51V solid-state NMR and density functional theory studies of eight-coordinate non-oxo vanadium complexes: Oxidized amavadin

Article abstract of DOI:10.1039/b820383k

Using ⁵¹V magic angle spinning solid-state NMR spectroscopy and density functional theory calculations we have characterized the chemical shift and quadrupolar coupling parameters for two eight-coordinate vanadium complexes, [PPh₄][V

Full text of DOI:10.1039/b820383k

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PAPER
www.rsc.org/dalton | Dalton Transactions
51V solid-state NMR and density functional theory studies of eight-coordinate
non-oxo vanadium complexes: oxidized amavadin
Kristopher J. Ooms,a Stephanie E. Bolte,a Bharat Baruah,b Muhammad Aziz Choudhary,b,c
Debbie C. Crans*b and Tatyana Polenova*a
Received 14th November 2008, Accepted 10th February 2009
First published as an Advance Article on the web 13th March 2009
DOI: 10.1039/b820383k
Using 51V magic angle spinning solid-state NMR spectroscopy and density functional theory
calculations we have characterized the chemical shift and quadrupolar coupling parameters for two
eight-coordinate vanadium complexes, [PPh4][V(V)(HIDPA)2] and [PPh4][V(V)(HIDA)2]; HIDPA =
2,2¢-(hydroxyimino)dipropionate and HIDA = 2,2¢-(hydroxyimino)diacetate. The coordination
geometry under examination is the less common non-oxo eight coordinate distorted dodecahedral
geometry that has not been previously investigated by solid-state NMR spectroscopy. Both complexes
were isolated by oxidizing their reduced forms: [V(IV)(HIDPA)2]2- and [V(IV)(HIDA)2]2-.
2-
V(IV)(HIDPA)2 is also known as amavadin, a vanadium-containing natural product present in the
Amanita muscaria mushroom and is responsible for vanadium accumulation in nature. The
quadrupolar coupling constants, CQ, are found to be moderate, 5.0–6.4 MHz while the chemical shift
anisotropies are relatively small for vanadium complexes, -420 and -360 ppm. The isotropic chemical
shifts in the solid state are -220 and -228 ppm for the two compounds, and near the chemical shifts
observed in solution. Presumably this is a consequence of the combined effects of the increased
coordination number and the absence of oxo groups. Density functional theory calculations of the
electric field gradient parameters are in good agreement with the NMR results while the chemical shift
parameters show some deviation from the experimental values. Future work on this unusual
coordination geometry and a combined analysis by solid-state NMR and density functional theory
should provide a better understanding of the correlations between experimental NMR parameters and
the local structure of the vanadium centers.
solid-state NMR spectra, are sensitive to the local vanadium
Introduction
environment and can be used to understand changes in the local
Vanadium has a diverse coordination chemistry and can be found
molecular orbital structure and ground state charge distribution
at the vanadium.9–24 Therefore, it is important to determine the
51V solid-state NMR parameters of less common coordination
geometries in vanadium complexes.
routinely in environments with coordination numbers ranging
from three to eight giving rise to many complexes that have useful
chemical and biochemical properties.1,2 Even though vanadium is
a trace element in biological systems, it is an essential co-factor in
a number of enzymes, also acts as an insulin enhancing agent and
has been found as a natural product in the mushroom Amanita
muscaria.2–8 The biological activity of vanadium complexes and
proteins is determined by the specific chemical environment of
the vanadium centers. The application of 51V NMR to study solid
vanadium-containing complexes that are of biological relevance
has grown rapidly in recent years9–25 due to the ability to acquire
high quality magic angle spinning spectra of both the central
and satellite NMR transitions of the 51V (I = 7/2). Studies
have demonstrated that both the quadrupolar coupling and the
chemical shift anisotropy, which can be determined from the
In nature, one of the highest concentrations of vanadium can
be found in the Amanita muscaria mushrooms.6,7 Vanadium levels
as high as 400 mg kg-1 of mushroom have been reported.7 The
vanadium is found in the mushroom as an eight coordinate
complex, amavadin (Fig. 1), in a highly distorted cubic geom-
26,27
=
etry that does not include the common V O bonds.
While
amavadin shows unique redox capabilities including peroxidative
halogenation28,29 and oxygen activation,30 its role in the biological
processes of the Amanita mushroom is unknown. Information
about the electronic properties of the vanadium center may be
important for understanding the chemical properties and the
function of this unusual molecule. Furthermore, it is known that
the redox potential of the amavadin is different by 100 mV from
that of the simpler analog, whereas electron transfer processes to
the vanadium are indistinguishable.31
The structure of the vanadium in amavadin does not change
dramatically upon oxidation. The oxidized amavadin complex
and its analog thus offer a unique opportunity to study an
eight coordinate vanadium geometry and to compare the effect
that such coordination has on the electric field gradient (EFG)
aDepartment of Chemistry and Biochemistry, 036 Brown Laboratories,
University of Delaware Newark, Delaware 19716. E-mail: tpolenov@mail.
chem.udel.edu; Fax: +302 831 6335; Tel: +302 831 1968
bDepartment of Chemistry, Colorado State University, Fort Collins, CO
80523. E-mail: crans@lamar.colostate.edu; Fax: +970 491 1801; Tel: +970
491 7635
cDepartment of Chemistry, The University of Azad Jammu & Kashmir,
Muzaffarabad, 13100, Pakistan
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Fig. 2 Solid-state 51V NMR spectra of the two complexes obtained at
14.1 T and different MAS rates. The signal observed at -600 kHz is from
the copper coil.
Fig.
1
The structure of (a) [PPh4][V(V)(HIDA)2]·CH2Cl2 and (b)
[PPh4][V(V)(HIDPA)2]·H2O. The structures were obtained from the Cam-
bridge crystal structure database; depository names YAPHOM and
YAPHUS, respectively. Hydrogen atoms are colour coded white, carbon
atoms = grey, nitrogen atoms = blue, oxygen atoms = red, and vanadium
atoms = green.
complexes and contain NMR signal from both the central and
satellite transitions. Using SIMPSON,33 numerical simulations
of the spectra were performed, and the 51V chemical shift and
quadrupolar coupling parameters were extracted, Table 1. The
high quality of the fits is illustrated in Fig. 3 using the spectra for
HIDA and HIDPA complexes acquired with the MAS frequency
of 18 kHz. The isotropic chemical shift in the HIDA complex,
diso = -220 ppm while in HIDPA diso = -268 ppm. These values
are significantly less shielded than those observed for seven-
coordinate complexes, where the isotropic shifts typically ranged
from -570 to -680 ppm. Similarly, in solution the 51V resonances
for the oxidized amavadin complex and its analog are reported
to be significantly less shielded than in the corresponding six-
and seven-coordinate complexes. Specifically, the oxidized form of
amavadin, [V(V)(HIDPA)2]-, showed chemical shifts ranging from
-281 ppm34 to -252 ppm;35 and for the analog, [V(V)(HIDA)2]- the
corresponding shift was -263 ppm.36 Therefore, these compounds
exhibit unusual chemical shifts both in solution and in the solid
state, and this behaviour has been attributed to the unique
geometric and electronic structure of these complexes.
Smaller isotropic shifts observed for [V(V)(HIDPA)2]- and
[V(V)(HIDA)2]- suggest that on average the mixing between the
high energy occupied and low energy unoccupied molecular
orbitals is better in these eight coordinate complexes than in
complexes with lower coordination numbers.37 The anisotropy of
the chemical shift is also smaller than that observed typically in
seven coordinate complexes, and resembles more the anisotropy of
a five- or six-coordinate complex. The smaller anisotropy indicates
that the magnitude of the vanadium shielding is similar in all
directions, suggesting a relatively symmetric molecular orbital
environment at the vanadium atom. Given the fact that these
and chemical shift anisotropy (CSA) in the solid state. Recently,
the role of each steric factor in the unique eight coordinate
geometry of the amavadin complex has been explored using DFT
calculations, structural analysis and solution 51V chemical shifts.32
These studies revealed that the unusual deshielding of the 51V
resonances for amavadin and analogs can be attributed to a
number of structural increments from regular substituent effects,
and that the largest contribution arises from the non-oxo nature
of these complexes.32 The variation that diastereomeric isomers
had on the chemical shifts was considered to be minor. In this
study, we present 51V solid-state NMR spectra of the oxidized
form of the natural product amavadin, (PPh4)[V(V)(HIDPA)2],
and one of its analogs, (PPh4)[V(V)(HIDA)2]; HIDPA and HIDA
stand for the basic forms of 2,2¢-(hydroxyimino)dipropionate and
2,2¢-(hydroxyimino)diacetate, respectively. The NMR parameters
for the two compounds are discussed in terms of coordination
environment and compared to the current body of 51V solid-
state NMR data for other vanadium complexes. Surprisingly, the
results reveal moderate quadrupolar coupling constants and small
chemical shift anisotropies for both oxidized amavadin and its
analog indicative of symmetric electronic charge distribution at
the vanadium center. The DFT calculations predict accurately
the electric field gradient tensor whereas the computed chemical
shift anisotropy tensors show discrepancies with the experiment
suggesting unusual electronic environment in these rare eight-
coordinate non-oxo vanadium complexes.
Results and discussion
=
complexes contain no V O groups, the high symmetry at the
distorted cubic vanadium atoms can be readily reconciled.
To rationalize the detected moderate quadrupolar coupling
constant, the electronic charge distribution at the vanadium site
needs to be analyzed. Based on simple point charge models,
vanadium in a site of cubic symmetry will experience no electric
51V solid-state NMR parameters
The NMR spectra of the two eight-coordinate complexes acquired
at 14.1 T and three magic angle spinning (MAS) frequencies are
shown in Fig. 2. The spectra are typical of vanadium coordination
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Table 1 Experimental and calculated 51V solid-state NMR parameters for the eight coordinate non-oxo amavadin-type vanadium(V) complexes
Compound
HIDA
Method (geometry)
CQ/MHz
hQ
d
iso/ppm
ds/ppm
hs
a/b/g
Experiment
5.0 0.1
-4.38
5.42
5.27
4.59
-3.51
5.34
4.66
-3.41
-4.48
-4.02
-2.96
6.4 0.2
-6.03
7.33
7.20
6.53
4.63
7.25
0.2 0.1
1.0
-220
-615
-561
-557
-611
-577
-609
-673
-638
-340
-363
-354
-268 0.1
-630
-572
-573
-630
-591
-628
-695
-655
-353
-376
-364
5
-420 20
-131
-131
-136
-136
-141
-115
-115
-120
245
0.5 0.05
0.426
0.03
0.04
0.04
0.03
0.02
0.02
0.02
0.08
0.08
0.09
0.5 0.1
0.47
0.03
0.03
0.03
0.03
-45/0/90
b3lyp/6–311 + G(d,p) (X-ray)
b3lyp/6–311 + G(d,p) (optimized)
b3lyp/6–311 + G(d,p)
b3lyp/6–311++G
0.89
0.91
0.99
0.65
0.89
0.96
0.64
0.93
0.85
0.34
0.45 0.1
0.49
0.45
0.46
0.49
0.55
0.45
0.49
0.57
0.53
0.59
0.72
b3lyp/tzvp
PBE1PBE/6–311 + G(d,p)
PBE1PBE/6–311++G
PBE1PBE/tzvp
PBEPBE/6–311 + G(d,p)
PBEPBE/6–311++G
PBEPBE/tzvp
246
245
HIDPA
Experiment
-360
-102
-103
-109
-108
-114
-85
8
90/0/90
b3lyp/6–311 + G(d,p) (X-ray)
b3lyp/6–311 + G(d,p) (optimized)
b3lyp/6–311 + G(d,p)
b3lyp/6–311++G
b3lyp/tzvp
PBE1PBE/6–311 + G(d,p)
PBE1PBE/6–311++G
PBE1PBE/tzvp
PBEPBE/6–311 + G(d,p)
PBEPBE/6–311++G
PBEPBE/tzvp
0.03
0.02
0.04
0.07
0.07
0.07
6.44
4.47
6.00
5.27
-85
-90
251
254
257
3.46
(1) The chemical shift parameters are defined such that |dzz - diso| ≥ |dxx - diso| ≥ |dyy - diso|, diso = (dzz + dyy + dxx)/3, ds = dzz - diso, and hs = (dyy
-
-
d
xx)/ds. The components of the chemical shift tensor are dii = (vii - vref.)/vref. (ii = zz, xx, yy). (2) The EFG parameters are CQ = eQVzz/h and hQ = (Vxx
V
yy)/Vzz where |Vzz| ≥ |Vyy| ≥ |Vxx|, e is the electron charge and h is Planck’s constant.
the high coordination number, the distribution of charge in the
vicinity of the vanadium atom deviates from spherical symmetry.
In addition to the chemical shift and EFG tensor parameters,
the Euler angles that define the relative orientation of the two
tensors can be found by fitting the spinning side bands envelope
of the satellite transitions. Due to the low signal-to-noise ratio in
the outer satellites as well as the lack of well-defined features in
the envelope the error on the angles is large. Despite this, it is clear
that b is close to 0◦ in both complexes, placing Vzz along dzz; the
angles used in the simulations are listed in Table 1.
DFT calculations of the NMR parameters
Quantum chemical calculations of 51V NMR parameters have
provided a valuable link between experimental NMR observables
and the structure of vanadium complexes.17,18,23,24,32,38–42 Table 1
contains the results of calculations performed on the HIDA and
HIDPA complexes using three functionals, b3lyp, PBE1PBE, and
PBEPBE, and three basis sets, 6–311++G(d,p), 6–311++G, and
tzvp as available in the Gaussian03 program.43 The results ob-
tained for the quadrupolar coupling constant and the asymmetry
parameters of the EFG tensor for the HIDPA complex are in
good agreement with experiment. The CQ calculated for the HIDA
complex is also well reproduced while the hQ parameter shows a
greater deviation from experiment. Calculations using b3lyp and
PBE1PBE functionals and 6–311++G(d,p) or 6–311++G basis
sets yield good agreement with experiment while calculations
performed with PBEPBE functional (all basis sets) and those
using a tzvp basis set (all functionals) underestimate the CQ.
All calculations were performed using a single molecule without
the inclusion of counterions. Given the good agreement of the
EFG calculations with the experiment, we conclude that the
Fig. 3 Experimental (black) and simulated (red) spectra of the two
complexes obtained while spinning the samples at 18 kHz at 14.1 T.
field gradient. However, when the ideal symmetry is broken an
electric field gradient will be created at the vanadium site. The
magnitude of these changes correlates directly to the magnitude
of the electric field gradient and therefore, to the measured
quadrupolar interaction parameters. Based on the NMR spectra
it is determined that the quadrupolar coupling constants for the
HIDA and HIDPA complexes are 5.0 and 6.4 MHz, respectively.
The asymmetry parameters of the EFG tensor are 0.2 and
0.45 for the HIDA and HIDPA, respectively. These values can
be considered average in size; typically the 51V CQ values for
coordination complexes studied to date range from ca. 3 MHz
to as high as 8.3 MHz. Therefore, it can be concluded that despite
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EFG at the vanadium is almost completely determined by the
vanadium complex. Longer-range influences from counterions
and neighbouring complexes are negligible and need not be
considered further.
Interpreting 51V NMR parameters in terms of molecular structure
From an NMR and chemistry perspective, the results for the
non-oxo eight-coordinate oxidized amavadin-type complexes pre-
sented here provide a key data set that allows us to expand our
understanding of the dependence of the 51V anisotropic solid-state
NMR parameters on the vanadium coordination environment.
These studies are the first in which vanadium compounds lacking
Unlike the EFG calculations, the calculated magnetic shieldings
(where the shieldings have been converted to chemical shifts as
described in the methods section) show greater deviations from
the experimental values. The calculated diso values are significantly
more shielded than the experimental results while the calculated
anisotropies of the tensor are smaller than the experimental
values. The worst agreement between experiment and theory is
reached when PBEPBE functional is used: the sign of the chemical
shift anisotropy is incorrect (Table 1). Further analysis of the
computational results reveals that the largest discrepancies arise
from the dxx and dyy components of the tensor, both of which are
much more shielded than experimentally observed. To understand
the possible source of the error we assume that the calculated
orientation of the EFG in the molecular frame is correct; this is a
good assumption given the success of the EFG calculation. Using
our experimental Euler angles we note that dzz should be parallel to
Vzz, i.e. b = 0◦. According to the calculations, Vzz is oriented along
the approximate N–V–N axis, see Fig. 4. The shielding along this
axis, dzz, is dictated by the molecular orbitals in the perpendicular
plane. In the case of HIDA, this plane consists mostly of bonds
between the vanadium and the carboxy oxygen atoms. Conversely,
the dxx and dyy components of the chemical shift tensor contain
contributions from the oxyamine portion of the ligands. Since it
is dxx and dyy that are reproduced less well in the calculations,
it is likely that it is the coordination of the oxyamines to the
vanadium that is the source of the discrepancies in the calculations.
Similar behavior was reported in a previous study of vanadium
complexes with oxyamine ligands where substantial errors were
observed for dzz, the chemical shift component perpendicular
to the oxyamine plane.24,32 The fact that even at this level of
theory difficulties are encountered in describing the interaction
between the three-membered oxyamine-vanadium is consistent
with the well-known problems with calculations of these strained
compounds. The lower calculated anisotropies suggest that the
molecule is less symmetric than the models used and indicate that
the calculations underestimate the bonding between the vanadium
and the oxyamine unit when using the X-ray structures.
=
a V O group and exhibiting eight-coordination geometry are
investigated by solid-state NMR, and thus will test the possibility
that NMR can assist chemists in the characterization and identifi-
cation of new vanadium-containing compounds. For the 51V NMR
to be used as a predictive tool, typically the NMR parameters for
related series of compounds are compiled into a database, and
empirically observed trends are subsequently used for building
structural models for new compounds. In order to establish
such correlations between 51V NMR parameters and chemical
structures a wide range of vanadium compounds of different
coordination geometries and ligand sets has to be investigated.
We have started building such a database using 51V solid-state
NMR to characterize vanadium(V) in an extensive range of
chemical and geometric environments from various classes of
compounds and with different coordination geometries,17,23,24 and
can therefore examine the current results together with those
from our prior work. The most fundamental correlations that
are often made are those between the chemical shift anisotropy
(or the isotropic chemical shift), or the CQ, and the coordination
number of the atom of interest. For example, such correlations
for 29Si have provided a valuable method for characterizing
the local environment of different silicon atoms in a range of
silicate materials; similarly correlations for aluminum have been
observed.44 Additionally, geometric factors such as bond angle and
length often produce trends in the NMR parameters such as those
observed for 17O in glasses and 31P in phosphates.45,46 The situation
becomes more complex for transition metals which can have a
wider range of geometries and coordination numbers, but some
trends have also been suggested such as for 139La coordination
complexes and even 51V in inorganic vanadia compounds.47,48
In solution state a number of correlations between isotropic
chemical shifts and coordination geometry have been reported for
various vanadium complexes, although the empirical relationships
are typically observed within classes of related molecules.1,9,49–51
For example, the chemical shifts of oxovanadates generally follow
the degree of polymerization with the systematic changes in the
51V shifts between monomer, dimer, tetramer, and pentamer.1
Similarly, the chemical shifts for oxovanadium(V) complexes
with diethanolamine and derivatives follow a pattern that varies
with the substitutions on the diethanolamine.49,50 Chemical shift
changes for vanadium alkoxides have been observed to vary
with the steric bulk of alkoxides, and are found to be indicative
of the coordination number of the vanadium.9 Solution studies
of peroxovanadium(V)1 and hydroxylamine1,51 complexes have
demonstrated that both of these classes of compounds are
significantly different from the more common six-coordinate
coordination complexes and are akin to molecules investigated
in this work.
Fig. 4 The orientation of the EFG tensor with respect to the HIDA
complex as determined by the DFT calculations. Hydrogen atoms are
color coded white, carbon atoms = grey, nitrogen atoms = blue, oxygen
atoms = red, and vanadium atoms = green.
All of the above correlations in the solid state and in solution
are typically established in a series of related compounds or in
materials where the identity of the coordinating atoms or ligands
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is preserved, and only geometric-structural changes occur. In
an attempt to decipher possible empirical correlations between
the 51V solid-state NMR parameters and the molecular structure
features we have analyzed the aggregate of solid-state NMR data
that we have generated in this work and in our several prior
studies.17,23,24 These studies span a broad range of bioinorganic
vanadium(V) complexes with different coordination numbers and
different ligand sets, and thus represent the existing information
available without any attempt to divide the molecules into the
different classes of complexes.
shift anisotropy, ds decreases systematically as coordination
number increases; excluding the eight coordinate complexes. This
would be consistent with what has been reported for vanadia-
based inorganic solids, where it has been observed that typically
six-coordinate vanadium has larger chemical shift anisotropies
than the four-coordinate species.47 The trend is broken by the eight-
coordinate complexes studied here, and may reflect the fact that
the electronic and geometric environments are different from those
in most compounds studied previously. Alternatively, the current
findings might indicate that the bonding environment in the
vicinity of the vanadium is better described as six-coordinate, i.e.,
the oxyamine fragment should be treated as a single coordination
position, an argument also put forth previously in the literature
for oxyamine-type complexes.51 While clustering of the diso values
is also present, there is no linear trend in diso as a function of
coordination number. The results suggest that for vanadium ds,
and not diso, is the more useful measure of the vanadium local
structure, as has been observed previously.17,18,24
Given the spread in the NMR parameter database as a
function of coordination number, it is desirable that calculations
be obtained as a link between the experimental parameters and
the local structure in the vicinity of the vanadium. As has been
shown above and in previous studies, DFT calculations of NMR
parameters can be reasonably accurate when standard basis sets
and functionals are used. Fig. 6 shows a plot of experimental CQ
and dii (ii = xx, yy, zz) values for all the complexes that we have
addressed previously by solid-state NMR and DFT calculations;
calculations were performed using b3lyp functional and 6–311 +
G** or 6–311 + G basis sets in Gaussian.17,23,24 From these plots it is
evident that good agreement can be achieved between experiment
and calculation. Typically the CQ is predicted to within 15% of the
experimental values, while the dii values are underestimated by 50
to 200 ppm. It is interesting that for the chemical shift calculations
the errors seem to be systematic. It has been previously reported
that the typical agreements between experimental results and
Fig. 5 shows a plot of a derived quadrupolar parameter vs. the
coordination number for a majority of the vanadium complexes
for which 51V solid-state NMR data are available.9,17,23–25,52,53 The
2
derived quadrupolar parameter,
, takes into account
CQ 1- h /3
the asymmetry parameter of the EFG tensor as well as CQ and is
useful because it can be extracted based solely on the second-order
quadrupolar shift.9 Perhaps not surprisingly, there is no clear trend
in these graphs that would provide a useful correlation between the
NMR observables and coordination number. For the quadrupolar
coupling, the lowest values are observed for four and six coordinate
complexes. This is likely the result of the fact that vanadium
coordination geometry is close to tetrahedral and octahedral in
these complexes. However, quadrupolar parameters close to 7
and 8 MHz can also be observed for four and eight coordinate
vanadium, similar magnitudes as seen for the other coordination
numbers. Clearly, the ability of vanadium to form non-ideal
coordination geometries leads to a wide range of quadrupolar
values.
Fig. 5 Plots of the NMR parameters as a function of the vanadium
coordination number (CN).
Fig. 6 Plots of the experimental vs. calculated NMR parameters for all
vanadium(V) complexes studied by solid-state NMR and DFT compu-
tational methods. The solid lines represent perfect agreement between
calculation and experiment.
Fig. 5 also shows the chemical shift parameters, ds and diso, vs.
the coordination number. Interestingly, these graphs show more
clustering as a function of the coordination number. The chemical
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theoretical chemical shift predictions are of the order of 100 ppm
for 51V. 23,39 Previous investigations using different levels of theory
have also revealed that, after a certain threshold, larger basis
sets did not significantly improve the chemical shift parameters.23
This suggests that the systematic deviation either arises from a
fundamental ill representation in the calculations of a particular
structural unit, or from a systematic issue in the atom positions
obtained from single crystal X-ray diffraction. The relatively low
resolution of the X-ray structure of amavadin complexes34 may be
one possible source of the discrepancy between the experimental
and calculated shieldings. However, the rigidity of the ligand leaves
little room for changes in the geometry of this complex. Indeed,
DFT calculations using geometry-optimized structures have not
improved the accuracy of the computed shieldings (Table 1). The
possibility that some of the error is due to the calculated shielding
of the reference compound VOCl3, should also be considered,
although this error is not expected to account for all of the
observed discrepancies. Combined these results provide important
benchmark data for which the analysis using both theory and
experiment can be developed for future applications.
fraction containing the product was dried in a rotary evaporator
at ambient temperature and then over P2O5. The 1H NMR spectra
were similar to those reported previously.34,35,54–57
HIPDA = 2,2¢-(hydroxyimino)dipropionate. HIPDA was syn-
thesized using the above procedure except that 2-bromopropionic
acid was employed in place of bromoacetic acid. The product was
1
confirmed using H NMR spectroscopy; this is a low yielding
reaction (<10% yield), and the material formed is less pure than
in the reaction producing the HIDA ligand.
[Ca(H2O)5][V(IV)(HIDA)2]·H2O. The compound was pre-
pared with [VO(acac)2] (2.65 g, 10.0 mmol), HIDA (3.15 g,
21.1 mmol) and CaCl2 (1.11 g, 10.0 mmol) in H2O (100 mL).
Blue prismatic crystals were obtained from H2O–isopropyl alcohol
using the slow liquid-diffusion technique at 293 K as described
previously.58
[Ca(H2O)5][V(IV)(HIPDA)2]·H2O. The compound was pre-
pared following the above procedure for [Ca(H2O)5][V(IV)-
(HIDA)2]·H2O except that HIPDA was used in place of
HIDA. Blue crystalline product was obtained from H2O–MeOH-
isopropyl alcohol using the slow liquid-diffusion technique at
293 K as reported before.27 This product was stable in the solid
state and stored at -20 ◦C.
Experimental
Materials and methods
[PPh4][V(V)(HIDA)2]·CH2Cl2. [Ca(H2O)5][V(IV)(HIDA)2]·H2O
was oxidized in H2O by addition of one equivalent of
[NH4]2[Ce(NO3)6] following the reported synthesis.34 The
red [V(V)(HIDA)2]- formed in solution was isolated by the
addition to the CH2Cl2 solution of two equivalents of PPh4Br.
[PPh4][V(V)(HIDA)2] thus formed was more soluble in the organic
layer, which was separated from the aqueous layer using a
separatory funnel. Solid product was isolated by evaporating
CH2Cl2, and the purity of the material was confirmed by NMR
Vanadyl acetylacetonate, [VO(acac)2] (99.99%), 2-bromopropionic
acid (≥99%), bromoacetic acid (≥99%), tetraphenylphospho-
nium chloride (98%), zinc acetate dihydrate (98%) and Dowex
HCR-W2 strong acid resin were purchased from Sigma-
Aldrich. Sodium hydroxide, hydrochloric acid, ceric ammo-
nium nitrate ([NH4]2[Ce(NO3)6], 98%) and hydroxylamine hy-
drochloride (NH2OH·HCl, 98%) were purchased from Fisher
Scientific. All compounds were used as received, and dis-
tilled deionized water was used in all syntheses. The ligands
HIDPA = 2,2¢-(hydroxyimino)dipropionate and HIDA = 2,2¢-
(hydroxyimino)diacetate and corresponding vanadium(IV) and
vanadium(V) complexes were prepared with slight modifications
of the reported syntheses.34,35,54–58
1
spectroscopy (51V diso = -263 ppm).34 The H NMR spectra and
elemental analysis showed that excess PPh4Br was present in
this sample. The solid-state NMR spectra were recorded within
one month after preparation and before evidence for any sample
decomposition.
Ligand synthesis: HIDA = 2,2¢-(hydroxyimino)diacetate. Hy-
droxylamine hydrochloride (3.47 g, 0.05 mol) was neutralized with
10 mL of 5.0 M NaOH. Bromoacetic acid (13.90 g, 0.10 mol) was
slowly neutralized with 20 mL of 5.0 M NaOH maintaining the
solution temperature at or below 0 ◦C. The hydroxylamine solution
was then chilled to ~0 ◦C in a salt–ice bath. The bromoacetic acid
solution was then added dropwise to the hydroxylamine solution,
and care was taken to maintain the solution mixture at 0 ◦C. An
additional 20 mL of 5.0 M NaOH were slowly added to the
mixture, which was then allowed to stand for ~72 h in a refrigerator
(~4 ◦C).
[PPh4][V(V)(HIPDA)2]·H2O. [PPh4][V(V)(HIPDA)2] was pre-
pared using the procedure reported previously,34 and the purity
was determined by NMR spectroscopy.34 This sample contained
<5% of an impurity that formed vanadate in solution, and
the rest of the compound had the characteristic triplet–peak
pattern with the central peak at 51V diso = -269 ppm as reported
previously for the samples containing [V(V)(HIPDA)2]- with dif-
ferent stereoisomers.34 The solid-state NMR spectra were recorded
within one month after preparation and before evidence for any
sample decomposition.
The mixture was then acidified to pH 4.0 with 1.0 M HCl.
To this solution zinc acetate dihydrate (11.0 g, 0.0684 mol) was
added. The mixture was allowed to stand overnight at ambient
temperature. The resulting zinc complex was then filtered off,
washed with 10 mL of ice-cold water and dried over P2O5.
The zinc complex (3.0 g, 0.014 mol) obtained as described
above was dissolved in 25 mL of water by dropwise addition of
concentrated HCl until the solution was clear. This solution was
loaded onto an acidic Dowex HCR-W2 cation exchange resin
(column size 2.5 cm ¥ 30 cm) and eluted with 0.2 M NaOH. The
Solid-state NMR spectroscopy
Solid-state 51V NMR spectra were acquired on a Tecmag Dis-
covery spectrometer, interfaced with a wide bore 9.4 T Magnex
magnet, and operating at a frequency of 105.23 MHz (9.4T)
for 51V. Spectra were also collected on a Varian InfinityPlus
spectrometer operating at 157.61 MHz (14.1 T). Vanadium
chemical shifts were referenced to neat VOCl3, diso = 0.0 ppm, used
as an external reference.59 This sample was also used to calibrate
the 90◦ pulse widths, which were set to 4.0 ms (g B1/2p ª 62 kHz)
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and 3.2 ms (g B1/2p ª 76 kHz) at 9.4 and 14.1 T, respectively. The
magic angle was set by maximizing the number of rotational echoes
observed in the 23Na NMR free induction decay of solid NaNO3.
The angle was set for each sample; accurate setting of the MA
is critical when acquiring spectra from the satellite transitions of
quadrupolar nuclei over a broad frequency range.60 A 4 mm Doty
XC4 MAS probe was used for acquiring all the spectra at 9.4 T
while a 3.2 mm Varian T3 MAS probe was used at 14.1 T. The
Examination of the available data of solid-state NMR parameters
for vanadium(V) complexes of different coordination geometries
reveals no simple all-inclusive empirical trends between NMR
parameters and coordination number. The overall success of
the DFT calculations, however, provides a reliable link between
experimental solid-state NMR and structure that makes 51V solid-
state NMR a valuable characterization method.
◦
temperature was 20 C during all experiments and controlled to
Acknowledgements
within 1 degree with a Varian VT controller. For each sample,
spectra at several MAS frequencies were acquired. The MAS
frequency was controlled by the Tecmag and Varian automated
control units to within 5 Hz. All spectra were acquired using a
one-pulse experiment with either 1 ms or 0.8 ms pulse, a spectral
width of 2 MHz, and a recycle delay of 1 s. Proton decoupling did
not significantly improve the spectra and was therefore not used.
The NMR spectra were processed with Gaussian line broadening
functions of 100 Hz, and baseline corrections. Spectra of the entire
manifold of central and satellite transitions were simulated with
SIMPSON,33 using finite excitation pulses corresponding to the
experimental conditions.
T. P. acknowledges financial support of the National Science
Foundation (CHE-0750079) and the National Institutes of Health
(P20-17716, COBRE individual sub-project). K. J. O. thanks the
Natural Sciences and Engineering Research Council of Canada for
financial support. D. C. C. acknowledges financial support of the
Institute of General Medicine at the National Institutes of Health
(GM40525) and the National Science Foundation (CHE-0628260
and CHE-0750079).
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