A New Type of .mu.4-Oxo-Bridged Copper Tetramer: Synthesis, X-ray Molecular Structure, Magnetism and Spectral Properties of (.mu.4-Oxo)tetrakis(.mu.-bromo)bis(.mu.-2,6-bis(morpholinomethyl)-4-methylphenolato)tetracopper(II) and (.mu.4-Oxo)tetrakis(.mu.-benzoato)bis(.mu.-2,6-bis(morpholinomethyl)-4-methylphenolato)tetracopper(II)