Structural and NMR characterization of Sm(III), Eu(III), and Yb(III) complexes of an amide based polydentate ligand exhibiting paramagnetic chemical exchange saturation transfer abilities

Complexes of Sm(III), Eu(III), and Yb(III) with a new polydentate ether ligand with amide arms were synthesized. Solid state X-ray structures of the complexes reveal all three complexes crystallize In monocllnic unit cells. The mononuclear complexes have nine coordinate tricapped trigonal prismatic geometries with coordination of all four amide carbonyl oxygen and all three of the backbone ether oxygen atoms. The molecules possess a pseudo C₂ symmetry axis. The complexes were characterized by solution and solid state emission spectroscopy and IR spectroscopy. Solution state behavior of the complexes was further explored using NMR. The ¹H NMR spectra show 16 peaks suggesting the complexes are slow in exchanging, on the NMR time scale and that the C₂ symmetry axis is maintained. The NMR spectra were assigned using ¹H, ¹³C, COSY, and HMQC experiments. The Eu(III) complex was tested for the recently explored Magnetic Resonance Imaging phenomenon called paramagnetic chemical exchange saturation transfer (PARACEST). At physiological pH and temperature two CEST peaks were observed that caused a decrease In the bulk water molecule signal Intensity of 10 and 16%.