High-Energy-Density Material with Magnetically Modulated Ignition.

Preparation of a redox-frustrated high-energy-density energetic material is achieved by gentle protolysis of Mn[N(SiMe3)2]2 with the perchlorate salt of the tetrazolamide [H2NtBuMeTz]ClO4 (Tz = tetrazole), yielding the Mn6N6 hexagonal prismatic cluster, Mn6(µ3-NTztBuMe)6(ClO4)6. Quantum mechanics-based molecular dynamics simulations of the decomposition of this molecule predict that magnetic ordering of the d5 Mn2+ ions influences the pathway and rates of decomposition, suggesting that the initiation of decomposition of the bulk material might be significantly retarded by an applied magnetic field. We report here experimental tests of the prediction showing that the presence of a 0.5 T magnetic field modulates the ignition onset temperature by +10.4 ± 3.9 °C (from 414 ± 4 °C), demonstrating the first example of a magnetically modulated explosive.

A soft co-crystalline solid electrolyte for lithium-ion batteries.

Alternative solid electrolytes are the next key step in advancing lithium batteries with better thermal and chemical stability. A soft solid electrolyte, (Adpn)2LiPF6 (Adpn, adiponitrile), is synthesized and characterized that exhibits high thermal and electrochemical stability and good ionic conductivity, overcoming several limitations of conventional organic and ceramic materials. The surface of the electrolyte possesses a liquid nano-layer of Adpn that links grains for a facile ionic conduction without high pressure/temperature treatments. Further, the material can quickly self-heal if fractured and provides liquid-like conduction paths via the grain boundaries. A substantially high ion conductivity (~10-4 S cm-1) and lithium-ion transference number (0.54) are obtained due to weak interactions between 'hard' (charge dense) Li+ ions and the 'soft' (electronically polarizable) -C≡N group of Adpn. Molecular simulations predict that Li+ ions migrate at the co-crystal grain boundaries with a (preferentially) lower activation energy Ea and within the interstitial regions between the co-crystals with higher Ea values, where the bulk conductivity is a smaller but extant contribution. These co-crystals establish a special concept of crystal design to increase the thermal stability of LiPF6 by separating ions in the Adpn solvent matrix, and also exhibit a unique mechanism of ion conduction via low-resistance grain boundaries, which contrasts with ceramics or gel electrolytes.

Metal-Free Reversible Double Cyclization of Cyanuric Diazide to an Asymmetric Bitetrazolate via Cleavage of the Six-Membered Aromatic Ring.

Crystallization of the reaction mixture of 2-amino-4,6-diazido-1,3,5-triazine and excess tert-butylamine results in the isolation of tert-butylammonium N,N-[1'H-(1,5'-bitetrazol)-5-yl]cyanamidate, suggesting a complex decyclization/cyclization rearrangement involving breakage of the six-membered aromatic ring and the formation of two new five-membered azole rings mediated by deprotonation of the precursor by the amine. The addition of tert-butylamine to 2-amino-4,6-diazido-1,3,5-triazine gives spectroscopic indication of thermodynamically unfavorable reactivity in low-dielectric solvents, and high-level quantum chemical computations also suggest its formation to be unfavorable. A computed interconversion pathway describes the likely reaction mechanism and supports the general thermodynamic unfavorability of the reaction and the requirement for a high-dielectric environment to template formation of the ionic product and its trapping by crystallization.

Titanium(II) as a Fuel Atom in Energetic Materials.

The energetic content of the compounds MgTp2, FeTp2, MnTp2, and TiTp2 is measured by bomb calorimetry and compared to theoretical calculations (Tp = trispyrazoylborate). TiTp2 had the largest heat of combustion of the four compounds. Comparison of the heat of combustion of the Ti complex to those of Mg and Mn complexes suggests an effective combustion energy of TiII of between 1400 and 3000 kJ/mol, affirming the role of TiII as a strong fuel atom.

Properties and Promise of Catenated Nitrogen Systems As High-Energy-Density Materials.

The properties of catenated nitrogen molecules, molecules containing internal chains of bonded nitrogen atoms, is of fundamental scientific interest in chemical structure and bonding, as nitrogen is uniquely situated in the periodic table to form kinetically stable compounds often with chemically stable N-N bonds but which are thermodynamically unstable in that the formation of stable multiply bonded N2 is usually thermodynamically preferable. This unique placement in the periodic table makes catenated nitrogen compounds of interest for development of high-energy-density materials, including explosives for defense and construction purposes, as well as propellants for missile propulsion and for space exploration. This review, designed for a chemical audience, describes foundational subjects, methods, and metrics relevant to the energetic materials community and provides an overview of important classes of catenated nitrogen compounds ranging from theoretical investigation of hypothetical molecules to the practical application of real-world energetic materials. The review is intended to provide detailed chemical insight into the synthesis and decomposition of such materials as well as foundational knowledge of energetic science new to most chemists.

Reaction Mechanism and Energetics of Decomposition of Tetrakis(1,3-dimethyltetrazol-5-imidoperchloratomanganese(II)) from Quantum-Mechanics-based Reactive Dynamics.

Energetic materials (EMs) are central to construction, space exploration, and defense, but over the past 100 years, their capabilities have improved only minimally as they approach the CHNO energetic ceiling, the maximum energy density possible for EMs based on molecular carbon-hydrogen-nitrogen-oxygen compounds. To breach this ceiling, we experimentally explored redox-frustrated hybrid energetic materials (RFH EMs) in which metal atoms covalently connect a strongly reducing fuel ligand (e.g., tetrazole) to a strong oxidizer (e.g., ClO4). In this Article, we examine the reaction mechanisms involved in the thermal decomposition of an RFH EM, [Mn(Me2TzN)(ClO4]4 (3, Tz = tetrazole). We use quantum-mechanical molecular reaction dynamics simulations to uncover the atomistic reaction mechanisms underlying this decomposition. We discover a novel initiation mechanism involving oxygen atom transfer from perchlorate to manganese, generating energy that promotes the fission of tetrazole into chemically stable species such as diazomethane, diazenes, triazenes, and methyl azides, which further undergo exothermic decomposition to finally form stable N2, H2O, CO, CO2, Mn-based clusters, and additional incompletely combusted products.

Metal-Binding Q-Proline Macrocycles.

We introduce the efficient Fmoc-SPPS and peptoid synthesis of Q-proline-based, metal-binding macrocycles (QPMs), which bind metal cations and display nine functional groups. Metal-free QPMs are disordered, evidenced by NMR and a crystal structure of QPM-3 obtained through racemic crystallization. Upon addition of metal cations, QPMs adopt ordered structures. Notably, the addition of a second functional group at the hydantoin amide position (R2) converts the proline ring from Cγ-endo to Cγ-exo, due to steric interactions.

Multinuclear Clusters of Manganese and Lithium with Silsesquioxane-Derived Ligands: Synthesis and Ligand Rearrangement by Dioxygen- and Base-Mediated Si-O Bond Cleavage.

The synthesis of manganese cluster complexes templated by polyhedral oligomeric silsesquioxane-derived ligands is described. MnII3(Ph7Si7O12)2Pyr4 (1) and MnII4(Ph4Si4O8)2(Bpy)2(Py)2 (3) are prepared by replacement of the amide ligands of Mn(NR2)2 (R = SiMe3) via ligand protolysis by the acidic proton of the respective silsesquioxane-derived silanols. Complex 1 is shown to undergo ligand rearrangement by reaction with O2, which results in oxidation of the cluster to a mixed MnII/III cluster, concomitant with cleavage of the Si-O bonds of the ligand, releasing a [Ph2Si-O]+ unit, opening a new ligating siloxide group, and resulting in the formation of Mn3(Ph6Si6O11)2Pyr4 (2). The ligand framework of 1 can also be perturbed by a base. The addition of LiOH/BuLi delivers a soluble equivalent of Li2O to 1, resulting in cleavage of the Si-O bonds and linkage of the resulting exposed silicon atoms by the new oxide, giving a linked ligand variant that templates a Li2Mn3 cluster, Mn3Li2(Ph7Si7O12OPh7Si7O12)DMF5Pyr (4). These systems are characterized by single-crystal X-ray diffraction, absorption spectroscopy, Fourier transform infrared, cyclic voltammetry, and CHN combustion analysis. Mechanistic implications for the Si-O bond cleavage events are discussed.

Redox properties of birnessite from a defect perspective.

Birnessite, a layered-structure MnO2, is an earth-abundant functional material with potential for various energy and environmental applications, such as water oxidation. An important feature of birnessite is the existence of Mn(III) within the MnO2 layers, accompanied by interlayer charge-neutralizing cations. Using first-principles calculations, we reveal the nature of Mn(III) in birnessite with the concept of the small polaron, a special kind of point defect. Further taking into account the effect of the spatial distribution of Mn(III), we propose a theoretical model to explain the structure-performance dependence of birnessite as an oxygen evolution catalyst. We find an internal potential step which leads to the easy switching of the oxidation state between Mn(III) and Mn(IV) that is critical for enhancing the catalytic activity of birnessite. Finally, we conduct a series of comparative experiments which support our model.

Activation of C-H, N-H, and O-H Bonds via Proton-Coupled Electron Transfer to a Mn(III) Complex of Redox-Noninnocent Octaazacyclotetradecadiene, a Catenated-Nitrogen Macrocyclic Ligand.

Reaction of 1,3-diazidopropane with an electron-rich Mn(II) precursor results in oxidation of the metal center to a Mn complex with concomitant assembly of the macrocyclic ligand into the 1,2,3,4,8,9,10,11-octaazacyclotetradeca-2,9-diene-1,4,8,11-tetraido (OIM) ligand. Although describable as a Werner Mn(V) complex, analysis by X-ray diffraction, magnetic measurements, X-ray photoelectron spectroscopy, cyclic voltammetry, and density functional theory calculations suggest an electronic structure consisting of a Mn(III) metal center with a noninnocent OIM diradical ligand. The resulting complex, (OIM)Mn(NH tBu), reacts via proton-coupled electron transfer (PCET) with phenols to form phenoxyl radicals, with dihydroanthracene to form anthracene, and with (2,4-di tert-butyltetrazolium-5-yl)amide to extrude a tetrazyl radical. PCET from the latter generates the isolable corresponding one-electron reduced compound with a neutral, zwitterionic axial 2,4-di tert-butyltetrazolium-5-yl)amido ligand. Electron paramagnetic resonance and density functional theoretical analyses suggest an electronic structure wherein the manganese atom remains Mn(III) and the OIM ligand has been reduced by one electron to a monoradical noninnocent ligand. The result indicates PCET processes whereby the proton is transferred to the axial ligand to extrude tBuNH2, the electron is transferred to the equatorial ligand, and the central metal remains relatively unperturbed.

Solution and Solid State Properties for Low-Spin Cobalt(II) Dibenzotetramethyltetraaza[14]annulene [(tmtaa)CoII] and the Monopyridine Complex.

The single-crystal X-ray structure of solvent-free (tmtaa)CoII reveals three different π-π intermacrocyclic interactions between tmtaa units (tmtaa = dibenzotetramethyltetraaza[14]annulene). Pairs of inequivalent (tmtaa)CoII units in the unit cell link into a one-dimensional π-π stacked array in the solid state. Magnetic susceptibility (χ) studies from 300 to 2 K reveal the effects of intermolecular interactions between (tmtaa)CoII units in the solid state. The effective magnetic moment per CoII center is constant at 2.83 µB from 300 to 100 K and begins to significantly decrease at lower temperatures. The magnetic data are fit to a singlet ( S = 0) ground state with a triplet ( S = 1) excited state that is 13 cm-1 higher in energy (-2 J = 13 cm-1). Toluene solutions of (tmtaa)CoII have 1H nuclear magnetic resonance (NMR) paramagnetic shifts, a solution-phase magnetic moment µeff (295 K) of 2.1 µB, and toluene glass electron paramagnetic resonance spectra that are most consistent with a low-spin ( S = 1/2) CoII with the unpaired electron located in the d yz orbital. Pyridine interacts with (tmtaa)CoII to form a five-coordinate monopyridine complex in which the unpaired electron is in the d z2 orbital. The five-coordinate complex has been structurally characterized by single-crystal X-ray diffraction, and the equilibrium constant for pyridine binding at 295 K has been evaluated by both electronic and 1H NMR spectra. Density functional theory computation using the UB3LYP hybrid functional places the unpaired electron for (tmtaa)CoII in the d yz orbital and that for the monopyridine complex in the d z2 orbital, consistent with spectroscopic observations.

Magnetism and EPR Studies of Binuclear Ruthenium Hydride Binuclear Species Bearing Redox-Active Ligands.

The binuclear complex {[N3]Ru(H)}2(µ-η1:η1-N2) ([N3] = 2,6-(ArylN═CMe)2C5H3N and Aryl = mesityl or xylyl) contains two formally Ru(I), d7 centers linked by a bridging dinitrogen ligand, although the odd electrons are substantially delocalized onto the redox non-innocent pincer ligands. The complex exhibits paramagnetic behavior in solution, but is diamagnetic in the solid state. This difference is attributed to intermolecular "π-stacking" observed in the solid state, which essentially couples unpaired electrons on each half of the complex to form delocalized 22-center-2-electron covalent bonds. Introduction of a bulky t-butyl group on the ligand pyridine ring prevents this intermolecular association and allows further investigation of the magnetic behavior and electronic structure of the binuclear species. The interaction of the unpaired electrons in the two halves of the complex has been probed with magnetic susceptibility and perpendicular and parallel mode EPR measurements, revealing a weakly antiferromagnetically coupled system with a thermally accessible triplet excited state. In addition, the mixed valent, S = 1/2, {[N3]Ru(H)}(µ-η1:η1-N2){[N3]Ru} system has also been observed via perpendicular mode EPR and was used to quantify the growth of the thermally accessible triplet state of the dihydride complex using parallel mode EPR.

Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis.

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O2/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.

Reactive Pendant Mn═O in a Synthetic Structural Model of a Proposed S4 State in the Photosynthetic Oxygen Evolving Complex.

The molecular mechanism of the Oxygen Evolving Center of photosystem II has been under debate for decades. One frequently cited proposal is the nucleophilic attack by water hydroxide on a pendant Mn═O moiety, though no chemical example of this reactivity at a manganese cubane cluster has been reported. We describe here the preparation, characterization, and a reactivity study of a synthetic manganese cubane cluster with a pendant manganese-oxo moiety. Reaction of this cluster with alkenes results in oxygen and hydrogen atom transfer reactions to form alcohol- and ketone-based oxygen-containing products. Nitrene transfer from core imides is negligible. The inorganic product is a cluster identical to the precursor, but with the pendant Mn═O moiety replaced by a hydrogen abstracted from the organic substrate, and is isolated in quantitative yield. 18O and 2H isotopic labeling studies confirm the transfer of atoms between the cluster and the organic substrate. The results suggest that the core cubane structure of this model compound remains intact, and that the pendant Mn═O moiety is preferentially reactive.

Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction.

We investigated the dependence of the electrocatalytic activity for the oxygen evolution reaction (OER) on the interlayer distance of five compositionally distinct layered manganese oxide nanostructures. Each individual electrocatalyst was assembled with a different alkali metal intercalated between two nanosheets (NS) of manganese oxide to form a bilayer structure. Manganese oxide NS were synthesized via the exfoliation of a layered material, birnessite. Atomic force microscopy was used to determine the heights of the bilayer catalysts. The interlayer spacing of the supported bilayers positively correlates with the size of the alkali cation: NS/Cs+/NS > NS/Rb+/NS > NS/K+/NS > NS/Na+/NS > NS/Li+/NS. The thermodynamic origins of these bilayer heights were investigated using molecular dynamics simulations. The overpotential (η) for the OER correlates with the interlayer spacing; NS/Cs+/NS has the lowest η (0.45 V), while NS/Li+/NS exhibits the highest η (0.68 V) for OER at a current density of 1 mA/cm2. Kinetic parameters (η and Tafel slope) associated with NS/Cs+/NS for the OER were superior to that of the bulk birnessite phase, highlighting the structural uniqueness of these nanoscale assemblies.

Manganese-Mediated Linkage of Perchlorate to Aminotetrazoles Produces Twice the Energy Density of the Unmetalated Salt.

New regimes of energy density in energetic materials may be achieved by replacement of oxidizing nitro groups with stronger oxidants such as oxychlorine species. We report the energetic material 1,3-di-tert-butyltetrazolium-5-imidoperchloratomanganese(II) 2, which crystallizes as an oligomeric cubane cluster. Bomb calorimetry on 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 traditional energetic nitramines, and ≈70 % higher than that of the unmetalated tetrazolium perchlorate salt without metal linkage. The effects of solid-state structure, charge, and lattice energy on the energy of 2 are discussed.

Covalent Metal-Metal-Bonded Mn4 Tetrahedron Inscribed within a Four-Coordinate Manganese Cubane Cluster, As Evidenced by Unexpected Temperature-Independent Diamagnetism.

The electronic structures of the manganese(IV) cubane cluster Mn(µ3-NtBu)4(NtBu)4 (1) and its one-electron-oxidized analogue, the 3:1 MnIV/MnV cluster [Mn(µ3-NtBu)4(NtBu)4]+[PF6]- (1+[PF6]), are described. The S = 0 spin quantum number of 1 is explained by a diamagnetic electronic structure where all metal-based d electrons are paired in Mn-Mn bonding orbitals. Temperature- and power-dependent studies of the S = 1/2 electron paramagnetic resonance signal of 1+ are consistent with an electronic structure described as a delocalized one-electron radical.

Synthesis and Structure of 2,5-Bis[N-(2,6-mesityl)iminomethyl]pyrrolylcobalt(II): Evidence for One-Electron-Oxidized, Redox Noninnocent Ligand Behavior.

The title complex Co(Mes2pyr)Cl2 is prepared from CoCl2 and (Mes2pyr)H. Instead of the expected (Mes2pyr)CoCl complex, a complex with formula (Mes2pyr)CoCl2 is isolated wherein the angle strain of the Mes2pyr ligand results in metal ligation by only two of the three ligand donor atoms. Careful examination of structural, spectroscopic, and magnetic features indicates this compound is best described as a complex of high-spin Co(II) with a neutral radical ligand.

Electronic Structure of Manganese Complexes of the Redox-Non-innocent Tetrazene Ligand and Evidence for the Metal-Azide/Imido Cycloaddition Intermediate.

The first synthetic manganese tetrazene complexes are described as a redox pair comprising anionic [Mn(N4 Ad2 )2 ](-) (1) and neutral Mn(N4 Ad2 )2 (2) complexes (N4 Ad2 =[Ad-N-N=N-N-Ad](2-) ). Compound 1 is obtained in two forms as lithium salts, one as a cationic Li2 Mn cluster, and one as a Mn-Li 1D ionic polymer. Compound 1 is electronically described as a Mn(III) center with two [N4 Ad2 ](2-) ligands. The one-electron oxidized 2 is crystalized in two morphologies, one as pure 2 and one as an acetonitrile adduct. Despite similar composition, the behavior of 2 differs in the two morphologies. Compound 2-MeCN is relatively air and temperature stable. Crystalline 2, on the other hand, exhibits a compositional, dynamic disorder wherein the tetrazene metallacycle ring-opens into a metal imide/azide complex detectable by X-ray crystallography and FTIR spectroscopy. Electronic structure of 2 was examined by EPR and XPS spectroscopies and DFT calculations, which indicate 2 is best described as a Mn(III) ion with an anion radical delocalized across the two ligands through spin-polarization effects.

Concise Syntheses of bis-Strychnos Alkaloids (-)-Sungucine, (-)-Isosungucine, and (-)-Strychnogucine†B from (-)-Strychnine.

The first chemical syntheses of complex, bis-Strychnos alkaloids (-)-sungucine (1), (-)-isosungucine (2), and (-)-strychnogucine B (3) from (-)-strychnine (4) is reported. Key steps included (1) the Polonovski-Potier activation of strychnine N-oxide; (2) a biomimetic Mannich coupling to forge the signature C23-C5' bond that joins two monoterpene indole monomers; and (3) a sequential HBr/NaBH3 CN-mediated reduction to fashion the ethylidene moieties in 1-3. DFT calculations were employed to rationalize the regiochemical course of reactions involving strychnine congeners.