Dinuclear vanadium, copper, manganese and titanium complexes containing O,O,N-dichelating ligands: Synthesis, crystal structure and catalytic activity

Article abstract of DOI:10.1016/j.ica.2013.04.044

O,O,N-or O,N-dichelating ligands, H₄L¹, H ₄L², derived from new bis(hydroxybenzylidene)-substituted terephthalo and oxalo hydrazides were prepared and characterized by FT-IR and NMR spectroscopy (H₄-L¹ = dicondensation of salicylaldehyde and terephthalic acid dihydrazide, H₄L² = dicondensation of sali-cylaldehyde and oxalic acid dihydrazide). [{VO ₂}₂(H₂L¹)] (1), [{Cu(CH ₃OH)}₂(L¹)] (2), [{Mn(CH₃OH)} ₂(L¹)] (3) and [TiO(H₃L¹) ₂] (4) complexes of the potentially hexadentate 2[ONO] donor hydrazone ligand [L¹]⁴ were synthesized by reaction of the appropriate metal salts with H₄L¹. The complexes [{VO(CH₃O)(CH₃OH)}₂(L²)] (5), [{Cu(CH₃OH)}₂(L²)] (6), [{Mn(H ₂O)₂(CH₃OH)}₂(L²)] (7) and [{TiO(CH₃OH)}₂(L²)]·2H₂O (8) were synthesized by the reaction of corresponding metal salts with H ₄L². All complexes were characterized by microanalyses, FT-IR, UV-Vis and NMR spectroscopic methods. The crystal structures of 1 and 5 were established by X-ray analyses. Compound 1 is a one-dimensional coordination polymer. The catalytic potential of the complexes was evaluated for oxidation of hydrocarbons including cycloalkenes, cyclic alkanes and benzylalcohol using H₂O₂ as terminal oxidant. Of the studied aroylhydrazone complexes, 4 showed the best selectivity and activity as catalyst.

Full text of DOI:10.1016/j.ica.2013.04.044

Accepted Manuscript
Dinuclear vanadium, copper, manganese and titanium complexes containing
O,O,N-dichelating ligands: synthesis, crystal structure and catalytic activity
Hassan Hosseini-Monfared, Nasim Asghari-Lalami, Aleksandra Pazio,
Krzysztof Wozniak, Christoph Janiak
PII:
S0020-1693(13)00243-0
http://dx.doi.org/10.1016/j.ica.2013.04.044
ICA 15449
DOI:
Reference:
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Inorganica Chimica Acta
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24 April 2013
24 April 2013
Please cite this article as: H. Hosseini-Monfared, N. Asghari-Lalami, A. Pazio, K. Wozniak, C. Janiak, Dinuclear
vanadium, copper, manganese and titanium complexes containing O,O,N-dichelating ligands: synthesis, crystal
structure and catalytic activity, Inorganica Chimica Acta (2013), doi: http://dx.doi.org/10.1016/j.ica.2013.04.044
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Dinuclear vanadium, copper, manganese and titanium
complexes containing O,O,N-dichelating ligands: synthesis,
crystal structure and catalytic activity
Hassan Hosseini-Monfareda,*, Nasim Asghari-Lalamia, Aleksandra Paziob, Krzysztof
Wozniak b, Christoph Janiak c
a Department of Chemistry, University of Zanjan 45195-313, Zanjan, I. R. Iran.
Email: monfared@znu.ac.ir
b Chemistry Department, University of Warsaw, Pasteura 1, 02093 Warszawa, Poland. E-
mail: kwozniak@chem.uw.edu.pl
cInstitut für Anorganische Chemie und Strukturchemie, Universität Düsseldorf,
Universitätsstr. 1, D-40225 Düsseldorf, Germany. E-mail: janiak@uni-duesseldorf.de
Abstract
O,O,N- or O,N-dichelating ligands, H4L1, H4L2, derived from new
bis(hydroxybenzylidene)-substituted terephthalo and oxalo hydrazides were prepared and
characterized by FT-IR and NMR spectroscopy (H4L1 = dicondensation of
salicylaldehyde and terephthalic acid dihydrazide, H4L2= dicondensation of
salicylaldehyde and oxalic acid dihydrazide). [{VO2}2(H2L1)] (1), [{Cu(CH3OH)}2(L1)]
(2), [{Mn(CH3OH)}2(L1)] (3) and [TiO(H3L1)2] (4) complexes of the potentially
hexadentate 2[ONO] donor hydrazone ligand [L1]4– were synthesized by reaction of the
1

appropriate metal salts with H4L1. The complexes [{VO(CH3O)(CH3OH)}2(L2)] (5),
[{Cu(CH3OH)}2(L2)] [{Mn(H2O)2(CH3OH)}2(L2)]
(6),
(7)
and
[{TiO(CH3OH)}2(L2)]·2H2O (8) were synthesized by the reaction of corresponding metal
salts with H4L2. All complexes were characterized by microanalyses, FT-IR, UV–Vis and
NMR spectroscopic methods. The crystal structures of 1 and 5 were established by X-ray
analyses. Compound 1 is a one-dimensional coordination polymer. The catalytic potential
of the complexes was evaluated for oxidation of hydrocarbons including cycloalkenes,
cyclic alkanes and benzylalcohol using H2O2 as terminal oxidant. Of the studied
aroylhydrazone complexes, 4 showed the best selectivity and activity as catalyst.
Keywords: Terephthalohydrazide; Oxalohydrazide; Transition metal; Structure;
Oxidation; Hydrocarbon
1. Introduction
Coordination chemistry of aroylhydrazones has been a subject of competitive research
since they exhibit a wide range of catalytic oxidation reaction properties, especially
aroylhydrazones which are derived from heterocyclic aldehydes or ketones [1,2,3].
Aroylhydrazone complexes show the potential to be good oxidation catalysts because of
bearing the necessary condition of stability towards oxidation. Also, the world-wide
demand for valuable oxidation products that can be produced in high yields and purities
grows substantially year after year [4,5]. The driving force is a strong economic incentive
2

to find better and more cost effective catalyst systems that can produce these value added
oxidation products. New catalysts can provide the potential to shift the production of
expensive niche products to value added commodity products [6]. The demand for more
environmentally acceptable methods in fine chemistry that allow elimination of common
but otherwise hazardous or costly oxidants, such as organic peroxyacids, is pushing
forward the use of cleaner oxidants like hydroperoxides (particularly hydrogen peroxide)
under catalytic conditions [7,8].
Acyl- and aroylhydrazones are multipurpose class of ligands (Scheme 1) having a
range of biological and chemical activities. Hydrazones exhibit physiological and
biological activities in the treatment of several diseases such as tuberculosis [9], for the
treatment of Fe overload disease [10,11], and as inhibitors for many enzymes [12]. Their
metal complexes have also found applications in various chemical processes like non-
linear optics, sensors, and medicine [13]. They are of interest in the field of
electrochromism where a change in the oxidation state of the metal is possible [14]. The
acyl and aroylhydrazones –CO–NH–N=CR´R´´ (R, R´, R´´=H, alkyl, aryl) contain
trigonal N- and O-donor atoms that can coordinate to metal ions [15,1]. Hydrazone
ligands can act as bidentate, tridentate, or tetradentate ligands depending on the nature of
heterocyclic ring substituents attached to the hydrazone unit. These ligands exhibit a
facile keto–enol tautomerization (Scheme 1b) which can modulate the coordination to the
metal as a uni-negative or di-negative ligand in the keto or enol form, respectively
(Scheme 2b). In the field of hydrazone chemistry, the study of bis(aryl)hydrazone
complexes is of interest for several reasons: (1) the study of the bis(aryl)hydrazone
complexes is justified by the presence of two hydrazone coordinating units in ditopic
3

ligands, which may yield supramolecular architectures or coordination polymers [16], (2)
the coordination of a metal ion by one unit may induce changes in the coordinative
properties of the other unit, and (3) binuclear complexes of these ligands are suitable for
weak spin–spin exchange interaction studies [17].
Herein we report the synthesis, crystal structure, spectroscopic properties and
catalytic activity of terephthalo- and oxalohydrazide complexes of V(V), Cu(II),
Mn(II)and Ti(IV) with 1D polymeric, dinuclear and mononuclear structures (Scheme 3).
2. Experimental
All 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 cm-1. UV–Vis spectra of solution were
1
recorded on a Shimadzu 160 spectrometer. H NMR spectra in DMSO-d6 and CDCl3
solution were recorded on a Bruker 250 MHz spectrometer and chemical shifts are
indicated in ppm relative to tetramethylsilane. The oxidation products were determined
and analyzed by a HP Agilent 6890 gas chromatograph equipped with a HP-5 capillary
column (phenyl methyl siloxane 30 m long × 320 μm i.d.× 0.25 μm film thickness). The
elemental analyses (carbon, hydrogen, and nitrogen) of compounds were obtained from
Perkin Elmer Model 2400 analyzer. Copper, manganese, titanium and vanadium
percentages of complexes were measured by a Varian spectrometer AAS-220. Electrical
conductivity of complexes was measured by a Metrohm 712Conductometer with cell
constant 0.8 cm-1.
4

2.1. Synthesis of H4L1
2.1.1. Synthesis of diethyl terephthalate
Diethyl terephthalate was produced as reported in the literature [18]. Terephthalic acid
(0.50 g, 3.0 mmol) in dry ethanol (20 mL) containing 2-3 drops of concentrated H2SO4
was refluxed for 24 h till it dissolved. Then the reaction mixture was poured in ice-cold
water. Immediately a white solid started separating from the clear solution. Then sodium
bicarbonate was used for the neutralization of excess acid. Addition of sodium
bicarbonate was continued till the effervescence seized. The ester was filtered, washed
several times with water, and dried in air. Yield: 84% (0.57 g). M.p. 44.5°C.1H NMR
(250.13 MHz, DMSO-d6): δ = 1.31 (t, J 7.0, 6H; CH3), 4.32 (q, J 7.7, 4H; CH2), 8.05
ppm (s, 4H, aryl).
2.1.2. Synthesis of terephthalohydrazide
A mixture of diethyl terephthalate (0.10 g, 0.45 mmol) and hydrazine hydrate (98%, 2
mL) in toluene (10 mL) was refluxed for 9-10 h. A white precipitate of
terephthalohydrazide was formed, filtered, washed with toluene and then dried at air.
Yield: 52% (45 mg). 1H NMR (250.13 MHz, DMSO-d6): δ = 4.51 (s, 4H, –NH2), 7.83 (s,
4H, aryl), 9.86 ppm (s, 2H, –NH–).13C {1H} NMR (62.89 MHz, DMSO-d6): δ = 127.4-
129.5 (4C), 135.9 (2C), 165.6 (2C, C=O).
5

2.1.3. Synthesis of N'1,N'4-bis(2-hydroxybenzylidene)terephthalohydrazide (H4L1)
A mixture of terephthalohydrazide (0.26 g, 0.88 mmol) and salicylaldehyde (0.216 g,
1.77 mmol) was refluxed in ethanol (10 mL) for 5 hours. The resulting yellow precipitate
was filtered, washed with ethanol and dried at air. Yield: 87% (0.47 g). IR (KBr, cm-1):
3443 (w, OH), 3221 (s, N-H), 3044 (m), 1655 (vs, C=O), 1640 (s, C=N), 1551 (m), 1488
(s), 1364 (s), 1287 (s), 1204 (m), 1148 (m), 1119 (m), 1081 (m), 1035 (m), 963 (m, N-N),
1
918 (m), 871 (s), 740 (s), 656 (s),483 (m), 439 (m). H NMR (250.13 MHz, DMSO-d6,
ppm): δ = 6.90-8.06 (m, 12 H, aryl), 8.65 (s, 2H, CH=N), 11.18 (s, 2H, aryl-OH), 12.21
13
(s, 2H, NH). C {1H} NMR (62.89 MHz, DMSO-d6, ppm): δ = 116.9, 119.1, 119.8,
128.3, 129.8, 132.0, 136.2 (16C, aryl rings), 149.0 (2C, CH=N), 157.9 (2C, C-OH), 162.5
(2C, C=O). UV–Vis (in CH3OH, c = 1×10-5 mol L–1, very light yellow solution, λmax [nm]
with ε [L mol–1 cm–1]): 220 (53900), 301 (38500), 339 (36900).
2.2. Synthesis of H4L2
2.2.1. Synthesis of dimethyl oxalate
Dimethyl oxalate was synthesized according to the reported procedure [19]. Concentrated
sulfuric acid (0.5 mL) was slowly added to a solution of oxalic acid (0.90 g, 10 mmol) in
methanol (2 mL). The mixture was heated near to boiling point and then filtered. After a
few hours, the colorless crystals of dimethyl oxalate settled down in the filtrate. They
were filtered, washed with cold methanol and dried in air. Yield: 69% (0.82 g). M.p.
51°C.1H NMR (250.13 MHz, DMSO-d6): δ = 3.89ppm (s, 6H, CH3).
6

2.2.2. Synthesis of oxalohydrazide
A mixture of dimethyl oxalate (0.06 g, 0.05 mmol) and hydrazine hydrate (98%, 2 mL) in
methanol was refluxed for 8 h. The white precipitate of oxalohydrazide was filtered,
1
washed with methanol and dried in air. Yield: 75% (0.045 g). H NMR (250.13 MHz,
DMSO-d6): δ = 4.47 (s, 4H, NH2), 9.85 (s, 2H, NH). 13C{1H} NMR (62.89 MHz, DMSO-
d6): δ = 158.3 (2C, C=O).
2.2.3. Synthesis of N'1,N'2-bis(2-hydroxybenzylidene)oxalohydrazide (H4L2)
Solution of oxalohydrazide (236 mg, 0.2 mmol) and salicylaldehyde (488 mg, 0.4 mmol)
in methanol (10 mL) was refluxed for 10 h. A milky precipitate was formed which was
filtered, washed with methanol and dried at air. Yield: 92% (60 mg). IR (KBr, cm-1):
3281 (m, OH), 3210 (m, NH), 3171 (m), 3071 (m), 1667 (vs, C=O), 1604 (s, C=N), 1536
(s), 1487 (m), 1459 (m), 1361 (s), 1308 (m), 1263 (s), 1224 (m), 1163 (m), 1056 (m), 968
1
(m), 876 (m), 829 (s), 758 (s), 733 (m), 635 (m), 531 (m). H NMR (250.13 MHz,
DMSO-d6): δ = 6.90-7.54 (m, 8H, aryl), 8.79 (s, 2H, CH=N), 10.97 (s, 2H, OH), 12.65 (s,
2H, NH). 13C{1H} NMR (62.89 MHz, DMSO-d6): δ = 116.9, 119.1, 119.9, 129.8, 132.5,
(10C, aryl rings), 151.4 (2C, CH=N), 156.3 (2C, (aryl)C-OH), 158.0 (2C, C=O). UV–Vis
(in CH3OH, c = 1×10-5 mol L–1, very light yellow solution, λmax [nm] with ε [L mol–1 cm–
1]): 214 (13100), 240 (8600), 302 (18400), 342 (18200).
7

2.3. Synthesis of metal complexes
General procedure: The appropriate ligand (H4L1 or H4L2) (1.0 mmol) was dissolved in
methanol (10 mL). The desired metal salt (vanadium(IV) oxide acetylacetonate
[VO(acac)2], Cu(OAc)2·H2O or Cu(NO3)2·5/2H2O, Mn(OAc)2·4H2O, TiO(acac)2 (2.0
mmol) was added and the solution was gently refluxed for 6 h. After cooling, the
resulting solid was filtered off, washed with cooled absolute methanol and then dried in
air. The analytical and physical data of the complexes are given in Table 1.
2.4. X-ray structure determination
The single crystal X-ray scattered intensities of reflections were collected for 1 and 5
using a BRUKER KAPPA APEXII ULTRA controlled by APEXII [20] software,
equipped with an APEX-II CCD detector and a MoKα rotating anode X-ray source (λ =
0.71073 Å, 50.0 kV, 22.0 mA) mono chromatized by multi-layer optics. Experiments
were carried out at 100(2) K for a single crystal of 1 (red plate) and 90(2) K for 5 (red-
brown plate) using the Oxford Cryo stream cooling device. Single crystals were mounted
on a nylon loop with a droplet of Paratone-N oil and immediately cooled. Indexing,
integration and initial scaling were performed with SAINT and SADABS software
(Bruker, 2007). Details of data collection procedure are given in Table 2.
Both structures were solved by direct methods using the SHELXS-97 program and
refined with SHELXL-97. The multi-scan absorption correction was applied in the
scaling procedure.
8

The refinement was based on F2 for all reflections except those with negative intensities.
Weighted R factors wR and all goodness-of-fit values were based on F2, whereas
conventional R factors were based on the amplitudes, with F set to zero for negative F2.
2
2
The Fo > 2σ( Fo ) criterion, applied only for R factors calculation, was not relevant to the
choice of reflections for the refinement. The hydrogen atoms were located in idealized
geometrical positions except H10 in structure 5 which was found from the residual
electron density. Scattering factors were taken from Tables 4.2.6.8 and 6.1.1.4 from the
International Crystallographic Tables Vol. C [20f]. All illustrations of crystal and
molecular structures of 1 and 5 were drawn with DIAMOND [21].
CCDC 904953 and 904952 contains the supplementary crystallographic data for 1 and 5,
respectively. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif
2.5. General oxidation procedure
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 at 80 or 60 ºC. In a typical experiment, a mixture of 30% H2O2 (3
mmol), solvent (3 mL), NaHCO3 (0.25 mmol), chlorobenzene (0.1 g) as internal standard
and cis-cyclooctene (1 mmol) was added to a flask containing the catalyst 1-8 (1.70
μmol). 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 retention times with those of authentic samples or
9

1
alternatively by H 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, H2O2 and NaHCO3 under the same conditions as the catalytic runs verifying
that no products, or only trace yields, were detected.
3. Results and discussion
Ligands H4L1and H4L2 were synthesized in three-steps by a similar method. Esterification
of terephthalic and oxalic acid was followed by the reaction with hydrazine hydrate to
form the corresponding hydrazide. Finally the condensation reactions of hydrazide with
two equivalents salicylaldehyde resulted in the formation of H4L1 and H4L2 in high yield
and purity, respectively (Table 1, Scheme 2a). Ligands H4L1 and H4L2 are potentially
hexadentate and could coordinate to two different metal centers by their symmetric two
tridentate unites. There is a similar keto-enol tautomerism for both H4L1and H4L2
(Scheme 1) and their anionic forms (Scheme 2b). V(V), Cu(II), Mn(II) and Ti(IV)
complexes 1-8 (Scheme 3) of hexadentate hydrazone Schiff base ligands were prepared
by treating a methanolic solution of the ligand with two equivalents of the appropriate
metal salt. Analytical and physical data of the complexes are summarized in Table 1. All
complexes are binuclear except 4 which is a mononuclear complex (cf. Scheme 3).
Electrical conductivity measurements gave ΛM values of 21–83 (Ω-1 cm2 mol-1)
and confirmed that they are neutral and non-electrolytes [22].
1H NMR data of the ligands in DMSO-d6 confirm their proposed structure
(Scheme 1). The principal peaks of the 1H NMR spectra of ligands H4L1-2 and complexes
1, 4, 5, 8 are listed in Table 3. Due to the complexity of the aromatic region (δ 8–6), no
10

band assignments have been made for the aromatic protons. The signal at δ 11.41–12.21
ppm in the spectra of the ligands and complexes is assigned to the common NH-group,
concomitant with the observation of a rapid loss of these signals when D2O is added to
the solution. Also the H4L1-2 ligand signals between δ 10.97-11.18 ppm are lost upon
addition of D2O to the solution. Hence, this signal is assigned to the aromatic –OH group
which is not present in 1 and 5. The resonances in the range of δ from 8.65 to 8.98 ppm
are assigned to the azomethine proton (–CH=N–) in the spectra of the ligands and
complexes. In a study of ligands of somewhat similar type the azomethine resonance
have been reported in the region δ 7–9 ppm [23,24].
3.1. Infrared spectra
A list of selected IR spectral data of the ligands and complexes is presented in Table 4.
Appearance of N–H (3200 cm-1) and C=O (about 1660 cm-1) stretching vibration bands
confirms the presence of the free ligands H4L1 and H4L2 in the keto form (Scheme 1). A
comparison of the spectra of the complexes with the ligands provides evidence for the
coordination mode of the ligands in the complexes. Presence of the ν(C=O) and ν(N–H)
bands in the complexes 4 and 5 proves the coordination of H4L1-2 to metal center in the
keto form. Appearance of ν(C=O) in complex 5 (1670 cm-1) almost at the same frequency
as ligand H4L2 (1665 cm-1) suggests that the –N-C=O bond is delocalized to a certain
extent in the free and the coordinated ligand [H2L2]2-. The lack of the ν(C=O) bands,
present in the ligands at 1651–1665 cm-1, indicates the enolization of the amide
functionality upon coordination to the metal center in 2, 3, 6, 7 and 8. Instead very strong
11

bands at ca. 1607–1622 cm-1 are observed, which can be attributed to the asymmetric
stretching vibration of the conjugated CH=N–N=C group [25], characteristic for
coordination of the enolate form of the ligands. On complexation the absence of N–H
band, red shifts in azomethine (–C=N–) and carbonyl bands of the ligands show
coordination of H4L1-2 as tetraanionic ligand in the enol form (Scheme 1) through the
deprotonated hydroxyl group (Ophenolate), azomethine nitrogen (Nazomethine) and the enol
oxygen groups in the complexes 2, 3, 6-8 (Table 3). The shifting of the ν (C=N) stretch
towards the lower wavenumbers from the free ligand also supports the coordination of
azomethine nitrogen [26] to the metal. The strong ν(V=O) band around 894 and 972 cm-1
could be clearly identified for the complexes 1 and 5, respectively [27].
3.2. Electronic spectra
Yellow H4L1 and milky H4L2 are soluble in polar and common organic solvents. The
electronic spectra of the ligands H4L1-2 in methanol show π→π* and n→π* transitions at
214-217 and 238-340 nm, respectively (Table 5). The complexes 1-8 are soluble in
methanol. Table 4 provides electronic spectral data of the ligands H4L1-2 and complexes
along with their assignments. The electronic spectra of the complexes in CH3OH exhibit
two groups of band about 208-241 and 289-436 nm. With respect to their molar
absorptivity they were assigned to n→π* and π→π* of the ligands, respectively.
Electronic transitions between metal orbitals (d-d transitions) are covered by broad π
→π* bands and thus not observed. DFT calculations on a similar hydrazone complex
showed that the LMCT transitions contribute also to the other bands [28].
12

With respect to microanalyses and spectral data, the formula for synthesized
complexes can be summarized as in Scheme 3.
3.3. Description of the structures
The molecular structures and labeling of the atoms for the complexes 1 and 5 are
displayed in Figs. 1 and 2, respectively, and selected bond lengths and angles are given in
Table 6.
In the crystal lattice, the dinculear moiety [{VO2}2(H2L1)] of compound 1 lies on a center
of symmetry coinciding with the center of the C6-terepthalate ring of [H2L1]2–and, thus,
one crystallographically unique vanadium atom is present. The [H2L1]2– ligand functions
as a hexadentate unit to two vanadium atoms. The two dioxidovanadium(V) or {VO4N}
chromophores in the molecular unit, together with the two monoanionic, tridentate
domains of the ligand, are oriented trans to each other. The atoms of the {VO4} group are
disordered over two positions (Fig. 1a). There is one major position with 76.3%
occupancy and a minor site with 23.7% occupancy. In the dinuclear molecular unit, the
coordination number of the vanadium atom is five due to close contacts to two oxido, a
phenolate-O, imine-N and amide-O atoms with the V-O distances ranging from 1.554(6)
(for V=O) to 1.933(6) Å (Table 6). This coordination number is increased to six in a
distorted octahedron when one considers an additional elongated the V-O bond (2.335(6)
Å) to the O4-oxido atom of the adjacent unit (Fig. 1b). This way, the dinculear units are
linked into one-dimensional coordination polymeric strands. The distortion from
octahedral geometry is due to the V–O–C–N–N and V–N–C–C–C–O chelate rings and a
long V1---O4 bond.
13

Unfortunately, the charge-dictated necessary H-atom in compound 1 could not be
found. Because of the disorder of the VO3 part in this structure we also refrained from
placing the H-atom at a calculated position which may have been either or both of the
disordered oxygen atoms O2/O2A and O4/O4A which then become C-O2-H and V-
O4(H)-V groups, respectively (cf. Fig. 1a). Formation of C-O2-H is in agreement with
the C8-O2 single bond length (1.309(10) Å cf. Fig. 1a). In part, the H-atom may even be
present at N2, even though from bond length arguments we suggest the presence of a
C=N double bond between C8 and N2 (cf. Fig. 1a, Table 6). The problem is that both
these atoms (O and N) have lone pairs of electrons so small maxima of electron density
will coincide with a possible position of this hydrogen atom. In general it is difficult to
discern lone-pair electron density from H-atom electron density. Hence, in summary,
delocalization of the H-atom in the solid-state at least over the oxygen atoms is likely,
and we prefer to leave this hydrogen position as undefined [29].
The intermolecular packing in 1 shows no significant ··· or C-H··· contacts
(calculated by PLATON [30]) despite the presence of the aromatic systems in the
[H2L1]2– ligand [31,32,33].
Structural analysis shows that, similar to 1, the dinculear moiety
[{VO(CH3O)(CH3OH)}2(L2)] of compound 5 lies on a center of symmetry coinciding
with the center of [L2]4–and, hence, one crystallographically unique vanadium atom (Fig.
2) is present. The [L2]4– ligand functions as a hexadentate unit to two vanadium atoms.
The two oxidovanadium(V) or {VO4N} chromophores in the molecular unit together
with the two dianionic, tridentate domains of the ligand adopt the trans configuration.
The vanadium atom is coordinated by the two oxygen atoms and one nitrogen atom from
14

the [L2]4– ligand, two oxygen atoms of a cis-positioned methanol and methoxy ligand and
an oxido atom. The oxido ligand is trans to the methanol ligand. V-O distances range
from 1.584 (for V=O [3]) to 2.268 Å (V---OCH3OH) (Table 6). The V-O– single bonds lie
between 1.762 and 1.959 Å. The C-O and C-N bond lengths of amide moiety in 5 (C8-O2
1.299(8) and C8-N2 1.293(9) Å) are almost the same (Table 6). In agreement with the
corresponding IR spectra. This finding also supports that the –N-C=O bond is delocalized
2-
to a certain extent in the coordinated ligand [H2L2] in 5. Highest delocalization has
been reported for a brown Cu-aroylhydrazone complex which shows almost the same
C-O and N-C bond lengths [29]. The equal bond lengths of around 1.30 Å in the –NCO–
unit in
5
agree with those in the fully deprotonated (2-oxy-3-
methoxybenzylidene)benzoylhydrazide ligand in a copper complex (1.31 Å each) for
which a valence structure description of –N=C–O(–) ↔ –N(–)–C=O was assigned based on
the calculated Mulliken charge densities for the amide N = –0.410 and O = –0.529 [29].
The V–O(amide) and V–O(phenolate) bond lengths for 1 and 5 are in agreement with
other previously known acylhydrazone-oxidovanadium(V) complexes in the literature
[2,2]. The average V=O bond length of the oxidovanadium is approximately 1.6 Å [3].
In 5 the phenyl ring C1-C6 is part of a weak ··· stacking interaction to an exactly parallel
(by symmetry) counterpart with a centroid-centroid contact of 3.802(5) Å, interplanar
separation of 3.476(3) Å and a slippage of 1.540 Å [31,32].
3.4. Catalytic reactivity
15

The catalytic oxidation of cis-cyclooctene was studied as a representative substrate with
H2O2 in the presence of complexes 1-8. Cis-cyclooctene oxide was the sole product. The
results of control experiments revealed that the presence of catalyst and oxidant are
essential for the oxidation. The oxidation of cis-cyclooctene in the absence of H2O2 does
not occur and in the absence of catalyst the product was trace.
Bare metal M ions (M = V(IV), Cu(II), Mn(II), Ti(IV)) did not activate H2O2 for cis-
cyclooctene oxidation (not shown in the Table). Effects of solvent, NaHCO3
concentration as co-catalyst, the ratio of oxidant/substrate and the temperature were
studied in order to find suitable reaction conditions to achieve the maximum conversion
of cis-cyclooctene. The presence of a hydrogen peroxide activator NaHCO3 with the
catalysts 2, 4 and 8 increased the catalysts activity but it did not affect the activity of the
other catalysts. Fig. 3 illustrates the influence of solvent in the catalytic epoxidation of
cis-cyclooctene by 1. Methanol, ethanol, acetonitrile, chloroform and dichloromethane
were tried as solvent. The highest conversion, 85% after 3 hrs, was obtained in
acetonitrile. The catalytic activity of all four complexes decreased in order acetonitrile
(relative dielectric constants [34], ε/ε0 = 37.5) > methanol (32.7) > ethanol (26.6) >
chloroform (4.9) ~dichloromethane. Overall, reactivity of the catalysts in other solvents
was much lower than in acetonitrile. The catalytic activities of 1–8 were examined in
oxidation of various hydrocarbons under the optimized conditions and the results are
shown in Table 8. The oxidation was not achieved by 3, 6 and 7 because of fast hydrogen
peroxide decomposition by these complexes. The decomposition of hydrogen peroxide is
an activated process. The presence of catalysts such as the Schiff base complexes of
transition metal ions is effective in catalyzing the decomposition of hydrogen peroxide.
16

In the decomposition of hydrogen peroxide, the formation of HOO¯ ions has been
proposed, which is subsequently consumed through the formation of a peroxido-metal
adduct. Finally, the adduct is decomposed to form dioxygen by reaction with hydrogen
ions generated in the first step [35]. The effect of various transition metal ions on the
decomposition of H2O2 has been studied and it was found that the complexes of Cu(II)
ions were more active in comparison to Co(II) and Ni(II) ions [36]. Many manganese
catalysts are also known to be particularly effective in the decomposition of H2O2 [37].
H2O2 dismutation by the manganese and copper complexes 3, 6 and 7 was fast during the
reaction and evolution of O2 gas bubbles were clearly seen. However, cis-cyclooctene
epoxidation was successfully catalyzed by the complexes 3, 6 and 7 when H2O2 was
replaced by tert-butyl hydroperoxide (TBHP) which resulted after 24 hrs in 74, 60 and
51% conversion, respectively.
Results of the oxidation of various substrates with complexes 1, 2, 4, 5 and 8
under optimum conditions are summarized in Table 7. These catalysts were able to
oxidize various substrates including cycloalkenes (cis-cyclooctene 60-96% conversion,
cyclohexene 73-96% conversion), cycloalkanes (cyclooctane 18-50% conversion,
cyclohexane 14-93% conversion) and benzyl alcohol (16-59% conversion) with aqueous
30% hydrogen peroxide. While conversion percent of the various substrates and products
selectivities was different for different catalysts, the products were the same. The catalyst
4 showed the highest activity in the oxidation of cycloalkenes and cycloalkanes.
Cis-cyclooctene was converted to the corresponding epoxide with 100%
selectivity. For cyclohexene in addition to cyclohexene oxide, allylic oxidation products
17

(2-cyclohexene-1-ol and 2-cyclohexene-1-one) were formed. Allylic oxidation has been
reported by the metalloporphyrin systems in the oxidation of alkenes such as cyclohexene
and is due to the radical nature of the active oxidizing species [3]. Oxidation of benzyl
alcohol to benzaldehyde was investigated and medium conversions (16-59 %) were
obtained. In the oxidation of cyclohexane the only product was cyclohexanol but
oxidation of cyclooctane resulted in cyclooctanol and cyclooctanone.
Catalytic activities of five catalysts 1, 2, 4, 5, 8 are compared in Fig. 4. There is
no single trend in the catalysts activities toward different hydrocarbons. The catalyst 4
showed the highest activity in the oxidation of cis-cyclooctene, cyclohexene and
cyclohexane.
All of the studied catalysts, 1, 2, 4, 5, 8, showed high selectivity in the
oxidation of cyclohexane. The oxidation of cyclohexane under mild conditions is a topic
of great interest [38]. In the literature better results have been reported by Shul'pin et al.
[39] using a binuclear manganese(IV) complex ([LMnIV(O)3MnIVL]2+, L: 1,4,7-trimethyl-
1,4,7-triazacyclononane) as catalyst, in the presence of acetic acid that prevents the
decomposition of H2O2 to water and dioxygen. However, the selectivity of
manganese(IV) complexes is not comparable with 4 (Tables 7,8). Furthermore, various
V(IV)/V(V)-catalysts have been used for the oxidation of cis-cyclooctene and
cyclohexene with H2O2 and TBHP (Table 8), yet none of the reported catalysts show
such high activity and/or selectivity as 1 and 5 complexes.
Table 8
18

Oxidation characteristics of cis-cyclooctene and cyclohexane by different transition metal
complexes
TOF
(h−1)
Time
(h)
5
Selec.
(%)
Conv.
(%)
92
Substrate
to product
Ref.
Oxidant/Catalyst
No.
This work 108
This work 102
H2O2/[{VO2}2(H2L1)]
H2O2/[{VO(CH3O)(CH3OH)}2(L2)]
TBHP/VIVO(L3)2
H2O2/[VVOL4Cl] b
H2O2/[VVOL5Cl] c
H2O2/NaHCO3/[TiO(H3L1)2]
100
1
2
3
4
5
6
7
Cis-cyclooctene
to cyclooctene
oxide
4
100
70
a
[40]
[41]
[41]
7
5
7
4.5
6
100
84
100
29
6
100
39
This work 137
4
100
93
H2O2/acetic acid/
1
58% A
42% K
23% A
77% K
22% A
66% K
31
[42]
[43]
640
53
d
Cyclohexane
[(Me3tacn)Mn(O)3Mn(Me3tacn)](PF6)2
H2O2/MnSO4/Me3tacn/Na-oxalate d
to cyclohexanol (A)
/ cyclohexanone (K)
3
73
82
8
9
d
[44]
74-
H2O2/acetic acid/[LMn(µ-O)3MnL][PF6]2
1.5-
4.5
222
a TBHP = tert-butylhydroperoxide, L3 = N-salicylidin-2-chloroethylimine
b L4 =Me2NCH2CH2N(CH2-2-O-3,5-C6H2 Bu2)2
t
c L5 = Me2NCH2CH(CH3)N(CH2-2-O-3,5-C6H2 Bu2)2
t
d L = Me3tacn =1,4,7-trimethyl-1,4,7-triazacyclononane
4. Conclusion
Eight new complexes of V(V), Cu(II), Mn(II) and Ti(IV) were prepared and
characterized and crystal structures of two V(V)complexes were determined. Bis(2-
hydroxybenzylidene)terephthalohydrazide complex of V(V) forms 1D coordination
polymer in the solid state. It was demonstrated that these binuclear hydrazone Schiff base
complexes are active and selective catalysts for oxidation of various hydrocarbons by
environmentally benign and clean oxidant H2O2 under mild conditions. Of the studied
aroylhydrazone complexes, [TiO(H3L1)2] (4) showed the best selectivity and activity as
catalyst.
Acknowledgments
19

The authors are grateful to the University of Zanjan, University of Warsaw and DFG
grant Ja466/23-1 (for CJ and HHM) for financial assistance.
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22

Scheme / Figure / Table captions
Scheme 1. (a) Acyl- and aroylhydrazones general formula, (b) keto–enol tautomerization
in H4L1 and H4L2
Scheme 2. (a) The ligand H4L1synthesis and (b) deprotonation of H4L2 relevant to the
complex formation form of [H2L2]2– and [L2]4– within the tautomeric equilibrium with
CH3OH/CH3O– in 5.
Scheme 3. Molecular structural formula of synthesized complexes.
Fig. 1. (a) Asymmetric unit of compound 1 with atom numbering scheme (50% ADPs);
the major conformer (68.4% occupancy) of the disordered {VO4} group is shown with
the solid ellipsoids and bonds, the minor conformer (31.6% occupancy) in the transparent
mode and with 'A' atom labels. The bond lengths and valence angles are given in Table 6.
(b) Polymeric strands in 1 through connection of the dinuclear molecular units by
elongated (dashed) V1---O4 bonds; only the major conformer is shown; symmetry
transformations i = –x, 1–y, 1–z; ii = –x, 2–y, 2–z.
Fig. 2. Dinculear molecular structure of 5 (50% thermal ellipsoids). Bond lengths and
angles are given in Table 6; symmetry transformation i = 0.5–x, 0.5–y, 1–z. The L2-
ligand in 5 is part of a protic equilibrium between mostly [L2]4–but also some [H2L2]2–
(Scheme 2).
23

Fig. 3. Influence of solvent in the catalytic epoxidation of cyclooctene by 1.
Reaction conditions: catalyst 1 1.70 μmol, cyclooctene 1.0 mmol, H2O2 3.0 mmol,
internal standard (chlorobenzene) 0.1 g, solvent 3.0 mL.
Fig 4. Activities of the catalysts 1, 2, 4, 5 and 8 in the oxidation of various substrates with
H2O2.
Table 1
Analytical and physical data of the complexes
Table 2
Crystal data and structure refinement for 1 and 5.
Table 3
1H-NMR data of the ligands H4L1-2 and complexes in DMSO-d6
Table 4
Selected characteristic IR bands of the ligands H4L1-2 and complexes
Table 5
Electronic transitionsof the ligands H4L1-2 and their complexes 1-8 in methanol
24

Table 6. Selected bond distances (Å) and angles (deg) for complexes 1 and 5.
Table 7
Oxidation of various hydrocarbons with 1, 2, 4, 5, 8 using H2O2 in acetonitrile.a
Table 8
Oxidation characteristics of cyclooctene and cyclohexane by different transition metal
complexes
25

(a)
(b)
Scheme 1
26

(a)
(b)
Scheme 2
27

Scheme 3
28

(a)
(b)
Fig. 1
29