cis Donor Influence on O-O Bond Lability in Iron(III) Hydroperoxo Complexes: Oxidation Catalysis and Ligand...

The Fe^III/Fe^II redox potentials for [Fe(tpen)]^2+/3+, [Fe(tpena)]^+/2+, and [Fe(tpenO)]^+/2+ (N-R-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine, where R = CH₂C₆H₄N, CH₂COO^-, CH₂CH₂O^-, respectively) span 470 mV with the oxidation potentials following the order [Fe^II(tpenO)]⁺ (MeOH) < [Fe^II(tpena)]⁺ (MeCN) < [Fe^II(tpen)]²⁺ (MeCN). In their +3 oxidation states the complexes react with 1 equiv of H₂O₂ to give the purple [Fe^III(OOH)(HL)] (n = 2 for L = tpena, tpenO; n = 3 for L = tpen). A pyridine arm is decoordinated in these complexes, furnishing a second coordination sphere base which is protonated at ambient pH. The lifetimes of these transient species depend on how readily the substrate (sometimes the solvent) is oxidized and reflect the trend in both the O-O bond lability and oxidizing potency of the putative iron-based oxidant derived from the iron(III) peroxides. In methanol solution, [Fe^III(tpenO)]²⁺ and [Fe^III(tpena)]²⁺ exist in their Fe(III) states and hence the formation of [Fe^III(OOH)(Htpena)]²⁺ and [Fe^III(OOH)(HtpenO)]²⁺ is instantaneous. This is in contrast to the short lag time that occurs before adduct formation between [Fe^II(tpen)]²⁺ and H₂O₂ due to the requisite prior oxidation of the solution-state iron(II) complex to its iron(III) state. Stabilization of the +3 iron oxidation state in the resting state catalysts affords complexes that activate H₂O₂ more readily with the consequence of higher yields in the oxidation of the C-H bonds using H₂O₂ as terminal oxidant. The presence of a cis monodentate carboxylato donor increases the rate of oxidation by hydrogen atom transfer in comparison to the systems with an alkoxo or pyridine in this position. Competing with substrate oxidation is the oxidative modification of the alkoxido group in [Fe^III(tpenO)]²⁺, converting it to a carboxylato group in the presence of H₂O₂: in effect, transforming tpenO to tpena.