Copper(II) Complexes with Chiral Diaminodiamido Ligands: Solution and Structural Studies

Abstract

Three diaminodiamido ligands (S,S)-N,N′-bis(prolyl)ethanediamine (ProNN-2), (S,S)-N,N′-bis(N-methylvalyl)ethanediamine (Me2ValNN-2), and (S,S)-N,N′-bis(N-methylphenylalanyl)-ethanediamine (Me2PheNN-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], Me2PheNN-2 forms the complexes [CuLH]3+, [Cu2L2H−2]2+ and [CuLH−2], whereas Me2ValNN-2 forms the monomer [CuLH−1]+ but not the dimer. The dimeric cation [Cu2L2H−2]2+, of Me2PheNN-2 has severe steric requirements, as demonstrated by the X-ray crystal structure of the complex [Cu2L2H−2]Cl2· 12H2O, 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.