Irreversible Reduction Wave Research Articles

Reductive Cleavage of Halides in Decomposition of Substituted Benzonitrile Pesticides

The reduction mechanism of chloroxynil (3,5 dichloro 4 hydroxy- benzonitrile), bromoxynil (3,5 dibromo 4 hydroxybenzonitrile) and ioxynil (3,5-diiodo-4-hydroxybenzonitrile) was studied in dimethylsulfoxide by the electrochemical methods combined with spectroelectrochemical and GC/MS identification of the products. DC polarography shows two irreversible reduction waves for ioxynil and only one reduction wave for bromoxynil and chloroxynil. Cyclic voltammetry at different scan rates and at various concentrations of pesticides is consistent with the radical anion formation followed by the subsequent chemical reactions. The main decomposition pathway includes the cleavage of the halogen atom and the dimerization of the dehalogenated intermediate. The differences in the mechanism of ioxynil, bromoxynil and chloroxynil are caused by the mutual differences in the rates of the bond cleavage and the dimerization process.

Synthesis of new ‘end-off’ μ-phenoxo and bis-μ-acetato tri-bridged copper(II), nickel(II) and zinc(II) complexes: Spectral, magnetic, electrochemical and catalytic studies

A new end-off type acyclic ligand with two N-methyl piperazine arms, 2,6-bis[(4-methyl piperazin-1-yl)]-4-formyl phenol [L], has been synthesized by a simple Mannich reaction. The mono and binuclear complexes of Cu(II), Ni(II) and Zn(II) have been prepared. The complexes were characterized by elemental and spectral analysis. The EPR spectrum of the mononuclear copper complex shows four hyperfine splittings and the binuclear complex shows a broad signal due to an anti-ferromagnetic interaction. The room temperature magnetic moment of the mono and binuclear copper complexes are found to be 1.72 μ B ( μ eff) and 1.58 μ B ( μ eff), respectively. The mononuclear Ni(II) complex is square planar and diamagnetic, whereas the six-coordinated binuclear Ni(II) complex shows a magnetic moment value of 2.98 μ B ( μ eff). Cyclic voltammetric studies evidenced one irreversible reduction wave for all the mononuclear complexes and two irreversible one-electron reduction waves for the binuclear complexes are obtained in the cathodic region. In the anodic region, one irreversible oxidation wave for the mononuclear nickel(II) complex and two irreversible oxidation waves for the binuclear nickel(II) complexes are obtained. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the mono and binuclear copper(II) complexes as catalysts and on the hydrolysis of 4-nitrophenylphosphate using the mono and binuclear copper(II), nickel(II) and zinc(II) complexes as catalysts were carried out. The binuclear complexes have higher rate constant values than those of the corresponding mononuclear complexes. The rate constant values for the complexes for the hydrolysis are in the order nickel(II) > copper(II) > zinc(II).

Unexpected New Chemistry of the Bis(thioimidazolyl)methanes

[reaction: see text] The synthesis of the linkage isomers of the bis(thioimidazolyl)methane family of compounds, namely CH(2)(N-tim)(2) (1) and CH(2)(S-tim)(2) (2) (where tim = thio(methyl)imidazolyl) has been reinvestigated in order to optimize the yields, to complete the characterization of these known compounds, and also to ascertain the effect of varying heteroatom binding on their electrochemistry. During the course of these studies, the reactive intermediate ClCH(2)(S-tim) (3) was isolated and characterized. The chloromethyl derivative 3 readily decomposes on warming to give the ionic compound [CH(2)(mu-C(4)H(5)N(2)S)(2)CH(2)](Cl)(2) (4), which was converted to the hexafluorophosphate salt (5) and then was characterized by single-crystal X-ray diffraction. It was also shown that CH(2)(S-tim)(2) (2) could be converted at temperatures greater than 120 degrees C to CH(2)(N-tim)(2) (1) by a thermal isomerization that proceeds via the remaining possible linkage isomer CH(2)(S-tim)(N-tim). Electrochemical studies on 1-3 in acetonitrile reveals that each undergoes irreversible (one electron per ring) oxidations above 0.7 V versus Ag/AgCl, while the ionic compound 5 shows an irreversible reduction wave centered at -1.09 V.

Multinuclear Fe(III) Complexes with Polydentate Ligands of the Family of Dicarboxyimidazoles: Nuclearity- and Topology-Controlled Syntheses and Magneto-Structural Correlations

Two polydentate ligands of the family of dicarboxyimidazoles, H(2)MeDCBI (= 4,5-dicarboxy-1-methyl-1H-imidazole) and H(3)DCBI (= 4,5-dicarboxyimidazole), have been used in reactions with the [Fe(3,5-(t)()Bu(2)salpn)](+) species {3,5-(t)Bu(2)salpn = the dianion of 1,3-bis-[(3,5-di-tert-butylsalicylidene)amino]propane} to synthesize selectively complexes of different nuclearities. Four complexes have been synthesized: the mononuclear complex [Fe(3,5-(t)Bu(2)salpn)(HMeDCBI)] (1), the two binuclear but topologically different complexes [Fe(3,5-(t)Bu(2)salpn)(MeDCBI)Fe(3,5-(t)Bu(2)salpn)] (2) and {[Fe(3,5-(t)Bu(2)salpn)](2)(HDCBI)} (3), and the trinuclear complex {[Fe(3,5-(t)Bu(2)salpn)](2)(DCBI)Fe(3,5-(t)Bu(2)salpn)} (4). The structures of these complexes have been determined by X-ray crystallography. Variable-temperature direct-current magnetic susceptibility measurements were conducted for all compounds to obtain information about their electronic structure and to investigate the extent of magnetic communication among the Fe(III) centers. The results of these measurements allowed us to correlate the different structural motifs with the possible magnetic interactions that arise in multinuclear complexes of dicarboxyimidazoles. For 1, the room-temperature chi(M)T value reveals an S = (5)/(2) ground state. The data for the binuclear but topologically different complexes 2 and 3, and the trinuclear complex 4 suggest that weak intramolecular antiferromagnetic interactions are present, with interaction parameters ranging from -3.6 to -5.1 cm(-1). Differences in the extent of the magnetic communication between the metal centers through the two different interaction pathways of the ligands MeDCBI and DCBI (either through the imidazole ring or through the carboxylate groups) have been observed in complexes 2-4 that can be explained by the structural differences observed in the crystal structures of these compounds (the separation of the metal centers and the coplanarity of the metal ion orbitals with the pi system of the ligands). Cyclic voltammetry measurements for the mononuclear compound 1 show an irreversible reduction wave that is attributed to Fe(3+) + e(-) --> Fe(2+). The electrochemical behavior of the multinuclear complexes 2-4 is more complicated; however, it indicates that there is a degree of electronic communication between the Fe(III) centers.

Ditopic Macropolycyclic Complexes: Synthesis of Hybrid Phthalocyaninoclathrochelates

A remetalation (a capping group exchange) reaction of the boronantimony-capped iron(II) clathrochelates with zirconium and hafnium(IV) phthalocyanines in CH2Cl2/CH3OH medium afforded the hybrid phthalocyaninoclathrochelates in a practically quantitative yield. The complexes obtained have been characterized both on the basis of elemental analysis, PD mass spectrometry, IR, UV-vis, 57Fe Mössbauer, and NMR spectroscopies, and crystallographically. An encapsulated iron(II) ion in an intermediate between a trigonal-prismatic and a trigonal-antiprismatic environment of six nitrogen atoms of the macrobicyclic ligand was found to be in a low-spin state. The cyclic voltammograms show irreversible oxidation and reduction waves assignable to Fe+/Fe2+ couples of macrobicyclic fragments and to phthalocyanine macrocycles.

Mononuclear oxovanadium complexes of tris(2-pyridylmethyl)amine

Mononuclear oxovanadium(IV) and dioxovanadium(V) complexes of tris(2-pyridylmethyl)amine (tpa) have been prepared for the first time. Crystal structure determinations of three oxovanadium(IV) complexes, [VO(SO4)(tpa)], [VOCl(tpa)]PF6, or [VOBr(tpa)]PF6, and a dioxovanadium(v) complex [V(O)2(tpa)]PF6 disclosed that the tertiary nitrogen of the tpa ligand always occupies the trans-to-oxo site. The structures of an oxo-peroxo complex [VO(O2)(tpa)]Cl that was prepared previously and of a mu-oxo vanadium(III) complex [{VCl(tpa)}2(mu-O)](PF6)2 have also been determined. The tertiary nitrogen is located at a trans site to the peroxo and chloride ligands, respectively. The total sums of the four V-N bond lengths from the tpa ligand are remarkably similar among the six complexes, indicating that the vanadium oxidation states become less influential in tpa bonding due primarily to the coordination of electron-donating oxo ligand(s). Absorption spectra of [VOCl(tpa)]+ in acetonitrile showed a significant change upon addition of p-toluenesulfonic acid and HClO4, but not on addition of benzoic acid. Protonation at the oxo ligand by the former two acids is suggested. Cyclic voltammetric studies in acetonitrile verified the proton-coupled redox behavior of the V(III)/V(IV) process involving the oxo ligand for the first time. From the dependence of the added p-toluenesulfonic acid to the CV, redox potentials for the following species have been estimated: [V(IV)OCl(tpa)]+/[V(III)OCl(tpa)](E1/2=-1.59 V vs. Fc+/Fc), [V(IV)(OH)Cl(tpa)]2+/[V(III)(OH)Cl(tpa)]+(Epc=-1.34 V), [V(IV)(OH2)Cl(tpa)]3+/[V(III)(OH2)Cl(tpa)]2+(Epa=-0.49 V), and [V(IV)Cl2(tpa)]2+/[V(III)Cl2(tpa)]+(E1/2=-0.89 V). The reduction of [V(V)(O)2(tpa)]+ in 0.05 M [(n-Bu)4N]PF6 acetonitrile showed a major irreversible reduction wave V(V)/(IV) at -1.48 V. The metal reduction potentials of the oxovanadium(IV) and dioxovanadium(V) species are very close, reinforcing the significant influence of the oxo ligand(s).

Electrochemical reduction of oxygen and hydrogen peroxide catalyzed by a surface copper(II)–2,4,6-tris(2-piridil)-1,3,5-triazine complex adsorbed on a graphite electrode

A graphite electrode irreversibly adsorbed by 2,4,6-tris(2-piridil)-1,3,5-triazine (abbreviated as TPT) was examined by cyclic voltammetry. The adsorbed TPT exhibited two irreversible reduction waves in the potential range of −0.7 and −1.0 V (versus SCE). Upon strong adsorption, TPT can serve as a coordination ligand for copper ions to form a surface complex. Its three adjacent nitrogen positions provide strong affinity to the metal ions and bond copper(II) to an electrode surface. A 1:1 coordination between Cu(II) or Cu(I) and the TPT ligand to form [Cu(II)(TPT)] 2+ or [Cu(I)(TPT)] + is the predominant process, evidenced by spectrophotometry, surface cyclic voltammetry, and coordinated structural feasibility of Cu(II)/Cu(I)–TPT complexes. The predominant copper(II)–TPT surface complex shows a reversible redox wave, which is identified as one-electron process of [Cu(II)(TPT)] 2+ ↔ [Cu(I)(TPT)] +. The electrode adsorbed by [Cu(II)(TPT)] 2+ complex showed electrocatalytic activity towards oxygen and/or hydrogen peroxide reductions. The catalyzed reduction of oxygen and hydrogen peroxide were identified as four-electron and two-electron process to form water. It is suggested that the possible electrocatalytic reductions were due to an inner-sphere mechanism, which involved a coordination between substrate (O 2 or H 2O 2) and [Cu(I)(TPT)] +. The reduction kinetics were also investigated by a rotating disk electrode method.

Spectral and electrochemical studies of axial ligand binding reactions of carbonylruthenium(II) meso-tetramesitylporphyrin

The spectral and electrochemical properties of carbonylruthenium(II) meso-tetramesitylporphyrin, Ru(CO)(TMP) in the presence of imidazole, N-methylimidazole and hydroxide anion as axial ligands were investigated. Spectrophotometric titration of Ru(CO)(TMP) of the nitrogeneous bases caused the absorption spectrum to shift to longer wavelengths. Larger shifts in wavelength were observed in the titration using tetrabutylammonium hydroxide. The formation constants for these ligands coordinated to the ruthenium center were calculated. The effect of axial ligand ligation caused the decrease of vibrational frequency of CO as detected from FT-IR spectroscopy. The CO stretching frequency ( ν CO) of Ru(CO)TMP in CH 2Cl 2 is 1940 cm −1, which is lowered to 1936 and 1913 cm −1 upon coordination of nitrogenous bases and hydroxide anion, respectively. Cyclic voltammetry of Ru(CO)(TMP) in CH 2Cl 2 showed an irreversible reduction wave at −1.63 V and two reversible oxidations at E 1/2 = 0.78 and 1.27 V vs. Ag|AgCl, respectively. Addition of imidazole and hydroxide into Ru(CO)(TMP) solution caused shifts in the redox potential and accordingly, the binding constants of the ligands to the one- and two-electron oxidized ruthenium porphyrins were estimated and compared. Spectroelectrochemical methods were used to characterize the above electron-transfer reactions.

Investigation of the electrochemical behavior of metallo-tetraazaporphyrin modified silver and pyrolytic graphite electrodes in aqueous nitrite solution

Five new complexes of the type [M(OBTTAP)] (where M = Mn 2+, Co 2+, Ni 2+, Cu 2+ and Zn 2+; OBTTAP = 2,3,7,8,12,13,17,18-octakis(benzylthio)-5,10,15,20-tetraazaporphyrin(2−)) were used to prepare modified silver and pyrolytic graphite electrodes and their electrochemical behavior towards the reduction of aqueous nitrite solution was investigated in acidic, neutral and alkaline medium. Of these electrodes, [Co(OBTTAP)] and [Cu(OBTTAP)] modified electrodes showed affinity for NO 2 - ions and thereby exhibited significantly altered cathodic responses. The [Co(OBTTAP)]/Ag electrode in acidic, neutral and also in alkaline nitrite solutions, exhibited a new one electron irreversible reduction wave at E p,c ∼ −0.2 V vs. Ag|AgCl. The [Cu(OBTTAP)]/Ag electrode in acidic and neutral nitrite solutions also behaved in a similar manner. However, in alkaline aqueous nitrite solution it showed two successive one electron reductions, the first reversible reduction at E 1/2 = −0.26 V and the second irreversible reduction at E p,c = −0.73 V vs. Ag|AgCl. Controlled potential electrolysis of alkaline nitrite solutions using a [Cu(OBTTAP)]/Ag cathode led to isolation of a nitrosyl complex. The peak current, particularly with [Cu(OBTTAP)]/Ag microelectrode in alkaline medium, showed a linear response to [ NO 2 - ] and could be exploited in the electrochemical sensing and determination of NO 2 - .

Synthesis of new octakis(benzylthio)tetraazaporphyrin complexes with bivalent transition metal ions and an investigation of their spectral, redox and catalytic properties

Transition metals in the +2 oxidation state form complexes with octakis(benzylthio)tetraazaporphyrin [M{TAP(SBn)8}] (M=Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and 2H+; Bn=CH2Ph; TAP=tetraazaporphyrin) and were prepared via demetallation of the template-synthesized complex [Mg{TAP(SBn)8}] in ClCH2COOH, followed by insertion of the metal ions in a PhCl–DMF mixture. All compounds were characterized by u.v–vis, 1H-n.m.r., e.s.r. and mass spectra, microanalyses and magnetic susceptibility data. PM3 geometry optimization showed near coplanar arrangement of five phenyl rings vis-a-vis the TAP ring plane, while the remaining three phenyl rings lie bent over it. The Mn2+, Co2+ and Cu2+ complexes exhibited reversible one electron oxidation waves, that were observed at E1/2 -0.33, -0.29 and 0.58 V versus Ag/AgCl respectively. They all exhibited an irreversible reduction wave at Ep -1.1 V versusAg/AgCl followed by appearance of an irreversible oxidation wave at Ep 1.1–1.2 V versus Ag/AgCl. This has been interpreted in terms of reductive dissociation, leading to removal of the benzyl substituents, and subsequent oxidation of thiolate products. The Mn2+, Co2+ and Cu2+complexes exhibited metal ion-based e.s.r. signals. The [Co{TAP(SBn)8}] complex in a homogeneous DMF solution was found to catalyze electro-reduction/oxidation of nitrite ions. In the presence of a 1/5 molar quantity of the complex [Co{TAP(SBn)8}], the nitrite reduction peak was observed to be shifted by 0.1 V towards the positive potential side and the peak current rose by a factor of 1.33. Similarly a sharp rise in the anodic current, corresponding to oxidation of nitrite ions, was observed in the presence of the complex.

Spectroscopic and electrochemical characterization of di- tert-butylated sterically hindered Schiff bases and their phenoxyl radicals

A series of sterically hindered N-arylsalicylaldimines (SAs) previously prepared from substituted salicylaldehydes (X-Sal, where X = H, Cl, Br, NO 2, OH, OCH 3) and 2,6-di- t-butyl-1-hydroxyaniline (L x H) and 2,5-di- t-butylaniline (L x ′H) were characterized by 1 H and 13 C NMR, UV-Vis and electrochemical methods. The electronic spectra (ES) (X = OH, OCH 3) in alcoholic solvents (MeOH, EtOH, PrOH, iso-PrOH) unlike other solvents exhibit a new absorption band in the region 630–675 nm ( ε=19–242 M −1 cm −1), which are not characteristic for other SA known in literature. The ESR studies of primary phenoxyl radicals generated from L x H by their oxidation with PbO 2 reveal that some of them with the time are converted to more stable secondary Coppinger’s radical. The cyclic voltammograms of L x H and L x ′H except NO 2-substituted ones in CH 3CN are similar and along with two or three irreversible anodic waves at the potentials ranging from 0.0 to +1.9 V versus Ag/AgCl, also display one or two irreversible reduction waves at potentials −0.6 to +0.5 V. A series of a new SA prepared from 3,5-di- t-butylsalicylaldehyde and mono-substituted anilines (X = H, o-, p-F, Cl, Br, OCH 3, p- t-butyl, 5,6-benzo) were characterized by analytical, spectroscopic (IR, UV-Vis, 1 H and 13 C NMR), and electrochemical techniques. The ES spectra of o-, p-Cl, p-Br, o-CH 3 and 5,6-benzo-substituted SA did not exhibit expected absorptions at 400–500 nm in alcoholic solutions.

2,6‐Di‐tert‐butyl‐4‐(3,3‐diarylpropadienylidene)‐2,5‐cyclohexadien‐1‐ones, the First Stable p‐Quinopropadienes

AbstractTreatment of 2,6‐di‐tert‐butyl‐4‐ethynylphenol with 2 equivalents of nBuLi, followed by addition of diaryl ketones, afforded the corresponding p‐quinopropadienes 2a−f upon appropriate dehydrative treatments. These compounds are fairly stable, orange to purple, crystalline substances, exhibiting intense absorptions at longer wavelengths. In particular, the bis(dimethylamino) derivative 2a absorbs across the whole of the visible spectrum and into the near infrared. Unlike the quinone methides 1, compounds 2 exhibited irreversible oxidation waves together with irreversible reduction waves on cyclic voltammetry. The very small E1sum value of 2a also indicates the highly amphoteric redox properties of 2a; however, both the anion and cation radicals of 2 are highly reactive. The molecular structures of 2a and 2e exhibit relatively small dihedral angles between two aromatic ring planes (2a for 47° and 2e for 52°), and each has a short central double bond and long side double bonds. The short central double bond (2a for 1.228 Å and 2e for 1.235 Å) indicates the importance of the resonance contribution of the dipolar structure with an acetylenic bond. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Reaction of Mixed-Ligand Iron–Sulfur Cluster [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2)] (Cp* = C5Me5) with Methyl Iodide. Synthesis, Structure, and Redox Behavior of [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2Me)

Abstract The reaction of the mixed-ligand iron–sulfur cluster [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2)] ([3]) (Cp* = C5Me5) with iodomethane gave a methylated cluster, [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2Me)]I ([4]I), in 60% yield. The anion exchange of [4]I with KPF6 gave [4]PF6 as a pure sample. A similar treatment of [3] using iodoethane afforded an ethylated cluster, [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2Et)]I ([5]I), and oxidized parent cluster [3]I. The reaction of [3] with 2-iodopropane did not give an alkylated cluster. An unreacted cluster [3] was recovered with a small amount of oxidized cluster [3]I. The molecular structure of [Fe4(Cp*)3(Ph2C2S2)(μ3-S)3(μ3-S2Me)] ([4]), which was obtained by the reduction of [4]PF6 with cobaltocene, was determined by an X-ray diffraction study. Crystallographic data are: Monoclinic, P21/a, a = 21.707(2) Å, b = 21.382(2) Å, c = 11.433(1) Å, β = 98.98(1)°, V = 5241.5(9) Å3, Z = 4, and R = 0.067 for 7412 reflections with |Fo| > 3σ(Fo). The methyl group transferred from MeI is located on the singly ligating sulfur atom in the disulfide ligand of the parent cluster [3]. A cyclic voltammogram of [4]PF6 in 0.1 mol dm−3 tetrabutylammonium tetrafluoroborate–CH3CN showed five reversible one-electron redox waves at E1/2 = +1.42 V, +0.92 V, +0.41 V, −0.38 V, and −1.22 V vs SCE, corresponding to 4+/3+, 3+/2+, 2+/1+, 1+/0, and 0/1− charged cluster couples, respectively. An irreversible two-electron reduction wave was also observed at Epc = −1.97 V vs SCE.

Macrocyclic Unsymmetrical Binuclear Copper(II) Complexes as Ligands: Spectral, Structural, Magnetic and Electrochemical Studies

A new series of macrocyclic unsymmetrical binuclear copper(II) complexes was prepared by using [CuLa,b] (La=[6,6′-piperazine-1,4-diyldimethylenebis(4-methyl-2-formyl)phenol] and Lb=[6,6′-piperazine-1,4-diyldimethylenebis(4-bromo-2-formyl)phenol]) and [Cu(oxen)](ClO4)2 or [Cu(oxpn)](ClO4)2, where oxen=N,N′-bis(2-aminoethyl)oxamide and oxpn=N, N′-bis(3-aminopropyl)oxamide. The binuclear copper(II) complexes contain three different compartments: the first compartment has two piperazinyl nitrogens and two phenolic oxygens (N2O2), the second compartment has two phenolic oxygens and two imine nitrogens (O2N2) and the third compartment has two imine nitrogens and two amide nitrogens (N4) as coordinating sites. Apart from the three compartments the complex has two oxamide oxygen atoms for further coordination. The copper(II) ion in the complex is tetracoordinated with two tertiary nitrogen atoms and two phenoxy oxygen atoms coordinated in the equatorial plane to the copper(II) ion and the geometry around the copper nuclei is distorted square planar. The binuclear copper(II) complexes were characterized by elemental and spectral analysis. Electronic spectra of the complexes show a single broad band for the d–d transition in the region around 585–635 nm. The mono- and binuclear copper(II) complexes show four-line hyperfine splitting in the ESR spectra. For the mononuclear complexes the observed g ‖ values are in the range 2.21 to 2.31, g ⊥ values vary from 2.10 to 2.11 and A ‖ values from 175 to 180 G. For the binuclear complexes the observed g ‖ values are in the range 2.20 to 2.10, g ⊥ values vary from 2.06 to 2.01 and A ‖ values from 165 to 175 G. Room temperature magnetic moments of the mono- and binuclear copper(II) complexes show a μ eff value around 1.72 BM, which is near to the spin-only value of 1.73 BM. Electrochemical studies of the mono- and binuclear copper(II) complexes are discussed. The mononuclear copper(II) complexes show one irreversible reduction wave in the region -0.54 to -0.89 V and the binuclear copper(II) complexes show two irreversible single-step one-electron transfer processes in the range , at the cathodic potential region.

Synthesis, structure, and formation mechanism of the halogen-bridged tricobalt clusters, [Co3Cp3(μ3-CPh)2(μ-X)]+ (X=Cl, Br, and I)

Reactions of a benzylidyne-capped tricobalt cluster, [Co3Cp3(μ3-CPh)2] (1), with halogens (X2 = Cl2, Br2, and I2) in CH2Cl2 afforded halogen-adducts of 1. The structure of four isolated salts [Co3Cp3(μ3-CPh)2(μ-Cl)]PF6·MeCN (2PF6·MeCN), [Co3Cp3(μ3-CPh)2(μ-Br)]SbF6 (3SbF6), [Co3Cp3(μ3-CPh)2(μ-I)]SbF6·CH2Cl2 (4SbF6·CH2Cl2), and [Co3Cp3(μ3-CPh)2(μ-I)]I3 (4I3) determined by X-ray diffraction can be regarded formally as halide-adducts of 12+. The halogen atom in each structure lies in the Co3 plane. The halogen-bridged Co–Co edge was elongated (in 2PF6·MeCN = 2.6072(4), in 3SbF6 = 2.6106(7), in 4SbF6·CH2Cl2 = 2.622(2), and in 4I3=2.6718(9) Å), and the Co–Co distances that had no halogen-bridge remained unchanged from the Co–Co distance of 1 (2.382(8) Å), (in 2PF6=2.4037(8) and 2.3948(7), in 3SbF6=2.3888(6) and 2.4017(7), in 4SbF6·CH2Cl2 = 2.393(2) and 2.388(1), and in 4I3=2.397(1) and 2.3868(9) Å). The UV–Vis absorption spectra of 2+, 3+, and 4+ had characteristic absorption peaks at 796, 819, and 844 nm, respectively. Cyclic voltammograms of 2PF6 in CH2Cl2 with 0.1 M nBu4NPF6 as the supporting electrolyte showed a chemically reversible oxidation (at a potential of 0.75 V versus Fc/Fc+), and an irreversible reduction wave at −0.57 V. The irreversible reduction resulted in the recovery of 1. The redox properties of 3+ and 4+ are very similar to that of 2+. Cyclic voltammetry of 1 in 0.1 M nBu4NCl/MeCN indicates that the formation of 2+ is a multi-step reaction. Initially, 1 is oxidized to 1+, and then, 1+ is coordinated by Cl− followed by immediate oxidation to 2+.

Electrochemistry of 4′-hydroxy and acetoxy flavones CV and NPP study

Electrochemical reduction behavior of 3-acetoxyflavone was compared with 3-hydroxyflavone on glassy carbon electrode with DMF and 60% DMF/Britton Robinson buffer at pH 3.8 & 10.1 using cyclic voltammetry (CV) and on dme using normal pulse polarography (NPP). Single irreversible reduction waves were observed due to the reduction of keto moiety. The effects of change in medium, pH and sweep rate were evaluated. The electrode process was found to be diffusion controlled and enhanced substituent effect was noticed due to extended conjugation. Kinetic parameters were calculated from CV & NPP measurements using R.S. Nicholson and I. Shain equation and Meites & Isreal equation, respectively.

Pyridine-2,6-dicarboxylate and perchlorate bridged hydrogen bonded 1D chains involving manganese(III)-cyclam moiety: synthesis, X-ray crystal structures and magnetic study

Two compounds H[Mn(cyclam)(pyridine-2,6-dicarboxylate) 2] · H 2O ( 1) and [Mn III(cyclam)Cl 2]ClO 4 ( 2) (where cyclam=1,4,8,11-tetraazacyclotetradecane) have been synthesized and structurally characterized. The complex 1 crystallizes in the monoclinic space group P2(1)/ c with a=11.4328(11), b=14.4275(14), c=8.5782(9) Å, V=1343.1(2) A ̊ 3 , Z=2, R=0.0453. Complex 1 is octahedral in which [Mn(cyclam)] 3+ unit occupies the basal plane having two pyridine-2,6-dicarboxylate anions in the axial positions. Molecular packing of the crystal is dominated by string of molecules along the b-axis. The strings are held together by extensive intermolecular hydrogen bonds involving N–H ⋯ O, N–H ⋯ N and O–H ⋯ O which impart it an infinite 1D chain. Complex 2 on the other hand crystallizes in the space group P2 12 12 1 (No. 19) of the orthorhombic system. Mn(III) ions occupy the center of a distorted octahedron and two chloride ions occupy the axial positions. The packing diagram of 2 reveals that the complex is percholrate-bridged hydrogen bonded 1D chain along a-axis. Cyclic voltammogram of complex 1 shows a reduction wave at −0.25 V coupled to an oxidation wave at −0.05 V versus SCE in aqueous solution. The complex 2 is characterized by an irreversible reduction wave at −0.11 V versus SCE and is identical to that observed for [Mn III(cyclam)Cl 2]Cl · 5H 2O. The magnetic measurements in the temperature range 1.9–300 K have been carried out for complex 1 which exhibits a very weak ferromagnetic interaction at low temperature. Complex 2 shows room temperature magnetic moment value of 4.92 BM consistent with the high spin d 4 electronic configuration.

Specificity in template syntheses of hexaaza-macrobicyclic cages: [Pt(Me5-tricosatrieneN6)]4+ and [Pt(Me5-tricosaneN6)]4+.

The racemic C3 hexadentate cage complex, [Pt(Me5-tricosatrieneN6)]Cl4 (1,5,9,13,20-pentamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosa- 3,14,18-triene)platinum(IV) tetrachloride), was synthesised stereospecifically and regiospecifically from a reaction of the bis-triamine template [Pt(tamc)2]Cl4 (bis[1,1,1-tris(aminomethyl)ethane]- platinum(IV) tetrachloride) with formaldehyde and then propanal, in acetonitrile under basic conditions. Largely, one racemic diastereoisomer was obtained in a surprisingly high yield (approximately 50%), even though the molecule has seven chiral centres. The origins of the stereoselective synthesis are addressed. The crystal structure of [Pt(Me5-tricosatrieneN6)]-(ZnCl4)1.5Cl.2H2O showed that all three imines were attached to one tame fragment with a chiral amine site ([symbol: see text] SSS, delta RRR) and a chiral methine carbon site ([symbol: see text] RRR, delta SSS) on each ligand strand. The PtN6(4+) moiety had a slightly distorted octahedral configuration with the two types of Pt-N bonds related to the imine and the amine donors, 2.050(7) and 2.072(6) A, respectively. Treatment with sodium borohydride (15 s, 20 degrees C) at pH approximately 12.5 reduced the imine groups, but not the Pt(IV) ion, producing a C3 saturated ligand complex [Pt(Me5-tricosaneN6)]Cl4 ((1,5,9,13,20- pentamethyl-3,7,11,15,18,22- hexaazabicyclo[7.7.7]tricosane)platinum(IV)tetrachloride). X-ray crystallographic analysis showed that the average Pt-N bond distance in the cation increased upon imine reduction to 2.10 (av) A. The cyclic voltammograms of the two cage complexes displayed irreversible two-electron reduction waves in aqueous media and a approximately 0.3 V shift to more positive potentials compared to that of the smaller cavity sar (3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) analogue. After reduction, net dissociation of one strand of the cage was also evident, to give unstable square planar Pt(II) macrocyclic products.

Direct determination of praziquantel in pharmaceutical formulations and human plasma by cathodic adsorptive stripping differential-pulse voltammetry

The polarographic and cyclic voltammetric behaviour of praziquantel was studied in B.R. buffers of different pH values. Contradictory to that mentioned in a previously published work, praziquantel is an electro-active compound. Its polarogram exhibited a single 2-electron irreversible reduction wave in B.R. buffer of pH 5, the wave height decreased on the increase of pH till it disappeared in solution of pH >7. This wave was attributed to the reduction of the CO double bond. The quantitative trace determination of bulk praziquantel was studied at a hanging mercury drop electrode by cathodic adsorptive stripping differential-pulse voltammetry. A fully validated sensitive procedure based on controlled adsorptive accumulation of the drug onto a HMDE was developed for its direct determination without derivatization. Accumulation of praziquantel was found to be optimized in 0.1 M Na 2SO 4 solution as supporting electrolyte under the following conditions: accumulation potential, −1.2 V (vs. Ag/AgCl/KCl s); accumulation time, 30 s; scan rate, 10 mV/s and pulse height 100 mV. The proposed procedure was applied successfully for determination of praziquantel in its pharmaceutical formulations and human plasma. The mean recoveries of the drug were 98.85–99.42% and 99.12–100.47% with RSD of 0.49–0.95% and 0.45–0.52% in pharmaceutical formulations and human plasma, respectively. Limits of detection and quantitation of 1.14×10 −9 and 3.80×10 −9 M praziquantel, respectively, were achieved.

Electrochemical Investigation of Metal-Free and Nickel-Containing Porphyrazines Carrying Eight Tosylaminoethylthia Groups

In this study, the electrochemical properties of metal-free and nickel-containing porphyrazines with eight tosylaminoethylthia groups were investigated using cyclic voltammetry, double potential step chronoamperometry, double potential step chronocoulometry, and controlled potential coulometry. Cyclic voltammetry measurements showed that both compounds exhibit two quasi-reversible and an irreversible reduction waves. The first electron transfer reaction was followed by an irreversible chemical reaction, the second one by a reversible chemical reaction for both species. The electrode processes of metal-free and nickel-containing porphyrazines are diffusion controlled, but the double potential step chronocoulometry measurements indicated that the nickel porphyrazine is adsorbed at the electrode. However, a small adsorption current has no significant effect on the mass transport mechanism of the system. Diffusion coefficients of both compounds were determined by both cyclic voltammetric and chronocoulometric measurements. The diffusion coefficients of the reduced forms of the porphyrazines were found to be smaller than those of the neutral forms.