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, IPBIMCl, 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 IPBIMCl 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).
Zhi et al. (Fri,) studied this question.