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. The eight-coordinate La[sup 3+] 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 [angstrom]) and weakly with the four nitrogen atoms within the macrocyclic ring (average distance = 2.718 [+-] 0.009 [angstrom]). The hydrogen atoms of the amide groups form a hydrogen-bonded system with the triflate counteranions. In acetonitrile or methanol solutions, [sup 13]C 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 (O[degrees]C) the ethylenic groups of the 12-membered tetraazamacrocyclic are rigid on the NMR time scale. Variable-temperature [sup 13]C NMR studies suggest that rigidity of the macrocyclic is lost at higher temperatures. A dynamic process involving conformational changes about the cyclen ethylenic groups is consistent with [sup 13]C NMR data, and an activation barrier of 58.9 [+-] 0.3 kJ mol[sup [minus]1] is calculated for this process. A second dynamic process thatmore » 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[sup [minus]1] is calculated for this process for the lanthanum complex in dimethyl-d[sub 6] sulfoxide. The activation barrier calculated for the free ligand 1 is 70.5 [+-] 0.5 kJ mol[sup [minus]1]. The close correspondence of these values in conjunction with [sup 13]C NMR data for the amide C(O) resonance suggests that, in dimethyl sulfoxide solution, the amide groups of the complex are not bound.« less