Lanthanide complexes of an oxazoline-phenoxide hybrid chelate

The synthesis of 2-(2′-hydroxy-3′-allylphenyl)-4,4-dimethyl-2-oxazoline, H-Allox (3), and lanthanide tris chelate complexes, mer-Ln(Allox)3 (Ln = La (4), Ce (5), Sm (6), Er (7), and Y (8)), derived from it are reported. A six-coordinate mer geometry without alkene coordination was confirmed in the solid state by X-ray crystallography for 5 and 7. Variable-temperature NMR experiments suggested that this is the most stable isomer in solution as well, although the inequivalent ligand environments undergo rapid averaging at room temperature for all five complexes. A mechanistic investigation indicated that this fluxional process is an intramolecular six-coordinate rearrangement, but it was not possible to distinguish between a Bailar (trigonal) or Rây–Dutt (rhombic) twist. Kinetic parameters for the fluxional process were determined by line shape analysis for 8 yielding ΔH‡ = 24 ± 2 kJ mol–1 and ΔS‡ = –99 ± 10 J mol–1 K–1. The structural and dynamic features of 4–8 were compared with the related In, Ga, and Al tris(2-oxazolylphenoxides).Key words: oxazoline, phenoxide, lanthanide, Group 3, Rây–Dutt twist, Bailar twist, Eyring plot, X-ray crystallography, mer isomer, variable-temperature NMR, line shape analysis, dynamic NMR, paramagnetic NMR, bidentate ligands.