A novel five-coordinate Cu(II)-based primary explosive engineered by positional isomerism for laser ignition

The development of green primary explosives that combine high energy with low sensitivity represents a central challenge in the field of energetic materials. This paper reports a novel energetic complex, Cu(1-MPCA)₂(H₂O)(ClO₄)₂, based on 1-methyl-1H-pyrazole-4-carbohydrazide (1-MPCA). Single-crystal X-ray diffraction analysis reveals that the Cu(II) center in this complex adopts a rare square-pyramidal five-coordinate geometry, in sharp contrast to its isomeric analogue based on 3-methyl-1H-pyrazole-4-carbohydrazide (3-MPCA), which exhibits a classic octahedral structure. Theoretical calculations indicate that the Jahn–Teller distortion induced by the five-coordinate geometry, along with the altered intermolecular hydrogen-bonding network, negatively impacts sensitivity characteristics, leading to nonlinear differences in macroscopic performance between the two complexes. Consequently, Cu(1-MPCA)₂(H₂O)(ClO₄)₂ displays lower thermal stability (T_d = 157 °C) and higher mechanical sensitivity (IS = 0.8 J, FS < 5 N). However, this declining trend does not extend to energy-related properties such as laser-ignition capability (E_l = 9 mJ) and detonation performance. This study clearly demonstrates that the precise position of a substituent in energetic complex can decisively influence the coordination geometry of the metal center, thereby exerting a far-reaching impact on the overall performance balance. These findings provide key insights for the rational design of high-performance energetic materials through “positional isomeric engineering”.