Hydrogen Atom Abstraction by a Mononuclear Ferric Hydroxide Complex:  Insights into the Reactivity of Lipoxygenase

The lipoxygenase mimic [Fe^III(PY5)(OH)](CF₃SO₃)₂ is synthesized from the reaction of [Fe^II(PY5)(MeCN)](CF₃SO₃)₂ with iodosobenzene, with low-temperature studies suggesting the possible intermediacy of an Fe(IV) oxo species. The Fe(III)−OH complex is isolated and identified by a combination of solution and solid-state methods, including EPR and IR spectroscopy. [Fe^III(PY5)(OH)]²⁺ reacts with weak X−H bonds in a manner consistent with hydrogen-atom abstraction. The composition of this complex allows meaningful comparisons to be made with previously reported Mn(III)−OH and Fe(III)−OMe lipoxygenase mimics. The bond dissociation energy (BDE) of the O−H bond formed upon reduction to [Fe^II(PY5)(H₂O)]²⁺ is estimated to be 80 kcal mol^-1, 2 kcal mol^-1 lower than that in the structurally analogous [Mn^II(PY5)(H₂O)]²⁺ complex, supporting the generally accepted idea that Mn(III) is the thermodynamically superior oxidant at parity of coordination sphere. The identity of the metal has a large influence on the entropy of activation for the reaction with 9,10-dihydroanthracene; [Mn^III(PY5)(OH)]²⁺ has a 10 eu more negative ΔS^⧧ value than either [Fe^III(PY5)(OH)]²⁺ or [Fe^III(PY5)(OMe)]²⁺, presumably because of the increased structural reorganization that occurs upon reduction to [Mn^II(PY5)(H₂O)]²⁺. The greater enthalpic driving force for the reduction of Mn(III) correlates with [Mn^III(PY5)(OH)]²⁺ reacting more quickly than [Fe^III(PY5)(OH)]²⁺. Curiously, [Fe^III(PY5)(OMe)]²⁺ reacts with substrates only about twice as fast as [Fe^III(PY5)(OH)]²⁺, despite a 4 kcal mol^-1 greater enthalpic driving force for the methoxide complex.