Sustainable Production of Reduced Phosphorus Compounds: Mechanochemical Hydride Phosphorylation Using Condensed Phosphates as a Route to Phosphite

Phosphorus removal and recovery technologies have been implemented to tackle the anthropogenic eutrophication caused by phosphate runoff into waterways. In pursuit of a better utilization of the phosphates recovered from waste water treatment, we herein report that condensed phosphates can be employed to phosphorylate hydride reagents under solvent-free mechanochemical conditions to furnish phosphite (HPO3)2−, a versatile chemical with phosphorus in the +3 oxidation state. Hydride phosphorylation, as a two-electron one-proton reduction of a main group element oxide, constitutes a direct parallel with CO2 reduction to formate. Using potassium hydride as the hydride source, sodium trimetaphosphate (Na3P3O9 ), triphosphate (Na5P3O10), and pyrophosphate (Na4P2O7) engendered phosphite in 44, 58, and 44% yields based on total P content, respectively, under their optimal conditions. Formation of overreduced products including hypophosphite (H2PO2−) was identified as a competing process, and mechanistic investigation revealed that hydride attack on in situ generated phosphorylated phosphite species is a potent pathway for overreduction. The phosphite generated from our method could be easily isolated in the form of barium phosphite, a useful intermediate for production of phosphorous acid. This method circumvents the need to pass through white phosphorus (P4) as a high energy intermediate and mitigates involvement of environmentally hazardous chemicals. A bioproduced polyphosphate from baker’s yeast was demonstrated to be a viable starting material for the production of phosphite. This example demonstrates the possibility of accessing reduced phosphorus compounds in a more sustainable manner, and more importantly, closing the modern phosphorus cycle.