Metal Template Synthesis of 'Broken' Aromatic Preorganized Terdentate Hosts for the Recognition of Lanthanide Tris-beta-Diketonate Guests

The binding of the terdentate precursor 2,2 '-(4-methyl-3,5-divinylpyridine-2,6-diyl)bis(1-allyl-5-bromo-1H-benzo[d]imidazole) (1) to the lanthanide container [Ln(hfac)(3)] (Ln=La, Eu, Gd, Y, Er; H-hfac=1,1,1,5,5,5-hexafluoropentane-2,4-dione) ensures the cis-cis orientation of the two adjacent a,a '-diimine units that is required for the successful intramolecular Grubb ring-closing metathesis generating the target rigid 6-methyl-9,11-dihydro-1H,3H-2?(2),10?(2)-pyrido[2,3-c:6,5-c ']bis(azepine) scaffold decorated with two terminal 5-bromo-1H-benzo[d]imidazole in ligand L7. The bond valence analysis of the crystal structures of the associated nine-coordinate adducts [L7Ln(hfac)(3)] (Ln=La, Eu, Gd, Er, Y) reveals a satisfying match between the rigid terdentate cavity and the size of the bound lanthanide metal, with a pronounced preference for the largest lanthanum cation. Thermodynamic studies in dichloromethane confirm the formation of [L7Ln(hfac)(3)] adducts with unprecedented stabilities due to the removal of the energy penalty associated with trans-trans to cis-cis reorganization. The introduction of saturated methylene groups within the polyaromatic ligand backbone breaks extended aromatic delocalization and clears the visible part of the electromagnetic spectrum from emission arising from low-energy ligand-based excited states.