Kinetics and mechanism of formation, acid catalysed aquation, reversible anation and photochemical reaction of trans-(aqua)(sulfito-S)[N,N′-ethylenebis(salicylidiniminato)]cobaltate(III) in aqueous media

Abstract

The reaction of trans-[Co(salen)(OH₂)OH] with SO₂ yields trans-[Co(salen)(OH₂)(SO₃-S)]⁻ (S-bonded isomer) for which the rate and activation parameters at 25 °C (I = 0.3 mol dm⁻³) are k^SO₂ = (5.9 ± 0.1) × 10¹⁰ dm³ mol⁻¹ s⁻¹, ΔH^‡ = 66 ± 4 kJ mol⁻¹ and ΔS^‡ = 183 ± 14 J K⁻¹ mol⁻¹. One possibility for the S^IV substitution is that Co–S bond formation is concerted with Co–O bond breaking. An alternative mechanism, involving a fast equilibrium between SO₂ and trans-[Co(salen)(OH₂)OH] forming an O-bonded sulfito species which then undergoes sulfite ligand linkage isomerisation, is also possible. An estimated value of the isomerisation rate constant for the trans-[Co(salen)(OH₂)(OSO₂H)] at 25 °C is ca. 10⁶ s⁻¹. The trans-[Co(salen)(OH₂)(SO₃-S)]⁻ (pK = 10.1 ± 0.1 at 25 °C, I = 0.3 mol dm⁻³) undergoes acid catalysed aquation to yield the parent diaqua complex and S^IV with k_H = 29.5 ± 1.1 dm³ mol⁻¹ s⁻¹, ΔH^‡ = 72 ± 3 kJ mol⁻¹, ΔS^‡ = 24 ± 9 J K⁻¹ mol⁻¹ at 25 °C (I = 0.3 mol dm⁻³). Steady state photolysis (254 nm) of trans-[Co(salen)(OH₂)(SO₃-S)]⁻ resulted in the reduction of Co^III. The redox rate constant and ϕ(Co²⁺) decreased with increasing pH. Attempts to detect an O-bonded sulfito complex as a transient in the conventional flash photolysis of this aqua-sulfito complex proved unsuccessful.

The aqua ligand replacement reactions of trans-[Co(salen)(OH₂/OH)(OH₂)]^+/0 with imidazole and that of the corresponding aqua-sulfito complex with N₃⁻, NCS⁻, imidazole, and S^IV in a large excess of the entering ligands have been studied at 25 °C. A comparison of the rate constants with the analogous data for trans-[Co(AA)₂(OH₂)(SO₃-S)]⁺ (AA = 1,2-diaminoethane; 1,3-diaminopropane) clearly shows that the kinetic trans-effect of the S-bonded sulfite is substantially attenuated in trans-[Co(salen)(OH₂)(SO₃-S)]⁻.