Synthesis and characterization of a macrocycle containing different functional groups and its non-cyclic counterpart

The macrocycle 9-methyl-3,6,9,12,15,22,31-heptaaza-25,28,38,41-tetraoxa-hexacyclo [31.2.2.3,62.17,20822,31]-heptatetraconta-1,17,18,20,33,34-hexaene (L1) containing different functional groups and its non-cyclic counterpart bis-[piperazinyl-4-(4-carboxy-benzyl)ethyl)]methylamine (L) were synthesized. The protonation behavior of both compounds, i.e. their basicity constants and protonation sequence, was investigated both by potentiometry (25°C, I=0.15 mol dm−3, NaCl) and 1H, 13C NMR spectroscopy. The results obtained were log K1=8.48(1), 8.73(1); log K2=7.25(1), 8.20(1); log K3=6.66(1), 6.93(1); log K4=4.01(1), 6.29(1); log K5=3.28, 5.49(1); log K6=1.7(1) for L and L1, respectively. The complex formation equilibria between L and L1 with metal ions such as Ga(III), Co(II), Cu(II), Zn(II) and Cd(II) were investigated by potentiometry using the same experimental conditions, revealing only mononuclear complexes as well as the presence of many protonated species. L forms a less charged and more stable complex with Ga(III) than does L1. Log K=15.5(1) for [GaL]+ and 13.8(1) for [GaL1]3+, respectively.