Vanadium Schiff Base Complex Research Articles

μ2-Oxido bridged dinuclear vanadium(V) complex: Synthesis and characterization

μ2-Oxido bridged dinuclear vanadium(V) Schiff base complex with the general formula [VO2(L)]2·2CH3OH was synthesized and characterized. The X-ray results show the complex is dinuclear with two μ-O bridging ions, and the vanadium centers have an octahedral N2O4 coordination sphere. The tridentate Schiff base ligand L coordinated to vanadium(V) center as a monoanion in the basal plane. The basal plane is completed by one of the μ-O bridges. The other μ-O bridge coordinated to vanadium(V) center as apical direction. The two oxido bridged connected the two Vv centers together with the Vv-O-Vv angle of 101.21(6)°, the VO distances of 1.70(6) and 2.31(7) Å and the V⋯V separation of 3.113(5) Å. Finally, the complex was calcinated at 600 °C for 3 h. The FT-IR spectrum of the product shows the formation of the V2O5 particles.

Camphor‐Based Schiff Base Of 3‐Endo‐Aminoborneol (SBAB): Novel Ligand for Vanadium‐Catalyzed Asymmetric Sulfoxidation and Subsequent Kinetic Resolution

AbstractNovel in situ formed vanadium Schiff base complexes, namely vanadyl Schiff base of 3‐exo‐aminoisoborneol (V(O)‐SBAIB) and vanadyl Schiff base of 3‐endo‐aminoborneol (V(O)‐SBAB), were demonstrated to catalyze an enantioselective sulfoxidation reaction. Among the twenty substitutionally and electronically different (1R)‐camphor based Schiff base ligands, 6‐bromo‐2‐hydroxy‐1‐naphthaldehyde Schiff base of 3‐endo‐aminoborneol, 48 (SBAB‐14), produced (S)‐ethyl 2‐naphthyl sulfoxide in excellent enantioselectivity (>99 %) with good yield (72 %). In an optimized reaction condition, 1.5 mol% 48 (SBAB‐14) catalyzed asymmetric sulfoxidation with concomitant kinetic resolution of various alkyl aryl sulfides and rendered corresponding sulfoxides with high ee (up to 97 %) with moderate yield (up to 74 %).

Synthesis, characterization, crystal structure determination and catalytic activity in epoxidation reaction of two new oxidovanadium(IV) Schiff base complexes

The five coordinated vanadium(IV) Schiff base complexes of VOL1 (1) and VOL2 (2), HL1 = 2-{(E)-[2-bromoethyl)imino]methyl}-2- naphthol, HL2 = 2-{(E)-[2-chloroethyl)imino]methyl}-2- naphthol, have been synthesized and they were characterized by using single-crystal X-ray crystallography, elemental analysis (CHN) and FT-IR spectroscopy. Crystal structure determination of these complexes shows that the Schiff base ligands (L1 and L2) act as bidentate ligands with two phenolato oxygen atoms and two imine nitrogen atoms in the trans geometry. The coordination geometry around the vanadium(IV) is distorted square pyramidal in which vanadium(IV) is coordinated by two nitrogen and two oxygen atoms of two independent ligands in the basal plane and by one oxygen atom in the apical position. The catalytic activity of the Schiff base complexes of 1 and 2 in the epoxidation of alkenes were investigated using different reaction parameters such as solvent effect, oxidant, alkene/oxidant ratio and the catalyst amount. The results showed that in the presence of TBHP as oxidant in 1: 4 and 1:3 ratio of the cyclooctene/oxidant ratio, high epoxide yield was obtained for 1 (76%) and 2 (80%) with TON(= mole of substrate/mole of catalyst) of 27 and 28.5, respectively, in epoxidation of cyclooctene.

Vanadium–Schiff base complex-functionalized SBA-15 as a heterogeneous catalyst: synthesis, characterization and application in pharmaceutical sulfoxidation of sulfids

VO2(picolinichydrazone) complex as a catalyst was stabilized on a SBA-15 mesoporous silica as a catalytic support by using (3-chloropropyl)triethoxysilane as a connector. SBA-15 is nanoporous and has a high ratio of surface area to volume. The immobilization of a metal–Schiff base complex to the surface area of SBA-15 can improve its catalytic effects by increasing the catalytic surface area. Unlike homogeneous catalysts, heterogeneous catalysts can be recovered and reused several times without any significant loss of catalytic activity. A vanadium–Schiff base complex-functionalized SBA-15 was synthesized by covalency connected by a pre-synthesised VO2(picolinichydrazone) complex to silanated SBA-15. The synthesized vanadium–Schiff base complex was characterized by proton nuclear magnetic resonance (1H NMR) spectroscopy, carbon nuclear magnetic resonance (13C NMR) spectroscopy and Fourier transform infrared spectroscopy (FT-IR), and the final V/SBA-15 was characterized by FT-IR, ultraviolet–visible spectrophotometry and X-ray powder diffraction. The morphology of V/SBA-15 was also obtained by scanning electron microscopy and transmission electron microscopy. The catalytic effect was examined by using V/SBA-15 as a heterogeneous catalyst in sulfoxidation reactions. The synthesis of modafinil and modafinic acid by pharmaceutical sulfoxidation of solfides was carried out and the effects of different solvents, reaction times and also recoverability and reusability of the heterogeneous catalyst were investigated. This catalyst showed high yield of sulfide conversion, stability and recyclability in the sulfoxidation of sulfides .

Supported vanadium Schiff bases complex on nano silica: a heterogeneous catalyst for the selective oxidation of sulfides and alcohols

A vanadium Schiff base complex on nano silica was prepared by reaction of nano silica NH2-functionalized, 2,4-dihydroxy benzaldehyde and VO(acac)2. The VSBC@NS was used as an efficient and reusable heterogeneous catalyst for the selective oxidation of sulfides to sulfoxides and alcohols to aldehydes with H2O2 as an oxidant in methanol.

Synthesis, crystal structure and catecholase activity of a vanadium(V) Schiff base complex

An X-ray structurally characterized vanadium(V) complex, [VO2(L–H)] [L–H=H2L derived ligand (H2L=N,N′-(salicyaldimine)-1,3-diaminopropan-2-ol], was found to behave as an effective catalyst towards the oxidation of 3,5-di-tert-butylcatechol in methanol to its corresponding quinone derivative in aerial oxygen. The reaction follows Michaelis–Menten enzymatic reaction kinetics with a turnover number (Kcat) of 2.063×103h−1.

Electronic, Conjugation, and Confinement Effects on Structure, Redox, and Catalytic Behavior of Oxido-Vanadium(IV) and -(V) Chiral Schiff Base Complexes

Change in oxophilicity, redox, and catalytic behavior with the modification of ligand system was observed for four oxido-vanadium Schiff base complexes. Mononuclear V(IV) complexes were obtained with methyl-substituted Schiff base ligand and one having higher conjugation. Electronic and conjugation effects reduce the oxophilicity of vanadium and led to mononuclear (LVIV═O, L = Schiff base ligand) species. However, μ-oxido bridged dinuclear [O═VVOL]2 complexes were formed with Schiff base ligands without such electronic effect under identical condition. Such structural variation also changed the electronic and redox properties of the metal complexes and brought about substantial differences in the catalytic activities of the vanadium Schiff base complexes. Confinement effect imparted by the zeolite-Y framework on the metal complexes synthesized inside the cavity by the “ship in a bottle synthesis” method was also found to alter the catalytic behavior of the metal complexes. All of the mononuclear and μ-oxi...

Vanadium-Schiff base complex covalently bonded on modified MCM-41 as catalyst for asymmetric oxidation of methyl phenyl sulfide

The vanadium-Schiff base complex was immobilized on mesoporous MCM-41 by a covalent grafting method using 3-aminopropyltriethoxysilane as a reactive surface modifier. The formation and integrity of the complex have been studied using XRD, FT-IR, UV–Vis, ESR, 29Si-MAS-NMR and N2 adsorption–desorption. The heterogeneous VO(Schiff)-MCM catalyst shows an activity and an enantioselectivity comparable to those obtained with the homogeneous catalyst VO(Schiff), for the asymmetric oxidation of methyl phenyl sulfide. The combination of the heterogenous catalyst with CHP as oxidant offers a selective catalytic system for oxidation of methyl phenyl sulfide with a significant enantioselectivity in the range of 17–23 % ee. In addition, the heterogeneized catalyst could be easily separated from the products and reused.

Stereospecificity in vanadium Schiff base complexes: Formation, crystallization and epimerization processes

The structures of two stereoisomers of the chiral anion [VO2(N-salicylidene-isoleucinato)]− possessing three centers of chirality, the vanadium atom (configuration A/C) and the isoleucine moiety (configuration R/S on alpha and beta carbons), are presented. The absolute configuration of all available stereosiomers, CSS, ARR, CSR and ARS, was determined by electronic circular dichroism (ECD), which allows distinguishing between diastereomers, and by vibrational circular dichroism (VCD) capable of differentiating between all four stereoisomers. The comparison of experimental VCD and infrared (IR) spectra with simulated spectra for band assignment revealed the IR spectra of the diastereomers differing significantly in the CH stretching region of the aromatic part in the molecule. Crystallization from binary systems composed of equal ratio of two stereoisomers of isoleucine, unveiled the lower solubility of CSS and ARR stereoisomers, while a longer crystallization time of the CSR and ARS stereoisomers allowed proceeding the vanadium-catalyzed epimerization, leading to the subsequent presence of the CSS and ARR stereoisomers in the product obtained.

Immobilization of an oxovanadium(IV) tetradentate Schiff base complex on clay as a recyclable heterogeneous catalyst for the epoxidation of olefins

The vanadyl Schiff base complex of N,N′-bis(5-[(triphenylphosphoniumchloride)-methyl-salicylidine) meso-stilben-diamine)] was synthesized and immobilized on sodium montmorillonite. The supported complex was characterized by X-ray diffraction, IR spectroscopy, diffuse reflectance spectroscopy and atomic absorption spectroscopy techniques. The infrared and diffuse reflectance spectra of the immobilized complex show that the vanadium(IV) Schiff base complex was physically entrapped within sodium montmorillonite. The new heterogeneous catalyst shows excellent catalytic activity in the epoxidation of olefins using tert-butylhydroperoxide in acetonitrile. In this report, the supported vanadyl complex exhibited 100 % selectivity for epoxidation with 98 % conversion for cyclohexene. The catalytic activity and selectivity of heterogeneous catalyst have not changed after four times of reusing.

Synthesis, characterization and crystal structure determination of a new vanadium(IV) Schiff base complex (VOL2) and investigation of its catalytic activity in the epoxidation of cyclooctene

The vanadium(IV) Schiff base complex VOL2, L=2-{(E)-[2-chloroethyl)imino]methyl}-6-methoxy phenol, has been synthesized by the reaction of a methanolic solution of VO(acac)2 with a methanolic solution of the Schiff base ligand, and it has been characterized using single-crystal X-ray crystallography, elemental analysis (CHN) and FT-IR spectroscopy. The crystal structure determination of this complex shows a deformed tetragonal pyramidal N2O3 coordination sphere of the vanadium center. The Schiff base ligand acts as a bidentate ligand with the two phenolato oxygen atoms and the two imine nitrogen atoms in trans positions. Non-classical inter- and intra-molecular hydrogen bonds of the type CH⋯O have been found in the structure, the latter connecting the monomeric VOL2 units. The catalytic activity of the VOL2 Schiff base complex in the epoxidation of cyclooctene was investigated using different reaction parameters, such as solvent effect, oxidant, alkene/oxidant ratio and the catalyst amount. The results showed that with a catalytic amount of the VOL2 Schiff base complex and a 1:3 ratio of the cyclooctene/TBHP, the cyclooctene was effectively and selectively converted into the corresponding epoxide with CHCl3 as the solvent.

Oxido-bridged dinuclear vanadium(V) Schiff base complexes derived from 1R,2S(−)-norephedrine: Synthesis, structure, characterization and catalytic studies

A series of vanadium(V) complexes with chiral tridentate Schiff base ligands, obtained by a single condensation of 1R,2S(−)-norephedrine with salicylaldehyde and its derivatives, were prepared. The complexes were characterized by elemental analysis and by their IR, CD, UV–Vis, 1D (1H, 51V) and 2D (COSY, gHSQC) NMR spectra. The X-ray analysis of μ-oxido-bis({1R,2S(−)-2-[(1-oxido-1-phenylpropyl)iminomethyl]-3,5-dimethoxyphenolato-κ3N,O,O′}oxidovanadium(V)), V2O3(C18H19NO4)2, 4, revealed oxido-bridged dinuclear structure with, unexpectedly, two five-coordinate square-pyramidal geometries about metal atoms. The two O-bonded oxidovanadium units are in the twist-angular configuration with the VV–O–VV angle of 108.75(8)° and V–O distances of 1.802(2) and 1.818(2)Å. The vanadium(V) complexes have ability to catalyze the oxidation of sulfides [PhSR (R=Me, Bz)] in good yields and enantiomeric excesses, utilizing aqueous 30% H2O2 or cumene hydroperoxide (CHP) as oxidant.

Heterotrinuclear manganese(II) and vanadium(IV) Schiff base complexes as epoxidation catalysts

The catalytic epoxidation of styrene using urea-hydrogen peroxide and heterotrinuclear Cu(II) complexes with general formula (ML n )2Cu(acac)2, where n = 1–3 and M = VO2+ or Mn2+ is reported. Schiff base complexes ML n involving a 3,4-diaminopyridine bridge with free coordination site were used as the ligand, where (Ln)2− is [(5-x-Sal)2Py]2 and x = H, Br or NO2. The complexes were characterized by physico-chemical and spectroscopic methods. The electrochemical properties of M were modified upon trinuclear complex formation. The trinuclear complexes show high catalytic activity, with up to 86% conversion and 93% selectivity, while no catalytic properties were observed for the monomeric complexes. The catalyst could be reused with some loss of activity.

A new vanadium Schiff base complex as catalyst for oxidation of alcohols

The monoanionic bidentate Schiff base, N-(phenolyl)-benzaldimine (HL), has been employed to synthesize a new vanadium(IV) complex of general composition [VO(L)2] (where L = O, N donor of Schiff base). The ligand and complex have been fully characterized by elemental analyses, molar conductance data, FT-IR, 1H- and 13C-NMR, and UV-Vis spectroscopies. Oxidation of alcohols to their corresponding aldehydes and ketones was conducted by this complex catalyst using Oxone as oxidant under biphasic reaction conditions (CH2Cl2/H2O) and tetra-n-butylammonium bromide as phase transfer agent under air at room temperature.

An improved high yielding immobilization of vanadium Schiff base complexes on mesoporous silica via azide–alkyne cycloaddition for the oxidation of sulfides

Azide–alkyne [3+2] cycloaddition “click reaction” was found to be a simple yet improved approach for the efficient immobilization of oxo–vanadium(IV) tridentate Schiff base complexes to mesoporous silica via covalent attachment as it occurred under mild reaction conditions and provided high catalyst loading compared to the direct immobilization of oxo–vanadium(IV) tridentate Schiff base complex to 3-chloropropylsilyl functionalized silica support.

Vanadium‐Catalyzed Asymmetric Oxidation of Sulfides Using Schiff Base Ligands Derived from β‐Amino Alcohols with Two Stereogenic Centers

AbstractNovel Schiff base ligands derived from β‐amino alcohols with two stereogenic centers were prepared and used in the preparation of optically pure sulfoxides by using aqueous hydrogen peroxide as the oxidant. A variety of sulfides were smoothly converted into the corresponding sulfoxides catalyzed by the chiral vanadium–Schiff base complex. Good yields (>80 %) with excellent enantioselectivities (>99 % ee) were obtained in most cases. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Electrocatalytic oxidation and determination of nitrite on carbon paste electrode modified with oxovanadium(IV)-4-methyl salophen

Electrocatalytic oxidation and detection of nitrite at a carbon paste electrode chemically modified with N, N′-bis(salicylaldehyde)-4-methyl-1,2-phenylenediimino oxovanadium(IV) as a Schiff base complex ([VO(SB)]), was thoroughly investigated. Anodic oxidation of nitrite (in pH 4) at the [VO(SB)]-modified carbon paste electrode occurred at low overpotential (0.87 V versus Ag/AgCl), and treatment of the voltammetric data showed that it was a purely diffusion-controlled reaction involving one-electron in the rate-determining step. The mechanism for the interaction of nitrite with the vanadium(IV) Schiff base complex is proposed to involve the V IV(SB) 2+/V III(SB) + redox process. The modified electrode exhibits good catalytic activity for the oxidation of nitrite with good sensitivity over the wide concentration range of 3.90 × 10 −6–4.05 × 10 −3 M nitrite, and a detection limit of 6.13 × 10 −7 M ( S/ N = 3). The heterogeneous rate constant, k′, and the standard heterogeneous rate constant, k 0, for the oxidation of nitrite at the surface of the modified electrode were determined by rotating disk electrode voltammetry using the Koutecky–Levich plot. The transfer coefficient ( α) for electrocatalytic oxidation of nitrite and the diffusion coefficient of this substance under the experimental conditions were also investigated in this study.

Studies on some oxovanadium(IV) complexes of acyl pyrazolone analogues

A series of novel bidentate pyrazolone based Schiff base ligands were synthesized by interaction of 4-benzoyl-3-methyl-1-(4′-methylphenyl)-2-pyrazolin-5-one with various aromatic amines like aniline, o-, m-, p-chloroaniline and o-, m-, p-toluidine in a ethanolic medium. All of these ligands have been characterized on the basis of elemental analysis, IR and 1H NMR data. The molecular geometries of five of these ligands have been determined by single crystal X-ray study. Crystallographic study reveals that these ligands exist in the amine-one tautomeric form in the solid state. NMR study also suggests the existence of the amine-one form in solution at room temperature. Ab initio calculations for representative ligand HL 1 has been carried out to know the coordination site of the ligand. Novel vanadium Schiff base complexes of these ligands with general formula [OV(L 1–7) 2(H 2O)] have been prepared by interaction of aqueous solution of vanadyl sulfate pentahydrate with DMF solution of the appropriate ligands. The resulting complexes have been characterized on the basis of elemental analysis, vanadium determination, molar conductance and magnetic measurements, thermo gravimetric analysis, infrared and electronic spectral studies. Suitable distorted octahedral structures have been proposed for these complexes.

Electroless deposition of vanadium–Schiff base complex onto carbon nanotubes modified glassy carbon electrode: Application to the low potential detection of iodate, periodate, bromate and nitrite

The multiwall carbon nanotubes (MWCNTs) were used as an immobilization matrix to incorporate vanadium-Schiff base complex as electron transfer mediator onto a glassy carbon electrode surface. First, the preheated glassy carbon was subjected to abrasive immobilization of MWCNTs by gently rubbing the electrode surface on a filter paper supporting the carbon nanotubes. Second, the electrode surface was modified with casting 100 μL of V-complex solution (0.01 M in ACN). The cyclic voltammograms of the modified electrode in aqueous solution is shown a pair of well defined, stable and nearly reversible redox system of (V(IV)/V(V)) couple with surface confined characteristics. Combination of unique electronic and electrocatalytic properties of MWCNTs and vanadium–Schiff base complex, results in a remarkable synergistic augmentation on the response. The electrochemical behavior and stability of the modified electrode in aqueous solution at pH range 1–7 was characterized by cyclic voltammetry. The apparent electron transfer rate constant ( K s) and transfer coefficient ( α) were determined by cyclic voltammetry and they were about 7 s −1 and 0.55, respectively. The modified electrodes showed excellent catalytic activity toward reduction of BrO 3 - , IO 4 - , IO 3 - and NO 2 - at unusually positive potential in acidic solution. The electrochemical reversibility, stability and catalytic activity of modified electrode prepared with incorporation of V-complex into CNTs film was compared with the film of adsorbed complex on the surface of preanodized GC electrodes. The modified electrode shows inherent stability at wide pH range, fast response time, high sensitivity, low detection limit and especially remarkable positive potential reduction of oxoanions that decreasing the effect of interferences in analysis. This modified electrode can be used as an amperometric detector for monitoring oxoanions in chromatography or flow injection systems.

Vanadium‐Catalyzed Enantioselective Sulfoxidation and Concomitant, Highly Efficient Kinetic Resolution Provide High Enantioselectivity and Acceptable Yields of Sulfoxides

AbstractSimple, inexpensive, preformed vanadium‐Schiff base complexes were facilely prepared and used in enantioselective sulfoxidation. Both the amount of aqueous H2O2 and reaction time greatly influenced the ee values and yields of chiral sulfoxides. High enantioselectivities (up to 99% ee) and reasonable yields (>40%) for various chiral sulfoxides were achieved by combining enantioselective sulfoxidation and appropriate concomitant kinetic resolution.