POTENTIOMETRIC, CALORIMETRIC AND NUCLEAR-MAGNETIC-RESONANCE STUDIES OF THE PROTONATION IN AQUEOUS-SOLUTION OF 1-THIA-4,7-DIAZACYCLONONANE-N,N'-DIACETIC ACID, 1-THIA-4,8-DIAZACYCLODECANE-N,N'-DIACETIC ACID AND RELATED OPEN-CHAIN DIAMINOCARBOXYLIC ACIDS

The acid-base properties in aqueous solution of 1-thia-4,7-diazacyclononane-N,N'-diacetic acid (TNODA) and 1-thia-4,8-diazacyclodecane-N,N'-diacetic acid (TDEDA) have been investigated in 0.1 mol dm-3 KNO3 at 25-degrees-C. The protonation constants were determined by conventional pH potentiometry. The protonation enthalpy changes were obtained by adiabatic calorimetry. The corresponding entropy changes were calculated. For TNODA: log K1H = 11.63, log K2H = 4.05, log K3H = 1.8; DELTA-H1-degrees = -41.4 kJ mol-1, DELTA-H2-degrees = -0.5 kJ mol-1, DELTA-H3-degrees = 4.2 kJ mol-1; DELTA-S1-degrees = 83.5 J mol-1 K-1, DELTA-S2-degrees = 75.8 J mol-1 K-1, DELTA-S3-degrees = 48.5 J mol-1 K-1. For TDEDA: log K1H = 12.9, log K2H = 3.95, log K3H = 2.0; DELTA-H2-degrees = 0.2 kJ mol-1, DELTA-H3-degrees = 3.2 kJ mol-1; DELTA-S2-degrees = 76.2 J mol-1 K-1, DELTA-S3-degrees = 50 J mol-1 K-1. The protonation sequence of the cyclic diaminocarboxylates was established by H-1 NMR spectroscopy. The results indicate that the first proton is added to a tertiary amino group, whereas the second and the third are added to the two carboxylate groups. The second amino group of TDEDA is only protonated in strong acid medium. Reduced solvation of the cyclic diaminocarboxylates as compared with their open-chain counterparts explains the increase in basicity at the first protonation step upon acetate substitution on the second amino group of the parent cyclic diamines. The acid-base properties of the open-chain diaminocarboxylates 2,5-diazahexane-N,N'-diacetic acid and 5-thia-2,8-diazanonene-N,N'-diacetic acid were also investigated under the same experimental conditions for comparison purposes.