High-valent diiron species generated from N-bridged diiron phthalocyanine and H2O2

Abstract

N-bridged diiron tetra-tert-butylphthalocyanine activates H₂O₂ to form anionic hydroperoxo complex [(Pc)Fe^IVN–Fe^III(Pc)–OOH]⁻ prone to heterolytic cleavage of O–O bond with the release of OH⁻ and formation of neutral diiron oxo phthalocyanine cation radical complex, PcFe^IVN–Fe^IV(Pc⁺˙)O. ESI-MS data showed stability of the Fe–N–Fe binuclear structure upon formation of this species, capable of oxidizing methane and benzenevia O-atom transfer. The slow formation kinetics and the high reactivity preclude direct detection of this oxo complex by low temperature UV-vis spectroscopy. However, strong oxidizing properties and the results of EPR study support the formation of PcFe^IVN–Fe^IV(Pc⁺˙)O. Addition of H₂O₂ at −80 °C led to the disappearance of iron EPR signal and to the appearance of the narrow signal at g = 2.001 consistent with the transient formation of PcFe^IVN–Fe^IV(Pc⁺˙)O. In the course of this study, another high valent diiron species was prepared in the solid state with 70% yield. The Mössbauer spectrum shows two quadrupole doublets with δ₁ = −0.14 mm s⁻¹, ΔE_Q1 = 1.57 mm s⁻¹ and δ₂ = −0.10 mm s⁻¹, ΔE_Q2 = 2.03 mm s⁻¹, respectively. The negative δ values are consistent with formation of Fe(IV) states. Fe K-edge EXAFS spectroscopy reveals conservation of the diiron Fe–N–Fe core. In XANES, an intense 1s → 3d pre-edge feature at 7114.4 eV suggests formation of Fe(IV) species and attaching of one oxygen atom per two Fe atoms at the 1.90 Å distance. On the basis of Mössbauer, EPR, EXAFS and XANES data this species was tentatively assigned as (Pc)Fe^IVN–Fe^IV(Pc)–OH which could be formed from PcFe^IVN–Fe^IV(Pc⁺˙)O by hydrogen atom abstraction from a solvent molecule. Thus, despite unfavourable kinetics, we succeeded in the preparation of the first dirion(IV) phthalocyanine complex with oxygen ligand, generated in the (Pc)Fe^IVN–Fe^III(Pc) – H₂O₂ system capable of oxidizing methane.