Electron paramagnetic resonance study of doped synthetic crystals of struvite and its zinc analogue

The electron paramagnetic resonance (EPR) technique has been used to study the Mn²⁺ paramagnetic impurity complexes in synthetic struvite (MgNH₄PO₄β6H₂O) and the zinc isomorph (ZnNH₄PO₄β6H₂O). EPR of VO²⁺ ion complexes in vanadyl doped crystals of the zinc isomorph of struvite has also been studied. Two differently oriented, but otherwise identical complexes of both Mn²⁺ ion and VO²⁺ ion are found in these crystals. The spin Hamiltonian parameters indicate a large orthorhombic distortion of the [Mn²⁺(H₂O)₆] octahedra and an axial symmetry of the vanadyl complexes. The results indicate that in both manganese and vanadyl complexes, the metal ions have covalent bonding with the ligands.     

Synthesis and spectral characterization of some investigated thiocarbohydrazone binuclear complexes with an illustrated EPR study for d<Superscript>1</Superscript> complexes

Some new complexes were synthesized using pyrol-2-carbaldhyde thiocarbohydrazone ligand and characterized using physicochemical techniques. The i.r. data reveal that, the ligand acts as a mononegative tetradentate in Ni^II and Cu^II binuclear complexes. Also, they coordinate as mononegative tridentates with Co^II, Cd^II and VO²⁺ ions but in the neutral state with Pt^IV ion in a mononuclear monomer structure for Co^II and Cd^II complexes, but binuclear dimer structure through bridged sulphate for the VO²⁺ complex. The magnetic moment beside the electronic spectral data proposed the complex geometries as a mixed stereochemistry for Ni^II, square-planar for Cu^II, square-pyramidal for VO²⁺ and octahedral for the other complexes. The thermal analysis supports the proposal of the presence or absence of water molecule in or outside the coordination sphere. The EPR spectra of Cu^II and VO²⁺ complexes were illustrated elaborately and some theoretical data were abstracted from EPR curves to support the proposed structures.     

Vanadium complexes with thiosemicarbazones: Synthesis,characterization, crystal structures and anti-Mycobacterium tuberculosis activity

The development of more efficient anti-tuberculosis drugs is of interest. Three oxovanadium(IV) and three cis-dioxovanadium(V) complexes with thiosemicarbazone derivatives bearing moieties with different lipophilicity have been prepared and had their inhibitory activity against Mycobacteriumtuberculosis H₃₇Rv ATCC 27294 evaluated. The analytical methods used by the complexes’ characterization included IR, EPR, ¹H, ¹³C and ⁵¹V NMR spectroscopies, elemental analysis, cyclic voltammetry, magnetic susceptibility measurement and single crystal X-ray diffractometry. [VO(acac)(aptsc)], [VO(acac)(apmtsc)] and [VO(acac)(apptsc)] (acac = acetylacetonate; Haptsc = 2-acetylpyridinethiosemicarbazone; Hapmtsc = 2-acetylpyridine-N(4)-methyl-thiosemicarbazone and Happtsc = 2-acetylpyridine-N(4)-phenyl-thiosemicarbazone) are paramagnetic and their EPR spectra are consistent with the monoanionic N,N,S-tridentate coordination of the thiosemicarbazone ligands, resulting in octahedral structures of rhombic symmetry and with the oxidation state +IV for the vanadium atom. As result of oxidation of the vanadium(IV) complexes above, the diamagnetic cis-dioxovanadium(V) complexes [VO₂(aptsc)], [VO₂(apmtsc)] and [VO₂(apptsc)] are formed. Their ¹H, ¹³C and ⁵¹V NMR spectra were acquired and support a distorted square pyramidal geometry for them, in accord with the solid state X-ray structures determined for [VO₂(aptsc)] and [VO₂(apmtsc)]. In general, the vanadium compounds show comparable or larger anti-M. tuberculosis activities than the free thiosemicarbazone ligands, with MIC values within 62.5–1.56 (μg/mL).     

Synthesis,characterization and interaction with DNA of mononuclear metal complexes with oxolinic acid

Seven new neutral mononuclear metal complexes of VO²⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺ and Cd²⁺ with the quinolone antibacterial agent oxolinic acid (=Hoxo) have been prepared and characterized with physicochemical and spectroscopic techniques. In all the complexes, oxolinic acid acts as a bidentate deprotonated ligand bound to the metal through the pyridone oxygen and one carboxylate oxygen. The metals in all the complexes are six-coordinate with slightly distorted octahedral geometry. The lowest energy model structures of the complexes Fe(oxo)₃, VO(oxo)₂(H₂O) and Mn(oxo)₂(H₂O)₂ have been determined with molecular modeling calculations. The ability of all the complexes to bind to calf-thymus DNA has been investigated with diverse spectroscopic techniques.     

Vanadium(V) Complexes of O,N,O-Donor Tridentate Ligands Containing the {VVO(OMe)}2+ Unit: Syntheses,Structures and Properties

Abstract

Syntheses, characterisation and properties of two complexes containing the oxovanadium(V) methoxide unit have been described. Deprotonated benzoylhydrazones of 2–hydroxy–5–methoxy‐benzaldehyde (H₂bhsOMe) and 2–hydroxy–5–chlorobenzaldehyde (H₂bhsCl) were used as coligands. Crystal structures of both the complexes were determined. In solid state one of them is a dinuclear species [VO(bhsOMe)(OMe)]₂ (1) whereas the other one is a mononuclear complex [VO(bhsCl)(OMe)(HOMe)](2). The dinegative ligands coordinate the metal ions via phenolate–O, imine–N and deprotonated amide–O atoms. In 1, the metal ions of two square pyramidal VO(bhsOMe)(OMe) units share the methoxide groups to form a dinuclear species. The oxygen of a methanol molecule completes the hexacoordination of the metal centre in 2. In each of the two distorted octahedral VO₅N moieties of 1 the bridging methoxide oxygen and in that of 2 the methanol oxygen is trans to the corresponding oxo group. Both the complexes are redox active. The VO³⁺ to VO²⁺ reduction potentials (vs Ag/AgCl) of 1 and 2 are observed at ?0.25 and ?0.04 V, respectively. The band positions in the electronic spectra and the redox potentials reflect the influence of the substituents present on the ligands.     

Mononuclear Pervanadyl (VO2+) Complexes with Tridentate Schiff Bases: Self‐assembling via C–H…oxo and π‐π Interactions

Pervanadyl (VO₂⁺) complexes with N‐(aroyl)‐N′‐(picolinylidene)hydrazines (HL = Hpabh, Hpath and Hpadh; H stands for the dissociable amide hydrogen) are described. The Schiff bases were obtained by condensation of 2‐pyridine‐carboxaldehyde with benzhydrazide (Hpabh), 4‐methylbenzhydrazide (Hpath) and 4‐dimethylaminobenzhydrazide (Hpadh), respectively. The reaction of [VO(acac)₂] and HL in acetonitrile in air affords the complexes of general formula [VO₂L]. The diamagnetic nature and EPR silence confirm the +5 oxidation state of vanadium in these complexes. Infrared spectra of the complexes are consistent with the enolate form of the coordinated ligands. Electronic spectra show charge transfer bands in the range 486–233 nm. The complexes are redox active and display an irreversible reduction (–0.64 to –0.72 V vs. Ag/AgCl). The crystal structures of all the complexes have been determined. In each complex, the metal centre is in a distorted trigonal‐bipyramidal N₂O₃ coordination sphere formed by the pyridine‐N, the imine‐N and the deprotonated amide‐O donor L^– and two oxo groups. The planar ligand satisfies one equatorial and two axial positions. The other two equatorial positions are occupied by the two oxo groups. In the solid state, the molecules of each of the three complexes form a chain‐like arrangement via the azomethine‐H…oxo interactions. Interchain weak π‐π interactions lead to two dimensional networks for [VO₂(pabh)] and [VO₂(path)]. On the other hand, [VO₂(padh)] forms a two‐dimensional network through interchain N‐methyl‐H…oxo interactions.     

Vanadium(IV,V) complexes of D-saccharic and mucic acids in aqueous solution

The vanadium(IV,V) complexes formed with two aldaric acids (D-saccharic or D-glucaric acid, and mucic or galactaric acid) in aqueous solution were characterised by employing pH-potentiometry, EPR, multinuclear NMR and UV-VIS spectroscopy. The stoichiometry and stability constants of the complexes formed were determined at 25 degrees C and ionic strength I= 0.2 mol dm(-3)(KCl). The spectral measurements revealed that vanadium(IV,V) coordinates first at the terminal COO(-) functions, forming mononuclear complexes. At pH > 3, through the metal ion-induced deprotonation and coordination of the neighbouring alcoholic functions, (COO(-), O(-)) coordinated dinuclear complexes are formed, which predominate in the pH range 4-8. In the basic pH range, the ligand molecules are displaced and binary metal hydroxo and oxo complexes are present. EPR measurements at room temperature and at 140 K proved that formation of the VO(iv) dimers is more enhanced at room temperature, but at 140 K their dissociation is favoured. An interesting pH-dependent cis-trans isomeric equilibrium was assumed and analysed by EPR and molecular modelling in the case of the complexes [(VO)(2)L(2)H(x)](x=-2 and -4). Joint evaluation of the pH-potentiometric and (51)V NMR measurements revealed that both aldaric acids are able to bind an excess of vanadium(V), through the formation of oligomeric 2:1 and 3:2 species, besides the 2:2 species formed with VO(IV).     

Vanadium-binding protein in a vanadium-rich ascidian Ascidia sydneiensissamea: CW and pulsed EPR studies

Some of the ascidians belonging to the suborder Phlebobranchia accumulate vanadium ion efficiently from seawater. Clarification of the mechanism of this surprisingly efficient metal-accumulation system is desirable. Two mutually similar vanadium-binding proteins (vanabin1 and vanabin2) have recently been isolated from a vanadium-rich ascidian Ascidia sydneiensis samea. In this study, the vanadium-binding properties of vanabin2 have been investigated by X-band CW EPR and pulsed EPR spectroscopy. CW EPR spectra of samples containing various ratios of VO2+ and vanabin2 invariably exhibited a usual mononuclear-type VO2+ EPR signal with the intensity dependent on the ratio [vanabin]/[V]. EPR titration has shown that vanabin2 can bind up to approximately 23.9 vanadium ions per one molecule, almost all of which ( approximately 84%) are in a mononuclear VO2+ state as estimated by EPR quantitation. Electron spin-echo envelope modulation (ESEEM) spectra of VO-vanabin2 exhibited reasonably intense peaks attributable to amine nitrogen. This is consistent with the fact that vanabin2 is a lysine-rich protein (14 lysines out of 91 amino acids). The present study reveals the uniqueness of vanabin2, which can bind a large number of metal ions in a mononuclear fashion in contrast to the situation for ferritin and metallothionein.     

Synthesis and structural characterization of oxovanadium(IV) complexes of dimedone derivatives

We have successfully synthesized new oxovanadium (IV) complexes with dimedone derivatives and their structure were confirmed by elemental analyses, spectroscopic techniques (FT-IR, UV–visible, EPR) and thermal analysis. The reaction of [VO (acac)₂] with the azo dimedone ligands ( HL _n ) produced mononuclear oxovanadium (IV) complexes with formula [VO (L_n)₂]H₂O. Results of the molar conductance proved that VO²⁺ complexes are non-electrolytes and fall in the range 14–16 Ω-¹cm²mol⁻¹. The coordination geometry of VO (IV) complexes is square-pyramidal, where vanadium (IV) ion is coordinated by oxygen atom of the carbonyl (C=O) group, and nitrogen atom of the deprotonating hydrazone moiety (–NH–), while the fifth position is occupied by an oxo group. Moreover, the optimized structure, bond angles, bond lengths, as well as the calculated quantum chemical parameters of the complexes have been estimated. DNA binding activities of the complexes were investigated using electronic absorption titration and viscosity measurements. The obtained results showed groove binding of the complexes to CT-DNA accompanied with a partial insertion of the ligand between the base stacks of the DNA with a binding constant of 2.07–5.51 x 10⁵ M⁻¹ range. Evaluation results of the synthesized complexes against the human cancer cell lines HepG-2 and MCF-7, as compared to the positive controls in the viability assay of vinblastine and colchicine have been reported. The in vitro anti-oxidant activity of all the complexes is determined by DPPH free radical-scavenging assay. Finally, the anti-microbial activities of the complexes have been investigated against fungal (Candida albicans), gram negative bacteria (Escherichia coli), and gram positive bacteria (Staphylococcus aureus) using the disc-diffusion method.     

Paramagnetic resonance in imidazolate-bridged homobinuclear (copper-copper) and heterobinuclear (copper-zinc) complexes

Bridged homobinuclear (copper-copper) and heterobinuclear (copper-zinc) complexes of diethylenetriamine have been prepared with 2-methyl-imidazole as bridging ligand. EPR spectra of the polycrystalline complexes have been studied at room temperature and also at liquid nitrogen temperature. Low temperature EPR and electronic spectroscopic studies of 50% aqueous DMSO of [(dien)Cu-(Melm)-Zn(dien)]3+ solutions show the imidazolate bridged complex to exist mainly over the pH range approximately 7.0 < pH < 10.0. At low pH the 2-MelmH+ ion and mononuclear copper and zinc complexes are formed. Above pH > 10.0 hydroxide ion splits the imidazolate bridge.     

Iron(III) complexes of the tris-(3-aminopropyl) derivative of a 14-membered tetraazamacrocycle: Potentiometric,spectroscopic and electrochemical studies

The properties of the iron(III) complexes of the ditopic macrocyclic ligand with three aminopropyl pendant arms, L¹ = 3,7,11-tris-(3-aminopropyl)-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene were investigated in aqueous solution. Potentiometric studies indicated the presence of mononuclear [FeH_hL¹]^h+3 (h = 0–3), and dinuclear [Fe₂L¹]⁶⁺, [Fe₂L¹(OH)]⁵⁺ and [Fe₂L¹(OH)₂]⁴⁺ complexes, and their stability constants were determined at 298.2 K and ionic strength 0.10 mol dm⁻³ in KNO₃. The log K values of mononuclear protonated species indicated the consecutive deprotonation of the aminopropyl arms, suggesting the nitrogen donor atoms from the macrocycle as the preferred coordination environment for the first metal centre, and the amines from the pendant arms for the second one. The dinuclear complex is formed at about 85% of the total amount of the metal ion for 2:1 Fe:L¹ ratio solutions at pH 4.0–4.5. The log K values of the deprotonation of dinuclear hydrolysed species are consistent with the presence of two water molecules directly bound to the metal centres. Spectroscopic UV–Vis and IR data for 2:1 Fe³⁺:L¹ ratio samples confirmed the existence of dinuclear and hydroxo dinuclear species. EPR spectra of these solutions were interpreted by an equilibrium of two high-spin d⁵ state of iron(III) species with different rhombic E/D distortions. Electrochemical studies also established the formation of mono- and dinuclear complexes, showing irreversible redox behaviour. The two metal centres on the dinuclear complexes have only weak interactions.     

Nano-sized, ternary oxovanadium(IV) complexes containing electron-rich oxygen-based ligands

Some mononuclear mixed-carboxylato ??-diketonato oxovanadium(IV) complexes of the general formula [VO(??-dike)(RCOO)] (where H??-dike?=?acetylacetone; benzoylacetone or dibenzoylmethane, R?=?C₁₅H₃₁ or C₁₇H₃₅) have been synthesized from VO(acac)₂ by stepwise substitutions of acetylacetonate ion with straight chain fatty acids (RCOOH) and ??-diketones in p-xylene under reflux. The substituted acetylacetone could be fractionated out with p-xylene as an azeotrope. These were characterized by elemental analyses, molecular weight determinations, spectral (electronic, infrared, ¹H NMR, EPR and powder XRD) studies, magnetic susceptibility measurements and cyclic voltammetry. Molar conductance values indicated the complexes to be non-electrolytes in nitrobenzene. Bidentate chelating nature of ??-diketonate and carboxylate ligands in the complexes was established by infrared and NMR spectra. Molecular weight determinations confirmed mononuclear nature of the complexes. The EPR spectra illustrated coupling of the unpaired electron with ⁵¹V nucleus (I?=?7/2). Cyclic voltammograms of all the complexes displayed one-step oxidation processes. The oxidation peak potential corresponded to the quasireversible one-electron oxidation process of the metal center, yielding V(V) species. Powder XRD and transmission electron microscopy (TEM) studies indicated the particles of these were lying in the nano-size range. The synthesized complexes are a new type of mixed-ligand complexes in which vanadium is having coordination number 5. A square pyramidal geometry around vanadium has been assigned in all the complexes.     

Synthesis,characterization, EPR,electrochemical, and in situ spectroelectrochemical studies of salen-type oxovanadium(IV) and copper(II) complexes derived from 3,4-diaminobenzophenone

A new half unit and some new symmetrical or asymmetrical VO(IV) and Cu(II) complexes of tetradentate ONNO Schiff base ligands were synthesized. The probable structures of the complexes have been proposed on the basis of elemental analyses and spectral (IR, UV–Vis, electron paramagnetic resonance, ESI-MS) data. VO(IV) and Cu(II) complexes exhibit square pyramidal and square-planar geometries, respectively. The complexes are non-electrolytes in dimethylformamide (DMF) and dimethylsulfoxide. Electrochemical behaviors of the complexes were studied using cyclic voltammetry and square wave voltammetry. Half-wave potentials (E _1/2) are significantly influenced by the central metal and slightly influenced by the nature of substituents on salen. While VO(IV) complexes give VO^IV/VO^V redox couples and a ligand-based reduction process, Cu(II) complexes give only a ligand-based reduction. In situ spectroelectrochemical studies were employed to determine the spectra of electrogenerated species of the complexes and to assign the redox processes. The g-values were calculated for all these complexes in polycrystalline state at 298?K and in frozen DMF (113?K). The evaluated metal–ligand bonding parameters showed strong in-plane σ-bonding for some Cu(II) complexes.     

Structure and Catalytic Properties of a Cu2+-Containing Hydrogel of Zirconium Dioxide

The distribution of Cu²⁺ ions in ZrO₂ and sulfated ZrO₂ hydrogel phases was studied by EPR spectroscopy and voltammetry. The formation of the following three types of copper structures was observed: mononuclear Cu²⁺ complexes (A), magnetic associates (B), and Cu²⁺ compounds (C) that gave no EPR signals under the conditions used in the spectroscopic measurements. The specific catalytic activity of various Cu²⁺ compounds in the liquid-phase reaction of 2,3,5-trimethyl-1,4-hydroquinone oxidation was determined. The copper complexes C were found to exhibit the highest catalytic activity.     

An EPR and optical absorption study of VO2+ ions in di-ammonium hydrogen citrate [(NH4)2C6H6O7] single crystals

Single crystal and powder EPR studies of VO2+ doped di-ammonium hydrogen citrate [(NH4)2C6H6O7] are carried out at room temperature. The angular variation of the EPR spectra show three different VO2+ complexes that are located in different chemical environment, and each environment contains two magnetically inequivalent VO2+ sites in distinct orientations occupying substitutional positions in the lattice. Crystalline field around the VO2+ ion is nearly axial. The optical absorption spectrum shows two bands centred at 16,949 and 12,345cm(-1). Spin Hamiltonian parameters and molecular orbital coefficients are calculated from the EPR and optical data, and results are discussed.     

A single crystal EPR study of VO ions doped in Cs2Co(SO4)2.6H2O Tutton salt

EPR spectra of VO²⁺ ions doped in single crystals of Cs₂Co(SO₄)₂.6H₂O single crystals have been studied at various temperatures (390–103 K) on X-band frequency. The detailed EPR analysis shows three vanadyl complexes with differing intensities. The g and A tensors are found to be axially symmetric. The intense vanadyl complexes in the lattice are found to occupy the Co²⁺ substitutional sites, whereas the weak vanadyl complex at the interstitial sites. The optical absorption spectrum at room temperature shows three absorption bands characteristic of VO²⁺ ions in tetragonal symmetry. By correlating the EPR and optical data, the molecular bonding coefficients and the Fermi contact interaction terms have been evaluated and discussed. The line broadening of VO²⁺ spectra on cooling the crystal is explained on the basis of spin-lattice relaxation narrowing. The spin-lattice relaxation time for the host Co²⁺ ions has been estimated at various temperatures.     

Metal Complexes of Pentadentate Macrocyclic Ligands Containing Oxygen and Nitrogen as Donor Atoms

Four macrocyclic ligands have been synthesized: 1-oxa-4,7,10,13-tetraazacyclopentadecane ( 1 ), 1,4-dioxa-7,10,13-triazacyclopentadecane ( 2 ), 1,4-dioxa-7,11,14-triazacyclohexadecane ( 3 ), 1,4-dioxa-7,11,15-triazacycloheptadecane ( 4 ), one of them 3 , for the first time. The protonation constants of the ligands and the stability constants of the complexes formed by the four ligands with some divalent first-series transition-metal ions, Cd²⁺ and Pb²⁺, were determined by potentiometric methods, in aqueous solution, at 25° and I = 0.10M (KNO₃). The sequence of protonation of ligand 1 was studied by ¹H-NMR spectroscopy. The Irving-Williams' order of stability is obeyed for the complexes of all the ligands, and the metal complexes of 1 present the higher values of stability. A drop in the stability of all the metal complexes studied is observed when the metal complexes of 1 are compared with the corresponding complexes of 2 . The effect of the increase of the ring size of the macrocycle can be observed for metal complexes of the series of ligands 2 4 , where, in spite of the slight increase of the overall basicity of the ligands (20.28, 22.25, and 24.96 for 2, 3 , and 4 respectively), small differences in stability are found for the corresponding complexes of 2 and 3 , but a significant drop occurs for all the metal complexes formed with the 17-membered ligand, specially for the larger metal ions like Mn²⁺ and Pb²⁺.     

Transition metal complexes derived from N′-(4-fluorobenzylidene)-2-(quinolin-2-yloxy) acetohydrazide: Synthesis,structural characterization,and biocidal evaluation

Mononuclear Mn²⁺ and Cu²⁺, - VO²⁺, Co²⁺, Ni²⁺, - and Zn²⁺ complexes of a synthetic novel hydrazone containing a quinoline moiety were prepared. The composition and structure of the prepared compounds were elucidated by spectral and analytical techniques. The results reveal that all complexes were formed in 1:1 mole ratio except Mn²⁺ and Cu²⁺ complexes ( 3 ) and ( 7) , which were formed in 1 M:2 L mole ratio. It was also found that the ligand binds the metal ions via NO donor sites as a monobasic bidentate chelator in all complexes through the enolic carbonyl oxygen and azomethine nitrogen atoms. The electronic absorption spectra and magnetic moment data demonstrated square pyramidal and octahedral geometries for the VO²⁺ and Ni²⁺ complexes, respectively, while the other complexes adopted tetrahedral geometry. The thermal decomposition of the complexes was discussed in relation to structure. The thermal analysis data demonstrated that all complexes were decomposed in one, two, three or four stages starting with the dehydration process, removal of coordination water molecules or elimination of anions and ended with the formation of the metal oxide. The bactericidal activities of the prepared compounds demonstrated that hydrazone ( 1) exerted a highly inhibitory effect against B. subtilis while VO²⁺, Co²⁺, and Cu²⁺ complexes ( 2) , ( 4) , and ( 7) showed an inhibitory effect against E. coli more than the tetracycline. Additionally, the inhibitory effect of the prepared compound against A. niger showed that the Cu²⁺ complex ( 6) is the most active while the Ni²⁺, Cu²⁺, and Zn²⁺ complexes ( 5–8) exhibited an extremely inhibitory effect against C. albicans.