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

Abstract

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 initial II conformation 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 the II type 2:1 Pd(II) complex for those OO type R-BTBP ligands, however which would not prevent them from forming the 1:1 type complex.