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-<i>N</i>-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

Synthesis and Reactivity of Tripodal Complexes Containing Pendant Bases

The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris­(amido)­amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt­(II) and zinc­(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes <b>[Co­(L)]</b>^<b>–</b> with dioxygen reversibly generates a small amount of a cobalt­(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn­[N­{CH₂CH₂NC­(O)­CH₂N­(CH₂Ph)₂}₃)]⁻ (<b>[Zn­(TN</b>^<b>Bn</b><b>)]</b>^<b>–</b>) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes <b>[M­(L)]</b>^<b>–</b> results in complete proteolysis and formation of the ligands <b>H</b>_<b>3</b><b>L</b>. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative <i>and</i> acidic conditions