Anadi C. Dash

Synthesis, characterization and electrochemistry of a binuclear copper(II) complex of schiff base derived from 2-aminomethyl-benzimidazole and salicylaldehyde

Abstract A dinuclear Cu II compound (Cu 2 L 2 2+ ) with the phenoxide form of N-salicylidine 2-aminomethylbenzimidazole has been synthesized and characterized on the basis of elemental analysis, UV-visible, IR and variable temperature ESR spectra, magnetic moment and electrochemical studies. Thermal studies indicate that the decomposition takes place in two stages. Lowering of ESR peak intensity with lowering in temperature indicates the dimeric nature of the compound. A stopped flow kinetic study indicated that the acid catalysed decomposition of the compound occurred via two consecutive steps, thus providing further evidence for the dimeric nature of the compound. Evidence of equilibrium NH deprotonation of the benzimidazole moiety of Cu 2 L 2 2+ at 0.005 ⩽ [OH − ], mol dm −3 ⩽ 0.02 is presented.

Interaction of cis-(amine)-bis-(ethylene diamine) cobalt(III) salicylates with Fe(III), Al(III) and Cu(II): Stability constants of [(NH3)(en)2CoCO2C6H3(X)OM](n+1)+ species

Abstract The cis -(amine)-bis-(ethylene diamine) cobalt(III) salicylates, [(NH 3 )(en) 2 CoCO 2 C 6 H 3 ( X )OH] 2+ ( X = H, 5-Br, 5-SO 3 − , 5-NO 2 and 3-NO 2 ), have been synthesised. The association equilibria involving the deprotonated form of these complexes and Fe(III), Al(III) and Cu(II) have been studied spectrophotometrically in aqueous medium. The p K OH of the salicylate complexes and the stability constants of the binuclear species. [(NH 3 )-(en) 2 CoCO 2 C 6 H 3 ( X )OM] ( n +1)+ , are reported at 29·5(±0·2)°C and μ = 1·0 M (adjusted with NaClO 4 ). For a given cobalt(III) substrate, the stability constants of the binuclear species ( K ML ) decrease in the sequence: Fe(III)>Al Al(III)>Cu(II). Both p K OH and log K ML values decrease in the sequence: H > 5-Br > 5-SO 3 − > 5-NO 2 > 3-NO 2 . Log K ML vs p K OH plots are linear with slopes close to unity for all the three metal ions. The metal ions in their binuclear complexes are presumed to be chelated by the phenate oxygen and the carboxyl group bound to the Co(III) centre.

Effect of solvent on the reactions of coordination complexes. Part 1.—Kinetics of solvolysis of cis-(bromo)(benzimidazole)bis(ethylenediamine)cobalt(III) in methanol–water media

The kinetics of the solvolytic aquation of cis-(bromo)(benzimidazole)bis(ethylenediamine)cobalt(III) have been investigated at 35, 40, 45, 50.2 and 54.8 °C in aqueous methanol media of methanol content 0–80 wt %. The pseudo-first-order rate constant decreases with increasing methanol content. Plots of log ksaqvs. D–1s(where Ds is the dielectric constant of the solvent mixture) and log ksaqvs. the Grunwald–Winstein Y parameter or revised YCl(1-adamantyl Chloride) parameter are non-linear. The plot of log ksaqvs. the molefraction of methanol (XMeOH), which deviates from linearity at 35, 40 and 45 °C, is an excellent straight line (r= 0.998) at 50 and 54.8 °C over the entire composition range studied. It is evident that the solvation phenomenon plays dominant role and that the rate of aquation is mediated by the dual solvent vectors, the overall basicity and acidity of the solvent mixtures. At 25 °C the calculated values of the transfer free energy of the dissociative transition state, cis-{[Co(en)2(bzmH)]3+}*, relative to that of the initial state, cis-[Co(en)2(bzmH)Br]2+, for the transfer of the ions from water to the mixed solvent were positive at all solvent compositions (except 80 % MeOH), exhibiting a maximum at XMeOH= 0.46. This reflects the propensity of the said tripositive transition state and the dipositive initial state towards solvation interaction with the medium. In contrast to the activation free energy, which varies linearly with XMeOH, the plots of activation enthalpy and entropy against XMeOH exhibit maxima at XMeOH= 0.12 and 0.46 and minima at XMeOH= 0.02 and 0.25; thereafter both ΔH‡ and ΔS‡ decrease steeply to significantly low values at XMeOH= 0.69 (ΔH‡= 80.1 ± 4.3 kJ mol–1 and ΔS‡=–91 ± 13 J K–1 mol–1), manifesting the effect of solvent-shell reorganisation around the complex ion both in the initial state and in the transition state as the composition is varied.

Acid hydrolysis of carboxylatopentaamminecobalt(III) complexes Kinetics and mechanism of aquation of malonato, succinato, o-phthalato and o-methoxy benzoato pentaamminecobalt(III) complexes

Abstract The kinetics of aquation of malonato, succinato, o-phthalato and o-methoxybenzoato pentaamminecobalt(III) complexes have been studied in aqueoys acidic media adjusted to 0·3 M ionic strength. At [H+] = 0·005 − 0·3 M and 55 − 70°C, the observed pseudofirst-order rate constants are described by the relationship: kobs = k1 + k2 [H+] where k1 and k2 stand for the rate constants of spontaneous and acid catalysed equation of (NH3)5CoCO2(CH2)nCO2H2+ (n = 1,2), (NH3)5CoCO2C6H4CO2H2+ and (NH3)5CoCO2C6H4OCH32+ species respectively. The rate and the activation parameters for the k1 and k2 paths have been determined. The results indicate that the unbound carboxyl group does not exert a specific catalytic effect on the spontaneous and acid catalysed paths of aquation of the cobalt(III) substrates derived from the dicarboxylic acids. CoO bond fission appears to be rate limiting in both k1 and k2 paths of all complexes. The possibility of CO bond breaking, however, can not be ruled out in the k2 path of the o-phthalato complex.

Kinetics of oxidation of ascorbic acid by the [Mn3IV(μ-O)4(bipy)4(H2O)2]4+ ion (bipy=2,2′-bipyridine)

Abstract In the pH range 1.91–5.24, an aqueous solution of the complex [Mn3IV(μ-O)4(bipy)4(H2O)2]4+ (1) oxidises ascorbic acid quantitatively to dehydroascorbic acid and is itself reduced to MnII. In the presence of excess ascorbic acid, the reaction follows first-order kinetics. The first-order rate constant k0 measured in the presence of excess ascorbic acid increases linearly with increasing concentration of the reductant but decreases as more and more 2,2′-bipyridine is added. k0 also increases with [H+] but tends to saturate at higher [H+]. The proposed rate-determining step involves simultaneous transfer of one electron and one proton from the ascorbate ion to [Mn3IV(μ-O)4(bipy)3(H2O)4]4+, an aquated form of 1. Subsequent rapid reactions via a dinuclear {Mn2O2}3+ intermediate lead to the final products.

Effect of solvent on the reactions of coordination complexes. Part 5.—Kinetics of solvolysis of cis-(bromo)-[(2-aminothiazole)-bis(ethylenediamine)cobalt(III) in methanol–water, propan-2-ol–water and ethylene glycol–water

The rates of solvolysis of the cis-(bromo)(2-aminothiazole)(ethylene-diamine)cobalt(III) ion have been investigated in acidic methanol–water, propan-2-ol–water and ethylene glycol–water (0–80 wt% of organic cosolvent) at 30–50 °C. A good linear correlation was observed between log ksobs and the Grunwald–Winstein Y parameter for MeOH–water and propan-2-ol–water. The log ksobsvs. 1/Ds(Ds= bulk dielectric constant of the mixed solvent) plots at 25 °C exhibited distinct curvature; the effect is pronounced for propan-2-ol–water. log ksobsvs. mole fraction of organic solvent component (Xorg) plots for MeOH–water were linear at 30–50 °C and XMeOH= 0–0.692, while similar plots for ethylene glycol–water and propan-2-ol–water were biphasic (two intersecting lines); the effect is relatively more pronounced for ethylene glycol–water. The dependence of the solvolysis rate on Xorg has been interpreted in terms of solvation of the initial state and the transition state, which is controlled by the overall acidity and basicity of the mixed solvent. Activation enthalpy and entropy vs.Xorg plots display maxima and minima, indicating that the solvent structural changes play a significant role in the activation process. The solvolysis reaction is isoenthalpic and more or less isentropic at Xorg= 0.225 for all three aqua-organic solvent mixtures. The observed mutual compensation effect of activation free energy shows that the perturbations in the solvent network causes proportionate perturbations in the reaction zone so that the solvational components of ΔH≠ and ΔS≠ correlate with each other linearly.

Effect of solvent on the reactions of coordination complexes. Part 2.—Kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) in methanol–water and ethylene glycol–water media

The kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) have been investigated in aqueous methanol (MeOH) and aqueous ethylene glycol (EG) media (0–80% by weight of MeOH or EG) at 45–64.7 °C. The logarithm of the pseudo-first-order rate constants for MeOH–water media exhibits linear dependence with the reciprocal of the bulk dielectric constant (D–1s), the mole fraction of MeOH (XMeOH) and the solvent ionizing power Y(Y1-AdCl) as determined by the solvolysis rates of 1-adamantyl chloride. Similar plots (log ksobsvs.xEG or D–1s) for EG–water media are non-linear. It is evident that the solvation phenomenon plays dominant role and the rate of solvolysis is mediated by the dual solvent vectors, the overall acidity and basicity of the solvent mixtures. The relative transfer free-energy calculations indicate that the mixed solvent media exert more destabilizing effect on the transition state as compared to the initial state. The activation enthalpy and entropy vs.Xorg(where Xorg is the mole fraction of the organic solvent component) plots display maxima and minima indicating that the solvent structural changes play significant role in the activation process. The activation free energy at a given temperature, however, increases only marginally and linearly with increasing Xorg. The mutual compensatory effect of activation enthalpy and entropy on the activation free energy is in keeping with the fact that the perturbations of the reaction zone and the solvent network remain approximately proportional to each other with increasing Xorg so that the isodelphic and the lyodelphic components of ΔH± and ΔS± correlate well with each other.

Kinetics of aquation and base hydrolysis of cis bioxaltoammine bis(ethylenediamine) cobalt(III) ion

Abstract Cis bioxalatoammine bis (ethylenediamine) Co(III) cation has been synthesized and its aquation and base hydrolysis reactions have been investigated in 1·0 M NaCl04 medium. In the range of hydrogen ion concentration 0·02−1·O M and temperature 55–70°C, the rate law for the aquation of (en)2(NH3) CoC204H2+ is given by with k 1 (70°C) = 1·82 (±0·10) x 10 −5 sec −1 ΔH ∗ = 26·9 (±1·6) kcal/mole and ΔS ∗ = -2·4 (±5) e.s.u. : k 2 (70°C) = 3·14 (±0·22) × 10 −5 sec −1 M −1 ΔH ∗ = 24·0 (± 1·7) kcal/mole and ΔS ∗ = −9·7 (±5) e.s.u. : K 1 (70°C) = 3·0(±0·5) × 10 −2 M . The alkaline hydrolysis of (en)2(NH3)CoC204+ was studied at [OH−] with k 2 (30°C) = 2.75(±0·10) × 10 −3 sec −1 M −1 , ΔH ∗ = 30·0(± 1 ·1) kcal/mole and ΔS* = 28·6(±3·6) e.s.u.

Studies on ion-association: Thiocyanate ion-pairs of Al(III), Ga(III) and In(III)

Abstract The thermodynamic instability constants of GaSCN++ and InSCN++ were determined by spectrophotometry and potentiometry. AlSCN++ could not be detected in solution in the concentration range in which the present work was done. The stabilities follow the order In > Ga > Al. Possible explanation for the sequence has been given.

Effect of solvent on the reactions of coordination complexes. Part 8.—Kinetics of solvolysis of cis-(chloro)(ethanolamine)bis(ethylenediamine)cobalt(III) in methanol–water, propan-2-ol–water and t-butyl alcohol–water

The solvolysis of cis-(chloro)(ethanolamine)bis(ethylenediamine) cobalt (III) in aqueous methanol, propan-2-ol and t-butyl alcohol results in the formation of the chelated ethanolamine complex, cis-[Co(en)2(NH2CH2CH2OH)]3+. The pseudo-first-order rate constant (ksobs) of this reaction decreases with increasing mole fraction (Xorg) of the organic cosolvent, the effect being less marked with increasing hydrophobicity of the alcohols. The plots of log ksobsvs.D–1s(Ds= bulk dielectric constant) are curved. The plots of log ksobsvs. Grunwald Winstein ‘Y’ parameter are nearly linear for CH3OH–H2O and propan-2-ol-H2O, but appreciably curved t-butyl alcohol–H2O. Under isodielectric conditions (Ds= 50) at 50 °C the plot of log ksobsvs.Xorg for CH3OH–H2O, C2H5OH–H2O, (CH3)2CH—OH–H2O, (CH3)3C—OH–H2O and (CH3)2CO–H2O in water-rich media is a straight line from which the data point for ethylene glycol–H2O (XEG= 0.36) shows positive deviation indicating its better solvating ability. The observed variation of the rate constant on solvent composition (log ksobs= log kwobs– a Xorg+bX2org) for a given aquo-organic cosolvent is interpreted in terms of a preferential solvation model. The plots of activation enthalpy and entropy vsXorg exhibit maxima and minima, thereby indicating that solvent structural changes play a significant role even for this intramolecular dissociative interchange process. The solvolysis of the complex in 99 % D2O yielded a solvent isotope effect (kH2O/kD2O= 12.6) at 50 °C, which is slightly lower than the value for several other analogous alkylamine complexes.