Comparison of high-spin and low-spin nonheme Fe(III)-OOH complexes in O-O bond homolysis and H-atom abstraction reactivities.
The geometric and electronic structures and reactivity of an S = 5/2 (HS) mononuclear nonheme (TMC)Fe(III)-OOH complex are studied by spectroscopies, calculations, and kinetics and compared with the results of previous studies of S = 1/2 (LS) Fe(III)-OOH complexes to understand parallels and differences in mechanisms of O-O bond homolysis and electrophilic H-atom abstraction reactions. The homolysis reaction of the HS [(TMC)Fe(III)-OOH](2+) complex is found to involve axial ligand coordination and a crossing to the LS surface for O-O bond homolysis. Both HS and LS Fe(III)-OOH complexes are found to perform direct H-atom abstraction reactions but with very different reaction coordinates. For the LS Fe(III)-OOH, the transition state is late in O-O and early in C-H coordinates. However, for the HS Fe(III)-OOH, the transition state is early in O-O and further along in the C-H coordinate. In addition, there is a significant amount of electron transfer from the substrate to the HS Fe(III)-OOH at transition state, but that does not occur in the LS transition state. Thus, in contrast to the behavior of LS Fe(III)-OOH, the H-atom abstraction reactivity of HS Fe(III)-OOH is found to be highly dependent on both the ionization potential and the C-H bond strength of the substrate. LS Fe(III)-OOH is found to be more effective in H-atom abstraction for strong C-H bonds, while the higher reduction potential of HS Fe(III)-OOH allows it to be active in electrophilic reactions without the requirement of O-O bond cleavage. This is relevant to the Rieske dioxygenases, which are proposed to use a HS Fe(III)-OOH to catalyze cis-dihydroxylation of a wide range of aromatic compounds.
Department, of Chemistry, Stanford University, Stanford, California 94305, USA.
Lei V
LV
Liu
Seungwoo
S
Hong
Jaeheung
J
Cho
Wonwoo
W
Nam
Edward I
EI
Solomon
D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D007700:Kinetics; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
D005290:Ferric Compounds; D006859:Hydrogen; D010100:Oxygen