Syntheses and properties of energetic cyclo-pentazolate cocrystals

As a new type of polynitrogen species that is stable at room temperature, 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 includes metal-N5− solvates, cocrystals composed of metal-N5− compounds and amines/MSM, and metal containing composite salts; The cocrystals of nonmetallic pentazolate salts includes 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 cocrystals of cyclo-N5− salts and high-density, high oxygen balance, high-energy oxidizer; and exploration of formation mechanism.