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

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⁺ (2X). 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-d₈), establishing ligand-controlled access to reactive oxoiron states for C-H activation.