Model compounds for iron proteins. Structures and magnetic, spectroscopic, and redox properties of Fe^IIIM^II and [Co^IIIFe^III]₂O complexes with (μ-carboxylato)bis(μ-phenoxo)dimetalate and (μ-oxo)diiron(III) cores

Abstract

A series of heterobimetallic complexes of the type [Fe^IIIM^IIL(μ-OAc)(OAc)(H₂O)](ClO₄)·nH₂O (2-5) and [{Fe^IIICo^IIIL(μ-OAc)(OAc)}₂(μ-O)](ClO₄)₂·3H₂O (6) where H₂L is a tetraaminodiphenol macrocyclic ligand and M^II = 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)-Ni^IIFe^III 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 Co^IIIFe^III 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. [Fe₂^III(HL)₂(μ-OH)₂](ClO₄)₂ (1) and the Fe^IIIMn^II complex (5), exhibit weak antiferromagnetic exchange coupling with J =-4.5 cm^-1 (1) and -1.8 cm^-1 (5). The Fe^IIINi^II (3) and Fe^IIICo^II (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.