Effect of ligand initial conformation and counteranion on complexation behaviors of R-BTBP toward Pd(Ⅱ)contained in highly active liquid waste

Soft N-donor bis-triazin bipyridines derives(R-BTBP)are a type of very promising extratant for extraction and complexation with long-lived trivalent minor actinides over lanthanides from highly active liquid waste(HLW).In addition to minor actinides,R-BTBP also holds very strong complexation ability toward fission palladium.However,few studies have been focused on the separation and complexation with the fission product Pd(Ⅱ)by R-BTBP.Herein,the complexation behaviors of Pd(Ⅱ)with four typical R-BTBP ligands were systematically studied by single crystal X-ray diffraction,1H NMR titration and theoretical calculation.The effects of R-BTBP initial conformation and nitrate anions on the complexation behaviors of R-BTBP with Pd(Ⅱ)were thoughtfully analyzed.Both the 1:1 and 2:1 binuclear complexes could be formed between Pd(Ⅱ)and R-BTBP with initialⅡconformation in the presence of nitrate anions,while only one 1:1 type Pd(Ⅱ)complex could be formed for those with initial OO conformation.Without nitrate anion,only one 1:1 type complex was formed in solution.The structure of the 1:1 Pd(Ⅱ)/R-BTBP complex was firstly characterized by single crystal crystallography.DFT calculation results showed that a significant large rotational energy barrier(21.8~22.6 kcal/mol)must be overcome to form theⅡtype 2:1Pd(Ⅱ)complex for those OO type R-BTBP ligands,however which would not prevent them from forming the 1:1 type complex.