Polynuclear complexes of copper(II). Synthesis, characterization, crystal and molecular structure of the cations bis[oxamidate-N,N′-dipropylamine-2,2′-dimethyl-N,N′, N1,N1′-copper(II)]copper(II) ([CuN4CuO4CuN4]2+) and bis[oxamidate-N,N′-dipropylamine-2,2′-dimethyl-N,N′,N1, N1′-copper(II)]dicopper(II)-O,O1,O2, N1,N2-μ-(oxamidate-N,N′-dipropylamine-2,2′-dimethyl-N,N′,O,N1,N1′, O1)([CuN4(CuN2O3)2CuN4]2+) packed in the same crystal as their tetranitrate dihydrate salts