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

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.