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

Abstract

[(FeFeBPLNP)-Fe-II-B-III(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-pyridplmethyl)(6-methyl-2-pyridylme no)methyl]-4-nitrophenol (HBPLNP). Complex 1 has been studied by electronic spectral, NMRI EPR, SQUID, and electrochemical methods. Complex 1 exhibits two strong bands at 498 nm (epsilon = 2.6 x 10(3) M(-1)cm(-1)) and 1363 nm (epsilon = 5.7 x 10(2) M(-1)cm(-1)) in CH3CN. These are assigned to phenolate-to-Fem 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 Fen and Fem centers is faster than NMR time scale at room temperature. Complex 1 undergoes quasireversible one-electron redox processes. The Fe-2(III)/(FeFeIII)-Fe-II and (FeFeIII)-Fe-II/Fe-2(II) redox couples are at 0.807 and 0.167 V versus SCE, respectively. It has K-comp = 5.9 x 10(10) representing that BPLNP/bis(acetato) ligand combination sta bilizes a mixed-valence (FeFeIII)-Fe-II complex in the air. Interestingly, complex 1 exhibits intense EPR signals at g = 8.56, 5.45, 4.30 corresponding to mononuclear high-spin Fem 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 = -2JS(1)(.)S(2)) between Fe-II and Fe-III centers. Thus, we can suggest that complex I has a very weak antiferromagnetic coupling between the iron centers due to the electronic effect of the nitro group in the bridging phenolate ligand.