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

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Title: Hydrogen Atom Abstraction by a Mononuclear Ferric Hydroxide Complex: Insights into the Reactivity of Lipoxygenase.
Authors: Goldsmith, Christian R.1, Stack, T. Daniel P.1 stack@stanford.edu
Source: Inorganic Chemistry. 7/24/2006, Vol. 45 Issue 15, p6048-6055. 8p. 3 Charts, 4 Graphs.
Subjects: Atomic hydrogen, Ferric hydroxides, Complex compounds, Lipoxygenases, Reactivity (Chemistry)
Abstract: 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(lll)-OH complex is isolated and identified by a combination of solution and solid-state methods, including EPR and IR spectroscopy. [FIII(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(lll)-OH and Fe(lll)-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(lll) 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(lll) 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. [ABSTRACT FROM AUTHOR]
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Abstract: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(lll)-OH complex is isolated and identified by a combination of solution and solid-state methods, including EPR and IR spectroscopy. [FIII(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(lll)-OH and Fe(lll)-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(lll) 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(lll) 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. [ABSTRACT FROM AUTHOR]
ISSN:00201669
DOI:10.1021/ic060621e