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 [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. [Fe-III(PY5)(OH)](2+) reacts with weak X-H bonds in a manner consistent with hydrogenatom 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 Delta S-double dagger value than either [Fe-III(PY5)(OH)](2+) or [FeIII(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 [MnIII(PY5)(OH)](2+) reacting more quickly than [Fe-III(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.