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.     

Synthesis, catalytic and biological activity of novel dinuclear copper complex with Schiff base

Noncyclic polyether-amino acid Schiff base con- taining some O and N atoms is a new important bio- logical ligand and it shows some interesting biological properties, such as antibacterial, antiphlogistic, anti- cancer and high catalytic activities[1―3], so the chemi- cal behavior of their transition metal complexes has drawn our attention[4]. We report here the synthesis of the title Cu(II) complex with polyether–phenylalanine Schiff base and its characterization by elemental analysis, I…     

Catalytic efficacy of an oxido-peroxido tungsten(VI) complex: synthesis,X-ray structure and oxidation of sulfides and olefins

An oxido-peroxido tungsten(VI) complex [WO(O₂)L(CH₃OH)] using salicylidene benzoyl hydrazine as a tridentate ONO donor Schiff base (H₂L) has been synthesized and characterized by elemental analysis, IR, ¹H NMR, molar conductance data, and single-crystal X-ray analysis. The complex was used as a catalyst for epoxidation of olefins and oxidation of sulfides. The results show that epoxides and sulfoxides were produced in high yield, turnover number, and selectivity.     

A reusable zirconium(IV) Schiff base complex catalyzes highly efficient synthesis of quinoxalines under mild conditions

A zirconium(IV) complex of a bidentate Schiff base (ZrL₂Cl₂) has been synthesized by the reaction of (z)-N-benzylidene-2-hydroxypropane-1-amine (HL) and ZrCl₄. Spectroscopic data and elemental analyses are consistent with a monomeric complex with a ligand:Zr ratio of 2:1. The catalytic activity of ZrL₂Cl₂ has been investigated for the efficient synthesis of a wide variety of quinoxaline derivatives under mild conditions. The employment of ethanol as an environmentally benign solvent in this high yield method, along with high turnover numbers and reusability of the catalyst providing ready scalability, makes it appropriate for practical applications.     

Synthesis,characterization, crystal structure determination,catalytic activity and thermal study of a new oxidovanadium(IV) Schiff base complex: production of V2O5 nano-particles

A new oxidovanadium(IV) Schiff base complex, VOL₂ (1), containing furfuryl pendant group was synthesized by the reaction of the related bidentate O, N-type Schiff base ligand and VO(acac)₂ in the ratio of 2:1 in methanol in the reflux conditions. The Schiff base ligand and its vanadyl complex were characterized by ¹H-NMR and FT-IR spectra and elemental analysis. The crystal structure of 1 was also determined the single-crystal X-ray analysis. It showed that the metal center located in a distorted tetragonal pyramidal (N₂O₃) geometry in which the two bidentate Schiff base ligands were coordinated to the vanadium(IV) ion in four equatorial positions, and one oxygen atom in its axial position. The catalytic activity of the vanadyl Schiff base complex was elucidated in the epoxidation of cyclooctene as a model substrate. Different reaction parameters were investigated in this reaction and the results showed that it was an effective and selective catalyst in these optimal conditions. Thermogravimetric analysis of 1 showed that it was decomposed in two stages by losing two methoxy groups and other organic residuals, respectively, in the temperature range of 253–532 °C. In addition, the vanadyl Schiff base complex (1) was thermally decomposed in air at 660 °C and the XRD pattern of the obtained solid showed the formation of the V₂O₅ nano-particles with the average size of 52 nm.     

Synthesis,characterization, crystal structure,catalytic activity in oxidative bromination,and thermal study of a new oxidovanadium Schiff base complex containing O,N-bidentate Schiff base ligand

A new oxidovanadium(IV) Schiff base complex, VOL₂ (1), HL = 2-{(E)-[2- (bromoethyl)imino]methyl}-6-methoxy phenol, containing ethyl bromide pendant group was synthesized by direct reaction of HL and VO(acac)₂ in the ratio of 2 : 1 in methanol at reflux. The Schiff base ligand and its vanadyl complex were characterized by FT-IR spectra and CHN analysis. Additionally, the Schiff base ligand has been characterized by ¹H NMR spectroscopy. The crystal structure of 1 was also determined by single-crystal X-ray analysis, showing the distorted square-pyramidal N₂O₃ coordination around vanadium(IV). The catalytic activity of 1 was studied in the oxidative bromination of 2-nitrophenol as a model substrate, and different reaction parameters were investigated. The oxidative bromination of some organic compounds in the presence of 1 in optimal conditions showed that it was an effective and selective catalyst in those optimal conditions. Thermogravimetric analysis of 1 showed that it decomposed in two stages. 1 was thermally decomposed in air at 660 °C, and the XRD pattern of the obtained solid showed the formation of the V₂O₅ nanoparticles with average size of 34 nm .     

Nanosilica‐anchored Pd(II)‐Schiff base complex as efficient heterogeneous catalyst for activation of aryl halides in Suzuki–Miyaura cross‐coupling reaction in water

A nanosilica (derived from rice husk)‐anchored Pd(II)–Schiff base complex has been synthesized and characterized. This immobilized complex has been found to be a very effective and recyclable heterogeneous catalyst for the Suzuki–Miyaura cross‐coupling reaction of various aryl halides with arylboronic acid in aqueous medium under mild conditions. The products were identified using ¹H NMR and mass spectral studies. This complex can be easily filtered out from the reaction medium and reused up to six times without significant loss of catalytic activity. Since the reaction proceeds under mild conditions in aqueous medium as well as the catalyst being recyclable, it provides an environmentally benign alternative route to the existing protocols for the Suzuki–Miyaura reaction.     

Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and palladium(II) Schiff base complexes: Heterogeneous catalysts for oxidation of sulfides with hydrogen peroxide

The condensation reaction of 1,2-bis(2′-aminophenoxy)benzene with 2-pyridinecarbaldehyde in a mole ratio of 1:2 gives a new Schiff base ligand (L). Four Schiff base complexes, CoL(NO₃)₂ (1), NiLCl₂ (2), ZnL(NO₃)₂ (3) and Pd₂LCl₄ (4) have been prepared by direct reaction of the ligand (L) and appropriate metal salts. The Schiff base ligand (L) has been characterized by IR, ¹H NMR and ¹³C NMR spectroscopy and elemental analysis. Also, all complexes have been characterized by IR and XRD spectroscopy techniques and elemental analysis. The synthesized complexes have very poor solubility in all polar and non-polar solvents such as: H₂O, MeOH, EtOH, CH₃CN, DMSO, DMF, CHCl₃, CH₂Cl₂, THF, etc; therefore, they have been used as heterogeneous catalysts. Catalytic performance of the complexes was studied in oxidation of thioanisole using hydrogen peroxide (H₂O₂) as the oxidant. Various factors including the reaction temperature, amount of oxidant and catalyst amount were optimized. The palladium Schiff base complex, Pd₂LCl₄ (4), shows better catalytic activity than other complexes. Therefore, the Pd(II) Schiff base complex has been used as a catalyst for oxidation of different sulfides to their corresponding sulfones in acetonitrile with hydrogen peroxide as the oxidant. The palladium Schiff base complex, Pd₂LCl₄ (4), has shown a very good recyclability, up to five times, without any appreciable decreases in catalytic activity and selectivity.     

Iron and cobalt salicylaldimine complexes as catalysts for epoxide and carbon dioxide coupling: effects of substituents on catalytic activity

The synthesis and characterization of substituted ONNO-donor salen-type Schiff base complexes of general formula [M^III(L)Cl] (L = Schiff base ligand, M = Fe, Co) is reported. The complexes have been applied as catalysts for the coupling of carbon dioxide and styrene oxide in the presence of tetrabutylammonium bromide as a co-catalyst. The reactions were carried out under relatively low-pressure and solvent-free conditions. The effects of the metal center, ligands, and various substituents on the peripheral sites of the ligand on the coupling reaction were investigated. The catalyst systems were found to be selective for the coupling of CO₂ and styrene oxide, resulting in cyclic styrene carbonate. The cobalt(III) complex with no substituents on the ligand showed higher activity (TON = 1297) than the corresponding iron(III) complex (TON = 814); however, the iron(III)-based catalysts bearing electron-withdrawing substituents on the salen ligands (NEt₃, TON = 1732) showed the highest catalytic activity under similar reaction conditions. The activity of one of the cobalt(III) complexes toward the coupling of 1-butene oxide, cyclohexene oxide and propylene oxide with CO₂ was evaluated, revealing a notable activity for the coupling of 1-butene oxide.     

Synthesis and study of binuclear calix[4]arene Schiff base Co(II) complexes as catalyst in the presence of PhIO for the catalytic oxidation of olefin

The binuclear Co(II) complexes of calix[4]arene substituted 2-vanillin (R₁) and 2-hydroxy naphthaldimine (R₂), Schiff bases (Co₂L¹ and Co₂L²) have been synthesized, characterized and employed as models to mimic monooxygenase in the catalytical oxidation of olefins. The kinetic mathematical model (oxygen rebound mechanism) for olefin cleavage catalyzed by the complexes has been proposed. The results show that, compared to the calix[4]arene-free analogous, the mono and multinuclear complexes of calix[4]arene Schiff bases as catalyst exhibit high activity in the olefin catalytic oxidation.     

An Fe3O4 nanoparticle-supported Mn (II)-azo Schiff complex acts as a heterogeneous catalyst in alcoholysis of epoxides

In this paper, an azo-containing Schiff base complex of manganese [Mn²⁺-azo ligand@APTES-SiO₂@Fe₃O₄] immobilized on chemically modified Fe₃O₄ nanoparticles has been used as a magnetically retrievable catalyst for the alcoholysis of different epoxides to their corresponding alkoxy alcohols with methanol, ethanol and n-propanol. The newly magnetic nanoparticles (MNPs) were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and vibrating sample magnetometry (VSM).     

Porous polymer supported palladium catalyst for cross coupling reactions with high activity and recyclability

Porous polymer supported palladium catalyst for cross coupling reactions with high activity has been successfully prepared by coordination of Pd 2+ species with Schiff bases functionalized porous polymer. The catalyst has been systemically investi-gated by a series of characterizations such as TEM, N 2 adsorption, NMR, IR, XPS, etc. TEM and N 2 isotherms show that the sample maintains the nanoporous structure after the modification and coordination. XPS results show that chemical state of palladium species in the catalyst is mainly +2. More importantly, the catalyst shows very high activities and excellent recycla-bility in a series of coupling reactions including Suzuki, Sonogashira, and Heck reactions. Hot filtration and poison of catalysts experiments have also been performed and the results indicate that soluble active species (mainly Pd(0) species) in-situ gener-ated from the catalyst under the reaction conditions are the active intermediates, which would redeposit to the supporter after the reactions.     

α-Pinene-type chiral Schiff bases as tridentate ligands in asymmetric addition reactions

A group of tridentate Schiff bases derived from (+)-α-pinene were synthesized. The steric effects in the transition state, the importance of π-π stacking interactions as well as the electronic effects of aryl aldehydes according to Hammett constant values in the enantioselective addition of Et₂Zn to aldehydes with the use of Schiff bases as chiral ligands are described. Also, a variety of aldehydes were cyanated using a catalyst prepared in situ from titanium tetraisopropoxide and chiral Schiff bases. The influence of a conjugated double-bond in the cyanation substrates on enantioselectivity was observed. The chemical structures of the chiral Schiff base-titanium alkoxide complexes are discussed based on their ¹H and ¹³C NMR spectra. 3D models of the Zn₂-complex catalyst and Ti-complex catalyst containing α-pinane-type Schiff bases based on X-ray diffraction experiments are postulated. The models presented were consistent with the reported chirality of the addition product and observed ee.     

Encapsulation of a binuclear manganese(II) complex with an amino acid-based ligand in zeolite Y and its catalytic epoxidation of cyclohexene

A Schiff base ligand was synthesized by the condensation of salicylaldehyde with l-tyrosine. Interaction of this ligand with Mn(II)-exchanged zeolite Y leads to encapsulation of the ligand within the zeolite and complexation of the metal. The encapsulated complex has been characterized by spectroscopic studies and chemical analyses. This material serves as a catalyst for the oxidation of cyclohexene to cyclohexene epoxide and 2-cyclohexene-1-ol using H₂O₂ as oxidant. The reaction conditions have been optimized for solvent, temperature and amount of oxidant and catalyst. The catalyst shows high activity and selectivity toward production of cyclohexene epoxide in acetonitrile at 60 °C with [H₂O₂]/[C₆H₁₀] = 2.5 molar ratio. Comparison of the encapsulated catalyst with the corresponding homogeneous catalyst showed that the heterogeneous catalyst had higher activity and selectivity than the homogeneous catalyst.     

Oxidation of styrene and cyclohexene with TBHP catalyzed by copper(II) complex encapsulated in zeolite-Y

Reaction of monobasic tridentate Hacpy-oap (Hacpy-oap?=?Schiff base derived from 2-acetylpyridine and o-aminophenol) with Cu^IICl₂ in refluxing methanol results in formation of [Cu^II(acpy-oap)Cl]. DFT calculations have been used to optimize structure of the complex. [Cu^II(acpy-oap)Cl] has also been encapsulated in the nanocavity of zeolite-Y and its encapsulation ensured by various physico-chemical techniques. Neat as well as encapsulated complexes are active catalysts for oxidation of styrene and cyclohexene using tert-butylhydroperoxide. Reaction conditions for oxidation of these substrates have been optimized by concentration of oxidant, amount of catalyst, volume of solvent and temperature of the reaction mixture. [Cu^II(acpy-oap)Cl] does not leach metal ion during catalytic activity and is recyclable.     

High‐Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single‐Atom Catalysts with Tetradentate N2O2 Coordination in a Three‐Phase Flow Cell

Carbon‐supported Ni^II single‐atom catalysts with a tetradentate Ni‐N₂O₂ coordination formed by a Schiff base ligand‐mediated pyrolysis strategy are presented. A Ni^II complex of the Schiff base ligand (R,R)‐(?)‐N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine was adsorbed onto a carbon black support, followed by pyrolysis of the modified carbon material at 300 °C in Ar. The Ni‐N₂O₂/C catalyst showed excellent performance for the electrocatalytic reduction of O₂ to H₂O₂ through a two‐electron transfer process in alkaline conditions, with a H₂O₂ selectivity of 96 %. At a current density of 70 mA cm^?2, a H₂O₂ production rate of 5.9 mol g_cat.^?1 h^?1 was achieved using a three‐phase flow cell, with good catalyst stability maintained over 8 h of testing. The Ni‐N₂O₂/C catalyst could electrocatalytically reduce O₂ in air to H₂O₂ at a high current density, still affording a high H₂O₂ selectivity (>90 %). A precise Ni‐N₂O₂ coordination was key to the performance.     

High-Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single-Atom Catalysts with Tetradentate N2O2 Coordination in a Three-Phase Flow Cell

Carbon-supported Ni^II single-atom catalysts with a tetradentate Ni-N₂O₂ coordination formed by a Schiff base ligand-mediated pyrolysis strategy are presented. A Ni^II complex of the Schiff base ligand (R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine was adsorbed onto a carbon black support, followed by pyrolysis of the modified carbon material at 300 °C in Ar. The Ni-N₂O₂/C catalyst showed excellent performance for the electrocatalytic reduction of O₂ to H₂O₂ through a two-electron transfer process in alkaline conditions, with a H₂O₂ selectivity of 96 %. At a current density of 70 mA cm⁻², a H₂O₂ production rate of 5.9 mol g_cat.⁻¹ h⁻¹ was achieved using a three-phase flow cell, with good catalyst stability maintained over 8 h of testing. The Ni-N₂O₂/C catalyst could electrocatalytically reduce O₂ in air to H₂O₂ at a high current density, still affording a high H₂O₂ selectivity (>90 %). A precise Ni-N₂O₂ coordination was key to the performance.     

Solvent-free chemoselective oxidation of alcohols by hydrogen peroxide using a new synthesized copper complex as reusable heterogeneous nanocatalyst

Single crystal of a new copper(II) Schiff base complex [Cu(HL)(H₂O)NO₃] (1) (H₂L = {2-[(2-hydroxy-1,1-dimethyl-ethylimino)methyl]phenol}) and its nanostructure have been synthesized by slow evaporation of a methanol solution and sonochemical process, respectively. The structure of complex 1 was determined by FTIR, UV–Visible spectra and single-crystal X-ray diffraction. Nanostructure of the complex was characterized by X-ray powder diffraction (XRD), FTIR spectroscopy and scanning electron microscopy (SEM). The synthesized complex was successfully applied as a new chemoselective and recyclable heterogeneous catalyst for the selective oxidation of alcohols to the corresponding carbonyl compounds in the presence of H₂O₂ as a green oxidant under solvent-free conditions. A suitable mechanism for the reaction is proposed.     

The Template Synthesis of the New Macrocyclic and Acyclic Metal Ion Complexes Derived from Putrescine

The template reaction of 2,6-diacetylpyridine with biogenic diamine–putrescine in the presence of cadmium(II), mercury(II) or lead(II) ions produces the complexes of 22-membered macrocyclic ligand L¹ with an N₆ set of donor atoms as a result of [2 + 2] Schiff base cyclocondensation. The lead(II) complex containing Schiff base acyclic ligand L² terminated by one carbonyl and one amine group as product of the partial [2 + 2] condensation has been also isolated and might be regarded as possible intermediate in the formation of the macrocyclic L¹ complex. Analogous reaction involving the uranyl nitrate generates the complex containing the same Schiff base acyclic ligand L² as a final product of template reaction. The complexes were characterized by spectral data (IR, ¹H NMR, FAB-MS), thermogravimetric and elemental analyses. A notable feature of the FAB mass spectrum of the uranyl complex is the appearance of the clusters of the form [(UO₂) _n O]⁺(n= 1–7) along with the peak corresponding to molecular ion.     

Heterogeneous Green Catalyst for Oxidation of Cyclohexene and Cyclooctene with Hydrogen Peroxide in the Presence of Host (Nanocavity of Y‐zeolite)/Guest (N4‐Cu(II) Schiff Base Complex) Nanocomposite Material

Copper(II) complex of a Schiff base ligand derived from pyrrolcarbaldehyde and o‐phenylenediamine (H₂L) has been synthesized and encapsulated in Y‐zeolite matrix. The hybrid material has been characterized by elemental analysis, IR and UV‐Vis spectroscopic studies as well as X‐ray diffraction (XRD) pattern. The encapsulated copper(II) catalyst is an active catalyst for the oxidation of cyclooctene and cyclohexene using H₂O₂ as oxidant. Under the optimized reaction conditions 81% conversion of cyclohexene with 65% selectivity for 2‐cyclohexenone formation and 87% conversion of cyclooctene with 46% selectivity for epoxide formation were obtained.