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

Abstract

The ¹³ C and ¹ H 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 (L ¹ ) 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 ¹³ C NMR spectra for the ring inversion in [LaL ¹ ] ³⁺ , [EuL ¹ ] ³⁺ , [LuL ¹ ] ³⁺ in CD ₃ OD and [EuL ¹ ] ³⁺ in CD ₃ CN: 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 [LnL ¹ ] ³⁺ in solution is compared with that of complexes with similar 12-membered tetraaza macrocycles bearing pendant arms.