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. β-H elimination
and direct attack at the phosphorus atom are the most common degradation
pathways observed in an alcoholic solvent, while α-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.