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Spin State Tunes Oxygen Atom Transfer towards Fe IV O Formation in Fe II Complexes
Author(s) -
Castillo Carmen E.,
Gamba Ilaria,
Vicens Laia,
Clémancey Martin,
Latour JeanMarc,
Costas Miquel,
Basallote Manuel G.
Publication year - 2021
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.202004921
Subject(s) - chemistry , ligand (biochemistry) , kinetics , medicinal chemistry , ferrous , spin states , crystallography , stereochemistry , inorganic chemistry , organic chemistry , biochemistry , physics , receptor , quantum mechanics
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 (CH 3 CN) 2 L](SbF 6 ) 2 ( [1](SbF 6 ) 2 and [2](SbF 6 ) 2 ) and [Fe II (CF 3 SO 3 ) 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 Bu 4 NIO 4 to form the corresponding [Fe IV (O)(CH 3 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)(CH 3 CN)L] 2+ is kinetically controlled by substitution in the starting complex to form [Fe II (IO 4 )(CH 3 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.