4,6-Diacetyl Resorcinol Based Vanadium(V) Complexes: Reactivity and Catalytic Applications

Article abstract of DOI:10.1002/ejic.201801243

Four ONO donor ligands are isolated from the condensation of 4,6-diacetyl resorcinol with isonicotinoyl hydrazide (H₂dar-inh, I), nicotinoyl hydrazide (H₂dar-nah, II), benzoyl hydrazide (H₂dar-bhz, III), and 2-furoyl hydrazide (H₂dar-fah, IV) on refluxing in MeOH. The reaction of in situ generated aqueous K[H₂V^VO₄] with ligands I–IV at neutral pH gives complexes [K(H₂O)₂][VO₂(dar-inh)] (1), [K(H₂O)₂][VO₂(dar-nah)] (2), [K(H₂O)₂][VO₂(dar-bhz)] (3), and [K(H₂O)₂][VO₂(dar-fah)] (4), respectively. The reaction of [V^IVO(acac)₂] (acac = acetylacetonato) with these ligands (I–IV) under aerobic conditions in methanol yields oxidomethoxidovanadium(V) complexes [VO(OMe)(MeOH)(dar-inh)] (5), [VO(OMe)(MeOH)(dar-nah)] (6), [VO(OMe)(MeOH)(dar-bhz)] (7), and [VO(OMe)(MeOH)(dar-fah)] (8). All the isolated complexes are characterized by elemental, thermal, electrochemical, and spectroscopic techniques [FTIR, UV/Vis, NMR (¹H, ¹³C and ⁵¹V NMR)], and single-crystal X-ray diffraction analysis (for 1, 6, 7, and 8). X-ray analysis confirms the coordination of the ligands through O_phenolate, N_azomethine, and O_enolate to the metal center. In the molecular structure of [K(H₂O)(EtOH)][V^VO₂(dar-inh)] (abbreviated as 1a where one molecule of water is replaced by EtOH), water molecules act as bridges between two K⁺ ions and the complex shows a dimeric structure due to the presence of electrostatic interactions between V=O oxygen atoms with K⁺ ions. These complexes are active catalysts for the oxidative bromination of thymol in the presence of KBr, HClO₄, and H₂O₂ and give 2-bromothymol, 4-bromothymol, and 2,4-dibromothymol as major products. Complexes 1–4 were also tested as catalysts for the epoxidation of various alkenes (namely styrene, cyclohexene, cis-cyclooctene, 1-hexene, 1-octene, cyclohexenone, and trans-stilbene) with H₂O₂ in the presence of NaHCO₃ as promoter, giving the corresponding epoxides selectively.

Full text of DOI:10.1002/ejic.201801243

A Journal of
Accepted Article
Title: 4,6-Diacetyl Resorcinol Based Vanadium(V) Complexes:
Reactivity and Catalytic Applications
Authors: Mannar R. Maurya, Nancy Jangra, Fernando Avecilla, and
Isabel Correia
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801243
Link to VoR: http://dx.doi.org/10.1002/ejic.201801243

European Journal of Inorganic Chemistry
10.1002/ejic.201801243
4,6-Diacetyl Resorcinol Based Vanadium(V) Complexes: Reactivity and
Catalytic Applications
[a]
[a]
[b]
[c]
Mannar R. Maurya,* Nancy Jangra, Fernando Avecilla and Isabel Correia
AUTHORS ADDRESS
[
a] Prof. Dr. Mannar R. Maurya, Ms. Nancy Jangra
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
E-mail: rkmanfcy@iitr.ac.in; Fax: +91 1332 273560; Tel: +91 1332 285327:
https://www.iitr.ac.in/CY/rkmanfcy
[
b] Prof. Dr. Fernando Avecilla
Grupo Xenomar, Centro de Investigacións Científicas Avanzadas (CICA), Departamento de
Química, Facultade de Ciencias, Universidade da Coruña, Campus de A Coruña, 15071 A
Coruña, Spain
[
c] Dr. Isabel Correia
Centro de Química Estrutural, Instituto Superior Técnico, Universidade Lisboa, 1049–001 Lisboa,
Portugal
Keywords: Vanadium complexes · Haloperoxidases models · Structure elucidation ·
Homogeneous catalysis · Oxidative bromination · Catalytic epoxidation of alkenes
Abstract: Four ONO donor ligands are isolated from the condensation of 4,6-diacetyl resorcinol with
isonicotinoyl hydrazide (H
2
dar-inh, I), nicotinoyl hydrazide (H
dar-fah, IV) on refluxing in MeOH. The reaction of in situ
] with ligands I-IV at neutral pH gives complexes, [K(H O) ][VO (dar-
][VO (dar-fah)]
] (acac = acetylacetonato) with these ligands (I-IV)
2
dar-nah, II), benzoyl hydrazide
(
H
2
dar-bhz, III) and 2-furoyl hydrazide (H
2
V
generated aqueous K[H
2
V O
4
2
2
2
inh)] (1), [K(H O) ][VO
2
2
2
(dar-nah)] (2), [K(H
2
O)
2
][VO
2
(dar-bhz)] (3) and [K(H
2
O)
2
2
IV
(
4), respectively. The reaction of [V O(acac)
2
under aerobic conditions in methanol yields oxidomethoxidovanadium(V) complexes,
[
VO(OMe)(MeOH)(dar-inh)] (5), [VO(OMe)(MeOH)(dar-nah)] (6), [VO(OMe)(MeOH)(dar-bhz)]
(
7) and [VO(OMe)(MeOH)(dar-fah)] (8). All the isolated complexes are characterized by elemental,
thermal, electrochemical and spectroscopic techniques [FT-IR, UV-Vis, NMR ( H, 13C and 51
1
V
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
NMR)] and single crystal X-ray diffraction analysis (for 1, 6, 7 and 8). X-ray analysis confirms the
coordination of the ligands through Ophenolate, Nazomethine and Oenolate to the metal center. In the molecular
V
structure of [K(H
2
O)(EtOH)][V O
2
(dar-inh)] (abbreviated as 1a where one molecule of water is
+
replaced by EtOH), water molecules act as bridges between two K ions and the complex shows a
dimeric structure due to the presence of electrostatic interactions between V=O oxygen atoms with
+
K ions. These complexes are active catalysts for the oxidative bromination of thymol in the presence
of KBr, HClO
4
and H
2
2
O and give 2-bromothymol, 4-bromothymol and 2,4-dibromothymol as major
products. Complexes 1-4 were also tested as catalysts for the epoxidation of various alkenes (namely
styrene, cyclohexene, cis-cyclooctene, 1-hexene, 1-octene, cyclohexenone and trans-stilbene) with
H
2
O
2
in the presence of NaHCO as promoter giving the corresponding epoxides selectively.
3
Introduction
A remarkable research growth on mononuclear oxidovanadium(V) complexes has been observed
in recent decades due to interesting discoveries on vanadium bioinorganic chemistry.[1]
Biologically important compounds such as mononuclear oxidovanadium(V) complexes have also
served as structural and functional models of vanadate-dependent haloperoxidases.[2,3] These have
also been explored as homogeneous catalysts for numerous catalytic transformations[4] such as
oxidative bromination,[5] sulphoxidation,[6] epoxidation of olefins,[7] oxidation of alcohols,[8]
oxidation of aromatic hydrocarbons/cycloalkanes,[9] hydroamination and oxidative amination,[10]
aromatization of 1,4-dihydropyridines,[11] etc. Our research group has been actively engaged in the
characterization of structural and functional models of haloperoxidases and has made efforts in
exploring their catalytic potential towards some of the above transformations. Herein, we report
the syntheses and characterization of dioxidovanadium(V) and oxidomethoxidovanadium(V) of
ligands I-IV derived from 4,6-diacetyl resorcinol and various hydrazides, Scheme 1.
Difunctional 4,6-diacetyl resorcinol and its derivatives have attracted attention due to their
biological and particullarly pharmaceutical activity. Various hydrazones derived from 4,6-diacetyl
resorcinol and their coordination complexes of nickel(II), cobalt(II), copper(II) and zinc(II) have
been investigated against gram-negative (Salmonella Typhi and Escherichia Coli), and gram
positive (Bacillus Subtilis and Staphylococcus Aureus) bacteria, as well as antifungal activity
against Candida Albicans, Aspergillus Niger and Cladosporium oxy-sporum, with positite
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
results.[12] In copper(II) complexes derived from the products of condensation of 4,6-diacetyl
resorcinol with 3-hydrazino-5,6-diphenyl-1,2,4-triazine or isatin monohydrazone the antimicrobial
activity of the ligands was enhanced by Cu-chelation.[13]
Only a few reports have been made on the catalytic activity of 4,6-diacetyl resorcinol
complexes. The activity of a copper(II) complex of a chitosan derivative showed high efficiency
towards the decomposition of H
2
O
as heterogeneous catalyst.[14] Pd(II) complexes from Schiff
2
bases derived from 4,6-diacetyl resorcinol provided high activity and substrate selectivity in the
oxidation of 1-octene.[15] Vanadium complexes of hydrazones of 4,6-diacetyl resorcinol have not
yet been reported. Thus, we have explored the catalytic potential of vanadium complexes derived
from the condensation of 4,6-diacetyl resorcinol with hydrazides towards the oxidative
bromination of thymol (a monoterpene) and epoxidation of various alkenes (namely styrene,
cyclohexene, cis-cyclooctene, 1-hexene, 1-octene, cyclohexanone and trans-stilbene).
Scheme 1. Synthetic scheme used to prepare ligands I, II, III and IV and their structural formulae.
Epoxidation reactions are extremely important transformations in organic chemistry since
epoxides are key building blocks in organic synthesis[16] and many vanadium complexes, such as
VO(acac)
(acac = acetylacetonate) have been used to catalyse this reaction.[17] However, there is
2
still room for improvement. The catalytic halogenation of organic substrates, and particularly
bromination reactions[5] are very important as brominated compounds have interesting biological
properties. The classical methods involving molecular bromine need to be replaced by safer and
greener reactions.
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European Journal of Inorganic Chemistry
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Results and Discussion
Synthesis and Solid-State Characterization of Complexes
Condensation of 4,6-diacetyl resorcinol with various hydrazides (Scheme 2) in equimolar ratio in
MeOH resulted in the formation of the analytically pure ligands H dar-inh (I), H dar-nah (II),
dar-bhz (III) and H
dar-fah (IV) in good yields (68-72% (Scheme 1).[18] An aqueous solution
of in situ generated potassium vanadate (predominantly K[H VO ] at pH 7.0), obtained by
dissolving V in aqueous KOH, reacted with the potassium salt of these ligands giving the
potassium salts of the corresponding dioxidovanadium(V) anions [K(H O) ][VO (dar-inh)] (1),
K(H O) ][VO (dar-nah)] (2), [K(H O) ][VO (dar-bhz)] (3) and [K(H O) ][VO (dar-fah)] (4),
respectively – see equation (1) considering 2 as a representative complex.
2
2
H
2
2
2
4
2
O
5
2
2
2
[
2
2
2
2
2
2
2
2
2
V
V
K[H
2
V O
4
] + K
2
dar-nah + 2HCl→ [K(H
2
O)
2
][V O
2
(dar-nah)] + 2KCl
(1)
IV
[
V O(acac)
ratio in methanol under reflux, followed by aerial oxidation to yield the mononuclear
oxidomethoxidovanadium(V) complexes [VO(OMe)(MeOH)(dar-bhz)] (7) and
VO(OMe)(MeOH)(dar-fah)] (8), respectively. Equations (2) and (3) present a possible reaction
2
] (Hacac = acetylacetone) also reacted with ligands III and IV in 1:1 molar
[
sequence considering H dar-fah (IV) as a representative ligand.
2
Complexes [VO(OMe)(MeOH)(dar-inh)] (5) and [VO(OMe)(MeOH)(dar-nah)] (6) were
obtained by aerial oxidation of complexes 1 and 2, respectively, in methanol - equation (3). All V-
complexes were obtained in moderate to good yields (68-85%).
IV
IV
[
V O(acac)
2
] + H
2
dar-fah + H
2
O → [V O(dar-fah)(H
2
O)] + 2 Hacac
(2)
IV
V
2
[V O(dar-fah)(H
2
O)] + ½ O
2
+ 4 MeOH → 2 [V O(dar-fah)(OMe)(MeOH)] + 3H
2
O (3)
For all complexes the molecular ion peak was found both in the positive and in the negative
ionization mode. An overview of the complexes reported in this paper is presented in Scheme 2,
based on elemental, thermal and electrochemical analyses and spectroscopic characterization data
1
13
51
(
ESI-MS, FT-IR, UV-Vis, H and C NMR and V NMR).
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
Structures of complexes 1, 6, 7 and 8 have been further indorsed by single-crystal X-ray
diffraction studies. These complexes exist as monomers and are soluble in MeOH, EtOH, CH
DMF, and DMSO, to different extents.
3
CN,
Scheme 2. Structural formulae of the complexes described in this work.
Thermal Studies
The thermal stability of the complexes was studied under oxygen atmosphere and relevant data
and TGA profiles are presented in Table S1 and Figure S1. The profiles obtained for complexes
1
-4 are consistent with i) the loss of one weakly coordinated water molecule; ii) the loss of L+H
2
O-
(
O) moiety and iii) KVO as the final product. Complexes 5-8 undergo mass losses roughly
3
equivalent to i) one methanol molecule; ii) two overlapping steps equivalent to (OMe+L)-1.5O
group, indicating the presence of a strongly coordinated methoxide group as well as the ligand and
iii) V
2
5
O as the final product.
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
Description of Molecular Structures
ORTEP diagrams of 1a, 6, 7 and 8 are shown in Figures 1, 2, 3 and 4, respectively. Selected bond
distances and angles are given in Tables 1 and 2. In all crystal structures coordination of the ligands
to the metal center occurs through Ophenolate, Nazomethine and Oenolate
.
Dihedral angles between the planes defined by the rings of the resorcinol moiety and the
rings of hydrazides were measured in all structures. Angles of 5.69(22)º for 1a, 12.26(12)º for 6,
1
0.17(23)º for 7 and 13.39(17)º for 8 evidence the versatility of the ligands to adopt different
conformations by rotation around the C(6)-C(9) bond, forming different atropoisomers.
Compound 1a shows a dimeric structure due to the presence of electrostatic interactions
+
+
between V=O oxygen atoms with K ions. Water molecules act as bridges between two K ions
see Figure 1). ··· stacking interactions are present in the structures in the crystal packing. The
molecules are grouped in pairs through π-π interactions forming dimers in antiparallel positions
see Figure 4). The distance between centroids [c1, C(1H)-C(2H)-C(3H)-C(4H)-C(5H)-C(6H); c2,
(
(
C(12B)-C(13B)-C(14B)-C(15B)-C(16B)-N(3AA)] is: dc1-c2 = 3.758(3) Å for compound 1a. In
compound 6, π-O interactions seem to be the governing factors and the molecules are grouped in
intercalation positions through π-π interactions between R C-OH oxygen atoms and aromatic rings
2
of the resorcinol moiety (see Figure S2). In 6 the distance between centroids [c5, C(1C)-C(2C)-
C(3C)-C(4C)-C(5C)-C(6C), c6, O(4G)] is dc5-c6 = 3.332(3) Å. Dihedral angles, , between the
planes formed by the C=O groups and aromatic rings, can be used to determine if the close contacts
between carbonyl oxygen atoms and aromatic centers correspond to lone-pair···π or π···π
interactions.[19] These angles are 3.4(2)º, close to 0º, indicating that they are π···π interactions.
The other two compounds present different crystal packing. The molecules are grouped in
chains through π-π interactions between C=O groups of resorcinol moiety and phenyl rings and π-
π interactions between rings of resorcinol groups (see Figures S3 and S4). The distances between
centroids are: for 7, dc7-c8 = 3.651(2) Å for the centroids situated in resorcinol groups [c7, C(1C)-
C(2C)-C(3C)-C(4C)-C(5C)-C(6C); c8, C(1N)-C(2N)-C(3N)-C(4N)-C(5N)-C(6N)] and dc9-c10
.265(2) Å for π-O interactions [c9, C(12A)-C(13A)-C(14A)-C(15A)-C(16A)-C(17A); c10,
O(5K)]. Dihedral angles, , between the planes formed by the C=O groups and aromatic rings, are
.4(2)º, for 8, dc11-c12 = 3.749(3) Å for the centroids situated in resorcinol groups [c11, C(1C)-
=
3
5
C(2C)-C(3C)-C(4C)-C(5C)-C(6C); c12, C(1O)-C(2O)-C(3O)-C(4O)-C(5O)-C(6O)] and dc13-c14
=
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
3
.303(3) Å for π-O interactions [c9, C(12B)-C(13B)-C(14B)-C(15B)-O(4B); c10, O(6M)].
Dihedral angles, , between the planes formed by the C=O groups and aromatic rings, 8.6(3)º,
also close next to 0. Hydrogen bonding details of compounds 1a, 6, 7 and 8 are given in Table 3.
Table 1. Bond lengths [Å] and angles [°] for the compound 1a.
Bond lengths
V(1)-O(1)
V(1)-O(2)
V(1)-O(3)
V(1)-O(4)
V(1)-N(1)
V(1)-K(1)
K(1)-O(6)#1
K(1)-O(7)
K(1)-O(8)
K(1)-O(8)#2
K(1)-O(1)
K(1)-O(2)
K(1)-O(7)#3
Bond angles
1.605(4)
1.638(3)
1.958(3)
1.908(3)
2.149(4)
3.5488(15)
2.739(3)
2.786(4)
2.800(4)
2.862(4)
2.907(4)
3.004(4)
3.062(4)
O(1)-V(1)-O(2)
O(1)-V(1)-O(4)
O(2)-V(1)-O(4)
O(1)-V(1)-O(3)
O(2)-V(1)-O(3)
O(4)-V(1)-O(3)
O(1)-V(1)-N(1)
O(2)-V(1)-N(1)
O(4)-V(1)-N(1)
O(3)-V(1)-N(1)
107.71(19)
104.41(17)
93.29(15)
101.95(17)
94.37(15)
148.78(16)
108.21(17)
143.92(18)
80.98(13)
75.10(12)
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European Journal of Inorganic Chemistry
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Table 2. Bond lengths [Å] and angles [°] for the compounds 6, 7 and 8.
Bond lengths
6
7
8
V(1)-O(1)
1.5785(19)
1.9524(17)
1.8652(17)
2.139(2)
2.3597(19)
1.7718(18)
6
1.589(3)
1.856(3)
1.934(3)
2.142(4)
2.306(3)
1.771(3)
7
1.5902(14)
1.9484(14)
1.8596(13)
2.1526(16)
2.3559(15)
1.7723(14)
8
V(1)-O(2)
V(1)-O(3)
V(1)-N(1)
V(1)-O(1S/M)
V(1)-O(2S/M)
Bond angles
O(1)-V(1)-O(2)
100.86(9)
100.09(9)
150.14(8)
94.18(10)
75.17(7)
100.29(14)
98.95(13)
151.76(12)
94.10(14)
82.76(12)
75.42(12)
100.15(14)
103.18(12)
93.51(13)
163.22(13)
175.84(15)
83.74(13)
77.27(13)
82.18(12)
82.29(13)
97.65(7)
98.92(7)
153.64(7)
98.01(7)
75.19(6)
82.30(6)
101.52(8)
95.16(7)
101.41(6)
159.24(7)
176.87(7)
81.57(6)
81.49(6)
80.85(6)
78.86(6)
O(1)-V(1)-O(3)
O(2)-V(1)-O(3)
O(1)-V(1)-N(1)
O(2)-V(1)-N(1)
O(3)-V(1)-N(1)
82.31(8)
O(1)-V(1)-O(2S/M)
O(2S/M)-V(1)-O(2)
O(2S/M)-V(1)-O(3)
O(2S/M)-V(1)-N(1)
O(1)-V(1)-O(1S/M)
101.69(10)
96.03(8)
100.29(8)
163.13(8)
175.56(9)
O(2S/M)-V(1)-O(1S/M) 81.78(8)
O(2)-V(1)-O(1S/M)
O(3)-V(1)-O(1S/M)
N(1)-V(1)-O(1S/M)
75.89(7)
81.87(8)
82.11(8)
#
1 x,y+1,z #2 -x+1,-y+1,-z+1 #3 -x+2,-y+1,-z+1
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European Journal of Inorganic Chemistry
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Table 3. Hydrogen bonds in the compounds 1a, 6, 7 and 8.
D-H...A
Compound
d(D-H)
0.82
d(H...A)
1.83
d(D...A)
2.556(5)
2.828(6)
2.916(5)
3.036(4)
2.830(3)
2.569(3)
2.762(5)
2.574(5)
2.787(2)
2.550(2)
<(DHA)
146.2
O(5)-H(5)...O(6)
O(7)-H(7O)...O(2)#1
1a
1a
0.74
2.14
156.9
O(8)-H(8OA)...N(3)#2 1a
O(8)-H(8OB)...O(4)
1a
O(1M)-H(1M)...N(3)#5 6
0.97
1.95
176.4
0.97
2.21
142.8
0.80(4)
0.92(4)
0.67(4)
0.72(5)
0.80(3)
0.74(3)
2.07(4)
1.78(4)
2.10(4)
1.90(5)
1.99(3)
1.84(3)
160(4)
143(3)
173(5)
155(5)
175(3)
161(3)
O(4)-H(4O)...O(5)
O(1S)-H(1S)...O(5)#6
O(4)-H(4O)...O(5)
6
7
7
O(1M)-H(1M)...O(6)#7 8
O(5)-H(5O)...O(6)
8
Symmetry transformations used to generate equivalent atoms:
#
#
1 -x+1,-y+1,-z+1 #2 x+1,y,z+1
6 -x+1,-y,-z #7-x+1,-y+1,-z
#3 -x+1,-y+2,-z+1 #4 x,y-1,z-1 #5 x+1/2,-y+1/2,-z+2
Figure 1. ORTEP plot for compound [K(H
2
O)(EtOH)][VO (dar-inh)] (1a). All the non-hydrogen
2
atoms are presented by their 50% probability ellipsoids.
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European Journal of Inorganic Chemistry
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Figure 2. ORTEP plot for compound 6. All the non-hydrogen atoms are presented by their 50%
probability ellipsoids.
Figure 3. ORTEP plot for compound 7. All the non-hydrogen atoms are presented by their 50%
probability ellipsoids.
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European Journal of Inorganic Chemistry
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Figure 4. ORTEP plot for compound 8. All the non-hydrogen atoms are presented by their 50%
probability ellipsoids.
Figure 5. Crystal packing of compound 1 (left). The molecules are grouped in pairs through π-π
interactions forming dimers in an antiparallel position (right). Drawings were done with SHELXL
package.
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European Journal of Inorganic Chemistry
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IR Spectral Study
The details of IR spectral data of all ligands and their corresponding dioxidovanadium(V) anions
and oxidomethoxidovanadium(V) complexes are given in Table S2. The characteristic bands in
the spectra of ligands due to ν(N–H) and ν(C=Ohydrazide) appear in the range 3097-3129 and 1658-
–
1
1
687 cm , respectively. These bands indicate a ketonic nature for the ligands in the free state.
Disappearance of these bands in the spectra of the complexes and appearance of new bands in the
–
1
region 1260-1280 cm indicates enolization followed by coordination to the metal center after
–
1
deprotonation. A strong band displayed by the ligands in the region 1561-1592 cm is likely
associated with ν(C=N) stretching and this band shifts to lower wavenumbers in the spectra of the
V
]+ complexes are
complexes, indicating coordination through the azomethine nitrogen. [V O
2
–
1
characterized by the presence two sharp peaks in the region 884-940 cm due to symmetric and
–
1
antisymmetric ν(O=V=O) stretching frequencies. A sharp band in the range 938-975 cm is
V
3+
assigned to V=O stretching frequency of the [V O] complexes. The ligand band assigned to ν(N-
–
1
N) appearing at 936-988 cm shows a bathochromic shift in the spectra of the complexes which
further reinforces the assignment of the azomethine coordination. The broad band appearing at ca.
–
1
3
086-3436 cm in the spectra of the complexes is possibly due H
2
O/MeOH associated with the
complexes as well as phenolic-OH groups, which were also present in the ligands’ spectra at ca.
–
1
3
267-3448 cm .
UV-Vis Spectroscopic Studies
UV-vis spectral data (absorption maxima and respective extinction coefficients) of ligands and
complexes are given in Table S3. Spectral studies of these ligands have been reported by us earlier
[
18]
and spectra of the complexes are presented in Figure 6. The transitions φ→φ*, π→π* and
n→π* show slight variations upon coordination to vanadium due to rearrangement and electronic
donation of the ligands. Additionally, the complexes’ spectra exhibit a new broad band of low
intensity at 400-472 nm, which is assigned to ligand to metal charge transfer transition (LMCT),
due to transfer of electron density from filled p-orbital of coordinated phenolate oxygen atoms to
0
2
3
vacant d-orbitals of d vanadium metal. The molar extinction coefficient of these bands (10 -10 )
0
further reinforces the LMCT character. All the vanadium complexes are in d state, hence no d–d
transitions are expected.
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European Journal of Inorganic Chemistry
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–
4
Figure 6. UV-Vis spectra of (a): dioxidovanadium(V) complexes 1-4 (2.30×10 M) and (b):
–
5
–5
–5
oxidomethoxidovanadium (V) complexes 5 (2.71×10 M), 6 (3.47×10 M), 7 (2.70×10 M) and 8
–
5
(
4.60×10 M) recorded in MeOH.
1
13
H and C NMR Spectroscopic Studies
V
V
Further evidence for the coordination modes of the ligands and their corresponding V O/V O
2
1
complexes was obtained from H NMR spectral studies. The relevant spectroscopic data of free
ligands and their respective complexes, recorded in DMSO-d are presented in Table 4, along with
6
the values of coupling constants for aromatic protons. Representative spectra of ligand H dar-bhz
2
1
(
III) and its complex [K(H
2
O)
2
][VO
2
(dar-bhz)] (3) are presented in Figure 7. In the H NMR
spectra of the ligands, two distinguishable singlets between δ = 12-14 ppm show the presence of
two phenolic protons in different environments. Disappearance of one of these signals confirms
the coordination of only one phenolate oxygen atom to the vanadium ion after deprotonation; the
other -OH group remains uncoordinated. The spectra of all the ligands exhibit signals in the range
δ = 11.18–11.63 ppm due to the presence of -NH protons while in the complexes these disappear,
suggesting the enolization of the CO group of the hydrazide moiety and its coordination to
vanadium. Signals due to methyl groups of coordinated methanol/methoxide appear at ca. δ = 3.1
ppm. Overall, resonances due to aromatic and methyl protons of ligands and complexes appear in
the expected regions with slight shifts in their positions.
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European Journal of Inorganic Chemistry
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1
Table 4. H NMR data (δ in ppm) of ligands and complexes
Compound[a]
dar-inh (I)
-OH
-NH
Aromatic Protons
Methyl Protons
H
2
14.34 (s, 1H),
11.63(s,
1H)
8.79 (d, J = 5.2 Hz, 2H), 8.11 (s, 1H), 7.84
(d, J = 5.7 Hz, 2H), 6.37 (s, 1H),
2.65 (s, 3H), 2.54
(s, 3H)
12.62 (s, 1H)
[
K(H
2
O)
2
][VO
2
(dar-inh)] (1)
(dar-nah)] (2)
(dar-bhz)] (3)
(dar-fah)] (4)
12.50 (s, 1H)
-
8.69 (d, J = 2.2 Hz, 2H), 8.33 (s, 1H), 7.90
2.08 (s, 3H), 2.67
(s, 3H)
(
d, J = 5.3 Hz, 2H), 6.18 (s, 1H)
H
2
dar-nah (II)
12.62 (br, 1H), 11.59(s, 9.08 (s, 1H), 8.7 (s, 1H), 8.27 (s, 1H), 8.16 (s, 2.66 (s, 3H), 2.56
14.25 (br, 1H)
1H)
1H), 7.57 (s, 1H), 6.38 (s, 1H)
(s, 3H)
[K(H
2
O)
2
][VO
2
12.50 (s, 1H)
-
9.17 (s, 1H), 8.67 (s, 1H), 8.31 (s, 2H), 7.50
2.66 (s, 3H), 2.93
(s, 3H)
(
d, J = 12.0 Hz, 1H), 6.17 (s, 1H)
H
2
dar-bhz (III)
12.65 (br, 1H), 11.41(s, 8.12 (s, 1H), 7.94 (d, J=6.7, 2H) , 7.62 (t,
2.66 (s, 3H), 2.55
(s, 3H)
14.50 (br, 1H)
1H)
-
J=6.7, 1H), 7.55 (t, J=7.2, 2H), 6.37 (s, 1H)
[K(H
2
O)
2
][VO
2
12.50 (s, 1H)
8.30 (s, 1H), 6.16 (s, 1H), 8.03 (d, J=5.9, 2H), 2.93 (s, 3H), 2.66
7.45 (m, 3H)
(s, 3H)
H
2
dar-fah (IV)
12.64 (br, 1H), 11.18(s, 8.11 (s, 1H), 7.80 (s, 1H), 7 .42 (s, 1H), 6.74
2.66 (s, 3H), 2.52
(s, 3H)
14.31 (br, 1H)
1H)
-
(s, 1H), 6.36 (s, 1H)
[K(H
2
O)
2
][VO
2
12.51 (s, 1H)
6.07 (s, 1H), 8.12 (s, 1H), 7.80 (s, 1H),
2.86 (s, 3H), 2.54
(
s, 3H)
6.62 (s, 1H), 6.97 (d, J = 3.1 Hz, 1H), 6.61 (d,
J = 1.5 Hz, 1H)
2
H dhap-inh (V)
13.36 (br, 1H), 11.40(s,
8.77 (d, J = 4.6 Hz, 2H), 7.81 (d, J = 5.7 Hz,
2H), 7.47 (d, J = 8.7 Hz, 1H), 6.35 (d, J = 8.7
Hz, 1H), 6.29 (s, 1H),
2.39 (s, 1H)
9.98 (s, 1H)
1H
[VO(OMe)(MeOH)(dar-inh)] (5) 12.52 (s, 1H)
-
6.26 (s, 1H), 8.38 (s, 1H), 8.86 (d, J = 5.2 Hz, 2.69 (s, 3H), 2.96
2H), 8.22 (d, J = 5.2 Hz, 2H),
(s, 3H)
[VO(OMe)(MeOH)(dar-nah)] (6) 12.53 (s, 1H)
-
6.18 (s, 1H), 9.18 (s, 1H), 8.67 (s, 1H), 8.32
s, 2H), 7.50 (dd, J = 7.0, 4.9 Hz, 1H)
2.67 (s, 3H), 2.93
(s, 3H)
(
[
VO(OMe)(MeOH)(dar-bhz)] (7) 12.58 (br, 1H)
-
-
8.37 (s, 1H), 6.39 (s, 1H), 8.06 (d, J = 7.9 Hz, 2.67 (s,3H), 2.9
2H), 7.52 (m, 3H)
0(s, 3H)
[VO(OMe)(MeOH)(dar-fah)] (8) 12.58 (br, 1H)
8.35 (s, 1H), 6.38 (s, 1H), 7.90 (s, 1H), 7.13
d, J = 3.4 Hz, 1H), 6.68 (d, J = 3.3 Hz, 1H),
2.84 (s, 3H), 2.70
(s, 3H)
(
[
[
[
a] Signals due to the methoxy protons in the spectra of the oxidovanadium (V) complexes:
VO(OMe)(MeOH)(dar-inh)] (5) at 3.17 ppm, [VO(OMe)(MeOH)(dar-nah)] (6) at 3.12 ppm,
VO(OMe)(MeOH)(dar-bhz)] (7) 3.21 ppm and [VO(OMe)(MeOH)(dar-fah)] (8) at 3.20 ppm
were identified.
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European Journal of Inorganic Chemistry
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1
Figure 7. H NMR spectra of ligand III and complex 3 recorded in DMSO-d
6
. * indicates the
proton impurity signal present in DMSO-d at δ = 2.5 ppm and that of moisture at δ = 3.25 ppm.
6
1
3
The coordination modes of the ligands were further endorsed by C NMR studies. Table
1
3
5
reveals the complete details of C NMR chemical shift values of each carbon atom of ligands
and respective complexes. Figure 8 depicts representative spectra of ligand H dar-fah (IV) and
complex [VO(OMe)(MeOH)(dar-fah)] (8). A significant chemical shift variation, Δδ =
δ(complex) − δ(ligand)], was noticed in the signals of the carbon atoms which are present in the
vicinity of the coordinating atoms O, N and O [i.e. phenolic oxygen (C3), azomethine nitrogen
2
[
(
C1) and enolic oxygen atom (C11)] assuring the coordination of Ophenolate, Nazomethine and Oenolate
to the vanadium.[20] Signals due to other carbon atoms of phenyl ring and methyl groups were
observed in the expected δ values for the ligands as well as complexes, with slight variations,
possibly due to electronic rearrangements. In addition, a new signal was observed in the spectra of
complexes 5-8 at ca. δ = 49 ppm, due to coordinated methoxy carbon atoms.
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European Journal of Inorganic Chemistry
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1
3
Table 5. C NMR spectroscopic data ( in ppm) of ligands and complexes
Compounds
dar-inh (I)
C9
C1
C3
C11
Other carbons
C8
C10
H
2
203.56 166.43 164.43 163.32 164.98 (C5), 150.91 (C14/14’), 141.19 14.81 27.36
C12), 134.55 (C7), 122.58 (C13/13’),
13.86 (C6), 113.09 (C2), 104.44 (C4)
(
1
2 2 2
[K(H O) ][VO (dar-inh)] (1) 203.55 166.99 165.44 164.53 165.44 (C5), 150.47 (C14), 142.75 (C12), 16.26 27.30
1
1
36.35 (C7), 122.35 (C13), 114.96 (C6),
12.77 (C2), 105.78 (C4)
(
Δδ)
(
0.56)
(1.01)
(1.21)
2
H dar-nah (II)
203.01 165.99 164.89 163.96 164.24 (C5), 158.32 (C16), 152.46 (C15), 12.58 26.24
1
1
49.0 (C13), 137.99 (C7), 12.89 (C12),
128.75 (C14), 123.59 (C6), 113.27 (C2),
12.64 (C4)
[
K(H
2
O)
2 2
][VO (dar-nah)]
202.86 171.28 166.92 165.27 163.48 (C5), 151.54 (C16), 149.40 (C15), 16.13 27.65
136.34 (C13), 135.75 (C7), 129.26 (C12),
(
2)
1
1
24.06 (C14), 116.65 (C6), 113.18 (C2),
05.90 (C4)
(
Δδ)
(
5.29)
(2.03)
(1.31)
2
H dar-bhz (III)
203.18 165.98 164.32 162.87 164.45 (C5), 133.52 (C12), 132.82 (C15), 13.91 27.47
28.49 (C7), 128.19 (C14/C16), 118.71,
C13/C17), 113.20 (C6), 112.80 (C2),
03.83 (C4)
1
(
1
[
K(H
2
O)
2 2
][VO (dar-bhz)]
203.00 171.19 168.47 165.14 164.84 (C5), 136.14 (C12), 133.64 (C15), 16.22 27.42
131.02 (C7), 128.70 (C14/C16), 128.25
(
3)
(
(
C13/C17), 116.82 (C6), 113.02(C2), 105.97
C4)
(
Δδ)
(
5.21)
(4.15)
(2.27)
2
H dar-fah (IV)
203.15 164.96 163.46 163.03 164.72 (C5), 155.65 (C12), 145.36 (C15), 13.88 27.81
1
1
40.39 (C7), 137.13 (C6), 133.39 (C13),
23.08 (C14), 113.65 (C2), 112.36 (C4)
[
K(H
2 2 2
O) ][VO (dar-fah)] (4) 203.44 168.28 163.20 161.61 148.32 (C5), 145.66 (C12), 146.17 (C15), 16.69 27.25
1
1
36.20 (C7), 133.49 (C6), 115.86 (C13),
13.82 (C14), 112.39 (C2), 107.59(C4)
(
Δδ)
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European Journal of Inorganic Chemistry
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(
3.32)
(-0.26) (-1.42)
2
H dhap-inh (V)
-
163.15 161.13 161.36 160.98 (C5), 150.65 (C14), 140.34 (C12), 14.56
-
1
1
30.78 (C7), 122.02 (C13), 112.10 (C2),
07.75 (C6), 103.77 (C4)
[
VO(OMe)(MeOH)(dar-
202.13 166.82 164.52 165.35 150.51 (C5), 136.66 (C14), 132.47 (C12), 16.28 27.27
136.66 (C7), 122.22 (C13), 116.88 (C6),
inh)] (5)
(
-1.4)
(0.39)
(0.09)
(2.03)
113.50 (C2), 105.84 (C4)
(
Δδ)
[
VO(OMe)(MeOH)(dar-
203.25 169.89 166.91 165.25 163.58 (C5), 151.51 (C16), 149.20 (C15), 16.69 27.72
136.34 (C13), 135.66 (C7), 124.08 (C12),
nah)] (6)
(
3.9)
(2.02)
(1.29)
1
1
16.98 (C14), 114.78 (C6), 113.20 (C2),
06.02 (C4)
(
Δδ)
[
VO(OMe)(MeOH)(dar-
202.45 170.92 167.73 164.84 161.63(C5), 136.87 (C12), 136.14 (C15), 17.84 28.45
133.64 (C7), 130.70 (C14/C16), 128.25
bhz)] (7)
(
4.94)
(3.41)
(1.97)
(
C13/C17), 116.00 (C6), 113.02 (C2),
06.10 (C4)
(
Δδ)
1
[
VO(OMe)(MeOH)(dar-
203.31 169.33 165.85 162.24 159.61 (C5), 146.42 (C12), 146.18 (C15), 17.07 27.83
136.13 (C7) 116.85 (C6), 115.38 (C13),
fah)] (8)
114.99 (C14), 112.67 (C2), 103.61 (C4)
(
Δδ)
(
4.37)
(2.39)
(-0.79)
1
3
Figure 8. C NMR spectra of H
fah)] (8) recoded in [DMSO]-d6.
2
dar(fah)(IV) and its vanadium complex [VO(OMe)(MeOH)(dar-
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European Journal of Inorganic Chemistry
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51V NMR Spectroscopic Study
5
1
51
Solutions for V NMR were prepared in MeOH and DMSO. Figure 9 shows the V NMR spectra
measured for all complexes and Table 6 collects spectral bands. All complexes, except 4 in MeOH
and 8 in DMSO, show the presence of only one peak with δ values between –533.7 and –547.8
V
ppm. In MeOH complex 4 shows two overlapped peaks at –546.5 and –547.75 ppm, while 8 in
DMSO shows two peaks with ratios of 1:4 at –534.0 and –547.3 ppm, respectively. These chemical
shifts are upfield from the ones found for the 4-benzoyl-3-methyl-1-phenyl-2-pyrazoline-5-one
vanadium complexes,[11] implying that the electronic density in the metal center is lower in this
case, which is in agreement with the involvement of the phenolate donor.
Regarding the different substituents, in MeOH the order is from downfield to upfield: bhz
<
nah = inh < fah. The compounds with the benzene substituent appear more downfield and the
ones containing the furane ring upfield, as expected due to electronic effects. However, in DMSO
this does not happen and even complexes 5 and 6 have rather different chemical shifts (as well as
the VO complexes derived from nah and inh, 1 and 2).
2
5
1
Table 6. Chemical shift of the peaks found in the V NMR spectra of vanadium complexes in
MeOH and DMSO containing 10% of deuterated solvent.
Compound
MeOH
–543.8
DMSO
–533.7
[
[
[
[
[
[
[
[
K(H
K(H
K(H
K(H
2
2
2
2
O)
O)
O)
O)
2
2
2
2
][VO
][VO
][VO
][VO
2
2
2
2
(dar-inh)] (1)
(dar-nah)] (2)
(dar-bhz)] (3)
(dar-fah)] (4)
–544.1
–536.6
–536.8
–536.6
–546.5, –547.8
–544.1
–538.0
VO(OMe)(MeOH)(dar-inh)] (5)
VO(OMe)(MeOH)(dar-nah)] (6)
VO(OMe)(MeOH)(dar-bhz)] (7)
VO(OMe)(MeOH)(dar-fah)] (8 )
–536.7
–544.4
–544.6
–540.9
–545.1
–545.9
–534.0, –547.3
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European Journal of Inorganic Chemistry
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Figure 9. 51V NMR spectra measured for solutions of the complexes dissolved in (a) MeOH
containing 10% MeOD and b) DMSO containing 10% DMSO-d .
6
Electrochemical Study
The electrochemical responses of all the ligands and complexes have been studied by measuring
cyclic voltammograms (CV) in dry DMF, in order to investigate the redox stability of the
complexes. Cyclic voltammograms of redox active ligands (I-IV) as well as complexes (1–8) were
recorded in the range of 2.0 V to -2.0 V vs. SCE at room temperature and the CV results are given
in Tables S4 and S5. As a representative example, the cyclic voltammograms of ligand H
2
dar-bhz
(
III) and its complex [K(H O) ][VO (dar-bhz)] (3) are shown in Figure 10 (for others see Figure
2
2
2
S5). All the ligands exhibit one peak at ca. +1.0 V, which has been attributed to hydrazine-based
ligand oxidation.[21] Furthermore, in the cyclic voltammograms of all the complexes, a reductive
response at ca. –0.8 V could be related to V(V) →V(IV) reduction.[22] This reductive response
observed in the spectra of all the complexes is irreversible in nature. No further reduction responses
were observed in any voltammogram of the complexes. Additionally, one oxidation peak in the
potential window 1.09 –1.24 V is also observed in all the complexes which could be attributed to
V(IV) →V(V) oxidation.[23] All vanadium complexes (1-8) showed identical redox processes,
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
which confirm similar structures and electronic features of the synthesized complexes. All these
redox processes were also supported by DPV (see Tables S4 and S5).
Figure 10. Comparative study of cyclic voltammograms and differential pulse voltammetry for
H
2
dar-bhz (III) and [K(H
2
O)
2
][VO (dar-bhz)] (3) at a scan rate of 100 mV/s recorded in absolute
2
dry DMF in the range 2.0 V to –2.0 V vs. Ag/AgCl at 298 K with 0.1 M TBAPF6 as a supporting
electrolyte.
Catalytic Studies
Oxidative Bromination of Thymol: The oxidative bromination of thymol was carried out in the
presence of KBr, 70 % aqueous HClO
4
, and 30 % aqueous H
2
2
O in aqueous medium. The catalytic
reaction led to the formation of three products, namely, 2-bromothymol, 4-bromothymol and 2,4-
dibromothymol.[24] Among these products 2,4-dibromothymol is formed due to further
bromination of monobrominated products. The reaction takes place on the most activated site as a
result of electrophilic aromatic substitution in the phenolic ring.
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European Journal of Inorganic Chemistry
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Thymol
2-Brth
4-Brth
2,4-dBrth
Scheme 3. Products of the oxidative bromination of thymol (2-Brth = 2- bromothymol, 4-Brth =
4
-bromothymol, 2,4-dBrth = 2,4-dibromothymol).
Further, to obtain the maximum yield of brominated products, several parameters were
changed, such as the amount of catalyst, KBr, H
2
O
2
and HClO using 3 as a representative catalyst.
4
The conversion of thymol obtained in the different assays and the selectivity for the different
brominated products, under particular conditions, are summarized in Table 7 and Figure S6
presents time-conversion plots obtained different conditions. It is clear from the data shown in
Table 7 that the conversion and the selectivity for the products differ on varying the reagents. Thus,
reaction conditions presented in entry 5 are suitable for obtaining the highest selectivity for 4-
bromothymol (ca. 90%) while conditions in entry 9 are suitable for the highest selectivity of 2,4-
dibromothymol (ca. 84%). However, the best reaction conditions for the maximum oxidative
conversion (98 % conversion) of thymol with minimum amount of different reagents are: catalyst
–
3)
(
0.001 g, 2.1×10 mmol), H
2
O
2
(1.1 g, 10 mmol), KBr (1.2 g, 10 mol), and HClO (1.2 g, 10 mol)
4
in 20 mL water (Entry 1 of Table 7). Under these conditions, the selectivity observed follows the
order 2,4-dibromothymol (49%) > 4-bromothymol (46%) > 2-bromothymol (5%).
The other three catalysts, i.e. complexes 1, 2 and 4 were also tested under the above reaction
conditions for the maximum conversion of thymol in 2 h of reaction time and the results are
summarized in Table 8. The catalytic potential of these complexes compares well with results
V
obtained with other metal complexes,
such as: [V O(acac)(6,6’-(2-(pyridin-
yl)ethylazanediyl)bis(methylene)-bis(2,4-di-tertbutylphenol)],[5e]
hydroxyphenyl)-1-phenyl-1,2,4-triazole)][5b]
and [Mo O
Cs(H
O)[V O
V
(3,5-bis(2-
2
2
2
VI
2
(8-formyl-7-hydroxy-4-
methylcoumarin-bhz)(MeOH)][25] which gave 99% conversion after 2h of reaction time.
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European Journal of Inorganic Chemistry
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Table 7. Conversion of thymol (for 1.5 g, 0.010 mol), TOF and product selectivity using complex
as a catalyst precursor in 2 h of reaction time under different reaction conditions
3
Entry
No.
Catalyst
[g (mmol)]
H
2
O
2
KBr
HClO
4
Conv TOF
Selectivity [%]b
a
[g (mmol)] [g (mmol)] [g (mmol)]
[%]
-1
[h ]
2-Brth
4-Brth
2,4-dBrth
–
3
1
2
3
4
5
6
7
8
9
0.001 (2.1×10 )
1.10 (10)
1.10 (10)
1.10 (10)
1.70 (15)
2.27 (20)
1.10 (10)
1.10 (10)
1.10 (10)
1.10 (10)
1.10 (10)
1.10 (10)
1.2 (10)
1.2 (10)
1.2 (10)
1.2 (10)
1.2 (10)
1.7 (15)
2.3 (20)
1.2 (10)
1.2 (10)
1.2 (10)
1.2 (10)
1.4 (10)
1.4 (10)
1.4 (10)
1.4 (10)
1.4 (10)
1.4 (10)
1.4 (10)
2.1 (15)
2.8 (20)
1.4 (10)
1.4 (10)
98
99
99
98
99
98
99
99
99
98
32
2289
154
5
6
7
3
2
6
7
8
7
5
8
46
46
43
91
92
38
30
16
9
49
48
50
6
–
2
0.015 (3.2×10 )
–
3
0.002 (4.8×10 )
1156
2289
2313
2289
2313
2313
2313
2289
……
–
3
0.001 (2.1×10 )
–
3
0.001 (2.1×10 )
6
–
3
0.001 (2.1×10 )
56
63
76
84
49
24
–
3
0.001 (2.1×10 )
–
3
0.001 (2.1×10 )
–
3
0.001 (2.1×10 )
–
3
1
1
0
1
0.001 (2.1×10 )
------
46
68
a) TOF values calculated at 2 h of reaction time.
The other three catalysts, i.e. complexes 1, 2 and 4 were also tested under the above reaction
conditions for the maximum conversion of thymol in 2 h of reaction time and the results are
summarized in Table 8. The catalytic potential of these complexes compares well with results
V
obtained with other metal complexes,
such as: [V O(acac)(6,6’-(2-(pyridin-
yl)ethylazanediyl)bis(methylene)-bis(2,4-di-tertbutylphenol)],[5e]
hydroxyphenyl)-1-phenyl-1,2,4-triazole)][5b]
and [Mo O
Cs(H
O)[V O
V
(3,5-bis(2-
2
2
2
VI
2
(8-formyl-7-hydroxy-4-
methylcoumarin-bhz)(MeOH)][25] which gave 99% conversion after 2h of reaction time.
Table 8. Conversion of thymol (1.5 g, 0.010 mol), TOF and selectivity of different products
using different catalysts in 2 h of reaction time under optimized reaction conditions.
-
1 a
Entry
Complex (g)
Conv. [%]
TOF [h ]
Selectivity [%]
2
-Brth
4-Brth
2,4-dBrth
1
2
3
4
.
.
.
.
1 (0.001)
2 (0.001)
3 (0.001)
4 (0.001)
98
90
98
95
2300
2112
2289
2188
5
46
49
14
9
68
30
40
18
61
42
18
aTOF values calculated at 2 h of reaction time.
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
Oxidation of Alkenes
Our previous research has shown that vanadium complexes of hydrazone ligands are good
homogeneous catalysts for the oxidation of olefins, hence, we continued our studies with the
complexes synthesized here and examined their ability to catalyze the epoxidation of various
alkenes, namely styrene, cyclohexene, cis-cyclooctene, 1-hexene, 1-octene, cyclohexanone and
trans-stilbene as model substrates. The oxidation of cis-cyclooctene, as a representative substrate
of olefins, was carried out in the presence of [K(H
2
O)
2
][VO (dar-inh)] (1) as representative
2
homogeneous catalyst to find the optimal conditions for the epoxidation of cis-cyclooctene.
To achieve suitable reaction conditions various parameters were screened, taking 5 mmol
(
0.55 g) of cis-cyclooctene and 6 h of reaction time: the amount of catalyst, i.e. complex 1; amount
of oxidant, i.e. aqueous 30% H , amount of NaHCO ; nature of solvent, the solvent volume; and
2
O
2
3
the temperature. Table 9 summarizes all the conditions and conversions obtained and Figure S7
presents conversion-time curves. The optimal reaction conditions for the epoxidation of 5 mmol
of cis-cyclooctene are (entry 5): catalyst (0.0005 g), 30% H
2
O
2
(1.70 g, 15 mmol), NaHCO (2
3
mmol), MeCN (7 mL) and 80 ºC of reaction temperature. Under these conditions, the reaction
requires 6 h to give a maximum of 85% conversion with 100% selectivity for cyclooctene oxide.
Other solvents were tested under the optimised reaction conditions, and in general gave lower
conversions, when compared to MeCN, following the order: EtOH (78%) > MeOH (56%) > CH
29%) > CH Cl (20%) > n-hexane (12%) > toluene (10%). Low solubility of the complexes in
non-polar solvents is possibly the reazon for the poor results in this type of solvents. We have also
tested K CO and NaOH as promoters, instead of NaHCO , but both of them did not perform well
39% conversion using K CO and only 2% using NaOH). The blank reaction (entry 12) under the
3
Cl
(
2
2
2
3
3
(
2
3
optimized conditions gave only 30% conversion.
Various other alkenes were screened, namely styrene, 1-octene, 1-hexene, trans-stilbene,
cyclohexene and 2-cyclohexenone in the above reaction conditions after 6 and 24 h of reaction
time using all catalysts and the results are summarized in Table 10; conversion profiles for cis-
cyclooctene as a representative substrate, with all four catalysts are presented in Figure 11.
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
Table 9. Conversion of cis-cyclooctene (0.55 g, 0.005 mol) using [K(H
2
O)
2
][VO
2
(dar-inh)] (1) as
catalyst precursor under different reaction conditions in the presence of NaHCO
time.
3
in 6 h of reaction
Entry Catalyst (g)
No.
H
2
O
2
NaHCO
g (mmol)
0.168 (2)
3
Temp.
[ C]
MeCN
[mL]
7
Conv.
[%]
85
Select.
%
100
o
g (mmol)
2.27 (20)
–
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
0.0005(1.06 ×10 )
80
80
80
80
80
80
80
60
70
80
80
80
80
80
–
3
0.001 (2.13 × 10 )
2.27 (20)
2.27 (20)
1.10 (10)
1.70 (15)
1.70 (15)
1.70 (15)
1.70 (15)
1.70 (15)
1.70 (15)
1.70 (15)
1.70 (15)
1.70 (15)
1.70 (15)
0.168 (2)
0.168 (2)
0.168 (2)
0.168 (2)
0.084 (1)
0.252 (3)
0.168 (2)
0.168 (2)
0.168 (2)
0.168 (2)
0.168 (2)
------
7
7
7
7
7
7
7
7
5
9
7
7
7
86
100
100
100
100
100
100
100
100
100
100
100
–
3
0.002 (4.26 × 10 )
87
–
3
0.005 (1.06 × 10 )
72
–
3
0.005 (1.06 × 10 )
85
–
3
0.005 (1.06 × 10 )
73
–
3
0.005 (1.06 × 10 )
86
–
3
0.005 (1.06 × 10 )
60
–
3
0.005 (1.06 × 10 )
75
–
3
0
1
3
4
5
0.005 (1.06 × 10 )
58
–
3
0.005 (1.06 × 10 )
71
------
------
30
------
------
–
3
0.005 (1.06 × 10 )
------
Figure 11 Plots representing conversion of cis-cyclooctene in the presence of different vanadium
complexes as catalysts.
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
In general, after 6 h the conversion of aromatic olefins is above 80% and reaches ca. 99%
in 24 h. Conversion of aliphatic olefins (1-octene and 1-hexene) is lower with all these catalysts
and 1-hexene shows better results than 1-octene (65-73% vs. 32-52%) in 6 h; and after 24 h the
conversion reaches 97-99%. In general, the epoxide is obtained selectively, as no additional signals
are observed in the GC for any other product, except in the oxidation of styrene and cyclohexene
that form two reaction products, with the major product being the epoxide (see Table 10). Blank
reactions (i.e. in the presence of H
2
O
2
and NaHCO
3
but without catalyst) with all alkenes provided
not only enhanced the
only 10-32% conversion. Using NaHCO
3
as promoter along with H
2
O
2
catalytic efficiency but the catalyst also became selective for the epoxidation product. It has been
−
observed earlier that NaHCO
3
in the presence of H
2
O
2
instantly produces HCO
4
(
peroxymonocarbonate) which is the key compound responsible for the fast formation of an
oxidoperoxidovanadium(V) intermediate.[25] Since the peroxido species is actually responsible for
the oxidation of the substrates, the role of the promoter is very important. Thus, these studies
indicate that the complexes have very good catalytic potential and that the promoter NaHCO
a very important role in enhancing the catalytic activity of these complexes.
3
plays
Table 10. Oxidation of olefins (5 mmol) by different vanadium catalyst (0.0005 g) and 30%
aqueous H
2
O
2
(15 mmol) as a terminal oxidant and NaHCO
3
(2 mmol) as promoter under the
optimized reaction conditions (see text).
[a]
Entry Substrate
Cataly Product(s)
st
Conv.
[6h/24h]
%
TOF
Selectivity
[6h/24h]
%
1
.
1
85/99
668
628
652
668
100/100
2
80/99
83/99
85/99
30
100/100
100/100
100/100
100
3
4
Blank
[
b]
2
.
1
2
3
4
91/99
80/99
93/99
92/99
715
628
730
723
100/76
[b]
1
00/78
[b]
1
00/92
+
[b]
100/95
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
Blank
32
100
[
c]
3
.
1
2
3
4
90/99
85/98
94/99
91/99
707
668
738
715
100/81
[c]
1
1
1
00/80
00/89
00/85
+
[c]
[c]
Blank
28
1
00
4
5
.
.
1
95/99
94/99
97/99
96/99
30
746
738
762
754
100/100
100/100
100/100
100/100
100
2
3
4
Blank
1
86/98
80/98
82/97
83/98
32
675
628
644
652
100/100
100/100
100/100
100/100
100
2
3
4
Blank
6
7
.
.
1
73/99
65/99
66/99
66/99
18
573
510
518
518
100/100
100/100
100/100
100/100
100
2
3
4
Blank
1
52/97
44/97
32/98
42/98
10
408
345
251
330
100/100
100/100
100/100
100/100
100
2
3
4
Blank
1
[
[
[
a] Calcd at 6 h as [h− ].
b] The selectivity of epoxide after 24 h. Rest is phenyl acetaldehyde formed.
c] The selectivity of epoxide after 24 h. Rest is 2-cyclohexenol.
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
The catalytic potential of the vanadium complexes studied here, particularly for the
epoxidation of cis-cyclooctene using H in the presence of NaHCO (80-85% conversion in 6h
2
O
2
3
of reaction time) compareswell with the catalytic potential of other vanadium complexes:
V
[
V O
2
{(E)-(E)-2-(2-(phenyl(pyridin-2-yl)methylene)hydrazinyl)benzo[d]thiazol}](96%
time),[7f]
[V O
V
{(E)-2-(2-(1-(pyridin-2-
L= isonicotinic acid(2-
conversion
in
5hof
reaction
2
yl)ethylidene)hydrazinyl)benzo[d] thiazole}] (97%, 5h),[7g] [V O
V
(L)][H
2
2
hydroxy-benzylidene)-hydrazide)] (80%, 5h),[26]PS-[V O(2-(2-aminoethoxy)ethanol)] [PS
IV
chloromethylated polystyrene] (83%, 6h),[27][V O(2-{(E)-[2-chloroethyl)imino]methyl}-2-
IV
=
naphthol)] (80%, 2h)[7h], [VO(Me
NCH
CH
N(CH
-2-O-3,5-C
H
2
(tBu
) (98%, 6h),[7i]
2
2
2
2
6
2 2
)
IV
[
V O(E)-N’-(phenyl(pyridin-2-yl)methylene)isonicotinhydrazide)Cl ]
2
(83%,
h),[28]
2[V OL(H
IV
O)]·CH
OH[HL=6,6'-((1E,1’E)-((2-hydroxypropane-1,3-
5
2
3
diyl)bis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol](90%, 4h)[29] and [V O(N-
IV
(
5-bromosalicylidene)-4,5-dinitro-1,2-phenylenediamine)] (64% 6 h).[30]
Overall our results also compare well with the ones obtained with a few Mo-catalysts which
have shown the following values: [MoO
2
Cl
(1,4-bis(2-hydroxy-benzyl)-(S,S)-2,2’-bipyrrolidine)](95%,
6h/24 ((S)-4-(1-Phenylpropyl)-1,2,4-
h),[33] [Mo
[Mo (4,4′-di-tert-butyl-2,2′-bipyridine) (93/100%,
]·H (trz
1,2,4-triazole)(92/99%, 6h/24h),[36]
] (99/100% 2h/24h),[37] and [Mo (57/82 %, 6/24 h).[38]
(HpypzA)]
2
(N,N-dimethylbenzamide)
2
](99/100% conversion at
6
1
h/24h of reaction time),[31] [MoO
h),[32][MoO
Cl (pbim)] (82/98%,
](98/100%
,6h/24h),[34]
h/24h),[35] (Htrz)
[Mo (O (trz)
ptk(inh)
2
2
2
2
O
6
triazole)
2
2
O
6
2
]
6
2
3
O
6
2
)
4
2
2
O
=
VI
[
{Mo O
2
(H
2
O)}
3
3
2
O
6
n
Reactivity of Vanadium(V) Complexes with H
It has been observed that the performance of aqueous H
epoxidation of olefins is moderate and various oxidation products form.[39] Performing these
reactions in the presence of NaHCO , not only improves the performance of the catalysts but the
2
O
2
2
2
O as terminal oxidant in the catalytic
3
reaction also becomes selective towards the epoxide product.[4] To clarify this and obtain
information on the intermediate species formed during the catalytic reaction, the reactivity of
[
K(H
2
O)
2
2
][VO (dar-bhz)](3) with H
2
O
2
in the absence and presence of NaHCO
3
was evaluated.
–5
Thus, the dropwise addition of H
2
O
2
(2.5 M) to a solution of 3 (6.1× 10 M) in MeOH resulted in
a decrease in the intensity and a red shift of the 378 nm band to 385 nm, along with the development
of an isosbestic point at 352 nm (Figure 12). Simultaneously, the intense 280 nm band changes to
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
a shoulder along with a slight increase in intensity. A similar behaviour was observed for
complexes 2 and 4 (Figures S8 and S9) and clear isosbestic points were noted for each complex
upon reaction with H
2
2
O . In the case of complex 1(Figure S10), the isosbestic points are not as
clear but the changes are stronger. The development of an isosbestic point suggests the formation
of a new species, possibly the oxidoperoxidovanadium(V) one, upon reaction of complexes with
H
2
2
O .
Figure 12. Spectral changes observed during the reactivity of [K(H
2
O)
solution to 15 mL of 6.1×
0 M solution of 3 in MeOH. Inset shows the expanded view of the 300-500 nm region.
2
][VO (dar-bhz)] (3) with
2
H
2
O2.The spectra were recorded upon stepwise addition of 2.5 M H
2
O
2
–
5
1
V
2+
We have also checked the reactivity of two [V O(OMe)] type complexes,7 and 8, (Figure
S11) with H . Thus, the progressive addition of H to a solution of 7 in MeOH (10 mL)
2
O
2
2
O
2
resulted in a strong decrease in intensity of the LMCT band at 380 nm (Figure 13) along with the
simultaneous formation of an isosbestic point at 348 nm. Two new bands of medium intensity also
start developing at 332 and 311 nm. The band at 271 nm only gains intensity while the 210 nm
band shifts to 224 nm with the intensity gain. Similar trends have also been observed with complex
8
upon its reaction with H
different from those observed for anionic complexes 1-4. The reaction of [V O(OMe)] type
complexes with H may involve the formation of dioxidovanadium(V) species as an
2
O
2
. The change in spectral patterns of these complexes are somewhat
V
2+
2
O
2
intermediate before converting into oxidoperoxidovanadium(V) species.
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European Journal of Inorganic Chemistry
10.1002/ejic.201801243
Figure 13. Spectral changes observed during the reactivity of [VO(OMe)(MeOH)(dar-bhz)] (7)
with H
2
O
2
. (a) The spectra were recorded upon stepwise addition of one drop portions of 1.82×
–
1
–5
1
0 M H
2
O
2
solution to 10 mL of 2.26× 10 M solution of 7 in MeOH.
To understand the combined effect of NaHCO
mixture of NaHCO and H dissolved in MeOH was added dropwise to [K(H
bhz)] (3) and [VO(OMe)(MeOH)(dar-bhz)] (7)solutions. Surprisingly, after adding only a few
drops of the mixture of NaHCO and H , very similar and immediate spectral changes could be
obtained (Figures 14andS12) as the ones observed by adding higher amounts of H alone to the
above complexes, hinting the quick formation of a similar oxidoperoxido species (Scheme 4) in
solution. It has been reported in the literature that NaHCO reacts with H instantly producing
HCO − (peroxymonocarbonate).[39]
3
and H
2
O
2
during the catalytic reaction, a
3
2
O
2
2
O) ][VO (dar-
2
2
3
2
O
2
2
O
2
3
2
O
2
4
Scheme 4. Proposed mechanism for the formation of oxidoperoxidovanadium(V) complex by
reaction of complex with H in the presence of NaHCO
2
O
2
3
.
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