Pyridinderivate als Komplexbildner. XI. Die Thermodynamik der Metallkomplexbildung mit Bis‐, Tris‐ und Tetrakis[(2‐pyridyl)methyl]‐aminen

Abstract

Pyridine Derivatives as Complexing Agents XI. Thermodynamics of Metal Complex Formation with Bis‐, Tris‐ and Tetrakis[(2‐pyridyl)methyl]‐amines. The equilibria between H+, Mn 2+, Fe 2+, Co 2+, Ni 2+, Cu 2+, Cd 2+, Pb 2+, Hg 2+ and Ag+, and the ligands bis(2‐pyridylmethyl)‐amine (=DPA), tris(2‐pyridylmethyl)‐amine (=TPA), tris(6‐methyl‐2‐pyridylmethyl)‐amine (=TLA) and N,N,N′,N′‐tetrakis(2‐pyridylmethyl)‐ethylenediamine (=TPEN) have been studied. Only the stability constants of DPA and TLA with almost all these cations were obtained using the pH method. For the other ligands, the complexes are already formed in acid solutions and only the use of different ligand‐ligand or metal‐metal exchanges as well as of pM methods were successful. The protonation constants indicate that for DPA the protonation occurs firstly at the aliphatic nitrogen atom whereas in all other cases only the pyridine groups can be protonated. The thermodynamic functions of protonation are in agreement with this interpretation. The stability constants of the complexes are often similar in magnitude to those of the analogous aliphatic amines, in spite of the much lower basicities of the pyridine derivatives. The Fe(II)N 6 species of DPA and TPEN are appreciably more stable than those of the corresponding aliphatic ligands. This is due to the formation of low‐spin complexes with an unexpected Δ H value. Comparison of the thermodynamic data of formation of the complexes with TPA and TLA shows the effect of the three bulky methyl groups of the second ligand. As a consequence of steric hindrance and of the major dehydration, Δ H and less Δ S are more positive for M(TLA)2+ than for M(TPA)2+. Therefore M(TLA)2+ is normally much less stable than M(TPA)2+. The data for MnTPA 2+ and ZnTPA 2+ appear to indicate that in these complexes the coordination number of the metal ion is seven and four respectively. In addition to the complexes ML 2+, with these two ligands hydroxo complexes ML(OH)+ are formed at remarkably low pH. Further TPEN seems to be sexidentate in the 1:1 complexes with Mn 2+, Co 2+ and Ni 2+ but quinquedentate in those with Cu 2+ and Zn 2+, also in agreement with the spectra in solution and of the solid complex salts. The reaction: M(DPA)22+ + TPEN → M(TPEN)2+ + 2DPA is for all metal ions favoured by Δ H and Δ S, whereas in the case of the corresponding aliphatic ligands only by the second term. This result is explained in terms of a different magnitude of hydration of the two sexidentate ligands as a consequence of the presence of the hydrophobic aromatic rings in TPEN.