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

Abstract

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 (k_H2O/k_D2O=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 (X_org) of the cosolvent and the effect was pronounced at relatively high values of X_org 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 k_s vs. 1/ε_s plots (k_s is the rate constant and ε_s, 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, [ΔΔG_t^≠](s←w), decreased linearly with X_org for all mixed-solvent media, indicating that the preferential solvation effect is not significant. The activation enthalpy and entropy vs. X_org 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.