Synthesis, structure, and catalytic bromination of supramolecular oxovanadium complexes containing oxalate

Article abstract of DOI:10.1080/00958972.2013.765561

Three supramolecular complexes, [VO(phen)(C₂O ₄)(H₂O)]·CH₃OH (1) [(VO) ₂(u₂-C₂O₄)(C₂O ₄)₂(H₂O)₂]·L·H ₂O (2), and [(4,4′-bipyH₂)_0.5] ⁺[VO₂(2,6-dipic)]^-·2H₂O (3) (phen = 1,10-phenanthroline 4,4′-bipy = 4,4′-bipyridine, 2,6-dipic = 2,6-pyridinedicarboxylic, L = 1,4-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl) benzene), have been prepared and characterized by elemental analysis, IR, and UV-vis spectroscopy and single-crystal diffraction analysis. Structural analysis shows that the three complexes all contain carboxylate and V=O moiety; vanadium of 1 and 2 are six coordinate with distorted octahedral geometry with N ₂O₄ and O₆ donor sets, respectively, while 3 is five coordinate with distorted trigonal bipyramidal geometry with a NO ₄ donor set. The complexes exhibit catalytic bromination activity in the single-pot reaction for the conversion of phenol red to bromophenol blue in H₂O-DMF at 30 ± 0.5 C with pH 5.8, indicating that they can be considered as functional model vanadium-dependent haloperoxidases. In addition, electrochemical behaviors are also studied.

Full text of DOI:10.1080/00958972.2013.765561

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Synthesis, structure, and catalytic
bromination of supramolecular
oxovanadium complexes containing
oxalate
a
b
a
a
a
Chen Chen , Feng-Ying Bai , Rui Zhang , Ge Song , Hui Shan
a
a
,
a
Na Xing & Yong-Heng Xing
College of Chemistry and Chemical Engineering, Liaoning Normal
University, Dalian, ꢀP.R. China
b
College of Life Science, Liaoning Normal University, Dalian,
P.R.China
Accepted author version posted online: 17 Jan 2013.Version of
record first published: 05 Mar 2013.
To cite this article: Chen Chen , Feng-Ying Bai , Rui Zhang , Ge Song , Hui Shan , Na Xing & Yong-
Heng Xing (2013): Synthesis, structure, and catalytic bromination of supramolecular oxovanadium
complexes containing oxalate, Journal of Coordination Chemistry, 66:4, 671-688
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Journal of Coordination Chemistry
Vol. 66, No. 4, 20 February 2013, 671–688
Synthesis, structure, and catalytic bromination of
supramolecular oxovanadium complexes containing oxalate
CHEN CHEN†, FENG-YING BAI‡, RUI ZHANG†, GE SONG†, HUI SHAN†,
NA XING† and YONG-HENG XING*†
†
College of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian,
P.R. China
‡
College of Life Science, Liaoning Normal University, Dalian, P.R. China
(
Received 31 August 2012; in final form 2 November 2012)
Three supramolecular complexes, [VO(phen)(C
2
O
4
)(H
2
O)]·CH
3
OH (1) [(VO)
O (3) (phen = 1,10-phenanthroline
2 2 2 4 2 4 2
(u -C O )(C O )
+
ꢀ
(
H O)
2 2
]·L·H
2
O (2), and [(4,4′-bipyH
2
)
0.5] [VO
2
(2,6-dipic)] ·2H
2
4,4′-bipy = 4,4′-bipyridine, 2,6-dipic = 2,6-pyridinedicarboxylic, L= 1,4-bis((3,5-dimethyl-1H-pyra-
zol-1-yl)methyl)benzene), have been prepared and characterized by elemental analysis, IR, and
UV–vis spectroscopy and single-crystal diffraction analysis. Structural analysis shows that the three
complexes all contain carboxylate and V=O moiety; vanadium of 1 and 2 are six coordinate with
distorted octahedral geometry with N
with distorted trigonal bipyramidal geometry with a NO
bromination activity in the single-pot reaction for the conversion of phenol red to bromophenol
blue in H O–DMF at 30 ± 0.5 °C with pH 5.8, indicating that they can be considered as functional
2
O
4
and O
6
donor sets, respectively, while 3 is five coordinate
4
donor set. The complexes exhibit catalytic
2
model vanadium-dependent haloperoxidases. In addition, electrochemical behaviors are also
studied.
Keywords: Vanadium complex; Hydrogen bonds; Electrochemistry; Bromination reaction activity
1. Introduction
Coordination chemistry of vanadium has received attention because of the biological and
catalytic properties of relevant systems [1–8], such as haloperoxidation, nitrogen fixation
[9,10], phosphorylation, glycogen metabolism, and insulin mimicking [11–13]. In particu-
lar, vanadate-dependent haloperoxidases (V-HPOs) have developed in catalyzing different
oxidation reactions like oxidation of halides [14,15], which could catalyze the two-electron
+
+
ꢀ
oxidation of halides to an {X } species, where {X } is X , X and/or XOH, that is, hyp-
2
3
ohalous acid in the presence of hydrogen peroxide, resulting in halogenation of certain
organic substrates [16].
Structural characterization of vanadium-dependent haloperoxidases has revealed that
ɛ
vanadium is covalently linked to the N of imidazolyl of a histidine, and further through
extensive hydrogen bonding to a variety of amino acid side chains (Arg, His, Ser, Lys)
and interstitial water in the proximity of the active centre [17,18]. Bromoperoxidation was
*
Corresponding author. Email: xingyongheng@lnnu.edu.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online Ó 2013 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2013.765561

6
72
C. Chen et al.
influenced by interactions of covalents and also affected by non-covalent interactions [19],
such as hydrogen-bonding and metal-ligand coordination, as well as π-π stacking, hydro-
phobic, ionic, and van der Waals forces [20,21]. The role of hydrogen bonding in stabiliza-
tion of structures as well as in the functions of various metalloproteins is well known in
biochemistry [22]. Hydrogen-bonding interactions influence catalytic reactions to some
extent.
Numerous supramolecular vanadium complexes containing N, O-donor ligands (as
acceptors for participating in inter- and intramolecular hydrogen bonds) have been pre-
pared [23,24], such as (i) [VO(OMe)(tbhsR)] (H tbhsR: the O, N, S-donor Schiff bases
2
derived from thiobenzhydrazide and 5-substituted salicylaldehydes; R = H, OMe, Cl, Br
and NO ) [25], which form dimeric, 1-D and 2-D self-assembled structures via intermolec-
2
ular C–H···O and O–H···O interactions. (ii) [VO(L) ] (L= N-salicylidin-2-bromoethyli-
2
mine), which form a zigzag chain via only two intermolecular hydrogen bonds (C–H···O)
[26]. However, study on these complexes in catalytic reactions of bromoperoxidation is
comparatively rare. Our group has a long-standing interest in investigation of the influence
of oxovanadium complexes containing N, O-donors (especially carboxylates) on bromoper-
oxidation. We have reported a family of crystal structures of oxovanadium complexes with
⁄
⁄
⁄
carboxylate ligands: TpVO(L ) and Tp VO(pzH )(L ) (Tp = hydrotris(pyrazolyl)borate,
1
2
HL = 5-methyl-1H–pyrazole-3-carboxylic acid, Tp = hydrotris(3,5-dimethyl-pyrazolyl)
1
⁄
borate, pzH = 3,5-dimethylpyrazole, HL = 5-phenyl-1H-pyrazole-3-carboxylic acid) [27],
2
⁄
⁄
⁄
⁄
⁄
⁄
Tp VO(pzH )(CH COO), Tp VO(pzH )(C H COO) and Tp VO(pzH )(m-NO C H
2 6 4-
3
6
5
COO)·CH CN [28], which pertain to the mechanism of bromoperoxidation. However,
3
hydrogen-bonding interaction for influence on this catalytic activity is rarely investigated.
In this manuscript, we report the syntheses, crystal structures and properties of three
supramolecular complexes, [VO(phen)(C O )(H O)]·CH OH (1), [(VO) (u C O )
2
4
2
3
2
2 2 4
+
ꢀ
(
C O ) (H O) ]·L·H O (2), and [(4,4′-bipyH ) ] [VO (2,6-dipic)] ·2H O (3) (phen = 1,
2 4 2 2 2 2 2 0.5 2 2
1
0-phenanthroline, 4,4′-bipy = 4,4′–bipyridine, 2,6-dipic = 2,6-pyridinedicarboxylic, L= 1,4-
bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)benzene). Bromination activity and influence of
hydrogen-bonding interaction on bromination have also been investigated.
2
2
. Experimental
.1. Materials and methods
All chemicals were of analytical grade and used without purification. L was synthesized
according to the literature method [29,30]. Elemental analyzes for C, H, and N were
carried out on a Perkin Elmer 240C automatic analyzer. Infrared spectra were recorded on
ꢀ
1
a JASCO FT/IR-480 spectrometer with pressed KBr pellets from 200 to 4000 cm
.
UV–vis spectra were recorded on a JASCO V-570 spectrometer (200–2500 nm, as solid
samples).
2
2
.2. Synthesis of the complexes
.2.1. [VO(phen)(C O )(H O)]·CH OH (1). VO(acac) (0.27 g, 1.00 mmol), 1,10-phen
2
4
2
3
2
(
0.2 g, 1.00 mmol) and H C O (0.13 g, 1.00 mmol), were dissolved in CH OH (10 mL)
2 2 4 3

Supramolecular oxovanadium
673
and the mixture was stirred for 4 h at room temperature to give a green solution. Then, the
solution was left at room temperature for a few days with green crystals obtained in ca.
3
7% yield based on V(IV). Anal. Calcd for C H O N V: C, 46.87; H, 3.65; N, 7.29.
15 14 7 2
ꢀ
1
Found: C, 46.55; H, 3.46; N, 7.21%. IR (KBr, ν, cm ): 3408, 3060, 1715, 1693, 1649,
1
5
386, 1517, 1424, 1230, 1142, 1105, 983, 451, 356. UV–vis (λmax, nm): 262, 342, 440,
38, 722.
2
.2.2. [(VO) (u -C O )(C O ) (H O) ]·L·H O (2). 2 was synthesized by procedure
2
2
2
4
2
4 2
2
2
2
similar to that for 1 but 1,10-phen was replaced by L in the reaction (0.29 g, 1.00 mmol)
in C H OH (95%). Primrose blue crystals of 2 were obtained in ca. 33% yield (based on
2
5
V(IV)). Anal. Calcd for C H O N V : C, 37.68; H, 3.92; N, 7.33. Found: C, 37.58; H,
2
4 30 18 4 2
ꢀ
1
3
.88; N, 7.30%. IR (KBr, ν, cm ): 3426, 3148, 2976, 1715, 1672, 1630, 1399, 983, 479.
UV–vis (λmax, nm): 260, 342, 586, 728.
+
ꢀ
2
.2.3. [(4,4′-bipyH2)0.5] [VO (2,6-dipic)] ·2H O (3). V (SO ) (0.39 g, 1.00 mmol),
2
2
2
4 3
4
,4′-bipy (0.16 g, 1.00 mmol), 2,6-dipic (0.17 g, 1.00 mmol) and H O (10 mL) were mixed
2
and stirred for 2 h, the mixture sealed into a bomb and heated at 160 C for 4 days, cooled
to room temperature and yellow crystals of 3 were obtained in ca. 45% yield (based on V
(
3
1
III)). Anal. Calcd for C H O N V: C, 39.65; H, 3.30; N, 7.71. Found: C, 39.56; H,
.32; N, 7.83%. IR (KBr, ν, cm ): 3439, 3104, 3076, 1677, 1640, 1344, 1489, 1433,
252, 1180, 1079, 958, 934, 470, 428, 360. UV–vis (λmax, nm): 262, 304, 322.
12 12 8 2
1
ꢀ
2
.3. Single-crystal structural determinations
The crystals of 1, 2, and 3 were mounted on glass fibers for X-ray measurement, respec-
tively. Reflection data were collected at room temperature on a Bruker AXS SMART
APEX II CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073
Å) and a ω scan mode. All independent reflections (I > 2σ (I)) were used in the structural
analyzes and semi-empirical absorption corrections were applied using SADABS [31]. The
structures were solved by direct methods using SHELXL-97 [32]. All non-hydrogen atoms
were refined anisotropically. Hydrogens were fixed at calculated positions geometrically
and refined by using a riding model, except hydrogens of coordinated water in 1 and
lattice waters and 4,4′-bipy in 3 were found in difference Fourier map. Crystallographic
data and structure refinement details are given in table 1. Drawings were made with
Diamond 3.2.
2
.4. Measurement of bromination activity in solution
Bromination activity tests were carried out in mixed H O–DMF solution at 30 ± 0.5 C.
2
Oxidovanadium complexes were dissolved by addition of 25 mL H O–DMF in volume
2
ratio of 4 : 1. Solutions used for kinetic measurements were maintained at a constant con-
+
centration of H (pH 5.8) by addition of the buffer solution of NaH PO –Na HPO [14].
2
4
2
4
Reactions were initiated by addition of phenol red solution. Oxidovanadium complexes
with five different concentrations were prepared in five cuvettes. The cuvettes were put
in the constant temperature for 10 min, and spectral changes were recorded using a

6
74
C. Chen et al.
Table 1. Crystallographic data and structure refinement for 1–3.
Complexes
Formula
1
2
3
C
15
H
12
N
2
O
7
V
C
24
H
30
N
764.40
4
O
18
V
2
12 12 2 8
C H N O V
ꢀ1
M (g mol
)
383.21
363.18
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P2(1)/c
7.6720(15)
13.338(3)
15.593(3)
90
97.499(3)
90
1582.0(5)
4
Triclinic
P-1
8.938(2)
9.607(3)
9.641(2)
85.289(3)
78.700(4)
77.113(4)
790.7(3)
1
Monoclinic
C2/c
25.904(3)
6.9066(8)
16.2790(19)
90
106.164(2)
90
2797.4(6)
8
α (°)
β (°)
γ (°)
V (Å )
3
Z
D
calc
1.609
1.605
1.725
Crystal size (mm)
0.54 ꢁ 0.16 ꢁ 0.11
0.4 ꢁ 0.25 ꢁ 0.1
0.47 ꢁ 0.24 ꢁ 0.12
F(000)
μ(Mo-Kα) / mm
h (°)
780
392
1480
ꢀ1
0.669
2.02–25.36
7877
2882 (2251)
234
0.678
2.16–23.86
3589
2402 (1585)
217
0.756
1.64–25.00
4369
2339 (1794)
220
Reflections collected
Independent reflections (I >2σ(I))
Parameters
ꢀ
3
D(q) (e Å
)
0.658, ꢀ0.573
0.345, ꢀ0.323
0.631, ꢀ0.491
Goodness-of-fit
1.073
1.036
0.0531 (0.1181)
0.0905 (0.1350)
1.088
0.0431 (0.1118)
0.0597 (0.1188)
a
b
b
b
R
0.0497 (0.1331)b
a
b
b
wR
2
0.0653 (0.1447)
P
P
P
P
a
2
2
2
2 2 1=2
R ¼
kFoj ꢀ jFck= jFo, wR2 ¼ ½ ðwðjFoj ꢀ jFcj Þ =½ ðwðjFoj Þ ꢂ ; ½jFoj > 4aðjFojÞꢂ.
7
21 UV–vis spectrophotometer every 5 min. The resulting data were collected and fitted
using curve-fitting software in Microsoft Excel.
The rate of this reaction is described by the rate equation: dc=dt ¼ kc c c , giving the
x
1
y
z
2 3
equation “log(dc/dt) = logk + xlogc + ylogc + zlogc ,” corresponding to “ꢀlog(dc/dt)
1
2
3
=
ꢀxlogc ꢀ b(b = logk + ylogc + zlogc )” where k is the reaction rate constant, c , c , c
1
2
3
1
2
3
are the concentrations of the complex, KBr and phenol red, respectively; x, y, and z are
the corresponding reaction orders. According to Lambert-Beer’s law, A= ɛ·d·c, where A is
the measurable absorbance of the resultant, ɛ is molar absorption coefficient, which of bro-
mophenol blue is measured as 14,500 M cm at 592 nm and d is 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 complexes (dA/dt) was given by the
slope of this line. By changing the concentration of oxovanadium complexes in the reac-
tion system, a series of dA/dt data can be obtained. The reaction rate constant (k) can be
ꢀ
1
ꢀ1
obtained from a plot of ꢀlog(dc/dt) versus ꢀlogc , fitted using the curve-fitting software
1
in 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 oxovanadium complexes in this
reaction and “b” is the intercept of the line. In the experiment, considering that the reaction
orders of KBr and phenol red (y and z) are 1 according to the literature; c and c are 0.4
2
3
ꢀ4
and 10 mol/L, respectively. Based on the equation “b = logk + ylogc + zlogc ,” the reac-
2
3
tion rate constant (k) can be obtained. Bromination of phenol red was monitored by mea-
surement of the absorbance at 592 nm for reaction aliquots which were extracted at
specific times and diluted into pH 5.8 phosphate buffer.

Supramolecular oxovanadium
.5. Electrochemical determinations
675
2
Electrochemical determinations were performed by cyclic voltammetry in DMF in the
potential range from 1000 to ꢀ1200 mV using a MCP-1 Potentiostat/Galvanostat and a
home-built computer-controlled instrument based on the ZF-10B data acquisition board
(National Instruments). A three-electrode cell was used, in which a platinum wire electrode
was the working electrode, a saturated Ag/AgCl electrode was the reference electrode, and
another platinum wire was the auxiliary electrode. The electrodes were rinsed with deion-
ꢀ
4
ꢀ1
ized water between each grade. The concentration of the complex is 1.0 ꢁ 10 mol L
.
Cyclic voltammetric current-potential (i–E) curves were used to assess heterogeneous elec-
tron transfer, where i is the Faradaic current at the formal reduction potential (E).
3
3
. Results and discussion
.1. Synthesis
Both 1 and 2 were synthesized by reaction of VO(acac) as starting material at room tem-
2
perature with methanol and ethanol. To obtain single crystals suitable for X-ray diffraction,
we have screened solvents, showing that methanol was best for 1, while ethanol was suit-
able for 2. Although the molecular structure of 3 was been reported, the synthetic method
and starting material used in this work were different from those in the literature [33].
Starting materials of V(III) used in the synthesis of 3, while oxidation state of vanadium
of the corresponding complex is +5, were further confirmed by Bond-Valence Theory
[34,35]. It is easy to oxidize V(III) by oxygen from the air [36]. 1, 2, and 3 are all quite
stable at room temperature; 1 and 2 are easily soluble in DMF and CH Cl , slightly in
2
2
methanol and water, and insoluble in hexane and ether, while 3 is slightly soluble in DMF
and insoluble in other solvents.
3
.2. Spectra properties
3
3
.2.1. IR spectra. For 1–3 (Supplementary material), spectra have bands at 3408–
ꢀ
1
ꢀ1
439 cm assigned to O-H stretches and bands at 3060–3148 cm assigned to =C–H
ꢀ
1
stretches. The band at 2976 cm for 2 is assigned to –CH stretch. The asymmetric ν
3
as
ꢀ
and symmetric ν (COO ) stretches of deprotonated carboxylates are 1640–1693 and
(
COO–)
s
ꢀ1
Table 2. Characteristic IR bands (cm ) for 1–3.
Complex
1
2
3
ν
ν
ν
ν
ν
ν
(O–H)
3408
3060
–
3426
3148
2976
1715
1672, 1630
1399
–
3439
3104, 3076
(=C–H)
as(–CH3)
(C=N)
–
1677
1640
1344
1715
1693, 1649
ꢀ
ꢀ
as(COO )
s
(COO )
1386
Pyridine/pyrazolyl
1517, 1424, 1230, 1142, 1105
1489, 1433 , 1252, 1180, 1079
ν
(V=O)
ν
(V–O)
ν
(V–N)
983
451
356
983
479
–
958, 934
470, 428
360

6
76
C. Chen et al.
ꢀ
1
ꢀ1
ꢀ1
1
344–1399 cm , respectively. V=O bands were 934–983 cm , V–O at 428–480 cm
,
ꢀ1
and V–N at 350–360 cm [37]. Detailed assignments of IR spectra for 1–3 are shown in
table 2.
3
.2.2. UV–vis absorption spectra. UV–vis absorption spectra of 1, 2, and 3 (Supple-
mentary material) are recorded as solid samples and their characteristic UV–vis bands are
⁄
listed in table 3. The high-frequency absorptions at 260–262 nm are assigned to π–π tran-
sitions of the aromatic-like chromophore from 1,10-phenanthroline, L, 2,6-dipic acid and
4
,4′-bipy [38]. For vanadyl(IV) complexes, it is generally considered that transitions occur
2
2
2
from dxy to (d , d ) (ν ), d ꢀ d (ν ) and d (ν ) orbitals with increasing energies. The
xz
yz
1
x
y
2
z
3
absorptions at 304–440 nm are attributed to LMCT (ligand to metal charge transfer) transi-
tion. Complexes 1 and 2 exhibit two sets of absorptions with maximum wavelength at
7
22–728 and 538–586 nm, which can be assigned to (ν ) and (ν ) bands of oxovanadium
1 2
(IV) [39].
3
.3. Structural description of 1–3
The molecular structures of 1 and 2 are depicted in figures 1 and 3, respectively. The prin-
cipal bond distances and angles for 1–3 are summarized in table 4. Deviations of the coor-
dinated atoms out of the equatorial plane for 1–3 are listed in Supplementary Material.
There are abundant inter-and intra-molecular hydrogen bonds in the three complexes. Rele-
vant H-bond parameters of 1, 2, and 3 are listed in table 5.
Table 3. Characteristic UV-vis bands (nm) for 1–3.
⁄
Complex
π–π transition
LMCT
d–d transition
1
2
3
262
260
262
342, 440
342
304, 322
538, 722
586, 728
–
Figure 1. The molecular structure of 1 (all hydrogens are omitted for clarity).

Supramolecular oxovanadium
Table 4. Selected bond lengths (Å) and angles (°) for 1–3.
677
Complex 1
V–O1
V–O2
V–O3
V–O4
1.588(3)
2.048(3)
1.979(2)
2.001(2)
2.322(3)
2.123(3)
107.43(13)
99.73(13)
80.86(10)
95.45(14)
90.51(13)
O2–V–N2
O1–V–N1
O3–V–N1
O4–V–N1
O2–V–N1
N2–V–N1
O4–V–O2
O1–V–N2
O3–V–N2
O4–V–N2
96.65(13)
165.14(12)
86.39(10)
87.62(10)
78.61(11)
73.81(10)
164.21(13)
93.59(13)
157.07(11)
86.76(10)
V–N1
V–N2
O1–V–O3
O1–V–O4
O3–V–O4
O1–V–O2
O3–V–O2
Complex 2
V1–O1
V1–O2
V1–O3
V1–O4
V1–O7
1.571(3)
2.027(3)
1.975(3)
2.000(3)
2.020(3)
O4–V1–O7
O1–V1–O2
O3–V1–O2
O4–V1–O2
O7–V1–O2
O1–V1–O8
O3–V1–O8
O4–V1–O8
O7–V1–O8
O2–V1–O8
92.04(14)
99.80(17)
89.61(14)
160.18(15)
89.45(13)
171.86(16)
82.62(13)
78.06(13)
75.15(12)
83.24(13)
V1–O8
2.284(3)
O1–V1–O3
O1–V1–O4
O3–V1–O4
O1–V1–O7
O3–V1–O7
Complex 3
V–O1
V–O2
V–O3
V–O4
V–N1
104.87(16)
99.62(17)
81.58(14)
97.25(16)
157.70(13)
1.602(2)
1.614(2)
1.993(2)
1.990(2)
2.089(2)
109.37(13)
99.59(11)
74.72(9)
O2–V–O4
O1–V–O3
O2–V–O3
O4–V–O3
O1–V–N1
O2–V–N1
O4–V–N1
97.41(11)
100.29(11)
98.78(10)
148.46(9)
117.47(11)
133.14(11)
74.44(9)
O1–V–O2
O1–V–O4
O3–V–N1
For 1, vanadium is coordinated by a terminal oxygen (O6), one oxygen (O5) from coor-
dinated water, two oxygens (O1, O2) from oxalate, and two nitrogens (N1, N2) from 1,10-
phenanthroline to form a distorted octahedral vanadium with O2, O3, O4, and N2 in the
equatorial plane, O1 and N1 in axial positions. The deviations of O2, O3, O4, and N2
from the least-squares plane are 0.0494, ꢀ0.0597, 0.0603, and ꢀ0.0500 Å, respectively,
showing that these atoms are almost in one plane. The V, O1, and N1 from axial positions
lay 0.3121, 1.8887, and ꢀ1.9865 Å out of the equatorial plane, with V toward the oxo
O1, trans to N1, a consequence of the strong trans influence of the terminal oxo group.
The O1ꢀVꢀN1 angle is 165.14(12)°. By comparison, the order of the bond lengths of V–
O is V–O2
> V–O4
> V–O3
, which indicates that the coordination ability
carboxyl
water
carboxyl
of oxalate is stronger than that of coordinated water.
There are two types of hydrogen bonds in 1 as shown in figure 2, O–H···O (2.9151–
2
.9869 Å) and C–H···O (3.1861 Å). Hydrogen bonds of O–H···O are between coordinated
#
1
#1
water (O2) and carboxyl oxygen (O5 and O6 , #1: 1 + x, y, z) of oxalate; hydrogen
bond of C–H···O is between 1,10-phenanthroline (C2) and terminal oxygen (O1). By C2–
H2A···O1 hydrogen bond, molecules are linked to form a 1-D chain along the b-axis.
Adjacent chains are connected through O2–H14A···O5 and O2–H14A···O6 hydrogen
bonds to form a 2-D supramolecular structure.
#
1
#1
Structural analysis for 2 shows that there are two vanadiums, three oxalates, two
coordinated waters, and one free L in the asymmetric unit. Complex 2 consists of L and

6
78
C. Chen et al.
Table 5. Hydrogen bonds (Å) of 1–3.
D–H···A
d(D–H) (Å)
d(H···A) (Å)
d(D···A) (Å)
\D–H···A (°)
Complex 1
#
1
1
O2–H14A···O5
O2–H14A···O6
C2–H2A···O1
Complex 2
0.7862
0.7862
0.9300
2.3840
2.2093
2.5450
2.9869
2.9151
3.1861
134.34
149.67
126.40
#
O1w–H1wB···O5
O1w–H1wB···O6
O2–H23B···O1w
0.9000
0.9000
0.7500
0.9000
0.9600
0.9300
0.9300
2.3332
2.2774
1.9230
1.8303
2.5501
2.4363
2.5984
3.1756
2.9084
2.6412
2.7249
3.3295
3.3591
3.4974
155.77
126.92
160.32
172.32
138.39
171.64
162.74
#2
O2–H23A···O3
#
1
C2–H2A···O1
C3–H3A···O7
C9–H9···O5
Complex 3
#3
#4
#4
#4
N2 –H2 ···O2w
0.9462
0.8500
0.8633
0.7349
0.7349
0.9300
0.9300
0.9300
1.7312
2.3746
2.1566
2.3500
2.5100
2.3565
2.5087
2.4583
2.6649
2.9610
3.0077
2.9943
2.9504
3.2674
3.3313
3.3828
168.44
126.59
168.58
147.18
120.35
166.30
147.58
172.68
#2
#3
#3
#2
O1w –H1wB ···O6
#4
#4
O2w –H2wB ···O5
#4
#4
#1
O2w –H2wA ···O1w
O2w–H2wA···O5
#4
#4
#2
C12 –H1 ···O4
C3–H3···O1
#4
#4
#4
C9 –H7 ···O1w
Symmetry transformation used to generate equivalent atoms: 1 (#1 : 1 + x, y, z); 2 (#1: ꢀx, ꢀy + 1, ꢀz + 1; #2: ꢀx
1, ꢀy, ꢀz; #3: ꢀx, ꢀy, 1 ꢀ z); 3 (#1: ꢀx, y, 0.5 ꢀ z; #2: x, ꢀy, 0.5 + z; #3: x, y ꢀ 1, z; #4: ꢀx, ꢀy, 1 ꢀ z).
+
Figure 2. (a) Fragment of 1D chain network of hydrogen bonding along the b-axis; (b) A view of two-
dimensional hydrogen bonding network in 1. (All hydrogens except for hydrogen bonds are omitted for clarity).
binuclear V-oxalate complex moiety with hydrogen-bonding interaction. The binuclear
anion is centrosymmetric with the center of inversion lying on the CꢀC bond of the
bridging oxalate [33]. The same coordination environment of the two vanadiums is
observed and we take one as an example. The oxidation state of vanadium is +4,

Supramolecular oxovanadium
679
Figure 3. The molecular structure of 2 (all hydrogens are omitted for clarity).
confirmed by Bond-Valence Theory. Vanadium is coordinated by a terminal oxygen (O1),
one oxygen (O2) from coordinated water, and four oxygens (O3, O4, O7, O8) from oxa-
late to form an axial trans octahedron with O2, O3, O4, and O7 in the equatorial plane
and O1 and O8 in axial positions. The deviations of O2, O3, O4, and O7 from the least-
squares plane are ꢀ0.0233, 0.0255, ꢀ0.0249, and 0.0226 Å, respectively, with V1, O1,
and O8 from axial positions lying ꢀ0.3594, ꢀ1.9281, and 1.9167 Å out of the equatorial
plane, indicating that vanadium is displaced toward the oxo oxygen O1, forming the angle
of O1–V1–O8 of 171.86(16)°. The bond length order of V–O (V–O2, V–O3 and V–O4)
is V–O2water > V–O4carboxyl > V–O3carboxyl, similar to those in 1. The order of the bond
lengths of V–Ocarb from oxalate is V–O8 > V–O7 > V–O4 > V–O3, because O7 and O8 are
bridging, and O8 is trans to terminal oxo.
There are two types of hydrogen bonds, O–H···O (2.6412–3.1756 Å) and C–H···O
(
3.3295–3.3591 Å) in 2, as illustrated in figure 4(a). O–H···O hydrogen bonds are between
#
2
coordinated water (O2) and carboxyl oxygen (O3 , #2: ꢀx + 1, ꢀy, ꢀz) of coordinated
oxalate, coordinated water (O2) and lattice water (O1w), lattice water (O1w), and carboxyl
(
O5 and O6), respectively; C–H···O hydrogen bonds come from C2 in methyl of the unco-
#
1
ordinated L and terminal oxygen (O1 , #1: ꢀx, ꢀy + 1, ꢀz + 1), C3 in pyrazolyl of L and
#
3
oxygen (O7 , #3: -x, -y, 1-z) in bridging positions, respectively. The coordinating vana-
#
2
dium via O2–H23A···O3 along the b-axis becomes a 1-D chain, and then, adjacent
#
1
#3
chains are connected to organic ligands (L) via C2–H2A···O1 and C3–H3A···O7 to
form a 2-D sheet structure. Numerous O–H···O hydrogen-bonding interactions involving
coordinated water of one layer and coordinated oxalate of the adjacent layer via lattice
water are established (O1w–H1wB···O5, O1w–H1wB···O6 and O2–H23B···O1w), as
shown in figure 4(b). Consequently, these hydrogen bond interactions combine to form a
3-D hydrogen-bonding network.
For 3, the structure can be evaluated by the Addison distortion index τ [40]: τ = |β ꢀ α|/
6
0°, β is the greater of the basal angles of the axial ligand. In a five-coordinate system,
values for τ are 1.0 for trigonal bipyramidal and 0.0 for square pyramidal [41]. The value
of τ for 3 is 0.65, indicating that the coordination environment of V(V) is close to
distorted trigonal bipyramidal. Comparison of related bond lengths of 3 found that (i) the
V–O1 bond length (1.606(2) Å) is shorter than those in C(NH ) [VO (dipic)]·2H O,
2
3
2
2
NH [VO (dipic)] [42], [CH NHC(NH ) ] [V O (dipic) ] [43] and K[VO (dipic-OH)]·H O
4
2
3
2 2 2
2
4
2
2
2

6
80
C. Chen et al.
V-oxalate
chain
(
a)
(
b)
Figure 4. (a) A view of two-dimensional hydrogen bonding network in 2; (b) A view of 3D hydrogen bond
network in 2. (All hydrogens except for hydrogen bonds are omitted for clarity).
Table 6. Comparison of the bond lengths (Å) in related complexes.
Complex
C(NH [VO
V=O
V-Ocarb1
V-Ocarb2
V-N
Ref.
[42]
[42]
[43]
This work
[45]
[46]
[44]
[47]
2
)
3
2
(dipic)]·H
2
O
1.614(7)
1.624(2)
1.606(1)
1.602(2)
1.579(2)
1.598(1)
1.616(5)
1.594(3)
1.983(2)
1.974(2)
1.995(1)
1.993(2)
2.053(2)
2.144(1)
1.990(5)
2.026(3)
1.988(2)
1.978(2)
1.984(1)
1.990(2)
2.064(2)
2.045(1)
2.033(5)
2.051(4)
2.086(2)
2.091(2)
2.097(2)
2.089(2)
2.088(2)
2.169(2)
2.089(6)
2.163(4)
NH [VO (dipic)]
NHC(NH
4,4’-bipy)[VO
[VO(O )(H
K[VO(O )(DL-cmhist)]·H
(dipic-OH)]·H
O) (dipic)·2H
4
2
[
(
CH
3
2
)
2
]
[V
2 2
O
4
(dipic)
2
]
2
(dipic)]
2
·2H
2
O
NH
4
2
2 2
O)(dipic)]·xH O (x = 1.3)
2
2
O
K[VO
VO(H
2
2
O
2
2
2
O
[
44] and slightly longer than NH [VO(O )(H O)(dipic)]·xH O (x ≈ 1.3) [45], K[VO(O )
4 2 2 2 2
(
DL-cmhist)]·H O (H cmhist = DL-N-carboxymethylhistidine) [46] and VO(H O) (dipi-
2
2
2
2
c)·2H O [47]. (ii) The V–N bond length is similar to those corresponding to the complexes
2

Supramolecular oxovanadium
681

6
82
C. Chen et al.
above, as shown in table 6. (iii) The N–V=O and O=V–Ocarb angles are 99.59(11)° and
00.29(11)° in 3, respectively. Both angles are larger than those in C(NH ) [VO (dipic)]·
1
2
3
2
H O, NH [VO (dipic)], NH [VO(O )(H O)(dipic)]·xH O (x ≈ 1.3) and K[VO (dipic-
2
4
2
4
2
2
2
2
OH)]·H O, while the N–V–O
angles are close to those found in other vanadium
2
carb
complexes (table 7).
In the molecular packing of 3, there are three types of hydrogen bonds, N–H···O
2.6649 Å), O–H···O (2.9504–3.0077 Å) and C–H···O (3.2674–3.3828 Å). The N–H···O
hydrogen bond comes from 4,4′-bipy (N2 , #4: -x, -y, 1-z) and lattice water (O2w ). The
(
#
4
#4
Figure 5. (a) A view of two-dimensional hydrogen bonding network in 3; (b) A view of 3D hydrogen bond
network in 3. (All hydrogens except for hydrogen bonds are omitted for clarity).

Supramolecular oxovanadium
683
Figure 6. Cyclic voltammograms for 1–3 in DMF; scan rate = 80 mV/s
#
1
#4
O–H···O hydrogen bonds come from lattice water (O1w , #1: ꢀx, y, 0.5ꢀz and O2w ),
#
4
#2
#2
the lattice water (O2w and O1w , #2: x, ꢀy, 0.5 + z) and carboxyl oxygen (O5 and
#
3
#4
O6 , #3: x, yꢀ1, z) of 2,6-dipic; C–H···O hydrogen bonds are from C12 of 4,4′-bipy
#
2
and carboxyl oxygen (O4 ) of 2,6-dipic, C3 in pyridyl ring of 2,6-dipic and terminal oxy-
#
4
#4
gen (O1), C9 of 4,4-bipy and lattice water (O1w ), respectively. Independent molecules
are linked to form a 2-D sheet structure with C–H···O and O–H···O hydrogen bonds
#
4
#4
#2
#4
#4
#4
#2
#3
#3
#4
(
C12 –H1 ···O4 , C3–H3···O1, C9 –H7 ···O1w , O1w –H1wB ···O6 , O2w –
#
4
#2
#4
#4
#1
#4
#4
#4
H2wB ···O5 , O2w –H2wA ···O1w , N2 –H2 ···O2w ). Adjacent sheets are con-
nected through hydrogen bonds O2w–H2wA···O5, C9 –H7 ···O1w
H1 ···O4 to form a 3-D crossed supramolecular structure, as illustrated in figure 5.
#
2
#2
#2
#2
and C12 –
#
2
#4
3
.4. Analysis of redox behavior
The redox behaviors of 1–3 were studied using cyclic voltammetry. As shown in figure 6,
the cyclic voltammogram of 2 exhibits one redox couple with the oxidation wave at
7
45 mV and the corresponding reduction peak at ꢀ849 mV, which could be assigned to the
process V(IV/V). The reduction peak of 1 is observed at ꢀ702 mV. However, it is hard to
observe the oxidation wave due to electron donating 1,10-phen. The redox couple was not
observed in 3, due to the highest oxidation state of vanadium(V) already established. The
peak shape of 2 is stable, explaining that reduction product could be oxidized sequentially
on the electrode, so it was a reversible process; oxidation and reduction of 1 and 3 were
irreversible.
3
3
.5. Functional mimics of the vanadium haloperoxidases
.5.1. 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 H O . For example, the bromination
2
2

6
84
C. Chen et al.
Figure 7. Oxidative bromination of phenol red catalyzed by 2. Spectral changes at 10 min intervals. The
ꢀ1
ꢀ4
ꢀ1
reaction mixture contained phosphate buffer (pH 5.8), KBr (0.4 mol L ), phenol red (10 mol L ) and 2
ꢀ1
(
0.1 μmol L ).
þ
of trimethoxybenzene [48], benzene, salicylaldehyde and phenol catalyzed by the VO
2
+
moiety [49] and the bromination of phenol red by [VO(O2)H O] and related species [50]
2
have been reported. Herein, the bromination reaction activities of 1, 2, and 3 using phenol
red as substrate is shown by conversion of phenol red to bromophenol blue. The reaction
is rapid and stoichiometric, producing halogenated product by reaction of oxidized halogen
species with the organic substrate.
Addition of solution of 2 to bromide in phosphate buffer with phenol red as a trap for
oxidized bromine resulted in color change of the solution from yellow to blue. As shown
in figure 7, decrease in absorbance of the peak at 443 nm due to the loss of phenol red
and increase in the absorbance of the peak at 592 nm characteristic of the bromophenol
blue product show that 2 performs significant catalytic activities. The results of the mimic
catalytic activities for 1 and 3 are similar to that of 2.
3
.5.2. Kinetic studies of bromination. A series of dA/dt data for 2 was obtained (figure
8
) by changing the concentration of the oxovanadium complex. (The absorbance depen-
dence of time for 1 and 3 are shown in Supplementary material).
The plot of ꢀlog(dc/dt) versus ꢀlogc for 2 gave a straight line with a slope of 1.0025
and b = ꢀ1.3, as shown in figure 9. The former confirms the first-order dependence on
vanadium. Based on “b = logk + ylogc + zlogc ,” the reaction rate constant, k, is determined
2
3
by the concentrations of KBr and phenol red (c and c ), the reaction orders of KBr and
2
3
phenol red (y and z) and b. Considering the reaction orders of KBr and phenol red (y and
ꢀ
4
ꢀ1
4
z) are 1 according to the literature [51,52], c and c are 0.4 and 10 mol L , respec-
2
3
ꢀ2
tively, so the reaction rate constant (k) for 2 can be calculated as 1.253 ꢁ 10 (mol/L)
ꢀ
1
s
. (–log(dc/dt) dependences of –logc for 1 and 3 are shown in Supplementary Material).
Kinetic data for 1, 2, and 3 in DMF–H O at 30 ± 0.5 °C are shown in table 8. The
2
reaction orders of the oxidovanadium complex in bromination reaction are all close to 1,
confirming the first-order dependence on vanadium; the order of the reaction rate constant
is 1 > 2 > 3.

Supramolecular oxovanadium
685
Figure 8. The measurable absorbance dependence of time for 2. Conditions used: pH = 5.8, c(KBr) = 0.4 mol/
ꢀ
4
ꢀ2
ꢀ2
L, c(H
2
O
2
) = 1 mmol/L, c(phenol red) = 10 mol/L. c(complex 2/mmol/L) = a: 1.13 ꢁ 10 ; b: 2.25 ꢁ 10 ; c:
ꢀ2 ꢀ2 ꢀ2
3.38 ꢁ 10 ; d: 4.50 ꢁ 10 ; e: 5.63 ꢁ 10
.
2
Figure 9. –log(dc/dt) dependence of –logc for 2 in DMF–H O at 30 ± 0.5 °C (c is the concentration of 2.
4
ꢀ
Conditions used: c(phosphate buffer) = 50 mmol/L, pH = 5.8, c(KBr) = 0.4 mol/L, c(phenol red) = 10 mol/L.
⁄
Table 8. Kinetic data for the complexes in DMF–H
2
O at 30 ± 0.5°C .
ꢀ2
ꢀ1
Complex
x
b
k (mol/L)
s
Ref.
4
4
1
2
3
4
5
6
7
8
1.1256
1.0025
1.0255
1.0076
0.9986
1.02
ꢀ1.0663
ꢀ1.3
2.146 ꢁ 10
1.253 ꢁ 10
This work
This work
This work
[54]
[55]
[55]
3
ꢀ1.4756
ꢀ2.3524
ꢀ2.7355
ꢀ2.7758
ꢀ1.1672
ꢀ1.4858
8.36 ꢁ 10
3
1.116 ꢁ 10
3
0.462 ꢁ 10
3
0.421 ꢁ10
2.347 ꢁ 10
1.003 ꢁ 10
3
3
1.1153
0.9921
[56]
[56]
4
: VO(HB(3,5-Me
phen)]·0.5H
derived from salicylaldehyde and DL-α-alanine).
2
pz)
3
)(3,5-Me
2
pz)(HOOCCH
2
CH
2
2
COO); 5: [VO(sal-ala)(2,2’bipy)]·H
2
O; 6: [VO(sal-ala)(1,10-
2
O; 7: [VO(C
2
O
4
)(2,2′bipy)(H
2
O)]·C
H
5
OH; 8: [VO(C )(phen)]·H O (H sal-ala: Schiff base
2
O
4
2 2

6
86
C. Chen et al.
Scheme 1. The catalytic bromination reaction mechanism.
The catalytic bromination reaction mechanism is shown in scheme 1, in which in step a
vanadium complexes are easily oxidized to [VO(O )L] with H O as a oxidation reagent;
2
2 2
ꢀ
+
in step b Br is oxidized rapidly by [VO(O )L], forming Br and [VO L]. Catalytic reac-
2
2
tion rate would be mainly based on the stability of the formed [VO(O )L] (from step a to
2
b) [50]. Comparison of kinetic data for the complexes reported previously is listed in table
8
. The order of the reaction rate constant is 1 > 2 > 3 > 7 > 4 > 8 > 5 > 6. The complexes 4–8
were synthesized according to references [53,18,54]. The complexes 1, 2, 3, 7, and 8 con-
tain oxalate, and 1–3 and 7 have higher catalytic activities on bromination. Complex 8 is
relatively lower, possibly due to its molecular structure being stable and more difficult to
form [VO(O )L]. Complexes 5 and 6 have the lowest activities, probably because of the
2
effect of the Schiff base ligands.
Catalytic activities of 1–3 are higher than the others, and the amounts and kinds of the
hydrogen bonding should be also considered. We presume that lower dimension hydrogen-
bonding network could favor formation of the intermediate of [VO(O )L], further enhanc-
2
ing catalytic bromination activity.
4
. Conclusion
To explore model oxovanadium complexes for the active centre of VHPO, we synthesized
three supramolecular oxovanadium complexes and tested the bromination reaction activity
with phenol red as organic substrate in the presence of H O , KBr, and phosphate buffer.
2
2
Structural analysis shows that vanadium of 1 and 2 are six coordinate with distorted octa-
hedral geometry, while 3 is five coordinate with distorted trigonal bipyramidal geometry.

Supramolecular oxovanadium
687
There are abundant inter-molecular hydrogen bonds to form 1-D, 2-D and 3-D supramolec-
ular structures. These intermolecular hydrogen bond networks may influence catalytic bro-
mination activity.
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 numbers CCDC: 873784 for 1, 873785 for 2, from the Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223 336033; Email: deposit@ccdc.
cam.ac.uk or http://www.ccdc.cam.ac.uk).
Acknowledgements
This study was supported by the grants of the National Natural Science Foundation of
China (No. 21071071) and Dr start Foundation of Liaoning province in China (No.
2
0111067) for financial assistance.
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[
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