A spectroscopic study on the coordination and solution structures of the interaction systems between biperoxidovanadate complexes and the pyrazolylpyridine-like ligands

Article abstract of DOI:10.1039/c3dt51986d

In order to understand the substitution effects of pyrazolylpyridine (pzpy) on the coordination reaction equilibria, the interactions between a series of pzpy-like ligands and biperoxidovanadate ([OV(O₂)₂(D ₂O)]^-/[OV(O₂)₂(HOD)]^-, abbrv. bpV) have been explored using a combination of multinuclear ( ¹H, ¹³C, and ⁵¹V) magnetic resonance, heteronuclear single quantum coherence (HSQC), and variable temperature NMR in a 0.15 mol L^-1 NaCl D₂O solution that mimics the physiological conditions. Both the direct NMR data and the equilibrium constants are reported for the first time. A series of new hepta-coordinated peroxidovanadate species [OV(O₂)₂L]^- (L = pzpy-like chelating ligands) are formed due to several competitive coordination interactions. According to the equilibrium constants for products between bpV and the pzpy-like ligands, the relative affinity of the ligands is found to be pzpy > 2-Ester-pzpy ≈ 2-Me-pzpy ≈ 2-Amide-pzpy > 2-Et-pzpy. In the interaction system between bpV and pzpy, a pair of isomers (Isomers A and B) are observed in aqueous solution, which are attributed to different types of coordination modes between the metal center and the ligands, while the crystal structure of NH₄[OV(O₂)₂(pzpy)]·6H ₂O (CCDC 898554) has the same coordination structure as Isomer A (the main product for pzpy). For the N-substituted ligands, however, Isomer A or B type complexes can also be observed in solution but the molar ratios of the isomer are reversed (i.e., Isomer B type is the main product). These results demonstrate that when the N atom in the pyrazole ring has a substitution group, hydrogen bonding (from the H atom in the pyrazole ring), the steric effect (from alkyl) and the solvation effect (from the ester or amide group) can jointly affect the coordination reaction equilibrium. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3dt51986d

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
A spectroscopic study on the coordination and
solution structures of the interaction systems
between biperoxidovanadate complexes and
the pyrazolylpyridine-like ligands†
Cite this: Dalton Trans., 2014, 43,
1524
Xian-Yong Yu,*a Lin Deng,a Baishu Zheng,a Bi-Rong Zeng,b Pinggui Yi*a and
Xin Xu*c,d
In order to understand the substitution effects of pyrazolylpyridine (pzpy) on the coordination
reaction equilibria, the interactions between a series of pzpy-like ligands and biperoxidovanadate
([OV(O2)2(D2O)]−/[OV(O2)2(HOD)]−, abbrv. bpV) have been explored using a combination of multinuclear
(1H, 13C, and 51V) magnetic resonance, heteronuclear single quantum coherence (HSQC), and variable
temperature NMR in a 0.15 mol L−1 NaCl D2O solution that mimics the physiological conditions. Both the
direct NMR data and the equilibrium constants are reported for the first time. A series of new hepta-
coordinated peroxidovanadate species [OV(O2)2L]− (L = pzpy-like chelating ligands) are formed due to
several competitive coordination interactions. According to the equilibrium constants for products
between bpV and the pzpy-like ligands, the relative affinity of the ligands is found to be pzpy > 2-Ester-
pzpy ≈ 2-Me-pzpy ≈ 2-Amide-pzpy > 2-Et-pzpy. In the interaction system between bpV and pzpy, a pair
of isomers (Isomers A and B) are observed in aqueous solution, which are attributed to different types
of coordination modes between the metal center and the ligands, while the crystal structure of
NH4[OV(O2)2(pzpy)]·6H2O (CCDC 898554) has the same coordination structure as Isomer A (the main
product for pzpy). For the N-substituted ligands, however, Isomer A or B type complexes can also be
observed in solution but the molar ratios of the isomer are reversed (i.e., Isomer B type is the main
product). These results demonstrate that when the N atom in the pyrazole ring has a substitution group,
hydrogen bonding (from the H atom in the pyrazole ring), the steric effect (from alkyl) and the solvation
effect (from the ester or amide group) can jointly affect the coordination reaction equilibrium.
Received 22nd July 2013,
Accepted 21st October 2013
DOI: 10.1039/c3dt51986d
www.rsc.org/dalton
Introduction
In the last four decades, vanadium complexes, in particular,
heteroligand peroxidovanadate complexes, have received
increasing attention because they could act as an inorganic
aKey Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of
Education, Hunan Province College Key Laboratory of QSAR/QSPR, School of
Chemistry and Chemical Engineering, Hunan University of Science and Technology,
Xiangtan 411201, China. E-mail: yu_xianyong@163.com, pgyi@hnust.cn;
Fax: +86-731-58290509
co-factor or a catalytic center involved in many enzymatic
processes such as haloperoxidation, nitrogen fixation, etc.1–4
As peroxidovanadates and the corresponding complexes
can inhibit protein tyrosine phosphatase (PTPase) and cause
DNA cleavage both in vitro and in vivo, they have been tested as
potential insulin-mimetic agents,1–3,5 as well as anti-tumor
drugs.6 Being model compounds for haloperoxidases, peroxido-
vanadates could also oxidize organic or inorganic substrates
to the corresponding products under mild conditions with
high yield and remarkable selectivity. Therefore, the synthesis
and characterization of peroxidovanadates and the investi-
gation of their biological mechanisms have recently become
the subject of extensive studies.1–12 In these studies, the
coordination chemistry of the peroxidovanadate complexes is
bDepartment of Materials Science and Engineering, Key Laboratory for Fire
Retardant Materials of Fujian Province, Xiamen University, Xiamen 361005, China
cShanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
MOE Laboratory for Computational Physical Science, Department of Chemistry,
Fudan University, Shanghai, 200433, China
dState Key Laboratory of Physical Chemistry of Solid Surfaces, College for Chemistry
and Chemical Engineering, Xiamen University, Xiamen 361005, China.
E-mail: xxchem@fudan.edu.cn
†Electronic supplementary information (ESI) available: The experimental
procedures and spectroscopic data of ligands 1–5 and complex 6. CCDC 898554.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt51986d
1524 | Dalton Trans., 2014, 43, 1524–1533
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Scheme 1 The structures of some classic heteroligands in the peroxido-
vanadate complexes.5,10,11c,d,12f
Scheme 2 The structures of the pyrazolylpyridine (pzpy)-like ligands
1–5.
the main concern.3,9–12 For example, Matsugo and co-workers
have synthesized several novel peroxidovanadate(V) compounds
containing amino acid derivatives and have studied their physio-
logical effects.7a A novel vanadium(V) oxoperoxido complex
containing the 2-acetylpyridine thiophene-2-carboxylic hydra-
zone ligand has been synthesized and characterized by Zhou’s
group.7b And their study indicated that the crystal structure is
stabilized by intermolecular hydrogen bonding.7b A unique
dinuclear mixed V(V) oxo–peroxo complex in the structural
speciation of the ternary V(V)–peroxo–citrate system has been
explored by Salifoglou’s group.7c At the same time, they have
also studied the possible mechanism and structure to explore
the aqueous synthetic chemistry of a dinuclear V(V)–citrate–
H2O2 system. Mayer and co-workers have synthesized and
studied the reactivity of the oxo–peroxo–vanadium(V) bipyri-
dine compounds.7d A series of peroxidovanadate complexes
containing heteroligands such as nitrilotriacetic acid, 2,2′-
bipyridine, 1,10-phenanthroline, and picolinate have been syn-
thesized and characterized as functional models for the
that mimics the physiological conditions by means of multi-
nuclear (1H, 13C, and 51V) magnetic resonance, heteronuclear
single quantum coherence (HSQC), and variable temperature
NMR. For the pzpy (1) ligand, a new single crystal (NH4-
[OV(O2)2(pzpy)]·6H2O, CCDC 898554) was obtained and has
been characterized using spectroscopic methods. By combin-
ing the above mentioned methods and comparing the crystal
structures of all the different species, their solution structures
and coordination modes in the interacting systems have been
determined; thereafter
a better understanding has been
achieved which suggests that the coordination equilibria
in solution are influenced by hydrogen bonding (from the
H atom in the pz ring), the steric effect (from alkyl) and the
solvation effect (from the ester or amide group).
Results and discussion
Solid state structure of 6
vanadium haloperoxidase enzymes9 or as potential insulin- Crystal 6 consists of ammonium ions, pzpy ligand-coordinated
mimetic agents.5,10 Furthermore, peroxidovanadates contain- biperoxidovanadate(V) complexes and water of crystallization,
ing 2-(2-pyridyl)-imidazole or picolinamide ligands have been held together by ionic and hydrogen bond interactions.
successfully synthesized and characterized by Chen’s group.11c,d
The structure of [OV(O2)2(pzpy)]− is depicted in Fig. 1,
The structures of some classic heteroligands in peroxidovana- which shares a distorted pentagonal bipyramidal geometry.
date complexes are shown in Scheme 1.5,10,11c,d,12f As revealed The four peroxido oxygens (O1, O2, O3 and O4), as well as the
by the studies from Pettersson’s or Rehder’s groups,12a–g it is nitrogen atom (N2) in the pz ring, define an equatorial plane
particularly important to choose these chelating ligands which with respect to the axial VvO bond within the limits of experi-
contain the pyridine (py) building block in synthesizing the mental error. The nitrogen atom (N1) in the pyridine ring
peroxidovanadate complexes.
binds to V in the axial position. Listed in Table 1 are some
The pyrazolylpyridine (abbr. pzpy) ligand, which has selected bond lengths of complex 6, and the corresponding
both the py ring and the pyrazole (pz) ring, is analogous to values of other biperoxidovanadate complexes are also shown
2-(2-pyridyl)-imidazole in terms of the chelating site and is in the table for comparison. More detailed structural data of
expected to be a good ligand for peroxidovanadate com- complex 6 can be found in ESI as presented in Table S1.†
plexes. There are reports in the literature showing that pzpy
could coordinate with a series of metal ions such as Mn2+ 13a
The axial VvO length is 1.597 Å for 6 as shown in Table 1.
This value is typically found for a double bond between V and
,
Fe2+,13b Co2+,13c Ni2+,13c,d Cu2+,13e Zn2+,13c,f Mo6+,13g Rh+,13h O atoms and is within the range observed in similar geome-
Ru2+,13i Cd2+,13f,i Pt2+,13j and Au3+.13k In the present work, a tries such as those of [OV(O2)2(pic)]− and [OV(O2)2(2-NH2-
series of the pzpy-like ligands (1–5, shown in Scheme 2) have pprd)]−.10,12f The 0.33 Å displacements of the vanadium atom
been synthesized, which have been used to investigate the sub- from the equatorial plane are comparable to that of ∼0.3 Å,
stitution effects of the pzpy ligand on the coordination reac- found in other hepta-coordinated biperoxidovanadate com-
tion equilibrium with peroxidovanadate. We have explored the plexes.11c,14 The V–Operoxido bond lengths are around
interactions between [OV(O2)2(D2O)]−/[OV(O2)2(HOD)]− (abbr. bpV) 1.865–1.922 Å which are also within the range of the normal
and the pzpy-like ligands in a 0.15 mol L−1 NaCl D2O solution V–Operoxido bond distances as reported in [OV(O2)2(py-im)]−
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1524–1533 | 1525

View Article Online
Paper
Dalton Transactions
Fig. 2 Solid-state 51V NMR spectra of 6. The asterisk indicates the 51V
NMR signal; other bands are spinning side-bands.
1
spectra. The details of H and 13C NMR spectroscopic data for
6 in solutions can be found in ESI.†
Fig. 3 shows the variable temperature 1H NMR spectra of
complex 6. From Fig. 3 it is clear that: (1) the spectra are
quite broad at room temperature, and become much
more sharp at the temperature range of 1–6 °C, (2) the low
temperature signals can be grouped mainly into two sets, one
stronger and the other weaker, and (3) the signals of the trace
free pzpy are very weak and will be explored in the following
section.
Fig. 1 Structures of the pzpy-like ligand-coordinated peroxidovanadate
complexes in the NH4[OV(O2)2(pzpy)]·6H2O (6) crystals. Ammonium ions
and waters of crystallization are not shown.
and [OV(O2)2(pprd)]−.11c,12f The lengths of the (O–O)peroxido
bonds are within the range of 1.451–1.456 Å, similar to the
corresponding values in [OV(O2)2(pic)]− and [OV(O2)2(bipy)]−.10,14e
The bond lengths for both V–Nequatorial (2.086 Å) and V–Naxial
(2.370 Å) are in good agreement with the corresponding values
found for other peroxidovanadate complexes such as
[OV(O2)2(pic)]−, [OV(O2)2(py-im)]−, and [OV(O2)2(bipy)]−.10,11c,14e
The 51V MAS NMR spectra of 6 are shown in Fig. 2 at the
spinning rates of 3, 5, and 7 kHz, respectively. Its chemical
shift is located at −796 which is different from that of Isomer
A in solution (chemical shift located at −774 at 20 °C,
see below). The shielding differences may be imparted by
the nature of the ligands/the ligand sphere in crystals or in
solutions.
A comparison between 13C NMR spectra of 6 in solution
(a) and that in the solid state (b) is shown in Fig. 4. As seen in
Fig. 4, this comparison seems to be not so straightforward.
Obviously, the signals in solution can also be grouped into two
sets, one stronger (labeled with *) and the other weaker. We
will assume that when complex 6 is dissolved in solution,
there exists a pair of isomers (A and B). The set of stronger
signal peaks are to be attributed to Isomer A, which is the
main product in solution and may adopt the same coordi-
nation mode as that in the solid state of 6. Isomer B, which
gives rise to the set of weaker signal peaks, would adopt a
different coordination mode as compared to that in the solid
state and it is the minor product in solution. By a careful exam-
ination of the crystal structure in Fig. 1, we propose that
Isomer B may have an interchanged coordination mode
with Nequatorial and Naxial, i.e., it is the nitrogen atom (N2) in
the pz ring, instead of that (N1) in the py as in Isomer A, that
Solution state structures of 6
X-ray diffraction provides conclusive results for the coordi-
nation modes of 6 in the solid state. However, its solution state
structures need to be further explored by using solution NMR
Table 1 Selected bond lengths of 6 as well as those of the other peroxidovanadate complexes
bpV(pzpy)
bpV(pprd)
bpV(2-NH2-pprd)
bpV(pic)
bpV(py-im)
bpV(bipy)
VvO
V–Operoxo(trans)
1.597(3)
1.877(3)
1.865(3)
1.922(3)
1.920(3)
1.456(4)
1.451(4)
2.086(4)
2.370(4)
This work
1.611(3)
1.895(2)
1.606(3)
1.882(4)
1.878(4)
1.912(4)
1.901(3)
1.473(6)
1.465(5)
2.128(4)
2.400(4)
12f
1.599(4)
1.895(4)
1.881(4)
1.917(4)
1.899(4)
1.464(5)
1.458(6)
2.123(5)
2.290(4)
10
1.614(2)
1.891(2)
1.879(2)
1.918(2)
1.899(2)
1.462(3)
1.449(3)
2.102(3)
2.356(3)
11c
1.619(3)
1.883(3)
1.880(3)
1.911(3)
1.909(3)
1.471(4)
1.465(4)
2.149(4)
2.288(3)
14e
V–Operoxo(cis)
(O–O)peroxo
1.887(2)
1.472(3)
V–Nequatorial
V–N/Oaxial
Ref.
2.142(3)
2.332(3)
12f
1526 | Dalton Trans., 2014, 43, 1524–1533
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Fig. 5 13C spectra of the bpV–pzpy interaction systems in aqueous
solution at 1 °C. The molar ratio of bpV and pzpy is 1 : 1 and 1 : 1.2 in (a)
and (b), respectively. A, B and C label the signals from Isomer A, Isomer
B and the free pzpy (1) ligand, respectively.
bpV solution was first prepared according to the procedure
described in the Experimental section and then varied
amounts of the pzpy-like ligands (2–5) were added.
Shown in Fig. 5 are the 13C spectra of the bpV–pzpy inter-
action systems in aqueous solution at 1 °C. The molar ratio of
bpV and pzpy is 1 : 1 and 1 : 1.2 in (a) and (b), respectively. In
Fig. 5(a), there exist mainly two sets of peaks. These are identi-
cal to those in Fig. 3(a) for the solution of 6. However, when
the amount of pzpy (1) was increased (from 1 to 1.2 equi-
valents), peaks of free pzpy ligands appeared. These are clearly
seen in Fig. 5(b). Therefore, we assign the set of stronger
signals as an indication of the formation of Isomer A, and
the set of weaker signals as an indication of the formation of
Isomer B. Based on the assignment of 13C NMR signals, the
1H NMR signals of the bpV–pzpy interaction systems can also
be assigned using the HSQC spectrum as displayed in Fig. 6.
NMR (1H, 13C and 51V) results for the other interaction
systems between bpV and the pzpy-like ligands (2–5) are sum-
marized in Table S2 in ESI.† The changes of the 1H and 13C
Fig. 3 Variable temperature 1H NMR spectra of complex 6 in aqueous
solution.
Fig. 4 13C NMR spectra: complexes 6 in solution (at 1 °C, a) and in
solid-state (at room temperature, b).
binds to V in the axial position in Isomer B. Similar phenomena
have been observed when bpV binds to the 4-(pyridin-2-yl)-
pyrimidine-like ligands.12f
To better understand the reasons for the coordination
differences in crystals and in solutions, we will ascertain the
coordination modes associated with Isomers A and B to
explore their formation mechanisms. In the following sections,
we will also discuss the substitution effects on the coordi-
nation reaction equilibria of a series of pzpy-like ligands (1–5)
to biperoxidovanadates.
1H and 13C NMR data of the interaction systems between bpV
and the pzpy-like ligands
In order to investigate the coordination ways between the
metal center vanadium and pzpy (1) in detail, the bpV–pzpy-
like ligand interaction systems were studied by using variable
Fig. 6 HSQC spectrum of the bpV–pzpy interaction systems in
aqueous solution. The molar ratio of bpV and pzpy is 1 : 1.2. A, B and C
label the signals from Isomer A, Isomer B and the free pzpy (1) ligand,
1
temperature H and 13C NMR spectra. For the pzpy (1) ligand,
we used the solution of complex 6 as the bpV–pzpy interaction
system (the molar ratio is 1 : 1). For other pzpy-like ligands, the respectively.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1524–1533 | 1527

View Article Online
Paper
Dalton Transactions
chemical shifts of organic ligands before/after coordination
reaction suggest that ligands (2–5) indeed coordinate to bpV
and have similar coordination models to that of pzpy in solu-
tion. According to the spectroscopic results from NMR or X-ray
diffraction, these N,N′-chelating biheteroaromatic ligands shall
form the hepta-coordinated [OV(O2)2L]− (L = pzpy-like ligands)
species as exemplified by [OV(O2)2(py-im)]−, [OV(O2)2(pprd)]−,
or [OV(O2)2(2-NH2-pprd)]− studied previously.11c,12f
51V NMR of the interaction systems between bpV and the pzpy-
like ligands
Table S2† summarizes the 51V NMR results for the interaction
systems between bpV and the pzpy-like ligands (1–5), which we
will discuss in detail in this section. It was reported that the
as-prepared bpV (see the Experimental section) has a 51V peak
located at −692 ppm at 20 °C (see Fig. 7(a)).15 When the pzpy
ligand was added to the bpV solution, new peaks appeared at
about −738 and −774 ppm (see Fig. 7(b)–(d)), which indicates
the formation of bpV–pzpy complexes [OV(O2)2(pzpy)]−. In the
bpV–pzpy interaction systems, Isomer A, corresponding to the
upfield peak, is the major product, while Isomer B, corres-
ponding to the low field peak, is the minor product. The inten-
sities of these newly-formed peaks increase with the increase
of the molar ratio of pzpy over bpV (from 0 to 0.5, 1, and
finally to 1.2 equivalents) before it reaches a maximum, as pre-
sented in Fig. 7(b)–(d). It is worth mentioning that the molar
ratio of Isomer A to B is still kept at 3.2 : 1 in the process of
adding more pzpy (1) into the solution at a temperature of
20 °C. These results are consistent with those of 1H and 13C
NMR spectra of 6 dissolved in solution as well as those of the
bpV + pzpy (1) systems. The conclusion that there are two
types of coordination modes existing when the pzpy ligand (1)
binds to the metal center as indicated by the appearance of
two sets of peaks for the species of [OV(O2)2(pzpy)]− in the
NMR spectra is, therefore, further supported.
Fig. 8 Variable temperature 51V NMR spectra of the complex
solution.
6
In order to investigate how temperature influences the
coordination equilibrium, the variable temperature 51V NMR
spectra of the solution of complex 6, which can be considered
as the interaction system between bpV and pzpy (1) with a
1 : 1 molar ratio, was studied in the temperature range of
1–40 °C. The results are displayed in Fig. 8, which show that:
(1) with the increase of temperature, all peaks move towards
low field. The chemical shift of bpV moves about 4.5 ppm
every 10 °C and that of Isomer A or B moves about 4.5 or
3.6 ppm, respectively, every 10 °C; (2) with the increase of
temperature, the quantity of [OV(O2)2(pzpy)]− species
decreases, due to its conversion into bpV. At the same time,
the ratio between Isomers A and B decreases (4 : 1 at 1 °C and
2.8 : 1 at 40 °C). This suggests that with an increase of temp-
erature, [OV(O2)2(pzpy)]− is converted back into bpV gradually,
and Isomer A is also converted into Isomer B. Coinci-
dently, the variable temperature 1H NMR spectra shown in
Fig. 3 also suggest that the peaks of free pzpy increase with the
increase of temperature, which agrees with the experimental
results of the variable temperature 51V NMR spectra.
Fig. 9 displays the respective 51V NMR spectra for all pzpy-
like ligands (1–5) at 1 °C, which reveals how the N-substitution
groups in the pz ring affect the coordination equilibrium with
bpV. Fig. 9 shows that when the corresponding pzpy-like
ligands are added, the intensity of the bpV peak decreases and
new peaks appear. For pzpy (1) shown in Fig. 9(a), peaks at
−745 and −783 ppm are assigned to [OV(O2)2(pzpy)]−. For
2-Me-pzpy (2) shown in Fig. 9(b), new peaks at about −743 and
−775 ppm are assigned to [OV(O2)2(2-Me-pzpy)]−. Similarly,
there are newly-formed peaks presented in Fig. 9(c)–9(e)
at −741 and −773 ppm, −737 and −761 ppm, −740 and
−760 ppm, respectively, which are assigned to [OV(O2)2(2-Et-
pzpy)]−, [OV(O2)2(2-Ester-pzpy)]−, and [OV(O2)2(2-Amide-
pzpy)]−, accordingly. At the same time, there are small new
Fig. 7 The 51V NMR spectra of the interaction systems between bpV
and pzpy (1) in aqueous solution at 20 °C. (a)–(d) correspond to the
molar ratios of pzpy/bpV = 0, 0.5, 1.0, and 1.2, respectively.
1528 | Dalton Trans., 2014, 43, 1524–1533
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Fig. 11 Fluorescence spectra of the Benesi–Hildebrand plot for the
interaction between pypz and bpV.
Table 2 The equilibrium constants for coordination between bpV and
the pzpy-like ligands. Note that the linear correlation coefficient R2 is
generally higher than 0.999 at 30 °C
Fig. 9 51V NMR spectra of the interaction systems of bpV and the pzpy-
like ligands (1–5) in aqueous solution at 1 °C. Peaks of the newly formed
[OV(O2)2L]− (L = 1–5) species are indicated by arrows.
Systems
K
R2
bpV + pzpy (1)
1375
1200
1100
1237
1175
0.9994
0.9992
0.9999
0.9994
0.9997
bpV + 2-Me-pzpy (2)
bpV + 2-Et-pzpy (3)
bpV + 2-Ester-pzpy (4)
bpV + 2-Amide-pzpy (5)
peaks at −756 ppm shown in Fig. 9(b), which might be
assigned to the ligand-free peroxidovanadate [HV2O3(O2)4]3−
according to a literature report.16 These assignments of the
pzpy-like ligand coordinated 51V peaks can be confirmed by
analyzing their fluorescence quenching with the increase in
bpV concentration (see below for further discussion). Based on
the ratios of bpV peak areas before and after coordination reac-
tions, the relative affinity of the pzpy-like ligands towards bpV
was determined to be 1 > 4 ≈ 2 ≈ 5 > 3.
concentration. The results, when the ligand is pzpy, are pre-
sented in Fig. 10.
Since the molar ratio between bpV and the pzpy-like
ligands in the newly-formed peroxidovanate complexes is 1 : 1,
the formation constant can be analysed by using the Benesi–
Hildebrand17 equation:
1
1
1
Quantitative measurements of the formation constants for the
interaction systems between bpV and the pzpy-like ligands
¼
þ
ð1Þ
I ꢀ I0 αðI′ ꢀ I0Þ KαðI′ ꢀ I0Þ½bpVꢁ
where K is the formation constant, I0 is the initial fluorescence
intensity of the free pzpy-like ligands, I′ is the minimum fluo-
rescence intensity, and I is the observed fluorescence intensity.
A plot of 1/(I − I0) vs. 1/[bpV] is shown in Fig. 11 for the pzpy
ligand. The formation constant can be obtained from the
intercept and the slope according to eqn (1). The results are
displayed in Table 2 for all ligands (1–5). The linear corre-
lations are all excellent for the coordination reactions with all
pzpy-like ligands. The data in Table 2 quantitatively evaluate
the relative affinities between bpV and various pzpy-like
ligands, which reconfirm the conclusion from the 51V NMR
spectra shown in Fig. 9 that the pzpy-like ligands (1–5) bind to
bpV with the trend in binding affinity as 1 > 4 ≈ 2 ≈ 5 > 3.
To achieve a quantitative evaluation of the coordination
affinity of the ligands, the formation constant of bpV and a
pzpy-like ligand is determined by analyzing the fluorescence
quenching of the pzpy-like ligand with the increase of the bpV
Coordination mechanisms between bpV and the pzpy-like
ligands
In order to explore the coordination mechanisms, we first
investigate the interactions between bpV and a series of
organic molecules, which are the building blocks of the pzpy-
like ligands, such as py, pz, and 1-methylpyrazole (1-Me-pz).
Fig. 10 The fluorescence emission spectra of pypz (1 × 10−4 mol L−1
)
with different concentrations of bpV. (a ∼ l): [bpV] = 0, 1, 2–12 × 10−4
mol L−1 (λex = 286 nm).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1524–1533 | 1529

View Article Online
Paper
Dalton Transactions
Scheme 3 The coordination modes between bpV and organic ligands such as pyridine, pyrazole or 1-methylpyrazole. ‘)(’ in 8 indicates the steric
interaction.
According to literature reports,12f,15a the relative binding of H3 in the N3 atom to O5 (2.781 Å). The hydrogen bond
affinity of the ligands towards bpV follows the trend as py ≈ interactions help to improve the complex stability which
pz > 1-Me-pz. The coordination modes between bpV and gives rise to Isomer A as the major product. On the other
py, pz, or 1-Me-pz are summarized in Scheme 3. The above hand, when the N-substituted pzpy-like ligands coordinate to
experimental results demonstrate that: (1) for these organic bpV, the py ligand is expected to favorably occupy the equa-
ligands, coordination to bpV leads to a series of hexa-co- torial position and to form a hexa-coordinated Intermediate B.
ordinated peroxidovanadate species as shown in 7–9, (2) for a This is due to the fact that the coordination ability of py is
pz ligand, coordination to bpV is always permissive, although stronger than that of 1-Me-pz, because of the steric effect in
the N-substitution group as shown in 8 will lead to a steric the latter. Isomer B is eventually formed by binding the N-sub-
effect which will decrease the binding affinity, and (3) for a py stituted pz building block to the metal center epaxially to form
ligand, however, coordination of an ortho-substituted ligand to the hepta-coordinated structure (i.e., Route B in Scheme 4).
bpV is permissive only when the substitution group is also a This explains why, for ligands 2–5, complexes of Isomer B type
coordination group, such that the chelate effect enhances the are the main products.
stability of the complex.12f
It is known that pz binds strongly to bpV, while an ortho
According to our previous study12f and the Conte group’s N-substituted group will reduce its binding affinity due to the
work,18 a neutral monodentate ligand such as py or pz can steric effect. Hence, for 2-Me-pzpy and 2-Et-pzpy, the relative
coordinate to [OV(O2)2]− in two possible ways: one is that the binding affinity follows the trend 2-Me-pzpy > 2-Et-pzpy.
ligand lies in the equatorial plane of the peroxide oxygens and When the N-substituted ligand contains the ester/amide
the other is that the ligand is on the axis trans to the oxo group, the additional effect plays a role in stabilizing the
group. Previous ab initio calculations suggested that the product. For 2-Ester-pzpy and 2-Amide-pzpy, both the steric
former is more favorable in energy than the latter.18 For the effect and the solvation effect have affected the coordination
pzpy-like bidentate ligands, we assume that the interaction reactions. To some extent, the favorable solvation effect can
process can be artificially decomposed into two steps (see offset the otherwise steric repulsion; the relative affinity of
Scheme 4). Because the coordination ability of the ligands is 2-Ester-pzpy or 2-Amide-pzpy is close to that of 2-Me-pzpy, while
py ≈ pz > 1-Me-pz, both the py and the pz building blocks are better than that of 2-Et-pzpy. These conclusions agree with
expected to occupy the favorable equatorial position to form a the results shown in Fig. 9 and Table 2 which shed light on
hexa-coordinated complex, giving Intermediate A or B. There- the coordination mechanisms between bpV and the pzpy-like
fore, in the interaction systems between bpV and the pzpy-like ligands in solutions. Hence, the ligand binding affinities
ligands, both isomers of A and B type shall be observed. There- follow the trend pzpy
fore, for the pzpy ligand, the molar ratio between Isomers A 2-Amide-pzpy > 2-Et-pzpy.
and B shall be similar, as py and pz possess similar binding
> 2-Ester-pzpy ≈ 2-Me-pzpy ≈
affinity to bpV. However, the observed ratio between Isomers A
and B is actually 2.8 : 1 when the temperature is increased to
Conclusions
40 °C (in Fig. 8). In the crystal structure shown in Fig. 1, hydro- In this work, the interactions between biperoxidovanadate
gen bond interactions are evident, as shown by the distances complexes (bpV) and a series of pyrazolylpyridine (abbr. pzpy)-
1530 | Dalton Trans., 2014, 43, 1524–1533
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
products from Sinopharm Chemical Reagent Company
without further purification. The pzpy-like ligands were syn-
thesized according to a method reported in the literature.19
The experimental procedures and spectroscopic data of these
compounds have been summarized in ESI.†
To form the interaction systems between bpV and pzpy-like
ligands, NH4VO3 and H2O2 were mixed with a 1 : 5 molar ratio
to the bpV solution first. The respective ligands were then
added to the solution. Unless otherwise stated, the total con-
centration of vanadate species is 0.2 mol L−1
.
The biperoxidovanadate crystals were prepared by adding
10 mL H2O2 (30%, w/v, solution) and 1.17 g NH4VO3 to 50 mL
distilled water. After NH4VO3 was dissolved, 1.47 g pzpy was
added to the mixture. The mixture was stirred in an ice water
bath at 273 K for 0.5 h. The reaction mixture was then filtered
and the solution was kept at 278–283 K for a week to crystal-
lize. The yellow block crystals were filtered and washed with
2 mL cold water and 7 mL cold ethanol and then dried on a
filter paper. The isolated yield was 50–70% based on the
vanadate.
Crystal structure determination
Determination of the unit cell and data collection for biper-
oxidovanadate compound 6 (yellow block crystal, 0.45 × 0.16 ×
0.10 mm) were performed on a Bruker SMART APEX CCD
single-crystal diffractometer using graphite-monochromatized
Mo Kα radiation (λ = 0.71073 Å) at 173 K. Crystal data and
details of the data collection and refinement are shown in
Table 3. The structures were solved using direct methods as
implemented in the program SHELXS97. The non-hydrogen
atoms were refined using the full-matrix least-square pro-
cedure on F2. Anisotropic displacement parameters were
assigned to non-hydrogen atoms.
Scheme 4 Possible interaction modes between bpV and the pzpy-like
ligands in solution.
like ligands in a 0.15 mol L−1 NaCl D2O solution have been
investigated by using several NMR experimental techniques.
The relative affinity of the pzpy-like ligands was found to
follow the trend pzpy
> 2-Ester-pzpy ≈ 2-Me-pzpy ≈
CCDC 898554 contains the crystallographic data for 6.
2-Amide-pzpy > 2-Et-pzpy. When the ligand was pzpy, a pair of
isomers (Isomers A and B) were observed in aqueous solution,
which have been attributed to different types of coordination
modes between the metal and the ligands. Isomer A (the main
product) has the same coordination structure as in the
NH4[OV(O2)2(pzpy)]·6H2O (CCDC 898554) crystal. For the
N-substituted ligands, both complexes of Isomers A and B types
have also been observed in solution. However, the molar ratio
of the two isomers is reversed with Isomer B type being the
main product. These results demonstrate that, for the N-sub-
stituted groups in the pyrazole ring, the hydrogen bonding
(from the H atom in the pyrazole ring), the steric effect (from
alkyl) and the solvation effect (from the ester or amide group)
can join to significantly affect the coordination reaction
equilibrium.
Table 3 Experimental crystal data and structure refinement parameters
for NH4[OV(O2)2(pzpy)]·6H2O (6)
Compound reference
6
Chemical formula
Formula mass
Crystal system
a/Å
b/Å
c/Å
α/°
C8H23N4O11
402.24
Monoclinic
7.5216(17)
26.293(6)
9.411(2)
90.00
V
β/°
γ/°
111.488(4)
90.00
1731.9(7)
173(2)
Unit cell volume/Å3
Temperature/K
Space group
P21/a
4
8593
3029
0.0284
0.0727
0.1357
0.0735
0.1362
898554
No. of formula units per unit cell, Z
No. of reflections measured
No. of independent reflections
Rint
Final R1 values (I > 2σ(I))
Final wR(F2) values (I > 2σ(I))
Final R1 values (all data)
Final wR(F2) values (all data)
CCDC number
Experimental section
Materials and preparation
The compounds NH4VO3, H2O2, 4-(2-hydroxyethyl)-1-piperazine-
ethane-sulfonic acid (HEPES), and NaCl were commercial
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1524–1533 | 1531

View Article Online
Paper
Dalton Transactions
Spectroscopic measurements
A. D. Keramidas, S. S. Amin, O. P. Anderson and
S. M. Miller, J. Chem. Soc., Dalton Trans., 1997, 2799–2812.
3 D. C. Crans, A. D. Keramidas, H. Hoover-Litty,
O. P. Anderson, M. M. Miller, L. M. Lemoine, S. Pleasic-
Williams, M. Vandenberg, A. J. Rossomando and L. J. Sweet,
J. Am. Chem. Soc., 1997, 119, 5447–5448.
4 A. Butler and J. V. Walker, Chem. Rev., 1993, 93, 1937–1944.
5 B. I. Posner, R. Faure, J. W. Burgess, A. P. Bevan, D. Lachance,
G. Y. Zhang-Sund, I. G. Fantus, J. B. Ng, D. A. Hall,
B. S. Lum and A. Shaver, J. Biol. Chem., 1994, 269, 4596–
4604.
6 (a) J. H. Hwang, R. K. Larson and M. M. Abu-Omar, Inorg.
Chem., 2003, 42, 7967–7977; (b) M. Sam, J. H. Hwang,
G. Chanfreau and M. M. Abu-Omar, Inorg. Chem., 2004, 43,
8447–8455; (c) D. W. J. Kwong, O. Y. Chan, R. N. S. Wong,
S. M. Musser, L. Vaca and S. I. Chan, Inorg. Chem., 1997,
36, 1276–1277.
7 (a) H. Sugiyama, S. Matsugo, T. Konishi, T. Takamura,
S. Kaneko, Y. Kubo, K. Sato and K. Kanamori, Chem. Lett.,
2012, 41, 377–379; (b) L. Z. Geng, J. Xing and Y. Z. Zhou,
Chin. J. Struct. Chem., 2012, 31, 185–190; (c) M. Kaliva,
C. Gabriel, C. P. Raptopoulou, A. Terzis, G. Voyiatzis,
M. Zervou, C. Mateescu and A. Salifoglou, Inorg. Chem.,
2011, 50, 11423–11436; (d) C. R. Waidmann, A. G. DiPasquale
and J. M. Mayer, Inorg. Chem., 2010, 49, 2383–2391.
8 (a) V. Conte, F. Di Furia and S. Moro, Tetrahedron
Lett., 1994, 35, 7429–7432; (b) M. Andersson, V. Conte,
F. Di Furia and S. Moro, Tetrahedron Lett., 1995, 36, 2675–
2678; (c) V. Conte, F. Di Furia, S. Moro and S. Rabbolini,
J. Mol. Catal., 1996, 113, 175–184; (d) V. Conte and B. Floris,
Inorg. Chim. Acta, 2010, 363, 1935–1946; (e) V. Conte,
F. Fabbianesi, B. Floris, P. Galloni, D. Sordi, I. W. C. E. Arends,
M. Bonchio, D. Rehder and D. Bogdal, Pure Appl. Chem.,
2009, 81, 1265–1277.
Room temperature solid-state NMR experiments were per-
formed on a 300 MHz Bruker DMX NMR spectrometer, operat-
ing at 78.8 MHz for 51V NMR and 75.5 MHz for 13C NMR. The
51V chemical shifts were referenced to VOCl3, while the 13C
chemical shifts were referenced to the carbonyl carbon of
glycine, which, in turn, is 173.2 ppm with respect to that of
tetramethylsilane. IR spectra were recorded on a Perkin-Elmer
spectrometer (Spectrum One).
The solution NMR spectra were recorded on a 500 MHz
Bruker AV II NMR spectrometer operating at 500.13 MHz for
1H NMR, 125.77 MHz for 13C NMR and 131.47 MHz for 51V
NMR. DSS (3-(trimethylsilyl)-propanesulfonic acid sodium
salt) was used as an internal reference for 1H and 13C chemical
shifts. 51V chemical shifts were measured relative to the exter-
nal standard VOCl3 with the upfield shifts considered as nega-
tive values. The signal-to-noise ratios were improved by a line-
broadening factor of 2 Hz and 10 Hz in the Fourier transform-
ation of all 13C and 51V spectra. In the NMR experiments, an
ionic medium, 0.15 mol L−1 NaCl D2O, was chosen in order to
represent physiological conditions.
The equilibrium constants for coordination between bpV
and the pzpy-like ligands were measured via a fluorescence
method. A stock solution of the pzpy-like ligand (1 × 10−2 mol
L−1) was prepared by ethanol, and diluted with HEPES buffer
solution (0.1 mol L−1, pH = 7.0) to 1 × 10−4 mol L−1 for testing,
while bpV was diluted by HEPES buffer solution to 2.5 × 10−2
mol L−1. A 2.5 mL pzpy-like ligand test solution was titrated by
successive additions (10 μL) of bpV solution with concen-
trations varying from 0 to 12 × 10−4 mol L−1 at 30 °C.
Detailed spectroscopic measurement results may be found
in ESI.†
9 (a) G. J. Colpas, B. J. Hamstra, J. W. Kampf and
V. L. Pecoraro, J. Am. Chem. Soc., 1994, 116, 3627–
3628; (b) G. J. Colpas, B. J. Hamstra, J. W. Kampf and
V. L. Pecoraro, J. Am. Chem. Soc., 1996, 118, 3469–3478;
(c) T. S. Smith and V. L. Pecoraro, Inorg. Chem., 2002, 41,
6754–6760; (d) G. Zampella, P. Fantucci, V. L. Pecoraro and
L. De Gioia, Inorg. Chem., 2006, 45, 7133–7143.
Acknowledgements
This work was supported by the NSF of China (21172066,
21201062, and 51103122), the Ministry of Science and Technol-
ogy (2011CB808505), Scientific Research Fund of Hunan Pro-
vincial Education Department (12K101), Hunan Provincial
Natural Science Foundation of China (11JJ2007), the Opening
Project of State Key Laboratory of Physical Chemistry of Solid
Surfaces (Xiamen University, 201309), and Aid Program for
Science and Technology Innovative Research Team in Higher
Educational Institutions of Hunan Province.
10 A. Shaver, J. B. Ng, D. A. Hall, B. S. Lum and B. I. Posner,
Inorg. Chem., 1993, 32, 3109–3113.
11 (a) X. Y. Yu, X. Xu and Z. Chen, Int. J. Mass Spectrom., 2008,
269, 138–144; (b) X. Y. Yu, S. H. Cai, X. Xu and Z. Chen,
Inorg. Chem., 2005, 44, 6755–6762; (c) X. Y. Yu, S. H. Cai
and Z. Chen, J. Inorg. Biochem., 2005, 99, 1945–1951;
(d) X. Y. Yu, J. Zhang, S. H. Cai, P. G. Yi and Z. Chen,
Spectrochim. Acta, Part A, 2009, 72, 965–969; (e) B. R. Zeng,
R. Q. Fu, S. H. Cai, J. Zhang and Z. Chen, Inorg. Chim. Acta,
2011, 365, 119–126; (f) B. R. Zeng, R. Q. Fu, S. H. Cai and
Z. Chen, Inorg. Chem. Commun., 2009, 12, 1259–1262.
References
1 (a) K. H. Thompson and C. Orvig, Science, 2003, 300, 936–
939; (b) K. H. Thompson and C. Orvig, J. Inorg. Biochem.,
2006, 100, 1925–1935; (c) K. H. Thompson, J. H. McNeill 12 (a) A. Gorzsas, I. Anderson and L. Pettersson, J. Inorg.
and C. Orvig, Chem. Rev., 1999, 99, 2561–2572.
2 (a) D. C. Crans, J. J. Smee, E. Gaidamauskas and
L. Yang, Chem. Rev., 2004, 104, 849–902; (b) D. C. Crans,
Biochem., 2009, 103, 517–526; (b) L. Pettersson, I. Andersson
and A. Gorzsas, Coord. Chem. Rev., 2003, 237, 77–87;
(c) I. Andersson, A. Gorzsas and L. Pettersson, Dalton
1532 | Dalton Trans., 2014, 43, 1524–1533
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Trans., 2004, 421–428; (d) H. Schmidt, I. Andersson, 14 (a) A. Shaver, D. A. Hall, J. B. Ng, A. M. Lebuis, R. C. Hynes
D. Rehder and L. Pettersson, J. Inorg. Biochem., 2000, 80,
149–151; (e) H. Schmidt, I. Andersson, D. Rehder and
L. Pettersson, Chem.–Eur. J., 2001, 7, 251–257; (f) A. Gorzsas,
I. Andersson and L. Pettersson, Dalton Trans., 2003, 2503–
2511; (g) M. Časný and D. Rehder, Dalton Trans., 2004,
839–846; (h) X. Y. Yu, P. G. Yi, D. H. Ji, B. R. Zeng, X. F. Li
and X. Xu, Dalton Trans., 2012, 41, 3684–3694.
and B. I. Posner, Inorg. Chim. Acta, 1995, 229, 253–260;
(b) P. Schwendt, M. Ahmed and J. Marek, Inorg. Chem.
Commun., 2004, 7, 631–633; (c) J. Tatiersky, P. Schwendt,
J. Marek and M. Sivak, New J. Chem., 2004, 28, 127–133;
(d) D. Begin, F. W. B. Einstein and J. Field, Inorg. Chem.,
1975, 14, 1785–1790; (e) S. Helga and S. Rolf, Acta Chem.
Scand., 1983, A37, 553–559.
13 (a) A. M. Kutasi, D. R. Turner, B. Moubaraki, S. R. Batten 15 (a) X. Y. Yu, R. H. Liu, H. L. Peng, H. W. Huang, X. F. Li,
and K. S. Murray, Dalton Trans., 2011, 40, 12358–12367;
(b) S. R. Batten, J. Bjernemose, P. Jensen, B. A. Leita,
K. S. Murray, B. Moubaraki, J. P. Smith and H. Toftlund,
Dalton Trans., 2004, 3370–3375; (c) X. T. Zhang, X. Y. Wu,
D. F. Sun, P. H. Wei, Z. H. Ni, J. M. Dou, D. C. Li, C. W. Shi
and B. Hu, Sci. China Chem., 2010, 53, 2285–2290;
(d) V. Mishra, F. Lloret and R. Mukherjee, Eur. J. Inorg.
Chem., 2007, 2161–2170; (e) J. S. Uber, Y. Vogels, D. van
B. S. Zheng, P. G. Yi and Z. Chen, Spectrochim. Acta, Part A,
2010, 75, 1095–1099; (b) X. Y. Yu, F. X. Yang, D. H. Ji,
J. M. Zhou, R. H. Liu;, G. B. Li, X. F. Li, J. Chen,
H. W. Huang and P. G. Yi, Spectrochim. Acta, Part A, 2010,
77, 816–820; (c) X. Y. Yu, D. H. Ji, R. H. Liu, F. X. Yang,
J. Xie, J. M. Zhou, X. F. Li and P. G. Yi, J. Coord. Chem.,
2010, 63, 1555–1562; (d) R. J. Zeng, Z. X. Wang, X. F. Li,
J. Xie and X. Y. Yu, J. Coord. Chem., 2010, 63, 498–504.
den Helder, I. Mutikainen, U. Turpeinen, W. T. Fu, 16 D. Rehder, T. Polenova and M. Bühl, Annu. Rep. NMR
O. Roubeau, P. Gamez and J. Reedijk, Eur. J. Inorg. Chem., Spectrosc., 2007, 62, 50–114.
2007, 4197–4206; (f) T. L. Hu, R. Q. Zou, J. R. Li and 17 (a) H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc.,
X. H. Bu, Dalton Trans., 2008, 1302–1311; (g) A. C. Coelho,
M. Nolasco, S. S. Balula, M. M. Antunes, C. C. L. Pereira,
F. A. A. Paz, A. A. Valente, M. Pillinger, P. Ribeiro-Claro,
J. Klinowski and I. S. Goncalves, Inorg. Chem., 2011, 50,
1949, 71, 2703–2707; (b) C. Yang, L. Liu, T. W. Mu and
Q. X. Guo, Anal. Sci., 2000, 16, 537–539; (c) R. Rajamohan,
S. Kothai Nayaki and M. Swaminathan, J. Solution Chem.,
2011, 40, 803–817.
525–538; (h) A. P. Martinez, M. J. Fabra, M. P. Garcia, 18 V. Conte, O. Bortolini, M. Carraro and S. Moro, J. Inorg.
F. J. Lahoz, L. A. Oro and S. J. Teat, Inorg. Chim. Acta, 2005, Biochem., 2000, 80, 41–49.
358, 1635–1644; (i) T. Jozak, D. Zabel, A. Schubert, Y. Sun 19 (a) F. Wang and A. W. Schwabacher, Tetrahedron Lett., 1999,
and W. R. Thiel, Eur. J. Inorg. Chem., 2010, 5135–5145;
( j) L. Holland, W. Z. Shen, P. von Grebe, P. J. S. Miguel,
F. Pichierri, A. Springer, C. A. Schalley and B. Lippert,
Dalton Trans., 2011, 40, 5159–5161; (k) K. Suntharalingam,
D. Gupta, P. J. S. Miguel, B. Lippert and R. Vilar,
Chem.–Eur. J., 2010, 16, 3613–3616.
40, 4779–4782; (b) J. S. Uber, Y. Vogels, D. van den Helder,
I. Mutikainen, U. Turpeinen, W. T. Fu, O. Roubeau,
P. Gamez and J. Reedijk, Eur. J. Inorg. Chem., 2007, 26,
4197–4206; (c) W. R. Thiel, M. Angstl and T. Priermeier,
Chem. Ber., 1994, 127, 2373–2379; (d) H. Zhang, Q. Cai and
D. W. Ma, J. Org. Chem., 2005, 70, 5164–5173.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 1524–1533 | 1533