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 kd (298.2 K) = 78.5 ± 1.0 and 26.1 ± 0.6 s-1, ΔHd‡ = 49.2 ± 0.3 and 57.7 ± 0.4 kJ mol-1, and ΔSd‡ = -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, ke (298.2 K) = 18 300 ± 3100, 7100 ± 220 and 7010 ± 200 s-1, ΔHe‡ = 41.3 ± 1.3, 24.6 ± 0.5 and 53.7 ± 0.6 kJ mol-1, and ΔSe‡ = -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.