Reversible Core-Interconversion of an Iron(III) Dihydroxo Bridged Complex

Complexes [Fe(Hhbi)2(NO3)]·2EtOH (1·2EtOH) and [Fe2(μ-OH)2(Hhbi)4]·2H2O·8EtOH (2·2H2O·8EtOH) crystallize in the orthorhombic Fdd2 and P42212 space groups, respectively (Hhbi− = the monoanion of 2-(2′-hydroxyphenyl benzimidazole). Complex 1 exhibits paramagnetic relaxation as evidenced by Mössbauer spectroscopy, and significant axial zero-field splitting (1.5 cm1 <D < 3 cm−1), along with a rhombic distortion (E = 0.205 cm−1), as shown by X-band electron paramagnetic resonance (EPR) spectroscopy. Complex 2 undergoes, upon drying, a core conversion toward an oxo-bridged diferric species and, to a smaller extent, decomposition toward a mononuclear ferric complex, as evidenced by Mössbauer, magnetic, EPR, and mass spectrometry data. The core conversion is reversible through recrystallization from wet EtOH, leading to the original complex, which crystallizes in the initial space group.