Tetranuclear complexes of 1,3,5,9,11,13-hexaketonates. 1. Synthesis and structure of bis[1,3-bis(2-methyl-4,6,8-octanetrion-8-yl)benzenato(4-)]tetrakis(pyridine)tetracopper(II), Cu4(MOB)2(py)4. Observation of four-electron-transfer electrochemistry