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 DeltaS++ 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.