Study of paraCEST response on six-coordinated Co(ii) and Ni(ii) complexes of a pyridine-tetraamide-based ligand

This study highlights the successful synthesis of a potential ligand, 2,2 ',2 '',2 '''-((pyridine-2,6-diylbis(methylene))bis(azanetriyl))tetraacetamide (PATA), along with its corresponding Co(II) and Ni(II) complexes for paraCEST-based agents. X-ray diffraction data confirmed that both the complexes are six coordinated with distorted octahedral geometries, but only the [Co(PATA)](2+) complex has a good structural feature to show paraCEST activity. After a thorough characterization of the ligand and both of its complexes, various studies, including solution-state magnetic properties, redox properties, temperature, and pH variation studies, were carried out. [Co(PATA)](2+) remained inert in the presence of competing ions, under acidic conditions, at high temperatures, and in the physiological pH range. The paraCEST response of [Co(PATA)](2+) has been measured in the presence of HEPES buffer medium, and a high paraCEST feature was discovered at both 37 and 25 degrees C. The pH variation paraCEST studies were carried out and the exchange rate constant of the probe at 37 and 25 degrees C was also determined. However, due to the fast exchange of water protons, the [Ni(PATA)(OH2)](2+) complex remained inactive in the CEST process.