Changes In Coordination Sphere Research Articles

First Example of Antiparasitic Activity Influenced by Thermochromism: Leishmanicidal Evaluation of 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine Metal Complexes.

The World Health Organization catalogues illnesses such as Leishmaniasis as neglected diseases, due to low investment in new drugs to fight them. The search of novel and non-side effects anti-parasitic compounds is one of the urgent needs for the Third World. The use of triazolopyrimidines and their metallic complexes has demonstrated hopeful results in this field. This work studies the antiparasitic efficacy of a series of 5,7-dimethyl-1,2,4- triazolo[1,5-a]pyrimidine first row transition metal complexes against three leishmania spp. strains. The in vitro antiproliferation of promastigote forms of different strains of leishmania spp. (L. infantum, L. braziliensis and L donovani) and the cytotoxicity in macrophage host cells are reported here. The antiparasitic assays have been complemented with enzymatic tests to elucidate the mechanisms of action. New crystal structure description, thermal analysis, magnetic susceptibility and magnetization experiments have also been carried out in order to present a whole characterization of the studied compounds and interesting physical properties besides the biological tests. The results of antiproliferation screening and cytotoxicity show great antiparasitic efficacy in the studied complexes. The superoxide dismutase enzymatic assays exhibit a different behaviour according to the thermochromic triazolopyrimidine form tested. Antiproliferative assays and enzymatic tests corroborate the synergetic leishmanicidal effect present in coordination triazolopyrimidine complexes. The changes in coordination sphere derived from thermochromism affect the physical properties as well as the biological efficacy.

Structural changes in copper based metal-organic framework catalyst induced by organic solvents

Abstract Metal-organic frameworks (MOFs) are an emerging class of catalysts that posess large surface area due to intrinsic porosity, and high metal loading. However, they often lack catalytic properties due to the saturated coordination sphere of metal centres. Some MOFs have been found to exhibit good to excellent catalytic activity in a limited number of reactions, resulting from solvent induced changes of the coordination sphere. In this work the coordination sphere changes of Cu(bpy)(H2O)2(BF4)2(bpy) (bpy = 4,4-bipyridine) affected by common solvents with different dipole moments were studied by means of solution-state 1H and 19F NMR. It was found that organic molecules interact with the copper(II) centre and cause structural transformations of Cu-MOF in proportion to both the dipole moment and concentration of solvent. The sensitive NMR resonances reveal an exchange of the BF4¯ anions to DMSO (dimethylsulfoxide) molecules at axial positions of Cu(II) and even breakage of Cu-N and Cu-O bonds in equatorial positions with increasing DMSO concentration. Therefore, solvents with suitable dipole moments (1.7–3.9) can promote structural changes and, thus release highly active sites in the reaction, which can efficiently enhance the activity of Cu-MOF catalysts.

Crystal structure, thermochromic and magnetic properties of organic-inorganic hybrid compound: (C7H7N2S)2CuCl4

The synthesis, thermal analysis, crystal structure and magnetic properties of (2-aminobenzothiazolium)2CuCl4, organic-inorganic hybrid compound, have been described. The compound crystallizes in the monoclinic space group P21/c with two formula units in a unit cell of dimensions a = 6.9522(4) Å, b = 9.6979(4) Å, c = 13.9633(6) Å, β = 97.849(3)° and volume 930.83(8) Å3 at 150(2) K. The structure consists of isolated nearly square planer [CuC14]2− units, with somewhat longer than normal CuCl bond lengths [CuCl (average) = 2.2711 Å]. The magnetic measurements of (2-aminobenzothiazolium)2CuCl4 using SQUID magnetometer show paramagnetic nature of the compound. Thermal measurements (TG-DTA and DSC) on this compound showed reversible phase transition at 83 °C. This transition is accompanied by the reversible change in colour of the prismatic crystal from green to dark brown, thermochromic behaviour. Temperature dependent EPR measurements on powdered sample ascertain change in coordination sphere around Cu(II) with shift in g|| = 2.150 and g⊥ = 2.071 at room temperature, typical of square planar, to g|| = 2.201 and g⊥ = 2.182 at 170 °C, typical of distorted tetrahedral geometry.

Reversible Single-Crystal to Single-Crystal Transformations of a Zn(II)-Salicyaldimine Coordination Polymer Accompanying Changes in Coordination Sphere and Network Dimensionality upon Dehydration and Rehydration.

A fluorescent Zn(II)-salicyaldimine coordination polymer, [Zn(L(salpyca))(H2O)]n (1; H2L(salpyca) = 4-hydroxy-3-(((pyridin-2-yl)methylimino)methyl)benzoic acid), showing a one-dimensional (1D) zigzag chain structure has been hydro(solvo)thermally synthesized. Removal of coordination water molecules in 1 by thermal dehydration gives rise to the dehydration product [Zn(L(salpyca))]n (1'), which has a dizinc-based two-dimensional (2D) gridlike (4,4)-layer structure. X-ray powder diffraction (XRPD) patterns, thermogravimetric (TG) analyses, and infrared (IR) spectra all clearly indicate that the structure of 1 is quite flexible as a result of a reversible 1D-2D single-crystal to single-crystal (SCSC) transformation upon removal and rebinding of coordination water molecules, which accompanies changes in coordination sphere and network dimensionality. Additionally, Zn(II)-salicyaldimine polymers 1 and 1' exhibit different solid-state photoluminescences at 458 and 480 nm, respectively. This is reasonably attributed to the close-packing effect and/or the influences of the differences on the conformation and the coordination mode of the L(salpyca) ligand and the coordination geometry around the Zn(II) center.

EPR Methods for Biological Cu(II): L-Band CW and NARS.

Copper has many roles in biology that involve the change of coordination sphere and/or oxidation state of the copper ion. Consequently, the study of copper in heterogeneous environments is an important area in biophysics. EPR is a primary technique for the investigation of paramagnetic copper, which is usually the isolated Cu(II) ion, but sometimes as Cu(II) in different oxidation states of multitransition ion clusters. The gross geometry of the coordination environment of Cu(II) can often be determined from a simple inspection of the EPR spectrum, recorded in the traditional X-band frequency range (9-10 GHz). Identification and quantitation of the coordinating ligand atoms, however, is not so straightforward. In particular, analysis of the superhyperfine structure on the EPR spectrum, to determine the number of coordinated nitrogen atoms, is fraught with difficulty at X-band, despite the observation that the overwhelming number of EPR studies of Cu(II) in the literature have been carried out at X-band. Greater reliability has been demonstrated at S-band (3-4 GHz), using the low-field parallel (gz) features. However, analysis relies on clear identification of the outermost superhyperfine line, which has the lowest intensity of all the spectral features. Computer simulations have subsequently indicated that the much more intense perpendicular region of the spectrum can be reliably interpreted at L-band (2 GHz). The present work describes the development of L-band EPR of Cu(II) into a routine method that is applicable to biological samples.

The Effect of the Second Coordination Sphere on Electrochemistry of Niobium Fluoride Complexes in Alkali Halide Melts: I. Diffusion Coefficients of Nb(V) and Nb(IV) Complexes

Diffusion coefficients of Nb(V) and Nb(IV) fluoride complexes in chloride and fluoride melts were obtained by linear sweep voltammetry, chronopotentiometry and chronoamperometry methods. The values found by these methods are in good agreement with each other. Decreasing of diffusion coefficients was obtained when the cation in the second coordination sphere changes from Na+ to Cs+. It was determined that diffusion coefficients decrease when chloride melts are converted to fluoride melts. Optimized geometrical structures and energies of such model particles as (M+)n[Nb(V)F7]2− and (M+)n[Nb(IV)F7]3− (M+ = Na+, K+, Cs+; n = 1–6) were obtained by quantum-chemical calculations. There most stable compositions of these particles have been determined. Energies of both the [NbF7] complexes for all outersphere compositions, of the [NbF7] complex bond with its outersphere shell, and the (Nb-F) and (Nb-M) bond lengths have been calculated. Variations of specified quantities in the Na+-K+-Cs+ and Nb(IV)-Nb(V) series have been compared with changes in diffusion coefficients. This has allowed to make some guesses concerning the mechanism of the [NbF7] complex diffusion in melts.

EPR investigation on synthesis of Lithium zinc vanadate using sol–gel-combustion route and its optical properties

The present work describes the synthesis of Lithium zinc vanadate (LiZnVO4) nanophosphor prepared by sol–gel-combustion method and its optical properties. The prepared sample was characterized by X-ray diffraction, SEM, electron paramagnetic resonance and photoluminescence spectroscopy. X-ray diffraction study showed the formation of pure LiZnVO4 at 600°C with distorted phenacite structure. SEM investigation revealed that the phosphor powder has spherical morphology with particle size of about 100–200nm. EPR study showed the change of coordination sphere around vanadium from axially distorted octahedral symmetry to tetrahedral geometry along with the change in oxidation state of vanadium ion from +4 to +5. The emission spectrum showed a broad emission at 543nm with λex=375nm. The decay time obtained on mono-exponential fitting was 8.3μs. The colour coordinates of the system were evaluated using CIE index diagram to be 0.31 and 0.41, which suggest that the prepared material is a potential green emitting phosphor. A bright green colour emission was also observed directly from this phosphor upon excitation with an UV source.

Influence of the Second Coordination Sphere on the Diffusion Coefficients of Niobium Fluoride Complexes in Chloride and Fluoride Melts

The diffusion coefficients of Nb(V) and Nb(IV) fluoride complexes in chloride and fluoride melts were obtained by linear sweep voltammetry, chronopotentiometry and chronoamperometry methods. The values found by these methods are in a good agreement with each other. The decreasing of diffusion coefficients was obtained when the cation in the second coordination sphere changes from Na to Cs. It was determined that the diffusion coefficients decrease when going from chloride melts to fluoride melts. Optimized geometrical structures and energies of such model particles as (M+)n[Nb(V)F7]2- and (M+)n[Nb(IV)F7]3- (M - Na, K, Cs; n=1 - 6) were obtained by quantum-chemical calculations. The most stable compositions of these particles were determined; energies of the [NbF7] complexes for all outersphere compositions, energy of the [NbF7] complex bonds with its outersphere shell, and the (Nb-F) and (Nb-M) bond lengths were calculated. Variations of the specified quantities in the Na-K-Cs and Nb(IV)-Nb(V) series are compared with a changes of diffusion coefficients. It has allowed making some guesses related mechanism of the [NbF7] complex diffusion in melts.

Influence of metal ions on thermal generation of carbohydrate radicals in native and modified starch studied by EPR

AbstractInfluence of enrichment with ions from alkali (K), alkali‐earth (Mg) and d transition (Cu, Fe) metal groups of native and modified starches on the process of carbohydrate radicals generation has been studied using electron paramagnetic resonance (EPR) spectroscopy. This technique allows following the character and amount of created radicals, as well as the changes in coordination sphere of transition metal ions incorporated into starch matrix. The main difference between transition metal ions and the ions of alkali and alkali‐earth metals consisted in the character of their interactions with functional groups of starch. Transition metal ions coordinated glucose units of starch and influenced the mechanism of carbohydrate radical formation independently of the kind of starch modification. Interaction of starch, native and modified, with ions of alkali and alkali‐earth metals did not change the mechanism of radical generation, however, influenced their number. Contents of total metal ions found analytically by spectroscopic methods and by EPR allowed determination of the amount of transition metal ions engaged in interactions with starch matrix, as well as the differentiating of native and modified starch in respect of their metal ions adsorptive ability.

Structure Comparison of Early and Late Lanthanide(III) Homodinuclear Macrocyclic Complexes with the Polyamine Polycarboxylic Ligand H8OHEC

AbstractThe solid‐state structures of two new homodinuclear chelate complexes with the late lanthanide(III) ions Yb and Lu, [Na2(Yb2OHEC)].14.5H2O (1), and [Na2(Lu2OHEC)].14.5H2O (2) (H8OHEC = 1,4,7,10,14,17,20,23‐octaazacyclohexacosane‐ 1,4,7,10,14,17,20,23‐octaacetic acid), have been determined by X‐ray crystal structure analysis. Each lanthanide(III) ion is coordinated by eight donor atoms of the ligand and the geometry of the coordination polyhedron approaches a bicapped trigonal prism. These structures are compared with those of the homodinuclear chelate complexes with the same ligand and the mid to early lanthanide(III) ions Gd, Eu, La and also Y. A distinctive structural change occurs across the lanthanide series. The centrosymmetric mid to early lanthanide(III) complexes are all ninefold‐coordinated in a capped square antiprismatic arrangement with a water molecule coordinated in a prismatic position. This structure is maintained in aqueous solution, together with an asymmetric minor isomer. The late lanthanide(III) OHEC complexes not only lack the inner‐sphere water, but the change of coordination sphere also results in a loss of symmetry of the whole complex molecule. The observed change of coordination mode and number of the lanthanide ion may offer a geometric model for the isomerization process in eight‐ and ninefold‐coordinated complex species that are isomers in a possible coordination equilibrium observed by NMR in aqueous solution. This model may also explain the intramolecular rearrangements necessary during water exchange in the inner coordination sphere of the complex [(Gd2OHEC)(H2O)2]2− through a slow dissociative mechanism. Protonation constants of the H8OHEC ligand and complex formation constants of this ligand with GdIII, CaII, CuII and ZnII have been determined by solution thermodynamic studies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Effects of Water and Ion-Exchanged Counterion on the FTIR Spectra of ZSM-5. II. (Cu+–CO)-ZSM-5: Coordination of Cu+–CO Complex by H2O and Changes in Skeletal T–O–T Vibrations

The 2157 cm−1 (strong) and 2108 cm−1 (very weak) ν(CO) IR bands due to Cu+–CO in ZSM-5 zeolite with 12C and 13C isotopes, respectively, are reversibly red-shifted by subsequent adsorption of H2O at 293 K. On the contrary, the locally perturbed internal (T–O–T) asymmetric stretching framework vibration [νasint(TOT)(Cu+–CO)=965 cm−1] is reversibly blue-shifted. The courses of the band shifts revealed notable features. Charge transfers from water to Cu+ ions, changes in coordination spheres of Cu+(CO)(H2O)n aqua complexes and secondary (solvent-like) effects were considered to explain the results.

Understanding the Facile Photooxidation of Ru(bpy)32+ in Strongly Acidic Aqueous Solution Containing Dissolved Oxygen

The previously observed facile photooxidation of Ru(bpy)32+ to Ru(bpy)33+ in oxygenated solutions of 9 M H2SO4 (Kotkar, D; Joshi, V.; Ghosh, P. K. Chem. Commun. 1987, 4; Indian Patent No. 164358 (1989)) is further studied. A similar phenomenon was observed with Ru(phen)32+ but not with Ru(bpy)2[bpy-(CO2H)2]2+. The reaction is strongly dependent on acid concentration, with a sharp change in the region of 2−7 M H2SO4. The quantum yield of Ru(bpy)33+ formation in 9 M H2SO4 is close to the quantum yield of steady-state luminescence quenching by O2. Photooxidation is accompanied by near-stoichiometric formation of H2O2 as reduced product. Chromatographic, spectroscopic, electrochemical and optical rotation studies reveal that Ru(bpy)32+ survives the strongly acidic environment with little evidence of either any change in coordination sphere or ligand degradation, even after repeated cycles of photolytic oxidation followed by electrolytic reduction. The high quantum yield and selectivity of the reaction is ascribed to (i) predominance of the electron transfer quenching pathway over all others and (ii) highly efficient trapping of O2•- by H+ followed by rapid disproportionation to H2O2 and O2. These are likely on account of the high ionic strength of the medium which favors the required shifts in the potentials of the O2/O2•- and O2/H2O2 couples. Upon storage of the photooxidized Ru(III) solution in dark, partial recovery of Ru(bpy)32+ occurs gradually. Studies with the electrooxidized complex over a range of acid concentrations indicate that Ru(bpy)32+ is regenerated by reaction of Ru(bpy)33+ with H2O2. The reaction is promoted by increasing concentrations of [H2O2] and inhibited by [O2] and [H+]. The fraction of Ru(III) remaining after the reverse reaction is allowed to plateau in solutions of varying acid concentrations follows a similar trend to that found after attainment of steady state in the photooxidation reaction, although in all cases the forward reaction produces more Ru(III) than what remains in the reverse reaction. These observations are consistent with the following equation 2Ru(bpy)32+ + O2 + 2H+ →(hν)/←(dark) 2Ru(bpy)33+ + H2O2 for which the equilibrium constant has been computed. Light helps overcome the activation barrier of the forward reaction by driving it via *Ru(bpy)32+, and to the extent that the photooxidation is driven past the equilibrium, there is conversion of light energy in the form of long-lived chemical products. Spectroscopic evidence rules out any significant shift in the redox potential of Ru(bpy)33+/2+, suggesting thereby that H2O2 is much more stable in the more strongly acidic medium and less capable of reducing Ru(bpy)33+ unlike at higher pH.

Effect of metal size on coordination geometry of N, N′, N″-tris(2-pyridylmethyl)- cis,cis-1,3,5-triaminocyclohexane: synthesis and structure of [M IIL](ClO 4) 2 (M=Zn, Cd and Hg)

The hexadentate ligand N, N′, N″-tris(2-pyridylmethyl)- cis, cis-1,3,5-triaminocyclohexane (tachpyr) forms stable metal complexes from the salts M(ClO 4) 2·6H 2O (M=Zn 2+ and Cd 2+) and Hg(ClO 4) 2 in methanol or ethanol. The complexes [Zn(tachpyr)](ClO 4) 2·CH 3OH ( 1·CH 3OH), [Cd(tachpyr)](ClO 4) 2 ( 2) and [Hg(tachpyr)](ClO 4) 2 ( 3) have been isolated, and their solid state structures have been determined and analyzed. The coordination sphere changes from octahedral to trigonal-prismatic geometry with increasing metal radius. The trigonal twist angles ( α) are 43.7(2)° in compound 1·CH 3OH, range between 11.0(4)° and 21.7(2)° in compound 2 and range between 3.1(3)° and 5.5(2)° in compound 3. To accommodate larger metal ions there is also a slight outward expansion of the triaminocyclohexane nitrogens, which leads to a reduction in the average cyclohexyl ring C–C–C–C torsion angles ( β) with concomitant flattening of the ring. The torsion angles ( β) in 1·CH 3OH are 52.3(6)° and 52.4(6)°, range between 46.1(9)° and 52.7(9)° in compound 2 and range between 45.1(6)° and 52.9(7)° in compound 3.

Reactions of diorganotin(IV) oxides with isatin 3- and 2-thiosemicarbazones and with isatin 2,3-bis(thiosemicarbazone): influence of diphenyldithiophosphinic acid (isatin = 1H-indole-2,3-dione)

Reaction between SnMe2O and isatin 3-thiosemicarbazone (HLI) in the presence of diphenyldithiophosphinic acid (HS2PPh2) afforded [SnMe2(S2PPh2)2] and [SnMe2LI{O(S)PPh2}]·EtOH, which were studied by X-ray diffraction and IR spectroscopy in the solid state and by multinuclear (1H, 13C, 31P and 119Sn) NMR spectroscopy in CDCl3 or dmso-d6 solution (isatin = 1H-indole-2,3-dione). In the bis(dithiophosphinate) derivative the tin atom is bound to two strongly anisobidentate ligands both in the solid state and in solution in chloroform. In the mixed-ligand complex the monothiophosphinate ligand is O-co-ordinated and the thiosemicarbazone (TSC) is bound primarily through its sulfur atom and the nitrogen atom of the azomethine group, although there is an additional weak bond with the remaining isatin oxygen atom [O(1)]. The NMR spectrum in dmso-d6 suggests weakening of the Sn–S bond and reinforcement of the Sn–O(1) bond. For comparison, the structure of HLI was also studied by X-ray diffraction. Reaction of SnMe2O with isatin 2,3-bis(thiosemicarbazone) (H2LL) afforded [SnMe2(LL)]. X-Ray diffraction shows that one TSC chain is bound to the metal through the sulfur atom and the azomethine nitrogen atom, forming a five-membered metallacycle. The other chain co-ordinates through the nitrogen of the hydrazine group following its deprotonation. The co-ordination sphere changes in dmso-d6 solution, in which a molecule of solvent probably occupies a co-ordination position to make the co-ordination number of the metal center up to six. The spectra of the other [SnR2(LL)] complexes prepared (R = Et or Bu) suggest that both have structures similar to that of the methyl derivative. Reaction of SnMe2O with isatin 2-thiosemicarbazone (H2LII) gave [SnMe2LII], the NMR spectra of which indicate a five-co-ordinated tin atom and an O,N,S-tridentate TSC ligand.

Characterization and crystal structure of zinc azurin, a by-product of heterologous expression in Escherichia coli of Pseudomonas aeruginosa copper azurin.

Azurin*, a by-product of heterologous expression of the gene encoding the blue copper protein azurin from Pseudomonas aeruginosa in Escherichia coli, was characterized by chemical analysis and electrospray ionization mass spectrometry, and its structure determined by X-ray crystallography. It was shown that azurin* is native azurin with its copper atom replaced by zinc in the metal binding site. Zinc is probably incorporated in the apo-protein after its expression and transport into the periplasm. Holo-azurin can be reconstituted from azurin* by prolonged exposure of the protein to high copper ion concentrations or unfolding of the protein and refolding in the presence of copper ions. An X-ray crystallographic analysis of azurin* at 0.21-nm resolution revealed that the overall structure of azurin is not perturbed by the metal exchange. However, the geometry of the co-ordination sphere changes from trigonal bipyramidal in the case of copper azurin to distorted tetrahedral for the zinc protein. The copper ligand Met121 is no longer co-ordinated to zinc which adopts a position close to the carbonyl oxygen atom from residue Gly45. The polypeptide structure surrounding the metal site undergoes moderate reorganization upon zinc binding. The largest displacement observed is for the carbonyl oxygen from residue Gly45, which is involved in copper and zinc binding. It moves by 0.03 nm towards the zinc, thereby reducing its distance to the metal from 0.29 nm in the copper protein to 0.23 nm in the derivative.

Zur Kristallstruktur von Sr6(CrO4)3Cl / On the Crystal Structure of Sr5(CrO4)3Cl

Abstract Single crystal X-ray investigation shows that Sr5(CrO4)3Cl is not isotypic with the apatite structure as being affirmed up to now. Chlorine occupies a position other than z = 0.250 implying the lower symmetry C6 6-P 63. As a consequence one strontium coordination sphere changes from octahedral to trigonal prismatic.

Electron transfer spectra of some solid lanthanide(III) complexes

Electron transfer spectra are reported for the compounds LnPO4,Ln2(CO3)3,3H2O,Ln2(oxalate)3,10H2O, and several compounds containing sulphate or halide ions (Ln = Sm, Eu, and Yb). The shifts in the band positions correlate with the number of co-ordinated ligands. All oxygen donors of the type AOnx–(sulphate, selenate, etc.) show similar optical electronegativities. The ytterbium spectra are sometimes difficult to correlate with those of the other metals because of changes in co-ordination sphere. Electron transfer spectra of lanthanide halides do not always show the same halide dependence as transition-metal spectra.