Spin State Tunes Oxygen Atom Transfer towards FeIVO Formation in FeII Complexes |
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Authors: | Dr Carmen E Castillo Dr Ilaria Gamba Laia Vicens Dr Martin Clémancey Prof?Dr Jean-Marc Latour Prof?Dr Miquel Costas Prof?Dr Manuel G Basallote |
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Institution: | 1. Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica, y Química Inorgánica, Facultad de Ciencias, Instituto de Biomoléculas (INBIO), Universidad de Cádiz, Puerto Real, Cádiz, 11510 Spain;2. Grup de Química Bioinspirada, Supramolecular i Catàlisi (QBIS-CAT), Institut de Química Computacional i Catàlisi (IQCC), Departament de Química, Universitat de Girona, Campus de Montilivi, Girona, 17071 Catalonia, Spain;3. CEA, CNRS, IRIG, DIESE, LCBM, Université Grenoble Alpes, pmb, 38000 Grenoble, France |
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Abstract: | Oxoiron(IV) complexes bearing tetradentate ligands have been extensively studied as models for the active oxidants in non-heme iron-dependent enzymes. These species are commonly generated by oxidation of their ferrous precursors. The mechanisms of these reactions have seldom been investigated. In this work, the reaction kinetics of complexes FeII(CH3CN)2L](SbF6)2 ( 1](SbF6)2 and 2](SbF6)2 ) and FeII(CF3SO3)2L] ( 1](OTf)2 and 2](OTf)2 ( 1 , L=Me,HPytacn; 2 , L=nP,HPytacn; R,R′Pytacn=1-(6-R′-2-pyridyl)methyl]-4,7- di-R-1,4,7-triazacyclononane) with Bu4NIO4 to form the corresponding FeIV(O)(CH3CN)L]2+ ( 3 , L=Me,HPytacn; 4 , L=nP,HPytacn) species was studied in acetonitrile/acetone at low temperatures. The reactions occur in a single kinetic step with activation parameters independent of the nature of the anion and similar to those obtained for the substitution reaction with Cl? as entering ligand, which indicates that formation of FeIV(O)(CH3CN)L]2+ is kinetically controlled by substitution in the starting complex to form FeII(IO4)(CH3CN)L]+ intermediates that are converted rapidly to oxo complexes 3 and 4 . The kinetics of the reaction is strongly dependent on the spin state of the starting complex. A detailed analysis of the magnetic susceptibility and kinetic data for the triflate complexes reveals that the experimental values of the activation parameters for both complexes are the result of partial compensation of the contributions from the thermodynamic parameters for the spin-crossover equilibrium and the activation parameters for substitution. The observation of these opposite and compensating effects by modifying the steric hindrance at the ligand illustrates so far unconsidered factors governing the mechanism of oxygen atom transfer leading to high-valent iron oxo species. |
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Keywords: | iron kinetics oxidation reaction mechanisms spin crossover |
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