Thermodynamics of lanthanide and uranyl complexes with tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA)

Abstract

We present the results of an investigation of the thermochemistry of the complexation of La³⁺, Nd³⁺, Eu³⁺, Dy³⁺, Tm³⁺, and UO₂²⁺ by tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA). This predisposed structural analog to oxydiacetic acid (ODA) has been previously shown both to exhibit greater sensitivity to lanthanide cation radius than complexes with the unconstrained ODA and to form anomalously weak complexes with UO₂²⁺. Our purpose is to interpret these observations in terms of the balance between enthalpy and entropy contributions to the overall complexation thermodynamics. Enthalpies have been calculated from titration calorimetry experiments both for the protonation of the free ligand and for the formation of selected 1∶1 and 1∶2 complexes in pH 2–3 acidic media (I = 0.1 M). The complexation entropies for the lanthanide complexes have been calculated using the previously reported stability constants for the MH₂L⁺, MHL, and M(H₂L)²⁻. The stability constants for the uranyl complexes have been determined by potentiometric titration and these values used to calculate the thermodynamic parameters. Complexation enthalpies for the 1∶1 lanthanide–THFTCA complexes (MH₂L⁺ species) are nearly identical to those of the lanthanide ODA complexes. Therefore, the size-selectivity observed in the lanthanide–THFTCA complexes arises from the complexation entropy. The comparative weakness of the uranyl complexes with THFTCA also is accounted for thermodynamically in the entropy term. Calculations based on an electrostatic model for complexation entropy and molecular mechanics modeling are used to help interpret the experimental results.