High-Pressure Synthesis of Cyclic Phosphazenes by Near-UV Photoinduced Reactivity of NH3 and Elemental Phosphorus

A comparison between the high-pressure (0.8 GPa) photoinduced reactivity of black and red phosphorus at ambient temperature in the presence of ammonia has been conducted in diamond anvil cells (DACs), using spectroscopy (IR and Raman) and X-ray diffraction (XRD). Reactivity has been triggered exploiting the two-photon absorption of near-UV radiation by ammonia. The infrared characterization showed a very complex reactivity in the case of red phosphorus, proceeding to a much higher extent with respect to the black allotrope. Furthermore, Raman spectra showed the formation of molecular hydrogen and phosphine besides three different solid products. Whereas one of them is air sensitive, the other two are recoverable at ambient conditions. IR, Raman, and XRD data for the obtained products have been compared to those acquired on known HxPyNz compounds: for one of the two stable products, a fair matching was found with the XRD pattern and the IR spectrum of P3N3(NH2)6·(NH3)0.5, whereas for the other one only the functional groups actually involved in the structure could be evinced from accurate Raman mapping of the sample, with no further information about composition or stoichiometry. High density conditions in combination with near-UV laser irradiation were thus proved to be effective in the formation of two stable reaction products featuring new P-N functionalities, both recoverable at ambient pressure. For the first time, a cyclic triphosphazene has been synthesized through the reaction of red phosphorus and ammonia triggered by UV light under moderate high-pressure conditions, possibly opening new perspectives about this topic.