Ligand driven heterolytic O–O bond cleavage in a non-haem phenolato-Fe(iii)–OOH complex to yield a formal Fe(v)O intermediate

Abstract

Fe(V)O species can be generated by the heterolytic cleavage of the O–O bond of corresponding Fe(III)–OOH species. In haem complexes the redox non-innocence of the ligand facilitates such heterolytic cleavage, however non-haem iron complexes generally show homolytic cleavage to form an Fe(IV)O species and a hydroxyl radical. The hydroxyl radical formed is undesirable due to its non-selective reactivity. Here we show that the redox non-innocence of a phenolato ligand moiety in the complex [LFe(III)(μ-O)Fe(III)L]²⁺, where L is 2-(((di(pyridin-2-yl)methyl)(pyridin-2-ylmethyl)amino)methyl)phenolate, facilitates heterolytic O–O bond cleavage, similar in manner to that observed with haem Fe(III)–OOH species, to yield a formal Fe(V)O intermediate. Although not observed directly, the intermediacy of an Fe(V)O species is manifested in the immediate appearance of a doubly oxidised bis-phenolato bridged complex observed by time resolved UV/vis absorption and resonance Raman spectroscopy. This complex is formed by C–C coupling at the para position of the phenolato moiety of the ligand. The pathways to form the final complex via various Fe(IV)O and Fe(V)O intermediates are investigated by DFT methods, which indicate that the impact of the phenolato moiety is due to its redox non-innocence primarily. The ability of the phenolato moiety to transfer charge and spin density induces a switch in the mechanism of O–O bond cleavage from homolytic to heterolytic manifested in the radical character at the para-position needed for C–C bond formation and the high oxidation state of the first observed product.