Synthesis, characterisation and catalytic potential of hydrazonatovanadium(v) model complexes with [VO]3+ and [VO 2]+ cores

Article abstract of DOI:10.1039/b416292g

Reaction between [VO(acac)₂] and H₂L (H₂L are the hydrazones H₂sal-nah I or H₂sal-fah II; sal = salicylaldehyde, nah = nicotinic acid hydrazide and fah = 2-furoic acid hydrazide) in methanol leads to the formation of oxovanadium(IV) complexes [VOL·H₂O] (H₂L = I: 1, H₂L = II: 4). Aerial oxidation of the methanolic solutions of 1 and 4 yields the dinuclear oxo-bridged monooxovanadium(V) complexes [{VOL}₂μ-O] (H ₂L = I: 2, H₂L = II: 5). These dinuclear complexes slowly convert, in excess methanol, to [VO(OMe)MeOH)L] (H₂L = I: 9, H ₂L = II: 10), the crystal and molecular structures of which have been determined, confirming the ONO binding mode of the dianionic ligands in their enolate form. Reaction of aqueous K[VO₃] with the ligands at pH ca. 7.5 results in the formation of [K(H₂O)][VO₂L] (H ₂L = I: 3, H₂L = II: 6). Treatment of 3 and 6 with H ₂O₂ yields (unstable) oxoperoxovanadium(V) complexes K[VO(O₂)L], the formation of which has been monitored spectrophotometrically. Acidification of methanolic solutions of 3 and 6 with HCl affords oxohydroxo complexes, while the neutral complexes [VO ₂(Hsal-nah)] 7 and [VO₂(Hsal-fah)] 8 were isolated on treatment of aqueous solutions of 3 and 6 with HClO₄. These complexes slowly transform into 9 and 10 in methanol, as confirmed by ¹H, ¹³C and ⁵¹V NMR. The anionic complexes 3 and 6 catalyse the oxidative bromination of salicylaldehyde in water in the presence of H ₂O₂/KBr to 5-bromosalicylaldehyde and 3,5- dibromosalicylaldehyde, a reaction similar to that exhibited by vanadate-dependent haloperoxidases. They are also catalytically active for the oxidation of benzene to phenol and phenol to catechol and p-hydroquinone.

Full text of DOI:10.1039/b416292g

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F U L L P A P E R
Synthesis, characterisation and catalytic potential of hydrazonato-
3+
+
2
vanadium(V) model complexes with [VO] and [VO ] cores†
a
a
b
b
b
Mannar R. Maurya,* Shalu Agarwal, Cerstin Bader, Martin Ebel and Dieter Rehder
a
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247 667, India.
E-mail: rkmanfcy@iitr.ernet.in
Institut f u¨ r Anorganische und Angewandte Chemie, Universit a¨ t Hamburg,
Martin-Luther-King-Platz 6, 20146, Hamburg, Germany.
E-mail: dieter.rehder@chemie.uni-hamburg.de
b
Received 21st October 2004, Accepted 14th December 2004
First published as an Advance Article on the web 10th January 2005
Reaction between [VO(acac)
nah = nicotinic acid hydrazide and fah = 2-furoic acid hydrazide) in methanol leads to the formation of
oxovanadium(IV) complexes [VOL·H O] (H L = I: 1, H L = II: 4). Aerial oxidation of the methanolic solutions of 1
and 4 yields the dinuclear oxo-bridged monooxovanadium(V) complexes [{VOL} l-O] (H L = I: 2, H L = II: 5).
These dinuclear complexes slowly convert, in excess methanol, to [VO(OMe)(MeOH)L] (H L = II: 10),
L = I: 9, H
the crystal and molecular structures of which have been determined, confirming the ONO binding mode of the
dianionic ligands in their enolate form. Reaction of aqueous K[VO ] with the ligands at pH ca. 7.5 results in the
formation of [K(H O)][VO L] (H L = II: 6). Treatment of 3 and 6 with H yields (unstable)
L = I: 3, H
oxoperoxovanadium(V) complexes K[VO(O )L], the formation of which has been monitored spectrophotometrically.
Acidification of methanolic solutions of 3 and 6 with HCl affords oxohydroxo complexes, while the neutral complexes
VO (Hsal-nah)] 7 and [VO (Hsal-fah)] 8 were isolated on treatment of aqueous solutions of 3 and 6 with HClO
These complexes slowly transform into 9 and 10 in methanol, as confirmed by H, C and V NMR. The anionic
complexes 3 and 6 catalyse the oxidative bromination of salicylaldehyde in water in the presence of H /KBr to
-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde, a reaction similar to that exhibited by vanadate-dependent
2
] and H
2
L (H
2
L are the hydrazones H
2
sal-nah I or H
2
sal-fah II; sal = salicylaldehyde,
2
2
2
2
2
2
2
2
3
2
2
2
2
2
O
2
2
[
2
2
4
.
1
13
51
2
O
2
5
haloperoxidases. They are also catalytically active for the oxidation of benzene to phenol and phenol to catechol and
p-hydroquinone.
Introduction
and II formed from salicylaldehyde (Hsal) and nicotinic acid
hydrazide (Hnah) or 2-furoic acid hydrazide (Hfah), Scheme 1.
Their conversion to complexes containing the {VO(OH)} and
The stabilization, under aerobic conditions, of vanadium(V)
complexes with a mixed ON coordination sphere
topical interest because of the potential of these complexes as
structural and/or functional models for vanadate-dependent
haloperoxidases (VHPO).
1
–3
is of
{VO(O
2
)} cores, and thus mimicking active site features of
VHPO, will be addressed. The isolation of complexes with the
{
VO(OMe)} core from {VO
2
} and {l-O(VO) } type complexes
2
4–9
In VHPO, vanadium is in a
is of particular interest in the context of oxo transfer catalysis.
The knowledge on alkoxo-oxovanadium(V) complexes so far is
trigonal-bipyramidal (native form) or tetragonal-pyramidal
(
peroxo form) environment, covalently linked to the Ne of a
20,22–29
sparse.
4,10,11
histidine.
These enzymes catalyse the oxidation, by perox-
ide, of halides to hypohalous acid which further halogenates
hydrocarbons non-enzymatically. Vanadium(V) complexes have
also been shown to act as electron acceptors and thus initiators
12
in the photo-cleavage of DNA. Further, many vanadium
complexes show catalytic activity in oxidation and oxygen
1
3,14
transfer reactions,
organic sulfides to (chiral) sulfoxides.
various organic substrates by peroxides, catalysed by vanadium
including the oxidation of (prochiral)
15–18
Oxidation reactions of
19
complexes, have been reviewed.
We recently reported on structural models of VHPO, viz.
dioxovanadium(V) complexes of ONO donor ligands based on
salicylidenehydrazides, and demonstrated their ability to trans-
fer, via an oxo-peroxo intermediate, an oxo group to a substrate
20
such as PPh
3
.
Anionic and neutral cis-dioxovanadium(V) com-
Scheme 1
plexes containing the tridentate N-salicylidenehydrazide ligand
system, and their model character for VHPO have recently
21
been reviewed by Plass. The objective of the present work
is to synthesise mononuclear dioxovanadium(V) and dinuclear
l-oxo-bis{oxovanadium(V)} complexes of the hydrazones I
Experimental
Materials and methods
V
2
O
5
, nicotinic acid hydrazide, 2-furoic acid hydrazide (Fluka
Chemie, Switzerland), acetylacetone (Hacac) (Aldrich, USA),
salicylaldehyde (Hsal), phenol, benzene, 30% aqueous H and
70% HClO (Qualigens, India) were used as obtained. Other
†
Electronic supplementary information (ESI) available: Fig. S1: Mag-
2
O
2
netic susceptibility vs. temperature for 1; detailed description of IR
1
13
spectra; Table S1: H NMR chemical shifts; Table S2: C NMR chemical
shifts; Table S3: Electronic absorption spectra. See http://www.rsc.org/
suppdata/dt/b4/b416292g/
4
chemicals and solvents were of analytical reagent grade. H sal-
2
30
31
fah and [VO(acac)
2
]
were prepared according to methods
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 5 3 7 – 5 4 4
5 3 7

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reported in the literature. H
2
sal-nah was prepared following the
(20 ml) was added with stirring to a solution of [VO(acac) ]
2
procedure reported for H sal-fah.
2
(0.530 g, 0.002 mol) dissolved in methanol (20 ml), and the
resulting reaction mixture was refluxed on a water bath for
6 h. After cooling to room temperature, a greenish-yellow
precipitate of 1 was filtered off, washed with methanol and dried.
Compound 1 was suspended in methanol (40 ml) and air was
passed through the suspension for up to 20 h with occasional
shaking, until precipitation of crystalline brown solid 2 occurred.
This was filtered off, washed with methanol and dried in vacuo.
For the preparation of 2, it is not necessary to isolate 1. Data for
Elemental analyses of the ligands and complexes were per-
formed by the micro-analytical section of the Central Drug
Research Institute, Lucknow, India. IR spectra were recorded
as KBr pellets on a Perkin-Elmer model 1600 FT-IR spectrome-
ter. Electronic absorption spectra were measured in methanol
or DMF with an UV-1601 PC UV-Vis spectrophotometer.
1
51
H NMR spectra were obtained on a Bruker 200, and
V
NMR spectra on a Bruker Avance 400 MHz spectrometer at
9
4.73 MHz with the common parameter settings. NMR spectra
1: Yield 0.38 g (62.3%) (Found: C, 48.38; H, 3.26; N, 13.13. Calc.
5
1
−1
were usually recorded in DMSO-d
enced relative to VOCl
and V NMR results have also been obtained in CD OD. Ther-
mogravimetric analyses of the complexes were carried out under
oxygen atmosphere using a TG Stanton Redcroft STA 780
instrument. The magnetic susceptibilities were measured by a Vi-
brating Sample Magnetometer model 155, using nickel as stan-
dard. Susceptibilities were measured in the temperature range
6
, and d( V) values are refer-
as external standard. Selected H NMR
for C13
H
11
N
3
O
4
V (324.19 g mol ): C, 48.15; H, 3.42; N, 12.97%).
1
−1
IR (KBr, mmax/cm ): 1608 (C=Nazomethine), 1285 (C–Oenolate), 1046
3
5
1
(N–N), 988 (V=O). leff (293 K) = 1.54 l
. Data for 2: Yield
3
B
(based on [VO(acac)
N, 12.12. Calc. for C26
H, 2.89; N 13.37%). IR (KBr, mmax/cm ): 1607 (C=Nazomethine),
1279 (C–Oenolate), 1048(N–N), 968 (V=O), 850 [V–(l-O)–V].
NMR (DMSO-d
2
]): 0.48 g (38.2%) (Found: C, 48.35; H, 3.0;
−
1
H
18
N
6
O
7
V (628.01 g mol ): C, 49.70;
1
2
−
5
1
V
6
, d/ppm): −542.4 and minor signal at −584;
2
98–78 K while applying a magnetic field of 0.5 T. Diamagnetic
(CD
3
OD): −541.7 and a minor signal at −550.1.
32
corrections were carried out using Pascal’s increments. EPR
spectra were recorded with a Bruker ESP 300E spectrometer
between 9.42 and 9.47 GHz, and EPR parameters were adjusted
by simulation with the Bruker program system SimFonia.
The reaction product obtained from the catalytically con-
ducted oxidative bromination of salicylaldehyde and oxidation
of benzene as well as phenol were analysed using a Hewlett
Packard 5890 A gas chromatograph fitted with FID detector
and a stainless steel column HP 1. The identities of the
brominated products were confirmed by the GC-MS reference
system Shimadzu QP-5000.
V
[K(H O)][V O (sal-nah)] 3. Vanadium(V) oxide (0.91 g,
2
2
0.005 mol) was suspended in aqueous KOH (0.28 g, 0.005 mol
in 10 ml H O) and stirred for 2 h at 50 C. The resulting solution
was filtered. A filtered solution of H sal-nah (1.20 g, 0.005 mol),
dissolved in 20 ml of aqueous KOH (0.56 g, 0.010 mol), was
added with stirring, and the pH of the reaction mixture was
slowly adjusted to 7.5 with 4 M HCl. After 2 h of stirring, a
yellow solid had formed, which was filtered off, washed with
cold water and dried. The solid material was redissolved in
◦
2
2
◦
ca. 25 ml of methanol and kept at 10 C. Within 2–4 days, 3
precipitated as a yellow solid, which was filtered off and dried in
vacuo over silica gel. Yield: 0.31 g (41%) (Found: C, 42.0; H, 2.76;
Crystal structure determinations
−
1
Data were collected on a Bruker SMART CCD Apex diffrac-
N, 10.95. Calc. for C13
H
11KN
3
O V (378.98 g mol ): C, 41.17;
1
5
−
tometer at 153(2) K, using graphite-monochromated Mo-Ka
H, 2.93; N 11.08%). IR (KBr, mmax/cm ): 1619 (C=Nazomethine),
+
˚
radiation (k = 0.71073 A). H atoms (except of the methanol-
1224 (C–Oenolate), 1028 (N–N), 917, 935 (sym and antisym VO
2
).
5
1
OH) were placed into calculated positions and included in the
last cycles of refinement. The proton on O5 was found. Crystal
data and details on the data collection and refinement are
collated in Table 1. The program system SHELXTL 5.1 was
used throughout. Disorder of the furfuryl ring atoms C3 and
O6 of compound 10 over two alternative positions was treated
with a 1 : 1 model.
V NMR (DMSO-d
6
, d/ppm): −532.4.
IV
V
[V O(sal-fah)H O] 4 and [{V O(sal-fah)} l-O] 5. Com-
2
2
plexes 4 and 5 were prepared analogously to 1 and 2, replacing
H sal-nah for H sal-fah. Data for 4: Yield: 0.40 g (68.0%)
2
2
(Found: C, 46.29; H, 3.02; N, 9.18. calc. for C H N O V
1
2
10
2
5
−
1
(313.00 g mol ): C, 46.01; H, 3.22, N, 8.95%). IR (KBr,
−
1
CCDC reference numbers 252201 (9) and 251548 (10).
See http://www.rsc.org/suppdata/dt/b4/b416292g/ for cry-
stallographic data in CIF or other electronic format.
mmax/cm ): 1613 (C=Nazomethine), 1222 (C–Oenolate), 1030 (N–
N), 966 (V=O). l (293 K) = 1.48 l . Data for 5: Yield
eff
B
0.50 g (41.0%) (Found: C, 46.06; H, 3.76; N, 7.54. Calc. for
−
1
C
24
H
16
N
4
O
9
V (605.98 g mol ): C, 47.53; H, 2.66; N, 9.24%).
2
Preparations
−
1
IR (KBr, mmax/cm ): 1611 (C=Nazomethine), 1281 (C–Oenolate), 1029
IV
V
51
[
VO (sal-nah)H
2
O] 1 and [{V O(sal-nah)}
2
l-O] 2. A fil-
(N–N), 964 (V=O), 757 [V–(l-O)–V]. V NMR (DMSO-d
d/ppm): −536.6, −548.4 and −585.6.
6
,
tered solution of H sal-nah (0.480 g, 0.002 mol) in methanol
2
Table 1 Crystal data and structure refinement for [VO(OMe)(MeOH)(sal-nah)] (9) and [VO(OMe)(MeOH)(sal-fah)] (10)
9
10
Empirical formula
M
Crystal system, space group
C
738.50
Monoclinic, P2(1)/c
8.0161(2)
14.9534(4), 92.9190(10)
13.2937(4)
1591.42(8)
2
1.541
0.654
30
H
32
N
6
O
10
V
2
28 28 4 12 2
C H N O V
714.42
Monoclinic, P2(1)/c
8.3650(3)
16.0241(5), 104.4460(10)
11.7780(4)
1528.83(9)
2
1.556
0.681
a/A˚
◦
b/ A˚ , b/
c/A˚
V/A˚
3
Z
−
3
q
calc/g cm
−
1
l/mm
F(000)
760
41950/5507
0.0436
5507/0/281
1.151
0.0378, 0.0962
0.0395, 0.0976
736
15273/2659
0.0338
2659/0/210
1.214
0.0467, 0.1505
0.0491, 0.1535
Reflections collected/unique
R(int)
Data/restraints/parameters
2
Goodness-of-fit on F
R indices, [I > 2r(I
R indices (all data): R1, wR2
0
)]: R1, wR2
5
3 8
D a l t o n T r a n s . , 2 0 0 5 , 5 3 7 – 5 4 4

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V
[
K(H
2
O)][V O
2
(sal-fah)] 6. This complex was prepared from
Oxidation of benzene. Complexes 3 and 6 have also been
KVO
3
and H
2
sal-fah by the method outlined for 3. The crude
employed to carry out the oxidation of benzene. Benzene (3.9 g,
material was dissolved by heating in a minimum amount of
methanol and filtered. On keeping 2–4 days at ca. 10 C,
50 mmol), 30% aqueous H
(0.025 g, ca. 0.07 mmol) were taken in 5 ml of acetonitrile and the
reaction mixture was heated at 80 C in an oil bath with stirring.
2
O (5.67 g, 50 mmol) and catalyst
2
◦
◦
6
precipitated as a brown solid. This was filtered off and
dried in vacuo over silica gel. Yield 0.29 g (39.4%) (Found:
The reaction product was analysed by GC, by withdrawing small
aliquots from the reaction mixture after specific intervals of time.
C, 38.91; H, 2.86; N, 7.38. Calc. for C12
mol ): C, 39.13; H, 2.74; N 7.61%). IR (KBr, mmax/cm ): 1620
H
10KN
2
O
6
V (367.96 g
−
1
−1
To evaluate the effect of the oxidant, the amount of H
varied while keeping the other conditions constant.
2
2
O was
(
C=Nazomethine), 1224 (C–Oenolate), 1037 (N–N), 905, 935 (sym and
+
51
antisym VO
2
). V NMR (DMSO-d
6
, d/ppm): −533.9.
Oxidation of phenol. Phenol (4.7 g, 50 mmol) and 30%
V
V
[
V O
2
(Hsal-nah)] 7 and [V O
2
(Hsal-fah)] 8. 1 mmol of
aqueous H O (5.67 g, 50 mmol) were mixed in 5 ml of
2
2
◦
complex 3 or 6 was dissolved in water (40 ml) and treated drop-
wise with aqueous HClO (2 ml of 70% HClO diluted with 5 ml
acetonitrile and the reaction mixture was heated to 80 C in an
oil bath with stirring. After addition of complex 3 or 6 (0.025 g),
the stirring was continued. The reaction product was analysed
as described above.
4
4
of water) with stirring until most of the complex had precipitated
as a yellow solid. After 1 h of stirring, the solution was filtered,
the residue washed with water and dried in vacuo over silica
gel. Similar results were also obtained with HCl. Data for 7:
Results and discussion
−
1
Yield: 0.14 g (43.3%). IR (KBr, mmax/cm ): 3090 (NH), 1606
(
C=Nazomethine), 1220 (C–Oenolate), 1024 (N–N), 909, 940 (sym and
Synthesis, reactivity and solid state characteristics
+
antisym VO
2
). Data for 8: Yield: 0.15 g (48.1%). IR (KBr,
Scheme 2 provides an overview of the complexes discussed in this
−
1
m
1
max/cm ): 3108 (NH), 1618 (C=Nazomethine), 1221 (C–Oenolate),
paper. Reaction of [VO(acac)
ligand H sal-nah (I) or H sal-fah (II) in dry refluxing methanol
yielded the oxovanadium(IV) complex [VO(sal-nah)H O], 1,
or [VO(sal-fah)H O], 4. The ligands coordinate out of their
2
] with an equimolar amount of the
+
51
016 (N–N), 942, 966 (sym and antisym VO ). V NMR of the
51
2
2
2
crude sample (DMSO-d
6
, d/ppm): −551.0, −584.0. V NMR
, d/ppm): −538.7,
2
of 8 recrystallised from methanol (DMSO-d
550.5 and −583.5.
6
2
−
dianionic (ONO(2−)) enolate tautomeric form; cf. Scheme 1.
V
V
[V O(OMe)(MeOH)(sal-nah)] 9 and [V O(OMe)(MeOH)-
The complexes 1 and 4 exhibit effective magnetic moments of
(
sal-fah)] 10. Crystals of 9 and 10 suitable for an X-ray analysis
1.47 and 1.54 l
B
, respectively, at 298 K, which is subnormal
1
were grown from a dilute solution of 1 and 4 or 7 and 8
in methanol. The solutions were allowed to stand for slow
evaporation at ca. 10 C, whereupon dark brown crystals of
9
with respect to the expected value of 1.73 l
B
for a d (S =
1
2
) system. We studied the magnetic susceptibility of 1 as
◦
a function of temperature in the 298–78 K range; Fig. S1
(see ESI†). The magnetic moment is slightly dependent on
or 10 slowly separated out within 2–4 weeks.
temperature, the range being 1.47–1.40 l
plot [1/v vs. T] is a straight line with a Weiss constant of
11 indicating weak antiferromagnetic interaction. A possible
explanation for the antiferromagnetic behaviour is dimerisation
or oligomerisation) of the complex molecules via V=O · · · V
interaction.
B
. The Curie–Weiss
Catalytic reactions
m
−
Oxidative bromination of salicylaldehyde. Complexes 3 and
were used as catalysts to carry out oxidative bromination.
6
(
In a typical reaction, salicylaldehyde (0.244 g, 2 mmol) was
added to an aqueous solution (4 ml) of KBr (0.476 g, 4 mmol),
followed by addition of aqueous 30% H
in a 50 ml reaction flask. The catalyst (0.02 g) and 70% HClO
1 mmol) were added, and the reaction mixture was stirred at
room temperature. Three additional 1 mmol portions of 70%
HClO were further added to the reaction mixture in three
2
O
2
(1.42 g, 15 mmol)
4
(
4
equal portions in half hour intervals under continuous stirring.
After 4 h, the white product that had separated was filtered off,
washed with water, followed by diethyl ether, and dried. The
crude mass was dissolved in CH
was removed by filtration, and the solvent evaporated. A CH
2
Cl
2
; insoluble material, if any,
Cl
2
2
solution of this material was subjected to gas chromatography,
and the identity of the products confirmed by GC–MS. Table 2
gives an overview of the percentage conversion as a function
of varying amounts of HClO
kept constant.
4
, with all of the other parameters
Scheme 2
On aerial oxidation of [VO(sal-nah)H
2
O] in methanol, the
l-O], 2, was ob-
tained. The amount of solvent and duration of crystallisation
plays an important role, in that an excess of solvent and
prolonged crystallisation periods cause partial deoxygenation of
dioxovanadium(V) complex [{VO(sal-nah)}
2
Table 2 Effect of the amount of HClO
4
on the oxidative bromination
a
of salicylaldehyde
b
Complex
[HClO
4
]/mmol
Conversion (%)
2
, leading to the species [VO(OMe)(MeOH)(sal-nah)], 9. Simi-
3
2
3
4
5
2
3
4
5
42.1
46.8
51.5
51.0
38.6
49.2
50.8
50.0
larly, [VO(sal-fah)H l-O],
2
O] can be oxidised to [{VO(sal-fah)}
2
which can further be transformed into [VO(OMe)(MeOH)(sal-
fah)], 10. Eqns (1), (2) and (3) represent the overall synthetic
procedures.
6
(
(
1)
2)
a
2 2
Reaction conditions: Salicylaldehyde (2 mmol), H O (15 mmol),
b
complex (0.02 g, ca. 0.05 mmol) and KBr (4 mmol). Average of three
different reactions using fresh catalyst each time.
D a l t o n T r a n s . , 2 0 0 5 , 5 3 7 – 5 4 4
5 3 9

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V
[
{V O(sal-nah)}
2
l-O] + 4 MeOH
EPR studies
2
Complexes 1 and 4 exhibit an axial low temperature spectrum
typical of tetragonal-pyramidal oxovanadium(IV) complexes, i.e.
two sets of eight lines (coupling of the single electron with
the V spin 7/2 nucleus) corresponding to the parallel (z) and
perpendicular (xy) components of the g and hyperfine coupling
V
→
2 [V O(OMe)(MeOH)(sal-nah)] + H
2
O
(3)
9
5
1
A solution of potassium vanadate (which, at ambient pH, is
actually present as a mixture of vanadates), generated in situ
by dissolving V in aqueous KOH, reacts with the potassium
salts of I and II at pH ca. 7.5 to give the potassium salts of
the dioxovanadium(V) anions, [K(H O)][VO (sal-nah)], 3, and
K(H O)][VO (sal-fah)], 6; eqn. (4).
(
A) tensors. No rhombic distortion was observed. The g values
of 1 and 4 (gxy = 1.979, g = 1.948) and the hyperfine coupling
constants (Axy = 58.0, A = 166.0 × 10 cm ) are in accord with
the O N donor sets. The parallel hyperfine coupling constant A
is close to the value calculated from the partial contributions
of the equatorial ligand set (H
O, enolate(1−), phenolate(1−),
imine-N), A
(calcd.) = 162.2 × 10 cm .
2
O
5
z
−
4
−1
z
2
2
3
z
[
2
2
K[VO ] + K
[K(H
Aqueous solutions of 3 and 6 further react with HClO
yield the neutral complexes [VO (Hsal-nah)], 7, and [VO
3
2
L
2
V
→
2
O)][V O
2
L] (H
2
L = I: 3, H
2
L = II: 6)
(4)
to
−4
−1 34
z
4
2
2
(Hsal-
Structure description
fah)], 8, in which one of the nitrogens of the =N–N= group is
28
protonated (see also Plass et al. ); eqn. (5).
The molecular structures of the oxo-methoxo complexes 9 and
1
0 along with the numbering schemes are shown in Fig. 1. Table 3
contains selected bond lengths and angles. The coordination
geometry around vanadium can be described as tetragonal-
bipyramidal, with the doubly bonded oxygen and methanol in
the axial positions. The O=V–O(methanol) axis is almost linear;
(
5)
Slow crystallisation of 7 and 8 from excess methanol causes the
removal of the proton from the NH group and conversion to the
methoxo-oxovanadium(V) complexes 9 and 10; eqn. (6).
◦
◦
the angle at vanadium is 175.39(5) in 9 and 173.4(2) in 10.
Because of the trans influence of the oxo group, the distance
˚
to methanol, d(V–O5) (2.2514(10) in 9, 2.3081(5) A in 10) is
considerably elongated, making the methanol a weakly coordi-
nated ligand. Such elongation has previously been observed in
(
6)
23–29
other complexes with similar structures.
The phenolate and
The formation of the complexes 9 (L = sal-nah(2−)) and 10 (L =
sal-fah(2−)) from the corresponding neutral mono- or dinuclear
dioxovanadium(V) complexes in methanol is of interest in the
context of vanadium complexes used as oxo-transfer agents both
enolate oxygens, the methoxide and the imine nitrogen define the
˚
tetragonal plane, from which vanadium deviates by 0.2790 A
˚
in 9 and 0.2884 A in 10. The O–C, N–C and N–N distances
are consistent with the enolate form of the corresponding
functionality. Other bond lengths, and the bond angles, are
33
in catalytic and stoichiometric oxygenation reactions.
All of the complexes are soluble in methanol, ethanol, DMSO
and DMF; the anionic complexes 3 and 6 are additionally
soluble in water. The structures of the complexes as depicted
in Scheme 2 are based on the spectroscopic (IR, UV-Vis,
2
3–29,35–37
similar to those observed in comparable complexes.
2
Except for the non-coordinated N1, the atoms maintain sp
planarity. In 10, N1 is involved in intermolecular hydrogen
bonding to the proton of the coordinated methanol trans to the
1
13
51
EPR, H, C and V NMR) data, thermogravimetric patterns,
magnetic susceptibility measurements, elemental analyses, and
X-ray diffraction analyses in the case of 9 and 10.
˚
oxo group, d(N1 · · · O5) = 2.795 A (10). In addition, there are
close intermolecular O · · · H–C contacts between the enolate-O
˚
(
O2) and C10 of the sal moiety in 10, d(O2 · · · C10) = 2.459 A.
IR patterns of the complexes correspond to those observed
The methanol-OH in 9 forms a hydrogen bond to the pyridine-N
1
in other oxo-hydrazonatovanadium complexes; for a detailed
˚
(
N3) of an adjacent molecule, d(O5 · · · N3) = 2.792 A.
description see ESI† (IR spectra).
Thermal studies
The complexes [VO(sal-nah)H
weight between 100 and 160 C equivalent to one water
molecule, indicative of relatively weakly coordinated water.
2
O] and [VO(sal-fah)H
2
O] lose
◦
Further increase of the temperature causes weight loss between
◦
2
50 and 450 C in two overlapping steps, corresponding to the
loss of the organic ligand minus one oxygen. The end product
of the thermal decomposition is V . The dioxovanadium(V)
complexes [K(H O)][VO (sal-nah)] and [K(H O)][VO (sal-fah)]
2
O
5
2
2
2
◦
2
lose weight in the temperature range 150–220 C due to the loss
of one water, which we attribute to water coordinated to K .
+
The water-free species K[VO
two overlapping steps in the temperature range 280–440 C,
2
(sal-nah)] further decomposes in
◦
while decomposition of K[VO
overlapping steps between 250 and 650 C to form potassium
2
(sal-fah)] occurs in multiple
◦
metavanadate K[VO
corresponds to the loss of ligand minus one oxygen. The
decomposition patterns of [{VO(sal-nah)} l-O] and [{VO(sal-
l-O] could not be followed in detail due to multiple
3
]. The total weight loss in both complexes
2
fah)}
2
overlapping decomposition steps in the temperature range 200–
◦
5
00 C. The end product is close to V
2
O
5
◦
in either case.
Complex [VO
decomposes in three steps yielding V
2
(Hsal-nah)] is stable up to 275 C, and thereafter
◦
2
O
5
at 480 C (obs: 27.7, cal:
Fig. 1 ORTEP plot (at the 30% probability level) of [VO(OMe)-
MeOH)(sal-nah)] (9) (top) and [VO(OMe)(MeOH)(sal-fah)] (10) (bot-
tom).
2
8.2%). Similarly, [VO
2
(Hsal-fah)] decomposes in several steps
(
◦
between 190 and 500 C to give V
2
O (obs: 28.3, cal: 29.2%).
5
5
4 0
D a l t o n T r a n s . , 2 0 0 5 , 5 3 7 – 5 4 4

View Article Online
◦
Table 3 Selected bond lengths [ A˚ ] and bond angles [ ] for 9 and 10
9
10
9
10
V1–O1
V1–O2
V1–O3
V1–O4
V1–O5
V1–N2
N1–C1
N1–N2
N2–C7/6
O2–C1
1.5943(10)
1.9559(9)
1.8598(9)
1.7764(9)
2.2514(10)
2.1282(11)
1.3081(15)
1.3929(14)
1.2939(15)
1.3004(14)
1.593(2)
1.976(2)
1.851(2)
1.769(2)
2.295(2)
2.122(3)
1.304(4)
1.394(3)
1.281(4)
1.302(3)
O1–V1–O5
O4–V1–O3
O4–V1–O2
O3–V1–O2
O4–V1–N2
O3–V1–N2
O2–V1–N2
C1–N1–N2
C7/6–N2–N1
N2–C7/6–C8/7
N1–C1–O2
175.39(5)
105.53(4)
91.84(4)
151.36(4)
162.09(4)
83.66(4)
175.66(9)
102.92(9)
93.55(9)
152.61(9)
161.73(10)
83.66(9)
74.56(9)
108.7(2)
116.9(2)
123.4(3)
123.6(3)
74.14(4)
107.52(9)
116.50(10)
123.111(11)
123.92(10)
Solution studies
1
−541.7 and −550.1, corresponding to the dinuclear complex 2
and the oxo-methoxo complex [VO(OMe)(MeOD)(sal-nah)], 3.
H NMR studies. The relevant data are presented in Table
S1 (ESI†). Both of the free ligands give rise to a signal at d =
The complex [{VO(sal-fah)}
2
l-O] (5) displays three signals at
d = −538.7 (5), −550.5 (10) and −583.5.
1
1.20 due to the –NH proton, indicating their existence in the
ketonic form. The absence of this signal in the complexes is in
accord with enolisation and subsequent replacement of H by the
metal ion. Similarly, the absence of the signal for the phenolic
OH (ca. 12.2 ppm in the ligands) indicates coordination of phe-
nolate oxygen. A significant downfield shift of the azomethine
Electronic absorption spectra. The data for the ligands and
complexes are listed in Table S3 (ESI†). The UV spectrum of
H sal-nah (I) exhibits four absorption bands at 214, 288.5, 297.5
2
and 328 nm, while H sal-fah (II) shows bands at 211.5, 288.5, 299
2
and 328 nm. The most probable assignments of the first and last
(
–CH=N–) proton signal in the complexes with respect to the
bands are ␾ → ␾* and n → p* transitions, respectively, while the
two remaining bands are p → p* transitions. A weak shoulder
associated with the first band is due to hydrogen bonding. In
all of the complexes, this shoulder disappears, and the split p
→ p* band merges into one, while the other bands invariably
are present. The ␾ → ␾* band has no definite trend, while the
other two bands are generally shifted towards lower wavelengths
in comparison to the uncoordinated ligands. In addition, all of
the complexes display a medium intensity band at 396–409 nm,
which is assigned to a ligand-to-metal charge transfer (LMCT)
from the phenolic oxygen to an empty d-orbital of the vanadium
ion. The oxovanadium(IV) complexes 1 and 4 exhibit one broad
but well-developed band at ca. 405 nm. This is similar to the
absorption reported for [VO(hydrox-atp)H O] (H hydrox-atp =
corresponding free ligands confirms the coordination of the
1
azomethine nitrogen. The H NMR data are thus consistent
with the ONO dibasic tridentate binding mode of the ligands.
The aromatic protons of ligands and complexes appear in the
expected region. In the case of the dinuclear complexes 2 and
5
, speciation appears to occur in CD OD, as two sets of signals
3
were observed for several of the protons. One set of signals
corresponds to 2 and 5, while the other, minor set (not shown
1
3
51
in Table S1), belongs to a second species (see also C and
V
NMR). Similarly, solutions of [VO (Hsal-fah)] (8) exhibit two
2
signals for the azomethine proton at d = 8.92 and 8.84 in the
ratio of 6.5 : 1 (freshly prepared) or 1 : 1.8 (recrystallised from
methanol), indicating slow conversion of 8 to 10.
2
2
Schiff base derived from 2-hydroxy-1-naphthaldehyde and 2-
1
3
13
C NMR studies. We have also recorded C NMR of
41
aminothiophenol) and is considered to be a d → d transition.
ligand II and its complexes to provide diagnostic tools for
the elucidation of the structures. Assignments of the peaks
are similar to those reported earlier, and are based on the
chemical shift and intensity patterns. Table S2 (ESI†) presents
the data along with the coordination induced shifts Dd =
d(complex) − d(ligand) of the signals for carbon atoms in the
vicinity of the coordinating functions. A large Dd observed for
carbon atoms in the vicinity of the phenolate (C3), enolate
Reaction of 3 and 6 with HClO4. Drop-wise addition of
HClO (dissolved in the minimum amount of methanol) to
4
[K(H O)][VO (sal-fah)] (6) in methanol (cf. Fig. 2) causes
2
2
darkening of the solution with an increase in intensity of the UV
bands, and a decrease in the intensity and shift of the 396 nm
band to 406.5 nm. The UV bands at 282.5, 301 and 324 nm
(
C8) and azomethine (C1) groups suggests their involvement
in coordination. Two new signals in sample 8 recrystallised
from methanol appear at d = 48.5 and 74.3 ppm. These
signals correspond to methanol and methoxy carbon atoms,
respectively, testifying partial conversion of [VO (Hsal-fah)] (8)
2
to [VO(OMe)(MeOH)(sal-fah)] (10).
5
1
51
V NMR studies. d( V) values of the complexes are con-
tained in the Experimental section. The resonances are some-
5
1
what broadened due to nuclear quadruple relaxation (I( V) =
7
/2)); line widths at half height are typically around 200 Hz. The
dioxovanadium(V) complexes 3 and 6 show one strong resonance
at d = −532.4 (3) and at d = −533.9 (6), which is close to the
3
8
one reported for [K(H
2
O)][VO
2
(sal-inh)] (−532.0 ppm) and
the expected shift for dioxovanadium(V) complexes containing
39,40
a mixed O/N donor set.
at d = −542.4 and −584.6 for [{VO(sal-nah)}
The appearance of two signals
l-O] (2) in a
ratio of 4 : 1 indicates the presence of two species. The major
signal at −542.4 ppm is assigned to 2. The nature of the
minor, high-field signal remains elusive. A possible candidate
is [VO(OH)(sal-nah)] formed by reaction of 2 with the water
2
2 2
Fig. 2 Titration of [K(H O)][VO (sal-fah)] 6 with a saturated solution
of HClO in MeOH; the spectra were recorded after addition of 2-drop
4
4
present in DMSO. In CD
3
OD, the two signals appear at d =
portions of MeOH–HClO
4
to 10 ml of ca. 10− M solution of 6 in MeOH.
D a l t o n T r a n s . , 2 0 0 5 , 5 3 7 – 5 4 4
5 4 1

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finally merge into two bands at 300 and 318 plus a shoulder
at 290 nm. Corresponding results have been obtained with a
saturated solution of HCl gas in methanol. A reduction in the
intensity of the 395.5 nm band and an increase of the 320 and
2
31 nm bands along with their shift have also been observed for
K(H O)][VO (sal-nah)] (3): The 395.5 nm band shifts to 405 nm
and broadens, while the 320 and 231 nm bands shift to 326 and
[
2
2
2
53 nm.
We interpret these result in terms of the formation of oxohy-
+
droxo complexes of composition [VO(OH)(HL)] via [VO
2
(HL)]
(
eqn. (7)),
+
+
H
H
−
+
[VO
2
(L)] → [VO (HL)] → [VO(OH)(HL)]
2
(7)
with one of the =N–N= nitrogens being the site of protonation;
see 7 and 8 in Scheme 2. This assumption is based on the
similarity between the electronic spectra of freshly prepared
methanolic solutions of 7 and 8, and the spectra obtained for
Fig. 3 Titration of [K(H
2
O)][VO
2
2 2
(sal-nah)] 3 with 30% H O ; the
3
and 6 after addition of the first portion of HClO
Protonation of the hydrazone nitrogen has been reported, e.g. for
the structurally characterised complex [VO(Hsal-bhz)] (H sal-
4
or HCl.
spectra were recorded after the successive addition of 2-drop portions
−4
of H
2
O
2
to 10 ml of a ca. 0.5 × 10 M solution of 3 in MeOH.
2
bhz derives from salicylaldehyde and benzoylhydrazide), which
forms on treatment of the corresponding anionic dioxo complex
Catalytic activity studies
28
Oxidative bromination of salicylaldehyde. As pointed out
in the Introduction, vanadate-dependent bromoperoxidases
catalyse the bromination of organic substrates in the presence
with HCl.
Oxo-hydroxovanadium complexes have previously been gen-
erated on acidification of K[VO (sal-inh)H O] (H
sal-inh =
hydrazone derived from salicylaldehyde and isonicotinic acid
hydrazide) and [K(H O) ][VO (Clsal-sbdt)] (H
Clsal-sbdt =
ligand derived from 5-chlorosalicylaldehyde and S-benzyl-
2
2
2
of H
2
2
O and bromide, and the ability of vanadium compounds
to mimic this reaction has been demonstrated. Examples are
the bromination of trimethoxybenzene by the VO
2
2
2
2
+
47
2
+
ion, and
2
0
+
the bromination of phenol red by [VO(O )H O] and related
2
2
dithiocarbazate). An oxo-hydroxo complex, [VO(OH)(LH)]
(
48
ꢀ
species. In the following section we show, for complexes 3 and
, that they catalyse oxidative bromination of salicylaldehyde,
where LH = N-{(o-hydroxyphenyl)methyl}-N -bis(2-hydroxy-
6
ethyl)ethylenediamine) in solution has also been reported to
form from a dinuclear dioxovanadium(V) precursor in a simi-
benzene and phenol, using aqueous H
2
O /KBr in the presence
2
42
V
43
IV
of HClO ; cf. eqn. (8).
4
lar manner. [V O(OH)(8-oxyquinolinate)
Tp(H
2
] and [V O(OH)-
44
2
O)] [Tp = tris(3,5-diisopropyl-1-pyrazolyl)borate(1−)]
have been characterised in the solid state. On addition of
+
a methanolic solution of KOH to [VO(OH)(Hsal-nah)] or
VO(OH)(Hsal-fah)] , the solution acquired the original spectra
+
[
of 3 or 6; the reaction is thus reversible. This reversibility is an im-
portant observation in the context of the active site structure and
the catalytic activity of vanadate-dependent haloperoxidases, for
which an apical hydroxo ligand at the vanadium centre has been
(
8)
Oxidative bromination with H
2
O
2
/KBr catalysed by V
2
O
5
and oxovanadium(V) complexes1 has shown that, during this
9b
45
derived from X-ray diffraction evidence, and also suggested on
process, vanadium reacts with one or two equivalents of H
2
O
2
,
7,8
+
−
the basis of kinetic investigations.
forming monoperoxo {VO(O
2
) }or bis(peroxo) {VO(O
2
)
2
}
species, which ultimately oxidise bromide, possibly via a hy-
−
droperoxo intermediate. The oxidised bromine species (Br
and/or HOBr) then brominates the substrate.
2
, Br
3
4,49
Reaction of 3 and 6 with H
complexes in methanol by treatment of [VO
could be established by electronic absorption and V NMR
2
O
2
.
The formation of peroxo
−
2
L] with H
2
O
2
In order to achieve optimum conditions, both catalysts (3
and 6) were studied in detail by varying parameters such as
5
1
−
4
spectroscopies: Treatment of 10 ml of a ca. 0.5 × 10 solution
the volume of HClO
of catalyst and of H
conversion of salicylaldehyde (ca. 51%) was achieved with
4 mmol of HClO , 2 mmol of substrate, 15 mmol of H O ,
4
(see Experimental section), the amount
of [K(H O)][VO (sal-nah)] (3) with 2-drop portions of 30% H
2
2
2
O
2
2
O
2
at ambient temperature. Maximum
dissolved in methanol resulted in the UV-Vis spectra recorded
−
in Fig. 3. The band for [VO
2
(sal-nah)] at 397.5 nm shifts to
4
2
2
4
04.5 nm along with a decrease in intensity. The two bands
20 mg (ca. 0.05 mmol) of catalyst and 0.476 g (4 mmol) of KBr.
GC and GC-MS analysis of the crude products obtained under
optimised conditions gave 85.8% 5-bromosalicylaldehyde, 9.0%
3,5-dibromosalicylaldehyde and 5.2% of other, unidentified,
products. The turn over rate per hour (TOF h = moles of
substance converted per mole of catalyst per hour) is about 5,
which compares to what has been reported for the complex
appearing at 318.5 and 282.5 nm slowly merge into one and
appear as a shoulder of a strong absorption at ca. 230 nm (not
shown). The amount of peroxo complex formed depends upon
the amount of H
for 6, where the 394.5 nm band shifts to 400.5 nm, and the 319.0,
−
1
2
O
2
added. Exactly the same pattern is observed
5
1
2
99.5 and 286.5 nm bands merge into a shoulder. The V NMR
spectrum of [K(H O)][VO (sal-fah)] (6) in DMSO/H O plus a
small amount of HClO and H , i.e. under conditions where
2
2
2
[K(H
pyridoxal and isonicotinic acid hydrazide) (46% conversion).
Examples for turn over rates previously reported for model
2
O)
3
][VO (pydx-inh)] (pydx-inh is the hydrozone from
2
20c
4
2
O
2
peroxo complexes should form and be fairly stable, resulted in
the appearance of an additional peak at −575 along with the
original signal at −538 ppm. The signal at −575 ppm should
reflect the presence of an oxomonoperoxo species. The peroxo
ligand usually gives rise to an upfield shift of 40–60 ppm with
2
+
systems are 0.15 (bromination of trimethoxybenzene by VO at
4
7
−1
pH 3) and 32 h (with polymeric oxovanadium(IV) complexes
ꢀ
of Schiff bases derived from 5,5 -methylenebis(salicylaldehyde)
and diamines). In the absence of the catalyst, the reaction
mixture did not produce any brominated product demonstrating
5
0
46
respect to the parent dioxovanadium(V) complex.
5
4 2
D a l t o n T r a n s . , 2 0 0 5 , 5 3 7 – 5 4 4

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the catalytic role of the dioxovanadium(V) complexes. Similarly,
acid (here HClO ) was found to be essential to carry out catalytic
bromination. The complexes slowly decompose during the
reaction when larger amounts of HClO (5 mmol or more) were
employed. This decomposition could, however, be minimised
if HClO was added successively in about 4–5 equal portions.
Non-oxidising acids, e.g. H
the conversion of salicylaldehyde.
4
4
(9)
4
i.e. the expected products according to the directing effect of
the phenolic OH group. In order to achieve suitable reaction
conditions for the maximum oxidation of phenol, we studied
the following parameters in detail: effect of amount of catalyst,
2
SO
4
, give rise to similar results in
+
The formation of oxohydroxo species such as [VO(OH)HL]
−
is likely to occur upon acidification of [VO
2
L] (see above), and
effect of H
phenol) and effect of phenol concentration (moles of phenol per
mole of H ). Under optimised reaction conditions (50 mmol
substrate, 50 mmol 30% aqueous H , 0.07 mmol catalyst in
CN at 80 C) and 15 h of contact time, 20.1%
2
O
2
concentration (moles of H
2
O
2
per mole of
−
the corresponding oxoperoxo species [VO(O
should form upon treatment with H
2
)L] /[VO(HO
in acidic methanol, thus
2
)L]
2
O
2
2
O
2
providing the active species necessary for the catalytic reaction.
Activation of peroxide by coordination to vanadium thus may
be succeeded by formation of an (intermediate) hydroperoxo
2
O
2
◦
52
5
ml of CH
3
conversion of phenol to 11.2% catechol and 8.9% hydroquinone
is observed with 3 as the catalyst, while the conversion rate is
2.6% (producing 18.0% catechol and 14.6% hydroquinone) in
the case of 6. The corresponding turn over rates per hour were 3
and 5, respectively, for 5 h of contact time, a time interval where
0% of the overall conversion was achieved. Only minor changes
4
complex, which allows nucleophilic attack of the bromide,
followed by release of hypobromous acid, which then brominates
organic substrates. A hypobromite intermediate may also be
involved, as evidenced by electron spray ionization mass spectra
3
51
on model systems. A catalytic cycle is proposed in Scheme 3.
9
in either conversion rate or product composition were observed
for reaction times beyond 15 h. In the absence of catalyst, both
of these oxidation reactions did not proceed.
Conclusion
IV
Vanadium complexes of general composition [V O(H
2
O)L],
V
V
V
V
[
(
{V OL}
2
l-O], [K(H
2
O)][V O
2
L], [V O HL] and [V O-
2
OMe)(MeOH)L] have been synthesised and characterised, and
the molecular structures of [VO(OMe)(MeOH)L] revealed by
single crystal X-ray diffraction analyses. L is a dianionic ONO
hydrazone ligand derived from salicylaldehyde (sal) and the
hydrazides of nicotinic acid (nah) and 2-furanecarbonic acid
(
fah), i.e. of common constituents of physiologically relevant
compounds and metabolites thereof. The general overall co-
Scheme 3
ordination environment is represented by an O N donor set:
4
the ONO ligand, the doubly bonded oxo group and an aqua,
methoxo or bridging oxo ligand The complexes thus model
structural features of the vanadate-dependent haloperoxidases
from marine algae (such as Ascophyllum nodosum and Corallina
officinalis) and the fungus Curvularia inaequalis. The model
character extends to the simulation of the functions of the
enzymes in that the complexes [K(H O)][VO L] catalyse the
Oxidation of benzene and phenol. Oxidation of benzene to
phenol was also catalysed by 3 and 6. The effect of the amount
of H
2
2
O on the oxidation of benzene to phenol is shown in
Table 4. At a molar ratio benzene : H
2
O
2
= 1 : 1, 24.2% benzene
is converted to phenol within 15 h in the presence of 3. Lowering
the ratio to 1 : 0.5 decreases the conversion considerably (8.6%),
while increasing the ratio to 1 : 2 increases this conversion to
2
2
oxidative bromination of salicylaldehyde by H O to afford 5-
2
2
bromo and 3,5-dibromosalicylaldehyde, and the oxidation of
(oxo transfer to) phenol to yield catechol and p-hydroquinone.
Turn over rate in strongly acidic solution is 5 mol of brominated
2
8.0%. However, the percentage efficiency of H
is lower than for a 1 : 1 molar ratio. Similarly, complex 6 gave
8.3% benzene conversion at a 1 : 1 (benzene to oxidant) molar
2
O for a 1 : 2
2
−
1
−1
2
compound (mol of vanadium complex) h ; the complex
is thus substantially less active than the enzyme which, at
ratio and 34.5% at a 1 : 2 molar ratio. No further conversion
was noticed after 15 h of contact time. It is interesting to note
that, under the reaction conditions applied here, no oxidation
beyond the formation of phenol was observed.
Complexes 3 and 6 are also able to catalyse the oxidation
of phenol to give two major products, viz. catechol and p-
hydroquinone; eqn. (9),
5
pH 6.5, exhibits ca.10 mol of brominated product (mol of
−
1
−1 47
enzyme) h . The formation of intermediates proposed for
+
the catalytic cycle, i.e. oxo-hydroxo complexes [VO(OH)HL]
and oxo-peroxo complexes [VO(O )L] , has been made plausible
−
2
on the basis of characteristics in the UV-Vis spectra of the
−
anionic dioxovanadium precursor compounds [VO L] treated
2
with acid and hydrogen peroxide, respectively.
a
2 2
Table 4 Effect of H O on percentage conversion of benzene
Acknowledgements
%
Conversion (%)
We acknowledge performance of the micro-analyses by the
Regional Sophisticated Instrumental Centre, CDRI, Lucknow
and GC analysis by the Institute Instrumentation Centre of IIT
Roorkee. Financial assistance from the Department of Science
and Technology, Government of India (Grant No. SP/S1/F-
Complex
Amount of oxidant/g (mmol)
2.83 (25)
3 h
6 h
15 h
3
3.0
5.9
8.6
24.2
28.0
10.3
28.3
34.5
5
.67 (50)
10.4
12.7
7.9
12.5
17.3
17.9
22.3
9.5
15.3
21.0
11.34 (100)
2.83 (25)
6
0
7/2001), New Delhi, and the Deutsche Forschungsgemein-
.67 (50)
schaft (grant RE 431/20-1) are gratefully acknowledged.
a
Reaction conditions: 50 mmol benzene, 25 mg (ca. 0.07 mmol) catalyst,
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