Dinuclear vanadium complexes with rigid phenylpolycarboxylate ligands: Synthesis, structure, and catalytic bromination reaction with potential detection of hydrogen peroxide

Article abstract of DOI:10.1080/00958972.2014.926007

Vanadium complexes (VO)₂(2,2′-bipy)₂(bta) (H₂O)₂ (1) and (VO)₂(1,10-phen) ₂(bta)(H₂O)₂ (2) (H₄bta = 1,2,4,5-benzenetetracarboxylic acid, 2,2′-bipy = 2,2-bipyridine and 1,10-phen = 1,10-phenanthroline) have been synthesized by the reaction of V ₂(SO₄)₃, H₄bta, 2,2′-bipy (for 1) and 1,10-phen (for 2) by hydrothermal methods. The complexes were characterized by elemental analysis, IR, UV-vis, thermogravimetric analyses, and single-crystal X-ray diffraction. Structural analyses indicate that 1 and 2 are both VO-bta-N-heterocycle system complexes. The central vanadium is coordinated by N₂O₄ donors to form a distorted octahedral geometry. The complexes exhibit catalytic bromination activity in a single-pot reaction with conversion of phenol red to bromophenol blue in a mixed solution of H ₂O-DMF at 30 ± 0.5°C with pH 5.8, indicating that they can be considered as a functional model of vanadium-dependent haloperoxidases. The practical application of H₂O₂ detection has also been studied. 2014

Full text of DOI:10.1080/00958972.2014.926007

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Dinuclear vanadium complexes with
rigid phenylpolycarboxylate ligands:
synthesis, structure, and catalytic
bromination reaction with potential
detection of hydrogen peroxide
Rui Zhanga, Xiao-Xi Zhanga, Feng-Ying Baib, Chen Chena, Qing-Lin
Guana, Ya-Nan Houa, Xuan Wanga & Yong-Heng Xinga
a College of Chemistry and Chemical Engineering, Liaoning Normal
University, Dalian, PR China
b College of Life Science, Liaoning Normal University, Dalian, PR
China
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Accepted author version posted online: 19 May 2014.Published
online: 13 Jun 2014.
To cite this article: Rui Zhang, Xiao-Xi Zhang, Feng-Ying Bai, Chen Chen, Qing-Lin Guan,
Ya-Nan Hou, Xuan Wang & Yong-Heng Xing (2014) Dinuclear vanadium complexes with rigid
phenylpolycarboxylate ligands: synthesis, structure, and catalytic bromination reaction with
potential detection of hydrogen peroxide, Journal of Coordination Chemistry, 67:9, 1613-1628, DOI:
10.1080/00958972.2014.926007
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 9, 1613–1628, http://dx.doi.org/10.1080/00958972.2014.926007
Dinuclear vanadium complexes with rigid
phenylpolycarboxylate ligands: synthesis, structure, and
catalytic bromination reaction with potential detection of
hydrogen peroxide
RUI ZHANG†, XIAO-XI ZHANG†, FENG-YING BAI*‡, CHEN CHEN†,
QING-LIN GUAN†, YA-NAN HOU†, XUAN WANG† and YONG-HENG XING**†
†College of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian, PR China
‡College of Life Science, Liaoning Normal University, Dalian, PR China
(Received 15 August 2013; accepted 11 March 2014)
Two oxovanadium complexes, (VO)2(2,2′-bipy)2(bta)(H2O)2 (1) and (VO)2(1,10-phen)2(bta)
(H2O)2 (2) (H4bta = 1,2,4,5-benzenetetracarboxylic acid, 2,2′-bipy = 2,2-bipyridine and 10-phen =
1,10-phenanthroline), have been synthesized and characterized. The catalytic bromination activity
and the practical application of H2O2 detection have been studied.
Vanadium complexes (VO)2(2,2′-bipy)2(bta)(H2O)2 (1) and (VO)2(1,10-phen)2(bta)(H2O)2 (2)
(H4bta = 1,2,4,5-benzenetetracarboxylic acid, 2,2′-bipy = 2,2-bipyridine and 1,10-phen = 1,
*Corresponding authors. Email: baifengying2000@163.com (F.-Y. Bai); xingyongheng@lnnu.edu.cn (Y.-H. Xing)
© 2014 Taylor & Francis

1614
R. Zhang et al.
10-phenanthroline) have been synthesized by the reaction of V2(SO4)3, H4bta, 2,2′-bipy (for 1) and
1,10-phen (for 2) by hydrothermal methods. The complexes were characterized by elemental analy-
sis, IR, UV–vis, thermogravimetric analyses, and single-crystal X-ray diffraction. Structural analyses
indicate that 1 and 2 are both VO-bta-N-heterocycle system complexes. The central vanadium is
coordinated by N2O4 donors to form a distorted octahedral geometry. The complexes exhibit cata-
lytic bromination activity in a single-pot reaction with conversion of phenol red to bromophenol
blue in a mixed solution of H2O-DMF at 30 0.5 °C with pH 5.8, indicating that they can be
considered as a functional model of vanadium-dependent haloperoxidases. The practical application
of H2O2 detection has also been studied.
Keywords: Dinuclear oxovanadium complex; 1,2,4,5-Benzenetetracarboxylic acid; Bromination
reaction activity; Crystal structure
1. Introduction
Vanadium has received a great deal of attention due to significant roles in biochemical and
catalytic properties, such as haloperoxidation, nitrogen fixation, and metalloproteins [1–3].
The metal also has the ability to lower plasma glucose levels, stimulate glucose uptake and
glycogen synthesis, and enhance insulin sensitivity [4–6]. Among these properties, vana-
dium haloperoxidases (V-HPOs) which are found in marine algae are able to accelerate the
oxidative halogenation of organic compounds [7–9] in the presence of hydrogen peroxide,
organic hydroperoxides, or molecular oxygen [10, 11]. The structure of the active centers of
V-HPO reveal the central vanadium to be in a trigonal-bipyramidal environment linked to
four oxygens and the axially binding Nε of a histidine residue, which is embedded in the
protein molecule, through an extensive hydrogen-bonding network. The active site is con-
sidered to consist of oxidovanadium-containing O and N donor ligands [12–16]. In particu-
lar, the interaction of simple vanadium species (VO2+ and VO3+) with various ON or NN
donors having bioactive ligands is of growing interest [17]. In order to explore the relation-
ship between the structure and the catalytic activity of the enzyme mimics, a large number
of vanadium complexes have been synthesized. For example, vanadium complexes with
poly(pyrazolyl)borate ligands, TpVO(acac) [18] and Tp*VO(Me-acac), etc. [19], peroxid-
evanadium complexes VO(O2)(pzH)(Tp) and VO(O2)(pz)(pzTp), etc. [20], and vanadium
complexes with Schiff base ligands, [VO2(C9H7NO3)](C10H10N2)0.5 and [VO(C10H8N2)
(C9H7NO3)]3, etc. [21]. The coordination environment of the center vanadium with different
–N or –O donors can be proposed as scheme 1, when dinuclear vanadium complexes with
N-heterocyclic ligands and rigid polycarboxylates are reported.
Most dinuclear vanadium complexes are bridged by C2O42−, such as (VO2)2(μ2-C2O4)
(C3H4N2)2 [22], V2(bpz*eaT)2(μ2-C2O4)(C2O4)2 [23], etc. Many efforts have been focused
to find a new bridged ligand for the synthesis of dinuclear vanadium complexes. As H4bta
is a flexible ligand with four carboxylic groups that can be partially or completely deproto-
nated, various coordination modes can be realized, while the aromatic ring can stabilize
crystal structures via π-stacking interactions. Studies of low valence dinuclear vanadium
(III/IV) complexes with H4bta as ligands were reported. Our group has interest in this fam-
ily of complexes with some related work having been reported [24]. In order to further
study the structure and the function of these vanadium complexes, we synthesized two di-
nuclear vanadium complexes, (VO)2(2,2′-bipy)2(bta)(H2O)2 (1) and (VO)2(1,10-phen)2(bta)
(H2O)2 (2) with H4bta as ligand, and studied the bromination activity. The detection of c
(H2O2) by the colorimetric method in the bromination reaction system is reported.

Dinuclear vanadium complexes
1615
O
O
V
O
V
N
N
N
N
O
O
O
N
N
V
N
N
N
N
N
N
O
V
O
V
O
V
N
O
N
O
O
O
O
O
O
N
N
N
N
N
O
Scheme 1. The coordinated environments of vanadium with different –N or –O ligands.
2. Experimental
2.1. Materials and methods
All chemicals used were of analytical grade and used without further purification. Elemental
analyses for C, H, and N were carried out on a Perkin Elmer 240C automatic analyzer.
Infrared spectra were recorded on a JASCO FT/IR-480 spectrometer with pressed KBr pel-
lets from 200 to 4000 cm−1. UV–vis spectra were recorded on a JASCO V-570 spectrometer
(200–1100 nm, in form of solid sample). Thermogravimetric analyses (TG) were performed
under air with a heating rate of 10 °C min−1 on a Perkin Elmer Diamond TG/DTA.
2.2. Synthesis of the complexes
2.2.1. (VO)2(2,2′-bipy)2(bta)(H2O)2 (1). V2(SO4)3 (0.25 mM, 0.098 g), 2,2′-bipy (0.25
mM, 0.042 g), and H4bta (0.25 mM, 0.064 g) were dissolved in methanol–water (3 : 1)
mixed solution, stirred for 1–2 h at room temperature, giving a yellow–green solution.
Then, the mixture was sealed in a Teflon-lined autoclave and heated at 100 °C for three
days, cooled to room temperature and green crystals of 1 were obtained in ca. 52.6% yield
based on V(IV). Anal. Calcd for C30H22O12N4V2: C, 49.16; H, 2.98; N, 7.67. Found: C,
49.23; H, 3.21; N, 7.76. IR (KBr, ν, cm−1): 3060, 1599, 1378, 1487, 1444, 1320, 1164,
1024, 981, 457, 400. UV–vis (λmax, nm): 256, 316, 388, 750.
2.2.2. (VO)2(1,10-phen)2(bta)(H2O)2 (2). The synthesis of 2 was similar to that of 1;
however, 2,2′-bipy was replaced by 1,10-phen (0.25 mM, 0.045 g). Green crystals of 2 were
obtained in ca. 63.4% yield based on V(IV). Anal. Calcd for C34H22O12N4V2: C, 52.32; H,
2.84; N, 7.18. Found: C, 52.41; H, 3.02; N, 7.26. IR (KBr, ν, cm−1): 3059, 1616, 1519,
1485, 1428, 1373, 1343, 1146, 1107, 982, 849, 726, 647, 458, 429, 396. UV–vis
(λmax, nm): 262, 304, 356, 444, 780.
2.3. X-ray single crystal structural determination
The crystals of 1 and 2 were mounted on a glass fiber for X-ray measurement. Reflection
data were collected at room temperature on a Bruker AXS SMART APEX II CCD diffrac-
tometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and a ω scan mode.
All the measured independent reflections (I > 2σ(I)) were used in the structural analyses,
and semi-empirical absorption corrections were applied using SADABS [25]. The structure

1616
R. Zhang et al.
Table 1. Crystallographic data and structure refinement for 1.
Formula
C30H22N4O12V2
732.4
M (g M−1
)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/n
10.8353(7)
11.8109(8)
12.1040(7)
90
α (°)
β (°)
102.7910(10)
90
1510.57(17)
1
1.610
0.58 × 0.32 × 0.16
744
γ (°)
V (Å3)
Z
DCalcd (Mg m−3
)
Crystal size (mm3)
F (0 0 0)
μ (Mo Kα) (mm−1
)
0.692
Reflections collected
Independent reflections (I > 2σ(I))
Parameters
9319
3648(2914)
214
Δ(ρ) (e Å−3
)
0.562 and −0.746
1.037
Goodness-of-fit
R1
0.0394(0.0538)b
0.1079(0.1172)b
a
a
wR2
aR1 = Σ||Fo| − |Fc||/Σ|Fo|, wR2 = {Σw(Fo2 − Fc2)2/Σw(Fo2)2}1/2
,
[Fo > 4σ(Fo)].
bBased on all data.
was solved by the direct method using SHELXL-97 [26]. All hydrogens of organic ligand
were fixed at calculated positions geometrically and refined using a riding model. Hydro-
gens of coordination water were located in difference Fourier maps. The non-hydrogen
atoms were refined with anisotropic thermal parameters. Crystallographic data and the struc-
ture refinements are given in table 1.
2.4. Measurement of bromination activity in solution
Brominations were carried out at 30 0.5 °C. Oxidovanadium complex was dissolved in a
mixed solution of 25 mL H2O-DMF (DMF: 2 mL; H2O: 23 mL). The solutions used for
kinetic measurements were maintained at a constant concentration of H+ (pH 5.8) by the
addition of NaH2PO4–Na2HPO4 buffer solution [27]. Reactions were initiated with the pres-
ence of phenol red solution. Five different concentrations of oxidovanadium complex were
placed in five cuvettes and then put into the constant temperature water bath. The spectral
changes were recorded using a 721 UV–vis spectrophotometer at 5 min intervals, and the
resulting data were collected during the reaction.
It was assumed that the rate of this reaction is described by the rate equation: dc/dt =
x
y
z
kc1 c2 c3 , from which the equation “log(dc/dt) = log k + xlog c1 + ylog c2 + zlog c3” was
obtained, corresponding to “−log(dc/dt)=− xlog c1 − b (b = log k + ylog c2 + zlog c3),” where
k is the reaction rate constant; c1, c2, and c3 are the concentrations of the complex, the KBr,
and the phenol red, respectively; while x, y, and z are the corresponding reaction orders.
According to Beer–Lambert’s law, A = ε·d·c, which is differential, dA/dt = ε·d·(dc/dt), where
A is the measurable absorbance of the resultant; ε is molar absorption coefficient, of which
the bromophenol blue is measured as 14,500 M−1 cm−1 at 592 nm; d is light path length of

Dinuclear vanadium complexes
1617
sample cell (d = 1). When the measurable absorbance data were plotted versus reaction time,
a line was obtained and the reaction rate of the complexes (dA/dt) was given by the slope
of the line. By changing the concentration of the oxovanadium complex in the reaction sys-
tem, a series of dA/dt data could be obtained. The reaction rate constant (k) could be
obtained according to a plot of −log (dc/dt) versus −log c1 and were fitted using the curve-
fitting software in Microsoft Excel by generating least squares fit to a general equation of
the form “y = mx − b,” in which “m” is the reaction order of the oxovanadium complex in
this reaction and “b” is the intercept of the line. In the experiment, considering that the reac-
tion orders of KBr and phenol red (y and z) are 1 according to the literature [12, 28]; c2 and
c3 are 0.4 and 10−4 M L−1, respectively. Based on the equation “b = logk + ylogc2 + zlogc3,”
and then the reaction rate constant (k) can be obtained. The bromination of phenol red was
monitored by measurement of the absorbance at 592 nm for the reaction solution, taken at
specific time points and diluted into the phosphate buffer solution (pH 5.8).
3. Results and discussion
3.1. Synthesis
Two vanadium coordination complexes with H4bta as ligand have been synthesized by
hydrothermal methods at 100 °C (scheme 2). Although the complexes have been synthe-
sized in the literature [29], single crystal structure data were not obtained. V2(SO4)3 (III)
was used in the synthetic process of the complexes, however, the valence state of vanadium
has been changed. The reason may be that the higher temperature led to V(III) oxidizing
into V(IV) by oxygen in the air. Because the starting material V2(SO4)3 (III) is not soluble
in common solvents, 1 and 2 could not obtained by conventional synthesis.
3.2. Spectra properties
3.2.1. IR spectra. IR spectra of 1 and 2 are shown in figure S1 (see online supplemental
material at http://dx.doi.org/10.1080/00958972.2014.926007) and selected data are listed in
table 2. The weak peaks observed at 3000–3100 cm−1 are attributed to the =C–H stretch
[24]. The strong peaks at 1600 cm−1 (1599 cm−1 for 1 and 1616 cm−1 for 2) and 1380 cm−1
(1378 cm−1 for 1 and 1373 cm−1 for 2) are attributed to asymmetric and symmetric stretches
of C=O group, respectively [30]. Peaks at 1000–1500 cm−1 are assigned to stretching vibra-
tions of pyridine [31]. The strong and sharp stretch of V=O are observed in the region of
981–982 cm−1 [32] and 457–458 cm−1 are due to the V–O stretch, while bands at 396–400
cm−1 are assigned to V–N stretch [33]. The infrared spectra of the complexes are consistent
with the structural characterization of 1 and 2.
O
C
O
C
O
O
O
O
N
N
N
N
O
O
O
O
V2(SO4)3, 100oC
Methanol-water
OH
OH
HO
HO
RN2
R
R
+
V
V
C
O
C
O
O
O
O
O
Scheme 2. The synthesis of 1 and 2 (for 1: RN2 = 2,2′-bipy; for 2: RN2 = 1,10-phen).

1618
R. Zhang et al.
Table 2. IR data (cm−1) for 1 and 2.
Complexes
1
2
ν(=C–H)
3060
1599
1378
3059
1616
1373
−
νas(COO
)
−
νs(COO
)
Pyridine ring
ν(V=O)
ν(V–O)
1487, 1444, 1320, 1164, 1024
1485, 1428, 1343, 1146, 1107
981
457
400
982
458
396
ν(V–N)
3.2.2. UV–vis absorption spectra. UV–vis absorption spectra of 1 and 2 (Supplementary
material) were recorded as solid samples (figure S2). The high-frequency absorptions at
256–262 nm are assigned to π–π* transitions of the aromatic-like chromophore from 2,2′-
bipy, H4bta and 1,10-phen [34]. Absorptions at 304–444 nm are attributed to LMCT (ligand
to metal charge transfer) transition. For vanadyl(IV) complexes, it is generally considered
that transitions occur from dxy to (dxz, dyz) (ν1), d2 −d2 (ν2), and d2 (ν3) orbitals with
x
y
z
increasing energies. Complexes 1 and 2 exhibit a set of absorptions with maximum wave-
length at 750–780 nm, which can be assigned to (ν1) bands of oxovanadium(IV) [35].
3.3. Structural description of 1 and 2
The molecular structure of 1 is depicted in figure 1. Selected bond distances, bond angles,
and hydrogen bonds are summarized in table 3.
Structural analysis shows that 1 crystallizes in the monoclinic system with P21/n space
group. The molecular structure of 1 contains two vanadiums, two terminal oxygens, one
H4bta, two 2,2′-bipy, and two coordinated waters. The binuclear molecule is centrosymmet-
ric, while the coordination environment of the two vanadiums is the same, so we take one
for an example. The oxidation state of vanadium is +4, confirmed by Bond-Valence Theory.
The vanadium is coordinated by one terminal oxygen (O1), N1 and N2 from 2,2′-bipy, O2
from coordinated water, and two oxygens (O3 and O4) from H4bta, to form a distorted
octahedron. The deviations of O2, O3, O4, and N1 that composed the least-squares plane
are 0.0389, −0.0421, 0.0396, and −0.0365 Å, respectively, showing that these atoms are
almost on one plane. The V, O1, and N2 from the axial positions lay 0.3205, 1.9120, and
−1.9217 Å out of the equatorial plane, indicating that V is toward O1, trans toward N2. The
distance of V=O (1.6030(17) Å) is within the range reported for VO2+ complexes [23, 33,
Figure 1. The molecular structure of 1 (hydrogens are omitted for clarity).

Dinuclear vanadium complexes
1619

1620
R. Zhang et al.
36, 47]. The V–Owater distance is 2.0507 Å, close to the typical bond lengths for V–OH2
bond [37]. V–Ocarboxyl distances are 1.9657 and 1.9949 Å, shorter than that of [V2(di-
pic)2(H2bta)(H2O)4]·2H2O (H2dipic = 2,6-pyridinedicarboxylic acid) [24], which can be
explained as the consequence of the different coordinated H4bta ligand. The bond lengths
of V–N are 2.142(2) and 2.2619(18) Å, which are close to reported values [38]. The angles
of the O–V–O and O–V–N are 86.74(6)–163.73(7)° and 81.59(7)–165.73(8)°, respectively,
and the angle of N–V–N is 72.89(8)°.
There are three kinds of hydrogen bonds (figure 2) in 1: (i) hydrogen bond between coor-
dinated oxygen and oxygen from the carboxylate of H4bta, O2#2–H2A#2⋯O5 (2.7025 Å,
161.03°, #2: x, y, −1 + z); (ii) hydrogen bond between oxygen from the carboxylate of
Figure 2. (a) A view of 2-D hydrogen bonding network in 1; (b) a view of 3-D hydrogen bonding network (all
hydrogens except for the hydrogen bonds are omitted for clarity).

Dinuclear vanadium complexes
1621
O
C
O
C
O
O
N
N
N
N
O
O
O
O
V
V
C
C
O
H2O
OH2
O
Figure 3. The proposed structure of 2.
H4bta and carbon of 2,2′-bipy: C4#1–H4#1⋯O5 (3.3220 Å, 159.32°, #1: 1 − x, −y, −z);
C9#2–H9#2⋯O5#3 (3.5045 Å, 170.57°, #3: −0.5 + x, 0.5 − y, 0.5 + z). The molecules are
connected to a 2-D superamolecular sheet by the hydrogen interactions of O2#2–
H2A#2⋯O5 and C4#1–H4#1⋯O5; (iii) neighboring sheets are connected to a 3-D network
through the hydrogen interactions of C9#2–H9#2⋯O5#3.
The structure of 2 is similar to that of 1 (figure 3), confirmed by IR and UV–vis spectros-
copy, elemental analyses, and TG. However, its crystal structure data was not obtained
because of poor quality of the crystal.
3.4. Thermal properties
To examine the thermal stabilities of 1 and 2, TG was carried out at a heating rate of 10 °C
min−1 under N2 from 30 to 1000 °C (figure 4). The TG curve of 1 shows two stages, the
first stage occurs from 30 to 576 °C with weight loss of 59.59% (Calcd 59.67%), which is
attributed to release of two coordinated waters, two 2,2′-bipy and two coordinated carboxyl.
The second stage is observed from 576 to 1000 °C, which is considered from two coordi-
nated carboxyl and the gradual decomposition of the framework of H4bta. The TG curve of
Figure 4. The TG curves for 1.

1622
R. Zhang et al.
2 can be divided into three stages. The initial weight loss of 5% (Calcd 4.62%) from 30 to
197 °C is due to the release of two coordinated waters. The second weight loss of 45.49%
(Calcd 46.18%) at 197 to 632 °C is from the release of two 1,10-phen. The last step of
decomposition occurred from 632 to 1000 °C, from loss of H4bta. The final residue corre-
sponds to vanadium oxide and nitride.
3.5. XRD analysis
The powder X-ray diffraction data of 1 was obtained and compared with the corresponding
simulated single-crystal diffraction data (figure 5). The complex is considered pure owing
to agreement of the peak positions. The different intensities may be due to preferred orienta-
tion of the powder samples.
3.6. Functional mimics of the V-HPOs
3.6.1. Mimicking bromination of the complexes. Oxidovanadium complexes are able to
mimic a reaction in which V-HPOs catalyze the bromination of organic substrates in the
presence of H2O2 and bromide [39, 40]. For example, bromination of trimethoxybenzene
[41], benzene, salicylaldehyde, and phenol by VOþ2 [42], and the bromination of phenol red
by [VO(O2)H2O]+ and related species [43]. Herein, the bromination activity of 1 and 2
using phenol red as substrate, which is shown by conversion of phenol red to bromophenol
blue, has been investigated. The reaction is rapid and stoichiometric, producing the haloge-
nated product by reaction of oxidized halogen species with the organic substrate, and the
reactive process is shown in scheme 3.
Addition of the solution of 1 to the standard reaction of bromide in a phosphate buffer
with phenol red as a trap for oxidized bromine resulted in color change of the solution from
yellow to blue. The electronic absorption showed a decrease in absorbance of the peak at
443 nm from loss of phenol red and an increase of the peak at 592 nm with production of
Figure 5. The simulation XRD (a) and experimental XRD (b) spectrum of 1.

Dinuclear vanadium complexes
1623
Scheme 3. The reactive process of bromination reaction for the complexes.
bromophenol blue (shown in figure 6), showing that 1 possesses better catalytic activity.
The catalytic activity for 2 is similar to that of 1.
3.6.2. Kinetic studies of bromination. We used 2 to carry out kinetic studies of bromina-
tion. A series of dA/dt data were obtained by changing the concentration of the oxovana-
dium complex, then the plot of −log(dc/dt) versus −logc for 2 was depicted according to
the data of figure 7, obtaining a straight line (figure 8) with a slope of 1.018 and an inter-
cept of −2.1331. The former confirms the first-order reaction is dependent on vanadium.
Based on the equation “b = log k + ylog c2 + zlog c3,” the reaction rate constant, k, is deter-
mined by the concentrations of KBr and phenol red (c2 and c3), the reaction orders of KBr
and phenol red (y and z), as well as b. Considering that the reaction orders of KBr and
phenol red (y and z) are 1 according to the literature [44]; c2 and c3 are known to be 0.4
and 10−4 M L−1, respectively, so the reaction rate constant (k) for 2 can be calculated as
0.184 × 103 (M L−1)−2 s−1.
Similar plots for 1 were generated in the same way (figures S3 and S4), and values of the
slope and the intercept are 1.0033 and −2.1251, respectively.
The results show that: (i) the reaction orders of the oxidovanadium complexes in bromin-
ation reactions are close to 1, confirming the first-order dependence on vanadium; (ii) the
reaction rate constants of the two complexes are close, which is probably due to similarity
of their structures.
The halide oxidation catalytic reaction mechanism for the oxovanadium complexes in this
work is shown in scheme 4. The peroxidovanadium intermediate [VO(O2)N2] was formed
Figure 6. Oxidative bromination of phenol red catalyzed by 1. Spectral changes at 10 min intervals. The reaction
mixture contained phosphate buffer (pH 5.8), KBr (0.4 M L−1), phenol red (10−4 M L−1), and 1 (0.1 μM L−1).

1624
R. Zhang et al.
Figure 7. A series of linear calibration plots of the absorbance at 592 nm dependence of time for different concen-
tration of 2. Condition used: pH 5.8, c(KBr) = 0.4 M L−1, c(H2O2) = 1 M L−1, c(phenol red) = 10−4 M L−1. c(com-
plex 2/mM L−1) = a: 1.08 × 10−5; b: 2.15 × 10−5; c: 3.23 × 10−5; d: 4.31 × 10−5; e: 5.38 × 10−5. Error bars shown are
three standard deviations for three independent measurements.
Figure 8. −Log(dc/dt) dependence of −logc for 2 in DMF-H2O at 30 0.5 °C (c is the concentration of 2).
by action of oxidovanadium complexes with H2O2 (step a). Then Br− is oxidized rapidly by
[VO(O2)N2], with creation of Br+ and [VO2N2] at the same time (step b). The oxidized bro-
mides can be used to perform a halogenation reaction when an organic substrate is present
(step c). The hydrogen peroxide was decomposed into water and dioxygen in the absence
of a substrate (step d) [45]. The catalytic reaction rate would be mainly based on the
stability of the formed intermediate [VO(O2)N2].
We compared the catalytic activity of some complexes reported previously, and the
kinetic data are listed in table 4. It is found that: (i) vanadium(III) complexes show
higher catalytic activity than those of vanadium(IV) complexes; (ii) different composition
of the vanadium(IV) complexes lead to different rates of formation of the intermediate

Dinuclear vanadium complexes
1625
O
O
V
N
N
N
N
H2O2
a
O
L
V
O
OH
H2O + O2 +X-
X- + H+
d
b
H2O2 + 2OH-
OH-
O
V
H+
RH
N
+ X+
RX
c
O
N
Scheme 4. The catalytic bromination reaction mechanism of the complex.
[VO(O2)N2], which can also influence the catalytic activity: (a) the complexes with coordi-
nated H2O have higher catalytic activity, such as 5, 6, and 10, attributed to low bond energy
of the V–OH2O and easy dissociation of water, resulting in easier formation of the interme-
diate; (b) conjugated tridentate ligand makes the complexes more stable and more difficult
to form the intermediate, e.g. 4 and 8; (c) it is easier for complexes, which contain weakly
coordinating ligands, to form the intermediate, such as 11 and 9 (the order of the coordina-
tion ability is C2O24ꢀ > SO42ꢀ); (d) it is difficult to form the intermediate if the complex has
two V=O bonds, such as 1; (e) different coordination pattern results in a significant
Table 4. Comparisons of the kinetic data for the complexes in DMF–H2O at 30 0.5 °C.
Complexes
m
b
k(M L
)
−1 −2s−1
Ref.
1
2
1.0033
1.018
−2.1251
−2.1331
−2.1349
−2.0537
−1.0857
−1.3284
−2.2581
−2.11
0.187 × 103
0.184 × 103
0.183 × 103
0.221 × 103
2.054 × 103
1.174 × 103
0.14 × 103
0.19 × 103
0.52 × 103
1.28 × 103
0.70 × 103
This work
This work
[34]
[34]
[21]
[21]
[20]
[20]
[20]
3a
4b
5c
6d
7e
8f
1.0458
1.0314
1.0381
1.0495
1.0527
1.0976
1.0666
1.0967
0.9808
9g
10h
11i
−1.6809
−1.2923
−1.5521
[20]
[20]
a[VO(Tp)(pzTp)]·2H2O.
b[VO(Tp4I)(pz)(SCN)]·1/2CH2Cl2.
c[V2(dipic)2(H2btec)(H2O)4]·2H2O.
dV(dipic)(Hbdc)(H2O)2.
e(VO)2(bpz*T-O).
fVO(bpz*eaT)(SCN)2.
gV2(bpz*eaT)2(μ2-C2O4)(C2O4)2.
h[VO(SO4)(bpz*P)(H2O)]·H2O.
iVO(SO4)(bpz*P-Me)(H2O).

1626
R. Zhang et al.
Figure 9. A series of lines of the absorbance at 592 nm dependent on c(H2O2) at 5 min intervals of 2 (a: 25 min;
b: 30 min; c: 35 min; d: 40 min; e: 45 min). Error bars shown are three standard deviations for three independent
measurements.
influence on stability of the complexes, for example, for 1 and 2, the V–OH2bta bond length
of the coordinated 1,2,4,5-benzenetetracarboxylic acid is shorter than that of 6, indicating
formation of the intermediate is more difficult. These results show that the catalytic activity
of the complexes may have great connection with their structural characterization.
3.6.3. The effect of the concentration of H2O2 on bromination. H2O2 is widely used in
our daily life and industry [46], and the determination of it is of great practical importance
in fields such as foodstuff, clinical physic, pharmaceutical production, and environment pro-
tection. So we need a measure to determine H2O2 in practical application. We have studied
the dependence of the concentration of H2O2 and the bromination catalytic reaction to
develop a new H2O2 detection method. The results show that the oxidation catalyzed by the
oxovanadium complexes is H2O2 concentration dependent, and the absorbance at 592 nm
linearly depends on concentration of H2O2 along with the formation of bromophenol blue.
A series of lines dependent on c(H2O2) at 5 min intervals (among 25–45 min) were
plotted for 2 (figure 9). The detection limit of the concentration H2O2 was estimated to be
1.0 M L−1 by extension of the line. All these observations further confirmed that the
catalytic reaction system can be used as a method for H2O2 detection, and the complex can
be used as a potential diagnostic kit for H2O2. In the literature [47], 30–40 min is needed
for the complexes to complete the reaction, while we could get good linear relations from
25 to 45 min in this work, perhaps from different structures of the complexes. These
experiment results are similar to those of the literature [48].
4. Conclusion
Two vanadium(IV) complexes with 1,2,4,5-benzenetetracarboxylic acid ligands have been
synthesized by hydrothermal reaction of V2(SO4)3(III) in methanol–water solution, and the
single crystal structure data have been obtained. To explore the model oxovanadium

Dinuclear vanadium complexes
1627
complexes for the active center of VHPO and the further study of the H2O2 detection sys-
tem, bromination activities were tested with phenol red as organic substrate in the presence
of H2O2, KBr, and phosphate buffer. By comparing with complexes in the literature, the
complexes are good candidates for H2O2 detection system upon phenol red bromination
since the reaction time is shorter. Based on the results, we expect that a fast, simple, and
sensitive H2O2 detection device will be exploited in the future. Further research of practical
application of the H2O2 detection is ongoing.
Supplementary material
Tables of atomic coordinates, isotropic thermal parameters, and complete bond distances
and angles have been deposited with the Cambridge Crystallographic Data Center. Copies
of this information may be obtained free of charge by quoting the publication citation and
deposition number CCDC: 952227 for 1, from the Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (Fax:+ 44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk).
Funding
This study was supported by the grants of the National Natural Science Foundation of China [grant number
21071071], [grant number 21371086] for financial assistance.
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