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,Nʹ,Nʹtetraalkylpyridine-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 pseudomonocapped 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(right arrow)LnIII 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.