On the conformation of the actinide-selective hydrophilic SO3-Ph-BTP ligand in aqueous solution. A computational study

We studied the conformation and hydration of the SO 3 -Ph-BTP ligand used as hydrophilic actinide and lanthanide separating agent using DFT methods. To avoid a large conformational space, changes in conformation and hydration were considered step-by-step. The central BTP system of the SO 3 -Ph-BTP ligand can be slightly non-planar in the gas phase, but in an aqueous solution it adopts the planar BTP- cc -aq conformation, known from metal complexes with BTP-like ligands. Out of a dozen conformations of protonated BTP, the most stable appeared to be the one protonated at the pyridine N-atom. The analysis of the conformation and hydration of the larger Ph-BTP system indicates that in water the proton dissociates from the protonated BTP core to form a hydronium ion, which additionally interacts with two water molecules H-bonded to triazine's N2-atoms. Considering 24 different protonated Ph-BTP conformers surrounded by five water molecules led us to the conclusion that the core of the most stable form is practically identical to the most stable hydrated and protonated BTP form. Modelling the hydration of both every part of the SO 3 -Ph-BTP ligand independently and the whole SO 3 -Ph-BTP ligand indicated that the hydronium ion formed near the SO 3 H groups can migrate and be stabilized near the center of the SO 3 -Ph-BTP ligand. Thus, in the process of actinide and lanthanide extraction from acidic solutions, the metal ions compete with the hydronium ion for the position at the ligand center.