Base-Assisted Formation of Ferryl Species from TMC with Pendant N-Methylimidazole: Ligand and Base Effects on FeIV-Oxo Generation via O-O Bond Heterolysis.

Abstract

Fe^IV-oxo species can be generated using hydrogen peroxide via peroxidases, where the iron heme active site has a trans-imidazole. Here, we report the synthesis of Fe^II(TMC) complexes bearing pendant N-methylimidazole or pyridine donors using a modified reductive amination method. The structural characterization of complex 1 using ESI-Ms spectrometry, ¹H-NMR spectroscopy, and X-ray crystallography revealed a distorted square-pyramidal geometry (τ = 0.45). The Fe-N_imidazole bond to N-methylimidazole was significantly shorter (2.066(6) Å) than that to pyridine (2.112 Å), indicating stronger electron donation from N-methylimidazole. For complex 2, the corresponding Fe^IV-oxo species was successfully generated using iodosylbenzene (PhIO)/hydrogen peroxide (H₂O₂) and characterized by UV-vis, ¹H-NMR, FTIR spectroscopy, ESI-Ms spectrometry, and X-ray crystallography. The second-order rate constants (k₂) of 2 with CHD and PPh₃. are 0.14(0) and 0.50(1) M^-1 s^-1, respectively. In addition, the Kinetic analysis demonstrated a 1000-fold increase in the Fe^IV-oxo formation rate (k_obs = 1.1(1)×10^-1 s^-1 for 2; 2.7(2) × 10^-1 s^-1 for 4) when 2,6-lutidine was replaced with triethylamine in the presence of H₂O₂. Eyring analysis revealed a heterolytic O-O bond cleavage mechanism for H₂O₂ (ΔH^‡ = 19(1) kJ mol^-1, ΔS^‡ = -197(4) J/mol K for 2; ΔH^‡ = 18(0) kJ/mol, ΔS^‡ = -188(2) J mol^-1K^-1 for 4).