NOPR Collection:

Kinetics and mechanism of the reaction of trans-(diaqua)(N,N' - ethylene bis-(salicylidineiminato)cobalt(III) with ascorbic acid

2017-02-10T08:56:43Z

Title: Kinetics and mechanism of the reaction of trans-(diaqua)(N,N' - ethylene bis-(salicylidineiminato)cobalt(III) with ascorbic acid Authors: Dash, Anadi C; Das, Arabinda; Bramha, Gouri S; Mohanty, Prakash Abstract: The kinetics of the reactions of trans-[Co(Salen)(OH₂)₂]⁺ (Salen = N.N¢-ethylene bis (salicylidineiminate) ·jth ascorbic acid (H₂Asc) have been studied under varying conditions of pH, [ascorbic acid]_T, and temperature at .5 mol dm^-3 ionic strength. The initial fast reactions observed in the stopped flow time scale are due to the complex formation between the reactants. This occurs in two phases i.e., the formation of the trans- [(2 ua)(ascorbato)Co^III(Salen)] and its transformation to the corresponding ascorbate chelate. The rate constants are the activation parameters for the formation of the monobonded and chelate ascorbate complexes are reported. T low values of ΔH⁺ and negative values of ΔS⁺ for the complexation reaction favour associative interchange mechanism(l_a). The hydroxide in trans-[Co(Salen)(OH)(OH₂)] marginally accelerates substitution of the aqua and by HAsc- and the trans-[Co(Salen)(OH)(AscH)]^- is considered to undergo fast internal proton transfer to .nerate trans-[Co(Salen)(OH₂)(Asc)]^- which undergoes chelation of the Co^lll centre by the bound ascorbate noiety; the latter reaction is, however, 15 times slower than the corresponding reaction of trans- [Co(Salen)(OH₂)(AscH)]. The faster complexation reactions are followed by the slow redox reactions. The rate constant for the internal reduction of Co^lll by the coordinated ascorbate in the chelate [Co(Salen)(AscH)] is 5 - 10 times (25°C – 45°C) faster than the same for [Co(Salen)(Asc)]^-. This trend in reactivity is due to the low value of ΔHfor the former although the high negative value of ΔS compensates at least partly the overriding effect of the activation enthalpy. The internal redox occurs via innersphere mechanism. We also have observed a redox path involving trans-[Co(Salen)(OH₂)(AscH₂)]⁺ and H₂Asc for which electron transfer most likely involves outersphere mechanism. Page(s): 947-960

Interaction of N,N'-ethylene bis(salicylamide) with iron (III): A magneto-structural, electrochemical, and mechanistic investigation

2017-02-10T07:28:09Z

Title: Interaction of N,N'-ethylene bis(salicylamide) with iron (III): A magneto-structural, electrochemical, and mechanistic investigation Authors: Dash, Anadi C.; Mishra, Achyutananda Abstract: The reaction of N,N¢-ethylene bis(salicylamide) (H₂SALM) with ion(lll) results in the formation of a 1: 1 complex in solution. The rate and activation parameters for the reactions, Fe(OH₂)+ H₂SALM ® Fe(OH₂)₂(SALMH)²⁺, and Fe(OH₂)₅,(OH)²⁺ + H₂SALM ® Fe(OH₂)₂(SALMH)²⁺ are: k(25°C)/dm³ mol^-1 s^-1 = 2.1 ± 0.6, (2.67 ± 0.1) x 10³; ΔH (kJ mol^-1) = 97 ± 8, 64 ± 1; ΔS'(J K^-1 mol^-1) = 87 ± 28, 35 ±4 (I= 1.0 NaCIO₄, 8% v/v MeOH/H₂O) respectively; the mechanism is I_a. for hexa-aquairon(III) and I_d for hydroxopentaaquairon(III). The substantially large values of the activation parameters as compared to those reported for the water exchange reactions of the iron (III) species are accountable in terms of the solvent shell reorganisation in the process. The species Fe(OH₂)₂(SALMH)²⁺ undergoes facile deprotonation of the phenolic and amide functions which strongly suggests that these remain coordinated to the iron(lll) centre (N bonding for -C=ONH-) in the complex. While the complexation reaction displays a single relaxation mode in the stopped flow time scale, the amide deprotonation of the complex obeys biphasic kinetics (Fe(SALM)⁺® Fe(SALM-H), k_t; Fe(SALM-H) ® Fe(SALM-2H)^-, k_s) with general base catalysis (k_H2O< k_OH< k_tris' < k_H2BO3^-, tris = trishydroxymethylaminomethane, k = k_t, or k_s,). The complex isolated in the solid state as [Fe(SALM)(NO₃)(OH₂)]H₂O displays I. R. bands characteristic of H₂O and NO₃; coordinated to iron(III) centre. Its m_eff = 5.96 B M and 4 line (broad) X-band ESR spectrum are characteristics of high spin iron(III) with five unpaired electrons. The cyclic voltamogram ( Ag/AgCI reference) of the insitu generated complex, Fe(SALM)⁺ (pH = 5.01, 10% v/v MeOH/H₂O, 1.0 KNO₃, 25°C) shows quasi-reversible redox process with E_pc and E_ac. as -0.391V and -0.326V respectively. Page(s): 961-972

Kinetics and mechanism of the oxidation of thiolactic acid by 12-tungstocobaltate(III) ion in acetate buffer

2017-02-10T07:12:16Z

Title: Kinetics and mechanism of the oxidation of thiolactic acid by 12-tungstocobaltate(III) ion in acetate buffer Authors: Mehrotra, Raj N.; Dholiya, Susheela (nee Gatiyala); Sharma, Kamla; Prakash, A. Abstract: The reaction between 12-tungstocobaltate(III) and thiolactic acid (TLA) is expressed by the stoichiometric equation, 2[Co(III )W]^5- + 2CH₃CH(SH)COOH = CH₃CH(COO)SS(OOC)HCH₃C + 2H ⁺ 2[Co(II)W]^6-,which is consistent with the stoichiometric ratio Δ[Co(III)W]^5-/Δ [TLA] = 1.00 ± 0.06. The reaction is first-order in [Co(III )W]^5-·, fractional order in [RSH] and shows a linear correlation between k_obs^-1and [H⁺] with an intercept on the rate ordinate. The formation of a complex between [Co(III)W]^5- and CH₃CH(SH)COO^- is evident both from rapid scanning of the reaction mixture and the kinetics. The formation of the complex from Co(III )W]^5-and CH₃CH(COOH)S^- , suggested in a previous study, is shown to be unlikely. The formation constant (0.025) for the complex is independent of temperature. The activation parameters for the rate limiting step are ΔH = 15 ± 1 kJ mol^-1 and ΔS =-153 ± 3 J K^-1 mol^-1. The small ΔH value is suggestive of a multiple process that included preequilibrium reactions. The rate determining intramolecular electron-transfer from COO^- is synchronous with the transfer of H atom, produced by the homolytic S-H fission, to COO resulting in the formation of CH₃CH(COOH)S· free radical which dimerises to the oxidation product. Thus the electron-transfer is theoretically inner-sphere. Since the central Co^lll is well protected, it is difficult to visualise the manner in which the thiolic proton could form a bridge. It is equally difficult to be specify whether the central Co^lll atom is directly reduced to Co^ll by the inner sphere electron transfer as suggested by the mechanism or it is reduced by the electron first transferred to outer tungstate framework (in outer-sphere pathway) which instantaneously transfers it to the central Co^lll atom. In either case the spectrum of the reduced reaction mixture would be identical with that of 12-tungstocobaltate(II). Hence, the theoretical aspect of the electron transfer in such a case might have some bearing on the nature of the reaction. Page(s): 973-979

Electrochemistry and spectra of six coordinated high-spin bis (tetrahydrofurane) protoporphyrinato IX-iron encapsulated in aqueous surfactant micelles

2017-02-10T07:07:36Z

Title: Electrochemistry and spectra of six coordinated high-spin bis (tetrahydrofurane) protoporphyrinato IX-iron encapsulated in aqueous surfactant micelles Authors: Das, Diganta Kumar; Medhi, Okhil K. Abstract: The optical spectra of the bis (tetrahydrofurane) adduct of iron(III) and iron(II) protoporphyrinato IX in aqueous solutions of anionic (SDS), cationic (CTAB), and neutral (Triton X-100) surfactant micelles are typical of six-coordinated high-spin (6 cHS) ferrous and ferric hemes. The characteristic spectral bands in the ferric form are observed at. 400, 575 nm and 598 nm while these in the ferrous form are found at 420, 540 nm and 567 nm. The 6 cHS ferrous heme is very rare and the complex reported here is the first example in aqueous solutions. The mid-point potential of the bis(thf) adducts of heme are -0.35 V, -0.36 V and -3.0V vs. Ag-AgCl in SDS, TX- 100 and CTAB micelles, respectively. Binding of thf to iron porphyrin in SDS micelles gives a 67 mV anodic shift of the potential. As the high-spin iron is brought into the porphyrin plane in the six-coordinated bis (thf) complexes, the ferrous porphyrin is preferentially stabilised compared to the ferric hemin. The displcement of the iron atom to the porphyrin plane influence the charge transfer bands and the mid-point potential to a significant extent. Page(s): 980-984