Direct NMR Evidence of Distinct Cation‐CO ₂ Adducts in Asymmetric Phosphonium‐Based Aprotic N ‐Heterocyclic Anion (AHA) Ionic Liquids

We report direct evidence of distinct cation‐CO 2 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‐CO 2 carbamate product. 13 C and 31 P NMR spectra of three triethyloctylphosphonium ([P 2228 ] + ) AHA ILs all reveal two distinct carboxylate species, resulting from regioselective CO 2 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 ([P 6666 ] + ) analog forms only a single cation‐CO 2 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‐CO 2 signals, detected only under undiluted, isotopically enriched conditions. These findings expand the mechanistic understanding of AHA IL‐CO 2 chemistry and demonstrate that cation structure, particularly asymmetry and proton acidity, critically governs reactivity. The results provide a framework for designing ILs with tunable CO 2 activation profiles for selective capture and release.