Spectroscopic and Electrochemical Analyses for Neodymium Complexes in Potassium Bis(trifluoromethylsulfonyl)amide Melts

Abstract

The coordination states of the multivalent neodymium complexes in potassium bis(trifluoromethyl-sulfonyl) amide; K{[}TFSA] were investigated by Raman spectroscopy. The concentration dependences of deconvoluted Raman spectra were investigated for 0.23-0.45 mol kg(-1) Nd(III), and mixed sample of Nd(II)/Nd(III) = 1/3 at the molar ratio in K{[}TFSA]. According to the conventional analysis, the solvation number; n of neodymium complexes were determined to be n = 4.06 for Nd(II), 5.02 for Nd(III). Moreover, thermodynamic properties such as Delta(iso)G, Delta H-iso, and Delta S-iso for the isomerism of {[}TFSA](-) from trans- to cis-coordinated isomer were evaluated from temperature dependence of Raman bands. In the first solvation sphere of Nd3+ cation, Delta H-iso(Nd) became remarkably negative, suggesting cis-{[}TFSA](-) isomers were stabilized by enthalpic contributions. Then, Delta H-iso(Nd) contributed to the remarkable decrease in Delta(iso)G(Nd), and this result clearly indicates cis-{[}TFSA](-) conformers bound to Nd3+ cations are preferred coordination state of {[}Nd-(III)(cis-TFSA)(5)](2-). Furthermore, the electrochemical analysis revealed the reduction process proceeded in two steps; {[}Nd-(III)(TFSA)(5)](2-) + e(-) -> {[}Nd-(II)(TFSA)(4)](2-)+ {[}TFSA](-) and {[}Nd-(III)(TFSA)(5)](2-) + 3e(-) -> Nd(0) + 5{[}TFSA](-). The diffusion coefficient of {[}Nd-(II)(TFSA)(4)](2-) was larger than that of {[}Nd-(III)(TFSA)(5)](2-) on the range of whole temperature. The activation energies of {[}Nd-(II)(TFSA)(4)](2-) and {[}Nd(III)(TFSA)(5)](2-) were 29.5 kJ mol(-1) and 49.8 kJ mol(-1), respectively. (C) The Author(s) 2017. Published by ECS. All rights reserved.