3D Energetic Metal–Organic Frameworks: Synthesis and Properties of High Energy Materials

Abstract

Metal–organic frameworks (MOFs) have attracted great attention because of their intriguing molecular topologies and potential applications in chemical separation, gas storage, drug delivery, catalysis and chemical sensor technology. Particularly, MOFs could also be potential energetic materials because of their high densities and high heats of detonation. For example, Hope-Weeks and co-workers recently reported two hydrazine-perchlorate 1D MOFs [(Ni(NH2NH2)5(ClO4)2)n (NHP), and (Co(NH2NH2)5(ClO4)2)n (CHP)] with linear polymeric structures, which were regarded as possibly the most powerful metal-based energetic materials known to date, with heats of detonation comparable with that of hexanitrohexaazaisowutzitane (CL-20; about 1.5 kcalg ). Unfortunately, these coordination polymers were highly sensitive to impact deriving from their low rigidity characteristic of such linear polymeric structures, which makes practical use infeasible. In order to decrease the sensitivities, the same authors also used a hydrazine derivative (hydrazine-carboxylate) as the ligand to construct MOFs with 2D sheet structures [((Co2(N2H4)4(N2H3CO2)2)(ClO4)2·H2O)n (CHHP) and ((Zn2(N2H4)3(N2H3CO2)2)(ClO4)2·H2O)n (ZnHHP)], which showed a considerable reduction to the sensitivity, however, concomitantly their heats of detonation decreased (Figure 1). Despite these advances, current coordination frameworks are only limited to be a 1D or 2D structure. Compared with 1D linear and 2D layered structures, three-dimensional (3D) frameworks possess more complicated connection modes, which could further enhance structural reinforcement, hence improve the stabilities and energetic properties. A lot of 3D MOFs have been synthesized with interesting magnetic, catalytic, and luminescent properties, some of them incorporate a variety of energetic moities such as nitrate anions (NO3 ), perchlorate anions (ClO4 ) into the 3D frameworks. However, their potential applications as energetic materials have not been disclosed or discussed; relevant data about energetic properties are also missing in the literature. Additionally, both reported 1D and 2D energetic MOFs based on the perchlorate anions, have been scrutinized by the US Environmental Protection Agency (EPA) because they promote thyroid dysfunction and are teratogenic. Continuing our interest in finding new highly energetic, eco-friendly energetic materials, we explore the preparation of halogen-free energetic 3D MOFs, for which two polymers [Cu(atrz)3(NO3)2]n (1) and [Ag(atrz)1.5(NO3)]n (2) were designed by replacing the hydrazine ligand with 4,4’-azo1,2,4-triazole (atrz). Here, we chose to use atrz as a ligand for the following reasons: 1) as a nitrogen-rich heterocyclic backbone, atrz possesses a high nitrogen content (N%= Figure 1. Energetic MOFs with different topologies.