A New Bis(aquated) High Relaxivity Mn(II) Complex as an Alternative to Gd(III)-Based MRI Contrast Agent.

Disclosed here are a piperazine, a pyridine, and two carboxylate groups containing pentadentate ligand H2pmpa and its corresponding water-soluble Mn(II) complex (1). DFT-based structural optimization implied that the complex had pentagonal bipyramidal geometry where the axial positions were occupied by two water molecules, and the equatorial plane was constituted by the ligand ON3O donor set. Thus, a bis(aquated) disc-like Mn(II) complex has been synthesized. The complex showed higher stability compared with Mn(II)-EDTA complex [log KMnL = 14.29(3)] and showed a very high r1 relaxivity value of 5.88 mM-1 s-1 at 1.41 T, 25 °C, and pH = 7.4. The relaxivity value remained almost unaffected by the pH of the medium in the range of 6-10. Although the presence of 200 equiv of fluoride and bicarbonate anions did not affect the relaxivity value appreciably, an increase in the value was noticed in the presence of phosphate anion due to slow tumbling of the complex. Cell viability measurements, as well as phantom MR images using clinical MRI imager, consolidated the possible candidature of complex 1 as a positive contrast agent.