Stereochemical features of the disulfoxides 1,3-bis(n-propylsulfinyl)propane (BPSP) and 1,2-bis(methylsulfinyl)ethane (BMSE), and their copper(II) complexes. Crystal and molecular structure of meso-BPSP, [trans-Cu(meso-BPSP)2(H2O)2](ClO4)2, trans-Cu(meso-BMSE)2(ClO4)2, and trans-Cu(rac-BMSE)2(ClO4)2