Manipulating Terminal Iron-Hydroxide Nucleophilicity through Redox

We report changes in the reactivity profile of a high-spin, terminal ferrous hydroxo complex (^EmL)Fe(OH) as a function of oxidation states (i.e., Fe^II/Fe^III). The terminal, high-spin Fe-OH adducts were isolated within a sterically hindered dipyrrin ligand scaffold. In the ferrous state, (^EmL)Fe(OH) exhibits nucleophilic reactivity toward carbon-based electrophiles (e.g., CS₂, CO₂, nitrile, isocyanate), highlighted by the reversible capture of CO₂ to generate (^EmL)Fe(κ²-O,O-HCO₃) (ΔG° = -2.0 kcal/mol) both in solution and solid state as characterized by single-crystal X-ray crystallography, ⁵⁷Fe Mössbauer spectroscopy, and IR spectroscopy. We probed the nucleophilic character of ferrous analogues with different terminal ligand motifs (X: -CH₃, -NH₂, -F, -SH, -H) through a comparison of their reactivity with CO₂. In contrast to the nucleophilic character exhibited by (^EmL)Fe^II(OH), its high-spin ferric analogue (^EmL)Fe^IIII(OH) exhibited electrophilic reactivity at the hydroxo ligand, undergoing radical recombination with carboradicals, akin to the radical recombination reactivity observed in hydroxylation from high-valent iron oxenoids. These results highlight the effect of the oxidation level, ligand electronegativity, and basicity on the resulting nucleophilic/electrophilic character of the terminal Fe-X pair.