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

Abstract

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 L5 < L6 < L7. [Ln(Li)]³⁺ and [Ln(Li)₂]³⁺ are formed with the three ligands, but 1 ∶ 3 complexes are strictly limited to [Ln(L5)₃]³⁺ and [Ln(L6)₃]³⁺ because of the significant steric congestion provided by the twelve benzyl groups located along the three-fold axis in [Ln(L7)₃]³⁺. Comparisons between L6 and L7 in the 1 ∶ 2 complexes evidence superimposable pseudo-monocapped square antiprismatic coordination spheres in the crystal structures of [Ln(Li)₂(H₂O)₂(CF₃SO₃)](CF₃SO₃)₂ (i = 6, Ln = Eu: 9; i = 7, Ln = Gd: 10). Photophysical properties of [Ln(L6)₂]³⁺ and [Ln(L7)₂]³⁺ (Ln = Eu, Gd, Tb, Lu) are similar except for improved quantum yields for [Ln(L7)₂]³⁺ (Ln = Eu, Tb) which can be assigned to a slightly more efficient L7 → Ln^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.