Peroxidative bromination and oxygenation of organic compounds: Synthesis, X-ray crystal structure and catalytic implications of mononuclear and binuclear oxovanadium(V) complexes containing Schiffbase ligands

Article abstract of DOI:10.1016/j.poly.2011.06.005

Treatment of of (R,R)-N,N-salicylidene cyclohexane 1,2-diamine(H ₂L¹) in methanol with aqueous NH₄VO₃ solution in perchloric acid medium affords the mononuclear oxovanadium(V) complex [VOL¹(MeOH)]·ClO₄ (1) as deep blue solid while the treatment of same solution of (R,R)-N,N-salicylidene cyclohexane 1,2-diamine(H₂L¹) with aqueous solution of VOSO ₄ leads to the formation of di-(μ-oxo) bridged vanadium(V) complex [VO₂L²]₂ (2) as green solid where HL ² = (R,R)-N-salicylidene cyclohexane 1,2-diamine. The ligand HL ² is generated in situ by the hydrolysis of one of the imine bonds of HL¹ ligand during the course of formation of complex [VO ₂L²]₂ (2). Both the compounds have been characterized by single crystal X-ray diffraction as well as spectroscopic methods. Compounds 1 and 2 are to act as catalyst for the catalytic bromide oxidation and C-H bond oxidation in presence of hydrogen peroxide. The representative substrates 2,4-dimethoxy benzoic acid and para-hydroxy benzoic acids are brominated in presence of H₂O₂ and KBr in acid medium using the above compounds as catalyst. The complexes are also used as catalyst for C-H bond activation of the representative hydrocarbons toluene, ethylbenzene and cyclohexane where hydrogen peroxide acts as terminal oxidant. The yield percentage and turnover number are also quite good for the above catalytic reaction. The oxidized products of hydrocarbons have been characterized by GC Analysis while the brominated products have been characterized by ¹H NMR spectroscopic studies.

Full text of DOI:10.1016/j.poly.2011.06.005

Polyhedron 30 (2011) 2286–2293
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Peroxidative bromination and oxygenation of organic compounds: Synthesis,
X-ray crystal structure and catalytic implications of mononuclear
and binuclear oxovanadium(V) complexes containing Schiffbase ligands
b
a,
Tapan Kumar Si a, , Michael G.B. Drew , Kalyan Kumar Mukherjea
⇑
⇑
a Department of Chemistry, Jadavpur University, Kolkata 700032, India
b Department of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Treatment of of (R,R)-N,N-salicylidene cyclohexane 1,2-diamine(H2L1) in methanol with aqueous NH4VO3
solution in perchloric acid medium affords the mononuclear oxovanadium(V) complex [VOL1(MeOH)]ꢀ-
ClO4 (1) as deep blue solid while the treatment of same solution of (R,R)-N,N-salicylidene cyclohexane
Article history:
Received 12 May 2011
Accepted 12 June 2011
Available online 25 June 2011
1,2-diamine(H2L1) with aqueous solution of VOSO4 leads to the formation of di-(
l-oxo) bridged vana-
dium(V) complex [VO2L2]2 (2) as green solid where HL2 = (R,R)-N-salicylidene cyclohexane 1,2-diamine.
The ligand HL2 is generated in situ by the hydrolysis of one of the imine bonds of HL1 ligand during the
course of formation of complex [VO2L2]2 (2). Both the compounds have been characterized by single crys-
tal X-ray diffraction as well as spectroscopic methods. Compounds 1 and 2 are to act as catalyst for the
catalytic bromide oxidation and C–H bond oxidation in presence of hydrogen peroxide. The representa-
tive substrates 2,4-dimethoxy benzoic acid and para-hydroxy benzoic acids are brominated in presence
of H2O2 and KBr in acid medium using the above compounds as catalyst. The complexes are also used as
catalyst for C–H bond activation of the representative hydrocarbons toluene, ethylbenzene and cyclohex-
ane where hydrogen peroxide acts as terminal oxidant. The yield percentage and turnover number are
also quite good for the above catalytic reaction. The oxidized products of hydrocarbons have been char-
acterized by GC Analysis while the brominated products have been characterized by 1H NMR spectro-
scopic studies.
Keywords:
Oxovanadium(V) Schiffbase complex
Crystal structure
Catalytic oxidation
Haloperoxidase activity
Hydrogen peroxide
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
reactions e.g. epoxidation of alkenes and allylic alcohols [11–15],
oxygenation of alkanes, arenes and primary or secondary alcohols
Research in oxovanadium chemistry containing the vanadium–
oxygen core moieties VO2+, VO3+ and VO2+ has gained a special
momentum in the field of homogeneous catalytic research as they
involve the conversion of crude oil and natural gas constituents into
their oxygenated derivatives in presence of hydrogen peroxide [1,2].
The discovery [3] of the core species present in the active site (VO4N)
of the enzymes of biological systems in the form of vanadium-
dependent haloperoxidases (VHPO) [4–7] and insulin mimicking
behavior of oxovanadium compounds have also increased the
importance in the field of research of oxovanadium chemistry. The
haloperoxidases are known to function as essential enzymes for
the biosynthesis of halogenated natural products [8] catalyzing
the oxidation of halides [9,10] by hydrogen peroxide to correspond-
ing HOX, X3ꢁ or X2 (X = halogen). The vanadium(V) complexes have
also been found to act as catalyst precursors in various oxidation
to the corresponding aldehydes or ketones [16–19] as well as oxida-
tion of organic sulfides [20–22] to sulfoxides and sulfones in pres-
ence of hydrogen peroxide. It may be noted that the tridentate and
tetradentate Schiffbase complexes of vanadium are well established
[23] but the report of these complexes that are used as catalyst for
hydrocarbon oxidation and halides oxidation are still very scanty
[24]. Mohebbi and Sererstini [25] reported a good yield with im-
proved turnover number (TON) (integrated) for olefin epoxidation
using VO-tetradentate Schiffbase complexes as catalyst and O2 as
an oxidant in CH3CN medium where the conversion of the substrates
were dependent on the nature of substituent in the Schiffbase li-
gand. The oxovanadium(V) complexes with (R,R)-N,N-salicylidene
cyclohexane 1,2-diamine was first reported by Nakajima et al. [26]
and later by some other groups [27,28] but none of them examined
their multifunctional catalytic activity. Herein, we are reporting the
synthesis and structural characterization of a new mononuclear
complex of oxovandium(V) [VOL1(MeOH)]ꢀClO4 (1) of tetradentate
Schiffbase ligand (R,R)-N,N-di-salicylidene cyclohexane 1,2-diami-
⇑
Corresponding authors. Tel./fax: +91 33 24146223.
E-mail addresses: tksi2002@yahoo.co.in (T.K. Si), k_mukherjea@yahoo.com (K.K.
Mukherjea).
ne(H2L1) and a di-( -oxo) bridged vanadium(V) complex [VO2L2]2
l
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.06.005

T.K. Si et al. / Polyhedron 30 (2011) 2286–2293
2287
and trans-1,2-cyclohexanediamine (10 mmol) in 25 ml dry metha-
nol for 5–6 h. Then yellow crystalline precipitate of H2L1 was depos-
ited. The solid mass was collected and dried in vacuo. Yield 2.80 g
(85%). Anal. Calc. for C20H20N2O2: C, 74.96; H, 6.29; N, 8.74. Found:
C, 74.83; H, 6.21; N, 8.96%. IR (KBr disc, cmꢁ1). 3481(b)[O–H],
2921(w), 2850(w), 1629(s)[C@N], 1581(w), 1500(m), 1461(w),
1421(w), 1382(w), 1340(w), 1280(s), 1215(w), 1147(m), 1095(w),
1043(w), 939(w), 846(m), 763(s), 667(w), 655(w), 364(w). The
trans-1-salicylideneimine 2-amine-cyclohexane (HL2) was pre-
pared in situ by the reaction of H2L1 ligand (2 mmol) in 10 ml meth-
anol with VOSO4ꢀH2O, (2 mmol) with constant stirring.
HO
OH
N
HO
HC
N
CH
HC
H2N
N
H2L1
HL2
B
A
Fig. 1. Ligand: H2L1 = (R,R)-N,N-di-salicylidene cyclohexane 1,2-diamine(A) and
HL2 = (R,R)-N-salicylidene cyclohexane 1,2-diamine (B).
2.3. Synthesis of [VO(L1)(CH3OH)]ꢀClO4 (1)
(2) of tridentate Schiffbase (R,R)-N-salicylidene cyclohexane 1,2-
diamine(HL2). Though the structural characterization of compound
2 has appeared in a recent literature [29] in 2009, but herein, the
same compound is being reported with different synthesis proce-
dure along with the catalytic implications of both the compounds
1 and 2 towards C–H bond oxidation as well as bromide oxidation
with good yield percentage and turnover number for such oxidation
reactions. The structure of the chiral Schiffbase ligands is repre-
sented in Fig. 1.
VOSO4ꢀH2O, 0.186 g (1 mmol) was dissolved in aqueous medium
and acidified by perchloric acid followed by addition of methanolic
solution of H2L1, 0.323 g (1 mmol) with constant stirring. A deep
blue colored compound precipitated. Solid products was collected
through filtration and dried in vacuo after washing it by diluted
methanol. Yield 0.268 g (52%). Anal. Calc. for C21H23ClN2O8V: V,
9.85; C, 48.69; H, 4.44; N, 5.41. Found: V, 9.71; C, 48.33; H, 4.21; N,
5.75%. IR(KBr disc, cmꢁ1). 3448(b), 2943(w), 2864(w), 2367(w),
2340(w), 1613(s)[C@N], 1553(s), 1447(s), 1390(m), 1345(w),
1305(s), 1274(s), 1228(w), 1114(s), 1073(s), 974(s)[V@O], 911(m),
864(w), 828(s), 766(s), 661(s), 625(m), 581(m), 552(w), 509(m),
2. Experimental
481(w), 454(m). UV–Vis; kmax nm
(e
Mꢁ1 cmꢁ1): 567(1800),
466(sh), 249 (27,100), 235(sh).
2.1. Materials and methods
2.4. Synthesis of [V2O4(L2)2] (2)
Ammonium metavanadate, cyclohexane, cyclohexanol and
cyclohexanoe were of extrapure variety and obtained from Sisco
Research Laboratory (India). Salicylaldehyde and Vanadyl Sulfate
were obtained from S.D. Fine-Chem. Ltd. (India) and trans-1,2-
cyclohxanediamine was obtained from Aldrich Chemical Co., Ltd.
Potassium bromide and methanol (G.R.) were the products of E.
Merck (India) and were directly used. Ethanol (95%) was obtained
from Bengal Chemical and Pharmaceutical Works (Calcutta), and
was lime distilled before use. All other chemicals needed were ob-
tained from E. Merck (India). Acetonitrile, dichloromethane and
acetone were further purified by literature method for physico-
chemical studies. Iolar II grade dioxygen, dihydrogen, zero air,
and dinitrogen gas used for chromatographic analysis were ob-
tained from Indian Refrigeration Stores, Calcutta. Triply distilled
(all glass) water was used whenever necessary. All the solvents
used for chromatographic analysis were either of HPLC, spectro-
scopic, or GR grade and in all cases their purity was confirmed
by GC analysis before use. The IR spectra were recorded as KBr pal-
Dissolved the ligand H2L1, 0.323 g (1 mmol) in 10 ml methanol
and then it was added to the methanolic solution of VOSO4ꢀH2O,
0.167 g (1 mmol) with constant stirring. Immediately a deep glass-
green color complex of V2O4(L2)2], 2 was obtained. The complex 2
is insoluble in water, methanol, ethanol, acetonitrile or other highly
polar solvents but soluble in dichloromethane and chloroform
slowly. Yield 0.33 g (55%). Anal. Calc. for C26H34N4O6V2: V, 17.00;
C, 52.00; H, 5.66; N, 9.33. Found: V, 16.51; C, 51.33; H, 5.85; N,
9.54%. IR (KBr disc, cmꢁ1). 3448(b), 3052(w), 2933(m), 2860(w),
2367(w), 2340(w), 1614(s)[C@N], 1543(s), 1447(s), 1392(m), 1347
(w), 1311(s)(C–Oenolate), 1200(m), 1147(m), 1093(w), 1064(w), 979
(s)[V@O], 908(m), 858(w), 810(m), 760(s), 680(w), 621(s), 570(w),
492(w), 424(w). UV–Vis; kmax nm
366(sh), 280(99,100), 267(sh).
(e
Mꢁ1 cmꢁ1): 592(5300),
2.5. X-ray crystallography
lets on a Perkin–Elmer 597 IR spectrophotometer (4000–200 cmꢁ1
)
and Electronic spectra on a Hitachi U-3410 UV/Vis–NIR spectro-
photometer. 1H NMR spectra were measured in CDCl3 on a Bruker
AM 360 (300 MHz) FT NMR spectrometer using TMS as an internal
standard. A systronics (India) model 335 digital conductivity
bridge with a bottle type cell was used to determine the molar con-
ductance values of the isolated complexes at 25 °C using a thermo-
static arrangement. A SUNVIC (UK) apparatus was used to measure
melting point of the organic substrates as well as their oxidized
products. Elemental analyses were performed with the help of a
Perkin–Elmer 240C elemental analyzer. GLC measurements were
done in an Agilent model 6890 gas chromatograph using HP-1
and INNOWAX capillary column in FID mode with dinitrogen as
carrier gas.
The good quality deep blue crystals of 1 were obtained by slow
diffusion of petroleum ether into acetone solution of the com-
pound 1 while glass-green crystals of 2 were obtained by slow
evaporation of chloroform solution of compound 2. The crystals
were very narrow needles and diffracted weakly. X-ray intensity
data were measured at 150 K using the Oxford X-Calibur CCD Sys-
tem [30], with Mo K
a radiation. The crystals were positioned at
50 mm from the CCD. 321 frames were measured with a counting
time of 3 s. The structures were solved using direct methods with
the program SHELXS and refined using the program SHELXL [31]. The
non-hydrogen atoms were refined with anisotropic thermal
parameters. The hydrogen atoms bonded to carbon were included
in geometric positions and given thermal parameters equivalent to
1.2 times those of the atom to which they were attached. Empirical
absorption corrections were carried out using the ABSPACK program
[32]. The structures were refined on F2 using SHELXL. Crystal data,
structure solution and refinement parameters for complexes 1
and 2 are summarized in Table 1.
2.2. Synthesis of ligand (H2L1)
The ligand N,N-bis(salicylidene)cyclohexane1,2-diamine (H2L1)
was prepared by refluxing the mixture of salicylaldehyde (20 mmol)

2288
T.K. Si et al. / Polyhedron 30 (2011) 2286–2293
Table 1
H2O2 (20 mmol) in a high-pressure reactor (100 mL capacity Parr
type hydrogenation apparatus) at 60–80 °C was stirred for definite
period as presented in Table 2. When required, an aliquot of the
reaction solution was withdrawn with the help of long needle syr-
Crystal data and structure refinement parameters for complexes 1 and 2.
1
2
Empirical formula
Molecular weight
T (K)
C
21H23ClN2O8V
C26H34N4O6V2
600.46
153(2)
0.71073
monoclinic
P21/a
inge and was subjected to multiple ether extractions and 1
concentrated ether extract was injected to the GC port with the
help of 10 L syringe. The retention times of the peaks were com-
lL of
517.18
153(2)
0.71073
triclinic
k (Å)
l
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
pared with those of commercial standards and the unknown peaks
were characterized by GC–MS analysis. Parr type apparatus was
used for the convenience of the physico-chemical studies but it is
not absolutely necessary for laboratory as well as industry.
ꢀ
P1
9.367(3)
11.316(3)
11.690(4)
108.61(3)
107.92(3)
90.23(3)
1110.2(6), 2
1.549
8.8637(8)
12.4952(18)
11.8915(13)
–
103.73(1)
–
1279.4(3), 2
1.559
624
a
(°)
b (°)
3. Results and discussion
c
(°)
V (Å3), Z
3.1. Synthetic aspects
Dcalc (g cmꢁ3
F(0 0 0)
)
534
H2L1 reacts with NH4VO3 solution acidified with perchloric acid
Crystal size (mm)
h Range for data collection (°)
Reflections collected
0.03 ꢂ 0.03 ꢂ 0.22 0.02 ꢂ 0.04 ꢂ 0.27
yielding [VO(L1)(CH3OH)]ꢀClO4 (1),
a six coordinate oxovana-
2.30–30.00
9641
2.90–30.10
7008
dium(V) complex possessing {VO4N2}+ coordination environment.
The VO3+ is coordinated by four donor atoms (ONNO) of di-nega-
tive tetradentate (L1)2ꢁ ligand in one plane (the equatorial plane
of the octahedron) and favors to form mononucler oxovana-
dium(V) complex with octahedral geometry where the sixth posi-
tion of octahedra is coordinated by CH3OH molecule instead of
ClO4ꢁ anion, because lowering of steric crowding with equatorially
coordinated tetradentate ligand. While the tetradentate Schiffbase
ligand (R,R)-N,N-di-salicylidene cyclohexane 1,2-diamine (H2L1) is
partially hydrolyzed [29] at pH 7 by VO2+ species of VOSO4 to form
tridentate HL2 ligand via oxidation of VO2+ species to VO3+ which
then forms oxovanadium(V) moiety of VO3N2 coordination envi-
ronment with tridentate (L2)ꢁ ligand and the two dioxovanadi-
um(V) moieties are bridged by two oxide ligands to form dimeric
oxo-bridged complex [(VO2L2)2] (2). The schematic representation
of the two reactions for the synthesis of 1 and 2 is shown below
(Schemes 1 and 2).
Independent reflections (Rint
Completeness to h = hmax (%)
Refinement method
Absorption correction (Tmin, Tmax
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
)
6106(0.040)
94.4
3626(0.077)
95.8
full-matrix-least-squares on F2
)
0.783, 1.243
6106/0/298
0.638
R1 = 0.0487
wR2 = 0.1046
R1 = 0.1518
wR2 = 0.1114
1.458, ꢁ1.206
0.761, 1.293
3626/0/172
0.995
R1 = 0.0988
wR2 = 0.2498
R1 = 0.1529
wR2 = 0.2771
1.554, ꢁ1.337
Final R indices [I > 2
r(I)]
R indices (all data)
Largest differences in peak and
hole (e Åꢁ3
)
2.6. Experimental setup of catalytic bromination
Catalyst and substrate (substituted benzoic acid) with a mole
ratio (1:1000) was dissolved in 5 ml CH3CN and then 3 ml aqueous
solution of KBr, 1.18 g (10 mmol) was added followed by the addi-
tion of 2.5 ml 30% of H2O2 with constant stirring at room temper-
ature. 1(N) nitric acid was added for maintaining the medium
acidic (pH 3). After 12 h the reaction was stopped and collected
the organic part with ether extract. After evaporation solid product
was isolated and studied by the 1H NMR spectroscopy for analysis
of the catalytic products.
3.2. General characterizations, infra-red and electronic spectroscopic
studies
The compound 1 in acetonitrile solution shows the molar con-
ductance value of 118 ohmꢁ1 cm2 molꢁ1 that value can be rational-
ized by assuming 1:1 electrolyte where the complex is of the
composition [VO(HOCH3)(L1)]ꢀClO4 but compound 2 is non electro-
lyte in acetonitrile. In compound 1 and 2, the vanadium ion exists
in +5 oxidation state (d0 system) and therefore magnetic suscepti-
bility measurement data indicate, both 1 and 2 are diamagnetic at
2.7. Experimental setup of catalytic oxidation of hydrocarbons
An acetonitrile solution (10 cm3) containing a given substrate
(5 mmol), oxovadium(V) catalyst (0.05–0.005 mmol) and 30%
Table 2
Details of catalytic hydrocarbon oxidation by compound 1 and 2 used as catalyst in presence of H2O2 and CH3CN as solvent.
Entry
1
Substrate
Product (s)
Time (h)
% Yields (mmol) for catalyst
% Selectivity/proportion for catalyst
Turn over number (TON)
1
2
1
2
1
2
10
100
78
A-30 B-70
A-40 B-60
990
780
O
OH
A
O
B
2
3
20
20
43
60
24
47
100
100
430
600
120
470
CH3
H3C
H
C-96 D-4
C-95 D-5
O
CH3
CH3 HO
D
C

T.K. Si et al. / Polyhedron 30 (2011) 2286–2293
2289
+
OH
O
HO
O
O
HClO4
-
ClO4
3-
CH
+
V
VO4
HC
N
N
CH
HC
N
N
aqueous MeOH,
Stirring
MeOH
H2L1
1
3ꢁ
Scheme 1. Formation of compound 1 from the reaction of H2L1 and VO4
.
O
O
O
OHC
aqueous MeOH,
Stirring
H2L1
VO2+
OH
+
V
HC
NH2
N
+
MeOH
O
H2N
O
HOMe
H2N
O
O
O
O
N
+
CH
+
2MeOH
HC
V
N
V
NH2
V
O
CH
V
NH2
N
HC
O
O
N
MeOH
O
O
O
2
Scheme 2. Probable formation mechanism of compound 2 from the reaction of H2L1 and VO2+ ion in neutral medium.
298 K. As shown in the experimental section the
m
(V@O) vibrations
the metal center where the VO3+ ion is bonded to the tetradentate
ligand (L1)2ꢁ, (N,N-bis(salicylidene)1,2-diaminatocyclohexane)
with the oxo ligand O(1) and CH3OH molecule occupying the axial
position of slightly distorted octahedron. The selected bond dis-
tances and angles in central metal coordination were listed in Ta-
ble 3. The vanadium center being displaced above the mean
equatorial plane defined by four donor atoms O(64), N(56), 0N(49)
in 1 (Fig. S1) and 2 (Fig. S2) appears at 974 cmꢁ1 and 979 cmꢁ1
respectively indicating the ligand effect on V@O vibrations varies
in the order of 2 > 1. The
1627 cmꢁ1 while in compound 1, the
creases to 1613 cmꢁ1 after chelation with the vanadium center. In
case of compound 2, the
(C@N) absorption band for the ligand HL2
m
(C@N) of H2L1 is observed at
m
(C@N) absorption band de-
m
is observed at 1628 cmꢁ1 while in compound 2 the
m
(C@N) absorp-
and O(41) of (L1)2ꢁ ligand towards the terminal O(1) by 0.29 ÅA with
0
tion band is observed at 1614 cmꢁ1 which can be rationalized
by assuming that the drainage of electron density from nitrogen
an r.m.s. deviation of 0.04 ÅA. The extent of deviation is nearly same
0
for typical monomeric oxovanadium(V) complexes (0.27–0.32 ÅA)
0
to vanadium(V) after coordination decreases the
p-overlap of
[33,34]. The V1–O1 bond length (1.581 ÅA) in [VO(MeOH)L1]+ lies
C–N bond. The strong C–O(phenolic) vibration band at 1279 cmꢁ1
in the range observed for typical oxovanadium(V) complexes
0
for the free H2L1 ligand is shifted to higher energy region
(1.57–1.59 ÅA) [35]. The longer axial V(1)–O(101) bond length
0
(m
(C–Ophenolate) at 1305 cmꢁ1) in compound 1 due to deprotonation
2.328(3) ÅA indicates that O(1 0 1) is being shared by coordinated
CH3OH molecule instead of methoxy anion (CH3Oꢁ). The molecular
packing in 1 show that the oxo groups of two vanadium centers are
closely spaced and the perchlorate ions are present in the vicinity
of methanol coordinated to vanadium centers indicating the
presence of weak interaction between perchlorate ions and meth-
anol (Fig. 3).
of phenolic group increases the C–O bond order by back donation
though it is coordinated with the metal ion.
The blue color acetonitrile solution of compound 1 exhibits
LMCT transition at wave length 567 nm (
two shoulders at 466 and 235 nm. The intra-ligand
tion is observed at the wave length 249 nm (
= 27 100 Mꢁ1 cmꢁ1).
While the compound 2 in chloroform solution exhibits a LMCT
transition at the wave length 592 nm (
= 5300 Mꢁ1 cmꢁ1) and a
intra-ligand
p⁄ transition of aromatic ring at 280 nm
= 99 100 Mꢁ1 cmꢁ1) with two shoulders at 366 and 267 nm.
e
= 1800 Mꢁ1 cmꢁ1) with
p
?
p⁄ transi-
e
The crystal structure of compound 2 revealed by X-ray crystal-
lographic determination showed that the molecule consists of two
[VO2L2] units (Fig. 4) making it a dimeric centrosymmetric struc-
ture (Fig. 5) with two six coordinated octahedral environment.
The selected bond distances and angles in central metal coordina-
tion were listed in Table 4. There is a striking difference in the two
metal–oxygen bond distances of the two bridging oxygen of com-
pound 2 which is formed through asymmetric bridging of two
e
p
?
(e
3.3. Molecular structure of 1 and 2
The crystal structure of compound 1 revealed by X-ray crystal-
lographic determination showed that the molecular structure of 1
contains discrete [VO(MeOH)L1]+ cations and perchlorate anions.
The geometry of the complex cation [VO(MeOH)L1]+ shown in
Fig. 2 is described as six coordinated octahedral environment of
[VO2L2] unit.
0
The V1–O2 bond distance is 1.669 ÅA and the O2 bridges to the
0
V1a (ꢁx, 1 ꢁ y, 1 ꢁ z) at the distance 2.303(3) ÅA. Similarly the
0
O2a linked with V1a (V1a–O2a distance is 1.669 ÅA) bridges to V1

2290
T.K. Si et al. / Polyhedron 30 (2011) 2286–2293
Fig. 2. The structure of [VO(MeOH)L1]+ in 1 with ellipsoids at 25% probability.
0
at the distance 2.303(3) ÅA. The vanadium atom(V1) is six coordi-
nated octahedral geometry (Fig. 5) formed by three donor atoms
O11, N19, N26 from the planner uni-negative tridentate chiral
Schiffbase ligand, (R,R) N-salicylidene cyclohexane 1,2-diamine
(HL2) and two oxo-oxygens O1, O2 and a bridging oxygen O2a.
The same octahedral geometry is occurred in the case of other
vanadium (V1a). The equatorial plane consist the donor atoms
O11, N19, N26 and O2 in case of V1 centered octahedral geometry.
In the dimmer the two chiral carbons of the ligand (L1)ꢁ linked
with V1 are C24, C25 possess configuration (R,R) while C24a,
C25a of the ligand linked with V1a have the configuration (S,S).
Fig. 4. Square-pyramidal assymetric unit of dioxovanadium (V), [VO2L1] of 2.
dimerization of two [VO2L2] unit. Therefore observed V(1)ꢀ ꢀ ꢀV(1))
0
0
distance (3.108(1) ÅA) and V1–O2a (2.303(3) ÅA) are the shortest
distances than the earlier reported di-(l-oxo) bridged vana-
dium(V)complex [V2O4L2] [36,37] (where L is any uni-negative
tridentate ligand). The apical oxo-vanadium V1–O1 double bond
0
A
pair of strong hydrogen bonds 0 (N26–H26bꢀ ꢀ ꢀO11a) and
distance is 1.608(4) [38]. The V1–O2 bond distance 1.669 ÅA which
0
(N26a–H26bꢀ ꢀ ꢀO11) of distance 2.169 ÅA in 2 stabilizes the tight
is slightly larger than the V1–O1 double bond (1.608 ÅA) and smaller
0
the V1–O11 single bond (1.91 ÅA) due to delocalization of
p(pi)
electrons of O2 atom by asymmetric oxo bridging of the two
penta-coordinated [VO2L2] units the V1–O2 double bond nature
is reduced. The molecular packing in 2 exhibits two different kinds
of hydrogen bonding networks (Fig. 6 and Table 5. In 2, a pair of
intra-molecular N26–H(26b)ꢀ ꢀ ꢀO11 hydrogen bonds stabilize the
dimeric form via tight dimerization of two asymmetric VO2L1
unit forming two R11(6) ring. The other two weak intermolecular
Table 3
Selected bond lengths (Å) and angles (°) for complex 1.
V(1)–O(1)
V(1)–O(41)
V(1)–O(64)
V(1)–O(101)
V(1)–N(49)
V(1)–N(56)
O(1)–V(1)–N(56)
O(64)–V(1)–N(56)
O(41)–V(1)–N(56)
N(49)–V(1)–N(56)
1.581(2)
1.849(2)
1.828(3)
2.328(3)
2.096(3)
2.099(3)
98.37(13)
87.04(13)
155.45(11)
76.77(13)
O(1)–V(1)–O(64)
O(1)–V(1)–O(41)
O(64)–V(1)–O(41)
O(1)–V(1)–N(49)
O(64)–V(1)–N(49)
O(41)–V(1)–N(49)
O(1)–V(1)–O(101)
O(64)–V(1)–O(101)
O(41)–V(1)–O(101)
N(49)–V(1)–O(101)
103.34(12)
99.68(12)
104.72(12)
91.51(12)
159.51(12)
86.25(12)
171.96(12)
83.70(10)
82.02(10)
80.73(10)
Fig. 5. The centrosymmetric structure of 2 ([VO2L2]2) together with the atomic
numbering scheme, ellipsoids at 25% probability.
Fig. 3. The molecular packing in the complex [VO(MeOH)L1]ꢀClO4, 1.

T.K. Si et al. / Polyhedron 30 (2011) 2286–2293
HOBr
2291
Table 4
O
Selected bond lengths (Å) and angles (°) for complex 2.
H2O2
V
V(1)–N(19)
2.147(4)
1.608(4)
1.912(4)
2.114(4)
1.669(3)
2.303(3)
106.25(19)
101.44(18)
99.21(16)
93.69(19)
91.58(17)
O(11)–V(1)–N(26)
O(11)–V(1)–N(19)
N(26)–V(1)–N(19)
O(11)–V(1)–O(2)$1
N(26)–V(1)–O(2)$1
N(19)–V(1)–O(2)$1
V(1)–O(2)–V(1)$1
O(1)–V(1)–N(19)
O(2)–V(1)–N(19)
O(1)–V(1)–O(2)$1
O(2)–V(1)–O(2)$1
158.06(17)
85.02(16)
76.81(16)
84.26(14)
79.29(15)
75.06(15)
101.77(16)
99.51(18)
152.39(17)
171.88(18)
78.23(16)
H2O
V(1)–O(1)
V(1)–O(11)
V(1)–N(26)
V(1)–O(2)
O
Br
H
O
O
V(1)–O(2)$1
O(1)–V(1)–O(2)
O(1)–V(1)–O(11)
O(2)–V(1)–O(11)
O(1)–V(1)–N(26)
O(2)–V(1)–N(26)
V
-OOH
V
OH
H+
Br-
H2O
$1 Symmetry element ꢁx, 1 ꢁ y, 1 ꢁ z.
O
O
O
V
O
O
O
H2O
V
Br-
Scheme 5. Probable mechanism of bromide oxidation by oxovanadium(V) complex
[39] in presence of KBr and H2O2.
3.4. Catalytic bromination of aromatic compounds
In presence of hydrogen peroxide, both the monomeric and
dimeric oxovanadium(V) complex 1 and 2 respectively forms
oxoperoxo-vanadium(V) species which subsequently oxidizes the
bromide ion in acidic condition (pH 3–4) as catalyst to form Br+
(exists as Br3ꢁ, Br2 or HOBr) (shown in Scheme 5) [39]. Thus, the
in situ generated bromonium species leads to formation of bromi-
nated organic products by reacting with suitable organic
substrates. It has been seen that in presence of very low pH (<1),
the suitable organic compounds are brominated by H2O2/Brꢁ to a
very small amount and at pH >3 the yields of brominated products
are very poor after a long period of reaction. Such as, the organic
compounds 4-hydroxy benzoic acid and 2,4-dimethoxy benzoic
acid are not brominated effectively (2–5% after 10 h reaction) by
H2O2/Brꢁ system in acidic condition (pH 3–4) in the absence of
the catalyst, but in presence of catalyst (1 or 2) both 4-hydroxy
benzoic acid (Scheme 3) and 2,4-dimethoxy benzoic acid (Scheme
4) yield 98% brominated products after 12 h of reaction period. The
reactions are shown in the Schemes 4 and 5. The catalytic effi-
ciency of compound 1 is slightly higher compared to that of com-
pound 2 may be (Table 6) due to the high stability of the dimeric
form of 2 that inhibits peroxidic attack to the metal center. The so-
lid brominated products are characterized by 1H NMR spectra.
Fig. 6. The intra- and inter-molecular H-bonded (N–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀO) figure of
[(VO2L2)2] (2) with the direction of the cell axes is (1 0 0).
Table 5
Relevant hydrogen bonds for complex 2.
D–Hꢀ ꢀ ꢀA
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
\(DHA)
N(26)–H(26B)ꢀ ꢀ ꢀO(11)(i)
C(18)–H(18)ꢀ ꢀ ꢀO(2)(ii)
C(21)–H(21B)ꢀ ꢀ ꢀO(2)
0.90
0.93
0.97
2.17
2.37
2.59
2.968(6)
3.205(6)
3.468(7)
147
150
151
Symmetry codes: (i) ꢁx, 1 ꢁ y, 1 ꢁ z (ii) 1/2 ꢁ x, 1/2 + y, 1 ꢁ z.
COOH
COOH
OCH3
OCH3
+
KBr H2O2
/
/
H
3.5. Characterizations of brominated products by 1H NMR
spectroscopic studies
[VO2L2]2
[VO(L1)(CH3OH)]+
/
Br
OCH3
OCH3
In the substrate molecule, 2,4-dimethoxy benzoic acid, the
chemical shifts for ortho- and para-methoxy hydrogen are ob-
served at d 3.88 (s, 3H) and 4.04 (s, 3H) respectively Chemical shifts
for three single H atom attached with C3, C5 and C6 carbon atoms
of aromatic ring observed at d 6.53 (s, 1H), 6.63 (d, 1H, J = 10.5 Hz)
and 8.12 (d, 1H, J = 8.64 Hz) respectively (Fig. S3) while in the
mono brominated (5-bromo 2,4-dimethoxy benzoic acid) product,
Scheme 3. Catalytic bromination of 2,4-dimthoxy benzoic acid using 1, and 2 as
catalyst.
hydrogen bonding interaction C(18)–H(18)ꢀ ꢀ ꢀO(2) and C(21)–
H(21B)ꢀ ꢀ ꢀO(2) (Fig. 6) in 2 produced infinite one-dimensional chain
propagating along the (1 0 0) direction.
COOH
COOH
COOH
+
KBr H2O2
/
/
H
+
[VO2L2]2
[VO(L1)(CH3OH)]+
/
Br
Br
OH
Br
OH
OH
Scheme 4. Catalytic bromination of p-hydroxy benzoic acid using 1 and 2 as catalyst.

2292
T.K. Si et al. / Polyhedron 30 (2011) 2286–2293
Table 6
Details of the catalytic bromination by compound 1 and 2 used as catalyst in presence of KBr and H2O2 in acid medium and CH3CN is used as solvent. Substrate:catalyst = 1000:1.
Entry
I
Substrate
Product/s
Time (h)
% Yields (mmol) for catalyst
% Selectivity/proportion for catalyst
Turn over number (TON)
1
2
1
2
1
2
15
85
68
A-67 B-33
A-69 B-31
850
680
COOH
COOH
COOH
Br
Br
Br
OH
OH
A
OH
B
II
12
98
78
100
100
980
780
COOH
COOH
OCH3
OCH3
Br
OCH3
OCH3
Fig. 7. The plot of products percentage with time for the catalytic oxidation of
Fig. 8. The proportion diagram of products and substrate during the course of
toluene and cyclohexane.
catalytic oxidation in case of ethylbenzene.
the chemical shifts of the two methoxy group are obtained at d
3.99 (s, 3H) and 4.09 (s, 3H). Another two singlets for two aromatic
protons are observed at d 6.51 (s, 1H) and 8.33 (s, 1H) which are
shown in the figure (Fig. S4) In the case of the substrate, para-
hydroxy benzoic acid, the chemical shifts for two ortho hydrogen
atoms is observed at d 7.32 (d, 2H, J = 8.86 Hz) and for two meta
hydrogen at d 6.73 (d, 2H, J = 8.67 Hz) respectively. The chemical
shift for –OH hydrogen is observed at d 5.30 (s, 1H). In mono bro-
minated products (3-bromo 4-hydroxy benzoic acid), the chemical
shifts are observed at d 5.30 (s, 1H for –OH group), 6.90 (d, 1H,
J = 8.62 Hz for meta- H of aromatic ring), 7.32 (d, 1H, J = 8.85 Hz
for one ortho(6-) H atom) and 7.77 (s, 1H of another ortho(2-) H
atom) respectively (Fig. S5). In the dibrominated (3,5-dibromo 4-
hydroxy benzoic acid) only two singlets are observed in the 1H
NMR spectrum (Fig. S6) and chemical shifts for these two are at
d 7.769 (s, 2H of two ortho aromatic H atoms) and 5.30 (s, 1H),
respectively. The magnitude of the NMR signal in the mixture of
products indicates that mono brominated is the major product.
ethyl benzene and toluene to their alcoholic and carbonyl func-
tions as shown in the Table 2. The catalytic efficiency of the two
oxovanadium(V) complexes 1 and 2 for hydrocarbon oxidation
are not same (shown in the Table 2) due to their different coordi-
nation geometry probably because of the tight dimerisation of two
oxovanadium(V) nucleus in complex 2 which makes it less reactive
towards H2O2 activation. After 10 h cyclohexane is totally oxidized
(Fig. 7) to cyclohexanol and cyclohexanone with high turnover
number (TON), while toluene is selectively oxidized to benzalde-
hyde (43%) after the period of 20 h without any side products
(Fig. 7). In case of ethyl benzene, the C–H bonds of benzyl carbon
are easily oxidized to form acetophenone (58%) as major products
(Fig. 8) instead of 1-phenyl ethanol (2%) up to a reaction period of
20 h. The lower yield of toluene oxidation compared to ethylben-
zene indicates that the C–H bond of secondary carbon is more la-
bile towards oxidation rather than that with primary carbon.
However, the Fig. 9 indicates that the cyclohexanol (oxidized prod-
uct of cyclohexane) is further oxidized to cyclohexanone by the
same catalyst in the reaction medium. So, during the course of
reaction, the proportion of cyclohexanone is increased with a high-
er degree than cyclohexanol. The mechanism of the peroxidative
hydrocarbon oxidation by oxovanadium(V) based catalyst is same
as appeared in our previous work [40].
3.6. Catalytic oxidation of hydrocarbons
Complexes 1 and 2 possess effective catalytic properties in the
oxofunctionalization of representative hydrocarbons cyclohexane,

T.K. Si et al. / Polyhedron 30 (2011) 2286–2293
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Acknowledgements
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2K10-EMRI dated 10.03.2010) for Research Associateship. Funding
by DST, INDIA for Agilent 6890N Gas Chromatograph with dual
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Appendix A. Supplementary data
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CCDC 805362 and 805363 contains the supplementary crystallo-
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it@ccdc.cam.ac.uk. The figures of IR spectra (Figs. S1 and S2) and 1H
NMR spectra (Figs. S3–S6) are available in the supplementary file.
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