Dioxo- and oxovanadium(V) complexes of biomimetic hydrazone ONO and NNS donor ligands: Synthesis, crystal structure and catalytic reactivity

Article abstract of DOI:10.1016/j.poly.2011.02.005

Oxo- and dioxo-vanadium(V) complexes of hydrazone ONO donor ligands with the formula [V^VO(μ₂-OCH₃)(L ¹)]₂ (1) and [V^VO₂(L ²)]·H₂O (2) were synthesized by th

Full text of DOI:10.1016/j.poly.2011.02.005

Polyhedron 30 (2011) 1375–1384
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Dioxo- and oxovanadium(V) complexes of biomimetic hydrazone ONO and NNS
donor ligands: Synthesis, crystal structure and catalytic reactivity
a
a
b
Hassan Hosseini Monfared a, , Samaneh Kheirabadi , Nasim Asghari Lalami , Peter Mayer
⇑
a Department of Chemistry, Zanjan University, 45195-313 Zanjan, Iran
b Fakultät für Chemie und Pharmazie, Ludwig-Maximilians-Universität, München, Butenandtstr. 5-13, Haus D, D-81377 München, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Oxo- and dioxo-vanadium(V) complexes of hydrazone ONO donor ligands with the formula
[VVO(
l
2-OCH3)(L1)]2 (1) and [VVO2(L2)]ꢀH2O (2) were synthesized by the reaction of [VO(acac)2] with pro-
Received 4 December 2010
Accepted 3 February 2011
Available online 17 February 2011
ton-transfer complexes of benzenetricarboxylic acid/benzoylhydrazide and benzenetricarboxylic acid/
isonicotinohydrazide, respectively (H2L1 = monocondensation of benzoylhydrazide and acetylacetone,
H2L2 = monocondensation of isonicotinohydrazide and acetylacetone). Dioxo complex of V(V), [VO2(L3)]
(3), was synthesized by the reaction of equimolar amounts of VO(acac)2, 2-acetylpyridine and thiosem-
icarbazide (H2L3 = hydrazone Schiff base of acetylpyridine and thiosemicarbazide and Hacac = acetylace-
tone). They were characterized by FT-IR, UV–Vis and NMR spectroscopic methods. The crystal structures
of 1 and 2 were determined by X-ray analyses. The 1H NMR spectrum of the complex 1 in CDCl3 solution
indicated that this dimeric complex is converted appreciably into its respective monomeric form. The
catalytic potential of the complexes has been tested for the oxidation of alkene, alkane and aromatic
hydrocarbons using H2O2 as the terminal oxidant. Good to excellent conversions have been obtained
for the oxidation of most of the hydrocarbons.
Keywords:
Methoxy-bridged vanadium complex
Hydrazone complexes
Crystal structure
Catalytic activity
Proton-transfer complex
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
pounds presenting this subunit is related to its relative acidity
and its capacity to stabilize free radicals [18]. It has been found
The coordination chemistry of vanadium has received consider-
able attention since the discovery of vanadium in the vanadium-
dependent haloperoxidases [1,2], nitrogenases [3] and nitrate
reductases [4]. The structures and properties of these vanadium
enzymes have stimulated the search for structural and functional
model compounds [5–8]. Oxovanadium(IV) and dioxovanadium(V)
complexes with N-, O- and S-donor chelating ligands have been
studied for their potential insulin-mimetic effects [9,10], tumor
growth inhibition and prophylaxis against carcinogenesis [11], to
inhibit several enzymes, including phosphatases, ATPases, nucleas-
es, kinases [12] and the antimicrobial activity against Mycobacte-
rium tuberculosis [13].
that oxovanadium(IV) and cis-dioxovanadium(V) complexes of thi-
osemicarbazone show comparable or larger anti-M. tuberculosis
activities than the free thiosemicarbazone ligands [19].
The use of oxovanadium complexes in oxidation and oxotrans-
fer catalysis has been noted [20]. Various organic substrates are
oxidized by peroxides in the presence of vanadium and dioxovana-
dium(V) complexes [21,22]. Dioxovanadium(V) complexes are
commonly synthesized (i) by the reaction of vanadates with
ONO-functional ligands in aqueous solution [23,24]; (ii) by the
reaction of triethyl vanadate, [VO(OEt)3], with appropriate ligands
[25] or (iii) by oxidation of oxovanadium(IV) complexes with KNO2
in H2O/MeOH [26] and (iv) by the reaction of [VO(acac)2]
(acacH = acetylacetone) or VOSO4 with the ligands in non-aqueous
or mixed solvent media followed by oxidation with O2 [5,27]. To
the best of our knowledge there is no report on the synthesis of
neutral dioxovanadium(V) complexes of hydrazone ligands via
proton-transfer complexes.
In view of importance of vanadium compounds, and also
extending the search for more efficacious compounds of vanadium
with neutral charge (one of the desirable qualities of vanadium
compounds to be useful as biomimetic drugs include neutral
charge [28]), we present here the synthesis and characterization
of oxovanadium(V) and dioxovanadium complexes of the
hydrazones H2L1, H2L2 and HL3 derived from acetylacetone/
On the other hand, hydrazones ANHAN@CRR0 (R and R0 = H, al-
kyl, aryl) are versatile ligands due to their applications in the field
of analytical [14] and medicinal chemistry [15]. Hydrazones exhi-
bit physiological activities in the treatment of several diseases such
as tuberculosis. This activity is attributed to the formation of stable
chelate complexes with transition metals which catalyze physio-
logical processes [16]. Hydrazone moieties are the most important
pharmacophoric cores of several antiinflammatory, antinocicep-
tive, and antiplatelet drugs [17]. The biological profile of com-
⇑
Corresponding author. Tel.: +98 241 5152576; fax: +98 241 5283203.
E-mail address: monfared@znu.ac.ir (H. Hosseini Monfared).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.02.005

1376
H. Hosseini Monfared et al. / Polyhedron 30 (2011) 1375–1384
benzoylhydrazide, acetylacetone/isonicotinohydrazide and acetyl-
pyridine/thiosemicarbazide; Scheme 1. Neutral oxo and dioxova-
nadium(V) complexes of hydrazone ligands were synthesized via
proton-transfer complexes. As many vanadium complexes show
catalytic activity in oxidation and oxygen transfer reactions [29],
including the oxidation of (prochiral) organic sulfides to (chiral)
sulfoxides [30], thus modeling the haloperoxidase activity, related
reactivity patterns have also been studied by testing the catalytic
activity of the complexes with respect to the oxidation of hydro-
carbons using H2O2 as an oxidant.
NH3+), 7.26–7.51 (m, 5H, benzoylhydrazide aryl), 7.86 (d, 3H, BTC3ꢁ
aryl), 8.46 (s (broad), 1H, amid ANHA).
2.2. Synthesis of [VVO( 2-OCH3)(L1)]2 (1)
l
To a solution of [Bzh+]3[BTC3ꢁ] (119 mg, 0.19 mmol) in metha-
nol (15 ml), [VO(acac)2] (50 mg, 0.19 mmol) was added. The reac-
tion mixture was stirred under reflux for 4 h. The obtained black
colored mixture was cooled to room temperature and the product
[VVO( 2-OCH3)(L1)]2 was removed by filtration, washed with
l
methanol and dried in air. Yield: 95% (113 mg). X-ray quality crys-
tals of [VVO( 2-OCH3)(L1)]2 could be grown from methanol. Anal.
l
2. Experimental
Calc. for C26H30N4O8V2 (628.42): C, 49.69; H, 4.81; N, 8.92; V,
16.21. Found: C, 49.45; H, 4.61; N, 9.00; V, 16.10%. IR (KBr,
cmꢁ1): 3457 (m, br), 2924 (m), 2856 (w), 1718 (w), 1600 (m),
1547 (s), 1495 (m), 1390 (s), 1276 (m), 1033 (m), 980 (s), 944
(m), 895 (w), 785 (m), 698 (m), 596 (s), 499 (m). 1H NMR
(250.13 MHz, CDCl3): d = 1.92 (s, 3H, 3ꢂH-8(bridged)), 2.30 (s,
3H, 3ꢂH-12(bridged)), 2.42 (s, 3H, 3ꢂH-8, monomer), 2.56 (s, 3H,
3ꢂH-12, monomer), 3.49 (s, 3H, V-OCH3, monomer), 5.12 (s, 3H,
OCH3 (bridged)), 5.52 (s, 1H, H-10, monomer), 5.71 (s, 1H, H-10
(bridged)), 7.21–8.10 (m, 5H, aryl (monomer and dimmer)). kmax
Bis(acetylacetonato)oxovanadium(IV), [VO(acac)2], alkenes, sol-
vents and other materials with high purity were purchased from
Merck and Fluka and used as received. IR spectra were recorded
as KBr disks with a Matson 1000 FT-IR spectrophotometer in the
range of 4000–450 cmꢁ1. UV–Vis spectra of solution were recorded
on a Shimadzu 160 spectrometer. 1H NMR spectra in DMSO-d6
solution were recorded on a Bruker 250 MHz spectrometer and
chemical shifts are indicated in ppm relative to tetramethylsilane.
The reaction products of oxidation were determined and analyzed
by HP Agilent 6890 gas chromatograph equipped with a HP-5 cap-
(e
, dm3 molꢁ1 cmꢁ1) = 214 (52 500), 271 (8700), 345 nm (4250).
illary column (phenyl methyl siloxane 30 m ꢂ 320
l
m ꢂ 0.25
lm)
2.3. Synthesis of [H2Inh2+][HBTC2ꢁ
]
and gas chromatograph–mass spectrometry (Hewlett-Packard
5973 Series MS-HP gas chromatograph with a mass-selective
detector). The elemental analyses (carbon, hydrogen, and nitrogen)
of compounds were obtained from Carlo ERBA Model EA 1108 ana-
lyzer. Vanadium percentages of complexes were measured by a
Varian spectrometer AAS-110.
For the synthesis of the isonicotinohydrazide–benzenetricarb-
oxylic acid proton-transfer complex ([H2Inh2+][HBTC2ꢁ]), a mix-
ture of isonicotinohydrazide (Inh) (40 mg, 0.29 mmol) and
benzene-1,3,5-tricarboxylic acid (BTC) (20 mg, 0.10 mmol) in
methanol (10 ml) was refluxed for 4 h. The white crystalline pre-
cipitate of proton-transfer complex ([H2Inh2+][HBTC2ꢁ]) obtained
on cooling the reaction media was filtered, washed with methanol
and dried in air. Yield: 32% (25 mg). 1H NMR (250.13 MHz, CDCl3):
d = 3.49 (very broad, 3H, NH3+), 4.20 (very broad, 1H, pyridinium-
NH+), 7.26–7.60 (m, 4H, aromatic of Inh), 7.72 (s, 3H, BTC3ꢁ),
8.78 (s, 1H, amid ANHA).
2.1. Synthesis of [HBzh+]3[BTC3ꢁ
]
At first the benzoylhydrazide–benzenetricarboxylic acid pro-
ton-transfer complex ([HBzh+]3[BTC3ꢁ]) was prepared. For the syn-
thesis of [HBzh+]3[BTC3ꢁ], a mixture of benzoylhydrazide (Bzh)
(0.38 g, 2.79 mmol) and benzene-1,3,5-tricarboxylic acid (H3BTC)
(0.20 g, 0.95 mmol) in methanol (10 ml) was refluxed for 5 h. The
white crystalline precipitate of proton-transfer complex
([HBzh+]3[BTC3ꢁ]) obtained on cooling the reaction media was fil-
tered, washed with methanol and dried in air. Yield: 34%
(199 mg). 1H NMR (250.13 MHz, CDCl3): d = 3.49 (s (sharp), 3H,
2.4. Synthesis of [VVO2(L2)]ꢀH2O (2)
To a solution of [H2Inh2+][HBTC2ꢁ] (84 mg, 0.10 mmol) in meth-
anol (10 ml), [VO(acac)2] (60 mg, 0.23 mmol) was added. The
H
N
N
N
N
N
N
OH
OH
O
O
OH
O
H2L1
N
N
N
N
H
N
N
N
N
N
OH
OH
N
OH
O
O
O
H2L2
NH2
H
N
N
NH2
N
N
SH
N
S
HL3
Scheme 1. The ligands H2L1–2 and HL3 tautomerism.

H. Hosseini Monfared et al. / Polyhedron 30 (2011) 1375–1384
1377
reaction mixture was stirred under reflux for 5 h. The obtained
black colored mixture was cooled to room temperature and
[VVO2(L2)]ꢀH2O was removed by filtration, washed with methanol
and dried in air. Yield: 90% (66 mg). Anal. Calc. for
in idealized geometry riding on their parent atoms. The O-bonded
hydrogen atoms were located from the difference map and refined
freely. Graphics were drawn with DIAMOND [34].
C
11H14N3O5VꢀH2O (319.19): C, 41.39; H, 4.42; N, 13.16; V, 15.96.
2.9. Experimental set up for catalytic oxidation
Found: C, 41.41; H, 4.39; N, 13.15; V, 16.10%. IR (KBr, cmꢁ1):
3450 (br, s, H2O), 3088 (w), 2925(s), 2854 (s), 1719 (m), 1618
(m), 1589 (m), 1534 (m), 1492 (vs) 1399 (vs), 1290 (w), 1215
9w), 1030 (s), 997 (s), 859 (m), 689 (m), 479 (s), 443 (s). 1H NMR
(250.13 MHz, CDCl3): d = 2.50 (s, 3H, 3ꢂH-5), 2.68 (s, 3H, 3ꢂH-1),
8.50 (s, 2H, pyridinium) and 8.80 (s, 2H, pyridinium). 1H NMR
(250.13 MHz, DMSO-d6): d = 5.35 (s, 1H, H-3), 8.02 (s, 2H, pyridini-
The liquid phase catalytic oxidations were carried out under air
(atmospheric pressure) in a 25 ml round bottom flask equipped
with a magnetic stirrer and immersed in a thermostated oil bath.
In a typical experiment, H2O2 (3 mmol) was added to a flask con-
taining the catalyst 1, 2 or 3 (about 3 lmol) and a representative
alkene, namely cyclooctene (1 mmol) in a solvent (2 ml). The
course of the reaction was monitored using a gas chromatograph
equipped with a capillary column and a flame ionization detector.
The oxidation products were identified by comparing their reten-
tion times with those of authentic samples or alternatively by 1H
NMR and GC–mass analyses. Yields based on the added substrate
were determined by a calibration curve. Control reactions were
carried out in the absence of catalyst, under the same conditions
as the catalytic runs. No products were detected.
um), 8.71 (s, 2H, pyridyl). kmax
274 (12 300) and 354 nm (2650).
(e
, dm3 molꢁ1 cmꢁ1) = 208 (38 800),
2.5. Preparation single crystals of [VVO2(L2)]ꢀH2O (2)
The charge transfer complex [Inh2+]3[BTC3ꢁ]2 (14 mg, 16 mmol)
and [VO(acac)2] (20.0 mg, 0.075 mmol) were placed in the main
arm of the branched tube (‘branched tube’ method). A mixture of
ethanol and water (50:50 v/v, 13 ml) was carefully added to fill
the arms, the tube was sealed and the reagents containing arm im-
mersed in an oil bath at 60 °C while the other arm was kept at
ambient temperature. After 1 week, red-brown crystals were
deposited in the cooler arm, which were filtered off, washed with
methanol and air dried. Yield: 87% (21 mg).
3. Results and discussion
The ligands H2L1 and H2L2 were synthesized in situ and coordi-
nated to vanadium by refluxing the proton-transfer complex of the
corresponding hydrazide with [VO(acac)2]. The proton-transfer
complexes [Bhz+]3[BTC3ꢁ] and [H2Inh2+][HBTC2ꢁ] were obtained
by direct reaction of benzene-1,3,5-tricarboxylic acid (H3BTC) with
2.6. Synthesis of 2-(1-(pyridin-2-
yl)ethylidene)hydrazinecarbothioamide, HL3
The ligand HL3 was synthesized as described in the literature
[31]. Yield: 93% (180 mg). Anal. Calc. for C8H10N4S (194.26): C,
49.46; H, 5.19; N, 28.84. Found: C, 49.43; H, 5.22; N, 28.83. 1H
NMR (250.13 MHz, DMSO-d6): d = 2.33 (s, 3H, CH3), 7.30–8.52 (m,
Table 1
Crystal data and structure refinement for [VVO(
l
2-OCH3)(L1)]2 (1) and [VVO2(L2)]ꢀH2O
(2).
Compound
[VVO(
l
2-OCH3)(L1)]2
[VVO2(HL2)]ꢀH2O
(2)a
(1)
4H, pyridyl), 8.10 (s, NH2), 10.27 (s, 1H, NH). kmax
(e,
dm3 molꢁ1 cmꢁ1) = 201 (104 300), 314 nm (57 900).
Net formula
C26H30N4O8V2
628.422
200(2)
C11H14N3O5V
319.188
173(2)
orthorhombic
Pnma
13.9893(6)
6.6960(4)
13.8666(8)
90
Mr (g molꢁ1
T (K)
)
2.7. Synthesis of [VO2(L3)]
Crystal system
Space group
a (Å)
b (Å)
c (Å)
orthorhombic
Pbca
10.8712(3)
12.1990(3)
21.1367(6)
90
[VO2(L3)] was also synthesized by branched tube method from
equimolar reaction of [VO(acac)2] with HL3 in methanol. After
4 days, needle brown crystals were deposited in the cooler arm,
which were filtered off, washed with methanol and air dried. Yield:
81% (79 mg). Anal. Calc. for C8H9N4O2SV (276.19): C, 34.79; H, 3.28;
N, 20.29; V, 18.44. Found: C, 34.70; H, 3.28; N, 20.22; V, 18.41%. IR
(KBr, cmꢁ1): 3304 (m), 3145 (m), 1638 (s) (C@N), 1598 (m), 1553
(m), 1504 (m), 1448 (vs), 1373 (m), 1325 (m), 1185 (s), 1043 (w,
V@O), 938 (vs, V@O), 883 (m), 810 (m), 775 (m), 741 (m), 621
(m), 473 (m). 1H NMR (250.13 MHz, DMSO-d6): d = 2.50 (s, 3H),
a
(°)
b (°)
90
90
90
90
c
(°)
V (Å3)
2803.10(13)
4
1298.92(12)
4
1.63222(15)
0.787
‘multi-scan’
0.96070–1.00000
3366
0.0268
0.0392
4.21–26.31
1082
0.0422, 0
mixed
1433
125
2
Z
Dcalc (g cmꢁ3
)
1.48911(7)
0.722
l
(mmꢁ1
)
Absorption correction
Transmission factor range
Reflections measured
Rint
Mean
h Range
Observed reflections
x, y (weighting scheme)
Hydrogen refinement
Reflections in refinement
Parameters
‘multi-scan’
0.92394–1.00000
12 786
0.0301
7.57–8.62 (m, 4H, pyridyl), 7.81 (s, 2H, NH2). kmax
(e,
dm3 molꢁ1 cmꢁ1) = 210 (64 700), 312 (54 000), 420 nm (2900).
r
(I)/I
0.0370
The crystal structure of 3 has been reported recently [19].
4.22–26.33
2086
0.0493, 0
constr.
2848
184
2.8. X-ray structure determination
Single crystals of 1 and 2 were carefully selected under a polar-
izing microscope. X-ray quality crystals of [VVO( 2-OCH3)(L1)]2, (1)
l
Restraints
0
(brown) could be grown from methanol. Dark brown crystals of 1
(0.22 mm ꢂ 0.20 mm ꢂ 0.19 mm) and 2 (0.26 mm ꢂ 0.15 mm ꢂ
0.10 mm) were investigated in diffraction experiments at
200(2) K or 173(2) K on an Oxford XCalibur/a KappaCCD diffrac-
R(Fobs
)
0.0296
0.0799
0.937
0.001
0.0307
0.0793
0.985
0.001
0.309
Rw(F2)
S
Shift/errormax
Maximum electron density
0.281
(e Åꢁ3
)
tometer and with monochromated Mo K
a radiation (k = 0.71073).
Minimum electron density
ꢁ0.326
ꢁ0.337
The structures were solved by Direct Methods with SIR97 [32],
and refined with full-matrix least-squares techniques on F2 with
SHELXL-97 [33]. The crystal data and refinement parameters are pre-
sented in Table 1. The C-bonded hydrogen atoms were calculated
(e Åꢁ3
)
a
C-bound H: constr., O-bound H: distance fixed to 0.82 Å with U(H) = 1.5 U(O),
N-bound H: distance fixed to 0.86 Å with U(H) = 1.2 U(N).

1378
H. Hosseini Monfared et al. / Polyhedron 30 (2011) 1375–1384
benzoylhydrazide (Bzh) and isonicotinohydrazide (Inh) in metha-
nol, respectively. Reaction of [VO(acac)2] with [Bhz+]3[BTC3ꢁ] led
to the condensation of benzoylhydrazide with one of the
O
V
O
O
O
O
H
N
[BTC3-] + 2HOCH3 + O2
+
2
2
H3N
acetylacetonato groups and formation of [VVO( 2-OCH3)(L1)]2 (1)
l
O
(where H2L1 = Schiff base derived from monocondensation of ace-
tylacetone and benzoylhydrazide). For the synthesis of complex
[VVO2(L2)]ꢀH2O (2) a method similar to complex 1 was followed
but a dioxovanadium(V) complex was formed. All the stated reac-
tions can be represented as Schemes 2 and 3 for complexes 1 and 2.
Complex 3 was formed in a straightforward fashion from solutions
of the respective ligand (HL3) and [VO(acac)2]. The oxidizing agent
for the formation of pentavalent state of vanadium is the aerial
oxygen.
3
#
H
N
N
O
OH
H2L1
3.1. Infrared spectra
In compound [VO(
observed with splittings at 1600 and 1547 cmꢁ1 are due to newly
formed AC@NAN@CA moiety. weak band observed at
1718 cmꢁ1 is assigned to the acetylacetonto C@C band. The dinu-
l
2-OCH3)(L1)]2 (1), a strong intensity bands
N
O
N
A
O
V
OCH3
O
clear complex exhibits a sharp band at 980 cmꢁ1 due to
m
(V@O)
l
H3CO
+ 4H2O + 2Hacac + 2H3BTC
V
O
and a broad but sharp band at 895–944 cmꢁ1 due to
V] mode.
m
[V–( -O)–
O
O
N
N
In IR spectrum of complex 2 exhibits a broad band at 3450 cmꢁ1
due to hydrogen bonded NAH of pyridinium group and OAH of
water molecule. The AC@NAN@CA moiety is observed as a med-
ium band at 1618 and 1589 cmꢁ1. The complex 2 shows strong
bands at around 997 and 859 cmꢁ1 which may be assigned to anti-
Scheme 2. Synthesis pathway of 1.
420 nm for 3. The other bands observed at 271 and 214 nm for 1,
274 and 208 nm for 2, 312 and 210 nm for 3 correspond mainly
to intraligand transitions. DFT calculations on the similar hydra-
zone complex show that LMCT transitions contribute also in the
other bands [35]. The LMCT energy of complexes derived from
Schiff base ligands decreases in the order: 1 > 2 > 3. The intense
208–214 nm and 271–312 nm bands were assigned to intraligand
symmetric and symmetric
group, respectively [10]. The spectrum shows
tions in the range 1399–1618 for 2. A band around 1719 cmꢁ1 is
probably due to (C@C) of acetylacetonato moiety.
m
(O@V@O) vibrations of the cis-VO2
m
(C@C, C@N) vibra-
m
In the compound 3, NH2 shows two bands at 3304 and
3145 cmꢁ1. The AC@NAN@CA moiety is observed as medium
bands at 1598–1638 cmꢁ1. The very strong band at 938 cmꢁ1 and
p
?
p⁄ and n ? p⁄ transitions, respectively [35,36].
weak band at 980 cmꢁ1 are assigned to
m(V@O) of dioxo. m(CS) ap-
pears at 810 cmꢁ1
.
3.3. 1H NMR spectra
3.2. Electronic spectra
1H NMR data of all the complexes are given in Section 2. The
coordinating modes of the ligands were also confirmed by record-
ing 1H NMR spectra of the complexes in CDCl3 and DMSO-d6. 1H
NMR signals were assigned on the basis of intensity, chemical shift
values and deuteration experiments. The 1H NMR spectra of all
complexes show the simultaneous occurrence of two sets of sig-
nals, which are attributable on one hand to the aromatic entities
and on the other hand to the aliphatic moiety. Absence of the NH
proton in the spectra of the complexes 1 and 2, suggests the con-
version of the keto to the tautomeric enol group and subsequent
The isolated complexes are usually dark in appearance and are
soluble in polar protic solvents but sparingly soluble in common
organic solvents. The electronic spectrum of ligand HL3 in metha-
nol showed the
p ?
p⁄ and n ? p⁄ transitions at 201 and
314 nm, respectively. For the complexes 1, 2 and 3, no d–d bands
are expected as there are no d electrons. Compounds 1, 2 and 3 dis-
play an intense electronic spectral band in the near-UV region,
Fig. 1. This band originates due to ligand to metal charge transfer
(LMCT) transition. The values are 345 nm for 1, 354 for 2 and
NH
O
V
O
O
O
O
H
N
[HBTC2-] + O2
+
H3N
O
[H2Inh2+][HBTC2-]
#
NH
NH
H
N
N
N
N
+ H2O + Hacac + 2H3BTC + 2Inh
O
O
V
O
O
OH
H2L2
O
Scheme 3. Synthesis pathway of 2.

H. Hosseini Monfared et al. / Polyhedron 30 (2011) 1375–1384
1379
1.4
1.2
1
coordinated ligand L2, the spectrum of 1 exhibited one singlet at
5.12 ppm corresponds to protons of the bridging OCH3 group in
addition to another singlet (with different intensity) at 3.49 ppm
attributed to the coordinated terminal OCH3 group [38]. These
two peaks indicate that this dimeric species is decomposed to
the corresponding monomeric species significantly (the ratio of di-
meric species to monomeric being approximately 1:1 in CDCl3
Complex 1
Complex 3
Complex 2
0.8
0.6
0.4
0.2
0
205
255
305
355
405
455
505
555
Wavelength / nm
Fig. 1. Electronic spectra of the complexes 1–3 in methanol (c = 2 ꢂ 10ꢁ5 M).
coordination of the enolate oxygen of the aroyl hydrazone Schiff
base ligands. Aromatic protons of the complexes appear well with-
in the expected range. The methyl region of the acetylacetonato
complexes exhibits two signals for methyl protons of 1 and 2 be-
tween 2.4 and 2.7 ppm [37]. Complex [VVO2(L2)]ꢀH2O (2) studied
both in DMSO-d6 and CDCl3 solution for solubility reason. Complex
[VVO( 2-OCH3)(L1)]2 (1) was identified by its NMR spectrum in
l
CDCl3 solution. Apart from all the aromatic proton signals of the
Fig. 3. Molecular structure of [VVO2(L2)]ꢀH2O (2). Symmetry codes: (e) = x, 0.5 ꢁ y,
z; (f) = x, 1.5 ꢁ y, z.
Fig. 2. Molecular structure of 1. Symmetry code: a = 1 ꢁ x, ꢁy, ꢁz.
Fig. 4. Crystalline structure of [VVO2(L2)]ꢀH2O (2). Hydrogen atoms except those
Table 2
that involved in hydrogen bonding are omitted for clarity. [N(3a)–
0
0
Selected bond distances (AÅ) and angels (°) for complexes 1 and 2.
H(3a)ꢀ ꢀ ꢀO(4b) =02.689(3) ÅA,
N(3a)–H(3a)ꢀ ꢀ ꢀO(4b) = 163(3)°],
[O(4b)–H(4b)ꢀ ꢀ ꢀO
(3c) = 2.737(2) ÅA, O(4b)–H(4b)ꢀ ꢀ ꢀO(3c) = 171(2)°]. Symmetry operations used: (a)
1 + x, y, 1 + z; (b) 0.5 ꢁ x, 1 ꢁ y, 0.5 + z; (c) 0.5 ꢁ x, ꢁy, 0.5 + z; (d) 0.5 ꢁ x, 0.5 + y,
0.5 + z.
Complex 1i
Complex 2ii
V1–O1
V1–O2
V1–O3
V1–O4
V1–N2
V1–O4a
1.9161(12)
1.8587(13)
1.5868(14)
1.8277(12)
2.0830(15)
2.3752(12)
V1–O1
V1–O2
V1–O3
V1–O3a
V1–N1
1.9352(19)
1.9626(18)
1.6255(14)
1.6255(14)
2.111(2)
90
80
70
60
O1–V1–O2
O1–V1–O3
O1–V1–O4
O1–V1–N2
O1–V1–O4a
O2–V1–O3
O2–V1–O4
O2–V1–N2
O2–V1–O4a
O3–V1–O4
O3–V1–N2
O3–V1–O4a
O4–V1–N2
O4–V1–O4a
O4a–V1–N2
153.59(6)
98.87(6)
88.86(5)
75.02(5)
81.35(5)
100.37(6)
104.01(5)
84.41(6)
80.24(5)
103.34(6)
98.34(6)
177.43(6)
154.74(6)
74.09(5)
84.20(5)
O1–V1–O2
O1–V1–O3
O1–V1–N1
O1–V1–O3a
O2–V1–O3
O2–V1–N1
O2–V1–O3a
O3–V1–N1
O3–V1–O3a
O3a–V1–N1
157.65(8)
96.67(6)
82.64(8)
96.67(6)
96.18(6)
75.01(8)
96.18(6)
125.16(5)
109.46(7)
125.16(5)
50
1 h
40
30
20
10
0
2 h
3 h
CH3CN
MeOH
EtOH
CHCl3
petroleum ether
Fig. 5. Effect of solvent in oxidation of cyclooctene with [VVO2(L2)]/H2O2. Reaction
i
conditions: [VVO2(L2)]ꢀH2O (2) 3
lmol, cyclooctene 1 mmol, chlorobenzene 0.1 g,
acetonitrile 2 ml, H2O2 3 mmol and temperature 80 1 °C.
Symmetry code: (a) = 1 ꢁ x, ꢁy, ꢁz.
Symmetry code: (a) = x, 1/2 ꢁ y, z.
ii

1380
H. Hosseini Monfared et al. / Polyhedron 30 (2011) 1375–1384
solution). Methyls and @CHA protons of the acetylacetonate group
also showed doubling of signals (see Section 2) which are attribut-
able to respective monomeric species.
ing two vanadium atoms. Hence the vanadium atom adopts a
distorted 0 octahedral geometry. The Vꢀ ꢀ ꢀV separation is
3.3708(1) ÅA and the angle V1–O4–V1a is 105.912(1)° which is very
similar to reported dimethoxy-bridged hydrazone complexes [39–
41]. The H2L1 ligand binds the vanadium meridionally in its fully
deprotonated (L1)2ꢁ form through the acetonate-O of acac, imine-
N and deprotonated amide-O in its enol form generating a folded
6-membered and an almost planar 5-membered chelate ring. The
corresponding bite angles are 84.41(6)° and 75.02(5)° which are
very similar with other reported hydrazone complexes [42,43].
So, the acetonate-O, enolate-O and the imine-N atoms of (L1)2ꢁ
species and the methoxide-O atom (lying trans to the0 imine-N)
form the square plane with an r.m.s. deviation of 0.0440 ÅA. Two ax-
ial positions are occupied by the oxido-O(3) atom and a bridging
3.4. Description of the structures
The dimeric complex 1, which lies on a crystallographic center
of symmetry is formed by two bridging methoxide-O atoms with
V–O(4) and V–O(4a) [symmetry code a = 1 ꢁ x, ꢁy, ꢁz] bond
0
lengths of 1.8277(12) and 2.3752(12) ÅA, respectively. The molecu-
lar structure of 1 is illustrated in Fig. 2 and the bond parameters are
listed in Table 2. The lattice is orthorhombic in nature with Pbca
symmetry and the unit cell consists of four molecules. The centro-
symmetric dimer is formed via the bridging methoxylatos connect-
Table 3
Oxidation of various hydrocarbons with [VVO(
l
2-OCH3)(L1)]2 using H2O2 and acetonitrilea.
Product(s)
Entry
1
Substrate
Yield (%)b
93
Conv. (%)b (time/h)
94 (4 h)
TONc
292
O
2
3
24
100 (5 h)
314
308
O
55
21
69
2-Cyclohexen-1-ol
2-Cyclohexen-1-one
97 (5 h)
OH
28
44
Cyclohexanone
4
87 (5 h)
274
OH
43
92
Cyclooctanone
5
6
92 (5 h)
5 (4 h)
289
16
CH2OH
5
O
7
8
11
67
11 (1 h)
67 (5 h)
35
CH3
CH2OH
211
O
a
b
c
Reaction conditions: catalyst 1.60 lmol, substrate 1 mmol, chlorobenzene 0.1 g, acetonitrile 2 ml, H2O2 3 mmol and temperature 80 1 °C.
Yields and conversions based on the starting substrate and determined by GC.
TON: turnover number = number of moles of product formed per mole of V in the catalyst.

H. Hosseini Monfared et al. / Polyhedron 30 (2011) 1375–1384
1381
methoxide-O(4a) atom of the neighboring molecule. The disposi-
tion of the oxovanadium groups is anti-coplanar which is also ob-
noted that since pyridyl nitrogen N3 protonates and forms pyridi-
nium, this leads to a total charge of ꢁ1 for the ligand L2. The molec-
ular structure of the dioxocomplex 2 along with the atom labels is
given in Fig. 3. The complex present five-coordinated vanadium(V)
center bonded to two oxo ligands in a cis arrangement, one nitro-
gen atom and two oxygen atoms from L2 (in enol form), which
coordinates dianionic in the O,N,O-tridentate mode, forming 5-
and 6-membered chelate rings around the vanadium atom. Diox-
ovanadium(V) complexes with planar tridentate ligands can have
either trigonal-bipyramidal or square pyramidal geometry
[45,46,25]. Bond lengths and angles of special interest are given
in Table 2. The geometry around V1 remains between trigonal-
served in other dimeric oxovanadium complexes [40]. The V@O
0
distance is only 1.5868(14) ÅA which is very closed to another re-
ported complex [44]. The five VAO bond lengths follow the order:
VAO(oxo) < VAO(methoxide)e
(e = equaterial) < VAO(acetona-
to) < V–O(enolate) < VAO(methoxide)a (a = axial).
The compound 2 was synthesized in situ by template method
and [VVO2(L2)]ꢀH2O crystallized as a dioxovanadium species due
to aerial oxidation into an orthorhombic lattice with Pnma symme-
try. Suitable crystals for X-ray diffraction were grown from the
reaction mixture by branched-tube method in a mixture of ethanol
and water (1:1 v/v). The in situ formed ligand H2L2 deprotonates
and acts as a dianionic tridentate chelating species. It should be
bipyramidal and square planar (
s = 0.5415) [47]. The geometry
around the vanadium center is better defined as considerably
Table 4
Oxidation of various hydrocarbons with [VVO2(L2)] using H2O2 and acetonitrilea.
Entry
1
Substrate
Product(s)
Yield (%)b
96
Conv. (%)b (time/h)
96 (5 h)
TONc
307
O
2
3
17
93 (3 h)
294
284
O
60
16
50
2-Cyclohexen-1-ol
2-Cyclohexen-1-one
89 (5 h)
OH
39
56
Cyclohexanone
4
88 (5 h)
281
OH
32
65
Cyclooctanone
5
6
65 (5 h)
10 (5 h)
208
32
CH2OH
10
O
7
8
49
83
49 (5 h)
83 (1 h)
157
140
CH3
CH2OH
O
a
b
c
Reaction conditions: catalyst 3.13 lmol, substrate 1 mmol, chlorobenzene 0.1 g, acetonitrile 2 ml, H2O2 3 mmol and temperature 80 1 °C.
Yields and conversions based on the starting substrate and determined by GC.
TON: turnover number = number of moles of product formed per mole of V in the catalyst.

1382
H. Hosseini Monfared et al. / Polyhedron 30 (2011) 1375–1384
distorted trigonal bipyramid [48,49]. Two oxo groups and the
imine-N atom of L2ꢁ form the equatorial plane. Two axial sites
are taken up by two oxygen atoms of L2. The V@O distance
V1AO3 1.6255(14) is not exceptional [50]. The 0 C(6)–O(2) bond
hydrogen bonds [VVO2(L2)]ꢀmolecules are connected together
through water molecules and form a chain along b axis. The chains
are connected along a axis by O(4b)ꢀ ꢀ ꢀN(3a) hydrogen bonds.
length due to deprotonation extends to 1.306(3) ÅA in the complex
3.5. Catalytic reactivity
0
from 1.232(3) ÅA in the keto form of the analog benzoyl hydrazone
0
ligand [31], making the V(1)–O(2) distance, 1.9626(18) ÅA.
The main driving force for the development of new efficient
oxygenation catalysts is the necessity to functionalize feedstock
hydrocarbons to raw oxygen-containing chemicals and the ability
to selectively hydroxylate non-activated CAH bonds in elaborate
chemicals in order to save many steps in the preparation of fine
chemicals. In addition, for obvious environmental constraints, clas-
sical stoichiometric oxidants, such as dichromate or permanganate,
In the complex 2, the crystal is stabilized by a net of intermolec-
ular hydrogen bonds, which are shown in Fig. 4. Strong N–Hꢀ ꢀ ꢀO
and O–Hꢀ ꢀ ꢀO hydrogen bonds are found. The nitrogen atom
N(3a), of the pyridinium NAH group, is H-bonded through H(3)
to the O(4b) atom of water, while H(4) interacts with the vanadyl
oxygen atom O(3d) of a second and third complex molecules. By
Table 5
Oxidation of various hydrocarbons with [VO2(L3)] using H2O2 and acetonitrilea.
Entry
1
Substrate
Product(s)
Yield (%)b
96
Conv. (%)b (time/h)
94 (5 h)
TONc
265
O
2
3
31
98 (2 h)
271
240
O
52
15
48
2-Cyclohexen-1-ol
2-Cyclohexen-1-one
87 (4 h)
OH
39
71
Cyclohexanone
4
100 (5 h)
276
OH
29
86
Cyclooctanone
5
6
86 (5 h)
71 (5 h)
238
196
CH2OH
71
O
7
8
31
63
31 (5 h)
63 (1 h)
86
CH3
CH2OH
174
O
a
b
c
Reaction conditions: catalyst 3.62 lmol, substrate 1 mmol, chlorobenzene 0.1 g, acetonitrile 2 ml, H2O2 3 mmol and temperature 80 1 °C.
Yields and conversions based on the starting substrate and determined by GC.
TON: turnover number = number of moles of product formed per mole of V in the catalyst.

H. Hosseini Monfared et al. / Polyhedron 30 (2011) 1375–1384
1383
should be replaced by new environment friendly catalytic pro-
cesses using clean oxidants like molecular oxygen or hydrogen per-
oxide [51,52].
by the three catalysts. This finding shows that the reaction prod-
ucts are controlled mainly by steric effects not electronic one. Cat-
alysts 1–3 were also active in oxidation of aromatic toluene,
ethylbenzene and tetrahydronaphthalene (Tables 3–5). The highest
conversion (63–83%) was observed for tetrahydronaphthalene and
the lowest for ethylbenzene (5–71%) by catalyst 1–3. Electron-rich
alkenes and alkanes displayed a greater activity than aromatic
compounds. Hydrocarbons with electron withdrawing phenyl sub-
stituent showed a lower product yield and turnover number than
the electron rich ones (Table 3–5, entries 6–8).
Catalytic activities of three catalysts 1–3 are compared in Fig. 6.
In general, electronic and steric effects of the ligands L1–3 are al-
most the same and the catalysts show nearly the same reactivity
towards various hydrocarbons except tetralene and cyclohexene.
These findings (Fig. 6) suggest the plausible presence of some type
of interactions between tetralene and the catalyst 2 that reinforce
its approach to the catalyst and fast oxidation (Fig. 6). Such situa-
tion is also seen for the catalyst 3 in the oxidation of cyclohexene.
Conclusive interpretation of this subject needs further studies in
the future. The total mechanism of the reaction is not fully clear.
However, on the basis of the oxidation products of the various
hydrocarbons (Tables 3–5), it is predicted that the oxidation reac-
tions proceed by intermediacy of a peroxo-vanadium-hydrazone
Schiff base species formed in the reaction mixture in the presence
of hydrogen peroxide [38].
The catalytic oxidation of cyclooctene with hydrogen peroxide
was studied in the presence of [VVO2(L2)]ꢀH2O (2). All reactions
were carried out with 1 mmol of cyclooctene in CH3CN and in
the presence of catalyst 2 at 80 °C. Cyclooctene oxide was the sole
product. The results of control experiments revealed that the pres-
ence of catalyst and oxidant are essential for the oxidation. The
oxidation of cyclooctene in the absence H2O2 does not occur and
in the absence of catalyst the oxidation proceeds only up to 6%
after 24 h.
In search of suitable reaction conditions to achieve the maxi-
mum oxidation of cyclooctene, the effect of solvent was studied.
Fig. 5 illustrates the influence of the solvent nature in the catalytic
epoxidation of cyclooctene by 2. Methanol, ethanol, acetonitrile,
chloroform and petroleum ether were used as solvents. The highest
conversion was obtained in acetonitrile, 79% after 3 h. It was ob-
served that the catalytic activity of complex 2 decreases in order
acetonitrile (relative dielectric constants [53], e/e0 = 37.5) > metha-
nol (32.7) > ethanol (26.6) > chloroform (4.9) > petroleum ether
(ꢃ2). In Fig. 5, the solvents are shown in order of their relative
dielectric constants (e/e0). In general, there was a good correlation
between the solvent polarity (e/e0) and cyclooctene conversion
percent. The highest conversion in acetonitrile possibly is caused
by its high dielectric constant.
The catalytic activities of 1–3 were examined in oxidation of
various hydrocarbons under the optimized conditions (H2O2/cyclo-
octene molar ratio = 3, acetonitrile, reaction temperature 80 °C)
and the results are shown in Tables 3–5. Cyclooctene was con-
verted to the corresponding epoxide with 100% selectivity. For
cyclohexene in addition to cyclohexene oxide, allylic oxidation
products (2-cyclohexene-1-ol and 2-cyclohexene-1-one) were also
formed. Allylic oxidation has been reported in the metalloporphy-
rin systems [54,55] in the oxidation of alkenes such as cyclohexene
and represents radical nature of the active oxidizing species.
Catalysts 1–3 oxidized various cyclic alkanes including cyclo-
hexane, cyclooctane and methylcyclohexane to the corresponding
alcohol and/or ketone with conversions 87–97% for 1, 65–89% for
2 and 86–100% for 3 (Tables 3–5). The profile of products show
the oxidation reaction is highly selective. The oxidation of tertiary
and secondary carbon atoms is favored electronically but the only
product of methylcyclohexane oxidation was cyclohexylmethanol
4. Conclusions
Our work has revealed that oxo- and dioxovanadium(V) com-
plexes of tridentate hydrazone Schiff base ligands could be synthe-
sized by a proton-transfer complex of hydrazide. Two new oxo-
and dioxo-vanadium(V) complexes of hydrazone ONO donor li-
gands with the formula [VVO( 2-OCH3)(L1)]2 (1) and
l
[VVO2(L2)]ꢀH2O (2) were synthesized by this method and character-
ized by spectroscopic and single crystal X-ray analyses. The cata-
lytic abilities of complexes 1–3 were investigated by using the
environmentally benign and clean oxidant H2O2 for oxidation of
various hydrocarbons. Alcohol and/or ketone were obtained with
good to excellent conversion as the main products of the hydrocar-
bon oxidation.
Acknowledgements
The authors are grateful to the Zanjan University, the Faculty of
Chemistry and Biochemistry of the Ludwig-Maximilians-Universi-
tät München, and the School of Chemistry for financial assistance.
160
Catalyst 1
Catalyst 2
120
Appendix A. Supplementary data
Catalyst 3
CCDC 801668, 801669, and 669245 contain the supplementary
crystallographic data for 1, 2, and 3. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
80
40
0
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