# para-Donor Effects in PyNO Push Ligands Control O–O Bond Cleavage of TPAFe(III)-Acylperoxo to High-Valent Fe(IV)O or Fe(IV)O Radical Cation Species

## TL;DR

This study shows how substituents on pyridine N-oxides influence the O–O bond cleavage in iron complexes, leading to different high-valent iron species with distinct reactivity.

## Contribution

The paper introduces para-donor effects in PyNO ligands as a new strategy to control O–O bond cleavage pathways in nonheme iron complexes.

## Key findings

- Para-donor-substituted PyNOs direct Fe(III)–acylperoxo intermediates toward homolysis or heterolysis.
- NMe2-PyNO produces a short-lived Fe(IV)–O radical cation capable of oxidizing alkanes with high C–H bond dissociation energies.
- The study confirms ligand-controlled access to reactive oxoiron states for C–H activation with a large kinetic isotope effect.

## Abstract

Selective O–O
bond cleavage in nonheme Fe­(III)–OOR
complexes supported by tetradentate N4 ligands has long
been challenging. Here we show that para-donor-substituted
pyridine N-oxides (PyNOs) act as powerful external
push ligands that direct Fe­(III)–acylperoxo intermediates toward
homolysis or heterolysis. Treatment of the μ-oxo dimer [(TPA)­(H2O)­FeIII(μ-O)­FeIII(H2O)­(TPA)]4+ (1) with X-PyNO (X = H, OMe, NMe2) and mCPBA at −40 °C generates
the elusive acylperoxo species [(TPA)­(X-PyNO)­FeIII–OOC­(Ar)]2+ (2

X
). PyNO and OMe-PyNO
promote homolytic O–O cleavage to afford high yields of the S = 1 Fe­(IV)O species, [(TPA)­(PyNO)­FeIV=O]2+ (3), corroborated by a λmax at 727 nm and low-temperature 1H NMR data supporting
PyNO binding. In contrast, strongly donating NMe2-PyNO
enforces heterolysis, producing a short-lived S =
1/2 oxoiron intermediate with near-isotropic EPR (g = 2.024, 2.011, 1.992) and intense UV–vis absorption at 586
and 637 nm. DFT analysis reveals antiferromagnetic coupling between
an FeIV=O unit and the NMe2-PyNO·
+ ligand, defining this species as an Fe­(IV)O–PyNO·
+ radical cation (4). Complex 4 oxidizes alkanes with C–H BDEs up to 99 kcal·mol–1 and exhibits a large KIE = 7.9 (toluene/toluene-d
8), establishing ligand-controlled access to
reactive oxoiron states for C–H activation.