Kinetic and equilibrium study of the reaction of (meso-tetrakis(p-sulfonatophenyl)porphyrinato)diaquocobaltate(III) with pyridine in aqueous solution

Article abstract of DOI:10.1021/ic50221a076

The reaction of pyridine (py) with (meso-tetrakis(p-sulfonatophenyl)porphyrinato)diaquocobaltate(III) (Co(TPPS)(H₂O)₂^3-) has been investigated between pH 2 and 13 at μ= 1.00 M (NaClO₄). Scheme I best depicts the reactions deduced to be important. The values of all of the parameters in Scheme I were determined and are reported. The k₁^py path is second 7 order with k₁^py = (9.56 ± 0.30) × 10² M^-1 s^-1 at 25°C and the values of the activation parameters are ΔH*₁ = 17.6 ± 0.4 kcal/mol and ΔS*₁ = 14.4 ± 1.3 cal/(deg mol). These parameters imply dissociative (d) activation. The k₄^py path is second order with k₄^py = (1.22 ± 0.12) × 10⁶ M^-1 s^-1 at 25°C, and the values of the activation parameters are ΔH*₄ = 11.5 ± 0.5 kcal/mol and Δ*₄ = 6.4 ± 1.6 cal/(deg mol). The parameters imply d activation and the OH^- ligand has labilized the Co(TPPS)(OH)H₂O^4- by a factor of 10³ compared to Co(TPPS)(H₂O)₂^3-. The k₂^py path has a pseudo-first-order rate law with the pseudo-first-order rate constant k_obsd = (E[py] + FG)/(G + [pyl]) at pH 13. This is interpreted to imply a dissociative (D) mechanism. The parameter E represents the first-order rate constant for formation of the 5-coordinate intermediate and has a value of (1.57 ± 0.13) × 10³ s^-1 at 25°C. The values of ΔH*₇ and ΔS*₇ are 11.6 ± 3.1 kcal/mol and -5.1 ± 10.1 cal/(deg mol). The values of K_a1, K_a2, and K_a1^py at 25°C are (9.4 ± 0.6) × 10^-8, (1.7 ± 0.2) × 10^-10, and (5.6 ± 0.2) × 10^-10 M, respectively. The values of K₁^py, K₂^py, K₄^py, K₅^py, and K₆^py at 25°C are (9.1 ± 0.9) × 10⁸ M^-1, (1.2 ± 0.1) × 10⁵ M^-1, (5.4 ± 0.8) × 10⁶ M^-1, (1.3 ± 0.2) × 10², and 3.4 ± 0.7, respectively. In summary, the kinetic and equilibrium properties of Co(TPPS)(H₂O)₂^3- are very similar to those of (meso-tetrakis(4-N-methylpyridyl)-porphyrin)diaquocobalt(III) and (meso-tetrakis(4-carboxyphenyl)porphyrinato)diaquocobaltate(III). However there are some significant differences.