Enantiomerization of Pendant-Arm Triaza Macrocyclic Lithium(I) and Sodium(I) Complex Ions

Abstract

The first reported enantiomerization of six-coordinate alkali metal complex ions, represented by 1,4,7-tris(2-hydroxyethyl)-1,4,7-triazacyclononanelithium(I) and its sodium(I) analogue in methanol, has been characterized by variable-temperature ¹³C{¹H} n.m.r. spectroscopy. The respective kinetic parameters are: k = (1·11 ± 0·05) × 10⁶ and (2·27 ± 0·09) × 10⁵ s^-1 at 298·2 K, ΔH = 27·2 ± 0·3 and 21·7±0·2 kJ mol^-1, and ΔS = –36·3 ± 1·3 and –69·6±1·2 J K^-1 mol^-1. Molecular orbital calculations show that these enantiomers have distorted trigonal prismatic structures consistent with the interpretation of the n.m.r. spectra. For the Li⁺, Na+, K⁺, Rb⁺ and Cs⁺ complex ions log(K/dm³ mol⁻¹) = 3·13±0·09, 3·52±0·05, 3·23±0·05, 2·78±0·10 and 2·47±0·08, respectively, at 298·2 K and I = 0·05 mol dm^-3 (NEt₄ ClO₄) in methanol, where K is the complex ion stability constant.