Analysis of Thermodynamic Properties for Rare Earth Complexes in Ionic Liquids by Raman Spectroscopy and DFT Calculation

Abstract

The coordination states of the divalent and trivalent rare earth complexes in ionic liquid, triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl) amide [P 2225 ][TFSA] were investigated by Raman spectroscopy and DFT calculation. The concentration dependences of the deconvoluted Raman spectra were investigated for 0.23–0.45 mol kg - 1 RE(III), RE=Nd and Dy, and the mixed sample of RE(II)/RE(III)=1/3 at the molar ratio in [P 2225 ][TFSA]. According to the conventional analysis, the solvation number; n of rare earth complexes in [P 2225 ][TFSA] were determined to be n =4.06 for Nd(II), 5.01 for Nd(III), 4.12 for Dy(II) and 5.00 for Dy(III). Thermodynamic properties such as I” iso G , I” iso H and I” iso S for the isomerism of [TFSA] - from trans- to cis- isomer in bulk and the first solvation sphere of the centered [RE 3+ ] cation in [P 2225 ][TFSA] were evaluated from the temperature dependence in the range of 298-398K. I” iso G (bulk), I” iso H (bulk) and T I” iso S (bulk) at 298 K were -1.06, 6.86, and 7.92 kJ mol - 1 , respectively. The trans -[TFSA] - was dominant in the enthalpy due to the positive value of I” iso H (bulk) and T I” iso S (bulk) was slightly larger than I” iso H (bulk), so that cis -[TFSA] - was revealed to be an entropy-controlled in [P 2225 ][TFSA]. On the other hand, in the first solvation sphere of [RE 3+ ] cation, I” iso H (Nd)(-47.39 kJ mol - 1 ) increased to the negative value remarkably and implied that the cis -[TFSA] - isomers were stabilized for enthalpy. I” iso H (Nd) contributed to the remarkable decrease in the I” iso G (Nd) and this result clearly indicated that the cis -[TFSA] - bound to Nd 3+ cation was preferred and the coordination state of [Dy (III) ( cis -TFSA) 5 ] 2 - was stable in [P 2225 ][TFSA] The optimized geometries and the bonding energies of [RE (II) ( cis -TFSA) 4 ] 2 - and [RE (III) ( cis -TFSA) 5 ] 2 - clusters were also investigated from DFT calculation with ADF package. The bonding energy; I” E b was calculated from I” E b = E tot (cluster) – E tot (RE 2,3+ ) – nE tot ([TFSA] - ). I” E b ([Nd (II) ( cis- TFSA) 4 ] 2 - ), I” E b ([Nd (III) ( cis- TFSA) 5 ] 2 - ), I” E b ([Dy (II) ( cis- TFSA) 4 ] 2 - ) and I” E b ([Dy (III) ( cis- TFSA) 5 ] 2 - ) were -2241.6, -4362.3, -2135.4 and -4284.2 kJmol - 1 , respectively. This result was revealed that [RE (III) ( cis- TFSA) 5 ] 2 - cluster formed stronger coordination bonds than [Dy (II) ( cis- TFSA) 4 ] 2 - cluster. The average atomic charges and the bond distances of these clusters were consistent with the thermodynamic properties.