Reinvestigation of the formation of a mononuclear Fe(iii) hydroperoxido complex using high pressure kinetics

Abstract

Previous stopped flow kinetic experiments suggested an interchange associative mechanism for the ligand substitution reaction, [Fe(bztpen)(OMe)]²⁺ + H₂O₂→ {[Fe(bztpen)(OMe)(HOOH)]²⁺}^‡→ [Fe(bztpen)(OOH)]²⁺ + MeOH (bztpen = N-benzyl-N, N',N'tris(2-methylpyridyl)-ethylenediamine). Thus a seven-coordinate transition state containing both the leaving methoxide and the incoming hydrogen peroxide ligands was proposed. On the basis of high pressure kinetic data we can now conclude that this is not the case since the rate of the reaction is independent of pressure for the formation of the purple low spin transient hydroperoxido complex, [Fe(bztpen)(OOH)]²⁺. [Fe(bztpen)(OOH)]²⁺ has so far proved to be too short-lived for solid state isolation. As part of our ongoing pursuit of this elusive species we have structurally characterised the nitrosyl and acetate iron(II) complexes, [Fe(bztpen)(NO)](OTf)₂ and [Fe(bztpen)(OAc)](BPh₄), as well as the air stable Co(II) complexes [Co(bztpen)Cl)]BF₄, [Co(metpen)Cl]SbF₆ and [Co(bztpen)(OAc)]BPh₄. We did not realise our aim of accessing stable Co(III) hydroperoxido or peroxido complexes by reaction of the cobalt complexes with H₂O₂.