Anal. Calcd for C33H37N6O17V3: C, 42.05; H, 3.96; N, 8.92. Found: C, 42.01; H, 3.92;
N, 8.89. IR (KBr; cm1): 4218 (O-H), 4061(O-H), 1602 (C=N), 1250 (C–O) enolic, 1036 (
N–N), 1007 (V=O), 986 (V=O) and 914 (V=O). 1H NMR (DMSO-d6, ): 8.62 (s, 3H, -CH=N),
7.65-6.94 (m, 15H, C6H4) 3.74 (s, broad, 9H, -CH3). 51V NMR (DMSO-d6, ): -529, -541 and -579.
m/z 907 [(M-2H2O)+H]+ (100 %).
X-ray measurements
X-ray single crystal of complex 3 was immersed in cryo-oil, mounted in Nylon loops and
measured at room temperature of 298 K. Intensity data were collected using a Bruker APEX-II
PHOTON 100 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
23

New Journal of Chemistry
Page 24 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
retrieved using Bruker SMART74 software and refined using Bruker SAINT74 on all the observed
reflections. Absorption corrections were applied using SADABS.74 Structures were solved by direct
methods by using SIR9775 and refined with SHELXL–2018/1.76 Calculations were performed using
the WinGX System–Version 2014.1.77 Least square refinements with anisotropic thermal motion
parameters for all the non-hydrogen atoms and isotropic for the remaining atoms were employed.
All hydrogen atoms bonded to carbon were included in the model at geometrically calculated
positions and refined using a riding model. Uiso(H) were defined as 1.2Ueq of the parent carbon
atoms for phenyl and benzylic residues and 1.5Ueq of the parent carbon atoms for the methyl groups.
The hydrogen atoms of the free and coordinated water molecules have been inserted in calculated
positions obtained with CALC-OH routine; the isotropic thermal parameter was set at 1.5 times the
average thermal parameter of the belonging oxygen atoms and refined with DFIX and DANG
restrains. The structure contains residual electron density (12 electrons) in a void of 61.7 Å3 which
was removed by means of PLATON SQUEEZE routine.78 CCDC 1880765 contains the
supplementary crystallographic data for 3. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Solvent-free microwave-assisted catalytic oxidation of cyclohexane and
cyclohexanol
The catalytic oxidations of cyclohexane or cyclohexanol were carried out in sealed
cylindrical Pyrex tubes (10 mL capacity reaction tube with a 13 mm internal diameter), under
focused microwave irradiation (MW) in an Anton Paar Monowave 300 reactor 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 equipped with a FID detector and a capillary column (DB-WAX, column
length: 30 m; column internal diameter: 0.32 mm) and run by the software Jasco-Borwin v.1.50.
The temperature of injection was 240 °C. After the injection, the reaction temperature was
maintained at either 100 °C (oxidation of cyclohexane) or 140 °C (oxidation of cyclohexanol) for 1
min, then raised, by 10 °C/min, either to 160 °C (oxidation of cyclohexane) or 220 °C (oxidation of
cyclohexanol) and held at this temperature for 1 min. Helium was used as the carrier gas.
GC-MS analyses were performed using a Perkin Elmer Clarus 600 C instrument (He as the carrier
gas). The ionization voltage was 70 eV. Gas chromatography was conducted in the temperature-
programming mode, using a SGE BPX5 column (30 m × 0.25 mm × 0.25 µm).
24

Page 25 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
Typical procedures for the catalytic oxidation of cyclohexane or cyclohexanol and
products analysis
9
Typical reaction conditions are as follows: catalyst 1 - 3 (1-10 mol, 0.04 - 0.4 mol% vs.
substrate) was added to 2.50 mmol of substrate, whereafter 5.00 mmol of 30% aqueous hydrogen
peroxide (oxidation of cyclohexane) or 70% aqueous tert-butyl hydroperoxide (TBHP, oxidation of
cyclohexanol) were introduced in the tube. For the assays in the presence of a radical trap, NHPh2 in
stoichiometric amount relative to the oxidant was added. The tube was placed in the microwave
(MW) reactor and the mixture stirred (800 rpm) and irradiated (10 - 25 W) for 0.25 ‒ 3 h at 60 ºC
(oxidation of cyclohexane) or 100 ºC (oxidation of cyclohexanol). After the reaction, the mixture
was allowed to cool down to room temperature.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Solution samples were analysed by GC (internal standard method) after addition of
nitromethane (as the standard compound). Reaction products were identified by comparison of their
retention times with those from known reference compounds, and by comparing their mass spectra
to fragmentation patterns obtained from the NIST spectral library stored in the computer software of
the mass spectrometer.
In the case of the oxidation of cyclohexane, the product yields were obtained after the reduction of
the reaction sample with an excess of triphenylphosphine, following a method developed by
Shul’pin.46-48 CyOOH was then reduced to cyclohexanol, with formation of O=PPh3 as a by-product.
Blank experiments, in the absence of any catalyst, were performed under the studied reaction
conditions. No considerable conversion of cyclohexane was observed and only traces (< 0.5%) of
cyclohexanone were generated in the V-free systems.
Computational Details
The full geometry optimization of all structures and transition states (TSs) has been carried
out at the DFT level of theory by using the M06 functional79 with the help of the Gaussian 09
program package.80 No symmetry operations were applied. The geometry optimization was carried
out by using a relativistic Stuttgart pseudopotential which describes 10 core electron (MDF10) and
the appropriate contracted basis set (8s7p6d1f)/[6s5p3d1f]81 for the vanadium atoms and the 6-31G*
basis set for other atoms. Single-point calculations were performed on the basis of the equilibrium
geometries found by using the 6-311+G** basis set for non-metal atoms.
25

New Journal of Chemistry
Page 26 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
6
7
8
9
The Hessian matrix was calculated analytically for the optimized structures to prove the
location of correct minima (no imaginary frequencies) or saddle points (only one imaginary
frequency) and to estimate the thermodynamic parameters, with the latter calculated at 25 °C. The
nature of all transition states was investigated by analysis of the vectors associated with the
imaginary frequency and by the calculations of the intrinsic reaction coordinates (IRC) by using the
method developed by Gonzalez and Schlegel.82-84
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The total energies corrected for solvent effects Es were estimated at the single-point
calculations on the basis of gas-phase geometries at the CPCM-M06/6-311+G**//gas-M06/6-31G*
level of theory using the polarizable continuum model in the CPCM version85,86 with cyclohexane as
solvent. The UAKS model was applied for the molecular cavity, and dispersion, cavitation and
repulsion terms were taken into account. The entropic term in cyclohexane solution (Ss) was
calculated according to the procedure described by Wertz87 and Cooper and Ziegler88 using eqn. 20–
23:
S1 = R ln(Vsm,liq/Vm,gas
)
(20)
(21)
(22)
S2 = R ln(V°m/Vsm,liq
)
 = [S°,sliq – (S°,sgas + S1)]/[S°,sgas + S1]
Ss = Sg + Ssol = Sg + [S1 + (Sg + S1) + S2] =
Sg + [(–10.77 cal/mol•K) – 0.19(Sg – 10.77 cal/mol•K) + 4.42 cal/mol•K]
(23)
where Sg is the gas-phase entropy of solute, Ssol is the solvation entropy, So,sliq, So,sgas and Vsm,liq are
the standard entropies and molar volume of the solvent in the liquid or gas phases (204.35 and
298.19 J/mol•K and 107.83 mL/mol, respectively, for cyclohexane), Vm,gas is the molar volume of
the ideal gas at 25 °C (24450 mL/mol), Vo is the molar volume of the solution that correspond to
m
the standard conditions (1000 mL/mol). The enthalpies and Gibbs free energies in solution (Hs and
Gs, respectively) were estimated using the expressions 24 and 25
Hs = Es(6-311+G**) + Hg(6-31G*) – Eg(6-311+G**)
Gs = Hs – TSs
(24)
(25)
26

Page 27 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
where Es and Eg are the total energies in solution and the gas phase and Hg is the gas-phase enthalpy
calculated at the corresponding level. Gibbs free energies in solution are discussed in this work if
not stated otherwise.
6
7
8
9
Conclusions
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In this study, we have successfully investigated and compared the catalytic activity of three
oxidovanadium(V) complexes (1-3) with azine based aroylhydrazone ligands having a similar type
of tridentate ONO chelate pockets, differing in their nuclearities, i.e., mononuclear (1), binuclear (2)
and trinuclear (3), towards MW-assisted homogeneous oxidation of cyclohexane and a secondary
alcohol (cyclohexanol). They act as homogeneous catalysts for the MW-assisted and solvent-free
(with aq. TBHP) oxidation of cyclohexane and cyclohexanol, via radical mechanisms, affording the
oxygenate products in high selectivity. Complex 1 exhibited the highest catalytic activity per V
centre (in terms of yield and TON values) for both alkane and alcohol oxidations. However,
complex 3 led to an almost quantitative conversion of cyclohexanol into cyclohexanone.
Considering the overall catalytic activity per mole of catalyst, the following order was observed:
3>2>1.
Other advantages of the present catalytic systems are the mild and environmentally benign
conditions concerning (i) a solvent- and additive-free protocol, (ii) the use of environmentally
acceptable oxidant (aq. TBHP) and energy source (MW irradiation), and (iii) a very short reaction
time. These are significant features towards the development of a sustainable chemical process for
both oxidations.
A mechanism of the HO• radical generation – the rate limiting step of the cyclohexane
oxidation by H2O2 – was investigated in detail using theoretical DFT methods. This mechanism is
different from those usually accepted for the Fenton or Fenton-like systems and it includes (i)
coordination of H2O2 to the catalyst molecule, (ii) proton transfer from ligated H2O2 to the methoxy
ligand and elimination of the formed methanol molecule and (iii) coordination of a second H2O2
molecule followed by HO–OH bond cleavage to give HO• (Scheme 7). The activation of the ligated
H2O2 in complex 18 towards O–O bond cleavage is associated with the redox active nature of the
ligand L in the catalyst molecule, whereby this azine based ligand (instead of the metal or of HOO–
ligand) acts as the reducing agent of the H2O2 ligand to cleave its O–O bond.
This study demonstrates the non-innocent character of this type of azine based
aroylhydrazone ligand, which provides a key role in the peroxide O–O bond cleavage in the
27

New Journal of Chemistry
Page 28 of 32
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
peroxidative alkane oxidation. It also concerns the perspective of varying the vanadium nuclearity in
catalysts for such oxidation reactions. These issues deserve to be further explored and can open up a
new window to the vanadium chemistry.
6
7
8
9
Conflicts of interest
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
There are no conflicts of interest to declare.
Acknowledgment
Authors are grateful to the Fundação para a Ciência e a Tecnologia (FCT, projects
UID/QUI/00100/2013, PTDC/QEQ-ERQ/1648/2014 and PTDC/QEQ-QIN/3967/2014), Portugal,
for financial support. M.S. acknowledges the FCT for a working contract "DL/57/2017" (NT
13/CQE). Authors are thankful to the Portuguese NMR Network (IST-UL Centre) for access to the
NMR facility and the IST Node of the Portuguese Network of mass-spectrometry for the ESI-MS
measurements.
Supporting information
Supplementary Tables TS1-TS6. CCDC 1880765 for 3 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. Supplementary data to this article can be
found online at .....
References
1 M. Sutradhar, L. M. D. R. S. Martins, M. F. C. Guedes da Silva and A. J. L. Pombeiro, Coord.
Chem. Rev., 2015, 301-302, 200-239.
2 J. A. L. da Silva, J. J. R. Frausto da Silva and A. J. L. Pombeiro, Coord. Chem. Rev., 2011, 255,
2232- 2248.
3 V. Conte, A. Coletti, B. Floris, G. Licini and C. Zonta, Coord. Chem. Rev., 2011, 255, 2165-2177.
4 M. Sutradhar and A. J. L. Pombeiro, Cord. Chem. Rev., 2014, 265, 89-124.
5 A. G. J. Ligtenbarg, R. Hage and B. L. Feringa, Coord. Chem. Rev., 2003, 237, 89–101.
6 A.E. Shilov, G.B. Shul’pin, Chem. Rev. 97 (1997) 2879–2932.
7 M. Sutradhar and A. J. L. Pombeiro, Vanadium Complexes in Catalytic Oxidations, in J. Reedijk,
(Ed.) Elsevier Reference Module in Chemistry, Molecular Sciences and Chemical Engineering.
Waltham, MA: Elsevier (2017). doi.org/10.1016/B978-0-12-409547-2.13525-5.
28

Page 29 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ00348G
1
2
3
4
5
8 G. B. Shul'pin, Catalysts. 2016, 6, 50(1-40).
9 A. M. Kirilov and G. B. Shul'pin, Coord. Chem. Rev., 2013, 257, 732-754.
10 G. B. Shul'pin, Org. Biomol. Chem., 2010, 8, 4217-4228.
11 A. J. L. Pombeiro, in Advances in Organometallic Chemistry and Catalysis, ed: A. J. L.
Pombeiro, John Wiley & Sons: Hoboken, NJ, USA, 2014, Ch. 2, pp. 15–25.
12 A. E. Shilov and G. B. Shul’pin, Activation and Catalytic Reactions of Saturated Hydrocarbons
in the Presence of Metal Complexes, Kluwer Academic Publishers, Dordrecht, 2000.
13 J.-E. Backvall, Modern Oxidation Methods, Wiley-VCH, Weinheim, 2004.
14 A. J. L. Pombeiro (ed.), Advances in Organometallic Chemistry and Catalysis, John Wiley &
Sons: Hoboken, NJ, USA, 2014
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
15 G. B. Shul’pin, in Transition Metals for Organic Synthesis, ed.: M. Beller, C. Bolm, Vol. 2, 2nd
edn, Wiley-VCR, New York, 2004, Vol. 2, Ch. 2 p. 215.
16 A. E. Shilov and G. B. Shul’pin, Chem. Rev., 1997, 97, 2879-2932.
17 G. B. Shul’pin, Mini-Rev. Org. Chem., 2009, 6, 95-104.
18 R. H. Crabtree, J. Chem. Soc., Dalton Trans., 2001, 2437-2450.
19 R. H. Liu, X. M. Liang, C. Y. Dong and X. Q. Hu, J. Am. Chem. Soc., 2004, 126, 4112-4113.
20 R. F. Nie, J. J. Shi, W. C. Du, W. S. Ning, Z. Y. Hou and F. S. Xiao, J. Mater. Chem. A, 2013, 1,
9037-9045.
21 S. J. Liu, N. Zhang, Z. R. Tang and Y. J. Xu, ACS Appl. Mater. Interfaces, 2012, 4, 6378-6385.
22 M. N. Kopylovich, A. P. C. Ribeiro, E. C. B. A. Alegria, N. M. R. Martins, L. M. D. R. S.
Martins and A. J. L. Pombeiro, Adv. Organomet. Chem., 2015, 63, 91-174.
23 Y. Y. Karabach, M. N. Kopylovich, K. T. Mahmudov and A. J. L. Pombeiro, in Advances in
Organometallic Chemistry and Catalysis, ed: A. J. L. Pombeiro, John Wiley & Sons: Hoboken, NJ,
USA, 2014, Ch. 18, pp. 233-245.
24 S. Horikoshi, N. Serpone, Microwaves in Catalysis: Methodology and Applications, John Wiley
& Sons, 2015.
25 M. Domarus, M. L. Kuznetsov, J. Marçalo, A. J. L. Pombeiro and J. A. L. da Silva, Angew.
Chem. Int. Ed., 2016, 55, 1489-1492.
26 D. Rehder, Bioinorganic Vanadium Chemistry, Wiley, Chichester, U. K., 2008.
27 G. B. Shul’pin and Y. N. Kozlov, Org. Biomol. Chem., 2003, 1, 2303–2306.
28 M. Sutradhar, N. V. Shvydkiy, M. F. C. Guedes da Silva, M. V. Kirillova, Y. N. Kozlov, A. J. L.
Pombeiro and G. B. Shul’pin, Dalton Trans., 2013, 42, 11791-11803.
29 M. Sutradhar, M. V. Kirillova, M. F. C. Guedes da Silva, L. M. D. R. S. Martins and A. J. L.
Pombeiro, Inorg. Chem., 2012, 51, 11229-11231.
30 M. Sutradhar, M. V. Kirillova, M. F. C. Guedes da Silva, C.-M. Liu and A. J. L. Pombeiro,
Dalton Trans., 2013, 42, 16578-16587.
31 M. Sutradhar, L. M. D. R. S. Martins, M. F. C. Guedes da Silva, E. C. B. A. Alegria, C.-M. Liu
and A. J. L. Pombeiro, Dalton Trans., 2014, 43, 3966-3977.
32 M. Sutradhar, L. M. D. R. S. Martins, S. A. C. Carabineiro, M. F. C. Guedes da Silva, J. G.
Buijnsters, J. L. Figueiredo and A. J. L. Pombeiro, ChemCatChem., 2016, 8, 2254-2266.
33 M. Sutradhar, L. M. D. R. S. Martins, M. F. C. Guedes da Silva and A. J. L. Pombeiro, Appl.
Catal. A: Gen., 2015, 493, 50-57.
34 M. Sutradhar, E. C. B. A. Alegria, T. Roy Barman, F. Scorcelletti, M. F. C. Guedes da Silva and
A. J. L. Pombeiro, Mol. Catal., 2017, 439, 224-232.
35 M. Sutradhar, L. M. D. R. S. Martins, M. F. C. Guedes da Silva, C.-M. Liu and A. J.
L. Pombeiro, Eur. J. Inorg. Chem., 2015, 3959-3969.
36 M. Sutradhar, E. C. B. A. Alegria, M. F. C. Guedes da Silva, L. M. D. R. S. Martins and A. J. L.
Pombeiro, Molecules, 2016, 425, 1-13.
29