CHEM. RES. CHINESE UNIVERSITIES 2012, 28(5), 768—774
Vanadium Haloperoxidases Model Compounds: Synthesis, Structural
Characterization and Mimic Catalytic Bromination
Activity of [VO(C2O4)(2,2′-bipy)(H2O)]·C2H5OH and
VO(C2O4)(phen)(H2O)
REN Dong-xue, CAO Yun-zhu, CHEN Chen and XING Yong-heng*
College of Chemistry and Chemical Engineering, Liaoning Normal University,
Dalian 116029, P. R. China
Abstract Two oxo-vanadium(IV) complexes, [VO(C2O4)(2,2′-bipy)(H2O)]·C2H5OH(1) and VO(C2O4)(phen)(H2O)
(2), where 2,2′-bipy=2,2′-bipyridyl, phen=1,10-phenanthroline, were synthesized as potential functional models of
vanadium haloperoxidases(VHPOs) in mixed solvent of ethanol and water at room temperature. The complexes were
characterized by elemental analysis, infrared(IR), UV-Vis and X-ray crystallography. Structural analyses showed that
vanadium atom was coordinated by a terminal oxygen, one oxygen atom from coordinated water, two oxygen atoms
from the carboxylate group of oxalic acid, and two nitrogen atoms(N1 and N2) from 2,2′-bipy/phen. Central vanadium atoms in complexes 1 and 2 were both in a distorted-octahedral environment, and some intermolecular hydrogen bonding linkages were also observed in each complex. Bromination reaction activity of the two complexes was
evaluated with phenol red as organic substrate in the presence of H2O2, Br– and phosphate buffer, indicating that they
can be considered as a potential functional model of VHPO. In addition, thermal analysis was also performed and
discussed in detail.
Keywords Oxo-vanadium complex; Bromination reaction activity; Vanadium haloperoxidase
Article ID 1005-9040(2012)-05-768-07
1
Introduction
The coordination chemistry of vanadium is a new field in
potentially biological application due to its vital roles in many
abiotic as well as biotic systems, such as haloperoxidation[1,2],
nitrogen fixation[3], phosphorylation[4], glycogen metabolism[5―7] and insulin mimicking[8,9]. Among these systems,
vanadium haloperoxidases(VHPOs) have received increasing
attention mainly due to their catalytic ability for the oxidation
of halides to the corresponding hypohalous acids in the
presence of hydrogen peroxide, resulting in the halogenation of
certain organic substrates[Reaction(1)][10―12]:
(1)
R―H+H2O2+X–+H+→R―X+2H2O
The vanadium bromoperoxidase enzymes from two kinds
of the red algal species, Corallina officinalis and Corallina
pilulifera, have been studied in detail[13,14]. Structural analysis
reveals that they all show a high degree of amino acid homology in their active centers and have nearly identical structural
features, with vanadium atom in a proved trigonal-bipyramidal
NO4 coordination geometry which is covalently bonded to
three oxygen atoms in the equatorial plane, Nε of the histidine
and an OH on axial positions[15,16]. The good structures and
properties of these vanadium enzymes have stimulated the
researches on structural and functional model compounds with
a V=O moiety containing O and N donor ligands[17,18]. In
particular, the interaction of simple vanadium species(VO2+ and
VO3+) with various O―N or N―N donor atoms having bioactive ligands has triggered growing interest recently. Based on
the points above, a number of vanadium complexes were designed and synthesized to study and understand the relationship
between structure and catalytic activity.
To our best knowledge, although the oxo-vanadium complexes with dicarboxylate ligands have been reported a
lot[19―25], the majority of them are about synthesis and structures, while the study on the catalytic activities about bromoperoxidation is relatively rare[26―29]. In order to extend the investigation of vanadate-dependent haloperoxidases on their
structures and properties, we synthesized two structural and
functional model complexes of vanadium haloperoxidases by
choosing small molecule carboxylic acids as oxygen donor
ligands(mimicking V―O, V=O of the active center in
enzymes) and 2,2′-bipy/phen(2,2′-bipy=2,2′-bipyridyl, phen=
1,10-phenanthroline) as nitrogen donor ligands(mimicking
V―N of the active center in enzymes). In addition, bromide
catalytic activities of the two model complexes were estimated
systematically by the catalytic kinetic experiment, which
———————————
*Corresponding author. E-mail: [email protected]
Received October 25, 2011; accepted December 9, 2011.
Supported by the National Natural Science Foundation of China(No.21071071) and the Education Foundation of Dalian City
in China(No.2009J21DW004).
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REN Dong-xue et al.
indicated that they can be considered as a potential
model of VHPO.
2
2.1
functional
Experimental
Materials and Methods
All the chemicals used were of analytical grade and
without further purification. VO(acac)2 was synthesized
according to the literature[30]. The synthetic manipulations were
carried out in the atmosphere at room temperature. Elemental
analyses(C, H and N) were performed on a PE 240C automatic
analyzer. Infrared(IR) spectra were determined on a JASCO
FTIR-480 PLUS Fourier transform spectrometer(200―4000
cm–1, with pressed KBr pellets). UV-Vis spectra were determined with a JASCO V-570 UV-Vis spectrometer(200―1100
nm, in the form of solid sample).
2.2
Syntheses of Complexes
2.2.1 Synthesis of VO(C 2 O 4 )(2,2′-bipy)(H 2 O)·
C2H5OH(1)
A solution of 15 mL of VO(acac)2(0.13 g, 0.5 mmol) in a
mixed solvent of ethanol and water(with volume ratio of 2:1)
was stirred for 1 h, then oxalic acid(0.045 g, 0.5 mmol) and
2,2′-bipyridyl(0.08 g, 0.5 mmol) were added to the solution
with continuous stirring for another 2 h at room temperature.
After the green solution was filtered off from the mixture and
left at room temperature for about 2 d, yellow-green crystals
suitable for X-ray diffraction were obtained. Yield(based on V):
0.15 g, 80%. Elemental analysis(%) calcd. for C14H16N2O7V:
C 44.77, H 4.26, N 7.46; found: C 44.58, H 4.21, N 7.58.
IR(KBr), ߥ෤/cm–1: 3420, 3144, 1716, 1654, 1388, 1229, 983,
767, 549, 473, 421, 393, 357. UV-Vis, λmax/nm: 258, 398, 424,
554, 720.
2.2.2 Synthesis of VO(C2O4)(phen)(H2O)(2)
An identical procedure with that of preparing complex 1
was used to prepare complex 2 except that 2,2′-bipyridyl was
replaced by 1,10-phenanthroline(0.10 g, 0.5 mmol). Yield
(based on V): 0.16 g, 90.65%. Elemental analysis(%) calcd. for
C14H10N2O6V: C 47.57, H 2.83, N 7.93; found: C 47.39, H 2.87,
N 7.85. IR(KBr), ߥ෤/cm–1: 3380, 3062, 1715, 1692, 1650, 1389,
983, 546, 474, 428, 382, 360. UV-Vis, λmax/nm: 264, 372, 440,
548, 738.
2.3 X-Ray Crystallographic Determination
Suitable single crystals of the two complexes were
mounted on glass fibers for X-ray measurement, respectively.
Reflection data were collected at room temperature on a Bruker
AXS SMART APEX II CCD diffractometer with graphite monochromatized Mo Kα radiation(λ=0.071073 nm) at an ω scan
mode. All the measured independent reflections[I>2σ(I)] were
used in the structural analyses, and semi-empirical absorption
corrections were made via SADABS program[31]. Crystal
structures were solved by the direct method. All the nonhydrogen atoms were refined anisotropically. All the hydrogen atoms were fixed at calculated positions with isotropic
769
thermal parameters. All the calculations were performed via the
SHELXS-97 program[32]. Crystal data, details of the data
collection and the structure refinement are given in Table 1.
Table 1 Crystallographic data and structure refinement
for complexes 1 and 2
Complex
Formula
1
C14H16N2O7V
M
375.23
353.18
Crystal system
Monoclinic
Triclinic
Space group
P2(1)/n
a/nm
0.7212(19)
Pī
0.7647(15)
b/nm
1.2562(3)
0.9807(2)
c/nm
1.6976(4)
0.9905(2)
α/(º)
β/(º)
γ/(º)
90
89.11
101.822(3)
74.08(3)
90
80.39(3)
V/nm3
1.5053(7)
0.7039(2)
Z
4
2
Dc/(Mg·m−3)
Crystal size/mm3
1.656
1.666
F(000)
0.46×0.26×0.10
772
0.10×0.14×0.16
1064
μ(Mo Kα)/cm−1
0.701
0.739
Reflection collected
8580
6915
Independent reflection[I>2σ(I)]
3465(2734)
3185(2049)
Δρ/(e·nm−3)
Goodness-of-fit on F2
472, –724
536, –611
1.140
1.048
Ra
0.0665(0.0800)b
wR2
b
a
0.1924(0.1998)
a. R=Σ ⎢⎢Fo ⎢− ⎢Fc ⎢⎢/ Σ ⎢Fo ⎢,
[Fo> 4σ(Fo)]; b. based on all data.
2.4
2
C14H10N2O6V
0.0552(0.1197)b
0.0989(0.1384)b
wR2={Σ[w(Fo −Fc ) ]/Σ[w(Fo2)2]}1/2;
2
2 2
Thermal Analysis
Thermogravimetric analysis(TGA) experiments were
carried out on a Perkin Elmer Diamond TG/DTA(DTA: differential thermal analysis) instrument. The samples were initially
heated for 1 h at 50 °C to remove the rudimental air. During the
simple ramping experiment, mass changes were recorded as a
function of temperature at a 10 °C/min temperature gradient
between 30 and 1000 °C in nitrogen atmosphere.
2.5 Bromination Reaction Activity Measurement
of the Complexes
Bromination reaction activity tests were carried out in the
mixed solution of H2O-DMF(DMF=N,N-dimethylformamid) at
a constant temperature of (30±0.5) °C. Oxidovanadium complexes were dissolved in 25 mL of H2O-DMF solution with a
volume ratio of 4:1. Solutions used for kinetic measurements
were maintained at a constant concentration of H+(pH=5.8)
by the addition of NaH2PO4- Na2HPO4[33]. Reactions were
initiated by the addition of a phenol red solution. The oxidovanadium complexes at five different concentrations were confected in five cuvettes, respectively. Then the cuvettes were put
in water with the constant temperature and heated for 10 min
and spectral changes were recorded on a 721 UV-Vis spectrophotometer at intervals of 5 min. Finally, the resulting data
were collected and fitted via the curve-fitting software in the
program Microsoft Excel.
It is assumed that the rate of this reaction is described by
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CHEM. RES. CHINESE UNIVERSITIES
x
y
z
the rate equation: dc/dt=kc1 c2 c3 , then the equation of
lg(dc/dt)=lgk+xlgc1+ylgc2+zlgc3 was obtained, it is corresponding to –lg(dc/dt)= –xlgc1–b(b=lgk+ylgc2+zlgc3), where k is
the reaction rate constant; c1, c2, c3 are the concentrations of
oxidovanadium complexes, KBr and phenol red, respectively;
while x, y, z are the corresponding reaction orders. According to
Lambert-Beer’s law, A=ε·d·c, viz. dA/dt= ε·d·(dc/dt), where A is
the measurable absorbance of the resultant; ε is molar absorption coefficient, which of bromophenol blue is measured as
14500 L·mol–1·cm–1 at 592 nm; d is the light path length of
sample cell(d=1). When the measurable absorbance data were
plotted versus reaction time, a line was obtained and the reaction rate of the oxidovanadium complex(dA/dt) was given by
the slope of this line. By changing the concentration of the
oxidovanadium complex, a series of dA/dt data can be obtained.
The reaction rate constant(k) can be obtained from a plot of
–lg(dc/dt) versus –lgc1 and fitted via the curve-fitting software
in the program Microsoft Excel by generating a least squares fit
to a general equation of the form y=mx–b, in which m is the
reaction order of the oxidovanadium complex in this reaction
and b is the intercept of the line. In the experiment, the reaction
orders of KBr and phenol red(y and z) are both 1 according to
the literature[34]; c2 and c3 are known as 0.4 and 10–4 mol/L,
respectively. Based on equation b=lgk+ylgc2+zlgc3, the reaction
rate constant(k) can be obtained. Bromination of phenol red
was monitored by the measurement of the absorbance at 592
nm for reaction aliquots which were extracted at specific time
points and diluted into phosphate buffer(pH=5.8).
3
3.1
Results and Discussion
Synthesis
Complexes 1 and 2 were all synthesized in the system of
ethanol and water(volume ratio of 2:1) at room temperature. In
order to synthesize similar structural complexes, we chose
2,2′-bipy/phen as the first ligand and other carboxylic acids
such as succinic acid, glutaric acid and adipic acid as the
second ligand replacing oxalic acid, but the final desired products can not be obtained. Complexes 1 and 2 are quite stable
in the solid state at room temperature, and can dissolve in DMF
and CH2Cl2 easily. Slightly dissolve in methanol and water and
can hardly dissovle in hexane and ether.
–1
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–1
cm and 1388―1389 cm , respectively. The bands of the
ν(V=O) are observed at 983 cm–1 for both complexes 1 and 2.
Besides these general features, some groups of the complexes
also manifest other features clearly, e.g., bands at 549 and 393
cm–1 for complex 1 and 546 and 382 cm–1 for complex 2 are
due to ν(V―Owater) , those at 473 and 357 cm–1 for complex 1
and 474 and 360 cm–1 for complex 2 are due to ν(V―Ocarboxyl)
and stretching vibration of V―N in a range of 421―428 cm–1.
The three bands substantiate the coordination of oxide acid to
the metal center in the complexes[35].
3.3
UV-Vis Spectra
The UV-Vis absorption spectra of complexes 1 and 2
(Figs.S3 and S4, see the supporting information of this paper,
http://www.cjcu.jlu.edu.cn) were recorded in the form of solid
sample. The high-frequency absorption bands in a range of
258―264 nm for the complexes are assigned to the π-π* transition of the aromatic-like chromophore from either of ligand
2,2′-bipyridyl or ligand 1,10-phenanthroline[36]. The absorption
bands in a range of 372―440 nm are attributed to the LMCT
(ligand to metal charge transfer) transition. For vanadyl(IV)
complexes, it is generally considered that transitions occur
from dxy to (dxz, dyz)(ν1), dx2–dy2(ν2) and dz2(ν3) orbitals with
increasing energies. In the visible range, complexes 1 and 2
exhibit two sets of absorption maximum at 720―738 nm and
548―554 nm which can be assigned to ν1 and ν2 bands of
oxovanadium(IV)[37].
3.4
Structural Description of Complexes 1 and 2
The molecular structures of complexes 1 and 2 are
depicted in Figs.1―3, and some selected bond lengths and
bond angles for the complexes are summarized in Tables 2 and
3. Single-crystal X-ray structure analyses reveal that complexes
Fig.1
Molecular structures of complexes 1(A) and 2(B)
All H atoms and lattice ethanol molecules are omitted for clarity.
3.2
IR Spectra
The IR spectra of complexes 1 and 2 are shown in Fig.S1
and S2(see the supporting information of this paper,
http://www.cjcu.jlu.edu.cn). It was found that there is a broad
band in the range of 3144―3420 cm–1 assignable to ν(O―H)
of the coordinated or uncoordinated water and/or ethanol molecules associated with the complexes which are confirmed by
elemental analyses. Also, the characteristic stretching vibrations of C=N from the 2,2′-bipyridyl for complex 1 and from
the 1,10-phenanthroline for complex 2 are at 1716 and 1715
cm–1, respectively. For complexes 1 and 2, the antisymmetric
νas(COO–) and symmetric νs(COO–) vibrations of the carboxyl
group from the oxalic acid are in the ranges of 1650―1654
Fig.2
Complex 1 connected with external molecules
(A) and ethanol(B) by hydrogen bonds
Symmetry transformation: #1. 1+x, y, z; #2. 1–x, 1–y, –z; #3.–1/2+x, 1/2–y,
–1/2+z; #4. –1+x, y, z.
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REN Dong-xue et al.
Fig.3
View of the hydrogen bonded dimer(A) and 1D
network(B) of complex 2
All H atoms except the hydrogen bonds are omitted for clarity.
Symmetry transformation: #5. –x, –y, 2–z.
Table 2
Selected bond lengths(nm) and bond angles(°)
of complex 1
V―O1
V―O2
0.1585(3)
0.2041(3)
V―O4
V―N1
0.2001(3)
0.2125(4)
V―O3
0.1974(3)
V―N2
0.2295(4)
O1―V―O3
O1―V―O4
105.93(16)
100.38(17)
O4―V―N1
O2―V―N1
89.95(13)
92.11(15)
O3―V―O4
80.83(13)
O1―V―N2
166.83(16)
O1―V―O2
95.03(17)
O3―V―N2
O3―V―O2
91.83(14)
O4―V―N2
O4―V―O2
164.24(15)
O2―V―N2
O1―V―N1
94.59(17)
N1―V―N2
O3―V―N1
158.67(14)
Table 3
Complex 2
86.85(13)
–0.0002
0.0002
84.53(14)
O4
–0.0051
–0.0002
81.14(14)
N1
0.0044
0.0002
73.07(14)
V
–0.0312
–0.0343
O1
–0.1889
–0.1920
N2
0.1963
0.1963
V―O3
0.1991(3)
V―N2
0.2325(3)
O1―V―O3
O1―V―O4
105.36(13)
101.22(15)
O4―V―N1
O2―V―N1
92.37(11)
91.56(11)
O3―V―O4
80.73(10)
O1―V―N2
167.24(13)
O1―V―O2
98.88(15)
O3―V―N2
87.37(11)
O3―V―O2
88.81(11)
O4―V―N2
81.20(11)
O4―V―O2
159.18(12)
O2―V―N2
80.38(11)
O1―V―N1
94.03(13)
N1―V―N2
73.30(12)
O3―V―N1
160.31(12)
1
2
Deviations(nm) from least-squares planes
of complexes 1 and 2
–0.0044
0.0051
0.1584(3)
0.2030(3)
Complex
Table 4
Complex 1
V―O1
V―O2
Table 5
1 and 2 are isomorphous, but there is a lattice ethanol molecule
in complex 1.
For the two complexes, vanadium atom is coordinated by
a terminal oxygen(O1), one oxygen atom(O2) from coordinated
water, two oxygen atom(O3 and O4) from the carboxylate
group of oxalic acid and two nitrogen atoms(N1 and N2) from
2,2′-bipyridyl for complex 1 or 1,10-phenanthroline for complex 2. They have a distorted octahedron vanadium center, with
O2, O3, O4 and N1 in the equatorial plane, O1 and N2 on the
axial positions. The deviations of V, O1 and N2 out of equatorial plane for complexes 1 and 2(Table 4) indicate that the
vanadium atoms are displaced toward the oxo oxygen O1, trans
to N2. The angles of O1―V―N2 for the two complexes are
166.83(16)° and 167.24(13)°, respectively. The order of the
bond lengths of V―O in complex 1 is V―O2water>
V―O4carboxyl>V―O3carboxyl, similar trend was observed in
complexes 2, which indicates that the coordination ability of
oxalic acid is stronger than that of water. In addition, there is
obvious difference between the coordination ability of the two
nitrogen atoms in 2,2′-bipyridyl/phen, as expected, the V―N
bond distanced trans to the V=O bond is considerably longer
than that of the V―N bond in the equatorial plane that is a
consequence of the strong trans influence by the terminal oxo
group.
O2
O3
Selected bond lengths(nm) and bond angles(°)
of complex 2
V―O4
V―N1
771
0.1994(3)
0.2126(3)
Atom
In addition, there are abundant inter-molecular hydrogen
bonds in the two complexes. Relevant H-bond parameters of
complexes 1 and 2 are listed in Table 5. As illustrated in Fig.2,
there are two types of hydrogen bonds in complex 1, O―H···O
(0.2608―0.2872 nm) and C―H···O(0.2955―0.3374 nm).
Particularly, hydrogen bonds of O―H···O come from coordinated water(O2) and carboxyl(O6#1), coordinated water (O2)
and lattice ethanol(O7), ethanol(O7) and carboxyl(O6), respectively; hydrogen bonds of C―H···O are from 2,2′-bipy (C4 and
C6) and carboxyl(O4#2) and O3#3 as well as lattice ethanol
(C13 and C14) and carboxyl(O6 and O5#4). With the help of
Hydrogen bond lengths and bond angles of complexex 1 and 2*
D―H···A
O2―H2A···O7
O2―H2B···O6#1
d(D―H)/ nm
0.0820
0.0960
d(H···A)/ nm
0.1788
0.1794
d(D···A)/ nm
0.2608
0.2728
∠D―H···A/(°)
178.06
163.61
O7―H7B···O6
0.0820
0.2100
0.2872
156.89
C4―H4A···O4#2
0.0930
0.2539
0.3374
149.60
C6―H6A···O3#3
0.0930
0.2593
0.3311
134.43
C13―H13A···O6
0.0970
0.2544
0.3290
133.68
C14―H14A···O5#4
0.0960
0.1996
0.2955
175.97
O2―H2A···O3#5
0.0845
0.1892
0.2712
162.88
C9―H9···O1#5
0.0900
0.1963
0.2696
137.52
* Symmetry transformation used to generate equivalent atoms: #1. 1+x, y, z; #2. 1–x, 1–y, –z; #3. –1/2+x, 1/2–y, –1/2+z; #4. –1+x, y, z; #5. –x, –y, 2–z.
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CHEM. RES. CHINESE UNIVERSITIES
complicated hydrogen bonds, the whole molecular structure is
more stable. For complex 2, two independent molecules form a
dimer by the inter-molecular hydrogen bonding of
O2―H2A···O3#5[Fig.3(A)]. These dimeric units are connected
to one another via the hydrogen bond of C9―H9···O1#5 to
form a 1D chain on the bc-face[Fig.3(B)].
3.5
Thermal Stability Analysis
To examine the thermal stability of the complexes, thermogravimetric analyses(TGA) were carried out in a range of
30―1000 °C. For complex 1, the TG curve indicated five steps
(Fig.4 curve a): the temperature range from 80 °C to 200 °C
with a mass loss of 10.77% accords with the loss of an ethanol
molecule(calculated value 12.27%). The decomposition
occurred within a range from 200 °C to 266 °C with a mass
loss of 4.65%, which was due to the loss of one-coordinated
water molecule(calculated value 4.8%). The stage in temperature range from 266 °C to 300 °C shows a mass loss of 11.04%,
due to the loss of a CO2 molecule from the oxalic acid agreed
with the DTG curve(Fig.4, curve b calculated value 11.74%).
For the temperature range of 300 °C to 350 °C, a mass loss of
31.22% was observed, which was due to the release of another
CO2 molecule of oxalic acid and 45% of 2,2′-bipy moiety
(calculated value 30.46%). The last step of decomposition
occurred within a range from 350 °C to 1000 °C with a mass
loss of 22.91%, which corresponded to the loss of the residue
of 2,2′-bipy moiety(calculated value 22.88%). The remaining
product(19.41%) is due to the mixture of VO2 and V2O5(calculated value 17.87%). The TGA curves of complex 2 (Fig.S5,
see the supporting information of this paper, http://www.cjcu.
jlu.edu.cn) is different from those of complex 1, which might
be due to the impact of lattice ethanol molecules.
Vol.28
have also investigated the bromination reaction activities of
complexes 1 and 2 with phenol red as organic substrate which
is shown by the conversion of phenol red to bromophenol blue.
The reaction is rapid and stoichiometric, producing the halogenated product by the reaction of oxidized halogen species with
the organic substrate, and the reactive process is as follows(Scheme 1).
Scheme 1
Reactive process of bromination reaction in
the presence of V complex
Addition of the solution of complex 1 to the standard reaction of bromide in phosphate buffer with phenol red as a trap
for oxidized bromine resulted in the visible color change of the
solution from yellow to blue. As shown in Fig.5, the electronic
absorption spectra were collected during such a color change
with complex 1 as catalyst. The spectrum recorded shows a
decrease in absorbance of the peak at 443 nm due to the loss of
phenol red and a peak at 592 nm, the characteristic of the
product bromophenol blue, showing that complex 1 performs
significant catalytic activity. The result of the mimic catalytic
activities of complex 2(Fig.S6, see the supporting information
of this paper, http://www.cjcu.jlu.edu.cn) is similar to that of
complex 1.
Fig.5
Electronic absorption spetra of oxidative bromination of phenol red catalyzed by complex 1
Spectral changes at 10 min intervals, c(phosphate buffer)=50 mmol/L,
pH=5.8, c(KBr)=0.4 mol/L, c(phenol red)=10–4 mol/L.
Fig.4
TG(a), DTG(b) and DTA(inset) curves of complex 1
3.6 Functional Mimics of the Vanadium Haloperoxidases
3.6.1 Mimicking Bromination Reaction of the Complexes
Oxidovanadium complexes are able to mimic the reaction
in which vanadium haloperoxidases catalyze the bromination of
organic substrates in the presence of H2O2 and bromide, for
example, the bromination of trimethoxybenzene[38], benzene,
salicylaldehyde and phenol by VO2+[39], and the bromination of
phenol red by [VO(O2)H2O]+ and related species[40]. Herein, we
3.6.2 Kinetic Studies of Mimicking Bromination
Reaction
As vanadate-dependent haloperoxidases catalyze the visible conversion of phenol red to bromophenol blue under peroxide conditions, the role of the oxidovanadium complexes in
the bromide oxidation reaction was investigated in the mixed
solution of H2O-DMF at a constant temperature of (30±0.5) °C.
Solutions used for kinetic measurement were maintained at a
constant concentration of H+ by the addition of NaH2PO4Na2HPO4. Reactions were initiated in the presence of H2O2 and
KBr. Take complex 1 for instance, a series of dA/dt data was
obtained(Fig.6) by changing the concentration of the oxovanadium complex. The measurable absorbance dependence of
No.5
REN Dong-xue et al.
time for complex 2 is shown in Fig.S7(see the supporting information of this paper, http://www.cjcu.jlu.edu.cn).
As is shown in Fig.7, the plot of –lg(dc/dt) versus –lgc for
complex 1 gives a straight line with a slope of 1.1289 and b=
–1.0275, the former confirms the first-order dependence on
vanadium. Based on the equation of b=lgk+ylgc2+zlgc3, the
reaction rate constant(k) is determined by the concentrations of
KBr and phenol red(c2 and c3), the reaction orders of KBr and
phenol red(y and z) and b. In the experiment, the reaction orders of KBr and phenol red are both 1 according to the literature[34]; c2 and c3 are known as 0.4 and 10–4 mol/L, respectively,
so the reaction rate constant for complex 1 can be calculated as
2.347×103 L2·mol–2·s–1. Similar plot for complexes 2(Fig.S8,
see the supporting information of this paper, http://www.cjcu.
jlu.edu.cn) was generated in the same way.
Fig.6
Dependence of measurable absorbance on time
for complex 1
Conditions used: pH=5.8, c(KBr)=0.4 mol/L, c(H2O2)=1 mmol/L,
c(phenol red)=10–4 mol/L. Concentration of complex 1/(mmol·L–1):
a. 2.38×10–2; b. 4.76×10–2; c.7.14×10–2; d. 9.51×10–2; e. 1.43×10–1.
773
complexes such as complexes 1 and 2 exhibit different catalytic
activity and they can be considered as potential functional
model vanadium-dependent haloperoxidases.
Table 6 Kinetic data for the complexes in H2O-DMF
at (30±0.5) °C*
m
b
k/(L2·mol–2·s–1)
1.1289
–1.0275
2.347×103
1
1.0415
–1.3967
1.003×103
2
* Conditions used: c(phosphate buffer)=50 mmol/L, pH=5.8, c(KBr)=
0.4 mol/L, c(phenol red)=10–4 mol/L. m is the reaction order of the oxidovanadium complex; b is the intercept of the line; k is the reaction rate constant for the oxidovanadium complex.
Complex
4
Conclusions
To obtain more efficacious complexes to model the active
center of VHPOs, we synthesized two small molecuar oxidovanadium complexes and tested the bromination reaction
activities of them with phenol red as organic substrate in the
presence of H2O2, KBr and phosphate buffer. The two complexes are easily obtained at room temperature. Structural
analysis shows that the center vanadium atoms are both six
coordination, with distorted octahedron geometry. In addition,
there are abundant inter-molecular hydrogen bonds in both the
complexes, by which an infinite outspread chain structure is
formed. In general, the synthesis of small molecular functional
model complexes may help to develop the study of such compounds and to understand the reaction machanism of vanadium
haloperoxidases.
Supplementary Material
Supplementary data associated with this article can be
obtained free of charge by quoting the publication citation.
CCDC No. for complexes 1 and 2 are 836165 and 836166,
respectively. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre at www.ccdc.cam.
ac.uk/data_request/cif.
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Fig.7
Dependence of –lg(dc/dt) on –lgc for complex 1
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Supporting Information
Fig.S1
Fig.S2
The infrared spectrum for complexe complex 1
The infrared spectrum for complexe complex 2
Fig.S3
The UV–Vis spectrum for complex 1
Fig.S4
The UV–Vis spectrum for complex 2
S1
S2
CHEM. RES. CHINESE UNIVERSITIES
Fig.S5
Fig.S6
TG/DTA curves of complex 2
Oxidative bromination of phenol red catalyzed by complex 2.
Spectral changes at 10 min intervals, c(phosphate buffer) = 50 mmol/L, pH = 5.8, c(KBr) = 0.4 mol/L, c(phenol red) =10-4 mol/L
Fig.S7
The measurable absorbance dependence of time for complex 2.
Conditions used: pH 5.8, c(KBr) = 0.4 mol/L, c(H2O2) = 1 mmol/L, c(phenol red) = 10-4 mol/L. c(complex 2/mmol/L) = a: 2.38×10-2;
b: 4.76×10-2; c: 7.14×10-2; d:9.51×10-2; e: 1.43×10-1.
Fig. S8
–log(dC/dt) dependence of –logc for complex 2 in DMF–H2O at 30 ± 0.5 °C
c is the concentration of the oxidovanadium complex 2; Conditions used: c(phosphate buffer) = 50 mmol/L, pH 5.8, c(KBr) = 0.4
mol/L, c(phenol red) = 10-4 mol/L.
Vol.28