RESEARCH STUDY: REACTING METAL BIS(TRIMETHYL)AMIDES WITH DOUBLE-BASE PROPELLANT STABILIZERS

Abstract

During World War II, it was discovered that when lead was added to double-base propellants, it produced beneficial burn rate phenomena. Specifically, the propellant burn rate first increased unexpectedly at low pressures, then the burn rate became independent of pressure, followed lastly by “mesa burning” where the burn rate actually decreased with increasing pressure. This results in a beneficial negative feedback mechanism. Over the past 75 years, researchers have explored different lead complexes to achieve better propellant performance. However, over the last decade, research has shifted to finding an alternative to using lead as an additive to reduce toxicity. Until the attempts detailed herein, researchers had not, to our knowledge attempted to combine double-base propellant stabilizers with various metals to achieve these desired results. In doing so, we prepared two lead complexes, Tetrakis (µ3-(4-methyl-3-nitrophenyl imido lead (II))) 1, and Bis(dinitrophenyl imido lead(II)) 2, that were synthesized by reacting lead bis(trimethylsilyl)amide with a common double-base propellant stabilizer 2-nitrodiphenylamine (NDPA) and 4-methyl-3-nitroaniline. Both complexes formed from protolysis of the trimethylsilylamide ligand by the acidic proton of the amine, and crystallized from tetrahydrofuran (THF). Bomb calorimetry coupled with crystal density structure determined that 1 has a very high energy density of 74.1 MJ/L, more than three times the energy density of conventional nitroamine explosives, whereas 2 was lower at 38.2 MJ/L. The structure, charge and characterization of 1 and 2 are discussed. However, each complex is air sensitive making burn rate experimentation infeasible, so any possible changes to the propellant as an additive remained undetermined. Attempts to use of tin, zinc, or bismuth bis(trimethyl)amides in place of lead, were unsuccessfully characterized, although reactions were likely observed.