Conformational Control in C2-Substituted Benzimidazolium-Based Ionic Liquids Enables Superior TcO ₄ – Separation from Simulated High-Level Liquid Waste

Selective extraction of pertechnetate (TcO₄⁻) from high-level liquid waste (HLLW) remains a formidable challenge due to intense competition from nitrate under harsh acidic conditions. Here, we introduce a rational design strategy based on conformational control in C2-substituted benzimidazolium ionic liquids to achieve unprecedented separation of TcO₄⁻. Through systematic steric tuning at the C2 position, single-crystal X-ray diffraction analysis reveals that the bulky phenyl-substituted extractant, [IPBIM]Cl, induces a conformational switch to a cis configuration. This contrasts with the trans arrangement observed in analogues with smaller substituents. NMR spectroscopy, supported by DFT-calculated chemical shifts, confirms that this conformational preference persists in solution. Thermodynamic analysis confirms this cis geometry acts as a preorganized binding site, evidenced by a significantly less negative entropy of extraction (ΔS = -2.35 J·mol⁻¹·K⁻¹), which minimizes the entropic penalty upon complexation. This preorganized structure enables optimal π-π stacking and electrostatic interactions, allowing [IPBIM]Cl to achieve distribution ratios DRe > 29 and DTc > 69 from simulated nuclear liquid waste containing 3 M HNO₃, a condition mimicking actual HLLW. Density Functional Theory (DFT) calculations further corroborate the superior binding affinity of the cis conformer (ΔE = -44.31 kJ·mol⁻¹) and its high selectivity over NO₃⁻ (ΔΔE = -16.38 kJ·mol⁻¹).