Copper(II) complexes with chiral diaminodiamido ligands: Solution and structural studies

Three diaminodiamido ligands (S,S)-N,N'-bis(prolyl)ethanediamine (ProNN-2), (S,S)-N,N'-bis(N-methylvalyl)ethanediamine (Me(2)ValNN-2), and (S,S)-N,N'-bis(N-methylphenylalanyl)ethanediamine (Me(2)PheNN-2) were synthesised and their complex formation equilibria with copper(II) investigated in aqueous solution by potentiometry and, For ProNN-2, by electronic spectrophotometry. ProNN-2 forms the species [CuLH](3+), [Cu2L2](4+), [Cu2L2H-2](2+) and [CuLH-2], Me(2)PheNN-2 forms the complexes [CuLH](3+), [Cu2L2H-2](2+) and [CuLH-2], whereas Me(2)ValNN-2 forms the monomer [CuLH-1](+) but not the dimer. The dimeric cation [Cu2L2H-2](2+), of Me(2)PheNN-2 has severe steric requirements, as demonstrated by the X-ray crystal structure of the complex [Cu2L2H-2]Cl-2 . 12H(2)O, of the corresponding non-methylated ligand. Since copper(II) complexes of the ligands examined are used as additives to the mobile phase to perform chiral resolution of D,L-amino acids in RP-HPLC, the present results provide valuable clues to an understanding of the mechanism of the enantiomeric separation.