Alkylene-functionality in bridged and fused nitrogen-rich poly-cyclic energetic materials: Synthesis, structural diversity and energetic properties

From the standpoint of chemical structures, the organic backbones of energetic materials can be classified into aromatic rings, nonaromatic rings, and open chains. Although the category of aromatic energetic compounds exhibits several advantages in the regulation of energetic properties, the nonaromatic heterocycles, assembling nitramino explosophores with simple alkyl bridges, still have prevailed in benchmark materials. The methylene bridge plays a pivotal role in the constructions of the classic nonaromatic heterocycle-based energetic compounds, e.g., hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), whereas ethylene bridge is the core moiety of state-of-the-art explosive 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20). In this context, it is of great interest to employ simple and practical bridges to assemble aromatic and nonaromatic nitrogen-rich heterocycles, thereby expanding the structural diversity of energetic materials, e.g., bridged and fused nitrogen-rich poly-heterocycles. Furthermore, alkyl-bridged poly-heterocycles highlight the potential for the open chain type of energetic materials. In this review, the development of alkyl bridges in linking nitrogen-rich heterocycles is presented, and the perspective of the newly constructed energetic backbones is summarized for the future design of advanced energetic materials.