Stability constants of macrocyclic tetrathioether complexes of Nickel and Cobalt

Abstract

Determining stability constants provides insight into metal-ligand affinity and complex stability, essential for understanding selectivity and potential applications. In this regard, we measured the stability constants of first row transition metal ions with macrocyclic tetrathioethers. namely, 1,4,7, 10-tetrathiocyclododecane ([12]aneS₄), 1,4,8,11-tetrathiocyclotetradecane ([14]aneS₄) and 1,5,9,13-tetrathiocyclohexadecane ([16]aneS₄). All three ligands have the same number of sulfur atoms but different number of atoms making the overall structure and the size of the cavity of the ligand increase from [12]aneS₄ to [16]aneS₄. The metal ions selected for the study were chromium, manganese, iron, cobalt and nickel. The stability constants of the complexes that showed observable complex formation in solutions were measured using the McConnell-Davidson method. Out of the metal ions studied, only nickel and cobalt show observable complex formation. Although nickel forms measurable complexes with all three macrocyclic ligands, cobalt forms a complex only with [14]aneS₄. Chromium, manganese and iron do not show any complex formation with the macrocyclic tetrathioethers investigated in this study. The stability constants determined are as follows: Ni^II([12]aneS₄) (4.0 M^-1), Ni^II([14]aneS₄) (75.0 M^-1), Ni^II([16]aneS₄) (166.0 M^-1) and Co^II([14]aneS₄) (133.0 M^-1). The nickel complexes show a slight increase in stability constants going from [12]aneS₄ to [16]aneS₄. The stability constant of Co^II( [14]aneS₄) (133.0 M^-1) exceeds that of Ni^II([14]aneS₄) (75.0. M^-1). To the best of our knowledge, this is the first report of determination of apparent stability constants for Ni^II([12]aneS₄), Ni^II([16]aneS₄) and Co^II([14]aneS₄). Job’s plots indicate that both metal ions, cobalt and nickel form 1:1 metal to ligand complexes with all three macrocyclic tetrathioethers in solution. The stability constants obtained in the present study are compared with reported stability constants of copper complexes with the same macrocyclic tetrathioethers.