Diastereomeric D-1,4,7,10-tetrakis((R)-2-hydroxy-2-phenylethyl)-1,4,7,10-tetraazacyclododecane and Its Alkali-Metal Complex Ions. A Potentiometric Titration, Nuclear Magnetic Resonance, and Molecular Orbital Study

Abstract

13C NMR studies are consistent with Δ-1,4,7,10-tetrakis((R)-2- hydroxy-2-phenylethyl)-l,4,7,10tetraazacyclododecane (ΔR-thpec 12) and its eight-coordinate alkali-metal complexes, Δ[M(R-thpec 12)]+, existing predominantly as single square antiprismatic Δ diastereomers in dimethylformamide. Molecular orbital calculations show the parallel square oxygen and nitrogen planes to delineate the square antiprismatic structure of ΔR-thpec12 and Δ[M(R-thpec12)]+ where the basket defined by the four phenyl groups of ΔR-thpec12 becomes increasingly shallow as the M+ radius increases from Na+ to Cs+ and the four oxygens move further apart. An intramolecular exchange process, involving double inversion of all four nitrogen centers, occurs in ΔR-thpec12 for which k(298.2 K) = 46300 ± 1800 s-1, ΔH(+) = 40.8 ± 0.4 kJ mol-1, and ΔS(+) = -18.8 ± 1.7 J K-1 mol-1. In [M(R-thpec12)]+ this process is characterized by k(298.2 K) = 233 ± 2, 98 ± 1, 4900 ± 100, 33500 ± 1000, and 34500 ± 1100 s-1, ΔH(+) = 34.6 ± 0.3, 46.1 ± 0.2, 42.7 ± 0.3, 39.1 ± 0.3, and 38.5 ± 0.3 kJ mol-1, and ΔS(+) = -83.5 ± 1.1, -52.2 ± 0.7, -31.1 ± 1.2, -27.2 ± 1.2, and -28.9 ± 1.3 J K-1 mol-1, respectively, when M+ = Li+, Na, K+, Rb+, and Cs+. For intermolecular ligand exchange on Δ[M(Rthpec12)]+, decomplexation is characterized by k(d)(298.2 K) = 396 ± 3, 156 ± 3, and 152000 ± 6000 s-1, ΔH(d)(+) = 46.0 ± 0.3, 62.3 ± 0.5, and 69.8 ± 0.5 kJ mo1-1, and ΔS(d)(+) = -40.9 ± 1.0, 6.0 ± 1.9, and 88.4 ± 2.1 J K-1 mol-1, respectively, when M+ = Li+, Na+, and K+. The stability constant, K, of Δ[M(R-thpec 12)]+ varies as M+ changes in the sequence Li+ (3.13 ± 0.05), Na+ (4.25 ± 0.05), K+ (4.10 ± 0.05), Rb+ (3.57 ± 0.05), Cs+ (3.47 ± 0.05), and Ag+ (8.14 ± 0.03), where the figures in parentheses are 1og(K/(dm3 mol+1)) determined in dimethylformamide by potentiometric titration at 298.2 K and I = 0.05 mol dm-3 (NEt4ClO4).