Iron(II) Complexes Supported by Sulfonamido Tripodal Ligands: Endogenous versus Exogenous Substrate Oxidation

High-valent
iron species are known to act as powerful oxidants in both natural
and synthetic systems. While biological enzymes have evolved to prevent
self-oxidation by these highly reactive species, development of organic
ligand frameworks that are capable of supporting a high-valent iron
center remains a challenge in synthetic chemistry. We describe here
the reactivity of an Fe­(II) complex that is supported by a tripodal
sulfonamide ligand with both dioxygen and an oxygen-atom transfer
reagent, 4-methylmorpholine-N-oxide (NMO). An Fe­(III)–hydroxide
complex is obtained from reaction with dioxygen, while NMO gives
an Fe­(III)–alkoxide product resulting from activation of a
C–H bond of the ligand. Inclusion of Ca2+ ions in
the reaction with NMO prevented this ligand activation and resulted
in isolation of an Fe­(III)–hydroxide complex in which the Ca2+ ion is coordinated to the tripodal sulfonamide ligand and
the hydroxo ligand. Modification of the ligand allowed the Fe­(III)–hydroxide
complex to be isolated from NMO in the absence of Ca2+ ions,
and a C–H bond of an external substrate could be activated
during the reaction. This study highlights the importance of robust
ligand design in the development of synthetic catalysts that utilize
a high-valent iron center.