Ground-State Electronic Structures of Binuclear Iron(II) Sites: Experimental Protocol and a Consistent Description of Moessbauer, EPR, and Magnetization Measurements of the Bis(phenolate)-Bridged Complex [Fe2(salmp)2]2-

Abstract

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 Fez* 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, Mbsbauer spectroscopy, and saturation magnetization. Here we report a consistent set of exchange, fine structure, and hyperfine structure parameters for the FezOsNz coordination unit of [Fe~(salmp)~]~-, where salmp is bis(salicylidenamino)-2-methylphenolate(3-). The techniques employed yield J = -14 cm-* (eex = JS&) for the exchange coupling constant. The Mbssbauer 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.