Ligand substitution and electron transfer reactions of trans-(diaqua)(salen)manganese(iii) with oxalate: an experimental and computational study

Abstract

The trans-Mn^III(salen)(OH₂)₂⁺ undergoes reversible aqua ligand substitution by HOX⁻ (H₂salen = N,N′-bis(salicylidene)ethane-1,2-diamine; HOX⁻ = ⁻O–COCO₂H) with k₁/dm³ mol⁻¹ s⁻¹ (k₋₁/s⁻¹) = 11.8 ± 0.7 (0.255 ± 0.02), ΔH^≠/kJ mol⁻¹ = 54.6 ± 0.8 (64.2 ± 6.7), ΔS^≠/J K⁻¹ mol⁻¹ = −41.2 ± 2.6 (−40.8 ± 22.7) at 25.0 °C and I = 0.3 mol dm⁻³. The low values of the activation enthalpy and nearly the same and negative values of the activation entropy are ascribed to an associative transition state for this interchange process (I_a mechanism). The redox reaction that follows involves several paths and the products are Mn^II and CO₂ identified by ESR spectroscopy and conventional test, respectively. The rate retardation by acrylamide monomer with no perceptible polymerization during the course of the redox reaction supports the involvement of the radical intermediate, C₂O₄⁻˙ (= CO₂ + CO₂⁻˙) which succeeds in reducing Mn^III species much faster than the dimerisation of its congener, CO₂⁻˙ in keeping with the stoichiometry, |[ΔMn^III]/Δ[OX]| = 2. The trans-[Mn^III(salen)(OH₂)(HOX) and its conjugate base, trans-Mn^III(salen)(OH₂)(OX)⁻ are virtually inert to intramolecular reduction of the Mn^III centre by the bound oxalate species but undergo facile electron transfer by H₂OX, HOX⁻ and very slowly by OX²⁻ following the reactivity sequence, k_H2OX > k_HOX ⋙ k_OX and featuring second order kinetics. The rate retardation by the anionic micelles of SDS (sodium dodecyl sulfate) and rate enhancement by N₃⁻ provide supportive evidence in favor of the proposed mechanistic pathways. The structure optimization of trans-Mn^III(salen)(OH₂)(HOX) (A), trans-Mn^III(salen)(HOX)₂⁻ (B), trans-Mn^III(salen)(OH₂)(OX)⁻ (C), trans-Mn^III(salen)(OH₂)(H₂OX)⁺ (E₁), and trans-Mn^III(salen)(HOX)(H₂OX) (E₂) {all high spin Mn^III(d⁴)} by Density Functional Theory (DFT) reveals that the structural trans-effect of the unidentately bonded OX²⁻ in C is the strongest and Mn^III assumes five coordination with the H₂O molecule (displaced from the Mn^III centre), hydrogen bonded to the phenoxide oxygen moiety. The computational study highlights different modes of H-bonding in structures A–E. The activation parameters for the redox reactions, A + HOX⁻ and A + H₂OX, ΔH^≠/kJ mol⁻¹ (ΔS^≠/J K⁻¹ mol⁻¹): 42.5 ± 6.2, (−106 ± 20) and 71.7 ± 7.7 (+12 ± 25), respectively, are indicative of different degrees of ordering and reorganization of bonds as expected in the case of a proton coupled electron transfer (PCET) process.