NMR Investigation of Lanthanoid (III) Complexes of bbppn (H2bbppn: N,N’-bis(2-hydroxybenzyl)-N,N’-bis(2-methylpyridyl)propylene-1,2-diamine)

Pseudocontact shift NMR spectroscopy is a powerful tool in integrative structural biology. Conformationally rigid, enantiopure chelators with extremely high affinity to lanthanoids play a crucial role for this technique. In this context, we have synthesized enantiopure complexes of Y, Eu, Dy, Tm and Lu of (S)-H₂bbppn (H₂bbppn: N,N’-bis(2-hydroxybenzyl)-N,N’-bis(2-methylpyridyl)propylene-1,2-diamine) and investigated their NMR properties. Strongly paramagnetic ¹H-NMR spectra showing extremely large chemical shift ranges were obtained for Dy (2280 ppm), Tm (380 ppm) and Eu (68 ppm) and the anisotropy of the magnetic susceptibility was determined in all cases. The axial components of the susceptibility tensors for the (S)-bbppn complexes were determined as 225, −32 and 6×10⁻³² m³ for Dy, Tm and Eu. For (S)-[Y(bbppn)Cl], a single crystal structure was obtained providing atom coordinates for the isostructural series of lanthanoid compounds. It was demonstrated that in all cases rigid, enantiopure lanthanoid complexes were formed, providing an attractive new scaffold for potential conjugatable lanthanoid chelating tags.