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

摘要

The crystal and molecular structure of 1, 4, 7, 10-tetrakis (N-benzylcarbamoylmethyl)-1, 4, 7, 10-tetraazacyclododecane, L~1, revealed an S_2-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 Eu~(III) and Lu~(III) display an averaged C_4-symmetry on the NMR timescale. One isomer only was detected, as reported for similar complexes, and the arrangement around Eu~(III) and Lu~(III) appears to be rigid while it is more fluxional around La~(III). 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 = 43.0 +- 0.1 kJ mol~(-1), #DELTA#HS = -49.3 +- 0.4 JK~(-1) mol~(-1) and k~(298) = 47.7 +- 1.1 s~(-1). In the solid state, anhydrous eight-co-ordinate complexes were isolated and high resolution luminescence spectroscopy of Eu~(III) in the europium and Eu-doped compounds of La, Gd and Lu confirmed a site symmetry close to C_4 along the lanthanideseries. Energy transfer from the L~(13) #pi# #pi#~* state to the Tb(~5D_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 photoinduced electron transfer processes. The Tb(~5D_4) and Eu(~5D_0) 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 H_2O: number of bound waters, q = 0.96 (Eu) and 0.71 (Tb) in MeCN+2MH_2O and 1.19 (Eu) and 1.30 (Tb) in water. The relaxivity r_1 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).