Spectroscopic and Catalytic Characterization of a Functional FeIIIFeII Biomimetic for the Active Site of Uteroferrin and Protein Cleavage

A mixed-valence complex, [FeIIIFeIIL1(μ-OAc)2]­BF4·H2O, where the ligand H2L1 = 2-{[[3-[((bis­(pyridin-2-ylmethyl)­amino)­methyl)-2-hydroxy-5-methylbenzyl]­(pyridin-2-ylmethyl)­amino]­methyl]­phenol}, has been studied with a range of techniques, and, where possible, its properties have been compared to those of the corresponding enzyme system purple acid phosphatase. The FeIIIFeII and FeIII2 oxidized species were studied spectroelectrochemically. The temperature-dependent population of the S = 3/2 spin states of the heterovalent system, observed using magnetic circular dichroism, confirmed that the dinuclear center is weakly antiferromagnetically coupled (H = −2JS1·S2, where J = −5.6 cm–1) in a frozen solution. The ligand-to-metal charge-transfer transitions are correlated with density functional theory calculations. The FeIIIFeII complex is electron paramagnetic resonance (EPR)-silent, except at very low temperatures (III2 complex showed an EPR spectrum due to population of the Stot = 3 state (J= −3.5 cm–1). The phosphatase activity of the FeIIIFeII complex in hydrolysis of bis­(2,4-dinitrophenyl)­phosphate (kcat. = 1.88 × 10–3 s–1; Km = 4.63 × 10–3 mol L–1) is similar to that of other bimetallic heterovalent complexes with the same ligand. Analysis of the kinetic data supports a mechanism where the initiating nucleophile in the phosphatase reaction is a hydroxide, terminally bound to FeIII. It is interesting to note that aqueous solutions of [FeIIIFeIIL1(μ-OAc)2]+ are also capable of protein cleavage, at mild temperature and pH conditions, thus further expanding the scope of this complex’s catalytic promiscuity.