Tetranuclear complexes of 1,3,5,9,11,13-hexaketonates. 2. Synthesis and electrochemistry of a series of heterotetranuclear complexes, bis {1,1'-(1,3-phenylene)bis[7-methyl-1,3,5-octanetrionato(4-)]}hexakis(pyridine)bis[di-oxouranium(VI)dimetal(II), M2(UO2)2(MOB)2(py)6