A novel diiron complex as a functional model for hemerythrin

Abstract

Diiron(II) complexes with a novel dinucleating polypyridine ligand, N,N,N′,N′-tetrakis(6-pivalamido-2-pyridylmethyl)-1,3-diaminopropan-2-ol (HTPPDO), were synthesized as functional models of hemerythrin. Structural characterization of the complexes, [Fe II 2(Htppdo)(PhCOO)](ClO 4) 3 ( 1), [Fe II 2(Htppdo)(( p-Cl)PhCOO)](ClO 4) 3 ( 2), [Fe II 2(Htppdo)(( p-Cl)PhCOO)](BF 4) 3 ( 2′) and [Fe II 2(tppdo)(( p-Cl)PhCOO)](ClO 4) 2 ( 3), were accomplished by electronic absorption, and IR spectroscopic, electrochemical, and X-ray diffraction methods. The crystal structures of 1 and 2′ revealed that the two iron atoms are asymmetrically coordinated with HTPPDO and bridging benzoate. One of the iron centers (Fe(1)) has a seven-coordinate capped octahedral geometry comprised of an N 3O 4 donor set which includes the propanol oxygen of HTPPDO. The other iron center (Fe(2)) forms an octahedron with an N 3O 3 donor set and one vacant site. The two iron atoms are bridged by benzoate ( 1) or p-chlorobenzoate ( 2). On the other hand, both Fe atoms of complex 3 are both symmetrically coordinated with N 3O 4 donors and two bridging ligands; benzoate and the propanolate of TPPDO. Reactions of these complexes with dioxygen were followed by electronic absorption, resonance Raman and ESR spectroscopies. Reversible dioxygen-binding was demonstrated by observation of an intense LMCT band for O 2 2− to Fe(III) at 610 ( 1) and 606 nm ( 2) upon exposure of dioxygen to acetone solutions of 1 and 2 prepared under an anaerobic conditions at −50°C. The resonance Raman spectra of the dioxygen adduct of 1 exhibited two peaks assignable to the ν(O–O) stretching mode at 873 and 887 cm −1, which shifted to 825 and 839 cm −1 upon binding of 18O 2. ESR spectra of all dioxygen adducts were silent. These findings suggest that dioxygen coordinates to the diiron atoms as a peroxo anion in a μ-1,2 mode. Complex 3 exhibited irreversible dioxygen binding. These results indicate that the reversible binding of dioxygen is governed by the hydrophobicity of the dioxygen-binding environment rather than the iron redox potentials.