Energetic Trinitro- and Fluorodinitroethyl Ethers of 1,2,4,5-Tetrazines

Probing the Reaction Mechanisms of 3,5-Difluoro-2,4,6-Trinitroanisole (DFTNAN) through a Comparative Study with Trinitroanisole (TNAN)

To probe the thermal decomposition mechanisms of a novel fluorinated low-melting-point explosive 3,5-difluoro-2,4,6-trinitroanisole (DFTNAN), a comparative study with trinitroanisole (TNAN) was performed under different heating conditions. The thermal decomposition processes and initial reactions were monitored by DSC-TG-FTIR-MS and T-jump-PyGC-MS coupling analyses, respectively. The results show that fluorine decreased the thermal stability of the molecular structure, and the trigger bond was transferred from the ortho-nitro group of the ether to the para-nitro group. The possible reaction pathway of DFTNAN after the initial bond breakage is the rupture of the dissociative nitro group with massive heat release, which induces the ring opening of benzene. Major side reactions include the generation of polycyclic compounds and fluorine atom migration. Fluorine affects the thermal stability and changes the reaction pathway, and fluorinated products appear in the form of fluorocarbons due to the high stability of the C-F bond.

Divergent Synthesis of Monosubstituted and Unsymmetrical 3,6-Disubstituted Tetrazines from Carboxylic Ester Precursors

Since tetrazines are important tools to the field of bioorthogonal chemistry, there is a need for new approaches to synthesize unsymmetrical and 3-monosubstituted tetrazines. Described here is a general, one-pot method for converting (3-methyloxetan-3-yl)methyl carboxylic esters into 3-thiomethyltetrazines. These versatile intermediates were applied to the synthesis of unsymmetrical tetrazines through Pd-catalyzed cross-coupling and in the first catalytic thioether reduction to access monosubstituted tetrazines. This method enables the development of new tetrazine compounds possessing a favorable combination of kinetics, small size, and hydrophilicity. It was applied to a broad range of aliphatic and aromatic ester precursors and to the synthesis of heterocycles including BODIPY fluorophores and biotin. In addition, a series of tetrazine probes for monoacylglycerol lipase (MAGL) were synthesized and the most reactive one was applied to the labeling of endogenous MAGL in live cells.

Bridge-Clamp Bis(tetrazine)s with [N]8 π-Stacking Interactions and Azido-s-Aryl Tetrazines: Two Classes of Doubly Clickable Tetrazines

Click chemistry at a tetrazine core is useful for bioorthogonal labeling and crosslinking. Introduced here are two new classes of doubly clickable s-aryl tetrazines synthesized by Cu-catalyzed cross-coupling. Homocoupling of o-brominated s-aryl tetrazines leads to bis(tetrazine)s structurally characterized by tetrazine cores arranged face-to-face. [N]₈ π-stacking interactions are essential to the conformation. Upon inverse electron demand Diels-Alder (iEDDA) cycloaddition, the bis(tetrazine)s produce a unique staple structure. The o-azidation of s-aryl tetrazines introduces a second proximal intermolecular clickable function that leads to double click chemistry opportunities. The stepwise introduction of fluorophores and then iEDDA cycloaddition, including bioconjugation to antibodies, was achieved on this class of tetrazines. This method extends to (thio)etherification, phosphination, trifluoromethylation and the introduction of various bioactive nitrogen-based heterocycles.

Pyrazole-Tetrazole Hybrid with Trinitromethyl, Fluorodinitromethyl, or (Difluoroamino)dinitromethyl Groups: High-Performance Energetic Materials

High-nitrogen-content compounds have attracted great scientific interest and technological importance because of their unique energy content, and they find diverse applications in many fields of science and technology. Understanding of structure-property relationship trends and how to modify them is of paramount importance for their further improvement. Herein, the installation of oxygen-rich modules, C(NO₂ )₃ , C(NO₂ )₂ F, or C(NO₂ )₂ NF₂ , into an endothermic framework, that is, the combination of a nitropyrazole unit and tetrazole ring, is used as a way to design novel energetic compounds. Density, oxygen balance, and enthalpy of formation are enhanced by the presence of these oxygen-containing units. The structures of all compounds were confirmed by XRD. For crystal packing analysis, it is proposed to use new criterion, Δ_OED , that can serve as a measure of the tightness of molecular packing upon crystal formation. Overall, the materials show promising detonation and propulsion parameters.

Synthesis and Characterization of 2,2'-Dinitramino-5,5'-bi(1-oxa-3,4-diazole) and Derivatives as Economic and Highly Dense Energetic Materials

2,2'-Dinitramino-5,5'-bi(1-oxa-3,4-diazole) (2) is a new highly energetic material with superior calculated detonation performance in comparison to cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) and penta-erythritoltetranitrate (PETN) and can be prepared by an economical and practical two-step synthesis. The starting material 2,2'-diamino-5,5'-bi(1-oxa-3,4-diazole) (1) is synthesized by the reaction of oxalyl dihydrazide with cyanogen bromide. Nitration of 1 yields the title compound in perfect yield and purity. The combination of its high density of 1.986 g cm^-3 , the positive heat of formation (+190 kJ mol^-1 ), and a slightly positive oxygen balance (+6.2 %) results in ideal calculated detonation parameters (e.g. detonation velocity 9296 m s^-1 ). The sensitivities toward impact and friction can be adjusted by deprotonation and formation of corresponding nitrogen-rich salts, for example, ammonium (3), hydroxylammonium (4), and guanidinium (5) salts.

Cooperative enhancement of the nonlinear optical response in conjugated energetic materials: A TD-DFT study

Conjugated energetic molecules (CEMs) are a class of explosives with high nitrogen content that posses both enhanced safety and energetic performance properties and are ideal for direct optical initiation. As isolated molecules, they absorb within the range of conventional lasers. Crystalline CEMs are used in practice, however, and their properties can differ due to intermolecular interaction. Herein, time-dependent density functional theory was used to investigate one-photon absorption (OPA) and two-photon absorption (TPA) of monomers and dimers obtained from experimentally determined crystal structures of CEMs. OPA scales linearly with the number of chromophore units, while TPA scales nonlinearly, where a more than 3-fold enhancement in peak intensity, per chromophore unit, is calculated. Cooperative enhancement depends on electronic delocalization spanning both chromophore units. An increase in sensitivity to nonlinear laser initiation makes these materials suitable for practical use. This is the first study predicting a cooperative enhancement of the nonlinear optical response in energetic materials composed of relatively small molecules. The proposed model quantum chemistry is validated by comparison to crystal structure geometries and the optical absorption of these materials dissolved in solution.