Synthesis, characterization, and study of octanuclear iron-oxo clusters containing a redox-active Fe4O4-cubane core

A one-pot synthetic procedure yields the octanuclear Fe-III complexes Fe-8(mu(4)-O)(4)(mu-PZ*)(12)X-4, where X = Cl and pz* = pyrazolate anion (PZ = C3H3N2-) (1), 4-Cl-pz (2), and 4-Me-pz (3) or X = Br and pz* = pz (4). The crystal structures of complexes 1-4, determined by X-ray diffraction, show an Fe4O4-cubane core encapsulated in a shell composed of four interwoven Fe(mu-pz*)(3)X units. Complexes 1-4 have been characterized by H-1 NMR, infrared, and Raman spectroscopies. Mossbauer spectroscopic analysis distinguishes the cubane and outer Fe-III centers by their different isomer shift and quadrupole splitting values. Electrochemical analyses by cyclic voltammetry show four consecutive, closely spaced, reversible reduction processes for each of the four complexes. Magnetic susceptibility studies, corroborated by density functional theory calculations, reveal weak antiferromagnetic coupling among the four cubane Fe centers and strong antiferromagnetic coupling between cubane and outer Fe atoms of 1. The structural similarity between the antiferromagnetic Fe-8(mu(4)-O)(4) core of 1-4 and the antiferromagnetic units contained in the minerals ferrihydrite and maghemite is demonstrated by X-ray and Mossbauer data.