Characterization of a Ferryl Flip in Electronically Tuned Nonheme Complexes. Consequences in Hydrogen Atom Transfer Reactivity.

Two oxoiron(IV) isomers ( 2a and 2b) of general formula [Fe (O)( PyNMe )(CH CN)] are obtained by reaction of their iron(II) precursor with NBu IO . The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between 2a and 2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that 2a reacts one order of magnitude faster than 2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in 2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand.