XAS characterization of end-on and side-on peroxoiron(iii) complexes of the neutral pentadentate N-donor ligandN-methyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine

Abstract

Peroxo intermediates are implicated in the catalytic cycles of iron enzymes involved in dioxygen metabolism. X-ray absorption spectroscopy has been used to gain insight into the iron coordination environments of the low-spin complex [Fe^III(Me-TPEN)(η¹-OOH)]²⁺ (1) and the high-spin complex [Fe^III(Me-TPEN)(η²-O₂)]⁺ (2) (the neutral pentadentate N-donor ligand Me-TPEN = N-methyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine) and obtain metrical parameters unavailable from X-ray crystallography. The complexes exhibit relatively large pre-edge peak areas of approximately 15 units, indicative of iron centers with significant distortions from centrosymmetry. These distortions result from the binding of peroxide, either end-on hydroperoxo for 1 (r_Fe–O = 1.81 Å) or side-on peroxo for 2 (r_Fe–O = 1.99 Å). The XAS analyses of 1 strongly support a six-coordinate low-spin iron(III) center coordinated to five nitrogen atoms from Me-TPEN and one oxygen atom from an end-on hydroperoxide ligand. However, the XAS analyses of 2 are not conclusive: Me-TPEN can act either as a pentadentate ligand to form a seven-coordinate peroxo complex, which has precedence in the DFT geometry optimization of [Fe^III(N4Py)(η²-O₂)]⁺ (the neutral pentadentate N-donor ligand N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), or as a tetradentate ligand with a dangling pyridylmethyl arm to form a six-coordinate peroxo complex, which is precedented by the crystal structure of [Fe₂^III(Me-TPEN)₂(Cl)₂(μ-O)]²⁺.