Rapid Analysis of Tetrakis(dialkylamino)phosphonium Stability in Alkaline Media

Hydroxide-stable organic cations are crucial components for ion-transport processes in electrochemical energy systems, and the tetrakis(dialkylamino)phosphonium cation is a promising candidate for this application. These phosphoniums are known to be highly resistant to alkaline media; however, very few investigations have systematically evaluated how these cations decompose in the presence of hydroxide or alkoxide anions. The excellent stability of several tetraaminophosphoniums in 2 M KOH/CH3OH at 80 C led us to design experiments for the rapid assessment of phosphonium degradation in homogeneous solution and under phase-transfer conditions. The analysis illustrated how substituents around the cation core affect both degradation pathways and rates. beta-H elimination and direct attack at the phosphorus atom are the most common degradation pathways observed in an alcoholic solvent, while alpha-H abstraction and direct attack are observed under phase-transfer conditions (PhCl and 50 wt % NaOH/H2O). The collected data provided a relative stability comparison for this family of cations to enable future design improvements and illustrated the utility of using multiple tests for degradation studies.