Title: Synthesis and Characterization of the Phosphorus Triazides OP(N₃)₃ and SP(N₃)₃

Authors:  Xiaoqing Zeng, Eduard Bernhardt, Helmut Beckers, and Helge Willner

Journal:  Inorganic Chemistry

Affiliation:  FB C − Anorganische Chemie, Bergische Universität, Wuppertal, Germany

Take-Home Importance according to the Authors:  Two explosive triazides of phosphorus(V), OP(N₃)₃ and SP(N₃)₃, have been prepared as neat substances and structurally characterized.
Both compounds can be handled in gas, liquid, and solid states in submillimolar quantities. Their single-crystal structures were obtained by X-ray diffraction and found to be significantly distorted from the predicted ideal C3 symmetry because of intermolecular interactions. The spectroscopic and structural properties are discussed in combination with density functional theory calculations.

Take-Home Importance according to the Blogger:  As chemists have successfully synthesized two very energetic phosphorus-azide compound, just want to emphasize that simple things are difficult to do sometimes. Kudos.

Summary: Organic azide compounds are known to be thermally unstable and sometimes extremely heat- or shock- sensitive. In 1975, Dr. Wolfgang Buder and Dr. Armin Schmidt at the University of Stuttgart isolated P(N₃)₃, OP(N₃)₃, and P(N₃)₅ by reacting sodium azide with the respective PCl_xO_y salts (DOI: 10.1002/zaac.19754150310). The compounds were characterized in concentration solutions by IR, Raman, and ³¹P NMR. From these techniques, deduction of molecular species’ structure is not difficult. Since these molecules are already in solution, handling may be much easier than their neat state, which SP(N₃)₃ in acetonitrile has been reported to be stable. A concept mentioned in the previous post, safe handling of unstable materials often involve in manageable small quantity, such as a dilute solution.

The lack of stability in azides often springs from the bond cleavage that will form nitrides and very thermodynamically favorable nitrogen gas. As the product is a fairly inert gas, the back reaction equilibrium constant is tiny and the kinetics must be very fast. If a compound rapidly produces gas and expands in volume, the most simple case is an explosion. So the isolation and the consequent crystallography of these materials demand advanced techniques and of course very steady and careful hands. The authors have warned about ampoule-key techniques for standard manipulations of volatile  materials that those typical handling may produce explosions upon mechanical stress.

Now, the synthetic scheme of these molecules is straightforward that the solvent (acetonitrile) and reagent (OPCl₃ or SPCl₃) are directly condensed into reaction vessel containing dry NaN₃ under liquid nitrogen (L.N.). By warming the reaction flask and stirring, reactions occur without external source of energy input. If you have read about the experimental setup, you might wonder why a simple Schlenk line is fitted with not one, but three U traps. Traps on the Schlenk line are usually used as a filtration device that prevent volatile or gaseous compounds from entering the vacuum source, typically a vacuum pump. The trap condense the materials that would otherwise damaging the internal parts of a vacuum pump by lowering the temperature and causing phase changes. The decreasing volume via cooling also often results in a better vacuum. However, these traps serve different functions. By setting traps at different temperatures ( –30, –100, –196 degrees Celsius in the case of OP(N₃)₃), product, unreacted starting materials and solvent, and noncondensed gases can be separated by different vapor pressures, as the reaction flask was warmed to room temperature slowly.

Using similar warming-cooling concept, Raman spectroscopy was performed to further confirm the product after gas phase infrared spectroscopy. Product in an argon matrix was deposited Rh-plated Cu block for infrared spectroscopy. In all cases, the measured vibrational frequencies matched up well with calculations. The flexibility in the azide moieties in these molecules has cause significant symmetry distortion evident in the spectroscopic characterization. Finally, crystal structure was obtained by slow-warming and condensing of the product. The crystal mounting process was a bit complicated, since those molecules melt at room temperatures and they are not quite stable.

So overall, the chemical concept presented in this paper is a simple metathesis reaction that generated NaCl and the chalcogenide-phosphoryl azide. However, to get there, specialized equipments and great techniques were necessary. The group have progressed very well, from the synthesis of O₂S(N₃)₂ to recent isolation of small molecules OPN and ONP. There are many more simple compounds in the family that are also quite unstable. However, skills of good chemists make their isolation no longer just wishful thinking.