# Manipulating Terminal Iron-Hydroxide Nucleophilicity through Redox

## TL;DR

This paper shows how changing the oxidation state of iron in a complex affects its chemical reactivity, especially with CO2 and radicals.

## Contribution

The study reveals how redox state and ligand properties control the nucleophilic or electrophilic behavior of iron hydroxo complexes.

## Key findings

- Ferrous iron hydroxo complexes show nucleophilic reactivity toward CO2, forming a reversible bicarbonate adduct.
- Ferric analogues exhibit electrophilic behavior, reacting with carboradicals through radical recombination.
- Ligand electronegativity and oxidation state influence the reactivity profile of terminal Fe–X pairs.

## Abstract

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., FeII/FeIII). 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)­FeII(OH), its high-spin ferric analogue (EmL)­FeIIII­(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.