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

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 FeIII2–FeIIIFeII 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 FeIIFeIII 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 FeIIFeIII 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.