Characterization of Mn(II) and Mn(III) binding capability of natural siderophores desferrioxamine B and desferricoprogen as well as model hydroxamic acids

Complexes formed between Mn(II) ion and acetohydroxamic acid (HAha), benzohydroxamic acid (HBha), N-methyl-acetohydroxamic acid (HMeAha), DFB model dihydroxamic acids (H₂(3,4-DIHA), H₂(3,3-DIHA), H₂(2,5-DIHA), H₂(2,5-H,H-DIHA), H₂(2,4-DIHA), H₂(2,3-DIHA)) and two trihydroxamate based natural siderophores, desferrioxamine B (H₄DFB) and desferricoprogen (H₃DFC) have been investigated under anaerobic condition (and some of them also under aerobic condition). The pH-potentiometric results showed the formation of well-defined complexes with moderate stability. Monohydroxamic acids not, but all of the dihydroxamic acids and trihydroxamic acids were able to hinder the hydrolysis of the metal ion up to pH ca. 11. Maximum three hydroxamates were found to coordinate to the Mn(II) ion, but presence of water molecule in the inner-sphere was also indicated by the corresponding relaxivity values even in the tris-chelated complexes. Moreover, prototropic exchange processes were found to increase the relaxation rate of the solvent water proton over the value of [Mn_aqua]²⁺ in the protonated Mn(II)-siderophore complexes at physiological pH. The much higher stability of Mn(III)-hydroxamate (especially tris-chelated) complexes compared to the corresponding Mn(II)-containing species results in a significantly decreased formal potential compared to the Mn(III)_aqua/Mn(II)_aqua system. As a result, air oxygen becomes an oxidizing agent for these manganese(II)-hydroxamate complexes above pH 7.5. The oxidation processes, followed by UV-Vis spectrophotometry, were found to be stoichiometric only in the case of the tris-chelated complexes of siderophores, which predominate above pH 9. ESI-MS provided support about the stoichiometry and cyclic-voltammetry was used to determine the stability constants for the tris-chelated complexes, [Mn(HDFB)]⁺ and [MnDFC].     

Novel dioxomolybdenum(VI) and oxomolybdenum(V) complexes with pyridoxal thiosemicarbazone ligands: Synthesis and structural characterisation

New molybdenum complexes were prepared by the reaction of [Mo^VIO₂(acac)₂] or (NH₄)₂[Mo^VOCl₅] with different N-substituted pyridoxal thiosemicarbazone ligands (H₂L¹ = pyridoxal 4-phenylthiosemicarbazone; H₂L² = pyridoxal 4-methylthiosemicarbazone, H₂L³ = pyridoxal thiosemicarbazone). The investigation of monomeric [MoO₂L¹(CH₃OH)] or polymeric [MoO₂L^1-3] molybdenum(VI) complexes revealed that molybdenum is coordinated with a tridentate doubly-deprotonated ligand. In the oxomolybdenum(V) complexes [MoOCl₂(HL^1-3)] the pyridoxal thiosemicarbazonato ligands are tridentate mono-deprotonated. Crystal and molecular structures of molybdenum(VI) [MoO₂L¹(CH₃OH)]·CH₃OH, and molybdenum(V) complexes [MoOCl₂(HL¹)]·C₂H₅OH, as well as of the pyridoxal thiosemicarbazone ligand methanol solvate H₂L³·MeOH, were determined by the single crystal X-ray diffraction method.     

Synthesis and characterization of a high spin d Fe(II) complex with the tridentate ligand dipyrido(2,3-a:3,2-j)phenazine (dpop)

A new bis-(N-tridentate) Fe(II) complex [Fe(dpop^′)₂](PF₆)₂ (dpop^′=dipyrido(2,3-a:3^′,2^′-j)phenazine) was prepared and studied. The magnetic moment of the solid was determined as μ=5.2-4.9 BM and in CH₃CN solution as μ=4.9 BM and indicate the high spin Fe(II) state. The electronic absorption spectrum displays a broad weak absorption MLCT transition at 602 nm (ε=3.8×10³ M⁻¹ cm⁻¹), consistent with CT absorptions of other Fe(II) HS complexes. The cyclic voltammogram of the complex shows an irreversible Fe^2+/3+ oxidation at +1.55 V and two dpop^′0/−1 centered reductions at −0.20 and −0.59 V versus Ag/AgCl.     

Synthesis, structure and solution chemistry of mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes incorporating tridentate ONO donor hydrazone ligands

In the title family, the ONO donor ligands are the acetylhydrazones of salicylaldehyde (H₂L¹) and 2-hydroxyacetophenone (H₂L²) (general abbreviation, H₂L). The reaction of bis(acetylacetonato)oxovanadium(IV) with a mixture of tridentate H₂L and a bidentate NN donor [e.g., 2,2′-bipyridine(bpy) or 1,10-phenanthroline(phen), hereafter B] ligands in equimolar ratio afforded the tetravalent complexes of the type [V^IVO(L)(B)]; complexes (1)-(4) whereas, if B is replaced by 8-hydroxyquinoline(Hhq) (which is a bidentate ON donor ligand), the above reaction mixture yielded the pentavalent complexes of the type [V^VO(L)(hq)]; complexes (5) and (6). Aerial oxygen is most likely the oxidant (for the oxidation of V^IV → V^V) in the synthesis of pentavalent complexes (5) and (6). [V^IVO(L)(B)] complexes are one electron paramagnetic and display axial EPR spectra, while the [V^VO(L)(hq)] complexes are diamagnetic. The X-ray structure of [V^VO(L²)(hq)] (6) indicates that H₂L² ligand is bonded with the vanadium meridionally in a tridentate dinegative fashion through its phenolic-O, enolic-O and imine-N atoms. The general bond length order is: oxo < phenolato < enolato. The V-O (enolato) bond is longer than V-O (phenolato) bond by ∼0.07 Å and is identical with V-O (carboxylate) bond. ¹H NMR spectrum of (6) in CDCl₃ solution indicates that the binding nature in the solid state is also retained in solution. Complexes (1)-(4) display two ligand-field transitions in the visible region near 820 and 480 nm in DMF solution and exhibit irreversible oxidation peak near +0.60 V versus SCE in DMSO solution, while complexes (5) and (6) exhibit only LMCT band near 535 nm and display quasi-reversible one electron reduction peak near −0.10 V versus SCE in CH₂Cl₂ solution. The VO³⁺-VO²⁺E_1/2 values shift considerably to more negative values when neutral NN donor is replaced by anionic ON donor species and it also provides better VO³⁺ binding via phenolato oxygen. For a given bidentate ligand, E_1/2 increases in the order: (L²)²⁻ < (L¹)²⁻.     

Oxidation of thiocyanate by iron(V) in alkaline medium

The oxidation of thiocyanate by iron(V) (Fe(V)) was studied as a function of pH in alkaline solutions by a premix pulse radiolysis technique. The rates decrease with an increase in pH. The rate law for the oxidation of SCN⁻ by Fe(V) was obtained as −d[Fe(V)]/dt = k₁₀{[H⁺]²/([H⁺]² + K₂[H⁺] + K₂K₃)}[Fe(V)][SCN⁻], where k₁₀ = 5.72 ± 0.19 × 10⁶ M⁻¹ s⁻¹, pK₂ = 7.2, and pK₃ = 10.1. The reaction precedes via a two-electron oxidation, which converts Fe(V) to Fe(III). Thiocyanate reacts approximately 10³× faster with iron(V) than does with iron(VI).     

Metal compounds for the treatment of parasitic diseases

The cysteine proteases of the trypanosomatid parasitic protozoa have been validated as targets for chemotherapy of Chagas’ disease and leishmaniasis. Metal complexes of gold, platinum, iridium, palladium, rhodium and osmium have been reported to have activity against a variety of trypanosomatids, but the molecular target of these compounds has not been defined. The activity of gold(III) and palladium(II) cyclometallated complexes, and oxorhenium(V) complexes against mammalian and parasitic cysteine proteases was investigated. All gold(III) complexes (1-6) inhibited cathepsin B with IC₅₀ values in the range of 0.2-1.4 μM. Of the six palladium compounds, aceto[2,6-bis[(butylthio-κS)methyl]phenyl-κC]-, (SP-4-3)-palladium(II) (11) was the most potent inhibitor of cathepsin B with an IC₅₀ of 0.4 μM. A clear structure-activity relationship was observed with the oxorhenium(V) complexes with chloro[2,2′-(thio-κS)bis[ethanethiolato-κS)]] oxorhenium(V) (16) being the most potent inhibitor of cathepsin B with an IC₅₀ of 0.009 μM. Six complexes were further tested against the parasite cysteine proteases, cruzain from T. cruzi, and cpB from L. major; the most potent inhibitors were the two rhenium complexes (2(1H)-pyridinethionato-κS²)[2,6-bis[(mercapto-κS)methyl]pyridine-κN¹] oxorhenium(V) (15) and chloro[2,2′-(thio-κS)bis[ethanethiolato-κS)]] oxorhenium(V) (16). The compounds were also evaluated in assays for parasite growth. Two oxorhenium(V) compounds ((p-methoxyphenylthiolato-S)[2,6-bis[(mercapto-κS)methyl]pyridine-κN¹] oxorhenium(V) (14) and (methanethiolato)[2,2′-(thio-κS)bis[ethanethiolato-κS)]] oxorhenium (V) (18)) and the palladium compound 11 inhibited T. cruzi intracellular growth, and compound 11 inhibited promastigote growth in three Leishmania species. In conclusion this preliminary data indicates that metal complexes targeted at parasite cysteine proteases show promise for the treatment of both Chagas’ disease and leishmaniasis.     

Synthesis, structure and solution chemistry of quaternary oxovanadium(V) complexes incorporating hydrazone ligands

[V^IVO(acac)₂] reacts with an equimolar amount of benzoyl hydrazone of 2-hydroxyacetophenone (H₂L¹) or 5-chloro-2-hydroxyacetophenone (H₂L²) in the presence of excess pyridine (py) in methanol to produce the quaternary [V^VO(L¹)(OCH₃)(py)] (1) and [V^VO(L²)(OCH₃)(py)] (2) complexes, respectively, while under similar condition, the benzoyl hydrazones of 2-hydroxy-5-methylacetophenone (H₂L³) and 2-hydroxy-5-methoxyacetophenone (H₂L⁴) afforded only the methoxy bridged dimeric [V^VO(L³/L⁴)(OCH₃)]₂ complexes. The X-ray structural analysis of 1 and 2 indicates that the geometry around the metal is distorted octahedral where the three equatorial positions are occupied by the phenolate-O, enolate-O and the imine-N of the fully deprotonated hydrazone ligand in its enolic form and the fourth one by a methoxide-O atom. An oxo-O and a pyridine-N atom occupy two axial positions. Quaternary complexes exhibit one quasi-reversible one-electron reduction peak near 0.25 V versus SCE in CH₂Cl₂ and they decompose appreciably to the corresponding methoxy bridged dimeric complex in CDCl₃ solution as indicated by their ¹H NMR spectra. These quaternary VO³⁺ complexes are converted to the corresponding -complexes simply on refluxing them in acetone and to the -complexes on reaction with KOH in methanol. An equimolar amount of 8-hydroxyquinoline (Hhq) converts these quaternary complexes to the ternary [V^VO(L)(hq)] complexes in CHCl₃.     

C4′ sugar oxidation of deoxyribonucleotide triphosphates by chromium(V) complexes

The Cr(V) complexes, bis(2-ethyl-2-hydroxybutyrato)oxochromate(V) ([OCr^V(ehba)₂]⁻) and (2,2-bis(hydroxymethyl)-2-(bis(2-hydroxyethyl)amino)ethanolato)oxochromate(V) ([OCr^V(BT)]²⁻), were reacted with a series of deoxyribonucleotide triphosphates. Oxidation of deoxyribose at C4′ was observed by measuring the amount of thiobarbituric acid reactive species (TBARS) produced in these reactions. For both compounds, the TBARS obtained with purine nucleotides was between 2.25 and 3.5 times greater than what was observed with pyrimidine nucleotide. This result suggests that the identity of the nucleic acid base can influence the hydrogen atom abstraction at C4′. Overall, the amount of product obtained with [OCr^V(BT)]²⁻ was significantly less than what was observed with [OCr^V(ehba)₂]⁻, indicating that these two Cr(V) model complexes may oxidize DNA differently.     

Functional model for intradiol-cleaving catechol 1,2-dioxygenase: Synthesis, structure, spectra, and catalytic activity of iron(III) complexes with substituted salicylaldimine ligands

A series of mononuclear iron(III) complexes with containing phenolate donor of substituted-salicylaldimine based ligands [Fe(L¹)(TCC)] · CH₃OH (1), [Fe(L²)(TCC)] · CH₃OH (2), [Fe(L³)(TCC)] (3), and [Fe(L⁴)(TCC)] (4) have been prepared and studied as functional models for catechol dioxygenases (H₂TCC = tetrachlorocatechol, or HL¹ = N′-(salicylaldimine)-N,N-diethyldiethylenetriamine, HL² = N′-(5-Br-salicylaldimine)-N,N-diethyldiethylenetriamine, HL³ = N′-(4,6-dimethoxy-salycyl-aldimine)-N,N-diethyl-diethylenetriamine, HL⁴ = N′-(4-methoxy-salicylaldimine)-N,N-diethyl-diethylenetriamine). They are structural models for inhibitors of enzyme-substrate adducts from the reactions of catechol 1,2-dioxygenases. Complexes 1-4 were characterized by spectroscopic methods and X-ray crystal structural analysis. The coordination sphere of Fe(III) atom of 1-4 is distorted octahedral with N₃O₃ donor set from the ligand and the substrate TCC occupying cis position, and Fe(III) is in high-spin (S = 5/2) electronic ground state. The in situ prepared iron(III) complexes without TCC, [Fe(L¹)Cl₂], [Fe(L²)Cl₂], [Fe(L³)Cl₂], and [Fe(L⁴)Cl₂] are reactive towards intradiol cleavage of the 3,5-di-tert-butylcatechol (H₂DBC) in the presence of O₂ or air. The reaction rate of catechol 1,2-dioxygenase depends on the redox potential and acidity of iron(III) ions in complexes as well as the substituent effect of the ligands. We have identified the reaction products and proposed the mechanism of the reactions of these iron(III) complexes with H₂DBC with O₂.     

Synthesis, structure and properties of di- and mononuclear ruthenium(III) complexes with N-(benzoyl)-N′-(salicylidene)hydrazine and its derivatives

The reactions of [Ru(PPh₃)₃Cl₂], N-(benzoyl)-N′-(5-R-salicylidene)hydrazines (H₂bhsR, R = H, OCH₃, Cl, Br and NO₂) and triethylamine (1:1:2 mole ratio) in methanol afford mononuclear ruthenium(III) complexes having the general formula trans-[Ru(bhsR)(PPh₃)₂Cl]. In the case of R = H, a dinuclear ruthenium(III) complex of formula [Ru₂(μ-OCH₃)₂(bhsH)₂(PPh₃)₂] has been isolated as a minor product. The complexes are characterized by elemental analysis, magnetic, spectroscopic and electrochemical measurements. The crystal structures of the dinuclear complex and two mononuclear complexes have been determined. In the dinuclear complex, each metal centre is in distorted octahedral NO₄P coordination sphere constituted by the two bridging methoxide groups, one PPh₃ molecule and the meridionally spanning phenolate-O, imine-N and amide-O donor bhsH²⁻. The terminal PPh₃ ligands are trans to each other. In the mononuclear complexes, bhsR²⁻ and the chlorine atom form an NO₂Cl square-plane around the metal centre and the P-atoms of the two PPh₃ molecules occupy the remaining two axial sites to complete a distorted octahedral NO₂ClP₂ coordination sphere. All the complexes display ligand-to-metal charge transfer bands in the visible region of the electronic spectra. The cryomagnetic measurements reveal the antiferromagnetic character of the diruthenium(III) complex. The low-spin mononuclear ruthenium(III) complexes as well as the diruthenium(III) complex display rhombic EPR spectra in frozen solutions. All the complexes are redox active in CH₂Cl₂ solutions. Two successive metal centred oxidations at 0.69 and 1.20 V (versus Ag/AgCl) are observed for the dinuclear complex. The mononuclear complexes display a metal centred reduction in the potential range −0.53 to −0.27 V. The trend in these potential values reflects the polar effect of the substituents on the salicylidene moiety of the tridentate ligand.     

Functional model for catecholase-like activity: Synthesis, structure, spectra, and catalytic activity of iron(III) complexes with substituted-salicylaldimine ligands

Four new mononuclear iron(III) complexes with the substituted-salicylaldimine ligands, [Fe(L¹)(TCC)] (1), [Fe(L²)(TBC)] (2), [Fe(L³)(TBC)] (3) and [Fe(L⁴)(TCC)](CH₃CN) (4) (HL¹ = N′-(5-OH-salicylaldimine)-diethylenetriamine, HL² = (N′-(5-Cl-salicylaldimine)-diethylenetriamine, HL³ N′-(5-Br-salicyl-aldimine)-dipropylenetriamine, HL⁴ = (N′-3,5-Br-salicylaldimine)-dipropylenetriamine, H₂TCC = tetrachlorocatechol, and H₂TBC = tetrabromocatechol), were prepared and characterized by XRD, EPR, and Mössbauer spectroscopy. The coordination sphere of the Fe(III) in complexes 1-4 is a distorted octahedral with N₃O₃ donors set which constructed by the Schiff-base ligands and the catecholate substrates of TBC or TCC. The in situ prepared Fe(III) complexes [Fe(L¹)Cl₂], [Fe(L²)Cl₂], [Fe(L³)(Cl₂)], and [Fe(L⁴)Cl₂] in absence of TBC or TCC show a high catecholase-like activity for the oxidation of 3,5-DTBC to the corresponding quinone 3,5-DTBQ.     

Synthesis, structural characterization and electrochemical activity of oxidovanadium(IV/V) complexes of a diprotic ONS chelating ligand

The present work reports the chemistry of a few oxidovanadium(IV) and (V) complexes of the ONS chelating ligand S-benzyl-β-N-(2-hydroxyphenylethylidine) dithiocarbazate (H₂L). Major objective of this work is to arrive at some general conclusions about the influence of binding environment generated by the replacement of an O-donor center by a S-donor point in a ligand (of a similar arrangement of the other O- and N-donor points) on the redox behavior and on the structural features of comparable [VO(OEt)(ONS)] and [VO(OEt)(ONO)] complexes. Synthesis, characterization by various physicochemical techniques (UV-Vis, IR, EPR and elemental analysis), exploration of electrochemical activity of the oxidovanadium(V) complex [V^VO(OEt)L] (1), the mixed ligand complex [V^VO(N-O)L] (3) (where N-O is the mono anion of 8-hydroxyquinoline) and a binuclear complex [V^VO(OEt)L]₂(μ-4,4′-bipy) (2) are reported. Similar studies on of mixed ligand oxidovanadium(IV) complexes of the formula [V^VO(N-N)L] (4,5) (where N-N = 2,2′-bipy and o-phen) are also presented here. The [V^VO(OEt)L] complex is pentacoordinated and distorted square pyramidal, while the [V^IV(N-N)L] complexes are hexacoordinated and octahedral. Structural features of the complex 1 were compared with the corresponding aspects of the previously reported analogous complex [V^VO(OEt)(ONO)] (1′).     

Synthesis and characterisation of dinuclear oxomolybdenum(V) complexes with thienyl carboxylate ligands

The first structurally characterised oxomolybdenum(V) complexes with thienyl carboxylate ligands were prepared by the reaction of [Mo₂O₃(C₅H₇O₂)₄] or (NH₄)₂[MoOCl₅] with the corresponding acid (2-thiophenecarboxylic, 5-methyl-2-thiophenecarboxylic or 3-(3-thienyl)acrylic acid). Complexes [Mo₂O₃(μ-OC₂H₅)(μ-O₂CR)(C₅H₇O₂)₂](R = -C₄H₃S (1), -C₄H₂S(CH₃) (2) or -CHCHC₄H₃S (3)) were obtained upon substitution of two acetylacetonate ligands from [Mo₂O₃(C₅H₇O₂)₄] with RCOO⁻ in dry ethanol. Reactions of (NH₄)₂[MoOCl₅] with the corresponding thienyl carboxylic acid in the presence of γ-picoline (C₆H₇N) yielded complexes (C₆H₇NH)[Mo₂O₄(μ-O₂CR)Cl₂(C₆H₇N)₂] (R = -C₄H₃S (4), -C₄H₂S(CH₃) (5) or -CHCHC₄H₃S (6)). All of the six new complexes were characterised as dinuclear. The molecular structures of 1, 3, 4·0.5CH₃CN and 5 were determined by the single crystal X-ray diffraction method. In the complexes the two molybdenum atoms are doubly bridged either by one oxygen and one ethoxy-oxygen, or alternatively by two oxo-oxygens, and are additionally bridged by the thienyl carboxylate ion in a didentate bridging manner. All complexes were further characterised by means of chemical analysis, IR spectroscopy, TG and in some cases by the one and two-dimensional NMR method.     

Electron transfer. Part 169 . Delayed reduction of mononuclear manganeses(IV) using vanadium(III)

The tris-chelate formed by biguanide, H₂NC(NH)NHC(NH)NH₂, and Mn(IV) is one of the rarely encountered examples of water-stable mononuclear complexes of this oxidation state. This cation is reduced in aqueous acid by both V(II) and V(III). In contrast to the kinetically straightforward bimolecular reduction by V(II) in 0.5 M HClO₄ (k = 7.4 × 10³ M⁻¹ s⁻¹ at 22 °C), reductions by excess V(III) yield profiles which are linear (zero-order in Mn^IV) until the last few percent reaction. Analyses of these composite curves are consistent with the sequences

     

High-coordinated lanthanum(III) complexes with new mono- and bidentate phosphoryl donors; spectroscopic and structural aspects

Monodentate and bidentate ligands PhNHP(O)(NC₄H₈O)₂ (1) and PhC(O)NHP(O)(NH(tert-C₄H₉))₂ (2) were used to prepare new 7, 9 and 10-coordinated lanthanum(III) complexes; La(1)₂Cl₃(H₂O)₂ (3), La(1)₂(NO₃)₃H₂O.La(1)₂(NO₃)₃CH₃CN (4) and La(2)₂(NO₃)₃ (5), respectively. Crystallization of compound 2 in CH₃OH:CH₃CN leads to one conformer in contrast to the crystallization result from CHCl₃:n-C₇H₁₆ (two conformers). Compound 4 contains two independent nine-coordinated La(III) complexes that are different in the solvated molecules (H₂O and CH₃CN). Some structural and electronic perturbations in coordinated ligand were occurred upon complexation, that are confirmed by increase of ²J_PH, ³J_PH and ⁶J_PH coupling constants from the free ligand 1 to complexes 3 and 4. The steric repulsions in the first coordination sphere of La³⁺ ion, metal-ligand (M-L) binding strength and PO stretching frequency are very influenced by changing the counter ion from Cl⁻ to . Comparing the X-ray crystallography data of free ligand 2 with bis-chelated complex 5, it is found that the phosphoryl group is more reactive than carbonyl counterpart. A blue shift of the ν(N-H) vibration is observed in line with the weakening of the hydrogen bond from N-H···O_Phosphoryl in 1 to N-H···Cl in 3. Three dimensional butterfly-shape structures are seen in the unit cell of complex 3, which are produced by O_Water-H···O_Morpholine hydrogen bonds.     

Syntheses and properties of new metal-carbon bonded heteroleptic complexes of ruthenium(II) containing terpyridine coligand

Reactions of 2-(arylazo)aniline, HL [H represents the dissociable protons upon orthometallation and HL is p-RC₆H₄N = NC₆H₄-NH₂; R = H for HL¹; CH₃ for HL² and Cl for HL³] with Ru(R₁-tpy)Cl₃ (where R₁-tpy is 4′-(R₁)-2,2′,6′′,2′′-terpyridine and R₁ = H or 4-N,N-dimethylaminophenyl or 4-methylphenyl) afford a group of complexes of type [Ru(L)(R₁-tpy)]·ClO₄ each of which contains C,N,N coordinated L⁻ as a tridentate ligand along with a terpyridine. Structure of one such complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, display characteristic ¹H NMR signals and intense dπ(Ru^II) → π∗(tpy) MLCT transitions in the visible region. Cyclic voltammetric studies on [Ru(L)(R₁-tpy)]·ClO₄ complexes show Ru(II)-Ru(III) oxidation within 0.63-0.67 V versus SCE.     

Mixed ligand complexes of osmium(II)-2,2-bipyridine: synthesis, spectral characterization and electrochemical properties of bis-chelated-arylazoimidazole-bipyridine-osmium(II) and X-ray crystal structure of [(2,2-bipyridine)-bis-{1-methyl-2-(p-tolylazo)imidazole}osmium(II) hexafluorophosphate

Reaction of ctc-OsBr₂(RaaiR^′)₂ [RaaiR^′=1-alkyl-2-(arylazo)imidazole, p-R-C₆H₄-NN-C₃H₂-NN-1-R^′, where R=H (a), Me (b), Cl (c) and R^′=Me (2), Et (3) and CH₂Ph (4)] with 2,2^′-bipyridine (bpy) in presence of AgNO₃ in EtOH followed by the addition of NH₄PF₆ afforded a mixed ligand complex [Os(bpy)(RaaiR^′)₂](PF₆)₂. The structure of the complex, in one case [Os(bpy)(MeaaiMe)₂](PF₆)₂ · 4H₂O, has been confirmed by X-ray crystallography. The complexes are diamagnetic (low spin d⁶, s=0) and they show intense MLCT transition in the visible region (480-525 nm) and a weak transition at longer wavelength (>850 nm) in CH₃CN solution. Cyclic voltammetry of the complexes show two metal oxidation, Os(II)/Os(III) at 0.72-0.76 V and Os(III)/Os(IV) at 1.34-1.42 V and three successive ligand reductions.     

Coordination chemistry of acrylamide 6: Formation and structural characterization of [Fe(O-OC(NH2)CHCH2)6][Fe2OCl6]

The new acrylamide iron(II)/iron(III) complex [Fe(O-OC(NH₂)CHCH₂)₆][Fe₂OCl₆] (1) was obtained by the reaction of a mixture of anhydrous FeCl₂ and anhydrous FeCl₃ with acrylamide (molar ratio 1:2:6) in 98% pure commercial nitromethane under nitrogen atmosphere. According to an X-ray structural analysis, the acrylamide ligands in the cation are coordinated via the amide-oxygen atoms. The formation of the (μ-oxo)bis[trichloroferrate(III)]²⁻ anion presumably resulted from partial hydrolysis of FeCl₃ or [FeCl₄]⁻ by small amounts of water in the nitromethane and/or by the nitromethane itself.     

Synthesis and characterization of some oxovanadium(V) complexes with internally functionalized oximes: Crystal and molecular structures of heptacoordinated [VO{ONC(CH3)(C4H3O-2)}3] and [VO{ONC(CH3)(C4H3S-2)}3] · 0.5C6H6

New oxovanadium(V) complexes with internally functionalized oximes of the type VO{OPrⁱ}_3−n{ONC(CH₃)(Ar)}_n] (where Ar = C₄H₃O-2, C₄H₃S-2 and C₅H₄N-2 and n = 1-3) have been prepared in quantitative yields by the reaction of VO(OPrⁱ)₃ with the corresponding oximes in various stoichiometric ratios in refluxing anhydrous benzene. The products have been characterized by elemental analyses and spectroscopic (FT IR, ¹H, ¹³C{¹H} and ⁵¹V NMR) studies. FAB mass spectral analysis of [VO{OPrⁱ}{ONC(CH₃)C₄H₃S}₂] indicates the monomeric nature of the complex. ⁵¹V NMR values for these complexes suggest the formation of tetra-coordinate species in solution. However, the single crystal X-ray diffraction studies of [VO{ONC(CH₃)(C₄H₃O-2)}₃] and [VO{ONC(CH₃)(C₄H₃S-2)}₃] · 0.5C₆H₆ exhibit the presence of vanadium(V) atoms in a unique hepta-coordination state with distorted pentagonal bipyramidal geometry in the solid state. The oxo- atom occupies the axial position while the oximato ligands are bonded in a dihapto (η²-N,O) manner with the formation of three membered rings.     

Structure-activity relationship on DNA binding and anticancer activities of a family of mixed-ligand oxidovanadium(V) hydrazone complexes

The title family of mixed-ligand oxidovanadium(V) hydrazone complexes are [V^VO(HL¹)(hq)] (1) and [V^VO(HL²)(hq)] (2), where (HL¹)^2? and (HL²)^2? are the dinegative form of 2-hydroxybenzoylhydrazone of acetylacetone (H₃L¹) and benzoylacetone (H₃L²), respectively, and hq^? is the mononegative form of 8-hydroxyquinoline (Hhq). Complexes were used to determine their binding constant with CT DNA using various spectroscopic techniques namely, electronic absorption, fluorescence and circular dichroism spectroscopy. The binding constant values suggest the intercalative mode of binding with the CT DNA and follow the order: 2 > 1. The bulky size as well as electron withdrawing property of the phenyl group (which is present in the β-diketone part of the hydrazone moiety in complex 2 in place of a CH₃ group in complex 1) is responsible for the higher activity of 2 than 1. Complexes were screened for cytotoxic activity on cervical cancer cells and were found to be potentially active (IC₅₀ value for 1 and 2 is 33 and 29 μM, respectively), even better than the widely used cis-platin (IC₅₀ = 63.5 μM) and carboplatin (IC₅₀ = > 200 μM) which is evident from the respective IC₅₀ value. Nuclear staining experiment suggests that these complexes kill the SiHa cancer cells through apoptotic mode. The molecular docking study also suggested the intercalative mode of binding of these complexes with CT DNA and HPV 18 DNA.