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

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 = CH 2C 6H 4N, CH 2COO -, CH 2CH 2O -, 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 H 2O 2 to give the purple [Fe III(OOH)(HL)] n+ ( 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 H 2O 2 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 H 2O 2 more readily with the consequence of higher yields in the oxidation of the C-H bonds using H 2O 2 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 H 2O 2: in effect, transforming tpenO to tpena.