Synthesis,spectral, thermal,potentiometric and antimicrobial studies of transition metal complexes of tridentate ligand

A series of metal complexes of Cu(II), Ni(II), Co(II), Fe(III) and Mn(II) have been synthesized with newly synthesized biologically active tridentate ligand. The ligand was synthesized by condensation of dehydroacetic acid (3-acetyl-6-methyl-(2H) pyran-2,4(3H)-dione or DHA), o-phenylene diamine and fluoro benzaldehyde and characterized by elemental analysis, molar conductivity, magnetic susceptibility, thermal analysis, X-ray diffraction, IR, ¹H-NMR, UV–Vis spectroscopy and mass spectra. From the analytical data, the stoichiometry of the complexes was found to be 1:2 (metal:ligand) with octahedral geometry. The molar conductance values suggest the non-electrolyte nature of metal complexes. The IR spectral data suggest that the ligand behaves as a dibasic tridentate ligand with ONN donor atoms sequence towards central metal ion. Thermal behaviour (TG/DTA) and kinetic parameters calculated by the Coats–Redfern and Horowitz–Metzger method suggest more ordered activated state in complex formation. To investigate the relationship between stability constants of metal complexes and antimicrobial activity, the dissociation constants of Schiff bases and stability constants of their binary metal complexes have been determined potentiometrically in THF–water (60:40%) solution at 25 ± 1 °C and at 0.1 M NaClO₄ ionic strength. The potentiometric study suggests 1:1 and 1:2 complexation. Antibacterial and antifungal activities in vitro were performed against Staphylococcus aureus, Escherichia coli and Aspergillus niger, Trichoderma, respectively. The stability constants of the metal complexes were calculated by the Irving–Rosotti method. A relation between the stability constant and antimicrobial activity of complexes has been discussed. It is observed that the activity enhances upon complexation and the order of antifungal activity is in accordance with stability order of metal ions.     

Total synthesis of attenols A and B

A concise approach for the total synthesis of attenols A and B is described involving MacMillan’s α-aminooxylation, Evan’s asymmetric alkylation, Brown’s allylation, and spiro-ketalization as key steps. This approach has successfully demonstrated the α-aminooxylation protocol for the construction of the anti-1,3-diol unit.     

Synthesis,characterization and antimicrobial studies of novel ONO donor hydrazone Schiff base complexes with some divalent metal (II) ions

Metal complexes of two general formulae [M(L)(Cl)(H₂O)₂] [M = Mn(II), Co(II), Ni(II) and Cu(II)] and [M(L)(H₂O)] [M = Zn(II) and Cd(II)] with pyrazine-2-carbohydrazone of 2-hydroxy-5-methylacetophenone (H₂L) are synthesized and characterized by microanalytical, thermal, magnetic susceptibility measurement, spectroscopic (IR, ¹H NMR, ¹³C NMR), mass, molar conductance, X-ray powder diffraction, ESR and SEM studies. While the molar conductance measurements in DMSO indicated their non-electrolytic nature, the spectroscopic studies confirmed a tridentate ONO donor behaviour of the ligand towards the central metal ion. Based on the physico-chemical studies monomeric octahedral geometry around Mn(II), Co(II), Ni(II) and Cu(II) ions (i.e. for the first series of complexes) whereas tetrahedral to Zn(II) and Cd(II) ions (i.e. for the second series of complexes) are suggested. Based on the thermal behavior of the complexes, various kinetic and thermodynamic parameters were evaluated using Coats-Redfern method. The ligand and its metal complexes were screened for in vitro antibacterial and antifungal activity against Gram +ve S. aureus, B. subtilis and Gram –ve E. coli and S. typhi. and fungal strains, C. albicans and A. niger. The observed data infer promising biological activity of some of these complexes compared the parent ligand against all bacterial and fungal species.     

Towards the rational design of palladium-N-heterocyclic carbene catalysts by a combined experimental and computational approach

A combined experimental and computational approach towards the development of Pd-NHC catalysts is described. A range of benzimidazolylidinium ligands incorporating electron-rich and electron-poor substituents were prepared and evaluated in the Suzuki reaction. The most electron-rich ligand showed the highest catalytic activity. Based on this information, the first alkyl-alkyl Negishi cross-coupling reaction protocol was developed. Evaluation of N,N′-diaryl-(4,5-dihydro)imidazolylilidinium ligands showed a strong dependence on the steric topography around the metal centre. A computational study of the most active ligand in the Negishi reaction, its Pd(0) and PdCl₂-complexes and related structures were modelled at the B3LYP/DZVP and HF/3-21G levels of theory. The potential energy hypersurfaces flattened with increase in ligand size. Binding energies were computed for carbene/Pd(0) adducts (in the range ∼31-40 kcal mol⁻¹), roughly double that for PH₃ (∼16 kcal mol⁻¹). Weak intramolecular interactions were found using AIM analyses.     

Syntheses,characterization and X-ray crystal structures of Co(III) and Mn(II) complexes of pyrimidine derived Schiff base ligands

Two pyrimidine based NNS tridentate Schiff base ligands S-methyl-3-((2-S-methyl-6-methyl-4-pyrimidyl)methyl)dithiocarbazate [HL₁] and S-benzyl-3-((2-S-methyl-6-methyl-4-pyrimidyl)methyl)dithiocarbazate [HL₂] have been synthesised by 1:1 condensation of 2-S-methylmercapto-6-methylpyrimidine-4-carbaldehyde and S-methyl/S-benzyl dithiocarbazate. One Co(III) and one Mn(II) complex of HL₁ and one Mn(II) complex of HL₂ have been prepared and characterized by elemental analyses, molar conductivities, magnetic susceptibilities and spectroscopic studies. All the bis-chelate complexes have a distorted octahedral arrangement with an N₄S₂ chromophore around the central metal ion. Each ligand molecule binds the metal ion using pyrimidyl nitrogen, azomethine nitrogen and the thiolato sulfur atoms. In the free ligand moieties, the pyrimidine nitrogen atoms, azomethine nitrogen atoms and thione sulfur atoms are in EEE orientation to each other. During chelation, all the donor sites of the ligands are reoriented to ZEZ configuration in order to facilitate the chelation process. In all the complexes, the respective ligand molecule functions as the monoanionic tridentate one. All complexes were analyzed by single crystal X-ray diffraction and significant differences concerning the distortion from octahedral geometry of the coordination environment were observed.     

The use of a new carboranylamidophosphite ligand in the asymmetric Pd-catalysed allylic alkylation in organic solvents and supercritical carbon dioxide

A novel P-monodentate ligand based on carboranyl alcohol and (S)-2-(anilinomethyl)pyrrolidine provides high enantioselectivities (ee’s up to 95%) in the Pd-catalyzed allylic alkylation of (E)-1,3-diphenylallyl acetate. The first example of the Pd-catalysed allylic alkylation in supercritical carbon dioxide is also described.     

New 5-bromo-2-hydroxybenzaldehyde S-ethylisothiosemicarbazone and its mixed-ligand Cu(II) complex with imidazole: synthesis, characterization and DFT calculation

A new Schiff base ligand of 5-bromo-2-hydroxybenzaldehyde S-ethyl-isothiosemicarbazone (H₂L) was synthesized and its mixed-ligand Cu(II) complex was also prepared by reaction of Cu(NO₃)₂·3H₂O with H₂L and imidazole. Their structures were fully characterized by elemental analysis, FT-IR, molar conductivity and UV-Vis methods. The analytical data suggest that the metal, H₂L and imidazole ratios in the Schiff base complex are 1:1:1. Single crystal diffraction was also used to better understand the molecular structure of the Cu(II) complex. The results of physico-chemical analyses of the Schiff base complex reveal the coordination geometry around the central atom is square planar. The H₂L ligand (NNO donor) is coordinated to the metal center as a tridentate bionegatively agent. Another position of the square planar geometry is occupied by the imidazole ligand. Furthermore, computational studies of the new complex were performed by carrying out DFT calculations. Geometry optimization and natural band analysis of the complex is discussed in further detail.      

Synthesis and structures of cadmium(II) complexes of a series of multinucleating N/S donor ligands

The coordination chemistry of a series of bis-bidentate ligands with cadmium(II) ions has been investigated. The ligands, containing two N,S-donor chelating (pyrazolyl/thioether) fragments, have afforded complexes of a variety of structural types (dinuclear M₂L₂ ‘mesocate’ complexes, a one-dimensional chain coordination polymer and a simple mononuclear complex) according to whether the bis-bidentate ligands act as bridges spanning two metal ions, or a tetradentate chelate to a single metal ion. The p-phenylene and m-biphenyl spaced ligands L¹ and L³ form dinuclear M₂L₂ complexes where the ligands are arranged in a ‘side-by-side’ fashion. In contrast the m-phenylene spaced ligand L² forms a one-dimensional coordination polymer where the ligands adopt a highly folded conformation. The 1,8-naphthalene spaced ligand L⁴ adopts a tetradendate chelating mode and affords a simple mononuclear complex.     

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.     

Crystal field aspects of vibrational spectra: VII. Derivation of a spectrochemical series of ligands from infrared spectra

The infrared spectra (700-200 cm^?1) of 52 complexes of general formula Na[ML₃] or [ML₂B] (where M = a divalent metal ion of the first transition series; L = α-thenoyltrifluoroacetonate or benzoyltrifluoroacetonate anion; B = 2H₂O, 2NH₃, 2pyridine, 2,2'-bipyridine or 1,10-phenanthroline) are discussed. Within each series of complexes with common L and B, the IR band near 400 cm^?1 which exhibits maximum sensitivity to the coordinated metal ion (the sensitivity being in the sequence of crystal field stabilization energies) and which generally occurs in a region free from ligand absorption, is assigned to the metal—oxygen stretching frequency (v(M—O)). For each series of complexes with common M and L, the magnitude of v(M—O) decreases progressively with increasing ligand field strength of B. This relationship enables the coordinated bases, B, to be arranged in a spectrochemical series which is practically identical with that obtained from electronic spectra.     

Stereoselective synthesis of β-glycosyl esters of cis-cinnamic acid and its derivatives using unprotected glycosyl donors

The β-glycosyl esters of cis-cinnamic acid were synthesized directly using Hannesian’s unprotected glycosyl donor and the carboxylic acid in toluene. This protocol does not require protecting groups on the glycosyl donors, and high stereoselectivity was achieved. The first synthesis of a potent allelochemical, 1-O-cis-cinnamoyl-β-d-glucopyranose, is also described.     

Observation of Main‐Group Tricarbonyls [B(CO)3] and [C(CO)3]+ Featuring a Tilted One‐Electron Donor Carbonyl Ligand

A combined experimental and theoretical study on the main‐group tricarbonyls [B(CO)₃] in solid noble‐gas matrices and [C(CO)₃]⁺ in the gas phase is presented. The molecules are identified by comparing the experimental and theoretical IR spectra and the vibrational shifts of nuclear isotopes. Quantum chemical ab initio studies suggest that the two isoelectronic species possess a tilted η¹(μ₁‐CO)‐bonded carbonyl ligand, which serves as an unprecedented one‐electron donor ligand. Thus, the central atoms in both complexes still retain an 8‐electron configuration. A thorough analysis of the bonding situation gives quantitative information about the donor and acceptor properties of the different carbonyl ligands. The linearly bonded CO ligands are classical two‐electron donors that display classical σ‐donation and π‐back‐donation following the Dewar–Chatt–Duncanson model. The tilted CO ligand is a formal one‐electron donor that is bonded by σ‐donation and π‐back‐donation that involves the singly occupied orbital of the radical fragments [B(CO)₂] and [C(CO)₂]⁺.     

Structural and electronic properties of small platinum metallorganic complexes

A theoretical first-principles study of Pt _n (ligand) _m (n = 1–3) metallorganic complexes is performed, by varying the number of metal atoms and the nature and number of organic coordinate ligands (specifically, vinylic and arylic ligands). For each system, the nature of the bonding, the structure and the energetics of the metal/organic-species interaction are analyzed to derive information on the growth of coated metal clusters in solution. It is found that two régimes can be distinguished: a “coordinatively saturated” régime, in which the ratio among the number of ligands and the number of metal atoms is high and a ligand/organic π-interaction mode is preferred, and a “coordinatively unsaturated” régime, in which the ligand/metal ratio is low and a ligand/organic σ-interaction mode is preferred. Reactive channels, such as oxidative insertion of Pt into C–H bonds with the corresponding formation of platinum hydride species, can be opened in the latter régime.     

Synthesis and characterization of an unsymmetric salicylaldimine ligand derived from 1-(2-Aminoethyl) piperazine and investigation of its analytical properties for the extraction and preconcentratoion of some divalent cations

The synthetic, structural, spectroscopic and analytical properties of steric hindered Schiff-base ligand [N-(3,5-di-tert-butylsalicylaldimine)-1-(2-Aminoethyl) piperazine (HL)] and its mononuclear Cu(II), Co(II) and Ni(II) complexes are described. The new unsymmetric steric hindered Schiff base ligand containing a donor set of NONO was prepared by the reaction of 1-(2-Aminoethyl) piperazine with 3,5-di-tert-butylsalicylaldehyde. Certain metal complexes of this ligand were synthesized by treating an ethanolic solution of the ligand with an equimolar amount of metal salts. The ligand and its metal complexes were characterized by FT-IR, UV-Vis, ¹H NMR, elemental analysis, molar conductivity and magnetic susceptibility techniques. The reaction of this ligand in a 1:2 mole ratio with metal acetate afforded mononuclear metal complexes. The molar conductivity (??_M) values of the metal complexes of Ni(II), Co(II) and Cu(II) were in the range of 6.4 to 9.8 ??^?1 cm² mol^?1 at room temperature. Preconcentration and separation of Cu²⁺ from aqueous solution using N-(3,5-di-tert-butylsalicylaldimine)-1-(2-Aminoethyl) piperazine (HL) as a new extractant were studied. The extraction experiments were carried out at various pHs. While Cu²⁺ showed the highest extractability and selectivity at pH 7.0, extractions of Co²⁺ and Ni²⁺ were unsuccessful due to precipitate formation.     

SEVERAL COORDINATION MODES OF 5-AMINO-1,3,4-THIADIAZOLE-2-SULFONAMIDE (HATS) WITH Cu(II), Ni(II) AND Zn(II): MIMETIC TERNARY COMPLEXES OF CARBONIC ANHYDRASE-INHIBITOR

Abstract

The coordination properties of 5-amino-1,3,4-thiadiazole-2-sulfonamide (Hats) with Cu(II), Ni(II) and Zn(II) ions, are analyzed. Although the ligand presents several donor atoms, we have only observed three coordination behaviors: (i) as a monodentate ligand through the N_sulfonamido atom, (ii) as a bridging ligand linking the metal ions through the N_sulfonamido and N_thiadizole atoms and (iii) as a bridging ligand linking metal ions through the N and O atoms of the sulfonamidate group. It is noteworthy that coordination mode (iii) is observed for the first time in heterocyclic sulfonamides complexes. In addition, the conformation of the Hats as counter-ion is analyzed and compared with the conformations that the ligand adopts in metal complexes.     

Understanding the electronic and π-conjugation roles of quinoline on ligand substitution reactions of platinum(II) complexes

A kinetic and mechanistic study of chloride substitution by thiourea nucleophiles, namely thiourea, N-methylthiourea, N,N-dimethylthiourea and N,N,N′,N′-tetramethylthiourea in the complexes chlorobis-(2-pyridylmethyl)amineplatinum(II) (Pt1), chloro N-(2-pyridinylmethyl)-8-quinolinamineplatinum(II) (Pt2), chloro N-(2-pyridinylmethylene)-8-quinolinamineplatinum(II) (Pt3) and chlorobis(8-quinolinyl)amineplatinum(II) (Pt4) was undertaken under pseudo-first-order conditions using UV–visible spectrophotometry. The study showed that lability of the chloro leaving group is dependent on the strength of π-interactions between the filled dπ-orbitals of the metal and the empty π*-orbitals of the chelating ligand in the following manner: Pt1 > Pt3 > Pt2 > Pt4. Introduction of the quinoline moiety within the non-labile chelated framework of the Pt(II) complexes results in a more electron-rich metal centre which retards the approach of the nucleophile through repulsion. Moreover, the net σ-effect of the ligand moiety plays a significant role in controlling the reactivity of the complexes. The experimental results are interpreted with the aid of computational data obtained by density functional theory (B3LYP(CPCM)/LANL2DZp//B3LYP/-LANL2DZp) calculations. The mode of substitution remains associative as supported by negative entropies and the dependence of the second-order rate constants on the concentration of entering nucleophiles.     

Intramolecular CH/OH Bond Cleavage with Water and Alcohol Using a Phosphine‐Free Ruthenium Carbene NCN Pincer Complex

Transition metal complexes that exhibit metal–ligand cooperative reactivity could be suitable candidates for applications in water splitting. Ideally, the ligands around the metal should not contain oxidizable donor atoms, such as phosphines. With this goal in mind, we report new phosphine‐free ruthenium NCN pincer complexes with a central N‐heterocyclic carbene donor and methylpyridyl N‐donors. Reaction with base generates a neutral, dearomatized alkoxo–amido complex, which has been structurally and spectroscopically characterized. The tert‐butoxide ligand facilitates regioselective, intramolecular proton transfer through a C?H/O?H bond cleavage process occurring at room temperature. Kinetic and thermodynamic data have been obtained by VT NMR experiments; DFT calculations support the observed behavior. Isolation and structural characterization of a doubly dearomatized phosphine complex also strongly supports our mechanistic proposal. The alkoxo–amido complex reacts with water to form a dearomatized ruthenium hydroxide complex, a first step towards phosphine‐free metal–ligand cooperative water splitting.     

Cu(II), Co(II), Ni(II), Mn(II), and Fe(II) metal complexes containing N,N′-(3,4-diaminobenzophenon)-3,5-But2-salicylaldimine ligand: Synthesis,structural characterization,thermal properties,electrochemistry, and spectroelectrochemistry

The synthesis, structure, spectroscopic and electro-spectrochemical properties of steric hindered Schiff-base ligand [N,N′-(3,4-benzophenon)-3,5-Bu^t₂-salicylaldimine (LH₂)] and its mononuclear Cu(II), Co(II), Ni(II), Mn(II) and Fe(II) complexes are described in this work. The new dissymmetric steric hindered Schiff-base ligand containing a donor set of NONO was prepared through reaction of 3,4-diaminobenzophenon with 3,5-Bu^t₂-salicylaldehyde. Certain metal complexes of this ligand were synthesized by treating an ethanolic solution of the ligand with an equimolar amount of metal salts. The ligand and its complexes were characterized by FT-IR, UV–vis, ¹H NMR, elemental analysis, molar conductivity and thermal analysis methods in addition to magnetic susceptibility, electrochemistry and spectroelectrochemistry techniques. The tetradentate and mononuclear metal complexes were obtained by reacting N,N′-(3,4-benzophenon)-3,5-Bu^t₂-salicylaldimine (LH₂) with some metal acetate in a 1:1 mole ratio. The molar conductance data suggest metal complexes to be non-electrolytes.     

Functional mimics of copper enzymes. Synthesis and stereochemical properties of the copper(II) complexes of a trinucleating ligand derived from l-histidine

The ligand piperazine-1,4-bis[4-(N-(1-acetoxy-3-(1-methyl-1H-imidazol-4-yl))-2-propyl)-N-(1-methyl-1H-imidazol-2-ylmethyl)aminobutyl] (PHI) was synthesized by a multistep procedure starting from N^τ-methyl-l-histidine, piperazine-1,4-bis[4-(4-oxo-4-butanoic) acid] and 1-methyl-1H-imidazole-2-carbaldehyde. This ligand has two potential tridentate, aminobis(imidazole) (A sites), and one bidentate, piperazine (B site), binding sites for metal ions and was employed for the synthesis of the binuclear [Cu₂PHI]⁴⁺ and the trinuclear [Cu₃PHI]⁶⁺ complexes, the latter of which features a coordination environment mimicking that present in the trinuclear clusters of the blue copper oxidases. For comparison purposes, the mononucleating ligand l-N^α-(1-methyl-1H-imidazol-2-ylmethyl)-N^τ-methylhistidine methyl ester (IH) and its complex [CuIH]²⁺ have been also prepared. These copper(II) model complexes are the first reported which are directly derived from chiral l-histidine residues. A detailed analysis of the UV–vis, CD and EPR spectra of the complexes has established that the Cu(II) centers bound to PHI A sites are square-pyramidal in solution, with the amino and one imidazole donor in the equatorial plane and the additional imidazole group bound axially. This arrangement implies the adoption of an unusual conformation of λ chirality by the l-histidine residue and is determined by the attempts to minimize steric interference between the substituents at the tertiary amine donor group and the histidine residue bearing the C-α substituent acetoxymethylene group of the bound PHI ligand. For the less sterically crowded secondary amine group of the bound IH ligand, the histidine C-α substituent can occupy a pseudoaxial position, so that in the complex [CuIH]²⁺ the `normal' arrangement with three equatorial nitrogen donors and δ chirality in the l-histidine chelate ring occurs.     

Synthesis,characterization, antimicrobial and diuretic study of Mg(II), Mn(II), Fe(II) and VO(II) complexes of chemotherapeutic importance

The synthesis, characterization and diuretic activity of four new biologically active complexes of Mg(II) and VO(II) with bidentate Schiff base ligand acetazolamide–salicylaldimine (L) obtained from the inserted condensation of 5-acetamido-1,3,4-thiadiazole-2-sulphonamide (acetazolamide) with salicylaldehyde in a 1:1 molar ratio have been reported. Using this bidentate ligand complexes of Mg(II), Mn(II), Fe(II) and VO(II) with general formula ML₂ have been synthesized. The synthesized complexes were characterized by several techniques using elemental analysis, FT-IR, electronic spectra, TGA, mass, particle size analysis and molar conductance measurements. The elemental analysis data suggest the stoichiometry to be 1:2 [M:L]. The molar conductance measurements suggest non-electrolytic nature of the complexes. Infrared spectral data agreed with the coordination to the central metal ion through deprotonated phenolic oxygen and azomethine nitrogen atoms. On the basis of spectral studies, octahedral geometry is suggested for Mg(II), Mn(II), Fe(II) and square pyramidal geometry is suggested for VO(II) complexes. The pure drug, synthesized ligand and metal(II) complexes were screened for their antimicrobial activities against Eschericia coli, Bacillus subtilis, Aspergillus niger and Aspergillus flavous. The results show that the metal complexes were more active than the ligand and pure drug against these microbial species as expected. The ligand and its Mg(II) complexes was screened for their diuretic activity also.