Step by Step Assembly of Polynuclear Lanthanide Complexes with a Phosphonated Bipyridine Ligand

The synthesis of the octadentate ligand L (LH₈ = ((([2,2'-bipyridine]-6,6'-diylbis(methylene))bis(azanetriyl))tetrakis(methylene))tetrakis(phosphonic acid)) is reported. The coordination of L with various lanthanide cations was monitored by absorption and luminescence spectrophotometric titration experiments (Ln = Tb, Yb), potentiometry (Ln = La, Eu, Lu), and mass spectrometry (Ln = Tb). It was found that L forms very stable mononuclear (LnL) species in aqueous solutions (log K = 19.80(5), 19.5(2), and 19.56(5) for La, Eu, and Lu, respectively) with no particular trend along the series. Spectroscopic data showed the Ln cations to be enclosed in the cavity formed by the octadentate ligand, thereby shielding the metal from interactions with water molecules in the first coordination sphere. When more than one equivalent of cations is added, the formation of polynuclear [(LnL)₂Ln_x] complexes (x = 1-3) can be observed, the presence of which could be confirmed by electrospray and MALDI mass spectrometry experiments. DFT modeling of the mononuclear (LnL) complexes indicated that the coordination of the cation in the cavity of the ligand results in a very asymmetric charge distribution, with a region of small negative electrostatic potential on the hemisphere composed of the chromophoric bipyridyl moiety and an electron-rich domain at the opposite hemisphere around the four phosphonate functions. DFT further showed that this polarization is most likely at the origin of the strong interactions between the (LnL) complexes and the incoming additional cations, leading to the formation of the polynuclear species. ¹H and ³¹P NMR were used to probe the possible exchange of the lanthanide complexed in the cavity of the ligand in D₂O, revealing no detectable exchange after 4 weeks at 80 °C and neutral pD, therefore pointing out an excellent kinetic inertness.