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) bis7-methyl-1, 3, 5-octanetrionato (4-) hexakis (pyridine) bis[di-oxouranium (VI) dimetal (II), M2 (UO2) 2 (MOB) 2 (py) 6