A New Model for the Reduced Form of Purple Acid Phosphatase: Structure and Properties of [Fe₂BPLMP(OAc)₂](BPh₄)₂

[Fe^IIFe^IIIBPLMP(OAc)₂](BPh ₄)₂ (1), a new model for the reduced form of the purple acid phosphatases. has been synthesized by using a dinucleating ligand, 2,6-bis[((2-pyridylmethyl)(6-methyl-2-pyridylmethyl)amino) methyl]-4-methylphenol (HBPLMP). Complex 1 has been characterized by X-ray diffraction method as having (μ-phenoxo)bis(acetato)diiron core. Complex 1 was crystallized in the monoclinic space group C2/c with the following cell parameters: a=41.620(6) Å, b=14.020(3) Å, c=27.007(4) Å, β=90.60(2)°, and Z=8. The iron centers in the complex 1 are ordered as indicated by the difference in the Fe-O bond lengths which match well with typical Fe^III-O and Fe^II-O bond lengths. Complex 1 has been studied by electronic spectral, NMR, EPR, SQUID, and electochemical methods. Complex 1 exhibits strong bands at 592 nm, 1380 nm in CH₃CN (ε= 1.0× 10³, 3.0× 10²). These are assigned to phenolate-to-Fe^III and intervalence charge-transfer transitions, respectively. Its NMR spectrum exhibits sharp isotropically shifted resonances, which number half of those expected for a valence-trapped species, indicating that electron transfer between Fe^II and Fe^III centers is faster than NMR time scale. This complex undergoes quasireversible one-electron redox processes. The Fe^III₂/Fe^IIFe^III and Fe^IIFe^III/ Fe^II₂ redox couples are at 0.655 and -0.085 V vs SCE, respectively. It has K_comp=3.3 × 10¹² representing that BPLMP/bis(acetate) ligand combination stabilizes a mixed-valence Fe^IIFe^III complex in the air. Complex 1 exhibits a broad EPR signal centered near g=1.55 which is a characteristic feature of the antiferromagnetically coupled high-spin Fe_IIFe^III system (S_total=l/2). This is consistent with the magnetic susceptibility study showing the weak antiferromagnetic coupling (J=- 4.6 cm^-1, H=-2JS₁·S₂) between Fe^II and Fe^III center.