Synthesis, Structural Characterization, and Electrochemical Studies of NewOxovanadium(V) Complexes Derived from 2-Furanoylhydrazon Derivatives

Five monooxovanadium(V) complexes [VO(L)(OCH3)(OHCH3)] (1), [VO(L )(OCH3)(OHCH3)] (2), [VO(L )(OCH3) (OHCH3)] (3), [VO(L )(OCH3)(OHCH3)] (4), and [VO(L )(OCH3)(OHCH3)] (5) were synthesized and characterized by IR, NMR UV-Vis, and single-crystal structure analysis [H2L 1 = (EEE-N-((2-hydroxynaphthalen-1-yl)methylene)furan-2-carbohydrazide, H2L 2 = (EEE-N-(2-hydroxybenzylidene)furan-2-carbohydrazide, H2L 3 = (EEE-N-(5-bromo-2-hydroxybenzylidene)furan2-carbohydrazide, H2L 4 = (EEE-N-(2-hydroxy-5-nitrobenzylidene)furan-2-carbohydrazide, H2L 5 = (EEE-N-(2-hydroxy-5iodobenzylidene)furan-2-carbohydrazide]. In all 1–3 structures the vanadium atom has a distorted octahedral coordination with the three meridional donor atoms from the Schiff base dianion (L)2 and one methoxylato group occupying the sites of the equatorial plane. e oxo group and one methanol molecule occupy the apical sites. In the complexes 1, 2, and 3 the conformation of 2-furanyl oxygen atom relative to the carbohydrazide oxygen atom is s-anti, s-anti/s-syn, and s-syn at 293K, respectively. Cyclic voltammetric experiments of the solution species 1–5 in DMSO revealed a quasi-reversible behavior.


Introduction
e chemistry of vanadium has received considerable attention due to the discovery that vanadium is an essential element in biological systems [1].ere is continuous interest in the chemistry of vanadium complexes due to its inhibitory capabilities for various enzymes [2], its ability to catalyze oxidation and oxo-transfer reactions [3], and its occurrence in many biological systems [4].is is particularly related with the discoveries of several medicinal properties of vanadium complexes, that is, insulin mimetic [5][6][7], antifungal/ antibacterial [8], antitumor [9], anticancer activities [10], and the presence of vanadium in the prosthetic group of certain haloperoxidases and nitrogenases [11].Structural and functional models for vanadate-dependent haloperoxidases, for vanadium nitrogenases and other biologically active vanadium compounds, have further stimulated vanadium coordination chemistry [12].Vanadium complexes are also important catalysts for several chemical reactions such as oxidation, epoxidation, and hydroxylation [13].Schiff base complexes of V (III, IV and V) have been used as catalysis for the electroreduction of O 2 to H 2 O in acetonitrile [14].Because of electrochemical reversibility of V-Schiff base complex derivations, they can be used as electron transfer mediator for modi�cation of different electrode materials, and preparation of chemically modi�ed electrodes with these compounds has been received increasing interest in the �eld of electroanalysis [15].
On the other hand, hydrazone ligands, a class of Schiff base, derived from the condensation of acid hydrazides (R-CO-NH-NH 2 ) with aromatic 2-hydroxy carbonyl compounds are important tridentate O, N, O-donor ligands.e coordination chemistry and biochemistry of aroylhydrazones, R-CO-NH-N=CH-R � , have attracted increasing interest due to their chelating ability and their pharmacological applications [16].Hydrazone ligands create environment similar to biological systems by usually making coordination through oxygen and nitrogen atoms [17].Furthermore, hydrazones have wide spread applications in �elds such as coordination chemistry [18,19], analytical chemistry [20,21], bioinorganic chemistry [22][23][24], and also in magnetic, electronic, nonlinear optically active, and �uorescent [25] compounds.
As part of our research in the study of the coordinating capabilities of aroylhydrazones and their coordination compounds [26][27][28][29][30], here, we report the synthesis, structure, and electrochemistry behavior of monooxovanadium(V) complexes of furancarbohydrazide Schiff bases (Scheme 1).

Materials and Instrumentations.
Vanadyl bis(acetylacetonate), 2-furancarboxylic acid hydrazide, 2-hydroxybenzaldehyde, 5-bromo-2-hydroxybenzaldehyde, 2-hydroxy-5nitrobenzaldehyde, and 2-hydroxy-1-naphthaldehyde were purchased from Merck and used as received.2-hydroxy-5iodobenzaldehyde was synthesized according to the reported literature procedure [31].Solvents of the highest grade commercially available (Merck) were used without further puri�cation.IR spectra were recorded in KBr disks with a Bruker FT-IR spectrophotometer.UV-Vis spectra of solution were recorded on a thermospectronic, Helios Alpha spectrometer. 1 H and 13 C NMR spectra of ligands and their complexes in DMSO-d 6 solution were recorded on a Bruker 250 and 62.9 MHz spectrometer, and chemical shis are indicated in ppm relative to tetramethylsilane.Voltammetric experiments were performed using an autolab voltammetric analyzer.

Synthesis of the Ligands
2.2.1.General Procedure.All ligands were prepared in a similar manner by re�uxing a mixture of 2-furancarboxylic acid hydrazide and o-hydroxybenzaldehyde with equivalent molar ratio in 20 mL methanol.e mixture was re�uxed for 2-3 h.e solution was then evaporated on a steam bath to 5 mL and cooled to room temperature.e obtained solids were separated and �ltered off, washed with 5 mL of cooled methanol, and then dried in air.Completion of the reactions was checked by TLC on silica gel plates.4.  dm 3 mol −1 cm −1 ): 239 (6 400 sh ), 290 (14 080), 300 (15 280), 332 nm (10 980).

Synthesis of (E)-N
(3) at 30% probability level, bond lengths and angles in Table 6.

Synthesis of the Complexes [VO(𝐿𝐿
(1-5).ese complexes were synthesized by the same method.General method: the appropriate ligand ( , or H 2 L 5 ) (1.0 mmol) was dissolved in a solution of methanol (20 mL) then VO(acac) 2 ⋅(0.265 g, 1.0 mmol) was added and the solution was re�uxed for 4 h.A�er cooling, the resulting solid was �ltered o�, washed with cooled absolute ethanol, and dried at 100 , and [VO(L 3 )(OCH 3 )(OHCH 3 )] (3) were prepared by the thermal gradient method.were changed periodically to avoid aqueous contamination from entering the cell via the Ag/AgCl electrode.e electrolytic medium consisted of 0.1 mol/L lithium perchlorate (LiClO 4 ) as supporting electrolyte in dimethyl sulfoxide, and all experiments were carried out at room temperature.e solutions were freshly prepared before use and were purged with N 2 saturated with solvent for 10 min prior to taking measurements in order to remove dissolved O 2 .Voltammograms were recorded in the range from 0.0 to +1.0 V versus Ag/AgCl.

�.1�. ����� �r�st���o�r�ph� D�t� �o��ection �n� �e�ne�ent.
Dark brown crystals of 1, 2, and 3 were investigated by X-ray diffraction at 200 K on an Oxford-Diffraction Xcalibur Nova E diffractometer equipped with a molybdenum microsource ( = 0.7107 Å). e structures were solved by Direct Methods with SIR97 [32] and re�ned with full-matrix least-squares techniques on  2 with CRYSTALS [33].e crystal data and re�nement parameters are presented in Table 1.e hydrogen atoms were found in successive Fourier difference analysis.All nonhydrogen atoms were re�ned anisotropically.Hydrogen atoms were �rst re�ned with restraints on the bond lengths and angles (C-H in the range 0.93-0.98Å and O-H = 0.82 Å) and U iso (in the range 1.2-1.5 times U eq of the parent atom), a�er which they were re�ned with riding constraints.e molecular structure plots were drawn with ORTEPIII [34][35][36].

Results and Discussion
e reaction of 2-furancarboxylic acid hydrazide with several aromatic o-hydroxy aldehydes with different substituents in methanol gave the desired tridentate Schiff base ligands in excellent yields and purity.Oxovanadium(V) complexes with tridentate hydrazone Schiff base ligands were prepared by treating a methanolic solution of the appropriate ligand with equimolar amount of VO(acac) 2 (Scheme 1).

Description of the Structures (1, 2, and 3).
In order to de�ne the coordination sphere conclusively, a singlecrystal X-ray diffraction study was made.A list of some crystallographic data of 1, 2, and 3 is given in Table 1.An ORTEP diagram with the atom numbering scheme of the 1, 2, and 3 is shown in Figures 1, 2, and 3 and selected bond lengths 74.17 (13) and angles are given in Tables 2, 3, and 4, respectively.In 1, 2, and 3, the vanadium atom has a six-coordinated structure as a VO 5 N with nitrogen and two oxygen atoms provided by the Schiff base ligand and three oxygen atoms from methoxy, methanol, and oxo ligands.An axial position is occupied by the oxygen atom from methanol, and another axial position is occupied by the oxygen atom from the oxo ligand.In these compounds the Schiff base ligands form a six-membered and a �ve-membered chelate ring with bite angles of about 84 ∘ (O phenolat -V-N) and 74 ∘ (N-V-O enolat ), respectively.is angles are the same with previously reported naphthohydrazone oxovanadium complexes [29].e double deprotonated form of the N-arylidene fouranohydrazide ligands is consistent with the observed O-C carbonyl and N=C carbonyl bond lengths of 1.29 and 1.31 Å, respectively in 1, 2, and 3. is is in agreement with the reported complexes containing the enolate form of N-arylidene hydrazone ligands [29,30,37], whereas the C=O bond is considerably short for reported complexes with the coordinated keto form of the N-arylidene benzohydrazide system [28,38].In furancarbohydrazide ligands the C-N and C=O bond  lengths are about 1.35 Å and 1.23 Å, respectively [39][40][41].e corresponding bond lengths in complex 1-3 are about 1.31 Å and 1.29 Å, respectively.Comparison of these bond lengths indicates the shortening of the C-N bond length and lengthening of the C=O bond due to coordination in enol form.e vanadium to oxygen bond lengths follows the order V-oxo oxygen < V-methoxy oxygen < V-phenolate oxygen < V-enolate oxygen < V-methanol oxygen.e oxovanadium (V) complexes under consideration crystallize in the monoclinic crystal system.In 1 and 2 the space group is P2 1 /c but in 3, the space group is P2 1 /n.In the complexes 1, 2, and 3 the conformation of 2furanyl oxygen atom relative to the carbohydrazide oxygen atom is s-anti, s-anti/s-syn, and s-syn at 293 K, respectively.ese �ndings suggest the presence of low barrier energy for rotation of the 2-furanyl group around the C-C bond between 2-furanyl and the carbohydrazide groups at room temperature.is rotation plausibly prevents 2-furanyl oxygen atom involvement in the coordination to the vanadium center of the adjacent molecule.In addition, the oxygen atom in furan is too poor for a donor to take part in metal binding.From steric consideration also, the furan group is not properly positioned to be involved in metal binding.
Hydrogen bonding is a common feature for vanadium(IV) and vanadium(V) compounds in the solid state, if appropriate hydrogen bonding donors are present [42,43].e type of complexes described in this work contains two major functionalities which can participate in intermolecular hydrogen bond interactions.ese are the N atom of the hydrazine fragment of the tridentate ligand and the O-H group of the metal coordinated methanol.In these complexes two molecules of complex are connected together by strong intermolecular O (methanol) -H⋅ ⋅ ⋅N (amide) hydrogen bonds and create a pseudodimer as depicted in Figure 4. is strong intermolecular hydrogen bond stabilizes the crystal structure of 1, 2, and 3. ese pseudodimers make a chain along  diagonal by intermolecular hydrogen bonds.Parameters of hydrogen bonding geometry are given in Table 5.On complexation the absence of N-H and O-H peaks of the ligands shows coordination of H 2 L 1 -H 2 L 5 as dianionic ligands in enol form (Scheme 1).In these ligands, a triplet peak and two doublet peaks should have been observed around  6.5-7.5 ppm for furan's hydrogens but because of the low coupling constant (ca. 2 Hz.) [44] and also not using a powerful instrument, only a singlet peak is observed for these hydrogens; however, in three cases (H 2 L 1 , H 2 L 2 and H 2 L 4 ) this splitting could be observed.We have also recorded 13 C NMR of ligands and their complexes to provide diagnostic tools for the elucidation of the structures.Assignments of the peaks are similar and are based on the chemical shi and intensity patterns.Δ observed for carbon atoms in the vicinity of the phenolate, enolate, and azomethine groups suggests their involvement in coordination.Two new signals in complexes appear at  about 49 and 75 ppm; these signals correspond to methanol and methoxy carbon atoms, respectively.

Infrared Spectra.
A list of the important vibrational frequencies (IR spectra) of the free ligands (H 2 L 1 -H 2 L 5 ) and their oxovanadium complexes, which are useful for determining the mode of coordination of the ligands, are given in the experimental part.A comparison of the spectra of the complexes with the ligands provides evidence for the coordination mode of the ligands in the complexes.Hence signi�cant frequencies are selected by comparing the IR spectra of the ligands with those of oxovanadium complexes.All hydrazone Schiff base ligands (H 2 L 1 -H 2 L 5 ) exhibit a broad band around 3167-3270 cm −1 due to NH-vibrations.Also in IR spectra of all the ligands very strong band appears around 1650-1680 cm −1 due to C=O-vibration.In addition a broad band is centered at 3400-3600 cm −1 in H 2 L 1 -H 2 L 5 due to the O-H of the phenol, probably involved in intramolecular hydrogen bonding.e infrared spectra of complexes display IR absorption band around 1610 cm −1 which can be assigned to the C=N stretching frequency of the coordinated hydrazone ligand, whereas for the free ligands the same band are observed around 1600 cm −1 .Strong C=N stretch (around 1600 cm −1 ) indicates the C=N group of the coordinated Schiff base ligands [45,46].On complexation the absence of N-H and C=O bands and red shis in azomethine (-C=N-) band [47] of the ligands shows coordination of H 2 L 1 -H 2 L 5 as three dentate dianionic ligands in enol form (Scheme 1).In all complexes very broad bond around 3380-3440 cm −1 expresses presentce of -OH and coordination of methanol to the vanadium.e band at 963-972 cm −1 is assigned to (V=O); this band is observed as a new peak for the complexes and is not present in the spectra of the free ligands.Similarity of the IR spectra of the complexes shows the similarity of their structures.) at the para position with respect to phenolic OH group in the aryloxy ring is very closet to each other and they have very similar shape (they have two  max in around 290 and 305 nm).e Oxovanadium(V) complexes have bands in the range 205-220 and 324-336 nm.ese bands are assigned as due to intraligand transitions.All bands shi in complexes indicating the coordination of ligands to the metal ions [50,51].e shoulder appeared at about 280 nm for 1-5 corresponds to LMCT band of V=O which is appeared at 274 nm for [VO(acac) 2 ]. e lowest energy transition lying around 420 nm is assigned to LMCT transition of the type O (phenolic)  V 5+ [52][53][54][55].Electronic spectra for these complexes in MeOH solutions are akin similar electronic structure in solution of these compounds.
3.2.4.Electrochemistry.e electrochemical behaviors of the complexes were studied by cyclic voltammetry techniques in the range of 0.00 to +1.0 V at a scan rate of 50 mV/s in DMSO on a glassy carbon electrode (GCE) and Ag/AgC1 reference electrode using lithium perchlorate (LiClO 4 ) as the supporting electrolyte.All the complexes exhibit a quasireversible reduction peak due to VO 3+ /VO 2+ couple.Cyclic voltammetry data for these complexes are collected in Table 7 and the Figure 5 displays a representative cyclic voltammogram of 2. e effect of the electronic nature of the substituent present on the salicylidene fragment of tridentate ligand is clearly re�ected on the trend of the  1/2 values for this reduction.For the strong electron withdrawing substituent (Y = NO 2 , complex 4) the reduction of the metal centre occurs at the highest potential while for the least (or without any) electron withdrawing substituent (X = H, complex 2) it occurs at the lowest potential.Also for complex 4 the oxidation of the metal centre occurs at the lowest potential, while for complex 2 it occurs at the highest potential.

Conclusion
is work revealed that coordination complexes of V 5+ and tridentate hydrazone Schiff base ligands obtained from the reaction of 2-furancarboxylic acid hydrazide and aromatic o-hydroxyaldehydes derivatives afford a new class of V 5+ complexes.Five monooxovanadium(V) complexes of tridentate Schiff base ligands were synthesized and characterized by spectroscopic methods and single crystal X-ray analysis.e crystal structures of 1-3 suggest the presence of lowbarrier energy for rotation of the 2-furanyl group around the C-C bond between 2-furanyl and the carbohydrazide groups at room temperature.Electrochemical studies by cyclic voltammetry technique indicated that these complexes are quasi-reversible electroactive.

F 4 :
e hydrogen-bonded structure of the complex 3.

Table 6 .
(1)(2)(3)(4)(5)13CNMRspectral data of the ligands in DMSO-d 6 con�rmed the proposed structure of the ligands (Scheme 1).e principal peaks of the 1 H NMR spectra of ligands H 2 L 1 -H 2 L 5 T 6: 1 H NMR of ligands (H 2 L 1 -H 2 L 5 ) and complexes(1)(2)(3)(4)(5). esignal at  12.12-12.33 in the spectra of H 2 L 1 -H 2 L 5 is assigned to the common NH-group, concomitant with the observation of a rapid loss of these signals when D 2 O is added to the solution.Also the signals between  10.62-12.22 in the spectra of H 2 L 1 -H 2 L 5 are lost upon addition of D 2 O to the solution.Hence, this signal is assigned to the phenolic OH group.e resonances between  8.57-8.67 are assigned to the azomethine (-CH=N-) in the spectra of H 2 L 1 -H 2 L 5 .In all ligands other aromatic protons appear between  6.60-7.85.e chemical shis for these complexes are comparable and very close to each other.
[20,48,49]tronic Spectra.esecomplexes are shiny dark brown in solid state, but their methanol solutions are brown in color.esesolutionshavebeenused to record the electronic spectra.For the oxovanadium(V) compounds, no d-d bands are expected because they have a 3d 0 con�guration and there are no d electrons[12].ehydrazoneligands have bands in the range 209-290 and 300-342 nm.Based on their extinction coefficients these are assigned as due to    * and n   * transitions, respectively[20,48,49]. e UV-Vis spectrum of H 2 L 3 , H 2 L 4 , and H 2 L 5 which they have electron-withdrawing group (e.g., Br, I, and NO 2 −