A route to magnetically separable nanocatalysts: Combined experimental and theoretical investigation of alkyl substituent role in ligand backbone towards epoxidation ability

We have prepared two chiral Schiff base ligands, H₂L¹ and H₂L², and one achiral Schiff base ligand, H₂L³, by treating 2,6‐diformyl‐4‐methylphenol separately with (R )‐1,2‐diaminopropane, (R )‐1,2‐diaminocyclohexane and 1,1′‐dimethylethylenediamine, in ethanolic medium, respectively. The complexes MnL¹ClO₄ ( 1 ), MnL²ClO₄ ( 2 ), MnL³ClO₄ ( 3 ), FeL¹ClO₄ ( 4 ), FeL²ClO₄ ( 5 ) and FeL³ClO₄ ( 6 ) have been obtained by reacting the ligands H₂L¹, H₂L² and H₂L³ with manganese(III) perchlorate or iron(III) perchlorate in methanol. Circular dichroism studies suggest that ligands H₂L¹ and H₂L² and their corresponding complexes have asymmetric character. Complexes 1 – 6 have been used as homogeneous catalysts for epoxidation of alkenes. Manganese systems have been found to be much better than iron counterparts for alkene epoxidation, with 3 as the best catalyst among manganese systems and 6 as the best among iron systems. The order of their experimental catalytic efficiency has also been rationalized by theoretical calculations. We have observed higher enantiomeric excess product with catalysts 1 and 4 , so they were attached to surface‐modified magnetic nanoparticles to obtain two new magnetically separable nanocatalysts, Fe₃O₄@dopa@MnL¹ and Fe₃O₄@dopa@FeL⁴. They have been characterized and their alkene epoxidation ability has been investigated. These catalysts can be easily recovered by magnetic separation and recycled several times without significant loss of catalytic activity. Hence our study focuses on the synthesis of a magnetically recoverable asymmetric nanocatalyst that finds applications in epoxidation of alkenes and at the same time can be recycled and reused.     

Synthesis,spectral and EPR studies of oxovanadium(IV) complexes incorporating tridentate ONO donor hydrazone ligands: Structural study of one oxovanadium(V) complex

Five oxovanadium(IV) complexes of 2-hydroxy-4-methoxybenzaldehyde nicotinic acid hydrazone (H₂L¹), 2-hydroxy-4-methoxyacetophenone nicotinic acid hydrazone (H₂L²) and a binuclear oxovanadium(V) complex of H₂L² have been synthesized. These complexes were characterized by different physicochemical techniques like electronic, infrared and EPR spectral studies. The complexes [VOL¹]₂ · H₂O (1) and [VOL²]₂ · H₂O (4) are binuclear and [VOL¹bipy] (2), [VOL¹phen] · 1.5H₂O (3) and [VOL²phen] · 2H₂O (6) are heterocyclic base adducts and are EPR active. In frozen DMF at 77 K, all the oxovanadium(IV) complexes show axial anisotropy with two sets of eight line patterns. The complex [VOL² · OCH₃]₂ (5) is an unusual product and has distorted octahedral geometry, as obtained by X-ray diffraction studies.     

Synthesis,crystal structures and antibacterial studies of oxidovanadium(IV) complexes of salen-type Schiff base ligands derived from meso-1,2-diphenyl-1,2-ethylenediamine

A series of new derivatives and previously reported Schiff base ligands and their oxidovanadium(IV) complexes were synthesized, characterized and tested as potential antibacterial agents against four human pathogenic bacteria. These N₂O₂ type Schiff base ligands were derived from the condensation of meso-1,2-diphenyl-1,2-ethylenediamine with different salicylaldehyde derivatives, and their metal complexes were obtained from the reaction of these ligands with bis(acetylacetonato)oxidovanadium(IV). Our studies showed that the metal complexes had moderate antibacterial activity, and this activity was higher than that of the free ligands against both Gram-positive and Gram-negative bacteria. Besides, it was found that the presence of more substituents on the ligands increases the antibacterial activities of both the free ligands and their complexes. The crystal structures of H₂L⁴ and its corresponding complex VOL⁴ were determined by X-ray crystallography.     

Synthesis and Thermal Studies of 2,4'-bipyridyl Complexes of Manganese(II) Salts

2,4'-Bipyridyl (2,4'-bipy or L) complexes of Mn(II) with the formulae MnL₂X₂·2H₂O (X_–=Cl, Br, NCS, NO₃), MnLSO₄·5H₂O and MnL₄(ClO₄)₂·2H₂O were synthesized and characterized via the IR spectra and magnetic, and conductivity measurements. The nature of the Mn(II)-ligand coordination is discussed. The thermal decompositions of these compounds were studied in air atmosphere. The mode of decomposition depends on the anion present, but the final product in all cases is Mn₃O₄. Some of the intermediates (MnL₂Cl₂, MnLCl₂, MnL₂Br₂, MnL₂(NCS)₂ and MnLSO₄) formed during the pyrolysis are isomeric with 2,2'-bipy and 4,4'-bipy complexes.This revised version was published online in November 2005 with corrections to the Cover Date.     

Studies on the Phenol Oxidation with H2O2 Catalyzed by Metallomicelle Made from Crowned Schiff Base Mn(III) Complexes

Metallomicelles made from two Schiff base manganese(III) complexes (MnL¹ and MnL²) and surfactants (CTAB and Brij35) were used as mimetic peroxidase in the catalytic oxidation of phenol by H₂O₂. The catalytic activity of the complexes (MnL¹ and MnL²) were investigated. The mechanism and a kinetic mathematic model of the phenol catalytic oxidation were also studied. The results show the optimum acidity of the enzyme-like system in the paper is ca. pH 7.0, the optimum temperature which is ca. 35°C and the optimum molar ratio of H₂O₂ to the complex is ca. 30 in the complexes-H₂O₂-buffered solution; the Schiff base manganese(III) complexes and their metallomicelles as peroxidase mimics exhibit good catalytic activity and similar catalytic character to natural enzyme.     

Synthesis,physico-chemical and biological studies on oxovanadium(IV) derivatives of mercaptotriazoles

Summary Complexes of 3-methyl- or 3-ethyl-4-amino-5-thiolato-1,2,4-triazole (LH) and its Schiff base derivatives with oxovanadium(IV), of the types [VOL₂(H₂O)], [VOL₂-(H₂O)] and [VOL(H₂O)₂] (where LH = Schiff bases derived from condensation of LH with benzaldehyde, 2-chlorobenzaldehyde, 4-methoxybenzaldehyde or acetophenone; LH₂ = Schiff bases derived from condensation of LH with salicylaldehyde or o-hydroxyacetophenone) have been prepared and characterized by elemental analyses, electrical conductance, magnetic moments and spectral data. The thermal behaviour of selected complexes was investigated by t.g., d.t.g. and d.s.c. techniques. The antifungal and antiviral activities of the Schiff bases and their corresponding complexes were also investigated.Author to whom all correspondence should be directed.     

Fixation of Sulphur Dioxide by Manganese(II)‐Schiff Base Complexes: Thermal Stability of these Adducts and the Possible Conversion of the Coordinated SO2 to Sulphate

A series of manganese(II) complexes of general formula MnLⁿ(H₂O)_m (where H₂Lⁿ are substituted N,N′‐bis(salicylidene)‐1,2‐diimino‐2,2‐dimethylethane) have been prepared by electrochemical synthesis and characterized by analytical and spectroscopic techniques, magnetism and by studying their redox reversibility character by cyclic and normal pulse voltammetry. The reactivity of these complexes with sulphur dioxide has been investigated in the solid state and in toluene slurries at room temperature. The studies of the reversibility of the reaction (desorption studies) by thermogravimetrical analysis (TGD) have shown a different behaviour among the SO₂‐adducts (from irreversible to totally reversible fixing), pointing to different SO₂ binding modes. Thus, adducts 10 , 12 and 14 , kept the SO₂ after TGD, signifying S–bridged SO₂ binding mode, while TGD for 8 , 9 and 13 revealed the lability of their SO₂, attributable to ligand bound SO₂ coordination. The manganese(II) precursor 4 is that one which has the ability of reversily fixing a major quantity of SO₂ and undergoes the sulphato reaction to form 11 also.     

Self-assembled biomimetic catalysts: studies of the catalase and peroxidase activities of Mn(III)-Schiff base complexes

Five Mn(III) nitrate complexes have been synthesized from dianionic hexadentate Schiff bases obtained by the condensation of 3-ethoxy-2-hydroxybenzaldehyde with different diamines. The complexes have been characterized by elemental analysis, ESI mass spectrometry, IR and ¹H NMR spectroscopy, r. t. magnetic, and molar conductivity measurements. Parallel-mode EPR spectroscopy of 1 is also reported. Ligand H₂L³ and complexes [MnL¹(H₂O)₂](NO₃)(CH₃OH) (1), [MnL³(H₂O)₂]₂(NO₃)₂(CH₃OH)(H₂O) (3), and [MnL⁴(H₂O)₂](NO₃)(H₂O)₂ (4) were crystallographically characterized. The X-ray structures show the self-assembly of the Mn(III)–Schiff base complexes through µ-aquo bridges between neighboring axial water molecules and also by π–π stacking interactions, establishing dimeric and polymeric structures. The peroxidase and catalase activities of the complexes have been studied. Complexes with the shorter spacer between the imine groups (1–2) behave as better peroxidase and catalase mimics, probably due to their ability to coordinate the hydrogen peroxide substrate to manganese.     

Synthesis,crystal structures,and biological activity of oxovanadium(V) complexes with similar tridentate hydrazone ligands

Two new oxovanadium(V) complexes, [VOL¹(OEt)(EtOH)] (1) and [VOL²(OMe)(MeOH)] (2), were prepared by reaction of [VO(acac)₂] (where acac?=?acetylacetonate) with N′-(3-bromo-2-hydroxybenzylidene)-4-methylbenzohydrazide (H₂L¹) in ethanol and N′-(3-bromo-2-hydroxybenzylidene)-4-methoxybenzohydrazide (H₂L²) in methanol, respectively. Crystal and molecular structures of the complexes were determined by elemental analysis, infrared spectra, and single-crystal X-ray diffraction. The V ions have octahedral coordination. Thermal stability and the inhibition of urease of the complexes were studied.     

Synthesis and characterization of copper(II), cobalt(II), nickel(II), and iron(III) complexes with two diamine Schiff bases and catalytic reactivity of a chiral diamine cobalt(II) complex

New transition metal complexes of Co(II), Cu(II), Ni(II), and Fe(III) of the ligands 6,6′-(1E,1′E)-(4,5-dimethyl-1,2-phenylene)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(7-hydroxy-5-methoxy-2-methyl-4H-chromen-4-one) H₂L¹ and 6,6’-(1E,1′E)-cyclohexane-1,2-diylbis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(7-hydroxy-5-methoxy-2-methyl-4H-chromen-4-one) H₂L² have been prepared and characterized using physio-chemical and spectroscopic methods. The results obtained for the complexes indicated that the geometries of the metal centres are either square planar or octahedral. Cyclopropanation reactions of unactivated olefins by ethyldiazoacetate (EDA) in the presence of [L¹Cu]·H₂O, [L²Cu]·2H₂O and [L²*Co]·2H₂O as catalysts were examined. The results showed that only [L²*Co]·2H₂O can act as a catalyst for the cyclopropanation reaction of unactivated olefins with very high selectivity (up to 99% based on EDA).     

Oxidation of a dinuclear manganese(II) complex to an oxide‐bridged dimanganese(IV) complex

Bis{μ‐2‐[bis(pyridin‐2‐ylmethyl)amino]acetato}bis[diaquamanganese(II)] bis(trifluoromethanesulfonate) monohydrate, [Mn₂(C₁₄H₁₄N₃O₂)₂(H₂O)₄](CF₃O₃S)₂·H₂O, (I), and bis{μ‐3‐[bis(pyridin‐2‐ylmethyl)amino]propionato}bis[aquamanganese(II)] bis(trifluoromethanesulfonate) dihydrate, [Mn₂(C₁₅H₁₆N₃O₂)₂(H₂O)₂](CF₃O₃S)₂·2H₂O, (II), form binuclear seven‐coordinate complexes. Oxidation of (II) with ammonium hexanitratocerate(IV), (NH₄)₂[Ce(NO₃)₆], gave the oxide‐bridged dimanganese(IV) complex di‐μ‐oxido‐bis(bis{3‐[bis(pyridin‐2‐ylmethyl)amino]propionato}manganese(IV)) bis[triaquatetranitratocerate(IV)], [Mn₂O₂(C₁₅H₁₆N₃O₂)₂][Ce(NO₃)₄(H₂O)₃]₂, (III). The manganese complexes in (II) and (III) sit on a site of symmetry.     

Urease inhibition of oxovanadium(V) complexes with hydrazone and hydroxamate ligands

Two new oxovanadium(V) complexes, [VOL¹(SHA)] (I) and [VOL²(BHA)] (II), were prepared by the reaction of [VO(Acac)₂] (Acac = acetylacetonate) with N′-(2-hydroxybenzylidene)isonicotinohydrazide (H₂L¹) and salicylhydroxamic acid (HSHA) and 4-chloro-N′-(2-hydroxy-3-methoxybenzylidene)benzohydrazide (H₂L²) and benzohydroxamic acid (HBHA), respectively, in methanol. Crystal and molecular structures of the complexes were determined by elemental analysis, infrared spectra and single crystal X-ray diffraction (CIF file CCDC nos. 978238 (I) and 978392 (II)). The V atoms are in octahedral coordination. Thermal stability and the inhibition of urease of the complexes were studied.     

DNA/BSA binding interactions and VHPO mimicking potential of vanadium(IV) complexes: Synthesis,structural characterization and DFT studies

Vanadium(IV) Schiff base complexes (VOL¹‐VOL³) were synthesized and characterized by elemental analysis, various spectral methods and single crystal XRD studies. Structural analysis of VOL² reveals that the central vanadium ion in the complex is six coordinate with distorted octahedral geometry. Density functional theory (DFT) and time dependent (TD‐DFT) studies were used to understand the electronic transitions observed in the complexes in UV–Vis spectra. The electrochemical behavior of the complexes was investigated in acetonitrile medium exhibit quasi‐reversible one electron transfer. The DNA and BSA protein binding interaction of vanadium complexes has been explored by UV–Vis and fluorescence spectral methods and viscosity measurements reveal that the complexes interact with CT‐DNA through intercalation mode and follows the order VOL¹ < VOL³ < VOL². The complexes exhibit binding interactions with BSA protein. The complexes act as chemical nuclease and cleave DNA in the presence of H₂O₂. The 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) assay was used to evaluate the radical scavenging activity demonstrate the antioxidant property of the complexes. The antimicrobial activity was screened for several microorganisms, Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli. The mimicking of vanadium haloperoxidase was investigated by the bromination of the organic substrate phenol red by vanadium complexes in the presence of bromide and H₂O₂.     

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes: Efficient and Simple Hydrogenation Catalysts

The use of bis(NHC) manganese(I) complexes 3 as catalysts for the hydrogenation of esters was investigated. For that purpose, a series of complexes has been synthesized via an improved two step procedure utilizing bis(NHC)-BEt₃ adducts. By applying complexes 3 with KHBEt₃ as additive, various aromatic and aliphatic esters were hydrogenated successfully at mild temperatures and low catalyst loadings, highlighting the efficiency of the novel catalytic system. The versatility of the developed catalytic system was further demonstrated by the hydrogenation of other substrate classes like ketones, nitriles, N-heteroarenes and alkenes. Mechanistic experiments and DFT calculations indicate an inner sphere mechanism with the loss of one CO ligand and reveal the role of BEt₃ as cocatalyst.     

Synthesis,spectral, thermal and crystallographic investigations on oxovanadium(IV) and manganese(III) complexes derived from heterocyclic β-diketone and 2-amino ethanol

Novel mononuclear oxovanadium(IV) and manganese(III) complexes [VO(L¹)₂·H₂O] (1); [VO(L²)₂·H₂O] (2); [VO(L³)₂·H₂O] (3); [Mn(L¹)₂]ClO₄·H₂O (4); [Mn(L²)₂] ClO₄·H₂O (5); [Mn(L³)₂]ClO₄·H₂O (6) were prepared by condensation of 1 mol of VOSO₄·5H₂O or Mn(OAc)₃· 2H₂O with 2 mol of ligand HL¹, HL² or HL³ (where HL¹ = 4-[(2-hydroxy-ethylamino)-methylene]-5-methyl-2- phenyl-2,4-dihydro-pyrazol-3-one; HL²=4-[(2-hydroxy-ethylamino)-methylene]-5-methyl-2-p-tolyl-2,4-dihydro-pyrazol-3-one; HL³=4-{4-[(2-hydroxy-ethyl-amino)-methyl]-3-methyl-5-oxo-4,5-dihydropyrazol-1-yl} benzene sulfonic acid). The resulting complexes were characterized by elemental analyses, molar conductance, magnetic and decomposition temperature measurements, electron spin resonance, FAB mass, IR and electronic spectral studies. From TGA, DTA and DSC, the thermal behaviour and degradation kinetic were studied. Electronic spectra and magnetic susceptibility measurements indicate distorted octahedral stereochemistry of oxovanadium(IV) complexes and regular octahedral stereochemistry of manganese(III) complexes. Hamiltonian and bonding parameters found from ESR spectra indicate the metal ligand bonding is partial covalent. The X-ray single crystal determination of one of the representative ligand was carried out which suggests existence of amine-one tautomeric form in the solid state. The ¹H-NMR spectra support the existence of imine-ol form in solution state. The LC-MS studies sustain the¹H-NMR result. The electronic structure of the same representative ligand was optimized using 6-311G basis set at HF level ab initio studies to predict the coordinating atoms of the ligand.     

Synthesis,electrochemical and antimicrobial studies of mono and binuclear iron(III) and oxovanadium(IV) complexes of [ONO] donor tridentate Schiff-base ligands

Tridentate Schiff bases (H₂L¹ or H₂L²) were derived from condensation of acetylacetone and 2-aminophenol or 2-aminobenzoic acid. Binuclear square pyramidal complexes of the type [M₂(L¹)₂]?·?nH₂O (M?=?Fe–Cl, n?=?0; M?=?VO, n?=?1) were accessed from interaction of H₂L¹ with anhydrous FeCl₃ and VOSO₄?·?5H₂O, respectively. A similar reaction with H₂L², however, produced mononuclear complexes [ML²(H₂O) _x ]?·?nH₂O (M=Fe–Cl, x?=?0, n?=?0; M=VO, x?=?1, n?=?1). The compounds were characterized using elemental analysis, FT-IR, UV-Vis, and NMR (for ligand only), and mass spectroscopies and solution electrical conductivity studies. Magnetic susceptibility measurements suggest antiferromagnetic exchange in binuclear Fe(III) and VO(IV) complexes. Thermo gravimetric analysis (TGA) provided unambiguous evidence for the presence of coordinated as well as lattice water in [VOL²(H₂O)]?·?H₂O. Cyclic voltammetric studies showed well-defined redox processes corresponding to Fe(III)/Fe(II) and VO(V)/VO(IV). In vitro antimicrobial activities of the compounds were investigated against Klebsiella pneumoniae, Staphylococcus aureus, Pseudomonas aeroginosa, Escherichia coli, Bacillus subtilis, and Proteus vulgaris. H₂L¹ and its binuclear complexes exhibited pronounced activity against all the microorganisms tested.     

Synthesis,structure and antibacterial activity of manganese(III) complexes of a Schiff base derived from furfurylamine

Two new mononuclear complexes of manganese(III) viz. [MnL₂(LH)₂]ClO₄ (1) and [MnL₂(N₃)]·0.5CH₃OH (2) have been synthesized by reacting manganese perchlorate with furfurylamine and salicylaldehyde (plus sodium azide in 2) where L = (2-hydroxybenzyl-2-furylmethyl)imine, an asymmetric bidentate Schiff base formed in situ to bind the Mn(III) ion. The complexes have been characterized by elemental analysis, IR spectroscopy, TGA and single crystal X-ray diffraction studies. Structural studies reveal that the complexes 1 and 2 adopt an octahedral and a square pyramidal geometry, respectively. The antibacterial activity of the complexes has been tested against Gram(+) and Gram(?) bacteria.     

Reactivity of Schiff Base Manganese(III) Complexes with Different Pendants toward the Hydrolysis of p‐Nitrophenyl Picolinate

This paper describes the catalytic performance of two manganese(III) complexes with mono‐Schiff base ligands as artificial hydrolases towards the hydrolysis of p‐nitrophenyl picolinate (PNPP). Observations reveal that the one complex (MnL₂²Cl) containing morpholine pendants exhibits 1.2–1.7 fold kinetic advantage over the other one (MnL₂¹Cl) containing benzoaza‐15‐crown‐5 group. Especially, optimum molecule structures using a Gaussian 03 software confirm that MnL₂²Cl indeed possesses a relatively open linked site for the approaching of PNPP, resulting in higher efficiency due to a convenient association between substrate (PNPP) and MnL₂²Cl. In addition, the steric hindrance of two pendants, i.e., benzoaza‐15‐crown‐5 and morpholine, may be a main influencing factor for tuning catalytic activities of the synthesized Mn(III) catalysts. Both Mn(III) catalysts used here were found to have fine tolerance to the operated temperature and pH. Related kinetic and thermodynamic analyses were also given to demonstrate their structure‐activity relationships (SAR) of both catalysts used.     

Oxovanadium(V) complexes incorporating tridentate aroylhydrazoneoximes: Synthesis, characterizations and antibacterial activity

Some new oxovanadium(V) complexes, [VOL^1-3(OEt)(EtOH)] (1-3), have been reported, which were obtained from the reaction of the Schiff bases H₂L^1-3 (where H₂L¹ = the salicylhydrazone of diacetyl monoxime; H₂L² = the 4-methoxy salicylhydrazone of diacetyl monoxime and H₂L³ = the 4-hydroxy salicylhydrazone of diacetyl monoxime) with VO(acac)₂ in a 1:1 molar ratio. Three 4-R-aroylhydrazoneoximes (V) have been used as ligands in the present study, differing in the inductive effect of the substituent R (R = H, OCH₃ and OH), in order to observe their influence, if any, on the redox potentials and biological activity of the complexes. All the synthesized ligands and metal complexes were successfully characterized by elemental analysis, IR, UV-Vis and NMR spectroscopy. An X-ray diffraction study of [VOL¹(OEt)(EtOH)] (1) reveals that the metal center has a distorted octahedral O₅N coordination sphere, where the O,N,O donor ligand and the ethoxo group constitute a satisfactory O₃N basal plane. Cyclic voltammetry of the complexes show a quasi-reversible cyclic voltammetric response in the potential range 0.29-0.36 V involving a single electron V(V)-V(IV) reduction. The complexes have also been screened for their antibacterial activity against Escherichia coli, Bacillus, Proteus and Klebsiella. Minimum inhibitory concentrations of these complexes and the antibacterial activities indicate compound 1 as the potential lead molecule for drug design.     

Binuclear vanadium(V) complexes of bis(aryl)adipohydrazone: synthesis,spectroscopic studies,crystal structure and catalytic activity

Three new binuclear vanadium(V) complexes of bis(aryl)adipohydrazones (H₄L¹ = bis((2-hydroxynaphthalen-1-yl)methylene)adipohydrazide, H₄L² = bis(5-bromo-2-hydroxybenzylidene)adipohydrazide, and H₄L³ = bis(2-hydroxy-3-methoxybenzylidene)adipohydrazide) were synthesized by direct reaction of [VO(acac)₂] with the hydrazone ligands. The ligands and complexes were characterized by FT–IR, UV–Vis, and NMR spectroscopic methods. The crystal structures of the complexes of L¹ and L³ were determined by X-ray analyses. The solid-state structure of the complex of L¹ features a 1D hydrogen-bonded chain from N⋯H–O hydrogen bonding. The catalytic activities of these complexes have been tested in the oxidation of various hydrocarbons using H₂O₂ as the terminal oxidant. Generally, good to excellent conversions have been obtained.