Syntheses, characterization, and X-ray crystal structures of two cis-dioxovanadium(V) complexes of pyrazole-derived, Schiff-base ligands

Article abstract of DOI:10.1080/00958972.2010.499457

Two mononuclear cis-dioxovanadium(V) complexes of pyrazole-derived, Schiff-base ligands have been synthesized and characterized. Single crystal X-ray analyses were performed with N '-[(3-methyl-1H-pyrazole-5-yl)carbonyl] pyridine-2-carbohadrazonamido cis-dioxovanadium(V), {[VO₂(PzOAP)] H₂O} (1), and 5-methyl-N-[(1E)-1-(pyridin-2-yl)ethylidene]-1H- pyrazole-3-carbohydrazonate cis-dioxovanadium(V), {[VO₂(PzCAP)]} (2). Both complexes crystallize in monoclinic crystal systems with different space groups. Complex 1 crystallizes in the space group P21/c, 2 in space group C2/C. In each complex, the vanadium sits within a distorted square pyramidal geometry with an N₂O₃ chromophore. The parameters of the complexes (0.33 for 1, 0.22 for 2) support their square pyramidal geometry. The interesting finding in the work is that the alkoxide oxygen, imino nitrogen, and pyridine nitrogen take part in the coordination process leaving the pyrazole rings inactive in coordination.

Full text of DOI:10.1080/00958972.2010.499457

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Syntheses, characterization, and
X-ray crystal structures of two cis-
dioxovanadium(V) complexes of
pyrazole-derived, Schiff-base ligands
a
a
b
Somnath Roy , Tarak Nath Mandal , Kinsuk Das , Ray J.
c
d
a
Butcher , Arnold L. Rheingold & Susanta Kumar Kar
a
Department of Chemistry , University College of Science , 92,
A.P.C. Road, Kolkata–700 009, West Bengal, India
b
Department of Chemistry , Haldia Government College, Debhog ,
Purba Midnapur–721657, West Bengal, India
c
Department of Chemistry , Howard University , 2400 Sixth
Street, N.W., Washington, DC 20059, USA
d
Department of Chemistry and Biochemistry , University of
California , San Diego, La Jolla, California 92093, USA
Published online: 13 Jul 2010.
To cite this article: Somnath Roy , Tarak Nath Mandal , Kinsuk Das , Ray J. Butcher , Arnold L.
Rheingold & Susanta Kumar Kar (2010) Syntheses, characterization, and X-ray crystal structures
of two cis-dioxovanadium(V) complexes of pyrazole-derived, Schiff-base ligands, Journal of
Coordination Chemistry, 63:12, 2146-2157, DOI: 10.1080/00958972.2010.499457
To link to this article: http://dx.doi.org/10.1080/00958972.2010.499457
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Journal of Coordination Chemistry
Vol. 63, No. 12, 20 June 2010, 2146–2157
Syntheses, characterization, and X-ray crystal structures of
two cis-dioxovanadium(V) complexes of pyrazole-derived,
Schiff-base ligands
SOMNATH ROYy, TARAK NATH MANDALy, KINSUK DASz,
RAY J. BUTCHERx, ARNOLD L. RHEINGOLD{ and
SUSANTA KUMAR KAR*y
yDepartment of Chemistry, University College of Science, 92, A.P.C. Road,
Kolkata – 700 009, West Bengal, India
zDepartment of Chemistry, Haldia Government College, Debhog,
Purba Midnapur – 721657, West Bengal, India
xDepartment of Chemistry, Howard University, 2400 Sixth Street, N.W.,
Washington, DC 20059, USA
Department of Chemistry and Biochemistry, University of California, San Diego,
La Jolla, California 92093, USA
{
(Received 15 October 2009; in final form 30 March 2010)
Two mononuclear cis-dioxovanadium(V) complexes of pyrazole-derived, Schiff-base ligands
have been synthesized and characterized. Single crystal X-ray analyses were performed
0
with
N -[(3-methyl-1H-pyrazole-5-yl)carbonyl]pyridine-2-carbohadrazonamido
cis-
O} (1), and 5-methyl-N-[(1E )-1-(pyridin-2-yl)ethyli-
dene]-1H-pyrazole-3-carbohydrazonate cis-dioxovanadium(V), {[VO (PzCAP)]} (2). Both
dioxovanadium(V), {[VO
2
(PzOAP)] ꢀ H
2
2
complexes crystallize in monoclinic crystal systems with different space groups. Complex 1
crystallizes in the space group P21/c, 2 in space group C2/C. In each complex, the vanadium sits
within a distorted square pyramidal geometry with an N O chromophore. The ꢀ parameters of
2 3
the complexes (0.33 for 1, 0.22 for 2) support their square pyramidal geometry. The interesting
finding in the work is that the alkoxide oxygen, imino nitrogen, and pyridine nitrogen take part
in the coordination process leaving the pyrazole rings inactive in coordination.
Keywords: Vanadium(V) complexes; Syntheses; Spectral properties; Crystal structures
1. Introduction
The coordination chemistry of vanadium is of interest because of its presence in abiotic
and biotic systems [1–6]. The element is also present in some sea squirts [1, 2],
mushrooms [3, 4], and vanadium-containing enzymes, such as nitrogenase [5] and
haloperoxidases [6]. Vanadium(V) complexes are effective inhibitors of various
enzymes. Momentum to the coordination chemistry of vanadium in medical applica-
tions arises from the ability of vanadium complexes to promote insulin mimetic activity
in the pathophysiological state of diabetes mellitus in humans [7–10]. This biological
*
Corresponding author. Email: skkar_cu@yahoo.co.in
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.499457

cis-Dioxovanadium(V) complexes
2147
and catalytic relevance of vanadium has prompted the synthesis of numerous vanadium
compounds containing O, N donor ligands whose spectroscopic, magnetic, and redox
properties have been widely investigated [11–21]. Other vanadium complexes of O, N
donor ligands have been reported [22]. Although work on vanadium with heterocyclic
thiosemicarbazones and other ligands has been reported [23], little research has been
centered on vanadium chemistry with polytopic ligands having pyrazole as the
heterocyclic ligand. Considering the broad spectrum of potentially useful biological
implications and the catalytic activity of the metal complexes with 5-methylpyrazole-3-
carbohydrazide derivatives [15–20], herein we report syntheses, spectroscopic charac-
terization, and X-ray crystallographic characterization of two cis-dioxovanadium(V)
complexes with PzOAP and PzCAP. We reported earlier the syntheses and coordinating
behaviors of these ligands toward Cu(II) [24, 25]. As an extension, we have used the
same ligands to study vanadium chemistry in an NNO donor environment. In our
previous work [24, 25], PzOAP and PzCAP formed homoleptic tetranuclear Cu(II)
2 ꢁ 2 square/rectangular grid complexes using alkoxide oxygen as a bridge in 1 : 1
ligand : metal proportion.
2
2
. Experimental
.1. Materials
All chemicals were of reagent grade, purchased from commercial sources and used
without purification. Ethanol was purified using the established method [26, 27].
2.2. Purification of ethanol
A 2 L round bottom flask was washed by chromic acid and then distilled water. The
flask was dried in an oven. Approximately 7.5 g magnesium turnings were weighed and
very dil. HCl (1 : 50 v/v) was used to clear the magnesium as silver white. The
magnesium turnings were again washed 10–15 times by distilled water and alcohol and
ꢃ
dried in an oven at ꢂ50 C. Five grams of dry magnesium was taken in the flask in
which the alcohol was to be dried; 0.5 g iodine and 50 mL dehydrated alcohol were
added to it. The reaction mixture was warmed until vigorous reaction occurred. After
complete reaction, the mixture was kept at room temperature. After the formation of
magnesium ethoxide as a white cake, 1 L of ethyl alcohol was added and refluxed for
2
h. Finally, the purified ethyl alcohol was distilled off.
2
.3. Physical measurement
Elemental analyses (carbon, hydrogen, and nitrogen) of the ligands and the metal
complexes were determined with a Perkin–Elmer CHN analyzer 2400 at the Indian
Association for the Cultivation of Science, Kolkata. Electronic spectra of the complexes
in DMF were recorded on a Hitachi model U-3501 spectrophotometer. Infrared (IR)
ꢄ1
spectra (KBr pellets, 4000–500 cm ) were recorded on a Perkin–Elmer model 883 IR
1
spectrophotometer. H-NMR spectra of the ligands were recorded in d -DMSO with
a Bruker AM 300L (300 MHz) superconducting FT-NMR.
6

2
148
S. Roy et al.
CH3
CH3
NH
X
X
NH
N
NH
N
N
N
N
N
OH
N
O
“
Ketoimine” form
“Imineol” form
X = NH ; PzOAP
2
X = CH ; PzCAP
3
Scheme 1. Tautomeric forms of the ligands.
2.4. Syntheses and characterization of the ligands
5-Methylpyrazole-3-carbohydrazide was prepared following established method [28].
The two ligands were synthesized as described below, following the same methods as
reported earlier [24, 25]. Schematic representations of the two ligands are shown in
scheme 1.
2.4.1. Synthesis of PzOAP (C H N O). The methyl ester of iminopicolinic acid was
11 12 6
prepared in situ by the reaction of 2-cyanopyridine (5.2 g, 0.05 mol) with sodium
methoxide solution, produced by dissolving sodium metal (2.5 g, 0.108 mol) in dry
methanol (25 mL). 5-Methylpyrazole-3-carbohydrazide (7 g, 0.05 mol) in 25–30 mL dry
methanol was added to the above solution with stirring for 1 h. The mixture was
refluxed for 5 h and cooled to room temperature. Excess methanol was removed in a
rotary evaporator to leave a red oily sticky liquid. It was then mixed with water (50 mL)
and neutralized with AcOH (pH ꢂ 5) affording a yellow powder. The solid was filtered
off, washed thoroughly with methanol, and dried in vacuo over fused CaCl . Yield:
2
ꢃ
þ
ꢄ1
7.32 g, 60%; m.p. 216 C (decomp.); MS (m/z) 244 (M , 100%); IR (cm ): ꢁNH 3353;
1
ꢁCO/CN 1651(s), 1539(s); ꢁN–Npz 1057(s); ꢁpy 1004(s). H-NMR ꢂ (d -DMSO): 2.23
6
(s, 3H), 6.31 (s, 1H), 6.79 (s, NH ), 7.43 (m, 1H), 7.82 (m, 1H), 8.12 (d, 1H), 8.54
2
(
d, 1H), 10.03 (s, Pz-NH), 12.96 (b, azo-NH). Anal. Calcd for C H N O: C, 54.09; H,
1
1
12
6
4
.92; N, 34.42. Found: C, 53.95; H, 4.61; N, 34.32%.
2
.4.2. Synthesis of PzCAP (C H N O). 5-Methylpyrazole-3-carbohydrazide (1.40 g,
12 13 5
0
.01 mol) in 20–25 mL methanol was added dropwise to the methanolic solution (5 mL)
of 2-acetyl pyridine (1.21 g, 0.01 mol) with constant stirring. The stirring was continued
for 20 min and the mixture was refluxed for 5 h and cooled to room temperature. Excess
methanol was removed by rotary evaporator giving a colorless viscous liquid which was
kept in an ice bath. After a while, the liquid froze to a microcrystalline white solid. The
solid was filtered off, washed thoroughly with cold methanol and dried in vacuo over
ꢃ
þ
fused CaCl . Yield: 2.09 g, 80%; m.p. 210 C (decomp.); MS (m/z) 243 (M , 100%);
2
ꢄ1
1
IR (cm ): ꢁNH 3325; ꢁCO/CN 1661(s), 1534(s); ꢁN–Npz 1047(s); ꢁ 1020(s). H-NMR
py
ꢂ (d -DMSO): 2.25 (s, 3H), 2.30 (s, 3H), 6.51 (s, 1H), 7.52 (m, 1H), 7.80 (m, 1H), 8.05
6
(d, 1H), 8.57 (d, 1H), 10.26 (s, Pz-NH), 13.10 (s, azo-NH). Anal. Calcd (%) for
C H N O: C, 59.26; H, 5.35; N, 28.80. Found (%): C, 59.12; H, 5.32; N, 28.78.
1
2
13
5

cis-Dioxovanadium(V) complexes
.5. H-NMR spectra of the ligands
2149
1
2
1
H-NMR spectra (ppm) in d -DMSO at 300 MHz of PzOAP and PzCAP give singlets
6
at ꢂ 2.23 (3H) and ꢂ 2.25 (3H) assignable to C –CH (ring pz) and singlets at ꢂ 6.31 (1H)
3
5
and ꢂ 6.51 (1H) due to C –H (ring pz). A singlet at ꢂ 6.79 (2H) for PzOAP is ascribed to
4
the terminal C–NH and a singlet at ꢂ 2.30 (3H) for PzCAP is ascribed to terminal
2
CꢄCH . A singlet at ꢂ 10.03 (1H) for PzOAP and a singlet at ꢂ 10.26 (1H) for PzCAP
3
are ascribed to the pyrazolyl –NH proton of the ligands. A broad singlet at ꢂ 12.96 (1H)
for PzOAP and a singlet at ꢂ 13.10 (1H) for PzCAP confirm the presence of ‘‘ketoimine’’
–
NH protons in the ligands.
2
2
.6. Preparation of the complexes
0
.6.1. Preparation of N -[(3-methyl-1H-pyrazole-5-yl)carbonyl]pyridine-2-carbo-hydrazo-
namido cis-dioxovanadium(V) {[VO (PzOAP)] H O} (1). To a solution of PzOAP
2
.
2
(0.447 g, 2.27 mmol) in hot purified ethanol (15 mL) was added VO(acac) (0.601 g,
2
2
.27 mmol). The green mixture was refluxed for 3 h turning greenish yellow. This mixture
was kept at room temperature for slow evaporation. Yellow crystals suitable for X-ray
diffraction so formed were filtered off after 72 h, washed with ethanol : water (1 : 1 v/v)
and dried over anhydrous CaCl . Yield: 0.733 g, 70%. Anal. Calcd for C H N O V (1):
2
11 13
6
4
ꢄ1
C, 38.36; H, 3.77; N, 24.40. Found (%): C, 38.38; H, 3.80; N, 24.36. IR (KBr, cm ): ꢁNH
3
1
309; ꢁCO/CN 1610(s), 1508(s); ꢁN–Npz 1140(s); ꢁpy 1005(s). H-NMR ꢂ (d -DMSO): 2.46
6
(s, 3H), 6.40 (s, 1H), 7.75 (s, NH ), 8.24 (m, 1H), 8.33 (m, 1H), 8.39 (d, 1H), 8.78 (d, 1H),
2
1
0.05 (s, Pz-NH). UV-Vis (CH CH OH): ꢃmax/nm: 263, 380.
3 2
2.6.2. Preparation of 5-methyl-N-[(1E )-1-(pyridin-2-yl)ethylidene]-1H-pyrazole-3-carbo-
hydrazonate cis-dioxovanadium(V), [VO (PzCAP)] (2). Complex 2 was prepared using
2
the same procedure and reaction stoichiometry as for 1. Yield: 0.807 g, 70%. Anal.
Calcd (%) for C H N O V (2): C, 44.28; H, 3.69; N, 21.52. Found (%): C, 44.31; H,
1
2
12
5
3
ꢄ1
3
1
.71; N, 21.50. IR (KBr, cm ): ꢁNH 3199; ꢁCO/CN 1600(s), 1505(s); ꢁN–Npz 1065(s); ꢁpy
1
030(s). H-NMR ꢂ (d -DMSO): 2.23 (s, 3H), 2.52 (s, 3H), 6.49 (s, 1H), 7.72 (m, 1H),
6
8
ꢃmax/nm: 295, 393.
.15 (m, 1H), 8.33 (d, 1H), 8.75 (d, 1H), 10.31 (s, Pz-NH). UV-Vis (CH CH OH):
2
3
2
.7. Single crystal X-ray crystallography
Selected crystal data for 1 and 2 are given in table 1 and selected metrical parameters of
these complexes are given in table 2. For 1 and 2, data collections were made using a
Bruker SMART CCD area diffractometer and Oxford Diffraction Gemini R equipped
˚
with graphite-monochromated Mo-Kꢄ radiation (ꢃ ¼ 0.71073 A) fine focus sealed tube
source in ’ and ! scan mode at 100(2) and 200(2) K, respectively. For 1, cell parameter
refinement and data reduction were carried out using the Bruker SMART [29] and
SAINT software [30], and for 2, these were done by CrysAlis CCD, Oxford Diffraction
Ltd., Version 1.171.32.15 (release 10-01-2008 CrysAlis171.NET) [31]. The structures
were solved by direct methods and refined by full-matrix least squares using
2
F
data. SHELXS-97 and SHELXL-97 [32] were used for structure solution and

2
150
S. Roy et al.
Table 1. Experimental data of crystallographic analysis for 1 and 2.
Compound
1
2
Empirical formula
Formula weight
Temperature (K)
C
11
H
13
N
6
O
4
V
C
12 12 5 3
H N O V
344.21
100(2)
0.71073
325.21
200(2)
0.71073
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions (A, )
Monoclinic
P21/c
Monoclinic
C2/C
˚
ꢃ
a
b
c
ꢄ
ꢅ
ꢆ
8.7298(8)
6.9842(7)
22.283(2)
90
96.232(2)
90
1350.6(2), 4
1.693
0.765
12.5105(3)
13.2484(3)
16.2139(3)
90
97.522(2)
90
2664.2(1), 8
1.622
0.763
3
˚
Volume (A ), Z
Calculated density (mg m
ꢄ3
)
ꢄ1
Absorption coefficient (mm
F(000)
)
704
1328
0.48 ꢁ 0.41 ꢁ 0.18
3
Crystal size (mm )
0.30 ꢁ 0.10 ꢁ 0.05
2.35 to 28.23
ꢄ11 ꢅ h ꢅ 11
ꢄ9 ꢅ k ꢅ 8
ꢄ29 ꢅ l ꢅ 29
1.056
ꢃ
ꢇ range for data collection ( )
Index ranges
4.50 to 34.91
ꢄ19 ꢅ h ꢅ 19
ꢄ20 ꢅ k ꢅ 21
ꢄ25 ꢅ l ꢅ 25
2
Goodness-of-fit on F
0.979
Completeness to ꢇ ¼ 25.00 (%)
Independent reflections
Absorption correction
Max. and min. transmission
Refinement method
100.0
3102 [R(int) ¼ 0.0308]
99.1
5541 [R(int) ¼ 0.0268]
None
0.931 and 0.883
Semi-empirical from equivalents
1.00000 and 0.92826
2
Full-matrix least-squares on F Full-matrix least-squares on F
2
Data/restraints/parameters
Reflections collected
3102/0/219
10,997
5541/0/192
22,244
Final R indices [I 4 2ꢈ(I)]
R indices (all data)
Largest difference peak and hole (e A
R
R
1
1
¼ 0.0403, wR
2
2
¼ 0.1019
¼ 0.1071
R
R
1
1
¼ 0.0352, wR
2
2
¼ 0.0944
¼ 0.0999
¼ 0.0484, wR
¼ 0.0534, wR
ꢄ3
˚
)
0.663 and ꢄ0.399
0.436 and ꢄ0.255
refinement, respectively. For 1 and 2, absorption corrections were done by multi-scan
method using SADABS [33] and CrysAlis RED [31], Oxford Diffraction Ltd., Version
1
.171.32.15 (release 10-01-2008 CrysAlis171.NET). Empirical absorption correction was
done using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.
Positions of some hydrogens are placed in calculated positions and some are determined
from dF map. Ortep-3 and Platon programs were used for calculations of geometrical
parameters and Diamond 3.1 software was used for drawing the crystal structures of 1
and 2.
3
3
. Results and discussion
.1. Syntheses
PzOAP and PzCAP are mono-anionic with NNO donors. Complexes 1 and 2 were
obtained from the reaction of PzOAP and PzCAP, with VO(acac) in 1 : 1 molar ratio in
2
ethanol. Elemental analyses (C, H, and N) and IR spectral data [characteristic bands
ꢄ1
(cm ) at 3353–3325 (ꢁNH), 1534–1539 (ꢁ
), 1057–1047 (ꢁN–Npz), 1661–1651 (ꢁCO),
CH¼N

cis-Dioxovanadium(V) complexes
2151
˚
Table 2. Selected bond distances (A) and angles ( ) in 1 and 2.
ꢃ
1
2
˚
Bond distances (A)
V(1)–O(2)
V(1)–O(3)
V(1)–O(4)
V(1)–N(3)
V(1)–N(1)
O(4)–C(5)
N(3)–N(4)
N(4)–C(5)
N(5)–N(6)
N(2)–C(6)
N(3)–C(6)
1.625(2)
1.635(2)
1.968(1)
2.036(2)
2.144(2)
1.320(2)
1.392(2)
1.294(3)
1.351(3)
1.324(3)
1.304(3)
V(1)–O(1)
V(1)–O(2)
V(1)–O(3)
V(1)–N(2)
V(1)–N(1)
O(3)–C(8)
N(2)–N(3)
N(2)–C(6)
N(3)–C(8)
N(4)–N(5)
C(6)–C(7)
1.619(9)
1.62(1)
1.943(9)
2.101(9)
2.104(1)
1.300(1)
1.380(1)
1.291(1)
1.309(1)
1.340(1)
1.483(2)
ꢃ
Bond angles ( )
O(2)–V(1)–O(3)
O(2)–V(1)–O(4)
O(3)–V(1)–O(4)
O(2)–V(1)–N(3)
O(3)–V(1)–N(3)
O(4)–V(1)–N(3)
O(2)–V(1)–N(1)
O(3)–V(1)–N(1)
O(4)–V(1)–N(1)
N(3)–V(1)–N(1)
110.01(8)
99.71(8)
103.33(7)
121.21(8)
128.47(8)
74.58(7)
95.10(8)
98.23(7)
147.69(7)
73.18(7)
O(1)–V(1)–O(2)
O(1)–V(1)–O(3)
O(2)–V(1)–O(3)
O(1)–V(1)–N(2)
O(2)–V(1)–N(2)
O(3)–V(1)–N(2)
O(1)–V(1)–N(1)
O(2)–V(1)–N(1)
O(3)–V(1)–N(1)
N(2)–V(1)–N(1)
110.29(5)
100.74(4)
103.99(5)
132.32(5)
117.03(4)
73.87(4)
93.76(4)
99.96(5)
145.47(4)
73.21(4)
and 1539–1534 (ꢁCN)] are in good agreement with the structures in scheme 1.
ꢄ1
The absence of a band in the 3150–3175 cm region of the IR spectra of the free ligands
suggests the absence of a ‘‘imineol’’ tautomer in the solid state [34, 35]. The elemental
analyses, IR, UV-Vis, and NMR spectroscopy are consistent with the proposed
mononuclear formulation of 1 and 2.
3
.2. Description of crystal structures
The molecular structures of 1 and 2, including atom labeling diagrams, are shown in
figures 1 and 2, respectively. Selected bond lengths and angles are given in table 2.
Complex 1 crystallizes in P21/c space group while 2 C2/C space group.
[
VO (PzOAP)] Á H O and [VO (PzCAP)] are neutral and mononuclear. The asymmetric
2
2
2
units of 1 and 2 contain four and eight molecules, respectively. The vanadium in each
complex is coordinated to two nitrogens, one alkoxo oxygen, and two terminal oxo
groups. PzOAP and PzCAP function as singly deprotonated tridentate ligands. The
deprotonation is accompanied by tautomerization to the ‘‘iminol’’ form as shown in
scheme 1. The Schiff bases are coordinated meridionally, occupying three equatorial
positions. The remaining equatorial position is occupied by one of the two oxo groups
(O3 for 1 and O1 for 2). The apical position has the other oxo (O2 for 1 and 2).
The coordination geometry around vanadium is best described as square pyramidal
with a slight distortion (addison parameters (ꢀ) 0.33 for 1 and 0.22 for 2) toward
trigonal bipyramidal. The ꢀ parameters of the complexes (0.33 for 1, 0.22 for 2) also
support their square pyramidal geometry [36, 37], with 1 showing greater distortion

2
152
S. Roy et al.
Figure 1. Structural representation and atom-numbering scheme of 1.
Figure 2. Structural representation and atom-numbering scheme of 2.
than 2. All bond angles comprising adjacent donors in the basal plane and V1 as well as
any one donor atom, V1 and O3, i.e., O2–V1–O3, O3–V1–O4, N1–V1–O3 (for 1) and
ꢃ
O1–V1–O2, O1–V1–O3 (for 2) deviate from the ideal value of 90 (table 2) and V1 is
˚
shifted from the square plane toward the apical oxygen (O2 for 1 and 2) by 0.499 A in 1
˚
˚
˚
and 0.556 A in 2. The average V¼O distances 1.630 A in 1 and 1.619 A in 2, slightly
˚
longer than typical V¼O double bond length of 1.595 A, indicate that this bond length
is associated with weak hydrogen-bonding interactions [38]. The V1–N1 and V1–N3
˚
˚
distances 2.144 and 2.037 A in 1 and V1–N1 and V1–N2 distances 2.104 and 2.101 A in
, respectively, are in accord with the literature values for V–N distances [38, 39]. For 1,
C5, C6, C7, C8, C9, C10, C11, N1, N3, N4, O4 are almost in the same plane (with
2
˚
˚
maximum deviation of 0.069 A) with the vanadium shifted by 0.021 A out of that plane
toward O3 while for 2, C1, C2, C3, C4, C5, C6, C8, N1, N2, N3, O3 are also in the same
˚
˚
plane (with maximum deviation of 0.109 A) with vanadium shifted by 0.348 A out of
that plane toward O2 [40]. Hence, for 1, due to this deviation, the conformation of the
five-membered chelate ring containing V1, N3, N4, C5, and O4 is an envelope at V1.
Cremer and Pople [41] puckering analysis shows that the other five-membered chelate
ring containing V1, N1, N3, C6, and C7 also adopts an envelope conformation but in
˚
that case on C6 (with a deviation of 0.056 A) from the mean plane containing V1, N1,
N3, and C7. For 2, conformation of the five-membered chelate ring containing V1, N2,
N3, C8, and O3 is proposed to be an envelope on V1. The other five-membered chelate

cis-Dioxovanadium(V) complexes
2153
Figure 3. Packing diagram of 2. View along the c-axis.
Figure 4. H-bonding pattern in 1.
ring containing V1, N1, N2, C5, and C6 is also in an envelope conformation on C6
(
˚
with a deviation of 0.107 A) from the mean plane containing V1, N1, N2, and C5.
Intermolecular contacts which lead to the observed packing diagram of 2 are shown
in figure 3. In the packing of 2, the molecules are arranged in an opposed fashion,
minimizing steric hindrance.
Complex 1 is stabilized through a network of weak H-bond interactions (figure 4).
The details are presented in table 3. The H-bond distances between N2 and N5
˚
˚
˚
2.820 A], O1 and O3 [2.798 A], and N6 and O1 [2.817 A] support a weakly H-bonded
[
structure in 1 (figure 4). Similarly, the H-bond distance between N5 and O2#1 in
˚
is 2.872 A, indicating the presence of weak H-bonding interaction (figure 5).
2
3
.3. Characterization of the complexes
The V(V) complexes have been prepared employing a V(IV) starting material,
VO(acac) . Molecular dioxygen from air serves as the oxidant during the reaction.
The two complexes gave satisfactory C, H, and N analyses.
2

2
154
S. Roy et al.
˚
Table 3. Details of hydrogen bond distances (A) and angles ( ).
ꢃ
Compounds
D–H ꢀ ꢀ ꢀ A
d(D–H)
d(H ꢀ ꢀ ꢀ A)
d(D ꢀ ꢀ ꢀ A)
ff(DHA)
1
N2–H2 ꢀ ꢀ ꢀ N5
O1–H3 ꢀ ꢀ ꢀ O3
0.88(3)
0.78(3)
0.89(3)
0.88(3)
0.88(3)
2.04(3)
2.03(4)
1.93(3)
2.20(3)
2.03(3)
2.820(3)
2.798(2)
2.817(2)
3.027(3)
2.872(3)
148(3)
170(3)
173(3)
156(3)
159(3)
N6–H5 ꢀ ꢀ ꢀ O1
N2–H1 ꢀ ꢀ ꢀ O1
2
N(5)–H(5A) ꢀ ꢀ ꢀ O(2)
Symmetry transformations used to generate equivalent atoms: 1: ꢄ1 þ x, y, z; ꢄx, 1 ꢄ y, ꢄz; 2: ꢄx, ꢄy, 1 ꢄ z.
Figure 5. H-bonding pattern in 2.
3.4. IR spectra
A comparative study of the IR spectral data of the reported complexes with those of the
uncomplexed ligand gives information regarding bonding sites of the ligand. The
ꢄ1
complexes show a broad band in the 3400–2900 cm region, probably arising from
H-bonding between one of the V¼O groups with one pyrazole N in 1 and between one
ꢄ1
of the V¼O groups with one water in 2. The IR band at 1539–1534 cm in free PzOAP
ꢄ1
ꢄ1
and PzCAP spectra, ꢁ shifts to a lower wavenumber by 31 cm in 1 and 29 cm for
CN
2
, due to the coordination of azomethine [30]. A strong two-band pattern at
ꢄ1
941–818 cm is the signature of cis-dioxovanadium(V) [31], assigned to symmetric
and asymmetric ꢁ(
vibrations [32]. The same observations were reported in the
O¼V¼O)
ꢄ1
literature [22]. IR spectra of the two complexes show an intense band at ꢂ1070 cm
,
ꢄ1
indicating the presence of ꢁ
complexes may be assigned to coordinated ꢁ , which is also evident from X-ray
vibration. A strong band at ꢂ1020 cm in both
N–Npz
py
ꢄ1
crystallographic studies of 1 and 2. An intense band at ca 1656 cm characteristic for
ꢄ1
ꢁCO in the two ligands shift to a lower wavenumber by ca 51 cm after complexation,
indicating that the oxygen of C¼O coordinates in its deprotonated ‘‘imineol’’ form [30].
3
.5. Electronic spectra and DRS of 1 and 2
The electronic absorption spectra of the complexes were carried out in DMF.
The complexes have absorptions at 263 and 380 nm for 1 and 295 and 393 nm for 2. The
intense absorption around 279 nm corresponds to an intraligand transition, whereas the
band around 386 nm is assignable to L–V (dꢉ) LMCT [42, 43]. The diffuse reflectance
spectra of the two complexes show one broad band at 410–425 nm.

cis-Dioxovanadium(V) complexes
2155
1
.6. H-NMR spectra of 1 and 2
3
1
The H-NMR spectra (dH ppm) in d -DMSO at 300 MHz of 1 and 2 give singlets at
6
ꢂ 2.46 (3H) and ꢂ 2.23 (3H) assignable to C –CH (ring pz) and singlets at ꢂ 6.40 (1H)
5
3
and ꢂ 6.49 (1H) due to C –H (ring pz), respectively. A singlet at ꢂ 7.75 (2H) for 1 and a
4
singlet at ꢂ 2.52 (3H) for 2 are ascribed to the terminal C–NH and C–CH , respectively.
2
3
A singlet at ꢂ 10.05 (1H) for 1 and a singlet at ꢂ 10.31 (1H) for 2 are ascribed to the
pyrazolyl –NH proton. The absence of a singlet around ꢂ 12.96 in 1 and ꢂ 13.10 in 2
confirms enolization prior to complexation followed by deprotonation during
complexation. This is also evident from the X-ray crystal structure studies.
4
. Conclusion
We have prepared two V(V) complexes with pyrazole-derived Schiff-base ligands
PzOAP and PzCAP. In complexes, the polytopic ligand in its ‘‘imineol’’ form is a
monoanionic ONN donor, where the heterocyclic pyrazole nitrogens remain
non-coordinated; the deprotonated ‘‘enol’’ oxygen participates in coordination.
In Cu(II) salts, the pyrazole nitrogen takes part in the coordination forming 2 ꢁ 2
grid complexes. To make the alkoxide a donor, the two ligands undergo tautomeriza-
tion to ‘‘imineol’’ form followed by a deprotonation during complexation with V(V).
In the two complexes, V(V) is five-coordinate with alkoxide oxygen, the imine nitrogen,
pyridine nitrogen, and two oxo groups. The two similar ligands produce identical
coordination environments toward V(V).
Supplementary material
CCDC 743347 and 743348 contain the supplementary crystallographic data for 1 and 2.
These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: (þ44) 1223-336-033; or E-mail: deposit@ccdc.cam.
ac.uk. Supplementary data associated with this article can be found in the online
version.
Acknowledgments
S. Roy thanks CSIR, New Delhi, India for financial support by awarding senior
research fellowship (Sanc. No. 09/028(0698)/2008-EMR-1, dated 14.03.2008).
T.N. Mandal also acknowledges University Grants Commission (UGC), New Delhi,
India for financial assistance.

2
156
S. Roy et al.
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