Biomimetic oxo-, dioxo- and oxo-peroxo-hydrazonato-vanadium(IV/V) complexes

Article abstract of DOI:10.1039/b202852m

[VO(acac)₂] reacts with HL (HL is the hydrazone Hacpy-inh I, or Hacpy-bhz II; acpy = acetylpyridine, inh = isonicotinic acid hydrazide, bhz = benzoylhydrazide) in dry methanol to yield the oxovanadium(IV) complexes [VOL(acac)] (HL = I: 1, HL =

Full text of DOI:10.1039/b202852m

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Biomimetic oxo-, dioxo- and oxo-peroxo-hydrazonato-
vanadium(IV/V) complexes†
Mannar R. Maurya,*a Shilpa Khurana,a Wenjian Zhangb and Dieter Rehderb
a Department of Chemistry, Indian Institute of Technology, Roorkee 247 667, 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
Received 20th March 2002, Accepted 17th June 2002
First published as an Advance Article on the web 11th July 2002
[VO(acac)2] reacts with HL (HL is the hydrazone Hacpy-inh I, or Hacpy-bhz II; acpy = acetylpyridine,
inh = isonicotinic acid hydrazide, bhz = benzoylhydrazide) in dry methanol to yield the oxovanadium() complexes
[VOL(acac)] (HL = I: 1, HL = II: 3). The dioxovanadium() complexes [VO2L] (HL = I: 2, HL = II: 4) are obtained by
aerial oxidation of 1 and 3 in methanol. Treatment of 1 and 3, or 2 and 4, with H2O2 yields the
oxoperoxovanadium() complexes [VO(O2)L] (HL = I: 5, HL = II: 6). In the presence of catechol or benzohydroxamic
acid, 1 and 3 give the mixed chelate complexes [VOL(cat)] (HL = I: 7, HL = II: 8) or [VOL(bha)] (HL = I: 9, HL = II:
10). The peroxo complexes 5 and 6 undergo oxygen transfer reaction with PPh3 in DMF. In DMF and DMSO, 7, 8, 9
and 10 slowly convert to the corresponding dioxo species 2 and 4. Acidification of 2 and 4 with HCl dissolved in
methanol affords oxo–hydroxo complexes. Reaction of 7 with -ascorbic acid yields 2. The crystal and molecular
structures of 2 and 4 have been determined, confirming the ONN binding mode of I and II from their enolate form.
Introduction
Schiff base and related complexes of vanadium, known in a
large variety, have received renewed attention in the context of
the role of vanadium in living organisms. This resurgence of
interest in vanadium chemistry stems from the discovery of
two classes of vanadium enzymes, viz. vanadium-nitrogenases,
containing vanadium in medium oxidation states, and
vanadate-dependent haloperoxidases.1,2 The latter catalyse the
oxidation by peroxide of halides to hypohalous acid.3 Under
laboratory conditions, thioethers are oxidized to sulfoxides.4
The enzymes lose their activity upon reduction or removal of
vanadate. Re-oxidation, or reconstitution of the apo-enzyme
with vanadate, fully restores their activity,5 demonstrating that
vanadium() is essential for the catalytic activity. Intermediate
species having {VO2}, {VO(OH)}, {VO(H2O)} and {VO(O2)}
cores have been proposed during catalytic turnover,6,7 in which
vanadium is in a trigonal-bipyramidal (native form) or
tetragonal-pyramidal (peroxo form) environment, covalently
Scheme 1
linked to the Nε of a histidine. The stability of VV complexes
under aerobic conditions has allowed the design of structural
{VO(OH)}cores, similar to those observed for the enzyme.
We also report on mixed ligand complexes containing, in
and/or functional models for the haloperoxidases.2 Vanadium-
() has also been shown to act as an electron acceptor and thus
addition to I and II, catecholate or benzohydroxamate.
initiator in the photo-cleavage of DNA.8 Further, many
Catacholates and hydroxamates are able to bind vanadium
vanadium() complexes show haloperoxidase activity9 and
activity in other oxidation and oxo transfer reactions,6,10 includ-
ing the enantioselective oxidation of prochiral thioethers to
sulfoxides.11 The insulin like effect of vanadium coordination
compounds is another intriguing and promising feature that
has further stimulated vanadium coordination chemistry.12,13
The objective of the present work is to introduce bio-mimetic
dioxovanadium() complexes of the hydrazone ligands I and II
(Scheme 1) having pyridine-N, enamine-N and amide-O donor
functions, and to study their reactivity with various substrates
to characterise species with {VO(H2O)}, {VO2}, {VO(O2)} and
rather strongly in both the ϩ and ϩ oxidation states.14,15
The relevance of catecholato-vanadium complexes in terms of
their biological significance arises from the role of tunichromes
in reduction, stabilisation and accumulation of vanadium by
certain ascidians (sea squirts);16 the iron binding siderophore
desferrioxamine, which contains hydroxamate functions, also
strongly binds to VO2ϩ.17
Experimental
Materials and methods
† Electronic supplementary information (ESI): table of 51V chemical
shifts for 2 and 4–10. See http://www.rsc.org/suppdata/dt/b2/b202852m/
V2O5, [NH4][VO3], isonicotinic acid hydrazide (H2inh), benzoyl
chloride, hydrazine hydrate, catechol (H2cat) and benzo-
DOI: 10.1039/b202852m
J. Chem. Soc., Dalton Trans., 2002, 3015–3023
This journal is © The Royal Society of Chemistry 2002
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Table 1 Crystal data and structure refinement for [VO2(acpy-inh)] (2) and [VO2(acpy-bhz)] (4)
2
4
Empirical formula
Formula weight
Temperature/K
Wavelength/Å
Crystal system, space group
a/Å, α/Њ
C13H11N4O3V
322.20
293(2)
C14H12N3O3V
321.21
153(2)
0.71073
0.71073
Monoclinic, C2/m
14.280(5), 90
6.834(2), 106.507(6)
14.297(5), 90
1337.7(8)
Monoclinic, C2/c
16.2827(15), 90
12.6396(12), 119.7930(10)
14.7731(14), 90
2638.5(4)
b/Å, β/Њ
c/Å, γ/Њ
Cell volume/Å3
Z
4
8
Absorption coefficient/mmϪ1
F(000)
0.757
656
0.766
1312
Crystal size/mm
Reflections collected/unique
R(int)
Data/restraints/parameters
R indices, [I > 2σ(I0)]: R1, wR2
R indices (all data): R1, wR2
0.50 × 0.2 × 0.05
3777/1299
0.1061
1299/0/130
0.0576, 0.1325
0.0771, 0.1442
0.40 × 0.15 × 0.10
15407/3097
0.0535
3097/0/190
0.0468, 0.1234
0.0569, 0.1286
hydroxamic acid (H2bha) were purchased from Loba Chemie,
India, acetylacetone (Hacac) and 2-acetylpyridine (acpy) were
obtained from Aldrich. [VO(acac)2] was prepared as
described.18 Benzoylhydrazide (H2bhz) was prepared by the
reaction of a two-fold excess of hydrazine hydrate with ethyl
benzoate, which in turn was prepared by refluxing benzoyl
chloride in an excess of absolute ethanol. All chemicals and
solvents were of analytical grade and used as obtained.
64.83, H, 5.10; N, 23.11. Calc. for C13H12N4O: C, 65.0; H, 5.0;
N, 23.33). IR (KBr, νmax/cmϪ1): 3200 (NH), 1661(C᎐O), 1608,
᎐
1573(C᎐N, C᎐C).
᎐
᎐
Hacpy-bhz II. This ligand was prepared by following the
same procedure outlined for Hacpy-inh. The pure product was
obtained as a white solid. Yield 1.55 g (65 %) on crystallisation
from methanol. M.p: 141 ЊC. (Found: C, 70.35; H, 5.80; N,
17.48. Calc. for C14H13N3O: C, 70.29; H, 5.86; N, 17.57). IR
(KBr, νmax/cmϪ1): 3230 (NH), 1653(C᎐O), 1622, 1581(C᎐N,
Microanalyses of the ligands and complexes were performed
by Regional Sophisticated Instrumentation Center, Central
Drug Research Institute, Lucknow, India. Magnetic suscepti-
bilities of the oxovanadium() complexes were recorded at
the Institute’s Instrumentation Centre IIT, Roorkee. Thermo-
grams of the complexes were recorded on a simple manually
operated thermo balance constructed in our laboratory.
Crystallised CuSO4ؒ5H2O was used to calibrate the instrument,
and thermograms were recorded under dynamic air atmosphere
with a heating rate of 3 to 5 ЊC minϪ1. IR spectra were recorded
as KBr pellets with a Perkin-Elmer model 1600 FT-IR, and
UV–Vis spectra (in DMF) on a UV-1601 PC spectro-
᎐
᎐
C᎐C).
᎐
[VO(acpy-inh)acac] 1. A stirred solution of Hacpy-inh (0.480
g, 2 mmol) in dry methanol (20 ml) was treated with [VO(acac)2]
(0.530 g, 2 mmol), and the resulting reaction mixture was
refluxed on a water bath for 4 h. After cooling to room temp.,
dark green 1 was filtered off, washed with methanol and dried
in vacuo. Yield 0.73 g (90%). (Found: C, 53.04; H, 4.48; N,
12.81. Calc. for C18H18N4O4V: C, 53.33; H, 4.44; N, 13.82). IR
(KBr, νmax/cmϪ1): 1591 (C᎐N, C᎐C), 1273 (C–O, enolic), 1022
᎐
᎐
1
(N–N), 954 (V᎐O), 469, 442, 411 (V–O, V–N).
᎐
photometer. H NMR spectra were run on a Bruker 200 MHz
spectrometer, 13C and 51V NMR spectra on a Bruker AM 360
with the common parameters settings. All δ(51V) values are
referenced relative to VOCl3 as external standard. EPR spectra
(in DMSO and MeCN) were run in the X-band mode (9.75
MHz) on a Bruker ESP 300E spectrometer.
[VO2(acpy-inh)] 2. Method 1. 1 (0.405 g, 1 mmol) was
dissolved in hot methanol (75 ml) and air was passed through
the solution with occasional shaking and heating at ca. 50 ЊC.
Residual solid material slowly dissolved, and the green solution
gradually changed to yellow; yellow crystalline 2 precipitated
within 2 d. This was filtered off, washed with methanol and
dried in vacuo. Yield 0.229 g (71%). (Found: C, 48.58; H, 3.47;
N, 17.19. Calc. for C13H11O3N4V: C, 48.45; H, 3.42; N, 17.39).
IR (KBr, νmax/cmϪ1): 1598, 1565 (C᎐N, C᎐C), 1256 (C–O,
Crystal structure determinations
Data were collected on a Bruker SMART CCD Apex
diffractometer using a graphite monochromator and Mo-Kα
radiation (λ = 0.71073 Å) at 153(2) K. Hydrogen atoms were
placed in calculated positions and included in the last cycles
of refinement. A disorder of C12 and C13 in 2 was accounted
for by a 1 : 1 model. Crystal data and details of the data
collection and refinement are collated in Table 1. The program
systems SHELXS 86 and SHELXL 93 were used throughout.19
CCDC reference numbers 182866 and 182867.
᎐
᎐
enolic), 1022 (N–N), 1025 (N–N), 946, 899 (sym and antisym
O᎐V᎐O), 507, 441, 421 (V–O, V–N).
᎐ ᎐
Method 2. V2O5 (0.500 g, 5 mmol) was suspended in 10 ml of
aqueous KOH (containing 0.336 g, 6 mmol) and stirred. The
vanadate solution thus generated was filtered after 2 h, and to
this was added, under continuous stirring, a filtered solution of
Hacpy-inh (1.20 g, 5 mmol) dissolved in 20 ml of aqueous KOH
(containing 0.280 g, 5 mmol). The pH of the reaction mixture
was slowly adjusted to 7.5 with 4 M HCl, and stirring was
continued. After 2 h, the precipitated yellow 2 was filtered
off, washed with water and dried. Recrystallisation from
DMF–MeOH 1/3 gave 0.810 g (50%) yield of 2.
See http://www.rsc.org/suppdata/dt/b2/b202852m/ for crystal-
lographic data in CIF or other electronic format.
Preparations
Hacpy-inh I. A mixture of 2-acetylpyridine (1.22 g, 10 mmol)
and isonicotinic acid hydrazide (1.37 g, 10 mmol) in 50 ml of
methanol was refluxed on a water bath for 6 h. After reducing
the solvent to ca. 15 ml and cooling to room temp. for 2 h, the
precipitated white solid was filtered off, washed with methanol
and dried. Recrystallisation from methanol yielded pure I
as a white solid. Yield 1.92 g (80%). M.p: 160 ЊC. (Found: C,
[VO(acpy-bhz)acac] 3. Complex 3 was prepared as described
for 1 by replacing Hacpy-inh for Hacpy-bhz. Yield 0.69 g
(50%). (Found: C, 56.31; H, 4.90; N, 10.25. Calc. for
C19H19N3O4V: C, 56.44; H, 4.70; N, 10.3). IR (KBr, νmax/cmϪ1):
1596 (C᎐N, C᎐C), 1267 (C–O, enolic), 1018 (N–N), 957 (V᎐O),
᎐
᎐
᎐
494, 442, 415 (V–O, V–N).
3016
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[VO2(acpy-bhz)] 4. This complex was prepared from
[VO(acpy-bhz)(acac)] as well as from KVO3 and Hacpy-bhz
by the methods 1 and 2, respectively, as outlined for 2, in
∼55% yield. (Found: C, 52.54; H, 3.96; N, 13.12. Cal. for
C14H12O3N3V: C, 52.34; H, 3.74; N, 13.08). IR (KBr, νmax/cmϪ1):
10 ЊC after reducing the volume to ca. 10 ml. Complex 9 was
filtered off, washed with methanol and dried in vacuo. Yield
0.24 g (55%). (Found: C, 54.21; H, 3.96; N, 15.68. Calc. for
C20H17N5O4V: C, 54.30; H, 3.85; N, 15.84). IR (KBr, νmax/cmϪ1):
1597, 1573 (C᎐N, C᎐C), 1235 (C–O, enolic), 1029 (N–N), 965
᎐
᎐
1598,1565 (C᎐N, C᎐C), 1260 (C–O, enolic), 1028 (N–N), 946,
(V᎐O), 552, 462, 432 (V–O, V–N).
᎐
᎐
᎐
909 (sym and antisym O᎐V᎐O), 509, 472, 441, 410 (V–O, V–N).
᎐ ᎐
[VO(acpy-bhz)bha] 10. This was prepared following the
procedure outlined for 9. Yield 0.21 g (48%). (Found: C, 57.35;
H, 4.16; N, 12.57. Calc. for C21H18N4O4V: C, 57.14; H, 4.08;
N, 12.70). IR (KBr, νmax/cmϪ1): 1600, 1564 (C᎐N, C᎐C), 1261
[VO(O2)(acpy-inh)] 5. Method 1. A 30% H2O2 (2 ml, 17.6
mmol) was added to an aqueous solution of KVO3 (5 mmol),
prepared as outlined in method 2 for 2, and the resulting
solution was stirred at ∼10 ЊC for 1/2 h. Separately, the
potassium salt of Hacpy-inh was prepared by reacting I (1.2 g,
5 mmol) and KOH (0.280 g, 5 mmol) in 20 ml of water, followed
by filtration. This solution was added dropwise to the above
solution with constant stirring. After 2 h of stirring, the
orange–yellow solid which had separated was filtered off,
washed with water and dried. The crude mass was dissolved
in a minimum amount of DMF while heating to ∼50 ЊC, and
filtered. After treatment with 15 ml of methanol, it was kept at
ca. 10 ЊC, whereupon orange crystals of 5 separated. These were
filtered off and dried in vacuo at ambient temperature. Yield
0.845 g (50%). (Found: C, 45.98; H, 3.19; N, 16.49. Calc. for
C13H11O4N4V: C, 46.15; H, 3.25; N, 16.57). IR (KBr, νmax/cmϪ1):
᎐
᎐
(C–O, enolic), 1026 (N–N), 959 (V᎐O), 509, 452, 422 (V–O,
᎐
V–N).
Reaction of 5 and 6 with PPh3
PPh3 (0.195 g, 0.75 mmol) was added to complex 5 or 6 (0.170 g,
0.5 mmol) dissolved in 15 ml of hot DMF, and the reaction
mixture was digested on a water bath for 8 h. After reducing
the volume of the solvent to ca. 5 ml, methanol (15 ml)
was added, and the solution kept at 10 ЊC, whereupon crystal-
line solid separated out within 4–6 h. This was filtered off
and dried in vacuo. Yield: 50%. Analytical and spectral data
of the complexes thus obtained matched well with 2 and 4,
respectively.
1597, 1570 (C᎐N, C᎐C), 1260 (C–O, enolic), 1020 (N–N), 956
᎐
᎐
(V᎐O), 926 (O–O), 701 (VO antisym), 574 (VO sym), 501, 475,
᎐
2
2
450, 421 (V–O, V–N).
Results and discussion
Method 2. 0.5 mmol of 1 (dissolved in 20 ml of MeOH) or 2
(dissolved in 10 ml of DMF by heating on a water bath) was
treated with 30% H2O2 (3 ml, 26.5 mmol) while stirred at room
temperature, which caused an immediate change of colour to
orange–yellow. After 2 h of stirring, the volume was reduced to
ca. 5ml. In the case of 2 as the precursor, 20 ml of MeOH was
added. These solutions were kept at 10 ЊC overnight. The
orange–red crystals were filtered off and dried in vacuo. Yield
0.085 g (40%). Analytical and spectral data of the isolated
complexes match well with those of compound 5 prepared
according to method 1.
Scheme 2 provides an overview of the complexes described in
this contribution. Structural formulae are based on structural
evidence (vide infra) and plausibility.
[VO(O2)(acpy-bhz)] 6. This complex was prepared by
analogy to 5 (method 1) replacing Hacpy-inh by Hacpy-bhz.
Crystallisation from DMF–MeOH as outlined above afforded
6. Yield 0.925 g (55%). (Found: C, 50.0; H, 4.48; N, 12.37. Calc.
for C14H12O4N3V: C, 49.85; H, 3.56; N, 12.47). IR (KBr, νmax
/
cmϪ1): 1598, 1565 (C᎐N, C᎐C), 1260 (C–O, enolic), 1022
᎐
᎐
(N–N), 968 (V᎐O), 921 (O–O), 714 (VO antisym), 562
᎐
2
(VO2 sym), 472, 440, 414 (V–O, V–N).
[VO(acpy-inh)cat] 7. 1 mmol of 1 was dissolved in hot
methanol (40 ml) and cooled to ambient temp. To this was
added catechol (0.110 g, 1 mmol) and the reaction mixture was
stirred for 5 h. After reducing the solvent to ca. 10 ml and
keeping overnight at 10 ЊC, dark brown complex 7 was filtered
off and dried in vacuo. Yield 0.21 g (50%). (Found: C, 55.22;
H, 3.49; N, 13.41. Calc. for C19H15N4O4V: C, 55.08; H, 3.62;
N, 13.53). IR (KBr, νmax/cmϪ1): 1591, 1572 (C᎐N, C᎐C), 1274
᎐
᎐
(C–O, enolic), 1029 (N–N), 951 (V᎐O), 467, 440, 412 (V–O,
᎐
V–N).
[VO(acpy-bhz)cat] 8. Complex 8 was prepared analogously
to 7, replacing 1 by 3. Yield 0.226 g (55%). (Found: C, 58.24; H,
3.76; N, 9.93. Calc. for C20H16N3O4V: C, 58.11; H, 3.87; N,
10.17). IR (KBr, νmax/cmϪ1): 1598, 1578 (C᎐N, C᎐C), 1272
Scheme 2
᎐
᎐
Synthesis and solid state characteristics
(C–O, enolic), 1022 (N–N), 951 (V᎐O), 468, 452, 417 (V–O,
᎐
V–N).
Synthesis. Reaction between equimolar amounts of
[VO(acac)2] and Hacpy-inh (I in Scheme 1) in dry, refluxing
methanol yields [VO(acpy-inh)acac], 1. Here, as in all of the
complexes formed with I and II, the ligand reacts out of its
enolic tautomeric form, i.e. in the NNO(1Ϫ) mode. On aerial
oxidation in methanol, the dioxovanadium() complex
[VO(acpy-inh)bha] 9. 1 mmol of 1 was dissolved in 40 ml of
hot methanol and to this, after cooling to ambient temp., was
added benzohydroxamic acid (0.137 g, 1 mmol). The obtained
reaction mixture was stirred for 4 h and then kept overnight at
J. Chem. Soc., Dalton Trans., 2002, 3015–3023
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[VO2(acpy-inh)], 2, is obtained, a reaction which requires water.
The intermediate green complex 1 can be isolated from the
methanolic solution prior to aeration. Eqns. (1) and (2) repre-
sent the synthetic procedures:
The peroxo complexes undergo oxygen transfer reactions with
PPh3 in DMF to give the corresponding dioxovanadium()
complexes [VO2L], 2 and 4; Scheme 3.
[VO(acac)2] ϩ Hacpy-inh
[VO(acpy-inh)acac] ϩ Hacac (1)
2[VO(acpy-inh)acac] ϩ 1/2 O2 ϩ H2O
2[VO2(acpy-inh)] ϩ 2Hacac (2)
Scheme 3
Treatment of 1 and 3 with catechol or benzohydroxamic acid
in methanol under aerobic conditions leads to the formation of
the mixed-ligand complexes 7, 8, 9 and 10 as represented by
eqns. (5) and (6). During this reaction, oxidative replacement
of the remaining acetylacetonato group by catecholate or
benzdydroxamate takes place.
Similarly, the complex [VO(acpy-bhz)acac], 3, can be oxi-
dised in methanol to [VO2(acpy-bhz)], 4. Further, 2 and 4 were
obtained from the reaction of vanadate, prepared in situ by
dissolving V2O5 in an aqueous solution of KOH, by addition of
solutions of the potassium salt of I or II, and after adjustment
to pH ∼ 7.5. Aqeuous vanadate solutions actually contain a
mixture of mono-, di-, tetra- and pentavanadates.20,21 Addition
of H2O2 to 1 and 3 yields the peroxo complexes 5 and
6, eqn. (3). The complexes [VO(O2)L] (HL = I or II, 5 or 6)
can also be prepared from aqueous solutions of potassium
metavanadate, hydrogen peroxide and the potassium salts of
the ligands, as symbolically depicted in eqn. (4). The actual
vanadium precursor probably is in situ generated [VO(O2)2(OH/
2[VIVOL(acac)] ϩ 2H2cat ϩ 1/2O2 ϩ 2Hϩ
2[VVOL(cat)] ϩ 2Hacac ϩ H2O (5)
2[VOL(acac)] ϩ 2H2bha ϩ 1/2O2 ϩ 2Hϩ
2[VOL(bha)] ϩ 2Hacac ϩ H2O (6)
Complexes 1 and 3 are paramagnetic with a magnetic
moment of 1.89 and 1.96 BM, respectively, while the other
complexes are diamagnetic as expected for 3dЊ systems. The
dioxo complexes 2 and 4, and the peroxo complexes 5 and 6 are
soluble in DMF and DMSO only, while the other complexes
also dissolve in methanol and ethanol. Complexes 7 to 10 are
also soluble in CH2Cl2.
H2O)x]nϪ 21
.
2[VO(acac)L] ϩ 3 H2O2
2 [VO(O2)L] ϩ 2 Hacac ϩ 2 H2O (3)
KL ϩ KVO3 ϩ H2O2
[VO(O2)L] ϩ 2 KOH (4)
Thermal studies. The TGA profiles of the VO2ϩ complexes 1
and 3 show that these complexes lose acetylacetone, one oxygen
remaining attached to vanadium, between 200 and 275 ЊC, with
concomitant formation of [VO2L] as shown by eqn. (7).
(HL = I: 5; HL = II: 6)
Oxo-monoperoxo complexes may further be generated by
treating cis-dioxo-vanadium() complexes with H2O2 at ice
temperature. This reaction was studied by electronic absorption
spectrophotometry. The spectral changes obtained during the
reaction (Fig. 1) include a shift of the 370 nm band of 1 to 385
[VIVO(acac)L]
[VVO2L] ϩ {acac Ϫ O}
(7)
On further heating, [VO2L] decomposes in the presence of air
to yield V2O5. The peroxo complexes 5 and 6 degrade in the
temperature range 120–220 ЊC, the decrease of weight by ∼9.5%
corresponding to the loss of the peroxo group (calcd. 9.5%).
Thermograms of the other complexes indicate continuous
degradation to V2O5 in the 250–700 ЊC temperature range.
Structure description. The crystal structure of [VO2(acpy-
inh)] (2) and [VO2(acpy-bhz)] (4) with atom numbering schemes
are shown in Fig. 2, selected bond lengths and angles in Table 2.
In 2, a mirror plane bisecting the angle O1–V1–O1# contains all
of the other atoms except of the pyrinine carbons C13 and C12,
which are disordered so as to satisfy the symmetry conditions
(O1# in 2 corresponds to O3 in 4). The N2O3 coordination
sphere around the vanadium atom is defined by the pyridine-N,
imine-N and the amide-O of the monobasic tridentate ligand
acpy-inh/bhz(1Ϫ), and by two cisoid oxo-groups. The three
ligand functions occupy meridional positions. The bond
distances N2–N3 [1.384(5), 1.381(3)], N3–C8 [1.311(6),
1.314(3)] and O3–C8 [1.296(6), 1.300(3) Å] are consistent
with the enolate form of the amide functionality. The V᎐O
᎐
distances of 1.615 and 1.618 Å are typical of non-hydrogen
Fig. 1 Titration of [VO(acpy-inh)acac] (1) with 30% H2O2; the spectra
were recorded after the successive addition of two-drop portions of
H2O2 to 5 ml of ca. 10Ϫ4 M solution of 1 in MeOH.
bonded V᎐O groups.22 Other bond lengths, e.g. d(V–N1),
᎐
d(V–N2) and d(V–O3), are in accord with reported values for
comparable complexes.23,24 The ligand forms two five-
membered rings with bite angles of 72.96Њ (N1–V–N2) and
73.56Њ (O2–V–N2). With an N1–V–O2 angle of 146.51Њ, N1
and O2 are inclined towards N2.
The coordination geometry around vanadium can best be
described as distorted square-pyramidal with one of the doubly
bonded oxo groups (O3), the enolate oxygen (O2) and the two
nm in 5, and the appearance of a weak shoulder at ∼410 nm.
The rate at which peroxo complex formation occurs depends
upon the amount of H2O2 added. The band at 243 nm becomes
relatively sharp while the band at 282 nm slowly shifts to 273.5
nm. The feature of the final spectrum is similar to that of
authentic 5 (see below).
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J. Chem. Soc., Dalton Trans., 2002, 3015–3023

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Table 2 Selected bonding parameters for [VO2(acpy-inh)] (2) and [VO2(acpy-bhz)] (4)
Bond lengths/Å
Bond anglesa/Њ
2
4
2
4
V–O(1)
V–O(3)
V–O(2)
V–N(1)
1.615(3)
1.615(2)
1.618(2)
1.955(2)
2.111(2)
2.104(2)
1.300(3)
1.294(3)
1.381(3)
1.314(3)
O1–V–O2
O1–V–O1#/O3
O3–V–O2
O1–V–N2
O3–V–N2
O2–V–N2
O1–V–N1
O3–V–N1
O2–V–N1
N1–V–N2
V–O2–C8
C6–N2–N3
C6–N2–V
N3–N2–V
O2–C8–N3
O2–C8–C9
102.95(10)
110.22(19)
102.97(10)
110.35(10)
102.29(8)
124.03(9)
125.17(9)
73.56(7)
1.961(3)
2.107(3)
2.107(4)
1.296(6)
1.294(5)
1.384(5)
1.311(6)
124.62(10)
V–N(2)
O(3)–C(8)
N(2)–C(6)
N(2)–N(3)
N(3)–C(8)
73.59(14)
95.92(11)
96.19(9)
96.29(9)
146.55(15)
72.97(15)
117.4(3)
119.9(4)
121.6(3)
118.5(3)
124.9(5)
117.7(4)
146.51(7)
72.96(7)
118.39(15)
119.62(19)
122.19(15)
118.18(14)
123.1(2)
118.4(2)
a Due to symmetry restrictions, there is no O3 in compound 2. O3 in 4 corresponds to O1# in 2.
complexes.29 The peroxo complexes 5 and 6 show three IR
active vibrational modes associated with the [V(O2)]2ϩ moiety at
562–574, 701–714 and 921–926 cmϪ1, which are assigned to the
symmetric V(O2) stretch (ν2), the antisymmetric V(O2) stretch
(ν3) and the O–O intra-stretching (ν1) mode, respectively.30 The
presence of these bands confirms the η2-coordination of the
peroxo group.
The IR spectra of the ligands exhibit two bands in the
regions 3200–3230 and 1653–1661 cmϪ1 due to ν(NH) and
ν(C᎐O) stretches. The absence of these bands in the spectra of
᎐
all complexes is consistent with the enolisation of the amide
functionality and subsequent proton replacement by the metal
ion. A new band appearing in the 1235–1274 cmϪ1 range is
assigned to the ν(C–O)(enolic) mode. Unequivocal assignment
of the ν(C᎐N) (azomethine) as well as ν(C᎐C) (ring) stretches of
᎐
᎐
the ligands could not be carried out due to the complexity of
the spectra in the 1600 cmϪ1 region. However, the appearance
of a sharp band between 1593 and 1608 cmϪ1 should indicate
the coordination of both nitrogen atoms. The band due to
ν(N–N) mode appears at 991 cmϪ1 in Hacpy-inh, and at 976
cmϪ1 in Hacpy-bhz. This band which, on coordination of the
ligands, shifts up to 50 cmϪ1 to higher wave numbers with
respect to the free ligand, falls within the range commonly
observed for the monodentate coordination of the >N–N<
residue.31 The high frequency shift is expected because of
diminished repulsion between the lone pairs of adjacent
nitrogen atom.32 The absence of the band corresponding to
the ν(NH) and ν(OH) stretches in the complexes 9 and 10 of
benzohydroxamic acid (Hbha) is indicative of the enolisation of
Fig. 2 ORTEP plots (35% probability level) and numbering schemes
for 2 and 4.
the –C᎐O group of Hbha and subsequent replacement of H
᎐
by the metal ion. In addition, these complexes exhibit a weak
but broad band covering the region 2500–2800 cmϪ1, which
suggests intermolecular hydrogen bonding. Catechol complexes
do not show any band due to ν(OH) and thus confirm the
coordination of catecholate to the metal ion.
N functions (N1, N2) forming the tetragonal plane. There
are substantial distortions towards a trigonal bipyramid,
quantified by a τ parameter of 0.37 (τ = 0 for an ideal tetragonal
pyramid; τ = 1 for an ideal trigonal bipyramid). The distortion
thus compares to that in [VO2L], where HL is a Schiff base
derived from 2-hydroxy-1-naphthaldehyde and 8-aminoquino-
line,23 or another type of penta-coordinated hydrazonate.25
In [VO2L] complexes with an ideally square-pyramidal geom-
etry, there is a distinct tendency towards dimerisation to
[VOL(µ-O)]2 with octahedral arrangement.22,26,27 Examples of
trigonal-bipyramidal vanadium complexes are also known.26,28
Solution studies
Electronic absorption spectra. Table 3 lists electronic spectral
data for the ligands and various complexes. The visible absorp-
tion spectra of all complexes are dominated by an intense
absorption at 378–392.5 nm and are assigned to a ligand-to-
metal charge transfer (LMCT) band. The intra-ligand band at
ca. 370 nm merges with this band. Benzohydroxamate and cat-
echolate usually induce strong charge transfer to a high valent
metal centre. The new band at ca. 550 nm in 9 and 10, and two
intense bands at ca. 778 and 516 nm in 7 and 8 are thus con-
IR spectra. The complexes display a sharp band in the 946–
968 cmϪ1 region due to the ν(V᎐O) mode. In the case of 2 and 4,
᎐
there are two such bands between 899 and 946 cmϪ1 due to cis-
[VO2] structure, as noted previously by other workers for similar
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Table 3 Electronic absorption spectra
Compound (solvent)
λmax/nm (ε/MϪ1 cmϪ1
)
Hacpy-inh (DMF)
[VO(acpy-inh)acac], 1 (DMF)
(MeOH)
[VO2(acpy-inh)], 2 (DMF)
[VO(O2)(acpy-inh)], 5 (DMF)
[VO(acpy-inh)cat], 7 (MeOH)
[VO(acpy-inh)bha], 9 (MeOH)
Hacpy-bhz (DMF)
[VO(acpy-bhz)acac], 3 (DMF)
(MeOH)
211.5(21387), 240(11140), 293(14895), 369(3038)
275(13783), 385(10661)
205(11200), 243(7100), 282(7238), 370(4450)
269.5(13783), 386.5(5221.6)
242(12030), 273.5(12270), 385(7053), 410(sh)(4890)
214.5(39522), 379(17010), 516.5(6460), 778(6608)
214(28414), 378(12025), 551(2958)
205.5(18060),229.5(11835),295.5(21660),367(2007)
275.5(15162), 389(11527)
379(6515), 292(16110), 205(23320)
[VO2(acpy-bhz)], 4 (DMF)
[VO(O2)(acpy-bhz)] 6 (DMF)
[VO(acpy-bhz)cat] 8 (MeOH)
[VO(acpy-bhz)bha] 10 (MeOH)
392.5(22041), 274(18999), 236(4703), 202.5(6683)
392.5(20310), 268.5(17538), 255.5(6613)
777(4880), 516(4087), 388.5(22830), 219.5(38040)
550(4325), 388.5(14551), 249.5(23160), 211.5(32632)
Table 4 1H NMR spectra (δ in ppm) of ligands and complexesa
Compound
NH
CH3
Aromatic protons
Hacpy-inh
11.19(s, 1H)
2.51(s, 3H)
2.80(s, 3H)
2.71(s, 3H)
2.71, 2.82(s each, 3H)
2.74, 2.82(s each, 3H)
2.51(s, 3H)
7.45–8.76(m, 8H)
[VO2(acpy-inh)], 2
[VO(O2)(acpy-inh)], 5
[VO(acpy-inh)cat], 7
[VO(acpy-inh)bha], 9
Hacpy-bhz
7.78(m, 5H), 8.76–8.83(m, 3H)
7.94–8.81(m, 7H), 9.60–9.63(d, 1H)
6.58–6.73(m, 4H), 7.79–8.82(m, 8H)
7.40–8.76(m, 13H)
10.90(s, 1H)
7.43–8.61(m, 9H)
[VO2(acpy-bhz)], 4
[VO(O2)(acpy-bhz)], 6
[VO(acpy-bhz)cat], 8
[VO(acpy-bhz)bha], 10
2.78(s, 3H)
2.77(s, 3H)
2.77, 2.78(s each, 3H)
2.71, 2.81(s each, 3H)
7.48–8.40(m, 9H)
7.48–8.84(m, 9H)
6.58–6.73(m, 4H), 7.48–8.83(m, 9H)
7.40–8.47(m, 14H)
a Letters given in parentheses indicate the signal structure: s = singlet, d = doublet, m = multiplet.
sidered LMCT transitions originating from oxamate oxygens33
or phenolate oxygen.34 As vanadium() complexes have a 3dЊ
configuration, d–d bands are not expected. However, such d–d
transitions appear at ca. 600 nm in complexes 1 and 3 as a very
weak band.
to some downfield shift. The coordination of catecholate is
reflected by the appearance of a new peak at 145.3 in 7 and at
148.6 in 8, while coordination of benzohydroxamate is con-
firmed by the appearance of at least two new peaks in the 165–
180 ppm region. Several additional peaks of low intensity in
the 120–130 ppm region for the complexes 7 to 10 correspond
to the respective dioxo complexes formed by partial decom-
position (see also above and below).
1H NMR spectra. The coordinating modes of the ligands
1
were also confirmed by recording H NMR spectra of the lig-
ands as well as complexes. As presented in Table 4, the sharp
signal appearing at 11.19 (in Hacpy-inh) and 10.90 ppm (in
Hacpy-bhz) due to the hydrazone NH proton, disappears in the
spectra of the complexes, which suggests the conversion of the
keto to the tautomeric enol group and subsequent coordination
of the enolate oxygen. Aromatic protons of the ligands as well
as the complexes appear well within the expected range. The
aromatic protons of the coordinated catecholate (in 7 and 8)
give rise to two multiplets in the 6.58–6.73 ppm region, while in
the benzohydroxamate complexes 9 and 10, the phenyl reson-
ances appear along with other aromatic protons, thus making
this region very complex. A downfield shift of ∼0.2 ppm of the
methyl proton signals in comparison to those of the free ligands
demonstrates the coordination of the azomethine nitrogen. The
methyl region of the catecholate and benzohydroxamate com-
plexes exhibits at least two signals for methyl protons. One of
them appears at the same position as observed in the corres-
ponding dioxo complexes, while the other one is new and is due
to the authentic mixed ligand complexes. Apparently, slow
decomposition of these complexes occurs to give dioxo species.
51V NMR spectra. Table S1 (ESI) presents 51V NMR spectral
data of the complexes. The resonances have line widths at
half-height of approximately 200 Hz, which is still considered
as narrow in 51V NMR spectroscopy. The dioxovanadium()
complexes 2 and 4 exhibit one strong resonance at ca. Ϫ507, an
expected value for the dioxovanadium() complexes having a
mixed O/N donor set.36 The catecholato complexes 7 and 8 dis-
play two bands: one in the 319–362 ppm region characteristic
of catecholate coordination,34 and a second peak in the range
of dioxo species, again showing that partial decomposition
occurs in solution. Similarily, complexes 9 and 10 display two
bands in the 51 to 71 and at Ϫ506 to Ϫ507 ppm region. The
former resonance belongs to the complexes 9 and 10 while
the latter is due to decomposition to 2 and 4. In some of the
solutions, there is an additional minor resonance around
Ϫ590, which coincides with the chemical shift of the authentic
oxo–peroxo complexes 5 and 6. These resonances do not appear
under strict exclusion of air, suggesting that some redox chem-
istry takes place between the vanadium complexes and oxygen
in solution.
13C NMR spectra. Assignments of the relevant peaks
recorded for the ligands and complexes are presented in Table 5.
A large coordination-induced shift ∆δ = [δ(complex) – δ(free
ligand)] of the signals for the carbon atoms in the vicinity of
the coordinating atoms (e.g. the azomethine carbon C1 and the
enolate carbon C7) demonstrate their coordination.35 Even the
methyl carbons associated with the azomethine group give rise
EPR studies. DMSO solutions of the complexes [VOL(acac)]
(1 and 3) provide EPR patterns with a g0 value of 1.957 and the
following hyperfine coupling constants: 1: A0 = 98.7, Az = 175.3,
Axy = 60.2 × 10Ϫ4 cmϪ1; 3: A0 = 99.7, Az = 180.3, Axy = 58.9 × 10Ϫ4
cmϪ1. Practically the same values have been obtained in MeCN.
If compared with EPR parameters of other VO2ϩ complexes
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J. Chem. Soc., Dalton Trans., 2002, 3015–3023

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Table 5 13C NMR chemical shift (δ in ppm); see scheme below for atom numbering
Compound
Hacpy-inh
CH3
C1
C7
C2
C6
C9/C12
C10/C11
C3/C4/C5/C8/C13
12.94
154.92
162.83
156.29
156.34
153.99
157.2
148.71
149.40
148.68
154.0
136.72
142.06
142.34
142.27
150.14
150.37
150.48
150.53
119.81, 120.58, 122.0,
122.97, 124.49, 141.3
122.35, 124.96, 126.97,
139.9
115.65, 119.24, 122.27,
126.08, 128.61, 145.26
122.32, 125.19, 126.40,
127.84, 128.85, 129.76,
131.35, 135.22, 139.56
120.39, 124.17, 131.63,
133.98
125.71, 127.03, 130.82,
132.36
125.76, 127.08, 130.86,
132.41
125.85, 126.26, 127.04,
128.16, 131.01, 131.4,
132.27, 132.38, 148.56
124.70, 125.88, 126.56,
127.77, 128.15, 129.79,
131.20, 132.50, 135.30
[VO(O2)(acpy-inh)], 5
(∆δ)a
13.29
(0.35)
13.74
(0.80)
13.42
(0.48)
155.62
(0.70)
162.20
(6.58)
163.70
(8.78)
174.54
(11.71)
173.5
(10.67)
173.5
[VO(acpy-inh)cat]b, 7
(∆δ)
[VO(acpy-inh)bha]b, 9
(∆δ)
(10.67)
Hacpy-bhz
12.60
155.14
164.21
147.82
152.98
153.03
153.0
148.65
154.68
154.73
154.29
136.64
143.15
143.30
142.21
128.30
128.66
128.71
128.91
[VO2(acpy-bhz)], 4
(∆δ)
13.63
(1.03)
13.68
(1.08)
13.52
(0.92)
163.37
(8.14)
163.43
(8.29)
159.95
(4.81)
175.99
(11.78)
176.03
(11.82)
175.53
(11.32)
[VO(O2)(acpy-bhz)], 6
(∆δ)
[VO(acpy-bhz)cat]b, 8
(∆δ)
[VO(acpy-bhz)bh)]b, 10
(∆δ)
13.25
(0.65)
161.21
(6.07)
175.54
(11.33)
154.20
156.08
142.70
128.89
a ∆δ = δ(complex) Ϫ δ(free ligand). b Another set of signals for all carbons also appears and their positions are nearly identical to those of the dioxo
species.
having a mixed ON donor set,37 the low g and high Az values are
noticeable, and possibly account for diminished delocalisation
of spin density into the ligand system as a consequence of the
substantial distortion in the molecules.
observed. Addition of water facilitates this conversion. In a
typical experiment, addition of 1 drop of H2O in 5 ml of ∼10Ϫ4
M solution of 7 gives rise to complete conversion into 2 within
8 h, as illustrated in Fig. 3. Similar observations have also been
noted in oxovanadium() complexes containing catecholates
along with Schiff bases derived from salicylaldehyde and
8-aminoquinoline.23 Eqn. (8) represents the decomposition
reaction.
Stability studies. The stability of complexes 7, 8, 9 and 10 in
various solvents were studied. Solutions of these complexes are
stable for about a week in dry CH2Cl2 and for about two days in
methanol, but – as already evidenced by the spectral studies –
they slowly decompose in DMF and DMSO. The conversion of
the [VO]2ϩ complexes 7 and 8 into the [VO2]ϩ species with the
loss of the bidentate ligand catecholate(2Ϫ) was established by
electronic absorption studies of DMSO solutions, Fig. 3. In dry
DMSO conversion is rather slow and takes about 24 h. During
this period, a gradual loss in intensity of the LMCT bands due
to coordinated catecholate and their final disappearance is
[VO(acpy-inh)cat] ϩ H2O
[VO2(acpy-inh)] ϩ H2cat (8)
Reaction of 7 with L-ascorbic acid. The reactivity of complex
towards -ascorbic acid was monitored by electronic
7
absorption spectroscopy. In a typical reaction, 7 (10Ϫ4
M
solution in CH2Cl2) was treated with -ascorbic acid dissolved
in the minimum amount of MeCN in an approximately 1 : 2
molar ratio. The two LMCT bands at 790 and 518 nm slowly
disappeared. The 379 nm band experiences a slight red shift,
while the 274 nm band sharpens. The spectra recorded at
different intervals of time are shown in Fig. 4. Since catecholato
and benzohydroxamato complexes are stable in CH2Cl2 and
CH2Cl2–MeCN for several days, any change in the absorption
spectra should come about by the reaction between -ascorbic
acid and the complex. As -ascorbic acid has reducing
properties, intermediate reduction of 7 with concomitant
removal of coordinated catecholate may occur. The matching
of the final spectrum with the dioxovandium() complex 2
indicates that VIV is reoxidised to form [VO2L]. The net reaction
is thus an oxidation of ascorbic acid by atmospheric oxygen,
catalysed by VV.
Reaction of 2 and 4 with HCl. Dioxovanadium() complexes
can react with acids to give oxo–hydroxo complexes, and the
corresponding reaction has also been accomplished for 2 and 4
with HCl. Addition of methanol saturated with HCl gas to 2
dissolved in DMF causes darkening of the solution, accom-
panied by a slight reduction in intensity of the 274 nm band, as
illustrated in Fig. 5. Further addition of HCl results in an
Fig. 3 Absorption spectra of [VO(acpy-inh)cat] (7) in DMSO (ca. 10Ϫ4
M solution) plus one drop of H2O as a function of time over a period of
8 h.
J. Chem. Soc., Dalton Trans., 2002, 3015–3023
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looked at as structural models for the active site in vanadate-
dependent haloperoxidases. Intermediate species containing
the [VO(OH)2ϩ] core can be obtained by protonation of dioxo
complexes. One of the oxo-groups can also be replaced by
peroxide to form oxoperoxo complexes, another feature which
is pertinent to the peroxidases. Electron transfer processes are
modeled with the complex [VO(acpy-inh)cat] in the presence of
ascorbic acid and air, yielding [VO2L] possibly via a [VOL]ϩ
intermediate.
Acknowledgements
M. R. M. and S. K. are thankful to the Council of Scientific
and Industrial Research, New Delhi 110 012, India for financial
support of the work. We also wish to thank the RSIC, Central
Drug Research Institute, Lucknow, India, for providing
elemental analyses of the complexes. This work was further
supported by the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie.
Fig.
4
Absorption spectra of ca. 10Ϫ4
M CH2Cl2 solution of
[VO(acpy-inh)cat] (7) in the presence of -ascorbic acid (dissolved in a
minimum amount of MeCN) in a 1 : 2 molar ratio as a function of time
over a period of 2 h.
References
1 Vanadium and its Role in Live, Metal Ions in Biological Systems, eds.
H. Sigel and A. Sigel, Marcel Dekker, New York, 1995, vol. 31;
J. Inorg. Biochem., 2000, 80 (special issue dedicated to biological
aspects of vanadium).
2 D. Rehder, Coord. Chem. Rev., 1999, 182, 297.
3 A. Butler, Coord. Chem. Rev., 1999, 187, 17.
4 H. B. ten Brink, A. Tuyman, H. L. Dekker, W. Hemrika, Y. Izumi,
T. Oshiro, H. E. Shoemaker and R. Wever, Inorg. Chem., 1998, 37,
6780; M. A. Andersson and S. G. Allenmark, Tetrahedron, 1998, 54,
15293.
5 A. Butler and C. J. Carrano, Coord. Chem. Rev., 1991, 109, 61.
6 B. J. Hamstra, G. J. Colpas and V. L. Pecorao, Inorg. Chem., 1998,
37, 949; Y. Zhang and R. H. Holm, Inorg. Chem., 1990, 29, 911;
A. Butler and A. H. Baldwin, Struct. Bonding (Berlin), 1997, 89,
109.
7 V. Conte, F. Di Furia and G. Licini, Appl. Catal., 1997, 157, 335.
8 C. Hiort, J. Goodisman and J. C. Dabrowiak, Biochemistry., 1996,
35, 12354.
9 M. J. Clague, N. L. Keder and A. Butler, Inorg. Chem., 1993, 32,
4754; G. J. Colpas, B. J. Hamstra, J. W. Kampf and V. L. Pecoraro, J.
Am. Chem. Soc., 1996, 118, 3478; M. Bhattarcharjee, S. Ganguly
and J. Mukherjee, J. Chem. Res. (S), 1995, 80; R. M. Tótaro,
P. A. M. Williams, M. C. Apella, M. A. Blesa and E. J. Baran,
J. Chem. Soc., Dalton Trans., 2000, 4403.
10 I. W. C. E. Arends, M. P. Birelli and R. A. Sheldon, Stud. Surf.
Catal., 1997, 110, 1031.
Fig. 5 Titration of [VO2(acpy-inh)] (2) with a saturated solution of
HCl in methanol; the spectra were recorded after the addition of five
drops portions of methanol–HCl to 10 ml of a ca. 10Ϫ4 M DMF
solution of 2.
11 C. Bolm and F. Bienewald, Angew. Chem., Int. Ed. Engl., 1995, 34,
2640; A. H. Vetter and A. Berkessel, Tetrahedron Lett., 1998, 39,
1741; D. A. Cogan, G. Liu, K. Kim and B. J. Ellmon, J. Am. Chem.
Soc., 1998, 120, 8011; W. Adam, D. Golsch, J. Sundermeyer and
G. Wahl, Chem. Ber., 1996, 129, 1177; K. Nakajima, K. Kojima,
M. Kojima and J. Fujita, Bull. Chem. Soc. Jpn., 1990, 63, 2620;
D. Rehder, G. Santoni, G. M. Licini, C. Schulzke and B. Meier,
Coord. Chem. Rev., in the press.
12 K. H. Thompson, J. H. McNeil and C. Orvig, Chem. Rev., 1999, 99,
2561; M. Melchior, K. H. Thompson, J. M. Jong, S. J. Retting,
E. Shuter, V. G. Yuen, Y. Zhou, J. H. McNeil and C. Orvig, Inorg.
Chem., 1999, 38, 2288; S. S. Amin, K. Cryer, B. Zhang, S. K. Dutta,
S. S. Eaton, O. P. Anderson, S. S. Miller, B. A. Reul, S. M. Brichard
and D. C. Crans, Inorg. Chem., 2000, 39, 406.
13 D. Rehder, J. Costa Pessoa, C. F. G. C. Geraldes, M. M. C. A.
Castro, T. Kabanos, T. Kiss, B. Meier, G. Micera, L. Pettersson,
M. Rangel, A. Salifoglou, I. Turel and D. Wang, J. Biol. Inorg.
Chem., 2002, 7, 384.
14 C. R. Cornman, G. J. Colpas, J. D. Hoeschele, J. Kampf and
V. L. Pecoraro, J. Am. Chem. Soc., 1992, 114, 9925.
increase in intensity of the 274 nm band along with its broaden-
ing. In addition, a shoulder starts to appear at ca 324 nm. The
384 nm band remains nearly constant. Following the formation
of an oxo–hydroxo complex, viz. [VO(OH)HL]ϩ, where H2L =
N-[{(o-hydroxyphenyl)methyl}-NЈ-(2-hydroxyethyl)ethylene-
diamine], generated with HCl from the respective dixoxvanad-
ium() dimer,38 and based on established hydroxo–oxovanadium
complexes,39,40 the formation of [VO(OH)(acpy-inh)]ϩ is also
proposed here. On allowing the solution to stand overnight, or
on addition of KOH dissolved in methanol, the reaction is
reversed, i.e. the solution acquires the original spectral pattern
of 2. The formation of a hydroxo species is an important
observation in the context of the vanadate-dependent halo-
peroxidases, for which a hydroxo group in an apical position
has been confirmed.41
15 S.-X. Liu and S. Gao, Inorg. Chim. Acta, 1998, 282, 149 and
literature cited therein.
16 S. W. Taylor, B. Kammerer and E. Bayer, Chem. Rev., 1997, 97,
333.
Conclusion
17 I. Batinic´-Haberle, M. Birusˇ and M. Pribanicˇ, Inorg. Chem., 1991,
30, 4882.
18 R. A. Row and M. M. Jones, Inorg. Synth., 1957, 5, 113.
19 G. M. Sheldrick, SHELXS 86, University of Göttingen, 1986;
G. M. Sheldrick, SHELXS 93, University of Göttingen, 1993.
Monomeric, distorted square-pyramidal dioxovanadium()
complexes have been obtained by using bulky monobasic
tridentate ONN ligands based on hydrazones. These can be
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View Article Online
20 O. W. Howarth, Progr. Nucl. Magn. Reson Spectrosc., 1991, 22, 453.
21 H. Schmidt, I. Andersson, D. Rehder and L. Pettersson, Chem. Eur.
J., 2001, 7, 251.
22 C. J. Carrano, C. M. Nunn, R. Quan, J. A. Bonadies and V. L.
Pecoraro, Inorg. Chem., 1992, 29, 944.
23 G. Asgedom, A. Sreedhara, J. Kivikoski, E. Kolehmainen and
C. P. Rao, J. Chem. Soc., Dalton Trans., 1996, 93.
24 S. N. Pal and S. Pal, Acta Crystallogr., Sect. C, 2001, 57, 141.
25 M. R. Maurya, S. Khurana, W. Zhang and D. Rehder, Eur. J. Inorg.
Chem., 2002, 1749; M. R. Maurya, S. Khurana, C. Schulzke and
D. Rehder, Eur. J. Inorg. Chem., 2001, 779; D. Wang, M. Ebel,
C. Schulzke, C. Grüning, S. K. S. Hazari and D. Rehder, Eur. J.
Inorg. Chem., 2001, 935.
26 G. Asgedom, A. Sreedhara, J. Kivikoski and C. P. Rao, Polyhedron,
1997, 16, 643.
27 H. Schmidt, M. Bashirpoor and D. Rehder, J. Chem. Soc., Dalton
Trans., 1996, 3865.
28 L. M. Mokry and C. J. Carrano, Inorg. Chem., 1993, 32, 6119.
29 S. N. Pal and S. Pal, J. Chem. Crystallogr., 2000, 30, 329; X. Li,
M. S. Lah and V. L. Pecoraro, Inorg. Chem., 1988, 27, 4657;
A. G. J. Ligtenbarg, A. L. Spek, R. Hage and B. E. Feringa, J. Chem.
Soc., Dalton Trans., 1999, 659.
30 A. D. Westland, F. Haque and J.-M. Bouchard, Inorg. Chem., 1980,
19, 2255.
32 S. P. Perlepes, D. Nicholls and M. R. Harrison, Inorg. Chim. Acta,
1985, 102, 137.
33 S. Gao, Z.-Q. Weng and S.-X. Liu, Polyhedron, 1998, 15, 3595.
34 G. Asgedom, A. Sreedhara and C. P. Rao, Polyhedron, 1996, 15,
3731; C. R. Cornman, J. Kampf and V. L. Pecoraro, Inorg. Chem.,
1992, 31, 1981.
35 A. D. Keramidas, A. B. Papaioannou, A. Vlahas, T. A. Kabanos,
G. Bonas, A. Makriyannis, C. P. Rapropoulou and A. Terzis, Inorg.
Chem., 1996, 35, 357.
36 D. Rehder, C. Weidemann, A. Duch and W. Priebsch, Inorg. Chem.,
1988, 27, 584; D. Rehder, in Transtion Metal Nuclear Magnetic
Resonance, ed. P. S. Pregosin, Elsevier, New York, 1991 p. 1.
37 A. J. Tasiopoulos, A. N. Troganis, A. Evangelou, C. P. Raptopoulos,
A. Terzis, Y. Deligiannakis and T. A. Kabanos, Chem. Eur. J., 1999,
5, 910; C. R. Cornman, J. Kampf, M. S. Lah and V. L. Pecoraro,
Inorg. Chem., 1992, 31, 2035.
38 G. J. Colpas, B. J. Hamstra, J. W. Kampf and V. L. Pacoraro, Inorg.
Chem., 1994, 33, 4669.
39 M. Kosugi, S. Hikitchi, M. Akita and Y. Moro-oka, Inorg. Chem.,
1999, 38, 1173.
40 P. Knopp, K. Wieghardt, B. Nuber, J. Weiss and W. S. Sheldrick,
Inorg. Chem., 1990, 29, 363; A. Giacomelli, C. Floriani, A. O. de
Souza Duarte, A. Chiesi-Villa and C. Guastino, Inorg. Chem., 1982,
21, 3310.
31 M. R. Maurya, S. Gopinathan, C. Gopinathan and R. C. Maurya,
Polyhedron, 1993, 12, 159.
41 M. Weyand, H.-J. Hecht, M. Keiß, M.-F. Liaud, H. Vilter and
D. Schomburg, J. Mol. Biol., 1999, 293, 595.
J. Chem. Soc., Dalton Trans., 2002, 3015–3023
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