NMR and theoretical study on interactions between diperoxovanadate complex and 4-substituted pyridines

Article abstract of DOI:10.1016/j.saa.2008.01.019

To understand the substituting group effects of organic ligands on the reaction equilibrium, the interactions between diperoxovanadate complex [OV(O₂)₂(D₂O)]^-/[OV(O ₂)₂(HOD)]^- and a series of 4-substituted pyridines in solution were explored using multinuclear (¹H, ¹³C, and ⁵¹V) magnetic resonance, DOSY, and variable temperature NMR in 0.15 mol/L NaCl ionic medium for mimicking the physiological condition. Some direct NMR data are given for the first time. The reactivity among the 4-substituted pyridines is pyridine > isonicotinate > N-methyl isonicotinamide > methyl isonicotinate. The competitive coordination results in the formation of a series of new six-coordinated peroxovanadate species [OV(O₂)₂L]^n- (L = 4-substituted pyridines, n = 1 or 2). The results of density functional calculations provide a reasonable explanation on the relative reactivity of the 4-substituted pyridines. Solvation effects play an important role in these reactions.

Full text of DOI:10.1016/j.saa.2008.01.019

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 71 (2008) 644–649
NMR and theoretical study on interactions between diperoxovanadate
complex and 4-substituted pyridines
Xianyong Yua,b, Jun Zhanga, Birong Zenga, Pinggui Yia,b
,
Shuhui Caia, Zhong Chena,∗
a Department of Physics, State Key Laboratory of Physical Chemistry of Solid Surface,
Xiamen University, Xiamen 361005, China
b School of Chemistry and Chemical Engineering, Hunan Province College Key Laboratory of QSAR/QSPR,
Hunan University of Science and Technology, Xiangtan 411201, China
Received 20 November 2007; received in revised form 6 January 2008; accepted 12 January 2008
Abstract
To understand the substituting group effects of organic ligands on the reaction equilibrium, the interactions between diperoxovanadate complex
[OV(O2)2(D2O)]−/[OV(O2)2(HOD)]− and a series of 4-substituted pyridines in solution were explored using multinuclear (1H, 13C, and 51V)
magnetic resonance, DOSY, and variable temperature NMR in 0.15 mol/L NaCl ionic medium for mimicking the physiological condition. Some
direct NMR data are given for the first time. The reactivity among the 4-substituted pyridines is pyridine > isonicotinate > N-methyl isonicoti-
namide > methyl isonicotinate. The competitive coordination results in the formation of a series of new six-coordinated peroxovanadate species
[OV(O2)2L]n− (L = 4-substituted pyridines, n = 1 or 2). The results of density functional calculations provide a reasonable explanation on the relative
reactivity of the 4-substituted pyridines. Solvation effects play an important role in these reactions.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Diperoxovanadate; 4-Substituted pyridine; Interaction; NMR; Theoretical calculation
1. Introduction
tion chemistry of vanadium has great versatility for adjustment
of pharmacological characteristics [11,12]. Detailed and thor-
ough potentiometric and 51V NMR spectroscopic investigations
of H+/H2VO4−/H2O2/Ligand systems have been performed
by Pettersson and co-workers [13,14]. They also proposed a
model describing the distribution of pentavalent vanadium in
human blood based on speciation studies performed in the
physiological medium of 0.15 mol/L NaCl with various blood
constituents [15]. Conte and co-workers [16,17] have studied
the NH4VO3/H2O2/histidine-like ligand systems that imitate
the active site of haloperoxidases by using the electrospray
ionization-mass spectrometry (ESI-MS), 51V NMR, and density
functional calculations. In our previous works [18–23], we have
performed experimental and theoretical studies on the reactions
of diperoxovanadate and a series of organic ligands in solutions.
Especially, we explored how the substituting groups of picoline-
like ligands affect the reaction equilibrium in the interaction
systems in detail.
Peroxovanadate complexes play an important role in the bio-
sphere and are receiving renewed attention recently as a new
kind of powerful insulin mimic or anticancer agents based on
their biological response both in vitro and in vivo that may be
developed into new oral drugs for anti-diabetes or tumor sup-
pression [1–7]. However, the biochemical response mechanism
of vanadium is very complex due to the numerous different
possible ways vanadium can complex under physiological con-
ditions. It is therefore not surprising to see that coordination
chemistry and biological mechanism of vanadium compounds
have recently stimulated much interest. For example, the Orvig
[8,9] and Posner [10] groups have synthesized and characterized
many vanadium complexes and studied their insulinomimetic
activities in vitro or in vivo. Crans and co-workers have studied
various vanadium compounds and reported that the coordina-
In this work, we used multinuclear (1H, 13C, and 51V) mag-
netic resonance, DOSY, and variable temperature NMR to study
the interaction systems NH4VO3/H2O2/L (L = 4-substituted
∗
Corresponding author. Tel.: +86 592 2181712; fax: +86 592 2189426.
E-mail address: chenz@xmu.edu.cn (Z. Chen).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.01.019

X. Yu et al. / Spectrochimica Acta Part A 71 (2008) 644–649
645
tal parameters for a DOSY spectrum were as follows: gradient
duration δ = 2.0 ms, gradient strength G = 30 G/cm, spin lock
time τSL = 1.0 ms and time interval τ = 2.0 ms. Diffusion coef-
ficient was generally achieved by stepwise ramping up of the
amplitudes of pulsed field gradients (PFGs), and diffusion times
were optimized for every experiment. Reference deconvolution
and baseline correction were adopted to compensate experimen-
tal imperfections.
Scheme 1. Structural formula of pyridine (1), isonicotinate (2), methyl isoni-
cotinate (3), and N-methyl isonicotinamide (4).
2.3. Computational method
The geometries of the newly formed complexes were opti-
mized using the B3LYP hybrid density functional, which
includes a mixture of Hartree-Fock exchange with Beck88
exchange functional under generalized gradient approxima-
tion plus a mixture of Vosko–Wilk–Nusair local correlation
functional and Lee–Yang–Parr non-local correlation functional
[25–27]. The Wadt and Hay core-valence effective core poten-
tial [28] was used for the metal center (13 explicit electrons
for neutral V) with the valence double zeta contraction of the
basis functions (denoted as Lanl2dz in Gaussian [29]). For O,
N, C, and H, the standard 6–31 + G* basis sets developed by
Hariharan and Pople were used [30]. The single point solva-
tion energy was calculated using Polarizable Continuum Models
(PCM [31,32]) at each optimized gas phase geometry. Vibra-
tional frequencies were calculated to ensure that each minimum
is a true local minimum (only real frequencies). All calcu-
lations were carried out with the Gaussian 03 program suite
[29].
pyridines) in 0.15 mol/L NaCl ionic medium to probe the
substituting group effects of organic ligands on the reaction
equilibrium. Theoretical calculations are performed to provided
a reasonable explanation on the experimental observations.
Through the combined use of these methods, solution structures
and coordination fashion of all species in the interaction systems
can be determined and a better understanding of experimental
phenomena can be achieved.
2. Experimental
2.1. Materials and preparations
The compounds D2O, H2O2 (30%), NaCl, NaOH, NH4VO3,
pyridine (abbr. Py, 1), and isonicotinate acid, were commer-
cial products (Sinopharm Chemical Reagent Company) used
without further purification. The isonicotinate (abbr. i-nic, 2)
can be obtained by mixing the NaOH and isonicotinate acid
with 1:1 molar ratio online. The ligands, methyl isonicotinate
(abbr. Me-i-nic, 3) and N-methyl isonicotinamide (abbr. N-Me-
i-nicamide, 4), were synthesized according to the literature [18]
(Scheme 1). The ionic medium was chosen to represent the
physiological condition, 0.15 mol/L NaCl D2O solution at 20 ◦C
in all NMR experiments except for the variable temperature
NMR.
3. Results and discussion
3.1. 51V NMR of the interaction system of
NH4VO3/H2O2/Ligands
The mixture of NH4VO3 and H2O2 with 1:5 molar ratio
(0.2 mol/L vanadate concentration) has a 51V peak locating
at −692 ppm (Fig. 1), which is assigned to bpV according to
previous reports [18–23]. When pyridine is added to bpV solu-
tion, a new single peak appears at about −711 ppm, which is
assigned to the species [OV(O2)2(Py)]−. Its intensity increases
with the increasing of the quantity of pyridine (from 0.5 to
1.0, 2.0, and finally 3.0 equiv.) before reaching a maximum, as
shown in Fig. 1. Moreover, with the addition of pyridine, the
peak at −692 ppm slightly moves toward high field, whereas
the peak at −711 ppm hardly moves. These may result from
the change of pH value [22]. When the molar ratio between
Py and bpV reaches 2:1, almost all bpV is converted to
[OV(O2)2(Py)]−.
To form the ternary NH4VO3/H2O2/L (L = 1, 2, 3, and 4)
systems, NH4VO3 and H2O2 were first mixed in D2O to produce
the species [OV(O2)2(D2O)]−/[OV(O2)2(HOD)]− (abbr. bpV),
then the ligands were separately added to the solution.
2.2. Spectroscopies
All spectra were recorded on Varian Unity plus 500 MHz
NMR spectrometer, operating at 500.4 MHz for 1H, 125.7 MHz
for 13C, and 131.4 MHz for 51V NMR. DSS (3-(trimethylsilyl)-
propanesulfonic acid sodium salt) was used as an internal
1
reference for H and 13C chemical shifts. 51V chemical shifts
were measured relative to the external standard VOCl3 with
upfield shift being considered as negative. Signal-to-noise ratios
were improved by a line-broadening factor of 2 or 10 Hz in the
Fourier transformation of all 13C or 51V spectra.
DOSY spectra were recorded by using a z-gradient probe,
which delivers a maximum gradient strength of 30 G/cm. The
gradient compensated stimulated echo spin lock (GCSTESL)
[24] was used to acquire DOSY spectra. The typical experimen-
Analogously, when 1.0 equiv. of a 4-substituted pyridine
is added to the peroxovanadate solution, the area of bpV
peak decreases and a new peak appears at about −710 ppm
for isonicotinate {assigned to [OV(O2)2(i-nic)]2−}, −703 ppm
for methyl isonicotinate {assigned to [OV(O2)2(Me-i-nic)]−},
and −705 ppm for N-methyl isonicotinamide {assigned to
[OV(O2)2(N-Me-i-nic)]−}, as shown in Fig. 2. With the increase
of ligand quantity, the area of new 51V peak increases. From the

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X. Yu et al. / Spectrochimica Acta Part A 71 (2008) 644–649
Fig. 1. 51V NMR spectra of the interaction systems of bpV and Py, (a)–(e) cor-
responding to the molar ratios of Py/bpV = 0, 0.5, 1.0, 2.0, and 3.0, respectively.
The total concentration of vanadate species is 0.2 mol/L.
Fig. 2. 51V NMR spectra of the interaction systems of NH4VO3/H2O2/4-
substituted pyridine with 1:5:1 molar ratio in aqueous solution.
ratio of bpV peak areas before and after reaction, the order of
interaction strength of 4-substituted pyridines with diperoxo-
vanadate is deduced to be pyridine > isonicotinate > N-methyl
isonicotinamide > methyl isonicotinate.
and isonicotinate are weakened and the species is converted
to bpV little by little. The chemical shift of bpV moves about
3.3 ppm every 10 ◦C, and the chemical shift of [OV(O2)2(Me-
i-nic)]− moves about 1.9 ppm every 10 ◦C, shown in Fig. 3(A);
(2) for different ligand, the thermal stability of the newly formed
species is different. For example, about 25% [OV(O2)2(Me-
i-nic)]− formed at 20 ◦C is converted to bpV at 50 ◦C, while
the conversion efficiency of [OV(O2)2(i-nic)]2− is only 5.7%,
shown in Fig. 3(B). Therefore, the order of thermal stability
is [OV(O2)2(i-nic)]2− > [OV(O2)2(Me-i-nic)]−, in agreement
with the variation of interaction strength between diperoxovana-
date and the ligand; (3) the equilibrium is reversed when the
temperature decreases, as reflected from the behaviors of the
peaks in the spectra. The relative peak areas only depend on the
Variable temperature 51V NMR was employed to study
the influence of temperature on the equilibrium of the sys-
tems. As examples, the 51V NMR spectra of the mixture of
NH4VO3/H2O2/Me-i-nic and NH4VO3/H2O2/i-nic with 1:5:1
molarratio and0.2 mol/L vanadateconcentrationin the tempera-
turerangeof20–50 ◦CisshowninFig. 3. Itcanbefoundfromthe
experimental results that: (1) the quantity of the newly formed
species decreases and all the peaks in the spectra move towards
low field when the temperature is increased. This implies that
with the increase of temperature, the bonds between vanadium
Fig. 3. Variable temperature 51V NMR spectra of the interaction systems of NH4VO3/H2O2/Me-i-nic (A) and NH4VO3/H2O2/i-nic (B) with 1:5:1 molar ratio in
aqueous solution.

X. Yu et al. / Spectrochimica Acta Part A 71 (2008) 644–649
647
Table 1
1H and 13C NMR spectral data of the interaction systems of diperoxovanadate and 4-substituted pyridines (molar ratio 1:1)
Systems
Species
Chemical shifts
1H (ppm)
13C (ppm)
bpV + Py
[OV(O2)2(Py)]−
Py
7.79 (s, 2H, Py–H), 8.22 (s, 1H, Py–H), 8.85 (s, 2H, Py–H)
7.61 (s, 2H, Py–H), 8.06 (s, 1H, Py–H), 8.60 (s, 2H, Py–H)
152.1, 143.8, 129.0
149.7, 142.2, 127.7
bpV + i-nic
[OV(O2)2(i-nic)]2−
i-nic
8.06 (s, 2H, Py–H), 8.95 (s, 2H, Py–H)
7.94 (s, 2H, Py–H), 8.71 (s, 2H, Py–H)
174.3, 152.8, 151.0, 127.6
174.7, 150.5, 149.5, 126.6
bpV + Me-i-nic
bpV + N-Me-i-nic
[OV(O2)2(Me-i-nic)]−
Me-i-nic
4.03 (s, 3H, CH3), 8.21 (s, 2H, Py–H), 9.02 (s, 2H, Py–H)
4.00 (s, 3H, CH3), 7.89 (s, 2H, Py–H), 8.69 (s, 2H, Py–H)
168.6, 153.4, 143.5, 128.0, 56.3
169.8, 152.1, 140.8, 126.1, 56.0
[OV(O2)2(N-Me-i-nic)]−
N-Me-i-nic
2.99 (s, 3H, CH3), 8.02 (s, 2H, Py–H), 9.00 (s, 2H, Py–H)
2.99 (s, 3H, CH3), 7.70 (s, 2H, Py–H), 8.67 (s, 2H, Py–H)
169.8, 153.1, 147.6, 126.3, 29.2
171.2, 152.0, 144.7, 124.3, 29.3
temperature; and (4) the newly formed species is stable within
the experimental temperature range.
˚
pyramid. The V–N bond length ranging from 2.20 to 2.23 A for
different 4-substituted pyridine, which are within the range of
reported ones for diperoxovanadate complexes [19,20].
The reactivity of 4-substituted pyridine in solution depends
on the intrinsic bonding strength between [OV(O2)2]− and L
and the solvation effects. The free energies of the four reactions
studied here are as follows:
3.2. 1H and 13C NMR data of the interaction systems
1
The H and 13C NMR spectral data of the interaction sys-
tems of bpV (0.2 mol/L) and 4-substituted pyridines with 1:1
molar ratio in sodium chloride deuterium oxide solution are
listed in Table 1. There are two groups of 4-substituted pyri-
dine peaks in each 1H and 13C NMR spectra, respectively. One
group was assigned to the free 4-substituted pyridine and the
other to the coordinated 4-substituted pyridine, i.e., the ligand
of the new species formed in the interaction system. According
Py + [OV(O2)2(H2O)]− → [OV(O2)2(Py)]− + H2O,
ꢀG (298 K) = 2.35 kcal/mol (in gas phase) and
−9.34 kcal/mol (in solution)
(1)
1
to the chemical shifts and the relative areas of the H and 13C
peaks, we suggest that the newly formed species [OV(O2)2L]−
is seven-coordinated.
3.3. DOSY spectra of the interaction systems
Different molecules have different self-diffusion coefficients
while different groups in a molecule have almost the same self-
diffusion coefficients. Therefore, we can use DOSY spectra to
identify the components and their corresponding structures of
the interaction systems. The DOSY spectra of NH4VO3/H2O2/i-
nic and NH4VO3/H2O2/Me-i-nic in solution are shown in
Fig. 4(A) and (B), respectively. There are two components con-
tainingprotonexceptforthesolvent. Thecomponentwithslower
diffusion rate marked by a dash line was assigned to the newly
formed species [OV(O2)2(i-nic)]2− or [OV(O2)2(Me-i-nic)]−.
The component marked by a solid line was assigned to the free
ligand. Therefore, the results of DOSY experiments support the
1D spectroscopic assignments mentioned above.
3.4. Theoretical study on the reaction products
The structures of the newly formed species were optimized
using the B3LYP method. The results confirm above analysis
that the newly formed species [OV(O2)2L]− (L = 4-substituted
pyridines)aresix-coordinated. Thestructurehasanoxygenatom
attheapex andthe Natomin pyridinering with the oxygenatoms
of the peroxide groups at the equatorial plane. The V atom near
the center of the pentagonal base and form a distorted pentagonal
Fig. 4. DOSY spectra of the pyridine ring of the interaction systems of
NH4VO3/H2O2/4-substituted pyridine (1:5:1) in aqueous solution. The 4-
substituted pyridine is isonicotinate (A) or methyl isonicotinate (B). The peaks
marked by a solid line are assigned to the free ligand. The peaks marked by a
dash line are assigned to the coordinated ligand.

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X. Yu et al. / Spectrochimica Acta Part A 71 (2008) 644–649
pyridine (abbr. L) attacks the vanadium of bpV from the oppo-
site site of the terminal oxygen and forms a seven-coordinated
(pentagonal bipyramidal) transition state TS1 (route a). Accom-
panied by the leaving of the water molecule, TS1 turns into a
six-coordinated species [OV(O2)2L]− (route b); (2) a similar
process can occur between [OV(O2)2L]− and a water molecule
and forms TS2 (route c) and finally bpV (route d).
4. Conclusions
Several NMR experimental techniques were employed to
study the interactions between diperoxovanadate complex
and a series of 4-substituted pyridines in the 0.15 mol/L
NaCl D2O solution. The experimental results indicate that
new six-coordinated peroxovanadate species is formed. The
relative activity of these organic ligands is as follows: pyri-
dine > isonicotinate > N-methyl isonicotinamide > methyl isoni-
cotinate. Solvation effects play an important role in the reactions.
Scheme 2. Possible interaction modes between bpV and 4-substituted pyridines.
i-nic + [OV(O2)2(H2O)]− → [OV(O2)2(i-nic)]2− + H2O,
ΔG (298 K) = 38.2 kcal/mol (in gas phase)and
Acknowledgments
This work was supported by Key Project of Health and Sci-
ence and Technology of Xiamen (3502Z20051027), National
Natural Science Foundation of China (20772027), Hunan
Provincial Natural Science Foundation of China (06JJ30004),
China Postdoctoral Science Foundation (20070410805), and
State Key Laboratory of Physical Chemistry of Solid Surface
of China.
−7.98 kcal/mol (in solution)
(2)
Me-i-nic+[OV(O2)2(H2O)]−→ [OV(O2)2(Me-i-nic)]−+H2O,
ΔG (298 K) = 3.15 kcal/mol (in gas phase) and
−5.97 kcal/mol (in solution)
(3)
References
N-Me-i-nicamide + [OV(O2)2(H2O)]− →
[OV(O2)2(N-Me-i-nicamide)]− + H2O,
ΔG (298 K) = 3.07 kcal/mol (in gas phase) and
−6.21 kcal/mol (in solution)
These indicate that the reactions are impossible in the gas
phase but thermodynamically favorable in the solution. The free
energy changes of these four reactions resulting from solvation
effects are 11.69, 46.18, 9.28, and 9.12 kcal/mol, respectively.
Obviously, solvation effects play an important role, especially
for reaction (2). The large free energy variation for reaction (2) is
duetothenegativechargedcharacterofi-nicligand. Comparison
of the free energies of reactions (1)–(4) in solution shows that:
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