Model Compounds for Iron Proteins. Structures and Magnetic, Spectroscopic, and Redox Properties of FeIIIMII and [CoIIIFeIII]2O Complexes with (μ-Carboxylato)bis(μ-phenoxo)dimetalate and (μ-Oxo)diiron(III) Cores

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Abstract
A series of heterobimetallic complexes of the type [FeIIIMIIL(μ-OAc)(OAc)(H2O)](ClO4)·nH2O (2−5) and [{FeIIICoIIIL(μ-OAc)(OAc)}2(μ-O)](ClO4)2·3H2O (6) where H2L is a tetraaminodiphenol macrocyclic ligand and MII = Zn(2), Ni(3), Co(4), and Mn(5) have been synthesized and characterized. The 1H 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 coordination environments. A disordered crystal structure determined for 3 reveals the presence of a (μ-acetate)bis(μ-phenoxide)−NiIIFeIII core, in which the two metal ions have 6-fold coordination geometry and each 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 CoIIIFeIII units are bound by an Fe−O−Fe linkage. 6 crystallizes in the orthorhombic space group Ibca with a = 17.577(4) Å, b = 27.282(7) Å, c = 28.647(6) Å, and Z = 8. The two iron(III) centers in 6 are strongly antiferromagnetically coupled, J = −100 cm-1 (H = −2JS1·S2), whereas the other two S1 = S2 = 5/2 systems, viz. [Fe2III(HL)2(μ-OH)2](ClO4)2 (1) and the FeIIIMnII complex (5), exhibit weak antiferromagnetic exchange coupling with J = −4.5 cm-1 (1) and −1.8 cm-1 (5). The FeIIINiII (3) and FeIIICoII (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 redox behavior between −0.44 and −0.48 V vs Ag/AgCl associated with reduction to iron(II). The oxidation of cobalt(II) in 4 occurs quasi-reversibly at 0.74 V, while both nickel(II) and manganese(II) in 3 and 5 undergo irreversible oxidation at 0.85 V. The electrochemical reduction of 6 leads to the generation of 4.