Unsymmetrical Schiff Base Ligand Research Articles

Microwave assisted synthesis and characterization of unsymmetrical tetradentate Schiff base complexes of VO(IV) and MoO(V)

Microwave synthesis, is green chemical method, simple, sensitive, reducing solvent amount and reaction time. The attempt was made to synthesize the unsymmetrical tetradentate N 2O 2 ligands and their VO(IV) and MoO(V) unsymmetrical tetradentate Schiff base complexes by classical and microwave techniques using domestic microwave oven. The resulting unsymmetrical Schiff base ligands L 1–L 3 characterized by different spectral methods. Their complexes with oxocations of VO(IV) and MoO(V) have been synthesized and characterized by elemental analyses, conductometric measurements, infrared and electronic absorption, 1H NMR spectra, mass spectrometry, ESR spectra, magnetic susceptibility measurement and thermal study. The study suggests that the oxo metal ion is bonded to the ligand through the oxygen and imino nitrogen and the geometry around metal ion is distorted octahedral.

Synthetic, spectroscopic and thermal studies of some complexes of unsymmetrical Schiff base ligand

New unsymmetrical Schiff base ligand (H2L) is prepared via condensation of 2-hydroxy-5-methyl acetophenone, 2-hydroxy-5-chloro-3-nitro acetophenone and carbohydrazide in 1:1:1 ratio. Metal complexes of VO(IV), Cr(III), Mn(III), Fe(III), Zr(IV), MoO2(VI), WO2(VI) and UO2(VI) have been prepared. These complexes were characterized by elemental analysis, UV–Vis and IR spectroscopy and magnetic moment and thermogravimetric analysis. The purity of the ligand and the metal complexes is confirmed by microanalyses, while unsymmetrical nature of ligand was further corroborated by 1H NMR. All the complexes are air stable and insoluble in water and common organic solvents but fairly soluble in DMSO. The elemental analysis shows 1:1 metal to ligand stoichiometry for all the complexes. Thermal behaviour of the complexes was studied, the complexes were found to be quite stable and their thermal decomposition was generally via partially loss of the organic moiety and ended with respective metal oxide as a final product. Comparison of the IR spectrum of ligand and its metal complexes confirm that Schiff base behave as a dibasic tetradentate ligand towards the central metal ion with an ONNO donor sequence. The dc electrical conductivity is studied and data obtained obeyed the relation σ = σ 0 exp(−E a/kT) over the temperature range 40–130 °C. X-ray diffraction study of VO(IV) complex shows its crystalline nature with triclinic crystal system.

Synthesis, spectroscopic characterization, and 3D molecular modeling of lead(II) complexes of unsymmetrical tetradentate Schiff-base ligands

Solid complexes of Pb(II) with unsymmetrical Schiff-base ligands (H2L) derived from 2-aminobenzophenone, thiosemicarbazide, semicarbazide, salicylaldehyde, 2-hydroxynaphthaldehyde, and o-hydroxyacetophenone have been synthesized and characterized by elemental analysis, conductance measurements, molecular weight measurement, and UV–Vis, FTIR, 1H NMR, and 13C NMR spectroscopy. The spectral studies suggest the ligands behave as dibasic tetradentate ligands with ONNO/ONNS donor atom sequences toward the central metal ion. From the microanalytical data, the stoichiometry of the complexes was found to be 1:1 (metal:ligand). The physicochemical data suggest a tetracoordinated environment around the metal ion. Three-dimensional molecular modeling and analysis of bond lengths and bond angles have also been conducted for a representative compound, [PbL1], to substantiate the proposed structures.

Synthesis, structure and electrochemistry of Co(III) complexes with an unsymmetrical Schiff base ligand derived from 2-aminobenzylamine and pyrrole-2-carboxaldehyde

A series of cobalt(III) complexes with a new unsymmetrical tetradentate Schiff base ligand N,N’-bis(pyrrol-2-ylmethylene)-2-aminobenzylamine, H 2pyrabza, formed by the condensation of 2-pyrrole carboxaldehyde and 2-aminobenzylamine, has been synthesized and characterized by elemental analyses, IR, UV–Vis and 1H NMR spectroscopy. These complexes have the general formula trans-[Co III(L)(amine) 2]X {L 2− = pyrabza, and amine = N-methylimidazole (N-MeIm) ( 1), 3-methylpyridine (3-Mepy) ( 2), 4-methylpyridine (4-Mepy) ( 3), pyridine (py) ( 4), X = BPh 4 - }. The structure of compound 1 has been determined by X-ray crystallography. The geometry around the central cobalt ion is distorted octahedral. The electrochemical behavior of these complexes was also investigated. The reduction potential of Co(III), ranging from −0.79 V for 1 to −0.44 V for 4, shows a relatively good correlation with the σ-donor ability of the axial ligands.

Reactivity of tris(acetylacetonato) iron(III) with tridentate [ONO] donor Schiff base as an access to newer mixed-ligand iron(III) complexes

Two new mixed-ligand iron(III) complexes, [Fe(L n )(acac)(C 2H 5OH)] incorporating coordinated ethanol from the reaction solvent were accessed from the reaction of [Fe(acac) 3] with [ONO] donor dibasic tridentate unsymmetrical Schiff base ligands derived from condensation of 2-hydroxy-1-napthaldehyde with 2-aminophenol (H 2L 1) or 2-aminobenzoic acid (H 2L 2). The thermal study (TGA–DTA) provided evidence for weakly bound ethanol which is readily substituted by neutral N-donor molecule imidazole, benzimidazole or pyridine to produce an array of newer complexes, [Fe(L n )(acac)X] ( n = 1, 2; X = Im, Bim, Py). The compounds were characterized by elemental analyses, FT-IR, UV–vis, solution electrical conductivity, FAB mass, 1H and 13C NMR spectroscopy. Room temperature magnetic susceptibility measurements ( μ eff ∼ 5.8 B.M.) are consistent with spin-free octahedral iron(III) complexes. Cyclic voltammetry of ethanol complexes revealed a quasi-reversible one electron redox response (Δ E p > 100 mV) for the Fe(III)/Fe(II) couple. Low half wave redox potential ( E 1/2) values suggested easy redox susceptibility. The ground state geometries of the ethanol and imidazole complexes have been ascertained to be distorted octahedral by density functional theory using DMol3 program at BLYP/DNP level.

New copper(ii)-centered complexes with organometallic donor–acceptor substituted unsymmetrical Schiff base ligands

The synthesis and full characterization using various spectroscopic techniques and single-crystal X-ray diffraction analysis of a family of three robust copper(II)-centered complexes with organometallic donor–acceptor substituted unsymmetrical Schiff base ligands, namely, the binuclear derivatives Cu[Fc–C(O)CHC(CH3)N–X–NCH-(2-O,5-Br–C6H3)] (3: X = o-C6H4, 4: X = CH2CH2), and the corresponding ionic trinuclear counterpart of 4, [Cu{Fc–C(O)CHC(CH3)N–CH2CH2–NCH-(η6-2-O,5-Br–C6H3)RuCp*}][PF6] (5) (Fc = (η5-C5H5)Fe(η5-C5H4), Cp* = η5-C5Me5), have been explored. Single crystal X-ray diffraction analysis of complex 3 indicates a bowed structure of the unsymmetrical Schiff base scaffold. The Cu2+ ion is tetracoordinated in a square planar environment, with two nitrogen atoms and two oxygen atoms as donors. The EPR spectra of solid trinuclear complexes 5 and 7 (unsubstituted salicylidene ring and X = o-C6H4) at low temperature (68 K) consisted of fine-structure transitions (ΔMs = ±1) with zero-field splitting (ZFS) values of 0.0458 and 0.0415 cm−1, respectively, and a half-field signal (ΔMs = ±2) at ca. 1600 G, suggesting the formation of dimeric species (S = 1). In those dimers, Cu–Cu distances of 3.97 and 4.14 A, respectively, are computed from ZFS terms. The variable-temperature (2–300 K) magnetic susceptibilities of powdered samples of 5 and 7 have been measured, and an intermolecular antiferromagnetic interaction with exchange couplings of J = −1.05 and −0.65 cm−1, respectively, were found. The electrochemical behavior of both the neutral binuclear and ionic trinuclear compounds was investigated by cyclic voltammetry. Notably, the ability of the copper Schiff base core to transmit electronic effects between the two organometallic termini has been demonstrated by the observed anodic shift of the reversible voltammetric signal ascribed to the FeII/FeIII couple of the ferrocenyl donor fragment.

Synthesis, characterization, and antimicrobial activities of nickel(II) and copper(II) Schiff-base complexes

Ni(II) and Cu(II) metal complexes of simple unsymmetrical Schiff-base ligands derived from salicylaldehyde/5-methylsalicylaldehyde and ethylenediamine or diaminomaleonitrile (DMN) were synthesized. The ligands and their complexes were characterized by elemental analysis, 1H NMR, FT IR, and mass spectroscopy. The electronic spectra of the complexes show d–d transitions in the region at 450–600 nm. Electrochemical studies of the complexes reveal that all mononuclear complexes show a one-electron quasi-reversible reduction wave in the cathodic region. ESR spectra of the mononuclear copper(II) complexes show four lines, characteristic of square-planar geometry, with nuclear hyperfine spin 3/2. The copper(II) complexes show a normal room temperature magnetic moment value μ eff = 1.70–1.74 BM which is close to the spin only value of 1.73 BM. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the copper(II) complexes as catalysts were also carried out. The in vitro antimicrobial activity of the investigated compounds was tested against human pathogenic bacterias such as Staphylococcus aureus, Bacillus subtilis, Klebsiella pneumonia, Pseudomonas aeruginosa and Escherichia coli. The antifungal activity was tested against Candida albicans. Generally, the metal complexes have higher antimicrobial activity than the free ligands.

Syntheses, characterization and crystal structures of two square-planar Ni(II) complexes with unsymmetrical tridentate Schiff base ligands and monodentate pseudohalides

Two new square-planar Ni(II) complexes, [NiL1(NCS)] (1) and [NiL2(N3)] (2) have been synthesized with the unsymmetrical tridentate Schiff base ligands [(CH3)2NCH2CH2N=C(CH3)CH=C(OH)(C6H5)], L 1 H, derived from benzoylacetone and 2-dimethylaminoethylamine and [(CH3CH2)2NCH2CH2N=C(CH3)CH=C(OH)(C6H5)], L 2 H, derived from benzoylacetone and 2-diethylaminoethylamine, respectively. The complexes have been characterized by elemental analysis, FT-IR, UV-Vis spectroscopy, electrochemical and thermal methods (where applicable). Structures have been established by the single-crystal X-ray diffraction technique which reveals the discrete nature of the complexes in which the metal centers adopt a distorted square planar geometry. Coordination environments of the metal ions in the complexes are satisfied with two different unsymmetrical Schiff base ligands having similar N2O donor sets and a terminal pseudohalide anion (thiocyanate for 1 and azide for 2).

Aluminum and Yttrium Complexes of an Unsymmetrical Mixed Fluorous Alkoxy/Phenoxy-Diimino Ligand: Synthesis, Structure, and Ring-Opening Polymerization Catalysis

The coordination chemistry of the new unsymmetrical Schiff base ligand {(3,5-tBu2-1-OC6H4)CH═N(trans-1,2-cyclo-C6H10)N═C(Me)CH2C(CF3)2O}2− ({ArONCyNOCF3}2−) onto Al(III) and Y(III) centers has been studied. Pro-ligand {ArONCyNOCF3}H2 (1) reacts with AlMe2Cl, Al(OiPr)3 and Y(N(SiHMe2)2)3·(THF)2 to give the corresponding complexes {ArONCyNOCF3}AlX (X = Cl, 5; OiPr, 6) and {ArONCyNOCF3}Y(N(SiHMe2)2)(THF) (7) in high yields, with concomitant alkane, alcohol, and amine elimination, respectively. Single-crystal X-ray diffraction studies revealed that complexes 5 and 7 are mononuclear in the solid state with, respectively, five-coordinated distorted square-pyramidal and six-coordinated distorted octahedral geometries. Complexes 5−7 were also characterized in C6D6 solution by 1H, 13C, and 19F NMR spectroscopy, which indicated the existence of only one isomer at room temperature. Al-OiPr complex 5 is an effective, though sluggish, initiator for the ring-opening polymerization of racemic lactide, giving polymers with a highly isotactic-enriched microstructure (Pmeso = 0.87), molecular weights that match well the calculated values, and narrow polydispersities (Mw/Mn = 1.04−1.18).

Synthesis and Thermal Decomposition Kinetics of Uranium(VI) Complex with Unsymmetrical Schiff Base Ligand

AbstractA new unsymmetrical tetradentate Schiff base ligand N‐(5‐chloro‐2‐hydroxy‐α‐phenylbenzylidene)‐N′‐(2‐hydroxy‐3‐methoxybenzylidene)‐o‐phenylenediamine (H2L) was synthesized using 5‐chloro‐2‐hydroxybenzophenone, o‐phenylenediamine and o‐vanillin. A solid UO2(II) complex of this ligand [UO2(HL)(NO3)(H2O)]·H2O was prepared and characterized by elemental analyses, IR, UV, 1H NMR spectra and molar conductance. The thermal decomposition kinetics of the complex for the third stage was studied under a non‐isothermal condition by TG and DTG methods. The kinetic equation may be expressed as: dα/dt=A·e−E/RT·3/2[(1−α)−1/3−1]−1. The kinetic parameters (E, A), activation entropy ΔS≠ and activation free‐energy ΔG≠ were also gained: E=69.08 kJ/mol, ln A=20.63, r=0.9867, ΔS≠= −79.84 J·mol−1·K−1 and ΔG≠=120.6 kJ/mol.

Synthesis, characterization, and antibacterial activity of new rare-earth ion complexes with unsymmetrical Schiff base ligand

A new unsymmetrical Schiff base ligand (H2LLi) was synthesized using L-lysine, salicylaldehyde, and 2-hydroxy-1-naphthaldehyde. Three rare-earth ion complexes of this ligand, [REE(H2L)(NO3)]NO3 · 2H2O (REE = La, Sm, Ho), have been prepared and characterized by elemental analyses, IR spectra, UV spectra, TG-DTG, and molar conductance. The antibacterial activity of the ligand and its complexes are also studied. The antibacterial experiments indicate that this ligand and its complexes possess antibacterial activity against Escherichia coli, Staphylococcus aureus and Bacillus subtilis and the complexes have higher activity than that of the ligand.

Synthesis, characterization and antibacterial activity of new Ln(III) complexes with an unsymmetrical schiff base ligand

A new unsymmetrical Schiff base ligand (H2LLi) was synthesized using L-lysine, salicyladehyde and 2-hydroxy-1-naphthaldehyde. Three solid metal complexes of this ligand [Ln(H2L)(NO3)] NO3·2H2O (Ln=La, Sm, Ho) have been prepared and characterized by elemental analyses, IR spectra, UV spectra, TG-DTG and molar conductance. The antibacterial activities of the ligand and its complexes are also studied. The antibacterial experiments indicate that the ligand and its complexes possess antibacterial activity against Escherichia coli, Staphylococcus aureus and Bacillus subtilis and that the complexes have higher activity than those of the ligand.

Nickel(II) and copper(II) complexes of tetradentate unsymmetrical Schiff base ligands: First evidence of positional isomerism in such system

The 1:1 condensation of 2,4-pentanedione and 1,2-diaminopropane gives a mixture two positional isomers of tridentate mono-condensed product 7-amino-4-methyl-5-aza-3-octene-2-one (HAMAO) and 7-amino-4,6-dimethyl-5-aza-3-heptene-2-one (HADAH) that reacted readily with Ni(II) thiocyanate to yield exclusively a single product, [Ni(AMAO)NCS] ( 1) in which the methyl substituent of diamine is ‘remote’ from the imino nitrogen. The mixture of terdentate ligands has been used for further condensation with pyridine-2-carboxaldehyde or 2-acetylpyridine to obtain the unsymmetrical tetradentate Schiff base ligands. The tetradentate ligands formed by the condensation of it and pyridine-2-carboxaldehyde readily yielded complexes with Cu(II) and Ni(II) ( 2 and 3, respectively). Crystal structure analysis shows that in 2 the condensation site of the diamine with 2,4-pentanedione is the same as in 1 but that in 3 is different (the methyl group of the diamine is located in the vicinity of 2,4-pentanedione), i.e., the tetradentate ligand is in two different isomeric forms in complexes 2 and 3. Another tetradentate ligand, obtained by the condensation of the tridentate ligands and 2-acetylpyridine yielded a Ni(II) complex ( 4) where the methyl group is in the vicinity of 2,4-pentanedione as in 3. The isomerization in the Ni(II) complexes has been studied by NMR spectroscopy.

Synthesis and thermal decomposition kinetics of Th(IV) complex with unsymmetrical Schiff base ligand

A new unsymmetrical Schiff base ligand (H2LLi) was synthesized using L-lysine, o-vanillin and salicylaladyde. Thorium(IV) complex of this ligand [Th(H2L)(NO3)](NO3)2 .3H2O have been prepared and characterized by elemental analyses, IR, UV and molar conductance. The thermal decomposition kinetics of the complex for the second stage was studied under non-isothermal condition by TG and DTG methods. The kinetic equation may be expressed as: dα/dt=A .e-E/RT.1/2 (1-α).[-ln(1-α)]-1. The kinetic parameters (E, A), activation entropy ΔS ¹ and activation free-energy ΔG ¹ were also calculated.

Synthesis and Characterization of some Unsymmetrical Schiff Base Ligands and their Nickel(II) Complexes Incorporating o‐Phenylenediimine Units

AbstractSix new nickel(II) complexes of the unsymmetrical Schiff base ligands derived from o‐phenylenediamine were synthesized. These complexes were prepared by template and non‐template reactions of the precursor 3‐acetyl‐4‐[N‐(2'‐aminophenyl)‐amino]‐3‐buten‐2‐one (HL°) with appropriate o‐hydroxycarbonyl aromatic compounds, aromatic 1, 3‐oxo aldehydes and 1, 3‐diketones.The nickel(II) compounds were characterized by analytical and spectroscopic methods. Crystal structure of complex [3‐acetyl‐(6, 7)‐benzo‐8‐salicylidene‐5, 8‐diazahepta‐3‐ene‐2‐onato(2‐)]nickel(II) (NiL1) has been determined by X‐ray powder diffraction method, revealed that the molecules are almost flat, and there are no forces other than van der Waals interactions between molecules. The structure was solved by global optimisation technique and refined by the Rietveld method, obtained RF and Rwp are 11.6 and 17.4%, respectively. The synthesis of a new unsymmetrical nickel(II) tetraazamacrocyclic complex is also described.

Coordination properties of unsymmetrical Schiff base ligands containing N 3 donor sets when restricted to bidentate chelate bonding modes. Crystal structures of fac-[PtIMe 3(PMAMP)] (PMAMP=2-{(2′-pyridylmethylene)aminomethyl}pyridine) and fac-[PtIMe 3(PMAQ)] (PMAQ=8-{(2′-pyridylmethylene)amino}quinoline)

The N 3 ligands 2-{(2′-pyridylmethylene)aminomethyl}pyridine, (PMAMP), 8-{(2′-pyridylmethylene)amino}quinoline, (PMAQ) and 2-[{(6′-methyl-2′-pyridyl)methylene}aminomethyl]pyridine (MPMAMP) are analogous to 2,2′:6′,2″-terpyridine in respect of their spatial arrangement of N donor atoms. When acting as bidentate chelates PMAMP and PMAQ bind exclusively to the Pt IV moiety PtIMe 3 such that the five-membered chelate rings incorporate the imine bond. This was deduced by detailed solution NMR studies and X-ray crystal structures. In contrast, the ligand MPMAMP, with 6-methyl substitution of the pyridyl ring attached to the carbon of the imine bond, preferentially, but not exclusively, coordinates to the nitrogen of the other pyridyl ring. The population ratio of the two complexes formed was 6:1 (303 K, CDCl 3 solution), but there was no NMR evidence of exchange between these species, in the temperature range 303–413 K, resulting from 1,4-metallotropic shifts, as occurs in analogous complexes such as fac-[PtIMe 3(2,2′:6′,2″-terpyridine)].

Synthesis, Characterization of An Unsymmetrical Cu(II) Complex Liquid Crystal of a Schiff Base Containing a Terminal Double Bond

The unsymmetrical Schiff base ligand 4-undecenyloxy-salicylidene-4-dodecyloxyaniline (Im-11–12) containing a terminal double bond derived from 4-undecenyloxysalicylaldehyde and 4-dodecyloxyaniline and its Cu(II) complex [Cu(Im-11–12)2] were synthesized and characterized by elemental analyses, molar conductivity, IR, electronic spectra and 1H NMR. The Schiff base functions as a uninegative bidentate ligand and bonds through oxygen and imino-nitrogen. The complex is a non-electrolyte and has a square planar geometry. The mesogenic behavior of the ligand and the complex was determined by a hot-stage polarized microscope and DSC. The transition processes of the ligand and the complex were tentatively proposed to include K→S→I, but their liquid crystalline texture structures were different.

A mechanism for the rearrangement of unsymmetrical tetradentate (N2O2) ligands bound to manganese(III): the isolation and crystal structure of a manganese(III) complex containing a ten-membered cis-chelated ring

The unsymmetrical Schiff-base ligand L1(where H2L1 is the condensation product of 8-amino-4-methyl-5-azaoct-3-en-2-one and 3-ethoxysalicylaldehyde) has been found to rearrange into the symmetrical ligand L2[H2L2=N,N′-bis(3-ethoxysalicylidene)propane-1,3-diamine] in the presence of manganese(III). The manganese(III) complexes of both the unsymmetrical and symmetrical ligands have been characterised by elemental analyses, infrared spectroscopy, magnetic susceptibility measurements and fast atom bombardment (FAB) mass spectrometry and have been assigned the stoichiometries [Mn(HL1)2][ClO4]·H2O and [MnL2][ClO4]·3H2O based on these data. The FAB mass spectrum of the former exhibits an interesting phenomenon, with two distinct metal co-ordination environments being observed. The ion of [Mn(HL1)2]+ at m/z 661 is observed, as would be expected from the experimental data; however, the parent ion peak at m/z 357 corresponds to the monomeric [ML1]+ ion with the amine nitrogen atom of the ligand deprotonated. The manganese(III) complex of the unsymmetrical ligand [Mn(HL1)2][ClO4] has been crystallographically characterised, no water of crystallisation being observed in the selected crystal. This monomeric complex consists of a pseudo-octahedral metal centre with an N2O4 donor environment. The non-co-ordination of the amine nitrogen atom, confirmed by the Mn ⋯ N(2) separation of 3.51 A, produces a ten-membered chelate ring about the manganese ion, believed to be the first example of such a system. The crystal structure has enabled a mechanism for the rearrangement of Schiff bases about manganese(III) to be proposed. It involves a series of reversible hydrolysis reactions which are stabilised by manganese(III) acting as a template.