Expanding the Limits of Organic Energetic Materials: High-Performance Alliance of 1,3,4-Thiadiazole and Furazan Scaffolds.

A majority of known and newly synthesized energetic materials comprise polynitrogen or nitrogen-oxygen heterocycles with various explosophores. However, available structural combinations of these organic scaffolds are finite and are about to reach their limits. Herein, we present the design and synthesis of a series of sulfur-containing polyazole structures comprising 1,3,4-thiadiazole and furazan rings linked by C-C bonds and enriched with energetic nitro and azo functionalities. In terms of detonation performance, all synthesized 1,3,4-thiadiazole-furazan assemblies (D = 7.7-7.9 km s-1; P = 26-28 GPa) lie between the powerful explosive TATB (D = 8.0 km s-1; P = 31 GPa) and melt-cast material TNT (D = 6.9 km s-1; P = 23 GPa). In the synthesized series, azo-bridged derivative 5 seems to be most practically interesting, as it combines a relatively high energetic performance (D = 7.9 km s-1; P = 28 GPa), a very high thermal stability (271 °C), and insensitivity to friction. By these functional properties, 5 outperforms the benchmark heat-resistant explosive hexanitrostilbene (HNS). To the best of our knowledge, this is the first example of an energetic alliance of furazan and 1,3,4-thiadiazole scaffolds and a rare case of sulfur-containing high-energy materials, which can certainly be considered as an evolutionary step in energetic materials science.

Triiodide-Based Chair-Like Copper Complex Assembled by Halogen Bonding.

Cocrystallization of the dimeric [Cu2(µ-I)2(CNXyl)4] (Xyl = 2,6-Me2C6H3, 1) and polymeric catena-[Cu(µ-I)(CNC6H3-2-Cl-6-Me)2] (2) complexes with I2 at different molar ratios between the reactants resulted in a series of (RNC)2CuI-based crystal polyiodides formed along with gradual accumulation of iodine, namely the cocrystals [1·I2]·[Cu(µ1,1-I3)(CNXyl)2]2 followed by the generation of [Cu(µ1,3-I3)(CNXyl)2]2·2I2 (5·2I2) or [Cu(µ1,1-I3)(CNC6H3-2-Cl-6-Me)2]2 and then [Cu(µ1,3-I3)(CNC6H3-2-Cl-6-Me)2]n·n/2I2. The polyiodide 5·2I2 exhibits a novel supramolecular motif─a purely inorganic halogen-bonded Cu2(µ1,3-I3)2 core in the chair conformation. The X-ray structure of 5·2I2 featuring I···I contacts was analyzed by a set of theoretical methods and attributed to moderately strong halogen bonding (from -3.2 to -3.9 kcal/mol); these interactions determine the supramolecular architecture of 5·2I2.

Electron density-based protocol to recover the interacting quantum atoms components of intermolecular binding energy.

A new approach for obtaining interacting quantum atoms-defined components of binding energy of intermolecular interactions, which bypasses the use of standard six-dimensional integrals and two-particle reduced density matrix (2-RDM) reconstruction, is proposed. To examine this approach, three datasets calculated within the density functional theory framework using the def2-TZVP basis have been explored. The first two, containing 53 weakly bound bimolecular associates and 13 molecular clusters taken from the crystal, were used in protocol refinement, and the third one containing other 20 bimolecular and three cluster systems served as a validation reference. In addition, to verify the performance of the proposed approach on an exact 2-RDM, calculations within the coupled cluster formalism were performed for part of the first set systems using the cc-pVTZ basis set. The process of optimization of the proposed parametric model is considered, and the role of various energy contributions in the formation of non-covalent interactions is discussed with regard to the obtained trends.

Regioselective Synthesis of NO-Donor (4-Nitro-1,2,3-triazolyl)furoxans via Eliminative Azide-Olefin Cycloaddition.

A facile and efficient method for the regioselective [3 + 2] cycloaddition of 4-azidofuroxans to 1-dimethylamino-2-nitroethylene under p-TSA catalysis affording (4-nitro-1,2,3-triazolyl)furoxans was developed. This transformation is believed to proceed via eliminative azide-olefin cycloaddition resulting in its complete regioselectivity. The developed protocol has a broad substrate scope and enables a straightforward assembly of the 4-nitro-1,2,3-triazole motif. Moreover, synthesized (4-nitro-1,2,3-triazolyl)furoxans were found to be capable of NO release in a broad range of concentrations, thus providing a novel platform for future drug design and related biomedical applications of heterocyclic NO donors.

New Boron Containing Acridines: Synthesis and Preliminary Biological Study.

The synthesis of the first conjugates of acridine with cobalt bis(dicarbollide) are reported. A novel 9-azido derivative of acridine was prepared through the reaction of 9-methoxyacridine with N3CH2CH2NH2, and its solid-state molecular structure was determined via single-crystal X-ray diffraction. The azidoacridine was used in a copper (I)-catalyzed azide-alkyne cycloaddition reaction with cobalt bis(dicarbollide)-based terminal alkynes to give the target 1,2,3-triazoles. DNA interaction studies via absorbance spectroscopy showed the weak binding of the obtained conjugates with DNA. The antiproliferative activity (IC50) of the boronated conjugates against a series of human cell lines was evaluated through an MTT assay. The results suggested that acridine derivatives of cobalt bis(dicarbollide) might serve as a novel scaffold for the future development of new agents for boron neutron capture therapy (BNCT).

Impact of regiochemistry in energetic materials science: a case of (nitratomethyl-1,2,3-triazolyl)furazans.

The preparation of multipurpose high-energy materials for space technologies remains a challenging task and such materials usually require special precautions and fine tunability of their functional properties. To unveil new opportunities en route to high-performance energetic materials, novel potential melt-castable explosives and energetic plasticizers incorporating a (1,2,3-triazolyl)furazan scaffold enriched with nitro and nitratomethyl explosophoric functionalities were synthesized. The successful implementation of the regiodivergent approach enabled the preparation of regioisomeric (nitratomethyltriazolyl)furazans that possessed significantly different physicochemical properties classifying the target materials as melt-castable substances or energetic plasticizers. Hirshfeld surface calculations supported by energy framework plots were also performed to better understand the relationship between the molecular structure and sensitivity. All the prepared (1,2,3-triazolyl)furazans show high nitrogen-oxygen contents (76-77%), good experimental densities (up to 1.72 g cm-3) and high positive enthalpies of formation (180-318 kJ mol-1) resulting in good detonation performances (D = 7.1-8.0 km s-1; P = 21-29 GPa). Overall, this work unveils novel strategies for the construction of balanced energetic melt-castable substances or plasticizers for various applications.

Heteroligand α-Diimine-Zn(II) Complexes with O,N,O'- and O,N,S-Donor Redox-Active Schiff Bases: Synthesis, Structure and Electrochemical Properties.

A number of novel heteroligand Zn(II) complexes (1-8) of the general type (Ln)Zn(NN) containing O,N,O'-, O,N,S-donor redox-active Schiff bases and neutral N,N'-chelating ligands (NN) were synthesized. The target Schiff bases LnH2 were obtained as a result of the condensation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with substituted o-aminophenols or o-aminothiophenol. These ligands with combination with 2,2'-bipyridine, 1,10-phenanthroline, and neocuproine are able to form stable complexes upon coordination with zinc(II) ion. The molecular structures of complexes 4∙H2O, 6, and 8 in crystal state were determined by means of single-crystal X-ray analysis. In the prepared complexes, the redox-active Schiff bases are in the form of doubly deprotonated dianions and act as chelating tridentate ligands. Complexes 6 and 8 possess a strongly distorted pentacoordinate geometry while 4∙H2O is hexacoordinate and contains water molecule coordinated to the central zinc atom. The electrochemical properties of zinc(II) complexes were studied by the cyclic voltammetry. For the studied complexes, O,N,O'- or O,N,S-donor Schiff base ligands are predominantly involved in electrochemical transformations in the anodic region, while the N,N'-coordinated neutral nitrogen donor ligands demonstrate the electrochemical activity in the cathode potential range. A feature of complexes 5 and 8 with sterically hindered tert-butyl groups is the possibility of the formation of relatively stable monocation and monoanion forms under electrochemical conditions. The values of the energy gap between the boundary redox orbitals were determined by electrochemical and spectral methods. The parameters obtained in the first case vary from 1.97 to 2.42 eV, while the optical bang gap reaches 2.87 eV.

Novel family of nitrogen-rich energetic (1,2,4-triazolyl) furoxan salts with balanced performance.

Nitrogen-rich energetic materials comprised of a combination of several heterocyclic subunits retain their leading position in the field of materials science. In this regard, a preparation of novel high-energy materials with balanced set of physicochemical properties is highly desired. Herein, we report the synthesis of a new series of energetic salts incorporating a (1,2,4-triazolyl) furoxan core and complete evaluation of their energetic properties. All target energetic materials were well characterized with IR and multinuclear NMR spectroscopy and elemental analysis, while compound 6 was further characterized by single-crystal X-ray diffraction study. Prepared nitrogen-rich salts have high thermal stability (up to 232°C), good experimental densities (up to 1.80 g cm-3) and high positive enthalpies of formation (344-1,095 kJ mol-1). As a result, synthesized energetic salts have good detonation performance (D = 7.0-8.4 km s-1; p = 22-32 GPa), while their sensitivities to impact and friction are quite low.

Renaissance of dinitroazetidine: novel hybrid energetic boosters and oxidizers.

Nitrogen-oxygen organic materials constitute an important family of multipurpose high-energy materials. However, the preparation of energetic boosters and oxidizers for various civil and space technologies remains a challenging task and such materials usually require special precautions and fine tunability of their functional properties. To find a balance between energy and safety while retaining the oxidizing ability of target energetic materials, novel hybrid organic compounds comprising furoxan and 3,3-dinitroazetidine scaffolds enriched with additional nitro groups were synthesized. The prepared 3-(3,3-dinitroazetidinoyl)-4-nitrofuroxan and 3,3-dinitro-1-(2,2,2-trinitroethyl)azetidine have high nitrogen-oxygen contents (75-79%), positive oxygen balance to CO (up to +10.3%) and good experimental densities (1.75-1.80 g cm-3). A combination of superior detonation performance (D = 8.3-8.5 km s-1 and P = 32-33 GPa) and moderate mechanical sensitivity enables the application potential of these energetic materials as booster explosives or oxidizers. Additionally, their functional properties remain essentially competitive with other oxygen-rich energetic materials (pentaerythritol tetranitrate, ammonium dinitramide, and tetranitratoethane). Hirshfeld surface calculations supported by energy framework plots were also performed to better understand the relationship between the molecular structure and stability/sensitivity. This work unveils novel directions in the construction of balanced energetic boosters and oxidizers for various applications.

Mechanical stimulation of energetic materials at the nanoscale.

The initiation of energetic materials by mechanical stimuli is a critical stage of their functioning, but remains poorly understood. Using atomic force microscopy (AFM) we explore the microscopic initiation behavior of four prototypical energetic materials: 3,4-dinitropyrazole, ε-CL-20, α-PETN and picric acid. Along with the various chemical structures, these energetic compounds cover a range of application types: a promising melt-cast explosive, the most powerful energetic compound in use, a widespread primary explosive, and a well-established nitroaromatic explosive from the early development of energetics. For the softest materials (picric acid and 3,4-dinitropyrazole), the surfaces were found to behave dynamically, quickly rearranging in response to mechanical deformation. The pit created by nanoscale friction stimulation on the surface of 3,4-dinitropyrazole doubled in volume upon aging for half an hour. Over the same time frame, a similar pit on the picric acid surface increased in volume by more than seven-fold. Remarkably, increased humidity was found to reduce the rate of surface rearrangement, potentially offering an origin for the desensitization of energetic materials when wetted. Finally, we identify an inverse correlation between the surface dynamics and mechanical sensitivity of our test energetic compounds. This strongly suggests that surface dynamics influence a material's ability to dissipate excess energy, acting as a buffer towards mechanical initiation.

Way to Enforce Selectivity via Steric Hindrance: Improvement of Am(III)/Eu(III) Solvent Extraction by Loaded Diphosphonic Acid Esters.

Hybrid donor extractants are a promising class of compounds for the separation of trivalent actinides and lanthanides. Here, we investigated a series of sterically loaded diphosphonate ligands based on bipyridine (BiPy-PO-iPr and BiPy-PO-cHex) and phenanthroline (Phen-PO-iPr and Phen-PO-cHex). We studied their complex formation with nitrates of trivalent f-elements in solvent extraction systems (Am and Eu) and homogeneous acetonitrile solutions (Nd, Eu, and Lu). Phenanthroline extractants demonstrated the highest efficiency and selectivity [SF(Am/Eu) up to 14] toward Am(III) extraction from nitric acid solutions among all of the studied diphosphonates of N-heterocycles. The binding constants established by UV-vis titration also indicated stronger binding of sterically impaired diphosphonates compared to the primary substituted diphosphonates. NMR titration and slope analysis during solvent extraction showed the formation of 2:1 complexes at high concentrations (>10-3 mol/L) for phenanthroline-based ligands. According to UV-vis titrations at low concentrations (10-5-10-6 mol/L), the phenanthroline-based ligands formed 1:1 complexes. Bipyridine-based ligands formed 1:1 complexes regardless of the ligand concentration. Luminescence titrations revealed that the quantum yields of the complexes with Eu(III) were 81 ± 8% (BiPy-PO-iPr) and 93 ± 9% (Phen-PO-iPr). Single crystals of the structures [Lu(µ2,κ4-(iPrO)2P(O)Phen(O)2(OiPr))(NO3)2]2 and Eu(Phen-PO-iPr)(NO3)3 were obtained by chemical synthesis with the Phen-PO-iPr ligand. X-ray diffraction studies revealed a closer contact of the f-element with the aromatic N atoms in the case of sterically loaded P═O ligands compared with sterically deficient ligands. Density functional theory calculations allowed us to rationalize the observed selectivity trends in terms of the bond length, Mayer bond order, and preorganization energy.

Nitrogen-rich metal-free salts: a new look at the 5-(trinitromethyl)tetrazolate anion as an energetic moiety.

A series of energetic nitrogen-rich salts comprised of a 5-(trinitromethyl)tetrazolate anion and high-nitrogen cations was synthesized by simple and efficient chemical routes from readily available commercial reagents. These energetic materials were fully characterized by IR and multinuclear NMR (1H, 13C, 14N) spectroscopy, elemental analysis, and differential scanning calorimetry (DSC). Additionally, the structure of an energetic salt containing the 3,6,7-triamino-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazolium cation was confirmed by single-crystal X-ray diffraction. The synthesized compounds exhibit decent experimental densities (1.648-1.845 g cm-3) and positive enthalpies of formation (up to 725.5 kJ mol-1) and, as a result, superior detonation performance (detonation velocities 8.2-9.2 km s-1 and detonation pressures 28.5-37.8 GPa), which is comparable to or even exceeding those of commonly used booster explosive PETN. On the other hand, high mechanical sensitivity of several novel 5-(trinitromethyl)tetrazolate salts along with their high combined nitrogen-oxygen content (>81%) and excellent detonation performance render them environmentally friendly alternatives to lead-based primary explosives.

Bifurcated Halogen Bonding Involving Two Rhodium(I) Centers as an Integrated σ-Hole Acceptor.

The complexes [RhX(COD)]2 (X = Cl, Br; COD = 1,5-cyclooctadiene) form cocrystals with σ-hole iodine donors. X-ray diffraction studies and extensive theoretical considerations indicate that the d z 2-orbitals of two positively charged rhodium(I) centers provide sufficient nucleophilicity to form a three-center halogen bond (XB) with the σ-hole donors. The two metal centers function as an integrated XB acceptor, providing assembly via a metal-involving XB.

Design and Synthesis of Nitrogen-Rich Azo-Bridged Furoxanylazoles as High-Performance Energetic Materials.

A series of novel energetic materials comprising of azo-bridged furoxanylazoles enriched with energetic functionalities was designed and synthesized. These high-energy materials were thoroughly characterized by IR and multinuclear NMR (1 H, 13 C, 14 N) spectroscopy, high-resolution mass spectrometry, elemental analysis, and differential scanning calorimetry (DSC). The molecular structures of representative amino and azo oxadiazole assemblies were additionally confirmed by single-crystal X-ray diffraction and X-ray powder diffraction. A comparison of contributions of explosophoric moieties into the density of energetic materials revealed that furoxan and 1,2,4-oxadiazole rings are the densest motifs while the substitution of the azide and amino fragments on the nitro and azo ones leads to an increase of the density. Azo bridged energetic materials have high nitrogen-oxygen contents (68.8-76.9 %) and high thermal stability. The synthesized compounds exhibit good experimental densities (1.62-1.88 g cm-3 ), very high enthalpies of formation (846-1720 kJ mol-1 ), and, as a result, excellent detonation performance (detonation velocities 7.66-9.09 km s-1 and detonation pressures 25.0-37.7 GPa). From the application perspective, the detonation parameters of azo oxadiazole assemblies exceed those of the benchmark explosive RDX, while a combination of high detonation performance and acceptable friction sensitivity of azo(1,2,4-triazolylfuroxan) make it a promising potential alternative to PETN.

Synthesis and Structure of Nido-Carboranyl Azide and Its "Click" Reactions.

Novel zwitter-ionic nido-carboranyl azide 9-N3(CH2)3Me2N-nido-7,8-C2B9H11 was prepared by the reaction of 9-Cl(CH2)3Me2N-nido-7,8-C2B9H11 with NaN3. The solid-state molecular structure of nido-carboranyl azide was determined by single-crystal X-ray diffraction. 9-N3(CH2)3Me2N-nido-7,8-C2B9H11 was used for the copper(I)-catalyzed azide-alkyne cycloaddition with phenylacetylene, alkynyl-3ß-cholesterol and cobalt/iron bis(dicarbollide) terminal alkynes to form the target 1,2,3-triazoles. The nido-carborane-cholesterol conjugate 9-3ß-Chol-O(CH2)C-CH-N3(CH2)3Me2N-nido-7,8-C2B9H11 with charge-compensated group in a linker can be used as a precursor for preparation of liposomes for Boron Neutron Capture Therapy (BNCT). A series of novel zwitter-ionic boron-enriched cluster compounds bearing a 1,2,3-triazol-metallacarborane-carborane conjugated system was synthesized. Prepared conjugates contain a large amount of boron atom in the biomolecule and potentially can be used for BNCT.

Direct Synthesis of N-(1,2,5-Oxadiazolyl)hydrazones through a Diazotization/Reduction/Condensation Cascade.

A straightforward synthesis of a series of previously unknown N-(1,2,5-oxadiazolyl)hydrazones through the diazotization/reduction/condensation cascade of amino-1,2,5-oxadiazoles was accomplished. The described protocol was suitable for a wide array of target hydrazones, which were prepared in good to high yields under smooth reaction conditions with very good functional group tolerance. Importantly, the presented approach unveils a direct route to in situ generation of previously inaccessible (1,2,5-oxadiazolyl)hydrazines. In addition, a first example of the ionic structure incorporating a protonated hydrazone motif linked to the 1,2,5-oxadiazole 2-oxide subunit was synthesized, indicating the stability of prepared compounds toward acid-promoted hydrolysis. Overall, this method provides a direct access to the isosteric analogues of drug candidates for treatment of various neglected diseases, thus enabling their potential application in medicinal chemistry and drug design.

Structure-directing sulfur...metal noncovalent semicoordination bonding.

The abundance and geometric features of nonbonding contacts between metal centers and `soft' sulfur atoms bound to a non-metal substituent R were analyzed by processing data from the Cambridge Structural Database. The angular arrangement of M, S and R atoms with ∠(R-S...M) down to 150° was a common feature of the late transition metal complexes exhibiting shortened R-S...M contacts. Several model nickel(II), palladium(II), platinum(II) and gold(I) complexes were chosen for a theoretical analysis of R-S...M interactions using the DFT method applied to (equilibrium) isolated systems. A combination of the real-space approaches, such as Quantum Theory of Atoms in Molecules (QTAIM), noncovalent interaction index (NCI), electron localization function (ELF) and Interacting Quantum Atoms (IQA), and orbital (Natural Bond Orbitals, NBO) methods was used to provide insights into the nature and energetics of R-S...M interactions with respect to the metal atom identity and its coordination environment. The explored features of the R-S...M interactions support the trends observed by inspecting the CSD statistics, and indicate a predominant contribution of semicoordination bonds between nucleophilic sites of the sulfur atom and electrophilic sites of the metal. A contribution of chalcogen bonding (that is formally opposite to semicoordination) was also recognized, although it was significantly smaller in magnitude. The analysis of R-S...M interaction strengths was performed and the structure-directing role of the intramolecular R-S...M interactions in stabilizing certain conformations of metal complexes was revealed.

Interatomic exchange-correlation interaction energy from a measure of quantum theory of atoms in molecules topological bonding: A diatomic case.

The potential relations between the measure of topological interatomic bonding-integrals of electron density with respect to internuclear axis over the corresponding quantum theory of atoms in molecules (QTAIM)-defined interatomic surface (IAS)-and interatomic exchange-correlation contributions from the interacting quantum atoms approach are discussed. The quantum chemical computations of 38 equilibrium diatomic systems at different levels of theory (HF, MP2, MP4SDQ, and CCSD) are invoked to support abstract considerations. Parameters of excellent correlations between IAS integrals and interatomic exchange-correlation energy are found by the optimization. The performance of these trends depends on the accuracy of the electronic correlation treatment. The resulting trends are a unique feature of equilibrium states, whereas more complicated dependencies are explored for several systems at non-equilibrium conditions. The relations of established trends with other IAS-based estimations of strength of bonding interactions between topological atoms and issues explored for multiatomic systems are briefly discussed.

π-Hole···dz2[PtII] Interactions with Electron-Deficient Arenes Enhance the Phosphorescence of PtII-Based Luminophores.

Two phosphorescent PtII-based cyclometalated complexes were co-crystallized with perfluorinated arenes to give 1:1 co-crystals. The X-ray study revealed that each of the complexes is embraced by arenesF to give infinite reverse sandwich structures. In four out of six structures, a dz2 orbital of PtII is directed to the arenesF ring via π-hole···dz2[PtII] interactions, whereas in the other two structures, the filled dz2 orbital is directed toward the arene C atoms. Computed molecular electrostatic potential surfaces of the arenesF and the complexes, noncovalent interaction indexes for the co-crystals, and natural bond orbital calculations indicate that π-hole···dz2[PtII] contacts (and, generally, the stacking) are of electrostatic origin. The solid-state photophysical study revealed up to 3.5-fold luminescence quantum yield and 15-fold lifetime enhancements in the co-crystals. This increase is associated with the strength of the π-hole···dz2[PtII] contact that is dependent on the π-acidity of the areneF and its spatial characteristics.

Revisiting van der Waals Radii: From Comprehensive Structural Analysis to Knowledge-Based Classification of Interatomic Contacts.

The front cover artwork is provided by the TheoMAT group of ITMO University (Russia) and the Inorganic Systems Engineering Group of Delft University of Technology (The Netherlands). The image illustrates how one can find the most probable interatomic distance and determine the van der Waals parameters for interatomic interaction from extended and diverse structural datasets. The new approach for background elimination and analysis of extended bulk structural datasets is reported in our paper. Read the full text of the Article at 10.1002/cphc.201901083.