The difference of uranyl (UO22+) complexes with Nitrilotri–3–propanoic acid and Tris(2–carboxyethyl) phosphine: N–tricarboxylate versus P–tricarboxylate

Nitrilotri–3–propanoic Acid (NTPA) and Tris(2–carboxyethyl) phosphine (TCEP) possess similar skeleton. This pair of isostructural ligands is a perfect set to assess the differences of uranyl (UO 2 2+ ) complexes with N–tricarboxylate versus P–tricarboxylate. The protonation reactions of NTPA and TCEP were investigated by potentiometry and calorimetry. Complexation of uranyl with the both ligands was studied by potentiometry and structural analysis. The results indicate that central trialkylamine in NTPA has a higher affinity for the proton than trialkylphosphine in TCEP. Crystal structures and NMR analyses indicate that central amine of NTPA and central phosphine of TCEP are not involved in the bonding. NTPA/UO 2 2+ complex is more stable than TCEP/UO 2 2+ complex. This difference cannot be explained by electronic effect of central N and P atoms because of nonbonding interactions. The higher stability of UO 2 2+ /NTPA versus UO 2 2+ /TCEP may be due to the fact that the central N atom of NTPA has a lower steric hindrance than the central P atom of TCEP. Another possible reason is that UO 2 2+ /TCEP complexes have different coordination modes from UO 2 2+ /NTPA complexes.