A new bis(μ-alkoxo) diiron(III) complex and its implications regarding the number of Fe(III)–phenolate bonds and the redox potential in uteroferrin

Abstract

The first diiron(III) complex of the ligand N,N′-bis(2-hydroxybenzyl)-2-hydroxypropane-1,3-diamine (H₃bbpnol), [Fe₂(bbpnol)₂]·2H₂O 1, was prepared and fully characterized in order to model diiron metalloenzymes. The X-ray crystal structure determination together with the electrochemical data of 1 gave us sufficient information to infer the number of phenolates terminally bonded to the iron(III) centers in the active sites of Purple Acid Phosphatases (PAPs). Each iron(III) center in complex 1 has two O-phenolate atoms terminally coordinated. By comparison of redox potentials for the process Fe^III₂–Fe^IIIFe^II of the diiron PAP uteroferrin, complex 1 and complexes containing one, two and no phenolate oxygen atoms terminally bonded to the iron(III) centers, we confirmed the presence of only one terminal tyrosine group coordinated to the non-reducible iron(III) center, in agreement with the proposed structure of such metalloenzymes. The mixed-valence Fe^IIFe^III derivative of 1 has been characterized by spectroelectrochemistry in the UV–VIS region. The E°′ value obtained from the Nernst plot is in good agreement with CV data, and solutions of the Fe^IIFe^III species are stable under argon on the time scale of the spectropotentiostatic experiment. Magnetic susceptibility measurements in the range 4.5 to 300 K indicate very weak antiferromagnetic coupling between the iron ions with J = −2.2 cm⁻¹, consistent with the long Fe–O distance and the small Fe–O–Fe angle within the bis(μ-alkoxo)diiron(III) entity.