Ortho-phosphite (PO33−): Mechanochemical Synthesis of a Missing Oxoanion and Precursor to Value-Added Organophosphorus Compounds

Despite the ubiquity of the well-known phosphorus polyanion phosphite <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>(</mml:mo> <mml:msubsup><mml:mrow><mml:mi>HPO</mml:mi></mml:mrow> <mml:mrow><mml:mn>3</mml:mn></mml:mrow> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> <mml:mo>)</mml:mo></mml:math> , it appears to be the case that no simple salt of the corresponding conjugate base <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>(</mml:mo> <mml:msubsup><mml:mrow><mml:mi>PO</mml:mi></mml:mrow> <mml:mrow><mml:mn>3</mml:mn></mml:mrow> <mml:mrow><mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> , "ortho-phosphite") has ever been reported. We report the synthesis and characterization of this elusive species as a major component of a mixture obtained upon mechanochemical reduction of condensed phosphates, as evidenced by solid-state ³¹P NMR and Raman spectroscopy as well as subsequent reactivity studies. To corroborate the ³¹P NMR spectroscopic assignment, we independently generated Na₃PO₃ and K₃PO₃ by deprotonation of Na₂HPO₃ with NaCH₂SiMe₃ and of K₂HPO₃ with KCH₂Ph, respectively, providing an orthogonal route to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>PO</mml:mi></mml:mrow> <mml:mrow><mml:mn>3</mml:mn></mml:mrow> <mml:mrow><mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> salts whose spectroscopic signatures match those observed in the mixture obtained by mechanochemical reduction. We further found that ortho-phosphite can act as a precursor for various phosphorus chemicals, such as P-(OSiMe₃)₃ (46%), which is already well established as a precursor to a plethora of useful organophosphorus compounds. Therefore, our results not only establish the first formal pathway from P-(V) phosphate starting materials to P-(OSiMe₃)₃ without the intermediacy of white phosphorus, but also open the door to a broad range of downstream transformations based on this sustainable pathway. Additionally, BaHPO₃·H₂O (66%), OP-(OMe)₂Me (DMMP), and OP-(OBn)₂Bn (DBBP) have been generated directly from ortho-phosphite, all traditionally synthesized from white phosphorus.