GROUND-STATE ELECTRONIC-STRUCTURES OF BINUCLEAR IRON(II) SITES - EXPERIMENTAL PROTOCOL AND A CONSISTENT DESCRIPTION OF MOSSBAUER, EPR, AND MAGNETIZATION MEASUREMENTS OF THE BIS(PHENOLATE)-BRIDGE COMPLEX [FE2(SALMP)2]2-

A class of proteins containing oxygen-bridged exchange-coupled binuclear iron sites can exist in various oxidation states, including the Fe2+.Fe2+ form. A variety of techniques such as magnetic circular dichroism, EPR, Mossbauer spectroscopy, and magnetization measurements are generally being used to study the electronic structure of the low-lying levels of these clusters. Because Fe2+ sites exhibit large zero-field splittings, large quadrupole splittings, and substantial anisotropies of the magnetic hyperfine interactions, researchers are faced with solving a difficult multiparameter problem. In order to develop an experimental protocol for the study of iron-oxo proteins, we have investigated a structurally well-defined Fe2+.Fe2+ center with EPR, Mossbauer spectroscopy, and saturation magnetization. Here we report a consistent set of exchange, fine structure, and hyperfine structure parameters for the Fe2O6N2 coordination unit of [Fe2(salmp)2]2-, where salmp is bis(salicylidenamino)-2-methylphenolate(3-). The techniques employed yield J = -14 cm-1 (H(ex) = JS1.S2) for the exchange coupling constant. The Mossbauer data indicate that the two ferrous sites are equivalent. A ligand field analysis shows that the ferrous sites experience a trigonal distortion of the octahedral coordination unit. The data obtained by the three techniques have been fitted with a spin Hamiltonian over a wide range of applied magnetic fields and temperatures.