Water exchange reaction of a manganese catalase mimic: oxygen-17 NMR relaxometry study on (aqua)manganese(III) in a salen scaffold and its reactions in a mildly basic medium

The kinetics of water exchange for trans-[Mn^III(salen)(OH₂)₂]⁺ has been investigated by the ¹⁷O NMR line broadening technique in ¹⁷OH₂ enriched aqueous medium. The rate and activation parameters for the exchange reaction, trans-[Mn^III(salen)(OH₂)₂]⁺ + ¹⁷OH₂ → trans-[Mn^III(salen)(OH₂)(¹⁷OH₂)]⁺ + H₂O are: k²⁹⁸_ex/s⁻¹ = 3.8 × 10⁶, ΔH^≠/kJ mol⁻¹ = 18.3 ± 5.9, ΔS^≠/J K⁻¹ mol⁻¹ = −57.6 ± 20.2, which support the associative interchange mechanism (I_a). The kinetic parameters for the corresponding reaction of trans-[Mn^III(salen)(OH₂)(OH)] are but tentative (k²⁹⁸_ex/s⁻¹ = 3.5 × 10⁷, ΔH^≠/kJ mol⁻¹ = 54.1 ± 4.3, ΔS^≠/J K⁻¹ mol⁻¹ = +80.9 ± 17.0) due to its dimerization, and thermal instability of the dimer in a basic medium (pH ∼ 10). The kinetic parameters for the dimerisation of the (aqua)(hydroxo) complex are: k²⁹⁸₁/dm⁻³ mol⁻¹ s⁻¹ = 1.7 ± 0.4, ΔH^≠/kJ mol⁻¹ = 52 ± 4, ΔS^≠/J K⁻¹ mol⁻¹ = −64 ± 14; the reaction is reversible with a virtually temperature insensitive equilibrium constant, K²⁹⁸_eq./dm³ mol⁻¹ = 1.5 ± 0.3 × 10⁵. The negative value of the activation entropy is also consistent with an associative interchange mechanism (I_a) for the dimerisation reaction. The reaction mixture is EPR silent in X-band mode thus indicating that the +3 oxidation state of Mn is preserved. The dimer (OH₂)Mn^III(salen)(μ-OH)–Mn^III(salen)(OH) decays to hydrolysed product(s) obeying first order kinetics with k²⁹⁸₂/s⁻¹ = (0.8 ± 0.1) × 10⁻⁵. The relevance of water exchange kinetics of [Mn^III(salen)(OH₂)(OH₂/OH)]^+/0 with catalase and SOD (superoxide dismutase) activities of [Mn^III(salen)] has been discussed.