Another Variation On P–N Chemistry

Main-group inorganic chemists have a fascination with mixing and matching elements to see how many stable combinations they can come up with. In one of the latest examples, Christian Hering, Axel Schulz, and Alexander Villinger of the University of Rostock, in Germany, have isolated and characterized the first example of an azidophosphenium cation. The molecule has an azide group attached to a two-coordinate phosphorus atom (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201201851). The Rostock team treated the Lewis base [(CH₃)₃Si]₂N–PCl₂ with the Lewis acid GaCl₃ at low temperature to obtain the intermediate phosphenium cation [(CH₃)₃Si]₂N=PCl⁺. Subsequent reaction of the intermediate with (CH₃)₃SiN₃ led to the azidophosphenium salt. These compounds aren’t stable. The intermediate decomposes at room temperature to form a new type of ionic liquid, [(CH₃)₃SiN≡P][GaCl₄]. The azidophosphenium salt is stable in solution only below –40 °C, and as a solid it must be stored in an inert atmosphere at freezing temperatures. The researchers tracked all the reactions using variable-temperature ³¹P NMR spectroscopy and characterized the isolated compounds using low-temperature single-crystal X-ray analysis.