Structural studies of lanthanide nitrate–N,N′-dimethyl-N,N′-diphenylpyridine-2,6-dicarboxyamide complexes

The tridentate ligand N,N′-dimethyl-N,N′-diphenylpyridine-2,6-dicarboxyamide (DMDPhPDA) and the corresponding lanthanum complex [La(NO3)3(DMDPhPDA)2] have been prepared and structurally characterised. The crystal structure of DMDPhPDA shows syn–anti conformation. In the lanthanum complex, two DMDPhPDA molecules coordinated to La(III) in a tridentate fashion and to three nitrate ions in a bidentate fashion make the lanthanum atom 12-coordinate. The crystal structure of [La(NO3)3(DMDPhPDA)2] has a C2 symmetry. The stability constants determined by spectrophotometric titration suggest that [Ln(DMDPhPDA)2]3+ is the primary product in CH3CN solution and [Ln(DMDPhPDA)3]3+ is difficult to form. However, [Ln(DMDPhPDA)2]3+ could not be distinguished in 1H NMR spectra. The 1H NMR titration results imply that a fast ligand exchange process takes place.