Air-stability of aqueous solutions of (HOCH2)3P and (HOCH2CH2CH2)3P

The air-stabilities of buffered solutions of (HOCH2)3P [tris(hydroxymethyl)phosphine, THP] and the hydroxypropyl analogue [HO(CH2)3]3P (THPP) under ambient conditions are reported. Both phosphines have a stability zone where no decomposition or oxidation is observed: THP (of industrial importance) is stable between pH 7 and 8 for 7 days, while THPP is stable at pH 10–12 for 3 days. At pH <7, THP disproportionates to bis(hydroxymethyl)phosphine (HOCH2)2PH = BHP) and the tetrakis(hydroxymethyl)phosphonium cation (HOCH2)4P+), while the rate of aerial-oxidation of THP and of BHP then increases with decreasing pH. At pH >10, a known, base-promoted oxidation of THP takes place, and the rate of this increases with increasing pH; in a 0.1 N NaOH solution, THP is oxidized to the phosphine oxide within ∼10 min with concomitant generation of H2. Aerial oxidation of THPP is maximized at pH ∼8, while at lower pH the oxidation is slower due to conversion of THPP into the phosphonium cation [HO(CH2)3]3PH+; at pH ⩽4, no oxidation products are detected, while at pH >12, the base-promoted oxidation of THPP takes place.

Graphical abstractThe illustration (R = CH2OH) shows the behaviour of aqueous solutions of (HOCH2)3P at various pHs in the presence of air; conditions have been found for air-stability, and reactivity that includes oxidation by O2 or OH− to phosphine oxides, and disproportionation to R2PH and R4P+.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Water-soluble, commercial phosphine, (HOCH2)3P (THP). ► THP in homogeneous catalysis and metal-based drugs. ► THP solution, stable at pH 7.8. ► THP solution – decomposition products.