Jean-Claude Bunzli - 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

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      Bunzli,J.C.G., Charbonniere,L.J., Ziessel,R.F. Journal of the   Chemical Society-Dalton Transactions 2000  (12) 1917-1923

      Abstract: 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

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