MODEL COMPOUNDS FOR IRON PROTEINS - STRUCTURES AND MAGNETIC, SPECTROSCOPIC, AND REDOX PROPERTIES OF FE(III)M(II) AND [COIIIFEIII](2)O COMPLEXES WITH (MU-CARBOXYLATO)BIS(MU-PHENOXO)DIMETALATE AND (MU-OXO)DIIRON(III) CORES

A series of heterobimetallic complexes of the type [Fe(III)M(II)L(mu-O Ac)(OAc)(H2O)](ClO4). nH(2)O (2-5) and [{Fe-III-Co(III)L(mu-OAc)(OAc)} (2)(mu-O)](ClO4)(2) . 3H(2)O (6) where H(2)L is a tetraaminodiphenol m acrocyclic ligand and M(II) = Zn(2), Ni(3), Co(4), and Mn(5) have been synthesized and characterized. The H-1 NMR spectrum of 6 exhibits all the resonances between 1 and 12 ppm. The IR and UV-vis spectra of 2-5 indicate that in all the cases the metal ions have similar coordinati on environments. A disordered crystal structure determined for 3 revea ls the presence of a (mu-acetate)bis(mu-phenoxide)-(NiFeIII)-Fe-II cor e, in which the two metal ions have 6-fold coordination geometry and e ach have two amino nitrogens and two phenolate oxygens as the in-plane donors; aside from the axial bridging acetate, the sixth coordination site of nickel(II) is occupied by the unidentate acetate and that of iron(III) by a water molecule. The crystal structure determination of 6 shows that the two heterobinuclear Co-III-Fe-III units are bound by an Fe-O-Fe linkage. 6 crystallizes in the orthorhombic space group Ibc a with a = 17.577(4) Angstrom, b = 27.282(7) Angstrom, c = 28.647(6) A ngstrom, and Z = 8. The two iron(III) centers in 6 are strongly antife rromagnetically coupled, J = -100 cm(-1) (H = -2JS(1) . S-2), whereas the other two S-1 = S-2 = 5/2 systems, viz. [Fe-2(III)(HL)(2)(mu-OH)2] (ClO4)(2) (1) and the (FeMnII)-Mn-III complex (5), exhibit weak antife rromagnetic exchange coupling with J = -4.5 cm(-1) (1) and -1.8 cm(-1) (5). The (FeNiII)-Ni-III (3) and (FeCoII)-Co-III (4) systems, however , exhibit weak ferromagnetic behavior with J = 1.7 cm(-1) (3) and 4.2 cm(-1) (4). The iron(III) center in 2-5 exhibits quasi-reversible redo x behavior between -0.44 and -0.48 V vs Ag/AgCl associated with reduct ion to iron(II). The oxidation of cobalt(II) in 4 occurs quasi-reversi bly at 0.74 V, while both nickel(II) and manganese(II) in 3 and 5 unde rgo irreversible oxidation at 0.85 V. The electrochemical reduction of 6 leads to the generation of 4.