Diastereomeric discrimination in 1,4,7-tris((S)-2-hydroxypropyl)-1,4,7-triazacyclononane and its lithium(I), sodium(I) and zinc(II) complexes

Abstract

In methanol, interchange between equivalent forms of a single diastereomer of 1,4,7-tris((S)-2-hydroxypropyl)-1,4,7- triazacyclononanelithium(I), [Li(S-thpc9)]⁺, and a similar interchange in [Na(S-thpc9)]⁺ have been characterised by variable temperature ¹³C{¹H} NMR spectroscopy. The respective kinetic parameters are: k = 33.5 ± 0.8 and 3030 ± 80 s⁻¹ at 298.2 K, ΔH‡ = 50.6 ± 1.0 and 50.9 ± 1.3 kJ mol⁻¹, and ΔS‡ = -46.1 ± 3.4 and -7.6 ± 4.7 J K⁻¹ mol⁻¹, with the corresponding log(K/dm³ mol⁻¹) = 3.39 ± 0.05 and 2.50 ± 0.05 at 298.2 K and I = 0.05 mol dm⁻³ (NEt₄ClO₄), where K is the complex stability constant. In N,N-dimethylformamide (DMF), log(K/dm³ mol⁻¹) = 3.29, 2.29, 2.28, 2.27, 2.29 and 7.59 (all ± 0.05) for [M(S-thpc9)]⁺ where M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Ag⁺, respectively. For the interchange between equivalent forms of a single diastereomer of 1,4,7-tris((S)-2-hydroxy-2- phenylethyl)-1,4,7-triazacyclononanelithium(I), [Li(S-thpec9)]⁺, and a similar interchange in [Na(S-thpec9)]⁺, the kinetic parameters obtained in DMF are: k = 20.5 ± 0.8 and k = 8600 ± 200 s_1 at 298.2 K, ΔH‡ = 32.8 ± 1.8 and 54.7 ± 1.2 kJ mol⁻¹, and ΔS‡ = -110 ± 3.2 and 13.8 ± 4.2 J K⁻¹ mol⁻¹, respectively. In DMF, log(K/dm³ mol⁻¹) = 3.64, 2.03, 1.91, 1.91, 1.62 and 7.59 (all ± 0.05) for [M(S-thpc9)]⁺ where M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Ag⁺, respectively. Gas phase ab initio modelling shows S-thpc9, S-thpec9 and their Li⁺, Na⁺ and Zn(II) complexes to assume single distorted trigonal prismatic diastereomeric conformations, as do X-ray crystallographic, ab initio modelling and ¹³C{¹H} NMR spectroscopic solution studies of [Zn(S-thpc9)]⁺.