SYNTHESIS, CRYSTAL-STRUCTURE, AND PROPERTIES OF A POLYIMIDAZOLE DIIRON(III) COMPLEX - A NEW MODEL OF THE ACTIVE-SITE OF PURPLE ACID-PHOSPHATASE

The synthesis and characterization of a new dinuclear iron(III) comple x formed during the reaction of the septadentate ligand s[((2-hyroxybe nzyl)((1-methylimidazol-2-yl)methyl) amino)methyl]-4-methylphenol (H-3 -BIOMP) are reported. The crystal structure of the complex has been so lved. The mu-phenolate bis(mu-acetate) diiron(III) complex [Fe-2(BIOMP )(mu-O2CCH3)(2)](ClO4). H2O (5) crystallizes in the space group P2(1)/ n with cell dimensions a = 14.983(14) Angstrom, b = 12.341(13) Angstro m, c = 21.391(16) Angstrom, beta = 90.79(7)degrees, V = 3955(6) Angstr om(3), and Z = 4 and was solved for 3493 reflections [I > 3 sigma(I)], giving R = 6.7% and R(W) = 8.3%. The compound consists of two iron(II I) ions bridged by a phenolate oxygen atom and coordinated each to phe nol, amine, and imidazole pendants of the septadentate ligand as well as bridging acetate ions. The cation has approximately C-2 symmetry. T he iron(III) ions are weakly coupled antiferromagnetically (J = -6.34 cm(-1)) and undergo quasireversible one-electron redox processes. The Fen(2)(II)/(FeFeIII)-Fe-II and (FeFeIII)-Fe-II/Fe-2(III) couples are a t -0.785 and -0.225 V vs Ag/AgCl, respectively. Compound 5 also displa ys an intense phenolate-to-iron(III) charge transfer transition at 586 nm (epsilon = 7400 M(-1) cm(-1)/Fe-2). The Fe-57 Mossbauer spectra of 5 were recorded at 295 and 125 K, and both display a single quadrupol ar split doublet. The quadrupole splitting (Delta E(Q)) is equal to 1. 083(23) mm/s at 295 K and 1.055 (9) mm/s at 125 K. The isomer shift (d elta) parameters at 295 and 125 K are 0.406(13) and 0.508(5) mm/s, res pectively.