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 (L C ) reacts with the entire series of Ln III ions to yield a new class of neutral lanthanide carboxylates, the triple-stranded dimetallic helicates [Ln 2 (L C -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 Eu III helicate to have a stability comparable to that of [Eu(dota)] - . According to the X-ray structures of isotypical [Ln 2 (L C -2H) 3 ]·20.5H 2 O (Ln = Eu, Tb) (C 93 H 72 Ln 2 N 18 O 12 ·20.5H 2 O, monoclinic, P 2 1 / 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 Ln III ions, leading to pseudo- D 3 symmetries around the metals. The Ln III −Ln III 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 D 3 symmetries along the entire Ln III series, as shown by 1 H and 13 C 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) Ln III 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 Ln III ion, and L C is revealed to be a good sensitizer of Eu III . The flexible route used to synthesize L C 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.