High-valent diiron species generated from N-bridged diiron phthalocyanine and H(2)O(2)

Abstract

N-bridged diiron tetra-tert-butylphthalocyanine activates H(2)O(2) to form anionic hydroperoxo complex [(Pc)Fe(IV)=N-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(IV)=N-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 benzene via 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(IV)=N-Fe(IV)(Pc(+)˙)=O. Addition of H(2)O(2) 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(IV)=N-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 δ(1) = -0.14 mm s(-1), ΔE(Q1) = 1.57 mm s(-1) and δ(2) = -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(IV)=N-Fe(IV)(Pc)-OH which could be formed from PcFe(IV)=N-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(IV)=N-Fe(III)(Pc) - H(2)O(2) system capable of oxidizing methane.