Joan Frances Brennecke

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.

Butyrobetaine bis(trifluoromethylsulfonyl)imide, ([Hbbet][Tf ₂ N]), is a systematic variation of the task-specific ionic liquid betaine bis(trifluoromethylsulfonyl)imide, ([Hbet][Tf ₂ N]). [Hbbet][Tf ₂ N] displays improved properties with lower water solubility and a wider electrochemical window while maintaining...

The identity of electrolyte cations is known to strongly influence electrocatalytic activity, but the relationship between their interfacial arrangement and observed performance remains poorly understood. Organic cations, with their molecular tunability, provide a powerful platform for systematically probing these effects. Here, we leverage phosphonium-based geminal dications to control interfacial cation arrangement and identify the variables that most strongly influence catalytic rates. As a case study, we examine CO₂ reduction to CO over polycrystalline silver electrodes in dry aprotic acetonitrile. Through a combination of rotating disk electrode measurements, electrochemical impedance spectroscopy, and molecular dynamics simulations, we decouple the effects of cation-electrode distance and interfacial cation density on catalytic rates. We find that smaller, more densely packed cations induce stronger interfacial electric fields, which lower the activation barrier for CO₂ adsorption and increase reaction rates. Using geminal phosphonium dications [C_n(P_mmm)₂][ClO₄]₂, we demonstrate that both the vertical and lateral positioning of organic cations within the electrical double layer independently affect reactivity. These results demonstrate that electrolyte cation identity primarily influences catalytic kinetics by determining how efficiently charge can be arranged at electrochemical interfaces. Overall, our findings support an electrostatic view of cation effects in catalysis and provide design principles for next-generation electrolytes.

Carboxylate-based ionic liquids have been proposed as promising candidates for rare earth element (REE) separation. However, identifying the chemical basis for separation is challenging due to multiple complexes forming between the REEs and the carboxylate ligands. We developed two semi-quantitative methods to identify changes in speciation across the lanthanide series: (ⅰ) paramagnetic NMR (pNMR) that exploits lanthanide magnetic properties applying the Bleaney theory with the Evans method, and (ⅱ) scoring thermal decomposition curves obtained from thermogravimetric analysis (TGA) using dynamic time warping (DTW). Herein, we investigated Ln3+ speciation in aqueous protonated betaine bis(trifluoromethylsulfonyl)imide ([HBet][Tf2N]) ionic liquid systems using these methods in concert with cloud point temperatures of Ln3+ containing aqueous [HBet][Tf2N] systems and density function theory (DFT) calculations for isomerization energies between two known lanthanide/betaine complexes ([Ln2(Bet)4(H2O)8]6+ with coordination numbers of 8 and 9). Descriptors from pNMR and TGA classify solutions of lanthanides in aqueous [HBet][Tf2N] into three groups with structural transitions at Gd3+ and Er3+, consistent with the UCST trends. Small differences in energies for the complexes with heavy lanthanides suggest thermodynamics governs the exchange between multiple different complexes, whereas larger energy differences for the light lanthanides complexes kinetically controlled changes in speciation, according to the Bell-Evans-Polanyi relation. Thus, this multimodal framework provides unique insight into lanthanide speciation in complex carboxylate-based extraction systems.

The identity of electrolyte cations is known to strongly influence electrocatalytic activity, but the relationship between their interfacial arrangement and catalytic performance remains poorly understood. Organic cations, with their molecular tunability, provide a powerful platform for systematically probing these effects. Here, we leverage phosphonium-based geminal di-cations to control interfacial cation arrangement and identify the variables that most strongly influence catalytic rates. As a case study, we examine CO2 reduction (CO2R) to CO over polycrystalline silver electrodes in dry aprotic acetonitrile. Through a combination of rotating disk electrode measurements, electrochemical impedance spectroscopy, and molecular dynamics simulations, we decouple the effects of cation–electrode distance and interfacial cation density on catalytic rates. We find that smaller, more densely packed cations induce stronger interfacial electric fields, which lower the activation barrier for CO2 adsorption and increase reaction rates. Using geminal phosphonium di-cations, we demonstrate that both the vertical and lateral positioning of organic cations within the electrical double layer independently influence reactivity. These results demonstrate that electrolyte cation identity primarily influences electrocatalytic kinetics by determining how efficiently charge can be arranged at the electrocatalytic interface. These findings support an electrostatic view of cation effects in catalysis and provide principles for designing next-generation electrolytes.