Spin State Tunes Oxygen Atom Transfer towards FeIVO Formation in FeII Complexes. Issue 15 (15th February 2021)

• Record Type: Journal Article
• Authors: Castillo, Carmen E.; Gamba, Ilaria; Vicens, Laia; Clémancey, Martin; Latour, Jean‐Marc; Costas, Miquel; Basallote, Manuel G.
• Is Part Of: Chemistry. Volume 27:Issue 15(2021)
• Page Start: 4946
• Page End: 4954
• Publication Date: 2021-02-15
• Subjects: iron -- kinetics -- oxidation -- reaction mechanisms -- spin crossover; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202004921
• Languages: English
• ISSNs: 0947-6539

Abstract

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 [Fe II (CH3 CN)2 L](SbF6 )2 ([1](SbF6 )2 and [2](SbF6 )2 ) and [Fe II (CF3 SO3 )2 L] ([1](OTf)2 and [2](OTf)2 (1, L= Me, H Pytacn; 2, L= nP, H Pytacn; R, R′ Pytacn=1‐[(6‐R′‐2‐pyridyl)methyl]‐4, 7‐ di‐R‐1, 4, 7‐triazacyclononane) with Bu4 NIO4 to form the corresponding [Fe IV (O)(CH3 CN)L] 2+ (3, L= Me, H Pytacn; 4, L= nP, H Pytacn) 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 [Fe IV (O)(CH3 CN)L] 2+ is kinetically controlled by substitution in the starting complex to form [Fe II (IO4 )(CH3 CN)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. Abstract : Spin control : The formation of oxoiron(IV) complexes by the reaction of iron(II) precursors bearing tetradentate N ligands with IO4 − occurs through a fast spin‐crossover pre‐equilibrium followed by rate‐determining substitution to form an intermediate that leads rapidly to the Fe IV =O species. The experimental kinetic and activation parameters highlight a key role played the spin in controlling the reaction.