Base-catalysed hydrolysis of cis-(imidazolato)-, cis-(benzimidazolato)- and cis-(N-methylimidazole)-bis-(ethylenediamine)halogenocobalt(III) cations. A comparison of the reactivities of the deprotonated imidazole and benzimidazole complexes

The base hydrolysis of the complexes cis-[Co(en)2B(X)]n+ [en = ethylenediamine; B = imidazole (Him), N-methylimidazole (mim), or benzimidazole (Hbzim); X = Cl or Br] has been investigated at 20-35°C and [OH-]τ=(4.7-99.7)×10-3 mol dm-3(I= 0.10 mol dm-3) under which conditions the co-ordinated imidazole and benzimidazole undergo complete NH deprotonation. The activation parameters (85≤ΔH‡/kJ mol-1 ≤ 97, +84 ≤ ΔS‡/J K-1 mol-1 ≤ +97), and the rate dependence on the leaving groups and the non-labile amine ligands are consistent with a SN1 CB mechanism. The electron-displacement properties of the N-co-ordinated imidazolate (im) and benzimidazolate (bzim) ions appear to enhance the pKNH of the co-ordinated ethylenediamine, the effect being relatively more significant for the former anion. Analysis of the activation entropy data in terms of the dissociative activation model for the conjugate bases, cis-[Co(en)(en - H)B(X)]n+[B = im or bzim (n= 0); mim (n= 1)] indicates that the configurational rearrangement at the cobalt(III) centre most likely occurs in the transition state of the actual acta of the substitution for the less reactive chloro complexes. The presumed trigonal-bipyramidal intermediate is efficiently scavenged by azide

Decarboxylation of Hydrogencarbonatopentamminecobalt(III) in Aquo-organic solvent media

The decarboxylation of hydrogencarbonatopentaamminecobalt(III) has been investigated in aqueous, 99% D2O and aquo-organic solvent media (0-70 wt.% of cosolvent) at 15 ≤ t/°C ≤ 40 (I = 0.02 mol dm-3), using methanol, propan-2-01, tert-butyl alcohol, ethylene glycol, acetone, acetonitrile, DMSO and ethylene carbonate as cosolvents. The solvent isotope effects on rate (kH2O/kD2O=O 1.0 at 1535°C) and activation parameters (ΔH≠ = 77.7 ± 1.0, 77.8 ± 0.9 kJ mol-1 and ΔS≠ = 16 ± 3, 16 ± 3 J K-1 mol-1 for aqueous and 99% D,O media, respectively) were negligible. The decarboxylation rate constant increased with increasing mole fraction (Xorg) of the cosolvent and the effect was pronounced at relatively high values of Xorg for the dipolar aprotic cosolvents. This was attributed to a greater degree of destabilisation of the initial state as compared to the transition state with increasing mole fraction of the cosolvent. The In ks vs. 1/εs plots (ks is the rate constant and εs, the bulk relative permittivity) showed marked dependence on the nature of the cosolvents; the gradients of such plots generally increased with increasing dipole moment of the cosolvent molecules, indicating thereby that the solvation of the initial state and the transition state of the substrate is governed by the ion-dipole interactions between the water and cosolvent molecules. The relative transfer free energy of activation, [ΔΔGt≠](s←w), decreased linearly with Xorg for all mixed-solvent media, indicating that the preferential solvation effect is not significant. The activation enthalpy and entropy vs. Xorg plots displayed extrema suggesting that these thermodynamic parameters are sensitive to the structural changes in the bulk solvent phase. The solvent effects on ΔH≠ and ΔS≠ are mutually compensatory

Micellar effects upon the reactions of complex ions in solution. Part 4. Kinetics of aquation and base hydrolysis of some cis-(chloro)(amine)bis(ethylenediamine)cobalt(III) complexes in the presence of neutral and anionic surfactants in an aqueous medium

The binding of the substrates cis-[Co(en)2BCl]2+(en = 1,2-diaminoethane, B = alkylamines, imidazole, N-methylimidazole) to the micellar surface of sodium dodecyl sulphate resulted in the retardation of their dissociative aquation rates, the effect being sensitive to the hydrophobicity of the nonlabile amine ligand B. A contrastingly small rate acceleration for the corresponding ethanolamine and propan-2-ol amine complexes was observed under similar conditions. Triton X-100 (0.02 ≤ [Triton X]T/mol dm-3 ≤ 0.1) had virtually no effect on the aquation rates of such complexes except for cis-[Co(en)2(C6H11NH2)Cl]2+, in which case a small rate retardation was also observed. The rates of base hydrolysis of the cobalt(III) substrates were strongly retarded by the anionic micelles of SDS; the neutral micelles of Triton X-100 were effective in decelerating the rate of base hydrolysis of the cyclohexylamine complex cis-[Co(en)2(C6H11NH2)Cl]2+ only. The pseudo-phase ion-exchange equilibrium model satisfactorily explained the binding of the cationic substrates to the anionic micellar pseudo-phase of SDS. The values of the ion-exchange equilibrium constant and the relative base hydrolysis rates (kW/kM) indicated that both micellar binding and retardation of hydrolysis are governed by hydrophobic and electrostatic interactions

Effect of solvent on reactions of coordination complexes: Part 11†―Kinetics of solvolysis of cis-(chloro) (cyclohexylamine) bis(ethylenediamine) cobalt (III) in acetone + water media

167-170The kinetics of solvolysis of cis-(chloro)(cyclohexyl-amine)bis(ethylenediamine) cobalt (III) ion have been investigated in acetone + water (0-50 wt% acetone) in the temperature range of 35-50°C. The plots of log k versus the reciprocal of the bulk dielectric constant (Ds-l) of the mixed solvent and log k versus solvent ‘Yt-Bucl' parameter were nonlinear. The plots of log k versus Xorg (Xorg = mol fraction of acetone) at 35 ≤ tᵒ, C ≤ 50 are strikingly linear with the common gradient = - 3.14 ± 0.10. These facts are in keeping with the importance of the solvent structural effects and the lack of appreciable preferential solvation of the substrate under the conditions of rate measurements. It is striking to note that unlike for the alcohol-water solvent systems and a variety of cobalt (III) substrates, H≠ (and S≠) solvent composition profile in the present case, displaying a weak maximum at XMe2CO  0.01 is little sensitive to the medium beyond XMe2CO  0.05

Effect of Solvent on Reactions of Coordination Complexes: Part 6 - Kinetics of Solvolysis of cis-(Bromo)(2-aminothiazole)bis(ethylenediamine)cobalt (III) in Acetone + Water Media

772-77

Reactivity of coordinated anlido base in base hydrolysis of cis-(bromo)(2-aminothiazole)-bis-(ethylenediamine)cobalt(III) cation

601-603Evidence is presented in support of cobalt(Ill) promoted NH-deprotonation of coordinated 2-aminothiazole in cis-[Co(en)2(2-aminothiazole)Br]2+ which undergoes base hydrolysis via SN1 (CB) mechanism. The pKNH of coordinated 2-aminothiazole is 12.8 ± 0.1 at 25°C and I =0.1 mol dm-3. The amido conjugate base undergoes H2O or Br- substitution at cobaIt (ID) centre 107 times faster than its conjugate acid analogue. This dramatic rate enhancement is predominantly due to considerably low activation enthalpy (∆H≠ =46 ± 5 kJ mol-1) for the substitution reaction of amido base. The negative value of the activation entropy (∆S≠ = - 39 ± 20 J K-1 mol-1) is consistent with the fact that the solvation sphere of the transition state is relatively more ordered than that of the initial state

Effect of solvent on reactions of coordination complexes-Thorium (IV)-catalysed aquation of oxalatopentaamminecobalt (III) in aqueous-alcohol media†

379-382The kinetics of title reaction have been investigated in aqueous alcohol media using methanol, propan-2-ol, t-butanoi and ethylenegiycol. The efficiency of Th4+ in promoting water for C2O4OTh2+ substitution at Co(III) centre of the binuclear species (NH3)5CoC2OTh5+ is 20 times larger (kM/kH  20 at 50°C and fully aqueous medium) than that for H+ indicating importance of the charges and Lewis basicities of the leaving groups. With increasing proportion of any of the cosolvents (vol % of alcohols 10, 20 and 50) the rate constant for the binuclear complex (kM) decreases only to a small extent and the title reaction in D2O-H2O medium is only slightly slower than in fully aqueous media (kM(D2O)/kM(D2O) = 1.12 for 50% (v/v)D2O/H2O at 65°C

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 9. Kinetics of solvolysis of cis-(chloro)(cyclohexylamine) bis(ethylenediamine)cobalt(III) in methanol -water, propan-2-ol-water, ethylene glycol-water and t-butyl alcohol-water media

The kinetics of the solvolytic aquation of cis-(chloro)(cyclohexylamine)-bis(ethylenediamine)cobalt(III) ion were investigated in water-methanol, water-propan-2-ol, water -ethylene-glycol (0-80 wt % alcohol) and water-t-butyl alcohol (0-50 wt % alcohol) media at 35 -60 °C. Plots of log ksobsvs. D−1s(where Ds is the bulk dielectric constant of the solvent mixture), log ksobsvs. the Grunwald -Winstein solvent parameter, YButCl, and log ksobsvs. Xorg(= mole fraction of cosolvent) tended to be curved. Under isodielectric conditions (Ds≈ 50) at 50 °C, the plot of log ksobsvs. Xorg for methanol -water, ethanol -water, propan-2-ol-water, t-butyl alcohol-water, acetone-water, ethylene glycol -water also did not yield a satisfactory straight line. The observed dependence of solvolysis rate on Xorg has been interpreted in terms of preferential solvation of the initial and transition states which is believed to be at least partly controlled by the overall acidity and basicity of the mixed solvent media as well as the hydrophobic interaction effect. Variation of enthalpies and entropies of activation with solvent composition was correlated with changes in the physical properties of the solvent mixtures. The solvent isotope effect (kH2O/kD2O= 1.38 ± 0.16 at 50 °C) was, however, normal as observed for several other cis-[Co(en)2(primary amine)Cl]2+ complex ions

Medium effects on the electron-transfer processes: Reduction of aquapentaamminecobalt(III) by hexacyanoferrate(II) in ethyleneglycol-water, ethylene carbonate-water and urea-water media

824-831The outer-sphere electron transfer between Co(NH3)5OH and Fe(CN) - which proceeds via precursor ion-pair formation has been investigated in ethylene glycol-water (0-47.6 wt% of EG), ethylene carbonate-water (0-40 wt% of EC) at 20-35ᵒC and in urea-water (0-19.4 wt% of EG) at 25ᵒC (I = 0.50 mol dm-3 NaNO3)· The electron transfer rate constant for the ion pair (k, s-1) follows the order H2O ~ U-H2O 2O 2O. The specific solvent effect on the rate constant for EGH2O and EC-H2O media can be account d for in the frame work of the Marcus theory by considering the ion solvent dipole interaction in the solvent cospheres of the precursor and the successor ionpairs. Both H≠ and S≠ decrease linearly with increase in mole fraction of EC(XEC) while for the EG-H2O media a broad maximum s discernible in the plot of H≠ (or S≠)­ versus XEG indicating thereby that the solvent structural effects might mediate the activation process