Amide group coordination to the Hg2+ ion. Potentiometric, 1H NMR and structural study on Hg2+–N-protected amino acid systems †

Abstract

The binary complexes of Hg2+ formed by N-carbonyl and N-sulfonyl amino acids, which are ligands containing peptide and sulfonamide groups respectively, are investigated in aqueous solution by 1H NMR, UV spectroscopy and potentiometry. The corresponding ternary systems with 2,2′-bipyridine are studied in aqueous solution by potentiometry and in DMSO solutions by 1H-NMR. All the amino acids behave as simple carboxylate ligands at acid pH, while, around neutrality, N-p-tolylsulfonylglycine (tsglyH2), N-p-tolylsulfonyl-β-alanine (ts-β-alaH2) and N-2-nitrophenylsulfonylglycine (NO2psglyH2) switch to dianionic N,O-bidentate chelating ligands due to the involvement of the deprotonated amide nitrogen as an additional donor site. The Hg2+ ion is ineffective in promoting peptide nitrogen deprotonation in N-benzoylglycine (bzglyH). The binary and ternary species formed in aqueous solution and their stability constants are determined and compared with those of the homologous complexes of Pd2+, Cu2+, Cd2+ and Pb2+. The molecular structure of [Hg(bpy)2(NO2-psgly-N,O)]·0.5H2O is determined by X-ray crystallography. It represents a rare example of Hg2+ N,O coordination by an amino acid molecule. In the complex Hg2+ shows a distorted octahedral environment with a N5O donor set. Four nitrogen atoms are derived from the two bpy ligands, while the oxygen and the fifth nitrogen are from the NO2-psgly dianion. New information on the solution and solid state chemistry of Hg2+ with ligands of biological interest is provided which may be of great relevance in understanding the mechanism of metal toxicity.