Alkali-metal complexes of a pendant-arm tetraaza macrocycle. Equilibrium, inter- and intra-molecular exchange processes

Abstract

The stabilities log(K/dm³ mol^–1) of [ML¹]⁺ formed by 1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane (L¹) were found to vary with M⁺ in the sequence Li⁺(8.07 ± 0.05 and 8.90 ± 0.05), Na⁺(6.66 ± 0.05 and 7.49 ± 0.05), K⁺(3.40 ± 0.05 and 5.91 ± 0.05), Rb⁺(3.00 ± 0.05 and 4.23 ± 0.05), Cs⁺(2.90 ± 0.05 and 4.04 ± 0.05) and Ag⁺(9.35 ± 0.04 and 14.00 ± 0.05), in acetonitrile and propylene carbonate, respectively, determined by potentiometric titration at 298.2 K and I= 0.05 mol dm^–3(NEt₄ClO₄). Complexes of the parent macrocycle 1,4,7,10-tetraazacyclododecane (L²) are less stable. For the monomolecular decomplexation of [NaL¹]⁺ in acetonitrile and propylene carbonate the values k_d(298.2 K)= 78.5 ± 1.0 and 26.1 ± 0.6 s^–1, ΔH_d^‡= 49.2 ± 0.3 and 57.7 ± 0.4 kJ mol^–1, and ΔS_d^‡=–43.7 ± 0.9 and –24.0 ± 1.0 J K^–1 mol^–1, respectively, were determined by ²³Na NMR spectroscopy. Carbon-13 NMR spectroscopy showed that for the enantiomerisation of square-antiprismatic [LiL¹]⁺, [NaL¹]⁺ and [KL¹]⁺ in methanol, k_e(298.2 K)= 18 300 ± 3100, 7100 ± 220 and 7010 ± 200 s^–1, ΔH_e^‡= 41.3 ± 1.3, 24.6 ± 0.5 and 53.7 ± 0.6 kJ mol^–1, and ΔS_e^‡=–24.8 ± 5.9, –88.6 ± 1.8 and 8.8 ± 2.3 J K^–1 mol^–1, respectively. For [LiL¹]⁺ and [NaL¹]⁺ enantiomerisation occurs much more rapidly than intermolecular exchange of L¹, but for [KL¹]⁺ enantiomerisation occurs predominantly through intermolecular exchange of L¹.