Electronic Pre-Activation of -COOH Ligands by O-H···O in Natural Deep Eutectic Solvents for Enhanced Rare-Earth...

Natural deep eutectic solvents (NaDESs) are commonly regarded as passive solvation media. However, their local hydrogen bonds may act as electronically structured environments that can perturb embedded ligands. Herein, using quantum-chemical analysis, finite-cluster calculations suggest that selected O-H···O motifs can polarize RCOOH in a manner consistent with partial RCOO- like redistribution. This effect is resolved through complementary analyses of the structural, vibrational, density-based, orbital, and magnetic descriptors, including the H-bond metrics, DFT-IR, NCI/RDG, and ETS-NOCV charge-transfer channels, and the 17O NMR responses. Among the designed finite-cluster models, the ternary NaDES induced substantially stronger preactivation than the binary system, in which ternary menthol:borneol:benzoic acid (M:Bo:BA) had the strongest overall response across the complementary descriptors. To probe possible coordination consequences of this finite-cluster polarization, tris(carboxylate) complexes of La3+, Pr3+, and Eu3+ (κ2(O,O') were used as model systems. Analysis of the energetics indicated that the NaDES-induced ligand preorganization was consistent with the enhanced intrinsic propensity of the lanthanide ions for Ln-O coordination under the given dielectric conditions. The results support local H-bond-induced ligand preactivation as a finite-cluster mechanistic hypothesis for tuning coordination-relevant donor polarization in NaDES and provide a quantum-chemical basis for evaluating coordination-relevant donor polarization in selected NaDES-derived local motifs.