Laser-Induced Luminescence Studies and Crystal Structure of the Europium(III) Complex of l,4,7,10-Tetrakis(carbamoylmethyl)-l,4,7,10-tetraazacyclododecane. The Link between Phosphate Diester Binding and Catalysis by Lanthanide(III) Macrocyclic Complexes

Solution and solid-state properties of the Eu(III) complex of the tetraamide macrocyclic ligand TCMC (TCMC = 1,4,7,10-tetrakis(carbamoylmethyl)-l,4,7,10-tetraazacyclododecane) are investigated as part of an effort to develop lanthanide(III) macrocyclic catalysts. [Eu(TCMC)(H₂O)](CF₃SO₃)₃•2CH₃OH crystallizes in the centrosymmetric monoclinic space group P2₁/In with a = 12.0571(15) Å,b= 13.2196(20) Å, c = 25.4250(40) Å; β = 91.673-(11)°, V = 4050.8 ų, and Z = 4. The structure was solved and refined to R = 4.02% and R_w = 4.33% for 3510 reflections with F > 6σ(F). (R = 7.04%, R_w = 6.48% for all 5322 independent reflections.) Four stereoisomers (two enantiomeric pairs) of the [Eu(TCMC)(H₂O)]³⁺ cation appear in the crystal. The structure is disordered, containing two overlapping diastereomers that are interrelated by different conformations of the 1,4,7,10-tetraazacyclododecane ring defining the asymmetric unit. Two enantiomers are related to these by operations of the second kind (i, n-glide). The nine-coordinate Eu(III) center has a 4:4:1 coordination geometry formed by the octadentate TCMC ligand and a single bound water molecule. The laser-induced luminescence excitation band of a single crystal of [Eu(TCMC)(H₂O)](CF₃SO₃)₃•2CH_3OH resolves into two peaks, suggesting that each diastereomer gives rise to a separate excitation peak. Similarly, the luminescence excitation spectrum of a solution of [Eu(TCMC)]³⁺ in water indicates that two species are present in solution. Luminescence lifetime measurements in H₂O and in D₂O indicate that the europium(III) ion in the TCMC complex has 1.5 ± 0.5 bound water molecules. The ³¹P NMR resonance of diethyl phosphate is monitored at 18 ± 2 °C, pH 7.4, upon addition of [Eu(TCMC)]³⁺ as well as upon addition of several lanthanide(III) complexes that are catalysts for RNA cleavage. No binding of diethyl phosphate to [Eu(TCMC)]³⁺ is observed. The hexadentate Schiff-base complex [La(L¹)]³⁺ binds to diethyl phosphate with a binding constant of 47.7 ± 0.5 M⁻¹.