Quadridentate Schiff Base Ligands Research Articles

Experimental and Theoretical Evaluation of Four NLO‐Active Divalent Transition‐Metal Complexes Supported by an Enantiomerically Pure Tetradentate Schiff Base Ligand

AbstractThis paper reports on a series of five chiral Schiff base push‐pull compounds derived from enantiomerically pure (1R,2R)‐(−)‐1,2‐diaminocyclohexane. The NiII, CuII, ZnII and PdII complexes supported by a ferrocene‐containing unsymmetrically‐substituted N2O2 quadridentate Schiff base ligand were prepared via template reactions and isolated in 74–87 % yields. A combination of EA, IR, UV/vis and 1H/13C{1H} NMR spectroscopy, HRMS spectrometry, cyclic voltammetry and single‐crystal X‐ray analysis (for the proligand 2 and its NiII derivative 3), together with computational methods (DFT and TD‐DFT) was used to fully characterize and study the properties of all products. Both 2 and 3 crystallize in the orthorhombic non‐centrosymmetric space group P212121, with two (R,R)‐(−)‐chiral carbon atoms in their structure. Second‐order nonlinear polarizabilities β have been measured by using Harmonic Light Scattering at 1.91 μm, with the Pd(II) species showing the higher NLO response of 460×10−30 esu.

Synthetic, Characterization, Biological, Electrical and Catalytic Studies of Some Transition Metal Complexes of Unsymmetrical Quadridentate Schiff Base Ligand

Unsymmetrical tetradentate Schiff base N-(2-hydroxy-5-methylacetophenone)-N'-(2-hydroxy acetophenone) ethylene diamine (H₂L) and its complexes with Cr(III), Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) have been synthesized and characterized by elemental analyses, magnetic susceptibility measurements, IR, electronic spectra and thermogravimetric analyses. ¹H, ¹³C-NMR and FAB Mass spectra of ligand clearly indicate the presence of OH and azomethine groups. Elemental analyses of the complexes indicate that the metal to ligand ratio is 1:1 in all complexes. Infrared spectra of complexes indicate a dibasic quadridentate nature of the ligand and its coordination to metal ions through phenolic oxygen and azomethine nitrogen atoms. The thermal behavior of these complexes showed the loss of lattice water in the first step followed by decomposition of the ligand in subsequent steps. The thermal data have also been analyzed for the kinetic parameters by using Horowitz-Metzger method. The dependence of the electrical conductivity on the temperature has been studied over the temperature range 313-403 K and the complexes are found to show semiconducting behavior. XRD and SEM images of some representative complexes have been recorded. The antimicrobial activity of the ligand and its complexes has been screened against various microorganisms and all of them were found to be active against the test organisms. The Fe(III) and Ni(II) complex have been tested for the catalytic oxidation of styrene.

Ruthenium(II) carbonyl complexes containing S-methylisothiosemicarbazone based tetradentate ligand: synthesis, characterization and biological applications

A series of hexa-coordinated ruthenium(II) complexes of the type [Ru(CO)(B)L(n)] (n = 1-4; B = PPh3, AsPh3 or Py) have been synthesized by reacting dibasic quadridentate Schiff base ligands H2L(n) (n = 1-4) with starting complexes [RuHCl(CO)(EPh3)2(B)] (E = P or As; B = PPh3, AsPh3 or Py). The synthesized complexes were characterized using elemental and various spectral studies including UV-Vis, FT-IR, NMR ((1)H, (13)C and (31)P) and mass spectroscopy. An octahedral geometry was tentatively proposed for all the complexes based on the spectral data obtained. The experiments on antioxidant activity showed that the ruthenium(II) S-methylisothiosemicarbazone Schiff base complexes exhibited good scavenging activity against various free radicals (DPPH, OH and NO). The in vitro cytotoxicity of these complexes has been evaluated by MTT assay. The results demonstrate that the complexes have good anticancer activities against selected cancer cell line, human breast cancer cell line (MCF-7) and human skin carcinoma cell line (A431). The DNA cleavage studies showed that the complexes have better cleavage of pBR 322 DNA.

Synthesis, Crystal Structures, and Magnetic Properties of Two Tetrahedral MnIIMnIII3 Complexes

AbstractTwo tetranuclear manganese complexes, [NaMnIIMn3III(μ4‐O2–)(HL)3(SCN)4] (1) and [NaMnIIMn3III(μ4‐O2–)(HL)3Cl4][NaMnIIMn3III(μ4‐O2–)(HL)3Cl3(H2O)]ClO4·3.5H2O (2) were obtained from the reaction of manganese perchlorate with a quadridentate Schiff base ligand, 3‐(2‐hydroxybenzylideneamino)propane‐1, 2‐diol (H3L) derived from condensation of 2‐hydroxybenzaldehyde with 3‐amino‐1, 2‐propanediol, as well as the coligand KSCN or NaCl under basic conditions. Single‐crystal X‐ray studies reveal that those two complexes all have a mixed‐valent tetrahedral core, which contains an apical MnII ion and three basal MnIII ions situated in the [Mn3(μ4‐O2–)]7+ equilateral triangle plane. Fitting of the magnetic susceptibility data to the theoretical χmT vs. T expression, revealed that the presence of only antiferromagnetic interactions between the central metal atoms in 1, while both antiferromagnetic and ferromagnetic interactions are present in 2.

Synthetic, characterization and catalytic studies of some coordination compounds derived from unsymmetrical quadridentate Schiff base ligand

The stable complexes of VO(IV), Cr(III), Mn(III), Fe(III), MoO2(VI), and WO2(VI), with an unsymmetrical tetradentate Schiff base ligand derived from 2-hydroxy-5-methylacetophenone, 2-hydroxy-5-chloroacetophenone and carbohydrazide were synthesized and characterized by the elemental analysis, UV-Vis and IR spectroscopy, magnetic measurements and thermal analysis. The VO(IV) and Mn(III) complexes were tested for the catalytic oxidation of styrene. The conversion of styrene increases with use of VO(IV) catalyst and decreases with use of Mn(III) catalyst.

Synthesis and crystal structure of one-dimensional azide-bridged Mn(III) polymer [Mn(Sal2-1,2Pn)(N3)] n (Sal2-1,2Pn = N,N′-bis(salicylidene)-1,2-propanediamine)

The new one-dimensional azide-bridged manganese(III) polymer, [Mn(Sal2-1,2Pn)(N3)]n (I), where Sal2-1,2 Pn = N, N,N′-bis(salicylidene)-1,2-propanediamine, was prepared from a reaction mixture containing Sal2-1,2 Pn, MnCl2 · 2H2O and NaN3 (2: 1: 8 molar ratio) in methanol-chloroform (v/v 2: 1) and has been characterized by elemental analyses, FT-IR spectroscopy, and X-ray single-crystal diffraction. In the structure of I, Mn3+ ion is in a distorted octahedral geometry with an obvious Jahn-Teller distortion. The quadridentate Schiff-base ligand Sal2-1,2Pn is located in the equatorial plane. The azide ion acts as an end-to-end bridge to form the one-dimensional manganese(III) polymer.

Aerobic Oxidation of Some Arylthiols Catalyzed by a New Manganese (III) Schiff-Base Complex

Bis(2-hydroxybenzene)phthaldiimine (BHBPDI) as a new quadridentate Schiff base ligand and its [Mn(III)(BHBPDI)Cl] complex were synthesized and characterized by analytical and spectral data (MS, FT-IR, UV-Vis, 1H NMR). The aerobic oxidation of thiols catalyzed by Mn(III)-complex using molecular oxygen was studied at room temperature. The reaction proceeded to yield the corresponding symmetric disulfides in good to high yields. The reaction was retarded when the reaction mixture was saturated with nitrogen gas. This system provides an efficient and convenient method for the oxidation of thiols to symmetric disulfides.

Catalytic Oxidative Decarboxylation of Some Benzylcarboxylic Acid Derivatives by a New Iron(III) Schiff Base Complex

AbstractA quadridentate Schiff base ligand of N,N′‐bis(2‐hydroxy‐α‐methylbenzylidene)ethylenediamine (HMBEDA) and its new iron(III) complex were synthesized and identified by analytical, spectral data (1H NMR, 13C NMR FT‐IR and UV‐visible) and molar conductance. A rapid and efficient homogeneous oxidative decarboxylation of some benzylcarboxylic acid derivatives was carried out by a catalytic amount of iron(III) Schiff base complex in chloroform, using tetrabutylammonium periodate as a mild oxidant in good to excellent yields at room temperature.

Catalytic oxidative decarboxylation of arylacetic acids promoted by new Mn(III) and Fe(III) Schiff base complexes

Bis(2-hydroxybenzene)phthaldiimine (BHBPDI) as a new quadridentate Schiff base ligand and its Mn(III) and Fe(III) complexes were synthesized and characterized by analytical and spectral data. The Mn(III) and Fe(III) complexes were employed as catalysts in the oxidative decarboxylation of a variety of arylacetic acids, using tetrabutylamonium periodate as oxidant. The reactions were carried out in dichloromethane at room temperature. Relatively rapid and efficient oxidative decarboxylation of arylacetic acids to corresponding aldehydes or ketones in good to high yields were observed in the presence of the system, (Mn (III) (BHBPDI))Cl)/imidazole/(n-Bu)4NIO4. The Fe(III) analog system showed less catalytic activity at the same conditions.

Copper(II) complexes of N 4 tetradentate ligands with flexible alkyl spacers: Crystal structure, DNA binding and cleavage studies

Mononuclear copper(II) complexes, [Cu L1] (ClO 4) 2 ( 1), [Cu L2] (ClO 4) 2 ( 2) and [Cu L3] (ClO 4) 2 ( 3) with quadridentate Schiff base ligands L1 ( N, N′-bis-pyridin-2-ylmethyl-butane-1,4-diimine), L2 ( N, N′-bis-pyridin-2-ylmethyl-pentane-1,5-diimine) and L3 ( N, N′-bis-pyridin-2-ylmethyl-hexane-1,6-diimine) have been synthesized and characterized. The crystal structure data of 1 reveals the existence of the complex in two different geometries, namely a square pyramid and a distorted octahedron, which eventually leads to the packing of the molecule into helical and anti-parallel structures respectively. Absorption titration studies with calf thymus DNA for all three complexes are suggestive of groove binding with binding constant values for 1, 2 and 3 being 2.6 ± 0.2 × 10 4 M −1, 11.5 ± 0.2 × 10 4 M −1 and 1.83 ± 0.2 × 10 4 M −1 respectively. Control cleavage experiments using pBR 322 plasmid DNA and distamycin suggest minor groove binding for these complexes. In the presence of ascorbic acid, the complexes show efficient DNA cleavage, the order of efficiency being 1 > 2 ≅ 3.

Fluorous biphasic oxidation of sulfides catalysed by (salen)manganese(III) complexes

Abstract Quadridentate Schiff base ligands derived from 1,2-diamines and fluorous derivatives of salicylaldehyde were prepared and the corresponding manganese(III) complexes were tested as catalysts in the selective oxidation of alkyl aryl sulfides with PhIO. Complexes bearing two fluorinated ponytails were soluble in standard organic solvents and were used under classical homogeneous conditions, whereas heavily fluorinated complexes could be used in a CH3CN/perfluorooctane biphasic system. In both cases, sulfoxides were obtained as the main products, together with variable amounts of sulfones (≤10%), depending on the nature of the substrate and the catalyst. When reactions were carried out under fluorous biphasic (FB) conditions, the selectivity for sulfoxides was improved and the catalyst could be easily recovered by simple phase separation and reused up to four times. Despite their good chemoselectivity, catalytic efficiency and recyclability, chiral fluorous (salen)manganese(III) complexes showed low enantioselectivities in preliminary experiments run under fluorous biphasic conditions.

Synthesis, characterisation and X-ray crystal structure of copper(II) complexes with unsymmetrical tetradentate Schiff base ligands: first evidence of Cu(II) catalysed rearrangement of unsymmetrical to symmetrical complex

The mononuclear copper(II) complexes, [CuL 1](ClO 4) ( 1), and [CuL 2](ClO 4) ( 2) with unsymmetrical quadridentate Schiff base ligands derived from the 1:1:1 condensation of 2,4-pentanedione, pyridine-2-carboxaldehyde and 1,2-ethanediamine (HL 1) or 1,3-propanediamine (HL 2) have been prepared and characterised. Complex 2 undergoes Cu(II)/H + catalysed rearrangement to [CuL 3](ClO 4) 2 ( 3) where L 3 is a symmetrical tetradentate Schiff base involving 1,3-propanediamine and pyridine-2-carboxaldehyde. Structures of all the three compounds have been verified by single crystal X-ray analysis. The geometry around Cu(II) is square-planar in 1 and 2 whereas that in 3 is distorted octahedral with two axially coordinated perchlorate ions.

Synthesis, structure, and reactivity of bis( N, N′-bis(2-hydroxybenzylidene)-2-hydroxyphenylmethanediaminato)zirconium(IV), a Schiff base complex with 6,4,6-membered chelate rings

The title complex Zr(dshpm) 2 has been successfully prepared by heating a 1:2 mole ratio of Zr(OBu n ) 4 and the free ligand in absolute ethanol. It is mononuclear and eight-coordinate in the solid state with a dodecahedral coordination sphere. The two O,N,N,O donor atom quadridentate Schiff base ligands each form 6,4,6-membered chelate rings. As expected, the change in a chelate ring sizes from 6,5,6- to 6,4,6- resulted in significantly more labile complex, as indicated by TLC and NMR of heated solutions and its subsequent chemical reactions. The 0.010 Å shorter average ZrN bond distance was not anticipated, and appears to result from the zirconium atom's preference that the oxygen atoms occupy certain locations. Preparations of two heteroleptic complexes from the title complex were undertaken to evaluate potential of the title compound as a synthon. A new fluorine-substituted homoleptic Schiff base Zr complex was also prepared, for comparison purposes. Despite the reasonable stability of Zr(dshpm) 2 in air, the decomposition of labilized Hdshpm(2-) ligand on silica gel and in heated solution interfered with the purifications of the desired heteroleptic products.

Conformational flexibility and role of aromatic–aromatic interactions in the crystal packing of the coordination compounds of some novel quadridentate Schiff bases

The synthesis and characterisation of two novel quadridentate Schiff base ligands N,N′-ethylenebis(1,5-diphenyl-4-iminopentan-2-one) (2a) and N,N′-ethylenebis[1,5-di(1-naphthyl)-4-iminopentan-2-one] (2b) and five of their Ni(II)-, Co(II)- and Cu(II) complexes are described. The crystal structures of 2a and complexes 3–7 were established by single-crystal X-ray diffraction. Although the coordination geometries of 3–7 were found to be similar, the conformations of the aromatic side chains differed dramatically: very small changes in the coordination geometries, together with the high flexibility of the ligands, as confirmed by quantum mechanical ab initio calculations, seem to lead to remarkable conformational differences as well as to differences in the intermolecular aromatic–aromatic interactions and packing in the crystals.

Mixed Ligand Complexes of Chromium(III) containing some Bidentate Amines and Quadridentate Schiff Base Ligands

Mixed Ligand Complexes of Chromium(III) containing some Bidentate Amines and Quadridentate Schiff Base Ligands

Ligand-field control in the self-assembly of polymeric metal complexes: copper(II) complexes with quadridentate Schiff-base ligands involving an imidazole moiety

Ligand-field control in the self-assembly of polymeric metal complexes has been studied. A series of seven ‘ligand complexes’[Cu(HL)]+ containing an imidazole moiety have been prepared, where H2L are the quadridentate Schiff-base ligands of the 1 : 1 : 1 condensation products of 4-formylimidazole, a diamine (ethylenediamine, 1,3-diaminopropane or trans-cyclohexane-1,2-diamine), and a salicylaldehyde derivative (salicylaldehyde, 5-bromosalicylaldehyde, 5-methoxysalicylaldehyde, 3-methoxysalicylaldehyde, o-hydroxyacetophenone) or benzoylacetone. The single-crystal X-ray analyses of three complexes have confirmed that the co-ordination geometries are essentially square planar. From the deprotonation behaviour of the imidazole proton and the d–d band maxima, the complexes can be classified into two types; type A complexes composed of salicylaldehyde derivatives and type B complexes composed of benzoylacetone. Under alkaline conditions, the type A complexes easily produce a self-assembly process giving insoluble imidazolate-bridged polymeric species and their potentiometric pH titrations are irreversible in the forward and reverse titrations. The variable-temperature magnetic data of the deprotonated type A complexes are well reproduced using the Ising model based on H=–2ΣJijSiSj, and J values of –8.6, –1.8, –5.2, –13.0 and –4.5 cm–1 are obtained. On the other hand, under alkaline conditions, the type B complexes give mononuclear species soluble in common organic solvents and act as good ‘ligand complexes’. The potentiometric pH titration curves are almost the same in the forward and reverse titrations, indicating that protonation and deprotonation were reversible. The type A complexes exhibiting weaker ligand-field strength in the equatorial ligand are ‘self-assembly complexes’ and the type B complexes with stronger fields are ‘ligand complexes’.

Synthesis and magnetic properties of iron(III) complexes bridged by 1,1′-tetramethylenediimidazole exhibiting an infinite zigzag-chain structure

Three polymeric iron(III) complexes [Fe(tdim)Ln]CIO4(L = L11, L2 or L33) consisting of a bridging ligand (tdim = 1,1′-tetramethylenediimidazole) and a quadridentate Schiff base ligand have been prepared and characterized, where H2Ln= RN(CH2)2NR′[R = R′= MeC(OH)CHCMe, n= 1; R = R′=o-HOC6H4CH=,n=3;R = MeC(OH)CHCMe, R′=o-HOC6H4CH=,n= 2]. Magnetic susceptibility data (4.2–300 K) showed that 1 is low spin (S= 1/2), 2 of spin equilibrium (S= 5/2⇌½), and 3 essentially high spin (S=5/2) and exhibiting an unusual behaviour below ca. 100 K. The spin equilibrium of 2 occurs over a wider temperature range than that of the corresponding mononuclear complex [Fe(mim)2L2]CIO4(mim = 1-methylimidazole). A crystal structure analysis of 3 confirmed that it has an infinite zigzag-chain structure and the iron(III) ion is six-co-ordinated by N2O2 donor atoms of the quadridentate ligand, an imidazole nitrogen atom of a bridging ligand and a nitrogen atom of the adjacent bridging ligand. The crystal structure also shows that 3 suffers from disorder and there are two configurations A and B with respect to the orientation of the imidazole moiety, having populations of 0.65 and 0.35, respectively. The unusual magnetic behaviour is rationalized by the crystal structure, Mossbauer and ESR spectra.

Nickel(II) complexes with unsymmetrical quadridentate Schiff bases having a pendant N-acyl substituent

The condensation of N-acylated derivatives of o-aminocarbonyl aromatic compounds with the precursor 7-amino-4-methyl-5-azahept-3-en-2-one has been found to depend on the acidity of the amide proton and the nature of the substituent on the carbonyl carbon atom. Three new quadridentate Schiff-base ligands and their nickel(II) complexes having a pendant N-acetyl or N-trifluoroacetyl group have been obtained and characterized by spectroscopic techniques. A very unusual temperature-dependent proton NMR spectrum has been found for [4-methyl-9-(o-trifluoroacetylaminophenyl)-5,8-diazadeca-3,8-dien-2-onato(2–)]nickel(II) indicating a substantial energy barrier for enantiomer interconversion of the chiral complex molecule at moderate temperature. The crystal and molecular structure of this complex has been determined and shows that a fluorine–nickel interaction may be responsible for the inhibited enantiomer interconversion in N-trifluoroacetyl substituted nickel complexes. Crystallographic data: space group P21/c, a= 9.057(2), b= 13.769(4), c= 14.664(4)A, β= 108.43(2)°, final R= 0.0434.

Preparation and Characterization of Optically Active Quadridentate Schiff Base-Titanium(IV) Complexes and the Catalytic Properties of These Complexes on Asymmetric Oxidation of Methyl Phenyl Sulfide with Organic Hydroperoxides

Abstract Titanium(IV) complexes, [TiCl(Schiff base)]2O, with quadridentate Schiff base ligands derived from optically active 1,2-diamines and salicylaldehyde were prepared by treating TiCl4 with an equimolar amount of the Schiff base ligand in wet pyridine. The complexes were characterized by their 13C NMR, absorption, and circular dichroism spectra, and conductivity measurements. The complexes in dichloromethane take a μ-oxo dinuclear structure, and in methanol the complexes break up into a mononuclear species, [Ti(OMe)(MeOH)(Schiff base)]Cl. The complexes catalyze the asymmetric oxidation of methyl phenyl sulfide into the sulfoxide with organic hydroperoxides; the highest enantiomeric excess (ee) was ca. 60%. The kinetics conform to a rate law which is first order in the concentration of each sulfide, hydroperoxide, and complex catalyst.

Preparation and Characterization of Optically Active Schiff Base-Oxovanadium(IV) and -Oxovanadium(V) Complexes and Catalytic Properties of These Complexes on Asymmetric Oxidation of Sulfides into Sulfoxides with Organic Hydroperoxides

Abstract Oxovanadium(V) complexes, VVO (Schiff base)Y (Y=ClO4, Cl, NO3), with quadridentate Schiff base ligands derived from optically active 1,2-diamines and salicylaldehyde or its derivatives were prepared by oxidizing the corresponding VIVO (Schiff base) with CeIV in acetonitrile followed by the addition of HY. The complexes were characterized by their 1H NMR, IR, UV-Vis, and circular dichroism spectra, and electrochemistry. Reversible reduction potentials (V5+/V4+) obtained by cyclic voltammetry for a series of oxovanadium(V) complexes with Schiff base ligands derived from 5-substituted salicylaldehydes showed a linear dependence on the Hammett parameter. Both the vanadium(IV) and the vanadium(V) complexes catalyze asymmetric oxidation of sulfides into the corresponding sulfoxides with organic hydroperoxides. The oxovanadium(IV) complex catalysts gave much better asymmetric induction than the oxovanadium(V) complex catalysts and in most cases the enantiomeric excess (ee) ranged between 20 and 40%. In the reaction of VIVO (Schiff base) with an organic hydroperoxide the complex is finally converted into the oxovanadium(V) species, and a dark brown intermediate is suggested to be the catalytically active species.