Coordination Environment Modulation of Laser Sensitivity in Copper(II) APTTz Energetic Complexes: From Aqua to Perchlorate Ligation

Abstract Coordinated water is a major obstacle in the design of high-performance energetic coordination complexes because inert aqua ligands lower energy density and suppress laser sensitivity. Herein, we report a simple strategy for removing coordinated water by changing the Cu:ligand (6-(1H-pyrazol-1-yl)-[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazin-3-amine, APTTz) molar ratio from 1:1 to 5:1 during crystallization, which drives thermodynamic displacement of coordinated water by perchlorate anions. This yields hydrated ECP-1 ([Cu(APTTz)2(H2O)2](ClO4)2) with outer-sphere perchlorate at 1:1 and anhydrous ECP-2 ([Cu(APTTz)2(ClO4)2]) with inner-sphere perchlorate at 5:1. The structural transformation enhances laser initiation performance, with the ignition delay time and threshold both reduced by 58.6%. Both complexes reliably detonate HMX and penetrate 5 mm lead plates. Theoretical analysis reveals that water removal lowers crystal symmetry (P-1 → Pn), narrows the band gap (0.56 eV → 0.34 eV), and transforms the lowest-energy excitation from a symmetry-forbidden local excitation into a strongly allowed metal-to-ligand charge-transfer transition. Coordinated water is identified as a parasitic hole acceptor, while perchlorate acts as a structural enabler. This work reveals a trade-off between laser sensitivity and detonation performance, guiding the rational design of next-generation laser-ignitable energetic materials.