Bromoperoxidase mimic as catalysts for oxidative bromination - Synthesis, structures and properties of the diversified oxidation state of vanadium(iii, iv and v) complexes with pincer N-heterocycle ligands

Article abstract of DOI:10.1039/c3ce40410b

Novel oxovanadium complexes (VO)₂(bpz*T-O) (1), VO(bpz*eaT)(SCN)₂ (2), V₂(bpz*eaT) ₂(μ₂-C₂O₄)(C₂O ₄)₂ (3), [VO(SO₄)(bpz*P)(H ₂O)]·H₂O (4) and VO(SO₄)(bpz*P-Me) (H₂O) (5) (bpz*T-O: 4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5- triazin-2-olate, bpz*eaT: 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6- diethylamino-1,3,5-triazine, bpz*P: 2,6-bis(5-methyl-pyrazol-3-yl) pyridine, bpz*P-Me: 2,6-bis(1,5-dimethyl-pyrazol-3-yl)pyridine), were synthesized by the reaction of V₂(SO₄)₃- VOSO₄-VO(acac)₂ and various pincer N-heterocyclic ligands with solution or hydrothermal methods. The structures of all the complexes were characterized by elemental analysis, IR and UV-vis spectroscopy and single-crystal diffraction analysis. Furthermore, thermogravimetric analyses (TG) and quantum chemistry calculations were also performed. Structural analyses reveal that the vanadium atom has a distorted trigonal bipyramidal geometry with a N₃O₂ donor set in 1; distorted octahedral geometry in 2, 4 and 5 with donor sets of N₅O, N₃O₃ and N₃O₃, respectively; a distorted pentagonal bipyramidal geometry with a N₃O₄ donor set in 3. In addition, the five new complexes with abundant intro- and inter-hydrogen bonding interactions exhibited bromination catalytic activity in a single-pot reaction of the conversion of phenol red to bromophenol blue in a mixed solution of H ₂O-DMF at a constant temperature of 30 ± 0.5°C with a buffer solution of NaH₂PO₄-Na₂HPO₄ (pH = 5.8), indicating that they can be considered as a potential functional model of bromoperoxidase. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ce40410b

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PAPER
Bromoperoxidase mimic as catalysts for oxidative
bromination—synthesis, structures and properties of
the diversified oxidation state of vanadium(III, IV and V)
complexes with pincer N-heterocycle ligands3
Cite this: CrystEngComm, 2013, 15,
5561
Chen Chen,b Qiao Sun,c Dong-Xue Ren,b Rui Zhang,b Feng-Ying Bai,*a Yong-
Heng Xing*b and Zhan Shid
Novel oxovanadium complexes (VO)2(bpz*T-O) (1), VO(bpz*eaT)(SCN)2 (2), V2(bpz*eaT)2(m2-C2O4)(C2O4)2
(3), [VO(SO4)(bpz*P)(H2O)]?H2O (4) and VO(SO4)(bpz*P-Me)(H2O) (5) (bpz*T-O: 4,6-bis(3,5-dimethylpyr-
azol-1-yl)-1,3,5-triazin-2-olate, bpz*eaT: 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-diethylamino-1,3,5-tria-
zine, bpz*P: 2,6-bis(5-methyl-pyrazol-3-yl)pyridine, bpz*P-Me: 2,6-bis(1,5-dimethyl-pyrazol-3-yl)pyridine),
were synthesized by the reaction of V2(SO4)3–VOSO4–VO(acac)2 and various pincer N-heterocyclic ligands
with solution or hydrothermal methods. The structures of all the complexes were characterized by
elemental analysis, IR and UV-vis spectroscopy and single-crystal diffraction analysis. Furthermore,
thermogravimetric analyses (TG) and quantum chemistry calculations were also performed. Structural
analyses reveal that the vanadium atom has a distorted trigonal bipyramidal geometry with a N3O2 donor
set in 1; distorted octahedral geometry in 2, 4 and 5 with donor sets of N5O, N3O3 and N3O3, respectively;
a distorted pentagonal bipyramidal geometry with a N3O4 donor set in 3. In addition, the five new
complexes with abundant intro- and inter-hydrogen bonding interactions exhibited bromination catalytic
activity in a single-pot reaction of the conversion of phenol red to bromophenol blue in a mixed solution
of H2O–DMF at a constant temperature of 30 ¡ 0.5 uC with a buffer solution of NaH2PO4–Na2HPO4 (pH =
5.8), indicating that they can be considered as a potential functional model of bromoperoxidase.
Received 7th March 2013,
Accepted 8th May 2013
DOI: 10.1039/c3ce40410b
www.rsc.org/crystengcomm
that the central vanadium is covalently linked to the Ne of the
imidazolyl moiety of a histidine and through extensive
Introduction
The interest in the coordination chemistry of vanadium has
increased in the last few decades because of its catalytic and
biological importance.1–7 Besides the insulin-enhancing activ-
ity of oxidovanadium(V) and oxidovanadium(IV) compounds,8,9
vanadium haloperoxidases (VHPO)3,10,11 are enzymes capable
of catalyzing the oxidative bromination of a variety of organic
substrates. Thereinto, the crystal structure of the vanadium
bromoperoxidase (V-BrPO) from Ascophyllum nodosum shows
hydrogen bonding to a variety of amino acid side chains
(Arg, His, Ser, Lys) and interstitial water in the proximity of the
active centre.12,13 This determination of vanadium in the
active sites of biological systems of bromoperoxidase and the
recognition of its environment has increased the interest in
vanadium complexes with organic ligands for mimicking the
biological activity in natural systems. Oxidovanadium(IV) and
dioxovanadium(V) complexes with N-, O- and S-donor chelat-
ing ligands have been studied for their potential in insulin-
mimetic effects,14 tumor growth inhibition and prophylaxis
against carcinogenesis,15 inhibiting several enzymes, includ-
ing phosphatases, ATPases, nucleases, kinases16 as well as the
antimicrobial activity against Mycobacterium tuberculosis.17
In particular, the application study of oxidovanadium com-
plexes in oxidation and oxotransfer catalysis has recently
attracted more attention, with various organic substrates being
oxidized by peroxides in the presence of vanadium or
dioxovanadium(V) complexes.18,19
aCollege of Life Science, Liaoning Normal University, Dalian 11602, P. R. China
bCollege of Chemistry and Chemical engineering, Liaoning Normal University,
Huanghe Road 850#, Dalian 116029, P. R. China.
E-mail: xingyongheng2000@163.com; Tel: +86-0411-82156987
cARC Centre of Excellence for Functional Nanomaterials, Australian Institute for
Bioengineering and Nanotechnology, the University of Queensland, Brisbane,
Australia
dState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun 130012, P. R. China
Electronic supplementary information (ESI) available: Tables of atomic
3
coordinates, isotropic thermal parameters and complete bond distances and
angles. CCDC 893011 for 1, 893015 for 2, 893012 for 3, 893013 for 4 and 893014
for 5. For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ce40410b
In order to get a better understanding of the catalytic
function mechanism of V-BrPO, to determine the role of
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vanadium and to explore the relationship between the
structure and catalytical activity, it is necessary to synthesize
a series of vanadium complexes containing oxygen and
nitrogen donors as a functional model for V-BrPO.10,11
A
variety of ligands that yield active functional model systems for
V-BrPO, including Schiff base ligands,20 poly(pyrazolyl)bo-
rates21 and so on, have so far been well developed in the
coordination chemistry of vanadium. In the mean time, in our
group, vanadium haloperoxidases functional model complexes
with O-, N- or S-donor ligands have been tested for their
bromination catalytic reaction activity in a single-pot reaction
of the conversion of phenol red to bromophenol blue. This
reaction is considered to have some important strong points:
(i) the starting phenol red is catalyzed by H2O2 to the
corresponding bromophenol blue, as observed by the immedi-
ate color change of the reaction; (ii) the low toxicity of KBr; (iii)
there are no side reactions in the process, etc. To our
knowledge, the study of synthesis, structure and catalytic
activity of some oxidovanadium complexes with pincer
N-heterocycle ligands is still rare. In addition, it is significant
to further expand and apply other organic bromide reactions
in industry because molecular bromine is a favorable agent for
the synthesis of brominated organic compounds but it is
corrosive and toxic, and moreover, the transport, storage and
handling of bromine therefore requires strict safety standards
in traditional industry.22 By checking the catalytic function
and various structures of oxidovanadium complexes, we have
found that oxidovanadium complexes with pincer
N-heterocycle ligands could possess better catalytic activity
than other oxidovanadium complexes.
Scheme 1
two types, as depicted in Scheme 1b. Compared with type i,
type ii can display more coordinated sites and produce various
complexes with different coordination modes easily.26 To the
best of our knowledge, many transition metal complexes with
type ii ligands have been studied in recent decades26a,b but
only a few kinds of coordination complexes with the 2,6-bis(5-
methyl-1H-pyrazol-3-yl)pyridine (L3) ligand have been synthe-
sized and in particular, oxidovanadium complexes have not
been reported up to now. Furthermore, there is also much less
research of complexes with the 2,6-bis(1,5-dimethyl-pyrazol-3-
yl)pyridine (L4) ligand.
In this paper, we successfully synthesized five novel
different oxidation state vanadium(III, IV and V) complexes,
(VO)2(bpz*T-O) (1), VO(bpz*eaT)(SCN)2 (2), V2(bpz*eaT)2(m2-
C2O4)(C2O4)2 (3), [VO(SO4)(bpz*P)(H2O)]?H2O (4) and
VO(SO4)(bpz*P-Me)(H2O) (5), with pincer N-heterocycle ligands
for the first time and defined their structures by the X-ray
crystallography method. In particular, the five complexes were
tested for their bromination catalytic activity and the results
indicate that they can be considered as a potential functional
model of bromoperoxidase (V-BrPO).
In recent years, pyrazole and its derivatives have been
widely used in coordination chemistry owing to the good
coordination ability of heterocyclics containing nitrogen
atoms. Some complexes with pyrazolyl derived ligands have
attracted considerable attention in catalysis, biomimetics, ion
exchange and photophysical properties areas. As nitrogen
containing heterocycles, pyrazolyl–triazine and bis(pyrazolyl)–
pyridine ligands are two significant branches of pyrazole
derivatives which have received great interest recently due to
the fact that they are straightforward to synthesize and in
particular, those substituted at the 3, 4 and 5-position on the
pyrazolyl moiety are also readily accessible. In addition, they
can bind to both low- and high-oxidation state metal ions,
generally in a tridentate fashion.23 In general, for pyrazolyl–
triazine ligands, there are mainly two types that will be used in
this work, as depicted in Scheme 1a, namely, 2,4,6-tri(3,5-
dimethylpyrazol-1-yl)-1,3,5-triazine (L1) and 2,4-bis(3,5-
Experimental section
All the chemicals used were of analytical grade and used
without further purification. L1 (tpz*T), L2 (bpz*eaT),27,28 L3
(bpz*P-H) and L4 (bpz*P-Me) were synthesized according to the
modified literature method.26b,29,30 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 pellets in the range of 200–4000
cm21. UV-vis spectra were recorded on a JASCO V-570
spectrometer (200–2500 nm, in the form of a solid sample).
Thermogravimetric analyses (TG) were performed under
atmosphere with a heating rate of 10 uC min21 on a Perkin
Elmer Diamond TG/DTA.
dimethyl-1H-pyrazol-1-yl)-6-diethylamino-1,3,5-triazine
(L2).
The triazine ring is a particular aromatic moiety with a very
strong electron-deficient character that allows for the forma-
tion of p–p stacking and anion–p interactions.24 Furthermore,
this ring contains three nitrogen atoms that can give rise to
hydrogen bonds and can also coordinate to metal ions, such as
tridentate terpyridine-like, bidentate bipyridine-like, mixed
tridenate and bidentate types and bis-bipyridine-like.25 For
bis(pyrazolyl)pyridine derivative ligands, there are also mainly
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n(C–H); 1616, 1570, 1449, 1238, 1011, n(CLN), n(CLC) or d(C–C);
976, n(VLO); 1173, n(S–O); 607, d(S–O). UV-vis (l max, nm): 260,
p–p*; 336, LMCT; 596, 764, d–d.
(VO)2(bpz*T-O) (1)
VO(acac)2 (0.5 mmol, 0.082 g) and L1 (0.5 mmol, 0.182 g) were
dissolved in CH3OH (10 mL), instantaneously giving a green
solution which was stirred at room temperature for 4 h. Then,
the solution was left at room temperature for a few days and
yellow crystals were obtained in ca. 65% yield based on V(IV).
Anal. calc. for C13H14O3N7V: C, 42.48; H, 3.81; N, 26.68. Found:
C, 42.46; H, 3.84; N, 26.65%. IR (KBr, n, cm21): 3439, n(O–H);
2994, 2929, n(C–H); 1637, n(CLN) or n(CLC); 1427, 770, d(C–H);
958, n(VLO); 444, n(V–N). UV-vis (l max, nm): 208, 280, p–p*;
340, LMCT.
X-ray crystallographic determination
Suitable single crystals of the five complexes were mounted
onto glass fibers for X-ray measurements. The reflection data
were collected at room temperature on a Bruker AXS SMART
APEX II CCD diffractometer with graphite monochromatized
Mo–Ka radiation (l = 0.71073 Å) and a v scan mode. All the
measured independent reflections (I . 2s(I)) were used in the
structural analyses and semi-empirical absorption corrections
were applied using the SADABS program.31 The structures
were solved by the direct method using SHELXL-97.32 All non-
hydrogen atoms were refined anisotropically. The hydrogen
atoms of the organic frameworks were fixed geometrically at
calculated positions and refined using a riding model but the
hydrogen atoms of the lattice water molecules in 3, 4 and 5
were found in a difference Fourier map. The structure of 2 (Rint
= 0.0609) is not of perfect quality as a result of the low theta
diffraction angle (hmax = 22.8u), probably due to the small
crystal size or poor crystal quality. The crystallographic data
and the structure refinements are given in Table 1. The
drawings were made with Diamond 3.2.
VO(bpz*eaT)(SCN)2 (2)
VOSO4 (0.5 mmol, 0.082 g), L2 (0.5 mmol, 0.17 g) and KSCN
(0.5 mmol, 0.041 g) were dissolved in CH3CN (10 mL) with
refluxing for 3 h at a temperature of 80 uC to get a blue
solution. After filtration, the precipitate was dissolved in
CH2Cl2. After two days, blue crystals suitable for X-ray
diffraction were obtained in ca. 64.86% yield based on V(IV).
Anal. calc. for C19H26N10OS2V: C, 43.38; H, 4.95; N, 26.64.
Found: C, 43.43; H, 4.89; N, 26.60%. IR (KBr, n, cm21): 3433,
n(O–H); 2978, 2932, n(C–H); 2071, n(SCN); 1649, n(CLN) or
n(CLC); 1477, 766, d(C–H); 964, n(VLO); 448, n(V–N). UV-vis (l
max, nm): 262, p–p*; 350, LMCT; 768, d–d.
V2(bpz*eaT)2(m2-C2O4)(C2O4)2 (3)
Quantum chemical calculation
V2(SO4)3 (0.5 mmol, 0.195 g), L2 (0.5 mmol, 0.17 g) and H2C2O4
(2.00 mmol, 0.18 g) were mixed and stirred for 2 h in a solution
of H2O and CH3OH (12 mL, 2 : 1). Then, the mixture was
sealed into a bomb and heated at 100 uC for 3 days, cooled to
room temperature and brown crystals of 3 were obtained in ca.
27% yield (based on V(III)). Anal. calc. for C40H52O14N16V2: C,
44.33; H, 4.80; N, 20.69. Found: C, 44.38; H, 4.92; N, 20.61%. IR
(KBr, n, cm21): 3421, n(O–H); 2982, n(C–H); 1712, 1696,
nas(COO2); 1399, ns(COO2); 1652, n(CLN) or n(CLC); 1483,
792, d(C–H); 469, n(V–N). UV-vis (l max, nm): 262, p–p*; 316,
382, LMCT; 526, 676, d–d.
First-principles density-functional theory (DFT) calculations
have been carried out using the DMOL3 module in Materials
Studio.33,34 Double numerical basis sets for all the atoms with
effective core potentials (ECP)35,36 were used for the optimiza-
tion of the three complexes. The PBE version of the general-
ized gradient approximation (GGA)37 was used for the electron
exchange and correlation. The convergence criteria for the
charge density of self-consistent iterations were set to 1026
,
which allows the binding energy to converge to 1026 Ry.38 The
Brillouin zone was sampled by 2 6 2 6 2 k-points using the
Monkhorst–Pack scheme. The charge distribution on the
system was analysed by the Mulliken method.39 Based on
the optimized structures obtained above, we calculated their
energy levels and plotted the electron densities of the highest
occupied molecular orbital (HOMO) and the lowest unoccu-
pied molecular orbital (LUMO) at the Gamma point.
[VO(SO4)(bpz*P)(H2O)]?H2O (4)
V2(SO4)3 (0.25 mmol, 0.0975 g), L3 (0.25 mmol, 0.0593 g), H2O
(10 mL) and EtOH (2 mL) were mixed in a 25 mL glass beaker.
After stirring for 3 h, the mixture was sealed in a Teflon-lined
autoclave and heated at 160 uC for three days, followed by
slowly cooling to room temperature. After filtration, the filtrate
was left at room temperature for several days until blue
crystals suitable for X-ray diffraction were obtained in ca.
80.37% yield (based on V(III)). Anal. calc. for C13H17O7N5SV: C,
35.59; H, 3.89; N, 15.97. Found: C, 35.62; H, 3.92; N, 15.78%. IR
(KBr, n, cm21): 3609, 3487, n(O–H); 3135, n(N–H); 3074, n(Ar–
H); 2955, 2925, n(C–H); 1617, 1553, 1445, 1270, 1024, n(CLN),
n(CLC) or d(C–C); 989, n(VLO); 1148, n(S–O); 585, d(S–O). UV-vis
(l max, nm): 264, p–p*; 322, LMCT; 582, 706, d–d.
Measurement of the bromination activity in solution
Bromination reaction activity tests were carried out in a mixed
solution of H2O–DMF at a constant temperature of 30 ¡ 0.5
uC. The oxidovanadium complexes were dissolved by the
addition of 25 mL of H2O–DMF in the volume ratio of 4 : 1.
The solutions used for the kinetic measurements were
maintained at a constant concentration of H+ (pH 5.8) by the
addition
a
buffer solution of NaH2PO4–Na2HPO4.40 The
reactions were initiated by the addition of a phenol red
solution. The oxidovanadium complexes with five different
concentrations were placed in five different cuvettes and then
the cuvettes were placed in a constant temperature water bath
for 10 minutes and the spectral changes were recorded using a
721 UV-vis spectrophotometer at 5 min intervals. Finally, the
resulting data were collected during the reaction and fitted
VO(SO4)(bpz*P-Me)(H2O) (5)
The synthesis of 5 is similar to that of 4 but L3 was replaced by
L4 (0.25 mL, 0.0668 g). Green crystals of 5 were obtained in ca.
80.37% yield (based on V(III)). Anal. calc. for C15H19O6N5SV: C,
40.15; H, 4.24; N, 15.61. Found: C, 40.19; H, 4.15; N, 15.70%. IR
(KBr, n, cm21): 3422, 3329, n(O–H); 3097, n(Ar–H); 2926, 2857,
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Table 1 Crystallographic data for complexes 1–5
Complexes
1
2
3
4
5
Formula
C13H14O3N7V
367.25
293(2)
C19H26N10OS2V
523.54
293(2)
C40H52O14N16V2
1082.86
293(2)
C13H17O7N5SV
438.32
293(2)
C15H19O6N5SV
448.35
296(2)
M (g mol21
)
Temperature (K)
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/n
Triclinic
P1
Monoclinic
P21/n
Monoclinic
P21/n
¯
a (Å)
b (Å)
c (Å)
9.378(2)
12.245(3)
13.488(3)
90
92.744(3)
90
13.447(3)
8.7943(18)
22.021(4)
90
105.599(3)
90
9.3562(10)
11.9681(12)
11.9868(12)
60.215(2)
84.303(2)
81.998(2)
1152.9(2)
1
8.0090(17)
12.111(3)
18.060(4)
90
92.664(3)
90
12.260(2)
8.0264(15)
19.192(4)
90
102.580(2)
90
a (u)
b (u)
c (u)
V (Å3)
1547.1(6)
4
410.85(14)
1
1749.8(6)
4
1843.3(6)
4
Z
Dcalc
1.594
1.386
1.560
1.664
1.616
Crystal size (mm)
0.30 6 0.23 6 0.20 0.46 6 0.09 6 0.08 0.48 6 0.26 6 0.14
0.30 6 0.18 6 0.09 0.28 6 0.13 6 0.11
F(000)
768
0.671
2.25–25.00
7548
1084
0.594
1.92–22.80
9995
562
0.491
1.96–28.21
5645
900
0.736
2.03–27.90
10 503
924
0.696
2.45–25.00
8938
m(Mo–Ka)/mm21
h (u)
Reflections collected
Independent reflections(i . 2s(i)) 2726(2369)
3408(2206)
0.0609
4010(3062)
0.0199
4108(2599)
0.0518
3239(2649)
0.0286
Rint
0.0189
Parameters
217
298
329
244
253
Dr (e Å23
)
0.526, 20.636
1.077
0.440, 20.400
1.023
0.519, 20.461
1.014
0.549, 20.612
1.003
0.500, 20.384
1.087
Goodness of fit
Ra
wR2
0.0455(0.1367)b
0.0519(0.1421)b
0.0538(0.1201)b
0.0946(0.1401)b
0.0507(0.1240)b
0.0531(0.0927)b
0.0443(0.0565)b
0.1194(0.1263)b
0.0737(0.1332)b, (0.0674)b 0.1334(0.1566)b
a
a
2
b
R = S||Fo | 2 |Fc ||/S |Fo |, wR2 = (S(w(Fo 2 Fc2)2/S(w(Fo2)2)1/2; [Fo . 4s(Fo)]. Based on all the data.
using the curve-fitting software in the Microsoft Excel
program.
be obtained. The bromination of phenol red was monitored by
the measurement of the absorbance at 592 nm for reaction
aliquots which were extracted at specific time points and
diluted into a pH 5.8 phosphate buffer.
It is assumed that the rate of this reaction is described by
x
y
z
the rate equation: dc/dt = kc1 c2 c3 , from which the equation
‘‘log(dc/dt) = logk + x logc1 + y logc2 + z logc3’’ was obtained,
corresponding to ‘‘2log(dc/dt) = 2x logc1 2 b (b = logk + y logc2
+ z logc3)’’, where k is the reaction rate constant; c1, c2, c3 are
the concentrations of the complex, KBr and phenol red,
respectively, while x, y, z are the corresponding reaction
orders. According to Lambert–Beer’s law, A = edc, which as a
differential is dA/dt = ed(dc/dt), where A is the measurable
absorbance of the resultant; e is the molar absorption
coefficient, which for bromophenol blue is measured as
14 500 M21 cm21 at 592 nm; d is the light path length of
the sample cell (d = 1). When the measurable absorbance data
were plotted versus the 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 the
oxovanadium complexes in the reaction system, a series of dA/
dt data can be obtained. The reaction rate constant (k) can be
obtained according to a plot of 2log(dc/dt) versus 2logc1 and
was fitted using the curve-fitting software in the Microsoft
Excel program by generating a least squares fit to a general
equation of the form ‘‘y = mx 2 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,32 c2 and c3 are known
as 0.4 and 1024 mol L21, respectively. Based on the equation
‘‘b = logk + y logc2 + z logc3’’, the reaction rate constant (k) can
Results and discussion
Synthesis
We have successfully synthesized complexes 1–5 with a
hydrothermal reaction for 3, 4 and 5, a solution reaction for
1 and a refluxing reaction for 2 (Scheme 2a). To meet the
coordination conditions of the corresponding ligands and
vanadium atom together in the reaction system, herein, we
have endeavored to search for the most optimal reaction
conditions. Hereinto, complex 1 was synthesized from the
starting material VO(acac)2(IV), while the experimental results
indicate that the oxidation state of the vanadium atom of 1 is
+5, which is confirmed by bond-valence theory.41,42 This is
because an unsaturated ketone conjugate structure in complex
1 is formed by an in situ reaction of one 3,5-dimethylpyrazolyl
group in the 2-position of the L1 ligand,43 the central
vanadium metal being oxidized via [O] during the reaction
(Scheme 2b). Its lower density cloud electron draws the
d-electrons of the vanadium(IV) atom, which leads to a change
in the valence of the central metal atom (from +4 to +5). We
initially used a solution method to synthesize complex 2,
however, we could not obtain the corresponding complex.
Then we attempted to use a refluxing method with CH3CN and
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Scheme 2 a: The synthetic routes of complexes 1–5; b: the detail reaction
mechanism routes of complex 1.
Fig. 1 (a): The molecular structure of 1; (b): a view of the two-dimensional
hydrogen bonding network in 1. (All H atoms expect for the hydrogen bonds
are omitted for clarity.)
only got the powder of 2, which was recrystallized in CH2Cl2 to
get nice single crystals. The synthesis of complex 3 was subject
to experimentation in a mixed system of water and methanol
at temperatures of 120 uC, 140 uC and 160 uC. Unfortunately, it
failed so we readjusted the temperature at 100 uC and
successfully got single crystals of the corresponding complex
3. According to the experiences of previous experiments,44 we
successfully obtained complexes 4 and 5 in a mixed solution
system of ethanol and water (1 : 5).
It is interesting to note that starting materials of V(III) were
used in the synthesis process of complexes 3, 4 and 5.
However, the valence of the central metal of 4 and 5 has been
changed. A possible reason is that the higher temperature
could lead to V(III) atom being oxidized into a V(IV) or V(V)
atom by the oxygen in air.
stood in the axial positions (Fig. 1a). The structure of 1 can be
evaluated by the Addision distortion index t:45,46 t = (b 2 a)/
60u, where b and a are the largest angles (b . a) around a five-
coordinate metal center. In a pentacoordinated system, the
values for t in an ideal polyhedra are 1.0 for a trigonal
bipyramid and 0.0 for a square pyramid. The value of t for the
metal center of complex 1 is 0.60, indicating that the
coordination environment of the vanadium atom is close to
distorted trigonal bipyramidal. The deviations of the N1, O2
and O3 atoms that comprise the least-squares plane are all
0.0000 Å, showing that these atoms are on one plane. The V,
N5 and N6 from the axial position lie 20.0088, 1.9807 and
21.9961 Å out of the equatorial plane, indicating that the V
atom is located in the middle of the N5 and N6 atoms.
Furthermore, the pyrazolyl rings from two sides are distorted
to the least-squares plane (central triazine ring), with the
dihedral angle varying from 1.3(4) to 1.8(4)u. The bond lengths
of V–N and V–O are in the range of 2.060(3)–2.085(3) Å and
1.604(2)–1.609(2) Å, respectively. The O–V–O, O–V–N and N–V–
N angles are in the range of 109.65(13)u, 99.36(12)–128.08(12)u
and 72.55(10)–145.37(10)u, respectively.
Crystal structure analysis
The molecular structures of complexes 1–5 are depicted in
Fig. 1–5, respectively. The principal bond distances and angles
for complexes 1–5 are summarized in Table 2. The relevant
H-bond parameters of complexes 1–5 are listed in Table 3. The
deviations of the coordinated atoms out of the equatorial
plane for 1–5 are listed in Table S1, ESI.
3
Structure of complex 1. Complex 1 was synthesized with the
L1 ligand to link one vanadium atom and adopt a pincer
tridentate coordination mode. This vanadium atom is coordi-
nated by two terminal oxygen atoms (O2, O3) and three
nitrogen atoms (N1, N5 and N6) from the L1 ligand, with N1,
O2 and O3 located in the equatorial plane and N5 and N6
In addition, in the molecule of complex 1, there are
…
important C–H O hydrogen bonds (3.3184, 3.3874 Å), as
illustrated in Fig. 1b. The hydrogen bonds come from between
the pyrazolyl ring (C21#1 and C22#1, #1: 20.5 + x, 0.5 2 y, 0.5 +
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20.5 + x, 0.5 2 y, 20.5 + z) comes from the carbon in the
dimethylamino group of the L2 ligand and the terminal
oxygen, which forma a 1D chain. In the mean time, the other
intra-hydrogen bonding interactions could make the structure
of complex 2 more stable.
Structure of complex 3. The asymmetric unit of complex 3
consists of two V atoms, two L2 ligands and three oxalic acid
ligands. The coordination environment of the vanadium atom
is shown in Fig. 3a. V(III) is seven-coordinate with three N
atoms (N1, N5 and N7) from the L2 ligand with V–N bond
lengths in the range of 2.129(3)–2.259(3) Å, four oxygen atoms
(O1, O2, O3 and O4) from the oxalic acid ligands, hereinto, O1
and O2 come from a bridging oxalic acid with V–Obridging bond
lengths in the range of 2.094(2)–2.127(2) Å, and O3 and O4 are
from a terminal oxalic acid with V–Oterminal bond lengths in
the range of 1.990(2)–1.996(2) Å, to form
a distorted
pentagonal bipyramidal geometry. The molecular structure is
centrosymmetric and the centre of inversion is located on the
…
C–C bond of the bridging oxalate moiety with a V V contact of
5.5024(11) Å. The deviations of the O1, O3, N1, N5 and N7
atoms that comprise the least-squares plane are 0.5165,
20.2415, 0.4646, 20.6358 and 20.1038 Å, respectively,
showing that these atoms are close on one plane. The V1, O2
and O4 atoms from the axial position lie 20.0878, 1.9732 and
22.0678 Å out of the equatorial plane. Furthermore, the
pyrazolyl rings from two sides are distorted to the least-
squares plane (central triazine ring) with the dihedral angle
varying from 10.1(2) to 11.9(2)u. The O–V–O, O–V–N and N–V–
Fig. 2 (a): The molecular structure of 2; (b): a fragment of the 1D chain with
hydrogen bonding interactions along the b-axis in 2. (All H atoms expect for the
hydrogen bonds are omitted for clarity.)
z) and the unsaturated ketone group of the triazine ring (O1).
Independent molecules are linked to form a 2D sheet structure
via the two types of hydrogen bonds.
Structure of complex 2. The asymmetric unit of 2 is
completed by one vanadium atom, one coordinated L2 ligand
and two –SCN groups and the coordination mode of the L2
ligand is similar to that of 1, acting as a pincer tridentate
coordination mode. The environment around the V atom can
be described as a distorted octahedral geometry and it is
coordinated by one terminal oxygen (O), two nitrogen atoms
(N5, N6) from two –SCN groups and three nitrogen atoms (N2,
N3 and N7) from the L2 ligand (Fig. 2a). The deviations of the
N2, N3, N5 and N6 atoms that comprise the least-squares
plane are 20.2226, 20.2321, 0.2260 and 0.2287 Å, respectively,
showing that these atoms are close on one plane. The V, N7
and O from the axial position lie 0.4435, 21.7060 and 2.0320 Å
out of the equatorial plane, indicating that V is towards the O
2
and trans to N7. The bond distances of V–NSCN and V–NL are
in the range of 2.027(5)–2.039(5) Å and 2.120(4)–2.150(4) Å,
respectively. The N–V–N bond angles are in the range of
71.54(15) to 167.62(17)u. Furthermore, the pyrazolyl rings from
two sides are distorted to the least-squares plane (central
triazine ring) with the dihedral angle varying from 3.2(3) to
3.0(3)u. The V–N bond length is in the range of 2.027(5)–
2.150(4) Å and the V–O bond length is 1.589(3) Å, respectively.
The O–V–N and N–V–N angles are in the range of 95.62(17)–
178.56(18)u and 71.54(15)–167.62(17)u, respectively.
There are abundant intra- and inter-hydrogen bonds in the
…
…
structure, such as C1–H1A
…
O
(3.4143 Å), C6–H6A
…
O
(3.3470 Å), C9–H9A N9 (2.9666 Å), C14–H14A O (3.4772 Å)
and C16–H16C N8 (2.9836 Å) in complex 2, as illustrated in
Fig. 2b. Here, the inter-hydrogen bond of C14–H14A
Fig. 3 (a): The molecular structure of 3; (b): a view of the 1D chain from the
hydrogen bonding interactions in 3. (All H atoms expect for the hydrogen bonds
are omitted for clarity.)
…
#2
…
O
(#2:
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Fig. 5 (a): The molecular structure of 5; (b): the dimer from the hydrogen
bonding interactions; (c): a view of the 1D chain from the hydrogen bonding
interactions in 5. (All H atoms except for the hydrogen bonds are omitted for
clarity.)
N angles are in the range of 77.34(10)–165.19(10)u, 73.22(10)–
152.20(11)u and 69.20(10)–130.47(10)u, respectively.
In addition, in the molecular structure, there are three kinds
#3
…
of hydrogen bonds: (i) hydrogen bond (O–H O: O1W
–
H1WA#3 O6 and O1W#3–H1WB#3 O6 , #3: 2x, 1 2 y, 1 2
#4
#4
…
…
z. #4: 21 + x, y, z) between the oxygen from the lattice water
and the carboxylate oxygen from the oxalic acids; (ii) hydrogen
#3
…
…
…
bond (C–H O: C22–H22A O1W
and C30–H30A O3)
between the carbon atoms from the L2 ligand and the oxygen
Fig. 4 (a): The molecular structure of 4; (b): the dimer from the hydrogen
bonding interactions; (c): a view of the 1D chain from the hydrogen bonding
interactions in 4. (d): A view of the two-dimensional hydrogen bonding
network. (All H atoms expect for the hydrogen bonds are omitted for clarity.)
atoms from the lattice water and oxalic acid molecules; (iii)
hydrogen bond (C–H N: C29–H29A N3) between the carbon
and the nitrogen atoms from the L2 ligand. Through the
…
…
#3
#3
#3
#4
…
…
hydrogen bonds of C22–H22A O1W , O1W –H1WA
…
O6
and O1W#3–H1WB#3 O6#4, the molecules of the complex 3 are
connected into an infinite 1D chain structure (Fig. 3b) and the
lattice water is very important to link the vanadium complex
moiety via these hydrogen bonding interactions.
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Table 2 Crystallographic data for complexes 1–5
Complex 1
Table 3 Hydrogen bonds (Å) of complexes 1–5
…
… … …
d(D–H)/Å d(H A)/Å d(D A)/Å /D–H A/u
D–H
A
V–O2
V–O3
V–N1
V–N5
1.604(2)
1.609(2)
2.060(3)
2.081(3)
N1–V–N5
O2–V–N6
O3–V–N6
N1–V–N6
N5–V–N6
O2–V–N5
O3–V–N5
72.55(10)
101.04(11)
99.36(12)
72.92(10)
145.37(10)
99.38(11)
99.69(12)
Complex 1
C22#1–H22C#1 O1
0.9600
0.9300
2.5516
2.5494
3.3874
3.3184
145.60
140.28
…
C21#1–H21A#1 O1
…
V–N6
2.085(3)
O2–V–O3
O2–V–N1
O3–V–N1
109.65(13)
122.26(12)
128.08(12)
Complex 2
…
C1–H1A
C6–H6A
C9–H9A N9
O
O
0.9600
0.9600
0.9600
0.9700
0.9600
2.5479
2.4775
2.2155
2.5223
2.2700
3.4143
3.3470
2.9666
3.4772
2.9836
150.16
150.57
134.30
168.08
130.44
…
…
#2
…
…
Complex 2
V–O
V–N6
V–N5
V–N3
V–N2
V–N7
O–V–N6
O–V–N5
N6–V–N5
O–V–N3
N5–V–N3
C14–H14A O
1.589(3)
2.027(5)
2.039(5)
2.120(4)
2.127(4)
2.150(4)
96.61(18)
95.62(17)
167.62(17)
109.36(17)
87.49(16)
O–V–N2
107.48(17)
89.19(17)
88.95(17)
143.16(15)
178.56(18)
84.54(16)
83.26(15)
71.54(15)
71.63(15)
86.67(16)
C16–H16C N8
N6–V–N2
N5–V–N2
N3–V–N2
O–V–N7
N6–V–N7
N5–V–N7
N3–V–N7
N2–V–N7
N6–V–N3
Complex 3
#4
#4
O1W#3–H1WA#3 O6
0.8500
0.7781
0.9600
0.9600
0.9600
2.1572
2.3757
2.5210
2.2300
2.4245
2.9459
3.1134
3.3833
2.9703
3.1494
154.23
158.67
149.47
133.15
132.07
…
O1W#3–H1WB#3 O6
…
#3
…
C22–H22A O1W
…
C29–H29A N3
…
C30–H30A O3
Complex 4
…
O1w–H1wA O3
0.8500
0.8500
0.8600
0.8600
0.9300
0.9300
0.9300
0.9600
2.4745
2.1179
1.9579
2.0342
2.5853
2.5613
2.3713
2.5099
3.1792
2.9313
2.8028
2.7222
3.3186
3.3018
3.2586
3.2741
140.84
160.04
167.13
136.38
136.09
136.88
159.46
136.54
#5
…
O2–H2A O5
#6
…
Complex 3
V–O4
V–O3
V–O2
V–O1
V–N1
V–N5
V–N7
O4–V–O3
O4–V–O2
O3–V–O2
O4–V–O1
O3–V–O1
N5–V–N7
N1–V–N7
N3–H3B O5
…
1.990(2)
1.996(2)
2.094(2)
2.127(2)
2.129(3)
2.229(3)
2.259(3)
79.97(10)
165.19(10)
86.26(10)
104.53(10)
77.34(10)
130.47(10)
69.64(10)
O2–V–O1
O4–V–N1
O3–V–N1
O2–V–N1
O1–V–N1
O4–V–N5
O3–V–N5
O2–V–N5
O1–V–N5
N1–V–N5
O4–V–N7
O3–V–N7
O2–V–N7
O1–V–N7
77.44(9)
N5–H5A O6
…
110.30(11)
152.20(11)
79.81(10)
122.03(10)
79.96(10)
138.58(11)
114.34(10)
73.22(10)
69.20(10)
89.62(11)
85.10(10)
83.76(10)
154.94(10)
C17–H17A O1w
C19–H19A O1
C22–H22A O6
C25–H25B O1
#8
…
…
…
#9
#7
Complex 5
…
O1w–H1wB O2
0.8500
0.9300
0.9600
0.9600
2.3527
2.4901
2.4900
2.5045
2.9156
3.2869
3.3649
3.2003
124.11
143.82
151.50
131.79
#6
…
C11–H11A O2
…
C15–H15A O3
#10
…
C15–H15C O5
#1: 20.5 + x, 0.5 2 y, 0.5 + z; #2: 20.5 + x, 0.5 2 y, 20.5 + z; #3: 2x,
1 2 y, 1 2 z; #4: 21 + x, y, z; #5: 2x, 2 2 y, 2z; #6: 1 2 x, 2 2 y,
2z; #7: 21 + x, y, z; #8: 2x, 1 2 y, 2z; #9: 0.5 + x, 1.5 2 y, 0.5 + z;
#10: 1 2 x, 20.5 + y, 0.5 2 z.
Complex 4
V–O1
V–O3
V–N1
V–N2
V–N4
V–O2
O1–V–O3
O1–V–N1
O3–V–N1
O1–V–N2
O3–V–N2
1.593(3)
1.968(2)
2.085(3)
2.110(3)
2.110(3)
2.223(3)
99.26(13)
97.15(12)
162.76(11)
97.95(13)
96.30(11)
N1–V–N2
O1–V–N4
O3–V–N4
N1–V–N4
N2–V–N4
O1–V–O2
O3–V–O2
N1–V–O2
N2–V–O2
N4–V–O2
76.13(11)
95.82(13)
108.75(11)
74.65(12)
148.97(12)
176.86(12)
83.39(11)
80.37(11)
83.36(11)
81.70(11)
coordinated by a terminal oxygen (O1), one oxygen atom (O2)
from the coordinated water, one oxygen atom (O3) from the
sulfate group and three nitrogen atoms (N1, N2, N4) from the
L3 ligand (Fig. 4a), forming a slightly distorted octahedral
vanadium center, with N1, N2, N4 and O3 in the equatorial
plane and O1 and O2 in the axial positions. The deviations of
the N1, N2, N4 and O3 atoms that comprise the least-squares
plane are 0.0164, 20.0139, 20.0130 and 0.0105 Å, respectively,
showing that these atoms are almost on one plane. The V, O1
and O2 atoms from the axial position lie 20.2738, 21.8658
and 1.9485 Å out of the equatorial plane. The O1–V–O2 angle
for complex 4 is 176.86(12)u. The L3 ligand adopts a pincer
tridentate chelate mode to coordinate with vanadium. For the
sulfate group anion, one oxygen atom is coordinated to the
vanadium with a monodentate coordination mode and more-
over, the distance of the coordinated S–O (1.509(2) Å) is slight
longer than the uncoordinated S–O (1.436(3) Å, 1.438(3) Å,
1.450(3) Å). The V–O1 bond length is 1.593(3) Å, while the V–
O2 and V–O3 distances are 2.224(3) and 1.968(2) Å, respec-
tively, which indicates that V–O1 is a double bond and V–O2
and V–O3 are single bonds. Hereinto, it is found that the
length of V–O2 is obviously longer than V–O3, which shows
that the coordination ability of the sulfate group is stronger
Complex 5
V–O1
V–O3
V–N1
V–N2
V–N4
V–O2
O1–V–O3
O1–V–N1
O3–V–N1
O1–V–N2
O3–V–N2
1.592(2)
2.001(2)
2.074(3)
2.100(3)
2.101(3)
2.228(2)
101.45(12)
107.45(12)
151.06(10)
96.99(12)
99.79(10)
N1–V–N2
O1–V–N4
O3–V–N4
N1–V–N4
N2–V–N4
O1–V–O2
O3–V–O2
N1–V–O2
N2–V–O2
N4–V–O2
75.65(10)
97.08(12)
102.51(10)
75.56(10)
150.62(10)
167.86(11)
66.45(9)
84.67(9)
86.53(10)
85.08(10)
Structures of complexes
4 and 5. Single-crystal X-ray
structure analyses revealed that each vanadium atom of the
two complexes shows a distorted octahedral geometry defined
by the N3O3 donor set. For complex 4, the vanadium atom is
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than water. Similarly, the pyrazolyl rings from two sides are
distorted to the least-squares plane (central pyridyl ring), with
the dihedral angle varying from 2.8(3) to 4.9(3)u.
independent molecules form a dimer via the hydrogen
#6
…
bonding interaction of C11–H11A O2 (Fig. 5b) and the
#10
…
dimer is further linked through C15–H15C O5
hydrogen
…
bonds to generate a one-dimensional chain on the b-axis, as
illustrated in Fig. 5c.
There are three kinds of hydrogen bonds, O–H O (2.9313–
…
…
3.1792 Å), C–H O (3.2586–3.3186 Å) and N–H O (2.7222–
…
2.8028 Å), in 4. Hereinto, the O–H O hydrogen bond comes
It is found that the bond lengths of V–N1, V–N2 and V–N4 of
4 are slightly longer than those of 5, which could be due to the
electron donor group of ‘‘–CH3’’ increasing the electron cloud
density of the center metal for 5. The same reason results in
the V–O2 distance of 5 being longer than the V–O3 distance of
4. However, the bond length of V–O3 in 5 is longer than that in
4, which attributed to the trans effect of the terminal oxygen
atom in complex 5.
from the lattice water molecules and coordinated sulfate
…
groups: O1w–H1wA O3 (3.1792 Å), and between the coordi-
nated water molecules and uncoordinated sulfate groups: O2–
#5
…
…
H2A O5 (2.9313 Å, #5: 2x, 2 2 y, 2z); the C–H O hydrogen
bonds are from the carbon atoms of the L3 ligand, lattice water
molecules, terminal oxygen atoms and uncoordinated sulfate
#8
…
…
groups: C17–H17A O1w (3.3186 Å), C19–H19A O1
#9
…
(3.3018 Å, #8: 2x, 1 2 y, 2z), C22–H22A O6 (3.2586 Å, #9:
Quantum chemistry calculations of complexes 1–5
0.5 + x, 1.5 2 y, 0.5 + z) and C25–H25B O1#7 (3.2741 Å, #7: 21
…
…
+ x, y, z); the N–H O hydrogen bonds come from the
According to molecular orbital theory, the frontier orbitals and
nearby molecular orbitals are the most important factors for
the stability. The larger the difference between the frontier
orbitals, the more stable the molecular structure.47–49 The
frontier molecular orbital symmetry and eigenvalues in
uncoordinated N atoms of the L3 ligand and the uncoordi-
#6
…
nated sulfate groups: N3–H3B O5 (2.8028 Å, #6: 1 2 x, 2 2
y, 2z) and N5–H5A O6 (2.7222 Å). Two independent
…
#5
…
molecules form a dimer via a O2–H2A O5
hydrogen
bonding interaction (Fig. 4b). The dimer is further linked
Hartrees for complexes 1–5 are listed in Table S2, ESI. The
3
#6
#7
…
…
through the N3–H3B O5 and C25–H25B O1 hydrogen
bonds to generate a 1D chain structure, as illustrated in
Fig. 4c, and then the adjacent chains are connected through
highest occupied molecular orbitals (HOMO): 20.2625,
20.1819, 20.1850, 20.1829 and 20.1773 a.u; the lowest
unoccupied molecular orbitals (LUMO): 20.1495, 20.1818,
20.1834, 20.1827 and 20.1769 a.u. The gap (eV) is 3.074,
0.002, 0.037, 0.005 and 0.009, respectively. This shows that the
order of thermodynamic stability for the complexes is 1 . 3 .
5 . 4 . 2. The atomic net charges for the complexes calculated
C19–H19A O1#8 hydrogen bonds to form a 2D sheet structure
…
#9
…
(Fig. 4d). In addition, via the corresponding C22–H22A O6
hydrogen bond, adjacent sheets form a 3D structure.
For 5, the coordination environment of the central
vanadium atom is similar to that of 4. The vanadium atom
is coordinated by a terminal oxygen (O1), two oxygen atoms
(O2, O3) from the sulfate group and three nitrogen atoms (N1,
N2, N4) from the L4 ligand (Fig. 5a), forming a slightly
distorted octahedral vanadium center, with N1, N2, N4 and O3
in the equatorial plane and O1 and O2 in the axial positions.
The deviations of the N1, N2, N4 and O3 atoms that comprise
the least-squares plane are 0.1662, 20.1367, 20.1343 and
0.1048 Å, respectively, showing that these atoms are almost on
one plane. The V, O1 and O2 from the axial position lie
20.3733, 21.9640 and 1.8224 Å out of the equatorial plane.
The O1–V–O2 angle for the complex is 167.86(11)u. The
coordination mode of the L4 ligand is the same as complex 4
but the coordination mode of the sulfate group shows a
bidentate chelate mode, while a similar trend for the bond
lengths of S–O was observed in complex 5, namely, S–O
(coordinated) . S–O (uncoordinated). In complex 5, the length
of V–O1 is 1.592(2) Å, while the distances of V–O2 and V–O3
are 2.001(2) and 2.228(2) Å, which indicates that V–O1 is a
double bond and V–O2 and V–O3 are single bonds.
Furthermore, the pyrazolyl rings from two sides are distorted
to the least-squares plane (central pyridyl ring) with the
dihedral angle varying from 7.7(2) to 6.6(2)u.
at the PBE level are shown in Table S3, ESI. The net charges of
3
the central vanadium in complexes 1–5 are in the range of
0.422 to 1.113 eV, deviating from their +3, +4, +5 valences,
respectively. This shows that the d orbit of the metal vanadium
partly obtained electrons from the ligand (L1, L2, L3 or L4) and
the sulfate group. The average net charges of the oxygen atom
from the coordinated water (20.608 eV for 4) is a little more
negative than those of the coordinated oxygen atoms from the
sulfate ion (20.541 eV for 4, 20.569 eV for 5) and more
negative than that of the coordinated oxygen atoms from the
oxalic acid ligands (20.477 eV for 3). The average net charges
of the terminal oxygen atoms for 1–5 are 20.481, 20.471,
20.539, 20.571, 20.604 eV, respectively. In complexes 1–3, the
average net charges of the coordinated nitrogen atoms from
the pyrazolyl rings are 20.198, 20.205 and 20.202 eV,
respectively. In complexes 4 and 5, the average net charges
of the coordinated nitrogen atoms from the pyrazolyl rings are
20.302 and 20.209 eV, showing that the order of the average
net charge of the nitrogen atoms in complexes 1–5 is:
Npyrazolyl(triazine
. Npyrazolyl(pyridine ring), which may be
ring)
attributed to the larger degree of conjugation of the triazine
ring.
Thermal properties
Similarly, there are also two kinds of hydrogen bonds: O–
…
…
…
H
O and C–H O, in 5. The O–H O hydrogen bond comes
To examine the thermal stability of complexes 1–5, thermo-
gravimetric analysis (TG) was carried out at a heating rate of 10
uC min21 under nitrogen in the temperature range of 30–1000
uC (Fig. 6). In 1, the results show that the initial weight loss of
49.2% before 582 uC is due to the release of two 3,5-dimethyl
pyrazolyl moieties (calc. 51.6%). The second weight loss occurs
in the temperature range of 582–1000 uC, which is ascribed to
from the lattice water molecule and coordinated sulfate group:
…
…
O1w–H1wB O2 (2.9156 Å); the C–H O hydrogen bond is from
among the carbon atoms of the L4 ligand, the coordinated
sulfate groups and the uncoordinated sulfate groups: C11–
#6
…
…
…
H11A O2 (3.2869 Å), C15–H15A O3 (3.3649 Å) and C15–
#10
H15C O5
(3.2003 Å, #10: 1 2 x, 20.5 + y, 0.5 2 z). Two
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rapid and stoichiometric, producing the halogenated product
by the reaction of the oxidized halogen species with the
organic substrate.
The addition of a solution of 1 to the standard reaction of
bromide in a phosphate buffer with phenol red as a trap for
the oxidized bromine resulted in a visible color change of the
solution from yellow to blue. As shown in Fig. 7, a decrease in
the absorbance of the peak at 443 nm due to the loss of phenol
red and an increase in the absorbance of the peak at 592 nm
characteristic of the bromophenol blue product is seen,
showing that complex
1 possesses significant catalytic
activities. The results of the mimicking catalytic activities for
2–5 are similar to that of 1.
Kinetic studies of the mimicking bromination reaction.
Taking complex 1 as an example, a series of dA/dt data was
obtained (Fig. 8) by changing the concentration of the
oxovanadium complex. (The measurable absorbance depen-
Fig. 6 TG curves of complexes 1–5.
the release of the framework of the triazine ring of the L1
ligand, and the final residue corresponds to vanadium oxide
and nitride. In 2, the first weight loss of 15.26% is attributed to
the reduction of the diethylamine of the L2 ligand (calc.
13.75%) in the temperature range of 323–352 uC. The second
weight loss of 59.88% was attributed to the release of two –SCN
groups and two 3,5-dimethyl pyrazolyl moieties (calc. 62.28%)
in the temperature range of 352–1000 uC. The final residue
corresponds to the vanadium atom and triazine ring (obsd:
22.83%; calc.: 24.64%). In 3, the initial weight loss of 3.33%
before 113 uC corresponds to the release of two lattice water
molecules (calc. 3.32%). The second weight loss of 80.13% is
attributed to the reduction of two L2 ligands and two terminal
oxalic acid molecules (calc. 79.42%) in the temperature range
of 195–1000 uC, and the final residue corresponds to two
vanadium atoms and oxalic acid molecules in the bridge site.
In 4, the results show an initial weight loss of 8.42% before 156
uC which is due to the release of a lattice water and a
coordinated water molecule (calc. 8.21%). The second weight
loss occurs in the temperature range of 156–1000 uC, which is
ascribed to the release of the framework of the L3 ligand and
the sulfate iron. The thermal stability of complex 5 is similar to
that of complex 4. The first stage occurs in the temperature
region of 40–120 uC, with a mass loss of 3.97% corresponding
to the loss of one lattice water (calc. 4.01%). The last step is
observed in 120–1000 uC, which is considered to be the
gradual decomposition of the framework of the ligands. The
residues correspond to the vanadium nitride compound for
both 4 and 5.
dence on time for complexes 2–5 is shown in Fig. S1, ESI. )
3
According to the data in Fig. 8, the plot of 2log(dc/dt) vs.
2logc for complex 1 is depicted and gave a straight line with a
slope of 1.0527 and b = 22.2581, as shown in Fig. 9. The
former confirmed that the reaction order is first-order reaction
dependence on vanadium. Based on the equation ‘‘b = logk + y
logc2 + z logc3’’, the reaction rate constant, k, is determined by
the concentrations of KBr and phenol red (c2 and c3), the
reaction orders of KBr and phenol red (y and z) and b. In the
experiment, considering that the reaction orders of KBr and
phenol red (y and z) are 1 according to the literature,53,54 c2
and c3 are known to be 0.4 mol L21 and 1024 mol L21
respectively, so the reaction rate constant (k) for 1 can be
calculated as 0.14 6 103 (mol L21 22 s21 (2log(dc/dt)
,
)
dependence of 2logc for 2–5 is shown in Fig. S2, ESI ).
3
Similar plots for 2–5 were generated in the same way. The
kinetic data for 2–5 in DMF–H2O at 30 ¡ 0.5 uC are shown in
Table 4. It is found that: (i) the reaction orders of the
oxidovanadium complexes in the bromination reaction are all
close to 1, confirming the first-order dependence on vana-
dium; (ii) the order of the reaction rate constants for them is 4
. 5 . 3 . 2 . 1.
Functional mimic of vanadium haloperoxidase
Mimicking bromination reaction of the complexes. To our
knowledge, oxidovanadium complexes are able of mimicking a
reaction in which vanadium haloperoxidases catalyze the
bromination of organic substrates in the presence of H2O2 and
bromide. For example, the bromination of trimethoxyben-
zene,50 benzene, salicylaldehyde and phenol is catalyzed by the
VO2 moiety,51 and the bromination of phenol red by
+
[VO(O2)H2O]+ and its related species.52 Herein, the bromina-
tion reaction activities of 1–5 using phenol red as an organic
substrate, which is shown by the conversion of phenol red to
bromophenol blue, have been investigated. The reaction is
Fig. 7 Oxidative bromination of phenol red was catalyzed by 1. Spectral
changes at 10 min intervals. The reaction mixture contained a phosphate buffer
(pH 5.8), KBr (0.4 mol L21), phenol red (1024 mol L21) and complex 1 (0.1 mmol
L
21).
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Table 4 Kinetic data for the complexes in DMF–H2O at 30 ¡ 0.5 uC
Complex
x
b
k (mol L21 22 s21
)
1
2
3
4
5
1.0527
1.0976
1.0666
1.0967
0.9808
22.2581
22.11
21.6809
21.2923
21.5531
0.14 6 103
0.19 6 103
0.52 6 103
1.28 6 103
0.70 6 103
Conditions used: c(phosphate buffer) = 50 mmol L21, pH = 5.8,
c(KBr) = 0.4 mol L21, c(phenol red) = 1024 mol L21. ‘‘x’’ is the
reaction order of the oxidovanadium complex; ‘‘b’’ is the intercept of
the line; ‘‘k’’ is the reaction rate constant for the oxidovanadium
complex.
Fig. 8 The measurable absorbance dependence on time for complex 1.
Conditions used: pH = 5.8, c(KBr) = 0.4 mol L21, c(H2O2) = 1 mmol L21, c(phenol
red) = 1024 mol L21. c(complex 1/mmol L21) = a: 2.12 6 1022; b: 4.25 6 1022
;
c: 6.37 6 1022; d: 8.50 6 1022; e: 1.06 6 1021
.
of the reaction rate constant for them is 9 . 4 . 6 . 10 . 5 .
3 . 7 . 8 . 2 . 1. Complexes 6–10 were synthesized
according to their corresponding references.56–58 6 is an
oxidovanadium complex with a poly(pyrazole)borate ligand, 7
and 8 are oxidovanadium complexes with Schiff base ligands,
The cyclic catalytic brominated reaction mechanism is
shown in Scheme 3, in which step a: the vanadium complex
is easily oxidized to form an intermediate compound of
[VO(O2)L] with H2O2 as an oxidation regent; step b: Br2 is
oxidized rapidly by the [VO(O2)L], while at the same time Br+
and [VO2L] are formed. The catalytic reaction rate would be
mainly based on the stability of the formed intermediate
compound [VO(O2)L] (from step a to b).55 The experimental
results show that the order of the catalytic activity is 4 . 5 . 3
. 2 . 1. This is because the formation of the intermediate
compound [VO(O2)L] could be influenced by the different
molecular structures of the complexes. The catalytic reaction
activity of the complexes may correspond to the different
molecular structures. Hereinto, the catalytic activity of com-
plexes 4 and 5 with L3 and L4 are higher than those of
complexes 1, 2 and 3 with L1 and L2, which is explained by the
fact that pyridine–pyrazole complexes form intermediate
species more easily than triazine–pyrazole complexes.
while
9 and 10 are oxidovanadium complexes with a
2,29-pyridine and 1,10-phenanthroline ligand, respectively. It
is found that the catalytic activities of 1 and 2 are low, which
may be due to the fact that the conjugated structure of the
triazine ligands makes the complexes more stable and the
formation of the intermediate peroxidovanadium [VO(O2)L]
more difficult. The catalytic activity of complex 3 is higher
than that of 1 and 2, which is because the structure of 3 is a
binuclear complex. The complexes with coordinated H2O
molecules all indicate higher catalytic activity, such as
complexes 4, 9 and 10, which is attributed to the lower bond
energy of the V–O single bond, resulting in the easier
formation of the intermediate species [VO(O2)L], and among
them, the conjugation behavior of the ligand in the complexes
is 10 . 4 . 9, so the order of the reaction rate constant is 9 . 4
. 10. Compared with 5 and 6, owing to the carboxylate group
Furthermore, comparisons of the kinetic data for the
complexes reported previously are listed in Table 5. The order
Fig. 9 2log(dc/dt) dependence of 2logc for 1 in DMF–H2O at 30 ¡ 0.5 uC (c is
the concentration of complex 1; conditions used: c(phosphate buffer) = 50
mmol L21, pH = 5.8, c(KBr) = 0.4 mol L21, c(phenol red) = 1024 mol L21
.
Scheme 3 The cyclic catalytic brominated reaction mechanism.
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2 C. Wikete, P. Wu, G. Zampella, L. D. Gioia, G. Licini and
D. Rehder, Inorg. Chem., 2007, 46, 196–207.
Table 5 Comparison of the kinetic data for the complexes in DMF–H2O at 30 ¡
0.5 uC
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8 (a) K. Kawabe, T. Sasagawa, Y. Yoshikawa, A. Ichimura,
K. Kumekawa, N. Yanagihara, T. Takino, H. Sakurai and
Y. Kojima, J. Inorg. Biochem., 2003, 8, 893–906; (b)
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Complex
m
b
k (mol L21 22
)
s21
Reference
1
2
3
4
1.0527
1.0976
1.0666
1.0967
0.9808
1.0076
0.9986
1.02
22.2581
22.11
0.14 6 103
0.19 6 103
0.52 6 103
1.275 6 103
0.699 6 103
1.116 6 103
0.462 6 103
0.421 6 103
2.347 6 103
1.003 6 103
This work
This work
This work
This work
21.6809
21.2923
21.5531
22.3524
22.7355
22.7758
21.1672
21.4858
5
This work
6a
7b
8c
9d
10e
42
43
43
44
44
1.1153
0.9921
a
b
6: VO(HB(3,5-Me2pz)3)(3,5-Me2pz)(HOOCCH2CH2COO). 7: [VO(sal-
c
d
ala)(2,29-bipy)]?H2O. 8: [VO(sal-ala)(1,10-phen)]?0.5H2O. 9:
[VO(C2O4)(2,29-bipy)(H2O)]?C2H5OH. 10: [VO(C2O4)(phen)]?H2O
(H2sal-ala: Schiff base derived from salicylaldehyde and DL-
a-alanine).
e
in 6 adopting a monodentate mode, it is easier for the
intermediate species of the peroxidovanadium to form and the
cleavage of the peroxide bond is easier, so the catalytic activity
of complex 6 is higher.
9 (a) S. Karmaker, T. K. Saha, Y. Yoshikawa, H. Yasui and
H. Sakurai, J. Inorg. Biochem., 2006, 100, 1535–1546; (b)
´
´
¨
T. Kiss, T. Jakusch, D. Hollender, A. Dornyei, E. A. Enyedy,
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Conclusions
In this work, five new complexes, supported by pincer-
N-heterocyclic ligands, have been successfully synthesized for
the first time. X-ray analysis reveals that the molecules in
complexes, 2, 3 and 5 form 1D chain structures, respectively,
and that complex 1 forms a 2D sheet structure via intra- or
intermolecular hydrogen bonds. However, two adjacent
molecules in complexes 4 and 5 generate supermolecular
structural dimers through intermolecular hydrogen bonds and
the dimers could form a 1D and 3D structure via other
intermolecular hydrogen bonding interactions, respectively.
To explore more efficacious model oxovanadium complexes
with the active centre of VHPO, we tested the bromination
reaction activity with phenol red as an organic substrate in the
presence of H2O2, KBr and a phosphate buffer solution, and
complexes 4 and 5, with L3 or L4 pincer N-heterocyclic ligands,
have a higher catalytic activity than complexes 1, 2 and 3.
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Acknowledgements
This work was supported by the grants of the National Natural
Science Foundation of China (No. 21071071) and the State key
Laboratory of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University, Changchun 130012, P.
R. China (Grant No. 2013-05).
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