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

Abstract

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 Eu^III and Lu^III display an averaged C₄-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 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 Eu^III 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 Eu^III 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.