Synthesis and Dynamic Properties of Kinetically Inert Lanthanide Compounds: Lanthanum(III) and Europium(III) Complexes of 1,4,7,10-Tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane

The synthesis and dynamic properties of La(THED)(CF₃SO₃)₃ and Eu(THED)(CF₃SO₃)₃ are described. (THED is 1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane.) Variable-temperature ¹Hand ¹³CNMRstudies indicate that the ethylenic groups of the 12-membered tetraaza macrocycle are rigid on the NMR time scale at low temperatures (−40 °C), and all adopt the same conformation. Macrocycle rigidity is lost at elevated temperatures. A dynamic process involving conformational changes of the ethylenic groups in the tetraazamacrocycle is consistent with NMR data. An energy barrier of 52 (±0.7) kJ mol⁻¹ is calculated for this process from line-shape analysis of ¹³C NMR spectra of La(THED)(CF₃SO₃)₃ at four temperatures. Both complexes are stable in water at nearneutral pH. The dissociation of La³⁺ or Eu³⁺ from their respective complexes is investigated in the presence of excess Cu²⁺ by monitoring an absorbance at 312 nm attributed to the Cu(THED)²⁺ complex. For La(THED)(CF₃SO₃)₃, dissociation is first-order in lanthanum complex and independent of Cu²⁺ with a first-order rate constant of 9.2 (±0.5) × 10⁻⁶ s⁻¹ at 37 °C, pH 6.0. Eu(THED)(CF₃SO₃)₃ is substantially more inert to metal ion release than is the lanthanum complex. The first-order rate constant for dissociation of the europium complex at pH 6.0 and 37 °C is 7.1 (±0.4) × 10⁻⁷ s⁻¹; there is a slight dependence of the rate constant on pH in the pH range 3.0–6.8.