Intriguing structural chemistry of neutral and anionic layered monoalkylphosphates: single-source precursors for high-yield ceramic phosphates

Abstract

Building up on an available synthetic methodology, phosphate monoesters ROPO₃H₂ have been synthesized in good yields. The synthetic procedure employed features acetic anhydride mediated activation of phosphoric acid in the presence of alcohols, leading to the formation of phosphate monoesters. The products have been isolated as their cyclohexyl amine salts, [CyNH₃]₂[(MeO)PO₃]·3H₂O (1) and [CyNH₃][(RO)PO₃H] (Cy = cyclohexyl; R = Et (2), iPr (3), or tBu (4)). Neutralization of 1–4 by readily available inexpensive ion exchange resin Amberlite produces monoalkylphosphates (RO)P(O)(OH)₂ (R = Me (5), Et (6), iPr (7), or tBu (8)). Thermally labile 1–4 and 7 have been structurally characterized by single crystal X-ray diffraction studies. Due to their intrinsic thermal instability due to β-H elimination, these compounds can be used as ligands for the preparation of single-source precursors for ceramic phosphates by reacting them with suitable metals ions. It is also possible to isolate co-crystals of the anionic and neutral forms of these phosphates as it has been demonstrated in the isolation and structural characterization of [(iPrO)PO₃H₂]·{[CyNH₃][(iPrO)PO₃H]} (9). To demonstrate the utility of these monoalkylphosphates in the low-temperature synthesis of metal phosphate bioceramics, isopropyl phosphate 7 has been employed to prepare calcium phosphate [{Ca((iPrO)PO₃)(OH₂)}·H₂O]_n (10), which undergoes neat thermal decomposition in two stages to lose water and propene to yield β-Ca₂P₂O₇ at low temperatures (280 °C).