Structural and photophysical properties of LnIII complexes with 2,2′-bipyridine-6,6′-dicarboxylic acid: surprising formation of a H-bonded network of bimetallic entities

Abstract

The ligand H₂L = 2,2′-bipyridine-6,6′-dicarboxylic acid reacts with Ln(NO₃)₃·xH₂O (x = 6, Ln = Eu, Tb; x = 5, Ln = Gd) in MeOH/Et₃N to give complexes with 1∶2 and 2∶3 metal∶ligand stoichiometry, (Et₃NH)[LnL₂] and [Ln₂L₃(H₂O)₃]·xH₂O (x = 1, Ln = Eu, Tb; x = 0, Ln = Gd) which have been isolated and characterised. A sizeable quantum yield is obtained for the 1∶2 Eu∶Ligand complex in aqueous solution (Q^Eu_abs = 11.5 ± 2.3% at pH = 6.6), pointing to an efficient ligand-to-metal energy transfer. The presence of some inner-sphere interaction with water was deduced from Eu(⁵D₀) lifetime measurements in water (0.86 ± 0.01 ms vs. 1.55 ± 0.02 ms in the solid state between 10 and 295 K, q_est = 0.3–0.4 water molecule). For [TbL₂]⁻, sensitisation of Tb^III also occurs (Q^Tb_abs = 6.3 ± 1.3% at pH = 6.6) but the Tb(⁵D₄) excited level is de-populated at room temperature by a back-transfer process to the ligand. The crystal structure obtained for the 2∶3 Tb∶ligand complex evidences two distinct terbium sites, one Tb^III being complexed to two ligands affording a mono-anionic complex, itself linked to the second terbium ion with a μ-carboxylate bridge; the generic formulation of the crystallised complex is [TbL₂-μ-TbL(H₂O)₃]·2H₂O· 2MeOH. Consecutive dimers are linked by an elaborate network of H-bonds involving interstitial solvent molecules. A photophysical study of the 2∶3 Eu∶Ligand complex in the solid state points to the same structural features, revealing two metal ion sites with essentially no bonded water (q = 0.3, site I) and with 3 co-ordinated water molecules (q = 2.8, site II), respectively. The H₂L synthon is therefore an interesting building block for the design of elaborate compartmental ligands and/or of supramolecular functional assemblies.