N,N′-substituted dithiooxamides derived from alkyl-α-amino acids or from glycylglycine: acid dissociation properties in aqueous solution and crystal and molecular structures of N,N′-bis(carboxymethyl) dithiooxamide (GLYDTO) and N,N′-bis(1-carboxyethyl) dithiooxamide (ALADTO)

Abstract

The acid-base properties of N,N′-disubstituted dithiooxamides derived from alkyl-α-aminoacids or glycylglycine were studied in aqueous solutions of pH 2.5-10.8. For each dithiooxamide, protonation constants were obtained from potentiometric data for three dilute aqueous solutions [I=0.15 M (NaClO4), 37.0°C] using the program HYPERQUAD, and are reported as conventional pKa values. For compounds I, II and III (derived respectively from glycine, d,l-α-alanine and d,l-α-valine) only the protonation constants of the carboxylate groups could be accurately determined under the experimental conditions used. For the glycylglycine derivative (IV) the protonation constants of the carboxylate and peptide amide groups were determined, but not those of the thioamide groups. N,N′-bis(carboxymethyl)dithiooxamide (I) and racemic N,N′-bis(1-carboxyethyl) dithioxamide (II) were also studied by X-ray crystallography. Both have centrosymmetric molecules and rather similar structures in which the typical trans configuration of the dithiooxamide moiety is stabilized by two symmetrically related intramolecular hydrogen bonds linking each thioamide N–H the sulfur atom of the other half-molecule [N…S=2.94(1) Å, <(N–H–S)=122(3)° or 116(2)°]. In the crystal lattice each molecule is linked to four adjacent ones by intermolecular O–H…O hydrogen bonds.