Ligand control on the reactions of oxygen- and sulphur-bonded (sulphito)pentaminecobalt(III) complexes in solution

The kinetics of formation, acid-catalysed aquation, ligand isomerisation (CoIIIOSO2+→CoIIISO3+), intramolecular electron-transfer, and base-catalysed hydrolysis and isomerisation of O-bonded sulphito complexes, cis-[Co(en)2(B)(OSO2–O)]+[B=benzimidazole (bzimH), N-methylimidazole (N-meim)] and the anation of cis-[Co(en)2(B)OH)]2+ [B=bzimH, N-meim and imH (imidazole)] by oxSO2−3 are reported. Steric acceleration is observed in the formation and acid-catalysed aquation of the O-sulphito complexes. The ligand isomerisation leads to loss of the monodentate amine with the formation of trans-[Co(en)2(SO3–S)2]− (in an excess of sulphite). Steric acceleration is more pronounced in the isomerisation and base hydrolysis than in the redox process. The results indicate cis labilisation of the coordinated O-sulphite. The [(tetraethylenepentamine)Co(OSO2–O)]+ cation undergoes base hydrolysis 103 times faster than the corresponding (en)2(B) complexes; base-catalysed ligand isomerisation for the former is not observed unlike in the latter. The anation of cis-[Co(en)2(B)OH]2+ (B=imH, bzimH, N-meim) by SO2−3 in a mild alkali pH range (pH=7.9–9.6) and in an excess of SO2−3, yields exclusively trans-[Co(en)2(SO3–S)2]− with no evidence for the formation of the cis-[Co(en)2(B)(SO3–S)]+ or its O-sulphito analogue. The intramolecularly generated amido conjugate base of the sulphite ion-pair, {cis-[Co(en)2(B)OH]2+,SO32−}.41cm{cis-[Co(en)(en-H) (B)- OH2]2+,SO32−}, is believed to generate a five-coordinate intermediate (TBP) that captures the S-end of SO2−3 selectively from a site trans-to the amine B so that the amine is labilised by the trans effect of the sulphite. The NH-deprotonated coordinated imidazolate or benzimidazolate species, cis-[Co(en)2(bzm/im)OH]+, do not undergo anation by SO2−3. This revised version was published online in June 2006 with corrections to the Cover Date.     

Schiff base complexes of zirconium(IV) derived from Zr(acac)4

Summary Zr(acac)₄ undergoes ligand exchanges with various tri- and tetradentate Schiff base ligands, forming compounds of the Zr(L)₂ type (H₂L=tetradentate H₂Sal₂en, H₂Sal₂pn, H₂Sal₂ o-phen and the tridentate H₂SAP) and Zr(acac)₂L (H₂L=H₂SAN. H₂SAE). Upon reaction with a combination of tri- and tetradentate ligands, Zr(acac)₄ yields Zr(L)(L) complexes (H₂L=H₂Sal₂en or H₂Sal₂ o-phen; H₂L=H₂SAN, H₂SAE, or H₂SAP), which have been characterised by analytical data, m.ps, electrical conductivities, i.r. and n.m.r (¹H and¹³C) spectra, they have a coordination number of 6, 7 or 8.     

Steric effects on complex formation between nickel(II) and (2-imidazoleazo) benzene, 2,2′-biimidazole and 2,2′-bibenzimidazole

Summary The kinetics and mechanism of reversible complexation of Ni^II with (2-imidazoleazo)benzene (IAB), 2,2-biimidazole (Biim) and 2,2-bibenzimidazole (Bibzm) have been investigated at 15–35 °C, I = 0.30 mol dm^–3. The stability constants, K _M, of the [NiL]²⁺ species vary in the sequence: [Ni(IAB)]²⁺ < [Ni(Bibzm)]²⁺ < [Ni(Biim)]²⁺. The values of the spontaneous dissociation rate constant (k _r) at 25 °C decrease in the sequence: [Ni(IAB)]⁺ > [Ni(Biim)]²⁺ > [Ni(Bibzm)]²⁺. The aquation of [Ni(IAB)]²⁺ is insensitive to acid catalysis, whilst [Ni(Biim)]²⁺ is relatively more susceptible towards acid-catalysed aquation than [Ni(Bibzm)]²⁺. The chelate ring in [NiL]²⁺ (L = IAB, Biim or Bibzm) is sterically strained. The formation of [Ni(IAB)]²⁺ and [Ni(Bibzm)]²⁺ may be chelation controlled while the normal I _d mechanism is supported by our data for [Ni(Biim)]²⁺.     

Kinetics of aquation of pentaammine(substituted salicylato)cobalt(III) complexes by ferric ion

Summary The aquation of pentaammine (substituted salicylato) cobalt(III) complexes [(NH₃)₅CoO₂CC₆H₃(X)OH]²⁺,X = 5-SO₃, 5-Br, 5-NO₂, and 3-NO₂ in the presence of ferric ion was studied spectrophotometrically in the 65°–80° range. Ferric ion catalyses the aquation of the substratesvia formation of a reactive binuclear species.Reprints of this article are not available.     

Solubility studies in aqueous media: I. Solubility product of silver permanganate and standard electrode potentials of silver—silver permanganate electrode in aqueous media

The solubility and solubility product of silver permanganate in water have been determined at the temperatures ranging from 15 to 35°C over 5°C intervals in the presence of an added electrolyte, sodium perchlorate. The solubility of silver permanganate ranges from 0.966 x 10^?5 mol 1^?1 at 15°C to 1.420x10^?5 moll^?1 at 35°C and the corresponding solubility product 0.933 x 10^?10 mol² 1^?2 at 15°C to 2.017 x 10^?10 mol² 1^?2 at 35°C. The standard potentials of the Ag(s)/AgMnO₄(s)/ MnO^?₄ electrode have been calculated at these temperatures. The mean activity coefficients of silver permanganate at various rounded molarities of sodium perchlorate solutions, and the standard thermodynamic quantities for the process AgMnO₄(s)→Ag⁺ (aq)+MnO^?₄(aq) have been calculated at these temperatures.     

Solubility studies in formamide: II. Solubility product of silver iodate and standard electrode potential of silver—silver iodate electrode in formamide

The solubility and solubility product of silver iodate in formamide in sodium perchlorate solutions have been determined at 25, 30 and 35°C. The solubilities in pure formamide are found to be 2.169 × 10^?4, 2.488 × 10^?4, and 2.943 × 10^?4 mole I^?1, respectively, at these temperatures, and the corresponding solubility products are 4.609 × 10^?8, 6.053 × 10^?8, and 8.448 × 10^?8 mole² 1^?2. The standard potential of the Ag_(s)/AgIO_3(s)/IO₃^? electrode has been calculated and found to be 0.2562 V at 25°C. The mean activity coefficients of silver iodate at various rounded molarities of sodium perchlorate solutions, and the standard thermodynamic quantities for the process AgIO_3(s)åAg⁺ (solvated) + IO₃^? (solvated) have been calculated at these three temperatures.     

Mechanistic Study of the Reactions of l-ascorbic Acid and Oxalic Acid with an Octahedral Manganese(IV) Complex of 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane

The Mn^IV complex of 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane (Mn^IVL) with phenolate-amido-amine coordination is reduced by l-ascorbic acid and oxalic acid obeying overall 1:1 stoichiometry. The reactions are biphasic and Mn^IIIL⁻ is the reactive intermediate. The product of oxidation of ascorbic acid (H₂Asc) is dehydroascorbic acid and that of oxalic acid (H₂OX) is CO₂, while Mn^II is the end product from Mn^IV. Both Mn^IVL and Mn^IIIL⁻ form outer sphere adducts with H₂Asc and H₂OX with high values of equilibrium constants of formation (Q>10² dm³ mol⁻¹, I = 0.5 mol dm⁻³, 25.8 °C, 1.5% v/v MeOH+H₂O). The adduct formation is diffusion controlled and is attributed to hydrogen bonding interactions between the reactants. The rate constants for the electron transfer in (Mn^IV/^IIIL, H₂A), (Mn^IV/^IIIL, HA⁻) (H₂A = H₂Asc, H₂OX) and for (Mn^IVL, H₂Asc)+H₂Asc, (Mn^IIIL⁻, HAsc⁻)+HAsc⁻ are reported. There was no evidence of direct coordination of the reductants to the Mn^IV/III center indicating an outer sphere (ET) mechanism.     

Synthesis of a novel, highly symmetric bis-oxa-bridged compound

A highly oxygenated, novel pentacyclic bis-oxa-bridged compound 8 was synthesized with remarkable efficiency starting from readily available tetrachloro-5,5-dimethoxyclopentadiene and 1,4-cyclohexadiene in three steps. The ruthenium-catalyzed oxidation of 2:1 bis-adduct 1 followed by a one-pot transformation of the resulting bis-alpha-diketone 3 furnished (after esterification) the title compound in an overall yield of 29.1%. The versatility of this simple method was further demonstrated with other norbornyl alpha-diketones to obtain the corresponding strained oxa-bridged derivatives.     

Thermal degradation studies of substituted conducting polyanilines

The synthesis of conducting polymers based on m-nitroaniline, m-chloroaniline and m-aminophenol by aniline initiated ammonium peroxydisulfate oxidation, has been attempted. The IR spectra of the polymers have been studied. Thermogravimetric analysis of the conducting polymers has been followed using a computer analysis method LOTUS PACKAGE, developed by us for assigning the degradation mechanism. A number of equations have been used to evaluate the kinetic parameters. The mechanism of degradation of the conducting polymers has been explained on the basis of their kinetic parameters.     

The(q\bar q) - pion and its decay constant in a chiral potential modeland its decay constant in a chiral potential model

Pion mass and its decay constant have been studied in a chiral symmetric potential model of independent quarks. The non-perturbative multi-gluon interaction which is responsible for quark confinement in a hadron is phenomenologically represented here by an effective potentialU(r) = 1/2(1 +γ ⁰)(ar ² +V ₀). The residual interactions due to quark-pion coupling arising out of the chiral symmetry preservation and that due to quark-gluon coupling arising out of single-gluon exchange are treated as low order perturbations. The centre of mass correction is also taken into account appropriately. This leads to the mass in consistency with that of the PCAC-pion and the pion decay constant in reasonable agreement with experiment.