Ligand substitution reactions of a phenolic quinolyl hydrazone; oxidovanadium (IV) complexes
© Seleem et al 2011
Received: 5 May 2011
Accepted: 16 August 2011
Published: 16 August 2011
Quinoline ring has therapeutic and biological activities. Quinolyl hydrazones constitute a class of excellent chelating agents. Recently, the physiological and biological activities of quinolyl hydrazones arise from their tendency to form metal chelates with transition metal ions. In this context, we have aimed to study the competency effect of a phenolic quinolyl hydrazone (H2L; primary ligand) with some auxiliary ligands (Tmen, Phen or Oxine; secondary ligands) towards oxidovanadium (IV) ions.
Mono- and binuclear oxidovanadium (IV) - complexes were obtained from the reaction of a phenolic quinolyl hydrazone with oxidovanadium (IV)- ion in absence and presence of N,N,N',N'- tetramethylethylenediamine (Tmen), 1,10-phenanthroline (Phen) or 8-hydroxyquinoline (Oxine). The phenolic quinolyl hydrazone ligand behaves as monobasic bidentate (NO- donor with O- bridging). All the obtained complexes have the preferable octahedral geometry except the oxinato complex (2) which has a square pyramid geometry with no axial interaction; the only homoleptic complex in this study.
The ligand exchange (substitution/replacement) reactions reflect the strong competency power of the auxiliary aromatic ligands (Phen/Oxine) compared to the phenolic quinolyl hydrazone (H2L) towards oxidovanadium (IV) ion; (complexes 2 and 3). By contrast, in case of the more flexible aliphatic competitor (Tmen), an adduct was obtained (4). The obtained complexes reflect the strength of the ligand field towards the oxidovanadium (IV)- ion; Oxine or Phen >> phenolic hydrazone (H2L) > Tmen.
Results and discussion
Effect of solvent on the spectra of the hydrazone
Electronic spectral data*of the hydrazone in various solvents.
Competition study (ligand substitution reactions)
Physical and analytical data of the complexes.
H2L + VO SO4
[(VO)2 (HL)2 (MeOH)2 SO4].2MeOH (970.78)
H2L + VO SO4 + Oxine
[(VO) (oxinate)2].1/4 H2O (359.74)
H2L + VO SO4 + Phen
[(VO) (Phen)2(SO4)].21/2 H2O.41/4 MeOH (704.63)
H2L + VO SO4 + Tmen
[(VO)2 (HL)2 (Tmen) SO4].6H2O (1066.92)
Conductivity and magnetic measurements
Magnetic, conductivity and electronic spectral data of the complexes.
Electronic Spectral Bands (nm)
Ohm-1 cm2 mol-1
282, 341, 435, 460, 483
268, 341 (sh.)
267, 336 (sh.), 430 (sh.), 456, 486 (sh.)
Solution electronic spectra of the complexes in DMF (Table 3) are more or less similar and show a series of bands within the range 267-341 nm due to intra-ligand transitions. Unfortunately, although the oxidovanadium (IV)- complexes (1-4) are the easiest of the d1- systems to use experimentally, the interpretation of the spectra has been complicated by the deviation of the complexes from regular octahedral stereochemistry . Three bands are normally observed around 485, 460 and 430 nm in case of the phenolic complexes (1) and (4). On a simple crystal field model, these bands would be interpreted as the transitions from the 2B2- ground term to the 2E, 2B1 and 2A1- terms , respectively. However, such a treatment does not give a good quantitative fit to the spectra and the axial π- bonding must be taken into account. In case of the oxinato (2) and Phen (3) complexes, only the transition 2B2 → 2A1 was observed at 412 and 341 nm, respectively.
Thermal properties of the complexes
Thermodynamic and kinetic parametersa of complex 3.
A × 10-9 sec-1
The chemicals used in this investigation were of the highest purity available (Merck, BDH, Aldrich and Fluka). They included vanadyl sulfate monohydrate, N,N,N',N'- tetramethylethylenediamine (Tmen), 1,10-phenanthroline (Phen) or 8-hydroxyquinoline (Oxine), o-toluidine, ethyl acetoacetate, POCl3, sulfuric acid, hydrazine hydrate and 2,4-dihydroxybenzaldehyde. Organic solvents were reagent grade chemicals and were used without further purification.
Microanalyses were carried out on a Perkin- Elmer 2400 CHN elemental analyzer. Thermal analyses (TG-DSC) were carried out on a Shimadzu- 50 thermal analyzer in nitrogen atmosphere and a heating rate of 20°C/min using the TA-50 WS1 program. Electronic spectra were recorded on a Jasco V-550 UV/VIS spectrophotometer. IR spectra were recorded on a Bruker Vector 22 spectrometer using KBr pellets. ESR spectra were recorded on a Bruker Elexsys, E 500 operated at X- band frequency. Mass spectra were recorded at 70 eV on a gas chromatographic GCMSQP 1000- EX Shimadzu mass spectrometer. Molar conductance was measured as DMF solutions on the Corning conductivity meter NY 14831 model 441. Magnetic susceptibility was measured at room temperature using a Johnson Matthey, MKI magnetic susceptibility balance. Melting points were determined using a Stuart melting point apparatus.
Preparation of the phenolic hydrazone
An ethanolic mixture of 2-hydrazinyl-4,8-dimethyl quinoline (0.01 mol) and 2,4-dihydroxybenzaldehyde (0.012 mol) was refluxed for 1/2 h. The formed yellow compound was filtered off, washed with ethanol and crystallized from ethanol as described in our previous publication .
Synthesis of the complexes (1-4)
A methanolic solution of VOSO4.H2O was added gradually to a methanolic solution of the phenolic hydrazone in absence (complex 1) and in presence of Oxine, Phen or Tmen (complexes 2-4) at the mole ratio 1: 1: 1. The reaction mixture was refluxed until the solid complex was precipitated. Then, the isolated complexes were filtered off, washed with methanol, then diethyl ether and finally dried in vacuo. The obtained complexes (Table 2) are colored and quite stable in atmospheric conditions. The complexes are insoluble in water and most common solvents; but they are soluble in DMF and DMSO solvents.
Conclusion and comments
Trials to prepare mixed ligand complexes via the reaction of the phenolic hydrazone (H2L) with VOSO4.H2O in presence of Oxine or Phen were unsuccessful. Instead, an Oxinato complex (2) or a Phen complex (3) were obtained. In contrast, these trials were successful in case of the more flexible aliphatic base (Tmen) where a mixed ligand complex (4) was obtained. The obtained complexes have an octahedral arrangement except complex 2 which has a square pyramid arrangement. Also, they reflect the competency power towards the oxidovanadium (IV)- ion, and the following order holds; Oxine or Phen >> phenolic hydrazone (H2L) > Tmen. This order reflects that the nucleophilicity of Oxine or Phen is stronger than that of phenolic hydrazone (H2L).
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