Crystal structure, luminescence and other properties of some lanthanide complexes of the polypyridine ligand 6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine

Abstract

Europium(III) and gadolinium(III) form 1 : 1 complexes with the octadentate polypyridine ligand 6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine L ² . The crystal structure of the complex [GdL ² Cl ₂ ]Cl·2H ₂ O·0.5EtOH has been determined. The gadolinium atom is co-ordinated to seven nitrogen atoms of the L ² ligand and two chloride ligands. One of the pyridine groups of L ² is dangling unco-ordinated. Irradiation of water or MeOH solutions of the [EuL ² ]Cl ₃ complex with UV light led to a metal-centred luminescence. The luminescence lifetimes at 296 K in water, D ₂ O, MeOH and CD ₃ OD were 0.78, 1.28, 0.98 and 1.25 ms, respectively. From these data the average number of co-ordinated solvent molecules (water or MeOH) was calculated to be 0.5 according to earlier empirical correlations. The luminescence consists of a series of narrow lines corresponding to the envelopes of ⁵ D ₀ → ⁷ F _J transitions, the most intense occurring at 613 nm (→ ⁷ F ₂ ). The integrated intensities of these emission lines give the respective quantum yields of 0.046 and 0.089 in water and D ₂ O. The equilibrium constant for formation of the europium complex in MeOH, I = 0.100 mol dm ^-3 NBu ₄ ClO ₄ , was determined to be 10 ^{7.09 ± 0.09} dm ³ mol ^-1 .