Synthesis and characterization of a series of mixed transition metal oxido cubanes (MMnO_4)

Mixed transition metal oxides are central to the development of materials for oxygen evolution reaction, oxygen redn. reaction, and energy storage. Discreet clusters contg. mixed transition metal-oxido moiety were targeted. A series of heterometallic trimanganese-tetraoxido cubanes (M'Mn_3O_4) supported by a trisalkoxy-hexapyridyl ligand with a 1, 3,5-triarylbenzene spacer were previously reported where M' is a redox-inactive metal. Transmetallation of the Ca^2+ center of [CaMn_3O_4] with M^2+ salts (M = Fe, Co, Ni, Cu) yielded the mixed transition metal manganese-oxido cubanes (MMn_3O_4). The series of complexes was characterized by single crystal X-ray diffraction, dispersive difference diffraction anal., cyclic voltammetry, Mossbauer spectroscopy, and X-ray absorption spectroscopy. Dispersive difference diffraction anal. confirmed that the Mn centers remain at their original sites within the cubane moiety upon transmetallation of the Ca by other metals. Cyclic voltammetry studies show significant changes in the Mn^(IV)_3/Mn^(III)Mn^(IV)_2 redox potentials as a function of metal M. Through Mossbauer spectroscopy, XANES, and X-ray diffraction studies, it was detd. that redox reactions occur at Mn centers instead of M. These well-defined complexes have been employed as precursors for heterogeneous water oxidn. electrocatalysts.