Mimicking vanadium haloperoxidases: Vanadium(iii)-carboxylic acid complexes and their application in H2O2 detection

Article abstract of DOI:10.1039/c5nj01905b

Vanadium(iii) complexes [V(2,6-pdc)₂(H₂O)₂]·2H₂O (1) and V(2,6-pdc)(htba)(H₂O)₂ (2), (2,6-pdc = 2,6-pyridinedicarboxylic acid, htba = 2-acetoxy-4-trifluoromethylbenzoic acid) have been synthesized by the reaction of V₂(SO₄)₃ with 2,6-pdc (for 1) or 2,6-pdc and htba (for 2) under hydrothermal conditions at 120 °C for 36 hours. Because the vanadium(iii) was easily oxidized into higher oxidation states, we included the reducing agent vitamin C to protect the vanadium(iii) center. The complexes were characterized by elemental analysis, IR and UV-vis spectroscopy, and single-crystal X-ray diffraction. Structural analysis revealed that the central metal V atoms in the complexes 1 and 2 were seven-coordinate, forming pentagonal bipyramid geometries. The complexes catalyzed the bromination of the organic substrate phenol red in the presence of H₂O₂, bromide and buffer. Compared with vanadium complexes having other oxidation states, the vanadium(iii) complexes had better catalytic activity (the maximum reaction rate constant was 2.424 × 10²k(mol L^-1)^-2 s^-1). The mimicking vanadium haloperoxidases also overcame some serious disadvantages of natural enzymes. Therefore, the reaction system described herein can be considered as an effective model for hydrogen peroxide determination.

Full text of DOI:10.1039/c5nj01905b

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Mimicking vanadium haloperoxidases:
vanadium(III)–carboxylic acid complexes and their
Cite this:DOI: 10.1039/c5nj01905b
application in H O detection†
2
2
a
a
a
a
a
Xiao Dong Feng, Xiao Xi Zhang, Zhi Nan Wang, Jian Song, Yong Heng Xing*
b
and Feng Ying Bai*
Vanadium(III) complexes [V(2,6-pdc)
dicarboxylic acid, htba = 2-acetoxy-4-trifluoromethylbenzoic acid) have been synthesized by the reaction
of V (SO with 2,6-pdc (for 1) or 2,6-pdc and htba (for 2) under hydrothermal conditions at 120 1C for
6 hours. Because the vanadium(III) was easily oxidized into higher oxidation states, we included the
2
(H
2
O)
2
]Á2H
2 2 2
O (1) and V(2,6-pdc)(htba)(H O) (2), (2,6-pdc = 2,6-pyridine-
2
4 3
)
3
reducing agent vitamin C to protect the vanadium(III) center. The complexes were characterized by
elemental analysis, IR and UV-vis spectroscopy, and single-crystal X-ray diffraction. Structural analysis
revealed that the central metal V atoms in the complexes 1 and 2 were seven-coordinate, forming
pentagonal bipyramid geometries. The complexes catalyzed the bromination of the organic substrate
Received (in Montpellier, France)
2
1st July 2015,
2 2
phenol red in the presence of H O , bromide and buffer. Compared with vanadium complexes having
Accepted 10th November 2015
other oxidation states, the vanadium(III) complexes had better catalytic activity (the maximum reaction
2
À1 À2 À1
rate constant was 2.424 Â 10 k(mol L
)
s ). The mimicking vanadium haloperoxidases also
DOI: 10.1039/c5nj01905b
overcame some serious disadvantages of natural enzymes. Therefore, the reaction system described
herein can be considered as an effective model for hydrogen peroxide determination.
www.rsc.org/njc
1
5–20
chemiluminescence, etc.
Titanium-based assays (Ti-PAPS
Introduction
reagents) were developed in the 1980s for spectrophotometric
9
Hydrogen peroxide is an integral part of atmospheric chemistry detection of H
2 2
O . The Fox assay was developed in the 1990s
and biological systems. In the atmosphere, it is an oxidant that is based on ferrous ion oxidation in the presence of the ferric ion
ꢀ
10
produced from the combination of hydroperoxyl radicals (HO ) indicator xylenol orange under acidic conditions. Recently,
2
1
and their hydrated form. Hydrogen peroxide is exceptionally Luo and co-workers developed a detection method based on the
soluble in water and it is thought to be the most efficient oxidant oxidation of methyl orange using an iron-catalyzed Fenton
1
2
in the formation of H
2
SO
4
from dissolved SO
2
. This implies reaction system under acidic conditions.
Other methods, using fluorescent probes, have recently been
rainwater. Furthermore, hydrogen peroxide is of great practical developed to monitor hydrogen peroxide production in living
that hydrogen peroxide could have some role in the acidity of
2
1
3
importance in the food and pharmaceutical industries and cells. Chemiluminescence methods have also been developed
clinical and other industrial fields. Hence, there is an urgent recently. These methods are highly sensitive, but they are
need to establish a convenient, effective and rapid method for limited by the complicated apparatus setup and sensor prepara-
3
–6
14
the detection of H O .
There are numerous detection methods for H
of which the colorimetric method is one of the most
tion, and interference from metal ions. Based on the points
above, we developed a colorimetric method to detect H
Vanadium haloperoxidases (V-HPOs) which are found in
2
2
7
–14
2
O
2
,
2 2
O .
popular. Others include HPLC, electrochemistry detection, marine algae are able to accelerate the oxidative halogenation
of organic compounds in the presence of hydrogen peroxide,
2
1–23
a
organic hydroperoxides or molecular oxygen.
Unfortunately,
College of Chemistry and Chemical Engineering, Liaoning Normal University,
Huanghe Road 850#, Dalian City, 116029, P. R. China.
natural enzymes are proteins and inherently bear some serious
disadvantages, such as easy denaturation by environmental
changes, digestion by proteases, as well as time-consuming and
expensive preparation and purification. Therefore, much effort
has been put into developing stable artificial enzyme mimics of
E-mail: xingyongheng2000@163.com; Tel: +86-411-82156987
b
College of Life Science, Liaoning Normal University, Dalian 11602, P. R. China
Electronic supplementary information (ESI) available: Tables of atomic coordi-
†
nates, isotropic thermal parameters, and complete bond distances. CCDC
056936 (1) and 1056937 (2). For ESI and crystallographic data in CIF or other
1
24–28
electronic format see DOI: 10.1039/c5nj01905b
vanadium complexes as functional models for V-HPOs.
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a
Vanadium complexes are able to mimic a reaction in which Table 1 Crystallographic data and structure refinement for complexes 1–2
vanadium catalyzes the bromination of organic substrates in
Complexes
Formula
M (g mol
Crystal system
1
2
2
6–31
the presence of H O and bromide.
Through experimental
results, it is found that the oxidation reaction catalyzed by the
vanadium complexes is H O concentration-dependent, and
2
2
C
14
H
15
N
2
O
12
V
C
15 9 3
H11NO F V
À1
)
454.22
Triclinic
P1
%
8.540(2)
9.813(3)
11.707(3)
82.404(4)
73.547(4)
68.028(4)
872.2(4)
2
457.19
2
2
Monoclinic
P2(1)/c
15.3219(17)
17.734(2)
6.6947(8)
90
90.641(2)
90
1818.9(4)
4
the absorbance at 592 nm along with the formation of bromo- Space group
a (Å)
phenol blue presents a linear dependence on the concentration
b (Å)
2 2 2 2
of H O . The linear dependence of absorbance on H O concen-
c (Å)
a (1)
b (1)
2 2
tration can be used in the detection and quantification of H O .
In order to study the functional properties of vanadium(III)
complexes, here we synthesized two vanadium(III) complexes,
V(2,6-pdc) (H O) Á2H O (1) and V(2,6-pdc)(htba)(H O) (2),
g (1)3
V (Å )
Z
D
2
2
2
2
2
2
À3
calc (Mg m
)
1.730
1.670
(2,6-pdc = 2,6-pyridinedicarboxylic acid, htba = 2-acetoxy-4-
Crystal size (mm)
F(000)
m (Mo-Ka)/mm
y (1)
Reflections collected
Independent
reflections
0.12 Â 0.08 Â 0.05 mm 0.32 Â 0.24 Â 0.09 mm
trifluoromethylbenzoic acid) and studied their bromination
reaction activity and their ability to detect hydrogen peroxide.
464
0.642
920
0.626
1.76 to 28.44
11358
4462
À1
1.81 to 25.95
4712
3313
Experimental
Materials and methods
(
I 4 2s(I))
Parameters
262
312
À3
D(r) (e Å
)
0.690 and À0.705
0.511 and À0.467
Elemental analyses for C, H, and N were carried out on a PerkinEl-
mer 240C automatic analyzer. The infrared spectra were recorded on
Goodness-of-fit
1.044
1.037
a
b
b
R
1
0.0555(0.0835)b
0.0387(0.0567)
0.0975(0.1066)
a
b
a JASCO FT/IR-480 spectrometer with pressed KBr pellets in the wR
2
0.1371(0.1534)
À1
P
P
P
P
range 200–4000 cm . UV-vis spectra were recorded on a JASCO
a
2
2 2
2 2 1/2
R = ||F
o
o
| À |F
c
||/ |F
o
|, wR
2
= [ [w(|F
o
| À |F
c
| ) ]/ [w(|F
o
| ) ]]
;
b
V-570 spectrometer (200–1500 nm, using solid samples). The X-ray [|F | 4 4a(|F |)]. Based on all data.
powder diffraction data were collected on a Bruker AXS D8 Advance
o
diffractometer using Cu-Ka radiation (l = 1.5418 Å) in the 2y range
graphite-monochromated Mo-Ka radiation (l = 0.7107 Å) and
a o scan mode. All measured independent reflections (I 4 2s(I))
were used in the structural analysis, and semi-empirical absorp-
À1
of 5–601 with a step size of 0.021 and a scanning rate of 31 min
.
Synthesis of the complexes
3
2
tion corrections were applied using the SADABS program. The
3
3
[
V(2,6-pdc)
,6-pdc (0.0334 g, 0.2 mmol) and vitamin C (0.0180 g, 0.1 mmol)
were mixed and stirred for 3 h in a solution of H O (15 mL) at
room temperature, forming a kind of yellow turbid solution
pH = 6.5). Then, the mixture was sealed into a 23 mL Teflon-
2
(H
2
O)
2
]Á2H
2 2 4 3
O (1). V (SO ) (0.0390 g, 0.1 mmol),
structures were solved by the direct method using SHELXL-97.
2
The non-hydrogen atoms were refined with anisotropic thermal
parameters. Hydrogen atoms of the organic frameworks were
fixed at calculated positions with isotropic thermal parameters
and refined using a riding model. Crystal data and structure
refinements are shown in Table 1. The selected bond lengths
and bond angles are listed in Table S1 (ESI†). Hydrogen bonds
of the complexes 1–2 are given in Table S2 (ESI†).
2
(
lined stainless steel autoclave and heated at 120 1C. After 36 h,
it was cooled to room temperature and yellow crystals suitable
for X-ray diffraction were obtained. Yield (based on V): 0.0346 g,
7
5.14%. Anal. calc. (%). For C14
N, 12.31. Found: C, 36.91; H, 3.36; N, 12.30.
V(2,6-pdc)(htba)(H O) (2). The synthesis method of complex
was similar to that for complex 1. V (SO (0.0390 g, 0.1 mmol),
15 2
H N O12V: C, 36.94; H, 3.33;
Measurement of bromination activity in solution
2
2
Each vanadium complex was dissolved in a mixed solution of
2
2
4 3
)
2
5 mL H
initiated by the mixing of phenol red solution (10 mol L ),
buffer solution of NaH PO –Na HPO (pH 5.8), KBr
2 2
O–DMF (DMF: 2 mL; H O: 23 mL). The reactions were
htba (0.0206 g, 0.1 mmol), 2,6-pdc (0.0167 g, 0.1 mmol) and
vitamin C (0.0180 g, 0.1 mmol) were mixed and stirred for 2 h in
À4
À1
=
2
4
2
4
a solution of H O (15 mL) at room temperature, forming a yellow
À1
À1
2
(0.4 mol L ), vanadium complex (0.1 mmol L ) and H O2
2
turbid solution (pH = 6.5). After 1 day of heating at 120 1C, the
mixture was cooled to room temperature and yellow crystals
of 2 were obtained. Yield (based on V): 0.0368 g, 80.49%.
(
30%), and the bromination reaction activity tests were carried
out at the constant temperature of 30 Æ 0.5 1C. Vanadium
complexes at five different concentrations (measured 0.5 mL,
Anal. calc. (%). For C15
H
9
NO
9
F
3
V: C, 39.37; H, 1.97; N, 3.06.
À1
1
mL, 1.5 mL, 2 mL, 2.5 mL respectively from 0.1 mmol L
Found: C, 39.38; H, 1.99; N, 3.05.
vanadium complex) were placed in five cuvettes, and then put
into the constant temperature water bath. The spectral changes
were recorded using a UV1000 spectrophotometer at 592 nm
X-ray single crystal structural determinations
Suitable single crystals of the two complexes were mounted and 5 min intervals, and the resulting data were collected
on glass fibers for X-ray measurement. Reflection data were during the reaction. The bromination reaction rate constants
collected at room temperature with a Bruker AXS SMART APEX of the vanadium complexes could be obtained according to the
3
0
II CCD diffractometer (Bruker AXS, Karlsruhe, Germany) with literature method.
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Results and discussion
UV-vis spectral analysis
Synthesis discussion
The UV-vis spectra of complexes 1 and 2 (Fig. S3 and S4, ESI†)
were recorded for solid state samples, and the characteristic
UV-vis bands are listed in Table S4 (ESI†). The complexes have
similar absorption patterns. Bands at 274 and 324 nm for 1 and
Two vanadium(III) complexes have been successfully synthe-
sized by a hydrothermal reaction at 120 1C (Scheme 1). Because
the starting material V (SO ) (III) is not soluble in common
2 4 3
274 and 320 nm for 2 are attributed to the p–p* transition of the
solvents (methanol, ethanol, and water), instead forming a
kind of yellow turbid solution in distilled water, we used a
hydrothermal reaction at 120 1C to synthesize the complexes.
In the synthetic process for the complexes, in order to
prevent the metal vanadium(III) center from being oxidized into
vanadium(IV) or a higher state, we tried various reducing agents,
such as KI, hydrazine hydrochloride, hydrazine hydrate and
vitamin C. Finally, we found that vitamin C had the best effect
for the synthesis, providing an optimum pH (6.5) and reduction
effect. Meanwhile, the temperature of the reaction could not be
too high, or the vanadium(III) atom could be oxidized into
higher states. In addition, we found that the ethanoyl group of
the htba was removed because of the acidic conditions and hot-
water backflow.
ligands. The bands at 386 nm for 1 and 378 nm for 2 are assigned
to the LMCT (ligand to metal charge transfer) transition. The
broad peak at 710 nm for 1 and 825 nm for 2 can be caused by the
d - d* transition of the central metal vanadium atom.
Structural description of complexes 1–2
[
V(2,6-pdc) (H O) ]Á2H O (1). Structural analysis shows that
2
2
2
2
complex 1 is crystallized in the triclinic system with P1% space
group. The molecular structure of 1 consists of a vanadium
atom, two 2,6-pdc ligands, two coordinated water molecules
and two free water molecules (Fig. 1). The oxidation state of
vanadium is +3. The vanadium atom displays a pentagonal
bipyramid geometry; it is coordinated by one nitrogen atom
(N1) from a 2,6-pdc ligand, four oxygen atoms (O1, O3, O5, O6)
IR spectral analysis
from the 2,6-pdc ligands and two oxygen atoms (O9, O10) from
the coordinated water molecules. The two 2,6-pdc ligands adopt
tridentate chelating coordination and bidentate coordination
modes respectively, with a V–N bond length of 2.123(3) Å.
The coordinated water molecules adopt terminal monodentate
coordination, and the lengths of the V–O bonds are in the range
The IR spectral data of the complexes 1–2 (Fig. S1 and S2, ESI†)
are listed in Table S3 (ESI†). It is clear that broad absorption
bands appearing at 3417 and 3413 cm indicate the presence
À1
of water molecules. Absorption occurring at 3274, 3083, and
À1
2
973 cm should be assigned to the stretching vibrations of
2.056(3)–2.139(3) Å. The angles of the N–V–O bonds are in
CQC–H from the pyrazolyl ring and the benzene ring. The
bands at 1623 for 1 and 1672 cm for 2 are attributed to the
asymmetrical stretching vibration and symmetrical stretching
vibration of CQO bonds, respectively. For complex 1, peaks
appear at 1433, 1395, 1209, 1182 and 1076 cm because of the
stretching vibrations of the pyrazolyl ring. Equivalent peaks are
at 1472, 1347, 1224, 1125 and 1060 cm for complex 2. The
bands at 934 and 875 cm for 1, and 925 and 878 cm for 2
are characteristic of the stretching vibrations of QC–CH. The
À1
the range 72.99(12)–151.05(12)1. The angle of the O9–V–O10
bonds is 177.10(11)1.
In addition, there are three kinds of intermolecular hydrogen
bonds in the structure of complex 1, as illustrated in Table S2
À1
(
ESI†). (i) Hydrogen bonds of O–HÁÁÁO between the oxygen atoms
À1
(O1, O3, O5, O9) from the 2,6-pdc ligands and the oxygen atoms
#1
À1
À1
(O9, O10) of the coordinated water (O9–H9BÁÁÁO1 : 2.721(4) Å,
#1 #1
1
62.71; O9–H9BÁÁÁO5 : 3.009(4) Å, 119.51; O10–H10BÁÁÁO3 :
#1
À1
À1
2.787(4) Å, 161.5; O10–H10BÁ ÁÁO6 : 3.016(4) Å, 124.9, #1: 1 À x,
Ày, 1 À z). The molecules are connected to a 1D chain structure by
these four hydrogen bond interactions (Fig. 2a). (ii) Hydrogen
bands at 746 and 679 cm for 1 and 753 and 684 cm for 2 are
characteristic of the stretching vibrations of –C–H. The bands
À1
À1
at 600 and 562 cm for 1 and 585 cm for 2 are characteristic
À1
of the stretching vibrations of V–Owater. The bands at 506 cm
for 1 and 490 cm
vibration of V–Ocarboxyl. Absorptions at 477 and 457 cm are
characteristic of the stretching vibration of V–N.
À1
for 2 are attributed to the stretching
À1
Scheme 1 The reaction processes for the synthesis of the complexes 1
and 2.
Fig. 1 The molecular structure of 1.
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Fig. 3 The molecular structure of 2.
73.11(6)–149.85(6)1 and 61.21(6)–178.69(7)1, respectively. The
angle of the O1W–V–O2W is 178.69(7)1.
There are two kinds of hydrogen bonds in the structure of
complex 2. (i) The intermolecular hydrogen bonds of O–HÁ Á ÁO
between two oxygen atoms (O4, O6, O1W, O2W) from the
2
,6-pdc ligand and the coordinated water molecule (O1W–
#1
#2
H1WÁÁÁO4 : 2.6224(19) Å, 1751; O2W–H2WÁÁÁO6 : 2.7187(19) Å,
1
691; #1: 1 À x, 1 À y, 1 À z; #2: x, 0.5 À y, À0.5 + z). The hydrogen
Fig. 2 (a) 1D infinite chain structure of 1; (b) a view of a 2D supramolecular
network structure formed by the hydrogen bonds; (c) the 3D packing
framework of the complex 1.
#1
bonds O1W–H1WÁ Á ÁO4 connect the molecules forming a bio-
polymeric structure (Fig. 4a), and the dipolymers further form a
#
2
2
(
D sheet structure with the hydrogen bond O2W–H2WÁ Á ÁO6
Fig. 4b). (ii) Intramolecular hydrogen bonds O–HÁÁÁO between
two oxygen atoms from the htba ligand (O3–H3AÁÁÁO1: 2.588(3) Å,
bonds of O–HÁ Á ÁO between an oxygen atom (O2) from the
34
1
48.31), specified as S(6), six atoms forming a closed six-
2
,6-pdc ligand and an oxygen atom (O10) from the coordinated
membered ring.
#
1
water (O10–H10AÁ Á ÁO2 : 2.802(4) Å, 141.91). Then, the neigh-
bor chains are connected to a 2D sheet structure by the
XRD analysis
#
1
hydrogen bond interactions of O10–H10AÁ Á ÁO2 (Fig. 2b). (iii)
Hydrogen bonds of O–HÁ Á ÁO between the oxygen atoms from
free water (O1W, O2W) and coordinated water (O9) or 2,6-(pdc)
The powder X-ray diffraction data of the complexes 1 and 2 were
obtained and compared with the corresponding simulated
single-crystal diffraction data (Fig. S5 and S6, ESI†). The phase
of the corresponding complex is considered to be pure owing to
#
1
ligands (O4, O8) (O9–H9AÁ Á ÁO2W : 2.724(4), 158.81; O8–
#
1
#1
H8AÁ Á ÁO2W : 3.240(5), 140.91; O1W–H1WAÁ Á ÁO8 : 2.785(5),
#
2
1
1
33.91; O1W–H1CWÁ Á ÁO4 : 2.780(5), 128.61, #2 = 1 À x, 1 À y,
À z). Finally, the neighbor sheets are connected to a 3D
network through four hydrogen bond interactions (Fig. 2c).
2 2
V(2,6-pdc)(htba)(H O) (2). Complex 2 crystallizes in the
monoclinic system with P2(1)/c space group. The molecular
structure of 2 consists of one vanadium atom, one 2,6-pdc
ligand, one htba ligand and two coordinated water molecules
(Fig. 3). The vanadium atom may be best described as having
pentagonal bipyramid geometry; it is coordinated by a nitrogen
atom (N) and two oxygen atoms (O5, O7) from the 2,6-pdc
ligand, two oxygen atoms (O1, O2) from the htba ligands, and two
oxygen atoms (O1W, O2W) from coordinated water molecules.
The 2,6-pdc ligand adopts a tridentate chelating coordination
mode and the htba ligand adopts a bidentate coordination mode.
The bond length of V–N(1) is 2.1050(17) Å. The two coordinated
water molecules adopt terminal monodentate coordination;
the lengths of V–O are in the range of 2.0339(15)–2.1201(16) Å.
Fig. 4 (a) The biopolymer structure of 2; (b) a view of a 2D supramole-
The angles of the N–V–O and O–V–O bonds are in the range cular network structure formed by the hydrogen bonds.
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the agreement of the peak positions. The different intensities
may be due to the preferred orientation of the powder samples.
Mimicking the catalytic bromination reaction
To our knowledge, vanadium complexes are able to mimic
a reaction in which vanadium haloperoxidases catalyze the
bromination of organic substrates in the presence of H O and
2
2
bromide. Herein, the catalytic bromination reaction activities of
complexes 1 and 2 were investigated using phenol red as an
organic substrate, which is converted to bromophenol blue during
the reaction. The reaction is rapid (the maximum reaction rate
2
À1 À2 À1
Fig. 6 A series of linear calibration plots showing the dependence of the
constant was 2.424 Â 10 k(mol L
)
s ) and stoichiometric,
absorbance at 592 nm on reaction time for different concentrations of
producing the halogenated product by the reaction of the oxidized
À1
À1
complex 1. Conditions used: pH = 5.8, c(KBr) = 0.4 mol L , c(H
2
O
2
) =
À1
À4
À1
halogen species with the organic substrate. The reactive process is 1 mol L , c(phenol red) = 10 mol L . c(complex 0.1/mmol L ) = (a): 1.2 Â
10À3; (b) 2.6 Â 10À3; (c) 4 Â 10À3; (d) 4.5 Â 10À3; (e) 7.6 Â 10À3
.
shown in Scheme 2.
The addition of a solution of complex 1 to the standard reaction
of bromide with phenol red (as a trap for oxidized bromine) in
phosphate buffer resulted in the visible color change of the solution
from yellow to blue. The electronic absorption spectra recorded a
decrease in absorbance of the peak at 443 nm with the loss of
phenol red and an increase of the peak at 592 nm with the
production of bromophenol blue (shown in Fig. 5).
was depicted using the data from Fig. 6, to obtain a straight
line (Fig. 7) with a slope of 1.09 and an intercept of 2.0134. The
data confirms that the first-order reaction is dependent on
vanadium. Based on the equation b = log k + y log c + z log c ,
2
3
the reaction rate constant, k, is determined by the concentra-
tions of KBr and phenol red (c and c ), the reaction orders of
2
3
Complex 1 was used to carry out kinetic studies of the
bromination reaction. A series of dA/dt data were obtained
KBr and phenol red ( y and z), as well as b. Since the reaction
orders of KBr and phenol red ( y and z) are 1 according to
(where A = absorbance at 592 nm), changing the concentration
À1
the literature, and c
2
and c
3
are known to be 0.4 mol L and
of the vanadium complex. Then the plot of Àlog(dc/dt) vs.
À4
À1
10
mol L , respectively, the reaction rate constant (k) for
Àlog c for complex 1 (where c = concentration of complex 1)
2
À1 À2 À1
complex 1 can be calculated as 2.424 Â 10 (mol L
)
s .
Similar plots for 2 were generated in the same way (Fig. 8
and 9), and values of the slope and the intercept were 1.0048
and 2.2403 respectively.
The cyclic catalytic bromination reaction mechanism is
shown in Scheme 3. Centre V atom is coordinated by two water
molecules as shown in (a) ([V(H
easily loses a H O forming a compound (b) ([V(O
with H as an oxidation regent. The formation process of (c)
([VO L L ]) is the same as that of (b). (c) Is oxidized to form an
2
O)
2
L
1
L
2
]). The vanadium complex
2
2 1 2 2
)L L H O])
2 2
O
Scheme 2 Reactive process for the bromination reaction of phenol red,
catalyzed by the complexes.
2
1 2
À
intermediate compound (d) ([V(O )L L H O]). Br is oxidized
2
1 2 2
rapidly by (d), while at the same time HOBr and (c) are formed
À
with the loss of OH . The following process is similar to that in
the previous reaction, and the other coordinated water is oxidized
À
with H
2
O
2
forming an intermediate (d). Thus (c) oxidizes Br into
Fig. 5 Oxidative bromination of phenol red catalyzed by 1. Spectra
obtained at 10 min intervals. The reaction mixture contained phosphate
À1
À4
À1
À1
buffer (pH 5.8), KBr (0.4 mol L ), phenol red (10 mol L ), H
O
2 2
(1 mol L )
Fig. 7 Àlog(dc/dt) dependence of Àlog c for complex 1 in DMF–H
30 Æ 0.5 1C.
2
O at
À1
and complex 1 (0.1 mmol L ).
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HOBr which further brominates oxidized phenol red. The reaction
cycle involving (c) and (d) continues until the phenol red is
completely transformed into bromophenol blue.
The experimental results showed that: (i) the reaction orders
of the vanadium complexes in the bromination reaction are
all close to 1, confirming the first-order dependence of the
reaction on vanadium; (ii) the reaction rate constants of the two
complexes show the order 1 4 2 (Table 2).
Analysis of the results revealed that the vanadium(III) com-
plexes showed slightly different but high catalytic activity. The
order of the reaction rate constant for complexes is 1 4 2. We
thought that the catalytic reaction rate was influenced by the
formation of intermediate species. In complex 1, the bond
lengths of V–O between the central metal vanadium atom and
coordinated water were 2.076 and 2.070 Å, while they were
Fig. 9 Àlog(dc/dt) dependence of Àlog c for complex 2 in DMF–H
0 Æ 0.5 1C.
2
O at
3
2.056 and 3.034 Å in 2. We know that the longer the bonds are,
the weaker the bond energy is. Hence, the V–O bonds in 1 were
easier to break to form an intermediate species than those in 2,
so the reaction rate of 1 was faster than that of 2. Furthermore,
there was the same ligand (2,6-pdc) and the same coordination
model in the complexes, but the structures differed as a result
of the other ligand, 2,6-pdc for 1 and htba for 2. The space steric
effect of htba was bigger than that of 2,6-pdc, because there is a
vicinal hydroxyl on the benzene of htba. Therefore, the coordinated
2
H O was more easily oxidized to form an intermediate species and
so the reaction rate constants showed the order 1 4 2.
Scheme 3 The cyclic catalytic bromination reaction mechanism.
Colorimetric detection of hydrogen peroxide
As we can see, hydrogen peroxide plays an important role in the
food industry, clinical medicine, pharmaceutical production
and environmental protection, especially because it has great
practical effects in catalytic reactions. Based on this view, we
Table 2 Kinetic data for catalytic bromination by the complexes in DMF–
a
H
2
O at 30 Æ 0.5 1C
À1 À2 À1
Complexes
m
b
k (mol L
)
s
studied the effect of the concentration of H
2 2
O upon our
2
2
1
2
1.09
1.00
2.0134
2.2403
2.424 Â 10
catalytic reaction to develop a new H O detection method.
2
2
1.437 Â 10
Through the experimental results, it was found that the oxida-
tion reaction catalyzed by the vanadium complexes is H O
a
À1
Conditions used: c(phosphate buffer) = 50 mmol L , pH = 5.8, c(KBr) =
2
2
À1
À4
À1
0
.4 mol L , c(phenol red) = 10 mol L ‘‘m’’ is the reaction order of the
vanadium complex; ‘‘b’’ is the intercept of the line; ‘‘k’’ is the reaction
with the formation of bromophenol blue presents a linear rate constant for the vanadium complex.
dependence on the concentration of H in a reaction time
concentration-dependent, and the absorbance at 592 nm along
2 2
O
of 25 min (Fig. 10 and 11). The detection limit for the concen-
À1
tration H
0
2
O
2
À1
was estimated to be 0.94 mol L
for 1 and
.98 mol L for 2 by extrapolation of the graphs. All these
observations further confirm that the catalytic reaction system
can be used as a potential method for the detection of H
2 2
O .
Furthermore, comparisons of the detection limit for com-
plexes reported previously are listed in Table 3. Complexes 3
and 4 were synthesized according to their corresponding
ref. 30. 3 is an oxidovanadium(IV) complex with a VQO bond
coordinated by a 1,10-phenanthroline and an oxalic acid ligand.
4
is also an oxidovanadium(V) complex with two VQO bonds
coordinated by a bpz*T–O ligand (bpz*T–O: 4,6-bis(3,5-dimethyl-
pyrazol-1-yl)-1,3,5-triazin-2-olate). The order of the reaction rate
constants for these complexes is 4 4 3 4 2 4 1. It is found
that the 1 and 2 have similar detection limits, which is related
to the structure of the complexes. The increasing valence of the
Fig. 8 A series of linear calibration plots showing the dependence of the
absorbance at 592 nm dependence on reaction time for different concentra-
À1
tions of complex 2. Conditions used: pH = 5.8, c(KBr) = 0.4 mol L , c(H
2
O
2
) =
À1
À4 À1 À1
1
mol L , c(phenol red) = 10
mol L . c(Complex 0.1/mmol L ) =
(
a) 1.6 Â 10À3; (b) 3.1 Â 10À3; (c) 4.4 Â 10À3; (d) 6.7 Â 10À3; (e) 7.9 Â 10À3
.
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2 2
Furthermore, a new colorimetric detection method for H O ,
based on the catalytic bromination reaction was discovered
and this detection system was simple and sensitive. We will
investigate in detail the application of the detection reaction
mechanism in the future.
Acknowledgements
This work was supported by financial assistance from the National
Natural Science Foundation of China (Grant No. 21071071,
21371086), and the Commonweal Research Foundation of Liaoning
province in China (No. 2014003019).
Fig. 10 The linear calibration plot of the absorbance at 592 nm vs. the
2 2 2
concentration of H O in the catalysis reaction of complex 1 in DMF–H O
at 30 Æ 0.5 1C.
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