Electronic Structure and Reactivity of Mononuclear Nonheme Iron-Peroxo Complexes as a Biomimetic Model of Rieske Oxygenases: Ring Size Effects of Macrocyclic Ligands.

We report the macrocyclic ring size-electronic structure-electrophilic reactivity correlation of mononuclear nonheme iron(III)-peroxo complexes bearing N -tetramethylated cyclam analogues ( n -TMC), [Fe III (O 2 )(12-TMC)] + ( 1 ), [Fe III (O 2 )(13-TMC)] + ( 2 ), and [Fe III (O 2 )(14-TMC)] + ( 3 ), as a model study of Rieske oxygenases. The Fe(III)-peroxo complexes show the same δ and pseudo-σ bonds between iron and the peroxo ligand. However, the strength of these interactions varies depending on the ring size of the n -TMC ligands; the overall Fe-O bond strength and the strength of the Fe-O 2 δ bond increase gradually as the ring size of the n -TMC ligands becomes smaller, such as from 14-TMC to 13-TMC to 12-TMC. MCD spectroscopy plays a key role in assigning the characteristic low-energy δ → δ* LMCT band, which provides direct insight into the strength of the Fe-O 2 δ bond and which, in turn, is correlated with the superoxo character of the iron-peroxo group. In oxidation reactions, reactivities of 1 - 3 toward hydrocarbon C-H bond activation are compared, revealing the reactivity order of 1 > 2 > 3 ; the [Fe III (O 2 )( n -TMC)] + complex with a smaller n -TMC ring size, 12-TMC, is much more reactive than that with a larger n -TMC ring size, 14-TMC. DFT analysis shows that the Fe(III)-peroxo complex is not reactive toward C-H bonds, but it is the end-on Fe(II)-superoxo valence tautomer that is responsible for the observed reactivity. The hydrogen atom abstraction (HAA) reactivity of these intermediates is correlated with the overall donicity of the n -TMC ligand, which modulates the energy of the singly occupied π* superoxo frontier orbital that serves as the electron acceptor in the HAA reaction. The implications of these results for the mechanism of Rieske oxygenases are further discussed.