Synthesis, Structure, and Dynamic Properties of the Lanthanum(III) Complex of 1,4,7,10-tetrakis(2-carbamoylethyl)-1,4,7,10-tetraazacyclododecane

The synthesis, structure and dynamic properties of the lanthanum(III) complex of macrocycle 1 (1 = 1,4,7,10-tetrakis(2-carbamoylethyl)-1,4,7,10-tetraazacyclododecane) are reported. [La(1)³⁺][CF₃SO₃⁻]₃·MeCN crystallizes with the complex lanthanum cation, three triflate counteranions, and one acetonitrile solvent molecule in the noncentrosymmetric orthorhombic space group P2₁2₁2₁ (No. 19) with a = 10.6937(13) Å, b = 17.6299(26) Å, c = 22.3554(36) Å, V = 4214.7(11) ų, and Z = 4. The structure was solved and refined to R = 5.39% and R_w = 4.56% for all 5538 independent reflections and R = 3.37% and R_w = 3.32% for those 4130 reflections with |F_o| > 6σ|F_o|. The eight-coordinate La³⁺ cation is encapsulated by the macrocyclic ligand and has a distorted square antiprismatic coordination geometry. The macrocyclic ligand coordinates strongly with the four oxygen atoms of the amide groups (average distance = 2.423 ± 0.032 Å) and weakly with the four nitrogen atoms within the macrocyclic ring (average distance = 2.718 ± 0.009 Å). The hydrogen atoms of the amide groups form a hydrogen-bonded system with the triflate counteranions. In acetonitrile or methanol solutions, ¹³C NMR resonances for the amide C(O) carbons in the complex are shifted by greater than 2 ppm from those of 1, suggesting that the amide groups are coordinated in these solvents. At low temperatures (0°C) the ethylenic groups of the 12-membered tetraazamacrocycle are rigid on the NMR time scale. Variable-temperature ¹³C NMR studies suggest that rigidity of the macrocycle is lost at higher temperatures. A dynamic process involving conformational changes about the cyclen ethylenic groups is consistent with ¹³C NMR data, and an activation barrier of 58.9 ± 0.3 kJ mol⁻¹ is calculated for this process. A second dynamic process that averages protons on the amide nitrogens may involve free rotation about the amide C-N bond. An activation barrier of 69.8 ± 0.5 kJ mol⁻¹ is calculated for this process for the lanthanum complex in dimethyl-d₆ sulfoxide. The activation barrier calculated for the free ligand 1 is 70.5 ± 0.5 kJ mol⁻¹. The close correspondence of these values in conjunction with ¹³C NMR data for the amide C(O) resonance suggests that, in dimethyl sulfoxide solution, the amide groups of the complex are not bound. The complex dissociates rapidly in water at neutral pH and 37°C.