Diastereomeric 1,4,7,10-tetrakis((S)-2-hydroxypropyl)- 1,4,7,10-tetraazacyclododecane and its alkali metal complex ions. A nuclear magnetic resonance, potentiometric titration, and molecular orbital study

Abstract

13C NMR studies are consistent with 1,4,7,10-tetrakis((S)-2- hydroxypropyl)-1,4,7,10-tetraazacyclododecane (ΔS-thpcl2) and its eight- coordinate alkali metal complex ions (Δ[M(S-thpcl2)]+) existing predominantly as single distorted cubic diastereomers in methanol in accord with structures predicted through molecular orbital calculations. Intramolecular exchange in ΔS-thpcl2 is characterized by k(298.2 K) = 34 800 ± 1600 s-1, ΔH(≠) = 53.9 ± 0.6 kJ mol-1 and ΔS(≠) = 22.8 ± 2.5 J K-1 mol-1 in methanol. This process is slowed in [M(S-thpc12)]+, for which k(298.2 K) = 332 ± 6, 125 ± 2, and 3020 ± 30 s-1 ΔH(≠) = 21.4 ± 0.2, 26.3 ± 0.5, and 46.3 ± 0.2 kJ mol-1, and ΔS(≠) = -125 ± 1, -116 ± 2, and -23.1 ± 0.9 J K-1 mol-1, respectively, when M+ = Li+, Na+, and K+. For intermolecular ligand exchange on Δ[M(S-thpc12)]+, decomplexation is characterized by k(d)(298.2 K) = 2200 ± 10, 64.3 ± 1.6, and 11 900 ± 300 s-1, ΔH(d)(≠) = 35.3 ± 0.5, 62.8 ± 0.5, and 41.8 ± 0.4 kJ mol-1, and ΔS(d)(≠) = -62.6 ± 2.1, 0.3 ± 2.0, and -26.8 ± 1.6 J K-1 mol-1, respectively, when M+ = Li+, Na+, and K+. The stability constant, K, of [M(S-thpc12)]+ varies as M+ changes in the sequence Li+ (4.0 ± 0.1), Na+ (4.8 ± 0.1), K+ (3.5 ± 0.1), Rb+ (3.4 ± 0.1), Cs+ (3.2 ± 0.1), and Ag+ (12.8 ± 0.1), where the figures in parentheses are 1og(K/dm3 mol-1) determined in methanol by potentiometric titration at 298.2 K and I = 0.05 mol dm-3 (NEt4ClO4). Stability constants in acetonitrile, propylene carbonate, and dimethylformamide are also reported.