Lanthanide Helicates Self-Assembled in Water: A New Class of Highly Stable and Luminescent Dimetallic Carboxylates

The segmental ligand bis{1-ethyl-2-[6'-carboxy)pyridin-2'-yl]benzimidazol-5-yl}methane (LC) reacts with the entire series of LnIII ions to yield a new class of neutral lanthanide carboxylates, the triple-stranded dimetallic helicates [Ln2(LC-2H)3], which are stable in water in the pH range 4−13. A competitive titration with 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (dota) shows the EuIII helicate to have a stability comparable to that of [Eu(dota)]-. According to the X-ray structures of isotypical [Ln2(LC-2H)3]·20.5H2O (Ln = Eu, Tb) (C93H72Ln2N18O12·20.5H2O, monoclinic, P21/c, Z = 8), the helicates are comprised of two types of slightly different molecules (A, B) within the asymmetric unit. The three ligand strands are helically wrapped around the two nine-coordinate LnIII ions, leading to pseudo-D3 symmetries around the metals. The LnIII−LnIII distances lie in the ranges 8.81−8.83 Å for molecules A and 9.04−9.07 Å for molecules B, while the helical pitch amounts to 23.7−23.8 Å (A) and 24.5−24.7 Å (B). In solution, the helicates retain time-averaged D3 symmetries along the entire LnIII series, as shown by 1H and 13C NMR. The derivation of equations adapted to heterodimetallic complexes allows the separation of contact and dipolar contributions to the isotropic paramagnetic shift. Somewhat different structures are evidenced between larger (Ln = Ce−Tb) and smaller (Ln = Er−Yb) LnIII ions, attributed to a lengthening of the helix on going toward heavier lanthanides. The photophysical properties of the helicates depend on the nature of the LnIII ion, and LC is revealed to be a good sensitizer of EuIII. The flexible route used to synthesize LC opens interesting perspectives for the fine-tuning of the chemical and photophysical properties of lanthanide-containing homodimetallic helicates with potential applications in labeling and sensing technologies.