Aminoacid-derivatized oxidovanadium complexes: Synthesis, structure and bromination reaction activity

Article abstract of DOI:10.1016/j.ica.2011.01.031

A new family of aminoacid-derivatized oxidovanadium complexes as potential functional model of vanadium haloperoxidases: [V^VO(sal-phe)(OMe) (MeOH)] (1), [V^IVO(sal-ala)(2,2′-bipy)]·H₂O (2), [V^IVO(sal-ala)(1,10-phen)]·0.5H₂O (3) and [V^IVO(sal-his)(1,10-phen)]·2MeOH (4) (H₂sal-phe = Schiff base derived from salicylaldehyde and dl-β-phenylalanine, H ₂sal-ala = Schiff base derived from salicylaldehyde and dl-α-alanine, H₂sal-his = Schiff base derived from salicylaldehyde and l-histidine) have been synthesized. All the complexes were characterized by elemental analysis, IR spectra and UV-Vis spectroscopy. In particular, molecular structures of three representative complexes (1, 2 and 3) were determined by X-ray crystallography. In addition, bromination reaction activity of the starting material and the complexes has been tested by a method with phenol red as organic substrate in the presence of H₂O ₂, Br^- and phosphate buffer, the studying results indicate that the vanadium complexes can catalyze the visible conversion of phenol red to bromophenol blue under weak-acid conditions and the different structural characterizations of the complexes exhibit different catalytic activity, therefore, the complexes can be considered as a potential functional model of VHPO.

Full text of DOI:10.1016/j.ica.2011.01.031

Inorganica Chimica Acta 368 (2011) 223–230
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Aminoacid-derivatized oxidovanadium complexes: Synthesis, structure
and bromination reaction activity
a
Yun-Zhu Cao a, Hai-Yan Zhao a, Feng-Ying Bai a, Yong-Heng Xing a, , Dong-Ming Wei ,
⇑
Shu-Yun Niu a, Zhan Shi b
a College of Chemistry and Chemical Engineering, Liaoning Normal University, Huanghe Road 850#, Dalian 116029, PR China
b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new family of aminoacid-derivatized oxidovanadium complexes as potential functional model of
vanadium haloperoxidases: [VVO(sal-phe)(OMe) (MeOH)] (1), [VIVO(sal-ala)(2,20-bipy)]ꢀH2O (2),
[VIVO(sal-ala)(1,10-phen)]ꢀ0.5H2O (3) and [VIVO(sal-his)(1,10-phen)]ꢀ2MeOH (4) (H2sal-phe = Schiff base
derived from salicylaldehyde and DL-b-phenylalanine, H2sal-ala = Schiff base derived from salicylalde-
Received 27 September 2010
Received in revised form 5 January 2011
Accepted 11 January 2011
Available online 22 January 2011
hyde and DL-a-alanine, H2sal-his = Schiff base derived from salicylaldehyde and L-histidine) have been
synthesized. All the complexes were characterized by elemental analysis, IR spectra and UV–Vis spectros-
copy. In particular, molecular structures of three representative complexes (1, 2 and 3) were determined
by X-ray crystallography. In addition, bromination reaction activity of the starting material and the com-
plexes has been tested by a method with phenol red as organic substrate in the presence of H2O2, Brꢁ and
phosphate buffer, the studying results indicate that the vanadium complexes can catalyze the visible con-
version of phenol red to bromophenol blue under weak-acid conditions and the different structural char-
acterizations of the complexes exhibit different catalytic activity, therefore, the complexes can be
considered as a potential functional model of VHPO.
Keywords:
Oxidovanadium complexes
Aminoacid derivatives
Bromination reaction activity
Crystal structure
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
structural features, with vanadium atom in a proved trigonal–bipy-
ramidal NO4 coordination geometry which is covalently bonded to
It has long been known that vanadium being an essential bioel-
ement is involved in various catalytic and inhibitory processes
[1,2]. It is present in many abiotic as well as biotic systems, where
it plays several roles such as cofactors in metalloenzymes [3] (e.g.,
haloperoxidases [4] and nitrogenases [5]) and metalloproteins
(e.g., vanadium chromagen) [6]. It was also discovered to be in-
volved in insulin mimicking [7] and production [8]. Among these
systems, vanadium haloperoxidases (VHPOs) have received
increasing attention due to their unique characteristics and poten-
tial use in oxidations and epoxidation [9,10], which are able to oxi-
dize halides (chloride, bromide and iodide) in the presence of
peroxides, the basic reaction catalyzed by the enzymes is the
two-electron oxidation/oxygenation of halide Xꢁ to an {X+}, where
three oxygen atoms in the equatorial plane, Ne of the histidine and
an OH in axial positions [14,15]. The active site is shown to consist
of oxidovanadium with V@O moiety and one N-donor (the apical
His), and the other O-donors being from the oxido and hydroxido
ligands. In order to extend the search for more efficacious com-
plexes and finally fully understand the exact role of vanadium
complexes in VHPOs, great efforts have been made to prepare oxi-
dovanadium complexes derived from appropriate ligands. In the
respect, well-chosen aminoacid derivatives have been considered
to be suitable candidates, which allow easy artificial linkage of var-
ious functional groups with N-terminal to generate a variety of
multidentate oxidovanadium complexes [16]. To our best knowl-
edge, although oxidovanadium complexes of aminoacid deriva-
tives appear in the contemporary literature [16–23], such as:
{X+} is X2, X3 and/or XOH, i.e., hypohalous acid, and if an organic
ꢁ
nucleophilic acceptor is present, halogenated compounds can be
[V(O)(NMet)(
l
-OMe)]2ꢀMeOH and [V(O)(NThr)( -OMe)]2ꢀMeOH
l
produced [11–13]: Xꢁ + H2O2 + R–H + H+ ? R–X + 2H2O.
(HNMet = N-(2-pyridylmethyl)-DL-methionine, HNThr = N-(2-pyri-
dylmethyl)-DL-threonine) [16] [VO(Pic-AaR)2] (Pic = picolyl-5-
amide, AaR = HisH, HisMe, IleH, IleMe, TrpH, TrpMe, HTyrEt,
Irrespective of their origin from brown algae, red algae, or fungi,
in this class of enzymes they all show a high degree of amino acid
homology in their active centers and have almost identical
tBuTyrMe, HThrMe, tBuThrMe) [17]; [VO(pyr-D,
L
-Met)-(bipy)]
-Met made from DL
-Thr made from DL-threonine and
pyridoxal) [18]; and so on, the stable oxidovanadium complexes
and [VO(pyr-D,
L
-Thr)(bipy)]ꢀH2O (pyr-D,
L
-
methionine pyridoxal; pyr-D,
L
⇑
Corresponding author.
E-mail address: yhxing2000@yahoo.com (Y.-H. Xing).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.031

224
Y.-Z. Cao et al. / Inorganica Chimica Acta 368 (2011) 223–230
containing aminoacid–salicylaldehyde ligands are very limited so
far [24,25]. For quite sometime we have been engaging the study
on the coordination behavior of glycine–salicylaldehyde derived li-
gands with diverse transition metal ions [25–27]. Here, as a part of
our fundamental studies on aminoacid–salicylaldehyde vanadium
chemistry, in order to obtain some more efficacious complexes to
model the active center of the VHPO and elucidate the interrelation
between the configuration of aminoacid-derivatized oxidovanadi-
um complexes and the bromination reaction activity, in this paper
we report for the first time, the syntheses and characterization of
some vanadium complexes containing aminoacid–salicylaldehyde
(3): C, 59.07; H, 4.06; N, 9.39. Found: C, 59.10; H, 4.03; N, 9.43%. IR
(KBr,
, cmꢁ1) for 2: 3429, 2923, 2852, 1619, 1539, 1443, 1402,
1312, 1156, 957, 912, 805, 770, 545, 467, 388; for 3: 3433, 1619,
1539, 1442, 1400, 1348, 1312, 1200, 1152, 1107, 954, 546, 437,
385. UV–Vis (kmax, nm) for 2: 256, 398, 532, 722; for 3: 256, 400,
542, 770.
m
Complex 4 was prepared with
L-histidine (0.5 mmol, 0.08 g) in-
stead of DL -alanine according to the same procedures as those
-a
employed for 2. Furthermore, 1,10-phenanthroline (C12H8N2)
(0.5 mmol, 0.10 g) (4) was used as auxiliary ligand in the reaction.
The solution filtrate was placed in room temperature, after a few
days deep-red (4) microcrystalline compounds were isolated. But
all attempts to prepare X-ray quality crystals were futile. Yield
(based on V): 0.15 g, 74.38% (4). Anal. Calc. for C27H27N5O6V (4):
C, 57.05; H, 4.79; N, 12.32. Found: C, 57.09; H, 4.82; N, 12.28%. IR
(DL-a-alanine/DL-b-phenylalanine/L-histidine) and bipyridyl/phe-
nanthroline: [VVO(sal-phe)(OMe)(MeOH)] (1), [VIVO(sal-ala)(2,20-
bipy)]ꢀH2O (2), [VIVO(sal-ala)(1,10-phen)]ꢀ0.5H2O (3) and [VIVO(-
sal-his)(1,10-phen)]ꢀ2MeOH (4). Particularly, bromination reaction
activity of the starting material (VOSO4ꢀnH2O) and the complexes
has been evaluated, revealing that the structural information of
the complexes play a very important role on considering the differ-
ences in catalytic activity and the complexes can be considered as
potential functional model vanadium haloperoxidases.
(KBr, m
, cmꢁ1) for 4: 3421, 3134, 1618, 1538, 1445, 1425, 1370,
1303, 1113, 962, 849, 726, 539, 433, 380. UV–Vis (kmax, nm) for
4: 260, 430, 520, 736.
2.3. X-ray data collection and refinement of crystal structures
The crystals of 1–3 were mounted on glass fibers for X-ray mea-
surement, respectively. Reflection data were collected at room
temperature on a Bruker AXS SMART APEX II CCD diffractometer
2. Experimental
2.1. Reagents and instruments
with graphite-monochromated Mo K
and a scan mode. All the measured independent reflections
(I > 2 (I)) were used in the structural analyses, and semi-empirical
a radiation (k = 0.71073 Å)
x
Elemental analyses (C, H and N) were performed on P. E 240C
automatic analyzer. The IR spectra were determined by JASCO
r
FT/IR-480 PLUS Fourier Transform spectrometer (200–4000 cmꢁ1
,
absorption corrections were applied using SADABS program [28].
The structures were solved by the direct method using SHELXS-86
and refined using SHELXL-97 [29]. All hydrogen atoms were
positioned geometrically and refined using a riding model. The
non-hydrogen atoms were refined with anisotropic thermal
parameters. The crystallographic data and experimental details of
the data collection and the structure refinement are given in
Table 1. The drawings were made with Diamond 2.1c program.
with pressed KBr pellets); UV–Vis spectra were determined by JAS-
CO V-570 UV–Vis spectrometer (200–1100 nm, in form of solid
sample). All the chemicals used were of analytical grade and used
without further purification. The reactions were carried out under
room temperature.
2.2. Preparation of complexes 1–4
2.4. Bromination reaction activity measurement of the complexes [25]
2.2.1. Preparation of [VVO(sal-phe)(OMe) (MeOH)] (1)
A
mixture of DL-b-phenylalanine (0.5 mmol, 0.08 g), VO-
Bromination reaction activity tests were carried out in the
mixed solution of H2O–MeOH at the constant temperature of
SO4ꢀnH2O (0.3 mmol, 0.08 g, based on 71%VOSO4) in methanol
(12 mL) and water (1 mL) was stirred for 3–4 h, then a few drops
of salicylaldehyde (0.5 mmol, 0.06 g) was added with continuous
stirring. The mixture was filtered off to get a deep orange solution
and less blue precipitate. The solution filtrate was placed in room
temperature for about seven days, deep-red crystals suitable for
X-ray diffraction were obtained, then dried under vacuum. Yield
(based on V): 0.09 g, 67.29%. Anal. Calc. for C18H20NO6V: C, 54.42;
Table 1
Crystallographic data and structure refinement for complexes 1, 2 and 3.a
Formula
C18H20NO6V
397.30
C20H19N3O5V
432.32
C22H18N3O4.50
447.33
V
M (g molꢁ1
)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
C2/c
26.481(5)
8.0557(13)
19.572(3)
90.00
Monoclinic
Cc
18.784(13)
20.521(13)
12.259(8)
90.00
Monoclinic
C2/c
18.719(5)
22.649(6)
12.408(6)
90.00
H, 5.07; N, 3.53. Found: C, 54.37; H, 5.03; N, 3.48%. IR (KBr, m,
cmꢁ1): 3427, 2928, 1659, 1631, 1601, 1551, 1450, 1350, 1292,
1222, 1151, 1056, 974, 916, 819, 747, 634, 558, 459, 396. UV–Vis
(kmax, nm): 256, 436.
a
(°)
b (°)
116.687(2)
90.00
3730.5(11)
8
125.248(8)
90.00
3859(4)
8
127.963(2)
90.00
4148(3)
8
c
(°)
V (Å3)
2.2.2. Preparation of complexes [VIVO(sal-ala)(2,20-bipy)]ꢀH2O (2),
[VIVO(sal-ala)(1,10-phen)]ꢀ0.5H2O (3) and [VIVO(sal-his)(1,10-phen)]ꢀ
2MeOH (4)
Z
Dcalc
1.418
1.488
1.433
Crystal size (mm)
F(0 0 0)
0.54 ꢂ 0.44 ꢂ 0.03 0.56 ꢂ 0.12 ꢂ 0.11 0.31 ꢂ 0.26 ꢂ 0.17
1656
1784
1840
A
methanolic (10 mL) solution of VOSO4ꢀnH2O (0.3 mmol,
l(Mo K
a
) (cmꢁ1
)
5.65
5.52
5.15
0.08 g, based on 71% VOSO4), DL -alanine (0.5 mmol, 0.05 g) and
-a
h (°)
2.19–27.30
10 542
4177
1.94–28.41
11 867
4680
1.80–23.56
8817
3048
2,20-bipyridyl (C10H8N2) (0.5 mmol, 0.08 g) (2), 1,10-phenanthro-
line (C12H8N2) (0.5 mmol, 0.10 g) (3) was stirred for 2 h, then
salicylaldehyde (0.5 mmol, 0.06 g) was added dropwise with con-
tinuous stirring, and then 1 mL water was added. After separation
of the solids by filtration, the deep orange mother liquor was left in
room temperature, several days later, deep-red crystals suitable for
X-ray diffraction were collected. Yield (based on V): 0.12 g, 79.43%
(2); 0.13 g, 81.41% (3). Anal. Calc. for C20H19N3O5V (2): C, 55.56; H,
Reflections collected
Independent
reflections(I > 2
Parameters
) (e Åꢁ3
r(I))
246
287
280
D
(q
)
0.487, ꢁ0.372
0.453, ꢁ0.316
0.923, ꢁ0.311
Goodness-of-fit
1.027
1.068
1.089
Ra
wR2
0.0478(0.0854)b 0.0507(0.0863)b 0.0727(0.1063)b
0.1094(0.1260)b 0.1253(0.1434)b 0.2077(0.2293)b
a
2
2
1=2
a
R ¼
R
jjFoj ꢁ jFcjj=
Based on all data.
R
jFoj; wR2 ¼ ½
R
wðF2o ꢁ F2c Þ ꢃ=
R
wðF2oÞ ꢃ
.
b
4.43; N, 9.72. Found: C, 55.52; H, 4.48; N, 9.69%; for C22H18N3O4.50
V

Y.-Z. Cao et al. / Inorganica Chimica Acta 368 (2011) 223–230
225
30 0.5 °C. Oxidovanadium complexes were dissolved by the addi-
tion of 50 mL MeOH–H2O in the volume ratio of 1:4. Solutions used
for kinetic measurements were maintained at a constant concen-
tration of H+ (pH 5.8) by the addition of NaH2PO4–Na2HPO4 [30].
Reactions were initiated by addition of KBr solutions. Spectral
changes were recorded using a 721 UV–Vis spectrophotometer in
1 cm path length quartz cells with the constant temperature keep-
ing in a HH-S thermostat, and the resulting data were collected in a
short time of the reaction and fitted using the curve-fitting soft-
ware in the program Microsoft Excel.
starting material VOSO4ꢀnH2O, which shows the addition of ancil-
lary ligands with bigger steric bulk coordinated to the central me-
tal (e.g., 2,20-bipyridyl and 1,10-phenanthroline) in the reaction is
probably hinder the oxidation of VO2+ moiety. After determining
the structures of them, the systematic experiments have been car-
ried out by changing different starting materials (e.g., VOSO4ꢀnH2O,
VO(acac)2, V2O5, NH4VO3 and Na3VO4) and different solvents (e.g.,
mixed solution of water–methanol, water–ethanol, water–acetoni-
trile, methanol, ethanol and acetonitrile) to find out their optimum
reaction conditions. By doing amount of experiments, it is found
that: (i) in the reaction course, when V2O5, NH4VO3 and Na3VO4
were used as a vanadium source instead of VOSO4, the target prod-
ucts cannot be obtained; while VO(acac)2 is used, the final desired
products can be obtained; (ii) addition of water is in favor of being
crystalline for compounds, which may due to H2O is much more
polar than the organic solvent; (iii) growing crystal of complexes
in methanol is much more suitable for X-ray than that in acetoni-
trile or ethanol, but it can easily be reproduced in good yield no
matter either of the solvents. In addition, the color of complexes
1–4 is deep red and they are quite stable in the solid state at room
temperature. They are insoluble in hexane, ether, etc; whereas
highly soluble in other organic solvents such as acetonitrile, dichlo-
romethane and chloroform.
It is assumed that the rate of this reaction is described by the
rate law dc/dt = kc1xc2yc3z, then the equation of log(dc/dt) = log k +
xlog c1 + ylog c2 + zlog c3 was obtained. It is corresponding to
ꢁlog(dA/dt) = ꢁx log c1 ꢁ b (b = log k + ylog c2 + zlog c3), where A is
the measurable absorbance of the resultant; k is the reaction rate
constant; c1, c2, c3 are the concentrations of oxidovanadium com-
plexes, KBr and phenol red, respectively; while x, y, z are the corre-
sponding reaction orders. When the measurable absorbance data
was plotted versus time, a line was obtained and the slope of this
line gave the reaction rate of oxidovanadium complexes (dA/dt). By
changing the concentration of oxidovanadium complexes, a series
of dA/dt data can be obtained. The reaction rate constant (k) can be
obtained from a plot of ꢁlog (dA/dt) versus ꢁlog c1 and were fitted
using 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 oxidovanadi-
um 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 [31]; c2
and c3 are known as 0.4 mol/L and 1 ꢂ 10ꢁ4 mol/L, respectively.
Based on the equation of b = logk + ylogc2 + zlogc3, so the reaction
rate constant (k) can be obtained. Bromination of phenol red was
monitored by measurement of the absorbance at 592 nm for reac-
tion aliquots which were extracted at specific time points and di-
luted into pH 5.8 phosphate buffer.
3.2. IR spectra
The IR spectral data of complexes 1–4 are listed in Table 2. Com-
parison of the IR spectra of the metal complexes revealed that all
complexes showed a broad band in the range 3421–3433 cmꢁ1
assignable to
m
(OꢁH) of the coordinated or uncoordinated water
and/or methanol molecules associated with the complexes which
are confirmed by elemental analyses. Also, the characteristic
stretching vibration of C@N from the aminoacid derivatives is
found in the range of 1618–1659 cmꢁ1. For complexes 1–4, the
antisymmetric
m m
as(COOꢁ) and symmetric s(COOꢁ) vibrations of
the carboxyl group from the aminoacid derivatives are in the
ranges of 1538–1601 cmꢁ1 and 1442–1450 cmꢁ1, respectively.
3. Results and discussions
The large difference between m m
as(COOꢁ) and s(COOꢁ) indicates a
monodentate coordination of this group. The bands observed in
3.1. Synthesis
the range of 1056–1156 cmꢁ1 are due to C–N stretching vibrations.
The bands of the V@O are observed in the range of 954–974 cmꢁ1
.
For complexes 1–4, their synthesis routes are similar, which is
simplified as shown in Scheme 1.
Beside these general features, some groups of the complexes also
manifest other features clearly, e.g., bands at 539–558 cmꢁ1 are
due to the V–O(phenoxyl) stretch while those at about 433–467 and
380–396 cmꢁ1 are due to the V–N and V–O(carboxyl) bonds. The
three bands substantiate the aminoacid derivatives coordination
to the metal center for the complexes [32].
At room temperature in mixed solution of water–methanol, the
four complexes [VVO(sal-phe)(OMe) (MeOH)] (1), [VIVO(sal-
ala)(2,20-bipy)]ꢀH2O (2), [VIVO(sal-ala)(1,10-phen)]ꢀ0.5H2O (3) and
[VIVO(sal-his)(1,10-phen)]ꢀ2MeOH (4) were synthesized in moder-
ate yields by original self-assembly reaction. In general, the low
potentials for the vanadium(V/IV) couple suggest that in all likeli-
hood the aerial oxygen acts as the oxidant for the oxidation of the
metal center during the synthesis of oxidovanadium(V) complexes
from oxidovanadium(IV) starting material. It is worthy to note that
the oxidation state of vanadium in complex 1 (+5) is different from
that of other complexes (+4), but they are all obtained from the
3.3. UV–Vis spectra
The UV–Vis absorption spectra of complexes 1–4 are recorded
in form of solid sample. The sharp and strong high-energy absorp-
tion bands at about 256/260 nm for the complexes are assigned as
Scheme 1. The reaction process of complexes 1–4.

226
Y.-Z. Cao et al. / Inorganica Chimica Acta 368 (2011) 223–230
Table 2
Characteristic IR bands (cmꢁ1) for oxidovanadium complexes 1–4.
Complex
m(OꢁH)
m(C@N)
m
as(COOꢁ)
m
s(COOꢁ)
m
(V@O)
m(CꢁN)
m
(VꢁO)
m(VꢁN)
1
2
3
4
3427
3429
3433
3421
1659
1619
1619
1618
1601
1539
1539
1538
1450
1443
1442
1445
974
957
954
962
1056
1156
1152
1113
558,396
545,388
546,385
539,380
459
467
437
433
p
–
p
⁄ transition of the aromatic-like chromophore from the ligands
bipyridyl (phenanthroline) and the amino acid derivative. For van-
adyl (IV) complexes, it is generally considered that transitions oc-
cur from dxy to (dxz, dyz) (
increasing energies. In the visible range, complexes 2–4 exhibit
m
1), d2 ꢁ d2
(
m
2) and d2
(
m
3) orbitals with
x
y
z
two set of absorption maximum at 722–770 and 480–542 nm
which can be attributed to (m1) and (m2) the bands of oxovana-
dium(IV), while it is not observed in the range for complex 1, which
indicate oxidation state of the vanadium is different from that in
other complexes (Table 3). The intense band at 398–426 nm is
probably due to a oxo ? vanadium(IV) charge-transfer transition
Fig. 2. The molecular structure of complex 2 and a view of the hydrogen bonded
dimer of complex 2. (The lattice water and all H atoms except for the hydrogen
bonds are omitted for clarity.)
admixed with the dxy ? d2 transition[11,33]. While in the visible
z
range, complex 1 exhibits one intense absorption maximum at
436 nm, it is assumed to be a oxo ? vanadium(V) charge-transfer
transition, because central metal atom has not no d electron.
3.4. Structural description of complexes 1–3
The molecular structures of [VVO(sal-phe)(OMe)(MeOH)] (1),
[VIVO(sal-ala)(2,20-bipy)]ꢀH2O
(2),
[VIVO(sal-ala)(1,10-phen)]ꢀ
0.5H2O (3), with the atom-numbering scheme are depicted in Figs.
1a, 2a, 3a, respectively. The principal bond lengths and angles of
the three complexes are listed in Table 4. All the compounds are
oxidovanadium–tridentate aminoacid derivative complexes and
each vanadium atom possesses a distorted octahedral geometry
defined by the NO5 or N3O3 donor set. The deviations from least-
squares planes for complexes 1–3 are listed in Table 5. In addition,
there are several inter-molecular hydrogen bonds in molecular
packing. Relevant H-bond parameters of complexes 1–3 are listed
in Table 6.
Fig. 3. The molecular structure of complex 3 and a view of the hydrogen bonded
dimer of complex 3. (The lattice water and all H atoms except for the hydrogen
bonds are omitted for clarity.)
Table 4
Table 3
Selected bond lengths (Å) angles (°) for complexes 1–3.
Characteristic UV–Vis bands (nm) for oxidovanadium complexes 1–4.
Complex 1
Complex
p
–
p
ꢄ transition
LMCT
d–d transition
VꢁO1
1.580(2)
1.771(2)
101.3(4)
96.2(2)
98.6(9)
75.7(8)
82.9(9)
VꢁO2
1.848(2)
2.337(2)
100.0(1)
93.2(9)
158.1(9)
176.3(1)
80.2(8)
VꢁO4
1.967(2)
2.114(2)
100.9(1)
155.9(8)
84.4(8)
80.0(8)
VꢁO5
VꢁO6
VꢁN1
1
2
3
4
256
256
256
260
436
398
400
430
–
O1ꢁVꢁO5
O1ꢁVꢁO4
O1ꢁVꢁN1
O4ꢁVꢁN1
O2ꢁVꢁO6
O1ꢁVꢁO2
O5ꢁVꢁO4
O5ꢁVꢁN1
O1ꢁVꢁO6
O4ꢁVꢁO6
O5ꢁVꢁO2
O2ꢁVꢁO4
O2ꢁVꢁN1
O5ꢁVꢁO6
N1ꢁVꢁO6
532,722
542,770
520,736
79.5(8)
Complex 2
VꢁO1
1.597(2)
2.049(2)
101.8(1)
104.4(1)
95.3(1)
VꢁO2
1.952(2)
2.146(2)
98.9 (1)
88.5(9)
90.4(8)
166.7(9)
88.3(1)
VꢁO4
2.013(2)
2.327(3)
158.1(9)
79.3(9)
94.8(9)
82.2(9)
71.8(9)
VꢁN1
VꢁN2
VꢁN3
O1ꢁVꢁO2
O1ꢁVꢁN1
O1ꢁVꢁN2
N1ꢁVꢁN2
O4ꢁVꢁN3
O1ꢁVꢁO4
O2ꢁVꢁN1
O2ꢁVꢁN2
O1ꢁVꢁN3
N1ꢁVꢁN3
O2ꢁVꢁO4
O4ꢁVꢁN1
O4ꢁVꢁN2
O2ꢁVꢁN3
N2ꢁVꢁN3
160.0(1)
79.2(9)
Complex 3
VꢁO1
1.600(4)
2.057(5)
101.2(2)
104.4(2)
94.8(2)
VꢁO2
1.944(4)
2.133(5)
99.1(2)
89.3(2)
91.4(2)
167.2(2)
87.9(2)
VꢁO4
1.995(4)
2.364(5)
158.6(2)
79.1(2)
93.5(2)
82.1(2)
72.6(2)
VꢁN1
VꢁN2
VꢁN3
O1ꢁVꢁO2
O1ꢁVꢁN1
O1ꢁVꢁN2
N1ꢁVꢁN2
O4ꢁVꢁN3
O1ꢁVꢁO4
O2ꢁVꢁN1
O2ꢁVꢁN2
O1ꢁVꢁN3
N1ꢁVꢁN3
O2ꢁVꢁO4
O4ꢁVꢁN1
O4ꢁVꢁN2
O2ꢁVꢁN3
N3ꢁVꢁN2
Fig. 1. The molecular structure of complex 1 and a view of the hydrogen bonded
dimer of complex 1. (All H atoms except for the hydrogen bonds are omitted for
clarity.)
160.2(2)
79.5(2)

Y.-Z. Cao et al. / Inorganica Chimica Acta 368 (2011) 223–230
227
Table 5
The deviations (Å) from least-squares planes for complexes 1–3.
1
2
3
O2
N1
O4
O5/N2
V
0.0314(09)
ꢁ0.0350(10)
0.0345(10)
ꢁ0.0309(09)
0.3057(11)
1.8846(22)
ꢁ2.0295(23)
ꢁ0.0078(11)
0.0087(12)
ꢁ0.0080(11)
0.0072(10)
ꢁ0.3534(12)
ꢁ1.9454(27)
1.9460(28)
ꢁ0.0071(22)
0.0079(25)
ꢁ0.0075(24)
0.0066(21)
ꢁ0.3449(25)
ꢁ1.9395(47)
1.9960(53)
O1
O6/N3
Fig. 4. Fragment of 1D network of hydrogen bonding found in complex 2 along a-
axis. (All H atoms except for the hydrogen bonds are omitted for clarity.)
Table 6
Hydrogen bonds (Å) of complexes 1–3.*
dehyde ligands coordinated to VO2+ moiety, the structural charac-
terization of the oxidovanadium complexes do not change
obviously.
D–Hꢀ ꢀ ꢀA d(D–H) (Å) d(Hꢀ ꢀ ꢀA) (Å) \D–Hꢀ ꢀ ꢀA (°) d(Dꢀ ꢀ ꢀA) (Å)
Proposed hydrogen bonds for complex 1
O6–H6ꢀ ꢀ ꢀO3a
0.807(2)
1.891(7)
175.6(8)
2.697(3)
By comparison of the selected bond lengths and angles among
the three complexes, it is indicated that: the bond lengths of
V@O1(terminal), V–O2(phenoxyl), and V–O4(carboxyl) in 1 are all shorter
than that of 2 and 3, while the bond length of V–N1(aminoacid derivative)
in 1 is longer than that of 2 and 3; the orders of the angle of O1–V–
O2, O1–V–O4, O1–V–N1, O2ꢁVꢁO4, O2ꢁVꢁN1 and O4–V–N1 are
all 1 < 2 ꢅ 3. The dihedral angle between the V–N1–C8–C10–O4
plane and the V–N1–C7–C6–C1–O2 plane is 21.92(06)°
(1) > 17.37(17)° (2) > 13.43(46)° (3), which indicate that the struc-
ture of complex 1 is more distorted than that of 2 and 3.
In addition, there are abundant inter-molecular hydrogen bonds
in complexes 1–3. For complex 1, two independent molecules form
a dimer by inter-molecular hydrogen bonding of O6–H6ꢀ ꢀ ꢀO3a
(a = ½ ꢁ x, 3/2 ꢁ y, ꢁz; 1.891(7) Å, 175.6(8)°) (Fig. 1b). As illus-
trated in Fig. 2b, two independent molecules of complex 2 form a
dimer by inter-molecular hydrogen bonding of C17–H17ꢀ ꢀ ꢀO4b
(b = 1/2 ꢁ x, 1/2 ꢁ y, 2 ꢁ z; 2.777(6) Å, 164.7(3)°). These dimeric
units are connected to one another via hydrogen bond of
Proposed hydrogen bonds for complex 2
C17– H17ꢀ ꢀ ꢀO4b
0.928(2)
0.928(7)
2.571(9)
2.506(8)
144.0(3)
143.8(8)
3.367(5)
3.302(8)
C19– H19Aꢀ ꢀ ꢀO3c
Proposed hydrogen bonds for complex 3
C15–H15Aꢀ ꢀ ꢀO4d
C20–H20Aꢀ ꢀ ꢀO1e
O1W–
0.930(0)
0.930(0)
0.850(0)
2.508(5)
2.540(1)
1.825(7)
143.8(0)
135.6(9)
176.4(4)
3.305(0)
3.270(8)
2.674(6)
H1WAꢀ ꢀ ꢀO3f
*
Symmetry transformation used to generate equivalent atoms: a = 1/2 ꢁ x, 3/2 ꢁ y,
ꢁz; b = 1/2 ꢁ x, 1/2 ꢁ y, 2 ꢁ z; c = 1/2 + x, 1/2 ꢁ y, 1/2 + z; d = 1/2 ꢁ x, 1/2 ꢁ y, 1 ꢁ z;
e = ꢁx, y, 1/2 ꢁ z; f = 1/2 ꢁ x, 1/2 ꢁ y, ꢁz.
In all cases, vanadium atom is coordinated by a terminal oxygen
(O1), two oxygen atoms and one nitrogen atoms (O2, O4 and N1)
from aminoacid derivative, two oxygen atom (O5(methoxy) and
O6(methanol)) from methanol group for complex 1, while two nitro-
gen atom (N2 and N3) from 2,20-bipy/phen for complexes 2–3.
They have a distorted octahedron vanadium center, where four
equatorial positions are satisfied by O2, O4, N1 and O5/N2, while
two axial positions are occupied by O1 and O6/N3. The deviations
of V, O1 and O6/N3 out of equatorial plane for complexes 1–3
(Table 5) indicate that the vanadium atoms are displaced toward
the oxo oxygen O1, trans to O6/N3. The angles of O1–V–O6/N3
for the three complexes are 176.0(1)°, 166.7(9)° and 167.2(2)°,
respectively. It is found that: for complex 1, the V–O6(methanol)
(2.337(2) Å) bond distance trans to the V@O bond is longer than
the V–O5(methoxy) (1.771(2) Å) bond in the equatorial plane, which
is similar to those complexes [{VO(L1)(OCH3)(CH3OH)}
{VO(L1)(OCH3)}] (2.331(3) Å, 1.800(3) Å and [VO(L2) (OCH3)
(CH3OH)] (2.314(1) Å, 1.763(1) Å) [HL1 = N0-(2-hydroxybenzylid-
ene)-5-methyl-1-(pyridin-2-yl)-1H-pyrazole-3-carbohydrazide,
HL2 = 1-(4,6-dimethylpyrimidin-2-yl)-N0-(2-hydroxybenzylidene)-
5-methyl-1H-pyrazole-3-carbohydrazide] [34]. The order of the
Fig. 5. Fragment of 1D network of hydrogen bonding found in complex 3 along a-
axis. (All H atoms except for the hydrogen bonds are omitted for clarity.)
bond lengths of V–O in complex 1 is V–O1(terminal) < V–O2(phenoxyl)
<
V–O4(carboxyl), the similar trend was also observed in complexes 2
and 3; and for complexes 2 and 3, the length of V–N(aminoacid derivative)
0
is shorter than that of V–N(2,2 -bipy/phen), which indicates the coordi-
nation ability of aminoacid derivative is stronger than that of 2,20-
bipyridyl/phen. In addition, there is obvious difference between
the coordination ability of two nitrogen atoms of 2,20-bipyridyl/
phen: as expected, the V–N bond distance trans to the V@O bond
is considerably longer than the V–N bond in the equatorial plane
and is a consequence of the strong trans influence of the terminal
oxo group. By comparison of the selected bond lengths and angles
with the similar aminoacid-derivatized oxidovanadium complexes
[VO(C10H8N2)(C9H7NO3)]3 and VO(C12H8N2)(C9H7NO3)ꢀ2.33H2O
[25], the corresponding bond lengths and angles of the complexes
are very close, indicating that when different aminoacid–salicylal-
Fig. 6. A view of two-dimensional hydrogen bonding network in complex 3. (All H
atoms except for the hydrogen bonds are omitted for clarity.)

228
Y.-Z. Cao et al. / Inorganica Chimica Acta 368 (2011) 223–230
Scheme 2. The reactive process of bromination reaction for the complexes.
C19–H19Aꢀ ꢀ ꢀO3c (c = 1/2 + x, 1/2 ꢁ y, 1/2 + z; 2.506(8) Å, 143.8(3)°)
form a 1D chain on the ac-face (Fig. 4). Similarly, for complex 3, the
dimer was formed by inter-molecular hydrogen bonding of C15–
H15Aꢀ ꢀ ꢀO4d (d = 1/2 ꢁ x, 1/2 ꢁ y, 1 ꢁ z; 2.508(5) Å, 143.8(0)°)
(Fig. 3b). And the ribbon chain is formed with the dimeric mole-
cules by the hydrogen interaction of C20–H20Aꢀ ꢀ ꢀO1e (e = ꢁx, y,
1/2 ꢁ z; 2.540(1) Å, 135.6(9)°) on the ac-face (Fig. 5). Then the adja-
cent chains are connected via hydrogen bond of O1W–H1WAꢀ ꢀ ꢀO3f
(f = 1/2 ꢁ x, 1/2 ꢁ y, ꢁz; 1.825(7) Å, 176.4(4)° to generate a 2D
layer supramolecular structure (Fig. 6).
as catalyst. The spectrum recorded showed a decrease in absor-
bance of the peak at 443 nm due to loss of phenol red and a peak
at 592 nm characteristic of the product bromophenol blue, show-
ing that the complex 4 possess significant catalytic activity.
The results of mimicking bromination reaction activities for the
other complexes are similar to that of 4, it is probably due to the
oxidovanadium complexes are all with V@O moiety containing tri-
dentate aminoacid derivative ligands.
3.5.2. Kinetic studies of mimicking bromination reaction
A mechanism of halide oxidation and a detailed catalytic cycle
are proposed for the vanadium haloperoxidase enzyme based on
this work, assumed in Scheme 3. On the reaction of the oxidovana-
dium complexes with H2O2, the peroxidovanadium intermediate
[VO(O2)L2] formed (reaction a). It can then rapidly oxidize bromide
(reaction b), in which this species (‘‘Br+’’) is an equilibrium mixture
of HOBr/Br2/Br3ꢁ, with the relative amounts determined by the
acid/base and bromide concentrations. And then if an organic sub-
strate capable of electrophilic aromatic substitution is present, the
same reaction conditions used to oxidize bromides can be used to
perform halogenations. The overall reaction observed was followed
by monitoring the visible conversion of phenol red to the tetrabro-
minated derivative bromophenol blue (reaction c). While in the ab-
sence of a substrate (such as halide), the hydrogen peroxide was
decomposed into water and dioxygen (reaction d) [31]. Anyway,
it is easily found that the forming speed of the peroxidovanadium
intermediate [VO(O2)L2] is the most determinative step of the
overall reaction [34].
3.5. Functional mimics of the vanadium haloperoxidases
3.5.1. Mimicking bromination reaction of the complexes
It is well known that oxidovanadium complexes are able to mi-
mic the reaction in which vanadium haloperoxidases catalyze the
bromination of organic substrates in the presence of H2O2 and bro-
mide. For example, the bromination of trimethoxybenzene [35],
þ
benzene, salicylaldehyde and phenol by the VO2 ion [36], and
the bromination of phenol red by [VO(O2)H2O]+ and related species
[37]. Herein, we have also investigated the bromination reaction
activity of complexes 1–4 using 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 halo-
genated product by the reaction of oxidized halogen species with
the organic substrate, and the reactive process is as shown in
Scheme 2.
It is found that addition of the solution of complex 4 to the stan-
dard 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 is shown in Fig. 7, the electronic
absorption spectra collected during such a color change using 4
To investigate the role of the aminoacid-derivatized oxidovana-
dium complexes in the bromide oxidation reaction, we take com-
plex
4 for instance, by changing the concentration of the
oxidovanadium complex, a series of dA/dt data was obtained
(Fig. 8).
And then as is shown in Fig. 9, the plot of ꢁlog(dA/dt) versus
ꢁlog c1 for complex 4 gives a straight line with a slope of 0.9958,
confirming the first-order dependence on vanadium. It is also
Fig. 7. Oxidative bromination of phenol red catalyzed by complex 4. Spectral
changes at 10 min intervals, c(phosphate buffer) = 50 mmol/L, pH = 5.8, c(KBr) = 0.4 mol/L,
c(phenol red) = 1 ꢂ 10ꢁ4 mol/L.
Scheme 3. The catalytic cycle proposed for the vanadium haloperoxidase enzyme.

Y.-Z. Cao et al. / Inorganica Chimica Acta 368 (2011) 223–230
229
due to the steric hindrance of the ligands in the complexes. Never-
theless, it is well known that when the central vanadium com-
plexed with organic ligands, it can give more soluble and less
toxic compounds than the inorganic vanadium material [38,39],
which is being considered less toxic from the biological point of
view [40–42]; (iv) complex 4 is active markedly in comparison to
other oxidovanadium complexes, probably because containing L-
histidine–salicylaldehyde in 4 makes it more close to the active
center of the VHPO; (v) the order of the reaction rate constant
for other complexes is 1 > 2 > 3, which is may related to that the
difference of aminoacid-derivatized ligands and the steric bulk of
ancillary ligands. Since the real mechanism by which the bromide
oxidation takes place is not very clear at the moment, it is wise to
consider aminoacid-derivatized oxidovanadium complexes as
good structural models for VHPO.
Fig. 8. The measurable absorbance dependence of time for complex 4. Conditions
used: pH 5.8, c(KBr) = 0.4 mol/L, c(H2O2) = 1 mmol/L, c(phenol red) = 1 ꢂ 10ꢁ4 mol/
4. Conclusions
L. c(complex 4/mmol/L) = a: 7.56 ꢂ 10ꢁ3
;
b: 1.51 ꢂ 10ꢁ2
;
c: 2.27 ꢂ 10ꢁ2
; d:
3.03 ꢂ 10ꢁ2; e: 5.98 ꢂ 10ꢁ2
.
To obtain more efficacious complexes to model the active center
of the VHPO, we synthesized a family of aminoacid-derivatized
oxidovanadium complexes and tested the bromination reaction
activity of them by the method with phenol red as organic sub-
strate in the presence of H2O2, Brꢁ and phosphate buffer. All the
complexes are easily obtained by original self-assembly reaction
under room temperature in mixed solution of CH3OH and H2O.
Structural analysis show that the complexes are all with V@O moi-
ety containing tridentate aminoacid-derivatives, and there are
abundant inter-molecular hydrogen bonds among them to form di-
meric (1), one-dimensional (2) and two-dimensional (3) self-
assembled structures. It is noteworthy that the oxidation state of
the central metal atom in complex 1 (+5) is different from that of
complexes 2–4 (+4). In general, the successful synthesis of com-
pounds 1–4 may suggest a new series of aminoacid derivative li-
gands to synthesize mixed-ligand oxidovanadium complexes
which may help develop the study of such compounds and under-
standing the exact role of vanadium compounds in VHPOs.
Acknowledgements
The authors express our sincere thanks to National Natural Sci-
ence Foundation of China (No. 21071071) and the Education Foun-
dation of Dalian City in China (Grant No. 2009J21DW004) for
financial assistance.
Fig. 9. ꢁlog(dA/dt) dependence of ꢁlog c for complex 4 in MeOH–H2O at 30 0.5 °C
(c is the concentration of the oxidovanadium complex 4; conditions used:
c(phosphate buffer) = 50 mmol/L, pH 5.8, c(KBr) = 0.4 mol/L, c(phenol red) = 1
ꢂ 10ꢁ4 mol/L.
Appendix A. Supplementary material
obtained from the line that b = 2.2086. Basing on the equation of
b = log k + ylog c2 + zlog c3, the reaction rate constant, k, is defined
by the concentrations of KBr and phenol red (c2 and c3), the reac-
tion orders of KBr and phenol red (y and z) and b. While in the
experiment, considering that the reaction orders of KBr and phenol
red (y and z) are 1 according to the literature[34]; c2 and c3 are
known as 0.4 mol/L and 1 ꢂ 10ꢁ4 mol/L, respectively, so the reac-
CCDC for complexes 1–3: 748104, 748102, 748103, respec-
tively, contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-
ta_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.01.031.
tion rate constant (k) for complex
4 can be calculated as
4.06 ꢂ 106 (mol/L)ꢁ2 sꢁ1
.
Similar plots for the starting material (VOSO4ꢀ1.5H2O) and other
complexes (1–3) were generated in the same way. It is found that
(Table 6): (i) the reaction orders of the oxidovanadium complex in
bromination reaction are all close to 1, confirming the first-order
dependence on vanadium; (ii) the order of the reaction rate con-
stant for them is VOSO4ꢀ1.5H2O > 4 > 1 > 2 > 3; (iii) mimcing catal-
ysis activity of the starting material (VOSO4ꢀ1.5H2O) is more higher
than that of the aminoacid-derivatized oxidovanadium complexes,
revealing that the forming speed of peroxidovanadium intermedi-
ate [VO(O2)L2] moiety through VOSO4ꢀ1.5H2O is faster than that of
the oxocomplexes with aminoacid derivative, which is probably
References
[1] D. Rehder, Angew. Chem., Int. Ed. 30 (1991) 148.
[2] A. Butler, C.J. Carrano, Coord. Chem. Rev. 109 (1991) 61.
[3] T.L. Fernández, E.T. Souza, L.C. Visentin, J.V. Santos, A.S. Mangrich, R.B. Faria,
O.A.C. Antunes, M. Scarpellini, J. Inorg. Biochem. 103 (2009) 474.
[4] D. Rehder, G. Santoni, G.M. Licini, C. Schulzke, B. Meier, Coord. Chem. Rev. 237
(2003) 53.
[5] R.L. Robson, R.R. Eady, T.H. Richardson, R.W. Miller, M. Hawkins, J.R. Postgate,
Nature 322 (1986) 388.
[6] T. Ueki, T. Adachi, S. Kawano, M. Aoshima, N. Yamaguchi, K. Kanamori, H.
Michibata, Biochim. Biophys. Acta 1626 (2003) 43.

230
Y.-Z. Cao et al. / Inorganica Chimica Acta 368 (2011) 223–230
[25] H.-Y. Zhao, Y.-H. Xing, Y.-Z. Cao, Z.-P. Li, D.-M. Wei, X.-Q. Zeng, M.-F. Ge, J. Mol.
[7] D.C. Crans, L. Yang, J.A. Alfano, L.H. Chi, W. Jin, M. Mahroof-Tahir, K. Robbins,
M.M. Toloue, L.K. Chan, A.J. Plante, R.Z. Grayson, G.R. Willsky, Coord. Chem.
Rev. 237 (2003) 13.
[8] K.H. Tompson, J.H. McNeil, C. Orvig, Chem. Rev. 99 (1999) 2561.
[9] A.G.J. Ligtenbarg, R. Hage, B.L. Feringa, Coord. Chem. Rev. 237 (2003) 89.
[10] V.M. Dembitsky, Tetrahedron 59 (2003) 4701.
Struct. 938 (2009) 54.
[26] Y.-H. Xing, J. Han, G.-H. Zhou, Z. Sun, X.-J. Zhang, B.-L. Zhang, Y.-H. Zhang, H.-Q.
Yuan, M.-F. Ge, J. Coord. Chem. 61 (2008) 715.
[27] J. Han, Y.-H. Xing, X.-J. Zhang, G.-H. Zhou, Y. An, M.-F. Ge, Chem. J. Chin. Univ.
28 (2007) 1431.
[11] D. Rehder, Bioinorganic Vanadium Chemistry, John Wiley & Sons, Chichester,
2008.
[12] P.M. Reis, J.A.L. Silva, J.J.R.F. da Silva, A.J.L. Pombeiro, Chem. Commun. 19
(2000) 1845.
[28] G.M. Sheldrick, SADABS, Program for Empirical Absorption Correction for Area
Detector Data, University of Göttingen, Göttingen, Germany, 1996.
[29] G.M. Sheldrick, Program for Crystal Structure Refinement, University of
Göttingen, Göttingen, Germany, 1997.
[13] G. Zampella, J.Y. Kravitz, C.E. Webster, P. Fantucci, M.B. Hall, H.A. Carlson, V.L.
Pecoraro, L.D. Gioia, Inorg. Chem. 43 (2004) 4127.
[30] E. Verhaeghe, D. Buisson, E. Zekri, C. Leblanc, P. Potin, Y. Ambroise, Anal.
Biochem. 379 (2008) 60.
[14] W. Plass, Coord. Chem. Rev. 237 (2003) 205.
[15] C. Wikete, P. Wu, G. Zampella, L.D. Gioia, G. Licini, D. Rehde, Inorg. Chem. 46
(2007) 196.
[16] H.-J. Yue, D. Zhang, Z. Shi, S.-H. Feng, Inorg. Chim. Acta 360 (2007)
2681.
[17] H. Esbak, E.A. Enyedy, T. Kiss, Y. Yoshikawa, H. Sakurai, E. Garribba, D. Rehder,
J. Inorg. Biochem. 103 (2009) 590.
[18] H.-J. Yue, D. Zhang, Z. Shi, S.-H. Feng, Solid State Sci. 8 (2006) 1368.
[19] M. Ebel, D. Rehder, Inorg. Chim. Acta 356 (2003) 210.
[20] I. Cavaco, J.C. Pessoa, D. Costa, M.T. Duarte, R.D. Gillard, P. Matias, J. Chem. Soc.,
Dalton Trans. 2 (1994) 149.
[31] G.J. Colpas, B.J. Hamstra, J.W. Kampf, V.L. Pecoraro, J. Am. Chem. Soc. 118
(1996) 3469.
[32] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, John Wiley & Sons, New York, 1986.
[33] S. Mohanta, K.K. Nanda, S. Ghosh, M. Mukherjee, M. Helliwell, K. Nag, J. Chem.
Soc., Dalton Trans. 22 (1996) 4233.
[34] T.N. Mandal, S. Roy, A.K. Barik, S. Gupta, R.J. Butcher, S.K. Kar, Polyhedron 27
(2008) 3267.
[35] R.I. de la Rosa, M.J. Clague, A. Butler, J. Am. Chem. Soc. 114 (1992) 760.
[36] M.R. Maurya, S. Agarwal, C. Bader, M. Ebel, D. Rehder, J. Chem. Soc., Dalton
Trans. 3 (2005) 537.
[21] I. Cavaco, J.C. Pessoa, M.T. Duarte, R.T. Henriques, P.M. Matias, R.D. Gillard, J.
Chem. Soc., Dalton Trans. 9 (1996) 1989.
[22] M.A. Neelakantan, F. Rusalraj, J. Dharmaraja, S. Johnsonraja, T. Jeyakumar, M.S.
Pillai, Spectrochim. Acta A 71 (2008) 1599.
[23] J.C. Pessoa, I. Cavaco, I. Correia, D. Costa1, R.T. Henriques, R.D. Gillard, Inorg.
Chim. Acta 305 (2000) 7.
[24] J.C. Pessoa, I. Cavaco, I. Correia, M.T. Duarte, R.D. Gillard, R.T. Henriques, F.J.
Higes, C. Madeira, I. Tomaz, Inorg. Chim. Acta 293 (1999) 1.
[37] M.R. Maurya, A. Kumar, M. Ebel, D. Rehder, Inorg. Chem. 45 (2006) 5924.
[38] I. Goldwaser, S. Qian, E. Gershonov, M. Fridkin, Y. Shechter, Mol. Pharmacol. 58
(2000) 738.
[39] T. Jakusch, W. Jin, L. Yang, T. Kiss, D. Crans, J. Inorg. Biochem. 95 (2003) 1.
[40] J.L. Domingo, Mol. Cell. Biochem. 203 (2000) 185.
[41] A.K. Srivastava, Mol. Cell. Biochem. 206 (2000) 177.
[42] G.R. Noleto, C.L.O. Petkowicz, A.L.R. Mercê, M.D. Noseda, S.C. Méndez-Sánchez,
F. Reicher, M.B.M. Oliveira, J. Inorg. Biochem. 103 (2009) 749.