Pentadentate Schiff Base Ligand Research Articles

PCET-Driven Reactivity of Neptunyl(VI) Yields Oxo-Bridged Np(V) and Np(IV) Species.

Two unconventional polynuclear complexes of neptunium (Np) featuring mono- -oxo motifs have been accessed by proton-coupled electron transfer (PCET) reactivity involving the dissolution of neptunyl(VI) diacetate dihydrate ( HOAc) in methanol followed by addition of a pentadentate Schiff-base ligand. One complex is a mixed-valent [Np ,Np ,Np ] trimer with two bridging -oxos and the other is a [Np ,Np ] dimer featuring a single -oxo. In both complexes the outer Np centers are capped with terminal oxo ligands. Spectroscopic and spectrokinetic studies aimed at elucidating mechanistic details of complex formation in this system show that intermediate multinuclear [Np (OAc)] species form prior to metal chelation by the ligand; electrolysis experiments demonstrate that production of Np(V) gives rise to asynchronous proton transfer that does not occur otherwise (in the Np(VI) state) as well as condensation with loss of and formation of the polynuclear complexes. We attribute the oxo-deficient nature of these products, with respect to conventional actinyl ([AnO ] ) species, to the reduction/condensation reaction sequence of PCET.

Homo- and heterometallic complexes of Zn(II), {Zn(II)Au(I)}, and {Zn(II)Ag(I)} with pentadentate Schiff base ligands as promising anticancer agents.

Two families of homo- and heterometallic complexes, [Zn2L1(μ-OH)(H2O)2](ClO4)2, [Zn2L2(μ-OH)(H2O)2](ClO4)2, [Zn2L3(μ-OH)(H2O)2](ClO4)2, 1∞[{L1Zn2(μ-OH)}{μ-[Ag(CN)2]}](ClO4), [{L1Zn2(μ-OH)}2{μ-[Au(CN)2]}{[Au(CN)2]2}](ClO4)·H2O, 1∞[{L2Zn2(μ3-OH)}2(H2O){μ-[Ag(CN)2]}](ClO4)3·THF·0.5MeOH, 1∞[{L2Zn2(μ3-OH)}2(H2O){μ-[Au(CN)2]}](ClO4)3·THF·H2O, and 1∞[{L3Zn2(μ-OH)}{μ-[Ag(CN)2]}][Ag(CN)2]·H2O, respectively, have been synthesized and characterized. The Schiff bases used as ligands were obtained by condensation reactions of 2,6-diformyl-p-cresol with N,N-dimethyl-ethylenediamine (HL1), 2-aminomethyl-pyridine (HL2), and 2-aminoethyl-pyridine (HL3), respectively. The cytotoxic/cytostatic and genotoxic effects in cultured human MCF-7 (luminal type A breast cancer), MDA-MB-231 (triple negative breast cancer), HeLa (cervical carcinoma), and Lep-3 (non-tumor embryonal fibroblastoid cells) were studied. The investigations were performed by thiazolyl blue tetrazolium bromide test (MTT test), neutral red uptake cytotoxicity assay, crystal violet staining, hematoxylin and eosin staining, double staining with acridine orange and propidium iodide, AnnexinV/FITC, and Comet assay in short-term experiments (24-72 h, with monolayer cell cultures) as well as by 3D colony-forming method in long-term experiments (28 days, with 3D cancer cell colonies). The results obtained revealed that: (i) applied at a concentration range of 0.1-100 μg mL-1, the compounds investigated decrease in a time- and concentration-dependent manner the viability and/or proliferation of the treated cells; (ii) complexes of {Zn(II)Au(I)} show relatively higher cytotoxic/genotoxic activity and antitumor potential as compared to {Zn(II)Ag(I)}; (iii) some of the complexes demonstrate more pronounced cytotoxic potential than commercially available antitumor agents cisplatin, oxaliplatin, and epirubicin.

Self-assembly of Schiff base anions and the trapping of HO−, O2− and Piv− bridges in a family of Ni3Ln4 complexes: synthesis, structures and magnetic properties

Pentadentate Schiff base ligands bind three NiII–LnIII centres in cyclic order to yield a unique family of propeller-shaped heptanuclear Ni3Ln4 coordination aggregates which show overall antiferromagnetic interactions between the NiII and LnIII ions.

Dinuclear and trinuclear cyano-bridged {DyIIIMIV} (M = W, Mo) single-ion magnets supported by pentadentate Schiff-base ligands

Four cyano-bridged {DyIIIMIV} complexes 1–4, with various structures and magnetic behaviors, were obtained by adjusting the pyridyl N-sites of the pentadentate Schiff-base ligands.

Formation of dimethoxy bridged dinuclear iron(III) complex of pyridoxal Schiff base with iron-catalyzed oxidative C[sbnd]N bond cleavage – Structure, magnetic properties, and DFT calculations

Mononuclear Fe(III) complex [FeL(N3)]·CH3OH, where L2– is the dianion of an unsymmetrical propyl-ethyl pentadentate Schiff base ligand condensed from pyridoxal and N-(2-aminoethyl)propane-1,3-diamine, and dinuclear Fe(III) complex [{FeL’}2(μ-OCH3)2]·2.5CH3OH·H2O, where L’2– is the dianion of tetradentate Schiff base ligand based on pyridoxal and ethylenediamine, have been synthesized and characterized by elemental analysis, X-ray structural analysis, FT-IR and mass spectrometry. Tetradentate pyridoxal Schiff base ligand L’2– of the dinuclear complex was generated in situ via iron-catalyzed oxidative CN bond cleavage of the pentadentate Schiff base. The coordination geometry around the iron(III) centers of the complexes can be described as adistorted octahedron. Magnetic investigations of [FeL(N3)]·CH3OH complex revealed that the octahedral Fe(III) center retains high-spin in the entire temperature range (2–300 K) and enabled the evaluation of the axial zero-field splitting (ZFS) parameter D = –1.78 cm−1, intermolecular exchange parameter zj́ = –1.35 cm−1 and temperature-independent paramagnetism χTIM = 0.00029 cm3mol−1. Magnetic investigations of [{FeL’}2(μ-OCH3)2]·2.5CH3OH·H2O showed that Fe(III) centers are coupled by a strong antiferromagnetic interaction (J = –94.5 cm−1). The experimental magnetic properties of the complexes were compared with the data obtained from the density functional theory (DFT) calculations.

Activity of a New Chromium(III) Complex with a Pentadentate (N3O2) Schiff-Base Ligand in the Reaction of Carbon Dioxide with Propylene Oxide

Activity of a New Chromium(III) Complex with a Pentadentate (N3O2) Schiff-Base Ligand in the Reaction of Carbon Dioxide with Propylene Oxide

Chiral crystallization of a zinc(II) complex.

The compound, {6,6'-dimeth-oxy-2,2'-[(4-azaheptane-1,7-di-yl)bis-(nitrilo-meth-an-yl-idyne)]diphenolato}zinc(II) methanol monosolvate, [Zn(C22H27N3O4)]·CH3OH, at 298 K crystallizes in the ortho-rhom-bic space group Pna21. The Zn atom is coordinated by a penta-dentate Schiff base ligand in a distorted trigonal-bipyramidal N3O2 geometry. The equatorial plane is formed by the two phenolic O and one amine N atom. The axial positions are occupied by two amine N atoms. The distorted bipyramidal geometry is also supported by the trigonality index (τ), which is found to be 0.85 for the mol-ecule. In the crystal, methanol solvent mol-ecule is connected to the complex mol-ecule by an O-H⋯O hydrogen bond and the complex mol-ecules are connected by weak supra-molecular inter-actions, so achiral mol-ecules generate a chiral crystal. The Hirshfeld surface analysis suggests that H⋯H contacts account for the largest percentage of all inter-actions.

Iron(III) and cobalt(III) complexes with pentadentate pyridoxal Schiff base ligand – structure, spectral, electrochemical, magnetic properties and DFT calculations

Three complexes, [FeLCl]Cl·H2O, [Co2L2H3O2]Cl·4.8H2O, and [CoLN3], where L2– is the dianion of an asymmetric propyl-ethyl pentadentate Schiff base ligand condensed from pyridoxal and N-(2-aminoethyl)propane-1,3-diamine, have been synthesized and characterized by elemental analysis, FT-IR and mass spectroscopy. Crystal structures of the complexes along with the Schiff base were determined by X-ray diffraction. 1HNMR spectrum of the Schiff base has been obtained. The coordination polyhedra of all three complexes can be expressed as distorted octahedra. Magnetic investigations of [FeLCl]Cl·H2O complex confirmed high-spin state over the whole temperature range (5–300 K) and allowed the evaluation of the axial zero-field splitting (ZFS) parameter D = –0.97 cm−1, intermolecular exchange parameter zj́ = –0.88 cm−1 and temperature-independent paramagnetism χTIM = 0.00025 cm3mol−1. The low-spin state of Co(III) was stabilized in diamagnetic [Co2L2H3O2]Cl·4.8H2O and [CoLN3] complexes. Redox potentials of the Schiff base and the complexes were investigated by cycling voltammetry on the GC electrode in dry acetonitrile. The experimental magnetic properties of the complexes [FeLCl]Cl·H2O and [CoLN3] were compared with the theoretical ones calculated using density functional theory (DFT).

Synthesis, characterization and biological evaluation of novel octahedral Ru(III) complexes containing pentadentate Schiff base ligands

Synthesis, characterization and biological evaluation of novel octahedral Ru(III) complexes containing pentadentate Schiff base ligands

Spin-crossover in an iron(III) complex showing a broad thermal hysteresis.

A pentadentate Schiff-base ligand 3,5Cl-L2- and NCSe- form an iron(iii) mononuclear complex, namely [Fe(3,5Cl-L)(NCSe)], which shows a thermally induced spin crossover with a broad hysteresis width of 24 K between 123 K (warming) and 99 K (cooling). Analogous complexes of the [Fe(3,5X-L)(Y)] type, where X = Cl or Br and Y = Cl-, N3-, NCS-, and NCSe-, are high-spin over the whole temperature interval.

Synthesis, thermal, photo-physical, and biological properties of mononuclear Yb3+, Nd3+, and Dy3+ complexes derived from Schiff base ligands

Six new lanthanide complexes, [Ln(NO3)2(L1H2)](NO3).xCHCl3.yH2O {Ln = Yb (1), x = 1, y = 2; Nd (3), x = 2, y = 3 and Dy (5), x = 2, y = 2} and [Ln(NO3)2(L2H2)](NO3).xCHCl3 {Ln = Yb (2), x = 1; Nd (4), x = 2 and Dy (6), x = 2}, were prepared by NNNOO-donor pentadentate Schiff base ligands N,N′-bis(3-methoxysalicylidene)−2,2′-diaminoethylamine (L1H2) and N,N′-bis(2-hydroxynaphthalidene)−2,2′-diaminoethylamine (L2H2). These synthesized complexes were characterized by elemental analysis, FT-IR, 1H NMR, UV-vis spectroscopy, and mass spectrometry. The thermal characteristics of complexes were studied by TGA technique. The observation of the fluorescence emission properties of ligands L1H2 and L2H2 and its Ln(III) complexes in DMSO solution at excitation wavelength of 320 nm shows that the ligands favor energy transfers to the emitting energy level of Ln(III) complexes. The computational calculations using density functional theory of ligands and its Yb((III) complexes were performed to obtain optimized molecular geometry, the highest occupied molecular orbital, the lowest unoccupied molecular orbital and other parameters. The Schiff bases, metal salts, and its Ln(III) complexes were screened for their in vitro antibacterial studies against Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, and Salmonella typhi bacterial strains. The results of the antibacterial investigation show that the lanthanide complexes possess more potent bactericide properties than ligands and metal salts.

Lanthanum complexes stabilized by a pentadentate Schiff-base ligand: synthesis, characterization, and reactivity in statistical copolymerization of ε-caprolactone and l-lactide.

Developing catalysts that are capable of catalyzing the copolymerization of ε-caprolactone and l-lactide to give random CL/LA copolymers is of great importance. One-pot reaction of La[N(SiMe3)2]3 with 1 equiv. of LH2 (LH2 = NH(CH2CH2N[double bond, length as m-dash]CHC6H2-3,5-tBu2-2-OH)2) in THF at room temperature, followed by protolysis with one equivalent amount of ROH (R = C6H2-2,6-tBu2-4-CH3, tBu, iPr, Bn, and Et) at 60 °C gave the mono-Schiff-base-ligated lanthanum aryloxide complex LLa(OC6H2-2,6-tBu2-4-CH3)(THF) (1), and lanthanum alkyloxide complexes LLaOtBu(THF) (2), [LLaOiPr]2 (3), [LLaOBn]2 (4), and [LLaOEt]2 (5) in 59-69% isolated yields. These lanthanum complexes were capable of initiating the homopolymerization of l-lactide and rac-lactide with extremely high activity, and the copolymerization of ε-caprolactone (ε-CL) and l-lactide (l-LA) to give statistical CL/LA copolymers via a transesterification reaction.

One-dimensional lanthanide coordination polymers supported by pentadentate Schiff-base and diphenyl phosphate ligands: single molecule magnet behavior and photoluminescence

Four one-dimensional lanthanide coordination polymers constructed from Ln2 dimers showing single molecule magnet behavior and photoluminescence are reported.

Synthesis and physicochemical, DFT, thermal and DNA-binding analysis of a new pentadentate N3S2 Schiff base ligand and its [CuN3S2]2+ complexes.

A new N3S2 pentadentate Schiff base ligand derived from 5-bromothiophene-2-carbaldehyde, (E)-N1-((5-bromothiophen-2-yl)methylene)-N2-(2-((E)-((5-bromothiophen-2-yl)-methylene amino) ethyl ethane-1,2-diamine, is prepared. The ligand and its complexes are subjected to extensive physical and theoretical analyses and the results are consistent with their predicted compositions. Dicationic Cu(ii) complexes ([CuN3S2]X2) with a coordination number of 5 are proposed on the basis of the spectral data with N3S2 serving as a pentadentate ligand. The prepared complexes display a square pyramidal geometry around the Cu(ii) center. TG shows different thermal behavior for the N3S2 ligand and its complexes. Solvatochromism of the complexes is promoted by the polarity of the solvent used. A one-electron transfer Cu(ii)/Cu(i) reversible redox reaction is promoted by CV. SEM and EDS of the free ligand and its complexes support the morphology and composition changes observed upon the complexation of Cu(ii). As an outstanding goal to develop anticancer new metal chemotherapy, preliminary studies of the binding of the desired complexes with DNA were carried out, as it is through judging the strength of interactions that a future drug can be designed and synthesized. The viscosity and absorption results obtained for complex 1 indicated its enhanced CT-DNA binding properties as compared to those of complex 2 with Kb values of 3.2 × 105 and 2.5 × 105 M−1, respectively.

Synthesis, crystal structure and antibacterial activity of manganese(III) and cobalt(III) complexes containing pentadentate Schiff bases of a common amine and thiocyanate

Four new mononuclear Schiff base manganese(III) and cobalt(III) complexes viz. [Mn(L1)(NCS)] (1), [Mn(L2)(NCS)] (2), [Co(L3)(NCS)] (3), and [Co(L4)(NCS)]·0.5CH3OH·0.5H2O (4), containing thiocyanate as a common pseudohalide ion are reported. The pentadentate Schiff base ligands H2L1, H2L2, H2L3, and H2L4 were obtained by the condensation of substituted salicylaldehydes with N-(3-aminopropyl)-N-methylpropane-1,3-diamine. The syntheses of the complexes have been achieved by the reaction of manganese(II) perchlorate or cobalt(II) perchlorate with the respective Schiff bases in the presence of thiocyanate in methanol medium. Complexes 1–4 have been characterized by microanalytical, spectroscopic, single-crystal X-ray diffraction and other physicochemical studies. Structural studies reveal that 1–4 adopt nearly similar structures containing the MN4O2 (M = Mn, Co) chromophore in which each central M(III) ion adopts a distorted octahedral geometry. Weak intermolecular H-bonding interactions are operative in these complexes to bind the molecular units. The antibacterial activity of 1–4 and their constituent Schiff bases has been tested against some common bacteria.

Spin Crossover in Three Mononuclear Iron (III) Schiff Base Complexes

The synthesis, crystal structure, and magnetic properties of three new mononuclear complexes [Fe(R-LA)(L1)](BPh4), where R-LA2− is a doubly deprotonated pentadentate Schiff base ligand and L1 is a monodentate benzimidazole or furopyridine ligand, are reported. Ligand- and anion-driven changes in crystal structures and magnetic behavior were investigated in terms of the magnetic susceptibility measurements and theoretical calculations.

High-Spin Mononuclear Iron(III) Complexes with Pentadentate Schiff Base Ligands: Structural Analysis and Magnetic Properties.

Various substituted 2-hydroxybenzophenones were combined with aliphatic linear triamines to form pentadentate Schiff base ligands. Twelve new iron(III) complexes with the general formula [Fe(Ln )X].mCH3 CN (n=1-10; X=N3- , NCS- or NCSe- ; m=0-2) have been synthesized, and spectrally as well as structurally characterized. The structural analysis revealed a notable dependence of coordination polyhedra deformation as well as the spatial configuration of donor atoms on the length and symmetry of the Schiff base ligands. The magnetic properties of the compounds were investigated and the permanent high-spin state (S=5/2) for all reported compounds was established, and allowed calculation of zero-field-splitting parameters as well as coupling constants, which were further confirmed with DFT calculations. The solid-state EPR spectra were recorded at 293 K and 98 K, and in accordance with the magnetic measurements, showed a high-spin state in the measured temperature range.

Synthesis, crystal structure, and antimicrobial activity of a series of cobalt(III) Schiff base complexes

A series of six new mononuclear Schiff base complexes, 1–6 of cobalt(III) of the general formula, [CoLX] or its adduct with methanol, is reported. The pentadentate Schiff base ligand (H2L) was obtained by the condensation of N-(3-aminopropyl)-N-methylpropane-1,3-diamine with 1-(2-hydroxyphenyl)ethanone (H2L1) or 1-(2-hydroxyphenyl)propan-1-one (H2L2). X stands for the pseudohalides, N3–, N(CN)2− , and NCS–. The complexes have been synthesized by the reaction of equimolar amounts of cobalt(II) nitrate with H2L1 or H2L2 in the presence of the respective pseudohalide in methanol medium. All the complexes have been characterized by microanalytical, spectroscopic, single crystal XRD (except 3), and other physicochemical studies. Structural studies reveal that the central Co(III) ion in 1, 2, 4, 5, and 6 adopts a distorted octahedral geometry with a CoN4O2 chromophore. Weak intermolecular H-bonding and/or π-interactions are operative in these complexes to bind the molecular units. The antimicrobial activity of all the complexes and their constituent Schiff bases has been tested against some common bacteria and fungi.

Oxidation of Alcohols to Carbonyl Compounds Catalyzed by Oxo-Bridged Dinuclear Cerium Complexes with Pentadentate Schiff-Base Ligands under a Dioxygen Atmosphere

Ionic mononuclear and neutral dinuclear complexes of cerium(III) 3-L1–3-L9 bearing a series of dianionic pentadentate Schiff-base ligands were synthesized, characterized, and used as catalysts for N-oxyl radical-free aerobic alcohol oxidation. Reactions of Ce(NO3)3·6H2O with o-tert-butyl-substituted sterically hindered ligands NH(CH2CH2N═CHC6H2-3-(tBu)-5-R2-2-OH)2 (for L1H2, R2 = tBu; for L2H2, R2 = OMe; and for L3H2, R2 = H) in the presence of triethylamine afforded the corresponding anionic cerium complexes [HNEt3][Ce(L1–3)(NO3)2] (3-L1–3-L3), whereas complexation with sterically less hindered ligands, such as NH(CH2CH2N═CHC6H2-3-R1-5-R2-2-OH)2 (for L4H2, R1 = OMe and R2 = H; for L5H2, R1 = H and R2 = tBu; for L6H2, R1 = H and R2 = OMe; for L7H2, R1 = H and R2 = H; for L8H2, R1 = H and R2 = NO2; and for L9H2, R1 = tBu and R2 = NO2), afforded neutral dinuclear complexes [Ce(L4–9)(NO3)]2 (3-L4–3-L9). Among these newly prepared complexes, complex 3-L1 was selected as the best catalyst for oxidizing primary...

Impact of Substituent Variation on the Presence of Thermal Spin Crossover in a Series of Mononuclear Iron(III) Schiff Base Complexes with Terminal Pseudohalido Co-ligands.

A series of novel iron(III) complexes of the general formula [Fe(L)X] (where L is a dianion of pentadentate Schiff base ligand N,N'-bis({2-hydroxy-3,5-dimethylphenyl}phenyl)methylidene-1,6-diamino-3-azapentane=H2 L1 for 1 and 2; N,N'-bis({2-hydroxy-3-ethoxyphenyl}methylidene)-1,6-diamino-3-azapentane=H2 L2 for 3 and 3⋅C3 H6 O) and X is terminal pseudohalido ligand (X=N3 for 1, X=NCS for 2, and X=NCSe for 3 and 3⋅C3 H6 O) were synthesized and thoroughly characterized. Magnetic measurements revealed the above room temperature spin crossover for isomorphic complexes 1 and 2 (T1/2 =441 K and T1/2 =435 K, respectively), whereas the solvent-free complex 3 showed a half complete spin crossover (T1/2 =250 K), which was detected by variable temperature crystallography as well. On the other hand, solvated complex 3⋅C3 H6 O exhibited permanent high spin state behaviour and either recrystallization or in situ thermal desolvation converts 3⋅C3 H6 O to solvent-free and spin-crossover-active form 3. Magnetic properties of all the reported complexes were also supported by EPR spectroscopy experiments and in addition, DFT and ab initio calculations were employed for the evaluation of the g-factor and zero field splitting parameters.