title: Hydrogen atom abstraction by a mononuclear ferric hydroxide complex: insights into the reactivity of lipoxygenase.
abstract: 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.
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
authors.forename: Christian R
authors.initials: CR
authors.lastname: Goldsmith
authors.forename: T Daniel P
authors.initials: TD
authors.lastname: Stack
mesh_terms: D000873:Anthracenes / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D008084:Lipoxygenase / Q000737:chemistry*; D008670:Metals / Q000737:chemistry; D008956:Models, Chemical; D015394:Molecular Structure; D010087:Oxides / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry; D013696:Temperature; D013816:Thermodynamics
chemical_list: D000873:Anthracenes; D005290:Ferric Compounds; D008024:Ligands; D008670:Metals; D010087:Oxides; C021024:ferric hydroxide; D006859:Hydrogen; D008084:Lipoxygenase; D010100:Oxygen