Catalytic oxidation of aromatic hydrocarbons by mono-oxido- alkoxidovanadium(V) complexes of ONNO donor ethylenediaminebis(phenolate) ligands

Article abstract of DOI:10.1016/j.poly.2013.07.013

Two oxidovanadium(V) complexes, [V^VO(L¹)(OMe)] (1) and [V^VO(L²)(OMe)] (2), with ONNO donor ethylenediamine-bis(phenolate) ligands (H2L¹ and H2L²) have been readily synthesized by the reaction between the ligand precursors and VOSO₄·5H₂O in MeOH, and characterized by physico-chemical techniques and single crystal X-ray diffraction studies. Both complexes 1 and 2 are hexa-coordinated with a pseudo-octahedral geometry in an N₂O₄ coordination environment and are found to catalyze the oxidation of toluene to benzoic acid and isomers of xylene to the corresponding hydroxy acids, with turnover numbers (TON) over 200, except for o-xylene. On changing the ligand fragments and coordination geometry around the metal centre there is an improvement in the catalytic efficiency, selectivity and also TON of the reaction as compared with the previously reported systems. The reactions were monitored using ⁵¹V NMR spectroscopy. Based on mass spectra analysis and ⁵¹V NMR studies, the mechanism of the catalytic process has been proposed, employing the formation of oxido-hydroperoxido/hydroxido-peroxido [V(ONNO)(O)(OOH)] ? [V(OH)(ONNO)(O₂)] intermediates.

Full text of DOI:10.1016/j.poly.2013.07.013

Polyhedron 63 (2013) 189–198
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Catalytic oxidation of aromatic hydrocarbons by mono-oxido-
alkoxidovanadium(V) complexes of ONNO donor ethylenediamine-
bis(phenolate) ligands
Mainak Debnath , Arpan Dutta , Surajit Biswas , Kalyan Kumar Das , Hon Man Lee , Jan Vícha c,d,
a
a
a
a
b
c,d
c
a,
⇑
Radek Marek , Jaromir Marek , Mahammad Ali
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
National Changhua University of Education, Department of Chemistry, Changhua 50058, Taiwan
CEITEC – Central European Institute of Technology, Masaryk University, Kamenice 5, CZ-625 00 Brno, Czech Republic
National Centre for Biomolecular Research, Faculty of Science, Masaryk University, Kamenice 5/A4, CZ-62500 Brno, Czech Republic
b
c
d
a r t i c l e i n f o
a b s t r a c t
V
1
V
2
Article history:
Two oxidovanadium(V) complexes, [V O(L )(OMe)] (1) and [V O(L )(OMe)] (2), with ONNO donor ethy-
Received 22 March 2013
Accepted 2 July 2013
Available online 18 July 2013
1
2
lenediamine-bis(phenolate) ligands (H
between the ligand precursors and VOSO
2
L
and H
2
L ) have been readily synthesized by the reaction
4
ꢀ5H O in MeOH, and characterized by physico-chemical tech-
2
niques and single crystal X-ray diffraction studies. Both complexes 1 and 2 are hexa-coordinated with a
pseudo-octahedral geometry in an N coordination environment and are found to catalyze the oxida-
2 4
O
Keywords:
Ethylenediamine-bis(phenolate)
Oxidovanadium(V)
Catalytic oxidation
Aromatic hydrocarbons
DFT calculation
tion of toluene to benzoic acid and isomers of xylene to the corresponding hydroxy acids, with turnover
numbers (TON) over 200, except for o-xylene. On changing the ligand fragments and coordination geom-
etry around the metal centre there is an improvement in the catalytic efficiency, selectivity and also TON
of the reaction as compared with the previously reported systems. The reactions were monitored using
V NMR spectroscopy. Based on mass spectra analysis and V NMR studies, the mechanism of the cat-
alytic process has been proposed, employing the formation of oxido-hydroperoxido/hydroxido-peroxido
V(ONNO)(O)(OOH)] , [V(OH)(ONNO)(O )] intermediates.
5
1
51
Hydroperoxido intermediate
[
2
Ó 2013 Elsevier Ltd. All rights reserved.
1
. Introduction
The very rich and vivid chemistry of vanadium in all oxidation
solution studies on such vanadium compounds in the physiologi-
cally relevant oxidation states (III to V) under assorted O/N donor
environments [18,19,21,22] have been reported.
states, ranging from ꢁI to +V, is gaining more and more research
Interest towards the exploration of the chemical and electro-
chemical reactivity of oxidovanadium(V) complexes and dioxygen
activation, along with the ability to form oxido-peroxido vana-
dium(V) species, an important reactive intermediate in in vivo
enzymatic processes [14,23–25], is now apparent. Oxidoperxido–
vanadium(V) complexes are also known to exist in some catalytic
cycles in the oxidation of various organic substrates [26–28]. Due
to their high charge/radius ratio, vanadium(V) centers are usually
strong Lewis acids, which lead them to activate peroxidic reagents
interest because of its widespread involvement in enzymatic
[
1–3] and physiological activities [4–6]. Although numerous bio-
chemical and physiological functions have been suggested for this
element, all of these are limited to bacteria (Azotobacter, Strepto-
myces etc.) [7–12], algae, lichens and fungi (haloperoxidases)
[
3,7,8,13–15]. Knowledge regarding a clearly defined role of vana-
dium in the higher forms of life is still lacking [7,8,10,14–16]. The
minimal structural requirements of vanadium active sites in
haloperoxidases, as delineated by EXAFS and other spectroscopic
investigations, have shown a distinct preference of this metal cen-
2 2 2
like H O , molecular O , peroxyacetic acid, etc., [29] making them
act as catalyst precursors in various oxidation reactions, like bro-
mination, epoxidations of alkenes and allylic alcohols, hydroxyl-
ations of alkanes and arenes, and oxidation of primary and
secondary alcohols to their corresponding aldehydes and ketones
[30,31].
The bio-degradation of hydrocarbons (aromatic and aliphatic) is
catalyzed by some bacteria and yeast [32]. Catalytic oxidation of
hydrocarbons to their alcoholic, phenolic, carbonyl or carboxylic
acid functions [33,34] finds a wide range of applications in many
ter for N- and/or O-donor atoms provided by the N(e)-histidine
moiety of the protein residues [17–20]. All the above findings have
drawn the attention of chemists and pharmacologists to the
coordination chemistry of vanadium involving N/O donor ligands,
and subsequently a good number of structural, spectroscopic and
⇑
Corresponding author. Tel.: +91 33 2414 62230 (O), 9433249716 (M).
E-mail address: mali@chemistry.jdvu.ac.in (M. Ali).
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.07.013

1
90
M. Debnath et al. / Polyhedron 63 (2013) 189–198
synthetic organic reactions as well as in industry. The first catalytic
hydrocarbon oxidation using oxidovanadium(V) compounds was
furnished by Walter et al. [35]. Many vanadium compounds, in
2 2 2
combination with H O or O have also been used as catalyst for
the oxidation of hydrocarbons [33,27,35–37], but so far the yields
and TON (turnover number) obtained are not remarkable. Recently,
vanadium(V), as well as group 4 transition metal complexes of tri-
(
ONO) and tetra-(ONNO) dentate amine-bis(phenolate) ligands,
have proven to be highly active olefin polymerization catalysts
38–40].
All the above findings prompted us to design and synthesize
[
structural and functional models of biomolecules with the poten-
tial ability to catalyze hydrocarbon oxidation reactions. The model
complexes, described here, are found to catalyze the oxidation of
aromatic hydrocarbons through the formation of oxido-hydro-
peroxido/hydroxido-peroxido [V(ONNO)(O)(OOH)]/[V(ONNO)(OH)
2
(O )] intermediates, as observed previously [26–28]. We report
here the synthesis, NMR characterization and crystal structures
of two oxidovanadium(V) complexes (1 and 2), and the catalytic
oxidation of aromatic hydrocarbons by H O . We have been
2 2
Fig. 2. ORTEP view of complex 2 with 30% thermal ellipsoids; H atoms are omitted for
clarity.
successful in slightly improving the turnover number (TON) in
comparison to our previous work [41] by slightly modifying the
ligand frameworks.
ligand and a terminal oxido oxygen atom, O1. Similarly, in complex
2
2
the equatorial plane is occupied by N1, O1, O2 (from H
2
L ) and
O4 (from methoxide), while the apical positions by N2 and O3 (ter-
minal oxido). The trans angles in the basal plane for both com-
plexes (O2–V1–N9 = 162.74(11)° and O3–V1–O3 = 162.40(10)°
2
. Results and discussion
0
for complex 1; O4–V1–N1 = 162.30(17)° and O1–V1–O2 =
2.1. Syntheses and structural characterization
1
62.84(16)° for complex 2) are somewhat compressed and force
the central vanadium atom to move out of the least square basal
plane by 0.258 and 0.265 Å, respectively for complexes 1 and 2, to-
wards the apical oxido atom, with V@O distances of 1.595(2) and
i
The tetradenate H
2
L (i = 1 or 2) ligands were synthesized by the
0
simple Mannich condensation of N,N -dimethyl ethylenediamine
with 2,4-ditertiarybutyl phenol or 2,4-dimethyl phenol and form-
aldehyde in MeOH. The straightforward reactions of these ligands
1
.588(3) Å, which are slightly shorter than that expected for a six
coordinated V@O bond (ꢂ1.62 Å [42]). The distances of the apical
1
2
(
H
2
L and H
2
L ) with one equivalent of VOSO
4
2
ꢀ5H O in MeOH un-
0
donor nitrogen atoms, N9 for complex 1 and N2 for complex 2,
der refluxing conditions in the presence of two equivalents of tri-
ethylamine (TEA) lead to complexes 1 and 2, respectively.
The molecular structures of complexes 1 and 2 are depicted in
Figs. 1 and 2, and some selected bond distances and bond angles
are summarized in Table 1. Complex 1 crystallizes in the triclinic
from the central vanadium atoms are 2.397(3) and 2.403(4) Å,
respectively, which are slightly larger than the reported values
(
2.12–2.33 Å [43]). Both the complexes have an overall butterfly-
like molecular view.
ꢀ
space group P1, while complex 2 in the orthorhombic space group
Pbca. Both the complexes have a similar distorted octahedral
geometry. In complex 1 the equatorial positions are occupied by
2.2. Electrochemical study
0
1
the donor atoms N9, O3 and O3 from H
2
L and O2 from the meth-
The electrochemical behavior of complexes 1 (green line) and 2
(blue line) were studied by cyclic voltammetry in the range +0.50
0
oxide ion, while the axial positions are being taken up by N9 of the
Fig. 1. ORTEP view of complex 1 with 30% thermal ellipsoids; H atoms are omitted for clarity.

M. Debnath et al. / Polyhedron 63 (2013) 189–198
191
Table 1
2.4. NMR spectroscopy
Selected bond lengths and bond angles in complexes 1 and 2.
1H, 13C and 15N NMR studies of both complexes were performed
in toluene-d and the corresponding NMR chemical shifts are sum-
Complex 1
Complex 2
8
X-ray
DFT
X-ray
DFT
marized in Table 2. In contrast to the symmetric free ligands, each
complex produced two resolved sets of NMR signals. However, the
differences in NMR chemical shifts are vanishingly small for atoms
more distant from the metal center. The H– C correlations in
long-range 2D NMR chemical shift correlation spectra indicate that
both sets of signals originate from the same molecule. They are
caused by breaking the symmetry of the ligand part by complexa-
tion with vanadium, which bears a non-symmetrically placed
methoxy group.
Bond lengths (Å)
V–O(1)
V–O(2)
1.595(2)
1.777(2)
1.881(2)
1.883(2)
2.296(3)
2.397(3)
1.586 V(1)–O(1)
1.789 V(1)–O(2)
1.860 V(1)–O(3)
1.880 V(1)–O(4)
2.369 V(1)–N(1)
2.506 V(1)–N(2)
1.880(4)
1.857(4)
1.588(3)
1.821(4)
2.250(4)
2.403(4)
1.877
1.850
1.587
1.788
2.349
2.524
1
13
V–O(3)
0
V–O(3 )
V–N(9)
0
V–N(9 )
Bond angles (°)
O(1)–V–O(2)
O(1)–V–O(3)
106.06(12) 105.9 O(1)–V(1)–O(2) 162.84(16) 156.4
97.75(11)
95.06(11)
90.40(12)
99.0
95.9
91.4
O(1)–V(1)–O(3) 98.26(18)
O(1)–V(1)–O(4) 89.48(16)
O(1)–V(1)–N(1) 81.76(16)
97.8
91.4
80.70
80.40
99.6
95.2
87.3
79.7
0
O(1)–V–O(3 )
The largest differences between NMR chemical shifts for the
free ligands and complexes 1 and 2 were detected for the atoms
near the expected complexation sites. In the case of complex 1,
O(1)–V–N(9)
0
O(1)–V–N(9 )
165.96(11) 165.0 O(1)–V(1)–N(2) 82.88(16)
92.55(10)
95.46(10)
162.74(11) 162.0 O(2)–V(1)–N(1) 88.90(15)
87.96(11) 88.9 O(2)–V(1)–N(2) 80.90(15)
162.40(10) 160.8 O(3)–V(1)–O(4) 106.06(18) 106.2
79.86(9)
82.07(9)
88.05(9)
O(2)–V–O(3)
93.6
93.9
O(2)–V(1)–O(3) 96.17(17)
O(2)–V(1)–O(4) 95.5(3)
0
the differences in NMR chemical shifts (
D
d) were 8.4 ppm for C2,
O(2)–V–O(3 )
0
O(2)–V–N(9)
7.2 ppm for C11 (and 8.6 ppm for C11 ) and 5.6 ppm for N9
0
0
0
O(2)–V–N(9 )
(15 ppm for N9 ).
O(3)–V–O(3 )
The 1H NMR chemical shifts of the individual hydrogen atoms
O(3)–V–N(9)
79.0
81.9
88.7
80.6
74.0
O(3)–V(1)–N(1) 90.48(17)
O(3)–V(1)–N(2) 166.13(17) 164.6
O(4)–V(1)–N(1) 162.30(17) 162.9
O(4)–V(1)–N(2) 87.74(16)
N(1)–V(1)–N(2) 75.97(15)
90.0
from the CH
lent after complexation (see Fig. 5), resulting in two resolved sets
of resonances for each CH group (noted as and b in Table 2).
2
groups, H8 and H10, became magnetically inequiva-
0
O(3)–V–N(9 )
0
O(3 )–V–N(9)
0
0
O(3 )–V–N(9 ) 82.58(9)
89.1
74.7
2
a
0
N(9)–V–N(9 )
75.72(11)
Moreover a significant change in the NMR chemical shifts of H8
and H10 (Table 2) relative to the ligand signals was also observed.
Our findings unequivocally characterize the presence of the com-
plexes in solution.
As a next step, a study of the complexes in the presence of
hydrogen peroxide was performed directly in an NMR tube using
to 0.0 V at a scan rate of 100 mV s 1 in acetonitrile using a plati-
num tip working electrode, Pt rod as counter electrode and SCE
as reference electrode. tert-Butyl-ammonium perchlorate (TBAPC)
was used as the supporting electrolyte. The cyclic voltammograms
for complexes 1 and 2 (Fig. 3) are found to be quasi-reversible in
nature with E1/2 = 0.235 V (Epa = 0.272 V, Epc = 0.189 V,
and 0.216 V (Epa = 0.260 V, Epc = 0.172 V, E = 0.088 V), respectively.
A controlled coulometric experiment shows that the waves are one
electron transfer processes. Since the phenolate ligand as well as
the methoxy group could not be reduced in this potential range
ꢁ
8
toluene-d as the solvent. Toluene offers good solubility for the
complexes, could also function as substrate for oxidation and is
immiscible with water, which gives us the ideal opportunity to
study the process of the catalyst activation using NMR, because
the reaction of complexes with hydrogen peroxide occurs only at
the interface of the two phases, resulting in a dramatic increase
of the reaction time due to the rate determining diffusion step.
DE = 0.083 V)
D
[
44], we assign them as metal centered reduction potentials for the
51
A series of V NMR spectra was measured over 2 h (with 15 min
3+
2+
VO /VO couple.
2 2
steps), starting with the injection of H O into the NMR tube. Subse-
quent measurements were performed after 4, 8, 16 and 24 h, and the
sample composition was checked again after 48 and 72 h. Selected
2
.3. Electronic spectra
5
1
V NMR spectra for complex 1 are shown in Fig. 6.
5
1
The absorption spectra of the complexes show two peaks at
The intensity of the V NMR signal of the original complex
ꢁ1
ꢁ1
ꢁ1
ꢁ1
5
1
98 nm (
e
/M cm = 2800) and 347 nm (
e
/M cm = 4760) for
(d = ꢁ471 ppm) clearly decreases with time, while a new signal ap-
pears at d = ꢁ690 ppm (ꢁ689 ppm for complex 2) after 15 min.
After 2 h the signal corresponding to the original complex has van-
ished completely and subsequent measurements show no further
ꢁ1
ꢁ1
ꢁ1
and 526 nm
(
e
/M cm = 3140) and 355 nm
(
e/M
-
ꢁ1
cm = 2600) for complex 2, which may be attributed to ligand
(
phenoxido) to metal [V(V)] charge transfer (LMCT) bands (Fig. 4).
-
6
6
2
.0x10
3
2
-
1
.0x10
0
.0
Complex 2
Complex 1
-
6
6
6
-
-
-
1.0x10
1
0
-
2.0x10
-
3.0x10
0
.00
0.10
0.20
0.30
0.40
E/ V (vs. SCE)
300
400
500
600
700
800
Wavelength(nm)
Fig. 3. CV of complexes
TBAPC] = 0.10 M, scan rate = 100 mV s
1
and 2 in MeCN solvent at 25 °C, [C] = 1.0 mM;
1
ꢁ
[
.
Fig. 4. Electronic spectra of complexes 1 and 2 in MeCN at [C] = 0.50 mM at 25 °C.

1
92
M. Debnath et al. / Polyhedron 63 (2013) 189–198
Table 2
6
NMR chemical shifts (ppm) of 1 and 2 at 303 K in toluene-d .
Complex 1
H
Complex 2
H
1
1
Atom number
Chemical shift (ppm)
Multiplicity/J-coupling (Hz)
Atom number
Chemical shift (ppm)
Multiplicity/J-coupling (Hz)
0
0
4
6
8
8
1
1
1
1
1
1
1
2
, 4
, 6
a
7.51, 7.52
6.86, 6.85
4.41, 4.53
2.85, 3.15
2.64, 2.65
1.31, 1.12
2.15, 2.26
1.36
1.41
1.67
1.78
5.17
triplet, 3.0
doublet, 2.2
doublet, 12.3
doublet, 12.3
doublet, 10.0
doublet, 10.0
singlet, –
singlet, –
singlet, –
singlet, –
singlet, –
1, 21
3, 17
5, 20
6,15
2.36, 2.38
6.92, 6.86
2.21, 2.28
6.52, 6.48
4.56, 4.73
2.73, 3.02
2.50, 2.55
1.14, 1.06
2.06, 2.16
5.13
singlet, –
doublet, 1.4
singlet,-
doublet, 1.4
doublet, 14.2
doublet, 14.2
doublet, 12.5
doublet, 12.5
singlet, –
0
0
, 8 a
b, 8 b
0
0
0
a
, 10
a
9
a
, 13
9b, 13b
10 , 12
a
0b, 10 b
1, 11
4, 15, 16
4 , 15 , 16
7, 18, 19
0
a
a
10b, 12b
11, 11
22
0
0
0
0
0
singlet, –
0
0
7 , 18 , 19
0
singlet, –
13C
13C
0
0
0
0
0
0
0
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
2
, 2
, 3
, 4
, 5
, 6
, 7
, 8
162.4, 162.4
140.7, 141.2
122.8, 123.5
135.7, 136.2
123.1, 124.7
120.8, 124.4
64.4, 67.0
51.5, 54.6
48.0, 49.4
35.4, 35.7
34.3
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1, 21
2, 18
3, 17
4, 16
5, 20
6,15
7, 14
8, 19
9, 13
10, 12
11, 68
22
17.0, 17.1
124.8, 125.4
130.4, 130.8
126.6, 128.0
20.5, 20.5
127.0, 128.3
119.8, 125.0
160.4, 160.9
63.0, 66.7
50.2, 53.2
47.6, 48.8
72
–
–
–
–
–
–
–
–
–
–
–
–
0
0
0
0
0, 10
1, 11
2, 12
3, 13
4, 15, 16
31.7
31.8
30.7
30.9
0
0
0
0
4 , 15 , 16
7, 18, 19
0
0
7 , 18 , 19
0
72.9
1
5
15N
1,2
N
0
9
,9
37.2, 46.6
–
–
absent
–
–
51V
1
51V
1
ꢁ471.3
ꢁ469.3
bond lengths and bond angles are shown in Table 1. The V–N(9)
0
and V–N(9 ) bond lengths in complex 1 are over estimated in the
gas phase by about 0.07–0.08 Å. A similar trend is observed for
the V–N bonds in complex 2, where the difference is somewhat lar-
ger. The V–O bond lengths determined from the DFT calculations
are comparatively much closer to the X-ray crystallographic val-
ues. The discrepancies in the V–O bond lengths for the apical oxido
atom of both complexes are less than 0.01 Å. A similar bond incre-
ment was also observed in V(IV) complexes [46]. The V–Oi bond
lengths are all in good agreement with the values of the parent
complexes (Table 1).
The MO pictures of several frontier orbitals of both complexes 1
and 2 in their ground-state optimized geometries are shown in
Fig. 7. The doubly occupied highest molecular orbital (HOMO) of
both the complexes show that they have almost no vanadium char-
acter. The orbital is localized on either of the two phenyl groups of
Fig. 5. 1H–13C HSQC NMR spectrum of complex 2 in toluene-d
at 303 K.
8
z
the ligand. The HOMO has only 12% non-bonding p character of O3
for complex 1 and 14% of O1 for complex 2. No other donor atoms
have any role in the formation of the HOMO. The HOMO-1 orbitals
of the two complexes look very similar to those of the correspond-
ing HOMO, only there are changes in the phases of the orbitals, and
change in the composition of the solution. The deep violet color of
the solution turned to light yellow during this reaction. It is rather
difficult to determine the structure of the new compound just on
the basis of V NMR chemical shift, however peroxidovanadium
compounds are among the candidates whose resonance lines can
be found in the region around ꢁ700 ppm [45].
5
1
ꢁ1
the energy difference between them is only 335 cm for complex
ꢁ1
1
, but 1650 cm for complex 2. The LUMO of complex 1 has 68%
V(d) character and it is partly delocalized with the
p orbitals of
the oxygen atoms other than the terminal O1 atom. Similarly,
the vanadium character in the LUMO of complex 2 is about 70%
and delocalization with the terminal O3 atom is also present. The
calculated HOMO–LUMO energy gap for complexes 1 and 2 are al-
2
.5. DFT optimized structures
The B3LYP/LANL2DZ/6-31+G(d) optimized geometric parame-
ꢁ1
ters of complexes 1 and 2 are comparable with the values obtained
from the X-ray crystallographic data. Some selected optimized
most same (23885 and 23799 cm , respectively). The LUMO+1
orbitals of both complexes are characterized by V(d) and p
p

M. Debnath et al. / Polyhedron 63 (2013) 189–198
193
Fig. 6. 51V NMR spectra of complex 1 as a function of time in toluene-d
8
.
pation of the coordinating oxygen atoms is large. The vertical exci-
tation energies and oscillator strengths of the lowest few excited
singlets of both complexes in acetonitrile solvent obtained from
TDDFT calculations using the PCM model at the corresponding
ground state geometries are given in Table 3. The largest oscillator
complex 1
complex 2
LUMO+2
strength has been noted for S
2
at 535 nm for complex 1 compared
state is
to the observed absorption maximum at 598 nm. The S
2
dominated by the HOMO ? LUMO excitation along with two other
transitions, namely HOMOꢁ1 ? LUMO and HOMO ? LUMO+1.
1
The excitation energy of the S state corresponds to 595 nm with
f = 0.0228 only and it is mainly characterized by the HOMO-
1 ? LUMO transition. For complex 2, the maximum value of
the oscillator strength (f = 0.2339) is obtained at 496 nm (S ), while
the observed absorption peak is at 526 nm. The excitation energy
of the first excited singlet (S ) is 605 nm. Both S and S are char-
acterized by HOMO ? LUMO and HOMOꢁ1 ? LUMO transitions.
The former is dominated in S , while the latter in S . It may be
ꢁ
LUMO+1
2
1
1
2
1
2
noted that for both the complexes, the computed absorption max-
imum is at a higher energy end than the experimental one (SUP
Fig. 5a and b).
The natural atomic charges on the atoms of both complexes at
the optimized geometry are obtained from the natural bond orbital
LUMO
HOMO
Table 3
Vertical excitation energies (Ecal), oscillator strengths (fcal), and type of excitations of
the lowest three excited singlets obtained from TDDFT calculations of complexes 1
and 2 in acetonitrile solvent (
e
= 36.64).
Compd
State
Ecal/nm
f
cal
Excitation
Complex 1
S
1
595
535
0.0228
0.1924
HOMO-1 ? LUMO (0.66),
HOMO ? LUMO (0.21)
S
2
HOMO ? LUMO (0.61),
HOMO-1 ? LUMO (0.19),
HOMO ? LUMO+1 (0.15)
HOMO ? LUMO + 1 (0.61),
HOMO-1 ? LUMO + 1 (0.29),
HOMO ? LUMO (0.11),
HOMO ? LUMO + 2 (0.11)
HOMO-1 ? LUMO+2 (0.67),
HOMO ? LUMO+2 (0.15)
HOMO ? LUMO (0.67),
HOMO-1 ? LUMO (0.16)
HOMO-1 ? LUMO (0.63),
HOMO ? LUMO (0.15)
S
3
472
0.0484
HOMO-1
S
6
S
1
S
2
S
5
430
605
496
429
0.0476
0.0415
0.2339
0.0551
Fig. 7. Schematic diagram of selected frontier orbitals for complexes 1 and 2 in
their ground state geometries.
Complex 2
orbitals of the terminal oxygen atoms (O1 for complex 1 and O3 for
complex 2). The contribution of the oxygen orbitals is 16–17%. The
LUMO+2 orbitals have also strong metal character and the partici-
HOMO ? LUMO+2 (0.68)

1
94
M. Debnath et al. / Polyhedron 63 (2013) 189–198
Table 4
100
NBO charges on some selected atoms in the ground state.
90
Atom
Complex 1
Atom
Complex 2
V
0.738
V
0.747
7
5
O1
O2
O3
O4
N9
ꢁ0.270
ꢁ0.513
ꢁ0.560
ꢁ0.558
ꢁ0.456
ꢁ0.487
O3
O4
O1
O2
N1
N2
ꢁ0.276
ꢁ0.503
ꢁ0.570
ꢁ0.538
ꢁ0.457
ꢁ0.486
60
%
45
0
N9
3
0
.
22 0882
696.3292
6
.
5
96 2314
.
(
NBO) analysis. Table 4 shows the NBO charges on the central
698 3167
1
5
.
5
98 2157
.
vanadium atom, four coordinating oxygen atoms and two coordi-
nating nitrogen atoms. The charge distribution clearly favors an
electrostatic interaction between the positively charged V atom
and negatively charged coordinating atoms. The amount of partial
negative charges on the terminal oxygen atoms (O1 for complex 1
and O3 for complex 2) is almost half that of the other three oxygen
atoms. There is almost no effect of the tert-butyl group on the par-
tial charges of the two nitrogen atoms.
623 1742
0
575 600
650
700
750
m/z
Fig. 8. Mass spectra of the toluene oxidation reaction mixture (some of the
intermediates were characterized).
Å
of one OOH radical [31] from the intermediate III. In the subse-
2 2
quent steps, the oxido species (IV) reacts with H O to give the
hydroxido-hydroperoxido species (V), which after the loss of one
OH radical gives the oxido-hydroxido species (VII) through the
2.6. Catalytic oxidation and proposed mechanism
Å
transient intermediate (VI). The species (VII) further reacts with
another H O molecule to generate transiently the oxido-hydroxi-
2 2
do-hydroperoxido species (VIII), which subsequently gives the
intermediate (III) and completes the catalytic cycle. In this cata-
2 2
Hydroxyl radicals, very efficiently generated from H O (or less
efficiently from O
been successfully used for the in vitro oxidation of aliphatic [47–
0], olefinic [51] and aromatic [52] hydrocarbons. The catalytic
reaction mixture contains the hydrocarbon substrate, H and
complex 1 or 2 as a catalyst, which is converted to [VO(L)(OOH)]
2
) under the action of vanadium catalysts, have
5
lytic cycle, two H
HO and one HOO radical. The involvement of the proposed perox-
ido intermediates in the catalytic cycle was characterized by
NMR spectra and ESI-MS studies (Fig. 8), viz. species III (MW
2
O
2
molecules are consumed, generating one
2 2
O
Å
Å
51
V
(
(
III) through the transient intermediate [V(L)(OH)(OOH)(OMe)]
II) by the reaction with H (Scheme 1). This intermediate might
+
2 2
O
+
6
22), IV (MW 589 + Li = 596) and V (MW 623), The two radicals
be in equilibrium with the hydroxide-peroxido intermediate
Å
Å
0
51
(OOH and OH ), which were formed during the catalytic cycle,
now react with aromatic hydrocarbons to give the corresponding
oxidized products.
[
2
V(OH)(L)(O )](III ), as indicated by V NMR studies (vide infra).
It is subsequently transformed to the oxido species (IV) by the loss
The involvement of radicals in the reaction mechanism was
confirmed by the acceleration of the rate of reaction when the
experiment was conducted with a trace amount of azoisobutyro-
nitrile (AIBN) and deceleration of the same using benzoquinone
+
during the course of the reaction. An ESI-MS study of the toluene
oxidation reaction mixture shows the existence of benzyl alcohol
and benzaldehyde.
In Table 5, results of the catalytic activity as well as TON (turn-
over number) for the catalytic runs are given for complexes 1 and
2. The major products for the oxidation of toluene are benzoic acid
(ꢂ70%), benzyl alcohol and benzaldehyde (ꢂ15%). In the case of o-
and p-xylenes, the major products are o- and p-hydroxymethyl
benzoic acids (ꢂ46–60%). Figs. 9 and 10 give the % oxidation of
the substrates as a function of time for all the substrates by com-
plexes 1 and 2, respectively. Fig. 11 shows that with an increase
in catalyst concentration, the % yield also increases, but it soon
reaches saturation level and thereafter the % yield remains almost
constant with a further increase in the catalyst concentration.
When we fit the % yield versus time data to a single exponential
growth equation we get the pseudo-first-order rate constant for
formation of products (Table 5). The temperature 50 °C was found
to be the optimum temperature for these catalytic reactions and at
above or below this temperature the % yield of products was found
to decrease. Recovery of the catalyst complexes provides an oppor-
tunity to reuse the catalysts, and this was achieved, but with re-
duced activities.
Critical data analysis reveals that there was a slight improve-
ment in the TONs, for example it was 203 for the oxidation of tol-
uene catalyzed by complex 1, whilst it was 188 for the analogous
oxidovanadium complex reported earlier [41], and this may be
Scheme 1.

M. Debnath et al. / Polyhedron 63 (2013) 189–198
195
Table 5
Catalytic parameters for the oxidation of aromatic hydrocarbons by H
2 2
O using complexes 1 and 2 as catalysts at 50 °C. Other conditions are mentioned in Figs. 9 and 10.
TONa
10 ꢃ k (h
2
ꢁ1
)
Catalyst
Substrate
Reaction time (h)
Major Products
Conversion (%)
Yield
Major
Minor
1
1
.[VO(L )OMe]
24
24
COOH
85
82
70
71
15
11
213
205
13.29 ± 0.74
10.7 ± 0.85
2
2
.[VO(L )OMe]
1
1
2
.[VO(L )OMe]
24
24
COOH
83
80
57
60
26
20
208
200
12.10 ± 1.40
9.21 ± 1.07
2
.[VO(L )OMe]
CH OH
2
1
1
2
.[VO(L )OMe]
24
24
COOH
70
68
46
50
24
18
175
170
12.04 ± 1.10
7.73 ± 1.07
2
.[VO(L )OMe]
CH OH
2
a
TON is determined by the ratio of the No. of moles of substrate converted to product and the number of moles of catalyst used.
1
00
o-xylene
p-xylene
toluene
80
60
40
20
0
8
0
0
0
0
6
4
2
0
0
6
12
time/h
18
24
0
.00
0.03
0.06
0.09
Catalyst Concentration (mM)
Fig. 9. Plots of % yield vs. time (h) for the catalytic oxidation of toluene, o-xylene
and p-xylene by H using complex 1 as a catalyst with a substrate:catalyst ratio
of 250:1 and H
Fig. 11. Plots of % yield vs. catalyst concentration (mM) for the catalytic oxidation
of toluene by H O using complex 1 as a catalyst (substrate taken = 20 mM) in 24 h
2 2
at 50 °C.
2 2
O
2
O
2
:substrate ratio of 13:1 (at the beginning of the reaction) at 50 °C.
present complexes are already 6 coordinated and the coordination
of H imposes steric congestion around the metal center, thereby
making the intermediates unstable and hence more reactive to-
wards the oxidation.
8
6
4
2
0
2 2
O
0
0
0
0
3
. Conclusion
o-xylene
p-xylene
toluene
We have synthesized and structurally characterized two oxi-
dovavadium(V) complexes of ONNO donor ethylenediamine-
bis(phenolate) ligands with octahedral geometries, using NMR
spectroscopy and X-ray diffraction studies. Both the complexes ex-
hibit efficient catalytic activity towards the oxidation of aromatic
hydrocarbons with TONs over 200, with moderate selectivity. The
results of the NMR study might indicate the formation of oxido–
0
6
12
time/h
18
24
i
i
hydroperoxido/hydroxido-peroxido [V(L )(O)(OOH)] , [V(L )(OH)
(O
2
)] intermediates during the reaction. The presence of such inter-
mediates was further supported by ESI-MS studies.
+
Fig. 10. Plots of % yield vs. time (h) for the catalytic oxidation of toluene, o-xylene
and p-xylene by H using complex 2 as a catalyst with a substrate:catalyst ratio
of 250:1 and H :substrate ratio of 13:1 (at the beginning of the reaction) at 50 °C.
2 2
O
2 2
O
4
. Experimental
attributed to the fact that the earlier pentacoordinated complexes
can easily extend their coordination number to 6 by the coordina-
tion of the hydroperoxide anion without much change in the sta-
bility of the intermediate compared to its parent. However, the
4.1. Physical measurements
Elemental analyses were carried out using a Perkin-Elmer 240
elemental analyzer. Infrared spectra (400–4000 cm
ꢁ1
) were

1
96
M. Debnath et al. / Polyhedron 63 (2013) 189–198
V
2
recorded from KBr pellets on a Nickolet Magna IR 750 series-II FTIR
spectrophotometer. Electronic spectra were recorded on an Agilent
[V O(L )(OMe)] (2). Yield: 82%. Anal. Calc. For C23
33 2 4
H N O V: C,
61.00; H, 7.29; N, 6.19. Found: C, 61.36; H, 7.39; N, 6.25%. FTIR
(KBr disc, cm ): m(V@O) 954.
ꢁ1
8
453 UV–Vis diode-array spectrophotometer. GC analyses were
carried out in an Agilent 7890A GC instrument with an FID detec-
tor. Mass spectra were recorded on a Micromass Q-TOF spectrom-
eter (Waters) with an electrospray ionization source.
Electrochemical measurements were carried out using a computer
controlled AUTOLAB (Model: AutoLab 302) cyclic voltammeter
with a platinum working electrode and a saturated calomel elec-
trode (SCE) as the reference electrode.
4.5. NMR spectroscopy in solution
The H, 13C, 15N and 51V NMR spectra were recorded on a Bruker
1
1
Advance 500 spectrometer operating at frequencies of 500.13 ( H),
1
3
15
51
125.77 ( C), 50.68 ( N) and 131.55 MHz ( V). The compounds
were dissolved in toluene-d at concentrations of 5 mg/mL for 1
and C NMR measurements and 10 mg/mL in acetonitrile for
NMR measurements. The H, C and N NMR spectra were mea-
H
V
8
1
3
51
1
13
15
1
13 15
sured using 5-mm inverse probehead ( H{ C, N}) equipped with
5
1
x,y,z-gradient coils. V NMR spectra were recorded using 10 mm
1
13
broad-band-optimized probe head BBO. The H and C NMR
chemical shifts (d in ppm) were externally referenced to TMS.
1
5
The N NMR chemical shifts were referenced to external 1 M urea
in DMSO-d (77.0 ppm) and are reported relative to liquid ammo-
nia [54]. The V NMR chemical shifts were externally referenced
to VOCl . All NMR spectra were obtained at a temperature of
03 K. A set of 2D NMR experiments was used to assign the indi-
6
5
1
R = tert-butyl, H L
1 2
; R = Me, H L
2
2
3
3
1
13
1
13
vidual H and C resonances: H– C gs-HSQC [55,56] adjusted
1
1
13
for one-bond coupling, JH–C = 180 Hz; H– C gs-HMBC [57,58] ad-
n
1
13
justed for long-range coupling, JH–C = 7.5 Hz; H– C GSQMBC [54]
adjusted for long-range coupling,
GSQMBC with JH–C = 3 Hz.
4
.2. Materials
n
1
15
JH–C = 7.5 Hz and H– N
n
Starting materials for the synthesis of the ligands (H
2
L1 and
2
L ), namely 2,4-di-tert-butylphenol (Aldrich), 2,4-di-methylphe-
2
H
4.6. X-ray crystallography
nol (Aldrich) and formaldehyde (Merck India) were of reagent
grade and used as received. Solvents, like methanol, ethanol and
acetonitrile (Merck India), were of reagent grade and dried by stan-
dard methods before use. Substrates, namely toluene and isomers
of xylene (Merck India), used in the catalytic reactions were of
spectroscopic grade and used as received. 30% H O (Merck India)
2 2
was used in the catalytic oxidation reactions. Triethylamine (TEA)
was dried by refluxing over Na before use.
V
1
Intensity data of complex 1 [V O(L )(OMe)] were collected at
93(2) K on a smart CCD diffractometer with the
in the range 2.75 < 2h < 27.50°). No decomposition of the crystal
2
x
ꢁ2h scan mode
occurred during the data collection. The intensities were corrected
for Lorentz and polarization effects and for absorption using the
w-
scan method. The diffraction data for complex 2 were collected
with a KM4CCD diffractometer with a four-circle area-detector
4.3. Synthesis of the ethylenediamine-bis(phenolate) ligands
Table 6
Summary of crystallographic data.
The ligands (H
a reported method [53] and further characterized by CHN analysis.
L1and H L2) have been synthesized according to
2 2
Complex 1
Complex 2
1
H
2
L : Yield 80%. Anal. Calc. for Molecular formula, C34
56
H N
2
O
2
:
Formula
C35H57N O V
C23H33N O V
2 4
452.45
2
4
C, 77.81; H, 10.76; N, 5.34. Found: C ,78.34; H, 10.39; N, 5.46%.
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
620.77
triclinic
P 1ꢀ (No. 2)
2
orthorhombic
Pbca (No. 61)
15.2629(10)
12.8916(6)
22.7412(18)
90.00
H
2
L : Yield 83%. Anal. Calc. for Molecular formula, C22
32 2
H N O
2
:
C, 74.12; H, 9.05; N, 7.87. Found: C, 74.86; H, 9.20; N, 7.63%.
10.0360(7)
14.3083(8)
14.4449(10)
107.631(6)
101.786(6)
108.640(5)
1766.0(2)
2
4
4
.4. Preparation of the complexes
a
(°)
b (°)
(°)
1
.4.1. Synthesis of [VO(L )(OMe)](1)
c
1
.0 mmol (0.524 g) of the ligand H
MeOH and refluxed with 2.0 mmol (0.202 g) of TEA for 10 min.
The solution was then cooled. solution of VOSO
ꢀ5H
1.0 mmol, 0.253 g) in MeOH was added drop wise to the previous
2
L1 was dissolved in 15 mL
V (Å )
3
4474.6(5)
8
1.343
0.475
1920
Z
D
l
calc (g/cm3)
(Mo K ) (mm
1.167
0.318
A
4
2
O
ꢁ1
a
)
(
F (000)
T (K)
672
293
ligand solution and the resulting mixture was refluxed for 3 h. On
cooling to room temperature, the initial slurry was filtered off and
the filtrate was subjected to slow evaporation at room tempera-
ture, whereupon dark brown block shaped single crystals suitable
for an X-ray study were obtained within 3 days.
293
Radiation (Å)
h Minimum–Maximum (°)
Dataset
0.71073
2.76, 25.0
ꢁ11:11; ꢁ17:17;
0.71073
2.67, 26.5
ꢁ19:19; ꢁ16:16;
ꢁ28:28
ꢁ
17:17
Total, unique data, Rint
Observed data [I > 2r(I)]
12339, 6219, 0.0357
3804
12095, 4634, 0.0879
3181
Refinement Nreflection
Nparameter
,
6219, 394
4443, 277
2
4
.4.2. Synthesis of [VO(L )(OMe)] (2)
Complex 2 was synthesized following the same procedure as for
Minimum and Maximum
Resd. Density (e/Å )
ꢁ0.61, 1.21
ꢁ0.88, 1.55
3
2
1
2 2
complex 1, using the H L ligand instead of H L .
V
1
R, wR
2
, S
0.0527,0.1411,0.95
0.0984, 0.1830, 1.102
[
V O(L )(OMe)] (1). Yield: 78%. Anal. Calc. For C35
57 2 4
H N O V: C,
2
2
2
2
2) + (0.0959P)2 + 80.0842P]
6
(
7.65; H, 9.18; N, 4.51. Found: C, 67.25; H, 9.28; N, 4.38%. FTIR
w = 1/[s (F
o
) + (0.0800 P) ] (for complex 1) and 1/[s (F
o
2
2
ꢁ
1
(for complex 2) where P = (Fo + 2Fc )/3.
KBr disc, cm ): m(V@O) 959.

M. Debnath et al. / Polyhedron 63 (2013) 189–198
197
(
KUMA Diffraction, Poland), and equipped with an Oxford Cryo-
stream Cooler (Oxford Cryosystems, UK). MoK radiation
k = 0.71073 Å) (monochromator Enhance, Oxford Diffraction, UK)
then 1
l
L of the concentrated ether extract was injected to the GC
a
port with the help of a 10
peaks were compared with those of commercial standards and
the unknown peaks were characterized by ESI-MS analysis. The
lL syringe. The retention times of the
(
+
was used in all measurements. The
x
-scan technique with differ-
ent
j
and
u
offsets for covering an independent part of the reflec-
same experiment was carried out separately in the presence of
traces of azoisobutyronitrile (AIBN) and benzoquinone to check
the involvement of radicals in the reaction mechanism. The exper-
iments were carried out in triplicate runs and errors were found to
fall within ±5%.
tions in the 3.2–25.0° h range was performed. The cell parameters
were refined from all strong reflections. The data reductions were
carried out using the CrysAlis RED (Oxford Diffraction, UK) pro-
gram, and analytical absorption corrections were applied. Both
structures were determined by direct methods using SHELXS-97
[
59] refined anisotropically on F2 using the full-matrix least-
4.9. Recovery of catalyst
squares procedure of SHELXL-97 [60]. Hydrogen atoms were in-
cluded in the model at their calculated positions with C–
H = 0.95–0.99 Å and with Uiso(H) values of 1.2Ueq(C) [1.5 Ueq(C)
for methyl carbon atoms]. For complex 2, the H atoms attached
to C atoms were positioned geometrically, with C–H = 0.95–
Upon addition of diethyl ether to the reaction mixture contain-
ing the catalyst, products and unreacted substrate, all of them were
quantitatively extracted into the ether layer. The ether layer was
then separated off, added to MeOH and refluxed for 1 h in air.
The resulting brown solution was then kept for slow evaporation
in air. After 2 days shiny crystals of the catalyst (complex) were
deposited. The crystals were separated, dried in air and character-
ized by comparing the elemental data as well as the IR spectrum
with the data from the original complex, and the results were
found to be identical.
0
.98 Å and with Uiso(H) values of 1.2 Ueq(C) [1.5 Ueq(C) for methyl
carbon atoms]. The data for publication were prepared by SHELXL
and PARST [61] and the Figures by ORTEP-III [62]. The crystal data col-
lection and the procedure for data refinement are listed in Table 6
for the two complexes.
CCDC 775686 and 775687 contain the supplementary crystallo-
graphic data for complexes 1 and 2, respectively. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge crystallographic Data Cen-
ter, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336
Acknowledgements
Financial support from CSIR [02(2490)/11/EMR-II] and DST (ref.
SR/S1/IC-35/2006 and SR/S1/IC-20/2012) are gratefully acknowl-
edged. This research was financially supported by a grant from
the Czech Science Foundation (P206/12/0539) and the project
0
33; or e-mail: deposit@ccdc.cam.ac.uk.
4.7. DFT calculations
‘
1
‘CEITEC – Central European Institute of Technology ‘‘(CZ.1.05/
.1.0/02.0068) from European Regional Development Fund.
All the computations reported here have been carried out with
the GAUSSIAN 03 program [63]. The electron correlation is partially
0
taken into consideration by using the DFT method based on Becke s
Appendix A. Supplementary data
three parameter hybrid method [64] and the Lee–Yang–Parr corre-
lation functional (B3LYP) [65]. The LANL2DZ effective core poten-
tials and basis functions are used for the vanadium atom, while
-31+G(d) is the basis set for the other atoms, namely H, C, N
and O. Starting with the geometries provided by the crystal struc-
CCDC 775686 and 775687 contains the supplementary crystal-
lographic data for complexes 1 and 2. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or
e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.poly.2013.07.013.
6
tures, we have fully optimized the geometries of complexes 1 and
. Following the geometry optimization, calculations of all the
2
vibrational frequencies have been made to ensure their positive
values. Therefore, global minima on the potential energy surfaces
of the two molecules are obtained. The time dependent density
functional theory (TD-DFT) method coupled with the polarized
continuum model (PCM) [66] at the same level have been em-
ployed to calculate the excitation energies and intensities of the
twenty lowest excited singlets of both complexes in the presence
References
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(
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[
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4
.8. Experimental set up for catalytic oxidation
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[
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turned yellow. The solution was then heated up to 50 °C on an
oil bath for 24 h and 0.5 mL (17 mmol) of H was added inter-
mittently at a time intervals of 60 min. When required, an aliquot
0.1 mL) of the reaction solution was withdrawn with the help of a
long needle syringe and was subjected to multiple ether extraction,
2 2
O and the resulting solution immediately
357.
2 2
O
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