Transmetallation reactions of tetranuclear copper(II) complexes. I. Crystal and molecular structures of an intermediate and a final product of reaction of di-μ-oxo-tetra[chloro(DENC)copper(II)],(DENC = N,N-diethylnicotinamide) with [Zn(N2S2], (N2S2 = diacetyl-bis-hydrazonato-S-methyl-carboditithioate) in aprotic solvents