The oxidation of bromide ion by [(FeTPP)2O]+SbF6− in dichloromethane

Abstract

The oxidation of cetyltrimethylammonium bromide (CTAB) by mono-oxidised iron(III) tetraphenylporphyrin μ-oxo-dimer, [(FeTPP)₂O]⁺SbF₆⁻, has been found to be much slower than the corresponding oxidation of iodide ion, but can still be studied by stopped-flow techniques. The stoichiometry of the reaction has been established as 2 [Fe(TPP)₂O]⁺ + 3Br⁻ → 2(FeTPP)₂O + Br₃⁻. The rate law for the reaction is d [(FeTPP)₂O]/dt = [Fe(TPP)₂O⁺]·

(k_f [CTAB] − k_r). The rate constant for the first term has been identified with the rate determining forward process, the formation of the unstable Br₂⁻, and the second term with the attack of Br₂⁻ on the uncharged μ-oxo-dimer. A subsequent process involves what appears to be the rapid oxidation of Br₂⁻ in the presence of Br⁻ to give Br₃⁻. At 298 K k_f is 737(±30) M⁻¹ s⁻¹ and k_r is 0.80(±0.16) s⁻¹. For k_f, ΔH^‡ = 58.8(±1.6) kJ mol⁻¹, ΔS^‡ = 8.62(±0.45) J K⁻¹ mol⁻¹; for k_r, ΔH^‡ = 43.6(±10.8) kJ mol⁻¹, ΔS^‡ = −97.8(±36.0) J K⁻¹ mol⁻¹. The addition of cetyltrimethylammonium perchlorate (inert to reaction with the oxidised μ-oxo-dimer) slowed the reaction in a manner which indicated competition between perchlorate and bromide for the formation of ion pairs with the oxidised iron(III) dimer, the bromide ion pair being the reactive one.

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