Syntheses and properties of energetic cyclo-pentazolate cocrystals

As a new type of polynitrogen species that is stable at room temperature, the pentazolate anion (cyclo-N₅⁻) has attracted much attention in the field of high-energy density materials, but its energy and stability are unbalanced. Cocrystallisation can balance their properties to some extent by forming new chemical compositions from existing cyclo-N₅⁻ compounds through non-covalent interactions. This article reviews the research progress of cyclo-N₅⁻ cocrystals in recent years, including synthetic methods, cocrystals of metal-N₅⁻ compounds, and cocrystals of nonmetallic pentazolate salts. The cocrystals of metal-N₅⁻ compounds mainly include metal-N₅⁻ solvates, cocrystals composed of metal-N₅⁻ compounds and amines/MSM, and metal-containing composite salts. The cocrystals of nonmetallic pentazolate salts include cocrystals composed of cyclo-N₅⁻ salts and solvents, cocrystals composed of cyclo-N₅⁻ salts and N-heterocyclic molecules, and non-metallic composite salts. The fascinating crystal structures (in some cases topological structures), stable forms, and physicochemical properties of representative cocrystals were highlighted. In addition, the future directions that need to be focused on in this field were pointed out, including the development of more preparation methods, especially those suitable for scaling up; higher precision calculation or testing of enthalpy of formation; improvement of their thermal stabilities; creation of cocrystals of cyclo-N₅⁻ salts and high-density, high-oxygen balance, high-energy oxidizers; and exploration of the formation mechanism.