Mononuclear non-heme iron(iii) peroxide complexes: syntheses, characterisation, mass spectrometric and kinetic studies

Abstract

A series of transient interconvertible protonated and deprotonated mononuclear Fe(III) peroxo species are derived from the pH dependent reaction of dihydrogen peroxide with mononuclear iron(II) or iron(III) complexes of general formulation [Fe(Rtpen)X](A)_n, n = 1, 2; X = Cl, Br; Rtpen = N-alkyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine, alkyl = R = CH₃CH₂, CH₃CH₂CH₂, HOCH₂CH₂, (CH₃)₂CH, C₆H₅, and C₆H₅CH₂; A = ClO₄, PF₆. The low-spin iron(III) hydroperoxide complex ions [Fe(Rtpen)(η¹-OOH)]²⁺ are purple chromophores and the high-spin iron(III) peroxide complexes, [Fe(Rtpen)(η²-OO)]⁺ are blue chromophores. The spectroscopic observation (ESR, UV-vis, ESI MS) of a low-spin iron(III) precursor species [Fe(Rtpen)(η¹-OCH₃)]²⁺ and kinetic studies show that formation of [Fe(Rtpen)(η¹-OOH)]²⁺ from iron(II) solution species is a two step process. The first step, the oxidation of the iron(II) complex to [Fe(Rtpen)(OCH₃)]²⁺, is faster than the subsequent ligand substitution during which [Fe(Rtpen)(η¹-OOH)]²⁺ is formed. The kinetic data are consistent with an interchange associative mechanism for the ligand substitution, and a role for the proton bound to the uncoordinated hydroperoxide oxygen atom is suggested. The stability of [Fe(Rtpen)(η¹-OOH)]²⁺ R = HOCH₂CH₂, is significantly lower than for the peroxide complexes generated from the other alkyl substituted ligands (t_1/2ca. 10 min vs. several hours). Tandem MS/MS experiments with the [Fe(Rtpen)(η¹-OOH)]²⁺ ions show fragmentation via O–O cleavage to give the novel ferryl species [Fe(Rtpen)(O)]²⁺. By contrast the [Fe(Rtpen)(η²-OO)]⁺ ions are stable under the same gas phase conditions. This indicates a weaker O–O bond in the Fe(III) hydroperoxide complex ions, and that [Fe^IIIOOH]²⁺ rather than [Fe^IIIOO]⁺ species are the precursors to, at least, the ferryl Fe^IVO species. Crystal structures of four starting iron(II) compounds, [Fe(Rtpen)Cl]PF₆, R = HOCH₂CH₂, CH₃CH₂CH₂, C₆H₅CH₂, and [Fe(bztpen)Br]PF₆ show the iron atoms in distorted octahedral geometries with pentadentate Rtpen coordination with the halide ion as the sixth ligand. The structure of [Fe(etOHtpen)Cl]PF₆ shows an intermolecular H-bonding interaction between the dangling hydroxyethyl group and the chloride of a neighbouring molecule with O–H⋯Cl, 3.219(2) Å.