Electronic and Molecular Structure Studies of a (Bio)Inorganic Complex – A Multi-Technique Approach

The electronic structure of the dimeric iron complex [Fe2(bbpo)Cl4]n− with the tetranionic ligand 1,2-bis(3,5-di-tert-butyl-2-hydroxyphenyl)oxamide was studied in two different oxidation states. Combined Mössbauer and EPR investigations in conjunction with magnetic susceptibility measurements are used to demonstrate the complementarity of the techniques. The dianion [Fe2(bbpo)Cl4]2−(2) is a high-spin iron(III) compound with magnetic properties resulting from competing exchange and zero-field interactions of the iron sites. Unusual integer-spin EPR spectra could be recorded from an excited spin septet, St=3, of the coupled system. Electrochemical oxidation yields the monoanion [Fe2(bbpoox1)Cl4]−(2ox) which also is an iron(III) compound; the ligand is redox non-innocent and forms a π radical that is bound to one of the iron centers and the total spin St=1/2 ground states of the molecule originates from exchange coupling of the three paramagnetic centers. A simplified spin coupling scheme assuming infinitively strong radical-iron(III) interaction yields a reasonable and rational interpretation of the magnetic and electronic properties of 2ox.