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

Dalton Transactions Pub Date: 2010-11-11 DOI: 10.1039/C0DT00958J

Abstract

N-bridged diiron tetra-tert-butylphthalocyanine activates H2O2 to form anionic hydroperoxo complex [(Pc)FeIV[double bond, length as m-dash]N–FeIII(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, PcFeIV[double bond, length as m-dash]N–FeIV(Pc+˙)[double bond, length as m-dash]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 PcFeIV[double bond, length as m-dash]N–FeIV(Pc+˙)[double bond, length as m-dash]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 PcFeIV[double bond, length as m-dash]N–FeIV(Pc+˙)[double bond, length as m-dash]O. In the course of this study, another high valent diiron species was prepared in the solid state with 70% yield. The M?ssbauer spectrum shows two quadrupole doublets with δ1 = ?0.14 mm s?1, ΔEQ1 = 1.57 mm s?1 and δ2 = ?0.10 mm s?1, ΔEQ2 = 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 ? distance. On the basis of M?ssbauer, EPR, EXAFS and XANES data this species was tentatively assigned as (Pc)FeIV[double bond, length as m-dash]N–FeIV(Pc)–OH which could be formed from PcFeIV[double bond, length as m-dash]N–FeIV(Pc+˙)[double bond, length as m-dash]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)FeIV[double bond, length as m-dash]N–FeIII(Pc) – H2O2 system capable of oxidizing methane.

Graphical abstract: High-valent diiron species generated from N-bridged diiron phthalocyanine and H2O2
Recommended Literature