Isomerization kinetics of lanthanide(III) complexes with the pendant-arm macrocyclic ligand 1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane

The ¹³C and ¹H NMR spectra of the complexes of La^III, Eu^III and Lu^III with 1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane (L¹) in acetonitrile or methanol indicated the presence of two enantiomers the interconversion of which proceeds through both a ring inversion and a rearrangement of the pendant arms. The following kinetic parameters were extracted from temperature-dependent ¹³C NMR spectra for the ring inversion in [LaL¹]³⁺, [EuL¹]³⁺, [LuL¹]³⁺ in CD₃OD and [EuL¹]³⁺ in CD₃CN:k(298 K) = 1396, 1055, 1288 and 880 s^-1; ΔH^‡ = 37.6, 41.1, 48.2 and 47.7 kJ mol^-1; ΔS^‡ = -58.5, -49.2, -23.8 and -28.4 J K^-1 mol^-1, respectively. The lanthanide(III) substitution induces a continuous variation of the kinetic parameters, implying the same pathway for the enantiomerization. The behaviour of [LnL¹]³⁺ in solution is compared with that of complexes with similar 12-membered tetraaza macrocycles bearing pendant arms.