Structural and photophysical behaviour of lanthanide complexes with a tetraazacyclododecane featuring carbamoyl pendant arms

The crystal and molecular structure of 1,4,7,10-tetrakis(N-benzylcarbamoylmethyl)-1,4,7,10-tetraazacyclododecane, L1, revealed an S2-symmetric molecule. One pair of pendant arms is involved in intramolecular hydrogen bonding while the other interacts with two other ligand molecules through strong intermolecular hydrogen bonds. In solution, the ligand adopts a similar geometrical arrangement at low temperature but complexes with EuIII and LuIII display an averaged C4-symmetry on the NMR timescale. One isomer only was detected, as reported for similar complexes, and the arrangement around EuIII and LuIII appears to be rigid while it is more fluxional around LaIII. Rotation of the benzyl group around the amide function was observed for EuIII and a dynamic NMR study in the range 253–333 K yielded ΔH ‡ = 43.0 ± 0.1 kJ mol–1, ΔS ‡ = –49.3 ± 0.4 J K–1 mol–1 and k298 = 47.7 ± 1.1 s–1. In the solid state, anhydrous eight-co-ordinate complexes were isolated and high resolution luminescence spectroscopy of EuIII in the europium and Eu-doped compounds of La, Gd and Lu confirmed a site symmetry close to C4 along the lanthanide-series. Energy transfer from the L13ππ* state to the Tb(5D4) excited state is sizable leading to an absolute quantum yield of 6.4% in MeCN, while the EuIII luminescence is only poorly sensitized, possibly because of photoinduced electron transfer processes. The Tb(5D4) and Eu(5D0) lifetimes for both solid state samples and solutions in MeCN are consistent with no water molecule in the inner co-ordination sphere but addition of water results in the ninth co-ordination site being bound to H2O: number of bound waters, q = 0.96 (Eu) and 0.71 (Tb) in MeCN + 2 M H2O and 1.19 (Eu) and 1.30 (Tb) in water. The relaxivity r1 of the gadolinium(III) complex is pH dependent and varies between 4.08 (pH 0.8), 1.88 (6.7) and 6.14 (11.1) mM–1 s–1.