Direct NMR Evidence of Distinct Cation-CO₂ Adducts in Asymmetric Phosphonium-Based Aprotic N-Heterocyclic Anion (AHA)...

We report direct evidence of distinct cation-CO₂ adduct formation in ionic liquids (ILs) based on aprotic N-heterocyclic anions (AHAs), using a custom-designed in situ NMR (iNMR) method under conditions verified to achieve equilibrium speciation. These complexes are in addition to the primary anion-CO₂ carbamate product. ¹³C and ³¹P NMR spectra of three triethyloctylphosphonium ([P₂₂₂₈]⁺) AHA ILs all reveal two distinct carboxylate species, resulting from regioselective CO₂ addition at both α-carbons of the asymmetric cation. The product ratios vary with temperature, and DFT-predicted pK_a values confirm subtle differences in α-proton acidity. In contrast, a symmetric tetrahexylphosphonium ([P₆₆₆₆]⁺) analog forms only a single cation-CO₂ adduct, highlighting that multiple-site reactivity arises from chemical differentiation of α-positions rather than asymmetry alone. Surprisingly, ammonium and guanidinium-based AHA ILs previously considered completely inert toward carboxylation also exhibit weak but reproducible cation-CO₂ signals, detected only under undiluted, isotopically enriched conditions. These findings expand the mechanistic understanding of AHA IL-CO₂ chemistry and demonstrate that cation structure, particularly asymmetry and proton acidity, critically governs reactivity. The results provide a framework for designing ILs with tunable CO₂ activation profiles for selective capture and release.