Structural and photophysical properties of Ln(III) complexes with 2,2 '-bipyridine-6,6 '-dicarboxylic acid: surprising formation of a H-bonded network of bimetallic entities

摘要

The ligand H2L=2,2'-bipyridine-6,6'-dicarboxylic acid reacts with Ln(NO3)(3). xH(2)O (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)[LnL(2)] and [Ln(2)L(3)(H2O)(3)]. xH(2)O (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(abs)(Eu)=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(D-5(0)) 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(abs)(Tb)=6.3 +/- 1.3% at pH=6.6) but the Tb(D-5(4)) 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 mu-carboxylate bridge; the generic formulation of the crystallised complex is [TbL2-mu-TbL(H2O)(3)]. 2H(2)O . 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. [References: 26]