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+]·
(kf [CTAB] − kr). 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 kf is 737(±30) M−1 s−1 and kr is 0.80(±0.16) s−1. For kf, ΔH‡ = 58.8(±1.6) kJ mol−1, ΔS
‡ = 8.62(±0.45) J K−1 mol−1; for kr, 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.