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-benzylcarbamoylm ethyl)-1,4,7,10-tetraazacyclododecane, L-1, revealed an S-2-symmetric molec ule. One pair of pendant arms is involved in intramolecular hydrogen bondin g while the other interacts with two other ligand molecules through strong intermolecular hydrogen bonds. In solution, the ligand adopts a similar geo metrical arrangement at low temperature but complexes with Eu-III and Lu-II I display an averaged C-4-symmetry on the NMR timescale. One isomer only wa s detected, as reported for similar complexes, and the arrangement around E u-III and Lu-III appears to be rigid while it is more fluxional around La-I II. Rotation of the benzyl group around the amide function was observed for Eu-III and a dynamic NMR study in the range 253-333 K yielded Delta H doub le dagger = 43.0 +/- 0.1 kJ mol(-1), Delta S double dagger = -49.3 +/- 0.4 J K-1 mol(-1) and k(298) = 47.7 +/- 1.1 s(-1). In the solid state, anhydrou s eight-co-ordinate complexes were isolated and high resolution luminescenc e spectroscopy of Eu-III in the europium and Eu-doped compounds of La, Gd a nd Lu confirmed a site symmetry close to C-4 along the lanthanide-series. E nergy transfer from the L-1 (3)pi pi* state to the Tb(D-5(4)) excited state is sizable leading to an absolute quantum yield of 6.4% in MeCN, while the Eu-III luminescence is only poorly sensitized, possibly because of photoin duced electron transfer processes. The Tb(D-5(4)) and Eu(D-5(0)) lifetimes for both solid state samples and solutions in MeCN are consistent with no w ater molecule in the inner co-ordination sphere but addition of water resul ts in the ninth co-ordination site being bound to H2O: number of bound wate rs, q = 0.96 (Eu) and 0.71 (Tb) in MeCN + 2 M H2O and 1.19 (Eu) and 1.30 (T b) in water. The relaxivity r(1) of the gadolinium(III) complex is pH depen dent and varies between 4.08 (pH 0.8), 1.88 (6.7) and 6.14 (11.1) mM(-1) s( -1).