Mixed-ligand oxidovanadium(V) complexes with N′- salicylidenehydrazides: Synthesis, structure, and51V solid-state MAS NMR investigation

Article abstract of DOI:10.1002/ejic.200800063

The synthesis and spectroscopic characterization of a series of three oxidovanadium(V) complexes with 8-hydroxyquinoline and Schiff-base ligands derived from salicylaldehyde and ω-hydroxy-functionalized carbohydrazides with different chain lengths are reported. The complex with the hydrazone ligand containing the shortest chain length was crystallographically characterized. This complex crystallizes in the triclinic space group P1^- with two structurally similar but crystallographically independent oxidovanadium(V) complexes. Each vanadium atom is six-coordinate in a distorted-octahedral geometry. The two molecules are assembled through hydrogen-bonding interactions between the hydroxyl groups of the side-chain substituted Schiff-base ligand and the oxido group of one of the two complexes. Electrochemical measurements performed in acetonitrile solution reveal two reversible one-electron reduction steps. The observed pre-wave feature of the second reduction step indicates the presence of dissociation equilibria related to the 8-hydroxyquinoline coligand. Magic-angle spinning solidstate ⁵¹V NMR spectroscopy allowed to characterize the full series of complexes with alkyl and hydroxy alkyl-substituted hydrazone ligands that were used. The quadrupolar coupling constants are small with a value of about 4 MHz and show little variation within the series. The asymmetry of the chemical shift tensor indicates a rather axial symmetric environment around the vanadium(V) center. The isotropic chemical shifts observed in the solid state occur at about 30 ppm, which is in the same order of magnitude as the solvent induced variations, about 10 ppm, found for different solvents. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800063

FULL PAPER
DOI: 10.1002/ejic.200800063
Mixed-Ligand Oxidovanadium(V) Complexes with NЈ-Salicylidenehydrazides:
51
Synthesis, Structure, and V Solid-State MAS NMR Investigation
Simona Nica,[a] Axel Buchholz,[a] Manfred Rudolph,[a] Annika Schweitzer,[b]
Maria Wächtler,[b] Hergen Breitzke,[b] Gerd Buntkowsky,[b] and Winfried Plass*[a]
Dedicated to Professor Achim Müller on the occasion of his 70th birthday
Keywords: 8-Hydroxyquinoline / Schiff bases / Square-wave voltammetry / Vanadium / 51V NMR spectroscopy
The synthesis and spectroscopic characterization of a series
The observed pre-wave feature of the second reduction step
of three oxidovanadium(V) complexes with 8-hydroxyquino- indicates the presence of dissociation equilibria related to the
line and Schiff-base ligands derived from salicylaldehyde
and ω-hydroxy-functionalized carbohydrazides with dif-
ferent chain lengths are reported. The complex with the hy-
drazone ligand containing the shortest chain length was
crystallographically characterized. This complex crystallizes
8-hydroxyquinoline coligand. Magic-angle spinning solid-
state 51V NMR spectroscopy allowed to characterize the full
series of complexes with alkyl and hydroxy alkyl-substituted
hydrazone ligands that were used. The quadrupolar coupling
constants are small with a value of about 4 MHz and show
little variation within the series. The asymmetry of the chemi-
cal shift tensor indicates a rather axial symmetric environ-
ment around the vanadium(V) center. The isotropic chemical
shifts observed in the solid state occur at about 30 ppm,
which is in the same order of magnitude as the solvent in-
duced variations, about 10 ppm, found for different solvents.
¯
in the triclinic space group P1 with two structurally similar
but crystallographically independent oxidovanadium(V)
complexes. Each vanadium atom is six-coordinate in a dis-
torted-octahedral geometry. The two molecules are as-
sembled through hydrogen-bonding interactions between
the hydroxyl groups of the side-chain substituted Schiff-base
ligand and the oxido group of one of the two complexes.
Electrochemical measurements performed in acetonitrile
solution reveal two reversible one-electron reduction steps.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
plored leading to a series of derivatives viewed as inorganic
analogs of carboxylic acids.[21]
Introduction
There is a continuous interest in the coordination chem-
istry of vanadium due to its catalytic and medicinal impor-
tance.[1] Besides the insulin-like activity of oxidovanadi-
um(V) and oxidovanadium(IV) compounds,[2,3] its presence
in vanadium-dependent haloperoxidases[4–6] in particular
has stimulated the search for structural and functional
We could recently show that NЈ-salicylidenehydrazides
are versatile ligand systems supporting different types of
vanadium(V) complexes,[15] which is a result of this ligand
system being prone to variable protonation states at the
amide group.[22–24] Moreover, these systems can easily be
modified by the introduction of various functional groups
attached to the ligand core.[25–27] A particular feature is the
formation of intramolecular hydrogen-bonding interactions
with the vanadate moiety, in this case a hydroxy function-
alized side chain.[28,29]
In the present work, we report on a series of mixed-li-
gand oxidovanadium(V) complexes containing the ω-hy-
droxy functionalized NЈ-salicylidenehydrazide ligand sys-
tem together with the electrochemically interesting 8-hy-
droxyquinoline as a coligand. In addition to their electro-
chemical properties we also investigated these compounds
by 51V solid-state NMR spectroscopy. The latter has proven
to be an excellent probe to gain quantitative site-specific
models. In this context pentavalent-vanadium complexes
+
with the {VO3+
}
[7–12] and {VO2
}
[13–16] core motifs have re-
ceived considerable attention. Moreover, the ability of as-
cidians to accumulate vanadium has initiated a rich chemis-
try related to its reduced states.[1,17] This particularly in-
cludes 8-hydroxyquinoline complexes, which exhibit an
interesting electrochemistry.[18–20] The corresponding bis(8-
hydroxyquinolinato)vanadium(V) complexes have been ex-
[a] Institut für Anorganische und Analytische Chemie, Friedrich-
Schiller-Universität Jena,
Carl-Zeiss-Promenade 10, 07745 Jena, Germany
Fax: +49-3641-948132
E-mail: Sekr.Plass@uni-jena.de
[b] Institut für Physikalische Chemie, Friedrich-Schiller-Uni- information for amorphous and crystalline solids, in bulk,
on surfaces, and at the interfaces.[30–34]
versität Jena,
Helmholtzweg 4, 07743 Jena, Germany
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Mixed-Ligand Vanadium(V) Complexes
Results and Discussion
aromatic C–C stretching vibrations together with the disap-
pearance of the ν(OH) vibration of the free 8-hydroxyquin-
oline at 3152 cm–1.
Synthesis and Characterization
For all complexes synthesized, strong bands are observed
in the electronic absorption spectra of their acetonitrile
solutions at 241, 273, 321, and 540 nm. In comparison with
the electronic spectra of the corresponding free ligands an
additional band is apparent in the visible region at 540 nm
(ε ≈ 7000 cm–1 –1), which is attributed to a ligand-to-metal
charge-transfer band.
The Schiff-base ligands with a ω-hydroxy-functionalized
side chain, used in the present work, have been prepared by
condensation of salicylaldehyde with the appropriate car-
bohydrazides (H2salhyhb, H2salhyhp, and H2salhyhh; see
Scheme 1). This affords tridentate-chelate ligands with a
variable protonation state at the amide function, which can
therefore act as mono- and dianionic systems.[22–24] The re-
action of equimolar amounts of the Schiff-base ligand with
vanadyl acetylacetonate in methanol under reflux and aero-
bic conditions leads to a color change of the initially green
reaction mixture to a brown solution, indicating the forma-
tion of a vanadium(V) species. Subsequent addition of the
coligand 8-hydroxyquinoline affords dark-violet reaction
solutions from which the mixed-ligand oxidovanadium(V)
complexes [VO(salhyhb)(hq)], [VO(salhyhp)(hq)], and [VO-
(salhyhh)(hq)] can be isolated. Alternatively, this reaction
sequence can also be performed in acetonitrile leading to
somewhat higher overall reaction yields. The obtained com-
plexes show very good solubility in a broad range of or-
ganic solvents like alcohols, DMF, DMSO, chloroform,
dichloromethane, and acetonitrile. It should be noted here
that attempts to utilize ammonium metavanadate in meth-
anol solution directly as a vanadium(V) precursor did not
lead to the desired complexes, but instead afforded a mix-
ture of the corresponding ammonium salt of the cis-diox-
idovanadium(V) complexes[15,28] and bis(8-hydroxyquinoli-
nato)methoxyoxidovanadium(V).
1
The H and 13C NMR spectroscopic data for all com-
plexes are consistent with the formation of the proposed
mixed-ligand oxidovanadium(V) complexes. In particular,
the twofold deprotonation of the NЈ-salicylidenehydrazide
ligands is evident from the absence of the downfield reso-
nances corresponding to the O–H and N–H protons in the
1H NMR spectra of the complexes. Moreover, upon coordi-
nation the resonance of the azomethine proton is shifted
downfield to 9.15 ppm, whereas the resonances of the hy-
drogen atoms of the methylene group next to the carbonyl
group are slightly shifted upfield when the vanadium atom
is coordinated. The 13C NMR spectroscopic data confirm
the simultaneous coordination of both chelate ligands, the
Schiff base, and the 8-hydroxyquinoline coligand.
The 51V NMR spectra confirm the presence of one spe-
cies for solutions of the complexes in DMSO and chloro-
form with observed resonances at –472 and –483 ppm,
respectively. This indicates a considerable solvent depen-
dence for the 51V NMR resonances of the mixed-ligand
complexes. The observed hydrolytic stability of these com-
plexes in DMSO/water mixtures is consistent with the de-
scribed pronounced stability of phenolate substituted ox-
idovanadium(V) complexes.[10] Nevertheless, they can be
converted into the corresponding cis-dioxidovanadium(V)
complexes by treatment with an aqueous sodium hydroxide
solution. Monitoring this reaction by 51V NMR spec-
troscopy shows that 1 equiv. of alkaline base is required for
a complete cleavage of the 8-hydroxyquinolinate coligand,
with a resonance at –536 ppm for the final cis-dioxidovana-
dium(V) complex[28] that is stable even in the presence of
excess base.
Scheme 1.
Electrochemical Properties
The formation of an oxidovanadium(V) moiety in the
synthesized mixed-ligand complexes is confirmed by their
The electrochemical properties of the complexes
IR spectra, which reveal a strong band at 969 cm–1 for [VO- [VO(hq)(salhyhb)], [VO(hq)(salhyhp)], and [VO(hq)(salhyhh)]
(salhyhb)(hq)] and 972 cm–1 for [VO(salhyhp)(hq)] and [VO- in acetonitrile solutions have been investigated by cyclic
(salhyhh)(hq)], characteristic of the stretching vibration of square-wave voltammetry. As all three complexes exhibit a
the V=O moiety. Moreover, upon complex formation the similar electrochemical behavior, a typical series of cyclic
bands that are characteristic of the N–H and C=O stretch- square-wave voltammograms measured for [VO(hq)-
ing vibrations of the ligand disappear in the IR spectra and (salhyhp)] with square-wave frequencies ranging from 25 to
a new strong band shows up at around 1605 cm–1, which is 1500 Hz is depicted in Figure 1. The two observed one-elec-
typical for the conjugated –CH=N–N=C– system with the tron reduction steps correspond to the reactions shown in
NЈ-salicylidenehydrazide ligand present in its enolate Equation (1) and Equation (2).
form.[23] A broad band attributed to the O–H stretching
vibration of the hydroxy group of the side chain is observed [VO(salhyhp)(hq)] + e– h [VO(salhyhp)(hq)]–
at around 3440 cm–1. Further evidence for the complexation
(1)
(2)
of the coligand is given by the presence of its characteristic [VO(salhyhp)(hq)]– + e– h [VO(salhyhp)(hq)]2–
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W. Plass et al.
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Figure 2. Height of the peak current for the first (open squares)
and the second reduction step (open circles) as well as the plateau
current of the pre-wave (filled circles) associated with the second
reduction step of complex [VO(salhyhb)(hq)] as a function of the
square root of the square-wave frequency.
Figure 1. Square-wave voltammograms of complex [VO-
(salhyhb)(hq)] in acetonitrile with applied square-wave frequencies
of 25, 50, 100, 200, 400, 800, and 1500 Hz (with increasing peak
current).
The first reduction step, which occurs at a half-wave po-
tential of U11/2 = –0.452 V, appears to be fully reversible for
square-wave frequencies up to 1500 Hz, whereas the second
charge transfer reactions given in Equations (1) to (3) is
necessary. This is also required for the underlying reaction
scheme to satisfy the condition of an early diffusion con-
trolled peak current for the second reduction step, exclud-
ing the option of a single chemical equilibrium linking the
product of the first reduction step with the species responsi-
ble for the pre-wave feature. Moreover, this is corroborated
by the observed frequency-dependent shift of the pre-wave
towards more positive potentials for a decrease in the
square-wave frequency (see Figure 1), which is exactly the
opposite shift theoretically expected for a simple pre-equi-
librium.
1/2
reduction step at a potential of U2 = –2.428 V is ac-
companied by a pre-wave. These electrochemical features
are basically similar to what has been observed for mixed-
ligand oxidovanadium(V) complexes based on non-func-
tionalized NЈ-salicylidenehydrazide ligands,[35] although the
two half-wave potentials for the latter cases are somewhat
1/2
1/2
shifted to more negative values (U1 = –0.460 V and U2
= –2.453 V).
The frequency dependence of the peak current of the re-
duction steps related to Equation (1) and (2) as well as that
of the accompanying pre-wave of the second step is de-
picted in Figure 2. This shows that the first reduction step
is diffusion controlled over the entire frequency range. For
the second reduction step the peak current shows a similar
behavior and stays at least close to this limit, whereas its
accompanying pre-wave reaches a steady-state current for
very high square-wave frequencies. Even for low square-
wave frequencies this peak current deviates significantly
from the diffusion controlled limit, indicating that this pre-
wave is associated with the reduction of a species generated
by a preceding chemical equilibrium. Species potentially re-
sponsible for the observed pre-wave feature are the 8-hy-
droxyquinolinate anion hq– and the deprotonated
Hsalhyhp– ligand with reversible one-electron reduction
processes at potentials of about –2.33 V and –2.10 V,
respectively. Given the potential range observed for the pre-
wave we prefer an interpretation based on the 8-hydroxy-
quinolinate anion hq– [Equation (3)].
[VO(salhyp)(hq)] h [VO(salhyp)]+ + hq–
(4)
[VO(salhyp)(hq)]– h [VO(salhyhp)]+ + hq2–
(5)
The simulation of the electrochemical data is depicted in
Figure 3, showing a reasonable agreement with the experi-
mental data based on the assumed addition/elimination re-
actions given in Equation (4) and (5) for the vanadium(V)
complex and its singly reduced vanadium(IV) analog gener-
ated according to Equation (1), respectively. The equilibria
given in Equation (4) and (5) are found to be far on the
side of the intact complexes. Nevertheless, for potentials be-
tween the first reduction step and the pre-wave the equilib-
rium in Equation (5) is somewhat shifted to the side of the
dissociated products, as even small amounts of hq2– are in-
stantly reoxidized to hq–. Together with Equation (4) this
can account for the virtually diffusion-controlled behavior
of the second reduction step, simply by regeneration of the
corresponding [VO(salhyhp)(hq)]– species. Consequently,
the difference in the equilibrium constants for the reactions
given in Equation (4) and (5) results in the accumulation of
hq– + e– h hq2–
(3)
In order to sufficiently describe the observed data a com- effects leading to the generation of a pre-wave for the sec-
plex system of addition/elimination reactions given in ond reduction step. In addition, this can also account for
Equations (4) and (5) together with the corresponding the unusual frequency dependence of the potential shift of
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Mixed-Ligand Vanadium(V) Complexes
this pre-wave as well as the observed virtual diffusion con-
trol of the peak current of the second reduction step. It
should be mentioned here that basically similar and/or ad-
ditional effects could be expected from appropriate differ-
ences in the diffusion coefficients of the underlying species.
Nevertheless, for practicability reasons this has not been in-
cluded in the present simulations.
Figure 3. Measured (top) and simulated (bottom) square-wave vol-
tammograms of complex [VO(salhyhb)(hq)] in acetonitrile with ap-
plied square-wave frequencies of 25, 50, 100, 200, 400, 800, and
1500 Hz (with increasing peak current).
Figure 4. Molecular structures of the two independent complexes
in the crystal structure of [VO(salhyhb)(hq)]. Thermal ellipsoids of
non-hydrogen atoms are drawn at the 50% probability level.
ligand (O15 or O25) forms a tetragonal basal plane. Five-
and a six-membered-chelate rings are established by the hy-
Structure of [VO(hq)(salhyhb)]
Crystals suitable for X-ray diffraction could be isolated drazone ligand with bite angles of 75° (O13–V1–N11, O23–
for complex [VO(hq)(salhyhb)] by crystallization from a V2–N21) and 84° (O12–V1–N11, O22–V2–N21), respec-
methanol/acetonitrile (1:1) mixture upon slow evaporation tively. The V=O moiety in both molecules shows a bond
of the solvents at room temperature. The complex crys- length of about 159 pm, which is at the lower end of the
tallizes in the triclinic space group P1 with two crystallo- typically observed range for VO3+ complexes,[9–12] but con-
¯
graphically independent molecules in the asymmetric unit. sistent with other complexes containing hydrazone ligands
The molecular structures are depicted in Figure 4 and se- together with appropriate coligands such as 8-hydroxyquin-
lected bond lengths and angles are given in Table 1.
oline,[36] catechol,[37] hydroxamate,[38,39] and polyols.[40] The
In both independent molecules of [VO(hq)(salhyhb)] the trans influence of the oxido group leads to a rather long
vanadium atom is six-coordinate in a distorted-octahedral V–N bond to the nitrogen atom of the 8-hydroxyquinoline
geometry with similar structural features. The NЈ-salicylid- coligand (235 pm), and consequently to a displacement of
enehydrazide ligand is fully deprotonated and acts as a tri- the vanadium atom towards the oxido group out of the te-
dentate-chelate system coordinating through its ONO do- tragonal basal plane by about 30 pm. The observed bond
nor set (O12, N11, O13 or O22, N21, O23), which together lengths of the amide group of the hydrazone ligand are con-
with the oxygen atom of the bidentate 8-hydroxyquinoline sistent with its dianionic form,[23,28] whereas for complexes
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W. Plass et al.
FULL PAPER
Table 1. Selected bond lengths [pm] and angles [°] for [VO- their pendant side chains possess different orientations
(salhyhb)(hq)].
given by the Ci8–Ci9–Ci10–Ci11 torsion angles (172° for i
= 1, –68° for i = 2).
Distances
These chains associate in the solid state by π–π-stacking
interactions of the 8-hydroxyquinoline coligands from both
types of molecules present. This stacking occurs in such a
fashion that an alternating pairwise core-to-core [V2(hq)···
(hq)V2] and tail-to-tail [V1(hq)···(hq)V1] interaction of ad-
jacent chains is observed leading to a two-dimensional ar-
V1–O11
V1–O12
V1–O13
V1–O15
V1–N11
V1–N13
O13–C18
N12–C18
159.38(15)
186.09(14)
195.77(14)
184.28(14)
207.89(17)
235.44(17)
130.8(2)
V2–O21
V2–O22
V2–O23
V2–O25
V2–N21
V2–N23
O23–C28
N22–C28
159.79(14)
185.04(14)
195.09(14)
184.97(14)
207.91(17)
235.35(16)
131.3(2)
¯
ray within the crystallographic (011) plane. Figure 6 shows
130.3(3)
130.1(3)
the resultant packing with the view approximately along the
hydrogen-bonded chains.
Angles
O11–V1–O12 98.88(7)
O11–V1–O13 99.02(7)
O11–V1–O15 101.26(7)
O11–V1–N11 98.05(7)
O11–V1–N13 176.51(7)
O12–V1–O13 153.98(6)
O12–V1–O15 103.33(6)
O12–V1–N11 83.83(6)
O12–V1–N13 84.26(6)
O13–V1–O15 91.50(6)
O13–V1–N11 75.09(6)
O13–V1–N13 78.51(6)
O15–V1–N11 158.01(6)
O15–V1–N13 76.46(6)
N11–V1–N13 83.73(6)
O21–V2–O22
O21–V2–O23
O21–V2–O25
O21–V2–N21
O21–V2–N23
O22–V2–O23
O22–V2–O25
O22–V2–N21
O22–V2–N23
O23–V2–O25
O23–V2–N21
O23–V2–N23
O25–V2–N21
O25–V2–N23
N21–V2–N23
99.59(7)
97.04(7)
99.21(7)
99.95(7)
173.81(7)
155.27(6)
101.34(6)
83.79(6)
85.47(6)
93.89(6)
75.37(6)
79.33(6)
159.03(6)
76.18(6)
84.05(6)
Figure 6. Packing diagram for [VO(salhyhb)(hq)] (view approxi-
mately down the a axis).
with the coordinated keto form of the ligand the C–O bond
is considerably elongated.[22]
The hydrogen-bonding interactions induced by the hy-
droxy group of the side-chain substituted NЈ-salicylidenehy-
51V MAS NMR Spectroscopy
51V NMR spectra of the complexes [VO(hq)(salhyhb)],
drazide ligand leads to the formation of a chainlike ar- [VO(hq)(salhyhp)], and [VO(hq)(salhyhh)] as well as of their
rangement as depicted in Figure 5. The different roles of analogs with the hydroxy-free side chain [VO(hq)(salhyb)],
the two crystallographically independent molecules within
this chain define their structural differences. Basically this
chain is formed by the molecules containing the vanadium
atom V2 with a hydrogen-bonding relay between the oxido
group O21 and the hydroxy group O24 from an adjacent
molecule of the same type. The hydrogen-bonding relay is
established by the hydroxy group (O14) of the other crystal-
lographically unique molecules (V1) present in the struc-
ture, which form tails sticking out on one side of the chain
core. This difference between the two independent mole-
cules also reflects in their molecular structure such that
Figure 5. Representation of the hydrogen bonding interactions be-
tween the two independent molecules of [VO(salhyhb)(hq)] leading
to chains along the a axis; hydrogen atoms attached to carbon
atoms are omitted for clarity; broken lines represent hydrogen
bonds; relevant distances (in pm): O14···O24 286, O14···O21A 288.
Figure 7. 51V solid-state MAS NMR spectra of [VO(salhyhp)(hq)]
measured at 9.4 T and spinning rates of 7 kHz (bottom) and
10 kHz (top). Insets show an expansion of the central transition
region.
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Mixed-Ligand Vanadium(V) Complexes
[VO(hq)(salhyp)], and [VO(hq)(salhyh)] were measured at
The experimental 51V magic-angle spinning NMR spec-
9.4 T applying different rates for the magic-angle spinning. tra of the examined complexes are depicted in Figure 8 and
In Figure 7 the experimental spectra acquired for the determined spectral parameters are listed in Table 2. Be-
[VO(hq)(salhyhp)] with spinning rates of 7 and 10 kHz are cause of the second-order quadrupole induced shift, the ob-
depicted. The comparison of spectra, measured at different served isotropic shift δobsd is corrected according to Equa-
spinning rates, enables the unambiguous determination of tion (6)[41]
the observable isotropic peak (δobsd). In addition, this in-
creases the reliability of the parameters determined by nu-
merical simulations, the latter being essential for a quadru-
polar nucleus like 51V (I = 7/2).
(6)
Figure 8. 51V solid-state MAS NMR spectra of (a) [VO(salhyhb)(hq)], (b) [VO(salhyhp)(hq)], (c) [VO(salhyhh)(hq)], (d) [VO(salhyb)(hq)],
(e) [VO(salhyp)(hq)], and (f) [VO(salhyh)(hq)] measured at 9.4 T and a spinning rate of 10 kHz. The calculated spectra were obtained
with the parameters summarized in Table 2.
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Table 2. Solid-state NMR parameters for mixed-ligand oxidovanadium(V) complexes.[a]
Quadrupole
CQ
ηQ
δσ
ησ
δobsd
induced shift[b] δiso
δiso(solution)
[MHz]
[ppm]
[ppm]
[ppm]
[ppm]
[ppm]
[VO(salhyhb)(hq)]
[VO(salhyhp)(hq)]
[VO(salhyhh)(hq)]
[VO(salhyb)(hq)]
[VO(salhyp)(hq)]
[VO(salhyh)(hq)]
4.0
4.3
3.5
3.5
3.8
3.5
0.4
0.8
0.6
0.7
0.8
0.6
–449
–409
–407
–431
–436
–438
0.4
0.3
0.3
0.4
0.2
0.2
–454.2
–452.3
–436.7
–461.7
–463.9
–454.0
–3.9
–5.2
–3.2
–3.3
–4.0
–3.2
–450.1
–446.7
–433.1
–458.4
–460.1
–450.6
–472
–472
–472
–472
–472
–472
[a] Quadrupolar couplings (CQ, ηQ), chemical shift anisotropies (δσ, ησ), and the 51V chemical shifts (δobsd, δiso); for details see Exp. Sect.;
the following error margins apply: CQ Ϯ0.2 MHz, ηQ Ϯ0.1, δσ Ϯ20 ppm, ησ Ϯ0.1, δobsd Ϯ3 ppm, δiso Ϯ1 ppm. [b] Calculated according
to Equation (6).
where (e2qQ/h) is the nuclear quadrupolar coupling con- form induces an additional shielding effect of about 10 ppm
stant CQ given in MHz, I is the spin number (I = 7/2 for and that this solvent induced difference is in the same order
51V), m is the magnetic spin number (m = 1/2 for the central of magnitude as the observed variation of the isotropic
transition), ηQ is the asymmetry of the EFG tensor, and ν0 chemical shift values in the solid state (δ = 27 ppm).
the resonance frequency of the 51V nucleus (105.19 MHz
The anisotropy parameter δσ of the chemical-shielding
at 9.4 T). As can be seen from Table 2 this second-order anisotropy (CSA) tensor for the three complexes with the
correction is found to be within –3 and –5 ppm. This is alkyl side chains are within 7 ppm indicating rather subtle
because of the comparatively small values of the nuclear differences in the chemical environment, which is consistent
quadrupolar coupling constant in the order of about with their observed structural similarities.[35] Somewhat
4 MHz, which is at the lower end of the usually observed larger effects are observed for the hydroxyl-functionalized
range for oxidovanadium(V)[42,43] and dioxidovana- side chains with an observed range of about 40 ppm (see
dium(V)[44] complexes going up to about 7 MHz. Neverthe- Table 1). Interestingly, the two complexes [VO(salhyhp)(hq)]
less, the spectra are characterized by a considerable asym- and [VO(salhyhh)(hq)] exhibit a value of δ = –408 ppm
metry of the EFG tensor with values between 0.4 and 0.8. about the same anisotropy, whereas for the complex with
The determined nuclear quadrupolar coupling constants the shortest alkyl spacer [VO(salhyhb)(hq)] a value of δ =
CQ and asymmetry parameters ηQ of the EFG tensor are –449 ppm is observed. Unfortunately, we could not obtain
related to the overall width and shape, respectively, of the suitable crystals of the former two complexes to better elu-
spectra given in Figure 8.
cidate possible structural correlations. Nevertheless, this
The NMR parameters given in Table 2 show a significant could suggest a variation in the hydrogen bonding, as an
variation for the series of complexes under investigation, elongation of the alkyl spacer might prevent the analogous
although the differences in geometry of the first coordina- assembly in the solid state.
tion sphere of the vanadium centers for the structurally
Finally, the observed rather low asymmetry of the CSA
characterized vanadium(V) complexes (see Table 1 and tensor (ηQ in Table 2), which is in the range of 0.2 to 0.4, is
ref.[35]) are very small (bond lengths Ͻ 2 pm; bond angles consistent with a predominant role of the V=O bond for
Ͻ 5°). In addition, for all complexes large discrepancies determining the magnetic shielding at the vanadium(V) cen-
have been observed between the isotropic chemical shifts in ter. The strong influence of the V=O bond is in agreement
the solid state and in solution, which ranges from about 10 with earlier reports on magnetic-shielding effects on chelate
to 40 ppm. It is tempting to attribute this to a high sensitiv- rings in oxidovanadium(V) complexes, which indicate a
ity of the chemical-shielding interaction at the vanadium strong anisotropy around this functional group.[11]
nucleus with respect to changes in the local environment.
However, this can only be attributed to subtle differences
introduced through secondary coordination effects, e.g. hy-
drogen bonding (see Figure 5) or crystal packing, caused by
Conclusions
a variation of the side chain.
Mixed-ligand oxidovanadium(V) complexes containing
Consequently, the solution chemical shifts are the same hydroxyl-functionalized NЈ-salicylidenehydrazides and 8-
for all six complexes in a given solvent, despite their actual hydroxyquinoline have been synthesized. These complexes
side-chain characteristics. This can be rationalized in terms exhibit an electrochemistry with two one-electron reduction
of the solvent molecules interacting with the complexes in steps, which can only be explained on the basis of a coupled
a similar fashion. Moreover, in solution the side chains can reaction system including addition/elimination reactions of
be expected to freely relax their orientation away from the the 8-hydroxyquinoline ligand with regard to the vanadi-
vanadium center, which leads to a separation of the side- um(V) and the corresponding reduced vanadium(IV) com-
chain functionality preventing any kind of interference with plexes. The crystal structure of complex [VO(salhyhb)(hq)]
the vanadium atom. It is interesting to note that even reveals hydrogen-bonding interactions of the side chain of
changing the solvent environment from DMSO to chloro- one molecule with the oxido group of an adjacent indepen-
2356
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2350–2359

Mixed-Ligand Vanadium(V) Complexes
C12H16N2O3 (236.27): calcd. C 61.00, H 6.83, N 11.86; found C
60.92, H 6.82, N 11.74. M.p. 122–123 °C. 1H NMR (200 MHz, [D6]-
DMSO): δ = 1.40–1.49 (m, 2 H, CH2CH2OH), 1.56–1.63 (m, 2
dent second complex molecule. This supramolecular differ-
entiation does not lead to a significant alteration of the
structural parameters of the two crystallographically inde-
pendent molecules. This also reflects in the measured solid-
state MAS NMR spectra, which do not allow a differentia-
tion of these two vanadium nuclei. Nevertheless, these
NMR spectra indicate an influence of the crystal environ-
ment on the EFG and CSA tensors, which is only somewhat
larger than the observed solvent induced changes for the
3
H, CH2CH2C=O), 2.36 and 2.57 (both t, J = 7.5 Hz, total 2 H,
CH2C=O), 3.37–3.42 (m, 2 H, CH2OH), 4.38–4.42 (m, 1 H,
CH2OH), 6.83–6.90 (m, 2 H, arom. CH), 7.19–7.27 (m, 1 H, arom.
CH), 7.46–7.61 (m, 1 H, arom. CH), 8.24 and 8.33 (both s, total 1
H, CH=N), 10.13, 11.20, 11.57 (all s, total 2 H, NH and OH) ppm.
Selected IR data (KBr): ν = 3185 (s, NH, OHass), 1673 (s, C=O),
˜
1623 (s, CH=N) cm–1. UV/Vis (CH3CN solution): λmax (ε) = 278
isotropic chemical shift. Nevertheless, the CSA tensor is (16,600), 288 (14,800), 319 nm (7800 cm–1 –1).
very sensitive towards changes in the local environment of
vanadium(V) nuclei and in the presence of an oxido group
dominated by the V=O bond.
H2salhyhh: 6-Hydroxyhexanohydrazide (5.00 g, 34 mmol) and sali-
cylaldehyde (4.18 g, 34 mmol). Yield 7.10 g (28 mmol, 82%).
C13H18N2O3 (250.30): calcd. C 62.38, H 7.25, N 11.19; found C
62.38, H 7.28, N 11.23. M.p. 130–131 °C. 1H NMR (200 MHz, [D6]-
3
DMSO): δ = 1.23–1.47 (m, 4 H, CH2CH2CH2OH), 1.57 (tt, Jav
=
Experimental Section
7.3 Hz, 2 H, CH2CH2C=O), 2.20 and 2.56 (both t, 3J = 7.5 Hz,
total 2 H, CH2C=O), 3.34–3.41 (m, 2 H, CH2OH), 4.37 (br. s, 1
H, CH2OH), 6.85–6.91 (m, 2 H, arom. CH), 7.21–7.30 (m, 1 H,
arom. CH), 7.44–7.60 (m, 1 H, arom. CH), 8.24 and 8.32 (both s,
total 1 H, CH=N), 10.13, 11.19, 11.58 (all s, total 2 H, NH and
Materials: Abbreviations used throughout the text: H2salhyb =
NЈ-[(2-hydroxyphenyl)methylidene]butanohydrazide, H2salhyp
=
NЈ-[(2-hydroxyphenyl)methylidene]pentanohydrazide, H2salhyh =
NЈ-[(2-hydroxyphenyl)methylidene]hexanohydrazide, H2salhyhb =
4-hydroxy-NЈ-[(2-hydroxyphenyl)methylidene]butanohydrazide,
H2salhyhp = 5-hydroxy-NЈ-[(2-hydroxyphenyl)methylidene]penta-
nohydrazide, H2salhyhh = 6-hydroxy-NЈ-[(2-hydroxyphenyl)methyl-
idene]hexanohydrazide, Hhq = 8-hydroxyquinoline. The ω-hy-
droxycarbohydrazides were prepared by the treatment of the corre-
sponding lactone with 2 equiv. hydrazine hydrate and have been
recrystallized from absolute ethanol. The complexes [VO-
(salhyb)(hq)], [VO(salhyp)(hq)], and [VO(salhyh)(hq)] have been
prepared according to reported procedures.[35] All other reagents
were used as received without further purification.
Instrumentation: Solution 1H, 13C, 51V, 1H{1H} COSY, and
1H{13C} heteronuclear correlation NMR spectra were recorded
with Bruker AVANCE 200 and 400 MHz spectrometers. IR spectra
were measured with a Bruker IFS55/Equinox spectrometer on sam-
ples prepared as KBr pellets. Elemental analysis (C, H, N) were
determined with Leco CHNS-932 and El Vario III elemental ana-
lyzers. UV/Vis spectra were recorded with a Varian Cary 5000 UV/
Vis/NIR spectrophotometer using spectral grade acetonitrile.
OH) ppm. Selected IR data (KBr): ν = 3185 (s, NH, OHass), 1673
˜
(s, C=O), 1623 (s, CH=N) cm–1. UV/Vis (CH3CN solution): λmax
(ε) = 278 (16,300), 288 (15,000), 319 nm (7500 cm–1 –1).
Synthesis of Vanadium Complexes: The appropriate ω-hydroxy hy-
drazide ligand (2 mmol) was dissolved in methanol (25 mL) and
added to a stirred suspension of [VO(acac)2] (0.53 g, 2 mmol) in
methanol (10 mL). The resulting reaction mixture was heated un-
der reflux for 2 h with continuous stirring. The color changed from
green to brown. Over a period of 30 min a solution of 8-hydroxy-
quinoline (0.29 g, 2 mmol) in methanol (10 mL) was added drop-
wise and the stirred reaction solution was heated under reflux for
an additional 2 h. This reaction scheme can also be performed in
acetonitrile, which generally leads to somewhat better yields. The
solvent was removed to dryness under reduced pressure and the
residue recrystallized from a mixture of methanol and acetonitrile
(1:1), which yielded dark violet needles.
[VO(salhyhb)(hq)]: Yield 0.57 g (1.3 mmol, 65%) in methanol and
0.63 g (1.4 mmol, 70%) in acetonitrile. C20H18N3O5V (443.40):
1
Synthesis of ω-Hydroxy-NЈ-salicylidenecarbohydrazides: Salicylal-
dehyde (1 equiv.) was added dropwise to a solution of the ω-hy-
droxycarbohydrazide (about 40 mmol) in methanol (100 mL) under
continuous stirring at room temperature. The resulting solution was
stirred at room temperature for an additional 12 h. The colorless
precipitate that formed was separated by filtration. Additional ma-
terial can be obtained from the remaining solution by removal of
the solvent under reduced pressure.
calcd. C 56.69, H 4.21, N 9.74; found C 56.29, H 4.26, N 9.92. H
3
NMR (400 MHz, [D6]DMSO): δ = 1.26 (tt, Jav = 7.0 Hz, 2 H,
CH2CH2OH), 2.09 (t, 2 H, 3J = 7.4 Hz, CH2C=O), 3.09 (t, 2 H,
3J = 6.4 Hz, CH2OH), 4.26 (br. s, 1 H, OH), 6.72–6.74 (m, 1 H,
arom. CH), 6.98–7.00 (m, 1 H, arom. CH), 7.17–7.18 (m, 1 H,
arom. CH), 7.45–7.80 (m, 5 H, arom. CH), 8.09–8.10 (m, 1 H,
arom. CH), 8.50–8.52 (m, 1 H, arom. CH), 9.14 (s, 1 H,
CH=N) ppm. 13C NMR (100 MHz, [D6]DMSO):
δ = 26.3
(CH2CH2OH), 29.1 (CH2C=O), 59.6 (CH2OH), 110.9, 115.4,
119.5, 120.4, 122.5, 123.0, 128.2, 129.0, 133.2, 135.2, 138.2, 138.5,
146.1 (all arom. CH and C), 154.2 (CH=N), 162.3, 163.3 (both
arom. CO–V), 176.1 (CO–V) ppm. 51V NMR (105 MHz, [D6]-
DMSO): δ = –472 ppm (∆ν1/2 = 358 Hz). 51V NMR (105 MHz,
H2salhyhb: 4-Hydroxybutanohydrazide (5.00 g, 42 mmol) and sali-
cylaldehyde (5.17 g, 42 mmol). Yield 8.30 g (37 mmol, 88%).
C11H14N2O3 (222.24): calcd. C 59.45, H 6.35, N 12.61; found C
59.40, H 6.50, N 12.58. M.p. 122–123 °C. 1H NMR (200 MHz, [D6]-
3
DMSO): δ = 1.71 (tt, Jav = 6.9 Hz, 2 H, CH2CH2OH), 2.25 and
2.60 (both t, 3J = 7.5 Hz, total 2 H, CH2C=O), 3.37–3.47 (m, 2 H,
CH2OH), 4.49–4.58 (m, 1 H, CH2OH), 6.85–6.91 (m, 2 H, arom.
CH), 7.22–7.26 (m, 1 H, arom. CH), 7.44–7.60 (m, 1 H, arom. CH),
8.24 and 8.32 (both s, total 1 H, CH=N), 10.15, 11.20, 11.60 (all s,
CDCl3): δ = –483 ppm (∆ν1/2 = 148 Hz). Selected IR data (KBr): ν
˜
= 3446 (br., O–H), 1605 (s, CH=N–N=C), 969 (s, V=O) cm–1. UV/
Vis (CH3CN solution): λmax (ε) = 241 (43,500), 273 (20,800), 321
(9100), 540 nm (6800 cm–1 –1).
total 2 H, NH and OH) ppm. Selected IR data (KBr): ν = 3185 (s,
˜
[VO(salhyhp)(hq)]: Yield 0.45 g (1.0 mmol, 50%) in methanol and
NH, OHass), 1673 (s, C=O), 1623 (s, CH=N) cm–1. UV/Vis
(CH3CN solution): λmax (ε) = 278 (16,400), 288 (14,900), 319 nm
(7700 cm–1 –1).
0.69 g (1.6 mmol, 80%) in acetonitrile. C21H20N3O5V (445.35):
1
calcd. C 56.64, H 4.53, N 9.44; found C 57.00, H 4.43, N 8.97. H
NMR (400 MHz, [D6]DMSO):
δ = 1.04–1.17 (m, 4 H,
3
H2salhyhp: 5-Hydroxypentanohydrazide (5.24 g, 43 mmol) and sali-
cylaldehyde (5.67 g, 43 mmol). Yield 9.67 g (41 mmol, 95%).
CH2CH2CH2OH), 2.07 (t, 2 H, J = 6.6 Hz, CH2C=O), 3.09 (t, 2
H, CH2OH, 3J = 6.4 Hz), 4.21 (br. s, 1 H, OH), 6.72–6.76 (m, 1
Eur. J. Inorg. Chem. 2008, 2350–2359
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2357

W. Plass et al.
FULL PAPER
H, arom. CH), 6.97–7.04 (m, 1 H, arom. CH), 7.16–7.21 (m, 1 H,
actual sample temperature is a little higher depending on the spin-
arom. CH), 7.45–7.50 (m, 1 H, arom. CH), 7.55–7.60 (m, 1 H, ning speed. The magic angle was adjusted using potassium bro-
arom. CH), 7.70–7.73 (m, 2 H, arom. CH), 7.79–7.82 (m, 1 H, mide. The spectral width was 1.6 MHz and the pulse width was
arom. CH), 8.08–8.11 (m, 1 H, arom. CH), 8.50–8.54 (m, 1 H, 1 µs. For each of the compounds, spectra were recorded at two
arom. CH), 9.15 (s, 1 H, CH=N) ppm. 13C NMR (50 MHz, [D6]- different spinning speeds of 7 and 10 kHz to uniquely determine
DMSO):
δ = 22.3 (CH2CH2C=O), 29.2 (CH2C=O), 31.3 the isotropic peak and to increase the reliability of the parameters
(CH2CH2OH), 59.9 (CH2OH), 110.8, 115.4, 119.4, 120.4, 122.5,
123.0, 128.2, 129.0, 133.2, 135.2, 138.3, 138.5, 146.2 (all arom. CH
determined from the simulations. The data were processed by Fou-
rier transformation and baseline correction using the MestReC
and C), 154.2 (CH=N), 162.3, 163.2 (both arom. CO–V), 176.0 NMR spectroscopic data processing software.[45] Isotropic chemical
(CO–V) ppm. 51V NMR (105 MHz, [D6]DMSO): δ = –472 ppm
shifts are reported relative to neat VOCl3 whose 51V NMR spec-
trum was recorded and used as an external referencing standard.
The spectral line shape of the 51V NMR spectra is dominated by
the quadrupolar interaction. Compared to this, the influence of the
(∆ν1/2 = 357 Hz). 51V NMR (105 MHz, CDCl3): δ = –483 ppm
(∆ν1/2 = 148 Hz). Selected IR data (KBr): ν = 3435 (br., O–H),
˜
1604 (s, CH=N–N=C), 973 (s, V=O) cm–1. UV/Vis (CH3CN solu-
tion): λmax (ε) = 241 (43,900), 273 (21,400), 321 (9500), 540 nm chemical shielding anisotropy (CSA) is relatively small. Therefore,
(6800 cm–1 –1).
the evaluation of the spectra necessitates numerical simulations,
which were performed with the SIMPSON software package.[46]
The relationship of the quadrupolar and CSA parameters to their
principal tensor elements is given by Equation (7).
[VO(salhyhh)(hq)]: Yield 0.51 g (1.1 mmol, 55%) in methanol and
0.67 g (1.4 mmol, 70%) in acetonitrile. C22H22N3O3V (459.37):
calcd. C 57.52, H 4.83, N 9.15; found: C 57.33, H 4.83, N 8.37. 1H
NMR (400 MHz, [D6]DMSO): δ = 0.80–0.92 (m, 2 H, CH2), 1.02–
3
1.10 (m, 4 H, CH2), 2.15 (t, 2 H, J = 6.8 Hz, CH2C=O), 3.10 (t,
3
2 H, CH2OH, J = 6.6 Hz), 4.26 (br. s, 1 H, OH), 6.73–6.75 (m, 1
H, arom. CH), 6.97–7.03 (m, 1 H, arom. CH), 7.16–7.21 (m, 1 H,
arom. CH), 7.39–7.51 (m, 1 H, arom. CH), 7.56–7.59 (m, 1 H,
arom. CH), 7.60–7.69 (m, 2 H, arom. CH), 7.70–7.79 (m, 1 H,
arom. CH), 8.09–8.10 (m, 1 H, arom. CH), 8.51–8.53 (m, 1 H,
arom. CH), 9.15 (s, 1 H, CH=N) ppm. 13C NMR (50 MHz, [D6]-
(7)
where δiso = 1/3(δxx + δyy + δzz) is the isotropic chemical shift and
the principal tensor elements of the CSA (δ) and electric field-gra-
DMSO):
δ
=
24.6, 25.6 (CH2), 29.5 (CH2C=O), 31.9
dient (V) tensors are defined according to the convention[47] |λzz
–
(CH2CH2OH), 60.3 (CH2OH), 110.9, 115.4, 119.5, 120.4, 122.4,
122.9, 128.2, 129.1, 133.2, 135.2, 138.2, 138.6, 146.2 (all arom. CH-
C), 154.1 (CH=N), 162.3, 163.3 (both arom. CO–V), 176.0 (CO–
1/3Tr(λ)| Ն |λxx – 1/3Tr(λ)| Ն |λyy – 1/3Tr(λ)| (λ = δ, V). CQ is the
nuclear quadrupolar coupling constant, Q the nuclear quadrupole
moment (–4.8 fm2),[48] e the electronic charge, and h the Planck
constant. δσ characterizes the anisotropy of the CSA tensor,
whereas ηQ and ησ are the asymmetry parameters of the EFG and
CSA tensors, respectively. The full parameter set of the EFG and
CSA tensors as well as the Euler angles α, β, and γ describing the
relative orientation of the principle axis systems of the two tensors
at the vanadium center are obtained by least-squares fittings of the
simulated and experimental spectra (for details see ref.[44]).
V) ppm. 51V NMR (105 MHz, [D6]DMSO): δ = –472 ppm (∆ν1/2
=
=
358 Hz). 51V NMR (105 MHz, CDCl3): δ = –483 ppm (∆ν1/2
148 Hz). Selected IR data (KBr): ν = 3430 (br., O–H), 1605 (s,
˜
CH=N–N=CH), 973 (s, V=O) cm–1. UV/Vis (CH3CN solution):
λmax (ε)
= 241 (43,200), 273 (20,100), 321 (9100), 540 nm
(6600 cm–1 –1).
Electrochemical Measurements: Cyclic square-wave voltammetric
measurements have been carried out by a three-electrode technique
using a home-built computer-controlled instrument based on the
PCI 6110-E data acquisition board (National Instruments). The
experiments were performed in acetonitrile solutions containing
0.25  tetra-n-butylammonium hexafluorophosphate under a blan-
ket of solvent saturated with argon. The ohmic resistance that had
to be compensated for was determined by measuring the impedance
of the system at potentials where the faradaic current was negligi-
ble. Background corrections were applied only for cyclic voltammo-
grams by subtracting the current curves of the blank electrolyte
containing the same concentration of the supporting electrolyte
from the experimental cyclic voltammograms. Ag/AgCl was used
as the reference electrode in acetonitrile containing 0.25  tetra-n-
butylammonium chloride. All potentials reported in this paper re-
fer to the ferrocenium/ferrocene couple, which has been measured
at the end of each experiment. The working electrode was a hang-
ing mercury drop (mHg–drop ≈ 4 mg) generated by a CGME instru-
ment from Bioanalytical Systems Inc., West Lafayette, USA. Theo-
retical square-wave voltammograms were simulated using the Digi-
Elch simulation software developed in our group (available via
www.DigiElch.de).
Table 3. Crystallographic data and structure refinement parameters
for [VO(salhyhb)(hq)].
Empirical formula
C20H18N3O5V
Formula mass [g/mol]
Crystal size [mm]
Crystal system
Space group
a [pm]
b [pm]
c [pm]
431.31
0.3ϫ0.3ϫ0.2
triclinic
¯
P1
1010.08(2)
1042.23(3)
1779.17(6)
88.9150(10)
89.170(2)
88.790(2)
1.87206(9)
183(2)
α [°]
β [°]
γ [°]
V [nm3]
T [K]
Z
4
ρcalcd. [g/cm3]
1.530
0.569
µ [mm–1]
θ range [°]
2.78–27.49
8519
6635
531
1.029
0.0401
0.0996
Unique data
Observed data [IϾ2σ(I)]
Parameters
Goodness-of-fit on F2
R1 for observed data
wR2 for all data
MAS NMR Spectroscopy and Data Evaluation: Solid-state 51V
MAS NMR experiments were performed at a resonance frequency
of 105.19 MHz (9.4 T) with a Bruker AMX 400 spectrometer. The
measurements were performed at room temperature, although the
2358
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2350–2359

Mixed-Ligand Vanadium(V) Complexes
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a mixture of methanol and acetonitrile (1:1) afforded violet needle-
like crystals suitable for X-ray measurements. The crystallographic
data were collected with a Nonius KappaCCD diffractometer using
graphite-monochromated Mo-Kα radiation (71.073 pm). A sum-
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Received: January 20, 2008
Published Online: April 11, 2008
Eur. J. Inorg. Chem. 2008, 2350–2359
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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