Oxovanadium(IV) complexes of halogenated oxines

Summary Six VO²⁺ complexes of 8-hydroxyquinoline (oxine) and of some of its mono- and dihalogenated derivatives have been prepared. The complex of 5-chloro-oxine is very unstable and oxidizes rapidly, generating a V(V) complex of stoichiometry VO(QCl)₂OH which could also be prepared in pure form. The infrared spectra of all complexes have been recorded and are discussed in detail. The complexes containing halogenated ligands appear as polymeric species, interacting through V=O...V=O bridges. The magnetic moments, investigated at room temperature, indicate completely quenched orbital contributions. The analysis of the electronic spectra reveals very complex solution behaviour including, oxidation phenomena, ligand loss, and interaction with the solvent.

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Oxovanadium(IV)-Komplexe von halogenierten Oxinen

Zusammenfassung Sechs VO²⁺-Komplexe von 8-Hydroxyquinolin (Oxin) und einige seiner mono- und dihalogenierten Derivate wurden dargestellt. Der Komplex des 5-Chloroxins erwies sich als besonders instabil und oxidiert sehr schnell unter Bildung eines V(V)-Komplexes der Stöchiometrie VO(QCl)₂OH, welcher auch in reiner Form dargestellt werden konnte. Die Infrarotspektren aller Komplexe wurden aufgenommen und werden eingehend diskutiert. Die Komplexe mit halogenierten Liganden erscheinen als polymere Spezies, welche über V=O...V=O-Brücken wechselwirken. Die bei Raumtemperatur gemessenen magnetischen Momente zeigen die totale Abwesenheit von Orbitalbeiträgen. Die Analyse der Elektronenspektren weist auf ein besonders kompliziertes Lösungsverhalten hin, welches Oxidationsphänomene, Ligandenabspaltung und Wechselwirkung mit den Lösungsmitteln einschließt.

     

Mixed Ligand Complexes of Oxovanadium (IV) Oxalate

The oxovanadium(IV) complexes VOC₂O₄ · 2DMF, VOC₂O₄ · 2 formamide, VOC₂O₄ · 2 ethylene urea, VOC₂O₄ · 2 urea and VOC₂O₄ · en have been prepared and characterized. The oxalato group functions as a bidentate ligand in all these complexes. Except ethylene diamine the remaining ligands – DMF. formamide, ethylene urea and urea – act as monodentate ligand coordinating through their carbonyl oxygen. Ethylene diamine is bound to the metal through nitrogen. In all the complexes vanadium is five coordinated in distorted tetragonal-pyramidal structures.     

Oxovanadium(IV) complexes with polydentate nitrogen ligands

Summary Biacetyldihydrazone (BdH) and 2,2-6, 2-terpyridine (terpy) complexes of oxovanadium(IV) have been prepared and characterized by chemical analysis, conductance measurements, electronic, i.r. and e.p.r. spectral studies and magnetic susceptibilities measurements. Polymeric and monomer structures are proposed for the BdH and terpy complexes, respectively.     

Oxovanadium(IV) Complexes of tetradentate SCHIFF Bases

The oxovanadium(IV) complexes of the Schiff bases derived from 1,3-diamino-propane-2-ol and salicylaldehyde, 3-aldehydosalicylic acid, 5-chloro-salicylaldehyde, acetyl acetone, and also of the Schiff base derived from 1.3-diaminopropane and salicylaldehyde have been isolated in the pure state and characterized. The structure of these complexes is discussed on the basis of elemental analyses, electronic spectra and magnetic moment values. These complexes are characterized as five co-ordinate compounds having the general formula [VO](L), where, LH₂ = a molecule of dibasic, tetradentate Schiff base. Electronic spectra of the complexes are found to be solvent independent. The compounds are paramagnetic (μ_eff. = 1.76–1.79 B. M.), and are very stable in air.     

ERP-Untersuchungen an Oxovanadium(IV)-Bis-Komplexen mit 1,2-Dithioliganden

EPR Studies on Oxovanadium(IV) Bis-Complexes of 1,2-Dithio Ligands EPR investigations on oxovanadium(IV) bis-complexes containing the ligands 1,3-dithiol-2-thione-4,5-dithiolate, dmit^2?, 1,2-dithiol-3-thione-4,5-dithiolate, dmt^2?, and 1,2-dithiooxalate are reported. The parameters of the spin-Hamiltonian are used to characterize the bonding properties, and are compared with those obtained for the corresponding VOS₄ chelates containing four-membered chelate rings. According to this a successful characterization of VOS₄ type mixed-ligand complexes should be possible.     

Oxovanadium(IV) complexes of salicyl-L-aspartic acid and salicylglycyl-L-aspartic acid

The dipeptide and tripeptide analogues salicyl-L-aspartic acid (Sal-L-Asp) and salicylglycyl-L-aspartic acid (SalGly-L-Asp) were synthesized and their protonation and complex formation with V(IV)O2+ were studied in aqueous solution through the use of pH-potentiometry and spectroscopic (UV-Vis, CD and EPR) techniques. The phenolate terminus proved to be a good anchoring site to promote (i) the metal ion-induced deprotonation and subsequent coordination of the peptide amide group(s) in the pH range 4-5 for the dipeptide analogue, (ii) and in the pH range 5-6 in a very cooperative way for the tripeptide analogue. The results suggest that the presence of good anchoring donors on both sides of the amide groups is responsible for the cooperative deprotonation of the two amide-NH groups.     

Some Oxovanadium(IV) Complexes with C2v Symmetry

Synthesis of few oxovanadium (IV) complexes of C_2v symmetry with low symmetry β-diketones is described. Magnetic, infrared and electronic spectral studies of the complexes are reported. Splitting of the xy → xz, yz transition is observed at 77°K and is believed to be of vibrational origin.     

Oxovanadium(IV) complexes of cyclohexanone semicarbazone and cyclohexanone thiosemicarbazone

Summary Oxovanadium(IV) complexes, VOL₂X₂ (X = Cl, Br and 0.5 SO₄), have been synthesised and characterised by elemental analysis, room temperature magnetic moment, electronic, i.r. and electron spin resonance studies. The complexes are hexacoordinate and have a distorted octahedral structure.     

Oxovanadium(IV) based hypocrellin B complexes with enhanced photodynamic activity

Hypocrellin B (HB), a naturally occurring photosensitizer, has been extensively and intensively studied as a promising photodynamic therapy (PDT) agent. In this work, three new oxovanadium(IV) complexes were designed and synthesized with HB as a bridging ligand and phen (1,10-phenanthroline, complex 1), tmp (3,4,7,8-tetramethyl-1,10-phenanthroline, complex 2) and dpq (dipyrido[3,2-f:2'3'-h]quinoxaline, complex 3) as terminal ligands. The use of a diimine terminal ligand avoids the formation of polymeric complexes and ensures the three VO(2+)-HB complexes possess a definite molecular formula and molecular weight to meet the single component requirement for an ideal PDT agent. Compared to HB, the VO(2+)-HB complexes exhibit improved water solubility, enhanced absorptivity in the phototherapeutic window, increased binding affinity toward dsDNA, and similar singlet oxygen quantum yield, therefore advanced DNA photocleavage activity. Both the DNA binding constants and photo nuclease activities of the complexes follow the order 2 (tmp) > 3 (dpq) > 1 (phen), demonstrating the importance of the binding affinity to biomolecules, which improves the bioavailability of reactive oxygen species. Our work opens a new avenue for the development of HB-based PDT agents.     

Oxovanadium(IV) and (V) complexes of acetylpyridine-derived semicarbazones exhibit insulin-like activity

Reaction of 2-acetylpyridine semicarbazone (H2APS), 3-acetylpyridine semicarbazone (H3APS) and 4-acetylpyridine semicarbazone (H4APS) with [VO(acac)₂] (acac = acetylacetonate) gave [VO(H2APS)(acac)₂] (1), [VO(H3APS)(acac)₂] (2) and [VO(4APS)(acac)(H₂O)] · 1/2H₂O (3). Oxidation of complex 1 in acetonitrile gave [VO₂(2APS)] (4). The crystal structures of complexes 1 and 4 have been determined. Complexes 1–3 were able to enhance glucose uptake and to inhibit glycerol release from adipocytes, which indicate their potential to act as insulin-mimics.     

Catalytic Asymmetric Coupling of 2-Naphthols by Chiral Tridentate Oxovanadium(IV) Complexes

A series of chiral oxovanadium(IV) complexes derived from tridentate N-3,5-substituted-, N-3,4-benzo-and N-5,6-benzo-salicylidene-α-amino acids can serve as efficient catalysts for the enantioselective oxidative couplings of various 3-, 6-, and 7-substituted 2-naphthols under O2. The best scenario involves the use of a vanadyl complex arising from 2-hydroxy-l-naphthaldehyde and valine (or phenylalanine) in CCl4, leading to BINOLs in good yields (75-100%) and with enantioselectivities of up to 68%.     

Oxovanadium(IV), manganese(II) and manganese(III) carbodithioates exhibiting abnormal magnetic behaviour

Summary New carbodithioate complexes of the oxovanadium(IV), manganese(II) and manganese(III) ions have been prepared and studied by i.r. and electronic spectral and variable temperature magnetic susceptibility (77K to room temperature) measurements. The carbodithioate ligands, 4-methylpiperazine-1-carbodithioate (4-MPipzcdt) and 4-phenylpiperazine-1-carbodithioate (4-PPipzcdt), were derived from heterocyclic secondary amines. The VO(4-MPipzcdt)₂ and VO(4-PPipzcdt)₂ complexes possess C _4v symmetry; Mn(4-PPipzcdt)₂ is tetrahedral and Mn(4-PPipzcdt)₃ is octahedral. All exhibit abnormal room temperature magnetic moments and the variable temperature magnetic moments suggest antiferromagnetism for the oxovanadium(IV) and the manganese(II) complexes and the occurrence of low spin (³ T _1g ) high spin (⁵ E _g ) equilibrium in addition to antiferromagnetic interactions in the manganese(III) complex. The spin-spin exchange parameter (-2J) value for the VO(4-MPipzcdt)₂ complex has been calculated using variable temperature magnetic susceptibility data.     

Thermal Behaviour of Oxovanadium(IV) Complexes of Halogenated Derivatives of 8-hydroxyquinoline

The thermal behaviour of a series of oxovanadium(IV) complexes of halogenated derivatives of 8-hydroxyquinoline was investigated by means of TG and DTA measurements in oxygen atmosphere. V₂O₅ was the final pyrolysis residue in all cases.This revised version was published online in November 2005 with corrections to the Cover Date.     

Oxovanadium(IV) complexes of bromo-and methoxy substituted N1,N4-diarylidene-S-methylthiosemicarbazones

Four new oxovanadium(IV) compounds were prepared by template reaction of salicyl-, 5-bromosalicyl-and 3-methoxysalicyl-aldehyde S-methylthiosemicarbazones with 2-hydroxy-, 5-bromo-2-hydroxy-and 3-methoxy-2-hydroxy-benzaldehyde in various combinations. The compounds were isolated as stable solid compounds with general formula [VO(L)] and characterized by elemental analysis, conductivity and magnetic measurements, electronic, IR and EPR spectroscopy. The X-band EPR signals recorded from powder forms of all samples have a single asymmetric line shape and theoretical fit studies proved the presence of axial symmetry around the paramagnetic vanadium ions. The anisotropic Lande splitting factors take values of g_‖ < g_⊥ < g_e = 2.0023. Orbital energy levels for magnetic electrons were determined from theoretically well fitted Spin Hamiltonian parameters. The EPR spectra recorded from solution forms almost have isotropic character.     

ESR Characterization of Two Oxovanadium (IV) Schiff Base Complexes Derived from Tris(2-aminoethyl)amine

The compounds, C₂₁H₂₇N₄O₃(L¹) and C₂₁H₂₇N₇(L²), is a tripodal Schiff base that was obtained from the reaction of tris(2-aminoethyl)amine (tren) and furan-2-carbaldehyde and pyrole- 2- carbaldehyde. The tripodal Schiff bases and their oxovanadium complexes have been characterized on the basis of the results of the elemental analysis, magnetic susceptibility measurements and spectroscopic studies FT-IR, ¹H-NMR, UV–Vis, ESR, magnetic moment and thermal analysis (TGA). Job's method of continuous variation shows 3:2 metal to ligand ratio.     

Oxovanadium(IV) complexes as molecular catalysts in epoxidation: Simple access to pyridylalkoxide derivatives

Reaction of bis(aryl)-2-pyridylmethanol ligands (1a-7a) with VO(SO₄) · 5H₂O results in the formation of metal-oxo complexes [VO(N-O)₂] (1-7), with N-O = bis(aryl)-2-pyridylmethanol. A molecular structure of (4) has been determined by single crystals X-ray diffraction study, which showed the expected square planar pyramidal geometry with the pyridine ring nitrogens in trans-position to each other. The metal-oxo complexes (1-4,6,7) demonstrated the ability to catalyse epoxidation reactions of alkenes with molecular oxygen.     

Synthesis,Spectral and Electrochemical Studies of Mixed-Ligand Oxovanadium(IV) and Oxovanadium(V) Complexes Incorporating the Tridentate ONO Donor Schiff Base Derived from Acetylacetone and Benzoylhydrazine

Mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes with a tridentate dinegative ONO donor Schiff base ligand [viz., 4-(1-hydroxybenzylidenehydrazono)-2-penten-2-ol (H₂L)] and bidentate NN [viz., 2,2-bipyridine (bipy) and 1,10-phenanthroline (phen): complexes (1) and (2), respectively] or OO^–[viz., ethylene glycol (H₂gol), salicylaldehyde (Hsal) and vanillin (Hvan): complexes (3)–(5), respectively] donor ligands have been prepared and characterized by elemental analyses and by i.r., e.p.r., n.m.r., and u.v.–vis. spectroscopies. The complexes with NN donor ligands are one electron paramagnetic, displaying axial e.p.r. spectra and exhibiting two ligand-field transitions in the visible region, whereas the complexes with OO^– donor ligands are diamagnetic and display only LMCT bands. ¹H n.m.r. spectral data indicate that the pentavalent complexes (4) and (5) exist in two isomeric forms [(4A), (4B) and (5A), (5B) in 1:1 and 4:1 ratios, respectively]. The vanadyl(IV) complexes display an irreversible oxidation peak near +0.67 V while among the vanadyl(V) complexes, (3), displays an irreversible reduction peak near –0.20 V and (4) and (5) display a quasi-reversible one electron reduction peak near +0.25 V versus s.c.e. The trend in redox potential values and the selective stabilization of VO²⁺ and VO³⁺ motifs have been explained on the basis of the basicity of the bidentate auxiliary ligands.     

Synthesis,Characterization, and Antibacterial Studies of Oxovanadium(IV) Complexes with Thiazole-Derived Schiff Bases

Oxovanadium(IV) complexes of Schiff bases, derived from 2-amino-4-phenyl thiazole/substituted 2-amino-4-phenyl thiazoles and thiophene-2-aldehyde have been synthesized and characterized on the basis of elemental analysis, molar conductance measurements, magnetic susceptibility data, and UV-visible, and IR spectral studies. All the complexes are monomeric possessing a 1:2 (metal:ligand) stoichiometry. On the basis of these data, a square pyramidal geometry has been assigned for the complexes. A few complexes have been subjected to thermal decomposition studies. The ligands and their metal complexes have been screened for their antibacterial activities.

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