A New Model for the Reduced Form of Purple Acid Phosphatase: Structure and Properties of <TEX>$[Fe_2BPLMP(OAc)_2](BPh_4)_2$</TEX>

<TEX>$[Fe^{II}Fe^{III}BPLMP(OAc)_2](BPh_4)_2$</TEX> (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 I 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 <TEX>$Fe^{III}-O; and; Fe^{II}-O$</TEX> 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 <TEX>$CH_3CN$</TEX> (ε = 1.0 × 103 , 3.0 × 102). These are assigned to <TEX>$phenolate-to-Fe^{III}$</TEX> 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 <TEX>$Fe^{II};and;Fe^{III}$</TEX> centers is faster than NMR time scale. This complex undergoes quasireversible one-electron redox processes. The <TEX>$Fe^{III}_2/Fe^{II}Fe^{III};and;Fe^{II}Fe^{III}/Fe^{II}_2$</TEX> redox couples are at 0.655 and -0.085 V vs SCE, respectively. It has <TEX>$K_{comp}=3.3{ imes}10^{12}$</TEX> representing that BPLMP/bis(acetate) ligand combination stabilizes a mixed-valence <TEX>$Fe^{II}Fe^{III}$</TEX> 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 <TEX>$Fe^{II}Fe^{III}$</TEX> system <TEX>$(S_{total}=1/2)$</TEX>. This is consistent with the magnetic susceptibility study showing the weak antiferromagnetic coupling <TEX>$(J= - 4.6;cm^{-1},; H= - 2JS_1{cdot}S2)$</TEX> between <TEX>$Fe^{II}; and ;Fe^{III}$</TEX>center.