Synthesis and characterization of the mixed-valence [Fe(II)Fe(III)BPLNP(OAc)₂](BPh₄)₂ complex as a model for the reduced form of the purple acid phosphatase

[Fe(II)Fe(III)BPLNP(OAc)₂](BPh₄)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-nitrophen ol (HBPLNP). Complex 1 has been studied by electronic spectral, NMR, EPR, SQUID, and electrochemical methods. Complex 1 exhibits two strong bands at 498 nm (ε = 2.6 x 10³ M^-1cm^-1) and 1363 nm (ε = 5.7 x 10² M.-^. in CH₃CN. These are assigned to phenolate-to-Fe(III) and intervalence charge-transfer transitions, respectively. NMR spectrum of complex 1 exhibits sharp isotropically shifted resonances, which number is 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 at room temperature. Complex 1 undergoes quasireversible one-electron redox processes. The Fe(III)₂/Fe(II)Fe(III) and Fe(II)Fe(III)/Fe(II)₂ redox couples are at 0.807 and 0.167 V versus SCE, respectively. It has K(comp) = 5.9 x 10¹⁰ representing that BPLNP/bis(acetato) ligand combination stabilizes a mixed-valence Fe(II)Fe(III) complex in the air. Interestingly, complex 1 exhibits intense EPR signals at g = 8.56, 5.45, 4.30 corresponding to mononuclear high-spin Fe(III) species, which suggest a very weak magnetic coupling between the iron centers. Magnetic susceptibility study shows that there is a very weak antiferromagnetic coupling (J = -0.78 cm^-1, H = -2JS₁· S₂) between Fe(II) and Fe(III) centers. Thus, we can suggest that complex 1 has a very weak antiferromagnetic coupling between the iron centers due to the electronic effect of the nitro group in the bridging phenolate ligand.