Metal complexes of propane-1,2,3-triamine. Potentiometric and calorimetric investigations

The complexes formed by the ions Co 2+ , Ni 2+ , Cu 2+ , and Zn 2+ with the ligand propane-1,2,3-triamine (L 1 ) in 0.15 mol dm −3 aqueous NaCl at 25 °C have been studied by potentiometry and calorimetry (ligand to metal molar ratio, 1–3; pH range from 3.0 to 6.5–11.0). Stability constants have been obtained from potentiometric data by means of the least-squares program HYPERQUAD for the following species: [M(L 1 )] 2+ and [MH(L 1 )] 3+ for all four metal cations; [M(L 1 ) 2 ] 2+ and [MH(L 1 ) 2 ] 3+ for Ni 2+ , Cu 2+ , and Zn 2+ ; [MH 2 (L 1 ) 2 ] 4+ for Ni 2+ and Cu 2+ ; and [M(OH)(L 1 )] + for Cu 2+ only. Stabilities increase in the order Zn < Co < Ni < Cu. The complexes of L 1 have approximately the same stability as the corresponding ones formed by the related triamines 2-methyl (2-methylamine) propane-1,3-diamine (L 3 ) and 2-ethyl (2-methylamine)propane-1,3-diamine (L 4 ). The stability constants have been used to obtain, from the calorimetric titration experiments, the formation enthalpies of all the metal complexes except those for cobalt(II) as well as the corresponding entropy changes. The thermodynamic quantities are compared with the corresponding values previously found for L 3 , in order to derive information about the coordination ability of the ligand. The analysis suggests that the unprotonated ligand forms a fac-tridentate complex in most cases, but for some species, such as [Cu(L 1 ) 2 ] 2+ , [CuH(L 1 ) 2 ] 3+ , and [Zn(L 1 )] 2+ , the thermodynamic quantities indicate that at least one of the nitrogen atoms of the triamine is either loosely bonded or not coordinated to the metal ion. A gas-phase molecular mechanics analysis shows that the strain energies of the fac-[Co(L 1 ) 2 ] 2+ and -[Ni(L 1 ) 2 ] 2+ symmetrical (s) and unsymmetrical (u) isomers are very similar. If the same solvation energy is assumed for the two isomers, each isomer will be present in roughly equal amounts in solution. Keywords: propane-1,2,3-triamine, potentiometry, calorimetry, complexation, cobalt, nickel, copper, zinc.