Exchange Interaction on Multinuclear Transition-Metal Complexes. Part 11. Synthesis and X-Ray Structure of Cu2L2(CH3COO)2·Cu2(CH3COO)4·2 C2H5OH (LH: N-Methyl-N′-(4,6-dimethoxysalicylidene)-1,3-propanediamine), a Novel Alternating-Chai