Monometallic lanthanide complexes with tridentate 2,6-dicarboxamidopyridine ligands. Influence of peripheral substitutions on steric congestion and antenna effect

The systematic investigation of steric and electronic effects on the formation of lanthanide complexes with the tridentate N,N,N',N'-tetraalkylpyridine-2,6-dicarboxamide ONO ligands (alkyl=ethyl: L5, isopropyl: L6 and benzyl: L7) shows a reduced affinity with increasing steric demand in the order L5Ln(III) energy transfer process. The removal of two benzyl groups in the analogous N,N'-dibenzylpyridine-2,6-dicarboxamide ligand (L8) restores the formation of stable triple-helical complexes as demonstrated by the crystal structure of [Tb(L8)(3)](2)(CF3SO3)(6) (11). However, the existence of intricate mixtures of isomers in solution which are blocked on the NMR time scale limits their use as building blocks for the design of polymetallic d-f and f-f helicates.