Accessing a synthetic Fe^IIIMn^IV core to model biological heterobimetallic active sites

Metalloproteins with dinuclear cores are known to bind and activate dioxygen, with a subclass of these proteins having active sites containing FeMn cofactors and activities ranging from long-range proton-coupled electron transfer (PCET) to post-translational peptide modification. While mechanistic studies propose that these metallocofactors access Fe^IIIMn^IV intermediates, there is a dearth of related synthetic analogs. Herein, the first well-characterized synthetic Fe^III-(μ-O)-Mn^IV complex is reported; this complex shows similar spectroscopic features as the catalytically competent Fe^IIIMn^IV intermediate X found in Class Ic ribonucleotide reductase and demonstrates PCET function towards phenolic substrates. This complex is prepared from the oxidation of the isolable Fe^III-(μ-O)-Mn^III species, whose stepwise assembly is facilitated by a tripodal ligand containing phosphinic amido groups. Structural and spectroscopic studies found proton movement involving the Fe^IIIMn^III core, whereby the initial bridging hydroxido ligand is converted to an oxido ligand with concomitant protonation of one phosphinic amido group. This series of FeMn complexes allowed us to address factors that may dictate the preference of an active site for a heterobimetallic cofactor over one that is homobimetallic: comparisons of the redox properties of our FeMn complexes with those of the di-Fe analogs suggested that the relative thermodynamic ease of accessing an Fe^IIIMn^IV core can play an important role in determining the metal ion composition when the key catalytic steps do not require an overly potent oxidant. Moreover, these complexes allowed us to demonstrate the effect of the hyperfine interaction from non-Fe nuclei on ⁵⁷Fe Mössbauer spectra which is relevant to MnFe intermediates in proteins.