Synthesis and properties of transition metal complexes based on cyanoborohydride anions as green hypergolic fuels

Exploring novel hypergolic complexes for propulsion is currently a research hotspot. However, many hypergolic complexes suffer from complex synthesis routes and long ignition delays. To enhance the performance of hypergolic ignition of complexes, this study constructed four types of high-energy complexes utilizing three transition metal ions (Mn, Ni, Cu) as the central components, alongside cyanoborohydride (CBH) anions, pyridine, and its derivatives. Cu(Py) 2 (CBH) 2 (compound 1), Mn(VPy) 2 (CBH) 2 (compound 2), Cu(BPy) 2 (CBH) 2 (compound 3) and Ni(BPy) 2 (CBH) 2 (compound 4) were comprehensively characterized. The results showed that all the four prepared complexes have high stability and high energy density (compound 2, E g  = 30.08 kJ·g −1 and E v  = 32.15 kJ·cm −3 ). The densities of complexes1-4, ranging from 1.069 to 1.367 g·cm −3 , are significantly higher than those of hydrazine fuels. To ascertain the factors influencing hypergolic activity, a comparative study was conducted on a series of combinations with analogous structures but varying metal elements and ligands. Experimental analysis, along with theoretical calculations, indicate that the synergistic interaction between the metal center and ligand plays a crucial role in adjusting the hypergolic performance of the compounds 1–4. Their suitable physicochemical properties for hypergolic reactions, in addition to excellent ignition performance, highlight the considerable potential of these high-energy complexes as hypergolic propellant fuels. This study provides fundamental insights for the advancement of new high-performance hypergolic fuels.