Bio-inspired iron complexes featuring secondary coordination sphere interactions: Ligand design strategies and dioxygen reactivity

The activation of dioxygen plays an important role in processes ranging from biological oxidation to energy storage and utilization. In Nature, metalloenzymes use complex architectures to promote these multi-electron and multi-proton reactions using first-row transition metal centers. Many enzymes rely on well-place hydrogen bonding networks in the secondary coordination sphere and protein superstructure to facilitate favorable reactivity at the metal center. These non-covalent interactions can contribute to the overall reactivity in a number ways including fine-tuning metal ligand interactions, supporting reactive intermediates, and controlling the movement of protons and electrons in the active site. Interested in modeling these interactions and promoting oxygen activation with bio-inspired iron complexes, our research group has designed new ligand frameworks featuring dynamic secondary coordination sphere interactions. The work reported herein describes the synthesis and derivatization of several ligand scaffolds and application of their iron complexes to O2 activation. Early work focused on the synthesis and metallation of a dipodal ligand framework capable of tautomerization from a pyrrole-imine to an azafulvene-amine upon metallation. Dative coordination of this ligand with iron(II) salts demonstrated the success of this approach, resulting in ligand tautomerization upon metallation, placing hydrogen bond donating amino groups in the secondary coordination sphere. Coordination of ancillary redox active fragments further demonstrated the flexible coordination modes of this complex, resulting in a change from a meridional to facial geometry of the ligand. Binding and dioxygenation of ortho-phenylene moieties is also described, mimicking the reactivity of ring-cleaving dioxygenase enzymes. Systematic modification of the related tripodal ligand was then carried out to examine the influence changes in the primary and secondary coordination spheres had on the properties of the iron complexes. Detailed analysis of the iron(II)-hydroxo and iron(III)-oxo complexes suggested that the ligands’ conjugated π-system results in delocalization of electron density across the ligand, as demonstrated by similar changes upon modification of either coordination sphere. However, structural analysis of the hydrogen bond donor-acceptor distances in the secondary coordination sphere indicated that the identity of the ligand capping group could be used to tune the strength of these interactions. Additional studies with these derivatives highlighted an additional role of the capping groups in determining the reactivity of the iron complexes. Switching from a bulky cyclohexyl ligand derivative to less sterically demanding aryl variants resulted in the formation of new dimeric species not previously accessible and altered the reactivity of the iron(II) complexes towards water. Having examined several different types of ligand dynamics, later work focused on applying this framework in O2 activation. Treatment of the iron(II)-triflate complex was found to result in formation of a terminal iron(III)-oxo complex. Further investigation of this reaction resulted in the identification of an iron(III)-hydroxo intermediate as well as a bridging iron(III)-oxo species derived from off-pathway decomposition. Electrochemical analysis of these complexes, and several control compounds, provided further insight, suggesting that dynamics in the ligand’s secondary coordination sphere play an important role in the dioxygen activation process. Finally, the design of ligand frameworks seeking to incorporate hydroxy groups in the secondary coordination sphere is described. These ligands are based on elaboration of the commercially available chelate, tris(2-aminoethyl)amine (tren). Several generations of the framework are described and metallated using late first-row transition metals.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste