THERMODYNAMIC STUDY OF THE INTERACTION OF LONG OPEN-CHAIN POLYAZAALKANES WITH COBALT(II) AND NICKEL(II) IONS

Complexation equilibria between the open-chain terminally methylated polyamines 1,14-bis(methylamino)-3,6,9,12-tetraazatetradecane (L1), 1,17-bis(methylamino)-3,6,9,12,15-pentaazaheptadecane (L2), 1,20-bis(methylamino)-3,6,9,12,15,18-hexaazaicosane (L3) and 1,23-bis(methylamino)-3,6,9,12,15,18,21-heptaazatricosane (L4) with Co2+ and Ni2+ have been studied by means of potentiometric and spectrophotometric methods in 0.15 mol dm-3 NaClO4 aqueous solution at 298.15 K. The stability constants for the complexes formed have been determined from e.m.f. data. All these ligands form mononuclear ML2+ complexes of similar stability with Co2+ and Ni2+. Binuclear Co2L4(4+), Ni2L3(4+) and Ni2L4(4+) are also formed. The equilibrium constants of the complexes of L1-L4, which are the open-chain counterparts of the macrocyclic ligands [3k]aneN(k) (k=6-9), reveal that only [18]aneN6 with Co2+ and Ni2+, and [21]aneN7 with Co2+ present enhanced stability (macrocyclic effect) with respect to L1-L4. However this effect has not been observed for binuclear complexes.