Electrochemical investigations on pentagonal bipyramidal complexes of manganese(II), iron(II), and cobalt(II) with a heptadentate Schiff-base ligand in non-aqueous solvents

Summary The voltammetric properties of the complexes formed by manganese(II), iron(II), and cobalt(II) ions with a heptadentate Schiff-base ligand have been investigated by cyclic voltammetry and controlled-potential coulometry at mercury and platinum electrodes in acetonitrile and dimethyl sulfoxide solvents.All the species undergo a single one-electron oxidation process leading to the corresponding stable metal(III) complexes which have been isolated and characterized.The cathodic behaviour of manganese(II) and iron(II) derivatives is very similar, in that the less cathodic process occurs at nearly equal potential values, indicating that the ligand moiety is reduced rather than the metal centre. The one-electron reduction process of the cobalt(II) complex leads to the corresponding cobalt(I) derivative, stable in the electrolysis solution.     

First-row transition metal complexes of the strongly donating pentadentate ligand PY4Im

The new ligand PY4Im, which incorporates an axial N-heterocyclic carbene and four equatorial pyridine donors, is readily prepared on a multigram scale. Six-coordinate first row transition metal complexes of the general formula [(PY4Im)M(MeCN)](2+) (M = Fe, Co, Ni, Cu), where the PY4Im ligand coordinates in a square pyramidal pentadentate fashion, have been prepared. Structural, spectroscopic, and electrochemical characterization of these compounds provides evidence that PY4Im is a strongly donating ligand that favors the formation of low-spin complexes. Chemical oxidation of the iron(II) complex provides a low spin iron(III) complex, which has also been structurally and spectroscopically characterized. In the case of manganese(II), the PY4Im ligand is unable to either enforce a low-spin state or fully accommodate the metal ion. Rather, the ligand binds in a tridentate, face-capping mode.     

Solution and structural characterization of iron(II) complexes with ortho-halogenated phenolates: insights into potential substrate binding modes in hydroquinone dioxygenases

The new ligand cis,cis-1,3,5-tris-(E)-(tolylideneimino)cyclohexane (TACH-o-tolyl) forms a 1:1 complex with iron(II). Addition of substituted phenolates forms 1:1:1 ligand:iron:phenolate complexes, which have been characterized both in the solid state and in solution. There is complete binding of the phenolate to the complex only when there are ortho-halogens on the phenolate. The tertiary complexes with ortho-halo-substituted phenolates exhibit short Fe-halogen distances, and the complex containing a non-coordinating but similarly sized ortho-methyl phenolate has a significantly different conformation and coordination geometry. Therefore, it is likely that the metal-halogen interaction stabilizes the complexes. The iron(II)-halogen interaction in these complexes may explain the substrate specificity of PcpA and LinE, enzymes that preferentially bind phenols and hydroquinones containing halogen substituents in ortho positions.     

Tuning the redox properties of manganese(II) and its implications to the electrochemistry of manganese and iron superoxide dismutases

Superoxide dismutases (SODs) catalyze the disproportionation of superoxide to dioxygen and hydrogen peroxide. The active metal sites of iron and manganese superoxide dismutases are structurally indistinguishable from each other. Despite the structural homology, these enzymes exhibit a high degree of metal selective activity suggesting subtle redox tuning of the active site. The redox tuning model, however, up to now has been challenged by the existence of so-called cambialistic SODs that function with either metal ion. We have prepared and investigated two sets of manganese complexes in which groups of varying electron-withdrawing character, as measured by their Hammett constants sigma Para, have been introduced into the ligands. We observed that the Mn(III)/Mn(II) reduction potential for the series based on 4'-X-terpyridine ligands together with the corresponding values for the iron-substituted 4'-X-terpyridine complexes changed linearly with sigma Para. The redox potential of the iron and manganese complexes could be varied by as much as 600 mV by the 4'-substitution with the manganese complexes being slightly more sensitive to the substitution than iron. The difference was such that in the case where the 4'-substituent was a pyrrolidine group both the manganese and the iron complex were thermodynamically competent to catalytically disproportionate superoxide, making this particular ligand "cambialistic". Taking our data and those available from the literature together, it was found that in addition to the electron-withdrawing capacity of the 4'-substituents the overall charge of the Mn(II) complexes plays a major role in tuning the redox potential, about 600 mV per charge unit. The ion selectivity in Mn and FeSODs and the occurrence of cambialistic SODs are discussed in view of these results. We conclude that the more distant electrostatic contributions may be the source of metal specific enzymatic activity.     

Synthesis, characterization and antimicrobial activity of 3d transition metal complexes of a biambidentate ligand containing quinoxaline moiety

A new series of oxovanadium(IV), chromium(III), manganese(II), iron(III), cobalt(II), nickel(II), and copper(II) complexes of the 3-hydrazino quinoxaline-2-one (HQO) were prepared and characterized. The ligand exhibits biambidenticity. It behaves as a bidentate ON donor in oxovanadium(IV), iron(III) and copper(II) complexes and as a bis bidentate ONNN donor in chromium(III), manganese(II), cobalt(II) and nickel(II) complexes. The nature of bonding and the stereochemistry of the complexes have been deduced from elemental analyses, thermal, infrared, ¹H NMR, electronic spectra, magnetic susceptibility and conductivity measurements. An octahedral geometry was suggested for all the complexes. All the complexes show subnormal magnetic moments. The ligand, HQO, and its complexes were tested against one strain Gram +ve bacteria (Staphylococcus aureus), Gram −ve bacteria (Escherichia coli). The prepared metal complexes exhibited higher antimicrobial activities than the parent ligand.     

Synthesis and characterization of transition metal complexes of N′-[(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)methylene]thiophene-2-carbohydrazide

A hydrazone ligand (HL) containing the thiophene moiety has been prepared via condensation of thiophene-2-carbohydrazide with 1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazole-4-carbaldehyde. The complexes of copper(II), nickel(II), cobalt(II), manganese(II), zinc(II), palladium(II), iron(III), ruthenium(III), uranyl(VI), and titanium(IV) with the ligand were prepared in good yield from the reaction of the ligand with the corresponding metal salts. The ligand and complexes were characterized using infrared, mass spectra, nuclear magnetic resonance, electronic absorption spectra, electron spin resonance, and magnetic moment measurements as well as elemental and thermal analyses. The results showed that the complexes are enolic by nature, whilst the ratio between the metal ion and the ligand depends on the acidity of the metallic ions and their oxidation numbers.     

Synthesis and cytotoxicity of dinuclear complexes containing ruthenium(II) bipyridyl units linked by a bis(pyridylimine) ligand

Enantiopure dinuclear ruthenium polypyridyl complexes of the form [Ru(2)(LL)(4)L(1)](PF(6))(4) (LL = 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen); L(1)= C(25)H(20)N(4) a bis(pyridylimine) ligand containing a diphenylmethane spacer) have been synthesized using the chiral building blocks cis-[Ru(bpy)(2)(py)(2)](2+) and cis-[Ru(phen)(2)(py)(2)](2+). These dinuclear ruthenium complexes have been characterised using NMR, mass spectrometry, UV-visible absorbance, circular dichroism and linear dichroism. The compounds exhibit good photo and thermal stability. The extinction coefficient for the bpy complex at 478 nm is epsilon(478) = 15,700 mol(-1) cm(-1) dm(3) and for the phen complex is epsilon(478) = 24,900 mol(-1) cm(-1) dm(3). Both complexes have their longest wavelength (metal to ligand charge transfer) transition predominantly x/y (short axis)-polarised while the transitions at shorter wavelength are a mixture of x/y and z-polarisations, similar to both the copper helicate and iron triple helicate studied previously. Cytotoxicity studies reveal that the compounds are dramatically less active against cancer cell lines than the recently reported supramolecular cylinders prepared from the same bis(pyridylimine) ligand.     

Synthesis,characterization, electrical and biological studies on some bivalent metal complexes

Metal complexes of manganese(II), iron(II), cobalt(II), nickel(II), copper(II), zinc(II), and cadmium(II) with Schiff base derived from 2,5-dihydroxyacetophenone and s-benzyldithiocarbazate have been synthesized and characterized by elemental analysis, thermogravimetric analysis, molar conductance, molecular weight, magnetic susceptibility measurements, and electronic and infrared spectra. The molar conductivity data show them to be nonelectrolytes. The Schiff base behaves as a tridentate dibasic ONS donor towards metal ions. Thermal analyses indicate the presence of water in the complexes, making them six and four coordinates. The solid state electrical conductivity of the ligand and its complexes has been measured in the temperature range 313–414 K and the complexes are found to show semiconducting behavior. The antibacterial activities of the ligand and its complexes have also been screened against various organisms and it is observed that the coordination of metal ions has a pronounced effect on the bacterial activity of the ligand.     

Family of cofacial bimetallic complexes of a hexaanionic carboxamide cryptand

A series of coordination compounds has been prepared comprising manganese, iron, nickel, and zinc bound by a hexaanionic cryptand where carboxamides are anionic N-donors. The metal complexes have been investigated by X-ray crystallography, and possess metal centers in trigonal monopyramidal geometries with intermetallic distances spanning d(Mn,avg) = 6.080 ? to d(Ni,avg) = 6.495 ?. All complexes featuring trigonal monopyramidal metal(II) ions crystallize in Cc, and feature extended three-dimensional networks composed of cryptate anions linked by bridging potassium countercations. We also report the first solid state structure of the free cryptand ligand, which features no guest in its cavity and which possesses an extended hydrogen-bonding network. SQuID magnetometry data of the metal complexes reveal weak antiferromagnetic coupling of the metal centers. Only the diiron(II) complex exhibits reversible electrochemistry, and correspondingly, its chemical oxidation yields a powder formulated as the diiron(III) congener. The insertion of cyanide into the intermetallic cleft of the diiron(II) complex has been achieved, and comparisons of its solid state structure to the recently reported dicobalt(II) analogue are made. The antiferromagnetic coupling between the diiron(II) and the dicobalt(II) centers when bridged by cyanide does not increase significantly relative to the unbridged congeners. A one-site model satisfactorily fits Mo?ssbauer spectra of unbridged diiron(II) and diiron(III) complexes whereas a two site fit was needed to model the iron(II) centers that are bridged by cyanide.     

Reductive Coupling of (Fluoro)pyridines by Linear 3d-Metal(I) Silylamides of Cr–Co: A Tale of C−C Bond Formation,C−F Bond Cleavage and a Pyridyl Radical Anion

Herein, we disclose the facile reduction of pyridine (and its derivatives) by linear 3d-metal(I) silylamides (M=Cr–Co). This reaction resulted in intermolecular C−C coupling to give dinuclear metal(II) complexes bearing a bridging 4,4′-dihydrobipyridyl ligand. For iron, we demonstrated that the C−C coupling is reversible in solution, either directly or by reaction with substrates, via a presumed monomeric metal(II) complex bearing a pyridyl radical anion. In the course of this investigation, we also observed that the dinuclear metal(II) complex incorporating iron facilitated the isomerisation of 1,4-cyclohexadiene to 1,3-cyclohexadiene as well as equimolar amounts of benzene and cyclohexene. Furthermore, we synthesised and structurally characterised a non-3d-metal-bound pyridyl radical anion. The reactions of the silylamides with perfluoropyridine led to C−F bond cleavage with the formation of metal(II) fluoride complexes of manganese, iron and cobalt along with the homocoupling or reductive degradation of the substrate. In the case of cobalt, the use of lesser fluorinated pyridines led to C−F bond cleavage but no homocoupling. Overall, in this paper we provide insights into the multifaceted behaviour of simple (fluoro)pyridines in the presence of moderately to highly reducing metal complexes.     

Synthesis, Characterization, Electrochemical Properties and Antibacterial Activity of Some Transiton Metal Complexes with [(2-hydroxy-1-naphthaldehyde)-3-isatin]-bishydrazone

A bishydrazone was prepared by reacting isatin monohydrazone with 2-hydroxy-1-naphthaldehyde and a series of metal complexes with this new ligand were synthesised by reaction with Mn^II, Fe^II, Co^II, Ni^II, Cu^II and Zn^II salts. The complexes were characterised on the basis of elemental analysis, molar conductance, magnetic susceptibility data, u.v.–visible, i.r., e.s.r. and n.m.r. spectral studies, wherever possible and applicable. Analytical data reveal that the nickel(II), copper(II) and zinc(II) complexes possess 1:1 metal–ligand ratios and that manganese(II), iron(II) and cobalt (II) complexes exhibit 1:2 ratios. Infrared spectral data suggest that the bishydrazone behaves as a monobasic tridentate ligand with ONO donor sequence towards the metal ions. X-ray diffraction study of the copper(II) complex indicated an orthorhombic crystal lattice. The e.p.r. spectral data show that the metal–ligand bond has considerable covalent character. The electrochemical behaviour of the copper(II) complex was investigated by cyclic voltammetry (CV). Antibacterial tests of the ligand and the metal complexes were also carried out and it has been observed that the complexes are more potent bactericides than the ligand.     

1-苯基-3-甲基-4-苯甲酰基-5-吡唑啉酮与对氨基苯乙酮的缩合反应、量化计算和抑菌活性

利用1-苯基-3-甲基-4-苯甲酰基-5-吡唑啉酮(PMBP)和对氨基苯乙酮(PAAP)制备了PMBP缩对氨基苯乙酮(PMBP缩PAAP)及其铜(Ⅱ)、钴(Ⅱ)、镍(Ⅱ)和锌(Ⅱ)配合物,根据元素分析、红外、紫外、MS和1H NMR谱表征了结构.利用量子化学和分子力学计算方法对缩合反应的机理进行了探讨,较好地解释了PMBP-PAAP配体的烯胺酮式构型及其配合物的亚胺烯醇式构型.测定了化合物对大肠杆菌和金黄色葡萄球菌的抑制活性,其中以钴(Ⅱ)和铜配合物(Ⅱ)的抑菌活性最佳.     

A comparative study of high-spin manganese and iron complexes

Different metal complexes of the general form M(OH) _n (H₂O)_{6– n} have been studied for manganese and iron. Oxidation states considered for manganese are Mn(III), Mn(IV) and Mn(V) and for iron Fe(II), Fe(III) and Fe(IV). Oxygen containing ligands are used throughout with varying numbers of hydroxyl and water ligands. Some metal-oxo and some charged complexes were also studied. Large Jahn-Teller distortions were found for the Mn(III) and Fe(IV) complexes. Consequences of these distortions are that water ligands have to be placed along the weak JT-axis and that five-coordination by a loss of one of these water ligands is quite competitive with six-coordination in particular for manganese. For Fe(II) and Fe(III) lower coordinations than six are preferred due to the presence of two repulsive e _g electrons. For the metal-oxo complexes five-coordination is also preferred due to the strong trans effect from the oxo ligand. All complexes studied have high-spin ground states. An interesting effect is that the spin is much more delocalized on the ligands for the iron complexes than for the manganese complexes. This effect, which is chemically important for certain iron enzymes, is rationalized by the large number of 3d electrons on iron. For manganese with only five 3d electrons no spin delocalization is needed to obtain the proper high-spin states. Received: 4 February 1997 / Accepted: 24 February 1997     

New route to the mixed valence semiquinone-catecholate based mononuclear FeIII and catecholate based dinuclear MnIII complexes: first experimental evidence of valence tautomerism in an iron complex

The semiquinone-catecholate based mixed valence complex, [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] x DMF (1), and catecholate based (H2bispictn)[Mn2III(Cl4Cat)4(DMF)2] (2) (bispicen = N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine, bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, Cl4Cat = tetrachlorocatecholate dianion, and Cl4SQ = tetrachlorosemiquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have been characterized by single crystal X-ray diffraction study. The electronic structures have been elucidated by UV-vis-NIR absorption spectroscopy, cyclic voltammetry, EPR, and magnetic properties. The structural as well as spectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distribution in 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence charge transfer (IVCT) band in the 1900 nm region both in solution and in the solid. The intramolecular electron transfer, a phenomenon known as valence tautomerism (VT), has been followed by electronic absorption spectroscopy. For 1, the isomeric form [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] is favored at low temperature, while at an elevated temperature, the [FeII(bispicen)(Cl4SQ)2] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for 2 suggest that the MnIII(Cat)2- moiety is admixed with its mixed valence semiquinone-catecholate isomer MnII(SQ)(Cat)-, and the electronic absorption spectrum is dominated by the mixed charged species. The origin of the intervalence charge transfer band in the 1900 nm range is associated with the mixed valence form, MnII(Cl4Cat)(Cl4SQ)-. The observation of VT in complex 1 is the first example where a mixed valence semiquinone-catecholate iron(III) complex undergoes intramolecular electron transfer similar to manganese and cobalt complexes.     

Effect of constrained donor atom orientations on the stabilities, complexation kinetics, redox potentials, and structures of macrocyclic polythiaether Complexes. Copper(II) complexes with cyclopentanediyl derivatives of [14]aneS4 in 80% methanol

The two ethylene bridges in the macrocyclic tetrathiaether 1,4,8,11-tetrathiacyclotetradecane ([14]aneS(4)) have been systematically replaced by cis- or trans-1,2-cyclopentane to generate a series of new ligands that exhibit differing preferences for the orientation of the sulfur donor atoms while maintaining constant inductive effects. The resulting five dicyclopentanediyl derivatives, along with two previously synthesized monocyclopentanediyl analogues, have been complexed with Cu(II), and their stability constants, formation and dissociation rate constants, and redox potentials have been determined in 80% methanol/20% water (by weight). The crystal structures of the Cu(II) complexes with the five dicyclopentanediyl-[14]aneS(4) diastereomers as well as the structures for a representative Cu(I) complex and one of the free ligands have also been determined. The properties of these complexes are compared to previous data obtained for the corresponding cyclohexanediyl derivatives in an attempt to shed additional light on the influence of sterically constraining substituents upon the properties of macrocyclic ligand complexes.     

Thiohydrazone copper(II) complexes. The relationship between redox properties and superoxide dismutase mimetic activity

The redox behaviour of copper(II) complexes with the open chain ligand, benzilbisthiosemicarbazone, and the macrocyclic one [3,4,10,11-tetraphenyl-1,2,5,8,9,12,13-octaazacyclotetradeca-7,14- dithione- 2,4,9,11-tetraene] has been explored by cyclic voltammetry. The half-wave potential values for the copper(II)/copper(I) redox couple and the spectral data obtained on dimethylsulfoxide (DMSO) solution agree with the superoxide dismutase (SOD)-mimetic activity of the complexes. The macrocyclic complexes show more positive reduction potential and more activity than the open chain derivatives. From our results it follows that the structure and conformation of ligand has influence on the redox potential of central atom in coordination compound. The changes in the coordination sphere are connected with the change of biological function of compounds represented by SOD-mimic activity. In addition, the L1H6 derivatives show quasireversible waves associated to Cu(II)/Cu(III) process.     

Relative binding strength of terpyridine model complexes under matrix-assisted laser desorption/ionization mass spectrometry conditions

The relative binding strength of a series of terpyridine metal complexes of the type [M(II)L(2)](+) was investigated by using variable laser intensities in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). A model terpyridine ligand, 4'-(1,4,7-trioxa-octyl)-2,2':6',2"-terpyridine, was prepared and complexed with a series of transition metal ions including cadmium, cobalt, copper, iron, manganese, nickel and ruthenium. The relative binding strength of these complexes can be obtained by measuring MALDI mass spectra of the prepared compounds at different laser intensities. The ratio of the signal intensities belonging to the ligand [LH](+) and the complex [ML(2)](+) ([LH](+) /[ML(2)](+)) depends on the laser intensity utilized for the spectrum acquisition. By considering an [LH](+)/[ML(2)](+) ratio > 10 as the point of complete complex dissociation, it is possible to establish a row of complex stabilities depending on the kind of metal ion.     

Synthesis and Biological Evaluation of some 3d Metal Complexes with a Novel Heterocyclic Schiff Base

A heterocyclic Schiff base was prepared by condensing 3‐acetylcoumarin with 2‐amino‐3‐carboxyethyl‐4,5,6,7‐tetrahydrobenzo[b ]thiophene. Such Schiff bases derived from two different heterocyclic moieties are rare and expected to have properties surpassing those of either of the parent compounds in effectiveness of complex formation and biological activities. This ligand formed a series of complexes with manganese(II), cobalt(II), nickel(II), copper(II), and zinc(II) ions. The ligand and the metal complexes were characterized by various physicochemical and spectral studies. These included elemental analysis, molar conductance, magnetic susceptibility, as well as UV–vis, IR, ¹H NMR, ¹³C NMR, and ESR spectral studies. The ESR spectral data adequately supported the covalent nature of the metal–ligand bonds. The ligand possessed a hexagonal crystal structure, but on complexation the crystallinity was lost. The fluorescence spectra of the ligand and its metal complexes in DMSO were also recorded. The ligand and the metal complexes were screened for their antimicrobial activities, and it was observed that the metal complexes are more active than the ligand. The α‐amylase inhibitory activity and the DNA cleavage activity of the ligand and the metal complexes were also examined. in vitro antitumor activity of the copper(II) complex was assayed against human cervical carcinoma cells (HeLa cell line), showing that the complex exhibited promising antitumor activity on the HeLa cell line.     

Crystal Structure of 2-pyridine-2-yl-3(pyridine-2-carboxylideneamino)quinazolin-4(3H)-one and its Ligating Diversity Towards Transition Metal(II) Ions

The synthesis of a new ligand 2-pyridine-2-yl-3(pyridine-2-carboxylideneamino)-quinazolin-4(3H)-one (PPCAQ) is described together with its manganese(II), cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) complexes. The single crystal X-ray diffraction studies of the ligand reveal the presence of two crystallographically independent molecules in asymmetric unit cell, which exhibit N…N attractive interaction. The PPCAQ and its metal complexes were characterized by analytical, spectroscopic (i.r., n.m.r and u.v.–vis), magnetic moment, conductance and thermal studies. The i.r. spectral studies reveal the ligational diversity of the PPCAQ towards different metal ions as NNN donor in cobalt(II), copper(II), zinc(II) and cadmium(II) complexes and as ONN donor in manganese(II) and nickel(II) complexes. The antimicrobial activity of all the compounds was tested; copper(II), zinc(II) and cadmium(II) complexes show enhanced antibacterial activity compared to the free ligand.     

Molten lithium carbonate—sodium carbonate—potassium carbonate eutectic: The reaction of six first row transition metal cations

The reactions of manganese(II), iron(II), iron(III), cobalt(II), nickel(II) and copper(II) chlorides with the molten alkali metal carbonate eutectic have been studied by thermogravimetry. The insoluble products (binary or ternary metal oxides) have been identified by X-ray diffraction and reaction stoichiometries are suggested. Nickel(II) solutions were sufficiently stable for electronic spectroscopy and indicated, as did thermogravimetry, that addition of potassium chloride did not cause significant formation of chloro complexes.