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

The lipoxygenase mimic [FeIII(PY5)(OH)](CF3SO3)2 is synthesized from the reaction of [FeII(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. [FeIII(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 [FeII(PY5)(H2O)]2+ is estimated to be 80 kcal mol-1, 2 kcal mol-1 lower than that in the structurally analogous [MnII(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; [MnIII(PY5)(OH)]2+ has a 10 eu more negative ΔS value than either [FeIII(PY5)(OH)]2+ or [FeIII(PY5)(OMe)]2+, presumably because of the increased structural reorganization that occurs upon reduction to [MnII(PY5)(H2O)]2+. The greater enthalpic driving force for the reduction of Mn(III) correlates with [MnIII(PY5)(OH)]2+ reacting more quickly than [FeIII(PY5)(OH)]2+. Curiously, [FeIII(PY5)(OMe)]2+ reacts with substrates only about twice as fast as [FeIII(PY5)(OH)]2+, despite a 4 kcal mol-1 greater enthalpic driving force for the methoxide complex.