Dioxovanadium(V) complexes of ONO donor ligands derived from pyridoxal and hydrazides: Models of vanadate-dependent haloperoxidases

Article abstract of DOI:10.1002/ejic.200400211

[VO(acac)₂] reacts with H₂L [H₂L are the hydrazones H₂pydx-inh (I), H₂pydx-nh (II), or H ₂pydx-bhz (III); pydx = pyridoxal, inh = isonicotinohydrazide, nh = nicotinohydrazide, bhz = benzohydrazide] in dry methanol to yield the oxovanadium(IV) complexes [VOL] (H₂L = I: 1; H₂L = II: 4) or [VO(pydx-bhz)]. These complexes, when exposed to air, convert into the corresponding dioxovanadium(V) complexes [VO₂HL] (H₂L = I: 2; H₂L = II: 5; H₂L = III: 7). Aqueous solutions of vanadate and the ligands at pH = 7.5 give rise to the formation of [K(H ₂O)₃][VO₂(pydx-inh)] (3), [K(H ₂O)₂][VO₂(pydx-nh)] (6) and [K(H ₂O)₂][VO₂(pydx-bhz)] (8). Treatment of 6 and 8 with H₂O₂ generates the oxo(peroxo)vanadium complexes [VO(O₂)L] (H₂L = II: 9; H₂L = III: 10). Complexes 9 and 10 are capable of transferring an oxo group to PPh₃. Acidification of 8 with HCl afforded a hydroxo(oxo) complex. The crystal and molecular structures of ligand I and complex 3 have been solved by single-crystal X-ray diffraction. In the anion 3, the vanadium atom is in a distorted tetragonal-pyramidal environment (τ = 0.23). The K⁺ ion is coordinated to four water molecules (two of which bridge to a neighbouring K⁺ ion), the pyridine nitrogen atom of an isonicotinic moiety, the equatorial oxo group of the VO₂⁺ fragment, and the alcoholic group of the pyridoxal moiety, which links adjacent layers in the three-dimensional lattice network. In the presence of KBr/H₂O ₂, the anionic complexes 3, 6 and 8 catalyse the oxidative bromination of salicylaldehyde in water to 5-bromosalicylaldehyde in ca. 40% yields with ca. 87% selectivity. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejic.200400211

FULL PAPER
Dioxovanadium(
V
) Complexes of ONO Donor Ligands Derived from Pyridoxal
and Hydrazides: Models of Vanadate-Dependent Haloperoxidases
Mannar R. Maurya,*[a] Shalu Agarwal,[a] Cerstin Bader,[b] and Dieter Rehder*[b]
Keywords: Vanadium / Hydrazones / Catalytic bromination / Pyridoxal
[VO(acac)2] reacts with H2L [H2L are the hydrazones
H2pydx-inh (I), H2pydx-nh (II), or H2pydx-bhz (III); pydx =
The crystal and molecular structures of ligand I and complex
3 have been solved by single-crystal X-ray diffraction. In the
pyridoxal, inh
= isonicotinohydrazide, nh = nicotinohy- anion 3, the vanadium atom is in a distorted tetragonal-pyr-
drazide, bhz = benzohydrazide] in dry methanol to yield the
oxovanadium(IV) complexes [VOL] (H2L = I: 1; H2L = II: 4)
or [VO(pydx-bhz)]. These complexes, when exposed to air,
amidal environment (τ = 0.23). The K+ ion is coordinated to
four water molecules (two of which bridge to a neighbouring
K+ ion), the pyridine nitrogen atom of an isonicotinic moiety,
the equatorial oxo group of the VO2+ fragment, and the alco-
holic group of the pyridoxal moiety, which links adjacent
layers in the three-dimensional lattice network. In the pres-
ence of KBr/H2O2, the anionic complexes 3, 6 and 8 catalyse
the oxidative bromination of salicylaldehyde in water to 5-
bromosalicylaldehyde in ca. 40% yields with ca. 87% select-
ivity.
convert into the corresponding dioxovanadium(V) complexes
[VO2HL] (H2L = I: 2; H2L = II: 5; H2L = III: 7). Aqueous solu-
tions of vanadate and the ligands at pH = 7.5 give rise to the
formation of [K(H2O)3][VO2(pydx-inh)] (3), [K(H2O)2][VO2-
(pydx-nh)] (6) and [K(H2O)2][VO2(pydx-bhz)] (8). Treatment
of 6 and 8 with H2O2 generates the oxo(peroxo)vanadium
complexes [VO(O2)L] (H2L = II: 9; H2L = III: 10). Complexes
9 and 10 are capable of transferring an oxo group to PPh3.
Acidification of 8 with HCl afforded a hydroxo(oxo) complex.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
HalϪ ϩ H2O2 ϩ RH ϩ Hϩ Ǟ RHal ϩ 2 H2O
(1)
Introduction
Recent interest in vanadium coordination chemistry[1,2]
with ON oligodentate ligands arises from the potential of
these complexes as insulin-enhancing or insulin-mimetic ag-
ents,[3] their model character for vanadate-dependent halo-
peroxidases occurring in fungi and marine algae[4] and, in
relation to these enzymes, their use in oxo-transfer catalysis
We have previously reported on dioxovanadium(v) com-
plexes with ONO, NNO and NNS functional ligands, de-
rived from hydrazones based on the carbonyl components
salicylaldehyde or 2-acetylpyridine, and the hydrazides of
isonicotinic or benzoic acid, or S-benzyldithiocarbazate,[11]
and reduced Schiff bases derived from salicylaldehyde and
various amino acids.[12] Anionic and neutral cis-dioxovana-
dium(v) complexes with tridentate (ONO) N-salicylidenehy-
drazide ligand systems as models for vanadate-dependent
haloperoxidases have recently been reviewed by Plass.[13]
The present work is an extension to hydrazones formed
from the biogenic carbonyl constituents pyridoxal (vitamin
B6; HpydxOH), and the hydrazides of nicotinic acid (H2nh),
isonicotinic acid (H2inh) or benzoic acid (H2bhz; see
Scheme 1), which reveals novel structural features, and reac-
tivity patterns that model the haloperoxidase activity.
and oxidative halogenation. Vanadate-dependent halo-
2Ϫ
peroxidases contain VO(OH)O2
(HVO42Ϫ) coordinated
to the Nε of a histidine in an overall trigonal-bipyramidal
coordination.[5] These enzymes, and models of their active
centre, catalyse the oxidation of halides by peroxide to hy-
pohalous acid [which further halogenate non-enzymatically
hydrocarbons; Equation (1)],[6] and the oxidation of organic
(prochiral) sulfides to (chiral) sulfoxides.[7,8] Peroxo and
hydroperoxo complexes have been proposed to act as the
active intermediates.[6,9] The peroxo form, containing per-
oxovanadate HVO3(O2)2Ϫ attached to histidine, and in a
tetragonal-pyramidal geometry, has been structurally
characterised.[10]
Results and Discussion
[a]
Department of Chemistry, Indian Institute of Technology
Scheme 2 provides an overview of the complexes reported
in this contribution. Structures of these complexes are
based on spectroscopic (IR, UV/Vis, EPR, 1H and 51V
NMR) data, thermogravimetric studies, elemental analyses
and X-ray diffraction analyses of the ligand H2pydx-inh (I)
and complex 3.
Roorkee,
Roorkee 247667, India
E-mail: rkmanfcy@iitr.ernet.in
[b]
Institut für Anorganische und Angewandte Chemie,
Universität Hamburg,
Martin-Luther-King-Platz 6, 20146 Hamburg, Germany
E-mail: dieter.rehder@chemie.uni-hamburg.de
Eur. J. Inorg. Chem. 2005, 147Ϫ157
DOI: 10.1002/ejic.200400211
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
147

M. R. Maurya, S. Agarwal, C. Bader, D. Rehder
FULL PAPER
solution prior to aeration. Equations (2) and (3) represent
the synthetic procedures.
(2)
(3)
Similarly, the complex [VO(pydx-nh)] (4), can be oxidised
in methanol to [VO2(Hpydx-nh)] (5). Further, complexes 2
and 5 were obtained from the reaction of potassium vana-
date (which, at ambient pH, is actually present as a mixture
of vanadates; vide infra), generated in situ by dissolving
V2O5 in an aqueous solution of KOH, with solutions of the
potassium salts of ligands I or II and adjustment of the pH
of the reaction mixture to 6.0. Adjustment of the pH of
these solutions to 7.5 results in the formation of a mixture
of [K(H2O)3][VO2(pydx-inh)] (3) and 2 [Equation (4)], or
[K(H2O)2][VO2(pydx-nh)] (6) and 5, respectively. The final
pH of the reaction mixture plays an important role in that
a decrease in pH (e.g. to pH ϭ 6.5) increases the yield of
the neutral complexes 2 or 5. This pH dependence is poss-
ibly due to the actual vanadate species present in solution:
Scheme 1. Tautomerism shown for I only
Ϫ
2Ϫ
At pH ϭ 6, H2VO4 is in equilibrium with H2V2O7
,
5Ϫ
V4O124Ϫ(dominant species), V5O15 and V10O286Ϫ; at
pH ϭ 7.5, decavanadate disappears, and monovanadate
(again in equilibrium with divanadate, tetravanadate as the
main species, and pentavanadate) is present in the mono-
Ϫ
and diprotonated forms (the pKa for H2VO4 is around
Scheme 2. 1 and 4 are idealised structural units; cf. text
8.1).[18] The two complexes 2 and 3, or 5 and 6, can be
separated by fractional crystallisation from methanol,
where the neutral complex crystallises first. Complexes 7
Synthesis and General Characteristics
Reaction between equimolar amounts of [VO(acac)2] and and 8 were prepared similarly from [VO(acac)2] and
ligand I or II (Scheme 1) in dry, refluxing methanol gave the ‘‘KVO3’’, respectively. Addition of H2O2 to the methanolic
oxovanadium(iv) complex [VO(pdyx-inh)] (1) or [VO(pdyx- solution of 6 and 7 yields the oxomono(peroxo)vanadi-
nh)] (4), respectively. According to elemental analyses and um(v) complexes 9 and 10 [Equation (5)].
thermogravimetric studies, there is no coordinated ad-
ditional ligand such as water or methanol present in the
isolated, solid compounds; 1 and 4 exhibit a magnetic mo-
(4)
ment, at ambient temperature of 1.32 and 1.41 µB, respec-
tively; the spin-only value for a d1 system is 1.73 µB. The
magnetic data can be interpreted in terms of antiferromag-
netic exchange interaction,[14] implying close contacts be-
tween the monomers or the formation of dinuclear com-
plexes such as [{VO(L)}2µ-O]. Although dinuclear com-
plexes of this type are well documented,[15] the size of the
magnetic moments and the low wave number for the VϭO
stretch (888 cmϪ1) suggest intermolecular interactions of
the kind (L)VϭO···VO(L),[16] and thus an effective coordi-
nation number of 5. Anisotropic EPR data of DMSO solu-
tions (see Exp. Sect.) correspond to those expected for the
equatorial donor set shown for 1 and 4 in Scheme 2, i.e.
[VO2L]Ϫ ϩ H2O2 Ǟ [VO(O2)L]Ϫ ϩ H2O
(H2L ϭ II: 9; H2L ϭ III: 10)
(5)
Potassium vanadate reacts with 30% aqueous H2O2 to
generate K[VO(O2)2(OH/H2O)x]nϪ. In the presence of the
potassium salt of ligand III, the peroxo complex
K[VO(O2)(pydx-bhz)] (10) forms; see idealised Equa-
tion (6).
KH2VO4 ϩ K2pydx-bhz ϩ H2O2 Ǟ
K[VO(O2)(pydx-bhz)] ϩ 2 KOH ϩ H2O
(6)
O
phenolateNimineOenolate, plus a relatively weakly bonding
equatorial ligand such as water or DMSO.[17]
The peroxo complex of ligand I could not be isolated in
On aerial oxidation of 1 in methanol, the dioxovanadi- the solid state by any of the above methods due to its insta-
um(v) complex [VO2(Hpdyx-inh)] (2) was obtained. The in- bility at ambient temperature. The formation of a peroxo
termediate complex 1 can be isolated from the methanolic complex in solution by treatment of 8 with H2O2 has also
148
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Models of Vanadate-Dependent Haloperoxidases
FULL PAPER
˚
been established by electronic absorption spectroscopy. The
spectral changes are depicted in Figure 1. The band for
[VO2(pydx-bhz]Ϫ (8) at 404.5 nm shifts to 424 nm along
with an increase in intensity on dropwise addition of H2O2,
while the band at 329 nm only marginally shifts to 333 nm
with partial reduction in intensity. The amount of peroxo
complex formed depends upon the amount of H2O2 added.
The final spectral pattern is similar to that obtained for the
isolated peroxo complex 10 in methanol.
Table 1. Bond lengths [A] and angles [°] for I and 3 (symmetry
transformations used to generate equivalent atoms: #1: Ϫx, Ϫy ϩ
1, Ϫz; #2: x, y ϩ 1, z; #3: x, y Ϫ 1, z; #4: x, y, z ϩ 1)
I
3
V1ϪO1
V1ϪO2
V1ϪO3
V1ϪO4
V1ϪN3
K1ϪO2
K1ϪO5
K1ϪO6
1.6318(12)
1.6332(12)
1.9906(11)
1.8886(11)
2.1233(13)
2.8309(12)
2.7184(13)
2.8867(13)
2.9463(13)
2.7893
K1ϪO7
K1ϪO8
K1ϪO8#
K1ϪN1
K1···K1#
O3ϪC6
2.8444(14)
2.8076(14)
3.8176
1.3053(18)
1.302(2)
O1ϪC6
N2ϪC6
N2ϪN3
N3ϪC7
O2ϪC9
C12ϪC13
O3ϪC13
1.223(3)
1.363(3)
1.362(3)
1.281(3)
1.354(3)
1.506(3)
1.428(3)
N2ϪC6
N2ϪN3
N3ϪC7
O4ϪC12
C9ϪC14
1.3969(18)
1.2984(19)
1.3185(18)
1.510(2)
1.427(2)
138.99(6)
152.65(5)
73.91(5)
82.40(5)
133.64(7)
156.19(4)
128.30(10)
85.31(4)
110.29(13)
123.48(14)
108.45(12)
124.02(14)
O5ϪC14
O2ϪV1ϪN3
O3ϪV1ϪO4
O3ϪV1ϪN3
O4ϪV1ϪN3
V1ϪO2ϪK1
O2ϪK1ϪO5
K1ϪO5ϪC14
K1ϪO8ϪK1#
C9ϪC14ϪO5
O3ϪC6ϪN2
C6ϪN2ϪN3
N3ϪC7ϪC8
Figure 1. Titration of [K(H2O)2][VO2(pydx-bhz)] (8) with 30%
H2O2; the spectra were recorded after successive addition of 2-drop
portions of H2O2 to 10 mL of a ca. 10Ϫ4 m solution of 8 in MeOH
C12ϪC13ϪO3
O1ϪC6ϪN2
C6ϪN2ϪN3
N3ϪC7ϪC8
113.5(2)
122.7(2)
116.0(2)
117.7(2)
The peroxo complexes 9 and 10 undergo oxygen transfer
reactions with PPh3 in methanol to give the corresponding
dioxovanadium(v) complexes 6 and 8 [Equation (7)].
[VO(O2)L]Ϫ ϩ PPh3 Ǟ [VO2L]Ϫ ϩ OPPh3; (H2L ϭ I or III)
(7)
prevails. This is further confirmed by an intermolecular hy-
drogen bond N2H···O3HCH2 (1.970 A). Additional close
˚
intermolecular contacts exist between adjacent molecules
via C3H and O1 (2.575 A) of the isonicotinic acid moieties,
˚
Structure Description
as well as isonicotinic N1 and pyridoxal O3HCH2 (2.749
A).
An ORTEP plot and cell drawing of the ligand H2pydx-
inh (I) along with the atom-labelling scheme is presented in
Figure 2 and selected structure parameters in Table 1. The
bond parameters are well within the expected range. From
˚
Figure 3 shows an ORTEP plot and a schematic drawing
for the vanadium and potassium coordination environ-
ments of 3; Figure 4 is a representation of the 3D arrange-
ment of the molecular units. Selected structure parameters
are collated in Table 1. The geometry of the anion can be
described in terms of a tetragonal pyramid, distorted
towards a trigonal bipyramid. The τ value {[Є(O3ϪVϪO4)
Ϫ Є(O2ϪVϪN3)]/60} is 0.23 (τ ϭ 0 vs. 1 for ideal tetrag-
onal-pyramidal vs. trigonal-bipyramidal arrangements), re-
flecting a common situation encountered with pentacoordi-
nate oxovanadium complexes. For rare examples of a dis-
torted trigonal-bipyramidal arrangement, see ref.[19] The
cis-dioxovanadium unit is coordinated through the phenol-
ate oxygen atom O4 of pyridoxal, the imine nitrogen atom
N3, and the enolate oxygen atom O3 of the isonicotinic acid
hydrazone. Together with the doubly bonded O2, bridging
to the potassium ion, these functions form the tetragonal
plane. The angle at O2 is 133.67(7)°. The bond lengths
˚
the bond lengths d(C6ϪO1) [1.223(2) A] and d(C6ϪN2)
˚
[1.363(3) A], and the angles at C6 (av.120.0°) and N2
[116.0(2)°] it is clear that in the free ligand the ketonic form
Figure 2. ORTEP plot (at the 50% probability level) of the ligand
H2pydx-inh (I)
Eur. J. Inorg. Chem. 2005, 147Ϫ157
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M. R. Maurya, S. Agarwal, C. Bader, D. Rehder
FULL PAPER
Figure 3. ORTEP plot (30% probability level) of [K(H2O)3](VO2(pydx-inh)] (3), and a schematic drawing of the vanadium and potassium
environments; bold parts refer (to connections) to planes above and below
Thermal Studies
The complexes [VO(pydx-inh)] (1) and [VO(pydx-nh)] (4)
lose about 75% of their mass between 250 and 450 °C in
two overlapping steps, which corresponds to the loss of the
organic components minus 1.5 oxygen atoms per molecule
(3 per two molecules) (calculated mass loss: 74.1%). Conse-
quently, the remaining product is V2O5. The TGA profiles
of the dioxovanadium(v) complexes [K(H2O)3][VO2(pydx-
inh)] (3), [K(H2O)3][VO2(pydx-nh)] (6) and [K(H2O)3][VO2-
(pydx-bhz)] (8) show that these complexes contain three (3)
Figure 4. Section from the crystal lattice of 3, showing the supra-
or two (6 and 8) water molecules per formula unit. The loss
of this water in the temperature range 95Ϫ220 °C is indica-
tive of coordinated water. On further increasing the tem-
perature, the water-free species K[VO2(pydx-inh)] and
K[VO2(pydx-nh)] decompose in one step between 220 and
350 °C to form KVO3. The total loss corresponds to the
loss of ligand minus 1 oxygen atom. A mass loss of 8.6%,
equivalent to two water molecules (calcd. 8.93%) for 7 in
the temperature range 80Ϫ135 °C, suggests that this is just
water of crystallisation. Complexes 2, 5 and the water-free
form of 7 have decomposition patterns similar to those of
the respective forms of 3, 6, and 8 after loss of water. They
all yield V2O5 as the final product.
molecular arrangement
d(VϪN3) [2.1233(13)], d(VϪO3) [1.9906(11)] and d(VϪO4)
˚
[1.8886(11) A] are in agreement with literature values for
imine, enolate and phenolate coordinating to the vanadium
centre. Further, the bond lengths d(N2ϪN3) [1.3969(18)],
˚
d(N2ϪC6) [1.302(17)] and d(O3ϪC6) [1.3053(18) A] are
consistent with the enolate mode of coordination, sup-
ported by a comparison with the respective parameters for
the free ligand (Table 1), in which the carbonyl form is pre-
sent. The apical VϭO bond, d(VϪO1) ϭ 1.6318(12) is in
the expected range, while the basal VϭO bond, d(VϪO2) ϭ
˚
1.66332(12) A, is slightly elongated as a consequence of co-
IR Spectroscopic Studies
valent bonding contact to the Kϩ ion.
The Kϩ ion links three complex anions through the basal
The IR spectra of the ligands exhibit two bands at 3230
ϩ
oxo group of VO2 (O2), the pyridine nitrogen atom of an and 1678 [H2pydx-inh (I)], 3250 and 1673 [H2pydx-nh (II)]
isonicotinic acid moiety (N1) and the alcoholic oxygen and 3210 and 1677 cmϪ1 [H2pydx-bhz (III)] due to ν(NH)
atom of a pyridoxal moiety (O5). In addition, four water and ν(CϭO) stretches, respectively, indicative of their ke-
molecules (three per molecular unit) are coordinated to Kϩ, tonic nature in the solid state; cf. also Figure 1. The absence
resulting in the coordination number 7 for each potassium of these bands in the spectra of all complexes is consistent
ion. In the dinuclear rhombohedral {K2(µ-OH2)2} core, the with enolisation and replacement of H by the metal ion. A
angles are 85.31(4) at O8 and 94.69(4)° at K1; d(K1···K1#) new band appearing in the region 1220Ϫ1262 cmϪ1 is as-
˚
amounts to 3.818 A. O5 on K1 and O5# on K1# link to signed to the ν(CϪOenolic) mode. The ν(CϭNazomethine
)
adjacent planes. Supramolecular links are further estab- stretch of the free ligands appears as a weak band at
lished by one of the terminal water molecules, which is hy- 1617Ϫ1637 cmϪ1 along with the ν(CϭN) stretches of the
drogen-bonded to the ring nitrogen atom of the pyridoxal pyridine rings. A very sharp band at 1596Ϫ1608 cmϪ1 in
˚
moiety, N4···H2O6 ϭ 2.848 A.
the complexes is indicative of the coordination of the azo-
For the structures of the related complexes 2, 5 and 7, 6 methine nitrogen atom. A ligand band appearing at 1017
and 8, and the peroxo complexes 9 and 10, we assume a (I), 1010 (II) and 1028 cmϪ1 (III) due to ν(NϪN) undergoes
ligand arrangement corresponding to that in 3; cf. a shift to higher wave numbers by 5Ϫ45 cmϪ1 upon com-
Scheme 2.
plex formation. The high frequency shift of the ν(NϪN)
150
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Models of Vanadate-Dependent Haloperoxidases
FULL PAPER
band is expected because of diminished repulsion between
the lone pairs of adjacent nitrogen atoms.[20] The ligands
exhibit a medium-intensity ν(OH) band covering the region
2300Ϫ2700 cmϪ1, which is due to intramolecular hydrogen
bonds. On complexation, this band broadens and gains
intensity due to the involvement of the CH2OH group in
hydrogen bonding. All of the dioxovanadium(v) complexes
exhibit two or three sharp bands in the 885Ϫ950 cmϪ1 re-
gion, corresponding to the cis-[VO2]ϩ structural unit. The
peroxo complexes 9 and 10 show three IR-active vibration
modes associated with the peroxo moiety [V(O2)3ϩ] at
894Ϫ917, 707Ϫ717 and 553Ϫ574 cmϪ1, which are assigned
to the OϪO intra-stretch (ν1), the antisymmetric V(O2)
stretch (ν3), and the symmetric V(O2) stretch (ν2).[21] The
presence of these bands confirms the common η2-coordi-
nation of the peroxo group.[21] In addition, both peroxo
Table 2. Electronic absorption spectra (in methanol, if not indi-
cated otherwise)
Compound
λmax. [nm] (ε [ mϪ1·cmϪ1])
H2pydx-inh (I)
216 (18541), 285.5 (18010), 342 (8270),
411.5 (2085)
[VO2(Hpydx-inh)] (2) in DMF
[VO2(Hpydx-inh)] (2)
272 (8881), 340 (7659), 421 (5769)
273 (19403), 308 (16178), 343 (16704),
416 (5073)
[K(H2O)3][VO2(pydx-inh)] (3)
H2pydx-nh (II)
[VO2(Hpydx-nh)] (5)
269.5 (18115), 337 (15146), 418 (11475)
211 (23417), 307 (23898), 342 (17817)
235 (23118), 279 (16339), 328 (16322),
415 (4644)
[K(H2O)2][VO2(pydx-nh)] (6)
K[VO)(O2)(pydx-nh)]·H2O (9)
H2pydx-bhz (III)
234 (21351), 280 (16569), 326 (4384),
416 (4453)
225 (19005), 297 (15205), 308 (14622),
407 (8579)
206 (20330), 296.5 (18113), 306 (17616),
335 (11085), 406 (2106)
complexes exhibit an intense ν(VϭO) at 946Ϫ970 cmϪ1
.
[VO2(Hpydx-bhz)] (4) in DMF
[VO2(Hpydx-bhz)] (4)
[K(H2O)2][VO2(pydx-bhz)] (8)
270 (31939), 337 (26968), 414 (18638)
272 (9358), 334 (5324), 407 (3164)
237 (18304), 265 (19310), 333 (15110),
404.5 (5381)
Electronic Spectra
The absorption maxima of the ligands and complexes
along with their extinction coefficients are listed in Table 2.
The UV spectra of H2pydx-inh (I) shows three absorption
bands at 216, 285.5 and 342 nm, while H2pydx-bhz (III)
K[VO(O2)(pydx-bhz)]·H2O (10)
266 (15573), 329 (11810), 424 (10282)
displays four bands at 206, 296.5, 306 and 335 nm, probably traced back to association by hydrogen bonding. All of the
belonging to the transitions ϕ Ǟ ϕ*, π Ǟ π* and n Ǟ π*, complexes invariably showed this band, indicating the exist-
with the π Ǟ π* band split into two components in the case ence of hydrogen bonding in these complexes in solution
of III. A weak shoulder associated with the second band is as well.[22] The ϕ Ǟ ϕ* transition is only observed in the
1
Table 3. H NMR spectroscopic data
Compound[a][b]
OH (phenolic)
13.40 (br, 1 H)
ϪCHϭNϪ
ϪCH2Ϫ
ϪCH3
Aromatic H
H2pydx-inh (I)
8.15 (s,1 H)
9.48 (s, 1 H)
4.76 (s, 2 H)
4.93 (s, 2 H)
2.60 (s, 3 H)
2.63 (s, 3 H)
9.13 (s, 1 H), 8.83 (d, 2 H), 7.99
(d, 2 H)
8.90 (br, 2 H), 8.28 (s, 1 H),
8.07 (d, 2 H)
[VO2(Hpydx-inh)] (2)
(∆δ)
(1.33)
9.10 (s, 1 H)
[K(H2O)3][VO2(pydx-inh)] (3)
4.58 (s, 2 H)
4.76 (s, 2 H)
4.83 (s, 2 H)
4.89 (s, 2 H)
2.30 (s, 3 H)
2.60 (s, 3 H)
2.51 (s, 3 H)
2.57 (s, 3 H)
8.52 (br, 2 H), 7.91 (s, 1 H),
7.75 (d, 2 H)
(∆δ)
H2pydx-nh (II)
(0.95)
8.15 (s, 1 H)
13.29 (br, 1 H)
9.17 (s, 1 H), 9.07 (s, 1 H), 8.81
(d, 1 H), 8.42 (d, 1 H), 7.63
(m, 1 H)
8.74 (br, 1 H), 8.38 (d, 1 H),
7.89 (s, 1 H), 7.55 (m, 1 H)
[VO2(Hpydx-nh)] (5)
9.32 (s, 1 H)
(∆δ)
(1.17)
9.33 (s, 1 H)
[K(H2O)2][VO2(pydx-nh)] (6)
9.22 (s, 1 H), 8.74 (d, 1 H), 8.40
(d, 1 H), 7.97 (s, 1 H), 7.56
(m, 1 H)
(∆δ)
H2pydx-bhz (III)
(1.18)
8.14 (s, 1 H)
13.14 (br, 1 H)
4.76 (s, 2 H)
4.85 (s, 2 H)
4.90 (s, 2 H)
4.89 (s, 2 H)
2.60 (s, 3 H)
2.56 (s, 3 H)
2.58 (s, 3 H)
2.55 (s, 3 H)
9.09 (s, 1 H), 8.04 (d, 2 H),
7.54 (m, 3 H)
8.04 (m, 3 H), 7.49 (m, 3 H)
[VO2(Hpydx-bhz)] (7)
(∆δ)
[K(H2O)2][VO2(pydx-bhz)] (8)
(∆δ)
K[VO(O2)(pydx-bhz)H2O] (10)
(∆δ)
9.28 (s, 1 H)
(1.14)
9.29 (s, 1 H)
(1.15)
9.28 (s, 1 H)
(1.14)
Ϫ
Ϫ
Ϫ
8.10 (m, 3 H), 7.50 (m, 3 H)
8.14 (m, 3 H), 7.52 (m, 3 H)
[a]
[b]
Letters given in parentheses indicate the signal structure: s ϭ singlet, d ϭ doublet, br ϭ broad (unresolved), m ϭ multiplet. ∆δ ϭ
δ(complex) Ϫ δ(ligand).
Eur. J. Inorg. Chem. 2005, 147Ϫ157
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151

M. R. Maurya, S. Agarwal, C. Bader, D. Rehder
FULL PAPER
oxovanadium(iv) complexes; the two other bands are signif- complexes 7 and 8 is commonly observed as an oxo group
icantly shifted towards lower wavelengths with respect to is replaced by the side-on coordinated peroxo group.[23,25]
the uncoordinated ligands. The dioxovanadium(v) com-
Reaction with HCl
plexes are dominated by an intense band at 404.5Ϫ421 nm,
which is assigned to a ligand-to-metal charge transfer
(LMCT) from the phenolate oxygen atom to an empty d-
orbital of the vanadium ion.
For the catalytic activity of vanadate-dependent halo-
peroxidases, the presence of a coordinated hydroxo ligand
has been proposed on the basis of kinetic investigations.[6]
The generation of hydroxo(oxo) species has been ac-
complished for [K(H2O)2][VO2(pydx-bhz)]·H2O (8) on reac-
tion with HCl: Addition of HCl-saturated methanol to a
methanolic solution of 8 results in a colour change from
yellow to orange with gradual shift of the 404.5-nm band
in the electronic absorption spectrum to 420 nm, along with
a slight broadening and decrease in intensity of the absorp-
tion maximum (Figure 5). Addition of further HCl results
in an increase in intensity of the 333-nm band, disappear-
ance of the 265-nm band, and appearance of two new bands
at 317 and 308 nm. In addition, a shoulder starts to appear
at ca. 360 nm. The intensity and position of the 237-nm
band remain constant. Corresponding results have been ob-
tained with HClO4 (dissolved in a minimum amount of
methanol and added dropwise to a methanolic solution of
8). We interpret this result in terms of the formation of an
hydroxo(oxo) complex of composition [VO(OH)(H2pydx-
bhz)]2ϩ via [VO2(Hpydx-bhz)] and [VO2(H2pydx-bhz)]ϩ on
acidification as shown in Equation (8). The formation of
the intermediate species, protonated at the pyridine nitrogen
atom, viz. [VO2(Hpydx-bhz)], is based on the fact that the
electronic absorption spectrum of a solution of 8 obtained
after addition of 4 drops of HCl nearly matches the spec-
trum of authentic complex 7. The protonation of the N
atom of the hydrazide moiety not involved in coordination
is documented by a newly arising medium-intensity band
in the IR at 3205 cmϪ1 (free Schiff base: 3210 cmϪ1) on
acidification of 8. A comparable protonated Schiff-base
complex, viz. [VO2(Hsal-bhz)] (where H2sal-bhz is the hy-
drazone derived from salicylaldehyde and benzohydrazide)
has been structurally characterised.[26]
NMR Spectroscopic Studies
The coordinating modes of the ligands were confirmed
by comparing 1H NMR patterns of the ligands and the
complexes. The relevant spectroscopic data are collected in
Table 3. The broad signal appearing at
δ
ϭ
13.14Ϫ13.29 ppm, due to the phenolic OH group, disap-
pears in the spectra of the complexes. A significant down-
field shift (∆δ ϭ 0.84Ϫ1.33 ppm) of the signal for the azo-
methine (ϪCHϭNϪ) proton in the complexes relative to
the corresponding ligands demonstrates the coordination of
the azomethine nitrogen atom. The signals due to the NH
group (which is hydrogen-bonded to the alcoholic pyridoxal
OH group, viz. NH···OHCH2; see the structure description
for I above) and OH protons could not be located in the
δ ϭ 0Ϫ15 ppm region in the spectra of the ligands. How-
ever, appearance of a broad signal at δ ഠ 5.8 ppm in all
complexes, which we allocate to CH2OH, suggests the
breaking down of hydrogen bonding and coordination of
the enolate oxygen atom of the hydrazone moiety, further
supported by the absence of the NH proton signal in the
complexes. The methylene and methyl protons of the pyri-
doxal moiety of the ligands resonate at δ ϭ 4.76 and
2.60 ppm, and these signals appear in the complexes with
slight shifts in their positions. Aromatic protons appear in
the expected regions in the spectra of the ligands as well as
of the complexes with minor variations in their positions.
All these data are consistent with the conclusions drawn
from the IR spectral studies and support the dibasic, triden-
tate ONO coordination mode.
Further characterisation of the complexes was obtained
from 51V NMR spectra; values of specific compounds,
where solubility permitted data to be recorded, are pre-
sented in the Exp. Sect.. The resonances are somewhat
broadened due to quadrupolar interaction (51V: nuclear
spin ϭ 7/2, quadrupole moment ϭ Ϫ0.05 ϫ 10Ϫ28 m2); the
line widths at half-height are approximately 200 Hz, which
is still considered comparatively narrow in 51V NMR spec-
troscopy.[23] The dioxovanadium(v) complexes 3, 6 and 8
show one strong resonance between δ ϭ Ϫ532 and Ϫ534
ppm in [D6]DMSO, an expected value for dioxovanadi-
um(v) complexes having a mixed O/N donor set.[23,24] Com-
plex 8 exhibits distinct solvent dependence. In [D6]DMSO,
the 51V NMR signal appears at δ ϭ Ϫ534.2 ppm, while in
CH3OH/CD3OD it appears at δ ϭ Ϫ546.8, possibly indicat-
ing the participation of the solvent in coordination. Simi-
larly, the neutral dioxovanadium(v) complex 7 gives rise to
a signal at δ ϭ Ϫ535.7 ppm in [D6]DMSO and at δ ϭ
Ϫ546.7 ppm in CH3OH/CD3OD. The peroxo complex 10
displays a 51V NMR signal at δ ϭ Ϫ586.0 ppm in
Figure 5. Titration of [K(H2O)2][VO2(pydx-bhz)] (8) with a satu-
rated solution of HCl in MeOH; the spectra were recorded after
addition of 2-drops portions of MeOH/HCl to 10 mL of ca. 10Ϫ4
[D6]DMSO. This upfield shift with respect to the dioxo m solution of 8 in MeOH
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Models of Vanadate-Dependent Haloperoxidases
FULL PAPER
reaction time, but relatively fast decomposition of the com-
plexes took place.
(8)
(9)
Hydroxo(oxo)vanadium complexes have previously been
generated on acidification of K[VO2(sal-inh)H2O] and
[K(H2O)2][VO2(Clsal-sbdt)] (H2Clsal-sbdt ϭ ligand derived
from salicylaldehyde and S-benzyldithiocarbazate).[11] An
hydroxo(oxo) complex, [VO(OH)(LH)]ϩ {where LH ϭ N-
(2-hydroxyethyl)-NЈ-[(o-hydroxyphenyl)methyl]ethylene-
diamine}, has been reported to form from a dinuclear
As hydroxo(oxo) species such as [VO(OH)(H2pydx-
bhz)]2ϩ very likely form upon acidification of [K(H2O)2]-
[VO2(pydx-bhz)] (8), and oxo(peroxo) species 10 are ob-
tained upon treatment with H2O2 in methanol, the forma-
tion of an active oxo(peroxo) via a hydroxo(oxo) complex
is likely to occur in the presence of acid as well as H2O2
during the catalytic reaction.[4] Activation of peroxide by
coordination to vanadium may then be succeeded by for-
mation of an intermediate hydroperoxo complex, which al-
lows nucleophilic attack of the bromide,[21] followed by re-
lease of hypobromous acid which then brominates salicylal-
dehyde. A catalytic cycle is proposed in Scheme 3. It should
be noted here that recent electrospray ionisation MS studies
in the peroxovanadium/bromide system indicated the for-
mation of a hypobromite species, [VO(OH)(H2O)3OBr]ϩ,
the stability of which is supported by ab initio calcu-
lations.[32]
dioxovanadium(v) precursor in
a
similar manner.[27]
[VVO(OH)(8-oxyquinolinate)2][28a] and [VIVO(OH)Tp(H2O)]
[Tp ϭ tris(3,5-diisopropyl-1-pyrazolyl)borate(1Ϫ)][28b] have
been characterised in the solid state. On addition of a meth-
anolic solution of KOH to [VO(OH)(H2pydx-bhz)]2ϩ, the
solution acquired the original spectrum of 8; the reaction is
thus reversible. This reversibility is an important obser-
vation in the context of the active-site structure and the
catalytic activity of vanadate-dependent haloperoxidases,
for which a hydroxo ligand at the vanadium centre has also
been made plausible on the basis of X-ray diffraction
data.[29]
Oxidative Bromination of Salicylaldehyde
Oxidative bromination catalysed by V2O5 and oxovanadi-
um(v) complexes has been reported earlier using H2O2/
BrϪ.[30] During this process, vanadium reacts with 1 or 2
equiv. of H2O2, forming mono(peroxo) [VO(O2)ϩ] or bis-
(peroxo) [VO(O2)2Ϫ] species, which ultimately oxidise bro-
mide, possibly by formation of a hydroperoxo intermediate.
Ϫ
The oxidised bromine species (Br2, Br3 or, most likely,
HOBr) then brominates the substrate.[4,31]
We have observed that the complexes 3, 6 and 8 satisfac-
torily catalyse the oxidative bromination of salicylaldehyde,
using H2O2/KBr in the presence of HClO4 in aqueous solu-
tion; cf. Equation (9). A maximum conversion of 40Ϫ46%
of salicylaldehyde was found. In the case of 3 as the cata-
lyst, GC analysis revealed the presence of three products in
Scheme 3
Conclusion
Pentacoordinated neutral and cationic dioxovanadium
the ratio 8.8:86.6:4.3%, with 5-bromosalicylaldehyde being complexes containing a ligand system providing a dianionic
the major product. Thus, based on GC, the selectivity of ONO donor set and thus modelling the active site of
the formation of 5-bromosalicylaldehyde is 86.6% with re- vanadate-dependent haloperoxidases have been synthesised
spect to salicylaldehyde conversion. On purification of the and characterised. The cationic complexes of the general
crude product by column chromatography, 5-bromosalicyl- composition [K(H2O)n][VO2(ONO)] can be converted in
aldehyde was obtained in 40% yield (related to salicylal- situ on acidification to hydroxo(oxo) complexes
dehyde). The average yields of pure 5-bromosalicylaldehyde [VO(OH)(H2ONO)]2ϩ and to the peroxo complexes
for 6 and 8 as catalysts were 34.7% and 36.5%, respectively; K[VO(O2)(ONO)], and thus to species assumed to be inter-
the selectivity of formation of 5-bromosalicylaldehyde was mediates in the bromoperoxidase activity of the enzymes.
again about 87%. In the absence of the catalyst, the reaction The complexes are in fact active in bromination of salicylal-
mixture did not produce any brominated product. Similarly, dehyde in the presence of H2O2, BrϪ and acid, and hence
acid (here HClO4) was found to be essential to carry out may also be considered functional models. The ONO li-
catalytic bromination. The complexes slowly decompose gands employed are hydrazones containing pyridoxal (vi-
during the reaction; decomposition is slowed down, if tamin B6) and nicotinohydrazides as components, and thus
HClO4 is successively added in portions (see Exp. Sect.). biogenic molecular moieties.
Using CH3COOH as solvent as well as the acid supplier,
The complex [K(H2O)3][VO2(pydx-inh)] (where pydx-inh
efficiently produced 5-bromosalicylaldehyde within 2 h of is the doubly deprotonated condensation product from
Eur. J. Inorg. Chem. 2005, 147Ϫ157
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153

M. R. Maurya, S. Agarwal, C. Bader, D. Rehder
FULL PAPER
were usually recorded in [D6]DMSO, and δ(51V) values are quoted
relative to VOCl3 as external standard. Selected 51V NMR spectro-
scopic results have also been obtained in CD3OD. Thermogravi-
metric analyses of the complexes were carried out under oxygen
using a TG Stanton Redcroft STA 780 instrument. Magnetic sus-
ceptibility measurements of oxovanadium(iv) complexes were car-
ried out at room temperature by the Scientific Instrumentation
Centre of the Indian Institute of Technology in Roorkee. EPR spec-
tra were recorded with a Bruker ESP 300E spectrometer between
9.42 and 9.47 GHz, and EPR parameters were adjusted by simu-
lation with the Bruker program system SimFonia. All reaction
products obtained from the catalytically conducted reactions were
pyridoxal and isonicotinohydrazide), which has been struc-
turally characterised, contains the ligand in the enolate
form (the free ligand constitutes the ketonic form). The
seven-coordinate potassium ion links to three different
[VO2(pydx-inh)]Ϫ anions and thus generates, together with
hydrogen bonds, a complex supramolecular, three-dimen-
sional network.
Experimental Section
1
Materials and Instrumentation: V2O5, NH4VO3, isonicotinohydraz-
ide, benzoyl chloride, hydrazine hydrate (Loba Chemie, India), pyr-
idoxal hydrochloride (Fluka Chemie, GmbH, Switzerland), acetyl-
acetone (Hacac) (Aldrich, U.S.A.), and 30% aqueous H2O2 (Quali-
gens, India) were used as obtained. Other chemicals and solvents
were of analytical reagent grade. Benzohyrazide was prepared by
the reaction of a twofold excess of hydrazine hydrate with ethyl
benzoate, which in turn was obtained by refluxing benzoyl chloride
in an excess of absolute ethanol. [VO(acac)2] was prepared accord-
ing to the method reported in the literature.[33] The microanalytical
section of the Central Drug Research Institute, Lucknow, India,
performed elemental analyses of the ligands and complexes. IR
spectra were recorded as KBr pellets with a PerkinϪElmer model
1600 FT-IR spectrometer. Electronic absorption spectra were meas-
ured in methanol or DMF with a UV-1601 PC UV/Vis spectropho-
identified by recording their m.p., H NMR and IR spectra after
purification and separation by column chromatography on silica
gel using CH2Cl2 as an eluant. The product mixture obtained be-
fore purification was additionally analysed with a Shimadzu 14B
gas chromatograph, fitted with an SE-52 packed column, coupled
with an FID detector, and the identity of the products confirmed
by checking against the GC-MS reference system Shimadzu QP-
5000. Crystal structure data were collected with a Bruker SMART
Apex CCD diffractometer, using graphite-monochromated Mo-Kα
˚
radiation (λ ϭ 0.71073 A) at 153(2) K. In the case of ligand I, all
hydrogen atoms (except H3A) were placed into calculated positions
and included in the last cycles of refinement. H3A of ligand I and
all H atoms of complex 3 were found. The program systems
SHELXS 86 and SHELXL 93 were used throughout. Crystal data
and details of the data collection and refinement are collated in
Table 4. CCDC-233587 (I) and Ϫ233586 (3) contain the sup-
plementary crystallographic data of this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving/
1
tometer. H NMR spectra were obtained with a Bruker 200, and
51V NMR spectra with a Bruker Avance 400 MHz spectrometer at
94.73 MHz with the common parameter settings. NMR spectra
Table 4. Crystal and refinement data for complex 3 and ligand I
3
I
Empirical formula
C14H10KN4O8V
C14H15N4O3
287.30
monoclinic
P21/n
Formula mass [g·molϪ1
Crystal system
]
452.30
triclinic
Pi
Space group
Unit cell dimensions:
˚
a [A]
7.3320(4)
10.9605(6)
12.6229(7)
8.0828(5)
12.9614(9)
12.9926(8)
˚
b [A]
˚
c [A]
α [°]
β [°]
γ [°]
64.7210(10)
81.113(2)
89.366(2)
904.54(9)
2
1.661
0.830
456
90
90.5000(10)
90
1361.11(15)
3
1.397
0.102
600
3
˚
V [A ]
Z
Calculated density [g·cmϪ3
]
Absorption coefficient [mmϪ1
F(000)
]
Crystal size [mm]
θ range for data collection [°]
Index ranges
0.80 ϫ 0.22 ϫ 0.12
2.06 to 32.56
Ϫ10 Յ h Յ 10
Ϫ16 Յ k Յ 16
Ϫ19 Յ 1 Յ 19
24828
0.43 ϫ 0.29 ϫ 0.19
2.22 to 28.00
Ϫ10 Յ h Յ10
Ϫ16 Յ k Յ17
Ϫ17 Յ 1 Յ16
16031
Reflections collected
Independent reflections
Completeness to θ [°]
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I0)]
R indices (all data)
6380 [R(int) ϭ 0.0435]
97.0%
6380/0/293
3182 [R(int) ϭ 0.0409]
96.6%
3182/0/192
1.052
1.111
R1 ϭ 0.0400, wR2 ϭ 0.1178
R1 ϭ 0.0453, wR2 ϭ 0.1206
0.936/Ϫ0.439
R1 ϭ 0.0608, wR2 ϭ 0.1330
R1 ϭ 0.0818, wR2 ϭ 0.1523
0.631/Ϫ0.623
Ϫ3
˚
Largest difference peak/hole [e A
]
154
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Models of Vanadate-Dependent Haloperoxidases
FULL PAPER
html or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK [Fax: (internat.) ϩ 44-
1223-336-033; E-mail: deposit@ccdc.cam.ac.uk].
of the filtrate to ca. 15 mL and keeping it at ca. 10 °C, yellow
crystalline 3 slowly precipitated within 2 d. This was filtered off,
washed with cold methanol and dried. Yield 1.58 g (68%).
C14H18KN4O8V (460.4): calcd. C 36.52, H 3.91, N 12.17; found C
Preparation of Ligands
36.33, H 3.85, N 12.26. IR (KBr): ν˜max. ϭ 1596 (CϭN), 1257
ϩ
H2pydx-inh (I): A mixture of pyridoxal hydrochloride (1.02 g,
5 mmol) and isonicotinohydrazide (0.685 g, 5 mmol) in 50 mL of
methanol was refluxed using a water bath for 4 h. After reducing
the solvent volume to ca. 15 mL, the mixture was cooled to room
temperature within 3 h. During this time, a light orange solid of I
precipitated, which was filtered off, washed with methanol and
dried. I was recrystallised from methanol to give a crystalline solid.
Yield 1.15 g (81%). C14H14N4O3 (286.3): calcd. C 58.74, H 4.90, N
19.58; found C 58.69, H. 4.82, N 19.41. IR (KBr): ν˜max. ϭ 3230
(NH), 1678 (CϭO), 1620, 1600 (ring CϭN and CϭN), 1017
(CϪO, enolate), 1055 (NϪN), 955, 909 (sym. and antisym. VO2
)
cmϪ1 51V NMR ([D6]DMSO): δ ϭ Ϫ532.0 ppm.
.
[VO(pydx-nh)] (4) and [VO2(Hpydx-nh)] (5): Complex 4 was pre-
pared analogously to 1, replacing H2pydx-inh for H2pydx-nh. Air
was slowly passed through the methanolic suspension (50 mL) of
4 at ca. 40 °C for ca. 24 h with occasional shaking. After cooling
to ca. 10 °C for 2 d, yellow crystalline 5 slowly preciptated, which
was filtered, washed with methanol and dried in vacuo.
Data for 4: Yield 0.61 g (86.8%). C14H12N4O4V (351.2): calcd. C
47.86, H 3.42, N 15.95; found C 47.94, H 3.31, N 15.98. IR (KBr):
ν˜max. ϭ 1608 (CϭN), 1248 (CϪO, enolate), 1050 (NϪN), 888 (Vϭ
(NϪN) cmϪ1
.
H2pydx-nh (II) and H2pydx-bhz (III): These ligands were prepared
according to the procedure outlined for I.
O) cmϪ1. EPR (DMSO, 98 K): gxy ϭ 1.984, gz ϭ 1.942; Axy
ϭ
56.7, Az ϭ 161.2 ϫ 10Ϫ4 cmϪ1
.
II: Yield 1.12 g (78%). C14H14N4O3 (286.3): calcd. C 58.74, H 4.90,
N 19.58; found C 58.82, H 4.85, N 19.48. IR (KBr): ν˜max. ϭ 3250
Data for 5: Yield 0.40 g (54%) based on VO(acac)2. C14H13N4O5V
(368.2): calcd. C 45.65, H 3.53, N 15.22; found C 45.83, H 3.36, N
15.17. IR (KBr): ν˜max. ϭ 1597 (CϭN), 1234(CϪO, enolate), 1064
(NH), 1673 (CϭO), 1617 (ring CϭN, CϭN), 1025 (NϪN) cmϪ1
.
(NϪN), 961, 942, 884 (sym. and antisym. VO2ϩ) cmϪ1
.
III: Yield 1.07 g (75%). C15H15N3O3 (285.3): calcd. C 63.18, H 5.26,
N 14.74; found C 63.00, H, 5.34, N 14.68. IR (KBr): ν˜max. ϭ 3210
(NH), 1677 (CϭO), 1637, 1624 (ring CϭN, CϭN), 1045 (NϪN)
[K(H2O)2][VO2(pydx-nh)] (6): This complex was prepared from
KVO3 and H2pydx-nh by the method outlined for 3. The crude
mass obtained on crystallisation from methanol gave a 1.45 g (66%)
yield of 6. C14H16KN4O7V (417.3): calcd. C 38.01, H 3.65, N 12.67;
found C 38.20, H 3.74, N 12.56. IR (KBr): ν˜max. ϭ 1606 (CϭN),
1258 (CϪO, enolate), 1042 (NϪN), 940, 923 (sym. and antisym.
cmϪ1
.
Preparation of Complexes
[VO(pydx-inh)] (1) and [VO2(Hpydx-inh)] (2): A stirred solution of
H2pydx-inh (0.570 g, 0.002 mol) in dry methanol (20 mL) was
treated with [VO(acac)2] (0.530 g, 0.002 mol), dissolved in dry
methanol (10 mL), and the resulting reaction mixture was refluxed
using a water bath for 5 h. After cooling to room temperature, a
brown precipitate of 1 was filtered off, washed with methanol and
dried. Compound 1 was suspended in methanol (50 mL), and air
was slowly passed through the suspension at ca. 40 °C for ca. 24 h
with occasional shaking. During this period of time, the brown
suspension slowly disappeared and crystalline orange-red solid 2
separated. This was filtered off, washed with methanol and dried
in vacuo. For the direct preparation of 2, it is not necessary to
isolate 1.
VO2ϩ) cmϪ1 51V NMR ([D6]DMSO): δ ϭ Ϫ532.0 ppm.
.
[VO2(Hpydx-bhz)]·2H2O (7): A stirred solution of H2pydx-bhz
(0.570 g, 2 mmol) in methanol (20 mL) was treated with
[VO(acac)2] (0.530 g, 2 mmol), and the reaction mixture was re-
fluxed using a water bath for 5 h to give a brown solution. After
cooling to ambient temperature, a current of air was passed
through this solution for ca. 20 h; during this procedure, yellow
complex 7 slowly precipitated. This was filtered off, washed with
methanol and dried in vacuo. Yield 0.68 g (84%) based on
[VO(acac)2]. C15H18N3O7V (403.3): calcd. C 44.66, H 4.47, N
10.42; found C 44.75, H 4.36, N 10.28. IR (KBr): ν˜max. ϭ 1599
(CϭN), 1220 (CϪO, enolate), 1063 (NϪN), 940, 918, 888 (sym.
and antisym. VO2
([D6]DMSO); Ϫ546.4 ppm (MeOH/CD3OD).
Data for 1: Yield 0.35 g (50%). C14H12N4O4V (351.2): calcd. C
47.86, H 3.42, N 15.95; found C 47.43, H 3.61, N 15.86. IR (KBr):
ν˜max. ϭ 1601 (CϭN), 1262 (CϪO, enolate), 1060 (NϪN), 888 (Vϭ
) .
cmϪ1 51V NMR: δ: Ϫ535.7 ppm
ϩ
O) cmϪ1. EPR (DMSO, 98 K): gxy ϭ 1.984, gz ϭ 1.949; Axy
ϭ
[K(H2O)2][VO2(pydx-bhz)] (8): Complex 8 was prepared from
KVO3 (generated in solution from V2O5 and KOH) according to
the procedure outlined for 3. Recrystallisation from methanol gave
8 in pure form as a yellow crystalline material. Yield 1.48 g (66%).
C15H17KN3O7V (441.4): 40.82, C 3.88, N 9.52; found C 40.73, H
3.97, N 9.41. IR (KBr): ν˜max. ϭ 1597 (CϭN), 1222 (CϪO, enolate),
55.1, Az ϭ 159.7 ϫ 10Ϫ4 cmϪ1
.
Data for 2: Yield 0.48 g (65%) based on [VO(acac)2]. C14H13N4O5V
(368.2): calcd. C 45.65, H 3.53, N 15.22; found C 45.39, H 3.72, N
15.44. IR (KBr): ν˜max. ϭ 1599 (CϭN), 1257 (C-O, enolate), 1055
(NϪN), 956, 908 (sym. and antisym. VO2ϩ) cmϪ1
.
1063 (NϪN), 938, 915, 888 (sym. and antisym. VO2ϩ) cmϪ1 51V
.
[K(H2O)3][VO2(pydx-inh)] (3): Vanadium(v) oxide (0.91 g, 0.005
mol) was suspended in aqueous KOH (0.30 g, 0.005 mol in 10 mL
H2O) and stirred with occasional heating at 50 °C for 2 h. The
potassium vanadate solution thus generated was filtered. A filtered
solution of H2pydx-inh (1.42 g, 0.005 mol), dissolved in 20 mL of
aqueous KOH (0.56 g, 0.010 mol), was added with stirring, and the
pH of the reaction mixture was cautiously adjusted to 7.5 with 4
m HCl. After 2 h of stirring, the precipitated yellow solid was fil-
NMR: δ ϭ Ϫ534.2 ppm ([D6]DMSO); Ϫ546.9 ppm (MeOH/
CD3OD).
K[VO(O2)(pydx-nh)]·H2O (9): Complex 6 (900 mg, 2 mmol), dis-
solved in 20 mL of MeOH, was treated with 30% aqueous H2O2
(3 mL, 26.5 mmol) while stirring the reaction mixture at 10 °C,
which caused darkening of the solution. After 2 h of stirring, the
volume was reduced to ca. 10 mL and the solution was kept at 10
tered off, washed with water followed by acetone, and dried. On °C overnight. Yellow crystals precipitated, which were filtered off
crystallisation from ca. 50 mL of methanol, 2 precipitated as an and dried in vacuo. Yield 0.30 g (34%). C14H14KN4O7V (440.33):
orange-red solid, which was filtered off and dried in vacuo over calcd. C 38.19, H 3.20, N 12.72; found C 38.0, H 3.33, N 12.61.
silica gel. For the data of 2, see above. After reducing the volume IR (KBr): ν˜max. ϭ 1603 (CϭN), 1250 (CϪO, enolate), 1043 (NϪN),
Eur. J. Inorg. Chem. 2005, 147Ϫ157
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155

M. R. Maurya, S. Agarwal, C. Bader, D. Rehder
FULL PAPER
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tion of H2O2 (2 mL, 17.6 mmol) was added to an aqueous solution
of KVO3 (2 mmol), prepared as outlined above. The potassium salt
of H2pydx-bhz was prepared separately by treating III (0.570 g,
2 mmol) with KOH (0.168 g, 3 mmol) in water (10 mL) followed
by filtration. This solution was added dropwise to the above solu-
tion with constant stirring. After 2 h of stirring, the orange solid
that had precipitated was filtered, washed with water and dried.
The crude mass was recrystallised from methanol. Yield 0.14 g
(16%). Method 2: This method parallels the one adopted for 9,
using 8 and H2O2. Yield 0.48 g (22%). C15H15KN3O7V (439.3):
calcd. 41.01, H 3.44, N 9.56; found C 40.87, H 3.41, N 9.45. IR
(KBr): ν˜max. ϭ 1596 (CϭN), 1246 (CϪO, enolate), 1033 (NϪN),
970 (VϭO), 917 (OϪO), 717 [V(O)2 antisym.], 553 [V(O)2 sym.]
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Early View Article
Published Online November 4, 2004
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157