Understanding the Complexation of Alkyl-Substituted Nitrilotriacetamides with Uranium: A Study by Absorption Spectroscopy and Microcalorimetry

Complexation thermodynamics of UO₂²⁺ ions with a series of alkyl-substituted nitrilotriacetamides (NTA) was investigated by absorption spectroscopy and microcalorimetry. The hexamethyl derivative of NTA (HMNTA) forms the weakest two successive complexes with UO₂²⁺ ions with stability constants of log β₁₁ = 3.5 ± 0.1 and log β₁₂ = 6.1 ± 0.1. The formation constant values increased linearly with increasing alkyl chain length of the substituents from hexamethyl NTA to hexabutyl NTA (HBNTA) and to hexahexyl NTA (HHNTA). The complexation with each ligand was both enthalpy and entropy driven with exothermic enthalpy changes of ΔH₁₁ = −14.7 ± 1.0 kJ/mol, ΔH₁₂ = −10.2 ± 0.8 kJ/mol for HMNTA, ΔH₁₁ = −19.2 ± 1.2 kJ/mol, ΔH₁₂ = −16.4 ± 1.1 kJ/mol for HBNTA, and ΔH₁₁ = −21.3 ± 1.4 kJ/mol, ΔH₁₂ = −19.4 ± 2.3 kJ/mol for HHNTA. Similarly, the positive entropy changes with each ligand were ΔS₁₁ = 18.1 ± 2.7 J/mol/K, ΔS₁₂ = 82.9 ± 3.8 J/mol/K for HMNTA, ΔS₁₁ = 14.4 ± 1.2 J/mol/K, ΔS₁₂ = 87.2 ± 4.2 J/mol/K for HBNTA, and ΔS₁₁ = 16.1 ± 2.4 J/mol/K, ΔS₁₂ = 92.6 ± 3.1 J/mol/K for HHNTA. Structural features of the complex suggest the participation of two ligands coordinating in a bidentate mode via the carbonyl oxygens. The [UO₂L₂]²⁺ complexes appear to be noncentrosymmetric with two ligands and one water molecule occupying the equatorial plane of the dioxo uranyl cation. The structure of the complex was confirmed by ¹H NMR titration, EXAFS measurements, and DFT calculations.