Sulphonamide nitrogen-containing, nitrogen-protected amino acids interacting with the Cd2+ ion. A cadmium-113 nuclear magnetic resonance and potentiometric study. X-Ray diffraction of [Cd(ts-β-alaO)2(H2O4)]

Abstract

α-Alanine and β-alanine, N-protected by a tosyl (toluene-p-sulphonyl) group, bind Cd²⁺ acting initially as a simple carboxylate at low pH, and, at increasing pH, as N,O-bidentate ligands, through the carboxylic oxygen and the deprotonated amide nitrogen atoms. In the latter case they give rise to five- and six-membered ring chelate species, respectively. Cadmium-113 n.m.r. and potentiometric measurements reveal that the stability of the complexes is almost unaffected by the chelate ring size. The five-membered ring chelate species has a more deshielded chemical shift and only a slightly greater stability than the six-membered species. The crystal and molecular structures of the [Cd(ts-β-alaO)₂(H₂O)₄](ts-β-alao =N-tosyl-β-alaninate monoanion) complex have been solved. Crystal data are: monoclinic, space group P2₁/c, a= 14.956 (6), b= 5.472(2), c= 16.860(7)Å, β= 107.36(3)°, Z= 2, and R= 0.039 for 1 635 reflections. The amina acid molecule binds through the unidentate carboxylate group and the structure consists of discrete units in which the Cd²⁺ exhibits a nearly regular octahedral geometry.