DEVELOPMENT OF NOVEL HIGH-ENERGY-DENSITY MATERIALS THROUGH METAL-MEDIATED COVALENT LINKAGES OF FUEL TO OXIDANT

Abstract

The main goal of this work is to synthesize novel high-energy density materials (HEDMs) that possess a balance of high heats of decomposition with kinetic stability. In contrast to the standard approach in the field of energetics research, we are looking into the inclusion of non-CHNO atoms (in particular, transition metals) into energetic materials. In general, the inclusion of metals into energetic materials is seen as “diluting” the energy density since most of the energy is derived from the formation of hot, lightweight gaseous molecules such as N2, CO2, and H2O whereas metals typically form solid metal oxides upon combustion. However, we propose the replacement of high energy ionic bonds with covalent-like bonds, through protolysis of acidic, amine containing fuels with metal bis(amides), will result in the decrease in lattice energy of the material and thus increase the energy output. To this end, a heterocubane cluster of 1,3-di-tert-butyltetrazolium-5-imidoperchlorato-manganese (II) was synthesized through protolysis of 5-amino-1,3-di-tert-butyltetrazolium perchlorate with Mn(N(SiMe3)2)2. Combustion analysis of the amorphous, solvent-free phase gives an energy density of at least 40 ± 3 MJ/L, higher than hydrocarbon combustion in air, more than twice that of the current state-of-the-art high-energy materials, and 86% higher than that of the tetrazolium perchlorate salt without metal linkage. Similarly, the reaction of 1-tert-butyl-3-methyltetrazolium perchlorate with Mn(N(SiMe3)2)2 results in a hexagonal prismatic M6N6 cluster of bridging tetrazolimido ligands and terminal perchlorates. The replacement of the 1-position tert-butyl group with a methyl group results in a detonatable material when heated rapidly using an incandescent wire. Combustion analysis of the amorphous solid again shows an increase in energy-density in comparison with the tetrazolium perchlorate salt. Further analysis of the decomposition event was performed through temperature jump time-of-flight mass spectrometry (T-jump TOF-MS) with varying heating rates to determine the activation energy for decomposition. In an effort to replace perchlorate due to its cytoxicity, periodate salts of the aforementioned ligands were synthesized. Periodate and perchlorate anions possess similar oxidative potentials however, periodate is more environmentally benign and therefore, could result in “greener” energetics. However, due to the lability of the iodine-oxygen bond, only undesirable, redox reaction byproducts have been isolated. Lastly, a series of multinuclear clusters were prepared using neutral 5-amino-tetrazole precursors rather than perchlorate or periodate salts. Reactions of Mn(N(SiMe3)2)2 with 1,3-di-tert-butyltetrazolium-5-aminide result in dimers Mn2(μ-NtBu2Tz)2(NR2)2L2 or Mn2(μ-NtBu2Tz)2(NR2)2 depending on the solvent used. Reaction of Mn(N(SiMe3)2)2 with 5-amino-2-tert-butyltetrazole results in a hexamanganese cluster Mn6(μ3-NtBuTz)2(μ2-NHtBuTz)4(NR2)4.