Diastereomeric 1,4,7,10-Tetrakis((<i>S</i>)-2-hydroxypropyl)- 1,4,7,10-tetraazacyclododecane and Its Alkali Metal Complex Ions. A Nuclear Magnetic Resonance, Potentiometric Titration, and Molecular Orbital Study

Abstract

13 C NMR studies are consistent with 1,4,7,10-tetrakis(( S )-2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane (Λ S -thpc12) and its eight-coordinate alkali metal complex ions (Λ[M( S -thpc12)] + ) existing predominantly as single distorted cubic diastereomers in methanol in accord with structures predicted through molecular orbital calculations. Intramolecular exchange in Λ S -thpc12 is characterized by k (298.2 K) = 34 800 ± 1600 s - 1, Δ H ⧧ = 53.9 ± 0.6 kJ mol - 1, and Δ S ⧧ = 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, Δ H ⧧ = 21.4 ± 0.2, 26.3 ± 0.5, and 46.3 ± 0.2 kJ mol - 1, and Δ S ⧧ = −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 Λ[M( S -thpc12)] +, decomplexation is characterized by k d (298.2 K) = 2200 ± 10, 64.3 ± 1.6, and 11 900 ± 300 s - 1, Δ H d ⧧ = 35.3 ± 0.5, 62.8 ± 0.5, and 41.8 ± 0.4 kJ mol - 1, and Δ S 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 log( K /dm 3 mol - 1 ) determined in methanol by potentiometric titration at 298.2 K and I = 0.05 mol dm - 3 (NEt 4 ClO 4 ). Stability constants in acetonitrile, propylene carbonate, and dimethylformamide are also reported.