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

摘要

A series of transient interconvertible protonated and deprotonated mononuclear Fe(III) peroxo species are derived from the pH dependent reaction of dihydrogne 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 = CH_3CH_2, CH_3CH_2CH_2, HOCH_2CH_2, (CH_3)_2CH, C_6H_5, and C_6H_5CH_2; A = ClO_4, PF_6. The low-spin iron(III) hyroperoxide complex ions [Fe(Rtpen)(#eta#~1-OOH)]~(2+) are purple chromophores and the high-spin iron(III) peroxide complexes, [Fe(Rtpen)(#eta#~2-OO)]~+ are blue chromophores. The spectroscopic observation (ESR, UV-vis, ESI MS) of a low-spin iron(III) precursor species [Fe(Rtpen)(#eta#~1-OCH_3)]~(2+) and kinetic studies show that formation of [Fe(Rtpen)(#eta#~1-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)(OCH_3)]~(2+), is faster than the subsequent ligand substitution during which [Fe(Rtpen)(#eta#~1-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 be uncoordinated hydroperooxide oxygen atom is suggested. The stability of [Fe(Rtpen)(#eta#~1-OOH)]~(2+) R = HOCH_2CH_2, is significantly lower than for the peroxide complexes generated from the other alkyl substituted ligands (t_(1/2) ca. 10 in vs. several hours). Tandem MS/MS experiments with the [Fe(Rtpen)(#eta#~1-OOH)]~(2+) ions show fragmentation via O-O cleavage to give the novel ferry species [Fe(Rtpen)(O)]~(2+). By contrast the [Fe (Rtpen)(#eta#~2-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~(III)OOH]~(2+) rather than [Fe~(III)OO]~+ species are the precursors to, at least, the ferryl Fe~(IV)=O species. Crystal structures of four starting iron(II) compounds, [Fe(Rtpen)Cl]PF_6, R = HOCH_2CH_2, CH_3CH_2CH_2, C_6H_5CH_2, and [Fe(bztpen)Br]PF_6 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]PF_6 shows an intermolecular H-bonding interaction between the dangling hydroxyethyl group and the chloride of a neighboring molecule with O-H…Cl, 3.219(2) A.